Abstract:
In a non-friction texturing process, the filaments of a thermoplastic polymer yarn are heated along spaced zones to form a latent crimp in the filament. The filaments are then drawn and are subsequently heated above the glass transition temperature of the polymer while the filaments are under low enough tension to allow the crimp to form. The heating along spaced zones is preferably accomplished by passing the filaments over a heated rotating grooved roll.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of copending U.S. application Ser. No. 434,314, filed Jan. 17, 1974, now U.S. Pat. No. 3,949,041, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a crimped continuous filament yarn, having enhanced bulk level, and for a process of making such yarn. More particularly, this invention relates to a process at high speed giving excellent crimp uniformity and regularity. 
     In my patent application Ser. No. 434,314 an undrawn or partially drawn yarn is guided over a heated grooved roll allowing a specific contact time, which is related to the filament denier, and then drawn. The combination of heating for a critical time and draw produces the latent crimp. This phenomenon is discussed in the above application on page 10, lines 1-19. 
     SUMMARY OF INVENTION 
     Whereas, in the above application crimp is produced at a rather high draw ratio, the present invention deals with crimp at low draw ratios of melt-oriented yarns having specific orientation properties. While at higher draw ratios (1.05 and up) latent crimp can be produced by this method in unoriented yarns having extremely low birefringence. At lower draw ratios, latent crimp is produced only if the yarn has a certain amount of pre-orientation before heating for a critical time and subsequently drawing it. 
     It is believed that in a partially oriented fiber, such as high speed melt-oriented fiber, the crystallization process has already started and is in a stage, where rate of crystallization can be very fast at certain temperatures. Crystallization rate is further enhanced by minor amounts of tension and stretch. This explains why a completely amorphous and unoriented fiber, where crystal nucleation has not yet started, does not respond at very low levels of elongation and tension to form a latent crimp by this method. A further distinction in this invention is the fact that the yarn is stretched without the help of pairs of goudet rolls. The tension over the heated groove roll is sufficient to stretch the yarn to low draw ratios after it leaves the roll, as subsequently described. 
     The grooved roll can be replaced by a smooth heated roll, in which case the regular periodic crimp is converted into a spiral crimp. Critical contact time on the roll with regard to crimp intensity, however, is the same. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIGS. 1 to 5 are illustrated and described in said copending application Ser. No. 434,314, now U.S. Pat. No. 3,949,041. 
     FIG. 6 is a diagrammatic view of a prior art filament spinning process. 
     FIG. 7 is a diagrammatic view of apparatus for practicing the process of the invention. 
     FIG. 8 is a further diagrammatic view of apparatus for practicing the process of the invention. 
     FIG. 9 is a curve showing birefringence and elongation interrelation based on data from Table 11. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention which may be embodied in other specific structure. The scope of the invention is defined in the claims appended hereto. 
     FIG. 6 of the drawings shows the unmodified spinning line. having a spinnerette 19, a quench zone 26 and an air jet drawing filaments down and depositing them on a screen. This technique has been thoroughly described in U.S. Pat. No. 3,692,618 by Dorschner et al. FIG. 7 shows the improvement of this invention. 
     The air jet 20 is pulling filaments off the spinnerette 19 over a grooved roll A (FIG. 4) or a smooth heated roll A and a cold guide roll B. At high spinning speeds, molecular orientation is imparted in the quench zone 26. Heating the filaments over roll A for a specific time causes the filaments to yield or stretch to some extent between roll A and B, which results in the formation of a latent crimp, which can then be developed by heating the filaments above their glass transition temperature. The extent of yield or stretch between the freely rotating rolls A and B is measured by determining the rotations per minute (RPM) of the rolls with a stroboscope. From the RPM and the roll diameter, the yarn speed can be calculated, and from the yarn velocity and the spinning rate, the denier and denier per filament (dpf) can be calculated. Denier can also be measured on the product deposited on the screen 30. Thus, roll B is not an essential part of this invention with regard to forming latent crimp, but is used to conveniently determine stretch ratios which is necessary to understand the important parameters of this invention. 
     If a draw jet is used to draw down the yarn, yarn speed is determined by the interaction of spinneret temperature, quench efficiency, spinning rate and jet air pressure. In FIG. 8, speed of roll C determines positively the velocity of the filament as it leaves roll A. The stretch between rolls A and B is again measured with a stroboscope. 
     FIGS. 2 and 3 show the position of the filaments over the grooved roll A. FIG. 4 is a perspective view of the grooved roll and FIG. 5 shows schematically the relation of velocity, denier, roll speed and size, and stretch. 
     The yarn 2 is coming onto the grooved roll with the velocity V 1  and denier d 1 . The roll turns with the velocity V 1  on the surface. The contact time on the surface of the grooved roll is determined by the velocity V 1 , the roll radius R, and the wrap angle α, at point 6 the yarn is drawn off and yield to a velocity V 2 , and the denier is consequently reduced to d 2 . 
     To produce a latent crimp in a crystallizable synthetic fiber, it has been found that the contact time on the grooved roll has to be within a critical range. The critical contact time is dependent on the dpf: the higher the dpf, the longer the contact time has to be. The examples show that contact time t should be within 0.00001 and 0.001 times the dpf in seconds. It is further necessary that the temperature of the roll A be above a certain limit. The upper limit is determined by the point where the roll causes melting of the yarn. Furthermore, it was found that the filaments have to have a minimum of molecular orientation as measured by the fiber birefringence in order to develop crimp at the low stretch ratios used in this invention. Further details will become apparent from the following examples: 
     EXAMPLE 1 
     In this example, polypropylene of melt flow rate 35 (Exxon CD 523) was extruded through a 32 hole spinnerette. Quench air of 70°F and 0.8 m/sec. velocity was used. The spinning rate and air jet pressure was changed as indicated to give a variety of yarn speeds, deniers and roll A contact times. Best crimp development was observed at a t/dpf value of about 10 -   4  seconds. Crimping efficiency was evaluated by examining the crimp developed upon heating in an oven at 160°C for 30 seconds. The yarn was freely suspended and under no tension. During this heating, shrinkage of the filaments as well as contraction due to the developed crimped occurred. &#34;Texturing intensity&#34; or &#34;Crimp development&#34; was rated by measuring the extended length of the heated yarn sample, and then letting the yarn contract back into its crimped state while under a tension of 0.01 gram per denier. Contraction is then calculated as percent of extended length. Crimp development is defined as follows: 
     0 = no crimp development 
     1 = very slight crimp, crimped length = 99-95% of extended length 
     2 = slight crimp, 90 -95% 
     3 = marginal crimp, 80 -90% 
     4 = good crimp, 65 -80% 
     5 = excellent crimp, less than 65% 
     EXAMPLE 2 
     In this example the jet pressure was varied at constant spinning rate to result in various degrees of stretch at the optimum t/dpf range. At very low levels of stretch, crimp development is less intense. Extrusion and spinning conditions were identical to example 1, Exxon polypropylene CD 523 was used as in Example 1. Groove distance of roll A in Example 1 and 2 was 230 micron. Roll diameter of roll A and B = 1 inch. 
     EXAMPLE 3 
     This example is a rerun of Example 1, but using as roll A a smooth roll with a surface finish of 20 micro inches. Optimum crimp development was in the same range as in Example 1 with regard to t/dpf. 
     EXAMPLE 4 
     In this example the temperature of roll A (1 inch diameter, grooves at 230 micron distance) was varied from room temperature to 700°F. There was no stretch or crimp development at room temperature; crimp development became noticeable at 150°F. Polymer and spinning conditions were identical to Example 1. 
     EXAMPLE 5 
     Polymer and extrusion conditions were as in Example 1; spinnerette temperature and quench air temperature were changed to result in filaments of varying degrees of birefringence, at higher spinnerette and quency air temperature yarn velocity increased and denier decreased, however, the t/dpf value remained in the optimum range. Crimp development became very low at less than 0.0055 birefringence. 
     Examples 1-5 were run with the spinning arrangement as indicated in FIG. 6 using polypropylene; Examples 6-8 were run using the system as shown in FIG. 7. Polyethylene therephaphate of 0.65 intrinsic viscosity was used as the polymer. The extrusion spinnerette had 48 holes of 0.020 inches diameter. 
     Roll A = grooves at 230 micron distance 
     EXAMPLE 6 
     This example is the equivalent of Example 1 for polyester, establishing the same optimum range for t/dpf. 
     EXAMPLE 7 
     This example is the equivalent of Example 4 for polyester, showing the effect of roll A temperature in crimp development. 
     EXAMPLE 8 
     This example is the equivalent of Example 5 for polyester; quench air temperature was changed to result in filaments of different birefringence. At very low birefringence, no crimp development is seen. 
     EXAMPLE 1 
     
         __________________________________________________________________________Experiment No:      1    2    3    4    5    6    7    8__________________________________________________________________________SpinneretteTemperature, °F      550Spinning Rate,gram/min   40.4 27.0 13.3      141.0                               13.8 5.02 2.13Quench airtemperature °F      70Yarn velocity(roll &#34;A&#34;) m/min      3931 1859 2344 3931 5484 2602 1016 362.7Yarn velocity(roll &#34;B&#34;) m/min      4004 1896 2387 3969 5646 2660 1073 362.7Roll A, RPM      49140           23240                29300                     49140                          68550                               32520                                    12700                                         4177Roll B, RPM      50050           23700                29840                     49620                          70580                               33250                                    13410                                         4534% stretch  1.85 1.97 1.84 0.97 2.96 2.24 5.59 8.54Roll ATemperature °F      450Draw jet air psi      25   30   25   25   41   25   18   12denier perfilament(on roll &#34;A&#34;)(&#34;dpf&#34;)    2.89 4.08 1.60 2.89 7.23 1.49 1.39 1.79wrap angle(over roll &#34;A&#34;),degree     60   60   30   30   30   60   90   90contact length, cm      1.33 1.33 0.66 0.66 0.66 1.   2.00 2.00contact time,sec. .sup.. 10.sup.-.sup.4 (&#34;t&#34;)      2.03 4.29 1.69 1.01 0.72 3.07 11.81                                         35.9t/dpf .sup.. 10.sup.-.sup.4      0.70 1.05 1.05 0.35 0.10 2.06 8.50 20.01Birefringence (Δn)      0.0090           .0070                .0080                     .0090                          .0100                               .0080                                    .0075                                         .0065CrimpDevelopment      4    5    5    3    1    3    1    0__________________________________________________________________________ 
    
     EXAMPLE 2 
     
         __________________________________________________________________________Experiment No:  1    2    3    4    5    6__________________________________________________________________________Spinnerette Temperature, °F.           550→Spinning Rate, gram/min           32.0 32.0 32.0 32.0 32.0 32.0Quench air temperature °F           70Yarn velocity (roll &#34;A&#34;)m/min           6049 5287 4221 3130 2510 2046Yarn velocity (roll &#34;B&#34;)m/min           6358 5467 4328 3167 2533 2054Roll A, RPM     75610                66090                     52760                          39120                               31380                                    25580Roll B, RPM     79480                68350                     54100                          39580                               31670                                    25680% stretch       5.11 3.41 2.53 1.17 0.92 0.39Roll A Temperature °F           450→Draw jet air, psi           60   51   40   25   18   12denier per filament(on roll &#34;A&#34;) (dpf)           1.49 1.70 2.13 2.88 3.59 4.40wrap angle (over roll&#34;A&#34;) (degree)   60→contact length, cm           1.33→contact time,sec. .sup.. 10.sup.-.sup.4 (&#34;t&#34;)           1.32 1.51 1.89 2.54 3.18 3.90t/dpf .sup.. 10.sup.-.sup.4, sec.           0.89→Birefringence (Δ n)           0.0150                0.120                     .0100                          .0095                               .0080                                    .0065Crimp Development           5    4    4    2    1    0__________________________________________________________________________ 
    
     EXAMPLE 3 
     
         __________________________________________________________________________Experiment No:  1    2    3    4__________________________________________________________________________Spinnerette Temperature, °F           550→Spinning Rate, gram/min           41.0 28.7 139.0                          5.10Quency air temperature °F           70→Yarn velocity (roll &#34;A&#34;)m/min           3854 2040 5484 1022Yarn velocity (roll &#34;A&#34;)m/min           3930 2078 5640 1077Roll A, RPM     48180                25500                     68550                          12775Roll B, RPM     49120                25980                     70500                          13460% stretch       1.95 1.88 2.85 5.40Roll A temperature °F           450→Draw jet air, psi           25   30   40   17denier per filament(on roll &#34;A&#34;) (&#34;dpf&#34;)           2.99 3.96 7.12 1.40wrap angle (over roll &#34;A&#34;)(degree)        60   60   30   90contact length, cm           1.33 1.33 0.66 2.00contact time, sec. .sup.. 10.sup.-.sup.4 (&#34;t&#34;)           2.07 3.91 0.72 11.74t/dpf .sup.. 10 .sup.-.sup.4           0.69 0.99 0.10 8.39Birefringence (Δ n)           .0090                .0070                     .0100                          .0075Crimp Development           4    5    1    0__________________________________________________________________________ 
    
     EXAMPLE 4 
     
         __________________________________________________________________________Experiment No:  1    2    3    4    5    6__________________________________________________________________________Spinnerette Temperature, °F           500→Spinning Rate, gram/min           27.6→Quench air temperature °F           70→Yarn velocity (roll &#34;A&#34;)m/min           2510→Yarn velocity (roll &#34;B&#34;)m/min           2510 2511 2518 2525 2574 2624Roll A, RPM     31380→Roll B, RPM     31380                31390                     31470                          31560                               32170                                    32800% stretch       0    0.03 0.28 0.57 2.51 4.52Roll A Temperature °F           70   100  150  200  400  700Draw jet air, psi           50→denier per filament(on roll &#34;A&#34;) (&#34;dpf&#34;)           3.10→wrap angle (over roll&#34;A&#34;) (degree)   60→contact length, cm           1.33→contact time, sec. .sup.. 10.sup.-.sup.4(&#34;t&#34;)           3.18→t/dpf .sup.. 10 .sup.-.sup.4           1.03→Birefringence (Δ n)           0.0090                .0090                     .0085                          .0080                               .0075                                    .0070Crimp Development           0    0    1    2    3    5__________________________________________________________________________ 
    
     EXAMPLE 5 
     
         __________________________________________________________________________Experiment No:  1    2    3    4__________________________________________________________________________Spinnerette Temperature, °F           500  550  600  650Spinning Rate, gram/min           23.6Quench air temperature °F           50   200  400  600Yarn velocity (roll &#34;A&#34;)m/min           1626 1854 2688 4016Yarn velocity (roll &#34;B&#34;)m/min           1671 1902 2773 4159Roll A, RPM     20324                23180                     33600                          50200Roll B, RPM     20890                23770                     34660                          51990% stretch       2.80 2.54 3.15 3.56Roll A Temperature °F           450Draw jet air, psi           40   40   35   30denier per filament(on roll &#34;A&#34;) (&#34;dpf&#34;)           4.08 3.58 2.47 1.65wrap angle (over roll &#34;A&#34;)(degree)        60contact length, cm           1.33contact time, sec. .sup.. 10.sup.-.sup.4 (&#34;t&#34;)           4.91 4.30 2.97 1.99t/dpf .sup.. 10 .sup.-.sup.4           1.20 1.20 1.20 1.20Birefringence (Δ n)           0.0120                .0100                     .0055                          .0012Crimp Development           4    2    1    0__________________________________________________________________________ 
    
     EXAMPLE 6 
     
         __________________________________________________________________________Polyester, 48 hole spinneretteExperiment No:      1    2    3    4    5    6    7__________________________________________________________________________SpinneretteTemperature, °F      610→Spinning Rategram/min   211.5           106.3                91.4 64.0 56.7 42.5 17.1Quench airtemperature, °F      70→Yarn velocity(roll &#34;A&#34;) m/min      6200 5500 4450 4120 3510 3200 3020Yarn velocity(roll &#34;B&#34;) m/min      6231 5549 4504 4182 3577 3289 3164Roll A and Bdiameter, inches      1    1    1    1    1    3    3Roll A, RPM      77500           68750                55620                     51500                          43880                               1333 1258Roll B, RPM      77890           69370                56300                     52270                          44710                               1373 1318Roll C, m/min      6232 5550 4506 4184 3580 3290 3168% stretch  0.5  0.9  1.2  1.5  1.9  2.8  4.8Roll A Tempera-ture °F      600→denier perfilament(on roll &#34;A&#34;)(&#34;dpf&#34;)    6.39 3.63 3.85 2.91 3.03 2.49 1.06wrap angle(over roll &#34;A&#34;)      30   60   90   90   160  160  160contact length,cm         0.66 1.33 2.00 2.00 3.55 10.64                                    10.64contact time,sec. .sup.. 10.sup.-.sup.4 (&#34;t&#34; )      0.64 1.45 2.70 2.91 6.07 19.95                                    21.14t/dpf .sup.. 10 .sup.-.sup.4      0.10 0.40 0.70 1.0  2.0  8.0  20.0Birefringence      0.0120           .0120                .0100                     .0100                          .0100                               .0090                                    .0090(Δ n)Crimp Develop-ment       1    1    3    5    2    1    0__________________________________________________________________________ 
    
     EXAMPLE 7 
     
         __________________________________________________________________________Polyester, 48 hole spinnerette__________________________________________________________________________Experiment No:  1    2    3    4__________________________________________________________________________Spinnerette Temperature, °F           610→Spinning Rate, gram/min           64.0→Quench air temperature, °F           70→Yarn velocity (roll &#34;A&#34;)m/min           4120 4120 4120 4120Yarn velocity (roll &#34;B&#34;)m/min           4182 4166 4128 4120Roll A and B diameter,inches          1    1    1    1Roll A, RPM     51500                51500                     51500                          51500Roll B, RPM     52270                52070                     51600                          51500Roll C, m/min   4184 4170 4130 4124% stretch       1.5  1.1  0.2  0Roll A Temperature °F           600  400  200  70denier per filament(on roll &#34;A&#34;) (&#34;dpf&#34;)           2.91→wrap angle (over roll &#34;A&#34;)           90→contact length, cm           2.00→contact time, sec. .sup.. 10.sup.-.sup.4(&#34;t&#34;)           2.91→t/dpf .sup.. 10 .sup.-.sup.4           1.00→Birefringence (Δ n)           0.010→Crimp Development           5    2    1    0__________________________________________________________________________ 
    
     EXAMPLE 8 
     
         __________________________________________________________________________Polyester, 48 hole spinnerette__________________________________________________________________________Experiment No:  1    2    3    4__________________________________________________________________________Spinnerette Temperature, °F           610→Spinning Rate, gram/min           64.0→Quench Air temperature, °F           70   200  400  600Yarn velocity (roll &#34;A&#34;)m/min           4120 4108 4095 4084Yarn velocity (roll &#34;B&#34;)m/min           4182 4182 4182 4182Roll A and B diameter,inches          1    1    1    1Roll A, RPM     51500                51340                     51190                          51040Roll B, RPM     52270                52270                     52270                          52270Roll C, m/min   4185 4185 4185 4185% stretch       1.5  1.8  2.1  2.4Roll A temperature °F           600→denier per filament(on roll &#34;A&#34;) (&#34;dpf&#34;)           2.91 2.92 2.93 2.94wrap angle (over roll &#34;A&#34;)           90→contact length, cm           2.00→contact time, sec..sup.. 10 .sup.-.sup.4 (&#34;t&#34;)           2.91 2.92 2.93 2.94t/dpf .sup.. 10 .sup.-.sup.4           1.00 1.00 1.00 1.00Birefringence (Δ n)           0.0100                .0080                     .0045                          .0025Crimp Development           5    4    1    0__________________________________________________________________________ 
    
     EXAMPLE 9 
     The purpose of this example is to demonstrate the importance of the grooved roll contact time in relation to dpf (denier per filament) and texturing intensity for the polypropylene. Profax 6423, a product of Hercules Inc., was extruded at a spinnerette temperature of 280°C. Groove distance on the grooved roll was 230 microns. The draw ratio was 2.8. 
     
         __________________________________________________________________________Experiment    1    2    3    4     5     6__________________________________________________________________________Resin throughputg/min    2.2  4.4  44   37.7  75.4  70.9number offilaments    35   35   35   17    17    8dpf      15   15   15   15    30    60Feed rollspeed m/min    75.3 150.7              754  1330  1330  1330grooved rolldiameter (cm)    2.54 2.54 1.27 1.27  1.27  1.27wrap angle ofyarn (degree)    170  170  170  60    60    60contact time(seconds) &#34;t&#34;    0.030         0.015              0.0015                   0.0003                         0.0003                               0.0003t/dpf. 10.sup.4    20   10   1.0  0.2   0.1   0.05    0.002         0.001              0.0001                   0.00002                         0.00001                               0.000005Texturingintensity    0    1    5    4     2     0__________________________________________________________________________ 
    
     According to this table the workable t/dpf range lies between 0.002 and 0.00002 seconds. 
     EXAMPLE 10 
     The experiment number 3, of Example 9 was repeated, with the exception of the grooved roll temperature, which was varied in this series. 
     
                       TABLE 10______________________________________Grooved rolltemperature (°C)       70     100    150  200  250  280Texturingintensity   0      2      5    5    5    --(yarn                                    melting                                    on roll)______________________________________ 
    
     For polypropylene, the grooved roll temperature should be above 100 degrees Centigrade, but lower than 280°C to avoid melting of filaments. 
     EXAMPLE 11 
     The previous Examples, 1 and 8 showed that there is a relation between birefringence and stretch in regard to texturing intensity. Especially yarns drawn over the grooved roll at very low percentages show a sensitivity to the degree of melt orientation as measured by birefringence. 
     Example 11 has been run to define accurately the limits of stretch and orientation necessary to produce an acceptable level of texture or crimp. 
     Polypropylene yarn of various degrees of melt orientation was produced as feed yarn for the drawing experiments described in the table below. The yarn (polymer as in Example 1) was not mechanically drawn, but merely would at different speeds to produce different levels of melt orientation and birefringence: a winding speed of 2000, 1200, 800, 300 and 100 meter/minute produced yarn of 0.0125, 0.0068, 0.0041, 0.0022 and 0.0009 birefringence as measured with an interference microscope according to the procedure described in an article by Heyn, Textile Research Journal, 22, 513 (1952). Yarns of 15 denier per filament, 35 filaments per bundle, were produced and fed to a yarn draw apparatus as shown in FIG. 5, capable to apply a fixed mechanical draw ratio. The grooved roll temperature was kept at 190°C, the feed roll speed at 754 meters/minute. A grooved roll of 1.27 cm diameter and 230 micron groove distance was used. The yarn wrap angle was 170°. Under these conditions, the t/dpf factor was constant for all experiments at 0.0001. 
     
                                           TABLE 11__________________________________________________________________________Birefringence(Δ n) .sup.. 10.sup.4        125  68   41   22   9% Stretch**/Texturingintensity    1.8/4             1.2/4                  2.2/4                       5.0/5                            20/5        1.0/4             1.0/4                  1.8/4                       2.5/4                            10/5        0.5/2             0.5/2                  1.2/2                       2.2/1                            6/2        0.3/1             0.4/1                  1.0/1                       1.8/0                            5.0/1        0.1/0             0.3/0                  0.5/0                       1.2/0                            4.0/0__________________________________________________________________________ **% stretch = (draw roll speed - feed roll speed) × 100 / feed roll speed 
    
     In Table 11, for column 125, 1.8 is the percent stretch and 4 represents the texturing intensity. 
     These data are plotted on FIG. 9. FIG. 9 is a plot of data from Table 11 on a log-log scale with the birefringence Δ N and % stretch. The data with texturing intensity of 2 to 5 are indicated as &#34;acceptable&#34; with a circle; the data with texturing intensity 0-1 as &#34;unacceptable&#34; with an X. The curve 31 dividing the acceptable and unacceptable range fits the equation: 
     birefringence = 0.012 /(2 × % stretch + 1) or 
     % stretch = (0.006/birefringence) -0.5 
     which has been found empirically. 
     This means that at low levels of stretch, the yarns have to have a minimum level of birefringence or molecular orientation which is approximately inversely proportional to stretch, in order to produce an acceptable level of texturing intensity. In other words, at very low levels of stretch, the yarn must have a critical amount of pre-orientation in order to crimp. At less than 0.3% stretch, no crimp occurs. 
     At low stretch ratios, birefringence is very critical. At higher stretch ratios, birefringence is not critical. Commercially, it is not feasible to make yarn with a birefringence of less than 0.001 because this would require very slow spinning speeds.