Effects of Glass and Carbon Fiber on Nylon 6,6 Crystallization
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Krisda Siangchaew*, Theodore Davidson**, and Matthew Libera* Stevens Institute of Technology, Castle Point on the Hudson, Hoboken, N.J. 07030 * Department of Materials Science and Engineering. "**Polymer Processing Institute (PPI) and Design and Manufacturing Institute (DMI).
ABSTRACT
The effects of addition of glass fiber and HMS4 carbon fiber on the crystallization of nylon 6,6 has been investigated using DSC and polarized optical microscopy (POM). DSC observations indicate that HMS4 fiber lowers the supercooling required to initiate crystallization. A transcrystalline layer is also observed in the near-fiber region of carbon fiber-reinforced composites after DSC. The presence of transcrystallinity and the earlier onset of crystallization for this composite are due to heterogeneous nucleation on carbon fiber surfaces. DSC measurements of nylon 6,6 with glass fibers show supercoolings similar to those of neat nylon 6,6. Transcrystallinity is also absent in the glass-reinforced composites. These glass fibers appear to be weak nucleation catalysts. Different transcrystalline layer thickness is found to be influenced by thermal processing condition. In addition to transcrystallinity, the HMS4 carbon fiber also influences the bulk morphology of the nylon 6,6.
INTRODUCTION The presence of foreign matter within crystallizable materials can induce heterogeneous nucleation.' Thermoplastics, being semicrystalline polymers, have demonstrated heterogeneous crystallization in the form of substrate-induced morphology2 and surface crystallization. 3 Such processes produce a distinguishable morphology known as transcrystallinity which originated from a crowding of primary nuclei along a catalytic interface. Kinetically, heterogeneous nucleation lowers the degree of supercooling needed to initiate the liquid-crystal transformation. The presence of a transcrystalline microstructure and a lower supercooling required to initiate crystallization serve as indications of heterogeneous nucleation. The intent of this paper is to investigate the effect of addition of fiber on the heterogeneous crystallization of both the bulk and the near-fiber region in nylon 6,6 thermoplastic polymer.
EXPERIMENTAL PROCEDURE Neat nylon 6,6 pellets of a scientifically pure grade (Scientific Polymer Product Cat. No.033) and the glass fiber and HMS4 carbon fiber were used in this study. HMS4 (Hercules 651 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
Inc.) fiber was stored in air in a closed container after heat treatment in flowing ultrapure N2 gas at 400 TC for three hours. The glass fiber was obtained by pyrolysis at 550 °C in air overnight of Verton long fiber pellets. Neat and composite specimens were made by vacuum compression molding. During molding, nylon 6,6 pellets were held at 280 TC for 10 minutes before pressure was applied. Films were produced with thicknesses ranging from 60 to 100 gim. Samples were then cooled to room temperature under vacuum. To make composite specimens, the same procedure was followed except t
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