FR2631037A1 - ADHESIVE COMPOSITIONS BASED ON PHOSPHORIC ESTER - Google Patents

Abstract

A one-component, visible light curable liquid adhesive composition is provided. The composition of the invention contains (a) 2 to 20 parts by weight of at least one substantially pure phosphoric ester of formula CH2 = C (R ** 1) .CO.OR ** 2.OP (O) (OH ) 2 in which R ** 1 is hydrogen or methyl and R ** 2 is -CH2 -CH2 -, -CH2 -CH (CH3) - or -CH (CH3) -CH2 - and (b ) 98 to 80 parts by weight of at least one ethylenically unsaturated monomer copolymerizable with the phosphoric ester, (c) 1 to 9% by weight of a fine inorganic filler, and an effective amount of an activated catalyst by visible light. Application: dentistry.

Description

The present invention relates to compositions

  adhesives, and it relates, in particular, to composi-

  Polymerizable compositions able to adhere strongly to hard, rigid body tissues, such as teeth and bones. The compositions of the invention can be used as

  fittings to which composites can be applied

  sealing compositions for dental restorations and finishing compositions to which dental adhesives can be applied for the placement of, for example, orthodontic hooks or crowns. The present invention thus also relates to the use of the novel compositions as finishing compositions in the art for restoration purposes,

  adhesion, or modification of the position of teeth.

  Amalgam and filling compositions for dental restorations do not adhere well to the surface of the teeth. In filling a dental cavity, the problem of poor adhesion can be mitigated by the preparation of cavities so that

  improve the mechanical retention of the filling load.

  In the case of the enamel surface, the mechanical bond can be improved on a microscopic scale by etching

  corrosive surface with phosphoric acid.

  However, a system involving genuine adherence would allow the dentist to improve operating procedures. The use of phosphoric acid could be greatly reduced or even suppressed in some cases; restoration methods involving dentin removal could be implemented with a sharp reduction in the rate of removal of vital tissue; the failure

  Marginal sealing could be reduced.

  It must be remembered that an adhesive system is associated with bones and teeth that are real materials, that is, it is associated with a living dynamic tissue, and as such, any membership is likely to be the result of a compromise between competing influences. For example, to adhere to the surface of a tooth, the presence of hydrophilic groups is necessary, but their presence increases the likelihood that the adhesive will be unstable to hydrolysis. Nevertheless, the practitioner has a number of adhesives some of which are based on organic phosphoric esters. It is therefore highly desirable that the active adhesive component has a low concentration in the adhesive composition and, therefore, this component must have pronounced adhesive properties. The composition must be very viscous so that it flows easily over the

surface to be bound.

  European Patent No. 0 074 708 discloses a considerable number of patents in which various ethylenically unsaturated phosphorus esters are described as adhesives. In the aforementioned patent, it is question of particular phosphoric esters based on a long-chain alkyl ester of acrylic and methacrylic acids, and it is indicated that these esters have very good adhesive properties in dental applications. This

  document mentions that dihydrogenophosphate

  Methacryloylloxyethyl is a relative adhesive component

  mediocre and blistering a paint film. European Patent Application No. 115,948 discloses the use of organic pyrophosphoric esters as polymerizable components of adhesives. European Patent Applications Nos. 0 058 483 and 0 132 318 disclose the use of halophosphorus acid esters as polymerizable monomers in a dental adhesive; these esters contain at least one ethylenically unsaturated functional group and one chlorine or

  bromine attached directly to the phosphorus atom.

  U.S. Patent No. 4,044,044 discloses the use of phosphoric esters of hydroxyacrylates as components of compositions

  anaerobic adhesives. It is considered that these composi-

  These remain in the liquid state as long as they remain in contact with air, while it is stated that they are rapidly taken up by polymerization in the absence of air. These adhesive compositions are useful as preventing materials

  loosening especially under high pressure.

  The Applicant has just found that the adhesive properties related to polymerizable esters of a phosphorus acid depend not only on the degree

  impurity of the particular ester, but also of

  clusion of a small quantity of a relatively fine charge. According to the present invention, there is provided a composition. single-component, visible-curable liquid adhesive which comprises: (a) 2 to 20 parts by weight of at least one substantially pure phosphoric ester having the formula CH2 = C (R1) .CO.O.R2 . OP (O) (OH) 2 wherein R1 is an atom

  of hydrogen or a methyl group and R2. is a group -CH2-

  CH 2 -, - CH 2 -CH (CH 3) - or -CH (CH 3) -CH 2 - and (b) 98 to 80 parts by weight of at least one ethylenically unsaturated monomer copolymerizable with the phosphoric ester, and c) 1 to 9% by weight, based on a + b, of at least one inorganic filler with particles of diameter less than 0.1 μm, and an effective amount of an activated catalyst

by visible light.

  The phosphorus ester component present in

  compositions of the invention is preferably dihydro-

  2-methacryloyloxypropyl genophosphate. The phosphoric esters used in the compositions of the present invention may be prepared for example by reacting a hydroxyalkyl acrylate (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 dihydrogenophosphorus ester is then purified by a series of washing and extraction operations so that the ester is substantially pure, i.e. free of other phosphorus acid esters, so that the impurity level is less than 5% by weight and preferably less than 2% by weight. The adhesive compositions of the present invention contain at least one ethylenically unsaturated monomer copolymerizable with the phosphoric ester. The most frequently used monomers include those of (meth) acrylate, vinyl urethane and styrene, and vinyl acetate. However, other monomers such as (meth) acrylamides, vinyl ethers,

  fumarates, maleates, vinyl ketones, vinyl

  nitriles, vinylpyridines and vinylnaphthalenes can also be used, alone or in combination, as long as the viscosity parameter of the composition is satisfied. The concentration of the phosphoric ester in the adhesive composition is preferably equal to or greater than 5 parts by weight and it is preferable

  it does not exceed 15 parts by weight.

  Advantageous vinyl esters for use in the process of the invention include, for example, vinyl acetate and acrylic acid esters of the formula CH 2 = CH-COOR 3, wherein R 3 is alkyl, aryl, alkaryl, aralkyl or cycloalkyl. For example, R3 may be an alkyl group having 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-propyl and isopropyl acrylates and

  n-butyl, isobutyl and tert-butyl.

  Other suitable vinyl esters include, for example, esters of formula

CH2 = C (R 4) COOR 3

  in which R4 is a methyl radical. In the ester of formula

CH2 = C (R 4) COOR 3

  R3 and R4 may be the same or different.

  Particular vinyl esters which may be mentioned include, for example, methyl methacrylate, ethyl methacrylate, n-propyl and isopropyl methacrylates and n-butyl, isobutyl and tert-butyl methacrylates. and vinyl esters such as n-hexyl, cyclohexyl and tetrahydrofurfuryl acrylates and methacrylates. The monomers must have low toxicity. Suitable aromatic vinyl compounds of the styrene type include, for example, styrene and its derivatives, for example, α-alkyl derivatives of styrene such as α-methylstyrene and vinyltoluene. Suitable vinyl nitriles include, for example, acrylonitrile and its derivatives, for example methacrylonitrile. Other ethylenically unsaturated monomers which can be advantageously used include vinylpyrrolidone and hydroxyalkyl acrylates and methacrylates, for example hydroxyethyl acrylate,

  hydroxypropyl acrylate, hydroxypropyl methacrylate

  ethyl and hydroxypropyl methacrylate.

  The polyfunctional monomers can also be used as polymerizable materials, i.e., monomers containing two or more vinyl groups. Suitable monomers include, for example, glycol dimethacrylate, diallyl phthalate and

triallyl cyanurate.

  The ethylenically unsaturated material can

  comprise at least one ethylenically unsaturated polymer

  that advantageously in association with at least one

  monomer with ethylenic unsaturation.

  The composition of the invention is advantageous

it is devoid of volatile solvent.

  These polymerizable materials are preferably ethylenically unsaturated liquid materials such as vinyl urethane, for example materials described in British Pat. Nos. 1,352,063, 1,465,097, 1,498,421 and in the Federal Republic of Laid-Open Patent Application. German Offenlegungsschrift No. 2,419,887, or the reaction product of a diol such as glycol, but especially a bisphenol, with a glycidyl alkacrylate such as those

  which are described, for example, in US patents.

  United States of America Nos. 3,066,112 and 4,131,729 (the description of which

  this document is incorporated herein by reference).

  A preferred product of reaction between a glycidyl alkacrylate and a diol corresponds to the formula:

CH3 OH CH3

HH

2-C-COH 2 O.O 2 -CH-CH 2

  Preferred vinyl urethanes disclosed in the British patents and the aforementioned DE-OS patent application include the reaction product of a prepolymer

  of urethane and an ester of acrylic or methacrylic acid

  with a hydroxyalkanol having at least 2 carbon atoms, the urethane prepolymer being the reaction product of a diisocyanate of the formula OCN-R5-NCO and a diol of the formula HO-R6-OH, in which R5 is a divalent hydrocarbyl group and R6 is the residue of a condensation product of an alkylene oxide with a compound

  organic compound containing two phenolic or alcoholic groups

c.

  Other suitable vinylurethanes include

  those prepared by the reaction of alkyl and aryl diisocyanates, preferably alkyl, with hydroxyalkyl acrylates and alkacrylates such as those described in British Pat. Nos. 1,401,805,

  1,428,672 and 1,430,303 (incorporated herein by reference

  silent). The dental compositions of the present invention are cured by irradiation with visible radiation, preferably those having a wavelength in the range of 400 m4m to 500 mgm. In order that the curing of the compositions can be achieved in this range, the compositions contain a catalyst which is activated by visible light. Such a catalyst preferably contains at least one ketone chosen from

  fluorenone and diketone and at least one organic peroxide

  than. Preferred ketones for use in the composition of the present invention also themselves produce a certain photosensitive catalytic effect without the presence of an organic peroxide. This activity of the ketone is promoted by the addition of a reducing agent as described in German Offenlegungsschrift No. 2 251 048. Ketones are

  therefore chosen between fluorenone and

  diketones and their derivatives which, when mixed with a similar amount of organic amine which is capable of reducing this ketone when the latter is in an excited state but in the absence of organic peroxide, catalyze the maturation of a material to ethylenic saturation. The signs of ripening can conveniently be detected by examining the change in viscosity of a mixture of ethylenically unsaturated material containing the ketone and organic amine each in a proportion of 1% by weight based on the ethylenic unsaturation using an oscillating rheometer, samples of 2 mm thickness, while irradiating the mixture with light of a wavelength in the range of 400 to 500 mum. Such an examination may be carried out by the method described in British Standard 5199: 1975, paragraph 6.4, provided that arrangements are made to allow the incidence of visible light on the mixture. Preferably, the ketone has a maturation time of less than 15 minutes for an irradiation rate of 1000 w / m2, measured at 470 mim, over a bandwidth of 8 mpm, using for example a Macam radiometer (Macam Photometrics Ltd., Edinburgh,

Scotland).

  The dicetones respond to the formula:

A - C - C - A

  Wherein groups A, which may be the same or different, are hydrocarbyl or substituted hydrocarbyl groups and these groups A may be further bonded together by a divalent linkage or by a hydrocarbyl or substituted hydrocarbyl group, or groups A can jointly form an aromatic ring

  condensed. Groups A are preferably the same.

  Groups A may be aliphatic or aromatic. Included in the definition of the term aliphatic are cycloaliphatic groups and aliphatic groups which carry aromatic substituents, i.e., aralkyl groups. In the context of the definition of the aromatic group, it is likewise to consider groups bearing alkyl substituents, that is to say alkaryl groups. The expression groups

  aromatics also covers heterocyclic groups.

  The aromatic group may be a benzene aromatic group, for example the phenyl group, or it may be a non-benzene ring group which

  is technically recognized as possessing the characteristics

  of a benzene aromatic group.

  Groups A, especially when they are aromatic, may carry substituent groups other than hydrocarbyl groups, for example a halogen or an alkoxy group. Substitutions other than groups

  hydrocarbyl may cause inhibition of the polymer

  of ethylenically unsaturated materials, and whether the

  diketone contains such substituents, it is preferable

  that it is not present in the photopolymer composition

  at a concentration such that a significant inhibition of the polymerization of the

  ethylenic unsaturation in the composition.

  Groups A may furthermore be bonded to each other by a direct bond or a divalent group, for example a divalent hydrocarbyl group, that is, in addition to bonding by the group

-C - C -

  Groups A may further be linked to form a cyclic ring. For example, when the A groups are aromatic, the adicetone can have the structure (Y) m

/ \ I

  Ph - C - C - Ph II II 0 0 wherein Ph is a phenylene group, Y is a group> CH2, or a derivative of this group of which one or both of the hydrogen atoms are replaced by a hydrocarbyl group , and ma the value 0, 1 or 2. Preferably, the group Y is attached to the aromatic groups in the

ortho positions.

  Groups A may jointly form a

  condensed aromatic ring system.

  In general, α-diketones are capable of being excited by radiation in the visible region of the spectrum, i.e. by wavelength light greater than 400 mpm, for example in the length range. from 400 m4m to 500 m4m. For the present invention, the adicetone must have low volatility so as to minimize odor and concentration variations. Suitable α-diketones include benziline in which

  the two groups A are phenyl groups,

  diketones in which the two groups A are fused aromatic rings, for example? -naphthil and beta-naphthilic and adicetones whose A groups are alkaryl groups, for example ptolile. As an example of suitable α-diketone whose A groups are non-benzene aromatic, there may be mentioned a furile, for example 2: 2'-furile. Α-Diketone derivatives in which groups A carry non-

  hydrocarbyl such as, for example, a p, p'-dialkoxy-

  benzyl, for example p, p'-dimethoxybenzil or a p, p'-

  dihalobenzyl, for example p, p'-dichlorobenzil, or pnitrobenzil can be incorporated. Groups A may be linked to each other by a single bond or a divalent hydrocarbyl group to form a ring nucleus. For example, when the A groups are aliphatic, the a-diketone may

to be camphorquinone.

  An example of a-dicetone of formula I is phenanthraquinone in which the aromatic groups A are attached by a single ortho bond to the group

C - C-

He I1 0 0

  Suitable derivatives include 2-

  bromo-, 2-nitro, 4-nitro, 3-chloro-, 2,7-

  dinitro, 1-methyl-7-isopropylphenanthraquinone.

  Α-diketone may be acenaphtenequinone in which groups A together form a system of

condensed aromatic rings.

  The ketone may also be fluorenone and its derivatives such as, for example, lower alkyl derivatives (C1 to C6), halogenated, nitrated, carboxylic acid derivatives and their esters, especially in the positions

2 and 4.

  Ketone may, for example, be present in

  composition at a concentration within the

  from 0.01% to 2% by weight of the polymerizable material

  in the composition, although concentrations are

  both of this range can be used as appropriate.

  The ketone is advantageously present at a concentration of 0.1 to 1% and especially 0.5 to 1% by weight of the material & ethylenic unsaturation in the composition. The ketone must be soluble in the polymerizable material and the above concentrations refer to the solution concentration. Organic peroxides which can be used advantageously in the composition of the invention include those which correspond to the formula

R7 - 0 - 0 - R7

  wherein the R7 groups, which may be the same or different, represent hydrogen or alkyl, aryl or acyl groups, not more than one of R representing hydrogen. The term acyl means a group of formula

R8 - CO -

  wherein R8 is an alkyl, aryl, alkoxy or aryloxy group. The terms alkyl and aryl have the definitions given above for groups A and

  substituted alkyl and aryl groups. -

  Examples of organic peroxides that may be used in the composition of the present invention include diacetyl peroxide, dibenzoyl peroxide, di-tert-butyl peroxide,

  dilauroyl, tert-butyl perbenzoate,

  di-tert-butyl-cyclohexyl bonate.

  The organic peroxide may be, for example, present in the composition in the range of 0.1 to 10% by weight of the polymerizable material in the composition, although a concentration deviating from this range may be present in the composition.

used where appropriate.

  The reactivity of a peroxide is often measured

  according to the half-life temperature of ten hours, that is,

  say that in ten hours at this temperature, half of the oxygen has been made available. The peroxides present in the compositions of the invention preferably have half-life temperatures of ten hours less than 150 C and

preferably less than 100C.

  The rate at which the composition of the invention matures under the influence of visible light can be accelerated by incorporation in the composition of a reducing agent at a concentration of 0.1 to 10% by weight of the polymerizable material. which is able to reduce the ketone when the latter is in an excited state. Suitable reducing agents are described for example in German Offenlegungsschrift DE-OS-2 251 048 and include amines.

  organic, phosphites, sulfinic acids.

  In general, non-basic reducing agents are preferable because they are likely to react with the phosphoric ester. Suitable non-basic reducing agents include aldehydes and organic tin compounds having the formula:

(R9) n Sn (OR10) m -

  wherein n and m are integers having the value 1, 2 or 3 and the sum n + m = 4, R9 is an alkyl or alkenyl group containing 1 to 18 carbon atoms and R10 is R9 or a group R9.CO - or a group of formula ([R913Sn) 20O The filler is present in an amount of 1 to 9% by weight of the composition, preferably 4 to 6%. The filler is any form of silica, including divided forms of crystalline silica, e.g.

  sand, but it is preferably in a col-

  loidal such as silica formed by pyrogenation or precipitated. The components can be mixed by stirring together the polymerizable material and the filler. It may be useful to first dissolve the catalyst components in the polymerizable material; the latter may conveniently, but less advantageously, be diluted with a suitable diluent so as to improve the dissolution of the catalyst components. When the mixing has been carried out, the diluent may be removed if necessary, for example by evaporation. The composition of the invention is advantageously virtually devoid of solvent, because its presence can hinder adhesion and can

  cause toxicity problems.

  Since the photosensitive catalyst renders the polymerizable material and the light-sensitive monomer in the visible range of 400 to 500 mgm, that part of the preparation of the composition of the invention in which the photosensitive catalyst is added and the subsequent handling, for example container conditioning, must be carried out essentially away from the light entering this range. The preparation can very advantageously be carried out using light outside this range, for example the light emitted by electric discharge lamps in the vapor

sodium.

  According to another embodiment of the present invention, there is provided a maturing process which consists in irradiating the composition of the invention with visible radiation having a wavelength of between 400 and 500 mgm. The method can be implemented at

  all suitable temperatures, provided that the composi-

  does not crystallize or fragment, in which case the temperature is too low, or does not volatilize

  undesirably, in which case the temperature is too high.

  The process is preferably carried out at temperatures

  ambient, that is to say between 15 C and 40 C.

  It is contemplated to coat the surface of a tooth with the composition of the invention which is then matured under a visible light source; the composition is advantageously matured using a portable light source. However, it is preferable that the surface of the tooth is primed before being so coated; a preferred finish is a bisbiguanide, a polybiguanide, or a long chain quaternary salt as described in US Pat.

  British Patent Application No. 8,810,412.

  A dental composition according to the present invention is advantageously packaged in small individual containers (for example with a capacity of 10 g) so as to facilitate the surgical manipulation and to reduce the risk of inadvertent maturation, by

  example under the effect of scattered light.

  For aesthetic purposes, such a dental composition can take the appearance of a tinted, colored tooth

  natural and, consequently, the composition of the invention

  may contain small amounts of pigments, opalescent agents, etc. The composition may also contain small amounts of other materials such as antioxidants and stabilizers, of the moment

  that they do not significantly affect maturation.

  The invention is illustrated by the following examples:

Example 1

  (a) Preparation of Phosphoric Ester Phosphorus oxychloride (127.9 g, 0.83 mol) was mixed with methylene chloride (600 cm3). The mixture was stirred and cooled to 0 ° C. A mixture of hydroxypropyl methacrylate (120 g, 0.83 mol), pyridine (65.8 g, 0.83 mol) and methylene chloride (400 cm 3) was added dropwise to the mixture containing the phosphorus oxychloride over a period of 45 minutes while maintaining the reaction temperature in the range of 0 to 3 C. The mixture was stirred for a further 2 hours in this range. temperature. The reaction mixture was then poured into 1 liter of cold water and the chloromethylenic phase was separated and washed twice with water. The chloromethylenic solution was then mixed with water and the methylene chloride was rotary evaporated leaving an aqueous phase and a small amount of water insoluble inorganic material. The aqueous phase was washed two

  methylene chloride and then acidified with precau-

  (208 cc hydrochloric acid) with stirring. The aqueous phase was then extracted with ethyl acetate (1 liter) and the new aqueous phase was discarded. Phosphoric ester concentration was then estimated and analyzed by acid-base titration methods, and the product was found to contain less than 2% impurity.

  by weight of 2-dihydrogenphosphate ester

  méthacryloyloxypropyle. Dimethacrylate was then added

  triethylene glycol late (TEGDM) to obtain a 90:10 TEGDM: phosphate mixture. The ethyl acetate was then removed on a rotary evaporator at 65 ° C. and under a vacuum of

6.67 kPa.

  (b) Several formulations having a filler concentration and composition were prepared as shown in Table 1. The catalyst components were added to the monomer mixture as follows:% w / w Camphoquinone 0.73 Dibutyltin Dilaurate 0.49 "Topanol" O sufficient quantity for 200 mg / kg per

to the composition

  Final "Aerosil" A130 Variable Quantity This addition and subsequent operations of use of the composition were made at the

  light from a discharge in sodium vapor.

  Evaluation of adhesion strength on samples having the above formulations was performed using the methods described in British Dental Journal 1984, pages 93 to 95, except that the composite restorative material used in conjunction with the composition adhesive was "Occlusin" (trademark of Imperial Chemical Industries PLC). The surface of the dentin was brushed with "Vantocil" solution (0.1% by weight in water), the operation being followed by drying under a stream of air, before the formulation was applied. . The product "Vantocil" is a polybiguanide having the formula 4 (CH2) 6-NH.C (: NH) .NH.C (: NH) NH.n wherein n is 4 to 7. "Vantocil" is a trademark of Imperial Chemical Industries PLC. The samples were cured by radiation emitted from a tungsten filament halogen lamp having a tuned reflector and a dichroic filter to remove ultraviolet rays; the intensity was 1000 Wm-2 and the duration

  maturation was 30 seconds.

  "Topanol" is a trademark of Imperial Chemical Industries PLC and "Topanol" O

  refers to 2,6-ditertiobutyl-4-methylphenol.

  Aerosil "is a trademark of the

  Firm Degussa AG; "Aerosil" A130 is a hydro-quality

  phile of siiice in impalpable particles of a diameter

average of 0.02 μm.

TABLE 1

Load,% Link Strength (MPa)

O 7.0

13.0

7.5 11.0

7.0

  The results above are reproduced on the

single figure of the appended drawing.

Example 2

  A similar bond strength was obtained using an adhesive of the following composition:% by weight Phosphoric ester (as described in Example 1) 9.48 Vinyl urethane resin * 15.17

TEGDM 70.16

  Camphorquinone 0.71 Dibutyltin dibutyltin 0.48 "Aerosil" A130 4.00 * As described in Example 1 of the European patent

No. 59,649.

  The vinyl urethane resin and the phosphoric ester are incorporated under. form of solutions in the TEGDM, the amount of TEGDM thus added being included

in total TEGDM content.

Claims (10)

  1. A one-component, visible-light-curable liquid adhesive composition, characterized in that it comprises: (a) 2 to 20 parts by weight of at least one substantially pure phosphoric ester having the formula CH 2 = C ( R1) .CO.O.R2.0P (O) (OH) 2 wherein R1 is an atom   of hydrogen or a methyl group and R2 is a group -CH2-   CH 2 -, - CH 2 -CH (CH 3) - or -CH (CH 3) -CH 2 - and (b) 98 to 80 parts by weight of at least one ethylenically unsaturated monomer copolymerizable with the phosphoric ester, and   (c) 1 to 9% by weight of a fine inorganic   that and an effective amount of a catalyst activated by the visible light.   The composition according to claim 1, wherein the phosphoric ester has the formula CH2 = C (CH2) .CO.O.R2.OP (O) (OH) 2, wherein R2 is a isopropylene group.   3. Composition according to one of the claims   1 and 2, characterized in that the concentration of the ester   phosphoric acid is 5 to 15 parts by weight.   4. Composition according to any one of   Claims 1 to 3, wherein the monomer to   Ethylene saturation comprises a glycol dimethacrylate. 5. Composition according to claim 4, characterized in that the monomer also comprises a vinylurethane. 6. Composition according to any one of   Claims 1 to 5, characterized in that the weight of   present charge is in the range of 4 to 6%.   7. Composition according to any one of   Claims 1 to 6, characterized in that the load consists of colloidal silica.   8. Composition according to any one of   Claims 1 to 7, characterized in that the catalyst   activated by visible light includes camphorquinone and a reducing agent.   9. A composition according to claim 8, characterized in that the reducing agent is an organic tin compound of formula (R9) nSn (OR10) m in which n and m are integers having the value 1, 2 or 3 and the sum n + m is equal to 4, R9 is an alkyl or alkenyl group containing 1 to 18 carbon atoms   carbon and R10 represents R9 or a group R9CO-.   10. Composition according to Claim 8, characterized in that the reducing agent is an organic tin compound of formula ([R 9] 3 Sn) 2 in which R 9 is an alkyl or alkenyl group containing 1 to 18 carbon atoms.