CA1092824A - Borosilicate glass articles and method of making - Google Patents
Borosilicate glass articles and method of makingInfo
- Publication number
- CA1092824A CA1092824A CA286,601A CA286601A CA1092824A CA 1092824 A CA1092824 A CA 1092824A CA 286601 A CA286601 A CA 286601A CA 1092824 A CA1092824 A CA 1092824A
- Authority
- CA
- Canada
- Prior art keywords
- glass
- range
- composition
- silica
- sio2
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C23/00—Other surface treatment of glass not in the form of fibres or filaments
- C03C23/008—Other surface treatment of glass not in the form of fibres or filaments comprising a lixiviation step
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C3/00—Glass compositions
- C03C3/04—Glass compositions containing silica
- C03C3/076—Glass compositions containing silica with 40% to 90% silica, by weight
- C03C3/089—Glass compositions containing silica with 40% to 90% silica, by weight containing boron
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C3/00—Glass compositions
- C03C3/04—Glass compositions containing silica
- C03C3/076—Glass compositions containing silica with 40% to 90% silica, by weight
- C03C3/089—Glass compositions containing silica with 40% to 90% silica, by weight containing boron
- C03C3/091—Glass compositions containing silica with 40% to 90% silica, by weight containing boron containing aluminium
Landscapes
- Chemical & Material Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Materials Engineering (AREA)
- Organic Chemistry (AREA)
- Glass Compositions (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE A method for producing a glass article which comprises melting and forming a preshaped glass article having a composition in the phase separable regions of the alkali-boro-silicate or alkali-boro-germania-silicate systems, inducing the article to phase separate, leaching out a silica-poor phase from the surface layers only to form a structure having porous surface layers surrounding a solid region of substantially the original glass com-position, washing this structure with an organic media which dis-solves oxides of boron and other leaching reaction products, drying and heating to collapse the outer porous structure to form a glass having a silica-rich surface layer surrounding a solid region having substantially the original glass composition. The method substantially reduces the breaking tendency. A glass material and preshaped glass articles are also disclosed.
Description
The invention relates to producing glass articles having a composition range which reduces the breaking tendency substantially.
A further embodiment of this invention discloses a heat-treatment proced-ure which also reduces the breaking tendency of partially leached articles.
The present invention describes a method of producing glass articles in which the composition of the surface layers of the walls is richer in silica than the central layers. According to this invention a phase separable composition in alkali borosilicate or germanate systems having 0 to 5 mole percent alumina is melted and formed in the conventional manner. Suitable compositions have been described in previous patents:
U.S. Patents Nos. 2,106,744; 2,215,039; 2,355,746 (Hood and Nordberg);
Nos. 3,843,341 (Hammel) and 3,938,974 (Macedo and Litovitz).
Depending on the composition, the glass is heat-treated at a temperature from about 500 C. to 700 C. for a few seconds to several weeks such that it separates into two phases having a totally interconnected microstructure with an average thickness of 100 - 2000 A preferably be-tween 150 and 500 A. These dimensions are much smaller than the wall thick-ness of the preformed article.
One of the phases is mostly covalent and will be referred to as the "hard" phase. The other is predomonantly ionic in nature and will be referred to as the "soft" phase. It has been found advantageous to heat treat the glass at as low a temperature as possible but one which is consistent with reasonably short heat treatment times. This reduces the possibility of any deformation of the preform due to viscous flow.
The soluble phase is then leached out by a suitable leaching solution only up to a definite depth leaving an unleached central layer in the glass walls. Since the glass article is not leached throughout its mass but only in the surface layers, we call it "the partial leaching technique" in contrast to other processes where the leaching is carried out throughout the mass of the article.
A further embodiment of this invention discloses a heat-treatment proced-ure which also reduces the breaking tendency of partially leached articles.
The present invention describes a method of producing glass articles in which the composition of the surface layers of the walls is richer in silica than the central layers. According to this invention a phase separable composition in alkali borosilicate or germanate systems having 0 to 5 mole percent alumina is melted and formed in the conventional manner. Suitable compositions have been described in previous patents:
U.S. Patents Nos. 2,106,744; 2,215,039; 2,355,746 (Hood and Nordberg);
Nos. 3,843,341 (Hammel) and 3,938,974 (Macedo and Litovitz).
Depending on the composition, the glass is heat-treated at a temperature from about 500 C. to 700 C. for a few seconds to several weeks such that it separates into two phases having a totally interconnected microstructure with an average thickness of 100 - 2000 A preferably be-tween 150 and 500 A. These dimensions are much smaller than the wall thick-ness of the preformed article.
One of the phases is mostly covalent and will be referred to as the "hard" phase. The other is predomonantly ionic in nature and will be referred to as the "soft" phase. It has been found advantageous to heat treat the glass at as low a temperature as possible but one which is consistent with reasonably short heat treatment times. This reduces the possibility of any deformation of the preform due to viscous flow.
The soluble phase is then leached out by a suitable leaching solution only up to a definite depth leaving an unleached central layer in the glass walls. Since the glass article is not leached throughout its mass but only in the surface layers, we call it "the partial leaching technique" in contrast to other processes where the leaching is carried out throughout the mass of the article.
-2-- ~ :
.
~9~8Z4
.
~9~8Z4
3.
We h~ve discovered that in order to increase the survival proba~ility in the subsequent stages of the process, it is ad- ' vantageous to wash the partially leached article with a suitable organic solution which should contain less than 5~ weight per-cent water, preferably less than 25~ water. The organic contain- j ing solvent media should ~e one that preferentially keeps in solu-tion the oxides of boron and other leaching reaction products of the silica-poor phase which'are formed during leaching, without substantially attacking the silica~poor phase. While not intend-ing to be-bound by the veracity of the mechanism, we theorize that removal of the oxides of boron and other leaching reaction pro- I'~
ducts from the porous layer greatly increases the probability for the survival of the glass article through the process. If only water is used as a washing media, the silica-poor phase is attacked l~
during washing, generating constantly new reaction products and j, preventing the porous glass fxom bei'ng cle~ned. The organic components of the media are w~ter ~iscible and include the lower molecular ~eight' aliphatic alcohols, containing from 1 to 5 carbon atoms, or acetone or mixtures thereof, ~ere found to be especially`good, methanol and ethanol being preferable. The organics in an aqueous media decrease the rate of attack of 5;/ic~
water on the slilca-poor phases' while still permitting the l porous glass to be washed. Merely washing with water as de- ~¦
scribed by Hood and Nordberg results in excessively large breakage rates in the later stages of the process, making it economically impractical. By using the approach of the present invention, i.e., washing with methanol at room temperature for times varying from 5 minutes to 24 hours, depending on the thickness of the leached layer, it is possible to reduce the breakage rate to a level where the process becomes economically feasible. The washing temperature is not critical. For economic reasons alone, it is desirable to carry out the washing at or near the room temperature.
After washing, the article is dried as usual and consolidated at tempera'tures in the range of 800-1000C, depending on the com-8'~
position of the porous glass in the surface layers on solid glass objectssuch as tubes and bottles. The thickness of the silica-rich surface layer, which has greater strength than the remainder of the article, varies with the intended use. For example, the thickness may range from 0.1 mm for uses where minimum abrasion is present up to several mm for uses where severe con-ditions will be encountered. In each case, silica-rich surface layers of the desired thickness can be produced on all exposed surfaces of the art-icle regardless of shape and the resultant article will have improved phy-sico-chemical properties associated with the presence of silica-rich sur-faces. The glass articles produced according to this invention can be usedto make cooking utensils, radar domes, air and space craft windshields, windows for structural applications, containers such as jars, bottles and pipes, and chemical ware such as reaction vats.
We have further discovered that the breakage rate is substan-tantially reduced provided the composition is selected from the following ranges (in mole per cent):
Broad Preferred Most Preferred SiO2 50-66 58-65 60-62.5 B2O3 28-42 30-35 32.35 R20 3.5-9 4-7 4.5-5.5 2 3 0-1 0-0.5 f~ 0.4-0.85 0.5-0.75 0.6-0.7 where R2O is the total concentration of all alkali metal oxides and f is the ratio of the sum of K20, Rb2O and Cs20 contents to R2O content.
We have also discovered that:
i) The most preferred composition range yields the highest survival rates for a given article. As one goes to the preferred and then to the broad range, good survival rates are still obtained. However, the survival probability decreases from the preferred range to the broad range.
ii) The survival probability increases as the ratio, x, of thickness of porous layer to wall thickness of the article increases for any given composition and processing conditions.
We h~ve discovered that in order to increase the survival proba~ility in the subsequent stages of the process, it is ad- ' vantageous to wash the partially leached article with a suitable organic solution which should contain less than 5~ weight per-cent water, preferably less than 25~ water. The organic contain- j ing solvent media should ~e one that preferentially keeps in solu-tion the oxides of boron and other leaching reaction products of the silica-poor phase which'are formed during leaching, without substantially attacking the silica~poor phase. While not intend-ing to be-bound by the veracity of the mechanism, we theorize that removal of the oxides of boron and other leaching reaction pro- I'~
ducts from the porous layer greatly increases the probability for the survival of the glass article through the process. If only water is used as a washing media, the silica-poor phase is attacked l~
during washing, generating constantly new reaction products and j, preventing the porous glass fxom bei'ng cle~ned. The organic components of the media are w~ter ~iscible and include the lower molecular ~eight' aliphatic alcohols, containing from 1 to 5 carbon atoms, or acetone or mixtures thereof, ~ere found to be especially`good, methanol and ethanol being preferable. The organics in an aqueous media decrease the rate of attack of 5;/ic~
water on the slilca-poor phases' while still permitting the l porous glass to be washed. Merely washing with water as de- ~¦
scribed by Hood and Nordberg results in excessively large breakage rates in the later stages of the process, making it economically impractical. By using the approach of the present invention, i.e., washing with methanol at room temperature for times varying from 5 minutes to 24 hours, depending on the thickness of the leached layer, it is possible to reduce the breakage rate to a level where the process becomes economically feasible. The washing temperature is not critical. For economic reasons alone, it is desirable to carry out the washing at or near the room temperature.
After washing, the article is dried as usual and consolidated at tempera'tures in the range of 800-1000C, depending on the com-8'~
position of the porous glass in the surface layers on solid glass objectssuch as tubes and bottles. The thickness of the silica-rich surface layer, which has greater strength than the remainder of the article, varies with the intended use. For example, the thickness may range from 0.1 mm for uses where minimum abrasion is present up to several mm for uses where severe con-ditions will be encountered. In each case, silica-rich surface layers of the desired thickness can be produced on all exposed surfaces of the art-icle regardless of shape and the resultant article will have improved phy-sico-chemical properties associated with the presence of silica-rich sur-faces. The glass articles produced according to this invention can be usedto make cooking utensils, radar domes, air and space craft windshields, windows for structural applications, containers such as jars, bottles and pipes, and chemical ware such as reaction vats.
We have further discovered that the breakage rate is substan-tantially reduced provided the composition is selected from the following ranges (in mole per cent):
Broad Preferred Most Preferred SiO2 50-66 58-65 60-62.5 B2O3 28-42 30-35 32.35 R20 3.5-9 4-7 4.5-5.5 2 3 0-1 0-0.5 f~ 0.4-0.85 0.5-0.75 0.6-0.7 where R2O is the total concentration of all alkali metal oxides and f is the ratio of the sum of K20, Rb2O and Cs20 contents to R2O content.
We have also discovered that:
i) The most preferred composition range yields the highest survival rates for a given article. As one goes to the preferred and then to the broad range, good survival rates are still obtained. However, the survival probability decreases from the preferred range to the broad range.
ii) The survival probability increases as the ratio, x, of thickness of porous layer to wall thickness of the article increases for any given composition and processing conditions.
-4-.
: , 10~824
: , 10~824
5.
iiil The ~urvival probability is high, provided the heat treatment ~o cause phase separation is chosen according to the following table:
- ~ - road Prefexred Temp. (C) 480-550 500~530 Time (hrs.) 1/2-200 5-1~0 Finally, it is a well-known fact that the~survival probability decreases as the size of the article increases. Therefore, whether in practice one uses the most preferred composition and/or heat-treatment ranges or the broadest ranges depends on the ratio,x, ar.d the size of the article. For small siæe and larger x, the broad range may be most economical. On the other hand, large article size and small values of x clearly require the most preferred range.
EXAMPLES
1. A mixture having the composition (in mole %) 4 Na2O, 4 K2O, 36 B2O3 and 56 SiO2 was melted-and stirred to produce a homogeneous melt from which rods were drawn having a diameter in the range of 7 to 8 mm. The rods were heat-treated at 550C for 1.5 hrs. to cause phase separation and then the furnace was cooled.
The rods were partially leached at 95C with 3N HCl. The leaching time was chosen to be two hrs. which corresponded to a leached layer of about 1 mm in thickness. The partially leached rods were washed in methanol at room temperature for a period of 24 hrs. and dessicated at room temperature for 24 hrs. The rods were heated at a rate of 1C/min. up to 150C and then at a rate of 2C/min. up to about 850C when they consolidated. The glass article thus produced was clear in appearance and had a surface layer 1 mm thick containing more than 90 mole % of SiO2.
Photoelastic measurements showed that the surface layex had a uniform compression of 24,000 psi. The process may also be carried out using ethanol, propanol, acetone and aqueous mixtures containing 50 wt. % methanol and 75 wt. ~ methanol instead of methanol in the above example.
109'~8'~:4
iiil The ~urvival probability is high, provided the heat treatment ~o cause phase separation is chosen according to the following table:
- ~ - road Prefexred Temp. (C) 480-550 500~530 Time (hrs.) 1/2-200 5-1~0 Finally, it is a well-known fact that the~survival probability decreases as the size of the article increases. Therefore, whether in practice one uses the most preferred composition and/or heat-treatment ranges or the broadest ranges depends on the ratio,x, ar.d the size of the article. For small siæe and larger x, the broad range may be most economical. On the other hand, large article size and small values of x clearly require the most preferred range.
EXAMPLES
1. A mixture having the composition (in mole %) 4 Na2O, 4 K2O, 36 B2O3 and 56 SiO2 was melted-and stirred to produce a homogeneous melt from which rods were drawn having a diameter in the range of 7 to 8 mm. The rods were heat-treated at 550C for 1.5 hrs. to cause phase separation and then the furnace was cooled.
The rods were partially leached at 95C with 3N HCl. The leaching time was chosen to be two hrs. which corresponded to a leached layer of about 1 mm in thickness. The partially leached rods were washed in methanol at room temperature for a period of 24 hrs. and dessicated at room temperature for 24 hrs. The rods were heated at a rate of 1C/min. up to 150C and then at a rate of 2C/min. up to about 850C when they consolidated. The glass article thus produced was clear in appearance and had a surface layer 1 mm thick containing more than 90 mole % of SiO2.
Photoelastic measurements showed that the surface layex had a uniform compression of 24,000 psi. The process may also be carried out using ethanol, propanol, acetone and aqueous mixtures containing 50 wt. % methanol and 75 wt. ~ methanol instead of methanol in the above example.
109'~8'~:4
6.
2. A-m~xture having the composition ~ir. mole per cent? ~' 61-.3 SiO2, 33.4-B203, 1.8 Na20 and 3.5 K20 was melted and stirred t to~p'roducê' a~homogeneous melt from which rods were drawn having diameters in the range of 7 to 8 mm. The rods were heat-treated at~ 515C fo'r 100 hrs. to cause phase separation and the furnace wa~s-cooled.
The rods were par~ially leached, washed, dried and consoli-.
da-ted as described in Example 1. Breakage rate was found to be l-ess than 5%. The finished rods had a surface layer about 0.50 mm' t~ick in compression of about 30,000 psi. These rods were abraded an~ tested in three point bending. The modulus o~ rupture was fo'un~d to be 40,000 psi.
`. _ ll
2. A-m~xture having the composition ~ir. mole per cent? ~' 61-.3 SiO2, 33.4-B203, 1.8 Na20 and 3.5 K20 was melted and stirred t to~p'roducê' a~homogeneous melt from which rods were drawn having diameters in the range of 7 to 8 mm. The rods were heat-treated at~ 515C fo'r 100 hrs. to cause phase separation and the furnace wa~s-cooled.
The rods were par~ially leached, washed, dried and consoli-.
da-ted as described in Example 1. Breakage rate was found to be l-ess than 5%. The finished rods had a surface layer about 0.50 mm' t~ick in compression of about 30,000 psi. These rods were abraded an~ tested in three point bending. The modulus o~ rupture was fo'un~d to be 40,000 psi.
`. _ ll
Claims (18)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A glass material comprising a body having a silica-poor inner layer and a silica-rich surface layer having a col-lapsed pore structure, the material having been prepared from an alkali-boro-silicate or alkali-germania-silicate preform; the inner layer having substantially the same composition as the pre-form.
2. A material according to claim 1, wherein the pre-form has a composition in mole percent in the range of SiO2 50-66 R2O 3.5 - 9.0 A1203 o-3 wherein R2O represents the total alkali metal oxide content, said composition having the ratio, g , of the sum of K2O, Rb2O, and Cs2O contents to the R2O content, in the range 0.4 to 0.85.
3. A material according to claim 2, wherein the comp-osition in mole percent is in the range SiO2 58-65 AL2o3 0-1 g 0.5-0.75
4. A material according to claim 3, wherein the comp-osition in mole percent is in the range SiO2 60-62.5 R2O 4.5-5.5 A12O3 0-0.5 P 0.6-0.7, and wherein R2O stands for the sum of Na2O and K2O contents.
5. A material according to claim 1, 2 or 3, wherein the surface layer has a wall thickness in the range of from 0.1 millimetres to several millimetres.
6. A material according to claim 1, 2 or 3, wherein the surface layer has a compression of at least approximately 24,000 psi.
7. A material according to claim 1, 2, or 3, in the form of a preshaped glass article.
8. A method for producing a glass article which comprises melting a composition in the phase separable regions of the alkali-boro-silicate or alkali-germania-silicate systems and forming a preshaped glass article, inducing the preshaped art-icle to phase separate by heat treating it at 480°C. to 550°C.
for a period of time to leach a silica-poor phase from the surface layers only to form a structure having porous surface layers sur-rounding an unleached region of substantially the original glass composition, washing this structure with an organic media which is miscible with and which contains less than 50% water to dis-solve and remove oxides of boron and other leaching reaction products without substantially attacking the silica-poor phase, drying and heating the washed glass article to collapse the outer porous surface layers to form a glass having a silica-rich sur-face layer surrounding said unleached region of substantially the original glass composition.
for a period of time to leach a silica-poor phase from the surface layers only to form a structure having porous surface layers sur-rounding an unleached region of substantially the original glass composition, washing this structure with an organic media which is miscible with and which contains less than 50% water to dis-solve and remove oxides of boron and other leaching reaction products without substantially attacking the silica-poor phase, drying and heating the washed glass article to collapse the outer porous surface layers to form a glass having a silica-rich sur-face layer surrounding said unleached region of substantially the original glass composition.
9. A method according to claim 8, wherein said heat treatment is continued for a period of from 0.5 hour to 200 hours.
10. A method according to claim 8, wherein said comp-osition in mole percent is in the range SiO2 50-66 R2O 3.5-9.0 A12°3 0-3 ? 0.4-0.8.5 wherein R2O represents the total alkali metal oxide content, and wherein ? represents a ratio the value of the numerator being the sum of the K2O, Rb2O and Cs2O contents and the value of the denom-inator being the R2O content in the composition.
11. A method according to claim 10, wherein said comp-osition in mole percent is in the range SiO2 58-65 ? 0.5-0.75
12. A method according to claim lO wherein said comp-osition in mole percent is in the range SiO2 60-62.5 R2O 4.5-5.5 A12O3 0-0.5 ? 0.6-0.7
13. A method according to claim 11, wherein Na2O and K2O are the only R2O components present.
14. A method according to claims 8, 10 or 13, wherein the organic media is methanol.
15. A method according to claim 13, wherein the heat treatment is in the temperature range of from 500-530°C. for a period of time in the range of from 5-150 hours.
16. A method according to claim 8, wherein said organ-ic media contains less than 25% H2O.
17. A method according to claim 8, where said organic media are low molecular weight aliphatic alcohols containing 1-5 carbon atoms, acetone, and mixtures thereof.
18. A method according to claim 16, or 17 where said organic media is methanol.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US72485276A | 1976-09-20 | 1976-09-20 | |
| US724,852 | 1976-09-20 | ||
| US82744877A | 1977-08-25 | 1977-08-25 | |
| US827,448 | 1977-08-25 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1092824A true CA1092824A (en) | 1981-01-06 |
Family
ID=27111058
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA286,601A Expired CA1092824A (en) | 1976-09-20 | 1977-09-13 | Borosilicate glass articles and method of making |
Country Status (8)
| Country | Link |
|---|---|
| AU (1) | AU514242B2 (en) |
| CA (1) | CA1092824A (en) |
| DE (1) | DE2741759A1 (en) |
| DK (1) | DK415777A (en) |
| GB (1) | GB1587844A (en) |
| IE (1) | IE46043B1 (en) |
| NL (1) | NL7710249A (en) |
| SE (1) | SE429548B (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE4443172A1 (en) * | 1994-12-05 | 1996-06-13 | Ilmenauer Glaswerke Gmbh I L | Porous glass body with layered structure |
-
1977
- 1977-09-13 CA CA286,601A patent/CA1092824A/en not_active Expired
- 1977-09-16 AU AU28883/77A patent/AU514242B2/en not_active Expired
- 1977-09-16 DE DE19772741759 patent/DE2741759A1/en active Pending
- 1977-09-16 GB GB38652/77A patent/GB1587844A/en not_active Expired
- 1977-09-19 SE SE7710489A patent/SE429548B/en unknown
- 1977-09-19 NL NL7710249A patent/NL7710249A/en not_active Application Discontinuation
- 1977-09-20 DK DK415777A patent/DK415777A/en not_active Application Discontinuation
- 1977-09-20 IE IE1922/77A patent/IE46043B1/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| IE46043L (en) | 1978-03-20 |
| GB1587844A (en) | 1981-04-08 |
| AU2888377A (en) | 1979-03-22 |
| IE46043B1 (en) | 1983-02-09 |
| AU514242B2 (en) | 1981-01-29 |
| SE7710489L (en) | 1978-03-21 |
| SE429548B (en) | 1983-09-12 |
| DE2741759A1 (en) | 1979-05-23 |
| NL7710249A (en) | 1978-03-22 |
| DK415777A (en) | 1978-03-21 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry | ||
| MKEX | Expiry |
Effective date: 19980106 |