Wool–Poly(lactic acid) Biocomposite Mats as Degradable Sulphur Carriers: Effects on Forage Elemental Composition and Material Degradation

Low-grade sheep wool is an underused, keratin-rich by-product whose cystine-derived sulphur could be returned to agricultural soils. Carded wool was bonded with sprayed poly(lactic acid) (PLA) hot-melt to obtain porous wool–PLA biocomposite mats, which were exposed in field soil, in a pasture-grass composter and under soil-free outdoor conditions. The elemental composition of the wool, soil, forage, composting material and exposed mats was determined by ICP-OES after 60 days of exposure; each reported value is the mean of at least three analytical determinations, whereas the field layout comprised one plot per treatment. Forage from the plot with buried mats contained 47.8% more S and 21.9% more K than forage from the untreated plot, whereas Ca, Mg, B and Mo concentrations were lower; because the plots were not replicated, these differences are reported as preliminary observations rather than as demonstrated treatment effects. After composting, the mats retained only 17–23% of their initial storage modulus between −30 and 50 °C, while the relaxation of the PLA phase shifted by 7–10 °C towards higher temperatures. The water contact angle decreased from 72.3 ± 10.0° to 55.7 ± 12.1° (six droplets on one mat per condition), while the stylus roughness did not change appreciably. Digital microscopy revealed swelling, fissuring, cavities and fibre exposure along wool–PLA interfaces. A coupled mechanism of PLA ester hydrolysis and keratinolytic release of sulphur is proposed as a working hypothesis that is consistent with, but not yet proven by, the present data. The results support wool–PLA mats as temporary, degradable carriers of keratin-bound sulphur; replicated field trials with yield, nutrient uptake and soil measurements, together with direct degradation markers, are required before any fertilising or soil-improving effect can be claimed.