CA1098232A - Coating compositions - Google Patents

Abstract

RS(CASE/202)MSdc NOVEL COATING COMPOSITIONS

Abstract of the Disclosure - Protective and decorative coatings for metal are prepared using aqueous polymer compositions consisting essentially of a thermosetting vinyl or acrylic polymer which has been polymerized in the presence of a preformed, crosslinked, elastomeric copolymer. Substantially no unreacted monomer is present during polymerization of the thermosetting polymer, which contains pendant hydroxyl groups and a latent crosslinking agent.

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Description

B~CKGROUND OF ~ IliVLNTION

l'his invention relates to aqueous coating composikions.
~his invention further relates to agueous coating compositions containing dispersed particles of acrylic poly~ers having j incorporated therein a latent heat activated curing agen~ which i yields cross-linked, solvent-resistan-t coatings of sufficient flexibility to be employed as protective and decorative coatings ! for metal-¦ Thermosetting addition type polymers, including acrylic I and vinyl polymers, have been disclosed as useful materials for coating me-tal substrates. The polymer chains contain reactive groups such as hydroxyl groups, and an external latent cross~ ~
linking agen-./ such as the N-methylol melamines, N-alkoxy melamine ;, N--methylol ureas or M-alkoxy ureas which is not part o~ the j polymer chains, is included in the coating formulation. The formulation is coated onto a substrate and then heateG. The - latent crosslinking agent decomposes to the active species, which then reacts with hydroxyl or other functional groups on adiacent ~ol ~rmer chains ~o ^rm 2 c~^sslinked coatlng e~hlbl'in~ the j properties o~ hardness and solvent-resistance. The shortcoming i~ of this type of coating i5 that the flexibilitY associated with j~ good impact resistance usuall~r re~uires that the coating be relatively soft. Additionally, the aforementioned external latent cross-linking agents undergo spontaneous and gradual I~ decomposition at ambient temperature with a resultant increase !i in viscosity of the coating formulation. These types of formu-I' lations therefore cannot be stored lor e~tended periods cf time.

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~9~z ¦i United States Patent No 3~7~3~2 discloses heat-'I hardenable acrylic type graft copolymers wherein the latent crosslinking agent, an ~-methoxyme-thyl amide of acrylic or methacrylic acid, is present in the polymer. At elevated temperature this compound decomposes to yield methylol groups i which react with hydroxyl groups that are also present on the polymer ch~in to yield a cross-linked structure. The graft copolymers are prepared by polymerization of a hard, brittle ~ copol~mer in the presence of a solubilized or emulsified elas-` tomeric copolymer obtained by solution or emulsion polymerizatiG-n i~ in water or an organic liquid. The patent discloses adding the monomers for the hard, brittle copolymer as a single portion to the prefor~ed solution or dispersion of the elastomeric component.
~ Under these conditions -the product is believed to be a graft ' copolymer wherein sections of the brittle polymer are bonded to a "~ackbone" of elastomeric copolyr,er. Alternatively, t~.e monomers of the brittle component may ~enetrate into particles of the elas-tomeric component prior to polymerization, resulting 'I in an internin~ling of brittle and elastomeric polymers in each i particle of polymer~ Rega~dless of which type of structure is ' actually formed, coatings prepared using the polymers disclosed ", in the afore~entioned U~ S. Patent 3,793,282 do not exhibit the ~i high impact stren~th desired for protective ana d~corati~e metal ~l coating~O
¦ One objective cf this invention is to provide an improved method for preparing copo'ymers containing a latent~
heat-activated crosslinking agent. The copolymers do not exhibit the undesirable characteristics of the aforementioned prior art ! materials. Coatings incorporating the present copolymers exh.i~it ! superior solvent and impact resistance.

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Tl~is invention provides a heat-curable coating composition consisting essentially of an aqueous medium containing dispersed polymer particles which, in turn, consist essentially of an elastomeric emulsion copolymer A and a thermosetting copolymer B, wherein (1) copolymer B is prepared in the presence of preformed copolymer A, (2) the rate of addition of the constituent monomers of copolymer B is substantially equal to the reaction rate of said monomers to form copolymer B~

(3) copolymer B constitutes from 10 to 70% of the combined weight of copolymers A and B, exhibits a glass transition temperature from 40 to 75C., (~) the repeating units of copolymer B are derived from (a) at least one monoethylenically unsaturated compound containing from 2 to 20 carbol~ atoms, (b) from 2 to 20%, based on the weight of copolymer B, of at least ~:
one monoester of a diol containing from 2 to 4 carbon atoms and acrylic or methacrylic acid, said monoester being copolymerizable with said monoethyl-enically lmsaturated compound, and (c) from 5 to ~0%, based on the weight of copolymer B, of at least one latent curing agent selected from the group consisting of N-alkoxymethyl acrylamides and N-alkoxymethyl methacrylamides wherein the alkoxy group contains from 1 to 8 carbon atoms, ~5) copolymer A exhibits a glass transition temperature of from -15 to ~25C., (6~ the repeating units of copolymer A are derived from (a) at leas~ two monoethylenically unsaturated compounds containing from 2 to 20 carbon atoms, and (b) from about 0.5 to 5.0%s based on the weight of copolymer A, of a polyfunctional compound selected from the group consisting of diesters and triesters derived from acrylic or methacrylic acid and a diol containing from % to 8 carbon atoms or a triol containing from 3 to 8 carbon atoms, divinyl benzene and allyl methacrylate.

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. ~, D~T~ r~SCRIPTION OF THE INVENTION

A. The E]astomeric Copolymer The elastomeric component of the present polymer composltion~ is a slightly crosslinked polymer o~ at least two monoethylenically unsaturated compounds and exhibits a second ' order glass transition temperature, hereinafter referred to 1l as Tg, o~ ~rom -15 to ~25C. It will be understood that the same monomers can be employed to form both components of the present polymer compositions. The proportions of "hard" and "soft" mono~ers are adjusted to obtain the desired Tg~ as is known in the art. Suitable monoethylenically unsaturated compounds include olefins, such as ethylene, propylene, butylene, iso butylene and cyclohexene; vinyl monomers such as vinyl chloride, virlyl acetate, styrene and ~-methylstyrene3 ethylenica~lly unsaturated acids such as acrylic~ methacrylic, maleic and itaconic acids and clerivatives of these acids, including acrylonitri7e~ methacrylonitrile and acrylamide. Among the preferred e~hylenically unsaturated compounds are esters of I acrylic and methacrylic acids wherein the alcohol resiaue is l monofunctional and contains from 1 to 8 carbon atoms. Polymers !obtained fr~m these esters have been shown to yield superlor I coatings. ~referred esters include methyl methacrylate~
ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate~ In ~addition to esters of acrylic and methacrylic aclds with mono-functional alcohols containing from 1 to 8 carbon atoms9 one or l'more of the aforementioned copolymerizable com~ounds containing one carbon to carbon double bo~1 can be employed as comonomers.

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I The monomers of the elastomeric component are polymerized as ¦¦ an aqueous emulsion using conventional catalysts, emulsifiers I and reaction conditions to obtain an average molecular weight of ! at least 20~00 The elastomeric component also includes from ¦ 0.1 to 15.0~ preferably from 0.5 to 5%, based on the weight of the elastomeric componentg of a bi- or polyfunctional monorr.er which is capable of uniformly crosslinking the copolymer. Pre-ferred crosslinklng agents are the alk~lene glycol diacrylates, ~ including ethylene glycol diacrylate, butylene glycol~diacrylate, !I propylene glycol diacrylate and the corresponding esters of meth-acrylic acid. Other conventional polyfunctional, copolymerizable monomers such as divinyl benzene and allyl methacrylate can be employed in place of the aforementioned diacrylates.
Ii Any of a variety of common emulsifiers well ~nown I in the art for emulsion polymerization of acrylates and meth acrylates can be used. A low level of emulsifier is desirable9 preferably below three percent by weight based on the total weight of polymerizable monomers charged in all stages. Useful ~1 emulsifying agents include common soaps, alkylbenzenesulfonates, ¦' such as sodium dodecyl penzene sulfonate, alkylphenoxypolyethylene ! sulfonates, sodium lauryl sulfate, salts of long~chaln amines and salts of long-chain carboxylic and sulfonic acids. In general, the emulsifier should be a compound containing hydrocarbon groups of 8 - 22 carbon atoms coupled to highly polar solubilizing groups such as alkali metal and ammonium carboxylate groups, sulfate half ester groups, sulfonate groups and phosphate partial ester groups~
¦ The polymerization medium in each of the two stages or steps will preferably contain an effective amount of a suitable ¦ water-soluble, free radical generating polymerization initiator, I' , ;

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whlch is actlvated either thermally or by an oxidation-reduction (redox) reaction. The preferred initiators are 5hose which generate free radicals by a redox reactlon, since they allow ~or efficient po~lymerization at moderate reaction temperaturesO
Examples of suitable initiators are combinations of ammonium or alkali metal persulfate and sodium formaldehyde sulfoxolate~ An organic peroxide such as c~mene hydroperoxide can be added to ensure the complete conversion required during preparation of the elastomeric component of the present polymer composition.

I B. The "Glassy" or Thermosetting Copolymer The monoethylenically unsaturated compounds employed to prepare the first or thermosetting component of the present cornpositions are simllar to those used in the elastomeric copolymer. As is true for the elastomeric copolymer~ esters f acrylic or methacrylic acid wherein the alcohol residue contains from 1 to 8 carbon atoms are the preferred Tr.onomers for the thermosetting phase. The glassy component contains a higher percentage of monomers ylelding "glassy" homopolymers, such as methyl methacrylate than the elastomeric phase to ' a^h.iel~e the desired g ~of ~rom 40 to 75c I In addition to the aforementioned monofunctional mono-mers, the thermosetting copolymer also contains from 5 to 40% by weight of repeating units derived from at least one N-alkoxymethyl~

I acrylamide or N-alkoxy methacrylamide and from 2 to 20% by weight of at least one hydroxyalkyl acrylate or methacrylate. At elevated temperatures the alkoxymethyl acrylamide or methacryla-mide decomposes to yield a methylol group that reacts relatively I rapidly with hydroxyl groups on ad~acent polymer molecules to ';10~ 3~ , yield a cross~link2d fllm exhibitlng the deslred resistance to organic solvents. Since decompositlon of N--alkoxyacrylamides occurs in acidic med:La~ lt ls preferable that a number of free ~j carboxylic acid groups be present ln the thermosetting polymer.
5 !~ This is readily achleved by lncludlng from 0.1 to 5.0%, based on the total weight of monomers for the thermosetting polymer~
of a copolymerizable ethylenically unsaturated acid. Acrylic, j, methacrylic~ maleic and itaconic acids are preferred for this ¦l purpose.
¦I To achieve optimum film forming properties it is desirab]e that the molecular weight of the thermosetting copolymer be from 2,000 to 2009000. This can be accomplished by includlng 1l an efficacious amount, usually from 0.05 to 5.0%, based on total ,' monomer weight of a conventional chain transfer agent in the , monomer mixture. Mercaptans, mercaptocarboxylic acids, j mercaptocarboxylic esters and ha]ogen-containing hydrocarbons are suitable chain trans~er agents~ , ¦ The present polymer compositions are prepared using I a two step procedure. In the first stage, a s~all portion 20 il of the monomer mixture for the elastomeri^, copolymer is combined with water, suri-actant and initiator under an inert aimosphere.
l~ Preferably an inorganic buffer such as ammonium bicarbonate is ¦¦ also present to maintain the pH of the reaction mixture from 3.5 to 7.5. The resultant mixture is heated to a temperature ~ of 40 - 60C., at which time the remaining portion of first stage j monomers together with additional inltiator are gradually added ¦ to the reaction mixture~ Heatlng is continued until substantially i all of the first stage monomer has been po]ymerized. The degree ¦l of monomer conversion is readily determined by measuring the con~
¦ centration of non-volatile material present in the reaction I mixture~ ~ ' ~ . ~

~ ollowi~g completion of the first stage one or more sur~actants are added. The mixture of monomers ~or the ther-mosetting "glassy'l copolymer and an efficacious amount of ~I polymeri~ation initiator are then gradually added and rapidly ~~ polymerized. The addition rate is substantially equal to the ! rate of polymerization and the concentration of unreacted monomer in the reaction mixture is virtually æero. This is believed to result in an attachrnent and layering of the ther-i mosetting polymer onto the preformed particles of the elastomeric 1i polymer in contrast to the intimate mixture which result~ whenthere is significant penetration of the "glassy" phase monomers into the particles of elastomeric polymer prior to polymeriza-tion. The coatings obtained using this latter type of polymer exhibit considerably lower impact strength than those prepared 1~ ~ using polymers prepared in accordance with the present method `! as set forth in the accompanying examples. The addition o~ the monomers generally requires from four to sixteen hours, depending , upon the size of the charge~
¦I Polymer compositions prepared in accordance wit~ the 2~ I present method are blended with suitable coalescing solven~ to Il obc~irl Gne film-formirl~ component OI pro'vective and decora~ive ¦' coatings for use on metal surfaces. The coalescing solven~s ¦ are present at a concentration of from 1.0 to about 20%, based 1 on polymer weight. Suitable coalescing solvents include 1l monoethers of diols and dimers thereofg such as diethylene glycol, in addltion to the acetic acld esters of these monoethers.
Specific preferred coalescing solvents are ethylene glycol monobutyl ether, diethylene glycol monobutyl ether and ethylene 1~9823~

glycol manomethyl ether. Other 501vents for the polymer that will lower the minimum film-forming temperature thereof can .
also be employed as coalescing solvents. Suitable coalescing solvents for the various copolymers that can be employed as the "glassy" and thermosetting phases are well known in the art and, as such are not part of the present invention. The minimum film-forming temperature of the final polymer in the presence of the coalescing solvent should be from O to 80C. ~ preferably l from O to Z5~C.
Upon heating at temperatures from 90 to 1~0C., the i coatings spontaneously crosslink to yield durable~ solvent resistant coatings exhibiting exceptionally high levels of impac ;
strength. This desirable combination of properties cannot be Il achieved using the polymerization techniques disclosed in the ~il prior art relating to coatings.
! The following ex.ample discloses a representative polymer composition encompassed by the accompanying claims.
~' EXAMPLE 1 Il A reactor equipped with a mechanically driven s~irrer, !I thermometer, addition funnel, reflux condenser and nitrogen inlel ;
was charged with 6900 cc. of deionized water, 161 g. o~ a sur-factant CGmpOSition c~nsLs~rlg e~s~nti~lly of a mixture of an Il ethoxylate nonyl phenol and lauryl alcohol ethoxylate sulfate, available from Stepan Chemical Company as Polystep~ J-3, 75 g. o the sodium salt of a branched alkylbenzene sulfonate avialable ¦ as Polystep~ A-].6 from Stepan Chemical Company, 9 g. of ammonium bicarbonate, 1.4 g. of a 5% by weight aqueous solution o~ a ferrous-ethylene dlamine tetraacetlc acid complex and a ¦, 984 cc. portion of a monomer mixture containing 2305 cc.
3 ¦ f methyl methacrylate, 2461 cc. of butyl acrylatei 54 cc.
of N-isobutoxymethyl acrylamide, 22 cc. of hydroxypropyl methacrylate, 22 cc. of acrylic acid and 45 cc. of 1,4-butanedio I 33'-~

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dimethacrylate. The resultant liquid mixture was heated to 40C.
~i while nitrogen was bubbled through it for 0.5 hour~ Polymeriza-Il tion was then initiated by adding a solution containing 7.7 g.
,~ ammonium persulfate and 50 cc. of deionized water~ followed by !i a solution containing 1.4 g. of sodium hydrosulfite and 25 cc, ! deionized water. The temperature of the reac-tion mixture in-creased spontaneously -to 55C.~ at which time the remainder of the aforementioned monomer mixture together with a solution ~ containing 8.1 g. ammonium persulfate~ 24 g. of the aforementioned , surfactant composition and 100 cc. of deionized water were gradually added over a period of 1 to 1~5 hours, during whlch time the temperature of the reaction mixture was maintained between 52 and 55C. by controlling the temperature of the heating ' bath. The reaction mixture was maintained at from 50 to 52C.
~ for 1.5 hours following completiorl of the monomer addition ko ensure substantially complete conversion to polymerg at which ' time a solution containing 250 cc. of deionized water~ 75 g. of Polystep~ A-16 and 402 g. of Polystep~ J-3 was added to the ~ reaction mixture and stirring continued for an additional 0.5 1 hour, at which time the following mixtures were added gradually ~~ and concurrently over a period of from 4 to 5 hours while the ¦ll temperature was maintained from 52 to 55C, ~methyl methacrylate2566 cc.
Ibutyl acrylate 878 cc.
¦ N-isobutoxymethyl acrylamide 1026 cc.
Mixture I hydroxypropyl methacrylate 417 cc.
acrylic acid 22 cc, butyl mercaptoproplonate 20 cc~
t-butyl hydroperoxide5 cc.

~ammonium persulfake15.8 g.
Mixture II ~deionized water500 cc.

rsodium hydrosulfite13.5 g~
Mixture III~arnmoniurn bicarbonate 13.5 g-~eionized water 500 cc.

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, 3L 139B~32 Following completion of this addition the reaction mixture was heated for from 1 ko 1.5 hours at a temperature of 5Q-55C. The re~ction mlxture was then cooled to 30C. and passed through a 200 mesh filter. The pH of the mixture was then ad~usted to f'rom 9.0 to 9.4 using dimethylaminoethanol.
The resultant polymer emulsion W2S blended with 7~
(based on polymer weight) of the monobutyl ether of diethylene ~ glycol and then coated as a o.oo6 inch (0.015 cm.)-thick film ¦l onto a steel panel. The coating was dried at ambient temperatur 1¦ for 10 minutes and at 93C. for 10 minutes. Curing of the coating was achieved by heating it at 175C. for 15 minutes.
The thickness of the cured film was 0.0015 inch (0.0039 cm.)~
Il The film was equivalent in hardness to an H or 2H pencil (the ,I hardest pencil which did not penetrate the surface of the 1I coating). The coating was not visibly damaged when sub~ected to an impact of 120 inch-pounds (92-13$ cm.-kg.) on th~e coated side of the panel or 80 120 inch-pounds (92-138 cm.-kg.) on the reverse side.
For purposes of comparison a second polymer compositio was prepared using the foregoing procedure, with the exception that the monomer mixture for the thermosetting polymer was added o-ver a 2 '~o 2.5 hour period rather than the ll to 5 hours employe~ i in the foregoing procedure. Increasing the rate of monomer l addition had a profound and adverse effect on the impact resist-l ance of the cured coating. The values were 30 inch-pounds (34.5 cm.-kg.) for the coated side and 10 inch-pounds (11.5 cm.
kg.) for the uncoated side. This demonstrates that the rate of monomer addition for the thermosetting phase strongly influences the properties of the final polymer. One 3 explanation for the reduction in impact strength is that the monomers of the thermosetting phase penetrate the dispersed I ~i"~
A.,~
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'' , I partl.c].es of elastomeric polymer prior to being poly}neriæed~
~I resulting ln an intermingling of the t-Yo polymers rather than !~ an encapsulation of the elastomeric copolymer by the thermosetting !I phase. The physical properties of polymers prepared in accordance ` witll the present method are indicative of an encapsulated type il of structure.

Claims (11)

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

1. A heat-curable coating composition consisting essentially of an aqueous medium containing dispersed polymer particles which, in turn, consist essentially of an elastomeric emulsion copolymer A and a thermosetting copolymer B, wherein (1) copolymer B is prepared in the presence of preformed copolymer A, (2) the rate of addition of the constituent monomers of copolymer B is substantially equal to the reaction rate of said monomers to form copolymer B, (3) copolymer B constitutes from 10 to 70% of the combined weight of copolymers A and B, exhibits a glass transition temperature from 40° to 75°C., (4) the repeating units of copolymer B are derived from (a) at least one monoethylenically unsaturated compound containing from 2 to 20 carbon atoms, (b) from 2 to 20%, based on the weight of copolymer B, of at least one monoester of a diol containing from 2 to 4 carbon atoms and acrylic or methacrylic acid, said monoester being copolymerizable with said monoethyl-enically unsaturated compound, and (c) from 5 to 40%, based on the weight of copolymer B, of at least one latent curing agent selected from the group consisting of N-alkoxymethyl acrylamides and N-alkoxymethyl methacrylamides wherein the alkoxy group contains from 1 to 8 carbon atoms, (5) copolymer A exhibits a glass transition temperature of from -15° to +25°C., (6) the repeating units of copolymer A are derived from (a) at least two monoethylenically unsaturated compounds containing from 2 to 20 carbon atoms, and (b) from about 0.5 to 5.0%, based on the weight of copolymer A, of a polyfunctional compound selected from the group consisting of diesters and triesters derived from acrylic or methacrylic acid and a diol containing from 2 to 8 carbon atoms or a triol containing from 3 to 8 carbon atoms, divinyl benzene and allyl methacrylate.

2. A heat-curable coating composition as set forth in claim 1 wherein copolymers A and B are both derived from monoethylenically unsaturated compounds selected from the group consisting of esters of acrylic or methacrylic acid and monofunctional alcohols containing from 1 to 8 carbon atoms.

3. A heat-curable coating composition as set forth in claim 1 wherein said monoester of a diol is selected from the group consisting of hydroxy-propyl methacrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxyethyl acrylate.

4. A heat-curable coating composition as set forth in claim 1 wherein said polyfunctional compound is a diester derived from (1) acrylic or methacrylic acid and (2) a diol containing from 2 to 8 carbon atoms or a triol contain-ing from 3 to 8 carbon atoms.

5. A heat-curable coating composition as set forth in claim 4 wherein said polyfunctional compound is an ester of ethylene glycol, 1,2-propanediol, butylene glycol or trimethylol propane.

6. A heat-curable coating composition as set forth in claim 1 wherein the minimum film-forming temperature of the composition is from 0° to 80°C.

7. A heat-curable coating composition as set forth in claim 1 wherein the copolymers A and B are both derived from polyfunctional compounds selected from the group consisting of methyl methacrylate, butyl acrylate, 2-ethyl-hexyl acrylate, ethyl methacrylate and butyl methacrylate.

8. A heat-curable coating composition as set forth in claim 1 wherein copolymer B contains from 0.5 to 5%, based on the weight of said polymer, of repeating units derived from acrylic, methacrylic, maleic or itaconic acids.

9. A heat-curable coating composition as set forth in claim 1 wherein copolymer B is prepared using a mercaptan, mercaptocarboxylic acid, mercapto-carboxylic ester or a halogenated hydrocarbon as a chain transfer agent.

10. A heat-curable coating composition as set forth in claim 1 wherein the latent curing agent for copolymer B is N-butoxymethylacrylamide or N-isobutoxymethacrylamide.

11. A heat-curable coating composition as set forth in claim 1 wherein, with the exception of any monoethylenically unsaturated carboxylic acid, the same monomer residues are present in copolymers A and B.