CURABLE COMPOSITIONS WHICH CONTAIN NETWORK POLYMERS

Abstract: A network polymer that contains: (a) a plurality of polymer chains which are derived from one or more ethylenically-unsaturated monomer(s); and (b) at least one linking group which links the polymer chains and which contains anhydride, carbonate or uretidinedione moieties. The network polymer and other network polymers which contain similar dissociating groups are useful in curable formulations which contain a curable compound that contains: (a) a central moiety; and (b) a plurality of reactive groups which are bonded to the central moiety and which react with acid, hydroxyl or isocyanate moieties liberated by dissociation of the anhydride, carbonate or uretidinedione moieties. The curable composition builds viscosity more slowly and more contrallably during curing than equivalent systems which do not contain the network polymer. It is useful in coatings and other uses for curable compositions.

Description: CURABLE COMPOSITIONS WHICH CONTAIN NETWORK POLYMERS The present invention relates to resins useful in curable formulations, particularly for coating applications. Coating formulations that contain curable epoxy resins, isocyanates and other resins, and their curing agents, are commonly used to put decorative and/or protective coatings on indoor and outdoor articles. These formulations are usually applied as powders or as solutions of curable resin in aqueous or organic solvents. Solutions are commonly applied by spraying, brushing, dipping, rolling and other methods. Powders are commonly applied by f luidized bed or electrostatic spraying of powder onto a heated substrate which causes the powder to melt, flow, consolidate and cure. Viscosity (or melt viscosity in the case of powder-coating formulations) is a critical characteristic in solvent-borne and powder-coating formulations. The viscosity is ideally low enough so that the formulation flows and spreads evenly before it cures, in order to provide a smooth, even appearance and a surface which is free of holes and other flaws. High viscosity also makes it more difficult to obtain adequate wetting using the formulation. Inadequate wetting may reduce coating adhesion or hurt other coating properties. Viscosity control is becoming even more important because of current attempts to limit emissions of volatile organic compounds (VOC). In order to reduce their use of VOC, coaters either use formulations which contain higher solid concentrations and lower solvent concentrations than previously used or substitute powder-coatings into applications where they were not used previously because of uneven appearance. In both instances, the higher viscosity of the formulation, or melt, makes it more difficult to apply a smooth coating with even appearance. The viscosity of many formulations decreases as the formulation is heated for curing, providing an opportunity for the formulation to flow and cover the surface. However, the formulation builds molecular weight rapidly as it cures, so that the viscosity quickly rises again. The coating formulation would have more opportunity to spread evenly before it cures if the build-up of viscosity were slowed. Several methods are known to slow the reaction of curable resin and curing agent. For instance, the formulation may contain less reactive reagents or latent catalysts. Those methods control the build-up of viscosity, but also lengthen the time needed to complete the curing reaction. This is economically undesirable, because the cure requires longer residence time and more energy in curing ovens. What is needed is a relatively low viscosity curable composition which builds viscosity in a slower, more controlled fashion, but completes the curing cycle in about the same total time. One aspect of the present invention is a network polymer which contains: (1) a plurality of polymer chains which are derived from one or more ethyienically- -unsaturated monomer(s); and (2) one or more linking groups, which: (a) contain anhydride, carbonate or uretidinedione moieties, and (b) link two polymer chains, characterized in that the network copolymer: (A) contains less than 0.5 equivalents of pendant carboxylic acid per equivalent of anhydride moiety when the linking group contains an anhydride moiety; (B) is soluble up to at least 20 weight percent solids in at least one aromatic or aprotic polar organic solvent; and (C) has an average weight per equivalent of linking anhydride, carbonate or uretidinedione moieties of no more than about 5,000. A second aspect of the present invention is a process to make a network polymer, comprising the step of polymerizing: (a) one or more ethylenically-unsaturated monomers; and (b) in the presence of one or more linking compounds, which contain at least two ethylenically-unsaturated aliphatic moieties linked by anhydride, carbonate or uretidinedione moieties, using a free-radical initiator, characterized in that: (1) the weight ratio of linking monomers to other monomers is from 5:95 to 60:40, and (2) the reaction is carried out under starved reaction conditions. third aspect of the present invention is a curable composition which contains: (a) a curable compound which contains a plurality of second reactive groups; and (b) a network polymer, characterized in that the network polymer contains: (1) a plurality of polymer chains which are derived predominantly from one or more ethylenically-unsaturated monomer(s), and (2) at least one linking group which: (i) links two polymer chains; and (ii) contains a dissociating group that dissociates to form a plurality of first reactive groups which react with the second reactive groups to cure the composition. Forthe purposes of this application, "dissociate" and "dissociating" mean that either: (1) the dissociating group divides by action of heat, solvent, reagent and/or catalyst to form two reactive sites, or (2) a part of the dissociating group reacts with the curable compound in the same manner as a first reactive group, and that reaction cleaves the dissociating group to form another first reactive group. A fourth aspect of the present invention is a method to use a composition as previously described comprising the steps of: (1) coating a substrate with the composition of the present invention; and (2) maintaining the coated substrate at a temperature and for a time sufficient to cure the coating. A fifth aspect of the present invention is a coated article comprising: (1) a substrate; and (2) a coating, which contains a cured composition of the present invention, adhered directly or indirectly to the substrate. Network polymers of the present invention may be used in compositions of the present invention. Compositions of the present invention exhibit slower growth in molecular weight and viscosity than equivalent compositions which use conventional curable agents instead of the network polymer. Some compositions may exhibit a reduction in viscosity during the initial stages of cure. The compositions can be used in the method of use of the invention in order to make the coated articles of the invention. (A) Network Polymers The present invention uses a network polymer which contains chains of polymer derived from ethylenically-unsaturated monomers. The polymer chains are linked by pendant linking groups which contain dissociating moieties. (For further information and illustration of network polymers, see Encyclopedia of Polymer Science & Engineering, Volume 4, "Cross- -linking," pages 350 to 395 and "Cross-linking, Reversible," pages 395 to 417, (J. Wiley & Sons 1986); and Encyclopedia of Polymer Science & Engineering., Volume 10, "Networks," pages 95 to 112, (J. Wiley & Sons 1987).) An example of such a network polymer is illustrated in the following formula: FORMULA I wherein : 1 - represents a polymer chain derived from one or more ethylenically-unsaturated monomers ; each R independently represents a bond or a divalent organic moiety; each D represents a dissociating moiety; and the arrows "→" indicate that the network polymer may optionally continue further in either dimension with further polymer chains and further cross-links to other polymer chains. The group: R-D-R 7 is collectively the linking group. The polymer chains are derived from ethylenically-unsaturated monomers, such as lower (C l to C 6 ) alkenes (for instance, ethylene, propylene or butylene), vinyl chloride, vinyl acetate, vinylidene chloride, styrenes (for instance, styrene or alkylstyrene), acrylic or methacrylic acid, esters of acrylic or methacrylic acid, nitriles of acrylic or methacrylic acid, or amides of acrylic or methacrylic acid, and mixtures thereof. The polymer chains are preferably polymers or copolymers of lower (C 1 to C 6 ) alkenes, lower (C 1 to C 6 ) alkyl esters of acrylic or methacrylic acid and/or styrenes. The polymer chains are most preferably copolymers of two or more of those preferred monomers. More specific examples of the monomers are given in the discussion on processes to make the network polymer. The polymer chains are preferably substantially inert with respect to the curable compound under curing conditions. Preferably, no more than 5 percent of the repeating units in the polymer chain contain pendant acid, amine, amide or hydroxyl groups; more preferably no more than 1 percent do and most preferably no more than 0.1 percent do. The equivalent ratio of (pendant acid, amine, amide or hydroxyl groups):(dissociating moieties) in the network polymer is preferably less than 0.5: 1, more preferably less than 0.2: 1, more highly preferably less than 0.1 : 1 and most preferably less than 0.05: 1. Ideally, the equivalent ratio is 0: 1. The polymer chains are linked together by linking groups (L) which contain dissociating moieties (D). The dissociating moiety (D) dissociates to provide two reactive sites (hereinafter called "first reactive groups"). The first reactive groups are preferably: isocyanate groups, carboxylic acid groups, sulfonic acid groups, amine groups or hydroxyl groups. The first reactive groups are more preferably isocyanate groups or carboxylic acid groups, and are most preferably carboxylic acid groups. The dissociating moieties (D) are preferably: anhydride moieties, carbonate moieties, uretidinedione moieties, peroxide moieties, disulfide moieties, sulfate diester moieties, siloxy moieties, phosphonic diester moieties, carbonate anhydride moieties, N-hydroxyl imide moieties, ketene-acetal moieties, metal carboxylate moieties, or combinations thereof. The dissociating moieties are more preferably anhydride moieties, carbonate moieties or uretidinedione moieties, and are most preferably anhydride moieties. Examples of suitable dissociating moieties (D) are illustrated in the following formula: FORMULA II C - 0 anhydride 0 - C 0 - carbonate uretidinedione o o c - 0 o 0 M 0 c metal carboxylate o s - o - sulfate ester o Rl - o Si - O siloxy Rl - O P - O phosphonic diester ORl wherein M+ + is a divalent metal such as zinc, iron, magnesium, copper, calcium or nickel; R 1 is hydrogen or a lower alkyl group; and all other characters have the meanings previously given. The dissociating moiety may optionally be bonded directly to the polymer backbone, as exemplified in Formula lll(a); or it may optionally be linked to the polymer backbones by a pendant divalent organic moiety (R), as exemplified in Formula lll(b). (In Formula lll(a) and (b), the dissociating group is an anhydride group. Other dissociating groups may be substituted in its place.) FORMULA III ( a ) FORMULA IΙI (b) The divalent organic moiety (R) is preferably an alkylene ester moiety (-COOR 2 -, wherein R 2 is an alkylene group), an alkylene moiety or an arylene moiety. The divalent organic moiety is highly preferably an alkylene ester moiety or a phenylene moiety, is more preferably a lower alkylene ester moiety and is most preferably a 2-ethylene-ester moiety. R 1 is hydrogen or a lower (C1 to C β ) alkyl group and is preferably hydrogen or a methyl group. For example, Formula lll(c) illustrates a linking group (L) in which the dissociating moiety (D) is an anhydride group and the divalent organic moieties (R) are 2-ethylene-ester moieties. FORMULA IIKc l c — o - CH 2 CH 2~ -C— 0 — C — CH 2 CHJ- 0 The basic network copolymer structure may have several more complex variations. For instance: (1) A single linking group (L) may optionally contain two or more dissociating moieties (D) linked by a divalent organic moiety (R2), as illustrated in the following formula: FORMULA IV The divalent organic moiety is preferably alkyl, aromatic or alkyl-aromatic and preferably contains 1 to 12 carbon atoms and more preferably contains 2 to 6 carbon atoms. Each linking group (L) preferably contains a single dissociating moiety (D) and no R2 moiety. (2) The network polymer may optionally further contain linking groups which do not dissociate. For instance, polymer chains may be linked by groups derived from divinylbenzene, allyl methacrylate or vinyl ester resin. Preferably, the concentration of non-dissociating cross-links is minimized. The molar ratio of (non-dissociating cross-links):(dissociating cross-links) is preferably from 0: 1 to 1 : 1, more preferably from 0:1 to 0.5:1, more highly preferably from 0:1 to 0.2:1 and most preferably from 0: 1 to 0.1 : 1. Optimally, the network copolymer contains essentially no non-dissociating cross-linkers. (3) The network polymer may contain two or more different types of linking groups (L), such as containing both anhydride linking groups and carbonate linking groups. Other structural variations which are also within the scope of the invention will be obvious to persons of ordinary skill in the art. The number average molecular weight of the network polymer is preferably at least 1,000 and more preferably at least 1,500. It is preferably no more than 5,000. The weight o average molecular weight of the network polymer is preferably at least 2,000, more preferably at least 5,000 and most preferably at least 15,000. It is preferably no more than 100,000. The ratio of weight average molecular weight to number average molecular weight is preferably at least 3, more preferably at least 5 and most preferably at least 10. (Molecular weights for the purposes of this discussion are measured by gel permeation chromatography (GPC) using 5 polystyrene standards at 35°C, tetrahydrofuran as the effluent, μ-STYRAGEL τ " columns and a differential ref ractometer detector.) Within network polymers of the present invention, preferably at least 1 percent of repeating units in the polymer chains are bonded to linking groups (L), more preferably at least 5 percent of repeating units in the polymer chains are bonded to linking groups (L) and 0 most preferably at least 15 percent of repeating units in the polymer chains are bonded to linking groups (L). Preferably no more than 50 percent of repeating units in the polymer chains are bonded to linking groups (L) and more preferably no more than 25 percent of repeating units in the polymer chains are bonded to linking groups (L). The average weight of network polymer per equivalent of dissociating moiety is preferably at least 300, more preferably at 5 least 900 and most preferably at least 1 ,100. It is preferably no more than 5,000, more preferably no more than 2,000 and most preferably no more than 1 ,500. For some purposes, it may be useful to know the average molecular weight or equivalent weight of the network polymer after the dissociating groups have been dissociated. Dissociating may be accomplished by known methods. For instance, anhydride groups are 0 dissociated by treating with aqueous hydrogen chloride and methanol, and uretidinedione groups are dissociated by heating to at least 125°C to 135°C. The weight average molecular weight of the dissociated network polymer is preferably at least 1 ,000 and more preferably at least 3,000. The weight average molecular weight of the dissociated network polymer is preferably no more than 5,000 and more preferably no more than 3,000. The average weight 5 per equivalent of first reactive group is preferably at least 150, more preferably at least 450 and most preferably at least 550. It is preferably no more than 2,500, more preferably no more than 1,000 and most preferably no more than 750. An important use for the network polymer is in curable formulations with other compounds. Therefore, the network polymer is preferably soluble in a curable compound or in a solvent which will dissolve a curable compound. For instance, the network polymer is preferably, completely soluble in a stoichiometrically equivalent quantity of molten or liquid glycidyl methacrylate (GMA) copolymer, epoxy resin, polyamine, polyol, polyester or polyisocyanate. Alternatively, the network polymer is preferably soluble up to at least about 30 weight percent solids in at least one organic solvent, preferably a polar, aprotic solvent or an aromatic solvent. (B) Processes to Make Network Polymers Network polymers used in the present invention are preferably made by free- -radical copolymerization of: (1) ethylenically-unsaturated monomers; and (2) a linking monomer that contains two ethylenically-unsaturated moieties linked by a linking group (L) which contains a dissociating moiety (D). The ethylenically-unsaturated monomers were previously discussed in describing the polymer chains of the network polymer. Examples of suitable monomers include ethylene, propylene, butylene, styrene, vinyltoluene, vinyl chloride, vinylidene chloride, 4-vinylanisole, acrylonitrile, vinyl oxazoline, vinyl acetate, vinyl propionate, n-butyl vinyl ether, lauryl vinyl ether, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, butyl acrylate, hexyl acrylate, octyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, benzyl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, hexyl methacrylate, octyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, benzyl methacrylate, cyclohexyl methacrylate and methyl crotonate. The linking monomer is preferably represented by Formula V(a) and more preferably represented by Formula V(b): FORMULA V O O C (R 1 ) 2 =CR 1 -C-0-R 3 -D-R 3 -0-C-CR1=C (R1 ) 2 ( a ) C (R 1 ) 2 =CR 1 -R-D-R-CR1=C (Rl ) 2 (b) wherein R 3 is a lower alkylene group .(preferably an ethylene group) and all other moieties have the definitions previously given. These linking monomers can be made by the processes set out in the following forulae: FORMULA VI C (R 1 ) 2=CR 1 -R-C0 2 H + dehydrating anhydride (a) agent 2 C (R 1 ) 2=CR 1 -R-OH + COCl 2 carbonate (b) 2 C (R 1 ) 2=CR 1 -R-NCO uretidinedione ( c ) C (R 1 ) 2=CR 1 -R-C0 2 H IT 2 + + OH metal acid salt (d) sulfate ester (e) o Rl / C (R 1 ) 2=CRl-R-0-Si-OR+C (Rl) 2=CRl-R-OH ► siloxy ( f ) / Rl phosphonic diester (g) C (R1 ) 2=CR1-R-O-P-OH+C (R1 ) 2 =CR1 -R-OH ORl The foregoing reactions are generally described in the following references: (a) Loudon, G. M., Organic Chemistry, pages 1015 to 1016, (Addison Wesley Publishing 1984). (b) March, J., Advanced Organic Chemistry (2d Edition), page 361 and 649, (McGraw Hill Book Co. 1977). (c) Oertel, G., Polvurethane Handbook, page 15 and 79, (Hanser Publishing 1985). (d) Morrison, R., and Boyd, R., Organic Chemistry (5th Edition), pages 822 to 829, (Allyn & Bacon, Inc. 1987). (e) Vogel, A., Practical Organic Chemistry (3d Edition), page 303, (J. Wiley & Sons 1966). (f) Colvin, E., Silicon in Organic Synthesis, (Butterworths). (g) Kosoloapoff, G., and Maier, L, Organic Phosphorus Compounds, (J. Wiley & Sons 1972). Examples of suitable linking monomers include methacrylic anhydride, acrylic anhydride, crotonic anhydride, bis-(2-methylallyl)carbonate and the uretidinedione of 2- -cyanoethyl acrylate or 2-cyanoethyl methacrylate. The molar ratio of linking monomers to ethylenically-unsaturated monomers is preferably at least 1 :200, more preferably at least 1 : 10 and most preferably at least 1 :4. It is preferably no more than 1 : 1 and most preferably no more than 1 :2. The weight ratio of linking monomers to other monomers is preferably at least 5:95, more preferably at least 15:85 and most preferably at least 30:70. The weight ratio is preferably no more than 60:40. The reaction conditions for free-radical polymerization of unsaturated monomers are well known. See Encyclopedia Polymer Sci. & Engineering, "Radical Polymerization," Volume 13, pages 708 to 867, (J. Wiley & Sons 1988). The mixture of monomers is heated in the presence of a free-radical initiator, preferably in a solvent or in an emulsion. The initiators are usually peroxide compounds or azo-compounds. Suitable initiators, solvents, chain transfer agents and emulsif iers are commercially available and familiar to persons of ordinary skill in the art. The concentration of initiator is preferably 0.5 to 10 weight percent, based upon the weight of the unsaturated monomers. For a higher molecular weight network polymer, the concentration of initiator is more preferably 1 to 4 weight percent and for a lower molecular weight network polymer, the concentration of initiator is more preferably 4 to 8 weight percent. Special polymerization conditions may be needed to make some of the network polymers. When methacrylic anhydride and related monomers are used in ordinary free-radical polymerization, they frequently do not form a network polymer of the present invention. Instead, the methacrylic anhydride forms a linear polymer chain having a cyclic anhydride structure incorporated within the backbone. See Charle et al., U.S. Patent 3,762,863 (October 2, 1973). These polymers do not provide the same effect as the network polymers of the present invention. Cyclization is minimized and formation of the network structure is maximized by running the polymerization with vigorous agitation under "starved" conditions, in which monomers mixed with initiators are added to the reaction mixture continuously or intermittently in portions over a length of time. The time period is preferably at least about 2 hours, more preferably at least about 4 hours and most preferably at least about 8 hours. The time period is preferably no more than about 20 hours, more preferably no more than about 16 hours and most preferably no more than about 10 hours. The reaction temperature is preferably high enough to encourage a quick reaction of the monomers once they are added to the reaction mixture. The reaction temperature is preferably 100°C to 200°C, more preferably 125°C to 175°C and most preferably 150°C. Examples of suitable solvents include high boiling aromatic compounds (such as xylene, ethylbenzene, or AROMATIC 100 solvent from Exxon Chemical Corporation) and liquid, high-boiling esters (such as ethyl acetate, propyl acetate, butyl acetate, amyl acetate, propylene glycol methyl ethyl acetate or DOWANOL PM* solvent). (* - trademark of The Dow Chemical Company.) Alternative processes to make the network polymers are described in several publications. See Rees, U.S. Patent 3,312,676 (April 4, 1967), and Encyclopedia Polymer Science & Engineering, "Cross-linking, Reversible," Volume 4, pages 395 to 417. For instance, the Rees patent describes a process to link ethylene-methacrylic acid copolymers by reacting with acetic anhydride. (C) Formulations Containing Network Polymers The network polymer is preferably mixed into a composition with a curable compound which can cure by reaction with the network polymer. The curable composition optionally further contains any of the following: solvents, catalysts, fillers, pigments, flow- -modifiers, wetting agents, stabilizers, flame retardants and other common additives for curable formulations. The curable compound contains: (1) a plurality of reactive groups (hereinafter called "second reactive groups") which are selected to cure by reaction with the first reactive groups formed when the dissociating groups in the network polymer dissociate; and (2) a central moiety to which a plurality of the second reactive group is bonded. Examples of preferred second reactive groups include: epoxy and glycidyl moieties, isocyanate moieties, amine moieties, hydroxyl moieties, thiol moieties, carboxylic acid moieties and amide moieties. The preferred selection within this group depends upon the first reactive groups formed by dissociation of the network polymer. The second reactive groups are preferably epoxy groups, when the first reactive groups are acid, isocyanate or hydroxyl groups. The second reactive groups are preferably amine groups or hydroxyl groups, when the first reactive groups are isocyanate or acid moieties. The curable compound preferably contains on average more than 1.5 second reactive groups per molecule, more preferably at least 1.8 and more highly preferably at least 2. Many curable compounds most preferably contain on average more than 2 second reactive groups per molecule. Curable compounds which are not polymers or oligomers preferably contain no more than 10 second reactive groups, more preferably no more than 4 second reactive groups and most preferably no more than 3 second reactive groups. The second reactive groups are bonded to a central moiety. The central moiety is preferably essentially inert under curable conditions. Examples of suitable central moieties include phenylene rings, biphenylene moieties, bis(phenylene)alkane moieties, lower alkylene moieties, poly(alkylene oxide) moieties, polyester or polyacrylate oligomers, polymers or copolymers and combinations thereof. The central moiety is preferably an alkyl group, an aromatic group, an alkaryl group or a copolymer containing alkyl acrylate or alkyl methacrylate. The following are preferred examples of curable compounds: glycidyl methacrylate (GMA) copolymers, epoxy resins, polyamines, polyols and polyisocyanates. Of course, the best selection within those preferred examples shall vary depending upon the network polymer selected. The molecular weight of the curable compound is preferably selected so that the curable compound is liquid or is soluble in common solvents with the network polymer at temperatures at which the composition is used. The number average molecular weight of the curable compound is preferably at least 100 and preferably no more than 10,000. GMA copolymers preferably contain at least 1 weight percent GMA, more preferably at least 10 weight percent and most preferably at least 30 weight percent. They preferably contain no more than 80 weight percent GMA, more preferably no more than 60 weight percent and most preferably no more than 40 weight percent. The GMA is preferably copolymerized with ethylenically-unsaturated comonomers as described previously in describing the network polymer and methods to make it. The comonomers are more preferably selected from lower alkyl acrylates or methacrylates and styrene. The number average molecular weight of the GMA copolymer is preferably at least 1 ,500 and more preferably at least 2,500. It is preferably no more than 6,000 and more preferably no more than 4 ' 000 - Liquid epoxy resins and advanced epoxy resins are preferably poly(glycidyl ethers) or poly(glycidyl esters). The poly(glycidyl ether) is preferably derived from a polyhydric phenol (such as resorcinol, hydroquinone, catechol, bisphenol A, bisphenol F, a novolac resin, a trisphenol resin or a halogenated variation of one of those resins), but it may also be derived from an aliphatic polyol (such 2,2-bis-(4-hydroxycyclohexyl)propane or a poly(alkylene oxide)). The glycidyl ester is preferably derived from a diacid such as 1,2-cyclohexane-dicarboxylic acid. The formula weight of non-halogen atoms per epoxide equivalent in the epoxy resin is preferably at least 90 and more preferably at least 160. It is preferably no more than 500 and more preferably no more than 250. Many suitable epoxy resins are commercially available and others can be made by known processes such as those described in Tanaka, Epoxy Resins (2d Edition), "Synthesis & Characteristics of Epoxides," page 9 to 283, (Marcel Dekker 1988). Polyisocyanates are preferably derived from polyamines by phosgenation. Examples of suitable polyisocyanates include: methylene di(phenylene isocyanate) (MDI), tolylene diisocyanate (TDI), naphthalene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate. Some useful polyisocyanates are commercially available. Useful polyisocyanates and processes to make them are described in many publications, such as Oertel, G., Polyurethane Handbook, pages 12 to 15 and 62 to 73, (Hanser Publishing 1985); and Encyclopedia Polymer Science & Engineering, "Isocyanate-Derived Polymers," Volume 8, pages 448 to 453, (J. Wiley & Sons 1987). A wide variety of diamines and other polyamines which are useful as curable compounds are commercially available, and others can be made by known processes such as by hydrogen reduction of nitrated compounds. Examples of suitable polyamines include phenyienediamine, methylene dianiline, ethylenediamine, propylenediamine, triaminobenzene, toluenediamine, 1,6-hexanediamine, isophoronediamine and amine- -containing oligomers, such as are described in Encyclopedia Polymer Science & Engineering, "Polyamines and Polyquaternary Ammonium Salts," Volume 11 , pages 489 to 507, (J. Wiley & Sons 1988). A wide variety of polyols which are useful as curable compounds are commercially available, and others can be made by known processes. Examples of suitable polyols include lower molecular weight polyols, such as glycerine, butanediol, ethylene glycol, resorcinol, hydroquinone, catechol, bisphenol A, bisphenol F or 2,2-bis-(4-hydroxycyclohexyl)propane, and higher molecular weight polyols, such as poly(alkylene oxide), polyester polyols, polycaprolactone, acrylic polyols, phenoxy resin, novolac resin and trisphenols. The curable composition preferably contains either: (a) a network polymer which contains anhydride linking groups and either an epoxy resin or a polyol; (b) a polyamine or polyol and a network polymer which contains uretidinedione linking groups; or (c) a network polymer which contains carbonate linking groups and either a polyamine or a polyol. It most preferably contains a network polymer which contains anhydride linking groups and an epoxy resin. The composition optionally further contains chain extenders or curing agents for the curable compound. These chain extenders and curing agents preferably contain on average at least about 2 first reactive groups. They preferably contain on average no more than about 10 first reactive groups and most preferably no more than about 4 first reactive groups. The first reactive groups may be either the same as, or different from, the first reactive groups in the dissociated network polymer. The chain extender or curing agent preferably has a lower number average molecular weight than the network polymer. Its weight per equivalent of first reactive group is preferably no more than 500, more preferably no more than 300 and most preferably no more than 250. Its molecular weight is preferably at least 100. When the curable compound is epoxy resin, then examples of chain extenders and curing agents include: bisphenols, polyphenols, polyamines, polyamides, polyanhydrides, polyacids and guanidine compounds. When the curable compound is a polyamine or polyol, then a preferred curing agent is a polyisocyanate. The composition preferably contains 0 to 0.7 equivalents of chain extender or curing agent per equivalent of curable compound, more preferably 0 to 0.5 equivalent and most preferably 0 to 0.2 equivalent. The equivalent ratio of (curable compound):(the combination of first reactive groups on the dissociated network polymer and the curing agent and chain extender) is preferably at least 0.5:1, more preferably at least 0.7: 1 and most preferably at least 0.9: 1. The ratio is preferably no more than 2: 1, more preferably no more than 1.5: 1 and most preferably no more than 1.2:1. The equivalent ratio of (curable compound): (first reactive groups on the dissociated network polymer) is preferably at least 1 :0.1, more preferably at least 1 :0.5 and most preferably at least 1 :0.7. Compositions of the present invention preferably further contain a catalytic amount of catalyst which catalyzes the reaction of the curable compound and the network polymer. For example, when either the first or second reactive group is an epoxy group, then the catalyst is preferably a tertiary amine compound, a quaternary ammonium compound, a phosphine or phosphonium compound, an imidazole, oxazole or thiazole compound or any of several other known epoxy curing catalysts. When the reactive groups are isocyanate groups and hydroxyl groups, then the catalyst is preferably a polyamine. When the network polymer is a polyanhydride and the curable compound is a polyol, then the catalyst is preferably an organotin compound. The optimum concentration of catalyst varies depending upon the reactive groups and the catalyst selected. However, in most cases, the concentration of catalyst is preferably 0.01 to 100 milliequivalents (meq) catalyst per equivalent of second reactive group. Compositions of the present invention optionally further contain a solvent. Appropriate solvents vary widely depending upon the curable compound and the network polymer used. When the curable compound is an epoxy resin or when the reactive groups contain isocyanate groups, then the solvent is preferably a polar, aprotic organic solvent or an aromatic hydrocarbon. The solids content in the composition is preferably at least 30 weight percent and more preferably at least 50 weight percent. It is preferably no more than 95 weight percent and more preferably no more than 70 weight percent. When the curable compound is precipitated or isolated from a solvent, then the composition may also be used as a powder-coating or a neat solution. Compositions of the present invention may optionally further contain numerous additives known to be useful for coatings and other uses. Examples of such additives include fillers, pigments, stabilizers, flow modifiers, wetting agents and latency generators. Except for fillers and pigments, such additives preferably make up 0 to 5 percent of the solids in the compositions by weight. Fillers and pigments preferably make up 0 to 60 weight percent of the solids in the composition. (D) Use of Compositions to Coat Articles Compositions which contain network polymers are preferably applied to a substrate and cured to form a coating. The substrate is preferably wood, plastic, concrete or metal, and is more preferably iron, steel, tin or aluminum. Liquid compositions and solvent- -borne compositions are preferably applied by spraying, rolling or dipping. Solid powder compositions are preferably applied byfluidized bed or by electrostatic spraying. The composition is heated in contact with the substrate to a temperature at which the composition cures. The optimum curing temperature varies depending upon the substrate and the components in the composition in a manner familiar to persons of ordinary skill in the art. The conditions must be sufficient to at least dissociate the dissociating moieties and cause reaction between the first and second reactive groups. In most cases, a preferred temperature is between 25°C and 250°C. When the curable compound is an epoxy resin, the curing temperature is preferably at least 20°C, more preferably at least 60°C and most preferably at o least 90°C; and the curing temperature is preferably no more than 250°C, more preferably no more than 200°C and most preferably no more than 160°C. When the network polymer contains anhydride linking groups and the curable compound is a polyol, the curing temperature is preferably at least 60°C, more preferably at least 100°C and most preferably at least 125°C; and the curing temperature is preferably no more than 250°C, more preferably no 5 more than 200 C C and most preferably no more than 180°C. When the reactive groups are isocyanate and hydroxyl groups, the curing temperature is preferably at least 80°C, more preferably at least 100°C and most preferably at least 120°C; and the curing temperature is preferably no more than 250°C and more preferably no more than 200°C. When the network polymer contains uretidinedione moieties, the temperature is preferably at least 120°C and 0 more preferably at least 150°C. The resulting coated article contains a substrate and a coating of cured composition adhered directly or indirectly to the substrate. The coated article may optionally contain one or more primer layers between the substrate and the cured coating. It may also optionally contain one or more additional layers on top of the cured coating. The coating layer 5 is preferably between about 0.2 mil (5 μm) and about 5 mil (130 μm) thick. It has been observed with preferred components that when the curing temperature is continually increased, compositions containing network polymers show reduced viscosity at a temperature 10°C to 20°C lower than equivalent compositions without the network polymer and become non-flowing at a temperature 5°C to 10°C higher than 0 equivalent compositions that do not contain the network copolymers. It is theorized, without intending to be bound, that dissociation of the linking groups causes the molecular weight of the network polymer to decrease early in the reaction, thereby reducing the viscosity early in the reaction. Continuing dissociation holds the viscosity down through the end of the time period when ordinary compositions would be substantially non-flowing. These advantages are 5 illustrated more fully in the following examples. Examples The following examples are for illustrative purposes only. They should not be taken as limiting the scope of either the Specification or the Claims. Examples 1-7- Preparation of Copolymers Linked by Anhydride Moieties The following components were mixed (in the proportions shown in Table I) with 100 g of solution containing 4 to 6 weight percent ethyl 3,3-di-(t-amyl-peroxy)butyrate (sold as LUPERSOL™ 533-M75) in SC-100 aromatic solvent to make 900 g of solution: methacrylic anhydride (MAAn), n-butylacrylate (n-BA), n-butylmethacrylate (n-BMA), styrene and methyl- methacrylate (MMA). A 900 g quantity of the mixture was fed into a reaction vessel which contained SC-100 aromatic solvent at 142°Cto 161 "Cover a 7 to 9 hour time frame. When addition was completed, the solid content is as shown in the following table: Table I Monomer Concentration Used In Preparation Of Network Copolymer Wt% Example MAAn N-BA n-BMA Styrene MMA Solids 1 5.01 15.02 15.01 29.99 34.98 70.21 2 7.52 13.99 18.51 29.02 30.96 71.23 3 10.1 14.13 20.18 25.23 31.26 73.34 4 19.99 16.96 17.13 19.97 25.95 60.02 5 24.83 17.87 16.90 19.85 20.55 64.00 6 29.95 18.05 19.03 13.96 19.00 49.74 7 29.97 16.96 17.13 20.02 25.92 47.84 a MAAn, n-BA, n-BMA, and MMA are abbreviations for methacrylic anhydride, n-butyl acrylate, n-butyl methacrylate, and methyl methacrylate. The resulting copolymers were analyzed by infrared spectroscopy. They showed strong absorption at 1,802 cm-L and a weaker absorption at 1,782 cnrH and 1,762 cm- 1 . The results were consistent with a polymer that contained low quantities of 5-membered cyclic anhydride and high quantities of noncyclic anhydride. The molecular weight of each example was measured by gel permeation chromatography using polystryrene standards at 35°C, tetrahydrofuran eluent, μ-STYRAGEL columns and a differential refractometer detector. The results are shown in Table II. Anhydride groups in the network polymer were then hydrolyzed using water and hydrogen chloride, and the resulting product was esterif ied with trimethylsilyl diazomethane. o The molecular weight was remeasured and is shown in Table II. The significant reduction in M w and M z and in lv M n caused by hydrolysis showed that polymer chains were linked together by anhydride moieties. Hydrolysis of cyclic anhydrides in the backbone of a polymer would not cause the molecular weight of the polymer to go down substantially. The glass transition temperature for each example was also measured using differential scanning calorimetry, and 5 the Brookf ield viscosity of each example was measured at 40 percent solids in SC-100 aromatic solvent using a Brookf ield LVT model viscometer. The results of these tests are all shown in Table II. 0 5 0 5 Table II Brookfield Before Hydrolysis After Hydrolysis Tg, Viscosity, Example °C cps a Mn Mw Mz Mw/Mn Mn Mw Mz Mw/Mn 28.5 28.96 1821 7377 19700 4.05 1967 3476 5581 1.77 23.5 28.51 1686 8179 23107 4.85 1700 2918 4698 1.72 25.7 30.98 1624 8487 25572 5.23 1667 2756 4344 1.65 31.3 141.2 1640 16791 83716 10.235 1176 3316 7901 2.82 40.6 343.2 1650 19345 71108 1173 1254 2885 5076 2.30 r O I 26.1 33.7 1468 10713 37901 7.3 914. 2199 4303 2.40 40.2 110 2064 50816 248587 24.6 1162 3116 5945 2.68 a cps is an abbreviation for centipoise; viscosity measured at 40% solids. b Network copolymer prepared with 4 wt% initiator. Other network copolymer prepared with 6 wt% initiator. Example 8 - Composition of Network Polymer with Epoxy Resins and Resulting Coatings The network polymer from Example 6 was formulated into two compositions. Composition A contained 28.48 g of glycidyl methacrylate-acrylic acid copolymer, 50 g of network polymer and 21.61 g of DOWANOL PM* acetate solvent (* - trademark of The Dow Chemical Company). Composition B contained 12.1 g of bis(glycidy) ester) of 1,2-cyclohexane- dicarboxylic acid, 50 g of network polymer and 21.61 g of DOWANOL PM* acetate solvent. Each composition was stored for 14 days at a temperature of about 38 C C and the Zahn No. 2 cup viscosity was tested periodically. The results are shown in Table III. The 1-K stability of each composition as shown in Table III was very good. Benzyltriphenylphosphonium chloride catalyst was added to Composition B. The composition was applied to zinc phosphated cold- -roll steel panels and cured at 140°C for 30 minutes. The coating that resulted withstands more than 100 MEK double rubs. Table III Viscosity In Accelerated Aging Test Zahn #2 Cup Viscosity (sec . ) at Wt. Ratio Epoxy: Sample Epoxide Type Anhydride 1 days 5 days 7 days 10 days 14 days 6(A) GMA Copolymer 28.48:50 32.45 34.01 38.49 43.47 48.73 6(B) Resin 12.1:50 27.96 28.71 29.42 35.81 39.62 I to I Example 9 - Composition of Network Polymer with Solid Epoxy Resins to Make Powder- Coating Compositions and Resulting Coatings A 72 weight percent quantity of network polymer from Example 4 as powder isolated by precipitation with hexane was formulated with 25 weight percent ALMATEX PD-7610 glycidyl methacrylate/acrylic copolymer resin and 3 weight percent benzyltriphenylphosphonium chloride catalyst. The powder-coating composition was tested for cure rheology at a temperature from 70°C to 180°C and a temperature increase of 7°C per minute using RHEOLAB MC20™ equipment (made by Physica Corporation). The composition showed a relatively slow decrease in viscosity from 95°C to 140°C and then a sharper increase in viscosityfrom 140°C to 150°C. In comparison, the standard co-curing curve of a similar composition containing dodecanedioic acid showed a very sharp increase in viscosityfrom 110°C to 120 C C, a relatively stable viscosity to 138°C and then a very sharp increase in viscosity from 138°C to 142°C. The experiment showed that powder compositions of the present invention have a broader processing window than similar commercially available compositions. Example 10 - Composition of Network Polymer with Polyol and Resulting Coating An acrylic polyol is made by reacting 35 weight percent hydroxyethylacrylate, 15 weight percent methylmethacrylate, 15 weight percent styrene and 35 weight percent N- -butylacrylate. A composition was made containing 61.3 weight percent network polymer from Example 4, 38.7 weight percent (0.06 equivalent) of the acrylic polyol resin and 0.077 weight percent FASCAT™ 4202 catalyst, dissolved in SC-100 aromatic solvent at a concentration of 14.7 percent solids. The composition is applied to zinc phosphated cold-rolled steel using No. 22 and No. 34 draw-down bars. The compositions were cured for 30 minutes, 1 hour and 2 hours at 238°C. The solvent resistance of each composition is tested using the MEK double rub test. The results are shown in Table IV. Table IV No. 22 Bar No. 34 Bar Oven Time Thickness MEK Double Thickness MEK Double Sample (min) (mils/um) Rubs (mils/um) Rubs (A) 30 0.419 30 0.542 25 (10.6) (13.7) (B) 60 0.385 100 0.432 140 (9.7) (10.9) (C) 120 0.275 >250 0.517 >250 (6.93) (13.0) Example 11 - Powder-Coating Formulation Containing Network Polymer Network Polymer Preparation A first mixture of vinyl monomers was prepared by mixing 160.33 g of methacrylic anhydride, 209.78 g of methyl methacrylate, 136.06 g of n-butyl acrylate, 136.62 g of n-butyl methacrylate and 165.38 g of styrene. A second mixture was prepared containing 64.11 g of LUPERSOL 533-M75 initiator and 57.3 g of SC-100 solvent. A 679.2 g quantity of SC-100 aromatic solvent was heated to 144°C in a stirred reaction vessel. After the reactor was purged with nitrogen, the solutions of vinyl monomers and initiator are pumped into the reactor with a syringe pump over a period of 8 hours and 20 minutes. The polymerization temperature was between 137°C and 145°C. When addition of the reactants is completed, the reaction mixture was held for 48 minutes at 145°C and followed by the addition of 1 mL of LUPERSOL 533-M75 initiator. The reaction mixture was then allowed to cool to room temperature. Approximately 682 g of the reaction mixture was added slowly, with stirring, to 1 ,100 g of hexane. The network polymer coagulated and the hexane was decanted. The resulting viscous liquid was spread out in a glass tray, covered with aluminum foil containing holes and then dried under vacuum (1 to 2 mm Hg) at 60°C for at least 24 hours. Using a manual grinder, the resulting solid was ground to a white powder. The theoretical anhydride equivalent weight was 781. Powder-Coating Formulations Tne dried network polymer is dry-blended with ALMATEX'" PD-7610 glycidyl methacrylate-acrylic copolymer resin and RESIFLOW'" P-67 flow modifier. The blend is melt- -mixed in a Buss Condux PLK-46 single-screw extruder under the following conditions: Zone 1 (next to feed zone) - 70°C Zone 2 (next to outlet) - 70°C Screw interior - 70°C Screw Speed - 250 rpm As a comparative, a similar formulation was melt blended using dodecanedioic acid (DDDA) in place of the network polymer. The formulations are shown in Table V. Table V Dodecanedioic Acid and Anhydride Powder Formulations Reagents DDDA* Network Polymer DDDA/ 18 59 Network Polymer (g) ALMATEX PD7610 (g) 82 41 RESIFLOW-P-67 (g) 0.8 0.8 Total (g) 100.8 100.8 *Not an example of the invention The extrudate was ground and sieved with 150-mesh brass sieves (105 micron opening). The ground and sieved formulations were bagged in plastic and taped shut. They were applied to 4 inch x 12 inch x 0.032 inch (10 cm x 30 cm x 0.081 cm) polished, untreated, cold-rolled steel panels using a WAGNER EPB'" electrostatic spray gun with 70 kilovolts applied. The panels were baked at about 330°F (165 C C) for 20 minutes. The properties of the resulting coating are set out in Table VI. Table VI Summary of Powder-Coating Performance DDDA* Network Polymer Gel Time at 166°C 119 sec. 140 sec. a Thickness (mils/um) 2.1-2.3 2.1-2.3 (53-58) 60 °Gloss 122 119 20 °Gloss 102 92 BYK Wave Scan: Mean Rating 5.5 2.7 Longwave 18.7 49.3 Shortwave 0.4% 0.4% Thickness (mils/μm) 1.2-1.5 2.1-2.3 (30-38) Pencil Hardness HB Pass HB Pass c Thickness (mils/μm) 1.5-2.0 1.8-1.9 (38-50) MEK DR 120 Pass 80 Pass a Thickness of coatings used in gloss and wave scan measurements. Thickness of coatings used in hardness testing. c Thickness of coatings used in MEK double rub test. *Not an example of the invention.

Claims: 1. A network polymer which contains: (1) a plurality of polymer chains which are derived from one or more ethylenically- -unsaturated monomer(s); and (2) one or more linking groups, which: (a) contain anhydride, carbonate or uretidinedione moieties, and (b) link two polymer chains, characterized in that the network polymer: (A) contains less than 0.5 equivalent of pendant carboxylic acid per equivalent of anhydride moiety when the linking group contains an anhydride moiety; (B) is soluble up to at least 20 weight percent solids in at least one aromatic or aprotic polar organic solvent; and (C) has an average weight, per equivalent of linking anhydride, carbonate or uretidinedione moiety of no more than 5,000. 2. A network polymer as described in Claim 1 wherein the linking groups contain anhydride moieties. 3. A network polymer as described in Claim 1 wherein the linking groups contain uretidinedione moieties. 4. A network polymer as described in Claim 1 wherein the linking groups contain carbonate moieties. 5. A network polymer as described in any of Claims 1 to 4 which has a weight average molecular weight of 2,000 to 100,000. 6. A network polymer as described in Claim 5 which dissociates to provide a polymer having a weight average molecular weight of 500 to 5,000. 7. A network polymer as described in Claim 5 wherein the polymer chains contain repeating units derived predominantly from one or more of the following monomers: ethylene, propylene, butylene, styrene, vinyltoluene, vinyl chloride, vinylidene chloride, 4- -vinylanisole, acrylonitrile, vinyl oxazoline, vinyl acetate, vinyl propionate, n-butyl vinyl ether, lauryl vinyl ether, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, butyl acrylate, hexyl acrylate, octyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, benzyl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, hexyl methacrylate, octyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, benzyl methacrylate, cyclohexyl methacrylate or methyl crotonate. 8. A process to make a network polymer, comprising the step of polymerizing: (a) one or more ethylenically-unsaturated monomers; and (b) in the presence of one or more linking compounds, which contain at least two ethylenically-unsaturated aliphatic moieties linked by anhydride, carbonate or uretidinedione moieties, using a free-radical initiator, characterized in that: (1) the weight ratio of linking monomers to other monomers is from 5:95 to 60:40, and (2) the reaction is carried out under starved reaction conditions. 9. A curable composition which contains: (a) a curable compound which contains a plurality of second reactive groups; and (b) a network polymer, characterized in that the network polymer contains: (1) a plurality of polymer chains which are derived predominantly from one or more ethylenically-unsaturated monomer(s), and (2) at least one linking group which: (i) links two polymer chains; and (ii) contains a dissociating group that dissociates to form a plurality of first reactive groups which react with the second reactive groups to cure the composition. 10. A curable composition as described in Claim 9 wherein the network polymer is a network polymer as described in any of Claims 1 to 7. 11. A method to use a composition as described in Claim 10 comprising the steps of: (1) coating a substrate with the composition of Claim 10; and (2) maintaining the coated substrate at a temperature and for a time sufficient to cure the coating.

Inventors: BORIACK LINTON J, TREYBIG DUANE S, BORIACK, LINTON, J., TREYBIG, DUANE, S.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1996
Main classification: C08F246
IPC codes: C08F2, C08F8, C08F10, C08F14, C08F16, C08F18, C08F20, C08F212, C08F246, C08L101, C09D5, C09D133