Effect of process- And annealing-induced shrinkage on the thermomechanical properties of glass fiber-reinforced polypropylene

Thermoplastic composites undergo shrinkage during the cooling phase of the process and the annealing post-process step. We investigate the thermomechanical behavior of laminates made of glass fiber reinforced with a copolymer polypropylene matrix. The shrinkage is monitored using optical fiber Bragg gratings embedded in unidirectional laminates that are manufactured with a static hot-press (SHP) and a double-belt press (DBP). The processes have very different cooling rates and pressure levels. Results show, unexpectedly, that laminates processed with SHP are subjected to much lower shrinkage during cooling phase as compared to laminates manufactured with the DBP. The metallic mold used in this process has a constraining effect which inhibits shrinkage. This effect induces the build of residual stresses that cannot be entirely removed by an annealing treatment. Consequently, the transverse expansion coefficient of the laminates manufactured with the SHP is lower than that of the laminates manufactured with the DBP.