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
1995-06-06
2001-11-06
Gulakowski, Randy (Department: 1746)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S444000, C525S448000, C525S934000, C528S272000, C528S291000, C528S292000
Reexamination Certificate
active
06313234
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to coating systems settable by heat on the basis of linear carboxyl-functional polyester (PES) resins and polyfunctional epoxy compounds and/or &bgr;-hydroxy alkyl amides, their production and use, as well as protective coatings made of these coating systems.
TECHNOLOGY REVIEW
In general, thermosetting coating systems consist of a binder resin and a cross-linking agent, pigments and additives and/or fillers. Cross-linking and thus setting under heat takes place by means of polyaddition or polycondensation reactions between functional groups contained in the binder systems. Epoxy resin/hardener, polyester/epoxide, polyester/isocyanate, polyester/&bgr;-hydroxyl alkyl amide, acrylate/isocyanate are typical binder systems.
The macroscopic properties of hardened powder coating films change over time. This phenomenon in polymeric materials has been known for some time by the term physical aging (L.C.E. Struik: Physical Aging in Amorphous Polymers and Other Materials, Elsevier, Publ., 1978).
Physical aging is understood to be the change over time of the macroscopic properties of polymers in the glass state under constant ambient conditions, caused by a lack of an equilibrium state in the polymers. Physical aging is reversible, in contrast to chemical aging, wherein irreversible changes in the material occur, such as decomposition reactions or chain breaks, caused by thermal decomposition or photo-oxidation.
Examples of changes through physical aging are changes in the electrical and optical characteristic values as well as changes in mechanical properties, which are the most important for coating films. Although generally known in connection with coating films, up to now this phenomenon did not appear to present too great problems.
This phenomenon and the means to overcome it have created more interest because of the increased employment of organic pigments and the high demands made on powder coatings by the precoating metal and coil coating methods, particularly if the coated parts are post-formed. Transparent coatings used as protective layers against crack formation constitute another field of application of interest for powder coatings having increased resistance to physical aging.
In the previously mentioned book “Physical Aging in Amorphous Polymers and Other Materials”, L.C.E. Struik generally describes the phenomenon of physical aging by the example of polymers. To-date there are few references in the literature addressing the phenomenon of physical aging of organic coatings or, in particular, powder coatings.
Only the fundamental physical principles and the effects of physical aging are measured and described, however, no solution of this problems is proposed.
With powder coatings on the basis of carboxyl-functional polyesters and polyepoxides and/or &bgr;-hydroxyl alkyl amides as cross-linking agents, the phenomenon of physical aging can be forced back by increasing the curing temperature, by increasing the curing time, by an excess of cross-linking agent or by the installation of so-called branchers, i.e. acids or alcohols of a functionality greater than 2, in the basic polyester resin. An increase in the curing temperature or an extension of the curing time is connected with increased energy consumption and partially also with color changes. An excess of cross-linking agent must also be rejected from an economical point of view and is connected with increased costs. The installation of branchers in the polyester resin cannot be recommended from a technical viewpoint, since it is connected with a deterioration of the film leveling properties.
SUMMARY OF THE INVENTION
Thus, the object on which the invention is based is to overcome the above mentioned disadvantages of the prior art.
In particular, the object is attained by a coating system of carboxyl-functional polyester resins made of linear monomeric structural elements and cross-linking agents, wherein isophthalic acid constitutes maximally 10 mol-parts of all structural acid elements.
DETAILED DESCRIPTION OF THE INVENTION
It was noted that it was possible in a surprising manner to obtain a clear improvement of the resistance to physical aging of powder coatings on the basis of linear carboxyl-functional polyester, i.e. those in which the functionality of the acids and alcohols used is less than or equal to 2, and polyfunctional epoxides and/or &bgr;-hydroxy alkyl amides as cross-linking agents or hardeners.
Thus, the thermosetting coating system of the invention consists of a specific saturated polyester as binder resin a), which is distinguished in a particular way in that it is constructed from aliphatic and/or cyclo-aliphatic diols and aliphatic and/or cyclo-aliphatic and/or aromatic dicarboxylic acids, wherein the amount of isophthalic acid is less than 10 mol-parts referring to the entire amount of acid.
The carboxyl-functional polyester resin a) minimally contains 90 mol-parts of other aromatic, aliphatic or cyclo-aliphatic dicarboxylic acids, wherein terephthalic acid is preferred as the aromatic dicarboxylic acid, adipic acid, azelaic acid, sebacic acid and dodecane dicarboxylic acid as the aliphatic, or cyclohexane dicarboxylic acid as the cyclo-aliphatic dicarboxylic acid. In a preferred embodiment the polyester resins are made of 3 to 9 mol-parts of aliphatic dicarboxylic acid with at least 6 C-atoms, wherein adipic acid is particularly preferred, and/or 3 to 9 mol-parts of cyclo-aliphatic dicarboxylic acid, wherein 1,4-cyclohexane dicarboxylic acid is particularly preferred.
The diols of the polyester resin consist of at least 50 mol-parts of at least one branched aliphatic diol with 4 to 12 C-atoms, as well as maximally 50 mol-parts of at least one linear aliphatic diol with 2 to 22 C-atoms and at least one cyclo-aliphatic diol with 6 to 16 C-atoms, wherein 2,2-dimethyl-1,3-propane diol is preferred as the branched diol.
Epoxy compounds with at least two epoxy groups and/or &bgr;-hydroxy alkyl amides with at least two hydroxy alkyl amide groups are suitable as a cross-linking component b). Glycidyl ethers of cyanuric or isocyanuric acid or glycidyl esters of polycarboxylic acids are preferred monomeric polyepoxy compounds. Terephtalic acid, trimellitic acid or mixtures thereof are preferred here. Trisglycidyl isocyanuric acid (TGIC) is particularly preferred. Bis[N, N′-di(&bgr;-hydroxy-ethyl)]adipamide (Primid XL 552 of the firm Rohm and Haas) is particularly suited as the &bgr;-hydroxy alkyl amide compound.
In a preferred embodiment of the coating system, the polyester resin has an acid value of 15 to 100 [mg/KOH/g], an OH value of maximally 10 [mg/KOH/g] and a glass transition temperature T
g
of higher than 45° C.
The additives (c) which are customary for producing and using powder coatings can be additionally present in the coating system in accordance with the invention.
These are additives from the group of accelerators, pigments, fillers, leveling and degassing agents, heat, UV and/or HALS stabilizers or tribo-additives as well as matting agents such as waxes, if required.
The production of the carboxyl-functional polyester resins takes place in a known manner by the common heating of all monomeric components in the presence of customary esterification catalysts to a temperature of approximately 250° C. and removal of the reaction water which was generated, or in a two-stage process, wherein in a first stage a hydroxyl-functional polyester is formed in the presence of excess polyol and, in a second stage, it is reacted with one or several polybasic carboxylic acids or their anhydrides to form a carboxyl-functional polyester resin.
The production of the powder coatings of the invention preferably takes place in the melt by the common extrusion of all compound components at temperatures between 90 and 130° C. Subsequently the extrudate is cooled, milled and sieved to a particle size of less than 100 &mgr;m. In principle, other processes are also suitable for producing the powder coatings, for example mixing the compound c
Hoppe Manfred
Kaplan Andreas
Kinkelin Eberhard
EMS Inventa-AG
Gulakowski Randy
Schneller Marina V
Venable
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