MODIFICATION. A scientist at Du Pont's experimental station, Wilmington, Del., tries a new process modification to improve properties of fiber
we'll see new polyester fibers within six months." Currently, all polyester fibers being produced are polyethylene terephthalate (PET) polymers except for three types of Eastman Chemical Products' Kodel. These fibers are polymerized with DMT and 1,4-cyclohexylenedimethanol ( C H D M ) . Eastman introduced these fibers in 1958 after developing them outside the domain of ICI's polyester patents which expired in the early 1960's. The polymer has closely repeating cyclic groups which impart a high bending modulus or stiffness to the spun fiber. Because of the fiber's resiliency, Eastman markets the three types principally as carpet fibers and fiber fill. Celanese Fibers Marketing Co.'s Howard Elsom, director of commercial development, questions whether changing the basic polymer chemistry will satisfy various market needs in ways that modifying and engineering existing polyester fibers can't satisfy them. To be sure, polyester fiber producers have been meeting the performance requirements of myriad consumer products by making chemical and physical modifications on the basic PET polymer. For example, Du Pont sells 32 types of Dacron, Celanese 22 types of Fortrel, and Eastman nine types of Kodel. These many types vary in filament and staple form, cross section geometry, molecular weight, tenacity, degree of crystallinity, brightness, disperse and/or basic dyeability.
In spite of the numerous modifications, though, today's polyester fibers still have shortcomings. Some of these deficiencies bear on the textile mill's processing economics; others bear on consumer acceptance. What these shortcomings are give some clues to the chemical approaches fiber producers are taking in their developments. Some of the more obvious shortcomings: • Polyester fibers do not have inherent permanent press properties but require finishing treatments with crosslinking resins. • Polyester fibers have slow dye uptake rates with disperse and basic dyes and completely lack dyeability with the bright, inexpensive acid dyes. • Polyester fibers sometimes pill or form tiny fuzzballs after abrasive wear or laundering. • Polyester fibers lack sufficient surface adhesion for tire cord constructions without special adhesive priming. • Polyester fibers tend to generate static in apparel, home furnishing fabrics, and carpets. The new generation polyester fibers should alleviate many of these shortcomings by providing new polymer backbone structures—new starting points—from which producers can develop new sets of modifications. The most logical chemical approach in developing such fibers, several polymer chemists say, is to vary and test the many different glycol candidates while holding the TPA or DMT constant. ( Goodyear experimented in the early 1960's with isophthalic acid in making polyester fiber—a development which circumvented ICI's polyester patents but which showed a major disadvantage: The polymer chain did not propagate linearly.) The higher aliphatic glycols, in the series, say, from butanediol to dodecanediol, would impart greater flexibility to the polymer chain. The increased flexibility would enhance the fiber's hand and wear esthetics. At the same time, it would minimize the problem of fiber pilling—a function of polymer brittleness—which is now controlled by reducing molecular weight at the sacrifice of tenacity. Most important, these higher glycols offer hundreds of substitution possibilities along their chain lengths. Polar groups, such as methyl, hydroxyl, chloride, and acetate, would serve as dye sites to increase dye uptake rates and effect acid dyeability. These groups would impart greater adhesion characteristics to the fiber surface through hydrogen bonding and electrostatic forces. And finally, they would also increase the hydrofelicity of the polyester fiber so that ambient moisture would collect and dissipate static buildup.
Molecular parameter index is aid to processors As an aid to processors, Phillips Petroleum will now supply what it calls a molecular parameter index (MPI) in designating each of its Marlex highdensity polyethylene resins. MPI, a three-letter descriptive index, will give an indication of branching, molecular weight, and molecular weight distribution for each resin. The number designation, which specifies density and melt index, will continue to be part of the resin description. With the increasing proliferation of resins tailored to specific needs, Phillips explains, the processor must have information which tells him how a resin will process and how it will perform. Sometimes, Phillips says, density and melt index are not only insufficient information; they can actually be misleading. With branching, for example, the type of branching as well as the concentration can exert a pronounced effect on performance. In general, with an increase in short-chain branching, there is a decrease in tensile strength, flexural modulus, softening properties, and hardness. Environmental stress crack resistance, elongation, and toughness increase. With hexene as a comonomer rather than butène, a marked increase in environmental stress crack resistance occurs at the same density or crystallinity level. Similarly, molecular weight and molecular weight distribution affect both processability and end product performance. The properties desirable for injection molding, for example, differ from those best for extrusion. As a result, Phillips has devised a three-letter code to indicate these parameters. The first letter indicates branching: Ε for ethylene homopolymer, Β for ethylene-butene copoly mer, and Η for ethylene-hexene copol ymer. The second letter indicates mo lecular weight: M for average molec ular weight of less than 110,000 (me dium), H for between 110,000 and 250,000 (high), X for greater than 250,000 but less than 1.5 million (ex tra high), and U for greater than 1.5 million (ultrahigh). Molecular weight distribution is indicated by Ν for a molecular weight-to-number ratio of less than 7 (narrow), M for between 6 and 13 (medium), Β for 10 to 20 (broad), and V for greater than 18 (very broad). Molecular weight dis tribution ranges overlap, Phillips ex plains, because accuracy of molecular weight determination is generally ± 10 to 15%. Resin number designation will remain the same. For example, 5402 indicates a resin with density of 0.954 and melt index of 0.2. DEC. 2, 1968 C&EN 43
