A method for determining tire damage due to insufficient sulfur or thermal degradation

Abstract

The present invention belongs to the field of tire production and testing, and relates to a method for determining whether a tire is damaged by undersulfurization or thermal degradation. The method comprises the following steps: obtaining a rubber sample, freezing it, crushing and grinding it to obtain a uniform sample; extracting a portion of the sample using an extracting solution, testing the unextracted sample for sulfur content to obtain the total sulfur content, and testing the extracted sample for sulfur content to obtain the bound sulfur content; and determining the cause of stickiness in returned tires based on the test results and the formula (bound sulfur percentage = each bound sulfur content / (bound sulfur content of normally vulcanized new rubber) * 100%): when the bound sulfur percentage of the returned tire is greater than or equal to 95%, it is considered normally vulcanized thermally degraded rubber; and when the bound sulfur percentage of the returned tire is less than 95%, it is considered undersulfurized thermally degraded rubber. The method is simple and easy to operate, and provides visual determination data. By combining the sulfur content test data, it distinguishes and determines between undersulfurized thermally degraded rubber and normally vulcanized thermally degraded rubber, providing strong evidence support for determining the cause of stickiness in returned tires on the market.

Description

Technical Field

[0001] The present invention belongs to the field of tire production and testing, and in particular relates to a method for determining whether a tire is damaged by undersulfurization or thermal degradation, and is applied to the identification or determination of the cause of damage to tires returned to the market. Background Art

[0002] Due to harsh operating conditions and environments, heavy loads, and high speeds, tires can experience various damage symptoms, such as bulging, delamination, chipping, and zipper bursts. One common symptom of damage is the stickiness of the rubber compound inside the tire, which is the stickiness of the rubber compound at the damaged site. There are generally two reasons for the stickiness of the rubber compound at the damaged site of the tire. One is that the normally vulcanized rubber undergoes thermal degradation due to harsh operating conditions or environments, causing damage and resulting in stickiness. In other words, the rubber compound is a normally vulcanized-thermally degraded rubber, and the stickiness of the rubber compound is caused by customer use. The other is that the tire rubber compound undergoes thermal degradation due to insufficient vulcanization during production (called under-vulcanized rubber), causing damage and resulting in stickiness. In other words, the rubber compound is an under-vulcanized-thermally degraded rubber, and this type of damage is the responsibility of the tire manufacturer. It can be seen from this that these two types of damage directly determine who is responsible for the tire damage: the tire manufacturer or the actual user of the tire.

[0003] The industry currently generally uses empirical judgment to determine the causes of the above two types of damage. This is based on whether the damaged area is located in the upper or lower mold of the vulcanizer where the tire is vulcanized, the difference in hardness of the rubber around the damaged area compared to the normal area, whether the damage occurs all around or locally, and whether the tire is damaged in the early, middle, or late stages of use. These methods require the technicians to have extensive experience in formulas, production processes, and markets, and lack direct visual data for judgment.

[0004] In view of the above situation, there is an urgent need to develop a scientific judgment method with visual data to provide technical support for technicians. Summary of the Invention

[0005] In order to accurately determine whether tire damage is caused by thermal degradation due to insufficient sulfurization of the rubber or thermal degradation of normal vulcanized rubber caused by heat accumulation during use, that is, to accurately determine and distinguish between insufficient sulfurization-thermally degraded rubber and normal vulcanization-thermally degraded rubber, the present invention proposes a method for determining whether a tire is damaged by insufficient sulfurization or thermal degradation. The method is simple and easy to operate, and has visual determination data. By combining sulfur content test data, it can distinguish and determine insufficient sulfurization-thermally degraded rubber and normal rubber-thermally degraded rubber, providing strong evidence support for the determination of the cause of tire stickiness when the market returns.

[0006] The technical solution of the present invention is:

[0007] A method for determining whether a tire is damaged by undersulfurization or thermal degradation comprises the following steps:

[0008] (1) Obtain a rubber sample, freeze it in liquid nitrogen for 8 to 15 minutes, and then grind it to obtain a uniform sample; wherein the rubber sample includes normally vulcanized new rubber, normally vulcanized thermally degraded rubber, under-vulcanized new rubber, under-vulcanized thermally degraded rubber, and tire rubber to be determined;

[0009] (2) Divide the sample into two parts of equal mass. Extract one part of the sample with the extraction solution. After the extraction is completed, take out the sample and dry it;

[0010] The extracting liquid is a mixture of ethyl acetate and cyclohexane, and the volume ratio of ethyl acetate to cyclohexane is (20-40): (80-60). The polarity of cyclohexane is similar to that of the polycyclic sulfur compounds and the S-accelerator intermediates, which is more conducive to the extraction of free sulfur and unreacted S-accelerator intermediates in the rubber compound.

[0011] The volume ratio of ethyl acetate to cyclohexane is any ratio within the range of (20-40):(80-60), and the sum of their volume ratios is 100, for example, it can be 20:80, 40:60, 30:70, 25:75, 35:65, 22:78, 27:73, 33:67 or 38:62, etc., or it can be any other ratio that meets the conditions within the range.

[0012] (3) The result of the sulfur content test on the unextracted sample is the total sulfur content, that is, the total proportion of all sulfur elements in the rubber compound; the result of the sulfur content test on the extracted sample is the bound sulfur content, that is, the content of sulfur elements that participate in the cross-linking reaction and are bound to the macromolecular chain;

[0013] Under-vulcanized-bound sulfur content of new rubber: The bound sulfur content of under-vulcanized-bound new rubber is less than the bound sulfur content of normally vulcanized-bound new rubber. The bound sulfur content of normally vulcanized-bound new rubber is a, and the total sulfur content is e. The bound sulfur content of under-vulcanized-bound new rubber is b, and the total sulfur content is f. Then a*95%≥b, (the instrument test error is within 2.0%, and the rubber compound mixing production process control error is within 3%), e≈f, ("≈" represents the difference within 5%, the same below). The sulfur content of different formulas is different, and the absolute value of the bound sulfur content at the foaming point will be different. When the tire is under-vulcanized, it is generally concerned with whether the rubber compound has exceeded the foaming point during demolding. According to statistical data, the bound sulfur content of the rubber compound at the foaming point is ≤85%*a;

[0014] The bound sulfur content of normally vulcanized and thermally degraded rubber is c, and the total sulfur content is d, then c≈a, d≈e. This is because the thermal degradation of the vulcanized rubber starts from the weak bond of the molecular chain, that is, the breakage starts from the polysulfide bond -C-Sx-C-. However, when the SS at the polysulfide bond breaks to form free radicals, the free radicals will transfer. The free radicals will further trigger the C=C double bond reaction on the rubber molecular chain, causing the free radical cleavage reaction of the C=C double bond, depolymerizing monomers one by one from the rubber molecular network, while all the bound S elements are always retained on the rubber molecular chain. Therefore, the total sulfur and bound sulfur contents of normally vulcanized and thermally degraded rubber are kept at the same level as those of normally vulcanized and new rubber.

[0015] The total sulfur content of under-sulfurized thermally degradable rubber is g, and the bound sulfur content is h, then g≈e≈f≈d, h≈b≤95%a, and the bound sulfur content of the rubber at the foaming point is k≤85%a. This is because the total sulfur content is the sum of all sulfur elements contained in the formula components. For the same formula, the total sulfur content is maintained at a certain level, so g≈e≈f≈d. For under-sulfurized thermally degradable rubber, when the thermal degradation process occurs, the thermal degradation starts from the weak bond of the molecular chain, that is, from the polysulfide bond -C-Sx-C-. However, when the SS at the polysulfide bond breaks to form free radicals, the free radicals will transfer. The free radicals will further trigger the reaction of the C=C double bond on the rubber molecular chain, causing a free radical cleavage reaction of the C=C double bond, depolymerizing monomers one by one from the rubber molecular network, while all the bound S elements are always retained on the rubber molecular chain, so h≈b≤95%a. Based on the conclusion drawn from a large amount of test data, the bound sulfur content of the rubber at the foaming point is k≤85%a.

[0016] The calculation formula for the bound sulfur percentage of each rubber compound is: each bound sulfur percentage = each bound sulfur content / (normal vulcanization - bound sulfur content of new rubber)*100%; the judgment is made based on the test results and the bound sulfur percentage, and the judgment basis is: when the bound sulfur percentage of the returned tire rubber is greater than or equal to 95%, the returned tire rubber is a normally vulcanized and thermally degraded rubber; when the bound sulfur percentage of the returned tire rubber is less than 95%, the returned tire rubber is an under-vulcanized and thermally degraded rubber.

[0017] Furthermore, the normally vulcanized new rubber, normally vulcanized thermally degraded rubber, undervulcanized new rubber, undervulcanized thermally degraded rubber, and the tire rubber to be determined all use the same formula. For undervulcanized rubber, the bound sulfur content (sulfur that reacts with the rubber molecular chain is called bound sulfur) in its vulcanized rubber network is lower than the bound sulfur content in normally vulcanized rubber, which is well known in the industry. For undervulcanized rubber or normally vulcanized rubber, after thermal degradation occurs, traditional theory suggests that the crosslinking density of its crosslinked network will decrease, and the bound sulfur content in the three-dimensional network of rubber macromolecules that reacts with the rubber molecular chain will also decrease. Currently, there is no public research on using bound sulfur test data to distinguish between undervulcanized thermally degraded rubber and normally vulcanized thermally degraded rubber.

[0018] Furthermore, under normal vulcanization conditions, the formula t90 = 30 min, the normal vulcanization-new rubber adopts the normal vulcanization conditions of the rubber compound of the formula, and the vulcanization conditions are 150 ° C * 30 min, and the under-vulcanization-new rubber vulcanization conditions are 150 ° C * 12 min;

[0019] The normally vulcanized-thermally degraded rubber is taken from a normally vulcanized tire after a durability test until the tread becomes sticky. The normally vulcanized tire adopts the normal vulcanization conditions of the tire with this formula, and its vulcanization time is 48 minutes. The under-vulcanized-thermally degraded rubber is taken from a under-vulcanized tire after a durability test until the tread becomes sticky. The vulcanization time of the under-vulcanized tire is 42 minutes.

[0020] The above-mentioned normally vulcanized tire and under-vulcanized tire have the same rubber formula and the same vulcanization conditions. The only difference is the vulcanization time. The vulcanization time of the under-vulcanized tire is 42 minutes, which is the normal foaming time of the tire. At this time, the tire is not fully vulcanized and is in an under-vulcanized state. The vulcanization time of the normally vulcanized tire is 48 minutes, which is the sum of the normal foaming time and the safety time. At this time, the tire is fully vulcanized.

[0021] Furthermore, in step (1), a mixed freezing ball mill is used for crushing and grinding.

[0022] In step (1), the rubber sample is placed in a ball milling jar of a hybrid freezing ball mill, and the ball milling jar containing the rubber sample is placed in liquid nitrogen and frozen for 8 to 15 minutes, preferably 10 minutes; then, the rubber sample is removed and crushed and ground using a hybrid freezing ball mill to prepare a uniform sample. Using a freezing ball mill for crushing and grinding can make the prepared sample more uniform, thereby improving extraction efficiency and uniformity.

[0023] Furthermore, the extracting solution in step (2) is a mixed solution of ethyl acetate:cyclohexane=30:70 (v / v).

[0024] Furthermore, in step (2), the sample is extracted under reflux at 85-95°C for 7-9 hours, allowed to stand for 7-9 hours, extracted for another 1.5-2.5 hours, taken out, dried in an oven at 90°C for 2 hours, taken out to dry, weighed, and set aside.

[0025] Preferably, the sample to be extracted is refluxed and extracted at 90° C. for 8 hours, then allowed to stand for 8 hours and extracted for another 2 hours; taken out, the solvent is dried in an oven at 90° C. for 2 hours, and then placed in a desiccator to cool, weigh, and set aside.

[0026] The weighing in the above steps is used to calculate the bound sulfur content, and the bound sulfur content (%) = the tested sulfur content of the rubber compound (rubber compound after extraction) * the mass after extraction / the mass of the rubber compound before extraction * 100%.

[0027] The sample mass in the above steps should not be too much or too little, as too little will lead to large errors; the volume of the extract only needs to ensure that reflux can occur.

[0028] Furthermore, the sulfur content testing equipment used in step (3) is ELTRA CS580.

[0029] Furthermore, the sulfur content test temperature in step (4) is 1250-1500° C. Preferably, the sulfur content test temperature is 1350° C. At this temperature, sulfur can burn quickly to form sulfur dioxide, and the combustion is complete, which minimizes the impact on test data fluctuations.

[0030] Furthermore, the determination method is used to determine vulcanized rubber. The types of rubber in the vulcanized rubber include natural rubber, synthetic polyisoprene rubber, polybutadiene rubber, styrene-butadiene rubber and other common hydrocarbon rubbers.

[0031] The principle of the present invention is:

[0032] When the rubber material undergoes thermal degradation, the thermal degradation of the vulcanized rubber starts from the weak bond of the molecular chain, that is, it starts to break from the polysulfide bond -C-Sx-C-. When the SS at the polysulfide bond is broken to form free radicals, due to the high electron cloud density at the C=C double bond on the rubber molecular chain, the free radical will be transferred to the C=C double bond on the rubber molecular chain, causing a free radical cracking reaction of the C=C double bond, depolymerizing each rubber molecule monomer from the rubber molecular network, while all the sulfur elements bound to the molecular chain remain on the rubber macromolecular chain. Therefore, the bound sulfur of the undersulfurized-thermally degraded rubber remains at the same level as that of the undersulfurized-new rubber before and after thermal degradation, and the bound sulfur of the normally vulcanized-thermally degraded rubber remains at the same level as that of the normally vulcanized-new rubber before and after thermal degradation. The bound sulfur content of the undersulfurized-new rubber with different degrees of vulcanization is less than 95% of the bound sulfur content of the normally vulcanized-new rubber, and at the foaming point, the bound sulfur content of the undersulfurized-new rubber is less than 85% of the bound sulfur content of the normally vulcanized-new rubber.

[0033] Beneficial effects of the present invention:

[0034] The present invention conducts extraction experiments on unused under-sulfurized rubber (under-sulfurized-new rubber), under-sulfurized-thermally degraded rubber, normally vulcanized unused rubber (normally vulcanized-new rubber), normally vulcanized-thermally degraded rubber, and tire rubber to be determined, and by combining sulfur content test data, distinguishes and determines whether the tire rubber to be determined belongs to under-sulfurized-thermally degraded rubber or normally vulcanized-thermally degraded rubber, providing strong evidence support for the determination of the cause of tire stickiness after market returns. DETAILED DESCRIPTION

[0035] To further understand the present invention, the present invention will be further described below in conjunction with the embodiments of the present invention, and the technical solutions of the present invention will be clearly and completely described. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0036] The present invention provides a method for determining whether a tire is damaged by undersulfurization or thermal degradation, comprising the following steps:

[0037] (1) Using the same formula as the tire rubber to be judged, prepare normal vulcanization-new rubber (vulcanization conditions are 150℃*30min, where the formula t90=30min), normal vulcanization-thermal degradation rubber (normal vulcanization-tire prepared with a vulcanization time of 48min obtained after durability test), undervulcanization-new rubber (vulcanization conditions are 150℃*12min, where the formula t90=30min), undervulcanization-thermal degradation rubber (undervulcanization-tire prepared with a vulcanization time of 42min obtained after durability test);

[0038] Weigh the five rubber samples mentioned above and place them into the ball mill of a hybrid freezing ball mill. Place the ball mill containing the samples into liquid nitrogen and freeze for about 10 minutes. Then take them out and crush and grind them using a hybrid freezing ball mill to prepare uniform samples.

[0039] (2) The sample was divided into two parts of equal mass. One part of the sample was set aside for later use. The other part of the sample was wrapped into a loose bag with filter paper and extracted with a mixture of ethyl acetate and cyclohexane in a volume ratio of (20-40): (80-60) as the extractant. After reflux extraction at 85-95°C for 7-9 hours, the mixture was allowed to stand for 7-9 hours. After extraction for another 1.5-2.5 hours, the bag was removed and placed in a 90°C oven to dry for 2 hours. The bag was removed and placed in a desiccator to cool and dry for about 0.5 hours. The bag was weighed and set aside for later use.

[0040] (3) The sulfur content of the unextracted sample and the extracted sample was tested using the Eltra carbon / sulfur analyzer CS580 at a test temperature of 1250-1500°C. The test results of the unextracted sample were the total sulfur content, and the test results of the extracted sample were the bound sulfur content.

[0041] Among them, total sulfur ratio = total sulfur content / (normal vulcanization - total sulfur of new rubber) * 100%;

[0042] Bound sulfur percentage = bound sulfur content / (normal vulcanization - bound sulfur of new rubber) * 100%;

[0043] The judgment is made based on the test results and the percentage of bound sulfur. The judgment basis is: when the percentage of bound sulfur in the returned tire rubber is greater than or equal to 95%, the returned tire rubber is normally vulcanized and thermally degraded rubber; when the percentage of bound sulfur in the returned tire rubber is less than 95%, the returned tire rubber is under-vulcanized and thermally degraded rubber.

[0044] Example 1

[0045] A method for determining whether a tire is damaged by undersulfurization or thermal degradation comprises the following steps:

[0046] Using the same formula as the tire rubber to be judged, prepare normal vulcanization-new rubber (vulcanization conditions are 150°C*30min, where the formula t90=30min), normal vulcanization-thermal degradation rubber (normal vulcanization-tire prepared with a vulcanization time of 48min and obtained after durability test), undervulcanization-new rubber (vulcanization conditions are 150°C*12min, where the formula t90=30min), and undervulcanization-thermal degradation rubber (undervulcanization-tire prepared with a vulcanization time of 42min and obtained after durability test);

[0047] Weigh the five rubber samples mentioned above and place them in the ball mill of a hybrid freezing ball mill. Place the ball mill containing the samples in liquid nitrogen and freeze for 10 minutes, then take it out and crush and grind it using a hybrid freezing ball mill to prepare a uniform sample. Then weigh two samples of the five rubber samples, each 0.5 g, one of which is not processed and set aside, and the other is wrapped with filter paper to form a rubber bag, which is placed in an extraction cup, and 40 ml of a 30:70 (v / v) mixture of ethyl acetate and cyclohexane is added. After reflux extraction at 90°C for 8 hours, let it stand for 8 hours, and then extract for another 2 hours, take out the rubber bag, dry the solvent in a 90°C oven for 2 hours, then cool it in a desiccator for 0.5 hours, weigh it, and set aside.

[0048] The sulfur content of the unextracted and extracted samples was tested using an Eltra Carbon / Sulfur Analyzer CS580 at a temperature of 1350°C. The test results for the unextracted samples are the total sulfur content, while the test results for the extracted samples are the bound sulfur content.

[0049] Among them, total sulfur ratio = total sulfur content / (normal vulcanization - total sulfur of new rubber) * 100%;

[0050] Bound sulfur percentage = bound sulfur content / (normal vulcanization - bound sulfur of new rubber) * 100%;

[0051] The judgment is made based on the test results and the percentage of bound sulfur. The judgment basis is: when the percentage of bound sulfur in the returned tire rubber is greater than or equal to 95%, the returned tire rubber is normally vulcanized and thermally degraded rubber; when the percentage of bound sulfur in the returned tire rubber is less than 95%, the returned tire rubber is under-vulcanized and thermally degraded rubber.

[0052] Example 2

[0053] A method for determining whether a tire is damaged by undersulfurization or thermal degradation comprises the following steps:

[0054] Using the same formula as the tire rubber to be judged, prepare normal vulcanization-new rubber (vulcanization conditions are 150°C*30min, where the formula t90=30min), normal vulcanization-thermal degradation rubber (normal vulcanization-tire prepared with a vulcanization time of 48min and obtained after durability test), undervulcanization-new rubber (vulcanization conditions are 150°C*12min, where the formula t90=30min), and undervulcanization-thermal degradation rubber (undervulcanization-tire prepared with a vulcanization time of 42min and obtained after durability test);

[0055] Weigh the above five rubber samples and place them in the ball mill of a hybrid freezing ball mill. Place the ball mill containing the samples in liquid nitrogen and freeze for 8 minutes, then take it out and crush and grind it using a hybrid freezing ball mill to prepare a uniform sample. Then weigh two samples of the five rubber samples, each 0.5 g, one of which is not processed and set aside, and the other is wrapped with filter paper to form a rubber bag, which is placed in an extraction cup, and 40 ml of a 20:80 (v / v) mixture of ethyl acetate and cyclohexane is added. After reflux extraction at 85°C for 9 hours, let it stand for 9 hours, and then extract for another 2.5 hours, take out the rubber bag, dry the solvent in a 90°C oven for 2 hours, then cool it in a desiccator for 0.5 hours, weigh it, and set aside.

[0056] The sulfur content of the unextracted and extracted samples was tested using an Eltra Carbon / Sulfur Analyzer CS580 at a temperature of 1250°C. The test results for the unextracted samples are the total sulfur content, while the test results for the extracted samples are the bound sulfur content.

[0057] Among them, total sulfur ratio = total sulfur content / (normal vulcanization - total sulfur of new rubber) * 100%;

[0058] Bound sulfur percentage = bound sulfur content / (normal vulcanization - bound sulfur of new rubber) * 100%;

[0059] The judgment is made based on the test results and the percentage of bound sulfur. The judgment basis is: when the percentage of bound sulfur in the returned tire rubber is greater than or equal to 95%, the returned tire rubber is normally vulcanized and thermally degraded rubber; when the percentage of bound sulfur in the returned tire rubber is less than 95%, the returned tire rubber is under-vulcanized and thermally degraded rubber.

[0060] Example 3

[0061] A method for determining whether a tire is damaged by undersulfurization or thermal degradation comprises the following steps:

[0062] Using the same formula as the tire rubber to be judged, prepare normal vulcanization-new rubber (vulcanization conditions are 150°C*30min, where the formula t90=30min), normal vulcanization-thermal degradation rubber (normal vulcanization-tire prepared with a vulcanization time of 48min and obtained after durability test), undervulcanization-new rubber (vulcanization conditions are 150°C*12min, where the formula t90=30min), and undervulcanization-thermal degradation rubber (undervulcanization-tire prepared with a vulcanization time of 42min and obtained after durability test);

[0063] Weigh the above five rubber samples and place them in the ball mill of a hybrid freezing ball mill. Place the ball mill containing the samples in liquid nitrogen and freeze for 15 minutes, then take it out and crush and grind it using a hybrid freezing ball mill to prepare a uniform sample. Then weigh two samples of the five rubber samples, each 0.5 g, one of which is not processed and set aside, and the other is wrapped with filter paper to form a rubber bag, which is placed in an extraction cup, and 40 ml of a 40:60 (v / v) mixture of ethyl acetate and cyclohexane is added. After reflux extraction at 95°C for 7 hours, let it stand for 7 hours, and then extract for 1.5 hours. Take out the rubber bag, dry the solvent in a 90°C oven for 2 hours, then cool it in a desiccator for 0.5 hours, weigh it, and set aside.

[0064] The sulfur content of the unextracted and extracted samples was tested using the Eltra Carbon / Sulfur Analyzer CS580 at a temperature of 1500°C. The test results for the unextracted samples are the total sulfur content, while the test results for the extracted samples are the bound sulfur content.

[0065] Among them, total sulfur ratio = total sulfur content / (normal vulcanization - total sulfur of new rubber) * 100%;

[0066] Bound sulfur percentage = bound sulfur content / (normal vulcanization - bound sulfur of new rubber) * 100%;

[0067] The judgment is made based on the test results and the percentage of bound sulfur. The judgment basis is: when the percentage of bound sulfur in the returned tire rubber is greater than or equal to 95%, the returned tire rubber is normally vulcanized and thermally degraded rubber; when the percentage of bound sulfur in the returned tire rubber is less than 95%, the returned tire rubber is under-vulcanized and thermally degraded rubber.

[0068] Practical application examples

[0069] Tires returned from the market, symptoms: The rubber on the bottom of the tread is sticky and damaged with pieces falling off; it remains to be determined whether the rubber on the bottom of the tread of the tires returned from the market is normally vulcanized and thermally degraded rubber, or under-vulcanized and thermally degraded rubber.

[0070] The tread formulation of the tires returned to the market is: 100 parts NR, 500 parts N134, 5.0 parts zinc oxide, 2.0 parts stearic acid, 1.5 parts antioxidant RD, 2.0 parts antioxidant 6PPD, 1.5 parts sulfur, 1.2 parts accelerator NS, and 0.1 part scorch retarder CTP. All raw materials are commercially available.

[0071] Tire specifications: 12R22.5S667;

[0072] Tire vulcanization conditions: nitrogen vulcanization, vulcanizer: mold sleeve temperature 150℃, upper and lower hot plate temperature 145℃.

[0073] The tires returned to this market have been used for 7 months. The area of use is the hot African region with good road conditions.

[0074] (1) Sample preparation process:

[0075] The tread rubber of the sticky part of the tire returned from the market is the unknown vulcanization state to be determined - thermally degraded rubber;

[0076] The same tread formula as the market return tire was used to prepare the rubber compounds of normal vulcanization-new rubber, under-vulcanization-new rubber, under-vulcanization-tire and normal vulcanization-tire. Except for the vulcanization time, the other vulcanization conditions of these four rubber compounds are the same as the market return tire. The differences are as follows:

[0077] Normal vulcanization - new rubber: use 2mm thick physical property test specimen, vulcanization conditions: 150℃*30min;

[0078] Unvulcanized - New rubber: Use 2mm thick test specimens, vulcanization conditions: 150℃*12min

[0079] Normal vulcanization-tire: vulcanization time 48min;

[0080] Under-cured tire: curing time 42 minutes;

[0081] Durability test conditions for under-cured tires and normally cured tires were respectively carried out (according to the durability test conditions in GB / T 4501-2016 "Indoor Test Methods for Performance of Truck Tires"): test pressure 830kPa, test speed 50km / h, running until the tread rubber material becomes sticky, and the tread rubber with stickiness is taken as under-cured thermal degradation rubber and normally cured thermal degradation rubber;

[0082] (2) Sample processing process

[0083] Weigh 0.5 g of each of the five rubber samples for later use; then take 0.5 g of each rubber sample and place it in the ball mill of a hybrid refrigerated ball mill. Place the ball mill containing the sample in liquid nitrogen and freeze it for 10 minutes, then remove it and grind it using the hybrid refrigerated ball mill to prepare a uniform sample. Then wrap it with filter paper to form a rubber bag; then add it to an extraction cup, add 40 ml of a 30:70 mixture of ethyl acetate and cyclohexane, and reflux extract at 90°C for 8 hours, let it stand for 8 hours, and then extract for another 2 hours; remove the rubber bag, dry the solvent in a 90°C oven for 2 hours, then cool it in a desiccator for 0.5 hours, weigh it, and the rubber sample is ready for use;

[0084] (3) Sulfur content test

[0085] Sulfur content was measured on both unextracted and extracted rubber samples using an ELTRACS580 sulfur analyzer at 1350°C. The unextracted sample reported total sulfur content, while the extracted sample reported bound sulfur content. Each sample was tested five times, yielding the results shown in Table 1.

[0086] Table 1 Statistics and calculation results of sulfur content test data of various rubber compounds

[0087]

[0088] Note: Total sulfur percentage = total sulfur content / (normal vulcanization - total sulfur of new rubber) * 100%

[0089] Bound sulfur percentage = bound sulfur content / (normal vulcanization - bound sulfur of new rubber) * 100%

[0090] Conclusion: The total sulfur content of the tires returned to the market, that is, the unknown vulcanization state - thermally degraded rubber, is at the same level as the total sulfur content of the normally vulcanized - new rubber. The combined sulfur content is 98.1% of the new rubber. Therefore, this tire returned to the market is normally vulcanized - thermally degraded rubber, and the damage is caused by the use conditions, which is consistent with the conclusion of experience.

[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art may still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modifications, equivalent replacements, and modifications made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for determining whether a tire is damaged by insufficient sulfur or thermal degradation, characterized in that: The following steps are involved: (1) Obtain a rubber sample, freeze it in liquid nitrogen for 8-15 minutes, and then grind it to obtain a uniform sample; Among them, rubber samples include normally vulcanized new rubber, normally vulcanized thermally degraded rubber, under-vulcanized new rubber, under-vulcanized thermally degraded rubber, and tire rubber to be determined; (2) Divide the sample into two parts of equal mass. Extract one part of the sample with the extraction solution. After the extraction is completed, take out the sample and dry it; The extract is a mixture of ethyl acetate and cyclohexane, and the volume ratio of ethyl acetate to cyclohexane is (20-40): (80-60); (3) The sulfur content of the unextracted sample is tested to obtain the total sulfur content, and the sulfur content of the extracted sample is tested to obtain the bound sulfur content. The formula for calculating the bound sulfur ratio of each rubber material is: each bound sulfur ratio = each bound sulfur content / (normal vulcanization - bound sulfur content of new rubber) * 100%; judgment is made based on the test results and the bound sulfur ratio, and the judgment basis is: when the bound sulfur ratio of the returned tire rubber is greater than or equal to 95%, the returned tire rubber is a normally vulcanized-heat-degraded rubber; when the bound sulfur ratio of the returned tire rubber is less than 95%, the returned tire rubber is an under-sulfurized-heat-degraded rubber.

2. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: The normally vulcanized new rubber, the normally vulcanized heat-degraded rubber, the under-vulcanized new rubber, the under-vulcanized heat-degraded rubber and the tire rubber to be determined all adopt the same formula.

3. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 2, characterized in that: The normal vulcanization-new rubber adopts the normal vulcanization conditions of the rubber compound of the formula, and its vulcanization conditions are 150℃*30min, and the under-vulcanization-new rubber vulcanization conditions are 150℃*12min; The normally vulcanized-thermally degraded rubber is taken from a normally vulcanized tire after a durability test until the tread becomes sticky. The normally vulcanized tire adopts the normal vulcanization conditions of the tire with this formula, and its vulcanization time is 48 minutes. The under-vulcanized-thermally degraded rubber is taken from a under-vulcanized tire after a durability test until the tread becomes sticky. The vulcanization time of the under-vulcanized tire is 42 minutes.

4. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: In the step (1), a mixed freezing ball mill is used for pulverization and grinding.

5. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: The extracting solution in step (2) is a mixed solution of ethyl acetate:cyclohexane=30:70 (v / v).

6. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: In the step (2), the sample is refluxed and extracted at 85-95° C. for 7-9 hours, then allowed to stand for 7-9 hours, extracted for another 1.5-2.5 hours, taken out, dried in a 90° C. oven for 2 hours, taken out and dried, weighed, and set aside.

7. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: The sulfur content testing equipment used in step (3) is ELTRACS580.

8. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: The sulfur content test temperature in step (3) is 1250-1500°C.

9. The method for determining tire undersulfurization damage or thermal degradation damage according to claim 1, characterized in that: The determination method is used for determining vulcanized rubber.

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