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Factors that influence elastomeric coating performance: the effect
of coating thickness on basal plate morphology, growth and critical
removal stress of the barnacle Balanus amphitrite
D. E. WENDT
1
, G. L. KOWALKE
1
, J. KIM
2
& I. L. SINGER
2
1
Biological Sciences Department and Center for Coastal Marine Science, California Polytechnic State University, San Luis
Obispo and
2
Code 6176 US Naval Research Laboratory, Washington, DC, USA
Abstract
Silicone coatings are currently the most effective non-toxic fouling release surfaces. Understanding the mechanisms that
contribute to the performance of silicone coatings is necessary to further improve their design. The objective of this study was
to examine the effect of coating thickness on basal plate morphology, growth, and critical removal stress of the barnacle
Balanus amphitrite. Barnacles were grown on silicone coatings of three thicknesses (0.2, 0.5 and 2 mm). Atypical (‘‘cupped’’)
basal plate morphology was observed on all surfaces, although there was no relationship between coating thickness and i) the
proportion of individuals with the atypical morphology, or ii) the growth rate of individuals. Critical removal stress was
inversely proportional to coating thickness. Furthermore, individuals with atypical basal plate morphology had a significantly
lower critical removal stress than individuals with the typical (‘‘flat’’) morphology. The data demonstrate that coating
thickness is a fundamental factor governing removal of barnacles from silicone coatings.
current understanding of the mechanisms by which
Introduction
elastomeric coatings provide easy release is at best
There has been considerable effort over the past 25 incomplete. It has been demonstrated that a variety of
years to develop non-toxic, foul-release coatings to factors influences the performance of elastomeric
aid in the control and prevention of biofouling coatings. For example, silicone fluids or the in-
(Swain, 2004). This effort has been driven mainly corporation of oils enhance the performance of
by environmental regulations that have reduced or in elastomeric coatings (e.g. Truby et al. 2000;
some cases eliminated the use of highly effective Kavanagh et al. 2001; 2003; Stein et al. 2003). It is
toxic paints (e.g. triorganotin-based paints) (Walker, also known that certain factors are important such as
1998). In contrast to toxic paints, which prevent the surface energy (e.g. Baier et al. 1968; Finlay et al.
recruitment and growth of organisms on surfaces, 2002), coating modulus (e.g. Brady & Singer, 2000;
non-toxic coatings allow biofouling to occur and Singer et al. 2000; Berglin et al. 2003; Chaudhury
instead rely on the inability of organisms to adhere et al. 2004), frictional slippage (Newby et al. 1995;
well to surfaces. Weak adhesion by organisms Newby & Chaudhury, 1997), and coating thickness
facilitates their removal through factors such as biotic (Kohl & Singer, 1999; Brady & Singer, 2000).
disturbance or hydrodynamic forces (e.g. Swain et al. However, the practical importance of the aforemen-
1998; Schultz et al. 1999). tioned factors has not been thoroughly examined in
The most effective non-toxic surfaces to date are situ using live organisms. It is thus not clear whether
silicone-based elastomeric coatings. Much of the such factors are applicable to the release of hard-
knowledge of best performing coatings has been fouling from elastomeric coatings under natural
derived empirically through extensive laboratory conditions. The primary objective of this study was
and field testing of emerging coatings (e.g. Swain to examine using live barnacles the effect of coating
et al. 1992; Swain & Schultz, 1996). However, thickness on the performance of elastomeric coatings.
The notion that coating thickness is important to
performance is based on a fracture mechanics model
developed by Kendall (1971). Kendall showed that
the force required to pull off a rigid cylinder attached
to a thick elastomer is given by
1=2
2
8Ew
a
P
c
¼ pa ð1Þ
pað1 n
2
Þ
where P
c
, E,w
a
, and a are crack initiation force,
elastic modulus, Dupre’s work of adhesion between
epoxy and elastomer and contact radius, respectively.
However, if the contact radius is much larger than
the elastomer thickness, i.e. a h, where h is the
thickness of the elastomer, then the crack initiation
force is given by
1=2
2
2w
a
K
P
c
¼ pa ð2Þ
h
where K, the bulk modulus of the elastomeric film is
related to the elastic modulus by
K ¼ E=½3ð1 2nÞ ð3Þ
Since K E, because n approaches 0.5 for an
elastomer, a thin coating has a higher P
c
than a
more compliant thick coating.
Indeed, pull-off tests of epoxy bonded to silicone
coatings (often called pseudobarnacle tests) have
verified the behavior predicted by Kendall’s model
(Kohl & Singer, 1999; Brady & Singer, 2000; Singer
et al. 2000). However, the only two published reports
to date using live animals (in contrast to pseudo-
barnacles) failed to find an inverse relationship
between coating thickness and pull-off force (Singer
et al. 2000; Sun et al. 2004). The absence of a
thickness dependence found by Singer et al. (2000)
was likely to be due to the fact that most of the
barnacle basal plates broke during removal. In this
case the mechanics of detachment should not follow
Kendall’s model, which describes a fracture process
where a basal plate would peel from an elastomer.
A proportion of animals growing on silicone
coatings have been shown to have atypical basal
plate morphology (sometimes referred to as
‘‘cupped’’) that consists of a thick callus of cement
present between the calcareous basal plate and the
substratum; the basal plate often forms a cup over
this thick callus (Watermann et al. 1997; Berglin &
Gatenholm, 2003; Wiegemann & Watermann,
2003). Moreover, Holm et al. (2005) have shown
that the occurrence of the atypical basal plate
morphology has both an environmental and a genetic
underpinning and that there is a significant interac-
tion between these factors. Sun et al. (2004) suggest
that atypical basal plate morphology may form as a
result of increased production of adhesive as animals
try to maintain contact with a PDMS substratum. In
the present study tests were carried out to see if the
atypical morphology may be in part related to the
effective compliance of the substratum, and there-
fore, it was predicted that the frequency of the
atypical morphology may increase on the thicker,
more compliant coatings.
An initial study was conducted to determine which
of two commercially available silicones, Sylgard
184
TM
or a cross-linked polydimethylsiloxane elas-
tomer by Gelest
TM
, hereafter referred to as
PDMSdp125, gave the more effective foul-release
surface. After determining that the PDMSdp125 was
the more effective of the two, the earlier work with
pseudobarnacles was then extended by using live
barnacles on PDMSdp125 to investigate the effect of
coating thickness on i) the rate of growth and size of
individuals, ii) the frequency of atypical basal plate
and adult cement morphology, and iii) critical release
stress (sometimes referred to as adhesion strength).
Attempts were also made to determine whether the
critical removal stress differed between animals with
typical (‘‘flat’’) and atypical (‘‘cupped’’) basal plate
and adult cement morphology.
Methods
Silicone materials
Sylgard 184
TM
(Dow Corning Corp.) is a two-
component, high-temperature-vulcanized (HTV) si-
licone elastomer. Following the product instructions,
the base resin and curing agent provided in two
separate containers were thoroughly mixed in a ratio
of 10:1 by weight and cured at 858C for 2 h. The
PDMSdp125 coatings were prepared from a base
resin (vinyldimethylsiloxy-terminated polydimethyl-
siloxane, Gelest
TM
catalog DMS-V22, 200 cSt, 9400
g mole
71
(125 dp), 0.4 – 0.6 wt% vinyl) using a
crosslinker (25 – 30% methylhydrosiloxane-dimethyl-
siloxane copolymer, Gelest
TM
catalog HMS-301,
25 – 35 cSt), a catalyst (platinum-divinyltetramethyl-
disiloxane complex, Gelest
TM
catalog SIP6830.0,
3 – 3.5% platinum concentration in vinyl terminated
polydimethylsiloxane) and a curing control agent
(Maleate, from Dow Corning). The mixture was
cured at 758Cfor 1h.
Coating preparation
Two sets of coatings were prepared at NRL for
barnacle adhesion and removal studies. The first set
was 10 slides each with 1 mm thick coatings of
Sylgard 184
TM
and of PDMSdp125. The second set
was PDMSdp125 coatings at thicknesses of 0.1 mm,
0.5 mm and 2 mm. To achieve the correct thickness,
the silicone mixtures were poured into a cast
consisting of a microscope glass slide at the base and
spacers on four sides of fixed height. The surface of
the glass slide was coated with a silane coupling agent
to promote bonding to the coating. After pouring the
mixture, it was carefully covered by a hydrophobic
glass slide to constrain the thickness of the mixture
during curing; then the cast assembly was secured
with six clamps. The assembly was placed in an oven
where the mixture cured. After curing, the hydro-
phobic glass slide was removed. It should be noted
that the process of using a hydrophobic glass slide
can affect the surface properties of a cross-linked
PDMS, and can differ from an air cured PDMS. The
coatings were air-cured for several weeks before
leaching. Measurements via indentation showed that
the cured coatings acted as true elastomeric surfaces,
indicating complete curing.
Ten slides of each thickness were sent to Cal Poly
for barnacle studies and two slides remained at NRL
for pseudobarnacle testing (to be reported in a
separate paper). Both sets of coatings were leached
in 0.2 mm filtered natural sea water for 6 d.
Larval settlement and growth conditions
Balanus amphitrite cypris larvae were obtained from
the Duke University Marine Laboratory. A 500 ml
drop of seawater containing 20 – 50 cypris larvae was
placed on each treated microscope slide in a covered
Petri dish. Cypris larvae were allowed to settle in
these conditions for 72 h at 258C in a constant
temperature incubator under a 12 h light/dark cycle.
After that time, each slide was transferred to a
separate 100 mm 6 20 mm Petri dish and immersed
in 40 ml of natural, 0.2 mm filtered seawater. For the
pilot study on two different commercially available
PDMS formulations, settlement was performed on
28 April 2003. For the coating thickness experiment
this was done on two separate occasions: 27
December 2003 and 27 February 2004.
Newly metamorphosed barnacles were fed the
unicellular alga Duneliella tertiolecta and the diatom
Skeletonema costatum. Nauplius larvae of the brine
shrimp, Artemia sp., were added to their diet 2 weeks
following settlement. Feeding was done every
Monday, Wednesday and Friday, and involved a
complete replacement of the seawater and food
suspension within each Petri dish. Animals were
reared at 258C in a constant temperature incubator
under a 12 h light/dark cycle.
Growth of basal plate
Barnacles growing on 5 slides of each thickness were
monitored monthly from 31 December 2003 to 31
March 2004 to determine if there were any differ-
ences in growth rate of the basal plates as a function
of coating thickness. Photography was performed
using a Canon
TM
EOS 10D digital camera attached
to an Olympus
TM
SZX12 dissecting microscope.
The basal plates of the developing barnacles were
photographed through the transparent substratum,
and the area of the basal plate was calculated using
NIH’s ImageJ image analysis software. Basal plate
areas were compared by analysing data from 31
March 2005 in one- and two-factor ANOVAs.
Shear testing
The shear test apparatus consisted of an IMADA
TM
AXT 70 digital force gauge (2 kg) mounted on an
IMADA
TM
SV-5 motorised stand. The force gauge
was attached to a motorised stand that moved a
shearing head parallel to the coating surface at an
average speed of approximately 67 um s
71
. The glass
slides were clamped into a custom-built Plexiglas
chamber that allowed complete submersion of coat-
ings during release tests. Prior to each test, basal
plates were photographed using a Canon
TM
EOS
10D digital camera attached to an Olympus
TM
SZX12 dissecting microscope, and areas were
calculated using NIH’s ImageJ. As the coating
surface was flat, barnacle adhesive was assumed to
be in contact with the substratum over the entire
basal plate surface; therefore the size of the basal
plate was measured to determine area. Only barna-
cles with a diameter 43 mm were shear tested. The
force at which the barnacle detached from the
coating, the maximum force measured, is hereafter
called the critical removal force; the critical removal
stress is obtained by dividing the critical removal
force by the measured basal area. The shear test was
discarded in cases where any fraction of the basal
plate remained attached to the surface. Procedures
for performing shear tests differed from ASTM D
5618 in that the shear force was applied by an
automated test stand and the barnacle release was
performed in water. In June 2003, shear tests were
performed on the first set of PDMSdp125 and
Sylgard 184
TM
(1 mm thick) coatings. The second
set (0.2, 0.5 and 2 mm thick) of coatings was shear
tested on two occasions: mid-February and mid-June
2004. Data from shear testing satisfied the assump-
tions for an ANOVA, and were analysed using one-
and two-factor ANOVAs and Fisher’s PLSD post-hoc
tests.
Photographs were also used to determine the
morphology of the barnacle basal plates. The atypical
or altered cement produced by barnacles growing on
silicone surfaces could be observed visually as a white
mass that obscured the radial structures charac-
teristic of the barnacle’s basal plate (Berglin &
Gatenholm, 2003; Wiegemann & Watermann,
2003). Barnacles exhibiting any ‘‘clouding’’ of the
basal plate when viewed from below were designated
as having the atypical or ‘‘cupped’’ morphology, with
all others being designated ‘‘flat’’. The rate of
occurrence of atypical basal plates was determined
for each slide, and compared via a one-way ANOVA.
Results
Critical removal stress for PDMSdp125 vs Sylgard
184
TM
Only 44% of the barnacles removed from Sylgard
184 exhibited complete release from the surface,
without leaving all or part of the basal plate on the
surface of the coating. By contrast, 87% of the
barnacles removed from PDMSdp125 exhibited
complete release from the surface. Mean critical
removal stresses for barnacles removed from 1 mm
thick coatings of Sylgard 184
TM
and PDMSdp125
were 0.10 + 0.02 MPa and 0.069 + 0.007 MPa,
respectively; although technically not significant
(one-factor ANOVA; n ¼ 21, F ¼ 4.11, p ¼ 0.057)
the p-value was only marginally higher than a 0.050
critical value.
Occurrence of atypical basal plate and adult cement
morphology
The formation of atypical basal plates and adult
cement were observed on all coatings tested (see
Figure 1). The proportion of individuals with the
atypical morphology was significantly different
among coatings of different thickness (one-factor
ANOVA; n ¼ 15, F ¼ 3.9, p 5 0.05) (Figure 2). The
lowest frequency of occurrence was on the 0.5 mm
coating. The atypical morphology was observed in
essentially equal frequency on the thinnest (0.1 mm)
and thickest (2.0 mm) samples. Many of the
individuals displayed an ‘‘intermediate’’ morphol-
ogy; that is, the basal plate had the cupped
appearance in the centre and a flat morphology
closer to the perimeter (Figure 1C). The reverse
situation was not observed.
Growth rate and size as a function of coating thickness
Growth rate, as measured by the increase in basal
plate area, did not differ among coating thicknesses
(see Figure 3). Likewise, the areas of basal plates on
the three thicknesses were statistically indistinguish-
able throughout the three months of the experiment
(one-factor ANOVA; n ¼ 13, F ¼ 0.52, p ¼ 0.61)
(Figure 4A). The size of individuals with ‘‘cupped’’
basal plates was not significantly different from
individuals with ‘‘flat’’ basal plates (two-factor
ANOVA; n ¼ 37, thickness F ¼ 1.23, p ¼ 0.30; mor-
phology F ¼ 0.04, p ¼ 0.83) (Figure 4B).
Critical removal stress as a function of coating thickness
Critical removal stress was significantly different and
inversely proportional to coating thickness (one-factor
ANOVA; n ¼ 44, F ¼ 6.681, p ¼ 0.0031) (Figure 5).
The average critical removal stress was 0.093 + 0.008
MPa, 0.074 + 0.005 MPa, and 0.055 + 0.006 MPa
on 0.1 mm, 0.5 mm, and 2 mm thick coatings,
respectively. A post hoc test showed a significant
difference in critical removal stress between 2 mm
and both 0.5 mm and 0.1 mm, but not between
0.1 mm and 0.5 mm (Figure 5A). The data also show a
lower critical removal force for barnacles with an
atypical basal plate than for barnacles with a typical
basal plate (two-factor ANOVA: n ¼ 44, thickness,
F ¼ 7.437, p ¼ 0.0019; basal plate morphology, F ¼
4.182, p ¼ 0.0478; interaction, F ¼ 0.478, p ¼ 0.6338)
(Figure 5B).
Discussion
Growth rate and size as a function of coating thickness
The hypothesis that the growth rate of the basal
plate might differ significantly among coating
thickness was not supported (Figures 3 and 4).
However, using the basal plate area as a proxy for
growth does not take into account the actual mass
of the animal. Thus differences among barnacles
growing on different thicknesses may not have been
detected.
Occurrence of atypical basal plate and adult cement
morphology
The appearance of atypical basal plates and adult
cement has been observed when barnacles grow on
silicone coatings (Watermann et al. 1997, Berglin &
Gatenholm, 2003; Wiegemann & Watermann,
2003; Holm et al. 2005; Kavanagh & Swain,
personal communication) (Figure 1). Individuals
exhibiting the atypical morphology were observed to
synthesise a thick, paste-like cement that was
granular in nature, which was similar to the atypical
morphology reported by Berglin and Gatenholm
(2003). The exact mechanism by which silicone
coatings disrupt the typical process of basal plate
growth and adult cement formation is not under-
stood. Sun et al. (2004) suggest that atypical basal
plate morphology may form as a result of increased
production of adhesive as animals try to maintain
contact with a PDMS substratum. Berglin and
Gatenholm (2003) showed using electron probe
microanalysis and infrared spectroscopy that cal-
cium was incorporated as calcite in barnacles with
typical basal plate and adult cement morphology
grown on polymethlymethacrylate; in contrast, no
Figure 1. Examples of the various morphologies of a barnacle basal plate grown on an elastomeric surface. A ¼ typical or ‘‘flat’’ morphology;
B ¼ atypical or ‘‘cupped’’ morphology; C ¼ transitional basal plate exhibiting both ‘‘cupped’’ and ‘‘flat’’ characteristics.
calcium could be detected in the adult cement of
animals with the atypical morphology grown on a
polydimethylsiloxane (silicone) coating. In the same
study the authors did not report on the frequency of
occurrence of the atypical morphology in their
experiments. The present results showed that some
individuals grown on silicone coatings exhibit the
typical basal plate and adult cement morphology.
It was also observed that individuals may initially
have an atypical (‘‘cupped’’) morphology, but
then change to the typical (‘‘flat’’) morphology
as the animal grows. Holm et al. (2005) have
demonstrated genetic underpinnings that at least in
part determine the frequency of occurrence of the
atypical morphology. In their experiments different
genetic families of the barnacle B. amphitrite ex-
hibited different rates of occurrence of the atypical
morphology when grown on the same silicone
coatings. That a monotonic relationship between
coating thickness and occurrence of the atypical
morphology was not found suggests that coating
thickness (and therefore effective compliance) does
not directly influence the frequency of occurrence of
the atypical morphology (Figure 2).
Figure 2. The proportion of atypical, or ‘‘cupped’’ basal plates occurring on three different thicknesses of PDMS.
Figure 3. Growth rate of barnacle basal plates grown on three thicknesses of PDMS: 0.1 mm, 0.5 mm, and 2 mm.
Critical removal stress as a function of coating thickness
The results showed that critical removal force
decreased as a function of coating thickness,
although not as 1/h
1/2
, where h is coating thickness,
as predicted by Kendall (1971) for pull-off removal.
There are several reasons why this particular power-
law behaviour might not apply to shear of barnacles.
First, no theoretical model of shear currently exists
that predicts a 1/h
1/2
behaviour. Secondly, the
deviation could be an experimental artifact, e.g.
due to the application of non-shearing forces (e.g.
torque). In addition, Chaudhury has calculated the
thickness dependence of the shear removal stress for
both solid and flexible plates attached to elastomeric
coatings (Chaudhury, personal communication,
2005). He found that the thickness dependence
weakened as the flexibility of the plate decreased to
that of the coating. The present results also show that
the removal stress was lower for barnacles with
atypical basal plates than for barnacles with typical
basal plates (Figure 5B). These data suggest that the
functionality of the adhesive has been compromised
in some capacity, which is consistent with the
observations and data of Berglin and Gatenholm
(2003) and Sun et al. (2004).
Basal plate breakage during removal in shear
It is known that on non-easy release surfaces the
basal plates of barnacles will often fracture as animals
are dislodged. If the integrity of the basal plate is
compromised during removal, then it is likely that
the adhesion between the surface and the barnacle
cement is so great that the basal plate cannot support
the forces needed to dislodge the animal. As the
animal grows, the basal plate structural integrity
increases (e.g. Berglin et al. 2001). Berglin et al.
(2001) showed a gradual transition during barnacle
growth in failure mode, viz. i) the smallest barnacles
showed a total cohesive failure leaving the entire
basal plate on the surface, ii) as the animals grew they
shifted to a mixed failure mode where a portion of
the basal plate is removed (i.e. breaking of the basal
plate), and iii) complete removal of the barnacle
Figure 4. Mean area of the basal plates of barnacles grown on three thicknesses of PDMS, as measured during shear testing. A ¼ mean area
for all barnacles on each of three thicknesses; B ¼ mean area of barnacles exhibiting the typical (‘‘flat’’) basal plate and the atypical
(‘‘cupped’’) basal plate.
including the entire basal plate, indicating failure of
the adhesive bond to the surface. This transition,
which they suggest is a measure of the balance
between the cohesive strength of the barnacle basal
plate and the adhesion bond to the surface, occurs
earlier (i.e. for smaller barnacles) on better perform-
ing foul-release surfaces. Earlier observations of
Singer et al. (2000) showed that the barnacle B.
improvisus often broke during removal in tensile
from Sylgard 184
TM
. In the present study with
B. amphitrite a similar situation was observed;
only 44% of the barnacles grown on Sylgard 184
TM
were removed from the surface without cohesive
failure, whereas animals of the same size grown
on PDMSdp125 showed 87% complete removal.
In the coating thickness experiment using only
PDMSdp125 coatings total cohesive failure was
never observed and greater than 90% of the animals
showed failure of the adhesive bond to the surface
(i.e. complete basal plate removal). These data
clearly demonstrate i) that not all silicone coatings
are easy release, and ii) that a pure PDMS such as
PDMSdp125 can be easy release without additives
such as oils.
Figure 5. Mean critical removal stress of barnacles removed in shear from three thicknesses of PDMS. A ¼ overall mean critical removal
stress; B ¼ mean critical removal stress of barnacles exhibiting the two different basal plate morphologies: typical (‘‘flat’’) and atypical
(‘‘cupped’’).
Summary
No relationship was found between coating thickness
and the rate of growth or the size of barnacle
basal plates. There was additionally no discernable
relationship between coating thickness and the
occurrence of atypical basal plate and adult cement
morphology (i.e. ‘‘cupped’’ vs ‘‘flat’’). Critical
release stress qualitatively obeyed fracture mechanics
model, i.e. release force decreased with increasing
thickness in confinement regime. Indeed, critical
removal stress was significantly lower on 2 mm
coatings than on either 0.1 mm or 0.5 mm coatings.
Moreover, critical removal stress was lower for
barnacles with atypical basal plates than for barnacles
with typical basal plates. Although 2 mm is thicker
than is practical for most commercial coatings, the
data demonstrate that coating thickness is an
important parameter governing removal of barnacles
from elastomeric coatings; coating thickness should
be considered when optimising the design of
elastomeric foul-release surfaces.
Acknowledgements
We thank Dan Rittschoff and Beatriz Orihuela-Diaz,
both of Duke University, for providing us with
barnacle cypris larvae, and Emily Wilson, Ruth
Armour, Danielle Castle, and Lisa Needles for their
assistance with the culture of juvenile barnacles at
Cal Poly. The US Office of Naval Research is
gratefully acknowledged for financial support
(Grants no. N00014-02-0935 to DEW; N00014-
04-WX-2-0311 to ILS). Jongsoo Kim gratefully
acknowledges funding support of Lehigh University
and ONR (2002 – 2004) and North Dakota State
University (2004 – 2005). We also thank two anon-
ymous reviewers for their helpful comments and
suggestions.
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