Process of preparing textured fluoropolymer films

Details Process of preparing textured fluoropolymer films Process of preparing textured fluoropolymer films

1997-10-06

2002-02-19

Silbaugh, Jan H. (Department: 1732)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S246000, C264S544000, C264S316000, C264S320000, C264S322000, C264S331140, C264S294000

Reexamination Certificate

active

06348117

ABSTRACT:

FIELD OF INVENTION
The subject matter of the instant invention relates to a method of texturing fluoropolymer film and the textured product produced, which product retains the texture imparted even after further processing, such as in thermoforming or molding processes.
BACKGROUND OF THE INVENTION
Fluoropolymer films have long been used as a protective overlay for a variety of substrates such as metal, wood, and thermoplastic and thermoset polymers. With its excellent chemical resistance and weathering properties, a thin layer of fluoropolymer film can protect less resilient substrates from damage in both exterior and interior use. Fluoropolymer films, especially polyvinylfluoride (PVF) are consistently used in preparing decorative laminates for aircraft interiors because these films exhibit good flame resistance and low smoke generation. In addition, PVF films can be printed with a variety of inks so that the resulting laminates are both aesthetically pleasing and functional.
Surface texturing is another desirable aesthetic quality of these laminate structures. Due to the difficulty of permanently texturing fluoropolymer films, especially polyvinylfluoride, such decorative laminates usually include an embossable resin layer. Inclusion of these embossing resins increases the cost and manufacturing complexity of such laminates.
Fluoropolymer-clad decorative laminates are rarely used in non-aircraft applications, such as mass transit vehicles (trains, buses, and subways). Reasons include less stringent flammability requirements, little concern for lightweight parts, and the high cost of fluoropolymer films. The incumbent materials for mass transit vehicle interiors have been metal and plastics, the latter including both thermoplastic and thermoset polymers. The most commonly used thermoplastics for this application are polyvinylchloride (PVC), acrylic resins, or blends of these two polymers. These materials are typically supplied as flat, embossed sheets that are subsequently thermoformed to produce the desired part, e.g. seat backs, arm rests and tray tables. Both the thermoplastic and thermoset systems, however, lack the chemical and stain resistance of fluoropolymer films. Recent interest in easier-to-maintain materials for mass transit applications, rather than merely low cost, has created renewed interest in fluoropolymer films, providing that texture issues can be resolved effectively.
Flexible PVC has been used for many years to produce textured vinyl fabrics such as synthetic leather. Although PVC and PVF are chemically similar, those skilled in the art recognize that flexible films of these two compounds have very different properties and formulations and are manufactured by different processes. In order to produce flexible material, PVC must be compounded with high-boiling, compatible solvents (generally referred to as plasticizers). The solvents employed have very high boiling points (b.p. >350° C.) in order to minimize evaporation during processing and in the final end use. The plasticizer remains in the PVC fabric, film or sheet, typically comprising 25 to 50% of the total weight of the resulting product. In contrast, cast fluoropolymer films such as PVF employ a latent solvent, for example, N,N-dimethylacetamide or propylene carbonate. The latent solvent does not actually dissolve the polymer; it swells the PVF so that the polymer particles coalesce into a continuous film. Once coalescence has occurred, the latent solvent is removed; it is not required for film flexibility as is the case with PVC. To facilitate removal, the boiling point of the latent solvent is relatively low. The most commonly used latent solvents for PVF boil at temperatures below 250° C. Consequently, a key distinction between PVF and flexible PVC is that the plasticizer (latent solvent )is removed from the PVF after coalescence. The level of residual solvent in PVF film is typically on the order of 1% or less by weight. Excessive residual latent solvent is undesirable since it may retard adhesion between the PVF film and an underlying substrate. Unlike PVC film, PVF film retains its flexibility without the need for a plasticizer.
Three different approaches have been used in the past to produce textured polyvinyl fluoride film laminate constructions. The first of these involves the aforementioned embossing resins, wherein a laminate construction of polyvinyl fluoride film includes an embossable resin, typically a polyurethane, sandwiched between the PVF layer and the supporting substrate, e.g., refer to U.S. Pat. No. 4,693,926 assigned to Boeing. The second technique involves mechanically embossing laminate constructions of preformed sheets of polyvinyl fluoride film fused to thermoplastic substrates by using, for example, grooved or heated embossing rolls and then quenching the laminate construction against a chill roll. Quenching “freezes in” the texture. The third technique involves mechanical embossing of preformed sheets of polyvinyl fluoride film itself as described in U.S. Pat. No. 4,721,592 (Fruehauf et al). According to this method, embossing is performed at temperatures in the range of 360° F. (180° C.) to 390° F. (200° C.). The reference discloses that an embossed film pattern so formed will survive deep vacuum forming of shaped panels. However, this method of embossing uses such elevated temperatures that have been associated with discoloring the polyvinyl fluoride film.
These approaches to texture polyvinyl fluoride film laminates and subsequently thermoform the laminate into parts have resulted in discolored laminates or parts with insufficient texture retention after the thermoforming process. This difficulty has severely limited the utility of polyvinyl fluoride films in non-aircraft applications.
SUMMARY OF THE INVENTION
The instant invention solves problems associated with conventional methods by providing a method for making textured fluoropolymer films. The textured films of this invention may then be subsequently processed such as in thermoforming or molding techniques wherein the textured nature of the fluoropolymer film is retained after the treatment process. The ability to obtain textured fluoropolymer film, which make it useful for aircraft interiors, is also important in “mass transit” applications, such as subways, buses, trains, among others. For example, a polyvinyl fluoride film surface is desirable due to its low maintenance, e.g., graffiti can be removed more easily than from other polymers.
The process of the present invention relates to preparing a textured fluoropolymer film as the film is being cast by (a) preparing a fluid composition comprising a fluoropolymer resin; (b) casting the fluoropolymer composition onto a textured support; (c) forming a coalesced fluoropolymer film layer upon the support; (d) drying the fluoropolymer film layer while in contact with the support; and (e) removing the support from the fluoropolymer film wherein the film contains the textural features of the textured support.
In one preferred aspect of the invention, the process further comprises thermoforming the textured film. Preferably, the process also comprises laminating the film to a substrate to form a laminate structure prior to thermoforming.
In accordance with another preferred aspect of the invention, process further comprises laminating the dried fluoropolymer film while still in contact with the textured support onto a substrate to form a laminate structure prior to removal of the support. Preferably, the process further comprises thermoforming the laminate structure after removal of the support.
In accordance with another preferred aspect of the invention, the textured film prepared by the process has a modified texture retention measure at 15% area expansion of greater than about 0.9.
In accordance with another preferred aspect of the invention, thermoformable textured fluoropolymer film and a laminate of such film to a thermoformable substrate are provided have a modified texture retention measure at 15% area expansion of greater than about 0.9.
DETAILED DESCRI

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