Abstract:
A transparent iridescent thermoplastic resinous laminate film having at least 10 very thin layers of substantially uniform thickness, said layers being generally parallel, the contiguous adjacent layers being of different transparent thermoplastic resinous materials of which one is a naphthalate-based polyester or copolyester resin, the contiguous adjacent layers differing in refractive index by at least about 0.03.

Description:
FIELD OF THE INVENTION 
     The present invention relates to multilayer coextruded light-reflecting films which have a narrow reflection band because of light interference and contain a layer of naphthalate-based polyester. 
     BACKGROUND OF THE INVENTION 
     Iridescent multilayer films are composed of a plurality of generally parallel layers of transparent thermoplastic resinous material in which the contiguous adjacent layers are of diverse resinous material whose index of refraction differs by at least about 0.03. The film contains at least 10 layers and more usually at least 35 layers and, preferably, at least about 70 layers. 
     The individual layers of the film are very thin, usually in the range of about 30 to 500 nm, preferably about 50-400 nm, which causes constructive interference in light waves reflected from the many interfaces. Depending on the layer thickness and the refractive index of the polymers, one dominant wavelength band is reflected and the remaining light is transmitted through the film. The reflected wavelength is determined by the sum of the optical thickness of a pair of layers. 
     The quantity of the reflected light (reflectance) and the color intensity depend on the difference the two refractive indices, on the ratio of optical thicknesses of the layers, on the number of layers and on the uniformity of the thickness. If the refractive indices are the same, there is no reflection at all from the interfaces between the layers. In multilayer iridescent films, the refractive indices of contiguous adjacent layers differ by at least 0.03 and preferably by at least 0.06 or more. For first order reflections, reflectance is highest when the optical thicknesses of the layers are equal, although suitably high reflectances can be achieved when the ratio of the two optical thicknesses falls between 5:95 and 95:5. Distinct color reflections are obtained with as few as 10 layers. However, for maximum color intensity, it is desirable to have between 35 and 1000 or more layers. High color intensity is associated with a reflection band which is relatively narrow and which has high reflectance at its peak. It should be recognized that although the term “color intensity” has been used here for convenience, the same considerations apply for the invisible reflection in the ultraviolet and infrared ranges. 
     The multilayer films can be made by a chill-roll casting technique using a conventional single manifold flat film die in combination with a feedblock which collects the melts from each of two or more extruders and arranges then into the desired layer pattern. Feedblocks are described for instance in U.S. Pat. Nos. 3,565,985 and 3,773,882. The feedblocks can be used to form alternating layers of either two components or more (e.g. ABABAB . . . , ABCABC . . . or ACBCACBC . . . ). The very narrow multilayer stream flows through a single manifold flat film die where the layers are simultaneously spread to the width of the die and thinned to the final die exit thickness. The number of layers and their thickness distribution can be changed in inserting a different feedblock module. Usually, the outermost layer or layers on each side of the sheet are thicker than the other layers. This thicker skin may consist of one of the components which makes up the optical core, may be a different polymer which is utilized to impart desirable mechanical, heat sealing, or other properties, or may be a combination of these. 
     Examination of iridescent films of desirable optical properties revealed deficiencies in certain mechanical properties. For example, the adhesion between individual layers of the multilayer structure may be insufficient, and the film may suffer from internal delamination or separation of layers during use. The iridescent film is often adhered to paper or board for its decorative effect, and is then used for greeting cards, cartons, wrapping paper and the like. Delamination of the film is unsightly and may even lead to separation of the glued joints if carton. In addition, the solvent resistance and heat stability of such films are not as great as desired for widespread utilization. 
     In U.S. Pat. No. 4,310,584, these deficiencies are significantly overcome by using a thermoplastic terephthalate polyester or copolyester resin as the high refractive index component of the system in which two or more resinous material form a plurality of layers. While a substantial improvement was realized, it also required the use of two polymers from significantly different polymer families. That fact, in turn, means that there are inherent significant differences between the two polymers and their relative adhesion to each other, chemical resistance, toughness, etc. As a result, the film itself is generally no better with regard to a particular characteristic than the weaker or poorer of the polymers employed. If two polymers closely related were employed in order to maximize relative adhesion to one each other, or toughness, or chemical resistance, etc., the polymers involved did not have a sufficient difference in refractive index so as to create the desired iridescent color. 
     Schrenk and Wheatly (Co-extruded Elastomeric Optical Interference Film, Antec &#39;88, 1703-1707) have reported the preparation of a multilayer light reflecting film co-extruded from two thermoplastic elastomers. The film which had one thermoplastic elastomer based on nylon and the other based on urethane, exhibited reversible changes in reflection spectra when deformed and relaxed. That is, this very specific combination had the ability of stretching without losing appearance characteristics. This type of films has been described in more detail in U.S. Pat. No. 4,937,134. 
     U.S. Pat. No. 5,089,318 discloses that further improvements in adhesion, solvent resistance and the like can be obtained by employing a thermoplastic elastomer (TPE) as one of the resinous materials. Such materials are copolymers of a thermoplastic hard segment such as polybutyl terephthalate, polyethylene terephthalate, polycarbonate, etc., and a soft elastomeric segment such as polyether glycols, silicone rubbers, polyetherimide and the like. 
     While prior art structures represented significant improvement in the areas of delamination resistance and better solvent stability, there were still some limitations with regard to these properties. In addition, iridescent films of the prior art still had deficiencies relative to their temperature stability, tensile strength and UV stability. The present invention surprisingly provides significant improvements over current known structures with regard to these properties. 
     SUMMARY OF THE INVENTION 
     It is, therefore an object of the invention to provide a transparent thermoplastic resinous laminate having good heat and solvent stability, good tensile strength, good delamination resistance, and good UV stability. 
     In one embodiment, the present invention provides a transparent thermoplastic resinous laminate film of at least 10 very thin layers of substantially uniform thickness, said layers being generally parallel, the contiguous adjacent layers being of different transparent thermoplastic resinous materials of which one is a naphthalate-based polyester or copolyester resin, the contiguous adjacent layers differing in refractive index by at least about 0.03. 
     In another embodiment, the present invention provides a transparent thermoplastic resinous laminate film of at least about 70 very thin layers of substantially uniform thickness, said layers being generally parallel, the contiguous adjacent layers being of different transparent thermoplastic resinous materials of which one is a polyethylene naphthalate polyester or copolyester, and the other is a polybutylene terephthalate polyester or copolyester, wherein the outermost layers are polybutylene terephthalate polyester. 
     Other objects and advantages of the present invention will become apparent from the following description and appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     It has now been found that the objectives of this invention are realized by employing a naphthalate-based polyester or copolyester resin as a component in the contiguous adjacent layers in the optical core of a transparent thermoplastic resinous laminate film. Preferably, the naphthalate-based polyester or copolyester is based on naphthalene dicarboxylate. Examples of usable polyester resin include polyethylene naphthalate and polybutylene naphthalate. Examples of usable copolyesters include copolyesters comprising ethylene naphthalate and/or butylene naphthalate. Preferably, the copolyester consists of ethylene naphthalate and butylene naphthalate. 
     The iridescent film of the present invention can be obtained by coextruding the naphthalate-based polyester or copolyester resin with a different transparent thermoplastic resin which is selected to differ in refractive index by at least 0.03 and preferably by at least 0.06. Among the other resinous materials which can be used are transparent thermoplastic polyester or copolyester resins characterized by a refractive index of about 1.55 to about 1.61. Examples of usable thermoplastic polyester resins include polyethylene terephthalate (PET) which is made by reacting either terephthalic acid or dimethyl terephthalate with ethylene glycol; polybutylene terephthalate (PBT) which is made by the catalyzed combination of 1,4-butanediol with either terephthalic acid or dimethyl terephthalate; and the various thermoplastic copolyesters which are synthesized using more than one glycol and/or more than one dibasic acid. PETG polyester, for example, is a glycol modified PET made from ethylene glycol and cyclohexanedimethanol (CHDM) and terephthalic acid; PCTA copolyester is an acid-modified copolyester of CHDM with terephthalic and isophthalic acids. Additional other resinous materials that can be coextruded with the naphthalate-based polyester or copolyester resin are listed in Table 1. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Approximate 
               
               
                   
                   
                 Refractive 
               
               
                   
                 Polymer name: 
                 Index 
               
               
                   
                   
               
             
             
               
                   
                 Poly(tetrafluoroethylene-co-hexafluoropropylene) 
                 1.338 
               
               
                   
                 Poly(pentadecafluorooctyl acrylate) 
                 1.339 
               
               
                   
                 Poly(tetrafluoro-3-(heptafluoropropoxy)propyl 
                 1.346 
               
               
                   
                 acrylate) 
               
               
                   
                 Poly(tetrafluoro-3-(pentafluoroethoxy)propyl 
                 1.348 
               
               
                   
                 acrylate) 
               
               
                   
                 Poly(tetrafluoroethylene) 
                 1.35 (−1.38) 
               
               
                   
                 Poly(undecafluorohexyl acrylate) 
                 1.356 
               
               
                   
                 Poly(nonafluoropentyl acrylate) 
                 1.360 
               
               
                   
                 Poly(tetrafluoro-3-(trifluoromethoxy)propyl 
                 1.360 
               
               
                   
                 acrylate) 
               
               
                   
                 Poly(pentafluorovinyl propionate) 
                 1.364 
               
               
                   
                 Poly(heptafluorobutyl acrylate) 
                 1.367 
               
               
                   
                 Poly(trifluorovinyl acetate) 
                 1.375 
               
               
                   
                 Poly(octafluoropentyl acrylate) 
                 1.380 
               
               
                   
                 Poly(pentafluoropropyl acrylate) 
                 1.385 
               
               
                   
                 Poly(2-(heptafluorobutoxy)ethyl acrylate) 
                 1.390 
               
               
                   
                 Poly(2,2,3,4,4,4-hexafluorobutyl acrylate) 
                 1.392 
               
               
                   
                 Poly(trifluoroethyl acrylate) 
                 1.407 
               
               
                   
                 Poly(2-(1,1,2,2-tetrafluoroethoxy)ethyl acrylate) 
                 1.412 
               
               
                   
                 Poly(trifluoroisopropyl methacrylate) 
                 1.4177 
               
               
                   
                 Poly(2,2,2-trifluoro-1-methylethyl methacrylate) 
                 1.4185 
               
               
                   
                 Poly(2-(trifluoroethyoxy)ethyl acrylate) 
                 1.419 
               
               
                   
                 Poly(trifluorochloroethylene) 
                 1.42-1.43 
               
               
                   
                 Poly(vinylidene fluoride) 
                 1.42 
               
               
                   
                 Poly(dimethylsilylene(poly(dimethyl siloxane)) 
                 1.43 
               
               
                   
                 Poly(trifluoroethyl methacrylate) 
                 1.437 
               
               
                   
                 Poly(oxypropylene) 
                 1.4495 
               
               
                   
                 Poly(vinyl isobutyl ether) 
                 1.4507 
               
               
                   
                 Poly(vinyl ethyl ether) 
                 1.4540 
               
               
                   
                 Poly(oxyethylene) 
                 1.4563 
               
               
                   
                 Poly(vinyl butyl ether) 
                 1.4563 
               
               
                   
                 Poly(vinyl pentyl ether) 
                 1.4581 
               
               
                   
                 Poly(vinyl hexy ether) 
                 1.4591 
               
               
                   
                 Poly(4-methyl-1-pentene) 
                 1.459-1.465 
               
               
                   
                 Cellulose acetate butyrate 
                 1.46-1.49 
               
               
                   
                 Poly(4-fluoro-2-trifluoromethylstyrene) 
                 1.46 
               
               
                   
                 Poly(vinyl octyl ether) 
                 1.4613 
               
               
                   
                 Poly(vinyl 2-ethylhexyl ether) 
                 1.4626 
               
               
                   
                 Poly(vinyl decyl ether) 
                 1.4628 
               
               
                   
                 Poly(2-methoxyethyl acrylate) 
                 1.463 
               
               
                   
                 Poly(butyl acrylate) 
                 1.4631 
               
               
                   
                 Poly(butyl acrylate) 
                 1.466 
               
               
                   
                 Poly(tert-butyl methacrylate) 
                 1.4638 
               
               
                   
                 Poly(vinyl dodecyl ether) 
                 1.4640 
               
               
                   
                 Poly(3-ethoxypropyl acrylate) 
                 1.465 
               
               
                   
                 Poly(oxycarbonyl tetramethylene) 
                 1.465 
               
               
                   
                 Poly(vinyl propionate) 
                 1.4665 
               
               
                   
                 Poly(vinyl acetate) 
                 1.4665 
               
               
                   
                 Poly(vinyl methyl ether) 
                 1.467 
               
               
                   
                 Poly(ethyl acrylate) 
                 1.4685 
               
               
                   
                 Poly(ethylene-co-vinyl acetate) 
                 1.47-1.50 
               
               
                   
                 (30%-20% vinyl acetate) 
               
               
                   
                 Cellulose propionate 
                 1.47-1.49 
               
               
                   
                 Cellulose acetate propionate 
                 1.47 
               
               
                   
                 Benzyl cellulose 
                 1.47-1.58 
               
               
                   
                 Phenol-formaldehyde resins 
                 1.47-1.70 
               
               
                   
                 Cellulose triacetate 
                 1.47-1.48 
               
               
                   
                 Poly(vinyl methyl ether) (isotactic) 
                 1.4700 
               
               
                   
                 Poly(3-methoxypropyl acrylate) 
                 1.471 
               
               
                   
                 Poly(2-ethoxyethyl acrylate) 
                 1.471 
               
               
                   
                 Poly(methyl acrylate) 
                 1.472-1.480 
               
               
                   
                 Poly(isopropyl methacrylate) 
                 1.4728 
               
               
                   
                 Poly(1-decene) 
                 1.4730 
               
               
                   
                 Poly(propylene) (atactic, density 0.8575 g/cm 3 ) 
                 1.4735 
               
               
                   
                 Poly(vinyl sec-butyl ether) (isotactic) 
                 1.4740 
               
               
                   
                 Poly(dodecyl methacrylate) 
                 1.4740 
               
               
                   
                 Poly(oxyethyleneoxysuccinoyl) 
                 1.4744 
               
               
                   
                 (poly(ethylene succinate)) 
               
               
                   
                 Poly(teradecyl methacrylate) 
                 1.4746 
               
               
                   
                 Poly(ethylene-co-propylene) (EPR-rubber) 
                 1.4748-1.48 
               
               
                   
                 Poly(hexadecyl methacrylate) 
                 1.4750 
               
               
                   
                 Poly(vinyl formate) 
                 1.4757 
               
               
                   
                 Poly(2-fluoroethyl methacrylate) 
                 1.4768 
               
               
                   
                 Poly(isobutyl methacrylate) 
                 1.477 
               
               
                   
                 Ethyl cellulose 
                 1.479 
               
               
                   
                 Poly(vinyl acetal) 
                 1.48-1.50 
               
               
                   
                 Cellulose acetate 
                 1.48-1.50 
               
               
                   
                 Cellulose tripropionate 
                 1.48-1.49 
               
               
                   
                 Poly(oxymethylene) 
                 1.48 
               
               
                   
                 Poly(vinyl butyral) 
                 1.48-1.49 
               
               
                   
                 Poly(n-hexyl methacrylate) 
                 1.4813 
               
               
                   
                 Poly(n-butyl methacrylate) 
                 1.483 
               
               
                   
                 Poly(ethylidene dimethacrylate) 
                 1.4831 
               
               
                   
                 Poly(2-ethoxyethyl methacrylate) 
                 1.4833 
               
               
                   
                 Poly(oxyethyleneoxymaleoyl) 
                 1.4840 
               
               
                   
                 (poly(ethylene maleate)) 
               
               
                   
                 Poly(n-propyl methacrylate) 
                 1.484 
               
               
                   
                 Poly(3,3,5-trimethylcyclohexyl methacrylate) 
                 1.485 
               
               
                   
                 Poly(ethyl methacrylate) 
                 1.485 
               
               
                   
                 Poly (2-nitro-2-methylpropyl 
                 1.4868 
               
               
                   
                 methacrylate) 
                 1.4889 
               
               
                   
                 Poly(triethylcarbinyl methacrylate) 
               
               
                   
                 Poly(1,1-diethyipropyl methacrylate) 
                 1.4889 
               
               
                   
                 Poly(methyl methacrylate) 
                 1.4893 
               
               
                   
                 Poly(2-decyl-1,3-butadiene) 
                 1.4899 
               
               
                   
                 Poly(vinyl alcohol) 
                 1.49-1.53 
               
               
                   
                 Poly(ethyl glycolate methacrylate) 
                 1.4903 
               
               
                   
                 Poly(3-methylcyclohexyl methacrylate) 
                 1.4947 
               
               
                   
                 Poly(cyclohexyl α-ethoxyacrylate) 
                 1.4969 
               
               
                   
                 Methyl cellulose(low viscosity) 
                 1.497 
               
               
                   
                 Poly(4-methylcyclohexyl methacrylate) 
                 1.4975 
               
               
                   
                 Poly(decamethylene glycol dimethacrylate) 
                 1.4990 
               
               
                   
                 Poly(urethanes) 
                 1.5-1.6 
               
               
                   
                 Poly(1,2-butadiene) 
                 1.5000 
               
               
                   
                 Poly(vinyl formal) 
                 1.50 
               
               
                   
                 Poly(2-bromo-4-trifluoromethylstyrene) 
                 1.5 
               
               
                   
                 Cellulose nitrate 
                 1.50-1.514 
               
               
                   
                 Poly(sec-butyl α-chloroacrylate) 
                 1.500 
               
               
                   
                 Poly(2-beptyl-1,3-butadiene) 
                 1.5000 
               
               
                   
                 Poly(ethyl α-chloroacrylate) 
                 1.502 
               
               
                   
                 Poly(2-isopropyl-1,3-butadiene) 
                 1.5028 
               
               
                   
                 Poly(2-methylcyclohexyl methacrylate) 
                 1.5028 
               
               
                   
                 Poly(propylene) (density 0.9075 g/cm 3 ) 
                 1.5030 
               
               
                   
                 Poly(isobutene) 
                 1.505-1.51 
               
               
                   
                 Poly(bornyl methacrylate) 
                 1.5059 
               
               
                   
                 Poly(2-tert-butyl-1,3-butadiene) 
                 1.5060 
               
               
                   
                 Poly(ethylene glycol dimethacrylate) 
                 1.5063 
               
               
                   
                 Poly(cyclohexyl methacrylate) 
                 1.5066 
               
               
                   
                 Poly(cyclohexanediol-1,4-dimethacrylate) 
                 1.5067 
               
               
                   
                 Butyl rubber (unvulcanized) 
                 1.508 
               
               
                   
                 Poly(tetrahydrofurfuryl methacrylate) 
                 1.5096 
               
               
                   
                 Gutta percha (5) 
                 1.509 
               
               
                   
                 Poly(ethylene) ionomer 
                 1.51 
               
               
                   
                 poly(oxyethylene) (high molecular weight) 
                 1.51-1.54 
               
               
                   
                 Poly(ethylene) (density 0.914 g/cm 3 ) 
                 1.51 
               
               
                   
                 (density 0.94-0.945 g/cm 3 ) 
                 1.52-1.53 
               
               
                   
                 (density 0.965 g/cm 3 ) 
                 1.545 
               
               
                   
                 Poly(1-methylcyclohexyl methacrylate) 
                 1.5111 
               
               
                   
                 Poly(2-hydroxyethyl methacrylate) 
                 1.5119 
               
               
                   
                 Poly(vinyl chloroacetate) 
                 1.512 
               
               
                   
                 Poly(butene) (isotactic) 
                 1.5125 
               
               
                   
                 Poly(vinyl methacrylate) 
                 1.5129 
               
               
                   
                 Poly(N-butyl-methacrylamide) 
                 1.5135 
               
               
                   
                 Gutha percha (α) 
                 1.514 
               
               
                   
                 Terpene resin 
                 1.515 
               
               
                   
                 Poly(1,3-butadiene) 
                 1.5154 
               
               
                   
                 Shellac 
                 1.51-1.53 
               
               
                   
                 Poly(methyl α-chloroacrylate) 
                 1.517 
               
               
                   
                 Poly(2-chloroethyl methacrylate) 
                 1.517 
               
               
                   
                 Poly(2-diethylaminoethyl methacrylate) 
                 1.5174 
               
               
                   
                 Poly(2-chlorocyclohexyl methacrylate) 
                 1.5179 
               
               
                   
                 Poly(1,3-butadiene) (35% cis; 56% trans; 7% 
                 1.5180 
               
               
                   
                 1,2-content) 
               
               
                   
                 Natural rubber 
                 1.519-1.52 
               
               
                   
                 Poly(allyl methacrylate) 
                 1.5196 
               
               
                   
                 Poly(vinyl chloride) + 40% dioctyl phthalate 
                 1.52 
               
               
                   
                 Poly(acrylonitrile) 
                 1.52 
               
               
                   
                   
                 1.5187 
               
               
                   
                 Poly(methacrylonitrile) 
                 1.52 
               
               
                   
                 Poly(1,3-butadiene) (high cis-type) 
                 1.52 
               
               
                   
                 Poly(butadiene-co-acrylonitrile) 
                 1.52 
               
               
                   
                 Poly(methyl isopropenyl icetone) 
                 1.5200 
               
               
                   
                 Poly(isoprene) 
                 1.521 
               
               
                   
                 Poly(ester) resin, rigid (ca, 50% styrene) 
                 1.523-1.54 
               
               
                   
                 Poly(N-(2-methoxyethyl)methacrylamide) 
                 1.5246 
               
               
                   
                 Poly(2,3-dimethylbutadiene) (methyl rubber) 
                 1.525 
               
               
                   
                 Poly(vinyl chloride-co-vinyl acetate) (95/5-90/10) 
                 1.525-1.536 
               
               
                   
                 Poly(acrylic acid) 
                 1.527 
               
               
                   
                 Poly(1,3-dichioropropyl methacrylate) 
                 1.5270 
               
               
                   
                 Poly(2-chloro-1-(chloromethyl)ethyl methacrylate) 
                 1.5270 
               
               
                   
                 Poly(acrolein) 
                 1.529 
               
               
                   
                 Poly(1-vinyl-2-pyrrolidone) 
                 1.53 
               
               
                   
                 Hydrochlorinated rubber 
                 1.53-1.55 
               
               
                   
                 Nylon 6: Nylon 6,6: Nylon 6, 10 (moulding) 
                 1.53 
               
               
                   
                 (Nylon-6-fiber: 1.515 transverse, 
               
               
                   
                 1.565 in fiber direction) 
               
               
                   
                 Poly(butadiene-co-styrene) (ca, 30% styrene) 
                 1.53 
               
               
                   
                 black copolymer 
               
               
                   
                 Poly(cyclohexyl α-chloroacrylate) 
                 1.532 
               
               
                   
                 Poly(2-chloroethyl α-chloroacrylate) 
                 1.533 
               
               
                   
                 Poly(butadiene-co-styrene) (ca, 75/25) 
                 1.535 
               
               
                   
                 Poly(2-aminoethyl methacrylate) 
                 1.537 
               
               
                   
                 Poly(furfuryl methacrylate) 
                 1.5381 
               
               
                   
                 Proteins 
                 1.539-1.541 
               
               
                   
                 Poly(butylmercaptyl methacrylate) 
                 1.5390 
               
               
                   
                 Poly(1-phenyl-n-amyl methacrylate) 
                 1.5396 
               
               
                   
                 Poly(N-methyl-methacrylamide) 
                 1.5398 
               
               
                   
                 Cellulose 
                 1.54 
               
               
                   
                 Poly(vinyl chloride) 
                 1.54-1.55 
               
               
                   
                 Urea formaldehyde resin 
                 1.54-1.56 
               
               
                   
                 Poly(sec-butyl α-bromoacrylate) 
                 1.542 
               
               
                   
                 Poly(cyclohexyl α-bromoacrylate) 
                 1.542 
               
               
                   
                 Poly(2-bromoethyl methacrylate) 
                 1.5426 
               
               
                   
                 Poly(dihydroabietic acid) 
                 1.544 
               
               
                   
                 Poly(abietic acid) 
                 1.546 
               
               
                   
                 Poly(ethylmercaptyl methacrylate) 
                 1.547 
               
               
                   
                 Poly(N-allyl methacrylamide) 
                 1.5476 
               
               
                   
                 Poly(1-phenylethyl methacrylate) 
                 1.5487 
               
               
                   
                 Poly(vinylfuran) 
                 1.55 
               
               
                   
                 Poly(2 -vinyltetrahydrofuran) 
                 1.55 
               
               
                   
                 Poly(vinyl chloride) + 40% trictesyl phosphate 
                 1.55 
               
               
                   
                 Epoxy resins 
                 1.55-1.60 
               
               
                   
                 Poly(p-methoxybenyl methacrylate) 
                 1.552 
               
               
                   
                 Poly(isopropyl methacrylate) 
                 1.552 
               
               
                   
                 Poly(p-isopropylstyrene) 
                 1.554 
               
               
                   
                 Poly(chloroprene) 
                 1.554-1.558 
               
               
                   
                 Poly(oxyethylene)-α-benzoate-ω-methacrylate) 
                 1.555 
               
               
                   
                 Poly(p,p′-xylylenyl dimethacrylate) 
                 1.5559 
               
               
                   
                 Poly(1-phenylallyl methacrylate) 
                 1.5573 
               
               
                   
                 Poly(p-cyclohexylphenyl methacrylate) 
                 1.5575 
               
               
                   
                 Poly(2-phenylethyl methacrylate) 
                 1.5592 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenylene-1-propyl 
                 1.5602 
               
               
                   
                 butylidene-1,4-phenylene) 
               
               
                   
                 Poly(1-(o-chlorophenyl)ethyl methacrylate) 
                 1.5624 
               
               
                   
                 Poly(styrene-co-maleic anhydride) 
                 1.564 
               
               
                   
                 Poly(1-phenylcyclohexyl methacrylate) 
                 1.5645 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenylene-1,3-dimethyl- 
                 1.5671 
               
               
                   
                 butylidene-1,4-phenylene) 
               
               
                   
                 Poly(methyl α-bromoacrylate) 
                 1.5672 
               
               
                   
                 Poly(benzyl methacrylate) 
                 1.5680 
               
               
                   
                 Poly(2-phenylsulfonyl)ethyl methacrylate) 
                 1.5682 
               
               
                   
                 poly(m-cresyl methacrylate) 
                 1.5683 
               
               
                   
                 Poly(styrene-co-acrylonitrile) (ca, 75/25) 
                 1.57 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenyleleneisobutylidene- 
                 1.5702 
               
               
                   
                 1,4-phenylene) 
               
               
                   
                 Poly(o-methoxyphenyl methacrylate) 
                 1.5705 
               
               
                   
                 Poly(phenyl methacrylate) 
                 1.5706 
               
               
                   
                 Poly(o-cresyl methacrylate) 
                 1.5707 
               
               
                   
                 Poly(diallyl phthalate) 
                 1.572 
               
               
                   
                 Poly(2,3 -dibromopropyl methacryate) 
                 1.5739 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenylene-1-methyl- 
                 1.5745 
               
               
                   
                 butylidene-1,4-phenylene) 
               
               
                   
                 Poly(oxy-2,6-dimethylphenylene) 
                 1.575 
               
               
                   
                 Poly(oxyethyleneoxyterephthaloyl) (amorphous) 
                 1.5750 
               
               
                   
                 (poly(ethylene terephthalate)) 
               
               
                   
                 (crystalline fiber: 1.51 transverse; 
               
               
                   
                 1.64 in fiber direction) 
               
               
                   
                 Poly(vinyl benzoate) 
                 1.5775 
               
               
                   
                 poly(oxycarbonyloxy-1,4-phenylenebutylidene-1,4- 
                 1.5792 
               
               
                   
                 phenylene) 
               
               
                   
                 Poly(1,2-diphenylethyl methacrylate) 
                 1.5816 
               
               
                   
                 Poly(o-chlorobenzyl methacrylate) 
                 1.5823 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenylene-sec-butylidene- 
                 1.5827 
               
               
                   
                 1,4-phenylene) 
               
               
                   
                 Poly(oxypentaerythritoloxyphthaloyl) 
                 1.584 
               
               
                   
                 Poly(m-nitrobenyl methacrylate) 
                 1.5845 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenyleneisopropylidene- 
                 1.5850 
               
               
                   
                 1,4-phenylene) 
               
               
                   
                 Poly(N-(2-phenylethyl)methacrylamide) 
                 1.5857 
               
               
                   
                 Poly(4-methoxy-2-methylstyrene) 
                 1.5868 
               
               
                   
                 Poly(o-methylstyrene) 
                 1.5874 
               
               
                   
                 Poly(styrene) 
                 1.59-1.592 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenylenecyclohexylidene- 
                 1.5900 
               
               
                   
                 1,4-phenylene) 
               
               
                   
                 Poly(o-methoxystyrene) 
                 1.5932 
               
               
                   
                 Poly(diphenylmethyl methacrylate) 
                 1.5933 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-phenyleneethylidene-1,4- 
                 1.5937 
               
               
                   
                 phenylene) 
               
               
                   
                 Poly(p-bromophenyl methacrylate) 
                 1.5964 
               
               
                   
                 Poly(N-benzyl methacrylamide) 
                 1.5965 
               
               
                   
                 Poly(p-methoxystyrene) 
                 1.5967 
               
               
                   
                 Hard rubber (32% S) 
                 1.6 
               
               
                   
                 Poly(vinylidene chloride) 
                 1.60-1.63 
               
               
                   
                 Poly(sulfides (“Thiokol”) 
                 1.6-1.7 
               
               
                   
                 Poly(o-chlorodiphenylmethyl methacrylate) 
                 1.6040 
               
               
                   
                 Poly(oxycarbonyloxy-1,4-(2,6-dichloro)phenylene- 
                 1.6056 
               
               
                   
                 isopropylidene-1,4-(2,6-dichloro)phenylene)) 
               
               
                   
                 Poly(oxycarbonyloxybis(1,4-(3,5-dichiorophenylene)) 
                 1.6056 
               
               
                   
                 Poly(pentachiorophenyl methacrylate) 
                 1.608 
               
               
                   
                 Poly(o-chlorostyrene) 
                 1.6098 
               
               
                   
                 Poly(phenyl α-bromoacrylate) 
                 1.612 
               
               
                   
                 Poly(p-divinylbenzene) 
                 1.6150 
               
               
                   
                   
               
             
          
         
       
     
     The iridescent film of the present invention can also be obtained by coextruding the naphthalate-based polyester or copolyester resin with a different transparent naphthalate-based polyester which is selected to differ in refractive index by at least about 0.03 and preferably at least 0.06. 
     The outermost layers of the iridescent film of the present invention can be the same or different from resins in the optical core. For example, the outermost layers can comprise a polyester or copolyester resin such as polybutylene terephthalate polyester or glycol modified polyethylene terephthalate like PETG polyester. 
     The number of layers in the iridescent film of the invention is at least 10 layers, preferably at least 35 layers and more preferably at least about 70 layers. 
     A preferred combination in accordance with this invention involves an iridescent film having the contiguous adjacent layers in the optical core being of different transparent thermoplastic resinous materials of which one is polyethylene naphthalate polyester or copolyester, and the other is polybutylene terephthalate polyester or copolyester, wherein the outermost layers are polybutylene terephthalate or PETG polyester. 
     The delamination resistance of a film is tested by restraining one surface of the film with adhesive tape. A second piece of adhesive tape is applied to the other surface of the film. This second piece of tape is then pulled away and any indications of delamination is noted. If no delamination is observed, the tape is reapplied and the test repeated until failure is noted. Different tapes with different tack levels can be used to more fully differentiate between various film structures. Additionally, the film sample being tested can be immersed in solvent prior to testing or may be scored to provide a more severe form of this test. The number of pulls to failure using a particular tape is typically recorded. A description of this test method can be found in U.S. Pat. No. 5,089,318. 
     To test the solvent resistance of the film, samples of the film are immersed in the challenge solvent. The sample is observed for any color change, for the time at which the solvent begins to affect the iridescent color of the film and the nature of the color change. The time to initial color change and the nature of the color change at set time intervals are recorded. This test is typically run for a period of seven days with observations taken throughout the seven-day period. At the end of the seven days, the film sample is removed from the challenge solvent and allowed to dry for twenty-four hours. After the twenty-four hour drying period, the tester notes how the iridescent film color has changed. This data is referred to as the “Color Recovery”. 
     To test the heat shrinkage of the film, a 2″×2″ piece of film is cut from the film in question. The color of this piece is measured using a spectrophotometer. Data measured include the dominant wavelength (DWL), peak wavelength (PWL), and % peak reflection of the sample. The test specimen is then placed in an oven at the test temperature for a period of fifteen minutes. The sample is then removed from the oven. The sample is measured using a ruler to determine the percentage of shrinkage experienced by the film. The color of the sample is re-measured and the changes in DWL, PWL and peak reflection are recorded. The color of the sample after heat exposure is also compared to the original color of the test material by the person performing the test. Using all of this data, the temperature at which the film color begins to change is determined. This value is referred to as the “Color Shift Temperature”. 
    
    
     EXAMPLE 1 
     Iridescent Films IF(1-4) 
     Polybutylene terephthalate thermoplastic polyester was fed to the feedblock from one extruder and polymethyl methacrylate from a second extruder to form four films of varying thickness (IF(1)=1.1 mils (28 μm), IF(2)=1.2 mils (30 μm), IF(3)=1.3 mils (33 μm), IF(4)=1.4 mils (36 μm)), each consisting of a 226 optical layer film and two polyethylene terephthalate polyester skin layers (see Table 2). The films were brightly iridescent and shared the same properties. These films&#39; solvent resistance and temperature stability are described below in Tables 3 and 4. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Color and thickness of IF (1-4) 
               
             
          
           
               
                   
                   
                 Colors when seen 
                   
                   
               
               
                   
                   
                 by reflection at 
                   
               
               
                   
                   
                 perpendicular 
                 Dominant 
                 Thickness 
               
               
                   
                 Film 
                 incidence 
                 Wavelength (nm) 
                 (mils/μm) 
               
               
                   
                   
               
               
                   
                 IF (1) 
                 Blue/Violet 
                 465 
                 1.1/28 
               
               
                   
                 IF (2) 
                 Blue/Green 
                 485 
                 1.2/30 
               
               
                   
                 IF (3) 
                 Red/Green 
                 555 
                 1.3/33 
               
               
                   
                 IF (4) 
                 Red/Red 
                 590 
                 1.4/36 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Properties of IF (1-4) 
               
             
          
           
               
                   
                 Property 
                 Nominal Value 
               
               
                   
                   
               
               
                   
                 Maximum Process Temp. (° F./° C.) 
                 180/81  
               
               
                   
                 (Under Tension) 
               
               
                   
                 Color Shift Temperature (° F./° C.) 
                 225/107 
               
               
                   
                 Color Loss Temperature (° F./° C.) 
                 425/218 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Solvent Resistance of IF (1-4) 
               
             
          
           
               
                   
                 Time to 
                 Immersion Time 
                   
               
             
          
           
               
                   
                 initial 
                   
                 24 
                 7 
                 Color 
               
               
                 Challenge Solvent 
                 change 
                 5 min 
                 Hours 
                 Days 
                 Recovery 
               
               
                   
               
               
                 Alcohols 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 (Ethanol, Isopropanol) 
               
               
                 Aliphatic 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 Hydrocarbons 
               
               
                 (Hexane, Naptha) 
               
             
          
           
               
                 Aromatic 
                 1 
                 Hour 
                 N 
                 CC 
                 CC 
                 CL 
               
               
                 Hydrocarbons 
               
               
                 (Benzene, Toluene) 
               
               
                 Ketones 
                 &lt;5 
                 min 
                 CC 
                 CC 
                 CC 
                 CL 
               
               
                 (Acetone, MEK) 
               
               
                 Esters 
                 &lt;5 
                 min 
                 CS 
                 CC 
                 CC 
                 CL 
               
               
                 (Butyl Acetate, 
               
               
                 Ethyl Acetate) 
               
             
          
           
               
                 Fully Halogenated 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 Hydrocarbons 
               
               
                 (Carbon Tet., 
               
               
                 Perchloroethylene) 
               
             
          
           
               
                 Partially Halogenated 
                 &lt;5 
                 min 
                 CC 
                 CC 
                 CC 
                 CL 
               
               
                 Hydrocarbons 
               
               
                 (Ethylene Dichloride, 
               
               
                 Trichloroethylene) 
               
               
                   
               
               
                 Key  
               
               
                 N - No Change  
               
               
                 CS - Color Shift (Sample has started to change color due to solvent immersion)  
               
               
                 CC - Color Change (Total color change due to solvent immersion)  
               
               
                 CL - Color Loss  
               
               
                 Color Recovery  
               
               
                 The appearance of the material after removal from solvent immersion and the sample has dried.  
               
               
                 Excellent - Material recovers to initial appearance  
               
               
                 Good - Color recovery with some color shift  
               
               
                 Fair - Some color recovery with significant color shift  
               
               
                 None - No color recovery  
               
               
                 Color Loss - Material loses iridescent color upon removal from solvent.  
               
             
          
         
       
     
     IF(1-4) were also tested for delamination using Scotch 396 pull tape according to the above-described procedure. The skin layers delaminated after the first pull while delamination of the optical core occurred after 5 pulls. 
     EXAMPLE 2 
     Iridescent Films IF(5-8) 
     Polybutylene terephthalate thermoplastic polyester was fed to the feedblock from one extruder and ethylene vinyl acetate copolymer from a second extruder to form four films of varying thickness (IF(5)=1.1 mils (28 μm), IF(6)=1.2 mils (30 μm), IF(7)=1.3 mils (33 μm), IF(8)=1.4 mils (36 μm)), each consisting of a 226 optical layer film and two polymethyl methacrylate skin layers (see Table 5). The films were brightly iridescent and shared the same properties. These films&#39; solvent resistance and temperature stability are described below in Tables 6 and 7. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Color and thickness of IF (5-8) 
               
             
          
           
               
                   
                   
                 Colors when 
                   
                   
               
               
                   
                   
                 seen by 
               
               
                   
                   
                 reflection at 
               
               
                   
                   
                 perpendicular 
                 Dominant 
                 Thickness 
               
               
                   
                 Film 
                 incidence 
                 Wavelength (nm) 
                 (mils/μm) 
               
               
                   
                   
               
               
                   
                 IF (5) 
                 Blue/Violet 
                 465 
                 1.1/28 
               
               
                   
                 IF (6) 
                 Blue/Green 
                 485 
                 1.2/31 
               
               
                   
                 IF (7) 
                 Red/Green 
                 555 
                 1.3/33 
               
               
                   
                 IF (8) 
                 Red/Red 
                 590 
                 1.4/36 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Properties of IF (5-8) 
               
             
          
           
               
                   
                 Property 
                 Nominal Value 
               
               
                   
                   
               
               
                   
                 Maximum Process Temp. (° F./° C.) 
                 170/75  
               
               
                   
                 (Under Tension) 
               
               
                   
                 Color Shift Temperature (° F./° C.) 
                 225/107 
               
               
                   
                 Color Loss Temperature (° F./° C.) 
                 420/215 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Solvent Resistance of IF (5-8) 
               
             
          
           
               
                   
                 Time to 
                 Immersion Time 
                   
               
             
          
           
               
                   
                 initial 
                   
                 24 
                 7 
                 Color 
               
               
                 Challenge Solvent 
                 change 
                 5 min 
                 Hours 
                 Days 
                 Recovery 
               
               
                   
               
               
                 Alcohols 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 (Ethanol, Isopropanol) 
               
             
          
           
               
                 Aliphatic 
                 15 
                 min 
                 N 
                 CS 
                 CC 
                 None 
               
               
                 Hydrocarbons 
               
               
                 (Hexane, Naptha) 
               
               
                 Aromatic 
                 &lt;5 
                 min 
                 CS 
                 CC 
                 CC 
                 Good 
               
               
                 Hydrocarbons 
               
               
                 (Benzene, Toluene) 
               
               
                 Ketones 
                 &lt;5 
                 min 
                 CS 
                 CS 
                 CS 
                 Excellent 
               
               
                 (Acetone, MEK) 
               
               
                 Esters 
                 &lt;5 
                 min 
                 CS 
                 CS 
                 CS 
                 Excellent 
               
               
                 (Butyl Acetate, 
               
               
                 Ethyl Acetate) 
               
               
                 Fully Halogenated 
                 &lt;5 
                 min 
                 CS 
                 CC 
                 CC 
                 Fair 
               
               
                 Hydrocarbons 
               
               
                 (Carbon Tet., 
               
               
                 Perchloroethylene) 
               
               
                 Partially Halogenated 
                 &lt;5 
                 min 
                 CC 
                 CC 
                 CC 
                 Good 
               
               
                 Hydrocarbons 
               
               
                 (Ethylene Dichloride, 
               
               
                 Trichloroethylene) 
               
               
                   
               
               
                 Key - see Table 4  
               
             
          
         
       
     
     IF(5-8) were also tested for delamination using Scotch 396 pull tape according to the above-described procedure. Both the skin and the optical core delaminated after the first pull. 
     EXAMPLE 3 
     Iridescent Films IF(9-12) 
     Polybutylene terephthalate thermoplastic polyester was fed to the feedblock from one extruder and polyethylene naphthalate polyester from a second extruder to form four films of varying thickness (IF(9)=1.1 mils (28 μm), IF(10)=1.2 mils (30 μm), IF(11)=1.3 mils (33 μm), IF(12)=1.4 mils (36 μm)), each consisting of a 226 optical layer film and two polybutylene terephthalate polyester skin layers (see Table 8). The films were brightly iridescent and shared the same properties. These films&#39; solvent resistance and temperature stability are described below in Tables 9 and 10. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Color and thickness of IF (9-12) 
               
             
          
           
               
                   
                   
                 Colors when 
                   
                   
               
               
                   
                   
                 seen by 
                   
               
               
                   
                   
                 reflection at 
                   
               
               
                   
                   
                 perpendicular 
                 Dominant 
                 Thickness 
               
               
                   
                 Film 
                 incidence 
                 Wavelength (nm) 
                 (mils/μm) 
               
               
                   
                   
               
               
                   
                 IF (9) 
                 Blue/Violet 
                 465 
                 1.1/28 
               
               
                   
                 IF (10) 
                 Blue/Green 
                 485 
                 1.2/31 
               
               
                   
                 IF (11) 
                 Red/Green 
                 555 
                 1.3/33 
               
               
                   
                 IF (12) 
                 Red/Green 
                 590 
                 1.4/36 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Properties of IF (9-12) 
               
             
          
           
               
                   
                 Property 
                 Nominal Value 
               
               
                   
                   
               
               
                   
                 Maximum Process Temp. (° F./°C.) 
                 N/A 
               
               
                   
                 (Under Tension) 
               
               
                   
                 Color Shift Temperature (° F./° C.) 
                 380/193 
               
               
                   
                 Color Loss Temperature (° F./° C.) 
                 450/232 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Solvent Resistance of IF (9-12) 
               
             
          
           
               
                   
                 Time to 
                 Immersion Time 
                   
               
             
          
           
               
                   
                 initial 
                   
                 24 
                 7 
                 Color 
               
               
                 Challenge Solvent 
                 change 
                 5 min 
                 Hours 
                 Days 
                 Recovery 
               
               
                   
               
               
                 Alcohols 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 (Ethanol, Isopropanol) 
               
               
                 Aliphatic 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 Hydrocarbons 
               
               
                 (Hexane, Naptha) 
               
             
          
           
               
                 Aromatic 
                 48 
                 Hours 
                 N 
                 N 
                 CS 
                 Excellent 
               
               
                 Hydrocarbons 
               
               
                 (Benzene, Toluene) 
               
               
                 Ketones 
                 3 
                 Hours 
                 N 
                 CC 
                 CC 
                 Excellent 
               
               
                 (Acetone, MEK) 
               
               
                 Esters 
                 5 
                 Hours 
                 N 
                 CC 
                 CC 
                 Excellent 
               
               
                 (Butyl Acetate, 
               
               
                 Ethyl Acetate) 
               
             
          
           
               
                 Fully Halogenated 
                 N/A 
                 N 
                 N 
                 N 
                 N/A 
               
               
                 Hydrocarbons 
               
               
                 (Carbon Tet., 
               
               
                 Perchloroethylene) 
               
             
          
           
               
                 Partially Halogenated 
                 &lt;5 
                 min 
                 CS 
                 CC 
                 CC 
                 Fair 
               
               
                 Hydrocarbons 
               
               
                 (Ethylene Dichloride, 
               
               
                 Trichloroethylene) 
               
               
                   
               
               
                 Key - see Table 4  
               
             
          
         
       
     
     IF(9-12) were also tested for delamination using Scotch 396 pull tape according to the above-described procedure. Both the skin and the optical core showed no delaminated after 10 pulls. 
     EXAMPLE 4 
     Iridescent Film IF(13-18) 
     Resin A will be fed to the feedblock from one extruder and resin B from another extruder to form five films, IF(13-18) according to Table 11. Each film will consist of 100 layers and will be about 1.1 mils (28 μm) in thickness. 
     
       
         
               
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                 Film 
                 Resin A 
                 Resin B 
               
               
                   
               
             
             
               
                 IF (13) 
                 Fluorinated 
                 Polymethyl 
               
               
                   
                 ethylene-propylene 
                 acrylate 
               
               
                   
                 copolymer 
               
               
                 IF (14) 
                 Polybutylene 
                 Ethylene vinyl 
               
               
                   
                 terephthalate 
                 acetate 
               
               
                 IF (15) 
                 Polybutylene 
                 Propylene-ethylene 
               
               
                   
                 terephthalate 
                 copolymer 
               
               
                 IF (16) 
                 Polybutylene 
                 Butylene 
               
               
                   
                 terephthalate 
                 naphthalate 
               
               
                   
                   
                 polyester 
               
               
                 IF (17) 
                 Polybutylene 
                 Butylene 
               
               
                   
                 terephthalate 
                 naphthalate and 
               
               
                   
                   
                 ethylene 
               
               
                   
                   
                 naphthalate 
               
               
                   
                   
                 copolyester 
               
               
                 IF (18) 
                 Butylene 
                 Butylene 
               
               
                   
                 naphthalate 
                 naphthalate and 
               
               
                   
                 polyester 
                 ethylene 
               
               
                   
                   
                 naphthalate 
               
               
                   
                   
                 copolyester 
               
               
                   
               
             
          
         
       
     
     When compared to IF (13-15), it will be demonstrated that IF (16-18) has better delamination resistance, temperature stability and solvent resistance (with regard to at least one solvent). 
     The invention has been described in terms of preferred embodiments thereof, but is more broadly applicable as will be understood by those skilled in the art. The scope of the invention is therefore limited only by the following claims.