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In order to enable an undestroyed curing process, shear loading should be low, for example at the shear rate of γ ̇ = 1 s-1. Figure 3.47 presents a temperature-dependent viscosity function of a material showing gel formation, hardening or curing.

Minimum viscosity , gelation temperature or gel temperature, gel point or gelation point

Mostly, the viscosity curve shows a minimum value ηmin. This point is sometimes called the softening or melting temperature. Evaluating coatings such as paints or powder coatings, the following information my be important for practical users: At this point a wet coating layer may show optimum flow, spreading and leveling behavior. However, if the value of ηmin is too low, a wet layer may be too thin, and it may show sagging and edge creep, giving not enough edge protection finally. On the other hand, if ηmin is too high, a layer may not level out smoothly enough, and de-aeration may be not sufficient to obtain a surface without so-called pinholes, craters or air bubbles finally. All these effects may reduce the gloss of the surface after all.

Often, as a definition is taken: The gel temperature or gel point is reached if the viscosity has increased to a certain upper limiting value which was pre-defined by the user. The terms gelation temperature and gel temperature are often used with the same meaning.

Information given in Chapter 3.4.3 on the test conditions and on the possible differences arising when using the different test modes controlled shear rate (CSR) or controlled shear stress (CSS) also applies here. Among others, one of the advantages of oscillatory tests is the accurate determination of the sol/gel transition temperature after the onset of gel formation (see Chapter 8.6.3b).

Example 1: Testing epoxy resins

1 Temperature at the viscosity minimum: at T = +165 °C (e. g. showing ηmin = 10 Pas)

2 Gel temperature (when reaching the pre-defined value of η = 100 Pas): at T = +175 °C

Example 2: Gelation point when cooling mineral oils (acc. to ASTM D5133 and D7110)

Here, oils are cooled down in the range of T = -5 to -40 °C, at a constant cooling rate of

ΔT/Δt = 1 K/h, which corresponds to one degree Celsius per hour (according to D7110 with 3 K/h). The gelation point is defined as the temperature value at which the oil viscosity is reaching

η = 40,000 mPas = 40 Pas. Note: Here, a Brookfield viscosity is usually measured at the rotational speed of n = 0.3 min-1, and the shear rate is assumed to be γ ̇ = 0.2 s-1. In order to evaluate this relative viscosity values: see Chapter 10.6.2.

Example 3: Gelation temperature of reaction resins when reaching the thousendfold viscosity value

Gelation temperature Tgel is reached when ηgel = 1000 ⋅ ηmin with ηmin as minimum viscosity. Measuring via a temperature ramp with a gradient of ΔT/Δt = 2 K/min in the range of T = 50 °C until (Tgel + 10 K) , e. g. using a PP-geometry (gap 0.5 mm) at a constant shear rate of 10 s-1.

Note 1: Gelation index GI, and GI temperature (according to ASTM D341 and D7110)

When cooling oils, the gelation index GI is determined in temperature steps of ΔT = 1K (Kelvin; acc. to ASTM D5133 in the range of T = -5 to -50 °C) or of ΔT = 3 K (acc. to ASTM D7110 in the range of T = -5 to -40 °C) between two measuring points. Calculation of the GI after each step as

GI = (-1) ⋅ [(lg lg η1) – (lg lg η2) / (lg T1 – lg T2)], with η in mPas and T in K,

until the maximum of the GI function is exceeded finally. See also ASTM D341:

Empirical η(T) equation by MacCoull, Walther and Wright [3.72].

Presentation in a diagram as [lg lg y(lg x)] curve, the so-called gelation curve.

Analysis: The GI value is reached at the maximum of the GI curve, and the corresponding GI temperature (GIT) is determined. GI is therefore the value at the maximum viscosity increase.

Comment: Corresponding modern testing methods are oscillatory tests to determine the sol/gel transition temperature (see Chapter 8.6.3b).

Note 2: Cloud point , pour point , freezing point , dropping point of petrochemicals

In order to analyze cooling behavior of petrochemicals there are many, and often very simple, measuring and analysis methods. Samples include fuels such as kerosene, gasoline, diesel oils and heating oils, lubricants such as mineral oils and lubricating greases, paraffins and waxes.