US3564088A

US3564088A - Process for flash spinning an integral web of polypropylene plexifilaments

Info

Publication number: US3564088A
Application number: US854920A
Authority: US
Inventors: Rudolph Woodell
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1968-10-15
Filing date: 1969-09-03
Publication date: 1971-02-16
Anticipated expiration: 1988-02-16

Abstract

A WEB OF ENTANGLED PLEXIFILAMENTS OF ISOTACTIC POLY PROPYLENE IS OBTAINED BY FLASH SPINNING A SOLUTION OF THE POLYMER THROUGH TWO OR MORE CLOSELY-SPACED SPINNERET ORIFICES. DEPENDING UPON ARRANGEMENT OF THE ORIFICES, THE WEB MAY BE A YARN OR TOW OF SHAPED CROSS-SECTION, OR IT MAY BE A RIBBON OR SHEET PRODUCT. THE SOLVENT MAY BE 1,1,2-TRICHLORO - 1,2,2 - TRIFLUOROETHANE, TRICHLOROFLUOROMETHANE OR A MIXTURE THEREOF.

Description

R. WOODELL I 3,564,088 PROCESS FOR FLASH'SPINNING INTEGRAL WEB 0F POLYPROPYLENE PLEX LAMEN'I'S Sheets-Sheet 1 'Fiied-Sept., 5, 1969 v INVENTOR f \\\\j r S RUDOLPH woooeu.

'ZI BY I ATTORNEY 16, 1971 R WQQDE LL 3,564,088 PROCESS FOR FLASH SPINNING AN INTEGRAL POL - WEB OF YPROPYLENE PLEXIFILAMENTS Filed Sept. :5, 1969 r 2- Sheets-Sheet 2 Y 500 IOOO I |5OO 3; ;4 3 3 |4 E X X/ /J if Q. j L L 21o I j TEMPERATURE C ATTORNEY United States Patent O US. Cl. 264205 7 Claims ABSTRACT OF THE DISCLOSURE A Web of entangled plexifilaments of isotactic polypropylene is obtained by flash spinning a solution of the polymer through two or more closely-spaced spinneret orifices. Depending upon arrangement of the orifices, the web may be a yarn or tow of shaped cross-section, or it may be a ribbon or sheet product. The solvent may be 1,1,2-

trichloro

1,2,2 trifluoroethane, trichlorofiuoromethane or a mixture thereof.

RELATED APPLICATIONS This application is a continuation-in-part of US. patent application Ser. No. 768,617, filed Oct. 15, 1968, now US. Pat. 3,467,744, which is in turn a continuation-inpart of US. Ser. No. 506,304, filed Nov. 4, 1965, now abandoned.

BACKGROUND OF THE INVENTION In the US. Pat. 3,081,519 of Blades and White a method is described for preparing a fibrillated web or pleXifilament by flash spinning. In this process a polymer solution at a temperature above the boiling point of the solvent and at a pressure at least autogenous is extruded into a medium of lower temperature and substantially lower pressure. The sudden boiling which occurs at this point causes either microcellular structures or fibrillated networks to form. The fibrillated materials tend to be formed when the pressure changes are most severe, or when more dilute solutions are used. Under these circumstances the vaporizing liquid within the extrudate forms bubbles, breaks through confining walls, and cools the extrudate causing solid polymer to form therefrom. The resulting multifibrous yarn-like strand has an internal fine structure or morphology characterized as a three-dimensional integral plexus consisting of a multitude of essentially longitudinally extended, interconnecting, random-length, fibrous elements, referred to as film-fibrils.

Previous investigation with linear polyethylene has shown that under certain conditions the spinning solution forms a cloudy dispersion which, if allowed to stand without adequate agitation, settles into two distinct layers, one layer being rich in polymer and the other layer being lean in polymer. This phenomenon is described in Anderson and Romano.U.S. Pat. 3,227,794, issued Jan. 4, 1966.

Further. problems were encountered in developing an eflicient process for spinning all species of isotactic polypropylene by the process of Anderson and Romano. These problems were overcome by the improved technique 3,564,088 Patented Feb. 16, 1971 claimed in US. Pat. 3,467,744. In essence, the improvement consisted of using a specific solvent and maintaining temperatures and pressure considerably above those specified by Anderson and Romano. In the course of developing this improvement, I found that several strands of the highly fibrillated products can be spun simultaneously from several closely-spaced orifices to prepare an integral cohesive web.

SUMMARY OF THE INVENTION The purpose of the present invention is to provide an efiicient process for preparing an integral cohesive web from several continuous strands of flash-spun fibrillated isotactic polypropylene. The cohesive web may be in the form of continuous yarn or tow having a shaped crosssection, or may be a ribbon, or a sheet product. The strands are aligned principally in the lengthwise direction of the web and the fibrils of adjacent strands are entangled, thereby providing a single web which cannot be separated into constituent strands without tearing. The process of this invention involves forming a homogeneous single-phase polymer solution at a temperature which is above the critical temperature of the lowest boiling solvent constituent, and at a pressure which is above the two-liquid-phase pressure boundary for the solution, then passing the solution into a pressure let-down zone for lowering the pressure of the solution to about 10 to 400 p.s.i. below the two-liquid-phase pressure boundary for the solution, and finally, discharging the solution through two or more closely spaced spinneret orifices of restricted size to an area of substantially atmospheric pressure and ambient temperature. The orifices are of about equal size and are between 0.01 and 0.05 inch in diameter. The distance between any orifice and the next adjacent orifice measured center-to-center is 5 to 30 times the diameter of the orifices. The solution which is extruded through the orifices comprises 4 to 20% by weight isotactic polypropylene and the solvent is a chlorofluoroaliphatic material with cirtical temperature preferably between and 220 C.

FIG. 1 is a transverse cross-sectional view of an integral web of trilobal cross-sectional shape configuration (enlarged about 4- to 10-fold).

FIGS. 2a, 2b, 2c are diagrams of spinneret faces showing arrangement of orifices for spinning webs of trilobal, tetralobal, or ribbon cross-sections, respectivley.

FIG. 3 is a longitudinal cross-sectional view of a solution supply tube, a letdown chamber, and a spinneret.

FIG. 4 is a graph of pressure and temperature conditions for solutions of 1,1,2-trichloro-1,2,2-trifluoroethane and isotactic polypropylene, showing the location of twoliquid-phase pressure boundaries.

FIGS. 5a, 5b, and 5c are detail drawings of the spinneret portion of FIG. 3 when 4 orifices are present.

FIGS. 6a, 6b, and 6c are detail drawings of another spinneret portion usable with the apparatus of FIG. 3 and having 3 orifices arranged in a triangular pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the invention provides a variety of products when process conditions are controlled within the scope of the invention. For example, one may attain a continuous strand 1 having a trilobal shape such as shown in the FIG. 1. The entire strand is comprised of film-fibril elements 2 as described in Blades & White US. 3,081,519. The film-fibrils are interconnected by points along and across the lobes 3 of the strand forming an integral network in three dimensions. The individual lobes are tied to each other simply by entanglement. This entanglement occurs only when a high degree of fibrillation is obtained during spinning and when the orifice dimensions and spacings are properly controlled.

Other shapes such as tetralobal may be obtained by the process of the invention. In addition, tapes may be spun by the use of a number of holes arranged in a straight line, and a sheet product may be obtained by use of a large number of holes. All of the coherent integral web products are characterized by the same extremely thin film-fibrils. The film-fibrils are less than 4 microns thick and are molecularly oriented in the general direction of the longitudinal axis of the web. The degree of fibrillation is high, and the products have a surface area greater than 2 m. g.

The solvents which are useful in the invention are chlorofluoroaliphatic compounds or mixtures of such compounds. The following solvents are included:

1,1,2-trichloro-1,2,2-trifluoroethane (Freon-1 13) boiling point: 47.6 C.

critical temperature: 214 C.

critical pressure: 480 lbs./in. (gage) trichlorofiuoromethane (Freon1 l) boiling point: 24 C.

critical temperature: l98200 C.

critical pressure: 620-640 lbs/in. (gage) mixtures of Freonl1 and Freon-1l3.

The preferred mixture contains equal weights of Freon-11 and Freonl13. When using solvent mixtures, it is important that the solution temperature upstream of the let-down chamber be above the critical temperature of the lowest boiling constituent, i.e. above 198-200 C. for the Freonl1/Freon-l13 mixture. The pressure must be above the two-liquid-phase pressure boundary for the solution. A technique for locating the boundary is disclosed in Us. Pat. 3,467,744. The stated pressures and temperatures for the spinning solution upstream of the let-down orifice must be maintained throughout the spinning operation in order to obtain continuously a high degree of fibrillation throughout the length of the integral product. In addition, in order to obtain an entangled integral web with tenacity above 0.3 gram per denier, the process of the present invention requires passing the solution which is at a pressure above the two-liquid-phase pressure through an orifice into a let-down chamber having pressure about 30 to 400 p.s.i. below the two-liquid-phase pressure. The extent to which the pressure in the let-down chamber should fall below the two-liquid-phase pressure boundary varies with the solvent used. The optimum pressure in the let-down chamber can readily be determined for a given solvent system. If the pressure in the let-down chamber falls too far below the tWo-liquid-phase pressure boundary the product will be discontinuous, foamy particles, called fly. If the pressure does not fall sufliciently far below the two-liquid-phase pressure boundary the fibrils will be poorly separated and little or no entangling will take place, so the product will not be integral.

The size of the final orifices and the distance between the orifices are critical in determining the degree of entanglement between plexifilaments from any two adjacent orifices. Round orifices having a diameter between 0.01 and 0.05 inch are preferred. The center-to-center distance between any orifice and the adjacent orifice should be 5-30 times the diameter of the orifices. In the examples, the orifice diameters are indicated by Q and the distance between orifices (center-to-center) is indicated by the symbol P (see FIGS. 2a, 2b, and 2c). When the ratio of P/Q is greater than 30, the flash-spun plexifilaments fail to entangle under the influence of the evaporating solvent. It should be understood that with the particular P conditions specified, the solvent evaporates at a very high rate and creates much turbulence around the spinneret. This tends to cause entanglement of adjacent filaments when two strands are close enough. On the other hand, when the P/ Q ratio is less than 5, the integral strand seems to lose its shape definition and become similar to a single strand in appearance. The shape of the integral web is largely determined by the placement of the spinneret orifices relative to one another. A tetralobal strand is obtained from the spin orifice arrangement shown in FIG. 2a, a trilobal strand from 212, and a tape from 20.

The temperature and pressure combinations for operation of the process with Freon-l 13 are shown in FIG. 4. In the figure, curve A is the vapor pressure curve for the solvent. Line B is the critical temperature of the solvent, and Line C is the critical pressure, this being the pressure of the solvent under autogenous pressure at the critical temperature. The two-liquid-phase pressure boundary at various concentrations is indicated by curves H, J, and L. In FIG. 4, these boundaries H, I, and L apply to solutions containing 13, 10, and 4% by weight, respectively, of polypropylene in Freon-113. Temperature and pressure conditions to the right and below line H give a singlephase solution for a 13% solution. Temperature and pressure conditions to the left and above line H give cloudy solutions having tWo-liquid-phases, one rich in polymer and the other lean with respect to polymer. Further details on the location of the two-liquid-phase pressure boundary can be obtained by reference to my parent applications.

For 13% solutions, for example, the particular operating conditions in FIG. 4 which give a sufiicient degree of fibrillation for entanglement of the isotactic polypropylene plexifilaments are those in which the solution upstream of the first orifice is at a temperature above the critical temperature shown by line B and at a pressure above the two-liquid-phase pressure boundary shown by line H. The process of the invention further requires that the pressure he reduced by passage of the solution through a pre-flash chamber of the type shown, for example, in FIG. 3. In the pre-fiash chamber the pressure must be reduced so that it falls to the left and above line H, but not below the critical pressure. Finally, the solution passes through the spinneret orifice into the surrounding atmosphere. At this point, the flash-spun plexifilaments from adjacent orifices combine to give an integral product.

The particular spinning conditions which are preferred give a high degree of fibrillation and avoid fusion of film-fibrils. No additional air-jets are needed for entangling the adjacent lines and no drying process is needed, since the evaporation is adiabatic and since adequate heat is applied upstream of the pre-fiash chamber to evaporate the preferred solvents.

In determining the minimum operable solution pressures and temperatures for spinning solutions containing mixtures of solvents, one uses the critical temperature of the lowest boiling constituent as a minimum and the critical pressure of the highest boiling constituent as a minimum. For this purpose, one should ignore small quantities of extraneous materials in the solvent. If less than 10% of such material is present, it is not included as a constituen In preparing the solution the polymer and solvent are mixed by any of a number of known methods. For example, powdered isotactic polypropylene may be blended with liquid 1,1,Z-trichloro-1,2,2-trifluoroethane at room temperature to form a dispersion. The resulting dispersion (slurry) may then be heated with stirring in the vessel which is to serve as a supply reservoir for spinning, or it may be continuously pumped through a heat exchanger to a spinneret or spinning cell. In either case the solution must be delivered to the let-down chamber at a temperature above the critical temperature and at a pressure greater than the two-liquid-phase boundary pressure. The additional pressure can be created by pressurizing with an inert gas such as nitrogen. Such an inert gas should preferably not be mixed with the solution but rather should be present as a force pressing against it. Alternatively, it can be generated (1) by mechanical means such as one or more pumps, or (2) by heating the blend to the desired temperature in a vessel with a volume that is small enough to enable the solution to generate sufficient pressure to eliminate any gas phase, above the solution at the desired temperature. The blend should contain between 4 and 20% polymer and 96 to 80% solvent. These percentages, as well as others referred to in the description which follows, are on a weight basis.

The polymer used in the solution should have a melt flow rate between 0.09 and 10.0, the units as used throughout being in g./ min. The method for determining melt flow rate is ASTM Method 123 8T, Condition L for polypropylene.

The polymer used for preparing the solution is not necessarily composed of 100% propylene. The polymer should contain at least 85% of propylene, but may have as much as by weight of units derived from other ethylenically unsaturated monomers such as isobutylene, vinyl acetate, methyl methacrylate, or mixtures such as efhylene/octened. The term isotactic polypropylene as used herein refers to such polymers containing a high proportion, e.g. over 80% by weight, of isotactic macromolecules. A further description thereof is given by Natta et al. in U.S. Pat. 3,166,608.

In the examples which follow, a batch process is used for preparing solutions. For this purpose it is important to determine the amount of polymer and solvent which is needed to provide in the autoclave a homogeneous single-phase solution at a desired operating pressure and temperature. In other words, sufficient solution must be present in the autoclave to prevent the formation of a solvent vapor phase. The approximate amount of material may be calculated from the density of the solution at the desired spinning temperature and pressure. The solvent is added while the autoclave is under vacuum. The autoclave is then closed. The agitator is turned on and the autoclave heated as rapidly as possible while a graph of the temperature and pressure is made during the heatup cycle. Excessive pressure (due to minor errors in calculation or inaccurate density values) may be released by bleeding 01f small portions of the material from the autoclave from time to time.

When the solution is ready for batch flash-spinning, the agitator is stopped and the atmosphere above the solution is pressurized with nitrogen to a level 100 to 200 p.s.i.g. above the autoclave pressure. Stirring is avoided to prevent mixing of the nitrogen gas with the solution. The nitrogen pressure within the autoclave is maintained at this level so that no pressure drop will occur during spinning. The total pressure is recorded as the solution pressure and the 100-200 p.s.i.g. increment is included as though it were solvent-generated.

Although the use of nitrogen or other inert gas as above described will be illustrated in the examples which follow, it will be understood that for a commercial operation a piston or other mechanical means would be preferable.

Suitable spinnerets for use with the process of this invention are shown in FIGS. 3, 5a, 5b, 5c, 6a, 6b, and 60. FIG. 3 shows a longitudinal cross-section of a spinneret assembly which is attached to a solution supply by means of pipe thread 32. The spinneret assembly comprises a cylindrical tube 31 provided with an integral cap 37 and containing a spinneret 24, a let-down chamber insert 33 with a hollow cylindrical core, a let-down orifice insert 34 having a small orifice 35 and a hollow spacefilling insert 36 to provide adequate support for the other portions. All of the parts are machined to fit inside the outer tubular portion 31 and are gasketed to provide a pressure-tight system. One spinneret 11 for use in the spinneret assembly is shown in FIG. I511. The spinneret is held in place by means of shoulder 14. FIG. 5a is a cross-sectional side view showing a deep slot 10. FIG. 5b is a top view of the slot potrion of FIG. 5a. Both figures show the four orifices 12 aligned in a row at the bottom of the slot. FIG. 5c is an enlarged cross-section of a single orifice 12. The inner end of each orifice passage is countersunk to provide conical taper 13. The land length 15 includes only the tiny cylindrical portion of the orifice.

Another spinneret 24 which may be used in the same spinneret assembly is shown in FIG. 6a. The portion of this spinneret which is upstream of the orifices contains a wide bore cylindrical portion 20. Three spinneret orifices 21 are bored in the bottom of the wide cylindrical portion. These are equi-spaced around the axis of the cylinder on circle 22. In this case the orifices are not countersunk.

EXAMPLE I A solution of isotactic polypropylene was prepared from polymer having a melt flow rate of 0.8 g./ 10 min. using l,1,2-trichloro-1,2,2-trifluoroethane (Freon-11 3) as solvent. The solution was flash-spun using the conditions specified in Table l. The solution contained 10% polymer by weight and was spun through a spinneret having four orifices as shown in FIGS. 5a, 5b, and 5c. The cylindrical portion of the let-down chamber for this spinneret assembly was 0.5 inch in diameter and 3.31 inches long. The slot 10 was 0.7 inch long (measured transversely) and 0.75 inch deep. The slot width was 0.25 inch.

The product was a tape in which the original four strands were barely visible, being somewhat denser than the connecting film-fibril web. The four strand residues ran side-by-side through the length of the tape. The tape had a tenacity of 1.62 g./denier, elongation of 99% at break and denier of 56.

TABLE 1 Item No. I

Polymer melt flow rate 0.8 Solution:

Percent polymer 10 Temp. C. 219

Pressure lbs./in. (gage) 1300 Let-down chamber Inlet orifice diameter, inches .018

Pressure lbs/in. (gage) 900 Spinneret:

Extrusion rate, lbs. polymer hour 6 No. of orifices 1 4 Distance P, in. .130

Diameter Q, in. .010

Land length, in. .021

EXAMPLE II A solution was prepared containing 8% by weight isotactic polypropylene (melt flow rate 0.85 g./l0 min.) and 92% by weight trichlorofiuoromethane (Freon-11). A homogeneous solution was obtained by heating the solvent and polymer in an autoclave to 2l9222 C. which is above the critical temperature 198-200 C. Five different spinning conditions were tested as shown in Table 2. In each case the solution was spun from the same autoclave through a cylindrical pre-flash chamber 0.5 inch in diameter and 3.31 inches long. The round orifice at the inlet to the let-down chamber was 0.031 inch in diameter. The final orifices at the spinneret face consisted of three holes arranged in an equilateral triangular pattern. These holes were 0.014 inch in diameter and the distance between holes was 0.250 inch (P/Q: 17.9). The landlength through spinneret face was 0.020 inch. Since a vessel of limited size was used, the pressure rose to between 1630 and 1685 pounds per square inch, which is above the twoliquid-phase pressure boundary for an 8% solution of this polypropylene in Freon1 l. The products which were obtained at various spinning temperatures and pressures are indicated in Table 2. Each of the products had denier between 115 and 194, tenacity between 1.7 and 2.7 g.p.d. and elongation at break of 54 to 73%. Apparently for item II-D the pressure in the let-down zone was not sufliciently far below the two-liquid-phase pressure boundary for this 8 was between 1290 and 1730 lbs./in. (gage). Pressures in this range are above the two-liquid-phase pressure boundary of the mixture and above the critical pressure of Freon-l13 (480 lbs./in.

The solution was flash extruded through a spinneret 5 solution. similar to the one shown in FIG. 5a, but having 5 orifices TABLE 2 arranged in a straight line and having a conical lead-in instead of the oval slot 10. The narrow portion of the Item cone 1s 0.5 in dlameter and the cone widens as it ap- 11-13 proaches the orifice. The solution was supplied from the Solution temp, O c 220 221 221 222 autoclave at temperatures and pressures indicated in Table Solution pressure in autoela ,lbs.

(gage) f 1,670 1,660 1,630 1,685 there being four different parts to the experiment. In ghailebounriaryylbg/mt (gage)l 1,540 1,560 1,560 1,580 each case, the solution passed from the autoclave through etown 0 amber e, b (gage) 1,310 1,445 1,340 1,480 an inlet orrfice to a pre flash chamber. The inlet orifice Extrusion rat polymer 1 h 23 19 21 19 was 0.036 inch in diameter. The cylindrical portion of Remarks Integral 0) Integral (2) the pre-flash chamber was 0.5 inch in diameter and 3.31

1 Borderline integral. inches in length. 2 Not mtegml- The distance between spinneret orifices center-to-center EXAMPLE III Was 0.250 inch for each part of the experiment. The orifice diameters were each 0.014 inch. P/ Q was therefore Solutions were prepared from isotactic polypropylene 17.9. The land length was 0.020. Under each of the condihavlpg a melt ew rate of 0.7 g- 1( y h g m tions described in Table 4, a bulky tape was obtained. The arnixture containing equal quantities by welght of 1,1, five components were thoroughly entangled to provide a

tflehlol'o

1,2,2 tflfillofoethane and coherent integral web which could not be separated again ehlofofllloromethahe ease the Sohl- 25 into the components. When the tape was spread transtion composed y Welght P y e and 90% y versely, a uniform sheet about two inches wlde was obvflelght y Ihlxture- A homogeneous slngle'phase Solo" tained. The edges of the five original strands were faintly PP- h lbdlvldbal lobes of the Web Could not be P visible and were oriented along the length of the strand ditions 1nd1cated1n Table 3. i d l fa hion Thesolutior} e pasjscdthroughacylingiricallet-down A sheet or much greater width can be obtained by chamber 0.5 inch in diameter and 3.31 ln g, the spinning through a larger number of orifices arranged in orifice at the inlet to th1s chamber b ing .020 to Q lheh a straight line. One may also spin through several rows in diameter as indicated in Table 3. The solution was f rifi to b i Planar Sheets or webs having extruded through splnnelets eaoh havlng three c1osely selected cross-sectional shape or having greater density spaced holes arranged in an equilateral triangular pattern. unif rmity The spinneret of Item III-A had orifices arranged to give TABLE 4 a P/ Q ratio of 7.15. Under the temperature and pressure conditions indicated in Table 3 for Item III-A, this spinneret gave an integral tow which was trilobal in cross-sec- Solution temp., C 1 2% 1 Egg 1 1 3 1 st ,.n. e tron. The indlvidual lobes of the web could not be sepa- 40 j g rated without tearing the film-fibrils. The tow from Item lb5./il1. 1,080 1, 000 1,120 1,140 III-A had a denier of 231, tenacity of 2.3 g./denier, and lbs/m 2 950 870 1,015 1,075 elongation 69% at break. Extrusionrate, polymer lbs. 43 37 43 Items III-D through III-G each gave a tow with trilobal 23 53,; 393 i 7?, $3 3'72 cross-section. These products were very bulky. The denier 45 Elon ation, percent at break 71 73 74 70 for these yarns was between 360 and 400, the tenacity 1.33 to 1.61 g./denier, and the elongation 70 to 74%. What is cla1med1s: Items IIIB and III-C with P/ Q 35.8 did not give an inte- 1. In the process of flash spinning isotactic polygral cohesive product. proylene plexifilaments by the steps of (a) forming a TABLE 3 Item III-A III-B III-C III-D III-E III-F III-G Solution:

Temp, C 210 208 208 209 210 2 212 Pressure, lbs./in. 2, 020 1,650 1,780 1,615 1,790 1, J 1,980 Two-liquid-phase pressure boundary, lbs./in. 1,120 1,080 1,080 1,100 1,120 1,140 1,160 Inletorifice diam.,in .020 026 .026 .026 026 .026 .026 Let-down chamber:

Pressure. 1bs./in. 840 000 1,050 905 955 1,000 1,025 Extrusion rate, lbs. polymer/hr 24 22 22 27 30 31 30 spinneret:

Distance P, in 0.100 0,500 0.500 O. 250 0. 250 0.250 0. 250 Diamet r Q, in 0. 014 0. 014 0.014 0. 014 0. 0145 0.0145 0.0145 Ratio P/Q 7,15 35,8 35.8 17.3 17.3 17.3 17.3 Land length, in 0.20 .020 .020 .020 .020 .020 .020

EXAMPLE IV homogeneous single-phase solution of polypropylene, having a melt flow rate between 0.09 and 10.0 g./ 10 min., in A Solution of isotaetie P yp py (melt flow rate a chlorofluoroaliphatic solvent selected from the group 0.7 g./10 min.) was prepared using a mixture of 50% consisting of 1,1,2-trichloro-1,2,2-trifluoroethane, trichloy Weight (boiling Point and 50% rofluoromethane and mixtures thereof having a critical Freo11-11 as Solvent- The Solution temperature between 190 and 220 C., bringing it to a tained 10% polypropylene and solvent mixture. A homogeneous single phase was obtained by heating a slurry of the stated composition in an autoclave to between 207 and 210 C. as indicated in Table 4. Temperatures in this range are above the critical temperature of Freon-11 (l98200 (3.). The pressure in the autoclave temperature above the critical temperature of the lowest boiling component of the solvent and to a pressure above the two-liquid-phase pressure boundary for the solution, the said solution having a concentration of between 4 and 20% by weight of the polymer, (b) passing the solution into a pressure letdown zone for lowering the pressure of the solution to between about 10 and 400 p.s.i. below the two-liquid-phase pressure boundary for the solution, and (c) discharging the solution through a spin neret orifice into an area of substantially atmospheric pressure and ambient temperature to provide a continuous, highly fibrillated strand, the improvement which comprises the step (c), discharging the solution through at least two orifices of about equal diameter between about 0.01 and 0.05 inch, the center-to-center distance between any orifice and the next adjacent orifice being between about 5 and 30 times the orifice diameter, whereby there is formed a single cohesive integral web composed of a plurality of entangled plexifilamentary strands.

2. Improvement of claim 1 wherein the solvent is 1, 1,2-trichl0ro-1,2,2-trifluoroethane.

3. Improvement of claim 1 wherein the solvent is trichlorofluoromethane.

4. Improvement of claim 1 wherein the solvent is a mixture of 1,1,2-trichloro-1,2,2-trifluoroethane and trichlorofluoromethane.

5. Improvement of claim 1 wherein the solution is References Cited UNITED STATES PATENTS 3,504,076 3/1970 Lee 264205 JULIUS FROME, Primary Examiner H. MINTZ, Assistant Examiner US. Cl. X.R.