Abstract:
There is disclosed a novel apparatus and process for melt-blowing from fiberforming thermoplastic molten polymers to form fine fibers by extruding through orifices in nozzles the molten polymer at low melt viscosity at high temperatures where the molten fibers are accelerated to near sonic velocity by gas being blown in parallel flow through small orifices surrounding each nozzle. The extruded molten polymer is passed to the nozzles through a first heating zone at low incremental increases in temperature and thence rapidly through said nozzles at high incremental increases in temperature to reach the low melt viscosity necessary for high fiber acceleration at short residence time to minimize or prevent excessive polymer degradation.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to new melt-blowing processes for producing non-woven or spun-bonded mats from fiberforming thermoplastic polymers. More particularly, it relates to processes in which a thermoplastic resin is extruded in molten form through orifices of heated nozzles into a stream of hot gas to attenuate the molten resin as fibers, the fibers being collected on a receiver in the path of the fiber stream to form a non-woven or spun-bonded mat. Various melt-blowing processes have been described heretofore including those of Van A, Wente (Industrial and Engineering Chemistry, Volume 48, No. 8 (1956), Buntin et al. (U.S. Pat. No. 3,849,241), Hartmann (U.S. Pat. No. 3,379,811), and Wagner (U.S. Pat. No. 3,634,573) and others, many of which are referred to in the Buntin et al. patent. 
     Some of such processes, e.g. Hartmann, operate at high melt viscosities, and achieve fiber velocities of less than 100 m/second. Others, particularly Buntin et al. operate at lower melt viscosities (50 to 300 poise) and require severe polymer degradations to achieve optimum spinning conditions. It has been described that the production of high quality melt blown webs requires prior degradation of the fiber forming polymer (U.S. Pat. No. 3,849,241). At an air consumption of more than 20 lb. of air/lb. web substantially less than sonic fiber velocity is reached. It is known, however, that degraded polymer leads to poor web and fiber tensile strength, and is hence undesireable for many applications. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a novel apparatus and process for melt-blowing fiberforming thermoplastic polymers. 
     Another object of the present invention is to provide a novel apparatus and process for melt-blowing fiberforming thermoplastic polymers to form fine fibers. 
     A further object of the present invention is to provide a novel apparatus and process for melt-blowing fiberforming thermoplastic polymers to form fine fibers having a diameter of less than 2 microns. 
     Still another object of the present invention is to provide a novel apparatus and process for melt-blowing fiberforming thermoplastic polymers to form fine fibers exhibiting little polymer degradation. 
     A still further object of the present invention is to provide a novel apparatus and process for melt-blowing fiberforming thermoplastic polymers to form fine fibers with reduced air requirements. 
     Yet another object of the present invention is to provide a novel apparatus and process for melt-blowing fiberforming thermoplastic polymers to form fine fibers with improved economics. 
     SUMMARY OF THE INVENTION 
     These and other objects of this invention are achieved by extruding through orifices in nozzles the molten polymer at low melt viscosity at high temperatures where the molten fibers are accelerated to near sonic velocity by gas being blown in parallel flow through small orifices surrounding each nozzle. The extruded molten polymer is passed to the nozzles through a first heating zone at low incremental increases in temperature and thence rapidly through said nozzles at high incremental increases in temperature to reach the low melt viscosity necessary for high fiber acceleration at short residence time to minimize or prevent excessive polymer degradation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention as well as other objects and advantages thereof will become apparent upon consideration of the detailed disclosure thereof, especially when taken with the accompanying drawings, wherein like numerals designate like parts throughout; and wherein 
     FIG. 1 is a partially schematic cross-sectional elevational view of the die assembly for the melt blowing assembly of the present invention; 
     FIG. 2 is an enlarged cross-sectional view of the nozzle configuration for such die assembly, taken along the line 2--2 of FIG. 1; 
     FIG. 3 is another embodiment of a nozzle configuration; 
     FIG. 4 is an exploded view of the nozzle assembly; 
     FIG. 5 is a side elevational view of the nozzle assembly of FIG. 4; 
     FIG. 6 is an enlarged cross-sectional view taken along the lines 6--6 of FIG. 5; 
     FIG. 7 is a bottom view of a portion of the nozzle configuration of FIG. 4; 
     FIG. 8 is a cross-sectional side view of the nozzle configuration of FIG. 7; 
     FIG. 9 is a schematic drawing of the process of the present invention; and 
     FIG. 10 is a plot of Space mean Temperature versus the Fourier Number. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been found that fine fibers can be produced by the present invention which suffered very little thermal degradation by applying a unique heat transfer pattern, or time-temperature history at high resin extrusion rates. This is accomplished at a very low consumption of air per lb. of web, by having very small air orifices surrounding each polymer extrusion nozzle. By reducing the air orifice area per resin extrusion nozzle, higher air velocities can be achieved at low air consumption with concomitant considerable energy savings. 
     In order to produce very fine fibers by the melt-blowing process, it is necessary to reduce the resin extrusion per nozzle. This can be understood by the following considerations: Assuming that the maximum fiber velocity is sonic velocity (there has been no practical design exceeding this), than minimum fiber diameter is related to resin extrusion rate by the following equation: 
     
         D.sup.2 =4Q/πV,                                         (1) 
    
     wherein 
     D=fiber diameter, 
     Q=resin flow rate (cm 3  /sec.) and 
     V=fiber velocity. 
     To produce a 1 micron fiber at 550 meter/second, the resin extrusion rate can not exceed 0.023 cm 3  /minute/orifice. Since sonic velocity increases with temperature, the higher the air temperature, the lower the potential fiber diameter. It becomes obvious from the above, that, in order to produce fine microfibers economically, there have to be many orifices. Conventional melt-blowing systems have about 20 orifices/inch of die width. To reduce resin rate to the above mentioned level, means uneconomically low resin rate/extrusion die and a long resin residence time in the die causing unexceptably high resin degradation. 
     Heat transfer in cylindrical tubes is described by the basic Fourier equation as follows: ##EQU1## wherein T=Temperature in °C.; 
     r=radius in centimeters 
     t=time in seconds, and 
     a=thermal diffusivity. 
     Thermal diffusivity is calculated by the following equation: 
     
         a=η/cd (cm.sup.2 /sec),                                (3) 
    
     η=thermal conductivity (cal/°C.sec. cm 2  /cm) 
     c=heat capacity (cal/gram °C.) 
     d=density (gram/cm 3 ). 
     Referring now to FIG. 1, the die consists of a long tube 1 having a chamber connected to a thick plate 2 into which nozzles 3 are inserted through holes in plate 2, as shown, and silver soldered in position to prevent slipping and leaking. The tubes 3 extend through the air manifold 4 through square holes in the plate 5 in a pattern shown in FIG. 2. The four corners of the square 6 around the tubes 3 are the orifices through which air is blown approximately parallel to the fibers exiting tubes 3. The nozzle assembly consisting of plates 2 and 5 and nozzles 3 can be replaced with assemblies of different size nozzles and air orifice geometry (FIG. 3). The air manifold 4 is equipped with an air pressure gauge 8, thermocouple 9 and air supply tube 10 which in turn is equipped with an in line air flow meter 11 prior to the air heater 12. Some of the hot air exiting air heater 12 is passed through a jacket surrounding tube 1 to preheat the metal of the transition zone to the air temperature. The tubular die 1 is fed with hot polymer from an extruder 13. Tube 1 is equipped with three thermocouples 14, 15, 16 located 3 cm apart as shown. The thermocouples are jacketed and are measuring the polymer melt temperature rather than the steel temperature. A pressure transducer 17 measuring polymer melt pressure is located at cavity 18 near the spinning nozzle inlet. There is a resin bleed tube 19 and valve 20 to bypass resin from the extruder and thus reduce resin flow rate through the nozzles. By adjusting the bleed valve 20, different temperature and heat transfer patterns can be established in the tube section and nozzle zone. 
     Referring now to FIGS. 4 to 7, the die consists of a cover plate 22 and a bottom plate 23 into which half-circular grooves are milled to form a circular cross section resin transfer channel as shown in FIG. 5, Resin flowing from the extruder is fed into channel 24 and is divided into two streams in channels 25, which is divided into two channels 26 and again in channels 27, which lead to 8 holes 28 through plate 23. 
     The holes 28 lead to a cavity 29 feeding the nozzles 30 which mounted in the nozzle plate 31. The nozzles lead through the air cavity 32 which is fed by the inlet pipe 33. The nozzles 30 protrude through the holes of screen 35 mounted on the screen plate 34. The sides of the air cavity 32 are sealed by the side plates 36. The assembly is held together by bolts 37 (not all shown). FIG. 7 gives an enlarged sectional view of the nozzle and screen geometry, resin and air flow. FIG. 9 gives a perspective view of the total assembly. 
     FIG. 10 is a graph wherein &#34;Space mean Temperature&#34; (T m ) is plotted against the dimensionless &#34;Fourier Number&#34; (at/r 2 ). At constant radius (r), this shows the increase of temperature of a cylinder with time from the initial temperature T 1 , when contacted from the outside with the temperature T 2 . Although the basic heat transfer equation (2) covers only ideal situations and does not take into account influences of mixing temperature variations, boundary conditions and resin flow channel cross section variations, it has been found useful and a good approximation to describe process variables and design features. The dimensionless expression at/r 2 , which applies to fixed or motionless systems, can be converted into one applying for flowing systems such as polymer flow through die channels, when we consider that: 
     
         V.sub.p =l/t                                               (4) 
    
     and 
     
         A=Q/V.sub.p,                                               (5) 
    
     Since 
     
         A=πV.sup.2,                                             (6) 
    
     then 
     t=Al/Q, wherein 
     V p  =polymer flow velocity in channel length &#34;l&#34;, 
     t=residence time in channel of length &#34;l&#34;, 
     A=channel cross sectional area, and 
     Q=resin flow rate (volume/time) through A. 
     Then, 
     
         at/r.sup.2 =πal/Q (dimensionless terms)                 (7) 
    
     For non-cylindrical resin flow channels, the approximation r=2A/P is used, where P is the wetted perimeter. 
     EXAMPLES OF THE INVENTION 
     The following examples are included for the purpose of illustrating the invention and it is to be understood that the scope of the invention is not to be limited thereby. 
     For Examples 1 to 8, the apparatus of FIG. 1 is used equipped with the bleed tube 19 and bleed valve 20 whereby adjusting of the bleed valve 20, different temperature and heat transfer patterns can be independently established in the tube section (transition zone) and nozzle zone with the resulting effect observed and measured on spinning performance at various air volumes and pressures. 
     The die is a 12 cm. long tube 1 having a 0.3175 cm. inside diameter connected to a 0.1588 cm. thick plate 2 into which 16 nozzles 3 are inserted through holes in plate 2 and silver soldered into position to prevent slipping and leaking. The nozzles 3 extend through the air manifold 4 through square hole in the 0.1016 cm. thick plate 5 in a pattern, as shown in FIG. 2. The nozzles 3 are of Type 304 stainless steel and have an inside diameter of 0.03302 cm. and an outside diameter of 0.0635 cm. The squares in plate 5 are 0.0635 cm. in square and 0.1067 cm. apart from center to center. 
     EXAMPLE I 
     In this example, the length of the nozzles 3 is 1.27 cm. The total air orifice opening 6 around each nozzle is 0.086 mm 2 . The length of the nozzle segment 7 protruding through plate 5 is 0.2 mm. 
     The experiment was started at a low temperature profile using polyproplylene of melt flow rate 35 gram/10 min. resulting in a melt viscosity of 78 poise. Under these conditions, the air accelerated the fibers to 45 m/sec. The air temperature was increased to 700° and 750° F. (run b and c) resulting in a higher temperature profile and severe polymer degradation (reduced intrinsic viscosity of 0.3). Fiber acceleration was up to 510 m/sec. but was then increased from 8 to 16 and 20 cm 3  /min. which reduced the al/Q factor and residence time in tube 1. Run (f) had the lowest melt viscosity and highest fiber velocity at little thermal polymer degradation as seen from the following Tables 1 and 2: 
     
                       TABLE 1______________________________________run            (a)    (b)    (c)  (d)  (e)  (f)______________________________________total resin flow rate          8      8      8    16   20   20(cm.sup.3 /min) &#34;Q&#34;al/Q in tube die 1          0.150  0.150  0.150                             0.075                                  0.060                                       0.060residence time in          7.13   7.13   7.13 3.56 2.85 2.85tube die 1 (seconds)Temperature (°F.)at extruder exit          550    600    600  600  600  550at T.sub.1 (after 3 cm) (14)          610    660    690  675  668  650at T.sub.2 (after 6 cm) (15)          635    685    725  710  705  705at T.sub.3 (after 9 cm) (16)          645    695    740  730  725  740air temperature (9) in          650    700    750  750  750  775cavity 4resin flow rate through          0.5    0.5    0.5  1.0  1.25 1.25nozzle 3(cm.sup.3 /min/nozzle)al/Q in nozzle 3          0.254  0.254  0.254                             0.127                                  0.102                                       0.102residence time t(sec)          0.131  0.131  0.131                             0.066                                  0.053                                       0.053in nozzle 3resin pressure (psi)          410    163    47   158  223  144at gauge 17calculated apparent          78     31     9    15   17   11melt viscosity (poise) innozzle 3reduced intrinsic viscosity          1.3    0.8    0.3  1.1  1.3  1.1of fiber web______________________________________ 
    
     
                       TABLE 2______________________________________Fiber diameters at various air rates:                               Calculate                      Average fiber                               maximumrun  Air Volume Air Pressure                      diameter fiber velocity#    (gram/min) (psi)      (micron) (m/sec)______________________________________(a)  28         30         15       45(b)  9          10         13       6514         17         11       9021         21         9.5      12026         30         8.5      150(c)  9          10         6.5      25014         17         5.3      41021         21         5.0      45026         30         4.7      510(d)  9          10         12.3     15014         17         10.7     20021         21         8.1      35026         30         7.5      400(e)  9          10         14.8     13014         17         12.6     18021         21         9.0      34026         30         8.5      400(f)  9          10         9.0      35014         17         8.4      40021         21         8.0      45026         30         7.5      500______________________________________ 
    
     EXAMPLE 2 
     In this example, the resin flow rate from the extruder was set to give an al/Q factor of 0.06 in the tube 1, resulting in a low temperature profile at only 2.85 seconds residence time. This condition causes little thermal resin degradation in this section. The bleed valve 20 was then opened to reduce the resin flow rate in the nozzles and increase resident time. At 2.6 seconds nozzle resident time, thermal degradation was severe at 0.3 reduced intrinisc viscosity, the web had considerable amoutns of &#34;shot&#34;. Air pressure was 17 psi at gauge 8. The results are set forth in Table 3. 
     
                       TABLE 3______________________________________run #           (a)        (b)     (c)______________________________________total resin flow rate Q           20         20      20from extruder (cm.sup.3 /min)al/Q in tube die 1           0.060      0.060   0.060residence time t in tube           2.85       2.85    2.85die 1 (sec)Temperature (°F.)at extruder exit           600        600     600at T.sub.1 (after 3 cm) (14)           670        670     670at T.sub.2 (after 6 cm) (15)           705        705     705at T.sub.3 (after 9 cm) (16)           725        725     725air temperature 9 in           750        750     750cavity 4resin flow rate through bleed           18.4       19.2    19.6valve 20 (cm.sup.3 /min)resin flow rate Q through           0.1        0.05    0.025nozzle 3 (cm.sup.3 /min/nozzle)al/Q in nozzle 3           1.27       2.54    5.0residence time t(sec)           0.65       1.3     2.6in nozzle 3resin pressure (psi)           14.7       11.5    6.3at gauge 17calculated apparent           14         11      6melt viscosity (poise)in nozzle 3reduced intrinsic viscosity           1.0        0.7     0.3of fiber webaverage fiber diameter           2.5        1.7     1.0(micrometer)calculated average maximum           350        400     480fiber velocity (m/sec)______________________________________ 
    
     EXAMPLE 3 
     In this experimental series, the tube 1 was replaced by tubes of larger diameter (ID). This did not change the temperature profile, but increased the residence time at constant resin flow rate. Residence time in the nozzles was kept short to avoid degradation there. At 45 seconds residence time in the tube 1, resin degradation was severe (0.4 reduced intrinsic viscosity), the resin stayed in the hot section of the tube too long. Air pressure was 17 psi at gauge 8. The results are set forth in Table 4. 
     
                       TABLE 4______________________________________run #           (a)       (b)      (c)______________________________________total resin flow rate Q           16        16       16from extruder (cm.sup.3 /min)diameter (cm) of tube die 1           0.635     0.9525   1.27al/Q in tube-die 1           0.075     0.075    0.075residence time t (sec)           11.4      25.7     45in tube die 1Temperature (°F.)at extruder exit           600       600      600at T.sub.1 (after 3 cm) (14)           675       675      680at T.sub.2 (after 6 cm) (15)           710       710      680at T.sub.3 (after 9 cm) (16)           730       730      735air temperature 9 in           750       750      750cavity 4resin flow rate Q through           1.0       1.0      1.0nozzle 3 (cm.sup.3 /min/nozzle)al/Q in nozzle 3           0.127     0.127    0.127residence time t(sec)           0.066     0.066    0.066in nozzle 3resin pressure (psi)           137       116      63at gauge 17calculated apparent           13        11       6melt viscosity (poise)in nozzle 3reduced intrinsic viscosity           1.0       0.9      0.4of fiber webaverage fiber diameter           8.3       8.0      7.5(micrometer)calculated average maximum           330       360      450filament velocity (m/sec)______________________________________ 
    
     EXAMPLE 4 
     This example used a die assembly of larger dimension than in Examples 1 and 2. 
     Tube 1 had an inside diameter of 0.3167 cm. The nozzles had in inside diameter of 0.0584 cm. and an outside diameter of 0.0889 cm. and had a total length of 1.27 cm. The holes in plate 5 were triangular as shown in FIG. 3, resulting in an air orifice opening of 0.40 mm 2  per nozzle. 
     In this series, a through e, the resin flow rate was increased to result in decreasing al/Q factors in the nozzles, while leaving the temperature profiles in tube 1 near optimum. At al/Q of 0.1 and lower, the melt viscosities and fiber diameters at constant air rate (17 psi.) increased significantly, indicating that the resin temperature in the nozzles did not have enough time to equilibrate with the air temperature, as seen in Table 5. 
     
                       TABLE 5______________________________________run #           (a)    (b)     (c)  (d)   (e)______________________________________total resin flow rate Q           16     20      24   32    48from extruder (cm.sup.3 /min)al/Q in tube die 1           0.075  0.060   0.05 0.376 0.025residence time t(sec)           14.2   11.4    9.5  7.1   4.75in tube die 1Temperature (°F.)at extruder exit           600    600     600  600   600at T.sub.1 (after 3 cm)(14)           675    670     665  655   645at T.sub.2 (after 6 cm)(15)           710    705     700  690   677at T.sub.3 (after 9 cm)(16)           730    725     720  715   700air temperature 9 in           750    750     750  750   750cavity 4resin flow rate Q through           1.0    1.25    1.5  2     3nozzle 3 (cm.sup.3 /min/nozzle)al/Q in nozzle 3           0.127  0.102   0.085                               0.064 0.043residence time t(sec)           0.204  0.16    0.13 0.102 0.065in nozzle 3resin pressure (psi)           17     23      56   118   274at gauge 17calculated apparent           16     17      35   55    85melt viscosity (poise)in nozzle 3reduced intrinsic viscosity           0.9    1.0     1.05 1.2   1.4of fiber webaverage fiber diameter           8      9.7     17   24    41in micrometer (micron)calculated average maximum           350    300     120  80    40filament velocity (meter/sec)______________________________________ 
    
     EXAMPLE 5 
     The die assembly of Example 4 is used under the same air flow conditions. The bleed valve 20 was opened to increase the al/Q factor and residence time in the nozzles. At al/Q=0.1 fiber formation was good. Resin degradation became severe at residence times above 1.36 seconds, as seen from Table 6. 
     
                       TABLE 6______________________________________run #           (a)    (b)     (c)  (d)   (e)______________________________________total resin flow rate Q           48     48      48   48    48from extruder (cm.sup.3 /min)al/Q in tube die 1           0.025  0.025   0.025                               0.025 0.025residence time t(sec)           4.75   4.75    4.75 4.75  4.75in tube die 1Temperature (°F.)at extruder exit           600    600     600  600   600at T.sub.1 (after 3 cm)(14)           645    645     645  645   645at T.sub.2 (after 6 cm)(15)           675    675     675  675   675at T.sub.3 (after 9 cm)(16)           700    700     700  700   700air temperature 9 in           750    750     750  750   750cavity 4resin flow rate through bleed           28.0   40      44.8 45.6  46.5valve 20 (cm.sup.3 /min)resin flow rate Q through           1.25   0.5     0.2  0.15  0.10nozzle 3 (cm.sup.3 /min/nozzle)al/Q in nozzle 3           0.102  0.25    0.635                               0.85  1.27residence t(sec)           0.16   0.41    0.102                               1.36  2.04in nozzle 3resin pressure (psi)           28     11      3.4  2.1   0.85at gauge 17calculated apparent           21     20      16   13    8melt viscosity (poise)in nozzle 3reduced intrinsic viscosity           1.3    1.2     0.9  0.7   0.4of fiber webaverage fiber diameter           9.5    5.7     3.5  2.8   2.2(micrometer)calculated average maximum           310    350     380  420   480filament velocity (meter/sec)______________________________________ 
    
     EXAMPLE 6 
     In this example, a tube die assembly of small nozzles was used under conditions to make small fibers of high molecular weight. The tube 1 of Example 1 (12 cm. long, 0.3175 cm. diameter) is fitted with a nozzle assembly of the following dimensions: outside diameter--0.0508 cm., inside diameter--0.0254 cm., 0.7 cm. long. The holes in plate 5 were squares of 0.0508 cm. resulting in a total air orifice opening of 0.055 mm 2  per nozzle. The results are set forth in Table 7. 
     
                       TABLE 7______________________________________run #          (a)    (b)    (c)  (d)  (e)  (f)______________________________________total resin flow rate Q          20.0   10.0   16   16   16   16from extruder (cm.sup.3 /min)al/Q in tube die 1          0.060  0.12   0.075                             0.075                                  0.075                                       0.075residence time t(sec)          2.85   5.70   3.56 3.56 3.56 3.56in tube die 1Temperature (°F.)at extruder exit          600    600    600  600  600  600at T.sub.1 (after 3 cm)(14)          668    690    675  675  675  675at T.sub.2 (after 6 cm)(15)          705    725    715  715  715  715at T.sub.3 (after 9 cm)(16)          725    740    738  738  738  738air temperature 9 in          750    750    750  750  750  750cavity 4resin flow rate through          0      0      0    14.4 15.2 15.7bleed valve 20 (cm.sup.3 /min)resin flow rate Q through          1.25   0.625  1.0  0.10 0.050                                       0.020nozzle 3 (cm.sup.3 /min/nozzle)al/Q in nozzle 3          0.056  0.112  0.070                             0.70 1.4  3.51residence time t(sec)          0.017  0.034  0.021                             0.21 0.42 1.06in nozzle 3resin pressure (psi)          1344   176    661  25   12.4 5.0at gauge 17calculated apparent          65     17     40   15   15   15melt viscosity (poise)in nozzle 3reduced intrinsic viscosity          1.0    0.6    0.9  0.8  0.8  0.7of fiber webaverage fiber diameter          15.5   6.7    8.4  2.5  1.7  1.05(micrometer)calculated average maxi-          110    320    320  360  380  410mum filament velocity(m/sec)______________________________________ 
    
     Run (a) had a low temperature profile at high resin rate and too short a residence time in the nozzles, resulting in high melt viscosity and course fibers at relatively slow fiber velocity. Run (b) at 10 cm 3  /minute and al/Q of 0.12 had a temperature profile in the tube resulting in significant resin degradation (reduced intrinsic viscosity=0.6) and undesirable &#34;shot&#34; in the web. Run (c) had optimum fiber quality and little resin degradation. In runs (d), (e) and (f), the bleed valve 20 was opened to reduce flow through the 16 nozzles and produce small fibers of relatively high molecular weight. 
     EXAMPLE 7 
     In this example, the die assembly described in Example 1 is used. The resins were commercially available polystyrene, a general purpose grade of melt index 12.0, measured in accordance of ASTM method D-1238-14 62T. The polyester (polyethylene terephthalate) was textile grade of &#34;Relative Viscosity&#34; 40. &#34;Relative Viscosity&#34; refers to the ratio of the viscosity of a 10% solution (2.15 g. polymer in 20 ml. solvent) of polyethylene terephthalate in a mixture of 10 parts (by weight) of phenol and 7 parts (by weight) of 2.4.6-trichlorophenol to the viscosity of the phenol-trichlorophenol mixture per se. The results are set forth in Table 8. 
     The effect of the differences of thermal diffusivity &#34;a&#34; between polystyrene and polyester can be readily noticed by comparing runs (b) and (d). Fiber formation and velocities were similar in these two runs at approximately the same melt viscosities (22 and 18 poise), however, polyester had a substantially higher resin flow rate (12 vs. 7 cm. 3  /min. for polystyrene). 
     
                                           TABLE 8__________________________________________________________________________run #            (a)    (b) (c)    (d)__________________________________________________________________________polymer          polystyrene                   as (a)                       polyester                              as (c)Thermal diffusivity &#34;a&#34; (cm.sup.2 /sec)            5.6 × 10.sup.-4                   as (a)                       1.23 × 10.sup.-3                              as (c)total resin flow rate Q            20     7   20     12from extruder (cm.sup.3 /min)al/Q in tube die 1            0.02   0.058                       0.044  0.074residence time t(sec)            2.85   8.1 2.85   4.75in tube die 1Temperature (°F.)at extruder exit 550    550 560    560at T.sub.1 (after 3 cm)(14)            585    620 590    602at T.sub.2 (after 6 cm)(15)            612    657 615    625at T.sub.3 (after 9 cm)(16)            635    680 630    640air temperature 9 in            700    700 660    660cavity 4resin flow rate Q through            1.25   0.44                       1.25   0.75nozzle 3 (cm.sup.3 /min/nozzle)al/Q in nozzle 3 0.034  0.97                       0.075  0.125residence time t(sec)            0.053  0.151                       0.053  0.088in nozzle 3resin pressure (psi)            985    101 1115   142at gauge 17calculated apparent            75     22  85     18melt viscosity (poise)in nozzle 3average fiber diameter            20     5.0 22     6.3(micrometer)calculated average maximum            65     380 53     410filament velocity (m/sec)__________________________________________________________________________ 
    
     EXAMPLE 8 
     This example demonstrates the importance of the temperature profile in the transition zone with the results set forth in Table 9. Resin flow rate of Example 1 (d) was used in all 6 runs. In runs (a), (b) and (c) the extruder temperature was raised from 620° to 680° F., resulting in increased resin degradation and severe &#34;shot&#34; in run (c). In runs (d), (e) and (f) the air and extruder temperature was lowered maintaining the temperature defference at 40° F. This decreased resin degradation but increased melt viscosity to result in coarse fibers and slow fiber velocities. To obtain an optimum balance of low thermal resin degradation and high fiber velocity (=minimum fiber diameter), it becomes apparent that the melt-blowing process has to be run at a melt viscosity below approximately 40 poise and a temperature difference between air (=nozzle) and extruder temperature of more than 40° F., under heat transfer conditions (al/Q) defined in the previous Examples. 
     
                       TABLE 9______________________________________run #          (a)    (b)    (c)  (d)  (e)  (f)______________________________________Temperature (°F.)extruder exit  620    660    680  660  640  620at T.sub.1 (after 3 cm)(14)          670    690    700  680  660  640at T.sub.2 (after 6 cm)(15)          695    705    710  690  670  650at T.sub.3 (after 9 cm)(16)          712    714    715  695  675  655air temperature 9 in          720    720    720  700  680  660cavity 4resin pressure (psi)          263    210    105  525  1050 1840at gauge 17calculated apparent          25     20     10   50   85   175melt viscosity (poise)in nozzle 3reduced intrinsic viscosity          0.9    0.6    0.4  1.0  1.1  1.6of fiber webAverage fiber diameter          8.0    7.8    6.8  14   20   33(micrometer)calculated average          340    350    460  110  50   21maximum filament velocity(m/sec)______________________________________ 
    
     In the following examples, a 4&#34; die is used, as illustrated in FIGS. 4 through 7. The transition zone is designed to provide an optimum al/Q factor for a specific resin flow rate without using a bleed system. Instead of a bleed system, there is a resin distribution system to feed more nozzle for maximum productivity of the unit. 
     EXAMPLE 9 
     Example 9 demonstrates the effect of the heat transfer pattern on the thermal degradation of polypropylene in the multiple row 384-nozzle die. Polypropylene of Melt Flow Rate 35 and a Number Average Molecular Weight of 225,000 is used. The extruder exit temperature is 600° F., and the die and air temperature is 750° F. The results are set forth in Table 10. In run (a) melt-blowing is performed at high resin flow rate and optimum heat transfer pattern, i.e. low Σ al/Q in the transition zone, high Σ al/Q in the nozzle zone at short residence time in the die and nozzles. As resin flow rate is reduced in run (b) and (c), increased polymer degradation occurred. In run (c) the Σ al/Q reached 0.171 in the transition zone, and degradation and web quality became unacceptable. 
     
                       TABLE 10______________________________________Melt Blowing polypropylene in 4 inch/384 nozzle Die:run #               (a)     (b)      (c)______________________________________total resin flow rate Qfrom extruder: (cm.sup.3 /min)               610     66.4     23.96(cm.sup.3 /sec)     10.18   1.11     0.40residence time t(sec) in               0.663   6.00     16.88sections 24 through 29sum of all al/Q     0.0067  0.062    0.171sections 24 through 29resin flow rate Q through               0.0265  0.00288  0.00104single nozzle 30residence time t(sec)               0.041   0.378    1.04in single nozzle 30al/Q in nozzle 30   0.080   0.737    2.04Weight AverageMolecular Weight ----MW.sub.w ** of web               175,000 125,000  55,000reduced intrinsic viscosity               1.6     0.9      0.4of webaverage fiber diameter               8.0     2.6      1.6***(micrometer)calculated average maximum               520     540      550filament velocity (m/sec)______________________________________ **obtained by Gel Permeation Chromatography (performed by Springborn Laboratories, Inc. Enfield, Conn.) ***severe &#34;shot&#34; in web 
    
     EXAMPLE 10 
     The effect of heat transfer rate (thermal diffusivity) of different polymers on resin flow rates at optimum heat transfer pattern is shown in this example, using nylon-66 and polystyrene (the nylon-66, polyhexamethylene adipamide, was a staple textile grade, DuPont&#39;s &#34;Zytel&#34; TE, the polystyrene was the same as used in Example). The results are set forth in Table 11. Runs (a) and (c) were done at high resin flow rates, resulting in an al/Q factor in the nozzle zone too low for high fiber velocities. The fibers were rather coarse. Conditions in runs (b) and (d) were optimum for good web quality of fine fibers. This condition was reached for polystyrene at a higher resin flow rate than for nylon-66, due to the difference in heat transfer rates (thermal diffusivity &#34;a&#34;) for the two polymers. 
     
                       TABLE 11______________________________________run #           (a)      (b)     (c)   (d)______________________________________polymer         Nylon-66 Nylon-  poly- poly-                    66      styrene                                  styrenethermal diffusivity &#34;a&#34;           1.22     1.22    0.56  0.56(10.sup.3 × cm.sup.2 /sec)Extruder outlet temperature           550      550     610   610(°F.)Die Temperature (°F.)           630      630     730   730Air temperature (°F.)           630      630     730   730total resin flow rate Qfrom extruder (cm.sup.3 /sec)           5.45     2.28    11.98 7.45residence time t(sec) in           1.24     2.96    0.563 0.9sections 24 through 29sum of all &#34;al/Q&#34;           0.0093   0.021   0.0019                                  0.0031sections 24 through 29resin flow rate Q through           0.0142   0.0059  0.0312                                  0.0195single nozzle 30residence time t(sec)           0.076    0.184   0.035 0.056in single nozzle 30al/Q in nozzle 30           0.050    0.120   0.050 0.080average fiber diameter           12       4       26    9(micrometer)calculated average maximum           90       350     60    320filament velocity (m/sec)______________________________________ 
    
     Apparent melt viscosity is calculated from Poisseuille&#39;s equation: ##EQU2## where: Q=polymer flow through a single nozzle (cm. 3  /sec.), 
     p=polymer pressure (dynes/cm. 2 ), 
     r=inside nozzle radium (cm.), 
     l=nozzle length (cm.), and 
     η=apparent melt viscosity (poise); and 
     by measuring the polymer melt pressure above the extrusion nozzle or in more convenient form 
     
         η=2747 P A.sup.2 /Q l                                  (9) 
    
     where: 
     P=polymer pressure in psi. 
     A=extrusion nozzle cross section area (cm 2 ). 
     Intrinsic viscosities [η] as used herein are measured in decalin at 135° C. in Sargent Viscometer #50. Melt Flow Rates were determined according to ASTM Method #D 1238 65T in a Tinium Olsen melt indexer. 
     While the invention has been described in connection with several exemplary embodiments thereof, it will be understood that many modifications will be apparent to those of ordinary skill in the art; and that this application is intended to cover any adaptations or variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equivalents thereof.