OF

JOHN TIPPETT

FOR

SYSTEMS AND METHODS FOR PROVIDING

SHAPED COMPOSITES THAT INCLUDE FLUOROPOLYMER FILMS

PRIORITY

The present application claims priority to U.S. Provisional Patent Application Ser. No. 62/103,324 filed January 14, 2015, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND

This present invention generally relates to the forming of a composite that includes a metal structure and a fluoropolymer film on at least a portion of the metal structure.

Most non-stick fluoroplastic surfaces on three-dimensional metal shapes are applied using fluoroplastic spray coating methods and technology. Fluoroplastic materials give excellent chemical protection, release properties, and lubrication to metal surfaces. A common product found in many households is a metal frying pan with a non-stick fluoroplastic surface.

The formation of such an article may conventionally involve the following. The uncoated metal is first formed into a desired three-dimensional shape of, for example, a frying pan, using conventional metal forming techniques. A fluoroplastic coating is then typically applied to the formed metal frying pan using fluoroplastic spray-coating methods that usually involve metal etching and/or a priming stage and high temperature fusing of the fluoroplastic to the metal. In some applications, the fluoroplastic coating is comprised of polytetrafuoroethylene (PTFE) dispersion with filler. The filler may include pigments for color and/or enhanced properties such as improved abrasion resistance.

There are several undesirable properties and cost factors associated with PTFE coatings on metal surfaces. The spray coating is limited in thickness and longevity. Coating thicknesses on metal may range, for example, from 0.00025 in (0.006 mm) to 0.004 in (0.1 mm), but it is difficult to achieve high coating thicknesses. It is difficult and to build PTFE coating thickness on metal surfaces. A more practical thickness range is 0.001 in - 0.003 in (1-3 mils).

Thin PTFE spray coatings are inherently weak and brittle. Over time, these coatings may develop poor adhesion to metal and may become susceptible to removal upon slight agitation. For example, many households have experienced common failure of PTFE coatings on metal frying pans when using metal spatulas with sharp metal edges. The metal edges of a spatula may apply forces during use that result in the removal of the PTFE coating on a metal frying pan, ruining the non-stick properties of the frying pan to food.

Further, the brittle PTFE coatings on metal have very little elongation capability. If a metal part with a PTFE surface deforms, it is likely that the PTFE coating will not maintain a uniform and uninterrupted surface. If there is an interruption, such as a crack in the PTFE surface, the release properties may be comprised. Also, the PTFE coating may flake off the metal. It is for these reasons that most PTFE coatings are applied to metal parts after the forming process from a flat sheet into a three-dimensional shape.

Thin PTFE coatings may also have permeation concerns. A PTFE coating is inherently permeable. Liquids may permeate into PTFE surfaces over time and repeated use. As a PTFE surface is permeated by a foreign material, long-term release properties may be diminished. For example, the non-stick surface of a frying pan will eventually fail to release after long-term exposure to cooking. Oils from food permeate into the surface of the thin PTFE coating on metal, compromising its release properties. Food cooked on such a compromised surface may be permeated with oils and residue from previous cooking sessions. Thus, the non-stick properties are therefore, inferior to when the frying pan was first used without the permeants in the coating surface.

Also, the bond between the PTFE coating and metal may be diminished from permeation. A build-up of permeants can form between metal and the PTFE coating. This build-up may cause delamination or flaking-off of the PTFE coating from the metal.

To minimize permeation in PTFE surfaces, thicker PTFE surfaces are desired. A thicker coating surface will not prevent permeation but it may delay permeation to a point that is agreeable to the user. The cost to make a thicker PTFE coating on metal can be prohibitively high, and as discussed above, may be impractical given the limitations on PTFE coatings to stick to PTFE coatings There is also the additional cost for a metal forming company in transporting a three-dimensional metal object to a fluoroplastic coating vendor, although some metal forming companies may also have internal fluoroplastic coating capabilities. A ready-made metal/PTFE flat sheet could eliminate the need for internal coating capability, which can be costly for the forming company. An internal fluoroplastic coating operation may also require the use of chemicals with environmental compliance issues.

There remains a need for a more resilient fluoroplastic surface for metals that is formed from a flat-sheet, two-dimensional shape into a three-dimensional object.

SUMMARY

In accordance with an embodiment, the invention discloses a formed composite that includes a metal structure that has been shaped to a non-flat shape, said formed composite including a fluoropolymeric film bonded to a surface of the metal structure such that a peel strength between the metal structure and the fluoropolymeric film is at least about 1 lb/in of width.

In accordance with another embodiment, the invention provides a method of forming a shaped composite that includes the steps of: providing a metal structure; bonding at a bond interface, a fluoroplolymeric film to the metal structure to form a composite structure, said bond interface including in at least some areas, a mingling of molecules of the fluoropolymeric film with molecules of the metal structure; and shaping the composite structure to form the non-flat shaped composite, the step of shaping involving maintaining a substantial amount of the areas of mingled molecules of the fluoropolymeric film and the molecules of the metal structure.

In accordance with a further embodiment, the invention provides a method of forming a shaped composite that includes the steps of: providing a metal structure; bonding at a bond interface, a fluoropolymeric film to the metal structure to form a composite structure, the bond interface being characterized as having a peel strength of at least about 1 lb/in of width; and shaping the composite structure to form the shaped composite, the step of shaping involving substantially maintaining the bond interface having a peel strength of at least about 1 lb/in of width.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description may be further understood with reference to the accompanying drawings in which:

Figure 1 shows an illustrative diagrammatic view of the formation of a fluoroplastic film for use in an embodiment of the invention that includes a laminate of cross-oriented fluoroplastic layers;

Figure 2 shows an illustrative diagrammatic view of the laminated fluoroplastic film of Figure 1;

Figure 3 shows an illustrative diagrammatic view the formation of a cast fluoroplastic film for use in accordance with an embodiment of the present invention;

Figure 4 shows an illustrative diagrammatic view of the cast fluoroplastic film of Figure 3;

Figure 5 shows an illustrative diagrammatic view of the formation of a skived fluoroplastic film for use in accordance with an embodiment of the present invention;

Figure 6 shows an illustrative diagrammatic view of the skived fluoroplastic film of Figure 5;

Figures 7 A and 7B shows an illustrative diagrammatic views of the bonding of the laminated fluoroplastic film of Figures 1 and 2 to a metal structure;

Figures 8A and 8B shows an illustrative diagrammatic views of the bonding of the cast fluoroplastic film of Figures 3 and 4 to a metal structure;

Figures 9A and 9B shows an illustrative diagrammatic views of the bonding of the skived fluoroplastic film of Figures 5 and 6 to a metal structure;

Figure 10 shows an illustrative diagrammatic view of a composite in accordance with embodiments of the present invention in a press prior to application of heat and pressure;

Figure 11 shows an illustrative diagrammatic view of the composite of Figure 10 during the application of heat and pressure;

Figure 12 shows an illustrative diagrammatic view of a portion of the composite structure of Figure 10;

Figure 13 shows an illustrative diagrammatic view of a portion of the shaped composite of Figure 11;

Figure 14 shows an illustrative diagrammatic view of a curved portion of the shaped composite of Figure 11; and

Figure 15 shows an illustrative diagrammatic view of a portion of a shaped composite in accordance with an embodiment of the invention undergoing an externally applied force.

The drawings are shown for illustrative purposes only and are not to scale.

DETAILED DESCRIPTION

In accordance with various embodiments, the invention incorporates a barrier PTFE film surface that has been bonded to a flat metal surface. The metal component may be prepared in advance for bonding. A separate high-temperature adhesive material may be used to facilitate the bond between the prepared metal and fluoroplastic (e.g., PTFE) film surface. The barrier PTFE film surface may be made prior to bonding to metal or during the bonding to metal process. In the bonding to metal process, high temperature and pressure are used to melt the high-temperature adhesive and enact a bond between the barrier PTFE film and the prepared metal.

Any metal surface may be considered a candidate for composites of the invention. The sole limitation in metal is that it must be able to withstand the high-temperature bonding process used to make the laminate. Typical bonding temperatures will range from 400°F to 900°F.

One or both of the metal surfaces are prepared for bonding using typical industry methods such as sanding, sand blasting, grinding, and chemical treatment. Depending upon the metal, chemical primers may be coated to the metal for further bonding improvement to PTFE.

Once the metal has been prepared, a high-temperature adhesive layer may be provided in between the metal and PTFE surfaces. The adhesive layer may consist of a perfluoroalkoxy (PFA) film or coating, fluorinated ethylene propylene (FEP) film or coating, or PTFE film or coating. Blends of the three materials in film or coating form may also be used as an adhesive layer. Typical film thicknesses may range from a 1/4 mil to 200 mils. Liquid coatings of PTFE, PFA, or FEP dispersion may be sintered or unsintered.

Barrier PTFE films incorporated of an embodiment may be made using many types of processes. Candidate barrier PTFE films include those made from fine powders and liquid dispersion. Overall barrier PTFE film thickness may range from 1 mil to 300 mils.

Candidate barrier PTFE films made from fine powders include extruded PTFE films, PTFE Crossfilms (as disclosed, for example in U.S. Patent No. 5,466,531, the disclosure of which is hereby incorporated by reference in its entirety), cast PTFE films, expanded PTFE films, and modified PTFE films.

With reference to FIGS. 1 and 2, a laminated fluoroplastic film 10 for use in a composite of the invention may include a plurality of axially oriented PTFE films 12, 14, 16 laminated directly together, i.e., without the interposition of an adhesive or bonding agent therebetween. The PTFE films are preferably unsintered prior to lamination, and preferably are uniaxially oriented, with at least one of the films having its direction of orientation disposed angularly with respect to that of at least one other of the films. An example of a layout of film orientations is shown in FIG. 1. PTFE film thicknesses will typically range between 1-10 mils, and preferably from 2-5 mils. Prior to being oriented, the unsintered PTFE films may be prepared by a process that involves mixing powdered PTFE resin with a solvent to produce a paste, and the paste is preformed to remove air, extruded into a film, and then calendared to a desired thickness. The calendared film is then dried to evaporate the solvent. The films are then oriented and laminated in accordance with the embodiment of FIGs. 1 and 2.

Lamination is effected between heated platens under conditions of elevated pressure and temperature for varying time intervals. Lamination pressures need only be sufficient to expel entrapped air from between the plies while promoting intimate face-to-face contact. Pressures at or above 1 p.s.i. have been deemed adequate, with the preferred pressure range being between about 40-60 p.s.i. Lamination temperatures (measured as the temperatures of the platens in contact with the laminate) are selected to accommodate numerous variables, including differing laminator designs and thermal capabilities, the type of PTFE film being processed, e.g., sintered or unsintered, the number and thickness of the films making up the laminate, the residence time of the films in the laminator, etc. In such cases, the entire cross section of the laminate is heated above the melt temperature of the constituent films, which for unsintered PTFE is about 650°F, and for sintered PTFE is somewhat lower at about 621 °F. This results in the formation of interphase zones z at the bond lines where the molecules of adjacent films have commingled. Lamination temperatures are kept below about 900 °F. to avoid degrading or thermally disturbing the surface films of the laminate. Typically, lamination temperatures will range from about 660 °F - 760 °F, preferably between 710 °F - 730 °F.

Lamination times are selected to achieve uniform cross sectional heating of the laminate, and are otherwise minimized in order to promote production efficiencies. Typical lamination times range between 20-70 seconds, depending on the other process and equipment variables described above. Thicknesses of these films (prior to bonding to a metal substrate as discussed below) range from 1/4 mil to 200 mils.

In further examples, and with reference to FIGs. 3 and 4, a cast fluoroplastic film 20 may be used to form a composite of the invention. The fluoroplastic (e.g., PTFE) material may be extruded from a die 22 onto a cooling roll 24. The cast PTFE film may also, for example, have a thickness of about 1/4 mil to 200 mils. The cast films are produced in sintered form.

With reference to FIGs. 5 and 6, a skived fluoroplastic film 30 may be used to form a composite of the invention. The fluoroplastic (e.g., PTFE) material may be skived from a block 32 using a blade 34. The skived PTFE film is produced in a sintered form. In order to laminate the film, the film is heated to about 650 °F.

The invention and its advantages are illustrated by the following examples wherein laminates comprising various combinations of unsintered PTFE films are laminated and sintered. The extruded PTFE films were CROSS FILM PTFE films sold by Textiles Coated International of Manchester, New Hampshire. Skived PTFE was obtained through McMaster Carr, an industrial catalog.

Candidate PTFE films made from liquid dispersion include cast PTFE films. These films have a thickness range of 1/4 mil to 200 mils prior to bonding. A typical maximum single layer of cast PTFE film is around 6 mils. Single layers of cast PTFE film may be stacked to achieve thicknesses far greater than 6 mils. The films were sintered during lamination, and retained their respective directions of orientation following lamination.

As shown in FIGs. 7A and 7B, the laminated fluoroplastic film (10) may be bonded to a metal substrate 40 either directly or using an intermediate PFA or FEP film (as shown in FIG. 7B) to form a composite structure. In an embodiment, the PTFE film includes the intermediate layer of PFA. As shown in FIGs. 8A and 8B, the cast fluoroplastic film (20) may be bonded to a metal substrate 40 either directly or using an intermediate PFA or FEP film (as shown in FIG. 8B) to form a composite structure. In an embodiment, the PTFE film includes the intermediate layer of PFA. As shown in FIGs. 9A and 9B, the cast fluoroplastic film (20) may be bonded to a metal substrate 40 either directly or using an intermediate PFA or FEP film (as shown in FIG. 9B) to form a composite structure. In an embodiment, the PTFE film includes the intermediate layer of PFA. In addition, the barrier PTFE film surface may be prepared for adhesion to metal using various forms of etching processes typically used for bonding PTFE to dissimilar materials. These processes include corona etching, sodium naphthalene etching, and sodium ammonium etching, to name three processes

Temperature and pressure are required to achieve an effective bond between PTFE and metal surfaces. As discussed above, the typical bonding temperature will range from 400°F to 900°F. The temperature must be sufficient to activate the beneficial properties of the separate adhesive layer. Pressure will be delivered using any piece of equipment with flat surfaces that will force a high bond between the PTFE film and flat sheet of metal.

Once manufactured, the flat-sheet PTFE/metal laminate (composite structure) is capable of surviving conventional metal forming techniques without comprising the integrity of the PTFE film surface. As shown in FIGs. 10 and 11, as the metal changes shape from a two-dimensional sheet to a three-dimensional object by application in a press 50 including top and bottom molds 54, 56, the fluoroplastic film 58 (e.g., laminated, cast or skived) surface remains uninterrupted and uncompromised. The desired attributes of the composite structure 52 associated with a PTFE surface are unchanged and intact in the composite structure of 52' after the arduous metal-forming processes.

This invention relates to metal forming processes that are related to the stretching of metal. These processes include deep drawing, stretch forming, hydro forming, and bladder forming.

In the forming processes, lubricants such as silicone oil or PTFE spray coating may be used as a mold aid to reduce the exposure to significant shearing forces. The lubricant may be applied to all contacted PTFE surfaces in the forming processes. Since a PTFE surface is self-lubricating, the spray coating may or may not be needed as a molding aid.

To those skilled in the art, it is surprising to find that the compressive and shearing forces associated with metal forming do not destroy the bond between the barrier PTFE film and metal. In this invention, the typical bond strength between a barrier PTFE film and metal in a flat-sheet laminate ranges from 1 lbs/in of width to 200 lbs/in of width. As the metal is formed into a three-dimensional shape, the adhesion of the barrier PTFE film to metal undergoes an inconsequential decrease in adhesion value. Adhesion value decrease may range from 1% to 50%. This measurement is deemed inconsequential because a proper initial design of required adhesion levels in the flat-sheet laminate can offset decreases that may occur during the metal reshaping process. A flat-sheet laminate can be over-engineered to provide long-term adhesion between barrier PTFE film and metal components in a three- dimensional object. Most importantly, the physical properties of the PTFE surface remain undamaged and uncompromised after the arduous metal reshaping process.

The barrier PTFE surface of composites of the present invention is a significant improvement over the typical PTFE spray coatings that are used on metal. For example, typical PTFE spray coatings are abraded off the surface of the metal using slight scraping action, such as contact from the edge of a metal spatula. The PTFE coating is easily scraped in one or two tries with the edge of the metal spatula and reasonable human strength, revealing an obvious metal surface below the thin coating. In a similar test using a tough barrier PTFE film instead of a thin PTFE coating, the PTFE film surface cannot be scraped off after numerous tries using the edge of a metal spatula and reasonable human strength. In laboratory tests of this type with many samples, the tough barrier PTFE film thickness ranged from 2 mils to 10 mils.

It is believed that the increased thickness, toughness, tensile strength, and tear resistance properties of the barrier PTFE films on the surface of metal significantly improves the abrasion-resistance of the PTFE/metal laminate. These improved physical properties can act independently or cumulatively to assist the adhesion between the PTFE and metal surfaces. Because a thin and brittle PTFE coating does not have significant toughness, tensile strength, and tear-resistance properties, it is not able to contribute in strength properties to the adhesion between PTFE and metal surfaces. The thin and brittle PTFE coating is easily removed with surface agitation or metal deformation.

Thick and capable barrier PTFE film surfaces also may have elongation properties that contribute to the integrity of the PTFE surface. If a thick and capable PTFE film surface is contacted by the sharp edge of a metal spatula, a capable PTFE film may have the elongation capability to resist tearing, delamination, or flaking. A thin and brittle PTFE coating is more likely to delaminate or flake off rather than elongate when agitated.

In addition to maintaining the physical integrity of the PTFE film surface during the forming processes, an aesthetically-pleasing PTFE surface is achieved after the metal forming process. To those knowledgeable of the reshaping of PTFE surfaces, it is not expected that a quality PTFE appearance is maintained during a reshaping or forming process without heat. It is generally expected that a PTFE surface will undergo an appearance transition during the reshaping process to a heavily whitened and/or stressed condition. The whitened/and or stressed condition may preclude the use of the three-dimensional product in commercial applications where aesthetics are important. Contrary from the expected, it is found that the compressive forces during the metal reshaping process eliminate the presence of stressed or whitened PTFE surfaces.

Whitening may occur in a stressed or elongated barrier PTFE film due to the formation of fissures or voids. As a PTFE film is elongated without temperature or lubricant, fissures or voids may form, resulting in a whitish or stressed appearance. During the forming processes, the compressive forces may eliminate the presence of voids, which in turn eliminates the whitish or stressed appearance. The barrier PTFE film surface remains aesthetically-pleasing, smooth, and capable for long-term release after the deep drawing or stretch forming processes.

As shown in FIG. 12, the bond interface between the metal film 40 and the fluorolastic film 58 (laminated, cast or skived, and with or without the intermediate PFA or FEP layer) in the composite structure may be characterized has having a peel strength at the location shown at 60 of at least about 1 lb / inch of width as tested in accordance with ASTM D.1876. In further embodiments, the peel strength at the bond interface of the composite structure may be at least 5 lb / inch of width, and in certain embodiments, it may be at least 10 lb / inch of width. All peel strength testing herein may be provided in accordance with ASTM D.1876.

With reference to FIG. 13, the bond interface shown at 62 between the metal film 40 and the fluoroplastic film 58 (laminated, cast or skived, and with or without the intermediate PFA or FEP layer) in the shaped composite 52' may be characterized has having a peel strength of at least about 1 lb / inch of width as tested in accordance with ASTM D.1876. In further embodiments, the peel strength at the bond interface of the shaped composite may be at least 5 lb / inch of width, and in certain embodiments, it may be at least 10 lb / inch of width.

With reference to FIG. 14, the bond interface 64 of the shaped composite 52' includes areas of mingling of the metal and polymeric materials such that regardless of whether the metal structure is in compression or tension when shaped, the polymeric material remains firmly attached to the molecules of the metal structure. As shown at A, when the shaped composite is bent, the outer layer will be stretched due to tension if the material is able to stretch, while the inner material in the bend will be compressed as shown at B. While the PTFE film may be able to change shape to some extent, the metal substrate would not be able to undergo the same amount of change (and may not change its length at all). Depending on the type of shaping therefore, it may be preferred to include the layer of PFA film between the PTFE film (particularly if a cross-film is used) and the metal substrate to provide a material with sufficient yield between the PTFE film and the metal substrate.

With reference to FIG. 15, a result of this is that when a foreign disruptive forced is applied as shown at C using, for example a sharp metal tool, the PTFE film is disturbed and moves responsive to the force as shown at 68, but the strength of the bond at the bond interface 66 is not compromised. Again, this bond interface may be between the cross-film PTFE or cast film PTFE and the metal substrate, or may be between the PFA film (or FEP film) and the metal substrate.

Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention. What is claimed is