The Properties of Polyetheretherketone PEEK Film

The Properties of Polyetheretherketone PEEK Film

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, and has attracted extensive attention from domestic and foreign scholars. The characteristics and modification of materials have been studied in depth, and they have been applied in the fields of machinery, petrochemical, nuclear power, rail transit, and biomedical materials.

PEEK film is an important form of PEEK in addition to being used as engineering plastics and fibers. In the domestic application field, it is mainly the diaphragm of electronic products, the heat insulation film layer of aircraft sound insulation and heat insulation blanket, heat-resistant insulating tape and flexible printed circuit board. . At present, PEEK films mainly rely on imports, and only a handful of domestic enterprises have realized the industrial production of PEEK films.

A series of PEEK films were prepared by extrusion casting method. The thickness tolerance of the prepared films was small and the film surface flatness was high. The basic properties, optical properties, mechanical properties and factors affecting the mechanical properties of PEEK films were discussed to provide some basis Supported by data, it is hoped to speed up the industrialization of PEEK film in China, and then expand the application range of PEEK film.



Basic properties of PEEK film

PEEK is a semi-crystalline polymer. The light transmittance and physical properties of the film are closely related to the structure and shape of the polymer molecular chain. The PEEK film is opaque brown in the state of high crystallinity, and in the state of amorphous to low crystallinity. It is transparent and light brown, and the picture below is the photo of the prepared PEEK film.

The density of PEEK film is 1.26~1.30 g/cm3, the density of amorphous form is 1.26g/cm3, the glass transition temperature of PEEK film is 145°C, the cold crystallization peak temperature is 174°C, and the melting point is 341°C.

The PEEK film with a light transmittance of 84.6% and a thickness of 95 μm was heated at 250 °C for 20 min for heat treatment. The figure below shows the wide-angle XRD spectrum of the PEEK film before and after heat treatment. PEEK belongs to the orthorhombic crystal system, and the crystallinity of the film before heat treatment is low. After heat treatment, the PEEK film has four main diffraction peaks, which appear at 18.7 °, 20.6 °, and 22 . 6 °, 28.6 °, corresponding to 110, 111, 200, 211 and 202 directions respectively, in which the peaks of 211 and 202 directions overlap. High background of diffraction peaks.

The picture below is the SEM photo of the cross-sectional structure of the PEEK film. The PEEK film can maintain a high degree of toughness in liquid nitrogen. For the PEEK film prepared by extrusion casting method, the temperature of the casting roll is higher than the glass transition temperature of PEEK. Since the casting roll has a certain speed, the film has a certain stretch in the longitudinal direction, and the longitudinal direction of the film can be observed from the figure. On the cross-section, the polymer molecular chains exhibit a high degree of orientation along the stretching direction.

The relationship between film crystallinity and light transmittance

The temperature and rotation speed of the casting roll directly affect the structure and thickness of the film. The relationship between light transmittance and crystallinity is shown in Table 1. When the film thickness is the same, the higher the crystallinity, the lower the light transmittance.

The haze of the film can also reflect the color state of the film, the crystallinity is high, and the haze value increases. When the crystallinity is below 5%, the film is highly transparent and clear.

Analysis of Mechanical Properties of Cast Film

Since there is no domestic standard for PEEK films, the methods of tensile testing of PEEK films in academic literature are not uniform, and the influence of stretching rate on the mechanical properties of films needs to be explored. The 18 μm PEEK cast film was used for testing, and the stretching 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 showed a common point, that is, the tensile rate had little effect on the tensile strength and elongation at break, and the yield strength increased significantly when the tensile rate was increased. When the stretching rate is 100 mm/min, the tensile strength of the film is still high. In this paper, the mechanical properties of the PEEK film are tested at a stretching speed of 100 mm/min.

The influence of casting roll speed on mechanical properties is shown in Table 3. Under the same extruder temperature and screw speed, different casting roll speeds will lead to changes in film thickness. The higher the casting roll speed, the thinner the film, so the film has a certain stretch in the longitudinal direction. With the increase of the roll speed, the stretch becomes larger. From the longitudinal tensile strength data, the tensile strength increases with the increase of casting roll speed, from 129 MPa of 95 μm film to 155 MPa, and the elongation at break decreases with the increase of casting roll speed. Corresponding to the trend, the yield strength increased. From the transverse tensile data, the transverse tensile strength increases with the increase of casting roll speed, but the elongation at break and yield strength have no obvious fixed trend.

The extrusion temperature has an influence on the mechanical properties of the film. Due to the high processing temperature, side reactions such as partial degradation occur during the melt extrusion process. The peak melting point of PEEK is around 340°C, and the DSC data shows that it is completely melted at 350°C, so the processing temperature can be processed above 370°C. The processing stability of PEEK materials was verified by comparing the mechanical properties at extrusion temperatures of 390, 400, and 420°C. It can be seen from Table 3 that the mechanical properties of PEEK are stable at 390-400 °C, and even when the temperature reaches 420 °C, it can still maintain high mechanical strength. PEEK has a wide processing temperature, which is conducive to production and processing.

Analysis of Mechanical Properties of Biaxially Stretched Film

The elongation at break of PEEK film is relatively high at room temperature, but the tensile strength of cast film is low, which may be limited in some fields. The key to control the thickness of the cast film lies in the die head. The thickness tolerance of the film prepared by the cast method is relatively large, and the biaxial stretching method will reduce the thickness tolerance of the film.

At the same time, the tensile strength of the film can be improved by biaxial stretching. The sample 1# was subjected to a biaxial tensile test at 170°C, and the stretching rate was 0.3 m/min. When the stretching ratio exceeded the longitudinal × transverse direction = 2.5×2.5 times, the film was prone to rupture, so the stretching ratio was adjusted to Longitudinal × transverse = 2 × 2, in order to study the effect of biaxial stretching on film properties.

Compare the mechanical properties of the biaxially stretched film sample 12# with the sample 3# before stretching and the sample 8# with the same thickness. It can be seen from Table 4 that the biaxial stretching can greatly improve the mechanical properties of the film. The 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 Thin Film Mechanical Properties

In the experiment, it was found that the die head is the key factor restricting the surface quality of the film. If the screw and die head are not cleaned cleanly, it will affect the membrane surface of PEEK film. After the extruder is shut down, it is difficult to clean the screw and die head when the PEEK raw material is turned on again, and there will be more burnt materials and gel points on the film, which will affect the film surface quality and performance. The film after rinsing the PEEK raw material for 2 h was tested to obtain samples 10#, 11#, and 13#, and compared with sample 8#, the results are shown in Table 5. The gel point and coking material affect the quality of the film surface, but a small amount of gel point has little effect on the mechanical properties of the film.

Thin film UV resistance analysis

The PEEK film has been tested by ultraviolet absorption spectrum, which exceeds the detection range, and the absorption spectrum cannot be obtained. The UV transmission spectrum is shown in the figure.

In the figure (a) is the ultraviolet transmission spectrum of the 95 μm film in the range of 200-400 nm. It can be seen that the ultraviolet light transmittance below 380 nm is 0, indicating that the PEEK film has anti-ultraviolet properties.

The 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 is 0.1% at 300nm, while the film with a thickness of 95μm transmits UV light at 300nm. The ratio is 0.02%, and the transmittance decreases with the increase of film thickness.

Figure 5(c) is the UV transmission spectrum of PEEK films with different thicknesses in the range of 350-400nm. Table 6 shows the relationship between the 380nm UV transmittance and the film thickness. Reduced transmittance at ~380 nm and better UV resistance.

Figure 5(d) is the ultraviolet transmission spectrum of PEEK films with the same thickness and different crystallinity. As shown in Figure 5(d) and the data in Table 7, there is a slight difference in the 380nm ultraviolet light transmittance. When the crystallinity is high, the anti-ultraviolet Performance is better.

In summary, the conclusions are as follows:

(1) The light transmittance of PEEK film decreases with the increase of crystallinity; the tensile rate has little effect on the tensile strength and elongation at break, the tensile rate is fast, and the yield strength is significantly improved; the tensile strength increases with the flow casting The increase of the roll speed; PEEK extrusion experiment at 390~400 ℃, the mechanical properties of the film are stable, 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 tensile strength is 157MPa, and the elongation at break is 156%;

(3) PEEK film has anti-ultraviolet properties, the thickness of the film increases, the crystallinity increases, the transmittance of ultraviolet light at 360-380 nm decreases, and the anti-ultraviolet performance is better.