Mooney Viscometer

The Mooney viscosity method uses a Mooney Viscometer to measure the plasticity of rubber compounds. The testing principle is to measure the shear resistance produced by the sample to the rotation of the rotor under a certain temperature, time and pressure, and use the difference in torque to express the plasticity of the rubber material.

The test process is to first heat the die cavity to the test temperature, then put the sample into the cavity and preheat it for a certain period of time. The rotor starts to rotate, and the torque value is continuously collected and displayed on the display. The displayed value is the Mooney viscosity value of the rubber compound. The greater the Mooney viscosity, the higher the viscosity of the rubber material (The smaller the Mooney viscosity, the lower the plasticity of the rubber material). Mooney viscosity values ​vary due to different test conditions, so the test report must indicate the test conditions. Usually , Mooney viscosity is expressed as ML1+4 100℃. Among them, M means Mooney, L1+4, which means the torque value measured when the large rotor (diameter is 38.1mm) is preheated at 100℃ for 1 minute and rotated for 4 minutes. The measurement range of Mooney viscosity is 0 to 200.

Mooney viscosity reflects the viscosity of the rubber material under specific conditions and can be directly used as an indicator to measure the rheological properties of the rubber material. However, since the measurement speed of this method is slow and the shear rate is small, it can only reflect the rheological properties of the rubber material at low shear rate. If the test speed is increased, the results will be closer to the rheology in actual processing.

The most valuable property of raw rubber is high elasticity, but this property also brings some obstacles to subsequent processing. People need to mechanically process the raw rubber, heat treat it or add certain chemical additives to change it from an elastic state to a soft and easy-to-process plastic state. This is beneficial to the uniform mixing and dispersion of the ingredients, improves fluidity, facilitates molding, and makes it easier to control processing speed and quality. However, the plasticity of raw rubber is not higher, the better. Excessive plasticity will reduce the physical and mechanical properties of the material. Therefore, in the process of masticating raw rubber, we need a way to grasp whether the current degree of plasticity is within the range we want. Mooney viscometer was born specifically to solve this problem. Under certain conditions of temperature, time and pressure, the sample is subjected to continuous torsion in the die cavity. It can produce quantitative values as key indicators for evaluating rubber physical properties and processing performance, and can ensure the quality and stability of rubber products.

The vulcanization index (also called Mooney vulcanization speed) is often used as an indicator of the vulcanization rate of the rubber compound. A small value indicates a fast vulcanization speed; a large value indicates a slow vulcanization speed. It can be calculated using the following either formula:

When using the big rotor:

t35 – t5 =△30t

t35 – the time from the start of the test to the point where the viscosity of the compound drops to its lowest point and then rises by 35 values of Mooney viscosity.

t5 – the time from the start of the test to the point where the viscosity of the compound drops to its lowest point and then rises by 5 values of Mooney viscosity.

When using the small rotor:

t18 － t3 = △15t

t18 – the time from the start of the test to the point where the viscosity of the compound drops to its lowest point and then rises by 18 values of Mooney viscosity.

t3 – the time from the start of the test to the point where the viscosity of the compound drops to its lowest point and then rises by 3 values of Mooney viscosity.

The optimum cure time can be approximately calculated using the following formula:

Optimum cure time (large rotor) = t5 + 10 (t35 – t5)= t5 + 10 △30 t

Optimum cure time (small rotor) = t3 +10 (t18 – t3)= t3 + 10 △15 t

Stress relaxation means that after the material is fixedly deformed, the stress will gradually decrease over time. Stress relaxation is an important property of materials.

Mooney viscosity only reflects the fluidity of rubber after heating, but the speed at which it returns to its original state after the external force disappears cannot be known. Only through the stress relaxation curve can the size of the processed material be more accurately controlled. The Mooney stress relaxation test is that after the viscosity test is completed, the motor brakes within 0.1 seconds, and the twisted rubber in the die cavity will gradually relax. The torque changes are continuously measured to obtain the stress relaxation curve.

When doing the Mooney scorch test, a large rotor with a diameter of (38.10 ± 0.03) mm is generally used. When the sample is a high-viscosity rubber material, a small rotor with a diameter of (30.48 ± 0.03) mm is allowed. The Mooney scorch test temperature is generally 120±1°C. Its purpose is to simulate the temperature of the rubber material during processing and measure the early vulcanization characteristics of the rubber material at this processing temperature. If there are special needs, other tests can be used Experimental temperature.

Sample preparation

1) The sample is two pieces of circular discs, 45~55mm diameter, 6 ~ 8 mm thick, one of which has a ￠8~10 mm hole punched in the middle.​

2) The test surface should be free of impurities and pores.​

3) After processing, the sample should be stored at room temperature for 2 hours, but should not exceed 10 days.​

Operating steps

1) Power on and turn on the machine and computer respectively, and check whether the required tools and accessories are in place.​

2) Open the computer testing software and set relevant parameters, such as test temperature, formula selection, test time (preheating for one minute, testing for 4 minutes), etc.​

3) preheat the die cavity and rotor to the test temperature and allowing it to reach a stable state. When the Mooney viscometer rotates without load on the rotor, the Mooney value reading on the PC should be within the range of ± 0.5.​

4) Open the die cavity, pass the rotor shaft through the center hole of the holed sample, then insert the shaft into the bore of the lower die, finally accurately place the other sample on the rotor. (When testing low-viscosity rubber, you can place a high-temperature resistant film between the sample and the upper and lower dies to prevent the rubber from contaminating the die cavity.) Then quickly close the die cavity, and the system automatically starts timing from the moment the die cavity is closed. The general preheating time for the sample is 1 minute, but other preheating times can also be used as needed.​

5) After the sample reaches the preheating time, the system automatically starts the motor to rotate the rotor at a speed of (2.00 ± 0.02) r/m, and immediately automatically records the Mooney values during the test.

6) After 60 minutes of testing, the test can be stopped when the sample still does not appear scorched or its Mooney value does not rise from the minimum value to 18 or 35 Mooney values.​

7) After the test is completed, the system will open the die cavity automatically. You can take out the sample, and store or print the testing data.

Result expression

Mooney scorch time is t3 or t5 on the curve.

t3 – the time from the start of the test to the point where the viscosity of the compound drops to its lowest point and then rises by 3 values of Mooney viscosity in the case of a test with a small rotor.

t5 – the time from the start of the test to the point where the viscosity of the compound drops to its lowest point and then rises by 5 values of Mooney viscosity in the case of a test with a big rotor.