FLEXIBLE COATED SUBSTRATES AND METHOD FOR THEIR MANUFACTURE

Abstract: A composite comprising a fabric substrate which is coated with a blend comprising a halocarbon polymer and a silicone elastomer, and a method of manufacturing a flexible coated substrate which method includes the steps of forming a fibre fabric (10) coating the fabric with a coating material (11, 12, 14) comprising a blend of a halocarbon polymer and a silicone elastomer and heat treating the coating material to bond the same to the fabric.

Description: FLEXIBLE COATED SUBSTRATES AND METHOD FOR THEIR MANUFACTURE This invention relates to new and useful composites comprising coated substrates. Technical Field In particular the present invention is concerned with coating a substrate, such as a knitted or woven fabric and in particular a glass fibre fabric, to provide a product that is flexible and also resistant to chemical attack. More particularly the present invention relates to a new silicone elasto er/halocarbon polymer matrix useful as a coating for reinforced woven fabrics to produce a product which is flexible, exhibits good matrix integrity, and has good adhesion or bonding of the coating matrix to the substrate. The invention includes composites which have good chemical resistance at elevated temperatures and in humid environments. The invention further relates to a method of manufacturing such composites where the desirable high temperature chemical inertness of halocarbon polymers is combined with the desirable mechanical properties of silicone elastomers in such a way as to retain a desirable fabric-like flexibility. Perhaps the most well-known subclass of halocarbon polymers are the fluoropolymers called "perfluoroplastics" which are generally recognised to have excellent electrical characteristics and physical properties, such as a low coefficient of friction, a low surface free energy (i.e. they are non-wetting to many organic fluids), and a very high degree of hydrophobicity. Fluoroplastics, and pai— ticularly perfluoroplastics (i.e. those fluoroplastics which do not contain hydrogen), such as polytetrafluoro- ethylene (PTFE), fluoro (ethylene-propylene) copolymer (FEP) and copolymers of tetrafluoroethylene and perfluoro- propyl vinyl ether (PFA), are resistant to a wide range of chemicals, even at elevated temperatures, making them particularly useful in a variety of industrial and domestic applications. However, due to the partially crystalline nature of these fluorocarbon polymers they exhibit a degree of stiffness or lack of compliance which is detrimental to the utilisation of these desirable properties. This shortcoming is particularly noticeable and objectionable in a reinforced composite where some degree of flexibility, elasticity, and/or conformabi1ity is necessary. The broad class of fluoropolymers also includes fluoroelastomers which are not only elastomeric, but also possess, to a lesser degree the same physical and electri¬ cal properties as fluoroplastics. Discussion of Prior Art European Patent Application 125,955A - discloses a composite comprising a substrate coated with a coating comprising a blend of a fluoroplastic and a fluoroelas- tomer. This European patent application exploits the advantageous properties of perfluoroplastics, such as low co-efficient of friction, low surface free energy, a high degree of hydrophobicity and resistance to a wide range of chemicals at elevated temperatures, and combines this with the advantageous properties of flexibility (low flex modulus and conformabi!ity) which fluoroelastomers have. However, fluoroelastomers which contain hydrogen (i.e. those which are partially fluorinated) generally degrade rapidly at high temperatures and result not only in the loss of physical integrity but also in the formation of hydrofluoric acid. Hydrofluoric acid is highly corrosive to most mater¬ ials including those normally used to reinforce textile fabrics and particularly to fabric substrates such as glass fibre fabrics. There is however, a need for a similar composite which is coated with a coating which will resist chemical attack and which will be flexible at elevated temperatures but which will not generate hydrofluoric acid that will have a deleterious effect on the substrate. Summary of the Invention This invention in terms of a composite is set out in the following claim 1, and in its method aspect it is set out in the following claim 15. An object of preferred embodiments of the present invention is to provide a composite having a fabric sub¬ strate coated with a coating which will resist chemical attack and be flexible at high temperatures but will not degrade and react adversely with the fabric substrate. The invention meets this objective in that flexibility is imparted by the use of a blend of a halocarbon polymer and a silicone elastomer, desirably a blend which does not break down to produce hydrofluoric acid. Brief Description of the Drawing The present invention will now be described, by way of example, with reference to the accompanying drawing, the sole figure of which is an enlarged schematic cross-sec¬ tional side view of a woven composite constructed in accordance with the present invention. Description of Preferred Embodiments Referring to the drawing a woven glass fabric sub¬ strate 10 is precoated with a silicone oil or a blend of silicone oil and a halocarbon polymer such as PTFE and overcoated on both sides with a fluoroplastic layer 11. The layer 11 is itself overcoated with a layer 12 which is a blend of a silicone elastomer and a halocarbon polymer. The resulting composite may be further coated on one or both sides with an optional polyfluoroethylene (or a blend of a halocarbon polymer and a silicone elastomer) top layer 14. In greater detail, the method of manufacture of the composite shown in the drawing is as follows: The substrate 10 is manufactured by weaving, or knitting aramid fibres (such as for example that known by the trademark "KEVLAR") or glass fibre filaments 10a to form the fabric substrate 10. Surface finishes or sizes are removed from the substrate by heat-cleaning it. A saturant or lubricating agent, preferably poly(methylphenylsiloxane) oil, typically in a mixture containing 2 to 14 parts by weight of lubricating agent is applied as a first layer 11 to the substrate. This first layer is provided to minimise the stiffness of the com- pleted composite and may be a relatively thin covering. This first layer may be put down separately or incorporated with a halocarbon polymer such as PTFE. In one embodiment of the invention a blend of a silicone elastomer and a halocarbon polymer is applied to the first layer by dipping the substrate in an aqueous dispersion comprising a blend of a silicone elastomer (e.g. in the form of a linear polymer) and polytetrafluoro¬ ethylene. The substrate is dried and baked in an oven at a temperature of between 200°F to 500°F (93°C to 260°C). The substrate was repeatedly dipped, dried and sintered until a second layer 12 of about 0.05 to 0.25 mm thickness was applied on top of the first layer 11. The coating of the second layer 12 contained between 10 to 50 per cent by weight of silicone elastomer (preferably 13 per cent by weight of silicone elastomer). The coated substrate was further coated with a thin outer layer 14 of a PTFE by dipping it in the aqueous dispersion, drying and sintering at 700°F (370°C) for 1 minute. More specifically, 564 g/m and 1050 g\m 2 woven glass fibre substrate were heat cleaned to remove residual sizing. A combination of a fluorocarbon polymer such as PTFE (as an aqueous dispersion containing 609. solids) and poly(methylphenylsiloxane) oil (as an aqueous dispersion containing 35* solids) was applied to each of the two substrates by dipping, drying and fusing the coating in a two-zone coating tower at drying temperatures of between 200 to 350°F (93 to 177°C) and sintering at a temperature of 700°F (370°C). The first layer formed by this coating contained 93 parts w/w PTFE and 7 parts w/w poly(methylphenylsiloxane) oil. This first layer was applied as a very light undercoat (typically 5 oz/square yard (170 g/m 2 )) so as not to induce stiffness. The interstices between the fibres of the substrate are unblocked at this stage. A second layer was then applied to this first under¬ coat layer. The second layer of approximatel 680 g/m 2 was applied from a blend of a perhalocarbon polymer such as PTFE as an aqueous dispersion containing 60 solids, and a silicone elastomer. The silicone elastomer was a linear poly(diorganosiloxane) emulsion which contains structural units of the formula: R i - Si - 0 where R and R , which may be the same or different (and which may vary from one structural unit to another), are monovalent hydrocarbon radicals. The preferred silicone elastomer is linear poly(dimethylsiloxane) latex in the form of an emulsion. The emulsifier was anionic and the emulsion comprised 35 per cent by weight of polymer solids. The weight of vinyl in the polymer was 1.77 per cent. This second layer was applied in several passes by dipping, drying and sintering in a two-zone coating tower with drying temperatures of between 200 to 350°F (93 to 177°C) and a sintering temperature of 700°F (370°C). The blend of the second layer comprised 60% perhalocarbon polymer and 40% silicone elastomer, by weight. A third layer was then applied to the second layer. This third layer comprised a perhalocarbon polymer (for example PTFE) applied as an aqueous dispersion containing 60% solids, and was dried and sintered at 700°F (370°C). If a final smoothing of the coated substrate is deemed desirable, the final drying and sintering at.700°F (370°C) may be preceded by calendering (e.g. with a 300°F (149°C) calender). The final drying and sintering may be carried out by means of a final dry pass through the coating tower.

Claims: CLAIMS 1. A composite comprising a fabric substrate which is coated with a blend comprising a halocarbon polymer and a silicone elastomer. 2. A composite according to claim 1, wherein the halocarbon polymer is a perhalocarbon polymer. 3. A composite according to claim 1 or claim 2, wherein the elastomer is a linear silicone elastomer. 4. A composite according to any one of claims 1 to 3, wherein the silicone elastomer is a linear poly(diorgano- siloxane) which contains structural units of the formula: R - Si - 0 R 1 where R and R , which may be the same or different (and which may vary from one structural unit to another), are monovalent hydrocarbon radicals. 5. A composite according to any one of claims 1 to 4, wherein the fabric substrate is formed of woven or knitted glass fibre filaments. 6. A composite according to any one of claims 1 to 4, wherein the fabric substrate is formed of woven or knitted aramid fibres. 7. A composite according to any one of claims 1 to 6, wherein the fabric substrate is coated with a first layer which comprises a silicone oil. 8. A composite according to claim 4, wherein the first layer comprises a blend of a halocarbon polymer and a silicone oil . 9. A composite according to claim 8, wherein the halocarbon polymer is pσlytetrafluoroethylene and the silicone oil is poly(methylphenylsiloxane). 10. A composite according to any one of claims 7 to 9, wherein the substrate is coated with a second layer com¬ prising a blend of a halocarbon polymer and a silicone elastomer. 11. A composite according to any one of the preceding claims wherein the silicone elastomer is in the form of an emulsion comprising a poly(dimethylsiloxane) latex and an anionic emulsifier. 12. A composite according to claim 10, or claim 11 as dependent on claim 10, wherein a third layer comprising a halocarbon polymer is applied to the second layer. 13. A composite according to claim 12, wherein the halocarbon polymer in at least the third layer is poly- tetraf1uoroethy1ene. 14. A method of manufacturing the composite claimed in any one of the preceding claims, which method includes the step of applying to the substrate a coating comprising a blend of a halocarbon polymer and a silicone elastomer. 15. A method of manufacturing a flexible coated substrate which method includes the steps of forming a fibre fabric, coating the fabric with a coating material comprising a blend of a halocarbon polymer and a silicone elastomer and heat treating the coating material to bond the same to the fabric.

Inventors: SALLIS ROGER, SALLIS, ROGER
Applicants: COURTAULDS PLC
Application year: 1990
Main classification: D06N7
IPC codes: D06M13, D06M15, D06M101, D06N3, D06N7