The influence of isotacticity, ethylene comonomer content, and nucleating agent additions on the structure and properties of melt-spun isotactic polypropylene filaments

Abstract

SYNOPSISThe influence of isotacticity, ethylene comonomer content, and nucleating agent additions on the structure and properties of melt-spun polypropylene filaments was studied for a series of polypropylenes having similar resin melt flow rates (MFR z 35), average molecular weights, and polydispersities. In general, increasing the degree of isotacticity increases crystallinity and tensile modulus of the spun filaments, while increasing the copolymer content has the opposite effect. Nucleating agent additions also lead to greater crystallinity, but, under certain conditions, the addition of a nucleating agent can lead to lower tensile modulus in spite of higher crystallinity. For given spinning conditions, the tensile strength increased slightly with increased copolymer content but was little affected by tacticity in the range studied. Nucleating agent additions lowered the tensile strength of spun filaments. The effects of nucleating agents on the filament modulus and tensile strength were traced to their ability to raise the crystallization temperature and reduce the level of molecular orientation generated in the filaments. The reasons for the observed behavior are discussed. I N T R O D U CTlO NThis is the fifth paper in a series from our group dealing with the influence of resin characteristics on the structure and properties of melt-spun isotactic polypropylene filaments. Three of the earlier papers'-3 dealt extensively with the influence of molecular weight and polydispersity and the interaction of these variables with the spinning process variables. It was shown that the processing, structure, and properties of polypropylene filaments are highly influenced by both molecular weight and polydispersity. A fourth paper4 briefly examined the effects of changing the crystallization kinetics by adding a

The Influence of Calcium‐Stearate‐Coated Calcium Carbonate and Talc on the Quiescent and Flow‐Induced Crystallization of Isotactic Poly(propylene)

“…The graphs in Figure 10 show that the increase is more dramatic for the pure polymer than for the calcium‐carbonate‐containing polymers, which is in agreement with the findings from literature 21, 28–33. Even for these moderate shear intensities, both the nucleation density and the kinetics are already quite similar at the highest shear rate‐strain combinations.…”

“…As both the addition of (nucleating) particles and the application of flow increase the nucleation density, the competition between the contribution of both is a central issue. Hence the observation made in many studies involving a range of (moderate) shear conditions, that the relative acceleration of the crystallization kinetics with increasing shear intensity is less drastic for polymers that contain nucleating particles 21, 28–33. As under quiescent conditions, the particle‐containing polymer already has a higher nucleating density than the pure polymer, the extra (homogeneous) nuclei induced by moderate flow will have relatively less effect on the crystallization kinetics of the particle‐containing polymer.…”

“…As to morphology, many studies concerning the influence of (intense) flow on nucleated polymers suggest that particles enhance the orienting effect of shear to a certain extent 21, 28–30, 33–37. The extent of this “synergistic” effect may be linked with the intensity of the contact between the particles and their matrix 38.…”

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“…An explanation to the pathway of shish-kebab formation was recently reported by Hashimoto et al (2010). A factor that can affect the crystallization kinetics is nucleating agents such as organoclays (Lagasse and Maxwell 1976;Jerschow and Janeschitz-Kriegl 1997;Spruiell et al 1996;Nakajima and Yamaguchi 1996;Huo et al 2004;Somwangthanaroj et al 2003). Moreover, the kinetics of crystallization significantly changes under flow conditions at temperatures near the melt peak temperature and is a function of both strain and strain rate (Kitoko et al 2003;Hadinata et al 2006;Dai et al 2006;Tanner and Qi 2009).…”

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“…Spruiell et al [17,18] have investigated the effects of tacticity on crystallization kinetics in melt spinning iPP. They found slower crystallization rates in lower tacticity iPPs.…”