Wood-Plastic Composites—Performance and Environmental Impacts

Abstract

This chapter reviews and discusses the performance and environmental impacts of wood-plastic composites (WPCs) used in a variety of applications ranging from construction and automotive sectors to consumer goods. Performance is considered in terms of fitness for use, manufacturing methods, material components of WPCs, and user perceptions of the material. Recent research related to matrix components and their relation to mechanical properties are covered in detail, especially regarding effects of the wood component. Manufacturing processes are also significant contributors to the suitability of WPCs for a given use, and the impact of various aspects of manufacturing are discussed as well. The environmental impacts of WPCs are reviewed and contain comparisons to solid wood alternatives, different matrix components, and future considerations for performing environmental impact assessments of WPCs. Finally, critical aspects of further innovation and future research are covered that are necessary to improve WPCs use as suitable replacements for solid plastic products and materials.

Full text

Wood-Plastic Composites—Performance
and Environmental Impacts
Matthew John Schwarzkopf and Michael David Burnard
Abstract This chapter reviews and discusses the performance and environmental
impacts of wood-plastic composites (WPCs) used in a variety of applications
ranging from construction and automotive sectors to consumer goods. Performance
is considered in terms of fitness for use, manufacturing methods, material compo-
nents of WPCs, and user perceptions of the material. Recent research related to
matrix components and their relation to mechanical properties are covered in detail,
especially regarding effects of the wood component. Manufacturing processes are
also significant contributors to the suitability of WPCs for a given use, and the
impact of various aspects of manufacturing are discussed as well. The environ-
mental impacts of WPCs are reviewed and contain comparisons to solid wood
alternatives, different matrix components, and future considerations for performing
environmental impact assessments of WPCs. Finally, critical aspects of further
innovation and future research are covered that are necessary to improve WPCs use
as suitable replacements for solid plastic products and materials.
Keywords Applications Fibres Manufacturing Matrix components 
Renewable composites User perceptions Wood-plastic composites
1 Introduction
Wood-plastic composites (WPCs) are a product class that has been developing over
the last 40 years resulting in increased applications and expanded market share.
More specifically, WPCs are composites containing a wood component in particle
form (wood particles/wood flour) and a polymer matrix. They are used in a variety
M.J. Schwarzkopf (&)M.D. Burnard
Andrej MarušičInstitute, University of Primorska,
Muzejski Trg 2, Koper, Slovenia 6000, Slovenia
e-mail: matthew.schwarzkopf@iam.upr.si
M.D. Burnard
Research and Development, Abelium, d.o.o, Ljubljana, Slovenia
©Springer Science+Business Media Singapore 2016
A. Kutnar and S.S. Muthu (eds.), Environmental Impacts of Traditional
and Innovative Forest-based Bioproducts, Environmental Footprints
and Eco-design of Products and Processes, DOI 10.1007/978-981-10-0655-5_2
19

of structural and non-structural applications ranging from component and product
prototyping to outdoor decking. However, construction and automotive applications
are the most common worldwide (La Mantia and Morreale 2011; Eder and Carus
2013). WPCs can be used outdoors as well as indoors, and some common appli-
cations include construction materials, garden and yard products, automotive
applications (interior and engine), household items, packaging and consumer
goods.
The decision to use a WPC product in place of another, generally speaking,
should be predicated on achieving greater performance, reduced price, or reduced
environmental impact. Using exterior decking as an example of improved perfor-
mance, a homeowner may choose to use WPC decking instead of pressure-treated
timber because of the ease of maintenance and improved durability (i.e. coatings do
not need to be applied), or aesthetic reasons (e.g. lack of knots and splits likely to be
found in solid wood).
The benefits are not always clear though, as there are many trade-offs to consider
when choosing WPCs—especially when replacing responsibly sourced renewable
materials. Including any non-renewable materials in the product can significantly
increase its environmental impact. However, the case for WPCs may be clearer
when the competing product is made entirely of non-renewable polymers, as the
wood fraction of many WPCs can approach 50 % of the product volume, and
therefore reduce the resource pressure on non-renewable materials (e.g. polymers
derived from fossil sources). The environmental impact of WPCs is directly affected
by the ratio of renewable to non-renewable materials in the product. Not only do
WPCs have lower environmental impacts than unfilled plastics (but higher than
solid wood or most other wood composites), use of sustainably harvested and
recovered wood products in long-life products sequesters atmospheric carbon and
can produce a positive environmental impact (Hill et al. 2015).
Wood is often used in plastics as a means to reduce price compared to a solid
plastic product. Wood used in WPCs often comes from side streams such as
sawdust produced while manufacturing lumber or recovered wood products, and is
much cheaper to produce than the plastic that it replaces in many products. This
often helps to reduce prices for consumers.
Promising progress and research into bioplastics [i.e. plastics made from
biopolymers such as Polylactic acid (PLA)] reinforced with natural fibres (including
wood) indicate the potential for these renewable materials to eventually enter the
WPC market (Mukherjee and Kao 2011). However, these composites still have
significant environmental impact due to processing and production, and steps to
reduce the energy demands and water use should be taken (Qiang et al. 2014).
WPCs already have a significant market share (260,000 t in Europe in 2012) and
the trend is increasing (Eder and Carus 2013). The current reliance on plastics,
especially those derived from fossil sources, means that this demand is likely to
continue increasing especially as developing economies continue to grow.
However, to meet industrial, consumer, and environmental demand’s research must
continue to improve the processes for making WPCS, the component materials, and
final product functionality.
20 M.J. Schwarzkopf and M.D. Burnard

This chapter will give an overview of the material properties, manufacturing
processes, applications, and current developments related to WPCs.
2 Components of WPCs
WPCs are a group of composite materials and products comprised of two primary
and distinct phases. One of these phases is the matrix which holds the different
components together, binding them and providing load transfer between them. The
matrix in WPCs is either a thermoset or more commonly a thermoplastic polymer.
The other primary phase is the wood component. The wood component can be of
any shape or size and acts as a filler and/or reinforcement to the composite. Making
up a relatively small proportion of the total composite are additives which are added
to aid in processing and affect a variety properties of the final product.
2.1 Matrix Component
2.1.1 Thermosets
Thermosets are a class of polymers that upon curing cannot be remelted or
reprocessed for the same type of usage. From a liquid state these polymers are cured
into rigid solids that are chemically cross-linked. The mechanical properties of these
polymers come from initial molecular units and the density of cross-links formed
during curing (Hull and Clyne 1996). The wood adhesives industry often uses
thermosets to take advantage of this cross-linked form which provides a solid and
durable bond. When used with wood, the liquid polymer penetrates the wood
microstructure to varying degrees and is then cured, forming a three dimensionally
dispersed interphase region. Common thermosets being used are urea–formalde-
hyde, phenol–formaldehyde, epoxy, and polyamides. One of the first WPCs using a
thermosetting polymer was a phenol–formaldehyde—wood composite which is
branded Bakelite by Rolls Royce in 1916, used for the shifting knob in their
vehicles (Clemons 2002).
2.1.2 Thermoplastics
Thermoplastics are a class of polymers that can be heated and softened, cooled and
hardened, and then resoftened while maintaining their characteristic properties from
their first usage. Thermoplastics are used for a variety of everyday products like
plastic soda bottles, single use shopping bags, milk jugs, etc. Unlike thermosets,
these polymers are not cross-linked and rely on the properties of their monomer
units, large molecular weights, and polymer chain entanglement for their
Wood-Plastic Composites—Performance and Environmental Impacts 21

mechanical performance (Hull and Clyne 1996). When heated, these polymer
chains disentangle and allow them to slide past each other, allowing for repro-
cessing. High density polyethylene (HDPE), polypropylene (PP) and polyvinyl
chloride (PVC) are the most common thermoplastic polymers used in WPCs
(Klyosov 2007). HDPE accounts for the majority of the thermoplastics used in
WPCs at 83 %, followed by PP at 9 % and PVC making up 7 % of the total WPC
thermoplastic volume (Caulfield et al. 2005). Due to the thermal stability of wood,
thermoplastics are used because they can be processed at relatively low tempera-
tures below wood’s thermal degradation temperature (180–200 °C). These poly-
mers are also attractive for WPCs because they can be cut, screwed, and nailed with
tools already used for wood construction.
2.2 Wood Component
Polymer manufacturers have historically used minerals and synthetic materials like
talc, calcium carbonate, mica, glass fibres, and carbon fibres as extenders and
reinforcing materials (Eckert 2000). Wood as a filler or reinforcing material has
been used in composite materials for thousands of years (Bodig and Jayne 1982)
and the introduction of a natural filler like wood particles in polymers was
appealing to polymer manufacturers. Wood has many advantages to traditional
fillers like lower cost, relatively high strength to weight ratio, low density, is
relatively soft and easily integrated into existing plastic production lines, can offset
the amount of polymer used, and is a renewable resource (Wolcott and Englund
1999; Clemons 2002; Farsi 2012). English et al. (1996) found that wood flour used
in PP composites offers similar performance to that of talc and other mineral fillers
but with a lower specific gravity providing for lighter composites.
In its own right, wood is a naturally occurring composite utilising polymers in a
highly structured cellular construction. A thorough treatment of wood and its
constituents related to composite materials can be found in Bodig and Jayne (1982).
The wood material used in WPCs can be from a virgin source or is often a
post-industrial co-product like trimmings from sawmills, breakdown of urban and
demolition wood, or logging trimmings/slash. These materials are then chipped and
ground into their final form as wood particles. Typical methods for the com-
minution of wood into wood particles are through the use of hammer and attrition
mills. Unlike clear (free of defects) and undamaged wood, the wood particles often
used in WPCs have been heavily altered. Using this type of mechanical breakdown,
the finished product (Fig. 1) is heavily damaged and its properties are far from those
of defect free, clear wood.
Particles are typically less than 1 mm in length and have a wide distribution of
aspect ratios (Wang 2007). These particles are comprised of bundles of short fibres
rather than long individual wood fibres. The complex morphology of the particle
cellular structure, its irregular geometry, and the damage caused through processing
should be acknowledged when designing and manufacturing WPCs (Teuber et al.
22 M.J. Schwarzkopf and M.D. Burnard

2015). WPC properties are significantly influenced by the wood species and particle
size characteristics of the wood particles being used (Stark and Rowlands 2003).
2.2.1 Wood Species
The wood species used in WPCs is typically determined by geographical location,
availability, and price. The wood species used affects important aspects of WPC
production like chemical compatibility and mechanical contributions to the com-
posite. Common wood species used are pine, maple, and oak. Berger and Stark (1997)
tested a variety of wood species in injected-moulded WPCs. They found that hard-
wood species provide improved tensile properties and heat deflection when compared
with softwoods, and that ponderosa pine wood flour provided the optimum blend of
mechanical property enhancements. These results are by no means the answer for all
situations but show that wood species selection is an important factor to consider.
One aspect dependent on the wood species selection is the microstructure of the
wood itself. The effective surface area for interaction with the polymer and the
degree of polymer penetration into the wood structure both affect the composite
properties. Escobar and Wolcott (2008) investigated the influence of different
species on WPCs. Part of this study looked at the effect that different anatomical
features of wood had on the polymer penetration through the wood structure. For
example, Fig. 2shows a cross-sectional view of a wood specimen with cell lumens
filled with HDPE.
Gacitua and Wolcott (2009) found that wood species with higher interfacial areas
(Fig. 2) may increase the amount of mechanical interlocking of the polymer with the
wood structure. Circled in yellow are examples of the interfacial areas where the
wood and polymer interact. This area also includes the entire perimeter of the wood
particle. Depending on which wood species is used, the microstructure may be
much different and increase or decrease this interaction area. It is also important to
note that the degree of polymer penetration within the wood structure is also affected
by the polymer’s composition with respect to molecular weights. Low molecular
weight components can more easily penetrate the wood structure, but contribute less
to mechanical properties and an optimum balance must be found.
Fig. 1 SEM image of a wood
particle used in WPCs
(Schwarzkopf 2014)
Wood-Plastic Composites—Performance and Environmental Impacts 23

2.2.2 Wood Particle Size
The size and geometry of the wood particles being used in WPCs affect the
flow/handling characteristics and mechanical properties. Wood particles obtained on
an industrial scale often have a large-size distribution (Fig. 3) making it more dif-
ficult to design for certain properties. However, the more milling and screening that
are done to make the particles smaller or narrow this distribution increase the cost
Fig. 2 Wood particle
embedded in PE showing cell
lumen penetration and
highlighting areas making up
the entire wood–polymer
interaction area. Photo credit
Muszyński, L
Fig. 3 Particle size distribution scatter plot from a 40-mesh sample (Wang 2007)
24 M.J. Schwarzkopf and M.D. Burnard

significantly. Particle size is characterised by the size of spaces in a mesh screen the
particles pass through. Mesh size refers to how many openings there are in a screen
within 2.54 cm
2
. For example, a 100-mesh screen has 100 openings in 2.54 cm
2
. The
higher the mesh number, the smaller the particles. Mesh sizes of particles used in
WPCs will vary depending upon the desired product properties and finish, and are
most commonly from 10 to 80 mesh (Patterson 2001; Clemons 2002).
Stark and Rowlands (2003) investigated the effect of particle size on the
mechanical properties of wood/PP composites. They manufactured WPC test
specimens using particle mesh sizes 35–235 and performed a variety of mechanical
tests. They found WPCs with larger particle sizes contained more stress concen-
trations which affected the impact energy of the product. They also found that even
more important than particle size was the aspect ratio of the particles which is the
length divided by the width of the largest minor axis of the particles. Generally,
when there are particles with a larger aspect ratio, there is the potential for more
effective load transfer between the matrix and the particles leading to better
mechanical properties (Schwarzkopf and Muszynski 2015). Based on a 40-mesh
sample of commercially obtained wood particles, Wang (2007) investigated the
distribution of particle sizes. Using optical measurements of micrographs, Wang
found that the median aspect ratio was 2.8. Using fillers with greater aspect ratios
like wood fibre (10–20) can improve mechanical properties of the WPC, but dif-
ficulties in processing occur when feeding and metering the fibres into extruders
(Patterson 2001).
3 Manufacturing Methods
WPCs started being produced by the plastics industry which had prior expertise in
processing and manufacturing of plastic products (Clemons 2002). This industry
had used filler materials in the past and when wood became a viable option, it was
integrated into their existing production lines. While other wood-based composites
are typically made in a panel or beam like geometry. WPCs starting in a molten
state can be formed into highly detailed, linear profiles using extrusion processes or
can be formed into complicated shapes via injection moulding. In any thermoplastic
composite, the components must first be blended together and then later formed into
the desired product.
3.1 Compounding
Mixing or compounding is the act of combining the wood and polymer components
together. During the compounding procedure it is critical to evenly disperse the
wood particles throughout the molten polymer. This dispersion is especially
important with highly filled WPCs (Schirp and Stender 2009). It is also important in
Wood-Plastic Composites—Performance and Environmental Impacts 25

this step to wet or encapsulate the wood particles with the polymer. Proper
dispersion and wetting allow uniform and more effective load transfer to occur
throughout the composite. If not compounded properly, the composite will have
reduced mechanical properties compared with an optimally compounded blend and
increases the risk of durability issues. After compounding, the material can go
directly to shape formation of the final product or can be chipped into pellets for
later use.
3.2 Extrusion
The majority of WPCs are extruded into long linear profiles to use as decking
planks, siding, fences, etc. Extruders serve the two main purposes of compounding
the wood and filler, and then forming the shape of the extruded profile.
The wood and polymer components are metered and fed into the extruder and
mixed using single or twin-screw configurations. The screws act to mix and move
the material forward. Throughout the barrel of the extruder, the mix is heated
through friction between the barrel, screw, and wood–polymer mix as well as by
heated zones along the length. At the end of the extruder is a die through which the
material is fed, forming the desired profile. Twin screw extruders are sometimes
used as compounding units for producing pre-blended pellets. Manufacturers using
a single screw extruder or injection moulding process often purchase these
pre-blended pellets which are more easily fed into the machine and do not require
an extra compounding step.
3.3 Injection Moulding
Injection moulding is used much less for WPCs, but can be used to make more
complex shapes for a variety of products. The first steps in injection moulding are
similar to extrusion, but instead of being forced through a die, the mixed material is
injected into a mould. The wood-plastic mixture fills the mould, is cooled, and is
then ejected in the preparation for the next piece to be formed.
3.4 Wet Processes for Sheet Formation
Sheets of WPC, which are often used in the automotive industry (e.g. doors or
shelving applications), are either extruded or formed by a wet process. In wet process
fabrication, a slurry of water and wood is created and mixed with chemical additives
before being hot pressed into sheets (Pritchard 2004). These sheets may use a plastic
scrim to help holding the board together (Pritchard 2004).
26 M.J. Schwarzkopf and M.D. Burnard

3.4.1 3D Modelling with WPCs
3D modelling (or additive manufacturing) is a manufacturing technique that allows
for complex shapes to be created by depositing or removing materials (such as
WPCs and other plastics, but a variety of materials can be used) in a customisable
pattern to make three-dimensional objects. For most of these processes object
models are created in a 3D modelling environment (such as computed-aided
drafting (CAD) software), then processed in software that splits the 3D object into a
collection of layered elements which can be created by the modelling technique.
3D modelling is most frequently used as a rapid prototyping method. Prototyping
allows the users to create and test variations of product designs. Commercial appli-
cations for 3D modelling are expanding in a range of fields, however. Furthermore,
the affordability of non-commercial 3D printers (particularly Fused Deposition
Modelling (FDM) systems) has allowed researchers, hobbyists, and small scale
component manufacturers to explore a variety of materials, methods, and products.
WPCs are used in 3D modelling to reduce material costs and reduce the environ-
mental impact using fossil-based plastics. As in other WPC applications, waxes,
photostabilisers, lubricants, and other additives are used in the material matrix to alter
the properties of the final product and aid in the manufacturing process.
FDM is a leading 3D modelling method in many manufacturing areas (Nikzad
et al. 2011). The WPC used in this method is a filament that is fed into a nozzle that
heats and deposits the WPC according to the product design. The WPC must be
heated to a pliable state without exceeding the thermal degradation temperature of
the wood fibre in the filament, which can limit the types of plastics used in these
applications.
Selective laser sintering (SLS) is another 3D modelling technique which is in
developmental stages for use with WPCs (Guo et al. 2011). SLS utilises powders
which melt at different temperatures and that are fused together by laser radiation
and form solids as the temperature of the combined material decreases. The
methods for preparing WPCs for SLS are underdevelopment, but the wood com-
ponent must be treated (alkalised) and mixed with a thermoplastic adhesive powder.
3.5 Reinforcement of Plastic Matrices with Renewable
Materials
The primary purpose of using renewable fibre reinforcement in plastics is to reduce
material cost, which has a secondary effect of reducing ecological impacts, espe-
cially when replacing non-renewable reinforcement (e.g. metals and glass)
(Corbière-Nicollier et al. 2001). However, reinforcing plastics with particulates and
fibres impacts material properties (strength, durability, appearance, etc.) as well as
their ecological impact (Corbière-Nicollier et al. 2001; Zhong et al. 2001; Bouafif
et al. 2009; Westman et al. 2010; Mukherjee and Kao 2011). Using renewable
Wood-Plastic Composites—Performance and Environmental Impacts 27

fibres and particulates alter the properties of the composite material in a variety of
ways based on the geometry of the reinforcement, the type and components of the
matrix the renewable components are part of, and the type of fibre or particle
embedded in the matrix (Mukherjee and Kao 2011). Materials such as wood, reeds,
kenaf, grasses (like bamboo), cotton, carbon fibres, rayon, nylons, and many other
renewables allow reduced demand on fossil and other non-renewable (e.g. metals
and glass) matrix components.
4 Physical Characteristics
Composite materials are often optimised by selecting components for their strength,
stiffness, flexibility, and durability. When compared with individual materials,
composites may also offer more consistent performance, lower production costs,
and create an avenue for the utilisation of renewable resources. WPCs are no
different and are formulated to meet the needs of the consumer by finding the right
balance of these properties. Mechanical properties and durability are among the
most important to WPCs.
4.1 Mechanical Properties
With WPC decking making up the largest share of the WPC market (Clemons
2002), we can look at mechanical properties important to this market. WPC deck
boards are subjected to bending when they span a gap between supports and are
being dynamically loaded when walked on and supporting the static loads (e.g.
furniture and grills). Both the ultimate tensile stress (UTS) and modulus of elasticity
(MOE) are important properties to consider. UTS is the maximum stress that a
material can be subjected to before breaking. MOE refers to a material’s ability to
resist deformation and in a general sense is the stiffness of the material. For decking,
this is important for limiting deflection of the product. It should be mentioned that a
true elastic response in plastic composites is debatable, and the response of the
material is highly dependent on the testing rate, temperature, previous history of the
specimen, etc. Comparing values between different profiles, WPC formulations and
specimens from different testing facilities is difficult, but for research and devel-
opment purposes determining these values as a comparison is helpful. A study done
by Karas (2010), assessed a variety of mechanical properties including MOE and
UTS for wood-HDPE composites. Commercial pine flour was used as a filler as
well as a variety of wood fillers from “low-grade”sources including: whole-tree
juniper (WJ) (including bark), forest thinning material (FT), and urban wood from
demolition (UW).
In the left plot in Fig. 4, UTS is plotted against the filler loading ratio. There is a
horizontal line at 20 MPa which represents the UTS value for the HDPE used in this
28 M.J. Schwarzkopf and M.D. Burnard

experiment that contains no filler. The other lines represent composites made from
HDPE and the various filler types mentioned above. When increasing the wood
filler loading ratio there is a slight increase in UTS, but with higher loading levels
near 60 % wood, the UTS decreases. This behaviour is expected because when
more and more of the composite is wood, the particles are often not entirely
encapsulated by the polymer and optimal load transfer is not possible. The right plot
in Fig. 4is showing the MOE plotted against the loading ratio of wood fillers. As
the filler ratio increases from 0 to 60 %, the MOE increases for all of the samples.
This stiffening behaviour is also present in composites using fillers other than wood.
Filling WPCs above 60 % requires care in particle dispersion and increases the
likelihood of problems with not fully encapsulated particles, water absorption, crack
formation, and biological attack.
WPCs have found success in a variety of markets including outdoor decking,
railings, fences and landscaping timbers, but the number of applications for WPCs
is limited to service not requiring high-mechanical performance (Clemons 2002).
Hull and Clyne (1996) stated that understanding load transfer is the key to
understanding the composite’s mechanical behaviour. In WPCs, commonly used
thermoplastic polymers like PP are hydrophobic (water-hating) while the con-
stituent polymers of wood, like cellulose, are hydrophilic (water loving) in nature
and have reactive hydroxyl groups along the length of their chains (Sjöström1993).
This results in an incompatibility between the polar wood component and the
non-polar thermoplastic materials resulting in poor adhesion between the two (Lu
et al. 2000) and lower mechanical properties than properly bonded components.
Attempts to improve the quality of these bonds have been made in the past by
experimenting with additives known as coupling agents. Coupling agents are
defined by Pritchard (1998)as“substances that are used in small quantities to treat a
surface so that bonding occurs between it and other surfaces, e.g., wood and
thermoplastics.”The effects of coupling agents on the mechanical properties of
WPCs have been studied extensively (Woodhams et al. 1984; Maldas and Kokta
Fig. 4 Left Ultimate tensile stress versus wood filler loading ratio. Right Secant modulus of
elasticity versus wood filler loading ratio; PF pine flour, WJ whole-tree juniper, FT forest thinning,
UW urban wood, HDPE reference specimen with no filler (Karas 2010)
Wood-Plastic Composites—Performance and Environmental Impacts 29

1991; Raj and Kokta 1991; Stark and Rowlands 2003) and have shown that
coupling agents increase the strength and stiffness of the bulk composite. This
approach has been the topic of much research and a detailed review of coupling
agents used in WPCs has been compiled by Lu et al. (2000). One commonly used
coupling agent is maleic anhydride-grafted polypropylene (MAPP). This type of
coupling agent reacts with the wood component on one end, and on the other end
entangles a modified PP with the bulk PP polymer. Stark and Rowlands (Stark and
Rowlands 2003) study showed that the addition of MAPP had the greatest effect on
the properties of wood fibre composites containing wood particles with greater
aspect ratios (≈16). As they pointed out, wood particles commonly used in WPCs
have low aspect ratios (3–5). This being the case, coupling agents can only assist in
interfacial bonding to a limited extent. While one would expect better performance
from fibres with larger aspect ratios, this method adds cost and complexity to the
manufacture and processing of WPCs. Whether or not a WPC manufacturer decides
to use a coupling agent, the interaction between the polymer matrix and the
embedded particle still requires attention. Unlike measuring mechanical properties
of WPCs like creep or bending strength at a macroscale, understanding the inter-
action between the particle and matrix requires a look at the microscale. In the past,
a variety of methods have been used to explain and predict the interactions between
the wood and polymer phases using idealised analytical and numerical techniques
(Clyne 1989). These methods held some common assumptions including: the
embedded particles which are homogenous and isotropic, impermeable, cylindrical
in shape, have a large aspect ratio, have a perfect bonding interface with the matrix,
and have no transfer of load on their ends. Such assumptions can hardly be applied
to irregular, porous bio-based particles like wood (Raisanen et al. 1997). These
methods provided approximations of load transferred with an embedded inclusion
in a thermoplastic matrix but lacked the complexity of the actual system to be
satisfactory. Recently, Schwarzkopf and Muszynski (2015) investigated these
interactions using optical measurement techniques based on the digital image
correlation (DIC) principle. This study aimed to develop a methodology for the
efficient measurement of strain distribution patterns in the matrix material sur-
rounding embedded wood particles. Wood particles and reference wire particles
were embedded in a HDPE matrix. The specimens were pulled in tension and
imaged throughout the test. By comparing successive images to one another, the
displacements and strains on the surface of the specimen could be determined. The
results from their study showed that there is a good agreement between theoretical
(Clyne 1989) and observed strain distribution patterns (Fig. 5). However, a quan-
titative analysis of the load transfer between the two needs to happen using mor-
phologically informed predictive modelling tools. This approach has been used for
analysing load transfer in adhesively bonded specimens by Kamke et al. (2014) and
gives a unique look at the internal stress transfer in wood-based composites.
30 M.J. Schwarzkopf and M.D. Burnard

4.2 Durability
When compared with solid wood materials, WPCs have lower mechanical prop-
erties in strength, stiffness, and creep resistance. On the other hand, WPCs are less
susceptible to moisture absorption and absorb at a slower rate, providing for better
resistance to fungal attack and dimensional changes (Caulfield et al. 2005). This
being the case, it is important to remember that the wood particles within the WPC
are still a nutrient source for microbial decay and have the potential to become
degraded. While in theory the encapsulation of wood particles by a polymer should
provide some level of protection from biological decay and moisture intake,
external forces may compromise this protective layer.
Fig. 5 Comparison of observed strains (obtained from DIC) and a theoretical representation of the
interaction between an embedded particle and a polymer matrix. Theoretical model adapted from
Clyne (1989)
Wood-Plastic Composites—Performance and Environmental Impacts 31

WPCs are often marketed as highly durable, low maintenance, and a good
alternative to solid wood as an exterior product. One area under investigation for
using WPCs in exterior applications is in highway signs and markers. There are
many types of roadway markers, signs, and fixtures that exist along every kilometre
of the roadway.
Based on only two of these markers, tubular markers and inroad reflectors
(Fig. 6), Thompson et al. (2010) estimated that approximately 870 t of plastic is
being used annually in eight western states of the US. By introducing any amount
of wood filler into these products, a substantial volume of plastic could be displaced
by a renewable resource. While highway markers do not generally demand
high-structural performance, they are subjected to harsh environmental conditions,
mechanical abrasion, and ground contact throughout their entire service life. Rain
will soak the materials, temperatures will drop below and well above freezing
temperatures, vehicle tyres will drive over inroad markers, and the sun will expose
them to UV rays. Karas (2010) investigated the use of WPCs in highway appli-
cations and assessed their durability. In this study, durability was assessed by
measuring selected mechanical properties of WPCs before and after accelerated
weathering treatments as well as weight loss due to ground contact tests. To sim-
ulate the effects of environmental conditions that WPCs used in highway appli-
cations encounter, an accelerated weathering unit was used that applied UV light,
heat, and moisture. A soil contact test was also used to assess the durability of the
WPCs with respect to mass loss from biological decay.
As expected, WPC specimens that had been exposed to accelerated weathering
experienced lower mechanical performance than those that were unweathered.
Specimens that had higher loading levels of wood filler experienced more mass loss
and in specimens that were exposed to accelerated weathering, the mass loss was
almost double that of unweathered specimens. This was believed to be due to the
degradation of the polymer at the surface of the specimens. While the HDPE was
not substantially degraded due to biological decay in the soil contact tests, it may
have been degraded when exposed to UV in the weathering treatments. By
degrading the surface matrix material and allowing cracks and pathways to form
throughout the structure, more wood particles were exposed to biological attack.
This breakdown of the surface polymer encapsulating the wood fillers can also
occur during freeze thaw cycles and due to physical wear by car and truck tyres
abrading the surface of roadway markers. While WPCs used in decking materials
will not need to protect against vehicles driving over them, the same durability
concerns exist.
Fig. 6 Some examples of
in-road markers found along
the roadway. Photo Credit
Karas, M
32 M.J. Schwarzkopf and M.D. Burnard

Schirp et al. (2008) provides a state of the art summary of the biological
degradation of WPCs and provides some strategies for improving the durability of
WPCs. One of these strategies is to limit the loading ratio of wood filler to under
50 % unless using an antimicrobial treatment such as zinc borate which is effective
against wood-decay fungi and insects. This again touches on the incomplete
encapsulation of wood particles at higher loading ratios. Essentially, any cracks or
openings in the WPC or between the polymer and the wood provide a pathway for
moisture and biological decay to enter, as well as for crack propagation. Another
method used to improve the durability is to apply a cap stock layer. Most of the
durability issues in WPCs start at the surface and work inwards. Instead of dis-
tributing expensive additives like antimicrobial, colourants, and anti-UV agents
throughout the volume of the WPC, a thin layer of harder plastic with these pro-
tective additives is added to the outside (Hanawalt 2012). While effective, the cost
of making the production process more complicated must be assessed.
5 WPC Applications and Use
In Europe, WPCs account for nearly 11 % (260,000 t) of composite products pro-
duction, which includes product categories ranging from construction to consumer
goods (Carus et al. 2015). Table 1lists common products, their product categories,
and the associated manufacturing processes typically used to create them. Outdoor
decking and automotive components account for the greatest share of WPC pro-
duction in Europe (67 and 24 %, respectively), while other uses including siding and
fencing, furniture, consumer goods, and technical applications account for the
remainder (Carus et al. 2015). WPCs are considered a growth market, with increases
in major markets like North America and Europe estimated to be around 10 % while
in China growth is estimated to reach 25 % in 2015 (Eder and Carus 2013).
Table 1 Some common WPC products, their product categories, and associated manufacturing
products
Product examples Product category Manufacturing process
Decking boards and tiles, siding, and window frames Construction,
outdoors
Extrusion and injection
moulding
Garden furniture and fencing Garden/yard and
outdoors
Extrusion
Automotive interior trims and engine components
(exposed to temperatures less than 110 °C)
Automotive Extrusion, injection
moulding and sheet
forming
Furniture parts and furniture Housing and interior Extrusion and injection
moulding
Packaging (e.g. corner protectors), components for:
games, household electronics and other devices
Consumer goods,
interior and outdoor
Extrusion and injection
moulding, FDM
Table adapted from Eder and Carus (2013)
Wood-Plastic Composites—Performance and Environmental Impacts 33

5.1 Construction
WPCs are often used in the built environment, both indoors and outdoors. Although
exterior decking is the most common application worldwide, other construction-
related products are also important. These include railing system components,
stairs, window and door applications, flooring, exterior siding, fencing and land-
scape materials, interior moulding and trim work (Clemons 2002; Klyosov 2007;
Eder and Carus 2013; Carus et al. 2015). WPCs for construction purposes are
typically extruded and made to function and look like their solid wood counterparts.
These WPCs support conventional fasteners like screws, nails, brackets, and others
which allow them to be used without significant adjustment to typical building
practices (Pritchard 2004). The appearance of WPCs depend on the material con-
tents and finishing measures which include imprinting “wood-like”properties such
as grain structure and coatings, as in Fig. 7.
5.2 Automotive
The automotive industry uses WPC and other renewable-based fibre composites in
increasing quantities both to offset costs, reduce weight, and, in Europe, in an effort
to meet the demands for high recyclability contents of vehicles. Wood fibres in
fibre–plastic composites used in the automotive industry account for only about
38 % of the total renewable fibre usage (Carus et al. 2015). Other fibres in common
use are cotton, flax, kenaf and hemp. The primary use of WPC’s in automobiles is
for storage (trims in trunks, shelves for spare tyres, etc.) and interior door trims,
while other renewable fibre-based composites are used for higher value interior
Fig. 7 WPC coloured and formed to imitate the appearance of wood
34 M.J. Schwarzkopf and M.D. Burnard

trims including in dashboards and doors (Carus et al. 2015). In contrast to other
fields, WPCs in the automotive field are often used to replace metal or fibreglass
components, as opposed to wood or plastics.
European regulations stipulate that by 2015 reuse and recovery of all end-of-life
vehicles in the European Union must reach a minimum of 95 % by average vehicle
weight, and reuse and recycling of materials from end-of-life vehicles must reach a
minimum of 85 % by average vehicle weight (The European Parliament and the
Council of the European Union 2000). The use of renewable fibre-based composites
significantly reduces the weight of many vehicle components, and also increases the
recyclability. These factors, along with the existing technological capabilities of
vehicle manufacturers, provide for strong future growth in the automotive sector,
particularly in Europe where renewable fibre use could increase from 60,000 to
300,000 t each year (Carus et al. 2015).
5.3 Furniture, Highway Materials, Consumer Goods,
and Other WPC Applications
WPCs are already in use by major furniture producers, such as IKEA (2015a,b).
Applications in packaging and consumer products, including instruments, toys,
tableware are not yet mainstream, but are areas of likely growth (Haider and Eder
2010).
Highway construction in the United States utilises several materials which are
suitable for substitution by WPCs (e.g. treated wood and pure plastics), and may
become a growth sector in the future if certain barriers are overcome (Thompson
et al. 2010).
5.4 Perceptions of WPCs
User perceptions, knowledge, and preferences regarding materials have a large
impact on the overall utilisation of a product and vary in importance and impact
based on the needs and expectations of the user (Clemons 2002; Thompson et al.
2010). Material specifiers, manufacturers, and product users (both builders and
building occupants) all interact with materials and form opinions from their
experiences, which ultimately impact later material specification decisions.
Manufacturers of products, especially of products that are exposed to public
view like furniture, interior trim work, exterior decking, etc., may perceive WPCs
as ‘fake’and undesirable despite their workability and potential to imitate the
appearance of wood (Pritchard 2004). If manufacturers do not approve of the
material and refuse to use it (or do so begrudgingly), WPCs are unlikely to gain
traction in this subsector of the wood products market. However, manufacturers
Wood-Plastic Composites—Performance and Environmental Impacts 35

who typically deal with plastics are more likely to view the material in positive
terms because it reduces material costs compared to solid plastics and can be
considered to improve appearances, and reduce environmental impacts (Pritchard
2004; Bismarck et al. 2006; Klyosov 2007).
In the highway construction sector in the western U.S., WPCs were perceived
favourably by highway contractors compared to other more frequently used
materials despite low utilisation and relatively low familiarity (Thompson et al.
2010). Given the favourable perception of these products, the authors conclude that
once other challenges related to product certification are met, WPCs are well placed
to enter a potentially lucrative and sizeable market (Thompson et al. 2010).
In contrast to manufacturers and builders, lay persons often have different needs
and expectations from materials, especially related to aesthetics and maintainability.
For example, day-to-day building users have both active and passive responses to
their built environment. In both cases, the materials, design, and perceived qualities
of the environment impact user’s subjective and biological responses to the envi-
ronment (Burnard and Kutnar 2015). In the case of subjective reactions users may
decide that they dislike a building’s (or products) design and in some cases may
choose not to use it which has direct economic impacts. In the case of biological
responses, human well-being can be impacted by the user’s reaction to their envi-
ronment, especially over long periods of time. Material selection is a critical part of
designing buildings for human well-being (Burnard and Kutnar 2015). In new
building design paradigms that emphasise providing positive human health impacts,
including nature and natural elements in the built environment is important because
positive health impacts have been demonstrated repeatedly in outdoor environments
with greater degrees of naturalness (less apparent human intervention). User per-
ceptions of material naturalness are then a key to determine which materials should
be used in buildings designed to produce positive health impacts for occupants
(Burnard et al. 2015). A study comparing user perceptions of building material
naturalness conducted in three countries (Finland, Norway, and Slovenia) demon-
strated that users were quickly able to identify WPCs with imitated wood-like fea-
tures as significantly less natural than wood products that had undergone varying
degrees of transformation (solid wood, wood-based composites, stone, metal,
ceramics, and textiles were included in the study) (Burnard et al. 2015). However,
the WPC in the study was one of the few materials where differences in perceptions
of naturalness between countries were apparent, indicating that participants from two
of the countries (Finland, Norway) were able to distinguish between the natural and
imitated materials more readily than their counterparts (Slovenia). In addition to less
favourable ratings of naturalness, the study found that users in Finland and Norway
view the material as natural in a binary decision task about half as often as
Slovenians (25 and 50 %, respectively) (Burnard et al. 2015). Compared to other
materials in the study only seven materials were viewed as not natural by more
respondents overall (Fig. 8). In these cases, the difference in perceptions may be
related to the more prevalent use of wood materials in the built environment in
Finland and Norway than in Slovenia (Burnard et al. 2015).
36 M.J. Schwarzkopf and M.D. Burnard

6 Environmental Impacts of WPCs
Environmental impacts of processes and products have become an increasingly
publicised and are used by scientists, marketers, material selectors, and the public to
gain insight into how different technologies and products affect our environment.
Life cycle assessment (LCA) is means of estimating the environmental impact of a
product and the process used to create it. LCAs examine inputs and outputs of a
system used to create a product over a specified portion of its life. For example, a
cradle-to-grave LCA would use detailed information about the production and
extraction of raw materials, transportation, manufacturing, throughout the product's
useful life, and finally disposal (with wood products final disposal includes sig-
nificant energy recovery through combustion). Cradle-to-gate is another common
assessment period, which begins with raw material production but ends when the
product leaves the producers facility and does not include the use phase or disposal.
However, with the increased focus on recycling and reuse accounting for the
environmental impact of products from cradle-to-cradle is increasingly important,
especially in Europe where recycling and environmental targets are quite aggressive
(Gärtner et al. 2012;Höglmeier et al. 2013). In cradle-to-cradle scenarios for
renewables, materials are “cascaded”and reused after each life cycle, preferably
with minimal transformation in sequential uses, until finally the materials are
burned for energy reclamation (Gärtner et al. 2012;Höglmeier et al. 2013). WPCs
fit well into the wood cascade as repeat processing continually produces material
side streams which are suitable for use in WPCs, and reduction in material quality
and size (from solid timber to particles and fibres) after each life cycle will
Fig. 8 Percent of respondents stating the listed specimen was natural in a binary decision
assessment (Adapted from Burnard et al. 2015). *OSB Oriented Strand Board; **MDF Medium
Density Fibreboard
Wood-Plastic Composites—Performance and Environmental Impacts 37

eventually produce materials suitable for use in WPCs. Recycled plastics are also
used in WPCs which can significantly reduce their environmental impact (Vidal
et al. 2009; Oneil et al. 2013). Not only does wood use offset the quantity of
non-renewable material in WPCs, using wood in long-life products also sequesters
atmospheric carbon and mitigates the climate change potential connected to the
energy use of material extraction and processing (Hill et al. 2015).
The environmental impact of WPC matrix components varies greatly based on
the type of matrix used. Commonly used petroleum-based polymers (HDPE, PP,
PVC) produce negative environmental impacts throughout their life cycle, largely
because they rely on non-renewable raw materials (Rajendran et al. 2012; Qiang
et al. 2014). Using alternative, renewable-based, biodegradable polymers such as
PLA may produce smaller environmental impacts once improved manufacturing
techniques are developed that reduce water and energy utilisation (Qiang et al.
2014).
Studies directly comparing solid timber decking products against alternative
WPCs conclude that solid wood produces a significantly lower environmental
impacts than in both cradle-to-gate and cradle-to-grave scenarios (Bolin and Smith
2011; Oneil et al. 2013). Bolin and Smith (2011), following ISO 14040 and ISO
14044 standards (ISO/IEC 2006a,b), compared treated lumber used for decking
(alkaline copper quaternary (ACQ) lumber, an unspecified southern pine from the
US) to a WPC composed of 50 % recycled wood with a matrix mixture of virgin
and recycled HDPE (25 % of the total each). Comparing a theoretical deck of
approximately 30 m
2
, the ACQ-treated lumber deck used 14 times less fossil fuels
and nearly 3 times less water, produced 4 times less acid rain, 3 times less
greenhouse gas emissions, half of the smog and ecological toxicity, and nearly
equal eutrophication (Bolin and Smith 2011). The authors note that using biomass
or renewable energy sources for WPC production could significantly reduce the
striking difference in fossil fuel consumption between the ACQ-treated lumber and
the WPC (Bolin and Smith 2011). Assuming production in regions where renew-
able energy sources are more common (Europe, or the west coast of the US) could
significantly alter the outcomes of this type of comparison.
Another study comparing 9.3 m
2
of decking made from virgin PVC, virgin
WPC, recycled WPC, and redwood lumber found that recycled virgin WPC pro-
duced less global warming potential and ozone depletion than PVC, more smog,
acidification, eutrophication and respiratory effects using the US Environmental
Protection Agency’s TRACI methodology (Oneil et al. 2013). Recycled WPC
produced reduced impacts in comparison to virgin WPC in all categories except
ozone depletion, in which it produced similar effects. Compared to PVC, recycled
WPC produced significantly reduced impacts except eutrophication which was
greater for recycled WPC and smog impacts, which were approximately equal for
both (Oneil et al. 2013). In all categories, redwood lumber produced significantly
lower impacts between 10 and 30 % of the most significant impacts of the com-
pared products, except for global warming potential that produced a negative effect
(approximately 140 % reduced impact compared to PVC) (Oneil et al. 2013).
38 M.J. Schwarzkopf and M.D. Burnard

6.1 Product Category Rules and Environmental Product
Declarations
Product category rules (PCR) are defined methods for creating environmental
product declarations (EPD) for product groups such as “wood materials”. EPDs are
standardised statements of products environmental impacts that are meant to be
comparable to other similarly produced products that have EPDs following the
same PCR. The international standard for producing PCRs is ISO 14025, which
defines the necessary elements of PCRs, and, in effect, of EPDs (ISO/IEC 2006c).
Though no specific PCR or EPD could be found for a WPC product of any kind at
the time of this writing, a PCR for general wood materials can be used to produce a
WPC EPD. The PCR for wood materials developed by the firm Institut Bauen und
Umwelt e.V., specifies that their PCR can be used for a variety of wood composites,
including special wood materials which could include WPCs (Institut Bauen und
Umwelt e.V. 2009). Going forward using actual products for comparison, using
LCAs or EPDs-produced following a published PCR will provide more comparable
results. However, these tools are still developing and authors of LCAs and EPDs
should utilise the latest standards and versions of PCRs to conduct their
comparisons.
7 Summary
WPCs are a product category with many existing and emerging applications. The
vast majority of WPCs are used for exterior decking and other exterior board
applications. This market continues to grow in North America, is beginning to grow
in Europe, and rapid growth is expected in China. Automotive applications con-
tinue to grow as well, however, competition in this subsector from other fibre
sources poses a risk for WPCs. In both these primary uses, wood is used to reduce
the cost and weight of plastic products as well as to improve material properties
such as strength.
An added benefit of using wood as a filler product in plastics is the reduced
environmental impact that can be achieved by offsetting the amount of
non-renewable materials used in a product with renewably sourced wood. Often, the
wood used in WPCs can come from primary production side streams, forest slash, or
recovered wood products, which reduces the strain on raw forest resources.
However, competition for these products from energy producers, as well as fibre-
board and particleboard producers, has an impact on material costs for WPC man-
ufacturers which may steer them towards other renewable sources instead of wood.
Current development is focused on improving material qualities ranging from
strength to durability, as well as improving the compatibility of matrix components
for longer lasting products. Other current work is focused on examining the envi-
ronmental and performance impacts of using WPCs as substitution products in
Wood-Plastic Composites—Performance and Environmental Impacts 39

various new applications, using renewable-based plastics with wood reinforcement,
and examining new manufacturing methods for WPCs.
Future research and innovation must overcome challenges related to matrix
compatibility, rising costs related to increased demand for forest resources as a fuel
source, product shortcomings such as exterior durability, impact strength, as well as
knowledge gaps and negative perceptions amongst manufacturers in some sectors
and consumers. Other areas for innovation and development are rapid manufac-
turing with WPCs either for prototyping or bespoke product manufacturing. In each
case new innovations and developments must consider the environmental impacts
associated with the raw materials, manufacturing process, product utilisation,
recycling, and end-of-life scenarios. Improved data collection and quality amongst
manufacturers and consumers will help to improve the quality of LCAs conducted
on WPCs. Furthermore, using PCRs to produce comparable EPDs may alleviate
negative consumer and manufacturers perceptions about the suitability and envi-
ronmental impact of WPCs in a variety of product categories.
Acknowledgments The authors are pleased to acknowledge the support of WoodWisdom-Net+
and the Slovenian Ministry of Education, Science, and Sport of the Republic of Slovenia for their
support of the What We Would believe and cascading recovered wood projects; European
Commission for funding the project InnoRenew CoE under the Horizon2020 Widespread-2015
program, and infrastructure program IP-0035.
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