Pyrex Glass

CHEMICAL COMPOSITION
The composition of borosilicate glass used for chemical plants has following approximate composion.
SiO2 - 80.6% B2 O2 - 12.5%
Na2O - 4.2% Al2O3 - 2.2%

RESISTANCE TO CHEMICAL
Borosilicate glass is inert to almost all materials except hydroflouric acid (HF) phosphoric acid((H3Po4) and hot strong caustic solutions. Of these Hydroflouric acid has the most serious effect, even when it is present in PPM (parts per million) in solutions. Where as phosphoric acid and caustic solutions cause no problems when cold but at elevatrd Temperature corrosion occurs. In case of caustic solutions upto 30% concentration can be handled safely at ambient temperature.
Under actual operating conditions, the effect of turbulence, and traces of other chemicals in the solution may increase of decrease the rate of attack. So it is not possible to give exact figures for corrosion by caustic solutions.

THERMAL PROPERTIES
Linear coefficient of thermal expansion
The coefficient of thermal expansion of borosilicate glass over the temperature 0 - 300C is 3.3 x 10-6C. This is very low when compared with other glasses and metals. That is why, borosilicate glass is often called low expansion borosilicate glass.
Specific heat
Specific heat between 25C and 300C is average to be 0.233Kcal/Kg, C
Thermal Conductivity
Thermal conductivity is 1.0 Kcal/hr, mC. Over the permissible operating temperature range.

ANNEALING
Annealing of glass is the process where the glass is heated and kept for a defined period of time to relive internal stresses. Careful cooling under controlled conditions is essential to ensure that no stresses are reintroduced by chilling/cooling.

RESHAPEING
In the given below are shown characteristic temperature at a determined viscosity, essential for glass reshap.

Lower cooling temperature 1024 poise 515C
Upper cooling temperature 1013 poise 565C
Softening point 107 poise 795C
Reshapeing point 104 poise 120C

MECHANICAL PROPERTIES
The lack of ductility of glass prevents the equalization of stresses at local irregularities or flaws and the breakage strength varies considerably about a mean value. This latter is found to occur at a tensile strength of about 700kg/cm2
In order to allow for the spread of breaking stress, a large factor of safely is applied when determining the wall thickness requirement to allow operation up to values given in the table of working pressure.

OPTICAL PROPERTIES
Borosilicate glass show no appreciable absorption in the visible region of spectrum and therefore appears clear and clour less.
In photo chemical processes the transparency of ultra violet is of particular importance. It follows from the transmittance of material in UV region that photo chemical reactions such as chlorination & sulpho chlorination can be performed in it.

WORKING TEMPERATURE
Borosilicate glass retains its mechanical strength and will deform only at temperature which approach its strain point. The practical upper limit for operating temperature is much lower and is controlled by the temperature differentials in the glass which depends on the relative Temperature defferentials in the glass which depends on the relative Temperature of the contens of the equipment and the external surroundings. Provided borosilicate glass is not subject to rapid change in temperature, creating undue thermal shock, it can be operated safety at temperatures up to 250 C.
It must be realised that in complete plants, composed not only of borosilicate glass, but also includ other materials such as PTFE the recommended max. Operating temperature is 200C. Operating temperatures may have to be modified so as to compensate for the effects of other factors such as pressure, thermal cycling, rapid heating cooling etc.