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
1999-04-23
2002-08-13
Sellers, Robert E. L. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S934000
Reexamination Certificate
active
06433084
ABSTRACT:
The present invention relates to powder coating compositions based on solid poly(meth)acrylic resins having free carboxyl groups, i.e. on resins which are obtainable by polymerisation of monomers which contain acrylic monomers and/or methacrylic monomers, as well as to the use thereof as automotive lacquers.
Powder coating compositions based on poly(meth)acrylic resins are used whenever powder coating compositions of such high fastness to weathering and hardness are desired as cannot be achieved with other binder systems customarily used for powder coatings, such as polyesters. Poly(meth)acrylic resins containing specific solid carboxyl groups are, for example, sold commercially as binders for powder coatings which are intended especially for cures with triglycidyl isocyanurate and which give powder coatings having the cited properties (Johnson Wax Speciality Chemicals Product Application Bulletin, Powder Coatings). However, these powder coatings can only be used at temperatures above 160° C. because they have only very bad flow at temperatures lower than that, resulting in a pronounced orange peel effect. In spite of good fastness to weathering they are therefore e.g. not suitable for the preparation of automotive lacquers as in this case the coating composition is required to harden at a temperature of at maximum 140-150° C. in order not to adversely affect the coats of base lacquer during the hardening of the finishing coating composition coated thereon.
This invention provides a solution for the problem described above by providing special powder coatings based on poly(meth)acrylic resins which harden quickly and completely already at a temperature of 140-150° C. and which have good flow properties even at these relatively low temperatures as well as good fastness to weathering. This is achieved by using epoxy resins having a molecular weight of below 1500 as hardeners for powder coatings based on poly(meth)acrylic resins having free carboxyl groups, the epoxy resins containing at least 70% by weight of aliphatic and/or cycloaliphatic polyglycidyl polycarboxylates.
Accordingly, the invention relates to a powder coating composition comprising as binder one or more than one poly(meth)acrylic resin having free carboxyl groups and, as hardeners for these poly(meth)acrylic resins, one or more than one epoxy resin having a molecular weight of up to 1500, in which powder coating composition the epoxy resins contain at least 70% by weight of glycidyl esters which are selected from the group consisting of aliphatic and cycloaliphatic polyglycidyl polycarboxylates. More particularly, the instant invention relates to an improved coating composition, which is free of carboxyl-functional polyesters, comprising a binder consisting of one or more than one poly(meth)acrylic resin having free carboxyl groups and, as hardener for the poly(meth)acrylic resin, one or more than one epoxy resin having a molecular weight of up to 1500, wherein the epoxy resin is a diglycidyl ester of a cycloaliphatic polyglycidyl polycarboxylate or mixtures thereof, the cycloaliphatic polyglycidyl polycarboxylate being selected from the group consisting of diglycidyl hexahydrophthalate, diglycidyl hexahydroisophthalate, diglycidyl hexahydroterephthalate, diglycidyl methylhexahydrophthalate, diglycidyl 2,5-dimethylhexahydrophthalate, diglycidyl N endomethylenehexahydrophthalate, diglycidyl 1,8-decalindicarboxylates, diglycidyl 2,3-decalindicarboxylates, diglycidyl 2,6-decalindicarboxylates, triglycidyl cyclohexanetricarboxylates, tetraglycidyl hexahydromellophanate and tetraglycidyl hexahydropyrromellitate.
In addition to their excellent flow properties, the powder coating compositions according to this invention also have good storability at room temperature or at moderately elevated temperatures, such as in the temperature range from 10 to 40° C., and accordingly they also have a good reactivity/stability ratio.
Another advantage achieved in accordance with the present invention is that the powder coating compositions cure, preferably at temperatures in the range of about 100° C. to about 150° C., to a smooth form. The smoothness of coating surfaces can for example be objectively measured using the “Wave Scan” profilometer of Byk-Gardner, which is a commercially available portable flow measuring instrument for characterizing “orange peel” by optically scanning the brightness pattern reflected from a surface when it is irradiated with light emitted from a laser diode. The Byk-Gardner catalogue describing the profilometer is incorporated herein by reference,. The profilometer that is disclosed in U.S. Pat. No. 5,596,412, incorporated herein by reference, provides different measurement parameters, i.e., the parameter “Long-Wave” and “Subnote” being in particular indicative of the surface roughness and flow properties of the coating systems. A parameter “Long-Wave” of less than 50 and a parameter “Subnote” of less than 120 measured for a coating derived from a coating composition is, for the purpose of the instant invention, considered to be a sufficiently smooth surface and accordingly of good flow properties shown by said composition. The powder coating compositions according to the instant invention provide such smooth coatings having a parameter “Long-Wave” of less than 50, preferably less than 45, and a parameter “Subnote” of less than 120, preferably less than 100, even when cured at a temperature of below 160° C., in particular at a temperature of about 100° to about 150° C.
Poly(meth)acrylic resins having free carboxyl groups can be obtained by copolymerisation of acrylic monomers and/or methacrylic monomers, such as C
1
-C
12
alkyl(meth)acrylates, typically methyl(meth)acrylates, ethyl(meth)acrylates, propyl(meth)acrylates, butyl(meth)acrylates, pentyl(meth)acrylates, hexyl(meth)acrylates, octyl(meth)acrylates, 2-ethylhexyl(meth)acrylates, decyl(meth)acrylates and dodecyl(meth)acrylates, C
1
-C
4
alkyl(meth)acrylates being preferred, or (meth)acrylamide with acrylic acid and/or methacrylic acid and, optionally, other additional ethylenically unsaturated comonomers, typically vinylaromatics, e.g. styrene, &agr;-methylstyrene, vinyltoluene or also &bgr;-halogenated styrenes. This copolymerisation can be carried out according to known methods, for example by dissolving the monomers in suitable organic solvents and thermally reacting them in the presence of a suitable initiator which is soluble in the solvent, such as dicumyl peroxide, as well as in the presence of a suitable chain transfer reactant such as thioglycolic acid (solution polymerisabon), or by suspending and polymerising the monomer mixture together with a solution of the initiator in an organic solvent in water, or also by emulsifying the monomer mixture using surfactants, e.g. sodium lauryl sulfate in water and reacting it in the presence of a water-soluble polymerisation initiator, such as K
2
S
2
O
8
(emulsion polymerisation). The ready poly(meth)acrylic resin is then isolated in solid form from the solvent or water. The reaction can also be carried out without using solvents or water, for example in accordance with JP-A-Sho 53-140395. Suitable poly(meth)acrylic resins are solid in the temperature range from room temperature (15 to 25° C.). They usually have a molecular weight from 1000 to 50000 (medium weight M
w
), preferably from 5000 to 20000. The Tg value (glass transition temperature) of the poly(meth)acrylic resins, determined by DSC (heating rate: 10° C./minute), is preferably from 40 to 75° C. The acid number of the resins, indicated in mg equivalents of KOH per g of poly(meth)acrylic resin, is preferably from 30 to 160, more preferably from 35 to 80.
The aliphatic polyglycidyl polycarboxylates are preferably the aliphatic polyglycidyl polycarboxylates containing 2 to 50, particularly preferably 2 to 20, carbon atoms which may, in addition to the carboxyl groups, also contain other functional groups. Typical examples of suitable aliphatic polycarboxylic acids are oxalic acids, malonic acid, succinic acid, glutaric acid, adipic acid,
Doyle, Esq. Charles M.
Neuman, Esq. Kristin H.
Sellers Robert E. L.
Shalek, Esq. James H.
Vantico Inc.
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