Ethylene tetrafluoroethylene copolymer (ETFE) is a semi crystalline fluorinated polymer formed by copolymerization of ethylene and tetrafluoroethylene. It has excellent properties such as corrosion resistance, weather resistance, high temperature resistance, aging resistance, and radiation resistance. Due to its lightweight, high transparency, and environmental friendliness, it is widely used in various fields such as aerospace, deep-sea, weapons and equipment, nuclear power plants, oil fields, construction, agriculture, chemical industry, transportation, etc. after being processed into coatings, films, and cables. Compared to other application fields, the current focus of ETFE applications is mainly on wires and cables. ETFE has excellent physical, chemical, electrical, mechanical, and environmental aging resistance properties, which can solve problems encountered during cable use such as flame retardancy, weather resistance, corrosion resistance, vacuum escape, etc. It is an ideal choice for special cable insulation materials used in aerospace. Special communication cables made from ETFE have become an indispensable key component for modern defense and aerospace equipment to achieve reliable signal and energy transmission.

At the same time, ETFE also has the disadvantages of low surface energy and poor adhesion performance, which limits its application in bonding, potting, and other fields. Therefore, it is necessary to modify the surface of ETFE. Modifying ETFE insulation materials through surface engineering is an effective way to comprehensively improve the performance of ETFE insulated cables. How to use surface modification technology to efficiently and high-quality prepare ETFE insulated cables that meet the requirements of modern and future national defense and aerospace equipment has always been a problem that troubles researchers in related fields. There are many surface treatment methods for ET-FE, including chemical treatment, irradiation treatment, plasma treatment, mechanochemical treatment, gas thermal oxidation, etc. Among them, plasma treatment has become a research hotspot for surface modification of ETFE materials due to its characteristics of low processing temperature and short time, no impact on the inherent properties of the substrate, and no pollution, and has made certain progress.

This article uses atmospheric pressure air plasma to treat the surface of ETFE, achieving surface modification of ETFE, and studying the adhesive properties of ETFE after treatment. The changes in the composition, structure, wettability, and adhesive properties of ETFE before and after plasma treatment were analyzed using total reflection infrared spectroscopy (ATR-FTIR), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), water contact angle (WCA), and mechanical property testing; Explored the effects of different processing times, powers, and placement times on the wettability and adhesion properties of ETFE surfaces.

Characterization analysis of ETFE

Characterize and analyze the structure of ETFE before and after plasma treatment using ATR-FTIR and XRD. As shown in Figure 1 (a), the peaks at 2976 and 2881cm-1 represent the stretching vibrations of the - CH2 and - CH3 groups, while the peak at 1452cm-1 is the bending vibration peak of the C-H bond. In addition to the characteristic peaks attributed to polyethylene mentioned above, additional characteristic stretching vibration peaks of - CF2 groups can also be clearly detected in the infrared spectrum of the sample at 1250 and 1165 cm-1. The characteristic peaks of the infrared spectrum confirm that this sample is indeed ETFE, and no new characteristic peaks appeared after plasma treatment. Similarly, the XRD spectrum information of ETFE before and after ion treatment is basically consistent, with characteristic peaks of typical semi crystalline polymers appearing around 19 ° and 40 °, indicating that plasma treatment did not damage the structure of ETFE.

Figure 1 FTIR and XRD spectra of ETFE before and after plasma treatment

The influence of plasma treatment conditions on the wettability of ETFE surface

The surface of ETFE was treated with plasma, and the variation of contact angle with treatment time is shown in Figure 2. It can be seen that the contact angle of untreated ETFE is 94.2 °. After plasma treatment, the contact angle on the surface of ETFE significantly decreases. After 650W treatment, the contact angle rapidly decreases to 59.5 ° within 5 seconds, and after 30 seconds of treatment, the contact angle is 56.8 °. Then, with the extension of plasma treatment time, the change in contact angle is not significant, fluctuating around 54 ° and tending to stabilize. The change trend of the surface contact angle of ETFE after changing the plasma power to 950W is consistent with that at 650W, except that the surface contact angle decreases relatively significantly after the same treatment time, and finally stabilizes around 53 °.

Figure 2 (a) The effect of plasma treatment conditions on the contact angle of ETFE, ETFE (b) without plasma treatment, surface contact angle after 650 W treatment (c) 30 s, (d) 300 s, and 950 W treatment (e) 10 s, (f) 30 s, (g) 300 s

When modifying ETFE with plasma, high-energy particles such as electrons and ions in the plasma violently bombard the surface, causing surface chemical bond breakage and etching. From the SEM images of the ETFE surface before and after plasma treatment with different powers, it can be seen that the untreated ETFE surface is relatively smooth and flat, with only some traces caused by processing (Figure 4 (a)). After 10 seconds of 650W plasma treatment, there was no significant change in the surface, and the roughness only slightly increased. However, after 950W treatment, a more obvious rough structure was produced (Figure 3). After 30 seconds of treatment, obvious etching marks began to appear on the surface of ETFE, with irregular etched grooves. As the treatment time increased, the etching gradually became more severe, until a very serious etching groove appeared on the surface after 300 seconds. It is worth noting that under the same processing time, the surface etching after 950W plasma treatment is more severe than that of 650W treatment, and a dense "ablative" structure is formed on the surface after 300 seconds of 950W treatment, which was not observed before 650W treatment. The main reason for this phenomenon is that the ETFE surface exhibits intrinsic non-uniformity, with a certain degree of crystallization. The molecular segments in the crystalline region are relatively regular and dense, making it difficult to etch, while the segments in the amorphous region are arranged in an unordered manner, relatively loose, and have low density, making them easy to etch. In addition, cross-linking reactions may occur during the processing, and the cross-linked area will be relatively stable and less prone to etching reactions. In the initial stage of treatment, etching reaction mainly occurs, and the cross-linking reaction rate is slow. However, with the increase of treatment time, the amorphous region is severely etched, the degree of surface cross-linking becomes higher, the content of crystalline region increases relatively, and the etching rate slows down. Finally, only some difficult to etch structures remain on the surface. At the same time, the thermal accumulation effect can also cause thermal deformation on the surface of ETFE after long-term treatment, and the interaction of the two factors together forms a special structure on the surface after EFTE plasma treatment.

Figure 3 SEM images of the surface of ETFE treated with (a) 650 W and (b) 950 W plasma for 10 s, 30 s, 120 s, and 300 s (from left to right)

From the surface energy spectrum photos before and after ETFE plasma treatment (Figure 4), it can be seen that there are only two elements, C and F, on the surface before and after treatment, which are consistent with their chemical composition. After treatment, O elements that do not belong to its composition were detected on the surface, confirming the introduction of active groups such as free radicals on the ETFE surface through plasma treatment, thereby increasing surface energy, enhancing wettability, and reducing contact angle. However, due to the thermal accumulation effect of plasma, ETFE samples began to undergo thermal deformation after being treated with 950W for 60 seconds, and the deformation gradually worsened with the extension of treatment time. Correspondingly, thermal deformation began to occur after 120 seconds of treatment with 650W, and the degree of deformation of the sample was relatively mild after 300 seconds of treatment.

Figure 4 ETFE (a) SEM and energy spectrum photos without plasma treatment, (b) surface energy spectrum photos after 30 seconds of 650 W treatment

The influence of plasma treatment conditions on the surface adhesion of ETFE

As shown in Figure 5, the bonding strength of untreated ETFE after bonding and curing with adhesive (epoxy resin) was 0.22MPa. After plasma treatment, the bonding strength of the samples was significantly improved. After 30 seconds of 650W plasma treatment, the bonding strength increased to a maximum of 1.78MPa, which is about 8.1 times that of the untreated sample. Continuing to increase the processing time, the bonding strength slightly decreased. The significant improvement in the adhesion performance of the sample can be mainly attributed to: 1) After plasma treatment, the surface energy and surface wettability of ETFE surface increase, making the adhesive spread more evenly while utilizing chemical reactions with high surface energy groups on the surface to enhance the adhesion performance; 2) After plasma treatment, the surface roughness of ETFE increases, creating a mechanical "riveting effect" and improving the bonding performance.

Figure 5 Adhesive strength of ETFE before and after plasma treatment