A process for flash spinning polyolefins

Abstract: The invention relates to a process for flash-spinning plexifilamentary film-fibril strands of polyolefins. More particularly, the strands are flash-spun from mixtures of carbon dioxide, water and the polyolefin. The invention also relates to the film-fibril strands produced by the inventive process.

Description: FIELD OF THE INVENTION The invention relates to a process for flash-spinning plexifilamentary film-fibril strands of polyolefins. More particularly, the invention relates to plexifilamentary film-fibril strands that are flash-spun from mixtures of carbon dioxide, water and the polyolefin. BACKGROUND OF THE INVENTION Blades and White, United States Patent 3,081,519 describe flash-spinning plexifilamentary film-fibril strands from fiber-forming polymers. 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. According to Blades and White, the following liquids are useful in the flash-spinning process: aromatic hydrocarbons such as benzene, toluene, etc.; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fluorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids. The patent further states that the flash-spinning solution additionally may contain a dissolved gas, such as nitrogen, carbon dioxide, helium, hydrogen, methane, propane, butane, ethylene, propylene, butene, etc. Preferred for improving plexifilament fibrillation are the less soluble gases, i.e., those that dissolve to a less than 7% concentration in the polymer solution under the spinning conditions. Blades and White state that polymers which may be flash spun include those synthetic filament-forming polymers or polymer mixtures which are capable of having appreciable crystallinity and a high rate of crystallization. A preferred class of polymers is the crystalline, non-polar group consisting mainly of crystalline polyhydrocarbons, such as polyethylene and polypropylene. U.S. Patent 3,169,899 lists polyester, polyoxymethylene, polyacrylonitrile, polyamide, polyvinyl chloride, etc. as other polymers that may be flash spun. Still other polymers mentioned in the patent are flash spun as mixtures with polyethylene, including ethylene vinyl alcohol, polyvinyl chloride, polyurethane, etc. Example 18 of U.S. Patent 3,169,899 illustrates flash spinning from methylene chloride of a mixture of polyethylene and ethylene vinyl alcohol in which polyethylene is the predominant component of the polymer mixture. Flash spun polyethylene products have achieved considerable commercial success. "Tyvek®" is a spunbonded polyethylene sheet product sold by E. I. du Pont de Nemours and Company. These sheets are used in the construction and packaging industries. "Tyvek®" is also used in protective apparel since the flash spun product provides a good barrier to particulate penetration. However, the hydrophobic nature of polyethylene results in a garment which tends to be uncomfortable during hot, humid weather. A more hydrophilic flash spun product is clearly desirable for garment and some other end uses. Additionally, flash spinning of any of the polyolefins would preferably be achieved from an environmentally safe, non-toxic solvent. Trichlorofluoromethane (Freon-11) has been a very useful solvent for commercial manufacture of plexifilamentary film-fibril strands of polyethylene. However, the escape of such a halocarbon into the atmosphere has been implicated as a serious 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). The substitution of environmentally safe solvents for trichlorofluoromethane in a commercial flash spinning process should minimize the ozone depletion problem. There now has been discovered in accordance with this invention, flash spun polyolefin products desirable for uses such as garments, construction and packaging, which are flash spun from an environmentally acceptable mixture comprising carbon dioxide and water. SUMMARY OF THE INVENTION According to this invention plexifilamentary film-fibril strands of a polyolefin are flash spun by a process comprising the steps of forming a spin mixture of water, carbon dioxide and the polyolefin at a temperature of at least 130°C, at a pressure that is greater than the autogenous pressure of the mixture and then flash spinning the mixture into a region of substantially lower temperature and pressure, wherein the carbon dioxide is present in the range of from 30 to 90 percent based on the total weight of the spin mixture, and wherein the polymer constituent of the spin mixture comprises ethylene vinyl alcohol copolymer and an additional polyolefin present in the range from 0 to 6.5 percent based on the total weight of the spin mixture. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The term "plexifilamentary film-fibril strand" or simply "plexifilamentary strand", 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. Polyolefins particularly useful in the practice of this invention are polyethylene, polypropylene, copolymers of ethylene and vinyl alcohol (hereinafter sometimes referred to as "EVOH") and combinations thereof. The copolymers of ethylene and vinyl alcohol have a copolymerized ethylene content of about at least 20 mole % and generally a vinyl alcohol content of at least about 50 mole %. The ethylene vinyl alcohol copolymer may include as an optional comonomer other olefins such as propylene, butene-1, pentene-1, or 4-methylpentene-1 in such an amount as to not change the inherent properties of the copolymer, generally in an amount of up to about 5 mole%, based on the total copolymer. The melting points of these ethylene vinyl alcohol polymers are generally between about 160 and 190 degrees C. Ethylene vinyl alcohol polymers are normally prepared by copolymerization of ethylene with vinyl acetate followed by saponification of the acetate groups to the hydroxyl groups. At least about 90% of the acetate groups should by saponified, this being necessary to achieve sufficient mixing of the polymer. This process is well known in the art. The process requires forming a spin mixture of the polyolefin, water and carbon dioxide. The water is present in the range from 5 to 50 percent based on the total weight of the spin mixture. The carbon dioxide is present in the range from 30 to 90 percent based on the total weight of the spin mixture. The polyolefin is present in the range from 1.5 to 25 percent based on the total weight of the spin mixture. As noted above, the spin mixture may also comprise ethylene vinyl alcohol copolymer and an additional polyolefin present in the range from 0 to 6.5 percent based on the total weight of the spin mixture. Conveniently, polyethylene and polypropylene are the preferred additional polyolefins. The spinning mixture may optionally contain a surfactant. The presence of such a surfactant appears to assist in emulsifying the polymer, or in otherwise aiding in forming a mixture. Examples of suitable nonionic surfactants are disclosed in U. S. Patent No. 4,082,887, the contents of which is herein incorporated by reference. Among the suitable, commercially available, nonionic surfactants are the "Spans", which are mixtures of the esters of the monolaurate, monooleate and monostearate type and the "Tweens", which are the polyoxyethylene derivatives of these esters. The "Spans" and the "Tweens" are products of ICI Americas, Wilmington, DE. The required temperatures for preparing the spin mixture and for flash-spinning the mixture are usually about the same and usually are in the range of 130 to 275°C. The mixing and the flash-spinning are performed at a pressure that is higher than the autogenous pressure of the mixture. The pressure during the spin mixture preparation is generally in the range from 1,200 to 6,000 psi. Conventional flash-spinning additives can be incorporated into the spin mixtures by known techniques. These additives can function as ultraviolet-light stabilizers, antioxidants, fillers, dyes, surfactants and the like. EXAMPLES Equipment Two autoclaves were used in the following non-limiting examples. One autoclave, designated a "300cc" autoclave (Autoclave Engineers, Inc., Erie, PA) was equipped with a turbine-blade agitator, temperature and pressure measuring devices, heating means, a means of pumping in carbon dioxide under pressure and inlets for loading the ingredients. An exit line from the bottom of the autoclave was connected through a quick-acting valve to a spin orifice 0.079 cm in diameter. The spin orifice had a length to diameter ratio of 1 with a tapered conical entrance at an angle of 120 degrees. The second autoclave, designated a "1 gallon" autoclave (again made by Autoclave Engineers, Inc.), was equipped in an analogous manner to that of the "300cc" autoclave. Test Procedures The surface area of the plexifilamentary film-fibril strand product is a 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, Journal of American Chemical Society , Vol. 60, pp. 309-319 (1938) and is reported as m²/g. Tenacity and elongation of the flash-spun strand are determined with an Instron tensile-testing machine. The strands are conditioned and tested at 70°F (21.1°C) 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). In the non-limiting examples which follow, all parts and percentages are by weight unless otherwise indicated. The conditions of all Examples are summarized in Table I. Example 1 The "300 cc" autoclave was loaded in sequence with 7 g of an ethylene vinyl alcohol copolymer, 43 g crushed ice and 50 g crushed solid carbon dioxide. The copolymer contained 30 mole% ethylene units, had a melt flow rate of 3 g/10 min by standard techniques at a temperature of 210°C and a pressure of 2.16 kg, a melting point of 183°C and a density of 1.2 g/cc. The resin was a commercially available product from E. I. du Pont de Nemours and Company sold as SELAR® 3003. The autoclave was closed and the vessel was pressurized to 850 psi (5861 kPa) with liquid carbon dioxide for 5 minutes while stirring until the mixture reached room temperature (24°C). The amount of carbon dioxide added was then obtained from the difference of volumes (the densities of the polymer (1.2 g/cc), water (1.0 g/cc) and liquid carbon dioxide (0.72 g/cc) at 24°C assuming complete filling of the autoclave. The amount of carbon dioxide added to this point was 166 g. The stirrer was rotated at 2000 rpm, and heating was begun. When the temperature of the contents of the autoclave reached 175°C, the internal pressure was adjusted by venting approximately 10% of the carbon dioxide and 10% of the water to reduce the pressure to 2500 psi (17,238 kPa). The spin mixture, after venting, contained 3.6% ethylene vinyl alcohol copolymer, 19.8% water and 76.6% carbon dioxide as shown in Table I. The stirring was continued for 30 minutes at a temperature of 175°C and a pressure of 2500 psi. Agitation was stopped followed by prompt opening of the exit valve to permit the mixture to flow to the spin orifice which also had been heated to 175°C. The mixture was flash spun and collected. Scanning Electron Microscopy (SEM) revealed a finely fibrillated continuous plexifilamentary strand. The strand was noticably elastomeric and had recovery properties. Example 2 The procedure of Example 1 was followed except that an ethylene vinyl alcohol copolymer was used with 44 mole% ethylene units. The 44 mole% copolymer was obtained from E. I. du Pont de Nemours and Company as SELAR® 4416. It had a melt flow rate of 16 g/10 min (210°C, 2.16 kg) a melting point of 168°C and a density of 1.15 g/cc. The result as determined by SEM was a finely fibrillated plexifilamentary strand. The strand was noticably elastomeric and was similar in appearance to the strand of Example 1. Example 3 The procedure of Example 2 was followed except that the spin pressure was 2550 psi. The result again was an elastomeric plexifilamentary strand. SEM analysis showed the strand to be coarser than the strand of Example 2. Example 4 The procedure of Example 1 was followed except that the polymer concentration was increased and the spin pressure was 3300 psi. The result was a strand similar to that of Example 3. Example 5 The procedure of Example 1 was followed except that the spin pressure was 3500 psi and 0.5%, based on the total weight of the spin mixture, high density polyethylene (HDPE) was added to the mixture. The polyethylene used has a melt index of ca. 0.8, and is commercially available from Cain Chemical Co., Sabine, TX as ALATHON® 7026A. The result was a high quality finely fibrillated plexifilamentary strand. The strand was elastomeric but less so than the strand of Example 1. Example 6 The procedure of Example 5 was followed except that the amount of polyethylene was increased. The result as determined by SEM was a continuous finely fibrillated strand of slightly more coarse fibrillation than the strand of Example 5. The strand showed a further loss in elastomeric properties over the strand of Example 5. Example 7 The procedure of Example 5 was followed except that the amount of polyethylene was further increased. SEM analysis revealed a coarse plexifilamentary strand. The strand had no elastomeric properties. Example 8 The procedure of Example 1 was followed with the various component changes as shown in Table I. In this example, 2 g of a nonionic surfactant mixture containing 65% by weight "Span" 80 and 35% by weight "Tween" 80 was added to the spin mix. The autoclave was not vented in this example, but was allowed to reach the spin pressure by heating and holding the temperature at 177°C. The result was a continuous, somewhat coarsely fibrillated mat of plexifilamentary fibers. The fibers were elastomeric. Example 9 The procedure of Example 8 was followed with the various component changes as shown in Table I. The result was a strand similar to that of Example 8. Example 10 The procedure of Example 1 was followed with the various component changes as shown in Table I. The result was a plexifilamentary yarn of very fine, continuous white fibers. Example 11 The procedure of Example 5 was followed except that linear low density polyethylene (LDPE) was used instead of high density polyethylene, as shown in Table I. The linear low density polyethylene (melt index of 25) is sold commercially by Dow Chemical Corp., Midland, MI as Aspun® 6801. The result was fine, discontinuous plexifilamentary fibers 1/4 to 1/2 inch in length. Example 12 A "300 cc" autoclave was used. Through an addition port, the autoclave was loaded with 15g. Selar® OH 4416 resin, 15g ASPUN® 6801 resin and 56 g water. Most of the air was removed from the autoclave by brief evacuation to 20 in. mercury. The autoclave was then pressurized with 146 g carbon dioxide, the agitator set to 2000 rpm and heating begun up to a goal temperature of 170°C. When the goal temperature was reached, the pressure was adjusted by venting small amounts of the mixture to give 4,700 psi. The mixture was then agitated an additional 15 minutes. The exit valve was opened and the mixture spun through the spin orifice. A very finely fibrillated continuous yarn, soft and with fibers that are easily separated from the yarn bundle, was produced. Example 13 The procedure of Example 1 was followed, except that the charge consisted of 4 g Huntsman 7521 polypropylene (Huntsman Polypropylene Corp., Woodbury, NJ), an injection molding grade homopolymer of melt flow 3.5 g/10 minutes and melting point of 168°C, 6 g Selar® OH 4416 ethylene vinyl alcohol copolymer, 43 g ice and 50 g crushed solid carbon dioxide (i.e., dry ice). The autoclave was heated to a goal temperature of 175°C, a pressure of 3,500 psi and agitated at 2,000 rpm for 15 minutes. When the discharge valve was opened, a mass of discontinuous, coarsly fibrillated fibers was obtained. Example 14 The procedure of Example 13 was followed except that the autoclave was charged with 10 g Selar® OH 4416 resin, 4 g Huntsman 7521 polypropylene resin, 43 g ice and 50 g crushed solid carbon dioxide. A finer fibrillated semi-continuous mass of fibers was made. Although particular embodiments of the present invention have been described in the foregoing description, it will be understood by those skilled in the art that the invention is capable of numerous modifications, substitutions and rearrangements without departing from the spirit or essential attributes of the invention. Reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention. Conversions 1 psi = 6.895 kPa 1 gallon = 3.79 litres 1 denier = 1.11 d tex 1 inch = 2.54 cm

Claims: A process for flash spinning plexifilamentary film-fibril strands of a polyolefin by the steps of forming a spin mixture of water, carbon dioxide and the polyolefin at a temperature of at least 130°C, at a pressure that is greater than the autogenous pressure of the mixture and then flash spinning the mixture into a region of substantially lower temperature and pressure, wherein the carbon dioxide is present in the range of from 30 to 90 percent based on the total weight of the spin mixture, and wherein the polymer constituent of the spin mixture comprises ethylene vinyl alcohol copolymer and an additional polyolefin present in the range from 0 to 6.5 percent based on the total weight of the spin mixture. The process of claim 1 wherein the water is present in the range from 5 to 50 percent based on the total weight of the spin mixture. The process of claim 1 or 2 wherein the total polyolefin is present in the range from 1.5 to 25 percent based on the total weight of the spin mixture. The process of any one of claims 1 to 3 wherein the spin mixture is formed at a temperature in the range of 130 to 275°C and a pressure in the range from 8274 to 41370 KPa (1,200 to 6,000 psi). The process of any one of claims 1 to 4 wherein the additional polyolefin is selected from polyethylene and polypropylene. The process of any one of claims 1 to 5 wherein the spin mixture further comprises a surfactant present in the range from 0 to 2 percent based on the total weight of the spin mixture. The process of any one of claims 1 to 6 wherein the ethylene vinyl alcohol copolymer comprises at least 20 mole% of ethylene units.

Inventors: COATES DON MAYO, MCMILLIN CARL KENNETH, COATES, DON MAYO, MCMILLIN, CARL KENNETH
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: D01D5
IPC codes: D01D5, D01F6