Thermosetting acrylic resins - a literature review

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  • 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...

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