Author: Bradley Donnelly
Donnelly, Bradley, 2024 The development, manufacture and evaluation of novel antifouling coatings for acoustic applications, Flinders University, College of Science and Engineering
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Fouling is a persistent and ongoing problem for maritime vessel and platforms. The exact impact that fouling has on submersed sensors is currently under-explored, specifically the effect that it has on acoustic surfaces. Additionally, it is unclear to what extent antifouling techniques interfere with the transmission of acoustic signals.
To examine this effect, neoprene samples were coated with three commercial antifouling coatings, namely, Intersmooth 7460HS, HempaGuard X7 and Hempasil X3. Other neoprene samples were left uncoated but were imbedded with the biocide, 4,5-dichloro-2-octyl-4-isothiazolin-3-one (DCOIT). Uncoated samples that had been immersed in Port Phillip Bay, Australia, for 92, 156 and 239 days were also obtained. The acoustic properties of these samples were measured using an acoustic insertion loss test and compared to each other. It was found that the application of antifouling coatings had little effect on the transmission characteristics of the neoprene with approximately 1 dB loss. The embedment of DCOIT, however, has a chance of causing aeration in the neoprene, which can heavily hamper transmission. An assessment of the effect of the fouling growth found that light and medium fouling levels produced little transmission loss, whereas more extreme fouling can lead to a 9 dB transmission loss.
The previous results suggested that there may be value in designing an antifouling system specifically for acoustic surfaces. As such two Polyurethane (PU) systems were synthesised. Both contain copper microspheres, one in which the microspheres are evenly mixed into the PU matrix and the other which has the microspheres poured onto its surface. Samples were made with 3, 5 and 10 percent loading by weight. In order to determine their suitability as antifouling acoustic surfaces, both their acoustic properties were determined along with any antifouling properties. Transmission loss was determined again using the insertion loss method. While the antifouling efficacy was determined using a static settlement assay using untreated seawater sample from Brighton, South Australia.
In regard to antifouling performance both samples had exhibited similar amounts of antifouling activity and showed that efficacy increased with copper concentration. However, they differed quite significantly in acoustic performance. The mixed samples had roughly a 1 dB loss, whilst the poured samples both had transmission losses above 10 dB. This was determined to be a result of increased porosity in the poured samples. The elucidation of this difference can provide insight to improve the manufacture and design process of antifouling protection for acoustic surfaces.
This thesis also shows the value of using untreated seawater in static biofouling assays, it allows bioassays to include a broad spectrum of fouling organisms whilst maintaining high degrees of control and replicability. This could prove useful in tuning antifouling systems that have well-understood mechanisms, by exposing them to a community of organisms that they are unlikely to encounter in traditional single species assays. Additionally, for systems that are unsuited for field trials these assays can elucidate fouling phenomena without the requirement of a robust substrate.
Keywords: Antifouling, Biofouling, Acoustics, Copper Microspheres, polymer coatings, rubber coatings, acoustic transmission loss, underwater sensors.
Subject: Engineering thesis
Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Youhong Tang