- The global motivation for this work comes from the desire to fabricate "plastic lumber" with improved mechanical properties (particularly creep resistance) from recycled plastics and wood composites. The present studies have utilized "virgin plastics" of polystyrene (PS), an amorphous, stiff material that exhibits brittle fracture, and high density polyethylene (HDPE), a crystalline, tough material, that exhibits yield behavior but no fracture, with wood flour as a filler material. The goal is to achieve materials of high modulus of elasticity (MOE), reasonable strength, and very little creep.
The first phase of the work involved PS/HDPE blends with ratios: 100:0, 75:25, 50:50, 25:75, 0:100. The second phase of the work involved the addition of wood flour (aspect ratio 3.0) at levels of 10 -40wt% to the various plastic blends to make PS/HDPE/WF composites. In both cases, the materials were melt blended in a Banbury mixer and then processed through a single-screw extruder, with a shaping die attachment to make test bars. In some cases, the melt blended samples were compression molded in order to study processing effects. The samples were characterized using rheological, thermal (DSC), and morphological (SEM) techniques, and the mechanical properties (MOE, strength, creep) were measured.
Differential scanning calorimetry (DSC) indicates that the PS/HDPE blends are phase separated at all compositions, with the major and minor phases changing with composition and processing history. As was determined from SEM measurements, the HDPE remains the continuous phase up to 75% PS and a ribbon-like PS phase is observed in extruded samples. The MOE of the blends can be estimated by a weighted average of the blend components while the strength values generally fall below the weighted average value. Creep resistance is generally increased with increasing PS content. Processing history also has a significant effect on the blend mechanical properties, as evidenced by an elongated PS dispersed phase from an extruded blend which increased the strength by more than 50% and decreased the MOE by 25% as compared to a compression molded sample.
The PS/HDPE/WF composites exhibit changes in MOE, strength, and deformation behavior (rupture to yielding) with blend composition, wood flour content, and processing history. In general, MOE increased with increasing WF and PS content as was expected. The strength increases slightly with WF content for "HDPE-rich" composites, up to about 30wt% WF. Poor mixing affects properties at higher %WF content. Strength decreases with WF content for "PS-rich" blends, with the largest decreases for pure PS composites. The nature of the fracture also changes from yielding to brittle in these extruded "PS-rich" composites.
SEM images show that HDPE adequately coats the WF in the melt phase, but that there is very little adherence of HDPE to WF in the solid state. However, solvent extracted PS/HDPE/WF composites indicate that the WF does preferentially adsorb to the PS. The strength of the PS/WF cannot be determined. The SEM images also indicate that the aspect ratio of the WF is decreased with processing, which has implications the effectiveness of WF as a filler material. The previous "ribbon-like" structure observed in PS/HDPE extruded blends is not seen in extruded composites of similar composition, which helps to explain the strength change from a yielding to a more brittle nature.
The creep response of the composites has been evaluated with a three-parameter power model, from which "creep speed" can be determined. The creep speed is reduced with increasing PS content, and to a lesser extent, with increasing WF content. Samples of 75%PS-25%HDPE with varying WF content exhibited the lowest creep speed. The result is encouraging, indicating that PS/HDPE/WF composites may indeed lead to improved mechanical properties. Further studies using compatibilizers to increase adherence of the WF to the plastic matrix, higher aspect ration wood filler, and processing which allows for improved mixing (static mixers) are recommended.