**4. Properties**

The physical and mechanical properties of plastic lumber differ from natural wood parts with the same dimensions [6, 16, 17]. One of the most significant differences is the lower stiffness (modulus of elasticity) of plastic lumber. Pine and oak typically have a modulus of at least 6.9 GPa, which is higher than that of plastic lumber without a filler.

Mechanical properties of polymers depend on the time and temperature at which stress is applied. Plastic lumber is subject to permanent deformation (creep) under long-term loads [17, 18]. The strain rate depends on the amount and duration of the stress applied, as well as temperature. Moreover, changes in size as a result of temperature are more marked in plastic lumber than natural wood [8].

Drawbacks of plastic lumber include its low resistance to heat and flame in relation to the slow burn of natural wood, intense heating when exposed to direct sunlight and slow cooling. These problems can be mitigated by placing a small opening between adjacent planks, allowing air to flow around them and generating a cooling effect [17] or by producing hollow or foamed profiles.

These differences mean plastic lumber is generally unsuitable as a direct replacement for natural wood of a similar shape and size, since the resulting structures may exhibit unacceptable deformation under load or buckle over time due to their own weight [17, 43].

The abovementioned features limit the use of plastic lumber in structural applications such as support posts for decks. Most plastic lumber in decks is used as flooring, where the flexural modulus is less critical. The properties of this synthetic material change with the addition of fillers or compatibilizers or by promoting crosslinking of the base polymer [32].

Different compositions have been used to modify the physical and mechanical properties of plastic lumber and thereby ensure a larger number of applications with better results, the most noteworthy being those containing wood waste. One of the benefits of wood-plastic composites (WPC) is that they provide an alternative for waste from the lumber industry, which requires special attention as a low density material that needs a significant amount of storage space. Furthermore, using plant fibers in polymer composites improves the mechanical performance of conventional plastics, reduces environmental impacts, ensures recyclability and lowers costs [9, 44].

Wood fillers also increase the stiffness of composites, improve their machinability and are less expensive than polymer resin. Given the increasing use and importance of WPC, different authors have studied the effects of adding wood fiber on the mechanical properties of plastic lumber [7, 9, 43, 45–47]. **Table 2** shows flexural modulus results of three composites made with the same filler, however in three different polymer matrices.

Research by Solís and Lisperguer [48] indicates that adding wood waste reduced the impact strength of WPC (**Table 3**).

**7**

*Processing and Properties of Plastic Lumber DOI: http://dx.doi.org/10.5772/intechopen.82819*

*Impact strength of HDPE and wood flour composite [48].*

**Table 2.**

**Table 3.**

natural wood decking [17, 18, 43].

natural wood [10, 11].

Carroll et al. [43] evaluated the shear strength of Duraboard® plastic lumber planks made from a compound of recycled plastic and sawdust under load and high temperatures. Mechanical tests were also conducted simulating winter (−23.3°C) and summer conditions (40.6°C). The results under winter temperatures showed that plastic lumber exhibits tension, compression and flexure properties comparable to those of natural wood, but lower strength under simulated summer conditions, albeit with acceptable values. The findings demonstrate that plastic lumber pieces should be larger than their natural wood counterparts to compensate for these differences. The high temperature modulus of the plastic lumber was lower than that of natural wood, which increases structural deformation when submitted to loading. This behavior is evident in a number of decking applications. The low stiffness of plastic lumber planks makes them prone to buckling under their own weight. In the case of long plastic lumber planks, the distance between the support posts should be smaller and/or thicker planks should be used when compared to

**Type of polymeric matrix in composite Flexural modulus (GPa)**

*Flexural modulus values as a function of plastic matrix in lumber samples with 25 wt% of wood flour [27].*

**Amount of wood flour (wt%) Impact strength (kJ/m2**

0 10.0 20 2.3 10 1.9

**)**

Polyethylene 1.7 Polypropylene 2.1 Polystyrene 4.9

Glass fiber has also been used as a filler to reinforce WPC, which can significantly increase the elastic modulus and stiffness of PL, albeit not to the extent of

Breslin et al. [27] studied long-term variations in the mechanical properties of recycled plastic lumber made from HDPE with 20% fiberglass used to build a pier. The results of dimensional stability assessment indicated no significant variation in sample dimensions. Additionally, hardness showed no significant change over time. However, individual hardness measurements of the exposed cross-section varied

considerably along the surface (22 ± 16–36 ± 9 units on the Shore D scale). The authors attributed this result to the porous internal structure of the plastic lumber, making it denser closer to the surface. As such, since the properties of the porous core differ from those at the outer surface, plastic lumber should always be tested at different internal and external points in order to obtain conclusive results [43]. There was no significant difference in compressive strength until the nineteenth month of exposure to extreme weather conditions. The flexural modulus showed a high degree of variation between duplicate samples of plastic lumber profiles. Although the flexural modulus measured in the cross-sectional direction did not vary significantly between the first and nineteenth month of exposure, a substantial variation was measured over time, with the greatest change recorded in the summer months. According to the authors [43], varying cross-sectional bending

*Processing and Properties of Plastic Lumber DOI: http://dx.doi.org/10.5772/intechopen.82819*


#### **Table 2.**

*Thermosoftening Plastics*

alternative.

**4. Properties**

lumber without a filler.

own weight [17, 43].

crosslinking of the base polymer [32].

three different polymer matrices.

the impact strength of WPC (**Table 3**).

The fourth WPC production process uses wet wood and separate feeding of the polymers and additives (wood first, split feed), whereby the wet wood residue is fed into the extruder first and the polymer and additive are subsequently introduced into the barrel via a side feeder. However, this process typically requires longer extruders (L/D 44 or 48:1) with degassing zones located close to the feeding zone to

remove moisture from the wood, which is not always possible [42].

Changes in moisture can lead to melt consistency problems in processes where an extruder is used to dry the wood fiber, making pre-dried wood a safer

The physical and mechanical properties of plastic lumber differ from natural wood parts with the same dimensions [6, 16, 17]. One of the most significant differences is the lower stiffness (modulus of elasticity) of plastic lumber. Pine and oak typically have a modulus of at least 6.9 GPa, which is higher than that of plastic

Mechanical properties of polymers depend on the time and temperature at which stress is applied. Plastic lumber is subject to permanent deformation (creep) under long-term loads [17, 18]. The strain rate depends on the amount and duration of the stress applied, as well as temperature. Moreover, changes in size as a result of

Drawbacks of plastic lumber include its low resistance to heat and flame in relation to the slow burn of natural wood, intense heating when exposed to direct sunlight and slow cooling. These problems can be mitigated by placing a small opening between adjacent planks, allowing air to flow around them and generating

These differences mean plastic lumber is generally unsuitable as a direct replacement for natural wood of a similar shape and size, since the resulting structures may exhibit unacceptable deformation under load or buckle over time due to their

The abovementioned features limit the use of plastic lumber in structural applications such as support posts for decks. Most plastic lumber in decks is used as flooring, where the flexural modulus is less critical. The properties of this synthetic material change with the addition of fillers or compatibilizers or by promoting

Different compositions have been used to modify the physical and mechanical properties of plastic lumber and thereby ensure a larger number of applications with better results, the most noteworthy being those containing wood waste. One of the benefits of wood-plastic composites (WPC) is that they provide an alternative for waste from the lumber industry, which requires special attention as a low density material that needs a significant amount of storage space. Furthermore, using plant fibers in polymer composites improves the mechanical performance of conventional plastics, reduces environmental impacts, ensures recyclability and lowers costs [9, 44]. Wood fillers also increase the stiffness of composites, improve their machinability and are less expensive than polymer resin. Given the increasing use and importance of WPC, different authors have studied the effects of adding wood fiber on the mechanical properties of plastic lumber [7, 9, 43, 45–47]. **Table 2** shows flexural modulus results of three composites made with the same filler, however in

Research by Solís and Lisperguer [48] indicates that adding wood waste reduced

temperature are more marked in plastic lumber than natural wood [8].

a cooling effect [17] or by producing hollow or foamed profiles.

**6**

*Flexural modulus values as a function of plastic matrix in lumber samples with 25 wt% of wood flour [27].*


#### **Table 3.**

*Impact strength of HDPE and wood flour composite [48].*

Carroll et al. [43] evaluated the shear strength of Duraboard® plastic lumber planks made from a compound of recycled plastic and sawdust under load and high temperatures. Mechanical tests were also conducted simulating winter (−23.3°C) and summer conditions (40.6°C). The results under winter temperatures showed that plastic lumber exhibits tension, compression and flexure properties comparable to those of natural wood, but lower strength under simulated summer conditions, albeit with acceptable values. The findings demonstrate that plastic lumber pieces should be larger than their natural wood counterparts to compensate for these differences. The high temperature modulus of the plastic lumber was lower than that of natural wood, which increases structural deformation when submitted to loading. This behavior is evident in a number of decking applications. The low stiffness of plastic lumber planks makes them prone to buckling under their own weight. In the case of long plastic lumber planks, the distance between the support posts should be smaller and/or thicker planks should be used when compared to natural wood decking [17, 18, 43].

Glass fiber has also been used as a filler to reinforce WPC, which can significantly increase the elastic modulus and stiffness of PL, albeit not to the extent of natural wood [10, 11].

Breslin et al. [27] studied long-term variations in the mechanical properties of recycled plastic lumber made from HDPE with 20% fiberglass used to build a pier. The results of dimensional stability assessment indicated no significant variation in sample dimensions. Additionally, hardness showed no significant change over time.

However, individual hardness measurements of the exposed cross-section varied considerably along the surface (22 ± 16–36 ± 9 units on the Shore D scale). The authors attributed this result to the porous internal structure of the plastic lumber, making it denser closer to the surface. As such, since the properties of the porous core differ from those at the outer surface, plastic lumber should always be tested at different internal and external points in order to obtain conclusive results [43]. There was no significant difference in compressive strength until the nineteenth month of exposure to extreme weather conditions. The flexural modulus showed a high degree of variation between duplicate samples of plastic lumber profiles. Although the flexural modulus measured in the cross-sectional direction did not vary significantly between the first and nineteenth month of exposure, a substantial variation was measured over time, with the greatest change recorded in the summer months. According to the authors [43], varying cross-sectional bending


**Table 4.**

*Plastic lumber tensile modulus of HDPE reinforced with glass fiber and/or wood fiber [10].*

modulus values may be due to the heterogeneity of the material. Significant bending modulus variations should be taken into account when designing plastic lumber structures.

George and Dillman [10] analyzed glass fiber filler used to reinforce plastic lumber made from recycled HDPE. Additionally, the authors tested different formulations containing wood fiber and compared the effect of the content of each filler on the mechanical properties of the composites. The results showed that glass fiber significantly improved stiffness and promoted a greater increase in tensile and flexural moduli when compared to wood fiber (**Table 4**).

In some cases, adding glass fiber to plastic lumber for applications that come into contact with the skin, such as benches and handrails, can cause skin irritation. Glass fiber is also associated with a disease that affects the lungs in a manner similar to asbestos [41]. As such, these types of applications should be avoided.

Ledur et al. [49] developed a plastic lumber made from a mixture of polyethylene (PE) urban waste and ethylene-vinyl acetate (EVA) copolymer industrial waste filled with calcium carbonate. It was compounded in a Drais batch mixer and hot compression molded as rectangular-shaped sheets. An urban trash container prototype was prepared from the plastic sheets and a hundred trash containers passed a pilot test.

Other types of fillers have been studied for use in plastic composites, particularly natural components such as sisal, jute, hemp and coconut fiber [9, 11, 50, 51]. Wambua et al. [11] produced polypropylene composites reinforced with lignocellulosic fibers and found mechanical property values similar to those reported in the literature for glass fiber-reinforced PP composites.

Natural fibers offer a number of advantages over mineral or synthetic fillers as reinforcement in polymer composites, including less equipment abrasion, lower density of the final product, low cost and greater abundance.

One of the difficulties encountered in plastic lumber technology is the wide range of raw polymers, whose composition is beyond industrial control. This diversity results in polymer blends with coarse phase separated morphologies, which inevitably lead to incompatibility of properties. Possible solutions include adding a compatibilizer to the polymer blend or generating in situ molecular changes in the components through reactive extrusion to allow bonds to form between the polymer chains of the different phases [34].

Wood fiber/polyolefin composites widely used to obtain plastic lumber are incompatible because the thermoplastic material is nonpolar, while cellulose is polar, thereby requiring compatibilization using coupling or interfacial adhesion agents. Processing aspects, compatibilization and properties have been investigated by several authors [34, 42, 47].

Initially, the compatibilizer or coupling agent in contact with the filler surface should interact strongly with the fibers through strong covalent bonds, acid-base

**9**

*Processing and Properties of Plastic Lumber DOI: http://dx.doi.org/10.5772/intechopen.82819*

tion process.

biocides.

the fiber [9, 34, 50–52].

because it is less brittle.

decreases properties in general [54].

interactions or hydrogen bonds. The compatibilizing agent should contain a sufficient number of functional groups to allow reaction with cellulose hydroxyl groups. Another aspect to consider is the length of compatibilizer chains, which should be long enough to allow molecular entanglement with the polymer matrix in the interphase through mechanical anchoring [42]. Some authors [34, 52] have reported surface treatment of the fiber as a means of optimizing the compatibiliza-

Studies demonstrate that the modulus values of WPC increase slightly in relation to neat polymers, but with no statistically significant differences. However, there is almost no variation in tensile strength between neat HDPE and maleinized-polypropylene (MAPP) composites, although the former exhibited lower impact strength in relation to the other samples. This result is attributed to better adhesion between the fiber and polymer matrix, allowing better stress transfer to

In addition to compatibilizers, other additives are used to improve the properties and appearance of the final product, such as impact modifiers, colorants, flame retardants, antioxidants, UV stabilizers, lubricants, stabilizers and

One way of altering the properties of plastic lumber is by modifying the molecu-

lar structure of one or more component polymers through exposure to ionizing radiation. Irradiation has the advantage of being a clean, continuous easy-to-control process [53–56]. It promotes the formation of crosslinks parallel to chain scission as well as double bonds. Crosslinking causes an increase in molecular weight, which typically improves properties, whereas chain scission reduces molecular weight and

Martins et al. [54] assessed the effects of ionizing radiation on IMAWOOD®

One alternative to improve the mechanical properties of wood fiber-reinforced

Studies have shown that crosslinked composites obtained by addition of silane display greater toughness, impact strength and interfacial adhesion than composites without silane [57, 58]. Additionally, the formation of crosslinks in the polymer

Bengtsson and Oksman [58] analyzed the effects of silane on the mechanical properties of wood-HDPE composites. The silane-grafted composites were stored at different moisture levels (in a sauna and temperature chamber) to determine how this parameter affects the degree of crosslinking in the composites. Experiments conducted with different amounts of vinyltrimethoxysilane in the presence of small amounts of peroxide indicated that the samples stored in the sauna showed the highest degree of crosslinking, which was calculated by measuring the gel content and swelling ratio. The results demonstrated that the samples exposed to the highest moisture level exhibited a higher degree of crosslinking. Water was responsible for the hydrolysis of methoxy groups to silanol, increasing the degree of crosslinking in the material stored in the sauna. The flexural modulus and flexural strength of neat HDPE were higher than those of the silane-grafted composites. In contrast to

plastic lumber made from recycled polyethylene (around 75% low-density polyethylene—LDPE and 25% HDPE), using irradiation produced by industrial equipment with a 60Co source. The specimens received total doses of 10, 500, 1000 and 2000 kGy. The authors concluded that irradiation in air increased the tensile strength of IMAWOOD®, although a decrease in elongation at break was observed with ductile-brittle transition. Additionally, from an engineering standpoint, IMAWOOD® offers the best conditions for certain applications after irradiation

plastic lumber is through crosslinking using silane [57–60].

matrix reduces buckling when the material is overloaded [57].

#### *Processing and Properties of Plastic Lumber DOI: http://dx.doi.org/10.5772/intechopen.82819*

*Thermosoftening Plastics*

**Glass fiber Wood fiber**

structures.

**Table 4.**

modulus values may be due to the heterogeneity of the material. Significant bending modulus variations should be taken into account when designing plastic lumber

**Composition (wt%) Tensile modulus (MPa)**

0 0 810 25 0 2310 0 8 980 25 8 2900

*Plastic lumber tensile modulus of HDPE reinforced with glass fiber and/or wood fiber [10].*

George and Dillman [10] analyzed glass fiber filler used to reinforce plastic lumber made from recycled HDPE. Additionally, the authors tested different formulations containing wood fiber and compared the effect of the content of each filler on the mechanical properties of the composites. The results showed that glass fiber significantly improved stiffness and promoted a greater increase in tensile and

In some cases, adding glass fiber to plastic lumber for applications that come into contact with the skin, such as benches and handrails, can cause skin irritation. Glass fiber is also associated with a disease that affects the lungs in a manner similar to

Ledur et al. [49] developed a plastic lumber made from a mixture of polyethylene (PE) urban waste and ethylene-vinyl acetate (EVA) copolymer industrial waste filled with calcium carbonate. It was compounded in a Drais batch mixer and hot compression molded as rectangular-shaped sheets. An urban trash container prototype was prepared from the plastic sheets and a hundred trash containers passed a pilot test. Other types of fillers have been studied for use in plastic composites, particularly natural components such as sisal, jute, hemp and coconut fiber [9, 11, 50, 51]. Wambua et al. [11] produced polypropylene composites reinforced with lignocellulosic fibers and found mechanical property values similar to those reported in the

Natural fibers offer a number of advantages over mineral or synthetic fillers as reinforcement in polymer composites, including less equipment abrasion, lower

One of the difficulties encountered in plastic lumber technology is the wide range of raw polymers, whose composition is beyond industrial control. This diversity results in polymer blends with coarse phase separated morphologies, which inevitably lead to incompatibility of properties. Possible solutions include adding a compatibilizer to the polymer blend or generating in situ molecular changes in the components through reactive extrusion to allow bonds to form between the poly-

Wood fiber/polyolefin composites widely used to obtain plastic lumber are incompatible because the thermoplastic material is nonpolar, while cellulose is polar, thereby requiring compatibilization using coupling or interfacial adhesion agents. Processing aspects, compatibilization and properties have been investigated

Initially, the compatibilizer or coupling agent in contact with the filler surface should interact strongly with the fibers through strong covalent bonds, acid-base

flexural moduli when compared to wood fiber (**Table 4**).

literature for glass fiber-reinforced PP composites.

mer chains of the different phases [34].

by several authors [34, 42, 47].

density of the final product, low cost and greater abundance.

asbestos [41]. As such, these types of applications should be avoided.

**8**

interactions or hydrogen bonds. The compatibilizing agent should contain a sufficient number of functional groups to allow reaction with cellulose hydroxyl groups. Another aspect to consider is the length of compatibilizer chains, which should be long enough to allow molecular entanglement with the polymer matrix in the interphase through mechanical anchoring [42]. Some authors [34, 52] have reported surface treatment of the fiber as a means of optimizing the compatibilization process.

Studies demonstrate that the modulus values of WPC increase slightly in relation to neat polymers, but with no statistically significant differences. However, there is almost no variation in tensile strength between neat HDPE and maleinized-polypropylene (MAPP) composites, although the former exhibited lower impact strength in relation to the other samples. This result is attributed to better adhesion between the fiber and polymer matrix, allowing better stress transfer to the fiber [9, 34, 50–52].

In addition to compatibilizers, other additives are used to improve the properties and appearance of the final product, such as impact modifiers, colorants, flame retardants, antioxidants, UV stabilizers, lubricants, stabilizers and biocides.

One way of altering the properties of plastic lumber is by modifying the molecular structure of one or more component polymers through exposure to ionizing radiation. Irradiation has the advantage of being a clean, continuous easy-to-control process [53–56]. It promotes the formation of crosslinks parallel to chain scission as well as double bonds. Crosslinking causes an increase in molecular weight, which typically improves properties, whereas chain scission reduces molecular weight and decreases properties in general [54].

Martins et al. [54] assessed the effects of ionizing radiation on IMAWOOD® plastic lumber made from recycled polyethylene (around 75% low-density polyethylene—LDPE and 25% HDPE), using irradiation produced by industrial equipment with a 60Co source. The specimens received total doses of 10, 500, 1000 and 2000 kGy. The authors concluded that irradiation in air increased the tensile strength of IMAWOOD®, although a decrease in elongation at break was observed with ductile-brittle transition. Additionally, from an engineering standpoint, IMAWOOD® offers the best conditions for certain applications after irradiation because it is less brittle.

One alternative to improve the mechanical properties of wood fiber-reinforced plastic lumber is through crosslinking using silane [57–60].

Studies have shown that crosslinked composites obtained by addition of silane display greater toughness, impact strength and interfacial adhesion than composites without silane [57, 58]. Additionally, the formation of crosslinks in the polymer matrix reduces buckling when the material is overloaded [57].

Bengtsson and Oksman [58] analyzed the effects of silane on the mechanical properties of wood-HDPE composites. The silane-grafted composites were stored at different moisture levels (in a sauna and temperature chamber) to determine how this parameter affects the degree of crosslinking in the composites. Experiments conducted with different amounts of vinyltrimethoxysilane in the presence of small amounts of peroxide indicated that the samples stored in the sauna showed the highest degree of crosslinking, which was calculated by measuring the gel content and swelling ratio. The results demonstrated that the samples exposed to the highest moisture level exhibited a higher degree of crosslinking. Water was responsible for the hydrolysis of methoxy groups to silanol, increasing the degree of crosslinking in the material stored in the sauna. The flexural modulus and flexural strength of neat HDPE were higher than those of the silane-grafted composites. In contrast to

neat plastics, the crosslinked composites showed better flexural strength than the material without silane.

The improved flexural strength is likely due to greater wood-polymer adhesion, enabling stress transfer from the polymer matrix to the wood fibers when the material is overloaded. The authors [58] attributed the superior adhesion to the covalent bond between wood and polyethylene to condensation or free-radical reactions. Furthermore, hydrogen bonding between the silanol groups grafted onto polyethylene and the hydroxyl groups of wood, as well as van de Waals forces between condensed silane in the wood and the polyethylene matrix, can improve adhesion between phases.

Lower creep was observed in the crosslinked composites when compared to those without crosslinking. This behavior may be related to the reduced viscous flow in the matrix due to crosslinking and better adhesion between the polymer matrix and wood flour [57–60].

Bengtsson et al. [57] evaluated mechanical property variations in WPC treated with silane containing different wood fiber concentrations. The stress-strain curves of the silane-treated composites indicated increased stiffness of the material with the addition of wood flour, in addition to a decline in ultimate strength. There was a significant increase in tensile strength with a rise in wood flour concentration, in contrast to the behavior reported by other authors [33, 61], whereby tensile strength declined with an increase in wood flour content. The authors attributed this behavior to greater interfacial adhesion between the wood and plastic promoted by silane addition.
