Stock material or miscellaneous articles – Composite – Of polyamidoester
Reexamination Certificate
2000-07-12
2003-09-09
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of polyamidoester
C428S424800, C428S500000, C428S516000, C428S519000, C428S520000
Reexamination Certificate
active
06617033
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method for coating a thermoplastic work piece with an in-mold composition. More particularly, the present invention relates to a process for in-mold coating of thermoplastic work pieces, such as polyolefin work pieces made by injection molding, one step of which comprises injecting a thermoset in-mold coating into the mold after the polyolefin substrate is solidified, or partially solidified, to provide a polyolefin work piece having a topcoat with excellent adhesion and requisite surface qualities. The polyolefin work pieces of the present invention require fewer exterior protective coatings or additional steps to prepare the work piece surface and, depending on the in-mold coating selected, may be suitable for use as is in an end use application. The present invention also relates to a polyolefin work piece having in-mold thermoset coatings bonded thereto.
BACKGROUND OF THE INVENTION
Molded thermoplastic materials, including polyolefins, are used in a variety of applications, such as the transportation, automotive, marine, recreation, construction, office products, and lawn and garden equipment manufacturing industries. Their use, however, is not without problems. In many instances, molded thermoplastic work pieces may need to be coated to facilitate paint adhesion, or to satisfy other surface property requirements, such as durability and weather resistance. Because of the inherent low surface energy of thermoplastics, generally, and in particular, polyolefins, they are difficult to paint or coat. Moreover, in view of the variation among the surface properties of individual polyolefins and the coating compositions to be applied, a method that works with one specific thermoplastic may not work with another. Hence, a variety of methods have been developed to achieve adhesion of coatings to the surface of molded thermoplastics materials, including materials such as polyolefins.
One of the most common methods is to micro-etch the surface of the thermoplastic to generate micro-roughness that will provide adhesion-anchoring sites for the paint or other top and primer coatings. Etching may be done by solvents, which may be incorporated in the paint or coating being applied. The selection of solvent is critical because different solvents etch thermoplastics at different rates. Both over-etching and under-etching must be avoided. Insufficient etching does not provide proper adhesion; excessive etching can damage the thermoplastic. Excessive etching, exposing the coating to bleeding from the thermoplastic, or exposing the thermoplastic to attack by the solvent may warp thermoplastic parts. If thermoplastics have areas that are highly stressed by the molding process, use of etching solvents can form visible cracks in these areas.
Another method of preparing the surface of a thermoplastic part for painting or coating is through de-glazing. When some thermoplastics are molded, a highly crosslinked (glazed) skin is formed which is resistant to solvent etching. Tumbling with a moderately abrasive media, or blasting with a mildly aggressive grit material, may de-glaze the thermoplastic surface sufficient to allow satisfactory adhesion of the paint or coating.
Creating micro-roughness through etching or de-glazing may not be desirable and, in some instances, not effective, depending on the particular thermoplastic surface involved. Other methods to prepare a thermoplastic surface utilize a chemical reaction to create polar oxidized groups on the thermoplastic surface. These surface polarizing methods include coating with an adhesion promoter, or subjecting the polyolefin work piece to flame or plasma treatment, in order to make the thermoplastic surface chemically polar so it will bond with the coating. Low polarity thermoplastics can also be oxidatively surface treated using light sensitive chemicals called photosensitizers, followed by exposure to ultraviolet light. UV light cracks the molecules of the photosensitizers for form free radicals. Free radicals are extremely reactive species that combine with oxygen in the air. Oxygen free radicals, in turn, react with the thermoplastic to produce polar groups on the thermoplastic surface.
Previously, molded thermoplastic work pieces were formed in a mold, the molded product removed, and a coating was then applied on the surface of the molded work piece by a coating process, such as a surface treatment, primer coating, top coating, painting, etc. Hence, all of the foregoing methods required an additional step to achieve a finished surface on a thermoplastic work piece, which is treating the surface of the pre-formed thermoplastic work piece prior to applying a paint or coating. These methods required additional steps and increased costs of preparing the molded work piece surface.
It became desirable, therefore, to have a method by which a coating could be applied to a thermoplastic work piece in the mold, resulting in a coated thermoplastic work piece the surface of which would be finished and suitable for use as is in an end use application, or which would require less surface preparation treatment than heretofore utilized.
Application of in-mold coatings (IMC) to thermoplastic materials to provide generally smooth surfaces, improve durability and other surface properties, and to reduce or eliminate substrate porosity is known. A number of in-mold coating methods have been employed for applying primer coatings, in compression molding methods or injection molding methods employing molding materials of thermosetting resins, such as SMC (sheet molding compound) and BMC (bulk molding compound) (e.g., U.S. Pat. Nos. 4,076,788; 4,081,578; 4,331,735; 4,366,109; and 4,668,460).
Typical in-mold coatings are set forth in U.S. Pat. No. 4,189,517 and U.S. Pat. No. 4,222,929, which have been applied to fiber reinforced thermoplastics (FRP), such as sheet molding compounds, and which are the reaction products of an unsaturated fumarate polyester diol, a saturated polyester diol flexibilizer, a crosslinking aliphatic polyol, having from 3 to 6 hydroxyl groups, a diisocyanate, and an ethylenically unsaturated crosslinking compound, such as styrene. U.S. Pat. No. 4,331,735 sets forth a liquid crosslinkable composition having an average molecular weight of up to about 5,000 and a plurality of polymerizable ethylenic double bonds, being essentially free of active hydrogen atoms or being essentially free of isocyanate groups; a material such as a polyisocyanate or a reaction product of a polyisocyanate and an ethylenically unsaturated compound having —NH2 groups, —NH and/or —OH groups, said reaction product being free of active hydrogen atoms; and an organic free radical peroxide initiator.
Other coatings relate to those comprising at least one polymerizable epoxy-based oligomer having two acrylate groups thereon, at least one copolymerizable ethylenically unsaturated monomer such as styrene, and at least one copolymerizable monoethylenically unsaturated compound having a —CO— group and a —NH2, —NH—, and/or —OH group, as well as polyvinyl acetate, as set forth in U.S. Pat. No. 4,414,173 and U.S. Pat. No. 4,515,710 to Cobbledick et al.
Still other coatings include a conductive, thermoset in-mold coating for molded FRP parts, the binder of which comprises at least one polymerizable epoxy-based oligomer having at least two acrylate groups and at least one copolymerizable ethylenically unsaturated monomer, which provides good flow and coverage during molding, good adhesion, uniform color, good surface quality, and good paintability, as set forth in U.S. Pat. No. 5,614,581. Still other in-mold coatings include free radical peroxide initiated thermosetting compositions comprising an epoxy-based oligomer having at least two acrylate end groups and a hydroxy or amide-containing monomer, as set forth in U.S. Pat. Nos. 5,391,399; 5,359,002; and U.S. Pat. No. 5,084,353 to Cobbledick et al.
In-mold coating compositions, which have appearance or paint-like properties, are also known. Appearance in-mold coating composition
McBain Douglas S.
Straus Elliott J.
Burleson David G.
Fay Sharpe Fagan Minnich & McKee
Nakarani D. S.
OMNOVA Solutions Inc.
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