Pine-Polyethylene (Wood -Polymer) Composites: Synthesis and Mechanical Behavior Characterization.

Abstract

Processing of recycled wood chips, recently produced after extraction of creosote from telephone posts and railroad crossties, and combining them with a recycled polymer for the synthesis of novel composite materials would initiate a new trend toward preservation of natural resources. The challenge is taken in this work to produce and study the properties of such materials for further advancement of science. In the present study, addition of pine wood chips to HDPE reduced the tensile strength and break strain in tensile loading. Smaller wood chips generally resulted in smaller reductions. Peak load, modulus of rupture and stiffness were slightly higher at 40% pine wood chip concentration in the composite than the polymer alone; they later decreased with increasing concentration of wood chips. Optimal mechanical properties in these composites were produced by pine wood chips smaller than 0.125 inch in size, at around 40 vol. % in concentration. Pre-treatment of wood chips in a suitable solution of NaOH caused an increase in the coupling strength between protruding wood fibers and polymer. Such a treatment followed by a second one with vinyltrimethoxysilane was found to be the best for obtaining maximum bonding strength. Impact testing of the prepared samples showed that more fracture resistant wood-polymer composites were those with larger wood chips at the higher concentration range of 50 to 60 vol. %. In contrast, composites with 60% fine wood chips would fail easily at energy levels far less than those required to break the polymer alone. Variation of (Keng)2/E vs. crack length, the R curve, indicates three regions. The point of transition from region I (elastic) to region II is considered as a critical point of fracture process initiation, KIif,com. The transition from the state of stable crack growth, region II, to region III is considered to be at the point of instability. Likewise, the point of inflection on the plot of Ktrue vs. crack length, corresponding to a large change in slope, indicates also the point of instability. Keng corresponding to this critical point of inflection is proposed to be KIc, the real fracture toughness of these composites.