Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Bryan E. Koene et al.
SELF HEALING COATINGS FOR CORROSION
CONTROL
Bryan E. Koene, Shi-Hau Own, Kristen Selde
Luna Innovations, Incorporated. Blacksburg, VA 24060 USA
Tel: 1-540-558-1699
Fax: 1-540-951-0760
koeneb@lunainnovations.com
http://www.lunainnovations.com
In the US alone, the annual estimate for corrosion related costs is $276 billion. Much of these costs are
associated with the maintenance of metal materials; 20% of the estimated corrosion-related costs are related to
scraping and repainting steel structures. Because of the staggering costs stemming from corrosion of steel
infrastructure, there is a tremendous need to develop intelligent coatings that can perform numerous functions
above those historically demanded of coatings. Several research efforts, including those of Luna Innovations
Incorporated, have demonstrated the ability for coatings and other polymer materials to “self-heal” after a
damage event. In many cases, however, the protective properties of the repaired area of the coating are often
degraded from those of the qualified paint system itself. This is particularly true with respect to corrosion
resistance and foul resistance. Luna is addressing this need to enhance the service life of metal structures for
aerospace, marine, and surface applications. Luna’s coating system will self-heal damaged areas and supply
corrosion inhibitors to halt corrosion damage. This multifunctional coating will extend the service life of
metallic structures and vehicles. The technology described in this paper will decrease life cycle costs, reduce
maintenance, and increase readiness by limiting equipment down-time.
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Introduction
Organic coatings primarily perform as a barrier to protect the underlying substrate. Advances
over the past 50 years in organic / polymeric coating technology have certainly improved
coating durability in surface protection. However, damage to these coatings through
mechanical abrasion, scratches, impact, etc. can result in a point of breach for penetration to
the surface. Corrosive entities such as acid rain, coastal salt water, or industrial effluents can
enter through the damaged sites to degrade the substrate. Metals are particularly susceptible
to these corrosive materials, due to their natural vulnerability to oxidative attack. Because of
the staggering costs stemming from corrosion of steel infrastructure, there is a tremendous
need to develop intelligent coatings with extended performance in corrosion prevention. In
the past five years, there has been much attention focused on the research of ‘self healing’
coatings that have the ability to repair a coating in situ after the coating has been damaged.
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Self healing coatings
Recent research in self healing coatings is inspired by natural healing processes. That is, the
ability for a surface to repair itself is mimicked after naturally occurring or biological systems.
This type of approach is generally termed biomimetic or bioinspired, although non-living
systems have also shown similar repair mechanisms.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Bryan E. Koene et al.
The very use of protective coatings over various substrates is based upon natural systems. For
example, animal skin, cell membranes, bark on trees, wax on plants, etc. all protect the
underlying substrates.
In a similar way, synthetic coatings have been developed to protect the substrates on which
they are deposited. The main difference with natural protective systems and manmade
analogues is their ability to repair themselves after damage. The self healing of skin occurs
naturally to reproduce a basically identical surface. Certain plants continually renew the
surfaces of their leaves with waxy residue to prevent waterborne contaminant growth such as
fungus. Thus, the research in the area of self healing coatings was inspired by these natural
systems to add this functionality to protective surfaces.
The main class of self healing coatings described herein includes those prepared with the
incorporation of an encapsulated ‘healing’ agent, which is subsequently released upon
damage of the coating. These encapsulation methods have been demonstrated to successfully
heal materials after selected events of cuts, scribes, impacts, and deep scuffs on the surface of
the coatings. These healing materials will re-form the coating over the damaged area,
resulting in a protective surface comparable to that of the original undamaged surface. In
these coatings, the only initiation source to heal the material is the initial damage event.
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Encapsulation for self healing coatings
The use of encapsulated self healing materials was originally developed by White et al from
University of Illinois. The idea is to embed an encapsulated monomer, dicyclopentadiene
(DCPD), into a thermoset composite system. Once cracks were formed in the composite, the
capsules rupture, and the DCPD flows into the crack plane via capillary action. The monomer
then comes in contact with a catalyst present in the resin, which enables the polymerization
into a solid. This innovative concept has been described to be similar to the ‘self healing’ of
cracks in bones (a composite of rigid inorganic hydroxyapatite, collagen, and other flexible
organic components). Since the initial publication, several other articles have further
developed this concept.,,,
The transfer of the successful encapsulated monomer process to a coating application has
unique complications. Whereas a similar concept can perform well in a coating, it is
somewhat more difficult due to the low coating thickness, typically 25-200 µm thick
compared to the thickness of a composite, typically 0.15-2.5 cm thick. The composite
thickness allows large capsules 50-500 µm diameter to be incorporated without negatively
affecting its mechanical properties. Larger capsules will hold more volume fluid per mass,
and therefore have a greater healing ability. In a coating, the sizes of the capsules are limited
to the thickness of the resultant dry coating. Furthermore, if the diameters of the capsules are
on the same order of magnitude as the thickness of the coating, physical properties may be
degraded. The optimum size of capsules for coating properties, yet to have enough material
to heal a coating is about 5-50 µm.
US Army researchers Kumar and Stephenson recently demonstrated a self-healing, corrosioninhibiting coating system for use on outdoor steel structures.,, This was expanded to include a
lead dust suppression feature for painting existing infrastructure in a subsequent work by the
same authors., Similar research in the area of encapsulated self healing coatings is currently
performed at Luna Innovations under a Small Business Innovation Research (SBIR) program.
This concept is similar to the self healing composites with encapsulated monomers, except
that their use will be for protective coating applications. The capsules are broken under stress
such as a scratch in the coating.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Bryan E. Koene et al.
The monomers must be able to flow and polymerize under ambient conditions to form a
protective layer, with similar functionality as the undamaged coating.
One of the more promising class of materials for this application are drying oils, which are
already well known for use in paints and varnishes. The term drying is misleading since the
film forming is via an oxidative process. The monomers first oxidize across the unsaturated
alkene bonds to form polymeric chains. Higher degrees of unsaturation allow further
crosslinking between adjacent to occur. These materials are ideal since they are produced
from naturally occurring starting materials, inexpensive, susceptible to autoxidation in air, and
they will form a dense, continuous film that can prevent penetration by contaminants. Also,
these hydrophobic oils are easily encapsulated using conventional water-oil emulsion
encapsulation techniques.
Drying oils are typically unsaturated fatty acid tri-esters of glycerol as in Figure 1. The
degree of unsaturation on the fatty acid determines its autoxidation potential and quality of the
resultant films. The more unsaturated character, particularly in conjugation, increases its
ability to be oxidized. Further, more unsaturation will increase the level of crosslinking of the
resultant coating. The polymerization and subsequent crosslinking of the drying oils is
relatively slow and may take days to weeks to form a protective layer. Catalysts incorporated
within the coating matrix are commonly used to speed up this reaction. These include ionic
complexes of cobalt, lead, manganese, zirconium, and others.
O
CH2 O
C
CH
CH
CH
CH CH
CH (CH2)3-CH3
7
CH
CH
CH
CH CH
CH (CH2)3-CH3
7
CH
CH CH
CH CH
CH (CH2)3-CH3
CH2
7
CH2
CH2
O
CH
O
C
O
CH2
O
C
Figure 1: Example of a drying oil: glycerol trimester of eleosteric acid - main component of tung oil
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Encapsulation process
As described previously, monomers flow out of ruptured capsules into damaged areas to
repair coatings via autoxidation processes. Besides being a vessel to contain the liquid
components, the shell wall of the capsule can protect the monomers. The shell walls provide
a barrier to either permeants (water, oxygen) entering or ingredients exiting the capsule, as
well as potentially blocking light or UV that may cause premature polymerization.
The capsules must possess significant structural and chemical integrity in order to withstand
normal stress of application and use in a coating. For example, capsules must be resistant to
solvents in the paint or coating solution, as well as be able to endure the shear of application
e.g. high pressure spray equipment. The capsules must also be able to endure normal
handling and incidental impacts as well as weathering. However, under shear experienced
when a coating is scratched down to the metal substrate, the capsules must rupture to release
their contents into the damaged area.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Bryan E. Koene et al.
Encapsulation is a widely used technique for incorporating desired components into protective
shells. Applications of encapsulated materials include food additives, pharmaceuticals,
industrial, agriculture, and consumer products. Encapsulation methods are divided into two
general categories – physical and chemical encapsulation. Chemical methods have been used
almost exclusively for self healing coatings due to the desired attainable size range.
There is some question as to the optimum size to achieve good coating properties and have
enough fluid to heal a damaged area. Generally, the capsules should be less than about
100μm (smaller for thin coatings) and larger than 5 µm for healing.8,
The common chemical methods that produce capsules of an appropriate size include
interfacial polymerization, in situ polymerization, solvent evaporation, and complex
coacervation. All of these methods involve the use of oil in water micro-emulsions generated
by physical agitation or homogenization (Figure 2). Each of these methods has advantages
and disadvantages depending upon the desired end properties. For example, the hardness,
modulus, and permeability of the shell can have an effect on the resultant healing properties.
Also, the chemistry of the shell can be used such that it has compatibility with the coating
matrix to which it is added. The encapsulation of self healing materials is shown in Figure 3.
Size range for these materials can easily be varied to achieve the desired size between about
1-500 µm with a narrow size distribution.
Figure 2: Oil in water emulsion via mechanical agitation
Figure 3: Optical microscopic images of encapsulation by complex coacervation11
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
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Bryan E. Koene et al.
Incorporation of microcapsules into coatings
Microcapsules containing the healing ingredients have been mixed directly into selected
waterborne and solvent borne paint systems (epoxy and polyurethanes).8,11 In the case of
corrosion prevention, the co-encapsulation of corrosion inhibitors have been included to
ensure that the resultant healed coating possesses the same inhibition to corrosion as the
original coating.8,11,16, This blend can be dispersed and applied via conventional coating
methods.
A Scribe damage
B
Damaged area is
completely healed
50 μm
Figure 1: Environmental Scanning Electron Micrograph (ESEM) showing the self-healing of a damaged polymer
A. Damaged polymer surface; B. Healed polymer by self-healing
In an effort to observe the healing process, coatings were analyzed using an environmental
scanning electron microscope (ESEM). A scratching probe was set up in the ESEM for
evaluation of the self healing characteristics of the coatings. Figure 4 shows two captured
images from a video taken showing the healing event. Control coatings were also evaluated
that showed that this healing effect was not a result of polymer swelling or artifacts of
imaging at different temperatures.
Salt fog corrosion testing (ASTM B117; ASTM D5894) was performed on steel panels coated
with primers containing self healing microcapsules. The panels were scribed to bare metal
prior to testing to allow corrosive salt water to corrode the damaged area. Evaluation of the
panels indicate that the inclusion self healing material significantly delays the time to initial
corrosion. Figure 5 shows that after 500 hours in the salt fog environment, the sample with
the control primer has significant generation of iron oxide at the scribe due to reaction with
salt water, whereas the ones loaded with self healing capsules have minimal visible
corrosion.11
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Bryan E. Koene et al.
Coating containing selfhealing microcapsules
Control coating without
microcapsules
Figure 5: Salt fog corrosion testing per ASTM B117 results in delayed corrosion behavior with microcapsule
incorporation in epoxy primed steel panels
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Conclusions
There has been significant research in this exciting area of self healing coatings, particularly
in the past five years. Synthetic methods are being developed to produce materials with
desirable healing properties while maintaining high performance as protective coatings.
Accelerated aging testing has shown that corrosion is considerably delayed and reduced.
Developments in self healing technology have opened a new area of multifunctional coatings
with the potential to increase the lifetime and reduce the enormous costs associated with
maintenance of protective coatings.
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© Springer 2007