Patent GB-1076602-A (GB)
Title: Glass articles having surface compressive stress
IPC: C03C21/00
Filed: 1965-12-10
Abstract: A glass article has a surface compressive zone (A), a central interior tensile stress zone (C) and an intermediate tensile stress zone (B) between (A) and (C).  The maximum tensile strength of zone (B) is greater than that of zone (C) and less than the compressive stress of zone (A).  Zone (B) may have a composition different from zones (A) and (C).  The article may be prepared by (i) contacting the glass with an inorganic compound of an electropositive element (b) of atomic diameter less than that of the predominant alkali metal (c) in the glass; and thereafter (ii) contacting the glass with a salt of an alkali metal (a) of atomic diameter greater than that of (b); optionally, (iii), the glass may now be contacted with a molten salt of a metal (d) differing from (a).  Contact (i) is generally performed at above the strain point of the glass, the glass article preferably having been preheated to within 50 DEG  F. of the treatment temperature; alternatively, contact (i) may be performed at below the strain point, the temperature being raised thereafter above the strain point.  Contact (ii) is performed at below the strain point. Usually the glass is cooled to about room temperature and given an aqueous rinse between heat treatment steps.  Steps (i) and (ii) may be performed using molten salt baths, or by coating, e.g. by spraying, the glass with the salt and raising the temperature to melt the salt.  Examples of element (b) are Li and H, and of (c) are Na and Li.  The diameter of (a) may be equal to or greater than that of (c); thus (a) and (c) may, for example, both be Na. Other examples of (a) are K, K/Na mixture, Li, Cu, Cs, Rb and Ag. If (a) and (c) are Na, and (b) Li, then in step (iii), (d) may be K.  The salts used may be e.g. sulphates, nitrates, chlorides, fluorides or phosphates.  If (b) is H, acids or acid salts may be employed.  Acid or alkali build-up in the treating baths must be prevented by appropriate neutralization.  The thickness of zone (A) may be 10 m  to 200 m  or more.  Zone (B) is usually not thicker than (A), e.g. 10-90% of the thickness of (A).  The maximum surface compressive stress may be as high as or higher than 100,000 p.s.i. and the maximum centre tension may be as high as or higher than 25,000 p.s.i.  The articles have an inverted W-shaped stress profile, exemplified in the Figure, usually having (A) high surface compressive-(B) medium to low tension-(C) centre tension lower than the maximum tension of (B)-(B) medium to low tension-(A) high surface compression, wherein the major thickness of the glass is in zone (C).  The ratio of the maximum compressive stress in (A) to maximum tensile stress in (C) is from 10:1 to 10,000:1 or more; and the ratio of the maximum tensile stress in (B) to that in (C) is from 2:1 to 500:1 or more.  The articles may subsequently be subjected to conventional processing techniques, e.g. thermal tempering, preferably performed between steps (i) and (ii), cutting operations, and laminating operations. Bending may be performed during step (1), the glass being coated with the salt.  The glass of the invention is stronger, more flexible, and less subject to failure under impact than untreated glass, and may readily be cut without shattering. Various uses of the glass are exemplified, including doors, safety glass, constructional material, engine parts and dental articles. Glasses which may be treated by the method described include silicate glasses, e.g. lime-soda glass and lithium containing glasses.  Examples of constituents of glasses are Na2O, K2O, CaO, SiO2, MgO, B2O3, Al2O3, Na2SO4, Fe2O3, Li2O, ZrO2, unspecified alkali and alkaline earth oxides, TiO2, ZnO, BaO, PbO, P2O5, Sb2O3, Bi2O3, As2O3, F.