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Progress in Organic Coatings 72 (2011) 52– 57

Contents lists available at ScienceDirect

Progress in Organic Coatings

j ourna l ho me p ag e: www.elsev ier .com/ locate /porgcoat

elf-healing microcapsules and slow release microspheres in paints

amás Szabóa, Lívia Molnár-Nagyb, János Bognárb, Lajos Nyikosa, Judit Telegdia,∗

Institute of Surface Chemistry and Catalysis, Chemical Research Center, Hungarian Academy of Sciences, H-1025, Pusztaszeri út 59-67, Budapest, HungaryResearch Institute for Paint Industry Ltd., Hungary

r t i c l e i n f o

rticle history:eceived 16 December 2010eceived in revised form 22 March 2011ccepted 22 March 2011

eywords:icrocapsulesicrospheres

a b s t r a c t

Special microcontainers for producing self-healing and antifouling paints for corrosive and biocorro-sive environment were elaborated. Our work aimed to improve the efficiency of anticorrosion paints incases of abrasion and scratching with application of microcapsules filled with film former and corrosioninhibitor and, as well as of antifouling paints in underwater environment with application of slow-releasepolymeric microspheres filled with antifouling agent. First the preparation and characterization of micro-capsules/spheres were in the focus, then application in paints in order to see their anticorrosion andantifouling efficiency.

elf-healinglow-releasenticorrosionntifouling paints

The preparation of microcapsules and microspheres has been carried out by emulsion polymeriza-tion by traditional techniques. The characterization of microcontainers with different shells and coreswere done by complementary techniques. The morphology and the shell-thickness was visualized andmeasured by scanning and transmission electron microscopy (SEM, TEM), by fluorescence and opticalmicroscopy (FM, OM). The presence and the amount of encapsulated corrosion inhibitor and antifouling

nducspher

agent was measured by imicrocapsules and micro

. Introduction

All over the world the corrosion is a serious problem thatould be controlled by different methods. Classical methods thatan reduce the corrosion rate are cathodic protection [1], passi-ation [2] and barrier protection caused by coatings [3]. Coatingsn metal surfaces provide a barrier against the corrosive species.he effectiveness of the coating depends on its chemical composi-ion, on the parameters they are developed and on the permeabilityowards corrosive species. In the last decades water-borne paintsre replacing the solvent-borne ones because of the volatile compo-ent with health risk. Anticorrosion protection could be improvedy combination of different anticorrosion techniques. Corrosion

nhibitors built-in the water-borne paints help in diminishing theorrosion mainly in time of the drying period. Smart coatings thatould be alternatives to traditional ones can improve the protec-ion of metals, alloys and provide superior resistance to corrosion

specially then, when the coating is mechanically or chemicallytressed. When the paint is damaged either by mechanical forcesr by changes in the environment (pH, heat, light) the anticorro-ion additives involved into the dry paint cannot further controlhe corrosive processes. That is why researchers and industrial

∗ Corresponding author.E-mail address: telegdi@chemres.hu (J. Telegdi).

300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2011.03.014

tively coupled plasma spectrometer (ICP). The efficiency of coatings withes was checked on sandblasted mild steel and on glass in natural water.

© 2011 Elsevier B.V. All rights reserved.

experts try to find resolution for the extended protection. One res-olution is when the active anticorrosion chemicals are involvedinto the coating [4]. Another possibility is when the inhibitorsand sealants are encapsulated. The idea of microencapsulation ofactive ingredients was taken from the nature. In the pharmaceuticaland agricultural industry the capsulation is a well-known tech-nique. In case of paints microcapsules are added to the coatingmaterial [5,6]. When a mechanical damage weakens the coatings,the microcapsules will release their protective agents and healthe coating by preserving the barrier properties [7,8]. The dif-ferent copolymers (often urea-formaldehyde ones) can form thewall of containers; the corrosion inhibitors involved into it arewell studied [9,10]. In the case of paints there are several stud-ies [11–14] and patents [15] that describe the possible applicationof microcapsules as microcontainers in coatings which has a bigadvantage, as the active component i.e. the corrosion inhibitor, willbe available in the proper case and on the very place of injury. Theactive components will be free on pressure (mechanical destroy,abrasion, stress), on shell degradation (change in pH/temperature/ionic strength) and on rupture after swelling (solvent effect).

There are a number of advantages in using microencapsu-lated chemicals. A primary advantage is the ability to extend

the life expectance over a period of time to avoid the need forfrequent treatments. The other benefit of application of micro-capsules in coating systems is that the core material is protectedfrom the environment and their release is controlled, sustained ortimed.

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The microencapsulation is a combination of materials. The twoain components of a capsule are the shell and the core. The shellust be compatible with the components of paints; it could be

ermeable, semi-permeable or non-permeable. A permeable shellllows slow release of active additives from the core. This is goodn those cases when elongated active period is expected (e.g. inase of antifouling paints). When the shell is non-permeable, thective ingredient will get freed on special environmental effectsike mechanical injury, change in temperature, pH, etc. The chemi-als of core could be pigments, catalysts, polymeric substances, etc.he variety of chemicals in microcapsule shells is very wide whichllows the application of the most suitable ones in all cases. The sizef the microcapsules influences the permeability, diffusion and theontrolled release of active components of the core. The diversity ofhe capsulation techniques offers broad ranges of chemicals built-inhe capsules.

The other problem our research was focused on, is the controlf fouling process with special coatings. There are a lot of additivessed as antifoulants [16,17]. The most effectives are tin-derivativesut because of their toxicity they are banned. Nowadays severalommercial products (e.g. Zn pirithyon) and paints with high Cu2Ooncentration (which limits its use because of the copper accu-ulates in the natural waters) are used. The application of other

dditives in a so-called slow-release form was a challenge for us.hat is why our interest turned to the silver (its antibacterial activitys well-known) and to its involvement in permeable microspheres.n this case the concentration of the applied active material coulde decreased to a low level and a continuous antifouling effect is

nsured.Our work aimed to elaborate cost-effective, self-healing mate-

ials with low toxicity, to improve the efficiency of anticorrosionaints with application of microcapsules filled with film formerss well as with corrosion inhibitors. On the other hand, to developpecial microspheres for paints with slow release of the antifoulingdditive.

. Experimental

.1. Materials

Chemicals used for preparation of microcapsules are the fol-ows: urea, aldehydes as crosslinking agents, polyvinyl alcoholPVA), vegetable oil, inhibitors.

Chemicals used for preparation of microspheres for antifoulingurpose: natural polymer, crosslinking agent, urea, silver com-ound, vegetable oil.

The anticorrosion efficacy was followed on sand blasted mildteel plates.

To evaluate the effectiveness of paints with microspheres, atandard paint without any antifouling agent and a commercialntifouling paint mixture were used.

The organisms adhered onto the coated surfaces were observedith fluorescence microscope.

.2. Preparation of the microparticles

.2.1. Self-healing microcapsulesMicrocapsules for self-healing coatings were prepared by emul-

ion polymerization. The two non-miscible liquid phases wereir-drying vegetable oil and an aqueous solution. In a beaker urea

as dissolved in aqueous PVA solution and the vegetable oil was

mulsified with stirring or with sonication (Hielscher Ultrasoundechnology UP200S). To the emulsion crosslinking agent was addednder continuous stirring. The temperature of the reaction mixtureas raised to 70 ◦C and the reaction mixture was stirred at this

c Coatings 72 (2011) 52– 57 53

temperature for 5 h. As the reaction mixture cooled down to roomtemperature the capsules were separated by centrifugation (Sigma3–16), washed with diluted ethanol and with aqueous surfactantsolution to remove the uncapsuled oil. The cleaned capsules werestored in a dense suspension. When inhibitors were encapsulated,the procedure was similar to the previously described one, except,that the anticorrosion additives were mixed into the oily phase.

2.2.2. Microspheres of antifouling propertyThe preparation method is w/o type emulsion polymerization.

In distilled water natural polymer, urea and the silver compoundwere dissolved and the crosslinking agent was added with intensivestirring. Then this aqueous solution was poured into vegetable oilto form a stable emulsion. The emulsion was heated and stirred at80 ◦C for 5 h. The microspheres were dispersed in the oil withoutsedimentation. When the reaction mixture cooled down to roomtemperature, the oil was washed out, the dry microspheres wereground.

2.3. Techniques used for characterization and evaluation of grainsize and for chemical composition analysis

The structure and the diameter of the microcapsules were ana-lyzed by transmission electron microscope (Fei Morgagni 268D100 keV). Samples for TEM measurements were prepared by freeze-fracturing microtome (Balzers – BAF400). The morphology of themicrospheres was visualized by scanning electron microscope(Zeiss EVO 40 XVP) as well as by optical microscope (OM, Olym-pus BH-2). After acridine orange staining the presence of micro-and macroorganisms was visualized by fluorescence microscope(FM, Zeiss Axio Imager A1).

2.4. Functioning of microcapsules in paints

The microcapsules with self-healing additives were mixed intothe primer and then spread onto sandblasted metal plates. Thethickness of the dry coating was about 50 �m. The coated plateswere let to dry for a week. The painted surfaces were scratchedand placed into corrosive medium; the corrosion phenomena werevisually followed.

2.5. Functioning of microspheres in paints

Glasses coated by paints with microspheres mixed withantifouling additives (applied in two different concentrations) weredried for 2 days, dipped into natural lake water. Samples wereremoved from time to time and the macro- and microorganismswere fixed with ethyl alcohol and then stained by acridine orange.

3. Results and discussion

In the first set of experiments the formation of the microcapsuleswas followed and the reproducibility of the process was proven.Important question was the average diameter and the grain sizedistribution of the microcapsules.

3.1. Self-healing microcapsules and coating

3.1.1. Diameter, appearanceThe effect of emulsification type on the diameter of the emul-

sified droplets and the capsule size is shown in Fig. 1. It proves

that both ways of emulsification (stirring and sonication) result innarrow diameter distribution, but by sonication the average micro-capsule diameter is about 1–10 �m which is significantly smallerthan in the case of a stirring (25 �m). These date illuminate theimportance of the emulsification. In the further experiments the

54 T. Szabó et al. / Progress in Organic Coatings 72 (2011) 52– 57

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ig. 1. Diameter distribution of microcapsules prepared by stirring (a), and by son-cation (b).

onication technique was applied which resulted in very smallicrocapsules with proper size distribution.The other important factor which influences the application

ossibilities of capsules in paints is the shape of the capsules asell as the wall thickness. The SEM images in Fig. 2 show the micro-

apsules during the formation. The images present round shapedapsules of smooth surfaces at the beginning of the polymer filmormation. In the course of the shell formation more and more small

olymer particles cover the outer surface.

Scanning electron microscopy was performed to analyze notnly the surface morphology but to evaluate the shell thickness.n the SEM micrograph (Fig. 3) capsules with single wall (Fig. 3(a)),

Fig. 2. Microcapsules u

ig. 3. SEM image of microcapsules (filled with oil and inhibitors) with different wall thichell wall (c).

Fig. 4. TEM image of a replica copied from a freeze-fractured microcapsule whichis filled with oil.

with a shell built from massive excess of polymer (Fig. 3(b)), andwith double wall (Fig. 3(c)) are presented. A single shell wallseems to be fragile. The increase in the monomer concentrationsresulted in a little thicker but more granular microcapsule wallwhich showed that the excess of polymer does not increase thewall thickness significantly. Instead of that small polymer particlesare deposited onto the outer part of the shell. This is represented inFig. 3(b). On the other hand, when the polymerization process wasdoubled (in each case with the same monomer concentration) theshell thickness increased as the Fig. 3(c) demonstrates on a brokenmicrocapsule. The capsules with different wall thickness were sup-posed to elucidate the importance of the wall strength during theabrasion of paints filled with microcapsules.

By the SEM method the wall thickness was semi-qualitativelyevaluated. The TEM technique allowed us to measure the thicknessof the shell wall precisely. As Fig. 4 shows the single walled shellthickness is about 10 nm. The other information derived from theTEM measurement is that the core is not empty but filled with oil.

The core material was coloured for visualization of the inner partof the microcapsules. For colouring methyl red and fluorescent dyese.g. acridine orange and fluorene were used. The colours were veryuseful in the preliminary experiments when we wanted to prove

nder formation.

kness; single wall (a), capsule with massive wall thickness (b), capsule with double

T. Szabó et al. / Progress in Organic Coatings 72 (2011) 52– 57 55

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ig. 5. Microcapsules filled with the film-former; the stain is in case of (a): metil-filled microcapsules coloured with acridine orange. (For interpretation of the rrticle.)

he presence of the oil with or without anticorrosion additives inhe core (Fig. 5).

.1.2. Healing performance: microcapsules in paintsAfter mechanical crack the microcapsules incorporated into

aint should break immediately and release the healing core mate-ial. This self-healing process when a mechanical injury is followedy the escape of the core material from the capsule is recorded inig. 6. Fluorescence microscopic images of injured capsules showow the core material invades its environment, fill the gap, and healhe destroyed surface. The release of the oil from microcapsules cane seen in Fig. 6 fluorescence microscopic image. Capsules have areen halo because of the fluorescence. There are two perpendicu-ar scratches (cut with lancet) as deep as the thickness of the paint.s the figure shows, when the paint is scratched the self-healinggent leaks and additionally fills the gap.

This experiment proved us that these microcapsules will servehe purpose, i.e. when the coating with microcapsules will be

echanically damaged, along the scratch line the gap will be cov-red by the core material and it can prevent the metal surface from

direct contact with aggressive environment and so from a furtherorrosive deterioration.

.1.3. Effectiveness of self-healing coatings

The corrosion of a metal is caused by aggressive materials and

xygen in presence. When the coating cracks, the accessibility fororrosion accelerating components to the metal-coating interfaces easier. But when microcapsules are involved into the paint, due tohe healing process, the oil and the inhibitor from the capsules will

ig. 6. Release of film-forming material: optical (a) and fluorescence microscopicb) image of oil-filled microcapsules in colorless paint.

d, in case (b): transmission optical/reflection fluorescence microscopic image ofces to color in this figure legend, the reader is referred to the web version of the

be freed, activated and the consequence is that they prevent themetal from the corrosion. In our experiments microcapsules filledwith oil and corrosion inhibitor were mixed into the primer andcarbon steel coupons were coated by it. After complete drying thesurface was cross-cut up to the metal and the injured surface wascovered by electrolyte with sodium chloride and sodium sulphatesolution, pH 6.5. The control specimen were covered with the samepaint without microcapsules. Results are presented in Fig. 7. Longerinteraction of electrolyte with the injured metal surface withoutmicrocapsules caused serious deterioration of the coupon, but thepresence of the capsules could control the corrosion reactions. Thisis the first result, in the near future experiments with microcapsuleswith different properties (wall thickness, grain size, etc.) will beperformed to elaborate the preparation of capsules with the mostproper properties.

3.2. Antifouling microspheres and coating

The ban of tin-based antifouling paints increased the applicationof formulas with Cu2O more and more. But it also has a disad-vantage, i.e. in these paints the concentration of Cu2O is high. Theconsequence is that the copper load of the natural waters increases.This is why we tried to replace the Cu2O with other additives thatcould replace the Cu O and their efficiency could be similar to its.

In order to achieve this result the topic we worked on was to con-trol the adhesion of micro- and microorganisms onto coatings withmicrospheres filled with special additives which were supposed toinhibit the adhesion of different organisms by slow-release mech-

Fig. 7. Coatings with and without microcapsules after different exposure periods.

56 T. Szabó et al. / Progress in Organic Coatings 72 (2011) 52– 57

Table 1Dissolution of the antifoulant into natural water.

2 week 3 week 4 week 5 week1.11 1.59 1.81 2.01

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Concentration of silver in release experiment [mg/l]

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Ag concentration in dist. water [mg/l] 0.79

nisms. The active ingredient applied in our experiments interactsith the outer membrane of the organisms; it inhibits the enzymes

esponsible for the cell wall synthesis and, after its penetration,nzymes within the cell.

.2.1. Diameter, appearanceFig. 8 shows the SEM image of microspheres. The microspheres

ave a narrow diameter distribution with an average value of–4 �m. In fact, the shape of the ‘spheres’ is more polyhedral thanpherical because of the preparation method: before drying processhe spheres are wet and swollen, soft enough to become deformed.he drying process fixes this deformed shape.

.2.2. Release of antifouling agentThe question arisen was how quick is the dissolution of the

ctive material from the inert microsphere? In the first set of exper-

ments the slow release process of the antifoulant was followedTable 1). As ICP measurement data show, there is a continuousissolution of the antifoulant which means that a continuous fluxf inhibitor material is expected. Glass panels were coated by paintithout and with microspheres and placed into natural waters. At

ig. 9. Antifouling experiments: coating without any antifouling additive (a), with tradndicate 1 mm. (For interpretation of the references to color in this figure legend, the read

Fig. 8. SEM image of microspheres with silver load.

itional additive (b) and with microspheres (c). The yellow scales on the pictureser is referred to the web version of the article.)

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ifferent exposure periods the coated surfaces were stained and thedhesion of micro- and macroorganisms were visualized by fluo-escence microscope. Fig. 9 summarizes the images that presenthe adhesion of organisms on coated surfaces.

The images prove that on the coated surface without antifoulantdditive the mass of the adhered organisms increase in time dra-atically (Fig. 9(a)). The traditional antifoulant has inhibitive effect,

ut the effectiveness deceases with increasing time (Fig. 9(b)). Theed spots that represent the adhered organisms appear much latern the surface than on uninhibited paint panels but after 15 weekshousand and thousand organisms adhere to the painted panel.he presence of microspheres in the coating inhibits the adhesionith a constant rate (Fig. 9(c)), i.e. the efficacy does not decrease

n time which proves the slow-release of the active componentwhich was demonstrated by the preliminary dissolution experi-

ents (Table 1)). The dark spots on the pictures of Fig. 9(c) markhe microspheres in the paint. The continuous dissolution of thective inhibitor of adhesion – proven by ICP measurement results

are reflected in the dramatically decreased amount of adheredicro- and macroorganisms, i.e. in the fouling. It also shows that

here are significant differences between the types of applicationf active materials. In the case when the same active ingredients distributed in the paint, the inhibitor is quicker dissolved in thenvironment and the coating is earlier exhausted. In the other casehe microspheres allow a continuous dissolution of the inhibitor,nd this slow-release process allows a longer antifouling activity ofhe coating.

In the future experiments the effectiveness of the additives andhe permeability of the microspheres will be varied in order to findhe best composition for migration of the active ingredients fromhe microspheres.

. Conclusion

The microencapsulation experiment results show that micro-apsules with both film-forming material and corrosion inhibitors well as microspheres with anti-fouling agent were successfullyroduced.

The physical and chemical characteristics of microcapsules and

icrospheres that could influence their application were deter-ined. Experiments that elucidate their applicability prove that

n case of mechanical injury the microcapsules built in paint looseheir core material which fills the scratch line and so the corrosion isecreased by a self-healing mechanism. The fouling of the coatings

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c Coatings 72 (2011) 52– 57 57

built from the same paint depends significantly on the ingredi-ents. Films formed from paints without antiadhesion additiveswere invaded by organisms after 15 weeks. When the antifoulantwas homogeneously dissolved/distributed in the paint, the coat-ing was earlier exhausted than in the other case, when the activematerial was masked in microspheres which allowed a continuousslow-release of the antifoulant. In summary, we can conclude thatthe microspheres with antifoulant built in coating can successfullycontrol/reduce the adhesion of micro- and macroorganisms. Theelongated inhibition is due to a slow-release process.

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