Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
1998-12-14
2001-01-30
Cameron, Erma (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S410000, C427S412100, C427S412300
Reexamination Certificate
active
06180181
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for forming coating films on metallic and polymeric substrates and, more particularly, to composite coatings including a primary layer, basecoat and clearcoat which are applied in a wet-on-wet-on-wet process which when cured provide good chip resistance and a smooth finish.
BACKGROUND OF THE INVENTION
Over the past decade, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents which are emitted during the painting process. However, it is often difficult to achieve high quality, smooth coating finishes, such as are required in the automotive industry, without using organic solvents, which contribute greatly to flow and leveling of a coating. In addition to achieving near-flawless appearance, automotive coatings must be durable and chip resistant, yet economical and easy to apply.
Currently, in the automotive industry the coating system which provides a good balance between economy, appearance and physical properties is a system having four individual coating layers. The first coating is a corrosion resistant primer which is applied by electrodeposition and cured. The next coating is a primer/surfacer which is spray applied and then cured. The third coating is a spray-applied colored basecoat. The basecoat is generally not cured before the application of the final coating, the clear coat which is designed to provide toughness and high gloss to the system. The process of applying one layer of a coating before the previous layer is cured is referred to as a wet-on-wet (“WOW”) application.
U.S. Pat. No. 5,262,464 discloses a primer which can be dried at ambient conditions for 60 minutes and coated with a waterborne basecoat and two component, low VOC clearcoat (column 7, line 60 to column 8, line 44). The primer coating composition includes an aqueous dispersion of a thermoplastic anionic polyacrylate or polyurethane. The polyacrylate has functional carboxylic acid or anhydride groups which are neutralized with ammonia. The polyurethane is also neutralized with ammonia or an amine to be dispersible in water.
It is desirable, however, to use a thermosettable primer/surfacer coating to provide better adhesion to the substrate. Unfortunately, conventional thermosettable waterborne primer/surfacer compositions need to be cured before the basecoat is applied, increasing cost by requiring major capital investment in ovens and large amounts of energy.
The automotive industry would derive a significant economic advantage from an inexpensive coating process which provides a coated composite having good adhesion, chip resistance and smoothness, yet which can be applied wet-on-wet-on-wet (“WOWOW”), i.e., a process in which the primer/surfacer is not heated or is heated only for a short time at a low temperature to evaporate some of the water and/or solvent remaining in the primer/surfacer after it has been applied without significant crosslinking thereof.
SUMMARY OF THE INVENTION
The present invention provides a method for forming a composite coating comprising the steps of: (A) applying an aqueous primary coating composition to at least a portion of a surface of a substrate, the primary coating composition comprising: (1) at least one thermosettable dispersion comprising polymeric microparticles having functionality adapted to react with a crosslinking material, the microparticles comprising: (a) at least one acid functional reaction product of ethylenically unsaturated monomers; and (b) at least one hydrophobic polymer having a number average molecular weight of at least about 500; and (2) at least one crosslinking material, to form a substantially uncured primary coating thereon; (B) applying a secondary coating composition to at least a portion of the primary coating formed in step (A) without substantially curing the primary coating to form a substantially uncured secondary coating thereon; and (C) applying a clear coating composition to at least a portion of the secondary coating formed in step (B) without substantially curing the secondary coating to form a substantially uncured composite coating thereon.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method of the present invention provides a composite coating having good smoothness and aesthetic appearance, as well as good adhesion to the substrate and chip resistance. The methods comprise a first step (A) of applying an aqueous primary coating composition to at least a portion of a surface of a substrate.
The shape of the metal substrate can be in the form of a sheet, plate, bar, rod or any shape desired, but is preferably is in the form of an automobile part, such as a body, door, fender, hood or bumper. The thickness of the substrate can vary as desired. Suitable substrates can be formed from inorganic or metallic materials, thermoset materials, thermoplastic materials and combinations thereof.
The metal substrates coated by the methods of the present invention include ferrous metals such as iron, steel, and alloys thereof, non-ferrous metals such as aluminum, zinc and alloys thereof, and combinations thereof. Most load bearing components of automobile bodies are formed from metal substrates. Useful thermoset materials include polyesters, epoxides, phenolics, polyurethanes and mixtures thereof. Useful thermoplastic materials include polyolefins, polyamides, thermoplastic polyurethanes, thermoplastic polyesters, acrylic polymers, vinyl polymers, copolymers and mixtures thereof. Car parts typically formed from thermoplastic and thermoset materials include bumpers and trim. It is desirable to have a coating system which can be applied to both metal and non-metal parts.
To better understand the aforesaid important aspects of the invention, a metal coating operation in which such methods are useful will be discussed. One skilled in the art would understand that the methods of the present invention are not intended to be limited to use in coating metal substrates, but also are useful for coating polymeric substrates as discussed above.
Before depositing the coatings upon the surface of the metal substrate, it is preferred to remove foreign matter from the metal surface by thoroughly cleaning and degreasing the surface by physical or chemical means such as are well known to those skilled in the art. Preferably, a pretreatment coating, such as BONAZINC zinc-rich pretreatment (commercially available from PPG Industries, Inc.), is deposited upon at least a portion of the surface of the metal substrate.
An electrodeposited coating is preferably applied to the surface of an electroconductive substrate prior to applying the primary coating composition of step (A), which is discussed in detail below. Useful electrodepositable coating compositions include conventional anionic or cationic electrodepositable coating compositions. Methods for electrodepositing coatings are well known to those skilled in the art and a detailed discussion thereof is not believed to be necessary. Useful compositions and methods are discussed in U.S. Pat. No. 5,530,043 (relating to anionic electrodeposition) and U.S. Pat. Nos. 5,760,107; 5,820,987 and 4,933,056 (relating to cationic electrodeposition) which are hereby incorporated by reference.
In the methods of the present invention, an aqueous primary coating composition is applied to at least a portion of the substrate (which can be pretreated and/or electrocoated, as discussed above). The aqueous primary coating composition comprises, as a film former, at least one thermosettable or crosslinkable dispersion comprising polymeric microparticles having functionality adapted to react with a crosslinking material in an aqueous medium. As used herein, the term “dispersion” means that the microparticles are capable of being distributed throughout water as finely divided particles, such as a latex. See
Hawley's Condensed Chemical Dictionary,
(12th Ed. 1993) at page 435, which is hereby incorporated by reference. The uniformity of the dispersion can be increased by the addition of wetting, dispersing or emul
Crano Nicholas J.
Faler Dennis L.
Hartman Marvis E.
Temple Rodger G.
Trettel Victoria A.
Cameron Erma
Cannoni Ann Marie
PPG Industries Ohio Inc.
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