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

Abstract

Knot-pull strength̶the tensile breaking stress of a knotted fiber̶is one of the most important fiber properties. It is often regarded as a more important property than tensile strength, particularly with regard to ships, fisheries, and civil engineering [1]. Polypropylene (PP) monofilament, that is, a thick single fiber, is commonly used for these applications because it is highly flexible. The tensile strength of PP fibers has increased recently, but their knot-pull strength has barely increased. This sluggish increase in the knot-pull strength has now become a critical problem. The sluggish increase may be attributed to the complex breakage of knotted fibers. For example, the draw ratio at which the knot-pull strength reached a maximum is often lower than the draw ratio of the tensile strength maximum [2-4]. This indicates that a knotted fiber is not only broken by the tensile force but also by other forces, such as bending, compressional, twisting, shearing, and frictional forces applied to the knotted fiber [5, 6]. Yamaki explained the knot breakage mainly by the tensile force under the influence of radial compressional force [1], Pieranski et al. explained the knot breakage with computer simulations mainly by the bending force [7], and Uehara et al. interpreted the influence of twisting force on breakage [8]. Because the mechanism by which knotted fiber breaks is complex, the fiber breaking behavior also varies depending on the crosssectional shape of the fiber or fiber bundle, the material, and the operating environment. The location of the fiber breakage varies, for example, most knotted fibers tend to break in the vicinity of the knot entrance [9], but fibers also break within the knot [3]. There have been many other studies on the breaking mechanism of knotted fibers. Konda et al. [10, 11] investigated the knot breaking mechanism using tensile stress‒strain curves. They ignored the shearing and lateral compressional forces and assumed that the fiber breaks according to the sum of the tensile and bending strains. Yabe suggested that a knotted fiber is broken by the weakest of three 【Transaction】