Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
:~2~
The present invention relates to the technlcal
field oF ceramo-metallic dental recons-tructions. More
par-ticularly, it relates to the composition of the ceramic
glass employed to form the transparent surface layer of a
dental reconstruction such as crowns, inlays, bridges, and
the like.
This type of dental reconstruc-tion is usually
produced from a series of ceramic layers coa-ted on a metal
coping, deposited, for example, on the stump of a tooth
reduced after shaping with a drill.
This series of ceramic layers consists of a basal
opacifying layer, two intermediate layers known under the
terms of "dentine" and "incisal" and a transparent surface
layer intended to resemble the gleam of the enamel of the
natural tooth.
The subject of the present invention is aimed
precisely at producing this last transparent surface layer.
In the present state of the art no ceramic layer
is available which is endowed with a perfect translucency
comparable to that of a glass pane. A really translucent
ceramic surface layer would enable the user -to give -the
prosthesis the esthetic appearance of a natural -tooth.
The present invention improves the quality of
transparent ceramic layers of this type.
In accordance with the present invention, the
ceramic layer is produced From a mixture containing at
least two separate glass frits, namely:
approximately 90 % by weight of a first glass frit
which is fusible and does not devitrify, and
~$
30S
approximately 10 % by weight of a second glass
frit which is more refractory and devitrifies partially to
leucite.
Other features and advantages of the present
invention will become apparent from the reading of the
detailed description which follows, particularly relying on
an illustrative example of practical implementation.
The transparent ceramic layer in a preferred form
has expansion co-efficients which are stable and match all
the precious or base alloys.
The glass frit employed is produced by mixing two
separate glasses, one fusible and non-devitrifying, the
other slightly more refractory and devitrifying partially
to leucite, a mineral with high thermal expansion.
The first glass frit, which is fusible and does
not devitrify 9 is employed as such. In contrast, the
second glass frit is devitrified, for example, for
approxima-tely 12 hours at a temperature of the order of
900C. During this thermal devitrifying treatment, an in
situ crys-tallization phenomenon makes its appearance in a
conventional manner.
It will also be noted that, during successive
bakings, the fusible glass, at the limit of saturation with
leucite, does not dissolve the leuci-te in the second glass
and consequently its proportion remains cons-tant. In this
way, a stable expansion is produced.
The hardness of the transparent ceramic layer can
also be reduced by an increase in the percentage of fluxes,
favoring Na20. The ceramic layer is formed from a
,~ ,
36)~
- 3 --
mixture of two glass frits containing, in to-tal, from
approximately 20 to approxima-tely 30 % by weight of fluxes,
among which soda Na20 is present in a proportion of
approximately 7 to approximately 8 % by weight.
The following overall composition of transparent
ceramic layers produces satisfactory results in practice
and is given below by way of an illustrative example.
SiO2 59 to 59.5 % by weight
A1203 15 to 16 % by weight
CaO 0.5 to 1.5 % by weight
MgO O to 0.2 % by weight
K20 11 to 13 % by weight
Na20 7 to 8 % by weight
B203 2.5 to 3.5 % by weight
BaO O to 3 % by weight
CaF2 0.5 to 3 % by weight
TiO2 0.2 to 0.5 % by weight
The ceramic layers according to the invention have
also been adapted so as to be capable of avoiding any
background coloring phenomenon due to the metal
substrates. These metal substrates are usually made from
an alloy of various metals, among which silver is, in
particular, responsible for such a background coloring. In
fact, the Ag~-~ ion migrates fairly readily in the ceramic
and causes a greenish yellow coloring when reduced to the
metal s-tate. It is precisely to avoid a phenomenon o-F this
type that approximately 0.2 % by weight of cerium oxide
CeO2 7 which is an active oxidizing agent which keeps the
silver in the non-coloring oxide state 9 is added to the
~,
~ 2513~
-- 4 --
mixtures of glass frits according to the invention. This
cerium oxide is added after the grinding operation which
produces the glass frit. The glass frits are obtained in a
conventional manner by melting and homogenizing a powdered
mixture of the basic components required for producing the
abovementioned compositions.
To reduce shrinkage during the baking, the
particle size distribution of the mixture of -the two glass
frits has been adjusted as a result of the following
observation. The voidage of a population of large
particles (40 to 65 microns) is of the order of 40 %. By
introducing particles of medium size (12 to 40 microns) in
a proportion of approximately 5û % by weight relative to
the large particles, the voidage drops to approximately
20 %. Lastly, when particles of much smaller sizes (below
5 microns) are added, the voidage becomes still smaller.
Consequently, this results in a lower shrinkage on baking.
A particle size distribution of this type also makes it
possible to obtain better plasticity of the paste at the
time of its use.
In order to produce a glass frit according to the
invention, intended for producing a transparent surface
layer, it is thus desirable to make use of a particle size
distribution of the following type:
40 to 65~ ...... 40 to 50 % of the particles
12 to 40 ~ ..... 30 to 35 % of the particles
~ 12~u ......... 15 to 30 % of the particles.
Glass frits required for the production of
transparent ceramic surFace layers are presented in the
. '
~ :,
3~S
form of a paste ready for use and intended to facilitate
the work of the user whlle making it possible to achieve a
saving in active product. Such frits are presented in -the
form of a paste containing a suitable quantity of a
plas-ticizer~ so as to enable them -to be packaged in a tube
or a dispenser for paste of the aerosol type.
A typical example of formulation of glass frits
according to the invention, packaged in an aerosol
dispenser, is given below:
100 parts by weight of active products,
30 to ~0 parts by weight of plasticizer,
100 parts by weigh-t of a propellant agent 7 such as
a Freon .
Qccording to a particular embodiment of glass
frits of this type, the plasticizer is preferably chosen
from diethylene glycol diethyl ether and propylene glycol
methyl ether.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or -the scope of the appended claims.
