Experimental investigation of the behaviour and fate of block copolymers in fouling-release coatings

Fouling-release coatings (FRC) were developed as an environmentally friendly alternative after the ban of highly toxic antifouling coatings based on tributyltin. Poly(dimethylsiloxane) (PDMS) has been the most widely used polymer for FRC, and its fouling-inhibition properties have been enhanced by addition of copolymers. Examples of these copolymers include phenyl-modified PDMS or poly(ethylene glycol) (PEG)-based copolymers (e.g. triblock PEG-b-PDMS-b-PEG). These copolymers diffuse from the bulk to the surface of the coating upon immersion and modify the physicochemical properties of the surface. FRC provide superior fuel savings to the shipping industry on the first stages of immersion compared to other current technologies, albeit its performance declines over time.
This project is mainly concerned with improving the understanding of FRC and identifying the causes that result in the worsening of the fouling-inhibition properties of these complex systems, specially focusing on the behaviour and fate of the block copolymers used as additives. The development of various methods to visualize and quantify processes involving these copolymers are presented. Chapter 1 provides an overview on marine biofouling, and the evolution and state-of-the-art in biofouling prevention. In Chapter 2, the scope, aims and hypotheses of this project are set. Chapter 3 studies the diffusion and biofouling-inhibition properties of PEG-based surfactants and copolymers added to PDMS and Chapter 4 analyses the distribution and behaviour of PEG-b-PDMS-b-PEG copolymers in PDMS coatings by fluorescence means. Chapter 5 investigates the degradation of PDMS-PEG-based copolymers in FRC immersed in seawater. Chapter 6 consists of a long-term field study regarding the release/loss of these block copolymers from fouling-release coatings.
A coating based on a PDMS binder has been employed as model system in the thesis. The effect of the addition of various PEG-based surfactants and copolymers (i.e. amphiphiles) was investigated by a novel method developed in this project, and the diffusion coefficient and biofouling-inhibition properties of the different amphiphiles were studied. The results showed that there is a moderate dependence of the diffusion coefficient on the molecular weight of the molecule. The diffusion coefficients obtained were relatively high for all the investigated compounds with molecular weights (Mw) ranging from 600 to 4000 g/mol. Moreover, the biofouling-inhibition properties are not dependant on the diffusion coefficient of the amphiphiles, but mainly depend on the chemistry of the hydrophobic block, with PDMS-PEG-based copolymers providing the best results. Hence, the anchoring capabilities and stability of the copolymer on the surface of the coating are proven to be a central aspect of the performance of these FRC.
A novel fluorescent-labelled triblock PEG-b-PDMS-b-PEG copolymer was synthesized in this project to visualize the distribution and behaviour of PDMS-PEG-based copolymers in PDMS coatings. Images obtained by confocal microscopy proved that the copolymer molecules assemble in spherical domains inside the PDMS coating. The domains are smaller close to the surface and larger in the bulk of the film (with domains as large as 7 µm in diameter). The diffusion of copolymer from the bulk to the interfaces of the PDMS film could be observed by following the fluorescence intensity at different depths over time.
The chemical stability of PDMS-PEG-based copolymers in PDMS coatings immersed for up to 30 months in seawater in Singapore was also investigated. The copolymer remaining in the coatings after exposure was extracted and isolated, and its chemical composition was analysed. An increase in the relative content of PDMS was observed, probably due to the imperfect isolation process. However, no traces of degradation products were found, and it was observed that the molecular weight of the copolymer did not change over time. Therefore, it was concluded that the copolymer molecules remaining in the coating after 30 months of immersion did not suffer significant degradation. Conversely, the studied copolymers could be degraded in the laboratory under a range of conditions and the degradation products were successfully identified, mainly consisting of esters arising from the oxidative degradation of the PEG block. Further experiments showed that degradation can also occur in the bulk of the coatings depending on the physical properties and chemical composition of some of the constituents of FRC, both for coatings immersed in seawater and others kept in the laboratory in dry conditions.
The release/loss of copolymer from FRC was also studied, and the effect of different variables such as seawater temperature and the addition of biocide were addressed. Approximately 300 experimental coatings exposed to seawater for up to 5 years were analysed and the results exhibited a large scatter in the data, mainly attributed to differences in the coatings compositions. However, some comparable formulations suggest that seawater temperature has an important effect on the loss of copolymer from PDMS-coatings, while the initial concentration of copolymer has no influence for copolymer concentrations as high as 7 wt%. In addition, the initial concentration of biocide showed diverse effects, with biocide-containing coatings leading to higher copolymer losses in the first stages of immersion and larger copolymer retention values in long-term exposure, indicating that the addition of biocide strongly influences the release profile of these copolymers from FRC.
In summary, the addition of block copolymers is a successful method to impart biofouling-inhibition properties to fouling-release coatings. The development of new methodologies has allowed the investigation of the behaviour and fate of these block copolymers in FRC. While the performance of these copolymers is not limited by its diffusion properties in the film, attention should be put on the physical properties and chemical composition of the copolymer and other coating constituents. These variables have an important influence on the anchoring, release and degradation of these copolymers, which dictate the long-term performance of fouling-release coatings.