SPECIFICATIONSoft liningsThis invention reiates to moulding compositions; to elastomeric compositions prepared therefrom; and to the preparation thereof. More particularly, the present invention relates to elastomeric compositions suitable for use as so-called "soft" liners for dentures, especially dentures formed of hard acrylic resins' as maxillo-facial and hearing aid ear mould exoprostheses; and as plastic surgery endoprostheses.
The manufacture of articles such as dentures, formed of acrylic polymers by dough moulding techniques, is well established. In dough moulding, a particulate or powdered solid polymer is mixed with a liquid monomer to form a paste or dough which is subsequently homo- or co-polymerized in moulds, formed for example of plaster of Paris, by the action of a polymerisation catalyst, for example a free radical catalyst such as benzoyl peroxide, optionally under the influence of heat or a polymerisation activator. Thus, for example by utilising this technique, articles such as dentures have been formed of acrylic polymers by forming a paste or dough from a mixture of a finely divided or powdered acrylic polymer (such as polymethyl methacrylate) and one or more polymerisable acrylic monomers (such as methyl methacrylate) and subsequently polymerising the dough.It has also been proposed to incorporate a plasticiser, for example a phthalate based plasticiser, in the dough which plasticiser serves to depress the glass transition temperature of the resultant polymer to give a soft and pliant material. This proposal has, however, the disadvantage that the plasticiser may be leached in a moist environment such as the mouth, and the product gradually loses its softness and pliancy. Furthermore, such materials usually have poor mechanical and elastic properties. [Bradent and Clarke, J. Dent Res, 57, 1525-1528 (1972)]. In order to improve the properties of such compositions it has been proposed (see British Patent Specification No. 983817) to use plasticisers containing ethylenically unsaturated groups so that they are copoiymerised into the dough but results have not been completely satisfactory.
British Patent Specification No. 867357 discloses a moulding composition of the kind used in dental prosthesis and for artifical tissue material for the human body, which comprises a polymerisable mixture of (i) a particulate copolymer comprising one or more C2 to C4 alkanol esters of methacrylic acid and one or more C2 to C,2 alkanol esters of acrylic acid and (ii) a monomeric polymerisable fluid containing a mixture of C2 to C1s alkanol esters of methacrylic and/or acrylic acid. While the definitionof (i) comprehends elastomeric materials none is, in fact, specifically disclosed therein. Moreoveracrylic elastomers do not have sufficiently good mechanical, thermal, elastic and chemical properties tomake them viable for the uses proposed herein.For example, their resilience is less than 1 5%, typically from 10% to only 5%.
According to one aspect of the present invention, there is provided a moulding composition which comprises:(i) an elastomer which has a resilience of at least 30%; and(ii) a liquid comprising at least one substituted or unsubstituted C, to C,8 alkanol ester of(meth)acrylic acid, a blend of (i) with (ii) being polymerisable to form an elastomer.
"Resilience" is defined as the ratio of the energy output after deformation to the energy input before deformation. It can be shown [R. S. Marvin, J. Eng. Chem., 44, p. 696 (1 952)] that the resilience, R, is related to the loss tangent, tan S, by the equation R=exp (7r tan S). In this specification tan S is determined at a frequency of 0.3 Hz (nom.) and at 250C.
Preferably, the resilience of (i) is from 35% to 80%, more preferably from 40% to 70%, especially from 45% to 60%. The Tg of (i) is desirably below --1 OOC, preferably below -200C, more especially below 500 C.
A wide variety of elastomers, either synthetic or natural rubbers, may be used as (i). A part, andpreferably at least a major amount (that is, at least 50%, and preferably at least 67% by weight), of (i) consists of polymerised residues of at least one bis-olefinically unsaturated unsubstituted or substituted hydrocarbon, such as an unsubstituted or halo-substituted poly(diene), for example polybutadiene. It is particularly preferred that (i) comprises at least one random or block elastomer of butadiene with at least one substituted, preferably halo-substituted, or unsubstituted styrene, vinyl toluene, or (meth) acrylonitrile; natural rubber; butyl rubber; or polybutadiene.
It has also been found that it is very suitable for at least a part of (i) to consist of ethylene/propylene rubber.
Elastomer (i) may also comprise a graft copolymer, blend or other mixture of an elastomer, for example as mentioned above, with a minor amount (that is, up to 50% and preferably no more than 33% by weight) of at least one substituted or unsubstituted C, to C15 alkanol ester of (meth)acrylic acid. The particle size of the elastomer (i) is preferably 1 to 500 microns, more preferably from 2.5 to 50 microns.
By "(meth)acrylic acid" is meant herein acrylic acid, methacrylic acid, or both.
The (meth)acrylic ester monomers in (ii) desirably should readily form a blend with (i) and suitably comprises at least one unsubstituted C5 to C15, preferably C5 to C alkanol ester of (meth)acrylic acid comprises 12' such as hexyl methacrylate, ethyl hexyl methacrylate, octyl methacrylate, nonyl methacrylate, lauryl methacrylate, and tridecyl methacrylate. Such unsubstituted monomers readily form blends with butadiene/styrene random or block rubbers and natural rubber. The monomers in (ii) may suitably comprise at least one substituted, desirably polar substituted, preferably alkoxy-, especially C, to C4 alkoxy-, substituted C5 to C,5 alkanol ester of (meth)acrylic acid such as 2-ethoxyethyl methacrylate which has high chemical reactivity. Such substituted monomers readily form blends with butadiene/acrylonitrile rubbers.Polymerised moulding compositions comprising 2-ethoxyethyl methacrylate tend, however, to be somewhat water soluble and to have a high water absorption. Such systems are not preferred for intra-oral or endoprosthetic use.
The weight ratio of particulate elastomer (i) to (meth)acrylic ester monomer in (ii) in the moulding composition may vary within wide limits, depending upon the nature of the elastomer and acrylic ester monomer. However, the ratio is suitably from 3:1 to 1:1 by weight, preferably from 3:1 to 2:1 by weight.
In order to improve the mechanical and elastic properties of the final product it is often desirable to include in (ii) a small amount, for example from 1 to 5% by weight, of a cross-linking agent, that is a compound containing two ethylenically unsaturated groups such as glycol dimethacrylate or glycol diacrylate. In general, the higher the content of cross-linking agent the harder the final product.
In accordance with this invention, (i) may be in solution or emulsion; alternatively, (i) may be a particulate solid, an embodiment particularly suited to dough moulding.
This invention also provides a process for the preparation of an elastomeric composition, which process comprises forming a blend, for example a dough moulding mixture, of (i) and (ii) as herein defined; and subsequently polymerising the blend so formed. This may advantageously be effected at a temperature not exceeding 1000C: for example in a water bath.
The moulding compositions of the invention are suitably polymerised by the action of a freeradical catalyst such as a peroxide, for example benzoyl peroxide, in which case heat will generally benecessary in order to effect polymerisation of the blend. The peroxide may be added to the composition as such, dampened with water, or masterbatched, for example with dicyclohexyl phthalate.
Alternatively, a high residual (3%) peroxide homo- or co-poly(meth)acrylate ester may be blended, in an amount of up to 50% by weight, with a rubber to form elastomer (i). In the alternative an activator, such as N,N-dimethyl p-toluidine may be added to the composition, already containing the free radical catalyst, in order to achieve polymerisation at room temperature. It has been found convenient to mix the catalyst, suitably in an amount from 1 to 4% by weight, preferably from 2 to 3% by weight, with the elastomer (i), and subsequently to mix this with the (meth)acrylic ester monomer (ii), optionally containing an activator, suitably in an amount from 1 to 3% by weight, preferably 2.5% by weight, to form the blend.
The components (i) and (ii) for preparing an elastomeric composition in accordance with the invention are suitably assembled as two-component packs, one part containing elastomer (i) and the other containing (meth)acrylic ester monomer (ii), and one or other containing a catalyst such as benzoyl peroxide.
It has been found that where the moulding compositions in accordance with the invention arepolymerised in contact with hard acrylic plastics, good bonding is achieved between the soft, pliantelastomer produced in accordance with the invention and the hard acrylic plastic, thereby rendering themoulding compositions of the invention particularly suitable for the production of soft linings for acrylicdentures. Further, since the compositions of the invention contain no added plasticiser, the curedcompositions prepared therefrom do not harden consequent on leaching of plasticiser.
The following Examples illustrate the invention; all percentages are by weight unless otherwisestated.
Example 1A powdered butadiene acrylonitrile copolymer (acrylonitrile content 34.5-37%, particle size 2.5microns) was thoroughly mixed with 1.5% benzoyl peroxide. This mixture was mixed (in a weight ratioof 2:1) with a monomer liquid comprising 99% of 2-ethoxyethylmethacrylate and 1% of ethylene glycoldimethacrylate, and found to form a workable dough. On placing the dough in a Plaster-of-Paris mould,and heating the mould in boiling water, a good elastomeric product was obtained, which had adynamic shear modulus at 0.25 Hz of 1.45 MN/m2 and a mechanical loss tangent of 0.467.
On repeating this procedure with the dough in contact with a hard acrylic resin, excellent bonding was obtained.
Example 2The procedure described in Example 1 was repeated except that the monomer liquid alsocontained 2+% of N,N-dimethyl p-toluidine. The dough was pressed out between glass plates; after 20 minutes the product had polymerised at room temperature to give a good elastomeric product.
Example 3Example 1 was repeated, replacing the butadiene-acrylonitrile polymer with a powdered butadiene/styrene copolymer containing 30% styrene and having a particle size of 200 ym. The resultant dough again gave a good heat-cured product, with good bonding to hard acrylic resins. The product had a dynamic shear modulus at 0.25 Hz of 1.05 MN/m2 and a mechanical loss tangent of 0.507.
Example 4The procedure of Examples 1,2 and 3 was repeated except that the benzoyl peroxide was omitted and 3% of cumene peroxide was mixed with the monomer liquid prior to forming the dough. In all cases a good cured elastomeric product was obtained.
Example 5A powdered butadiene acrylonitrile copolymer (Breon 1042 ex B.P.) was thoroughly mixed with benzoyl peroxide (AK20 Lucidol CH50, a 50/50 masterbatch of benzoyl peroxide with dicyclohexyl phthalate) in the amounts shown in Table 1. This mixture was then mixed (in a powder:liquid ratio shown in Table 1) with a monomer liquid comprising 95 vol.% of 2-ethoxyethyl methacrylate and 5 vol.% of ethylene glycol dimethacrylate to which N,N-dimethyl p-toluidine was added.
The room temperature curing of the dough so formed was studied by measuring the exotherm with a thermocouple and potentiometric recorder.
Table 1Powder/monomer % Benzoylratio (g/ml) peroxide {w/w} % DMPT AT(0C) tmax (mins) 1 g/1.0 ml 1 1.3 9.2 21.51 g/1.5 ml 1 1.3 7 231 g/1.5 ml 1.5 1.3 6.8 19 1g/1.0m1 1.5 1.7 9 12Where AT is the pour temperature rise and tmax is the time to reach this pour rise when mixed.
Ail give good elastomeric products.
Table 2 gives a comparison of products of the Example with conventional elastomers.
Table 2
Comparative Silicones Soft acrylics This Example Tearing energy (kJ.m-2) 0.3-2.8 2-20 35- 42 Peeling energy* (kJ.m-2) 1.0-6.0 10-30 1 60-200 *Energy to peel off PMM base.
Example 6A powdered butadiene-styrene block copolymer (BS.41 6 - Performance Polymers Ltd.,Swindon, U.K.) was thoroughly mixed with 1% benzoyl peroxide (2% Akzo Lucidol CH50 as in Example 5). This formed an easily workable dough with a monomer liquid comprising 95% v/v Tridecyl methacrylate (methacryl ester 13 - Roehm Chemicals) and 5% v/v ethylene glycol dimethacrylate.
Curing this in a gypsum mould by normal dental procedures for one hour at 1000C produced a good elastomeric product.
Six weeks in water at 370C produced no sensible change in viscoelastic properties as shown inTable 3.
Table 3 Treatment G(MN/m2) Tan a TOC Initial 1.51 0.11 23"C After 7 days in water at 370C 1.70 0.11 1 80C After3daysdryingat370C 1.69 0.11 220C After 6 weeks in water at 37 C 1.82 0.13 150CAfter 3 days drying at 370C 1.71 0.12 220CExample 7The same polymer as in Example 6, but with 22% w/w benzoyl peroxide, together with the same monomer with 23% N,N-dimethyl p-toluidine, gave a good elastomeric product in about 40 minutes at 200C; the T value was 4.90C attomax 39 minutes. Curing in a hydroflask in warm water at 400C reduced the curing time to 20 minutes.
Example 8The monomer liquid in Example 7 was replaced by one in which tridecyl methacrylate was replaced by 2-ethoxyethyl methacrylate; the resulting material cured in 12 minutes at 200C.
Example 9The block copolymer in Example 6 was replaced by a random copolymer (bis.1204 ofPerformance Polymers Ltd.). Good elastomeric properties were obtained, which remained unchanged after six months in water at 370C.
Table 4 Treatment G(MN/m2) Tan a TOC Initial 0.89 0.12 22.50CAfter 7 days in water at 370C 0.92 0.12 22.00C After3daysdryingat370C 0.87 0.13 23.00CAfter 4 weeks in water at 370C 0.97 0.13 20.00CAfter 5 days drying at 370C 0.99 0.15 1 8.00C After 24 weeks in water at 370C 0.89 0.14 27.0 C After 3 days drying at 370C 0.91 0.14 24.50CExample 10To avoid the direct use of benzoyl peroxide, the copolymers of Examples 6 and 9 were mixed with a butyl/ethyl methacrylate copolymer:: G(N/m2) Tan60% Butadiene styrene block copolymers+40% BM/EM copolymer 8.81 0.1950 Butadiene styrene block copolymers+50% BM/EM copolymer 9.2 0.2160% Butadiene styrene block copolymers+40% BM/EM copolymer 4.63 0.2050% Butadiene styrene block copolymers+50% BM/EM copolymer 2.68 0.43Examples 11 and 12 give two-component liquid formulations which, when simply blended together and cured in a hydroflask at 400 C, give good elastomeric mouldings.
Example 11 Component Component B BS416(seeEx.6): 25g 25gLucidol CH50 (see Ex. 5): 5 gMethacryl ester B1: 49 ml 49 mlMethyl methacrylate: 5.5 ml 5.5 mlEGDM2: 5.5 ml 5.5 ml DMPT3: 1.5 ml1Tridecylmethacrylate (ex Roehm Chemicals).
2Ethyleneglycol dimethylmethacrylate.
3N,N-dimethyl p-toluidine.
Example 12 Component Component BBS 416: 125g 5g Lucidol CH50: 31.5 g Methacryl ester B: 245 ml 245 mlMethyl methacrylate: 27.5 ml 27.5 mlEGDM: 27.5 ml 27.5 mlDMPT: 2.8 mlExample 13 Component Component BBS 416: 225g 25g Lucidol CH50: 31.5 g Methacryl ester B: 441 ml 49 mlMethyl methacrylate: 49.5 ml 5.5 mlEGMD: 49.5 ml 5.5 mlDMPT: 2.8 ml