Agricultural Waste Valorization for Sustainable Polymer Composites: Pathways Toward a Circular Bioeconomy

Valorizing agricultural residues in polymer composites is often presented as a way to tackle two problems at once: fossil plastics and unused crop residues. This review tests that claim against the evidence. It follows up a 2020 review by the first author and asks how far the directions proposed then have been pursued, validated and taken up by industry between 2021 and 2026. A PRISMA 2020-guided search identified 105 included studies, each coded for feedstock status (field residue, processing by-product, post-consumer residue or purpose-grown comparator), matrix class, valorization pathway, scale of demonstration and environmental assessment. Physical processing (milling) and chemical surface modification (mainly alkali treatment) dominated (87% of studies). Biological and chemical conversion into new polymers remained rare. Roughly half of the composites used non-biodegradable thermoset or thermoplastic matrices. Of the 105 coded studies, 102 were laboratory-only, two reported a scale-up step, one reported a life-cycle assessment, and none reported a techno-economic assessment. Across studies, mechanical optima clustered at 10–20 wt% residue, and the optimum depended on which property was measured. Six years on, the 2020 propositions have been confirmed technically at bench scale, but they have not been validated industrially. Market uptake is limited to low-transformation fillers in commodity polymers, and bioplastics still account for only about 0.5% of plastics production. The available life-cycle evidence suggests that composites are environmentally favourable mainly when they displace virgin polymer; avoid open burning; use little chemical treatment; and do not divert residues from soil, feed or established material uses. Claims of sustainability and circularity therefore need defined benchmarks, a stated legal status for the residue and attention to its intrinsic properties. On this basis, we propose a property-led and cascade-consistent research agenda.