**4. Natural fiber surface modification: Case study outline**

Natural fibers are amenable to modification as they bear hydroxyl groups from cellulose and lignin. The hydroxyl groups may be involved in the hydrogen bonding within the cellulose molecules thereby reducing the activity towards the matrix. Chemical modifications may activate these groups or can introduce new moieties that can effectively interlock with the matrix. Interfaces play an important role in the physical and mechanical properties of composites (Joseph et al., 2000). Simple chemical treatments can be applied to the fibers with the aim of changing surface tension and polarity of fiber surface.

#### **4.1 Most important surface modification**

The different surface chemical modifications of natural fibers have achieved various levels of success in improving fiber strength, fiber fitness and fiber-matrix adhesion in natural fiber composites. Brief descriptions of some important fiber chemical modifications witch applied in this research are summarized in the following sub-sections.

#### **4.1.1 Alkaline treatment**

The important modification done by alkaline treatment is the disruption of hydrogen bonding in the network structure, thereby increasing surface roughness. This treatment removes a certain amount of lignin, wax and oils covering the external surface of the fiber cell wall, depolymerizes cellulose and exposes the short length crystallites (Li et al., 2000) Addition of aqueous sodium hydroxide (NaOH) to natural fiber promotes the ionization of the hydroxyl group to the alkoxide (Agrawal et al., 2000). Thus, alkaline processing directly influences the cellulosic fibril, the degree of polymerization and the extraction of lignin and hemicellulosic compounds ( Jahn et al., 2002). Alkali treatment increases surface roughness resulting in better mechanical interlocking and the amount of cellulose exposed on the fiber surface. This increases the number of possible reaction sites and allows better fiber wetting. The following reaction, takes place as a result of alkali treatment.

$$\text{Fiber} \longrightarrow \text{OH} + \text{NaOH} \xrightarrow{\text{H}\_2\text{O}} \text{Fiber} \longrightarrow \text{ONa}^+ + \text{H}\_2\text{O}$$

Fig. 10. Alkaline treatment reaction (Pothan et al., 2006).

Many attempts have been done by researchers to increase mechanical properties of biocomposites by alkaline treatments. For example, the effect of alkali treatment on properties of hybrid fiber biocomposite was reported by John et al., (2008). It has been reported that alkali treatment leads to fiber fibrillation i.e. breaking down of fiber bundles into smaller fibers which increases the effective surface area available for contact with the matrix.

Partial removal of lignin and hemicellulose on the alkali modification of cellulose fibers was reported by Sreekala et al., (1997). Mukherjee et al., (1993) reported that the removal of hemicellulose produces less dense and less rigid interfibrillar region. Kokot et al., (1995) also noted that as lignin is removed, the middle lamella joining the ultimate cells is expected to be more plastic as well as homogeneous, due to the gradual elimination of microvoids. Alkaline treatment increases the amount of crystalline cellulose and removes natural and artificial impurities, producing a rough surface topography.

#### **4.1.2 Silane treatment**

234 Some Critical Issues for Injection Molding

short fibers with a relatively low fiber fraction. Natural fiber compounds offer numerous advantages over other injection molding compounds, for instance, low wear of manufacturing tools, often reduced cycle time, and ease of recycling. The final properties of natural fiber thermoplastic composites manufactured by injection molding depend not only on the properties of raw materials used and their compositions, but also on processing methods. Injection molding is a very suitable procedure to process natural fiber reinforced polymers into sophisticated 3-dimensional parts. In this case, the use of granular material, which already includes natural fibers and coupling agent, proves a success. But the incorporation of natural fibers to thermoplastics leads to flow limitation, which are increased by the incompatibility of natural fibers and thermoplastics. A lot of methods have been used to solve this incompatibility problem, and natural fiber chemical modification

Natural fibers are amenable to modification as they bear hydroxyl groups from cellulose and lignin. The hydroxyl groups may be involved in the hydrogen bonding within the cellulose molecules thereby reducing the activity towards the matrix. Chemical modifications may activate these groups or can introduce new moieties that can effectively interlock with the matrix. Interfaces play an important role in the physical and mechanical properties of composites (Joseph et al., 2000). Simple chemical treatments can be applied to

The different surface chemical modifications of natural fibers have achieved various levels of success in improving fiber strength, fiber fitness and fiber-matrix adhesion in natural fiber composites. Brief descriptions of some important fiber chemical modifications witch

The important modification done by alkaline treatment is the disruption of hydrogen bonding in the network structure, thereby increasing surface roughness. This treatment removes a certain amount of lignin, wax and oils covering the external surface of the fiber cell wall, depolymerizes cellulose and exposes the short length crystallites (Li et al., 2000) Addition of aqueous sodium hydroxide (NaOH) to natural fiber promotes the ionization of the hydroxyl group to the alkoxide (Agrawal et al., 2000). Thus, alkaline processing directly influences the cellulosic fibril, the degree of polymerization and the extraction of lignin and hemicellulosic compounds ( Jahn et al., 2002). Alkali treatment increases surface roughness resulting in better mechanical interlocking and the amount of cellulose exposed on the fiber surface. This increases the number of possible reaction sites and allows better fiber wetting.

the fibers with the aim of changing surface tension and polarity of fiber surface.

applied in this research are summarized in the following sub-sections.

The following reaction, takes place as a result of alkali treatment.

Fig. 10. Alkaline treatment reaction (Pothan et al., 2006).

and coupling agent have a better result that is explained below.

**4.1 Most important surface modification** 

**4.1.1 Alkaline treatment** 

**4. Natural fiber surface modification: Case study outline** 

Silane is a chemical compound with chemical formula SiH4. Silanes are used as coupling agents to let glass fibers adhere to a polymer matrix, stabilizing the composite material. Silane coupling agents may reduce the number of cellulose hydroxyl groups in the natural fiber–matrix interface. Silanes undergo hydrolysis, condensation and the bond formation stage. The reaction scheme is given as follows:

Fig. 11. Hydrolysis of silane and hypothetical reaction of fibers and silane (Sreekala et al., 2000).

Bledzki et al., (1996) concluded that in the process of interaction between natural fibers and silanes, alkoxy silanes are able to form bonds with hydroxyl groups. In presence of moisture hydrolyzable alkoxy group of silans leads to the formation of silanols. Silanols can form polysiloxane structures by reaction with hydroxyl group of the fibers.

John et al., (2008) and Mathew et al., (2004) investigated the effect of silane treatment on the mechanical properties of biocomposites. They observed a marked improvement in the properties after chemical modification. Sreekala et al., (1997) also suggested that the silane treated cellulose fiber composite showed an increase in nucleation density compared the untreated fiber composite. The increased nucleation yielded smaller crystals that result in a transcrystalline interphase region, with improved bonding between the fiber and the matrix (Agrawal et al., 2000).

Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 237

Many of maleic anhydride grafted polyolefin such as HDPE-*g*-MA, PP-*g*-MA, and LDPE-*g*-MA studied by Polec et al., (2010), Farsi, (2010) and Tasdemir et al., (2009) respectively. they observed strong interfacial strength by incorporation of the coupling

One of the most important methods for studying the effect of natural fiber chemical surface treatment on the interfacial strength of the composites is dynamic mechanical thermal analysis (DMTA). The effect of wood chemical surface modification on the interfacial strength was tracked using adhesion factor. This parameter is obtained from DMTA data. Effect of PP-g-MA as coupling agent on the interface adhesion of WPC investigated by Correa et al., (2007). They mentioned a method based on a simplified single rule of mixtures aiming to compare differences in interface adhesion in the presence of PP-g-MA is proposed in terms of relaxation spectra of polypropylene–wood composites obtained by DMTA. DMTA has been widely used to investigate the structures and viscoelastic behaviors of composite materials as determined by their storage modulus (E'), loss modulus (E'') and loss factor (tanδ). This analysis technique can provide information on the stiffness of the composites (Kim et al., 2005). Relaxation peaks (α, β and γ) are observed for the tanδ curves, which are caused by the onset of the various motions of the chain molecules. The dominant β peak represents the glass-to-rubber transition of the amorphous portion in composites and

For determination of adhesion factor as interfacial intraction criterion, Correa et al., (2007) has been used equation originated from Kubat et al., (1990) work, about high density polyethylene filled with 20 Vol. % glass fibers. They assumed that the mechanical loss factor

Where the subscript f,i and p denotes filler ,interphase, and matrix respectively and Φ is the corresponding volume fraction. By considering δ<sup>f</sup> ≈ 0 and since the volume fraction of the

*Tan Tan Tan Tan <sup>c</sup> f f i i p p* (1)

Fig. 14. PP-g-MA reaction with natrural fiber (Bledzki et al., 1996).

**4.2 Role of chemical treatment on thermo-mechanical behavior** 

is assigned to the glass transition temperature.

interphase is rather small, above equation can be rearranged as follows:

(tanδc) of the composite can be written:

agents.

#### **4.1.3 Benzoylation treatment**

Benzoylation is an important transformation in organic synthesis (Paul et al., 2003). Benzoyl chloride is most often used in fiber treatment. The Benzoyl clride forme an ester linkage to the natural fibers, reducing its hydrophylicity and making it more compatible with matrix. The reaction between the cellulosic–OH group of natural fiber and benzoyl chloride is shown as follows:

$$\begin{array}{ccccc} \text{O}^{\text{e}} + ^{+} \text{NaOH} & \longrightarrow & \text{O} \\ \text{H}^{\text{e}} \longrightarrow \text{O} \text{H} + ^{+} \text{NaCl} & \longrightarrow & \text{O} \\ \text{O} & & \begin{array}{ccccc} \text{O} & & \text{O} \\ \text{O} & & & \text{O} \\ \text{O} & & & \end{array} \end{array}$$

Fig. 12. Reaction between cellulosic-OH groups and benzoyl chloride (Joseph et al., 2000).

Abu bakar & Baharulrazi (2008) also indicated that the benzoylated oil palm empty fruit bunch was able to improve the tensile properties, impact strength and the increase of water resistance, and the reduction of glass transition temperature of composites when compared to the untreated fiber.

#### **4.1.4 Acrylation treatment**

Acrylation reaction is initiated by free radicals of the cellulose molecule. Cellulose can be treated with high energy radiation to generate radicals together with chain scission (Bledzki & Gassan, 1999). The reaction was accomplished between OH groups and acrylic acid as follows:

$$\text{Fiber}-\text{OH} + \text{CH2} = \text{CH}-\text{COOH} \rightarrow \text{Fiber}-\text{O}-\text{CH2}-\text{CH2}-\text{COOH}$$

Fig. 13. Reaction between cellulosic-OH groups and acrylic acide (Mohanty et al., 2001).

Sreekala et al., (2000, 2002) used acrylic acid in natural fiber surface modification. acrylation led to strong covalent bond formation and thereby, the tensile strength and Young's modulus of treated fibers were improved marginally.

#### **4.1.5 Polyolefin-g-MA coupling agents**

Coupling agents such as PP grafted with maleic anhydride (PP-g-MA) and PP grafted with acrylic acid (PP-g-AA) are usually employed to improve interfacial properties. Esterification reaction and H-bond interactions may take place at the interface of the cellulosic filler and the PP-g-MA as suggested in Fig. 14. The PP chain permits maleic anhydride to be cohesive and produce maleic anhydride grafted polypropylene (PP-g-MA). Then the treatment of cellulose fibers with hot PP-g-MA copolymers provides covalent bonds across the interface. After this treatment, the surface energy of cellulose fibers is increased to a level much closer to the surface energy of the matrix. This results in better wettability and higher interfacial adhesion of the fiber.

Benzoylation is an important transformation in organic synthesis (Paul et al., 2003). Benzoyl chloride is most often used in fiber treatment. The Benzoyl clride forme an ester linkage to the natural fibers, reducing its hydrophylicity and making it more compatible with matrix. The reaction between the cellulosic–OH group of natural fiber and benzoyl chloride is

Fig. 12. Reaction between cellulosic-OH groups and benzoyl chloride (Joseph et al., 2000).

Abu bakar & Baharulrazi (2008) also indicated that the benzoylated oil palm empty fruit bunch was able to improve the tensile properties, impact strength and the increase of water resistance, and the reduction of glass transition temperature of composites when compared

Acrylation reaction is initiated by free radicals of the cellulose molecule. Cellulose can be treated with high energy radiation to generate radicals together with chain scission (Bledzki & Gassan, 1999). The reaction was accomplished between OH groups and acrylic acid as

Fig. 13. Reaction between cellulosic-OH groups and acrylic acide (Mohanty et al., 2001).

modulus of treated fibers were improved marginally.

**4.1.5 Polyolefin-g-MA coupling agents** 

adhesion of the fiber.

Sreekala et al., (2000, 2002) used acrylic acid in natural fiber surface modification. acrylation led to strong covalent bond formation and thereby, the tensile strength and Young's

Coupling agents such as PP grafted with maleic anhydride (PP-g-MA) and PP grafted with acrylic acid (PP-g-AA) are usually employed to improve interfacial properties. Esterification reaction and H-bond interactions may take place at the interface of the cellulosic filler and the PP-g-MA as suggested in Fig. 14. The PP chain permits maleic anhydride to be cohesive and produce maleic anhydride grafted polypropylene (PP-g-MA). Then the treatment of cellulose fibers with hot PP-g-MA copolymers provides covalent bonds across the interface. After this treatment, the surface energy of cellulose fibers is increased to a level much closer to the surface energy of the matrix. This results in better wettability and higher interfacial

**4.1.3 Benzoylation treatment** 

shown as follows:

to the untreated fiber.

follows:

**4.1.4 Acrylation treatment** 

Many of maleic anhydride grafted polyolefin such as HDPE-*g*-MA, PP-*g*-MA, and LDPE-*g*-MA studied by Polec et al., (2010), Farsi, (2010) and Tasdemir et al., (2009) respectively. they observed strong interfacial strength by incorporation of the coupling agents.

Fig. 14. PP-g-MA reaction with natrural fiber (Bledzki et al., 1996).

#### **4.2 Role of chemical treatment on thermo-mechanical behavior**

One of the most important methods for studying the effect of natural fiber chemical surface treatment on the interfacial strength of the composites is dynamic mechanical thermal analysis (DMTA). The effect of wood chemical surface modification on the interfacial strength was tracked using adhesion factor. This parameter is obtained from DMTA data. Effect of PP-g-MA as coupling agent on the interface adhesion of WPC investigated by Correa et al., (2007). They mentioned a method based on a simplified single rule of mixtures aiming to compare differences in interface adhesion in the presence of PP-g-MA is proposed in terms of relaxation spectra of polypropylene–wood composites obtained by DMTA. DMTA has been widely used to investigate the structures and viscoelastic behaviors of composite materials as determined by their storage modulus (E'), loss modulus (E'') and loss factor (tanδ). This analysis technique can provide information on the stiffness of the composites (Kim et al., 2005). Relaxation peaks (α, β and γ) are observed for the tanδ curves, which are caused by the onset of the various motions of the chain molecules. The dominant β peak represents the glass-to-rubber transition of the amorphous portion in composites and is assigned to the glass transition temperature.

For determination of adhesion factor as interfacial intraction criterion, Correa et al., (2007) has been used equation originated from Kubat et al., (1990) work, about high density polyethylene filled with 20 Vol. % glass fibers. They assumed that the mechanical loss factor (tanδc) of the composite can be written:

$$Tran\delta\_c = \Phi\_f Tran\delta\_f + \Phi\_i Tran\delta\_i + \Phi\_p Tran\delta\_p\tag{1}$$

Where the subscript f,i and p denotes filler ,interphase, and matrix respectively and Φ is the corresponding volume fraction. By considering δ<sup>f</sup> ≈ 0 and since the volume fraction of the interphase is rather small, above equation can be rearranged as follows:

Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 239

The chemical structure of natural fibres was measured according to TAPPI standard and aspect ratio was obtained using optical microscopy. Triethoxy vinyl silane, acrylic acid;

The Wood flour was introduced in a stainless steel vessel and 2 wt.% solution of NaOH was added into the vessel and stirred well. Wood flour was immersed for half an hour. Once this time was over, the fillers were separated from the solution and washed with distilled water containing a few percent of acetic acid to remove residual of alkali. The washed fiber was

Wood flour was dipped in solution of 5% of NaOH and benzoyl chloride for 15 minutes. Ethanol solution was used to remove of extra benzoyl chloride for one hour. Finally the fiber

Wood flour (which was treated with NaOH) was immersed to acrylic acid solution at 50° C

The silane used was Triethoxy vinyl silane. 1% of the respective silane was prepared by mixing with an ethanol/water mixture in the ratio 60/40 and was allowed to stand for 1 h. The pH of the solution was maintained between 3.5- 4 with the addition of acetic acid. Wood flour dipped in this solution and was allowed to stand for 1.5 h. The ethanol/water mixture was drained out and the washing and drying steps were repeated as mentioned in previous

The modified and unmodified wood flour was dried at 103±2 °C to constant weight before mixing process. PP and the modified and unmodified wood flour were blended in a batch mixer (Haake Buchler) at 190°C and 60 rpm for 8 min. In all cases, the weight ratio of fiber and polymer was 40:60 (Table 2). From the compounds which had been granulated, specimens were injection molded into ASTM standard by an injection molder at a molding

Dynamic mechanical thermal analysis (DMTA) was carried out by using Triton instrument, Model Tritic 2000 made by UK in triple-point bending mode. The dimension of each sample

benzoyl chloride and sodium hydroxide were from Merck Co, Germany.

was washed with distilled water and dried in oven at 80° C for 24 hours.

for half an hour and was then washed and dried similar to previous steps.

**5.2 Preparation of natural fibers** 

**5.2.1 Modification with sodium hydroxide** 

then dried in the oven at 80° C for 24 hours.

**5.2.2 Modification with benzoyl chloride** 

**5.2.3 Modification with acrylic acid** 

**5.2.4 Modification with silane** 

**5.3 Processing of the composites** 

temperature of 190°C and injection pressure was 3 Mpa.

treatments.

**5.4 Measurements** 

**5.4.1 Thermo-mechanical test** 

$$\frac{Tam\mathcal{S}\_c}{Tam\mathcal{S}\_p} = (1 + \Phi\_f)(1 + A) \tag{2}$$

$$A = \left[ \left[ \frac{\Phi\_i}{(1 + \Phi\_f)} \right] \right] \left( \frac{T an \mathcal{S}\_i}{T an \mathcal{S}\_p} \right) \tag{3}$$

Where Equation (3) can be rewritten as:

$$A = \left[ \mathbf{I} \left( \frac{\mathbf{1}}{\left( 1 + \Phi\_f \right)} \right) \left( \frac{\operatorname{Tan} \delta\_c}{\operatorname{Tan} \delta\_p} \right) \right] \cdot \mathbf{1} \tag{4}$$

With calculating A factor from DMAT data, one can interpret the interaction in the interphase, where there is strong interaction between wood fiber and polymer matrix due to reduction of macromolecular mobility in the vicinity of the filler surface, A factor decreases. In other words, a low value of A factor is an indication of good adhesion or high degree of interaction between two phases. This factor presents a macroscopic quantitative measure of interfacial adhesion during dynamic loading.

#### **5. Experimental**

#### **5.1 Materials**

Polypropylene of Arak Petrochemical Company in Iran (Trade Name of V30S) with a density of 0.9 g/cm3, and the melt flow index (MFI) of 16 g/10 min was used in this study as matrix. 60-mesh virgin wood flour was used as filler. The characteristics of the natural fibres are shown in Fig. 15.

Fig. 15. Characteristics of the wood flour.

The chemical structure of natural fibres was measured according to TAPPI standard and aspect ratio was obtained using optical microscopy. Triethoxy vinyl silane, acrylic acid; benzoyl chloride and sodium hydroxide were from Merck Co, Germany.
