GLASS - by Atri Sikdar

GLASS

SAFETY  GLASS & HEAT REFLECTIVE GLASS

S A F E T Y   G L A S S 

Normal soda – lime glass or annealed glass has the surface strength that provides the wind load performance and thermal stress resistance needed in most architectural applications. But it has poor resistance to hard, blunt objects or projectiles and when broken may fracture into large sharp pieces.

But a new invention, almost by accident, the Toughened Glass, was made by heating a glass pane and rapidly cooling it by blowing cool air onto both surfaces, the outer surfaces contract and are pulled into compression. The resulting pane is four times as strong as annealed glass and, if shattered, breaks into rounded non-sharp pieces.

Toughened Glass is very strong, as much as 4-5 times stronger than annealed glass, but has been known to fracture spontaneously due to nickel sulphide stones. This problem can be over come by heat soaking the glass after toughening. Toughened glasses are heat soaked under controlled process of accelerating the NiS inclusions to their low temperature phase. It is the genuine safe glass as chances of spontaneous breakage is almost nil.

 

Heat-Strengthened Glass, which is not as much as stronger than of Toughened glass but 2-3 times stronger than annealed glass. It has less rapid cooling cycles than toughened glass. Here chances of breakage are less. For those applications, where thermal breakage is a concern, heat strengthened glass should be specified. Heat strengthened glass because of its compressive stresses resists thermal breakage.

 

Laminated Glass is produced by using interlayer of PVB    (Polyvinyl butyral) which is permanently bonded together with two or more sheets of glass under heat and pressure. This bonding of materials provides a variety of performance benefits in architectural applications. The most important characteristic is the ability of the interlayer to support and hold the glass when broken.

 

Laminated Glass itself is available in different form, such as, laminated annealed glass, laminated heat strengthened glass and laminated fully tempered glass.  

 

For, Bullet resistant glass, strong and transparent poly carbonates and poly urethane are used with normal laminated process.

 

HEAT REFLECTIVE GLASS

Low-e coatings

Much of the architectural glass produced is now coated with low-e (for low emissivity) coatings to enable the production of more energy efficient windows. As with any advanced technology, there are several different production methods and the products have different properties.

The two basic methods of producing low-e coatings are sputtering and pyrolytic deposition:

Sputtering - Soft coat and off-line coating: Sputtering uses a vacuum chamber to put several layers of coating on the basic glass and the total thickness of the coatings is around ten thousand times thinner than a human hair. Sputtered coatings are referred to as 'soft coats' and must be protected from humidity and contact. The sputtered coatings are very soft but inside a sealed unit, they will easily last for the life of the unit.

These sputtered 'soft coat' products can have emissivities ranging from 0.05 to 0.1 compared to uncoated glass that has a typical emissivity of 0.89. This means that 'soft coat' products will reflect between 95 and 90% of the long-wavelength radiant energy from the surface where uncoated glass will only reflect 11% of the radiant energy received by the surface.

Pyrolytic Deposition - Hard coat and on-line coating: Pyrolytic coating deposits a metallic oxide directly onto the glass surface whilst it is still hot. The low-e coating is effectively 'baked-on' to the surface and the resulting low-e coating is very hard and durable.

The pyrolytic coatings are often referred to as 'hard coats'. Pyrolytic coatings can be up to 20 times thicker than sputtered coatings (they are still 500 times thinner than a human hair) and the baking process makes them much harder and resistant to wear.

Pyrolytic 'hard coats' have a low emissivity but this is higher than those achieved for soft coats. Hard coat products have emissivities ranging from 0.15 to 0.2.

 

 

Glass Type

( 6 mm )

Solar Absorptance %

Risk of Thermal Breakage

Clear

15 %

Low

Tinted

30 – 50 %

Medium

Pyrolytic coating on

Tinted float

40 – 50 %

Medium – High

Sputter reflective

Coating on clear

60 – 70 %

High

Sputter reflective

Coating on tinted

70 – 85 %

Very High