DE68902140T2 - PROTECTIVE AND DECORATION LAYER MATERIAL WITH A LIGHT-TRANSFERABLE COATING. - Google Patents

Description

Field of the invention

This invention relates to protective and decorative coating materials. More specifically, it relates to flexible, thermoformable coating materials that can be bonded to various substrates, including external automotive panels, as a protective and decorative coating.

Background of the invention

The invention will be described with reference to providing protective and decorative surfaces on external automotive panels, but it should be understood that an automotive vehicle is only one of many substrates to which the inventive coating material may be applied.

As highlighted by the article by AJ Backhouse entitled "Routes To Low Pollution Glamour Metallic Automotive Finishes", Journal of Coatings Technology, Vol. 54, No. 693, pages 83-90, October 1982, there is a growing need to reduce the amount of atmospheric pollution caused by solvents released during industrial painting processes. Many Various approaches have been proposed. For example, efforts have been made to replace solvent-based automotive paints with water-based paints. Work has also been carried out using high solids formulations to reduce the emission of organic solvents. However, automotive surface decoration is a very demanding technique because of the requirement for very high quality finishes and the common use of metallic finishes to provide what Backhouse calls "high stylistic effects". Past efforts to replace the low viscosity, low solids dye formulations commonly used in automotive spray painting processes have met with only limited success.

A more promising approach is to eliminate the need for spray painting altogether. Eliminating or reducing the use of spray painting would not only reduce atmospheric pollution, but would also provide cost savings, since spray painting processes are so uneconomical that more than half of the paint can be lost. One means of achieving this goal is to use preformed thermoplastic sheet material that can be bonded to a panel to provide a protective and decorative coating. Such processes have been used for automotive interior trim, as described, for example, in U.S. Patent No. 3,551,232, issued December 29, 1970.

The aim of US Patent No. 3,551,232 was to solve the problems of bubble formation in resin layers, which occurs during vacuum-building process. This is achieved by using an adhesive that contains an inert particulate filler that minimizes air entrapment.

The use of a process of the type described in U.S. Patent No. 3,551,232 on exterior automotive panels (wraps) presents a greater challenge. The surface appearance of such panels is of particular importance, so it is necessary not only to avoid blistering caused by trapped air, but also to obtain a protective or decorative coating that equals or exceeds the quality of a spray-painted finish.

Furthermore, exterior automotive panels present a particular problem in terms of the difficulty of smoothly adhering a flexible sheet material to a curved substrate and the difficulty of doing so while maintaining a uniform color intensity across the entire surface.

Others have attempted to produce a paint-coated, flexible and stretchable sheet material with these properties. It is generally believed that these attempts have resulted in products with numerous coating defects and in which the paint layer does not have the high degree of uniformity that would enable the sheet material to be subjected to the stresses of thermoforming and ultimately meet the exacting standards for external automotive panels.

A new type of flexible and stretchable sheet material has been developed (not prepublished) that is (1) capable of meeting the requirements for external automotive surfaces and (2) capable of withstanding the stretching and bending forces to withstand the stresses and strains encountered when bonding the material to exterior automotive panels. This was produced by laminar flow coating processes that allow precise control of the thickness and uniformity of the coatings. Such coating processes allow for essentially defect-free coatings with a substantially uniform quality and appearance.

This new type of sheet material comprises a flexible carrier film, a protective and decorative coating layer (lacquer layer), also known as a base layer, adhered to a surface of the carrier film, and a transparent topcoat or clear coat over the base layer. The carrier film has heat-softening and stretch-elongation properties which make it suitable for use in a thermoforming process and the coating layer (lacquer layer) and the topcoat have compatible heat-softening and stretch-elongation properties. As a result, the sheet material can undergo substantial stretching without crazing or flaking of the layers.

The present invention provides a further improvement with the new type of sheet material, particularly with regard to the tendency of stretched material to elastic recovery.

Summary of the invention

The layer material (sheet material) of the invention comprises

a flexible carrier film;

a protective and decorative coating layer (lacquer layer) which adheres to a surface of the carrier film; the coating layer comprising a polymeric binder and a color former or contains light-reflecting scales or both; and

a transparent cover layer over the coating layer. At least a portion of the transparent cover layer comprises a rubbery cushioning layer of a stretchable and thermoformable polymeric composition which is non-crosslinked or of low crosslink density and has a TG value of less than 20°C. Preferably, the latter (composition) contains the same type of polymer as the polymeric binder of the coating layer.

The sheet material is in a substantially unexpanded state prior to thermoforming and is heat softenable to a substantially plastic state in which it is thermoformable over irregular surfaces in an expanded state having an area (expansion) at least 50% greater than that of the unstretched state. It has a substantially uniform quality and appearance in both its unstretched and stretched states. In a preferred embodiment, the thickness of the top layer over a metallic paint layer (coating layer) is at least 0.1 mm and the sheet material has improved gloss retention when stretched and a reduced tendency to elastic recovery.

Short description of the drawings

Figures 1 and 2 show schematically an enlarged cross section of the layer material according to the invention and

Fig. 3 is a schematic side view of an apparatus for producing the layer material according to the invention.

Detailed description

Fig. 1 of the drawings shows a layer material 10 according to the invention, which comprises a flexible thermoformable carrier film 11, a thin connecting layer 12 and a coating layer (lacquer layer) 13, which contains light-reflecting scales (flakes) 14, for example aluminum flakes.

As a result of the laminar flow coating process by which the coating layer is preferably formed (as discussed in more detail below), the light-reflecting flakes are oriented substantially parallel to the surface of the coating layer. In the preferred embodiment, at least 75% of the flakes have an orientation within 120 degrees of parallelism to the coating layer surface.

Above the coating layer there is a thick transparent cover layer 15, at least a part of which is a rubbery damping layer. The latter is formed from a stretchable and thermoformable polymeric composition which is not cross-linked or of low cross-link density and has a glass transition temperature Tg of less than 20°C.

In the preferred embodiment according to Fig. 1, the transparent layer 15 comprises two layers, a damping layer 16 and above this and forming the surface of the layer material 10 a cross-linked transparent layer 17. The latter has a significantly higher Tg value than the damping layer. The Tg value is at least about 40°C. It provides a relatively hard abrasion-resistant surface of the layer material. Preferably it is significantly thinner than the damping layer.

An adhesive layer 18 is optionally applied to the opposite surface of the carrier film 11. This can be, for example, a pressure-sensitive or heat-activated adhesive. An adhesive layer is desirable when the layer material is to be bonded to substrates such as automotive parts by vacuum thermoforming. In other bonding processes, such as in-mold processes, which is a known process for laminating a thermoplastic layer to a plastic article by fusion during the formation of the article, an adhesive layer is not required.

To protect the pressure sensitive adhesive layer, a release layer of polyester film 19 or the like may be adhered thereto and then peeled off before the layer material is thermoformed and bonded to the substrate. An optional release layer 20, which is a silicone coating, may be provided on the layer 19 to assist in peeling off the adhesive layer.

The rubbery cushioning layer in the transparent top layer of the new sheet material provides important advantages. Most notably, it reduces the tendency of the stretched sheet material to elastic recovery at elevated temperatures. This is demonstrated by a test in which the sheet material is stretched and bonded to a substrate, namely a metal plate, by vacuum thermoforming. A cut or strip is then made through the sheet material to the metal. The test cut is made with a razor blade in a straight line transverse to the direction of stretching or elongation of the sheet material. The plate with the stretched and bonded sheet material is then heated to 93ºC (200ºF) for one hour. After this heat treatment, the Width of the cut is measured to determine the extent, if any, to which the sheet material has retreated from the cut. The sheet material of the invention retreats much less than a material whose entire transparent cover layer is a cross-linked polymer with a higher Tg.

It is a property of the sheet material described, when the coating layer contains light-reflecting flakes, such as aluminum flakes, that it tends to lose gloss when stretched beyond certain limits during thermoforming. This occurs even when the material has a glossy clear thin topcoat. However, the loss of gloss with stretching is reduced or eliminated if the transparent topcoat is made sufficiently thick. Particularly when the topcoat is at least 0.1 mm (4 mils) thick, a marked improvement in the retention of gloss of such a sheet material when stretched occurs.

In accordance with the invention, it has been found that when the transparent topcoat is thick, gloss maintenance is achieved even when all or part of the thickness is provided by the rubbery cushioning layer. In addition, the benefit of a reduced tendency to elastic recovery is obtained even when the topcoat includes both the cushioning layer and an overlying higher Tg crosslinking layer. However, since such a crosslinked layer has a "memory" when stretched, it is preferred that the higher Tg crosslinked layer be thinner than the cushioning layer. For example, with a total layer thickness of 0.15 mm (6 mils), the dry thickness of the cushioning layer 16 and the crosslinked surface layer 17 of Figure 1 are 0.1 and 0.05 mm (4 and 2 mils), respectively.

It is a surprising property of the sheet material of the invention that when it has a cross-linked surface layer of substantial density, it does not have excessive "memory" or elastic recovery even when heated. Obviously, the damping layer reduces the tendency of the entire sheet material to elastic recovery, allowing the inclusion of a cross-linked layer which provides the desired surface hardness and abrasion resistance. The combination of the two types of layers enables unexpected and valuable results.

The inclusion of a low Tg rubbery polymeric layer as all or part of the transparent topcoat is also believed to provide additional benefits. These include improved thermoformability - as evidenced by lower tendency to crack and better conformability to small details of the substrate surface - and improved resistance to impact damage from gravel and the like.

Figure 2 in the drawing shows in cross-section a simpler type of the layer material 20 according to the invention. It comprises a flexible and stretchable carrier film 21 which is coated on one surface with a thin pigmented but non-metallic coating layer (lacquer layer) 22. Over the coating layer is arranged a transparent cover layer which, according to the embodiment of Figure 2, is composed entirely of rubbery non-pigmented polymer of low Tg, that is below about 20°C. This composition forms a damping layer. Since this embodiment is intended for uses where high abrasion resistance is not required, no cross-linked glass layer is needed over the damping layer.

In the embodiment of Fig. 4, the coating layer does not contain any metallic flakes and thus does not lose excessive gloss when stretched within reasonable limits during thermoforming (that is, 50-100% increase in area). Accordingly, the top layer can be relatively thin, that is, less than about 0.1 mm thick. Similarly, in the embodiment of Fig. 2, a lacquer composition is chosen that does not require a tie layer to adhere it to the carrier film. This embodiment also serves purposes where an adhesive bond is not required. Consequently, no tie layer is shown.

Figure 3 schematically illustrates a preferred coating and drying apparatus that can be used to make the layered material of the invention. The coating hopper 31 is an extrusion coating hopper, of the type disclosed in Beguin, U.S. Patent 2,681,294. The coating hopper extrudes in laminar flow through a narrow slot a horizontally extending bead of the coating liquid in contact with a continuously moving film web 30. The surface of the web is thus coated with the composition in a uniform layer. The layer first applied to the web can be the tie layer 12 or the overcoat layer 13, as shown in Figure 1. In any event, the carrier film web is delivered from the roller 29, whether it is precoated or not, in close proximity to the narrow extrusion slot 31' of the coating vessel 31. As the web passes through the hopper, a laminar flow of the coating composition is extruded onto the web, with the entire thickness of the coating for the particular run simultaneously delivered across the width of the web. Immediately following coating, the web is pulled through a drying device with the wet layer on its surface.

The coated web 32 is drawn through successive stages of the drying apparatus by take-up rolls, not shown in the figure. This includes initial drying stages 33, 34, 35 and the final curing stage 48. Preferably, the stages are designed at successively higher temperatures, with the temperatures of the initial stages of the drying zone being mild. This is particularly desirable when drying an aqueous coating containing water and organic solvents. However, the final stage or stages may be at higher temperatures to remove residual liquids and/or to cure the cross-linkable topcoat. For example, heated air may be supplied via lines 37 at 27°C, 43°C and 66°C to stages 33, 34 and 35 respectively. Baffles 41 divert the air flow from the top of the film and cause it to flow into lower regions of the drying chamber. Sensors 45 and 46 help to maintain the required air temperature and humidity via feedback mechanisms not shown in the figure. The speed of the web is such that the coating is exposed to elevated temperatures for a substantial time, i.e. 3 to 6 minutes, in the final stage 48 where air is supplied via supply 40 at about 80°C.

Preferably, the moving web is heated on its underside by contact with metal rollers 36 and 43 and other rollers in the various drying stages. These metal rollers of high conductivity are heated by the warm air and transfer heat to the underside of the web, further increasing its temperature. In the final stage 48, the web is heated to the highest temperature as it travels a vertical path in a section of fluttering loops, passing through rollers 50 and blowing air from manifolds 49 to keep the coated side of the film from contact with rollers 51. The dried film leaving stage 48 is rolled up on a take-up roller, not shown.

In the preferred method of applying the transparent topcoat over the overcoat layer, the lacquer layer is applied and dried and then the overcoat is applied using the same laminar flow coating method as previously used to apply the tiecoat and overcoat layers. In this method, the sheet having the dried overcoat layer on its surface is again passed in close proximity to the narrow extrusion slot of an extrusion coating vessel 31, as shown in Figure 3, from which the overcoat composition is extruded in laminar flow. Again, as with the overcoat layer, the entire thickness of the overcoat layer to be applied in the particular pass is applied simultaneously across the width of the dried overcoat layer. Immediately thereafter, the web with the wet overcoat layer on its surface is pulled through the drying apparatus.

Since the topcoat of the preferred inventive layer material is thick, it is preferred that such layers be applied in two or more passes to adequately dry them. For example, if the dry topcoat thickness is to be 0.12 mm, a dry layer having a dry thickness of 0.06 mm can be applied in a first pass and a second layer can be applied in a second pass with drying to raise the overall dry density to 0.12 mm. Advantageously, the cushioning layer and the higher Tg crosslinked layer can be applied in separate passes. For greater thicknesses, additional coating and drying passes can be carried out.

For optimum quality, a gradual increase in drying temperatures and other processes, such as moisture control, is also preferred for drying the top layer.

If the drying conditions are carefully controlled, that is, by providing small temperature increases from stage to stage, and with a longer dwell time in each stage, the entire thickness of the topcoat can be applied and dried in a single pass. However, since the topcoat in the preferred inventive layer materials is extremely thick, i.e., greater than 0.1 mm when dried, the highest quality is achieved when the topcoat is applied and dried in two or more passes.

It is also possible to apply the topcoat simultaneously with the overcoat layer. This wet-on-wet coating process can be carried out with laminar flow coating vessels suitable for the simultaneous coating of multiple layers. Examples of such coating vessels can be found in U.S. Patent Nos. 2,761,419, 2,761,791 and 3,005,440. Although these simultaneous multiple coating processes can be used, the best results are obtained when the overcoat layer is applied and dried separately and then successive layers of the topcoat layer are applied and dried in two or more separate passes using an extrusion vessel as described in U.S. Patent No. 2,681,294 and drying each layer after it is applied.

The composition of the damping layer may be selected from a variety of polymeric compositions provided they have the required physical properties. An essential requirement is that the damping layer has a relatively low Tg, namely below about 20ºC. It is a rubbery polymer which does not shrink or elastically relax to any significant extent when heated in a standard test which involves stretching the layer material 100% in area by vacuum thermoforming. and bonding to a metallic plate. Linear cuts are then made transverse to the direction of stretch of the sheet material and the plate is heated to 93ºC (200ºF) for one hour. The amount of peeling from the cut allows a measurement of the elastic recovery through the sheet material. A low degree of peeling is desirable.

Low Tg rubbery polymers that are not cross-linked have the least tendency to elastic recovery when heated and are therefore preferred in the cushioning layer. However, at least a low cross-link density may be desirable for other reasons and is acceptable provided the Tg remains below 20ºC.

An example of a polymer with a low degree of cross-linking is a polyurethane blended with a cross-linking or curing aminoplast, such as a melamine resin of the type disclosed in U.S. Patent No. 4,719,132 or U.S. Patent No. 4,147,679. A particularly preferred composition for the damping layer of this type comprises the same type of polymer that is present in the paint composition of the layer material, but of course omitting the pigment.

The topcoat of the inventive layer material, including both the cushioning layer and the cross-linked layer, if present, is transparent. Preferably, in order to provide the highest quality of gloss, image brightness (DOI) and "flop" or geometric metamerism when a metallic paint is used, a topcoat that is completely clear is provided. However, it may contain a small amount of colorant if desired, provided that it remains transparent.

As previously stated, the preferred surface for the transparent topcoat comprises a cross-linked polymer having a relatively high Tg. Preferably, it is formed by mixing a two-component cross-linking composition, e.g., a polymeric polyol solution and a diisocyanate solution, which reacts after mixing to form a cross-linked polyurethane. When coating with this composition, the preferred procedure is to mix the reactive ingredients just prior to coating the mixture onto the coating layer. The ingredients are fed to a mixing zone and the mixture is extruded with laminar flow through a narrow horizontal extrusion slot of a coating vessel within less than about 50 seconds of its formation in the mixing zone.

The layer material of the invention includes flexible carrier films. The latter preferably comprises a substantially planar, self-supporting, stretchable, thermoplastic polymeric film. This can be transparent, translucent or opaque. If desired, by incorporating a dye, the color of the dye can be adapted to the color of the protective and decorative coating layer.

Suitable materials for the carrier film include stretchable thermoplastic polymeric films having heat softening and tensile elongation properties that make them suitable for use in thermoforming processes. Preferred are those films known to be suitable for thermoforming and/or vacuum forming processes, such as polyvinyl chloride, polyvinyl acetate, ABS (acrylonitrile-butadiene-styrene) resins, polyethylene and polypropylene. Useful materials include polyacrylate blends with copolyesters as described in U.S. Patent No. 4,246,381 and the copolyesters derived from aromatic polyesters and glycol polyesters as described in U.S. Patent No. 4,287,325. Blends of rubber-modified styrene-maleic anhydrides with polycarbonates and/or polyesters are useful, as are blends of polyetheresters and polyesters. Most preferred carrier film polymers exhibit a yield elongation at break of at least 40% when measured according to ASTM D-638. Most preferred is a blend of polyesters based on poly(1,4-cyclohexylenedimethylene terephthalate) and rubber-modified styrene-maleic anhydride copolymers as described in U.S. Patent No. 4,582,076. Another preferred carrier film is the highly flexible film made from a blend of polyetheresters and polyesters. The carrier film may include fillers, UV absorbers, plasticizers, dyes, antioxidants, etc. known to be used in polymeric films.

The coating layer of the coating material contains, in addition to a polymeric binder, a dye or light-reflecting flakes or both and optionally further additives such as surfactants, covalescent agents and the like. The light-reflecting flakes may be those conventionally used in paints, in particular those used in automotive paints. These include metallic flakes such as aluminum flakes, copper flakes, bronze flakes, copper-bronze flakes, nickel flakes, zinc flakes, magnesium flakes, silver flakes, gold flakes and platinum flakes and other flakes such as mica, glass, coated mica, coated glass and aluminum-coated polyester film fragments. Mixtures of two or more types of such flakes may also be used.

Conventional lake dyes can be used alone or in combination with such flakes, including inorganic pigments such as titanium dioxide, iron oxide, chromium oxide, lead chromate, carbon, silicon oxide, tallow, kaolin, metallic oxides, silicates, chromates, etc. and organic pigments such as phthalocyanine blue, phthalocyanine green, carbazole violet, anthrapyrimidine yellow, flavanthrone yellow, isoindoline yellow, indanthrone blue, quinacridone violet, perylene red, diazo red and others.

The film-forming polymeric binder for the coating composition may comprise any of the film-forming polymers conventionally used as binders in flexible and stretchable basecoat compositions. In particular, acrylic polymers, polyesters, including alkyds, and polyurethanes are useful. Examples of such binders and processes for preparing them are described in U.S. Patent Nos. 4,681,811, 4,403,003, and 4,220,679.

Water-based base coats in dye-plus-clear compositions are disclosed in U.S. Patent No. 4,403,003, and the resinous compositions used to make these base coats can be used in the practice of the invention. Also, water-based polyurethanes such as those disclosed in U.S. Patent Nos. 4,147,679 and 4,719,132 can be used as resinous binders in the coating layer/paint layer.

Preferred paints of those described above have been used on external automotive parts made from deformable polyurethanes. Such paints are stretchable and deformable without delaminating or cracking.

The coating layer preferably has a density in the range of 0.012 to 0.080 mm. A particularly preferred average thickness is in the range of about 0.020 to 0.060 mm.

The heat softening and stretch properties of the coating layer must be compatible with those of the carrier film so as to allow symmetrical stretching. By symmetrical stretching is meant that the layer material is subjected to substantial stretching by application of stretching forces without the coating layer splitting, crazing or flaking. Applicant's layer material provides excellent symmetrical stretchability resulting in part from the thickness and uniformity of the thickness of the layers. The ratio of the average thickness of the coating layer to the average thickness of the carrier is preferably in a range of 1.00 to 0.04 and more preferably in a range of 0.50 to 0.08. Layer materials having a thickness ratio of about 0.2 are particularly preferred to obtain substantial symmetrical stretchability without chipping, crazing or flaking.

Crosslinked transparent topcoat compositions for use with basecoats are well known in the art. Particularly useful are such compositions comprising crosslinkable polyols such as polyester polyols, polyurethane polyols and acrylic polyols blended with polyisocyanate curing agents. To prepare the inventive topcoat material, the topcoat composition is extruded through a slot onto the coating layer of the topcoat material. This laminar flow process allows coating with a coating in one or more passes and provides improved smoothness, excellent gloss and image brightness (DOI) compared to topcoats applied by spraying or other non-laminar flow techniques. The topcoat is then dried to a non-tacky state.

If there is no problem with loss of gloss, such as if the coating layer contains no metallic flakes or if stretching of the coating material is minimal, the total thickness of the topcoat can be relatively small, that is, as small as 0.025 mm (1 mil). However, if a metallic coating layer is used and the coating material must be stretched substantially, the topcoat has a thickness of at least about 0.1 mm, preferably at least 0.125 mm. Particularly preferred is a clear coat having a dry thickness of at least 0.15 mm. Clear coat thicknesses up to 0.25 mm or even more can be obtained using the coating methods described above.

The carrier film, the coating layer and the top layer have heat softening and stretch elongation properties that make the layer material suitable for use in a thermoforming process and are mutually compatible to permit symmetrical stretching in which the layer material is subjected to substantial elongation by the presence of stretching forces without crazing or flaking of the base layer or the top layer. The carrier film, the coating layer and the top layer have a sufficiently uniform character that the layer material has substantially the same uniformity of appearance before and after the thermoforming process.

An adhesion promoting linking layer is applied to the carrier film prior to the base layer in a preferred embodiment of the invention and this produces improved bond strength and reduces the risk of flaking. Suitable linking layers can be formed from compositions comprising an adhesion promoting species and optionally a suitable solvent. Useful, Adhesion promoting species include polymers derived from acrylonitrile, vinylidene chloride and acrylic acid; polymers derived from methyl acrylate, vinylidene chloride and itaconic acid; cellulose nitrate and a variety of commercially available adhesion promoting species, for example those known under the trade names FORMVAR 7/95, FORMVAR 15/95, BUTVAR B-98 and BUTVAR B-72 sold by Monsanto, MOBAY M-50 sold by Mobay, VINAC B-15 sold by Air Products, ELVACITE sold by DuPont, and LEXAN sold by General Electric. Suitable solvents include methyl ethyl ketone, methylene chloride, tetrahydrofuran, toluene, methyl cellosolve, methanol, ethanol, propanol, butanol, mixtures thereof and the like. The adhesion promoting layer preferably has a thickness no greater than about 0.0025 mm. The thickness is preferably substantially less than the thickness of the base layer. For example, the base layer preferably has a thickness of at least 8 times the thickness of the adhesion-promoting layer.

It should be noted that the thermoforming process typically involves stretching in all directions rather than just longitudinal stretching of the sheet material. Thus, the sheet material is expanded in area. Prior to thermoforming, the sheet material may be in a so-called unstretched state in which it is substantially unstretched. The thermoforming process softens it to a substantially plastic state and renders it plastically stretchable to an expanded state. The sheet material of the present invention is capable of being expanded to an expanded area of at least 50% more than the area in the unstretched state.

The layer material according to the invention has a substantially uniform quality and appearance in both the non-expanded and expanded state of the layer material.

This means that in the unexpanded state it is substantially free of coating defects such as lines, dimples, bubbles, streaks, line structures, rejections, dye agglomerations and the like when visually inspected in room light, on a light box and in sunlight. This further means that the coating material provides a commercially acceptable finish when applied to exterior automotive panels. Moreover, an unexpanded coating material can be stretched to various degrees in various areas or stretched in certain areas or unstretched in other areas and the uniformity of appearance remains substantially the same in all areas including image brightness (image distinctiveness), gloss, optical density and flop.

The inventive layer material preferably exhibits an initial image brightness (DOI) of at least 80 as measured by ASTM test E-430. Such measurements can be made, for example, on a HUNTERLAB MODEL D47R-6 DORIGON GLOSSMETER (TM). Preferred inventive materials exhibit an initial DOI of at least 85 and more preferably at least 90 when measured by this technique.

The sheet material of the invention exhibits an initial 20° gloss of at least 80 when measured in accordance with ASTM test D-523. Preferred sheet materials of the invention exhibit an initial 20° gloss of at least 85 and more preferably at least 90 when determined by this method. When stretched to more than 50 percent of the original area, the gloss remains at least about 80.

The layer material according to the invention preferably has an average surface roughness (Ra) of less than 0.4 Micrometers when determined by conventional surface roughness measurement techniques known in the art. For example, Ra can be measured using a grid type instrument having a non-slip probe and a cut-off at about 0.0762 cm (0.030 in). One such instrument is the Micro-Topographer® sold by Federal Products Corporation. Particularly preferred layered materials of the invention exhibit an average surface roughness of less than 0.2 micrometers, and more preferably less than 0.1 micrometers.

Laminar flow coating techniques are used to produce the coating materials of the present invention. These include direct extrusion onto a substrate using an extrusion vessel that functions to apply the coating composition via a bead coating or stretch flow coating mechanism, extrusion of the coating composition through a slot followed by sliding down the sliding surface of a sliding hopper or free fall in a curtain coating process, and curtain coating processes in which the curtain is created using an overflow weir. The critical aspect of the coating process is its ability to provide a defect-free coating with a very high degree of uniformity of layer thickness. In particular, the process must achieve this result even with a thick coating of the cross-linking clear coat composition. The use of precision coating techniques in the manufacture of the coating material - such as the use of the high precision extrusion vessels - provides for a superior coating material according to the present invention. The coating process should be carried out under precise control of all critical parameters in order to achieve a high degree of thickness uniformity and freedom from coating defects, which preferred coating material of the invention. By controlling both the thickness uniformity and the coating defects, a coating material is obtained which is capable of being subjected to the stresses of a thermoforming process and yields a product with the very high quality required by the exterior finish of an automobile.

Laminar flow extrusion coating vessels such as those disclosed in the Beguin patent U.S. Patent No. 2,681,294 are particularly effective devices for use in the process of the invention. Such devices are well adapted for use with difficult-to-thin compositions such as those typical of the paint compositions described above. In such vessels, transverse pressure gradients are reduced and almost all of the pressure drop across the slot is toward the coating. These properties are believed to contribute to the excellent results obtained with extrusion vessels, particularly in the coating of paint compositions containing light-reflecting flakes and in the coating of cross-linking clear coat compositions, although applicant does not wish to be bound by any theoretical explanations.

Although laminar flow coating and particularly coating by means of an extrusion vessel with a narrow slot are preferred, the benefits of the invention can also be obtained when the layered material is obtained by other coating methods, such as spray and reverse roll coating.

The invention is explained in more detail in the following examples and comparative examples:

example 1

A carrier film was prepared from a mixture described in US Patent No. 4,582,876 as follows:

A blend of 55:45 parts by weight of polyester (I) and Dylark® Styrene Copolymer 600 was compounded by extrusion. Polyester (I) was derived from terephthalic acid, 19 mole % ethylene glycol and 81 mole % 1,4-cyclohexanedimethanol. Dylark® Styrene Copolymer 600 is a rubber-modified styrene-maleic anhydride copolymer, available from ARCO Chemical Company, a division of Atlantic Richfield Company.

The mixture was heated to 260ºC-290ºC and extruded through a die into a gap comprising two temperature-controlled chrome-plated stainless steel rolls. The extruded sheet was then cooled on a chilled roll. The thermoplastic film has a thickness of approximately 0.20 mm and a maximum thickness variation of less than 0.02 mm.

The carrier film obtained as before was then coated with a thin tie coat solution (diluted to 5% solids prior to coating). Before the composition was diluted, it comprised 75% water, 10% acrylic resin, 10% urethane resin, 1% methyl pyrrolidone, 1% diethylene glycol monobutyl ether, less than 1% ethylene glycol monohexyl ether, less than 1% N,N-dimethylethanolamine, and over 0.05% "Fluorad FC 170" surfactant (sold by the 3M Company). The diluted composition was applied through a 0.13 mm extrusion coating tank slot and dried at 40-90°C. All percentages are by weight. When dry, the coating thickness was 0.54 g/m², the dry thickness 0.0005-0.001 mm.

The metallic paint composition set forth in Table I was metered from a vessel pressurized with nitrogen gas through a coating vessel extrusion slot having a width of 0.25 mm onto the carrier film moving at a speed of 8 m/min. The coating roll was cooled with circulating water to reduce the tendency of the coating to streak and to minimize the formation of precipitates at the vessel spout. The coating process and apparatus were essentially as described in U.S. Patent No. 2,681,294 and included means for maintaining the pressure differential across opposing surfaces of the coating belt. The coating was dried at 27-80°C to a dry coating thickness of 43 g/m2 and a thickness of 0.04 mm (1.5 mils) in a multi-stage continuous flat bed dryer supplied with a moderate low velocity moist warm air stream substantially as disclosed in Fronheiser et al. Application No. 166,083, cited above.

Table I - Metallic paint composition

Component approx. wt.%

Deionized water 50

Urethane Resin 25

Aluminium Paste 5

Ethylene glycol monohexyl ether 5

N-methylpyrrolidone 5

Diethylene glycol monobutyl ether 1

N,N-Dimethylethanolamine 1

Xylene 1

Aliphatic Solvent Naphtha 1

Isopropyl alcohol < 1

The dried paint coated web was rolled up on a take-up roll and again passed through a coating station, forming over the coating layer a uniform layer of a non-pigmented cushioning layer having substantially the same composition as the paint composition of Table I, but not containing the aluminum paste or other pigment. The rubbery dried clear cushioning layer had a thickness of 0.06 mm and a Tg of about 30°C.

A cross-linked clear topcoat was then coated and dried in the same manner over the non-pigmented layer using the two-component polyurethane composition according to Table II. Table II - Clear topcoat composition Component A approx. % by weight Urethane Resin (polyol) Toluene Benzotriazole (diluted with acetone to a solids content of 51%) Component B* Polyfunctional aliphatic isocyanurate Resin based on 1,6-hexamethylene diisocyanate

* Available as Desmodur N-3300 from Mobay Corporation Coatings Division.

Components A and B were mixed using an in-line mixer before reaching the coating tank. The composition flowed from the mixer directly into the same type of extrusion coating tank as that used for the overcoat layer and was extrusion coated in a single pass through a narrow extrusion slot of the tank. The web with a wet topcoat on its surface was then passed directly through a drying chamber where the topcoat was dried and cured by exposure to a warm moderately humid air stream in a series of successively increasing temperatures. The dried clear topcoat obtained from this single pass had a thickness of 0.06 mm and had a Tg of about 60 to 70°C. It was coated with an aqueous acrylic pressure sensitive adhesive layer and dried on the back of the carrier film.

Comparison test A

Another coating material was prepared in the same manner, except that the topcoat consisted entirely of the crosslinking clear topcoat composition of Table II. The dry thickness of the topcoat was 0.06 mm.

Additional examples and comparison tests

Thereafter, Examples 2-7 according to the invention and Comparative Tests B, C, D were prepared essentially as described for Example 1, with the exception that different thicknesses of the cross-linked clear layer and the damping layer were applied.

Table III below lists the specific dry thicknesses. In this table, "CL" stands for the rubbery damping layer and "XL" stands for the cross-linked clear coat.

A sample of each layer material was heated by infrared heaters to temperatures of about 135ºC and then drawn into contact with and bonded to a 7.5 x 30 cm inclined metal plate by vacuum thermoforming. As the samples were drawn onto the inclined plates, they were stretched to an extent proportional to the distance along the plate, with the area at the top of the plate (top) showing little or no stretch and the area halfway down the plate (center) showing a stretch of about 70 percent.

Gloss and DOI (image brightness) measurements taken at the top and central areas of each panel show the effect of stretching on the surface properties of the layer material. Measurements were also taken at the bottom of the panel. However, because the amount of stretching in this area (approximately 200%) exceeded any normally intended use of the layer material, measurements for the bottom area were not included in the results.

Gloss was measured according to the ASTM D-523 method using a Hunter 200 ASTM Glossmeter and DOI was measured using the ASTM Test E-430 method using a Hunterlab Model D47R-6 Dorigon Glossmeter.

The results are presented in Table III. Table III Example or Test No. Coating Thickness (mm) CL XL Total Clear Coat Thickness Gloss Head Center %Loss Comp. A Comp. B Comp. C Comp. D

The measurement results show that as the total thickness of the clear layer exceeds 0.1 mm, a dramatic improvement in gloss retention is observed. Although not shown in the table, DOI retention improves significantly as the total thickness of the clear layer increases. It is evident that these improvements occur when the total thickness of the clear layer exceeds 0.1 mm, whether the layer comprises a combination of cross-linked or cushioning layers as in Examples 1-5 or is made of a low Tg polymer. alone, as in Examples 6 and 7. Comparative Tests B and D are nevertheless within the scope of the invention.

Examples 8-16

Examples 8-16 were prepared as in Example 1 so that the damping layer, CL, and the cross-linked topcoat had the respective dry thicknesses listed in Table IV below.

Each layer material was bonded to an inclined metal plate (substrate) by a vacuum thermoforming process in such a way that a normal slope of thermoforming stretch was provided along the length of the slope.

A 5 cm long cut was then made across the stretch gradient line and with a 1 cm transverse cut across each end with a razor blade through the laminate down to the substrate at the point corresponding to 100% of maximum stretch. After cutting, each sample was subjected to a high temperature test which consisted of keeping the sample at 93ºC for one hour. Each sample was evaluated after etching by measuring the width at the center of the cut. The wider the cut, the greater the tendency of the thermoformed laminate material to elastic recovery.

Results are shown in Table IV. Table IV Example Layer thickness (mm) CL XL Width of cut after high temperature exposure (mm)

These test results show that the damping layer, whether used alone as in Examples 8-12 or together with a higher Tg cross-linked layer as in Examples 9, 10 and 13-15, reduces the tendency to elastic recovery when the layer material, stretched and bonded to a metal plate, is exposed to an elevated temperature. The data also show that the greatest improvements occur when the cross-linked layer is significantly thinner than the damping layer in Examples 9 and 13.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it should be understood that deviations and modifications therefrom may be made within the inventive concept and scope of the invention.

Claims (13)

1. A protective and decorative sheet material for use in a thermoforming process in which it is stretched and bonded to a three-dimensional substrate, comprising: a flexible and stretchable, thermoformable polymeric carrier film; a stretchable, thermoformable coating layer adhered to a surface of the carrier film; the coating layer comprising a film-forming polymeric binder and a color former or substantially flat, light-reflecting flakes or both; a transparent polymeric topcoat layer over the coating layer, at least a portion of the topcoat layer comprising a rubbery cushioning layer of a stretchable and thermoformable polymeric composition that is non-crosslinked or of low crosslink density and has a Tg of less than 20°C; wherein the sheet material is in an unexpanded state prior to thermoforming; the sheet material is heat-softenable to a substantially plastic state in which it is thermoformable over irregular surfaces to an expanded state having an area at least 50% greater than the area of the unexpanded state; and wherein the sheet material has a practically uniform quality and a practically uniform appearance both in the unexpanded state and in the expanded state. 2. Sheet material according to claim 1, in which the carrier film is a thin, substantially planar stretchable thermoplastic polymer film of substantially uniform Thickness includes. 3. Sheet material according to claim 2, in which the coating layer has light-reflecting flakes and the thickness of the covering layer is at least 0.1 millimeters. 4. A sheet material according to claim 3, wherein the flakes are metallic flakes. 5. A sheet material according to claim 4, wherein the cover layer comprises the cushioning layer and a cross-linked surface layer of higher Tg. 6. The sheet material of claim 5, wherein the crosslinked layer is thinner than the cushioning layer and has a Tg of at least about 40°C. 7. A sheet material according to claim 6, in which the surface of the cover layer is a substantially clear layer of crosslinked polyurethane. 8. Sheet material according to claim 7, having a pressure-sensitive adhesive layer on the side of the carrier film opposite the coating layer. 9. Sheet material according to claim 8, with a thin adhesion-promoting layer between the carrier film and the coating layer. 10. Sheet material according to claim 1, in which the carrier film is composed of a mixture of copolyesters based on poly(1,4-cyclohexylenedimethylene terephthalate) and styrene-maleic anhydride copolymers. 11. A sheet material according to claim 1, in which the coating layer contains no flakes and the thickness of the covering layer is less than 0.1 millimeters. 12. A sheet material according to claim 1, in which the cover layer has a thickness of at least 0.1 millimeters, the cover layer (a) comprises a relatively thick cushioning layer of a non-pigmented polymer of substantially the same composition as the polymeric binder of the coating layer, and that (b) overlying the cushioning layer is a relatively thin cross-linked polymer layer having a Tg value substantially higher than the Tg value of the cushioning layer. 13. The sheet material of claim 12 wherein the cover layer further comprises a clear cross-linked layer over the cushioning layer, the cross-linked layer being thinner than the cushioning layer and having a substantially higher Tg than the cushioning layer.