Mechanical properties of plasma-irradiated and surface-coated wood plastic composites (WPCs) have been investigated in this paper. WPCs were developed by injection molding technique using wood fiber (WF) as reinforcement and polypropylene (PP) as matrix. The short, discontinuous WF was compounded with thermoplastic PP at varying weight fractions of 0 wt%, 25 wt% (WP25), and 50 wt% (WP50) to yield tensile test specimens in accordance with JIS K7139-A32 standards. Subsequently, plasma treatment was performed on the test-pieces, followed by surface coating by immersion in acrylic resin liquid containing homogeneously dispersed TEMPO-oxidized cellulose nanofibers (CNF). The results indicate an increase in surface roughness after plasma irradiation, but surface coating of the specimens with acrylic paint and CNF decreased their surface roughness by ∼50% in comparison to the untreated specimens. Plasma treatment and surface coating also increased the tensile strength of neat PP, WP25 and WP50 specimens by 5.4–7.1%, 3.5–3.7% and 3.0–3.6%, respectively, whereas their fracture strains tended to decrease. Compared to the untreated specimens, the surface-coated specimens generally displayed higher tensile strength. This finding is a corroboration that the observed increase in strength is highly contingent on the adhesion between the specimen surface and the coating layer than on the improvement in surface roughness. Thus, it is inferable that surface coating could be of great importance in enhancing the mechanical performance of WPCs.

In order to clarify the fracture behavior of a unidirectional CFRP under proportional loading along the fiber (0°) and fiber vertical (90°) directions, a biaxial tensile test was carried out using a cruciform specimen with two symmetric flat indentations in the thickness direction. Three fracture modes were observed in the specimens after the test. The first mode was a transverse crack (TC), and the second was fiber breakage (FB). The third mode was a mixture mode of TC and FB (TC&FB). According to the measured fracture strains, regardless of the magnitude of the normal strain in the 0° direction, TC and TC&FB modes occurred when the normal strain in the 90° direction, ${\epsilon }_y$, ranged from 0.08% to 1.26% (positive values), and the FB mode occurred when ${\epsilon }_y$ ranged from −0.19% to −0.79% (negative values). The TC&FB mode is a unique mode that does not appear as a failure mode under uniaxial tension; it only occurs under biaxial tensile loading. Biaxial tensile tests were also conducted under non-proportional loading. The result showed three fracture modes similarly to the proportional loading case, each of which was also determined by the positive or negative value of ${\epsilon }_y$. Thus, this study reveals that the occurrence of each fracture mode in a unidirectional CFRP is characterized by only one parameter, namely ${\epsilon }_y$.

The depletion of conventional materials and their adverse environmental impacts have prompted a shift toward sustainable alternatives in composite materials engineering. In pursuit of this objective, this study investigated the mechanical properties of polypropylene matrix composites reinforced with Cordenka, an artificial cellulose fiber, and compared them to those reinforced with ramie, a natural cellulose fiber. Continuous strand composites were developed using the Multi-Pin-assisted Resin Infiltration (M-PaRI) process. The strands were subsequently sectioned into 15 mm lengths and injection-molded into dumbbell and strip specimens for mechanical characterization. The results showed that 20 wt% Cordenka/PP composites exhibited a tensile strength of 68.7 MPa, 2.04 times higher than neat PP and 1.66 times greater than the 20 wt% ramie/PP composites. Impact testing further demonstrated that Cordenka/PP composites absorbed 2 to 2.5 times more impact energy than ramie/PP composites, regardless of the presence of notches. Fiber length analysis indicated that Cordenka fibers maintained their length beyond the critical fiber length, allowing for efficient stress transfer and acting as a more effective reinforcement compared to ramie fibers, which were below this threshold. Consequently, the Cordenka/PP composites exhibited significantly enhanced mechanical performance. Scanning electron microscopy (SEM) analysis revealed fewer fiber pullouts in ramie-reinforced composites, suggesting superior interfacial adhesion to the PP matrix, although it did not translate to higher mechanical properties. These findings underscore the potential of Cordenka as a sustainable alternative to synthetic, non-biodegradable fibers in PP composites, providing improved mechanical properties and promising prospects for advanced composite applications.

The mechanical performance of natural fiber-reinforced polymer composites depends on fiber characteristics, matrix properties, and interfacial modifications. Although surface treatments such as polishing, coatings, and film lamination are commonly applied to enhance appearance, durability, or functionality, their effects on mechanical properties remain underexplored. This study investigates the influence of polypropylene (PP) and polyethylene terephthalate (PET) film lamination on the tensile behavior of wood fiber-reinforced polypropylene composites. Dumbbell-shaped specimens of neat PP and PP reinforced with 25 wt% wood fiber were fabricated via injection molding, then laminated with 0.1 mm PET or PP films using a plastic-compatible adhesive. Laminations were applied in four configurations: unlaminated (0S), single-sided lamination covering half the specimen length (1S-H), single-sided full-length lamination (1S-F), and double-sided full-length lamination (2S-F). PET film significantly improved tensile strength, with the 2S-F configuration yielding increases of 12% and 21% for neat PP and wood-filled PP, respectively. In contrast, PP film generally had minimal or negative effects, except in the 2S-F wood-filled case, which exhibited a 5% gain. Enhancements were attributed to PET’s superior stiffness and to surface homogenization, which reduced surface defects and improved stress distribution—effects supported by rule-of-mixtures predictions and morphological analysis. Surface roughness measurements and SEM confirmed smoother profiles and sealed surface voids, while fractographic evidence indicated that lamination mitigated crack initiation at specimen corners. These findings demonstrate that film lamination, particularly with PET, is an effective post-processing strategy for improving the mechanical integrity of wood-plastic composites.

This study investigates the effect of polymer film lamination on the tensile performance of wood fiber-reinforced polypropylene (WP) composites. Neat polypropylene (PP) and WP containing 25 wt% wood fiber were injection-molded and laminated with 0.1 mm PP or polyethylene terephthalate (PET) films using a compatible adhesive. Four configurations were examined: unlaminated (0S), single-sided half-length (1S-H), single-sided full-length (1S-F), and double-sided full-length (2S-F). Mechanical properties and fracture morphology were characterized by uniaxial tensile tests and scanning electron microscopy (SEM), alongside measurements of surface roughness. PET lamination produced the greatest strength enhancements, with 2S-F specimens achieving gains of 12% for PP and 21% for WP, whereas PP lamination gave minimal or negative effects, except for a 5% increase in WP. Strength improvements were attributed to surface smoothing and suppression of crack initiation, as confirmed by roughness measurements and SEM observations. PET’s higher stiffness and strength accounted for its superior reinforcement relative to PP. Fractographic analysis revealed flat regions near specimen corners—interpreted as crack initiation sites—indicating that lamination delayed crack propagation. The results demonstrate that PET film lamination is an effective and practical post-processing strategy for enhancing the mechanical performance of wood–plastic composites.