**1. Introduction**

Marine biofouling refers to the colonization of submerged surfaces by marine micro- and macro-organisms, and is a worldwide problem affecting maritime and aquatic industries [1–5]. For a long time, painting antifouling coatings on seawater-impregnated substrates has been an effective way to inhibit the attachment of marine organisms. It has been widely applied, such as in shipping vessels, heat exchangers, offshore rigs, jetties, aquaculture cages, and other submerged structures in the marine environment [6,7]. Meanwhile, one statistical analysis indicated that the practice saves the global shipping industry an estimated 150 billion US dollars per year [8]. Among them, self-polishing antifouling coatings incorporating tributyltin-based compounds (TBT-based coatings) are the most efficient. Unfortunately, with the development of relevant research, these coatings have been revealed as toxicants toward non-targeted species [2]. Furthermore, TBT compounds have also been reported to accumulate in mammals and debilitate the immune defenses of fish [2,3]. Therefore, in 2001 the International Marine Organization (IMO) forbade the use of TBT-based coatings. Moreover, the biocide used in the marine environment is now under strict control in many countries, and it has become a driving force for the development of environmentally-friendly alternative coatings, which mainly include fouling-resistant coatings, fouling release coatings, and fouling degrading coatings [8–12].

It is known that marine biofouling involves a wide variety of organisms (more than 4000 species have been identified) [12]. It has the following characteristics: the fouling organisms change very quickly, attaching to the growth is very easy, there is no selective attachment to most of the matrix materials, and the environmental adaptability of organisms is very strong. Therefore, it is difficult to completely reject the adhesion of the fouling organisms on the substrate. For fouling release coatings, by minimizing the adhesion strength between fouling organisms and the material surface, fouling may be readily removed by simple mechanical cleaning or hydrodynamic stress during navigation [13]. These coatings use physical means to protect the substrate immersed in seawater, and are not causes of environmental pollution. They are now widely used as environmentally-friendly alternatives [8,13].

Polydimethylsiloxane (PDMS) has a linear structure and is the most common hydrophobic material. The molecular structure of PDMS is arranged in helixes, whereas the outwards-directed methyl groups provide hydrophobicity [14,15]. PDMS with low surface energy and low elastic modulus reduces the adhesion of marine organisms [16,17]. It has been prepared for fouling release coatings, and the first organic PDMS-based polymer antifouling coating came out in 1972 [18].

The incorporation of non-reactive silicone oil in fouling release coatings based on PDMS has been seen since 1977 [19]. Related studies have suggested that silicone oil additives in fouling release PDMS coatings could reduce the coefficient of friction and favor an easier release of fouling organisms [20,21]. Further studies showed that the increase of interfacial slippage with silicone oil leached from the coated surfaces improved the fouling release performance [22]. Although the potential of silicone oil to affect the ability to overcome cellular barriers has been pointed out [17], there is no statistical evidence to support these concerns [13,20]. The incorporation of silicone oil into fouling release coatings based on PDMS showed that the amount of silicone oil was negligible [22,23], and the low toxicity of silicone oil caused little harm in marine life [24–26].

The research on the incorporation of silicone oil into coatings is mainly focused on the evaluation of antifouling behavior on certain specific fouling organisms by the leaching of silicone oil [20–23]. There is no related research on the choice of silicone oil, including viscosity and added amount. A particularly significant research gap exists regarding the effect of these factors on the leaching speed and the leaching percent of silicone oil. In this experiment, the phenylmethylsilicone oil (PSO) with different viscosity and content was incorporated into fouling release coatings based on PDMS. The effect of the leaching amount of PSO on the antifouling performance of the coating is studied herein, and the leaching cycle of PSO is predicted by a reasonable measurement. The analysis of related properties can reveal the reasons for the improvement of the antifouling performance of the coating.

#### **2. Materials and Methods**

#### *2.1. Materials*

Hydroxyl-terminated polydimethylsiloxane (PDMS) was obtained from Dayi Chemical Industry Co., Ltd. (Yantai, China). The kinematic viscosity of PDMS was 10,000 mm2/s and the relative molecular weight was about 60,000. Phenylmethylsilicone oil (PSO) was purchased from Shanghai Hualing Resin Co., Ltd. (Shanghai, China). The kinematic viscosity of PSO was 30 mm2/s, 75 mm2/s, and 100 mm2/s for products known as Si-30, Si-75, and Si-100, respectively. Tetraethylorthosilicate (TEOS) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Bismuth neodecanoate (BiND) was obtained from Deyin Chemical Co., Ltd. (Shanghai, China). Xylene and ethyl acetate were also analytical grade and supplied by Yongda Chemical Reagent Co., Ltd. (Tianjin, China).

#### *2.2. Preparation of Coating Samples*

The coating was composed of three parts. Pre-dispersed slurry included PDMS and PSO. TEOS and xylene were mixed to make the curing agent, and the mixture of BiND and ethyl acetate was prepared into the catalytic agent. The two-stage process of the preparation method was as follows: PDMS (100 g) and PSO were added into a 500 mL stirring tank at 2000 rpm for 30 min. Afterward, coatings were prepared in a 20:4:1 weight ratio of PDMS (from the pre-dispersed slurry):curing agent:catalytic agent. The coating was brushed on glass slides with

dimensions of 75 mm × 25 mm × 1 mm, and also poured into a Teflon mold with dimensions of 150 mm × 150 mm × 2 mm for at least 8 h to form a cross-linked elastomer. The blank control sample without PSO was set as A, and other experimental samples were set as Ax-y, where x represents the viscosity of PSO and y represents the mass of PSO.

#### *2.3. Experimental Procedure*

The antifouling performance of the fouling release coating based on PDMS depends mainly on the surface and mechanical properties. Therefore, we analyzed the effect of the PSO on the above properties. In this study, the change in surface properties was due to the change of the chemical composition caused by the incorporation of PSO, which could be analyzed by Fourier transform infrared (FTIR) spectroscopy. The influence of the PSO on the crosslink density of the coating was beneficial to the analysis of the coating's mechanical properties. The focus of this study was on the observation of leaching PSO, including leaching amount and leaching percent. The leaching of the PSO with different viscosity and content needed to be characterized by reasonable parameters. Finally, the antifouling performance of the coating was analyzed. Error values are quoted as standard error of the mean (SEM), based on the number of samples analyzed (*n*).

### *2.4. Characterization*
