Silicone rubber is widely adopted as cable material thanks to its excellent weather resistance and insulating properties. In practical applications, markings are printed on cable surfaces to distinguish product models and manufacturers. Silicone rubber cables are generally stacked during storage and transportation. Under contact conditions, ink molecules on silicone rubber surfaces diffuse and permeate mutually via thermal motion, resulting in printing contamination. Accordingly, surface treatment is required for silicone rubber to enhance its anti-printing contamination performance without compromising insulation and other comprehensive properties.

At present, chemical and physical methods are the two mainstream solutions to this issue. Physical methods clean ink-contaminated silicone rubber surfaces, while chemical methods modify surfaces by introducing functional groups. Nevertheless, existing techniques still have drawbacks in terms of treatment efficiency, volatile organic compound (VOC) control and long-term effectiveness. Therefore, a novel surface modification technology is urgently needed to strengthen the anti-printing contamination capacity of silicone rubber surfaces.

Low-temperature plasma surface modification technology has attracted extensive attention due to its simple processing conditions, high efficiency and zero pollution. In recent years, substantial progress has been made in domestic and international research on improving the surface properties of insulating materials via low-temperature plasma treatment.

At present, dielectric barrier discharge and atmospheric pressure plasma jet are the two primary generation modes of atmospheric low-temperature plasma for material surface modification. Among them, atmospheric pressure plasma jet features spatial separation between plasma generation and modification zones, making it suitable for surface treatment of cables.

Plasma Modification Effects

To evaluate the anti-printing contamination performance of silicone rubber, a jet plasma processor was used to treat printed silicone rubber samples. The treated samples were then extruded against untreated blank silicone rubber sheets with a 10 kg weight. After standing for 24 hours, the surfaces were inspected for printing contamination. Variations in surface hydrophobicity before and after modification were characterized by a water contact angle measuring instrument.

Surface Water Contact Angle

Figure 1 presents the measured water contact angles of silicone rubber surfaces before and after jet plasma treatment. As shown in Figure 1, the water contact angle of untreated silicone rubber is 113°. When the sample moving speed is 1 mm/s, the surface water contact angle after jet treatment drops to 94.1°, representing a 16.7% reduction compared with the untreated sample. The surface water contact angle rises gradually with increasing moving speed; at a sample moving speed of 5 mm/s, the contact angle reaches 107.2°, only 5.1% lower than the untreated group.

This indicates that jet plasma treatment weakens the hydrophobicity of silicone rubber, because high-energy particles generated by plasma jet bombard the silicone rubber surface and alter the chemical structure of surface molecules. Combined with the FTIR test results in Figure 2, the content of nonpolar Si-CH₃ groups decreases after treatment, which accounts for the reduced hydrophobicity. Although the water contact angle of treated silicone rubber is lower than that of the original material, it can still reach 107° under high-speed processing, maintaining favorable hydrophobic performance. Furthermore, increasing the processing speed can effectively mitigate the decline in hydrophobicity. To guarantee the hydrophobic performance of silicone rubber cables, the processing speed should be maximized on the premise of meeting modification requirements.

Figure 1 Changes in water contact angle of silicone rubber before and after plasma treatment

Figure 2 FTIR spectra of silicone rubber surfaces before and after plasma jet treatment

Surface Anti-Printing Contamination Performance

Figure 3 shows the test results of anti-printing contamination performance of silicone rubber surfaces before and after air jet plasma treatment.

Figure 3 Variations in anti-printing contamination property of silicone rubber surfaces before and after jet plasma treatment

As illustrated in Figure 3(a), ink from untreated silicone rubber transfers to the contacted silicone rubber sheet and leaves white traces, which proves the occurrence of printing contamination. After plasma jet treatment, no ink transfer marks are observed on the contacted silicone rubber, and the original printed text on the treated silicone rubber remains intact, as shown in Figure 3(b). This result verifies that surface modification via air plasma jet can effectively eliminate printing contamination of silicone rubber.