CA1069239A

CA1069239A - Polymeric and/or monomeric dental composition comprising submicron inorganic filler

Info

Publication number: CA1069239A
Application number: CA218,407A
Authority: CA
Inventors: Roland Schaefer; Albert Gross; Peter Wollwage; Rudolf J. Michl
Original assignee: Perdent GmbH
Current assignee: Perdent GmbH
Priority date: 1974-01-23
Filing date: 1975-01-22
Publication date: 1980-01-01
Also published as: GB1488403A; ATA39175A; SE437120B; FR2272638A1; SE7500725L; SE7802777L; SE429609B; AU7741275A; JPS50124491A; AT338437B; JPS5936602B2; FR2272638B1

Abstract

A B S T R A C T
This invention relates to a dental material comprising a polymeris-able monomer and/or a polymer suitable for dental purposes and a microfine inorganic filler which is present in a proportion of from 10 to 90% by weight of the material and has a particle size in the range from 10 to 400 m µ. Themechanical properties and polishing qualities of the dental material of this invention are better than materials not having the microfine inorganic filler described in this invention.

Description

~69Z39 The invention relates to dental materials containing polymerisable monomers and/or polymers suitable for dental pur-poses, and finely-divided filler.
The term "dental material" includes, for example, fillings for cavities, fixing cements, sealing and protect-ive coatings, crown and bridge materials, and denture base materials and substances for manufacturing artificial teeth based on polymerisable monomers ~nd/or polymers.
Monomers or polymers suitable for dental purposes, include for example, polyvinyl chloride, polystyrene and copolymers thereof, polyamides, epoxy compounds, polyure-thanes and, more particularly, monomeric and polymeric acrylates and methacrylates ~cf. Ullmanns Enzyklop'adie der Technischen Chemiej Volume 5, 1954, pages 717-721).
Artificialteeth or parts of teeth are usually manufactured from polymethacrylates in the form of bead .
or splinter polymers, which can normally be processed ~ ;
togather with the corresponding monomers, by heating in moulds. Mixtures of monomeric and polymeric methacrylates are used, for example, as denture base material and crown `~

and bridge material, the monomer in the mixture being polymerised in the presence of the polymer. Monomers alone are normally used as fixing cements, sealing and protect-îve coatings, and, more recently, as filling materials.
The Eollowing description is applicable mainly, .1 . , .
but not exclusively to denture base material and filling : . .~., material, since special problems occur in these cases.
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il069;~39 ~hen the conventionally-used monomeric acrylic or metha-crylic acid esters are polymerised, considerable shrink-age occurs. For this reason, a paste consisting of bead polymer and monomer in the weight ratio of 2:1 was very early used in the manu~acture of denture base material.
Even so, the contraction is still 7~. In addition, there is a high linear thermal expansion coefficient of 81.0 x 10 /mm/mm C. By comparison, the coefficient for a natural tooth is only about l/8th thereof, i.e.
11.4 x 10-6 Use for tooth filling and similar purposes did ~-not become more widespread until long-chain monomers were used,Based on bisphenol A and glycidyl methacry-late, according to United States Specification No.

3 066 112. These monomers, somet~mes called Bis-GMA
for short, shrink to a lesser extent. In order to reduce the shrinkage further, these monomers are some-times mixed with about three parts by weight of inert inorganic fillers. Accordingly, they comprise about 25%
of a binder - a cold-polymerisable monomer mixture -and about 75% of inorganic fillers, preferably aluminium and silicon oxides, silicate glass or calcium carbon-ate on various forms, such as spheres and fibres. This reduces the aforementioned contraction on polymerisation to about 1%, which can be tolerated for all applications, and also reduces the coefficient of thermal expansion to about 20 - 30 x 10 6mm/ =C.
Instead of the aforementioned monomer Bis-GMA, the binder mixture can contain other derivatives of ~ . .

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bisphenol A or urethane addition derivatives, e.g.
from diisocyanates and hydroxy alkyl methacrylates.
Usually, short-chain methacrylic acid esters and/or known cross-linking agents such as t:riethylene glycol dimethacrylate are added in order to reduce the viscosity.
Fillings are usually prepared by mixing two pastes containing binders and fillers. The redox system ~e.g. peroxide-amine) used for polymerisation is distributed so that one paste contains the peroxide catalyst only and the other paste contains the amine initiator.
Before mixing, the inorganic filler may be silanised, i.e. coated with suitable unsaturated silane compounds, to improve the bond to the organic matrix.
These materials, which are known as "composite", are given a filler content appropriate for the particular application and are suitable for filling cavities in front ~ teeth, and as fixing cements, seals over fissures and means ; 20 for preventing caries, i.e. protective coatings for teeth; ~ `
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they are also used as crown and bridge materials. ~
It has been found that although these materials ;
have very good mechanical properties in general, they polish badly and also frequently have unsatisfactory transparency. Attempts have been made to improve the polishing qualities by using finely-divided inorganic `
fillers having a particle size of not more than about 30~u ~cf. German Offenlegungsschrift 2 126 419).
Unfortunately, the transparency decreases .~ ...... .

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:~0~9;~3~3 as the particle size is reduced. The surface is initially unhomogeneous and, after a short time, becomes rough owing to uneven wear and may therefore become discol-oured.
Even after the particle size of the inorganic filler particles has been reduced to a minimum of about 0.8 - 8 ~ and a maximum of about 3 - 20 ~ ~see German Offenlegungsschrift 2 312 258), the teeth fil-lings still have unsatisfactoly transparency and polishing properties.
It is also known from German Auslegeschrift 1 928 831 and German Offenlegungsschriften 2 126 419, 2 164 668 and 2 224 683 that silicon dioxide can be -;
- added having a particle size of less than 1 ~ in pro-portions of up to 8% of the total weight of the tooth filling material in addition to the normal inorganic fillers. The only purpose of adding silicon dioxide, -~
of this particle size is to thicken the monomer in order to prevent sedimentation of the rela*ively large particles of filler. German Offenlegungsschrift 2 164 668 states that particles smaller than 0.7 ~ must be removed by suitable methods, since otherwise there is an excessive -decrease in the transparency of the filling material.
Likwise, German Offenlegungsschrift 2 126 419 ' states that silicon dioxide having a particle of size of 50 - 2000 ~, used as a thickening agent, must be -;-added in only a very small porportion - i.e. about 5 - 8%
; by weight of the total material.
Possibly, the disadvantages mentioned in the aforementioned patent specifications when silicon :
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9;~9 dioxide is used having a particle size of less than 1 ~ are due to the fact that silicon dioxide is used together with larger quantities of a filler having a greater particle size.
It has now unexpectedly been found that ~he mechanical properties and the polishing qualities of dental materials can be improved, without impai~ing the transparency thereof, when an inorganic filler is used which has a particle size of less than ~00 m~.
This invention relates to a dental material comprising a poly-merisable monomer and/or a polymer suitable for dental purposes, selected from polyurethanes, monomeric and polymeric acrylates and methacrylates, and mixtures thereof, and a silanised microfine inorganic filler which is present in a proporation of from 20 to 80% by weight of the material and has a particle size in the range from 10 to 400 m~. Preferably, the microfine inorganic filler has a specific surface area of less than 200 m2/g.
Preferably, at least 50% of the microfine inorganic filler particles have a particle size in the range from 10 to 40 m~.
The present invention also provides artificial teeth, replacements for parts of teeth and outer layers for artificial teeth and replacements for parts of teeth which are manufactured from the material defined above.
The microfine inorganic filler thickens the polymerisable monomer of ~` the material and gives it thixotropic properties, so that the material can be re-liquefied simply by stirring.
Advantageously, when more highly viscous monomers or monomer-polymer mixtures are used, the microfine inorganic filler is added in smaller quantities within the aforementioned range, so that the mixture does not become too viscous~ Alternatively, the proportion of th0 microfine inorganic filler can be increased, even in the case of monomers or mixtures . .

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having a greater initial viscosity, if ~he filler selected is such that its specific surface area is less than 200 m2/g. Preferably, the specific surface area of the microfine filler is between 30 and 80 m2/g.
Advantageously, the-proportion of ~he microfine inorganic filler is in the r~nge from 20 to 80%, pre-ferably from 40 to 75~, by weight of the material. As already mentioned, the most favourable ranges depend inter alia on the specific surface area of the filler and on the viscosity of the polymerisable monomer and/or -polymer.
Preferably, the microfine inorganic filler is silicon dioxide and/or aluminium oxide. Alternatively, use can be made of glass such as borosilicate glass or lithium aluminium silicate glass or glass containing barium oxide or lanthanum oxide or similar fillers, provided that the particle size thereof is less than 400 m y. Mixtures of microfine fillers can ~ ~ -also be used; preferably the glass has a small thermal expansion coefficient and is added in a proportion of up to 25% by weight of the total filler content.
Preferably the microfine filler is silanised, e.g. by processing with trimethoxy-~-3-methacryloyl-oxypropyl)-silane. The silanisation process is usually performed with a silane containing polymerisable organic Y
groups. The polymerisable groups react with the po ~eris-able mo~omer; in the organic matrix producing a strong -bond bet:ween the organic components of the material and the inorganic filler. However, in contrast to the previously-used ,` - - ,' .-~ 7 ~' '-' ' .
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comparatively coarse fillers, it is not absolutely necessary to silanise the microfine inorganic fillers.
The colour of the dental material according to the invention can be assimilated to that of natural teeth by adding organic or inorganic pigments and/or opacifiers. The particle size of such additives should not be more than 400 m ~.
The invention also embraces artificial teeth and replacements,for parts of teeth, e.g. tooth fillings, crowns, bridges ~more particularly the outer l~ayer thereof), veneers and similar replacements, manufactured from a dental material according to the invention.
Owing to the presence of the microfine inorganic filler, the dental materials can be used to manufacture products having unusually high compressive strength, excel-lent transparency and a very smooth homogeneous surface.
Even the naked eye can observe the difference in surface . . .
quality between a commercial dental material, e.g. a filling material, and a filling material containing the ~icrof~ne inorganic filler. Under the microscope, the difference .
` is even more visible. No individual particles are .
observable in the filling material containing the micro-fine filler ~the surface appears as a homogeneous unit) whereas in commercial filling materials the individual particles are visible in the form of splinters or spheres.
The microfine filler helps to eliminate the previous con-flict between the requirements for high transparency and for good polishing qualities. In addition, dental mat-erials according to the invention are opalescent. This considerably improves the cosmetic effect, since the ,~ ~ ',`"' '., ' .

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~)6~39 resin layer is yellowish in transmitted light and bluish-white in inciden~ light, in the same manner as natural teeth. Consequently, the materials according to the invention are particularly suitable as front-tooth fillings.
Good results have been obtained more particularly with the following polymerisable monomeric binders:
mono-, di- and higher esters of methacrylic acid, more particularly Bis-GMA, if necessary with an admix-ture of diluting monomers such as methyl methacrylate.
Other examples of monomeric binders are bis-[4-~2-hydroxy-3-methacryloyloxypropoxy)-phenyl] -dimethyl methane, 2,2-bis-~4-~2-hydroxy ethoxy)-phenyl]-propane dimethacrylate and triethylene glycol dimethacrylate. ~"~
Alternatively, use-can-be made of other derivatives of bisphenol-A or of the reaction products of hydroxy alkyl methacrylates and isocyànates. These monomers mostly have relatively high viscosity, which is usually lowered by adding short-chain monomeric methacrylic acid esters. Difunctional es*ers of acrylic or meth-acrylic acid can be added as cross-linking agents. ~ -The inorganic filler is the microfine filler.
Polymerisation catalysts can be added, e.g.
organic peroxides such as dibenzoyl peroxide, tertiary butyl peroctoate or azo compounds such as 2,2 azo-bis isobutyroni-trile ~A[BN). Use can also be made of redox systems which are conventional for dental plastics, e.g. di-benzoyl peroxideldimethyl-~-toluidine or dibenzoyl peroxide/trimethyl barbituric acid. It is possible -~
to prepare a "composite" material for fillings and : :'' .'';

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::'~,' ~692~9 other purposes, comprising two separately-stored components A and B, preferably in paste form. Both contain an organic binder system and a filler, and in addition one contains the catalyst and the other contains the activator. The proportion of inorganic filler to be added can vary, depending on the application. It can be, for example, 60 - 65% for a composite material used for filling purposes, whereas the organic matrix comprises 20 - 22% bis-GMA and 15-18% ethylene glycol dimethacrylate.
The organic binder mixture may be thoroughly mixed with the microfine inorganic filler until a paste is produced. Subsequently, 0.5 - 2% benzoyl peroxide may be added to the first component and 0.5 - 1% dimethyl paratoluidine added to the second component.
A test-piece may be manufactured by taking approximately equal quantities of paste A and B and mixing them on a mixing block; a metal spatula can be used, in contrast to when normal filling materials are used. The working time of the material is about 2 minutes;
after 5 minutes the mixture has polymerised into a solid specimen. The measured compressive strength, ` depending on the proportion of microfine filler used and the variation in the organic matrix, is between 4000 and 6000 kg/cm2 which is even better than for amalgam.
The transverse strength reaches values bet~een 11 and 16 kg/mm2, and the water adsorption after 2 months is between 0.5 and 1.5%. The surface quality of a commercial filling material and of a material manufactured '':, .' :,' , ': :~ '':
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in the aforementioned manner were compared under a microscope after both surfaces had been polished to a high lustre by conventional methods. It was found that the filling material containing the microfine filler has a homogeneous surface completely free from pores, whereas in the case of the commercial filling material, individual splinters or spherical particles were visible, embedded in the matrix. Another unexpected effect is that the microfine filling material is opal-escent, i.e. yellowish in transmitted light and bluish-white in incident light, and thus has optical properties closely resembling natural enamel, which is particularly desirable for front-tooth fillings.
The microfine filler can also be used to manu-facture a substantially improved material for crowns, inlays and bridges. This can be done, for example, by dissolving a crystalline dimethacrylate of a modified bisphenol-A in an organic solvent such as chloroform or ether and adding the~microfine filler ,: :
to the solution with continuous agitation. A paste is obtained and is kneaded until the solvent has completely evaporated. The powder is crushed in a ball mill and a catalyst, e.g. benzoyl peroxide, is -added. The mixture is screened, yielding a material suitable for manufacturing crowns and bridges. ~
A crown, for example, is modelled as follows: ; ;
Powder produced in the aforementioned manner is heated ` above the melting-point of the crystalline monomer in -~
a porcelain dish, until a thinly-liquid slurry is ~
obtained which can be applied with a brush or spatula. -~ , ....
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1069~39 The slurry is applied in layers to an isolated model stump and polymerised one layer at a time, e.g. in a stream of hot air, thus obtaining a crown having an attractive transparency and extremely high resistance to abrasion. Alternatively, the crown can be formed in conventional manner in layers in a dental flask, using a polymer/monomer paste to which a suitable quantity of microfine filler has been added, and can then be polymerised by heating in a water bath.
Alternatively, the microfine filler can be mechanically mixed with a polymethyl methacrylate in bead form and monomer can be added to form a paste which is polymerised at elevated temperature, e.g. 100C, under pressure in a metal mould, to form an artificial tooth or veneer.
A tooth manufactuTed in this way had very good opalescen~e, excellent physical properties such as :
compressive and transverse strength, and was cLearly superior to traditional materials.
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The microfine filler can be ~ery uniformly dis-tributed in the polymer by forming the monomeric methyl methacrylate and the microfine filler into a paste ~ ~:
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which is polymerised under pressure and at elevated temperatuTe to form a block, which is then ground ~ -into a sp:Linter polymer. The resulting polymer is mixed with filler and can be used in a conventional manner : ..', ' .
for manufacturing artificial teeth or parts of teeth, or as a base material for dentures.
In general, the microfine filler can be used to -make improved dental materials which have much better ' ,'~ , '', compressive strength, good polishing properties, excellent transparency and low abrasion, and which are opalescent, thus closely resembling natural enamel.
The dental material according to ~he invention has given good results in the manufacture of tooth-filling material and is also suited for manu-facturing crowns, bridges, prefabricated veneers or artificial teeth; in the case of crowns, bridges or artificial teeth, at least the outer layer is made of dental material according to the invention.
The following Examples illustrate the invention.
Example 1 61.5 g of silicon dioxide (average particle size between 10 and 20 m ~; specific surface area 50 m2/g) -was silanised in conventional manner with methacryl-oxypropyl trimethyl silane and poured into a laboratory kneading machine. 22 g of Bis-GMA and 16.5 g of ethylene glycol dimethacrylate were added and the mixture was kneaded into a homogeneous paste free from specks.
0.6 g of 50% benzoyl peroxide was added to 40 g of the paste (paste A). 0.1 g of dimethyl paratoluidine was added to another 40 g of the original paste (paste B).
If equal quantities of paste A and paste B are mixed on a mixing block, a material is obtained suitable for filling tooth cavities. The working time is 2 minutes, and the material is hard after 5 minutes. A specimen was prepared in the aforementioned manner and stored in water at 37C for 24 hours.
The specimen and a sample for comparison, .. ..
prepared from commercial composite material for dental -:, : ' ~` . .

~:1169;~9 fillings and containing about 75% silicon dioxide having an averag~ particle size of 30 y, was polished with a rubber polisher for 5 m:inutes. The surfaces of both test pieces were examined under-a microscope.
A photomicrograph of the mater:ial containing microfine silicon dioxide showed a homogeneous, uniformly smooth surface without pores, whereas in the case of the commercial material, the individual particles were clearly visible, distributed in the matrix.
The following tables show some comparative results:

Filling material `
Commerical according filling to the material invention . . .
Compressive strength after 24 hours in water, 37CCkgtcm2) 2700 4700 Transverse strength ~g~mm2~ 11.0 11.7 ~ater absorption after 1 month (%) 1.0 1.4 ~ ~-Transparency after 24 hours in water at 37C ~) 32 80 ;
Example 2 .
20 g of 2,2 bis 4-~2-hydroxy-ethoxy)-phenyl -propane dimethacrylate was dissolved in 50 g chloroform. -21 g of silanised silicon dioxide having an ~average particle size of 30 m lu and a specific surface area o~ `~
less than 80 m2/g was added to the solution. The resulting paste was dried with continuous agitation until the crystalline monomer had~;re-solidified and the solvent had evaporated. The silicon dioxide was thus distributed in the monomer in a completely homogeneous .

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manner. 0.5% ben~oyl peroxide was distributed in the powder by grinding it in a ball mill and subse-quently scree~ing it. This mixture is stable provided it is not heated above 42C. It was processed by melting the powder in a porcelain dish at 50 - 60C
and applying it in layers, using a brush or spatula, to an isolated model stump, each layer being polymerised in a stream of hot air at approximately 150C. The resulting crown was compared with a crown made from a commercial material.
The cro~n containing the microfine filler was opalescent, i.e. it appeared bluish-white in incident light, and therefore had very nearly the same cosmetic effect as natural ena~el. Tests on the two crowns, by brushing them with precipitated chalk using tooth-brushes, showed that the polymer containing microfine silicon dioxide was much more abrasion-resistant. The difference is illustrated by the follo~ing few comp-arative results.

Cro~n Crown containing manufactured microfine from a silicon commercial dioxide material Compressive strength kg/cm2 3300 1360 Transverse strength kg/mm2 11.5 6.0 Ball-~ressure hardness kg/cm 2320 1600 ~ater:abso~tion a$ter 1 month ~%~ Q.8 Example 3 lQQ g of the same microfine filler used as in ';
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9Z3~3 Example 1 ~except that it hat not been silanised) was mixed with 30 g of non-coloured polymethyl methacrylate in the fo~m of a bead polymer, and 2 g of 50% benzoyl peroxide. A monomer mixture was prepared, comprising 35% monomeric methyl methacrylate and 35 g of a reaction product of hydroxyethyl dimethacrylate and hexamethylene diisocyanate The powder and the liquid were mechanically mixed in a vibrating mixer in a sealed container until a viscous paste was produced.
The paste was poured into a tooth mould and polymerised at 110C for 4 minutes. The resulting artificial tooth -had marked opalescence, i.e. appeared yellowish in ~ -~
transmitted light and blue~white and transparent in incident light. The ball pressure hardness was 2800 kg/cm2 compared with 1400 kg/cm2 for comparable teeth prepared on a conventional methacrylate basis. The artificial tooth containing the microfine filler was appreciably more resistant to monomers, chloroform and ~ -` boiling water.
Embodiments of the invention are shown in the ~ ;
accompanying drawings, in which~
Figure 1 is a view of an artificial tooth;
Figure 2 is a cross section along the plane A~A
in Figure 1. As Figure 2 shows, the tooth has a plastics core 1 not containing a filler and an outer layer 2 mad~ from a dental material according to the ` invention.
~ Figure 3 shows a veneer made from dental material `~ according to the invention.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A dental material comprising a polymerisable monomer and/or a polymer suitable for dental purposes, selected from polyurethanes, monomeric and polymeric acrylates and methacrylates, and mixtures thereof, and a silanised microfine inorganic filler which is present in a proporation of from 20 to 80% by weight of the material and has a particle size in the range from 10 to 400 mµ.

2. A material according to claim 1 in which at least 50% of the microfine inorganic filler particles have a particle size in the range from 10 to 40 mµ.

3. A material according to claim 1 in which the proportion of the microfine inorganic filler is in the range from 40 to 75% by weight of the material.

4. A material according to claim 1 in which the microfine inorganic filler comprises silicon dioxide, aluminiumoxide, a glass or a mixture thereof.

5. A material according to claim 4 in which the microfine inorganic filler comprises borosilicate glass, lithium aluminium glass or a glass containing barium oxide or lanthanum oxide.

6. A material according to claim 4 in which microfine glass in a proportion of up to 25% by weight of the total filler content is present in addition to microfine silicon dioxide filler.

7. A material according to claim 1 in which the microfine inorganic filler has a specific surface area of less than 200 m2/g.

8. A material according to claim 7 in which the microfine filler is silanised with trimethoxy-(3-methacryloyloxypropyl)-silane.

9. A material according to Claim 1 which comprises a methacrylate ester of a mono- or polyhydroxy organic compound as a polymerisable monomeric binder.

10. A material according to Claim 9 in which the said ester is bis- [4-(2-hydroxy-3-methacryloyloxypropoxy)-pheny] -dimethyl methane (Bis-GMA) or 2,2-bis [4-(2-hydroxyethoxy)-phenyl] propane dimethacrylate.

11. A material according to Claim 1 which comprises a reaction product of a hydroxy alkyl methacrylate and an iso-cyanate as a polymerisable monomeric binder.

12. A material according to Claim 1 which includes a pigment and/or an opacifier having a particle size of not more than 400 m,µ.

13. Artificial teeth manufactured from a dental material according to Claim 1.

14. Replacements for parts of teeth manufactured from a dental material according to Claim 1.

15. Artificial teeth and replacements of parts of teeth having outer layers manufactured from a dental material according to Claim 1.