The invention relates to a method for heat treating glass sheets, in which the glass sheet is heated and cooled by blowing gas on to it. The inven¬ tion also relates to an apparatus for carrying out the heat treatment method.

As is generally known, the characteristics of glasses can be altered by different methods of heat treatment. The most common and best known method is to heat the glass sheet throughout to a temperature of 580°C to 700°C, after which the glass sheet is cooled as efficiently and rapidly as possible by means of air jets directed at the surfaces. The temperatures of the lower and upper surfaces of the glass sheet fall faster than do those of the inner parts of the glass. As a consequence of this, the inner parts of the glass sheet congeal more slowly than the surface parts of the glass sheet. During cooling the surface temperatures of the glass sheet are lowered and the surface layers shrink. The inner parts of the glass sheet prevent the shrink¬ age of the surface layers which congeal faster. Since the inner part of the glass sheet has not congealed, stretching occurs in the inner part. As a result of the differences in temperature, a state of compressive stress is formed in the surface layers of the glass sheet and a state of tensile stress in the inner parts. The difference in magnitude between the direc¬ tions of stress of the glass sheet' s surface layers and inner part affects the strength of the glass. When the differences in the directions of stress are sufficient¬ ly great, the properties required of safety glasses are attained. Glasses heat treated in the above-described manner are known as tempered safety glasses. In loading tests that have been carried out, tempered glass has

proved to be 2.8 times stronger than float glass of a corresponding thickness.

Tempered safety glasses are characterized by the fact that their bending strength in tests has proved to be 2.8 times and their fast temperature changes 3.5 times greater than conventional float glasses; furthermore, their impact strength is signi¬ ficantly better.

A tempered safety glass can break as the result of an impact from a hard and sharp object. When tem¬ pered safety glass breaks, small pea-sized roundish and lightweight glass balls are formed which are harmless to a human being.

A measurable characteristic of tempered safety glass is the amount of balls that are formed upon breakage in a 50 mm ² x 50 mm ² area as well as the form and size of the balls. On the basis of the amount of balls and their forms and sizes, it is possible to deduce how successful the heat treatment has been and furthermore the bending strength of the tempered safety glass and its behaviour under a load. In the area to be examined, there may be 50 to 400 balls, the size of an individual sliver may not be greater than 3.0 cm ² , the length of the sliver may not be greater than 75 mm and the edges of slivers may not be sharp. In tests, tem¬ pered glasses intended for use in vehicles must achieve the above-mentioned requirements relating to the amount, size and form of the slivers before the name tempered safety glass can be used with reference to these glasses.

The method of tempering glasses is generally known and it is described, for example, in Finnish Pa¬ tent 62 043. A method of tempering coated glass is described in U.S. Patent 4 857 094.

When the differences between the state of compressive stress of the glass sheet's surfaces and the state of tensile stress of the inner part are smaller than in tempered safety glasses, the require- ments set on safety glasses in terms of their strength, toughness, and the amount, form and size of pieces of broken glass are not achieved. Nevertheless, the strength of heat-treated glass is greater than before the heat treatment. Thus heat-treated glass is called thermally toughened glass. Thermally toughened glass is used when the strength or other properties of tempered glasses are not a requirement of the glass.

Often the strength and behavioural requirements of thermally toughened glass are set on glass sheets which are smaller than 4 mm in thickness and for which the strength properties of conventional glass do not suffice.

When a thermally toughened glass sheet breaks, large sharp-edged pieces are formed, which cause a dan- ger of bodily injury if they strike a human being. The manufacture and properties of thermally toughened glass are described in greater detail in U.S. Patent 4 759 788.

By heating a glass sheet to about 700°C, the glass can be formed in well-known ways. When the cor¬ rect form has been imparted to the glass, it is allowed to cool slowly such that great temperature differences do not arise between the surfaces and the inner part of the glass sheet, since these would cause stress differ- ences between the surface layers and the inner part. The stress differences in formed glasses may subse¬ quently break the glass in and of themselves. Formed glasses have the strength properties of conventional glass. Their strength can be increased by carrying out rapid cooling, whereby the formed glasses acquire the

strength characteristics of tempered or thermally toughened glass. A correctly formed glass sheet will withstand more loading than a flat glass sheet. The forming of a glass sheet is described in greater detail, for example, in European Patent Application 0 162 264 and U.S. Patents 3 023 542 and 4 830 649.

If necessary, tempered and thermally toughened glasses can be normalized by carrying out heating of the glass sheet and by cooling the heated glass sheet so slowly that a strength-increasing stress difference does not arise between the surface layers and the inner part. A normalized glass sheet has the same strength, heat-withstanding and impact strength properties as glass has before heat treatment. With these glasses the requirements in respect of strength and the ability to withstand temperature differences, which are set on tempered and thermally toughened glasses, are not achieved.

Tempered and thermally toughened glasses are used in conditions in which it is assumed that the glasses will be subjected to such loads as might break conventional glasses. Tempered safety glasses are used when there is a risk of injury should the glasses break. Usage conditions and architectural design often place on glasses requirements whose realization calls for applying to conventional glasses, for example, a metal or metal oxide coating or a coating that is a composite of these, which coatings reflect heat rays; this imparts the desired property. In general, coated glasses are intended to reflect the thermal radiation of the sun or the long-wave thermal radiation of a room space. If necessary, coating of glasses produces col¬ oured glasses which are used in particular in the facades of buildings to create an aesthetic impression.

Combining metal and metal oxide coatings in the appropriate layer thicknesses and sequence renders a coated glass capable of reflecting solar thermal radia¬ tion and the long-wave thermal radiation of a room space. Coated glasses having the special property of reflecting long-wave thermal radiation are known by the trade name of low-emissivity glass, which has a greater thermal resistance and a lower thermal transmittance coefficient than do conventional glasses. These prop- erties of coated glass are used to reduce the travel of energy through the glasses.

With low-emissivity glasses, the electron structure of the surface layer contains free and mobile electrons, a consequence of which is that the surface layer of low-emissivity glasses conducts electricity. On the other hand, the above-mentioned property can be utilized inversely in electrically heated glasses in which the low-emissivity coating layer serves a resist¬ ive function. When the opposing edges of low emissivity glass are provided with electrons and connected to a voltage source, electric current flows in the coating layer and the glass warms up.

UK Patent Applications 2 116 590, 2 134 444, 2 156 339 and 2 209 176, European Patent Applications 0 275 662, 0 283 923 and 0 301 755 and U.S. Patents 4 239 379, 4 462 883 and 4 707 383 describe in greater detail methods of manufacturing coated glasses, the materials to be used in coated glasses and their layer structures and properties. Among the greatest drawbacks resulting from the heat treatment of coated glasses are the bowing of the glasses during the process. One significant cause of the bowing of a glass sheet is the temperature differ¬ ences between the edges and the middle part of the glass sheet as well as between the uncoated and coated

glass, these differences resulting from the different properties which the coated and uncoated glasses have in terms of their capability of reflecting heat rays. Since areas of the same glass sheet warm up and cool down in different ways and at different rates in the heat treatment process, stress differences are gener¬ ated between the edges and the middle part, which cause the parts of the glass sheet to diverge from a flat level. The divergences appear as clear and obtrusive optical disturbances.

The optical disturbances can be noticed par¬ ticularly clearly in the edge areas of a glass sheet. The optical disturbances of edge areas can be perceived clearly in coloured and mirror-like reflecting coated glasses that have been heat treated. The drawback is especially problematical in the kinds of glasses whose emissivity, 6, is lower than that of conventional glasses, and the problem increases in significance in low-emissivity coated glasses whose € < 0.25. The emissivities of the surface layers of glasses and the effect of differences in them on the heat transfer occurring during the steps of heating and cooling in the heat treatment process can be described by means of the radiative heat transfer coefficient. The radiative heat transfer coefficient should be con¬ strued as a quantity which is continuously a function of the temperature and which is used in comparisons of the properties of glasses. When the temperatures exam¬ ined rise, the emissivities of the surfaces increase in a manner that is characteristic of each coating mater¬ ial. The formula for calculating the radiative heat transfer coefficient at room temperature is

the radiative heat transfer coefficient (W/Km ² ) σ the Stefan-Boltzmann constant

(5.67 • 10 ^"8 W/m ² (°K) ⁴ )

T the average temperature ( °K) e the emissivities of the surfaces subjected to radiation

The radiative heat transfer coefficient is sig¬ nificantly dependent on the prevailing temperatures. Experience has shown that when the temperature differ¬ ence of surfaces that are subjected to the interaction of thermal radiation increases by 100°K from the normal room temperature, in the computational formula the value of the denominator that takes into account the effects of the emissivities of the surfaces grows 1.6- fold and the value of the radiative heat transfer coef¬ ficient grows 5-fold. The radiative heat transfer coef¬ ficient of conventional float glasses is five times larger than that of low-emissivity coated glasses. The effect of the emissivities of glass surfaces on the heating effect which they give off is described by the computational formula

6 σ (T _p ⁴ -T _s ^< ) _;

Φ the thermal flux given off by the glass surface

(W/m ² ) e the emissivity of the surface σ the Stefan-Boltzmann constant

(5.67 • lO" ⁸ W/m ² ( °K) ⁴ ) the surface temperature of the glass (°K) the counterradiation temperature ( ^C K).

While the emissivity of the surface layers of float glasses is 0.84 and that of low-emissivity coated glasses is 0.05 to 0.25, the amount of heat given off by float glasses and the uncoated surfaces of coated glasses is four to six times greater than that of a coated surface. As a consequence of this, the temperat¬ ures of the surface layers of coated glasses differ significantly from each other, a fact that is taken into account in the design of cooling air jets, and the bowing of a glass sheet can be prevented.

The factor which is more difficult to remove and which increases bowing and weakens the result of the heat treatment is the electron loss that occurs in the edge areas of glass sheets during air cooling. The air jets that are directed at the surface of the glass sheet remove electrons from the surface, these having a cumulative effect in the sheet's edge areas. In par¬ ticular, electron loss occurs in the kinds of coated glasses which have a high density of free electrons and in which the electrons are not bound permanently to the outer electron shell of an atom in the coating layer. In these glasses the free electrons migrate readily along with the air flows that have been added because of the coating and they become earthed via the sur- rounding conductive materials. The migration of the electrons is increased by the rise in the temperature, whereby the electrons gain in density and mobility with the result that the bonding forces between the electron shell and the electron weaken. Generally, the supporting and transport of glass sheets to be heat treated takes place by means of conveying rollers made from an insulating surface ma¬ terial. During the heat treatment process the glass sheet is insulated from the other materials which might give up a corresponding amount of new electrons to

replace those lost as a result of the air flow. Owing to the losses of electrons, positive charges several kilovolts in magnitude are built up in the glass sheet, and often if the glass sheet is touched, these are dis- charged via the human body.

When the electron densities of the edge areas of low-emissivity coated glasses diminish, the prop¬ erties of the coated glasses weaken in this area. This means that the ability of low-emissivity coated glasses to reflect long-wave thermal radiation weakens and the thermal resistance of the glasses is lowered, a con¬ sequence of which is that heat transfer through the glasses increases and there is a greater consumption of heating energy in the buildings. The loss of electrons in the edge areas is particularly undesirable also in view of the requirement that window glasses have good thermal insulation properties in order to eliminate the detrimental effects of thermal bridges at the edges of a glass sheet. In electrically heated coated glasses, the electron loss of the edge areas increases the surface resistance of this area. The greater surface resistance of the edge area increases the heating effect locally, this being perceived as undesirable temperature differ- ences between the middle part of the glass and the edge areas. The heated-up edge areas of electrically heated glass sheets have a taxing effect on the current supply electrons, the tightness of the hermetically sealing masses of insulating glass panels and the performance of seals.

In tempered low-emissivity coated safety glasses in particular, the electron loss of the edge areas causes changes in the strength between the edge and the middle parts of the sheet. In splinter tests that have been carried out, the fragment size in the

edge areas has been observed to have decreased, and this is interpreted to be a detrimental difference in strength between the sheet's edge parts and the middle. This is felt to be an especially significant problem in tempered safety glasses, which must meet strength re¬ quirements.

In coated heat-treated glasses, significant strength differences between the edge and middle areas weaken the strengths of tempered, thermally toughened and formed glasses, limiting their use owing to reasons of safety. Solutions to the problem have been developed but they do not yet guarantee a sufficiently good end result.

By means of a heat treatment method of coated glasses according to the invention, an improvement in respect of the above-mentioned drawbacks is provided. To this end, the heat treatment method for glass in accordance with the invention is characterized by what is set forth in the characterizing part of the accom- panying claim 1. A preferred embodiment of the method is disclosed in claim 2. The apparatus according to the invention is characterized by what is set forth in the characterizing part of the accompanying claim 3. Pre¬ ferred embodiments of the apparatus are disclosed in claims 4-7.

The most important advantage of the heat treat¬ ment method of coated glasses according to the inven¬ tion can be considered to be the fact that the physical differences of the edge and middle areas of the glass sheets diminish. There are no differences in the strengths of the edge and middle areas of heat-treated glasses, the bowing of glass sheets by themselves di¬ minishes, optical drawbacks are eliminated, and the emissivities and electrical properties of the edge areas of coated glasses do not differ from the middle

areas. The desirable and required properties of glasses manufactured by the method of the invention for heat treating glass sheets do not change. This is a signi¬ ficant factor that increases safety. In addition, the optical drawbacks of glass sheets can be eliminated.

The heat treatment method for glass sheets is described in detail with reference to the accompanying drawings:

Figure 1 is a schematic representation of air jets which are directed at coated glass and which remove from the surfaces electrons that become earthed, the glass sheet thereby becoming positively charged.

Figure 2 is a schematic representation of cool¬ ing jets which are directed at the surface of a glass sheet and which contain freely moving electrons that cancel the glass sheet's positive charge by replenish¬ ing the lost electrons.

Figure 3 shows a heat treatment process for a glass sheet in which, in accordance with the invention, free electrons that cancel the positive charges are introduced along with the jets that heat and cool the glass sheet.

Figure 1 is a schematic representation of a glass sheet 1, having both sides coated with a metal, metal oxide or semiconductor oxide coating and/or a composite 2 of these. The glass sheet is moved about on rollers 3, whose surface layers are made of an insulat¬ ing material 4. Heating and/or cooling jets 6 directed at the surfaces of the glass sheet are blown via nozzles 5. The jets remove electrons 7 from the coated surfaces. Part of the removed electrons move to the middle area of the glass sheet, cancelling a positive charge 8. The electrons 9 that have broken away from the coating atoms in the edge area of the glass sheet become earthed to the ground potential by means of the

parts of device 10, whereby the surface of the glass sheet acquires a positive charge 8.

Figure 2 shows a situation according to the invention in which the heating or cooling jets contain excess electrons 11 which take the place of the migrat¬ ing electrons 7 and 9.

Figure 3 is a schematic representation of the heat treatment method according to the invention, in which an ionizer 12 producing electrons has been posi- tioned in an air jet tubing 13. Positioned in a mani¬ fold 14 of the glass sheet's upper and lower surfaces are flow-guiding nozzles 5 arranged in a jet tubing 15 which is disposed transversally to the direction of travel of the glass. In the heat treatment method for glass sheets according to the invention, the velocities of the heat¬ ing or cooling air jets 6 are very high. When the jets 6 impinge on the coated surfaces 2 of the glass sheet 1, the density of the free electrons of said surfaces being high, the jets remove electrons 7, part 9 of which are earthed by means of the devices 10. A posit¬ ive charge 8 is built up in the surfaces 2 of the glass sheet. In the middle area of the glass sheet there occurs a movement of the electrons 7 from one part of the surface to another. The heating and/or cooling jets remove part of the freely mobile electrons 9 in the edge areas of the glass sheet, the electrons becoming earthed to the surfaces 10.

In this example describing a heat treatment method for glass sheets in accordance with the inven¬ tion, the ionizer producing negative charges 11 is po¬ sitioned in the jet tubing 13. The ionizer 12 can be positioned alternatively in the other parts 14 and 15 of the manifold or in the nozzles 5. It is possible to position the ionizer 12 also in the immediate vicinity

of the glass surfaces. The most important feature in the method according to the invention is that the elec¬ trons 9 that are lost through the action of the jets 6 are replenished with the electrons 11 produced in the ionizer, said electrons being attracted by the positive charge 8, the surface of the glasses thus becoming uncharged. The heat transfer and electrical properties of the surface parts 2 of all the areas of the uncharged glass sheet 1 are retained during the heat treatment process in the method according to the inven¬ tion.

The optical distortions and bowing of the edge areas of the heat-treated sheets produced by the method according to the invention are reduced substantially, the electrical properties and properties related to the reflection of thermal radiation are retained or improv¬ ed, there are no strength differences between the edge and middle areas of the glass sheets, and there is no safety risk in the heat-treated glass sheet. It should be noted in particular that the heat treatment method according to the invention can be used with other materials than glass sheets. In the method according to the invention the electron losses are re¬ plenished by a corresponding amount of new negative charges. If necessary, the process can be arranged such that the surfaces ' to be treated become negatively charged. In addition, it should be noted that the heat treatment method of glasses according to the invention has been described with reference to only one working example, in which the jets heat and/or cool the prod¬ uct. The invention can be used in connection with the blowing of various kinds of gases or vapours having effects similar to those described herein. The appar¬ atus producing negative charges need not be located in the air jet tubing of the apparatus producing and guid-

ing the heating and cooling jets, even though this is to be recommended. It can be envisaged that the appar¬ atus in question is disposed so that it is positioned directly above the glass sheet at a distance therefrom or in between the air jet tubing/manifold.