Instruments and principles
The rotational viscometer plays a central role in measuring the viscosity of liquid silicone. This instrument estimates viscosity by measuring the torque generated by the rotor rotating in the measured liquid. Liquid silicone is a non-Newtonian fluid; its viscosity changes with the shear rate, unlike pure water, which is not constant. So when you adjust the machine, you need to pay attention to the combination of rotor shape and speed. I've seen people directly use conventional rotors to measure high-viscosity silicone, and the values fluctuate dramatically. The standard states that when measuring such materials, the shear rate should ideally cover a reciprocal first square range of 0.1 to 100 seconds, which can capture the true nature of rheological behavior. Inside the instrument, there is a spring system. When torque is transmitted to the rotor, the deformation is recorded by the sensor and then converted into viscosity values—this conversion depends on Newton's fluid model, but for silicone, only apparent viscosity is obtained. In actual tests, the thixotropy of liquid silicone also causes readings to decline over time, especially at high speeds. When you operate, keep the temperature at 23°C ±0.5°C, because a 1-degree temperature fluctuation can cause viscosity changes of more than 5%.
Standards and Steps
Referring to domestic and international standards such as ISO 3219 and ASTM D2196, they provide detailed guidance on rotating viscometer measurement of non-Newtonian fluids. In terms of steps, you first need to calibrate the instrument and verify accuracy with standard silicone oil, keeping the error within ±2%. Next, take a sample of liquid silicone gel, making sure it is uniform and free of bubbles—bubbles can reduce torque readings and cause low viscosity. I tried pouring the sample directly, but when stirring, air got mixed in, and the data was so messy it was hard to read. After pouring into the measuring cup, let it sit for 1 minute to relax internal stress before starting the rotor. Speed settings start at low speeds, such as 10 revolutions per minute, gradually increasing to 200 rpm, recording torque at each point and calculating viscosity. The standard requires three sampling cycles and an average value; if the deviation exceeds 5%, the test must be retested. A real-life case: Once, after testing a batch of silica gel, viscosity dropped by 30% with shearing time. Later, it was found that the crosslinking agent in the formula was not mixed evenly, and only after remixing did the data stabilize. This phenomenon is also referred to in the literature as shear thinning, and is a typical characteristic of silica gel, so you must specify the shear conditions in your report.
Case studies and data
For example, in a laboratory, when measuring liquid silicone vulcanized at room temperature, the viscosity range is expected to be between 10 and 50 pa secs. A rotary viscometer is used with a standard drum, and the shear rate is set to the square of every 10 seconds. The initial reading showed 35 pa seconds, but after 10 minutes of operation, it dropped to 28 pa seconds, a 20% decrease. In another batch, viscosity dropped from 42 pa seconds to 31 pa sec, a decrease of about 26%. When analyzing, besides recording the maximum and minimum values, it's best to draw a viscosity-time curve, which visually reflects thixotropic recovery. Literature mentions that some silica gels partially recover to 80% of their original viscosity after 5 minutes of standing, which is related to molecular chain entanglement. However, there is another detail often overlooked in measurement: liquid silicone may contaminate the drum surface, causing changes in the friction coefficient. I recommend cleaning the rotor with acetone after each test and then air-drying; otherwise, the next reading will be higher. When organizing data, the median of three measurements represents the final value, because the mean may be biased by outliers—once, I didn't remove a single jump point, and the report was sent back for revision. Note that the case is just for reference; different silicone formulas vary greatly, so you can't use one template to cover all situations.
Influencing factors
Factors affecting silicone viscosity measurement with a rotational viscometer include temperature, shear history, and sample storage time. For every 1-degree increase in temperature, the viscosity of most silicone gels decreases by 2% to 3%, so constant temperature control is essential. For shear history, if the sample has been stirred before, its viscosity may decrease, so it needs to be left to rest long enough for recovery. Silicone stored for a long time tends to partially cure and viscosity increases. I've seen samples left open for a month 15% higher than new. Additionally, rotor shape is important. Same-rotor is suitable for low viscosity, wide rotors for high-viscosity liquids, but liquid silicone is usually chosen for same-rotor because it is easier to fill. Impurity issues: During sampling and transfer, tiny particles may be introduced that interfere with torque sensors and cause oversized readings. One processing technique is sieve, filtering with a 300-mesh filter before measurement, which improves repeatability. Also, wear of instrument bearings can cause zero offset. You should perform a zero check once a month to ensure the idle reading is zero—a small deviation multiplied by high viscosity magnifies the error.
Techniques and inspiration
A small practical tip: before measurement, wet the rotor surface with a small amount of silicone to reduce creep caused by surface tension. Some liquid silicone can climb up along the rotor shaft and interfere with readings. In such cases, you can apply a thin layer of silicone oil to the shaft but avoid touching the measurement area. Wall slip has been discussed in the literature, which is more noticeable at low shear rates. It's best to apply sandpaper to the inner wall of the drum to increase roughness, but this will change the shape of the measurement chamber and requires careful calibration. Just to say randomly, don't expect a one-time success when recording data; errors mostly come from instruments or operations, so testing several batches to find patterns is more practical than fixating on a single point. I once encountered a case where two people tested the same bottle of silicone with a 10% difference in operation, but later found that the uneven temperature was caused by the rotor not being preheated. So keeping experiments standardized and being cautious can actually reveal details and problems. If you develop a new product, samples with different scents may also have viscosity, but since I haven't seen the standards mention this, you can only rely on experience.
Common misconceptions
One common misconception is that the viscosity of liquid silicone is a single value. In fact, you need to specify the shear rate and temperature, otherwise the data is meaningless. Some people write reports based on the "viscosity" shown by the instrument, but the results are completely incorrect in the process—because the shear rate is high during infusion and low at rest, resulting in viscosity differences of several times. The second misconception is ignoring the instrument's range. The rotational viscometer has a torque limit; if the rotor is stuck or the speed is too low, the reading will be saturated or insufficient. When selecting a rotor, first estimate the viscosity range, then adjust using the test run method. I have also seen insufficient sample usage, causing the measuring cup not to fill the rotor, resulting in a low reading. The formula can be used to convert viscosity η into the relationship between torque M and speed n, such as η = M / (k * n), where k is the rotor constant. When you enter the k value in the software, you need to verify; factory settings may differ by a fraction of a fraction from the actual setting. Also, if liquid silicone contains solvents, volatilization will change its concentration. You need to measure within half an hour—don't dawdle. Just a quick reminder: hand sweat or hand cream on the device can cause errors, so wearing gloves is not unnecessary. These misconceptions are quite common, but speaking them out helps avoid pitfalls.