GB2136025A - Crimped polyester tow and staple - Google Patents

Abstract

Polyester staple comprising a major portion of staple fibers having a cross sectional area having a moment of inertia, in one direction, less than the moment of inertia in other directions and less than 3/4 the moment of inertia of a circular cross section of equal area provides staple yarns having pleasing appearance and tactile properties. The staple has a 'three dimensionality factor' less than 2. Crimp is inserted by feeding a tow of filament having the recessing cross-section into a stuffer box wherein the crimp is set by steam. <IMAGE>

Description

SPECIFICATION Tow and staple and processes for making the same The invention relates to polyester staple and to tow and processes useful for making the same. More particularly,the invention relates to staple characterized by a high degree ofthree dimensionality and comprising a major portion of staple fibers having a cross sectional shape having a minimum moment of inertia in one direction and a set crimp in that direction.

As is well known in the textile arts, polyester staple, alone or blended with other staple fibers, provides yarns suitable for use in a varietyof knit and woven fabrics. Polyester staple is normally prepared from a towwhich is drawn until tenacity and elongation values are atthe desired level, crimped and cut into staple lengths. Yarns and fabrics of improved softness and hand are obtained by use of progressively finer denierfibers. However, the minimum denier which can be employed is limited by practical difficulties encountered in spinning very small denier filaments; bythe need for minimum filament strength to permit satisfactory processing oftow into staple; and the tendency offabrics of fine denier staple yarns to exhibit high pilling.

The polyester staple ofthis invention is characterized in that it comprises a major portion of staple fibers having a cross sectional shape such that the moment of inertia of the cross sectional area in one direction islessthan inotherdirectionsand lessthan 3/4the moment of inertia of a circular section of equal area. The staple fibers are crimped in the direction of lower momentof inertia andthecrimp is set. The crimps in the staple fibers tend to be three dimen sionallyoriented.Thethreedimensional orientation is apparentthroughoutthe entire staple fiber length, being observable even in short (6.5 mm. or less) segments thereof. Such staple provides yarns which in turn provide fabrics of improved tactile properties and appearance.The polyesterstaple of this inven tion, thetowsfrom which the staple is prepared and processes for making such tows and staple wil I be understood from the following descriptions of the drawings and preferred embodiments.

FIGURES 1,2, and 3 illustrate representative cross sections of fibers constituting the polyester staple and tows ofthis invention. FIGURE 4 is a view of a crimping apparatus illustrating its use in the process of this invention.

Theterm "staple", as a noun, unless otherwise indicated by context, is used herein in a pluralistic sense to denote a quantity of individual staple fibers sufficientfor processing in staple processing operations, e.g., carding, spinning orto form staple products, e.g., sliver, yarn.

The term "polyester" refers to fiber-forming polyethylene terephthalate polymers, for example, polymers comprised of at least 85% by weight of an ester or esters of one or more dihydric alcohols and terephthalic acids, which if desired, may contain additives or modifying agents to provide desired dyeability, luster, thermal stability, flame retardancy, etc. The chemistry of fiber-forming polyester polymers is well understood bythose skilled in the art and will not be discussed in detail here.

The polyester staple of this invention is characterized in that it comprisescrimped staple fibers in which the crimps are highly three dimensionally oriented. More specifically, the crimp orientation is such thatthe polyester staple has a three dimensionalityfactor less than 2, preferably less than 1.6.

The crimps are three dimensionally oriented in that they extend into different planes probably as a result offrequenttwisting of the staple fibers along their longitudinal axis and/or crimping ofthefibers on a bias. Staple fibers having cross sectional configurations as shown in FIGURES 1,2, and 3, when prepared as hereinafter described,tend to crimp on a bias from a plane bisecting the major dimension, a, and, in many instances twisting of the fiber in the area of crimp is observed. The three dimensionalityfactor is deter- mined by measuring dimensions of images of the fibers circumferentially viewed from successive 45" angles.At each angle,thetotal length of the fiber image along its crimped path between a straight line distance of 5.77 mm. is determined. Percent crimp is calculated as crimped length - straight linedistance straight line distance x 100. The ratio of the maximum percent crimp measured at the four different views to the percent crimp measured at a view 90" thereto is the "three dimensionalityfactor" To assure reliable data, repeat measurements are made on each fiber and at least 10 fibers are measured, with the results being arithmeticallyaver- aged.Individual measurements which deviate excessively are discarded as "outliers" in accordance with the teachings of Statistical Methods in Research and Production, edited by Davies and Goldsmith, Fourth Revised Edition, published for Imperial Chemical Industries, Ltd., by Oliver and Boyd, Tweeddale Court, Edinburgh, England (1972), chapter 3, p49 To assure reliability, a minimum of eight individual fiberthree dimensionality measurements are averaged following discardofoutliers. Lowerthree dimensionalityfactors indicate higher degree ofthree dimensionality.

The polyester staple of this invention is further characterized in that it is cardable using techniques and equipment such as discussed in American Cotton Handbook, Third Edition, Volume 1, Chapter 8 "Carding and Combing", edited by Dame S. Hamby, published by Interscience Publishers-a Division of John Wiley & Sons (1965).

Further, the polyester staple ofthis invention comprises a major proportion (at least 50% by number) of staple fibers having the characteristics hereinaftersetforth. Preferably, the staple comprises a predominant proportion (at least 90% by number) of such staplefibers. It will be understoodthatthe polyester staple of this invention can, in accordance with conventional practice, be employed blended in admixture with other staple products, e.g., cotton, wool, or acrylic staple. In such case. the percentages above are percentages of the total polyester staple fibers unless otherwise indicated.

The staple fibers constituting at least a major portion of the polyester staple of this invention are characterized by a cross sectional shape which permitsthefiberto be more easily bent or "crimped" in one direction across such a shape than in other directions. That is, the cross sectional area has a moment of inertia, sometimes referred to as a "second moment", in one direction across the cross section which is lower than the moment of inertia in other directions across the cross sectional area.

Moments of inertia of cross sectional areas will be determined by calculations as described in ChapterXII Section 1, "Moments of Inertia of Areas", Analytical MechanicsforEngineers. Seelyand Ensign (John Wiley and Sons, second edition).

The cross sectional measurements employed in such calculations can be obtained by microscopy or other suitable techniques. Such lower moment of inertia will be less than 0.75 times the moment of inertia of a circular cross section of equal area.

Itwil! be recognized that a variety of cross sectional shapes will fulfill the foregoing requirements. For example, rectangular, eliptical, or sinusoidal shapes having appropriate major and minor dimensions may be utilized. Sigma cross sections as described in U.S.

patent 4,254,181 may be used. Particularly desirable properties have been obtained with staple comprising staple fibers having cross sectional shapes corresponding to the outline of intersecting circles as shown in FIGURES 1,2, and 3. The major dimensions, a, as compared to the minor dimensions, b, of such shapes can be adjusted to provide a shape having the desired percentage of the moment of inertia of a circular shape of equal area. This percentage is lowered by decreasing the ratio of minor dimension (thickness) to major dimension (width).

Depending on the shapes involved, employment of a mixture of shapes and sizes ofstaple fibers may be desirable. Sizes mayvary longitudinallyalongthe individual staple fibers orfrom fiberto fiber.

The staple fibers constituting the polyester staple of this invention will have a set crimp. The degree of crimp will be sufficient to permit processing on normal textile processing machinery, (e.g., to permit carding as previously discussed) and will be set sufficiently to retain the crimpthoughout such operations. (It is known that uncrimped fibers of uniform cross-section cannot be processed on normal staple processing machinery). The foregoing characteristics are believed to contribute, at least in part, to desirable characteristics of fabrics knit or woven from yarns comprised in whole or in part of such staple fibers.

Preferably, the staplefibers should have an average denier perfilament no greaterthan 6.

The polyester staple of this invention is prepared from a tow of continuous filaments having the cross sectional and crimp characteristics described above.

Thetowis prepared from a tow of continuous filaments having the requisite cross sectional shape and can be-prepared by extruding molten polyester polymerthrough spinnerette orifices of proper shape and quenching.

The tow may advantageously comprise filaments of generally peanut shaped cross section which vary in denier along their lengths. Such filaments and methods of making the same are described in British Patent No. 2,003,423.

Although not all of the characteristics of the filaments described in said copending application are retained in or believed important to the polyester staple ofthis invention, staple prepared from such filaments provides yarns and, in turn, fabrics of excellent tactile properties.

Filamentsofsimilarshape butwithoutsubstantial variations in denier along their length can be obtained by use of a spinnerette with appropriately shaped orifices.

Filamenttows of about 50,000 denier or greater (tows of over 200,000 denier are commonly utilized) are prepared bycombining asuitablenumberof continuous filamentyarns or individual continuous filaments. The tow is typicallyfed through the processing zone using two sets of motor driven rolls.

Each set comprises a multiplicity of rolls designedto provide positive traction onthetow and to prevent tow slippage. The roll sets maybe providedwith immersion baths which applywater or a finish solution to the tow as it proceedsthrough the roll set.

Ifthetowfilaments haves high level of continuous filament finish, as is generally the case with filaments spun at high speeds, such finish must be removed and replaced with an appropriate level of staple finish.

(The composition and use of continuous filament and staple finishes is well understood bythose skilled in the art and will not be discussed herein). This can be conviently accomplished by passing the towthrough a high velocity water jet apparatus as described in U.S.

patent number 3,791,788 prior to entry into the first roll set The second set of rolls is operated at higher speed than the first set, thus creating adrawzone having a draw ratio equivalentto the speed ofthe second roll set divided by the speed ofthefirst roll set. The tow is drawn and orientated between the first and second roll sets as it is simultaneously subjected to the action of hot finnish spray in an enclosed chamber positioned between the roll sets.

The draw ratio can be varied to produce different tenacity and elongation. The optimum draw ratio depends on degree of orientation already in the filaments being fed to the draw zone. This degree of orientation varies widely depending on filament spinning speed. Thus draw ratios as high as 3 may be desired if filaments comprising the tow were spun at low speeds, e.g., 1500 yards/min. (1370 meters/minue) or less. If the filamentswerespun at high speed, e.g., 5000 yards/min. (4500 meters'min.) a draw ratio of between 1.2 and 1.3woutd beappropriate.

Thefinish level appliedinthe drawn zone is normallyfrom 0.1 to 0.3% of fiber weight to facilitate final staple processing in cotton yarn system machinery. The drawn tow is then passed into a steam chamberwhich is a closedtrough in which saturated steam contactsthe tow. The temperature of the tow is raised to 90"C. or higher in this step.

The tow is then fed into a stuffer box crimper as shown in FIGURE 4. The operation of this apparatus is described in detail in U.S. patent 3,618,183.

In operating the crimper, steam is fed to chamber 24 via side ports 30 to setthe crimp. The use of saturated steam is satisfactory, but superheated steam may be utilized. The tow can then be dried and cut into staple fibers of desired length. Staple fibers obtained from tow processed in this manner are more hig h ly th ree dimensional (lowerthreedimensionality coefficient) than similarfibers from tow processed without use of steam in the stuffer box crimping.

Further, unless steam is utilized, the resulting staple maynotbe readily cardable. This higherthree dimensionalityis believedto contribute, at leastin part,tothe processability ofthestaple and the tactile properties ofyarnand fabrics made therefrom. Staple with a three dimensionality factor somewhat less than 1.6 is observed to provideyarn and fabrics having desirable properties and it is expected that similar results will be obtained with staple having threedimensionalityfactors upto 2.

While the process has been described by reference to preferred embodiments, those skilled in the art will recognize that a variety of modifications can be utilized. For example, tow washing, drawing and finishing can be accomplished by any available means and the use of particular apparatus or techniques is not believed necessary. Crimper designs otherthan the one described can be utilized.

However, stuffer box crimpers are preferred to allow the tow to crimp in the general direction of lowest moment of inertia. The crimp bends, probably due to bias bending and/ortwisting, will be observed to be, in many instances, somewhat laterally deflected from the direction of lowest moment of inertia. This may be, in part, responsible for bulkiness and fullness of yarns made from polyester staple of the invention.

The invention is further illustrated bythe following examples.

Example 1 Atow of about 50,000 denier composed of spun filaments of polyethyleneterephthalate polymer having an intrinsic viscosity ofabout 0.65 and a TiO2 level of 1% is prepared. The filaments constituting the tow are prepared by metering the polymer at a temperature of 293"C. through a spinnerette having 68 combined orifices. The spinnerette has capillary dimensionsanddesignassetforth in the detailed description of FIGURES 1 and 2 in British Patent No.

2,003,423.

The polymerthroughput is adjusted to produce filaments of 2.5 average denier per filament at a spinning speed of 5000 yards (about 4500 meters) per minute, the molten streams being conventionally quenched into filaments by a uniform flowof transversely directed quenching air.

Dueto the geometry of the spinnerette constructin, the polymerflowing through the smallercapil- laries hasa highervelocitythan thatflowing through the larger capillaries. The speeds and momenta ofthe paired streams issuing from each combined orifice and the angle at which the streams converge outside thespinnerette are suchthatthe slower streams travel in substantially straight lines afterthe points at which the pairedstreamsfirsttouch and attach,while each ofthe smallerand faster of the streamsforms sinuous loops back and forth between-successive points of attachment with its associated larger stream.This action can be readily observed using a stroboscopic light directed onto the streams immediately below the spinnerette face. As the molten streams accelerate away from the spinnerette, the slower stream attenuates between the points of attachment and the loops ofthe faster stream become straightened until the faster stream is brought into continuous contactwith the slower stream. The slower stream attenuates more between than atthe points of first attachment, so thatthe resulting combined stream has a cross section which is larger atthe points offirst attachmentthan in the regions between these points. The resulting combined stream is then further attenuated somewhat until it is solidified into a filament by flow of transverse quench air.Due to minor difference between combined orifices, temperature gradations across the spinnerette, and other like deviations from exactly the same treatment for each pair of streams, a multiple orifice spinnerette will typically provide somewhat different repetition rates among the several resulting combined streams and filaments.

Each solidified filament has non-round cross sectional areas which vary in size repetitively along its length. When using the above spinning conditions, the filament cross sectional area varies at a repetition rateofthe order of magnitude of about one per meter, although this can be varied somewhat by modifying the spinning conditions and the geometry of the spinnerette passages.

The resulting filaments have cross sectional shapes as illustrated in FIGURES 1 and 2 and an average denier perfilamentofabout2.4.

The lowest moment of inertia of the cross sectional area is in the general direction transversing the major cross sectional dimension of the fiber and is about one-halfthe moment of inertia of a circular section of equal area. Thetow is passed through a washing apparatus as described in U.S. patent 3,791,788 to remove excess spin finish and is then fed through two sets offorwarding rolls with the relative speeds ofthe firstand second set being adjusted to provide a draw ratio of about 1.26. Aspray chamber is positioned between thefirst and second set of rolls and hot oil/water emulsion staple finish (90"C.) is applied to the tow in this chamber. The elevated temperature facilitates drawing.

The tow is fed through a steam chamber apparatus having a design ofthetype disclosed in U.S. patent 3,791,788 and contacted with sufficient steam to raise the tow temperature to about 95"C.

The tow is then fed into a stuffer box crimper as shown in FIGURE 4. The clapper bar 22 pressure is adjusted to give 9 to 12 crimps per inch (3.5 to 4.7 crimps percentimeter). Steam is fed into pots 30 at 25 psig (0.035kg/meter2). About 20% filament shrinkage occurs in the crimper stuffer box. The crimped tow is fed onto a moving bed dryer and dried at 1 050C. The tow is then cut into 11/2" (3.8 cm) staple and baled.

Staple three dimensionalityfactor is determined using a Quantimet720-25 image analyzer system manufactured by Cambridge Instrument Co., Monsey, New York. Techniques of using apparatus of this type are described, for example, in Image Analysis Bibliography, Cambridge Instrument Co., Monsey, New York, s r7w3; R. A. Swenson and J. R. Attie, American Laboratory, Vol.50, April, 1979; and D. C.

Felty and T. Murayama. J. Applied Polymer Science, Vol 26,987-995,1981.

For measurement, fibers are held in a holderthat suspends the sample fiber in the transmitted light beam of a Leitz Orthoplan microscope fitted with a lx objective and a matching substage condenser. The holder is made so that the sample fiber can be rotated at right angles to its long axis and measurements can be made at increments of 45".

The holder is made up of two metal posts that are tapped to hold rotating clamping devices. The posts are attached atthe ends of a 1 " x 3" (2.5 x 7.5 cm) glass microscope slide with the clamping devices facing toward the center of the slide. For mounting a sample fiber, the holder is placed on a stereo nn'croscope croscope at approximately 9x magnification and the fiber attached at both ends by the clamping devices.Thefiberis held horizontally in free air between the clamps but is notundersufficient tension to "pul out" any ofthe crimps. The length of sample fiber between the clamps is approximately 11/2" (3.8cm) and a droplet of opaque acrylic paint is applied about in the center ofthefiberto mark a point forthe measurements.The holder assembly is then placed on the microscope stage and covered by an open-ended, bottomless optical glass cell to mini mize the sample fiber vibration from air currents. The sample is then focused and examined on the cathode raytube of the image analyzer and the microscope stage moved until the droplet of acrylic paint just touches the outside ofthe measuring frame. The length of the fiber segment measured is 0.227" (0.6 cm). For measurements, the length ofthe fiber is detected by approximately 250 lines of detection with the coordinates of the trailing edge registered at each line and stored in the computer memory.After the coordinates are stored, they are compared by the computer and if an individual measurement is greater or less than the preceding or following measurement, then it is considered to be a deflection point and will be visually marked on a "flag" on the cathode ray tube image ofthe sample. After a measurement, the holder is removed, rotated 45 and returned to the stageforthe additional measurements.

Percent crimp measurements on samples are shown in Table 1 belowwithtwo measurements at each angle being averaged and the three dimen sionality factor calculated as previously discussed.

Table 1 5 Crimp Three dimensionality 0 45 90 135 Factor 15.70 16.99 18.99 20.10 14.91 16.31 17.77 17.33 15.31 16.65 18.38 18.72 1.22 18.81 16.71 19.83 14.01 18.93 16.03 20.39 13.92 18.87 16.37 20.11 13.97 1.07 18.87 16.37 20.11 13.97 1.07 25.51 24.91 17.97 14.91 26.63 25.03 16.90 15.03 26.07 23.97 17.33 13.97 1.39 26.07 24.97 17.44 14.97 1.49 14.91 17.61 23.29 21.30 15.09 17.91 23.40 20.53 15.00 17.76 23.35 20.97 1.56 19.10 17.17 18.81 19.98 28.91 15.93 19.70 20.19 24.01 16.55 19.26 20.09 1.25 19.21 17.86 24.22 20.37 17.91 24.79 29.18 25.24 18.56 21.33 26.70 22.81 1.44 19.46 18.86 21.74 22.39 20.96 18.80 21.05 22.30 20.21 18.83 21.40 22.35 1.19 20.36 18.70 14.91 12.91 19.36 19.00 15.01 12.06 19.86 18.85 14.96 12.49 1.33 31.98 27.54 28.85 32.69 30.79 27.79 28.55 30.63 31.39 27.66 28.70 31.66 1.14 28.92 34.44 17.99 17.10 27.91 31.30 18.12 17.71 28.42 32.92 18.06 17.41 1.89 Average 1.36 A portion of the staple is processed using a conventional textile cotton processing system with ring spinning into 24/1 cotton count yarn with 1 6Z twist per inch (per 2.54 cm). The yarn is knitted into a 24 cut plain jersey fabric. As compared to a similar polyester fabric knit from round cross section staple of equivalentfiber denier, the fabric prepared of staple of this invention is softer, has higher cover, and more natural hand. The fabric exhibits a luster similar to that of mercerized cotton.

Example II The process of Example I is repeated except: a) a spinnerette with paired parallel holes of about 0.030 diameter separated by about 0.038 mm. from center to center. Spinning is at 3800 yds./min. (3400 meters/ min.). b) The resulting filament has a denier of about 2.25; isthegeneral shape as shown in FIGURE 1; and c) is of substantially uniform size along its length. d) The tow is drawn at a draw ratio of 1.60. Thethree dimensionalityfactor is determined to be 1.39.

Afabric of similar properties is obtained.

Example 111 For purposes of comparison,the procedurede- scribed in Example I is repeated except that no steam is utilized in the stuffer box crimper. The three dimensionalityfactor is determined to be 3.38. The staple is notcardable.

Claims (11)

1. Cardabie polyester staple characterised in that said staple has a three dimensionalityfactor less than 2 and comprises a major proportion of staple fibers having a cross sectional area shape which has a lower moment of inertia in one direction across the area than in other directions and less than 0.75 times the moment of inertia of a circular cross section of equal area, said staple fibers having set crimps in the general direction ofthe lower moment of inertia.

2. The staple of claim 1 wherein said three dimensionalityfactoris less than 1.6.

3. The staple of claim 1 wherein the cross sectional shape ofthe fibers corresponds to the outline of partially intersecting circular shapes.

4. A tow of polyester filaments, characterized in that said tow has a three dimensionality factor less than 2 and that a major proportion of filaments constituting said tow have a cross sectional area shape which has a lower moment of inertia in one direction across the area than in other directions and less than 0.75 times the moment of inertia of a circular cross section of equal area, said filaments having set crimps in the general direction of the lower moment of inertia and further characterized in that staple cut from said tow is cardable.

5. The tow ofclaim 4wherein said three dimen- sionalityfactoris less than 1.6.

6. The tow of claim 3 wherein the cross sectional shape ofthe filament correspondsto the outline of partially intersecting circles and the filaments vary regularly in denier along their lengths.

7. A process for making polyester staple, said process being characterized by crimping a tow comprising a major proportion of filaments having a cross sectional area shape which has a lower moment of inertia in one direction across the area than in other directions and less than 0.75 times the moment of inertia of a circular cross section of equal area in a stuffer box crimper, setting the crimp by the action of steam in said crimper, and cutting the tow into fibers of staple length.

8. The process of claim 7 wherein the cross sectional shape ofthefilaments corresponds to the outline of partially intersecting circles.

9. Polyester staple made by the process of claim 8.

10. The process of claim 8 wherein the denier of the filaments varies regularlyand repetitively along the filament lengths.

11. Polyester staple made by the process of claim 10.