CA2077192C - Halocarbons for flash-spinning polyethylene plexifilaments - Google Patents

Abstract

An improved process is provided for flash-spinning plexifilamentary film-fibril strands of fiber-forming polyethylene from a dichlorotrifluoroethane such as 1,1-dichloro-2,2,2-trifluoroethane.

Description

WO 91/13193 ' PGT/~J590/a0875 TITLE
Halocarbons for Flash-Spinning Polyethylene Plexifilaments BACRGROUIND OF THE INVEIdTIOTd Field of the Invention This invention relates to flash=spinning polyethylene film-fibril strands. More particularly, the invention concerns an improvement in such a process which permits flash-spinning of the strands from a liquid which, if released to the atmosphere, would not detrimentally affect the earth's oaone.
Description of the Prior Art Blades and White, United States Patent 3,081,519, describes a flash-spinning process for producing plexifilamentary film-fibril strands from fiber-forming polymers such as polyethylene. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the, liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash-spinning causes the liquid to vaporize.and thereby.cool the exudate which forms a plexifil~mentary film--fibril strand of the polymer.
Anderson and Romano, United States F~atent 3,227,794, discloses technology for selecting conditions for spinning plexifilamentary strands.. A graph is presented of spinning temperature versus spinning pressure for solutions of 10 to 16 weight percent of linear polyethylene in trichlorofluoromethane (or .
"F-11"). This patent also descrih~~ in detail the preparation of a solution of 14 wei.gh+;, percent high w density linear polyethylene in trs.c:hlorofluoromethane at a temperature of about 185°C and a pressure of about ., 1640 psig which is then flash-spun from a let-down .

WO 91/13193 ~ ~ rl ~ ~ ~ ~ P(.T/dJS986~9875 1 ,..::

chamber at a temperature of 185°C and a pressure of 1050 psig. Very similar temperatures, pressures and concentrations have been employed in commercial flash-spinning of polyethylene into plexifilamentary ;
film-fibril strands, which were then converted into sheet structures.
Although trichlorofluoromethane has been a very useful solvent for flash-spinning plexifilamentary ;
film-fibril strands of polyethylene, and has been the solvent used in commercial manufacture of polyethylene plexifilamentary strands, the escape of such a halocarbon into the atmosphere has been implicated as a source of depletion of the earth's ozone. A general discussion of the ozone-depletion problem is presented, for example, by P.S. Zurer, "Search intensifies for Alternatives to Ozone-Depleting Halocz~rbons", Chemical ~
Engineering News, pages 17-20 (February 8, 1988).
An object of this invention is to provide an improved process for flash-spinniing plexifilamentary film-fibril strands of fiber-forming polyethylene, wherein the solvent should not be a depletion hazard to the earth's ozone.
SUMRiARY O~' THE IN'lEPITIOTd The present invention provides an. improved process for flash-spinning plexifilamentary film-fibril strands wherein polyethylene having a melt index of at least 4 and a density of about 0.92-0.98 is dissolved in at least one isomer of dichlorotrifluoroethane, preferably~l,l-dichloro-2,2,2-trifluoroethane, to form a spin solution containing 10 to 20 percent of the polyethylene by weight of the solution at a temperature in the range of 130 to 210°C and a pressure that is greater than 2400 psi followed by LJ.ash-spinning the solution into a region of substantially lower temperature and pressure.
The present invention provides a novel 3 PCl"/11S9~D/00875 - 3 ~ ~ .f. ~
solution consisting essenti:..ly of 10 to 20 weight percent of polyethylene having a melt index of at least 4 and a density of about 0.92-0.98 and 90 to 80 weight percent of at least one isomer of dichlorotrifluoroethane, preferably 1,1-dichloro-2,2,2-trifluoroethane.
DETAILED DESCRIPTION OF PREFERRED EI~IBODIMEI~ITS
"Polyethylene" as used herein is intended to w embrace not only homopolymers of ethylene, but also copolymers wherein at least 85~ of the recurring units are ethylene units. One preferred polyethylene is a linear high density polyethylene which has an upper limit of melting range of about 130 to 135°C, a. density in the range of 0.94 to 0.98 g/cm' and a melt index (as defined :v:y ASTM D-1238-57T, Condition E) of greater than 4, and preferably below 100. Another preferred polyethylene is a linear ,low density polyethylene having a density of about 0.92-0.94 and a melt index of at least 4, preferably also below 100.
The term "plexifilamentary film-fibril strands" as used herein, means a strand..which is characterized as a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of,random length and.of less than about 4 microns . , average thickness, generally coextensi~rely aligned with the longitudinal axis of the strand. The film-fibril elements intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the strand to form the three-dimensional network. Such strands are described in further detail by Blades and White, United States Patent 3,081,519 and by Anderson and Romano, United States Patent 3,227,794.
A convenie~.°;' test to determine whether a given solvent would be su~:able for flash-spinning a given polymer is disclosed by Woodell, United States Patent WO 91/13193 ~ ~ ~ ~ ~ ~ ~ -4- pCf/1~~9~D968f 3,655,998. This test has been used extensively to determine the suitability of alternatives to the trichlorofluoromethane solvent for preparing plexifilamentary strands. In the test, a mixture of the polymer plus the amount of solvent calculated to give about a 10 weight percent solution, is sealed in a ' thick-walled glass tube (the mixture occupies about one-third to one-half the tube volume) and the mixture is heated at autogenous pressure. Test temperatures usually range from about 100°C to just below the critical temperature of. the liquid being tested.
Woodell states that if a singled-phase, flowable solution is not formed in the tube at any temperature below the solvent critical temperature, T~, (or the polymer degradation temperature, which is lower) the solvent power is too low.
It has been found that, contrary to the prior art of Woodell, when an isomer of dichlorotrifluoroethane such as 1,1-dichloro-2,2,2-trifluoroethane ("HC-123") is the solvent it is entirely practical to produce a solution of 10 to 20 weight percent of polyethylene having a melt index of at least 4 and a density of about 0.92-0.98 arad then to flash-spin the solution at temperatures of 130 to 210°C and comparatively low pressures to produce high quality products. For this combination it is not necessary that the solution be formed into a single phase, it is sufficient that a homogeneous two phase solution be formed and spun as such. Indeed at pressures below about 5000-8000 psi such solutions will usually be of two-phases but high quality products can nonetheless be produced.
In accordance with the F!:esent invention the -t ri. f luoroethane halocarbon is 1,1-dichloro-2,2,2 '35 ("HC-123"), 1,2-dichloro-1,2,2-trifluoroethane ("HC-123a"), or 1,1-dichloro-1,2,2-trifluoroethane ' WO 91/13193 PC'~'/~JS9~/gyG~~7a _5- , . . , ("HC-123b"). The parenthetic designation is used herein as an abbreviation for the chemical formula of the halocarbon. The following table lists the known normal atmospheric boiling points (Tbp), critical temperatures (Tcr) and critical pressures (Pcr) for thesehalocarbons and far some prior art solvents. Tn the column labeled "Solubility", the Table also lists whether a 10~
polyethylene solution can be formed as a single phase in the halocarbon or hydrocarbon at temperatures between 100 and about 225°C under autogenous pressures.
Tbp,°C Tcr,°C Pcr, psia Solubility HC-123 28.7 185 550 no HC-123a 28 HC-123b 30.2 Trichloro-fluoromethane 23.B 198.0 639.5 yes Methylene-chloride 39.9 237.0 894.7 yes Hexane 68.9 234.4 436.5 yes Cyclohexane 80.7 280.4 590.2 yes It is to be noted that the halocarbons of the present invention do not dissolve the polyethylene at autogenous pressures, in contrast to the prior art solvents shown above. In contrast to the flash spi~aa~i~ag fluids of the past, they do not form a single phase solution with polyethylene at the rewired concentrations and temperatures at a pressure of less than 5,000 Asia. Indeed it is not necessary that these halocarbons form a single phase solution even at the mixing temperature. Thus the polyethylenes of this invention can be dissolved in the various HC-123 isomers to form a uniform two phase solution which can be spun directly.
In forming a solution of fiher-forming polyethylene and HC-123 or one of _ts isomers, a mixture of the components is raised to a temperature in the ;

WO 91/13193 P~d'/iU~9~/~~~B'7L~
...
range of 130 to 210°C. Below pressures of about 5000-8000 psi the solution will usually be of two phases, whereas above that range there will usually be only one phase. The mixtures described above are held ' under the required pressure until a homogeneous one phase or two phase solution is formed. Usually, maximum pressures of less than 10,000 psi are satisfactory. The pressure may optionally be reduced somewhat and the mixture then flash spun to form the desired high qua:~aty plexifilamentary strand structure.
The spin solution preferably consists of HC-123 or its isomers and fiber-forming polyethylene.
However conventional flash-spinning additives can be incorporated into the spin mixture by known techniques.
These additives can function as ultraviolet-light stabilizers, antioxidants, fillers, dyes, and the like.
The various characteristics and properties mentioned. in the preceding discussion and in the examples below were determined by the following procedures.
Test Methods The quality of the plexifilamentary film-fibril.. strands produced in the examples was rated subjectively. A rating of "5" indicates that the strand had better fibrillation than is usually achieved in the commercial production of spunbonded sheet made from such flash-spun polyethylene strands. A sating of "4"
indicates that the product was as good as commercially flash-spun strands. A rating of "3" indicates that the strands were not quite as good as the commercially flash-spun strands. A "2" indicates a very poorly fibrillated, inadequate strand. r "J." indicates no strand formation. A rating of "3" i.s the minimum considered satisfactory for use in the process of the present invention. The commercial strand product is s WO 91/13193 1PC'f/~.JS90d~f~lS'9S
groduced from solutions of abaut 12.5 linear polyethylene in trichlorofluoromethane substantially as set forth in Lee, United States patent 4,559,207, column 4, line 63, through column 5, line L0.
The surface area of the plexifilamentazy film-fibril strand product is another measure of the ' degree and fineness of fibrillation of the flash-spun product. Surface area is measured by the BET nitrogen absorption method of S. Hzunauer, P.H. Emmett and E.
Teller, J. Am. Chem Soc., V. 60 p 309-319 (1939) and is reported as m2 /g.
Tenacity of the flash-spun strand is determined with an Instron tensile-testing machine. The strands are conditioned and tested at 70°F and 65~
relative humidity.
The denier of the strand is determined from the weight of a 15 cm sample length of strand. The sample is then twisted to 10 turns per inch and mounted in the jaws of the Instzon Tester. A 1-inch gauge length and an elongation rate of 60% per minute are used. the tenacity at break is recozded in grams per denier (gpd).
The .invention is illustrated in the Examples which follow with a batch process in equipment of relatively small size. Such batch processes can be scaled-up and convested to continuous flash-spinning processes that can be performed, for example, in the type of equipment disclosed by Anderson and ~tomano, United States Patent 3,227,799. Parts and percentages are by weight unless otherwise indicated.
EXAPqPLES
For each of Examples 1-?.?. a.solution of HC-123 and polyethylene was flash-spun into, satisfactory plexifilamentary film-fibril strands in accordance with the invention. Five different polyethylenes were used, differing in melt index (molecular weigh.t). LLDPE

2 ~cri~»~~~r~~~~a WO (1/13193 stands for linear low density polyethylene, HDPE for high density polyethylene.
The apparatus employed comprises a pair of high pressure cylindrical vessels, each fitted with a piston for applying pressure. The vessels are cylindrical and are connected to each other with a ' transfer line. The transfer line contains a series of fine mesh screens intended for mixing the contents of the apparatus by forcing the contents through the transfer line from one cylinder to the other. A
spinneret assembly having an orifice of 0.030-inch diameter is connected to the transfer lines with quick acting means for opening and closing the orifice. Means are included for measuring the pressure and temperature inside the vessel. For Exmple 1 the spinneret assembly consists of a pressure letdown orifice of 0.03375 inch (8.5X10-°m) diameter and a 0.030 inch length (7.62X10-°m), a letdown chamber of 0.25 inch (6.3X10-3m) diameter and 1.92 inch length, and a spinneret orifice of 0.30 inch (7.62X10-°m) diameter. In operation, the apparatus is charged with polymer and HC-123 and a high pressure is applied to the charge. The contents then are heated at the desired temperature for about an hour and a half during which.time a differential.pressure ok about 50 psi is alternately established between the two cylinders to repeatedly force the contents. through the, transfer line from one cylinder to the other to provide mixing and effect formation of a solution. The pressure desired for spinning is then set and the spinneret orifice opened. The .resultant flash-spun product is then collected.
All Examples were performed in a similar fashion under the specific conditi.c~n: and with the particular ingredients shown in the following summary ' table. The table also records characteristics of the strands produced by the flash-spinning.

WO PC('/tJ~96~AQ~08'7~9 a:. _9_ In all of the Examples solution the which is spun is composedof two phases.

Table I

Example too. 1 2 3 4 Polyethylene LLDPE LLDPE HDPE HDPE

Melt Index 12 12 55 33 Density, g/cm3 0.933 0.933 0.955 0.955 Conc, wt ~ 15.4 15 15 15 Mixing Temp, C 140 180 180 180 Press, psig 2400- 2550 3500 3500 3500 Spinning Temp, C 160 180 180 180 (.

Press, psig 1950 3500 3500 3500 Strand Product Denier 554 570 457 525 - Tenacity, gpd 1.15 1.3 1.05 1.6 Quality 4 4 4 4 ~O 91113193 1PCT/U~9~!/8~& 7e~

Table I (cont.) Example No. 5 6 7 8 9 10 11 Polyethylene HDPE HDPE HDPE HDPE HDPE HDPE HDPE
Melt Index 17.5 6 6 6 6 6 6 Density, g/cm3 0.948 0.96 0.96 0.96 0.96 0.96 0<.94 Conc, wt ~ 15 15 16 12 16 16 16 Mixing Temp, °C 180 180 160 180 140 380 140 Press, psig 3500 3500 3000 3500 3500 3500 2500 Spinning Temp, °C 180 180 160 180 140 180 140 Press, psig 3500 3500 3000 3500 3500 3500 2500 Strand Product Denier 561 624 853 686 852 607 968 Tenacity, gpd 1.8 2.5 2.3 2.3 2.1 2.5 2.2 Quality 4 4 4 4 4 4 4

Claims (5)

I Claim:

1. An improved process for flash-spinning plexifilamentary film-fibril strands wherein polyethylene having a melt index of at least 4 and a density of about 0.92-0.98 is dissolved in an isomer of dichlorotrifluoroethane to form a spin solution containing 10 to 20 percent of the polyethylene by weight of the solution at a temperature in the range of 130 to 210°C and a pressure that is greater than 2400 psi followed by flash-spinning the solution into a region of substantially lower temperature and pressure.

2. A process in accordance with Claim 1 wherein the isomer is 1,1-dichloro-2,2,2-trifluoroethane.

3. A process in accordance with Claim 1 wherein the pressure is such that a two phase solution is spun.

4. A solution consisting essentially of 10 to 20 weight percent of polyethylene having a melt index of at least 4 and a density of about 0.92-0.98 and 90 to 80 weight percent of an isomer of dichlorotrifluoroethane.

5. A solution according to Claim 4 wherein the isomer is 1,1-dichloro-2,2,2-trifluoroethane.