Transformation area

This article describes the glass transition especially for inorganic glasses; for the general case see glass transition temperature

The transformation range is that temperature range in which the mechanical behavior of a certain glass changes significantly: With glass as an amorphous material, the melt does not occur at a certain well-defined temperature, rather the viscosity changes gradually, with four points on the temperature that are important from a glassblowing point of view - A distinction can be made on a scale that is determined by the decadic logarithm (log ₁₀ (η) or lg (η)) of the viscosity η (measured in 1 dPa · s = 1 poise = 1 P = 0.1 Pa · s).

Limits

• lower relaxation limit at lg (η) = 14.5:

This limit represents the highest service temperature of a glass component: Above this limit temperature, internal mechanical stresses are gradually reduced and the properties are permanently (but not irreversibly ) changed.

• upper cooling point at lg (η) = 13.0:

At this temperature, the desired relaxation is achieved as quickly as possible without the component changing in shape, even if it cools down slowly

• Softening point at lg (η) = 7.6:

Here the glass begins to flow noticeably and thus to deform under the influence of its own weight.

• Processing point at lg (η) = 4.0:

Most of the glassblowing processes for changing the shape can be carried out optimally at this temperature.

If these points are close together on the temperature scale, one speaks of a short glass , in the opposite case of a long glass .

Measurement of T _g using dilatometry . The linear areas above and below T _g are marked in green. T _g is the temperature at the intersection of the regression line drawn in red.

Measurement of T _g using DSC . T _g is the temperature at point A.

Transformation point

In contrast to viscosity, the thermal expansion of some glasses suddenly changes in a comparatively narrow temperature range, which is known as the transformation point T _g : above this temperature, the thermal expansion is significantly higher than below. This characteristic is essential for the production of tempered glass , for example .

The heat capacity also increases by leaps and bounds at the transformation point, which is usually used to measure the transformation point using DSC . The transformation temperature can also be determined dilatometrically .

The transformation point is the temperature at which the glass changes from the plastic range to the rigid state during cooling. The viscosity at this point depends on the sample preparation (especially the cooling curve), the heating or cooling rate during the measurement and the chemical composition. As a rule, the transformation point near the upper cooling point is at 10 ¹³ poise = 10 ¹² Pa · s. For dilatometric measurements, heating rates of 3 to 5 K / min are usual, for DSC measurements 10 K / min, whereby the heating rate for the measurement should correspond to the cooling rate for the sample preparation.

Glass transition in polymers

The glass transition in polymers can be found under glass transition temperature .

References

1. ↑ ^{a b} T _g measurement for glasses (in English).
2. ↑ ^{a b} O. V. Mazurin, Yu. V. Gankin: "Glass transition temperature: problems of measurements and analysis of the existing data"; Proceedings, International Congress on Glass, July 1-6, 2007, Strasbourg, France.