Correlation between Mechanical Properties and Processing Conditions in Rubber-Toughened Wood Polymer Composites

Abstract

The use of wood fibers is a deeply investigated topic in current scientific research and one of their most common applications is as filler for thermoplastic polymers. The resulting material is a biocomposite, known as a Wood Polymer Composite (WPC). For increasing the sustainability and reducing the cost, it is convenient to increase the wood fiber content as much as possible, so that the polymeric fraction within the composite is thereby reduced. On the other hand, this is often thwarted by a sharp decrease in toughness and processability—a disadvantage that could be overcome by compounding the material with a toughening agent. This work deals with the mechanical properties in tension and impact of polypropylene filled with 50 wt.% wood flour, toughened with different amounts (0%, 10%, and 20%) of a polypropylene-based thermoplastic vulcanizate (TPV). Such properties are also investigated as a function of extrusion processing variables, such as the feeding mode (i.e., starve vs. flood feeding) and screw speed. It is found that the mechanical properties do depend on the processing conditions: the best properties are obtained either in starve feeding conditions, or in flood feeding conditions, but at a low screw speed. The toughening effect of TPV is significant when its content reaches 20 wt.%. For this percentage, the processing conditions are less relevant in governing the final properties of the composites in terms of the stiffness and strength.

A Review of Wood Polymer Composites Rheology and Its Implications for Processing

“…This aims at creating an effective load transferring interface between the natural fibers and the surrounding matrix [ 17 ]. Moreover, since WPCs are usually brittle, their formulation often contains toughening agents [ 30 , 31 , 32 ], such as styrene butadiene rubber (SBR), ethylene propylene diene monomer rubber (EPDM), or thermoplastic elastomers (TPE).…”

Tribological Behavior of a Rubber-Toughened Wood Polymer Composite

“…This discrepancy can be justified by the type of measurement that is being carried out, as profilometry readings can be affected by the mechanical properties of the material that is being analyzed as explained next. In Mazzanti et al [ 42 ], both composites were characterized in tension following the ISO 37-2011 standard, and the results in terms of stress vs. strain plot are pictured in Figure 6 . The t-WPC showed a more ductile behavior with a greater strain at break and lower strength and stiffness than WPC.…”

“…Before extrusion, all natural filler containing pellets were dried at 80 °C for 15 h. A flat temperature of 190 °C was set along the extruder barrel, while the die temperature was maintained at 180 °C. The complete mechanical characterization of WPC and t-WPC can be found in [ 41 , 42 ], respectively.…”

Properties of Starve-Fed Extrusion on a Material Containing a VHMWPE Fraction

“…An optimization model for starve-fed single-screw extrusion showed that the optimized mode in a single-screw extruder in terms of specific energy consumption is starve-fed mode [ 9 ]. Recently Mazzanti showed the improvement in mechanical properties of rubber-toughened wood composites through starve-fed extrusion [ 10 ]. Compared to conventional polyolefins, very high molecular weight polyethylene (VHMWPE) offers several advantages in terms of mechanical properties, such as very high impact strength and crack resistance.…”