WO1991013193A1

WO1991013193A1 - Halocarbons for flash-spinning polyethylene plexifilaments

Info

Publication number: WO1991013193A1
Application number: PCT/US1990/000875
Authority: WO
Inventors: Hyunkook Shin
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1990-02-26
Filing date: 1990-02-26
Publication date: 1991-09-05
Also published as: CA2077192A1; KR0126548B1; DE69030626T2; HK113797A; EP0517693A1; CA2077192C; EP0517693B1; JP2851943B2; JPH05503552A; KR930700705A; DE69030626D1

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

TITLE

Halocarbons for Flash-Spinning

Polyethylene Plexifilaments

BACKGROUND OF THE INVENTION

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 ozone.

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 plexifilamentary film-fibril strand of the polymer.

Anderson and Romano, United States Patent 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 describes in detail the preparation of a solution of 14 weight percent high density linear polyethylene in trichlorofluoromethane at a temperature of about 185°C and a pressure of about 1640 psig which is then flash-spun from a let-down 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 Halocarbons", Chemical & Engineering News, pages 17-20 (February 8, 1988).

An object of this invention is to provide an improved process for flash-spinning plexifilamentary film-fibril strands of fiber-forming polyethylene, wherein the solvent should not be a depletion hazard to the earth's ozone.

SUMMARY OF THE INVENTION

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 1,1-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 flash-spinning the solution into a region of substantially lower

temperature and pressure.

The present invention provides a novel 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 at least one isomer of

dichlorotrifluoroethane, preferably

1,1-dichloro-2,2,2-trifluoroethane.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

"Polyethylene" as used herein is intended to 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 135ºC, a density in the range of 0.94 to 0.98 g/cm³ and a melt index (as defined by 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 coextensively 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 convenient test to determine whether a given solvent would be suitable for flash-spinning a given polymer is disclosed by Woodell, United States Patent 3,655,498. 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_c, (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 and then to flash-spin the solution at temperatures of 130 to 210ºC 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 present invention the halocarbon is 1,1-dichloro-2,2,2-trifluoroethane

("HC-123"), 1,2-dichloro-1,2,2-trifluoroethane

("HC-123a"), or 1,1-dichloro-1,2,2-trifluoroethane ("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 (Per) for thesehalocarbons and for some prior art solvents. In 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 225ºC under autogenous pressures.

Tbp,°C Tcr,°C Per, psia Solubility

HC-123 28.7 185 550 no

HC-123a 28

HC-123b 30.2

Trichloro- fluoromethane 23.8 198.0 639.5 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 spinning fluids of the past, they do not form a single phase solution with polyethylene at the required

concentrations and temperatures at a pressure of less than 5,000 psia. 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 fiber-forming polyethylene and HC-123 or one of its isomers, a mixture of the components is raised to a temperature in the 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 quality 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 rating 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. A "1" indicates no strand formation. A rating of "3" is the minimum considered satisfactory for use in the process of the present invention. The commercial strand product is produced from solutions of about 12.5% linear

polyethylene in trichlorofluoromethane substantially as set forth in Lee, United States patent 4,554,207, column 4, line 63, through column 5, line 10.

The surface area of the plexifilamentary 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. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem Soc, V. 60 p 309-319 (1938) 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 Instron Tester. A 1-inch gauge length and an elongation rate of 60% per minute are used. The tenacity at break is recorded 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 converted to continuous flash-spinning processes that can be performed, for example, in the type of equipment disclosed by Anderson and Romano, United States Patent 3,227,794. Parts and percentages are by weight unless otherwise indicated.

EXAMPLES

For each of Examples 1-11 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 weight). LLDPE 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^-4m) diameter and a 0.030 inch length

(7.62X10^-4m), 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^-4m) 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 of 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 conditions and with the particular ingredients shown in the following summary table. The table also records characteristics of the strands produced by the flash-spinning. In all of the Examples the solution which is spun is composed of two phases.

Claims

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.