INCREASING

THE THERMAL-SHOCK

BY T H E R M O C H E M I C A L

I.A.

RESISTANCE

OF G L A S S

TREATMENT

Boguslavskii

Translated from Steklo i Keramika, No. 9, pp. 26-28, September, 1960

It is well known that the thermal-shock resistance of glass characterized by its capacity for resistance to sharp changes in temperature depends mainly on the mechanical strength and the coefficient of linear thermal expansion of the glass. An increase in the thermal-shock resistance is obtained by altering the chemical composition of the glass (with the aim of reducing the thermal expansion) and by hardening the glass (to increase its mechanical strength). However the complexity of the manufacture and the lack of mass production of glasses with a low expansion coefficient, and also the inadequate increases in thermal-shock resistance in a number of cases with distributed air-jet hardening, prevent the wide use of these methods in industry and necessitate the development of new methods for increasing the thermal resistance of glass. The present work, carried out by the GSPKB for glass, was concerned with the possibility of increasing the thermal-shock resistance of glass by the method of thermochemical treatment of the surface with silicoorganie compounds, which have been developed by the department of glass technology at the D.I. Mendeleev Moscow Chemical Technology Institute. With this method, the glass is heated in a furnace to its softening point and treated with the silieoorganie compounds which undergo thermal decomposition on the heated surface with the conversion from the silicoorganic polymers into silicic acid compounds.

--Si--O-Si--O-I I R R

~

[S'i02] n .

The latter "cement" the surface microcracks of the glass, which increases its mechanical strength and thermal resistance. The high temperature of the glass, creating favorable conditions for decomposing the silicoorganic c o m pounds,also increases the adhesion to the glass of the developed films and fixes them securely to the surface. For complete and uniform treatment of the glass with the organic compounds, the best method is to apply them on the heated surface by atomization of solutions, This method was the basis of the present work. A vertically drawn plate glass of the following composition was selected for the experiments: 71.9 • 1% SiO2; 7.6 e 0.50]0 CaO; 3.8 9 0.50]0 MgO; 15i 1% NazO; 1010AlzO3; 0.15~ Fe~O3; 0.50]0 SOa. Specimens in the form of discs 180 m m in diameter and 4.5 mm thick with polished surfaces were made. The chemical reagent with which the glass was treated was a solution of polymeric silicoorganic of the following structure.

l

--Si--O--Si--O--

I C=H6

I C2Ha

liquid

1 n

The glass specimens were heated to 660 ~ in a hardening furnace and after a three-minute soak were transferred to an atomizing chamber, where under pressure, onto the glass surface, was sprayed the solution of the selected liquid. After this preliminary treatment the glass either went to the hardening belt or was c o d e d in free-convection air currents. With their ends protected, the glasses after having been thus treated were tested for thermal-shock resistance by the method of sprinkling. 472

A

=

,

|

,5

B

~,,.-

zo

Spraying time, sec

z

.------

,oo

~ ~=

5

to

2

,~

.~

2o

Spraying, ~ime, sec

Fig. 1. Relation between thermal shock resistance (1) and degree of hardening (2) of glass and the spraying time of the solutions. A) Spraying with subsequent air hardening; B) without hardening; a) spraying with silicoorganic solutions; b) solvent sprayed.

% #00 <1 d ed

sop

/Af

..... ! /

b

200

~~176 o

o,?

o,0

o,6

o,~

Fig. 1 A shows the change in thermal resistance and the degree of hardening in relation to the time of the preliminary processing of the surface with the silicoorganic solution and with pure solvent with subsequent air cooling (20 specimens were tested for each schedule; the graphs show the averages). The graphs show that treating the glass before air coohng with silicoorganic compounds increases the thermal resistance in spite of the fall in the degree of hardening. With equal degrees of hardening the thermal resistance of the glass processed with the organics is higher than with the processing with the solvent. Pre-treatment of the glass with solvent leads to a reduction in the thermal resistance which is connected with the reduction in the degree of hardening. This is explained by the fall in temperature of the glass with preliminary treatment, in connection with which, somewhat cooled glass is already proceeding toward the final hardening (as is well known, the degree of hardening depends on the temperature of hardening).

a

/

During the experiments, the optimum concentration of silicoorganic solutions (10%) was established, which did not worsen the optical properties of the glass (large concentrations produced a certain turbidity).

to

Degree of hardening N/cm Fig. 2. Relation between thermal-shock resistance of glass and the degree of hardening; a) treated with silicoorganic solutions; b) solvent treated.

$ilicoorganic solutions fulfilling the role of refrigerants simultaneously enrich the glass surface with silica which increases its thermal-shock resistance even with a reduced degree of hardening. With an increase in the processing time, the thermal destruction of

the silicoorganic polymers proceeds more fully, and consequently, the difference between the degree of thermal-shock resistance of glasses treated with solvent and polymer solution is increased. Similar conditions are observed with the thermochemical treatment of glass without subsequent ak cooling. Fig. 1 B shows the change in shock resistance and the degree of hardening of the glass, on the surface of which is sprayed the solvent and the solution of silieoorganic liquid with subsequent cooling in air in free-convection conditions. Being a cooling agent, the atomized liquid hardens the glass. With an increase in the atomization time, the degree of hardening increases, together with its thermal-shock resistance. The thermal-shock resistance of the glass treated with silicoorganic compounds with the same degrees of hardening is higher than with glass treated with pure solvents (Fig. 2); this is associated with the enrichment of the surface with silica and the formation of a strong and thermal-resistant polyorganosiloxane film. Thus, the thermal resistance of hardened glass depends on the degree of hardening and the conditions of the surface treatment. The thermal treatment of glass with silicoorganic solutions and solvent proceeds similarly and is therefore equal to the process creating the hardening, thermoelastic strains in the glass. However, the chemical processing of the surface with silicoorganie solutions by helping the surface to increase its silica content, further increases its thermal resistance. The maximum value of thermal resistance obtained by this method and with subsequent cooling in air and without it proved to be the same; consequently, the final air hardening of the glass may be excluded. Together with this, the cooling carried out by spraying the glass with silicoorganic solutions reduces and in some cases excludes the formation of hardening spots, observed with air-jet hardening of glass experienced extensively in industry.

473

The uniform cooling with hardening by the new method,together with the silica concentration at the surface, makes it possible to obtain high-strength thermal-resistant glass without optical distortion. Especially effective is the t h e r m o c h e m i c a l t r e a t m e n t of glasses softened at higher temperatures. The destruction of the stlicoorganic compounds at the surface of such glasses proceeds more c o m p l e t e l y which facilitates an even greater increase in thermal resistance. The new method used with nonalkaline glass with a low coefficient of thermal expansion c~ = 42 x 10 "~ increases its resistance b y 3 t i m e s - t o 420-440 ~ A considerable increase is attained with plate glass m a d e by vertical drawing with a = 87 x 10 -T (330-360 ~ and for nonalkaline glass with c~ = 42 x 10 -7 (420 - 440 ~ with the new method, and opens up a big potential for silicoorganic compounds in the glass industry. IRON-FREE

TALCITES

N.L.

Polyakova,

FROM KIRGITEISK

P.P.

Smolin,

and

A.M.

Eidel'kind

Translated from Steklo i Keramika, No. 9, pp. 28-33, September, 1960 Owing to its various favorable properties, talc, as is well known, has found extensive use in m a n y regions of industry. C e r a m i c materials based on t a l c possess high m e c h a n i c a l strength, c h e m i c a l resistance, t h e r m a l shock resistance aria m a y be used therefore for the production of frost-resistant wall tiles, decorated tiles, pottery, c h e m i c a l l y resistant and h e a t - r e s i s t a n t apparatus, noncrazing glazes, etc. T a l c ceramics possess e x c e l l e n t e l e c t r o t e c h n i c a l properties. S t e a t i t e ceramics based on compositions consisting almost e x c l u s i v e l y of t a l c possess, with zero moisture c a p a c i t y and high m e c h a n i c a l strength, unusually low energy losses in a high frequency field. Forsterite ceramics obtained with the addition to talc bodies co magnesia have low energy loss in both high and low frequency fields and a number of other favorable e l e c t r o t e c h n i c a l properties. Cordierite ceramics obtained from t a l c compositions with additions of alumina have an e x t r e m e l y low coefficient of thermal expansion both in the porous and glassy state. In the d e v e l o p m e n t of capitalist countries, the c e r a m i c industry is the leading user of talc. Thus in the USA the d e m a n d for t a l c in c e r a m i c production from 1931 to 1956 increased by 120 times (1500 to 185,000 tons); with this, the specific contribution of ceramics to the general volume of t a l c requirements increased from 1 to 36% [1, 2]. In the USSR the development of talc ceramics has for a long time been held up by the absence of a raw material base, Exploited Russian sources of talc have not satisfied the basic demand presemed to talc production by the ceramic industry in relation to uniformity and low content of harmful impurities, particularly iron oxides. Marked use of talc in the ceramic industry began after World War 2 when there was organized the mining of iron-free talcRes at Onotsk in the Irkutsk region. In the period 1956-59, according to data of GNTK RSFSR, the demands of the Russian ceramic industry for Onotsk and imported Chinese talc increased by 4.5 times-from 2750 to 12,600 tons, The limited development of the production of talc ceramics to a large degree is explained by the high cost of imported and Onotsk talc. The high cost of Onotsk talc is due to the smallness of the undeveloped mines. Thus, the users are deterred by the high cost of iron-free talc, and its high cost to a large degree explains the absence of the users. Furthermore, the Miassk T a l c Combine produces a powder of inadequate fineness and purity. In connection with this the most stable Russian user of iron-free t a l c is the r a d i o c e r a m i c industry, while foreign users prefer to obtain lump t a l c and carry out high-grade grinding in their factories which cannot be standardized. In addition to this, the slow growth in the production of t a l c ceramics is due to the traditional tendency of the c e r a m i c industry to the exclusive use of clays and similar materials (kaolin and Pegmatite). Newly developed types of grinding equipment in Russia (injector mills) and technological processes for obtaining t a l c ceramics (in particular zonal firing of large products) allow the organization of the production of quality powders and the extensive manufacture of t a l c ceramics possessing a number of specific favorable properties.

474