Process‐induced internal stresses in phenol‐formaldehyde cellulose composites

Abstract Phenol and melamine‐formaldehyde (PF and MF) cellulose composites are being used extensively in the laminate industry. Despite their many advantages, flat panels produced from such materials have a tendency to warp immediately after processing as well as during subsequent service. This report presents results of an investigation into the thermoviscoelastic properties and the process‐induced internal stresses of PF cellulose composites. Thermoelastic properties were measured using Electronic Speckle Pattern Interferometry (ESPI), Thermo Mechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA). This latter was also employed in order to build a time‐temperature relaxation superposition for PF cellulose. Process‐induced internal stresses were quantified via a modified beam‐bending technique that allowed for consolidation pressure to be applied on the materials. Results from the study showed the following: anisotropy in thermoelastic properties due to the oriented cellulose fiber network, a clear viscoelastic response allowing time‐temperature superposition in a bi‐phase material, and high levels of internal stress build‐up on cure. Polym. Compos. 25:214–228, 2004. © 2004 Society of Plastics Engineers.