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The purpose of this work is to compare the weathering properties of different types of wood-plastic composites (WPCs) based on high-density polyethylene (HDPE), recycled high-density polyethylene (rHDPE-I and rHDPE-II), low-density polyethylene (LDPE), polypropylene (PP), recycled polypropylene (rPP), polystyrene (PS), and recycled polystyrene (rPS). The modulus of rupture (MOR) and modulus of elasticity (MOE) of all WPCs decreased with increasing exposure time of weathering. Of these, the rHDPE-II-based composite exhibited the highest MOR and MOE retention ratios after 2000 h of accelerated ultraviolet (UV) weathering, while the PS-based WPC had the lowest values. In addition, the carbonyl index difference (CID) of various WPCs increased significantly as a function of exposure time. Among them, the PS-based WPCs exhibited the most severe degradation due to photo-oxidation on the surface, while the degradation of PE-based WPCs was the mildest. These results are consistent with the change in the surface cracking and flexural properties of the composites. The PS-based WPCs also exhibited higher moisture diffusion coefficients. The mechanical behavior of WPCs after weathering is influenced by a combination of factors, such as surface oxidation, morphology changes, and moisture absorption.

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This study investigated the effectiveness of heat-treated wood particles for improving the physico-mechanical properties and creep performance of wood/recycled-HDPE composites. The results reveal that the composites with heat-treated wood particles had significantly decreased moisture content, water absorption, and thickness swelling, while no improvements of the flexural properties or the wood screw holding strength were observed, except for the internal bond strength. Additionally, creep tests were conducted at a series of elevated temperatures using the time–temperature superposition principle (TTSP), and the TTSP-predicted creep compliance curves fit well with the experimental data. The creep resistance values of composites with heat-treated wood particles were greater than those having untreated wood particles due to the hydrophobic character of the treated wood particles and improved interfacial compatibility between the wood particles and polymer matrix. At a reference temperature of 20 °C, the improvement of creep resistance (ICR) of composites with heat-treated wood particles reached approximately 30% over a 30-year period, and it increased significantly with increasing reference temperature.

The crystallization behavior of bamboo fiber (BF) reinforced polypropylene (PP) composites (BPCs) was investigated using a differential scanning calorimeter (DSC). The results showed that unmodified BF as a nucleation agent accelerated the crystallization rate of the PP matrix during cooling whereas there is no significant effect on the improved crystallization rate in BPCs with acetylated BFs. Based on the Avrami method, Avrami–Ozawa method, and Friedman method, the corresponding crystallization kinetics of PP reinforced with different acetylation levels of BFs were further analyzed. The results demonstrated that the crystal growth mechanism of the PP matrix for BPCs with unmodified and various acetylated BFs exhibited tabular crystal growth with heterogeneous nucleation. A higher cooling rate is required to achieve a certain relative crystallinity degree at the unit crystallization time for BPCs with a higher weight percent gain (WPG) of acetylated BFs (WPG >13%). Furthermore, based on the Friedman method, the lowest crystallization activation energy was observed for the BPCs with 19% WPG of acetylated BFs.