The composition of borosilicate glass 3.3 used in the manufacture of glass apparatus is approximately as follows:
- SiO2 80,6%
- B2O3 12,5%
- NaO 4,2%
- Al2O3 2,2%
- Trace elements 0,5%
The worldwide highly diversified application of this construction material in the chemical and pharmaceutical industries as well as in a great number of related areas relies especially on its chemical and thermal properties (also see DIN ISO 3585) and a number of additional advantages which distinguish borosilicate glass 3.3 from other construction materials. Among these advantages are in particular properties, such as:
- smooth, non-porous surface
- catalytic indifference
- physiological safety
- odour and taste neutrality
Borosilicate glass 3.3 is chemically resistant to almost all products. Its chemical resistance is thus more comprehensive than that of other well-known construction materials. For example, borosilicate glass 3.3 is resistant to water, salt solutions, organic substances, halogens like chlorine and bromine, and also many acids. However, at high temperatures, hydrofluoric acid, concentrated phosphoric acid, and strong bases cause noticeable erosion on the glass surface. At ambient temperatures, borosilicate glass 3.3 can nevertheless be exposed without difficulty to alkaline solutions in concentrations of up to 30%.
Classification of the construction material borosilicate glass 3.3 according to relevant analytical methods leads to the following result (see also ISO 3585):
|Hydrolytic resistance at 98°C:||Grieß hydrolytic resistance class ISO 719HGB 1|
|Hydrolytic resistance at 121°C:||Grieß hydrolytic resistance class ISO 720HGA 1|
|Acid resistance||Deposit of Na2O < 100 µg/dm2 according to ISO 1776|
|Alkali resistance||Alkali resistance class ISO 695A2|
Borosilicate glass 3.3 differs from other materials used in apparatus manufacturing not only by its almost universal corrosion resistance, but also by its very low coefficient of thermal expansion. No sophisticated measures are therefore required to compensate temperature-dependent thermal expansions.
The physical properties most essential to apparatus construction are listed below (see also DIN ISO 3585):
|Average linear thermal conductivity coefficient||α 20/300 = (3,3 + 0,1) ± 106 K1|
|Average thermal conductivity between 20 and 200°C||λ 20/200 = 1,2 W m1 K1|
|Average specific thermal capacity between 20 and 100°C||C p 20/100 = 0,8 kJ kg1 K1|
|Average specific thermal capacity between 20 and 200 °C||C p 20/200 = 0,9 kJ kg1 K1|
|Density at 20 °C||p = 2,23 kg dm3|
The permissible strength parameters of borosilicate glass 3.3 include a safety factor which accounts for experiences made in testing the stability of glass, in particular the circumstance that glass is a brittle construction material. In contrast to other commonly used construction materials glass is not capable of reducing maximum strains occurring at irregular transitions and minutest fissures, as is the case with a tough or tenacious material (e.g. metal). In addition, the safety factor accounts for the subsequent processing of the parts (ground flanges), their handling (signs of use on the surface), and permissible strains acting apart from pressure and temperature while the glass is in use.
Hence the general rules for design, manufacture and testing specified in EN 1595 apply to permissible exposures of glass components to tensile, bending, and compressive strengths, also accounting for the state of the surface expected under practical conditions:
Tensile and bending strength K/S = 7 N mm-2
Compression strength K/S = 100 N mm-2
Elasticity modulus E = 64 kN mm-2
Poisson's ratio v = 0,2
The UV transparency of borosilicate glass, which is of great significance in the event of photochemical reactions, lies in the medium range, somewhat above the value applying to common window-pane glass.
If light-sensitive substances are to be processed, it is recommended to use brown coated borosilicate glass. The permanent special coating reduces UV transparency to a minimum.