Effects of Thermal Aging on the Structure and Properties of Polyvinylidene Fluoride

Polyvinylidene fluoride (PVDF) is a polymorphic semi-crystalline polymer withα,β,γandδand many other different crystalline phase forms [1], among whichαPhase-dominated PVDF has excellent properties such as high mechanical strength, high temperature resistance, and chemical resistance, and is the best choice for the inner sheath material in flexible pipes [2,3]. It is affected by the high temperature of 130 ℃ in the actual working condition of the pipeline, which is easy to cause structural changes and performance degradation. Therefore, the research on high temperature thermal aging of PVDF is of great significance for the practical application and modification development of the material. At present, research reports on thermal aging of PVDF cannot fully explain its long-term thermal aging behavior when applied to flexible pipe jackets. Silva et al. [2] studied the aging process of PVDF exposed to ethanol fuel and found that at 60 °C temperature The induced effect will not have a great influence on its chemical structure and dynamic mechanical properties. Cirilo[3] et al. conducted a 30-day thermal aging study of PVDF, and found that the annealing effect will occur in the material under the condition of 130 ℃ for a short time. However, the thermal aging research under long-term low temperature[2] or short-term high temperature[3] conditions is different from the long-term high temperature environmental conditions for the practical application of sheath materials, and domestic PVDF thermal aging related research has been rarely reported. . Therefore, this paper selects a long period of time (60 d) to simulate the actual working temperature (130 ℃) to conduct thermal aging research on PVDF, and systematically analyzes the influence of high temperature induced effects on the structure and properties of PVDF at different times, including crystallinity, crystallization Structure and mechanical properties, etc., through multi-scale analysis methods, from macroscopic to microscopic testing to characterize the change law of the material to fully understand the thermal aging behavior of PVDF, provide a theoretical reference for its further modification research, and prevent the functional failure of flexible pipes. 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 100～140μm/(m·K-1), melting point 170 ℃, operating temperature -40～150 ℃, Shanghai Sanaifu Co., Ltd. Electronic universal testing machine: Model WDW-5E, Jinan Star Testing Technology Co., Ltd.; Fourier transform infrared spectrometer: Model Nicolet iN10MX, Thermo Fisher, USA; X-ray diffractometer: XRD-7000S, Shimadzu, Japan; Differential Scanning Calorimeter: Model TGA/DSC1, METTLER TOLEDO. 1.2 Sample preparation and aging process The experimental raw materials were taken from standard dumbbell-shaped specimens cut from PVDF core tubes. For size information, refer to the ASTM D638 standard. The batch of PVDF specimens were placed in an oven and thermally aged for 0-60 d at a high temperature of 130 °C. The specimens were divided into 7 groups according to different aging times. One group of specimens was taken out every 10 d, and allowed to cool to room temperature. , mark its heat-aging days for testing. 1.3 Performance test and characterization 1.3.1 Macroscopic mechanical property test: The tensile test was carried out according to ASTM D638 standard. Using an electronic universal testing machine and setting the tensile rate to 5 mm/min, the same batch of mechanical tensile tests were performed on 7 groups of specimens with different aging days (0-60 d), and 5 specimens were taken from each group. The average value is obtained to obtain the tensile strength and elongation at break data of PVDF with different heat aging time. 1.3.2 Characterization of microcrystalline phase structure: X-ray diffraction (XRD) analysis, the wavelength is 0.154 nm, and the scanning range is 10.0°~60.0°, the scan speed is 1 (°)/min. Fourier transform infrared spectroscopy (FT-IR) analysis was performed using attenuated total reflection, operating in the range of 500 to 4000 cm-1, with a spectral resolution of 4 cm-1. For differential scanning calorimetry (DSC) analysis, a sample of about 10 mg was weighed and subjected to 2 heating-cooling cycles at a rate of 10 °C/min under nitrogen protection. The temperature ranged from 25 °C to 210 °C, and the crystallization of PVDF was recorded. melting curve. 2 Results and discussion 2.1 The effect of thermal aging on the mechanical properties of PVDF at different times 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 prolongation of aging time, the tensile strength of PVDF increases first and then decreases. In the early stage of thermal aging, the tensile strength gradually increases, because short-term annealing can improve the structure of PVDF, the intermolecular arrangement is more compact, and the ability to resist external forces is enhanced, resulting in an increase in tensile strength [4,5]; After 40 d in the later period, the tensile strength decreased greatly, and until the end of aging, the tensile strength of PVDF had dropped from the maximum value to lower than the initial strength. It shows that the annealing effect in the later stage gradually disappears, and the attenuation effect of the high temperature aging on the tensile strength of PVDF occupies the main influence. Fig.2 shows the change of elongation at break of PVDF with aging.
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