Effect of Polymer Matrix Type and Particle-Size Distribution on the Mechanical, Microstructural, and Thermal Behavior of Waste Tire Rubber Composites

Recycling ground tire rubber (GTR) into polymer-matrix composites requires accounting for both matrix formulation and rubber particle-size distribution. Here, polyurethane, epoxy, novolac–hexamine, and unsaturated polyester systems were compared using four gradations of the same rubber source (16 formulations). A common main-resin-to-rubber mass ratio of 20% was used; differences in required auxiliary components and curing conditions were explicitly considered. The 28-day reference compression results ranged from 1.22 to 1.35 MPa for the polyester formulations to 8.21 MPa for epoxy–K1; epoxy–K1 also reached 2.21 MPa in flexure, whereas polyurethane–K4 reached 6.29 MPa in compression and 1.94 MPa in flexure. Thirteen available compression curves were reprocessed using the 70 × 50 mm loaded face and 70 mm loading length; derived deformation and work-density parameters are apparent, machine-displacement-based measures. Binder-specific UPV trends were exploratory (four gradations per binder). Selected SEM/EDS, ATR-FTIR, and representative TGA/DTG data supplied complementary evidence without establishing interfacial chemical bonding, cure inhibition, or fire performance. The results indicate formulation-dependent strength–deformation trade-offs within the tested materials rather than a universal optimum matrix or gradation.