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. 1 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. 2 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. 1 Springer 2007
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. 3 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. 2 Springer 2007
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 CH 2 O C CH 2 CH CH 7 CH CH CH CH (CH 2 ) 3 -CH 3 O CH O C O CH 2 CH CH 7 CH CH CH CH (CH 2 ) 3 -CH 3 CH 2 O C CH 2 CH CH CH CH CH CH 7 (CH 2 ) 3 -CH 3 Figure 1: Example of a drying oil: glycerol trimester of eleosteric acid - main component of tung oil 4 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. 3 Springer 2007
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 coacervation 11 4 Springer 2007
5 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 5 Springer 2007
Control coating without microcapsules Coating containing selfhealing microcapsules Figure 5: Salt fog corrosion testing per ASTM B117 results in delayed corrosion behavior with microcapsule incorporation in epoxy primed steel panels 6 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. 6 Springer 2007