GB2090257A - Use of transparent anhydride- curable epoxy casting resins - Google Patents

Abstract

A transparent, anhydride-curable epoxy casting resin composition comprises a) a polyepoxide compound containing on average more than one epoxy group in the molecule, b) a dicarboxylic acid anhydride, as hardener, in an amount such that the number of anhydride groups per epoxy group is 0.3 to 0.7, c) a tertiary phosphine or a quaternary phosphonium or ammonium salt as accelerator for the cure, and d) optionally further conventional additives. The compositions are used for the production of transparent cast mouldings and for coating or covering objects.

Description

SPECIFICATION Use of transparent anhydride-curable epoxy casting resins The present invention relates to the use of transparent anhydride-curable epoxy resins which contain tertiary phosphines or quaternary phosphonium or ammonium salts as accelerators for the anhydride cure, which epoxy resins are used for the production of transparent cast mouldings and for coating or covering objects.

Epoxy resins have long been used with success in the art for coating and covering objects to protect them from harmful external influences. Recently, the use of transport epoxy resins for covering and coating components have also attained importance, especially in the electrical and electronics field.

The coating of components such as opto-electrical displays may be cited by way of example.

Stringent demands are made of such coatings and coverings. For economic reasons they should be produced in short periods of time, so that a brief curing time for the starting resins is required and this in turn is usually only possible at elevated temperature. During the cure no discolorations which would impair the light permeability may occur. During manufacture and use, the components must also be adequately protected against mechanical damage, the action of moisture, and corrosion. High temperatures can occur in particular during the production of eiectrical and electronic equipment, for example when soldering. The epoxy resins employed must therefore, after the cure, have excellent stability to degradation caused by heat, oxidation and radiation, and to the discoloration and diminution of mechanical and optical properties attendant on such degradation.

Anhdride-curable epoxy resins for coating and covering opto-electrical displays and containing conventional accelerators such as tertiary amines, are described in US patent specification 3 544 827.

However, resins of the bisphenol A type tend to discolour under the action of heat. To avoid this disadvantage the proposal is made in US patent 4 1 78 274 to use an anhydride-curable epoxy casting resin composition which is based on cycloaliphatic epoxy compounds and which contains zinc octoate as accelerator for the cure. In order to obtain homogeneous casting resins it is necessary to use a solvent for the accelerator, and this gives rise to processing and industrial hygiene problems. It is also known [Journal of Applied Polymer Science, Vol.23,1385-1396(1979)] that quaternary phosphonium salts are very suitable accelerators for anhydride-curable epoxy resins of the bisphenol A type and cure rapidly at elevated temperatures.

The present invention has for its object to provide coatings or coverings based on anhydridecurable epoxy resins, especially those derived from polyhydric phenols, which coatings or coverings are stable to thermal oxidative degradation and to radiation-induced degradation.

Accordingly, the present invention relates to the use of transparent, anhydride-curable epoxy resin coasting compositions which comprise a) a polyepoxide compound containing on average more than one epoxy group in the molecule, b) a dicarboxylic acid anhydride, as hardener, in an amount such that the number of anhydride groups per epoxy group is 0.3 to 0.7, c) a tertiary phosphine, quaternary phosphonium salt or ammounium salt as accelerator for the cure, and d) optionally further conventional additives, for the production of transparent cast mouldings and for coating or covering objects.

The epoxy compounds preferably contain on average at least two epoxy groups in the molecule and the amount is preferably such that the number of anhydride groups per epoxide group is 0.4 to 0.6.

Suitable polyepoxide compounds for the use according to the invention are, in particular, those which contain, on average, more than one glycidyl group, P-methylglycidyl group of 2,3epoxycyclopentyl group bonded to a hetero-atom (for example sulfur and preferably oxygen or nitrogen).Particularly preferred compounds are bis-(2,3-epoxycyclopentyl) ether; di- or polyglycidyl ethers of polyhydric aliphatic alcohols, such as 1,4-butanediol, or polyalkylene glycols such as polypropylene glycol; di- or polyglycidyl ethers of cycloaliphatic polyols, such as 2,2-bis-(4hydroxycyclohexyl)propane; di- or polyglycidyl ethers of polyhydric phenols, such as resorcinol, bis-(p hydroxyphenyl)methane, 2,2-bis-(p-hydroxyphenyl)propane (= diomethane), 2,2-bis-(4'-hydroxy-3',5'- dibromophenyl)propane and 1,1 ,2,2-tetrakis-(p-hydroxyphenyl)ethane, or of condensation products of phenols and formaldehyde which are obtained under acid conditions such as phenol novolaks and cresol novoiaks; di- or poly-(p-methylglycidyl) ether of the above mentioned polyhydric alcohols or polyhydric phenols; polyglycidyl esters of polybasic carboxylic acids, such as phthalic acid, terephthalic acid, such A4-tetrahydrophthalic acid and hexahydrophthalic acid; N-glycidyl derivatives of amines, amides and heterocyclic nitrogen bases, such as N,N-diglycidylaniline, N,N-di-glycidyltoluidine and N,N,N',N'tetraglycidyl-bis-(p-aminophenyl)methane; triglycidyl isocyanurate; N,N'-diglycidylethylene urea; N,N' diglycidyl-5,5-dimethylhydantoin and N,N'-diglycidyl-5-isopropylhydantoin; and N,N'-diglycidyl-5,5- dimethyl-6-isopropyl-5,6-dihydrouracil.

If desired, active diluents can be added to the polyepoxides to lower the viscosity. Examples of such diluents are: styrene oxide, butyl glycidyl ether, isooctyl glycidyl ether, phenyl glycidyl ether, glycidyl esters of synthetic, highly branched, mainly tertiary aliphatic monocarboxylic acids ("CARDURA E").

Preferred groups of suitable polyepoxide compounds are, in particular, glycidyl ethers of polyhydric phenols as well as glycidyl esters of di- or tricarboxylic acids, cycloaliphatic polyepoxides, or N-glycidyl derivatives of a N,N-heterocyclic compound. Compounds meriting special mention are bisphenol A diglycidyl ether, hexahydrophthalic acid diglycidyl ester and N,N-diglycidyl-5,5-dimethylhydrantoin.

Dicarboxylic acids suitable as hardeners are, for example, those which are derived from linear of branched aliphatic dicarboxylic acids containing preferably 4 to 10 carbon atoms, and cycloaliphatic or aromatic dicarboxylic acids which can be substituted by one or two alkyl groups of preferably 1 to 4 carbon atoms and which preferably contain a total of 8 to 10 carbon atoms. Examples of such dicarboxylic acid anhydrides are: succinic, phthalic, tetrahydrophthalic, hexahydrophthalic and methylhexahydrophthalic anhydride, and also mixtures therof.

The tertiary phosphines and quaternary phosphonium salts and ammonium salts can have the formulae I to III R3P (I) R4PX (II) or R4NX (III) wherein the Rs are identical or different hydrocarbon radicals of aliphatic cr aromatic character and X is the anion of an inorganic or organic acid.

R can be e.g. linear or branched alkyl of preferably 1 to 1 8 carbon atoms, cycloalkyl of preferably 5 to 1 8 carbon atoms which is unsubstituted or substituted by 1 or 2 alkyl groups, aryl of preferably 6 to 1 8 carbon atoms or aralkyl of preferably 7 to 1 8 carbon atoms.

X can be the anion of the carboxylic acid or sulfonic acid, e.g. formic acid, acetic acid, propionic acid or benzoic acid, of a hydrohalic acid such as hydrochloric, hydrobromic or hydrofluoric acid, of a halosulfonic acid such as fluorosulfonic or chlorosulfonic acid, of sulfuric acid and other inorganic oxyacids.

The accelerator c) is advantageously used in amounts of 0.05 to 10% by weight, preferably 0.5 to 5% by weight, based on the hardener b). The accelerator c) can be, in particular, tri-(n-butyl)phosphine, triphenylphosphine, tetra-(n-butyl)ammonium chloride, (n-butyl)triphenylphosphonium bromide, tetra (n-butyl)phosphonium acetate or tri-(n-butyl)tetradecylphosphoniu m chloride.

The epoxy casting resin composirons can additionally contain 0,1 to 15coo by weight. preferably 0.5 to 10% by weight, based on the anhydride bi, of a soluble co-stabiliser.

Examples of suitable co-stabilisers are the soluble partial esters or triesters of phosphorous acid and the soluble carboxylic acid salts of zinc and divalent tin. The esters can contain identical or different hydrocarbon groups, e.g. alkyl, cycloakyl, aryl, aralkyl or alkaralkyl, which groups preferably contain 4 to 1 8 carbon atoms. Examples of such esters are: di-(n-octylphosphite), tri(nonylphenyl)-phosphite, phenyldidecylphosphite and diphenyloctadecylphosphite. Examples of the salts are: zinc and tin(ll)octoacte, decanoate, dodecanoate, isodecanoate, tetradecanoate or octadecanoate.

The epoxy casting resin composition of the invention can be prepared by mixing the individual components before application. It is preferred to use two-component systems which consist of a) the epoxy resin and b) a mixture of the anhydride hardener with the accelerator for the cure, and which are mixed and homogenised before processing.

In any phase before the ultimate use there can be added to the epoxy resin or hardener, or to the epoxy casting resin composition, conventional additives or modifiers, e.g. extenders or fillers, colorants, plasticers, flow control agents, thixotropic agents, flame retardants and mould release agents.

The objects to be coated or covered according to the invention can be of different meterial and different shape. The materials can be of metal, metal alloys, glass, ceramics, plastics, wood or some other organic material. Preferred fields of use are the coating of electronic components, opto-electrical systems or optical displays, and light-optically utilised mouldings, as well as the covering of preparations and demonstration objects. The coating can also be combined with bonding, e.g. the bonding of optical lenses.

The curing temperature after coating or covering (casting) depends on the heat sensitivity of the object and on the desired curing time. In general, temperatures of at least 500 C, preferably in the range from 500C to below the decomposition temperature of the casting resin, are employed. The upper temperature range is preferably 1 800 C. The curing temperature and the curing time can also be influenced by the amount of hardener c).

The coatings or coverings obtained according to the invention are transparent and, surprisingly, have only a very insignificant inherent colour, even those which are cured at elevated temperature. The stability of the casting resins employed in the prectice of this invention to thermal and oxidative degradation is so great that, even at more elevated curing temperatures, the desired transparency is not diminished by discoloration of the resin. The increase in discoloration on subjecting the cured resin to heat is also surprisingly small, so that a trouble-free further processing of components, e.g. in dip brazings, is possible.

The invention is described in more detail by the following Example.

EXAMPLE The accelerators and co-stabilisers listed in the table are mixed with the anhydride hardener at a temperature above the latter's melting point. The hardener is then mixed with the epoxy resin and the composition is poured into aluminium moulds (round mould having a height of 10 mm and a diameter of 70 mm) and cured in a hot-air oven. The samples obtained are subjected to heat and the colour stability is determined.

The colour stability is assessed visually and by measuring the difference in shape against a reference sample containing benzyl-dimethylamine as accelerator. The colour fastness is also determined in accordance with DIN 54 001.

TABLE (Amounts are in parts by weight) bisphenol epoxy resin (viscosity: 9000-13000 mPas epoxide equivalent weight: 180-1 90) 100 100 100 100 100 100 100 hexahydrophthalic anhydride 90 90 90 90 90 90 90 di(n-octylphosphite) - 10 - 10 - 10 n-butyl-triphenylphosphonium bromide 1 1 tri-n-butylphosphine 1 1 tetrabutylammonium bromide 1 1 benzyldirnethylamine - - ~~ - - - 1 gel time at 1 500C in sec. 100 170 170 210 120 145 85 glass transition temperature (OC) 143 121 14 118 142 128 139 shade after curing* (visual)** 0 0 0 0 0 0 2 shade after 72/h1 200C (visual)** 0-1 0 0 0 0 0 2 shade after 1000 h/1000C (visual)** 1-2 0-1 2-3 1-2 1-2 0-1 5 colourfastness according to DIN 54001 1000h/1000C 2 3 3 4 3 4 2 change in colour DE 1000h/1200C 12.4 8.9 - 13.5 38 9.2 51.8 * curing for 4 h at 600C, 4 h at 800C ** visual assessment: 0 = best rating 5 = poorest rating

Claims (11)

1. Use of a transparent, anhydride-curable epoxy casting resin composition comprising a) a polyepoxide compound containing on average more than one epoxy group in the molecule, b) a dicarboxylic acid anhydride, as hardener, in an amount such that the number of anhydride groups per epoxy group is 0.3 to 0.7, c) a tertiary phosphine or a quaternary phosphonium or ammonium salt as accelerator for the cure, and d) optionally further conventional additives, for the production of transparent cast mouldings and for coating or covering objects.

2. Use according to claim 1, wherein the polyepoxide compound a) is a glycidyl ether of 9 polyhydric phenol, a glycidyl ester of a di- or tricarboxylic acid, a N-glycidyl derivative of a N,Nheterocyclic compound or a cycloaliphatic epoxide compound.

3. Use according to claim 1, wherein the polyepoxide compound a) is a diglycidyl ether of bisphenol A, a diglycidyl ester of hexahydrophthalic acid or N,N-diglycidyl-5,5-dimethylhydantoin.

4. Use according to any preceding claim, wherein the hardener b) is an aliphatic, cycloaliphatic or aromatic carboxylic acid anhydride.

5. Use according to claim 4, wherein the hardener b) is the anhydride of succinic acid, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid or methylexahydrophthalic acid, or a mixture of such anhydrides.

6. Use according to any preceding claim, wherein the accelerator c) is present in an amount of 0.05 to 10% by weight, based on the hardener b).

7. Use according to any preceding claim, wherein the accelerator c) is tri-(n-butyl)phosphine, nbutyltriphenylphosphonium bromide or tetra-(n-butyl)-ammonium bromide.

8. Use according to any preceding claim, wherein the epoxy resin composition additionally contains 0.1 to 1 5% by weight, based on the anhydride b), of a soluble partiai ester or triester of phosphorous acid and/or of a soluble carboxylic acid salt of zinc or divalent tin.

9. Use according to claim 8, wherein the epoxy resin composition contains di-(n-octyl)phosphite, tri-(nonylphenyl)phosphite, didecylphenylphosphite, zinc octoate, zinc isodecanoate, and/or tin(ll)octoacte.

10. Use according to any preceding claim, which comprises applying the epoxy resin casting composition to an object, or coating or covering said object, and subsequently curing the epoxy resin composition at a temperature olf at least 500 C.

11. Use according to claim 1 substantially as hereinbefore described with reference to the foregoing Examples.