Composite vulcanization accelerator, preparation method and application of composite vulcanization accelerator in natural latex and rubber compound

Abstract

The invention discloses a composite vulcanization accelerator, a preparation method and application of the composite vulcanization accelerator in natural latex and rubber compound. The composite vulcanization accelerator comprises a thioketone derivative and a pyridine compound, the pyridine compound is selected from 3-hydroxypyridine, 2-methylpyridine, 4-ethylpyridine, 2-nitropyridine, 3-nitropyridine, 2-aminopyridine and 3-aminopyridine, and the thioketone derivative is selected from sodium pyrithione, 3-methylthiazoline-2-thioketone, N-hydroxypyridine-2-thioketone, 5-dodecyl dithio-3-phenyl-1, 3, 4-thiadiazole-2-thioketone, 1, 3, 4-thiadiazole-2-thioketone, 1, 3, 4-triazole-2-thioketone, 1, 3, 4-triazole-2-thioketone and 1, 3, 4-triazole-2-thioketone. The compound is prepared from 5-(4-methylbenzoyl)-3, 4-dihydropyrimidine-2-thioketone, 3-thiazolidine-2-thioketone, tetrahydropyrrole-2-thioketone and 5-(4-methylbenzoyl)-3, 4-dihydropyrimidine-2-thioketone. The composite vulcanization accelerator does not generate harmful substances, has the properties of safety, high efficiency, low toxicity and environmental protection, and can promote the vulcanization of natural latex and rubber compound and improve the physical and mechanical properties of a vulcanized rubber film.

Description

Technical Field

[0001] The present invention relates to the technical field of rubber vulcanization, and particularly to a composite vulcanization accelerator, a preparation method thereof, and an application thereof in natural latex and mixed rubber. Background Art

[0002] Natural rubber is an important industrial raw material, and concentrated natural latex and mixed rubber have a wide range of applications in many fields. Vulcanization is a process in which linear chain molecules of rubber form three-dimensional network macromolecules through chemical cross-linking reactions. Unvulcanized natural rubber has many defects, such as low strength, poor abrasion resistance, heat intolerance, and aging, which limit its scope of application. Vulcanization not only improves the physical and mechanical properties of rubber but also endows it with good dimensional stability and aging resistance. The vulcanization of rubber is a complex chemical process involving multiple components. Researchers have conducted extensive studies on it and established a systematic kinetic model to clarify the reaction mechanism. The vulcanization process can be divided into three stages: the vulcanization progress stage, the cross-linking reaction stage, and the network formation stage. Through the study of the kinetic model, the vulcanization process can be optimized to improve the properties of vulcanized rubber.

[0003] Vulcanization accelerators play a crucial role in the vulcanization process. They can accelerate the rate of vulcanization reactions, shorten the vulcanization time, and reduce the vulcanization temperature, thereby optimizing the physical and mechanical properties of vulcanized rubber. Selecting the appropriate vulcanization accelerator is crucial for improving the quality of rubber products. Common vulcanization accelerators on the market include sulfenamides, thiazoles, thiurams, etc. Each type has specific application scenarios and advantages. For example, sulfenamide vulcanization accelerators have good delayed vulcanization performance and are suitable for various rubber products; while thiazole vulcanization accelerators are known for their fast vulcanization characteristics and are suitable for high-speed vulcanization processes. However, these traditional vulcanization accelerators also have some defects, such as the possible presence of harmful substances and harsh operating conditions. For example, thiazole and sulfenamide accelerators are prone to generating nitrosamines during vulcanization, which is a strong carcinogen and poses a threat to human health. In addition, these accelerators are prone to decomposition at high temperatures, resulting in unstable properties of vulcanized rubber. Therefore, the development of new, efficient, low-toxic, and environmentally friendly composite vulcanization accelerators has become the focus of research. Summary of the Invention

[0004] In view of the above-mentioned defects of the prior art, the present invention provides a composite vulcanization accelerator, a preparation method thereof, and an application thereof in natural latex and mixed rubber to solve the problems raised in the above background art.

[0005] To achieve the above object, the present invention provides the following technical solutions: A compound vulcanization accelerator, comprising a thione derivative and a pyridine compound, and the pyridine compound is selected from any one of 3-hydroxypyridine, 2-methylpyridine, 4-ethylpyridine, 2-nitropyridine, 3-nitropyridine, 2-aminopyridine and 3-aminopyridine.

[0006] Preferably, the thione derivative is selected from any one of sodium pyrithione, 3-methylthiazoline-2-thione, N-hydroxypyridine-2-thione, 5-dodecyl dithio-3-phenyl-1,3,4-thiadiazole-2-thione, 1,3-thiazolidine-2-thione, pyrrolidine-2-thione, 5-(4-methylbenzoyl)-3,4-dihydropyrimidine-2-thione.

[0007] Preferably, the mass ratio of the thione derivative to the pyridine compound in the compound vulcanization accelerator is 1:15.

[0008] Preferably, the mass ratio of the thione derivative to the pyridine compound in the compound vulcanization accelerator is 1:3.

[0009] Preferably, the dosage of the compound vulcanization accelerator is 2.5 mmol / Kg to 7.5 mmol / Kg.

[0010] Preferably, the dosage of the compound vulcanization accelerator is 3 mmol / Kg to 6 mmol / Kg.

[0011] Preferably, the dosage of the compound vulcanization accelerator is 5.0 mmol / Kg.

[0012] Preferably, the preparation method of the compound vulcanization accelerator comprises the following steps: adding the thione derivative to the pyridine compound, and obtaining the compound vulcanization accelerator after stirring evenly.

[0013] Preferably, the compound vulcanization accelerator involved in the present application is used in promoting the vulcanization of natural latex and compounded rubber, accelerating the vulcanization speed, improving its physical and mechanical properties and antibacterial and mildew-proof applications.

[0014] Compared with the prior art, the beneficial effects of the present invention are: (1)The composite vulcanization accelerator TK-PY mainly includes thione derivatives and pyridine compounds. The thione derivatives include sodium pyrithione, and the pyridine compounds include 3-hydroxypyridine. TK-PY contains groups such as thione and pyridine, and the main groups that play a role in vulcanization promotion are the thione group and the pyridine group. In this study, TAA, ETU, and 3-Hp were introduced into concentrated latex as vulcanization accelerators. The results showed that when adding 5 mmol / Kg, although ETU and 3-Hp could also improve the film properties at the same dosage, their effects were slightly inferior compared to TK-PY, and TAA had less influence on the film properties. It indicates that the composite vulcanization accelerator TK-PY has a better effect on improving the mechanical strength of the film than ETU and 3-Hp, that is, better than the promotion effect of any single group. It shows that the thione group and the pyridine group have a superimposed effect on the vulcanization promotion of natural rubber. TK-PY can effectively enhance the tensile strength, modulus at a specified elongation, and hardness of the concentrated latex vulcanized film. The tensile strength reaches 34.18 MPa, and at the same time, it shortens the vulcanization time.

[0015] (2)As a new type of composite vulcanization accelerator, TK-PY can significantly enhance the mechanical properties of natural rubber films. The use of TK-PY is not only safe, environmentally friendly, and low-cost, but also can promote the rubber vulcanization reaction, improve the physical and mechanical properties of rubber and product quality, and has high application value.

[0016] (3)The raw rubber prepared by adding TK-PY to fresh latex through wet mixing has a relatively low number-average molecular weight and a wide molecular weight distribution; the weight-average molecular weight of the ETU sample is also relatively low, and the molecular weight distribution coefficient is relatively small; while the weight-average molecular weight of the 3-Hp sample is also relatively low, and its molecular weight distribution coefficient is the lowest. The results of infrared spectroscopy analysis show that the characteristic peaks of the five samples are basically the same, and no significant migration or intensity change is observed, indicating that the vulcanization accelerator has little effect on the main chain structure of natural rubber. The raw rubber samples added with TK-PY and 3-Hp have an increase in storage modulus (G'), indicating an increase in the rigidity of the rubber; at the same time, the loss factor (Tanδ) curve is also lower, especially the 3-Hp sample shows a lower Tanδ value. Description of the Drawings

[0017] Figure 1 It is the vulcanization curve of the concentrated latex compounded film of two preservation systems at 100 °C for 3000 s; Figure 2 It is the vulcanization curve of the compounded film after adding 5 mmol / Kg of different vulcanization accelerators at 143 °C; Figure 3 It is the influence of adding 5 mmol / Kg of different vulcanization accelerators on the structural properties of the dry film; among them, (a) is the relative molecular weight distribution diagram; (b) is the infrared curve diagram; (c) is the TG curve diagram; (d) is the DTG curve diagram; Figure 4 (a) is the radar chart of the performance indexes of the dry rubber film after adding different vulcanization accelerators; Figure 4 (b) is the radar chart of the vulcanization parameters of the compounded rubber film and the mechanical strength indexes of the vulcanized rubber film after adding different vulcanization accelerators; Figure 5 For the effects of 4 vulcanization accelerators on the (a)M of the mixed rubber L , (b)M H , (c)M H -M L and (d)T 90 vulcanization time; Figure 6 For the effects of (a) TK-PY, (b) TAA, (c) ETU, and (d) 3-HP on the vulcanization curves of the rubber mixed rubber at different addition amounts; Figure 7 For the change trend of the modulus at a specified elongation of the natural rubber vulcanizate with the increase of the dosage of four vulcanization accelerators; among them, (a) is the modulus at 100% elongation, (b) is the modulus at 300% elongation, and (c) is the modulus at 500% elongation; Figure 8 For the change trend of the physical and mechanical properties of the natural rubber vulcanizate with the change of the dosage of four vulcanization accelerators; among them, (a) tensile strength, (b) elongation at break, (c) tear strength, and (d) hardness; Figure 9 For the vulcanization curves of the mixed rubber of natural rubber raw rubber prepared with different vulcanization accelerators; Figure 10 For the molecular weight distribution maps of natural rubber raw rubber prepared with different vulcanization accelerators; Figure 11 For the infrared spectra of natural rubber raw rubber prepared by adding four vulcanization accelerators to fresh latex by wet mixing; Figure 12 For the TG and DTG spectra of the rubber raw rubber prepared by adding four different vulcanization accelerators by wet mixing; Figure 13 For the (a) G’ and (b) Tanδ curves of natural rubber raw rubber after adding four vulcanization accelerators. Specific embodiments

[0018] To enable those skilled in the art to better understand the technical content of the present invention, the technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments.

[0019] Examples 1 to 13 Prepare a composite vulcanization accelerator according to the following components and addition amounts, as shown in Table 1: Table 1 Components and addition amounts of different composite vulcanization accelerators

[0020] The preparation method of the compound vulcanization accelerator comprises the following steps: uniformly mixing a thione derivative and a pyridine compound in a certain proportion and stirring evenly to obtain the compound vulcanization accelerator.

[0021] Vulcanization effect of Experimental Example 1 on natural rubber latex 1 Experimental materials and testing methods 1.1 Experimental materials Natural fresh rubber latex NRL was collected from the experimental farm of Chinese Academy of Tropical Agricultural Sciences; natural rubber latex additives TAA (thioacetamide), ETU (2-imidazolidinethione), and 3-Hp (3-hydroxypyridine) were industrial grade, and their chemical structural formulas are shown in Table 2. They were purchased from Shandong Yousuo Chemical Technology Co., Ltd.; the compound vulcanization accelerator TK-PY (Example 1) was prepared in the laboratory. Analytical grade tetrahydrofuran and 25% ammonia water were purchased from Guangdong Xilong Chemical Co., Ltd.; vulcanization aids used in the experiment included KOH, Peregal "O", ZnO, sulfur, and accelerator ZDC.

[0022] Table 2 Chemical structural formulas of TAA, ETU, and 3-Hp

[0023] 1.2 Testing methods 1.2.1 Preparation of concentrated latex samples Take a certain amount of natural fresh rubber latex NRL and centrifuge it with a continuous centrifuge to prepare a blank ammonia-free concentrated latex (Blank). Preserve it with TK-PY, and at the same time prepare a high-ammonia concentrated latex (HA) as a control (the preparation method of HA is to add liquid ammonia to the natural fresh rubber latex after centrifugation until the ammonia content reaches 0.7 wt%), and label them as TK-PY and HA respectively. After storing for 3 months, they are ready for use. Take an appropriate amount of the blank ammonia-free concentrated latex and add drugs according to the formula in Table 3 to prepare concentrated latex samples.

[0024] Table 3 Components and dosages of vulcanization accelerators for concentrated latex

[0025] 1.2.2 Preparation of dry rubber films, compounded rubber films, and vulcanized rubber films of concentrated latex Preparation of dry rubber films of concentrated latex: Referring to ISO 498:1992, slowly inject natural fresh rubber latex NRL into the inner groove of a 3-mm glass mold (covered), and after standing for 1 min, scrape off the excess NRL with a ruler. The mold is dried at room temperature for 16 hours, and then dried in an oven at 35 °C until completely transparent; Preparation of compounded rubber film sample: Take an appropriate amount of concentrated latex sample, add vulcanization aids according to the following formula and stir slowly for 2 h, and dry at room temperature. The formula of the vulcanization aids (dry basis, parts by mass) is: 100 parts of concentrated latex, 1 part of sulfur, 0.1 part of KOH, 0.1 part of Peregal "O", 0.5 part of ZDC, and 0.4 part of ZnO. Detect the vulcanization parameters of the compounded rubber film; Preparation of vulcanized rubber film sample: First, place an appropriate amount of NRL in a clean beaker and dilute its concentration to 50% with deionized water; second, under a water bath environment at 40 °C, stir and disperse the vulcanization aids in the diluted NRL; third, heat the above mixture to 60 °C, then cool and filter to prepare pre-vulcanized latex; fourth, pour an appropriate amount of pre-vulcanized latex into a clean glass dish to level, then dry the pre-vulcanized latex at room temperature until it becomes transparent, and then soak the rubber film in deionized water for 24 h. Finally, obtain the vulcanized rubber film by heating the rubber film at 80 °C for 6 h until it becomes translucent.

[0026] 1.2.3 Determination of vulcanization rate of compounded rubber film The vulcanization rate of the compounded rubber film was measured using a high-speed MD-3000A rotorless rheometer. The thickness of the compounded rubber film was about 4 mm. The temperature for measuring the vulcanization curve of the compounded rubber film of different vulcanization accelerator systems was 100 °C, and the measuring time was 50 min.

[0027] 1.2.4 Testing of physical and mechanical properties of dry rubber film and vulcanized rubber film The determination of the tensile strength, elongation at break, modulus at a specified elongation and tear strength of the dry rubber film and the vulcanized rubber film was carried out with reference to ISO37:2017 and ISO 34-1:2015. The test samples for tensile strength were dumbbell-shaped, and five parallel specimens were prepared for each sample. The mean value was recorded after five tests. The test speed of the tensile machine was set at 500 mm / min.

[0028] 1.2.5 Infrared test of dry rubber film The test was directly carried out using a TENSOR 27 Fourier transform infrared spectrometer, and the detection range was set to 4000~370 cm –1 , and the resolution was 4 cm –1 , and the number of scans was 32 times.

[0029] 1.2.6 Thermogravimetric analysis of dry rubber film The raw rubber sample was cut into particles, 10 mg of the sample was weighed and placed in a crucible, and tested using a STA449 type thermogravimetric analyzer. The test conditions were: nitrogen was used as the external atmosphere, the set flow rate was 50 mL / min; the protective gas was high-purity nitrogen, and the flow rate was set at 25 mL / min; the test temperature range was 25~600 °C, and the heating rate was 10 K / min.

[0030] 1.2.7 Determination of Low-temperature Characteristics of Dry Rubber Film The glass transition temperature of the dry rubber film was determined by differential scanning calorimetry (DSC method). The temperature range was set from -90 to 100 °C, and the heating rate was 10 K / min.

[0031] 1.2.8 Test of Molecular Weight and Distribution of Dry Rubber Film The molecular weight and distribution of the rubber were detected by a GPC gel permeation chromatograph. 3 g of dry rubber was cut into fine strips, soaked in an appropriate amount of tetrahydrofuran for one week until completely dissolved, filtered with a needle filter, and detected by a gel permeation chromatograph at 30 °C.

[0032] 2. Test Results and Analysis 2.1 Influence of Different Preservation Systems on Vulcanization Properties of Concentrated Latex 2.1.1 Vulcanization Rate of Compound Rubber Film of Concentrated Latex with Different Preservation Systems As Figure 1 shown in and Table 4, the S' of both compound rubber film samples increased continuously with the prolongation of vulcanization time, but there were certain differences in the rising speed and amplitude. Among them, the compound rubber film sample with the TK-PY composite vulcanization accelerator system had the largest increase in S', and the maximum torque M H value and torque difference M H -M L were the highest. The torque difference M H -M L of the HA compound rubber film sample was 1.19, and the torque difference of the compound rubber film with the TK-PY composite vulcanization accelerator system was 1.68, which was 41.18% higher than that of the HA sample, and was basically consistent with the change of the vulcanization curve. It shows that compared with the HA system, the vulcanization speed of the compound rubber film with the TK-PY composite vulcanization accelerator system is faster, and the vulcanization degree is higher.

[0033] Table 4 Vulcanization Characteristics of Compound Rubber Film of Concentrated Latex with Different Preservation Systems at 100 °C

[0034] 2.1.2 Physical and Mechanical Properties of Vulcanized Rubber Film of Concentrated Latex with Different Preservation Systems As shown in Table 5, the vulcanized rubber film was prepared with pre-vulcanized latex, and the vulcanization degree reached the third-level primary level. There were significant differences in the mechanical properties of the two concentrated latex vulcanized rubber films. Among them, the hardness, modulus at a specified elongation, and tensile strength of the vulcanized rubber film of concentrated latex with the TK-PY composite vulcanization accelerator were all higher. The tensile strength reached 34.18 MPa, which was 30.26% higher than that of the vulcanized rubber film of high-ammonia concentrated latex; the modulus at 500% elongation reached 1.81 MPa, which was 32.12% higher than that of high-ammonia concentrated latex; the tear strength and elongation at break were also relatively high, and with Figure 1The changes in the medium vulcanization curves are basically the same. Through comprehensive analysis, it is found that the physical and mechanical properties of the vulcanized rubber film of the TK-PY composite vulcanization accelerator for the ammonia-free concentrated latex are the best.

[0035] Table 5 Physical and mechanical properties of the vulcanized rubber film of the concentrated latex with different preservation systems

[0036] 2.1.3 Influence of the dosage of the composite vulcanization accelerator TK-PY on the mechanical properties of the dry rubber film of the concentrated latex As shown in Table 6, the strength of the dry rubber film can, to a certain extent, characterize the gel strength of the latex and has a great influence on the production process performance of the dipped products. Through comparative analysis with the mechanical properties of the dry rubber film of the blank ammonia-free concentrated latex, it is shown that TK-PY has an obvious promoting effect on the mechanical strength of the dry rubber film of the concentrated latex. As the dosage of TK-PY increases, the tensile strength and elongation at break of the dry rubber film of the concentrated latex continuously increase, and the mechanical properties are the highest when the dosage of TK-PY is about 5 mmol / Kg. As the dosage of TK-PY continues to increase, the tear strength, modulus at a specified elongation, and hardness of the vulcanized rubber film all show a trend of first increasing and then decreasing. The analysis shows that the mechanical properties are the highest when the dosage of TK-PY is about 5 mmol / Kg.

[0037] Table 6 Influence of different dosages of the composite vulcanization accelerator TK-PY on the mechanical properties of the dry rubber film of the concentrated latex

[0038] Note: The dry rubber film was prepared after the concentrated latex added with TK-PY was stored for 3 months.

[0039] 2.2 Comparison of different vulcanization accelerators 2.2.1 Influence of different vulcanization accelerators on the physical and mechanical properties of the dry rubber film of the concentrated latex As shown in Table 7, after adding 5 mmol / Kg of TK-PY, TAA, ETU, and 3-HP, the tensile strength of the dry rubber film is improved, but usually the elongation at break, tear strength, and Mooney viscosity of the concentrated latex dry rubber film are reduced. The tensile strength of the concentrated latex dry rubber film increases, the modulus at a specified elongation and elongation at break generally decrease, and the tear strength all decreases.

[0040] TK-PY: Provided the highest tensile strength, which was 77% higher than that of the Blank control, but reduced the tear strength and Mooney viscosity. TK-PY forms cross-links in NR through covalent bonds / hydrogen bonds to form a network that enhances the tensile strength; TAA: Had a minimal impact on the physical and mechanical properties, and the results were comparable to those of the Blank control. The limited reactivity of TAA may not be able to produce sufficient cross-links; ETU: Compared with the Blank control, the tensile strength increased by 69%, but the tear strength and Mooney viscosity also decreased. ETU promotes crosslinking through an intermediate or free radical mechanism; 3-HP: It has a limited effect on tensile strength, but significantly reduces the elongation at break and tear strength. The hydroxyl and pyridine groups of 3-Hp disrupt the hydrogen bonds / molecular packing of NR through competition or electronic interactions, weaken the cohesion and reduce the tear strength; Table 7 Effects of different vulcanization accelerators on the physical and mechanical properties of concentrated latex dry rubber films

[0041] 2.2.2 Effects of different vulcanization accelerators on the vulcanization rate of compounded rubber films As Figure 2 shown, although the changes in the vulcanization curves of the 5 parts compounded rubber film samples are basically the same, there are obvious differences in the maximum torque peaks. TK-PY most significantly improves the vulcanization rate of the compounded rubber film, as can be seen from the highest peak and shorter vulcanization time of the vulcanization curve. ETU and 3-Hp also significantly promote the vulcanization process. In contrast, TAA has the least effect on the vulcanization rate and does not show an obvious enhancing effect. Table 8 shows the vulcanization characteristic parameters of the concentrated latex compounded rubber film after adding 4 vulcanization accelerators. It can be seen from Table 8 that TK-PY can significantly increase the minimum torque value M L and the maximum value M H , where the M L value continuously increases with the dosage of TK-PY, and the M H value and torque difference are the highest when the dosage of TK-PY is 5 mmol / Kg. In addition, ETU and 3-Hp can also significantly increase the M H and torque difference of the compounded rubber film at an addition amount of 5 mmol / Kg. TK-PY, ETU, and 3-Hp can all greatly reduce the vulcanization time and increase the vulcanization rate, but TAA has little effect on the vulcanization process. Among them, as the dosage of TK-PY increases, the vulcanization time first decreases and then increases, and the vulcanization time is the shortest and the vulcanization rate is the fastest when the dosage is 5 mmol / Kg; ETU and 3-Hp can also greatly reduce the vulcanization time and increase the vulcanization rate at an addition amount of 5 mmol / Kg, with a very small difference in vulcanization time from that of TK-PY at the same dosage. All 4 vulcanization accelerators can shorten the vulcanization time, but considering the torque difference analysis, TK-PY shortens the vulcanization time and increases the vulcanization rate while significantly increasing the torque difference.

[0042] Table 8 Characteristic vulcanization parameters of concentrated latex compounded rubber films with different additives

[0043] 2.2.3 Effects of different vulcanization accelerators on the physical and mechanical properties of vulcanized rubber films As can be seen from Table 9, TK-PY, ETU, and 3-Hp can all effectively improve the modulus at a given elongation, tensile strength, elongation at break, and hardness of the vulcanized rubber film. The sample with TK-PY added has the highest tensile stress, tensile strength, and hardness. Among them, the modulus at 500% elongation reaches 1.45 MPa, which is 12.4% higher than that of the Blank sample, and the tensile strength increases to 19.26 MPa, 31.38% higher than that of the Blank sample. The sample with ETU added has the highest tear strength, reaching 59.72 KN·m -1 , which is 8.9% higher than that of the Blank sample, and the modulus at a given elongation and tensile strength are also improved. 3-Hp significantly improves the tensile strength and modulus at a given elongation of the vulcanized film, and TAA has the least effect on the physical and mechanical properties of the vulcanized film.

[0044] TK-PY forms disulfide bonds by releasing active sulfur, increasing the crosslinking density and intermolecular interaction, thereby improving the properties of the rubber. This enhances the tensile strength and hardness, while maintaining the elongation at break through a uniform network distribution and reducing stress concentration. The sulfur groups further enhance the bonding through coordination and hydrogen bonding with the rubber double bonds. ETU promotes the formation of polysulfide bonds by reducing the vulcanization activation energy. Its moderate sulfur bridge length produces dense directional crosslinks, improving the tear strength and chain cohesion while maintaining the elongation. The hydroxyl group of Hp increases the tensile strength through hydrogen bonding, but it causes brittleness and non-uniform crosslinking, thus promoting crack propagation. Overall, the results show that TK-PY exhibits excellent mechanical properties in enhancing the mechanical properties of the film.

[0045] Table 9 Physical and Mechanical Properties of Concentrated Latex Vulcanized Rubber Films with Different Vulcanization Accelerators

[0046] 2.2.4 Effect of Different Vulcanization Accelerators on the Relative Molecular Mass of Concentrated Latex Dry Rubber Films The addition of vulcanization accelerators enhances the strength of the NRL film, probably due to the change in their relative molecular mass. Figure 3 Figure (a) shows that the molecular weight distribution patterns of the five NRL samples are basically the same, presenting a typical single-peak distribution. After adding the four vulcanization accelerators, the overall molecular weight distribution pattern of the rubber shifts to the left. It is worth noting that the molecular weight distribution peaks of the samples with TK-PY and ETU added increase, while the molecular weight distribution peaks of the samples with TAA and 3-Hp added shift further to the left, increasing the content of small molecules and reducing the average molecular weight. As Figure 3 shown in Figure (a) and Table 10, the number-average relative molecular masses of the dry rubber films after adding these four substances have very little difference within the normal error range. While the weight-average relative molecular masses all show a small decrease, indicating that the mechanism for improving their physical and mechanical properties is not due to an increase in the relative molecular mass. At the same time, the decrease in the distribution coefficient also indicates an increase in the content of the low-molecular-weight part.

[0047] Table 10 Characteristic relative molecular mass of dry rubber film of concentrated latex samples

[0048] 2.2.5 Infrared spectra of dry rubber films of concentrated latex treated with different vulcanization accelerators As Figure 3 shown in (b), no obvious migration of wave peaks was seen in the infrared spectra of the 5 dry rubber film samples of concentrated latex, nor was there any obvious change in the wave peak intensity. The characteristic peaks basically appeared at the same wave number, and the overall infrared curve spectra were relatively close with small differences. The wave number of the stretching vibration peak of the C=C double bond of natural rubber molecules is 1645 cm -1 , and the bending vibration peak is 840 cm -1 ; the stretching vibration wave numbers of -CH3 and -CH2 are 2959 cm -1 and 2851 cm -1 , and their bending vibration peaks appear near the wave numbers of 1443 cm -1 and 1375 cm -1 respectively. There are some slight differences in some characteristic peaks among the 5 spectra, and the results show that the incorporation of the 4 substances has the least impact on the structure of the NRL dry film, Figure 3 and (b) shows that there is no significant shift in the characteristic peaks, excluding the possibility of polymer chain length change as the main factor.

[0049] 2.2.6 Influence of different vulcanization accelerators on the thermal stability of dry rubber films of concentrated latex Figure 3 (c) and Figure 3 (d) are the TG and DTG curves of the dry rubber film samples of concentrated latex after adding 4 substances respectively. The improvement of the mechanical strength of the dry rubber film of concentrated latex is related to the change of its crosslinking density, and the increase of the crosslinking density will inevitably improve its thermal stability. Compared with the blank sample, there are some changes in the degradation curves of the dry rubber film samples after adding the four vulcanization accelerators, but they are basically the same with very small differences. The DTG degradation curve of natural rubber is mainly used to characterize its thermal stability and thermal decomposition process, showing the relationship between the sample mass loss rate and temperature, and the peaks on the curve represent the key steps in the thermal decomposition process. As can be seen from Figure 3 (d), the DTG curve of natural rubber basically shows a shoulder peak shape. The peak area is proportional to the mass lost in the corresponding temperature range and can be used to quantify the percentage of mass lost in each decomposition reaction step. Combining the analysis of Figure 3 (c) and Figure 3 (d), the initial decline rates of the samples are basically the same, but the maximum degradation rate of the TK-PY sample is higher and the secondary degradation peak rate is lower.

[0050] Table 11 shows the characteristic degradation temperatures of concentrated latex dry rubber film samples after adding different vulcanization accelerators. There are some variations in the characteristic degradation temperatures of the 5 samples. After adding four vulcanization accelerators, the initial degradation stability of the concentrated latex dry rubber film samples all increases, but there are some fluctuations in the subsequent maximum degradation stability and termination degradation stability. The starting degradation temperature of the dry rubber film added with TK-PY increases, but the T50% significantly decreases; in addition, ETU and 3-HP also increase the starting degradation temperature of the dry rubber film, and the T50% is also relatively high. The initial degradation temperature and the maximum degradation temperature of the TK-PY sample are almost the same, with a difference <1%. TK-PY (T f = 407.7 °C) is 1.7% lower than the blank, and this change falls within the experimental error range (±2%). These multi-parameter comparisons confirm that the TK-PY additive basically does not damage the thermal stability of the NRL film.

[0051] Table 11 Characteristic degradation temperatures of concentrated latex dry rubber film samples

[0052] 2.2.7 Influence of different vulcanization accelerators on the low-temperature performance of concentrated latex dry rubber film As shown in Table 12, if the vulcanization accelerator promotes the vulcanization of natural rubber to generate a cross-linked structure in the dry rubber film, the glass transition temperature of the rubber film will increase to a certain extent. Compared with the blank control sample, there is no obvious change in the glass transition temperature of the concentrated latex dry rubber film samples after adding vulcanization accelerators. In addition, the addition of ETU and 3-HP increases the thermal conductivity of the dry rubber film.

[0053] Table 12 Glass transition temperatures of concentrated latex dry rubber films with different vulcanization accelerators

[0054] 2.2.8 Influence on the physical and mechanical properties of concentrated latex rubber film Figure 4 (a) shows a radar chart comparing the performance indices of dry rubber films after incorporating various accelerators. The blank concentrated latex dry rubber film sample has the highest tear strength, 500% modulus at elongation, weight-average relative molecular mass, and distribution coefficient indices; compared with the blank dry rubber film sample, the concentrated latex dry rubber film added with Example 1 has the best tensile strength and elongation at break indices; the number-average relative molecular mass of the dry rubber film added with ETU is the highest, and the tensile strength is excellent; the dry rubber film added with 3-Hp only has a relatively high molecular weight distribution coefficient.

[0055] Figure 4 (b) gives a radar chart of the vulcanization parameters and mechanical strength indices of vulcanized rubber films with four different accelerators. The blank concentrated latex compounded rubber film and vulcanized rubber film have relatively poor indices in all aspects; the M L , M H , MH -M L , T 90 The tensile strength and the index exponents of the modulus at a given elongation are both the highest, having the highest physical and mechanical properties; adding ETU latex results in a relatively high difference in torque of the compounded film and a relatively high index exponent of T 90 The index coefficients of the tear strength and the elongation at break of the vulcanized film are the highest, indicating excellent physical and mechanical properties and reaching a relatively high level; on the contrary, the indexes of the sample with 3-HP added are relatively low, and the statistical results of the TAA sample show that the index coefficients are the lowest.

[0056] TK-PY can induce a uniform high crosslink density by gradually releasing active sulfur, resulting in the formation of extended sulfur bridges, which enhances network stability and leads to only a slight increase in the elongation at break. In contrast, ETU accelerates vulcanization, producing a dense network of short sulfur bonds, thus generating a flexible and strong network. This results in an 8.9% increase in tear strength, and its tensile enhancement is less than that of TK-PY. The selective crosslinking of 3-Hp leads to local over-densification and increases the tensile strength. However, the brittleness caused by hydrogen bonds results in a sharp 14.0% decrease in tear strength, and TAA shows no significant effect due to its low reactivity. Although the crosslink density has a positive effect on strength and hardness, the tear performance is affected by the length and uniformity of the sulfur bridges. The flexible structure of ETU is superior to the non-uniform distribution of 3-Hp, while TK-PY achieves a balance between high strength and moderate elongation. Among these four accelerators, TK-PY shows the most significant promotion of NR vulcanization, producing the highest M value of the film H , tensile strength and constant tensile stress values. Although both ETU and 3-Hp promote vulcanization, their effects are not very obvious.

[0057] Summary: This application studies the mechanism of the effect of the composite vulcanization accelerator TK-PY on the mechanical properties of natural rubber latex dry films, the vulcanization characteristics of compounded films, and the mechanical properties of vulcanized films. The results show that the composite vulcanization accelerator TK-PY can effectively promote the vulcanization process of concentrated latex compounded films, improve the degree of vulcanization, shorten the vulcanization time, and at the same time greatly improve the mechanical properties of dry films and vulcanized films. When the addition amount of TK-PY is 5 mmol / Kg, the mechanical properties of the film are the highest.

[0058] The vulcanization accelerator system TK-PY mainly includes thione derivatives and pyridine compounds. The thione derivatives include sodium pyrithione, and the pyridine compounds include 3-hydroxypyridine. TK-PY contains groups such as thione and pyridine, and the main groups that play a role in vulcanization promotion are the thione group and the pyridine group. In this study, TAA, ETU, and 3-Hp were introduced into the concentrated latex as vulcanization accelerators. The results showed that when adding 5 mmol / Kg, although ETU and 3-Hp could also improve the film properties at the same dosage, their effects were slightly inferior to those of TK-PY. It indicates that the vulcanization accelerator system TK-PY has a better effect on improving the mechanical strength of the film than ETU and 3-Hp, that is, better than the promotion effect of any single group. Therefore, it is speculated that the thione group and the pyridine group have a superimposed effect on the vulcanization promotion of natural rubber. In short, TK-PY is a new type of composite vulcanization accelerator, which not only improves the anti-corrosion efficiency of NRL, but also significantly improves the mechanical properties of the vulcanized film. The application of TK-PY has safety, ecological friendliness, and cost-effectiveness. In addition, it accelerates the rubber vulcanization process, thereby improving the physical and mechanical properties of natural rubber and enhancing the overall quality of the product. Therefore, TK-PY has great practical application potential in the rubber industry.

[0059] Vulcanization effect on the mixed rubber in Experimental Example 2 1. Experimental materials and testing methods 1.1 Experimental materials: The same as those described in 1.1 of Example 1; 1.2 Testing methods 1.2.1 Preparation of natural rubber raw rubber samples Take a certain amount of fresh natural latex in the rubber plantation and preserve it with 0.1% ammonia water. Subsequently, add the corresponding vulcanization accelerator according to the formula in Table 13, stir well and then let it stand for 24 hours. Determine the appropriate amount of acid to coagulate the fresh latex according to the calculation formula for the amount of acid required for coagulation. After standing for 1 day, perform corrugated dehydration treatment on it and completely dry it under hot air at 70 °C to prepare raw rubber samples.

[0060] Table 13 Composition and dosage of vulcanization accelerator for preparing raw rubber by wet mixing of fresh latex

[0061] Note: The vulcanization accelerator is an aqueous solution or a colloidal suspension.

[0062] 1.2.2 Preparation of mixed rubber and vulcanized rubber samples Using the ACSI formulation and mixing procedure in GB / T 15340—1994, the natural raw rubber was mixed on an open mill to obtain the mixed rubber. The vulcanization characteristics of the mixed rubber were tested using a rotorless vulcanization meter at a vulcanization temperature of 143°C and a test time of 60 min. The mixed rubber was vulcanized using a flat vulcanizer at a vulcanization temperature of 143°C and a vulcanization time of the optimum vulcanization time t90 + 5 min to obtain the vulcanized film.

[0063] 1.2.3 Preparation of test samples of mixed rubber According to the pure rubber formulation, 3.9 kg of mixed rubber was prepared and evenly divided into 13 portions, each portion being 300 g. Subsequently, vulcanization accelerators were added according to the formulation in Table 14 and thoroughly mixed. After standing for 12 h, the vulcanization characteristics of the mixed rubber were detected, and vulcanized rubber films were prepared to evaluate their physical and mechanical properties.

[0064] Table 14 Types and dosages of vulcanization accelerators for mixed rubber

[0065] 1.2.4 Detection of raw rubber Initial plasticity value (P0) and plasticity retention index (PRI): Measured using a rapid plasticity meter according to GB / T 3517—2014 and GB / T 3510—2006; Mooney viscosity ML(1+4) 100°C: Tested using a Mooney viscometer according to GB / T 1232.1—2016; nitrogen content: Measured using a Kjeldahl nitrogen analyzer according to GB / T 8088—2008. Direct testing was carried out using a TENSOR 27 Fourier transform infrared spectrometer, with the detection range set to 4000~370 cm –1 , with a resolution of 4 cm –1 , and the number of scans being 32 times. The raw rubber sample was cut into particles, 10 mg of the sample was weighed and placed in a crucible, and tested using a STA449 type thermogravimetric analyzer. The test conditions were: nitrogen as the external atmosphere, with a set flow rate of 50 mL / min; the protective gas was high-purity nitrogen, with a flow rate set to 25 mL / min; the test temperature range was 25~600°C, and the heating rate was 10 K / min. The glass transition temperature of the dry rubber film was determined using differential scanning calorimetry (DSC method), with the temperature segment set to -90~100°C and the heating rate being 10 K / min. The molecular weight and distribution of the rubber were detected using a GPC gel permeation chromatograph. 3 g of dry rubber was cut into fine strips, soaked in an appropriate amount of tetrahydrofuran for one week until dissolved, filtered through a syringe filter, and detected using a gel permeation chromatograph at 30°C.

[0066] 1.2.5 Determination of mixed rubber and vulcanized rubber The vulcanization rate of the mixed rubber was determined using a high-speed railway MD-3000A rotorless rheometer at a temperature of 143 °C for 50 min. The tensile strength, elongation at break, and modulus at a specified elongation of the vulcanized rubber film were tested using an electronic universal testing machine. The determination of the tensile strength, elongation at break, and modulus at a specified elongation referred to GB / T 528—2009, and the determination of the tear strength referred to GB / T 528—2008. Dumbbell-shaped and right-angled specimens required for testing were prepared according to ISO 527, and then the stress-strain curve and related values of the samples were tested using an electronic universal testing machine.

[0067] 2. Results and Analysis 2.1 Effects of Different Vulcanization Accelerators on the Vulcanization Characteristics and Physical and Mechanical Properties of Mixed Rubber 2.1.1 Effects of Different Dosages of Vulcanization Accelerators on the Vulcanization Characteristics of Mixed Rubber As Figure 5 and Figure 6 show the change trends of the vulcanization characteristics of the mixed rubber in the pure rubber formula with the dosages of 4 vulcanization accelerators. The minimum torque value M L reflects the viscosity or fluidity of the mixed rubber in the initial stage of vulcanization. The lower the M L value, the better its processability. The increase in the M H value will lead to an increase in the torque difference (M H -M L ). The torque difference directly affects the physical and mechanical properties such as the hardness and elasticity of the vulcanized rubber and is an important indicator for evaluating the degree of hardening in the vulcanization process of the mixed rubber. A larger torque difference usually indicates good vulcanization responsiveness of the mixed rubber. The vulcanization time can be used to judge the accelerating or delaying effect of additives on the vulcanization process.

[0068] As Figure 5 shown in (a), with the increase in the dosages of the four vulcanization accelerators, there is a trend of first decreasing, then increasing, and then decreasing to varying degrees. The four vulcanization accelerators can all reduce the M L value of the mixed rubber, indicating that adding the four vulcanization accelerators can optimize its processing performance. Among them, at the same additive content, M L are all TAA>3-HP>TK-PY>ETU, indicating that under the same conditions, the processing performance of TAA is the worst and that of ETU is the best. As L shown in (b) and (c), M Figure 5 has the same trend as M H and M H -M L , that is, with the increase in the contents of the four vulcanization accelerators, both M H and M H -M L show an upward trend, and at the same additive content, the M H of TAA and M H -ML The lowest is TK-PY and the highest is TK-PY, indicating that TK-PY has the best vulcanization responsiveness and can endow the material with more excellent properties. Figure 5 (d) indicates that TK-PY can significantly reduce the vulcanization time of the material and promote vulcanization.

[0069] Figure 6 As shown, the improvement effect of TK-PY on the vulcanization curve is the most significant. Its vulcanization degree increases at the fastest speed and the torque value reaches the highest level. At the same time, as the dosage of TK-PY increases, the torque value increases accordingly, and the vulcanization time is also significantly shortened, but the phenomenon of vulcanization reversion appears. Secondly, the promoting effect of 3-Hp on vulcanization is also obvious. The vulcanization speed is relatively fast and the torque value is also relatively high. Compared with TK-PY, the phenomenon of vulcanization reversion is less. In contrast, the promoting effect of ETU on the vulcanization of natural rubber is weak, and it may even reduce the vulcanization speed when the dosage is low. Finally, TAA has no promoting effect on the vulcanization of the mixed rubber, but instead reduces the vulcanization degree to a certain extent. Among the four vulcanization accelerators, only the vulcanization time of the mixed rubber with TK-PY shortens as the dosage of TK-PY increases, significantly accelerating the vulcanization process. The other three vulcanization accelerators have little effect on the vulcanization time.

[0070] 2.1.2 Influence of the addition amount of different vulcanization accelerators on the vulcanization curve of the mixed rubber The modulus at a given elongation of vulcanized rubber is closely related to its crosslink density. From Figure 7 it can be seen that as the dosage of the vulcanization accelerator increases, except for TAA, the other three vulcanization accelerators all improve the modulus at a given elongation of natural rubber vulcanizate to a certain extent. Among them, the modulus at a given elongation of the vulcanized rubber film in the TK-PY group and the 3-Hp group continuously increases as the dosage of the vulcanization accelerator increases, showing a similar change law, but the increasing speed and amplitude of the TK-PY group are more significant; while the modulus at a given elongation of the vulcanized rubber in the ETU group shows a trend of first increasing and then decreasing; after adding TAA, the modulus at a given elongation of natural rubber vulcanizate hardly changes. This shows that the improvement of the modulus at a given elongation mainly depends on the pyridine group, and other functional groups play an important auxiliary and synergistic role.

[0071] The physical and mechanical properties mainly include tensile strength, elongation at break, tear strength and hardness. According to Figure 8As shown in the figure, with the increase of the dosage of TK-PY, the tensile strength of natural rubber vulcanizate increases significantly; at lower dosages, 3-Hp also has a certain promoting effect on the tensile strength, but when the dosage exceeds 5 mmol / Kg, the tensile strength no longer increases; while TAA and ETU have almost no significant effect on the tensile strength. The elongation at break of the rubber vulcanizate first increases and then decreases with the increase of the dosage of TK-PY; the effect of TAA on it is minimal, and there is a slight increase at higher dosages; with the increase of the dosage of 3-Hp, the elongation at break continuously decreases, while increasing the dosage of ETU shows a trend of first decreasing and then increasing. The tear strength of natural rubber vulcanizate continuously increases with the increase of the dosage of TK-PY, with the largest increase; while when the dosage of 3-Hp increases to a certain extent, it tends to be stable; increasing the dosage of ETU shows a trend of first increasing and then decreasing, and the effect of TAA is smaller. The hardness of the vulcanizate continuously increases with the increase of the dosages of TK-PY and 3-Hp, while increasing the dosage of ETU shows a trend of first increasing and then decreasing; the effect of TAA on the hardness is also weak. It is analyzed and inferred that the improvement of the physical and mechanical properties of the material after adding TK-PY mainly depends on the pyridine group contained in TK-PY.

[0072] 2.1.3 Results Analysis of Different Vulcanization Accelerators on Vulcanization Characteristics and Physical and Mechanical Properties As shown in Table 15: The performance differences of the four vulcanization accelerators (TK-PY, 3-HP, ETU, TAA) mainly stem from the influence of their chemical structures on the vulcanization reaction and crosslinking network.

[0073] Table 15 Comprehensive Correlation Table

[0074] TK-PY: The synergistic effect of the thione group and the pyridine group quickly releases sulfur free radicals and coordinates Zn ions, forming a high crosslinking density (M H -M L highest) and the shortest vulcanization time (T 90 shortest), significantly improving the modulus at a specified elongation, tensile strength and tear strength. However, the high proportion of polysulfide bonds (-S X -) leads to easy fracture at high temperatures and poor stability.

[0075] 3-HP: The pyridine group coordinates to promote crosslinking, and the hydroxyl hydrogen bond enhances the network uniformity, but the release of sulfur free radicals is insufficient, and the crosslinking density is the second highest (M H -M L higher), and the vulcanization speed is slower (T 90 longer). The increase of short sulfur bonds (-S- / -S-S-) increases brittleness and the tear strength is lower.

[0076] ETU: The thione group rapidly releases sulfur free radicals, with the lowest initial viscosity (M L lowest), but lacks coordination groups and has a low crosslink density (M H -M L medium). Short sulfur bonds dominate the network, with medium tensile strength but strong energy dissipation ability under dynamic loading (higher tear strength).

[0077] TAA: The thioamide group has insufficient activity, the vulcanization reaction is incomplete, and the crosslink density is the lowest (M H -M L lowest), the vulcanization speed is the slowest (T 90 extended), and the performance is close to that of unvulcanized rubber.

[0078] 2.2 Properties of Raw Natural Rubber Prepared by Wet Mixing with Four Kinds of Vulcanization Accelerators 2.2.1 Conventional Indexes of Raw Natural Rubber Fresh latex is prepared into raw natural rubber by wet mixing with four additives. Mooney viscosity is an important parameter to evaluate the processing fluidity of rubber and reflects the shear characteristics of rubber during mixing. As shown in Table 16, the Mooney viscosity of the raw rubber sample added with ETU is the highest, while the Mooney viscosity decreases after adding TAA, 3-HP, and TK-PY. Plasticity can measure the plasticity of rubber, and the plasticity retention rate is an important index to evaluate the stability of rubber. According to the data in Table 16, the initial plasticity value of the raw rubber changes slightly after adding the four vulcanization accelerators. Except for a slight increase in TK-PY, the other three all decrease to a certain extent. All four vulcanization accelerators reduce the plasticity retention rate of the raw rubber, among which the reduction amplitudes of ETU and 3-HP are larger, while the reduction amplitudes of TK-PY and TAA are relatively smaller. Finally, the change in nitrogen content may be due to the nitrogen element contained in the additive itself or they inhibit the microbial degradation reaction, thus retaining a higher protein content. Specifically, the nitrogen content of the raw rubber sample added with TK-PY is the highest, which may be related to the good antibacterial and mildew-proof effect of TK-PY.

[0079] Table 16 Conventional Indexes of Raw Rubber Prepared by Wet Mixing Fresh Latex with Four Vulcanization Accelerators

[0080] 2.2.2 Influence of Raw Rubber Prepared with Different Vulcanization Accelerators on Vulcanization Characteristics of Mixed Rubber Figure 9 Table 17 shows the vulcanization characteristics of the pure rubber formula of the mixed rubber prepared by wet mixing fresh latex with four vulcanization accelerators to prepare raw natural rubber. As Figure 9 shown in Table 17, all four vulcanization accelerators have a promoting effect on the minimum viscosity (M L ) of the mixed rubber. Among them, ETU and TK-PY have an effect on M of the mixed rubberL The increase is the largest. The maximum viscosity (M H ), and the minimum viscosity difference (M H -M L ) are highly correlated. The compounded rubber with 3-HP added performs best in terms of M H and M H -M L values, followed by TK-PY, while ETU has a relatively small enhancing effect on M H and M H -M L values, and TAA causes a decrease in M H and M H -M L values. In terms of vulcanization time, the 3-HP sample shows the shortest vulcanization time and the fastest vulcanization speed in all vulcanization stages; TK-PY is the second; ETU and TAA also accelerate the vulcanization process, but the effect is relatively weak.

[0081] Table 17 Influence of Different Vulcanization Accelerators on the Vulcanization Characteristics of Compounded Rubber Prepared from Fresh Latex

[0082] 2.2.3 Influence of Raw Rubber Prepared with Different Vulcanization Accelerators on the Physical and Mechanical Properties of Vulcanized Rubber Table 18 shows the test results of the physical and mechanical properties of natural rubber vulcanizates prepared by wet mixing of fresh latex with 4 additives. For vulcanized rubber, its physical and mechanical properties are important indicators for evaluating the final quality of the product. The following table lists several key performance indicators including constant tensile stress, tensile strength, elongation at break, tear strength, and hardness. As can be seen from Table 18, at elongation rates of 100%, 300%, and 500%, the samples with TK-PY and 3-HP showed higher tensile stress. Especially for 3-HP, it reached the highest value at each definite elongation stress. This indicates that these two vulcanization accelerators can more effectively improve the load-bearing capacity of the material during the stress-deformation process. In contrast, the definite elongation stress of the sample with TAA decreased instead. The samples with TK-PY and 3-HP also showed relatively high tensile strength, reaching 25.90 MPa and 26.49 MPa respectively, while the tensile strength of the sample with TAA was 18.47 MPa, even lower than that of the blank control group. The elongation at break of the samples with the four vulcanization accelerators all maintained at a similar high level, indicating their good elastic recovery characteristics. Among them, the sample with ETU had the highest elongation at break, reaching 902.0%, but the difference from other samples was not significant. The sample with 3-HP had the highest tear strength, reaching 27.9 KN / m, followed by the TK-PY sample, reaching 26.4 KN / m, which was also at a relatively high level. The sample with ETU also performed well, while the tear strength of the TAA sample was relatively low. The samples with TK-PY and 3-HP had higher hardness, while the hardness of the samples with ETU and TAA changed less.

[0083] Combining the above vulcanization characteristics and physical and mechanical properties, the reasons for the above are analyzed as follows: TK-PY shows better performance in improving mechanical properties, proving that the pyridine group has a role in promoting vulcanization. TK-PY forms crosslinks in NR through covalent / hydrogen bonds, forming a network that enhances tensile strength. TK-PY can enhance rubber properties by releasing reactive sulfur to form disulfide bonds, increasing the crosslink density and intermolecular interaction. This not only improves the tensile strength / hardness but also reduces stress concentration through a uniform network distribution, thus maintaining the elongation at break. The sulfur group further strengthens the binding through coordination and hydrogen bonding with the rubber double bond.

[0084] The hydroxyl group of 3-Hp enhances the tensile strength through hydrogen bonds but causes brittleness and uneven crosslinking, promoting crack propagation. The structural stability of TAA hinders the formation of effective sulfur bonds, resulting in low participation in vulcanization. Compared with the strength-brittleness trade-off of 3-Hp and the inertness of TAA, ETU and TK-PY show better effects in network optimization.

[0085] ETU promotes the formation of polysulfide bonds by reducing the vulcanization activation energy, thus playing an efficient accelerating role. Its moderate sulfur bridge length forms a dense and oriented crosslinking, enhancing the tear strength and intermolecular cohesion while maintaining the elongation at break. Although ETU cures faster than the TK-PY ratio, its limited sulfur extension restricts the improvement of tensile stress.

[0086] TAA has the worst mechanical properties, corresponding to the previous lower vulcanization performance and lower hardness. The reason may be that the degree of crosslinking is relatively low, resulting in fewer crosslinked networks formed. The limited reactivity of TAA may not be able to produce sufficient crosslinks.

[0087] Table 18 Influence of different vulcanization accelerators on the mechanical properties of vulcanized natural rubber prepared from fresh latex

[0088] 2.2.4 Influence of different vulcanization accelerators on the relative molecular mass and its distribution of natural rubber raw rubber Molecular weight has an important influence on the processing properties of rubber materials and the mechanical properties of the final products. Figure 10 Figures and Table 19 respectively show the relative molecular mass distribution diagrams and molecular weight detection results of natural rubber raw rubber prepared by wet mixing with the addition of four vulcanization accelerators. As Figure 10 shown, the relative molecular mass distribution of natural rubber raw rubber after treatment with vulcanization accelerators shows little difference compared with the control group, and all show a similar trend, presenting the characteristics of bimodal distribution. Among them, the sample with the addition of 3-HP shows obvious differences in the distribution diagram, with a higher content in the low molecular weight part and a lower content in the high molecular weight part. In contrast, the sample with the addition of TK-PY has little difference in the low molecular weight distribution compared with the 3-HP sample, but its high molecular weight part also has a lower content.

[0089] The weight-average molecular weight of natural rubber raw rubber prepared by wet mixing with the addition of 4 vulcanization accelerators is lower than that of the control group. Among them, the decrease of TK-PY is not obvious, and the decrease trend of 3-HP is the most obvious. The addition of TAA makes the number-average molecular weight increase to a certain extent, but the number-average molecular weight decreases after the addition of ETU and TK-PY, with the largest decrease of TK-PY, while the number-average molecular weight of the sample with the addition of 3-HP is the same as that of the control group. The distribution coefficient increases after the addition of TK-PY, but decreases after the addition of the other three vulcanization accelerators, with the most obvious decrease of 3-HP. The results show that after the addition of TK-PY, the content of small molecular weight components increases, the molecular weight of large molecular weight components becomes larger, and at the same time the molecular weight distribution becomes wider, and the difference in the molecular weight of rubber molecular chains becomes larger; after the addition of 3-HP, the small molecular weight components become more and the molecular weight is smaller, and the number of large molecules is small and the molecular weight is low, and the molecular weight distribution is the most compact, all of which are Figure 9 consistent with.

[0090] Table 19 Effects of Different Vulcanization Accelerators on the Relative Molecular Mass and Distribution Coefficient of Raw Rubber Prepared from Fresh Latex

[0091] 2.2.5 Effects of Different Vulcanization Accelerators on the Infrared Spectra of Prepared Natural Rubber Figure 11 The infrared spectra of raw natural rubber prepared by wet mixing of fresh latex with 4 additives are shown as follows. As Figure 11 shown, no obvious migration of peaks was observed in the infrared spectra of 5 raw natural rubber samples, nor was there any obvious change in the peak intensity. The characteristic peaks basically appeared at the same wave numbers, and the overall infrared curve patterns were relatively close with small differences. The wave number of the stretching vibration peak of the C=C double bond in natural rubber molecules is 1645 cm -1 , and the bending vibration peak is 840 cm -1 ; the stretching vibration wave numbers of -CH3 and -CH2 are 2959 cm -1 and 2851 cm -1 , and their bending vibration peaks appear near the wave numbers of 1443 cm -1 and 1375 cm -1 respectively. There are some slight differences in some characteristic peaks among the 6 spectra, indicating that the addition of 4 vulcanization accelerators has little effect on the structure of natural rubber, and it is difficult to judge their effects on the properties of dry rubber films.

[0092] 2.2.6 Effects of Different Vulcanization Accelerators on the Thermal Stability of Prepared Natural Rubber Raw Rubber Figure 12 Figures and Table 20 show the TG and DTG spectra and their characteristic temperatures of raw natural rubber prepared by adding four different vulcanization accelerators during wet mixing. By analyzing Figure 12 and Table 20, in terms of the initial decomposition temperature (T0), the samples added with TK-PY and 3-Hp showed slightly higher initial decomposition temperatures, indicating that these two additives may help improve the initial thermal stability of natural rubber. In contrast, the initial decomposition temperatures of the samples using TAA and ETU were similar to those of the sample without any additive (Blank), meaning that their effects on improving the initial thermal stability of natural rubber were not obvious. For the 50% weight loss temperature (T 50% ), among all the vulcanization accelerators, 3-Hp made natural rubber have the highest T 50% , that is, 386.28 °C, showing its significant positive effect on the thermal stability of rubber, while TK-PY decreased the T 50% of natural rubber, proving that its mid-term thermal decomposition rate is faster, and the other additives had little effect on T 50%has less influence. The peak temperature (Tp) is a measure of the temperature at which the most active decomposition stage occurs during the heating of the material. It can be seen from the data that the sample with TK-PY has the highest T p , reaching 382.00 °C, followed by 3-HP, which once again proves their importance in improving the thermal stability of natural rubber. The sample with TAA has the lowest T p , and there is little difference between ETU and the blank control group. Finally, considering the final decomposition temperature (T f ), the samples of TAA and ETU are even slightly higher than the blank sample, the difference between 3-HP and the blank control group is not significant, while TK-PY is lower than the control group.

[0093] Based on the above phenomena, it can be seen that all vulcanization accelerators can increase the initial decomposition temperature to a certain extent. However, overall, the thermal stability of TK-PY is poor, while the other vulcanization accelerators have slightly better thermal stability than the control group. The reasons are as follows: TK-PY: The pyridine group improves the initial stability, but the degradation effect and uneven crosslinking lead to a decrease in thermal stability in the middle and late stages. 3-HP: Hydrogen bonds and pyridine groups synergistically enhance the molecular chain stability, and the thermal stability is optimal in the middle stage. TAA: The low reactivity results in no significant improvement in thermal stability, and the residue only delays the final decomposition. ETU may be prone to premature vulcanization with double bonds in the rubber at high temperatures to form short sulfur bonds, and the formed short sulfur bonds slightly enhance its thermal stability.

[0094] Table 20 Characteristic temperatures of raw natural rubber prepared with four different vulcanization accelerators

[0095] 2.2.7 Influence of different vulcanization accelerators on the RPA data of raw natural rubber prepared The changes in the G’ (storage modulus) and Tanδ (loss factor) curves are very important for understanding the dynamic mechanical properties of rubber materials. They reflect the elastic behavior and energy dissipation characteristics of the material at different frequencies or temperatures. From Figure 13 (a) and Figure 13As can be seen from (b), the addition of TK-PY, 3-HP, and ETU may lead to an increase in the G’ value throughout the test range, with 3-HP being the most obvious and TK-PY being the second most obvious. This indicates an increase in the rigidity of the rubber, probably because TK-PY and 3-HP promote the formation of crosslinking points, thereby enhancing the network structure. At the same time, the change in the Tanδ curve reflects the change in the viscoelasticity of the material; if the peak shifts to a higher temperature or frequency, it indicates an increase in the glass transition temperature (Tg) of the material, meaning a decrease in the flexibility of the material and an increase in the thermal stability. TAA has little effect on the G’ and Tanδ of the raw rubber sample. After adding ETU, the G’ curve increases, and at the same time, Tanδ decreases, with little difference from the change curve of TK-PY. For the sample with 3-HP added, the G’ curve is the highest and Tanδ is the lowest.

[0096] The reasons are analyzed as follows: (1) TK-PY: ① Thione group promotes crosslinking: The thione group (R-C=S) releases active sulfur radicals, accelerating crosslinking and increasing the crosslinking density (G’↑). ② Coordination of pyridine group: Pyridine coordinates with Zn 2+ to enhance the rigidity of the crosslinking network, but the crosslinking may be locally uneven, resulting in incomplete restriction of molecular chain movement (limited decrease in Tanδ). ③ Degradation side reaction: The thione group may cause partial chain scission during vulcanization, forming microdefects and slightly increasing energy dissipation.

[0097] (2) ETU (ethylene thiourea): ① Rapid crosslinking to form short sulfur bonds: ETU efficiently promotes sulfur crosslinking, but mainly generates monosulfide bonds (-S-) or disulfide bonds (-S-S-), resulting in a relatively brittle crosslinking network. ② Bond recombination under dynamic loading: The short sulfur bonds are easily broken and reconnected during deformation, generating a certain amount of viscous dissipation (the decrease amplitude of Tanδ is not as large as that of 3-HP). ③ Trade-off between crosslinking density and uniformity: Although the crosslinking density is high, the network uniformity is poor, limiting the optimization of elastic response.

[0098] (3) TAA (thioacetamide): ① Low reactivity limits crosslinking: The thioamide group of TAA has low activity and cannot effectively participate in the vulcanization reaction, having no significant effect on the crosslinking network. ② Molecular chain movement is not restricted: No additional crosslinking points are formed, and the rubber maintains its original viscoelastic behavior (G’≈blank, tanδ≈blank).

[0099] (4) 3-HP (3-hydroxypyridine): ① Hydrogen bonds enhance intermolecular interactions: The hydroxyl group (-OH) forms reversible hydrogen bonds with rubber molecular chains, increasing the intermolecular binding force and the crosslinking point density, thereby significantly enhancing the rigidity (G’↑). ② Coordination stabilization of pyridine group: The pyridine ring coordinates with Zn 2+ such as Zn(C5H5N) 2+), optimize the crosslinking network uniformity, reduce the local movement of molecular chains, and lower the energy dissipation (Tanδ↓). ③ Synergistic effect: Hydrogen bonds and coordination interactions jointly form a uniform and stable crosslinking network, which not only enhances the elastic energy storage capacity but also inhibits viscous dissipation.

[0100] Summary: Through systematic experiments, analyze the specific performances and mechanisms of different vulcanization accelerators (TK-PY, TAA, ETU, and 3-Hp) during the preparation of natural rubber compound and vulcanizate. The research results show that TK-PY can effectively shorten the vulcanization time and exhibits strong vulcanization acceleration ability; in contrast, TAA has little positive effect on vulcanization and may even inhibit the vulcanization process; while ETU and 3-Hp show different degrees of vulcanization acceleration effects. TK-PY and 3-Hp have significant promoting effects on the modulus at a specified elongation, tensile strength, tear strength, and hardness of the vulcanizate. Among them, the TK-PY sample shows a faster vulcanization speed and a higher degree of vulcanization at higher dosages; while the promoting effect of TAA on the properties of the vulcanizate is almost negligible, and the effect of ETU is also relatively limited. The raw rubber prepared by adding TK-PY to fresh latex through wet mixing has a lower number-average molecular weight and a wider molecular weight distribution; the weight-average molecular weight of the ETU sample is also lower, and the molecular weight distribution coefficient is relatively small; while the weight-average molecular weight of the 3-Hp sample is also lower, and its molecular weight distribution coefficient is the lowest. The results of infrared spectroscopy analysis show that the characteristic peaks of the five samples are basically the same, and no significant migration or intensity change is observed, indicating that the vulcanization accelerator has little effect on the main chain structure of natural rubber. The raw rubber samples added with TK-PY and 3-Hp have an increase in the storage modulus (G'), indicating an increase in the rigidity of the rubber; at the same time, the loss factor (Tanδ) curve is also lower, especially the 3-Hp sample shows a lower Tanδ value.

[0101] In summary, TK-PY can not only significantly accelerate the vulcanization speed of natural rubber, improve the degree of vulcanization, but also significantly enhance the physical and mechanical properties of vulcanized natural rubber.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A compound vulcanization accelerator TK-PY, characterized in that, It includes a thione derivative and a pyridine compound, and the pyridine compound is selected from any one of 3-hydroxypyridine, 2-methylpyridine, 4-ethylpyridine, 2-nitropyridine, 3-nitropyridine, 2-aminopyridine, and 3-aminopyridine.

2. The compound vulcanization accelerator according to claim 1, characterized in that, The thione derivative is selected from any one of sodium pyrithione, 3-methylthiazoline-2-thione, N-hydroxypyridine-2-thione, 5-dodecyl dithio-3-phenyl-1,3,4-thiadiazole-2-thione, 1,3-thiazolidine-2-thione, pyrrolidine-2-thione, 5-(4-methylbenzoyl)-3,4-dihydropyrimidine-2-thione.

3. The composite vulcanization accelerator according to claim 1, characterized in that, The mass ratio of the thione derivative to the pyridine compound in the composite vulcanization accelerator is 1:(1 - 15).

4. The compound vulcanization accelerator according to claim 3, characterized in that, The mass ratio of the thione derivative to the pyridine compound in the composite vulcanization accelerator is 1:

3.

5. The compound vulcanization accelerator according to claim 1, characterized in that, The dosage of the composite vulcanization accelerator is 2.5 mmol / Kg - 7.5 mmol / Kg.

6. The compound vulcanization accelerator according to claim 5, characterized in that, The dosage of the composite vulcanization accelerator is 3 mmol / Kg - 6 mmol / Kg.

7. The compound vulcanization accelerator according to claim 6, characterized in that, The dosage of the composite vulcanization accelerator is 5 mmol / Kg.

8. The preparation method of the composite vulcanization accelerator according to claim 1, characterized in that, It includes the following steps: Add the thione derivative to the pyridine compound, and the composite vulcanization accelerator is obtained after stirring evenly.

9. The application of the composite vulcanization accelerator according to any one of claims 1 - 7 or the composite vulcanization accelerator prepared by the preparation method according to claim 8 in promoting the vulcanization of natural latex, accelerating the vulcanization speed, improving its physical and mechanical properties, and antibacterial and mildew-proof.

10. The application of the composite vulcanization accelerator according to any one of claims 1 - 7 or the composite vulcanization accelerator prepared by the preparation method according to claim 8 in promoting the vulcanization of the mixed rubber, accelerating the vulcanization speed, improving its physical and mechanical properties, and antibacterial and mildew-proof.

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