GB2196624A - Method of making a toughened transparent glass - Google Patents

Abstract

A method of making toughened transparent glass comprises providing at least one surface of a soda- lime glass substrate with at least one glass-based coating, the said coating comprising powdered glass and a melting point lowering agent, heating the coated substrate to a first temperature sufficient to fuse the coating to the substrate, and subsequently heating the coated substrate rapidly to a higher second temperature followed by quenching to toughen the coated substrate. <IMAGE>

Description

SPECIFICATION Method of making a toughened transparent glass This invention relates to a method of making toughened transparent glass, and to a glass made by such method.

There are various transparent or translucent glass or glass-like products available today.

However, they all suffer from certain disadvantages as will be more particularly discussed hereafter. In particular, none combines the characteristics of high strength with high transparency at low cost and availabihty in a wide range of shapes, thicknesses and sizes.

It is therefore an object of the present invention to provide a glass which more nearly achieves these desired characteristics than the existing glasses.

Accordingly, the present invention provides a method of making toughened transparent glass, comprising providing at least one surface of a soda-lime glass substrate with at least one glassbased coating, the said coating comprising powdered glass and a melting point lowering agent, heating the coated substrate to a first termperature sufficient to fuse the coating to the substrate, and subsequently heating the coated substrate rapidly to a higher second temperature followed by quenching to toughen the coated substrate.

The coating(s) may be either clear or coloured or may have other light filtering properties.

The substrate may be either flat, curved or of any other shape capable of being toughened, whether of constant or varying thickness.

Coating(s) may be present on-either one or both sides of the substrate and may cover all or part of the substrate and different coatings may be applied to different parts of the same substrate or may be superimposed on each other, as desired.

Preferably, the coated substrate is allowed to cool slowly to below the first temperature before rapid heating to the second temperature.

The invention further comprises a glass article made by the method specified above.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a time/temperature diagram illustrating the toughening stage of a method according to the invention, and The method of making a toughened transparent glass will now be described.

The principle raw material used in the manufacturing process is soda-lime glass, usually in the form of flat sheets, but also in the form of tubes, rods or pressed or moulded parts.

The manufacturing process consists of the following stages: (1) Cutting and/or drilling, grinding, polishing of the glass substrates to the required size, shape and finish.

(2) Washing, i.e. thorough removal of dust, contamination grease etc.

(3) Application of one or more coating(s) to one or more surfaces of the substrate.

-The application of the coating(s) may be by printing, silk screening, stamping, spraying, dusting, by transfers rolling, stamping or brushing.

The coating is applied in the form of a mixture whose essential ingredient is powered or ground glass (either pre-coloured or clear), a colouring agent (as and where required-in compounds where pre-coloured glass powder is used in the colouring agent may not be necessary), and smelting point lowering agent (for example lead and/or an alkaline material).

The melting point lowering agent causes the glass powder to liquify or to semi-liquify at a temperature substantially below that where the glass substrate would soften to an unacceptable degree.

The coating usually (but not necessarily) also contains a binder and a solvent (for example water).

The coating is-normally applied in a liquid or semi-liquid (e.g. paint-like) form, but may also be applied dry.

(4) Drying (at ambient or increased temperature) (5) Firing (i.e. heating in an oven, furnace or kiln) at the temperature appropriate to the particular coating (usually between 540"C and 6000C-this varies depending primarily on the colouring agent used). During firing the glass particles fuse together and onto the substrate into a continous film. During this process the colouring agent (if any) dissolves or becomes suspended or finely distributed in the glass film. On completion on the manufacturing process the coating is securely bonded, and indeed is an integral part of, the glass substrate.The speed at which the required temperature is reached and the time for which the substrate is maintained at this temperature are not critical, however the substrate must be maintained at the firing temperature for a minimum of approximately 10 seconds.

(6) Slow cooling to under 450"C (e.g. in still air).

The intermediate cooling has been found to have a stabilising effect on the colour coating thereby rendering it substantially less prone to deterioration during the subsequent high (toughen ing) temperature heating stage than would be the case if the unfoired coating were to be exposed to the high temperature stage immediately. The substrate is removed from the oven for slow cooling.

(7) Rapid re-heating to the required toughening temperature (usually between 650"C and 700"C). The required toughening temperature must be reached in less than 2 to 4 minutes after insertion into the heating zone. The total time spent at or above the required toughening temperature should not exceed approximately 15 seconds. The correct time and temperature to be used varies with the chemical composition of the coating, the substrate thickness and the number and thickness of the coatings. The actual values may be established experimentally.

Adjustment and accurate control of temperature and timing are more critical than in a conventional toughening cycle.

Where, as is normally the case, only one side of the substrate is coated, most of the radiant heat is applied to the opposite side. This may be achieved for example, by unequal setting of the heating element controls for the respective oven walls. At the end of the reheat cycle the entire substrate must have acquired the required toughening temperature. The time spent at this temperature must be less than 15 seconds.

The unequal application of heat has the effect of reducing the coating's exposure to excess temperature (i.e. over and above the proper firing for the coating), as well as the length of time the coating is exposed to excess heat.

Even temperature distribution across the thickness of the substrate is achieved by conduction of heat through the substrate, and by convection of the air circulating within the oven.

As a further refinement the output of the elements or other heat sources facing the noncoated side of substrate may be reduced during the final stages of the re-heat stage. This is an added help in ensuring an even temperature distribution across the entire substrate by the time the material emerges from the heating zone. A typical example is shown in Fig. 1.

Firing at higher temperatures for extended periods (as would apply in a conventional glass toughening cycle) invariably leads to an unacceptable deterioration of the coating, manifesting itself in, among other things: A severe and generally unacceptable loss of transparency (i.e. the coating becomes wholly or partially translucent matt or opaque).

Serious lack of reporducability of the colour or shade or optical properties of the coating (i.e.

the colour etc. produced may differ significantly from that intgended and may vary unpredictably from one firing to the next).

For this reason it has not so far been possible in practice, nor has it been generally considered possible to produce ceramic colour coated toughened glass which is substantially transparent, (or, to do so commercially to a technically widely acceptable standard and optical tolerances, in adequate range of colours and shades, and at a widely acceptable cost).

(8) Quenching (i.e. rapid cooling) of the heated glass parts, e.g. by air blast or immersion in a coolant. This stage is substantially identical to the quenching stage in a conventional toughening cycle. The rapid cooling sets up a stress pattern in the glass (where the material near the centre is in tension). This stress pattern is an essential feature of toughened glass.

The manufacturing process may or may not include a pressing or forming operation, which, if required; would be conducted at a stage prior to the final quenching operation.

The above process is distinguished from known processes by the following: (1) The separation of (a) the firing stage where the coating is allowed fuse at a temperature and under conditions suitable for the particular coating from (b) the later stage, where the material is heated to a higher temperature specifically suited to the toughening process.

(2) The intermediate cooling between stages (a) and (b) above.

(3) The minimisation of the re-heat time during stage (b) above.

(4) The application of heat of unequal intensity during stage (b) above, to the coated and non-coated sides of the material respectively (where required).

(5) The variation of the intensity of the heat applied during the re-heat stage (b) above (if required).

(6) The adaption and control during stage (b) above of oven temperatures, timing and rate of heat application specifically to take account of different compositions of coatings, coatingthicknesses and substrate-thicknesses.

Existing transparent or translucent products have the following disadvantages: Through-coloured transparent glass (commercial quality) See below.

Non-ceramic (e.g. epoxy) coated toughened glass May be produced in transparent from but the coating has poor temperature resistance and limited resistance to abrasion, peeling and discoloration.

Ceramic-coated toughened glass So far it has only been possible toproduce this in a translucent or opaque form.

Transparent ceramin-coated non-toughened glass Does not have the superior physical properties of toughened glass.

Dichroic (metal-vapour coated) glass Cannot be toughened: limited availability; chromaticity difficult to control (varies with angle of incidence); expensive.

Perspex Poor heat resistance; limited resistance against hydro-carbon solvents etc. and abrasion.

Polycarbonate Poor heat resistance; limited resistance to hydro-carbon solvents etc. and abrasion; poor mechanical properties (strength/stiffness); expensive.

Purpose made through-coloured transparent glass (toughened ornon-toughened) Not readily available and then usually only in small sizes; extremely expensive.

The principal advantage of the product of the present invention is that it combines the desirable properties of toughened glass, i.e.

-high impact resistance -high (bending) strength -heat resistance -thermal shock resistance -protection against personal injury due to crumble type fracture characteristics -can be readily manufactured in a very wide range of thicknesses, sizes, shapes, finishes, etc.

-relatively low cost with the desirable properties of through-coloured transparent glass i.e.

-high transparency -high light transmission abrasion resistance -resistance against discolouration, fading, etc. but at the same time, eliminates the undesirable properties of through-coloured transparent glass, i.e.

-low impact resistance -low strength -poor heat and thermal shock resistance -splintering on fracture generally available in a limited number of thickness only (usually between 2.5 and 3.5 mm), and in one or very few sizes (usually around 1 mix 1 m) only thickness cannot be maintained within close tolerances available in certain colours only colours, shades and optical characteristics cannot be maintained with close tolerances -inclusions, air bubbles, lack of homogeneity --through-coloured glass cannot be successfully toughened -relatively high cost.

Note: All of the above comments may not necessarily apply to purpose-made coloured glass, but the cost of this is usually excessive.

Further advantages are: (a) The product can be manufactured economically in almost any quantity or batch size (from very small to very large).

(b) The product can be readily manufactured in an almost infinite variety of colours and shades.

(c) The product can be manufactured towithin close optical (i.e. light transmission and chromaticity) tolerances.

(d) The product can be manufactured economically in almost any length and width (subject primarily to the capacity of the available toughening plant).

(e) The product can be produced inalmost any thickness (subject to the inherent limitations of the toughening process and capacity of the toughening plant, and the availability of the required thickness of glass substrate material). Normally thickness ranging from 3 to 19mm can be readily produced.

(f) The product has physical properties superior to those of many other transparent or translucent products.

(g) The product can be manufactured at comparatively low cost.

A summary of the characteristics of glass made according to the present invention and the known glasses is set out in Table 1.

Typical applications where the product can offer distinct economic technical and safety advantages are in, or as part of: Lighting equipment, signalling equipment, beacons, etc. (e.g. as lenses, colour filters or cover glasses).

Windows, interior and exterior.

Doors, interior and exterior.

Advertising signs.

Domestic appliances.

Vending and gaming machines.

Protective eye glasses.

Welding shields or masks.

Inspection windows.

Ultra-violet and infra-red filters.

Radio frequency shielding.

Protective shields for visual display units (VDU's).

Laboratory or medical equipment, fittings and enclosures.

Decorative glass products, kitchen or table ware.

As a means of sealing and protecting underlying decorative artwork on glass.

As a means of building up the thickness of existing glass parts.

General artistic or decorative use, transparent pictures, trophies, table mats (coasters).

Furniture, shelving, room dividers.

Photographic workshops.

Transparency a a M - a a a a a Light Transmission a a M - a a a a a Accuracy of Chromaticity a a - M - a M a a Choice of Chromaticities a - a a a M M G (Colours and Shades) ss oozo o loW woxxl O O O :E: Z :E: X I 1 2 X Z :E: t &verbar; &verbar; X I Choice of Thickness X O O I I z I X X Choice of Lengths & Widths G M a a a - a M "v" Temperature Resistance I I 1 t7 1 0 Thermal Shock Resistance a - a a - - - - (Bending) strength x - x x x - M 14 eoao O O O to Fracture Characteristics a - a a a - M X 6 a. Acids M a G a - 14 a D o I I I o X O X O O X 14 X O X O 14 a Hydro-carbon Solvents x x M x x a - - a 4 \ 4 t Fo\ Z Z z O X O X O O X O Z S Z z I Z O Peeling t) a X 14 a a a x x x Economy a 14 a a a - 14 - % e 2 I Z I I t I I O O X O X X 4tP e O O o CN X O X,O O X O Z Z X O X O O c > , a v 7 e g u c c u 4J 3 o a s g v o â a a a a V n a a c X c a ~ a v ,c o X c o W o > O E w X Ç e Ç W 4 a ~ a c d a ~ O < z C ,C O d b 14 U O &commat; 01 :s ~ a s E fc x 4 c a a: c X a a a > < o a c o gn o b a o u o ,Y g u ~ U v ~ c v X v o s o D o > E ,c a ,c o 4 7 u o a W U C rH a - &commat; X ol c c o a m h ,C . 4 D O D D > O W O O p > g a a Z H C 4) < S nfl a :s v s u w E a v 14 u o o u u E t W < X c ,c D D c z H c < > u o o o o a E w < a o a u ,c u ,c =: E m s r u rf u u u w rH rf X U X

*) At increased tempertures ( 200 C) TABLE 1 X = Excellent; G = Good; M = Moderate; - = Poor.

Claims (8)

1. A method of making toughened transparent glass, comprising providing at least one surface of a soda-lime glass substrate with at least one glass-based coating, the said coating comprising powdered glass and a melting point lowering agent, heating the coated substrate to a first temperature sufficient to fuse the coating to the substrate, and subsequently heating the coated substrate rapidly to a higher second temperature followed by quenching to toughen the coated substrate.

2. A method as claimed in Claim 1, wherein the coated substrate is allowed to cool slowly to below the first temperature before the rapid heating to the second temperature.

3. A method as claimed in Claim 2, wherein the first temperature is between about 540"C and 600"C, and wherein the coated substrate is allowed to cool slowly to less than about 450"C before the rapid heating to the second temperature.

4. A method as claimed in Claim 3, wherein the second temperature is between about 650"C and 700"C.

5. A method as claimed in Claim 3 or 4, wherein the coated substrate is rapidly heated so as to reach the second temperature in less than about 4 minutes and is maintained at or above the said second temperature for less than about 15 seconds.

6. A method as claimed in any preceding Claim, wherein the substrate is coated only on one side, and the coated substrate is heated to the second temperature by heating in an oven with more radiant heat being directed at the uncoated side than at the coated side, at least for an initial part of the heating period.

7. A method as claimed in Claim 6, wherein the heat directed at the uncoated side is reduced as the substrate approaches the second temperature.

8. A method as claimed in Claim 1, substantially as described herein with reference to the accompanying Drawing.