Pretreatment method for testing cross-linking degree of XLPE (cross-linked polyethylene), device used by method and test method for cross-linking degree of XLPE

Abstract

The invention discloses an XLPE crosslinking degree test pretreatment method, a device used by the method and an XLPE crosslinking degree test method, and relates to the technical field of test methods. The pretreatment method for testing the crosslinking degree of the XLPE comprises the following steps that periodic tensile stress is applied to an XLPE sample in a solvent environment according to the set frequency and the set strain amplitude, the set frequency is 0.05-1 Hz, the set strain amplitude is 50%-250%, and when the change rate of the periodic tensile stress is smaller than or equal to 5% when the set strain amplitude is reached for five continuous times, the XLPE sample is subjected to crosslinking treatment. And ending the periodic stretching. According to the pretreatment method provided by the invention, the physically entangled part in the XLPE sample can be quickly and fully unentangled, and meanwhile, the chemically crosslinked part is prevented from being damaged, so that the influence of the physically entangled part on the XLPE crosslinking degree test result can be eliminated to the greatest extent, and the accuracy of the XLPE crosslinking degree test result is improved.

Description

Technical Field

[0001] This invention relates to the field of testing methods, specifically to a pretreatment method for testing the degree of crosslinking of XLPE, the apparatus used therein, and a method for testing the degree of crosslinking of XLPE. Background Technology

[0002] XLPE (cross-linked polyethylene) is widely used in high-voltage and ultra-high-voltage power cables due to its excellent electrical insulation properties. The overall performance of XLPE is highly dependent on the cross-linked network structure within the material. Highly cross-linked XLPE typically exhibits higher mechanical properties such as thermal elongation, elongation at break, and tensile strength, and has a wider range of applications in medium- and high-voltage power cables.

[0003] Currently, the xylene high-temperature extraction method is widely used in the industry to characterize the chemical crosslinking degree of XLPE. This method is based on existing gel content testing standards. By extracting the sample in xylene, the uncrosslinked portion dissolves in organic solvents such as xylene. The remaining insoluble matter is considered "gel," and the proportion of gel mass to the original sample mass is used as a representative indicator of the chemical crosslinking degree. However, XLPE is a typical semi-crystalline polymer. Its network structure consists of irreversible chemical crosslinking networks and reversible physical entanglement networks (caused by chain segment entanglement, topological interlocking, crystalline region constraints, etc.). The xylene extraction method treats all "extracted insoluble matter" as chemical crosslinking networks, completely ignoring the existence of physical entanglement. This leads to inaccurate measurement results and is not conducive to precise control of the XLPE crosslinking degree during subsequent production. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a pretreatment method for testing the crosslinking degree of XLPE, the apparatus used in the method, and a method for testing the crosslinking degree of XLPE.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution: A pretreatment method for testing the crosslinking degree of XLPE includes the following steps: In a solvent environment, cyclic tensile stress is applied to the XLPE sample at a set frequency and a set strain amplitude. The set frequency is 0.05-1 Hz and the set strain amplitude is 50%-250%. The cyclic tensile stress is terminated when the rate of change of the cyclic tensile stress is ≤5% for 5 consecutive times when the set strain amplitude is reached.

[0006] The pretreatment method for testing the crosslinking degree of XLPE provided by this invention applies a reciprocating tensile-release oscillating force field of a certain frequency and amplitude to the sample in a solvent environment. This promotes the active slippage of XLPE chain segments, rapidly untangling physically entangled parts. Simultaneously, the chemically crosslinked parts remain intact during the stretching process under a strain amplitude ≤250%. Therefore, this invention can minimize the influence of physically entangled parts on the XLPE crosslinking degree test results, improving the accuracy of the test results. It should be noted that the periodic tensile stress described in this invention needs to meet a frequency of 0.05-1 Hz and a strain amplitude of 50%-250%. The inventors found that the above frequency and strain amplitude can adequately match the slippage of molecular chains in XLPE. If the frequency is too high, the interval between each stretch is too short, and the XLPE molecular chains fail to slip sufficiently, resulting in greater damage to the crosslinked structure during the stretching process and an increased risk of sample breakage. If the frequency is too low, the untangling time is too long. If the strain amplitude is too low, the molecular chains cannot slip sufficiently after a single stretching; if the strain amplitude is too high, the sample is prone to premature breakage and failure to fully untangle, which also affects the test results of the subsequent degree of crosslinking.

[0007] In a specific embodiment of the present invention, the total number of times the periodic tensile stress is applied is 40-200.

[0008] In a specific embodiment of the present invention, the set strain amplitude is the maximum strain amplitude of the XLPE specimen to be tested (within the cycle) after each application of periodic tensile stress.

[0009] Preferably, the step of pre-testing the XLPE specimen before applying the cyclic tensile stress includes testing the elongation at break of the XLPE specimen.

[0010] More preferably, the set frequency for applying the periodic tensile stress is set according to the elongation at break of the XLPE sample obtained from the pre-detection: When the elongation at break of the XLPE sample is ≤130%, the frequency is set to 0.6-1 Hz; When the elongation at break of the XLPE sample is >130% and ≤180%, the frequency is set to 0.4-0.8 Hz; When the elongation at break of the XLPE sample is >180% and ≤250%, the frequency is set to 0.2-0.4 Hz; When the elongation at break of the XLPE sample is >250%, the frequency is set to 0.05-0.2 Hz.

[0011] More preferably, when testing the elongation at break of the XLPE specimen, the tensile speed is 80-120 mm / min.

[0012] More preferably, the XLPE specimen with an elongation at break ≤130% has a tensile strength >16 MPa and a density of 0.910-0.950 g / cm³. 3 Shore hardness D40-70.

[0013] XLPE samples with an elongation at break of ≤130% can be considered as highly cross-linked XLPE samples, and their composition usually contains more than 1.8 wt% cross-linking agent.

[0014] More preferably, the XLPE specimen with an elongation at break >130% and ≤180% has a tensile strength >15 MPa and ≤16 MPa, and a density of 0.910-0.950 g / cm³. 3 Shore hardness D40-70.

[0015] XLPE samples with an elongation at break of >130% and ≤180% can be considered as medium-to-high crosslinking XLPE samples, and their composition usually contains 1.6-1.8 wt% crosslinking agent.

[0016] More preferably, the XLPE specimen with an elongation at break >180% and ≤250% has a tensile strength >14 MPa and ≤15 MPa, and a density of 0.910-0.950 g / cm³. 3 Shore hardness D40-70.

[0017] XLPE samples with an elongation at break of >180% and ≤250% can be considered as medium-low crosslinking XLPE samples, and their composition usually contains 1.4~1.6wt% crosslinking agent.

[0018] More preferably, the XLPE specimen with an elongation at break >250% has a tensile strength <14 MPa and a density of 0.910-0.950 g / cm³. 3 Shore hardness D40-70.

[0019] XLPE samples with an elongation at break >250% can be considered as low-crosslinking XLPE samples, and their composition usually contains <1.4wt% crosslinking agent.

[0020] More preferably, the set strain amplitude is set according to the elongation at break of the XLPE sample obtained from the pre-test: When the elongation at break of the XLPE specimen is ≤130%, the strain range is set to 50%-90%; When the elongation at break of the XLPE specimen is >130% and ≤180%, the strain range is set to 90%-120%. When the elongation at break of the XLPE specimen is >180% and ≤250%, the strain range is set to 120%-140%. When the elongation at break of the XLPE specimen is greater than 250%, the strain range is set to 140%-250%.

[0021] Preferably, the solvent environment is provided by cyclohexane.

[0022] Preferably, the mass ratio of cyclohexane to XLPE sample is >500:1.

[0023] More preferably, the mass ratio of cyclohexane to XLPE sample is (550-1200):1.

[0024] Preferably, the temperature of the solvent environment is 60-90°C.

[0025] Preferably, before applying cyclic tensile stress to the XLPE sample, the step of swelling the XLPE sample is further included, wherein swelling of the XLPE sample includes: The XLPE sample was swollen in a solvent environment at 60-90℃ until the change rate of the length of the XLPE sample in any three-dimensional direction within 5 minutes was ≤5%.

[0026] This invention also protects a method for testing the crosslinking degree of XLPE, comprising the following steps: Weigh the XLPE sample to be tested to obtain m0. Process the XLPE sample to be tested using the above method to obtain the pretreated sample. Then extract it in xylene at 90-130℃ for 12-36 h. After extraction, separate the solid and liquid and take the insoluble matter. Dry it to constant weight to obtain the dried sample. Weigh the dried sample to obtain m1. Calculate the degree of crosslinking C of the XLPE sample to be tested according to the formula C=m1 / m0×100%.

[0027] The sample obtained by the pretreatment method provided by this invention has been fully untangled physically. At this time, the untangled part can be directly removed by xylene extraction to obtain the real chemical cross-linked part.

[0028] Preferably, the mass ratio of xylene to the pretreated sample is >500:1.

[0029] More preferably, the mass ratio of xylene to the pretreated sample is (550-1200):1.

[0030] Preferably, the drying temperature is 90-130°C.

[0031] This invention also protects a pretreatment device for testing the crosslinking degree of XLPE, comprising a vertical frame, an upper clamp, a lower clamp, and a tank. The vertical frame includes a fixed part (5) at the bottom and a lifting and pulling part (6) slidably connected to the inner wall of the vertical frame. The lifting and pulling part is parallel to the fixed part. The tank 4 is disposed between the fixed part and the lifting and pulling part. The upper clamp 2 and the lower clamp 3 have the same structure, including a connecting rod (7) and a clamping surface (claw) at one end of the connecting rod. The end of the upper clamp connecting rod away from the clamping surface is fixedly connected to the end face of the lifting and pulling part facing the fixed part. The end of the lower clamp connecting rod away from the clamping surface is fixedly connected to the end face of the fixed part facing the lifting and pulling part. The upper clamp and the lower clamp are both located inside the tank and the clamping surfaces are arranged opposite each other, so that a clamping space for clamping the test sample is formed between the upper clamp and the lower clamp. The upper clamp can slide along the inner wall of the tank by sliding the lifting and pulling part. The tank body includes a sealed cavity (a cavity formed by the tank body sidewalls) and a sealing element threadedly connected to the sealed cavity. The sealing element includes a lower sealing element 8 disposed at the bottom of the sealed cavity and an upper sealing element 9 disposed at the top of the sealed cavity. The upper sealing element is connected to the connecting rod of the upper clamp, and the lower sealing element is connected to the connecting rod of the lower clamp.

[0032] The device in this invention differs from a conventional universal tensile testing machine in that, based on the original design, a container that can be filled with solvent is introduced outside the specimen clamp, allowing the tensile test of the specimen to be performed in the presence of solvent.

[0033] Preferably, the vertical inner wall of the vertical frame is provided with 10 slide rails, and the lifting and pulling part is slidably connected to the slide rails through a slider.

[0034] Preferably, the upper sealing member is provided with a first insertion hole, and the lower sealing member is provided with a second insertion hole; the connecting rod of the upper clamp is fitted and connected to the first insertion hole, and the connecting rod of the lower clamp is fitted and connected to the second insertion hole.

[0035] Preferably, the pretreatment device for testing the crosslinking degree of XLPE further includes a drive system (11) and a control module (12). The drive system is connected to the lifting and pulling part via a tensioning linkage, and the drive system is electrically connected to the control module.

[0036] More preferably, a heating system electrically connected to the drive module is provided on the outside of the tank body, and the heating system is an alloy resistance wire tightly wrapped around the outer wall of the tank body.

[0037] The usage process of the pretreatment device for testing the crosslinking degree of XLPE provided by this invention is as follows: Connect the sealed cavity in the tank to the lower seal with a threaded connection. Place the lower clamp inside the sealed cavity, ensuring that the end of the lower clamp connecting rod away from the clamping surface passes through the second insertion hole of the lower seal and is connected to the end face of the fixed part facing the lifting and pulling part via a pin, so that the clamping surface of the lower clamp is located inside the sealed cavity. Place the upper clamp inside the sealed cavity, ensuring that the clamping surfaces of the upper and lower clamps hold the two opposite ends of the XLPE sample. Fill the sealed cavity with solvent, ensuring that the end of the upper clamp connecting rod away from the clamping surface passes through the upper seal. The first insertion hole of the seal is then used to connect the upper seal to the sealed cavity via a threaded connection. The control module drives the drive system through the tension linkage, causing the lifting tension part to move down to the appropriate position along the vertical slide rail on the inner wall of the vertical frame. The end of the upper clamping linkage away from the clamping surface is connected to the end face of the lifting tension part facing the fixed part by a pin, thus completing the sample loading. Subsequently, the control module drives the drive system through the tension linkage, causing the lifting tension part to slide up and down along the slide rail and apply periodic tensile stress to the sample.

[0038] In this invention, the temperature of the solvent environment is controlled by the heating system in the pretreatment device for testing the degree of crosslinking of XLPE. During use, an infrared thermometer can be used to test the temperature of the solvent inside the tank in real time.

[0039] In this invention, the mass ratio of solvent (xylene, cyclohexane) to XLPE sample is controlled by changing the size of XLPE sample and the volume of the container.

[0040] Compared with the prior art, the present invention has the following beneficial effects: The pretreatment method provided by this invention can quickly and fully untangle the physically entangled parts in the XLPE sample while keeping the chemically cross-linked parts intact. Therefore, this invention can minimize the influence of the physically entangled parts on the XLPE cross-linking degree test results and improve the accuracy of the XLPE cross-linking degree test results. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the pretreatment device for testing the crosslinking degree of XLPE provided by the present invention. In the figure: 1 is a vertical frame, 2 is an upper clamp, 3 is a lower clamp, 4 is a tank, 5 is a fixing part, 6 is a lifting and pulling part, 7 is a clamp connecting rod, 8 is a lower seal, 9 is an upper seal, 10 is a slide rail, 11 is a drive system, and 12 is a control module.

[0042] Figure 2 The stress-strain curve is a graph obtained when the XLPE sample is pretreated using the method provided in this invention. Detailed Implementation

[0043] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents. The raw material information used in each embodiment and comparative example is as follows: Polyethylene insulating granules: LDPE, 2426H, China Petroleum Lanzhou Petrochemical Company.

[0044] High-crosslinking XLPE samples: Polyethylene insulating granules were mixed with DCP (dicumyl peroxide) at a mass ratio of 100:2, and then added to a flat vulcanizing agent. High-crosslinking XLPE samples were prepared by hot pressing using a flat vulcanizing machine. The hot pressing process was as follows: preheating at 120℃ and normal pressure for 5-10 min, then pressurizing to 5 MPa for 1 min, then pressurizing to 10 MPa for 1 min, then pressurizing to 15 MPa for 4 min, then raising the flat vulcanizing machine to 180℃ and pressing at 17 MPa for 18 min, finally maintaining constant pressure and water cooling for 10 min to cool the samples, resulting in strip-shaped samples with a thickness of 0.5 mm, a width of 5 mm, and a length of 40 mm. The theoretical crosslinking degree of the high-crosslinking XLPE samples prepared by this method was 81%. Twelve groups of high-crosslinking XLPE samples were prepared using the same method, and the elongation at break of each group was tested. The elongation at break of all 12 groups of samples was between 110% and 112%.

[0045] Medium-to-high crosslinking degree XLPE sample: The preparation method is basically the same as that of high crosslinking degree XLPE sample, the only difference being that polyethylene insulating granules are mixed with DCP (dicumyl peroxide) at a mass ratio of 100:1.7. The resulting medium-to-high crosslinking degree XLPE sample has a theoretical crosslinking degree of 77% and an elongation at break of 150%.

[0046] Medium-low crosslinking degree XLPE samples: The preparation method is basically the same as that of high crosslinking degree XLPE samples, the only difference being that polyethylene insulating granules are mixed with DCP (dicumyl peroxide) at a mass ratio of 100:1.5. The resulting medium-low crosslinking degree XLPE samples have a theoretical crosslinking degree of 70% and an elongation at break of 200%.

[0047] Low-crosslinking XLPE sample: The preparation method is basically the same as that of high-crosslinking XLPE sample, the only difference being that polyethylene insulating granules are mixed with DCP (dicumyl peroxide) at a mass ratio of 100:1.3. The resulting low-crosslinking XLPE sample has a theoretical crosslinking degree of 59% and an elongation at break of 260%.

[0048] Example 1 A method for testing the degree of crosslinking of XLPE includes the following steps: The highly cross-linked XLPE sample was weighed to obtain m0. The sample was then swollen in cyclohexane at 70°C until the change rate of the sample's length in any three-dimensional direction within 5 minutes was ≤5% [calculated as (changed size - original size) / original size]. Periodic tensile stress was applied to the XLPE sample (mass ratio of cyclohexane:sample = 1000:1) in cyclohexane at 70°C at a set frequency and strain amplitude. The set frequency was 0.8 Hz and the set strain amplitude was 70%. When the change rate of the periodic tensile stress was ≤5% for 5 consecutive cycles at the set strain amplitude [calculated as (changed stress - original stress) / original stress], the periodic stretching was stopped (a total of 40 stretches were performed), yielding the pretreated sample. This sample was then extracted in xylene (mass ratio of xylene:pretreated sample = 1000:1) at 110°C for 24 minutes. h, after extraction, solid-liquid separation is performed to obtain the insoluble matter, which is dried at 110℃ to constant weight to obtain the dried sample; the dried sample is weighed to obtain m1, and the degree of crosslinking C of the XLPE sample to be tested is calculated according to the formula C=m1 / m0×100%.

[0049] In the aforementioned method for testing the degree of crosslinking of XLPE, the pretreatment step is performed using a pretreatment device for testing the degree of crosslinking of XLPE. The device includes a vertical frame, an upper clamp, a lower clamp, and a tank. The vertical frame includes a fixed part located at the bottom and a lifting and pulling part slidably connected to the inner wall of the vertical frame. The lifting and pulling part is parallel to the fixed part. The tank is located between the fixed part and the lifting and pulling part. The upper clamp and the lower clamp have the same structure, including a connecting rod and a clamping surface located at one end of the connecting rod. The end of the upper clamp connecting rod away from the clamping surface is fixedly connected to the end face of the lifting and pulling part facing the fixed part. The end of the lower clamp connecting rod away from the clamping surface is fixedly connected to the end face of the fixed part facing the lifting and pulling part. The upper clamp and the lower clamp are both located inside the tank and the clamping surfaces are arranged opposite each other, so that a clamping space for clamping the test sample is formed between the upper clamp and the lower clamp. The upper clamp can slide along the inner wall of the tank by sliding the lifting and pulling part. The tank body includes a sealed cavity and a sealing element threadedly connected to the sealed cavity. The sealing element includes a lower sealing element disposed at the bottom of the sealed cavity and an upper sealing element disposed at the top of the sealed cavity. The upper sealing element has a first insertion hole, and the lower sealing element has a second insertion hole. The connecting rod of the upper clamp is fitted into the first insertion hole, and the connecting rod of the lower clamp is fitted into the second insertion hole.

[0050] Example 2 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: Replace the high-crosslinking XLPE sample with the same mass of medium-high crosslinking XLPE sample; set the frequency to 0.6 Hz and the strain amplitude to 100%.

[0051] Example 3 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: Replace the high-crosslinking XLPE sample with a medium-low crosslinking sample of the same mass; set the frequency to 0.3 Hz and the strain amplitude to 130%.

[0052] Example 4 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: Replace the high-crosslinking XLPE sample with the same mass of low-crosslinking XLPE sample; set the frequency to 0.1 Hz and the strain amplitude to 200%.

[0053] Example 5 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The frequency is set to 0.6 Hz.

[0054] Example 6 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The frequency is set to 0.4 Hz.

[0055] Example 7 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The strain amplitude is set to 100%.

[0056] Example 8 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The strain amplitude is set at 50%.

[0057] Comparative Example 1 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The frequency was set to 0.02 Hz. A total of 50 stretches were performed.

[0058] Comparative Example 2 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The frequency was set to 2 Hz. The sample broke after 30 tensile cycles.

[0059] Comparative Example 3 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The strain amplitude was set at 300%. The specimen fractured after 22 tensile cycles.

[0060] Comparative Example 4 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The strain amplitude was set to 40%. A total of 50 tensile tests were performed.

[0061] Comparative Example 5 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The periodic tensile test is terminated when the rate of change of the periodic tensile stress is ≤10% for five consecutive cycles when the set strain amplitude is reached. A total of 30 tensile tests are performed.

[0062] Comparative Example 6 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The periodic tensile test was terminated when the rate of change of the periodic tensile stress was ≤5% for two consecutive tests at the set strain amplitude. A total of 34 tensile tests were performed.

[0063] Comparative Example 7 A method for testing the degree of crosslinking of XLPE, wherein the only difference from Example 1 is: The pretreatment step involves applying cyclic tensile stress to the XLPE specimen in an air environment. This pretreatment step is performed in a conventional universal tensile testing machine.

[0064] Performance testing Accuracy Calculation: The detection results of the methods provided in the examples and comparative examples were compiled, and the accuracy was calculated according to the formula: Accuracy = |(Theoretical Crosslinking Degree - Test Crosslinking Degree) × 100% / Theoretical Crosslinking Degree|. The results are shown in Table 1 below: Table 1. Note: In Table 1, " / " indicates that the sample is broken and cannot be detected.

[0065] As can be seen from Table 1 above, the detection method provided by this invention, based on the thorough untangling of the physically entangled parts in the XLPE sample before detection, can achieve accurate detection of the chemical crosslinking degree of XLPE, with an accuracy of over 97.9%.

[0066] According to Examples 1-4, the pretreatment steps of the present invention are applicable to XLPE samples of any degree of crosslinking.

[0067] According to Examples 5-6, the set frequency in the pretreatment step has a significant impact on the untangling of the physically entangled parts in XLPE. For highly cross-linked samples with an elongation at break of ≤130%, setting the frequency below the preferred 0.6 Hz (Example 6) will decrease the accuracy of the detection results.

[0068] According to Examples 7-8, setting the strain amplitude also plays an important role in untangling the physically entangled parts in the XLPE sample, and an appropriate strain amplitude can ensure the accuracy of the test.

[0069] According to Comparative Example 1, while applying cyclic tensile stress at a low frequency can still achieve good accuracy, the long intervals between cyclic stretching and the tendency for the XLPE sample to recover its original shape during each stretching interval necessitate increasing the number of stretching cycles, resulting in excessively long untangling time and significantly reduced efficiency. According to Comparative Example 2, an excessively high frequency and short intervals between each stretching result in insufficient slippage of the XLPE molecular chains, leading to greater damage to the cross-linked structure during the stretching process and premature sample breakage.

[0070] According to Comparative Example 3, excessively high strain amplitude can easily lead to premature sample fracture, failing to fully detangle and affecting the subsequent crosslinking degree test results. According to Comparative Example 4, excessively low strain amplitude prevents sufficient slippage of molecular chains after a single stretch, directly damaging the crosslinked structure during the stretching process and consequently causing sample fracture.

[0071] According to Comparative Examples 5-6, the critical judgment condition of "the rate of change of periodic tensile stress ≤ 5% when the set strain amplitude is reached for 5 consecutive times" is the optimal judgment condition obtained through a large number of experiments. If this condition is not met, the periodic tensile pretreatment will end, and the accuracy of the test results will decrease.

[0072] According to Comparative Example 7, the periodic tensile testing performed using a conventional universal tensile testing machine in the art could not achieve the accuracy of the test results of this application. This is because, in the presence of solvent, the sample can be fully swollen, which is more conducive to the untangling of the physically entangled parts.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A pretreatment method for testing the degree of crosslinking of XLPE, characterized in that, Includes the following steps: In a solvent environment, cyclic tensile stress is applied to the XLPE sample at a set frequency and a set strain amplitude. The set frequency is 0.05-1 Hz and the set strain amplitude is 50%-250%. The cyclic tensile stress is terminated when the rate of change of the cyclic tensile stress is ≤5% for 5 consecutive times when the set strain amplitude is reached.

2. The pretreatment method for testing the crosslinking degree of XLPE as described in claim 1, characterized in that, The process of applying cyclic tensile stress includes a pre-testing step on the XLPE specimen, which includes testing the elongation at break of the XLPE specimen.

3. The pretreatment method for testing the crosslinking degree of XLPE as described in claim 2, characterized in that, Includes at least one of the following (a)-(b): (a) The set frequency for applying the periodic tensile stress is set according to the elongation at break of the XLPE specimen obtained from the pre-test: When the elongation at break of the XLPE sample is ≤130%, the frequency is set to 0.6-1 Hz; When the elongation at break of the XLPE sample is >130% and ≤180%, the frequency is set to 0.4-0.8 Hz. When the elongation at break of the XLPE sample is >180% and ≤250%, the frequency is set to 0.2-0.4 Hz; When the elongation at break of the XLPE sample is >250%, the frequency is set to 0.05-0.2 Hz; (b) When testing the elongation at break of the XLPE specimen, the tensile speed is 80-120 mm / min.

4. The pretreatment method for testing the crosslinking degree of XLPE as described in claim 2 or 3, characterized in that, The set strain amplitude is determined based on the pre-detected elongation at break of the XLPE sample: When the elongation at break of the XLPE specimen is ≤130%, the strain range is set to 50%-90%; When the elongation at break of the XLPE specimen is >130% and ≤180%, the strain range is set to 90%-120%. When the elongation at break of the XLPE specimen is >180% and ≤250%, the strain range is set to 120%-140%. When the elongation at break of the XLPE specimen is greater than 250%, the set strain range of the XLPE specimen after each application of cyclic tensile stress is 140%-250%.

5. The pretreatment method for testing the crosslinking degree of XLPE as described in claim 1, characterized in that, The solvent environment is provided by cyclohexane; preferably, the mass ratio of cyclohexane to XLPE sample is >500:

1.

6. The pretreatment method for testing the crosslinking degree of XLPE as described in claim 1 or 5, characterized in that, The temperature of the solvent environment is 60-90℃.

7. The pretreatment method for testing the crosslinking degree of XLPE as described in claim 1, characterized in that, Before applying cyclic tensile stress to the XLPE sample, the method further includes a step of swelling the XLPE sample, wherein swelling of the XLPE sample includes: The XLPE sample was swollen in a solvent environment at 60-90℃ until the change rate of the length of the XLPE sample in any three-dimensional direction within 5 minutes was ≤5%.

8. A method for testing the degree of crosslinking of XLPE, characterized in that, Includes the following steps: Weigh the XLPE sample to be tested to obtain m0. Treat the XLPE sample to be tested by the method described in any one of claims 1-7 to obtain a pretreated sample. Then extract it in xylene at 90-130℃ for 12-36 h. After extraction, separate the solid and liquid and take the insoluble matter. Dry it to constant weight to obtain a dried sample. Weigh the dried sample to obtain m1. Calculate the degree of crosslinking C of the XLPE sample to be tested according to the formula C=m1 / m0×100%.

9. The method for testing the degree of crosslinking of XLPE as described in claim 8, characterized in that, The mass ratio of xylene to the pretreated sample is >500:1; the drying temperature is 90-130℃.

10. A pretreatment apparatus for testing the degree of crosslinking of XLPE, characterized in that, The device includes a vertical frame, an upper clamp, a lower clamp, and a tank. The vertical frame includes a fixed part at the bottom and a lifting and pulling part slidably connected to the inner wall of the vertical frame. The lifting and pulling part is parallel to the fixed part. The tank is located between the fixed part and the lifting and pulling part. The upper and lower clamps have the same structure, including a connecting rod and a clamping surface at one end of the connecting rod. The end of the upper clamp connecting rod away from the clamping surface is fixedly connected to the end face of the lifting and pulling part facing the fixed part. The end of the lower clamp connecting rod away from the clamping surface is fixedly connected to the end face of the fixed part facing the lifting and pulling part. The upper and lower clamps are both located inside the tank and the clamping surfaces are arranged opposite each other, so that a clamping space for clamping test samples is formed between the upper and lower clamps. The upper clamp can slide along the inner wall of the tank by sliding the lifting and pulling part. The tank body includes a sealed cavity and a sealing element threadedly connected to the sealed cavity. The sealing element includes a lower sealing element disposed at the bottom of the sealed cavity and an upper sealing element disposed at the top of the sealed cavity. The upper sealing element is connected to the connecting rod of the upper clamp, and the lower sealing element is connected to the connecting rod of the lower clamp.

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