Value-added composite bioproducts reinforced with regionally significant agricultural residues

Abstract

Biopolymers, such as polyhydroxybutyrate-co-hydroxyvalerate (PHBV), combined with natural fiber into biocomposites have potential as sustainable alternatives to traditional plastics and composites for which recycling is challenging. The addition of natural fibers, such as hemp, kenaf, and jute can increase the stiffness and strength of biopolymers at low weight and cost without compromising composite biodegradability. Because production of many natural fibers is limited by climate or geography, local and regional fiber sources collected as residues from agricultural crop production have potential to further reduce composite environmental impact by reducing embodied energy related to transportation and fiber cultivation. In this study four agricultural residue fibers (AF) were assessed: (i) hollow stem wheat, (ii) solid stem wheat, and (iii) barley as regionally significant food crop residues compared to (iv) hemp residue from seed and oil production as an industrially relevant control. These fibers were compounded into PHBV composites at fiber weight fractions of 0%, 10%, 20%, and 30%. Two fiber compatibilizing treatments were investigated for their potential to enhance the mechanical performance of AF-PHBV composites: (i) silane vapor deposited at room temperature and (ii) PHBV grafted to the fibers using reactive extrusion (gPHBV). Mechanical properties including flexural modulus and ultimate flexural strength were used to evaluate the impact of fiber fraction and treatments on biocomposites. Statistical analysis from our design of experiments indicated that some combinations of fiber, weight fraction, and treatment clearly outperformed others. In particular, samples with 30% silane treated hemp had the highest modulus and high flexural strength, while 30% gPHBV hemp had high modulus and the highest strength. Among residue composites, hollow stem wheat is most comparable to hemp, with similar modulus but lower flexural strength in treated high fiber samples. Solid stem wheat and barley composites generally had lower modulus, lower strength, and less consistent mechanical properties. Increasing fiber fraction consistently increased flexural modulus. Grafted samples had inconsistent flexural strength due to deleterious effects to the gPHBV matrix, as observed with scanning electron microscopy and differential scanning calorimetry. The mechanical properties of the different AF-composites occupy a similar application space, indicating potential for robust composite processing using AF.