Recycled Polyurethane Glycolysate and Glycerolysate as Sustainable Plasticizers for Lignin-Filled NBR Composites

Glycolysate and glycerolysate—organic substances recovered from the chemical recycling of polyurethane waste—were investigated as sustainable plasticizers for acrylonitrile-butadiene rubber composites filled with 30 phr of calcium lignosulfonate or kraft lignin. The study evaluated the impact of these recycled plasticizers (added at 10 and 15 phr) on the curing process, morphology, rheology, mechanical and dynamic mechanical performances. Rheological analysis confirmed that both plasticizers significantly reduced the complex viscosity of the rubber compounds, with the effect being most pronounced at the 15 phr loading. While the incorporation of glycolysate and glycerolysate slightly extended the optimum cure time and decelerated the curing process, the cross-link density remained consistently within the range of 3.5–4 × 10−4 mol·cm−3. Morphological studies revealed that the plasticizers facilitated better dispersion of both lignin types and improved interfacial adhesion. However, the mechanical response differed significantly depending on the filler type. A consistent increase in elongation at break was observed only for composites filled with kraft lignin, where values rose from 341% for the reference up to 571% for the sample with 15 phr of glycolysate. In contrast, the application of plasticizers to calcium lignosulfonate-filled composites led to an initial decrease in both tensile strength and elongation at break. Notably, kraft lignin-filled composites exhibited superior overall mechanical performance, with glycolysate effectively maintaining tensile strength levels comparable to the reference. While both recovered substances performed effectively as processing aids, they had a negligible effect on the glass transition temperature. The results demonstrated that these recovered polyurethane derivatives are highly effective, sustainable alternatives to conventional plasticizers, showing a clear synergistic effect particularly with kraft lignin.