CA1041910A - Dental filling method and composition formed thereby - Google Patents

Abstract

DENTAL FILLING METHOD AND COMPOSITION FORMED THEREBY

Abstract of the Disclosure A novel dental filling system is described which permits the curing of a dental filling composition while excluding oxygen. The curable filling composition comprises (1) a free-radical polymerizable acrylate monomer, (2) a free-radical polymerization initiator and (3) an inorganic filler.
This composition is used to fill a dental cavity and the exposed surface has applied thereto an aqueous solution of waterglass, which dries to a solid, essentially oxygen-impermeable coating of waterglass. After the curable com-position has cured, the waterglass coating is washed off with water.

Description

V

BiC~GROUND ~F TÇIE INVENTION
This invention relates to fillings for dental cavities.
Canadian patent 1,020,687 issued November 8, 1977, by the inventors hereof relates to an improved dental filling com-position comprising certain monomeric compositions which are described in detail herein. Briefly, these compositions comprise polymerizable urethane-acrylate resins which cure by a free-radical mechanism, along with a free-radical initiator. While these compositions do function well for their intended purpose, because of the polymerization in-hibiting effect of oxygen on compositions of this nature, the surface cure of the compositions when applied to a dental cavity is sometimes not reliably completed. It has now been found that surface cure properties can be improved by use of the present system.
SUMMARY OF T~IE INVEI~TION
This invention deals with a novel means for sealins the surface of a composition for dental filling, whicil composition utilizes a polymerizable acrylate monomer, pre-ferably a urethane-acrylate monomer containing at least two reactive acrylic functional groups and at least two urethane linkages per molecule. It has been found that a monomer of this preferred type, when polymerized in situ in a dental application of the type described herein, will produce a polymer which has superior properties as a tooth filling.
Polymerization is induced by a free-radical method, involving either the use of an ultra-violet (hereinafter "UV" ) ~" .
, ' .... , ; . ~ . : ~

lV~ lV

activated free-radical generator, or a peroxy compound in combination with a known activator for said peroxy compound.
The composition may also be incorporated into a dental filling composite which utilizes the above-described urethane-acrylate monomer and curing agents, in combination with an inorganic filling material. This composition or com-posite is applied to a dental cavity which has been prepared according to s'tandard techniques of dentistry.
The novel feature of the present invention involves application to the composition or composite described above of an aqueous solution of waterglass, i.e., sodium meta-silicate, l~a2SiO3. The water is allowed to evaporate leaving a solid film or coating of sodium metasilicate which is essentially impermeable to oxygen. Thus, oxygen is excluded from the surface of the curable composition, thereby permitting the surface to cure fully. The water-glass film is then readily washed away by rinsing with water.
! The invention also involves a composition comprising a curable dental filling composition or composite having a waterglass coating thereon.
DESCRIPTION OF THE PREE'13RRED EMBODIMENTS
The critical feature of the invention is the creation of an essentially oxygen-impermeable film over the surface of the curable filling composite or composition. meoret-ically, any material which is capable of forming such a film in the environment of the mouth should be suitable provided it does not mix with or adsorb the filling material, can be easily removed without damaging the cured surface, and in non-toxic. Preferably, such material is inorganic.
Water-soluble organic polymers such as polyvinyl alcohol and its copolymers with polyvinyl chloride are not recommended.

. .

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It has been found that the most desirable material for this purpose is waterglass, i.e., sodium metasilicate, Na2SiO3. This material appears to be virtually unique in its exceptional ability to meet all the criteria described in the preceeding paragraph.
Waterglass is bland and non-toxic and will not mix with the cur-able monomer. It also has the very desirable property of being highly water soluble, which facilitates its application and per-mits easy removal. This property also permits the waterglass ~Ifilm to be readily and completely removed from the cured surface of the filling material by merely rinsing with water, preferably tepid water. Useful concentrations of waterglass are in the ''range of approximately 25 to 60 per cent by weight of the aqueous !, solution. The lower concentrations are preferred because of their ~relatively low viscosity. The preferred concentration range is ~, approximately 30 to 55 per cent by weight.
Il It is also desirable to include in the waterglass solution a ¦I small amount of a surface active agent which serves to improve 3 llthe wetting properties of the sodiurn metasilicate solution, there-~Iby enhancing its ability to cover the filling material. Obviously, 20 iI the surface active agent should be non-toxic. Theoretically, any ¦I non-ionic surface active agent with adequate wetting properties ¦can be used; however, the sulfates, sulfonates, phosphates, or llmixtures of these are preferred. Useful surface active agent i~ concentrations range from approximately 0.05 to approximately 1 " per cent by weight of the aqueous solution, preferably approxi-3~ mately 0.1 to approximately 0.5 per cent by weight.
, The waterglass solution can be applied to the composition or composite surface by any convenient means sucn as painiing or 'l spraying. I
.. ij .

~n general, any free-radical polymerizable acrylate monomer .' ~

'. 10~
can be use~ in this invention, provided that it is otherwise acceptable for dental use. It is known in the art that the polymerization o~ such monomers tends to be inhibited to some ,, extent by the presence of air or oxygen, thus resulting in incom-, plete cure at the exposed surface of the polymerizing mass. This problem is solved bv the present invention by covering such sur-face with an oxygen-excluding film or coating of waterglass.
!l The urethane~acrylate monomers preferably used in the compo-sitions and process of this invention are substances having structures which allow them to be regarded as the reaction product`
'of an organic polyisocyanate with a polymerizable acrylate ester having a hydroxy or a primary or secondary amino group in the l~alcoholic moiety thereof. The active ydrogen atom in the alco-I holic portion of the ester reacts with the isocyanate group, ~roducing the polymerizable urethane-acrylate monomer used herein.
It is ~.lnderc_ood that certain of these monomers may, in fact, ¦! be oligomers or other low polymers of the urethane-acrylate ¦monomer which contain at least two acrylic functional groups, and l!all such materials are considered to meet the definition of " urethane-acrylate monomer as discussed and used herein. Suitable ¦Imonomers are disclosed in U. S. Patent 3,425,988 to Gorman et al.
The acrylates which may be used in making the urethane-acrylate mOnGmer are substances of the general formula 11 CH2=CR . CoOR3 (I) iin which R2 is H, CH3, C2H5 or Cl and R3 is one of the following:
(a) a Cl_8 hydroxyalkyl or aminoalkyl group, (b) a Cl_6 alkyl-amino-Cl_8 alkyl group; or (c) a hydroxyphenyl, an aminophenyl, , ,, '~

"
, _ 4 ., . .:

..
.- -. . ... , . . ; . - .~ . .

~ l~Ml~10 a hydroxyn~phthyl or an aminonaphthyl group which may be further substituted b~ an al~yl, alkylamino or dialkylamino group, each alkyl group in this sub-part (c) containing up to about 3 carbon atoms.
These acrylates are exemplified by, but not limited to,

2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-amino-propyl methacrylate, hydroxyhcxyl acrylate, 2-tert-butylaminoethyl methacrylate and hydroxyoctyl methacrylate.
In a convenient and known process for making the poly-merizable monomer used in the polymerizable composition according to this invention, an acrylate of general formula I is reacted with a di- or other polyisocyanate, preferably in the presence of a solvent, at a temperature in the range of 0-200C as will be described in more detail later, chosen to suit the specific react-ants involved.
The polyisocyanates which may be used in making the polymerizable monomer may be generally represented by the formula (~=C=N)nQ, in which n is an integer from 2 to about 20, prefer-ably 2 to about 5, and Q is an organic radical having a molecular weight up to about 5000 and a bonding capacity equal to n. A
. preferred class of isocyanates are those of the formula:
; (o=C=N)nR4 (II) wherein n is 2 and R is a C2_20 alkylene, alkenylene or cyclo-alkylene radical or a C6 40 arylene, alkarylene, aralkarylene, alkyloxyalkylene or aryloxyarylene radical which may be substi-tuted by 1-4 chlorine atoms or by 1-3 amino or mono- or di-Cl 3-alkylamino or Cl 3 alkoxy groups.
¦ Ty cal examples of such isocyanates are toluene '-diisocyanates, 4,~'-diphenyl diisocyanate, 4,4~-diphenyl methane diisocyana~c, dianisidine diisocyana~.es, 1,5-naphthalene diiso-cyanate, 4,~ clipherlyl ether diisocyanate, _-phenylene diiso-cyanate, trimethy].ene di.isocyanate, tetramethylene diisocyanate, hexamethylen~ diisocyanate, ethylene diisocyanate, cyclohexylene diisocyanates, nonamethylene diisocyana~e, octadecamethylene diisocyanate, 2-chloropropane diisocyanate, 2,2'-diethyl-ether diisocyanate, 2-(dimethylamino) pentane diisocyanate, tetra-chlorophenylene-1,4-diisocyanate, 3-heptene diisocyanate and .
. 10 transvinylelle diisocyanate.
Other polyisocyanates which may be used are the higher molecular weight polyisocyanates obtained by reacting polyamines containing terminal primary or secondary amine groups, or poly--. hydric alcohols, for example, the al~ane and alkene polyols such as glycerol, 1,2,6-hexanetriol, 1,5-pentenediol, ethylene glycol, olyethy]ene slycol, "bisphenol-~." and su~stituted "bisphenol A~' . with an excess of any of the above-named diisocyanates. These : higher molecular weight urethane or ureide polyisocyanates may be : represented by the formula:
(o=C=N-R4-NH.Co.X-)nR5 ~III) .
: . in which R4 has the meaning given above; X represents O or NR6 where R6 is H or a Cl 7 alkyl group; and R5 is the non-functional residue of a polyamine or a polyhydric alcohol having at least n primary or secondary amino or hydroxyl groups respectively; and n i.s an integer from 2 to 20.
Accordingly, when the monomer is derived from one of t.he ~imple diisocyanates defined above, it has the general formula:
( 2=CR2.co.o.R7 o~cl~NH-i2R4 (IV) 1~
jl ' .

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in which R2 an~ R4 havc t]~e meallings given above and ~7 represents R less on~ .h~ro~en at~m. Preferred monomers conforming to thls definition include derivat-~ès of higher alkylene diisocyanates such as octame~hylene diisocyanate, and the aromatic diisocyanates containing more than 8 non-isocyanate-group carbon atoms, such as durene diisocyanate, i.e., tetramethylphenyl-1,4 diisocyanate, and 4,4'-diphenyl diisocyanate. When, on the other hand, the monomer is derived from one of the higher molecular weight urethane or ureide polyisocyanates aforesaid, it has the general formula:

(CH2=CR2.CO.O.R .O.CO.NH.R .NH.CO.X-)nR ~V) in which R2, R4, R5, R7, X and n hav~ the meanings given above.
As used herein, the term urethane denotes a compound . having in the molecule the characteristic group -O-CO-NH- and the term ureide denotes a compound having in the moleculc ~he ¦ characteristic group -NH-CO-NH-.
¦ A typical and preferred monomer useful in the polymeriz-. ¦ able composition of this invention.is the monomer of formula IV
¦ in which R is CH3, R is n-C3H6 and R is .

20 l ¦ . H3C ~

An acrylate is, as stated abo e, reacted with a poly-isocyanate to form a monomer for use in the polymerizable compo-.sition of the invention. While the proportions of the reactants used are not c.ritical, it is genexally preferred to use about a 0.1 equivalent excess of polyisocyanate above the amount needed to furnish one isocyanate group for each hydroxyl or amino group in the acrylate molecule.
The rea~tion may be carried out in thc presence or ~ ¦¦absence of solvent. Preferably solvents selected from the ¦~

_ 7 _ ... !

~ ~ 10. "

aliphatic, cycloaliphatlc and aromatic hydrocarbons, for example, benzene, toluene, cyclohexane, hexane and heptane, are employed, ; but other solvents such as methyl isobutyl ketone, diamyl ketone, _ butyl methacrylate, and cyclohexyl methacrylate can also be utilized if desired, especially where complete compatibility with the dental adhesive is desired. The chief reason for usiny a solvent is to prevent the reaction mixture from becoming too viscous.
The temperature employed in the reaction may vary over a wide ran~e. Where the reac.ants are present in approximate chelnically equivalent amounts or with slight excess of the isocyanate reactant, useful temperatures lie in a range extending : from about 10 to 175~C.
When the simpler isocyanates are used the reactants are preferably at or near room temperature, e.g., from 20C to 30C. In the preparation of the high molecular weight monomers using an excess of the isocyanate, the reactants may be combined at room temperature but it is preferable to allow them to react at a temperature in the range 40 to 150C, a specially preferred range having been found to extend from 90 to 120C. Further detail may be found in U. S. Patent 3,425,988.
Reaction proceeds with a slight elevation of the temperature and is complete when heat ceases to be evolved. 5'he reaction mixture is then cooled at room temperature; if the solvent used is suitable for incorporation in the polymerizable composition according to this invention, the reaction product wil~
need no extraction or purification and is ready for use.
To provide the necessary curing ability to the above-described urethane-acrylate monomer, a suitable initiator system is used in conjunction therewith. If the monomer is to be cured by a uv activation mechanism, a uv activated ree-radical gener-~ 1041~11(~ .

¦ator is selected, and generally may be incorporated directly into ¦ the urcthane-acrylate monomer. For example, it is possible to ¦ use a metal carbonyl of the formula Mx(CO)y, wherein M is a metal ¦atom, preferably Cr, Mn, Fe, Co, Ni or Mo, x is 1 or 2, and y is ¦an integer determined by the total valence of the meial atoms, ¦generally from 4 to 10. The preferred uv activated free-radical ¦generators are selected from: (a) Cl 16 strai~ht or branched ¦chain alkyl diones; and (b) carbonyl compounds of the general ¦ formula: R(CO)Rl ¦in which R is a Cl 10 alkyl, aryl, aralkyl or alkaryl radical, and ¦ Rl is R or H. R or Rl can contain any substituents which do not ¦adversely affect the compound in serving its intended function~
¦For example, R or Rl can be alpha-substituted with an alkyl, aryl, ¦alkaryl alkoxy or aryloxy radical, or with an amino or a mono-jor dialkylamino derivative thereof, each of the above sub-¦stitueilts containing up to about six carbon atoms. In addition, ¦R and Rl, taken together with the carbonyl group to which they ¦are attached, may represent an aromatic or heterocyclic ketone I containing up to about sixteen (16) carbon atoms.
¦ Preferred polymerization initiators are acetophenone, benzophenone and 1- and 2- acetonaphtone. Others are 2,3-buta-dione, 2,4-dimethyl-3-pentanone, 1- and 2-naphthaldehyde, l p-phenylacetophenone, n-proprionophenone, fluoren-9-one, ; I xanthen-9-one and ~,4'-bis-dimethylaminobenzophenone. Uv initi--~5 ¦ ators generally are used ai a level between about 0.1% and 10%
¦ by weight of the urethane-acrylate monomer, and preferably between ¦ about 1.5% and 7.5% by weight.
When cure ~s to be initiatkd hy fr~e-radical mechallism ¦ which is not uv dependent, the urethane-acrylate monomer is used ¦ in combination with a peroxy com2cund and an activator for said peroxy compound. The~specific peroxy compound and activator _ 9 _ ,..
. - - - -- ,, ~ ~
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1041Yil~-therefor are deterlllined by the speed of cure desired in the final dontal sealill~ or filling composition. This speed of cure al50 can be vari~ by the appropriate balancing of concentrations of monomer, peroxy compound and activator therefor. Organic hydro-pcroxides and organic peresters can be used in the compositionsand processes of this invention, particularly those having a molecular weight bet~een about 90 and about 800, preferably between 90 and 400. Typical examples are t-butylperbenzoate, cumene hydroperoxide and t-butylhydroperoxide. These initiators generally are used at a level between about 0.5 and 10% by weight of the urethane-acrylate monomer, preferably l~ to 5% by weight.
The peroxy compounds of the preceding paragraph frequentl Y
can be activated by the use of organic sulfimides (such as benzoic sulfirnide) and/or primary, secondary or tertiary amines (preferably those to be described hereafter). Typically, these activators are used at a level between about 0.] and abo-lt 7%
by wei~ht of the urethane-acrylate monomer, preferably 0.2% to 4%
by weight. Alternatively, low levels of transition metal com-pounds frequently can be used, commonly at a level between about l and 1000 p~rts per million by weight of the urethane-acrylate monomer. Most typically the transition metal is selected from the class consisting of copper, iron, manganese and cobalt.
The most highly preferred peroxy initiator system is obtained by the use of a peroxy initiator selected from the class consisting of acyl peroxides and silyl peroxides. The acyl peroxides have the general formula:
~r.CO.O2.CO.Ar where each Ar is an aryl radical containing up to about 10 carbon atoms and preferablY is C6H5, ClC6H4, NO2C6H4, or C12C6H3. The

3~ silyl perox ~es have the general ~ormula:

~ I . . l 1~41~10 , CH2=CII-~i(OOR )3 in which R is a Cl 6 straight or branchcd chain alkyl radical.
The preferrcd compoun ds from each group are benzoyl peroxide and vinyl tris(tert-butyl peroxy~silane, respectively. The most high-ly preferred class is the acyl peroxides. The peroxides of thisparagraph g~nerally are used at a level between about 0.05% and 5~ by weight of the urethane-acrylate monomer, preferably 0.1%
to 3% by weight.
The peroxides of the preceding paragraph, and most particularly the acyl peroxides, are most commonly activated by the use of an organic amine, generally having a molecular weight less than about 800. While primary, secondary or tertiary alkyl, aryl or alkyl/aryl amines can be used, the preferred amines are amines of the formula: ArNR9R10 where ~r is as defined above, preferably being C6H5 or a Cl-C4 alkyl substituted C6H5; and each of R9 and R10 is hydrogen or a Cl to C4 alkyl group. Preferably each of R9 and R10 is methyl or ethyl. - i : -The amines are used at the same level described in the preceding paragraph for the peroxides. In addition to the aboveingredients, other ingredients ~nown in the dental filling compo-sition art ~ay be added. The common additive, and one which is essential in most dental filling compositions, is an inorganic filler material, such as finely ground glass powder. The pre-ferred filler is an aluminum borosilicate glass, most preferablyhaving an average particle size which is less than about 40 microns. The inorganic filler frequently comprises a signifi-cant, and even a major, portion of dental filling composites.
For e~:ample, they can comprise from about 40 to about 95 percent by weight of the total composition, preferably 70 to 90~ by ¦ wei t.

l l 10~
it is frequel-tly desirable to add low levels, such as up t~ about 500 ~arts per million by weight, of a free-radical or uv stabili~r, many o~ which are shown in the art, to prevent spurious polymerization o~ the composition prior to the time of its intendecl use. Suitable free-radical stabilizers are hydroquinon~, p-benzoquinone, butylate of hydroxy toluene and butylate of h~droxyanisole. It also may be desirable to modify the compositions by the addition of lower viscosity polymerizable ingredients, most commonly lower viscosity acrylate esters.
Typical exam~]es are hydroxyethyl methacrylate, hydroxypropyl methacrylate, trimethylolpropane trimethacrylate, butyleneglycol dimethacrylate and polyethyleneglycol dimethacrylate. ~any other acrylate esters are known in the art, and essentially any of said esters can be used for purposes herein. An amount or low-lS er viscosity acrylate ester is used which is necessary to produce th~ d~sired viscosity, but whcn 'hc ester s~lected conta~r.s ~ly one acrylic functional group, the amount used should not exceed the weight of the urethane-acrylate ester since the hardness or durability of the final product could be affected.
Other materials, such as adhesive agents, plasticizers, pigmenting agents, etc., can be used if desired.
In discussing the use ratios of the various components, the bulk of the composition should be composed of at least 5%
by weight urethane acrylate monoMer, plus initiator system and 2S inorganic filler. All other ingredients preferably do not comprise more than about 30 percent by weight of the composition.
For a system which is to be initiated by a uv mechanism, ~t ;S gen~rally preferab]~ to adA the uv activated free-radica1 initiator directly to the mixture of ureth~ne acrylate monomer and othsr gredien ~9 as dcscri~ed herei". This provide- a - . .

lU41~J10 one-compon ~i system which can be used directly to fill the apert~res o~ cavities in the teeth and immcdiately activated with uv l;ght. In those systems which are not to be uv activated, it is pos~ible ~o prepare systems which can contain all necessary ; curing agents as a single componellt composition, such as one in which the cure is via a hydroperoxide/amine initiator system.
Such one-component peroxy systems tend to be excessively slow;
a far more preferable approach is to separate either the pero~
compound or the activator therefor into a second component which 1~ is to be added immediately prior to use. In this fashion, compounAs can be selected and used in such amount as to provide much more rapid cure. It is for this reason that a certain number of systems enumerated above have been specified as pre-ferred cGmpositions.
~hen the two-component system is to be utilized, it is preferable that the peroxy compound be used as an additive at the time of use since in this way the most highly stable compositions can be prepared. The dentist then can measure out the appro-priate guantities of each of the two components and mix them, directly apply them to the aperture or cavity, and within a short time such as one to 30 minutes, a hard and durable filling compo-sition will be formed in the tooth. When the two-component sys-tem is utilized, it is generally desirable to mix the peroxy ingredient with a plasticizer or a polymerizable acrylate ester, including a urethane acrylate monomer, to facilitate mixing at the time of use. The balance of weight of use between the two components is a matter of choice dependent upon the systems used, and determination thereof is well within the province of the reasonabiy skilled chemist.
The compositions o this invention have been found to b~ easily prep,red ~r,d ~lsed. ~hen placed and cured in a tooth ~ - - 1 3 -ll '10~
l cavity, th~ colnposil~-ion forms hard and durable adhesive bonds to ¦ normal tooth structure. The cured composition is abrasion resist-¦ ant, and can easily be formulated to match the tooth color and I texture.
- I
l EXAMPLES

¦ The invention will now be illustrated by the following description of specific embodiments thereof, given by way of example only.
The monomers used in the Examples are monomers A and B, and have the formulae:

~ CH2=c(cH3)cooc3H6ocoNH~ ~

Il ~ NH.CO.O { ~ :(CH3)2 A
and ¦¦ ~ l2=C(CH3 COOCH2.CH2.O.CO,NH

15. in which PR represents a propylene triol oligomer (average molecu-lar weight 2500) devoid of its three hydroxyl groups.

EXAMPLE I.
A dental filling composition was prepared by blending the following ingredients, expressed on a parts by weight basis:

... . . . . : .
-- ~-- - , - -. ~ . ~ - , 1 Monomer A 14 2 Monomer B 7 3 Methacrylic Acid 0.8

4 Methacryloxy trimethoxysilane 0.66 Naphthoquinone 0.33 6 Cumene hydroperoxide 0.166 7 Benzophenone 1.6 8 Hydroxyethyl methacrylate 14 9 Uniflex 330 ~ 15 Glass powder (dental quality) q.s.ad. 100 Ingredients 3 and 4 are adhesion promoters, 5 is a stabilizer, 6 is a free-radical inhibitor, 7 is the photo-initiated carbonyl compound, 8 and 9 are viscosity regulators, 9 being a proprietary product of Union Camp Corporation, Wayne, New Jersey, U.S.A. which comprises poly(butylene sebacate), and 10 is a mechanical extender and toughener as used in conventional dental filling compositions.
The composition was applied to a prepared tooth cavity, molded into the required shape, and painted with a solution made up as follows:

Waterglass 100 ml (as54% by weight aqueous solution) "Neutrobrite" 0.5 gm Water 50 ml.
In this formulation the water acts as a diluent and viscosity modifier. "Neutrobrite" is the trade mark of a mixture of sodium hexametaphosphate and sodium lauryl sulfate sold by Albright & Wilson, Ltd.. The water was allowed to evaporate, leaving a film of waterglass.
The covered composition was then irradiated for 60 seconds at 12 inches distance using a lighted Philips 125 watt HPK ~ high pressure mercury vapor ultraviolet-emitting lamp . Under this treatment the composition rapidly cured, yielding a highly satisfactory dental filling. The water-glass film was then readily removed by rinsing with tepid water, leaving the filling intact.
EXAMPLE II
A two-part dental filling composition was prepared using the following ingredients, expressed as parts by weight:
- FIRST PART
l. Monomer A 14 2. MGnomer B 8 3. Methacrylic acid 4. Methacryloxy trimethoxysilane 0.66

5. Naphthoquinone 0.3

6. N,N-dimethyl-p-toluidine 0.5

7. Hydroxypropyl methacrylate14

8. Uniflex 330 3 10

9. Glass Powder, q.s.ad. 100 SECOND PART

10. Dibenzoyl peroxide 5

11. Uniflex 330 ~ 45

12. Glass Powder 50 Ten parts by weight of the First Part were mixed with one part by weight of the Second Part. The mixture was applied to a prepared tooth cavity, where it was molded into the desired shape. The waterglass solution of Example I
was then applied and the water was permitted to evaporate.
After several minutes the filling was sufficiently hard for normal use, and was a highly satisfactory dental filling by standard professional criteria. The waterglass film was removed by rinsing with tepid water, leaving the filling intact.

. - .- .

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The combination which comprises: (a) a curable com-position comprising (1) a urethane-acrylate monomer formed by the reaction of an organic polyisocyanate with a poly-merizable acrylate ester having a hydroxy or amino group in the alcoholic moiety thereof; (2) a free-radical poly-merization initiator and (3) an inorganic filler and (b) an aqueous solution of about 25% to about 60% by weight waterglass applicable to said curable composition to form an oxygen-impermeable coating thereon.

2. The combination of claim 1 wherein the oxygen-impermeable coating is a surface film of sodium metasilicate.

3. The combination which comprises: (a) a curable dental filling composition comprising: (1) a urethane-acrylate monomer formed by the reaction of an organic polyisocyanate with a polymerizable acrylate ester having a hydroxy or amino group in the alcoholic moiety thereof; (2) a free-radical polymerization initiator; (3) an inorganic filler and (b) as an oxygen-excluding means on said composition, a surface film of sodium metasilicate.

4. The composition of claim 3 wherein the acrylate ester has the formula CH2=CR2?COOR3 wherein R2 is H, CH3, C2H5 or Cl and R3 is one of the following: (a) a C1-8 hydroxy-alkyl or aminoalkyl group, (b) a C1-6 alkylamino-C1-8 alkyl group; or (c) a hydroxyphenyl, an aminophenyl, a hydroxynaphthyl or an aminonaphthyl group, or such group further substituted by an alkyl, alkylamino or dialkylamino group, each alkyl group in this sub-part (c) containing up to about 3 carbon atoms.

5. The composition of claim 4 wherein the polyisocyanate has the formula (O=C-N)nR4 wherein n is 2 and R4 is a C2-20 alkylene, alkenylene or cycloalkylene radical or a C6-40 arylene, alkarylene, aralkarylene, alkyloxyalkylene or aryloxyarylene radical or such radical substituted by 1-4 chlorine atoms or by 1-3 amino or mono- or di-C1-3-alkyl-amino or C1-3 alkoxy groups.

6. The composition of claim 5 wherein the free-radical polymerization initiator is a UV activated free-radical generator selected from: (a) C1-16 straight or branched chain alkyl diones; and (b) carbonyl compounds of the general formula R(CO)R1 in which R is a C1-10 alkyl, aryl, aralkyl or alkaryl radical and R1 is R or H.

7. The composition of claim 5 wherein the free-radical polymerization initiator is a peroxy initiator selected from (a) acyl peroxides of the formula ArC(O)O2C(O)Ar wherein each Ar is an aryl radical containing up to about 10 carbon atoms and (b) silyl peroxides of the formula CH2=CH-Si(OOR8)3wherein each R8 is a C1 to C6 alkyl radical.

8. The combination of claim 1 in which the aqueous water-glass solution also contains about 0.05 to about 1.0 percent by weight of a water-soluble surface active agent.