FIELD OF THE INVENTION  
         [0001]    This invention relates to improvements in permitting greater efficiency for low denier polypropylene fiber and yarn production. Generally, spinning speeds are limited for polypropylene fibers and yarns as such materials tend to break easily upon exposure to excessively high tensions associated with low- to medium-spinning speeds. Low production speeds negatively impact the economics of producing such low denier fibers which prevents the widespread utilization of such fibers and yarns in various end-use applications, particularly applications for which low denier provides desirable hand characteristics. Thus, it has surprisingly been determined that such low denier manufactured fibers and yarns can be produced with certain nucleating additives that permit high tension levels in the quench stack as required for high-speed spinning procedures to be followed. Additionally, low-shrink and/or better resiliency properties are also available with the addition of such nucleating compounds within the target low denier polypropylene resins.  
         DISCUSSION OF THE PRIOR ART  
         [0002]    There has been a continued desire to utilize low denier polypropylene fibers in various different products, such as apparel (due to highly effective soft hand properties), and the like. Polypropylene fibers exhibit excellent strength characteristics, highly desirable hand and feel, and do not easily degrade or erode when exposed to certain “destructive” chemicals. However, even with such impressive and beneficial properties and an abundance of polypropylene, which is relatively inexpensive to manufacture and readily available as a petroleum refinery byproduct, such fibers are not widely utilized in products that are exposed to relatively high temperatures during use, cleaning, and the like. This is due to the high and generally non-uniform heat- and moisture-shrink characteristics exhibited by typical polypropylene fibers and, most importantly, the lack of reliable high-speed spinning manufacturing procedures available with polypropylene fibers and/or yarns in general especially for low denier fibers. Although polyesters (such as polyethylene terephthalate, or PET) and polyamides (such as nylons) are generally more expensive to manufacture, such fibers do not exhibit the same unacceptable high shrinkage, undesirable resiliency, and manufacturing efficiency problems as polypropylenes. Thus, it is imperative to provide remedies to such issues to permit utilization of such lower cost polymer materials in greater varieties of end-uses. Such a need has heretofore gone unattained.  
           [0003]    Such issues are quite prominent. For example, such polypropylene fibers are not heat stable and when exposed to standard temperatures (such as 150° C. and 130° C. temperatures), the shrinkage range from about 5% (in boiling water) to about 7-8% (for hot air exposure) to 12-13% (for higher temperature hot air). These extremely high and varied shrink rates thus render the utilization and processability of highly desirable polypropylene fibers very low, particularly for end-uses that require heat stability (such as apparel, carpet pile, carpet backings, molded pieces, and the like). Likewise, as noted above, high speed spinning for quicker fiber and/or yarn manufacturing (e.g., greater than 1500 m/min speeds) are basically unavailable for easily breakable polypropylene materials. Thus, there is room to improve in terms of manufacturing efficiencies for such polymer materials. Unfortunately, to date, there have been no simple or effective solutions to such problems.  
         DESCRIPTION OF THE INVENTION  
         [0004]    It is thus an object of the invention to provide improved manufacturing efficiencies for polypropylene fiber and/or yarn production by permitting highly reliable high speed spinning processes to be followed without appreciable fiber breakage concerns. A further object of the invention is to provide a class of additives that, in a range of concentrations, will permit such efficiency improvements in low denier polypropylene fibers. A further object of the invention is to provide a specific method for the production of nucleator-containing polypropylene fibers exhibiting low shrink properties than for standard polypropylene fibers and/or yarns. Additionally, another object of this invention is to provide a polypropylene fiber and/or yarn that can withstand such necessary and desirable high speed spinning procedures.  
           [0005]    Accordingly, this invention encompasses a method of producing polypropylene fibers exhibiting deniers per filament of at most about 5, preferably at most 3, more preferably at most about 1.5, and most preferably at most about 1.0, comprising the sequential steps of a) providing a polypropylene composition in pellet or liquid form comprising at least 100 ppm by weight of a nucleator compound; b) melting and mixing said polypropylene composition of step “a” to form a substantially homogeneous molten plastic formulation; c) extruding said plastic formulation to form a fiber structure; and d) spinning said extruded fiber (optionally while exposing said fiber to a temperature of at most 105° C.) at a minimum winding speed which varies with the fiber denier as shown in Table 1. Alternately, this minimum winding speed is given by the equation  
             y= 500+1400 *x,    
           [0006]    or the equation,  
             y′= 1000+1400 *x.    
           [0007]    Here, y is the preferred minimum winding speed (in m/min), x the denier per filament, and y′ the most preferred minimum winding speed. The same basic method is within this invention for yarns with a given denier per yarn with a minimum winding speed which depends on the yarn denier and is given in Table 2. Alternately, this minimum winding speed is given by the equation  
             y= 500+40  m /min* z+ 0.1 *z   2    
           [0008]    Or the equation,  
             y′= 1000+40* z+ 0.1* z   2    
           [0009]    Here, y and y′ are again the preferred minimum winding speed and most preferred minimum winding speed (in m/min) and z is the total yarn denier. Preferably, step “b” will be performed at a temperature sufficient to effectuate the melting of all polymer constituent (e.g., polypropylene), and possibly the remaining compounds, including the nucleating agent, as well (melting of the nucleating agent is not a requirement since some nucleating agents do not melt upon exposure to such high temperatures). Thus, temperatures within the range of from about 175 to about 300° C., as an example (preferably from about 200 to about 275°, and most preferably from about 220 to about 250° C., are proper for this purpose. The extrusion step (“c”) should be performed while exposing the polypropylene formulation to a temperature of from about 185 to about 300° C., preferably from about 210 to about 275° C., and most preferably from about 230 to about 250° C., basically sufficient to perform the extrusion of a liquefied polymer without permitting breaking of any of the fibers themselves during such an extrusion procedure. The drawing step may be performed at a temperature which is cooler than normal for a standard polypropylene (or other polymer) fiber drawing process. Thus, if a cold-drawing step is followed, such a temperature should be below about 105° C., more preferably below about 100° C., and most preferably below about 90° C. Of course, higher temperatures may be used if no such cold drawing step is followed. The final heat-setting temperature is necessary to “lock” the polypropylene crystalline structure in place after extruding and drawing. Such a heat-setting step generally lasts for a portion of a second, up to potentially a couple of minutes (i.e., from about {fraction (1/10)} th  of a second, preferably about ½ of a second, up to about 3 minutes, preferably greater than ½ of a second). The heat-setting temperature must be greater than the drawing temperature and must be at least 110° C., more preferably at least about 115°, and most preferably at least about 125° C. The term “spinning” is intended to encompass any number of procedures which basically involve placing an extensional force on fibers in order to elongate the polymer therein. Such a procedure may be accomplished with any number of apparatus, including, without limitation, godet rolls, nip rolls, steam cans, hot or cold gaseous jets (air or steam), and other like mechanical means.  
                       TABLE 1                           Preferred minimum   Most preferred minimum       Denier per filament   spinning speed   spinning speed                   0.5   1000 m/min   1500 m/min       0.8   1500 m/min   2000 m/min       1.0   2250 m/min   2750 m/min       1.5   2500 m/min   3000 m/min       2.0   3000 m/min   3500 m/min       3.0   4000 m/min   4500 m/min       4.0   4000 m/min   4500 m/min       5.0   4000 m/min   4500 m/min                  
 
           [0010]    [0010]                       TABLE 2                           Preferred minimum   Most preferred minimum       Denier per yarn   spinning speed   spinning speed                    15 denier   1000 m/min   1500 m/min        25 denier   1500 m/min   2000 m/min        50 denier   2250 m/min   2750 m/min        60 denier   2500 m/min   3000 m/min        70 denier   3000 m/min   3500 m/min       100 denier   3500 m/min   4000 m/min       140 denier   4000 m/min   4500 m/min       200 denier   4500 m/min   5000 m/min                    
           [0011]    Such fibers (or yarns comprising such fibers) require the presence of certain compounds that quickly and effectively provide rigidity and/or tensile strength to the target polypropylene fiber to a level heretofore unavailable, particularly in terms of permitting high-speed spinning for greater efficiency in fiber and/or yarn manufacturing. Generally, these compounds include any structure that nucleates polymer crystals within the target polypropylene after exposure to sufficient heat to melt the initial pelletized polymer and upon allowing such a melt to cool. The compounds must nucleate polymer crystals at a higher temperature than the target polypropylene without the nucleating agent during cooling. In such a manner, the nucleator compounds provide nucleation sites for polypropylene crystal growth which, in turn, appear to provide thick lamellae within the fibers themselves which, apparently (without intending on being bound to any specific scientific theory) increase the processability of the target fibers to such a degree that the tensions associated with high-speed spinning can easily be withstood. The preferred nucleating compounds include dibenzylidene sorbitol based compounds, as well as less preferred compounds, such as sodium benzoate, certain sodium and lithium phosphate salts (such as sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise known as NA-11 or NA-21), zinc glycerolate, and others. Sodium benzoate, in general, is not preferred because it is known to outgas corrosive benzoic acid, among other deficiencies.  
           [0012]    All shrinkage values discussed as they pertain to the inventive fibers and methods of making thereof correspond to exposure times for each test (hot air and boiling water) of about 5 minutes. The heat-shrinkage at about 150° C. in hot air is, as noted above, at most 11% for the inventive fiber; preferably, this heat-shrinkage is at most 9%; more preferably at most 8%; and most preferably at most 7%. Also, the amount of nucleating agent present within the inventive fiber is at least 10 ppm; preferably this amount is at least 100 ppm; and most preferably is at least 1250 ppm. Any amount of such a nucleating agent should suffice to provide the desired shrinkage rates after heat-setting of the fiber itself; however, excessive amounts (e.g., above about 10,000 ppm and even as low as about 6,000 ppm) should be avoided, primarily due to costs, but also due to potential processing problems with greater amounts of additives present within the target fibers.  
           [0013]    The target fibers and/or yarns may also be textured in any manner commonly followed for polypropylene materials. One example of this is false twist texturing, in which a twist is imparted to the fiber through the use of spindles, and while the fiber is in the twisted state it is heated and then cooled to impart into the individual filaments a memory of the twisted state. The yarn is then untwisted, but retains bulk due to the imparted memory. In another texturing embodiment, known as bulked continuous filament (BCF), the yarn is pushed with air jets into a stuffer box where it is crowded in a non-uniform state with other fibers and heated to retain the memory of this non-uniform state. The yarn is then cooled, but again retains bulk due to the imparted memory. Of course, other texturing methods, such as air texturing, gear texturing, etc., may be used. Polypropylene polymer containing nucleators retains the imparted memory of these texturing techniques better than polymer without nucleators because of the increased crystallization rate that the polypropylene undergoes when at elevated temperatures.  
           [0014]    The term “polypropylene” is intended to encompass any polymeric composition comprising propylene monomers, either alone or in mixture or copolymer with other randomly selected and oriented polyolefins, dienes, or other monomers (such as ethylene, butylene, and the like). Such a term also encompasses any different configuration and arrangement of the constituent monomers (such as syndiotactic, isotactic, and the like). Thus, the term as applied to fibers is intended to encompass actual long strands, tapes, threads, and the like, of drawn polymer. The polypropylene may be of any standard melt flow (by testing); however, standard fiber grade polypropylene resins possess ranges of Melt Flow Indices between about 2 and 50. Contrary to standard plaques, containers, sheets, and the like (such as taught within U.S. Pat. No. 4,016,118 to Hamada et al., for example), fibers clearly differ in structure since they must exhibit a length that far exceeds its cross-sectional area (such, for example, its diameter for round fibers). Fibers are extruded and drawn; articles are blow-molded or injection molded, to name two alternative production methods. Also, the crystalline morphology of polypropylene within fibers is different than that of standard articles, plaques, sheets, and the like. For instance, the dpf of such polypropylene fibers is at most about 5000; whereas the dpf of these other articles is much greater. Polypropylene articles generally exhibit spherulitic crystals while fibers exhibit elongated, extended crystal structures. Thus, there is a great difference in structure between fibers and polypropylene articles such that any predictions made for spherulitic particles (crystals) of nucleated polypropylene do not provide any basis for determining the effectiveness of such nucleators as additives within polypropylene fibers.  
           [0015]    The terms “nucleators”, “nucleator compound(s)”, “nucleating agent”, and “nucleating agents” are intended to generally encompass, singularly or in combination, any additive to polypropylene that produces nucleation sites for polypropylene crystals from transition from its molten state to a solid, cooled structure. Hence, since the polypropylene composition (including nucleator compounds in certain cases) must be molten to eventually extrude the fiber itself, the nucleator compound will provide such nucleation sites upon cooling of the polypropylene from its molten state. The only way in which such compounds provide the necessary nucleation sites is if such sites form prior to polypropylene recrystallization itself. Thus, any compound that exhibits such a beneficial effect and property is included within this definition. Such nucleator compounds more specifically include dibenzylidene sorbitol types, including, without limitation, dibenzylidene sorbitol (DBS), monomethyldibenzylidene sorbitol, such as 1,3:2,4-bis(p-methylbenzylidene) sorbitol (p-MDBS), dimethyl dibenzylidene sorbitol, such as 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (3,4-DMDBS); other compounds of this type include, again, without limitation, sodium benzoate, NA-11, and the like. The concentration of such nucleating agents (in total) within the target polypropylene fiber is at least 100 ppm, preferably at least 1250 ppm. Thus, from about 100 to about 5000 ppm, preferably from about 500 ppm to about 4000 ppm, more preferably from about 1000 ppm to about 3500 ppm, still more preferably from about 1500 ppm to about 3000 ppm, even more preferably from about 2000 ppm to about 3000 ppm, and most preferably from about 2500 to about 3000 ppm.  
           [0016]    Also, without being limited by any specific scientific theory, it appears that the required nucleators which perform the best are those which exhibit relatively high solubility within the propylene itself. Thus, compounds which are readily soluble, such as 1,3:2,4-bis(p-methylbenzylidene) sorbitol provides the lowest shrinkage rate for the desired polypropylene fibers. The DBS derivative compounds are considered the best shrink-reducing nucleators within this invention due to the low crystalline sizes produced by such compounds. Other nucleators, such as NA-11, also impart acceptable characteristics to the target polypropylene fiber in terms of withstanding high speed spinning tensions; however, apparently due to poor dispersion of NA-11 in polypropylene and the large and varied crystal sizes of NA-11 within the fiber itself, the performance is less consistent than for the highly soluble, low crystal-size polypropylene produced by well-dispersed 3,4-DMDBS or, preferably, p-MDBS.  
           [0017]    It has been determined that the nucleator compounds that exhibit good solubility in the target molten polypropylene resins (and thus are liquid in nature during that stage in the fiber-production process) provide more effective fiber properties for withstanding high speed spinning tension levels. Thus, substituted DBS compounds (including DBS, 3,4-DMDBS, and, preferably p-MDBS) appear to provide fewer manufacturing issues as well as lower shrink properties within the finished polypropylene fibers themselves. Although 3,4-DMDBS is preferred for such low denier fibers, any of the above-mentioned nucleators may be utilized within this invention. Mixtures of such nucleators may also be used during processing in order to provide such spinning efficiencies and low-shrink properties as well as possible organoleptic improvements, facilitation of processing, or cost.  
           [0018]    In addition to those compounds noted above, sodium benzoate and NA-11 are well known as nucleating agents for standard polypropylene compositions (such as the aforementioned plaques, containers, films, sheets, and the like) and exhibit excellent recrystallization temperatures and very quick injection molding cycle times for those purposes. The dibenzylidene sorbitol types exhibit the same types of properties as well as excellent clarity within such standard polypropylene forms (plaques, sheets, etc.). For the purposes of this invention, it has been found that the dibenzylidene sorbitol types are preferred as nucleator compounds within the target polypropylene fibers.  
           [0019]    The closest prior art references teach the addition of nucleator compounds to general polypropylene compositions (such as in U.S. Pat. No. 4,016,118, referenced above). However, some teachings include the utilization of certain DBS compounds within limited portions of fibers in a multicomponent polypropylene textile structure. For example, U.S. Pat. Nos. 5,798,167 to Connor et al. and 5,811,045 to Pike, both teach the addition of DBS compounds to polypropylene in fiber form; however, there are vital differences between those disclosures and the present invention. For example, both patents require the aforementioned multicomponent structures of fibers. Thus, even with DBS compounds in some polypropylene fiber components within each fiber type, the shrink rate for each is dominated by the other polypropylene fiber components which do not have the benefit of the nucleating agent. Also, there are no thick lamellae that can potentially provide the desired high tensile strengths formed within the disclosed polypropylene fibers. Of further importance is the fact that, for instance, Connor et al. require a nonwoven polypropylene fabric laminate containing a DBS additive situated around a polypropylene internal fabric layer which contained no nucleating agent additive. The internal layer, being polypropylene without the aid of a nucleating agent additive, dictates the shrink rate for this structure. Furthermore, the patentees do not discuss any high speed spinning possibilities for any low denier fibers at all.  
           [0020]    In addition, Spruiell, et al,  Journal of Applied Polymer Science,  Vol. 62, pp. 1965-75 (1996), reveal using a nucleating agent, MDBS, at 0.1%, to increase the nucleation rate during spinning. However, after crystallizing and drawing the fiber, Spruiell et al. do not expose the nucleated fiber to any heat, which is necessary to impart the very best shrinkage properties, therefore the shrinkage of their fibers was similar to conventional polypropylene fibers without a nucleating agent additive. Furthermore, no mention of tensile strength increases are discussed at all, not to mention at levels that are necessary to withstand high speed spinning tensions to prevent breakage of such fibers during processing thereby.  
           [0021]    Furthermore, such fibers may also be colored to provide other aesthetic features for the end user. Thus, the fibers may also comprise coloring agents, such as, for example, pigments, with fixing agents for lightfastness purposes. For this reason, it is desirable to utilize nucleating agents that do not impart visible color or colors to the target fibers. Other additives may also be present, including antistatic agents, brightening compounds, clarifying agents, antioxidants, antimicrobials (preferably silver-based ion-exchange compounds, such as ALPHASAN® antimicrobials available from Milliken &amp; Company), UV stabilizers, fillers, and the like. Furthermore, any fabrics made from such inventive fibers may be, without limitation, woven, knit, non-woven, in-laid scrim, any combination thereof, and the like. Additionally, such fabrics may include fibers other than the inventive polypropylene fibers, including, without limitation, natural fibers, such as cotton, wool, abaca, hemp, ramie, and the like; synthetic fibers, such as polyesters, polyamides, polyaramids, other polyolefins (including non-low-shrink polypropylene), polylactic acids, and the like; inorganic fibers such as glass, boron-containing fibers, and the like; and any blends thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate a potentially preferred embodiment of producing the inventive low-shrink polypropylene fibers and together with the description serve to explain the principles of the invention wherein:  
         [0023]    [0023]FIG. 1 is a schematic of the potentially preferred method of producing low denier polypropylene fibers through high speed spinning machinery. 
     
    
     DETAILED DESCRIPTION OF THE DRAWING AND OF THE PREFERRED EMBODIMENT  
       [0024]    [0024]FIG. 1 depicts the non-limiting preferred procedure followed in producing the inventive low denier polypropylene fibers. The entire fiber production assembly  10  comprises an extruder  11  including a metering pump (not illustrated) for introduction of specific amounts of polymer into the extruder  11  (to control the denier of the ultimate target manufactured fiber and/or yarn) which also comprises five different zones  12 ,  14 ,  16 ,  18 ,  20  through which the polymer (not illustrated) passes at different, increasing temperatures. The molten polymer is mixed with the nucleator compound (also molten) within a mixer zone  22 . Basically, the polymer (not illustrated) is introduced within the fiber production assembly  10 , in particular within the extruder  11 . The temperatures, as noted above, of the individual extruder zones  12 ,  14 ,  16 ,  18 ,  20  and the mixing zone  22  are as follows: first extruder zone  12  at 205° C., second extruder zone  14  at 215° C., third extruder zone  16  at 225° C., fourth extruder zone  18  at 235° C., fifth extruder zone  20  at 240° C., and mixing zone  22  at 245° C. The molten polymer (not illustrated) then moves into a spinneret area  24  set at a temperature of 250° C. for strand extrusion. All such temperatures may be modified as needed, and these levels are non-limiting and simply potentially preferred. The fibrous strands  28  then pass through an air-blown treatment area  26  and then through a treatment area  29  whereupon a lubricant, such as water or an oil, is applied thereto the strands  28 . The strands  28  are then collected into a bundle  30  via a take-up roll  32  to form a multifilament yarn  33  which then passes to a series of tensioning rolls  34 ,  36  prior to drawing. The yarn  33  then passes through a series of two different sets of draw rolls  38 ,  40 ,  42 ,  44  which increase the speed of the collected finished strands  33  as compared with the speed of the initially extruded strands  28 . The finished strands  33  extend in length due to a greater pulling speed in excess of such an initial extrusion speed within the extruder  11 . The strands  33  are then passed through a series of relax rolls  46 ,  48  and ultimately to a winder  50  for ultimate collection on a spool (not illustrated). The speed of the winder  50  ultimately dictates the speed and efficiency of the entire apparatus in terms of permitting high speed manufacturing and spinning (drawing) with minimal, if any, breakage of the target fibers during such a procedure. The draw rolls are heated to a very low level as follows: first draw rolls  38 ,  40  68° C. and the second set of draw rolls  42 ,  44  88° C., as compared with the remaining areas of high temperature exposure as well as comparative fiber drawing processes. The draw rolls  38 ,  40 ,  42 ,  44  individually and, potentially independently rotate at a speed of from about 1000 meters per minute to as high as about 5000 meters per minute. The second draw rolls  42 ,  44  generally rotate at a higher speed than the first in excess of about 800 meters per minute up to 1000 meters per minute over those of the first set.  
       Inventive Fiber and Yarn Production  
       [0025]    The following non-limiting examples are indicative of the preferred embodiment of this invention:  
         [0026]    68 Filament Yarns  
         [0027]    Yarns were produced at a draw ratio of 3.5× using a 68 filament spinneret with a control resin, Amoco  7550  (an 18 Melt Flow Index homopolymer polypropylene resin). Inventive samples were made by compounding the given amount of each additive in the control resin with the addition of 500 ppm of Irganox® 1010, 1000 ppm of Irgafos® 168 (both antioxidants available from Ciba), and 800 ppm of calcium stearate. The fiber line was configured with the roll speeds set such that the relax roll speed divided by the feed roll speed was equal to 3.5. With the line running, the throughput of the polymer melt metering pump was slowly lowered giving lower yarn deniers until the target yarn broke. The throughput of the metering pump was then slightly increased incrementally until a yarn sample could be produced without breaking. The yarn samples produced were considered to have the minimum yarn denier possible for the given resin types under the given processing conditions. The minimum denier difference between the resin samples was most remarkable at the higher relax roll speeds. At 2380 m/min relax speeds, the minimum denier per filament for the control sample was 1.54 g/9000 m vs. 0.80 g/9000 m for M3940 at 2750 ppm and 0.86 g/9000 m for M3988 at 2500 ppm. Any breakage was considered a failure, thus the measurements below all reflect the maximum spinning speeds for the lowest possible deniers of unbroken fibers. Any higher speeds or lower deniers and the target fibers broke.  
         [0028]    The results are tabulated below:  
                                                           TABLE 1                           Samples Fibers and Measurements (3.5 Draw Ratio)                    Relax   Minimum                   Roll   Yarn   Minimum       Sample       Speed   Denier   DPF       #   Nucleator (ppm)   (m/min)   (g/9000 m)   (g/9000 m)                    1   Control   1211   56.4   0.83       2   Control   1795   72.1   1.06       3   Control   2380   104.5   1.54       4   p-MDBS (2750 ppm)   1211   63.2   0.93       5   p-MDBS (2750 ppm)   1795   61.7   0.91       6   p-MDBS (2750 ppm)   2380   54.3   0.80       7   p-MDBS (1450 ppm)   1211   49.5   0.73       8   p-MDBS (1450 ppm)   1795   62   0.91       9   p-MDBS (1450 ppm)   2380   69.5   0.94       10   3,4-DMDBS (2500 ppm)   1211   55.2   0.81       11   3,4-DMDBS (2500 ppm)   1795   54.3   0.80       12   3,4-DMDBS (2500 ppm)   2380   58.6   0.86                  
 
         [0029]    This same experiment was repeated at a draw ratio of 2.5× with the following results  
                                                           TABLE 2                           Sample Fibers and Measurements (2.5 Draw Ratio)                    Relax    Minimum                   Roll   Yarn   Minimum       Sample       Speed   Denier   DPF       #   Nucleator (ppm)   (m/min)   (g/9000 m)   (g/9000 m)                    13   Control   1000   57.5   0.85       14   Control   1563   63.6   0.94       15   Control   2125   56.2   0.83       16   p-MDBS (2750 ppm)   1000   45.7   0.67       17   p-MDBS (2750 ppm)   1563   30.6   0.45       18   p-MDBS (2750 ppm)   2125   39.9   0.59       19   3,4-DMDBS (2500 ppm)   1000   42.7   0.63       20   3,4-DMDBS (2500 ppm)   1563   32.1   0.47       21   3,4-DMDBS (2500 ppm)   2125   32.6   0.48                  
 
         [0030]    At a draw ratio of 2.5×, lower minimum denier yarns were produced with the resin compounded with the nucleators than for the control resin at all relax roll speeds.  
         [0031]    140 Filament Yarns  
         [0032]    Additional work was performed to determine the effect of PP nucleating agents on minimum yarn denier using high speed fiber spinning equipment using a 140 filament spinneret. Initially, a theoretical minimum yarn denier in view of spinning speed was first determined (at a draw ratio of 3.5×). The control resin was Basell PDC 1302 HPP (compounded with and without nucleators with 1000 ppm calcium stearate in all samples). The minimum polymer melt throughput needed at take-up roll speeds of 1500, 2000, 2500, and 3000 m/min to give high quality yarn at the take-up position (POY) was first determined and considered the theoretical minimum denier with maximum spinning speeds. Once determined, the maximum roll speeds for the two series of draw rolls were determined by increasing incrementally until the speed was determined just below the speed where the yarn broke. The relax roll speeds and the winder speed were determined by adjusting the speeds to reach relax and winding tensions of ˜20-30 grams of force. Any breakage was considered a failure, thus the measurements below all reflect the maximum spinning speeds for the lowest possible deniers of unbroken fibers. Any higher speeds or lower deniers and the target fibers broke. The results are tabulated below:  
                                                           TABLE 3                           Sample Fibers and Measurements (140 Filaments)                    Relax   Minimum                   Roll   Yarn   Minimum       Sample       Speed   Denier   DPF       #   Nucleator (ppm)   (m/min)   (g/9000 m)   (g/9000 m)                    22   Control   3900   52.7   0.38       23   Control   4600   113.5   0.81       24   p-MDBS (3000 ppm)   4200   50.2   0.36       25   p-MDBS (3000 ppm)   4900   64.0   0.46       26   3,4-DMDBS (2650 ppm)   3250   62.0   0.44       27   3,4-DMDBS (2650 ppm)   4745   85.0   0.61                  
 
         [0033]    Once again, we see a marked decrease in the minimum yarn denier with the nucleated samples, significantly different for higher spinning speeds as well.  
         [0034]    Taking the results for these low denier high-speed-spun fibers, a Cartesian graphical equation has been generated in terms of spinning speed over fiber denier. Thus, the inventive fiber can also be defined in these terms via the following equation: y=379.9x+559.45, again with y being the winding speed (rn/min) and x being the denier.  
         [0035]    There are, of course, many alternative embodiments and modifications of the present invention which are intended to be included within the spirit and scope of the following claims.
Improvements in permitting greater efficiency for low denier polypropylene fiber and yarn production are provided. Generally, spinning speeds are limited for polypropylene fibers and yarns as such materials tend to break easily upon exposure to excessively high tensions associated with low- to medium-spinning speeds. Low production speeds negatively impact the economics of producing such low denier fibers which prevents the widespread utilization of such fibers and yarns in various end-use applications, particularly applications for which low denier provides desirable hand characteristics. Thus, it has surprisingly been determined that such low denier manufactured fibers and yarns can be produced with certain nucleating additives that permit high tension levels in the quench stack as required for high-speed spinning procedures to be followed. Additionally, low-shrink and/or better resiliency properties are also available with the addition of such nucleating compounds within the target low denier polypropylene resins.