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In the process of manufacturing ePTFE film, it is found that different factors lead to different structures, and the strength of the film is different, which in turn affects the waterproof performance of the film.
1. The waterproof and breathable film has a significant effect on the tensile strength and elongation at break of the film under actual conditions. Compared with the unstretched polytetrafluoroethylene tape, the strength of the film is doubled. . The tensile strength of the microporous film increases as the stretching temperature increases, and the elongation at break of the microporous film decreases as the stretching temperature increases. This is because as the stretching temperature increases, the molecular chain activity is higher, the wafer is more easily unwound, and the degree of orientation of the microfibers in the waterproof gas-permeable membrane is more ambiguous. When the draw ratio is small, the length of the microfibers is small, and the degree of orientation is low, and when the draw ratio is large, micropores having a more uniform size distribution can be obtained. Although the degree of orientation of the microfibers is increased, the larger microfiber size causes the microfibers to be in an orientated orientation in the subsequent heat setting treatment, and thus the strength becomes small.
Second, the biaxial stretching process of the polytetrafluoroethylene film makes the film in the waterproof and breathable film have good void characteristics, and the film in different directions is stretched, and the structure of the film micropores and agglomerates changes, and at the same time The mechanical properties of the film have anisotropic properties. This anisotropic property of the film is closely related to the stretching process of the film. Since the stretching ratio of the film in two directions is different, the arrangement state of the long agglomerates and the filaments in different directions is also different. The orientation of the microfibers is mainly along the transverse and longitudinal directions of the film, so that the film has relatively good mechanical properties in the longitudinal and transverse directions; while the film is more easily deformed in the oblique direction, the mechanical properties are significantly worse than the longitudinal and transverse directions of the film.
Third, the transverse stretching of the ePTFE film is non-uniform stretching, and the transverse stress along the transverse direction of the film exhibits a high and low intermediate variation on both sides, and the stress is gradually transmitted from both sides to the middle; Lateral to the center of the film, the lateral displacement continues to decrease. Due to the different degree of stretching in the transverse direction, the actual stretching ratio gradually increases from the middle to the two sides, and the actual stretching in the middle is the smallest, which ultimately leads to uneven structure and performance in the transverse direction of the film. In the transverse stretching process of the biaxially stretched PTFE microporous film, the transverse tensile stress is gradually transferred from the two sides to the middle. Due to the tensile stress transmission and the hysteresis of the material response, the transverse tensile stress is in the transverse direction. It has a large intermediate distribution on both sides, resulting in unevenness in stretching. In the case of applying the same stress to the ePTFE film, increasing the stretching temperature increases the deformation rate of the film, and the thickness is thinned; increasing the stretching rate is advantageous for improving the uniformity of the film.
4. This anisotropy of the film in various types of waterproof and breathable materials may affect the mechanical properties of the film material. The rational distribution of the film and the film substrate may also utilize the anisotropic properties of the film. The mechanical properties of the coating material are optimized.