Maritime University of Szczecin
Akademia Morska w Szczecinie
2010, 24(96) pp. 118–123 2010, 24(96) s. 118–123
The role of nanotechnology in improving marine antifouling
coatings
Rola nanotechnologii w ulepszaniu powłok
przeciwporostowych na wody morskie
Paweł Szewczyk
Silesian University of Technology, Faculty of Organisation and Management Institute of Production Engineering
Politechnika Śląska, Wydział Organizacji i Zarządzania, Instytut Inżynierii Produkcji 41-800 Zabrze, ul. Roosvelta 26, e-mail: pszewczyk@neostrada.pl
Key words: biofouling, hazardous substances, chemical substitution, nanostructured surfaces Abstract
To avoid the growth of organisms of different types on surfaces in a marine or freshwater environment usually coatings incorporating biocides (chemicals that kill organisms) are used. Instead of these biocide containing coatings there exist several approaches of nanostructuring the surfaces which prevents biofilm formation and bacterial adhesion as well as the attachment of larger organisms. In this paper results of a preliminary literature review on the potential role of nanotechnology in solving ecological problems concerning antifouling coatings are presented.
Słowa kluczowe: bioporastanie, substancje niebezpieczne, substytucja chemiczna, powierzchnie z
nano-strukturą
Abstrakt
W celu zapobieżenia wzrostowi organizmów różnych rodzajów na powierzchniach w środowisku wody mor-skiej lub słodkiej zwykle stosuje się powłoki zawierające biocydy (chemikalia, które zabijają organizmy). Zamiast powłok zawierających te biocydy można skorzystać z kilku sposobów, które na powierzchni tworzą nanostruktury zapobiegające powstawaniu biofilmu i adhezji bakterii, jak również przyczepianiu się więk-szych organizmów. W artykule przedstawiono wyniki wstępnego przeglądu literatury odnośnie roli nanotech-nologii w rozwiązywaniu ekologicznych problemów związanych z powłokami przeciwporostowymi.
Introduction
Biofouling is the colonisation of submerged surfaces by unwanted organisms such as bacteria, barnacles and algae, and has detrimental effects on shipping and leisure vessels, heat exchangers, oceanographic sensors and aquaculture systems [1]. The increase in frictional drag caused by the development of fouling on hulls of ships can reduce speed in excess of 10%. A vessel with a fouled hull burns 40% more fuel which has an impact on fuel costs and additional greenhouse gas production (estimated to be 20 million tonnes per annum). The estimated saving to the shipping industry through
the use of antifouling coatings is estimated to be ~20 billion Euros per year. Fouled hulls are also implicated in the spread of “alien species” around the world, potentially threatening the balance of sensitive ecosystems [2].
The influence of biofouling on coastal and oceanographic measuring instruments, which are routinely used in marine and coastal research and monitoring programmes, is very strong and the earliest stages of biofouling, within a few days of immersion, significantly affect data quality and instrument performance. There is a need to protect the instruments from biofouling so that they are able to gather better quality data and require less
maintenance. Currently there are no effective coatings to control this problem, the only solution involves expensive manual cleaning by divers [2].
Biofouling of intake structures, screens, sea-water piping systems and heat-exchanger tubes in desalination and power plants causes an overall decline in plant efficiency at great economic cost. For example the presence of a biofilm on transfer surfaces of heat exchangers cooled by seawater reduces the heat transfer rate by 20 to 50% and incurs a global expenditure of over $15 billions per annum to control the problem. The majority of current measures to control biofouling involve the use of biocides [2].
In this paper results of a preliminary literature review on the potential role of nanotechnology in solving ecological problems concerning antifouling coatings are presented.
Current technologies and novel solutions [3]
The two main approaches to creating commer-cial fouling-resistant coatings are a) “antifouling coatings” (i.e. no settlement (attachment) of the colonizing larvae, spores or cells) and, b) “fouling- -release” (i.e. organisms release under hydro-dynamic forces because they are weakly adhered). „Antifouling‟ coatings are typically based on the controlled release of biocides, to kill the colonising organisms. With the increasing environmental restrictions placed on biocides (e.g. the EC‟s Biocidal Products Directive 98/8/CE) the registra-tion of new active ingredients for use in antifouling paints is a very protracted business, increasing the cost and time required to develop a new coating. The emphasis in technological innovation is therefore now on non-toxic coatings that do not release materials into the environment and which function through their physico-chemical surface properties, either to deter organisms from settling, or which reduce their adhesion strength. The development of such coatings and an understanding of how they work may be accelerated through nanotechnology.
Biofouling is the outcome of surface coloniza-tion and adhesion processes used by the responsible organisms. The critical biointerfacial processes resulting in fouling are nanoscale / microscale in dimension: it follows therefore, that surface properties to control biofouling need to be on the same length scales. An area of particular interest in recent years is the impact of surface patterning (nano- and micro-scale) on fouling organisms. Surface patterns may be purely topographical,
chemical, or combinations of the two since they are often inter-related, and can be generated by either “top-down” or “bottom-up” approaches. The former are most suitable for small scale surfaces that enable specific hypotheses to be tested in laboratory studies. The latter are more suited to practical coating designs through, for example phase-segregating polymer blends or self-assemb-ling block copolymers. The importance of studying surfaces underwater is also becoming more important, especially for those surfaces that reconstruct in water, since it is the hydrated surface that the settling organisms / cells encounter.
Potential role of nanotechnology
From its early beginning chemistry has been confronted with the problem of handling toxic substances. During a long learning process, with errors leading partly to catastrophic consequences, reasonable precautionary measures have been developed [4]. One of the consequences is that chemistry is one of the most regulated fields. Nevertheless, there are still a large number of chemicals that, apart of their beneficial effects, produce negative side effects. The most effective way to avoid these side effects is to substitute the hazardous substances by less dangerous ones, offering the same or even more benefits. This is why the substitution principle plays a crucial role in the new EU chemicals policy to regulate substances manufactured in or imported into the EU (compare: EU regulatory system called Registration, Evalu-ation, Restriction and Authorisation of Chemicals – REACH).
The original meaning of chemical substitution is quite clear and narrow: one chemical substance is replaced by another, for whatever reason (availa-bility, costs, technical requirements). The ideal case with hazardous substances is that the substitute has the same functions as the original substance but without its hazardous potential. In practice, the substitute usually changes the properties of the whole product or of the process or even of the whole production chain and is much more expensive [4]. The meaning of substitution in the context of hazardous substances has become broader.
“In this context the scope ranges from simple substitution (e.g. exchanging substances) to risk management as a whole (i.e. prevention of hazar-dous substances, reduction or prevention of expo-sure, etc.).” [4, reference: Ahrens 2006]. Another definition is provided by Joachim Lohse et al. 2003 [ref. in 4]: “Substitution means the replacement or
reduction of hazardous substances in products and processes by less hazardous or non-hazardous subs-tances, or by achieving an equivalent functionality via technological or organisational measures.”
This definition implies not only the replacement of a hazardous substance by a less hazardous one but also the use of another technology or the reorganisation of the process in order to reduce or avoid the hazardous substance.
It is often assumed that nanotechnology (NT) holds the potential to provide a substantial contribution to the solution of various ecological problems, including high consumption of energy and materials and the generation of waste. However, problems surrounding the use and release of hazardous substances remain largely unexplored [4]. Since most nanotechnologies are at an early stage of development and due to the fact that NT is an enabling technology there are only a few publications and only very few research projects directly addressing the substitution of hazardous substances by nanotechnology.
The European Scientific Technology Option Assessment (STOA) panel has initiated a project called: “The role of Nanotechnology in Chemical Substitution” [5]. The aim of the project was to give an overview of already used and conceivable applications of nanotechnology in order to replace hazardous chemicals. The overall idea behind this project was to identify new applications of nanotechnologies which could help to reduce the risks related to hazardous substances and chemical processes. One prominent example is the substitution of anti-fouling coatings used in the ship industry by nanotechnological based coatings, which are already under investigation.
Related to the objective of this study three questions had to be addressed at the beginning of the project:
• Which substances are considered as „hazardous chemicals‟?
Since this project was focused on determining the potential of nanotechnology, the issue of how a substance can be identified as hazardous was discussed only briefly. Nevertheless, to estimate the potential of substitution it must be clear what is to be substituted. Here, a pragmatic solution was chosen. Only substances which are already known as toxic and dangerous to humans and the environment were considered.
• What is meant by the term nanotechnology and how can it be distinguished from biology and chemistry respectively?
There is no clear definition of nanotechnology nor is it possible to assign precisely an application to NT or to chemistry or to biology. The term nanotechnology implies a great variety of different techniques, analytic tools and materials including their production [6, 7]. Nevertheless, to identify relevant applications of NT serving the aim of this project presupposes a certain concept of NT.
The first approach used for this study was that everything is considered as NT what is claimed by proponents to be NT. In detail this means that all publications, which are published in journals carrying “Nano” in their title (e.g. “Journal of Nanoparticles Research”), and all projects carrying „Nano‟ in their title are considered to belong to NT. Publications and projects dealing with typical NT objects such as fullerenes1 or nanotubes2 are also
attributed to NT even if “Nano” is not the headline. This is a pragmatic solution to start with. The question of whether a certain technical concept of substitution can be attributed to NT could not be explicitly discussed within the scope of this project. For all presented examples the assignment to NT was accepted by the experts during the validation workshop.
• What is the meaning of “chemical substitution” in relation to NT?
The original meaning of chemical substitution is quite clear and narrow: one chemical substance is replaced by another, for whatever reason (availa-bility, costs, technical requirements). Due to the fact that NT is neither a group of substances nor a group of products but an enabling technology, the way NT can provide solutions is more fundamental than just replacing the function of the substitute. It is assumed that NT provides new effects which are not based on chemical properties of the related material but on the physical properties caused by its size and shape. It can be used to develop com-pletely different processes or different products which serve the same purpose but in a completely different way [5].
The research for the findings was based on two approaches [5]: a literature research and interviews with experts.
The relevant literature was identified by the following criteria:
reports from governmental departments, re-search institutes, industrial associations, and other stakeholder groups which addresses NT and environmental issues;
journals, carrying “Nano” in their titles;
journals addressing chemistry and environ-mental issues (e.g. Green Chemistry);
broad keyword based searches in several data bases.
This keyword based search was performed in: − journals of the American Chemical Society
(ACS)
− journals covered by Science Direct IP/A/STOA/ ST/2006-029 iii PE 383.212
The experts for the interviews were chosen due to publications on NT related to environmental issues or even related to a case of substitution. They are all experts in the field of NT but with different backgrounds. The experts included representatives from industry, science, research management, and NGOs.
To validate the findings of the project nine experts from different fields of NT or chemistry with nanotechnological background were invited to a workshop at the European Parliament. Prior to the workshop a summary of the preliminary results of the project was sent to the experts. The workshop focused on the discussion among the experts but was open to Members of the European Parliament in order to give them the opportunity to give their views in the evaluation of the preliminary findings and related policy options. In addition the workshop was open to other persons active in the field of nanotechnology.
Conclusions from the STOA project [5]
In addition to the fact that it is not obvious which substances in which contexts are to be considered as hazardous, there were two central questions, which had to be addressed if the original question was to be answered:
• What is meant by the term nanotechnology, and how can it be distinguished from related technologies derived from physics, biology, and chemistry?
• What is the meaning of chemical substitution in relation to NT?
Because there exist no clear answers for either question that are scientifically sound and accepted broadly within the scientific community, the origi-nal question cannot be answered in a systematic and quantitative manner.
The results of this project suggest that the contribution of NT with respect to the reduction of hazardous substances is manifold but incremental. From the examples and concepts that have been identified in the course of the project, one cannot conclude that NT can contribute in an exceptional
manner to a large increase in substitution of hazar-dous substances. It is unclear whether the current use of NT really provides new opportunities for the avoidance of hazardous substances. Nevertheless, most of the experts involved in the project assigned NT a considerable potential for substitution in the future. For a comprehensive assessment of this potential, each identified example has to be assessed case by case and in more detail than was done in this project. Because such an assessment is complex and time consuming, proposals for substitution should only be analyzed if the benefit would be outstanding or if no existing solution is already available. In the search for solutions concerning the use of hazardous substances, it is important to keep in mind that technical issues are only one aspect of the success of substitution.
In February 2010 a 5-year project AMBIO (Advanced Nanostructured Surfaces for the Control of Biofouling) was finished [1]. It was an R&D Integrated Project funded by the EC, through its 6th Framework Programme. The project operated at the frontiers of diverse disciplines, including nanotech-nology, polymer science, surface science, coating technology, hydrodynamics and marine biology.
The overall goal of the project was to provide a combination of fundamental and application-oriented research that will lead to the development of novel coatings that will prevent or reduce the adhesion of fouling organisms through the physico – chemical properties of the surface, rather than the release of biocides. The research on nanoscale interfacial properties of different surfaces and how organisms adhere will allow understanding how anti- biofouling systems can work at the nanoscale. To achieve this goal the project aimed to take advantage of the new opportunities for designing and manipulating antifouling surfaces provided by nanotechnology. Nanostructuring of a coating controls many surface and bulk properties that are relevant to an antifouling, “non-stick” surface, such as surface energy, charge, conductivity, porosity, roughness, wettability, friction, modulus, physical and chemical reactivity, and compatibility with organisms.
An additional goal of the project was to improve our understanding (theoretical and empirical) of how surface properties influence the adhesion processes of fouling organisms. This was achieved by hypothesis – driven experimentation that takes advantage of new technologies for creating surfaces with controlled and precisely known nano- and micro-scale properties.
The technical objectives of the project were formulated as 4 key questions:
• Which nano-designs show good “antifouling” properties against a range of test organisms? • How does nanostructure influence adhesion of
organisms?
• How do coatings respond to the aquatic environ-ment and how do nanofillers influence this? • Which design concepts show most promise as
durable antifouling coatings for a range of “real- -world” applications?
Some of the results of the project were as follows [8]:
Approximately 500 different nanostructured coatings, representing 64 generic coating chemi-stries were prepared at laboratory-scale and evaluated for their antifouling and fouling- -release performance. Of these, 15 were down- -selected for testing in a range of field and representative end-user tests. Several coatings showed promise in these trials, and have either been commercialized, or have the potential to be so after further development.
A range of surface-sensitive tools, including novel methods developed within the project, were applied to the study of nanoscale properties of coatings and how these change during immersion.
Fundamental advances were made in the under-standing of the influence of surface nanostruc-ture on the settlement and adhesion of fouling organisms. In particular there is a growing reco-gnition that an appropriate level of heterogeneity and pattern, in topography, or in surface chemi-stry, may be more effective than a homogeneous surface, leading to the concept of “ambiguous” coatings.
Novel methods for nanostructuring surfaces were developed with potential benefits for many technologies other than antifouling.
Fundamental advances were made in under-standing the relationship between the structure and properties of a coating and its biological performance.
Novel imaging methods including digital holography and imaging Surface Plasmon Resonance3 were developed and applied for the first time to the analysis of how fouling organisms explore surfaces and how coating properties affect this.
There was extensive knowledge and technology transfer between Partners working in different disciplines and between academia and industry. New cooperative, trans-national relationships were established between Universities and companies, and between academic Partners from
different institutions. Some of these have spaw-ned new enterprises and cooperative projects. 5 novel coating technologies were patented and
several coatings are either ready for practical commercial exploitation or will be so after further development work.
Final remarks
In case of biofouling to avoid the growth of organisms of different types on surfaces in a marine or freshwater environment usually coatings incor-porating biocides (chemicals that kill organisms) are used.
Instead of these biocide containing coatings there exist several approaches of nanostructuring the surfaces which prevents biofilm formation and bacterial adhesion as well as the attachment of larger organisms.
It is unclear whether the current use of nanotechnologies really provides new opportunities for the avoidance of hazardous substances.
Nevertheless nanotechnologies were assigned a considerable potential for substitution in the future. For a comprehensive assessment of this potential, each identified example has to be assessed case by case and in more detail than was done in the described above STOA project.
Notes
1) Fullerenes – any of a series of hollow carbon molecules that form either a closed cage (“buckyballs”) or a cylinder (carbon “nano-tubes”) (Encyclopædia Britannica).
2) Nanotubes – Carbon nanotubes – minute string-like structures of carbon atoms bonded together in a hexagonal framework (Encyclopædia Britannica).
3) Surface Plasmon Resonance – is an emerging tool in the bio and life – science markets. It offers a new generation in the Label-free analysis of bio-molecules, providing information on kinetic processes (association and disso-ciation), binding affinity and even concentration and real-time target molecule detection (Horiba Scientific).
References
1. AMBIO (Advanced Nanostructured Surfaces for the Con-trol of Biofouling), an R&D Integrated Project funded by the EC, 6th Framework Programme; http://www.ambio. bham.ac.uk/index.shtml; (Accessed 26 April 2010). 2. AMBIO: The biofouling problem. Jun 2010.
3. AMBIO: Current Technologies and Novel Solutions. Feb. 2010.
4. FIEDELER U.: Using nanotechnology for the substitution of hazardous chemical substances. Journal of Industrial Eco-logy, 2008, 12/3, 307–315.
5. European Technology Assessment Group (ETAG): The role of Nanotechnology in Chemical Substitution. Study IPOL/A/STOA/ST/2006-029 (PE 383.212). Manuscript completed in April 2007. See: www.europarl.europa. eu/stoa/default_en.htm (Accessed 26 April 2010).
6. Springer Handbook of Nanotechnology. Bhushan Bh. (Ed.), 2nd revised and extended edition. Springer Science + Business Media, Inc. Berlin, Heidelberg, New York 2007. 7. Gennesys. White Paper. Grand European Initiative on
Nanoscience and Nanotechnology using Neutron – and Synchrotron Radiation Sources. Dosch H. and Van de
Voorde M.H. (Eds), Max-Planck-Institut für Metallfor-schung, Stuttgart 2009; www.mpi-stuttgart-mpg.de (Acces-sed 26 April 2010).
8. AMBIO: Results of the project; http://www.ambio.bham. ac.uk/index.shtml (Accessed 26 April 2010).
Recenzent: dr hab. inż. Bartosz Powałka, prof. ZUT Zachodniopomorski Uniwersytet Technologiczny w Szczecinie
