TITLE:
A system for fluoride based glass melting and method for the same.
FIELD OF INVENTION:
This invention relates to a system for fluoride based glass melting.
This invention also relates to a method for fluoride based glass melting.
BACKGROUND OF THE INVENTION:
A Glass lonomer Cement (GIC) is a dental restorative material used in
dentistry for filling teeth and luting cements. These materials are based on
the reaction of silicate glass powder and polyalkenoic acid. These tooth-
coloured materials were introduced in 1972 for use as restorative materials
for anterior teeth (particularly for eroded areas, Class III and V cavities).
The material acquires its name from its formulation of a glass powder and an
ionomer that contains carboxylic acids. The solid portion is usually made by
formulating a typical composition containing fluorides and melting the same
in a conventional furnace to form a glass. The glass powder is then used with
the polymer liquid to form a cement.

In the conventional method of glass formation, the powder mixture is put in
a refractory crucible and subject to heat. The limitation of heating rate is
dependent on the thermal shock of crucible material used and the capability
of the furnace since the heating is from outside to inside. This process takes
enormous time and special provision has to be made to pour the melt at hot
condition. Therefore, this method is slow, creates temperature gradient
during heating, needs dedicated set up for melting and energy intensive.
Further, there is the possibility of contamination from heating element
during long exposure of fluoride based materials which also eventually
reduces the furnace like. Further, the accuracy in temperature measurement
is not confirmed since the thermocouple measures only the surface
temperature of the crucible from the outside giving erroneous result and
interpretation of data. Therefore, alternative method for melting the glass
composition containing fluorides has been explored. One such method is
Microwave assisted melting.
Microwave melting of glass has been experimented and in many cases
continuous systems developed. However, the studies of melting are mostly
limited to treat radioactive toxic waste or materials of volcanic origins.
Sawada et al. (US Patent 4,330,698; 1982) have described a microwave
melter for glass melting using metal crucibles with rotatable containing.
Johnson et al. (US Patent 4,940,865; 1990) have developed a 100 kW
continuous microwave melting system for waste materials. Similarly, two
patents from France (Fr-A-2-674-939 and Fr-A-2- 671-392) and two US
Patents (5,254,818; 1993,5,597, 504; 1997, 5, 822, 879, 1998, 7,297, 909

B2; 2007 etc) have described microwave melting equipment for mainly
radioactive waste materials processing. In some cases e.g. 5, 822,879, 1998,
monomodal microwave source is used at 915 MHz frequency. In some other
cases, e.g. Fr-A-2-674-939, microwave leakage during pouring was one of
the main limitation in the system. In one recent US Patent 6, 938, 441 B1 by
Hajek et al. in 2005 reports the melting of glass of volcanic origin using an
inert additive for assisting microwave absorption, since most of the glasses
are transparent to microwave at room temperature. This experiment is
significant since adding any foreign material to glass for microwave
absorption may change the glass properties. Further, in many such systems,
metallic crucibles are used which may corrode at high temperature altering
the properties of glass. Most of the systems developed are for continuous
operation and hence any new development in contaminant free condition is
difficult and can not practiced in the laboratory. Finally, all the glass systems
studied in these reports do not contain fluorides which are essential for
biological applications. The microwave melting of special types of materials
such as Lunar materials have also been reported by NASA on a synthetic
glass of Apollo 11 soil composition.
Various glass compositions have been reported which are suitable for Dental
glass ionomer cement (For ex. US Patent 4,775,592; 1988 by Akahane et al.
and US Patent 6, 136,737; 2000 by Todo et al.). However, the application of
microwave energy in heat treatment of such class of materials used for
dental applications has been limited mainly to curing the polymer glass
composition to form cement. In the US Patent 6,254,389 Bl; 2001 by

Seghatol, a hand-held microwave intra-oral dental system has been
developed to cure polymer materials intra-orally to produce improved dental
composites. To the best of our knowledge and belief, the microwave heat
treatment of mixtures to form a glass required for glass ionomer cement for
dental applications is not available in the published literature. Therefore, this
method is novel and new for processing glass for glass ionomer cement
using an innovative casket design and contact less temperature measurement
system.
The microwave energy has been used to cure the polymer resins. For
example, in US Patent 6,737,619 (Seghatol et al. 2004), an effort has been
made to overcome the long curing times associated with the conventional
thermal water bath technique, a technique of using commercial microwave
ovens to heat and cure polymer resins to form dental prosthetics. The
controlled microwave energy enables a higher degree of conversion of
monomers into polymer chains in the polymerization process, thereby
enhancing the physical and biocompatibility characteristics of both dental
prosthetics and dental composites made from polymers.
Therefore, it is essential to develop a process to melt the fluoride containing
glasses for biomedical applications. Due to the nature of applications, it is
essential that melting process should not contaminate the melt composition
and hence proper care has to be taken to chose inert crucibles and furnace
conditions. In the microwave heating method, employed in this study, we
have used re-crystallized inert alumina crucibles for melting. The design of

casket is important and using the same, any standard microwave furnace can
be used to melt the glass containing such fluorides. The process is very fast
since it undergoes volumetric heating of the materials. Further, since the
melt assembly is totally enclosed in a casket assembly and since microwave
cavity has no heating elements, hence exposure of fluoride containing
chemicals is limited in such process. Further, the process requires very less
energy compared to the same in conventional process. The process is also
environment friendly since it does not use any fossil fuels for melting such
compositions. Hence, the key technology involved is designing a proper
casket, proper temperature measurement system and a fast suitable
temperature profile for melting. Further the properties of such glasses need
to be studied in order to understand the superiority of this material compared
to its conventionally processed counterparts.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a system for melting fluoride based
glass;
Another object of this invention is to propose a method for melting fluoride
based glass;
Still another object of this invention is to propose a microwave assisted fast
method to melt fluoride based glass containing fluorides for dental
restoration application;
Further object of this invention is to propose a system and a method which
reduces the energy consumption during melting.

BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for melting a fluoride
based glass composition comprising: heating the glass composition in a
casket assembly till it melts; subjecting the melted glass to the step of
quenching; milling the quenched transparent glass process.
In accordance with this invention there is also provided a microwave
apparatus for heating and melting materials, comprising: a microwave
energy furnace comprising an applicator (9) for delivering microwave
energy via an inlet (10); a housing (2) having a front opening (4) to
accommodate an alumina-silicon carbide composite casket (3) with an
alumina lid (8), the housing (2) receiving said microwave energy through
said input (10); a recrystallized alumina crucible (5) containing a powder
mixture (7) disposed within said casket (3); and the microwave applicator
(9) comprising a mode stirrer (13) and a wave guide part (12), and internally
accommodating the housing with the casket (2, 3) to allow receiving of said
microwave energy which is enabled to heat and melt the powder material
within a pre-defined time.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig 1 is the general assembly of casket used for fluoride based glass melting
including the position of ceramic crucible.

Fig. 2 is the general assembly of the casket and exhaust system, wave guides
etc. in the microwave melting system used in this experiment.
Fig. 3 depicts the fluoride release from the microwave melted glass and
compared with that from commercially available glass.
Fig. 4 compares the strength of the glass ionomer cement of the microwave
assisted glass powder with that of conventionally processed powder.
DETAILED DESCRIPTION OF THE INVENTION:
According to the present invention, a casket assembly (2) is provided with a
front opening (4) as described in fig 1. A temperature port (1) in a casket (3)
is in alignment with an inlet (10) in the microwave chamber (9). A non-
contact infra red pyrometer (11) is used for temperature measurement on the
alumina plate with known emissivity. A recrystallized alumina crucible (5)
containing a powder mixture (7) is kept inside the alumina-silicon carbide
composite casket (3) with an alumina lid (8) of known emissivity which is
surrounded by ceramic blankets (6). The alumina lid (8) is focused through
the top opening (10) in the casket (3) to infra red pyrometer (11) for
temperature measurement. Fig. 2 describes the casket system which is
capable of reaching 1600 deg C and for any melting operations. The casket
(3) is placed in the middle of a 6 Kw microwave furnace comprising an
applicator (9) connected to an exhaust (15), and to a water source (16). A
provision for inlet air (14) is provided in the microwave applicator (9). The

applicator (9) is part of a standard 6 kW microwave furnace with a mode
stirrer (13) and a wave guide port (12) and without a turn table. The water
cooling system is used to take the reflected power load which is tuned with
stub tuners attached to the system.
In the typical microwave melting run, the front opening of the casket is
closed with insulation boards and blankets and the microwave power is
supplied. Since, the raw materials used in such compositions are not
microwave absorbing in nature, initial heating is provided through radiation
of microwave susceptible casket material. After a certain temperature, the
materials start interacting with the microwaves and heat the material rapidly
and melt the composition. Typically within 2h of heating including 30 min.
soak at peak temperature, the composition can be fully melted. The
temperature is monitored by the on line non-contact infra red pyrometer
following the emissivity matching method of known material. A calibration
was done to ascertain the temperature difference between the known
alumina material and the melt glass composition during microwave melting
prior to actual experiments using the pyrometer.
After the composition is melted, the microwave is switched off and the glass
is immediately poured into water to quench. The quenched transpart glass
pieces were milled manually in a mortar and pastle followed by milling for
one hour in a planetary mill prior to further testing. The milled powder was
mixed with a known commercial polymer and studied the setting
characteristics to form glass ionomer cement (GIC). The fluoride analysis

was carried out of the microwave melted glass and compared the same with
that obtained in conventionally processed glass carried out in this study and
also from the commercially available glass.
In the conventional melting process employed in this study, the glass
composition was melted in a raising hearth furnace using an alumina
crucible and quenched in water prior to further testing. It was observed that
the melting temperature could be brought down by 25°C and the heating
time to reach the peak melting temperature could be brought down by 50%.
Further, the soaking at peak temperature could also be brought down by 50%
during microwave assisted melting. Thus, the cost for melting could be
brought down by 50% using this new method, which is very significant in
terms of energy requirement for processing.
The microwave melted glass on testing resulted one important observation.
The fluoride release from the new method of processing was significantly
higher compared to either from the conventionally processed glass or from
the commercially available glass. This can be explained based on the fact
that during microwave assisted melting, the heat treatment time is very low
and hence the amount of fluoride lost during is significantly reduced. This is
beneficial during actual application where the fluoride can be released and
the dentine can be strengthened. Fig. 3 summarizes the results of fluoride
release from different glasses tested in this study. The other important
advantage of the new method is that there is no contamination of the glass

from the furnace atmosphere in the new method unlike that reported for
conventionally processed glasses.
The compression strength of the glass ionomer cement as measured by
following IS 12797:1989 revealed that the microwave melted glass has
similar strength compared to that of conventionally melted sample inspite of
the fact that the average particle size of former was nearly double than that
of the later. Therefore, if the average particle size of microwave processed
glass samples can be reduced, then the strength can also be improved
significantly. The strength result is comparable to the commercial sample
available in the market.
Regarding the energy requirement, since the microwave processing can be
completed within 2h compared to nearly six hours in conventional
processing, significant reduction in energy is possible during the new
process. Further, the average peak temperature is lower by -25 deg C
resulting marginal lowering of energy cost. Therefore, the overall energy
cost by processing using microwave energy will be in the order of 50 to
60%.
EXAMPLES:

Example 1:
A glass composition containing major constituents of 42 SiO2, 28 A12O3 and
16 CaF2 as the major composition could be melted at 1350°C with 30 min.
soaking at peak temperature in microwave assisted method in contrast to
1400°C with 120 min soaking in the conventional melting method. Further,
the heating time to reach the peak temperature was 2h in the new method
compared to 4h in the conventional method.
Example 2
A glass composition containing major constituents of 42 SiO2, 28 A12O3 and
8 CaF2 could be melted at 1325°C with 30 min. soaking at peak temperature
in microwave assisted method in contrast to 1350°C with 120 min soaking in
the conventional melting method. Further, the heating time to reach the peak
temperature was 2h in the new method compared to 4h in the conventional
method.
Example 3
A glass composition containing 34SiO2, 24A12O3 and 19 CaF2 could be
melted at 1275°C with 30 min. soaking at peak temperature in microwave
assisted method in contrast to 1300°C with 120 min soaking in the
conventional melting method. Further, the heating time to reach the peak

temperature was 2h in the new method compared to 4h in the conventional
method.

WE CLAIM:
1. A method for melting a fluoride based glass composition comprising:
heating the glass composition in a casket assembly till it melts;
subjecting the melted glass to the step of quenching;
milling the quenched transparent glass process.
2. The method as claimed in claim 1, wherein the said glass composition was
heated in the casket assembly for about 2 hours.
3. The method as claimed in claim 1, wherein the said melted glass in
quenched in water.
4. The method as claimed in claim 1, wherein the milling was done in a
mortar and pastle and also for an hour in a planetary mill.
5. A microwave apparatus for heating and melting materials, comprising:

- a microwave energy furnace comprising an applicator (9) for
delivering microwave energy via an inlet (10);
- a housing (2) having a front opening (4) to accommodate an alumina-
silicon carbide composite casket (3) with an alumina lid (8), the
housing (2) receiving said microwave energy through said input (10);
- a recrystallized alumina crucible (5) containing a powder mixture (7)
disposed within said casket (3); and

- the microwave applicator (9) comprising a mode stirrer (13) and a
wave guide part (12), and internally accommodating the housing with
the casket (2, 3) to allow receiving of said microwave energy which is
enabled to heat and melt the powder material within a pre-defined
time.
6. The apparatus as claimed in claim 5, wherein the applicator (9) is
connected to a water source (16) via an exhaust (15).
7. The apparatus as claimed in claim 5, wherein the casket assembly (2)
comprises a temperature port (1).
8. The apparatus as claimed in claim 5, comprising a non-contact infra red
pyrometer (11) adopted via said temperature port (1) to measure the
temperature emission within said applicator (9).
9. The apparatus as claimed in any of the preceding claims, wherein the
applicator (9) comprises an air inlet port (14).
10. The apparatus as claimed in any of the preceding claims, wherein the
casket (3) comprises ceramic blanket (6).

A method for melting a fluoride based glass composition comprising:
heating the glass composition in a casket assembly till it melts; subjecting
the melted glass to the step of quenching; milling the quenched transparent
glass process.