Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-05-08
2002-06-11
Copenheaver, Blaine (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S421000, C428S422000, C428S458000, C428S461000, C428S473500, C428S474400
Reexamination Certificate
active
06403213
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to primers and base coats for use with non-stick fluoropolymer coatings.
BACKGROUND OF THE INVENTION
Fluoropolymer resins, and especially perfluoropolymer resins, are known for their low surface energy and non-stick properties as well as thermal and chemical resistance. It has long been desirable to achieve durable non-stick polymer coatings on a variety of substrates. As discussed in U.S. Pat. No. 5,562,991 to Tannenbaum, a primer was developed for use on a variety of substrates, including smooth substrates, which has adequate adhesion both to the substrate and to a fluoropolymer overcoat. Tannenbaum achieved this objective by applying a primer comprising fluoropolymer and polymer binder in the weight proportion of 0.5 to 2.0:1 and in addition, from 5 to 30 weight % of an inorganic film hardener. There still remains a need for fluoropolymer coatings that have improved adhesion to substrates as well as improved scratch, abrasion, and wear resistance.
However in order to achieve a better level of adherence, it would be desirable to develop a coating system which exhibits reduced shrinkage during the drying and curing processes normally associated with the application of fluoropolymer systems. More specifically, such processes which require the application of heat to remove volatile components of the coating system as well as to melt flow and cure organic polymeric components, create dimensional changes in the coating and adversely affect adherence to substrate materials.
A particular concern in the cookware industry is decorative cookware where ceramic coating covers at least a portion of the interior of the cookware. This can occur when a ceramic “frit” is intentionally applied to the interior of the cookware to harden the surface. This can also occur when a ceramic coating is applied to the exterior of a metal preform that is geometrically complex and the masking of the interior is difficult and expensive. In these cases, ceramic over-spray can partially coat the interior of the preform and interfere with the adhesion of a non-stick coating which is subsequently applied.
SUMMARY OF THE INVENTION
The present invention provides a composition which is applicable as a undercoat for a non-stick coating on a substrate, comprising (a) inorganic filler of ceramic particles, the particles having an average particle size of 1 micrometer or less and an aspect ratio of not greater than 2.5, (b) adhesion promoter wherein the weight proportion of inorganic filler to adhesion promoter is in the range of 5:1 to 1.7:1 and (c) from 0 to 25 wt. % of a fluoropolymer based on the weight of the composition of the undercoat layer after baking.
In another embodiment the present invention includes a coated substrate having a non-stick coating, wherein the coating comprises an undercoat and a fluoropolymer overcoat, the undercoat containing (a) inorganic filler of ceramic particles, the particles having a particle size of 1 micrometer or less and an aspect ratio of not greater than 2.5, (b) at least one adhesion promoter wherein the weight proportion of inorganic filler to adhesion promoter is in the range of 5:1 to 1.7:1 and (c) from 0 to 25 wt. % of a fluoropolymer based on the weight of the composition of the undercoat layer after baking.
DETAILED DESCRIPTION
The present invention, in certain of its embodiments, provides a coating system comprising a substrate with a multilayer non-stick coating of a primer, and a top coat with one or more intermediate layers and optionally a base coat applied directly to the substrate beneath the primer. The base coat and primer are referred to as undercoats. The intermediate coating(s) and topcoat are referred to as overcoats.
Inorganic Filler
The major component of the undercoat composition of the present invention is an inorganic filler of ceramic particles. The inorganic filler together with a minor amount of adhesion promoter surprisingly produces a smooth, continuous film with superior adhesion to a variety of substrates as well as to fluoropolymer applied as an overcoat to the undercoat. By “undercoat” it is meant either the base coat applied directly to a substrate or a primer applied either to the base coat or to the substrate. By “overcoat” it is meant one or more intermediate coatings or a topcoat which are applied over the primer layer. The high level of inorganic filler present reduces the level of shrinkage that occurs during normal drying and coating operations. By reducing the levels of stress in a coating, adhesion of the coating to a substrate is increased. This reduction results because the levels of stress that contribute to shrinkage are in direct opposition to the force needed to secure the coating layer to the substrate. By increasing the amount of inorganic filler that contributes no or very little shrinkage, the coating comprised of inorganic and organic components undergoes decreased dimensional change upon heat application. The inorganic filler is composed of ceramic particles, the particles having an average particle size of 1 micrometer or less and the weight proportion of inorganic filler to adhesion promoter is in the range of 5:1 to 1.7:1. Thus the inorganic filler is usually present as 45-85 wt. %, preferably 60-85 wt. %, and more preferably 48-72 wt. % of the undercoat composition based on the baked weight, depending on the amount of optional fluoropolymer component. It is to be recognized that the levels of inorganic filler in the present invention are considerably higher than conventionally known compositions such as those disclosed in the prior art represented by U.S. Pat. No. 5,562,991 to Tannenbaum, and U.S. Pat. No. 5,250,356 to Batzar. Additionally, the high level of inorganic filler contributes to increased scratch, wear and abrasion resistance.
The inorganic filler component is one or more filler type materials that are inert with respect to the other components of the composition and thermally stable at its eventual baking temperature that fuses the undercoat composition and/or subsequent fluoropolymer overcoats. The inorganic filler is water insoluble so that it is typically uniformly dispersible but not dissolved in the aqueous dispersion form of the composition of the invention. Typically, the inorganic filler of this invention preferably comprises ceramic particles having an average particle size of 1 micrometer or less and preferably an average particle size in the range of 0.1 micrometer to 1.0 micrometer
It is also preferred that the ceramic particles have an aspect ratio of not greater than 2.5 and preferably not greater than 1.5. By aspect ratio is meant a ratio of the longest diameter of the particle to the greatest distance of a dimension measured perpendicular to the longest diameter (major axis) of the particle. The aspect ratio is a means of quantifying a preferred particle shape. Particles with a high aspect ratio are whisker-like or rod shaped unlike the preferred particles of this invention, which are preferably more spherical and more closely approach an ideal aspect ratio of 1. Also the particle size distribution is preferably narrow with a monodisperse distribution being most preferred. The particle shape and size distribution are important in order to maximize the packing density of the resulting coating.
The inorganic filler is preferably selected from a group of inorganic nitrides, carbides, borides and oxides. Examples of ceramic particles that may be especially useful include particles of inorganic oxides such as silicon oxide, aluminum oxide, zinc oxide, and tin oxide, and inorganic carbides such as silicon carbide.
Adhesion Promoter
The adhesion promoter component of the undercoat composition is preferably composed of organic polymer that is film-forming upon heating to fusion and is also thermally stable. This component is well known in undercoat applications for non-stick finishes, for adhering a fluoropolymer-containing undercoat layer to substrates and for film-forming within and as part of a undercoat layer. The fluoropolymer by itself has lit
Copenheaver Blaine
Paulraj Christopher
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