|Title of Invention||
"A SPONTANEOUS,ESSENTIALLY NOM-CRYSTALLINE OPAL GLASS"
|Abstract||The present invention relates to a spontaneous, essentially non-crystalline opal glass exhibiting a very dense, milky-white appearance, a softening point in excess of 700°C, a coefficient of thermal expansion (25°-300°C) between about 65-85 x 10-7/°C, a density of at least 2.4 g/cm3, an opal liquidus no higher than 1200°C, and excellent resistance to weathering and attack by alkaline detergents consisting essentially, expressed in terms of weight percent on the oxide basis as calculated from the batch, of 1.9-3.6% K2O, 4.2-7.3% Na2O, 0.2-3.0.% Li20, 0-1.2% MgO, 0-4.9% CaO, 0-12.5% BaO, 0-0.1% NiO, 0-4-4% ZnO, 5.3-9.6% B2O3, 8.8-13.5% Al2O3, 57.2-64.4% Si02, 1-6% P2O5, and 1.0-2.2% F.|
|Full Text||The present invention relates to a spontaneous, essentially non-crystalline opal glass
Background of the Invention
This application claims the benefit of U.S. Provisional Application No. 60/0 0 6,729 express mailed November 14, 1995, entitled PHASE-SEPARATED, NON¬CRYSTALLINE OPAL GLASSES, by John L. Stempin and Dale R. Wexell.
The invention is directed to the production of spontaneous opal glasses which contain amorphous particles as the opacifying or light-diffusing phase. The glasses are phase separated, but non-crystalline, exhibit a very dense, millcy-white appearance (unless colorants are added thereto), and strongly resist weathering and attack by acids and alkaline detergents, thereby recommending their utility for culinary ware and tableware. This type of glass has been termed an immiscible opal, i.e., an opal glass wherein the opacifying phase is a glass which is insoluble in the basic glass matrix. Numerous attempts have been pursued in the past to develop an example of this type of opal glass which combines good chemical durability and resistance to weathering with a dense, white-opacity.
Several attempts have been made in the past to develop opal glasses having the above properties. Examples of such glasses are disclosed in U.S. Pat. Nos. 4,309,219 (herein incorporated by reference); 3,498,810; 3,506,464; 3,661,601; 3,723,144; and 3,728,139.
There continues to be a need for opal glasses demonstrating dense opacity and good chemical durability and resistance to weathering, as well as good aesthetic appearance. Accordingly, the object of the present invention is to develop an improved opal glass, exhibiting excellent opacity and good chemical durability.
Summary of the Invention
Briefly, the present invention relates to a spontaneous, essentially non-crystalline
opal glass exhibiting a very dense, milky-white appearance, a softening point in excess of
750°C, coefficients of thermal expansion (25°-300°C) over the range of about 65-85 x
10~7/°C, a density of at least 2.4, an opal liquidus higher than 1200°C, consisting
essentially, expressed in terms of weight percent on the oxide basis as calculated from the
batch, of 1.9-3.6% K2O, 4.2-7.3% Na2O, 0-3% Li20, 0-1.2% MgO, 0-4.9% CaO,
0-12.5% BaO, 0-0.1% NiO, 5.3-9.6% B203, 0.4^4% ZnO, 8.8-13.5% A12O3, 57.2-64.4%
SiO,, 1-6% P205, and 1.0-2.2% F, with the sum of (MgO+CaO+BaO) being preferably in
the range of 4.5 to 12.5%; and Li2O being preferably, at least 0.2%.
Detailed Description of the Invention
The present invention is designed to produce spontaneous, essentially noncrystalline
opal glasses especially suitable for culinary and table ware exhibiting a very
dense, milky-white appearance in thin cross section and excellent resistance to attack by
alkaline solutions, particularly hot aqueous detergent solutions. The glasses are operable
in microwave applications because of their non-crystalline nature and remain noncrystalline
even after heat treatments to the annealing and softening points thereof.
Hence, the glass articles can be air tempered without the development of crystallization
therein. Finally, the glasses demonstrate coefficients of thermal expansion (25°-300°C)
over the range of about 65-85 x 10"7/°C, density of at least 2.4 g/cm3 and softening points
above 750 °C. This latter feature allows the use of high temperature decorating frits
where desired. The emulsion or opal liquidus greater than 1200 °C, preferably greater
than 1300 °C.
Glasses which satisfy those conditions and which are capable of being melted and
formed utilizing techniques conventional in the glass art, consist essentially, expressed in
weight percent on the oxide basis, of 1.9-3.6% K20, 4.2-7.3% NaA 0-3.0% Li20,
0-1.2% MgO, 0-4.9% CaO, 0-12.5% BaO, 0-0.1% NiO, 0.4-4% ZnO,5.3-9.6% B203,
8.8-13.5% A12O3, 57.2-64.4% SiOj, 1-6% P2O$, and 1.0-2.2% F. Preferably, the sum of
(MgO+CaO+BaO) is in the range of 4.5-12.5 weight percent; and most preferably, Li20
is at least 0.2 weight percent.
We have found that the best non-crystalline opal glass for the invention is a
Quorophosphate-borosilicate opal glass having 0.2-3% Li2O. The improved opal glass of
the invention exhibits increased opacity, less quench sensitivity, and uniform tintablity.
The addition of Li2O results in an opal glass having an increased phase separation which
in turn increases the opacity of the glass. With Li20 present, the phase separation consists
of many more small particles in contrast to a fewer number but larger particles without
Li,O present. This increase in opacity is a result of two effects: an increase in emulsion
liquidus temperature which produces greater opacity in glasses, and the generation of a
separated phase which has more smaller size particles. A key aspect of the invention is
that we have found that the present composition yields increased emulsion temperature
and consequently, high opacity without undesirable side effects such as gross phase
separation and "mother-of-pearl" surface finish.
As a result of the high emulsion temperatures of the present opal glass compositions,
we have found that the glasses can be processed by methods such as spinning
which results in better opacity than observed in pressed glass products. Generally, spun
ware exhibit increased density or opacity over pressed ware of the same thickness.
Spinning also produces glossier finish than pressed glass. This is a key aspect of the
present invention. Because of the higher emulsion temperature of the inventive glass, the
glass exhibits the desired opacity while having appropriate viscosity for spinning.
The necessary components for the development of the light -diffusing, liquid-liquid
phase separation comprise 1.0-2.2% F, 1-6% P2O5, and 5.3-9.6% B203. Interspersed
phosphate-rich and fluorine-rich non-crystalline glass phases occur when the B203, P2O5,
and F are combined with the other oxides in the inventive system. The temperature of the
working viscosity (103-104 poises) for these glasses is about 1360°C or less and can be
adjusted by varying the quantities of B2O3, Na^, K2O, and F in the composition. B2O3
encourages the constituents of the inventive glasses to form interspersed amorphous
phases manifesting substantial differences in refractive index. These differences provide
the very dense white opacity of the opals.
A concentration of F less than about 1.0%, leads to unacceptably high viscosity
and the resulting opal glass will commonly contain a crystalline phosphate phase. As the
F level is increased, the softening point and working temperature of the glass are
significantly reduced. For the present opal glass, we have found that amounts of F greater
than about 2.2% do not improve phase separation noticeably and the softening point is
reduced to such an extent that decorating with durable, high temperature enamels
becomes impossible. Moreover, excessive quantities of F lead to severe mold corrosion
and air pollution problems because of the volatility of fluorine compounds during melting
of the batch materials. The most preferred level of F is about 1.7%.
The alkaline earth metal oxides are utilized to enhance opacity. Those materials
preferentially enter into the glassy matrix of one of the amorphous phases and assist in
developing liquid phases with significantly dissimilar refractive indices. High
concentrations of CaO can have a very deleterious effect upon the acid and alkali
durability exhibited by the glasses since such lead to the formation of a non-durable,
amorphous phase in the base glass or by causing extensive surface crystallization on the
glass. Consequently, the level of CaO will be held below about 5%.
MgO may be present in relatively small amounts to intensify opacity or to adjust
physical properties, but such additions must be carefully controlled to avoid adversely
altering other physical properties of the glass to any substantial extent and/or initiating
gross phase separation.
BaO is unique among the alkaline earth metal oxides in not only enhancing opacity
but also exerting a positive effect upon the acid and alkaline durability of the glasses.
Because A1203, BaO, and Si02 improve the chemical durability of the glass, the preferred
glasses will contain about 66-90% total of those materials. BaO demonstrates the side
effects of lowering the softening point and increasing the density of the glass.
Furthermore, at high levels of P205, large concentrations of BaO hazard gross phase
A1203 is beneficial in stiffening the glass for forming purposes increasing the
chemical durability and improving the decorability thereof. It is believed that A1203 also
acts to densify the glassy matrix, thereby preventing any gross migration of Na+ and Fions
to the surface. Whereas A1203 plays a vital role in achieving excellent resistance to
and alkali attack, more than about 14% tends to flatten the viscosity of the glass
which, in turn, causes an undesirable increase in the working temperature of the glass.
The level of NajO is carefully regulated in order to optimize the chemical and
physical properties of the glass. K2O behaves as a fluxing agent and, in conjunction with
the other components, provides chemically durable, non-weathering glasses.
ZnO is beneficial in regulating the size of the separated phosphate rich phase in the
glass. It is known that ZnO appears in both the bulk phase and the separated phase and
alters the surface tension of the separated phase.
We have found that the incorporation of up to 4 wt. % weight ZnO to
compositions similar to those disclosed in U.S. Patent 4,309,219 can substantially alter
the degree of separation of the secondary phase in the formation of the opal glass. The
presence of ZnO significantly lowers the high temperature emulsion liquidus and thereby
results in the formation of a separated phase with smaller droplets when no ZnO is
present. In effect, the presence of ZnO at a minimu level of about 0.4% increases the
surface tension of the separated phase thereby allowing the formation of the smaller
spherical amorphous moities. The amount of ZnO required to be present depends on the
total Li 2O content, the speed of ware manufacturing, and the forming temperatures. The
addition of ZnO is most beneficial when the Li,0 content is greater than 02% and the
forming temperatures are below 1340°C.
Scanning electron microscopy and x-ray emission data indicate that the zinc enters
the separated phase and is also present in the bulk glassy phase. Whereas the mechanism
of that action has not been fully elucidated, it is believed that the presence of ZnO causes
the development of a separated amorphous phase with a larger population of smaller
particles. Increasing the surface tension of the separated phase indicates that it is more
difficult for the smaller particles to combine to form larger particles which might result in
break sources on heating of the glass and rapid cooling in tempering.
The presence of ZnO, if appropriate for a given composition, can offset the effects
of a large population of particles derived from a glass containing Li20 The addition of
Li,O above 0.2% significantly increases the emulsion liquidus of the glass and increases
the range over which the separated phase can form. As the lithium glass resides longer in
the emulsion range, there is a greater tendency to form larger amorphous separated phase
particles which can result in breakage of ware. The addition of ZnO serves to mitigate or
eliminate this phenomenon. In short, ZnO in the opal glass compositions can essentially
eliminate the undesirable large separated phase particles which can result in ware
breakage in thermal upshock.
Description of Preferred Embodiments
Table I records glass compositions, expressed in terms of parts by weight as
calculated from the batch, illustrating the inventive products. Because the sum of the
tabulated components totals or approximately totals 100, for all practical purposes the
reported figures may be deemed to represent the compositions in terms of weight percent.
Inasmuch as it is not known with which cation(s) the fluoride is combined, it is merely
recited as F in accordance with conventional glass analysis practice.
The actual batch ingredients may comprise any materials, either the oxides or
other compounds, which, when melted together, will be converted into the desired oxide
in the proper proportions. Although the following description is drawn to laboratory
scale melting and forming, it will be appreciated that the illustrative compositions could
also be utilized in large scale melting units.
The batch ingredients were compounded, rumble mixed to aid in securing a
homogeneous melt, and deposited into platinum crucibles. The crucibles were introduced
into an electrically-heated furnace operating at 1450°C-1550°C and the batches melted for
four hours. The melts were cast into steel molds to produce slabs having dimensions of
about 6" x 6" x 0.5" or manually pressed into discs of about 3-4" in diameter and about
0.125-0.25" in thickness. Pressing of the discs was undertaken to simulate roughly the
quick quenching of the glass taking place during commercial automatic pressing
operations. The glass slabs were transferred to a furnace operating at about the annealing
temperature, that temperature sustained for about one hour, and then cooled to room
temperature retained within the furnace (approximately 30°C/hour).
The annealed slabs demonstrated no translucency. The density of opacification
exhibited by the pressed discs varied in accordance with the proximity of the forming
temperature utilized to the emulsification temperature or opal liquidus of the glass. Most
preferably to ensure dense opacity, the pressing temperature will not be in excess of about
80°C above the emulsification temperature.
(Table Removed) Samples of several glasses were screened for potential weathering problems by
boiling in distilled water for one hour and analyzing the water for NajO content. Where
less than 4 ug Na^/cm2 were extracted (Ext.) from the specimens, the glass was
considered to be desirably resistant to weathering.
Detergent resistance (D.R.) was investigated via immersing specimens of the
passes into a 0.3% aqueous solution of SUPER SODLAX* brand detergent, marketed by
Economics Laboratories, St. Paul, Minn., operating at 95°C for periods of 24, 48, 72, and
96 hours. An exposure of 96 hours has been estimated to be equivalent to about 10 years
of use in a household dishwasher in an average home. The surface areas of the specimens
were limited to a ratio of 12 square inches of glass to one pound of the solution. The
samples were removed periodically from the hot solution, rinsed in tap water, and wiped
dry. Thereafter, a portion of each specimen was coated with DYE-CHEK* brand dye
penetrant, marketed by Magna-Flux Corporation, Chicago, 111., the dye permitted to
remain in contact therewith for 20 seconds, and the sample then wiped dry.
Examples 1 through 16 of Table II represent the preferred compositions since they
exhibit high emulsion liquidus, relatively high softening points, relatively low coefficients
of thermal expansion, and are entirely free from crystallization, with Examples 12 through
16 representing the more preferred glass compositions because they are more opaque and
whiter than the glasses of Examples 1-11. It is believed that the high opacity of the
glasses of Examples 12-16 is due to greater phase separation. The most preferred glass
compositions are represented by Examples 12 and 13.
(Table Removed)It will be appreciated that, where desired, conventional glass colorants, such as
CoO, Cr203, CuO, FeA, MnO2, NiO, and V20S, may be incorporated into the base glass
composition in quantities customary in the glass art, normally less than 5% by weight.
1. A spontaneous, essentially non-crystalline opal glass exhibiting a very dense, milky-white appearance, a softening point in excess of 700°C, a coefficient of thermal expansion (25°-300°C) between about 65-85 x 10-7/°C, a density of at least 2.4 g/cm3, an opal liquidus no higher than 1200°C, and excellent resistance to weathering and attack by alkaline detergents consisting essentially, expressed in terms of weight percent on the oxide basis as calculated from the batch, of 1.9-3.6% K2O, 4.2-7.3% Na20, 0.2-3.0.%) Li20, 0-1.2%) MgO, 0-4.9% CaO, 0-12.5%) BaO, 0-0.1%) NiO, 0-4-4%) ZnO, 5.3-9.6% B2O3, 8.8-13.5% Al2O3, 57.2-64.4% Si02, 1-6% P2O5, and 1.0-2.2%) F.
2. The opal glass as claimed in Claim I, wherein the sum of MgO+CaO+BaO is in the range of 4,5 to 12.5 wt. %.
3. The opal glass as claimed in Claim 2, wherein the glass consists essentially, expressed in terms of weight percent on the oxide basis as calculated from the batch, of
BaO 6.25 - 6.65
B2O3 6.25 - 6.65
CaO 1.45- 1.75
K2O 2.55 -2.85
Li2O 0.95- 1.05
MgO 0.95 - 1.25
Na2O 4.75 - 5.05
4. A spontaneous, essentially non-crystalline opal glass substantially as hereinbefore described with reference to the foregoing examples.
|Indian Patent Application Number||1707/DEL/2005|
|PG Journal Number||13/2009|
|Date of Filing||30-Jun-2005|
|Name of Patentee||CORNING INCORPORATED|
|Applicant Address||HOUGHTON PARK, CORNING, NEW YORK, 14831, UNITED STATES OF AMERICA|
|PCT International Classification Number||C03C3/04|
|PCT International Application Number||N/A|
|PCT International Filing date|