TPU molecules contain a large number of alkyl chains, therefore TPU materials exhibit strong hydrophobicity. By treating TPU with plasma, hydrophilic groups such as hydroxyl, carboxyl, and amino groups can be grafted onto the surface of TPU materials, thereby improving their hydrophilicity.

The influence of plasma treatment time on the hydrophilic properties of TPU materials

In order to investigate the effect of plasma treatment time on the hydrophilic properties of TPU materials, we prepared TPU films and tested the static contact angle of water on the TPU films before and after plasma treatment for 1, 2, 3, 4, and 5 minutes The experimental results showed that before plasma treatment, the water contact angle of TPU film was 113 °, indicating hydrophobicity (Figure 1a). After plasma treatment, the water contact angle of TPU was less than 90 °, and the hydrophilicity was significantly improved Among them, after 1 and 2 minutes of treatment, the water contact angle of TPU material decreased to 89 ° and 63 °, respectively (Figure 1b, 1c). As the treatment time increased, the water contact angle tended to stabilize (61 ° at 3 minutes; At 60 ° C for 4 minutes (Figure 1d, 1e), this may be because after 2 minutes of plasma treatment, the hydrophilic groups on the surface of TPU material have become saturated After 5 minutes of treatment, the water contact angle decreased to 51 ° (Figure 1f). The decrease in water contact angle may be due to the increase in surface roughness of TPU after plasma etching, which further reduces the water contact angle Overall, plasma treatment can effectively graft a large number of hydrophilic groups onto the surface of TPU materials, significantly improving the surface hydrophilicity of TPU materials.

Figure 1 (a) Changes in water contact angle of untreated TPU film and TPU film treated with plasma for (b) 1 min, (c) 2 min, (d) 3 min, (e) 4 min, and (f) 5 min

Principle of TPU plasma treatment to improve hydrophilicity

In order to further investigate the types of functional groups introduced on the surface of TPU materials by plasma treatment and verify the reasons for the changes in water contact angle, we conducted X-ray photoelectron spectroscopy (XPS) scans on the samples before plasma treatment and after treatment for 1, 2, and 5 minutes Figure 2 shows the spectral images of each sample, and Table 1 contains the relative composition ratios of each functional group in the sample based on peak area In the C1s fine spectrum, peaks at 284.8, 286.5, and 289.0 eV correspond to C-C bonds, C-O/C-N bonds, and O-C=O bonds, respectively In the 1s fine spectrum, the TPU sample before plasma treatment only has a peak located at 399.8 eV, corresponding to the amide bond in the TPU structure After plasma treatment, a peak at 401.5 eV was added to the 1s fine spectrum of the sample, indicating the introduction of N-O bonds during plasma treatment [24]. TPU material has two peaks in the O1s fine spectrum, with the peak at 532.1 eV corresponding to carbonyl oxygen and hydroxyl oxygen, and the peak at 533.5 eV corresponding to ester oxygen. The peak at 532.6 eV added after plasma treatment corresponds to the introduction of N-O bonds.

Figure 2 XPS spectra of TPU films treated with plasma (a) before treatment, (b) for 1 min, (c) for 2 min, and (d) for 5 min

After plasma treatment for 1 and 2 minutes, the proportion of C-O/C-N bonds in the C1s fine spectrum significantly increased, indicating the introduction of O and N elements in plasma treatment At the same time, the content of N-O bonds in the 1s fine spectrum and the content of C=O, C-OH, and N-O bonds in the O1s fine spectrum increased significantly, indicating that carboxyl, hydroxyl, and hydroxylamine groups were successfully grafted onto the surface of TPU material after 2 minutes of plasma treatment (Figure 2) Hydroxyl and carboxyl groups are more likely to form hydrogen bonds with hydrogen atoms attached to them due to the strong electron withdrawing effect of oxygen The increase of hydrophilic groups mentioned above can effectively improve the hydrophilicity of TPU materials.

Comparing the XPS results of the sample treated with plasma for 5 minutes with those of the sample treated with plasma for 2 minutes, there was no significant difference in the fine spectra of C1s, N1s, and O1s, indicating that the content of various chemical bonds in the sample remained basically unchanged compared to the sample treated with plasma for 2 minutes (Figure 2) This indicates that after 2 minutes of plasma treatment, the hydrophilic groups grafted on the surface of TPU material reach saturation Further increasing the plasma treatment time (2, 3, 4 minutes) did not significantly increase the number of hydrophilic groups on the surface of TPU material, resulting in no significant change in the water contact angle After 5 minutes of plasma treatment, the water contact angle decreased while the content of each hydrophilic group remained stable, indicating that the improvement in hydrophilicity of TPU material is not due to the increase of hydrophilic groups, but rather the plasma etching effect on the surface of TPU material, which increases the surface roughness and improves the hydrophilicity.