Note: Descriptions are shown in the official language in which they were submitted.
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.
-- 2 --
. .
.V ~.
i~
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
- 3
.' ~
'. 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 ' .
, ~
lU41~
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 _ ,..
. - - - -- ,, ~ ~
- .
. ~ :
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,
- 15 -
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.
- 16 -
. - .- .
