In this study some of the important properties of experimentally manufactured wood– plastic composites (WPC) were determined. Specimen having 60% and 80% particle and fibre of radiata pine (Pinus radiata ) were mixed with polypropylene (plastic) and four different additives, namely Struktol TR 016 which is coupling agent, CIBA antimicrobial agent (IRGAGUARD F3510) as fungicide, CIBA UV filter coating (TINUVIN 123S), CIBA blue pigment (IRGALITE), and their combinations.
Based on the initial finding of this work static bending properties of the samples enhanced as above chemicals were added into both particle and fibre-based specimens. Thickness swelling of the samples were also improved with having additives in the panels. Micrographs taken on scanning electron microscope (SEM) revealed that coupling agent and pigment resulted in more homogeneous mixture of wood and plastic together. Two surface roughness parameters average roughness (Ra) and maximum roughness (Rmax) used to evaluate surface characteristics of the samples showed that particle based samples had rougher surface characteristics than those of fibre based ones. No significant influence of chemicals added in the samples was found on surface roughness values of the samples manufactured from particle and fibre of radiata pine.
Wood-plastic composites (WPC) are widely used in USA, the most common type of such panels are produced by mixing wood flour and plastics to produce a material that can be processed similar to 100% plastic based products (Ballerini 2004, Charrier 1999, Groom, Shaler and Mott.1996, Simonsen 1995). Some of the major advantages of WPC include their resistance against biological deterioration for outdoor applications where untreated timber products are not suitable. The sustainability of this technology becomes more attractive when the low cost and high availability of fine particles of wood waste is considered.
These composites are transformed by extrusion processes to obtain applications including profiles, sheathings, decking, roof tiles, and window trims, with improved thermal and creep performance compared with unfilled plastics (English and Falk 1995, Tangram Technology 2002, Verhey Steven and Laks 2002). However, it is necessary to improve their physical and mechanical properties as well as appearance of such products to have a strong market share in wood composite panel industry. There are several ways to improve overall properties of WPC panels, namely using right size of raw material, optimum mixture and preparation of the elements in the product, and adding small amounts of additives such as coupling agents, pigments, antimicrobials or light stabilizers during their production (Nielsen and Landen 1994, Plueddemann 1982).
Most of the physical and mechanical properties WPC depends mainly on the interaction developed between wood and the thermoplastic material. One way to improve this interaction is incorporating a coupling agent as additive. In general, the additives help the compatibility between hydrophilic wood and hydrophobic plastic allowing the formation of single-phase composite. WPC also have problems when they are exposed to UV rays, their natural wood or pigmented colour may tend to fade away. Therefore, depending on the final application, UV filters have to be added to stabilize their colours for a longer time. When designing a commercial composite, the effect of particle size is one of the most important parameters affecting overall products properties. The use of optimum large particles might improve the mechanical properties of a composite, but the incorporation of a preservative should also be considered if it will be used for an application where biological resistance of this product is important (Verhey, Lacks, Richter, Larkin 2002).
Radiata pine is one of the main species having with an annual production of 27 million of cubic meters in Chile. It’s an excellent prime source for pulp and paper manufacture in Chile and many other countries. Additional products from radiata pine include interiors and exteriors panels, furniture manufacture, trimming, and structural lumber. Although research and technological development in the area of WPC in Chile has been increasing. But no comprehensive work has been done in this area (Ballerini 2004). Therefore the main objective of this work is to investigate some of the properties of WCP panels manufactured from radiata pine furnish and plastic, which is virgin polypropylene, with addition of various chemicals to provide an initial data in this area.
Wood plastic composite Materials and methods
Commercially produced polypropylene in the form of pellets and wood material of radiate pine (Pinus radiata) were used to manufacture experimental panels. Wood particles were manually screened on a sieve and classified into two portions, 10 mesh particles and 50 mesh fibres. Four different types of chemicals Struktol TR 016 which is coupling agent, CIBA anti-microbial agent (IRGAGUARD F3510) as fungicide, CIBA UV coating (TINUVIN 123S) and CIBA blue pigment (Irgalite) were added into the samples. Table 1 shows the list of the chemicals and their percentages used for panel production. Wood particles and fibres were dried in an oven before they were mixed with polypropylene. First plastic material was put into mixer rotating at 75 rpm having a temperature of 165°C for 2 minutes followed by adding the chemicals for each type of mat. In the next step particles or fibre were added into the mixture and rotated for another 3 minutes completing a total mixing time to 5 minutes. Figure 1 illustrates the mixer used for panel manufacture. Mixed samples then were pressed in a hot press with a 20 cm by 20 cm platen capacity. Each batch of sample was pressed using a temperature of 165° and a pressure of 40 bar for 5 minutes. The press was cooled off while the samples were still under compression before they were removed and conditioned in a climate chamber with a temperature of 20°C and a relative humidity of 55%. Average target thickness of the panel was 2.5 mm. Modulus of elasticity (MOE) and modulus of rupture (MOR) of the samples were determined on a Comten Testing Unite equipped with a load cell with a capacity of 2,000 kg Figures 2A, 2B, and 3 show some of fibre and particle based samples and bending test set-up, respectively.
Four samples with a size of 5 cm by 5 cm were used to determined thickness swelling (TS) of the panels. The thickness of each sample was measured at four points. Then samples were submerged in distilled water for 2 hours and 22 hours before thickness measurements were taken from the same location to calculate swelling values. Surface roughness of the samples was also determined using a stylus type profilometer. A portable stylus equipment consisted of a main unit and pick-up which had a skid-type diamond stylus with 5μm tip radius and 90° tip angle (Figure 4). The vertical displacement of the stylus is converted into electrical signal and digital information. Different roughness parameters such as average roughness and maximum roughness can be calculated from that digital information and profile of the surface can be developed as shown in Figure 5. Description of these parameters is discussed in previous studies [5,6]. Six random measurements were taken from the surface of tested bending samples to evaluate surface characteristics of the panels. Micrographs were also taken from the cross section of the samples with 3 mm by 3 mm face surface area to evaluate effect of wood plastic interaction on both particle and fibre based samples. Figures 9A through 9D show typical micrographs taken from the samples.