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Advanced characterization techniques of antifouling paints
Ameessa Viliam Tulcidas
Thesis to obtain the Master of Science degree in
Chemical Engineering
Supervisors: Dr. Elisabete Ribeiro Silva
Dr. Amaya Igartua
Examination committee
Chairperson: Prof. João Carlos Moura Bordado
Supervisor: Dr. Elisabete Ribeiro Silva
Members of the Committee: Dr. Raquel Bayón
Prof. António José Boavida Correia Diogo
December 2014
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i
Acknowledgments
Firstly, I would like to express my gratitude towards Dr. Elisabete Ribeiro Silva and Professor
João Bordado for giving me this opportunity to learn more about the antifouling paints’ industry and for
the patience to mentor this thesis. Some words of gratitude go also to Master Denise Afonso for
performing the roughness measurements and Dr. Elisabete and Master Olga Ferreira for carrying out
the stirring test with the alternative procedure.
I would also like to thank Dr. Raquel Bayón, Dr. Amaya Igartua and especially Olatz
Areitioaurtena, for kindly receiving me at IK4-Tekniker and giving me all the knowledge necessary to
complete my thesis. Thank you Olatz for performing the biodegradability, wettability and alga toxicity
tests. I am also grateful to my other colleagues from Tekniker, who helped me overcome any difficulty,
although being extremely busy. It was an excellent experience, both personally and professionally,
which allowed me to meet people from different countries (French, Spanish, Italian, Americans,
Germans, Belgians, Costa-Ricans…). To all of them an enormous “Muchas gracias/ Eskerrik asko”!
To my extremely conservative parents, for allowing me to stay away from home for 4 months
and supporting my academic decisions. Also, a few words of appreciation to my rebellious brother,
who helped me to construct some sentences in English and expressed his opinion towards the images
I used in this thesis.
At last, but not the least, I would like to thank my friends, especially Diogo, for all the support
and motivation given, Gonçalo and Rui, who have been two good lads who accompanied my journey
at IST and my friends from ISEL. These words of gratitude will never be enough to thank all the
kindness shown towards me.
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iii
Abstract
Marine biofouling, the settlement of marine organisms on immersed surfaces, has been a
severe hindrance for the shipping industry for more than 2000 years. Despite causing structural
damages on ships’ hulls, this phenomenon reduces the maximum speed attainable by the ship,
leading to a higher fuel consumption to compensate this effect and consequently up raise the fuel
cost. Several antifouling technologies have been developed to combat this inconvenience, being the
antifouling paints the most conventional. Unfortunately, these paints release biocides, being nefarious
to the marine environment, especially to non-target organisms, leading to the development of biocide-
free antifouling paints. However, these are not mechanically resistant and to improve these properties,
biocides were immobilized into the paints’ matrix through covalent bonds, to impede the biocide
release.
In this thesis, biocidal paints were characterized with respect to their durability, drag friction
effect and toxicity and further compared with biocide-free antifouling paints.
The inclusion of biocide (Econea) increased the hydrophobic properties of the paints,
especially of the silicone based paints. Additionally, Econea seems to decrease the resulted surface
roughness of the polyurethane and silicone based paints, which translated positively in the drag
friction, by reducing this effect by 0 - 16% and 9 - 20% for each paint, respectively, at different speeds
(200 – 1500 rpm; 4 – 30 knots). The immobilization of Econea seems to improve the scrubbing
resistance of the polyurethane based paints.
The silicone based paint containing Econea seems to be toxic to all the tested organisms.
Keywords: Antifouling paints, Biofouling, Drag friction, Biodegradability, Toxicity.
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v
Resumo
A bioincrustação marinha, a adesão de organismos marinhos em superfícies submersas, tem
sido um obstáculo severo para a indústria naval, desde há mais de 2000 anos. Para além de causar
danos estruturais no casco dos navios, este fenómeno reduz a velocidade máxima atingível,
traduzindo-se num elevado consumo de combustível para compensar este efeito, aumentando,
consequentemente, o seu custo. Diversas tecnologias anti-incrustantes têm sido desenvolvidas para
combater esta barreira, sendo o uso de tintas anti-incrustantes o mais convencional. No entanto,
estas tintas libertam biocidas, causando efeitos nefastos no ambiente marinho, nomeadamente em
organismos não-alvo, levando ao desenvolvimento de tintas anti-incrustantes sem biocidas. Contudo,
estas tintas não possuem resistência mecânica e para melhorar estas propriedades, foram
imobilizados biocidas na matriz das tintas através de ligações covalentes.
Na presente tese, tintas com biocidas foram submetidas a testes de caracterização, com o
intuito de avaliar a sua durabilidade, resistência por fricção e análise da toxicidade e comparadas
posteriormente com tintas anti-incrustantes sem biocidas.
A inclusão de biocida (Econea) aumentou a hidrofobicidade das tintas, especialmente as de
silicone. Adicionalmente, a presença de Econea diminuiu a rugosidade da superfície resultante das
tintas de poliuretano e de silicone, traduzindo-se positivamente na resistência por fricção, reduzindo
este efeito entre 0 - 16% e 9 – 20%, respetivamente, nas diferentes velocidades testadas (200 – 1500
rpm; 4 – 30 Nós).
A imobilização de Econea aparenta melhorar a resistência à esfrega das tintas de base de
poliuretano.
A tinta de silicone contendo Econea aparenta ser mais tóxica para todos os organismos
testados.
Palavras-chave: Tintas anti-incrustantes, Bioincrustação, Resistência por fricção, Biodegradabilidade,
Toxicidade
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vii
Table of Contents
1. Introduction .......................................................................................................................................... 1
1.1. The scale of industrial biofouling problem ................................................................................ 1
1.2. Main goals ................................................................................................................................ 2
1.3. Thesis outline ........................................................................................................................... 3
2. State of the art ..................................................................................................................................... 4
2.1. Biofouling .................................................................................................................................. 4
2.1.1. Stages of biofouling ............................................................................................................ 5
2.1.2. Biofouling prevention .......................................................................................................... 7
2.2. Antifouling methods .................................................................................................................. 7
2.2.1. Archaic antifouling methods ............................................................................................... 7
2.2.2. Modern antifouling methods: protective paints ................................................................... 9
2.3. General characteristics of antifouling paints ........................................................................... 16
2.3.1. Anticorrosiveness ............................................................................................................. 16
2.3.2. Durability ........................................................................................................................... 17
2.3.3. Adhesion ........................................................................................................................... 17
2.3.4. Abrasion ........................................................................................................................... 17
2.3.5. Smoothness ...................................................................................................................... 18
2.3.6. Drag friction ...................................................................................................................... 18
2.3.7. Wettability ......................................................................................................................... 18
2.3.8. Environmental risk assessment ........................................................................................ 19
3. Experimental methods ....................................................................................................................... 21
3.1. Surface properties assessment .............................................................................................. 23
3.1.1. Wettability test .................................................................................................................. 23
3.1.2. Roughness assessment ................................................................................................... 24
3.2. Mechanical tests ..................................................................................................................... 25
3.2.1. Thickness test ................................................................................................................... 25
3.2.2. Adhesion test – Cupping test ........................................................................................... 26
3.2.3. Hardness tests .................................................................................................................. 27
3.2.4. Abrasion tests ................................................................................................................... 29
3.2.5. Washability test ................................................................................................................ 30
3.2.6. Stirring test ....................................................................................................................... 33
3.2.7. Drag friction test ............................................................................................................... 34
3.3. Environmental compatibility tests ........................................................................................... 35
3.3.1. Biodegradability in sea water ........................................................................................... 35
3.3.2. Toxicity tests ..................................................................................................................... 36
viii
4. Results and discussion ...................................................................................................................... 42
5. Conclusions and Future work ............................................................................................................ 43
References ............................................................................................................................................ 46
Appendix ............................................................................................................................................... A-1
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List of Figures
Figure 2.1. Biofouling on a boat’s hull (Courtesy of Hempel) ..................................................................4
Figure 2.2. Man scraping off the fouling accumulated on the hull of a boat at Deba’s port, Spain (left);
Fouling scraped from the previous boat, after an operational period of 4 years (right). ..........................5
Figure 2.3. Stages of biofouling and the organisms involved in each stage (Adapted from [10]). ..........6
Figure 2.4. Chronogram summarizing all the antifouling methods used in the past (Adapted from [10]).
..................................................................................................................................................................9
Figure 2.5. Mechanism of action of soluble matrix paints and their efficiency loss observed during the
ship’s operational period (Adapted from [10]). ...................................................................................... 10
Figure 2.6. Mechanism of action of insoluble matrix paints and their efficiency loss observed during the
ship’s operational period (adapted from [10]). ....................................................................................... 11
Figure 2.7. Mechanism of action of TBT-SPC paints (adapted from [10]). ........................................... 12
Figure 2.8. Chronogram representing the antifouling paints used during the 19th and 20th centuries
(adapted from [10]). ............................................................................................................................... 13
Figure 2.9. Action mechanism of tin-free SPC paints when using copper acrylate [16]. ...................... 15
Figure 2.10. Schematic diagram of contact angle ................................................................................ 19
Figure 2.11. Behaviour of a liquid droplet on a flat solid surface .......................................................... 19
Figure 3.1. Characterization tests performed on antifouling paints ...................................................... 21
Figure 3.2. Goniometer used for the wettability measurements ............................................................ 23
Figure 3.3. Perthometer M1 instrument used to measure the roughness of the coatings (Courtesy of
IST) ....................................................................................................................................................... 25
Figure 3.4. Cupping test equipment ...................................................................................................... 26
Figure 3.5. Pencil scratch tester (Courtesy of IK4-Tekniker) ................................................................ 27
Figure 3.6. Persoz pendulum test equipment........................................................................................ 28
Figure 3.7. Taber Abrasion tester (Courtesy of IK4-Tekniker). ............................................................ 29
Figure 3.8. PVC black panels painted coated with the antifouling paints ............................................. 31
Figure 3.9. Coated naval steel panel ..................................................................................................... 32
Figure 3.10. Washability test equipment (Courtesy of IK4-Tekniker). ................................................... 32
Figure 3.11. Stirring test: rotating sample. ............................................................................................ 33
Figure 3.12. Leaching test at IST: static sample conditions. (Courtesy of IST) .................................... 34
Figure 3.13. Kit used to carry out the toxicity tests using the algae Phaeodactylum Tricornutum [38]. 38
Figure 3.14. Kit used to carry out the toxicity tests using the fresh water flea Daphnia magna [40]. ... 40
Figure 3.15. Daphnia magna fleas [40]. ............................................................................................... 40
Figure 3.16. The multi-well plate used in the Daphnia magna toxicity test [40]. .................................. 41
Figure 3.17. Thermostat (right) and the photometer (left) used in the Vibrio fisheri test. ..................... 42
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xi
List of Tables
Table 3.1 - Main characteristics of the paints provided by Hempel and the tests performed for each of
them ....................................................................................................................................................... 22
Table 3.2 - Chemical composition of the standard sea water containing heavy metals [25]. ............... 24
Table 3.3 - Abrasion test conditions ...................................................................................................... 29
Table 3.4 - Main characteristics of the primer and tie-coat paints provided by Hempel ....................... 30
Table 3.5 - Main characteristics of the topcoat paints provided by Hempel .......................................... 31
Table 3.6 - Characteristics of the paints tested in the drag friction test ................................................ 35
Table 3.7 - Main characteristics of the leaching products and the tests where they were obtained ..... 42
xii
Acronyms
BOD - Biological oxygen demand
CAP - Copper acrylate polymer
CDPs - Tin-free Controlled depletion paints
EC50 – Effective concentration
EL50 - Effect load
EPS - Extracellular polymeric substances
IMO - International Maritime Organization
MIC - Microbial Induced Corrosion OD - Optical density
PDMS – Polydimethylsiloxane
PU – Polyurethane
PVC – Polyvinyl chloride
SP – Self-polishing
SPCs - self-polishing copolymers
Spp – Subspecies
SRB - Sulphur-reducing bacteria
TBT – Tributyltin
TBTF - Tributyltin fluoride
TBTO - Tributyltin oxide
TBT-SPC - Tributyltin self-polishing copolymer
ThOD - Theoretical oxygen demand
TOC – Total organic carbon
xiii
List of Symbols
𝑎 – Radius of the internal cylinder
𝑏 – Radius of the container
𝐶 𝑚𝑐 – Drag friction coefficient
∆𝐶 𝑚𝑐 – Drag friction coefficient deviation
ϴ - Contact angle
∅ - Diameter
𝐻 – Height of the coated cylinder
𝜌 – Fluid density
𝛺 – Container’s rotating speed
𝜇 – Viscosity of the fluid
𝑅𝑒 – Reynold’s number
𝑇𝑞 – Friction torque generated between the coating and the fluid
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1
1. Introduction
1.1. The scale of industrial biofouling problem
Biofouling is defined as the attachment and growth of aquatic organisms on a total or partially
submerged surface in an aqueous environment [1].
The shipping industry is, particularly, the most affected, due to the occurrence of this
phenomenon on ships’ hulls, which mainly penalizes the fuel consumption and subsequent operational
costs. The settlement of aquatic organisms on the hull, leads to the modification of the surface
roughness, increasing the skin frictional drag between the surface and the sea water. Several papers
have studied this effect of marine fouling on the hydrodynamic performance of a surface. For instance,
Bohlander (1991) performed full-scale trials on a frigate and concluded that microfilms of biofouling
increased the drag friction by 8 to 18% [2]. In addition, the higher fuel consumption also leads to
higher emission of greenhouse gases, for example harmful gases such as CO2, NOx and SOx. For
instance, the International Maritime Organization (IMO) estimated an increase of at least 50% of CO2
emissions until 2030, under extreme scenarios [3]. Therefore, biofouling is, in fact, the major
inconvenience for the shipping industry.
Despite the aforementioned drawbacks, biofouling can also lead to the introduction of alien
species into certain areas, overpopulating and acting as predators to local species.
Since the ancient times, several methods have been used to combat this hindrance. From
metal sheathing to the incorporation of heavy metals (copper, arsenic and mercury) into coatings, the
latter prevailed and led to the development of potent and durable antifouling coatings [4]. After mid-
20th century, Tributyltin (TBT) was used in the antifouling paints, being considered the most effective
antifoulant, allowing a dry-docking period of five years. However, its use in antifouling paints was
banned due to its undesired effects on marine non-target organisms, such as the induction of imposex
in female gastropods [1]. As consequence of the severe measures applied on TBT, copper
compounds replaced the banned compound, being effective against barnacles, tube worms and the
majority of algae. The lack of efficiency towards several algal species (Enteromorpha spp, Ectocarpus
spp, Achnanthes spp) and the urge to develop a complete broad-spectrum antifouling paint, led to the
use of booster biocides in conjunction with copper, such as Irgarol 1051, Diuron and Zineb [5].
However, the mechanism of action of these paints relies on the release of biocides into the
sea, resulting into high concentrations of these booster biocides in areas with abundant shipping
activities. As consequence, increased concerns about their use have been leading to severe
restrictions, including the ban of some of these booster biocides in order to protect the environment
[6].
To overcome this hurdle, novel biocide-free technologies have been investigated to replace
the biocide releasing based coatings. Non-stick “fouling-release” coatings, containing fluoropolymers
and silicone, have been developed and appear to possess the desired properties to promote
antifouling by release. Nevertheless, the high cost and poor mechanical properties of these coatings
require more improvements [7].
2
Recently, researchers have been focusing in combining “fouling-release” coatings with
hydrogel technology. For example, Hempel has been investing in this technology modifying the
surface of commercial PDMS (polydimethylsiloxane) coatings in order to generate a hydrogel layer in
contact with water, with weak adhesion properties. This layer promotes its detachment from the former
paint layer together with any attached biofouling (eg. slime or algae) on the coating. Experiments were
also performed on ships, showing that this new coating is able to keep the surface clean even at low
speeds [8].
Besides possessing effective antifouling properties and being environmentally compatible, a
successful antifouling coating must also be durable and aesthetically presentable. The general
requirements for an optimal antifouling coating are [9]:
Anticorrosive
Antifouling
Environmentally acceptable
Economically viable
Durable
Compatible with the underlying system
Resistant to abrasion/biodegradability/erosion
Smooth
The need to achieve the aforementioned parameters leads to the submission of antifouling
coatings to advanced characterization techniques, in order to analyze which factor should be improved
for further application in ships. Considering this, the accomplishment of characterization tests is, in
fact, a relevant procedure and thus, it should be performed by antifouling paint manufacturers to
assess the quality of the paints, before being introduced in the market.
1.2. Main goals
The purpose of this thesis is to perform advanced characterization techniques of antifouling
paints, with a major focus on the newly Drag friction test rig developed at IK4-Tekniker, to measure the
friction generated between the antifouling coating and the sea water, being a decisive factor which
influences the fuel cost of the ships. In addition, the durability of the coatings was assessed by
carrying out mechanical tests to measure the scrubbing and abrasion resistance, as well as the
adhesion properties.
The toxicity assessment of these antifouling coatings is also a relevant aspect of this thesis, by
performing toxicity tests on leaching products obtained during the mechanical tests.
These characterization techniques were employed on biocide-free antifouling paints and on
newly developed biocidal paints, where the biocide was immobilized in the paint matrix through
covalent bonds, aiming to select the best antifouling paint to protect ships in the future, based on each
paint’s performance.
3
1.3. Thesis outline
This thesis presents the following outline: Chapter 2 provides information regarding to the
major problem that motivates the development of antifouling paints – Biofouling - and describes the
progress of antifouling technologies and the main characteristics required for an optimal antifouling
paint, simultaneously enhancing the importance of accomplishing characterization techniques.
Chapter 3 describes the characterization tests employed on the provided paint samples. Chapter 4
exposes the results obtained in all the tests mentioned in the previous chapter, along with the
interpretation of each result. Ultimately, Chapter 5 presents the main conclusions and the future work
to be developed in this field.
4
2. State of the art
Paint is a liquid dispersion, which hardens (curing) forming a solid film that can be used as a
protective coating. This coating provides an aesthetic appearance due to the colour and gloss applied
and the ability to reflect or absorb the desired amount of heat and light. The surface properties such as
friction and hardness can also be changed by applying the adequate paint. It can also prevent the
corrosion of a surface and decrease its exposure to other environmental factors such as moisture,
temperature, bacteria and fungi (antifouling). This prevention of the attachment of bacteria and fungi is
more usual on marine paints (e.g. for ships’ hull protection). The most conventional strategy to provide
antifouling ability is the incorporation of biocides to be gradually released and consequently repel/kill
the micro and macroorganisms.
In particular, ships and boats owners face many problems, since the wide structure of the ship
is exposed to several conditions, being the hull the most affected. As known, the hull is the bottom of
the ship and thus, permanently submerged in the sea water. It also possesses some zones on the top
such as an upper area, which is exposed to alternating immersion conditions, splash area (above the
water line) and the top sides, mostly in contact with the atmosphere. Therefore, it is necessary an
antifouling paint that can resist to all of the previously mentioned operating conditions and
simultaneously prevent biofouling [10].
2.1. Biofouling
Biofouling is a complex process which involves the attachment and growth of a community of
organisms on a surface in contact with an aqueous medium [1].
For the shipping industry in particular, biofouling is a critical problem, leading to the reduction
of the maximum speed and upraise of the fuel and maintenance costs. Consequently, as known,
higher fuel consumption also translates into higher emission of greenhouse gases such as NOx and
SOx. Also, an increase of at least 50% of CO2 emissions until 2030, under extreme scenarios, was
estimated by the International Maritime Organization (IMO) [3]. The settlement and accumulation of
marine organisms also leads to the increase of the drag created between the ships’ hull’s surface and
the sea water. Drag friction increases up to 40% can be achieved [11].
Figure 2.1. Biofouling on a boat’s hull (Courtesy of Hempel).
5
Biofouling is also associated to biocorrosion of surfaces, reducing the lifetime of the structures
under a marine environment, which is also promoted by the corrosive effect of sea water itself.
Microbiological fouling should be strictly controlled since it can create microbial induced corrosion
(MIC). For example, sulphur-reducing bacteria (SRB) come from the marine sediment and gain energy
using electrons from the steel structures, chemically reducing the sulphate from the sea water to
sulphide, causing the pitting corrosion of steel surfaces [9], [12].
Additionally, biofouling also contributes to the emigration of certain marine species to other
areas, as occurred in Ponta Delgada (São Miguel Island, Azores, Portugal), where alien species of
fouling organisms such as barnacles were found. Amphibalanus amphitrite was one of the species of
barnacles detected and it is assumed that it is originated from the Indo-Pacific Ocean. Therefore, it is
presumed that due to the fouling propensity of the reported species and given their origin, this
reallocation was caused by the increasing boat traffic in the last years, in Azores [13].
Figure 2.2 shows one of the effects caused by biofouling: hull discoloration and wear.
Figure 2.2. Man scraping off the fouling accumulated on the hull of a boat at Deba’s port, Spain (left); Fouling
scraped from the previous boat, after an operational period of 4 years (right).
2.1.1. Stages of biofouling
Biofouling is characterized by four main stages throughout the time. The first stage initiates
after the earliest minutes of immersion, where the physical adhesion of organic molecules of proteins,
polysaccharides, glycoproteins and others, occurs. In this stage, Van der Waal’s forces and
electrostatic interactions promote this adsorption phenomenon.
The movement of water leads to the contact and colonisation between the microorganism and the
surface. This attachment leads to the second phase, after 24 hours of immersion, where the reversible
adsorption of bacteria and unicellular algae occurs. Bacteria and other colonising microorganisms
secrete extracellular polymeric substances (EPS) to enclose and hold the substrate, forming a
microbial film. Consequently, the local surface chemistry is altered, being propitious to stimulate
further growth and settlement of macroorganisms.
This microbial film feeds spores of microalgae, allowing their attachment, which will constitute
a biofilm (1 week, third stage). The biofilm generated is a mass of microorganisms and the EPS
secreted creates a gel matrix providing enzymatic interaction and high resistance to biocides. Also, the
arrangement of the microorganisms in the biofilm protects them from the predators and from
environmental variations, facilitating the obtainment of the nutrients necessary for the settlement of
6
other microorganisms. This biofilm is capable of attracting more particles and organisms as larvae of
marine macroorganisms, characterizing the fourth stage, after 2 or 3 weeks of immersion. The
roughness of the surface created by the irregular microbial communities will also help the
accommodation of the new attracted organisms. All of these conditions will contribute to the adhesion
and attachment of macroalgae and marine invertebrates [9], [10].
Figure 2.3 summarizes the aforementioned stages and the marine organisms involved in each
stage, throughout the immersion time.
Figure 2.3. Stages of biofouling and the organisms involved in each stage (Adapted from [10]).
Figure x. Stages of biofouling and the organisms involved in each stage. (artigo elisabete)
0 – 1 min
1 – 24 hours
1 week
2 – 3 weeks
- Organic molecules
Proteins,
polysaccharides and
proteoglycans)
- Some inorganic
molecules
- Bacteria
Pseudomonas
putrefaciens and
Vibrio alginolyticus
- Diatoms
Achnanthes brevipes,
Amphora
coffeaeformis,
Amphiprora paludosa,
Nitzschia pusilla and
Licmophora
abbreviata
- Spores of microalgae
Ulothrix zonata and
Enteromorpha
intestinalis
- Protozoans
Vaginicola sp.,
Zoothamnium sp. and
Vorticella sp.
- Larvae of
macroorganisms
Balanus amphitrite
(Crustacea), Laomedea
flexuosa (Coelenterata),
Electra crustulenta
(Briozoa), Spirorbis
borealis (Polychaeta),
Mytilus edulis (Mollusca)
and Styela coriacea
(Tunicata)
Licmophora abbreviata
Balanus amphitrite
Ulothrix zonata
7
2.1.2. Biofouling prevention
The type and severity effects of biofouling depend on diverse parameters such as
temperature and salinity of the water, light, geography, depth and ship speed. Generally, biofouling is
more aggressive in high water temperature areas, since it is the prevailing condition for the breeding
and growth of fouling organisms. All of these factors can hardly be modified, being necessary to
develop inexpensive and environmentally friendly antifouling methods to solve this problem [7].
2.2. Antifouling methods
The urge to protect the ship hulls from marine biofouling, to avoid material damages and
excessive fuel consumption, has led to an intensive research for economical and environmentally
friendly solutions.
A description of the progress of the antifouling technologies since 5th century B.C. until the
modern developments is presented in this chapter.
2.2.1. Archaic antifouling methods
Marine biofouling has been a nuisance for more than 2000 years [14]. At that time due to the
lack of advanced technology and in order to overcome this inconvenience, natural products were used
to resist corrosion and biofouling.
For example, Phoenicians and Carthaginians were said to have used pitch and possibly
copper sheathing on ship hulls, whereas other ancient cultures used wax, tar and asphalt. The use of
coatings made with arsenic and sulphur mixed with oil were also used to resist shipworms, in the 5 th
century B.C. [14].
A brief description of the ancient antifouling methods is presented below.
A) First technologies and lead sheathing
As mentioned above, pitch, copper sheathing, wax, tar, asphalt or a mixture of arsenic and
sulphur with oil, were applied to protect ship hulls. Alternatively, lead sheathing was also used for
this purpose.
Lead sheathing consisted of covering the ship hulls with lead patches in order to protect
them from biofouling and corrosion. Ancient cultures such as Phoenicians employed this prevention
method in 700 B.C., while the Greeks were reported to use lead sheathing and tar and wax, in the 3rd
century B.C. Greeks and Romans also used copper nails to secure the sheathing.
In the period between 45 and 125 A.D., Plutarch mentioned the method of scraping the ship’s
sides to remove weeds, slime and filth, in order to facilitate the motion of the ship on the water.
8
Latterly, in 10 A.D, Vikings occasionally used seal tar. From the 13th to 15th century, the use of
pitch was abundant, being sometimes mixed with oil, resin or tallow.
In the 16th century, lead sheathing was largely adopted, being employed by Spain, France and
England, although wood sheathing was more usual.
However, the British Admiralty discarded the use of lead sheathing, in 1682, due to the
corrosion caused on the iron components of the ships. Subsequently, lead sheathing was then
alternated with wood sheathing. After applying the latter, it was painted with several mixtures such as
tar, grease, pitch and brimstone and then nailed with large headed copper or iron nails very
adjacently, in order to form a metallic sheathing [14].
B) Copper sheathing
The first reference regarding to the underwater use of copper was in 1618, during the reign of
the Danish King Christian IV, who used a coppered keel.
However, the first reference regarding to the use of copper as an antifouling agent was
patented by William Beale, who used a mixture of cement, powdered iron and a copper compound
(copper sulphide or copper arsenic ore).
The use of copper sheathing was firstly reported in 1758 on HMS Alarm frigate, whose
success motivated other ships to use copper, mostly the British Navy, around 1780. The application of
copper on wooden ships was so successful that England prohibited the exportation of this metal.
Only in the 19th century, Humphrey Davy showed that the fouling prevention was attained due
to the dissolution of copper in the sea water.
Anyhow, the use of copper sheathing on iron ships (introduced late in the 18th century) was
discontinued, due to the uncertainty of its antifouling action and corrosion effects on iron [14].
C) Other alternatives
Due to the introduction of iron ships and the abandonment of copper sheathings on this type of
boats, more alternatives were tried, to obtain protection against biofouling.
Sheathings of zinc, lead, nickel, arsenic, galvanized iron and alloys of antimony, zinc and tin
and coppered wooden sheathings were the alternatives tested. Non-metallic alternatives such as
soaking felt in tar or using cork, rubber and plain brown paper were often applied to separate the
copper sheathing from the iron hull. Wooden sheathings were also tested on these ships, although
without success, due to its high cost [14].
Figure 2.4 summarizes the above mentioned strategies used in ancient times for biofouling
control.
9
Figure 2.4. Chronogram summarizing all the antifouling methods used in the past (Adapted from [10]).
As a whole, the unavailability of effective antifouling methods capable of protecting the hull of
iron ships, motivated the research and development of new solutions such as antifouling paints.
2.2.2. Modern antifouling methods: protective paints
Novel antifouling systems were developed to overcome the limitations of the ancient methods.
These systems consisted of paints such as enamels, varnishes, primers, sealers and many others.
The majority of antifouling coatings is mainly composed by a primer (anticorrosive) and a topcoat. The
latter incorporates antifoulants to protect the hull from biofouling [9].
A) First antifouling paints
In the mid of 1800, different paints were created, by dispersing a toxicant in a polymeric base.
These toxicants consisted of copper oxide, arsenic and mercury oxide, whereas the solvents used
were turpentine oil, naphtha and benzene. Linseed oil, shellac varnish, tar and diverse types of resin
were employed as binders [14].
19th cent.
Figure x. Chronogram condensing all the antifouling methods used in the past. (artigo elisabete)
- Approximate
period:
Oldest
- Civilisation:
Oldest
- Antifouling
product
Wax, tar and
asphalt
- Civilisation:
Phoenicians,
Carthaginians
- Antifouling
product
Pitch, possibly
copper and lead
sheathing and
tallow
- Civilisation:
Phoenicians
- Antifouling
product
Coatings of
arsenic and
sulphur
mixed with
oil
- Civilisation:
Greeks
- Antifouling
product
Wax, tar
and lead
sheathing
- Civilisation:
Romans,
Greeks
- Antifouling
product
Lead
sheathing with
copper nails
- Civilisation:
Vikings
- Antifouling
product
Seal tar
- Navigator:
Plutarch
- Antifouling
product
Scraping of
algae, slime
and pitch
(...) 13th cent.
15th cent.
- Civilisation/Navigator:
Several/ Columbus
- Antifouling product
Pitch and mixtures with
oils, resin or tallow /
Pitch and tallow
- Period:
1618-1625
- Civilisation:
Various
- Antifouling
product
Copper,
possibly with
a mixture of
cement, iron
dust and
copper
sulphide or
arsenic ore
18th cent.
17th cent.
- Civilisation:
Various; English
- Antifouling product:
Sacrificial wood sheathing
on a layer of pitch and
animal hair; wood sheathing
covered with mixtures of tar,
fat, sulphur and pitch, with
metallic sheathing formed
with nails; metallic
sheathings; non-metallic
sheathing (1758 – 1816) /
copper sheathing using nails
of copper and zinc alloy
(1786)
- Civilisation: Various
- Antifouling product
Abandonment of wood sheathing
covered with copper sheathing
(1862); first paints with a toxicant
(Cu, As or mercury oxide)
dispersed in a polymeric binder
(linseed oil, shellac, colophony)
10
Nevertheless, these paints required the application of a primer in order to protect the steel hull
from the pigments used, since its direct utilization on the hull caused corrosion.
In the meantime, more paints were launched, such as “hot plastic paints” consisting of copper
sulphate in a metallic soap composition, shellac based paints (rust preventive) and “cold plastic paints”
which used diverse synthetic resins or natural products either solely or in mixtures. The latter
effectively decreased fouling and were easily applicable due to “airless” spraying, enabling dry docking
periods of up to 18 months [10], [14].
However, the antifouling industry revamped after the Second World War, leading to the
appearance of new synthetic petroleum based resins with improved mechanical characteristics. Also
during this period, organometallic paints were introduced and contained tin, arsenic, mercury and
many others, which after several developments, led to tributyltin (TBT) based paints [10], [14]. The
TBT based paints revealed to be remarkably efficient against biofouling.
Additionally, more paints’ technologies were developed to overcome the environmental issues
of organometallic based paints, and classified according to the chemical properties of the binder and
by their water solubility: soluble matrix and insoluble matrix paints [10], [14].
A.1) Soluble matrix antifouling paints
Soluble matrix antifouling paints contain rosins and their derivatives as binders and toxic
pigments (copper, iron, zinc oxides, arsenic and mercury). The toxic compounds can dissolve in sea
water, forming a thin leached layer which easily releases the toxic material as the sea water
penetrates. The thickness of the leached layer decreases when the ship speed increases, which
consequently leads to an exponential increase of the release rate. On the other hand, at static
conditions, the settlement of organisms is accentuated and the insoluble salts can block the coating’s
pores, which consequently decreases the release rate of the biocides. In addition, these paints are
less mechanically resistant than the insoluble paints due to the brittleness of the resin and its
instability to oxidation and as consequence, the life span of these paints is short (12 to 15 months) [7].
Nonetheless, they present the advantage of being easily applied on smooth bituminous-based primers
[10].
Figure 2.5 schematizes the loss of efficiency of soluble matrix antifouling paints.
Figure 2.5. Mechanism of action of soluble matrix paints and their efficiency loss observed during the ship’s
operational period (Adapted from [10]).
11
A.2) Insoluble matrix antifouling paints
Insoluble matrix antifouling paints have a polymeric matrix such as acrylic, vinyl or chlorinated
rubber, which are insoluble in sea water. When the coating is immersed in sea water, the soluble toxic
materials dissolve and consequently leave a multiporous layer known as leached layer, which enables
the further penetration of the water and the release of more poisonous compounds. The advantage of
this type of paint is the high mechanical resistance and stability to oxidation and photodegradation.
Although the coatings are thick to increase the content of toxic material, at some stage, the efficiency
will decrease due to the gradual release of the toxic compounds. Consequently, the empty space left
by the dissolved biocides will modify the roughness of the surface and capture pollutants from the sea
water, which will restrain the water penetration and as a result decrease the release rate, leading to
the reduction of the life span of this type of paint to 12 to 24 months [7], [10].
Figure 2.6 schematizes the mechanism of action and loss on efficiency for insoluble matrix
antifouling paints.
Figure 2.6. Mechanism of action of insoluble matrix paints and their efficiency loss observed during the ship’s
operational period (adapted from [10]).
A.3) Tributyltin self-polishing copolymer coatings
Since the insoluble and soluble matrix paints have some drawbacks, alternative coatings have
been developed in order to improve these paints.
The first tributyltin self-polishing copolymer (TBT-SPC) technology was patented by Milne and
Hails, in 1974, revolutionizing the entire shipping industry. Organic tin and its derivatives have been
generally used as antifoulants due to their broad-spectrum characteristic. Tributyltin oxide (TBTO) and
tributyltin fluoride (TBTF) were the organotin compounds used, also known as powerful fungicides,
completely capable of inhibiting the growth of most fouling organisms at a very low concentration [7].
As known, every paint contains pigments to confer the desired colour. Usually, metallic
copper, copper thyocyanate and cuprous oxide are the dominant copper pigments used in antifouling
paints. However, the copper ions as Cu2+ have a major role in antifouling, yet they can only target
specific fouling organisms. Biological indicators differ significantly according to the copper sensitivity,
being more selective to microorganisms than to macroorganisms (general decreasing order:
microorganisms > invertebrates > fish > bivalves > macroalgae) [7]. To alter the selectivity towards
macroorganisms, TBT was used in conjunction with copper, since it is highly toxic to oysters, molluscs
and crustaceans [10].
12
TBT-SPC paints were based on acrylic polymer (usually methyl methacrylate) with TBT
groups tethered to the polymer backbone by an ester. When immersed in water, the soluble pigment
particles such as zinc oxide (ZnO) or copper oxide (CuO) would begin to dissolve.
The water penetration was prevented by the hydrophobic nature of the polymer of TBT
methacrylate and methyl methacrylate. Thus, the water could only fill in the pores created by the
dissolution of the soluble pigment particles. Furthermore, the carboxyl-TBT bond is easily hydrolyzed
in slightly alkaline environments as sea water (pH of 7.5 to 8.5), which slit the TBT portion from the
copolymer and then released the biocides into the water. As the TBT portions were split, the partially
reacted brittle polymer backbone became prone to be washed off by the moving sea water, exposing a
fresh coating surface. The hydrolysis process provided a low hull roughness (about 100 μm), which
did not influence significantly the drag friction of the ship’s hull [7].
Figure 2.7 schematizes the action mechanism of this type of paint.
Figure 2.7. Mechanism of action of TBT-SPC paints (adapted from [10]).
One of the advantages of this kind of coating was the control of the polishing rate by the
manipulation of the polymer chemistry, being possible to balance the high effectiveness and a long life
span in function of the ships’ operating conditions and sailing speed. Studies have proven that the
release rate of TBT in the sea water is almost constant with the sailing speed, which confers a high
antifouling performance even at static or low speed. Additionally, TBT-SPC paints had high
mechanical resistance, high stability to oxidation and short drying times [7].
This type of paint was widely applied in the shipping industry due to its high efficiency and
versatility.
Consequently, the extensive use of TBT introduced high levels of contamination in the
environment and thus negatively affected the marine communities. TBT is extremely toxic to non-
target organisms ranging from bacteria to fish and mammals, affecting their growth, development,
reproduction and survival. For example, before the 1980’s, populations of gastropods were ceased
due to the presence of TBT compounds in the sea water. This disappearance is explained by the fact
that TBT causes a hormonal imbalance, which leads to the development of male sex organs on
female gastropods, which hinders the breeding of gastropods [4].
In 2001, IMO (International Maritime Organization) banned the use of TBT in the
manufacturing of paints from 1st January 2003 and the presence of these paints on ship hulls from 1st
January 2008. However, this ban did not apply to copper, since it is an essential element needed for
13
the growth of all plants and animals, besides being naturally present in the sea water. It is also
lipophilic and thus less bioaccumulative [10]. Nevertheless, it is possible that copper based paints may
end up facing the similar regulations as TBT. For instance, Sweden, Denmark and USA are planning
to strengthen the legislations regarding to the use of copper-based antifouling paints, since the
excessive boat traffic can lead to the contamination of the aquatic environment [3], [15].
Figure 2.8 shows all the aforementioned information condensed according to the respective
period.
Figure 2.8. Chronogram representing the antifouling paints used during the 19th and 20th centuries (adapted from
[10]).
19th cent.
- Period:
Mid 19th century
- Product:
First paints
- Binder: Linseed
oil, Shellac varnish,
tar, resins
- Pigment/biocide:
Copper, arsenic or
mercury oxides
- Characteristics:
Dispersion of a
toxicant in a
polymeric binder
- Product:
“Hot plastic paints”
- Binder:
metallic soap
composition or
colophony
- Pigment/biocide:
copper
compounds
- Product:
Spirit varnish paints
- Binder: Grade A
“Gum Shellac“
- Pigment/biocide:
Red mercury oxide
or zinc oxide, zinc
dust and India red
- Characteristics:
Contains alcohol,
turpentine essence
or pine tar oil
- Period: Late 19th
century
- Product: Rust
preventer
- Binder: Shellac
primer and
Shellac antifouling
paint
- Pigment/biocide:
Different toxicants
- Product:
“Cold plastic paints”
- Binder: Coal tar or
coal tar and
colophony; Shellac
varnish; Synthetic
resins
- Pigment/biocide:
Copper or mercury
oxides
- Characteristics:
Easy application by
airless spraying;
Some allowed dry
dock periods of up
to 18 months
- Product:
Soluble matrix
paint
- Binder:
Colophony and
others
- Pigment/biocide:
Copper, arsenic,
zinc, mercury or
iron oxides
- Product: Self-
polishing paints
containing tin (TBT-
SPC)
- Binder: Acrylic
polymer (normally
methyl meta-acrylate)
with TBT groups
bonded to main chain
by ester binders
(copolymer)
- Pigment/biocide: Zinc
oxide and insoluble
pigments or copper
oxide, tri-organo-tin
and co-biocides
- Product:
Application of
insulating primer
under the
antifouling paint
- Binder: Linseed
oil, Shellac varnish,
tar, resins with
preliminary
insulating varnish
coating
- Pigment/biocide:
Copper, arsenic or
mercury oxides
- Product:
Insoluble matrix
paint
- Binder: Acrylic
resins, vinyl resins
or chlorinated
rubber polymers
- Pigment/biocide:
Copper and zinc
oxides with or
without
organometallic
compounds
- Product:
Antifouling paints
- Binder: Tar
- Pigment/biocide:
Copper oxide
- Characteristics:
benzene and
naphtha used as
solvents
14
B) Environmentally friendly antifouling paints Due to the ban of the most efficient and versatile TBT-SPC paints, the paint producers felt the
urge to develop new and less environmentally harmful paints. Therefore, Tin-free SPC technology was
developed and commercially introduced [7].
The tin-free coatings can be divided into three categories: tin-free controlled depletion paints
(tin-free CDPs), tin-free self-polishing copolymers (tin-free SPCs) and hybrid paints (conjugation
between the CDPs and SPCs) [10].
Despite the fact that these paints are free of TBT, their mechanism consists of biocide release,
whose action has not always been fully explained. Considering this, the development of fully biocide-
free antifouling paints is still in course.
B.1) Tin-free controlled depletion paints (tin-free CDPs)
The tin-free CDPs are an improved version of the traditional soluble matrix paints, where
organically synthetized resins reinforce the binder, although presenting the same antifouling
mechanism as the conventional rosin matrix paints. The synthetized resins are more resistant than
rosins and control the dissolution of the soluble binder [7], [10].
These paints are also known as ablative/erodible paints, containing polymeric compounds
capable of controlling the relative rate of dissolution/erosion. They also contain a large proportion of a
non-toxic binder, which is soluble in sea water. The biocide content is high and dissolves in the sea
water, in conjunction with the soluble binder. The rate of erosion becomes constant after short time of
immersion [10].
However, these paints transform into an empty matrix, due to the dissolution of the soluble
compounds incorporated in the paint into the sea water, affecting their behaviour. Consequently, a
high amount of copper and co-biocide is needed, which rises the concern towards the environment
[10]. Also, as the compounds dissolve, the roughness of the coating increases, thus promoting
biofouling formation. The leached layer formed may be removed prior to recoating [16].
Regarding to the lifespan of these paints, these confer a protection a bit longer than 3 years.
They do not require a tie coat when repainted and are less expensive than TBT-based self-polishing
paints.
Usually, leisure boats and small ships with short service time apply these paints.
B.2) Tin-free self-polishing copolymer paints (tin-free SPCs)
The tin-free SPCs contain an acrylic copolymer matrix combined with booster biocides, where
different pendant groups are linked to the polymeric backbone and released after the contact with sea
water. This process resembles the hydrolysis of TBT-SPC paints [14].
In this type of paint, the antifouling activity is induced as the chemical reaction through
hydrolysis of copper, zinc or silyl acrylates occurs, forming an acidic polymer, which is soluble in sea
water and can be washed from the surface [7], [9].
15
The hydrolysis process is followed by the loss of the dissolved layer of polymer, smoothening
the surface [16].
These paints present a life span 3 to 5 years, due to their high polishing rate. Anyhow, they
are not as efficient as TBT-based-self-polishing paints [10].
For instance, when insoluble Zn acrylate is used, it hydrolyzes to soluble acidic polymer and
the following reaction is assumed:
Polymer−COO− Zn(solid) –X + Na+ → Polymer−COO−Na (solid) + X- + Zn2+
The Zn2+ is released into the sea water for antifouling properties and the soluble acidic
polymers can be washed from the surface. Currently, metallic copper, copper thyocyanate and
cuprous oxide are the dominant compounds used in antifouling paints.
Figure 2.9 schematizes the mechanism of tin-free SPC paints when copper acrylate polymer
(Cap) is used.
Figure 2.9. Action mechanism of tin-free SPC paints when using copper acrylate [16].
In comparison with the TBT antifouling paints and as mentioned before, copper-containing
coatings can only target specific fouling organisms. To improve the antifouling properties and thereby
the selectivity to macroalgae, barnacles and bryozoans, booster biocides such as Irgarol 1051, Diuron,
copper pyrithione, zinc pyrithione, isothiazolone, Zineb, Econea and many others are added, as an
alternative to TBT derivatives [7], [17]. Although the toxicity of the majority of the biocides
aforementioned has not been fully assessed, zinc pyrithione and Zineb seem to be the least toxic,
whereas Irgarol and Diuron are reported to be more poisonous [10].
B.3) Hybrid paints
The antifouling mechanism of hybrid paints is a conjunction of the mechanisms of both CDPs
and TF-SPCs. The leached layer, cost and the performance of these paints is intermediate.
An example of these hybrid paints are the microfibres incorporated in paints, by Hempel [10],
[16].
16
B.4) Biocide-free coatings
Due to the toxicity of the biocides used in the antifouling paints, novel biocide-free
technologies have been investigated to replace the biocide based coatings.
Non-stick “fouling-release” coatings, containing fluoropolymers and silicone, have been tested
regarding to the release of macrofouling organisms, using robust hydrodynamic conditions.
Apparently, fluoropolymers and silicone appear to possess the desired properties to promote
antifouling by release. Some low surface energy coatings have also been prepared with modified
acrylic resin and nano-SiO2. Moreover, accumulated fouling organisms are not easily released, being
difficult to develop an environmentally friendly and simultaneously effective coating.
Additionally, these methods have some drawbacks such as high cost, poor mechanical
properties and the difficulty of recoating [7].
Recently, researchers have been focusing in combining “fouling-release” coatings with
hydrogel technology. For example, Hempel has been investing in this technology modifying the
surface of commercial PDMS (polydimethylsiloxane) coatings in order to generate a hydrogel in
contact with water, with weak adhesion properties. This layer promotes its detachment from the former
paint layer together with any attached biofouling (eg. slime or algae) on the coating. Experiments were
also performed on ships, showing that this new coating is able to keep the surface clean even at low
speeds [8].
In summary, despite the fact that hydrogel based “fouling-release” coatings are showing
positive results on the biofouling prevention, their durability and effect on the environment are still
unknown, which should motivate a deep research in this field in order to develop an effective, durable
and non-toxic coating.
For this purpose, advanced characterization techniques should be performed to evaluate the
mechanical characteristics and the environmental compatibility, to proceed to further improvements
and finally introduce new potential benign products in the market.
2.3. General characteristics of antifouling paints After developing the desired polymeric matrix of the coating, it is necessary to proceed to
characterization tests in order to check if the coating is in accordance with the standards.
Generally, the requirements for an optimal antifouling coating consist of being anticorrosive,
environmentally acceptable, economically viable, durable, smooth, compatible with the underlying
system, resistant to abrasion, biodegradation and erosion [9].
2.3.1. Anticorrosiveness
If the substrate (ship’s hull) is steel, the paint should protect it from corrosion caused by the
exposure to the marine environment.
To prevent this problem, at least one coating of primer or anticorrosive paint is applied before
the layer of antifouling paint. The latter may contribute materially to the protection of the hull from
corrosion, depending primarily on the thickness of the antifouling coating and its resistance to the
17
ingress of sea water. Thus, thick paints impede corrosion and provide the necessary toxic storage for
extended fouling prevention [18]. The adequate thickness is generally specified by the paint providers
in the technical data sheet of the product. For instance, Jotun’s antifouling paints’ thickness ranges
from 75 to 150 μm, whereas the primers’ thickness ranges from 40 to 250 μm [19].
Also, the chemical compounds added into de coating should be taken into account, since they
might tend to quicken the corrosion effect. For instance, common toxic pigments such as metallic
copper and salts of copper and mercury have the tendency to intensify corrosion if they are applied
directly on the hull [18].
2.3.2. Durability
The durability of a coating is dependent on its resistance to mechanical damage, on the
erosion caused by the water motion and on the components present in the paint formulation. If it
contains any biocide, the coating’s disintegration must be also taken into account, since it degrades as
the sea water penetrates and releases the toxic biocide. Therefore, a balance between toxicity and
durability should be established.
The resistance to the erosive effect of the water motion is a notable problem in high speed
vessels, such as motor torpedo boats and hydroplanes. Considering this, it is necessary to develop
suitable paints, which can confer a hard and thus resistant surface to overcome this drawback.
The loss of durability is more accentuated near the waterline, due to the mechanical damage
caused by the floating debris and the alternation between the wet and dry conditions and also due to
the exposure to the sun. These factors intensify the coatings’ cracking, being necessary to develop a
paint which can resist to all of this harm [18].
2.3.3. Adhesion
The adhesion is an important property, since the paint should adhere adequately to the
substrate where it is applied, regardless of the natural conditions exhisting during this procedure. This
means that the paint should adhere suitably either when it is applied during winter (high moisture and
low temperature) or summer (low moisture and high temperature) [18].
Low adherence may lead to the desintegration of the coating and therefore expose the hull to
the marine environment, leaving it unprotected.
2.3.4. Abrasion
The assessment of the abrasion resistance of antifouling paints is a relevant factor, since it
can indicate the paints’ resistance to friction caused by moving particles transported by the wind or
water [18]. These particles can erode the paint, when the ship is in motion or in the port, compromising
the durability of the coating.
18
2.3.5. Smoothness
The paint must be applied uniformly to confer a smooth surface, which, therefore, will create
less frictional drag and biofouling attachment.
Additionally, it is also desirable that paints with a high viscosity (needed to form thick coatings)
have sufficient elasticity to fill up the minor irregularities present on the ship’s surface [18].
2.3.6. Drag friction
As known, a ship must be designed to move efficiently through the water with a minimum of
external force. However, when it is propelled through the water, it has drag associated with it, which
must be overcome by thrust to move forward.
Drag is defined as the force that opposes forward motion through the fluid and is parallel to the
direction of the free-stream velocity of the fluid flow. When moving on the water, the drag of a ship
presents two major components: wave-making drag and skin frictional drag. The frictional drag
typically accounts 60-90% of the total resistance and can be reduced by applying an appropriate
surface coating, which softens the surface. The roughness of the surface and, thereby, the frictional
resistance is influenced by different factors such as the age or condition of the ship hull, the surface
preparation, the paint application, the paint system and marine fouling [20], [21].
The skin frictional drag is increased when microbial communities, present in the sea, attach to
the surface of the coating, leading to extreme fuel and maintenance costs of the ships and CO2
emission. Several papers have studied the effect of marine fouling on the hydrodynamic performance
of a surface. For example, Bohlander (1991) performed full scale power trials on a frigate and
concluded that microfilms of biofouling increased the drag friction by 8 to 18% [2].
For this purpose, it is of utmost importance to assess the coating regarding to the drag friction
effect, to avoid excessive fuel consumption and subsequent penalties.
Several experiments have been applied to measure the drag friction of the coatings, including
a rough plate or a rotating disc or cylinders [20].
It is important to mention that the selection of the coating type does influence the drag friction
effect. Several studies have been carried out to compare the drag resistance of silicone based “foul-
release” coatings with tin-free self-polishing coatings. The former has shown positive results in
comparison with the latter, mainly due to its surface texture (less rough) [22].
2.3.7. Wettability
The wettability of a solid by a liquid can be determined by measuring the contact angle (or
wetting angle), ϴ. The contact angle is described as the angle between the surface of the liquid and
the outline of the contact surface, when an interface between a liquid and a solid exists (Figure 2.10).
19
Figure 2.10. Schematic diagram of contact angle.
The contact angle is specific for any given system and most often, the concept is illustrated
with a sessile or resting liquid droplet (small drop) on a horizontal solid surface. (Figure 2.11)
Figure 2.11. Behaviour of a liquid droplet on a flat solid surface.
In the case of complete wetting (spreading; hydrophilic), the contact angle is 0º. Between 0º
and 90º, the solid is wettable and above 90º it is not wettable (hydrophobic).
This test is useful to characterize antifouling paints, since it can indicate its
hydrophilicity/hydrophobicity. For instance, a hydrophilic surface has more affinity with water, which
can keep the surface clean as the ship sails. The constant washing can delay the slime settlement and
the following fouling process.
However, this behaviour is not constant nor applicable to all the marine organisms. For
example, barnacle Balanus improvisus prefers more hydrophilic surfaces whereas its relative Balanus
amphitrite appears to be fond of hydrophobic surfaces [8].
2.3.8. Environmental risk assessment Since the main mechanism of action of the majority of antifouling paints consists of releasing
biocide into de sea water, a severe environmentally compatibility assessment should be carried out
before the introduction of these paints in the market.
After the ban of TBT based paints, alternative biocides have been used in conjunction with
copper, which can be less or equally harmful.
For instance, the leaching of copper from the antifouling paints on Swedish boats tends to be
harmful to the Baltic Sea’s key-species bladderwrack and Fucus vesiculosus, leading the Swedish
20
Chemicals Agency to restrain the use of paints leaching excessive copper and prohibit copper based
paints on leisure boats (length < 12 m) [23].
Another example is regarding to a study carried out in Hong Kong, which consisted of testing
the toxicity of five commonly used booster biocides (Irgarol, diuron, zinc pyrithione, copper pyrithione
and chlorothalonil) on the growth or survival of 12 marine species, concluding that Irgarol is even more
toxic than TBT and copper pyrithione is as toxic as TBT [6].
Considering this, it is important to perform an accurate evaluation of the environmental risks
that these paints can pose to the marine species, by carrying out biodegradability tests or mechanical
tests that enable the collection of leachates for toxicity analysis.
A brief description of the characterization tests, performed in this work, on antifouling paints
developed by Hempel are presented in the next chapter.
21
3. Experimental methods
As previously mentioned, the antifouling paints need to be submitted to characterization tests,
in order to select a suitable paint with the needed technical requisites to be used in a marine
environment (e.g. ships).
In this chapter, the characterization techniques performed on antifouling paints, provided by
Hempel (Table 3.1), are divided in two parts: Surface/mechanical properties’ assessment and
environmental compatibility.
The former covers the mechanical tests carried out to evaluate the mechanical resistance and
thus, the durability of the coatings, as well as surface characteristics in terms of hydrophilicity
/hydrophobicity and the roughness. Additionally, due to the aggressiveness of certain mechanical
tests, leaching of the paints occurred and the resulting leachates were further collected for the
following environmental compatibility assessment.
The environmental compatibility assessment consisted in performing biodegradability tests on
paints and toxicity tests on the leachates obtained during the mechanical tests, in order to conclude
about their ecological risks.
Figure 3.1 summarizes the characterization tests performed on the antifouling paints.
Figure 3.1. Characterization tests performed on antifouling paints.
Figure x summarizes the characterization tests performed on the antifouling paints.
Biodegradability
test
Manometric
Respirometry
Test with Sea
water
Environmental Compatibility Mechanical/surface properties
assessment
Adhesion test
Hardness test
Abrasion test
Wettability test
22
Table 3.1 - Main characteristics of the paints provided by Hempel and the tests performed for each of them
Paint designation A B C D E F G
Paint supplier’s designation
Hempasil X3 Olympic + Hempaguard X7 F0027 F0032 F0033 F0042
Type Foul-release Commercial
SPC Commercial
Foul-release Commercial
New paint New paint (Blank
reference) New paint New paint
Polymeric matrix Silicone Acrylic Silicone Polyurethane Polyurethane Silicone Polyurethane
Biocide presence No Yes Yes Yes No Yes Yes
Wettability surface assessment
Adhesion test
Hardness test
Abrasion test
Washability test
Stirring test
Drag friction test
Biodegradability test
Toxicity tests
Alga growth inhibition test
Daphnia acute immobilization test
Luminescent bacteria Vibrio
fischeri test
23
The biocide-free paints A and E are the base for the development of biocide (Econea and
Irgarol) incorporated paints, in order to improve their mechanical characteristics. For instance, paint A
is the blank of silicone based paints, whereas paint E is the blank of polyurethane based paints. Paints
D and G contain biocides and the same polymeric matrix as paint E, while Paints C and F are silicone
based as paint A but contain biocide (Copper pyrithione and Econea, respectively). Paint B is a self-
polishing acrylic based paint containing biocide (Copper (I) Oxide and Zineb).
A brief description of the tests performed on each paint is presented below.
3.1. Surface properties assessment
3.1.1. Wettability test
The wettability of a solid by a liquid can be determined by measuring the contact angle (or
wetting angle), ϴ. The contact angle is described as the angle between the surface of the liquid and
the outline of the contact surface, when an interface between a liquid and a solid exists.
The testing samples were panels of naval steel Grade A coated with the commercial paints A
(biocide-free silicone based) and B (acrylic based with Copper (I) Oxide and Zineb) and the recently
developed paints D (polyurethane based with Econea), E (biocide-free polyurethane based) and F
(silicone based with Econea).
Procedure
To measure the contact angle, it was used a Goniometer Surftens UNIVERSAL automated
contact angle tester (Figure 3.2).
Figure 3.2. Goniometer used for the wettability measurements.
The method used for the angles measurements is the known Sessile Drop method. Sessile
drop method is an optical contact angle method based on the principle explained previously and
illustrated in the Figure 2.11.
24
A droplet of artificial seawater (electrolyte developed according with ASTM D1141 - 98) was
dispensed on the painted surfaces with the help of a syringe. The drop was illuminated with diffuse
light in order to obtain an image of the drop with sharp border. The image was recorded with a camera
and the angle between the baseline of the drop and the tangent at the droplet boundary was
measured by using a specific software SURFTENS 3.0.
The contact angle was measured throughout the time. The acquisition frequency was of 0.43
s, thus allowing, for a total time of 5 minutes for each test, to record 700 measurements. The error
associated to camera acquisition is 0.5º. The volume of the seawater drops used in all tests was of
approximately 4 L [24].
The standard sea water used as electrolyte was previously prepared, according to the ASTM
D1141 – 98 standard, using distillate water and the reactants shown in Table 3.2.
Table 3.2 - Chemical composition of the standard sea water containing heavy metals [25].
Compound Concentration (g/L)
NaCl 24.53
MgCl2 . 6H2O 5.20
Na2SO4 4.09
CaCl2 1.16
KCl 0.695
NaHCO3 0.201
KBr 0.101
H3BO3 0.027
SrCl2 . 6H2O 0.025
NaF 0.003
Ba(NO3)2 . 6H2O 9.94 x 105
Mn(NO3)2 . 6H2O 3.40 x 105
Cu(NO3)2 . 3H2O 3.08 x 105
Zn(NO3)2 . 6H2O 9.60 x 106
Pb(NO3)2 6.60 x 106
AgNO3 4.90 x 106
3.1.2. Roughness assessment
The coatings were submitted to roughness tests, in order to select the one that possesses less
roughness, since they are less prone to accommodate fouling organisms and therefore create less
drag friction effect. The main parameters measured were the average roughness (Ra), which is the
arithmetic average of the absolute values of the roughness profile and the maximum roughness depth
(Rmax), defined as the largest single roughness depth within the evaluation length [26].
25
Commercial paints A (biocide-free silicone based) and B (acrylic based with biocide) and
newly developed paints D (polyurethane based with biocide), E (biocide-free polyurethane based) and
F (silicone based with biocide) were tested on 50 x 50 mm and 100 x 100 mm sized samples.
Procedure
The procedure was performed according to the standard DIN EN ISO 3274, using the
Perthometer M1 (Figure 3.3). The probe can measure a maximum path length of 15.5 mm. At least
five measurements were performed for each sample.
Figure 3.3. Perthometer M1 instrument used to measure the roughness of the coatings (Courtesy of
IST).
3.2. Mechanical tests
The mechanical tests consisted in performing the following tests: washability test, stirring test,
drag friction test and abrasion test. Basic properties such as thickness, adhesion and hardness were
also measured.
The procedure of each test is described below in the following sub items.
3.2.1. Thickness test
The measurement of the paints’ thickness can be done by different methods and it should be
checked before each characterization test (washability, cupping test, etc.). A quick way to measure it
is by using a common specific device, which measures the thickness of the coating applied on a
metallic substrate. However, this method is limited, since it does not allow determining the thickness of
each individual layer which composes the coating (e.g. primer + top coat), it only gives the thickness of
a coating as a whole.
To determine the thickness of the individual layers that compose the paint, the coated
substrate is subjected to a metallographic preparation and further observed in an optical microscope.
26
The samples consisted of a transversally sliced substrate containing the coating. The size of
the samples was not rigorously taken into account, since any sized sample can be observed in the
microscope, as long as it enables the visualization of the cross section.
The metal substrates coated with the paints to be tested were prepared and sent by ENP
(Estaleiros Navais de Peniche, a Portuguese company specialized in painting hulls for ships). The
commercial paints A and B and the recently developed paints D, E and F were the samples tested.
Procedure
After transversally cutting the samples, each of the slices was observed in the optical
microscope Olympus Model SZX16 in order to measure the thickness of each layer.
3.2.2. Adhesion test – Cupping test
Adhesion tests were performed using the Cupping test method (Standard ISO 1520:2006).
The Cupping test allows the assessment of the elongation and deformability of a protective coating
applied on a metal substrate.
The testing samples were panels of naval steel Grade A (7 x 7 cm) coated with the
commercial paints A and B and the recently developed paints D, E and F.
Procedure
This test consisted in using a spherical nose punch to push upon the uncoated side of the
panel, thus stretching the material until the painted side deformed (cracked) or peeled off the coating.
The depth, to which the material was drawn to the punch until the coating cracks, is the measure of
the quality of the paint or coating material, expressing the durability, elongation and adhesive
properties [27]. Figure 3.4 shows the equipment used to perform this test.
Figure 3.4. Cupping test equipment.
After performing the test, the surface of each sample was observed on an optical microscope
LEICA provided with a digital camera DM2500MH.
27
3.2.3. Hardness tests
Hardness tests such as Pencil scratch test and Persoz pendulum test were performed to
measure the hardness of the paints. A brief description of each test is presented below.
A) Pencil scratch test
Scratch hardness tests are executed to determine the resistance of coatings to scratch effects.
Although being suitable for paints, it is also a useful aid in the development of synthetic resins or other
film forming materials.
Generally, the scratch hardness is measured by moving a sharp object under a known
pressure required to scratch through the test material if a scratching tool of constant hardness is used,
or the hardness of the scratching tool is varied while constant pressure is applied. The constant
pressure can be guaranteed by pulling the scratching tool with the pencil uniformly, on the sample,
without pressing it against the sample.
The sharp object consists of grading pencils in an assortment of hard and soft, ranging from
4H to 6B. The ‘H’ stands for hardness, ‘B’ for blackness and ‘HB’ for hard and black pencils. The
hardest is 9H, followed by 8H, 7H, 6H, 5H, 4H, 3H, 2H and H. F is the middle of the hardness scale.
Then comes HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B and 9B, which is the softest [24], [28].
Figure 3.5. Pencil scratch tester (Courtesy of IK4-Tekniker).
The testing samples were panels of naval steel Grade A (15 x 10 cm) coated with the
commercial paints A and B and the recently developed paints A, B and C.
Procedure
The procedure was based on the standard ISO 1518:1992 [28]. Each pencil, 4B, 3B, 2B, HB,
H, 2H, 3H, 4H, 5H and 6H was inserted into the scratcher, which was dragged by pulling the handle of
the scratching tool, using manual force, on each coated panel’s surface.
After performing the tests, the surface of each sample was also observed on an optical
microscope LEICA provided with a digital camera DM2500MH.
28
B) Persoz Pendulum test
The Persoz pendulum hardness test is based on the principle that the amplitude of the
pendulum’s oscillation will decrease more quickly when supported on a softer surface. The number of
oscillations made by the pendulum is measured within specified limits of amplitude by accurately
positioned sensors and is recorded by an electronic counter.
The pendulum leans on two tungsten carbide spheres (8 ± 0.005 mm of diameter), which rest
on the coating under test. Its total mass should be 500 ± 0.1 g [29].
The testing samples were panels of naval steel Grade A (15 x 10 cm) coated with the
commercial paints A and B and recently developed paints D, E and F.
Procedure
The procedure was based on the standard ISO 1522:2000 [29].
Primarily, each coated panel was placed on a specific surface on the top of the equipment,
where the tungsten carbide spheres of the pendulum were rested on. The pendulum was then
released and the oscillation started to be recorded. Three repetitions were performed. The higher the
number of oscillations obtained, the higher the hardness of the coating.
The equipment used is shown in the figure below.
Figure 3.6. Persoz pendulum test equipment
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3.2.4. Abrasion tests
The assessment of the abrasion resistance of antifouling paints is a relevant factor, since it
can indicate the paints’ resistance to friction caused by moving particles transported by the wind or
water, which can erode the paint, when the ship is in motion or in the port.
For this purpose, abrasion tests were performed in accordance with the standard ASTM
D4060 – Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser
tester. This test is among the most common tests to evaluate the abrasive resistance of coatings [30].
The testing samples consisted of rigid naval steel Grade A substrate 100 x 100 mm panel,
coated with the commercial paints A and B and the recently developed paints D, E and F.
Procedure
Each sample was placed in the turntable and rotated at a fixed speed under two weighted
abrasive wheels. (Figure 3.7)
Figure 3.7. Taber Abrasion tester (Courtesy of IK4-Tekniker).
The operating conditions applied on the testing samples are presented on Table 3.3.
Table 3.3 - Abrasion test conditions
Test conditions (ASTM 4060)
Abrasive wheels CS-10
Load (N) 5
Nº Cycles 9000
Radius (mm) 37.5
Rotational speed (rpm) 72
The loose debris generated during the tests can be removed using a vacuum system.
Before each test, the abrasive wheels were removed and resurfaced with an S-11 refacing
disc to standardize again the wheels’ surface.
30
The mass loss of each sample was calculated every 3000 cycles and the wear volume was
also estimated by observing the samples in the Confocal Microscope Nikon Eclipse ME600 and
tracing the profile track.
3.2.5. Washability test
The washability test was performed according to ISO 11998:2006 “Paints and varnishes –
Determination of wet-scrub resistance and cleanability of coatings”. This method was used in order to
test the paints’ resistance to wear caused by repetitive cleaning operations and penetration of soiling
agents [31]. After each test, it is possible to collect the leaching obtained and evaluate its toxicity.
Testing samples preparation
Inert PVC (Polyvinyl chloride) and Naval steel Grade A panels were used as substrates.
The PVC panels are free of chemical plasticizers susceptible to migrate, present a sufficient
rigidity to ensure a flat impermeable and inert surface to water and organic solvents. Each panel had a
nominal thickness of 0.25 mm, a length of 432 mm and a width of 165 mm.
The naval steel substrates already had the testing paints (primer and topcoat) applied on the
surface by ENP (Estaleiros Navais de Peniche). Each panel presented a length of 435 mm and width
of 165 mm.
The testing paints were manufactured and provided by Hempel. The main characteristics of
the paints (primer, tie-coat and topcoat), such as the polymeric matrix and the components weighed
which constitute the paint, can be found in Table 3.4 and Table 3.5.
Table 3.4 - Main characteristics of the primer and tie-coat paints provided by Hempel
Paint
type
Polymeric matrix to be
applied on Components Weight (g)
Primer Polyurethane P1 4.4
P2 1
Tie-coat Silicone
TC1 11.5
TC2 1.3
TC3 0.45
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Table 3.5 - Main characteristics of the topcoat paints provided by Hempel
Paint Components Weight (g)
A Unknown Unknown
B Unknown Unknown
D D1 1
D2 2.5
E Not weighted Not weighted
F
F1 38.2
F2 4.8
F3 1.2
G G1 97.7
G2 8.3
The first two paints (A and B) were previously tested by IK4-Tekniker in the past. They are
used in this work as reference paints in order to evaluate the paints’ behaviour with and without
biocides.
Procedure
For the paint application on PVC panels, the components of each paint were weighed in
accordance with the weight ratio recommended by the supplier, provided in the tables above, Table
3.4 and Table 3.5. Primarily, the components of the primer paint (or tie-coat for the silicone based
paint) were weighed, mixed and freshly applied on the PVC panel, forming one layer of primer (or tie-
coat), letting it dry for at least 24 hours. For the top coat layer, the same procedure was followed.
Using PVC panel as substrate, paints D, F and G were tested.
Figure 3.8. PVC black panels painted coated with the antifouling paints
Regarding to the naval steel panels, the manual application of both primer and topcoat was
executed and provided by ENP, since this requires professional paint procedures. The steel panels
were coated with the commercial paints A and B and recently developed paints D, E and F.
32
Figure 3.9. Coated naval steel panel
Subsequently, washability tests were performed for each painted panel in a washability tester
Braive Instruments (Figure 3.10) for a scrub rate of 10 000 and 50 000 cycles with a speed of 37
cycles per minute, using standard sea water as lubricant (prepared according to the standard D1141 –
98).
The samples applied on naval steel were tested for 10 000 and 50 000 cycles, at the same
speed and it was simultaneously applied a load of 254 g and 918 g (load + abrasive boar fur brush) on
each panel.
Additional tests were also carried out on PVC panels, for 50 000 cycles and at the same
speed, using fresh water fleas Daphnia magna’s mineral medium as lubricant, in order to collect a
leaching product containing a suitable medium for toxicity analysis. The leaching product obtained
using standard sea water would be harmful to these fresh water organisms, being necessary to use
the adequate medium as lubricant. Paint D and F were the paints tested with this lubricant, and used
as the representative ones for further comparative analysis. Paint G was not received in due time to
perform this wear resistance test using naval steel as substrate, since it is a recent developed paint in
the frame of a European collaborative project (FOUL-X-SPEL), where IST and IK4-Tekniker
participate.
The washability tester has a closed circuit, which allows the circulation of the lubricant. This
lubricant is pumped and spread along the sample, with a brush, which scrubs at the desired speed
(standard speed of 37 cycles per minute or any other personalized speed can be chosen). The
resulting lubricant (leachate) is returned to the lubricant’s container, being collected at the end of the
test.
Figure 3.10. Washability test equipment (Courtesy of IK4-Tekniker).
33
The thickness loss of each coated naval steel was also measured after the test with 10 000
cycles and the surface of the coating was also analyzed to evaluate the morphology of the scrubbed
surfaces. Three replicates were performed for each paint and the average thickness loss was
measured using a specific device from NEURTEK Instruments, which is only selective to metallic
substrates.
The gloss before and after the test was measured using the Glossmeter Rhopoint NEURTEK
in gloss units (GU).
3.2.6. Stirring test
The present test consisted in stirring immersed coated panels to evaluate if any biocide
release occured from the antifouling paints in a water environment. It was performed according with
standard ISO 15181: “Determination of release rate of biocides from antifouling paints” [32].
The samples consisted of acrylic panels (20 x 10 cm) coated with the testing paints C and F,
provided by Hempel.
Procedure
Coated acrylic panels with the testing paints, were immersed in approximately 3.5 L of
standard sea water (prepared according to ASTM D1141 - 98) and stirred for at least 45 days at 200
rpm. The painted area was 140 cm2. The leaching obtained was collected at the end of the stirring
process for toxicity analysis. The average pH of the standard sea water was around 8.2. The average
temperature ranged from 18 to 25 ºC. Figure 3.11 shows the stirring test apparatus.
Figure 3.11. Stirring test: rotating sample.
A second procedure was followed by IST and adapted from papers published among the
scientific community (e.g. [33]) and standards followed by those authors’ papers (e.g. [34]).This
procedure also aimed to assess the effect of different biocidal compounds and contents on the
leaching behavior of the obtained paint formulations, in order to guarantee that in all possible
scenarios the immobilized biocides remained attached to the polymeric paint matrix.
Briefly, the leaching test comprises the immersion of brush painted acrylic small prototypes in
simulated seawater (33 g/L of sea salt in distillate water-free of nitrate, phosphate and silicate, from
34
sera marin). The panels’ size were around 3.5 x 6 cm, where the painted area ranges from 70 to 85
cm2. The results are also further normalised in terms of mass of the paint used in each sample.
750 mL of simulated seawater contained in a glass is used in each test, remaining under
stirring for 24 hours for periods from three weeks to 45 days, at a stirring velocity of 60 rpm. The
average pH of the simulated water was around 8.5. Tests were performed under an average
temperature ranging from 18-25 ºC. Figure 3.12 illustrates the used apparatus for these leaching tests.
Leaching waters of those tests were further assessed in terms of toxicity in IK4-Tekniker. The
procedures and results can be found in the next subsections.
Figure 3.12. Leaching test at IST: static sample conditions (Courtesy of IST).
3.2.7. Drag friction test
In this test, the drag friction of antifouling paints was characterized by measuring the drag
force (torque) generated between the paint and the water (tap water and standard sea water). Some
of the paints were tested before and after 6 months exposure in real Atlantic water. These tests were
performed in a novel tribometer designed and built at IK4-Tekniker facilities. The samples were
mechanized according to the design specifications of the tribometer and sent to ENP (Estaleiros
Navais de Peniche) to be painted.
Confidential
35
3.3. Environmental compatibility tests
The environmental compatibility tests consisted of performing biodegradability tests and
toxicity tests of the leaching product obtained in the aforementioned mechanical tests.
A brief description of the procedure of each test is mentioned below.
3.3.1. Biodegradability in sea water
This biodegradability test was adapted from the Manometric Respirometry Test [35].
The procedure is similar, differentiating only on the inoculum used and the samples tested.
Two methods can be used to determine the biodegradability of chemicals in the sea water: Shake
flask method and Closed bottle method. The former is suitable for chemicals with high solubility in sea
water. Since the paint samples are viscous and hardly soluble in sea water, the Closed bottle method
was followed.
The samples consisted of some drops of fresh testing paints provided by Hempel: D and F.
Procedure
The procedure was followed in accordance with OECD Guideline 306 Biodegradability in sea
water [36]. In this test, the inoculum used was of marine origin, collected at the Deba’s port, where the
tide conditions were similar as the ones prone to create fouling: low and calm, with less agitation.
For the selection of the best inoculum, four samples of water were collected in distinct spots of
Deba to be analyzed regarding to the concentration of microorganisms.
The concentration of microorganisms was analyzed using marine agar, 1 mL of each diluted
sample of inoculum, which was incubated. The inoculum collected at the port of Deba presented more
microbiological activity, which justified its use in this experiment.
Overall, the Closed bottle method consisted of using a measured volume of inoculated mineral
medium where a pre-determined amount of the sample was dissolved to give a concentration of
usually 2-10 mg of test substance per liter. The samples were added in flask bottles containing the
inoculum, mineral stock solutions and standard sea water (prepared according to ASTM D1141-98)
and were stirred at a constant temperature of 20 ± 1 ºC, in the dark, during 28 days. The oxygen
consumed by the microorganisms present in the inoculum during biodegradation was quantified every
week, in the BOD-system OxiDirect BSB BOD, expressed as a percentage of ThOD (theoretical
oxygen demand). This uptake of oxygen was corrected for uptake by blank inoculum tested in parallel.
Sodium hydroxide pellets were also added on the top of each flask to absorb all the carbon dioxide
produced. The BOD (biological oxygen demand) unit consisted of the aforementioned closed bottles,
which contained a BOD sensor. As the bacteria in the samples consumed the dissolved oxygen, it was
replaced by the oxygen present in the air within the bottle. Simultaneously, as the released carbon
dioxide was being absorbed by the pellets, a decrease in the pressure was obtained and measured by
the BOD sensor, which was displayed as a BOD value in mg/L O2. A reference substance (Sodium
benzoate, NaC6H5CO2) was also tested to check the microbial activity of the sea water sample.
36
3.3.2. Toxicity tests
The toxicity of the leaching obtained for each mechanical test was assessed performing the
following described methods described.
A) Alga growth inhibition test
This test was based on the standard ISO 10253:2006 [37] and performed using MARINE
ALGALTOXKITTM, which contains all the material necessary to ensure the growth inhibition tests with
the marine diatom Phaeodactylum tricornutum. This type of diatom is among the most common type of
phytoplankton.
The sample analyzed was paint F obtained in the washability and stirring tests.
Procedure
The following procedure was performed in accordance with the manual provided with the
MARINE ALGALTOXKITTM kit (Figure 3.13).
Figure 3.13. Kit used to carry out the toxicity tests using the algae Phaeodactylum tricornutum [38].
The first step of this procedure consisted in preparing the mineral medium for the growth of the
algae Phaeodactylum tricornutum. For the preparation of the mineral medium, a 2 L volumetric flask
was filled in with approximately 1500 mL of deionized water and the vial containing pure NaCl was
added. The flask was agitated vigorously until the total dissolution of the salt and the other vials (2 to
7) with concentrated salt solutions were also added, in the sequence recommended. Subsequently, 30
mL of nutrient stock solution A, 1 mL of nutrient stock solution B and 2 mL of nutrient stock solution C
were also poured, filling up the flask mark up to 2 L with more deionized water. The content of the
flask was once again homogenized. All the aforementioned vials came with the kit, in exact quantities
necessary to prepare this medium.
After the preparation of the mineral medium, the tube containing the microalgae was rinsed
with 15 mL of mineral medium and transferred into the preculturing cell (10 cm long). The cell was
37
then closed and incubated for 3 days, in an climatic chamber with a controlled temperature of 20 ºC ±
2 ºC, under agitation, with a constant uniform sideway illumination supplied by cool white fluorescents
lamps.
Past 3 days of incubation, the optical density (OD) of the culturing cell was measured at 670
nm in a spectrophotometer Jenway 6300, equipped with a holder of 10 cm cells. The
spectrophotometer was calibrated using 25 mL of algal culturing medium.
Using the value of OD measured and the calibration curve included in the kit (Annex A.3), it
was possible to determine the algae density. If the algae density was equal to 1 x 106 cells/(mL
suspension), the algae growth was successfully achieved and apt to be used in the following steps.
After obtaining the desired value of algae density, the toxicants’ dilution serie was prepared.
200 mL of 100 vol.%, 50 vol%, 25 vol.%, 12.5 vol.% and 6.25 vol.% were the concentrations prepared
using the mineral medium and the leaching product of paint F obtained in the stirring test and
washability test. A reference test was also carried out, for which were prepared solutions of 1.8 mg/L,
3.2 mg/L, 5.6 mg/L, 10 mg/L and 18 mg/L of potassium dichromate (K2Cr2O7; reference substance).
Subsequently, 1 mL of the algae was added to each flask and after shaken, 25 mL of the content of
each flask was transferred into the 10 cm long cells.
The OD of each cell was measured (time = 0 hours) and then were incubated for 72 hours, in
the same conditions used for the algae pre-culturing step. Algal growth or inhibition was registered
every 24 hours.
After obtaining the optical density, it was possible to calculate the EL50 (Effect Load), which is
the concentration of the test substance that causes a decrease of 50% in the growth of the algae. This
terminology is used instead of the standard EC50 (Effective Concentration) when the test material is
not completely soluble at the test treat rates.
B) Daphnia magna acute immobilization test
This test was based on Test OECD 202 [39] and performed using DAPHTOXKIT F MAGNATM
kit (Figure 3.14). Daphnia magna are commonly known as water fleas, due to their saltatory swimming
resemblance to the movement of the fleas. Its short life span and reproductive capabilities make it an
ideal organism for analytical use.
38
Figure 3.14. Kit used to carry out the toxicity tests using the fresh water flea Daphnia magna [40].
The swimming capability of Daphnia Magna was assessed after 48 hours of exposure to the
diluted testing samples. The Daphnia were bred in the laboratory and should be no more than 24
hours old. The number of immobilized Daphnia was registered at 24 hours and 48 hours, for the
calculation of EL50 and comparison with the control values. The EL50 is the effective concentration of
the sample that is expected to cause immobilization to 50% of Daphnia.
Figure 3.15. Daphnia magna fleas [40].
The tested samples were regarding to the washability test, using PVC panel as substrate and
Daphnia’s mineral medium as lubricant, coated with paints D and F, whose characteristics are shown
in Table 3.1. It was only tested the leaching obtained for 50 000 scrubbing cycles, since it is more
concentrated than the leaching obtained for 10 000 scrubbing cycles.
Regarding to the stirring test, this toxicity test of the leaching was not possible to be done,
since the leaching contained standard sea water which can be harmful to Daphnia. For this purpose, it
would be necessary to start a new stirring test for more 45 days, immersing the painted panels in
Daphnia’s mineral medium instead of immersing it in standard sea water.
Procedure
The first step consisted in preparing the mineral medium for the growth of Daphnia magna. For
the preparation of the mineral medium, a 2 L volumetric flask was filled in with approximately 1500 mL
of deionized water and the vials with concentrated salt solutions were added. The vials came with the
39
kit, in exact quantities necessary to prepare this medium.The flask was then filled up, until the mark,
up to 2 L with more deionized water. The content of the flask was shaken vigorously to homogenize.
The next step consisted in hatching the Ephippia (Daphnia magna’s eggs) by incubating them
for 3 days, in the mineral medium prepared previously, at 20 ºC ± 2 ºC with constant illumination
supplied by cool white fluorescents lamps.
Past 3 days, the toxicants’ solutions were prepared, proceeding similarly as the preparation of
the solutions used in the algae toxicity test. 100 mL of solution with a concentration of 100 vol.%, 50
vol.%, 40 vol.%, 30 vol.%, 20 vol.% and 10 vol.% of sample were tested. A reference test was also
carried out, for which were prepared solutions of 1.8 mg/L, 3.2 mg/L, 5.6 mg/L, 10 mg/L and 18 mg/L
of potassium dichromate (K2Cr2O7; reference substance).
The Daphnias were fed with spirulina, two hours before being used in the toxicity tests.
Afterwards, 10 mL of each solution containing the testing paint sample were poured into each
well of the multi-well plate (Figure 3.16).
Figure 3.16. The multi-well plate used in the Daphnia magna toxicity test [40].
The first row of the multi-well plate corresponds to the control test, where the wells are filled
with 10 mL of mineral medium. In each well, five Daphnias were transferred with a micropipete and
then the whole multi-plate was incubated during 48 hours at 20 ºC ± 2 ºC, in the darkness.
The number of dead and immobilized Daphnias was counted every 24 hours.
C) Luminescent bacteria Vibrio fischeri test
Vibrio fischeri is a luminescent bacterium found globally in the marine environment. This
bacterium is bioluminescent, robust, nonpathogenic and easy to breed, which makes it an ideal
organism for laboratorial use. Vibrio fischeri uses riboflavin-5-phosphate to react with oxygen to
produce water and cold light emitted with a wavelength of 490 nm. The emission of luminescence is
directly proportional to the metabolic activity, thus any inhibition of the enzymatic activity causes a
corresponding decrease in the bioluminescence.
40
Testing samples
The samples consisted of the leachates obtained in the washability tests, stirring tests and
drag friction tests. Detailed information about these leachates is shown in Table 3.7.
Table 3.7 - Main characteristics of the leaching products and the tests where they were obtained
Paint Polymeric matrix Biocide presence Test where the leaching was obtained
A Silicone No - Drag friction test
B Acrylic Yes - Drag friction test
C Silicone Yes - Stirring test
47 days
D Polyurethane Yes
- Washability test
PVC substrate: 10 000 and 50 000 cycles
Naval steel substrate: 50 000 cycles;
- Drag friction test
E Polyurethane No
- Washability test
Naval steel substrate: 50 000 cycles
- Drag friction test
F Silicone Yes
- Washability test
PVC substrate: 10 000 and 50 000 cycles
Naval steel substrate: 50 000 cycles;
- Stirring test
47 days
- Drag friction test
G Polyurethane Yes - Washability test
PVC substrate: 50 000 cycles
Procedure
The procedure was followed according to the ISO 11348 standard [41].
A geometric dilution serie (1/2; 1/4; 1/8; 1/16; 1/32) of the leachates of each paint, obtained in
the mechanical tests, was prepared in cylindrical glass cells placed in a DR Lange LUMIstox
Thermostat (Figure 3.17). The pH of the leachates must be between 6 and 8.5. A 2 wt. % solution of
sodium chloride (NaCl) in deionized water was used as dilution medium. The amount of bacterial
solution added was of 0.5 mL.
The decrease of bioluminescence of a culture of liquid-dried luminescent bacteria of the strain
Vibrio Fischeri NRRL-B-11177A was measured after 15 and 30 minutes of exposure to the testing
samples of paint, using the DR Lange LUMIstox 300 photometer, at 15 ± 1ºC.
41
The bioluminescence of the bacteria was also tested in parallel with 22.6 mg/L of a reference
substance (potassium dichromate, K2Cr2O7) diluted in 2 wt.% NaCl solution. The test is considered
valid if it is obtained an EL50 of 11.3 mg/L [41].
To check if the composition of the leachates was stable during its conservation, a TOC (Total
Organic Carbon) test was simultaneously performed to indicate the amount of total organic carbon
present in the samples. A small amount of sample was added to the Dr Lange kit’s containers and
mixed in a Hach Lange TOC-X5 stirrer for 15 minutes. Afterwards, the containers were joined to the
indicator with bar codes and put into a digester Lange LT200 for 2 hours at 100ºC. Past 2 hours, the
TOC of the samples was measured in a HACH Lange Dr 600 colorimeter.
Since the presence of colour can inhibit the emission of the luminescence, the OD (optical
density) of the leachates was also measured in the DR Lange LUMIstox 300 photometer, before the
bioluminescence tests. If the OD obtained was higher or equal to 1.8, it would be necessary to
proceed to the colour correction, for example by filtration of the samples. In the present samples, it
was not necessary to correct the colour, since the OD obtained was lower than 1.8.
Figure 3.17. Thermostat (right) and the photometer (left) used in the Vibrio fischeri test.
42
4. Results and discussion
Confidential
43
5. Conclusions and Future work
Advanced characterization techniques were carried out on biocide-free antifouling paints and
on newly developed biocidal paints, where the biocide was immobilized in the paints’ matrix through
covalent bonds. The main aim of such detailed characterization, was to improve not only the
antifouling properties of paints, but also to guarantee their mechanical requisites, to further select the
best paint for future use on ships, based on the performance of each paint.
Regarding to the surface wettability assessment, silicone based paints exhibited both
hydrophobic (at the beginning of the test) and hydrophilic (at the end) properties, whereas the
polyurethane based paints showed a stable and hydrophobic response during the entire test. The
inclusion of biocide (Econea) in the polymeric matrix of the paint, increased the hydrophobic properties
of the paints, being more notable on silicone based paints.
After performing the mechanical tests, it was possible to verify that polyurethane based paints
showed better adhesion, hardness properties and abrasion resistance than the silicone based paints.
The presence of Econea seems to improve the scrubbing resistance of the polyurethane based paints.
However, the abrasion resistance and the hardness were not improved by the addition of this biocide,
neither on polyurethane and silicone based paints. In addition, it is relevant to note that the properties
are highly dependent on all the paint components used in each formulation, and they are important as
an indicator of the suitability of new formulation to the required technical requisites, to be further used
in real applications.
The drag friction of the coatings was evaluated in a novel tribometer (IK4-Tekniker), using
different assembling configurations (small gap of 5 mm and large gap of 10 mm between the coated
cylinder and the container). In every test, the drag friction decreased with the speed when using tap
water and standard sea water, which is explained by the fact that the friction coefficient is inversely
proportional to the speed and therefore to the Reynolds number (for constant test conditions). In
addition, as the speed increases (higher Reynolds number), the drag effect resulting from wave
contribution becomes predominant relatively to the skin frictional drag contribution. Comparing the
influence of the type of water, it was possible to conclude that using standard sea water as the tested
fluid leads to higher drag friction effect. This behaviour is due to the application of the downward force
for the total immersion of the samples, which opposed the buoyancy force created due to the higher
density of sea water. When using tap water, this effect is reduced since it has a lower density than sea
water.
In addition, the effect of the surface of each different coatings with and without biocide
(Econea) was also assessed, with different gap configurations (small gap and large gap) and using
standard sea water as fluid. The coated samples presented a significantly higher drag friction than the
uncoated reference PVC sample, due to the inherent roughness of the coated samples. The obtained
average roughness of silicone based paint F with biocide (Ra: 1.2 – 1.5 µm) is lower than the obtained
for the biocide-free silicone based counterpart A (Ra: 2.3 – 4.0 µm). The same behaviour was
observed for the polyurethane based paint D with biocide (Ra: 0.2 – 0.6 µm) and biocide-free
polyurethane based paint E (Ra: 0.6 – 1.8 µm). Consequently, the presence of biocide in paint
44
formulations led to a reduction on the drag friction effect, for both gap configurations, which can be
associated to the decrease in the surface roughness of those biocide based coatings. Drag friction
reduction at different speeds (4 - 30 knots) of 0 – 16 % was obtained when using polyurethane based
paint with biocide, and 9 – 20% when using a silicone based paint with biocide, for the small gap
configuration tester. Regarding to the large gap configuration, drag friction reduction of 2 – 15 % and 0
– 19% were obtained for those paint formulations, respectively, at the same speed range. Being aware
that the silicone reference paint is a commercial paint in use, such improvement on drag friction is
promising. In addition, comparing different technologies, commercial self-polishing acrylic based paint
B with the foul-release silicone based paint A, the latter presents slightly higher values of friction
coefficient (Cmc) than paint B, which can be decreased by the incorporation of biocide, as already
mentioned. This supports the prevalence of the use of silicone based coatings, as their characteristics
seem to be able to be improved.
The influence of biofouling on the drag friction effect was also evaluated. The exposed paints
formulations exhibited slightly high drag friction coefficients in comparison with the unexposed paints
formulations, which is associated to the modification of the surface roughness caused by the
attachment of marine organisms. It was possible to observe that foul-release silicone based paint A
(without biocide) suffered less biofouling than SP acrylic based paint B and this is in accordance with
the obtained lower drag friction ratios of the former. The lower attachment of organisms on the silicone
based paint A can be related to its low roughness when compared with the acrylic based paint B (Ra:
7.8 – 9.0 µm) and also due to its hydrophobicity/hydrophilicity properties, which enables the sea water
to spread uniformly on the coating, forming a hydrogel layer, which grants the ability to be easily
washed at high speeds when applied on a ship hull. The drag reduction caused by paint A in the
friction ratio relatively to paint B was also quantified and a reduction of 2 to 14% was obtained for the
small gap configuration and 14 to 29% for the large gap configuration, at the speeds tested (200 rpm –
1500 rpm; 4 knots – 30 knots). These results evidenced the profits provided by a foul-release paint
system, which is the reason why they are the most preferable nowadays. Such peculiar behaviour is
due to their low surface energy properties which impedes the adhesion of fouling organisms. However,
these paints are only efficient when used at high speeds.
The accomplishment of the drag friction experiments indicates that the new drag friction tester
is a promising tool to perform these tests in more new antifouling paints in the future.
Concerning the environmental compatibility assessment, none of the tested paints D
(polyurethane based with biocide) and paint F (silicone based with biocide) are biodegradable.
Regarding to the toxicity tests, the leachates obtained in the mechanical tests (washability test, stirring
test and drag friction test) of silicone based paint containing biocide (paint F) seem to be more toxic
than the polyurethane based paint with biocide (paint D). Comparing the toxicity of the tested paints on
the different organisms, paint F appears to be more toxic to alga Phaedactylum tricornutum, water
fleas Daphnia magna and luminescent bacteria Vibrio fischeri. However, the toxicity of the
polyurethane based paint G (containing two biocides Econea and Irgarol), should also be tested on
Phaedactylum tricornutum and Daphnia magna, in order to compare with the result obtained on Vibrio
fischeri. The toxic effect of the paints is either due to the biocide release or due to the releasing of
45
toxic compounds included in the paint’s composition. Unfortunately, the composition is confidential,
making it difficult to identify the potential toxic compound. However, only a further analysis of the
leachates can lead to a plausible conclusion that can justify the toxicity results.
Overall, it is possible to conclude that the immobilization of biocides in antifouling paints does
improve certain properties of the silicone based paints, especially the drag friction effect which can
result into high fuel savings. Unfortunately, these paints seem to be environmentally unfriendly for
more aggressive mechanical tests and therefore improvements on its properties to be suitable as an
antifouling paint is still in course. Alternatively, polyurethane based paints, especially paint D has also
shown promising results mechanically and environmentally.
Additional tasks need to be performed to accurately complete the experimental work of this
thesis, in order to complement and validate some of the achieved results and new ones, in particular
for recent newly paint formulation (e.g. paint G), which can boost the selection of the adequate
antifouling paint for ships’ hull protection. Future work can comprise the following tasks:
Performance of washability tests on the newly paints, using Daphnia magna’s mineral medium
as lubricant;
Performance of stirring tests with other velocities (150 - 250 rpm) in order to study its effect on
the leaching behaviour of the paints;
Analysis of the leachates obtained in the stirring test, to check if there was any biocide
release, by High Performance Liquid Chromatography (HPLC) or other suitable technique;
Performance of the drag friction tests of the newly developed paints D, E, F and G after
exposure in real conditions;
Performance of the toxicity tests with alga Phaeodactylum tricornutum using the leaching
products obtained in the washability, stirring and drag friction tests, for the paints D, E, F and
G;
Performance of all the surface assessment and mechanical tests on the newly developed
paint G, containing two immobilised biocides (Econea and Irgarol).
A scientific article will be submitted soon, as result of the experimental work carried out in this
thesis. A second article is also planned to be submitted in the near future. The first article and the
abstract of the second article are presented in the confidential annex.
46
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A-1
Appendix
Confidential