High Strength Metallocene Catalyst-Synthesized Polypropylene Fibers with High Stereoregularity and High Molecular Weight

Abstract

Isotactic polypropylene (iPP) has the excellent characteristics of light weight, high recycling efficiency, and chemical resistance, and it is used for molded products and films, as well as for fibers. iPP fiber is mainly used for industrial materials [1], for which high tensile strength is desired from the viewpoint of environmental loading and energy consumption [2]. Commonly used iPP is synthesized using Ziegler-Natta catalysts containing multiple active sites, which causes a wide molecular weight distribution [3, 4]. In comparison, metallocene catalysts with a single active site can synthesize iPP with a narrower molecular weight distribution [3, 5]. In addition, high stereoregularity iPP was recently synthesized using a metallocene catalyst [6‒11]. Using the high stereoregularity and narrower molecular weight distribution iPP, fibers with higher tensile strength than Ziegler-Natta iPP fibers can be fabricated [11]. It is also known that the high molecular weight polymer can be used to fabricate high strength fibers [2, 12]. However, unfortunately, the long relaxation time of the high molecular weight polymer requires a high melt-spinning temperature, which tends to promote a molecular weight decrease [12], and it often reduces the effect of the high molecular weight. Fortunately, the above narrow molecular weight distribution iPP reportedly suppresses the molecular weight decrease during melt-spinning [13]. A further increase of the tensile strength can then be expected by using this type of iPP with higher molecular weight. In this study, fibers fabricated using the above type of metallocene iPP with 4 g/10 min melt index were analyzed, including their tensile properties, creep behavior at high temperature, and fiber structure. The molecular weight decrease during melt-spinning at 270-290 ̊C was also investigated. To estimate the attainable maximum tensile strength, the extruded polymer was taken up at the lowest possible speed, and the as-spun fiber was drawn up to the maximum draw ratio at the highest possible temperature. The nanometer to micrometer scale fiber structures were analyzed by wide, small, and ultra-small angle X-ray measurements. The obtained results were compared with those of Ziegler-Natta iPP with equivalent molecular weight and 【Transaction】