Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-09-21
2002-08-27
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C528S085000, C524S590000, C524S904000, C524S928000
Reexamination Certificate
active
06441103
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a crosslinking agent prepared from a urethane group-containing adduct and an aminotriazine compound and to powder coating compositions containing such a crosslinking agent.
BACKGROUND OF THE INVENTION
In recent years, powder coatings have become increasingly popular because these coatings are inherently low in volatile organic content (“VOC”), which significantly reduces emissions of volatile organic compounds into the atmosphere during application and curing processes.
Hydroxyl, carboxyl, carbamate and/or epoxy functional resins, such as acrylic and polyester resins having relatively high glass transition temperatures (“Tg”), are commonly used as main film-forming polymers for these coatings. Because acrylic polymer systems can be more heat-resistant than condensation polymers, they can provide powder coating compositions having improved storage stability. However when exposed to the extreme temperatures which can be encountered during shipping and/or storage in many geographic areas, even better powder coating stability is desired.
Aminoplast resins are well known in the art as low cost crosslinking agents for hydroxyl, carboxyl and/or carbamate functional polymers in conventional liquid coating compositions. Common aminoplast resins are based on condensation products of formaldehyde with an amino- or amido-group carrying substance. Examples of these aminoplast resins include the methylol and alkoxymethyl derivatives of ureas, melamines and benzoguanamines which are most commonly used in liquid coating compositions. Such aminoplast resins provide enhanced coating properties such as exterior durability, chemical resistance and mar resistance.
Attempts to produce powder coating compositions based on conventional aminoplast resins which exhibit these desirable properties have been largely unsatisfactory because these materials are typically in liquid form and, as such, cause poor powder stability.
The methoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in powder coating compositions. Although solid in form, these materials nonetheless can depress the Tg of the powder coating composition significantly, even when combined with high Tg film-forming polymers such as the acrylic polymers described above. Such a depression in Tg also can result in poor powder stability.
Moreover, the use of conventional aminoplast resins in powder coating compositions can result in the phenomenon commonly referred to as “gassing”. “Gassing” occurs as a result of vaporization of the alcohol generated in the thermally induced aminoplast crosslinking reaction. The alcohol vapor is driven off through the coating film upon heating and, as the viscosity of the coating increases during the curing process, pinholes or craters are formed as the gas escapes through coating surface.
Carbamate functional polymers, that is, polymers having reactive pendent and/or terminal carbamate functional groups, are well known in the art as suitable film-forming resins for liquid coating systems where, for example, when combined with an aminotriazine curing agent, they provide coatings having excellent acid etch resistance. The carbamate N—H groups react readily with the methoxyl groups of the aminotriazine curing agent, thereby forming a urethane linkage which provides this acid etch resistance. These carbamate functional polymers further provide coatings which have excellent durability and adhesion properties.
Copending U.S. patent application Ser. No. 09/538,836 discloses powder coating compositions comprising a solid particulate mixture of a carbamate functional polymer, for example an acrylic, polyester and/or polyurethane polymer, in conjunction with a glycoluril resin. Due to the high glass transition temperature of the carbamate functional polymer, the powder coating compositions provide improved storage stability as well as coatings having excellent acid etch resistance. However, as discussed above, in some powder coating systems, the glycoluril crosslinking agent can depress the T
g
sufficiently to adversely affect powder stability.
It would, therefore, be advantageous to provide an aminotriazine-based crosslinking agent suitable for use in a powder coating composition which gives a highly stable powder as well as an acid etch resistant coating free of pinholes or crater resulting from “gassing” during the curing process.
SUMMARY OF THE INVENTION
In accordance with the present invention, two types of aminotriazine-based crosslinking agents are provided. The first type of crosslinking agent (“crosslinking agent I”) comprises the ungelled reaction product of (A) a reactive urethane group-containing reaction product and (B) an aminotriazine compound having one or less non-alkylated NH bond per triazine ring. The reactive urethane group-containing reaction product (A) comprises the reaction product of (1) a polyester polyol, and (2) a mono-isocyanate. The crosslinking agent is essentially free of urethane NH functionality and has a glass transition temperature of at least 25° C.
The present invention also provides a method for preparing the crosslinking agent I described above. The method comprises the steps of (1) reacting (a) a polyester polyol, and (b) a mono-isocyanate in a molar ratio of reactant (a) to reactant (b) ranging from 1:1.8 to 2.0 to form a urethane group-containing reaction product; (2) combining the reaction product of step (1) with an aminotriazine compound having one or less non-alkylated NH bond per triazine ring in a molar ratio of urethane group-containing reaction product to aminotriazine compound ranging from 1:1.5 to 3.2 to form a reaction admixture; and (3) heating the reaction admixture of step (2) to a temperature ranging from 95° C. to 135° C. for a time sufficient to form a powder crosslinking agent having a glass transition temperature of at least 25° C. which is essentially free of urethane NH functionality as determined by infrared spectroscopy.
Further provided is a powder coating composition comprising a solid particulate mixture of a reactive group-containing polymer having a T
g
of at least 30° C., and the crosslinking agent I described immediately above.
The second type of crosslinking agent (“crosslinking agent II”) comprises the ungelled reaction product of (A) a reactive urethane group-containing reaction product and (B) an aminotriazine compound having one or less non-alkylated NH bond per triazine ring. The reactive urethane group-containing reaction product (A) comprises the reaction product of (1) a polyisocyanate and (2) a monohydric capping agent. The crosslinking agent is essentially free of urethane N—H functionality and has a glass transition temperature of at least 10° C.
The present invention also provides a method for preparing the crosslinking agent II. The method comprises the steps of (1) reacting the following reactants: (a) a polyisocyanate, and (b) a monohydric capping agent, wherein the ratio of NCO equivalents of reactant (1) to OH equivalents of reactant (2) ranges from 0.8 to 1.0:1, to form a urethane group-containing reaction product; (2) combining the reaction product of step (1) with an aminotriazine compound having one or less non-alkylated NH bond per triazine ring in a molar ratio of urethane group-containing reaction product to aminotriazine compound ranging from 1.0:2.0 to 4.0 to form a reaction admixture; and (3) heating the reaction admixture of step (2) to a temperature ranging from 95° C. to 135° C. for a time sufficient to form a powder crosslinking agent having a glass transition temperature of at least 10° C. which is essentially free of urethane NH functionality as determined by infrared spectroscopy.
Further provided is a curable composition comprising a film-forming polymer having reactive functional groups and the crosslinking agent II described immediately above. Also provided is a curable powder coating composition comprising a solid particulate mixture of a reactive group-containing
Ambrose Ronald R.
Chasser Anthony M.
Hu Shengkui
Schneider John R.
Smith Jackie L.
Altman Deborah M.
Meyers Diane R.
PPG Industries Ohio Inc.
Yoon Tae H.
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