Adjusting the lengths of glass fibers and the relationship between fiber length and mechanical properties for reinforced poly(ethylene terephthalate)

The effects of fiber shape and compounding conditions on the distribution of glass fiber length of reinforced poly(ethylene terephthalate) are studied. The relationships between fiber length and mechanical properties are discussed. The length of the fibers pulled out from the rupture cross section and the critical fiber length are reported in different deformation modes. Fibers of finer diameter showed shorter average length and narrower distribution in the compound. Bundle size showed little effect on the fiber length distribution at the identical diameter. The weight average length, L-w, prepared by a dry blending method is shorter than that by a melt blending method. A compound with a Dulmage type screw has fewer short fibers than that with a full-flight type screw. Therefore, the fiber length compounded with a mixing type screw, such as Dulmage type, showed the sharp distribution. The flexural strength depends more on the aspect ratio of fiber, L-w/D, than on L-w. The Izod impact strength changed depending on L-w and on the content of fibers longer than 0.25mm. The L-w values of fibers pulled out from the rupture cross section were shorter in descending order for flexural, tensile and notched impact test specimens. The maximum length of fibers, L-m, leads to the approximate critical length, L-c, of the fiber fracture; 0.32mm for tensile, 0.50 mm for flexural and 0.24 mm for Izod impact. The value of L-c for tensile was comparable to the theoretical one. As for flexural fatigue under small strain at 23 degrees C or lower temperature, most of the glass fibers were not pulled out, but were broken at the rupture cross section.