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

29

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

31

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