Effect of 100 Days of Hot-Wet Conditioning on Mechanical Properties of PU-Coated Polymer Composites

Abstract Bidirectional silica fabric-based polymer composite laminates were fabricated using resin transfer molding followed by oven curing. To evaluate the effects of environmental exposure, samples were cut as per testing requirements, and a substrate was surface-coated with a 300-µm-thick polyurethane (PU) layer. Both PU-coated and uncoated samples were subjected to hot-wet conditioning in a climatic chamber for 100 days under critical conditions of 65 °C and 85% relative humidity (RH). In this study, density, thermal conductivity, pull-off adhesion, scratch resistance, and various mechanical strengths at room temperature and 100 °C were measured before and after conditioning. For uncoated composites, a significant effect of hot-wet conditioning on mechanical strengths, with reductions ranging between 15 and 30%, was observed. When compared between uncoated and coated composites, coated composites exhibited an additional reduction in tensile strength, flexural strength by 14–25%, and interlaminar shear strength decreased from 53 MPa (uncoated, after conditioning) to 46 MPa (after coating & conditioning), although compression and in-plane shear strengths were relatively unaffected. Furthermore, coated samples experienced over a 35% reduction in scratch resistance and pull-off adhesion strength (declined significantly from 8.1 MPa to 5.1 MPa), while density and thermal conductivity remained unchanged. These degradations were attributed to the formation of micro-crevices between the composite substrate and the PU coating, resulting in cavitation damage and matrix degradation. Based on these findings, PU coatings are not recommended for outdoor applications wherein environmental conditions are 65 °C and 85% RH.