ADHESION PROMOTERS FOR THIOLENE ADHESIVE FORMULATIONS

Abstract: Compounds useful as adhesion promoters for thermally cured thiolene adhesives include (a) acrylic or norbornene acid phosphate esters such as (I) and (II); b) maleic, acrylic, methacrylic, norbornene dicarboxylic, and fumaric acids and half acid esters and peresters of maleic fumaric and norbornene dicarboxylic acids; and c) norbornene, acrylic or methacrylic silanes having two or three hydrolyzable groups bound to the silicon atom thereof, such as 5-(triacetoxysilyl)norbornene. The adhesion promoters may be employed as surface primers for conventional thiolene compositions. Alternatively the thiolene compositions themselves may be modified by addition of 0.1 to 10 % by weight of the adhesion promoter. Preferred thiolene compositions employ norbornene functional ene ingredients.

Description: ADHESION PROMOTERS FOR THIOLENE ADHESIVE FORMULATIONS Background of the Invention In the field of adhesive bonding there is a substantial body of literature devoted to improving adhesion of various adhesive systems. The various techniques include altering substrate surfaces by cleaning, priming with inter- mediate adhesive materials, irradiating the surfaces, etc. ; subjecting curable adhesives to specifically sequenced cure cycles; priming the substrate surfaces with various cure in- itiators or accelerators; and altering the adhesive composi- tion by including specific compounds which improve the bond strength of the adhesive to the bonded substrates. Examples of the compounds in the latter category are silane compounds used in RTV silicone formulations which contain both polar groups compatible with metallic surfaces and hydrolyzable groups; and acrylic, maleic or fumaric compounds with various acid functionalities as well as silane compounds with acrylic and hydrolyzable groups, all of which have been used in acrylic adhesives of various types. Other examples are well known in the various curable adhesive arts. The utility of co-curable adhesion promoters, however, tends to be very system specific. The particular adhesion promoter must not only provide functionalities which effectively provide a bridge from the adhesive to the substrate surface, it must also not significantly deteriorate the performance of the adhesive. Consequently, the selection of adhesion promoting additives in new curable systems cannot be made on the basis of performance in another curable system. Thiolene curable systems involve addition of thiol (-SH) groups across an olefinic or acetylenic double bond. The reaction may be catalyzed by acids or free radi- cals. A detailed discussion of the mechanism of this reac- tion and of the academic literature in this field may be found in Oae, ed. , "Organic Chemistry of Sulfur", Plenum Press New York, New York, pp. 131-187 (1977) . Oswald et al reportedly disclosed photocrosslinkable compositions employ- ing tetraallyl ester compounds and dithiols at the April 1965 ACS meeting in Detroit, Michigan. See Oswald et al, Die Makromolekulare Che ie, 92, 258-266 (1966) . Formula- tions of di-or polythiols and dienes or polyenes which are curable to solids by chemical or photolytic radical genera- tors are also described in U.S. Patents 2,767,156; 3,661,744; 3,240,844; 4,119,617; and 4,157,421. All of these prior art curable formulations have involved polythiol additions to double bonds of linear olefins or acetylene compounds. Silicone polymers having alkyl thiol groups are disclosed in U.S. 4,289,867 and background references discussed therein. In U.S. 4,284,539 other silicones having linear and cyclic mercaptoalkyl groups are described as are the thiol-ene reactions of such materials with silicones having vinyl or silacyclopentene groups. Peroxide cured compositions of vinyl terminated silicones with polythiols having organic or silicone are described in U.S. 3,445,419. In copending application Ser. No. 099,676, filed September 21, 1987 and incorporated herein by reference, there are described improved thiolene curable systems based on compounds having a plurality of norbornenyl or analogous bicyclic ene groups. Despite the extensive literature on thiolene compositions applicants have been unable to locate any reference which teaches adhesion promoting techniques for such compositions and in particular, have been unable to lo- cate any references disclosing adhesion promoting co-curable additives for thiolene compositions. In U.S. 4,435,497 there are described polymerizable thiolene resist compositions in which the ene compound contains at least one each of (meth) cryloxy groups; allyl, ethallyl, or 1-propenyl groups bonded through an oxygen or oxycarbonyl group to an aromatic or cycloaliphatic nucleus; and carboxylic acid groups. The function of the carboxylic acid group is understood as a latent debonding agent, allowing removal of uncured formula- tion with a mild base wash and removal of the cured resist with a strong base wash. Summary of the Invention The invention in one aspect comprises a thiolene composition which includes an additive which promotes adhesion to metals and other polar substrates the adhesion promoting compound selected from the group consist- ing of a) Acrylic or norbornene acid phosphate esters; b) Maleic, acrylic, methacrylic, norbornene dicarboxylic and fumaric acids and half acid esters and peresters of maleic fumaric and norbornene dicarboxylic acids; and c) Norbornene, acrylic or methacrylic func- tional silanes having two or three hydrolyzable groups bound to the silicon atom thereof.- A further aspect of the invention comprises a composition as previously described wherein the ene compound contains a plurality of norbornene or similar bicyclic func- tional ene groups. A still further aspect of the invention co - prises an adhesively bonded article wherein the adhesive is a cured composition as described above. In a further embodiment of the invention an adhesion promoting compound as described above is used as a surface primer to improve adhesion of cured thiolene co - positions, especially of thermally cured thiolene adhesive compositions. Detailed Description of the Invention The ene resins useful in the invention are compounds which may be represented by the formula: [A] (X) n where A is an n-valent organic or polyorganosiloxane residue, X is a terminal group which includes an aliphatically unsaturated carbon-carbon double or triple bond and n is an integer of at least 2. Suitable X groups include those represented by the formulas: (b) -0-C=C-R RR (c) -S-C=C-R SUBSTITUTE SHEET Thiolene compositions based on ene compounds having terminal groups (a) to (e) above are described in U.S. 3,661,744. In the groups (a) to (e) , f is an integer from 1 to 9; R is a radical selected from the group consisting of hydrogen, fluorine, chlorine, furyl, thienyl, pyridyl, phenyl and substituted phenyl, benzyl and substituted benzyl, alkyl and substituted alkyl, alkoxy and substituted alkoxy, and cycloalkyl and substituted cycloalkyl. The substituents on the substituted members are selected from the group consisting of nitro, chloro, fluoro, acetoxy, aceta ide, phenyl, benzyl, alkyl, alkoxy and cycloalkyl alkyl and alkoxy groups have from one to nine carbon atoms and cycloalkyl groups have from three to eight carbon atoms. 3y3STiTUTE SHEET I3A/EP Ene compounds having terminal groups (f) and (g) , above are described in U.S. Patents 4,543,397 and 4,640,849, respectively. Thiolene compositions of such com¬ pounds are described in U.S. Ser. No. 001,498, filed January 5, 1987, corresponding to EP 229033. In formulas (f) and (g) at least one of R 1 , R 2 , and R 3 is a member of the group consisting of vinyl, 1-propenyl and isopropenyl and the remaining R 1 , R 2 , and R 3 groups are selected from H, lower alkyl and alkoxy. R 4 is alkyl. Thiolene compositions in which the ene com¬ pounds has terminal ene groups represented by formula (h) are described in U.S. Patents 4,119,617 and 4,157,421. In formula (h) , R 5 is H or alkyl and p is 0 or 1. Preferred ene compounds and thiolene composi¬ tions based thereon are described in WO 88/02902. These preferred compositions have plural norbornene groups repre¬ sented by formula (i) above, where R 6 is H or alkyl Q is CR 6 2 , O, S, NR 6 or S0 2 and q is an integer from 0 to 10. Most suitably q is 0 and is CR 6 . Preferred enes of for¬ mula (i) include compounds with two or more nadimide groups such as and ii) compounds derived from Diels-Alder reaction of cyclopentadiene with a plural acrylate or methaσrylate com¬ pound. The latter compounds may be represented by the for¬ mula: A' where A * is as defined above for A, L is O, S or NR 5 and n, R 5 , and Q are as previously defined. Particularly preferred are ene compounds represented by the formula: where ' is as defined above for , n' is 1-6 and x and y are integers of one or more. The polythiol component of the inventive compositions may be any compound having two or more thiol groups per molecule. Suitable polythiols are described in U.S. 3,661,744 at Col. 8, line 76 - Col. 9, line 46, in U.S. 4,119,617, Col. 7, lines 40-57, U.S. 3,445,419 and U.S. 4,289,867. Especially preferred are polythiols obtained by esterification of a polyol with an α- or β- mercaptocarbόxylic acid such as thioglycolic acid, and J-mercaptopropionic acid. Particularly preferred polythiols are pentaerythritol-tetrakis-mercaptoacetate and pentaerythritol-tetrakis- 3-mercaptopropionate. The ratio of the polyene to the polythiol component can be varied widely. Generally it is preferred that the ratio of thiol to * ene groups be between 1:4 and 4: 1 but ratios outside this range may occasionally be usefully employed if the numbers of ene or thiol groups per molecule of the excess component is particularly high. SUBSTITUTE SHEET The thiolene composition includes an effec- tive amount of a free radical catalyst in addition to the thiol and ene ingredients. The free radical catalyst may be a photoinitiator, however, it will most usually be a thermal catalyst since the benefits of the invention are most desirable in applications for adhering two substrates together. Such applications rarely permit photoinitiated curing. The advantages of the invention are especially ap- parent with zinc surfaces, such as galvanized steel sub- strates. The thermal initiator may be a peroxide compound suitably a diacyl peroxide such as benzoyl peroxide, a hydroperoxide such as cu ene hydroperoxide or a perester such as t-butyl perbenzoate. Other thermal radical in- itiators which may be employed include azonitrile compounds such as 2,2'-azobis(iso-butyronitrile) , and benzopinacol compounds, such as benzopinacol and the substituted ben- zopinacols disclosed in U.S. 4,330,283. Effective amounts of peroxy compounds are typically in the range of 0.1-5%. Effective amounts of azonitriles are typically in the range of 0.1-1% by weight of the compositions. Effective amounts of benzopinacols are typically in the range of 0.1-2% by weight. The thiolene composition may also include stabilizers, fillers, reinforcing agents-, thixotropic agents, and other conventional ingredients known in the thiolene and adhesive formulation arts. Useful stabilizers include conventional free radical stabilizers and certain vinyl compounds disclosed in U.S. 3,619,393. The inventive compositions are characterized by the presence of an adhesion promoting compound as described above. The adhesion promoting compound may be present as a component of the base thiolene adhesive for- mulation or as a separate ingredient, applied- to the sub- 1 strate as a primer prior to application of the base 2 adhesive, which diffuses into the surface contacting layers 3 of the base adhesive prior to or during the curing reaction. 4 The adhesion promoting compound may be an 5 acrylate or methacrylate acid phosphate ester, a correspon - 6 ing norbornenyl compound. The acrylate and methacrylate 7 acid phosphate esters are compounds which include at least 8 one acrylate or methacrylate group and a group of the for- 9 mula: 0 • o 1 -P-OH 2 ' 3 4 References which describe such acid phosphate _ (meth)acrylic esters and compositions thereof include U.S. 6 patents 3,754,972; 3,987,127; 4,044,044; 4,250,007; ? 4,223,115; 4,293,665; 4,322,509; 4,452,944; 4,433,124; 8 4,434,278; 4,259,117; 4,368,043; 4,499,251; 4,525,493; and 9 4,515,930. Other such references include Jpn Kokai 0 57/167364 (1982); Jpn Kokai 49/20238 (1974) ; Jpn Kokai 1 49/84964 (1984) , see Chem. Abst. 101: 153164k; Jpn Kokai 2 56/100803 (1981), see Chem. Abst. 95:221461q; EP 0058483; 3 and EP 0074708. Most preferably, the acid phosphate esters have the formula: where R 6 is H or methyl, R 7 is a divalent organic residue having from 2-40 carbon atoms and a is 1 or 2. Suitably, R 7 is C 2 -C 10 alkylene. Examples of commercially available phos- phate ester compounds include mono(methacryloxyethyl)phosphate; bis(methacryloxyethyl)phosphate; mono(acryloxyethyl)phosphate; bis(acryloxyethyl)phosphate; mixed mono and bis(acryloxyhexyl)phosphate and mixed mono and bis(methacryloxyhexyl)phosphate. Corresponding norbornene acid phosphate esters are acid phosphate esters as described above in which the (meth)acrylate group is replaced with a norbornene-5- carboxylate group: Such compounds can be prepared by reaction of P0C1 3 with a 2-hydroxyalkylnorbornene carboxylate, followed by hydrolysis, or by Diels-Alder reaction of cyclopentadiene and a (meth)acrylic acid phosphate ester. Other norbornene phosphate monomers can be prepared by P0C1 3 reaction with other norbornenyl functional alcohols such as norbornene methanol, followed by hydrolyis to the mono or bis 5-norbornene methylphosphate: A second class of adhesion promoting additive comprises carboxylic acids selected from the the group con¬ sisting of maleic, acrylic, methacrylic, norbornene dicar¬ boxylic and fumaric acids and half acid/esters or half acid/peresters of maleic, fumaric and norbornene dicar¬ boxylic acids. Suitable peresters include mono-t- butylperoxy maleate (peroxymaleic acid) and mono-t- butylperoxy fumarate. Suitable half esters are hydrocarbyl and hydrocarbylether maleates and fumarates, particularly monoalkylmaleates and monoalkyleneoxyalkyl maleates. The adhesion improving performance of these acidic compounds, particularly on zinc surfaces such as gal- vanized steel is surprising. Acrylic and methacrylic acids have been observed to reduce thermal resistance and actually lessen adhesion of anaerobic acrylic adhesives to zinc sub- strates. Although maleate double bonds will undergo ther- mally induced addition of thiol, the maleate double bond is significantly less susceptible to radical catalyzed thiol addition than allylic double bonds under ambient conditions. Norbornene dicarboxylic acid is not known to have been reported as an adhesion promoter in any system. At least one other unsaturated acid, 5-allylbarbituric acid, has been shown to provide some adhesion promotion on steel substrates but its performance on zinc substrates is much worse than controls employing no adhesion promoter. A final class of compounds useful as thiolene adhesion promoters are norbornene, or acrylate, functional silyl compounds which also have two or three hydrolyzable groups bound to the silicon atom. Suitable hydrolyzable groups are known in the art and include alkoxy, acetoxy, and oxi e groups. Acetoxy groups are preferred. Such compounds are useful for improving adhesion to glass and, again surprisingly, to zinc substrates. Examples of such com- pounds include 5-(triacetoxysilyl)norbornene and acryloxyethyltriacetoxysilane. The adhesion promoter may be used either as a component of the composition, uniformly dissolved suspended in the mixture of thiol and ene resins or as a surface primer which difuses into the adhesive compositions at the adhesive/substrate interface. When directly mixed into the composition the adhesion promoter may be usefully employed at levels of between about 0.1% and 10%, preferably between about 0.5% and 5% and more preferably between about 1% and 3%, based on total composition weight. Perester adhesion promoters can optionally be used as both initiator and adhe- sion promoter in such directly mixed formulations, eliminat- ing the need for a separate thermal initiator. When used as a surface primer the adhesion promoter is preferably dis- solved in a neutral solvent. The invention is illustrated by the following non-limiting examples: Example 1 (Preparative example of preferred ene resin Sartomer 351 w (diacrylate ester of ethoxylated bisphenol A, 1 mole) was stirred at 40 β C and freshly cracked cyclopentadiene (2.2 equivalents) was added at such a rate that the reaction temperature rose to ca. 75 β C. The reac- tion mixture was maintained at 75*C overnight. The reaction mixture was then stripped on a rotary evaporator. Analysis of the reaction mixture by HPLC showed the conversion to be complete. The product, ethoxylated bisphenol A-bis(norborn-2-ene-σarboxylate) may be represented by the formula: where x + y is an average 3.4 Example 2 An adhesive comprising a base composition of ethoxylated bisphenol A-bis(norbornene-2-ene-5-carboxylate) 70%, pentaerythitol tetra-(3-mercaptopropionate) 28% and t-butylperoxy benzoate 2%, and a primer composition of 1% by weight PA-21, a commercial mixture of mono and bis acryloxyhexylphosphate in methyl ethyl ketone was prepared. The primer composition was applied to pairs of 1 inch x 4 inch (2.54 cm x 10.16 cm) test specimens and the solvent was allowed to evaporate. The base composition was then applied and the test specimens were then joined and the adhesive cured by placing the joined assembly in a 250 β F (121 β C) oven for 2 hrs. Controls using no primer were run simul¬ taneously. Results on three test specimen materials at two different bond gaps are shown in table 1. The results in all tables are averages of three samples each. Table I ADHESION .PROMOTERS FOR THIOLENE HEAT CURED FORMULATIONS Gap TSS Substrate dhesi ro ter l m l 2 2 A control adhesive was prepared using the base formulation of the previous example without a primer. A modified adhesive using 2% peroxymaleic acid (POMA) in place of the 2% t-butylperoxy benzoate in the control was also prepared. Steel and galvanized specimens were bonded and tested as in the previous example. The results shown in Table II demonstrate that peroxymaleic acid functions effec¬ tively as an adhesion promoter as well as an initiator. Table II PEROXYMALEIC ACID INITIATED HEAT CURED THIOLENE FORMULATIONS Substrate Steel Steel Steel Steel Galvanized Galvanized Galvanized Galvanized Example 4 Adhesives were prepared as in Example 2 ex¬ cept that the primer used was a 1% solution of the compounds listed in Table III. The adhesives were applied to gal¬ vanized steel test specimens, cured, and tested as in Ex¬ amples 2 and 3. The results given in Table III demonstrate that 5-allylbarbituric acid, an unsaturated carboxylic acid outside the scope of this invention reduced adhesion and 3-norbornenyl propenoic acid, another unsaturated acid out¬ side the scope of the invention, provided relatively little adhesion improvement whereas the remaining compounds, all within the scope of the invention provided significant im¬ provement. Table III t-Butylperoxy Benzoate Heat Cured Norbornenes Thiol Formulation Tensile Shear Strength Adhesion Promoter Primed As Received Galvanized Steel Lap Shears t-butylperoxy Maleate (2017) 142 Example 5 Example 4 was repeated except that steel lap shear coupons were used as the test specimens. The results in Table IV are from two different test series. On this substrate the norbornenylpropenoic acid reduced adhesion whereas all other materials improved adhesion in varying de¬ grees.

Claims: WHAT IS CLAIMED IS: 1. In a thiolene composition curable free radi- cally under thermal conditions, the composition comprising a polythiol resin and a polyene resin, the improvement wherein the composition includes an effective amount of an adhesion promoter compound selected from the group consisting of a) Acrylic or norbornene acid phosphate esters; b) Maleic, acrylic, methacrylic, norbornene dicarboxylic and fumaric acids and half esters and half peresters of maleic, fumaric and norbornene dicarboxylic acids; and c) Norbornene, acrylic or methacrylic func- tional silanes having two or three hydrolyzable groups bound to the silicon atom thereof. 2. A composition as in claim 1 wherein the said adhesion promoter comprises a surface primer which has dif- fused into said thiolene composition after said primer and thiolene compositions have been successively applied to a substrate upon which the substrate is to be cured. 3. A composition as in claim 1 wherein the said adhesion promoter is a component uniformly mixed into said thiolene composition prior to application thereof to a sub- strate on which the composition is to be cured. 4. A composition as in claim 1 wherein the com- position includes an effective amount of a free radical in- itiator selected from the group consisting of peroxy co - pounds, azonitrile compounds and benzopinacol. 5. A composition as in claim 1 wherein the adhe- sion promoter is selected from the group consisting of maleic acid, fumaric acid, acrylic acid, methacrylic acid, and norbornene dicarboxylic acid and half esters and half peresters of said acids. 6. A composition as in claim 1 wherein the adhesion promoter is an acrylic or norbornene acid phosphate ester. 7. A composition as in claim 1 wherein the adhesion promoter is a norbornene functional silane. 8. A composition as in claim 7 wherein the adhesion promoter is 5-(triacetoxysilyl)norbornene. 9. A composition as in claim 5 wherein the adhesion promoter is a maleic acid monoperester. 10. A composition as in claim 6 wherein the adhesion promoter is selected from mono(methacryloxyethyl)phosphate; bis(methacryloxyethyl)phosphate; mono(acryloxyethyl)phosphate; bis(acryloxyethyl)phosphate; mixed mono and bis(aeryloxyhexyl; mixed mono and bis(methacryloxyhexylphosphate; mono and bis 5-norbornenecarboxyethylphosphate; mono and bis 5-norbornenecarboxyhexylphosphate; and 5-norbornenemethylphosphate. 11. A composition as in claim 1 wherein the polyene includes a plurality of norbornene groups. 12. A composition as in claim 11 wherein the polyene has the formula where 1 is CR 5 , O, S, NR 5 or S0 2 , R 5 is H or alkyl, n' is 1-6, and x and y are integers of one or more. 13. A composition as in claim 11 wherein the polythiol is an ester of a polyol and an a or J-mercaptocarboxylic acid. SUBSTITUTESHEET 14. An assembly comprising a pair of substrates adhesively bonded together with a cured composition as in claim 1. 15. A method of bonding a pair of substrates com- prising: applying a solution in a volatile solvent of a compound selected from the group consisting of a) Acrylic or norbornene acid phosphate esters; b) Maleic, acrylic, methacrylic, norbornene dicarboxylic and fumaric acids and half acid esters and peresters of maleic, fumaric and norbornene dicarboxylic acids; and c) Norbornene, acrylic or methacrylic functional silanes having two or three hydrolyzable groups bound to the silicon atom thereof; to one of said substrates, allowing the solvent to evaporate; applying a thermally curable composition of a polythiol and a polyene to one of said substrates; joining the substrates; and, subjecting the joined substrates to elevated temperatures for sufficient time to cause the said composition to cure. 16. A method as in claim 15 wherein at least one of said substrates has a zinc surface. 17. A method as in claim 16 wherein at least one of said substrates is galvanized steel. 18. An assembly as in claim 14 wherein at least one of said substrates is galvanized steel.

Inventors: CLARK PAUL J, GLASER DAVID M, JACOBINE ANTHONY F, CLARK, PAUL, J., GLASER, DAVID, M., JACOBINE, ANTHONY, F.
Applicants: LOCTITE CORP, LOCTITE CORPORATION
Application year: 1989
Main classification: C08G75
IPC codes: C08G75, C08L81, C09J4, C09J181