Coated transparent sheets

Abstract

A transparent structure comprising a transport sheet having a non-continuous coating of aluminum distributed over at least one surface of the sheet in a regular pattern, to provide means of controlling the percentage transmission of visible light and infra-red radiation.

Description

The invention relates to a sheet which is coated over discrete areas of its surface with aluminium in such a manner that when sun-light is transmitted by the sheet, the infra-red ray is effectively masked whereas the percentage transmission of the visible light ray may be adjusted by control of the aluminium coating.

Domes or windows particularly for exhibition buildings, and the like are often constructed from transparent glass and plastics to enable the maximum amount of sun-light to be transmitted to within the building. However the infra-red rays which pass into the building with the visible light rays contain radiant heat energy, which causes an increase in the temperature inside the building. The temperature increase is particularly noticeable inside buildings with large domes or windows and necessitates the costly installation of air-conditioning.

It is known to overcome these inconveniences by the erection of aluminium sunshades on the upper vault of the dome. Alternatively a translucent film obtained by vacuum-metallizing polyester film with a very thin coating of aluminium may be stuck to the dome or window.

These methods have technical disadvantages, and cannot be used for general purposes although they may be used for some special purposes. The sunshade used in the first method outlined consists of a number of aluminium plates and cannot therefore give a homogeneous percentage transmission across the entire area of the dome or sheet. Furthermore, the high erection costs tend to make this system uneconomical. The second method outlined above is used effectively. The translucent films are bonded onto a transparent base sheet but their application is limited to certain kinds of base sheet. Furthermore, the cost of fitting the sheets is high and the transmitted ray tends to have a bluish tinge and is different from natural light.

The biggest disadvantage common to both these methods is that the masking materials are structurally weak and have poor weathering resistance or durability and must therefore be fitted inside the glass or plastic transparent sheet. As a result, the infra-red ray is partly absorbed by the glass or plastic sheet itself. At the same time, the ray reflected by the aluminium is re-absorbed by the same sheet. Thus the sheet temperature rises sharply and, when used in closed type buildings, the temperature inside the building rises. In the case of a plastic sheet, which has a large heat expansion this provides a serious technical problem in the designing and erection of building structures.

Another possibility is to incorporate an infra-red absorber in the sheet; however as the temperature of the sheet itself rises the effect of the infra-red absorber is reduced significantly.

It is common knowledge that the reflectivity of infra-red rays (or near infra-red rays) by aluminium is high, and because of this aluminium sheet or some other sheet to which aluminium suspension paint has been applied is often used to mask infra-red rays. The suspension paint has, however, to be applied to the total surface area and it is not then possible to adjust the amount of the visible ray that is transmitted.

We have devised a novel inexpensive method whereby the percentage transmission of visible light and infra-red rays can be controlled.

The present invention provides a transparent sheet coated over only a part of its surface with aluminium whereby the percentage transmission of the visible light and the infra-red ray is controlled.

The infra-red ray absorption by the sheet itself is controlled by local coating with aluminium. In this way the heat expansion and contraction due to the temperature fluctuations may be minimized. This makes designing and erection work much easier.

The present invention also provides a method for controlling the transmission by a transparent sheet of infra-red rays and visible rays comprising coating directly certain areas of the sheet with aluminium. Transmission of infra-red rays and visible rays can thus be reduced to any desired value. The aluminium may be applied as paste, as a liquid medium such as an ink or a paint containing aluminium. The liquid medium containing aluminium paste may be securely coated on the base sheet by painting, spraying or printing.

If the liquid is applied by painting or spraying it is difficult to control the thickness of the coating and thus difficult to control the amount of light transmitted to a specified value, and it is unavoidable that the silhouette becomes distorted or dim. Printing and particularly screen printing methods are therefore preferred. Under this method, the amount of light transmitted can be adjusted for the whole surface of the sheet; furthermore the distortion of the silhouette viewed through the sheet can be avoided by providing a reticular print on the plate. The ratio of printed area to the total area (hereinafter referred to as the coated area ratio) can be changed so that the percentage transmission within the visible limit and percentage transmission of infra-red ray can be accurately reduced to a predetermined level.

In the production of the coated sheet a negative with the desired coated area ratio may be prepared by the phototype process generally used for printing. And thus using this negative a liquid medium containing aluminium paste may be printed on the base sheet, by screen printing techniques and a sheet having the desired surface coated area ratio obtained.

If the percentage transmission of visible and infra-red rays is to be reduced the coated area ratio should be increased, it must be understood that the percentage transmission of infra-red ray depends not only on the coated area ratio but also on the transmission spectrum of the sheet in the near infra-red limit and to the energy distribution of the light source.

For example, according to an experiment which we have performed in which a transparent sheet of polymethyl methacrylate (5mm thick) was used as the base sheet, and an infra-red ray bulb was used as the light source, the percentage transmission of the visible ray was adjusted from 20 percent to 80 percent and the percentage transmission of infra-red ray was adjusted from 5 percent to 50 percent by changing the coated area ratio. If the sun is the light source used in the above experiment, accurate values cannot be obtained, because the power of the light source does not remain constant. We found however that the percentage transmission of infra-red rays could be adjusted from about 10 percent to about 65 percent.

It is common practice to adjust the percentage transmission of the visible ray, by adding different amounts of translucent dyestuffs or pigments to the base sheet. These pigmented sheets are called smoked sheets but they do not mask infra-red rays. For instance, by using smoked sheets of polymethyl methacrylate (5 mm thick) as the base sheet, the percentage transmission of the visible ray may be adjusted from 20 percent to 80 percent when an infra-red ray bulb is used as the light source; however, the percentage transmission of infra-red ray remains constant at about 55 percent.

The temperature rise of the base sheet, and hence the absorption coefficient of the infra-red ray varies depending on the thickness of the base sheet, the transmission spectrum of the sheet in the near infra-red limit, the energy distribution of the light source, the type and particle size of aluminium paste contained in the liquid medium and the concentration of aluminium paste in the liquid medium. The temperature also depends on whether the light comes directly to the printed surface or first passes through the base sheet. In the latter case, a part of the infra-red rays are absorbed in the base sheet before the transmitted ray reaches the printed surface, and then part of infra-red rays reflected by the printed surface are also absorbed. Thus the absorption coefficient of infra-red rays of the coated sheet is larger if the rays pass through the sheet to reach the coating rather than first striking the coating.

In the case of translucent film vacuum-metallized with aluminium, which is generally known as half mirror, adhesion between aluminium and the film as well as between the metallized film and the base sheet is not strong enough to withstand the outdoor exposition, and therefore, the film must be adhered to the inner surface of the base sheet. Consequently, the base sheets absorption coefficient of infra-red rays is increased and the over-all insulating effect is reduced. The coatings of the present invention which are applied directly to the transparent sheet have good adhesion to the sheets and may be exposed to inclement weather.

It is therefore preferred to have the partially coated surface facing the outside for the purpose of insulating closed buildings or domes from the infra-red rays of sun-light. We have found that, application of the coating by screen printing, produces a good thickness of coating which has good surface weathering properties. If however it is only desired to adjust the transmission of the visible ray, it makes no difference which surface of the sheet is coated.

In this specification, percentage transmission of the visible ray means the total ray percentage transmission of light, through the sheet. The light being derived from a photo-electric photometer with an intergrating sphere provided with a green filter.

Percentage transmission of the infra-red ray and of the absorption coefficient were measured by the following methods.

Firstly the light absorption coefficient of a sooted copper sheet was considered to be 96 percent. The infra-red ray transmission of the sheet of the present invention was then determined from the relationship between the curve showing the temperature rise of the sooted copper sheet, where all the absorbed light was converted to heat energy and a curve showing the temperature rise of the sooted copper sheet when the coated sheet of the present invention was placed between the sooted copper sheet and the light source.

The absorption coefficient of the coated sheet of the present invention was measured by measuring the temperature rise of the inside surface of the partially surface coated sheet.

The partially surface coated sheet not only transmits and absorbs the light, but also reflects the infra-red ray, and the sum of its percentage transmission, absorption coefficient and reflectivity is unitary and thus the reflection may also be determined.

The spots of aluminium (number of lines per 1 inch) applied by screen printing can be selected arbitrarily. The greater the number of lines per inch the narrower the space becomes between the printed part and the blank part and the clarity of vision through the sheet is reduced; furthermore as the number of lines increase it becomes increasingly difficult to maintain uniformity of the printed surface because of the plugging of the screen with the liquid medium. Therefore, a smaller number of spots is desirable particularly if the sheet is to be used to view an object which is a long distance from the printed surface. Our preferred coating is made up of 10-50 spots.

Any transparent sheet may be used as the base sheet. For example, glass sheet and transparent plastic sheet such as polystyrene, polyvinyl chloride and polymethyl methacrylate which is our preferred material can be used.

The liquid coating medium may be a silver coloured ink which is made from aluminium powder which may be mixed with other inks made from yellow, red or other colour pigments or dyestuffs. For example gold and other coloured coatings can be obtained by mixing suitably coloured inks with the silver coloured ink. By the use of a liquid medium prepared in accordance with the invention a surface comprising a tightly adhered coating of aluminium paste is obtained. This coating has good weathering properties which may be improved still more if the coated surface is covered by a transparent lacquer by printing or painting.

If reticular printing is carried out by a screen printing method, any transparent coloured ink can be printed within the outline of the aluminium printing space by using a negative having the same number of spots as that of the negative used for printing with the aluminium paste, and having a coated area ratio which is equal to or less than that of the negative used for printing with the aluminium paste. Thus if this method is applied to a dome, white light is transmitted into the building and a decorative coloured metallic gloss is observed from the side of the light source.

Another application of reticulately surface printed sheet is as follows. If two surface printed sheets are placed back-to-back or if both sides of the base sheet are printed, what is called "moire pattern" is obtained, and this can be used for ornamental purposes.

The invention is illustrated but in no way limited by reference to the following examples:

EXAMPLE 1

A sheet of 5 mm thick polymethyl mathacrylate was printed by screen printing techniques using a negative designed to produce a pattern having 30 spots and a 50 percent coated area ratio, with a silver ink, containing 30.0 percent aluminium powder 5.0 percent pigment 5.0 percent polyethylene wax: and 60.0 percent organic solvent solution of polymethyl methacrylate and a very small amount of a foaming agent.

______________________________________ The sheet obtained had the following properties : Percentage transmission of visible ray: 56% Percentage transmission of infra-red ray: 33% Absorption coefficient: 21% ______________________________________

The percentage transmission of the infra-red ray was determined using an infra-red ray bulb with the printed surface of the sheet facing the bulb. Clear vision was obtained at a distance of more than 2 m away from the surface printed sheet.

EXAMPLE 2

The surface printed sheet produced in Example 1 was placed with the printed surface facing away from the light source, and the absorption coefficient of the infra-red ray was found to be 40 percent.

EXAMPLE 3

The process of Example 1 was repeated using a negative having 50 spots and a 35 percent coated area ratio. The printed surface of the finished sheet of polymethyl methacrylate was then printed with a transparent red lacquer, using the same negative in the same position as in Example 1. The sheet produced was placed with the printed surface facing the light source, and the percentage transmission of visible ray, infra-red ray and the absorption coefficient were observed as below.

______________________________________ Percentage transmission of visible ray: 74% Percentage transmission of infra-red ray: 46% Absorption coefficient: 21% ______________________________________

Clear vision is obtained at a distance of more than 1 m away from the surface printed sheet and a beautiful red metallic gloss is observed from the side of light source, while natural light is transmitted through the sheet.

Claims

We claim:

1. A transparent structure comprising a transparent sheet having a non-continuous coating of aluminum distributed over at least one surface of the sheet in a regular pattern, in the form of closely spaced parallel lines, there being from 10 to 50 lines of coating per inch, to provide a means of controlling the percentage transmission of visible light and infra-red radiation.

2. A transparent article as in claim 1 wherein the transparent sheet is a polymethyl methacrylate sheet.

3. A transparent article as in claim 1 wherein the transparent sheet is a glass sheet.

4. A transparent article as in claim 1 having a total luminous transmission of from 20 percent to 80 percent.

5. A transparent article as in claim 1 having a transmittance for infra-red rays of from 5 percent to 50 percent.

6. A window comprising a transparent article according to claim 1.

7. A window as in claim 6 mounted with the coated surface of the sheet facing outwardly from the building.

8. A transparent article as in claim 1 in which the areas coated with aluminum are overcoated with a transparent lacquer.

9. A transparent article as in claim 1 wherein the non-continuous aluminum coating comprises aluminum powder dispersed in a vehicle tightly adhering to the surface of said transparent sheet.

10. A transparent article as in claim 9 wherein the coating also contains a color component selected from the group consisting of pigments and dye.