CROSSLINKING AGENT

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a thermosetting polyester powder coating composition useful as a coating and a film forming material. More particularly, the present invention relates to a polyester powder coating composition containing glycidyl methacrylate copolymer as a crosslin ing agent.

Description of Related Art Thermosetting compositions, including polyester resin and various crosslinking agents, are known to be useful for coatings. The compositions can be in powder form, having particle size typically in the range of from 40 to 120 micrometers. The compositions should be physically and chemically stable during storage at ambient temperature.

They are electrostatically applied to objects and heated to temperatures in the range of from 120 to 240"C at which point they fuse and undergo chemical reactions forming a uniform, crosslinked and insoluble film.

U. S. Patent No. 4,065,438 describes thermosetting powder coating compositions comprising carboxyl functional polyesters and bisphenol A type epoxy resins. These compositions have been found to result in coatings which exhibit excellent mechanical properties, such as flexibility and impact resistance. However, their outdoor durability has been found to be unsatisfactory for many applications.

U. S. Patent Nos. 4,147,737 and 4,528,341 are directed to carboxyl functional polyester powder coatings crosslinked with triglycidyl isocyanurate. Such powder coating compositions containing the triglycidyl isocyanurate as a curing agent have good mechanical properties and satisfactory outdoor durability. However, recently there have been concerns about the potential health hazard connected with toxicity of triglycidyl isocyanurate.

U. S. Patent No. 3,781,379 discloses powder coating compositions containing glycidyl methacrylate copolymers and anhydride crosslinking agents. While these coatings have been found to exhibit excellent outdoor durability, their mechanical properties, especially impact resistance, has been poor.

U. S. Patent No. Re. 32,261 of U. S. Patent No. 4,042,645 discloses a process for producing thermosetting finishing powders. The composition contains a copolymer of at least 30 weight percent of an acrylic or methacrylic ester monomer, 3 to 20 weight percent of an α.,/3- ethylenically unsaturated carboxylic acid, or from 3 to 40 weight percent of a glycidyl acrylate or glycidyl methacrylate, and from 0 to 67 weight percent of a copolymerizable morfomer. Thermosetting finishing powders are disclosed to be produced by mixing 40 to 5 parts by weight of an epoxy resin containing 2 or more epoxy radicals in the molecule, and 0.002 to 2.0 parts by weight of a tertiary a ine with 60 to 95 parts by weight of the copolymer. Alternately, a thermosetting finishing powder is produced from 40 to 3 parts by weight of a crosslinking compound containing 2 or more equivalents of carboxyl radicals or acid anhydridic thereof in the molecule, or 40

„to 3 parts by weight of a crosslinking compound containing in the molecule at least 2 nitrogen atoms, with 60 to 97 parts by weight of the copolymer. The crosslinking agents

include a variety of carboxyl group containing hydrocarbons, as well as polyester resins having two or more carboxyl radicals in the molecule, which are obtained by the esterification of the polyvalent carboxylic acid compound with polyhydric alcohols. The crosslinking agent is indicated to be 3 to 40 percent by weight of the total of the crosslinking agent and the copolymer. If the amount of crosslinking agent is less than 3 percent by weight, there is insufficient thermosetting. When the amount of crosslinking agent exceeds 40 percent by weight, the durability against weathering is reduced and the thermal fluidity is greatly reduced in some cases.

SUMMARY OF THE INVENTION

The present invention is a thermosetting composition which is useful as a powder coating material. The composition is a thermosetting composition which comprises carboxylic polyester and a copolymer of glycidyl methacrylate, with an acrylic monomer, and optionally an ethylenically nonunsaturated comonomer referred to as a glycidyl methacrylate copolymer as a crosslinking agent.

Exclusive of flow control agents, the composition comprises from about 80 to about 92 weight percent of a polyester having an acid number of from 15 to 40, and preferably from

20 to 30 mg KOH/g and a softening point of from 105°C to about 125 ^β C measured according to ASTM E-28; and from about

8 to 20 weight percent of a glycidyl methacrylate copolymer.

The polyester is derived from at least one compound containing at least two carboxyl groups or anhydride thereof esterified with at least one polyhydric alcohol. In preferred embodiments of the present invention the polyester is derived from at last two alcohols having two hydroxyl groups, most preferably neopentyl glycol and 1,6-hexanediol and from at least two carboxylic acids selected from

terephthalic acid, isophthalic acid, adipic acid and trimellitic anhydride.

The crosslinking agent is a copolymer of a glycidyl methacrylate, an acrylic monomer and optionally an ethylenically monounsaturated comonomer different from glycidyl methacrylate and the acrylic monomer. The copolymer comprises from about 45 to about 70 weight percent of glycidyl methacrylate; from 5 to about 55 of the acrylic comonomer, and from 0 to 35 weight percent of the ethylenically monounsaturated comonomer. The acrylic monomer is preferably selected from the group consisting of esters of an α,,3-ethylenically unsaturated carboxylic acid having from 3 to 8 carbon atoms. The glycidyl methacrylate copolymer has an epoxy equivalent weight of from 200 to about 350, and preferably from 230 to 300; a softening point of about 105"C to about 120 ° C, measured according to ASTM E- 28. The copolymer has a number average molecular weight of from 2000 to about 8000.

Compositions of the present invention have been found to be useful as a powder coating material and can form film when crosslinked. The composition results in a powder coating having excellent mechanical properties and at the same time has a good outdoor durability and very good chemical resistance. Additionally, the composition avoids toxicity problems, such as experienced with compositions containing triglycidyl isocyanurate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a novel thermosetting polyester powder coating composition comprising polyester resin and a glycidyl methacrylate copolymer. The copolymer is present in minor amounts -and acts as a crosslinking agent.

The polyester useful in the present invention is generally characterized as carboxylic functional polyester. It can be prepared by any manner known in the art and is preferably made by condensing at least one polyfunctional organic acid, the methyl ester or anhydride thereto with at least one polyalcohol in the presence or absence of catalysts. Useful catalysts are selected from the group consisting of organo-tin compounds and organo-titanium compounds.

Useful alcohols from which the polyester can be made include alcohols having at least two hydroxyl groups with preferred alcohols including, ethylene glycol, propylene glycol, butanediol, hexanediol, cyclohexanediol, neopentyl glycol, tri ethylolethane, trimethylolpropane, and glycerine. The most preferred alcohols include 1,4- butanediol, 1,6-hexanediol and neopentyl glycol.

The carboxylic acids are preferably aromatic carboxylic acids with the most preferred carboxylic acids being dicarboxyl acids and tricarboxyl acids. Useful carboxyl acids included phthalic acid, terephthalic acid, isophthalic acid, tri ellitic acid, and the anhydrides thereof. The most preferred carboxylic acid is a mixture of terephthalic acid and trimellitic anhydride.

The polyester can comprise minor amounts, i.e., less than 30 mole percent of at least one other aliphatic or cycloaliphatic carboxylic acid. The presence of such comonomers is acceptable so long as the composition is useful in powder coating applications. Useful polybasic aliphatic and cycloaliphatic carboxylic acid include adipic acid, sebacic acid, tetrahydrophthalic acid, hexahydrophthalic acid and the anhydrides thereof.

The molar ratio of the acidic to alcoholic monomers in the polyester is selected in the manner to result in a polyester having a softening point measured in accordance with ASTM E-28 of from 105 ^β C to 125"C. The polyester preferably has an acid number of from about 15 to about 40, and preferably 20 to 30 g KOH/g. The number average molecular weight of the polyester resin measured by gel permeation chro otography (GPC) is from 2500 to 6000.

The polyester can have a linear or branched structure. The linear polyesters are produced from dicarboxylic acids and diols with a molar excess of from 1 to 10 percent of the dicarboxylic acid. The branched polyesters can contain minor amounts, typically from up to 10 mole percent of a tri-functional compound such as trimethylol propane or trimellitic acid. A useful branched polyesters is obtained by condensation of about one mole of neopentyl glycol, 0.4 mole of 1,6-hexanediol, 1.3 mole terephthalic acid, and 0.1 mole of trimellitic anhydride.

The crosslinking copolymer of the present invention comprises glycidyl methacrylate, an acrylic monomer, and optionally an ethylenically monounsaturated comonomer different than the glycidyl methacrylate and acrylic monomers.

The acrylic monomer is selected from the group consisting of the esters of an α,3-ethylenically unsaturated carboxylic acid having from 3 to 8 carbon atoms. A preferred acrylic monomer has the formula

i ¹ CH ₂ = C - COOR

wherein R]_ is H or CH ₃ and R ₂ is an alkyl radical containing 1 to 18 carbon atoms and preferably 1 to 6 carbon atoms.

Useful acrylic monomers include: ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate and lauryl methacrylate. The copolymer can optionally contain an ethylenically monounsaturated comonomer which is different from the glycidyl methacrylate monomer and the acrylic monomer. Any such comonomers can be used which do not prevent the composition from being used in powder coating applications. Useful ethylenically unsaturated comonomers include styrene, vinyl toluene, dimethyl styrene, alpha methyl styrene, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, acrylonitrile acrylamide and vinyl acetate.

The copolymer useful in the composition of the present invention comprises from about 45 to 70 weight percent of glycidyl methacrylate; from about 5 to 55, weight percent of the acrylic monomer; and from about 0 to about 35 weight percent of the ethylenically monounsaturated comonomer.

The copolymers can be prepared in any known manner, preferably by free-radical polymerization in bulk, solution, emulsion or suspension. Preferably, the reaction is conducted in the presence of free-radical initiators such an benzoyl peroxide, tert-butyl peroxide, decanoyl peroxide, azo compounds such as azobisisobutyronitrile, and the like. Useful initiators are present in from 0.1 to about 5 percent by weight of the total monomers.

The copolymers have a number average molecular weight in the range of from about 2000 to about 8000. Preferably the copolymers do not contain significant amounts of higher molecular weight fractions with less than 2 percent of the copolymer having a molecular weight greater than 20,000.

The molecular weight distribution can be measured by the

ratio of the weight average molecular weight to the number average molecular weight, and the range should be from 1.5 to about 2.2. The preferred molecular weight range distribution is from about 1.6 to 2. Generally, control of the molecular weight can be obtained by using chain transfer agents. Useful chain transfer agents include mercaptans, halides, disulfides or thioethers.

The copolymer preferably has an epoxy equivalent weight (EEW) of from 200 to 350, and preferably about 230 to 300. The copolymer preferably has a softening point of from about 100'C to 120 ^β C measured according to ASTM E-28.

The composition of the present invention contains from about 80 to 92, weight percent of the carboxylic polyester. The preferred polyester has an acid number of from about 15 to about 40 mg KOH/g, a softening point from about 105 ^β C to about 125 ^β C measured according to ASTM E-28. The composition additionally contains a corresponding amount from about 8 to 20 weight percent of the copolymer.

The composition of the present invention can also include conventional additives useful in powder coating compositions such as flow control agents, dyes and pigments, light and heat stabilizers, anti-static agents, plasticizers, fillers, catalysts, and the like. Flow control agents can be selected from the group consisting of polyacrylates, such as polybutylacrylate, fluoro compounds, silicones and the like. Flow control agents are typically added in the amount from about 0.1 to about 3.0 percent by weight. They are used to eliminate surface imperfections, such as orange-peel effect, pin holes, craters, etc. Catalysts can be added to help control the gel time of the

_^ composition. Such catalysts are typically present in amounts from 0.05 to about 1 percent by weight. Typically the catalyst is selected to produce a gel time for the

powder coating composition no less than 30 seconds and no longer than 5 minutes at the baking temperature of the composition. As used herein, the gel time of the coating composition is that time in which the composition develops elasticity and resistance to flow at the baking temperature.

In order to produce high quality powder coatings the ingredients are preferably dry blended, followed by melt blending. Melt blending can be conducted in an internal mixer, or in an extruder. Preferred melt blending temperatures are from 80"C to about 120'C. The homogeneous composition is then cooled to room temperature, crushed, ground in a mill and sifted. The powder must be free flowing and have a particle size of from 40 to 120 micrometers. The powder composition of the present invention is preferably physically and chemically stable at room temperatures for prolonged periods of time, up to 6 months to 2 years. The compositions are preferably applied as a coating on a base material such as a metal and then baked from 5 to 60 minutes within an oven at 160 to 220 ^β C to obtain a crosslinked film having excellent thermal stability, solvent resistance, metal adhesion, mechanical strength and durability against weathering.

The following examples illustrate the practice of the present invention. The examples should not be construed as limiting the invention to anything less than that which is disclosed or which would have been obvious to one of ordinary skill in the art therefrom. Percents and parts are by weight unless otherwise indicated.

EXAMPLES

Example 1

Preparation of a carboxyl functional polyester resin. Into a 3-liter reaction flask equipped with an agitator, thermometer, nitrogen inlet tube, fractionating column, condenser and water receiver, were charged 600 parts of neopentyl glycol, 115 parts of 1,6-hexanediol, 100 parts of adipic acid, 970 parts of terephthalic acid, 50 parts of trimellitic anhydride and 5 parts of dibutyltin oxide. The flask was heated to 150 ^β C, then the agitation was started with the nitrogen purge. The temperature was slowly increased to 240"C. The reaction was followed by testing the acid number and the softening point of the resin and the reaction water was collected as it was formed. The condensation was complete when the acid number was 24 mg KOH/g and the softening point was 112 " C . The hot, molten resin was then discharged onto an aluminum dish and cooled to room temperature. The brittle, solid resin was then crushed and ground to a free-flowing powder.

Example 2

Preparation of a glycidyl methacrylate copolymer having an epoxy equivalent weight of 300. Into a 3-liter reaction flask equipped with an agitator, thermometer, reflux condenser and a dropping funnel, 800 parts of toluene were charged and the solvent was heated to reflux. Using the dropping funnel a mixture consisting of 435 parts of glycidyl methacrylate, 248 part of methyl methylacrylate, 172 parts of butyl acrylate, 45 parts of styrene, 20 parts of tert. dodecyl mercaptane and 25 parts of 2,2'-azobis-(2-methylpropionitrile) were added to the refluxing toluene over a period of 3 hours. After the addition of the mixture was complete, the reflux was maintained for an additional 2 hours. The solution was then cooled to 50"C, the reflux condenser was exchanged fσr a distillation condenser, and the solvent was removed via

vacuum distillation. At the end of distillation the temperature in the flask was 160 ^β C. The hot resin, free of solvent, was discharged onto an aluminum dish and left for cooling at room temperature. The solid copolymer was ^' then crushed and ground to a free flowing powder. The softening point of the copolymer was 108"C and the EEW was 300.

Example 3

Preparation of a glycidyl methacrylate copolymer having EEW of 250.

Example 2 was repeated with a monomer composition consisting of 535 parts of glycidyl methacrylate, 210 parts of methyl methacrylate and 155 parts of butyl acrylate. The resulting copolymer had a softening point of 110'C and EEW of 250.

Example 4

Preparation of a powder coating composition

240 parts of the carboxyl functional polyester resin from Example 1, 30 parts of the glycidyl methacrylate copolymer from Example 2, 100 parts titanium dioxide and 3 parts of a polyacrylate flow control agent (Resiflow P-67 made by Estron Chemical, Inc.) were thoroughly blended. The compound powder mixture was then homogenized in an extruder at a temperature of 100"C. The extruded material was cooled to room temperature, crushed, ground and sifted. Particles below 75 microns diameter were electrostatically sprayed onto a clean steel panel and cured for a period of 10 minutes at 200"C. A smooth, glossy coating was obtained having an excellent adhesion, a good mechanical strength, (DuPont Impact Tester) , and a very good solvent resistance.

The adhesion resistance was measured using a cross-cut test. Eleven cuts were made at intervals of about 1 millimeter apart in the longitudinal and lateral direction on the surface of the coated panel. An adhesive tape was

pasted over the grid and lifted off. None of the squares peeled off; there was 100% adhesion.

The mechanical strength was measured by judging cracking of the coating after dropping a load of 500 grams onto the test panel from a height of 30 cm by using a striking core of a diameter of 1/2 inch with a DuPont Impact Tester. This sample had an impact resistance of 80 lbs/inch.

The solvent resistance was measured by rubbing the coated panel 100 times with cloth saturated with each of xylene, toluene and methyl ethyl ketone. The rubbed sample was observed for surface appearance and softening. Results with each solvent were excellent.

Example 5

Preparation of a powder coating composition.

Example 4 was repeated, but instead of 30 parts copolymer from Example 2, 25 parts copolymer from Example 3 were used. The cured coating had a smooth glossy appearance, excellent adhesion (100% rating using the above cross-cut test, i.e., no peeling), very good mechanical strength (DuPont Impact Tester resulting in 160 lbs/inch*) and a very good solvent resistance as tested using the procedure in Example 4.

While exemplary embodiments of the invention have been described, the true scope of the invention is to be determined from the following claims.