Preface

It is my utmost pleasure to write the preface for this book entitled "Fillers", which is written by different researchers in the world. Fillers, in general, are substances that can be added to various polymer systems to reduce the cost or improve their properties (mechanical properties, visual performance, and other physical-chemistry performances). It can be either organic or inorganic, and added into the main matrix in different ways as a solid, liquid, or gas with different dimension scales, including macro, micro, even nanoscale.

How they can be used efficiently in wood adhesive, plastic, poly (lactic acid, PLA)/ rubber composite, and packaging? The aim of this book is to present the most updated information on the fillers in different application fields. In addition, the most recent research achievements will also be covered.

I fully believe that the authors' excellent work will have significant academic value and a far-reaching impact, with extensive applicability and practical significance to the polymers industry.

> **Dr. Xiaojian Zhou** Southwest Forestry University, Panlong District, Kunming, Yunnan Province, China

**1**

**Chapter 1**

**Abstract**

**1. Introduction**

resistance, and biodegradability [1–4].

*Abbas Hasan Faris*

Fillers in Wood Adhesives

The introduction of a second component to polymers has been presented; this component is often used to modify the characteristics of the products and to acquire new polymer materials with improved properties. Composite materials have a pivotal role in industries that are now considered the most progressive worldwide. At present, synthetic adhesives based on formaldehyde such as phenolformaldehyde (PF), urea formaldehyde (UF), and melamine formaldehyde (MF) are predominantly used for wood composite production, and these adhesives are commonly used in the wood panel industry. These adhesives have some advantages and disadvantages. The use of PF adhesives is as important as UF adhesives in the wood panel industry. However, their application is still limited because of its brittleness, brown color, high curing temperature, long curing time, and toxicity due to liberation of phenol and formaldehyde. A variety of methods have been used to improve the performance of UF and PF adhesives as well as to expand their use. These methods are widely used in the industry; they include the simple addition of fillers. Moreover, the addition of fillers could reduce shrinkage and alleviate the

stress on the glue line, which improves the hardness and durability.

**Keywords:** bonding strength, PF, wheat flour, tannin adhesive, organoclay

The introduction of a second component to polymers has been presented; this component is often used to modify the characteristics of the products and to acquire new polymer materials with improved properties. Composite materials have a pivotal role in industries that are now considered the most progressive worldwide. Generally, the term "composite" is given to materials made of more than one component. Polymer composites are a mixture of polymers with inorganic or organic additives that have certain geometric forms (spheres, flakes, fibers, and particulates). A wide range of polymer characteristics can be improved by composite technologies, such as their mechanical, thermal, barrier, flame retardant, magnetic, optical, and electrical properties and durability, chemical stability, corrosion

Wood adhesives now represent a vital aspect of the wood-based panel industry, and synthetic condensation resins have become widely used. The synthetic resins used can be classified into four varieties, and all these adhesives are based on formaldehyde. These resins are urea formaldehyde (UF), phenol-formaldehyde (PF), resorcinol formaldehyde (RF), and melamine formaldehyde (MF). The PF resin is used in wood adhesives, thermal insulation materials, coatings, molding compounds, and other applications. The high stability of the C-C linkages between the aromatic ring and methylene bridge and the resistance to hydrolysis make it a favorite resin for

## **Chapter 1** Fillers in Wood Adhesives

*Abbas Hasan Faris*

### **Abstract**

The introduction of a second component to polymers has been presented; this component is often used to modify the characteristics of the products and to acquire new polymer materials with improved properties. Composite materials have a pivotal role in industries that are now considered the most progressive worldwide. At present, synthetic adhesives based on formaldehyde such as phenolformaldehyde (PF), urea formaldehyde (UF), and melamine formaldehyde (MF) are predominantly used for wood composite production, and these adhesives are commonly used in the wood panel industry. These adhesives have some advantages and disadvantages. The use of PF adhesives is as important as UF adhesives in the wood panel industry. However, their application is still limited because of its brittleness, brown color, high curing temperature, long curing time, and toxicity due to liberation of phenol and formaldehyde. A variety of methods have been used to improve the performance of UF and PF adhesives as well as to expand their use. These methods are widely used in the industry; they include the simple addition of fillers. Moreover, the addition of fillers could reduce shrinkage and alleviate the stress on the glue line, which improves the hardness and durability.

**Keywords:** bonding strength, PF, wheat flour, tannin adhesive, organoclay

#### **1. Introduction**

The introduction of a second component to polymers has been presented; this component is often used to modify the characteristics of the products and to acquire new polymer materials with improved properties. Composite materials have a pivotal role in industries that are now considered the most progressive worldwide. Generally, the term "composite" is given to materials made of more than one component. Polymer composites are a mixture of polymers with inorganic or organic additives that have certain geometric forms (spheres, flakes, fibers, and particulates). A wide range of polymer characteristics can be improved by composite technologies, such as their mechanical, thermal, barrier, flame retardant, magnetic, optical, and electrical properties and durability, chemical stability, corrosion resistance, and biodegradability [1–4].

Wood adhesives now represent a vital aspect of the wood-based panel industry, and synthetic condensation resins have become widely used. The synthetic resins used can be classified into four varieties, and all these adhesives are based on formaldehyde. These resins are urea formaldehyde (UF), phenol-formaldehyde (PF), resorcinol formaldehyde (RF), and melamine formaldehyde (MF). The PF resin is used in wood adhesives, thermal insulation materials, coatings, molding compounds, and other applications. The high stability of the C-C linkages between the aromatic ring and methylene bridge and the resistance to hydrolysis make it a favorite resin for

**Figure 1.**

*Synthesis of phenol-formaldehyde resins by polycondensation of phenols, in excess of formaldehyde developed by Baekeland.*

glue lines as well as boards, such as weatherproof plywood, oriented strand boards (OSB), medium-density fiber boards (MDF), or particleboards for use under exterior weather conditions. The global demand for PF resin (resol, novolac, and others) continues to increase; recent reports expect this demand to grow at an annual rate of 5.7% from 2014 to 2019 by \$19.31 billion. This increased demand is associated with the increased construction in developing regions [5]. The resol phenolic resin accounted for more than 75% of the global market in 2013. This market is expected to grow at a healthy rate of 4.1% until 2019. The PF resin is obtained via condensation reactions between phenol and formaldehyde in the presence of an acid or a base to produce novolac or resol, respectively. Resol-type PF resins (**Figure 1**) [6] have been widely used for decades to manufacture wood adhesives due to their high performance in terms of the mechanical and thermal properties as well as its water resistance.

The concern regarding crude oil supplies and the toxicity of the raw materials extracted from the fossil fuels has grown as they can cause climate change. According to findings conducted by the National Cancer Institute (NCI) on a group of workers who were exposed continuously to formaldehyde, a high possibility of cancer was diagnosed, prompting the World Health Organization to the reclassification of formaldehyde to group I. Wood-based panel industry relies heavily on this material, making it difficult to impose an outright ban on the use of formaldehyde.

Recently, the development of natural or green-based wood adhesives as successful substitutes for synthetic resins has become of interest because of the unwarranted increase in the prices of fossil fuels as well as environmental and health concerns. Lignin and tannin are materials rich in phenolic compounds; both were found to be successful alternatives for phenol in the manufacturing of bio-based PF resins.

#### **2. Fillers**

Fillers are substances that can be added to various polymer systems to reduce the cost or improve their properties [7, 8]. This material can be added as a solid, liquid, or gas. For example, the use of a minimal percentage of clay loading can lead to significant improvements in the mechanical and thermal properties [7, 8]. In general, the fillers used to modify the properties of polymers can be classified into two categories: inert fillers and active fillers. Inert fillers come from inorganic mineral powders, such as kaolin, diammonium phosphate, porcelain clay (which is frequently used), sodium silicate, and magnesium oxide. These fillers are used to reduce cost. These mostly hydrophilic materials can be dispersed in adhesives. Active fillers are organic compounds, which swell when dissolved in a solution.

**3**

**Figure 2.**

*Montmorillonite structure.*

*Fillers in Wood Adhesives*

nosilicate ([SiO4]

poly(ethylene oxide) [8].

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

<sup>4</sup><sup>−</sup> and [AlO4]

These fillers include the cellulose-type fillers (wood powder, bark powder, etc.), protein-type fillers (soy protein, blood powder, etc.), and starch-type fillers (wheat

Clays are one group of the additives that have been widely used in the preparation of polymeric composite materials. Recently, increasing attention has been given to the development of polymer/clay nanocomposites because of their dramatically improved properties as compared with conventional fillers after the addition of very low portions of the filler [9]. Clay minerals are used in soil science and sedimentology to refer to particles formed by the combination of tetrahedral and octahedral alumi-

dimensions, as shown in **Figure 2** [10]. Clay minerals are important materials that are mainly hydrated aluminosilicates with neutral or negative charges [11]. The internal

The hydrophilic nature of clays makes them poorly suited for mixing with most

For these reasons, clay requires pretreatment before it is used as filler in polymer composites. These piles of clay platelets have dimensions much larger than 1 nm. Therefore, untreated clay would be ineffective during use because most of the clay would be trapped inside and unable to react with the polymer matrix. Generally, the intercalation of clay with different organic species is the basic condition for compatible composite materials to ensure the interaction between the clay surfaces and the polymeric components. The intercalation or surface modifications decrease the surface energy of clay layers and match their surface polarity with polymer polarity. The low surface energy of organoclays is more compatible with organic polymers and can intercalate with the interlayer space under specific experimental conditions.

and external cations can be changed by inorganic or organic cations [10, 12].

hydrophobic polymer materials [13]. In addition, the poor physical interaction between the organic components in polymeric materials and the inorganic components in clay leads to their separation and the formation of discrete phases. Therefore their mixtures have poor properties, and this incompatibility prevents the dispersion of clay layers within polymer matrix, thereby leading to weak interfacial interactions [14–16]. Moreover, electrostatic forces tightly link the clay platelets with each other. Only some hydrophilic polymers are miscible with layers of clay; these materials are used in the preparation of polymer-clay composites such as poly(vinyl alcohol) and

<sup>5</sup><sup>−</sup>) layers that are 1 nm thick with 200–300 nm lateral

flour, etc.). These materials have the ability to react with resins.

#### *Fillers in Wood Adhesives DOI: http://dx.doi.org/10.5772/intechopen.92150*

*Fillers*

**Figure 1.**

*by Baekeland.*

glue lines as well as boards, such as weatherproof plywood, oriented strand boards (OSB), medium-density fiber boards (MDF), or particleboards for use under exterior weather conditions. The global demand for PF resin (resol, novolac, and others) continues to increase; recent reports expect this demand to grow at an annual rate of 5.7% from 2014 to 2019 by \$19.31 billion. This increased demand is associated with the increased construction in developing regions [5]. The resol phenolic resin accounted for more than 75% of the global market in 2013. This market is expected to grow at a healthy rate of 4.1% until 2019. The PF resin is obtained via condensation reactions between phenol and formaldehyde in the presence of an acid or a base to produce novolac or resol, respectively. Resol-type PF resins (**Figure 1**) [6] have been widely used for decades to manufacture wood adhesives due to their high performance in terms of the mechanical and thermal properties as well as its water resistance. The concern regarding crude oil supplies and the toxicity of the raw materials extracted from the fossil fuels has grown as they can cause climate change. According to findings conducted by the National Cancer Institute (NCI) on a group of workers who were exposed continuously to formaldehyde, a high possibility of cancer was diagnosed, prompting the World Health Organization to the reclassification of formaldehyde to group I. Wood-based panel industry relies heavily on this material, making it difficult to impose an outright ban on the use of formaldehyde. Recently, the development of natural or green-based wood adhesives as successful substitutes for synthetic resins has become of interest because of the unwarranted increase in the prices of fossil fuels as well as environmental and health concerns. Lignin and tannin are materials rich in phenolic compounds; both were found to be successful alternatives for phenol in the manufacturing of bio-based PF resins.

*Synthesis of phenol-formaldehyde resins by polycondensation of phenols, in excess of formaldehyde developed* 

Fillers are substances that can be added to various polymer systems to reduce the cost or improve their properties [7, 8]. This material can be added as a solid, liquid, or gas. For example, the use of a minimal percentage of clay loading can lead to significant improvements in the mechanical and thermal properties [7, 8]. In general, the fillers used to modify the properties of polymers can be classified into two categories: inert fillers and active fillers. Inert fillers come from inorganic mineral powders, such as kaolin, diammonium phosphate, porcelain clay (which is frequently used), sodium silicate, and magnesium oxide. These fillers are used to reduce cost. These mostly hydrophilic materials can be dispersed in adhesives. Active fillers are organic compounds, which swell when dissolved in a solution.

**2**

**2. Fillers**

These fillers include the cellulose-type fillers (wood powder, bark powder, etc.), protein-type fillers (soy protein, blood powder, etc.), and starch-type fillers (wheat flour, etc.). These materials have the ability to react with resins.

Clays are one group of the additives that have been widely used in the preparation of polymeric composite materials. Recently, increasing attention has been given to the development of polymer/clay nanocomposites because of their dramatically improved properties as compared with conventional fillers after the addition of very low portions of the filler [9]. Clay minerals are used in soil science and sedimentology to refer to particles formed by the combination of tetrahedral and octahedral aluminosilicate ([SiO4] <sup>4</sup><sup>−</sup> and [AlO4] <sup>5</sup><sup>−</sup>) layers that are 1 nm thick with 200–300 nm lateral dimensions, as shown in **Figure 2** [10]. Clay minerals are important materials that are mainly hydrated aluminosilicates with neutral or negative charges [11]. The internal and external cations can be changed by inorganic or organic cations [10, 12].

The hydrophilic nature of clays makes them poorly suited for mixing with most hydrophobic polymer materials [13]. In addition, the poor physical interaction between the organic components in polymeric materials and the inorganic components in clay leads to their separation and the formation of discrete phases. Therefore their mixtures have poor properties, and this incompatibility prevents the dispersion of clay layers within polymer matrix, thereby leading to weak interfacial interactions [14–16]. Moreover, electrostatic forces tightly link the clay platelets with each other. Only some hydrophilic polymers are miscible with layers of clay; these materials are used in the preparation of polymer-clay composites such as poly(vinyl alcohol) and poly(ethylene oxide) [8].

For these reasons, clay requires pretreatment before it is used as filler in polymer composites. These piles of clay platelets have dimensions much larger than 1 nm. Therefore, untreated clay would be ineffective during use because most of the clay would be trapped inside and unable to react with the polymer matrix. Generally, the intercalation of clay with different organic species is the basic condition for compatible composite materials to ensure the interaction between the clay surfaces and the polymeric components. The intercalation or surface modifications decrease the surface energy of clay layers and match their surface polarity with polymer polarity. The low surface energy of organoclays is more compatible with organic polymers and can intercalate with the interlayer space under specific experimental conditions.

**Figure 2.** *Montmorillonite structure.*

**Figure 3.** *Scheme of the modification of clay layers by organic onium cations.*

This spacing of organoclays is affected by a variety of factors, including the chemical structure of the surfactant, the degree of cation exchange, and the silicate layer thickness [17]. Organically modified clays have been extensively studied in various practical applications in the field of organic-inorganic hybrids, composites, and nanoscale composites [15]. The surface modification of clay layers can be performed through an ion exchange process via the replacement of cations, such as sodium and calcium, in the interlayer space by ammonium or phosphonium surfactants, which usually include benzyl groups and short aliphatic chains [18]. In addition to modifying the surface and increasing the hydrophobicity of clay layers, the introduction of alkylphosphonium (R4P+ X<sup>−</sup>) and alkylammonium (R4N+ X<sup>−</sup>) cations into the clay layers increases the distances and spacing of the clay layers, which facilitates the intercalation of the polymer chain during the preparation of nanocomposites [19]. Moreover, the cations (R4N+ X<sup>−</sup>) and (R4P+ X<sup>−</sup>) can provide the necessary functional groups that have the ability to interact with the polymer chains or to initiate polymerization processes, thereby increasing the interfacial interactions, as shown in **Figure 3**.

#### **3. Modification of wood adhesives using fillers**

At present, synthetic adhesives based on formaldehyde such as PF, UF, and MF are predominantly used for wood composite production. These adhesives have some advantages and disadvantages. For example, UF adhesives have advantages such as the lack of color in the cured polymer, low price, good mechanical properties, and so on. Consequently, these adhesives are commonly used in the wood panel industry. However, UF resin can only be used as an interior adhesive because of its poor water resistance. The use of PF adhesives is as important as UF adhesives in the wood panel industry. The good water resistance of PF adhesives allows them to be used under more stringent conditions than UF adhesives; thus, PF adhesives can be classified as exterior adhesives. However, their application is still limited because of its brittleness, brown color, high curing temperature, long curing time, and toxicity due to liberation of phenol and formaldehyde [20]. A variety of methods have been used to improve the performance of UF and PF adhesives as well as to expand their use. These methods are widely used in the industry; they include the simple addition of fillers. A variety of resin properties can significantly improve their performance as wood adhesives by the addition of fillers, which are most important for their lower cost. The mechanical performance of the resin is also improved in most cases. Moreover, the addition of fillers could reduce shrinkage and alleviate the stress on the glue line, which improves the hardness and durability. The flow of the adhesive and its smooth flow on the wood surface will be improved because of the penetration of water into the glue in the presence of a filler. The moderate permeability of the glue between the wood components has good mechanical effects by

**5**

*Fillers in Wood Adhesives*

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

strength between the wood products is improved.

by approximately 50% as compared with the control boards.

improving the durability and weather resistance of the resins.

improves the mechanical and thermal properties of polymers.

cases, the emission of free formaldehydes was significantly reduced.

Du et al. [26] used the synergistic effect of mineral filler (MgO and SiO2) with UF adhesives for the production of plywood. Plywood has better performance with mineral fillers as compared with the absence of filler or with wheat flour fillers. These results can be attributed to two reasons. (i) The (MgO and SiO2) filler prevents the glue from excessive permeation into the wood substrate and reduces the internal stress caused by the shrinkage. (ii) The oxygen from silicate and magnesium may be linked with the hydroxymethyl groups of the UF adhesive, thereby

Another newly developed type of mineral fillers are the nanomaterials, which differ physically and chemically from the normal fillers in terms of their thermal, light, radiation, and mechanical characteristics, among others [27]. Polymer nanocomposites have attracted considerable attention from researchers, especially in research related to polymer-layered silicate (PLS). This interest is attributed to the fact that the addition of small amounts of these materials often significantly

Some researchers have tried to use the nanomaterial fillers in wood adhesive systems, such as nano-SiO2, nano-CaCO3, and nano-montmorillonite (MMT) [28–31]. Their results showed that the adhesive bonding strength, gel time, and free formaldehyde emission were affected by the addition of nanomaterials. In most

The exfoliated nano-MMT was used by Zhang et al. [32] as a nanomineral filler with polymeric MDI to modify degraded soybean protein (DSP) and improve its water resistance and technical applicability. The DSP adhesives with MDI as a crosslinking agent showed improved water resistance and bond strength, but their shelf

physically interlocking the wood surface with the curing agents; thus, the bonding

Wheat flour is an activated filler, which is most commonly used in factories for the production of UF resins. The addition of approximately 10 to 20% of the wheat flour to the total solid resin can increase the weather resistance of the resin because this filler can reduce the shrinkage of the glue line [21]. According to Ma [22], the use of oxidized starch as a filler with UF resin at proportions of 5–10% will decrease the free formaldehyde emission from 3–7% to 0.2–0.5%. The hydroxyl group reacts with starch molecule chains and liberates formaldehyde to form acetals, thereby reducing the free formaldehyde emission. Compared with wheat flour, oxidized starch as a filler can further increase the bonding strength and water resistance of UF resins [23]. Mosiewicki et al. [24] studied the mechanical and thermal behavior of a composite formulated from a natural quebracho tannin adhesive with pine wood flour as well as the effect of moisture on its adhesive properties. The results showed that the mechanical and thermal properties of composite materials of wood flour/tannin adhesives have sufficient values for use in some industrial applications in terms of its higher stiffness, which is an important requirement. However, this composite material can be used in the dried state alone and must be avoided in humid atmospheres. Kargarfard and Jahan-Latibari [25] used recycled polyethylene with UF to reduce swelling fish and water absorption during particleboard production, thereby improving its performance in terms of weather resistance. In this work, different amounts of recycled polyethylene (5%, 10%, and 15%) were mixed with the lowest amount of UF resin (4%). Their results revealed that 5% of recycled polyethylene in the surface layer could improve the weather resistance of particleboards. The modulus of rupture (MOR) and modulus of elasticity (MOE) properties were marginally increased when the percentage of recycled polyethylene was increased to 5% in the surface layer. However, when 15% recycled polyethylene was added to the surface layer, the increased MOR was almost doubled, and the MOE was increased

#### *Fillers in Wood Adhesives DOI: http://dx.doi.org/10.5772/intechopen.92150*

*Fillers*

**Figure 3.**

alkylphosphonium (R4P+

X<sup>−</sup>) and (R4P+

*Scheme of the modification of clay layers by organic onium cations.*

**3. Modification of wood adhesives using fillers**

the cations (R4N+

This spacing of organoclays is affected by a variety of factors, including the chemical structure of the surfactant, the degree of cation exchange, and the silicate layer thickness [17]. Organically modified clays have been extensively studied in various practical applications in the field of organic-inorganic hybrids, composites, and nanoscale composites [15]. The surface modification of clay layers can be performed through an ion exchange process via the replacement of cations, such as sodium and calcium, in the interlayer space by ammonium or phosphonium surfactants, which usually include benzyl groups and short aliphatic chains [18]. In addition to modifying the surface and increasing the hydrophobicity of clay layers, the introduction of

X<sup>−</sup>) and alkylammonium (R4N+

ers increases the distances and spacing of the clay layers, which facilitates the intercalation of the polymer chain during the preparation of nanocomposites [19]. Moreover,

that have the ability to interact with the polymer chains or to initiate polymerization processes, thereby increasing the interfacial interactions, as shown in **Figure 3**.

At present, synthetic adhesives based on formaldehyde such as PF, UF, and MF are predominantly used for wood composite production. These adhesives have some advantages and disadvantages. For example, UF adhesives have advantages such as the lack of color in the cured polymer, low price, good mechanical properties, and so on. Consequently, these adhesives are commonly used in the wood panel industry. However, UF resin can only be used as an interior adhesive because of its poor water resistance. The use of PF adhesives is as important as UF adhesives in the wood panel industry. The good water resistance of PF adhesives allows them to be used under more stringent conditions than UF adhesives; thus, PF adhesives can be classified as exterior adhesives. However, their application is still limited because of its brittleness, brown color, high curing temperature, long curing time, and toxicity due to liberation of phenol and formaldehyde [20]. A variety of methods have been used to improve the performance of UF and PF adhesives as well as to expand their use. These methods are widely used in the industry; they include the simple addition of fillers. A variety of resin properties can significantly improve their performance as wood adhesives by the addition of fillers, which are most important for their lower cost. The mechanical performance of the resin is also improved in most cases. Moreover, the addition of fillers could reduce shrinkage and alleviate the stress on the glue line, which improves the hardness and durability. The flow of the adhesive and its smooth flow on the wood surface will be improved because of the penetration of water into the glue in the presence of a filler. The moderate permeability of the glue between the wood components has good mechanical effects by

X<sup>−</sup>) can provide the necessary functional groups

X<sup>−</sup>) cations into the clay lay-

**4**

physically interlocking the wood surface with the curing agents; thus, the bonding strength between the wood products is improved.

Wheat flour is an activated filler, which is most commonly used in factories for the production of UF resins. The addition of approximately 10 to 20% of the wheat flour to the total solid resin can increase the weather resistance of the resin because this filler can reduce the shrinkage of the glue line [21]. According to Ma [22], the use of oxidized starch as a filler with UF resin at proportions of 5–10% will decrease the free formaldehyde emission from 3–7% to 0.2–0.5%. The hydroxyl group reacts with starch molecule chains and liberates formaldehyde to form acetals, thereby reducing the free formaldehyde emission. Compared with wheat flour, oxidized starch as a filler can further increase the bonding strength and water resistance of UF resins [23].

Mosiewicki et al. [24] studied the mechanical and thermal behavior of a composite formulated from a natural quebracho tannin adhesive with pine wood flour as well as the effect of moisture on its adhesive properties. The results showed that the mechanical and thermal properties of composite materials of wood flour/tannin adhesives have sufficient values for use in some industrial applications in terms of its higher stiffness, which is an important requirement. However, this composite material can be used in the dried state alone and must be avoided in humid atmospheres.

Kargarfard and Jahan-Latibari [25] used recycled polyethylene with UF to reduce swelling fish and water absorption during particleboard production, thereby improving its performance in terms of weather resistance. In this work, different amounts of recycled polyethylene (5%, 10%, and 15%) were mixed with the lowest amount of UF resin (4%). Their results revealed that 5% of recycled polyethylene in the surface layer could improve the weather resistance of particleboards. The modulus of rupture (MOR) and modulus of elasticity (MOE) properties were marginally increased when the percentage of recycled polyethylene was increased to 5% in the surface layer. However, when 15% recycled polyethylene was added to the surface layer, the increased MOR was almost doubled, and the MOE was increased by approximately 50% as compared with the control boards.

Du et al. [26] used the synergistic effect of mineral filler (MgO and SiO2) with UF adhesives for the production of plywood. Plywood has better performance with mineral fillers as compared with the absence of filler or with wheat flour fillers. These results can be attributed to two reasons. (i) The (MgO and SiO2) filler prevents the glue from excessive permeation into the wood substrate and reduces the internal stress caused by the shrinkage. (ii) The oxygen from silicate and magnesium may be linked with the hydroxymethyl groups of the UF adhesive, thereby improving the durability and weather resistance of the resins.

Another newly developed type of mineral fillers are the nanomaterials, which differ physically and chemically from the normal fillers in terms of their thermal, light, radiation, and mechanical characteristics, among others [27]. Polymer nanocomposites have attracted considerable attention from researchers, especially in research related to polymer-layered silicate (PLS). This interest is attributed to the fact that the addition of small amounts of these materials often significantly improves the mechanical and thermal properties of polymers.

Some researchers have tried to use the nanomaterial fillers in wood adhesive systems, such as nano-SiO2, nano-CaCO3, and nano-montmorillonite (MMT) [28–31]. Their results showed that the adhesive bonding strength, gel time, and free formaldehyde emission were affected by the addition of nanomaterials. In most cases, the emission of free formaldehydes was significantly reduced.

The exfoliated nano-MMT was used by Zhang et al. [32] as a nanomineral filler with polymeric MDI to modify degraded soybean protein (DSP) and improve its water resistance and technical applicability. The DSP adhesives with MDI as a crosslinking agent showed improved water resistance and bond strength, but their shelf

life is very short. After the modification of MDI-modified DSP by nano-MMT, the produced adhesive had a much longer shelf life but slightly lower bonding strength.

Various PLSs have been successfully used with various types of resins, such as epoxy [33], polyacrylic ester, polyurethane [34], and even PF [35]. However, research on the feasibility and the mechanism of wood adhesive modification with MMT is rather limited.

#### **4. Preparation of water-resistant wood adhesives**

Glyoxalated lignin-tannin (GLT) adhesives are good candidates for the replacement of formaldehyde-based adhesives because of health and environmental concerns. Although glyoxalated lignin-tannin resins are low cost and have environment-friendly properties, these types of adhesives do not meet the fundamental required bonding strength and water resistance [36]. In addition, poor water-resistant property has limited their application. Ease of hydrolysis makes this adhesive useful for interior applications only. Consequently, appropriate measure must be taken to overcome the problem and to improve the water resistance of adhesives. In this work, we tried to develop and improve the properties of glyoxalated lignin-tannin resin to meet the fundamental requirement of bonding strength and water resistance.

#### **4.1 Preparation of green wood adhesives using modified tannin and hyperbranched poly(amine-ester)**

Due to the unique structure, chemical and physical characteristics, and various industrial applications, hyperbranched polymers (HBPs) have become the focus of considerable interest to chemists, biochemists, biologists, and biomedical experts [37]. This is due to high solubility, reduction of melt and solution viscosity, and abundance of functionalities as a result of the large number of reactive terminal groups within a molecule, nearly spherical molecular shape and the absence of chain entanglement [38]. These characteristics make dendrimers and hyperbranched polymers (HBPs) applicable as drug delivery agents, catalysis, Mitchell mimics, and nanoscale building blocks to artificial cells and coatings [39]. Both dendrimers and hyperbranched polymers are three dimensional and highly branched macromolecules. In this work, oligomeric precursors of poly(amine-ester) were synthesized, and then they were used to modify the tannin to improve the water resistance and mechanical properties of glyoxalated lignin-tannin (GLT)-based wood adhesives. **Figure 4** shows the synthesis of hyperbranched poly(amine-ester).

Previous studies by Lei [21] indicated the use of GL/PF/pMDI formulation with calcium lignosulfonate, and it turned out that the molecular mass decreased significantly during thermal treatment at 170°C for 90 min and pH of 12.7. This has been done to reduce the molecular mass of the lignin. Some linkages of lignin are broken by thermal treatment, and positions that are more reactive will be obtained. This indicates that the thermal treatment will make the material more suitable for such reactions. In this work, the two types of low molecular mass lignins extracted from oil palm empty fruit bunch were used. These are (i) kraft lignin and (ii) organosolv lignin. The adhesive resin properties are shown in **Table 1**.

The quality of wood and how the resin penetrates through the wood surface are important factors that affect the assessment of the adhesive joint in plywood. The bonding strength formed by TGKL and MTGK adhesives was studied via tensile strength test on wood substrates. For a comparative study, the same test was implemented for synthetic commercial phenol-formaldehyde resin. The data is displayed in **Table 2**.

**7**

*Fillers in Wood Adhesives*

**Figure 4.**

*a*

**Table 1.**

**Resin type**

*at 105°C to constant weight.*

*Synthesis of hyperbranched poly(amine-ester).*

**Viscosity at 30o**

**100 rpm (cP)**

**C,** 

*%Solid content = (weight of the solid resin/weight of the solution) × 100.*

*Variation of the physical properties of CPF, TGKL, and MTGKL resins.*

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

The maximum load, tensile strength, elastic modulus, and elongation at break of panels are direct measures of the performances. From **Table 2**, the trend shows that the dry MTGKL resin has the tensile strength of 39.72 MPa and elongation at break of 21.91%. This result indicates that MTGKL resin is stronger than that of the CPF resin. TGKL has the tensile strength of 28.79 MPa and elongation at break of 20.61%. That is also stronger than that of the CPF resin but lesser than MTGKL resin. The results of MTGKL and TGKL resins are within the requirement of the relevant international standard as per British Standard for dry tensile strength of plywood (≥0.35 MPa) [40]. Interestingly when comparing TGKL with MTGKL resins, the water resistance of MTGKL resin is improved after modification with poly(amine-ester). This is evident in the results of tensile strength (2.72 MPa) after soaking the plywood samples of MTGKL resin in tap water for 24 h at room temperature, where no delamination occurred in any of the specimens. However, delamination was observed in the plywood samples of TGKL resin after soaking in water for a period not exceeding 1 h, indicating the lack of water resistance for TGKL resin when it was used to bond plywood. This suggests that the modification of tannin by adding oligomeric precursors (hydroxyl-terminated) of a hyperbranched poly(amine-ester) is successful to boost the network structure of natural phenolic resin to prepare a water-resistant resin.

*Solid content of the resins was determined by measuring the weight before and after removing the solvent by heating* 

**Solid contenta**

CPF 190 59.80 11.30 480 TGKL 65 39.26 9.46 164 MTGKL 108 53.08 9.30 235

 **(%) pH Gel time at 100°C (s)**

*Fillers in Wood Adhesives DOI: http://dx.doi.org/10.5772/intechopen.92150*

*Fillers*

MMT is rather limited.

life is very short. After the modification of MDI-modified DSP by nano-MMT, the produced adhesive had a much longer shelf life but slightly lower bonding strength. Various PLSs have been successfully used with various types of resins, such as epoxy [33], polyacrylic ester, polyurethane [34], and even PF [35]. However, research on the feasibility and the mechanism of wood adhesive modification with

Glyoxalated lignin-tannin (GLT) adhesives are good candidates for the replace-

Due to the unique structure, chemical and physical characteristics, and various industrial applications, hyperbranched polymers (HBPs) have become the focus of considerable interest to chemists, biochemists, biologists, and biomedical experts [37]. This is due to high solubility, reduction of melt and solution viscosity, and abundance of functionalities as a result of the large number of reactive terminal groups within a molecule, nearly spherical molecular shape and the absence of chain entanglement [38]. These characteristics make dendrimers and hyperbranched polymers (HBPs) applicable as drug delivery agents, catalysis, Mitchell mimics, and nanoscale building blocks to artificial cells and coatings [39]. Both dendrimers and hyperbranched polymers are three dimensional and highly branched macromolecules. In this work, oligomeric precursors of poly(amine-ester) were synthesized, and then they were used to modify the tannin to improve the water resistance and mechanical properties of glyoxalated lignin-tannin (GLT)-based wood adhesives.

Previous studies by Lei [21] indicated the use of GL/PF/pMDI formulation with calcium lignosulfonate, and it turned out that the molecular mass decreased significantly during thermal treatment at 170°C for 90 min and pH of 12.7. This has been done to reduce the molecular mass of the lignin. Some linkages of lignin are broken by thermal treatment, and positions that are more reactive will be obtained. This indicates that the thermal treatment will make the material more suitable for such reactions. In this work, the two types of low molecular mass lignins extracted from oil palm empty fruit bunch were used. These are (i) kraft lignin and (ii) organosolv

The quality of wood and how the resin penetrates through the wood surface are important factors that affect the assessment of the adhesive joint in plywood. The bonding strength formed by TGKL and MTGK adhesives was studied via tensile strength test on wood substrates. For a comparative study, the same test was implemented for synthetic commercial phenol-formaldehyde resin. The data is displayed

ment of formaldehyde-based adhesives because of health and environmental concerns. Although glyoxalated lignin-tannin resins are low cost and have environment-friendly properties, these types of adhesives do not meet the fundamental required bonding strength and water resistance [36]. In addition, poor water-resistant property has limited their application. Ease of hydrolysis makes this adhesive useful for interior applications only. Consequently, appropriate measure must be taken to overcome the problem and to improve the water resistance of adhesives. In this work, we tried to develop and improve the properties of glyoxalated lignin-tannin resin to meet the fundamental requirement of bonding strength and water resistance.

**4.1 Preparation of green wood adhesives using modified tannin and** 

**Figure 4** shows the synthesis of hyperbranched poly(amine-ester).

lignin. The adhesive resin properties are shown in **Table 1**.

**4. Preparation of water-resistant wood adhesives**

**hyperbranched poly(amine-ester)**

**6**

in **Table 2**.

#### **Figure 4.** *Synthesis of hyperbranched poly(amine-ester).*


*a Solid content of the resins was determined by measuring the weight before and after removing the solvent by heating at 105°C to constant weight.*

*%Solid content = (weight of the solid resin/weight of the solution) × 100.*

#### **Table 1.**

*Variation of the physical properties of CPF, TGKL, and MTGKL resins.*

The maximum load, tensile strength, elastic modulus, and elongation at break of panels are direct measures of the performances. From **Table 2**, the trend shows that the dry MTGKL resin has the tensile strength of 39.72 MPa and elongation at break of 21.91%. This result indicates that MTGKL resin is stronger than that of the CPF resin. TGKL has the tensile strength of 28.79 MPa and elongation at break of 20.61%. That is also stronger than that of the CPF resin but lesser than MTGKL resin. The results of MTGKL and TGKL resins are within the requirement of the relevant international standard as per British Standard for dry tensile strength of plywood (≥0.35 MPa) [40].

Interestingly when comparing TGKL with MTGKL resins, the water resistance of MTGKL resin is improved after modification with poly(amine-ester). This is evident in the results of tensile strength (2.72 MPa) after soaking the plywood samples of MTGKL resin in tap water for 24 h at room temperature, where no delamination occurred in any of the specimens. However, delamination was observed in the plywood samples of TGKL resin after soaking in water for a period not exceeding 1 h, indicating the lack of water resistance for TGKL resin when it was used to bond plywood. This suggests that the modification of tannin by adding oligomeric precursors (hydroxyl-terminated) of a hyperbranched poly(amine-ester) is successful to boost the network structure of natural phenolic resin to prepare a water-resistant resin.


**Table 2.**

*Tensile strength, elastic modulus, and elongation at break of plywood using CPF, TGKL, and MTGKL resins.*

The improvement of water resistance of MTGKL resin is assumed to be due to the reaction of the terminal units of glutaraldehyde with the hydroxyl groups of tannin and with ▬NH2 and ▬COOH. Other exposed groups lead to an increase in the cross-linking density within the resin in the hot pressing process, and consequently, there will be an increase in the mechanical properties. In addition, the presence of furfuryl alcohol as a cross-linking agent and its participation in increasing the cross-linking reaction help to improve the water resistance of MTGKL-based wood adhesives. The whole process is represented in **Figure 5**.

#### **4.2 A combination of lignin polyol-tannin adhesives and polyethylenimine (PEI) as fillers**

Protein adhesive, which was secreted via marine mussels, called marine adhesive protein (MAP), is a good example of a renewable resource and free formaldehyde adhesive [42]. MAP has the ability to form strong linkages on wet surfaces and thus could be used as a strong and water-resistant wood adhesive [42]. However, they are costly and not readily available. MAP contains two functional groups (catechol and amino groups). The different reactions between the catechol group and the amino group lead to the cross-link and solidification of the MAP, thereby converting the MAP to a very strong and very water-resistant adhesive [42].

Condensed tannins and lignin are one of the few natural polymers containing a catechol moiety [43]. Previous studies showed the possibility of using a combination of condensed tannins or lignin and polyethylenimine to synthesize wood adhesives. **Figure 6** shows representative structures of polyethylenimine. This adhesive has high shear strength and exhibits significant water resistance [43]. In this work, the wood adhesive system, which consists of a tannin-glyoxalated lignin polyols (TGLP) and PEI, was evaluated for plywood and compared to plywood produced with a glyoxalated lignin/tannin and conventional phenol-formaldehyde resin.

It was observed that an increase in the total solid content of the TGLP-PEI adhesives leads to an increase in its viscosity. Moreover, when the total solid contents in adhesives reached 55%, the TGLP-PEI adhesives became too viscous. It can clearly be seen from **Figure 7** that the total solid contents increased from 48.54 to 56.92% when the ratio of PEI was changed from 10 to 20% in TGLP resin. The tensile strengths (dry, WSAD, and BWT/dry) of wood composites bonded with the adhesives have revealed a dramatic increase in its values (63.04, 59.48, and 53.53 MPa), respectively, and a linear relationship among all values. The TGLP-PEI

**9**

**Figure 7.**

*Fillers in Wood Adhesives*

**Figure 5.**

**Figure 6.**

*Representative structures of polyethylenimine (PEI).*

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

*Proposed mechanism for the participation of formulation components in cross-linking reactions and improvement of water-resistant properties for the resulting modified tannin-glyoxalated lignin [41].*

*Effect of total solid content of TGLP-PEI adhesives on tensile strength (dry, tap water soaking for 24 h, and* 

*boiling water soaking for 2 h) in comparison with TGLP and CPF.*

*Fillers in Wood Adhesives DOI: http://dx.doi.org/10.5772/intechopen.92150*

#### **Figure 5.**

*Fillers*

**Resin type**

**Table 2.**

The improvement of water resistance of MTGKL resin is assumed to be due to the reaction of the terminal units of glutaraldehyde with the hydroxyl groups of tannin and with ▬NH2 and ▬COOH. Other exposed groups lead to an increase in the cross-linking density within the resin in the hot pressing process, and consequently, there will be an increase in the mechanical properties. In addition, the presence of furfuryl alcohol as a cross-linking agent and its participation in increasing the cross-linking reaction help to improve the water resistance of MTGKL-based wood

*Tensile strength, elastic modulus, and elongation at break of plywood using CPF, TGKL, and MTGKL resins.*

**Tensile strength, MPa Elastic** 

CPF 1.39 --- --- 56.61 12.45

TGKL 2.89 Delamination --- 125.27 20.61 MTGKL 3.97 --- Delamination 185.30 21.91

**Boiling water soaking, 2 h**



**module, MPa**

**Elongation at break (%)**

**4.2 A combination of lignin polyol-tannin adhesives and polyethylenimine** 

Protein adhesive, which was secreted via marine mussels, called marine adhesive protein (MAP), is a good example of a renewable resource and free formaldehyde adhesive [42]. MAP has the ability to form strong linkages on wet surfaces and thus could be used as a strong and water-resistant wood adhesive [42]. However, they are costly and not readily available. MAP contains two functional groups (catechol and amino groups). The different reactions between the catechol group and the amino group lead to the cross-link and solidification of the MAP, thereby converting the

Condensed tannins and lignin are one of the few natural polymers containing a catechol moiety [43]. Previous studies showed the possibility of using a combination of condensed tannins or lignin and polyethylenimine to synthesize wood adhesives. **Figure 6** shows representative structures of polyethylenimine. This adhesive has high shear strength and exhibits significant water resistance [43]. In this work, the wood adhesive system, which consists of a tannin-glyoxalated lignin polyols (TGLP) and PEI, was evaluated for plywood and compared to plywood produced with a glyoxalated lignin/tannin and conventional phenol-formalde-

It was observed that an increase in the total solid content of the TGLP-PEI adhesives leads to an increase in its viscosity. Moreover, when the total solid contents in adhesives reached 55%, the TGLP-PEI adhesives became too viscous. It can clearly be seen from **Figure 7** that the total solid contents increased from 48.54 to 56.92% when the ratio of PEI was changed from 10 to 20% in TGLP resin. The tensile strengths (dry, WSAD, and BWT/dry) of wood composites bonded with the adhesives have revealed a dramatic increase in its values (63.04, 59.48, and 53.53 MPa), respectively, and a linear relationship among all values. The TGLP-PEI

adhesives. The whole process is represented in **Figure 5**.

**DRY Cold water** 

**soaking, 24 h**

MAP to a very strong and very water-resistant adhesive [42].

**(PEI) as fillers**

**8**

hyde resin.

*Proposed mechanism for the participation of formulation components in cross-linking reactions and improvement of water-resistant properties for the resulting modified tannin-glyoxalated lignin [41].*

**Figure 6.** *Representative structures of polyethylenimine (PEI).*

#### **Figure 7.**

*Effect of total solid content of TGLP-PEI adhesives on tensile strength (dry, tap water soaking for 24 h, and boiling water soaking for 2 h) in comparison with TGLP and CPF.*

adhesives with total solid contents (56.92%) were applied to plywood and used in subsequent experiments.

The production of wood composites does not favor the use of wood adhesives with low solid contents due to higher energy consumption and comparatively longer time of water evaporation through hot press. In the current study, a significant amount of the total solid contents of adhesives (56.92%) was obtained during the modification of lignin polyol-tannin resins with polyethylenimine. There was a substantial difference in the solid contents of modified (56.92%), unmodified adhesives (45.82%), and the commercial phenol-formaldehyde (59.50%) under the same experimental conditions. The solid content of the TGLP resin (45.82%) was lower than the solid contents of the CPF resin (59.50%). However, the solid contents of the TGLP-PEI resins were almost comparable to the CPF resin, especially when the ratio of PEI was 20% in TGLP-PEI resin. It is known that CPF resins contain large amount of urea, and for this reason, the solid content value is very high [44].

To date, the reactions between lignin and tannin and PEI are not fully understood. Some of the proposed potential reactions are given in **Figure 8**. The nature of TGLP-PEI resin curing mechanisms is comparable to the quinone-tanning methods as reported elsewhere. Moreover, the reaction mechanisms between lignin and PEI are mostly similar to the possible reactions between tannin and PEI. At elevated temperatures, the catechol moiety (2) in the demethylated lignin is highly vulnerable to oxidation process, leading to the formation of quinones (3). This occurs at higher temperatures (140°C) using a hot press during the formation of wood composites. The quinones (3) possibly react with the amino groups present in PEI structure, thus forming Schiff bases (4 and 5). Some other possible reactions between quinones and amino groups of PEI (such as Michael addition reaction) may also occur to form (6) further Schiff bases (7 and 8). A similar kind of reaction occurs during the oxidation process to form phenolic hydroxide in lignin structure during hot press, which further results in the formation of quinone. It further supports the increase of the reactions with amino groups of PEI to form Schiff bases.

**11**

**Figure 9.**

*(d) TGLP-ODA-BT composites.*

*Fillers in Wood Adhesives*

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

**4.3 Influence of bentonite (BT) clay on lignin**

between natural and organic bentonite.

It is well known that wood has lignin, which contains phenolic hydroxyl groups, and these groups can be oxidized to quinone during the hot press; hence they cannot be excluded from the covalent bond formation between TGLP-PEI adhesive and wood. In addition to the reaction of the quinone, the catechol moiety can react with the amino groups of PEI and form strong hydrogen bonds. Furthermore, it can also form strong hydrogen bonds with the hydroxyl groups in the wood components (see a representative structure 9 and 10 in **Figure 8**). It can be inferred from these reactions that the curing reactions through the hot press process lead to the formation of highly cross-linked TGLP-PEI network polymers, as well as water-resistant adhesives. These results are in close agreement to mechanical tests and proven through soaking plywood samples bonded with TGLP-PEI adhesives in the tap water and boiling water.

Improving the polymer properties through incorporating a second component such as fillers to obtain new materials has become common. Inorganic additives are frequently used in the wood adhesive industry that improves the mechanical and thermal properties of resins [36]. Recently, the utilization of the montmorillonite to modified phenolic resin attracts significant interest of researchers to improve some properties such as stiffness and toughness [45]. In this work, wood adhesive system, consisting of a tannin-glyoxalated lignin polyol, (TGLP) was modified using

Scanning electron microscope (SEM) is a significant technique used to interpret changes in morphology of bentonite during a modification process by surfactants. SEM micrographs of bentonite before and after modification with octadecylamine (ODA) as well as the SEM images of composites with adhesive are shown in **Figure 9**. The corresponding elemental analysis data are summarized in **Table 3**. It is worth mentioning that there are no substantial differences in morphology

bentonite and organo-bentonite clay and then evaluated on plywood.

*SEM micrographs of (a) unmodified BT, (b) ODA-modified BT, (c) TGLP-BT, and* 

**Figure 8.** *Some possible reactions between tannin and PEI.*

#### *Fillers in Wood Adhesives DOI: http://dx.doi.org/10.5772/intechopen.92150*

*Fillers*

subsequent experiments.

adhesives with total solid contents (56.92%) were applied to plywood and used in

The production of wood composites does not favor the use of wood adhesives with low solid contents due to higher energy consumption and comparatively longer time of water evaporation through hot press. In the current study, a significant amount of the total solid contents of adhesives (56.92%) was obtained during the modification of lignin polyol-tannin resins with polyethylenimine. There was a substantial difference in the solid contents of modified (56.92%), unmodified adhesives (45.82%), and the commercial phenol-formaldehyde (59.50%) under the same experimental conditions. The solid content of the TGLP resin (45.82%) was lower than the solid contents of the CPF resin (59.50%). However, the solid contents of the TGLP-PEI resins were almost comparable to the CPF resin, especially when the ratio of PEI was 20% in TGLP-PEI resin. It is known that CPF resins contain large amount of urea, and for this reason, the solid content value is very high [44]. To date, the reactions between lignin and tannin and PEI are not fully understood. Some of the proposed potential reactions are given in **Figure 8**. The nature of TGLP-PEI resin curing mechanisms is comparable to the quinone-tanning methods as reported elsewhere. Moreover, the reaction mechanisms between lignin and PEI are mostly similar to the possible reactions between tannin and PEI. At elevated temperatures, the catechol moiety (2) in the demethylated lignin is highly vulnerable to oxidation process, leading to the formation of quinones (3). This occurs at higher temperatures (140°C) using a hot press during the formation of wood composites. The quinones (3) possibly react with the amino groups present in PEI structure, thus forming Schiff bases (4 and 5). Some other possible reactions between quinones and amino groups of PEI (such as Michael addition reaction) may also occur to form (6) further Schiff bases (7 and 8). A similar kind of reaction occurs during the oxidation process to form phenolic hydroxide in lignin structure during hot press, which further results in the formation of quinone. It further supports the increase of the reactions with amino groups of PEI to form Schiff bases.

**10**

**Figure 8.**

*Some possible reactions between tannin and PEI.*

It is well known that wood has lignin, which contains phenolic hydroxyl groups, and these groups can be oxidized to quinone during the hot press; hence they cannot be excluded from the covalent bond formation between TGLP-PEI adhesive and wood. In addition to the reaction of the quinone, the catechol moiety can react with the amino groups of PEI and form strong hydrogen bonds. Furthermore, it can also form strong hydrogen bonds with the hydroxyl groups in the wood components (see a representative structure 9 and 10 in **Figure 8**). It can be inferred from these reactions that the curing reactions through the hot press process lead to the formation of highly cross-linked TGLP-PEI network polymers, as well as water-resistant adhesives. These results are in close agreement to mechanical tests and proven through soaking plywood samples bonded with TGLP-PEI adhesives in the tap water and boiling water.

#### **4.3 Influence of bentonite (BT) clay on lignin**

Improving the polymer properties through incorporating a second component such as fillers to obtain new materials has become common. Inorganic additives are frequently used in the wood adhesive industry that improves the mechanical and thermal properties of resins [36]. Recently, the utilization of the montmorillonite to modified phenolic resin attracts significant interest of researchers to improve some properties such as stiffness and toughness [45]. In this work, wood adhesive system, consisting of a tannin-glyoxalated lignin polyol, (TGLP) was modified using bentonite and organo-bentonite clay and then evaluated on plywood.

Scanning electron microscope (SEM) is a significant technique used to interpret changes in morphology of bentonite during a modification process by surfactants. SEM micrographs of bentonite before and after modification with octadecylamine (ODA) as well as the SEM images of composites with adhesive are shown in **Figure 9**. The corresponding elemental analysis data are summarized in **Table 3**. It is worth mentioning that there are no substantial differences in morphology between natural and organic bentonite.

#### **Figure 9.**

*SEM micrographs of (a) unmodified BT, (b) ODA-modified BT, (c) TGLP-BT, and (d) TGLP-ODA-BT composites.*


#### **Table 3.**

*EDX elemental analysis of unmodified BT, ODA-BT, and composites.*

The unmodified bentonite (**Figure 9a**) demonstrated huge aggregated morphology with large-sized particles in the structure. Some fragments of small-sized particles in BT structure were relatively regular. This may be due to the interactions of face-to-face and face-to-edge of particles. Comparing with the unmodified BT, the BT-ODA (**Figure 9b**) showed smaller particles and irregular shapes as well as more severely aggregated particles than BT. In addition, the SEM images also revealed the expansion that took place in layers of silica and a more open structure in the ODA-BT. As a result, further exfoliation during composite fabrication is expected.

SEM analysis of BT-TGLP composites in **Figure 9c** exhibited the lack of compatibility between the unmodified bentonite and matrix resin, which could lead to the formation of tactoids. During the modification of organic bentonite using appropriate modifiers, the interaction between the matrix resin and the organo-bentonite would be sufficient for a certain degree of exfoliation and/or intercalation even when forming the tactoid. SEM micrograph of ODA-BT containing the composite resin is given in **Figure 5d**. As shown in **Figure 9d**, ODA-BT particles were dispersed in the TGLP matrix, and the resulting composite structure (TGLP-ODA-BT) demonstrated a highly homogenous distribution.

The elemental analysis by EDX of the BT, ODA-BT, and composites is shown in **Table 3**. The energy spectra indicate that the elements present are carbon (C), silicon (Si), aluminum (Al), and oxygen (O). From **Table 3** the BT sample has the highest content of silicon (Si) and aluminum (Al). It is clear that the bentonite is a class of aluminum silicate minerals. For ODA-BT, it was observed that the amount of carbon (C) element (21.12 w%) has significantly increased compared with BT sample (12.79 w%), indicating the presence of alkylammonium cations as intercalant group in the bentonite gallery that additionally supports the modification that occurred. It is noteworthy to point out that the percentage of all the other elements such as Si and Al decreased significantly and this can be due to the presence of carbon (C).

The average tensile strength of unfilled TGLP resin and composite with corresponding bentonite and organo-bentonite loadings was determined from the tests conducted on at least five plywood specimens under dry conditions as shown in **Figure 6**. The tensile strength of TGLP composite has been considerably improved by incorporating both BT and ODA-BT.

The tensile strength of TGLP composite increases nonlinearly with BT and ODA-BT content. From **Figure 10**, the dry tensile strength was more affected by ODA-BT than BT. The reason in the strengthening of tensile properties is probably due to intercalation/exfoliation of the bentonite structure in the phenolic matrix of adhesives. As the silicate layer in bentonite structure has excellent mechanical characteristics compared with TGLP, the enhanced mechanical properties of resin may be linked to the dispersion level of the silicate layers into the matrix of resin. It is believed that the dispersion process of clay in polymer chain provides a large interfacial interaction that may cause a restriction on the mobility.

There are multiple factors that affect the mechanical properties including the ratio of filler, dispersion of the filler, and the adhesion at the filler matrix interface

**13**

ties of the resin.

**5. Conclusions**

*Fillers in Wood Adhesives*

**Figure 10.**

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

[46]. In addition, the exfoliation degree of the silicate layer in the polymer affects the modulus of composites. This reveals that the significant increase of the tensile strength modulus through the incorporation of low contents of ODA-BT (1, 2, and 3 wt%) can be due to the uniform dispersion of nanoparticles at such a low content. High content of particles (4 and 5 wt%) reduced the dispersion and thus restricts the improvement of tensile strength (**Figure 10**). The elastic modulus has the same sensitivity toward dispersion. Dispersion of filler particles has less effect when BT is used; the limitations of tensile strengths show at a loading of 5% (**Figure 10**). The decrease of the tensile strength at high content of BT and ODA-BT can be attributed to the agglomeration of particles. An agglomeration of particles could also serve as crack initiation sites, as well as the nonuniform distribution. Through the results, it could be concluded that the use of ODA-BT is better than BT to improve the mechanical properties, particularly tensile strength when used in low concentrations. This can possibly be due to lower silicate layers in the ODA-BT than BT because of the organic modification of bentonite that has a positive effect on

With the modification of bentonite, the ODA-BT became more compatible with the TGLP resin chains, due to the decrease of surface energy of bentonite layers; thus the surface polarity of ODA-BT is compatible with the resin polarity. The ODA-BT with lowered surface energy has more ability to interact and intercalate within the interlayer space of resin than BT and thus improves the various proper-

Composite materials have a pivotal role in industries that are now considered the most progressive worldwide. Polymer composites are a mixture of polymers with inorganic or organic fillers that have certain geometric forms. Wood adhesives now represent a vital aspect of the wood-based panel industry, and synthetic condensation resins have become widely used. At present, synthetic adhesives based on formaldehyde such as PF, UF, and MF are predominantly used for wood composite production. A variety of methods have been used to improve the performance of wood adhesives. These methods are widely used in the industry; they include the simple addition of fillers. In this chapter, it was demonstrated that a combination of lignin polyol-tannin (TGLP) and polyethylenimine (PEI) was an excellent

ODA-BT, thus increasing the cross-linking density.

*Effect of the clay loading of TGLP adhesives on tensile strength.*

*Fillers*

**Table 3.**

The unmodified bentonite (**Figure 9a**) demonstrated huge aggregated morphology with large-sized particles in the structure. Some fragments of small-sized particles in BT structure were relatively regular. This may be due to the interactions of face-to-face and face-to-edge of particles. Comparing with the unmodified BT, the BT-ODA (**Figure 9b**) showed smaller particles and irregular shapes as well as more severely aggregated particles than BT. In addition, the SEM images also revealed the expansion that took place in layers of silica and a more open structure in the ODA-BT. As a result, further exfoliation during composite fabrication is expected. SEM analysis of BT-TGLP composites in **Figure 9c** exhibited the lack of compatibility between the unmodified bentonite and matrix resin, which could lead to the formation of tactoids. During the modification of organic bentonite using appropriate modifiers, the interaction between the matrix resin and the organo-bentonite would be sufficient for a certain degree of exfoliation and/or intercalation even when forming the tactoid. SEM micrograph of ODA-BT containing the composite resin is given in **Figure 5d**. As shown in **Figure 9d**, ODA-BT particles were dispersed in the TGLP matrix, and the resulting composite structure (TGLP-ODA-BT)

**Sample C (w%) O (w%) Si (w%) Al (w%)** BT 12.79 61.05 17.96 5.73 ODA-BT 21.12 68.84 6.22 1.97 TGLP-BT 22.42 67.14 3.95 1.21 TGLP-ODA-BT 23.22 66.39 0.72 0.64

The elemental analysis by EDX of the BT, ODA-BT, and composites is shown in **Table 3**. The energy spectra indicate that the elements present are carbon (C), silicon (Si), aluminum (Al), and oxygen (O). From **Table 3** the BT sample has the highest content of silicon (Si) and aluminum (Al). It is clear that the bentonite is a class of aluminum silicate minerals. For ODA-BT, it was observed that the amount of carbon (C) element (21.12 w%) has significantly increased compared with BT sample (12.79 w%), indicating the presence of alkylammonium cations as intercalant group in the bentonite gallery that additionally supports the modification that occurred. It is noteworthy to point out that the percentage of all the other elements such as Si and Al decreased significantly and this can be due to the presence of carbon (C). The average tensile strength of unfilled TGLP resin and composite with corresponding bentonite and organo-bentonite loadings was determined from the tests conducted on at least five plywood specimens under dry conditions as shown in **Figure 6**. The tensile strength of TGLP composite has been considerably improved

The tensile strength of TGLP composite increases nonlinearly with BT and ODA-BT content. From **Figure 10**, the dry tensile strength was more affected by ODA-BT than BT. The reason in the strengthening of tensile properties is probably due to intercalation/exfoliation of the bentonite structure in the phenolic matrix of adhesives. As the silicate layer in bentonite structure has excellent mechanical characteristics compared with TGLP, the enhanced mechanical properties of resin may be linked to the dispersion level of the silicate layers into the matrix of resin. It is believed that the dispersion process of clay in polymer chain provides a large

There are multiple factors that affect the mechanical properties including the ratio of filler, dispersion of the filler, and the adhesion at the filler matrix interface

interfacial interaction that may cause a restriction on the mobility.

demonstrated a highly homogenous distribution.

*EDX elemental analysis of unmodified BT, ODA-BT, and composites.*

by incorporating both BT and ODA-BT.

**12**

**Figure 10.** *Effect of the clay loading of TGLP adhesives on tensile strength.*

[46]. In addition, the exfoliation degree of the silicate layer in the polymer affects the modulus of composites. This reveals that the significant increase of the tensile strength modulus through the incorporation of low contents of ODA-BT (1, 2, and 3 wt%) can be due to the uniform dispersion of nanoparticles at such a low content. High content of particles (4 and 5 wt%) reduced the dispersion and thus restricts the improvement of tensile strength (**Figure 10**). The elastic modulus has the same sensitivity toward dispersion. Dispersion of filler particles has less effect when BT is used; the limitations of tensile strengths show at a loading of 5% (**Figure 10**).

The decrease of the tensile strength at high content of BT and ODA-BT can be attributed to the agglomeration of particles. An agglomeration of particles could also serve as crack initiation sites, as well as the nonuniform distribution. Through the results, it could be concluded that the use of ODA-BT is better than BT to improve the mechanical properties, particularly tensile strength when used in low concentrations. This can possibly be due to lower silicate layers in the ODA-BT than BT because of the organic modification of bentonite that has a positive effect on ODA-BT, thus increasing the cross-linking density.

With the modification of bentonite, the ODA-BT became more compatible with the TGLP resin chains, due to the decrease of surface energy of bentonite layers; thus the surface polarity of ODA-BT is compatible with the resin polarity. The ODA-BT with lowered surface energy has more ability to interact and intercalate within the interlayer space of resin than BT and thus improves the various properties of the resin.

#### **5. Conclusions**

Composite materials have a pivotal role in industries that are now considered the most progressive worldwide. Polymer composites are a mixture of polymers with inorganic or organic fillers that have certain geometric forms. Wood adhesives now represent a vital aspect of the wood-based panel industry, and synthetic condensation resins have become widely used. At present, synthetic adhesives based on formaldehyde such as PF, UF, and MF are predominantly used for wood composite production. A variety of methods have been used to improve the performance of wood adhesives. These methods are widely used in the industry; they include the simple addition of fillers. In this chapter, it was demonstrated that a combination of lignin polyol-tannin (TGLP) and polyethylenimine (PEI) was an excellent

alternative for marine adhesive protein (MAP). The results revealed that the increase in PEI ratio led to an increase of the solid content of TGLP-PEI adhesives. The TGLP adhesives had poor water resistance compared with TGLP-PEI adhesive despite the high tensile strength (31.1 MPa). This is clearly shown when soaking the plywood specimens in the tap water and boiling water, where delamination occurred. However, at 10% of PEI with TGLP resin, the delamination of plywood specimens did not occur.

There are not a lot of studies conducted on the influence of clays on wood adhesives prepared from biomass. The mechanical and thermal characteristics of TGLP system were studied. The modification of bentonite clay was implemented using octadecylamine (ODA) salt. The mechanical properties (tensile strengths) of TGLP composites have been considerably improved by incorporating both BT and ODA-BT. The tensile strengths revealed that the use of low contents of clay is better than using high clay content that could be due to the decrease in dispersion at high contents of clay. These results are particularly clear when using modified bentonite with ODA. This is probably due to the decrease of some silicate layers in the ODA-BT compared with BT.

#### **Acknowledgements**

The author is grateful for the financial support of this chapter from the University Science Malaysia through the USM Research University Grant 1001/ PKIMIA/854002 and to the laboratories of Material Research Directorate in the Ministry of Higher Education and Scientific Research, Baghdad, Iraq.

#### **Author details**

Abbas Hasan Faris Materials Research Directorate, Ministry of Higher Education and Scientific Research, Baghdad, Iraq

\*Address all correspondence to: abbas\_hf@yahoo.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**15**

*Fillers in Wood Adhesives*

**References**

2010;**95**:2126-2146

2006;**91**:2951-2959

Forecast. 2013

1998;**12**:675-680

[4] Pandey JK, Reddy KR,

Kumar AP, Singh RP. An overview on the degradability of polymer nanocomposites. Polymer Degradation

[5] Transparency MR. Phenolic Resins (Resol, Novolac and Others) Market for Wood-Adhesives, Molding Compounds,

and Stability. 2005;**88**:234-250

Laminates, Insulation and Other Applications: Global Industry Analysis, Size, Share, Growth, Trends and

[6] Baekeland LH. The synthesis, constitution, and uses of bakelite. Journal of Industrial and Engineering

[7] Giannelis EP. Polymer-layered silicate nanocomposites: Synthesis, properties and applications.

Applied Organometallic Chemistry.

[8] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Progress in Polymer

Science. 2008;**32**:1119-1198

Chemistry. 1909;**1**:149-161

[2] Cosoli P, Scocchi G, Pricl S, Fermaglia M. Many-scale molecular

simulation for ABS-MMT

Materials. 2008;**107**:169-179

[3] Ma H, Xu Z, Tong L, Gu A, Fang Z. Studies of ABS-graft-maleic anhydride/clay nanocomposites: Morphologies, thermal stability and flammability properties. Polymer Degradation and Stability.

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

[1] Armentano I, Dottori M, Fortunati E, Mattioli S, Kenny JM. Biodegradable polymer matrix nanocomposites for tissue engineering: A review. Polymer Degradation and Stability.

[9] Thostenson ET, Li C, Chou TW. Nanocomposites in context. Composites Science and Technology. 2005;**65**:491-516

[10] Alexandre M, Dubois P. Polymerlayered silicate nanocomposites: Preparation, properties and uses of a new class of materials. Materials Science

[11] Murray HH. Traditional and new applications for kaolin, smectite, and palygorskite: A general overview. Applied Clay Science. 2000;**17**:207-221

Pinnavaia TJ. Polymer-layered silicate nanocomposites: An overview. Applied

and Engineering. 2000;**28**:1-63

[12] Lebaron PC, Wang Z,

Clay Science. 1999;**15**:11-29

University Malaysia; 2010

Materials. 1996;**8**:29-35

Wiley; 1977

2002;**14**:4654-4661

[18] Ahmad MB, Hoidy WH, Ibrahim NAB, Al-Mulla EAJ. Modification of montmorillonite by new surfactants. Journal of Engineering and Applied Science. 2009;**4**:184-188

[13] Sansuri AJB. Super absorbent polymer composites [thesis]. Pahang:

[14] Giannelis EP. Polymer layered silicate nanocomposites. Advanced

[15] Singla P, Mehta R, Upadhyay SN. Clay modification by the use of organic

[16] Van Olphen H. An Introduction to Clay Colloidal Chemistry. New York:

[17] Maiti P, Yamada K, Okamoto M, Ueda K, Okamoto K. New polylactide/ layered silicate nanocomposites: Role of organoclays. Chemistry of Materials.

[19] Chigwada G, Wang D, Jiang DD, Wilkie CA. Styrenic nanocomposites

cations. Green and Sustainable Chemistry. 2012;**2**:21-25

nanocomposites: Upgrading of industrial scraps. Microporous and Mesoporous

### **References**

*Fillers*

specimens did not occur.

the ODA-BT compared with BT.

**Acknowledgements**

**14**

**Author details**

Abbas Hasan Faris

Research, Baghdad, Iraq

Materials Research Directorate, Ministry of Higher Education and Scientific

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

alternative for marine adhesive protein (MAP). The results revealed that the increase in PEI ratio led to an increase of the solid content of TGLP-PEI adhesives. The TGLP adhesives had poor water resistance compared with TGLP-PEI adhesive despite the high tensile strength (31.1 MPa). This is clearly shown when soaking the plywood specimens in the tap water and boiling water, where delamination occurred. However, at 10% of PEI with TGLP resin, the delamination of plywood

There are not a lot of studies conducted on the influence of clays on wood adhesives prepared from biomass. The mechanical and thermal characteristics of TGLP system were studied. The modification of bentonite clay was implemented using octadecylamine (ODA) salt. The mechanical properties (tensile strengths) of TGLP composites have been considerably improved by incorporating both BT and ODA-BT. The tensile strengths revealed that the use of low contents of clay is better than using high clay content that could be due to the decrease in dispersion at high contents of clay. These results are particularly clear when using modified bentonite with ODA. This is probably due to the decrease of some silicate layers in

The author is grateful for the financial support of this chapter from the University Science Malaysia through the USM Research University Grant 1001/ PKIMIA/854002 and to the laboratories of Material Research Directorate in the

Ministry of Higher Education and Scientific Research, Baghdad, Iraq.

\*Address all correspondence to: abbas\_hf@yahoo.com

provided the original work is properly cited.

[1] Armentano I, Dottori M, Fortunati E, Mattioli S, Kenny JM. Biodegradable polymer matrix nanocomposites for tissue engineering: A review. Polymer Degradation and Stability. 2010;**95**:2126-2146

[2] Cosoli P, Scocchi G, Pricl S, Fermaglia M. Many-scale molecular simulation for ABS-MMT nanocomposites: Upgrading of industrial scraps. Microporous and Mesoporous Materials. 2008;**107**:169-179

[3] Ma H, Xu Z, Tong L, Gu A, Fang Z. Studies of ABS-graft-maleic anhydride/clay nanocomposites: Morphologies, thermal stability and flammability properties. Polymer Degradation and Stability. 2006;**91**:2951-2959

[4] Pandey JK, Reddy KR, Kumar AP, Singh RP. An overview on the degradability of polymer nanocomposites. Polymer Degradation and Stability. 2005;**88**:234-250

[5] Transparency MR. Phenolic Resins (Resol, Novolac and Others) Market for Wood-Adhesives, Molding Compounds, Laminates, Insulation and Other Applications: Global Industry Analysis, Size, Share, Growth, Trends and Forecast. 2013

[6] Baekeland LH. The synthesis, constitution, and uses of bakelite. Journal of Industrial and Engineering Chemistry. 1909;**1**:149-161

[7] Giannelis EP. Polymer-layered silicate nanocomposites: Synthesis, properties and applications. Applied Organometallic Chemistry. 1998;**12**:675-680

[8] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Progress in Polymer Science. 2008;**32**:1119-1198

[9] Thostenson ET, Li C, Chou TW. Nanocomposites in context. Composites Science and Technology. 2005;**65**:491-516

[10] Alexandre M, Dubois P. Polymerlayered silicate nanocomposites: Preparation, properties and uses of a new class of materials. Materials Science and Engineering. 2000;**28**:1-63

[11] Murray HH. Traditional and new applications for kaolin, smectite, and palygorskite: A general overview. Applied Clay Science. 2000;**17**:207-221

[12] Lebaron PC, Wang Z, Pinnavaia TJ. Polymer-layered silicate nanocomposites: An overview. Applied Clay Science. 1999;**15**:11-29

[13] Sansuri AJB. Super absorbent polymer composites [thesis]. Pahang: University Malaysia; 2010

[14] Giannelis EP. Polymer layered silicate nanocomposites. Advanced Materials. 1996;**8**:29-35

[15] Singla P, Mehta R, Upadhyay SN. Clay modification by the use of organic cations. Green and Sustainable Chemistry. 2012;**2**:21-25

[16] Van Olphen H. An Introduction to Clay Colloidal Chemistry. New York: Wiley; 1977

[17] Maiti P, Yamada K, Okamoto M, Ueda K, Okamoto K. New polylactide/ layered silicate nanocomposites: Role of organoclays. Chemistry of Materials. 2002;**14**:4654-4661

[18] Ahmad MB, Hoidy WH, Ibrahim NAB, Al-Mulla EAJ. Modification of montmorillonite by new surfactants. Journal of Engineering and Applied Science. 2009;**4**:184-188

[19] Chigwada G, Wang D, Jiang DD, Wilkie CA. Styrenic nanocomposites prepared using a novel biphenylcontaining modified clay. Polymer Degradation and Stability. 2006;**91**:755-762

[20] Gu JY. Adhesive and Paint. Beijing: China Forestry Publishing House; 1998

[21] Lei H. Synthetic and natural materials for wood adhesive resins [thesis]. Epinal: University Henri Poincare-Nancy; 2009

[22] Ma WW. The preparation of urea-formaldehyde resin modified by oxygenated amylum. Chemistry and Adhesion. 1995;**3**:147-148

[23] Shi YM. Development of the low cost urea-formaldehyde resin adhesive. China Adhesive. 2003;**12**:30-32

[24] Mosiewicki M, Aranguren MI, Borrajo J. Thermal and mechanical properties of wood flour/tannin adhesive composites. Journal of Applied Polymer Science. 2004;**91**:3074-3082

[25] Kargarfard A, Jahan-Latibari A. Application of recycled polyethylene in combination with urea-formaldehyde resin to produce water resistant particleboard. In: Proceedings of the 55th International Convention of Society of Wood Science and Technology. Beijing, China; 2012. pp. 1-7

[26] Du GB, Wu LZ, Yang QX. Development on mineral filler for UF resin. Adhesion. 1995;**16**:9-12

[27] Cavicchi RE, Silsbee RH. Coulomb suppression of tunneling rate from small metal particles. Physical Review Letters. 1984;**52**:1453

[28] Lin QJ, Yang GD, Liu JH. Study on the property of nano-SiO2/ureaformaldehyde resin. Scientia Silvae Sinica. 2005;**41**:129-135

[29] Yang GD, Lin QJ, Liu JH. The effect of nanometer silicon dioxide on properties of UF resin. Journal of Fujian Forestry College. 2004;**24**:114-117

[30] Yang G, Lin QJ, Liu JH. Study on using nanometer SiO2 for modifying UF resin. China Wood Industry. 2004;**18**:7-9

[31] Yu HW, Fu SY, Wen GF. The effect of nanometer calcium carbonate on properties of UF resin. China Adhesives. 2002;**11**:22-24

[32] Zhang W, Ma Y, Xu Y, Wang C, Chu F. Lignocellulosic ethanol residuebased lignin–phenol–formaldehyde resin adhesive. International Journal of Adhesion and Adhesives. 2013;**40**:11-18

[33] Lv JK, Ke YC, Qi ZN. Synthesis and mechanical properties of epoxy/ clay nanocomposites. Acta Materials Composed Sinica. 2002;**19**:117-121

[34] Tien Y, Chen TK, Wei KH. Synthesis and characterization of novel segmented polyurethane/clay nanocomposites. Polymer (Guildf). 2000;**41**:1345-1353

[35] Choi MH, Chung IJ, Lee JD. Morphology and curing behaviors of phenolic resin-layered silicate nanocomposites prepared by melt intercalation. Chemistry of Materials. 2000;**12**:2977-2983

[36] Pizzi A, Mittal KL. Handbook of Adhesive Technology. 2nd ed. New York: Marcel Dekker; 2003

[37] Magnusson H, Malmström E, Hult A. Structure buildup in hyperbranched polymers from 2,2-bis(hydroxymethyl) propionic acid. Macromolecules. 2000;**33**:3099-3104

[38] Fang K, Xu Z, Jiang X, Zhang X, Fu S. Preparation and characterization of hyperbranched polyesteramides. Polymer Bulletin. 2008;**60**:533-543

[39] Khan JA, Kainthan RK, Li MG. Water soluble nanoparticles from

**17**

*Fillers in Wood Adhesives*

London 9, 1985; part 8

Research. 2012;**19**:21

**97**:19-29

Toronto; 2013

*DOI: http://dx.doi.org/10.5772/intechopen.92150*

PEG based cationic hyperbranched polymer and RNA that protect RNA from enzymatic degardation. Biomacromolecules. 2006;**7**:1386-1388

[40] British Standard. Specification for Bond Performance of Veneer Plywood.

[41] Li X, Essawy H, Pizzi, Delmotte L, Rode K, Le Nouen D, et al. Modification of tannin based rigid foams using oligomers of a hyperbranched

poly(amine-ester). Journal of Polymer

[43] Li K, Geng X, Simonsen J, Karchesy J. Novel wood adhesives from condensed

[44] Zhao Y. Development of bio-based phenol formaldehyde resol resins using mountain pine beetle infested lodgepole pine barks [thesis]. University of

[42] Waite JH. The phylogeny and chemical diversity of quinone-tannin glues and varnishes. Comparative Biochemistry and Physiology. 1990;**B** 

tannins and polyethylenimine. International Journal of Adhesion and

[45] Jiang W, Chen SH, Chen Y. Nanocomposites from phenolic resin and various organo-modified montmorillonites: Preparation and thermal stability. Journal of Applied Polymer Science. 2006;**102**:5336-5343

[46] Wang K, Wu J, Chen L, He C, Toh M. Mechanical properties and

nanocomposites with highly exfoliated pristine clay. In: ANTEC Conference

fracture behavior of epoxy

Proceedings. Chicago; 2004.

pp. 1820-1824

Adhesives. 2004;**24**:327-333

*Fillers in Wood Adhesives DOI: http://dx.doi.org/10.5772/intechopen.92150*

*Fillers*

prepared using a novel biphenylcontaining modified clay. Polymer

[21] Lei H. Synthetic and natural materials for wood adhesive resins [thesis]. Epinal: University Henri

[22] Ma WW. The preparation of urea-formaldehyde resin modified by oxygenated amylum. Chemistry and

[23] Shi YM. Development of the low cost urea-formaldehyde resin adhesive.

China Adhesive. 2003;**12**:30-32

[24] Mosiewicki M, Aranguren MI, Borrajo J. Thermal and mechanical properties of wood flour/tannin

[25] Kargarfard A, Jahan-Latibari A. Application of recycled polyethylene in combination with urea-formaldehyde resin to produce water resistant particleboard. In: Proceedings of the 55th International Convention of Society of Wood Science and Technology. Beijing, China; 2012. pp. 1-7

[26] Du GB, Wu LZ, Yang QX.

resin. Adhesion. 1995;**16**:9-12

Letters. 1984;**52**:1453

Sinica. 2005;**41**:129-135

Development on mineral filler for UF

[27] Cavicchi RE, Silsbee RH. Coulomb suppression of tunneling rate from small metal particles. Physical Review

[28] Lin QJ, Yang GD, Liu JH. Study on the property of nano-SiO2/ureaformaldehyde resin. Scientia Silvae

[29] Yang GD, Lin QJ, Liu JH. The effect of nanometer silicon dioxide on

adhesive composites. Journal of Applied Polymer Science. 2004;**91**:3074-3082

Poincare-Nancy; 2009

Adhesion. 1995;**3**:147-148

[20] Gu JY. Adhesive and Paint. Beijing: China Forestry Publishing House; 1998

properties of UF resin. Journal of Fujian Forestry College. 2004;**24**:114-117

[30] Yang G, Lin QJ, Liu JH. Study on using nanometer SiO2 for modifying UF resin. China Wood Industry. 2004;**18**:7-9

[31] Yu HW, Fu SY, Wen GF. The effect of nanometer calcium carbonate on properties of UF resin. China Adhesives.

[32] Zhang W, Ma Y, Xu Y, Wang C, Chu F. Lignocellulosic ethanol residuebased lignin–phenol–formaldehyde resin adhesive. International Journal of Adhesion and Adhesives. 2013;**40**:11-18

[33] Lv JK, Ke YC, Qi ZN. Synthesis and mechanical properties of epoxy/ clay nanocomposites. Acta Materials Composed Sinica. 2002;**19**:117-121

[34] Tien Y, Chen TK, Wei KH. Synthesis

and characterization of novel segmented polyurethane/clay nanocomposites. Polymer (Guildf).

[35] Choi MH, Chung IJ, Lee JD. Morphology and curing behaviors of phenolic resin-layered silicate nanocomposites prepared by melt intercalation. Chemistry of Materials.

[36] Pizzi A, Mittal KL. Handbook of Adhesive Technology. 2nd ed. New York:

[37] Magnusson H, Malmström E, Hult A. Structure buildup in hyperbranched polymers from 2,2-bis(hydroxymethyl) propionic acid. Macromolecules.

[38] Fang K, Xu Z, Jiang X, Zhang X, Fu S. Preparation and characterization of hyperbranched polyesteramides. Polymer Bulletin. 2008;**60**:533-543

[39] Khan JA, Kainthan RK, Li MG. Water soluble nanoparticles from

2000;**41**:1345-1353

2000;**12**:2977-2983

Marcel Dekker; 2003

2000;**33**:3099-3104

2002;**11**:22-24

Degradation and Stability.

2006;**91**:755-762

**16**

PEG based cationic hyperbranched polymer and RNA that protect RNA from enzymatic degardation. Biomacromolecules. 2006;**7**:1386-1388

[40] British Standard. Specification for Bond Performance of Veneer Plywood. London 9, 1985; part 8

[41] Li X, Essawy H, Pizzi, Delmotte L, Rode K, Le Nouen D, et al. Modification of tannin based rigid foams using oligomers of a hyperbranched poly(amine-ester). Journal of Polymer Research. 2012;**19**:21

[42] Waite JH. The phylogeny and chemical diversity of quinone-tannin glues and varnishes. Comparative Biochemistry and Physiology. 1990;**B 97**:19-29

[43] Li K, Geng X, Simonsen J, Karchesy J. Novel wood adhesives from condensed tannins and polyethylenimine. International Journal of Adhesion and Adhesives. 2004;**24**:327-333

[44] Zhao Y. Development of bio-based phenol formaldehyde resol resins using mountain pine beetle infested lodgepole pine barks [thesis]. University of Toronto; 2013

[45] Jiang W, Chen SH, Chen Y. Nanocomposites from phenolic resin and various organo-modified montmorillonites: Preparation and thermal stability. Journal of Applied Polymer Science. 2006;**102**:5336-5343

[46] Wang K, Wu J, Chen L, He C, Toh M. Mechanical properties and fracture behavior of epoxy nanocomposites with highly exfoliated pristine clay. In: ANTEC Conference Proceedings. Chicago; 2004. pp. 1820-1824

**19**

**Chapter 2**

**Abstract**

Panel Products

wood or even give it new functions.

**1. Introduction**

**2. Filler species**

Wood Adhesive Fillers Used

*Long Cao, Xiaojian Zhou and Guanben Du*

during the Manufacture of Wood

During the manufacture of wood panel products, fillers are commonly added to wood adhesives to lower costs and give body to liquid adhesives and also reduce undesired flow or overpenetration into wood. The fillers used in wood adhesives are often neutral or weakly alkaline compounds that typically require no chemical reaction with curing agent, or other components. Fillers are mixed with other components prior to the application of resin on the surface of wood, wood veneer, or wood flakes. Fillers can be either organic (e.g., rye, wheat, walnut shell, and wood flours), or inorganic (e.g., calcium carbonate, calcium sulfate, aluminum oxide, or bentonites). Overall, fillers are low-cost materials for improving the properties of

**Keywords:** fillers, wood adhesives, performances, advantages, applications

Fillers are solid additives that are primarily used to lower the cost and give body to liquid adhesives or reduce undesired flow or overpenetration into wood. This leads to an improvement in properties of the adhesives and gives rise to new functions [1]. Fillers usually increase the rigidity of cured adhesives. They may also modify the coefficient of thermal expansion of a film to approximately that of the adjacent adherends. This can reduce thermal stresses in the joint generated during cooling following heat-curing conditions or when thermally cycled during service. Fillers, normally, are neutral or weakly alkaline compounds and do not chemically react with adhesives, curing agents, or other components in wood adhesive system.

There are many kinds of adhesives for manufacturing of wood panel products, and the type and the amount of filler greatly affect its performance [2]. The choice of filler depends on the materials and application, making fillers one of the most important components of adhesives during the production of wood-based boards. Adding an appropriate filler to an adhesive can reduce the amount and cost of glue and also improve the performance of the adhesive. Commonly used adhesive fillers are organic, such as flour, soybean powder, wood powder, and bark powder or other agroindustrial

#### **Chapter 2**

## Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products

*Long Cao, Xiaojian Zhou and Guanben Du*

#### **Abstract**

During the manufacture of wood panel products, fillers are commonly added to wood adhesives to lower costs and give body to liquid adhesives and also reduce undesired flow or overpenetration into wood. The fillers used in wood adhesives are often neutral or weakly alkaline compounds that typically require no chemical reaction with curing agent, or other components. Fillers are mixed with other components prior to the application of resin on the surface of wood, wood veneer, or wood flakes. Fillers can be either organic (e.g., rye, wheat, walnut shell, and wood flours), or inorganic (e.g., calcium carbonate, calcium sulfate, aluminum oxide, or bentonites). Overall, fillers are low-cost materials for improving the properties of wood or even give it new functions.

**Keywords:** fillers, wood adhesives, performances, advantages, applications

#### **1. Introduction**

Fillers are solid additives that are primarily used to lower the cost and give body to liquid adhesives or reduce undesired flow or overpenetration into wood. This leads to an improvement in properties of the adhesives and gives rise to new functions [1]. Fillers usually increase the rigidity of cured adhesives. They may also modify the coefficient of thermal expansion of a film to approximately that of the adjacent adherends. This can reduce thermal stresses in the joint generated during cooling following heat-curing conditions or when thermally cycled during service. Fillers, normally, are neutral or weakly alkaline compounds and do not chemically react with adhesives, curing agents, or other components in wood adhesive system.

#### **2. Filler species**

There are many kinds of adhesives for manufacturing of wood panel products, and the type and the amount of filler greatly affect its performance [2]. The choice of filler depends on the materials and application, making fillers one of the most important components of adhesives during the production of wood-based boards. Adding an appropriate filler to an adhesive can reduce the amount and cost of glue and also improve the performance of the adhesive. Commonly used adhesive fillers are organic, such as flour, soybean powder, wood powder, and bark powder or other agroindustrial

wastes (**Table 1**), such as palm kernel and starch material, etc. In addition, inorganic materials such as metal powders, metal oxides, and minerals, have also been used as adhesive fillers, typically to improve compression strength and dimensional stability.


**21**

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products*

According to their color, fillers can be divided into white fillers and color fillers. They can also be divided according to their preparation method into natural fillers and synthetic fillers. They can be divided according to their function into temperature-resistant fillers, conductive fillers, and anti-sink fillers. According to their particle sizes, they can be divided into natural fillers, ultra-fine fillers, and nanoscale fillers. According to their composition, they can be divided into compound

The adhesive composition mainly includes a matrix material, curing agent, toughening agent, diluent, filler, and modifier. However, fillers are solid materials that do not chemically react with the adhesive component but can change its performance [3, 4]. The main functions are summarized in **Table 1** and further discussed

Many polymers have weak intermolecular interactions and low cohesive energies, so their mechanical properties are inferior to other materials. Fillers with an appropriate particle size can enhance the adhesive strength, and the active surface of filler particles can be used to cross-link several large molecular chains to form a network structure. When one molecular chain is stressed, the stress can be dispersed and transferred to other molecules through cross-linking. Even if one chain fractures, the other chains remain intact, and it is unlikely that the entire structure will immediately fracture, leading to a substantial improvement in the mechanical properties of adhesives. Commonly used fillers, such as flour, metal powders, and metal oxides, can improve the compression strength of adhesives and their dimensional stability. Adding carbon black, silica, or calcium carbonate into silicone and rubber glue can improve the tensile strength, hardness, and wear resistance, etc.

Conductive and magnetic adhesives are obtained by the addition of silver powder and carbon-based iron powder into adhesives, respectively. The thermal conductivity of adhesives can be improved using copper powder, aluminum powder, alumina, and magnesium oxide as fillers. The thermal conductivity adhesives can be widely used in microelectronic assembly and bonding electronic products instead of spot welding. The magnetic adhesives can be improved production efficiency because of simple operation process in electrical machinery bonding field. In epoxy resins, zinc chromate and Zr(SiO3)2 can help retain strength and reduce water absorption. Flame retardant powders such as aluminum hydroxide can improve the flame retardancy of an adhesive. Some fillers can also improve the resistance of

Fillers can prevent local overheating near the bonding interface because the curing reaction is exothermic. In most cases, the curing shrinkage of wood adhesives often occurs during the glue bonding process, but filler can be used to adjust the shrinkage rate. The addition of wheat flour can reduce cracking, which is caused by curing shrinkage of urea-formaldehyde resins. The proper selection of filler can

*DOI: http://dx.doi.org/10.5772/intechopen.91280*

**3. The advantages of fillers for wood adhesives**

**3.1 Increase the mechanical properties of the adhesive**

adhesive joints to moisture and heat aging and salt spray.

**3.2 Give new functions of adhesives**

**3.3 Reduce joint stress**

fillers and mixture fillers.

in this section.

#### **Table 1.**

*The types, amount, and function of fillers in wood adhesives.*

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products DOI: http://dx.doi.org/10.5772/intechopen.91280*

*Fillers*

**Types Amounts** 

Flour, wood powder, soybean powder, bark

powder

Copper oxide, magnesium oxide, magnesium hydrate, iron sesquioxide, titanium dioxide, chromium hemitrioxide, zinc oxide, copper powder, silver powder, magnesium carbonate, iron powder

Calcium carbonate, cement, clay, talcum

Carborundum, mica

Synthetic cement, Ganister sand

Asbestos powder, glass fiber, talcum powder

powder

powder

**(%)**

wastes (**Table 1**), such as palm kernel and starch material, etc. In addition, inorganic materials such as metal powders, metal oxides, and minerals, have also been used as adhesive fillers, typically to improve compression strength and dimensional stability.

> increase ductile behavior, avoid overpenetration and heterogeneous spread

hardness, thermal conductivity, electrical conductivity, and processability

curing shrinkage and increase compression and hardness

> increase physical properties

> > performance

mechanical properties

increase viscosity

and fluid flow performance

performance, lubricity, and wear resistance

strength and heat resistance

20–50 Improve impact

10–30 Decrease cost,

25–100 Improve compression,

25–100 Reduce cost and

<100 Reduce cost and

10–20 Improve the fluid flow

White carbon black 20–100 Improve the

Land plaster 10–100 Reduce cost and

Kaolin <10 Improve thixotropy

Graphite <50 Increase thermal

*The types, amount, and function of fillers in wood adhesives.*

**Function Main application** 

**in wood adhesive**

Urea-formaldehyde, phenolformaldehyde, melamineformaldehyde resin, polyvinyl acetate resin

Epoxy, rubber, polyurethane glue, phenolformaldehyde resin, polyvinyl acetate resin

Polyurethane, epoxy, ureaformaldehyde, phenolformaldehyde, polyvinyl acetate resin

Epoxy, phenolformaldehyde resin

> Polyurethane, epoxy resin

Epoxy resin, ureaformaldehyde resin, polyvinyl acetate resin

Epoxy, rubber resin, polyvinyl acetate resin

Rubber, epoxy, urea-formaldehyde resin, polyvinyl acetate resin

Rubber, epoxy resin Musical

Epoxy resin Wood

**Application fields**

Wood-based panels, wood-glued products

> Wood structure products

> Wood structure products

> Wood structure products

> Wood structure products

> > device

bonding and coating

Wood-based panels

> Musical device

Musical device and coating

**20**

**Table 1.**

According to their color, fillers can be divided into white fillers and color fillers. They can also be divided according to their preparation method into natural fillers and synthetic fillers. They can be divided according to their function into temperature-resistant fillers, conductive fillers, and anti-sink fillers. According to their particle sizes, they can be divided into natural fillers, ultra-fine fillers, and nanoscale fillers. According to their composition, they can be divided into compound fillers and mixture fillers.

#### **3. The advantages of fillers for wood adhesives**

The adhesive composition mainly includes a matrix material, curing agent, toughening agent, diluent, filler, and modifier. However, fillers are solid materials that do not chemically react with the adhesive component but can change its performance [3, 4]. The main functions are summarized in **Table 1** and further discussed in this section.

#### **3.1 Increase the mechanical properties of the adhesive**

Many polymers have weak intermolecular interactions and low cohesive energies, so their mechanical properties are inferior to other materials. Fillers with an appropriate particle size can enhance the adhesive strength, and the active surface of filler particles can be used to cross-link several large molecular chains to form a network structure. When one molecular chain is stressed, the stress can be dispersed and transferred to other molecules through cross-linking. Even if one chain fractures, the other chains remain intact, and it is unlikely that the entire structure will immediately fracture, leading to a substantial improvement in the mechanical properties of adhesives. Commonly used fillers, such as flour, metal powders, and metal oxides, can improve the compression strength of adhesives and their dimensional stability. Adding carbon black, silica, or calcium carbonate into silicone and rubber glue can improve the tensile strength, hardness, and wear resistance, etc.

#### **3.2 Give new functions of adhesives**

Conductive and magnetic adhesives are obtained by the addition of silver powder and carbon-based iron powder into adhesives, respectively. The thermal conductivity of adhesives can be improved using copper powder, aluminum powder, alumina, and magnesium oxide as fillers. The thermal conductivity adhesives can be widely used in microelectronic assembly and bonding electronic products instead of spot welding. The magnetic adhesives can be improved production efficiency because of simple operation process in electrical machinery bonding field. In epoxy resins, zinc chromate and Zr(SiO3)2 can help retain strength and reduce water absorption. Flame retardant powders such as aluminum hydroxide can improve the flame retardancy of an adhesive. Some fillers can also improve the resistance of adhesive joints to moisture and heat aging and salt spray.

#### **3.3 Reduce joint stress**

Fillers can prevent local overheating near the bonding interface because the curing reaction is exothermic. In most cases, the curing shrinkage of wood adhesives often occurs during the glue bonding process, but filler can be used to adjust the shrinkage rate. The addition of wheat flour can reduce cracking, which is caused by curing shrinkage of urea-formaldehyde resins. The proper selection of filler can

reduce the difference between the thermal expansion coefficient and the expansion rate between adhesives and bonded materials. Additionally, it can reduce the internal stress of joints, the thermal expansion coefficient, and curing shrinkage ratio of adhesives.

Colloids form during the curing process due to chemical reactions and cause volume shrinkage. Thermal shrinkage will also occur due to the different thermal expansion coefficient of the adhesive. These two types of shrinkage will produce internal stresses in the rubber layer, resulting in stress concentration, cracking, or joint damage of the rubber layer, which directly affects the service life of rubber joints. Filler can be used to adjust shrinkage during curing, reduce the difference of thermal expansion coefficients between the wood adhesive and the object being glued, and can also prevent cracks from extending. Thus, fillers can significantly improve the bonding strength, especially the shear strength at high temperatures.

#### **3.4 Improve operation process**

Fillers in adhesives can adjust the curing speed, prolong the pot life, and facilitate manufacturing. During plywood manufacturing, wheat flour added into urea-formaldehyde resin can increase the viscosity of an adhesive to prevent it from excessively penetrating into wood pores. Fillers can also improve the thixotropy of liquid glues to control their fluidity, adjust the curing speed, extend the service life, and facilitate operation and construction.

Normally, adhesive bonding strength, adhesion, and heat resistance significantly increase when a certain amount of filler is added, especially polar fillers, such as metal powders, metal oxides, and minerals. This reduces adhesive curing shrinkage and coefficients of thermal expansion.

The addition of asbestos wool and glass fiber has been shown to improve the impact strength. Quartz, porcelain, and iron powders can increase the hardness and compression resistance of adhesives, while graphite and talcum powders can improve wear resistance. Alumina and titanium dioxide can increase the bonding strength. Flour is the most widely used filler in wood adhesives in the wood panel industry and is used to improve the mechanical properties, shrinkage, expansion behaviors, and other physical performances at the glue-wood interface. Most importantly, the use of flour reduces the cost.

An appropriate proportion of filler should be used to provide the desired function and to ensure the overall superior performance of an adhesive. A high proportion of filler will increase the viscosity of the adhesive, making it difficult to control and stir, leading to inferior wettability and a low bonding strength. It may also reduce the de-lamination strength and increase wood failure. Overall, the purpose of fillers is to enhance the physical and mechanical properties of woodbased panels.

Filler selection should meet the following requirements:

#### 1.Non-toxic


**23**

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products*

6.Easily dispersed and has good lubrication with adhesive

7.Should meet the specific requirements of the adhesive, such as electrical con-

There are many different types of fillers, which have different applications. The most widely used fillers include natural calcium carbonate, barite powder, quartz powder, talc powder, kaolin, mica powder, attapulgite (aluminum-silicon-magne-

Nanomaterials have quantum size effects, small size effects, surface effects, and macroscopic quantum effects. The addition of small amounts of nanoscale powders into an adhesive can significantly increase its viscosity, improve the bonding strength, and prevent caking. For example, it was found that nanoscale calcium carbonate prolonged the curing time of resin and the content of free formaldehyde decreased as the amount of calcium carbonate increased. Nanoscale montmorillonite was also shown to increase mechanical properties and reduce formaldehyde emission [5–7].

Calcium carbonate (CaCO3) is an odorless white powder and is one of the most widely used fillers [8]. Lightweight precipitated calcium carbonate can be synthesized by a chemical method with a whiteness of 90% and a relative density

chalk, and shells, which are ground to certain fineness by a mechanical method. In the adhesives industry, calcium carbonate is widely used as a filler because of its low price, non-toxicity, white color, abundant resources, easy mixing in formulas, and stable performance. The addition of nanoscale calcium carbonate to an adhesive can enhance the mechanical strength and increase the transparency, thixotropy, and spreading smoothness. Additionally, the adhesive easily provides a shielding effect, leading to

an anti-UV aging effect, as well as an improvement in its mechanical strength.

Kaolin, usually called clay or china clay, has a main mineral component of kaolinite, which is a variety of crystal rock with the molecular formula

. Heavy calcium carbonate is composed of natural calcite, limestone,

site containing water), and flours of different renewable biomaterials.

The selection of appropriate fillers in wood adhesive systems is important because different fillers have different effects, as show in **Table 1** and **Figure 1**.

*DOI: http://dx.doi.org/10.5772/intechopen.91280*

ductivity, heat resistance, etc.

**4. Applications of fillers**

*The function and selection of fillers.*

**Figure 1.**

**4.1 Calcium carbonate**

of 2.6 g/cm3

**4.2 Kaolin**

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products DOI: http://dx.doi.org/10.5772/intechopen.91280*

**Figure 1.** *The function and selection of fillers.*

*Fillers*

ratio of adhesives.

**3.4 Improve operation process**

and facilitate operation and construction.

and coefficients of thermal expansion.

importantly, the use of flour reduces the cost.

reduce the difference between the thermal expansion coefficient and the expansion rate between adhesives and bonded materials. Additionally, it can reduce the internal stress of joints, the thermal expansion coefficient, and curing shrinkage

Colloids form during the curing process due to chemical reactions and cause volume shrinkage. Thermal shrinkage will also occur due to the different thermal expansion coefficient of the adhesive. These two types of shrinkage will produce internal stresses in the rubber layer, resulting in stress concentration, cracking, or joint damage of the rubber layer, which directly affects the service life of rubber joints. Filler can be used to adjust shrinkage during curing, reduce the difference of thermal expansion coefficients between the wood adhesive and the object being glued, and can also prevent cracks from extending. Thus, fillers can significantly improve the bonding strength, especially the shear strength at high temperatures.

Fillers in adhesives can adjust the curing speed, prolong the pot life, and facilitate manufacturing. During plywood manufacturing, wheat flour added into urea-formaldehyde resin can increase the viscosity of an adhesive to prevent it from excessively penetrating into wood pores. Fillers can also improve the thixotropy of liquid glues to control their fluidity, adjust the curing speed, extend the service life,

Normally, adhesive bonding strength, adhesion, and heat resistance significantly

increase when a certain amount of filler is added, especially polar fillers, such as metal powders, metal oxides, and minerals. This reduces adhesive curing shrinkage

The addition of asbestos wool and glass fiber has been shown to improve the impact strength. Quartz, porcelain, and iron powders can increase the hardness and compression resistance of adhesives, while graphite and talcum powders can improve wear resistance. Alumina and titanium dioxide can increase the bonding strength. Flour is the most widely used filler in wood adhesives in the wood panel industry and is used to improve the mechanical properties, shrinkage, expansion behaviors, and other physical performances at the glue-wood interface. Most

An appropriate proportion of filler should be used to provide the desired function and to ensure the overall superior performance of an adhesive. A high proportion of filler will increase the viscosity of the adhesive, making it difficult to control and stir, leading to inferior wettability and a low bonding strength. It may also reduce the de-lamination strength and increase wood failure. Overall, the purpose of fillers is to enhance the physical and mechanical properties of wood-

Filler selection should meet the following requirements:

3.In a specific physical state, such as uniform particle size

4.Low-cost, a wide range of sources, and convenient processing

2.Unreactive toward other components in the wood adhesive system

5.Should not contain moisture, grease, or harmful gases; moisture absorption is

**22**

based panels.

1.Non-toxic

not easily changed


The selection of appropriate fillers in wood adhesive systems is important because different fillers have different effects, as show in **Table 1** and **Figure 1**.

#### **4. Applications of fillers**

There are many different types of fillers, which have different applications. The most widely used fillers include natural calcium carbonate, barite powder, quartz powder, talc powder, kaolin, mica powder, attapulgite (aluminum-silicon-magnesite containing water), and flours of different renewable biomaterials.

Nanomaterials have quantum size effects, small size effects, surface effects, and macroscopic quantum effects. The addition of small amounts of nanoscale powders into an adhesive can significantly increase its viscosity, improve the bonding strength, and prevent caking. For example, it was found that nanoscale calcium carbonate prolonged the curing time of resin and the content of free formaldehyde decreased as the amount of calcium carbonate increased. Nanoscale montmorillonite was also shown to increase mechanical properties and reduce formaldehyde emission [5–7].

#### **4.1 Calcium carbonate**

Calcium carbonate (CaCO3) is an odorless white powder and is one of the most widely used fillers [8]. Lightweight precipitated calcium carbonate can be synthesized by a chemical method with a whiteness of 90% and a relative density of 2.6 g/cm3 . Heavy calcium carbonate is composed of natural calcite, limestone, chalk, and shells, which are ground to certain fineness by a mechanical method.

In the adhesives industry, calcium carbonate is widely used as a filler because of its low price, non-toxicity, white color, abundant resources, easy mixing in formulas, and stable performance. The addition of nanoscale calcium carbonate to an adhesive can enhance the mechanical strength and increase the transparency, thixotropy, and spreading smoothness. Additionally, the adhesive easily provides a shielding effect, leading to an anti-UV aging effect, as well as an improvement in its mechanical strength.

#### **4.2 Kaolin**

Kaolin, usually called clay or china clay, has a main mineral component of kaolinite, which is a variety of crystal rock with the molecular formula

Al2O3·SiO2·nH2O. Kaolinite has a flake structure and can be divided into calcined kaolin and washed kaolin. Calcined kaolin generally has a higher oil absorption, opacity, porosity, hardness, and whiteness than washed kaolin.

Kaolin normally forms an unstable structure in water because of its charge distribution, with positively charged sheet edges and a negatively charged surface. If the kaolin dosage is high, it will form a gel, preventing an adhesive from flowing [9]. Clay is sometimes added to epoxy resins to thicken or modify coefficients of thermal expansion.

#### **4.3 Renewable bio-based materials**

Flour and other renewable bio-based materials include wood powder, starch, protein, and lignocellulose as well as the agroindustrial wastes. Adding a small amount of starch into wood adhesives can significantly increase the viscosity and effectively improve the solid content and initial viscosity of the adhesive [10–12]. Oxidized starch and palm kernel can also neutralize excess acidic substances in the rubber layer, prevent excessive decomposition of the cured rubber layer, and improve the aging resistance of urea-formaldehyde and melamine-urea-formaldehyde resin adhesives [13, 14]. It was also concluded that the stability and initial viscosity of a resin, its pre-compression behavior, and the bonding strength of adhesive products were improved. Walnut shell flour is a filler that is incorporated in urea or resorcinol adhesives to improve spreading or reduce penetration into open wood pores [15–17]. In addition, sorghum flour, protein, bark, and lignin, these kinds of agricultural, forestry, and industrial wastes as fillers have been used in plywood adhesives system [18–21].

In general, different fillers have different advantages. Although kaolin has better properties than flour and calcium carbonate, flour is renewable and sustainable. Most importantly, it is much cheaper, resulting in broader applications in the wood panel industry.

#### **5. Conclusions**

Fillers are low-cost additives for wood adhesives during the manufacture of wood composites. They undergo no chemical reactions with the components of wood adhesive systems and can improve some properties or even provide new functions. In the wood panel industry, almost all factories use kaolin clay or flour blended with other components in wood adhesive systems to reduce undesired flow and overpenetration into wood pores in the glue interphase. With the development of society, low-carbon economy, energy conservation, and environmental issues will drive future adhesive developments. Thus, we can predict that future fillers will be functional, differentiated, refined, nanosized, dust-free, and environmentally benign.

#### **Acknowledgements**

This work is supported by the following grants and programs: (1) National Natural Science Foundation of China (NSFC 31971595, 31760187); (2) Yunnan Provincial Applied and Basic Research Grants (2017FB060); (3) "Ten-thousand Program"-youth talent support program, and (4) Yunnan Provincial Reserve Talents for Middle & Young Academic and Technical Leaders.

**25**

**Author details**

, Xiaojian Zhou\*†

and gongben9@hotmail.com

Southwest Forestry University, Kunming, China

† These authors are contributed equally to this work.

provided the original work is properly cited.

\*Address all correspondence to: xiaojianzhou@hotmail.com

 and Guanben Du\* Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Long Cao†

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products*

*DOI: http://dx.doi.org/10.5772/intechopen.91280*

There is no conflict of interest in this field.

**Conflict of interest**

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products DOI: http://dx.doi.org/10.5772/intechopen.91280*

### **Conflict of interest**

*Fillers*

thermal expansion.

[18–21].

panel industry.

**5. Conclusions**

**4.3 Renewable bio-based materials**

Al2O3·SiO2·nH2O. Kaolinite has a flake structure and can be divided into calcined kaolin and washed kaolin. Calcined kaolin generally has a higher oil absorption,

Kaolin normally forms an unstable structure in water because of its charge distribution, with positively charged sheet edges and a negatively charged surface. If the kaolin dosage is high, it will form a gel, preventing an adhesive from flowing [9]. Clay is sometimes added to epoxy resins to thicken or modify coefficients of

Flour and other renewable bio-based materials include wood powder, starch, protein, and lignocellulose as well as the agroindustrial wastes. Adding a small amount of starch into wood adhesives can significantly increase the viscosity and effectively improve the solid content and initial viscosity of the adhesive [10–12]. Oxidized starch and palm kernel can also neutralize excess acidic substances in the rubber layer, prevent excessive decomposition of the cured rubber layer, and improve the aging resistance of urea-formaldehyde and melamine-urea-formaldehyde resin adhesives [13, 14]. It was also concluded that the stability and initial viscosity of a resin, its pre-compression behavior, and the bonding strength of adhesive products were improved. Walnut shell flour is a filler that is incorporated in urea or resorcinol adhesives to improve spreading or reduce penetration into open wood pores [15–17]. In addition, sorghum flour, protein, bark, and lignin, these kinds of agricultural, forestry, and industrial wastes as fillers have been used in plywood adhesives system

In general, different fillers have different advantages. Although kaolin has better properties than flour and calcium carbonate, flour is renewable and sustainable. Most importantly, it is much cheaper, resulting in broader applications in the wood

Fillers are low-cost additives for wood adhesives during the manufacture of wood composites. They undergo no chemical reactions with the components of wood adhesive systems and can improve some properties or even provide new functions. In the wood panel industry, almost all factories use kaolin clay or flour blended with other components in wood adhesive systems to reduce undesired flow and overpenetration into wood pores in the glue interphase. With the development of society, low-carbon economy, energy conservation, and environmental issues will drive future adhesive developments. Thus, we can predict that future fillers will be functional, differentiated, refined, nanosized, dust-free, and environmentally

This work is supported by the following grants and programs: (1) National Natural Science Foundation of China (NSFC 31971595, 31760187); (2) Yunnan Provincial Applied and Basic Research Grants (2017FB060); (3) "Ten-thousand Program"-youth talent support program, and (4) Yunnan Provincial Reserve

Talents for Middle & Young Academic and Technical Leaders.

opacity, porosity, hardness, and whiteness than washed kaolin.

**24**

benign.

**Acknowledgements**

There is no conflict of interest in this field.

### **Author details**

Long Cao† , Xiaojian Zhou\*† and Guanben Du\* Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming, China

\*Address all correspondence to: xiaojianzhou@hotmail.com and gongben9@hotmail.com

† These authors are contributed equally to this work.

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **References**

[1] Dunky M. Adhesives in the wood industry. Handbook of Adhesive Technology. 2003;**2**:50

[2] Frihart CR. Wood Adhesion and Adhesives. Boca Raton, FL: CRC Press; 2005

[3] Dunky M, Pizzi A. Wood adhesives. Adhesion Science and Engineering. Elsevier Science BV; 2002. pp. 1039-1103

[4] Qiao L, Easteal AJ, Bolt CJ, et al. The effects of filler materials on poly (vinyl acetate) emulsion wood adhesives. Pigment & Resin Technology. 1999;**28**(6):326-330

[5] Lei H, Du G, Pizzi A, Celzard A. Influence of nanoclay on ureaformaldehyde resins for wood adhesives and its model. Journal of Applied Polymer Science. 2008;**109**(4):2442-2451

[6] Zhou X, Pizzi A, Du G. The effect of nanoclay on melamine-ureaformaldehyde wood adhesives. Journal of Adhesion Science and Technology. 2012;**26**(10-11):1341-1348

[7] Ghosh PK, Patel A, Kumar K. Adhesive joining of copper using nanofiller composite adhesive. Polymer. 2016;**87**:159-169

[8] Liu D, Chen H, Chang PR, et al. Biomimetic soy protein nanocomposites with calcium carbonate crystalline arrays for use as wood adhesive. Bioresource Technology. 2010;**101**(15):6235-6241

[9] Zhang B, Chang Z, Li J, et al. Effect of kaolin content on the performances of kaolin-hybridized soybean mealbased adhesives for wood composites. Composites Part B: Engineering. 2019;**173**:106919

[10] Wang Z, Li Z, Gu Z, et al. Preparation, characterization and properties of starch-based wood adhesive. Carbohydrate Polymers. 2012;**88**(2):699-706

[11] Zhang Y, Ding L, Gu J, et al. Preparation and properties of a starch-based wood adhesive with high bonding strength and water resistance. Carbohydrate Polymers. 2015;**115**:32-37

[12] Imam SH, Gordon SH, Mao L, et al. Environmentally friendly wood adhesive from a renewable plant polymer: Characteristics and optimization. Polymer Degradation and Stability. 2001;**73**(3):529-533

[13] Zhao X, Peng L, Wang H, et al. Environment-friendly urea-oxidized starch adhesive with zero formaldehydeemission. Carbohydrate Polymers. 2018;**181**:1112-1118

[14] Ong HR, Khan MMR, Prasad DMR, et al. Palm kernel meal as a melamine urea formaldehyde adhesive filler for plywood applications. International Journal of Adhesion and Adhesives. 2018;**85**:8-14

[15] Khanjanzadeh H, Pirayesh H, Sepahvand S. Influence of walnut shell as filler on mechanical and physical properties of MDF improved by nano-SiO 2. Journal of the Indian Academy of Wood Science. 2014;**11**(1):15-20

[16] Pizzi A. Resorcinol adhesives. Handbook of Adhesive Technology. Taylor & Francis Group, LLC; 2003:599-613

[17] Zhou X, Pizzi A. Pine tannin based adhesive mixes for plywood. International Wood Products Journal. 2014;**5**(1):27-32

[18] Hojilla-Evangelista MP, Bean SR. Evaluation of sorghum flour as extender in plywood adhesives for sprayline coaters or foam extrusion.

**27**

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products*

*DOI: http://dx.doi.org/10.5772/intechopen.91280*

Industrial Crops and Products.

[19] Kang H, Wang Z, Wang Y, et al. Development of mainly plant proteinderived plywood bioadhesives via soy protein isolate fiber self-reinforced soybean meal composites. Industrial Crops and Products. 2019;**133**:10-17

[20] Aydin I, Demirkir C, Colak S, et al. Utilization of bark flours as additive in plywood manufacturing. European Journal of Wood and Wood Products.

[21] Olivares M, Aceituno H, Neiman G,

et al. Lignin-modified phenolic adhesives for bonding radiata pine plywood. Forest Products Journal.

2011;**34**(1):1168-1172

2017;**75**(1):63-69

1995;**45**(1):63

*Wood Adhesive Fillers Used during the Manufacture of Wood Panel Products DOI: http://dx.doi.org/10.5772/intechopen.91280*

Industrial Crops and Products. 2011;**34**(1):1168-1172

[19] Kang H, Wang Z, Wang Y, et al. Development of mainly plant proteinderived plywood bioadhesives via soy protein isolate fiber self-reinforced soybean meal composites. Industrial Crops and Products. 2019;**133**:10-17

[20] Aydin I, Demirkir C, Colak S, et al. Utilization of bark flours as additive in plywood manufacturing. European Journal of Wood and Wood Products. 2017;**75**(1):63-69

[21] Olivares M, Aceituno H, Neiman G, et al. Lignin-modified phenolic adhesives for bonding radiata pine plywood. Forest Products Journal. 1995;**45**(1):63

**26**

*Fillers*

**References**

2005

[1] Dunky M. Adhesives in the wood industry. Handbook of Adhesive

properties of starch-based wood adhesive. Carbohydrate Polymers.

[11] Zhang Y, Ding L, Gu J, et al. Preparation and properties of a starch-based wood adhesive with high bonding strength and water resistance. Carbohydrate Polymers. 2015;**115**:32-37

[12] Imam SH, Gordon SH, Mao L, et al. Environmentally friendly wood adhesive from a renewable plant polymer: Characteristics and

[13] Zhao X, Peng L, Wang H, et al. Environment-friendly urea-oxidized starch adhesive with zero formaldehydeemission. Carbohydrate Polymers.

Stability. 2001;**73**(3):529-533

2018;**181**:1112-1118

2018;**85**:8-14

2003:599-613

2014;**5**(1):27-32

optimization. Polymer Degradation and

[14] Ong HR, Khan MMR, Prasad DMR, et al. Palm kernel meal as a melamine urea formaldehyde adhesive filler for plywood applications. International Journal of Adhesion and Adhesives.

[15] Khanjanzadeh H, Pirayesh H, Sepahvand S. Influence of walnut shell as filler on mechanical and physical properties of MDF improved by nano-SiO 2. Journal of the Indian Academy of

Wood Science. 2014;**11**(1):15-20

[16] Pizzi A. Resorcinol adhesives. Handbook of Adhesive Technology. Taylor & Francis Group, LLC;

[17] Zhou X, Pizzi A. Pine tannin based adhesive mixes for plywood. International Wood Products Journal.

[18] Hojilla-Evangelista MP, Bean SR. Evaluation of sorghum flour as extender in plywood adhesives for sprayline coaters or foam extrusion.

2012;**88**(2):699-706

[2] Frihart CR. Wood Adhesion and Adhesives. Boca Raton, FL: CRC Press;

[3] Dunky M, Pizzi A. Wood adhesives. Adhesion Science and Engineering. Elsevier Science BV; 2002. pp. 1039-1103

[4] Qiao L, Easteal AJ, Bolt CJ, et al. The effects of filler materials on poly (vinyl acetate) emulsion wood adhesives. Pigment & Resin Technology.

[5] Lei H, Du G, Pizzi A, Celzard A. Influence of nanoclay on ureaformaldehyde resins for wood adhesives and its model. Journal of Applied Polymer Science. 2008;**109**(4):2442-2451

[6] Zhou X, Pizzi A, Du G. The effect of nanoclay on melamine-urea-

formaldehyde wood adhesives. Journal of Adhesion Science and Technology.

2012;**26**(10-11):1341-1348

2016;**87**:159-169

2019;**173**:106919

[10] Wang Z, Li Z, Gu Z, et al. Preparation, characterization and

[7] Ghosh PK, Patel A, Kumar K. Adhesive joining of copper using nanofiller composite adhesive. Polymer.

[8] Liu D, Chen H, Chang PR, et al. Biomimetic soy protein nanocomposites with calcium carbonate crystalline arrays for use as wood adhesive. Bioresource Technology. 2010;**101**(15):6235-6241

[9] Zhang B, Chang Z, Li J, et al. Effect of kaolin content on the performances of kaolin-hybridized soybean mealbased adhesives for wood composites. Composites Part B: Engineering.

Technology. 2003;**2**:50

1999;**28**(6):326-330

**29**

**Chapter 3**

**Abstract**

*Giovani Otavio Rissi*

nanofillers, circular economy

**1. Introduction**

brought home.

same place [1].

the place where you are.

Fillers for Packaging Applications

Packaging in general is frequently overlooked and demonized. The lack of educational programs and efficient waste treatment lead packaging to be treated as an environmental problem. However, packaging is an enabler of our society because it makes feasible the availability of any and every good, regardless of its production location. Furthermore, the packaging business plays a significant role in the global economy, following a continuous trend of growth. The use of fillers in various packaging types can be a valuable resource not only for reducing its cost but also improving its mechanical strength (therefore reducing the number of raw materials required for making that specific package), improving its visual properties to ensure customer attractiveness, creating new possibilities of use, and extending the shelf life of perishable foods. However, the use of fillers in packaging should be made in a

**Keywords:** packaging, packaging market, paper packaging, plastic packaging,

Before proceeding, it is worthwhile to make a little observation about the environment around you. Take a quick break and behold what surrounds you and

Unless you are reading this in the wild of nature, everything you see was made available through some sort of packaging, even the materials used in the construction

Also, everything you eat—regardless of being an ordinary meal or a gourmet delight—came to you via packaging entrusted to keep high food safety and hygiene standards. Even if one has a backyard or farm that provides a wide variety of fresh produce, at some point in time some packaged foods will be bought and

Now think about all medicines that are consumed by millions of people every day to keep their well-being and health. Without being noticed, packaging allows for one of the most noble uses: to provide a longer and healthier life. You may now have realized for the first time that life as we know it exists at current standards due to a powerful enabler: packaging. Some studies suggest an association between the quality of life in a certain location and the level of packaging development in the

The reason for this is simple: our society has developed in a way that knowledge and experimentation are part of everyday life. We are informed about the latest trends and we want to try new things. At the same time, just a small fraction of the goods we buy is produced in our vicinity. Most goods are produced thousands of

way that permits proper recovery and recycling after use.

of the place where you are were bundled, contained, or packed.

#### **Chapter 3**
