POWDER COATINGS BASED ON EPOXY RESINS

Abstract: A curing agent for an epoxy resin based coating composition comprises at least one polyfunctional carboxylic acid having not less than three carboxyl groups together with a nitrogen containing accelerator, provided that, in the case where the accelerator is an imidazoline, the accelerator is not present as a salt of the acid.

Description: POWDER COATINGS BASED ON EPOXY RESINS This invention relates to the production of powder coatings and in particular to the production of matt or semi gloss epoxy based powder coatings. Coatings based on epoxy resins which are thermally cured with a variety of cross linking agents are well known and the nature of the cross linking agent has a significant effect on the properties of the cured coating. These properties include chemical resistance, appearance, gloss value and mechanical properties. Of particular importance are methods of producing matt or semi gloss coatings. Such coatings have conventionally been produced by careful selection and incorporation into the epoxy based powder coating composition of specific fillers traditionally referred to as matting agents. Alternatively, particular cross linking agents chemically chosen to give matt or semi gloss coatings have been used. However, the use of such matting agents is not always desirable because of problems of incorporating such materials into the composition and because the high loadings required can have a detrimental effect on the physical properties of the coating. Furthermore, available cross-linking agents for the production of matt or semi gloss coatings often give inconsistent gloss values under the same curing conditions. By means of the present invention matt or semi gloss epoxy coatings can be made whilst avoiding the inherent difficulties associated with the previously known methods described above. The inventors have shown that by crosslinking epoxy resins by means of curing agents comprising a polyfunctional acid in combination with a suitable accelerator, semi-gloss or matt epoxy powder coatings can be consistently produced. According to a first aspect of the invention there is provided a curing agent for an epoxy resin based powder coating composition comprising at least one polyfunctional carboxylic acid having not less than three carboxyl groups and a nitrogen-containing accelerator provided that, in the case where the accelerator is an imidazoline, it is not present as a salt of the acid. According to a second aspect of the present invention there is provided a method of curing an epoxy resin based powder coating composition comprising mixing the epoxy resin composition with a curing agent as hereinabove defined, and curing the mixture at elevated temperature. Subject to the foregoing proviso, the nitrogen- containing accelerator may be, for example, a substituted urea, a substituted imidazole, a substituted imidazoline, a ureide, a urone, hexamethylene tetramine or a hydrazide. Preferably the mixture is cured at a temperature of 170 to 220°C. In an embodiment of the invention, the curing agent may further comprise at least one difunctional carboxylic acid. Examples of the polyfunctional acids which are suitable for use in the compositions of the invention include butanetetracarboxylic acid, cyclopentanetetracarboxylic acid, nitrilo triacetic acid, citric acid, aconitic acid, trimellitic acid, and mixtures of one or more thereof. Most dicarboxylic acids including oxalic, succinic, adipic, itaconic, isophthalic, terephthalic and citraconic acids may also be included. Compositions comprising polyfunctional acids chosen from one or more of the following acids are particularly preferred:- pyromellitic acid, trimesic acid ethylene diamine tetra acetic acid, diethylene triamine penta acetic acid and nitrilotriacetic acid. A substituted urea or hexamethylene tetramine or a combination thereof are the preferred accelerator. The proportions of the individual components of the composition have a significant effect on the final level of gloss. For example, a low level of gloss can be achieved by using a high level of tetra functional acid, or by using a trifunctional carboxyl compound when accelerated with a compound including an amine f nctionality. Certain mixtures of the polyfunctional acids are also preferred for economic and performance reasons. In use, the mixtures described, in combination with a suitable accelerator are pre-blended with epoxy resin, and if desired with fillers, pigments, and other additives often used in the coating industry. The blended materials are then extruded and finally ground. The final ground material can then be electrostatically sprayed and thermally cured to give a tough, high performance coating with varying levels of gloss depending upon the composition of the starting material. The ratio of curing agent to the epoxy resin component, the extrusion technique, the curing temperature and the curing times are chosen to attain specific and consistent properties of the coating. The curing temperature is preferably in the range of from 170°C to 220°C and is particularly preferably in the range of from 190°C to 200°C. The ratio of the curing agent to the epoxy resin is dependent upon the epoxy equivalent weight of the epoxy resin and is usually between 3 and 10 parts by weight of said curing agent to 100 parts by weight of the epoxy resin (referred to hereinafter as phr) . Suitable accelerators include 2-methyl imidazole, 2-phenyl imidazole, 2-ethyl-4-methylimidazole, 1-H- imidazole, 1-vinylimidazole, 2-aminoimidazole, 1- benzoylimidazole, 2-ethylimidazole, 2,4-dimethyl imidazole, 2-benzylimidazole, 1,2,-dimethylimidazole, certain hydrazides, certain ureides, certain urones including 3-(4-chlorophenyl)-1,1-dimethylurea, 3-(3,4- dichlorophenyl)-1,1-dimethylurea, 3-phenyl-l, 1- dimethylurea, 3-(3-chloro-4-methylphenyl)-1,1-dimethyl urea and toluene-2,4-bis (N,N-dimethylcarbamide) and hexamethyl tetramine. 3-phenyl-l,1-dimethylurea and hexamethyl tetramine are particularly preferred. The epoxy resins with which the curing agents of the invention are used will be ordinarily solid polyepoxides containing more than one epoxy group per molecule. Such polyepoxides are the polyglycidyl ethers of aromatic or aliphatic polyhydric compounds for example resorcinol, hydroquinone, pyrocatechol, bisphenol A, bisphenol F, glycerol, pentaerythritol, mannitol, sorbitol and trimethylol propane. The most suitable polyepoxides are those based on the reaction between bisphenol A or bisphenol F with epichlorhydrin. The following examples illustrate the invention. EXAMPLE 1 Curing agents A, B and C were prepared by blending the components as shown in Table 1 in a suitable mixer followed by grinding in a rotating hammer mill. Grinding was continued until at least 95% of the blended components passed through a 75 micron sieve. -Table 1 Pyromellitic acid Terephthalic acid 1,l-dimethyl-3-phenyl urea 100 100 100 The above curing agents were converted into epoxy powder paints using the formulation shown in Table 2 and the following method. Table 2 Paint Formulation Epoxy Resin (EEW 850-1000) 100.0 parts by wt Titanium dioxide 33.0 parts by wt Modaflow Powder 2 0.8 parts by wt Tripenta stearate 0.5 parts by wt Curing Agent Variable Method All the powdered components were mixed together and passed through an extruder at a temperature of 80- 100°C. The extruded mixture was then allowed to cool and ground using a rotating hammer mill until the particle size was less then 125 microns. Paints were made as indicated in Table 2 above using curing agents A, B and C at varying phr as shown in Table 3. These paints were then electrostatically sprayed onto steel panels and stoved for 10 mins at 200°C. The level of gloss achieved was measured using a 60° Specular Gloss Meter. Table 3 phr 5.5 6.0 6.5 7.0 As can be seen from Table 3, low levels of gloss can be achieved by means of the curing agents of the present invention when they are used in appropriate amounts. Example 2 The following curing agents (D-H) were made following the method of Example 1 using the components shown in Table 4. The above curing agents were converted into paints as in Example 1 and using a phr of 6.0. The gloss levels shown in Table 5 were obtained on the resultant stoved panels. Table 5 Curing Agent D E F G H % Gloss (60°) 6 7 10 5 7 Example 3 The following curing agents (I - L) were made following the method of Example 1 using the components shown in Table 6. Table 6 The above curing agents were converted into paints as in Example 1 and using a phr of 6.0. The gloss levels shown in Table 7 were obtained on the resultant stoved panels. Table 7 Curing Agent I J K L % Gloss (60°C) 14 28 30 29 Example 4 The following physical properties were obtained on the stoved film using curing agent C at a phr of 6.5 cured for 10 minutes at 200°C. Impact (Reverse) ASTM D2794.84 60 inch/pound Mandrel Bend Test (BS 3900:El) - Pass 3mm MEK Rub Test - 100 rubs/slight softening Cross hatch adhesion test (BS 3900:E6) - 0 Overbake properties - Good Taber Abrasion (ASTM lkg/100 revs/CS17 wheel) - 10.4 mgs Example 5 The following curing agents (M-P) were made following the method of Example 1, using the components shown in Table 8. Table 8 M N O P Pyromellitic acid 55 - - - Terephthalic acid 40 - - E.D.T.A. - 85 90 - Nitrilo Triacetic Acid - - - 91.2 1, 1-dimethyl 3-phenyl urea 5 - - - Hexamethylene tetramine - 15 10 8.8 100 100 100 100 Paints were prepared from the above curing agents as in Example 1 and when cured and tested, as shown below, gave the following properties as shown in Table 9. Table 9 phr Stoving Conditions Time (mins) Object temperature °C % Gloss (60°C) Impact (Reverse) inch MEK Rub test (100 rubs) Mandrel Bend Test (BS 3900:E1) Example 6 A matt black paint was made to the following formulation using the method given in Example 1. Parts by weight Epoxy Resin (E.E.W. 715-750) Calcite Carbon Black Modaflow Powder 2 Curing Agent '0' The paint was sprayed onto a metal panel and stoved for 20 mins, object temperature at 190°C, and had the following properties: phr 7.9 % Gloss (60°) 6 Reverse impact (80 inch pounds) Pass MEK double rubs (100) Pass

Claims: CLAIMS 1. A curing agent for an epoxy resin based powder coating composition characterised in that the curing agent comprises at least one polyfunctional carboxylic acid having not less than three carboxyl groups and in that the curing agent further comprises a nitrogen containing accelerator, provided that, in the case where the said accelerator is an imidazoline, the accelerator is not present as a salt of the acid. 2. A curing agent as claimed in claim 1 wherein the curing agent further comprises at least one difunctional carboxylic acid. 3. A curing agent as claimed in claim 1 or 2 wherein the polyfunctional carboxylic acid is selected from the group comprising butanetetracarboxylic acid, cyclopentanetetracarboxylic acid, nitrilo triacetic acid, citric acid, aconitic acid, trimellitic acid, pyromellitic acid, trimesic acid, ethylene diamine tetra acetic acid, diethylene triamine penta acetic acid, nitrilotriacetic acid and mixtures thereof. 4. A curing agent as claimed in claim 1, 2 or 3 wherein the accelerator is selected from the group comprising substituted ureas, substituted imidazoles, substituted imidazolines, ureides, urones, hydrazides and hexamethylene tetramine. 5. A curing agent as claimed in claim 4 wherein the accelerator is selected from the group comprising 2-methyl imidazole, 2-benzylimidazole, 1,2- dimethylimidazole, 3-(4-chlorophenyl)-l, 1-dimethylurea, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 3-phenyl-l,1- dimethylurea, 3-(3-chloro-4-methylphenyl)-l, 1- dimethylurea and toluene-2,4-bis (N,N- dimethylcarbamide) . 6. A curing agent as claimed in any of claims 2 to 5 wherein the difunctional carboxylic acid is selected from the group comprising isophthalic acid. terephthalic acid, oxalic acid, succinic acid, adipic acid, and itaconic acid. 7. A method of curing an epoxy resin based powder coating composition comprising admixing the epoxy resin composition with a curing agent and curing the mixture at elevated temperature, characterised in that the curing agent comprises at least one polyfunctional carboxylic acid having not less than three carboxyl groups and in that the curing agent further comprises a nitrogen containing accelerator, provided that, in the case where said accelerator is an imidazoline, the accelerator is not present as a salt of the acid. 8. A method as claimed in claim 7 wherein the mixture is cured at a temperature in the range of from 170°C to 220°C. 9. A method as claimed in claim 7 or 8 wherein the admixture comprises from 3 to 10 parts by weight of the curing agent per 100 parts by weight of the epoxy resin.

Inventors: RICHARDSON FRANK BARNETT, SPOOR JOSEPH, WATSON PETER JOSEPH, RICHARDSON, FRANK, BARNETT, SPOOR, JOSEPH, WATSON, PETER, JOSEPH
Applicants: SWAN THOMAS & CO LTD, THOMAS SWAN & CO., LTD.
Application year: 1990
Main classification: C09D163
IPC codes: C08G59, C09D5, C09D163