Polyetheretherketone (PEEK) has high temperature resistance, fatigue resistance, chemical resistance, wear resistance, hydrolysis resistance, and excellent mechanical properties, barrier properties, electrical properties, radiation resistance and other properties. It has attracted extensive attention from domestic and foreign scholars, and has conducted in-depth research on the characteristics and modification of the material. It has been used in machinery, petrochemicals, nuclear power, rail transportation, and biomedical materials. PEEK film is an important form of PEEK in addition to its application as engineering plastics and fibers. Its main domestic application areas are electronic product diaphragms, thermal insulation film layers of aircraft sound insulation and thermal insulation blankets, heat-resistant insulation tapes, and flexible printed circuit boards. At present, PEEK film mainly relies on imports, and only a very small number of domestic companies have achieved industrial production of PEEK film.     This paper prepares a series of PEEK films by extrusion casting. The thickness tolerance of the prepared films is small and the film surface flatness is high. The basic properties, optical properties, mechanical properties and factors affecting the mechanical properties of PEEK films are discussed to provide some basic data support, hoping to accelerate the industrialization of PEEK films in China and expand the application range of PEEK films. Basic properties of PEEK films PEEK is a semi-crystalline polymer. The light transmittance and physical properties of the film are closely related to the structure and morphology of the polymer molecular chain. PEEK films are opaque brown in a state of high crystallinity and transparent light brown in amorphous to low crystallinity. The following figure is a photo of the prepared PEEK film. The density of PEEK film is 1.26~1.30 g/cm3, the density in amorphous state is 1.26g/cm3, the glass transition temperature of PEEK film is 145℃, the cold crystallization peak temperature is 174℃, and the melting point is 341℃. The PEEK film with a transmittance of 84.6% and a thickness of 95μm was heat treated at 250℃ for 20min. The figure below shows the wide-angle XRD spectrum of PEEK film before and after heat treatment. PEEK belongs to the orthorhombic system. The crystallinity of the film before heat treatment is low. After heat treatment, the PEEK film has 4 main diffraction peaks, which appear at 18.7°, 20.6°, 22.6°, and 28.6°, respectively, corresponding to the directions of 110, 111, 200, 211, and 202, respectively. The peaks in the directions of 211 and 202 overlap. The test data is consistent with the literature reports. Since PEEK is a semi-crystalline polymer, the amorphous region causes a higher diffraction peak background. Wide-angle XRD spectrum of PEEK film PEEK film can maintain high toughness in liquid nitrogen. The PEEK film prepared by extrusion casting has a casting roller temperature higher than the glass transition temperature of PEEK. Since the casting roller has a certain rotation speed, the film has a certain stretch in the longitudinal direction. From the figure, it can be observed that the longitudinal section of the film shows a high degree of orientation of the polymer molecular chain along the stretching direction. SEM photo of PEEK film cross section Relationship between film crystallinity and transmittance The temperature and speed of the casting roll directly affect the structural morphology and thickness of the film. The relationship between transmittance and crystallinity is shown in Table 1. When the film thickness is the same, the higher the crystallinity, the lower the transmittance. The haze of the film can also reflect the color state of the film. The higher the crystallinity, the greater the haze value. When the crystallinity is below 5%, the film is highly transparent and clear. PEEK film Analysis of mechanical properties of cast film Since there is no standard for PEEK film in China, the method of tensile testing of PEEK film in academic literature is not unified, and the influence of tensile rate on the mechanical properties of the film needs to be explored. The 18μm PEEK cast film was tested, and the tensile rate of the film was tested from 40 to 100 mm/min. The experimental results are shown in Table 2. The tensile strength of the samples shows a common point, that is, the tensile rate has little effect on the tensile strength and elongation at break. Increasing the tensile rate significantly increases the yield strength. When the stretching rate is 100 mm/min, the tensile strength of the film is still high. The mechanical properties of the PEEK film in this paper are tested at a stretching speed of 100 mm/min. The effect of the casting roller speed on the mechanical properties is shown in Table 3. Under the same extruder temperature and screw speed, different casting roller speeds will lead to changes in film thickness. The higher the casting roller speed, the thinner the film. Therefore, the film has a certain stretching in the longitudinal direction. As the casting roller speed increases, the stretching becomes larger. From the longitudinal tensile strength data, the tensile strength increases with the increase of the casting roller speed, from 129 MPa of the 95μm film to 155 MPa, and the elongation at break tends to decrease with the increase of the casting roller speed. Corresponding to the trend, the yield strength increases. From the transverse tensile data, the transverse tensile strength increases with the increase of the casting roller speed, but the elongation at break and the yield strength do not have an obvious fixed trend. Table 3 Effect of casting roll speed and extrusion temperature on film mechanical properties Extrusion temperature has an effect on the mechanical properties of the film. Due to the high processing temperature, partial degradation and other side reactions occur during the melt extrusion process. The melting point peak of PEEK is around 340℃, and DSC data show that it is completely melted at 350℃, so it can be processed at a processing temperature above 370℃. By comparing the mechanical properties at extrusion temperatures of 390, 400, and 420°C, the processing stability of PEEK materials was verified. As can be seen from Table 3, the mechanical properties of PEEK are stable at 390-400°C. Even if the temperature reaches 420°C, it can still maintain a high mechanical strength. PEEK has a wide processing temperature range, which is conducive to production and processing. The elongation at break of PEEK film at room temperature is high, but the tensile strength of cast film is low, and its application in some fields may be subject to certain restrictions. The key to controlling the thickness of cast film lies in the die head. The thickness tolerance of the film prepared by the cast method is large. The thickness tolerance of the film will be reduced by the method of biaxial stretching. At the same time, the tensile strength of the film can be improved by the method of biaxial stretching. Sample 1# was subjected to biaxial stretching test at 170℃, with a stretching rate of 0.3 m/min. When the stretching ratio exceeded longitudinal × transverse = 2.5 × 2.5 times, the film was prone to rupture. Therefore, the stretching ratio was adjusted to longitudinal × transverse = 2 × 2 to study the effect of biaxial stretching on film performance. The mechanical properties of biaxially stretched film sample 12# were compared with those of sample 3# before stretching and sample 8# with the same thickness. As can be seen from Table 4, biaxial stretching can greatly improve the mechanical properties of the film. The longitudinal tensile strength can reach 203MPa, but the elongation at break is reduced to 76%, and the yield strength and elastic modulus of the film are also greatly improved, that is, the rigidity of the film is good. Analysis of surface defects and mechanical properties of films In the experiment, it was found that the die head is the key factor restricting the surface quality of the film. Incomplete cleaning of the screw and die head affects the film surface of the PEEK film. After the extruder is shut down, it is difficult to clean the screw and die head when the PEEK raw material is restarted. There will be more coke and gel points on the film, which will affect the film surface quality and performance. The film was tested after rinsing the PEEK raw material for 2 hours, and samples 10#, 11#, and 13# were obtained, and compared with sample 8#. The results are shown in Table 5. Gel point and char material affect the film surface quality, but a small amount of gel point has little effect on the mechanical properties of the film. Analysis of film UV resistance The PEEK film was tested by UV absorption spectrum, and the absorption spectrum could not be obtained because it exceeded the detection range. The UV transmission spectrum is shown in the figure. UV transmittance spectrum of PEEK film The UV transmittance spectrum of 95μm film in the range of 200~400 nm shows that the UV transmittance below 380 nm is 0, indicating that PEEK film has anti-UV properties. PEEK film has a weak UV transmittance at 300nm. Figure 5(b) shows that the UV transmittance of the film with a thickness of 15~21μm at 300nm is 0.1%, while the UV transmittance of the film with a thickness of 95μm at 300nm is 0.02%. As the thickness of the film increases, the transmittance decreases. The UV transmittance spectra of PEEK films with different thicknesses in the range of 350~400nm. Table 6 shows the relationship between the UV transmittance at 380nm and the film thickness. It can be seen that as the thickness of the film increases, the transmittance of UV light at 360~380nm decreases, and the anti-ultraviolet performance is better. The UV transmittance spectra of PEEK films with the same thickness and different crystallinity are shown in Figure 5(d) and Table 7. There is a slight difference in the UV transmittance at 380nm. When the crystallinity is high, the anti-ultraviolet performance is better. In summary, the conclusions are as follows: (1) The transmittance of PEEK film decreases with the increase of crystallinity; the stretching rate has little effect on the tensile strength and elongation at break, and the faster the stretching rate, the significantly higher the yield strength; the tensile strength increases with the increase of the casting roller speed; the mechanical properties of PEEK film are stable when extruded at 390~400℃, and even if the extrusion temperature reaches 420℃, the PEEK film can still maintain a high mechanical strength;   (2) Biaxial stretching after casting can greatly improve the tensile strength of PEEK film. The stretching ratio is longitudinal × transverse = 2×2. The longitudinal tensile strength can reach 203 MPa and the elongation at break is 76%. The transverse tensile strength is 157MPa and the elongation at break is 156%;   (3) PEEK film has anti-ultraviolet properties. As the film thickness increases and the crystallinity increases, the transmittance of ultraviolet light at 360~380 nm decreases, and the anti-ultraviolet performance is better.   (Articles and pictures are from the Internet)