15 Bewertungen

Basics of thixotropy

Many products used in daily life can be characterized by their thixotropic behavior. Thixotropy is the property that explains why personal care products like hair gels and toothpaste are liquid when squeezed out of the tube but recover to their initial solid state afterwards in order to remain in place. The perfectly adjusted rheological properties of structural decomposition and regeneration as a function of time are responsible for the quality of a product. This article describes how thixotropy testing can be performed with a rotational viscometer/rheometer to control/influence the application behavior of materials.

Definition of thixotropy

Thixotropy is the property of certain fluids and gels of becoming thinner when a constant force is applied and after reduction of the force the viscosity recovers fully to the initial state in an appropriate period of timei-ii. The higher the force that is applied, the lower the viscosity becomes. Thixotropy is a time-dependent phenomenon, as the viscosity of the substance must recover after a certain period of time when the applied force is removediii.The term thixotropy consists of the Greek words "thixis" (to touch) and "trepein" (to turn). It means the change or transition of a substance due to mechanical loadiv. Examples of thixotropic materials are lotions, gels, ketchup, paints, and gypsum. For example, ketchup flows out of the tube when it is pressed. Its viscosity becomes lower as force is applied. After the force lessens, the viscosity of the ketchup recovers to its initial state for perfect leveling on French fries. This means that thixotropic behavior is always combined with shear-thinning flow behavior. Shear thinning, also called ‘pseudoplastic’ flow behavior, is characterized by a decrease in viscosity due to an increasing applied force (shear load). All in all, there are three different types of time-dependent flow behaviors:

Thixotropic behavior

In thixotropic materials the structural strength decreases with a higher load (in rheological terms: while shearing) and recovers completely after a certain rest period. The rest period needed for recovery strongly depends on the application and has to be defined prior to the test. Thixotropic behavior is an important quality characteristic of, for example, paints and coatings. It influences the way paint levels out and prevents sagging but also ensures a sufficient and consistent wet layer thickness. 

Non-thixotropic behavior

In non-thixotropic materials the structural strength decreases while shearing but the viscosity does not fully recover after an appropriate rest period. It remains thinner than the initial state which means that the structure does not fully recover (<100 %). A typical sample which shows this behavior is yogurt. After stirring, the viscosity of yogurt remains thinner than initially. Learn more about the rheology of dairy products here

Rheopectic behavior

In rheopectic materials the structural strength increases while shearing and recovers after a certain rest period. This phenomenon is rare but can be found in suspensions with a high solid content like latex dispersions or ceramic casting slips. 

Test methods for thixotropy testing

Thixotropy testing can be carried out with a viscometer or rheometer in rotation or oscillation. Rotational tests are described in the next chapter. There are diverse test methods available for analyzing thixotropic behavior. The focus of this article lies on the most common test methods. It has to be noted that each of the following test methods is performed with a different test procedure and, therefore, the outputs will differ from each other. Only thixotropic behavior tests conducted with the same method under the same conditions can be compared to each other.

Step test (3 intervals thixotropy test, 3ITT)

A step test is usually performed with a rotational rheometer by fast speed changes. The step test consists of three intervals and is therefore called “3 intervals thixotropy test (3ITT)”. It can be either performed in a controlled shear rate (CSR) mode or in a controlled shear stress (CSS) mode: In CSR mode the shear rate or rotational speed is preset, whereas in CSS mode the shear stress or torque is preset on the viscometerv.

The test is performed at two different speeds/shear rates. The first and last intervals are performed at a low shear rate and the second interval is performed at a high shear rate (Figure 1). In CSS mode the first and last intervals are performed at a low shear stress and the second interval is performed at a high shear stress.

Figure 1: Step procedure of a rotational test consisting of a low-shear, high-shear, and low-shear phase. y ̇ = shear rate; t = time

Figure 1: Step procedure of a rotational test consisting of a low-shear, high-shear, and low-shear phase. ẏ = shear rate; t = time

Time-dependent changes in viscosity during the 3ITT test represent the sample’s real behavior before, during, and after the application (see Figure 2):

  • Low-shear phase: The aim of the first interval is to obtain a constant viscosity at a constant low shear rate. This interval provides the reference viscosity of the sample at rest. 
  • High-shear phase: In this interval the sample is strongly sheared at a constant high shear rate to simulate the sample’s behavior during application, e.g during stirring, rolling, painting, spraying, and pumping. The structural decomposition can be determined due to the sample’s shear-thinning behavior, also known as pseudoplastic behavior.
  • Low-shear phase: Here, the same constant low shear rate is preset as in the first interval. This interval allows the sample to recover its structure/viscosity. The structural regeneration of the sample can be determined with one of the following analysis methods.
Figure 2: Time-dependent viscosity of a sample with thixotropic behavior. ƞ = viscosity, t = time

Figure 2: Time-dependent viscosity of a sample with thixotropic behavior. ƞ = viscosity, t = time

Analysis methods for the step test

The third interval of the 3ITT test is used for analyzing the thixotropic behavior of the sample. There are different methods for analyzing the structural regeneration: 

  • Recovery ratio after a given time: Prior to starting the test, the user has to define the point of time at which the structural recovery should be analyzed. The points of time have to be set according to the requirements of the application. The viscosities at these points are then compared to the viscosity of the rest phase in the first interval. For example, the structure of the paint recovered up to 80 % after 60 seconds of the third interval (Figure 3).
Figure 3: Analyzing the recovery ratio after a given time. ƞ = viscosity, t = time

Figure 3: Analyzing the recovery ratio after a given time. ƞ = viscosity, t = time

  • Time for a given recovery ratio: The time needed for structural recovery (100 %) is often very long. For example, after shaking paraffin oil, it needs about eight hours to fully recover to its initial solid state. For this reason, the time for a lower recovery ratio is usually analyzed. The recovery ratio of interest is set prior to the test. Then the time needed to recover to the set recovery ratio is calculated. The time is measured from the beginning of the third interval, the recovery interval. In Figure 4 the time needed for 25 % and 50 % structural recovery is analyzed.

Figure 4: Analyzing the time for a given recovery ratio. ƞ = viscosity, t = time

Figure 4: Analyzing the time for a given recovery ratio. ƞ = viscosity, t = time

Hysteresis area method

Another simple method for analyzing the time-dependent flow behavior is the hysteresis area. In older literature that is not up to date anymore, this behavior is called thixotropic or rheopectic, respectively. However, according to modern standards such as DIN spec 91143-2 and ISO/WD 3219-1 they are no longer valid in principle. The reason is: This measuring method evaluates the amount of structural breakdown (or build-up) under high shear conditions, but there is no interval available to evaluate structural recovery under really low shear conditions. In this test the sample is sheared at different speeds. The viscometer/rheometer is first set to a low speed. The speed is increased stepwise to higher speeds, generating an upwards ramp (e.g. 1 rpm to 100 rpm). After reading the shear stress at the top speed, the speed is either kept for a certain holding time (e.g. 60 seconds) and finally decreased stepwise to the lowest speed, generating a downwards ramp (e.g.100 rpm to 1 rpm) or the downwards ramp is generated immediately without a holding period. The result is plotted as a flow curve diagram showing the shear rate on the x-axis and the shear stress on the y-axis. Usually the shear rate is preset on the rheometer and the torque/force needed to rotate the bob in the cup filled with sample is measured. The area between the upwards- and downwards ramp is called the hysteresis area (Figure 5).

Figure 5: Flow curve showing the hysteresis area. 1 = indication for structural breakdown; 2 = indication for structural build-up, Ԏ = shear stress, y ̇ = shear rate

Figure 5: Flow curve showing the hysteresis area. 1 = indication for structural breakdown; 2 = indication for structural build-up, Ԏ = shear stress, ẏ = shear rate

The flow curve diagram shows how the shear stress changes with increasing shear rate/speed. A decrease in shear stress during the holding interval at a constantly high speed indicates that the viscosity of the sample decreases. If the upwards and downwards ramp do not differ from each other the sample’s behavior is independent of time when shearing. If the upwards ramp shows a higher shear stress reading than the downwards ramp the sample’s behavior is time-dependent under shear load, showing shear-thinning behavior then. If the upwards ramp shows a lower shear stress reading than the downwards ramp, then the sample shows time-dependent behavior when shearing, showing shear-thickening behavior. 

The amount of the hysteresis area is calculated as follows: 

Difference between 

  • Area between the upwards ramp and ẏ-axis 
  • Area between the downwards ramp and ẏ-axis 

If the value is positive the sample shows structural breakdown and if the value is negative the sample shows structural build-up on shearing.

For very simple quality control tests some users perform the following method in order to evaluate thixotropic behavior. To analyze the time needed for recovery of the viscosity after shearing, the viscometer has to be stopped after the downwards ramp. After a certain waiting period, the viscometer is started again at the lowest speed available in order to see the build-up of the viscosity (structural regeneration). Comparing the viscosity of the sample before and after turning the viscometer off and on illustrates how quickly the sample’s viscosity returns to its initial state after shearing. If the viscometer shows the same viscosity value as before, the viscosity has fully recovered in the waiting period.  

"Thixotropic Index"

Sometimes the term “Thixotropic Index (TI)” is used in different ways concerning measurement methods and analysis. 

  1. Some call TI the ratio between the viscosity of a sample at a low (ƞ A) and at a high (ƞ B) rotational speeds. For example, a material’s viscosity was measured at 5 rpm (ƞ A) and at 50 rpm (ƞ B). Afterwards ƞ A is divided by ƞ B. If the value of TI = 1 the sample shows Newtonian flow behavior, i.e. it remains unchanged. If TI > 1 the sample shows speed-dependent shear-thinning flow behavior and if TI < 1 the sample shows speed-dependent shear-thickening flow behavior. However, here the term “thixotropic index” is misleading since this ratio quantifies time-independent non-Newtonian (shear-thinning or shear-thickening) behavior and not thixotropy. To quantify thixotropy, time-dependent structural decomposition and regeneration have to be measured. TI is sometimes also called the “Shear Thinning Index”vi, which is the better term in fact.

  2. Others may call TI the ratio between the viscosity values at two different points of time obtained at a constant rotational speed. For example a material’s viscosity is measured after 30 s (ƞ A) and after 600 s (ƞ B) at 20 rpm. Afterwards ƞ A is divided by ƞ B. If TI = 1 the material shows time-independent flow behavior. If TI > 1 the material shows time-dependent shear-thinning behavior and if TI < 1 the material shows time-dependent shear-thickening behavior. Also here, the term “thixotropic index” is misleading since this ratio quantifies time-dependent structural decomposition of a material but not its structural regeneration. 


"Thixotropic breakdown coefficient"

The “thixotropic breakdown (Tb) coefficient” is a simple test for analyzing the time-dependent behavior of samples. It is especially used for quick quality control checks with entry-level rotational viscometers. In this test the sample is sheared at a constant speed (or shear rate) for a certain period of time. The change in viscosity over time indicates the sample’s time-dependent behavior. If the viscosity decreases, the sample shows time-dependent shear-thinning behavior and if the viscosity increases over time the sample features a time-dependent shear-thickening behaviorvii

For example, paint is measured while in rotation for 10 minutes by constantly maintaining 50 revolutions per minute (rpm). The viscosity of the sample has to be recorded at regular intervals (e.g. every 30 seconds). The viscometer reading (viscosity) is then plotted against time. Afterwards the Tb is quantified by a single number using equation 1viii

$$Tb= (\frac{St_1 - St_2}{ln (\frac{t_2}{t_1})}) ⋅ F$$

St1 = Viscometer reading at t1 minutes
St2 = Viscometer reading at t2 minutes
F = Factor for spindle/speed combination

Equation 1: Formula for calculating the "thixotropic breakdown coefficient"

Tb has the unit of viscosity (Pa•s or mPa•s, or in old literature P or cP). 

Also here, “Thixotropic breakdown coefficient” is not a very suitable name: According to modern standards this ratio does not describe thixotropic behavior since there is no structure recovery interval available afterwards. This method can be compared to those mentioned in chapter 2.4

Conclusion

Thixotropy tests give an insight into the sample’s time-dependent flow behavior and can thereby be employed for quality control of various products. According to modern standards such as DIN spec 91143-2 and ISO/WD 3219-1 thixotropy is characterized by decreasing viscosity over time when a shear rate is applied and full structural regeneration after the shear rate is set to a very low value. Only materials which fully recover their structure after shearing, like most ketchup samples, are called thixotropic materials and can be analyzed using the step test. Simple methods, such as analyzing the hysteresis area, “thixotropic index”, and the “thixotropic breakdown coefficient” are often used as a simple and quick quality control method. However, according to state-of-the-art standards they do not entirely evaluate thixotropic behavior. 

Learn how thixotropy testing with a rotational viscometer/rheometer can support you in the automotive paint application process.

References

i DIN spec 91143-2 Modern rheological test methods – Part 2: Thixotropy - Determination of the time-dependent structural change - Fundamentals and interlaboratory test (2012)

ii ISO 3219/WD 3219-1: General terms and definitions for rotational and oscillatory rheometry (2019)

iii Mewis, J; Wagner, N J (2009). "Thixotropy". Advances in Colloid and Interface Science. 147-148, 214-227

iv Mezger, T. (2014). The Rheology Handbook. 4rd revised ed. Hanover: Vincentz Network.

v Mezger, T.G.: Applied Rheology, 2018 (5th edition).

vi ASTM D2196-10: Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational (Brookfield type) Viscometer

vii Basu, S.; Shivhare, US.; Raghavan, GSV. (2007) Time Dependent Rheological Characteristics of Pineapple Jam. International Journal of Food Engineering 3, 3

viii Shapiro, I. (1946) The characteristic shear value: A coefficient of thixotropic breakdown. J. Am. Chem. Soc. 68 (10), 2122 – 2123