Effects of hygrothermal aging and a silane coupling agent on the tensile properties of injection molded short glass fiber reinforced poly(butylene terephthalate) composites

The hygrothermal and mechanical properties of injection molded poly(butylene terephthalate) (PBT) composites containing 10, 20 and 30 wt.% short glass fiber (SGF) has been investigated. The kinetics of moisture absorption was investigated by immersion of PBT and SGF–PBT specimens in water at three different temperatures, i.e. 60°C, 80°C and 100°C. A single free-phase model of diffusion, which assumed Fickian diffusion and utilized Fickʹs second law of diffusion was used in analysing the data. A good agreement was observed between the experimental and theoretical values. The equilibrium moisture content, Mm and the apparent diffusivity, D, were found to be dependent on the volume fraction of the fibers. DSC measurements revealed that the thermal behavior, viz. the melting point and the degree of crystallinity of PBT and SGF–PBT composites were affected by hygrothermal aging process. This was attributed to the changes in the microstructure of PBT due to its interactions with water molecules. Hygrothermal aging reduced the tensile properties of PBT, the effect of which is strongly dependant on factors such as water immersion temperatures, volume fractions of fibers and also the chemical treatment of SGF. The presence of 3-aminopropyltriethoxysilane (3-APE) coupling agent has improved the retention of tensile properties of SGF–PBT composites, especially under adverse hygrothermal condition. Failure modes of both the matrix and the composites, assessed by fractographic studies in a scanning electron microscope (SEM) are discussed.