【Deep Coating Society Coating Technology】Study on the characteristics of polyetheretherketone PEEK film

Film haze can also reflect the color state of the film, high crystallinity, haze value increases
.
When the crystallinity is less than 5%, the film is highly transparent and high-definition
.

PEEK film

• Mechanical properties analysis of cast film

Since there is no standard for PEEK films in China, the methods of tensile testing of PEEK films in the academic literature are not uniform, and the influence of tensile rate on the mechanical properties of thin films needs to be explored
.
The 18 μm PEEK cast film was tested, and the tensile rate of the film was tested from 40 to 100 mm/min, and the experimental results were shown in
Table 2.

Table 2 Relationship between mechanical properties and tensile rate

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 tensile rate was improved, and the yield strength was significantly improved
.
When the tensile rate is 100mm/min, the tensile strength of the film is still high, and the mechanical properties of the PEEK film are tested
at a tensile speed of 100 mm/min.

The effect of cast roll speed on mechanical properties is shown in
Table 3.
Under the same extruder temperature and screw speed, the different speed of the cast roller will lead to a change in the thickness of the film, the higher the speed of the cast roll, the thinner the film, so the film has a certain stretch in the longitudinal direction, and with the increase of the speed of the cast roll, the stretch becomes larger
.
From the longitudinal tensile strength data, the tensile strength increases with the increase of the speed of the cast roll, from 129 MPa of 95 μm film to 155 MPa, and the elongation at break decreases with the increase of the speed of the cast roll, corresponding to the trend
, the yield strength is improved.
From the perspective of lateral stretching data, the lateral tensile strength increases with the increase of the speed of the cast roll, but there is no obvious fixed trend
in the elongation at break and yield strength.

Table 3 Effects of cast roll speed and extrusion temperature on the mechanical properties of thin films

The extrusion temperature has an impact on the mechanical properties of the film, and due to the high processing temperature, the side reactions such as partial degradation occur
during the melting extrusion process.
The peak melting point of PEEK is around 340 °C, and the DSC data shows that it has fully melted at 350 °C, so the processing temperature can be processed above 370 °C
.
The processing stability of PEEK materials is verified
by comparing the mechanical properties at extrusion temperatures of 390, 400 and 420°C.
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 high mechanical strength, and PEEK has a wide processing temperature, which is conducive to production and processing
.

• Analysis of the mechanical properties of biaxially oriented films

PEEK films have a high elongation at break at room temperature, but the tensile strength of cast films is low, and applications in some areas may be limited
.
The key to the control of the thickness of the cast film lies in the die, the thickness tolerance of the film prepared by the casting method is large, and the tolerance of the film thickness will be reduced by the biaxially oriented method
.

At the same time, the tensile strength of the film can be improved by the method of biaxial drawing
.
The biaxially tensile test of sample 1# at 170 °C was carried out with a tensile rate of 0.
3 m/min, and when the tensile magnification exceeded the longitudinal × transverse = 2.
5 × 2.
5 times, the film was prone to rupture, so the tensile ratio was adjusted to longitudinal × transverse = 2×2 to study the influence
of biaxial stretching on the film performance.

Biaxially stretched film sample 12# and pre-stretching sample 3# and thickness of the sample 8# for mechanical properties comparison, from Table 4 can be seen that biaxially oriented stretching can greatly improve the mechanical properties of the film, longitudinal tensile strength can reach 203MPa, but the elongation at break is reduced to 76%, the yield strength and elastic modulus of the film are also greatly improved, that is, the film rigidity is good
.

Table 4 Mechanical properties of biaxially oriented PEEK films

• Surface defects and thin film mechanical properties analysis

In the experiment, it was found that the die was a key factor
restricting the surface quality of the film.
Unclean screw and die cleaning has an impact
on the membrane surface of PEEK film.
After the extruder is stopped, it is difficult for PEEK raw materials to rinse the screw and die when starting again, and there will be more coke and gel spots on the film, which will affect the quality and performance
of the film film.
The film was tested after 2 h of flushing of PEEK raw materials, and samples 10#, 11#, 13# were obtained, and compared with sample 8#, and the results were shown in
Table 5.
Gel points and coke affect the film surface quality, but a small number of gel spots have little
effect on the mechanical properties of the film.

Table 5 Effect of surface defects on the mechanical properties of thin films

• Thin film UV resistance analysis

PEEK films have been tested for ultraviolet absorption spectroscopy, exceeding the detection range and cannot obtain an absorption spectrum
.
The ultraviolet transmission spectrum is shown in
Figure 5.

Figure 5(a) shows the UV transmission spectrum of a 95μm film in the range of 200 to 400 nm, and the UV light transmittance below 380 nm is 0, indicating that PEEK film has anti-ultraviolet characteristics
.

PEEK film has a weak UV light transmittance at 300nm, Figure 5(b) shows that the thin film with a thickness of 15 to 21 μm has an ultraviolet light transmittance of 0.
1% at 300 nm, while a film with a thickness of 95 μm has an ultraviolet light transmittance of 0.
02% at 300nm, and the transmittance decreases as the thickness of the film increases
.

Figure 5(c) is the ultraviolet transmission spectrum of PEEK films of different thicknesses in the range of 350~400nm, Table 6 shows the relationship between 380nm UV light transmittance and film thickness, it can be seen that with the increase of film thickness, the UV light transmittance at 360~380 nm decreases, and the anti-UV performance is better
.

Table 6 Relationship between ultraviolet light transmittance and film thickness

Figure 5(d) is the ultraviolet transmission spectrum of PEEK films of the same thickness and different crystallinity, as shown by the data of Figure 5(d) and Table 7, the UV light transmittance of 380nm is slightly different, and the UV resistance is better
when the crystallinity is high.

Table 7 Relationship between ultraviolet light transmittance and crystallinity

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 increase of the flow roll rate; PEEK extrusion experiment at 390~400°C, the mechanical properties of the film are stable, even if the extrusion temperature reaches 420°C, PEEK film can still maintain high mechanical strength;

(2) Biaxial stretching after casting can greatly improve the tensile strength of PEEK film, the tensile magnification is longitudinal × transverse = 2×2, the longitudinal tensile strength can reach 203 MPa, the elongation at break is 76%, the lateral tensile strength is 157MPa, and the elongation at break is 156%;

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

Source: Fang Tianxia

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