GB2111972A - A method of thermally prestressing glass - Google Patents

Abstract

Glass objects which have a low heat stress factor and/or a small thickness eg plates 11 are heated in an oven 10 to a temperature above their annealing temperature but below their fusing temperature. The heated plates 11' are then preliminarily quenched by being briefly sprayed with an air/water mixture from nozzles 15, 15'. The initially quenched plates 11' are then plunged into water as a main quenching operation. <IMAGE>

Description

SPECIFICATION Method of thermally pre-stressing glass The invention relates to a method of thermally pre-stressing glass objects which have a low heat-stress factor and/or are of small glass thickness. The heat-stress factor is represented by the expression, a . E/(1-y) [N/(mm2K)], wherein a is the linear coefficient of thermal expansion between 20 and 300"C (K-'), E is the modulus of elasticity (N/mm2), and is the Poisson's ratio of the glass.

Methods are already known in which pestressing is obtained by spraying with liquid and subsequent immersion in liquid. Such a method is described in US-PS 3 706 544.

However, this Patent Specification points out explicitly that the liquids which are used for the spraying and immersion operations should be free of water but could at most contain less than 5% water.

DE-OS 2 952 045 describes a method wherein glasses with a low heat stress factor are quenched by means of an air-liquid mixture. According to this known method, water may also be used as the liquid in the said mixture.

DE-OS 3 001 944 describes a method according to which the glass is plunged into water having a layer of oil thereon, for the quenching process. The oil layer has the function of effecting preliminary chilling of the glass.

The objective of the present invention is a new method of thermally pre-stressing glass objects which is particularly well suited for the pre-stressing of glasses having a low heatstress factor and/or of glass objects of small thickness.

In direct contrast with the state of the art (US-PS 3 706 544) in which it is alleged that for a combined spray-and immersion quenching process both applied liquids must be water-free, it was surprisingly found that, subject to the application of suitable spraying apparatus and a carefully selected spraying time, glasses with low heat-stress factor and/ or of small thickness can be successfully prestressed by spraying with water and immersion in water.

Tests have shown that the two-substanceor binary jet nozzles and air liquid mixtures described in Germany OS 2 952 045 may be successfully used for the preliminary chilling stage. The advantages of the claimed method by comparison with the known method reside first in a substantially shorter spraying time and secondly in the achievement of a higher pre-stressing in the treated object due to aftercooling in water. The shorter spraying time permits a substantially narrower spraying zone to be provided. The glass object, for instance a sheet of glass, may be conveyed through a spraying zone and subsequently immersed in water. Since quenching in water is more costeffective than spray-chilling, the claimed method is considerably cheaper. Besides achieving great economies in costs, the new method is also capable of achieving a higher pre-stressing in the treated glass object.

The method according to the invention is distinguished from the known method according to Germany OS 3 001 944 in that the preliminary chilling stage is applied by spraying with water instead of dipping in oil.

Quenching in water having a layer of oil thereon has two drawbacks. First, the oil, unless sheathed by an inert gas, begins to burn and secondly, the glass objects must be cleaned after pre-stressing. Both these disadvantages are obviated by the new method.

In the literature it was frequently alleged that, when hot glass was plunged into water, a steam jacket or envolope would form on the glass surface which would initially prevent fast heat-transmission between glass and water, and when this steam-jacket collapsed at lower glass temperature, there would follow a steep increase in heat transmission glass-water according to this idea.

However, tests which were carried out with thin (5 mm thick) plates of aluminium have shown that the heat transfer coefficient between aluminium and water becomes increasingly smaller the lower the temperature of the aluminium on immersion into water (Fig. 1).

All tests with glass indicate that this also applies equally for the heat-transfer between glass and water.

Thus the heat transfer between glass and water can be influenced by the pre-cooling of glass. Prolonged pre-cooling by spraying will therefore result in a lower heat transfer over a longer period of time as well as in a lower heat transfer in the water.

For the pre-cooling by spraying, the data given in German OS 2 952 045 in respect of air/water mixtures and distances (glass-nozzle or nozzle-nozzle) may be adopted according to the type and thickness of the glass to be treated. Accordingly, the amount of liquid per nozzle should be between 0.1 and 51/hand the amount of air per nozzle between 0.1 and 5 l/m3. The droplet-diameter of the atomised liquid should, on an average lie between 1 and 100 y. The distance between glass surface and spraying nozzle is from 30 to 200 mm and the distance between the nozzles from 30 to 60 mm.

However, in contrast with the teachings of German OS 2 952 045, only a narrow spray ing-nozzle zone is required. The actual width of the nozzle zone depends on the speed at which the glass sheet is conveyed through the spraying zone and also on the thickness of the glass. The length of preliminary chilling or cooling time within the nozzle zone should be approximately from 1 to 10 seconds for sheets which are from 2 to 7 mm thick. As an approximate guide, spraying time in seconds is chosen to be equal to glass thickness in mm. Spray-chilling tests have shown that, for example with a 5 mm thick glass sheet, the maximum temperature difference between glass surface and glass interior is reached after 4 to 6 seconds (Fig. 4).

According to the method of this invention, heat transfer increases by immersion in water at a point in time at which the glass surface begins to cool at the same rate as or more slowly than the glass interior. Thus, the tension which is temporarily set up at the glass surface can be kept low so that there will be no fracture of the glass sheet during the preliminary chilling stage. The increased heat transfer in the water has the effect that the high temperature gradient between glass surface and glass interior which has been created by the preliminary chilling stage is maintained, or even increased, up to the point at which the core of the glass has reached its solidification temperature.

By means of the method of the invention it is thus possible to increase compression prestress without having to accept a significant increase in temporary tension stresses.

The method of the invention is in direct contrast to the above-mentioned US-PS 3 706 544 in so far as the applied liquids are concerned.

In the method of the invention, the glass sheets may be conveyed through a vertical or a horizontal spraying plant to effect the prechilling process.

Organic, water-soluble liquids may be added to the spraying solution as well as to the dipping solution. However, in order to obtain the desired high degree of heat transfer, and also for obvious cost reasons, the amount of such added organic liquids should preferably not exceed 20 vol%. Furthermore, the percentage of organic solvents should be sufficiently low to prevent the formation of a complete vapour jacket during the whole of the spraying time.

In order to avoid premature cooling of the glass sheets by the spraying liquid, the spraying appratus is screened off relative to the furnace.

In Fig. 1, the heat transfer coefficient of aluminium-water is plotted against spraying time. It will be seen that the heat transfer coefficient in the water decreases with increasing spraying time.

Fig. 2 shows diagrammatically a vertically arranged pre-stressing plant: Inside a heating oven 10, a glass sheet 11 is vertically suspended by means of clamps (not shown) and heated to a temperature which is above its annealing temperature but below its fusing temperature by means of heating elements 1 3 controlled by a controller 14. After having been heated to the desired temperature, the heated sheet 11' is sprayed for a few seconds with a liquid (water) as it travels past twin jet nozzles 1 5 and 15'. A screening device 1 6 prevents premature cooling of the sheet 11 by spray-mist. Immediately after the spraying stage, the chilled sheet 11" is plunged into a tank 1 8 filled with water 1 9 in a dipping zone 17.

Alternatively the glass sheets may be heated horizontally. A suitable plant for this purpose is shown in Fig. 3. Glass sheet 11 is heated in a roller bed or air-cushion-furnace 20. By raising or lowering a roller assembly 21 the sheet 11' is transported on an inclined plane at predetermined speed on rollers 21' and 21" through the narrow spraying zone into the water 1 9 in dipping tank 8 in dipping zone 17'.

The effectiveness of the method was tested, among other materials, in relation to a borosilicate glass with the heat-stress factor a . E/(1-,u) = 0.25 N/(mm2K). This type of glass is commercially obtainable, for example under the Trade Mark "DURAN" from the firm Glaswerke Schott. Fig. 4 shows curves for the measured compression pre-stress data for sheets of 2mm and 5mm thickness in relation to spraying time in seconds. The spraying nozzles used in these tests were twin jet nozzles made by the firm Schlick (Model 970). The distance from nozzle-to-nozzle was 30 mm in both directions, the distance between nozzle and glass was 50 mm. For an 0.5 mm diameter nozzle bore and a depth of water of 500 mm, the amount of water per nozzle was 1.5 I/h, and the amount of air was 4 Nm3/h at 3 bar air pressure.At the end of 6 seconds spraying time with the 5mm sheets, and after slightly over 4 seconds with the 2mm sheets, the same compression prestress is obtained as that obtained by an exclusive spray-tempering process. By shortening spraying time and subsequent plunging in water, the pre-stressing can be increased.

The thinner the glass, the shorter can be the spraying time. The part of the curve shown in broken line in Fig. 4 indicates those spraying times for which surface cracking may be expected. The better the surface condition of the glass sheet under treatment, the shorter can be the effective spraying time.

With glasses in which the heat-stress factor is near to the upper limit value, a slightly longer spraying time will be needed for the same glass thickness. Whereas for a 5 mm glass with a heat stress factor of 0.25 N/(mm2K), the spraying time may be from 2 to 4 seconds, this should be increased to 4-6 seconds under otherwise identical conditions if the glass has a heat stress factor of 0.5 N/(mm2K).

Claims (8)

1. A method of thermally pre-stressing glass which has a low heat stress factor and/or is of small thickness by heating the glass to a temperature which is above its annealing temperature but below its fusing temperature, and quenching the heated glass in two stages, wherein preliminary chilling is applied for a short length of time in a spraying zone by means of an air-water mixture and the main quenching stage takes place by immersion in water.

2. A method according to claim 1, wherein the preliminary chilling takes place in a narrow spraying zone for a time of 1 to 10 seconds.

3. A method according to claim 1 or 2, wherein the spraying is effected using a binary nozzle, the amount of liquid per nozzle being between 0.1 and 5 I/h, the amount of air per nozzle being 0.1 to 10 m3/h and the ratio of liquid to air being between 0.1 and 5 l/m3.

4. A method according to any one of claims 1 to 3, wherein water-soluble compounds are added to the water used for spraying and/or in the quenching tank.

5. A method according to any one of claims 1 to 4, wherein the time in the spraying zone is correspondingly abbreviated the lower the heat stress factor and/or the thickness of the glass under treatment.

6. A method according to claim 5, wherein the spraying time in seconds approximately equals the glass thickness in millimetres.

7. A method of thermally pre-stressing glass substantially as hereinbefore described with reference to Fig. 2 or Fig. 3 of the accompanying drawings.

8. Glass which has been thermally prestressed by a method as claimed in any preceding claim.