SCIENCE
IN G L A S S
PRODUCTION
COMPOSITION DEPENDENCE OF SHEET-GLASS PROPERTIES (REVIEW) I.
N. Gorina and A. P. Zhil'tosv
UDC 666.151:666.11.01:666.113
The composition for sheet glass should be chosen mainly on the basis that the glass should melt sufficiently easily, homogenize readily, and have a high viscosity at the shaping temperature, since the last is the basis for high processing speeds. Also, the liquidus temperature should be relatively low, which eliminates the hazard of devitrification during processing. The finished glass should also have high chemical resistance. Glass compositions vary with the processing methods, furnace design, applications, and the specifications for their working properties. Sheet glass is made from compositions in the five-component SiO2-AI203-CaO-MgO-Na20 SiO 2 is the major component (sheet glass contains about 70% SiO2, here and subsequently mass contents). SiO 2 increases the viscosity, technical strength, and chemical resistance, but the melting behavior deteriorates, as does the capacity for homogenization, and the tendency to crystallize rises. system.
Calcium oxide is a good flux, particularly in glasses containing less than 75% SiO2, and has a favorable effect on the strength and resistance. Therefore, raising the CaO in the glass to 9-11% increases the melting rate by a factor 1.5. CaO also reduces the viscosity at high temperatures but raises it in the forming range, which favors increased working rates. A shortcoming of CaO is that it tends to favor crystallization. Magnesium oxide reduces the crystallization tendency and is usually added at a level of not more than 4%, since higher levels sharply raise the viscosity and reduce the water resistance. The MGO:CaO ratio is important. There is usually i-2% aluminum oxide in the glass, although higher levels would be favorable since they improve the resistance and the crystallization behavior. Elevated levels of AI203 are restricted by its making the glass much more refractory and reducing the capacity for homogenization, which is often the cause of swirls. Sodium oxide is a very effective component as it reduces the melting temperature, suppresses crystallization, and favors homogenization. It reduces the viscosity throughout the temperature range, so Na20 reduces the working rate. The optimum Na20 content of sheet glass is 13.5% [i]. Potassium oxide is a very useful additive, as it improves the crystallization behavior, gives the glass luster, and raises the setting rate, which enables one to increase the processing speed. However, until recently, not more than 0.5% K20 has been used in glass because of the lack of reliable raw-material sources that include K=O. Sheet glass is processed mainly by three methods: vertical boat pulling VBP, vertical boat-free pulling VBFP, and the flat method. Table i gives typical glass compositions [2].
TABLE 1 Mass content in glass, %
Working method VBP VBFP Float
sio:, 72,4+0,3 72.6zh0,3 71,8--73,0
Al,O~
l:e~O.,
2,0• 1,5• 0,9-- 1,9
0,2• 0,12~0,01 0, I --0,4
Technical Building Glass Cooperative. 2-4, October, 1991.
0361-7610/91/0910-0425512.50
1
CaO 6,6• 8,2--+_0,2 8,2--9,0
~4gO
Na~O+K~O as Na2 0
4,2• 3,5• 3,3--3,8
14.8zk(),2 13,0:L0,2 13,4-- 14,0
so~ " 0,6 0.4 0,3--0.5
Translated from Steklo i Keramika, No. i0, pp.
9
Plenum Publishing Corporation
425
TABLE 2 Mass content in glass, % SiO2
AI203 Fe~O~
CaO
MgO
65--75
0--5
5--15
0--10
70--75
I--2
5--10
France: St, Gobain (Patent 2082647)
70--73
0,2--2
St. Gobain (Patent 225965)
71.74
Patent 2058439
70,2
Country and firm
Japan: Asahi Glass K. K. (Patent 70-23221)
Nihon Itaglass K. K. (patent 71-
Na~O K~O
SO~
others
10--18 0--5
--
--
I--5
10--15
--
6,5--12,5
I--4,5
12--16
--
0,4 0,05 3,5
7,4
4,0
0,29
9,2
2,8
14 0,12 14 [3,47 0.21
0.32
- -
41579)
0,3
0 - - 2 ZnO - -
Sweden~ Tremplex
71,15
0,73 0,09
9,5
4,15
Britain: Patent 1423025
71,6
10.3
1.8
5,5--7,7
3,5--4,9
14,1 _. 15.5--19 0--0.5
8.97
3.18
13.6
0.23
11.5--13
--
12.9--14,4
0,35
65L-75
1,8 0.26 0.1--1.5 0.03--0,7 0,9 0,09 1,55--2.1 0,084 l- 3
I--6
i[o 3
USA (Patent 3846143)
74,1
9,4
--
12--17 ~.o I 14,7
--
1--10 1,8
Czechoslovakia
72,38
8.57
3,97
13,55
0.29
0,05 TiO~
Italy
72,44
8,19
3,77
13,42
0,37
Cu, Co
1,5-. 2
6,8--7.7
4.42--5
12.5-- 14
--
0.9 O.I 0.9 0,1 1.4 0.1 1
8,75
3,70
9,0
3,3
13.15 0.2 13,4
8,6
3,8
13,7
8,7
3,6
1,9 0,1 1.4
8,6
70--73,3
Patent 1400053 Pilkington
73
Turkey
71,45--72
GDR (Patent 220592)
1.23 O,]l ] ,3
--
0.13 0,2
--
- -
- -
- -
(1.27 [
USSR:
70--73 I
a.c. 8811024 Bohr glass plant
72,35
Salavat glass plant
73.0
Proletarian g l a s s
72,6
plant
Avtosteklo glass plant
72,8
Saratov glass plant
71.9
Tekhstroisteklo
71,8
Cooperative
J 0.33
E
0.8-- 1.5 As~O3. 0,4--IF --
0,3
--
0,4
--
13,4
0,5
--
3.7
13.4
0,4
--
8,2
3,6
14,0
i0,35
--
8.3
3,7
13,4
0.4
--
0,3 Unified composition
72,8
1.4
Much attention has been given [1-6] to improving the properties of glass by adjusting the composition. Table 2 gives compositions for sheet glasses worked mainly by horizontal methods. The ranges in % for the components in the glass are quite wide: 65-75 SiO 2, 1-3 AI=O 3, 5.5-12.5 CaO, 1.8-5 MgO, 10-19 Na20, up to 0.5 K20. The main attention in composition choice is directed to the chemical resistance, crystallization tendency, melting behavior, and viscosity. The chemical resistance is of primary significance in making float glass and can be raised by reducing the Na20 level and increasing the AI~03 [i, 2]. The A1203 content is restricted to 2%, since larger amounts hinder melting and markedly increase the tendency to crystallize. An Na20 content of 13% is probably the limit [7], and any further reduction is undesirable. High shaping speeds (over i000 m/h) have been attained recently and have been responsible for tightened specifications on glass melting, which is important also in reducing the energy consumption. 426
There are many ways in which the glass composition could be varied, particularly to raise the melting rate, but some lines deserve attention. In British Patent 1400953, it is proposed that the contents of the alkali oxides should be increased to 15.5-19%, with the viscosity at the melting temperature reduced by 10-20%, which favors more rapid maturation. However, the chemical resistance deteriorates considerably. It is promising to raise the alkaline-earth oxide level to more than 12.5% [8-10]; with the optimum MgO:CaO ratios fo 0.28 and 0.49, one gets the highest melting rates. Some researchers have sought to accelerate the melting by adding 0.5-8% ZnO (French Patent 2259065, Authors' Certification 1030329 and 1143701). Also, 0.5-3% of the Na20 may be replaced by Li20 [8, 9-11]. Other components such as As203, F, and CeO 2 have not been widely used because they are toxic and scarce (Authors' Certificate 881024). The following results have been obtained in tests on polished glass from various plants and firms, including vitrification rate, upper and lower crystallization-temperature limits, viscosity, expansion coefficient, density, strength, chemical resistance, light transmission, and thermophysical, electrical, and other parameters within the usual composition ranges: i) the thermophysical, electrical, and elastic parameters and the viscosity and microhardness are virtually the same; 100~
2) the tendency to crystallization varies somewhat, but the safe forming interval (60eliminates the crystallization hazard during working;
3) the melting parameters improve by about a factor 1.5 when the alkaline-earth oxide content is raised to 13.0-13.5%, but any further increase in the amount of RO raises the upper crystallization limit (to over I050~ 4) there is a tendency for the strength to increase by about a factor 1.3 with the alkaline-earth oxide content; and 5) as regards chemical resistance, the glasses belong to hydrolytic class III, where their chemical resistance can be raised by increasing the AI=03 content and reducing the amounts of alkali oxides or by partial replacement of Na20 by K20. LITERATURE CITED i. 2. 3. 4. 5. 6.
7. 8.
9.
I0. ii.
I. N. Gorina, Researches on Sheet Glass Synthesis with Reduced Alkaline Contents: Ph. D. Thesis [in Russian], Moscow (1977). T. D. Andryukhina, E. I. Raevskaya, E. I. Sanina, et al., Chemical Composition for Mass-Produced Glasses [in Russian], VNIIESM (1986). A. K. Lyle and P. V. Tooly, "Glass composition design and developments," The Handbook of Glass Manufacture, Reinhold, New York (1974), pp. 1-17. I. N. Gorina, B. E. Romanov, and G. I. Postnikova, "Melting properties of glasses worked by rolling," Steklo Keram., No. 5, 18-19 (1972). I. N. Gorina, V. I. Kondrashov, and B. E. Romanov, Heat-Absorbing Sheet Glass [in Russian], VNIIESM (1986). D. L. Orlov, T. D. Andryukhina, E. I. Raevskaya, et el., "Current trends in optimizing commercial glass compositions," in: Abstracts for the Second All-Union Conference [in Russian], Moscow (1983), p. 142. K. Bingham, "Low soda development," Ceram. Eng. Sci. Proc., ~, No. 3-4, 186-191 (1982). Esin Kazazogku, Galgin Albayrok, and Alev Jaraman, "Effect of MgO/CaO ration on the melting behavior of soda-lime silica glasses," XIII Intern. Glass Cong., Hamburg (1983). Glastechn. Ber., 56, 19-24 (1983). P. D. Sarkisov and V. A. Zavgorodnev, "New sheet-glass compositions and raw materials for making them," in: Advances in Glass Production, Abstracts for the Second All-Union Conference [in Russian], Moscow (1983), p. 123. A. J. Grabowski, "Chemical composition of glass for use in sheet rolling," Steklo Ceram., No. 15, 180-184 (1964). J. J. Angelo, "Lithium in glass and glass melting," Glass, No. 12, 447 (1986).
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