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
2002-09-17
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S162000, C525S406000, C525S438000, C525S440030, C525S934000
Reexamination Certificate
active
06451928
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a crosslinking agent based on aminotriazine compounds and to powder coating compositions containing these crosslinkers.
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. The relatively high Tg of such polymers provides powder coatings having good 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 heretofore have been 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 crystalline 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.
U.S. Pat. No. 3,759,854 discloses heat-fusible powder coating compositions prepared by pre-reacting a thermosetting polyester resin and a suitable crosslinking resin such as a condensation product of an aldehyde with melamine, urea or benzoguanamine. The reaction product typically is prepared from 1 to 50 weight percent of the amine-aldehyde material and the reaction is carried to an extent such that the resulting powder has a softening point of at least 65° C.
U.S. Pat. No. 5,302,462 discloses a process for preparing a partially cured, but non-gelled, powder coating composition. The powder coating composition is prepared by reacting a less than stoichiometric amount of methoxymethyl aminotriazine with a linear, hydroxyl-terminated polyester. The ratio of polyester to triazine ranges from 97:3 to 70:30 by weight. Methanol is stripped from the reaction mixture under reduced pressure. The powder coating composition provides a cured film free of surface defects caused by outgassing of methanol which volatilizes during the curing process.
U.S. Pat. No. 3,980,732 discloses a process for preparing a curable powder resin composition having a sintering temperature above 40° C. The method comprises condensing a methylolamino compound with an aliphatic alcohol and an aliphatic diamide to produce an aminoplast condensate with a T
g
ranging from −10° C. to 100° C. and blending the aminoplast condensate with an acrylic or polyester resin having a glass transition temperature ranging from 60° C. to 100° C. The methylolamino compound is selected from methylolureas and methylolaminotriazines and contains no more than one unmethylolated NH bond per molecule. At least half of the methylol groups of the methylolamino compound have been condensed with the aliphatic alcohol or the aliphatic diamide. The molar ratio of the aliphatic alcohol to the methylolamino compound is at least 2; and the molar ratio of the methylolamino compound to the aliphatic diamide ranges from 1:0.5 to 1:1.5.
U.S. Pat. No. 4,185,045 discloses a powder coating composition comprising a solid crosslinking agent having a softening point ranging from 50° C. to 120° C. and prepared by heating 40 to 75% by weight of an acrylic polyol and 60 to 25% by weight of an alkoxyaminotriazine at 50° to 120°, and a base resin having a softening point ranging from 60° C. to 130° C. The alkoxyaminotriazine has less than 1 non-methylolated NH bond per triazine ring and at least 80% of the methylol groups have been etherified with an aliphatic or alicyclic alcohol or ethylene glycol monoalkyl ether.
U.S. Pat. No. 4,230,829 discloses a solid crosslinking agent having a softening point of 50° C. to 120° C. and prepared by heating 40 to 70% by weight of a polyester polyol and 60 to 30% by weight of an alkoxyaminotriazine. The alkoxyaminotriazine has one or less non-methylolated NH bond per triazine ring and at least 80% of the methylol groups have been alkoxylated with an alcohol. Powder coating compositions are prepared from a mixture of the crosslinking agent and a base resin having a softening point of 60° C. to 130° C.
While the above-described prior art aminoplast-based crosslinkers for powder coating compositions provide some improvement in “gassing” and powder stability over their liquid aminoplast counterparts, the powder coating compositions containing these crosslinkers can, nonetheless, exhibit some of the aforementioned deficiencies. In addition, many of the crosslinkers disclosed in the prior art are high molecular weight, high viscosity, partially cured, and, thereby, unstable mixtures. Thus, there remains a need for an aminoplast crosslinking agent suitable for use in powder coating compositions which provides a storage stable powder composition having the desirable coating properties usually associated with aminoplast-based liquid coatings without causing coating surface defects due to “gassing”.
SUMMARY OF THE INVENTION
In accordance with the present invention, provided is a crosslinking agent comprising the ungelled reaction product of (a) a polyester polyol and (b) an aminotriazine compound having one or less non-alkylated NH bond per triazine ring. The polyester polyol (a) comprises the reaction product of (i) a cycloaliphatic polyol and (ii) a cyclic polycarboxylic acid or anhydride. The crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 10° C.
Also provided is a method for preparing the aforementioned crosslinking agent. The method comprises the steps of (I) combining the previously described polyester polyol and an aminotriazine compound having one or less non-alkylated NH bond per triazine ring to form a reaction admixture; (II) heating the reaction admixture to a temperature ranging from 90° C. to 135° C.; and (III) maintaining that temperature for a time sufficient to obtain an ungelled reaction product having a glass transition temperature of at least 15° C. which is essentially free of hydroxyl functionality as determined by infrared spectroscopy.
Further provided is a curable powder coating composition comprising a solid particulate film-forming mixture of (A) a polymer containing reactive functional groups and having a T
g
of at least 30° C. and (B) the crosslinkin
Ambrose Ronald R.
Chasser Anthony M.
Hu Shengkui
Schneider John R.
Smith Jackie L.
Altman Deborah M.
Meyers Diane R.
PPG Industries Ohio Inc.
Short Patricia A.
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