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Transparent wood composites (TWCs) are a new class of light-transmitting wood-based materials composed of a delignified wood template that is infiltrated with a refractive- index-matched polymer resin. Recent research has focused primarily on the fabrication and characterization of single-ply TWCs. However, multi-ply composite laminates are of interest due to the mechanical advantages they impart compared to the single ply. In this work, 1- and 2-ply [0°/90°] TWC laminates were fabricated using a delignified wood template (C) and an acetylated delignified wood template (AC). The optical and mechanical properties of resultant C and AC TWC laminates were determined using ultraviolet-visible spectroscopy (UV-Vis) and tensile testing (5× replicates), respectively. In addition, the ability of classical lamination plate theory and simple rule of mixtures to predict multi-ply tensile modulus and strength, respectively, from ply-level mechanical properties were investigated and are reported herein. Experimental results highlight tradeoffs that exist between the mechanical and optical responses of both unmodified and chemically modified TWCs. Template acetylation reduced the stiffness and strength in the 0° fiber direction by 2.4 GPa and 58.9 MPa, respectively, compared to the unmodified samples. At high wavelengths of light (>515 nm), AC samples exhibited higher transmittance than the C samples. Above 687 nm, the 2-ply AC sample exhibited a higher transmittance than the 1-ply C sample, indicating that thickness-dependent optical constraints can be overcome with improved interfacial interactions. Finally, both predictive models were successful in predicting the elastic modulus and tensile strength response for the 2-ply C and AC samples. 

Social, political, and environmental pressures continue to drive the development of sustainable alternatives to petroleum-based materials. Accordingly, natural fiber composites (NFCs) are being developed and used for a range of low- and high- performance applications, such as packaging, automotive parts, and construction materials. As the use of NFCs become more widespread, there is a rising need to investigate the effect of weathering on this emerging class of materials. Previous studies on the moisture and freeze-thaw induced deterioration of NFCs have focused primarily on composites with high fiber contents (>50% by volume). Due to factors, such as low cost, bio-renewability, and enhanced mechanical properties, most commercially available NFCs maximize the content of natural fibers. However, high fiber contents also increase susceptibility to the deleterious effects of environmental aggressors (e.g., moisture and temperature). Since limited data exists on the durability of low-fiber content NFCs, this study investigates the moisture-induced deterioration of NFCs with low fiber content and explicitly analyzes the added effects due to freezing and thawing. Results from a combination of environmental conditioning and X-ray tomography provide and visual evidence of the effect of moisture-induced damage in low-fiber NFCs. Results also show that this deterioration is exacerbated by below-freezing temperatures. Investigating the response of NFCs to such environmental aggressors as demonstrated in this study provides an evidenced-based approach for material design, which ultimately depends on both the intended application and expected environmental conditions. 

Natural fiber-reinforced polymers are currently used in a variety of low- to high-performance applications in the automotive, packaging, and construction industries. Previous studies have demonstrated that natural fibers (e.g., flax, hemp) exhibit good tensile mechanical properties and have positive environmental and economic attributes such as low cost, rapid renewability, and worldwide availability. However, natural fibers are inherently susceptible moisture-induced changes in physical and mechanical properties, which can be unfavorable for in-service use. This study illustrates how a micromechanics-based modelling approach can be used to help facilitate durability design and mitigate the deleterious effects of freeze-thaw deterioration in wood-plastic composites (WPCs). The model described in this study predicts the critical fiber volume fraction (V_fcrit) at which damage to the composite will occur under certain environmental conditions for different WPC formulations of hardwood and softwood fiber reinforcement and polymer matrix types. As expected, the results show that V_fcrit increases (a positive result) as anticipated in situ moisture content decreases. In addition, results suggest that fiber packing distribution directly influences V_fcrit and that V_fcrit increases as the mechanical properties of the polymer matrix increase. In sum, the study demonstrates how predictive modeling can be applied during the design phase to ensure the durability of WPCs.