Flash Spinning Process

Abstract

Process for preparing high quality plexifilaments from isotactic polypropylene by flash extruding trichlorofluoromethane solutions at temperatures from 200.degree. to 240.degree.C. and pressures in excess of 900 psig, provided further that (MFR/c) .gtoreq. 1.13-0.04 (T-220) where Mfr is the melt flow rate of the isotactic polypropylene at the instant of extrusion, with 2 .ltoreq. MFR .ltoreq. 30, c is the concentration of the isotactic polypropylene in the solution, expressed in weight percent, and T is the temperature of the solution in .degree.C.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for preparing a high quality plexifilamentary product from isotactic polypropylene. More particularly, the invention is concerned with the flash-extrusion of solutions of isotactic polypropylene in trichlorofluoromethane under certain specified conditions.

2. Description of the Prior Art

In U.S. Pat. No. 3,081,519 a method is described for preparing a fibrillated web or plexifilament by flash extrusion (flash spinning). In this process a polymeric solution at a temperature above the boiling point of the solvent and at a pressure at least equal to the autogenous pressure is extruded abruptly into a region of lower temperature and substantially lower pressure. In a selected concentration range and when the temperature of the solution exceeds the "self-nucleation temperature" of the solvent (approximately 45.degree.C. below the solvent critical temperature, T.sub.c), flash vaporization of the solvent generates extraordinarily large numbers of bubbles throughout the extrudate, breaks through the confining bubble walls, and instantaneously cools the extrudate, causing solid polymer to precipitate in highly subdivided skeletal form. The resulting multifibrous yarn-like strand has an internal fine structure of 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. These film-fibrils have the form of thin ribbons of a thickness less than four microns which intermittently unite and separate at irregular intervals called "tie points" in various places throughout the width, length, and thickness of the strand to form the integral three-dimensional plexus. The fibrous strand comprising a three-dimensional network of film-fibril elements is referred to as a plexifilament and has utilities similar to those of spun staple textile yarns. Alternatively, these interconnected fibrous networks may be spread laterally, as by extruding through slot-shaped post-orifice shrouds or by impinging the nascent strand on a solid deflecting surface, to form continuous plexifilamentary webs which may be deposited on a belt to form highly desirable nonwoven sheet structures.

Certain plexifilamentary structures are desirably prepared from isotacttc polypropylene, a relatively inexpensive polymer which provides improved creep resistance, higher resilience, and higher melting point as compared to plexifilaments of, e.g., linear polyethylene. However, in attempting to prepare continuous plexifilaments of polypropylene employing the above flash spinning technique, difficulties have been experienced in consistently obtaining strong, continuous strands with a high degree of fibrillation throughout their length. One operable flash spinning system has been discovered comprising isotactic polypropylene/1, 1, 2-trichloro-1, 2, 2-trifluoroethane solutions under prescribed conditions of temperature, pressure and concentration as described in U.S. Pat. No. 3,467,744. However, it has been an objective for some time to discover conditions for preparing high quality polypropylene plexifilaments from trichlorofluoromethane solutions, since the latter solvent is not only more economical, but is also eminently suitable as a flash extrusion solvent for linear polyethylene, and use of a common solvent would greatly simplify production of plexifilamentary products from either polyolefin at will, employing a common facility. Although experiments have domonstrated that good polypropylene plexifilaments can still be obtained when a minor proportion of the operable 1, 1, 2-trichloro-1, 2, 2-trifluoroethane solvent is replaced by a quantity of trichlorofluoromethane, this route is not attractive since it leads to the need for dual solvent supply systems, requires careful control of two-solvent feed ratios, and complicates the requirements for a solvent recovery system.

SUMMARY OF THE INVENTION

A process for the flash spinning of high quality plexifilamentary products of isotactic polypropylene has now been discovered which comprises:

1. preparing a solution of isotactic polypropylene and trichlorofluoromethane wherein the concentration of polypropylene is between about 2 and 20 percent by weight, the temperature of the solution is between about 200.degree.C. and 240.degree.C., the pressure of the solution is in excess of 900 psig; and the melt flow rate (MFR) of the isotactic polypropylene fulfills the following formula:

(MFR/c) .gtoreq. 1.13 - 0.04 (T-220)

in which c is the concentration of the isotactic polypropylene in the solution, expressed in weight percent and T is the temperature (.degree.C.) of the solution, and wherein MFR is equal to a numeral between 2 and 30, inclusive;

2. and extruding said solution abruptly into a region of lower temperature and pressure. Values of (MFR/c) at least 15 percent higher than the minimum value defined above are preferred. The most preferred extrusion temperatures are in the range from 220.degree. to 235.degree.C. The preferred pressures are those which will maintain the solution in a single phase condition at the corresponding temperature and concentration, as depicted in FIG. 2. It is also preferred that the solution be extruded via a small pressure letdown chamber just upstream of the terminal extrusion orifice wherein the pressure is reduced to a value which will yield a two-phase solution, again as depicted in FIG. 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the relationship between MFR/c and temperature.

FIG. 2 is a graph illustrating phase changes encountered under various conditions of temperature and pressure for solutions of isotactic polypropylene and trichlorofluoromethane at several concentrations.

FIG. 3 is a cross-sectional view of a spinneret having a letdown chamber suitable for spinning transient two-phase solutions, according to one of the preferred processes of this invention.

FIG. 4 is a graph showing combinations of temperature and pressure for a blend of isotactic polypropylene and trichlorofluoromethane when heated in a closed vessel.

FIG. 5 is a diagram of a convex slot shroud, as employed in several of the Examples hereinafter.

DESCRIPTION OF THE INVENTION

The key relationship between (MFR/c) and extrusion temperature which appears to govern whether high quality polypropylene plexifilaments having good morphology or plexifilaments having poor morphology will be produced, is empirical in nature, and has been discovered by examining the results of a large number of flash extrusion experiments. Thus, when flash extrusion of the system isotactic polypropylene/trichlorofluoromethane was attempted employing only the teaching of the prior art, the results-- particularly as regards production of high-quality plexifilamentary products-- appeared to be extremely erratic. Further work included studies of not only the known prior art variables of concentration, pressure, and temperature, but also such variables as polymer source (various manufacturers, various initial MFR's various molecular weight distributions, etc.) and incorporation of various additives, and attempts to duplicate deliberately those which might have been occasionally present adventitiously, as well as a survey of the affect of a wide variety of surfactant additives. Although no controlling parameter was found amongst these variables, the vast majority of the results of these varied experiments fell into a regular pattern, once the key parameters of (MFR/c) and T were discovered.

FIG. 1 is a graphical representation of the parameters (MFR/c) and T depicting the results for some 70 flash extrusion experiments detailed in the examples included hereinafter. To be sure, a small hand-full of experiments run under conditions quite close to the "key relationship boundary line" yield products whose morphology is predicted incorrectly, possibly indicating that this boundary-- at least as expressed solely in terms of the (MFR/c) and T parameters-- has some finite width rather than the infinite sharpness of a geometrical line. This feature is suggested in FIG. 1 by the shaded band which covers all conditions within 15 percent of the "key relationship boundary line." However, even within this boundary region, the morphology of the product is predicted correctly most of time. In the regions above and below this boundary region, the products' morphology is correctly predicted by the present key relationship in the overwhelming majority of cases. No special significance is attributed to the few exceptions to the general rule, other than the possibility that the recorded MFR's for these particular products may have been in error, since some difficulties with these measurements have occasionally been experienced.

This key relationship is quite unexpected since it teaches that successful operation with this system is possible only under conditions approximately an order of magnitude removed from the preferred conditions taught in the prior art for other similar flash spinning systems. For example, at an extrusion temperature of 220.degree.C. (the mid-point of the operable range from 200.degree.-240.degree.C.), the relationship requires that the MFR of the polymer in solution be at least 1.13 fold greater than the polymer concentration in the solution expressed in weight percent. In marked contrast, the art polyolefin flash extrusion procedures commonly employed polymer whose MFR <<1.0 (e.g., 0.2) at concentrations of 10 percent and higher, and whose (MFR/c) ratios would therefore be <<0.1 in contrast to the 1.13 minimum ratio required for the present trichlorofluoromethane/isotactic polypropylene system. Furthermore, since solution concentrations of at least 2 percent polymer, and preferably at least 4 percent polymer, are required in order to generate a continuous plexifilamentary product in the flash spinning process (and concentrations of at least 10 percent are preferred for commercially attractive processes) this newly discovered key relationship teaches that polymer of high MFR-- at least 2, and preferably at least 4-- is required in order to produce high quality plexifilamentary products from this system. It is surprising that plexifilamentary products of polymers with such high MFR's (low molecular weights) continue to exhibit attractive high tenacities particularly in view of the prior art teachings of a strong preference for employing polymers of MFRs substantially less than one.

The isotactic propylene polymer employed in this invention is not necessarily composed of 100 percent propylene units. The polymer may have as much as 15 percent by weight of units derived from other ethylenically unsaturated monomers such as ethylene, isobutylene, vinyl acetate, or methyl methacrylate. Furthermore, the polymer may contain some nonisotactic polypropylene units. Thus, the term "isotactic polypropylene" as used herein refers to such polymers containing at least 80 percent by weight, of isotactic polypropylene macromolecules. A further description is given by Natta et al. in U.S. Pat. No. 3,166,608.

The polymer in the solution to be flash extruded should have a MFR high enough to satisfy the above-described key relationship at the particular concentration and solution temperature chosen, but preferably not substantially in excess of MFR = 30, since otherwise the physical properties-- particularly the tenacities-- of the resulting plexifilamentary products drop off to unattractive levels simultaneously with a deterioration of the morphology of the yarn (degrees and uniformities of fibrillation). The MFR of the polymer is determined according to ASTM method 1238T Condition L. Isotactic polypropylene with MFR meeting the above conditions may be employed directly in making up the flash spinning solutions or alternatively and preferably, isotactic polypropylene of lower initial MFR may be employed and be deliberately thermally degraded to the required MFR during solution preparation, storage, and transfer to the extrusion orifice. In either case, the MFR of the isotactic polypropylene in solution just prior to extrusion is inferred by running the ASTM test on the quenched and dried (solvent free) plexifilamentary product produced on flash extrusion.

In this specification "high quality plexifilament" means a continuous, three-dimensional, interconnected strand or web of film-fibrils which is highly fibrillated, i.e., is substantially free of foamy material but not over-fibrillated, i.e., is not shredded or torn apart, has substantially all its film-fibrils interconnected at both ends, and is produced without generation of appreciable quantities of loose particles ("fly"). Such a "high quality plexifilament" is also described herein as a product having "good morphology."

To prepare the spinning 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 trichlorofluoromethane 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 should be delivered to the spinneret at a temperature of at least 200.degree.C. and at a pressure greater than the two-liquid-phase boundary pressure described in subsequent paragraphs. For all spinnable solutions of this invention this pressure is above 900 psig and is well above the vapor pressure of the solvent. This superautogenous pressure can be created by pressurizing the system with an inert gas such as nitrogen. The inert gas should preferably not be mixed with the solution but rather should be present as a force pressing against it. No upper pressure limit exists, of course, save those imposed by the equipment design. Alternatively the required superautogenous pressure 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 "filled" with the blend such that thermal expansion of the confined blend generates sufficient pressure to prevent formation of any gas phase above the solution at the desired extrusion temperature.

Since the pressure on the solution is well above the vapor pressure of the solvent, abrupt extrusion into an area of substantially lower temperature and pressure will cause the solution to "flash" from the container.

Trichlorofluoromethane has only limited solvent power for isotactic polypropylene, i.e., it dissolves appreciable quantities of polymer only at temperatures above its normal boiling point and at super-atmospheric pressures. However, at progressively higher temperatures the solvent power again decreases (as thermal expansion tends to decrease the solvent density) and such "super-hot" flash spinning solutions generally form cloudy dispersions which, if allowed to stand without adequate agitation, separate into two distinct layers, one being polymer-rich and the other being polymer-lean. This "partial dissolution" may be avoided by applying still higher super-autogenous pressure to the system to prevent the solvent density decrease and loss of solvent power and thus maintain a single phase solution. Such single phase solutions are preferred in order to avoid the undesirable discontinuities which otherwise could occur in attempting to store, transfer and extrude a gross two-phase system of nonuniform composition. However, it has been found desirable, in order to achieve superior fibrillation of the product, to pass such homogeneous single phase solutions through a small pressure letdown chamber to form a transient two-phase solution in the form of an extremely fine dispersion which promptly exits through the final spinning orifice before coalescence of the dispersion can occur.

The operation of this single phase solution/two-phase extrusion for the present trichlorofluoromethane/isotactic polypropylene system amy be better understood by referring to FIG. 2, where the ordinate for the graph is temperature, .degree.C.; and the abscissa is gauge pressure in pounds per square inch (psig.). Absolute pressure (psia) are 15 psi greater than gauge pressure. Line A of the graph gives the vapor pressure of the solvent trichlorofluoromethane at various temperatures. Line B shows the critical temperature of the solvent (198.degree.C.) while line C shows the critical pressure (625 psig). Line E shows the temperature limit (200.degree.C.) above which the polymer/solvent combinations of this invention will give strong continuously fibrillated products. Line G indicates the minimum pressure (900 psig) for obtaining strong continuously fibrillated products.

A family of curves H, J, K and L, corresponding to solution concentrations of 13 percent, 12 percent, 11 percent, and 10 percent, is also shown in FIG. 2. Each curve represents a series of pressure/temperature combinations which are herein referred to as the "two-liquid-phase pressure boundary." To better visualize the graph one should look for the curve in the family which corresponds to the solvent/polymer concentration of interest and should then imagine that the other lines are nonexistent. For example, the two-liquid-phase pressure boundary for a 10 percent solution of a given sample of isotactic polypropylene in trichlorofluoromethane at various temperatures is represented by line L. The system at equilibrium under all temperature-pressure combinations to the left of line L consists of two liquid phases: a polymer-rich liquid, and a polymer-lean liquid. On the other hand, under temperature-pressure combinations to the right of line L the system consists of a single liquid phase.

In flash spinning a 10 percent solution of isotactic polypropylene in trichlorofluoromethane, for example, the solution in the main reservoir and solution transfer lines upstream of the inlet orifice to the pressure letdown chamber should have a temperature-pressure relationship corresponding to a point within the area to the right of line L. For example, the solution might be maintained at a temperature of 220.degree.C. and a pressure of 1,600 psig as indicated by point Y on the graph. Under these conditions the solution consists of a single liquid phase. When such a solution passes through a properly sized inlet orifice into the pressure letdown chamber, its pressure can be made to drop, for example, to 1,200 psig, while the temperature drops only slightly, e.g., 1.degree. to 2.degree.C., as represented by point Z on the graph. Under instantaneous conditions at point Z, the solution consists of two-liquid phases in the form of a very fine dispersion. The continuous phase is a solution of isotactic polypropylene of relatively high concentration as compared to the dispersed phase. The dispersed phase is essentially pure solvent with a very small amount of polymer dissolved therein. The volume of the letdown chamber should be selected such that the residence time of the two-phase solution within the letdown chamber will be sufficiently brief to avoid separation of the two phases into distinct layers. In the absence of dispersion-stabilizing treatments, e.g., stirring, such residence time is preferably kept below 30 seconds. Under these preferred conditions, the solution passes through the final constriction, i.e., the spinneret orifice, into the atmosphere in a very finely divided dispersed form, and the solvent evaporates instantaneously giving a highly fibrillated strand of polypropylene. If the residence time in the letdown zone substantially exceeds 30 seconds, the two phases are likely to separate into layers or into large droplets. Commonly a strand produced under the latter conditions would be discontinuous or otherwise possess nonuniform morphology.

As will be apparent from FIG. 2, a wide variety of conditions can be used to obtain the desired highly-fibrillated strand. By experimentation it has been found that the location of the two-liquid-phase pressure boundary shifts upward and to the left when solutions of higher concentration are used and shifts downward and to the right for lower concentrations. The location of the two-liquid-phase pressure boundary for a given polymer concentration is not very sensitive to changes in polymer melt flow rate (melt flow rate being inversely related to molecular weight).

The location of the two-liquid-phase pressure boundary may be established for a given polymer batch and solvent combination by observing the solution at various temperatures and pressures through a high-pressure sight glass in an apparatus equipped with a mechanical pump or other means for providing the necessary superautogenous pressures. At pressures above the two-liquid-phase pressure boundary, the solution will be clear; at pressures below the two-liquid-phase pressure boundary the solution will be cloudy. The phase boundary (temperature and pressure) for a given set of conditions is read at the point where incipient cloudiness of the solution in the sight tube is first observed. When the data have been collected for a number of temperatures, a graph may be constructed by plotting the boundary pressure for each temperature as in line J of FIG. 2. It is desirable to observe the solutions under static conditions as well as under flow. For this observation a needle valve can be used in place of the spinneret, and this valve may be closed while the solution is being observed through the sight glass. By regulating the flow rate through the spinneret, further data can be obtained to establish the optimum residence time for the solution in the pressure letdown zone.

When a relatively insoluble inert gas such as nitrogen is dissolved in the system, the two-phase boundary line generally shifts downward and to the right. Although introduction of such a gas tends to increase the degree of fibrillation of the product, there are certain practical difficulties which ensue. Since the dissolved gas causes the two-liquid-phase pressure boundary to move to higher pressures, equipment which will hold higher pressures is required in such systems. Furthermore, quantitative regulation of the gas concentration is difficult. Thus, although batch extrusion experiments, as illustrated in the examples hereinafter, may for convenience employ nitrogen gas to maintain a selected superautogenous pressure during extrusion of the solution, preferably the gas simply exerts a pressure on the surface of the solution and dissolution of the gas in the system is minimized by avoiding stirring of the gas-pressurized system and minimizing the exposure time of the system to the gas. In order to completely avoid complications due to variable gas dissolutions, it is generally preferable to use mechanical means such as pistons or screw extruders for building up pressures in commercial processes.

For economic reasons the spinnable concentrations for flash spinning are advantageously above the level of 10 percent polymer in solution. However, at very low concentrations, (usually below 2 percent), another phase boundary will be found in which the phase relationships are reversed from those discussed in the preceding paragraphs. Thus, in this very low concentration area the dispersed phase will consist of a small percent polymer in solution while the continuous phase will consist mainly of clear solvent. At this end of the concentration scale the pressure boundary curves will move closer to the autogenous vapor pressure curve A as lower concentrations are used. However, solutions or dispersions having such low polymer concentration, i.e., less than 2 percent, ordinarily do not give continuous fibrillated strands of uniform morphology and hence are unsuitable in the practice of this invention. For convenience, an upper polymer concentration level of 20 percent is provided.

In the examples which follow, a batch process is used for preparing solutions, and a thermal-expansion technique is employed for generating the required superautogenous pressures. In using this technique, it is important to charge a sufficient quantity of polymer and solvent such that thermal expansion of the mixture will completely fill the autoclave when the temperature reaches some intermediate value. Further heating of the confined solution will generate the required superautogenous pressure when the desired extrusion temperature is reached. The required amount of material may be closely estimated if the density of the solution is known for the desired spinning temperature and pressure. Use of a slight excess of material is recommended, since any excess pressure may be released by venting a small amount of solution during the heating operation.

In FIG. 4 pressure is plotted versus temperature for a blend of 10 percent polypropylene and 90 percent trichlorofluoromethane heated while confined in an autoclave. Isotactic polypropylene weighing, for example, 2,050 g. is added to a steam jacketed, stirred autoclave containing a void space of 18,000 ml. The autoclave containing the polymer is then evacuated to remove the air and 18,450 grams of "Freon-11" trichlorofluoromethane 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 heat-up cycle.

Line Q of FIG. 4 represents the vapor pressure of solvent at various temperatures during the first stage of the heating cycle. Departure of the pressure level from the vapor pressure curve for "Freon-11" at R defines the temperature and pressure conditions at the point of "filling" the autoclave, i.e., the point at which the solvent vapor phase disappears. As the heating is continued, the pressure rises sharply. If no material is released from the autoclave the temperature and pressure combinations shown by line S will be recorded. It should be understood that a family of curves similar to line S will be generated by charging various amounts of ingredients. The temperature required for "filling" the autoclave will increase as the amount of ingredients decreases. Excessive pressure (due to minor errors in calculation or inaccurate density values) may be released by bleeding off small portions of the material from the autoclave from time to time. In order not to alter the relative quantities of polymer and solvent, it is desirable that this bleeding not be required until after the polymer has dissolved, which occurs rapidly at a temperature of about 110.degree.C. When the correct quantity of ingredients has been charged, bleeding the autoclave will not be necessary before reaching a temperature of approximately 180.degree.C. as indicated in FIG. 4.

When the solution is ready for flash spinning, the agitator is stopped and the solution is pressurized with nitrogen gas to the desired extrusion pressure, e.g., a pressure 100 to 200 psig above the two-phase boundary pressure. Stirring is avoided from this point on to prevent mixing and dissolution of the nitrogen gas in the solution. The applied nitrogen pressure within the autoclave is maintained constant during spinning.

In the following examples and elsewhere in the disclosure, parts and percentages are by weight unless otherwise indicated.

EXAMPLE I

A series of solutions of isotactic polypropylene and trichlorofluoromethane was prepared at various concentrations from several commercial sources of polymer, as described in Table 1A. These solutions were prepared in a five-gallon autoclave by the filled-system technique described above. The computed quantities of polymer and solvent were charged into the autoclave which was then sealed, and stirring and heating commenced to reach the fill-point temperature of approximately 160.degree.C. (corresponding to point R of FIG. 4) in about twenty minutes. Further heating to reach the spinning temperature of 220.degree.C. required a minimum of about 70 minutes additional time, and even longer heating times were sometimes employed when additional polymer degradation in solution was desired. Just prior to extrusion, nitrogen pressure was applied above the solution to hold the pressure during extrusion above the single phase boundary line (cf. H, J, K, and L in FIG. 2). A value of approximately 1,750 psig is preferred. The spinneret assemblies employed included a pressure letdown chamber as shown at 13 in FIG. 3 and preceded by a letdown orifice 12 and leading to a spinneret orifice 14 terminating in an exit (slot) shroud 16. All the runs except No. 6 employed a rectangular slot shroud, i.e., the exit face is flat and perpendicular to the axis of the cylindrical spinneret orifice, which is centrally located in the "bottom" of the slot shroud, as indicated in the FIG. 3 side view. Run No. 6 employed a convex shroud, i.e., the exit face is a spherical segment such that the ends of the slot are shallower than the central portion (which is directly in line with the spinneret axis), as illustrated in FIG. 5. The function of these shrouds is to spread the expanding vaporizing solution laterally in order to generate a web-shaped plexifilament. The pertinent dimensions of the spinnerets employed for each run are tabulated in Table 1B.

The 30 runs recorded in Table 1A were all extruded at 220.degree.C., and therefore should produce "good" and "bad" morphology products for (MFR/c) ratios above and below 1.13, respectively. Only five of these runs failed to fit the predicted morphology pattern and four of these five "exceptions" (runs 2, 4, 9, and 25) occur so close to the boundary line (within .+-. 15 percent) that they do not constitute meaningful "exceptions," allowing for the experimental uncertainties in the polymer MFR data. (Production of a "bad" morphology product in run 24 remains unexplained). The data for these runs are plotted in FIG. 1. Note from Table 1A the strong tendency for flexifilaments of good morphology to exhibit the higher tenacities.

EXAMPLE II

Another series of flash extrusion experiments employing isotactic polypropylene/trichlorofluoromethane was conducted in a 20-gallon autoclave employing filling and heating techniques similar to those described above. However, in order to explore extrusion performance at higher temperatures, the maximum autoclave solution temperature for this series was generally held between 170.degree.-200.degree.C. (where polymer degradation rates are not excessive) and the autoclave was connected to the spinneret by a heated 186-inch long 1/4-inch diameter transfer line whereby the solution temperature could be raised rapidly during flow through the line to extrusion temperatures between 220.degree.-240.degree.C. with minimum exposure time to these polymer-degrading temperatures. Although precise temperature control with this experimental system was difficult, data for higher temperature extrusions were obtained as reported in Table 2A. Points for these runs are also indicated on FIG. 1, and again-- except for three or four possibly significant exceptions-- the morphologies of the products are as predicted by the (MFR/c) relationship.

Pertinent data for the spinneret hardware employed in these runs are given in Table 2B.

EXAMPLE III

Nine 5-gallon autoclave flash extrusion experiments were run by the procedure of Example I to define the maximum operable polymer MFR, since high values of this parameter are required in order to exceed the (MFR/c) limit whenever higher concentrations are employed. All nine runs employed isotactic polypropylene of initial MFR equal 0.4, and the solutions were held at elevated temperature for varying periods of time in order to provide varying degrees of degradation, and hence MFR increases, prior to extrusion. Each product in this series would be predicted to have good morphology and high tenacity, since each run (except perhaps for No. 67) satisfied the required initial conditions of temperature, concentration, pressure and (MFR/c), as indicated by the data of Table 3A. However, the results of these experiments (e.g., runs 62, 66, 69) indicate that for polymer MFRs above about 30, polymer degradation has become so severe that the plexifilament morphology deteriorates, i.e., the film-fibrils are too weak to prevent web splitting during the violent flash extrusion process, and yarn tenacity as measured on plexifilamentary samples twisted to 10 tpi also falls to lower values. (No immediate explanation is at hand for run 65, which appears to be an exception to the general correlation.) Unfortunately, but not surprisingly, the data (e.g., runs 61, 68) do not indicate a sharply defined maximum operable polymer MFR, but simply indicate that MFR's below about 30 are to be preferred.

The pertinent spinneret hardware parameters for these runs are indicated in Table 3B. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6##

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the flash spinning of high quality plexifilamentary material which comprises in sequence:

1. mixing sufficient isotactic polypropylene with trichlorofluoromethane to obtain a solution wherein the concentration of polypropylene is between about 2 and 20 percent by weight,

2. adjusting the temperature and pressure of the solution until the temperature is between about 200.degree.C. and 240.degree.C., the pressure on the solution is in excess of 900 psig; and the melt flow rate (MFR) of the isotactic polypropylene fulfills the following formula:

(MFR/c) .gtoreq. 1.13 - 0.04 (T-220)

in which c is the concentration of the isotactic polypropylene in the solution, expressed in weight percent and T is the temperature (.degree.C.) of the solution, and wherein MFR is equal to a numeral between 2 and 30, inclusive; and

3. extruding said solution abruptly into a region of lower temperature and pressure so as to produce said plexifilamentary material.

2. The process of claim 1 wherein the minimum MFR/c value is 15 percent greater than the value 1.13 - 0.04 (T-220).

3. The process of claim 1 wherein the temperature of the solution in step (2) is between about 220.degree. and 235.degree.C.

4. The process of claim 2 wherein the temperature of the solution in step (2) is between about 220.degree. and 235.degree.C.

5. The process of claim 1 wherein the pressure on the solution in step (2) is high enough to maintain the solution as a single phase.

6. The process of claim 5 wherein, after step (2) and before step (3), the solution is passed through a pressure letdown chamber where the pressure on the solution is lowered to a point at which the solution forms a two-phase liquid, said solution remaining in said letdown chamber not more than 30 seconds.

7. The process of claim 6 wherein the minimum MFR/c value is 15 percent greater than the value 1.13 - 0.04 (T-220).

8. The process of claim 6 wherein the temperature of the solution in step (2) is between about 220.degree. and 235.degree.C.

9. The process of claim 7 wherein the temperature of the solution in step (2) is between about 220.degree. and 235.degree.C.

10. The process of claim 1 wherein the polypropylene has a MFR lower than that required to fit the equation in claim 1 and is thermally degraded in the solution to a value that does fit said equation.