FLOWABLE POWDER COATING COMPOSITION

Abstract: A powder coating composition, which comprises (A) an advanced epoxy resin prepared by reacting a mixture comprising a bisphenol diglycidyl ether of bisphenol (A1) and an epoxy resin having an epoxy functionality of > 2(A2) with a bisphenol (A3) in the presence of an advancement catalyst, and (B) a polymer containing epoxy-reactive groups is distinguished by good flow behaviour and increased storage stability.

Description: Flowable powder coating composition The present invention relates to a powder coating composition based on an epoxy resin mixture. Powder coating compositions based on epoxy resins and resins containing epoxy-reactive groups are known for diverse applications. EP-A 119 164, inter alia, describes powder coating compositions comprising an epoxy mixture consisting of a resin having an epoxy functionality of >2 and of another resin having an epoxy functionality of <2, as well as an epoxy-reactive component, which compositions are distinguished by high storage stability and short curing times and are suitable for coating vessels on the inside. According to EP-A 857 742 it is possible to improve the flow of powder coatings based on carboxyl-containing polyesters by using a mixture of two epoxy resins, the epoxy functionalities of which differ specifically. However, there is a need for powder coatings which, in addition to a good flow behaviour, have, in particular, increased storage stability, especially at higher temperatures. This invention relates to a powder coating composition which is improved in that respect and which comprises (A) an advanced epoxy resin prepared by reacting a mixture comprising a bisphenol diglycidyl ether of bisphenol (A1 ) and an epoxy resin having an epoxy functionality of >2 (A2) with a bisphenol (A3) in the presence of an advancement catalyst, and (B) a polymer containing epoxy-reactive groups. The advanced epoxy resin according to component A is a product which is solid at room temperature and which is obtainable by reacting the components A1 , A2 and A3 in the presence of an advancement catalyst and, optionally, of a chain terminator. As usually necessary in advancement reactions of epoxy resins, the glycidyl groups of the components A1 and A2 must in this reaction be in stoichiometric excess compared to the phenolic hydroxyl groups of component A3. Suitable components A1 are, for example, the diglycidyl ethers of bisphenol A and bisphenol F, 4,4'-dihydroxybiphenyl, 4,4'-dihydroxydiphenylsulfone, 4,4-dihydroxydiphenylketone, 1 ,2-bis(4-hydroxyphenyl)ethane and 1 ,1 -bis(4-hydroxyphenyl)ethane. It is of course also possible to use mixtures of two or more diglycidyl ethers. Preferred components A1 are the diglycidyl ether of bisphenol A and the diglycidyl ether of bisphenol F. The resin component A2 can embrace practically any epoxy resin which is liquid, and preferably solid, at room temperature and which has an average epoxy functionality of more than 2, i.e. every epoxy resin having on average more than 2 epoxy groups per molecule, such as corresponding polyglycidyl ethers or polyglycidyl esters. Examples of resins suitable as component A2 are: triglycidyl isocyanurate, trimellitic acid triglycidyl ester, hexahydrotrimellitic acid triglycidyl ester, 1 ,1 ,2,2-tetrakis(4-glycidyloxyphenyl)ethane, the N,N,O-triglycidyl derivative of 4-ami- nophenol, the glycidyl ether of polyfunctional spirobisindanes, epoxyphenol novolaks and epoxycresol novolaks. Also suitable as component A2 are the solid mixed phases of tri- and difunctional epoxy resins described in EP-A-0 536 085. Preferred as component A2 are triglycidyl isocyanurate, trimellitic acid triglycidyl ester, hexahydrotrimellitic acid triglycidyl ester, epoxycresol novolaks, epoxyphenol novolaks or polyglycidyl compounds having on average more than two glycidyl groups per molecule on the basis of polyfunctional 1 ,1 '-spirobisindanes of formula I wherein Z is a direct single bond or -O-; and wherein more than two of the radicals R 1 f R 2l R 3 and R 4 are -OH, -O-CO-R-CO-OH, -O-R-OH, -O-CO-NH-R-NH-CO-O-R-OH or -[O-C m H 2rn ] n -OH, wherein m is an integer from 2 to 4 and n is an integer from 1 to 20, and R is CrC 8 alkylene, C 5 -C 8 cycloalkylene, C 6 -C 14 - arylene or partially hydrated C 6 -C 14 arylene, and the remaining radicals R 1 f R 2 , R 3 and R 4 are hydrogen, -0-CrCg.alkyl, -O-C 5 -C 8 cyclo- alkyl, -O-C 6 -C 14 aryl, partially hydrated -O-C 6 -C 1 aryl or (meth)acryloxy, and R 5 , R 6 , R 7 and R 8 are each independently of one another d-C 8 alkyl, C 5 -C 8 cycloalkyl, C 6 - C 14 aryi, partially hydrated C 6 -Cι 4 aryl or hydrogen. The polyglycidyl compounds which are derived from the spirobisindanes of formula I are described in WO 99/03851. Component A3 in the novel compositions may, in principle, be any of the aromatic compounds containing two phenolic hydroxyl groups that are known in the advancement method. Examples thereof are mononuclear diphenols, such as resorcinol, naphthalenes containing two hydroxyl groups, such as 1 ,4-dihydroxynaphthalene, biphenyls and other dinuclear aromatic compounds containing two hydroxyl groups which have an alkylene, -O-, -CO-, -S- or -SO 2 - link. It is also possible to use halogenated bisphenols, for example tetrabromobis- phenol A. Preferred components A3 are bisphenol S (bis(4-hydroxyphenyl)sulfone), 4,4'-dihydroxybi- phenyl, bisphenol F and, in particular, bisphenol A. In addition to the components A1 , A2 and A3, the mixtures for the preparation of the advanced epoxy resin A can contain a monofunctional compound as chain terminator. This is particularly advantageous if an especially good flow behaviour is desired. Suitable chain terminators are monophenols and phthalimides, for example phthalimide, 3- methylphthalimide, 4-methylphthalimide or 3,3-dimethylphthalimide. Monophenols used are particularly preferably phenols containing one or more than one C C 12 alkyl substituent or a C 6 -C 10 aryl substituent, such as ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, tert-pentyl, neopentyl, n-hexyl, isohexyl, corresponding heptyl or octyl substituents, in particular tert-octyl, nonyl, dodecyl or phenyl substituents. Mono-C Cβ-alkylphenols are particularly preferred, especially mono-C 3 -C 8 - alkylphenols, in particular the corresponding para-alkylphenols, and p-phenylphenol. Other suitable monophenols are described in US-A-5,095,050. Accordingiy, this invention also relates to a powder coating composition containing as component A an advanced epoxy resin prepared from a mixture which, in addition to the components A1 , A2 and A3, contains a monophenol or a phthalimide as chain terminator. The amounts of the components A1 , A2, A3 and, optionally, of the chain terminator used for the preparation of the advanced epoxy resin A can vary within a wide range. The amounts of A1 and A2 used are preferably chosen such that per 100 epoxy equivalents of the component A1 there are 1-20, particularly preferably 2-10 and, more preferably, 3-8, epoxy equivalents of the component A2. The amount of the component A2 used is preferably from 1.0-5.0 % by weight, more preferably from 1.25-3.0 % by weight, based on the sum of the components A1 + A2. The amount of the component A3 used in the advancement reaction is preferably chosen such that per 100 epoxy equivalents there are 10-90 hydroxy equivalents, particularly preferably 30-80 hydroxy equivalents and, more preferably, 50-75 hydroxy equivalents. The reaction of the components A1 , A2 and A3 is usefully carried out in the presence of a customary advancement catalyst. Suitable catalysts are disclosed, inter alia, in US-A-5,095,050, which disclosure is explicitly referred to here. Preferred examples of catalysts are tertiary amines, such as triethylamine, tripropylamine, tributylamine, 2-methylimid- azole, 2-phenylimidazole, N-methylmorpholine, N,N-ethylmethylpiperidinium iodide, quarter- nary ammonium compounds and alkali metal hydroxides. It is also possible to use combinations of different catalysts. The catalysts are used in customary catalytic amounts, i.e. generally in amounts from 0.0001 to 10 % by weight, based on the epoxy resin. The reaction temperatures are preferably from 80 to 200 °C, more preferably from 130 to 200 °C. Suitable components B of the novel compositions are, in principle, all polymers containing epoxy-reactive groups that are known in powder coating technology. The use of polyesters, polyacrylates or polyethers is preferred. Carboxyl-terminated polyesters are particularly preferred. The polyesters preferably have an acid number (indicated in mg KOH/g polyester) of 10 to 100 and a molecular weight of 4000 to 15000, more preferably of 6500 to 11000 (weight average M w from GPC measurement with polystyrene calibration). The ratio MJM n in the case of these polyesters is usually from 2 to 10. The polyesters are usefully solid at room temperature and have a glass transition temperature from 35 to 120 °C, preferably from 40 to 80 °C. Such polyesters are known, inter alia, from US-A-3,397,254 or EP-A-0 600 546, which disclosure is explicitly referred to here. They are reaction products of polyols with dicarboxylic acids and, optionally, with polyfunctional carboxylic acids or with the corresponding carboxy- lic anhydrides. Suitable polyols are, for example, ethylene glycol, the propylene glycols, 1 ,3-butanediol, 1 ,4-butanediol, neopentanediol, isopentyl glycol, 1 ,6 hexanediol, glycerol, hexanetriol, tri- methylolethane, trimethylolpropane, erythritol, pentaerythritol, cyclohexanediol or dimethylol- cyclohexane. Suitable dicarboxylic acids are, for example, isophthalic acid, terephthalic acid, phthalic acid, methylphthalic acids, tetrahydrophthalic acid, methyltetrahydrophthalic acids, for example 4-methyltetrahydrophthalic acid, cyclohexanedicarboxylic acids, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid or 4,4'-diphenyldicarboxylic acid, and the like. Suitable tricarboxylic acids are, for example, aliphatic tricarboxylic acids, such as 1 ,2,3-propanetricarboxylic acid, aromatic tricarboxylic acids, such as trimesic acid, trimellitic acid and hemimellitic acid or cycloaliphatic tricarboxylic acids, such as 6-methylcyclohex-4-ene-1 ,2,3-tricarboxylic acid. Suitable tetracarboxylic acids are, for example, pyromellitic acid or benzophenone-3,3',4,4'-tetracarboxylic acid. Commercially available polyesters are particularly often based on neopentyl glycol and/or trimethylolpropane as essential alcoholic components and also on adipic acid and/or terephthalic acid and/or isophthalic acid and/or trimellitic acid as essential acid components. Powder coating compositions of this invention which comprise trimellitic acid-free polyesters often have a particularly good flow. The novel powder coating compositions preferably contain 20-80 % by weight of the component A and 80-20 % by weight of the component B, the sum of the components A + B always being 100 % by weight. The components A and B are preferably present in the novel powder coating compositions in such amounts that the ratio of epoxy-reactive groups to epoxy groups in the powder coating composition is from 0.5 to 1 and 2 to 1 , preferably from 0.8 to 1 and 1.2 to 1 , more preferably about 1 to 1 Jn this case, epoxy-reactive groups-containing polymers and epoxy resins are present in a weight ratio from 70 ± 5 to 30 ± 5 , 60 ± 5 to 40 ± 5 or 50 ± 5 to 50 ± 5 (70/30-; 60/40- and 50/50 hybrid systems). The novel powder coating compositions can additionally contain other additives customarily used in the paint industry, such as light stabilisers, dyes, pigments, e.g. titanium dioxide, degassing agents, e.g. benzoin, and/or flow control agents. The novel powder coating compositions can be prepared by simply mixing the components A and B and the other components, e.g. in a ball mill. Another possibility is to melt the components together, mixing them and homogenising them, e.g. using an extrusion machine, such as a Buss co-kneader, to cool the composition and to comminute it. The ready powder coating mixtures preferably have a particle size in the range of 0.015 to 500 μm, more preferably of 10 to 100 μm. After being applied to the article to be coated, the powder coating compositions are cured at a temperature of at least about 100 °C, preferably of 150 to 250 °C. Curing takes about 5 to 60 minutes. Suitable materials for coating are all those which are stable at the temperatures necessary for curing, in particular ceramics, glass and metals. Examples 1-4: Preparation of the advanced epoxy resins Example 1 : A mixture consisting of 630 g of diglycidyl ether of bisphenol A (epoxy content: 5.26 - 5.38 val/kg), 231 g of bisphenol A, 12.6 g of triglycidyl isocyanurate and 0J8 g of KOH (in the form of an aqueous solution comprising 7 % by weight of KOH) is heated to 170 °C and kept at this temperature for 3 h. The melt is cooled to 130 °C and poured on panels, upon which the product crystallises. Examples 2-4: In analogy to Example 1 , mixtures consisting of diglycidyl ether of bisphenol A (epoxy content: 5.26-5.38 val/kg), bisphenol A, triglycidyl isocyanurate, tert-butylphenol and KOH are reacted. The amounts used and the properties of the advanced resin are compiled in Table 1. The glass transition temperature T g is determined via differential scanning calorimetry (DSC, 2. Flow: -30 °C to 250 °C). The melt viscosity is measured at 150 °C using an ICI cone/plate viscosimeter. Table 1 : Example 5: Preparation of a masterbatch A mixture consisting of 630 g of diglycidyl ether of bisphenol A (epoxy content: 5.26 - 5.38 val/kg), 231 g of bisphenol A, 12.6 g of triglycidyl isocyanurate and 0.18 g of KOH (in the form of an aqueous solution containing 7 % by weight of KOH) is heated to 170 °C and is kept for 3 h at this temperature. After cooling to 140 °C, a sample is taken: the product has a glass transition temperature of 55 °C and molecular weights of M n = 1250 and M w = 3400. 87 g of polybutylacrylate (Acronal ® 4F) are then added to this melt. The reaction mass is homogenised for 0.5 h at 140 °C and is then poured on panels, upon which the product crystallises. Examples l-V: Preparation of powder coating compositions Uralac ® P 2127 is a commercially available polyester, of DSM (NL), based on phthalic acid/ isophthalic acid/adipic acid and neopentylglycol/trimethylolpropane. Resin and polyester are used in all powder coating compositions in such amounts that an about 10 % excess of epoxy group is obtained, based on the carboxyl groups of the polyester. After premixing the components, they are further mixed using a twin screw extruder (PRISM TSE 16 PC) at 1 10°C. The extrudate is cooled on a cooling roll, broken up into clumps and then rapidly ground in a centrifugal mill, of Retsch, to a fine powder which is then sieved through a sieve having a mesh width of 100μm. By means of an electrostatic spraying pistol, of ESB, the powder is coated onto 60μm thick Q-panels at a coating thickness of 60 μm and 65 μm, respectively. The panels are heated in an oven for 15 min to 200°C in order to melt and fully cure the coating. The surface structure is examined with respect to its texture using a "Wave Scan" profilo- meter (of Byk-Gardener). The k-parameters (longwave) measured are compiled in Table 2. k-Values (longwave) higher than about 50 indicate in this case a very uneven surface (orange peel effect) and thus an unsatisfactory flow, whereas values in the range of 30 indicate a very level surface and excellent flow. The impact strength r(reverse side) is determined by dropping a 2 kg die, on the bottom side of which is a ball 20 mm in diameter, bottom first from a specific height from the reverse side onto the coated area. The value indicated is the product of the weight of the die in kg and of the test height in cm at which no damage of the coating is yet found. An impact strength of > 160 cm kg is measured in all Examples. The composition of the powder coating compositions and the measured properties are compiled in Table 2. Table 2:

Claims: What is claimed is 1. A powder coating composition, which comprises (A) an advanced epoxy resin prepared by reacting a mixture comprising a bisphenol diglycidyl ether of bisphenol (A1) and an epoxy resin having an epoxy functionality of >2 (A2) with a bisphenol (A3) in the presence of an advancement catalyst, and (B) a polymer containing epoxy-reactive groups. 2. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from diglycidyl ether of bisphenol A or diglycidyl ether of bisphenol F as component A1. 3. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from a mixture containing as component A2 triglycidyl isocyanurate, trimellitic acid triglycidyl ester, hexahydrotrimellitic acid triglycidyl ester, an epoxycresol novolak, an epoxyphenol novolak or a polyglycidyl compound containing on average more than two glycidyl groups per molecule based on a polyfunctional 1 ,1'-spirobisindane of formula I wherein Z is a direct single bond or -O-; and wherein more than two of the radicals Rι, R 2 , R 3 and R 4 are -OH, -O-CO-R-CO-OH, -O-R-OH, -O-CO-NH-R-NH-CO-O-R-OH or -[O-C m H 2m ] n -OH, wherein m is an integer from 2 to 4 and n is an integer from 1 to 20, and R is CrC 8 alkylene, C 5 -C 8 cycloalkylene, C 6 - Cι 4 arylene or partially hydrated C 6 -C 14 arylene, and the remaining radicals Rι, R 2 , R 3 and R are hydrogen, -O-CrCβalkyl, -O-C 5 -C 8 cyclo- alkyl, -O-C 6 -C 14 aryl, partially hydrated -O-C 6 -C 14 aryl or (meth)acryloxy, and R5. Rε. 7 and R 8 are each independently of one another Cι-C 8 alkyl, C 5 -C 8 cycloalkyl, C 6 - Cι aryl, partially hydrated C 6 -Cι 4 aryl or hydrogen. 4. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from bisphenol A, bisphenol F, bisphenol S or 4,4'-dihydroxybiphe- nyl as component A3. 5. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from a mixture which, in addition to the components A1 , A2 and A3, comprises a monophenol or a phthalimide as chain terminator. 6. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from a mixture consisting of the components A1 , A2 and A3, the amounts of A1 and A2 used being chosen such that per 100 epoxy equivalents of the component A1 there are 1-20 epoxy equivalents of the component A2. 7. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from a mixture of the components A1 , A2 and A3, the component A2 being used in an amount from 1.0-5.0 % by weight, based on the sum of the components A1 + A2. 8. A powder coating composition according to claim 1 , wherein component A is an advanced epoxy resin prepared from a mixture of the components A1 , A2 and A3, the amount of the component A3 used being chosen such that per 100 epoxy equivalents there are 10-90 hydroxy equivalents. 9. A powder coating composition according to claim 1 , wherein component B is a polyester, polyacrylate or a polyether. 10. A powder coating composition according to claim 1 , wherein component B is a carboxyl- terminated polyester. 11. A powder coating composition according to claim 1 , which contains 20-80 % by weight of component A and 80-20 % by weight of component B, the sum of the components A + B always being 100 % by weight. 2. A powder coating composition according to claim 1 , wherein the ratio of the components A and B is chosen such that per one epoxy equivalent there are 0.5-1.5 equivalents of the epoxy-reactive groups.

Inventors: MALHERBE ROGER, PEYER ROBERT PETER, RICKERT CHRISTOPH, ROTH MARTIN, MALHERBE, ROGER, PEYER, ROBERT, PETER, RICKERT, CHRISTOPH, ROTH, MARTIN
Applicants: CIBA SC HOLDING AG, MALHERBE ROGER, PEYER ROBERT PETER, RICKERT CHRISTOPH, ROTH MARTIN, CIBA SPECIALTY CHEMICALS HOLDING INC., MALHERBE, ROGER, PEYER, ROBERT, PETER, RICKERT, CHRISTOPH, ROTH, MARTIN
Application year: 2000
Main classification: C09D163
IPC codes: C08G59, C09D5, C09D163