**2. Background**

The oldest records of plastic lumber date back to the early 1970s, when processing techniques for the material were developed in Europe and Japan. The resulting plastic lumber consisted primarily of post-industrial plastic scrap, which was the only source of low-cost plastic available at the time. Nevertheless, the plastic lumber industry did not initially experience significant growth [17].

The Klobbie intrusion system was developed in the 1970s and is based on a combination of conventional extrusion and injection processes. It consists of an extruder coupled to several rotating molds and a tank of cooling water. The plastic material is mixed and melted in the extruder and then forced into one of the molds. Once the

**3**

**3. Processing**

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

Sustainable Development (NERDES).

high-density polyethylene (HDPE) [16, 20].

highly toxic compounds have been identified [30].

lumber market [16, 21].

mold is filled, the carousel rotates to allow another mold to be filled. The filled mold is then cooled by passing it through the tank of chilled water before ejecting it. The process is capable of producing thick wall moldings and linear profiles [13, 17, 19]. Technologies developed from the 1980s onwards include Advanced Recycling Technology (Belgium), Hammer's Plastic Recycling (United States) and Superwood (Ireland). The equipment developed by Advanced Recycling Technology, denominated ET/1 *(*Extruder Technology 1), is an adiabatic extruder capable of processing mixed waste plastics with different densities to produce posts, rods, stakes, boards, etc. The process used by Hammer's Plastic Recycling differs somewhat from the Klobbie system and generates thick wall parts such as pallets, animal feeders and

Other processes have been developed for the continuous extrusion of profiles under cooling, such as Mitsubishi Petrochemical's Reverzer process to manufacture large cross-section products. There have also been historical experiments with

compression molding, such as the Recycloplast process developed in Germany [17, 19]

Following the abovementioned efforts in equipment design, attention turned

Despite the development of technology to obtain recycled PL, lack of standardization prevented its use by the construction industry in the early 1990s, particularly in structural applications. The Plastic Lumber Trade Association worked to establish a set of ASTM standards initially applicable to plastic lumber made from

In addition to their application in decking, American manufacturers use WPC to replace plywood in fencing, windows and panels [22], and the composites are also

The timber market and industry are searching for more sustainable products, and plastic lumber is a viable alternative in railway tie manufacturing, for example [31].

Although the composition of plastic lumber varies, the market consists primarily of companies that manufacture HDPE-based plastic lumber and those that use plastic composites and wood waste [16]. This has contributed to greater interest in

the search for wood-plastic composite (WPC) processing technologies.

Wood-plastic composites (WPC) are an important segment of the plastic

Plastic lumber has been used in marine environments as a replacement for natural wood treated with chromated copper arsenate (CCA) due to its rot resistance and high durability, as well as the environmental preservation it provides because no harmful chemicals are used in its manufacture [25, 26]. Recent studies have shown that copper, chromium and arsenic are continuously released by CCAtreated wood in marine environments [18, 27–29]. By contrast, the disposal of metal and organic contaminants from plastic lumber in river or seawater is low and no

being considered in the production of roofing and cladding [6, 23, 24].

to the composition of plastic lumber, and wood-plastic composites (WPC) emerged in the 1990s to replace wood with recycled plastic lumber in decking and fencing [16, 18]. Also in the 1990s, pioneering studies on plastic recycling for the development of plastic lumber began at the Institute of Macromolecules of the Federal University of Rio de Janeiro, under the supervision of Professor Eloisa Biasotto Mano, the first line of research in the area at university level. In 2009, at the same institute, a laboratory scale plastic recycling machine and pilot scale equipment were developed at the Center of Excellence in Recycling and

bench brackets, as well as linear profiles such as planks [19].

to produce thick wall parts such as pallets, benches and grates.

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

*Thermosoftening Plastics*

**Figure 1.**

different processes and recycling equipment are used to produce plastic lumber [12–15]. Factors such as the properties of the material to be processed, how plastic waste reaches the processing stage, the presence of additives and the moisture content of the material require extruders with specific technical characteristics when compared to processing virgin plastic [12]. These characteristics include shortening the residence time of the plastic inside the equipment, maintaining a constant feed rate inside the extruder and good degassing and homogenization of the material. Due to their natural origin, wood-based products may exhibit a series of structural defects, such as knots, cracks, warping, wormholes and fungal damage, as well as low-dimensional stability and other imperfections resulting from varying moisture content and drying, which significantly influence the final strength of

Plastic lumber has several advantages over natural wood in a number of applications and can be made from used plastics such as bottles, cups, packing and other products with a short useful life, thereby minimizing the accumulation of plastic material in the environment. It can be worked using conventional carpentry tools and planed, sawn, drilled and nailed in the same way as natural wood [6]. The advantages of plastic lumber over natural wood include being waterproof, resistant to weathering, mold and borers, and not requiring regular painting or maintenance, meaning it can be used in environments that natural wood would be unable to withstand for long periods. These include wet or underwater structures such as sea dikes in coastal areas [17, 18]. Plastic lumber can also be used to protect forests by preventing new trees from being felled to make furniture, decking, fencing and piers [6]. Different plastic lumber profiles are

The oldest records of plastic lumber date back to the early 1970s, when processing techniques for the material were developed in Europe and Japan. The resulting plastic lumber consisted primarily of post-industrial plastic scrap, which was the only source of low-cost plastic available at the time. Nevertheless, the plastic lumber

The Klobbie intrusion system was developed in the 1970s and is based on a combination of conventional extrusion and injection processes. It consists of an extruder coupled to several rotating molds and a tank of cooling water. The plastic material is mixed and melted in the extruder and then forced into one of the molds. Once the

industry did not initially experience significant growth [17].

products and are difficult to control [16].

*Examples of different plastic lumber profiles.*

**2**

shown in **Figure 1**.

**2. Background**

mold is filled, the carousel rotates to allow another mold to be filled. The filled mold is then cooled by passing it through the tank of chilled water before ejecting it. The process is capable of producing thick wall moldings and linear profiles [13, 17, 19].

Technologies developed from the 1980s onwards include Advanced Recycling Technology (Belgium), Hammer's Plastic Recycling (United States) and Superwood (Ireland). The equipment developed by Advanced Recycling Technology, denominated ET/1 *(*Extruder Technology 1), is an adiabatic extruder capable of processing mixed waste plastics with different densities to produce posts, rods, stakes, boards, etc. The process used by Hammer's Plastic Recycling differs somewhat from the Klobbie system and generates thick wall parts such as pallets, animal feeders and bench brackets, as well as linear profiles such as planks [19].

Other processes have been developed for the continuous extrusion of profiles under cooling, such as Mitsubishi Petrochemical's Reverzer process to manufacture large cross-section products. There have also been historical experiments with compression molding, such as the Recycloplast process developed in Germany [17, 19] to produce thick wall parts such as pallets, benches and grates.

Following the abovementioned efforts in equipment design, attention turned to the composition of plastic lumber, and wood-plastic composites (WPC) emerged in the 1990s to replace wood with recycled plastic lumber in decking and fencing [16, 18]. Also in the 1990s, pioneering studies on plastic recycling for the development of plastic lumber began at the Institute of Macromolecules of the Federal University of Rio de Janeiro, under the supervision of Professor Eloisa Biasotto Mano, the first line of research in the area at university level. In 2009, at the same institute, a laboratory scale plastic recycling machine and pilot scale equipment were developed at the Center of Excellence in Recycling and Sustainable Development (NERDES).

Despite the development of technology to obtain recycled PL, lack of standardization prevented its use by the construction industry in the early 1990s, particularly in structural applications. The Plastic Lumber Trade Association worked to establish a set of ASTM standards initially applicable to plastic lumber made from high-density polyethylene (HDPE) [16, 20].

Wood-plastic composites (WPC) are an important segment of the plastic lumber market [16, 21].

In addition to their application in decking, American manufacturers use WPC to replace plywood in fencing, windows and panels [22], and the composites are also being considered in the production of roofing and cladding [6, 23, 24].

Plastic lumber has been used in marine environments as a replacement for natural wood treated with chromated copper arsenate (CCA) due to its rot resistance and high durability, as well as the environmental preservation it provides because no harmful chemicals are used in its manufacture [25, 26]. Recent studies have shown that copper, chromium and arsenic are continuously released by CCAtreated wood in marine environments [18, 27–29]. By contrast, the disposal of metal and organic contaminants from plastic lumber in river or seawater is low and no highly toxic compounds have been identified [30].

The timber market and industry are searching for more sustainable products, and plastic lumber is a viable alternative in railway tie manufacturing, for example [31].

### **3. Processing**

Although the composition of plastic lumber varies, the market consists primarily of companies that manufacture HDPE-based plastic lumber and those that use plastic composites and wood waste [16]. This has contributed to greater interest in the search for wood-plastic composite (WPC) processing technologies.


#### **Table 1.**

*Mechanical properties of polypropylene (PP) composites filled with wood flour made using different manufacturing processes [33].*

In general, WPC are manufactured by extrusion, whereby the molten material is forced through a matrix and formed into a continuous profile in the desired shape. Extrusion is a process whereby plastic and other additives are melted, mixed, homogenized and formed into long continuous profiles typical of construction materials [21], in either simple solid shapes or complex hollow structures [6, 22, 32].

Wood-plastic composites can be produced in single-screw, co- or counterrotating conical or parallel twin-screw extruders or piggyback extruders [21, 32]. Manufacturing companies use different types of extruders and processing strategies [21], with some employing a single-screw extruder for the final shaping process [21], or use a twin-screw extruder for mixing and mold the final artifact in another extruder. Other manufacturing companies use a range of piggyback extruders, one to homogenize the mixture and others for shaping [21]. The screws are specifically designed to bind the wood residue to the polymer matrix in order to evenly disperse it in the polymer [24].

Various types of extruders used lead to significant differences in the properties of plastic lumber. Yang et al. [33] studied the properties of WPC and polypropylene (PP) composites filled with rice husk flour (RHF) made using different manufacturing processes. The authors used single- and twin-screw extrusion systems and found that WPC processed in a twin-screw extruder exhibited better mechanical properties when compared to the composite obtained by single-screw extrusion, attributing these results to better wood dispersion in the former process (**Table 1**). They also observed that the presence of a maleic anhydride-grafted polypropylene (MAPP) compatibilizer [34, 35] improved the mechanical properties of the RHF-filled PP composite when compared to the composite without the compatibilizing agent [33].

In addition to extrusion, processing technologies such as injection and compression molding can also be used to produce WPC [36], with the composite formulation adjusted according to processing requirements. For example, the low viscosity needed for injection molding may limit the wood residue content in the formulation [21]. Experts from the WPC industry claim that injection molding has significant potential, with the ability to produce complex shapes, whose growing number of applications includes products such as tiles and cladding [9, 37].

A number of aspects should be considered when processing WPC. Moisture content and particle size should be tightly controlled to prevent discontinuities and parts with defects due to the presence of bubbles or stains caused by thermo-oxidative processes [34, 38]. Thus, as a primary requirement, wood waste must be pre-dried, and degassing zones must be used to remove residual moisture during processing. One of the factors directly affected by the moisture content of the lignocellulosic reinforcement is the output of the extrusion line: the higher the moisture content of the particles, the lower the throughput due to the longer residence time needed to devolatilize the composite [39, 40]. As such, the longer the material remains inside the extruder, the more susceptible it is to thermomechanical degradation.

Additionally, the low thermal stability of cellulose (200–220°C) is a limiting factor in the process, except when residence times are minimal. Exposing wood waste

**5**

extruder [32].

wood fibers and releasing moisture.

and a range of other materials [32].

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

to temperatures above this range releases volatile compounds, causing discoloration and odor and making the composite brittle [36]. This has restricted the use of thermoplastics in WPCs to major commercial resins such as polyolefins (PE and PP), styrenics (PS, HIPS and ABS) and polyvinyl chloride (PVC), which can be

Another factor that hampers WPC processing is the low density of wood waste, which makes it difficult for the residue to pass through the small openings typical of

Processing WPC can be classified into four distinct categories. In pre-drying and premixing, wood waste is pre-dried at moisture levels below 1% and the material is fed into a counter-rotating twin-screw extruder along with the polymer, usually in the form of a powder. The dry blend of polymer, wood and additives is prepared in high-intensity Henschel mixers before being fed into the extruder [42]. The dry blend is then fed into the extruder using a Crammer feeder. Given the narrow residence time distribution of the material in the system and limited thermal energy generation, counter-rotating twin-screw extruders are used primarily for PVC

Pre-drying wood and feeding the polymer and wood residue into the extruder separately (pre-dry; split feed) allow better control of the residence time of the wood filler during processing [42]. High capacity twin-screw extruders with side feeders are generally used in this type of process, where the residue is mixed with the molten polymer, passing through distributive mixing and degassing zones. A third process involves feeding wet wood residue into the extruder first, followed by the molten polymer (wood first; melt feed). Two simultaneously operating extruders are needed, the first to dry the wood and a second smaller extruder to plasticize the polymer and additives [42]. An example of this type of system is the Woodtruder®, equipped with a counter-rotating twin-screw extruder designed to remove moisture from wood fiber even at high levels (1–8% moisture content). The process includes a primary counter-rotating parallel twin-screw extruder (L/D 28:1) and a satellite extruder with either one or two screws depending on the polymer used. The primary extruder dries the wood fiber and then mixes it with the polymer, while the satellite extruder melts the polymer and returns it to the primary

In many ways, processing wood waste in parallel twin-screw extruders is similar

to processing neat polymers [32]. Although standard feeders are generally used for polymers, gravimetric ones are needed to feed the wood waste into the twinscrew extruder. The feed rate is automatically adjusted by the controller to increase feeding efficiency, circumventing the problems caused by wood fiber bulk density fluctuation. The screws drive the residue forward as the heat from the barrel and screws is transferred to the material, heating both the wood and the water in the

The WoodTruder system uses a set of two extruders. The wood fibers enter the feeding zone of a counter-rotating parallel twin-screw extruder with a special venting section to draw moisture, while the molten polymer is added in sequence via a side mounted, single-screw extruder. The mixture then enters the compression section of the primary extruder to more easily blend the two components [32, 42]. Degassing occurs after compression in order to remove volatile components from the polymer or residual moisture from the wood fiber. The completely dried homogenized mixture then moves into a different zone to increase the pressure flow through the head. Melting temperatures are typically between 170 and 185°C. Temperatures above 200°C should be avoided in order to reduce wood degradation. In addition to wood fibers, the WoodTruder system can also process rice hulls, sisal, peanut shells

processed at temperatures below cellulose degradation [9, 21].

conversion due to its thermal instability [12, 40].

plastic processing equipment, leading to a decline in throughput [41].

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

*Thermosoftening Plastics*

PP/wood flour: 70/30 wt%

*manufacturing processes [33].*

**Table 1.**

**Composite Type of screw** 

**extrusion**

it in the polymer [24].

In general, WPC are manufactured by extrusion, whereby the molten material is forced through a matrix and formed into a continuous profile in the desired shape. Extrusion is a process whereby plastic and other additives are melted, mixed, homogenized and formed into long continuous profiles typical of construction materials [21], in either simple solid shapes or complex hollow structures [6, 22, 32]. Wood-plastic composites can be produced in single-screw, co- or counterrotating conical or parallel twin-screw extruders or piggyback extruders [21, 32]. Manufacturing companies use different types of extruders and processing strategies [21], with some employing a single-screw extruder for the final shaping process [21], or use a twin-screw extruder for mixing and mold the final artifact in another extruder. Other manufacturing companies use a range of piggyback extruders, one to homogenize the mixture and others for shaping [21]. The screws are specifically designed to bind the wood residue to the polymer matrix in order to evenly disperse

*Mechanical properties of polypropylene (PP) composites filled with wood flour made using different* 

**Tensile strength (MPa)**

Single-screw 25 7 Twin-screw 28 10

**Izod impact strength (kJ/**

**)—unnotched**

**m2**

Various types of extruders used lead to significant differences in the properties of plastic lumber. Yang et al. [33] studied the properties of WPC and polypropylene (PP) composites filled with rice husk flour (RHF) made using different manufacturing processes. The authors used single- and twin-screw extrusion systems and found that WPC processed in a twin-screw extruder exhibited better mechanical properties when compared to the composite obtained by single-screw extrusion, attributing these results to better wood dispersion in the former process (**Table 1**). They also observed that the presence of a maleic anhydride-grafted polypropylene (MAPP) compatibilizer [34, 35] improved the mechanical properties of the RHF-filled PP composite when compared to the composite without the compatibilizing agent [33]. In addition to extrusion, processing technologies such as injection and compression molding can also be used to produce WPC [36], with the composite formulation adjusted according to processing requirements. For example, the low viscosity needed for injection molding may limit the wood residue content in the formulation [21]. Experts from the WPC industry claim that injection molding has significant potential, with the ability to produce complex shapes, whose growing number of

A number of aspects should be considered when processing WPC. Moisture content and particle size should be tightly controlled to prevent discontinuities and parts with defects due to the presence of bubbles or stains caused by thermo-oxidative processes [34, 38]. Thus, as a primary requirement, wood waste must be pre-dried, and degassing zones must be used to remove residual moisture during processing. One of the factors directly affected by the moisture content of the lignocellulosic reinforcement is the output of the extrusion line: the higher the moisture content of the particles, the lower the throughput due to the longer residence time needed to devolatilize the composite [39, 40]. As such, the longer the material remains inside

Additionally, the low thermal stability of cellulose (200–220°C) is a limiting factor in the process, except when residence times are minimal. Exposing wood waste

applications includes products such as tiles and cladding [9, 37].

the extruder, the more susceptible it is to thermomechanical degradation.

**4**

to temperatures above this range releases volatile compounds, causing discoloration and odor and making the composite brittle [36]. This has restricted the use of thermoplastics in WPCs to major commercial resins such as polyolefins (PE and PP), styrenics (PS, HIPS and ABS) and polyvinyl chloride (PVC), which can be processed at temperatures below cellulose degradation [9, 21].

Another factor that hampers WPC processing is the low density of wood waste, which makes it difficult for the residue to pass through the small openings typical of plastic processing equipment, leading to a decline in throughput [41].

Processing WPC can be classified into four distinct categories. In pre-drying and premixing, wood waste is pre-dried at moisture levels below 1% and the material is fed into a counter-rotating twin-screw extruder along with the polymer, usually in the form of a powder. The dry blend of polymer, wood and additives is prepared in high-intensity Henschel mixers before being fed into the extruder [42]. The dry blend is then fed into the extruder using a Crammer feeder. Given the narrow residence time distribution of the material in the system and limited thermal energy generation, counter-rotating twin-screw extruders are used primarily for PVC conversion due to its thermal instability [12, 40].

Pre-drying wood and feeding the polymer and wood residue into the extruder separately (pre-dry; split feed) allow better control of the residence time of the wood filler during processing [42]. High capacity twin-screw extruders with side feeders are generally used in this type of process, where the residue is mixed with the molten polymer, passing through distributive mixing and degassing zones.

A third process involves feeding wet wood residue into the extruder first, followed by the molten polymer (wood first; melt feed). Two simultaneously operating extruders are needed, the first to dry the wood and a second smaller extruder to plasticize the polymer and additives [42]. An example of this type of system is the Woodtruder®, equipped with a counter-rotating twin-screw extruder designed to remove moisture from wood fiber even at high levels (1–8% moisture content). The process includes a primary counter-rotating parallel twin-screw extruder (L/D 28:1) and a satellite extruder with either one or two screws depending on the polymer used. The primary extruder dries the wood fiber and then mixes it with the polymer, while the satellite extruder melts the polymer and returns it to the primary extruder [32].

In many ways, processing wood waste in parallel twin-screw extruders is similar to processing neat polymers [32]. Although standard feeders are generally used for polymers, gravimetric ones are needed to feed the wood waste into the twinscrew extruder. The feed rate is automatically adjusted by the controller to increase feeding efficiency, circumventing the problems caused by wood fiber bulk density fluctuation. The screws drive the residue forward as the heat from the barrel and screws is transferred to the material, heating both the wood and the water in the wood fibers and releasing moisture.

The WoodTruder system uses a set of two extruders. The wood fibers enter the feeding zone of a counter-rotating parallel twin-screw extruder with a special venting section to draw moisture, while the molten polymer is added in sequence via a side mounted, single-screw extruder. The mixture then enters the compression section of the primary extruder to more easily blend the two components [32, 42]. Degassing occurs after compression in order to remove volatile components from the polymer or residual moisture from the wood fiber. The completely dried homogenized mixture then moves into a different zone to increase the pressure flow through the head. Melting temperatures are typically between 170 and 185°C. Temperatures above 200°C should be avoided in order to reduce wood degradation. In addition to wood fibers, the WoodTruder system can also process rice hulls, sisal, peanut shells and a range of other materials [32].

#### *Thermosoftening Plastics*

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 alternative.
