Moisture dynamics of WPC and the impact on fungal testing

Abstract

Wood-plastic composites (WPCs) are increasingly used in decking applications, where exterior exposure can lead to sufficient moisture for fungal deterioration. Standard tests recommended to assess fungal durability of WPC, but initially developed for wood or wood-based panels, are not applied in this study because the similarity in moisture behaviour for wood (-based panels) and WPC is questioned. The moisture dynamics of commercialised WPC versus wood-based panels were studied employing different moistening methods. The moisture sorption differences between various WPCs were minimal despite different wood contents, particle sizes, and plastics employed, but given sufficient time WPC wood particles gained sufficient water for fungal decay. To assay fungal durability of WPCs, immersion of the specimens for at least 1 week in water at 70 °C seems to be the most effective pre-treatment.

Introduction

Wood-plastic or wood-polymer composites, known as WPCs, are a combination of wood and thermoset or thermoplastic polymers (Ellis, 2000, Clemons, 2002). Bringing those two materials together moderates their weaknesses. Compared to wood, WPC products are less susceptible to moisture sorption, easier to design, and need less maintenance, while compared to plastic the material becomes stiffer, cheaper, and lighter, and shows a lower thermal deformation by incorporation of wood. On the other hand, by mixing both materials, new frailties are introduced in the resulting material. Compared to wood, WPCs have a lower E-modulus and a higher density, and are more expensive. Finally, a WPC component is an anisotropic material that is more brittle than plastic and susceptible to colour change and moisture uptake (English and Falk, 1996, Clemons, 2002, Van Acker, 2006).

An increasing amount of WPC material is used as decking because it has a relatively stable colour and no need for chemical treatment against microbial attack or attack by termites. It is, furthermore, easy to install, nail, screw, and clean (stains are more easily removed) and does not splinter (Mankowski et al., 2005, Manning et al., 2006). Mankowski et al. (2005) and Manning et al. (2006) mentioned that warranties typically range from 10 to 25 years and some up to “limited lifetime”. Eder et al. (2007) remark that the perception of WPC products is totally different in Europe and America. In the U.S. and Canada, WPC is considered a cheap material made of recycled plastic and recycled wood and intended to replace preservative-treated wood. In Europe, WPC products are introduced to replace tropical hardwood as a high technological product made of renewable, but mostly virgin, resources for specific applications. Furthermore, in America most WPC boards are solid boards and 90% of the boards are based on HDPE, while in Europe more than 80% are hollow profiles and a considerable portion are PVC-based. Concluding, in Europe more expensive raw material is used to produce hollow profiles and prices vary from 0.5 to 1.5 times the prices for the best tropical hardwood decking (Crez and Jezequel, 2007), while in America cheaper but more raw material is used to make solid boards with prices that are nearly equal to those for the best tropical hardwood and 1.6–1.8 times the price of pressure-treated pine decking (Anonymous, 2004).

When WPC was introduced, wooden particles were thought to be entirely encapsulated and therefore inaccessible to water and fungi. These initial assumptions were later rejected (Morris and Cooper, 1998, Mankowski and Morrell, 2000, Mankowski et al., 2005, Manning et al., 2006). According to Klyosov (2007), WPC has to be regarded as a porous material, not only because the adhesion between wood and polymer might be poor and cause little gaps, but also since the polymer itself can be porous when it is filled with ligno-cellulose fibre and other additives at high temperature. In this case, plastic undergoes a rather noticeable degradation or depolymerisation, which leads to volatile organic compounds. Eventually, these VOCs, together with steam originating from the heating of residual moisture in the ligno-cellulosic fibres, can make the material foam resulting in a porosity that is poorly controllable. This porosity and the defective encapsulation results in small channels, which act as pathways for moisture, even to the core of the material.

Fungal tests recommended by the European Committee for Standardization to assess biological durability (CEN/TS 15534-1, 2006) are originally developed for application to wood or wood-based panels. As moisture is indispensable for fungal growth, this is only justified if WPCs and wood-based panels have similar moisture behaviour and if the test methods also put WPC in a worst-case scenario. A considerable amount of research has been reported concerning WPCs from well-defined laboratory-fabricated materials (Falk et al., 2000, Rangaraj and Smith, 2000, Pendleton et al., 2002, Verhey and Laks, 2002, Gnatowski, 2003, Clemons and Ibach, 2004). Yet WPC is a commercial product already available on the market as decking, siding, cladding, benches, etc. The research presented here studied first the moisture dynamics of a number of commercialised WPC decking products. As durability tests in the laboratory environment want to simulate a worst-case scenario of realistic in-service situations, this study searches for the situation with the largest impact. If WPC decking is installed with little or no ventilation, a moist environment can be created. Furthermore, bad installation can cause water traps on the deck and lead to immersion of parts of the boards, and leaving a wet fabric—for example, a doormat—on a WPC deck can cause moisture problems, as can ground contact applications.

After observing differences between the tested WPC products, a comparison with the moisture behaviour of wood-based panels is made.