Low-cure powder coatings and methods for using the same

Details Low-cure powder coatings and methods for using the same Low-cure powder coatings and methods for using the same

2002-05-31

2004-05-18

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Composite

Of epoxy ether

C428S413000, C428S423100, C427S386000, C427S388100, C528S061000, C528S211000, C528S905000, C525S423000

Reexamination Certificate

active

06737163

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to powder coating compositions; more particularly, the present invention relates to low temperature cure thermosetting powder coating compositions. The compositions consistently produce coatings having desirable performance properties when cured, and that are stable when uncured.
BACKGROUND OF THE INVENTION
Coating compositions have long been used to provide the surface of articles with certain desired physical characteristics, such as color, gloss and durability. Many coating compositions rely on a liquid carrier, which evaporates after the composition is applied. In recent years, powder coatings have become increasingly popular; because these coatings are inherently low in volatile organic content (VOCs), their use reduces air emissions during the application and curing processes as compared with liquid coatings.
Powder coatings are typically cured by heating the coated substrate to an elevated temperature. These temperatures almost always exceed 125° C., and commonly reach about 190° C. to 205° C. During the curing process, the powder particles melt and spread, and the components of the powder coating react. In addition to not emitting any VOCs into the environment during the application or curing processes, powder coating systems are extremely efficient since there is essentially no waste (i.e., application yield is approximately 100 percent). Because of the relatively high (i.e., greater than 125° C.) cure temperatures of most powder coatings, their use, for practical purposes, is often limited to substrates that can withstand such high temperatures or that can be heated to an appropriate temperature long enough for cure to occur.
Despite the desirability of low-cure powder compositions, two problems have prevented their widespread production and use—their mechanical stability and their chemical stability. Powders that use resins with a glass transition temperature (“Tg”) lower than 60° C. usually encounter package stability problems, especially if exposed to prolonged heat exposure, and become fused, sintered or clumpy within days. Similarly, prolonged heat exposure can destroy the chemical stability of a powder if it includes crosslinkers that react at temperatures below about 170° C.; if a crosslinker with a lower cure temperature is used, cure may be initiated during storage even though the film has not been formed. The premature gelation that occurs in these powder formulations results in coatings having shortened gel times. It is not unusual for low-cure powders to lose >50 percent of their gel time as a result of the premature gelation.
Problems encountered when a powder loses either mechanical or chemical stability can be severe. Poor mechanical stability creates obvious handling, application and appearance issues. Poor chemical stability creates subtler yet just as problematic issues. For example, a powder that has poor chemical stability will fluidize and apply like virgin powder, but because it has advanced in reactivity (i.e. undergone some premature gelation), it demonstrates restricted flow or no flow at all during cure. The result can be a coating having an “orange peel” appearance, a rough texture or gel bodies.
Ideally, a powder should not lose its handling properties under elevated temperature storage and the gel time should remain the same as that of the virgin material. To achieve this, powders are typically formulated with resins having a Tg greater than about 60° C. and/or crosslinkers that react at temperatures of about 170° C. or greater. Such powders, however, are not low cure. Low-cure powders having lower Tg resins or lower temperature crosslinkers can require expensive storage under refrigeration and air-conditioned application facilities to overcome inherent lack of stability, or must be prepared using special techniques.
Thus, there is a need in the coatings art for low-cure powder coatings having a wide range of application, which also have an acceptable level of durability when cured on the finished product and good stability at room temperature.
SUMMARY OF THE INVENTION
The present invention is directed to powder coating compositions generally comprising a tertiary aminourea compound, a tertiary aminourethane compound, or mixtures thereof, and a film-forming polyepoxide resin. It has been surprisingly discovered that polyepoxide resins, when used with the present tertiary aminourea and/or aminourethane compositions, cure to form a suitable coating without the aid of crosslinkers, accelerators, or other additives typically regarded in the art as being necessary to cure a polyepoxide resin. The cured coatings that result from the present compositions have performance properties that are at least as good as powder coating compositions prepared with the same polyepoxides and conventional curing agents, but lacking the tertiary aminourea or aminourethane compositions described herein. Significantly, this desirable result is achieved by using curing temperatures much lower than those used for conventional products. Accordingly, the present compositions are low-cure. “Low-cure” as used herein refers to powder coating compositions that cure at a temperature between about 80° C. and 125° C. However, the present invention is not limited to this temperature range and also provides cured films at temperatures up to and even greater than 190° C.
As a result of being low-cure, the present compositions can be used on substrates that are not appropriately exposed to temperatures greater than about 125° C. Examples include, but are not limited to, plastics such as thermoset and thermoplastic compositions, wood, and pieces of thick metal that cannot be heated above about 95° C. because of their size. Also suitable are articles of manufacture that include a variety of substrates; for example, motors that contain both metal and rubber components can be suitably coated using the present, low-cure powder compositions.
The present compositions also overcome some of the difficulties that have been observed with other powder coating compositions, particularly other low-cure powders. For example, the present powder compositions are storage stable, and reduce, if not eliminate, the problems with chemical and mechanical stability seen with other low-cure powder compositions. The present compositions can be stored at room temperature, and they do not continue to catalyze the reaction of the polyepoxide molecules after the removal of heat. Moreover, the present powder compositions can be prepared using standard methods known in the art for preparing powder coating compositions; no special processing or handling is needed. Thus, the present compositions provide a significant advance in the low-cure powder coatings art.
Methods for coating substrates using the present powder compositions, and substrates coated thereby, are also within the scope of the present invention. Various low-cure catalysts are also included in the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a powder coating composition comprising: (a) a material having the structure of Formula I:
wherein R
1
is an organic radical having 6 to 25 carbon atoms, R
2
is an organic radical having 1 to 20 carbon atoms; R
3
and R
4
are independently alkyl or phenyl groups having 1 to 8 carbon atoms; Z is oxygen or nitrogen and when Z is oxygen R
5
is absent and when Z is nitrogen R
5
is hydrogen or
and n is 1 to 4; and (b) a polyepoxide. It will be understood that when Z is oxygen a tertiary aminourethane compound is represented and when Z is nitrogen, Formula I depicts a tertiary aminourea compound. It will be further understood that when R
5
is
there will be two each of R
2
, R
3
and R
4
. Each R
2
, each R
3
and each R
4
can be the same or different as the other R
2
, R
3
or R
4
. For example, one R
2
can have one carbon and the other have two carbons, and the like.
The material of Formula I can be an oligomer wherein R
1
is a monovalent, divalent, trivalent or tetravalent organic radical; divalent

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