Quick test methods for marine antifouling paints

Progress in Organic Coatings 42 (2001) 15–19
Quick test methods for marine antifouling paints
Ilva Trentin ∗ , Vittorio Romairone, Giuseppe Marcenaro, Giorgio De Carolis
Istituto per la Corrosione Marina dei Metalli, C.N.R. — Via De Marini 6, 16149 Genoa, Italy
Received 21 November 2000; accepted 25 January 2001
Abstract
Antifouling paints are used to protect ship and boat hulls from fouling settlement and to improve craft speed by reducing water friction.
All this, in shipping economics, translates into several advantages obtained through faster craft speed and fuel saving. In this paper,
laboratory tests are described that have been developed and tested in pilot plants with a view to speeding up testing time of new products,
compared with conventional field test methods. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Antifouling paints; Marine transports; Methodology tests; Fouling organisms
1. Introduction
Most antifouling products for marine applications feature
paints which release chemical substances (mainly copper
and its derivatives), thus preventing fouling organisms from
settling [1].
As a matter of fact, copper was used in the past with rather
satisfactory results, leading to new technologies that spurred
the development of the well known vinyl polymer matrix
paints in the 1940s–1960s [2]. Later on, in the 1980s–1990s,
in view of strict regulations and the ban on organic-tin
biocides in self-polishing copolymers (SPC), a new generation
of coatings was introduced, whereby more advanced
technologies were employed leading to a “copper revival”.
Plenty of different formulations were developed: products
with insoluble partially erodible vinyl matrix; products with
soluble and ablative matrix; hydrolizable acrylated products
in which tin has been replaced with copper, thus going
back to the SPC paint concept [3]. Therefore, today, copper
is the main player in the fight against biologic fouling on
ship hulls.
Efficiency, duration over time, and environmental compatibility
requirements in antifouling products depend on
the “performance mechanism” of copper inside the coating
applied on the structure to be protected.
Duration of an antifoulant paint depends on regular biocide
migration from the inside to the external paint film
surface. Hence, the active biocide at the surface must be
suitable to protect the underwater structure. At the same
time, it must be present in the right quantity, which is
∗ Corresponding author. Tel.: +39-010-64751; fax: +39-010-6475400.
slightly higher than the practical threshold level, but not too
much, to ensure minimum impact on the water environment.
Generally, this migration from inside to the outside takes
place in the following ways:
1. Through the porous paint film which has formed under
water. Water, which makes the matrix partially soluble,
penetrates into it allowing the copper to leach out (Fig. 1).
2. Through full matrix solubilization, obtained with chemical
and physical processes with sea water involvement.
In this case, the paint gets gradually detached into more
or less thick layers together with the biocide, until full
antifoulant coating depletion.
Efficiency tests of new antifoulant products are normally
done on painted specimens submerged in sea water
or through field tests by painting portions of the ship hull
directly during dry-docking.
These tests take quite a long time to be completed and,
specially field tests in dry-docks, are quite expensive. For
example, products applied on large ships, take from 3 to 5
years, namely the interval between dry-docks, to be tested
[4]. Therefore, for so long tests, testing costs are likely to
rise significantly, with a negative impact on the production
of the new coating.
For this reason, tests in pilot plants are necessary, where
the new paint undergoes operating stresses equal or greater
than those normally present under field conditions. It is
not too difficult to set a pilot plant for accelerated aging.
Conversely, the development of a suitable methodology in
order to obtain reproducible results and make a significant
assessment with reference to the real employment of the
product on ship hulls is undoubtedly more difficult. The
0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0300-9440(01)00150-3

16 I. Trentin et al. / Progress in Organic Coatings 42 (2001) 15–19
Fig. 1. Porosity of insoluble partially erodible matrix coating which is still effective after 3 years of aging at sea (static immersion test of painted specimens).
Sea water solubilizes the matrix soluble parts (Rosine) thus creating something which, at 4000× magnification, looks like a huge cavern structure.
goal of our work is indeed to develop these systems and
benchmark laboratory methods with conventional field testing,
by using, as a standard, paints that have already been
tested in many years of application on ship hulls.
2. Methods
In this study, three types of paints have been taken as standard:
the first is a partially erodible insoluble matrix paint
(a); the second is a self-polishing controlled polymer dissolution
paint (ablative paint) (b); the third is a self-polishing
paint with a less soluble matrix (c). Cuprous oxide was the
main biocide contained in all three coatings.
These paints have been previously tested with conventional
methods as follows:
1. in static immersion of painted specimens;
2. in field tests on the hull of a ship.
In the first case, product “a” showed a higher efficiency
than product “b”, since it inhibited fouling settlement for
over 3 years. Product “b”’ resulted to be effective for 2 years.
In the second case, product “b” had a duration nearly 3
years, whereas product “c” was no longer effective after 1.5
years.
Together with field tests, accelerated aging tests were also
carried out in the laboratory, namely,
1. accelerated aging tank;
2. accelerated aging turbine.
Two different pieces of equipment were employed: the
first was a 150 l tank with circulating water at a rate of
2.5 l/min; a propeller agitator, submerged at the center of the
tank, creating an ascending water current along the four tank
walls, that lapped painted specimens at a speed of 0.2 m/s
(Fig. 2).
In the second apparatus, specimens were arranged at the
center of another smaller tank filled with flowing sea water
and exposed to the flow of water agitated by a 680 rpm
rotary turbine. Specimens were rotating at the same speed
of the turbine but opposite to the water flow direction. The
tangential friction of water on the painted surface created a
highly, yet difficult to measure, dynamic situation (Fig. 3).
In the first test paints were kept in the aging tank for
7 months at a temperature which was adjusted to seasonal
variations and ranged from a 25 ◦ C maximum to 18 ◦ C minimum.
In the second test, in the more dynamic turbine aging
plant, specimens were tested for 35 days at 24 ◦ C operating
temperature.
Natural coastal sea water was employed for these tests,
which had varying salinity ranging from 36 to 38 ppt,
depending on weather conditions, with a stable pH of
approximately 8.2.
During testing we took the specimens out of their respective
tanks and temporarily (1.5 h) submerged them in
containers with 1.2 l of natural sea water which had been
standardized according to the following parameters: paper
filtered, 37 ppt salinity correction, 25 ◦ C temperature. The
aim was in fact to evaluate the biocide leaching rate and
consequently the accelerated aging level of the paint.
In addition, to confirm the accelerated aging level of the
paint another method was done. For product “a” we checked
by microprobe analysis, the amount of copper present on the

I. Trentin et al. / Progress in Organic Coatings 42 (2001) 15–19 17
Fig. 2. Accelerated aging tank: (a) propeller agitator; (b) descending current; (c) ascending current; (d) painted specimens; (e) water inlet; (f) drain.
Fig. 3. Accelerated aging turbine: (a) turbine rotation; (b) specimens rotation; (c) water flow direction; (d) painted specimens.
paint surface. The lower peak in this element (highlighted
by a small circle) may indicate a faster consumption and
quicker release into the water (Fig. 4).
A similar comparison was done for product “b”
(self-polishing) in lab test with the aging tank, with the
aging turbine and in the static immersion test at sea, but in
the last case no response was possible in that after 3 years
of water exposure the paint was fully depleted.
3. Results
The data reported in the graph of Fig. 5 show the amounts
of copper released by the paints per unit of time and surface
during the 7–month test period in the tank. Both benchmarked
products “a” and “b” show regular efficiency in the
central part of the graph. However, product “b” stops to
be effective (biocide below threshold) 1 month earlier than
product “a”.
Fig. 4. Microprobe analysis of paint “a”: (1) paint before tests; (2) tank
test: 7 months; (3) static immersion: 3 years.

18 I. Trentin et al. / Progress in Organic Coatings 42 (2001) 15–19
Fig. 5. Accelerated aging tank: (paint “a”) insoluble/erodible matrix; (paint
“b”) self-polishing (ablative).
Fig. 7. Accelerated aging turbine: (paint “b”) self-polishing; (paint “c”)
self-polishing (less soluble matrix).
In static immersion tests of specimens at sea (conventional
tests), when the two paints were compared, the same
results were obtained. However, the duration for product “a”
(partially erodible insoluble matrix) was over 3 years and
for product “b” (soft matrix self-polishing), was 2 years, as
indicated in the graph of Fig. 6.
The lower efficiency duration of product “b” is undoubtedly
due to higher film consumption rate. Indeed, from the
examination carried out both at the beginning and at the
end of the tank tests, the consumption rate was found to
be 10 m per month, whereas the partially erodible matrix
product had a consumption rate of only 4 m per month. It
is thus clear that, for self-polishing paints, the active film
thickness has to be accurately calculated, depending on
desired antifouling efficiency duration.
The graph of Fig. 7 reports data referred to benchmarked
paints “b” and “c”, which were tested in the high dynamic
turbine lab tank. It can be noted that in just 35 days of test,
copper release is more regular and greater than threshold
levels for product “b”, whereas for product “c” the release
Fig. 8. Ship hull test: (paint “b”) self-polishing; (paint “c”) self-polishing
(less soluble matrix).
rate drops more rapidly, and in only 25 days the amount
of copper released is no longer sufficient to inhibit fouling
settlement.
It had taken nearly 3 years to achieve the same outcome
in the field test, on a ship hull, for product “b” and 1.5 years
for product “c” (Fig. 8).
4. Discussion and conclusions
Fig. 6. Static immersion tests at sea: (paint “a”) insoluble/erodible matrix;
(paint “b”) self-polishing (ablative).
Laboratory tests have indeed shown an accelerated paint
“consumption” compared to conventional field tests.
The outcome obtained from laboratory tests has been interpreted
by examining the copper released by the paint film
into the water, as well as the amounts of copper still present
on the outermost layer of the antifoulant coating. Hence,
the residual ability to either inhibit or not fouling settlement
was verified on the basis of theoretical threshold values
obtained from the literature [5], as well as from practical
values obtained from field tests at sea. These values resulted
to fall within a 10 to 20 g/cm 2 per day range, and refer
to cuprous oxide as biocide and to the specific geographic

I. Trentin et al. / Progress in Organic Coatings 42 (2001) 15–19 19
field test area (Mediterranean area with mild temperate
climate).
Results from static immersion tests at sea of specimens
painted with the three standard coatings were obtained by
recording the time when fouling was first observed in significant
amounts on exposed surfaces. The real threshold value
mentioned above was obtained by matching them with analytical
data on the amounts of copper released into the water.
This is a fundamental value, because it is the link between
theoretical laboratory and practical field tests.
In fact a ship hull covered with fouling organisms means
that antifouling paint had exhausted or restricted its antifouling
activity, because the biocide amount dropped below the
threshold value.
In accelerated aging tests, the amounts of biocide released
over time indicate product efficiency, and when they drop
below threshold values, testing is finished.
Actually, by comparing field and laboratory test duration,
laboratory tests proved to be significantly shorter, with a
reduction in time from several years for field tests, to just a
few months and days for laboratory tests.
Furthermore, similar results were obtained from the laboratory
test in the tank and the static immersion test of
specimens at sea on one side, and between the turbine test
and the field test on the hull on the other side. In the first
two types of tests (tank and static immersion), which are
less affected by hydrodynamic conditions, chemical factors
are more likely to have an impact on a faster aging
of product “b” over product “a”. Conversely, in the second
type of tests (turbine and vessel), involving a much greater
mechanical action, physical factors prevail, accelerating
the consumption of product “c” (hard matrix) faster than
product “b” (self-polishing coating with soft matrix).
In conclusion, owing to the limited number of tests so far
conducted, the need for field tests on experimental products
cannot be ruled out yet in absolute terms. Our work is just a
contribution which adds to the work of other, not too many
yet, authors [6], who have tried to tackle this very complex
issue. Vessels often operate in geographic areas that are a
severe “test bench” owing to the aggressive and resistant
nature of fouling. Also, they often have differing and irregular
operating schedules. Just think of the different types
of operations between a military vessel, a cargo vessel,
an oil tanker, a passenger ferry boat, a cruiser vessel, etc.
Navigation under strong dynamic conditions, long stops in
ports with highly eutrophic water, in region with different
climate conditions, are all elements that play an important
role in maritime transport economics.
Therefore, antifoulant coatings are designed with an eye
to the chemical and/or physical processes that may take
place under certain environmental conditions. However,
guaranteeing the product efficiency in absolute terms is difficult,
specially when complex physical, chemical, biological
events, human events (e.g. sea water pollution) and restrictive
environment protection regulations, make the activity
of an antifouling coating a real challenge, not easy to win.
References
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[2] G. Torriano, Accademia Navale, Livorno, 4–7 April 1995, p. 21.
[3] J.E. Hunter, Antifouling coatings and the global environmental debate,
Protect. Coat. Eur. PCE 2 (11) (1997) 16.
[4] R. Chapman, Protect. Coat. Eur. PCE 3 (3) (1998) 32.
[5] F.H. de La Court, H.J. de Vries, Prog. Org. Coat. 1 (1973) 375.
[6] J.J. Caprari, O. Slutzky, Pitture e Vernici Eur. 74 (13) (1998) 7.