Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Reexamination Certificate
2001-05-29
2003-04-01
Sergent, Rabon (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
C525S440030, C528S073000, C528S080000, C528S081000, C528S083000
Reexamination Certificate
active
06541595
ABSTRACT:
The present invention relates to powder coating compositions which can be cured at low temperatures either with or without the use of a urethane catalyst. ore particularly, the present invention relates to branched hydroxyl terminated oligoesters which when crosslinked provide improved performance properties at low curing temperatures and which do not release blocking agents from crosslinker into the environment.
BACKGROUND OF THE INVENTION
Thermosetting powder coating compositions are well known in the art and are widely used as coatings for electric appliances, bicycles, garden furniture, accessories for the automotive industry, general metal parts and the like. Thermosetting powders consist of a mixture of a primary resin and one or more crosslinkers, often called hardeners or curing agents. The general approach associated with powder coating technology is to formulate a coating from solid components, mix them, disperse pigments (and other insoluble components) in a matrix of the major binder components, and pulverize the formulation into a powder. In so far as possible, each particle contains all of the ingredients in the formulation. The powder is applied to the substrate, usually but not limited to a metal, and fused to a continuous film by baking.
Compositions which include organic polyhydroxy compounds and blocked to include internal or self-blocked polyisocyanates and which are solid at room temperature, are important binding agents for thermally cross-linkable powder coatings (see for example, U.S. Pat. Nos. 3,857,818 and 4,375,539). Common to these Systems is the disadvantage that, during thermal cross-linking, the compounds used as blocking agents, excluding self/internal blocked agents, are split off and escape into the environment. Therefore, during cure and crosslinking special precautions must be taken to purify the waste air and/or to recover the blocking agent for reasons of ecology and work hygiene.
The elimination of emissions from the curing of powder coatings has been attempted with the use of blocking-agent-free, uretdione-group-containing polyurethane (PUR) powder coating hardeners. In these compositions cross-linking takes place with thermal cleaving of the uretdione groups. (See for example U.S. Pat. Nos. 5,621,064 and 4,413,079). Typically, films produced with these types of uretdione crosslinkers, however, do not have optimal film properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss.
Another problem with powder coating compositions is that they frequently have low glass transition temperatures (T
g
) and will agglomerate or sinter when stored at elevated temperatures for a prolonged duration of time. This phenomena causes an application problem when the powder coating composition taken from storage is agglomerated and requires remilling, which may or may not permit application of a powder coating having a suitable particle size.
It is an object of the invention to provide a powder coating composition which has a relatively high glass transition temperature and that will resist agglomeration during storage.
It is an object of the invention to provide a powder coating composition which will maximize film properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss, yet also provide a coating composition with a relatively high lass transition temperature.
It is another object of the invention to provide a powdered coating composition which can be cured at temperatures as low as about 160° C. without the use of an effective amount of urethane catalyst or at temperatures less than about 160° C. with the use of an effective amount of urethane catalyst such as 1,5-diazabicyclo(4.3.0)non-5-ene, 1,8-diazabicyclo(5.4.0)undec-7-ene, dibutyltin dilaurate, butane stannoic acid, dibutyltin oxide, and others known in the art.
It is yet another object of the invention to provide a thermosetting powder coating composition that includes crosslinker that does not release a blocking agent into the environment upon curing.
It is another object of the invention to provide a powder coating composition with 0T bend performance, and accelerated cure schedules at temperatures greater than about 160° C. which are typical requirements for coil coating applications.
It is another object of the invention to provide a powder coating composition with a desirable melt viscosity.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.
SUMMARY OF THE INVENTION
The present invention provides a powder coating composition that will not readily agglomerate during storage and can be cured at temperatures as low as about 160° C. without the use of an urethane catalyst, and at temperatures less than about 160° C. with the use of urethane catalyst. Moreover, the present invention has an additional advantage of utilizing crosslinking agents, which when unblocked, do not release blocking agents into the environment.
The powder coating composition of the invention comprises a unique combination of a branched oligoester polyol and crosslinking agent which when cured results in a coating with desirable hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss. The branched oligoester polyol has a unique combination of branched structure, number average molecular weight, hydroxyl number, and acid number which provides a relatively high glass transition temperature, and hence, agglomeration resistance. When the latter branched oligoester polyol is cured with an uretdione, the combination of branched oligoester polyol and uretdione provides a coating with good performance characteristics without the production of volatile organic compounds (VOCs) with or without the use of urethane catalysts. The invention provides an increase in reactivity and high rate of cure at lower temperatures without VOCs and without sacrificing storage stability because of agglomeration or sintering.
The branched oligoester polyol has a Tg of at least about 40° C. to about 80° C., a number average molecular weight of from about 1000 to about 7500 daltons, a hydroxyl functionality of about 1.5 to about 5.0, a hydroxyl number of from about 15 to about 250 and an acid number of about 1 to about 25, and in an very important aspect, an acid number of about 5 to about 7. In another important aspect, the branched oligoester will have a viscosity of from about 20 to about 90 poise at about 200° C.
The powder coating composition of the invention comprises the branched oligoester polyol and uretdione powder coating crosslinking agent each in relative amounts which are effective for providing crosslinked coating compositions with a pencil hardness of at least about HB, a direct impact resistance of at least about 80 in lb and a reverse impact resistance of at least about 80 in lb at a binder thickness of about 0.8 to about 4 mils when curing is conducted at temperatures below about 160° C. and up to about 350° C. The powder coating composition of the invention which comprises the branched oligoester polyol and uretdione has a Tg of from about 40° C. to about 80° C. In an important aspect the powder coating composition comprises from about 40 to about 97 weight percent of the branched hydroxyl terminated oligoester, based on the weight of branched oligoester polyol and crosslinking agent.
The branched oligoester polyol may be synthesized by forming a generally linear hydroxyl terminated oligoester diol by reacting a diol and a diacid and then reacting the resulting hydroxyl terminated oligoester diol with less than a stoichiometric amount (relative to the hydroxyls on the oligoester) of a polyacid having a carboxyl functionality of at least about 3. This less than stoichiometric amount provides some carboxyl groups to the oligomer, but its more important purpose is to generally provide complex branching of the oligoester polyol so that oligomer chains extend in some cases, from a
Bronk John Michael
Panandiker Kamlesh Pai
Spitler Franklin Paul
Fitch Even Tabin & Flannery
McWhorter Technologies Inc.
Sergent Rabon
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