One advantage of wood-plastic composites (WPCs) over traditional wood members is that they can be extruded in a variety of cross-sections. This creates the potential for the designer to tailor the product profile to the required stress distribution and tested material properties. Current design methodologies, however, do not make use of this method because the material behavior is not well characterized and the necessary material testing procedures have not been established. Since WPCs are visco-elastic, their mechanical behavior must be understood both in quasi-static loading and long-term response.
This study examines the short-term and long-term mechanical behaviors of seven WPC formulations (three HDPE, two Polypropylene, one LDPE, and one HDPE-Polystyrene blend) in uni-axial tension and compression. The shear responses were also evaluated through flexure tests at three different length/depth ratios. The results of the first 90 days of the creep experiments were functionally expressed using a time-dependent constitutive equation established for polymers, Findley’s power law. Since the materials were found to be nonlinear viscoelastic, that is, their mechanical response is both time- and stress-dependent, a stress dependency also was included in the functional representation. This allowed the response to the continuum of axial stresses in flexure to be properly replicated. The constitutive responses were coded into a user-defined subroutine in Abaqus, a finite-element analysis program, and the time-dependent deflections of full-size WPC products with a 2.1m span subjected to bending were predicted. Each prediction was compared to three-year experimental tests under identical conditions and found to be promising. This technique has potential to be a useful method for predicting long-term creep performance of extruded WPC products.
In spite of the excellent properties of wood-plastic composites (WPCs), their application has been limited to a few construction uses, such as decking boards and guard rails in North America and Europe and siding materials in Europe and Asia. In those applications, an extrusion process for the WPC production is efficient and powerful because of the design freedom in complicated shapes and engineering structures. In trials to produce WPC panels, however, the extrusion process is limited in productivity, product width, the size of the extrusion die due to the low flow-ability of WPC melts, and higher material cost compared to conventional wooden panel products. Moreover, many studies show that WPCs are not strong or stiff enough for structural applications if not appropriately reinforced. In this study, a novel WPC panel product produced with reinforcing laminations for structural applications will be discussed. WPC panel products can be easily upgraded with functional laminations of stronger materials and are very competitive with the manufacturing cost of conventional extrusion processes. The novel panel production was successful on the pilot-scale and the reinforcement was created with fiber-reinforced plastic (FRP) sheets. The mechanical properties of the WPC panels (over 12 GPa in flexural modulus of elasticity) indicate that the panels can be used for structural uses. This study addresses various issues about the FRP reinforcements, such as bondability of WPCs based on polyolefin plastics, and unique advantages of the processing methods used to create the WPC panel products.
Continued growth is expected for the WPC market in Japan, Korea, China, Thailand, Malaysia, Indonesia, and even Vietnam. In fact, it has already displayed double-digit growth in each country. The growth rate of WPC markets has been heavily dependent on deck material; however, WPC products have a much wider application usage. To create new markets, constant innovation is needed and we are now conducting many new development projects, including new usage of foam products as general purpose material.