Non-curling reinforced composite membranes with differing opposed faces, methods for producing and their use in varied applications

Abstract

A double-faced PTFE-silicone rubber reinforced composite with curling tendency controlled is achieved by coating one side of a balanced PTFE/glass composite with liquid silicone rubber. Each face of the composite can perform independent functions in a single application, thereby optimizing performance.

Description

BACKGROUND OF THE INVENTION

This invention relates to new and useful reinforced composite membranes for use in conveying, material handling, surface modifying, surface protection, and barrier applications, in which the two opposing faces of the membrane differ significantly in composition and physical characteristics, each face being constructed to perform independent functions in a given single application, thereby optimizing overall performance.

More specifically, this invention relates to reinforced composite membranes in which one face is a perfluoropolymer, such as polytetrafluoroethylene (PTFE), and the other face is an elastomer. Although the use of elastomeric materials of varied compositions is contemplated, silicone rubber is the preferred elastomeric component.

Polytetrafluoroethylene (PTFE) coated fiberglass fabrics and silicone rubber coated fiberglass fabrics are examples of reinforced composite membranes commonly used in many of the applications mentioned above. The two materials share several unique and valuable physical properties: flexibility, thermal stability in operating environments exceeding 350° F., and low-energy surfaces providing easy release to sticky, viscous, or adhesive materials. On the other hand, they may differ markedly in surface hardness, finish, frictional characteristics, and surface qualities difficult to specify but related to the way the surface adheres to other surfaces. PTFE has one of the lowest coefficients of friction possessed by any common material and exhibits minimal “stick-slip” behavior. On the other hand, silicone rubber, depending on its composition, finish, and hardness (durometer), often has the high coefficient of friction and pronounced stick-slip behavior or “grabby” quality typically associated with elastomeric materials.

The choice of whether to use a PTFE or a silicone rubber composite in a given application sometimes involves consideration of the materials' frictional and related surface characteristics. Certain applications may require a material with a low coefficient of friction, in which case PTFE composites would be expected to perform very well, while silicone rubber constructions would not. In other applications, a material with a high coefficient of friction or stick-slip characteristics may be required, in which cases a silicone rubber material would answer readily, while a PTFE construction would not.

However, in some applications, a membrane with the frictional and related surface characteristics of PTFE on one face and those of an elastomer on the other face may be needed. To address this need, efforts have been made to combine the two materials in a double-faced membrane, with PTFE on one face and silicone rubber on the opposite face. In the past, these attempts have yielded materials with a strong tendency to curl, making their handling extremely difficult and limiting their usefulness. The curling tendency is due to imbalanced stresses generated in manufacturing these composites, the result of differences in the curing characteristics, thermal coefficients of expansion, and modulae of the two components. It is the object of this invention to produce double-faced PTFE-elastomer reinforced composite membranes with curling tendency controlled to the extent that their handling characteristics and usefulness remain uncompromised.

SUMMARY OF THE INVENTION

The invention achieves a double-faced PTFE-silicone rubber reinforced composite with curling tendency controlled by coating one side of a balanced PTFE-coated glass composite with liquid silicone rubber. The composite comprises two opposing faces, wherein one face is composed of a perfluoropolymer, such as PTFE, and the other face is composed of an elastomer, such as silicone rubber. The composite consequently can perform independent functions in a single application, thereby optimizing performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective and cut away view of an exemplary composite of the invention.

DESCRIPTION OF THE INVENTION

In accordance with one embodiment the invention, a continuous web of a glass fabric-reinforced composite, with PTFE on one face and silicone rubber on the opposite face, is produced by dip-coating, knife-over-roll coating, metering and/or wiping, and thermal curing processes.

To produce the membrane in a continuous process, woven reinforcement, which may comprise fiberglass, aramid, or other fiber able to tolerate PTFE processing temperatures and suited to the end use of the membrane, is paid off a roll and saturated and/or coated with PTFE by dipping through an aqueous dispersion or latex of the resin, removing the excess dispersion by wiping or metering, drying to remove the water, baking to remove most of the surfactant, and finally heating sufficiently to sinter or fuse the PTFE. Typically, the coating operation will be repeated several times in order to apply the desired amount of PTFE free of cracks and other defects. Alternately, the fusing step is eliminated in the first several passes, and the multiple layers of unfused PTFE resin applied thus are smoothed and consolidated by passing the web through a calendar machine, prior to fusing and the application of one or more subsequent layers of fused PTFE resin to complete the composite. In any case, the PTFE resin is applied in such a way as to “balance” the mechanical forces or residual stresses within the PTFE resin on each coating face so that the composite lies flat and does not tend to curl. One way of achieving a balanced-coating is to apply equal amounts of PTFE to both faces and subject the faces to the same processing conditions. The coating of PTFE should be as thin as possible, while sufficient to achieve the desired function. Although a variety of coating thicknesses of PTFE to serve a variety of functions are contemplated by the invention, a coating thickness of about 1-5 mils (1 mil equals 0.001 in.) is preferred. All the technology involved is familiar to those skilled in the art of producing PTFE coated fabric reinforced composites, described for example in U.S. Pat. No. 5,141,800 to Effenberger et al., incorporated herein by reference.

In the next step in the process, one face of the PTFE/glass composite is rendered bondable by coating with a mixture of a colloidal silica dispersion, for example DuPont Ludox® 40, and a perfluorinated copolymer resin dispersion, such as fluorinated ethylene propylene (FEP) or perfluoroalkoxy-modified tetrafluoroethylene (PFA). This is applied by dipping, wiping, drying, baking, and fusing, essentially as the PTFE dispersions were applied. Alternately, one face may be rendered bondable by treatment, under appropriate conditions and with appropriate pre- and post-treatment processing, with a mixture of sodium metal, naphthalene, and a glycol ether, or alternately, a mixture of sodium metal and anhydrous ammonia. If applicable, other means of rendering the PTFE surface bondable, for example corona treatment in special atmospheres, chemical and electrochemical treatments, metal sputtering, and vacuum deposition of metals or metal oxides, might be employed. Once again, the technology involved in applying bondable treatments to PTFE surfaces is thoroughly described in the literature and is familiar to those skilled in the art of processing PTFE films, articles, and composites of all kinds.

In the final step, the face of the PTFE/glass composite rendered bondable in the previous step is coated with a controlled amount of a relatively low viscosity (ca. 10,000 to 200,000 centipoises) platinum catalyzed, addition cure, 100 percent solids, solventless, liquid silicone rubber (LSR) formulation. The LSR coating should be as thin as possible, only as thick as necessary to achieve the desired function. Although a variety of coating thicknesses of silicone rubber to serve a variety of functions are contemplated by the invention, a coating thickness of about 2-50 mils is preferred. The LSR formulation is composed of commercially available A and B components that are mixed in a specified ratio, typically 1:1 or 10:1. Each component contains vinyl-terminated polydimethylsiloxane polymers and may contain fumed silica as a reinforcing filler, and/or extending fillers. Typically, one component, for instance the A component, contains the catalyst and the B component contains a crosslinking agent and an inhibitor that is removed by heating to allow the LSR to cure into a solid rubber. The LSR formulation may incorporate pigments and/or other additives. The membrane thus coated is completed by passing it through a coating oven or other heating device, raising the temperature of the coating sufficiently to drive off or decompose the inhibitor allowing the LSR to cure into a solid rubber.

Alternately, the web may be coated with a silicone rubber incorporating an organic peroxide catalyst (“heat-curable” silicone) which may be applied from a solvent solution, dried if necessary to remove the solvent and heated appropriately to effect a cure, or with silicone rubber incorporating an atmospheric-moisture-activated acetoxy cure system (“one-package” RTV), once again perhaps from a solvent solution, dried if necessary to remove the solvent and allowed to remain exposed to moist air sufficiently long to effect a cure, perhaps being heated to accelerate the process.

The resulting composite is a durable, two-faced material with one face PTFE and one face silicone rubber. The composite exhibits flexural modulus about that of a plain PTFE/glass fabric composite comprising the same reinforcement fabric and percent PTFE, and has little tendency to curl.

Should it be desirable, ribs, lugs, cleats, or other protuberances composed of rubber that is the same or similar in composition to the rubber face of the composite may be formed on the relatively smooth rubber face by applying beads of flowable, uncured rubber by the use of robotically controlled applicators or by methods similar to those described in co-pending application Ser. No. 09/608,649, filed Jun. 30, 2000, the subject matter of which is incorporated by reference herein. The material applied is then cured, depending on its type, according to the procedures in the paragraphs above. The rubber must be sufficiently viscoelastic to permit retention of its shape without flowing excessively during the time required to apply the required number of cleats and transfer the work to an oven or other heating device or other environment in which the cure is effected.

Unlike other elastomers, silicone rubber has low surface energy. Surprisingly, the surface of silicone rubber is tacky when dry, yet very slippery when water is present on the surface. This feature of silicone rubber results in unique advantages and end uses which are contemplated by the invention.

FIG. 1 shows an exemplary composite of the invention in which a fiberglass reinforcement (1) is coated on both faces with PTFE (2). A mixture of a colloidal silica dispersion (3) is applied to one surface of the PTFE-coated fabric. The application of the colloidal silica dispersion (3) renders the surface bondable. The PTFE/fiberglass face rendered bondable by dispersion (3) is coated with silicone rubber (4,5) to achieve a composite with one face PTFE and one face silicone rubber (5).

EXAMPLE 1

One face of a roll of commercially available PTFE/glass fabric (Chemfab Chemglas® Basic 5), containing style 2116 glass fabric as a reinforcement and comprised of about 50 percent by weight PTFE resin and 50 percent by weight glass, is rendered bondable by applying a mixture of colloidal silica dispersion (DuPont Ludox® 40), PFA fluoropolymer resin solution (Dupont TE-9946), surfactants, stabilizers, and water; wiping off the excess; drying; baking; and fusing. The fabric weighs about 5.4 ounces per square yard (osy) and is about 0.005 inches thick.

Using conventional PTFE tower-coating equipment, a coating of an LSR formulation composed of 50 parts by weight each of Wacker Silicones Elastosil® LR6289A and LR6289B and about 12 parts by weight of a red iron oxide masterbatch containing about 35 percent iron oxide and about 65 percent vinyl terminated silicone polymer, is applied to the bondable face of the Chemglas Basic 5. The tower is operated to provide conditions of time and temperature sufficient to cause the rubber to cure. The end result is a composite with a smooth, glossy coating of silicone rubber on one face and a PTFE surface on the opposite face. The whole weighs about 7.5 osy; the silicone rubber coating is about 0.002 inches thick and is strongly adhered. The composite lays flat and can be handled easily without curling.

The example composite is fabricated into a belt for a combination weighing/packaging machine for meat and other food products. In the heat-sealing section of the machine, the non-working face of the belt, i.e., the face that does not contact the product being weighed and wrapped, must slide freely over a heated platen. In another section of the machine, the wrapped package must be conveyed up an incline without slipping back. An all-PTFE belt slides freely over the heating platen but allows the package to slip back at the incline. An all-silicone belt carries the package up the incline without slipping, but does not slide freely over the heated platen. The example belt functions optimally in both sections of the machine.

EXAMPLE 2

One face of another roll of the PTFE/glass fabric used in the above example is rendered bondable by treatment with a solution of sodium, naphthalene, and glycol ether. It is coated using the procedure described in Example 1, yielding a composite with physical properties almost identical to those of Example 1.

EXAMPLE 3

One face of a length of Chemltas 64-40916, a Chemfab product comprised of Style number 64 glass fabric saturated/coated with 40 percent by weight PTFE, is rendered bondable by application of the colloidal silica formulation as described in Example 1 and coated with 8 osy. of silicone rubber. The resulting product is fabricated into a belt 50 meters long and 1.5 meters wide. It is substituted for a conventional glass-reinforced silicone rubber belt used as a conveyor and release surface in the assembly of plastic wine bags by heat sealing. The silicone release surface of the conventional belt performs to the user's satisfaction, but the construction is difficult to drive on the user's equipment due to excessive frictional force generated when the rubber non-working face of the belt, i.e., the face that does not contact the product, slides over stationary components of the machine. The PTFE non-working face of the example belt generates minimal frictional force against the machine's stationary surfaces, allowing it to be easily driven, while the working face provides the silicone rubber release surface desired.

EXAMPLE 4

A conveyor belt with raised cleats for use in a fast-food-service toaster in which the bread products being toasted are slid across a heated platen or griddle by means of force transmitted by the moving belt, is produced as follows. A rectangle or belt “blank” of appropriate size is cut from the composite of Example 1. Using a robotic applicator, a pattern of many, identically-shaped raised cleats is laid down on the silicone rubber face of the blank. The cleats are composed of the same LSR formulation as the face itself. Each cleat is about 0.8 inches long and roughly simicircular in cross section, about 0.2 inches wide at the base and 0.04 inches high at the highest point. The longitudinal centerline of the cleat is a straight line oriented perpendicular to the direction of travel of the finished conveyor belt. The LSR forming the cleats has viscoelasticity that allows it to retain its shape during the time it takes to apply the entire pattern of cleats. After the pattern is applied, the blank is placed in an oven operating at 500° F. and allowed to remain for two minutes. When the blank is removed from the oven the silicone rubber surface bears a pattern of durable rubber cleats strongly adhered to the surface. The belt is completed by attaching lacings on two opposite ends. When installed on the toaster the cleats contacting the bread products being toasted, for example hamburger rolls, are found to drive the rolls more reliably, with less slippage, than a smooth-faced belt made of similar material.

Claims

1. A fiber-reinforced flexible composite comprising:

a reinforcement material;

a first exposed face on a first side of the reinforcement material, the first exposed face formed from a first material, the first material having, a low coefficient of friction, and thermal stability in operating environments exceeding 350° F.; and

a second exposed face on a second side of the reinforcement material opposing the first exposed face, the second exposed face formed from a second material, the second material having, a high coefficient of friction, and thermal stability in operating environments exceeding 350° F.;

wherein the flexible composite lies flat and does not tend to curl.

2. The composite of claim 1, wherein the composite comprises about a same flexural modulus when it includes the second material that it comprises when it does not include the second material.

3. The composite of claim 1, wherein the second material is a portion of a first layer having a thickness of up to about 50 mil.

4. The composite of claim 3, wherein the first layer has a thickness of at least about 2 mil.

5. The composite of claim 1, wherein the first material is present on the second side of the reinforcement material.

6. The composite of claim 5, wherein the first material is only located on the first side of the reinforcement material.

7. The composite of claim 1, wherein

the flexible composite comprises two compositionally distinct opposing faces;

the reinforcement material consists essentially of glass fibers;

the first material comprises a perfluoropolymer material, the first material being located on each side of the reinforcement material, the perfluoropolymer in a balanced state having mechanical forces within the perfluoropolymer equal on each side of the reinforcement such that it helps to prevent the membrane from curling; and

the second material comprises an elastomer disposed over the first material on one side of the reinforcement.

8. The composite of claim 1, wherein

the composite comprises two compositionally distinct opposing faces;

the first material comprises perfluoropolymer;

the composite comprises a first layer comprising the first material and a second layer of perfluoropolymer material;

the reinforcement material is a fibrous reinforcement material and is intermediate the first and second layers;

the second material comprises an elastomer;

the second material is disposed over the second layer of perfluropolymer material; and

the first and second layers have a thickness sufficient to inhibit the composite from curling.

9. The composite of claim 1, wherein

the composite comprises, two compositionally distinct opposing faces; a fibrous reinforcement material; and a perfluoropolymer material coating on each side of the fibrous reinforcement material;

the first material comprises a perfluropolymer material;

the perfluoropolymer material coating on each side of the fibrous reinforcement material is in a balanced state having mechanical forces within the perfluoropolymer equal on each side of the reinforcement to prevent the membrane from curling;

the second material comprises an elastomer;

the second material is disposed over the perfluoropolymer material on one side of the fibrous reinforcement material; and

the second material has a thickness of about 2 to about 50 mils.

10. The composite of claim 1, wherein

the composite comprises, two compositionally distinct opposing faces; a fibrous reinforcement material; and a perfluoropolymer material coating on each side of the fibrous reinforcement material;

the first material comprises a perfluropolymer material;

the perfluoropolymer material coating on each side of the fibrous reinforcement material is in a balanced state having mechanical forces within the perfluoropolymer equal on each side of the reinforcement to prevent the membrane from curling;

the second material comprises an elastomer;

the second material is disposed over the perfluoropolymer material on one side of the fibrous reinforcement material; and

the weight ratio of the reinforcement to the perfluoropolymer coating is 50:50.

11. The composite of claim 1, wherein the first material comprises a first layer having a thickness of at least 1 mil.

12. The composite of claim 11, wherein the first layer has a thickness of up to about 5 mil.

13. The composite of claim 1, wherein the first material comprises a layer having a thickness of up to 5 mil.

14. The composite of claim 1, wherein the first material and the second material both have low surface energies.

15. The composite of claim 1, wherein the second face is slippery when water is present and tacky when dry.

16. The composite of claim 1, wherein the second face has pronounced stick-slip and the first face has minimal stick-slip.

17. The composite of claim 1, comprising protuberances raised above the second exposed face.

18. The composite of claim 1, wherein the composite has a weight of about 7.5 ounces per square inch (osy).

19. The composite of claim 1, wherein the reinforcement member consists essentially of glass fiber.

20. The composite of claim 1, wherein the first material comprises a perfluoropolymer and the second material comprises an elastomer.

21. An article for modifying surface, comprising:

a flexible composite comprising a reinforcement material; a first exposed face on a first side of the reinforcement material, the first exposed face formed from a first material, the first material having, a low coefficient of friction, and thermal stability in operating environments exceeding 350° F.; and a second exposed face on a second side of the reinforcement material opposing the first exposed face, the second exposed face formed from a second material, the second material having, a high coefficient of friction, and thermal stability in operating environments exceeding 350° F.;

wherein the flexible composite lies flat and does not tend to curl; and

wherein the article is configured to be placed on the surface such that the properties of the face of the material not in contact with the surface are different than the properties of the surface.

22. The article of claim 21, wherein the first surface of the flexible composite is configured to be placed in contact with the surface such that the first face is not in contact with the surface.

23. The article of claim 21, wherein the first material is formed on the first surface in a layer at least about 1 mil thick.

24. The article of claim 21, wherein the second material is formed on the second surface in a layer up to about 50 mil thick.

25. The article of claim 21, wherein the first material consists essentially of polyfluoropolymer and the second material consists essentially of elastomer.

26. The article of claim 21, wherein the first material is also disposed on the second side of the reinforcement material.

27. The article of claim 21, wherein

the first material comprises polyfluoropolymer and the second material comprises elastomer;

the first material is also disposed on the second side of the reinforcement material; and

the second material is not disposed on the first side of the reinforcement material.

28. The article of claim 27, wherein

the first material is formed on the first surface in a layer at least about 1 mil thick; and

the second material is formed on the second surface in a layer up to about 50 mil thick.

29. A flexible composite based belt, the belt comprising:

a reinforcement material;

a first exposed face on a first side of the reinforcement material, the first exposed face formed from a first material, the first material having, a low coefficient of friction, and thermal stability in operating environments exceeding 350° F.; and

a second exposed face on a second side of the reinforcement material opposing the first exposed face, the second exposed face formed from a second material, the second material having, a high coefficient of friction, and thermal stability in operating environments exceeding 350° F.;

wherein the belt lies flat and does not tend to curl.

30. The belt of claim 29, wherein

the flexible composite comprises two compositionally distinct opposing faces;

the reinforcement material consists essentially of glass fibers;

the first material comprises a perfluoropolymer material, the first material being located on each side of the reinforcement material, the perfluoropolymer in a balanced state having mechanical forces within the perfluoropolymer equal on each side of the reinforcement such that it helps to prevent the membrane from curling; and

the second material comprises an elastomer disposed over the first material on one side of the reinforcement.

31. The belt of claim 29, wherein

the composite comprises two compositionally distinct opposing faces;

the first material comprises perfluoropolymer;

the composite comprises a first layer comprising the first material and a second layer of perfluoropolymer material;

the reinforcement material is a fibrous reinforcement material and is intermediate the first and second layers;

the second material comprises an elastomer;

the second material is disposed over the second layer of perfluropolymer material; and

the first and second layers have a thickness sufficient to inhibit the composite from curling.

32. The belt of claim 29, wherein

the composite comprises, two compositionally distinct opposing faces; a fibrous reinforcement material; and a perfluoropolymer material coating on each side of the fibrous reinforcement material;

the first material comprises a perfluropolymer material;

the perfluoropolymer material coating on each side of the fibrous reinforcement material is in a balanced state having mechanical forces within the perfluoropolymer equal on each side of the reinforcement to prevent the membrane from curling;

the second material comprises an elastomer;

the second material is disposed over the perfluoropolymer material on one side of the fibrous reinforcement material; and

the second material has a thickness of about 2 to about 50 mils.

33. The belt of claim 29, wherein

the composite comprises, two compositionally distinct opposing faces; a fibrous reinforcement material; and a perfluoropolymer material coating on each side of the fibrous reinforcement material;

the first material comprises a perfluropolymer material;

the perfluoropolymer material coating on each side of the fibrous reinforcement material is in a balanced state having mechanical forces within the perfluoropolymer equal on each side of the reinforcement to prevent the membrane from curling;

the second material comprises an elastomer;

the second material is disposed over the perfluoropolymer material on one side of the fibrous reinforcement material; and

the weight ratio of the reinforcement to the perfluoropolymer coating is 50:50.

34. The belt of claim 29, wherein the belt comprises about a same flexural modulus when it includes the second material that it comprises when it does not include the second material.

35. An apparatus comprising:

a machine; and

a belt capable of being driven by the machine, the belt comprising, a reinforcement material; a first exposed face on a first side of the reinforcement material, the first exposed face formed from a first material, the first material having, a low coefficient of friction, and thermal stability in operating environments exceeding 350° F.; and a second exposed face on a second side of the reinforcement material opposing the first exposed face, the second exposed face formed from a second material, the second material having, a high coefficient of friction, and thermal stability in operating environments exceeding 350° F.; wherein the belt lies flat and does not tend to curl.

36. The apparatus of claim 35, wherein

the machine comprises a heated platen;

the machine and belt are configured such that the first exposed face will move across the heated platen and the second exposed face will be in contact with objects.

37. The apparatus of claim 35, wherein the machine and belt are configured such that the second exposed face will be in contact with objects and the belt will move the objects along an incline.

38. A fiber-reinforced flexible composite comprising:

a fibrous reinforcement material;

a first exposed face on a first side of the fibrous reinforcement material, the first exposed face formed from a first material comprising perfluropolymer, the first material having, a low coefficient of friction, thermal stability in operating environments exceeding 350° F., minimal stick-slip, and a low surface energy; and

a layer on a second side of the fibrous reinforcement material, the layer formed from a second material comprising a perfluoropolymer, the second material having, a low coefficient of friction, thermal stability in operating environments exceeding 350° F., and a low surface energy; and

a second exposed face, on the second side of the reinforcement material, opposing the first exposed face, the second exposed face formed from a third material comprising an elastomer, the third material having, a high coefficient of friction, thermal stability in operating environments exceeding 350° F., pronounced stick-slip, and a low surface energy;

wherein the flexible composite lies flat and does not tend to curl;

wherein the composite comprises about a same flexural modulus when it includes the second material that it comprises when the composite does not include the second material; and

wherein the composite comprises two compositionally distinct opposing faces.

39. The composite of claim 38, wherein

the third material is not located on the first side of the reinforcement material;

the third material is a portion of a layer having a thickness of up to 50 mil; and

the first material is present on the first side with a thickness of 1 mil to 5 mil.

40. The composite of claim 38, wherein the first material and the second material are a same type of material and the third material is bonded to the second material.