Wood Handbook, Wood as an Engineering Material

improved the strength and stiffness. Generally, adding a
coupling agent to the mix also improved mechanical prop-
erties. Adding wood to polypropylene was not without
tradeoffs. Impact resistance of such composites decreased
compared with that of unfilled polypropylene.

In addition to these commercial deck products, wood–plas-
tic composites are being developed for structural applica-
tions such as foundation elements, deck substructures, in-
dustrial decking, and shoreline structures (Bender and others
2006). Table 12–10 shows the range of average mechanical
properties of extruded wood–plastic composites by polymer
type. In general, polyvinylcholoride and polyethylene for-
mulations produce higher mechanical properties than those
produced from polyethylene alone. Formulations that use
coupling agents with either polypropylene or high-density
polyethylene result in improved strength, stiffness, and re-
duced moisture absorption properties.

Properties of wood–plastic composites can vary greatly de-
pending upon such variables as type, form, weight fractions
of constituents, type of additives, and processing methods
(Stark and Rowlands 2003, Wolcott and others 2006). Be-
cause formulations from each commercial manufacture are
proprietary, design data should be obtained directly from the
manufacturer.

Inorganic-Bonded Composites

Inorganic-bonded wood composites are molded products or
boards that contain between 10% and 70% by weight wood
particles or fibers and conversely 90% to 30% inorganic
binder. Acceptable properties of an inorganic-bonded wood
composite can be obtained only when the wood particles are
fully encased with the binder to make a coherent material.
This differs considerably from the technique used to manu-
facture thermosetting-resin-bonded boards, where flakes or

particles are “spot welded” by a binder applied as a finely

distributed spray or powder. Because of this difference and

because hardened inorganic binders have a higher density

than that of most thermosetting resins, the required amount

of inorganic binder per unit volume of composite material

is much higher than that of resin-bonded wood composites.

The properties of inorganic-bonded wood composites are

significantly influenced by the amount and nature of the in-

organic binder and the woody material as well as the density

of the composites.

Inorganic binders fall into three main categories: gypsum,

magnesia cement, and Portland cement. Gypsum and mag-

nesia cement are sensitive to moisture, and their use is

generally restricted to interior applications. Composites

bonded with Portland cement are more durable than those

bonded with gypsum or magnesia cement and are used in

both interior and exterior applications. Inorganic-bonded

composites are made by blending proportionate amounts of

lignocellulosic fiber with inorganic materials in the presence

of water and allowing the inorganic material to cure or “set

up” to make a rigid composite. All inorganic-bonded com-

posites are very resistant to deterioration, particularly by

insects, vermin, and fire. Typical properties of low-density

cement-wood composite fabricated using an excelsior-type

particle are shown in Table 12–11.

Testing Standards

The physical and mechanical properties of wood-based

composite materials are usually determined by standard

ASTM test methods. The following are the commonly used

methods described in ASTM (2009):

ASTM C 208–08. Standard specification for cellulosic

fiber insulating board.

Chapter 12 Mechanical Properties of Wood-Based Composite Materials

Table 12–9. Selected properties of wood–plastic productsa

Composite

Specific

gravity

Tensile properties Flexural properties Izod impact

energy

Strength Modulus Strength Modulus (J m–1)

MPa (lb in–2) GPa

(×10^6

lb in–2) MPa (lb in–2) GPa

(×10^6

lb in–2) Notched Unnotched

Polypropylene

(PP)

0.90 28.5 (4,134) 1.53 (0.22) 38.30 (5,555) 1.19 (0.17) 20.9 656

PP + 40% wood

flour

1.05 25.4 (3,684) 3.87 (0.56) 44.20 (6,411) 3.03 (0.44) 22.2 73

PP + 40% wood

flour + 3%

coupling agent

1.05 32.3 (4,685) 4.10 (0.59) 53.10 (7,702) 3.08 (0.45) 21.2 78

PP + 40% wood

fiber

1.03 28.2 (4,090) 4.20 (0.61) 47.90 (6,947) 3.25 (0.47) 23.2 91

PP + 40% wood

fiber + 3%

coupling agent

1.03 52.3 (7,585) 4.23 (0.61) 72.40 (10,501) 3.22 (0.47) 21.6 162

aFrom Stark and Rowlands (2003).