Progress in Organic Coatings, 13 (1985) 333 - 345 333

THERMOSETTING ACRYLIC RESINS - A LITERATURE REVIEW

G. Y. TILAK

Faculty of Engineering, Ehime University, Matsuyama (Japan)

Contents

1 Introduction. ................... 2 Solvent-borne TSAs ...............

2.1 Amide type systems ............ 2.2 Carboxyl type systems .......... 2.3 Hydroxyl type systems .......... 2.4 Glycidyl type systems. .......... 2.5 Modified monomers ............ 2.6 TSAs with other film-formers ......

2.6.1 With amino resins ......... 2.6.2 With alkyd resins ......... 2.6.3 With phenolics ........... 2.6.4 With epoxy resins ......... 2.6.5 With cellulosics ........... 2.6.6 With isocyanates .......... 2.6.7 Miscellaneous ............

3 TSA emulsions .................. 4 Water-solubilised TSAs ............. 5 Conclusions .................... Acknowledgements ................. References .......................

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1 Introduction

Thermosetting acrylics (TSAs) are one of the fastest growing industrial finishes. This rapid growth is attributed to three factors: (i) the inherent superior properties of acrylics; (ii) the availability of lower cost monomers resulting from important advances in acrylic chemistry and manufacturing technology; and (iii) the development of new types of polymers with im- proved performance properties and a greater latitude in formulation.

Extensive information regarding TSAs is available in review papers and research articles on the subject [ 1 - 171. The purpose of this review is to bring together the majority of the work on the subject described in the patent literature.

The term ‘TSA resin’ describes those resins that are copolymers of functional acrylic monomer(s) and esters of acrylic acid and/or methacrylic acid, but which may also contains monomers with vinyl unsaturation such

0033-0655/85/$5.05 0 Elsevier Sequoia/Printed in The Netherlands

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as styrene or vinyl toluene. By suitable blending of the available monomer types, it is possible to tailor-make resins possessing a desired balance of physical properties and durability characteristics.

When TSA resins are stoved, the reactive groups from functional acrylic monomer units take part in crosslinking reactions which convert the low molecular weight polymer to a high molecular weight film-forming material. Some TSAs are self-reactive but usually other resins are also used mainly as crosslinkers. The general term ‘TSA enamel’ is at present used loosely in the paint industry to describe enamels that contain widely varying proportions of acrylic resin.

TSA resins are available in three forms and hence they can conveniently be grouped into three classes, viz. solvent-borne TSAs, TSA emulsions and water-solubilised TSAs.

2 Solvent-borne TSAs

These are usually supplied at about 50 - 70% solids content as clear solutions in mixtures of organic solvents such as butanol, glycol ethers, ace- tates, xylol and ketones. Several types of functionality can be built into acrylic monomers. The major functional groupings used commercially are amide, carboxyl, hydroxyl and epoxy types.

2.1 Amide type systems (Meth)acryl amide monomer is usually alkylated and etherified. Alkylo-

lation and etherification are usually undertaken after copolymerisation. However, in some cases alkylolation is carried out in the presence of como- nomers, followed by copolymerisation and subsequent etherification [ 181. In other cases etherified derivatives are made first in the presence of other comonomers and then copolymerised [ 191. N-3-Oxyhydrocarbon-substituted acrylamide is used as a comonomer [ 201.

Apart from non-functional monomers, various functional acrylic mono- mers are also sometimes incorporated in amide systems. The amide monomer may be copolymerised with a common hydroxyl monomer [21], a carboxyl monomer [ 22 - 241, a hydroxyl-terminated unsaturated polyurethane [25], vinyl ether derivatives [ 261, an unsaturated epoxy polyolefin [ 271, a hydroxy- alkyl polyallyl ether [ 281, 2-methacryloxy-2-methylethyl phosphoric acid [29], or ally1 alcohol [30] to yield excellent impact resistance, and with modified ally1 alcohol [31] and maleic anhydride to give a primer composi- tion to be used for sanitary can coatings and automobile finishes.

As such, this system does not require any crosslinking agent. However other film-formers may also be incorporated to enhance the properties. With amino resins they show improved compatibility [32], excellent weathering properties [ 331, and films having a microwrinkled surface [ 341.

Amide-containing TSAs are used with oil-modified -alkyds [32, 351. They show good compatibility with epoxy resins [32, 361. They are used

along with conventional epoxy resins [37] having an epoxy equivalent of 450 - 525 and a hydroxy equivalent of 145, and also with epoxide-free hydroxy esters of epoxide resins [38]. They may be reacted with a stoi- chiometric amount of vinyl cyclohexene dioxide [ 391, and are mixed with polyester [40] to obtain non-gelled heat-hardenable compositions.

The system is usually cured by baking in the presence of acid catalysts. The baking of acrylic resins crosslinked through an amide functionality has been described [ 41, 421. PTSA or phosphoric acid are typical catalysts. For rapid curing or for low-temperature curing, the use of substantially neutral salts such as ZnClz, SnC14 or other Lewis acids has been suggested [43].

2.2 Carboxyl type systems (Meth)acrylic acid is commonly used. Carboxyl TSAs are usually cross-

linked with epoxies [ 44 - 461. The reaction between carboxyl and epoxide is basecatalysed. A wide variety of bases are effective for the crosslinking reaction and inorganic bases, amines and quatemary ammonium compounds have been evaluated. Various catalysts such as quatemary monoimidazoline salts [ 471, peroxides [ 481 or dicyanodiamide [ 491 have also been suggested.

The basecatalysed TSA/epoxy blend system has a limited stability. In one case the blend with an epoxy has been stabilised by heating at tempera- tures of 50 - 150 “C with 1 - 2 mol equiv/COOH equiv of the copolymer of a (vinyl or) vinylidene compound [ 501 such as alkyl vinyl ether, isobutyl- ene, diisocyanates, 2,3-dihydropyran or 2,3-dihydrofuran. Other functional monomers have also been employed. The use of ally1 glycidyl ether as a comonomer has been mentioned [ 511.

Other film-forming materials apart from epoxies have also been used as crosslinkers. Carboxyl acrylics containing acrylamide derivatives may be blended with alkyl etherified phenolics [52]. Along with MF resins they show good adhesion to aluminium as well as good abrasion resistance [ 531. 1 - 5 equiv oxymethyl per carboxyl group has been suggested [54] when used with an alkylated aminoplast. Thermoplastic acrylics have also been incorporated to yield excellent adhesion, gloss and durability [ 551.

2.3 Hydroxyl type systems Hydroxy alkyl (meth)acrylate is the most commonly used functional

acrylic monomer. Heat-hardening hydroxyl type acrylics have been described [ 561, par-

ticularly those having high solvent resistance, hardness as well as good adhe- sion to metals especially aluminium [57]. They are often copolymerised with methacrylic acid and etherified N-methyl01 acrylamide [ 581. Unusual comonomers such as bicyclo(2,2,l)heptene-2 derivatives, having at least one hydroxyl group in the ring or in side chains, have also been described [59]. This can be crosslinked with polyisocyanate to yield excellent solvent resis- tance. They are usually combined with partially butylated MF resins [60] in a 3:l ratio [61].

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Conventional acidic catalysts may be used for curing the system. Thus for curing a hydroxy system, 1 - 5% wt/wt of magnesium perchlorate and/or zinc perchlorate catalysts have been suggested [ 621.

Copolymers derived from styrene, dialkyl fumarate, a hydroxy alkyl acrylate or methacrylate, an unsaturated carboxylic acid, and other mono- mers have been used in a thermosetting resin composition for metallic finishes [ 63 1.

2.4 Glycidyl type systems Glycidyl (meth)acrylate is used as a functional acrylic monomer. Though

this system is self-crosslinking, in many cases amino or epoxy resins have been recommended for improving certain properties such as flexibility, adhe- sion or corrosion resistance.

Unfortunately its ‘reactivity’ is a point of criticism as the glycidyl groups tend to react at normal temperatures in the presence of acid and amino groups or other trace materials. Consequently this system will often show poor stability unless very adequate precautions are observed. A further drawbask is the cost of the glycidyl monomer.

2.5 Modified monomers Apart from the usual hydroxyl, carboxyl, amide and glycidyl-type

acrylic functional monomers described above, various interesting functional monomers have been prepared to give special properties or to give built-in crosslinking agent-type compositions. On many occasions conventional acrylic monomers have been modified with suitable agents.

Organic epoxides which may be either monoepoxides or polyepoxides may be reacted with acrylamide [64]. The adduct is useful as a comonomer. Cure is enhanced in the presence of epoxy-functional monomers and amide monomers. For low-temperature curing, an amino resin is incorporated.

Reaction of an epoxidised oil (e.g. soybean oil) with acrylic acid pro- duces a useful comonomer [65]. The copolymer is useful for protective coatings, as a stabiliser or plasticiser, and in adhesives, etc.

Epoxy resin can be reacted with methacrylic acid in a suitable solvent using a polymerisation inhibitor. The reaction product is copolymerisable [66 3. Copolymers with (meth)acrylic acid as one of the comonomers give excellent hardness and resistance to alkalies.

The adduct of acrylic acid and vinyl ethyl ether is used as a comonomer with ethyl acrylate [67]. This is blended with acrylic resin containing acryl- amide derivatives and (meth)acrylic acid.

Reaction of an alkoxyalkyl melamine compound with acrylic monomer yields a copolymerisable compound. A typical example is the reaction of hexamethoxymethyl melamine with methacrylic acid [68]. Reaction of a monoepoxy compound not containing ethylenic unsaturation with cy - /3 unsaturated fatty acid per mole of epoxy compound yields a suitable copoly- merisable adduct [ 691.

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A modified comonomer having the formula HC=CR-COOCH+HMe- (OCHCHMe),OCH-CH(Me)OH, where R is H or methyl (Me) and n = 2 - 6, has been suggested [70], and a TSA polymer containing oxazoline groups described [ 711.

Polypropylene glycol esters of methacrylic or acrylic acid having hydroxyl values of 100 - 200 are used as comonomers along with hydroxyl, carboxyl and etherified amide monomers [ 721. Another composition which has been described contains polypropylene glycol methacrylate with carboxyl-type monomers [ 731.

Self-crosslinking polymers have been prepared by introducing small amounts of monomer bases capable of quaternisation, e.g. dimethyl amino ethylmethacrylate, or polymer&able halogen-containing monomers, e.g. chloromethyl acrylate, which can take part in quaternisation reactions [ 741.

Polymers having pendant amino ester groups and polymers containing oxazoline drying oil constituents have also been described [ 751.

The reaction of the glycidyl ester of a branched chain acid with acrylic acid results in a hydroxyalkyl ester [ 76, 771, which in turn may be copoly- merised with hydroxyethyl methacrylate, styrene and methyl methacrylate. The resulting polymer is useful in coatings.

Vehicles may be made by copolymerising tertiary amine derivatives of acrylic or methacrylic esters with 2-hydroxyethyl acrylate or acrylonitrile [ 781. These vehicles which can be cured at temperature of about 80 “C are crosslinked with aminoplast resins.

2.6 TSAs with other film-formers TSAs may be blended with various other film-formers to suit the needs

of the end application. Those systems which are not included earlier are mentioned in this subsection. Useful thermosetting acrylic compositions have been described [79 - 811, while one composition which gives flexibility to the system has also been described [SZ]. TSAs used for reflowable finishes [ 83,841 and TSA manufacture have been described [ 851.

2.6.1 With amino resins A TSA/aminoplast coating composition has been described [ 861. TSAs

containing methyl01 ether, alcoholic hydroxyl and carboxyl groups may be cured with amino resins [ 871. Such an amino resin cured TSA polymer [ 881 contained in addition to a carboxyl and a hydroxyl group, a vinyl ester of a 7 - 91 carbon carboxylic acid of formula HC=CHOCOC(Me)-C(RR’).Me, where Me = methyl, R = H, Ph or a 1 - 4 carbon alkyl, R’ being the same or different and n = 1 - 12.

TSA polymers can be cured with hexamethoxymethyl melamine [89, 901. A TSA polymer cured with benzoguanamine resin [ 911 contained car- boxy1 and glycidyl groups, and ally1 alcohol. A TSA copolymer containing etherified derivatives of acrylamide and acrylic acid has been cured with polyalkyl etherified methyl01 benzoguanamine [ 921. TSAs containing acrylic acid and the tetrahydrofurfural ester of acrylic or methacrylic acid have been

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cured by means of methoxylated and/or ethoxylated MF or benzoguana- mine formaldehyde resin [ 931. This latter composition is useful for coating paper and leather.

Amino-cured TSA resin can be plasticised [94] by incorporating small amounts of the vinyl ester of a carboxylic acid which is difficult to saponify, e.g. a vinyl ester of a t-carboxylic acid.

Amino-cured TSA polymers having better intercoat adhesion [95] employ a polymerisable monomer having a hydroxyl and a talky1 group, a typical example being the cardura E/methacrylic acid reaction product.

TSA is partially condensed with amino resins by heating above room temperature to give more reproducible film properties [ 961.

2.6.2 With alkyd resins TSA resins which contain modified acryl amide as a comonomer can be

blended with an oil-modified alkyd [ 971. Another composition uses a hydro- xyl monomer along with a derivative of acryl amide as a comonomer [98]. Polymer containing hydroxyl and carboxyl groups are blended with alkyl along with amino resins [ 991.

Blending with alkyd overcomes the drawbacks of TSAs such as high stoving temperature and bad odour [loo].

2.6.3 With phenolics Polymers containing acrylic acid and acrylonitrile may be blended with

phenolics and epoxies [loll. Optionally butylated MF resin may also be added.

Resin containing amide, carboxyl groups and olefinically unsaturated fatty oil is blended with normal phenolics [ 1021.

TSA resin containing carboxyl, hydroxyl and cyanide groups can be mixed with a potentially thermosetting resin such as a phenolic along with an alkylene oxide/trio1 adduct [ 1031.

2.6.4 With epoxy resins Polymers containing P-ethoxy ethyl groups have been blended with

epoxies [ 1041. TSA along with epoxy resin is used in ductile coating compo- sitions [ 1051.

2.6.5 With cellulosics Polymer containing acrylic acid and diepoxy compounds such as vinyl

cyclohexene diepoxide or dicyclopentadiene diepoxide has been blended with cellulosics along with aminoplast [ 1061. The composition exhibited no cracking during repainting and had good low temperature properties and resistance to swelling by solvents.

Acrylic polymer containing carboxyl and hydroxyl groups may be blended with CAB and amino resin [ 1071.

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2.6.6 With isocyanates Polymer containing acidic groups and reacted with 10 - 40% of a glyci-

dyl ester of a secondary or tertiary 9 - 11-monocarboxylic acid has been used with isocyanate [ 1081. A TSA polymer which can be used with poly- urethane has been described [ 1091.

2.6.7 Miscellaneous Polysulphonazide may be used as a crosslinking agent for TSAs having

the formula R(SONa), where x > 1 and R is an alkylene, arylene, acylkylene or alkrylene [ 1101.

Poly(vinylaceta1) resin can be blended with thermosetting acrylic resin along with epoxy resin [ 1111.

TSA polymer has been crosslinked with azalactone [ 1121, and blended with thermoplastic polymer for use in treating paper [ 1131.

TSAs utilising a new group of epoxy compounds which are derivatives of bis(hydroxy-4-phenyl)acetic acid have been described [ 1141.

3 TSA emulsions

Emulsions containing N-(meth)acrylamide derivatives as crosslink sites have been mentioned [ 1151, and aqueous dispersions containing alkoxy methyl acrylamide, (meth)acrylonitrile and (meth)acrylic acid together with phenol formaldehyde resin described [ 1161.

Emulsions containing N-methyl01 ether derivatives, carboxyl groups and a hydroxyl functionality having a uniform particle size for eliminating water popping have been claimed [ 1171.

Emulsions containing acrylamide and methacrylic acid exhibit intra- and inter-molecular interactions between the carboxyl and amide groups. One such polymer provides an excellent varnish with good Freon resistance [118].

Water-dispersible solderable wire coatings containing acrylonitrile and methacrylic acid have been described [ 1191.

Emulsions containing acrylonitrile and (meth)acrylic acid may be pre- pared [ 1201 using a mixture of surfactants derived from hydroxy-terminated polyethylene oxide adducts with monoalkyl glycol ether as a coalescing agent.

Aqueous dispersions to be used in paints containing acrylic acid and glycidyl methacrylate have been described [ 1211.

Water-dispersible copolymers with two sources of non-nitrogeneous hydroxyl functional unsaturated materials have been reported [122]. One material is a hydroxy functional monomer while the other is a hydroxy- terminated polyethylenically unsaturated polyester. Coatings with improved gloss and mar resistance combined with superior flexibility can thus be prepared.

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Aqueous dispersions having applications in coil coatings and domestic appliances have been mentioned [ 1231. They contain w-hydroxy carboxylic acid, (meth)allyl ester and (meth)acrylic acid and/or its derivatives.

Crosslinkable emulsions containing halogen-containing acrylates, hy- droxy and carboxyl type monomers may be crosslinked without thermal treatment by controlled addition of alkali [ 1241. They are used in lacquers, varnishes, adhesives, pigment binders and are suitable as pressure-sensitive adhesives.

Emulsion preparation in the presence of plasticising non-reactive modi- fier, e.g. dibenzyl diethylene glycol diadipate, has been described [ 1251.

Emulsions containing acrylic acid may be prepared [ 1261 using hydro- carbon sulphonates of ethoxylated fatty alcohols and optionally an ethoxy- lated alkyl phenol as an emulsifier. They are used with amino resins in metal coatings with good elasticity and weather resistance properties.

Surfactant-free emulsion coating systems having most of the advan- tages of both the water-soluble and emulsion systems without their disad- vantages, and containing hydroxyl and carboxyl groups, crosslinked with HMMM have been described [ 127,128].

Emulsions with ammonium polyacrylate as a thickener and with water- soluble resin have been described [ 1291.

Self-curing emulsions containing conventional epoxy resin uniformly dispersed throughout the latex particle may be prepared by blending an epoxy resin in a monomer mixture and conducting the polymerisation in a conventional manner [130]. The resulting emulsions displayed a fairly uni- form particle size and provided properties required in various adhesives, binders and paint applications.

Crosslinking emulsions based on an acrylate or methacrylate oligomer containing more than two acrylol or methacrylol radicals have been claimed [131]. The vehicle may be dispersed in water and pigmented to obtain a water-borne coating.

4 Water-solubilised TSAs

Thermosetting water-solubilised type acrylic compositions have been described [ 132 - 1341. Apart from a carboxyl-type monomer which is essen- tial for the solubilising action, other functional monomers may also be added in the polymer composition. Hydroxyl-type monomers and derivatives of acrylamide are usually incorporated to aid the water solubility and cross- linking, and to impart specific properties to the finished product.

Polymers containing acrylic acid and N-methyl01 acrylamide, prepared in isopropanol which cure at about 160 “C, have been described [ 1351.

Polymers which crosslink at room or elevated temperatures contain (meth)acrylamide methyl01 ally1 ethers as comonomer [136].

A copolymer containing alkylolated or methoxy alkylated (meth)acryl- amide and unsaturated carboxylic acid may be used along with amino or phenolic resin [ 1371.

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A copolymer is described which is comprised of N-methyl01 (meth)- acrylamide and/or alkoxy ether derivatives along with carboxyl- and hy- droxyl-type monomers [ 1381.

The TSA polymer contains acrylamide, methyl propane sulphonic acid, IV-methyl01 or N-alkoxy methyl acrylamide and o - 0 unsaturated carboxylic acid [139].

The copolymer containing acrylic acid and hydroxy ethyl methacrylate may be crosslinked with etherified MF resin [ 1401. The composition is suitable for low-temperature stoving for metallic finishes.

Polymer containing acrylic acid and hydroxy ethyl acrylate may be neutralised with ammonia or amine [ 1411.

Polymer containing hydroxyl and carboxyl groups can be used with partially etherified MF resin [ 142).

Acrylic copolymer containing hydroxyl and carboxyl groups which is partially dissolved and partially dispersed can be used together with HMMM [143].

Copolymer comprising hydroxyl and carboxyl groups contain dicyan- diamide or a guanidine salt [ 1441.

Acrylic copolymer containing methacrylic acid and hydroxy ethyl methacrylate can be used with a water-soluble aminoplast. The composition has improved anticorrosive, antisoiling and alkali-resistance properties [ 1451.

The TSA containing methacrylic acid and vinyl butyl ether may be solubilised with ammonia. The composition is used for printing inks having improved rheological properties [ 1461.

The product which is neutralised with ammonia or amine contains hydroxyl, carboxyl and amide groups, and a polyhydric alcohol preferably containing an ally1 group [ 1471.

The heatcurable coating composition for plywood contains water- soluble or water-dispersible aminoplast as a crosslinking agent. The composi- tion has a reduced tendency towards surface defects such as crossing and pinholing [ 1481.

A high stability electrodeposited coating composition containing amine neutralised acidic acrylic resin and MF resin has been described [ 1491.

Acidic copolymer neutralised with amines or ammonia may be cross- linked with capped polyisocyanates, epoxy compounds or amino resins [ 1501. The composition has good resistance towards water, detergents and weathering.

The TSA polymer crosslinked with MF resin has been used as a lubri- cant coating composition [ 1511.

The copolymer containing itaconic or acrylic acid may be neutralised with ammonia. This polymer along with epoxy/aniline resin has been used as an insulating coating in the electronics industry [ 1521.

A composition used for can coatings with improved impermeability has been described [ 1531. The composition containing acrylic acid has also been suggested for internal can coatings [154].

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Heat-curable particulate coating compositions of an elastomeric emul- sion copolymer and a thermosetting copolymer have been described [ 1553.

The copolymer containing the mono(meth)acrylate of a polyhydric alcohol and (meth)acrylic acid having an acid value of 5 - 150 and a hydroxyl value of 30 - 120 has been used for gravure or flexo inks and paints [ 1561.

The copolymer of butyl acrylate, methyl methacrylate and acrylic acid prepared in isopropanol can be used in protective coating compositions [ 1571.

Most of the above stated compositions are solubilised due to the pres- ence of anionic charges on the backbone. Those solubilised as a result of cationic charges, which are used in cathodic electrodeposition, have also been described.

A hot water resistant thermosetting metal coating composition com- prising an acrylic emulsion and a water-soluble acrylic copolymer resin derived from a dialkyl amino ethyl (meth)acrylate and other monomers [ 1581 has been described.

Acrylic polymer with a fatty acid and a polymer having an N base grouping within the molecule has been used for cathodic electrodeposition [159].

A composition suitable for cathodic electrodeposition is produced by grafting an epoxy ester of a fatty acid and a glycidyl ester of a carboxylic acid on to an amine/acrylate backbone copolymer derived from a secondary amine (meth)acrylate, a hydroxy alkyl (meth)acrylate, an alkyl methacrylate and a mercaptoethanol [ 160, 1611.

5 Conclusions

It can be seen from the results reported that continued attempts are being made towards better economy, good overall properties and adequate crosslinking mechanisms to suit the needs of the end application.

Although this review is not claimed to be exhaustive, the majority of the work described in the patent literature has been highlighted. The author hopes that this paper will be of some use to beginners and to those who are actively engaged in research on acrylics.

Acknowledgements

The author wishes to express his sincere thanks to Prof. Dr. Ju Kuma- notani for his encouragement in writing this paper and for a critical reading of the manuscript.

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