Three-point frequency scanning method for testing rubber

In tire processing and production, Mooney viscometer is usually used as a quick test method to detect the processability of raw rubber and mixed rubber. However, is it comprehensive to use a Mooney viscometer with a low shear rate (rotor speed 2 rpm) to determine the processing characteristics of the rubber compound during the entire rubber processing process? Let's take a look at the following examples (Figure 1, Figure 2 and Table 1).

From the above examples, it can be concluded that the processing characteristics of the same rubber compound at different shear rates are different, and the scorch time at different shear rates is also different. In actual production, after obtaining the desired Mooney viscosity value (rotor speed: 2rpm), the rubber compound may still scorch in advance during subsequent extrusion and calendering. The reason is that in the traditional Mooney viscosity detection method (rotor speed: 2rpm), the shear rate to which the rubber compound is subjected is only 1.58 s-1, while in subsequent processes such as extrusion and calendering, the shear rate to which the rubber compound is subjected is much higher than 1.58 s-1. Therefore, it can be further concluded that it is incomplete to use only the traditional Mooney viscosity test method (rotor speed: 2rpm) to reveal the rubber processing characteristics of the entire rubber processing process (such as: open mixing, mixing, extrusion, calendering, molding, etc.).

This article introduces a method that can dynamically reveal the processing characteristics of the entire rubber processing process, and is currently widely used by world-renowned tire manufacturers. The "three-point frequency scanning method" is used for rapid testing or quality control of raw rubber and mixed rubber.

ASTM D6204-01 standard requirements

ASTM D6204-01 standard aims to use rheological testing technology to detect the viscoelastic properties of rubber/polymers, and then analyze the intrinsic properties of rubber/polymers, including: modulus, molecular weight distribution, branching, etc. From the following test, we can understand the correlation between the Mooney viscosity test and the rheological test at a certain temperature, frequency and strain (Figure 3).

A rheometer that complies with the ASTM D6204-01 standard was selected, and its oscillation frequency was set to the required frequency value instead of the common rheometer with a fixed oscillation frequency (1.67Hz). We used this rheometer to test the composite torque S* of 23 different synthetic rubbers under the test conditions of an oscillation frequency of 0.1 Hz, a temperature of 100℃, and an oscillation amplitude of ±0.5°. The results showed that the composite torque S* value was correlated with its corresponding Mooney viscosity value MU at ML(1+4)100℃ at 2rpm, and the correlation coefficient R was as high as 0.95. Therefore, it is concluded that under certain conditions, the Mooney viscosity value MU has a good correlation with the composite torque S*. From the following formula, it can be seen that there is a certain relationship between frequency and shear rate: Shear rate =

strain x angular velocity (where: angular velocity = 2 x π x frequency)

The formula shows that the shear rate is proportional to the strain and angular velocity, and the angular velocity is proportional to the frequency, that is, the shear rate is proportional to the frequency. A change in frequency means a change in shear rate. In this way, the rheometer can be used to detect the viscoelastic properties (i.e., processability) of rubber materials at different shear rates at different frequencies, thereby overcoming the limitations of using Mooney viscosity testing at a fixed low shear rate.

In the "Summary of Test Methods" in Section 4 of the ASTM D6204-01 standard, there are clear provisions for "frequency sweep" and "frequency sweep test", requiring the shear rate to be gradually increased during the test to examine the processability of the rubber material, thereby overcoming the limitations of the traditional Mooney viscosity test (rotor speed is fixed at 2rpm).

In Section 5 of the ASTM D6204-01 standard, "Significance and Application", the "frequency sweep test" is discussed as follows: This test method is used to detect the viscoelastic properties of raw rubber and unvulcanized rubber. This property is related to the processing behavior of the factory; this test method can be used for quality control in rubber processing and the development of raw rubber and rubber compound formulas. This test method can also be used to judge the differences between different rubber materials.

In addition, in Section 6 of the ASTM D6204-01 standard, specific requirements are also made for the "instrument" of the "frequency sweep test". According to the relevant provisions and requirements of the standard, we need a "torsion strain rotorless oscillating rheometer with a sealed cavity, an oscillation angle of ±0.5° (7.0% shear strain), and an oscillation frequency that can continuously increase in certain increments between 0.03 and 30 Hz". In the past two decades, only a few countries such as the United States and Germany can produce such rheometers, and the prices are extremely expensive. However, after more than five years of dedicated research and development, Shanghai Nuojia Instrument Co., Ltd. has successfully developed and put on the market the "Multi-function Rheometer MFR100A", which fully complies with and meets the provisions and requirements of the ASTM D6204-01 standard and has a high cost-effectiveness. The advent of the Multi-function Rheometer MFR100A has filled the gap in my country's high-end applications of rubber/polymer rheological testing technology and has obtained a number of national patents.

The Norka Multifunctional Rheometer MFR100A is a newly designed rotorless rheometer with a sealed mold cavity. The oscillation angle can be automatically switched between 0.2°, 0.5° or 1.0°, and the oscillation frequency can be continuously increased in certain increments between 0.1~20Hz. In addition to the standard vulcanization test specified in ASTM D 5289, more practical functions have been added to more accurately reflect the viscoelastic properties of the rubber. These new features include: frequency sweep test, variable temperature simulated vulcanization, and user-defined isothermal vulcanization test. [page]

Now only the frequency sweep test specified in ASTM D6204-01 standard performed by the Norka Multifunctional Rheometer MFR100A is introduced in detail.

The ASTM D6204-01 standard defines the terminology [§3] as follows:

▲ Composite shear modulus G*: the ratio of peak shear stress to peak shear strain (comprehensively reflects the viscoelastic properties of the rubber itself, and is independent of the oscillation angle);

▲ Composite torque S*: the torque peak value of the applied sinusoidal strain measured by the torque sensor (comprehensive response of the viscoelastic properties of the rubber to the sinusoidal strain);

▲ Dynamic composite viscosity h*: the ratio of the composite shear modulus to the angular velocity of the oscillation frequency (reflecting the fluidity of the rubber);

▲ Elastic torque S': the peak torque in phase with the applied sinusoidal strain (reflecting the elastic characteristics of the rubber);

▲ Hysteresis angle δ: the phase angle at which the composite torque lags behind the applied sinusoidal strain (reflecting the processability of the rubber);

▲ Loss factor tanδ: the ratio of the loss modulus to the elastic modulus (the ratio of the viscous torque to the elastic torque) (reflecting the processability of the rubber);

▲ Loss modulus (viscous modulus) G”: the ratio of the peak viscous shear stress to the peak shear strain with a phase difference of 90° with the applied sinusoidal strain (reflects the viscous characteristics of the rubber itself, and has nothing to do with the oscillation angle);

▲ True dynamic viscosity h': the ratio of the loss modulus to the angular velocity of the oscillation frequency (reflects the fluidity of the rubber);

▲ Elastic shear modulus G': the ratio of the peak elastic shear stress to the peak shear strain with the same phase as the applied sinusoidal strain (reflects the elastic characteristics of the rubber itself, and has nothing to do with the oscillation angle);

▲ Viscous torque S”: the peak torque with a phase difference of 90° with the applied sinusoidal strain (reflects the viscous characteristics of the rubber).

It can be seen from the above test data points that the frequency sweep test pays more attention to the inherent viscoelastic properties or processability of the rubber and the dynamic characteristics of the rubber under different conditions.

Test and results:

The frequency sweep test was carried out on three kinds of rubber compounds using Norm multifunctional rheometer MFR100A according to the provisions and requirements of AS TMD6204-01. Each rubber compound was tested three times to examine the repeatability and reproducibility. The experimental data are shown in Table 2.

Table 2. Detailed report of experimental data

Figure 4: As the oscillation frequency increases, the composite modulus G* value increases, indicating that the increase in shear rate will lead to changes in processability; compounds B and C have similar viscoelastic properties, but compound A is quite different from compounds B and C, showing that the frequency sweep test has good sensitivity to differences between batches.

Figure 5: The elastic modulus G' value increases with increasing frequency; compounds B and C have similar elastic properties, but compound A is quite different from compounds B and C.

Figure 6: The viscosity modulus G” value increases with the increase of frequency, indicating that the fluidity of rubbers A, B, and C is changing; rubbers B and C have similar viscosity characteristics, while rubber A is quite different from rubbers B and C.

Figure 7: The elastic torque S' value increases with the increase of frequency; rubbers B and C have similar elastic characteristics, while rubber A is quite different from rubbers B and C.

Figure 8: The viscous torque S" value increases with the increase of frequency; rubber B and C have similar viscosity characteristics, while rubber A is quite different from rubber B and C.

Figure 9: The loss factor tanδ value decreases and tends to be stable with the increase of frequency, reflecting the relationship between processability and shear rate; rubber B and C basically maintain the same processing characteristics with the increase of frequency and do not change much with the increase of frequency; while rubber A changes greatly with the increase of frequency, reflecting the differences in molecular weight distribution, branching, etc. between rubbers; in the low-frequency stage, the processability of rubber B and C is quite different from that of rubber A; in the high-frequency stage, the processability of rubbers A, B, and C is basically similar, and may even be reversed.

Summary

From the test case results, it can be seen that: by testing the composite shear modulus G*, elastic shear modulus G', loss modulus G", elastic torque S', viscous torque S", loss factor tanδ and dynamic composite viscosity h* of the rubber under different oscillation frequency sweeps, much richer test results can be obtained than the traditional Mooney viscosity (rotor speed is 2rpm) test method; the three rubbers A, B, and C have different processability; in different frequency sweep tests, the processing characteristics of each rubber are different; the processing characteristics of rubber B and rubber C are relatively close; the frequency sweep test has good sensitivity to differences between batches.

Through the examples, it can be seen that the use of the "frequency sweep test fast test method" that meets the provisions and requirements of the ASTM D6204-0 1 standard for rubber quality control and batch release in the production process has the following benefits: overcoming the limitations of the traditional Mooney viscosity test method that only tests rubber at low shear rates. This limitation may lead to misjudgment or wrong judgment in quality management of the factory; dynamically reflect the viscoelastic properties or processing performance of the rubber in the whole process of rubber processing; the time of a "frequency sweep test" is not longer than that of a Mooney viscosity test, but the same test time is used to obtain the real, dynamic and comprehensive properties of the rubber; the frequency sweep test has good sensitivity to the differences between batches; the use of the Norka multifunctional rheometer MFR100A, in addition to obtaining data under the "frequency sweep test", can also obtain standard isothermal vulcanization test, user-defined isothermal vulcanization test, variable temperature simulated vulcanization and automatic switching of strain angle, which is an economical and practical solution; the management of testing instruments develops in a simple, professional and dedicated direction, reducing the factory's investment and management costs; get closer to world-renowned companies, establish advanced quality control standards and methods, optimize testing methods, improve product quality, and reduce production and management costs. The

above introduces a testing method that is currently the most advanced concept and meets international standards. We hope to share it with the tire and rubber companies, and test its advancement, rationality, scientificity and economy in practice, and point out possible problems during use, and continue to supplement and improve it. (end)