Method for testing stability of raw rubber

Abstract

The invention discloses a method for testing the stability of raw rubber, which comprises the following steps of: placing the raw rubber in a rubber processing analyzer, keeping the temperature at 100 DEG C for 30 seconds, and scanning at the frequency of 0.02-16Hz; raising the temperature from 100 DEG C to 180 DEG C at an interval of 10 DEG C, keeping the temperature of each temperature point constant for 30 seconds, and repeating frequency scanning; and then cooling to 100 DEG C, keeping the constant temperature for 30 seconds and scanning again. And mapping the hysteresis loss Tan.delta obtained by scanning and the frequency, and deducing the branching degree change of the crude rubber molecular chain by comparing the second and first 100 DEG C frequency-hysteresis loss curves in a low-frequency region: the increase of the hysteresis loss in the low-frequency region indicates that the molecular chain is broken and the branching degree is increased, otherwise, the molecular chain is crosslinked and the branching degree is reduced. The method can directly characterize the molecular structure change of the raw rubber at high temperature, effectively discriminates the difference of different batches of the same raw rubber, provides technical indexes for quality control and raw material purchase of rubber manufacturers, and has the characteristics of simplicity and convenience in operation and accurate result.

Description

Technical Field

[0001] The invention relates to the technical field of rubber production, in particular to a method for testing the stability of raw rubber. Background Art

[0002] In rubber production and processing, raw material quality control is the first step. The quality of raw rubber directly affects the performance of rubber compounds and rubber products such as tires. Thermal stability, in this context, refers to the stability of the raw rubber's molecular structure after undergoing thermal history. When exposed to high temperatures, some polymer chains break, while other polymer chains can crosslink to form higher molecular weight substances. If different batches of the same raw rubber have different thermal stabilities, more chain scissions or crosslinking will occur during the mixing process, affecting the final properties of the rubber compound.

[0003] A common method for controlling the quality of raw rubber is to test its Mooney viscosity. This is because the Mooney viscosity of raw rubber is only related to the average molecular weight of the polymer, and there are many factors that affect the rheological behavior of the polymer, such as molecular weight distribution, degree of branching, and polymer microstructure. In particular, the degree of branching in the molecular chain of raw rubber has a significant impact on the fluidity and processing properties of the polymer, and polymers with higher degrees of branching generally have better wear resistance and heat resistance. When mixing rubber compounds in an internal mixer, the mixing temperature can reach 160°C or even higher. Therefore, the differences in the structural stability of different batches of raw rubber after thermal history directly affect the performance of the rubber compound. Testing the changes in the degree of branching of different batches of raw rubber after thermal history becomes an important indicator for determining the thermal stability of raw rubber and thus controlling the quality of rubber products.

[0004] Frequency scanning of a rubber processing analyzer can quickly capture more structural change information about raw rubber. The frequency scanning result, hysteresis heat generation (Tan.δ), shows a good correlation with the degree of branching. Judging the quality of raw rubber materials by the performance of rubber compounds or even rubber products not only results in a large amount of raw material waste but also affects production efficiency. Therefore, a method is needed that can directly characterize the molecular structure changes of different raw rubbers exposed to high temperatures, and further characterize the differences between different batches of the same raw rubber. This can help rubber manufacturers control the quality of their products and provide technical indicators for raw material identification and procurement by tire or product companies.

[0005] The dynamic rheological properties of some common rubber raw rubbers, such as solution-polymerized styrene-butadiene rubber raw rubber, are that they will undergo cross-linking and reduce the degree of branching after a thermal history, while natural rubber raw rubber will undergo chain scission and increase the degree of branching after a thermal history.

[0006] Based on this, a method for testing the stability of raw rubber is now provided, which can eliminate the disadvantages of the existing device. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for testing the stability of raw rubber, which solves the problem of inconvenience in use in the prior art.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A method for testing the stability of raw rubber comprises the following steps:

[0010] Step 1 (Initial State Characterization): Place the raw rubber sample to be tested in a rubber processing analyzer and set a constant test strain (preferably 10% ± 0.5%). Maintain the sample at a constant temperature of 100°C for 30 seconds to eliminate the effects of thermal history. Then, perform a frequency sweep from 0.02 Hz to 16 Hz and record the hysteresis loss Tan.δ value corresponding to each frequency point.

[0011] Step 2 (thermal history simulation): The test temperature was gradually increased from 100°C to 180°C in 10°C intervals (i.e., 100°C → 110°C → ... → 180°C). After each temperature point was held constant for 30 seconds, the frequency sweep was repeated (0.02 Hz-16 Hz) and the Tan.δ value was recorded.

[0012] Step 3 (recovery test after thermal history): Lower the temperature to 100°C and hold it for 60 seconds (extending the recovery time to ensure thermal equilibrium), perform a frequency sweep again (0.02Hz-16Hz), and record the hysteresis loss data.

[0013] Step 4 (curve construction): With frequency as the horizontal axis and hysteresis loss Tan.δ as the vertical axis, draw the frequency sweep curves of step 1 (initial 100℃) and step 3 (100℃ after thermal history) respectively;

[0014] Step 5 (inference of branching degree change): Compare the low-frequency areas (0.02Hz-0.2Hz) of the two 100°C curves: If the Tan.δ value of the curve in the low-frequency area after the thermal history increases by ≥15% compared with the initial value, it is determined that the raw rubber molecular chain is broken and the branching degree is increased; if the Tan.δ value of the curve in the low-frequency area after the thermal history decreases by ≥10% compared with the initial value, it is determined that the raw rubber molecular chain is cross-linked and the branching degree is reduced.

[0015] On the basis of the above technical solutions, the present invention also provides the following optional technical solutions:

[0016] In an optional solution, the frequency scan adopts a logarithmic interval frequency reduction scanning mode, with the sequence being 16 Hz→8 Hz→4 Hz→…→0.02 Hz.

[0017] In one optional solution: during the heating process of step 2, after the frequency scan of each temperature point is completed, the system automatically performs a stress relaxation test (keeping the strain constant and recording the stress decay curve over time) to obtain a relaxation time spectrum as an auxiliary verification indicator of the change in branching degree.

[0018] In an optional scheme: the raw rubber includes any one or more blends of natural rubber (NR), solution styrene butadiene rubber (SSBR), and butadiene rubber (BR). Before testing, the sample needs to be pretreated for 24 hours at 23°C ± 2°C and a relative humidity of 50% ± 5%.

[0019] In one optional scheme: the degree of branching change is quantified by calculating the integral area difference (ΔA) of two 100°C curves in the low frequency region (0.02Hz-0.2Hz), and a mathematical model is established: branching change index (BCI) = ΔA / initial curve area × 100%. When BCI>0, it indicates an increase in branching degree, and when BCI<0, it indicates a decrease in branching degree. The larger the absolute value of BCI, the more significant the difference in thermal stability.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention provides a method for characterizing the thermal stability of raw rubber based on the dynamic rheological characteristics of raw rubber. The heat resistance of the raw rubber can be judged according to the change in the degree of branching after the raw rubber has undergone a thermal history. By comparing the changes in the molecular chains of the same raw rubber in different batches, the thermal stability of the raw rubber between different batches can be confirmed. The method is convenient, fast, reproducible and accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the frequency sweep curve of solution styrene butadiene rubber SSBR5271 raw rubber from continuous low temperature to high temperature and finally back to low temperature.

[0023] Figure 2 This is the frequency scanning curve of natural rubber 5#NR raw rubber from continuous low temperature to high temperature and finally back to low temperature. DETAILED DESCRIPTION

[0024] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0025] The specific test steps are as follows:

[0026] (1) The raw rubber was placed in a rubber processing analyzer, the test strain amplitude was fixed at 6.98%, and the test temperature was kept constant at 100°C for 30 seconds, and then a frequency sweep was performed at the test temperature in the range of 0.02-16 Hz;

[0027] (2) The test temperature was raised to 120°C, 140°C, 160°C, and 180°C in sequence, and the temperature was kept constant for 30 seconds before performing a frequency sweep. The frequency sweep range was 0.02-16 Hz.

[0028] (3) Lower the test temperature to 100°C, maintain the constant temperature for 30 seconds, and then perform a frequency sweep in the range of 0.02-16 Hz;

[0029] (4) Plotting the hysteresis loss Tan.δ measured at each frequency scanned within the frequency region against the frequency to obtain a frequency scanning curve, wherein the frequency serves as the abscissa of the curve and the hysteresis loss Tan.δ serves as the ordinate of the curve;

[0030] (5) The change in the frequency-hysteresis loss curve at the second 100°C and the first 100°C after the raw rubber is subjected to a continuous temperature increase frequency sweep is used to infer the change in the degree of branching of the raw rubber molecular chain. A significant increase in the hysteresis loss in the low-frequency region indicates that intramolecular friction is intensified, molecular chain motion increases, molecular chain breakage, branching degree increases, and the molecular network structure is irreversibly destroyed. Conversely, a significant decrease in the hysteresis loss indicates that the molecular chain is cross-linked and the degree of branching decreases.

[0031] Example 1

[0032] The branching degree of solution polymerized styrene butadiene rubber (SBR) was characterized.

[0033] Put the solution styrene butadiene rubber SSBR5271 into the rubber processing analyzer, and the frequency scanning curve is as follows Figure 1 shown.

[0034] Example 2

[0035] The branching degree of natural rubber 5#NR was characterized.

[0036] Put natural rubber 5#NR raw rubber into the rubber processing analyzer, and the frequency scanning curve is as follows Figure 2 shown.

[0037] from Figure 1 It can be seen from the medium frequency scanning curve that after heating, the frequency scanning curve of the solution-polymerized styrene-butadiene rubber SSBR5271 raw rubber sample is lower than the curve before heating in the low frequency region, which is consistent with the dynamic rheological properties of the solution-polymerized styrene-butadiene rubber after undergoing thermal history and cross-linking after heating. This method can characterize the change in the degree of branching of the raw rubber after undergoing thermal history.

[0038] from Figure 2 It can be seen from the medium frequency scanning curve that the frequency scanning curve of the natural rubber 5#NR raw rubber sample after heating is generally higher than the curve before heating, indicating that the molecular chain undergoes irreversible breakage, which is consistent with the dynamic rheological properties of natural rubber raw rubber after experiencing thermal history and decomposition after heating. This method can characterize the changes in the degree of branching of raw rubber after experiencing thermal history.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for testing the stability of raw rubber, characterized in that: The following steps are involved: Step 1: Initial state characterization: Place the raw rubber sample to be tested in a rubber processing analyzer, set a constant test strain, and maintain it at a constant temperature of 100°C for 30 seconds. After eliminating the influence of thermal history, perform a frequency sweep from 0.02Hz to 16Hz, and record the hysteresis loss Tan.δ value corresponding to each frequency point. Step 2: Thermal history simulation: The test temperature was gradually increased from 100°C to 180°C in 10°C intervals (i.e., 100°C → 110°C → ... → 180°C). After each temperature point was held constant for 30 seconds, the frequency was repeatedly scanned from 0.02Hz to 16Hz and the Tan.δ value was recorded. Step 3: Recovery test after thermal history: Lower the temperature to 100°C, hold the temperature for 60 seconds, perform a frequency sweep (0.02Hz-16Hz) again, and record the hysteresis loss data; Step 4. Curve construction: With frequency as the horizontal axis and hysteresis loss Tan.δ as the vertical axis, draw the frequency sweep curves of step 1 (initial 100℃) and step 3 (100℃ after thermal history) respectively; Step 5. Inference of the change in degree of branching: Compare the low-frequency area 0.02Hz-0.2Hz of the two 100℃ curves: If the Tan.δ value of the curve in the low-frequency area after the thermal history increases by ≥15% compared with the initial value, it is determined that the raw rubber molecular chain is broken and the degree of branching is increased; if the Tan.δ value of the curve in the low-frequency area after the thermal history decreases by ≥10% compared with the initial value, it is determined that the raw rubber molecular chain is cross-linked and the degree of branching is reduced.

2. The method for testing the stability of raw rubber according to claim 1, wherein The frequency scan adopts a logarithmic interval frequency reduction scan mode, and the sequence is 16 Hz→8 Hz→4 Hz→…→0.02 Hz.

3. The method for testing the stability of raw rubber according to claim 1, wherein During the heating process of step 2, after the frequency scan of each temperature point is completed, the system automatically performs a stress relaxation test (keeping the strain constant and recording the stress decay curve over time) to obtain the relaxation time spectrum as an auxiliary verification indicator of the change in branching degree.

4. The method for testing the stability of raw rubber according to claim 1, wherein The raw rubber includes any one or more blends of natural rubber, solution-polymerized styrene-butadiene rubber, and butadiene rubber. Before testing, the sample needs to be pretreated for 24 hours at 23°C ± 2°C and a relative humidity of 50% ± 5%.

5. The method for testing the stability of raw rubber according to claim 1, wherein The degree of branching change was quantified by calculating the integral area difference ΔA of the two 100°C curves in the low-frequency region of 0.02Hz-0.2Hz, and a mathematical model was established: branching degree change index BCI = ΔA / initial curve area × 100%. When BCI>0, it indicates an increase in branching degree, and when BCI<0, it indicates a decrease in branching degree. The larger the absolute value of BCI, the more significant the difference in thermal stability.

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