Abstract:
Heat-stable coatings on cookware can be made stain-resistant by including a pigmented brown intermediate layer and a clear topcoat layer containing mica or coated mica platelets for sparkle. Such coatings are particularly suitable for electrical cookware.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to coated cookware having a heat-stable multi-layer nonstick coating. 
     In modern cookware coatings, it has been found desirable to provide a sparkling appearance. However, sooner or later, food stains appear in most cookware coatings. These stains generally have a brown color and are formed by pyrolysis of particles of food, oil or other materials which penetrate into the more-or-less porous outer layers of the coating. This phenomenon occurs with most types of cookware coating, including coatings based on polytetrafluoroethylene (PTFE). 
     The problem of staining becomes more acute when the temperature profile across the face of the cooking surface is not uniform. This occurs particularly in cookware heated by an electrical resistance element in the shape of a pattern imbedded in or against the back of the cookware, i.e., the surface opposite that on which the coating system is applied. This staining phenomenon takes the form of a brown line following the pattern of the heating element. 
     It would be desirable to be able to coat electric cookware in such a way that no conspicuous stained pattern develops during use of the cookware. 
     SUMMARY OF THE INVENTION 
     This invention provides a cookware article comprising a substrate coated with a heat-stable coating system having three layers, each layer comprising a polymer stable at temperatures above 300° C., said layers including 
     a primer layer in direct contact with the substrate and which adheres the coating system to the substrate, 
     an intermediate layer in contact with the primer layer and containing pigments which mask the brown color of food stains that develop in the coating system during cooking, and 
     a light-transmitting topcoat layer in contact with the intermediate layer and containing mica or mica platelets coated with pigment, giving a sparkling appearance to the coating system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been found that food stains do not appear conspicuously on cookware coatings, even on cookware heated with a circular electrical heating element on the back, if a three-layer coating is used including a primer, an intermediate layer with enough pigment of an appropriate color to mask the brown color of food stains that develop in the coating, and a light-transmitting top layer containing mica or mica platelets coated with pigment to give a sparkling appearance. Dark brown or black pigments in the intermediate layer are desirable. The present invention draws on technology expressed in other U.S. Pat Nos., including 
     4,123,401--Berghmans and Vary (Oct. 31, 1978) directed to coating compositions containing fluoropolymer, mica, decomposable polymer, and a liquid carrier, particularly suited for use as topcoats; 
     4,049,863--Vassiliou (1977) directed to primers containing fluoropolymer, colloidal silica, the salt of a polyamic acid, and a coalescing agent, mica, and a liquid carrier; 
     4,087,394--Concannon (May 2, 1978) directed to aqueous coating compositions of fluorocarbons and a second film-forming material which can be the salt of a polyamic acid; 
     4,143,204--Fang (Mar. 6, 1979) directed to coated articles in which the coating comprises a copolymer of tetrafluoroethylene and hexafluoropropylene along with an auxiliary film-forming material; 
     and the following five U.S. Pat. Nos. which are concerned with heat-stable coatings containing oxidation catalysts and/or antioxidants: 
     4,054,704--Vassiliou (1977), 
     4,054,705--Vassiliou (1977), 
     4,064,303--Vassiliou (1977), 
     4,120,608--Vassiliou (Oct. 17, 1978), and 
     4,122,226--Vassiliou (Oct. 24, 1978); and 
     finally, application Ser. No. 881,668, filed Feb. 27, 1978 and allowed May 25, 1979, now U.S. Pat. No. 4,180,609--Vassiliou (Dec. 25, 1979), directed to coated articles having a multi-layer coating with fluorocarbon and mica in a basecoat and a topcoat, and with a defined relationship between the concentration of mica in the basecoat and that in the topcoat. 
     The above-mentioned patents and application are incorporated herein by reference. 
     Preferably by using techniques of the above-identified patents and the application, cookware is coated with a primer layer, an intermediate layer and a topcoat layer which have the defined relationships to obtain the advantages of the invention. 
     The following example teaches a preferred embodiment of the invention. Parts, percentages and proportions herein are by weight except where indicated otherwise. 
     The following coating compositions were used to make the respective coating layers in accordance with the teachings of Example 2 of U.S. Pat. No. 4,049,863 for the primer layer and Example 4 of U.S. Pat. Nos. 4,123,401 and 4,180,609 for the intermediate and topcoat layers. 
     Briefly, the procedure involved gritblasting an aluminum substrate, allowing the primer to air dry then spraying on the intermediate and topcoat layers, wet-on-wet, following by a bake at 425° C. for 5 minutes. The resulting coatings were a dark chocolate brown with a sparkling appearance, and they did not show food stain after extensive cooking when the substrate was an electric grill with a patterned electrical resistance heating element embedded in the aluminum. 
     
                       TABLE I______________________________________PRIMER COATING COMPOSITION                      % Wt.______________________________________PTFE Solids (60% solids in water, Du Pont T30)                        11.16Ludox® AM Colloidal Silica Solids (Du Pont)                        0.96Amide-Imide Resin (Polyamide acid of                        4.93Example 1A of U.S. Pat. No. 4,049,863)Octyl Phenol Polyether Alcohol                        0.67Deionized Water              67.76Furfuryl Alcohol             3.60Diethyl Ethanolamine         0.65Triethyl Amine               1.31N-Methyl Pyrrolidone         2.46Cobalt Aluminate Pigment     5.88Red Iron Oxide Pigment       0.59Afflair® Pigment (Du Pont) TiO.sub.2 Coated Mica                        0.05                        100.00______________________________________ 
    
     
                       TABLE II______________________________________INTERMEDIATE COATING COMPOSITION                      % Wt.______________________________________PTFE Solids (60% solids in water, Du Pont T30)                        42.39Phosphoric Acid (100%)       0.08Sodium Lauryl Sulfate        0.03Sodium Polynaphthalene Sulfonate                        0.01Acrylic Latex Solids (methyl methacrylate/ethyl                        5.04acrylate/methacrylic acid - 39/57/4terpolymer dispersion (40% in water) 0.2micron average particle size)Deionized Water              39.19Octyl Phenol Polyether Alcohol                        2.54Triethanolamine              2.82Oleic Acid                   1.14Toluene                      4.37Butyl Carbitol               1.49Red Iron Oxide Pigment       0.28Channel Black                0.10Afflair® Pigment (DuPont) TiO.sub.2 Coated Mica                        0.43Aluminosilicate Pigment      0.05                        100.00______________________________________ 
    
     
                       TABLE III______________________________________TOPCOAT COATING COMPOSITION______________________________________PTFE Solids (60% solids in water, Du Pont T30)                       43.05Sodium Lauryl Sulfate       0.03Acrylic Latex Solids        4.81(as in Table II)Deionized Water             42.93Triton® X-100           2.58Iriethanolmine              1.59Oleic Acid                  0.75Toluene                     2.85Butyl Carbitol              0.98Afflair® Pigment (Du Pont) TiO.sub.2 Coated Mica                       0.43                       100.00______________________________________ 
    
     Instead of the red iron oxide-carbon black combination of Table III, one can use equivalent amounts of burnt umber, having an approximate composition shown in Table IV. 
     
                       TABLE IV______________________________________COMPOSITION OF BURNT UMBER         %______________________________________Fe.sub.2 O.sub.3          40-73SiO.sub.2      10-20Al.sub.2 O.sub.3          10-20MnO.sub.2       4-19H.sub.2 O sol. salts          0.4Pb              0.02As              0.03Mg               0.0075______________________________________