Читать книгу The Rheology Handbook - Thomas Mezger - Страница 167

The gelation time tgel is reached if ηgel = 1000 ⋅ ηmin with ηmin as the minimum viscosity. The reaction time (in min) is defined as: tr = tgel – tmin with tmin at the time point of the viscosity minimum. Measured is under isothermal conditions, e. g. using a PP-geometry (gap 0.5 mm) at a constant shear rate of 10 s-1.

Some further, but often very simple test methods to determine whatsoever gel times or gelation times are briefly presented in Chapters 11.2.1d/e/f (formation and tackiness of resin filaments, temperature increase due to an exothermic reaction), 11.2.8e (falling ball), 11.2.11a/b (rotation until standstill) and 11.2.12a4/5/6 (reduction of an oscillation amplitude path, reaching the phase shift angle of 45°, deflection path by the formograph); see also Chapter 12 (guideline, 12.2.2, 12.2.4, 12.3.3, 12.3.5), and the Index. In Chapter 11.2.13 information can be found about the following terms: incubation time, vulcanization time, scorch time, rise time, cure time of uncured rubbers, crosslinked rubbers and elastomers.

Note 1: Pot life , open time , and gelation time of resins

The following terms are often used to analyze the period of time for the use of a resin in an open container at the processing temperature. The reason for the increasing gel formation may be drying, oxidation or air humidity, or a curing reaction with two-component resins. Typical evaluation by a rotational test is a viscosity measurement at a low constant shear rate, e. g. at γ ̇ = 1 s-1. Possible criteria for analysis are:

1 The pot life is the period of time as long as a material is applicable [3.73]. In daily practice, many users are choosing the time point when the initial viscosity value has doubled.

2 The open time is the period of time for which the material is still able to flow. This time is over if a previously defined, upper limiting value of the viscosity is reached; this time point is often also called the gelation time.

Tip: It is more meaningful to evaluate here the viscoelastic properties using oscillatory tests (see Note 1 in Chapter 8.5.3b).