modeling of spherical silver

Modeling of spherical silver nanoparticles in
silicone-based nanocomposites for marine
antifouling†
Mohamed S. Selim,ab
Sherif A. El-Safty,*ac
Maher A. El-Sockary,b
Ahmed I. Hashem,d
Ossama M. Abo Elenien,b
Ashraf M. EL-Saeedb
and Nesreen A. Fatthallahe
Since the use of organotin antifouling paints was prohibited in 2003, researchers have endeavored to
develop novel environment-friendly marine antifouling coatings. We report the successful fabrication of
model silicone foul-release (FR) coatings with elastomeric polydimethylsiloxane (PDMS)/spherical silver
(Ag) nanocomposites. This design integrates two inhibition modes of (1) chemical inertness and (2) the
physical repelling force of microfouling. The antifouling nanocomposite models were successfully
synthesized via the solution casting technique. In this approach, a series of filler concentrations of Ag
nanoparticles (NPs) with a particle size of <10 nm and spherical morphology facet dominantly controlled
on the {111} lattice plane was used to control the antifouling models. The surface hydrophobicity,
roughness, and free energy properties of the nanocomposites were systematically studied as fouling
non-stick factors. The physicomechanical properties were also assessed. Selected bacterial strains were
used as microfoulants for a laboratory assay investigation for 30 days. Our findings provide important
insights into how subtle structural changes in polymer nanocomposites can considerably improve
biological activity and simplify surface cleaning. Hydrophobicity, surface inertness, fouling resistance, and
surface easy-cleaning properties significantly improved in the nanocomposite design models fabricated
with nanofiller loadings of up to 0.1% spherical Ag NPs without changes in the bulk mechanical
properties. The fabricated models were subjected to a rigorous test in a field trial in Red Sea water. The
results show the potential of our models based on Ag nanofillers up to 0.1% for ecologically friendly
antifouling coatings as an alternative to traditional systems. The PDMS/Ag composite models have a
long-term durability and antifouling performance, which are important factors for developing effective,
stable, and eco-friendly nanocomposites.
1. Introduction
In recent years, the physical, chemical and mechanical prop-
erties of nanomaterials have considerably improved their
potential applications ranging from environment and energy to
healthcare compared with those of bulk materials.1
Interna-
tional ecological restrictions have directed research toward
developing environment-friendly nanomaterials for bioeld
applications.2
The environmental exposure of nanomaterials is
inevitable because of their widespread use in many technolog-
ical and biological elds.3,4
As various nanomaterials have been
developed for marine applications, marine fouling represents a
major economic and ecological problem worldwide.
Biofouling is a complex issue in marine elds, particularly in
marine transportation.5,6
Marine biofouling increases drag
resistance and decreases sailing speed to a minimum, thereby
increasing fuel consumption and maintenance leading to
poisonous gas emissions to the atmosphere.7,8
Antifouling (AF)
paints have been widely used to prevent and control the
biofouling of marine structures. AF coatings based on organotin
compounds pose a threat to the marine environment.9
As the
use of biocidal AF technologies has been restricted because of
their bioaccumulative properties and high toxicity to various
marine organisms, less or even nontoxic AF paints must be
developed.10
Many researchers have focused on TBT-free AF
paints, but these materials are also deleterious to the environ-
ment.11,12
Toxicity and concerns about the application of AF
a
National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukubashi,
Ibaraki-ken 305-0047, Japan. E-mail: sherif.elsay@nims.go.jp; Web: http://www.
nims.go.jp/waseda/en/labo.html
b
Petroleum Application Department, Egyptian Petroleum Research Institute, Nasr City
11727, Cairo, Egypt
c
Graduate School for Advanced Science and Engineering, Waseda University, 3-4-1
Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. E-mail: sherif@aoni.waseda.jp; Web:
http://www.nano.waseda.ac.jp/
d
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt
e
Processes Development Department, Egyptian Petroleum Research Institute, Nasr City
11727, Cairo, Egypt
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra07400b
Cite this: RSC Adv., 2015, 5, 63175
Received 23rd April 2015
Accepted 9th July 2015
DOI: 10.1039/c5ra07400b
www.rsc.org/advances
This journal is © The Royal Society of Chemistry 2015 RSC Adv., 2015, 5, 63175–63185 | 63175
RSC Advances
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biocidal paints have impelled the development of novel
environment-friendly materials based on non-stick physical
anti-adhesion mechanisms.13
Non-stick FR coatings, which are
a viable, commercial, and nontoxic alternative, function by
providing many factors including low surface free energy, low
glass transition temperature, low microroughness and high
mobility of the polymer chains; hence, fouling species cannot
securely attach on the surface and can be easily removed
hydrodynamically.14,15
Silicone FR coatings based on polydimethylsiloxane (PDMS)
feature non-toxicity, low microroughness, low surface tension,
low glass transition temperature (Tg), high molecular mobility,
water repellant and non-leachant properties, stability in marine
water, and excellent adhesion properties onto various hull
surfaces; these characteristics inhibit adhesion of fouling
organisms onto the surface.16,17
Although PDMS coatings
exhibit good FR effect, nanollers are added to improve FR
performance. As a trade-off exists between FR and durability,
the challenge is to improve the durability of FR coatings without
affecting their mechanical properties. Thus, this research
introduces novel FR coating models.
The use of noble metal nanoparticles (NPs), particularly
nano-Ag, has received great attention in the elds of physics,
chemistry, materials, and sensors.18
Although nano-Ag is less
expensive and presents excellent optical properties compared
with nanogold, it has rarely been studied and applied. Nano-Ag
particles have received great attention for many industrial
applications, particularly in the paint industry, because of their
superior physical and biological properties, such as antimicro-
bial characteristics.19
Among various antibacterial agents, Ag
NPs are highly favorable because of their high toxicity to a broad
spectrum of microorganisms but low cytotoxicity to higher
animals.20,21
The high surface-area-to-volume ratio of NPs
contributes to their unique physical, chemical, mechanical, and
quantum size effect properties. The anti-fungal,22
antiviral,23
antiangiogenisis,24
and AF activities of Ag NPs have been high-
lighted in several studies.25
Spherical Ag NPs with an average
particle size of 20 Æ 0.21 nm have been shown to exert strong
antimicrobial and antioxidant activities in biological applica-
tions against bacteria, such as Staphylococcus aureus and
Escherichia coli.26
Spherical Ag NPs that mainly contain {111}
facets exhibit the most signicant antibacterial activity over
other Ag nanostructures (cubic, wire, and triangular), which
contain few {111} planes, against E. coli and Bacillus microor-
ganisms.27
The polar properties of edged Ag spheres with
densely packed {111} lattice planes, which exhibit the lowest
surface energy per unit area and stability over the {100} and
{110} facets of other morphologies, contribute to the FR and
antibacterial properties.28
Small-sized particles also exhibit
antibacterial properties against bacteria. The smallest Ag NPs
with dimensions less than 10 nm exert the highest antibacterial
activity because they contain numerous {111} facets.29
The
stability and dispersion of Ag NPs could be enhanced through
functionalization of the hydrophobic hydrocarbon chain of
dodecylamine, thereby preventing aggregation.30
Ag NPs also
enhance the hydrophobicity of lms.31
Thus, the use of
modied nano-Ag lms to develop hydrophobic surfaces poses
a challenge for the current eld of study.
In this study, we fabricated low-cost, non-stick, and nontoxic
PDMS/spherical nanostructured surfaces through solution
casting to be used as novel FR coating surfaces. This study is the
rst to use solution casting to fabricate such composites. Stable
spherical Ag NPs with an average particle size of <10 nm were
synthesized via sol–gel technique. Various concentrations of Ag
NPs were prepared by directly blending the desired amount in
the PDMS matrix to compare and identify the most effective
concentration. The bulk mechanical properties were evaluated
using tensile modulus measurements. The surface properties
were also quantitatively determined through measurements of
contact angle, roughness values, and surface free energy.
Spherical Ag plays a novel role of increasing FR properties in
marine AF paints by improving the super-hydrophobicity and
producing an inert silicone nanocomposite surface with low
energy. Given these properties, fouling organisms cannot settle
on the surface without being killed. The well-dispersed nano-
llers in the polymer matrix enhance the surface property and
stability of the bulk mechanical properties, thereby producing
smooth surfaces, which can prevent fouling adhesion through a
failure adhesion mechanism. Several microorganisms were
used to investigate the FR performance of the prepared nano-
composites. A eld trial was also conducted to examine the AF
performance in natural sea water for 12 months. The proposed
nanocomposite system could be potentially used to completely
control fouling via a nontoxic method through a physical anti-
adhesion mechanism.
2. Experimental details
2.1. Chemicals
Octamethylcyclotetrasiloxane (D4, 98%), tetramethyldivinyldi-
siloxane (C8H18OSi2, 97%), polyhydromethylsiloxane (PMHS;
Mn ¼ 1700–3200), Karstedt catalyst (platinum (0), divinylte-
tramethyldisiloxane in solution; Pt content: 8–11%), silver
acetate (99%), phenyl hydrazine (97%), and dodecylamine
(DDA, 98%) were purchased from Sigma-Aldrich Chemical Co.
Ltd, United States. Trichloroethylene, KOH, methylbenzene,
ethanol, acetone, and methanol were obtained from Merck Co.,
Mumbai, India and used without further purication.
2.2. One-step fabrication of spherical Ag NPs
Spherical Ag NPs were synthesized through a one-step chemical
reduction-stabilizing technique by using phenyl hydrazine
(substituted hydrazine) and 1-dodecylamine as suitable
reducing and stabilizing agents, respectively, at a relatively low
temperature range of 50–60
C.32–35
In this approach, unique
and spherical Ag NPs with an average particle size of 10 nm
were obtained under specic synthesis conditions with
controlled concentrations of the silver precursor, reducing and
stabilizing agents, and pH solution as follows: phenyl hydrazine
(1 mmol) solution in 10 mL of methylbenzene was added
dropwise to a mixture of silver acetate (1 mmol) solution in 40
mL of methylbenzene and DDA (10 mmol) with continuous
63176 | RSC Adv., 2015, 5, 63175–63185 This journal is © The Royal Society of Chemistry 2015
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stirring at 50
C to 60
C (pH 10.5) for 1 h to prepare spherical Ag
NPs with particle sizes lower than 10 nm. Aer the addition of
methanol and acetone (50 : 50 in volume) to the mixture, black–
silver DDA NPs precipitated. The precipitates were ltered,
washed with a mixture of acetone and ethanol, and then dried
in air (Scheme 1).
2.3. Fabrication of FR model based on novel spherical
PDMS/Ag nanocomposites
Linear PDMS with vinyl terminal groups (VTPDMS) was
prepared to fabricate the novel spherical PDMS/Ag nano-
composites. The preparation was performed through the
anionic ring-opening polymerization of octamethylcyclote-
trasiloxane, as we reported recently.16
Ag NPs with different
concentrations (0.01%, 0.05%, 0.1%, 0.5%, 1%, 3%, and 5%)
were sonicated in methylbenzene using a VCX-750 (Sonics and
Materials Co., USA) with an ice bath for 15 min. A solution of the
VTPDMS resin in methylbenzene was slowly added, stirred for
20 min, and then sonicated for a further 15 min. A solution
containing VTPDMS (15 g in 50 mL of methylbenzene) and Ag
NPs was subjected to hydrosilation crosslinking using poly-
functional silicon hydride PMHS (curing agent, 0.5 g in 10 mL of
methylbenzene) in the presence of Karstedt catalyst (0.55 g in 20
mL of trichloroethylene) through the hydrosilation pathway of
curing.16,36
A fabricated coating formulation (VTPDMS 79.5%, 0.1% Ag
NPS, 10% ferric oxide pigment (95%, SDFCl, India), and 0.5%
surfactant) was applied over the epoxy primer rst coat and tie
coat layer aer drying. Free-standing lms with a thickness of
150 mm were applied. The top-coat spherical Ag/silicone nano-
composite lm was le for 24 h at RT for complete curing.
2.4. Characterization of the Ag NPs and FR model based on
PDMS/Ag NP nanocomposites
Various characterization techniques were performed on the
fabricated Ag NPs and FR model based on PDMS/Ag NP
nanocomposites.
The crystalline nature of the prepared Ag NPs was deter-
mined through XRD, and the data were collected using a
PANalytical instrument (X’Pert PRO, Netherlands) equipped
with monochromated CuKa radiation (l ¼ 0.15406 nm) at an
accelerating voltage of 30 kV. Scanning was performed in 2 theta
angle that ranged from 10
to 70
with d-spacing values between
0.144 and 0.235 nm, and the data obtained were identied in
accordance with the ICDD database.
Samples for high-resolution TEM were prepared by placing a
drop of the sonicated Ag NPs (in ethyl alcohol) on a carbon-
coated copper grid (300 mesh) to evaluate the morphology
and size. TEM images were collected with an electron micro-
scope (JEM2100 LaB6, Japan) at 200 kV accelerated voltage with
1.4 ˚A point resolution. The Ag/silicone nanocomposite lms
were cut into ultra-thin sections of 100–150 nm thickness using
a Leica Ultracut UCT ultracryomicrotome (Leica Microsystems,
Austria). These sections were cut with a 45
sharp diamond
cryoknife at À160
C. The cryoslices were collected serially onto
a copper-mesh TEM grid and analyzed at 160 kV acceleration
voltage.
The elemental composition of the Ag NPs was evaluated
using an EDS instrument (Oxford X-Max, Birth) directly con-
nected to an electron microscope (JEM2100 LaB6) at an accel-
eration voltage of 30 kV.
The morphology of the as-synthesized Ag NPs was captured
using a scanning electron microscope (JEOL JSM530) at an
accelerating voltage of 20 keV.
The optical micrographs of the immersed coated glass
samples were obtained using a polarized microscope (Olympus
BH-2, Japan).
2.5. Surface and release performance of the PDMS/Ag
nanocomposites
A Tantec line of contact angle goniometer (Germany) was used
to measure the static contact angles of liquids on the as-
synthesized cured polymer and Ag/silicone nanocomposites.
Measurements were performed on coated microscopic slides
through the sessile drop technique. The solid surface free
energy of the coated surfaces was determined based on the Fox–
Zisman theory using static contact angle measurements; the
Owens, Wendt, Rabel, and Kaelble (OWRK) method, also called
the geometric mean method, was employed for measurement.37
The liquids used were DI water and diiodomethane. The total
surface tension gtotal
S measurements of the cured polymer lms
and nanocomposite surfaces were calculated using the OWRK
method with the following (eqn (1) and (2)):
ð1 þ cos qLÞgL ¼ 2
ffiffiffiffiffiffiffiffiffiffiffiffi
gD
L gD
S
q
þ
ffiffiffiffiffiffiffiffiffiffiffi
gP
LgP
S
q
(1)
gtotal
S ¼ gD
S + gP
S (2)
where qL and gL are the static contact angle and surface tension
values of the testing liquids, respectively; gD
and gP
are the
dispersion and polar free energy of the testing liquids, respec-
tively; and gD
S and gP
S are the dispersion and polar free energy of
the testing surfaces, respectively.
A prolometer (Mitutoyo Surest SJ-301) with a standard
stylus device was used to determine the average surface
Scheme 1 Preparation of spherical Ag NPs through a one-step
chemical reduction-stabilizing technique by using phenyl hydrazine
and 1-dodecylamine as reducing and stabilizing agents, respectively, at
a relatively low temperature range of 50–60
C.
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roughness (Ra), which is the average distance from the prole to
the mean line over the length of assessment of the fabricated
polymer nanocomposites. Measurements were performed using
a stylus tip with a 4 mm diameter, 10 mm minÀ1
tracing speed,
and 90
tip angle of the tool and repeated at random points
several times during the tests.
2.6. Biodegradability and eld exposure testing
Different bacterial strains, including S. aureus, Bacillus subtilis,
Pseudomonas aeruginosa, and E. coli, were obtained from the
Faculty of Agriculture in Mircen, Cairo, Egypt. The strains were
grown in nutrient broth for the maintenance and cultivation of
the bacterial strains at 30
C for 30 days.38
Biodegradation tests
were carried out in 100 mL batch asks (containing 30 mL of
basal salts medium and at a pH of 7).39
The incubation was
performed in a (150 rpm) shaking incubator. Then the speci-
mens were taken away, subject to distilled water washing and
then dried. Biodegradability was evaluated under aerobic
conditions through weight loss determination.39
The exposure of painted panels to natural conditions is the
oldest and one of the most effective techniques to study AF
marine paints.40
A eld trial was applied at the Suez Canal
Marine shipyard, Suez, Egypt for the as-synthesized PDMS/Ag
nanocomposites. To ensure the visual assessments, the FR
evaluation in the eld test was conducted based on a screening
process and image analysis for 12 months in Red Sea water.40a,b
The Suez Canal was selected as the test eld because it is a
tropical area where fouling occurs throughout the year. The
painted panels were tested in water at temperatures of 23
C to
30
C, pH of 8.5–9.1, and salinity of 40%. The test steel panels
(300 mm Â 200 mm Â 1.5 mm) were cleaned and removed of
rust. The panels were then painted on two sides with anticor-
rosive coating followed by a tie coat. Aer drying, the developed
nontoxic FR nanocomposite coatings were applied on the two
sides of the panels. The panels were allowed to cure at RT, and
the thickness of the top coat dry lm was 100–150 mm.
2.7. Mechanical tests of the FR model design
The viscoelastic behavior of the designed spherical Ag/silicone
nanocomposites was studied. The rectangular samples were
investigated in tension mode using a TTDMA instrument (UK)
at RT and 1 Hz with a 2 N preload through the ASTM D412
protocol.
Three different tests, namely, cross cut, T-bending, and
impact, were performed to evaluate the mechanical properties
of the prepared silicone/Ag nanocomposite coatings. The tests
determined the adhesion strength, exibility, and elasticity of
the painted lms.
A 170 mm Â 90 m Â 1 mm steel panel was degreased and
coated with a primer coat of two-component epoxy resin. A tie
coat of silicon/epoxy was applied as a second coat. Aer
complete drying, a top coat of the PDMS/Ag nanocomposite
coating was applied with a dry lm thickness of 150 mm. The
adhesion degree of the coating was examined by a cross-cut test.
The coated panels were scribed with a sharp steel cutter (Sheen
750, UK), and the resulting grid size was 2 mm Â 2 mm.
Adhesive tape was used, and the adhesive strength was rated
according to ASTM D 3359. The T-bending test was performed
on the test panels (Ref 809 type model, Sheen Instruments Ltd,
Kingston, UK) through ISO 6272 to check the formability of the
prepared nanocomposites. The measurements were based on
the spindle diameter. Impact tests were carried out using an
impact tester (Ref BG5546, Sheen Instruments. Ltd, UK)
according to ISO 6272 to determine the height of sudden falling
load (1 kg weight) on the painted PDMS and resistance to
damage of the nanocomposite lms.
3. Results and discussion
3.1. Controlled particle and distribution sizes, morphology,
and stability of the Ag NPs
A facile and inexpensive synthesis of spherical Ag NPs was
successfully conducted, and the size of the particles could be
controlled at low temperatures within short reaction times. The
stability and particle size control of the Ag NPs are strongly
dependent on the pH of the prepared solution.41
Increasing the
molar ratio of the Ag source may inevitably result in small
particle sizes and a narrow particle size distribution.42
Consid-
erable changes in particle and distribution sizes, morphology,
and stability may occur during the synthesis of Ag NPs even with
minimal variations in the reaction parameters.
AF activity can be maximized by increasing the number of
particles per unit area at the nanoscale level. The mechanism of
Ag NP formation is discussed and illustrated in Scheme 1. The
crystalline information and morphological homogeneity of the
as-synthesized Ag NPs were obtained using XRD (Fig. 1). The
distinct peaks at the 2q values of 38.18, 44.34, and 64.54
correspond to the {1 1 1}, {2 0 0}, and {2 2 0} inter-planar
reections of the spherical crystal system; this nding showed
that the resulting Ag NPs were well crystallized. The {111} lattice
plane is more intense because of its predominant orientation
compared to the other peaks. The average crystallite size (D) of
10 nm was determined by substituting the parameters in the
Debye–Scherrer equation.41,43
The TEM images of the as-synthesized Ag NPs are presented
in Fig. 2A–C. Overall, the prepared NPs presented a mean
Fig. 1 XRD pattern of the prepared Ag NP spheres. Inset is the sche-
matic shape of a liquid droplet with super-hydrophobic character on
the surface of coated steel panels.
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particle size of 10 nm, a spherical shape, and single crystals
without agglomeration. The crystalline nature of the prepared
Ag NPs was further conrmed via selected-area (electron)
diffraction (SA-ED) (Fig. 2D). Distinct ring patterns were
observed, and the {111}, {200}, and {220} crystal planes were
indexed to conrm the polycrystalline nature of the prepared
NPs.41
The ndings indicated that the main facet is the {111}
crystal plane, which represents the desired FR properties, such
as low surface energy, antibacterial activity, and nearest
neighbour atoms per unit area. Fig. 2E shows the fringes of the
{111} lattice of the Ag NPs with 0.23 nm d-spacing. The reported
values are compatible with the XRD results. Fig. 2F illustrates
the elemental map of the synthesized NPs via the EDS spectrum.
The results reveal that the prepared NPs demonstrated clear Ag
peaks without impurities or contaminations. The peaks of Ag
NPs around 3.0 refer to the Ag Lu binding energies, whereas the
peaks around 3.2 and 3.4 keV belong to that of Ag Lb and Ag Lb2
energy, respectively. The other peaks of C and Cu elements
could be attributed to the use of the carbon-coated copper grid
in the analysis. The SEM images of Ag NPs (ESI, Fig. S1A and B†)
reect the well-dispersed Ag nanospheres with a smooth surface
nature, which explains the good surface properties of these
particles.
3.2. Ag–polymer nanocomposite lm formation and NP
diffusion
With the promising features of PDMS in FR coatings16
and the
advantages of applying the hydrosilation curing mechanism,32
VTPDMS was prepared successfully,16
and tailored toward
developing novel solutions for non-stick FR coatings to deter
fouling.
The spherical Ag NP lm showed enhanced surface hydro-
phobicity and self-cleaning properties and exerted biological
activity. The low surface energy (111) orientation of the spher-
ical Ag NPs stabilized and enhanced the biological activity (see
Section 3.4).8
Ag ions are particularly suitable for AF because of
their oligodynamic effect at relatively low concentrations. The
interactions of the microorganisms with the Ag NPs were
considerably affected by the morphology and size distribution
of the NPs.44
With this approach, novel PDMS/spherical Ag
nanocomposites were fabricated (Scheme 2) and various
concentrations of Ag nanospheres were embedded into the
prepared VTPDMS matrix to investigate chemical inertness
against fouling.
TEM observations showed that the Ag NPs appeared as dark
spheres, and the uniformly bright surroundings were the cured
PDMS. The TEM images (ESI, Fig. S1C–E†) of the prepared
spherical Ag/silicone nanocomposites (0.1% Ag nanospheres
concentration) showed well-dispersed spherical Ag NPs, which
were separated from one another without any agglomeration at
low concentrations. The retention of well-dispersed Ag NPs in
homogeneous and spherical size domains without aggregation
inside the PDMS matrices could signicantly enhance the
physical, surface and AF properties of the nanocomposites. By
contrast, a different trend was observed at high concentrations
(5% Ag nanosphere concentration); this nding demonstrated
the high-degree of agglomeration and aggregation of the Ag
NPs, as well as their condensation over one other (ESI, Fig. S1F–
H†). Agglomeration and aggregation enhanced the surface
Fig. 2 (A)–(C) are the TEM images of the synthesized spherical Ag NPs
at different magnifications; (D) corresponding SA-ED patterns of the
as-synthesized Ag NPs; (E) corresponding crystal lattice, which is
consistent with the XRD results; and (F) corresponding EDX image of
the as-synthesized Ag NPs.
Scheme 2 Preparation of a series PDMS/Ag nanocomposites with the
addition of different concentrations of Ag NPs to vinyl terminal groups
(VTPDMS) under sonication.
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roughness and reduced the surface hydrophobic properties,
thereby facilitating the adhesion of fouling organisms.
3.3. Wetting behaviour and surface investigation of the
fabricated nanocomposites
The main factor for the anti-adhesive behaviour of the silicone
FR surfaces is the low surface tension. Fouling could be
deterred by inhibiting the bonding ability on the FR surfaces
and eliminated by hydrodynamic or mechanical methods.
Surface hydrophobicity and free energy are the most important
factors for designing FR coatings. The surface hydrophobicity of
the hybrid nanocomposites was evaluated in the dry state and
aer submersion using static contact angle measurements, as
illustrated in Fig. 3a. The unlled PDMS showed a hydrophobic
characteristic with a measured contact angle of 102
Æ 1
. At
various Ag nanoller concentrations, the wetting disposal of the
prepared Ag/silicone nanocomposites shied toward higher
hydrophobicity up to 0.1% Ag and presented a contact angle of
148
Æ 1
in water. This nding indicated the superhydrophobic
surface nature (Lotus effect) and could be attributed to the well-
dispersed NPs that increased the surface area and smoothness
and enhanced the chemical bonding between the NPs and the
polymer matrix (Fig. 3a). The average surface roughness (Ra)
measurements indicated higher microroughness for the
unlled PDMS (0.76 mm) than for the spherical Ag-lled PDMS
nanocomposites (Fig. 3a). The microroughness value gradually
decreased up to the Ag nanoller (0.16 mm) concentration of
0.1%, resulting in an ultra-smooth surface with non-stick FR
characteristics. As the concentration of Ag NPs increased in the
matrix, hydrophobicity reduced until 5% nanoller concentra-
tion because of the enhanced agglomeration and particle clus-
tering of the Ag nanollers. Consequently, the chemical
bonding of the Ag nanospheres with the polymers was mini-
mized, which is the main problem at high lling concentra-
tions. The combination of the Ag NPs, the formation of strongly
bonded aggregates, and the loss of high surface area could
reduce the hydrophobic characteristics and enhance the surface
roughness (0.96 mm), thereby reducing the FR efficiency. Aer
submersion, the fabricated samples presented reduced contact
angles compared with the dry samples. The hydrophobic values
were restored aerward, thereby conrming the renewable
properties of the designed PDMS/Ag nanocomposites.
The surface tension values for the unlled PDMS and Ag-
lled silicone nanocomposites were calculated while dry and
aer submersion in water using the Zisman and OWRK equa-
tions as reported in Table 1. The data showed that
gtotal
S decreased at lower nanoller concentrations (21.34 mN
mÀ1
for unlled PDMS) up to 0.1% Ag nanoller, which
exhibited the lowest surface free energy (10.93 mN mÀ1
). By
contrast, gtotal
S gradually increased with increasing nanoller
loading up to 5% (17.49) because of the roughness caused by
agglomeration and aggregation. The surface free energy of the
dry samples with unlled and lled PDMS showed lower surface
Fig. 3 (a) Water contact angle measurements of the prepared unfilled
and Ag-filled silicone nanocomposites before and after immersion
(error bars represent Æ1 standard deviation based on three determi-
nations) and inside the roughness measurements (error bars represent
Æ0.01 standard deviation from three replications); (b) tensile modulus
of the prepared Ag nanocomposites.
Table 1 The measured dry and wet contact angles in diiodomethane (error bars represent the standard deviation from three measurements) and
also the values of gtotal
S calculated according to goniometer mean methods for the fabricated unfilled and filled silicone/Ag NPs (error bars
represent the standard deviation from three replications)
Sample design
(q) Diiodomethane Goniometer mean method
Dry Wet
gD
S gP
S gtotal
S (mN mÀ1
)
Dry Wet Dry Wet Dry Wet
PDMS blank 75
Æ 1
70
Æ 2
20.13 21.21 1.22 1.642 21.34 23.91
PDMS/Ag (0.01%) 83
Æ 2
78
Æ 3
15.98 18.53 0.11 1.101 16.09 19.63
PDMS/Ag (0.05%) 91
Æ 2
84
Æ 2
12.26 13.6 0.36 0.525 12.62 14.13
PDMS/Ag (0.1%) 98
Æ 1
92
Æ 2
9.41 11.83 1.52 0.57 10.93 12.4
PDMS/Ag (0.5%) 93
Æ 1
88
Æ 1
11.41 13.6 1.03 0.525 12.45 14.13
PDMS/Ag (1%) 89
Æ 2
83
Æ 2
13.15 15.98 0.66 0.004 13.81 15.98
PDMS/Ag (3%) 85
Æ 3
78
Æ 2
15.01 18.53 5.29 0.001 20.29 18.54
PDMS/Ag (5%) 80
Æ 1
73
Æ 2
17.49 21.21 0.03 0.39 17.49 21.6
RSC Advances Paper
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free energy compared with those of the immersed PDMS and its
nanocomposites. Our nding indicates that the low surface free
energy was caused by the embedding of Ag in the nano-
composites (up to 0.1%). In addition, the well-dispersion of the
Ag NPs is the key component factor for the inertness of the
surface toward biological attachment. The chemical interaction
between the polymer and the spherical Ag NPs was further
analyzed by evaluating the bulk mechanical properties of the as-
prepared unlled and lled spherical Ag/silicone nano-
composites (Fig. 3b). The tensile modulus values presented a
nearly steady state at lower concentrations because of the good
dispersion and gradually suffered from increased stiffness with
higher lling because of Ag NP agglomeration and clustering.
The size of the prepared Ag NPs of 10 nm is a major factor for
the increased surface area and {111} lattice plane, resulting in
pronounced surface properties. However, the surface area was
reduced because of agglomeration and the {111} faces were
reduced because of clustering.
3.4. Characteristics of the tailored FR coating system based
on spherical Ag NPs
We developed a novel model of a FR coating for ship hulls using
spherical Ag NPs. The model enhanced the surface character-
istics of silicone FR coatings by increasing the hydrophobicity
and decreasing the roughness and surface free energy, thereby
preventing the adhesion of fouling microorganisms through a
physical non-toxic mechanism (see Fig. 4).
To explore the real applicability of the PDMS/Ag NP nano-
ller model design, the FR performance of the newly fabricated
PDMS/spherical Ag nanocomposites (0.1% loading because of
its superior properties) in terms of surface free energy and
surface hydrophobicity was compared with the following
models; (1) two commercial FR silicone coatings, namely, Syl-
gard® 184 (hydrosilation cured PDMS) and RTV11 (condensa-
tion cured PDMS),45
(2) Sylgard 184/multi-wall carbon
nanotubes (MWCNT),46
and (3) Cu2O silicone nanocomposites
(0.1% of cubic Cu2O), repectively.16
These model compositions
were proven as effective FR coatings against certain bacterial
growth and fouling settlements.47,48
Sylgard® 184 and RTV11 FR
coatings were evaluated and compared based on surface
hydrophobicity and surface free energy, as reported previously
(see ESI†).45–48
Fig. 4 shows evidence that the silicone/spherical Ag nano-
composites fabricated using the preferred concentration (0.1%
Ag NPs) achieved the highest surface hydrophobicity, micro-
roughness and surface free energy, among the four FR models.
This nding indicates that the designed silicone/spherical Ag
nanocomposites exhibited superior FR properties and are
promising as environment-friendly materials.
Our nding might indicate that the low surface energy of the
silicone/spherical Ag nanoller design is mainly associated with
the existence of a high-density of {111} facets along the spher-
ical, face-centered-cubic (fcc) Ag nanocrystal domains, which
are different to other fcc metal NPs predominantly associated
along low-density {110} or {100} facets, as previously reported.49a
The Ag NPs orientation around {111} facets enabled a minimal
interfacial energy surface that effectively affected the selective
surface exposure properties and chemical activity of the nano-
ller coatings, leading to pronounced FR efficiency.49
The developed nanoller model also showed a profound
effect compared with our previous model of a Cu2O based sys-
tem.16a
The contact angle increased up to 148
Æ 1
, indicating
self-cleaning ability, whereas the surface free energy decreased
(approximately 10.73 mJ mÀ2
) and became highly resistant to
fouling. The biological and eld test results of the prepared
PDMS/Ag nanocomposites conrmed their extraordinary prop-
erties to retard fouling and prevent adhesion through a physical
adhesion failure mechanism without causing toxicity. The
promising AF adhesion and settlement results of the designed
FR nanocomposites indicate that they are an excellent eco-
friendly alternative to the existing AF systems.
3.5. Biological activity of the FR coating system based on
spherical Ag NPs
PDMS composites were believed to resist decomposition by
bacteria and living organisms. However, in the 20th century, a
study conrmed that biodegradation can be induced by living
organisms.50
PDMS biodegradation yields dimethylsilanediol
and, later, carbon dioxide and inorganic silicate.51
Nevertheless,
siloxane biodegradation remains a challenge because of the
lack of decomposition data available. Biodegradation studies
were conducted on the prepared spherical Ag/silicone nano-
composites by applying biodegradability tests for 30 days of
immersion with selected microorganisms. With various
concentrations starting from 0.01% up to higher loadings, the
biodegradability results were compared as shown in Fig. 5. The
biodegradability percentages were higher in the unlled PDMS
and decreased gradually until it reached nearly zero for the
0.1% loading in the PDMS/Ag nanocomposites. This nding
could be due to the good dispersion of the Ag NPs without any
agglomeration, which improved the surface hydrophobicity,
surface smoothing, and chemical bonding with the polymer
chains (as indicated by the Ra measurements in Fig. 3a). As a
result, the composites exhibited extraordinary FR characteris-
tics of resistance to adhesion of fouling organisms and
prevention of biofouling. By contrast, with increasing Ag
Fig. 4 Comparison of the prepared PDMS/Ag nanocomposites and
other commercial FR coatings.
Paper RSC Advances
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nanoller loading from 0.5% to 5%, biodegradability gradually
increased aer 30 days of immersion. This nding could be due
to agglomeration and aggregation at higher Ag nanosphere
concentrations.
Consequently, roughness increased and chemical bonding
with the polymer and hydrophobicity decreased to a lesser
extent because of the decreased surface area of the clustered
particles, thereby facilitating the ability of microorganisms to
attach to the surface. These results were conrmed by lm
coverage pictures taken using a polarized optical microscope
(ESI, Fig. S2†).
A eld exposure test was applied on the prepared PDMS/
spherical Ag nanocomposites (0.1% nanoller concentration
due to its superior FR properties), as illustrated in Fig. 6. In
these experimental studies, we used unlled PDMS as the
control (ESI, Fig. S3†). Fouling prevention against micro and
macro-fouling organisms occurred until the 12th month of
immersion. No adherence of fouling organisms and surface
deterioration were observed. In the formulation, aer 12
months of immersion, the few adhered microorganisms on the
edges were easily removed hydrodynamically because of the
superior FR performance of the novel PDMS/Ag nanocomposite
with renewable self-cleaning properties.
3.6. Mechanical feature tests
The cross cut test was performed without resulting visible
adhesion defects (ESI, Table S1†). The T-bend test was con-
ducted without visible cracking. No intrusion was identied
under a magnifying glass in any of the investigated coatings
aer penetration and bending on a 5 mm cylindrical spindle
(ESI, Table S1†). No cracks were observed during the impact
test, indicating the high elasticity and exibility of the tested
PDMS nanocomposites and the total formulation.
3.7. AF coating formulation for FR coating
The incorporation of pigments in marine coatings affects
various physical properties. Studies performed on pigmented
FR coatings revealed that the adhesion strength of the coating
increased with increasing pigment level up to 10 wt%.52
This
nding could be due to the increase in coating cohesive
strength and subsequent increase in tensile strength and
modulus caused by improved mechanical properties from
surface homogeneity. Increasing pigment levels reduces the
adhesion strength of the paint to the metallic surface because of
pigment aggregation and agglomeration at high percentages.53
Red iron oxide was selected because of its low toxicity, cheap
Fig. 5 Biodegradability measurements of the unfilled and filled PDMS/
Ag nanocomposites against various bacterial strains.
Fig. 7 (A)–(G) Field test results of the formulated coating for 12
months of immersion in natural seawater.
Fig. 6 (A)–(G) are the field exposure test results of the prepared Ag
(0.1%)/PDMS nanocomposites for 12 months of immersion in natural
sea water.
RSC Advances Paper
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cost, and anticorrosive properties.53
The pigment surfactant was
well-dispersed. A eld test of the total formulation was further
conducted in natural environments against various microor-
ganisms. The results showed superior fouling resistance and
prevention against macro-fouling of organism settlements
(Fig. 7). The rust around the edges was not found until 12
months.
4. Conclusions
This study demonstrates a simple, safe, cost-effective, and eco-
friendly preparation of spherical Ag NPs. To our knowledge, this
study is the rst to utilize Ag NPs in silicone FR coatings. This
study also elucidated the underlying mechanism, which is
based on developing a superhydrophobic surface that prevents
fouling from adhesion without leaching. The characterization
results of the prepared Ag NPs revealed a homogenous spherical
shape enclosed by {111} facets, particle sizes of less than 10 nm,
and well-distributed NPs without agglomeration. For the ship-
ping industry, this study provides insights regarding the capa-
bility of superhydrophobic PDMS/Ag nanocomposite coatings
to inhibit biofouling through their inertness and low surface
free energy. Advanced substrates were designed based on micro-
nano fabricated PDMS/Ag hybrid nanocomposites with various
nanoller loading concentrations. The surface properties rele-
vant to the developed Ag nanocomposites were reported on the
basis of contact angle determinations and OWRK equations.
The results demonstrated that a signicant increase was
observed at 0.1% loading, which exhibited the lotus effect, self-
cleaning, and FR property. By contrast, high loadings showed
severe deviation toward a lesser angle of contact, increased
surface coarseness, nonhomogeneity, and rough topology
because of agglomeration, thereby minimizing the FR ability.
Interestingly, no signicant effect on the viscoelastic properties
was observed at low concentrations of Ag NPs up to 0.1%. By
using a biological assay in the laboratory and eld exposure to
test seawater environments, the FR performance of the
prepared nanocomposites was studied.
The designed PDMS/Ag nanocomposite surfaces could ach-
ieve economical savings, considering the annual costs sus-
tained for controlling biofouling impact. The advantages of the
developed composite could contribute to prolonging longevity
and establishing strategies and investments in green technol-
ogies. This work strongly highlights that PDMS/Ag is a prom-
ising environment-friendly AF coating and exhibits high
potential for applications in FR silicon/noble metal nano-
composite technology.
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