ES353065A1 - A process and its corresponding apparatus to chemically temperate a body of vitreo or vitrocrystaline material. (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

In a process for the chemical tempering of glass or vitro-crystalline articles by ion exchange the ion exchange takes place under the influence of an electric field extending through the article/medium interface, and the field polarity is reversed at least once but always so that at the last phase the medium side of the interface is at the higher potential. The present process reduces the stress gradient through the glass by causing deeper penetration of the ions and hence increased strength. In one embodiment a glass-ceramic disc 1 has one surface traversed by a diametrically opposed pair of hollow graphite electrodes 12, 13. The electrodes are moved by a central axial member which also has conductor rings. A field of 80 volts D.C. is used which for each electrode is reversed after every 180 degrees of rotation, each electrode being of opposite polarity to the other. As they are rotated the electrodes are caused to advance step-wise inwardly towards the axis. They are filled with a solution of, by weight, 2% LiNO 3 and 98% KNO 3 as medium, the process being performed at 300 C. In a further example a vehicle windscreen is, on one surface, provided with an outer zone coating containing LiCl, NaCl, LiNO 3 and KNO 3 and a "safety" zone coating of 80% by wt. rubidium peroxide and 20% by wt. NaNO 3 insulated from the outer zone. One electrode of an A.C. supply of 30 v. at 5 c/s. is connected to each zone and the process performed at 530 C. for 15 minutes. Different degrees of temper are obtained for voltages down to 5 v. and frequencies up to 10,000 c/s. Differential tempering is also achieved by causing relative motion between the surface or surfaces of the article and the ion exchange medium or the electrodes. It has also been shown that the use of ultrasonic frequencies from 20,000 to 1,000,000 c/s. are beneficial when used during the process at from 0.3 to 5 watt/dm.2. They are preferably propagated parallel to the article surface because this reduces the tendency of "fall-off" in ion exchange over a period of time. This method is best for use with large ions, e.g. of cesium, rubidium, calcium, strontium, barium. An example of a glass ceramic is, in weight per cent, Li 2 O 15 Al 2 0 3 10 SiO 2 50 CaO 10 B 2 O 3 8 TiO 2 7. A flat glass composition is given as, by wt., SiO 2 80%, B 2 O 3 15%, Na 2 O 5%.