Heat Reflecting Pane and a Method of Producing It

1. Field of the Invention

This invention relates to a heat-reflecting pane or panel comprising a substantially transparent glass substrate, for example of soda-lime-silicate glass, and further comprising two layers of TiO.sub.2. The outside TiO.sub.2 layer comprises TiO.sub.2 in the rutile form with the intermediate layer TiO.sub.2 in the anatase form. Both TiO.sub.2 layers are formed in a similar manner by vapor deposition in vacuo and subsequent oxidation of the layer thus formed.

2. Description of the Prior Art

Heat-reflecting window glass panes of this kind in which a heat-reflecting TiO.sub.2 layer is applied by vapor-coating of a titanium layer in vacuo and subsequent oxidation of such layer at very high temperatures in air are known and have been described e.g., in a publication by G. Hass entitled "Preparation, Properties and Optical Applications of Thin Films of Titanium Dioxide" (G. Hass, Vacuum, Vol. 2, No. 4, pp. 331-345 (1952)). Depending upon the conditions in which the Ti layer is vapor-coated in vacuo, there are two TiO.sub.2 forms which may be produced in the subsequent oxidation of the Ti layer. When the titanium is vapor-coated rapidly in a good vacuum i.e., of approximately 10.sup.-5 mm Hg or even harder, the rutile form is produced, whereas if the vapor-coating proceeds relatively slowly in a less hard vacuum, e.g., of approximately 10.sup.-4 mm Hg, the anatase form is produced. TiO.sub.2 layers produced in this way have a variety of uses and optics to coat glass panes, e.g., as color filters and as sunshine-reflecting coatings, the thickness of the layer being made such as to act as a highly effective quarter-wave interference layer, over the spectral range for which a modification of the reflection properties of the substrate is required.

One particular purpose for which heat-reflecting glass panes of the kind described are used is in facade elements or infilling panels for buildings. The requirements in the case of infilling panels of this kind are for TiO.sub.2 -coated glass panels which have a high neutral reflection, possibly with a slight blue or yellow tint, in the visible spectral range. The usual practice with panels of this kind is for the TiO.sub.2 interference layer to be disposed on the outside of the building, the back of the glass substrate being treated with an opaque enamel or varnish, so that parts of the building behind the panel cannot be seen through it from the outside.

Films of TiO.sub.2 in the rutile form are very advantageous, more particularly for the last mentioned purpose, since such layers have a higher index of refraction than anatase layers and can therefore provide higher reflection values which are very desirable in the case of facade elements and infilling panels. It has also been found that rutile films are much harder and much more resistant to abrasion than anatase films. Consequently, panes of glass where the TiO.sub.2 interference film on the outside of the building has a rutile form can be exposed directly to the atmosphere without damage for a prolonged period of time. Also, the usual cleaning agents for outside surfaces of glass can be used to clean such panes or panels.

In some of the uses of TiO.sub.2 -coated glass panes or panels of the kind described, more particularly for use as a facade element or infilling panel, the glass must be toughened to comply with safety regulations. Toughening is necessary when infilling panels having rutile films are enameled on the back. Because of the presence of the enamel coating, which is impervious to radiation, the glass may become so hot in sunshine that heat cracks would occur in an untoughened glass substrate. The glass is prestressed or toughened in a known manner by being heated beyond its transformation temperature, the temperatures at which softening begins, and then cooled very abruptly. Temperatures of from about 570.degree. to 620.degree. C. are necessary for this purpose in the case of soda-lime-silicate glasses having the chemical composition of conventional flat glass.

The heating of the glass to toughen it in the manner described above may also be used to promote oxidation of the vacuum, vapor-coated Ti film. Alternatively, the vapor-coated Ti layer can first be oxidized at a temperature of from 400.degree. to 500.degree. C., in one furnace, the pane with its TiO.sub.2 coating being thereafter cooled and being subsequently heated again, for toughening, in another furnace to the temperature necessary to toughen the glass, between 570.degree. to 620.degree. C. in the case of soda-lime-silicate glass.

Theoretically, of course, oxidation of the Ti film to TiO.sub.2 should be carried out very rapidly to provide an acceptable layer of TiO.sub.2 in the rutile form. It is known from G. Hass, Vacuum Vol. 2, No. 4, p. 335, FIG. 3, that oxidation proceeds faster in proportion as the oxidation temperature is higher. Unfortunately, when in the known process the oxidizing temperature is increased to above 550.degree. C. in order to promote rapid oxidation, alterations occur in the rutile layers, which turn matt and dull and cause so much light scattering both in transmission and reflection that panes having such layers cannot be used for the purposes mentioned, that is as infilling panels and facade elements. A peculiarity is that these changes in the film occur only in rutile films. When other vacuum vapor-coating conditions are used, more particularly a softer vacuum and/or longer vapor-coating time, oxidation leading in the manner described to TiO.sub.2 layers having the anatase form and panes which have been coated in this way can be heated to relatively high temperatures, such as 550.degree. C. or more, without experiencing any of these changes in their coatings. Of course, the same difficulties of the rutile coatings undergoing changes always occur when the glass panes are toughened by heat treatment in the manner described, since for this purpose, and as already stated, the temperatures required are above 550.degree. C. and in the case of soda-lime-silicate glasses are preferably of from 570.degree. to 620.degree. C. These unwanted changes in the coating occur when the panes are heated to the temperatures required for toughening irrespective of whether oxidation of the Ti layer to TiO.sub.2 and heating to the glass toughening temperature are effected in a single step or whether the Ti layers are first oxidized at a relatively low temperature below 550.degree. C. and only subsequently, possibly after further processing, are the panes heated to the temperatures necessary for toughening.

The parent application solves the problem of manufacturing heat-reflecting panes of the aforementioned kind, suitable for use as spandrel or infilling panels or facade elements, and of providing a method of manufacturing them wherein the TiO.sub.2 films are mostly or entirely rutile and undesirable changes are effectively prevented from occurring in the films when heated to temperatures above 550.degree. C., as required for rapid oxidation of the Ti film, more particularly for thermal toughening. To this end, according to the parent application, a vapor-deposited silicon oxide layer which does not cause interference is disposed between the glass pane and the TiO.sub.2 film, the method according to the parent application being characterized in that a silicon oxide film which does not cause interference is vacuum-deposited on the glass pane before the Ti film is deposited, and the thus-coated glass pane is oxidized by heating in air.

According to the present invention, this problem is solved in a completely different manner from that used in the parent application. Correspondingly, a method according to the present invention for manufacturing a heat-reflecting pane of the kind according to this invention is also disclosed and claimed.

The invention is based upon the surprising discovery that the disadvantageous changes in a rutile TiO.sub.2 film after heating to temperatures above 550.degree. C., as required for thermal toughening, can be avoided if an intermediate film of TiO.sub.2 in the anatase form is disposed between the glass pane and the rutile layer. The two films are produced as follows: A first Ti film is deposited on the glass pane relatively slowly and at a relatively low vacuum, after which a second Ti film is deposited relatively fast at a relatively high vacuum; as a result of subsequent oxidation, the anatase intermediate film is formed from the first Ti film on the glass pane and the rutile film is formed from the second Ti film.

It has been shown that like the invention disclosed in the parent application the invention of providing an anatase film between the glass pane and the rutile film completely solves the problem presented in the prior art, i.e., changes occurring in the film at the temperatures required for oxidation or thermal toughening. In addition, however, the TiO.sub.2 coating does not crack or become loose even when it has a total thickness of up to 600 A, as is required when it is desired to obtain a neutral-color external appearance. Since the same titanium-filled evaporator devices are preferably used for depositing both Ti films, the total coating, according to this invention, can be applied even more easily and economically than in the case of the heat-reflecting pane and the method according to the parent application. In addition, titanium coating material is available in wire form, whereas silicon monoxide, when used for layer coating, is in granular form, so that it is easier to weigh the amount of material required for the evaporator devices when this invention is used instead of that disclosed in the parent application. The double coating according to the invention has the same hardness and wear resistance as a rutile film without an anatase intermediate film, provided the rutile film is applied in a thickness of at least 80 A. The total thickness of the rutile film portion of the total TiO.sub.2 coating should not be more than approximately 300 A, since otherwise the rutile film may crack. The maximum permissible thickness of the rutile film is slightly dependent on the temperature chosen for oxidizing the films or for thermal toughening, that is, the maximum permissible thickness is somewhat lower if the temperature is increased. In addition, the maximum permissible thickness of the rutile film before cracking is slightly influenced by the surface state of the glass before coating, i.e, if the glass surface is corroded or has not been carefully cleaned, the maximum permissible thickness is reduced.

Another special advantage of the coating according to the invention is that the glass pane is directly adjacent the TiO.sub.2 coating, resulting in particularly firm adhesion. In addition, the Ti films, even before being oxidized, adhere very firmly to the glass, so that it is easier to handle the Ti-coated panes before the Ti films have been oxidized.

The prior art could not give a person skilled in this art any inkling of this invention. German Offenlegungsschrift No. 1,596,816 describes an arrangement in which TiO.sub.2 and silicon dioxide layers are applied to a glass pane. The glass pane is covered first with a TiO.sub.2 film and then with a silicon dioxide film. Similarly, CA Patent specification No. 464,446 discloses film systems wherein a mixed layer of TiO.sub.2 and silicon oxide is deposited on a glass pane, followed by one or more pure TiO.sub.2 or silicon dioxide films. However, these publications do not provide any information regarding the teaching of the invention: avoiding changes in TiO.sub.2 films on glass panes by manufacturing the TiO.sub.2 coating by vacuum deposition and subsequent oxidation and by providing an intermediate layer of anatase TiO.sub.2 between the glass pane and the rutile TiO.sub.2 film. Furthermore, U.S. Pat. No. 2,478,817, which relates in general to the manufacture of TiO.sub.2 films using the immersion process, and Czechoslovak patent specification PV 4393-65, published in part in Glastechnische Berichte P 11, February 1968, do not come anywhere near the teaching of this invention, i.e., of manufacturing TiO.sub.2 films by oxidizing vapor-coated Ti films in vacuo, by choosing the deposition conditions so that an intermediate anatase film is first deposited on the glass pane and subsequently covered with an outer film in the rutile modification, which has the advantageous properties relating to the outer surface of the pane as hereinbefore described.

Embodiments of the invention will be described in detail hereinafter with reference to the accompanying diagrammatic drawing.

The single FIGURE forming the drawing is a view in section, showing the construction of a heat-reflecting pane embodying the invention.

As the drawing shows, a heat-reflecting pane embodying the invention comprises a substantially transparent glass substrate or glass pane 10, more particularly of soda-lime-silicate glass, covered in succession by an anatase TiO.sub.2 intermediate layer 12 and an outside rutile TiO.sub.2 layer 14 disposed on the side of the anatase layer 12 remote from the glass substrate 10. The system of films may be produced by depositing a first titanium layer relatively slowly at a vacuum of the order of 10.sup.-4 Torr and then depositing a second Ti layer relatively fast in a vacuum of the order of 10.sup.-5 Torr, after which the two layers are oxidized at a temperature sufficient to produce the anatase and rutile modification respectively. The glass substrate shown in the drawings and in the example described herein, consists of thermally toughened glass, wherein the glass pane is heated to a temperature in the range of from 570.degree. to 620.degree. C., (in the case of soda-silicate glass), and then thermally toughened by rapidly cooling the pane. Preferably, the glass pane, coated with the two Ti films, is first heated to a temperature in the range of from 570.degree. to 620.degree. C., (in the case of soda-silicate glass), thus converting the deposited Ti films into the anatase layer 12 and the rutile layer 14 and also heating the glass pane 10 to the temperature required for thermal toughening, so that it can immediately be chilled. No disadvantageous changes have been observed in rutile TiO.sub.2 films 14 prepared in this way. Even long-term tests do not show any changes in the TiO.sub.2 coating.

Two examples will now be given. Example 1 illustrates the result produced when the provision of an intermediate layer of rutile TiO.sub.2 is omitted. Example 2, illustrating by way of comparison, the result when an intermediate layer of rutile TiO.sub.2 is provided in accordance with the invention.

A float glass pane 8 mm thick and having external dimensions of 300 cm.times.245 cm was first cleaned in the conventional manner by a glow discharge at a pressure of 3.times.10.sup.-2 Torr in a vacuum deposition plant. Next, a Ti film was vacuum-deposited at a pressure of 5.times.10.sup.-5 Torr for 35 seconds. Next, the coated pane was heated to 620.degree. C. in a conventional thermal toughening furnace and then chilled. During the heating process, the titanium film was oxidized to a titanium film 470 A thick having a rutile structure. However, the film was turbid and matt and was thus unsuitable, for architectural reasons, for use as an infilling panel or facade element for a building or the like.

The procedure was the same as for Example 1 except with the difference that after the glow discharge purification, a first Ti film was deposited at a pressure of 1.5.times.10.sup.-4 Torr for two minutes, after which the deposition process was interrupted, the vacuum was made more intense, and a second Ti film was deposited at a pressure of 5.times.10.sup.-5 Torr for 20 seconds. The TiO.sub.2 -coated pane was then toughened in the same manner as in Example 1. The outer TiO.sub.2 layer, which was produced from the Ti layer applied at high vacuum, had a rutile structure, whereas the intermediate layer had an anatase structure. In contrast to the pane produced according to Example 1, the pane produced in Example 2, according to the invention, was completely transparent after toughening, and could be used very satisfactorily as an infilling or spandrel panel or as a facade element for a building or the like. The pane had a neutral-color with a silvery, shining appearance. Its reflective power, relative to the sensitivity of the human eye to brightness, was 40%. Utilization of this pane is therefore architecturally useful in conjuntion with insulating glass panes of a neutral color.

While I have disclosed an exemplary method and structure to illustrate the principles of my invention, it should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.