Stock material or miscellaneous articles – Composite – Of polyamidoester
Reexamination Certificate
2001-07-31
2003-12-16
Gorr, Rachel (Department: 1711)
Stock material or miscellaneous articles
Composite
Of polyamidoester
C428S413000, C428S428000, C428S500000, C544S088000, C544S089000, C544S096000, C528S406000, C528S407000, C525S329900, C525S452000, C525S437000, C525S523000, C525S403000, C525S480000, C525S502000
Reexamination Certificate
active
06663972
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to crosslinking agents based on modified aminoplast resins and to curable powder coating compositions containing such crosslinking agents.
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 (“T
g
”), are commonly used as main film-forming polymers for these coatings. The relatively high T
g
of such acrylic polymer systems provides powder coatings having good storage stability. However, when exposed to the extreme temperatures which can be encountered in many geographic areas during shipping and/or storage even better powder coating stability is desired. By “storage stability” is meant the ability of the individual powder particles which comprise the powder coating to resist the tendency to adhere to one another, thereby causing “clumping” or “fusing” of the powder coating composition upon storage prior to application. Powder coating compositions having very poor storage stability can be difficult, if not impossible, to apply.
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. Condensation products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are most commonly used in liquid coating compositions where they can provide enhanced coating properties such as exterior durability chemical resistance and mar resistance. Such aminoplast resins typically are in liquid form and, as such, generally may not be suitable for use in curable powder coating compositions.
The alkoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in powder coating compositions. Although in solid form, these materials nonetheless can depress the T
g
of the powder coating composition significantly, even when combined with high T
g
film-forming polymers such as the acrylic polymers described above. Such a depression in T
g
also can result in poor powder stability.
Moreover, the use of conventional aminoplast resins in curable powder coating compositions can result in the phenomenon commonly referred to as “gassing”. Gassing can occur 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 can be formed as gases escape through coating surface.
U.S. Pat. Nos. 3,904,623, 4,189,421 and 4,217,377 disclose a solid, non-gelled low molecular weight addition reaction product and a method for preparing the reaction product. The addition reaction product is suitable for use as a crosslinking agent in powder coating compositions when combined with polymers having various reactive functional groups. The crosslinking agent is the reaction product of 1.8 to 2.2 moles of a monohydroxy-, single-ring, aromatic compound, for example, phenol, and 1.0 mole of an alkoxymethyl aminotriazine compound, such as hexakis (methoxymethyl aminotriazine.
U.S. Pat. No. 4,393,181 discloses solid adducts prepared from aminotriazine compounds and a large excess of polyhydric phenols. The adducts, due to their phenolic functionality, are useful as crosslinking agents for epoxy resins in powder coating compositions when used in conjunction with a curing agent accelerator such as an imidazole or benzimidazole.
U.S. Pat. No. 3,759,854 discloses heat-fusible powder coating compositions comprising a crosslinker prepared by pre-reacting a thermosetting polyester resin and a suitable conventional aminoplast crosslinking resin such as a condensation product of an aldehyde with melamine, urea or benzoguanamine.
U.S. Pat. No. 5,302,462 discloses a similar process for preparing a partially cured powder coating crosslinker. The crosslinker is prepared by partially reacting a less than stoichiometric amount of methoxymethyl aminotriazine with a substantially linear, hydroxyl-terminated polyester.
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 partially reacting 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.
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.
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.
While the above-described prior art aminoplast-based crosslinkers for powder coating compositions provide some improvement for 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, most of the crosslinkers disclosed in the prior art can be high molecular weight, high viscosity and/or partially cured and, hence, unstable mixtures. Further, the crosslinkers described in U.S. Pat. Nos. 3,904,623, 4,189,421, and 4,217,377 can contain a significant amount of unreacted phenol impurity, which can significantly limit their use in the powder coatings industry.
Thus, there remains a need for an aminoplast crosslinking agent suitable for use in curable 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, a crosslinking agent having reactive benzoxazine groups is provided. The crosslinking agent comprises an ungelled reaction product of (A) one or more aminoplast resins, (B) one or more polyhydric aromatic compounds; and (C) at least one compound different from (B) having active hydrogen groups reactive with aminoplast resin (A). The compound (C) is selected from at least one of (i) compounds having the following structure (I):
wherein X is aromatic; R
1
, R
2
, and R
3
can be the same or different and each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group, provided that at least one of R
1
, R
2
, and R
3
represents an active hydrogen-containing group which is reactive with the aminoplast resin (A); and (ii) compounds having the following structure (II or III):
where R′ and R″ are the same or different and each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms; and (iii) compounds different from both (i) and (ii) and having a melting point of at least 80° C., and mixtures thereof. The crosslinking agent has a glass transition temperature of at least 10° C. and is essentially free of functional groups which are reactive with aminoplast resin.
T
Ambrose Ronald R.
Chasser Anthony M.
Hu Shengkui
Smith Jackie L.
Altman Deborah M.
Gorr Rachel
Meyers Diane R.
PPG Industries Ohio Inc.
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