3 Process‐controlling Properties
3.1 Viscosity
Among all the properties of a glass‐forming substance, the viscosity–temperature relationship is by far the most important practically in glass making. Referring to Chapter 4.1 for an in‐depth review of this topic, here we will consider it from a simpler technological point of view. The main feature then is that the viscosity of a glass‐forming liquid extends over a range of 12–14 orders of magnitude, which thus involves large rheology changes in the temperature range relevant to glass manufacturing (Table 2). Conventionally, technologists do not report viscosities in terms of values found at given temperatures, but rather in terms of temperatures at which given viscosity values are obtained. Thus, T(n.n), the isokom temperature, denotes the temperature in °C at which the melt assumes a value log η = n.n in poise CGS units (1 P = 0.1 Pa·s so that all values compiled in Table 1 have to be decreased by 1 if one retains instead the Pa·s SI unit used in many recent publications). The importance of reporting temperatures instead of viscosities rests on the facts that the (Newtonian) rheological response of all substances is by definition the same at the same viscosity and that only the temperature of a glass – not its viscosity – can be operationally regulated during the various forming, tempering, annealing, or cooling steps of glass making.
Table 2 Viscosity ranges of industrially manufactured glasses.
| Viscosity as log η,η in dPa·s | Process range, technological meaninga |
|---|---|
| Melting | |
| 2.0 | Typical of a soda‐lime silicate glass melt at 1450 °C |
| 3.0 | Transfer to forming area Volume relaxation time is <1 s Fixpoint T(3.0) = gob temperature Bushing tip temperature for fiber productionb |
| Forming | |
| 4.0. | Upper limit of mechanical working range Fixpoint TWP = T(4.0) = working point |
| 6.0 | Lower limit of mechanical working range The difference T(4.0) – T(6.0) is termed the “length” of a glass |
| Tempering, annealing, and cooling | |
| 7.6 | Upper limit of macroscopic shape stability Fixpoint TL = T(7.6) = Littleton softening point |
| 11.0c | Dilatometric softening point TD Above TD, temperature gradients in a glass object no longer cause thermal stresses A related temperature level is Td = T(11.5) = deformation point Glass objects deform under own weight at rates of a few μm/h |
| 13.0 | Technical definition of the glass transition Fixpoint Tg = T(13.0) = annealing point Volume strain relaxation time is 60 s |
| 14.5 | Technical definition of ultimate transition to a rigid state Fixpoint T(14.5) = strain point Volume strain relaxation time is 30 min |
a In the earlier days of mechanical forming, empirical indicators were in use. They remain worth to be consulted as empirical guidelines when the complex feature of “workability” is to be kept constant under a compositional change; these indicators read: gob temperature, GT = 2.63·(TL – Tg) + TL; working range index WRI = TL – Tg; RMS = relative machine speed = (TL – 450)/(WRI + 80); DI = devitrification index = WRI – 160.
b Some stonewool processes use T(1.5) as fibrization temperature.
c Approximation, the exact value depending on the load applied by the dilatometer.
Viscosity–temperature relationships do vary much with chemical composition: as illustrated in Figure 3 by the data for a soda‐lime float glass, a stonewool and silica glass, and those for a bulk metallic glass and pure Ag and Fe liquids. The fixed points of Table 1 are pinpointed for each glass melt and read in this graph, as just explained, starting from its Y axis. The complete viscosity curves can be reproduced by Vogel–Fulcher–Tamman (VFT) equations,