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1) Time-dependent viscosity ratio or “thixotropy index” (using a single test interval only)

Some users evaluate thixotropic behavior by the following simple testing and analysis method: with a single test interval, presetting a constant medium or high rotational speed n = const (or shear rate). Afterwards the thixotropy index (TI) is calculated as follows

TI = η1(t1) / η2(t2)

with the time points t1 and t2 [s], (e. g., t1 = 30 s, and t2 = 600 s). For flow behavior independent of time TI = 1, for time-dependent shear-thinning TI > 1, and for time-dependent shear-thickening TI < 1.

Comment: Here, the term thixotropy index is misleading since this ratio quantifies time-dependent structural decomposition of a material only. Thixotropic behavior, however, can only be quantified if – directly after the break of a material’s superstructure – also the subsequent time-dependent structural recovery under low-shear condition is evaluated (to TI, see also Note 3 in Chapter 3.4.2.2a, and Note 2 in Chapter 3.3.2). Therefore, instead of TI, this ratio should better be called time-dependent viscosity ratio under a constant shear load, or similar.

2) Bingham build-up (BBU) and rate of build-up (RBU) after a 20-minute gelation test consisting of two test intervals: first high, then low shear load [3.7].

Some users perform the following simple test and analysis method consisting of two intervals. In the first part, preset is a constantly high rotational speed nH [min-1] for a period of t10 = 10 min, and in the second part a constantly low speed nL for another 10min = t20 (e. g. for ceramic suspensions, with nL = nH /10, for example, at nH = 100 min-1 and at nL = 10 min-1). Please be aware that these η-values are relative viscosity values if the test is performed using a spindle (which is a relative measuring system; see also Chapter 10.6.2). Here, instead of the shear stress often is used dial reading DR (which is the relative torque value Mrel in %), and the viscosity values are calculated then simply as η = DR/n (with the rotational speed n in min-1). Usually here, all units are ignored.

Bingham build-up (BBU) indicates the change of the relative viscosity values between the end of the second, low-shear interval and the first, high-shear interval.

Calculation: BBU = ηL (nL, t20) – ηH (nH, t10) = (DRL/nL) – (DRH/nH).

Example: with nH = 100, nL = 10, DRH = 50, DRL = 40,

then: BBU = (40/10) – (50/100) = 4 – 0.5 = 3.5

Rate of build-up (RBU) is the partial change over the first two minutes in the second, low-shear interval related to the total change over the full ten minutes in this interval.

Calculation: RBU = [η(nL, t12) – η(nH, t10)] / [η(nL, t20) – η(nH, t10)]

= [(DRL, 12/nL) – (DRH/nH)] / [(DRL/nL) – (DRH/nH)]

with time point t12 after 12 minutes (or after two minutes in the second interval, resp.)

Example: with nH, nL, DRH and DRL as above, and DRL, 12 = 30, then:

RBU = [(30/10) – (50/100)] / [(40/10) – (50/100)] = (3 – 0.5) / (4 – 0.5) = 2.5/3.5

Thus: RBU = 0.71 = 71 %

Comment: Using this simple method, usually a too high rotational speed is applied in the low-shear phase to enable regeneration conditions which are related to practice. At these conditions, which are not really simulating low-shear conditions or even the state-at-rest, any regeneration – if at all – is merely possible to a partial, very limited degree.