COATED FABRIC

Abstract: A method of making a coated fabric to be used in a phase separation process comprising the steps of impregnating a cloth with at least one coagulable polymer latex and coagulating the polymer on/or in the cloth. The coagulation is carried from a water phase so as to provide a cloth having a porous polymer coating thereon and/or therein.

Description: COATED FABRIC The present invention relates to coated fabrics, particularly for use in phase separation processes such as papermaking and filtration. The coated fabrics of the invention have particular, but not exclusive, application in the filtration of ingestible materials. It is known to prepare coated filter fabrics by applying a solution of a polyurethane in DMF (dimethyl formamide) either by immersion in or by coating on one side and then immersing the treated cloth in water. The polyurethane is coagulated and is attached to the fabric as a result. DMF is a toxic substance. If the filter fabric is to be used in foodstuffs related areas any residual DMF remaining in the filter fabric may end up in the foodstuffs which come near or into contact with the filter fabric. This is obviously undesirable. The present invention seeks to provide an alternative method of making a coated filter fabric which avoids the use of toxic substances such as DMF. According to the present invention there is provided a method of making a coated fabric to be used in a phase separation process comprising the steps of impregnating a cloth with at least one coagulable polymer latex and coagulating the polymer on or in the cloth, wherein the method comprises carrying out the coagulation from the water phase so as to provide a cloth having a porous polymer coating thereon and/or therein. The polymer is preferably coagulated with steam or super¬ heated steam rather than water. The polymer used may comprise polyurethane. However, polyurethane is susceptible to hydrolytic degradation. In some instances it is desirable to use polymers having improved resistance to hydrolytic degradation. In such circumstances polymers containing hydrolysis-susceptible linkages such as esters or amide should be avoided. Suitable examples include polyisoprene, polybutadiene, polyvinylidene dichloride, polyvinyl chloride, polychloroprene and particularly styrene- butadiene polymers or mixtures thereof. Fabrics having such polymer coatings surprisingly exhibit vastly improved resistance to hydrolysis, acids/alkalis and oxidation. The cloth substrate may be woven or nonwoven, but is preferably a needlefelt. The cloth may be made from any suitable material, such as polypropylene. The coagulated polymer is preferably foamed, said foaming occurring simultaneously with or immediately after the coagulation. Foaming may be achieved either by physical means or by using a chemical foaming agent. The foaming agent preferably comprises a low boiling water insoluble halogenated hydrocarbon. The halogenated hydrocarbon preferably has a boiling point in the range from -40°C to 50°C and more preferably in the range from -20°C to 30°C. Preferred foaming agents include l,2-dibromo-l,l,2,2-tetrafluoroethane and trichlorofluoroethane. The foaming and/or the coagulation of the polymer may be achieved by heating the impregnated coated textile substrate, preferably in the presence of a heat coagulant. Suitable heat coagulants include vinyl alkyl ether and derivatives thereof, polyacetals, polythio ethers, poly(ethylene oxide) and derivatives thereof, and poly(propylene/ethylene oxide) and derivatives thereof. The heat coagulant may be built into the backbone of the polymer. Usually heating to a temperature of about 70 °C results solely in coagulation. Heating above this temperature will generally also result in foaming provided a foaming agent is present. Coagulation may also be achieved by means of adding a suitable electrolyte and/or varying the pH of the polymer latex. For example, with cationic polymers coagulation may occur at an alkaline pH and for anionic polymers coagulation occurs at an acid pH. This may be followed by heating to achieve satisfactory foaming. The coagulated coatings may be particle-reinforced or fibre-reinforced. The strength of the individual cells in the coagulated cellular polymer network can be improved by the addition of finely-chopped fibres such as RYTON fibres and/or finely dispersed particles such as PTFE particles. These particles and/or fibres should ideally have a chemical inertness, heat stability and acid and/or alkali resistance at least similar to that of the coagulated polymer. The particles and fibres would be incorporated into the polymer emulsion prior to coagulation. Preferably the filter cloths have a coating thickness in the range from 0.5 to 2.0mm, corresponding to 50 to 250 (preferably 120 to 180) g/m 2 addition by weight of polymer. The coating may be applied by any coating technique such as knife coating, dip coating, lick coating, screen printing or spraying. Reverse roller techniques may be employed. The method of the invention may also be used to provide coatings on papermachine clothing such as press felts. The coatings obtained would be tough, smooth and permeable.

Claims: Q1___I____. 1. A method of making a coated fabric to be used in a phase separation process comprising the steps of impregnating a cloth with at least one coagulable polymer latex and coagulating the polymer on or in the cloth, wherein the method comprises carrying out the coagulation from a water phase so as to provide a cloth having a porous polymer coating thereon and/or therein. 2. A method as claimed in claim 1, wherein the water phase comprises steam. 3. A method as claimed in claim 1 or claim 2, wherein the polymer comprises polyurethane. 4. A method as claimed in claim 1 or claim 2, wherein the polymer comprises any of the following:- polyisoprene, polybutadiene, polyvinylidene dichloride, polyvinyl chloride, polychloroprene or styrene-butadiene polymers or mixtures thereof. 5. A method as claimed in any preceding claim, wherein the fabric comprises polypropylene. 6. A method as claimed in any preceding claim, wherein the coagulated polymer is foamed. 7. A method as claimed in any preceding claim, wherein the coagulated polymer is foamed by using a chemical foaming agent. 8. A method as claimed in claim 7, wherein the foaming agent comprises a low boiling water insoluble halogenated hydrocarbon. 9. A method as claimed in claim 8, wherein the foaming agent has a boiling point in the range from -40 β C to 50°C. 10. A method as claimed in claim 8 or claim 9, wherein the foaming agent has a boiling point in the range from -20 °C to 30 β C. 11. A method as claimed in any of claims 7 to 10, wherein the foaming agent comprises l,2-dibromo-l,l,2,2-tetrafluoroethane and trichlorofluoroethane. 12. A method as claimed in any preceding claim, wherein the polymer is coagulated by heating the polymer in the presence of a heat coagulant. 13. A method as claimed in claim 12, wherein the heat coagulant includes any of the following:- vinyl alkyl ether and derivatives thereof, polyacetals, polythio ethers, poly(ethylene oxide) and derivatives thereof, and poly(propylene/ethylene oxide) and derivatives thereof. 14. A method as claimed in any of claims 1 to 10, wherein the polymer is coagulated by adding an electrolyte to the polymer latex and/or varying the pH of the polymer latex. 15. A method as claimed in any preceding claim, wherein the thickness of the coating is in the range from 0.5 to 20mm. 16. A phase separation fabric for use in filtration or paper aking made by the method of any of claims 1 to 15.

Inventors: LOFTS PETER, LOFTS, PETER
Applicants: LOFTS PETER, SCAPA GROUP PLC, LOFTS, PETER
Application year: 1995
Main classification: D06N3
IPC codes: D06M13, D06M15, D06M23, D06N3, D21F1