Abstract:
To make a monofilament yarn having synthetic polymeric materials, a multifilament or monofilament core yarn is coated with a polymer using a specially designed die during an extrusion process. Flow instabilities created in the die resulting in a yarn that develops a non-uniform cross-sectional beading of the coating material. The beaded material forms a helical pattern around the yarn core, producing a false twist effect.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Yarn comprises continuous strands of twisted threads of natural or synthetic materials, which are used in weaving or knitting. Threads used to make yarn are fine cords of fibrous material, such as cotton, flax, wool or nylon, which are twisted together to form the threads.  
         [0002]     With the advent of synthetic plastic coating materials, fabrics created from coated yarn can gain the benefit of increased durability, increased resistance to abrasion, weathering, burning, chemical attack, microbial and fungal attack, improved aesthetic appearance, increased blocking of sunlight and harmful ultraviolet radiation, and can be used effectively as a thermal insulator.  
         [0003]     Other fabrics can have a decorative appearance if they are woven from a beaded yarn. In the prior art, coated yarns have been created by running a monofilament or multifilament yarn through a plastic coating extruder. However, such yarns are not beaded. A plastic coated, beaded yarn can provide the aforementioned benefits of a coated yarn and additionally provide unique and exotic aesthetic qualities brought by new product patterns and designs in specialty and niche markets. Thus, there is a need for creating a plastic coated, beaded yarn.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     To make a monofilament yarn having synthetic polymeric materials, a multifilament or monofilament core yarn is coated with a polymer using a specially designed die during an extrusion process. Flow instabilities created in the die resulting in a yarn that develops a non-uniform cross-sectional beading of the coating material. The beaded material forms a helical pattern around the yarn core, producing a false twist effect.  
         [0005]     In one aspect, the present invention is directed to a polymeric coated yarn, comprising: a core yarn strand; and a polymeric coating over said core yarn strand; wherein said coating is non-uniform along a radial direction of said strand.  
         [0006]     In another aspect of the present invention, said coating forms a helical bead along an axial direction of said strand.  
         [0007]     In another aspect of the present invention, said helical bead is non-uniformly distributed along the axial direction, thereby forming a zigzag appearance.  
         [0008]     In another aspect of the present invention, an outer edge of a radial cross section of said coating is substantially circular.  
         [0009]     In another aspect of the present invention, an outer edge of a radial cross section of said coating is substantially circular.  
         [0010]     In another aspect of the present invention, said core yarn strand comprises artificial fibers.  
         [0011]     In another aspect of the present invention, said core yarn strand comprises more than one filament.  
         [0012]     In another aspect of the present invention, each filament comprises a different type of fiber.  
         [0013]     In another aspect of the present invention, said core yarn strand comprises a material different from a polymeric material.  
         [0014]     In another aspect of the present invention, said coating is extruded over said core yarn strand.  
         [0015]     In another aspect of the present invention, the yarn further comprises a pigment, wherein the coating is extruded with said pigment.  
         [0016]     In another aspect, the present invention is directed to a polymeric yarn, comprising a single polymeric strand having an axial axis, wherein an outer surface of said strand is non-uniform along a radial direction from said axial axis.  
         [0017]     In another aspect, the present invention is directed to a method of coating yarn, comprising: creating an instability in flow of a polymeric coating through a die attached to an extruder; extruding a core yarn strand with said coating through said die; and forming a non-uniform coating along a radial direction of said strand.  
         [0018]     In another aspect of the present invention, the method further comprises varying one or more processing parameters from the group of extruder pressure, extruder barrel temperature, die temperature, and yarn draw speed.  
         [0019]     In another aspect of the present invention, the method further comprises extruding a pigment with said core yarn strand and coating.  
         [0020]     In another aspect of the present invention, the method further comprises cooling said coated yarn in a chilled water tank.  
         [0021]     In another aspect of the present invention, the method further comprises rolling said coated yarn through nip rollers.  
         [0022]     In another aspect, the present invention is directed to a crosshead extrusion die for forming a polymeric coating over a strand of yarn, comprising: a die head; a feed zone to admit said strand and said coating; a flow balance zone to orient a flow of said coating; and a die cap having an orifice through which said coating and said strand is extruded, and a reservoir to create a flow instability in said coating; wherein a non-uniform coating along a radial direction of said strand is formed.  
         [0023]     In another aspect of the present invention, said orifice is circular.  
         [0024]     In another aspect of the present invention, said die comprises more than one orifice to simultaneously extrude more than one polymeric coated strand of yarn. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  shows a cross-section of a monofilament core yarn in a first embodiment of the invention;  
         [0026]      FIG. 2  shows a length of a helical beaded, false twist yarn from the first embodiment of the invention;  
         [0027]      FIG. 3  shows a length of yarn from a second embodiment of the invention;  
         [0028]      FIG. 4  shows a cross-section of a multifilament core in an embodiment of the invention;  
         [0029]      FIG. 5  shows a cross-section of a coreless polymeric yarn from an embodiment of the invention;  
         [0030]      FIG. 6  shows equipment which performs the extrusion process used to produce embodiments of the present invention;  
         [0031]      FIG. 7  shows an embodiment of a die used to produce coated yarns; and  
         [0032]      FIG. 8  shows an embodiment of a die cap used to produce coated yarns; and  
         [0033]      FIG. 9  shows a die cap in the prior art. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]      FIG. 1  shows a cross-section of a coated monofilament core yarn  100  in a first embodiment of the invention. As shown in  FIG. 1 , a monofilament core yarn  110  is preferably coated by a synthetic polymeric coating material  120 . Core yarn  110  may be any number or size of yarn, and may be polyester or other synthetic yarn, or may be natural yarn.  FIG. 1  illustrates a preferred embodiment where coating  120  has a circular shape, but the coating may also be formed into any number of shapes, such as flat, triangular, square, rectangle, hexagonal, etc. An outer edge of coating  120  has a non-uniform distance from the center of core yarn  110 . Essentially, as illustrated in the preferred embodiment shown in  FIG. 1 , the center of coating  120  is offset from the center of core yarn  110 .  
         [0035]     Coating  120  is any material that can be conventionally extruded, including polyvinyl chloride (PVC) compounds, polypropylene, polyester, and the like. Preferably, coating  120  is a compound containing several ingredients shown in Table 1 below:  
                                           TABLE 1                           Coating 120 Compound Ingredients                Ingredient   % Added                            PVC Resin 1   68.7           Plasticizer   22.7           Heat Stabilizer   4.1           PVC Resin 2   3.6           UV Stabilizer   .3           Biocide   .3           Lubricant   .2                      
 
         [0036]     The coating ingredients shown in Table 1 provides a combination of PVC resin, plasticizer, and lubricant that results in flow properties required in the production of false twist beaded yarn over a relatively wide range of processing conditions.  
         [0037]     Coating  120  may have a clear formulation, but may also have streaks of coloring material by mixing a pigment into the coating, preferably by using a secondary extruder. In addition, yarn can be extruded with two or more pigmentation colors at various percentages, using a typical coextrusion process.  
         [0038]      FIG. 2  shows a length of a helical beaded, false twist yarn from the first embodiment of the invention. Along the length, or axial direction of core yarn  110 , the center of coating  120 , which is a radially offset from the center of core yarn  110 , rotates about the center of core yarn  110 , thus preferably providing an eccentric effect, thereby forming a beaded surface having a helical, or false twist appearance. Although this preferred embodiment has a regular, periodic appearance, yarn  100  may be formed into any combination of cross-sectional dimensions, bead sizes, bead spacing, and helix angles by modifying the process or equipment used to create the yarn.  
         [0039]      FIG. 3  shows a length of yarn from a second embodiment of the invention. Along the length of core yarn  110 , the radial offset of the center of coating  120  rotates about the center of core yarn  110 . But the offset angle does not vary in proportion to the distance along the length of core yarn  110 . In this preferred embodiment, the offset direction of the center of coating  120  rotates about the center of core yarn  110  aperiodically, thus preferably providing a zigzag, beaded appearance. In an alternative preferred embodiment, the outer dimensions of coating  120  varies along the length of yarn  100 , forming different cross-sectional dimensions along the length, thereby forming a zigzag, beaded appearance.  
         [0040]      FIG. 4  shows a cross-section of a multifilament core yarn  400  of an alternative embodiment of the invention. As shown in  FIG. 4 , a multifilament core yarn  410 , is preferably coated by a synthetic polymeric coating material  420 . Core yarn  410  comprises more than one filament  450   460 . In a preferred embodiment, core yarn  410  comprises two 1000 denier multifilament yarns, coated by the customized PVC compound disclosed in Table 1. Alternatively, each yarn filament  450   460  may comprise different materials and sizes, compared to other filaments in core  410 .  
         [0041]      FIG. 5  shows a cross-section of a coreless polymeric yarn from another embodiment of the invention. As shown in  FIG. 5 , coreless polymeric yarn  500  is preferably created by extruding a synthetic polymeric material  510 , thereby forming a strand having an axial axis  520 , wherein an outer surface of said strand is non-uniform along a radial direction from axial axis  520 . Along the length, or radial direction of coreless yarn  500 , an offset direction of the center of the material rotates about axial axis  520 , thus preferably providing an eccentric effect, thereby forming a helical, beaded appearance. Although this preferred embodiment has a regular, periodic appearance, yarn  500  may be formed into any combination of cross-sectional dimensions, bead sizes, bead spacing, and helix angles by modifying the process or equipment used to create the yarn.  
         [0042]      FIG. 6  shows equipment that creates yarn in an extrusion process, which produces embodiments of the present invention.  FIG. 6  shows a set of creel  610  upon which the core yarn is wound, a 2-inch single screw, primary extruder  620  (preferably a Davis Standard, Model DS-20) equipped with a barrier screw and static mixing section, a die  630 , chilled water tank  640 , nip rolls  650 , and yarn winder  660 . Also shown in  FIG. 6  is an optional secondary 1-inch extruder  660 .  
         [0043]      FIG. 7  shows an embodiment of a die  700  used to produce coated yarns in the present invention. Preferably attached to extruder  620  is the crosshead yarn coating die  700  shown in  FIG. 7 .  
         [0044]      FIG. 8  shows a preferred embodiment of a die cap. As shown in  FIG. 8 , a small reservoir  810  is machined into die cap  800 , just before the orifice  820 .  
         [0045]      FIG. 9  shows a die cap in the prior art. As shown in  FIG. 9 , standard extruder cap  900  lacks the reservoir machined into the die cap illustrated in  FIG. 8 .  
         [0046]     Now, the extrusion process will be described in more detail. Referring to  FIG. 6 , a pre-compounded PVC formulation (see Table 1 above) is extruded on extruder  620 . As melt from the extruder is continuously forced into the die head, two 1000-denier multifilament polyester yarns of indeterminate length are continuously pulled off of a set of creels  610  and through the top of die  630  by a pair of downstream nip rolls  650 .  
         [0047]     Preferably, secondary extruder  660  optionally creates bi-color streaks on some false twist beaded yarns. In such case, a compound with a secondary pigment is extruded into the primary melt stream of main 2-inch extruder  620  just prior to flow into the static mixing section. This process is not limited to a single secondary extruder, and extrudate with multicolored streaks can be produced in this fashion. The final product is then run through a chilled water tank  640  before being pulled through nip rolls  650  and rolled onto a tube using a precision winder  660 .  
         [0048]     Referring to  FIG. 8 , the position of the polyester core yarns are adjusted as they pass through the die core and the melt flow of the PVC extrudate is oriented and balanced as it travels around the core. The core yarns come into contact with the PVC melt and the final yarn shape is created in a die cap. Upon entering reservoir  810 , the PVC melt is allowed to relax and it loses some of its flow orientation. This increases the shear stress on the melt as it enters orifice  820 , and causes melt flow instabilities that result in melt fracture over a wide range of processing conditions. As the final product is extruded downward from the bottom of the die, the melt fracture causes the yarn to form its beaded appearance.  
         [0049]     Processing conditions which affect the appearance of beaded yarn are listed below in Table 2. A wide range of processing conditions result in the production of false twist beaded yarn. Different process settings combined with a different sized die cap orifice or different coating compound materials could also result in the same yarn characteristics as those noted above. In addition, the following variables have been considered and have been determined to affect the characteristics of false twist beaded yarn to create a variety of patterns and textures:  
                         TABLE 2                       Processing Parameters                                    Parameter           Process parameters (screw speed, barrel and die temperatures,           and draw down speed)           Die orifice size and shape           Die reservoir size and shape           Core yarns (size, number, type, and materials)           Coating compound used           Screw and barrel design           Extruder size and throughput capacity           Downstream surface texturing equipment           Type of wind up system           Coextrusion (bi-colors, etc.)                      
 
         [0050]     More specifically, the following processing conditions illustrated in Table 3 below were found to create the first and second preferred embodiments:  
                                           TABLE 3                           Processing Conditions            Process Parameter   First Embodiment   Second Embodiment                    Screw Speed (rpm)   101   50       Line Speed (m/min)   80.5   91.4       Barrel Temperatures (° C.)   152, 158, 161   152, 158, 161       Die Temperatures (° C.)   164, 158   158, 156       Melt Temperature (° C.)   187   183       Water Temperature (° C.)   13   13       Head Pressure (MPa)   23.4   22.8       Die Orifice Shape   Circular   Circular       Die Orifice Size (Ø mm)   1.02   1.02       Coating Material   PVC   PVC                  
 
         [0051]     The extruder temperature is maintained according to a heat temperature profile. The heat temperature profile comprises 3 barrel temperatures shown in Table 3 for each of the two embodiments. The extruder barrel has three separate heating zones along its length. Each temperature value shown in the table corresponds to an individual zone. The first temperature ( 152 ) corresponds to the zone closest to the feed (entrance) end of the extruder and the last temperature ( 161 ) corresponds to the zone at the metering (exit) end of the extruder. Likewise, the die is maintained according to the temperature profile shown in Table 3.  
         [0052]     The parameters as shown in Table 3 are typical of conditions that were determined through experimentation to produce two distinctive yarn designs that represent points near the upper and lower limits that produce beaded yarn on the preferred extrusion equipment. Process conditions that also produce beaded yarn exist at many different setpoints, and at least include those setpoints between the first and second preferred embodiment setpoints shown.  
         [0053]     Multiple experiments were conducted to study the complex interaction of multiple processing, equipment, and material related variables. The results of these experiments are reflected in the parametric value setpoints disclosed in Table 3. Changes in any parameter affects the other settings, and can add or diminish the formation of beaded yarn  100 . Generally, the effect generated by changing each process parameter will now be explained.  
         [0054]     Screw and Line Speed: In general, plastic melt fracture depends on a specific combination of shear stress, melt viscosity, flow rate, melt elasticity, and flow channel geometry at the die orifice. Screw speed directly affects the first three listed conditions and a range of required screw speeds can be used to create beading. Deviating outside of the range results in a smooth coating rather then a fractured (beaded) coating. The actual range over which this phenomenon occurs depends on the material, extruder screw design, die design, and temperature set points. The die recommended in the preferred embodiment can be used over a wide range of screw speeds. Beaded yarn can be produced with screw speeds ranging from 40 rpm up to the maximum screw speed for the extruder (110 rpm). The exact appearance of the beaded yarn will depend on a combination of the screw speed and line speed with higher screw speed and lower line speed generating a thicker yarn with an appearance similar to that shown in  FIG. 2  and lower screw speed and higher line speed generating a thinner yarn with an appearance similar to that shown in  FIG. 3 .  
         [0055]     Temperature Settings: Temperature set points affect melt viscosity, shear stress, and flow rate at the die orifice and will thus have a similar affect as screw speed. In general, a range of temperature settings is required. Deviation from these settings, either below or above, will result in the production of a smooth coating rather then a fractured (beaded) coating. Because PVC compounds are temperature sensitive and require a relatively narrow temperature processing range, the absolute range of barrel temperatures that can be used to produce beaded yarn has not yet been determined.  
         [0056]     Having thus described at least illustrative embodiments of the invention, various modifications and improvements will readily occur to those skilled in the art and are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.