AU618805B2 - Adhesive compositions - Google Patents

Description

i

AUSTRALIA

Patents Act 618805 COMPLETE SPECIFICATION

(ORIGINAL)

Class Application Number: Lodged: Int. Class 0oo 0r Complete Specification Lodged: Accepted: Published: Priority Related Art: APPLICANT'S REF: PH1.34737/AU 0 0 0 Name(s) of Applicant(s): Address(es) of Applicant(s): Actual Inventor(s): Address for Service is: IMPERIAL CHEMICAL INDUSTRIES PLC Imperial Chemical House Milibank London SWiP 3JF UNITED KINGDOM BRIAN CAUSTON PHILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Nkcboutne, Australia, 300 Complete Spccificatlon for the invention entitled-, ADHESIVE COMPOSITIONS The following stptcmcnt Is i full dcscription of this Invention, inclding the best inethod of performing it known to applicant(s): P1913/8 UI I- U 1 ii Eu1~-~A~uNbWiu mrrsr~P~xrrmc~unrus~-~~ ~L I^ PII~Ylllf~-r-I- 1_1; 14 The methylene chloriAo solution Vag then mixed vith vater and the I 4 Z- O1 6- I n#l OULILL LJ.L11TLtZY1,4jLr=L6) "Ut VLILVY To: The Commissioner of Patents -la- ADHESIVE COMPOSITIONS The present invention relates to adhesive compositions and in particular to polymerisable compositions suitable for adhering strongly to rigid, hard tissues in the body such as teeth and bones.

The compositions of the invention may be used as liners to which restorative dental filling compositions may be applied and as primers onto which may be applied dental adhesives for mounting for example orthodontic brackets or crowns. The present invention therefore also relates to the use of the present compositions as primers in a method for repairing, adhering to, or altering the position of teeth.

o 9* o *0 Amalgam and restorative dental filling compositions do not adhere well to tooth surface. In filling a dental cavity, the problem of poor adhesion can be ameliorated by preparing undercut cavities so as improve mechanical retention of the filling. In the case of enamel surfaces, mechanical keying can be improved on a microscopic scale by etching the surface with phosphoric acid.

However a truly adhesive system would enable the dentist to improve ,o operative procedures. The use of phosphoric acid could be greatly reduced or even eliminated in some instances; restorative procedures 0° involving removal of dentin could be carrted out with a much reduced level of removal of vital tissue; marginal leakage might be reduced.

o It has to be borne in mind that an Adhksive system is associated with bone and teeth which are real waterials, that is, it is associated with living, dynamic tissue, and as such any adhesive is likely to be tho result of a compromise betweon competing influences. For example, to adhere to the surface of a tooth requires hydrophilic group! but their presence increases the probability that the adhesive will be hydrolytically unstable.

Nevertheless a number of adhesives are available to the practitioner some of which are based on organic phosphate esters. It is highly desirable therefore that the active adhesive component have a low r I -2concentration in the adhesive composition and hence that component needs to have high adhesive properties. The compositien should have low viscosity so that it readily flows over the surface to be bonded.

European patent specification 0 074 708 discusses a considerable number of patent specifications in which various ethylenically unsaturated phosphorus esters are described as adhesives, In that specification particular phosphate esters based on a long chain alkyl ester of acrylic and methbcrylic acids are claimed and said to have improved adheoive properties in derntal applications.

That specification states that 2-methacryloyloxyettyi dihydrogen phosphate is a comparatively poor adhesive component and causes blistering in a paint film. European patent application 0 115 948 describes the use of organic pyrophosphate esters as polymerisable adhesive components. European patent applications 0 058 483 and 0 132 318 describe the use of halophosphorus acid esters as polymerisable monomers in a dental adhesive; these esters contain at least one ethylenically unsaturated functional group and a chlorine or bromine atom attached directly to the phosphorus atom.

4 4* 4 United States patent specification 4044044 describes the use of phosphate esters of hydroxy acrylates as components in So*. anaerobic adhesive compositions. Those compositions are said to remain in a liquid state as long as they remain in contact with air whilst they are said to cure rapidly by polymerisation under the exclusion of air. Those adhesive compositions are useful as loosening prevention materials particularly at high pressure.

It has now been found that the adhesive properties associated with polymerisable esters of phosphorus acid are not only dependant upon the level of impurity in the particular ester but on the inclusion of a small quantity of a related fine filler.

-L 1; -i-l i u -L I I s-_I~ According to the present invention a single-component, visible light-curable liquid adhesive composition is provided which comprises 2 to 20 parts by weight of at least one substantially pure phosphate ester having the formula CH 2 -C(Rl).CO.O.R 2 .0P(0)(OH) 2 in which R 1 is a hydrogen atom or a methyl group, and R 2 is -CH 2

-CH

2

CH

2

-CH(CH

3 or -CH(CH 3

)-CH

2 and 98 to 80 parts by weight of at least one ethylenically unsaturated monomer copolymerisable .ith the phosphate estnr,and 1 to 9% by weight based on a b of at -4eat ne zr3ga z- -4&4i -having particle size less than 0.l1m, and an effective amount of a visible light activated catalyst.

The phosphorus ester component in the present compositions is preferably 2-methacrylyoylxypropyldihydrogen phosphate. The phosphate esters used in the compositions of the present invention may be prepared by for example reacting hydroxy alkylacrylate (or methacrylate) with at least an equimolar amount of phosphorus oxychloride in the presence of a tertiary amine, followed by hydrolysis of any remaining chlorine-phosphorus bonds. The dihydrogen phosphorus ester is then purified by a series of wash and extraction stages so that the ester is substantially pure, ie free of other esters of phosphorus acid so that such impurity level is less than 5% by weight, preferably less than 2% by weight.

The adhesive compositions of the present invention contain at least one ethylenically unsaturated monomer copolymerable with the phosphate ester. The most frequently used monomers include those of th (meth)acrylate, vinyl urethane and styrene types and vinyl acetate. However other monomers such as (meth)acrylamides, i -4vinyl ethers, fumarates, maleates, vinyl ketones, vinyl nitriles, vinyl pyridines and vinyl naphthalenes may also be used either alone or in combination provided that the viscosity parameter of the composition is fulfilled. The concentration of phosphate ester in the adhesive composition is preferably 5 parts by weight or more, and preferably no greater than 15 parts by weight.

Vinyl esters suitable for use in the method of the invention include, for example, vinyl acetate and esters of acrylic acid having the structure CH2=CH-COOR 3 where R 3 is an alkyl, aryl, alkaryl, aralkyl or cycloalkyl group. For example, R 3 may be an alkyl group having from 1 to 20, and preferably 1 to 10 carbon atoms.

Particular vinyl esters which may be mentioned include, for example, methyl acrylate, ethyl acrylate, n- and isopropyl-acrylates, and n-, iso- and tertiary butyl acrylates.

Other suitable vinyl esters include, for example, esters of the formula CH2=C(R 4

COOR

3 where R 4 is methyl. In the ester of formula

CH

2

-C(R

4

COOR

3

R

3 and R 4 may be the same or different.

Particular vinyl esters which may be mentioned include, for example, methyl methacrylate, ethyl methacrylate, n- and isopropylmethacrylate, and iso- and tertiary butyl methacrylate vinyl esters such as n-hexyl, cyclohexyl and tetrahydrofurfuryl acrylates and methacrylates. The monomers should have low toxicity. Suitable aromatic vinyl compounds of the styrene type include, for example, styrene and derivatives thereof, e.g. a-alkyl derivatives of styrene, e.g. a-methyl styrene, and vinyl toluene.

-4 5 Suitable vinyl nitriles include, for example, acrylonitrile and derivatives thereof, e.g. methacrylonitrile.

Other suitable ethylenically unsaturated monomers include vinyl pyrrollidone, and hydroxyalkyl acrylates and methacrylates, e.g. hydroxyethyl acrylate, hydroxypropyl-acrylate, hydroxyethylmethacrylate and hydroxypropyl-methacrylate.

Polyfunctional monomers are also suitable as polymerisation materials, that is, monomers containing two or more vinyl groups.

Suitable monomers include, for example, glycol dimethacrylate, diallyl phthalate, and triallyl cyanurate.

The ethylenically unsaturated material may include at least one ethylenically unsaturated polymer, suitably in combination with at least one ethylenically unsaturated monomer.

The present composition is preferably free from volatile solvent.

Such polymerisable materials are preferably liquid ethylenically unsaturated material such as vinyl u7pthane for example those described in British patent specifications 1352063, 1465097, ,1498421 and German Offenlegungsschrift 2419887 or the reaction product of a diol such as glycol but particularly a bisphenol with a glycidyl alkacrylate such as those described for example in United States patent specifications 3066112 and 4131729 (the disclosure in these specifications are incorporated herein by way of reference).

A preferred reaction product of a glycidyl alkacrylate and a diol has the formula: CH3 OH CH3

CH

2 2--CO. 0-CH 2

-CH-CH

2 CH3 -I ~p*glffi~lX~ -6- Preferred vinyl urethanes described described in the aforesaid British patent specifications and German Offenlegungsschrift are the reaction product of a urethane prepolymer and an ester of acrylic or methacrylic acid with a hydroxy alkanol of at least 2 carbon atoms, the urethane prepolymer being the reaction product of a diisocyanate of the structure OCN-R 5 -NCO and a diol of the structure HO-R 6

-OH

wherein R 5 is a divalent hydrocarbyl group and R 6 is the residue of a condensate of an alkylene oxide with an organic compound containing two phenolic or alcoholic groups.

Other suitable vinyl urethanes include those made by the reaction of alkyl and aryl, preferably alkyl, diisocyanates with hydroxy alkyl acrylates and alkacrylates such as those described in British patent specifications 1401805, 1428672 and 1430303 (the disclosures of which are included herein by way of reference).

The dental compositions of the present invention are cured by irradiating the composition with visible radiation, preferatly that having a wavelength in the range 400m4 to 500m. In order that curing of the compositions may be achieved in that range the compositions contain a catalyst which is activated by visible light.

Such a catalyst preferably contains at least one ketone selected from fluorenone and diketone and at least o',e organic peroxide.

Ketones suitable for use in the composition of the present invention also show some photosensitive catalytic action on their own without the presence of organic peroxide. Such activity of the ketone is enhanced by the addition of a reducing agent as described in the German Offenlegungsschrift 2251048. Accordingly the present ketones are selected from fluorenone and a-diketones and their derivatives which in admixture with a similar amount of organic amine which is capable of reducing that ketone when the latter is in an exicted state, but in the absence of organic peroxide, catalyse cure of an ethylenically unsaturated material. Evidence of cure may be conveniently detected by examining the change in viscosity of a mixture of the ethylenically unsaturated material containing the ketone and organic amine each at 1% by weight based on ethylenically unsaturated material using an oscillating rheometer, samples -7thickness 2mm., whilst the mixture is being irradiated with light having wavelength in the range 400 to 500m4. Such an examination may be carried out using the method described British Standard 5199: 1975, paragraph 6.4 provided that provision is made to allow visible light to be directed onto the mixture. Preferably the ketone has a cure time of less than 15 minutes at a radiation level of 1000 w/m 2 as measured at 470 mu, bandwidth 8m, for example using a Macam Radiometer (Macam Photometrics Ltd., Edinburgh, Scotland).

Diketones have the formula: A C C A II II 0 0 in which the groups A, which may be the same or different, are hydrocarbyl or substituted hydrocarbyl groups and in which the groups A may be further linked together by a divalent link or by a divalent Shydrocarbyl or substituted hydrocarbyl group or in which the groups A together may form a fused aromatic ring system. Preferably che groups A are the same.

The groups A may be aliphatic or aromatic. Within the o scope of the term aliphatic are included cycloaliphatic groups and aliphatic groups which carry aromatic substituents, that is, aralkyl groups. Similarly within the scope of the term aromatic group are 4 included groups which carry alkyl substituents, that is, alkaryl groups. Within the term aromatic groups are included baterocyclic groups.

The aromatic group may be a benzenoid aromatic group, e.g.

the phenyl group, e.g. the phenyl group, or it may be a non-benzenoid cyclic group which is recognised in the art as possessing the characteristics of a benzanoid aromatic group.

3 1111 -i

"P~

8 The groups A, especially when aromatic may carry substituent groups other than hydrocarbyl, e.g. halogen or alkoxy. Substituents other than hydrocarbyl may result in inhibition of polymerisation of ethylenically unsaturated materials, and if the a-diketone contains such substituents it preferably is not present in the photopolymerisable composition in such a concentration as to result in substantial inhibition of polymerisation of the ethylenically unsaturated material in the composition.

The groups A may be further linked together by a direct link, or by a divalent group, e.g. a divalent hydrocarbyl group, that is, in addition to the link through the group,

-C-C-

C 11 II o 0 The groups A may be further linked so as to form a cyclic ring system. For example, where the groups A are aromatic the a-diketone may have the structure (Y)m

I

Ph C C Ph II lI o 0 in which Ph is a phenylene group, Y is >CH2, or a derivative thereof wherein one or both of the hydrogen atoms are replaced by a hydrocarbyl group, and m is 0, 1 or 2. Preferably the group Y is linked to the aromatic groups in positions ortho to the group The groups A together may form a fused aromatic ring systems -9- In general, a-diketones are capable of being excited by radiation in the visible region of the spectrum, that is, by light having a wavelength greater than 400mL, e.g. in the wavelength range 400mu to 500m For the present invention, a- diketone should have low volatility so as to minimise odour and concentration variation.

Suitable a-diketo'es include benzil in which both groups A are phenyl, a-diketones in which both of the groups A are fused aromatic, e.g. anaphthil and p-naphthil, and a-diketones in which the groups A are alkaryl groups, e.g. p-tolil. As an example of a suitable adiketone in which the groups A are non-benzencid aromatic there may be mentioned furil, e.g. 2:2'-furil. Derivatives of the a-diketone in which the groups A carry non-hydrocarbyl groups as for example p,p'dialkoxy benzil, e.g. p,p'-dimethoxy benzil or dihalobenzil, e.g. p,p'-dichlorobenzil, or p-nitrobenzil may be incorporated.

The groups A may be linked together by a direct link or by a divalent hydrocarbyl group to form a cyclic ring system. For example, where the groups A are aliphatic the a-diketone may be camphorquinone.

example of an a-diketone having the structure I is phenanthraquinone in which the aromatic groups A being linked by a direct link ortho to the group C C II II 0 0 Suitable derivatives include 2-bromo-, 2-nitro, 4-nitro, 3- chloro-, 2,7-dinitro, l-methyl-7-isopropyl phenanthraquinone.

The a-diketone may be acenaphthene quinone in which the groups A together form a fused aromatic ring system.

10 The ketone may also be fluorenone and its derivatives such as for example lower alkyl halo, nitro, carboxylic acid and esters thereof, particularly in the 2- and 4-positions.

The ketone may, for example, be present in the composition in a concentration in the range 0.01% to 2% by weight of the polymerisable material in the composition although concentrations outside th.s range may be used if desired. Suitably the ketone is present in a concentration of 0.1% to 1% and more preferably 0.5% to 1% by weight of the ethylenically unsaturated material in composition.

The ketone should be soluble in the polymerisable material and the above concentrations refer to solution concentration.

The organic peroxides suitable for use in the present composition include those having the formula I R 7 0 0 R 7 in which the groups R 7 which may be the same or different, are hydrogen, alkyl, aryl, or acyl groups, no more than one of the groups R being hydrogen, The term acy! means having the formula i 4 R8 CO in which R 8 is an alkyl, aryl, alkloxy or aryloxy group. The term alkyl and aryl have the definitions given hereinbefore foi the groups A and include substituted alkyl and aryl.

Examples of organic peroxide suitable for use in th, composition of the present invention include diacetlt peroxide, dibenzoyl peroxide, di-tertiary butyl peroxide, dilatroyl peroxide, tertiary butyl perbenzoate, ditertiary butyl cyclohexyl perdicarbonate.

11 The organic peroxide may be, for example, present in the composition in the rpnge 0.1% to 10% by weight of the polymerisable material in the composition although concentration outside this range may be used if desired.

The reactivity of a peroxide is often measured in terms of a ten hour half life temperature, i.e. within ten hours at that temperature half of the oxygen has been made available. The peroxides in the present compositions preferably have ten hour half life temperatures of less than 150°C, more preferably less than 100 0

C.

The rate at which the composition of the invention cures under the influence of visible light can be increased by incorporation into the composition of reducing agent in concentration 0.1% to 10% by weight of the polymerisable material, which is capable of reducing the ketone when the latter is in an excited state. Suitable reducing agents are described, for example, in German Offenlegungsschrift 2251048 and include organic amines, phosphites, sulphinic acids.

In general non-basic reducing agents are preferred because they are less likely to react with the phosphate ester.

Suitable nonr-basic reducing agents include aldehydes and organotin compounds having the formula:-

(R

9 )n Sn (0R 0 )iO in which n and mn are integers having the value I, 2 or 3 and n-m-ia4 R 9 is an alkyl or alkenyl group containing 1 to 18 carbon atoms and R

I

O

is R 9 or R 9 .CO- or having the formula

(R

9 13sn$)20 based OA Q O The filler is present in amount I t9 9% by weighte-e4h- -eeompaitio4 preferably 4 to Thi, filler 4o- any form of silica including comminuted forms of crystalline silicat including for example 12 sand, but is preferably a colloidal fort u."h as pyrogenic, formed or precipitate silica.

Mixing of the components may be effected by stirring together the polymerisable material and the filler. It may be useful to dissolve the catalyst components first in Lhe polymerisable material; the polymerisable material may conveniently but less preferably be diluted with a suitable diluent so as to improve solution of the catalyst components. When mixing has been effected the diluent may be removed if desired, e.g. by evaporation.

Desirably the present composition is substantially free of solvent because its presence may interfere with adhesion and may give rise to toxicity problems.

Because the photosknsitive catalyst renders the polymerisable material and wonomer sensitive to light in the 4Omp to visible range, that part of the preparation of the present composition in which photosensitive catalyst is added and subsequent manipulation, e.g. filling of containers should be carried out in the substantial absence of light in that range. Most convenientlyj tha preparation can be carried out using light outside that range for example under that emitted by sodium vapour electric discharge lamps.

In accordance with a further embodiment of rhu p? sent invention a curing process is provided which comprises irradiating the compusition of the invention with visible radiation having wavelength between 4004m. and 500m. The process may be carried out at any convenient temperature provided that the composition does not crystallize and fragment, the temperature thereby bfiing too low, or unduly volatl.lic, the temperature thereby being too high. Preferably the process is carried out at ambient temperatures, ie. between 1500 and 40O0.

It is envisaged that a tooth surface will be coated with the present composition and then cured under a visible light aource; conveniently the composition is cured from a portable light source. However it in 13 preferred that the tooth surface is primed before being so coated; a preferred primer is a bisbiguanide, polybiguanide or a long chain quaternary salt as described in British Application 8810412.

A dental composition according to the present invention is preferably packed in single small containers lOg capacity) so as to facilitate handling in the surgery and reduce the risk of inadvertent curing by Ifor example stray light.

For cosmetic purposes, such a dental composition may have a stained, coloured or natural tooth appearance and hece the present composition may include small quantities of pigments, opalescent agents and the like. The composition may also include small quantities of other materials such as anti- oxidants and stabilisers S' provided that they do not substantially affect cure.

The invention is illustrated with reference to the following Examples:- Example 1 Preparation of phosphate ester Phosphorus oxychloride (127.9g 0.81 moles) was mixed with methylene chloride (600 cm 3 The mixture was stirred and cooled to 00°. A mixture of hydroxypropyl methacrylate (120g; 0.83 moles), pyridine (65.8g; 0,83 moles) and methylene chloride (400cm 3 was added dropwise to the phosphorus oxychloride mixture over a period of minutes whilst keeping the reaction temperature in the range 0 to The mixture was stirred for a further 2 hours within that temperature range, The reaction mixture was then poured into 1 litre cold water and the methylene chloride layer was separated, washed twice with water.

14 The methylene chloride solution was then mixed with water and the methylene chloride removed using a rotary evaporator to leave an aqueous phase and some water insoluble inorganic mate,.ial. The aqueous layer was washed twice with methylene chloride, and then carefully acidified (hydrochloric acid 208cm 3 with stirring, the aqueous phase was then extracted using ethyl acetate (1 litre) and the aqueous phase was rejected. The concentration of phosphate ester was then estimated and analysed by acid base titration methods and found to contain ester impurity less than 2% by weight of 2methacryloyloxy-propyl dihydrogen phosphate. Triethylene glycol dimethacrylate (TEGDM) was then added to provide a 90:10 TEGDM:phosphate ble d. Ethyl acetate was then removed using a rotary evaporator at 65 0 C and under a vacuum of A number of formulations were prepared having filler concentration and composition as given in the Table 1. To the monomer mixture were added the catalyst components as follows %w/w Camphorquinone 0.73 Dibutyl tin dilaurate 0.49 'Topanol' 0 to 200 ppm on final composition.

'Aerosil' A130 varying This addition and subsequent operations using the composition were carried out under sodium vapour discharge light.

Evaluation of adhesive strength on samples having the above formulations were carried out using the procedure described in British Dental Journal 1984, pages 93 to 95, except that the composite restorative material used in conjunction with the adhesive composition was 'Occlusin' (trade mark, Imperial Chemical Industries PLC). The surface of dentin was painted with a solution of 'Vantocil'

I

1 1 LI II-- -I Cli~n ~CIII~I*C~ 15 by weight in water), followed by drying with an air blast, before the adhesive formulation was applied. 'Vantocil' is a polybiguanide having the formula -(CH 2 6 -NH.C(:NH).NH.C(:NH)NH-,n where n is 4 to 7. 'Vantocil' is a trade mark of Imperial Chemical industries PLC. The samples were cured by radiation from a tungsten halogen lamp having a tuned reflector and a dichroic filter which eliminates ultra-violet radiation; the intensity was 1000 Wm 2 and cure time was 30 seconds.

'Topanol' is a trade mark of Imperial Chemical Industries PLC and 'Topanol' 0 is 2,6-ditertiarybutyl-4-methylphen.l.

'Aerosil' is a trade mark of Degussa AG; 'Aerosil' A130 is a hydrophilic grade of fumed silica having mean particle size 0.02km.

Table 1 fq 4 Filler 0 Bond Strength (MPa) 13.0 11.0 The above results are presented in the accompanying figure, Example 2 Similar bond strength was obtained using an adhesive having the following composition:i i 16 by weight Phosphate ester (as described in Example 1) Vinyl urethane resin

TP!GDM

Camphorquinone Dibtityl tin dilaurate' 'Aerosill A130 9.48 15,17 70.16 0.71 0.48 4.00

I.

*0 04 4 4 4 01 4* 4 0 O 9 as described in Example 1 of European Patent Specification 59649.

The vinyl urethane resin and the phc.sphate ester are incorporated as 8olutiorns in TEGDM, tho quantity of TEGD11 so-added being included in the TEGDI4 total content.

4 40 04 4*4

Claims (7)

1. A single-component, visible light-curable liquid adhesive composition which comprises 2 to 20 parts by weight of at least one substantially pure phosphate ester having the formula CH 2 =C(RI) .CO.O.R 2 (OH) 2 in which R 1 is a hydrogen atom or a methyl group, and R is -CH 2 -CH 2 -CH 2 -CH(CH 3 or -CH'CH) -CH 2 and 98 to 80 parts by weight of at least one ethylenically unsaturated monomer copolymerisable with the phosphate ester 1 to 9% by weight based on a b of a silica filler having a particle size less than 0.1 pm, and an effective amount of a visible light a ivated catalyst.

2. A composition as claimed in claim 1 in which the phosphate ester has the formula CH 2 =C(CH) .CO.OR OP(O) (OH) 2 in which R 2 is an isopropylene group. 3, A composition as claimed in either claim 1 or claim 2 in which the concentration of the phosphate ester in 5 to parts by weight.

4. A composition as claimed in any one of claims 1 to 3 in which the ethyleneically unsaturated monomer comprises a glycol dimethacrylate. A composition as ~.2aimed in claim 4 in which the monomer also includes a vinyl urethane.

6. A composition as claimed in any one of claims 1 to 5 in which the weight of filler present is in the range 4% to 6%.

7. A composition as claimed in any one of claims 1 to 6 in which the filler is colloidal silica. U. A composition as claimed in any one of claims 1 to 7 in Swhich the visible light activated catalyst comprises WN 17 V^Cir e camphorquinone and a reducing agent,

9. A composition as claimed in claim 8 in which the reducing agent is an organotin compound having the formula (R 9 )nSn(OR 10 m in which n and m are integers having the value 1,2 or 3 and n+m=4, R is an alkyl or alkenyl group containing 1 to 18 10 9 9 carbon atoms and R 0 is R or R CO-. A composition as claimed in claim 8 in which the reducing agent is an organotin compound having the formula ([R 9 3 Sn) 2 0 9 I in which R 9 is an alkyl or alkenyl group containing 1 to 18 carbon atoms.

11. A composition, according to claim 1, substantially as hereinbefore described with reference to any one of the examples. DATED: 23 October, 1991 PHILLIPS ORMONDE FITZPATRICK Attorneys for: IMPERIAL CHEMICAL INDUSTRIES PLC 7918N W©N 18