CA1260188A - Binder combinations for the coating of plastics - Google Patents

Abstract

Binder combinations for the coating of plastics A b s t r a c t The invention relates to a process for preparing, on surfaces on metal and plastics, coatings which, by com-bining OH-functional polyacrylate resins and polyiso-cyanates, can be hardened to give coats which, in addition to having excellent film appearance and chemical resist-ance, have an elasticity level so high as to make possible their use for example inter alia in the automotive sector on parts which are subject to a high impact and bending stress.

Description

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The invention relates to a process for preparing, on surfaces on metal and plastics, coatings whlch, by com-bining OH-functional polyacrylate resins and polyiso-cyanates, can be hardened to give coats which, in addi-tion to having excellent film appearance and chemicalresistance, have an elasticity level so high as to make possible their use for example inter alia in the automotive sector on parts which are subject to a high impact and bending stress.
- 10 It is known to use coatings to improve the pro-perties of plastics in such a way as to make possible their use for a specific field of use. If plastics parts are to be used for example in the automotive area in places which are particularly exposed to weathering, im-pact stress and stress due to environmental factors, as is the case for example in the area of the lower side parts (sills), and of the front and rear parts, then these parts need to be provided with coatings which need to meet a whole number of requirements. Also to be mentioned here are for example household appliances which need to be coated, such as washing machines, dish washers etc.
Owing to the h;gh mechanical stress to which for example the coatings are subject in the automotive sec-tor, in particular in the so-called bumper area, it is also necessary to obtain in addition to extremely high flexibility adequate surface hardnesses.
The plast;cs surfaces which are particularly exposed to dirt off the road, namely the sills, on the other hand, need to be protected by coatings which in addition to having the mechanical properties required (stone chip resistance) have a high resistance to salt and tar and oil spots~ All coatings have the common object of obtaining a good film appearance which in addi-tion to high degrees of gloss also guarantees the Le A 23 915 ~.~

~ . ' preservation of these visual properties over a period of many years despite weathering effects.
To coat plastics, a large number of b;nder sys-tems are used. For instance, DE-OS (German Published Specification 2,364,736) describes coating rubber products ~ith f;nishes which are composed of an acrylic finish based on styrene, alkyl tmeth)acrylates and hydroxyalkyl (meth)acrylates and are hardened by combination with alkylated melamine-formaldehyde resins or polyisocyana~es.
However, the resulting highly flexible coatings are ob-tained at the price of a number of disadvantages~ The basic property which is responsible for the mechanical properties, namely the adhesion of the coating to the plastics surface is obtained by surface-activating the rubber surface before coating and thereafter priming it with two acrylic resin finishes.
An additional restriction is given by the choice of substrate. The coat;ngs exhibit their special proper-ties only on pretreated rubber surfaces such as an ethylene-propylene copolymer rubber, styrene-butadiene copolymer rubber, a polybutadiene rubber, a polyisoprene rubber or a butyl rubber.
DE-OS (German Published Specification) 2,756,708 describes polyacrylate-based films which are capable of be;ng coated ~ith a reactive finish system based on polyol/polyisocyanate ~;thout adhesion problems and with-out further pretreatment. The adhesion problems are removed by adding to the copolymers monomers having func-tional end groups ;n amounts of 1 to 15% by weight, which can enter a bond with the reactive isoGyanate-based finish system. This process, despite the omission of a special pretreatment of the substrate surface, has the disadvan-tage that it is onLy applicable to the special case of where the film material is made of tmeth)acrylate copoly-mers and substrate surfaces based on different polymerscannot be coated in this way~
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lt has now been found that the binder combina-tions of OH-functional copolymer resins of a certain com-position and polyisocyanates give after hardening on sur-faces which have not been further pretreated, preferably plastics surfaces, firmly adhering films whose property profile such as elasticity, hardness, chemical resistance and weather resistance make possible the use for example in the mixed method of automotive construction and also in windows, household appliances etc~
The invention provides binder combinations of A~ 99 - 10 parts by weight, preferably 90 - 30 parts by weight, of OH-functional copolymer resins which con-ta;n per molecule at least two functional OH groups, prepared by copolymerization from a) O - 26.5 parts by weight of at least one unsaturated aromatic monomer such as styrene, vinyltolu-ene and/or methacrylic acid ester with ali-phatic hydrocarbon radicals of chain length C1 to C6, preferably methyl methacrylate, b) 40 - 65 parts by weight of at least one aliphatic acrylic acid ester with 1 - 12 C atoms in the alcohol component, preferably butyl acrylate, c) 25 - 40 parts by weight of at least one hydroxyalkyl ester of acrylic acid with 2 - 8 C atoms in the hydroxyalkyl radical, d) O - 50 parts by weight of at least one acrylic or m~thacrylic acid glycidyl ester or of reac-tion products thereof with aliphatic and/or aromatic monocarboxylic acids and amino com-pounds, e) O - 40 parts by weight of an acrylic acid or meth-:~ ~ acrylic acid amide with 1 - 20 C atoms in the amido radical, in particular alkoxylated ~ ~: hydroxymethyl-acrylic (or methacrylic) acid :~ 35 amides~ which can also be produced in situ, f) O - 60 parts by weight of acrylonitrile, ; Le A Z3 915 ::

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9) 0.05 - S parts by weight of at least one ~,~-monoole-finically unsaturated monocarboxylic or di-carboxylic acid having 3 - 7 C atoms and/or of at least one maleic acid half-ester or S fumaric acid half-ester with 2- 14 C atoms in the alcohol radical, preferably acrylic acid, and B) 1 - 90 parts by weight, preferably 5 - 45 parts by weight, of a pc,lyfunctional isocyanate, preferably oligomeric ;socyanates having a biuret, allophanate, urethane and/or isocyanurate structure, in each case the sum of the parts by weight of components A) and ~) and the sum of the parts by weight of components a) to g) being 100~
The preparation of the polyacrylate resins A can be effected by copolymerizing the constituents a - g by customary methods, of which preference is given to the free radical polymer;zation in the mass or in solution.
Therein monomeric units are copolymerized at temperatures 20 of:70 - 160C, preferably 100 - 160C, in the presence of free radical producers and, optionally, regulators.
Preferred starting compounds for the polyacrylate resins A consist of a) 0 - 26.5 parts by weight of styrene, vinyltoluene, : 25 methyl methacrylate, butyl methacrylate or mixtures thereof, b) 45 - 63 parts by weight of ethyl acrylate, butyl ~: acrylate, 2-ethylhexyl acrylate or mixtures ~: : thereof, 3û c) 25 - 39 parts by weight of 2-hydroxyethyl acrylate,

2-hydroxypropyl acrylate, 4-hydroxybutyl : acrylate or mixtures thereof, ;~ d) 0 - 30 parts by weight of glycidyl acrylate, gly-cidyl methacrylate or mixtures thereof, optionally after reaction with saturated and/
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monocarboxylic acids, the reaction being car-ried out before, preferably after the copoly-merization, at temperatures of 80 - ~60C with the use of customary catalysts, e) 0 - 30 parts by weight of methoxymethyLacrylamide or methoxymethylmethacrylamide or mixtures thereof, where the alkoxylated monomers can if desired be copolymerizeri, or where after copolymerization c,f acrylic acid or meth-acrylic acid amide or m;xtures thereof prep-aration of the alkoxylated compounds can if desired be eFfected by reaction with formal-dehyde in the presence of methanol, f) 0 - 45 parts by weight of acrylonitrile, 9) O.S - 4 parts by weight of acrylic acid~ methacrylic acid or maleic acid half-ester w;th 4 - 8 C
atoms in the alcohol component or m;xtures thereof.
The monomers a - g are incorporated in the copoly-mer in essentially the same ratios in which they are usedfor the polymerization. The copolymerized units can essentially be randomly distributed.
Suitable solvents are for example aromatics such as benzene, toluene, xylene, chlorobenzene, esters such as ethyl acetate, butyl acetate, methylglycol acetate~
ethylglycol acetate, ethers such as butylglycol, tetra-hydrofuran, d;oxane, ethylglycol etherr ketones such as acetone, methyl ethyl ketone, halogen-containing solvents such as methylene chloride or trichloromonofluoroetharle.
The preparation of the polyacrylate resins can be carried out either continuously or discontinuously.
Preferably the monomer mixture and the initiator are metered uniformly and continuously into a polymerization reactor and the corresponding amount of polymer will be continuously taken off at the same time.
Preferably this method can be used to prepare Le A 23 915 ~: :
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chemically uniform copolymers. Chemically almost uniform copolymers can also be prepared by letting the reaction mixture run at a csnstant speed into a stirred vessel without discharging the polymer.
Suitable initiators for preparing the polyacryl-ate resins are compounds whose free radical decay half-lives at 70 - 160C range from 0.01 to 400 minutes.
In general the copolymerization reaction is effec-ted within the temperature range from 70 to 160C, prefer-ably from 100 to 160C, under a pressure of 103 to 2 x 104 mbar. The initiators are used in amounts of 0.05 to 15 by weight, relative to the total amount of monomers. In general the polymerization is brought to a conversion of 98% of the starting monomers, but can also be ciiscontinued beforehand.
Suitable initiators for carrying out the process according to the invention are those compounds which de-compose thermally into free radicals, for example ali-phatic azo compounds such as azoisobutyronitrile, azo bis-2-methylvaleronitrile, 1,1'-azo-bis-1-cyclohexanonitrile and alkyl 2,2'-azo-bis-isobutyrates; symmetrical diacyl peroxides such as, for example, acetyl peroxide, propionyl peroxide, butyryl peroxide, bromine, nitro, methyl or methoxyl sub~tituted benzoyl peroxides and also lauroyl peroxide; symmetrical peroxydicarbonates such as, for example, diethyl, diisopropyl, dicyclohexyl and dibenzoyl peroxide carbonate; tert.-butyl peroctoate or tert.-butylphenyl peracetate and also peroxycarbamates such as tert.-butyl N-(phenylperoxy)-carbamate or tert.-butyl N-(2 , 3- or 4-chlorophenylperoxy)-carbamate. Also suit-able are for example tert.-butyl hydroperoxide, di-tert.-butyl peroxide, cumine hydroperoxide or dicumyl peroxide.
To regulate the molecular weight of the acrylate resin it is possible to use customary regulators, for example n-dodecylmercaptan, diisopropylxanthogen disulphide, di(methylenetrimethylolpropane)xanthogen disulphide Le_A 23 915 .
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and thioglycol. Thioglycol is preferred. The regulators are added in amounts of 0.1 to 10% by weight, relative to the monomeric mixture.
The monomers are generally incorporated in the copolymer in the same ratios as used for the polymeriza-tion, and the copolymerized units are generally randomly distributed. The carboxyl-containing acrylate polymers have average molecular weights MGpC from about 1000 to 60,0ûO, preferably 2000 to 50,0ûO, and their solut;ons in xylene and/or butyl acetate have at 20C a viscosity of about 10 to 100,000 mPa.s. This can depend on the con-centration and the amount of regulator added. Further-more, the hydroxyl-containing acrylate polymers have molecular inhomogeneities U of 0.5 - 10, preferably 0.5 - 5.
The molecular inhomogeneity U ~= heterogeneity factor U~ can be defined by the equation:
U = (MW/Mn) - 1 wherein Mw = weight average and Mn = number average of the molecular weight.
The average molecular weights and the molecular inhomogeneities of the copolymer resins were determined by gel permeation chromatography (GPC) over styr3gels using tetrahydrofuran as eluent. In this method, a uni-versal calibration is established (after Z. ~rubisic, P. Rempp and H. Benoit, J Polymer Sci. Part B. Polymer Letters 5 (1967) 745) and the molecular weight averages Mw and Mn are determined from the distribution of the chromatograms.
It is known that products having the same average molecular weight (number average Mn) but different mol-ecular inhomogeneities have different solution viscosities.
The product having the greater inhomogeneity always has a higher solution viscosity, since high-molecular portions make a significantly greater contribution to the Le A 23 915 . _ ' - ~
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viscosity than the same amount of low-molecular portionsO
A ~ide molecular weight distribution also means that the number of reactive groups per molecule and hence the reactivity o~ the individual chains is subject to large differences.
The carboxyl-containing monomers of the group A.g. can be reacted before, during or after the polymeri-zation with epoxides such as ethylene oxide, propylene oxide and glycide and with glycidyl esters of monocar-boxylic acids having 8 - 20 C atoms, preferably 10 - 16 C atoms.
The glycidyl compounds of the group A.d. make it possible, by way of the chemically reactive epoxide group-ing, to crosslink with carboxyl-containing compounds and serve to modify the polyacrylate resin by the addition of fatty acids, which, if they are unsaturated, make it pos-sible to obtain air-drying polyacrylate resins.
After their preparation and after any modifica-tion has been carried out, the copolymers can be freed from volatile constituents at temperatures of 100 - 200C.
This can be done for example either under atmospheric pressure in curled tube evaporators by injection o~ an inert gas, such as nitrogen or hydrogen, in amounts of 0.1 to 1 m3 per 1 kg of resin melt or in vacuo in evapor-ator apparatuses, such as falling film evapora~ors, thinfilm evaporators, screw evaporators, flash evaporators or spray evaporators.
If the solution is polymerized, it is possible to use inert solvents, such as, for example, ethanol, propanol, isopropanol, N- or iso-butanol, methyl ethyl ketone, toluene, xylene, butyl acetate or butylglycol.
The preparation of the polymer resins according to the invention can also be effected by the known principle of emulsion polymerization. In principle it is also pos-sible to carry out the polymeri~ation reaction in thepresence of so-called reactive diluents. These are in - Le A 23 915 :

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particular polyhydric alcohols of the molecular weight range 62 to 6000, preferably 62 to 400, ~hich on com-pletion of the polymerization reaction are present in the mixture with the polymer resins~ These liquid mixtures 5 can then be used directly as binder component, for example in two-component polyurethane finishes. Suitable reac-tive diluents are for example low-molecular polyols such as ethylene glycol, propylene glycol, butylene glycol, 1,6-dihydroxyhexane, trimethylolpropane, diethylene gly-col, triethylene glycol and/or tripropylene glycol orhigher-molecular polyols of the type known per se from polyurethane chemistry, i.e. polyesterpolyols based on the said simple alcohols and the customary, polybasic carboxylic acids such as adipic acid, phthalic acid, tetrahydrophthalic acid or hexahydrophthalic acid or fin-3lly higher-molecular polyetherpolyols of the type which is accessible in a manner known per se by alkoxylation of the said simple polyols which are free of ether groups using ethylene oxide and/or propylene oxide.
It is also possible to modify the polymer resins according to the invention after their preparation, for example by reaction ~ith epoxides such as, for example, glycide or glycide esters, with carboxylic anhydrides such as phthalic anhydride, with carboxylic acids such as benzoic acid, ethylhexanoic acid, fatty acids or oil acids, in order thereby to improve certain properties of the polyols such as, for example, the pigment compati-bility or of the eventually obtained coatings such as, for example, their adhesion and alkali resistance.
The polymer resins according to the invention are valuable binder components for two-component finishes.
"Two-component finishes" is in this context to be under-stood as mean;ng not only "one-pot systems" but also "two-pot systems". The fact that the polymer resins according to the invention are binder components for two-component finishes means that in the finishes a hardener Le_A 23 915 : :

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' component is present in addition to the polymer resins according to the invention (and optionally further poly-hydroxyl compounds). If this hardener is for example a polyisocyanate having free isocyanate groups, the ready-to-use coating agents can also be prepared shortly before their processing by mixing the components. In such a case this would be a "two-pot system". However, if the hard-ener is a compound which reacts with the polymer resins according to the invention only at elevated temperature, for example polyisocyanates having blocked isocyanate groups, the hardener can also be combined with the poly-mer resin even at room temperature to give a "one-pot system" which ;s storable at room temperature.
If the polymer resins according to the invention are used according to the invention, they can find utility in the form of mixtures with up to 90% by weight, prefer-ably up to 40% by weight, relat;ve to the total mixture, of other compounds having active hydrogen atoms.
These are for example the "reactive diluents", already mentioned above by way of illustration, i.e. poly-hydroxyl compounds which were already present in the pre-paration of the polymer resins or are subsequently mixed with the polymer resins, up to a maximum amount of up to 35 OH equivalent ~ in the mixtures. In addition to the higher-molecular polyhydroxyl compounds already mentioned above by way of illustration, the mixture component can also be for example oil-modified alkyd resins or urethane-modified polyesterpolyols. Suitable hydroxyl-containing mixture components for the polymer resins according to the invention, in part;cular those based on polyester, are described for example in relevant standard works such as Temple C~ Pattan, Alkyd Resin Technology, Interscience Publishers John Wiley ~ Sons, New York, London 1962;
2. Dr. Johannes Scheiber, Chemie und TechnoLogie der kunstlichen Harze~CChemistry and Technology of Synthetic Resins], Wissenschaftliche Verlagsgesellschaft Le A 23 915 :

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3~ Hans Wagner and Hans-Friedrich Sarx, Lackkunstharze [Synthetic Resins for Finishes], 4th edition, Karl Hanser Verlag, Munich 1959;

4. Ullmanns Encyclopar~ie der technischen Chemie [Ullmann s Encyclopaedia of Industrial Chemistry], Volume 14, pages 80 to 106 (1063~.
In the case of the previously mentioned prepara-tion of the polymer resins according to the invent;on in the presence of polyesterpolyols as reaction medium it is possible if weak solvents are used in the preparation of the polymer resins as addit;onal reaction medium such as, for example, white sp;r;t and/or aromatic hydrocarbon solvents which dissolve the polyesterpolyol but not the polyhydroxypolyacrylate to obtain the polyhydroxylpoly-acrylate in the form of 3 dispersion of polymer micro-particles which are stably dispersed ;n the polyesterpolyol or, more specifically, its solut;on. The use of such dispersions ;s advantageous in the process according to the inYention in particular ~hen the finishes are hardened by the action of heat. In such a case the dispersed poly-hydroxylpolyacrylate particles melt before.their cross-linking and simultaneously become homogeneously dispersed in the paint film, so that even in this case it is pos-sible to obtain clear coats.
Reactant 8 for the hydroxyl containing acrylateresins prepared according to the invention for the pre-paration of binders for surface coat;ngs can be commerci-ally availa~le paint polyisocyanates whose preparation is treated for example in the follow;ng patent specifications:
U.~. Patent Specif;cations 39124,605, 3,358,010, 3,903,126, 3,903,127, 3,976,622, 3,183,112, 3,394,111, 3,645,979, 3,919,218, British Patent Specifications 1,060,430, ~ ~ 1,234,972, 1,506,3703 anc~ 1,458,564.
; 35 They of commerce Preferably paint polyisocyanates wh;ch have biuret groups, urethane groups, allophanate groups Le A 23 915 : :

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or isocyanurate groups.
The pclyisocyanates containing biuret groups are preferably reac~ion products of alkylene diisocyanates having 4 to 6 C atoms in the alkylene group, and water.
The polyisocyanates containing wrethane groups are preferably composed of aliphatic triols and tetraols such as, for example, trimethylolpropane, glycerol, penta-erythritol and aromatic or cycloaliphatic diisocyanates such as 2,4- and/or 2,6-toluylene d;isocyanate, 2~4'-and/or 4,4'-diisocyanatodicyclohexylmethane, isophorone diisocyanate and the like, and essentially 1 hydroxyl group of the polyol having reacted with one 1 mole of diisocyanate.
The polyisocyanates which have isocyanurate groups are cyclotrimerization products of the abovementioned ali-phatic and aromatic diisocyanates, in particular from toluylene diisocyanate and hexanethylene di;socyanate.
In principle it is also possible to use the iso-cyanate component in the form blocked with blocking agents for isocyanate groups such as~ for example, phenols, ox-imes such as cyclohexanoneoxime, -caprolactam, diethyl malonate or ethyl acetoacetate, so that heat-crosslinkable stoving enamels result.
The hydroxyl-containing polyacrylates and poly-isocyanates are used in such mixing ratios as to appor-tion 0.5 to 2, preferably 0.7 to 1.3~ isocyanate grouPs per hydroxyl group.
In the use according to the invention further possible crosslinking agents are as follows:
for example amino resins which have been enabled with hydroxyl groups in ~he sense of a condensation reaction, for example corresponding melamine derivatives such as hexamethoxmethylmelamine or other melamine-formaldehyde condensation products, as described for example in French Patent Specification 943,411 or by D.H. Salomon in "The Chemistry of Organic Filmformers", pages 235-240~ John Le A 23 915 ,` :

, Wiley ~ Sons, Inc., New York, 1974.
It is also possible to use customary crosslinking agents which are known from paint technology, for example guanidine resins, urea resins, resol resins, phenoLic resins or compound having epoxy groups enabled with alco-holic hydroxyl groups in the sense of an addition or con-densation reaction. It is also possible to use any desired mixtures of the crosslinking agents mentioned by way of example.
The crosslinking agents are used in the use according to the invention of the polymer resins according to the invention or their mixtures with further compounds having alcoholic hydroxyl groups in general in such amounts that 0.8 to 2.5, preferably 0.9 to 1.5, reactive groups of the crosslinking agent are available for each alcoholic hydroxyl group.
In the use according to the invention, it is of course also possible to include the customary auxiliaries and additives known from paint technology.
~20 The auxiliaries include solvents such as, for example, methanol, ethanol, isopropanol, ethyl acetate, butyl acetate, ethylglycol acetate, ethylene glycol mono-ethyl ether, methyl ethyl ketone, methyl isobutyl ketone~
cyclohexanone, toluene, xylene, white spir;t or mixtures of such solvents. If free isocyanate groups are present it is preferable to dispense with the use of solvents having groups which are reactive towards isocyanate groups unless they are used not as inert solvents but as "reac-tive diluents".
Further auxiliaries and additives are for example pigments, viscosity-regulating substances, antifoam agents, catalysts for the NCO/OH addition reaction, UV
absorbers, antioxidants or other substances ~hich prevent polymer degradation, such as, for example, sterically hindered amines and the like.
The coating agents wh;ch are used according to Le A 23 915 ,.

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the invention can be hardened w;thin the temperature range from 0 to 250C depending on the nature of the crosslinking agent used. Coating agents which are used according to ~he invention and which contain compounds ~ith isocyanate groups as crosslinking agents can even be hardened at room temperature. Preferably such systems are hardened at 0 to 130C~ Stoving enamels which are used according to the invention and which contain for example polyisocyanates having blocked isocyanate groups as crosslinking agents are generally hardened within the temperature range from 60 to 250C. Herein it is occa-sionally advantageous to begin the hardening initially at temperatures below 120C and to complete it at tempera-tures above 120C.
The surface coating agents prepared using the polymer resins according to the invention as binder com-ponent in accordance with the invention g;ve on hardening coatings of h;gh extensibility and weathering and chemi-cal resistance.
They are therefore particularly suitable ~or coating articles which are exposed to the weather, such as metals~ for example the metal outer skin of motor vehicles, guttering, railings, bicycles, household appli-ances and the like.
Owing to the high extensibility and the good ad-hesion properties and also the good scratch resistance, they are also highly suitable for coating plastics parts such as, for example, car body parts made of plastic tfor example bumper trim, wings, bonnets, tail gates~
spoilers), window frames and household appliances and the like.
Owing to the good properties on plastics sub-strates on the one hand and metal surfaces on the other, the binders acrording to the invention are highly suit-able for the common coating of plastics and metal notonly ;n motor vehicles ;n mixed construct;on at Le_A 23 915 :

temperatures of 60 - 130C, preferably 80 - 100C.
It is of course also possible to coat other sub-strates, such as, for example, wood and mineral sub-strates, with the binders according to the invention.
The finishes are applied by the customary methods of paint technology, for example by spraying, casting, dipping or roLling. The finishes are generally applied in such amounts as to give dry film thicknesses of the finishes between 0.005 and 0.10 mm.
In the examples below, all percentages and all "parts" are by weight.
Example 1 A reactor which is equipped with reflux condenser, heating, cooling and metering means is charged ~ith 2250 Parts of butyl acetate and 1500 parts of xylene and is heated to 130C. At that temperature a mixture of 5988 parts of butyl acrylate, 95 parts of acrylic acid and 3817 parts of 2-hydroxyethyl acrylate ;s metered in under nitrogen in the course of ~ hours.
At the same time 600 parts of tert.-butyl per-octoate and 750 parts of xylene are metered in together in the course of 5 hours in a separate operation.
This is followed by 3 hours of stirring at 135C, heating to reflux temperature and maintenance of the reac-tion material at that temperature for about 1 hour. ~il-tration with suction leaves a copolymer res;n hav;ng a viscosity of 932 mPasec. (20C), a solids content of 69.6% by weight, an acid value of 7 mg of KOHtg of sub-stance, a colour value of 90 APHA and a hydroxy value of 121 mg of KOH/g of substance.
Example 2 A reactor which is equipped with reflux concdenser, heating, cooling and metering means is charged with 4500 parts of xylene and heated to 115C. At that temperature a mixture of 1852 parts of styrene, 5055 parts of butyl acrylate, 2451 parts of 2-hydroxyethyl acrylate and Le A 23 915 ., . ~ .
'. . : ' 92 parts of acrylic acid is metered in under nitrogen in the rourse of 4 hours~
At the same time 375 parts of tert.-butyl per-octoate and 675 parts of xylene are added together in the in the course of 5 hours in a separate operation.
This is followed by 3 hours of stirring at 115C, heating to the reflux temperature and maintenance of the reaction material at that temperature for about 1 hour.
Filtration with suction leaves a copolymer resin having 1û a viscosity of 2265 mPasec (20C), an acid value of 7 mg of KOH/g of substance, a solids content of 65~0% by weight, a hydroxyl value of 75 mg of KOH/g of substance and a Far value of 10 APHA.
The solids content was determined for all the copolymer resins described here by heating about 0~2 9 of resin solution on a metal lid of 7.5 cm diameter in a layer thickness of about 5 - 10 ~m at 125C in a circu-lating air cabinet for 1 hour.

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Example 1 Example 2 (60% TiO2/35% BM/NCO:
OH 1.2/0.2% zinc octoate)

5 Life at room temperature ca. 20 h ca. 5 h Drying time rec. ca. 4.5 h ca. 3 h 6loss (60 Gardner) 92 90 Pendulum hardness (after 14 d) 92 112 Dissolvability*
10 (1 min. treatment) toluene 1 0 - 1 acetone 1 0 - 1 Tar resistance*
15 Treatment time 1 h O O
DB tar solut;on 3 h O O
24 h Extensibility (on ~ayflex 91) 44% 44%

* O = best possible value 4 = least satisfactory value BM = binder EGA = ethylglycol acetate EA = ethyl acetate D8 tar solution~ = (Daimler-8enz) 25 Bayflex 91 ~ = (Bayer AG) APHA = unit of measurement of the Hazen colour value Le A 23 915 , ,_ .
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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A binder combination comprising 95-55 parts by weight of an OH-functional copolymer resin which contains at least two functional OH groups per molecule, prepared by copolymerization of a) 0-26.5 parts by weight of styrene, vinyltoluene, methyl methacrylate, butyl methacrylate or a mixture thereof, b) 45-63 parts by weight of butyl acrylate, 2-ethylhexyl acrylate or a mixture thereof, c) 25-39 parts by weight of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate or a mixture thereof, d) 0-30 parts by weight of glycidyl acrylate, glycidyl methacrylate or a mixture thereof, e) 0-30 parts by weight of methoxymethylacrylamide, methoxy-methylmethacrylamide or a mixture thereof, where the alkoxylated monomers can if desired be copolymerized, or where after copolymer-ization of acrylic acid or methacrylic acid amide or mixtures thereof preparation of the alkoxylated compounds can if desired be effected by reaction with formaldehyde in the presence of methanol, f) 0-45 parts by weight of acrylonitrile, g) 0.5-4 parts by weight of acrylic acid, methacrylic acid, maleic acid half-ester with 4 to 8 carbon atoms in the alcohol component or a mixture thereof; and 5-45 parts by weight of a polyfunctional isocyanate.

2. A method of coating a surface wherein the coating com-prises the binder combination according to claim 1.

3. A method according to claim 2 wherein the surface is the surface of a motor vehicle part.

4. Motor vehicle parts coated with binder combinations according to claim 1.