GB1584530A - Moulded dental fitments - Google Patents

Description

(54) MOULDED DENTAL FITMENTS (71) We, BAYER AKTIENGESELLSCHAFT, a body corporate organised under the laws of the Federal Republic of Germany, of Leverkusen Bayerwerk, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement': - The invention relates to moulded dental fitments, such as dentures, crowns or bridges, with improved mechanical properties.

In most cases, dentures made of plastic are prepared by the powder-liquid process [Ger man Patent Specification 737,058].

In this prior art procedure, preferably a bead polymer based on polymethacrylates is processed, with methacrylates, such as, for example, methyl methacrylate, to form a paste by stirring 2 to 3.parts of powder with 1 part of liquid. A peroxide has been added to the monomer before preparing the paste, so that after the paste has been moulded it can be cured by heating, the monomer being polymerised.

Because the preparation process for dentures, crowns and bridges is easy to carry out, the -powder-liquid process has become the standard technique for preparing plastic dentures.

Furthermore, it is known to improve the processability of dental beads in the powder-liquid process by using polymethyl methacrylate powder or, preferably, polymethyl methacrylate beads of a definite particle size, and it is also known to improve the processing spectrum of dental beads by using beads consisting of copolymers of methyl methacrylate with a major proportion of copolymerised methacrylic acid methyl ester as the powder; instead of polymethyl methacrylate beads. These variations make it possible to obtain the desired rapid processability together with the broad processing spectrum which is also desired.

A disadvantage of the dentures, crowns and bridges based on polymethyl methacrylates and prepared by the powder-liquid process is that the mechanical values of the raw material are not satisfactory for many structures. In particular, in many cases the toughness properties of the plastics under load are not sufficient for dentures, crowns and bridges. Improving the impact strength of the plastic would have the effect of lowering the tendency of the dentures to break and also therefore of making it possible to carry out the cleaning operation more reliably.

It has been found that moulded dental fitments prepared by the powder-liquid process, such as dentures, bridges, crowns and orthodontic appliances, based on polymethacrylates, have improved mechanical properties if polymethyl methacrylates which have been elasti casted with polyurethanes are used or co-used as the powder.

The same also applies to synthetic teeth prepared in this manner. Polymethyl methacrylates elasticated with polyurethanes are also a suitable component of materials for repairing dentures, bridges, crowns and orthodontic appliances.

Accordingly the present invention provides . a moulded dental fitment based on a polymethacrylate in which a methacrylate which is elasticated by a polyurethane is used. A "methacrylate which is elasticated by a polyurethane" means a heterogenous system, i.e. a system consisting of two or more phases, polyurethane being present as a phase with good elastic properties.

It is known to elasticate polymethyl methacrylate by polymerising the methyl methacrylate by the bulk polymerisation process, whilst simultaneously shaping. However, it was not to be expected that dentures with improved properties can be obtained if the powder-liquid process-is used and a polymethyl methacrylate which contains a polyurethane as an elastifying component is employed as the powder.

As' 'is generally known, dental plastics which are obtained by the powder-liquid process are characterised by a particular structure. A multi-phase system, which can be detected by special methods, exists in the cured plastic: only some of the original "liquid" has penetrated into the powder .particles during the initial swelling procedure. A large, if not predominant proportion of the liquid polymerises as a phase in itself and fills the intermediate spaces between-the swollen original powder particles. The structure of moulded fitments which consist of polymethacrylates or modified polymethyl methacrylates and which have been obtained by the powder-liquid process is thus substantially different to that of moulded fitments which consist of polymethyl methacrylates and which have been obtained by customary shaping processes.

It is indeed also known, from German Patent Specification 940,493, to improve the mechanical properties of moulded fitments consisting of methyl methacrylates, by using mixtures of different polymers or copolymers as the powder components. For example, copolymers consisting of 80% of methyl methacrylate and 20% of butadiene are used for improving the flexural endurance. However, because of the butadiene content, copolymers of this type have a poor fastness to light.

Furthermore, it is known, from German Patent Specification 940,493, to use post chlorinated polyvinyl chloride as an additive in order to improve the flexural impact strength and the flexural endurance of moulded fitments which are based on methyl methacrylate polymers and which have been obtained by the powder-liquid process. However, using post-chlorinated polyvinyl chlorides as an additive has the effect of lowering the resistance towards discolouration. Moreover, the stability of post-chlorinated polyvinyl chlorides is not sufficient when relatively active peroxides or relatively high polymerisation temperatures are used.

Moulded fitments for dental purposes, such as full and partial dentures, bridges, crowns or orthodontic appliances, based on organic plastics can be prepared by various procedures.

Thus, for example, it is possible to convert the plastic into the desired moulded fitment via an injection or extrusion process.

The dentures, bridges, crowns or teeth according to the invention are obtained by this process by shaping polyurethane-elasticated polymethacrylates, optionally mixed with cus tomary "injectable" polymethyl methacrylates, by means of an injection device or by means of an extrusion device.

However, a particularly versatile process for the preparation of dentures, crowns or bridges is the powder-liquid process. The moulded fitments according to the invention are obtained by.this process by using a polyurethane-elasticated polymethacrylate as the powder. These powders can be obtained by converting polyurethane-elasticated polymethacrylates into -so-called "acrylate chips" via a comminuting process. However, particularly good results are obtained when those polyurethane-elasticated polymethacrylate powders which have been prepared by the procedure of a bead polymerisation are used.

In addition to the better processability compared with the acrylate chips, the use according to the invention of the elasticated polymer beads additionally has the advantage that the elasticating component is better protected against degradation by components in the medium of the mouth and is generally protected against the action of components in the medium of the mouth. In the dental beads, the polyurethane present as a separate phase is enveloped by the base substance of the dental beads, that is to say the polymethacrylate, and is thus protected from such action. In addition, the dental beads are themselves also in turn embedded in a matrix of polymethacrylate and are thus protected.

A particular embodiment of the procedure according to the invention, for the preparation of dentures, crowns or bridges by the powder-/liquid process consists in setting up the desired processability and the required processing spectrum by using elasticated dental beads of a definite particle size, or by adjusting the initial swelling behaviour of the polymer beads by using comonomers in the bead polymerisation. However, it is very particularly advantageous to set up the characteristic quantities of processability and processing spectrum, which are particularly important for handling from the point of view of dentistry, by adding non- elasticated beads. It was surprising that the good elasticating activity of the dental beads is not lowered when the latter are used as a mixture with customary dental beads. The mixing ratios most favourable technologically must nevertheless be determined from case to case and depend on the construction and on the function of the denture or bridge.

Polymethacrylates in the sense of the present invention are understood as polymerisation products of methacrylic acid esters. In most cases, methacrylic acid methyl ester is the main component, but useful results are also obtained with polyfunctional esters of methacrylic acid, and for specific purposes, good results are given by, for example, bis-GMA or its modification products and also the comonomers mentioned in U.S. Patent 3,730,947.

Polyurethanes in the sense of the present invention are understood as reaction products of polyols and polyisocyanates. In particular those polyurethanes which are obtained from the diisocyanates below are of industrial interest: A) aliphatic diisocyanates having a branched carbon skeleton with 7 to 36 C atoms, for example 2,2,4- or 2,4,4-trimethyl-hexane -1,6-diisocyanate or industrial mixtures thereof, diisocyanates derived from esters of lysine or diisocyanates based on dimerised fatty acids, which are prepared in a known manner by conversion of dicarboxylic acids of this type with up to 36 C atoms into the corresponding diamines and subsequent phosgenation, B) cycloaliphatic diisocyanates, for example cyclobutane- 1,3-diisocyanate, cyclohexane-1,3- and - 1 ,4-diisocyanate, 2,4- or 2, 6-diisocyanato- 1 -methylcyclohexane or 4,4'-diisocyanatodicyclohexylmethane, either in the form of the pure geometric isomers or as industrial mixtures thereof, and furthermore l-isocyanato-3,3,5- trimethyl-5-isocyanato methylcyclohexane (isophorone diisocyanate), and finally C) aliphatic or cycloaliphatic diisocyanates which are modified by free radical graft copolymerisation with vinyl monomers and which are obtained by polymerising ia the presence of 100 parts of the diisocyanate of 10 to 100 parts, preferably methyl methacrylate, with the aid of a free radical polymerisation initiator, for example an organic peroxide, such as benzoyl peroxide and tert.-butyl peroctoate, or an aliphatic azo compound, such as azoisobutyronitrile. In addition to the diisocyanates already mentioned, aliphatic diisocyanates having a linear carbon chain, for example hexamethylene diisocyanate, are also suitable for use as the graft substrate. It has been shown that aliphatic diisocyanates modified in this manner lead to polyurethene-urea elastomers, which are soluble in monomeric methyl methacrylate to give a clear solution and give clear polymers when the refractive indices of the polymer phase and viscous phase are correctly matched.

Isophorone diisocyanate and hexamethylene diisocyanate or isophorone diisocyanate which have been modified by graft copolymerisation with methyl methacrylate and have a polymer content of up to 50%, preferably of up to 40%, are preferably used.

Suitable polyols which can be used, according to the present invention, for the preparation of the polyurethanes are longer-chain diols with 2 terminal hydroxyl groups. Polyesters, polyethers, polyacetals or polycarbonates with molecular weights of 400 to 6,000 and a glass transition temperature A20 C are preferably used.

Suitable polyesters containing hydroxyl groups are, for example, reaction products-of dihydric alcohols with dibasic carboxylic acids.

In the preparation of the polyurethanes to be used according to the invention, the hydroxyl component and the isocyanate component are not employed in equivalent amounts, but an excess of one or other component is used. In particular, in the prepolymer process, a polyurethane prepolymer which is free from OH groups and has NCO functional groups and which can still contain free diisocyanate is obtained in the first stage, and this is reacted in the second stage with the chain lengthener until the desired molecular weight is reached. A residue of free NCO groups usually remains in the product, and these are appropriately protected with the aid of a monofunctional chain stopper [component (C)]. Examples of suitable chain stoppers are lower aliphatic alcohols, such as methanol, ethanol, butanol or allyl alcohol.

Chain lengtheners for the polyurethanes to be used according to the invention suitable short-chain compounds with 2 hydroxyl compounds are, for example: ethylene glycol, propylene 1,2- and 1,3-glycol, butylene 1,4- 1,3- and 2,3-glycol, penta-1,5-diol, hexane-1,6diol, octane- octane-1,8-diol, neopentyl-glycol, 1 ,4-bis-hydroxymethyl-cyclohexarie, 2methyl-propane-1,3-diol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols with a molecular weight (400, dipropylene glycol, polypropylene glycols with a molecular weight 400,dibutylene glycol, polybutylene glycols with a molecular lar weight S400, 4,4'-dihydroxy-diphenylpropane or hydroquinone-bis-(2-hydroxyethyl ether).

For their processing by the powder-/liquid process in dentistry, the- polyurethaneelasticated polymethacrylates are mixed with a monomer to form a paste. Methyl methacrylate is preferably used as the monomer. Monomers which contain two or more double bonds in the molecule and which thus lead to crosslinking are added in order to increase the resistance to solvents and the abrasion resistance. The following compounds, for example, can be added as crosslinking agents in amounts of 0.1% by weight to 30% by weight, preferably 1% by weight to 15 % by weight: ethylene glycol dimethacrylate, triethylene glycol dimethacrylate butanediol dimethacrylate, tnmethyldlpropane trimethacrylate, bis-GMA, methylene-bis- acrylamide, triacrylformal and the bifunctional comonomers mentioned in U.S. Patent 3,730,947.

The resulting compositions consisting of bead polymer and monomer can be cured using initiator systems, based on peroxides or aliphatic azo compounds, which release free radicals.

Examples of suitable polymerisation initiators are diacyl peroxides, such as, -for example, dibenzoyl peroxide, or alkylacyl peroxides, such as, for example, tertiary butyl perpivalate, optionally in the presence of accelerators, such as aromatic tertiary amines, for example alkylated anilines, toluidines or xylidines. Cobalt salts or copper salts as well as compounds from the barbiturate group and sulphinic acids and sulphones can also be used as accelerators.

Whilst curing at elevated temperature can be carried out by means of peroxides alone, such as dibenzoyl peroxide, chloro-benzoyl peroxide, toluyl peroxide or lauryl peroxide, or by means of free radical initiators alone, such as, for example, azoisobutyric acid nitrile or azoisobutyric acid esters, it is necessary to add accelerators in the case of curing at low temperatures. 0.01% by weight to 2% by weight of polymerisation initiator are required in the case of curing at elevated temperature. 0.02% by weight to 5% by weight of polymerisa- tion initiators and 0.02%by weight to 5%by weight of accelerators are required in the case of curing at low temperatures.

Example 1 Dental beads are prepared, in the presence of a polyurethane, by a process for the bead polymerisation of methyl methacrylate.

MgCO3 was used as the dispersing agent in the bead polymerisation and a mixture of lauroyl peroxide and dicyclohexyl percarbonate in the ratio 1:1 was used as the peroxidic initiator in an amount of 0.73%, relative to methyl methacrylate used (% by weight). The methyl methacrylate contained 9.9% of dissolved polyurethane.

the polyurethane is a "diol"-lengthened polyester-polyurethane based on a mixture of two polyester-diols A and B.

Polyester-diol A consists of a polyester based on adipic acid, hexane- 1 ,6-diol and neopentylglycol, with a hydroxyl number of 66.

Polyester B is a polyester based on ethylene glycol, adipic acid and phthalic anhydride, with a hydroxyl number of 64.

Polyester A (0.35 equivalent) and polyester B (0.15 equivalent) are reacted with isophorone diisocyanate (0.75 equivalent), and the chain is lengthened to the extent of 85% by means of butane-1,4-diol and terminated by means of 2-hydroxy-ethyl methacrylate. The polyurethane formation is catalysed by tin dioctoate.

0.25% by weight of dibenzoyl peroxide are added to 15 parts by weight of the dental beads prepared in this manner and the mixture is made into a paste with 5.36 parts by weight of a liquid consisting of 94% by weight of methyl methacrylate and 6% by weight of ethylene glycol dimethacrylate. Sheets 2mm thick are pressed from this paste and polymerisation is then carried out.

The polymerisation is carried out as follows: the water bath is heated to 700C in the course of 30 minutes, the temperature is kept constant for 30 minutes, the bath is then heated to 1000C and this temperature is kept constant for a further 30 minutes. The cell is cooled in a water bath.

After removing from the cell, test pieces are cut out of the sheet without heating the sheet.

The test pieces thus obtained are subjected to the Dynstat test according to DIN 53,452.

Test results: (in each case the mean value from 5 test pieces) Impact strength 30.4 kp/cm2 Bending angle 12.6 Flexural strength 981 kp/cm2 Ball indentation hardness 10" 1,355 kp/cm2 60" 1,249kp/cm2 Dental beads elasticated by polyurethanes are also used in Examples 2, 3 and 4. These dental beads differ in that different polyurethanes are used as the elasticating agents in the bead polymerisation.

Example 2 The dental beads contain a polyurethane which was prepared using 1 equivalent of,, isophorone diisocyanate instead of 0.75 equivalent of isophorone diisocyanate. For lengthening, the chain was lengthened to the extent of 90% using butanediol.

0.5% by weight of lauroyl peroxide was added to the beads obtained in this manner, the mixture was polymerised with a liquid consisting of 97% by weight of methyl methacrylate and 3% by weight of triethylene glycol dimethacrylate and the polymer was subjected to the strength test according to DIN 53,452: Impact strength 32.0 kp/cm2 Bending angle 23.4 Flexural strength 1,315 kp/cm2 Ball indentation hardness 10" 1,249 kp/cm2 60" 1,137 kp/cm2 Example 3 The dental beads used are also obtained as described in Example 1; a polyesterpolyurethane which is prepared using 1.25 equivalents of isophorone diisocyanate and the chain of which is lengthened to the extent of 90% using butane-1,4-diol is employed as the elasticating polyurethane.

0. % by weight of dichlorodibenzoyl peroxide was added to the beads obtained in this manner, the mixture was polymerised with a liquid consisting of 90% by weight of methyl methacrylate and 10% by weight of trimethylolpropane trimethacrylate and the polymer was subjected to the strength test according to DIN 53,452: Impact strength 49.1 kp/cm2 Bending angle 16.6 Flexural strength 1,086 kp/cm2 Ball indentation hardness 10" 1,360kp/cm2 60" 1,252kp/cm2 Example 4 The dental beads used are elasticated with a polyurethane which has been prepared using 1.5 equivalents of isophorone diisocyanate and the chain of which has been lengthened to the extent of 90% using butane-1,4-diol.

1% by weight of Ditoluyl peroxide was added to the bead polymers obtained in this manner, the mixture was polymerised with a liquid consisting of 88% by weight of methyl methacrylate and 12% by weight of butanediol dimethacrylate and the polymer was subjected to the strength test according to DIN 53,452: Impact strength 27.3 kp/cm2 Bending angle 16.8 Flexural strength ,1,267kp/cm2 Ball indentation hardness 10" 1,517kp/cm2 60" 1,385 kp/cm2 Example 5 1% by weight of bis-4-chloro-benzoyl peroxide is added to 4 parts by weight of the dental beads prepared according to Example 1 and the mixture is made into a paste with 3 parts by weight of a liquid consisting of 94% by weight of methyl methacrylate, 6% by weight of ethylene glycol dimethacrylate and 0.7 % by weight of N,N'-dimethyl-p-toluidine. A pourable consistency is obtained with this mixing ratio. A kneadable consistency is obtained with a mixing ratio of 4.7 parts by weight of powder and 2 parts by weight of liquid. The polymerisation has ended after 16-17 minutes at 23"C.

The test pieces described in Example l are subjected to the Dynstat test according to DIN 53.452. Test results: (in each case the mean value from 5 test pieces) Impact strength 27.4 kp/cm2 Flexural strength 1,005 kp/cm2 Bending angle 28.8 Ball indentation hardness 10" 1,327kp/cm2 60" 1,137kp/cm2 Comparison As a control experiment, customary methyl methacrylate beads containing 0.25% by weight of dibenzoyl peroxide are polymerised with a liquid consisting of 94% of methyl methacrylate and 6% by weight of ethylene glycol dimethacrylate and the polymer is subjected to the strength test according to DIN 53,452: Impact strength 19.4 kp/cm2 Bending angle 18 Flexural strength 1,059 kp/cm2 Bdl indentation hardness 10" 1,249 kp/cm2 60" 1,158kp/cm2 WHAT WE CLAIM IS: 1. A moulded .dental fitment based on a polymethacrylate in which a methacrylate which is elasticated by a polyurethane (as hereinbefore defined) is used.

2. A moulded dental fitment according to claim 1 which is in the form of one or more synthetic teeth.

3. A moulded dental fitment according to claim 1 which is in the form of a full or partial denture, a bridge or a crown.

4. A moulded dental fitment according to claim 1 substantially as hereinbefore described.

5. A process for the production of a moulded dental fitment as claimed in claim 1 by the powder-liquid process (as hereinbefore defined) in which a fine-particled polymethyl methacrylate which is elasticated with a polyurethane is used as the powder.

6. A process according to claim 5 in which a polyurethane-elasticated fine-particled polymethacrylate which is obtained in the form of polymer beads by the bead polymerisation procedure is used as the powder.

7. A process according to claim 5. in which a polyurethane-elasticated polymethyl methacrylate which has been obtained in the form of acrylate chips (as hereinbefore defined) via a grinding process is used as the powder.

8. A process according to claim 6 or 7. in which a non-elasticated methacrylic acid methyl ester polymer is admixed to the polyurethane-elasticated polymethyl methacrylate in order to set up the desired processability and processing spectrum.

9. A process according to claim 5 in which a polyurethane-elasticated polymethyl methacrylate which has been obtained by the process of anyone of Examples 1 to 5 is used as the powder.

10. A moulded dental fitment when obtained by the process of any of claims 5 to 9.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. Impact strength 19.4 kp/cm2 Bending angle 18 Flexural strength 1,059 kp/cm2 Bdl indentation hardness 10" 1,249 kp/cm2 60" 1,158kp/cm2 WHAT WE CLAIM IS:

1. A moulded .dental fitment based on a polymethacrylate in which a methacrylate which is elasticated by a polyurethane (as hereinbefore defined) is used.

2. A moulded dental fitment according to claim 1 which is in the form of one or more synthetic teeth.

3. A moulded dental fitment according to claim 1 which is in the form of a full or partial denture, a bridge or a crown.

4. A moulded dental fitment according to claim 1 substantially as hereinbefore described.

5. A process for the production of a moulded dental fitment as claimed in claim 1 by the powder-liquid process (as hereinbefore defined) in which a fine-particled polymethyl methacrylate which is elasticated with a polyurethane is used as the powder.

6. A process according to claim 5 in which a polyurethane-elasticated fine-particled polymethacrylate which is obtained in the form of polymer beads by the bead polymerisation procedure is used as the powder.

7. A process according to claim 5. in which a polyurethane-elasticated polymethyl methacrylate which has been obtained in the form of acrylate chips (as hereinbefore defined) via a grinding process is used as the powder.

8. A process according to claim 6 or 7. in which a non-elasticated methacrylic acid methyl ester polymer is admixed to the polyurethane-elasticated polymethyl methacrylate in order to set up the desired processability and processing spectrum.

9. A process according to claim 5 in which a polyurethane-elasticated polymethyl methacrylate which has been obtained by the process of anyone of Examples 1 to 5 is used as the powder.

10. A moulded dental fitment when obtained by the process of any of claims 5 to 9.