IMPROVED METHACRYLATE POLYMERS AND COMPOSITIONS

Abstract: A terminally reactive diene polymer is dissolved in methylmethacrylate monomer and the solution is polymerised by an aqueous suspension technique to provide polymer beads. These may be used in an acrylic dough to form a moulded denture base of increased impact strength while retaining flexural modulus.

Description: IMPROVED METHACRYLATE POLYMERS AND COMPOSITIONS The invention relates to improved methacrylate polymers and moulding compositions thereof primarily for use by dental technicians in moulding denture bases. In making a moulded denture base, the dental technician will use a mould into which he will pack or pour a so-called acrylic doug consisting of a blend of finely divided preformed acrylic polymer together with methylmethacrylate monomer or methylmethacrylate containing a minor proportion of crosslinking monomer. It is known to enhance the impact strength of polymethylmethacrylate or of copolymers comprising a major proportion of units derived from methylmethacrylate, for use in denture bases, by the incorporation of rubbery polymers, see "Kirk-Othmer Encyclopaedia of Chemical Technology", volume 18 of the Third Edition, pages 443-78, and the book entitled "Toughene Plastics" by Bucknall, published 1977 by Applied Science Publishers, London, England. Elastomers derived from 1,3- ' conjugated dienes, especially 1,3-butadiene, may be used for this purpose. It has been found that such modified materials cause difficulties in the course of processing into denture bases. For example, the dough may not be prepared so readily as in the case of untoughened compositions, or the workability of the dough may be impaired, or the workability time may be reduced. Accordingly there is need for polymeric materials which can be used in the preparation of say moulded denture bases using established procedures to provide products having improved physical properties. The invention is based upon the realisation that by forming a diene polymer having terminal groups which can co-polymerise witl; methylmethacrylate the dental technician can use a dough moulding procedure to make a dental base of improved properties, in particular impact strength. In this description, the term "polymer" is to be construed to include the term "copolymer", and the term "polydiene" to include the term "diene copolymer". According to one aspect of the invention there is provided a method of making a poly(methylmethacrylate) powder, comprising, dissolving a diene polymer having terminal reactive groups in methylmethacrylate monomer and then carrying out polymerisation to form the polymer. Although different polymerisation techniques may be employed it is preferred to use aqueous suspension polymerisation. According to a more specific aspect of the invention there is provided a method of making a methylmethacrylate powder, comprising, (i) dissolving a diene polymer having acrylate or methacrylate end groups and a molecular weight of about 10,000 to about 60,000 in methylmethacrylate monomer in the presence of a polymerisation initiator therefor, e.g. benzoyl peroxide, (ii) forming a slurry of the mixture in water and polymerising to form polymer beads (iii) separating, cleaning and drying the beads to form a powder, and then, (iv) blending the formed poly (methylmethacrylate. with a polyacrylate powder e.g. a homopolymethylmethacrylate powder, and, optionally (v) mixing the powder with methylmethacrylate monomer to form a dough which is moulded into a predetermined shape and polymerised to form an article. The terminally reactive diene polymers preferably have an average molecular weight which is sufficiently low for the terminally o reactive diene polymer to dissolve readily in methylmethacrylate monomer to yield a solution of moderate viscosity. As a result, a subsequent aqueous suspension polymerisation will proceed without difficulty. Such a step would be inconvenient if the diene poϊy er was of high molecular weight. Preferably the average molecular weight is in within the range of about 10,000 to about 60,000 g/mole, most preferably about 25,000 to about 40,000. The diene polymers carrying the terminal reactive group may be prepared by anionic polymerisation utilising organo-lithium or organo-sodium initiators. The propagating species in this class of- polymerisation is the so-called "living" polymeric anion. It is possible to terminate the polymerisation, and at the* same time to introduce terminal groups such as acrylate terminal groups, by addition of suitable reagents capable of reaction with the polymeric carbanions. A general" review of the art of anionic polymerisation Is to be found in the book by Morton entitled "Anionic Polymerisation", published in 1983 by Academic Press Inc., U.S.A., Including descriptions of the preparation of block polymers, while descriptions of methods of preparation of terminally reactive polydienes are to be found in US patent specifications 3 862 101 (Example 4) and 3 862 102 (Example 11). Descriptions of the synthesis of terminally reactive diene block copolymers are to be found in for example US patent 3 862 267 and 3 842 146 (Example 59). Preferred 1,3-conjugated dienes which are amenable to anionic polymerisation to form elastomers are 1,3-butadiene, 1-methyl- l,3-butad!ene also known as * , piperylene, 2-methyl-l,3-butadiene also known as isoprene and 2,3-dimethyl-l,3-butadiene. Particularly preferred are butadiene and/or Isoprene to form the dental compositions of the invention. Preferred copolymers of dienes contain units derived from one or more vinyl aromatic monomers which are susceptible to copolymerisation with conjugated dienes by anionic polymerisation examples of which ar styrene, alpha-methyl styrene, nuclear substituted styrenes such as para-methyl styrene or mixed methyl styrenes, t-butyl styrene, vinyl naphthalene. Suitable terminal reactive groups may be introduced Into the polydiene according to known techniques by terminating the anionic polymerisation with reagents such as acryloyl or methacryloyl or clnnamoyl chloride, allyl or methallyl chloride, malelc anhydride, vinyl chloroacetate or vinyl 2-chloroethyl ether. For reasons of superior reactivity with methyl methacrylate under moderate conditions it is preferred that the terminal group be either acryloyl or methacryolyl. The diene polymers formed by anionic polymerisation may be either random or block copolymers. If the diene polymer also contains units derived from one of more vinyl aromatic monomers, together with units derived from a conjugated diene., the proportion of units of the former class will not normally exceed about 50% by. weight of the copolymer. It is surprising that terminally reactive diene polymers having comparatively low molecular weights are of relatively low viscosity and are also capable of enhancing the impact strength of polymethylmethacrylate. While we do not wish to be limited by this theory, we believe that these properties are because the terminal reactive groups of the diene polymer copolymerise with the methylmethacrylate, forming a polymer having a comblike structure, comprising a backbone of methylmethacrylate units with a plurality of branches derived from the diene polymer. If the diene polymer does not carry terminal reactive groups readily copolymerlsable with methylmethacrylate, a comparable enhancement of impact strength is not observed, as shown below. The polymerisation of methylmethacrylate, or of a mixture of monomers comprising a major proportion of methyl methacrylate, together with the terminally reactive diene polymer may be carried out in mass, in aqueous dispersion, or in solution in organic solvent, using an appropriate free radical initiator. It is preferred to use aqueous suspension polymerisation, since this method is highly convenient, the polymer formed consisting of fine particles which are readily separated by filtration or centrifugation. The proportion ' of diene polymer to methyl methacrylate in the mixture which is subjected to polymerisation to form one of the ingredients of compositions of the invention, designated ingredient A hereinafter, is normally in the range of about 5 to about 50 parts by weight of the total mixture. The proportion will vary according to the nature of the polymer and the degree of desired toughening. If the terminally reactive polydiene is a homopolymer, the proportion used in the polymerisation mixture may be in the lower part of the range indicated. If the polydiene is a copolymer with a vinyl aromatic monomer, the proportion will be in the upper part of the range, depending on the proportion of vinyl aromatic monomer units in the terminally reactive polydiene. The copolymer so formed is used to form a dough with methylmethacrylate monomer or methacrylate containing a polyfunctlonal methacrylate containing a polyfunctlonal methacrylate for crosslinking purposes as is well known In the art. This monomeric component is hereinafter designated ingredient B and the ratio of A : B will typically be In the range of 2 : 1 to 1 : 1 by weight. Ingredient B may optionally contain a polyfunctlonal monomer for the purpose of crosslinking such as ethylene glycol dimethacrylate, in the proportion of about 10. by weight. However, it is found that suitable denture base physical properties can be realised without such a crosslinking monomer and since these substances are expensive, the ability to omit them represents a further advantage of the invention. It will be understood that, in addition to ingredient A, other preformed polymers or copolymers, In finely-divided form, may also be employed with advantage in the dough. Where such polymers designated Ingredient C are present, then the ratio A/B/C in the dough may be typically 25/50/25 parts by weight, Ingredient C being in this case polymethylmethacrylate itself although higher levels of the preformed compounds may with advantage be used. The moulded compositions of the invention exhibit excellent physical properties, in particular Impact strength while retaining flexural modulus. It is an advantage of the compositions of the invention that these enhanced physical properties are obtained together with retention of good doughlng behaviour, and that there are no drawbacks in the suspension polymerisation step. In another aspect the Invention provides as a new composition of matter, each of the ingredients A and B alone or in combination, optionally with ingredient C, and also includes products made thorefrom. T'ιe invention has been described in relation to moulding compositions primarily for use in making moulded dental bases. The invention is not however limited to that purpose as mouldable or settable methylmethacrylate compositions are useful for other purposes e.g. as sealing or caulking compounds, car light covers, etc. The invention will now be illustrated by the following examples. Example I (preparation of a terminally reactive polydiene) A styrene butadiene diblock copolymer having a theoretical molecular weight of about 30,000 was prepared by anionic polymerisation using n-butyl lithium initiator in toluene as solvent. The proportion by weight of styrene units in the polymer was 30.. The reaction was terminated by addition of propylene oxide followed by methacryloyl chloride, each in the proportion of 2 moles per mole alkyl lithium. The polymer was precipitated from solution by addition of methanol, filtered and devolatilised. Example II (preparation of ingredient A by suspension polymerisation) The polydiene prepared according to Example 1 was dissolved in methylmethacrylate monomer to form solutions of the following composition: * Polydiene (parts by weight) (0) 15 20 30 Methylmethacrylate (parts by weight) (100) 85 80 70 * i.e. a control experiment Polymerisation was carried out by aqueous suspension polymerisation, as follows: To a suspension polymer reactor equipped with a heating/cooling jacket, stirrer and reflux condenser there was added de-ionised water 60kg poly inylpyrollidone 0.4kg and the mix was heated to 6θ°C with stirring. There was then added 30kg of the polydiene solution in solution in methyl methacrylate monomer and 0.4kg of benzoyl peroxide. The heating at 80°C was continued for 5 hours, the mix cooled and the bead polymer was recovered by filtration washing with water and drying. • , Example III (preparation of denture base from ingredient A and B) The polymer beads of Example 2 were each made into a dough in the usual way with methylmethacrylate monomer (B), the proportion of A to B was 50/50 by weight of A plus B. The doughs were processed to form test specimens appropriate to the tests, and the following data was obtained: Proportion of diene 15 20 30 copolymer in ingredient A, wt. % Young's modulus, G.Pa. 2.9 2.2 1.7 1.1 Impact Strength, J 0.027 0.065 0.128 0.271 * i.e. a control experiment In which poly (methyl methacrylate) was used,in the absence of any component A. Example IV (prepararatlon of denture base from ingredient A, B and C) Powders * prepared according to Example III, were mixed in the mould with preformed ' poly (methylmethacrylate) homopolymer (ingredient C) and properties were measured. The following data was obtained: Proportion 6 7-5 10 11 14 of diene copolymer in A plus C combined, wt. %. Young' s Modulus 2.36-2.54 2.31-2.46 2.25-2.35 2.3 approx 1.95-2.06 G.Pa. Impact 0.049- 0.051- 0.075- 0.070- 0.092- trength J 0.062 0.063 0.085 0.096 0.11 The glass transition temperature was over 100°C in each case. The dough comprising 10. polydiene in the combined weight of A plus C yielded a high impact dental moulding composition and was readily workable. o Example V Butadiene styrene copolymers with methacrylate end groups and formed from butadiene- and styrene in different proportions under, suitable conditions ' were each dissolved in methylmethacrylate together with benzoyl peroxide. The solution was slurried with water and heat in a known suspension polymerisation technique to form polymer beads. The beads were removed by centrifuging, washed, dried and sieved. The product was mixed with some homo (polymethylmethacrylate) to form a denture base powder. This was then mixed with methylmethacrylate monomer to form a dough which was moulded and heat polymerised. The products were tested for elastic modulus and impact, strength. The materials used and the results obtained are shown in the accompanying Table I. These show the effect of Increasing the butadiene/sty ene content. It will be observed from the results of Table I that by suitably adjusting the proportion of butadiene and styrene in the block copolymer one can obtain a permutation of valuable properties. Because sufficient block copolymer can be added to the monomer » without undue increase in monomer viscosity the customary manufacturing techniques involving bead polymers i.e. suspension polymerisation, can be used. Table 1 Butadiene styrene Modulus of Elasticity Hounsfield impact strength block copolymer (%) (GPa) notch test ft. lbs Joules 2.85 0.02 0.027 2.35-2.5 0.03 0.041 7.5 2.2-2.5 0.045 0.061 10.0 2.5 0.05 0.068 15 2.0 0.06 0.081 20 1.8 0.086 0.116 30 1.3 0.20 0.271

Claims: Claims 1. A method of making a pol (methylmethacrylate) powder by polymerising methylmethacrylate, characterised by dissolving a diene polymer having terminal reactive groups in methylmethacrylate monomer and then carrying out polymerisation to form the polymer. 2.' A method according to Claim 1 characterised in that the diene polymer has an average molecular weight within the range about 10,000 to about 60,000 g/mole, preferably about 25,000 to about 40,000. 3. A method according to Claim 1 or 2 characterised in that the diene polymer is prepared by anionic polymerisation. 4. A method according to Claim 3 characterised in that 1,3- conjugated dienes used in the anionic polymerisation to form the diene polymers are 1,3-butadiene and 2-methyl-l,3-butadiene. 5. A method according to any of Claims 1 to 4 characterised IT in that polymerisation of methylmethacrylate or of a mixture of monomers comprising a major proportion of methylmethacrylate and the terminally reactive diene polymer is carried out in mass, in aqueous dispersion, or In solution in organic solvent, using an appropriate free radical initiator. 6. A method according to Claim 5 characterised in that the proportion of diene polymer to methylmethacrylate in the polymerisation is in the range of about 5 to about 50 parts by weight of the total mixture. 7. A method according to Claim 6 characterised in that the * copolymer Is reacted with monomer or monomers to form a dough, the ratio of the product to the monomers being about 5 to about 50/about 95 to about 50 parts by weight respectively. 8. A method according to Claim 5, 6 or 7 characterised in that other preformed polymers or copolymers in finely-divided form are present and the ratio of the ingredients is about 25/50/25 parts by weight. 9. A method according to any preceding Claim characterised _____ (i) dissolving a diene polymer having acrylate or. methacrylate end 'groups and a molecular weight of 18 about 10,000 to about 60,000 in methylmethacrylate monomer in the presence of a polymerisation Initiator therefor, e.g. benzoyl peroxide, (ii) forming a slurry of the mixture in water and polymerising to form polymer beads (iii) separating, cleaning and drying the * beads to form a powder, and then, (iv) blending the formed polymethylmethacrylate with a polyacrylate powder, and, optionally (v) mixing the powder formed with methylmethacrylate monomer to form a dough and then moulding the dough into a predetermined shape and polymerising the dough to form an article. 10. A powder characterised in that the powder comprises a polymer bead formed of a polymer comprising a poly (methacrylate) backbone with diene polymer side chains, the beads being in admixture with a poly acrylate. 11. An. acrylic dough comprising an acrylic polymer and ' methymethacrylate monomer or methylmethacrylate and a crosslinking member characterised in that the acrylic polymer comprises a powder according to Claim 10. 12. A moulded product characterised in that it comprises a dcugh according to Claim 11 which has been placed in a mould and then polymerised.

Inventors: BRADEN MICHAEL, LUXTON ALAN, RODFORD ROSEMARY, WOOD LAURENCE GORDON, BRADEN, MICHAEL, LUXTON, ALAN, RODFORD, ROSEMARY, WOOD, LAURENCE, GORDON
Applicants: BONAR COLE POLYMERS LTD, REVERTEX LTD, BONAR COLE POLYMERS LIMITED, REVERTEX LIMITED
Application year: 1986
Main classification: C08F293
IPC codes: A61K6, C08F287