Mechanical Acceptability versus Material Stability in Hot-Plate-Welded PVC-U/Glass-Fibre Composite Joints

Abstract

This study assesses the material integrity of hot-plate-welded unplasticized PVC profiles reinforced with glass-fibre composite inserts, focusing on whether cycle shortening preserves the weld-zone state beyond satisfying macroscopic strength criteria. The constituent materials were characterised to define the mechanical and thermomechanical mismatch of the hybrid system. Welded corner specimens were produced under selected parameter sets and examined by ball indentation hardness testing, stereomicroscopy, differential scanning calorimetry, and Fourier-transform infrared spectroscopy in attenuated total reflectance mode. The composite contained 52.25 wt.% glass fibre and showed higher stiffness, strength, and heat deflection temperature than PVC-U, but substantially lower deformability, confirming pronounced material asymmetry in the joint region. The weld produced at 264 °C for 22 s exhibited a material response comparable to the reference condition of 264 °C for 30 s. Hardness, microscopy, DSC, and FTIR-ATR revealed no detectable microstructural, thermal, or chemical deterioration after reducing plasticisation time from 30 s to 22 s. Conversely, although the 270 °C/21 s variant remained mechanically acceptable in terms of the failure-load criterion, it showed light-brown discoloration extending from the bead into the joint interior, lower weld-zone hardness, and a broader DSC response, indicating local thermal overexposure. These results show that mechanical acceptability does not necessarily imply material stability and that cycle-time reduction should therefore be evaluated using complementary mechanical, morphological, thermal, and chemical criteria.