Properties of Glass
Most of today's glass production is float glass, with thicknesses usually ranging from 2 - 25 mm and a standard size of 3.21 x 6 m that is used for further processing.
The glass has the following physical properties.
The density of the material is determined by the proportion of mass to volume and is indicated using the notation "p". Float glass has a factor of p = 2,509 kg/m3. That means that the mass for a square meter of float glass with a thickness of 1 mm is 2.5 kg.
The elastic module is a material characteristic that describes the correlation between the tension and expansion when deforming a solid compound with linear elastic properties. It is designated with the formula symbol "E". The more a material resists deformation, the higher the value of the E-module. Float glass has a value of E = 7 x 10'” Pa and is defined in EN 572-'l.
Emissivity (e) measures the ability of a surface to reflect absorbed heat as radiation. A precisely defined "black compound" is used as the basis for this ratio. The emissivity of float glass is 89, which means 89 % of the absorbed heat is re-radiated.
As the term implies, this indicator demonstrates the resistance of a material to compressive stress. Glass is extremely resilient to pressure, as demonstrated by its 700 - 900 MPa. Flat glass can withstand a compressive load 10 times greater than the tensile load.
Tensile Bending Strength
The tensile bending strength of glass is not a speciﬁc material parameter, but rather an indicated value which, like all brittle materials, is inﬂuenced by the composition of the surface being subjected to tensile stress. Surface infractions reduce this indicated value, which is why the value of the flexural strength can only be defined using a statistically reliable value for the probability of fracture. This definition states that the fracture probability stress of 45 MPa for float glass maybe 5% on average, based on the likely hood of 95% as determined by statistical calculations methods.
Ꝺ = 45 MPa as measured with the double ring method in EN 1288-2.
Resistance to Temperature Change
The resistance of ﬂoat glass to temperature differences over the glass pane surface is 40 K (Kelvin).
This means that a temperature difference of up to 40 K over the glass pane has no effect. Greater differences can cause dangerous stress in the glass cross section, and this may result in glass breakage. Heating devices should therefore be kept at least 30 cm away from glazing. If this distance cannot be maintained, the insulation of single pane safety glass is recommended. The same applies in the cease of solid, permanent and partial shading of glazing, due, for example, to static building elements or to nearby plants.
The mechanical properties for ﬂoat glass vary within a deﬁned temperature range. This ranges between 520 - 550 °C and should not be compared with the pre-tempering and form shaping temperature, which is about 100 “C warmer.
The glass transition or softening temperature of ﬂoat glass is approx 600 °C.
Length Expansion Coefficient
This value indicates the minimum change in ﬂoat glass when the temperature is increased. This is extremely important for joining to other materials: 9 x 10‘ K" pursuant to ISO 7991 at 20 - 300 °C. This value gives the expansion of a glass edge of 1m when the temperature increases by 1 K
Speciﬁc Heat Capacity
This value determines the heat increase needed to heat 1 kg of ﬂoat glass by 1 K: C=800 J/ kg/K
Heat Transmittance Coefﬁcient (U value)
This value is calculated in accordance with DIN 4108-4 to EN 673. The value for ﬂoat glass with a thickness of 4 mm is 5.8 W/m²K.
Chart: Class 1 acc. to DIN 12116
Chart: Class 1-2 act. to ISO 695
Chart: Hydrolytic class 3-5 acc. to ISO 719
Fresh, Aggressive Alkaline Substances
These include substances washed out of cement which have not completely hardened and which when they come into contact with the glass, attack the silica acid structure that is part of the glass structure. This changes the surface as contact points become rougher. This effect occurs when the liquid alkaline substances dry and is completed after the cement has fully solidiﬁed. For this reason, alkaline leaching substances should never come into contact with glass and any points of contact should be removed immediately by rinsing them off with clean water.