Toward Biocomposites Recycling: Localized Interphase Degradation in PCL-Cellulose Biocomposites and its Mitigation

Tailoring the Holocellulose Fiber/Acrylic Resin Composite Interface with Hydrophobic Carboxymethyl Cellulose to Enhance Optical and Mechanical Properties

Interface engineering is essential for cellulosic fiber-reinforced polymer composites to achieve high strength and toughness. In this study, carboxymethyl cellulose (CMC) functionalized with hydrophobic quaternary ammonium ions (QAs) were utilized to modify the interface between holocellulose fibers (HF) and acrylic resin. The wet HF/CMC papers were prepared by vacuum filtration, akin to papermaking, followed by cationic ion exchange with different hydrophobic QAs. Subsequently, the modified papers were dried, impregnated with an acrylic resin monomer, and cured to produce transparent composite films. The effect of the hydrophobic QA moieties on the structure and optical and mechanical properties of the HF/CMC/acrylic resin composites were investigated. The composite film with cetyltrimethylammonium (CTA)-functionalized CMC showed high optical transmittance (87%) with low haze (43%), while the composite film with phenyltrimethylammonium (PTMA)-functionalized CMC demonstrated high Young's modulus of 7.6 GPa and high tensile strength of 180 MPa. These properties are higher than those of the composites prepared through covalent interfacial modification strategies. The results highlighted the crucial role of hydrophobic functionalized CMCs in facilitating homogeneous resin impregnation in the HF fiber network, producing a composite with enhanced interfacial adhesion strength, increased optical transparency, and mechanical strength. This facile use of hydrophobic CMCs as interfacial compatibilizers provides an energy-efficient route for preparing transparent, thin, and flexible composite films favorable in optoelectronic applications.

Highly reinforced and degradable lignocellulose biocomposites by polymerization of new polyester oligomers

Unbleached wood fibers and nanofibers are environmentally friendly bio-based candidates for material production, in particular, as reinforcements in polymer matrix biocomposites due to their low density and potential as carbon sink during the materials production phase. However, producing high reinforcement content biocomposites with degradable or chemically recyclable matrices is troublesome. Here, we address this issue with a new concept for facile and scalable in-situ polymerization of polyester matrices based on functionally balanced oligomers in pre-formed lignocellulosic networks. The idea enabled us to create high reinforcement biocomposites with well-dispersed mechanically undamaged fibers or nanocellulose. These degradable biocomposites have much higher mechanical properties than analogs in the literature. Reinforcement geometry (fibers at 30 µm or fibrils at 10–1000 nm diameter) influenced the polymerization and degradation of the polyester matrix. Overall, this work opens up new pathways toward environmentally benign materials in the context of a circular bioeconomy.

Cellulose biocomposites from nanocellulose or plant fibers with polymer matrix are often not degradable and suffer from insufficient mechanical properties to replace established materials. Here, the authors demonstrate the fabrication of hydrolytically degradable polymers through in-situ polymerization of new functionally balanced oligomers within high-content lignocellulose reinforcement networks.

Relatively Independent Motion of a Continuous Nanocellulose Network in a Polymer Matrix

Nanocellulose has been studied extensively in polymer composites as it can be employed as biobased reinforcement for synthetic polymers. However, the challenge to optimize the reinforcing component to consume applied energy as much as possible remains. This is related to the reacting force in the test sample and its extensibility. Prolonging the fracture strain of the material is one of the most effective strategies for such a purpose. The investigation on nanocellulose movement in a polymer matrix could shed light on the nanocellulose reinforcing mechanism's fundamental understanding. In this work, a continuous nanocellulose network was used to prepare nanocellulose/polymer composites. Different from using noncontinuous nanofillers, e.g., cellulose nanofibers and nanocrystal, the regenerated cellulose gel network used in this work could move together with the polymer under an axial signal force, serving as an excellent model advantageous in investigating the movement of nanocellulose in the polymer matrix. The deformation of the nanocellulose in the matrix was able to be evaluated by tracking the fracture strain of the materials. A series of chemical cross-linked nanoporous cellulose hydrogels (CCNCGs) were prepared, and their fracture strain increased first and then decreased as the molar ratio of epichlorohydrin (ECH) to the anhydroglucose unit (AGU) of cellulose increased. Two polymer matrices, polycaprolactone (PCL) and polyurethane (PU), were selected to be polymerized in CCNCGs in situ. The fracture strain of CCNCG/PCL and CCNCG/PU nanocomposites in the tensile test showed the same tendency as neat CCNCGs in the hydrated state, regardless of the surrounding environment. The relatively independent motion of the nanocellulose network in the polymer matrix was clearly demonstrated. Possible mechanisms of the nanocellulose's independent motion in the polymer matrix were discussed, implying the potential of independent deformation of the continuous nanocellulose network in the polymer matrix.