Influence of Fungal Decay and Moisture Absorption on Mechanical Properties of Extruded Wood-Plastic Composites

Anke Schirp, Michael P. Wolcott

Abstract


In previous research on the effects of fungal decay on the mechanical properties of wood-plastic composites (WPC), results were inconclusive due to the influence of moisture absorption. This study was conducted to clarify the contributions of moisture and wood decay fungi to WPC damage. Changes in flexural strength (MOR), modulus (MOE), and weight of two extruded wood-polyethylene composite (WPC) formulations, yellow-poplar sapwood and redwood heartwood, were compared following 3 months of incubation with wood decay fungi. All materials were evaluated using modified agar-block tests in which the white-rot fungus Trametes versicolor and the brown-rot fungus Gloeophyllum trabeum were employed as test fungi. In addition, soil-block tests were performed with yellow-poplar, one WPC formulation, and T. versicolor as test fungus only. It was determined that stiffness of WPC was affected more severely by moisture absorption than by fungal colonization. Strength of WPC was not affected by decay fungi but significantly (p = 0.0001) reduced by moisture absorption for a formulation containing 70% wood filler. Calculation of weight loss in WPC was based on the wood fraction only. Modified agar-block and soil-block tests were equally suited for determining weight loss in WPC, but agar-block tests could be completed in a shorter time span. Weight loss of a formulation with 70% wood filler and incubated with T. versicolor was twice as high as that of redwood in a modified agar-block test (6% versus 3%); however, only 1% weight loss was obtained when the formulation contained 49% wood filler. These results indicate that WPC can be designed to provide high fungal durability by controlling the material composition of the formulation. Weight loss is a more sensitive indicator of fungal decay than strength and stiffness measurements in WPC as well as in redwood.

Keywords


Fungal decay;mechanical properties;moisture absorption;natural fiber thermoplastic composites;redwood;weight loss;wood-plastic composites;yellow-poplar

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