FIELD OF THE INVENTION
This invention relates to 1,3-propanediol-based polyesters such as polytrimethylene terephthalate which are used to make textile fabrics and. More particularly, the invention relates to a new polytrimethylene terephthalate textile fabrics which exhibit very good stretch and very good hand.
BACKGROUND OF THE INVENTION
Stretch and hand are two highly desirable tactile properties for textile fabrics. Hand is the term used in the textile industry to describe the tactile qualities of a fabric, e.g., softness, firmness, elasticity, fineness, resilience, and other qualities perceived by touch. Stretch fabrics are typically made from (1) fibers with good elastic recovery such as nylons and polybutylene terephthalate (PBT), or (2) a mixture of typical nylon or polyester yarns and an elastic fiber such as the polyurethane elastomer Spandex. Soft hand derives partly from the intrinsic property of the polymer such as modulus and also from the yarn and fabric processing such as setting the yarn during draw-texturing and mercerizing the fabric with caustic sodium hydroxide solution.
Therefore, stretch and soft hand can be manipulated to some extent. However, improvement in one usually is accompanied by a negative effect on the other. For example, fiber stretch can be improved by increasing its crimp level or bulk and latent torque energy during draw-texturing, by increasing the yarn count and denier per filament (dpf), or by pre-twisting the yarn followed by texturing. However, the increase in stretch is achieved at the expense of hand. Higher crimp yarn and higher dpf fabrics feel coarse and have poorer hand. The most common way to improve the hand of a draw textured yarn (DTY) is to set the yarn using a secondary heater at a temperature usually lower than the temperature at which crimping is carried out (in the first or primary heater). This secondary heat set treatment turns the yarn into a set DTY. Although the hand is improved, the secondary setting process also removes some of the crimp and latent torque energy resulting in poorer stretch (see Lubos Hes and Petr Ursiny, Yarn Texturing Technology, Comett Eurotex, 1994 and Ali Demir, Hassan Mohamed Behery, Synthetic Filament Yarn: Texturing Technology,Prentice Hall, 1997, on the technology of producing high extensibility and low extensibility yarns)..
It is therefore difficult to obtain fibers or fabrics that have both good stretch and good hand without using elastic fibers such as Spandex. The only fiber currently that comes close to achieving the combination of good stretch and hand is PBT polyester. It is marketed by various fiber companies such as Hoechst-Celanese as extra stretch performance fiber. However, it is difficult to set PBT textured yarn because of its low glass transition temperature.
PTT fiber is known to have good elastic recovery and soft hand because of its low modulus. British patent no. 1254826, Japanese application 09078373, and WO patent application 00/22210 teach the spinning and texturing of PTT yarns for applications in carpets and stretch fabrics in sportswear, leisure wears, etc. The texturing of the yarns were done either by air texturing or false-twist texturing. Japanese application 09078373 teaches texturing of PTT yarns by manipulating the texturing heater temperature between 165 and 180° C. such that the textured yarn has a Young modulus <30 g/den to maintain a soft hand. WO patent application 00/22210 teaches that PTT fiber with a modulus, Q, and an elastic recovery, R, such that the ratio of Q/R satisfies the relationship of 0.18<=Q/R<=0.45, is useful for making textile fabrics. The Q/R ratio is merely a re-statement of teachings found in British patent no. 1254826 and Japanese application 09078373. None of the above references disclose further setting the textured yarn with a secondary heater.
They also used modulus as a measure of soft hand. Although hand is related to yarn modulus it is a much more complicated tactile property. It encompasses softness without feeling limpy, drapability, dry touch, and fabric fullness to touch. Although a robotic instrument (Kawabata Evaluation System) was developed to measure fabric hand, it has its advantages and drawbacks in simulating human touch. Therefore, touch by experienced textile personnel is still widely used to evaluate the hand of fabrics as in Japanese application 09078373. In this patent application, example 5 and its comparative example have Young moduli of 21 and 20 g/den., respectively, yet the hand of the example 5 was rated at 7 while the later was rated 5 on a scale of 1 to 8., 8 being the best and 1 being the worst. This shows that low modulus, although is a reasonable measure, is inadequate in evaluating hand.
BRIEF SUMMARY OF THE INVENTION
In this invention, we found unexpectedly that when PTT fiber is drawn and textured under certain conditions, the fibers and fabrics made from this texturing process possess stretch and hand which is much better than current commercially available PBT and PET stretch fabrics. Specifically, the invention describes the spinning of PTT polymer into a partially oriented yarn (POY) and draw-texturing the POY using a false-twist draw-textured yarn machine at a draw ratio of 1.02 to 1.6, preferably between 1.05 to 1.4, and a yarn temperature of 120 to 160° C., preferably 130 to 150° C. Alternatively, the soft stretch yarn made be made by spinning a polytrimethylene terephthalate polymer yarn and drawing it in-situ using heated godets to a final elongation of 60 to 150% at the same temperature. The yarn is further set, usually with a secondary heater, at a yarn temperature of 70 to 130° C., preferably 85 to 115° C., prior to winding. Then knitted fabrics are dyed, tentor-set, and finished with fabric softener. This invention is different from the above teachings in that the PTT partially oriented yarn is draw-textured in a false-twist machine. The yarn is crimped (textured) at a much lower first heater temperature of 130 to 160° C. than described in the above prior art.
DETAILED DESCRIPTION OF THE INVENTION
The polytrimethylene terephthalate polymer is prepared by the reaction of a molar excess of 1,3-propanediol (PDO) and terephthalic acid (TPA) (or dimethyl terephthalate) by esterification followed by polycondensation, with the important proviso that the reaction conditions include maintenance of relatively low concentration of PDO and TPA in the melt reaction mixture.
As used herein, “1,3-propanediol-based aromatic polyester” refers to a polyester prepared by the condensation polymerization reaction of one or more diols with one or more aromatic diacids or alkyl esters thereof (herein referred to collectively as “diacid”) in which at least 80 mole percent of the diol(s) is 1,3-propanediol. “Polytrimethylene terephthalate” refers to such a polyester in which at least about 80 mole percent of the diacid(s) is terephthalic acid. Other diols which may be copolymerized in such a polyester include, for example, ethylene glycol, diethylene glycol, 1,4-cyclohexane dimethanol, and 1,4-butanediol; and other aromatic and aliphatic acids which may be copolymerized include, for example, isophthalic acid and 2,6-naphthalane dicarboxylic acid.
The preparation of the invention composition can be conveniently described by reference to an esterification step, a prepolymerization step, and a polycondensation step. The process can be carried out in batch or continuous mode. Each step can be carried out in multiple stages in a series of reaction vessels if desired for optimum efficiency in the continuous mode or for product quality. Each step is preferably carried out in the absence of oxygen. The following will describe the process in terms of the preferred continuous mode.
An esterification catalyst is optional but preferred in an amount of about 5 ppm to about 100 ppm (metal), preferably about 5 ppm to about 50 ppm, based on the weight of final polymer. Because of the desirable lower temperatures under which the esterification is carried out, the esterification catalyst will be of relatively high activity and resistant to deactivation by the water byproduct of this step. The currently preferred catalysts for the esterification step are titanium and zirconium compounds. The currently preferred catalyst for esterification, prepolymerization, and polycondensation is titanium tetrabutoxide. The catalyst is preferably formulated and added to the monomer feed, prior to or during the esterification, as a dilute liquid solution in 1,3-propanediol. This catalyst feed will preferably contain 5 wt % or less titanium.
The invention 1,3-propanediol-based aromatic polyester prepared by the invention process has an intrinsic viscosity (IV) of at least 0.6, preferably 0.7 or greater, most preferably 0.8 or greater, and for some applications, preferably within the range of about 0.9 to about 1.3, as measured in a solution of 0.4 g polymer in 100 ml of a 60:40 solution of phenol:tetrachloroethane at 30° C. (or as a dilute solution in another solvent such as hexafluoroisopropanol, and converted by known correlation to the corresponding IV in 60:40 phenol:tetrachloroethane).
The first step of the process of the present invention is spinning the PTT polymer into a partially oriented yarn (POY) with an elongation at break of <160% by varying the winder take-up speed between 1,000 and 4,500 m/min.. Alternatively, the extruded filament bundle can be cooled and then drawn between a set of heated godets to a final elongation of 60 to 150%. The yarn is then wound into appropriate packages.
Yarn made with either one of the above methods is then draw-textured in a false-twist draw-texturing machine at a draw ratio of 1.02 to 1.6, preferably 1.05 to 1.4. The false-twist method is preferred for texturing into high elastic yarn for stretch fabrics because it gives higher twist density, high crimp, and latent torque energy needed for stretch yarn compared to other texturing methods such as edge-crimping and stuffer box crimping. The false-twist method is a continuous method for producing textured yarns which utilizes simultaneous drawing, twisting, heat setting, and untwisting. The yarn is taken from the supply package and fed at controlled tension through the heating unit, through a false-twist spindle or over a friction surface that is typically a stack of rotating discs called an aggregate or crossed friction belts, through a set of take-up rolls, and onto a take-up package. The twist is set into the yarn by the action of the heater tube and subsequently removed above the spindle or aggregate resulting in a group of filaments with the potential to form helical springs.
The yarn is heated to a temperature of 120 to 160° C., preferably 130 to 150° C. The goal is to make the yarn soft for drawing and twisting. If a contact heater is used, then the heater may be operated at the above temperature range. However, if a non-contact heater is used, then the heater may be operated at a higher temperature of 180 to 260° C., preferably 200 to 240° C.
The draw-textured yarn (DTY) is then subjected to a second heat set treatment before it is wound and further processed. The second heater is often part of the DTY machine but a separate heater may also be used. The second heat set is done at a lower temperature than the first heater by about 35° C. Thus the yarn is set in the second heater at 70 to 130° C., preferably 85 to 115° C. If a contact heater is used, it may be operated at this temperature range. If a non-contact heater is used, it should be operated at an appropriately higher temperature of 120 to 200, preferably between 140 to 180° C.
To finish the yarn and make a fabric, the standard steps of knitting, dyeing of fabric with disperse dyes at 100 to 110° C. without using a carrier, setting the fabric in a tentor frame with overfeed at 130 to 170° C., and finishing with a softener may be used. Some of the stretch of the yarn is lost during the finishing but when the above proper finishing parameters are used, the final fabric has good stretch and has very good hand. This is quite surprising since the conventional wisdom is that a second heat set will destroy the stretch of a polyester fabric.