Method for evaluating processability of natural rubber

Abstract

The invention provides a method for evaluating the processability of natural rubber, which uses RPA test to determine optimal test conditions and test means, and combines a nonlinear data fitting analysis method to overcome the problem that a corresponding relationship between conventional raw material test items and results of natural rubber and the viscosity of natural rubber in actual use cannot be established. And the viscosity of the natural rubber in actual use can be better, so that the viscosity processability difference of the natural rubber raw material can be better tested and represented, the purchase of the raw material and the adjustment of the processing technology are guided, and the quality of the rubber product is more sufficiently ensured. And particularly, the processing performance of the natural rubber when the Mw is relatively close can be evaluated.

Description

Technical Field

[0001] The present invention relates to the detection of natural rubber, and particularly to a method for evaluating the processing performance of natural rubber. Background Art

[0002] Natural rubber is an important strategic material in China. Due to its high strength and excellent resilience that synthetic rubber does not possess, it is widely used in various fields such as automobile tires, national defense and military industry, medical and health, damping materials, adhesives, etc. In the development process of tires, natural rubber has always played an indispensable role. Today, natural rubber is still the raw material with the largest consumption in truck and bus tires and engineering tires, with a weight ratio of more than 30% in a single tire. In truck and bus tires, the rubber compounds in key parts such as the tread, base compound, carcass, and belt layer are mainly produced with natural rubber as the main matrix. Therefore, the quality and stability of natural rubber have a crucial impact on the tire manufacturing process and product performance.

[0003] The main component of natural rubber is cis-1,4-polyisoprene, accounting for more than 95%. In addition to rubber hydrocarbons, it also includes non-rubber components such as proteins, acetone-soluble substances, esters, water-soluble substances, and inorganic salts. The non-rubber components in natural rubber have important effects on the vulcanization rate, crosslink density, tensile-induced crystallization ability, ozone resistance performance, etc. of rubber. Currently, the main producing countries of natural rubber include China, Thailand, Malaysia, Indonesia, Vietnam and other countries. The soil environment, sunlight, planting technology, processing technology, etc. in different regions will all have a certain impact on the performance of natural rubber. Natural rubber from different origins may cause fluctuations in the quality of the mixed rubber and the production process, and even affect the quality of tire products.

[0004] Generally, there is gel in natural rubber, which makes it difficult to test the molecular weight. At the same time, the existing Mooney viscometer test method has insufficient recognition accuracy, resulting in obvious differences in processing performance under the same viscosity. Therefore, new methods need to be introduced to strictly identify and control natural rubber to detect the fluctuations in the processing performance of natural rubber raw materials. Patent CN105527194B, a method for detecting the processing performance of rubber for tires, discloses using a Mooney viscometer combined with a torque rheometer to test and fit the power-law equation to characterize the processing performance of rubbers with similar viscosities. However, the power-law parameters of different rubbers obtained by fitting are relatively close, with poor recognition, which has a certain impact on the result judgment.

[0005] This patent uses a Rubber Processing Analyzer (RPA) to test and fit the rheological processing performance of natural rubber. The test results have a strong corresponding relationship with the viscosity of natural rubber in the steel wire bonding rubber formula, and have strong guiding significance for the adjustment of the process and formula in tire production. Summary of the Invention

[0006] To solve the above problems, the present invention provides a method for evaluating the processing performance of natural rubber, which can particularly evaluate the processing performance of natural rubber when Mw is relatively close.

[0007] The object of the present invention is achieved in the following way: A method for evaluating the processing performance of natural rubber, comprising the following steps: (1) Performing a temperature sweep test on a natural rubber sample using RPA: determining the viscous flow processing temperature of the natural rubber, and fixing the test shear frequency γ and strain ε within the range of the test temperature; (2) Performing an RPA strain sweep test with the RPA set at a fixed temperature T and a fixed shear frequency γ, determining the linear viscoelastic region from the strain sweep curve, and thus determining the fixed strain ε within the linear viscoelastic region; (3) According to the viscous flow processing temperature and the strain ε within the linear viscoelastic region obtained above, setting the RPA at a fixed temperature T and a fixed shear frequency γ for an RPA frequency sweep to obtain the corresponding shear frequency γ and complex viscosity η*; (4) According to the test results of the above RPA frequency sweep, taking the data of the shear frequency γ and the complex viscosity η* for Cross model fitting or Carreau - Yasuda model fitting, so as to obtain the zero - shear viscosity η0; Among them, Cross model fitting: In the formula, η* is the complex viscosity, η0 is the zero - shear viscosity, λ is the relaxation time, γ is the shear frequency, and n is the non - Newtonian index; Carreau - Yasuda model fitting: In the formula, η* is the complex viscosity, η0 is the zero - shear viscosity; ω is the frequency, that is, the data corresponding to γ; α is the Carreau constant, and n is the power index; (5) Fitting the Fox - Flory equation according to the obtained zero - shear viscosity η0 to obtain Mw: In the formula, K1, K2 are constants related to temperature and molecular structure; Mw is the weight - average molecular weight; Mc is the critical entanglement molecular weight; (6) The Mw can be calculated according to the test results of the Fox - Flory equation fitting, which is used to characterize the processing performance of the rubber; (7) Since the difference in Mw of different natural rubbers is within 2×10 4 and it is not possible to well judge the relative processing performance of natural rubber by comparing the magnitudes of Mw; when the Mw of the rubber measured for the sample is relatively close, with the difference in Mw within 2×10 4When it is within a certain range, the processing performance is compared according to the γ-η* curve obtained by fitting: the intersection value with the vertical axis is obtained according to the γ-η* curve fitted by the sample. After the intersection value drops by 5-20% on the vertical axis, a straight line parallel to the horizontal axis is made. The abscissa coordinate value corresponding to the intersection point of the straight line parallel to the horizontal axis and the γ-η* curve fitted by the sample is compared. For different samples with relatively close Mw values, the abscissa coordinate values are compared. The more to the left the abscissa coordinate value is, the better the processing performance.

[0008] Preparation of samples: Natural rubber is preheated and then made into samples on an open mill.

[0009] Preparation of samples: Natural rubber is heated in an oven at 60°C for 10 min, and then passed through an open mill with a roll temperature of 40-80°C and a roll gap of 2 mm for 3-6 passes.

[0010] Preparation of samples: Natural rubber is passed through an open mill with a roll temperature of 60°C for 4 passes.

[0011] When the measured Mw of the samples is relatively close, the processing performance is compared according to the γ-η* curve obtained by fitting: the intersection value with the vertical axis is obtained according to the γ-η* curve fitted by the sample. After the intersection value drops by 10% on the vertical axis, a straight line parallel to the horizontal axis is made. The abscissa coordinate value corresponding to the intersection point of the straight line parallel to the horizontal axis and the γ-η* curve fitted by the sample is compared. For different samples with relatively close Mw values, the abscissa coordinate values are compared. The more to the left the abscissa coordinate value is, the better the processing performance.

[0012] In the step (1), the temperature scanning test conditions are as follows: the fixed strain ε is 4-10%, the fixed frequency γ is 0.5~3 Hz, and the temperature scanning range is 30~180°C; In the step (2), the strain scanning test conditions are as follows: the fixed frequency γ is 0.5~3 Hz, the fixed temperature T is 90~120°C, and the strain scanning ε range is 1~100%.

[0013] In the step (3), the RPA is set to a fixed temperature T and a fixed shear frequency γ for RPA frequency scanning, and the frequency scanning range is 0.01~33.3 Hz.

[0014] In the step (1), the temperature scanning test conditions are as follows: the fixed strain ε is 7%, and the fixed frequency γ is 1 Hz; In the step (2), the strain scanning test conditions are as follows: the fixed frequency γ is 1 Hz, and the fixed temperature T is 100°C.

[0015] In the step (3), the frequency scanning test conditions are as follows: the fixed temperature T is 140°C, the fixed strain ε is 7%, and the frequency scanning range is 0.01~33.3 Hz.

[0016] Nonlinear fitting of data was performed using Origin or Matlab software.

[0017] The constant K2 for fitting the zero-shear viscosity to the Fox-Flory equation is 1×10 -16 ~1×10 -15 .

[0018] The present invention provides a method for evaluating the processing performance of natural rubber. Using RPA testing, the optimal testing conditions and means are determined. By combining the method of nonlinear data fitting analysis, the problem that there is no corresponding relationship between the conventional raw material test items and results of natural rubber and the viscosity during the actual use of natural rubber is overcome, and it can better correspond to the viscosity during the actual use of natural rubber. Therefore, it can better test and characterize the differences in the viscosity processing performance of natural rubber raw materials themselves, guide the procurement of raw materials and the adjustment of processing technology, and more fully ensure the quality of rubber products. In particular, it can evaluate the processing performance of natural rubber when Mw is relatively close. Description of the Drawings

[0019] Figure 1 Temperature-shear modulus curve of natural rubber.

[0020] Figure 2 Strain-shear modulus curve of natural rubber.

[0021] Figure 3 Frequency-complex viscosity curve of natural rubber.

[0022] Figure 4 Cross fitting curve of the frequency-complex viscosity curve of STR-1. In the figure, A1 is η0 in the cross model, t is λ, and m is 1-n.

[0023] Figure 5 Cross fitting curve of the frequency-complex viscosity curve of STR-2. In the figure, A1 is η0 in the cross model, t is λ, and m is 1-n.

[0024] Figure 6 Cross fitting curve of the frequency-complex viscosity curve of STR-3. In the figure, A1 is η0 in the cross model, t is λ, and m is 1-n.

[0025] Figure 7 Cross fitting curve of the frequency-complex viscosity curve of SMR. In the figure, A1 is η0 in the cross model, t is λ, and m is 1-n.

[0026] Figure 8 Processing performance comparison curve of similar Mw.

[0027] Figure 9 Stress relaxation curve of natural rubber. Specific embodiments

[0028] A method for evaluating the processing performance of natural rubber, comprising the following steps: (1) Performing a temperature sweep test on a natural rubber sample using an RPA: determining the viscous flow processing temperature of the natural rubber, and fixing the test shear frequency γ and strain ε within the test temperature range; (2) Performing an RPA strain sweep test with the RPA set at a fixed temperature T and a fixed shear frequency γ, determining the linear viscoelastic region from the strain sweep curve, and thereby determining the fixed strain ε within the linear viscoelastic region; (3) According to the viscous flow processing temperature and the strain ε within the linear viscoelastic region obtained above, performing an RPA frequency sweep with the RPA set at a fixed temperature T and a fixed shear frequency γ to obtain the corresponding shear frequency γ and complex viscosity η*; (4) According to the test results of the above RPA frequency sweep, taking the data of the shear frequency γ and the complex viscosity η* for Cross model fitting or Carreau-Yasuda model fitting, thereby obtaining the zero-shear viscosity η0; Among them, Cross model fitting: In the formula, η* is the complex viscosity, η0 is the zero-shear viscosity, λ is the relaxation time, γ is the shear frequency, and n is the non-Newtonian exponent; Carreau-Yasuda model fitting: In the formula, η* is the complex viscosity, η0 is the zero-shear viscosity; ω is the frequency, that is, the data corresponding to γ; α is the Carreau constant, and n is the power exponent; (5) Fitting the Fox-Flory equation according to the obtained zero-shear viscosity η0 to obtain Mw: In the formula, K1 and K2 are constants related to temperature and molecular structure; Mw is the weight-average molecular weight; Mc is the critical entanglement molecular weight; (6) The Mw can be calculated according to the test results of the Fox-Flory equation fitting, and is used to characterize the processing performance of the rubber; (7) Since the difference in Mw of different natural rubbers is within 2×10 4 , it is not possible to well use the comparison of Mw sizes to judge the relative processing performance of natural rubber; when the measured Mw of the sample rubbers is relatively close, with the difference in Mw within 2×10 4When it is within a certain range, the processing performance is compared according to the γ-η* curve obtained by fitting: the intersection value with the vertical axis is obtained according to the γ-η* curve fitted by the sample. After the intersection value drops by 5-20% on the vertical axis, a straight line parallel to the horizontal axis is made. The abscissa coordinate value corresponding to the intersection point of the straight line parallel to the horizontal axis and the γ-η* curve fitted by the sample is compared. For different samples with relatively close Mw values, the abscissa coordinate values are compared. The more to the left the abscissa coordinate value is, the better the processing performance.

[0029] Preparation of the sample: The natural rubber is preheated and then made into a sample on an open mill.

[0030] Preparation of the sample: The natural rubber is heated in an oven at 60 °C for 10 min, and then on an open mill with a roll temperature of 40-80 °C, a roll gap of 2 mm, and passed through the rolls 3-6 times.

[0031] Preparation of the sample: The natural rubber is passed through the rolls 4 times on an open mill with a roll temperature of 60 °C.

[0032] When the measured Mw of the samples is relatively close, the processing performance is compared according to the γ-η* curve obtained by fitting: the intersection value with the vertical axis is obtained according to the γ-η* curve fitted by the sample. After the intersection value drops by 10% on the vertical axis, a straight line parallel to the horizontal axis is made. The abscissa coordinate value corresponding to the intersection point of the straight line parallel to the horizontal axis and the γ-η* curve fitted by the sample is compared. For different samples with relatively close Mw values, the abscissa coordinate values are compared. The more to the left the abscissa coordinate value is, the better the processing performance.

[0033] In the step (1), the temperature scanning test conditions are: the fixed strain ε is 4-10%, the fixed frequency γ is 0.5~3 Hz, and the temperature scanning range is 30~180 °C; In the step (2), the strain scanning test conditions are: the fixed frequency γ is 0.5~3 Hz, the fixed temperature T is 90~120 °C, and the strain scanning ε range is 1~100%.

[0034] In the step (3), the RPA is set to a fixed temperature T and a fixed shear frequency γ for RPA frequency scanning, and the frequency scanning range is 0.01~33.3 Hz.

[0035] In the step (1), the temperature scanning test conditions are: the fixed strain ε is 7%, and the fixed frequency γ is 1 Hz; In the step (2), the strain scanning test conditions are: the fixed frequency γ is 1 Hz, and the fixed temperature T is 100 °C.

[0036] In the step (3), the frequency scanning test conditions are: the fixed temperature T is 140 °C, the fixed strain ε is 7%, and the frequency scanning range is 0.01~33.3 Hz.

[0037] The data is non-linearly fitted using Origin or Matlab software.

[0038] The constant K2 for fitting the zero-shear viscosity to the Fox-Flory equation is 1×10 -16 ~1×10 -15 .

[0039] The present invention provides a method for evaluating the processing performance of natural rubber. Using RPA testing, the optimal testing conditions and means are determined. By combining the method of non-linear data fitting analysis, the problem that there is no corresponding relationship between the conventional raw material test items and results of natural rubber and the viscosity during the actual use of natural rubber is overcome, and it can better correspond to the viscosity during the actual use of natural rubber. Therefore, it can better test and characterize the differences in the viscous processing performance of natural rubber raw materials themselves, guide the procurement of raw materials and the adjustment of processing technology, and more fully ensure the quality of rubber products. In particular, it can evaluate the processing performance of natural rubber when Mw is relatively close.

[0040] The following specifically describes the present invention with reference to specific embodiments. It is necessary to point out here that these embodiments are only used to further illustrate the present invention and should not be construed as limiting the protection scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the content of the present invention above.

[0041] The natural rubbers used in this application are natural rubbers from different manufacturers in Southeast Asia respectively.

[0042] Examples 1-4 are the tests on the processing performance of natural rubbers imported from different manufacturers in Southeast Asia, including the following steps: (1) Sample preparation Take 200 g of natural rubbers from different manufacturers in Southeast Asia respectively, and perform the following operations for sample preparation: Heat in a constant-temperature oven at 60 °C for 10 min, pass through the rolls 4 times on an open mill, with a roll gap of 2 mm and a roll temperature of 60 °C, and then take out the sheet. Samples STR-1, STR-2, STR-3, and SMR are obtained respectively.

[0043] (2) RPA temperature scan Take the natural rubber samples to test RPA respectively. The temperature scan program: Fix the strain at 7%, fix the frequency at 1 Hz, and the temperature scan range is 30~180 °C to determine the viscous flow processing temperature of natural rubber.

[0044] As the temperature increases, natural rubber gradually changes from a solid state to a liquid state that can flow, and the shear modulus will show a downward trend. The temperature scan curve is as Figure 1 shown. From the Figure 1 test curve, when the temperature exceeds 120 °C, the shear modulus of natural rubber begins to decrease significantly, indicating that when the temperature is higher than 120 °C, the rubber begins to change from a solid state to a viscous flow state; therefore, asFigure 1 As shown, the available viscous flow processing temperature of natural rubber is 120 - 160 °C.

[0045] (3) RPA strain sweep Take natural rubber samples to test RPA respectively. Strain sweep procedure: fix the frequency at 1 Hz, fix the temperature at 100 °C, strain sweep range 1 - 100%, and the strain sweep curve is as Figure 2 shown. According to Figure 2 shown, determine that the strain in the linear viscoelastic region is 4% - 10%.

[0046] (4) RPA frequency sweep Take natural rubber samples to test RPA respectively. The temperature of natural rubber during the internal mixing process is approximately around this temperature. Therefore, according to the viscous flow processing temperature obtained in step (2) and the strain ε in the linear viscoelastic region, set the fixed temperature T of RPA to 140 °C and set the fixed shear frequency γ to 7% for RPA frequency sweep. The frequency sweep range is 0.01 - 33.3 Hz, and the frequency sweep curve is as Figure 3 shown, and obtain the corresponding shear frequency γ and complex viscosity η*.

[0047] (5) Fitting of RPA frequency sweep data According to the test results of the above RPA frequency sweep, take the data of shear frequency γ and complex viscosity η*, and use the Cross model to perform non - linear data fitting with Origin software. Obtain the zero - shear viscosity η0 according to the fitting results; Among them, Cross model fitting: In the formula, η* is the complex viscosity, η0 is the zero - shear viscosity, λ is the relaxation time, γ is the shear frequency, and n is the non - Newtonian index; The fitting results of the Cross model for 4 samples are as Figures 4 - 7 shown.

[0048] (6) Fit the Fox - Flory equation based on the obtained zero - shear viscosity η0 to obtain Mw: In the formula, K1 and K2 are constants related to temperature and molecular structure; Mw is the weight - average molecular weight; Mc is the critical entanglement molecular weight.

[0049] Since it can be found in both literature and professional books that for low - molecular - weight polymers, Mw is less than the critical entanglement molecular weight Mc; for high - molecular - weight polymers, especially rubbers (with a molecular weight exceeding 1 million), their Mw is much greater than Mc. Therefore, in this application, η0 = K2M W 3.4 And it is found that the constant K2 takes 5×10 -15 .

[0050] Compare the Mw of different natural rubber samples to obtain the relative advantages and disadvantages of the processing performance of different natural rubbers: the larger the Mw molecular weight, the higher the viscosity, the worse the processing performance, and the faster the stress relaxation; the smaller the Mw molecular weight, the lower the viscosity, the better the processing performance, and the slower the stress relaxation.

[0051] The fitting results of the Fox-Flory equation for different samples are shown in Table 1: Table 1 Comparison of the processing performance of imported natural rubber by RPA fitting (7)Analysis of processing performance at similar Mw Since the difference in Mw of different natural rubbers is within 2×10 4 , it is not possible to well judge the relative processing performance of natural rubber by comparing the Mw size. From Table 1, since the Mw of STR-2 and STR-3 is similar and the difference in Mw is within 2×10 4 , for samples STR-2 and STR-3 with similar Mw: the intersection values of the γ-η* curves fitted by STR-2 and STR-3 with the vertical axis. After the intersection values decrease by 10% on the vertical axis, draw lines parallel to the horizontal axis, which are a1 and a2 respectively. The abscissa corresponding to the intersection point of line a1 and the γ-η* curve fitted by STR-2, and the abscissa corresponding to the intersection point of line a2 and the γ-η* curve fitted by STR-3. The intersection coordinate values are A and B respectively. The more to the left the coordinate value, the better the processing performance. As Figure 8 shown, the small coordinate in the figure is an enlarged view. B is to the left of A, so the processing performance of STR-3 is better than that of STR-2.

[0052] Principle analysis: When the fitted Mw is close, the shear frequency - complex viscosity curve obtained by fitting can be analyzed. For polymers with a wide molecular weight distribution, due to the presence of low molecular weight parts, they are more sensitive to shear and the phenomenon of decreasing complex viscosity will occur earlier; while for polymers with a narrow molecular weight distribution, the high molecular weight part is relatively more, and stronger shear is required to open the molecular chain entanglement, and the decrease in complex viscosity will occur at high frequencies. And polymers with a wide molecular weight distribution have better processing performance. As Figure 8 shown, at point B compared to point A, when the complex viscosity decreases by a certain percentage, the shear frequency is lower, the molecular weight distribution is wide, and the processing performance is good.

[0053] (7)In addition, perform RPA test stress relaxation on the samples Take natural rubber samples for RPA stress relaxation test. The test procedure: test temperature 100°C, preheating time 60s, test time 120s, to obtain the shear modulus G’-t curve. The shorter the stress relaxation time, the stronger the elasticity and the worse the processing performance. The results are as Figure 9 shown in and Table 2.

[0054] Table 2 Comparison of RPA stress relaxation performance of imported natural rubber The test results of the above-mentioned sample's conventional properties are shown in Table 3 as follows: Table 3 Comparison of performance tests of imported natural rubber In Table 3, MU, P0, and PRI here are respectively the pure rubber tests of natural rubber of STR-1, STR-2, STR-3, and SMR. Tensile strength, elongation at break, modulus at a specified elongation, ML, MH, and T90 are respectively the performance tests of the masterbatch compounded according to the basic formula with STR-1, STR-2, STR-3, and SMR as raw materials.

[0055] The basic formula of the masterbatch compound is: NR 300; ZnO 15; SA 1.5; S 9.0; accelerator M 2.1. The compounding steps of the masterbatch compounds based on STR-1, STR-2, STR-3, and SMR as raw materials are the same. The basic formula compounding steps here are as follows: Take 300 g of natural rubber, set the initial temperature of the open mill at 70 ± 5 °C, adjust the roll gap to 0.5 mm, add raw rubber and wrap it around the front roll, cut with a cutter twice for a duration of 2 min, adjust the roll gap to 1.65 mm, add stearic acid, cut with a cutter 2 times for a duration of 1.5 min; add accelerator and zinc oxide, cut with a cutter 2 times for a duration of 2 min; add sulfur, cut with a cutter 2 times for a duration of 2 min; adjust the roll gap to 0.8 mm, pass the coiled rubber through the open mill six times without wrapping the roll for a duration of 1 min, and then weigh; adjust the roll gap to 2.2 mm and take off the sheet for a duration of 0.5 min.

[0056] Table 3 comprehensively lists the basic test and semi-finished product surface tackiness test results of samples STR-1, STR-2, STR-3, and SMR. There is no corresponding relationship between the conventional raw material test items and results of natural rubber in Table 3 and the tackiness during actual use in the last row. Therefore, it can be seen that the conventional natural rubber raw material tests cannot well distinguish the differences in processing performance. Therefore, it is impossible to determine the process adjustment from the raw material test, nor can the appropriate tackiness be obtained from the raw materials for tire quality control.

[0057] Table 1 lists the results obtained by the RPA test fitting calculation of natural rubber samples STR-1, STR-2, STR-3, and SMR. The molecular weight sizes in Table 1 are: STR1 > STR2 > STR3 > SMR, indicating that the processing performance is STR1 < STR2 < STR3 < SMR. Since the molecular weights of STR-2 and STR-3 are relatively close, to prevent errors caused by calculation, this application further compares their processing performance according to the γ-η* curve obtained by fitting. According to Figure 8It can be clearly compared that the processing performance of STR3 is better than that of STR2. Therefore, when the molecular weights are relatively close, the inaccurate judgment of the processing performance is overcome.

[0058] The processing performance of the natural rubber samples determined by the method of this application is STR1 < STR2 < STR3 < SMR, which corresponds to the viscosity in Table 3: STR1 > STR2 > STR3 > SMR. Therefore, the method provided by this application conforms to the rule that the larger the Mw molecular weight, the higher the viscosity, and the worse the processing performance. Therefore, the method of this application can better identify the differences in processing performance between different natural rubber raw materials from the raw material end, guide the procurement of raw materials, and guide the process adjustment for different raw materials during processing, more fully ensuring the quality of rubber products.

[0059] Table 2 gives the test results of the stress relaxation of natural rubber. The percentage of the modulus drop at 20s in Table 2: SMR > STR3 > STR2 > STR1, which has a good correspondence with the RPA fitting results provided by this application and the processing performance and semi-finished product performance of the rubber compound in actual production. It conforms to the rule that the larger the Mw molecular weight, the higher the viscosity, the worse the processing performance, and the faster the stress relaxation; the smaller the Mw molecular weight, the lower the viscosity, the better the processing performance, and the slower the stress relaxation.

[0060] In summary, the method disclosed in this invention uses RPA testing to determine the optimal testing conditions and testing means, and combines the method of non-linear data fitting analysis to overcome the problem of insufficient identification accuracy of traditional natural rubber testing methods, and can better test and characterize the differences in the processing performance of natural rubber raw materials themselves, ensuring the quality and performance of rubber products.

[0061] The above are only the preferred embodiments of the present invention, but the protection scope of the present invention is not limited thereto. It should be pointed out that for those skilled in the art and anyone familiar with the technical field of the present invention, without departing from the overall concept of the present invention, equivalent substitutions or changes made according to the technical solution and inventive concept of the present invention, as well as several changes and improvements made, should also be regarded as the protection scope of the present invention.

Claims

1. An evaluation method for the processing performance of natural rubber, characterized in that: The following steps are included: (1) Perform temperature scanning test on natural rubber samples using RPA: determine the viscosity flow processing temperature of natural rubber, and fix the test shear frequency γ and strain ε within the test temperature range; (2) The RPA is set to a fixed temperature T and a fixed shear frequency γ to perform an RPA strain sweep test, and the linear viscoelastic region is determined by the strain sweep curve, thereby determining the fixed strain ε in the linear viscoelastic region; (3) According to the viscous flow processing temperature and the strain ε in the linear viscoelastic region obtained above, the RPA is set to a fixed temperature T and a fixed shear frequency γ to perform RPA frequency scanning to obtain the corresponding shear frequency γ and complex viscosity η*; (4) According to the test results of the above RPA frequency scan, the shear frequency γ and complex viscosity η* data are taken for Cross model fitting or Carreau-Yasuda model fitting to obtain the zero shear viscosity η0; Among them, Cross model fitting: Where η* is the complex viscosity, η0 is the zero shear viscosity, λ is the relaxation time, γ is the shear frequency, and n is the non-Newtonian index; Carreau - Yasuda model fitting: Where η* is the complex viscosity, η0 is the zero shear viscosity; ω is the frequency, i.e. the data corresponding to γ; α is the Carreau constant, and n is the power exponent; (5) The Fox-Flory equation is fitted according to the zero shear viscosity η0 to obtain Mw: Wherein, K1, K2 are constants related to temperature and molecular structure; Mw is the weight average molecular weight; Mc is the critical entanglement molecular weight; (6) Mw can be calculated based on the experimental results of the Fox-Flory equation and used to characterize the processing properties of rubber; (7) Since the difference in Mw of different natural rubbers is within 2×10 4 , it is not possible to effectively use the comparison of Mw to judge the relative processing performance of natural rubber; when the Mw of the rubber measured for the samples is relatively close, within a difference in Mw of 2×10 4 , the processing performance is compared according to the γ-η* curve obtained by fitting: the intersection value with the vertical axis is obtained from the γ-η* curve fitted for the sample. After the intersection value drops by 5 - 20% on the vertical axis, a straight line parallel to the horizontal axis is drawn. The abscissa coordinate value corresponding to the intersection of the straight line parallel to the horizontal axis and the γ-η* curve fitted for the sample is compared for different samples with relatively close Mw. The more to the left the abscissa coordinate value is, the better the processing performance.

2. The evaluation method for the processing performance of natural rubber according to claim 1, characterized in that: Sample preparation: Natural rubber is sampled in an open mixing mill after preheating.

3. The evaluation method for the processing performance of natural rubber according to claim 2, wherein: Sample preparation: Natural rubber was heated in an oven at 60°C for 10 min, and rolled in an open mill at a roller temperature of 40-80°C, with a roller gap of 2mm and 3-6 passes.

4. The evaluation method for the processing performance of natural rubber according to claim 3, characterized in that: In an open mill with a roller temperature of 60°C, the rolls were passed 4 times.

5. The evaluation method for the processing performance of natural rubber according to claim 1, characterized in that: When the rubber Mw of the samples measured are relatively close, their processing performance is compared according to the fitted γ-η* curve: the intersection value with the ordinate is obtained according to the γ-η* curve fitted by the sample, and a straight line parallel to the abscissa is drawn after the intersection value drops 10% on the ordinate. The abscissa coordinate value corresponding to the intersection of the straight line parallel to the abscissa and the γ-η* curve fitted by the sample is compared. The more the abscissa coordinate value is to the left, the better the processing performance.

6. The method for evaluating the processing performance of natural rubber according to claim 1, characterized in that: In the step (1), the temperature scanning test conditions are: fixed strain ε is 4-10%, fixed frequency γ is 0.5-3 Hz, and the temperature scanning range is 30-180°C; In step (2), the strain scanning test conditions are: fixed frequency γ is 0.5~3Hz, fixed temperature T is 90~120℃, and strain scanning ε interval is 1~100%; In the step (3), the RPA is set with a fixed temperature T and a fixed shear frequency γ to perform RPA frequency scanning, and the frequency scanning range is 0.01-33.3 Hz.

7. The evaluation method for the processing performance of natural rubber according to claim 6, characterized in that: In the step (1), the temperature scanning test conditions are: the fixed strain ε is 7%, and the fixed frequency γ is 1 Hz; In the step (2), the strain scanning test conditions are: the fixed frequency γ is 1 Hz, and the fixed temperature T is 100 °C.

8. The evaluation method for the processing performance of natural rubber according to claim 6, characterized in that: In the step (3), the frequency scanning test conditions are: the fixed temperature T is 140 °C, the fixed strain ε is 7%, and the frequency scanning range is 0.01~33.3 Hz.

9. The evaluation method for the processing performance of natural rubber according to claim 1, characterized in that: Data non-linear fitting is performed using Origin or Matlab software.

10. The evaluation method for the processing performance of natural rubber according to claim 1, characterized in that: The constant K2 for fitting the zero shear viscosity to the Fox-Flory equation is 1×10 -16 ~1×10 -15 .

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