Glass Manufacturing )..Borosilicate glass )

Borosilicate glass is a high silicate glass with at least 5% boron oxide bow handle threaded rod with a triangular shaped handle.

Borosilicate glass is a particular type of glass, better known under the brand names Pyrex and Kimax. It was first developed by German glassmaker Otto Schott in the late 19th century and sold under the brand name "Duran" in 1893. After Corning Glass Works developed Pyrex in 1924, it became a synonym for borosilicate glass in the English-speaking world. Borosilicate glass is the oldest type of glass to have appreciable resistance to thermal impact and higher temperatures, also has excellent resistance to chemical attack. In this glass structure, the first to carry the Pyrex trademark, some of the SiO² is replaced by boric oxide. Borosilicate glass has a low coefficient of thermal expansion and is, thus, suited for telescope mirrors and other precision parts. Also, because this glass can withstand thermal shock, it is used for oven and laboratory ware, headlamp lenses, and boiler gage glasses. Most borosilicate glasses have better resistance to acids than do soda-lime glasses, but poor resistance to alkalis. Glass fibers used in reinforcing plastic compounds are a modified borosilicate glass. Chemical Composition SiO2 = 80.6% B2O3 = 13.0% Na2O = 4.0% Al2O3 = 2.3% Physical Properties Coefficient of expansion (20°C–300°C) 3.3 x 10-6 K-1 Density 2.23g/cm3 Refractive index (Sodium D line) 1.474 Dielectric constant (1MHz, 20°C) 4.6 Specific heat (20°C) 750J/kg°C Thermal conductivity (20°C) 1.14W/m°C Poisson’s Ratio (25°C – 400°C) 0.2 Young’s Modulus (25°C) 6400 kg/mm2 Optical Information Refractive index (Sodium D line) = 1.474 Visible light transmission, 2mm thick glass = 92% Visible light transmission, 5mm thick glass = 91% Critical Temperatures 150°C - When working above this temperature care should be taken to heat and cool Borosilicate glass in a slow and uniform manner. 500°C - The maximum temperature that Borosilicate glass should be subjected to and then only for short period of no longer than a few minutes. 510°C - Temperature at which thermal stress can be introduced to Borosilicate glassware. 565°C - Annealing temperature. When uniformly heated in controlled conditions, such as a kiln or oven thermal stress’s can be removed. 820°C - Softening point at which Borosilicate may deform. 1252°C - Working point, the temperature that glassblowers need to attain in order to work Borosilicate glass. Working Temperatures Borosilicate glass retains its mechanical strength and will deform only at temperatures which approach its strain point. The practical upper limit for operating temperatures is much lower and is controlled by the temperature ditterentials in the glass, which depend on the relative temperatures of the contents of the equipment and the external surroundings. Provided borosilicate glass is not subjected to rapid change in temperature, creating undue thermal shock, it can be operated safely at temperatures up to 450°F (232°C). The normal limiting factor is actually the gasket material. The degree of thermal shock (usually defined as sudden chilling) which it can withstand depends on many factors, for example: stresses due to operating conditions; stresses imposed in supporting the equipment; the wall thickness of the glass, etc. It is therefore undesirable to give an overall figure but, as a general guide, sudden temperature changes of up to about 216°F (120°C) can be accommodated . At sub-zero temperatures, the tensile strength of borosilicate glass tends to increase and equipment can be used with safety at cryogenic temperatures. Further development in glassmaking continues to create new glass-ceramics that outperform borosilicate glass in various ways. article source http://www.glassonweb.com