Polyvinylalcohol Plexifibrils, Nonwoven Webs made of them and the Process for their Manufacturing The invention relates to a process for manufacturing of plexifibrils out of polyvinylalcohol and of nonwoven webs-made out of them. Polyvinylalcohol will be mixed with water in a weight ratio from 1 to 0. 5 up to 1 to 10 to form a gel or a solution which solution will be heated under pressure up to a temperature from 120 to 230 °C (spinning temperature) and subsequently forwarded by means of a spinning gear pump at a pressure corresponding at least to the equilibr-im water steam pressure-at the spinning-temperature and extruded through a heated spinneret in an expansion space kept at a pressure substantially below the solution pressure-mentioned above whereby the solved polymer will be fine fibrillated due to instant flash evaporation of the present water.

The invention relates further to a process, where the weight ratio of polyvinylalcohol to water will be kept between 1 to 0. 5 to 1 to 2. Further, the invention relates to a process where the spinning temperature will be kept among 130 to 200 °C. Furthermore, the invention relates to a process where the expansion space will be kept essentially at atmospheric pressure.

Further, the invention relates to a process as described above, whereby the resulting polyvinylalcohol plexifibrils will be deposited at a foramin conveyer belt evacuated from the bottom so, that they foi-in a fibrous nonwoven web.

Finally, the invention relates to polyvinylalcohol plexifibrils manufactured according to any of the processes described above, to nonwoven webs made out of such polyvinylalcohol plexifibrils, as well as such polyvinylalcohol pleifibrils or nonwoven webs which are crosslinked to be made water insoluble in a way which is known and which represents a state of the art.

State of the Art Polyvinylalcohol (PVAL) can not be manufactured by direct polymerisation because the monomeric vinylalcohol in fact does not exists as a stable chemical compound. Acetaldehyd as a tautomeric isomer to vinylalcohol will be largely preferred. Therefore, polyvinylalcohol can only be manufactured by means of polymer-analogical reactions as by hydrolysis of polyvinylacetates or, as in technical scale, by catalytic trans-esterification of polyvinylacetates by alcohols, preferably by methanol. Technical PVAL is available at the market place as a white or yellowish powder in a range of molecular weights of 20,000 to 100,000, i. e. in a range of degrees of polymerisation from 500 to 2500. Another important parameter of the technical products is the degree of saponification, as they are, in fact, partially saponifie polyvinylacetates. Depending on the desired use, the degree of saponiScation can vary from 99 to about 87 %, or in terms of the portion of residual non changed acetate groups in a range of 1 to 13 Glass transition temperature and melting temperature of technical polyvinylalcohol products depends at the portion residual acetate groups, their distribution along the polymer chain and tacticity. Fully saponifie PVAL has a glass transition temperature of S5 °C and melting temperature of 228 °C. Respective values for partially saponified products (87-89 %) would be substantially lower at 58 °C or 186 °C respectively.

From the point of view of toxicology, polyvinylacetate will be considered as non dangerous and it is at least partially bio-degradabe. It is resistent against hydrocarbons, chlor- hydrocarbons, esters, fiats and oils. They are soluble in-water and-in strong polar organic solvents like formamide, dimethylformamide or dimethylsulfoxide. The solubility in-water can be reduced by crosslinking with aldehydes (acetalisation), forming of complexes with nickel or copper salts or by treatment with bi-chromates, boric acid or borax.

Technologies for spinning of PVAL are known since the 1-930-ties. The original process used aqueous PVAL solution which have been"wet-spun"by extruding through a spinneret in a coagulation bath. As-coagulation bath-have-been used concentrated solutions of sodium or ammonium sulphate capable to remove water from the spinning solution and cause a coagulation process in the formed fibre. In-a-second step, the still wet (water containing) fibres have been drawn in air or in salt bath. Thermosetting of fully drawn PVAL fibres under strain at 210 °C or more allows a formation of a oriented crystalline structure in the fibres, which can reach excellent strength properties. The resistance against boiling water can be reached by partial crosslinking of a part of the hydroxyl groups in non crystalline areas by formaldehyde at 70 to 90 in. Residual formaldehyde will be washed out.

Another, more modem technology is based on "dry spinning"of concentrated polymer solutions, which are extruded at high temperature through the spinneret an-quenched by hot air so, that the coagulation occurs below the spinneret by evaporation of residual water. The following technological steps, drawing, thermosetting and crosslinking, will be performed as described above.

According to known spinning technologies it is not possible to receive microfibres or fibrils ofpolyvinylalcohol.

It is further known to reach very fine fibrilla structure by using flash spinning process.

According to this technology it is possible to spin high density polyethylene (HDPE) from a solution in trifluor-chlor-methane or methylenchloride [USP 3, 081, 519 from 19. 03.1963: H.

BLADES and J. R. WHITE for the E. I. DuPont de Nemours & Co., Inc.]. A polymer solution of 10-15 % will be heated to a temperature of 200 °C (spinning temperature) under a pressure of 45 bar and subsequently extruded through a spinneret into an expansion space. The spinning temperature is far above the boiling temperature of the solvent. Therefore, the solvent will flash out from the thread and leave behind-a fibrillated three dimensional-network of polymer, which will be addressed as plexifilament. The thickness of the fibrils will be about 4 pm or less, their length of about 400 to 1000-The mm. The three dimensionality results from the cross- linking interconnection of the fibrils. Thus a multitude of individual but interconnected fibrils are created from each singular spinneret hole.

The fibrillation process is based on forming of bubbles as the pressurised solution is depressurised during spinning. This bubbles may grow and fracture, forming the plexifibrillar network. Gases insoluble in the solvent may be added to the pressurised solution to promote high rates of bubble nucleation.

By using a multiplicity of spinning holes arranged across a foraminous conveyer belt, the plexifilaments may be used to form a uniform fibrous sheet, which can be thermally bonded to form a paper-or textile-like web. Such webs provide excellent strength properties and due to their fibrillar structure an excellent softness. They can be used for protective clothing, envelopes, bags etc.

However, Rash spinning process using organic solvents, particularly such dangerous as chlor- fluor-hydrocarbons, is a very complex technology because all solvent has to be recycled and any pollution of the environment has to be prevented. Therefore, the process must be performed in closed cabinet with sophisticated provisions against any contamination of the environment. Even than there will still remain a residual risk of environmental disaster.

On top, the recycling process for the organic solvent represents a substantial factor in cost calculation.

Objective of the Invention Objective of this invention is to simplify the technology of manufacturing of plexifibrils from polymer solutions and/or gels by using exclusively aqueous polymer solutions or gels, limiting the choice of polymers on such which are either water soluble or at least capable to form aqueous gels.

Scope of the Invention The objective of the invention will be preferably reached by using polyvinylalcohol (PVAL). the meaning of this invention, technicals PVAL represent partially saponifie polyvinylacetates with a degree of saponification of at least 25 %. They are, particularly at elevated temperature, either water soluble, or at least forming aqueous gels.

Technical PVAL are available at the market place in form of white or yellowish powder or granules with a degree of polymerisation of 500 to 2500 or in a-range of molecular weights of 20, 000 to 100, 000. Such commercial are further characterised by various degrees of saponification.

One particular group of PVAL products has a degree of saponification of 87 to 89 % and hence a residual content of acetate groups of 13 to 11 %. Their glass transition temperature will be about 58 °C, their melting temperature 186 ° (z.

An other particular group of PVAL products has a very high degree of saponification of 98 to 99 % and hence a residual content of acetate groups of only 2 to 1 %. Their glass transition temperature will be about 85 °C and their melting temperature 228 OC.

Technical PVAL products are partially crystalline polymers. Their degree of crystallinity depends on the residual content of acetate groups, their distribution along the polymer chains, their tacticity and their thermal treatment during and after their manufacturing. Residual acetate groups limit the degree of crystallisation. Their influence is more intense when they are randomly spread, than in case of their block arrangement.

Technical PVAL are water soluble. Polymers with higher degree of crystallinity, particularly the highly saponifie ones, require higher temperature to be solved, but their aqueous solutions remain-solved over a broad temperature range, even at lower temperature. Polymers with lower degree of saponification provide an inverse solubility, i e. they are better soluble in cold water than at higher temperatures. The objective of this invention will be reached in a way, that an aqueous solution or gel of polyvinylalcohol (spinning solution) containing 50 to 1000 % water based on polymer weight will be heated under pressure to a temperature of 120 to 230 °C (spinning temperature), forwarded by means of a spinning gear pup-under pressure corresponding a least to the equilibrium water steam pressure under the spinning temperature to a spinneret and extruded through the spinneret hole into an expansion space which will be kept at a pressure substantially below the above mentioned equilibrium pressure, preferably at the atmospheric pressure. Water present in the spinning solution in a super critical condition will be instantaneously evaporated, flashed out, after the spinning solution will be depressurised. Thereby bubbles will be rapidly formed which may grow and fracture leaving behind a three dimensional network of polyvinylalcohol plexifibrils. Water steam, evaporated during the Hash spinning process, can not spoil-the atmosphere of the expansion space as they are no noxious substances present. It can be removed by simple exhausting. The Rash spinning process according to this invention can be performed in an open air system under atmospheric pressure.

Both technical PVAL types described above are basically suitable to be used according to this invention.

The objective of this invention will be reached in a way, that an aqueous solution or gel of polyvinylalcohol (spinning solution) containing 50 to 1000 % water based on polymer weight will be heated under pressure to a temperature of 120 to 230 °C (spinning temperature), forwarded by means of a spinning gear pump-under pressure corresponding a least to the equilibrium water steam pressure under the spinning temperature to a spinneret and extruded through the spinneret hole into an expansion space which will be kept at a pressure substantially below the above mentioned equilibrium pressure, preferably at the atmospheric pressure. Water present in the spinning solution in a super critical condiiton will be instantaneously evaporated, flashed out, after the spinning solution will be depressurised.

Thereby bubbles will be rapidly formed which may grow and fracture leaving behind a three dimensional network of polyvinylalcohol plexifibrils. Water steam, evaporated during the Hash spinning process, can not spoil-the atmosphere of the expansion space as they are no noxious substances present. It can be removed by simple exhausting. The Rash spinning process according to this invention can be performed in an open air system under atmospheric pressure.

Nevertheless, it is of advantage to limit the water content in the spinning solution or gel. It has been found, that water content in the spinning solution can be kept among 50to 200 % based on the polymer weight.

According to a preferred embodiment the spinning temperature could be kept among 130 to 200 °C, which can help to prevent thermal degradation of the polymer.

Due to the impact of the inherent energy of the overheated water in the spinning solution, which will be free during the flash spinnin g process, the PVAL plexifibrils as formed are substantially oriented. Therefore, excellent strength properties of 1 daNldtex and more will be reached. The particular three dimensinal network structure of the plexifibrils provides high specific surface of about 2 m2/g, which, among other benefits, cause high light scattering without any dulling agent added. Therefore, plexifibrils and-webs made out of them appear opaque. Further, due to the super fine structure of the plexifibrils, webs made out of them are characterised by very soft touch. The degree of crystallinity of the PVAL plexifibrils is very high, their density reaches about 1.30 gear3.

Nevertheless, PVAL plexifibrils remain to be sensitive against water, particularly at elevated temperatures. To make them non soluble in boiling water, a part of free hydroxyl groups in non crystalline areas-has to be used for cross-linking. This can be reached by treatment with aldehydes, particularly formaldehyde at temperatures from 70 to 90 °C. After this treatment, residual formaldehyde has to be washed out.

In similar way it is possible to use complex formation by means of nickel or copper salts or by a treatment with bi-chromates, boric acid or borax.

Crosslinked plexifibrils, as well as fabrics made of them, are protected against boiling water and against polar organic solvents like formamide, dimethylformamide or dimethylsulfoxide.

Only at higher temperature the solvents mentioned above can cause damages. They provide excellent resistance against hydrocarbons, chlor-hydrocarbons, esters, fats and oils.

In dry conditions, PVAL pleifibrils are resistent against higher temperature up to about 180 °C. Substantial thermal degradation would take place at about 220 °C.

By arrangement of the spinnerets across the width of a foraminous conveyer belt it is possible to form an uniform sheet of plexifibrils, which can be thermobonded to provide a paper-or textile-like nonwoven plexifibril web. The thermobonding will be eased, when the ple, fibrils still contain small portion of water. Crosslinking treatment as described above follows the bonding process.

Nonwoven PVAL plexifibril sheets represent a new category of fabrics. Due to their chemical resistance and excellent strength they can be used for protective clothing, envelops, bags and similar end uses. They are water proof but permeable for air and water vapour, important for medical and surgical use-is-he fact, that they can be exposed to all known sterilisation procedures by steam, X-rays or by ethyleneoxide so.

According to this invention, PVAL plexifibrils and fabrics made of them are manufactured by an environmentally friendly technology by flash spinning of aqueous polymer solutions or gels which is free of any pollutin. This simplifies their technology and reduces manufacturing cost. This distinguish them against known polyethylene fabrics manufactured from polymer solutions in dangerous organic solvents, particularly chlor-fluor-hydrocarbons.

It is obvious, that similar technology can be used for other polymers which are either water soluble or at least foaming aqueous-gels : A suitable polymers-can be mentioned polyacrylic acid, polylactic acid, polyacrylamide, polyethylenoxide, polypropyleneoxide, polypetides, sodium carboxymethylcelulose and sodiumalginate.

Examples Example 1 Technical polyvinylalcohol with a molecular weight of about 80, 000 and a high degree of saponification of about 99 % has been solved in hot water in a ratio. of 1 weight part of polymer and 1.5 weight parts of water. The viscose spinning solution has been heated in an autoclave to 195 °C and the pressure has been kept above the equilibriumlevel of 14 bar. The spinning solution has been forwarded by a gear spinning pump to a one hole spinneret and extruded-with a throughput of 2.5 gAmn in an expansion-space. The depressurised spinning solution developed numerous bubbles which grew and finally fractured leaving behind a three dimensional fibrilla network structure of plexifibrils.

Example 2 Same spinning solution has been used as in Example 1. It has been heated in an autoclave to 210 °C under pressure reaching l9 bar and for-warded by means of a spinning gear pump to a primary air spinneret described in the German Patent DE-PS 14 35 461 [L. HARTMANN for Freudenberg on 22.02. 1964]. The spinneret and the primary air have been heated to 210 °C.

The spinneret had 10 spinning holes per 10 cm width. The throughput has been fixed at 1.5 g/min per hole. During the extrusion-connected with de-pressurising, threads of the spinning solution developed numerous bubbles, which grew and finally fractured. The-resulting plexifibrils have been forwarded by two hot air streams to-a foraminous conveyer belt evacuated from the bottom and deposited in form of a uniform nonwoven sheet. The nonwoven sheet has been thermobonded by a pair of calendar rollers and subsequently treated at 70 °C by a 35 % formaldehyde solution to be cross-linked. Residual formaldehyde has been washed out.

The pleifibrillar nonwoven fabric provided excellent strengths properties and has been used for protective overalls and envelopes.