WO2002024990A1

WO2002024990A1 - Polyvinylalcohol plexifibrils, nonwoven webs made of them and the process for their manufacturing

Info

Publication number: WO2002024990A1
Application number: PCT/EP2001/000044
Authority: WO
Inventors: Ivo Edward Ruzek
Original assignee: Ivo Edward Ruzek
Priority date: 1999-12-31
Filing date: 2001-01-02
Publication date: 2002-03-28
Also published as: DE19963983A1

Abstract

This invention relates to plexifibrils made by flash spinning of overheated pressurised aqueous solutions or gels of technical polyvinylalcohol. After the extrusion of threads of overheated pressurised aqueous polyvinylalcohol solution in an expansion space water steam is flashing out causing bubble formation in the thread, their growth and finally their fracture. As a result, the polymer will be transferred in very fine fibrils with characteristic three dimensional network structure, called plexifibrils. They can be deposited at a foraminous conveyer belt in form of plexifibrillar nonwoven sheets, which can be partially cross-linked to be made resistant against boiling water. The polyvinylalcohol plexifibrils and nonwoven plexifibrillar sheets made of them can be used for manufacturing of protective clothing, envelopes, bags and similar.

Description

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 equilibrium 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

will be kept between 1 to 0.5 to 1 to 2. Further, the invent on relates to a process where spinmng temperature wiHbe kept among 130-tβ-20θ °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 foraminous conveyer belt evacuated from the bottom so, that they form a fibrous nonwoven web.

Finally, the invention relates to polyvinylalcohol plexifibrils manufactured according to any of the processes describe above, t< nonwoven- webs- made out of such polyvinylalcohol plexifibrils, as well as such polyvinylalcohol plexifibrils 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 tauto eric iso er 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 alcoholes, preferably by ethanol. Technical PVAL is-avaϋable 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 25QG\! Another important parameter of the technical products is the degree of saponification, as they are, in fact, partially saponified polyvinylacetates. Dependiiig-on-the desired use, the degree of saponification can vary from 99 to about 87 %, or in terms of the portion of residual non changed acetate groups in a range of l to l3 %.

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 saponified PVAL has a glass transition temperature of 85 °C and melting temperature of 22S °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, fats and oils. They are soluble in water andr 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 1930-tiesτ 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 ste ^ 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 erossMnking of a part of the hydroxyl groups in non crystalline areas by formaldehyde at 70 to 90 °C. Residual formaldehyde will be washed out.

Another, more modern technology is- based on "dry spinning" of concentrated polymer solutions, which are extruded at high temperature through the spinneret and 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 of polyvinylalcohol.

It is further known to reach very fine fibrillar structure by using flash spinning process. According to this technology it is possible to spin high density- olyethylene (HOPE) from a solution in trifluor-chlor-methane or methylenchloride [USP 3,081,519 from 19. 03. 1963: H. BLADES and J. R WEBFΓE 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 i t©^ 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^ fibrillated three dimensional network of polymer, which will be addressed as plexifilament. The thickness of the fibrils wiM be- about 4 μm or less, their length of about 400 to 100& mnt 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, forcing 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 άu& to their fibrillar structure an excellent softness. They can be used for protective clothing, envelopes, bags etc. However, flash 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). In the meaning of this invention, technical PVAL represent partially saponified 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 °C.

An other particular group of PVAL product&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 °C.

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 saponified 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. 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 hi a super critiςal condition will be instantaneously evaporated, flashed out, after the spinning solution will be depressurised. Thereby bubbles- will be rapidly føxmed-wJύch-iiiay grow and fracture leaving behind a three dimensional network of polyvinylalcohol plexifibrils. Water steam, evaporated during the flash spinning- rocess, can not spoil the atmosphere of the expansion_^ space as they are no noxious substances present. It can be removed by simple exhausting. The flash 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 spinning, process, the PVAL plexifibrils as formed are substantially oriented. Therefore, excellent strength properties of 1 daN/dtex and more will be reached. The particular three dimensional 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- ebs 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 plexifibriis is very high, their density reaches about 1.30 g cm³.

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 areasf 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 plexifibrils 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 wilLbe eased, when the plexifibrils 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 pollution. 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 forming aqueous gels. As suitable polymers can be mentioned polyacrylic acid, polylactic acid, polyacrylamide, polyethylenoxide, polypropyleneoxide, polypetides, sodium carboxy ethylcelulose and sodiumalginate.

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 hi 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 hasbeen kept above e equifrbrium level 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 g/min- in an expansion- space. The depressurised spinning solution developed numerous bubbles which grew and finally fractured leaving behind a three dimensional fibrillar 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 19 bar and forwarded 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 plexifibrillar nonwoven fabric provided excellent strengths properties and has been used for protective overalls and envelopes.

Claims

Patent Claims

1. Process for manufacturing of polyvinylalcohol plexifibrils and of nonwoven sheets made of them wherein polyvinylalcohol will be mixed with water in a ratio among 1 to 0.5 and 1 to 10 at temperatures of 120 to 230 °C (spinning temperature) and under pressure to form solutions or gels which are forwarded by means of a spinning gear pump and at a pressure which at least corresponds to the equilibrium pressure of water steam at the spinning temperature to a spinneret and subsequently extruded through a spinneret hole in an expansion space kept at a pressure substantially lower than said equilibrium pressure and thereby fibrillated by flashing out of the water steam.

2. Process according to claim 1 wherein a technical polyvinylalcohol with a degree of saponification of at least 75 % will be used.

3. Process according to claim 1 or 2 wherein a technical polyvinylalcohol with a degree of saponification of at least 95 % will be used.

4. Process according to claim 1 to 3 wherein a technical polyvinylalcohol with a molecular weight of at least 60,000 will be used.

5. Process according to claim 1 to 4 wherein a technical polyvinylalcohol with a molecular weight of at least 75,000 will be used.

6. Process according to claim 1 to 5 wherein the weight ratio of polymer to water will be among 1 to 0.5 and 1 to 10.

7. Process according to claim 1 to 6 wherein the weight ratio of polymer to water will be among 1 to 0.5 and 1 to 2.

8. Process according to claim 1 to 7 wherein the expansion space, to where the thread of the spinning solution will be extruded, will be kept at atmospheric pressure.

9. Process according to claim 1 to 8 wherein polyvinylalcohol plexifibrils will be deposited at a foraminous conveyer belt evacuated form the bottom to form a fibrillar nonwoven sheet.

10. Process according to claim lto 9 wherein polyvinylalcohol plexifibrils or nonwoven fibrillar sheets made of them will be partially cross-linked.

11. Process according to claim 1 to 10 wherein polyvinylalcohol plexifibrils or nonwoven fibrillar sheet made of them will be cross-linked in wet condition by using aqueous solutions of aldehydes, as formaldehyde, nickel- or copper salts, bi-chromates, boric acid or borax,

12. Polyvinylalcohol plexifibrils made according any of the claims 1 to 11.

13. Nonwoven fibrillar sheets made of polyvinylalcohol plexifibrils manufactured according to claim 12.