COATING POWDERS

Abstract: A thermosetting coating powder comprises a carboxyl-terminated polyester resin and a curing agent for the resin, with an aluminium organic compound added in a concentration to modify the surface finish of the cured coating and preferably to produce a matt finish.

Description: COATING POWDERS This invention relates to thermosetting polyester resin coating powders. These are typically based on a comminuted resin/pigment formulation and are typically applied by fluidized bed coating or electrostatic spray coating followed by stoving. Coating powders have various advantages over liquid paints; they are essentially 100% non-volatile, no solvents or pollutants being given off during application or curing; they are easily applied by unskilled operators and automatic systems; the finish is tougher and more abrasion resistant than that of most liquid paints; the material is well utilised, overspray is easily collected and re-applied. An increasing share of the total paint market is now being supplied by powder coating processes. In this specification, the term coating powder refers to the material and the term powder coating to the process and the applied film. This invention is concerned with agents which modify the surface finish of the cured coating. There follows a brief review of the current situation relating to commercially available matting agents for the production of reduced gloss finishes. For many decorative powder applications the preferred finish has a lower level of gloss than that which could be obtained without a matting agent. It is felt by designers that a reduced gloss finish has a better aesthetic appeal than a highly reflective glossy surface which does tend to mirror all film defects. Reduction in gloss can be achieved by incorporation of certain fillers, polyethylene and other waxes, and by physical and chemical incompatibility. Generally, the effect of fillers is not marked since the matting effect which is apparent in wet paints cannot be simulated by the hot melt compounding technique. If sufficient filler is added to affect the gloss level this is usually accompanied by a reduction in film flow properties. The presence of cheap fillers can also affect the exterior durability of coatings. Waxy materials tend to sit in the surface of the stoved film where they impart slip and a gloss reduction due to light scattering of the individual particles. This is the technique of producing a very fine texture which has the visual effect of matting the film. But wax spoils mar resistance and gives rise to a thermal break problem if a sealant is applied. Physical incompatibility caused by the mixture of standard powder resins with such incompatible resins as neoprene, can reduce gloss but often adversely affects the mechanical properties of the cured film. Reducing the gloss level of exterior durable coatings based on polyester resins requires a different and often troublesome technique. It is necessary to combine polyester systems which use both carboxylated and hydroxylated polymers. The carboxylated types are cured with TGIC and the hydroxylated types with a blocked isocyanate. If compounded together in the extruder they do not produce the optimum matting effect. It is, therefore, necessary to extrude the two types separately and to premix the extrudate flakes prior to fine grinding. This technique will give a smooth low gloss finish of 20-30% but it is almost impossible to achieve a true matt finish (below 10%). This technique increases the labour costs during manufacture and, in the case of coloured coatings, the two components have to be exactly colour matched prior to blending. There is a need for better matting agents for thermosetting polyester resin coating powders. This invention results from the finding that aluminium organic compounds can fulfil that need. SU 730716 describes metal organic compounds, including aluminium organic compounds, as coaccelerators with zinc compounds for dicyandiamide hardeners in epoxy resin coating powders. Transition metal or aluminium compounds are used in JP 53073225 as curing agents for hydroxyl group containing polyester or acrylic resins. The polyester/epoxy system of European Patent Application EPO 165207 contains a combination of a wax and a metal salt or metal complex of an organic compound to give a matting effect. GB 1580358 describes coating powder compositions of hydroxyl terminated polyester resins with a low acid number, and metal organic compounds as additives. The subject of the present invention is the surprising discovery that aluminium organic compounds alone can act as matting agents in coating powders comprising carboxyl terminated polyester resins. The invention provides a thermosetting coating powder comprising a carboxyl terminated polyester resin and a curing agent for the resin, wherein there is present an aluminium organic compound in a concentration to modify the surface finish of the cured coating. The carboxyl terminated polyester resins used in the coating powders of this invention may be conventional, and may or may not be chosen from among the following: The carboxyl terminated polyester resin has an acid number of at least 15, preferably 20-55 and more preferably 30-40, (mg KOH/g resin) . Often, the resin will have a hydroxyl number of not more than 10 preferably less than 5. The thermosetting coating powders of this invention are preferably suitable to apply exterior durable paint coatings. In such compositions, which should preferably not contain epoxy resins, the polyester resins have traditionally been cured by means of modified epoxies such as TGIC (triglycidyl isocyanurate) . More recently so-called non-toxic curing agents based on β-hydroxyalkylamides have become available, such as that marketed by EMS Grillon under the Trade mark Primid XL552 having the structure (OH.CHR.CH_} _NC0.R„CO. (CH- .CHR.OH) „ Such curing agents are within the scope of this invention. The nature of the curing agent used is not material to the invention. The aluminium organic compound may be an aluminium alkoxide or a compound derivable from an aluminium alkoxide by a substitution or condensation reaction. There is preferably used an aluminium organic compound including at least one group - 0 - Al - 0 - where X is derived from HX which is an alcohol or phenol, a carboxylic acid, a 1 ,3-diketone, an alkyl acetoacetate or an alkyl half-ester of a dicarboxylic acid and may be unsubstituted or substituted. The aluminium organic compound (AOC) may include one or two hydroxyl groups attached to Al. Or the AOC may have the formula A1X-, where X is defined above. Or the AOC may contain more than one Al atom per molecule, by virtue of having the formula where p is less than q equals 3-2p, and n is 2 or more. X is derived from HX which may be water but is organic in at least one position in the molecule and is preferably: a C1 to C30 aliphatic alcohol or a phenol which may be substituted; a C1 to C30 carboxylic acid, which may be aliphatic, alicyclic or aromatic and which may be substituted; a 1, 3-diketone such as acetylacetone; - a C1 to C12 alkyl acetoacetate, which may be substituted in the alkyl group; a C1 to C12 alkyl half-ester of a C1 to C12 aliphatic, alicyclic or aromatic dicarboxylic acid, which may be substituted. In all cases, X may be the same or different at different positions of the molecule. As substituents in the above, C1 to C12 alkoxy groups and C1 to C4 dialkyl amino groups are envisaged among others. Alkyl acetoacetates have the formula CHgCOCH-COOR, where R is C1 to C12 alkyl. They are believed to co-ordinate to Al through both oxygen atoms, in similar manner to carboxylates and to acetylacetone. Examples of various AOC ' s within the above general classes are set out below in numbered paragraphs 1 to 8. 1. Aluminium tri-alkoxides 1.1 Al monobutoxyethoxide di-isopropoxide 1.2 Al monoisopropoxide (2-methylpentan-2,4-diolate) 1.3 Al triisopropoxide 1.4 Al tri sec. butoxide 1.5 Al tri ethoxypropoxide 1.6 Al tristearyloxide 1.7 Al tri 2-methylpentoxide 1.8 Al tri 2-methylbutoxide 1.9 Al triethoxyaminate 2. Monosubstituted Al.dialkoxides 2.1 Al diisopropoxide mono(ethylacetoacetate) 2.2 Al diisopropoxide mono(monoisopropylmaleate/ fumarate) 2.3 Al diisopropoxide mono(monoisopropylphthalate) 2.4 Al diisopropoxide mono(acetylacetonate) 3. ' Pisubstituted Al monoalkoxides 3.1 Al monoisopropoxide di(ethylacetoacetate) 3.2 Al monoisopropoxide mono(ethylacetoacetate)mono (monoisoprop lmaleate) 3.3 Al monomethoxypropoxide monoethylacetoacetate dimethylamino ethoxide 3.4 Al monoisopropoxide monoethylacetoacetate monoacetate 3.5 Al monoisopropoxide monohydroxide monoversatate 3.6 Al monoisopropoxide di(acetylacetonate) 4. Aluminium tri-acetYlacetQaate 5. Aluminium tri(ethylacetoacetate) - • Oxo aluminium alkoxides 6.1 Oxo aluminium ethoxypropoxide 6.2 Oxo aluminium butoxyethoxide 7. QXQ Al acyloxides and enolates 7.1 Oxo aluminium mono(ethylacetoacete) 7.2 Oxo aluminium 2-ethylhexoate 7.3 Oxo aluminium stearate 8. Aluminium acvloxides 8-1 Dihydroxy Al monostearate 8.2 Monohydroxy Al di stearate 8.3 Al monoacetate diethylacetoacetate 8.4 Al tristearate All these AOC ' s have at least one substituent, an alkoxide or an oxo linkage, which is capable of addition or substitution reaction with a carboxylic acid group of the resin with which it is compounded, to provide a reaction product resistant to hydrolysis or dissociation. These AOC ' s may also have an ester substituent which, we have discovered, can transesterify with any hydroxyl groups present in the resin. The AOC ' s may have the property of enhancing the gloss finish of the cured coating at low levels of AOC addition. At higher levels they will usually provide a reduced gloss surface finish, of the kind often described as satin, semi-gloss, semi-matt, matt or dead matt, and may thus be referred to as matting agents. The amount of AOC used is generally 0.1 - 20%, e.g. 0.5 - 10%, preferably 1 - 5%, by weight on the combined weights of resin and curing agent. If the concentration is too low, little surface modification results. If the concentration is too high, the physical properties of the cured coating may be adversely affected. The use of an AOC according to this invention may permit the amount of curing agent to be less than would otherwise be required. It is thus envisaged that in certain cases the AOC may be used in partial or even total replacement of the curing agent. Alternatively, use of an AOC together with a standard amount of curing agent may permit the time or temperature of cure to be somewhat reduced. It is preferred that the carboxyl terminated polyester be the only resin present in major amount. The invention is not concerned with mixed resin systems, such as epoxy-polyester or even hydroxypolyester - carboxypolyester systems. Similarly it is preferred that the AOC be the only texturing or matting agent present in major amount. In particular, waxes have disadvantages as noted above, and are best omitted. The coating powder is in particulate form. For fluidized bed coating, particles above 70 mesh (210 microns) are generally removed. For electrostatic spray methods, finer particles are generally preferred, e.g. which pass a 100 mesh (149 micron) sieve but are retained on a 325 mesh (44 micron) sieve. Curing agents are generally present in proportions from 1 to 25% or 40% by weight on the weight of the resin. Other conventional ingredients for thermosetting coating powders, such as pigments, mineral and organic extenders, flow additives, wetting agents and accelerators, may be present in conventional concentrations. Coating powders can be formulated by dry- blending or more preferably by melt-mixing. In melt- mixing the ingredients are intensively mixed, then melt compounded at 80-105 * C and rapidly cooled to prevent thermosetting reaction. The cooled ribbon is broken into small chips suitable for fine grinding. It is a surprising discovery that AOC ' s do not cause premature gelation at the elevated temperature required for the melt-mixing process. In liquid paint manufacture for example, AOC ' s are found to be highly reactive with carboxyl groups even at room temperature. AOC's which are liquid can be made more compatible with the remaining ingredients by adsorption onto the surface of a particulate extender or pigment such as silica, aluminium hydroxide or titanium dioxide; AOC's can act as dispersing aids by coating the particles of extenders or pigment and making them more compatible with the resin. Masterbatch techniques may be needed to achieve uniform dispersion of curing agents. AOC ' s may act as water scavengers, by reacting with any moisture adsorbed on pigment or extender. This may have several effects: oxo-aluminium organic compounds may be formed in situ rather than being added deliberately. solvent may be released by the reaction. This may lower the viscosity of the resin during compounding, which may permit the use of simpler machinery or higher filler loadings. Conventional coating techniques can be used, including fluidized bed coating, hot flocking, electrostatic fluidized bed coating, and preferably electrostatic spray coating. This latter technique is well adapted for automatic operation to apply uniform coatings of standard thickness from 25 to 2500 microns. In contrast to liquid paints, with electrostatic powder coating, too much material cannot be applied and a fully coated part is obtained even where the parts are complex in shape. The following examples illustrate the invention. Comparative Example Matt Control Formulation (Traditional Technique) Component A BkH P2504 480 B1530 120 CL2310 250 Resiflow PV5 10 P2504 is a commercially available hydroxylated polyester from DSM Resins. B1530 is a blocked isocyanate curing agent from Huels. CL2310 is titanium dioxide from Kronos. Resiflow PV5 is a flow agent from Worlee Chemie. The components were premixed in a polyethylene bag by shaking and melt compounded in a Baker Perkins MP 2000 twin screw extruder. Component B pbw P2400 558 PT810 42 CL2310 250 Resiflow PV5 10 P2400 is a commercially available carboxylated polyester from DSM Resins. PT810 is a triglycidyl isocyanurate (TGIC) curing agent from Ciba-Geigy. The components were compounded as for Component A. Mixed Polyester System Component A 100 pbw Component B 100 pbw Equal parts of extrudates of Component A and Component B were ground together in a pin disk mill and sieved through 125 μm screen. The resultant powder was applied to aluminium and steel panels using a Volstatic electrostatic spray gun and the coated panels were stoved at 200 * C for 10 minutes (peak metal temperature) . The stoved coatings had a smooth satin finish with good mechanical properties. pbw 558 42 42.8 (70:30 silica) 250 10 - 1 2 - AOC 3.3 aluminium monomethoxypropoxide monoethylacetoacetate dimethylaminoethoxide, is a viscous liquid and was absorbed onto Aerosil 200 fumed silica (Degussa) in the ratio 70/30 p.b.w., prior to compounding with the other ingredients. The components were melt compounded in a twin screw extruder at 95"C set barrel temperature, applied to panels and stoved as for Matt Control formulation. The coating had a matt, slightly textured appearance with good mechanical properties. For comparison, the formulation was made up using the equivalent amount of fumed silica without AOC (i.e. 13 parts by weight). The coating obtained did not have a matt finish. This confirms that the matting effect is due to the AOC rather than to the silica. Example 2 Various AOC ' s were tested as matting agents with two different carboxyl-terminated polyesters and a range of pigments. Results demonstrate improved matt finish (lower gloss %) with increasing quantity of AOC matting additive. Uralac P2400 used in General Formulation I and Table I below is the carboxylated polyester as above. Acid No. 30-37. Grilesta P7309.3 in General Formulation II and Table II is another commercially available carboxylated polyester. Acid No. 30-36. Components were premixed then melt compounded in a twin screw extruder with the barrel temperature set at 95"C, applied to panels and stoved, as previously. AOC Mattinσ Additives AOC 1 Aluminium diisopropoxide mono(ethylacetoacetate) (2.1) 2 Aluminium monoisopropoxide di(ethylacetoacetate) (3.1) 3 Aluminium tri(ethylacetoacetate) (5) 4 Aluminium tri-stearyloxide (1.6) 5 Aluminium tri-acetylacetonate (4) 6 Aluminium tri-stearate (8.4) 7 Aluminium monoisopropoxide monohydroxide monoversatate (3.5) 8 Aluminium monoisopropoxide di (acetylacetonate) (3.6) 9 Aluminium monomethoxypropoxide monoethylacetoacetate dimethylaminoethoxide (3.3) AOC ' s 1, 3 and 9 being viscous liquids were absorbed onto Aerosil 200 amorphous silica in the ratio 70pbw AOC:30 pbw silica prior to compounding with other ingredients. General Formulation I URALAC P2400 558 pbw PT810 42 pbw AOC 12-60 pbw (See Table 1) PIGMENT See Notes 1-4 RESIFLOW PV5 10 pbw 2. Bayerferrox 303 T 7 pbw (from Bayer AG) Bayerferrox 130 BM 25 pbw (from Bayer AG) 3. Αpart from the TiO_, all reagents are from Sando: TABLE 1 (RESIN: URALAC P2400) AOC pbw PIGMENT GLOSS(%) 30 18 24 30 18 24 30 TABLE 2 (RESIN: GRILESTA P7309.3) a) . Gloss (%) at 60° angle to BS3900 D5 (Full gloss = 100% Dead Matt = 0%). b) . Mandrel Flexibility (mm) to BS3900 E1 (Quoted as smallest Mandrel which can be used without cracking. Low values are best) . c) . Erichsen Slow Indentation to BS 3900 E4 (Quoted as Indentation until the onset of cracking. High values are best) . Example 3 The following formulation uses a different curing agent. GRILESTA V76.12 570 pbw PRIMID XL552 30 pbw AOC NO. 9 0-30 pbw PIGMENT See Note 1 RESIFLOW PV5 10 pbw Grilesta V76.12 is a commercially available carboxylated polyester. Acid No. 34, T 62 * C. Primid XL552 is a curing agent based on a β- hydroxyalkylamide and has the formula (OH.CHR.CH 2 ) 2 NCO.R 2 CO.N(CH 2 .CHR.OH) 2 . Grilesta V76.12 and Primid XL552 are supplied by EMS Grillon.

Claims: £LAI2 1. A thermosetting coating powder comprising a carboxyl-terminated polyester resin and a curing agent for the resin, wherein there is present an aluminium organic compound in a concentration to modify the surface finish of the cured coating. 2. A thermosetting coating powder as claimed in Claim 1, wherein the aluminium organic compound is an aluminium alkoxide or a compound derivable from an aluminium alkoxide by a substitution or condensation reaction. 3. A thermosetting coating powder as claimed in Claim 1 or Claim 2, wherein the aluminium organic compound including at least one group - 0 - Al - 0 - where X is derived from HX which is an alcohol or phenol, a carboxylic acid, a 1,3-diketone, an alkyl acetoacetate or an alkyl half-ester of a dicarboxylic acid, may be unsubstituted or substituted and may be the same or different at different positions of the molecule. 4. A coating powder as claimed in Claim 3, wherein the aluminium organic compound has the formula A1X 3 - 5. A coating powder as claimed in Claim 3 or Claim 4, wherein at least one group X is derived from an alkyl or alkoxyalkyl acetoacetate or acetylacetone. 6. A coating powder as claimed in any one of Claims 1 to 5, wherein the aluminium organic compound is present at a concentration of 1 - 10 parts by weight per 100 parts of resin. 7. A thermosetting coating powder as claimed in any one of Claims 1 to 6, wherein the nature and concentration of the aluminium organic compound is such as to provide a reduced gloss surface finish of the cured coating. 8. A thermosetting coating powder as claimed in any one of claims 1 to 7, wherein the polyester resin has an acid number greater than 20. 9. A method of powder coating a workpiece, which method comprises applying the thermosetting coating powder of any one of Claims 1 to 8 to the workpiece by an electrostatic spray coating technique and curing the resin to form a coating on the workpiece. 10. Use of the thermosetting coating powder claimed in any one of Claims 1 to 8 to form a cured coating having a reduced gloss surface finish.

Inventors: AZIZIAN FARID, HOPKINS ALAN CHARLES, AZIZIAN, FARID, HOPKINS, ALAN, CHARLES
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1992
Main classification: C09D5
IPC codes: C09D5, C09D167