- This study examined the decay resistance of the pine and maple components of
wood plastic composites (WPC' s) of varying thicknesses exposed on several culture
media to wood decay fungi under laboratory conditions. The ability of malt agar extract
(MEA), potato dextrose agar (PDA), amended basal salts, sawdust (maple and red
alder), soil (direct exposure, sandwich system and soil block tests), vermiculite, and
malt liquid broth (stationary and rotary shaker conditions) to enhance WPC decay was
compared with traditional soil block tests.
Modulus of elasticity (MOE), modulus of rupture (MOR), creep, ultimate tensile
strength (UTS), moisture content, weight loss or scanning electronic microscopy were used to assess fungal effects on the WPC. MOE, MOR, creep and UTS were poor
parameters for assessing decay because the plastic component tended to dominate WPC
properties and was not susceptible to fungal attack.
Agar and soil block tests were both suitable for assessing wood decay of the
WPC's. Decay rates were strongly influenced by media type and test fungus. WPC
specimens exposed to Trametes versicolor or Postia placenta on 1.5% PDA, and those
specimens exposed to Gloeophyllum trabeum and P. placenta on MEA reached higher
moisture contents and experienced greater weight losses than specimens exposed to the
same fungi in the soil block test. Liquid media was unsuitable for enhancing WPC
decay, probably due to oxygen limiting conditions.
Scanning electron microscopy (SEM) showed that surface wood particles
exposed directly to fungal attack were partially or totally degraded, increasing the void
volume and exposing more wood particles to fungal attack, regardless of fungus or
WPC specimens made of maple were more susceptible to fungal degradation
than those made with pine. Increasing WPC thickness reduced moisture content and
weight loss of the wood. Slow moisture uptakes sharply reduced fungal attack in thicker
specimens. A strong relationship was found between weight loss and moisture content;
higher weight losses were consistently associated with higher moisture contents.
The results indicate that WPC decay can be accelerated using both agar and soil
as media, provided the specimens are small enough to rapidly absorb moisture.
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