Abstract
The influence of nano-silica (nSiO2) and micro-silica (mSiO2) in the shell and wood fiber filler in the core on the thermal expansion behavior of co-extruded wood/polyethylene composites (Co-WPCs) was investigated to optimize the thermal expansion resistance. The cut Co-WPCs samples showed anisotropic thermal expansion, and the thermal expansion strain and linear coefficient of thermal expansion (LCTE) decreased by filling the shell layer with rigid silica, especially nSiO2. Finite element analysis indicated that the polymer-filled shell was mainly responsible for the thermal expansion. The entire Co-WPCs samples exhibited a lower thermal expansion strain than the cut Co-WPCs samples due to protection by the shell. Increasing the wood fiber content in the core significantly decreased the thermal expansion strain and LCTE of the Co-WPCs. The Co-WPCs whose core layer was filled with 70% wood fiber exhibited the greatest anisotropic thermal expansion.
Highlights
Wood polymer composites (WPCs) usually consist of moisture-sensitive hydrophilic wood fibers and a temperature-sensitive hydrophobic polymer
The thermal expansion strain of the shell layer without filler (S0) was 29.74 % at 90 ◦ C and decreased to 18.96% for nSiO2 (S20) and 8.84% for mSiO2 (S20) (Figure 2a,b). This indicates that the thermal expansion resistance of the shell was greatly improved by adding nSiO2, which has been shown to improve the mechanical properties by forming immobilization sites on High-density polyethylene (HDPE) chains via van der Waals forces [14,24,25]
The linear coefficient of thermal expansion (LCTE) was obtained from the slope of the linear portion of the thermal expansion strain curve (Figure 2c,d)
Summary
Wood polymer composites (WPCs) usually consist of moisture-sensitive hydrophilic wood fibers and a temperature-sensitive hydrophobic polymer. The co-extrusion of these two components can prevent moisture absorption by coating a high polymer content in the shell layer [1,2]. Co-extruded wood/polyethylene composites (Co-WPCs) with core–shell structures can achieve better weatherability [3,4] and fire retardation [5,6] than regular WPCs [1,7]. Co-WPCs contain much more polymer in the shell layer, which may increase the thermal expansion more than regular WPCs [8]. Adding fibers or spherical particles into the polymer matrix can mechanically restrain polymer chains during heating or cooling
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