GB2180240A - Curing cationically polymerisable compounds by irradiation - Google Patents

Abstract

Cationically polymerisable compounds can be polymerised by irradiation with electron beams or X-rays using a sulfoxium salt as catalyst. The process is particularly suitable for the rapid curing of coatings based on epoxy resins.

Description

SPECIFICATION Curing cationically polymerisable compounds by irradiation The present invention relates to a process for curing cationically polymerisable compounds, especially epoxy compounds, by irradiation.

Curing cationically polymerisablecompounds by irradiation is of great technical importance for preparing thin layers of organic polymers, for example coating compositions, printing inks, printed circuits orfor image recording methods. Curing such cationic resins by irradiation requires the use of a catalyst which is inactive in the dark or in longwave light, but which is able to initiate cationic polymerisation on exposure to shortwave light. The catalysts hitherto employed in the art have been in particular aromatic diazonium, sulfoxonium and iodonium salts containing complex anions.Aromatic sulfoxonium salts have also been proposed forthis utility, for example in US patent specifications 4299938,4339567,4383025 and 4398014. Examples of such saltsarephenyldimethylsulfoxonium hexafluorophosphateorphenyldiphenoxysulfoxonium hexafluorophosphate. Exposure is made with actinic light in the wavelength range from 200 to 600 nm, using light sources having a high portion of UV light. Exposure times of a few seconds suffice to obtain a non-tacky cure ofthin layers of unpigmented resins. However, appreciably longer exposure times are required for curing thick layers or pigmented resins, and only a superficial cure is obtained with the customary exposure time of a few seconds.

It has now been found that the curing of cationically polymerisable compounds by irradiation in the presence of a sulfoxonium salt catalyst can be accelerated by effecting irradiation with electron beams or X-rays. These rays have wavelengths below 100 nm.The advantages ofthis process are especially evidentfor curing pigmented resins and thick layers. This process is especially suitable for curing the epoxy compounds used as epoxy resins by irradiation. Epoxy resins are normally cured by mixing them with a chemical reactant known as the hardener. Unless hardening is accelerated by applying heat, the time required for hardening is many hours. In the process of this invention, however, hardening is effected in only fractions of a second without heating.This is of great importance in large-scale manufacturing processes. There are at the present time a number of devices for irradiating with electron beams and X-rays on an industrial scale by moving the substrate to be irradiated at constant speed beneath and past the light source.

Curing with electron beams and X-rays has hitherto been employed almost exclusively for radically polymerisable coating compositions, especially those based on acrylate compounds. Such curing must be effected in an inert gas atmosphere, as atmospheric oxygen interferes with the radical polymerisation.

However, cationic polymerisation is not adversely affected by oxygen and it is therefore possible to carry out the process of this invention in air.

The cationically polymerisable compounds can also be employed in admixture with radically polymerisable compounds, as these latter can also be polymerised without a catalyst. Examples are acrylates, unsaturated polyesters or acrylamides. Preferably such mixtures contain only a small amount of radically polymerisable compounds, as otherwise it is necessary to effect irradiation in an inert gas atmosphere.

The apparatus required for irradiating with electron beams issimplerandthetechnology is also more advanced than for X-ray irradiation. For this reason the preferred method of irradiation is electron beam curing. Both types of irradiation are equally efficient and in fractions of a second it is possible to cure films so that they are no longer tacky. After irradiation, an additional postcuring is effected because of the longevity of cationic reaction chains.

Compounds that can be cured by irradiation in the process of this invention are those compounds which are cationically polymerisable, e.g. certain heterocyclic ring systems, certain compounds having olefinic unsaturation, phenolic resins and amino resins.

Examples of heterocyclic ring systems are oxiranes, oxetanes, oxolanes, cyclic acetals, cyclic lactones, thiiranes orthietanes. Among these heterocyclic compounds, the oxiranes or 1,2-epoxides commercially available as epoxy resins are particularly important. These resins are mainly di- orpolyepoxideswhich are non-volatile at room temperature. Examples of such epoxy resins are the glycidyl ethers of aliphatic, cycloaliphatic or aromatic diols or polyols, e.g. the diglycidyl ethers of 1,4-butanediol, 1 ,4-bis(hydroxymethyl)cyclohexane, 2,2-bis(4-hydroxycyclohexyl)propane, resorcinol, 4,4'-dihydroxydiphenylmethane or2,2-bis(4-hydroxyphenyl)propane, orthetriglycidyl ethers of glycerol, trimethylolpropane ortris(4-hydroxyphenyl )methane.Further examples are the N-glycidyl compounds of ethyleneurea, 1 ,3-propyleneurea, 5,5-dimethylhydantion or 3,3'-methylene-5,5,5',5'-tetramethylhydantoin ortriglycidylisocyanurate.

Further technically important glycidyl compounds are the glycidyl esters of carboxylic acids, e.g. the glycidyl esters of adipic acid, phthalic acid, terephthalic acid, isophthalic acid, hexahydrophthalic acid or trimellitic acid.

Examples of curable polyepoxides which are not glycidyl compounds are vinylcyclohexene diepoxide, dicyclopentadiene diepoxide, (3,4-epoxycyclohexyl methyl )-3,4-epoxycyclohexanecarboxylate or epoxidised soybean oil.

Polymeric epoxides are e.g. glycidyl ethers of novolaks or of polyalkylene glycols, polymeric glycidyl acrylates or methacrylates, or epoxidised polybutadiene.

Often mixtures of epoxides are used to vary the properties of the resins. Monoepoxides, e.g. butylglycidyl ethers or oleylglycidyl esters, are also often used concurrently in mixtures.

Olefinically unsaturated compounds are normally radically polymerisable and can therefore be polymerised by irradiation without a catalyst. However, certain olefins are more readily cationicallythan radically polymerisable, in which case it is advisable to add a sulfoxonium catalyst. Examples of such olefins are isobutylene, allyl benzene, vinyl cyclohexane, vinyl alkyl ethers, N-vinylpyrolidone or 3,4-dihydro-2H-pyrane derivatives.

It is also possible to cure phenolic resins, melamine resins, urea resins and other aminoplasts bythe process of this invention. These resins are low molecular, soluble prepolymers which can be converted by irradiation into the high molecular crosslinked state.

The eligible sulfoxonium salts are known compounds. They consist of a sulfoxonium cation and the anion of an inorganic or organic protic acid. In particular, they are compounds ofthe general formula oriel

wherein R1, R2 and R3 are each independently C1-C6 alkyl, unsubstituted or substituted by halogen or phenyl or interrupted by-O- or-SO2-, or are C5-C6cycloalkyi, C6-C14aryl, unsubstituted or substituted by a member selected from phenyl, halogen, C1-C12aIkyI,C1-C12alkoxy, phenoxy and nitro, orC#C14aryIoxy, unsubstituted or substituted by a member selected from phenyl, halogen, Cl-Cl2alkyl, Cl-Cl2alkoxy, phenoxy and nitro, or R1 and R2, when taken together, are a divalent hydrocarbon radical of 3 to 10 carbon atoms which,together with the sulfur atom,forms a heterocyclic ring, R4 is C1-C1 2aIkyI, C2-C6alkenyl, C#C16araIkyI, C3-C8cycloal kyl, C4-C10cycloalkylalkyl, orC6-Cl4aryl which is unsubstituted or substituted by a member selected from C1-C12aIkyl, C1-C10alkoxy, halogen, phenyl, phenoxy and nitro, R5 has the same meaning as R4 or is C2-C8dialkylamino, phenylamino, tolylamino, piperidino or morpholino, or R4 and R5, when taken together, are a divalent hydrocarbon radical of 3 to 10 carbon atoms which, together with the sulfur atom, forms a heterocyclic ring, R8is a radical of valency n which is an aliphatic radical of 1 to 18 carbon atoms, a cycloaliphatic radical of 5to 12 carbon atoms, an aromatic radical of 6to 10 atoms which is unsubstituted or substituted by a member selected from Cl-Cl2alkyl, C1-C4alkoxy, halogen and nitro, or is an araliphatic radical of 7 to 15 carbon atoms, R7 is hydrogen, C1-C12aIkyI, C7-C12aralkyl, C6-Cl0aryl or a-CO-R8group,wherein R8 is C1-C12alkyl or C6-C,0aryl which is unsubstituted or substituted by a member selected from halogen, C1-C12aIkyI and C1-C4alkoxy,

nis an integerfrom 1 to 4, and A is the anion of an inorganic proticacid or of an organic sulfonic acid.

The anion A is preferablya complex anion offormula [ MXm ] -, wherein M is boron, phosphorus, arsenic, antimony or bismuth, X is a fluorine or chlorine atom and m is 4 or 6 and is greater by 1 than the valency of M.

Examples of anions are: [ BF4j-, [ PF6 ] -, [AsF6]- [ AsCI6 ] #, [ SbCl6 ] -, [ SbF6 ] or [ 8iCI6 ] #.

R1, R2 and R3 may be methyl, ethyl, isopropyl, n-butyl, sec-butyl, isopentyl, n-hexyl, 2-ethylbutyl, 2-chloroethyl, 2-methoxyethyl, 2-(butylsulfonyl)ethyl, 2-butoxyethyl or benzyl. R1, R2 and R3 as cycloalkyl may becyclopentyl orcyclohexyl.

R1, R2 and R3 as aryl may be phenyl, naphthyl, phenanthryl, tolyl, xylyl, 4-hexylphenyl, 4-chlorophenyl, 4-phenoxyphenyl, 3-nitrophenyl or 4-methoxyphenyl.

R1, R2 and R3 as aryloxy may be phenoxy, tolyloxy, 4-isopropoxy, naphthoxy or4-chlornphenoxy.

Adivalent radical formed by R1 and R2together or by R4 and R5 together may be saturated or unsaturated and,together with the sulfur atom, forms a heterocyclic ring, e.g. a thiophene, methylthiophene, tetrahydrothiophene, thietane orthiane ring.

R4, R5 and R7 may be methyl, ethyl, propyl, butyl, hexyl, octyl, nonyl, decyl or dodecyl.

R4 and R5 as alkenyl may be vinyl, ailyl, methallyl, butenyl or hexenyl.

R4and R5 as cycloalkyl orcycloalkylalkyl may be cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, methylcyclohexyl, cyclooctyl or cyclohexylmethyl.

R4 and R5 as aryl may be phenyl,tolyl, xylyl,tert-butylphenyl, methoxyphenyl, chlorophenyl,fluorophenyl, naphthyl orchloronaphthyl.

R4, R5 and R6 as aralkyl may be benzyl, 2-phenylethyl or 2-phenylpropyl. R5 as dialkylamino may be dimethylamino, diethylamino or dibutylamino.

R6 as a monovalent radical may be an alkyl, alkenyl, cycloalkyl, aryl or alkyl radical. Examples of such radicals are methyl, isopropyl, n-butyl, n-octyl, n-dodecyl, stearyl, oleyl, cyclohexyl, cyclooctyl, phenyl, naphthyl,tolyl, xylyl, ethoxyphenyl, chlorophenyl, nitrophenyl, chlorotolyl, benzyl or phenylethyl. R6 as a divalent radical may be an alkylene, alkenylene, cycloalkylene, arylene orxylylene radical, e.g. 1,2-ethylene, 1 4-butylene, 1 4-butenylene, hexamethylene, octamethylene, 1 ,4-cyclohexylene, 1 3-phenylene, 1 4-phenylene, 1 ,4-naphthylene, 1,5-naphthylene, 1,8-naphthylene, and m- or p-xylene.R6 as a trivalent radical may be an alkanetriyl or arenetriyl radical, e.g. propane-l ,2,3-triyl, butane-I ,2,3-triyl, pentane-1 ,3,5-triyl or benzene-1 ,2,4-triyl. R6 as a tetravalent radical is e.g. butane-1 ,2,3,4-tetryl or benzene-1 ,2,4,5-tetryl.

Compounds of formula I are disclosed in US patent specification 4299938 orthey can be prepared by methods analogous to those disclosed therein.

Examples of compounds of formula I are: diphenylphenoxysulfoxonium hexafluorophosphate, phenyldiphenoxysulfoxonium hexafluorophosphate, methyldiphenoxysulfoxonium hexafluorophosphate, p-tolyl-phenoxy-p-tolyoxylsulfoxonium hexafluorophosphate, ethyl(ethylsulfonylmethyl)-p-tolyloxysulfoxonium hexafluorophosphate, p-chlorphenoxy-p-tolylphenoxysulfoxonium hexafluorophosphate, 1 -phenoxy-1 - oxotetrahydrothiophenium hexafluorphosphate, 1 -phenoxy-1 -oxotetrahydrothiophenium tetrafluoroborate, 1 -phenoxy-1 -oxotetrahydrothiophenium hexafluoroantimonate.

Further compounds of formula I are disclosed in US patent specification 4339567 or may be prepared by methods analogous to those described therein. Examples of such compounds are: dimethylphenylsulfoxonium hexafluorophosphate, triphenylsulfoxonium tetrafluorborate, dodecylmethylbenzylsulfoxonium hexafluoroantimonate, dimethyl-p-chlorophenylsulfoxonium hexafluorophosphate or benzylbutylcyclohexylsulfoxonium methanesulfonate.

Aromatic compounds offormula II, wherein Yis a-CO- group, are disclosed in US patent specification4 339 567. Examples of such compounds are: dimethylphenacylsulfoxonium hexafluorophosphate, diphenylphenacylsulfoxonium hexafluoroantimonate, 1 ,4-bis [ 1 -oxo-2-(di methylsu If oxoni um)ethyl ] benzene di(hexafluoroarsenate) or 1 -phenacyl-1 -oxotetrahydrothiophenium tetrafluoroborate.

Aromatic compounds of formula ll, wherein Y is a -NH-CO- or-CO-NH-CO- group are disclosed in US patent specification 4 383 025. Examples of such compounds are: anilinocarbonylmethyldimethylsulfoxonium hexafluorophosphate, 3,4-dichloroanilinocarbonyl methyldiethylsulfoxonium hexafluoroarsenate, p-toluidinecarbonyl methyldimethylsulfoxonium tetrafluoroborate, benzamidocarbonylmethyldimethylsulfoxonium hexafluorophosphate or 2,4-bis(dimethylsulfoxoniummethylcarbamoyl)toluene di(hexafluorophosphate).

Aromatic compounds offormula Il, wherein Y is a -SO2- group, are disclosed in US patent specification 4398014. Examples of such compounds are: dimethyl-p-tolylsulfonylmethylsulfoxonium hexafluorophosphate, dibutylphenylsulfonylmethylsulfoxonium hexafl uoroantimonate, 1 ,3-bis(dimethylsulfoxoniummethylsulfonyl)benzene di(tetrafluoroborate) or dimethylbenzylsulfonylmethylsulfoxonium benzenesulfonate.

Compounds of formula llthatdo not contain an aromatic group are disclosed in European patent application EP-A-164.314. Examples of such compounds are: dimethylmethylsulfonylmethylsulfoxonium hexafluorophosphate, dimethylacetylmethylsulfoxonium hexafluorophosphate, dimethylbutyl aminocarbonylmethylsulfoxonium tetrafluoroborate, or dimethylethylsulfonylmethylsulfoxonium hexafluoroarsenate.

It is preferred to use as catalyst a compound offormula I, wherein Ra, R2 and R3 are each independently C1-C#alkyl, phenyl or naphthyl, or phenyl or naphthyl each substituted by a member selected from halogen, C1-C12alkyl, C1-C4alkoxy and nitro, or phenoxy which is unsubstituted or substituted by a member selected from halogen, C1-C12alkyl, C1-C4alkoxy and nitro, or R1 and R2, when taken together, are a divalent aliphatic radical of 3to 5 carbon atoms, and A is either BF4, BF4, PF6,AsF6orSbCl6.

It is especially preferred to use a compound offormula I, wherein R', R2 and R3 are phenyl unsubstituted or substituted by a member selected from chlorine, C1-C4alkyl and C1-C4alkoxy, or are phenoxy, unsubstituted or substituted by a mem ber selected from ch lorine, C1-C4alkyl and C1-C4alkoxy, or R1 and R2, when taken together, are C3-C5alkylene or C4alkadienylene, and A is either BF4 or PFG.

Examples of especially preferred compounds of formula 1 are: p-cholorophenoxy-p-tolyloxyphenylsulfoxonium hexafluorophosphate, bis(p4olyloxy)phenylsulfoxonium hexafluorophosphate, and 1 -phenoxy-1 -oxidotetrahydrothiophenium hexafluorophosphate.

The sulfoxonium salt catalysts are added to the resins to be cured in an amount from 0.1 to 7.5 %, preferably from 0.5to 5%, by weight. The catalysts can also be added in the form of solutions. Such catalyst solutions in the resin to be cured are stable over long periods of time in the dark and can therefore be marketed as such.

The resins to be cured may be transparent or pigmented. The curing of pigmented resins is of particular importance. The process ofthis invention for curing pigmented resins with electron beams or X-rays is markedly superiorto the known curing with UV light. This applies also to resins that fillers or reinforcing materials. Examples of fillers are kaolin, gypsum, talcum and silicatefillers. Examplesof reinforcing materials are glass, metal or carbon fibres.

Further modifiers which may be present in the resins to be cured are stabilisers to prevent ageing ofthe cured resins, e.g. antioxidants, metal deactivators or light stabilisers, or stabilisers for enhancing storage stability in the dark, e.g. specific nitriles, amides, lactams or sulfoxides. The resins may also contain additives that promote film formation, e.g. diluents, flow control agents orthixotropic agents.

The resins may be applied to any kind of material such as metal, wood, paper, glass, ceramic compositions or plastics materials. The exposure times depend on the thickness of the layer and on the presence of pigments orfillers. In general, exposure times of fractions of seconds suffice for obtaining a non-tackyfilm.

After irradiation, a slow post-curing of the film takes place, which can be accelerated by a thermal aftertreatment. A variant of the curing process ofthe invention thus also comprises carrying out a thermal treatment after the irradiation with electron beams or X-rays. This can be effected e.g. by passing thefilm through an oven or by irradiation with IR light or by inductive heating.

The process is susceptible of application to all kinds of coatings with cationically curable resins, e.g.for decorative finishes, anticorrosive finishes, for applying insulating layers, for applying patterns for integrated circuits and in the field of electronic reproduction.

The invention is illustrated by the following non-limitative Examples.

Example 1 An epoxy resin based on bisphenol A glycidyl ether having an epoxide equivalent of 185-196 g/eq. (Araldits GY 250, ex Ciba-Geigy AG) is mixed with different amounts of the sulfoxonium salt catalyst K-1

and the composition is applied in a thickness of about 120 Fm to glass plates. The catalyst is dissolved beforehand in equal parts ofpropylenecarbonate.

The samples are subjected to irradiation with electron beams in an Electrocurtain laboratory machine supplied by Eng. Science Internat./USA at an intensity of 20 Mrad. To determine the degree of curing,the pendulum hardness ofthefilms is measured after 10 and 30 minutes and after4and 24 hours. It is found thata maximum curing is obtained with a catalyst concentration of 2 %. A comparison sample prepared without catalyst remains fluid underthese conditions. The results are reported in the following table.

Amount of K-1 Pendulum hardness according to DIN 53157 (sec) after 10 min. 30 min. 4 hours 24 hours 0.5% - 13 50 122 1% 25 62 133 196 2% 29 70 182 216 5% 18 30 202 214 0% Example 2 The resins employed are a) a liquid epoxy resin based on bisphenol A glycidyl ether (Araldite GY250) and b) a liquid cycloaliphatic epoxy resin having an epoxide equivalent of 133-143 g/eq. (Arnldit~ CY 179). 2% by weight of a catalyst K-2

is dissolved in each resin by stirring with gentle heating. The solutions are applied in a thickness of 100 ~lm to glass plates. Irradiation of the samples is effected as described in Example 1 at an intensity of 20 Mrad. A wipe-resistantfilm is obtained on each plate. The pendulum hardness ofthe cured films is determined after 39 minutes.

Pendulum hardness according to the method of Koenig (DIN 53157) after irradiation at an intensity of 20 mrad: a)Araldit~GY250 213 b) Araldits CY 179 70 Comparison samples of both epoxy resins without a catalyst remain completely fluid when irradiated at this intensity.

Claims (9)

1. A process for curing cationically polymerisable compounds by irradiation in the presence of a sulfoxonium salt compound, which comprises carrying out said irradiation with electron beams orX-rays.

2. A process according to claim 1 for curing epoxy resins by irradiation.

3. A process according to claim 1 or 2, wherein the catalyst employed is a sulfoxonium salt of formula I or II

wherein R1, R2 and R3 are each independently C1-C6alkyl, unsubstituted or substituted by halogen or phenyl or interrupted by-O- or-SO2-, or are Cs-Cscycloalkyl, C6-Ca4aryl, unsubstituted or substituted by a member selected from phenyl, halogen, C1-C12alkyl, C1-C12alkoxy, phenoxy and nitro, or C6-Ca4aryloxy, unsubstituted or substituted by a member selected from phenyl, halogen, C1-C12alkyl, C1-C12alkoxy, phenoxy and nitro, or R1 and R2, when taken together, are a divalent hydrocarbon radical of 3 to 10 carbon atoms which,together with the sulfur atom, forms a heterocyclic ring, R4 is C1-C12alkyl, C2-C6alkenyl, C#C16aralkyl, C3-C8cycloalkyl, C4-C10cycloalkylalkyl, or C6-C,4aryl which is unsubstituted or substituted by a member selected from C1-C12aIkyl, C1-C10alkoxy, halogen, phenyl, phenoxy and nitro, R5 has the same meaning as R4oris C2-C8dialkylamino, phenylamino,tolylamino, piperidinoor morpholino, or R4and R5, when taken together, are a divalent hydrocarbon radical of 3to 10 carbon atoms which, together with the sulfur atom, forms a heterocyclic ring, R6 is a radical of valency n which is an aliphatic radical of 1 to 18 carbon atoms, a cycloaliphatic radical of 5to 12 carbon atoms, an aromatic radical of 6 to 10 atoms which is unsubstituted or substituted by a member selected from C1-C12alkyl, C1-C4alkoxy, halogen and nitro, or is an araliphatic radical of7 to 15 carbon atoms, R7 is hydrogen, C1-C12alkyl, CT-C12aralkyl, C6-C,Oaryl or a -CO-R8 group, wherein R8 is C1-C12alkyl or C6-CXOaryl which is unsubstituted or substituted by a member selected from halogen, C1-C12 and C1-C4alkoxy,

n is an integerfrom 1 to 4, and A is the anion of an inorganic protic acid or of an organicsulfonicacid.

4. A process according to claim 3, wherein the catalyst is a compound offormula I or II, wherein [ A ] - isan anion offormula [ MXm ] -, in which M is boron, phosphorus, arsenic, antimony or bismuth, X is a fluorine or chlorine atom and m is 4 or 6 and is greater by 1 than the valency of M.

5. A process according to claim 3, wherein the catalyst is a compound offormula I, wherein R', R2 and R3 are each independently C1-C6aIkyl, phenyl or naphthyl, or phenyl or naphthyl each substituted by a member selected from halogen, C1-C12alkyl, C1-C4alkoxy and nitro, or phenoxy which is unsubstituted or substituted by a member selected from halogen, C1-C12aIkyl, C1-C4alkoxy and nitro, or R1 and R2, when taken together, are a divalent aliphatic radical of 3to 5 carbon atoms, and A is either BF4, PF6,AsF6orSbCl6.

6. A process according to claim 3, wherein the catalyst is a compound offormula 1, wherein R1, R2 and R3 are each independently phenyl, unsubstituted or substituted by a member selected from chlorine, C1-C4alkyl and C1-C4alkoxy, or are phenoxy, unsubstituted or substituted by a member selected from chlorine, C1-C4alkyl and C1-C4alkoxy, or R1 and R2, when taken togehter, are C3-C5alkylene or C4alkadienylene, and A is either By4 or PF6.

7. A process according to any preceding claim, wherein irradiation is effected with electron beams.

8. A process according to any preceding claim, wherein the cationic compound to be cured is mixed with a pigment, a filler or a reinforcing material.

9. A process according to claim 1 substantially as hereinbefore described with reference to either of the frregoing Examples.