BACKGROUND OF INVENTION There has been increased interest in recent years in water-dispersible synthetic fiber, especially of polyester. Such water-dispersible fiber is used in various non-woven applications, including paper-making and wet-laid non-woven fabrics, sometimes as part of a blend, often with large amounts of wood pulp, or fiberglass, but also in applications requiring only polyester fiber, i.e., unblended with other fiber. This use, and the requirements therefor, are entirely different from previous more conventional use as tow or staple fiber for conversion into textile yarns for eventual use in woven or knitted fabrics, because of the need to disperse this fiber in water instead of to convert the fiber into yarns, e.g., by processes such as carding, e.g. in the cotton system. It is this requirement for water-dispersibility that distinguishes the field of the invention from previous, more conventional polyester staple fiber.

Most such water-dispersible polyester fiber is of poly(ethylene terephthalate), and is prepared in essentially the same general way as conventional textile polyester staple fiber, except that most water-dispersible polyester fiber is not crimped, whereas any polyester staple fiber for use in textile yarns is generally crimped while in the form of tow, before conversion into staple

fiber. Thus, waterdispersible polyester fiber has generally been prepared by melt-spinning the polyester into filaments, combining the filaments to form a tow, drawing, applying a suitable coating to impart water-dispersible properties, generally in the same way as a finish is applied to a tow of conventional textile filaments, and then, generally without any crimping (or with imparting only some mild wavy undulations in some cases to provide extra bulk and a three-dimensional matrix), converting the tow into staple. Some prior polyester staple fiber has been prepared in uncrimped form, e.g. for use as flock in pile fabrics, but for such use, water-dispersibility has not been required.

Polyester fibers are naturally hydrophobic, so it is necessary to apply to the polyester a suitable coating, as disclosed by Ring et al. in U.S. Patent No. 4,007,083, Hawkins in U.S. Patent Nos. 4,137,181, 4,179,543 and 4,294,883, and viscose Suisse in British Patent No. 958,350, to.overcome the inherent hydrophobic character of the polyester fiber without creating foam or causing the fibers to flocculate. It is this coating that has distinguished water-dispersible polyester fiber from more conventional polyester staple fiber, rather than any inherent characteristic feature of the polyester itself, or of its shape, such as the cross-section. Heretofore, so far as is known, the cross-section of all commercial water-dispersible polyester fiber has been round. Indeed the cross-section of most commercial polyester staple fiber has generally been round, because this has been preferred.

Although, hitherto, most synthetic polymeric water-dispersible fiber has been formed of polyester, being inexpensive and plentiful, increasing amounts of polyolefins and polyamides are beginning to be used for water-dispersible fibers, and so the invention is not

limited only to polyesters, but covers other synthetic polymers.

SUMMARY OF INVENTION According to the present invention, there is provided new synthetic polymer water-dispersible fiber, especially polyester fiber, characterized in that the fibers are of cruciform cross-section.

A cruciform cross-section has been used heretofore for other polyester fibers, as described herein. Other than the cross-section, the water-dispersible fiber of the invention may be essentially similar to prior water-dispersible polyester or other synthetic polymer fibers, although the advantages described hereinafter may provide the opportunity for additional modifications. The invention will be described hereinafter with special reference to polyester fiber, although it will be recognized that other synthetic polymers, such as polyamides and polyolefins, may also be used.

The fibers of the invention may be made conveniently by melt-spinning and drawing polyester ^• filaments of appropriate denier per filament (dpf), and applying thereto a suitable coating to impart water-dispersible characteristics. The filaments are then generally cut into staple of whatever length is desired for the end-use contemplated.

The use of a cruciform cross-section for the water-dispersible fiber of the invention has, surprisingly, been found to promote dispersibility, in comparison with a round cross-section, and this imparts to the resulting wet-laid fibers better uniformity, more opacity, good permeability, and an attractive flannel-like hand as will be seen in the Example.

BRIEF DESCRIPTION OF DRAWINGS Figure 1 shows a cruciform cross-section for a stylized fiber according to the invention.

Figure 2 shows a typical spinneret orifice for spinning filaments of the invention.

DISCLOSURE OF THE INVENTION As indicated above, a cruciform cross-section has already been used for more conventional polyester staple fiber, that has been spun into filaments and drawn, cut, converted into spun yarn, and used in woven or knitted fabrics. Such fiber had the water-dispersible characteristics required for this invention. Similarly, polyester filaments having a cruciform cross-section are already known from Lehmicke U.S. Pat. No. 2,945,739, which discloses a process for melt-spinning polyamide and polyester filaments of, inter alia cruciform cross-section, and woven and knitted fabrics from staple fibers, and from Jamieson U.S. Pat. No. 3,249,669, which discloses a process for making a multifilament yarn of polyester filaments of various cross-sections, including a cruciform cross-section. Oriented polyester filaments of non-round cross-section have also been described by Frankfort et al. in U. S. Pat. Nos. 4,134,882 and

4,195,051, having been prepared by spinning at a very high speed (6,000 yp ) , which high speeds could also be used to prepare oriented polyester filaments of cruciform cross-section as a substrate for applying thereto a suitable coating to impart water-dispersible characteristics, and thereby obtain water-dispersible fiber according to the invention. None of this art concerns the field of the present invention. However, the polyester filamentary substrates for making the water-dispersible fiber of the invention may be prepared by the techniques described therein, or by appropriate modifications of- these or other known techniques of making polyester filaments of non-round cross-section.

The prior art references disclose parameters for a cruciform cross-section and Figure 1 is essentially as shown therein.

The preparation of the polyester staple fiber is otherwise conventional, involving the steps of melt-spinning polymer into filaments, collecting the filaments into a tow, drawing the tow, and applying a suitable coating water-dispersible to impart characteristics. If low shrinkage is desired, the drawn filaments are generally annealed.

Selection of an appropriate coating to promote water-dispersibility is important, and more of such coating is generally required than for comparable weights of fiber of round cross-section of similar dpf, because of the larger surface area of the periphery of the cruciform cross-section. It is especially important to provide good boundary lubrication properties. For this reason, an ethoxylated coating is preferred.

Suitable coatings are disclosed in Hawkins, U. S. Pat. Nos. 4,137,181, 4,179,543 and 4,294,883 and also in copending USSN 721,344, filed simultaneously herewith in the names of van Issum and Schluter, disclosing the use of a synthetic copolyester of poly(ethylene terephthalate) units and poly(oxyalkylene) of groups derived from a poly(oxyalkylene) glycol having an average molecular weight in the range of 300 to 6,000, as disclosed, e.g. in Mclntyre, et al. U.S. Pat. Nos. 3,416,952, 3,557,039 and 3,619,269, referred to therein; other useful segmented copolyesters are disclosed in Raynolds U.S. Patent No. 3,981,807; all these disclosures are incorporated herein by reference.

Such polyester fiber is generally prepared first in the form of a continuous filamentary uncrimped tow or, if extra bulk is required, and a more three-dimensional matrix, the filaments may be provided with mild wave-like undulations by a mild crimping-type process, and the uncrimped or mildly wave-like filaments are cut to the desired cut length, i.e. to form the water-dispersible

fiber, which is generally sold in the form of bales, or other packages of cut fiber. Suitable cut lengths are generally from about 5 to about 90 mm (1/4 to 3 inches), generally up to 60 mm (2-1/2 inches), and of length/diameter (L/D) ratio from about 100:1 to about 2000:1, preferably about 150:1 to about 2000:1, it being an advantage of the invention that good performance has been obtainable with preferred water-dispersible fiber of the invention with an L/D ratio higher than we have considered satisfactory with prior art water-dispersible polyester fiber. For instance, machine manufacturers have generally recommended that the L/D ratio not exceed 500:1, and many operators have considered even this figure unrealistically high. A suitable denier per filament is generally from about 0.5 to about 20. The coating is generally present in amount about 0.04 to about 1.0% of the weight of fiber (OWF%), it being an advantage that smaller amounts may generally be used than we have considered satisfactory according to the prior art. There is also provided a process for preparing such water-dispersible polyester fiber, comprising the steps of melt-spinning the polyester into filaments of cruciform cross-section, forming a tow of such filaments, drawing, and then coating the filaments in the tow with such synthetic copolyester, and, at an appropriate time, converting such coated filaments into staple fiber.

The coating is preferably cured on the filaments by heating the coated filaments, or the resulting staple fiber, if desired, to a temperature of about 100° to about 190° to improve durability.

The invention is further illustrated in the following Example, in which all parts and percentages are by weight, unless otherwise indicated, and OWF is (solids) "of weight of fiber". Reference is made to several measurements of yarn properties, such as tensile

properties (tenacity and elongation-to break), which are measured according to the methods described in Frankfort et al. U.S. Patent No. 4,134,882. It will be understood that other conditions can be used e.g., other designs of 5 orifice, such as are shown in the art.

Example The following fibers, Fiber A, a comparison of round cross section, and Fiber N, a fiber of the invention of cruciform cross section, were both spun from

10 poly(ethylene terephthalate) of intrinsic viscosity 0.64, containing 0.3% TiO- as a delusterant.

Fiber A was spun at 1600 ypm into filaments with conventional radial air quenching using a 900 hole spinneret, with round holes 0.015 inches in diameter and

15 capillary length of 0.030 inches, a 270°C block, and polymer throughput 68.2 pounds/hour. Denier per filament was 3.67. Fiber A was then oriented by running over a set of feed rools at 29.3 ypm, followed by a set of draw rolls at 80.0 ypm, and delivered to a conveyer by puller rolls

20 at 80.1 ypm. Between feed roll sections the filaments were treated in a 45°C water bath. Between feed and draw rolls the rope was sprayed with water at 98°C. Between draw and puller rolls a commercial water-dispersible coating (50/50 mixture of potassium salt of mono and

25 diacid phosphate esters of lauryl alcohol/tallow alcohol ethoxylated with 25 moles of ethylene oxide) was applied. The filaments were then relaxed free in an oven at 150°C for 6 minutes.

Fiber N was produced in a similar manner to Fiber

30 A except that 625 filaments of 3.22 dpf and cruciform cross-section were spun through capillaries as shown in Figure 2, with block temperature 273°C, and throughput 42.9 pounds/hour. Roll speeds for the orientation were feed rolls 32.1 ypm, draw rolls 80.2 ypm and puller rolls

3579.2 ypm, and a somewhat higher level of water-dispersible

coating was used to offset approximately 57% higher surface area of the cruciform cross-section.

The properties of the drawn coated filaments are compared in Table 1.

Table 1

Sample A N

Cross-section Round cruciform dpf 1.47 1.56 coating OWF(%) 0.4 0.44

Boil-off shrinkage(%) 1.0 0

Dry heat shrinkage

(196°C) (%) 2.45 3.6

Tenacity at break (g/d) 4.5 4.8

Elongation at break(%) 42 26

Tenacity at

2% elongation(g/d) 0.93 0.93

Both types were cut to form water-dispersible fiber of 1/4, 3/8, 1/2 and 3/4 inch cut lengths and were ... tested on an inclined wire Fourdrinier machine. Fibers we ^' re dispersed for three minutes in a small pulper at 0.75% consistency (lbs. fiber per 100 lbs. slurry, or furnish). The cylindrical pulper was approximately 3 feet in diameter by 6 feet deep. Fibers were then mixed with unrefined sulphite pulp to form a 50% polyester blend and diluted to 0.1% consistency in a 10 cubic meter stock tank. This stock was further diluted in the headbox of the machine to 0.0143% consistency and formed into a 0.5 meter wide wet lay nonwoven fabric at 20 meters/minute. A spray of an acrylic binder, Acronyl 240D was spray applied at the end of the Fourdrinier wire. The fabric was then cured in a through air drier at 150 ^β C. Finished fabric weight averaged 40 grams/square meter.

Dispersion quality can be judged by the uniformity of the fabric produced from a given sample. As cut length increases, the uniformity of the fabric can generally be

expected to suffer significantly. However, great advantages can result from using a longer fiber because the fabric tear strength increases, for example. In practice, therefore, a fabric producer will generally wish to use the longest fiber that will meet his uniformity standards. Thus, a longer fiber with improved, or equivalent uniformity would be preferred.

The dispersion quality of fabrics from Fibers A and N were rated as they were produced on the machine by observing the fabrics as the water drained from them on the Fourdrinier wires. Results of this comparison are in Table 2 and indicate good dispersion for the cruciform in spite of its 57% greater surface area.

TABLE 2

DISPERSION DESCRIPTION

ROUND CRUCIFORM

CUT LENGTH ITEM A ITEM N

1/4 inch good dispersion good dispersion few log defects few log defects

3/8 inch some log defects good dispersion general quality not fair fabric cover so good as 1/4 inch (opacity)

1/2 inch fairly good dispersion normal dispersion 3/4 inch dispersion definitely very good dispersion poor, cover lower

Standard physical properties were measured for the set of fabrics at Herty Foundation, Savannah, GA. Compared each time to Fiber A as 100%, Fiber N had the following average properties:

Air Permeability, Gurley 112%

Opacity, ISO 2471 111%

Bulk, TAPPI T410 om-83 and T411 om-83 118%

Tensile Strength, TAPPI T494 om-81 100% Tensile Stretch, TAPPI T494 om-81 85%

Tear Strength, TAPPI T414 om-82 104%

On balance. Item N exhibited advantages in the important areas of higher permeability, opacity, bulk and tear strength compared to the control at equivalent tensile strength with a small reduction in stretch. The cover advantage is important because less fiber can be used for a nonwoven fabric with similar performance characteristics, thereby saving materials cost. The fabrics of Item N also have an attractive flannel-like hand.

When used with the appropriate water-dispersible coating in appropriate amount, the cruciform cross-section fiber of the invention has given a fabric with surprisingly good dispersion uniformity, and the properties indicated.

From theoretical considerations, water-dispersible fibers of conventional round cross-section would have been expected to give more uniform dispersions, and, therefore, more uniform wet-laid fabrics. This- is because the surface energy required to disperse a fiber (or other articles) is given by:

Energy - (Surface Tension) X (Dispersed surface area -

Undispersed surface area). The undispersed fiber exists in logs or clumps of many hundreds of fibers, most of which are on the inside of the logs. Therefore the undispersed surface area is negligible compared to the dispersed area, and the energy term can be expressed approximately as:

Energy - (Surface Tension) X (Number of fibers) X

(Surface area of a fiber). This energy term describes both the energy required to disperse the fiber, and the free energy driving force for reagglomeration. Therefore, for any given coating, and fiber dpf, fibers with lower area would be expected to provide a more uniform dispersion, hence more uniform fabric. The minimum surface area per unit weight for a given fiber occurs when the cross-section is round, which would be expected, therefore, to be preferred.

Surprisingly, however, these cruciform fibers, in spite of about 60% greater surface area gave more uniform fabrics. Without limiting the invention to any theory, this may result from the fiber's hydrodynamic sha.pe, which may more effectively use the energy available in the mixer shear field.