Introduction to glass
The main characteristics of glass are transparency, heat resistance, pressure and breakage resistance and chemical resistance.
A 4mm thick pane of glass weighs 10kg/m2
The hardness of float glass is established according to Knoop. The basis is the test method given in DIN 52333 (ISO 9385).
800 - 1000 MPa
The compression strength defines the ability of a material to resist a load applied vertically to its surface
|Modulus of elasticity||
70 000 MPa
The modulus of elasticity is either determined from the elastic elongation of a thin bar, or from bending a bar with a round or rectangular cross section.
The bending strength of a material is a measure of its resistance during deflection. It is determined by bending tests on glass plate using the double ring method according to DIN EN 1288-5.
|Transformation range||520 - 550°C|
Contrary to solid bodies of crystalline structure, glass has no defined melting point. It continuously transforms from the solid state to the viscous plastic state. The transition range is called the transformation range and according to DIN 52324 (ISO 7884), it lies between 520°C and 550°C. Tempering and bending require a temperature of a further 100°C.
0.8 J/g/K The specific heat (in joules) defines the amount of heat required to raise the temperature of 1g of float glass by 1K. The specific heat of glass increases slightly the temperature is increased up to the transformation range.
|Thermal conductivity||0.8W/mK Thermal conductivity determines the amount of heat required to flow through the cross sectional area of the float glass sample in unit time at a temperature gradient.|
9.10-6 K-1 There is a difference in the expansion behaviour of a body under the effect of heat between linear expansion and volumetric expansion. With solid bodies, the volumetric expansion is three times that of linear expansion. The temperature coefficient of expansion for float glass is given according to DIN 52328 and ISO 7991.
Glass has several strong points concerning optical properties:
- It can be produced in large and homogeneous panes
- Its optical properties are not affected by ageing
- It is produced with perfectly flat and parallel surfaces
n = 1.52
If light from an optically less dense medium (air) meets an optically denser medium (glass), then the light ray is split at the surface interfaces. The measure of deflection determines the refractive index. For float glass, this refractive index is n=1.52.
|Chemical resistance against||
Water;= class 3 (DIN 52296)
Acid = class 1 (DIN 12116)
Alkaline = class 2 (DIN 52322 and ISO 695)
The surface of glass is affected if it is exposed for a long time to alkalis (and ammonia gases in damp air) in conjunction with high temperatures. Float glass will also react to compounds that contain hydrofluoric acid under normal conditions. These are used for treating glass surfaces.
|Wear tests||Abrasion tests (DIN 52347 and ISO 3537) The scattering of light the transmission of directed light striking the surface is evaluated.|
Light scatter increase for float glass is approx. 1% (after 1000 abrasion cycles). The permitted light scatter increase for vehicle safety glass (windshield) is 2% in Europe (ECE R43) and the USA (ANSI Z 26.1).
Sand trickling process (DIN 52348 et ISO 7991). For this diagonal impact abrasion test, 3kg of sand with a 0.5/0.71mm particle size are trickled onto the surface to be tested, which is inclined at 45 y from a height of 1600mm. Measurement of wear is the reduced luminous density (according to DIN 4646 part 2).
|Reduced luminous density for float glass is approx. 4cd/m2lux.|
Micro scratch hardness for float glass is approx. 0.12N (Mar resistance-test)..
The test was originally designed to determine the scratch hardness of plastics. A diamond point with 50 y cone angle and 15 mm point radius is drawn over the glass surface by applying different loads. The load at which a scratch is produced in the surface is a measurement of scratch hardness. This is not an accurate method; the influence of the tester must not be neglected.