Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1996-08-23
1998-07-07
Gulakowski, Randy
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525533, 525934, C08F 2000, C08L 6300
Patent
active
057770450
DESCRIPTION:
BRIEF SUMMARY
This invention relates to thermally-stable, glycidyl-terminated polyesters from diglycidyl phthalates and carboxylated polyesters. The resulting resins, when blended and cured with other carboxylated polyesters, are improved binders for powder coating applications. The invention also relates to polyglycidyl ester-based powder coating compositions containing these resins.
BACKGROUND OF THE INVENTION
The binder resins of the present invention are used in powder coatings as a replacement for co-reactants in current systems which contain either hazardous components, such as triglycidyl isocyanurate, or curatives that produce volatile by-products, or an epoxy resin containing bisphenol-A. The powder coating formulation of the present invention contains a binder resin blend with a glass transition temperature (Tg) of about 50.degree. C. or greater, cures to a thermoset in about 20 minutes or less at about 200.degree. C. to about 250.degree. C. and preferably in less than about 10 minutes at about 200.degree. C. to about 205.degree. C., does not produce a volatile by-product during cure, displays resistance to UV exposure, and does not sinter or cake when stored under hot ambient conditions.
"Hybrid resins", which are commercial binders for powder coatings are mixtures of resins containing carboxylated polyesters and diglycidyl ethers of bisphenol-A (DGEBA). Their use in coatings has the disadvantage of being limited to interior applications because of the UV (ultraviolet irradiation) instability (poor weatherability) of the DGEBA. This instability has been attributed to the presence of bisphenol-A. For exterior applications, the DGEBA is replaced with other epoxy derivatives, such as triglycidyl isocyanurate (TGIC) or with .beta.-hydroxyalkylamides. However, these replacement products also have disadvantages. For example, TGIC is considered hazardous, and .beta.-hydroxyalkylamides give off water as a by-product during cure which can cause pinholes in the coating, thus rendering unsatisfactory. Powder coating formulations which emit a volatile by-product such as water vapor cannot be used for thicker coatings.
Different types of carboxylated polyester resins are used, depending on the type of co-curative. TGIC and .beta.-hydroxyalkylamides are trifunctional and tetrafunctional (or greater), respectively, so a carboxylated polyester with carboxyl substitution of 1.1 to 2.5 per chain, preferably 1.8 to 2.5 per chain, is used for crosslinking. However, a hybrid resin, which contains a diepoxide, requires a polyester containing an average of at least about 2.1 and preferably about 3 to about 4 or more carboxyl groups for crosslinking.
Carboxylated polyesters (i.e., carboxyl-terminated polyesters) which are used in powder coatings are prepared primarily from terephthalic acid and neopentyl glycol. Co-reactants, such as trimellitic anhydride, trimethylolpropane, and pentaerythritol are often added to give higher carboxyl functionality by chain branching. Other co-reactants such as isophthalic acid, adipic acid, ethylene glycol, etc. are also added to afford the desired physical properties of the final coating.
An important property which is desirable in a powder coating formulation is that it be stable when stored under hot ambient conditions. The formulation should not sinter into a non-friable mass at 45.degree. C. because many storage facilities may have an ambient temperature of 45.degree. C. and sintering or caking of the powder coating formulation on storage at this temperature would destroy its usefulness. Generally, resins with a glass transition temperature (Tg) of about 50.degree. C. or higher will not sinter at 45.degree. C. Therefore an important property desired for the blend of carboxylated polyester and the co-curative is that the glass transition temperature (Tg) be about 50.degree. C. or greater. This is to prevent caking or "blocking" of the powder while it is being stored under hot, ambient conditions or in the feed section of an extruder.
JP-188436 discloses a resin mixture for powder paint use c
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WPI Derwent Abstract, AN 84-072523.
Amoco Corporation
Gulakowski Randy
Kanady Mary Jo
Oliver Wallace L.
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