HEAT REFLECTIVE TEXTILE COMPOSITES

Abstract: A flexible heat-reflective textile composite (10) comprising a textile substrate (11) coated with at least one layer (13) of fluoropolymer which is pigmented by metal particles, preferably aluminium flakes, to form a composite having heat-reflective properties at at least one surface thereof.

Description: HEAT REFLECTIVE TEXTILE COMPOSITES The present invention relates to flexible heat-reflective textile composites which may be utilised for the production of conveyor belts or ducting compensators. Textile composites comprising a woven cloth substrate which is coated in a fluoropolymer, usually PTFE (polytetrafluoroethylene), are well known. Such materials are utilised in conveyor belting and in particular for conveyor belting which is utilised at elevated temperatures in, for example, commercial food cooking processes, polymer curing or for use on or in hot air or gas ducting (e.g. as ducting compensators). A problem with textile fluoropolymer composites utilised in high temperature environments is that the composite begins to break down after a period of time. The present invention provides a thermally reflective textile fluoropolymer composite sheet material, inter alia a conveyor belt. According to one aspect of the invention there is provided a flexible heat-reflective textile composite comprising a textile substrate coated with at least one layer of a fluoropolymer which is pigmented with metallic particles to form a reflective surface. Preferably the metallic particles are flakes of aluminium but other metallic particles such as copper, silver, gold, titanium, bronze and/or brass flakes may also be suitable. The textile substrate is preferably of woven glass fibres but could be a woven or non-woven layer of other temperature-resistant materials such as an aromatic polyamide (e.g. that known under the Trade Mark "Kevlar"), or a polyphenylene sulphide (e.g. that known under the Trade Mark "Nomex"). The fluoropolymer could be a fluoroelastomer or a fiuoroplastic or a combination of both. Preferably the fluoropolymer is a fiuoroplastic for example, polyethylene tetrafluoride, fluorinated ethylene polγpylene, polyvinγl fluoride or polvfluoroalkoxycopolymer (PFA). The composite, preferably comprises a plurality of layers of fluoropolymer. Having a pigmented layer as an inner layer which is further coated by at least one further coating of an unpigmented layer is a convenient arrangement. According to a further aspect of the invention there is provided a method of manufacture of a heat reflective textile composite in which a textile substrate is passed through a bath containing an aqueous dispersion of fluoropolymer which further includes an aqueous based metallic flake pigment. The flake pigment is preferably aluminium. Conveniently the bath containing the aqueous dispersion of the fluoropolymer and metallic flake pigment also includes a thickening agent for control of the viscosity of the dispersion. The fluoropolymer dispersion is preferably an aqueous dispersion of polytetrafluoroethylene (suitably with 35 to 60% solids content). According to still further aspect of the invention a method of heat treating a product in a radiant heat oven through which the product is passed on a conveyor belt, is characterised in that the conveyor belt comprises a heat reflective textile composite, the reflective belt directing radiant heat away from the belt and onto the product to be treated. The invention will now be further described, by way of example, with reference to the accompanying drawings in which:- Figure I is a schematic view of composite according to the invention. Figure 2 is a sketch of an apparatus for manufacture of the composite of Figure I, and Figure 3 is a graph in which the thermal properties of a natural PTFE composite are compared with the invention. With reference to Figure I a textile composite I0 comprises a central woven glass-fibre substrate II which is coated with a PTFE base layer 12, which may be pigmented or not as is required. Typical compositions for the base layer coatings are available from ICI under the designation GPI, and from DuPont UK under the designation 30-N. The coated substrate II, 12 is coated on each side thereof with a reflective PTFE layer 13 which is formed with PTFE having metallic flakes dispersed therein. An unpigmented PTFE outer layer 14 may be coated over the outerside of each reflective layer 13. Typical compositions for the outer layer coatings are available from ICI under the designation 639AD and from DuPont UK under the designation 3417-N. The compositions of the final piuriiayer composite will be in the order of PTFE between 30% and 80% by weight Glass between 70% and 20% by weight Aluminium between 0.5 and 20% by weight but is more preferably PTFE about 62%, glass about 37% and aluminium about 1.0% by weight. The composite 10 is manufactured (see Figure 2) by passing a continuous woven substrate 21 drawn off a feed roll 22 through a bath 23 of aqueous PTFE dispersion to leave a coating on both sides of the substrate 21. The substrate 21 then passes between metering bars 24 (to remove excess coating material), through a first set of ovens 25 (to remove water), a second set of ovens 26 (for sintering the PTFE) and is then wound on a collecting roll 27. The roll 27 can then form a new feed roll 22 so that the layers 12, 13, 14 of PTFE can be built up on the substrate II by passing the substrate through aqueous dispersions in the bath 23 a number of times to build up a number of coats of PTFE and sintering each coat in turn. For example for a woven glass-fibre substrate with an areal weight of between 80 and 1000 gm/m 2 , the base layer 12 may be built up of between I to 8 coats, preferably 6 coats, each reflective layer 13 may be built up of I or more coats (preferably two coats) of pigmented dispersion and each optional outer layer 14 may be built up of I or more coats, (preferably two coats), of unpigmented PTFE dispersion. The process can also be successfully conducted by omitting the sintering process on early coating passes and using a calender to flatten and smooth the coating, as is well known in the industry. In a preferred example a base layer having six coats will have a covering of about 200 to 220 gm/m 2 of PTFE, two two coat reflective layers will have a combined covering of about 60 gm/m 2 and two two coat outer layers will have a combined covering of about 50 gm m 2 . Each reflective layer will preferably contain about 7% by weight of aluminium dispersed in the PTFE. The PTFE base layers 12 and outer layers 14 are formed by passing the substrate 21 (or coated substrate) through a 57-60% solid dispersion of PTFE in water. Surfactant, stabilizers, and fillers (such as graphite) may be added to the dispersion as is required and as is known in the art. The two reflective layers 13 are formed by passing the coated substrate 21 through a bath containing an aqueous dispersion typically comprising (all by weight) :- PTFE 17.5-59% Aluminium Flake Pigment 0.5-18% Thickening Agent 0.1-0.3% and Water to make up to 100% The pigment is preferably Aluminium Aquavox l752-207 "S" (available from Silberline Limited of Leven in Scotland) and is mixed with water prior to blending with a PTFE dispersion which has been previously mixed with a thickening agent (preferably a poiyacylic acid) . The viscosity of the pigment- loaded mixture is adjusted to between 100 and 7000 centipoise, preferably between 200 and 250 centipoise. A preferred composition comprises (by weight) 96% PTFE dispersions, about 3.75% aluminium pigment, and 0.25% thickening agent. The textile composite manufactured by the above-noted process can be used to make conveyor belts for curing/cooking processes and other processes requiring transportation of goods at elevated temperatures. The thermal properties of a composite according to the invention have been compared with a prior art unpigmented textile composite. The belt structure for the two materials both comprise a woven glass-fibre substrate having an areal weight of 200 gm/m 2 and being an "E" glass fabric coated with approximately 300 gm/m 2 PTFE (total on both sides of the substrate) having a nominal thickness of 0.26mm. The composition according to the invention included 4.2 gm/m 2 aluminium and will have been manufactured by a separate build up of the required number of coatings of PTFE in the manner describe above. Thermal Conductivity This was measured for both materials by a modification of the standard Lees Disc method for the measurement of thermal conductivity by the absolute plane parallel plate technique. Prior Art 222 mW/m/°C Reflective Textile Composite 235 mW/m/°C Thermal Reflectivity The textile composite was placed 700 mm in front of a hot (400°C) electrically heated element so that it was subjected to radiant heat. A thermocouple was placed on the reverse side of the composite, away from the heater, and the temperature of the fabric, as recorded by this thermocouple, was noted as a function of time. Figure 3 shows the results obtained. The composite according to the invention was significantly cooler than the prior art material. The composite according to the invention reached a temperature of about I90°C whereas the prior art composite reached a maximum temperature of about 230°C. Further the reflective composite absorbed heat more slowly so that it was always cooler than the prior art material. A conveyor belt made of a textile composite according to the invention, when passing through an oven heated in some proportion by radiant heat, will run at a lower temperature than a prior art belt and will therefore have an extended belt life. Furthermore since the belt is reflective, the oven can be maintained at a desired running temperature for a reduced energy input since the belt reflects radiant heat back into the product. This may be particularly important in some cooking operations where the product requires to be cooked from both sides. Whilst the textile composite according to the invention has been described with a reflective aluminium-loaded layer 13 sandwiched between a PTFE base layer 12 and an outer layer 14 it will be appreciated that the aluminium-loaded layer may be a single aluminium layer built up from a number of coatings, or may be anywhere in the sequence of layers provided that its reflective properties are manifest at at least one outer surface of the composite. In a modification to produce a heat reflective textile a textile substrate is passed through a bath of fluoropolymer dispersion to precoat or coat the substrate. In a separate process a fluoropolymer film is produced, which preferably employs the methods covered by US 4883716 and related US and European patents and applications. Either the coated substrate or the film or both may contain one or more layers containing a metallic flake pigment, preferably aluminium. The coated substrate and the film, depending on the manufacturing method used to produce each, are brought together using a melt processable "adhesive" film, preferably as described in US 4943473 and related patents and applications, or using techniques described in US 5I4I800 and related patent applications. Further, whilst a thermally reflective composite according to the invention may preferably be used for a conveyor system, the material is applicable for uses in other areas requiring good thermal stability and/or reflective properties such as in ductings for hot air or gases where the composite may be used for ducting insulation and/or for flow dividers in ducts and for ducting compensators.

Claims: CLAIMS 1. A flexible heat-reflective textile composite comprising a textile substrate coated with at least one layer of a fluoropolymer which is pigmented with metallic particles to form a reflective surface. 2. A composite as claimed in claim I, where the metallic particles are aluminium flakes. 3. A composite as claimed in claim I or claim 2 wherein the textile substrate is formed from glass fibre. 4. A composite as claimed in any one of claims I to 3, wherein the composite comprises a plurality of layers of fluoropolymer and said at least one pigmented layer is an inner layer which is covered by at least one further coating of unpigmented fluoropolymer. 5. A composite as claimed in claim 4, wherein the substrate is coated in turn by at least one layer of unpigmented fluoropolymer, at least one said pigmented layer, and finally by at least one further unpigmented layer. 6. A composite as claimed in any one of claims I to 5, wherein the pigmented layer includes 2 to 30% by weight of metallic particles in the fluoropolymer; and preferably 5 to 15% by weight of aluminium flake, or more preferably 7% by weight of aluminium flake. 7. A flexible heat-reflective, textile composite comprising a textile substrate and a film one or both of which contain a metallic flake pigment. 8. A conveyor belt comprising a composite as claimed in any one of claims I to 7. 9. A method of manufacture of a heat reflective textile composite in which a textile substrate is passed through a bath containing an aqueous dispersion of fluoropolymer which further includes an aqueous based metallic flake pigment. 10. A method as claimed in claim 9, wherein the bath containing the aqueous dispersion of fluoropolymer further includes in the dispersion a thickening agent for control of the overall viscosity of the dispersion. 11. A method as claimed in claim I0, wherein the viscosity is between I00 and 7000 centipoise. 12. A method as claimed in claim II, wherein the viscosity of the dispersion is between 200 and 250 centipoise. 13. A method of manufacture of a heat reflective textile composite in which a substrate and film are separately formed, either or both of which may contain a metallic flake pigment and which are combined in an additional stage. 14. A method of heat treating a product in a radiant heat oven through which the product is passed on a conveyor belt, wherein the conveyor belt comprises a heat-reflective textile composite as claimed in any one of claims I to 7, the reflective belt directing radiant heat away from the belt and onto the product to be treated.

Inventors: HEWARD CHRISTOPHER MICHAEL, MOREY JASON, HEWARD, CHRISTOPHER, MICHAEL, MOREY, JASON
Applicants: HEWARD CHRISTOPHER MICHAEL, MOREY JASON, TYGAFLOR LTD, HEWARD, CHRISTOPHER, MICHAEL, MOREY, JASON, TYGAFLOR LIMITED
Application year: 1995
Main classification: D06N3
IPC codes: B32B27, D06N3