SYNOPSISClassical morphological analysis has been performed on novel PET fibers of high modulus and low shrinkage (HMLS). As expected, amorphous orientation controls the degree of shrinkage and tenacity. The uniqueness of these materials is derived from a hign "effective" crosslink density which results in a high retractive force during elevated temperature shrinkage and significant stress-amplification during room temperature extension. Although the morphological origin of the high effective crosslink density is unknown, it is speculated that the interfibrillar regions contribute to the observed behavior by suppressing yielding.

ABSTRACT:The orientation of the noncrystalline (amorphous) phase in poly (ethylene terephthalate) fibers is analyzed by separating the amorphous scattering in the wideangle x-ray diffraction patterns into isotropic and anisotropic components. Two parameters are used to characterize the amorphous orientation-the fraction of the anisotropic component and its degree of orientation. The x-ray amorphous orientation parameters are compared with the sonic modulus and the birefringence values. Our results illustrate that the intrinsic birefringence of the amorphous phase is not ''intrinsic'' but depends on its density. The role of the oriented amorphous phase in determining the strength (tenacity) and the dimensional stability (shrinkage) of the fibers is discussed. We conclude that although amorphous orientation determines the shrinkage, other factors such as the connectivity between the amorphous phase and the crystalline regions play an important role in determining the tenacity of the fibers.

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Poly(ethylene terephthalate) was separated into 12 fractions of equal size by a stepwise increase in the amount of solvent in the two‐phase liquid fractionation system of phenol‐tetrachloroethane (PTCE) (1:1) and n‐heptane. Various molecular weight parameters of the fractions were measured by osmotic pressure and gel‐permeation chromatography. Intrinsic viscosity‐molecular weight plots were made for three different solvents at 25°C. Mark‐Houwink constants for viscosity‐average molecular weight were measured and gave values of K of 2.50, 2.37, and 2.25 × 10−4 dl/g and values of a of 0.73 for 1:1 PTCE, 3:2 PTCE, and o‐chlorophenol, respectively. A comparison with the literature values for this polyester was made, and application of the Mark‐Houwink equation to the determination of number‐average (Mn) and weight‐average (Mw) molecular weight of whole polymers is considered.

The properties of cords made from fibers with superior modulus/shrinkage characteristics have been studied, and the implications of these properties on tire performance have been addressed. The cord-modulus, dimensional-stability, stress-ampUncation, and retractive-force trends are comparable to those of the precursor yams, indicating that the yarn morphology yielding these properties is maintained in tire-cord form. One would expect that these cords could be used to make tires with improved uaiformity, bruise-resistance, and handling, and reduced sidewall indentations. INTRODUCTIONThe recent introduction of the monoply radial tire and increased consumer preference for uniform high-performance tires have resulted in the need for improved reinforcement materials'"^. Tire companies also continually desire improvements such as elimination of the post-cure-inflation (PCI) step, which will simplify the tire-making process. Hence, the focus has been on the sidewalls of radial passenger tires, which in the U.S.A. and Europe are predominantly reinforced with po!y(ethylene terephthalate) (PET) fibre and viscose, respectively. Both PET fiber and viscose possess desirable characteristics for this type of application. PET fiber exhibits good tenacity and modulus, and viscose possesses good dimensional stability, a term used in the fiber and film industries to describe materials with the combination of high modulus and low shrinkage at elevated temperature (HMLS)''. Tire designers desire a fiber with the tenacity of PET fiber and the dimensional stability of viscose without a premium in price. Because of the higher costs and environmental concerns associated with viscose processing^, it is most reasonable to develop a' universal' tire yam by improving the properties of PET fiber, and this is the approach we have taken.In Part I of this investigation, the relationship between the morphology and properties of PET-fiber yams of advanced dimensional stability was described*. These yams were observed to possess a high degree of stress-amplification upon elongation as compared with conventional yams. In this paper, the properties of these yams and of newly developed yam of higher dimensional stability are studied in tire-cord form. It will be shown that the desirable yam characteristics of these materials are translated to treated cord. In addition, the results of tire-building-simulation tests strongly indicate that these treated-cord advantages will be realized in actual tire-reinforcement applications. For example, an important challenge to tire engineers is to reduce sidewall indentations (SWIs), a cosmetic defect caused by overlap of tire-reinforcement cords at the splice region. The advantage of these new HMLS yams with regard to SWIs will be shown to be significant.