Novel Polyurea Microcapsules Using Dendritic Functional Monomer: Synthesis, Characterization, and...

Novel Polyurea Microcapsules Using Dendritic Functional Monomer:Synthesis, Characterization, and Its Use in Self-healing andAnticorrosive Polyurethane CoatingsPyus D. Tatiya, Rahul K Hedaoo, Pramod P. Mahulikar, and Vikas V. Gite*

Department of Polymer Chemistry, School of Chemical Sciences, North Maharashtra University, Jalgaon, Maharashtra, India-425 001

*S Supporting Information

ABSTRACT: Polyamidoamine (PAMAM) dendrimer of zero generation was synthesized and characterized by Fouriertransform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopic techniques. A novel chemistry has beendeveloped to synthesize polyurea microcapsules containing solvent and linseed oil as the active healing agent by interfacialpolymerization of commercial methylene diphenyl diisocyanate (MDI) and dendritic 0.0 G PAMAM capable of cross-linking toform a shell material. Spherical with some irregular shape microcapsules were observed with average diameter from 20 to 270 μmat different agitation rates (3000−8000 rpm). Interfacial interaction between polyurea microcapsules and polyurethane (PU)coating were studied by FTIR, and this showed that chemical bonds were formed by the reaction between isocyanates and theamine groups present on the wall of microcapsules. The thermal stability of the microcapsules showed that preparedmicrocapsules experienced excellent stability up to 380 °C. The anticorrosive performance of PU coatingd loaded with differentpercentages of microcapsules was carried out in 5% NaCl aqueous solution. The results showed that the composite providessatisfactory anticorrosive properties at 5% capsule loading under an accelerated corrosion process. The idea and approachpresented in this work have the potential to fabricate microcapsules which could provide better anticorrosive and mechanicalproperties to coating composites.

■ INTRODUCTIONFor last three decades, highly branched dendritic polymers suchas hyperbranched or dendrimers have been paid great attentionbecause of their unique molecular architecture.1−4 Linearpolymers, synthesized by the classical polymerization processare usually random in nature and produces molecules ofdifferent size having some smaller or longer branches. Incontrast to traditional linear polymers, dendritic polymersexhibit significantly improved physical and chemical properties.As a result of controlled size, more functional end groups, andglobular and nonentangled structures, dendritic polymers showmany useful properties like interior cavities, high reactivity,decreased melt and solution viscosity, good solubility, etc., ascompared to their linear analogs of similar molecular weight.5

They can also be tuned with respect to functionality andpolarity to adjust the properties for certain applications.6

This special architecture and behavior make dendriticpolymers suitable for various applications in different fieldslike adhesives,7 coatings,8 catalysts,9 chemical sensors,10 lightharvesting materials,11 drug delivery,12 etc. Use of highlybranched polymers (hyperbranched and dendrimers) enhancedthe curing rate of epoxy resin,13a,b improved the flow andperformance characteristics of reduced volatile organiccompound (VOC) coatings,14 lead to excellent drying withlow viscosity in the field of alkyd, rapid curing in polyurethanes(PUs), and dramatically increased toughening in amine curedepoxies.15 Dendrimer (0 G polyamidoamine (PAMAM)) hasbeen recently reported as a cross-linking agent and inducedbetter gas separation performance for polyimide film.16 Che etal. have demonstrated the effect of single-walled carbonnanotubes (CNTs) functionalized with PAMAM dendrimer

in epoxy composite.17 The result revealed that the graft fromPAMAM acts as a covalent matrix binding agent and alsoenhances the dispersion of CNT which tends to increase themechanical properties of composite. In the recent years,considerable advances have been made in using the uniquearchitecture of a dendrimer in a host−guest system to protectand carry different materials.18

Microcapsulation19 in several fields, including agriculture,food, pharmaceuticals, cosmetics, printing, textiles, andprotective/smart coatings, has been used to encapsulate manydifferent active materials like pesticides, flavors, drugs, enzymes,inks, dyes, and healing agents.20,21 Microcapsules have beenprepared from a variety of material such as PU, polyurea, urea-formaldehyde (UF), phenol formaldehyde (PF), melamineformaldehyde (MF), etc.22 Although polyurethanes are a groupof versatile polymeric materials having excellent physical,chemical, and mechanical properties and are demanded in thefield of industrial coatings,23 the research on their performanceenhancement for anticorrosive nature is in progress. In the fieldof polymeric composites, microcapsules containing self-healingagents have received more attention as protective/smartcoatings. Huang et al. synthesized PU microcapsules,containing hexamethylene diisocyanate (HDI) as a corematerial and embedded in polymeric composites to achievethe self-healing properties and improved corrosion resistance ofthe coatings.24 The main features required in preparation of

Received: July 10, 2012Revised: December 5, 2012Accepted: December 18, 2012Published: December 18, 2012

Article

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© 2012 American Chemical Society 1562 dx.doi.org/10.1021/ie301813a | Ind. Eng. Chem. Res. 2013, 52, 1562−1570

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self-healing composites are that the microcapsules which act asself-healing reservoirs must possess sufficient strength in orderto withstand the stress of application, impermeable shell wall toprevent leakages, and diffusion of liquid healing agent alongwith high bond capacity to host polymer.25 Interfacialinteraction between microcapsules and polymer matrix alsoplays an important role to decide the good mechanicalproperties and self-healing function of a composite coating. Liet al demonstrated that PF microcapsules modified with a silanecoupling agent improve the interfacial performance of a PFmicrocapsule−epoxy composite.26 A striking contribution byKlug and Weisser27 reported the possibility of using guanidinecompounds and amine as cross-linkers for fabrication of robustpolyurea microcapsules which motivated us to use of dendriticpoly(amidoamine), due to the similarity in its chemicalfunctionality to guanidine compounds. Therefore, we decidedto use low generation PAMAM dendrimer for the preparationof polyurea microcapsules which can serve as a functionalmonomer as well as a cross-linker.The present work deals with a new approach to form

polyurea microcapsules via interfacial polymerization ofaromatic diisocyanate (MDI) and zero generation dendriticPAMAM with several amino groups capable to form a cross-linked polyurea shell wall. The main objective set in usingdendritic polymer as a one of the reactants for preparation ofmicrocapsules is to encapsulate a self-healing agent with a shellhaving amine functional groups that will provide good strengthand better interfacial interaction through cross-linked structureand chemical bonding with polymer binder (Figure 1). To our

knowledge, this is the first report of preparation of micro-capsules to encapsulate a healing agent by interfacial polymer-ization between diisocyanate and multifunctional PAMAMdendrimer. In service of this objective, hydrophobic linseed oil(as a self-healing agent) and suitable solvent was utilized as acore material. The performance of formed microcapsules forcorrosion protection in a PU coating by healing of cracks wasevaluated by immersion studies.

■ MATERIAL AND METHODS

Materials. The materials used in experiments includemethylene diphenyl diisocyanate (Kishore Polyurethanes Pvt.Ltd., Nasik, India) was of commercial grade and used as such.Ethylenediamine (EDA) and methacrylate was purchased froms.d. fine Chemicals Ltd., Mumbai, India. Poly(vinyl alcohol)and xylene were procured from Loba Chemicals, Mumbai,India. Linseed oil was used as received from Calf Brand, Pune,India. Acrylic polyol SETALUX 1196 VV-60YA and DesmodurW were used as received from Nuplex resin, USA, and BayerMaterial Science.

■ EXPERIMENTAL SYNTHESIS OF MICROCAPSULES

Synthesis of 0.0 G PAMAM Dendrimer. Synthesis ofPAMAM dendrimer was carried out in two-step process. Thefirst step was Michael addition reaction between ethylenedi-amine (EDA) as an initiator core and methacrylate resulting totetra ester, and the second step was amidation of the resultingesters with a large excess of ethylenediamine.28a,b

The first step involved in synthesis of half generation (−0.5G) PAMAM by attaching four acrylate moieties on each aminogroup of ethylenediamine (EDA) (Scheme 1). To a 150 mL,two neck flask equipped with condenser, stirrer, andthermometer 24.1 g of methacrylate (0.28 mols) was takenand to it 4.0 g of ethylene diamine dissolved in methanol addedat room temperature with stirring. The mixture was thenallowed to stand for 48 h at room temperature and at the endexcess methacrylate was removed by vacuum distillation.The second step involved synthesis of zero generation

PAMAM by amidation of four terminal carbomethoxy(−COCH3) groups of −0.5 G formed in first step with EDA.To a 1 L reaction flask containing 65.42 g of ethylenediaminedissolved in methanol was added the −0.5 G PAMAM (10 g)dissolved in methanol. The mixture was then allowed to stand

Figure 1. Interaction between microcapsule and PU matrix.

Scheme 1. Synthesis of PAMAM Dendrimer up to Zero Generation

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301813a | Ind. Eng. Chem. Res. 2013, 52, 1562−15701563

under stirring for 55 h at room temperature. After completionof reaction, excess EDA was removed by vacuum distillation.The compound obtained was referred as “0.0 G PAMAM”.Preparation of Microcapsules. Microcapsules were

prepared by interfacial polymerization of an oil-in-wateremulsion technique. About 50 mL of aqueous solutioncontaining 1.0 wt % of PVA as a protective colloid wasadded into a 100 mL flask. Then 1.5 g of MDI and 2 g oflinseed oil were dissolved into 10 mL of xylene in order toobtain organic phase. Subsequently, both organic and aqueousphases were mixed and subjected to high speed disperser atdefined repititions per minute (3000, 5000, and 8000 rpm) andtemperature of 25 °C for 5 min to obtain stable emulsion.Simultaneously PAMAM solution was prepared by adding 0.9 g(0.005 M) of PAMAM in 10 mL water containing 1 wt % ofPVA. Then, emulsion was stirred at 300 rpm in a three neckround-bottom flask and PAMAM solution was added dropwise.Addition was continued for 5 min under agitation. The reactionwas maintained at ambient temperature around 30 °C understirring for next 30 min and then for next 1.5 h at temperaturearound 45−50 °C. Scheme 2 represents the shell wall formingreaction. Formation of microcapsules was checked by observingreaction mixture time to time under optical microscope. After1.5 h when stable microcapsules were observed, reactionmixture was allowed to cool at room temperature understirring. Then, microcapsules from the suspension wererecovered by filtration under vacuum, rinsed with water,washed with xylene to remove suspended oil, and dried undervacuum.Preparation of PU Coating Loaded With Micro-

capsule. Polyurea/PU coatings were prepared by dispersingpolyurea microcapsules (from 0 to 5%) in 40 wt % solution ofacrylic polyol (Setalux 1196 VV-60YA) in xylene withDesmodur W (H12MDI) (20 w % of polyol) and catalystdibutyltin dilaurate. The coatings were applied by brush on allsides of steel panels of dimension (150 mm × 70 mm × 1 mm)and allow to cure at room temperature for 24 h.Characterization. Spectra of the samples were recorded on

a Fourier transform infrared (FTIR) spectrophotometer(Shimadzu 8400, Japan) by using KBr pellets. Preliminary

observations and morphological study of microcapsules werecompleted on optical microscope (Labomed Sigma, 2124001,Texas) on 40−100× resolutions. Microcapsules were observedfor their morphological study under scanning electronmicroscopy (JEOL JSM 6360 and JEOL JSM 5400, Japan).Particle size of microcapsules was analyzed by using laserparticle size analyzer (Mastersizer 2000, M41100167, Malvern,UK). Thermal decomposition of microcapsules was carried outusing thermo gravimetric analyzer (Shimadzu TGA 50, Japan)by heating the sample from room temperature to 800 °C at theheating rate of 10 °C/min in an inert nitrogen environment.

Determination of Core Content. The amount of activematerial in microcapsules was determined by xylene extractionof core material using a Soxhlet apparatus. First, a certainamount of dried microcapsules (Wm) was sealed in a filterpaper bag. Then the sample bag (Ws) was placed in a Soxhletapparatus. The weight of round-bottom flask was noted (Wo).Extraction was carried out with xylene as a solvent for linseedoil. After 6 h of extraction, the sample bag was carefully takenout from Soxhlet apparatus and after completely wearing thesolvent; it was dried in an oven for 12 h. Meanwhile the corematerial collected in the RB was subjected to vacuumdistillation to remove xylene and final weight of RB containingcore material noted (Wov). The final weight of the dried samplebag (Wsd) was also noted. Thus percentage of core content(αc), shell material (αs) and solvent entrapped (αsv) weredetermined with the help of following formulas.

(a) Core content (αc, %)

=−

×⎡⎣⎢

⎤⎦⎥

W WW

100ov o

m (1)

(b) Shell material (αs, %)

=−

×⎡⎣⎢

⎤⎦⎥

W WW

100s sd

m (2)

(c) Solvent entrapped (αsv, %)

α α= − +100 ( )c s (3)

Scheme 2. Illustration of Possible Reaction Mechanism for the Formation of Polyurea Shell of Microcapsule



Immersion Study. The performance of formed micro-capsules for corrosion protection in a PU coating composite byhealing of cracks was demonstrated through immersion studies.The steel panels coated with PU composites containingmicrocapsules loaded in different concentration were comparedagainst steel sample coated without loading microcapsules. Aninduced crack was developed by manual hand scribing with arazor blade in order to rupture the microcapsules, and after thescribing, the samples were allowed to heal at room temperaturefor 48 h. Immersion studies of these coatings panels werecarried out in salt solution (NaCl, 5 wt %). The corrosion ofthe damaged area was monitored at different intervals periodsby visual inspection using digital camera (Canon A 3100). Thesamples were tested for a total exposure time of 120 h.Adhesion Test. The loss of coating adhesion to the

substrate is one of the reasons for corrosion failure mechanism.To evaluate the change in adhesion induced by the introductionof microcapsules, a cross hatch adhesion test was carried out forabove prepared four coating samples. The adhesion test wasdone with a cross hatch cutter (Elcometer, 107) according toEN ISO 2409.

■ RESULTS AND DISCUSSIONSpectroscopic Techniques. FTIR spectra of −0.5 G and

0.0 G PAMAM dendrimers (Supporting Information) contain acharacteristic band at 1735 cm−1, which is assigned to the estergroup; the absence of an absorption band for >NHstretching at 3300−3500 cm−1 indicates all NH2 groupspresent in EDA are consumed during formation of −0.5 GPAMAM. Spectrum B shows absorption bands at 3475 cm−1

which is assigned to >NH stretching of primary amine, andthe 3288 cm−1 absorption band correspond to >NHstretching of secondary amine. The absorption bands at 2935and 2819 cm−1 corresponds to aliphatic >CH stretching. Thefrequency at 1641 cm−1 observed due to the >CO stretch ofamide. The absorption bands at 1567, 1483, and 1327 cm−1 aredue to >NH bending of N-substituted amide; at 1117 cm−1

due to CC bending indicates the formation of zerogeneration of polyamidoamine (PAMAM). The structure ofPAMAM also confirmed by NMR spectroscopy performed onVarian Mercury YH-300 MHz spectrometer using DMSO-d6 asa solvent and TMS as a standard; d 1.87 (NH2), d 2.18 (CH2CO); d 2.99 (CH2NH2); d 2.44 (CH2NHCO); d 7.99 (NHCO); d 2.6 (>NCH2); d 2.53(CH2CH2NH2) (Scheme 1).Figure 2 shows the FTIR spectra of the shell, linseed oil, and

microcapsules. For linseed oil, spectrum A revealed a stretchingvibration peak of ester >CO at 1745 cm−1, aliphatic CC stretching vibration at 1693 and 1546 cm−1, CH bendof methylene group at 1462 cm−1, and CO stretchingvibration peak at 1190 cm−1.The spectrum of polyurea shell (C) showed the strong band

at 3315 cm−1, which is assigned to NH stretching vibration.The CO stretching frequencies for CONH are presentin polyurea microcapsules observed at 1658 cm−1. Theabsorption peak at 1600 cm−1 corresponds to CCpresent in the aromatic ring. The absorption band at 2280 cm−1

is assigned to the NCO group from unreacted MDI (wall-forming monomer). Furthermore, absorption band for NH bending of N-substituted amide in polyurea can be observedat 1517 cm−1.The spectrum of microcapsules (B) showed the character-

istics absorption bands for >CO (1745 cm−1), aliphatic

CC (1693 cm−1), CH (1462 cm−1), and CO(1190 cm−1) present in the spectrum of linseed oil along withthe entire vibration bands observed in polyurea shell spectrum.Hence, from FTIR data of linseed oil, polyurea shell, andmicrocapsules, it can be concluded that linseed oil is present inpolyurea microcapsules.

Core Materials in Microcapsules. Viability of theencapsulated linseed oil dissolved in xylene was initiallyassessed by a simple visual inspection under optical microscopeand video recording (Supporting Information) by digitalcamera. A small amount of dry capsule powder was placedon the glass slide after filtration and drying. Under the opticalmicroscopic observations, the one specific selected micro-capsule was pricked with the help of a pointed capillary. It wasobserved and recorded that encapsulated liquid emerged fromthe burst capsule, which confirmed the encapsulation of linseedoil. From the Soxhlet extraction, it was concluded that about27% core material and 28% solvent encapsulated within themicrocapsule and remaining 42% mass was from polyurea shellwall material.

Microcapsule Size and Size Distribution. Microcapsulesize and size distribution are affected by a number of factorsincluding emulsifier concentration, temperature, agitation rate,geometry of mixing equipment, etc.24 In the presentcommunication, we studied the effect of agitation rate onmicrocapsule size and size distribution by keeping other factorsconstant. Figure 3 represents the polyurea microcapsulesprepared at different agitation rate from 3000−8000 rpm.Microcapsules primarily observed under an optical microscopefrom 40 to 100× resolution. The result showed that nearlyspherical shape microcapsules were obtained in all the cases.Further, it was observed that the microcapsule size decreaseswith increasing agitation rate applied during the emulsion step.This may be because at a high agitation rate, larger dropletsexperience strong shear force which results in breaking of largedroplets into smaller ones, while dominating interfacial tensionat a low agitation rate led to larger size microcapsules.29

Scanning Electron Microscopy. The surface morphologyof microcapsules synthesized under various agitation rates wasalso observed using SEM micrographs, and the results arepresented in Figure 4. From the SEM images, it was clearedthat the spherical shape microcapsules with a nonporous,

Figure 2. Illustrates the FTIR spectra of (A) linseed oil, (B) polyureamicrocapsules containing core material, and (C) the extractedpolyurea shell.



compact shell wall were obtained at all agitation rates withaverage diameter from 20 to 270 μm. A nonporous shell wallensures the impermeability of prepared capsules towardleakages and diffusion of liquid healing agent. Further, werealized that at low agitation rates, the surface of microcapsuleswas comparatively smooth with some contraction thanmicrocapsules at higher agitation rates [Figure 4i−iv]. Whileat higher agitation rate (8000 rpm), microcapsules showedwrinkled, damaged walls and intense contraction [Figure 4v andvi]. It is well-known from the literature that wrinkles observedon microcapsules may be due to the interaction of fluid inducedshear forces, shell determined elastic forces, inhomogeneousreaction kinetics, and compression forces acting on themembrane.30 At higher agitation rates, more surface area isavailable for interacting the above-mentioned forces and for this

reason intense contraction and wrinkles may formed onmicrocapsules.

Particle Size Analysis. Figure 5 shows the mean particlesizes and particle size distributions of the microcapsules filled

with linseed oil as core material prepared at different agitationrates at emulsion stage. The mean particle sizes of the preparedmicrocapsules were 266, 147, and 24 μm at 3000, 5000, and8000 rpm, respectively. In the present study, it has beenobserved from the graph that the particle size distributionbecame narrower and the average particle size became smallerwith increasing agitation rate. This finding is in favor with theobservations reported previously.31 Higher agitation rate tendsto form finer oil droplets in emulsion systems and also favoredhomogenization of the emulsion which results into narrowuniform size distribution of capsules. Therefore, it can bededuced that the microcapsules of a desirable particle size couldbe prepared through the optimization of agitation rate. Fromthe SEM and particle size analysis observation, we concludethat 5000 rpm could be proper agitation speed formanufacturing microcapsules. Moreover, the particle sizedistribution would be improved further by optimizing thesurfactant concentration along with agitation speed.

Thermogravimetric Analysis. The TGA themograms ofextracted shell and microcapsules loaded with linseed oil are

Figure 3. PU microcapsules obtained at various agitation rates (a) 3000 rpm (magnification: 10×), (b) 5000 rpm (magnification: 10×), and (c) 8000rpm (magnification: 40×).

Figure 4. SEM micrographs of polyurea microcapsules obtained at3000 rpm (i and ii), 5000 rpm (iii and iv), and 8000 rpm (v and vi).

Figure 5. Particle size histogram of PU microcapsules at differentagitation rate (a) 8000, (b) 5000, and (c) 3000 rpm.



shown in Figure 6. TGA of microcapsules showed that there isless than 3% weight loss observed down to 120 °C that may be

due to absorbed moisture. Further, microcapsules experiencedthree stages of weight loss. Initial stage weight loss observedfrom 125−270 °C presumably due to escaping entrappedxylene from the microcapsules. Second stage weight loss from270−375 °C attributed to degradation of linseed oil. The thirdstage weight loss observed above 375 °C was associated withdegradation of shell wall material which is in concord with theresults for extracted shell material. Extracted shell walldecomposed substantially above 380 °C with about 24% ofthe original mass remaining at 800 °C which exhibit theadvanced thermal stability of the polyurea shell wall preparedfrom PAMAM as compared to polyurethane, polyurea, PF, andUF shell walls.29,31−33 Therefore present work revealed thatnewly fabricated polyurea microcapsules have better ability topreserve the core material from surrounding environment.The compositional analysis of microcapsules in terms of

percent weight loss was also done through TGA data andcalculation of derivative weight loss. It was found that totalcontent of core was about 55% of which 28% was contributedby entrapped solvent and 27% through linseed oil andremaining about 42% was of shell material, which are in goodagreement with the results obtained from Soxhlet extractionmethod.Interfacial Interaction Study. It was demonstrated that

some coating adhesion will always be sacrificed by addition ofmicrocapsules.34 To reveal the change in adhesion induced bythe introduction of microcapsules, the adhesion test wasperformed and the results are shown in Figure 7. PU samples

without microcapsules and with 1% microcapsules showedbetter adhesion. Adhesion decreased for 3 and 5% micro-capsules containing PU coatings. In this study although weobserved declined adhesion of coating with rising microcapsulepercentage, it was still less than 15% for maximum micro-capsule loading (5%). The possible explanation for betteradhesion is high polarity exerted on the surface of microcapsuledue to the presence of free amino groups on the wall ofmicrocapsule results into increase in interaction between shellwall, PU, and substrate that would have conserved adhesionbetween matrix and substrateThe interfacial interaction as seen from the adhesion test can

also be confirmed by IR analysis, and the resulting spectrum isshown in Figure 8. Stronger intensity of the vibration at 1635

cm−1 (CO stretching frequencies for CONH) in the PUmatrix embedded with microcapsules compared with pristinePU matrix confirms additional urea character formed byreaction of free amine groups of the shell wall and diisocyanate.Further intensity of unreacted NCO stretching frequencyfrom MDI observed in the PU matrix was weakened in thecoating composite illustrating the consumption of NCOgroups due to reaction with the amine groups. From Figure 8, itwas illustrated that a broad absorption peak for NH

Figure 6. Thermogravimetric analysis of microcapsule and shell.

Figure 7. Illustrates the adhesion rating for coating with differentloading of microcapsules.

Figure 8. Illustrates the FTIR spectra of (A) microcapsules, (B)microcapsule + PU matrix, and (C) PU matrix.



stretching in the PU matrix shifted to higher wavenumber inmicrocapsule loaded coating. Probably it may be due tointroduction of microcapsule within PU matrix disturbedintermolecular hydrogen bonds formed in the PU matrix,which affects the distinct construction formed in pristine a PUmatrix.Immersion Study. The results from the accelerated

corrosion immersion test (Figure 9) in salt solution clearlydemonstrated that compared with the control sample, speci-mens with increasing microcapsule content from 2 to 5%(Figure 9j, k, and l) revealed decreasing order of corrosion andblister at the scribed lines after 120 h of immersion studies. Incontrast, rapid corrosion was seen in the control specimenwithin 24 h (Figure 9a) and exhibited severe corrosion after120 h, most prevalently within the groove of the scribed areaand also extending rusting across the substrate surface (Figure9i). From the images of the coated steel panels, it can also beillustrated that the scratched area of the steel panel coated withPU coating with 5% microcapsules showed practically leastcorrosion after 120 h of immersion in salt solution (Figure 9l).Corrosion resistance performance of the coatings may be dueto filling of cracks by newly formed film through oxidativepolymerization of linseed oil released from the rupturedmicrocapsules. The healed crack in this way restricts thediffusion of salt ions and thus protects the substrate from the

corrosion process even after 120 h, while severe corrosion wasobserved in and around the crack of the control specimen at thebeginning. Therefore, it could be concluded that polyureamicrocapsule containing linseed oil (healant) offered betteranticorrosion property at 5% capsules loading to the PUcoating on steel panels tested for accelerated corrosion process.

■ CONCLUSION0.0 G PAMAM dendrimer was synthesized and characterizedby FTIR and 1H NMR spectroscopy. Linseed oil filled polyureamicrocapsules were synthesized by interfacial polymerization of0.0 G PAMAM dendrimer and MDI. Encapsulation of linseedoil within polyurea shell wall confirmed by FTIR study ofmicrocapsules, core, and shell material. Spherical microcapsuleswith mean average diameter in the range of 20−300 μm wereprepared by adjusting agitation rate over the range of 3000−8000 rpm. Polyurea microcapsules prepared by using PAMAMas one of the reactant shows grater thermal stability up to 380°C. Core content of resultant microcapsule was about 55% asderived from derivative TGA. Microcapsules having an aminofunctional wall show improved adhesion performance with bothpolymer matrix and substrate through chemical bonding whichis confirmed by FTIR and the adhesion test. PU coatings onsteel substrate embedded with the polyurea microcapsulescontaining linseed oil showed escalating corrosion protection

Figure 9. Corrosion test results of PU coatings loaded with and without microcapsules after 24, 72, and 120 h.



capability with increasing microcapsules loading from 2 to 5%and proved better anticorrosive property at 5% capsule loadingunder accelerated corrosion testing.

■ ASSOCIATED CONTENT*S Supporting InformationIR and NMR characterization of 0.0 G PAMAM dendrimer;video confirmation of encapsulation of liquid core material.This information is available free of charge via the Internet athttp://pubs.acs.org/.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Phone No: + 91 2572257431. Fax No: + 91 257 2258403.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are very grateful to Department of Science andTechnology (DST), Govt. of India, for providing an INSPIREfellowship. We also acknowledge Kishore Polyurethanes Pvt.Ltd. Nasik, India, for providing the methylene diphenyldiisocyanate and Indofil Industries Ltd. Thane, India, forhelping in determination of particle size.

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(34) Kumar, A.; Stephenson, L. D. Self-Healing Coatings for SteelStructures. Tri-Service Corrosion Conference, Orlando, Florida, No-vember 14−18, 2005.



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