Rowell R.M. (ed.) Handbook of Wood Chemistry and Wood Composites
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12.4.3 TESTING PROTOCOLS FOR FILTERS
12.4.3.1 Kinetic Tests
A kinetic test determines the time it takes to remove a given concentration of a contaminant. Two
approaches are used. One uses a constant weight of sorbent that is applied to various concentrations
of solution, and the other uses a constant concentration of solution applied to variable weights of
the sorbent (Eaton-Dikeman 1960).
12.4.3.2 Isotherms
Adsorption occurs on the surface of the fibers and when the rate of adsorption equals the rate
of desorption, equilibrium has been achieved and the capacity of the fiber has been reached.
The theoretical adsorption capacity of the fiber for a particular contaminant can be determined
by calculating its adsorption isotherm. Wood samples are carefully weighed (M) and placed in a
known volume (V) of standard solution of a given metal ion and shaken at 150 rpm. After 24 hours
the mixture of the standard solution and fiber sample are filtered and the final concentration of
the solution is determined by ICP.
The quantity of adsorbate that can be taken up by an adsorbent is a function of both the
characteristics and concentration of adsorbate and the pH (some authors also include temperature,
but pH has more impact than temperature in wood fiber filtration). Generally, the amount of material
adsorbed is determined as a function of the concentration at a constant pH, and the resulting function
is called an adsorption isotherm. One equation that is often used to describe the experimental
isotherm data was developed by the isotherm models of Langmuir and Freundlich (Weber 1970).
A characteristic of the Langmuir isotherm is that the loading of sorbate onto the sorbent approaches
a limiting value, Q
max
, as the concentration increases. This corresponds to monolayer coverage of the
adsorbent surface with a Langmuir constant, b, related to the free energy of adsorption:
(12.1)
FIGURE 12.9 Plot of phosphorus removal from solution using juniper fiber filters.
q
bQ C
bC
e
e
e
=
+
max
,
1
1588_C12.fm Page 359 Friday, December 3, 2004 12:10 PM
© 2005 by CRC Press
where b is Langmuir constant (L/g) and Q
max
is maximum adsorbate loading (grams adsorbate/grams
adsorbent).
The Freundlich isotherm is empirical and is used for heterogeneous surface energies:
(12.2)
where K (L/mg), a measure of sorption capacity, and 1/n, a measure of sorption intensity, are
Freundlich constants. To obtain the best estimate of all constants for the Langmuir and Freundlich
isotherms, data were fitted with nonlinear regressions using a least-squares fitting program.
12.4.4 BIOFILTERS FOR ORGANIC COMPOUNDS
A biofilter is a filter that contains a microorganism capable of degrading the organic compound
that is trapped by the filter. The filter first sorbs the compound by physical or chemical entrapment
and then the microorganism decomposes the chemical into smaller chemicals such as carbon dioxide
and water. Langseth and Pflum (1994) reported that biofilters could be used to remove organic
compounds with an estimated 95% efficiency. The chemicals removed were alcohols, aldehydes,
organic acids, and small amounts of low–molecular weight volatile organics such as benzene and
toluene. The retention times for small simple volatile compounds are very short, with a 90%
reduction in low–molecular weight compounds in about 10 seconds. Higher molecular weight
organics decompose at a slower rate (English 1996).
12.5 SORBENTS
Wood-based sorbents have been used for years as cleaning aids. For example, bark has been used
to sorb oil from seawater, and treated sawdust and wood shavings have been used to clean industrial
floors. There are several important factors that influence sorptive capacity, including density,
porosity, surface area, selectivity, and retention (English 1996).
12.5.1 DENSITY
Bulk density is simply a measure of wood mass per unit volume. Wood bulk density varies from
about 0.2 gm/cm
3
to greater than 1 gm/cm
3
. In general, the lower the bulk density, the greater the
sorption capacity. The true density of the wood cell wall is between 1.3 and 1.5 and does not take
into consideration any internal voids.
12.5.2 POROSITY AND SURFACE AREA
Porosity can be defined in several ways. It can be a measure of the size of the voids in the wood
or it can be a measure of how quickly and easily a liquid or gas can penetrate through a piece of
wood of a given size. Porosity can be measured as the time it takes for a gas or liquid to travel
through a piece of wood or it can be a measurement of the sizes of internal voids in the wood.
Woods with a low bulk density have a very large internal void space and therefore large internal
surface area. One cubic inch of wood with a specific gravity of 0.4 has a surface area of 15 square feet.
12.5.3 SELECTIVITY
Wood is hydrophilic, so it strongly hydrogen bonds to polar liquids such as water. Selectivity is a
measure of the ability of wood to preferentially sorb one gas or liquid, or one chemical in a gas
or liquid, over another. Selectivity is also influenced by the wood pore size, ability to wet the wood
surface, and capillary pressure.
qKC
n
ee
=
1/
,
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© 2005 by CRC Press
12.5.4 RETENTION
Retention is the ability of a saturated wood sample, after it has been used as a sorbent, to retain
the sorbed gas, liquid, or specific chemical. Retention can be a critical factor in choosing a sorbent
system. If the wood has the ability to remove a given contaminant but quickly releases it upon, for
example, exposure to air, the sorbed contaminant will be released into the environment before it
can be disposed of.
12.6 MULCH MATS
Mulching materials have been used around plants for many years (Waggoner et al. 1960) and are
generally of two types: particle mulches and sheet mulches. Particle mulches, such as bark, wood
shavings, or sawdust, are used to heat and hold moisture close to the plant. Sheet mulches are
similar to geotextiles but are usually thicker and last longer in the environment. They are used to
enhance seedling survival in wood species where the seedling remains in a “grass-like” state for
several years, i.e., the southern pines (Figure 12.10). The mulch mats, like the loose mulches, hold
moisture and heat as well as reducing weed growth around the seedling. Mulch mats must remain
intact for as long as competition from unwanted plants remains. For this reason, many mulch mats
are made from plastics such as polyethylene or polypropylene.
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© 2005 by CRC Press
13
Wood Thermoplastic
Composites
Daniel F. Caulfield
1
, Craig Clemons
1
, Rodney E. Jacobson
2
,
and Roger M. Rowell
1,3
1
USDA Forest Service, Forest Products Laboratory, Madison, WI
2
A-J Engineering, LLC, Middleton, WI
3
Department of Biological Systems Engineering, University of Wisconsin,
Madison, WI
CONTENTS
13.1Brief History
13.2Materials
13.2.1Wood Flour
13.2.2Wood Fiber
13.2.3Thermoplastic Matrix Materials
13.2.4Added Compounds
13.2.4.1Compatibilizers
13.3Processing
13.4Performance
13.4.1Mechanical Properties
13.4.2Moisture Properties
13.4.3Biological Properties
13.4.4Weathering and Fire Properties
13.5Higher Performance Materials
13.6Markets
References
The term “wood-plastic composites” refers to any number of composites that contain wood (of any
form) and either thermoset or thermoplastic polymers. Thermosets or thermoset polymers are plastics
that, once cured, cannot be remelted by heating. These include cured resins, such as epoxies and
phenolics, plastics with which the forest products industry is most familiar (see Chapter 10). Wood-
thermoset composites date to the early 1900s. An early commercial composite marketed under the
trade name Bakelite was composed of phenol-formaldehyde and wood flour. Its first commercial
use was reportedly as a gearshift knob for Rolls Royce in 1916 (Gordon 1988). Thermoplastics are
plastics that can be repeatedly melted, such as polyethylene, polypropylene, and polyvinyl chloride
(PVC). Thermoplastics are used to make many diverse commercial products, such as milk jugs,
grocery bags, and siding for homes. In contrast to the wood-thermoset composites, wood-thermo-
plastic composites have seen phenomenal growth in the United States in recent years. This chapter
deals exclusively with wood-thermoplastic composites, which are now most often simply referred
to as wood-plastic composites (WPCs) with the common understanding that the plastic always refers
to a thermoplastic.
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The birth of the WPC industry involved the interfacing of two industries that historically knew
little about each other and had very different knowledge bases, expertise, and perspectives. The
forest products industry has greater experience and resources in the building products market and
its production methods center around the typical wood processes: sawing, veneering, chipping,
flaking, and gluing. The plastics industry has knowledge of plastics processing that centers around
extrusion, compression-molding, and injection-molding technologies. Not surprisingly, some of the
earliest companies to produce WPCs were window manufacturers that had experience with both
wood and plastics.
13.1 BRIEF HISTORY
In the United States, WPCs have been produced for several decades, but they were produced even
earlier in Europe. Major growth in the United States, however, did not occur until fairly recently.
In 1983, American Woodstock, now part of Lear Corporation in Sheboygan, Wisconsin, began
producing WPC panel substrates for automotive interiors using Italian extrusion technology (Schut
1999). Polypropylene with approximately 50% wood flour was extruded into a flat sheet that was
then formed into various shapes for interior automotive paneling. This was one of the first major
applications of WPC technology in the United States.
In the early 1990s, Advanced Environmental Recycling Technologies (AERT, Junction, TX)
and a division of Mobil Chemical Company that later became Trex (Winchester, VA) began pro-
ducing solid WPCs consisting of approximately 50% wood fiber in polyethylene. These composites
were sold as deck boards, landscape timbers, picnic tables, and industrial flooring (Youngquist
1995). Similar composites were milled into window and door component profiles. Today, the
decking market is the largest and fastest growing WPC market.
Also in the early 1990s, Strandex Corporation (Madison, WI) patented technology for extruding
high wood fiber content composites directly to final shape without the need for milling or further
forming. Strandex has continued to license its evolving technology.
Andersen Corporation (Bayport, MN) began producing wood fiber-reinforced PVC subsills for
French doors in 1993. Further development led to a wood-PVC composite window line (Schut
1999). These products allowed Andersen to recycle wastes from both wood and plastic processing
operations. The market for WPC window and door profiles has continued to grow.
In 1996, several U.S. companies began producing a pelletized feedstock from wood (or other
natural fibers) and plastic. These companies provide compound pellets for many processors who
do not want to blend their own material. Since the mid-1990s, activity in the WPC industry has
increased dramatically. Technology is developing quickly and many manufacturers have begun to
produce WPCs. Although the WPC industry is still only a fraction of a percent of the total wood
products industry (Smith 2001), it has made significant inroads in certain markets. Current end
product manufacturers are an interesting mix of large and small manufacturers from both the plastics
and forest products industries. According to a recent market study, the WPC market was 320,000
metric tons (700 million lb) in 2001, and the volume is expected to more than double by 2005
(Mapleston 2001b). The status and developments in the WPC industry are now also closely
monitored by Principia Partners, an international business consulting firm for the industry. In 2003,
Principia Partners reviewed recent developments and trends (Morton et al. 2003). In North America
more than 67 WPC enterprises produce 590,000 tons valued at $700,000. The vast majority of
production is extruded materials for decks, with lesser amounts of extruded profiles for windows,
railings, transportation, and other infrastructural uses. Demand is projected to increase at an annual
rate of 14% over the next few years, reaching a total increase of 290% by 2010. Significant
production increases in Europe and Japan are also anticipated.
In 1991, the First International Conference on Woodfiber-Plastic Composites was convened in
Madison, Wisconsin (about 50 attendees), with the intent of bringing together researchers and
industrial representatives from both the plastics and forest products industries to share ideas and
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technology on WPCs. A similar, sister conference (Progress in Woodfiber-Plastic Composites) began
in Toronto, Ontario, the following year and is being held in alternating years. These conferences
have grown steadily through the 1990s, and in 2003 in Madison the 7
th
International Conference
on Wood- and Natural-Fiber-Plastic Composites Conference was attended by over 400 conferees.
Additional conferences have been held in North America and elsewhere around the world as both
interest and the market have grown.
13.2 MATERIALS
13.2.1 W
OOD FLOUR
The wood used in WPCs is most often in particulate form (e.g., wood flour) or very short fibers
and bundled fibers rather than long individual wood fibers. Products typically contain approximately
50% wood, although some composites contain very little wood and others as much as 70%. The
relatively high bulk density and free-flowing nature of wood flour compared with wood fibers or
other longer natural fibers, as well as its low cost, familiarity, and availability, is attractive to WPC
manufacturers and users. Common species used include pine, maple, and oak. Typical particle sizes
are 10 to 80 mesh. There is a wide range of wood flour suppliers and they cater to a number of
different industries. These are both large companies that have broad distribution networks as well
as small, single source suppliers catering to single customers. Because of the varied and diverse
nature of wood flour suppliers, there are currently few good resources that list wood flour manu-
facturers. Wood-plastic composite manufacturers obtain wood flour either directly from forest
products companies such as lumber mills and furniture, millwork, or window and door manufac-
turers that produce it as a byproduct or from companies that specialize in wood flour production.
With a growing number of wood flour suppliers targeting the wood-plastic composites industry,
they are beginning to be listed in plastics industry resources.
As with most materials, wood flour costs are variable and depend on such factors as volume,
availability, particle size, and shipping distance. However, wood flour is typically about
$0.11–0.22/kg ($0.05–0.10/lb) in the United States. Narrow particle size distributions and fine wood
flour sizes tend to increase cost. Because there are many small manufacturers of wood flour and
the volume is small relative to other wood products (solid wood, wood composites, and paper),
reliable statistical information on wood flour availability is scarce.
13.2.2 WOOD FIBER
Because of the potential of improved mechanical properties with fillers of greater l/d ratios, there
has been a continuing interest in the use of individual wood fibers rather than wood flour as
reinforcement in WPC. Recently, two wood-pulp fiber suppliers, Rayonier Corp. (Savannah, GA)
and Creafill Fiber Corp. (Chesterton, MD) have been marketing wood-pulp fibers for use as
reinforcements for thermoplastics. The fibers that they supply have been promoted for use in WPC
of higher performance potentials. Adding fibers rather than flour increases mechanical properties
such as strength, elongation, and unnotched Izod impact energy. However, processing difficulties,
such as feeding and metering low-bulk-density fibers, have limited the use of fibers in WPCs. There
have been some developments in pelletizing fibers for ease of handling and processing (Jacobson
et al. 2002).
13.2.3 THERMOPLASTIC MATRIX MATERIALS
The WPC presently consumes over 685,000 tons of thermoplastic resins annually (Morton et al.
2003). There are some differences in processing technologies between Europe and North America
and this is reflected in the consumption totals for the various resins. Polyethylene resin (PE) is the
dominant resin in North America amounting to 83% of the 685,000 tons consumed. Polypropylene
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© 2005 by CRC Press
(PP) is 9% of the total, and 7% of the thermoplastic resin used in the WPC industry is polyvinyl
chloride (PVC). Only about 1% of other resins are used.
13.2.4 ADDED COMPOUNDS
Wood and thermoplastic are not the only components in WPCs. These composites also contain
additional materials that are added in small amounts to affect processing and performance. Although
formulations are highly proprietary, additives such as coupling agents, light stabilizers, pigments,
lubricants, fungicides, and foaming agents are all used to some extent. Some additive suppliers are
specifically targeting the WPC industry (Mapleston 2001a).
13.2.4.1 Compatibilizers
Since the wood component of a wood-thermoplastic composite is hydrophilic and the plastic is
hydrophobic, a compatibilizer is often used to improve interfacial bonding of the two different phases.
One of the most common compatibilizers used today is a maleic anhydride grafted polypropylene
(MAPP). There are many different types of MAPPs differing in molecular weight and the degree
of maleic anhydride substitution. One used most often had a number average molecular weight of
20,000, a weight average molecular weight of 40,000 and had about 6% by weight of maleic
anhydride in the polymer.
This compatibilizer it thought to work by two different mechanisms. First, the anhydride reacts
with a cell wall polymer hydroxyl group to form an ester bond then the PP polymer attached to
the anhydride intertangles into the PP or PE network in the melt (see Figures 13.1 and 13.2).
Figure 13.3 shows a composite where a compatibilizer has been used. It can be seen that when
the composite was broken in two pieces, the fiber shown broke rather than be pulled out of the
thermoplastic matrix. There is a small gap at the base of the fiber showing that there was some
slippage but the fiber broke rather than being pulled out of the matrix due to weak interfacial
bonding between the fiber and the matrix.
13.3 PROCESSING
The plastics industry has traditionally used inorganic materials like talc, calcium carbonate, mica,
and glass fibers to fill and to modify the performance of plastic; about 2.5 billion kg of fillers and
reinforcements are used annually (Eckert 2000). The industry was reluctant to use wood or wood
fiber as filler because of its low bulk density, low thermal stability, and tendency to absorb moisture.
The majority of thermoplastics arrive at a manufacturer as free-flowing pellets or granules with a
FIGURE 13.1 Reaction of an anhydride end group on MAPP with a cell wall hydroxyl group.
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© 2005 by CRC Press
bulk density of about 500 kg/m. The plastics processor is faced with the problem of how to
consistently meter and force the low bulk density wood fiber into small feed openings typical of
plastics processing equipment. In addition, the processing temperature for even low melting point
plastics is often too high for incorporating wood fiber without thermal degradation. The high
moisture content of wood and other natural fibers is also problematic to the plastics industry, which
considers about 1–2% moisture content high. Even plastics processors with vented equipment
capable of removing moisture during processing were averse to removing 5–7% moisture from
wood fibers. Resin dryers, which are occasionally needed to dry plastics, are not appropriate for
wood particles or fibers, and drying the fine wood particles poses a fire hazard. Plastics processors
who tried to use wood or other natural fibers often lacked knowledge about wood, and their failed
attempts made the industry generally skeptical of combining wood and plastic.
For the wood products industry, thermoplastics were a foreign world, albeit one that occasionally
intruded on traditional markets (e.g., vinyl siding). Competing in different markets, forest products
and plastics industries had few material and equipment suppliers in common and they processed
materials very differently and on entirely different scales (Youngquist 1995).
FIGURE 13.2 Entanglement of the PP on MAPP with the matrix PP.
FIGURE 13.3 Fiber broken in a test for compatibilization.
1588_C13.fm Page 369 Thursday, December 2, 2004 4:46 PM
© 2005 by CRC Press