This invention relates to the chemical treatment of glass and more particularly to a novel method for the chemical strengthening of glass articles such as bottles or rods by chemical reaction in a fluid bed.
A variety of techniques are known to the art whereby glass products are subjected to chemical treatment. An important function served by certain treatment processes is improvement of the mechanical strength of a glass object by a chemical modification which produces or induces compressive stress in a stratum or zone of the glass object adjacent the surface. One chemical treatment method which has been subjected to extensive research and development effort is ion exchange, the reaction technique in which the desired zone of compressive stress is provided by exchanging the principal alkali cations of the glass for cations having a different ionic diameter. Where the cations of the exchange medium are larger than the principal alkali ions of the glass, the process is often referred to as "ion stuffing" since compression directly ensues from occupation by the larger cations of the space vacated by the smaller cations of the glass. Ion stuffing must be performed under time and temperature conditions such that the stuffing induced compressive stress is not lost by thermally induced stress relaxation. Where cations of the exchange medium are smaller than the principal alkali ions of the glass, an ion-exchanged layer is produced which has a lower thermal coefficient of expansion than the main body of the glass. On cooling, the relatively greater contraction of the main body places the chemically treated layer under compression. In either type of exchange, the compressively stressed stratum imparts substantially improved strength to the glass object as a whole. Typically, bottles or other articles constituted by soda/lime glass are strengthened by exchange with an ion exchange medium comprising a potassium or lithium salt. Other exchange media include rubidium, cesium, silver or copper as the exchange ion.
Strengthening by ion exchange or other chemical treatment offers particular advantages in the manufacture of bottles for carbonated beverages. Such bottles are routinely exposed to internal pressures in a range of about 50 psig. When such a bottle is broken, the resulting fragments can be propelled at high velocity by the carbon dioxide which is released. Occasionally a beverage bottle explodes due to the force of internal pressure alone. Chemical strengthening of beverage bottles affords the potential advantage of reducing the incidence of breakage from either overpressure or percussion, and thus contributes materially to the safety of those who use or handle these bottles. An additional advantage of strengthening beverage bottles is the ability to reduce container weight, with associated cost reductions.
Prior to the present invention, however, commercial application of ion exchange or other chemical strengthening processes has been very limited due to the practical drawbacks associated with most previously known processes. Thus, the vast majority of known processes for ion exchange have involved immersion of glass objects in a molten salt bath containing ions that are exchanged with ions of the glass. The problems which may arise from incorporation of a molten salt bath dipping operation into a process for the high volume production of a low cost commodity such as glass bottles, are apparent. Moreover, aside from the difficulty and expense attendant a molten salt dipping operation as such, the bottles leaving the dipping bath bear a coating of frozen salt which must be removed before the bottles can be packaged or further processed. This is a particular problem where the bottles are to be subsequently dipped in a plastic coating bath to provide a plastic containment coating of the type that is desirable or necessary in the light weight nonreturnable bottles for which ion exchange strengthening is particularly desirable.
Although other methods have been devised in the art which avoid some of the more severe conditions necessitated by dipping in molten salt, essentially all prior art processes result in the formation of a salt residue on the bottle surface which must be washed off after the strengthening operation is complete. This result almost unavoidably follows from the basic approach of the prior art wherein a static coating is provided on the bottle surface so as to afford the time and area of contact considered necessary to carry out the chemical strengthening reaction. In fact, most of the prior art ion exchange processes have required that the exchange salt be ultimately brought to a molten state for carrying out the ion exchange reaction.
Thus, for example, a number of U.S. patents including Poole et al. U.S. Pat. No. 3,508,895, Graham U.S. Pat. No. 3,473,906, Grubb et al. U.S. Pat. No. 3,498,773, Poole et al. U.S. Pat. No. 3,607,172, and Poole et al. U.S. Pat. No. 3,743,491 disclose processes in which a coating of ion exchange salt is provided by spraying bottles with an aqueous solution of the salt. Subsequent to application of this coating the bottles are heated to a temperature at which the salt is molten and ion exchange proceeds. In a further improved process described in British Pat. No. 1,384,936 solid particulate ionic material is applied by an electrostatic process, after which the bottle or other glass object is heated to a temperature at which ion exchange takes place with a molten salt. Although superior in a number of important respects to the molten dipping and aqueous spray processes, the method of the British patent nonetheless leaves a residue which must be washed off before the ion exchange strengthened bottle can be plastic coated or packaged for sale.