INCREASING

THE

STRENGTH

I. N. Y a s h c h i s h i n , and

Z.

OF

FLAT

GLASS

V. V. S h e v c h e n k o ,

UDC 666.151.002.237

V. G o r b a i

The problem of improving the strength of glass is becoming m o r e and m o r e important. The development of methods which will not only produce the s t r o n g e r glass n e c e s s a r y for t r a n s p o r t applications but also for thin window glass is playing an important p a r t in this p r o b l e m . A m a r g i n a l increase lathe strength of window glass had a l r e a d y made it possible to reduce breakages during cutting, t r a n s p o r t a t i o n , and on the construction sites.

TABLE i Amount of MoisturereTreatment Na~SOv nag sistance, ml temperature, /I00 cm2 10.O1NHC1 /100 cm2of ~ of glass glass Untreated 300 450 500 550 600

2.05 0,92

3,37 4,90 5,40 6,00 6,90

o~7

0,54 0,43

TABLE 2 Heat-chemically Untreated glass Ireated glass

We have studied the interaction between the SO2 and the s u r face of the g l a s s . A connection was found between the amount of sodium sulfate deposited, the t e m p e r a t u r e of the heat and chemical t r e a t m e n t of the g l a s s , and the p r o p e r t i e s of the latter. Flat industrial glass made by the L ' v o v Mechanized Glass Plant was used; the composition of the g l a s s was as follows %: 72.42 SIO2, 0.12 Fe203, 1.66 A1203, 7.67 CaO, 3.76 MgO, 13.97 Na20 , 0.4 SO 3.

0 ~d2

t~ ~m

100 200 300 450 500 550 600

It is well known [1] that treating glass with SO2 produces a blueg r e y deposit, which is in fact sodium sulfate, on the s u r f a c e . The formation of Na2SO 4 is a s s o c i a t e d with the interaction between the alkali oxides and the SO2 resulting in the alkali-depletion of the s u r face. This in turn will cause a change in the strength of the g l a s s and an i n c r e a s e in its chemical r e s i s t a n c e [2].

2,2 2,2 2,2 2,2 2,2 2,2 2,2

502 502 502 502 502 502 562

2,2 2,3 2,5 3,1 3,4 3,5 3,8

522 520 586 612 622 665 664

TABLE 3 Heat-chemically Untreated glass ~reated glass ~P

I5 I 2,2 30 2,2 60 2,2 120 2,2

502 502 502 502

3,5 [ 665 4,1 665 3,9 646 4,0 595

The impact strength was d e t e r m i n e d on specimens m e a s u r i n g 120 x 15 • 3 m m using a PSV-0.4 pendulum impact t e s t e r to an a c c u r a c y of +0.2 kJ/m2; and the m i c r o h a r d n e s s on a P M T - 3 m i c r o hardness m e t e r at a load of 50 g with an a c c u r a c y of ~100 MPu. A chemical method was used to determine the amount of sodium sulfate deposited. In o r d e r to establish the w a t e r r e s i s t a n c e , s p e c i mens m e a s u r i n g 15 • 20 ram with a total a r e a of 100 cln z w e r e b o i l e d for 10 h. Specimens with a total a r e a of 450 em z were p r e p a r a t o r i l y h e a t - c h e m i c a l l y t r e a t e d in a closed muffle furnace. Sulfur dioxide supplied in cylinders with a m o i s t u r e content of 0.15 g / l i t e r was used in the study. As the gas was not specially moistened, h e n c e forth the t e r m "dry" gas is used. The consumption of gas in all c a s e s was 0.9 l i t e r / r a i n . T h e studies of the dependence of the amount of Na2SO 4 which is f o r m e d and the m o i s t u r e r e s i s t a n c e of the g l a s s specimens on the t e m p e r a t u r e of the h e a t - c h e m i c a l t r e a t m e n t suggests that the t e m p e r a t u r e is the m o s t important factor which determines the rate of the r e a c t i o n between the s u r f a c e of the g l a s s and the gas (Table 1 and Fig. i).

L ' v o v Polytechnic Institute. L ' v o v Mechanized Glass Plant. T r a n s l a t e d f r o m Steklo i Keramika, No. 8, pp. 6-7, August, 1974. 9 1975 Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

531

Q,k.l/m 2 Q, kJ/m2

H-10

Y]Pll

9

f

H-10MPu

~s

J,s ~0

~o Z

r~

o,..~

~0

~ot

,~o ~'~

2,6

NO

22 t

22

~5 ) ;

t

t

5~

6o 7 5 . ~ ' ~1o5

Time,, ~ n

Fig. 1

Fig. 2

Fig. 3

Fig. 1. The dependence of the amount of Na2SO 4 and the m o i s t u r e r e s i s t a n c e oa the h e a t - c h e m i c a l t r e a t m e a t : 1) Na2SO4; 2) m o i s t u r e r e s i s t a n c e . F i g . 2 . The dependence of the impact strength and m i c r o h a r d n e s s oa the t e m p e r a t u r e of the h e a t - c h e m i c a l t r e a t m e n t : 1) impact strength of the s p e c i m e n s ; 2) m i e r o h a r d n e s s of the s p e c i m e n s . Fig. 3. The dependence of the impact v i s c o s i t y and the m i c r o h a r d n e s s on the t i m e of h e a t - c h e m i c a l t r e a t m e a t : 1) i m p a c t strength; 2) m i c r o h a r d n e s s of s p e c i m e n s . The r e s u l t s of the study of the strength of g l a s s as a function of the t e m p e r a t u r e of the h e a t - c h e m i c a l t r e a t m e n t (Table 2 and Fig.2) can be used to c o n f i r m the fact that the t e m p e r a t u r e is one of the m o s t i m portant conditions for the r e a c t i o n . The t r e a t m e n t t i m e plays an important p a r t in the interaction between the s u r f a c e of the g l a s s and the SO2. The h e a t - c h e m i c a l t r e a t m e n t of the s p e c i m e n s for longer than 30 rain does not give an i n c r e a s e ia the impact s t r e n g t h while the m i e r o h a r d n e s s is lowered (Table 3, Fig. 3). Weil showed in [3] that as a r e s u l t of leaching out,the s u r f a c e of the g l a s s a c q u i r e s the p r o p e r t i e s c h a r a c t e r i s t i c of g l a s s e s with a low alkali concentration, i.e., it has a low coefficient of t h e r m a l expansion. Thus, on cooling, the s u r f a c e l a y e r is ia a state of c o m p r e s s i o n which m a y be the cause of the i n c r e a s e in m e chanical strength. Using " d r y " SO2 we e s t a b l i s h e d that the m i c r o h a r d n e s s of the g l a s s i n c r e a s e s a f t e r it has been t r e a t e d with the gas and depends on t e m p e r a t u r e . T h i s p r o v i d e s a b a s i s for maintaining that when the g l a s s is t r e a t e d by " d r y " SO 2 the strengthening does not o c c u r as a r e s u l t of the e m e r g e n c e of c o m p r e s s i o n s t r e s s e s in the s u r f a c e l a y e r but as the r e s u l t of a change in the s t r u c t u r e of the s u r f a c e . The m i c r o p h o t o g r a p h s of the s u r f a c e s t r u c t u r e visible at a depth of 108/~ (Fig.4) indicate the d i f f e r ence between the s t r u c t u r e of the s u r f a c e of the untreated and SO2-treated g l a s s e s . The m i c r o p h o t o g r a p h s w e r e obtained by photographing the s u r f a c e of the s p e c i m e n s of the u n t r e a t e d and h e a t - c h e m i c a l l y t r e a t e d g l a s s etched in 20% HF. A study of the m e c h a n i s m b y which the r e l i e f l a y e r is f o r m e d in the f o r m of indentations (Fig.4b) sugg e s t s that the r e l i e f l a y e r is f o r m e d a s the r e s u l t of the r e m o v a l of Na + ions f r o m the g l a s s during etching. It follows f r o m the above that s u r f a c e leaching o c c u r s as a r e s u l t of the h e a t - c h e m i c a l t r e a t m e n t of the g l a s s in an SO 2 a t m o s p h e r e . Thus the c h e m i c a l r e s i s t a n c e of the g l a s s is i n c r e a s e d . The amount of sodium sulphate f o r m e d on the s u r f a c e of the g l a s s i n c r e a s e s with an i n c r e a s e in the t e m p e r a t u r e and t i m e of the t r e a t m e n t . The amount of Na20 which e n t e r s the solution f r o m the g l a s s during boiling is in i n v e r s e p r o p o r t i o n to the amount of sodium sulfate f o r m e d . C l e a r l y the change in the c h e m i c a l composition of the s u r f a c e l a y e r s of the g l a s s leads to an i n c r e a s e in the m i c r o h a r d n e s s . Thus, as a r e s u l t of the interaction between the SO 2 and the s u r f a c e of the g l a s s and the f o r m a t i o n of the r e a c t i o n p r o d u e t s , t h e s t r u c t u r e of the s u r f a c e l a y e r s changes. The defects at the s u r f a c e of the g l a s s a f t e r the h e a t - c h e m i c a l t r e a t m e n t c l e a r l y acquire a f o r m l e s s dangerous for concentration of the s t r e s s e s and this p r o d u c e s an i n c r e a s e in the impact strength. M o r e o v e r , t h e r e is a b a s i s for a s s u m i n g that the r e a c t i o n p r o d u c t s t h e m s e l v e s have an important effect on the s t r e n g t h of the g l a s s .

532

Fig.4. Microphotography (x200) of the structure of the surface layer of fiat glass at a depth of 108 #: a) untreated; b) heat-chemically treated. A regime for the treatment of aglass s t r i p b y ~dry N SO2 suitable for plant conditions was developed on the basis of these studies. Tests carried out at the L'vov Mechanized Glass Plant showed that the t r e a t meat of the glass strip with SO2 in the drawing chamber of the aoadebiteuse glass-drawing system increases the impact strength by 50% and the microhardness by 15-20%. The chemical resistance of the glass is trebled in this case. LITERATURE 1. 2. 3.

CITED

R . W . Douglas and J. O. Isard, J. Soc. Glass Technol., 33 (1949); A. Sendt, Glastechn. Ber., 3__77,No.2 (1964). A . V . Gorokhovskii and V. P. Shcherbakov, Steklo i Keramika, No.3 (1970). W . A . Weil, Glass Industry, 26, No. 8 (1945).

533