Method for the pre-treatment of coated glasses before heat treat

Details Method for the pre-treatment of coated glasses before heat treat Method for the pre-treatment of coated glasses before heat treat

1995-05-15

1997-05-06

Beck, Shrive

Coating processes

Nonuniform coating

Deforming the base or coating or removing a portion of the...

427287, 4273835, 65 605, 65 61, B05D 512, C03C 1723, C03C 1900

Patent

active

056269110

DESCRIPTION:

BRIEF SUMMARY
CROSS REFERENCES TO RELATED APPLICATIONS

This application is related to PCT Application No. PCT/FI93/00255, filed Jun. 11th, 1993, which claims priority from Finnish Application No. 922754, filed Jun. 15, 1992.


Background of the Invention

1. Field of the Invention
This invention relates to a method for the pretreatment of coated glasses before heat treatment. In the heat treatment the glass plate is heated and then cooled preferably by air jets.
2. Description of the Prior Art
As is well-known, the resistance of glasses to various strains can be increased by thoroughly heating the glass up to a temperature ranging from 580.degree. C. to 700.degree. C. and then cooling it rapidly by air jets directed to its surfaces. The temperature of the upper and lower surfaces of the glass plate drops more rapidly than that of the interior of the glass plate. As a result, the surfaces of the glass plate solidify earlier than the interior of the glass. When the surface temperatures decrease, the surfaces tend to shrink. The interior of the glass plate prevents the shrinking of the solidified surface portions. As the interior of the glass has not solidified, it tends to stretch. As a result, there is a compression stress state on the surface and a tensile stress state in the interior.
The strength properties of glass plates depend on the differences between the surface and interior stresses. With sufficient stress differences, the glass will be provided with safety glass properties. A glass plate heat-treated in this way is known as hardened safety glass. Hardened safety glass has a bending strength about five times higher than that of glasses which have not been heat-treated.
It is typical of hardened safely glasses that they have a bending strength five times higher, a resistance to rapid temperature changes ten times higher, and an impact strength significantly higher than glasses not treated by heat. If hardened safety glass is broken, small roundish pea-size lightweight pieces are formed which are not injurious to people.
A further measurable property of hardened safety glasses is the number of pieces formed at breakage within an area of 50 mm.sup.2 .times.50 mm.sup.2 and the shape and size of the pieces. An allowable number of pieces within such an area is between 50 and 400. In a broken plate of hardened glass, none of the pieces must be sharp or have the shape of a dagger blade.
The glass hardening method is widely known. Hardening of a coated glass plate is described in U.S. Pat. No. 4,857,094.
When stress differences between the surfaces and the interior of the glass plate are smaller than in hardened safety glasses, the glass does not fulfil the requirements made of safety glass. However, the bending strength of such glass is about two times greater than that of glass not treated by heat, and the product is called heat-toughened glass. Similarly as with ordinary glass, large sharp-edged pieces injurious to people are formed at breakage. The production and properties of heat-toughened glass are described in greater detail in U.S. Pat. No. 4,759,788.
When a glass plate is heated to about 700.degree. C., it can be shaped in various known ways. After the glass has been shaped as desired, it is allowed to cool slowly so that no temperature differences causing stress differences are formed between the surfaces and the interior. The glass has strength properties similar the interior. The glass has strength properties similar to those of ordinary glasses. If required, hardened safety glasses and heat-toughened glasses can be normalized by repeating the heat treatment without the cooling air jets. Normalized glasses have strength, temperature resistance and impact strength properties similar to those of untreated glasses.
Hardened and heat-toughened glasses are used when it is assumed that ordinary glasses would not resist the strains to which they will be exposed. Hardened safety glasses are used when the breakage of such glasses would risk the safety of people.
Conditions at the point of installation and the archi

REFERENCES:
patent: 3843472 (1974-10-01), Toussaint
patent: 4240816 (1980-12-01), McMaster et al.
patent: 4300934 (1981-11-01), DeTorre

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