GB2136316A

GB2136316A - Coated Glazing Materials

Info

Publication number: GB2136316A
Application number: GB08307460A
Authority: GB
Inventors: Cauter Albert Van; Robert Terneu
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1983-03-17
Filing date: 1983-03-17
Publication date: 1984-09-19
Also published as: CH660477A5; BE899128A; FR2542728A1; GB8307460D0; FR2542728B1; NL8400793A; LU85252A1; GB2136316B

Abstract

Glazing material comprises a light- transmitting sheet 1 which has on one face a light-transmitting coating 2 of electrically conductive metal oxide which reduces infra-red emissivity from the coated face. The composition and thicknesses of the sheet 1 and coating 2 are such that alone they would have a luminous transmissivity of at least 70%. The coating 2 is overcoated by a dielectric coating 3 which has a thickness not exceeding 160 nm and which increases the luminous transmissivity of the glazing material. <IMAGE>

Description

SPECIFICATION Coated Glazing Material This invention relates to glazing material comprising a light-transmitting sheet having on a face thereof a light-transmitting electrically conductive metal oxide coating which reduces emissivity of infra-red radiation from that side of the material.

Such glazing materials are widely used where it is desirable to make use of as much solar radiant energy as possible and to reduce heat loss by emission of medium or long wavelength infra-red radiation. For example, for a glazed solar collector, it is clearly important for high efficiency that the glazing material should transmit as high a proportion of solar radiation as possible while emitting as low a proportion as possible of radiation at wavelengths due to the temperature of the collector itself.

Ideally therefore such a glazing should transmit all radiation in the solar spectrum while emitting no radiation at wavelengths longer than say 2000 nm or 3000 nm.

Such glazing materials are also used in vegetable enclosures, for example greenhouses, cold frames and cloches where it is advantageous to reduce heat loss while permitting maximum exposure of the vegetable to visible light, so as to promote growth.

A further use for such glazing material lies in buildings for human habitation, again with a primary objective of reducing heat loss, and here a slightly different consideration arises in that the quality of the visible light transmitted may be as important as its quantity. In particular it is desirable that the glazing material should permit clear vision through it, that is that it should transmit light non-diffusely.

It is a principal object of the present invention to enhance the luminous transmissivity of glazing material incorporating a coating of low infra-red emissivity.

According to the present invention there is provided glazing material comprising a light transm:tting sheet having on a face thereof a light-transmitting electrically conductive metal oxide coating which reduces emissivity of infra-red radiation from that side of the material, characterised in that said electrically conductive coating is overcoated by a dielectric coating which has a thickness not exceeding 1 60 nm and which increases the luminous transmissivity of the material.

As used in this specification the term "luminous transmissivity" denotes a ratio of the quantity of transmitted visible light to the quantity of incident visible light, such quantities being corrected integrations of the transmitted and incident light values respectively over the whole spectral range of visible light, the integrations being corrected to compensate for the spectral distribution of the radiant energy source and for the spectral sensitivity characteristics of the human eye. The measurements are made with light incident on the coated face of the material using a spectrophotometer and a light source whose spectral composition is that of Illuminant C as defined by the International Commission on Illumination. This illuminant may be taken to represent average daylight.The eye sensitivity correction factor applied is likewise that which is standardised by the International Commission on Illumination.

The invention affords the advantage of increasing light transmissivity through the glazing material. It is well recognised in that the art that the reduction in emissivity due to the conductive coating arises because of the presence of free charge carriers at the surface of the material. However we have now found that, although a said dielectric coating will increase the emissivity of the conductively coated surface, provided that the dielectric coating is below 160 nm, such increase in emissivity can be acceptable in that glazing material according to the invention has a favourable compromise as between its visible light transmitting and its infra-red emitting properties.

It has in fact been found that the infra-red emissivity of glazing material according to the invention increases substantially directly with increase in the thickness of the said dielectric coating. It is accordingly preferred, in order to keep the increase in infra-red emissivity small, that said dielectric coating has a thickness not exceeding 140 nm and preferably not exceeding 1 10 nm.

In the most preferred embodiments of the present invention, said dielectric coating has an optical thickness in transmission of at least 60 and preferably at least 70 nm. The optical thickness of a coating in transmission is defined as its absolute thickness multiplied by its refractive index. By making use of this feature of the invention, considerable benefits in luminous transmissivity can be achieved due to interference extinction of reflected visible light. It has been thought that to achieve the maximum benefit from such interference effects the coating should have an optical thickness equal to one quarter of the wavelength of the light whose transmission is to be maximised. In fact this has been found not to be true in the case where coatings of two different materials are applied as in this invention.The optimum thickness of the dielectric coating for increasing the luminous transmissivity is also a function of the thickness of the underlying conductive coating. Such variation in optimum thickness, which is substantially sinusoidal, is exemplified by the following table which gives the optimum absolute thickness of a silica dielectric coating deposited on coatings of doped tin oxide of various absolute thicknesses on a soda-lime glass substrate. The composition of the silica coating is SiOx where x lies between about 1.95 and 2, but for convenience it will be referred to as silicon dioxide (SiO2) coating.

Similarly the doped tin oxide coatings will be referred to as being of stannic oxide (SnO2).

SnO2 350 nm 385 nm 420 nm 455 nm 490 nm SiO2 100 nm 55 nm 100 nm 55 nm 100 nm The SiO2 coatings had a refractive index of approximately 1.41 so that the corresponding optical thickness values were 141 nm and 77.5 nm.

Preferably, the refractive index of the dielectric coating is within ten percent of the square root of the refractive index of the electrically conductive coating. Adoption of this preferred feature of the present invention reduces the total amount of light reflected at the boundaries between the dielectric coating and, on the one side air, and on the other side, the conductive coating. By way of example, the SnO2 coatings specified above had a refractive index of about 2.0. The refractive index of the SiO2 coatings, 1.41, is substantially equal to the square root of 2.

It is surprising that the observance of this condition also gives rise to significant benefits in reducing that proportion of the transmitted light which is transmitted diffusely.

The invention is especially valuable for promoting luminous transmission in products which already have high luminous transmissivity, and it is accordingly preferred that the compositions and thicknesses of the light-transmitting sheet and said electrically conductive coating are such that in the absence of said dielectric coating, they would have a luminous transmissivity of at least 70%.

In some preferred embodiments of the invention said electrically conductive coating comprises tin oxide. The use of low infra-red emissivity coatings of tin oxide is well known per se and such coatings can be formed which have good luminous transmission and infra-red emissivity. In addition such coatings can be highly resistant to abrasion.

In other preferred embodiments of the invention, said electrically conductive coating comprises indium oxide. Indium oxide coatings are also well known per se, and such coatings can have better properties than tin oxide in respect of high luminous transmissivity and low infra-red emissivity but suffer from being relatively easily abraded. For this reason indium oxide coatings as such are not widely used in circumstances where they are exposed. Since, according to the present invention the electrically conductive coating, for example of indium oxide, is over-coated, such a relative lack of abrasion resistance need not present any disadvantages.

In preferred embodiments of the invention, said dielectric coating has a higher abrasion resistance than the electrically conductive coating. The expression "abrasion resistance" is used herein to denote resistance to abrasion as measured by the method defined in American National Standard No. Z26. 1-1977 Test No. 18 in respect of safety glass.

Said dielectric coating is preferably composed of silica. Silica can be formed into highly transparent coatings which have excellent abrasion resistance and good chemical stability. In addition, silica coatings can readily be formed with a refractive index of about 1.41. This is especially useful as has been remarked earlier when the refractive index of the subjacent electrically conductive coating has a refractive index approximately equal to (1.41)2 as is the case with tin oxide.

Such a silica coating can be formed in various ways. In one method, a transparent sheet of glass bearing an electrically conductive coating is dipped in a solution of an organic silicon compound such as a solution of tetramethylorthosilicate Si(OCH3)4 in methanol. On exposure to air, Si(OCH3)4 is converted to Si(OH)4 and this in turn is converted to SiO2 by heating preferably at 500600 C, in air.

The desired thickness of the coating can be attained by regulating the speed at which the sheet is withdrawn from the solution. In another method particularly adapted for forming a continuous coating on hot glass bearing a conductive coating, for example a continuous ribbon of freshly formed and coated glass, the dielectric coating is formed by vapour deposition, for example by contacting the hot coated glass with silicon hydride vapour in the presence of atmospheric oxygen.

Coatings having very smooth surfaces can be formed in either of these ways. Surface smoothness is one of several factors which has an effect on the proportion of diffuse/non-diffuse luminous transmission. When fairly thick conductive costings are used, for example approaching 1 ym in thickness, it is difficult to eliminate all rugosity from the surface of the coating. Such rugosity promotes diffusion of transmitted light. . The application of an overcoating having a smoother surface can alleviate this so that a higher proportion of the light transmitted is non-diffuse. It is to be noted that this phenomenon is not dependent on the refractive index of the dielectric coating.

Among other materials which may be used for forming a said dielectric coating are fluorides of calcium and magnesium. It is to be noted however that these materials are not highly resistant to abrasion, though they may with advantage be used in the formation of a coating which is to be located at an interior surface of a multiple glazing unit.

In some preferred embodiments of the present invention, said dielectric coating is a coating of polymeric material. Such coatings, for example of a silicone or polyurethane may be formed by dipping a conductively coated glazing sheet in a solution of the desired dielectric coating material while the uncoated face of the glazing sheet is protected. Such coatings are best used for an interior surface of a multiple glazing unit so that they are protected against abrasion.

Preferred embodiments of the present invention will now be described by way of example and with reference to the accompanying diagrammatic drawings in which: Figure 1 is a detail cross sectional view of a sheet of glazing material according to the invention, and Figure 2 is a detail cross sectional view of a hollow glazing panel incorporating a sheet of glazing material according to the invention.

In Figure 1 a light transmitting sheet 1 e.g. of glass, has a coating 2 deposited on one face. This coating 2 is a light transmitting electrically conductive metal oxide coating which reduces the emissivity of the coated face in respect of infra-red radiation, in particular infra-red radiation having wavelengths longer than 3,us. The conductive coating 2 is overcoated by a dielectric coating 3 which has a thickness not exceeding 1 60 nm and which increased the luminous transmissivity of the glazing material.

In Figure 2, a light transmitting glazing sheet 4 bears on one face a light transmitting conductive coating 5 of metal oxide which reduces the emissivity of the glazing sheet in respect of infra-red radiation and which is in turn overcoated by a dielectric coating 6 which has a thickness less than 160 nm and increases the luminous transmissivity of the glazing material. The glazing sheet 4 is bonded via a marginal spacer member 7 to a second sheet 8 of glazing material which may be coated or not. The coatings 5, 6 are located within the panel so that they are protected against abrasion.

The following Table 1 relates to properties of sheets of 4 mm thick float glass 1 coated with fluorine doped tin oxide coatings 2 and overcoated with dielectric coatings 3 of silica.

Silica coatings are chemically stable and abrasion resistant so that they may be used exposed to the atmosphere. Such a glazing sheet may be incorporated into a hollow glazing panel for example as described with reference to Figure 2 with its coated face facing inwardly or outwardly of the hollow panel. The refractive index of the tin oxide coatings was about 2.0 and that of the silica coatings was about 1.41.

In Table 1, the various columns relate to the following properties: Actual thickness of SnO2 coating (nm) II Infra-red emissivity of SnO2 coated glass (%) Ill Luminous transmissivity of SnO2 coated glass (%) IV Optimum actual thickness of SiO2 coating (nm) V Infra-red emissivity of SnO2 and SiO2 coated glass (%) Vl Luminous transmissivity of SnO2 and SiO2 coated glass (%) TABLE 1 I II Ill IV V Vl 350 29 83 100 35 90 420 21 85 100 25 90.5 490 16 82 100 20 90.3 840 12 75 100 17 90.3 The deposition of tin oxide coatings as referred to above is so well known that it need not be detailed here. Specific examples of suitable processes for depositing such coatings are to be found in British Patent Application No. 82 12 670 (Specification No. GB A).

Examples of two processes for depositing silica coatings, one a batch process and the other a continuous process now follow.

Batch Process for Depositing Silica A solution is made up of the following proportions: Tetramethylorthosilicate Si(OCH3)4 0.55 mole Water 0.75 mL HCI 1.0 mL Methanol to 1.0 L Metal oxide coated glass is dipped in such a solution and withdrawn steadily. The coated glass is then heated in air to as high a temperature as is convenient (for example in the range 5000C to 6000C) to convert the Si(OCH3)4 in turn to Si(OH)4 and finally to SiO2.

The thickness of the resulting SiO2 coating is regulated by varying the speed of withdrawal of the sheet from the solution.

Such a process results in the metal oxide coated glass sheet being given a silica coating on both faces, unless of course the uncoated face is masked during dipping or cleaned after dipping and prior to heating. Such a second coating of silica makes negligible difference to the properties of the glass sheet.

Continuous Process for Depositing Silica Hot metal oxide coated glass sheets are, or a continuous ribbon of hot metal oxide coated glass is contacted with silicon hydride in the vapour phase. If the glass is hot enough, for example at a temperature above 5000 C, the silicon hydride pyrolises in the presence of oxygen to form an adherent coating of silica.

Example 6 of British Patent Specification No. 1,524,326 gives a process for forming a coating of silicon on a glass ribbon by exposing the ribbon to silicon hydride vapour in an oxygen-free atmosphere.

This process can readily be modified, simply by performing the coating step in air, to form a silica (SiO2) coating.

The effect on light transmissivity and infra-red emissivity of varying the thickness of an SiO2 dielectric deposited on a coating 840 nm thick of SnO2 is shown in the following Table 2.

TABLE 2 SiO2 Thickness Light Infra-red (nm) Transmissivity % Emissivity % 0 75 12 50 86.7 14.5 75 89.6 15.8 100 90.3 17 150 85.4 19.5 It will be noted that the infra-red emissivity of the glazing material increases in linear fashion for such thicknesses of dielectric coating whereas the light transmissivity reaches a maximum when the dielectric coating has a thickness of 100 nm (optical thickness 141 nm). This latter is due to interference effects.

Instead of forming a tin oxide conductive coating, it possible to use another metal oxide, for example indium oxide. Doped indium oxide coatings can be formed in the same way as tin oxide coatings by using a coating precursor solution of indium chloride. In fact indium oxide coatings as thin as 100 nm have excellent low infra-red emissivity. Indium oxide coatings themselves do not have high abrasion resistance, but they may be protected by a harder top coating, for example of silica which has a better resistance to abrasion. Otherwise they are best used as shown in Figure 2 within a multiple glazing unit.

In circumstances where the dielectric coating is not exposed, as is the case of the coating 6 at the interior of the hollow glazing panel shown in Figure 2, that coating may be of a soft material.

Examples of such relatively soft materials are fluorides of magnesium and calcium which can be deposited as thin coatings having a refractive index of 1.38 and 1.43 respectively.

Alternatively, such a dielectric coating may be of polymeric material. Many such materials have a refractive index of about 1.5. Such materials may be applied to metal oxide coated glass sheets by dipping in a solution of the required plastics material. Silicones and polyurethane dielectric coatings may be applied in this way.

It is to be emphasised however that such coatings must be not more than 160 nm thick in order not to lose the benefit of applying the low-emissivity conductive coating. By way of example, if a coating 840 nm thick of SnO2 is overcoated with a dielectric layer 1000 nm thick, the infra-red emissivity of the glazing material will be about 90%, substantially the same as that of uncoated glass.

Claims

1. Glazing material comprising a light-transmitting sheet having on a face thereof a lighttransmitting electrically conductive metal oxide coating which reduces emissivity of infra-red radiation from that side of the material, characteristed in that said electrically conductive coating is overcoated by a dielectric coating which has a thickness not exceeding 160 nm and which increases the luminous transmissivity of the material.

2. Glazing material according to claim 1 wherein said dielectric coating has a thickness not exceeding 140 nm and preferably not exceeding 110 nm.

3. Glazing material according to claim 1 or 2, wherein said dielectric coating has an optical thickness in transmission of at least 60 and preferably at least 70 nm.

4. Glazing material according to any preceding claim, wherein the refractive index of the dielectric coating is within ten percent of the square root of the refractive index of the electrically conductive coating.

5. Glazing material according to any preceding claim, wherein the compositions and thicknesses of the light-transmitting sheet and said electrically conductive coating are such that in the absence of said dielectric coating, they would have a luminous transmissivity of at least 70%.

6. Glazing material according to any preceding claim, wherein said electrically conductive coating comprises tin oxide.

7. Glazing material according to any of claims 1 to 5, wherein said electrically conductive coating comprises indium oxide.

8. Glazing material according to any preceding claim, wherein said dielectric coating has a higher abrasion resistance than the electrically conductive coating.

9. Glazing material according to any preceding claim, wherein said dielectric coating is composed of silica.

10. Glazing material according to any preceding claim wherein said light-transmitting sheet is of glass.

10. Glazing material according to any of claims 1 to 7, wherein said dielectric coating is a coating of polymeric material.

11. Glazing material according to any preceding claim wherein said light-transmitting sheet is of glass.

Supersed Claims 1 to 11 New or Amended Claims:

1. Glazing material comprising a light-transmitting sheet having on a face thereof a lighttransmitting electrically conductive metal oxide coating which reduces emissivity of infra-red radiation from that side of the material, characterised in that the compositions and thicknesses of the lighttransmitting sheet and said electrically conductive coating are such that alone they would have a luminous transmissivity of at least 70%, and in that said electrically conductive coating is overcoated by a dielectric coating which has a thickness not exceeding 160 nm and which increases the luminous transmissivity of the material.

2. Glazing material according to claim 1, wherein said dielectric coating has a thickness not exceeding 140 nm and preferably not exceeding 110 nm.

5. Glazing material according to any preceding claim, wherein said electrically conductive coating comprises tin oxide.

6. Glazing material according to any of claims 1 to 4 wherein said electrically conductive coating comprises indium oxide.

7. Glazing material according to any preceding claim, wherein said dielectric coating has a higher abrasion resistance than the electrically conductive coating.

8. Glazing material according to any preceding claim, wherein said dielectric coating is composed of silica.

9. Glazing material according to any of claims 1 to 6, wherein said dielectric coating is a coating of polymeric material.