Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-09-16
2002-09-24
Morris, Terrel (Department: 1771)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S406000, C523S407000, C523S439000, C525S117000
Reexamination Certificate
active
06455614
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an adhesion-promoting thermoset coating for plastic substrates comprised of polyolefinic polymers. Because of their low surface energy and non-polar nature, such polymers are very difficult to paint with decorative and/or protective coatings. More particularly it relates to adhesion-promoting coatings which are halogen-free. Even more particularly, it relates to an aqueous coating composition having zero VOC (volatile organic compound-free) which improves the adherence of such a polyolefinic article to subsequent coatings when the adhesion-promoting composition is applied to the surface and cured by baking it at an elevated temperature.
The use of polyolefins such as polypropylene and polypropylene containing compounds modified with rubber called thermoplastic olefins (TPO) has grown significantly in areas such as the automotive market since the 1970's because of their low cost, low weight, impact resistance, ease of molding and recyclability. They have been used in interior applications such as airbag covers and exterior ones such as fascia and trim.
The isotactic crystalline structure of polypropylene gives it a low surface energy of anywhere from 20 to 30 dynes/cm, resulting in a surface that most coatings will not wet out or adhere to. This surface must be physically or chemically altered to render it paintable with available coating technology. Plasma treatment and corona discharge employ ionized gases to oxidize the surface of the polypropylene or TPO which raises its surface energy and allows it to chemically and mechanically bond with the coating. However, the surface remains temporarily oxidized and must be coated before the energy lowers again. This process can be difficult and expensive in a production environment when complexly shaped automotive parts are involved. Flame treatment will also oxidize the surface but this requires a precise distance of flame to surface which is again hard to control on a production line that is coating components of different shapes and sizes. TCE (1,1,1 trichloroethane) is used to etch or dissolve the surface of the propylene or TPO but due to volatile organic compound (VOC) and toxicity concerns, this method is being phased out. A more common approach to the painting of polypropylene and TPO is the use of an adhesion promoting primer coating that uses a chlorinated polyolefin resin to gain adhesion to the surface. These coatings are easy to process in a production environment and can be topcoated after a short ambient flash or bake. However, solvent borne chlorinated polyolefin resins have limited solubility and are mainly compatible only with aromatic solvents such as xylene or toluene and must be sprayed at V.O.C.'s of 5.7 pounds per gallon or higher to properly coat the surface of the polypropylene or TPO. This can be a problem for a finisher that is required to comply with strict federal and state emissions standards.
Aqueous dispersions of chlorinated polyolefin resins have been formulated into adhesion promoter coatings with V.O.C.'s ranging from 1.0 to 0 pounds per gallon but the high cost of these materials has been a hindrance to their acceptance in the marketplace, despite the lower V.O.C.'s. The presence of chlorine in these aqueous adhesion promoting compositions is still an environmental concern.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a non-halogenated thermoset adhesion promoting coating for polypropylene-based substrates.
It is a related object of this invention to provide a zero VOC coating composition containing a non-halogenated resin which promotes adhesion of a subsequent coating to a polypropylene-based substrate.
It is a further related object of this invention to provide a zero VOC coating composition containing a non-halogenated adhesion promoting resin which is capable of sufficiently enhancing the adhesion of coatings on polypropylene based surfaces that such coatings comply with the standards, including those of humidity and gasoline resistance, required for using such coatings on the interior and exterior of automobiles as set by the automotive industry.
It is a still further object to provide such adhesion promoting resins that are relatively easy to formulate, store and use because they are sufficiently stable that they can be completely mixed to the composition in which they are intended to be applied and then stored in a single package for extended periods prior to use.
It is a related object of this invention to provide an article having a polypropylene-based surface which is coated with a zero VOC coating composition containing a non-halogenated resin which promotes adhesion of a subsequent coating to the surface.
It is a related object of this invention to provide an article having a polypropylene-based surface which is coated with an aqueous, zero VOC coating composition containing a non-halogenated resin which, when dry, promotes adhesion of a subsequent coating to the surface.
It is a related object of this invention to provide a polypropylene-based article having a coating which promotes adhesion of a subsequent coating in conditions of high humidity and when in contact with gasoline.
These and other objects which will become apparent from the following description thereof are provided by an article having a polyolefin resin surface which is coated with an aqueous composition comprising:
(a) a halogen-free copolymer of a higher &agr;-olefin and at least one monomer selected from the group consisting of ethylene and propylene, said copolymer having an &agr;, &bgr;-unsaturated dicarboxylic acid or anhydride grafted thereon;
(b) a base;
(c) an acid catalyst, and
(c) a water-dispersible resin containing at least two epoxy groups.
Other polymers, including acrylics, polyurethanes, and melamines, to name a few, can be blended with the above mentioned composition. These resins are used to improve the physical properties of the system. In addition, pigments and additives can be employed to provide desired aesthetic or functional effects. The polyolefin surface may be that of an article made from a polyolefin or it may be that of a film on an article made from another material. The articles of particular interest for the purposes of this invention are polypropylene-based automotive parts that may be exposed to high humidity and may be subject to repeated contact with gasoline.
DETAILED DESCRIPTION OF THE INVENTION
The olefin copolymer of component (a) in the aqueous coating composition of the invention is a random copolymer or block copolymer of the ethylene and/or propylene with a higher &agr;-olefin. Propylene/&agr;-olefin copolymers suitable for use in this invention are available from Eastman Chemical Products, Inc. and from the Rexene Company. Ethylene/&agr;-olefin copolymers suitable for use in this invention are available from Union Carbide under the trademark FLEXIMERS. They are exemplified by ethylene/butene, ethylene/hexene, and ethylene/butene/hexene copolymers. A random copolymer of a higher &agr;-olefin and propylene is preferred and the proportion of the propylene component is preferably not less than 10 weight percent. If it is less than 10 weight %, then adhesion to polypropylene is poorer, which is, of course, undesirable. Examples of the higher &agr;-olefins include, butene-1, pentene-1, and hexene-1. Among these, hexene-1 is preferable from the standpoint of physical properties of the polymer. Hexene-1 levels may range from 10 to 80 wt % but a preferable level is approximately 60 weight percent of the propylene/&agr;-olefin copolymer composition. Mixtures of two or more of the higher &agr;-olefins may be copolymerized with ethylene or propylene or with a mixture of ethylene and propylene.
The &agr;,&bgr;-unsaturated dicarboxylic acids or their acid anhydrides to be graft copolymerized onto the propylene/&agr;-olefin copolymer are exemplified by maleic acid, fumaric acid, itaconic acid, citraconic acid, allylsuccinic acid, mesaconic acid and aconitic acid, and their aci
Jackson Michael L.
Mecozzi Joseph M.
Miklos David J.
Neymark Alexander L.
Stubbs Frank A.
Guarriello John J.
Morris Terrel
Rohm and Haas Company
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