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Unique Silicone-Epoxy Coatings for Both Fouling- and Drag-
Resistance in Abrasive Environments
Robert Baier, Mark Ricotta, Vincent Andolina, Faraaz Siraj, Robert Forsberg, Anne Meyer,
Center for Biosurfaces, State University of New York at Buffalo, Buffalo, NY 14214
Abstract
Multiple years of international trials in both oceanic and fresh water sites led to successful easy-
release coatings based on the methyl-silicone polymers now widely employed as substitutes for
tri-butyl-tin and copper-based ship bottom paints. These have proven to be too soft for harsh
conditions, and especially during abrasion, but do serve for useful periods in commercial and
military circumstances where abrasion is not frequent. This paper reviews abrasion-related
research of the past twenty years that identifies a novel version of silicone-based coatings
having a retained easy-release value of Critical Surface Tension, of about 26 mN/m,
compounded with a tough epoxy component that allows the two-component coating to survive
and function well in extremely abrasive circumstances. This coating has been applied as a rake-
scraped trash rack coating subject to intense zebra mussel fouling, as an airfoil blade coating
showing significantly lower drag than competitive paints, as a turbine encasement seal layer
remaining functional in zebra mussel-infested waters, and as an easy-release surface for flash-
frozen ice while also resisting damage by transit through ice floes. The coating is formulated
using polymeric methyl-silicone granules that are dispersed within an oil-in-water multiple
emulsion in an epoxy base that maintains excellent substratum adhesion while allowing the
methyl-silicone-based matter to dominate and be continuously refreshed via minimum wear at
the environmental interface.
Keywords
Silicone Abrasion-Resistant Fouling Release Ice-Release Drag-Reduction Non-Toxic
Coating
Introduction
Although prior work has led to the commercial and military acceptance of methyl-silicone-based,
non-toxic and easy-release coatings (1,2) for the minimization of biofouling concerns of the
world’s shipping and power industries, the best of these have proven to be too susceptible to
abrasion for routine service in ice fields, dusty or turbid conditions, trash racks and turbine
encasement seals—among other harsh environments. While improving the wear-resistant
qualities of these coatings, one must also maintain, or improve, the drag-reduction properties (3,
4). The former coatings, when applied to racks that later were physically scraped and water-
lance scoured to release accumulated zebra mussel (Dreissena sp.) debris and ice frazil
required only 5psi water-lance pressure to completely clean the coatings. The underlying
mechanical trash raking-induced damage to the coating was seen easily. In contrast, requiring
only 50psi water-lance pressure, a specific meld of silicone and epoxy ingredients produced a
hardy paint that released all debris without visual (or measured surface property) damage to the
coating. Conversely, similarly challenged field-exposed trash racks fabricated from carbon-
loaded high-density polyethylene, required over 2000psi water-lance pressure to remove most
of their debris, always leaving behind the zebra mussel byssus threads attached by the still
retained, adhesive byssus discs. Mussel byssus threads, terminated in adhesive discs, are the
proteinaceous “beards” seen protruding from the shells of mussels in both seawater and fresh
water. The best of the easy-release methyl-silicone-based coatings required only 5psi for
complete biofouling removal, byssus discs and all other deposits, but these coatings were also
the most readily abrasion damaged and released from their substrata.
The intermediate 50psi fouling-release-strength coating, not as biofouling-resistant as a pure
polydimethylsiloxane coating, was chosen for testing of its drag reduction properties in both
stagnation point flow and as an airfoil coating in a large towing tank (3,4), and for its ease of
shedding of flash-frozen ice droplets (5) in comparison with numerous comparative materials.
Table 1 and references (1) and (2) identify the comparative test materials. Only the coating that
is the subject of this paper was demonstrably able to resist concrete, zebra mussel, and hard
rubber reciprocating-seal-induced wear on cyclically operating turbine enclosures at a major
power plant (6).
Here we describe only the characterization of the selected coating by both compositional and
surface characterization criteria.
Materials
Originally introduced in response to the zebra mussel invasion of the Great Lakes in the early
1980’s (2), numerous compositions of epoxy and silicone were developed into water-based
emulsions for application to critical lake infrastructures and to transiting ships, to provide easy-
release of the encrusting mussels. The compositions are described in a compilation of early
reports of the zebra mussel appearance and consequences (7), basically encompassing
mixtures of epoxy and silicone ingredients in what are called “inverse emulsions” to produce
strongly adhesive coatings with otherwise exposed, and renewable, waxy surfaces. During a
novel government-industry-university cooperative program (8), the particular composition called
Wearlon 2020.98 was selected by intensive testing from a larger group of nontoxic
compositions.
According to its manufacturer (John Smith, Plastic Maritime Corporation, PO Box 2131, Wilton,
NY 12831, private communication),
“The Wearlon chemistry is based on taking two incompatible products—silicone and
epoxy, that when emulsified become compatible. With the addition of a curing agent, an
exothermic reaction occurs resulting in the breakdown of the emulsion, the release of
water, and the formation of a silicone-epoxy block copolymer. Different mole % of silicone
to epoxy results in a variety of products. The higher the silicone content to epoxy, the
softer the coating becomes. The curing agents, catalysts, surfactants, and wetting
agents, are the company’s proprietary information.”
Table 2 summarizes the characteristics of the best-performing coating in this series, designated
Wearlon 2020.98. Testing was carried out with coated aluminum, steel, and concrete surfaces
with deposits of Wearlon 2020.98 of 75 to 100 micrometers dry film thickness.
Methods
Coatings of the various supplied and numerous comparative materials were all characterized by
a combination of physico-chemical methods and performance tests. Specific chemical
compositions were characterized by Multiple Attenuated Internal Reflection InfraRed
Spectroscopy (MAIR-IR) (9), and the Critical Surface Tensions —and theoretical surface
energies-- of the coatings were derived from contact angle measurements with a series of highly
purified test fluids by methods now widely used in fouling-release paint testing (10). Details of
the contact angle liquids utilized, their sizes and reactivities, and the interpretive methodology
are found in that easily retrieved publication. Scanning electron photomicroscopy, and light
microscopy, produced the illustrative images necessary to observe each specimen’s surface
heterogeneity. Energy-dispersive x-ray analysis provided spot-by-spot identification of the
elemental abundances in each visualized region. The specific instrumentation utilized is named
in the prior references.
For stagnation point drag testing, specific laboratory devices were constructed and operated as
earlier described (1, 3), and--for airfoil drag testing -- helicopter rotor blades were individually
surface-prepared and tested (both before and after coating application) in a large water towing
tank (4).
For friction and wear studies, coated steel coupons were mounted into laboratory devices
originally developed for long-term testing of dental (toothbrush, resin, tooth) wear, both by
rotating (1) units and linear reciprocating units employing hard rubber (turbine chamber seal)
partners (6). The rotating unit was also utilized to test the ice-shard wear resistance of the
various coatings after frozen water-droplet push-off testing had revealed the 2020.98
composition to be the easiest-release for flash-frozen ice (5). Earlier trials with rime ice
impaction on rapidly rotating rods in a hot-water-condensing freezer chest had previously
indicated qualitatively that the 2020.98 coating would be a good candidate for easy ice release.
Results
Figure 1 illustrates the near-surface internal distributions of the silicone-based and epoxy-based
ingredients of the dried 2020.98 composition, illustrating a dual emulsion (7) spheroidal
separation of the silicone-dominated ingredient from the continuous-phase epoxy-rich
component responsible for excellent substratum adhesion with no required primer coats. Figure
2 is the MAIR-IR spectrum of the methylsilicone-dominated surface zone of the self-cured
water-borne the final, water-borne 2020.98 formulation. Figure 3 is a plot of the contact angle
data (cosines) against the liquid/vapor surface tensions of pure diagnostic liquids, leading to an
extrapolated intercept at the zero contact angle value (cos=1) of 26 mN/m, called the Critical
Surface Tension (CST) by the originator of this surface characterization method (11). CST
values between 20 & 30 mN/m have historically been associated with the easiest shedding of
attaching biological debris at low shear stresses (12). In the case of the 2020.98 formulation,
vigorous wear against hard rubber and other articulation substances actually “polished” the
formulation to an improved, lower-energy surface state by attacking the softer silicone-rich
granules and smearing their low-energy contents over the easy-release interface.
From stagnation point chamber studies, the 2020.98 formulation coatings showed the second
best results in terms of minimizing flow drag of water moving from its stagnation point at 90
degrees from contact with the test surface at a constant water pressure of 27 psi, second only to
a soft polyvinylysiloxane replica of roughened glass in stagnation point chamber tests of 21
different materials (Figure 4). In the water tunnel-based coated-airfoil trials, the 2020.98
formulation on airfoil rotor blades showed as much as a 5% drag reduction over the values for
the clean, smooth, uncoated helicopter rotor blades (3). This exceptional effectiveness was
attributed to the presence of a microscopic roughness of about 20 micrometers for the blade-
applied 2020.98 surface coating, trapping water-released air bubbles in the hydrophobic micro-
topographic valleys so that the actual shear took place through a water-air boundary layer of
lower viscosity than wholly liquid water (4).
Regarding ice adhesion studies, quantitation of attachment strength was obtained by placing
10-20 microliter droplets of water, where the water contact angles were measured in advance,
onto the surfaces of well-characterized reference materials in contact with a liquid nitrogen bath,
producing an average temperature approaching -20C, and then pushing off the diameter-
measured droplets in accord with a published ASTM test method (5, 13), assuring that the
pusher was flush with the substratum surface so that no peeling force was experienced.
Characteristics of the test materials are summarized in Table 3. Again, the 2020.98 formulation
gave the easiest-release values. The concurrent observation that the push-off strength was
slightly lower than that for isolated polydimethylsiloxane (PDMS) was again attributed to the
micro-roughness of the 2020.98 coating, trapping gas at the droplet base.
Anticipating use of these coatings in newly opening Arctic shipping lanes, coated steel coupons
were rotated at 60 revolutions per minute (RPM) through ice cubes piled at the air/water
interface of the same device used earlier for wear testing. When the 2020.98 coating was
compared with numerous coatings of the methyl silicone formulations earlier advocated (2), the
paint damage was the least after equivalent icy-water passages of many miles. Actual Arctic
field trials are now in the planning process.
Discussion and Conclusion
It is important to consider the large scope of the opportunity presented by these tougher-than-
normal coatings; [1] at a National Soaring Society meeting in San Diego some years ago,
polydimethylsilicone gel was distributed to participants to smear over their glider airfoil leading
edges, to release impacting insect “guts” and thereby increase “aloft” time. A world record was
claimed soon thereafter, but the coating’s longevity was disappointing. A more-adhesive,
permanent coating is desired; [2] while testing foul-release coatings at Pearl Harbor, Hawaii,
small-boat operators ferrying tourists from shore to the USS Arizona Memorial asked for a more-
hardy paint that could not only be fouling-free but also sufficiently strong to survive frequent
impacts of these boats with the shoreline docking facilities. Many related options for selection of
abrasion-survivable paints are available. Toward that goal, these are the key surface qualities
determined for the 2020.98 composition that has performed best in the described studies, as
determined by both the Zisman technique and the theoretical surface energy component
method of Kaelble (11, 14): Critical Surface Tension = 26 mN/m; dispersive component of the
total surface free energy = 22.8 mN/m; polar component of the total surface free energy = 5.2
mN/m; calculated composite total surface free energy = 28.0 mN/m. The observed water contact
angle against this paint’s surface is hydrophobic, at 103 degrees.
The results briefly noted here describe a “compromise” coating of silicone admixed with epoxy
that is not as intrinsically “easy-release” for biomass as the pure silicone coatings alone, but is
sufficiently wear-resistant (while renewing its surface features continuously) to serve in high-
abrasion settings.
In such circumstances, the abrasive qualities of the impacting environment actually provide
mechanical forces that assist in the removal of the otherwise deposited biomass. The
simultaneous shedding of ice, concomitant with resistance to ice impact and frictional damage,
suggests these hardy easy-(but not easiest)-release coatings will find their most rapid
commercial adoption for service in the opening Arctic environment. A concern is that ships that
acquire adverse biofouling layers in warmer waters may carry these foulants into cold water
regions, with possibly negative environmental consequences. Current sufficiently abrasion-
resistant ship hull paints to meet the present abrasion challenge, however, are among the most
fouling-prone and retentive coatings known—so should become rapidly replaced by the less
fouling- retentive, and low-drag, coatings described here.
Acknowledgments
Aspects of the work reported here were developed with support from the Office of Naval
Research, Grant N00014-89-J-3101 and Sea Grant/NOAA Projects R/EMS-2 and E/IF-1. We
thank Mr. John Smith of Plastic Maritime Co. for supplies of the paint ingredients, and Mr. Lucas
Latini for supply of the helicopter rotor blades and towing tank fixtures. Dr. Claes Lundgren
provided access to the Project THEMIS towing tank at University at Buffalo, and supervised
those tests.
References
[1] Baier, RE, Meyer AE, Forsberg RL. Certification of Properties of Nontoxic Fouling-Release
Coatings Exposed to Abrasion and Long-Term Immersion. Naval Research Reviews, XLIX (4):
60-65, 1997.
[2] Wells AW, Meyer, AE, Matousek JA, Baier, RE, Neuhauser EF. Nontoxic Foul-Release
Coatings for Zebra Mussel Control. Waterpower ’97: Proceedings of the International
Conference on Hydropower, American Society of Civil Engineers, Vol 1, pp 451-460, 1997.
[3] Baier RE, Meyer AE, Forsberg, RL, Ricotta, MS. Intrinsic Drag Reduction of Biofouling-
Resistant Coatings, Proceedings, Emerging Nonmetallic Materials for the Marine Environment.
US-Pacific Rim Workshop, US Office of Naval Research, Honolulu, HI, pp1-36 through 1-40,
1997.
[4] Ricotta, MS. Investigation of Possible Mechanisms for Inherent Drag Reduction by
Biofouling-Release Coatings, MS Thesis, Department of Mechanical and Aerospace
Engineering, State University of New York at Buffalo, 1998 (expanded conclusions in Baier, RE,
Ricotta, MS, Meyer AE, Forsberg RL, Latini, LJ, Pendergast DR, Drag Reduction by Easy
Release of Surface-Attached Foulants, Proceedings 25th Annual Meeting of The Adhesion
Society, pp 352-355, 2002.
[5] Siraj FM, Baier RE. Water Droplet Contact-Adhesion of Ice on Reference Materials,
Proceedings 39th Annual Meeting of The Adhesion Society, 2016.
[6] Andolina VL. Evaluation of a Hydrophobic ‘Easy-Release’ Silicone-Epoxy Coating for
Maintaining Underwater Sealing of a Sliding Steel/Neoprene/Steel Interface Subject to
Biofouling, MS Thesis, Biomaterials Graduate Program, State University of New York at Buffalo,
2012.
[7] Garti N, Smith J. New Non-Stick Epoxy-Silicone Water0Based Coatings, Part I: Physical and
Surface Properties, Proceedings of The Fifth International Zebra Mussel and Other Aquatic
Nuisance Organisms Conference, Toronto, Canada, pp 151-169, 1995.
[8] Hogan JN. Sea Grant Scholars: Sea Grant Helps Educate Future Decision Makers,
Coastlines, New York Sea Grant Institute, 26 (1), 32-33, 1996.
[9] Baier RE, Loeb GI.Multiple Parameters Characterizing Interfacial Films of a Protein
Analogue, Polymethylglutamate, in Polymer Characterization: Interdisciplinary Approaches. CD
Craver (ed), Plenum Press, NY pp 79-96, 1971.
[10] Meyer A, Baier R, Wood CD, Stein J, Truby K, Holm E, Montemarano J, Kavanagh J,
Nedved B, Smith C, Swain G, Weibe D. Contact Angle Anaomalies Indicate that Surface-Active
Eluates from Silicone Coatings Inhibit the Adhesive Mechanisms of Fouling Organisms,
Biofouling 22: 411-423, 2006.
[11] Zisman WA. Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution, in
Contact Angle, Wettability, and Adhesion, Advances in Chemistry Series 43: 1-51, 1964.
[12] Baier RE. Surface Behaviour of Biomaterials: The theta surface for Biocompatibility, J Mater
Sci: Mater Med 17: 1057-1062, 2006.
[13] ASTM D5618-94. Standard Test Method for Measurement of Barnacle Adhesion Strength in
Shear. Am Stand Test Mat, Paint, Tests for Formulated Products and Applied Coatings. Vol
06.01, 1994.
[14] Kaelble DH. Dispersion-Polar Surface Tension Properties of Organic Solids, J. Adhesion 2:
66-81, 1970.
Figure 1 – Characterization of Formulation 2020.98 coating by backscattering scanning electron
microscopy and energy-dispersive X-ray analysis. Circular particulate inclusions (upper and
lower right frames) have higher silicon content than the bulk phase.
Figure 2 - MAIR-IR spectrum of the surface zone of Formulation 2020.98.
Absorption band positions indicate dominance by methylsilicone components, as
confirmed by Critical Surface Tension data [see Figure 3].
Figure 3 – Contact angle data plot (Zisman Plot) for Formulation 2020.98.
Critical surface tension = 26 mN/m.
Figure 4 - Plot of average fill times in stagnation point flow cell. See Table 1 for descriptions of
materials specified on X-axis
Table 1 – Summary of Materials Used in Testing
(Stagnation Point Flow, Towing Tank, and/or Ice Adhesion Experiments)
Material Abbreviation
(as used in Figure 4) Description
Glass Control Glass microscope slide (detergent-washed)
RFGDT Glass Radiofrequency glow-discharge cleaned
glass, water-wettable
PM4545-76 Blade Wearlon formulation 76-coated helicopter
blade
RFGDT Rough Glass Glass microscope slide, sandblasted with 50-
micrometer silica and washed before RFGDT
Rough Glass Glass microscope slide, sandblasted with 50-
micrometer silica and washed
ODS Octadecylsilane-coated glass
DMS Dimethylsilane-coated glass
F-150 Naval Primer Epoxy primer paint (Navy formulation F-150)
on glass
PM2020-98 Wearlon formulation 98 on glass
PDMS (BACK)
Polydimethylsiloxane reference standard
from National Institutes of Health; smooth
side
PDMS Blade Polydimethylsiloxane coating on helicopter
blade
3-HEPT Glass Fluorosilane-coated glass
PM4545-76 Wearlon formulation 76 on glass
F1-M Wearlon formulation F1-M on glass
PDMS (FRONT) Polydimethylsiloxane reference standard
from National Institutes of Health; rough side
DC3140
Dow Corning formulation 3140 methylsilicone
on
smooth glass
PVS Kerr Corporation polyvinylsiloxane on smooth
glass
3-HEPT Rough Glass Fluorosilane coating on sandblasted glass
F1-M Blade Wearlon formulation F1-M on helicopter
blade
PM2020-98 Blade Wearlon formulation 98 on helicopter blade
PVS Rough Kerr Corporation polyvinylsiloxane replica of
sandblasted glass
Table 2 – Characteristics of Wearlon® PM2020.98
General composition: water-borne, epoxy-based dual emulsion; indicative elements: Si, O
Attributes: Low surface energy; low friction; abrasion resistance;
chemical/water/weather/corrosion resistance; excellent adhesion to metallic substrata
Table 3 – Surface Characteristics of Materials Used in Ice Adhesion Experiments
Material Critical
Surface
Tension
[mN/m]
Polar
Component
of Surface
Free Energy
[mN/m]
Dispersion
Component of
Surface Free
Energy
[mN/m]
Ice Adhesion
(Shear Strength)
in Laboratory
Experiments
[psi]
Polytetrafluoroethylene** 18 1 19 7
Polydimethylsiloxane
[PDMS]* 22 3 20 27
PM2020-98 (Wearlon)* 26 5 26 9
Low Density Polyethylene 30 2 29 26
Polystyrene (bacterial grade) 30 4 30 25
Fused Silica 31 49 32 34
Glass Control* 33 24 26 40
Pyrolytic Carbon 37 10 35 45
Titanium (commercially pure) 37 51 32 50
Calcium Hydroxyapatite 39 55 39 78
Stainless Steel (316L) 40 45 38 137
Mica 41 65 38 55
*also in Table 1 and Figure 4
** similar to 3-HEPT in Table 1 and Figure 4
