High-Temperature Rotational Viscometry: Automatizing Standards and Understanding Temperature Effects on Measurements

Viscosity is a key property influencing the processability and performance of materials such as bitumen, hot melt adhesives (HMAs), and waxes. These materials are highly temperature-sensitive, making precise viscosity measurements essential for quality control, formulation optimization, and regulatory compliance. Rotational viscometers equipped with advanced temperature control mechanisms provide reliable and repeatable viscosity measurements, ensuring these materials perform as required under real-world conditions. This article explores how commonly used industry standards, such as ASTM D4402 and D3236, can be optimized using automated measurement methods. Additionally, it highlights how small variations in handling high-temperature measurement systems can significantly impact test results, leading to erroneous viscosity readings if not properly controlled.

The significance of viscosity measurements at high temperatures

Understanding the viscosity behavior of materials at elevated temperatures is crucial across various industries. The viscosity of bitumen, for instance, directly affects its pumpability, mixability, and workability in road construction and roofing applications. Similarly, HMAs require controlled viscosity to ensure proper application, adhesion strength, and setting time in industries such as packaging, bookbinding, and automotive manufacturing. Likewise, polyethylene (PE) wax, widely used in coatings, adhesives, and lubricants, requires viscosity control to maintain consistent product quality and performance across different temperature conditions.
For these material groups, standardized methods such as ASTM D4402, D3236, and D1986 provide globally recognized procedures for obtaining reliable viscosity measurements. These standards were originally developed in the late 1980s, at a time when manual intervention was an essential part of viscosity testing due to technological limitations. Modern advancements in viscometry, however, now allow for significant automation, increasing the efficiency of these tests and reducing operator involvement. In particular, powerful Peltier temperature devices enable both active heating and cooling, allowing for quick transitions between different measuring temperatures without excessive waiting time for cooling phases.

Optimized measurement techniques for different material groups Bitumen testing: ASTM D4402

Bitumen is a highly viscous, thermoplastic material that transitions from a brittle state at low temperatures to a semi-solid state at room temperature and flows easily at elevated temperatures. This behavior makes it highly versatile for applications such as pavement construction, roofing, sealing, and insulation. However, its viscosity must be carefully measured and controlled to ensure consistent performance in each of these applications.
The system of choice for the tested sample was an R – torque model viscometer equipped with a small sample adapter spindle (here, the SC4-21) and a Peltier temperature device (PTD 175). The PTD 175 is capable of reaching a maximum temperature of 175 °C while providing the advantage of rapid active cooling back to the starting temperature, allowing for immediate repeat testing. Alternatively, for applications requiring higher temperatures, an electrical heating chamber (ETD 300) can be used, accommodating tests up to 300 °C. However, it should be noted that cooling in an electric heating system is a more time-consuming process, requiring external cooling plugs to expedite the process. This means that this variant is more suitable for measurements where measurements are mainly taken at a constant temperature.    
The viscometer includes a pre-programmed automated D4402 method. This method follows all the steps outlined in the standard, with the added advantage that the ideal speed for high torque (and thus high measurement accuracy) is automatically detected and used by the viscometer. It should be noted that if the samples exhibit non-Newtonian behavior, higher speeds will result in lower viscosity values (typical shear-thinning behavior). If all samples are to be measured at a specific constant speed, the method must be adjusted accordingly.
The viscometer allows you to run the test with measurements taken at up to three different temperatures in a single run-through, in this case, 135 °C, 149 °C, and 175 °C, as can be seen in Figure 2. Throughout the entire test procedure, no user intervention is required. The green line shows the temperature steps, and the blue line shows the measured viscosity. At each of the three temperatures, the viscometer has determined and used a speed that results in an optimal torque of approx. 90 % of the instrument (21.4 rpm, 25.1 rpm and 33.5 rpm respectively).
The final report shows the average viscosity based on three consecutive viscosity readings taken each minute, with deviations kept below 0.5 % for each of the three specified temperatures (Table 1). The PTD automatically returns to the initial temperature of 135 °C within approximately three minutes after completion of the test.

HMA testing: ASTM D3236

Hot melt adhesives (HMAs) are solid formulations like sticks, granules, or plates at room temperature. They can be processed by heating above their softening point, e.g., to +100 °C. HMAs are used to bond different kinds of materials like metals, plastics, glass, and wood. Typically, solidification takes only a few seconds to reach full bonding strength. For that reason, HMAs are in use in various industries like packaging, bookbinding, automotive, electronics, textile, and the wood industry.
HMAs typically consist of a base material (polymer) and various additives like tackifying resins and waxes. One of the most important properties of HMAs is their melt viscosity. It influences how the product can be applied with e.g., a hot glue gun or roller and the resulting wetting of the surface.
To reduce the viscosity of a final product, waxes are added to the formulation. The viscosity of HMAs is highly temperature-dependent. For that reason, the viscosity must be tested at the application temperature(s). ASTM D3236 describes a test method for quality control of HMA.
The system of choice was the R – torque model with the small sample adapter SC4-27 and the disposable cups with the ETD 300. The corresponding sample amount of solid HMA was put into the measuring cup. The weight can be calculated from the density of the sample and the required sample volume for the measuring system. For repeatable measurement results, it is important to always use the same sample amount. The cup was placed in the temperature chamber, and as soon as the sample started to melt, the spindle was immersed. In a following chapter it will be demonstrated why it is recommended to give the spindle time to preheat in the temperature chamber itself instead of preheating in an oven (or not preheated at all). A test at a temperature of 175 °C was carried out. The graph in Figure 3 shows the temperature equilibration phase (green line) and three measurement intervals (blue line). As the final viscosity result, the mean value of three repeated measurements is reported. Also, in this case, further measuring steps with different temperatures can be added to the method.
The test measurement procedure for ASTM D1986, which covers tests on polyethylene wax, is very similar to the previously described standards. Therefore, the same methodology can be applied.

Using temperature scans to determine temperature-dependent behavior

A very effective method for analyzing the sample’s viscosity at a broad temperature range is to perform a temperature scan. This allows the determination of viscosity during, e.g., different stages of processing. In the test performed, as seen in Figure 4, the sample was measured with a steady speed of 20 rpm, starting from 135 °C and ending at 175 °C, whereby 10 data points were collected for linearly distributed temperatures. For each temperature step, the viscometer automatically waits for the sample temperature to stabilize before starting the measurement (T-Ready). It is of great importance that sufficient temperature equilibration time be allowed, especially for samples with low thermal conductivity. Note: ensure that a constant speed or constant shear rate is selected for tests with non-Newtonian samples, otherwise there is a risk of distortion of the values due to shear thinning effects.

Best practices for high-temperature measurements

Regardless of the precision of a viscometer, proper handling techniques are essential, improving both repeatability and accuracy of the results and also potentially shortening measurement times, thus increasing productivity. As expected, one of the most important aspects of high-temperature measurements is the temperature accuracy of the sample. In addition to a precisely calibrated temperature chamber for temperature control accuracy, the measuring system used (measuring spindle and cup) also plays a major role. In addition to the sample, this must also be sufficiently well pre-tempered.
Figure 5 illustrates a measurement in which the cup, sample, and spindle are pre-tempered in the chamber (15 min at 200 °C). After a quick pre-shear step the sample has basically equilibrated and shows a constant viscosity after just 1 min of measuring time. On the other hand, figure 6 shows the same sample with the same preconditions, with the difference that the spindle was preheated in an external oven, then removed and immersed in the preheated sample in the chamber after 1 min (time of transport and attaching the spindle to the device). The temperature loss of the spindle during this time causes a huge difference in the measurement, as it takes about another 10 min for the viscosity of the sample to stabilize.

Selecting the right spindle for high-temperature measurements
Typically, there are two spindle designs used for high-temperature measurements, spindles with a hook/link and solid-shaft spindles. The hooked-version spindle has the advantage that it centralizes itself with the rotation (wobbling back and forth is prevented), which ultimately leads to higher measuring accuracy. In addition, due to the almost-decoupled design (the thin hook as a connection between the measuring body and the coupling), heat transfer via the spindle and thus away from the sample to the outside hardly takes place (better temperature stability in the sample). However, with highly viscous samples, the advantage of self-centering with rotation becomes a disadvantage. In these cases, the weight of the spindle body is not sufficient, and it practically floats in the sample; a wobbling movement can be seen. In these cases, it is advisable to use a spindle with a rigid/solid shaft, as the spindle body is pressed into the sample and is forced to stay in place.

Conclusion

High-temperature rotational viscometry is critical for accurately characterizing materials such as bitumen, HMAs, and waxes. By leveraging modern automation, optimizing test handling,
and selecting the appropriate spindle, industries can enhance quality control, improve test efficiency, and obtain reliable viscosity data for high-temperature applications.