Polyvinylidene fluoride (PVDF) is a polycrystalline semi-crystalline polymer with a variety of different crystalline phase forms such as α, β, γ, and δ . Among them, PVDF with α phase as the dominant has high mechanical strength. , High temperature resistance, chemical resistance and other excellent properties, it is the best choice for the inner sheath material in the flexible pipeline [2,3]. It is affected by the high temperature of 130 ℃ in the actual working conditions of the pipeline, which can easily cause structural changes and performance degradation. Therefore, the high-temperature thermal aging research of PVDF is of great significance to the practical application and modification development of the material. At present, research reports on PVDF thermal aging cannot fully explain its long-term thermal aging behavior when it is applied to flexible pipe sheaths. Among them, Silva et al.  studied the aging process of PVDF exposed to ethanol fuel and found that the temperature is 60 ℃ The induction effect will not have a major impact on its chemical structure and dynamic mechanical properties. Cirilo  et al. conducted a 30-day study on the thermal aging behavior of PVDF, and found that the annealing effect of the material will be produced at a short time of 130 ℃. However, the thermal aging research conducted under long-term low temperature  or short-term high temperature  is different from the long-term high temperature environmental conditions actually used for sheath materials, and domestic PVDF thermal aging related research has been rarely reported. . For this reason, this paper selects a long period of time (60 d) to simulate the actual working temperature (130 ℃) to study the thermal aging of PVDF, and systematically analyze the influence of high temperature induction effects on the structure and performance of PVDF at different times, including crystallinity, crystallization Structure and mechanical properties, etc., through multi-scale analysis methods, from macro to micro testing to characterize the change law of materials to fully understand the thermal aging behavior of PVDF, provide a theoretical reference for its further modification research, and provide a way to prevent functional failure of flexible pipelines Theoretical support. 1 Experimental part 1.1 Reagents and instruments PVDF: model Kynar 400 COA, density 1.76～1.78 g/cm3, thickness 4 mm, thermal expansion coefficient 100～140 μm/(m·K-1), melting point 170 ℃, operating temperature -40 ～150 ℃, Shanghai Sanaifu Company. Electronic universal testing machine
: model WDW-5E, Jinan Star Testing Technology Co., Ltd.; Fourier transform infrared spectrometer: model Nicolet iN10MX, American Thermo Fisher Company; X-ray diffractometer: XRD-7000S, Japan Shimadzu Company; differential Scanning calorimeter: Model TGA/DSC1, METTLER TOLEDO. 1.2 Sample preparation and aging process The experimental raw materials are taken from standard dumbbell-shaped specimens made by cutting the PVDF core tube, and the size information refers to the ASTM D638 standard. Put batches of PVDF specimens in an oven and set them to heat aging at a high temperature of 130 ℃ for 0-60 d. The specimens are divided into 7 groups according to different aging time. One group of specimens is taken out every 10 d and left to cool to room temperature. , Mark its heat aging days for testing. 1.3 Performance test and characterization 1.3.1 Macro-mechanical performance test: The tensile test is carried out in accordance with ASTM D638 standard. Using the electronic universal testing machine, set the tensile rate to 5 mm/min, perform the same batch of mechanical tensile tests on 7 groups of specimens with different aging days (0～60 d), and take 5 specimens from each group. The average value obtains the data of tensile strength and elongation at break of PVDF for different heat aging time. 1.3.2 Characterization of microscopic crystal phase structure: X-ray diffraction (XRD) analysis, wavelength is 0.154 nm, scanning range is 10.0°～60.0°, scanning speed is 1(°)/min. Fourier transform infrared spectroscopy (FT-IR) analysis, using attenuated total reflection method for testing, operating in the range of 500 to 4000 cm-1, the spectral resolution is 4 cm-1. Differential scanning calorimetry (DSC) analysis. Weigh about 10 mg of sample and perform two heating and cooling cycles at a rate of 10 ℃/min under nitrogen protection. The temperature range is from 25 ℃ to 210 ℃, and the crystallization of PVDF is recorded. Melting curve. 2 Results and discussion 2.1 The effect of thermal aging for different time on the mechanical properties of PVDF. Fig. 1 shows the change of the tensile strength of PVDF with the aging time when the aging temperature is 130 ℃. It can be seen that with the extension of the aging time, the tensile strength of PVDF shows a trend of first increasing and then decreasing. In the initial stage of thermal aging, the tensile strength gradually increases. This is because short-term annealing can promote the perfect structure of PVDF, the intermolecular arrangement is closer, and the ability to resist external forces is enhanced, resulting in an increase in tensile strength [4,5]; thermal aging progresses After 40 days later, the tensile strength decreased significantly. Until the end of aging, the tensile strength of PVDF has dropped from the maximum value to lower than the initial strength. It shows that the annealing effect in the later period gradually disappears, and the high temperature aging of PVDF has a major influence on the attenuation of tensile strength. Fig.2 shows the change of the elongation at break of PVDF with the progress of aging. It can be seen from the figure that under the influence of continuous high temperature, the elongation at break of PVDF shows a single downward trend. In practical applications, in order to improve the processability of PVDF, it is usually necessary to add a plasticizer, and the plasticizer will be lost under the influence of temperature, which will reduce the toughness of PVDF. Different types of PVDF are used as pipes with different plasticizer content. The Coflon ○R type PVDF plasticizer developed by Technip in the early stage is about 12%. After thermal aging, a large amount of volatilization causes a serious decline in performance. (2%～4%) Plasticized PVDF, such as Kynar 400 COA, Coflon XD○R and other models, reduces the impact of the plasticization process in high temperature environments .
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