Effects of Draw Ratio and Additive on Knot-Pull Breaking Phenomenon in a Polypropylene Monofilament

Knot-pull strength is as important as tensile strength, but the mechanism by which the fiber in a knot breaks has not been sufficiently elucidated. In the present study, the effects of the draw ratio and a melt-kneaded additive on the knot-pull strength of polypropylene monofilament, that is, a thick single fiber, were discussed by comparing the tensile and loop strengths of the fiber. The knot deformation behavior during a knot-pull test was also investigated. In contrast to the increase in tensile strength at a high draw ratio, the knot-pull strength was hardly affected by the additive, and the loop strength was reduced by the additive. During a knot-pull test on a high-draw-ratio fiber, the knot thickness continued to decrease to breakage, even after the knot length stopped decreasing. This behavior indicates that the highly drawn fiber breaks at the buckled part in the knot owing to the lateral compressional forces.