Process for coating olefinic substrates

Details Process for coating olefinic substrates Process for coating olefinic substrates

1999-06-30

2002-07-23

Yao, Sam Chuang (Department: 1733)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S090000, C525S09200D, C525S09200D, C524S495000, C524S496000, C524S505000, C156S309600

Reexamination Certificate

active

06423778

ABSTRACT:

FIELD OF THE INVENTION
This invention concerns a process for applying coatings to olefinic substrates, particularly thermoplastic polyolefin (TPO) substrates.
BACKGROUND OF THE INVENTION
It is often desirable, for decorative or functional reasons, to apply a coating over a plastic substrate. It has been difficult to obtain good adhesion of paints to olefinic substrates, including thermoplastic polyolefin (TPO) substrates and other such modified polyolefin-based materials. As used in the art, “thermoplastic polyolefin” or “TPO” designates a rubber-filled polyolefin. The rubber domains may be, for example, EPDM rubber, but other rubbers may be used.
In addition to painted articles, TPO has been used in a color-in-mold process to produce articles of the desired colored that are not intended to be painted, so long as the requirements for appearance are not stringent. The color-in-mold process, however, is not adequate for producing high class (“class 1”) surfaces for a number of reasons. First, it is difficult to achieve high gloss finishes, and difficult to control the gloss of the finish reproducibly from one article to the next. Secondly, TPO is relatively soft and it would be desirable to coat the substrate with a coating composition that can offer resistance to marring and scratching. Finally, uncoated or unpainted TPO may have unsatisfactory weathering properties (i.e., may degrade, discolor, or chalk during outdoor exposure) in certain applications, whereas coatings can provide good weathering characteristics to many different substrates. For these reasons, then, it would be desirable to have a means of applying a coating composition to an olefinic substrate such as TPO to provide the properties lacking in the uncoated (raw) uncolored or colored substrate.
Plastic substrates may be coated with curable, or thermosettable, coating compositions. Thermosettable coating compositions are widely used in the coatings art, particularly for high-performance primers and topcoats. Color-plus-clear composite coatings have been particularly useful as topcoats for which exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired.
In the past, it has been necessary to include one or more additional, separate, manufacturing steps to prepare an olefinic substrate for painting so that the coating layer will be able to adhere to the olefinic substrate. According to one frequently used method, a thin layer of an adhesion promoter or tie layer is applied directly to the olefinic substrate. The desired coating layer or layers are then applied over the adhesion promoter. Such adhesion promoters typically include a chlorinated polyolefin. Adhesion promoters with chlorinated polyolefins are, however, expensive to use, often exhibit instability, and, if used in a clear (unpigmented) composition, produce colored, hazy films. In addition, the chlorinated polyolefin materials may produce coatings with poor exterior durability because they are susceptible to degradation when exposed to UV light. Another method that has been used to prepare an olefinic substrate to receive a coating layer is chemical modification of the substrate surface, for example by flame or corona pretreatment.
Recently, coating compositions that include significant concentrations of adhesion promoting agents in order to achieve good adhesion to olefinic substrates have been proposed. Published international application WO 97/35937 describes a composition that includes 5-45% by weight of resin solids of a substantially saturated polyhdroxylated polydiene polymer having terminal hydroxyl groups. International Publication Number WO 97/35937 and all of the references cited therein are hereby incorporated herein by reference. The international publication '937 discloses that such polymers are the hydrogenated product of dihydroxy polybutadiene produced by anionic polymerization of conjugated diene hydrocarbon capped with two moles of ethylene oxide and terminated with two moles of methanol. (The ethylene oxide produces the oxygenated anion, and the methanol provides the hydrogen cation to form the hydroxyl group.)
The large amount of the hydrogenated polyhydroxylated polydiene polymer adhesion promoting agent that must be included may adversely affect physical properties and appearance of the resulting coating. In addition, compositions that include significant concentrations of the adhesion promoting agent may separate into phases because the different components frequently are not very compatible. The '937 reference requires a specific solvent package that may be undesirable in many instances. The same problems are encountered with other prior art adhesion promoting agents such as chlorinated polyolefins. It is also known that including chlorinated polyolefins in some coating compositions, e.g., curable coating compositions that include acid catalysts, can result in adverse interactions between the different components of the coating composition.
Yamamoto et al., U.S. Pat. No. 4,874,818 discloses a primer composition for plastic and steel substrates that includes a polycaprolactone graft of a styrene-butadiene-styrene block copolymer or a hydrogenated styrene-butadiene-styrene block copolymer. The primer composition is applied to the desired substrate as a solventborne primer coating composition having at least 50% by weight organic solvent, generating unwanted emissions.
Co-pending patent application Ser. No. 09/143,156 describes a coating composition for olefinic substrates that includes an olefin block that is substantially saturated and at least one (poly)ester or (poly)ether block. The olefin-based block copolymer can be prepared by reacting a low molecular weight hydroxyl-functional, saturated or substantially saturated olefin polymer with a chain-extension reagent that is reactive with hydroxyl groups and will polymerize in a head-to-tail arrangement of monomer units. The olefin-based block copolymer of the invention provides excellent adhesion either used as an adhesion promoter layer or as an additive in a curable coating composition, even when used in very minor amounts compared to the amounts required for previously known agents.
One problem in coating plastic parts such as TPO has been the expense and space required to install and maintain a paint shop. The expense of a paint shop includes not only the cost of equipment for applying and curing the coating, but also abatement equipment and other expenses relating to regulatory compliance with extensive government rules for coatings operations. Such expenses are difficult for a smaller manufacturer to bear.
For these reasons, it would be desirable to provide a free paint film that can readily be laminated onto the article to be painted to produce a laminated article without the emissions problem.
Harvey, U.S. Pat. No. 5,389,178 discloses applying with heat and pressure a film of a crosslinked rubbery polymer to a polyolefin sheet or film that is not rubber modified. The crosslinked rubbery polymer is produced by a multi-stage reaction in which the first stage is formed predominantly from butyl acrylate and the second stage is formed predominantly from methyl methacrylate. The laminate is mechanically separated so that a thin layer of the crosslinked rubbery material remains on the surface of the polyolefin sheet or film. The modified polyolefin sheet or film is then laminated with a (meth)acrylic sheet or with a latex, solvent-containing paint, or ink. The Harvey method provides improved adhesion but still requires application of the latex or paint in its usual form. Moreover, the film must be separated and removed to leave the tie layer as a by-product of the process. It is a further drawback of the process that the method leaves small surface holes that produces a matte surface, making the process unworkable when a glossy finish is desired.
Marrs, U.S. Pat. No. 3,666,701 discloses a laminating adhesive having an A-B structure, with A being a block of polymerized, conjugated dienes and B being a polylactone block. The laminati

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