Effects of Molecular Weight of Polypropylene on the Correlation between Fracture Toughness and Mechanical/Impact Properties of Gfpp Composites

Abstract

The accurate prediction of scientific and statistical parameters depends, largely, on the functional correlation between the variables involved, as well as their definite and predictable behaviour around the line of best fit (the regression line). Hence, an accurate and reliable prediction of the fracture toughness and mechanical properties of glass fibre reinforced polypropylene (GFPP) composites is very important in order to prevent sudden failure of components fabricated from them. It also gives the opportunity to save cost on experimentation. Three grades of polypropylene (PP) of different molecular weights (MW) had glass fibre reinforcements each. The incorporation of maleic anhydride grafted PP (MA-g-PP) into the composites was for compatibilisation between the matrix and reinforcements. There was tensile testing on both the un-notched and notched dumbbell samples. The use of J-Integral analysis was to investigate the fracture toughness of the composites. There was an evaluation of the effect of different MW of PP on the fracture toughness and impact properties of the composites. The examination of the morphology of the materials was by (SEM), while charge coupled device (CCD) camera monitored crack propagation behaviour under tensile loading. There was further morphology study to analyse the fibre length distribution and interfacial shear strength of the composites. Investigations revealed that the composite with the lowest MW PP consistently showed better mechanical, impact and fracture toughness properties than those with higher MW PP. It was also observed that incorporating MA-g-PP as compatibilisers had greater interfacial bonding effects, resulting in higher interfacial shear strength in GFPP of lower MW and high melt flow rates than those of higher MW and low melt flow rate. The investigation further revealed that as the MW of PP decreased, the fracture toughness and other mechanical properties increased in a linear correlation. Crack propagation trailed the interface between PP matrix and GF because of weak bonding in the composite with higher MW PP. This study revealed that incorporating MA-g-PP into GFPP composites achieved better properties with PP of lower MW and higher melt flow rate. Hence the mechanical and fracture properties of GFPP depend largely on the MW of the polymer PP matrix. There was a strong linear correlation observed between the MW of PP, fracture toughness and other impact properties thereby making it possible to make a reliable prediction of any property of the composites from another.