Influence of Thermal Curing on Strength Development and Rock Reinforcement Performance of Low-Temperature Polyurethane Resins (PURs)

Abstract This study investigates selected mechanical properties and thermal curing response of three low-temperature polyurethane resins developed for application in coal mines and how they interact with weak sandstone when used as rock mass reinforcement. About 120 PURs specimens were tested in the laboratory under ISO 604 conditions at curing temperatures of 20–50 °C and curing durations from 1 h to 7 days. The results made the contrasts between the three materials very obvious. Force–deformation characteristics revealed a three-stage compression response: initial rapid compaction, a stable elastic zone, and non-linear hardening; however, the lowest temperature polyurethane transitioned directly to elastic compression. The maximum UCS was over 38 MPa at 20 °C and over 25 MPa at 50 °C proving that higher temperature of the environment reduces PUR strength considerably. The modulus of deformability was very low for all three resins, but this allowed them to accommodate relatively large strains without brittle failure. All resins, just after mixing, recovered their shape after unloading, which is useful when these materials are injected into fractured rock mass. Testing the composites made of crushed sandstone and PURs revealed that their mechanical performance is mainly influenced by the weaker component of the mixture. Elevated curing temperature (40 °C) reduced uniaxial compressive strength (UCS) for all composites. Polyurethanes effectively bind crushed rock and help restore its strength before cracking occurs, but only up to the strength level of the weaker component, either the sandstone itself or the polyurethane itself. All composites exhibited high residual strengths, up to 90% of peak stress, indicating highly desirable elastic–plastic behaviour and sustained load-carrying capacity after peak deformation.