Introduction Borosilicate Glas
A glass is an inorganic non metallic material that does not have a crystalline structure. Such materials are said to be amorphous and are virtually solid liquids cooled at such a rate that crystals have not been able to form. Typical glasses range from the soda-lime silicate glass for soda bottles to the extremely high purity silica glass for optical fibers. Glass is widely used for windows, bottles, glasses for drinking, transfer piping and recepticles for highly corrosive liquids, optical glasses, windows for nuclear applications etc. etc. In history most products have been blown glass. In recent times most flat glass has been produced using the float process. Mass produced bottles and decorative products are made using industrial scale blown glass process. Hand blown glass items are made in art/craft centres throughout the UK.
The main constituent of glass is silicon dioxide (SiO 2). The most common form of silica used in glassmaking has always been sand.
Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700o C. Adding other chemicals to sand can considerably reduce the temperature of the fusion. The addition of sodium carbonate ( Na 2 CO 3), known as soda ash,in a quantity to produce a fused mixture of 75% Silica (SiO 2) and 25% of sodium oxide (Na 2O), will reduce the temperature of fusion to about 800o C. However, a glass of this composition is water soluble and is known as water glass. In order to give the glass stability, other chemicals like Calcium Oxide (CaO) and magnesium oxide (MgO) are needed. The raw materials used for introducing CaO and MgO are their carbonates, limestone (CaCO 3) and dolomite (MgCO3), which when subjected to high temperatures give off carbon dioxide leaving the oxides in the glass.
Borosilicate glass is produced using 70% - 80% Silica (SiO 2) and 7% - 13% Boric oxide (B2O3 ) with small amounts of the alkali Sodium Oxide (soda) (Na 2O) and Aluminum Oxide (AI2O3). Glassware is often used in laboratories where repeated exposure to water vapour at high temperatures can leach out alkali ions. Borosilicate glass has a relatively low alkali content and with a resultant high resistance to attack by water. Borosilicate glass has exceptional resistance to thermal shock because it has a low coefficient of expansion (3.3 x 10 -6 K-1) and a high softening point. The maximum recommended working temperature (short time) for Borosilicate glass is 500oC Borosilicate glass has good optical properties with the ability to transmit light through the visible range of the spectrum and in the near ultra-violet range. It is therefore widely used in the field of photochemistry. Because of its thermal and optical properties it is widely used for high intensity lighting applications. This glass is used in the manufacture of glass fibres for used in plastic and textile reinforcement-- see below In the home Borosilicate glass is familiar in the form of oven-ware and other heat-resisting domestic receptacles e.g Pyrex. These items are generally used at temperatures up to 250oC Borosilicate glass has a very high resistance to attack from water, acids, salt solutions, halogens and organic solvents. It also has a moderate resistance to alkaline solutions. Only hydrofluoric acid, hot concentrated phosphoric acid and strong alkaline solutions cause appreciable corrosion of the glass. This glass is therefore widely used in chemical plants and for laboratory apparatus.
General Properties Of Glass
Glass has great inherent strength. It is weakened only by surface imperfections, which give everyday glass its fragile reputation. Special surface treatment can minimize the effect of surface flaws. The practical tensile strength of glass is about 27MPa to 62 MPa. However, glass can withstand extremely high compressive stresses . Therefore, most glass breakage is due to tensile strength failure. The reason that glass is weak in tensile strength is that it is normally covered in microscopic cracks which generate local stress concentrations. Glass does not possess mechanisms for reducing the resulting high localised stresses and so it is subject to rapid brittle fracture. There are two methods of reducing /eliminating this problem : Treating the glass thermally or chemically such that the outer surfaces are compressively stressed at relatively high levels, the middle region between the surfaces being under tensile stress. The cracks are therefore "held closed by the continuous residual stress...This is tempered/ toughened glass. The strength of the glass can be improved by a factor of up to 10 using this method. Ensuring that the glass surfaces have no cracks and ensuring that the glass in use is not in mechanical contact with anything which could scratch the surface. Glass produced with no surface flaws have strength values approaching the theoretical tensile strength values of 6,5 GPa. These have been produced using very fine fibres of glass. Hardness Borosilicate glass is about 2,3 x the hardness of plate glass. On the Moh's scale plate glass has a hardness value of about 5,7. Glass is harder than most grades of unhardened steel. Elasticity Gives under stress - up to a breaking point - but rebounds exactly to its original shape. Glass has virtually zero ductility. Youngs Modulus for fused Quartz glass is about 72 GPa
Affected by few chemicals. Resists most industrial and food acids.
Thermal Shock Resistance
Normal glass has low heat shock resistance but borosilicate glass has very good heat shock resistance, and withstands intense heat or cold as well as sudden temperature changes.
Retains heat, rather than conducts it. Absorbs heat better than metal.
- Reflects light
- Bends light
- Transmits light very efficiently
- Absorbs light with great accuracy.
- Electrical Insulation
- Strongly resists electric current. Stores electricity very efficiently.
Kennzeichnend für Borosilikatgläser sind der hohe Anteil von Kieselsäure (SiO 2 ) und Borsäure als Glasbildner. In der Regel liegt der SiO 2 -Gehalt zwischen 70 - 80 %. Solche Gläser mit einem B 2 O 3 -Gehalt bis maximal 13 % besitzen eine hohe Beständigkeit gegen chemische Einwirkungen und Temperaturunterschiede. Sie finden deshalb vorwiegend Verwendung in der chemischen Industrie für Produktionsanlagen, in der Pharmaindustrie als Packmittel von Arzneimitteln sowie als belastbare Glühlampenträger.
Die Höhe des Borsäuregehaltes beeinflußt die Glaseigenschaften dahingehend, daß neben den als chemisch nahezu inert bekannten Arten auch solche Verwendung finden, die infolge andersartigen strukturellen Einbaus der Borsäure nur noch geringe chemische Resistenz zeigen. Hier sind B 2 O 3 -Gehalte von über 15 % charakteristisch. Die Borosilikatgläser werden demzufolge weiter in erdalkalifrei, erdalkalihaltig und hochborsäurehaltig unterteilt.