Patent GB-1048580-A (GB)
Title: Glass articles having surfaces under compression
IPC: C03C21/00
Filed: 1963-12-17
Abstract: A glass article having an outer compressive zone (B) and a central interior tensile stress zone (C), where zone (B) has an alkali metal (b) content in at least a portion thereof which is higher than the content of the same metal (b) in zone (C), comprises a surface zone (A), at the surface of zone (B), which contains H or a cationic metal (a) of smaller atomic diameter than that of (b), the concentration of H or (a) being greater in zone (A) than in any interior zone.  The article is made by a process comprising one or more steps (i) wherein a glass article, which may or may not have been thermally tempered, containing an alkali metal (c) is contacted with a salt of an alkali metal (b) of atomic diameter greater than that of (c), followed by treatment step (ii) comprising contacting the glass with (1) an acid in the absence of F, or (2) a salt of metal (a), and (iii) removing the glass from such contact (ii) before the compressive stress is released.  If more than one treatment step (i) is performed, then preferably in the later such treatments (i) the metal (b) should be of larger atomic diameter than the metal (b) used in the preceding treatment (i).  Preferably the glass is preheated before step (i) to within 150 DEG  F. of the temperature of the molten salt, which may be e.g. 925 DEG -1050 DEG  F.  Before step (ii) the glass may be cooled, e.g. to 200 DEG -250 DEG  F., washed, and preheated.  Step (ii) is preferably halted before the compressive stress has been reduced by 50%, and the distance of incorporation of H or (a) is usually less than 1 m .  For treatment with H, inorganic or organic acids may be employed.  With inorganic acids, water may be present physically and/or chemically bound in the acid, or added to permit use of the acid as a liquid or solution. With an organic acid, water or an inert organic solvent may be used as appropriate.  Contact with the acid may be made with solutions of the acid, or with the molten acid, in which case, if the acid is organic, non-oxidizing atmospheres and temperatures below thermal decomposition point are preferably employed.  The contact of water-soluble salts of (c) is usually performed in the presence of water which may be physically and/or chemically bound to the salt which may be molten, or present as a solvent for the salt. Contact (ii) may be performed by e.g. dipping, spraying, flow-coating, transfer coating, or in a bed of acid or salt particles fluidized by gas or ultrasonic energy. Examples of metal (b) are Na, e.g. as NaCl, K, e.g. as KNO3, or mixtures thereof.  Examples of suitable acids for step (ii) are: inorganic: phosphoric, boric, hydriodic, nitric, sulphuric, permanganic, hydrobromic, iodic, phosphorous, hydrochloric, nitrous, sulphurous, selenious, arsenic, arsenious and pyrophosphoric acids; organic: formic, acetic, propionic, butyric, valeric, hexanoic, heptanoic, octanoic, nonanoic, carbonic, oxalic, citric, lactic, malonic, fumaric, salicylic, benzoic, adipic, glutaric and pyrogallic acids, and phenol. Suitable water-soluble salts of (c), which in the presence of water may also or alternatively act as sources of H, include simple and compound salts, including acid salts, or mixtures of simple and/or compound salts.  Examples of suitable salts are water-soluble chlorides, bromides, iodides, sulphates, bisulphates and nitrates of Zn, Ni, Al, Be, Cd, Cu, Na, Ba, Cr, Co, Fe, Ti, and zinc amino chloride, sodium citrate, barium perchlorate, and sodium dichromate.  The ratio of maximum compressive stress in zone (B) to maximum tensile stress in zone (c) for thermally tempered glass is 3: 1 to 40: 1, and for glass not thermally tempered, 100: 1 to 500: 1 for glass of thickness 0.06 inch or more.  The thickness of zone (B) is 1-200 m , and the ratio of the thicknesses of zones (B) and (C) is 10: 1 to 200: 1 or more.  The glass to be treated may be transparent, translucent or opaque.  Exemplified constituents of the glass, with ranges in percentage by weight, are BaO, 0-25; CaO, 0-25; Li2O, 0-20; MgO, 0-25; K2O, 0-10; Na2O, 2-20; SrO, 0-25; Al2O3, 0-40; Fe7O3, 0-2; PbO, 0-25; TiO2, 0-10; ZnO, 0-25; SiO2, 40-70; B2O3, 0-15; Na2SO4, 0-2.  The glass articles produced by the above method have improved durability, moisture resistance, and strain resistance compared to similar articles not treated as in step (ii).  The articles may be used as shields, closures and partitions.  Reference has been directed by the Comptroller to Specification 917,388.