Reexamination Certificate
2001-07-25
2002-10-29
Cameron, Erma (Department: 1762)
Reexamination Certificate
active
06471185
ABSTRACT:
FIELD OF INVENTION
This invention generally relates to two-pack coating compositions used in automotive coating applications, particularly in automotive refinish applications. More particularly, this invention relates to a coating composition that cures in two stages under ambient or low bake conditions.
BACKGROUND OF THE INVENTION
Solvent-based coating compositions generally include a binder polymer and a crosslinking agent, which cross link upon application to produce coatings having excellent coating properties. One problem associated with such coating compositions is the relatively longer time required to cure these compositions. Such longer cure times cut down the productivity in automotive refinish shops by requiring the automobile or truck to remain for a longer period of time in the area in which it was spray coated. A rapid initial cure is thus desirable to produce a coating that can be readily sanded or buffed without fouling sandpaper. Such a rapid initial cure permits the user to readily remove coated automobile or trucks bodies out of the spray booths and allow them to fully cure at other convenient locations. As a result, productivity of coating autobodies can be improved substantially.
Attempts have been made to decrease the curing time of these two pack-coating compositions by using more reactive components or catalysts. However, while the use of such components decreases curing time, the higher reactivity of such components usually reduces the time to gelation or pot life of the coating composition as well. In the most extreme case, instant gelation of the composition can occur when, for example, the polyisocyanate is mixed with a polymer having reactive amine groups. Thus, a need still exists for a two-pack coating composition that cures rapidly while still having an acceptable pot life.
Attempts have been made to provide for a coating composition that cures rapidly after application. A commonly assigned U.S. Pat. No. 5,859,136 discloses a coating composition containing a dispersed core-shell acrylic polymer having a core polymerized from ethylenically unsaturated monomers containing amine functional groups. These amine groups positioned in the core are not readily available to react with the crosslinking agent, such as an isocyanate, which has to penetrate the shell to reach the core to crosslink with amine functionality. Applicants have come up with a novel alternate approach to reduce the cure time of the coating composition while still providing acceptable pot life and coating properties.
SUMMARY OF THE INVENTION
The present invention is directed to a two-stage cure coating composition comprising a binder and a crosslinking component,
said binder component comprising an acrylic polymer having in the range of from 5.0 weight percent to 70.0 weight percent of hydroxyl moieties and in the range of from 0.5 weight percent to 8.0 weight percent of secondary amine moieties, all percentages based on the weight of binder component solids; and
said crosslinking component comprising a crosslinking agent having at least two isocyanate groups.
The present invention is also directed to a method of producing a coating on a substrate, said method comprising:
mixing a binder component and a crosslinking component of a two-stage cure coating composition to form a pot mix, wherein said binder component comprises an acrylic polymer having in the range of from 5.0 weight percent to 70.0 weight percent of hydroxyl moieties and in the range of from 0.5 weight percent to 8.0 weight percent of secondary amine moieties, all percentages based on the weight of binder component solids; and wherein said crosslinking component comprises a crosslinking agent having at least two isocyanate groups;
applying a layer of said pot mix over said substrate;
first stage curing said layer, wherein said layer has a Persoz hardness of at least 30 within 2 hours after said application;
second stage curing said first stage cured layer into said coating on said substrate.
One of the advantages of the coating composition of the present invention is that a coating resulting therefrom has rapid first stage cured state (defined below) obtained under ambient conditions when compared against the coatings obtained from conventional coating compositions.
Another advantage of the present invention is that a coating resulting therefrom still provides excellent second stage cured state (defined below) necessary to achieve desired long term coating properties, such as etch and mar resistance.
Still another advantage of the present invention is that it requires substantially low amounts of solvent needed for efficient spray application, thus enabling the formulator, especially in the United States, to meet the increasingly stringent legal requirements that deal with the acceptable level of release of VOC (volatile organic content) in the atmosphere.
Still another advantage of the coating composition of the present invention is its extended pot life as compared to conventional rapid curing coating compositions.
The novel coating composition of the present invention also advantageously permits a formulator to select from a wider selection of other film forming polymers that can be incorporated in the coating composition.
DETAILED DESCRIPTION OF THE INVENTION
As defined herein:
“Two-pack coating composition” means a thermoset coating composition comprising two components stored in separate containers. These containers are typically sealed to increase the shelf life of the components of the coating composition. The components are mixed prior to use to form a pot mix. The pot mix has a limited pot life typically of minutes (about 30 minutes to 60 minutes) to a few hours (1 hour to 2 hours). The pot mix is applied as a layer, typically through a spray nozzle, of desired thickness on a substrate surface, such as an autobody. After application, the layer is cured under ambient conditions or bake cured at elevated temperatures to form a coating on the substrate surface having desired coating properties, such as high gloss, mar-resistance and resistance to environmental etching.
“GPC weight average molecular weight” and “GPC number average molecular weight” means a weight average molecular weight (Mw) and a number average molecular weight (Mn), respectively measured by utilizing gel permeation chromatography. A high performance liquid chromatograph (HPLC) supplied by Hewlett-Packard; Palo Alto, Calif. was used. Unless stated otherwise, tetrahydrofuran was used as the liquid phase and polystyrene was used as the standard.
“Polydispersity” of a polymer is a ratio of Mw to Mn.
“(Meth)acrylate” means methacrylate and acrylate.
“Polymer solids” or “composition solids” means a polymer or composition in its dry state.
“First stage cure” under ambient conditions occurs when a layer of a coating composition upon application cures sufficiently within a very short duration of less than two hours to produce a surface that can be readily buffed or sanded without fouling the sanding paper. Such a first stage cured layer generally has a Persoz hardness of about 30 or more, preferably about 35 or more and most preferably in the range of about 40 to about 100. The procedure used for determining the Persoz hardness is described in the Examples section below.
“Second stage cure” of the first stage cured layer under ambient conditions occurs within several days (generally about 2 days to about 10 days) to produce a fully cured coating that is tough and hard and has the desired coating properties, such as etch and mar resistance. If desired, the second cure time can be shortened to about 12 hours to 48 hours by bake curing it for about 60 to 15 minutes at about 60° C. to 80° C.
The novel composition of this invention has an excellent pot life of about 1 to 2 hours. A layer from the novel composition of this invention dries rapidly to the first stage cure at ambient temperatures. Thereafter, within a few days, the first stage cured layer fully cures (the second stage cure) at ambient temperatures to produce a hard tough coating. This is partic
Lamb Douglas Matt
Lewin Laura Ann
Scopazzi Christopher
Cameron Erma
Deshmukh Sudhir G.
E. I. du Pont de Nemours and Company
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