In this study, MWCNTs-COOH and SiO2 nanoparticles were successfully added to the PDMS. With the increase in SiO2 content, Si-O-Si bonds in the samples also increased gradually. This explains the improvement of antifouling performance of the samples.

Baier curve was usually used to explain the relationship between surface properties and the amount of bio-organisms adhered on surface [9]. The amount of adhered bio-organisms on surface would be lowest when the critical surface free energy (SFE) reached 22~25 mN/m according to Baier curve, the SFE range was the fouling release area. The critical surface of PDMS can meet the conditions of the fouling release area [26], which gave PDMS excellent antifouling performance under dynamic conditions. The low surface energy of PDMS was determined by a large number of Si-O-Si bonds in the PDMS structure [13]. Brady and Singer [27] revealed that bio-adhesion was associated with (Eγc)1/2 positive correlation, where E is the elastic modulus and γc is the critical surface energy of the polymer. Adding nanoparticles reduced the elastic modulus of the material and the biological adhesion of the coating. Although the results showed that the improvement of antifouling performance is not obvious with the increasing in SiO2 amount, the non-toxic coating improved the antifouling performance of substances by reducing surface bio-adhesion. Several different antifouling coatings were selected for comparison, as shown in Table 5. In contrast, PCSi5 has lower antibacterial efficiency compared with some other coatings. However, PCSi5 is easy to prepare as it possesses a single-layer structure but not containing additional bactericide triclosan. V. natriegens, a typical marine fouling organisms, was selected to evaluate the antifouling performance of PCSi5. On the other hand, PCSi5 not only had satisfactory antifouling performance, but also had excellent underwater sound absorption performance, which was not covered by other studies.The sound absorption performance of the new coating is significantly improved. According to previous studies, after adding MWCNTs-COOH and surfactants into PDMS material, the underwater acoustic absorption coefficient of the material can be increased to more than 0.75 above 1500 Hz [16]. In this experiment, after the addition of SiO2, the underwater sound absorption coefficient of the material was not significantly improved above 1500 Hz. Only when 2.5% SiO2 was added did the sound absorption coefficient of the material slightly increase to more than 0.8. It can be concluded that MWCNTs-COOH contributed to most of the improvement of the underwater sound absorption performance of the materials. The improvement of the underwater sound absorption performance of MWCNTs-COOH materials can be attributed to the relative displacement of MWCNTs-COOH nanoparticles in the PDMS matrix under the action of acoustic waves and the dissipation of acoustic energy under the action of friction [17]. This is probably the reason why relatively more SiO2 nanoparticles improved the sound absorption performance of the material. In addition, during the addition of MWCNTs-COOH, some air microbubbles were introduced into the matrix, and the sound waves were dissipated when passing through these microbubbles [28]. In addition, the peak of the sound absorption coefficient of materials near 1000 Hz can also be explained by the resonance theory [29,30]. The experimental sample was a flexible nanocomposite material, which can be treated as a spring-mass system. When the frequency of the incident sound approached the natural frequency of the material, a peak of sound absorption was formed.In addition, the poor mechanical properties of PDMS also limit its application. The influence of nanofiller on the tensile properties of the samples is very complicated. The main factors affecting the tensile properties of nanocomposites are the distribution of nanoparticles across the matrix and the transfer form of load on the matrix caused by the overall bonding strength between polymer and nanoparticles [31,32]. On the one hand, nanoparticles will produce some voids in the polymer matrix, resulting in the reduction in Young’s modulus. On the other hand, the strong bonding between nanoparticles and polymers will increase the young’s modulus. However, nanoparticles at high content will cause agglomeration and stress concentration, which have an adverse impact on the tensile properties of the coating. In this study, when less SiO2 was added, the SiO2 nanoparticle was unevenly distributed and voids were generated in the PDMS matrix, resulting in the fracture of the composite under even small stress. In addition, the strong covalent bond [33] formed between MWCNTs-COOH and PDMS made positive effect on the UTS of the material. Therefore, when MWCNTs-COOH and SiO2 at a ratio of 1:1 were added, the UTS of the material was similar to that of pure PDMS. When 1.5% SiO2 was added, the UTS of the composite was the smallest and the tensile properties are the worst. In the study of Bahramnia et al. [34], a hybrid polymer nanocomposite coating containing epoxy resin/polyurethane mixture, MWCNTs, and SiO2 nanoparticles were prepared, and the tensile test was investigated. The optimum SiO2 content was 0.75%. In this study, it can be predicted that when the content of SiO2 increases (more than 2.5%), the UTS of the nanocomposite will also increase, and then decrease after reaching an optimal content. The optimum content of SiO2 can be supplemented by subsequent research. Meanwhile, the change in mass loss indicated that the water erosion resistance of the surfaces increased with the addition of MWCNTs-COOH and SiO2. It was worth noting that the water erosion resistance of the samples became stronger with the increase in SiO2 content, and erosion resistance after 12 h water erosion was significantly lower than that of PDMS. The erosion resistance of materials is influenced by many factors, including the reinforcement materials type, combination ratio of matrix and reinforcement materials, expansion rate, and the hardness of materials [33]. As a carbon-based material, the uniform distribution of CNTs in nanocomposites has a great influence on the erosion resistance of the materials. Herein, mass loss of PCSi3 increased after adding MWCNTs-COOH and SiO2, which due to uneven dispersion of MWCNTs-COOH as filler, resulting in negative erosion resistance effects. SiO2 has a positive effect on the erosion resistance of the material, which may be explained by the surface property of the material. The visible texture of the material surface became blurred with the increase in SiO2, and the texture of surface became completely invisible for 2.5% SiO2 added PDMS, which indicated that SiO2 was wrapped in PDMS. SiO2 made the material more compact and has better erosion resistance capacity under the encapsulation of PDMS, then the water containing sand was not able to enter into PDMS and caused surface crack. In addition, SiO2 may also compensate for some of the material’s flaws by creating a thin protective layer. The mass loss rate of pure PDMS is 6.92 g·m−2·h−1, while that of PCSi5 is 2.02 g·m−2·h−1. This represents a 70.8% increase in durability of PCSi5 compared to pure PDMS.

Table 5. Antifouling performance of different antifouling coatings.

Table 5. Antifouling performance of different antifouling coatings.

NumberComponentsBacteriaAntibacterial EfficiencyCharacteristicsReferences1PDMS/MWCNTs-COOH/SiO2V. natriegens74.69%Environmental friendly
Simple structure/2NH2-UiO-66/NH2-PDMS/epoxy resinE. coli79.42%Best antifouling property
Environmental unfriendly[35]3NH2-UiO-66/NH2-PDMS/epoxy resin/triclosan>99.98%4PDMS/HD-SiO237.8%Complex structure[36]5Composite coating with outer PDMS/PAA-ZnO and inner PDMS/HD-SiO2 layer81.1%