Liquid Handling

Materials: GlassMechanical ResistanceThermal stressesDuring the production and processing ofglass, hazardous thermal stresses may beintroduced. During the cooling of moltenglass, the transition from the plastic stateto the brittle state takes place in the rangebetween the upper and lower annealingpoints. At this stage, existing thermal stressmust be eliminated through a carefully controlledannealing process. Once the lowerannealing point is reached, the glass maybe cooled more rapidly, without introducingany major new stress.Glass responds in a similar way whenheated, e.g., through direct exposure toa Bunsen flame, to a temperature higherthan the lower annealing point. Uncontrolledcooling may result in the "freezing in" ofthermal stress which would considerablyreduce resistance to breakage and mechanicalstability.To eliminate inherent stress, glass must beheated up to a temperature between theupper and lower annealing point, be keptat this temperature for approx. 30 minutesand be cooled by observing the prescribedcooling rates.Resistance to temperature changesWhen glass is heated to a temperaturebelow the lower annealing point, thermalexpansion and the poor thermal conductivityresult in tensile and compressive stress.If, due to improper heating or cooling rates,the permissible mechanical strength isexceeded, breakage occurs. Apart fromthe coefficient of expansion α, which varieswith each kind of glass, the wall thickness,the geometry of the glass body, and any existingscratches must be taken into account.Therefore, it is difficult to state specific numericalvalues for thermal shock resistance.However, a comparison of the α valuesshows that Boro 3.3 is much more resistantto thermal changes than, e.g., AR-GLAS ® .Mechanical stressesFrom a technical viewpoint, glassesbehave in an ideally elastic way.This means that, after exceedingthe limits of elasticity, tensile andcompressive stress does not resultin plastic deformation, but breakageoccurs. The tensile strength isrelatively low and may be further diminishedby scratches or cracks. Forsafety reasons, the tensile strengthof Boro 3.3 in apparatus and plantdesign is calculated at 6 N/mm 2 .The compressive strength, however,is approximately ten times as high.Technical Information2018T maxfor glasses ofhigh shape-stability1614TransformationsrangeLower annealing pointUpper annealing point1210log η in dPas8642Working rangep.e. sintering, pressing,drawing, blowing,melting, castingSoftening point200 400 600 800 1000 1200Temperature in °CGlass viscosity curveTypical viscosity temperature curve using a borosilicate glass as an example.Upper annealing point(viscosity 10 13 dPas)Lower annealing point(viscosity 10 14.5 dPas)Linear expansion coefficientα 20/300 10 -6 K -1Densityg/cm 3Soda-lime glass (AR-GLAS ® ) 530 495 9,1 2,52Borosilicate glass 3.3 (Boro 3.3) 560 510 3,3 2,23www.brand.de333