Progress in Organic Coatings 63 (2008) 72–78

Contents lists available at ScienceDirect

Progress in Organic Coatings

journa l homepage: www.e lsev ier .com/ locate /porgcoat

Preparation and characterization of microcapsules-he

umal Am

il filulesil droope

differas res fou

containing linseed oil and its use in self

C. Suryanarayanaa, K. Chowdoji Raob, Dhirendra Ka Naval Materials Research Laboratory, Shil-Badlapur Road, P.O. Anand Nagar, Additioinb Department of Polymer Science and Technology, S.K. University, Anantapur, AP, India

a r t i c l e i n f o

Article history:Received 21 September 2007Received in revised form 4 April 2008Accepted 18 April 2008

Keywords:MicroencapsulationSelf-healing coatingsCrack healingLinseed oil

a b s t r a c t

Effectiveness of linseed opaint/coatings. Microcapsform shell over linseed oscanning electron microscstirring microcapsules inhealed when linseed oil wLinseed oil healed area wa

1. Introduction

Encapsulation of functionally active materials in hollow micro-spheres is an attractive way of storing as well as protectingthese from environment till required for fulfilling appropriate

applications. Microencapsulated substances have been utilized forsustained drug release [1,2], electro rheological fluids [3], intumes-cent fire retarding powders [4,5], preservation of flavours [6,7],electro phoretic display applications [8], textiles [9], biotechnol-ogy [10,11] and inorganic metal salt catalyst [12], etc. Recently,there has been growing interest in use of microencapsulated mate-rials for healing of cracks generated during service of a polymerbased composite materials [13,14]. Microcapsules containing dicy-clopentadiene were incorporated in the composite matrix. Thesecapsules rupture and release dicyclopentadiene during crack for-mation and reacts with Grubbs ruthenium catalyst present in thecomposites leading to crack repair to restore mechanical proper-ties.

Paints are extensively used for modification of substrates eitherfor aesthetic appearance or for corrosion protection. During its ser-vice life, the paint film undergoes changes in mechanical propertiesleading to formation of microcracks which subsequently propa-gates and exposes substrate to atmospheric moisture and oxygen.This action results in accelerated disbonding of the paint and flakeformation from the metal coating interface. Paint coatings can be

∗ Corresponding author. Tel.: +91 251 2620187; fax: +91 251 2620604.E-mail address: dhikumar@rediffmail.com (D. Kumar).

0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2008.04.008

aling coatings

ara,∗

bernath (East), Dist. Thane 421506, India

led microcapsules was investigated for healing of cracks generated inwere prepared by in situ polymerization of urea–formaldehyde resin toplets. Characteristics of these capsules were studied by FTIR, TGA/DSC,

(SEM) and particle size analyzer. Mechanical stability was determined byent solvents and resin solutions. Cracks in a paint film were successfullyleased from microcapsules ruptured under simulated mechanical action.nd to prevent corrosion of the substrate.

© 2008 Elsevier B.V. All rights reserved.

considered as a special class of composite materials, comprisingbinders and pigments. Hence, the concept of self-healing of cracks,as reported for composites, can be adopted for coatings to providelonger durability. An attempt for healing of scratches on automotivecoating using temperature dependant elastic properties of polymerhas been reported [15].

Here, we report our work on development of self-healing coat-ings with microencapsulated drying oil. In this study, linseed oil

along with driers has been selected as a healing agent due to itsfilm forming ability by atmospheric oxidation. Microcapsules withurea–formaldehyde as a shell and drying oil as a core were synthe-sized by in situ polymerization [13]. Efficacy of these microcapsulesin healing of cracks in an epoxy coating and corrosion protectionhas been demonstrated.

2. Materials and methods

Urea, formaldehyde, ammonium chloride, resorcinol, poly vinylalcohol (PVA) and red dye (Disperse red 1) were procured in ARgrade from Sigma Aldrich. Epoxy resin (XR-87) was purchased fromAtul Limited, India; epoxy hardener (Ancamine 2280) was obtainedfrom Air Products India Limited. Linseed oil (commercial grade)was purchased from Jayant Oil Mils Pvt. Limited, India. All chemi-cals/materials were used without any purification.

2.1. Experimental synthesis of microcapsules

Microcapsules were prepared by in situ polymerization in an oil-in-water emulsion. At room temperature, 260 ml of deionised water

C. Suryanarayana et al. / Progress in Or

Microcapsules, linseed oil and urea–formaldehyde resin wereanalyzed using thermo gravimetric analyzer (Auto TGA 2950HR,TA instruments) in nitrogen environment with a sample weight

Fig. 1. SEM micrographs: (a) shell thickness and (b) shell morphology of microcap-sules.

and 10 ml of 5 wt% aqueous solution of polyvinyl alcohol (PVA) weremixed in 1000 ml beaker. Under agitation 5 g urea, 0.5 g ammoniumchloride and 0.5 g resorcinol were dissolved in solution. The pH wasadjusted to approximately 3.5 by using 5 wt% solution of hydrochlo-ric acid in deionised water. One to two drops of octanol was added asan antifoaming agent. 60 ml of linseed oil containing 0.7 wt% cobaltnaphthenate and 2.5 wt% lead octoate driers was added slowly toform an emulsion and allowed to stabilize for 10 min under agi-tation. After stabilization, 12.67 g of 37 wt% aqueous solution offormaldehyde was added. The emulsion was covered and slowlyheated and maintained at 55 ◦C under stirring at 200 rpm for 4 h.Contents were cooled to ambient temperature. Microcapsules fromthe suspension were recovered by filtration under vacuum. Thesewere rinsed with water, washed with xylene to remove suspendedoil. The capsules were dried under vacuum.

Fig. 2. Particle size analysis of microcapsules.

ganic Coatings 63 (2008) 72–78 73

Fig. 3. FTIR spectrum: (A) urea–formaldehyde resin and (B) shell material of micro-capsule.

2.2. Analysis of microcapsule size and shell morphology

Microcapsule size analysis was carried out with a particle sizeanalyzer (Mastersizer 2000, Malvern). Surface morphology andshell thickness of microcapsules were determined by scanningelectron microscopy, (LEO1455). Microcapsules were mounted onadhesive tape and ruptured with a razor blade for shell thicknessmeasurement.

2.3. Thermal analysis of microcapsules

of about 3 mg. Heating rate was maintained at 20 ◦C/min in thetemperature range of 30–800 ◦C.

Similarly samples were also analyzed by using differential scan-ning calorimeter (SetsysTG-DSC 16, TA instruments) in oxygenenvironment with sample weight of about 3 mg at heating rate of20 ◦C/min, between 30 and 900 ◦C.

2.4. Linseed oil content in microcapsules

Amount of linseed oil present in microcapsules was determinedby extracting oil in a soxhlet apparatus. A known weight of micro-capsules (WC) was crushed using pestle and mortar and transferredto a thimble of known weight (Wti), pestle and mortar were rinsedwith xylene and added to thimble. Extraction was carried out usingxylene as a solvent for linseed oil. After 1 h of extraction, thimblewas carefully taken out of the soxhlet apparatus and after com-

Fig. 4. FTIR spectrum: (A) linseed oil and (B) core material of microcapsule.

74 C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78

hell capsule and linseed oil.

after every 10 min and the number of microcapsules broken were

Fig. 5. DSC curves of s

pletely draining the solvent, it was dried in oven. The final weight

of the thimble (Wtf) was noted.

Weight of linseed oil (%) = (WC + Wti) − Wtf

WC× 100

2.5. Infrared spectroscopy

Spectra of shell and core material extracted from soxhletapparatus were recorded on Fourier Transform Infrared spec-trophotometer (NICOLET 5700 Thermo Electron Corporation). Thesolid shell material collected after soxhlet extraction was mixedwith KBr and palette was prepared for recording spectra. Infrared spectra of extract after soxhlating, neat linseed oil andurea–formaldehyde resin were also recorded.

2.6. Mechanical stability of microcapsules

Mechanical stability was determined by subjecting microcap-sules to stress generated during mechanical stirring at 200 rpm inmineral turpentine oil (MTO) and epoxy resin solutions of differ-ent viscosities. Samples were observed under optical microscope

Fig. 6. TGA curves of capsule ( ), linseed oil (....), and UF resin ( ).

counted. Further stability was also studied at varying speeds in MTOand epoxy resin solution of viscosity 200 s (Ford cup no. B4 at 30 ◦C)for 1 h.

Fig. 7. Mechanical stability of microcapsules. (a) Effect of speed on mechanical sta-bility. (b) Effect of viscosity on mechanical stability. (c) Effect of time on mechanicalstability.

C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78 75

) In M

Fig. 8. Mechanical stability of microcapsules. (a) In MTO mixed @200 rpm for 1 h. (bmixed @500 rpm, (e) and (f) distribution of beads in clear films.

2.7. Preparation and performance evaluation in respect ofcorrosion resistance of coating with microcapsules

Microcapsules were incorporated in an epoxy resin solution,

which was prepared by diluting low molecular weight epoxy resin(epoxy equivalent weight ∼180 gm/eq.) in a solvent mixture ofxylene/butanol (4:1 ratio by volume). The solid content of epoxysolution was kept at 90 wt%. A cycloaliphatic polyamine hardnerwas used in stoichiometric amount for curing.

Microcapsules were added under slow agitation to epoxy resinsolution at ambient temperature. After mixing for 15 min the com-position was mixed with the required amount of hardner. Cleanmild steel panels, size 150 mm × 60 mm × 1 mm were coated on oneside by brush to obtain an average dry film thickness of approx-imately 150 �m. After 7 days of curing, cross-cut was made onpanels and kept at ambient for 24 h. A composition without micro-capsules was prepared as a control. Specimens coated with bothcompositions were exposed for a period 72 h in salt spray cabinetfor evaluation of corrosion protection.

2.8. Evaluation of self-healing process

The process of crack healing, on a coated surface, was carriedout by incorporating microcapsules into a solvent free anticorrosivepaint. This paint having solid content 98 wt%, density 1.4 g/cm3 was

TO mixed @200 rpm for 4 h. (c) In epoxy resin mixed @200 rpm. (d) In epoxy resin

used with the hardner described above. Colored linseed oil filledmicrocapsules were mixed in paint to produce contrast in the crackwhen observed under optical microscope to record healing process.A tinned panel, 15 mm × 50 mm × 0.4 mm was coated with single

coat to produce a dry film thickness of about 150 �m. After 7 days ofcuring a crack was generated in the coated panel by quick manualbending. Panel was immediately kept under objective lens of opticalmicroscope to record healing process initiated due to release oflinseed oil from ruptured microcapsules.

3. Results and discussion

The formation of urea–formaldehyde capsules has beendescribed by Park et al. [7]. The process comprises reaction of ureaand formaldehyde to obtain methylol ureas, which further con-denses under acidic conditions to form the shell material.

Encapsulation of linseed oil in the capsules takes place simul-taneously during formation of crosslinked urea–formaldehydepolymer. Reactants urea and formaldehyde are soluble in water.When the pH is changed to acidic and heated to 55 ◦C, ureaand formaldehyde reacts to from poly (urea–formaldehyde)as stated above. During the initial stage of polymerization,urea–formaldehyde molecule is rich with polar groups and is watercompatible. The number of polar groups will gradually reduceas molecular weight of polymer increases. Finally after attaining

76 C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78

s of se

Fig. 9. Microscope photo

certain molecular weight, hydrophylicity of urea–formaldehydepolymer molecule will reduce leading to separation from aque-ous phase and get deposited on the already emulsified oil droplets(hydrophobic organic phase). This process continues and a thinshell is formed over oil droplets. The thickness of the shell has beenoptimized in such a way that the shell contains maximum amountof core material. A shell thickness of 0.2 �m was obtained at rpmof 250 to contain 80% linseed oil. Fig. 1 shows the SEM micrographsof (a) capsule shell thickness and (b) microcapsule shell morphol-ogy. Fig. 2 shows the particle size distribution of microcapsules.Size ranges from 5 to 100 �m. However, most of the particles fall inthe size range around 50 �m. This is quite satisfactory for using inpaints.

Microcapsules prepared from urea–formaldehyde resin (shellmaterial) and filled with linseed oil (core material) were character-ized using different instrumental techniques. Core and shell of themicrocapsules were separated by soxhlet extraction. FTIR spectrawere recorded.

It is seen from the FTIR spectra of urea–formaldehyde resin andshell material (Fig. 3) that both are closely matching at charac-

lf-healing coating films.

teristic peaks of a N H stretching vibration at 1571 cm−1, a C Ostretching vibration at 1650 cm−1, and a C H stretching vibra-tion at 1460 cm−1. C N stretching vibrations are shown at 1286and 1142 cm−1. The O H peak is shown as a broad absorptionpeak at 3500–3200 cm−1. This spectrum confirms that shell mate-rial is made of urea–formaldehyde polymer. FTIR spectrum ofurea–formaldehyde resin is also matching with spectrum reportedin the literature [15]. Spectra of linseed oil and core materialhave also been found matching (Fig. 4) at characteristic peaksfor C O and C C stretching vibrations. In view of above it isestablished that linseed oil has been successfully encapsulated inurea–formaldehyde shell.

Thermal analyses of microcapsules as well as shell and corematerials were carried out for further characterization. DSC anal-ysis (Fig. 5) of shell material shows strong endothermic peakat 230 ◦C, which is attributed to the degradation of UF resin asconfirmed from TGA of shell material (Fig. 6). Exothermic peakobserved at 155 ◦C in the DSC curve of microcapsule correspondsto the curing of linseed oil as observed in DSC analysis of only oil(inserted figure in Fig. 5). Degradation of core (linseed oil) com-

s in Or

coati

C. Suryanarayana et al. / Progres

Fig. 10. Salt spray performance of

mences at 300 ◦C as also shown in TGA curve. From these resultsit is further established that microcapsules contain both materials,i.e. linseed oil (as core) and UF resin (as a shell).

3.1. Mechanical stability of microcapsules

Microcapsules should have sufficient mechanical strength towithstand shear stress generated during mixing and application ofpaint. Microcapsules should not break during mixing and appli-cation of paint. They should break and release healing materialas and when crack is generated in the paint film. Viscosity ofthe paint, stirring speed and stirring time are the main param-eters affecting the stability of microcapsules during mixing andapplication of paint. Effect of all the three parameters on the sta-bility of microcapsules was studied by stirring microcapsules inepoxy resin solutions of varying viscosities at different speeds forvarying time periods. Similar studies were also carried out by agi-tating microcapsules in MTO and epoxy solution at fixed speed of200 rpm.

It is seen from Fig. 7(a) that microcapsules were undam-aged up to 200 rpm. About 75% of microcapsules were brokenwhen stirred at 500 rpm in solvent (MTO). Microcapsule sta-bility was affected due to high shear stress, when viscosity ofthe medium was increased. Only 10% microcapsules were foundbroken at a viscosity of 250 s (Fig. 7(b)). As the viscosity ofcommercial paints range between 85 and 150 s, the microcap-sules are expected to withstand shear force during mixing andpaint application. Mechanical stability of microcapsule is alsofound affected when they are stirred continuously for longer peri-ods. From Fig. 7(c) it is seen that microcapsules are stable upto 120 min under stirring, and approximately 15% of microcap-

sules were broken after 4 h of continuous stirring at a speed of200 rpm. However, during incorporation of microcapsules intopaint composition it was observed that, microcapsules could beuniformly mixed without any damage into paint by stirring ataround 150 rpm for 15 min. Fig. 8 presents optical photographsmicrocapsules dispersed in different medium under varying stir-ring conditions.

3.2. Healing performance

For effective healing, the microcapsules incorporated in thepaint film, should break immediately to release healing materialwhen the cracks are generated in the paint film. It is observed thatthe shell surface of microcapsules is very rough (Fig. 1b), whichwill provide good bonding with the film matrix. This facilitatesin breaking of microcapsules under stress due to cracking. Thehealing of crack in a paint film recorded under optical microscopeat 100× magnification has been shown in Fig. 9. It is observed thatthe initial open length of the crack is maximum (Fig. 9, initial),which gradually reduced and completely filled after 90 s (Fig. 9,90 s). This healed material (linseed oil) has dried by oxidation with

ganic Coatings 63 (2008) 72–78 77

ngs at different exposure periods.

atmospheric oxygen and has formed continuous film in the crack.This is further confirmed from the superior corrosion resistanceperformance of healed films.

3.3. Corrosion resistance of self-healing coating films

Corrosion of metallic substrate takes place when moisture andoxygen are transported through the cracks to the metal–coatinginterface. Healing of cracks, thus, provides an effective method toprevent corrosion. Performance of linseed oil as a healing materialwas assessed by exposing specimens coated with paint containingfilled microcapsules to salt spray. Before exposure, coated sur-face was cross-cut up to the metal. Control specimens had thepaint without microcapsules. Up to 72 h of exposure, specimenswith paint containing capsules were found free from corrosion atthe scribed lines (Fig. 10). Control panels, however, suffered fromcorrosion after 48 h of exposure. Superior corrosion resistance per-formance of healed films is due to the reason that, linseed oilreleased from ruptured microcapsules filled the crack and formeda film by oxidative polymerization with atmospheric oxygen whichprevented the ingress of moisture and oxygen and thus preventedcorrosion.

4. Conclusions

Linseed oil along with driers has been successful encapsulatedin to urea–formaldehyde shell. Microcapsules having sufficientstrength to withstand shear generated during mixing in to paintand paint application. The rough morphology of microcapsule shellhas provided good anchoring between microcapsule and paintmatrix. Microcapsules in paint films released healing material,

which during cracking healed cracks efficiently with satisfactoryanticorrosive properties.

Acknowledgement

The authors sincerely thank to Dr. J. Narayana Das, Director,NMRL for his keen interest, constant encouragement and for givingpermission to publish this paper.

References

[1] J. Eukaszczyk, P. Urba, React. Funct. Polym. 33 (1997) 233–239.[2] W. Tiyaboonchai, G.C. Ritthidej, Songklanakarin, J. Sci. Technol. 25 (March–April

(2)) (2003) 252.[3] Y.H. lee, C.A. Kim, W.H. Jang, H.J. Choi, M.S. Jhon, Polymer 42 (2001) 8277–8283.[4] D. Saihi, I. Vroman, S. Girand, S. Bourbigot, React. Funct. Polym. 64 (2005)

127–138.[5] S. Girand, S. Bourbigot, M. Rochery, I. Vroman, L. Tighzert, R. Delobel, F. Pouch,

Polym. Degrad. Stabil. (88) (2005) 106–113.[6] X.D. Liu, T. Ataroshi, T. Furuta, H.F. Yoshii, S. Ashima, M. Okiawara, P. Linko,

Drying Technol. 19 (7) (2001) 1361–1374.[7] S.J. Park, Y.S. Shin, J.R. Lee, J. Colloid Interface Sci. 241 (2001) 502–508.[8] B.J. Park, J.Y. Lee, J.H. Sung, H.J. Choi, Curr. Appl. Phys. 6 (4) (July 2006) 632–635.[9] G. Nelson, Int. J. Pharm. 242 (2002) 55–62.

[

[[

78 C. Suryanarayana et al. / Progress in Or

10] G. Orive, R.M. Hernandez, A. Rodryguez Gascon, R. Calafiore, T.M.S. Chang, P. deVos, G. Hortelano, D.H.I. Lacyk, J.L. Pedraz, Trends Biotechnol. 22 (February (2))(2004) 87–92.

11] G. Sukhorukov, A. Fery, H. Mohwald, Prog. Polym. Sci. 30 (2005) 885–897.12] H.B. Ji, G.J. Kuang, Y. Qian, Catal. Today 105 (2005) 605–611.

[

[

[

ganic Coatings 63 (2008) 72–78

13] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, J. Microencapsul. 20 (6) (2003)719–730.

14] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, M.F. Montemor, P. Cecılio, M.G.S.Ferreira, Electrochem. Commun. 8 (3) (March 2006) 421–428.

15] C. Challener, JCT, January 2005, pp. 50–55.