The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene .2. Thermal properties

In this report we present the results from the second part of a study on the influence of fibre length and concentration on the properties of glass reinforced polypropylene laminates. The heat deflection temperature of these laminates is dependent on both fibre length and concentration. A maximum plateau level close to the polypropylene melting point was observed, longer fibres require a lower concentration to attain this plateau value. Elevated temperature stiffness retention was also enhanced by higher fibre concentration and longer fibres. The Cox-Krenchel equations gave a good prediction of the laminate stiffness over the temperature range -50 to 100 degrees C. Both the in-plane and out-of-plane linear coefficients of thermal expansion were strongly dependent on fibre concentration but relatively insensitive to the fibre length. We obtained excellent correlation between experimental values of the in-plane linear coefficients of thermal expansion and theoretical predictions based on the shear lag theory. Out-of-plane linear coefficients of thermal expansion were found to be much larger than predicted by the equations used for continuous fibre reinforced composites. An approach based on the in-plane compression of the matrix due to the restriction of the matrix expansion by the reinforcing fibres was found to give good agreement with the experimental data. Good correlation of the experimental and predicted data was only obtained when the effect of voids was included in the calculations. (C) 1996 Elsevier Science Limited.