- Email: [email protected]
Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings
Progress in Organic Coatings 63 (2008) 72–78
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings C. Suryanarayana a , K. Chowdoji Rao b , Dhirendra Kumar a,∗ a b
Naval Materials Research Laboratory, Shil-Badlapur Road, P.O. Anand Nagar, Additioinal Ambernath (East), Dist. Thane 421506, India 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 2007 Received in revised form 4 April 2008 Accepted 18 April 2008 Keywords: Microencapsulation Self-healing coatings Crack healing Linseed oil
a b s t r a c t Effectiveness of linseed oil filled microcapsules was investigated for healing of cracks generated in paint/coatings. Microcapsules were prepared by in situ polymerization of urea–formaldehyde resin to form shell over linseed oil droplets. Characteristics of these capsules were studied by FTIR, TGA/DSC, scanning electron microscope (SEM) and particle size analyzer. Mechanical stability was determined by stirring microcapsules in different solvents and resin solutions. Cracks in a paint film were successfully healed when linseed oil was released from microcapsules ruptured under simulated mechanical action. Linseed oil healed area was found to prevent corrosion of the substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Encapsulation of functionally active materials in hollow microspheres is an attractive way of storing as well as protecting these from environment till required for fulfilling appropriate applications. Microencapsulated substances have been utilized for sustained drug release [1,2], electro rheological fluids [3], intumescent fire retarding powders [4,5], preservation of flavours [6,7], electro phoretic display applications [8], textiles [9], biotechnology [10,11] and inorganic metal salt catalyst [12], etc. Recently, there has been growing interest in use of microencapsulated materials for healing of cracks generated during service of a polymer based composite materials [13,14]. Microcapsules containing dicyclopentadiene were incorporated in the composite matrix. These capsules rupture and release dicyclopentadiene during crack formation and reacts with Grubbs ruthenium catalyst present in the composites leading to crack repair to restore mechanical properties. Paints are extensively used for modification of substrates either for aesthetic appearance or for corrosion protection. During its service life, the paint film undergoes changes in mechanical properties leading to formation of microcracks which subsequently propagates and exposes substrate to atmospheric moisture and oxygen. This action results in accelerated disbonding of the paint and flake formation from the metal coating interface. Paint coatings can be
considered as a special class of composite materials, comprising binders and pigments. Hence, the concept of self-healing of cracks, as reported for composites, can be adopted for coatings to provide longer durability. An attempt for healing of scratches on automotive coating using temperature dependant elastic properties of polymer has been reported [15]. Here, we report our work on development of self-healing coatings with microencapsulated drying oil. In this study, linseed oil along with driers has been selected as a healing agent due to its film forming ability by atmospheric oxidation. Microcapsules with urea–formaldehyde as a shell and drying oil as a core were synthesized by in situ polymerization [13]. Efficacy of these microcapsules in healing of cracks in an epoxy coating and corrosion protection has been demonstrated. 2. Materials and methods Urea, formaldehyde, ammonium chloride, resorcinol, poly vinyl alcohol (PVA) and red dye (Disperse red 1) were procured in AR grade from Sigma Aldrich. Epoxy resin (XR-87) was purchased from Atul Limited, India; epoxy hardener (Ancamine 2280) was obtained from Air Products India Limited. Linseed oil (commercial grade) was purchased from Jayant Oil Mils Pvt. Limited, India. All chemicals/materials were used without any purification. 2.1. Experimental synthesis of microcapsules
∗ Corresponding author. Tel.: +91 251 2620187; fax: +91 251 2620604. E-mail address: [email protected] (D. Kumar). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.04.008
Microcapsules were prepared by in situ polymerization in an oilin-water emulsion. At room temperature, 260 ml of deionised water
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
73
Fig. 3. FTIR spectrum: (A) urea–formaldehyde resin and (B) shell material of microcapsule.
2.2. Analysis of microcapsule size and shell morphology Microcapsule size analysis was carried out with a particle size analyzer (Mastersizer 2000, Malvern). Surface morphology and shell thickness of microcapsules were determined by scanning electron microscopy, (LEO1455). Microcapsules were mounted on adhesive tape and ruptured with a razor blade for shell thickness measurement. 2.3. Thermal analysis of microcapsules
Fig. 1. SEM micrographs: (a) shell thickness and (b) shell morphology of microcapsules.
and 10 ml of 5 wt% aqueous solution of polyvinyl alcohol (PVA) were mixed in 1000 ml beaker. Under agitation 5 g urea, 0.5 g ammonium chloride and 0.5 g resorcinol were dissolved in solution. The pH was adjusted to approximately 3.5 by using 5 wt% solution of hydrochloric acid in deionised water. One to two drops of octanol was added as an antifoaming agent. 60 ml of linseed oil containing 0.7 wt% cobalt naphthenate and 2.5 wt% lead octoate driers was added slowly to form an emulsion and allowed to stabilize for 10 min under agitation. After stabilization, 12.67 g of 37 wt% aqueous solution of formaldehyde was added. The emulsion was covered and slowly heated and maintained at 55 ◦ C under stirring at 200 rpm for 4 h. Contents were cooled to ambient temperature. Microcapsules from the suspension were recovered by filtration under vacuum. These were rinsed with water, washed with xylene to remove suspended oil. The capsules were dried under vacuum.
Fig. 2. Particle size analysis of microcapsules.
Microcapsules, linseed oil and urea–formaldehyde resin were analyzed using thermo gravimetric analyzer (Auto TGA 2950HR, TA instruments) in nitrogen environment with a sample weight of about 3 mg. Heating rate was maintained at 20 ◦ C/min in the temperature range of 30–800 ◦ C. Similarly samples were also analyzed by using differential scanning calorimeter (SetsysTG-DSC 16, TA instruments) in oxygen environment with sample weight of about 3 mg at heating rate of 20 ◦ C/min, between 30 and 900 ◦ C. 2.4. Linseed oil content in microcapsules Amount of linseed oil present in microcapsules was determined by extracting oil in a soxhlet apparatus. A known weight of microcapsules (WC ) was crushed using pestle and mortar and transferred to a thimble of known weight (Wti ), pestle and mortar were rinsed with xylene and added to thimble. Extraction was carried out using xylene as a solvent for linseed oil. After 1 h of extraction, thimble was 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
Fig. 5. DSC curves of shell capsule and linseed oil.
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 × 100 WC
after every 10 min and the number of microcapsules broken were counted. Further stability was also studied at varying speeds in MTO and epoxy resin solution of viscosity 200 s (Ford cup no. B4 at 30 ◦ C) for 1 h.
2.5. Infrared spectroscopy Spectra of shell and core material extracted from soxhlet apparatus were recorded on Fourier Transform Infrared spectrophotometer (NICOLET 5700 Thermo Electron Corporation). The solid shell material collected after soxhlet extraction was mixed with KBr and palette was prepared for recording spectra. Infra red spectra of extract after soxhlating, neat linseed oil and urea–formaldehyde resin were also recorded. 2.6. Mechanical stability of microcapsules Mechanical stability was determined by subjecting microcapsules to stress generated during mechanical stirring at 200 rpm in mineral turpentine oil (MTO) and epoxy resin solutions of different viscosities. Samples were observed under optical microscope
Fig. 6. TGA curves of capsule ( ), linseed oil (....), and UF resin (
).
Fig. 7. Mechanical stability of microcapsules. (a) Effect of speed on mechanical stability. (b) Effect of viscosity on mechanical stability. (c) Effect of time on mechanical stability.
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
75
Fig. 8. Mechanical stability of microcapsules. (a) In MTO mixed @200 rpm for 1 h. (b) In MTO mixed @200 rpm for 4 h. (c) In epoxy resin mixed @200 rpm. (d) In epoxy resin mixed @500 rpm, (e) and (f) distribution of beads in clear films.
2.7. Preparation and performance evaluation in respect of corrosion 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 of xylene/butanol (4:1 ratio by volume). The solid content of epoxy solution was kept at 90 wt%. A cycloaliphatic polyamine hardner was used in stoichiometric amount for curing. Microcapsules were added under slow agitation to epoxy resin solution at ambient temperature. After mixing for 15 min the composition was mixed with the required amount of hardner. Clean mild steel panels, size 150 mm × 60 mm × 1 mm were coated on one side by brush to obtain an average dry film thickness of approximately 150 m. After 7 days of curing, cross-cut was made on panels and kept at ambient for 24 h. A composition without microcapsules was prepared as a control. Specimens coated with both compositions were exposed for a period 72 h in salt spray cabinet for evaluation of corrosion protection. 2.8. Evaluation of self-healing process The process of crack healing, on a coated surface, was carried out by incorporating microcapsules into a solvent free anticorrosive paint. This paint having solid content 98 wt%, density 1.4 g/cm3 was
used with the hardner described above. Colored linseed oil filled microcapsules were mixed in paint to produce contrast in the crack when 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 of curing a crack was generated in the coated panel by quick manual bending. Panel was immediately kept under objective lens of optical microscope to record healing process initiated due to release of linseed oil from ruptured microcapsules. 3. Results and discussion The formation of urea–formaldehyde capsules has been described by Park et al. [7]. The process comprises reaction of urea and formaldehyde to obtain methylol ureas, which further condenses under acidic conditions to form the shell material. Encapsulation of linseed oil in the capsules takes place simultaneously during formation of crosslinked urea–formaldehyde polymer. Reactants urea and formaldehyde are soluble in water. When the pH is changed to acidic and heated to 55 ◦ C, urea and 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 water compatible. The number of polar groups will gradually reduce as molecular weight of polymer increases. Finally after attaining
76
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
Fig. 9. Microscope photos of self-healing coating films.
certain molecular weight, hydrophylicity of urea–formaldehyde polymer molecule will reduce leading to separation from aqueous phase and get deposited on the already emulsified oil droplets (hydrophobic organic phase). This process continues and a thin shell is formed over oil droplets. The thickness of the shell has been optimized in such a way that the shell contains maximum amount of core material. A shell thickness of 0.2 m was obtained at rpm of 250 to contain 80% linseed oil. Fig. 1 shows the SEM micrographs of (a) capsule shell thickness and (b) microcapsule shell morphology. Fig. 2 shows the particle size distribution of microcapsules. Size ranges from 5 to 100 m. However, most of the particles fall in the size range around 50 m. This is quite satisfactory for using in paints. Microcapsules prepared from urea–formaldehyde resin (shell material) and filled with linseed oil (core material) were characterized using different instrumental techniques. Core and shell of the microcapsules were separated by soxhlet extraction. FTIR spectra were recorded. It is seen from the FTIR spectra of urea–formaldehyde resin and shell material (Fig. 3) that both are closely matching at charac-
teristic peaks of a N H stretching vibration at 1571 cm−1 , a C O stretching vibration at 1650 cm−1 , and a C H stretching vibration at 1460 cm−1 . C N stretching vibrations are shown at 1286 and 1142 cm−1 . The O H peak is shown as a broad absorption peak at 3500–3200 cm−1 . This spectrum confirms that shell material is made of urea–formaldehyde polymer. FTIR spectrum of urea–formaldehyde resin is also matching with spectrum reported in the literature [15]. Spectra of linseed oil and core material have also been found matching (Fig. 4) at characteristic peaks for C O and C C stretching vibrations. In view of above it is established that linseed oil has been successfully encapsulated in urea–formaldehyde shell. Thermal analyses of microcapsules as well as shell and core materials were carried out for further characterization. DSC analysis (Fig. 5) of shell material shows strong endothermic peak at 230 ◦ C, which is attributed to the degradation of UF resin as confirmed from TGA of shell material (Fig. 6). Exothermic peak observed at 155 ◦ C in the DSC curve of microcapsule corresponds to the curing of linseed oil as observed in DSC analysis of only oil (inserted figure in Fig. 5). Degradation of core (linseed oil) com-
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
77
Fig. 10. Salt spray performance of coatings at different exposure periods.
mences at 300 ◦ C as also shown in TGA curve. From these results it is further established that microcapsules contain both materials, i.e. linseed oil (as core) and UF resin (as a shell).
atmospheric oxygen and has formed continuous film in the crack. This is further confirmed from the superior corrosion resistance performance of healed films.
3.1. Mechanical stability of microcapsules
3.3. Corrosion resistance of self-healing coating films
Microcapsules should have sufficient mechanical strength to withstand shear stress generated during mixing and application of paint. Microcapsules should not break during mixing and application of paint. They should break and release healing material as and when crack is generated in the paint film. Viscosity of the paint, stirring speed and stirring time are the main parameters affecting the stability of microcapsules during mixing and application of paint. Effect of all the three parameters on the stability of microcapsules was studied by stirring microcapsules in epoxy resin solutions of varying viscosities at different speeds for varying time periods. Similar studies were also carried out by agitating microcapsules in MTO and epoxy solution at fixed speed of 200 rpm. It is seen from Fig. 7(a) that microcapsules were undamaged up to 200 rpm. About 75% of microcapsules were broken when stirred at 500 rpm in solvent (MTO). Microcapsule stability was affected due to high shear stress, when viscosity of the medium was increased. Only 10% microcapsules were found broken at a viscosity of 250 s (Fig. 7(b)). As the viscosity of commercial paints range between 85 and 150 s, the microcapsules are expected to withstand shear force during mixing and paint application. Mechanical stability of microcapsule is also found affected when they are stirred continuously for longer periods. From Fig. 7(c) it is seen that microcapsules are stable up to 120 min under stirring, and approximately 15% of microcapsules were broken after 4 h of continuous stirring at a speed of 200 rpm. However, during incorporation of microcapsules into paint composition it was observed that, microcapsules could be uniformly mixed without any damage into paint by stirring at around 150 rpm for 15 min. Fig. 8 presents optical photographs microcapsules dispersed in different medium under varying stirring conditions.
Corrosion of metallic substrate takes place when moisture and oxygen are transported through the cracks to the metal–coating interface. Healing of cracks, thus, provides an effective method to prevent corrosion. Performance of linseed oil as a healing material was assessed by exposing specimens coated with paint containing filled microcapsules to salt spray. Before exposure, coated surface was cross-cut up to the metal. Control specimens had the paint without microcapsules. Up to 72 h of exposure, specimens with paint containing capsules were found free from corrosion at the scribed lines (Fig. 10). Control panels, however, suffered from corrosion after 48 h of exposure. Superior corrosion resistance performance of healed films is due to the reason that, linseed oil released from ruptured microcapsules filled the crack and formed a film by oxidative polymerization with atmospheric oxygen which prevented the ingress of moisture and oxygen and thus prevented corrosion. 4. Conclusions Linseed oil along with driers has been successful encapsulated in to urea–formaldehyde shell. Microcapsules having sufficient strength to withstand shear generated during mixing in to paint and paint application. The rough morphology of microcapsule shell has provided good anchoring between microcapsule and paint matrix. Microcapsules in paint films released healing material, which during cracking healed cracks efficiently with satisfactory anticorrosive properties. Acknowledgement The authors sincerely thank to Dr. J. Narayana Das, Director, NMRL for his keen interest, constant encouragement and for giving permission to publish this paper.
3.2. Healing performance References For effective healing, the microcapsules incorporated in the paint film, should break immediately to release healing material when the cracks are generated in the paint film. It is observed that the shell surface of microcapsules is very rough (Fig. 1b), which will provide good bonding with the film matrix. This facilitates in breaking of microcapsules under stress due to cracking. The healing of crack in a paint film recorded under optical microscope at 100× magnification has been shown in Fig. 9. It is observed that the 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
[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 Organic Coatings 63 (2008) 72–78
´ ´ ´ R. Calafiore, T.M.S. Chang, P. de [10] G. Orive, R.M. Hernandez, A. Rodryguez Gascon, ´ J.L. Pedraz, Trends Biotechnol. 22 (February (2)) Vos, G. Hortelano, D.H.I. Lacyk, (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.
[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. Cec´ılio, M.G.S. Ferreira, Electrochem. Commun. 8 (3) (March 2006) 421–428. [15] C. Challener, JCT, January 2005, pp. 50–55.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings C. Suryanarayana a , K. Chowdoji Rao b , Dhirendra Kumar a,∗ a b
Naval Materials Research Laboratory, Shil-Badlapur Road, P.O. Anand Nagar, Additioinal Ambernath (East), Dist. Thane 421506, India 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 2007 Received in revised form 4 April 2008 Accepted 18 April 2008 Keywords: Microencapsulation Self-healing coatings Crack healing Linseed oil
a b s t r a c t Effectiveness of linseed oil filled microcapsules was investigated for healing of cracks generated in paint/coatings. Microcapsules were prepared by in situ polymerization of urea–formaldehyde resin to form shell over linseed oil droplets. Characteristics of these capsules were studied by FTIR, TGA/DSC, scanning electron microscope (SEM) and particle size analyzer. Mechanical stability was determined by stirring microcapsules in different solvents and resin solutions. Cracks in a paint film were successfully healed when linseed oil was released from microcapsules ruptured under simulated mechanical action. Linseed oil healed area was found to prevent corrosion of the substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Encapsulation of functionally active materials in hollow microspheres is an attractive way of storing as well as protecting these from environment till required for fulfilling appropriate applications. Microencapsulated substances have been utilized for sustained drug release [1,2], electro rheological fluids [3], intumescent fire retarding powders [4,5], preservation of flavours [6,7], electro phoretic display applications [8], textiles [9], biotechnology [10,11] and inorganic metal salt catalyst [12], etc. Recently, there has been growing interest in use of microencapsulated materials for healing of cracks generated during service of a polymer based composite materials [13,14]. Microcapsules containing dicyclopentadiene were incorporated in the composite matrix. These capsules rupture and release dicyclopentadiene during crack formation and reacts with Grubbs ruthenium catalyst present in the composites leading to crack repair to restore mechanical properties. Paints are extensively used for modification of substrates either for aesthetic appearance or for corrosion protection. During its service life, the paint film undergoes changes in mechanical properties leading to formation of microcracks which subsequently propagates and exposes substrate to atmospheric moisture and oxygen. This action results in accelerated disbonding of the paint and flake formation from the metal coating interface. Paint coatings can be
considered as a special class of composite materials, comprising binders and pigments. Hence, the concept of self-healing of cracks, as reported for composites, can be adopted for coatings to provide longer durability. An attempt for healing of scratches on automotive coating using temperature dependant elastic properties of polymer has been reported [15]. Here, we report our work on development of self-healing coatings with microencapsulated drying oil. In this study, linseed oil along with driers has been selected as a healing agent due to its film forming ability by atmospheric oxidation. Microcapsules with urea–formaldehyde as a shell and drying oil as a core were synthesized by in situ polymerization [13]. Efficacy of these microcapsules in healing of cracks in an epoxy coating and corrosion protection has been demonstrated. 2. Materials and methods Urea, formaldehyde, ammonium chloride, resorcinol, poly vinyl alcohol (PVA) and red dye (Disperse red 1) were procured in AR grade from Sigma Aldrich. Epoxy resin (XR-87) was purchased from Atul Limited, India; epoxy hardener (Ancamine 2280) was obtained from Air Products India Limited. Linseed oil (commercial grade) was purchased from Jayant Oil Mils Pvt. Limited, India. All chemicals/materials were used without any purification. 2.1. Experimental synthesis of microcapsules
∗ Corresponding author. Tel.: +91 251 2620187; fax: +91 251 2620604. E-mail address: [email protected] (D. Kumar). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.04.008
Microcapsules were prepared by in situ polymerization in an oilin-water emulsion. At room temperature, 260 ml of deionised water
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
73
Fig. 3. FTIR spectrum: (A) urea–formaldehyde resin and (B) shell material of microcapsule.
2.2. Analysis of microcapsule size and shell morphology Microcapsule size analysis was carried out with a particle size analyzer (Mastersizer 2000, Malvern). Surface morphology and shell thickness of microcapsules were determined by scanning electron microscopy, (LEO1455). Microcapsules were mounted on adhesive tape and ruptured with a razor blade for shell thickness measurement. 2.3. Thermal analysis of microcapsules
Fig. 1. SEM micrographs: (a) shell thickness and (b) shell morphology of microcapsules.
and 10 ml of 5 wt% aqueous solution of polyvinyl alcohol (PVA) were mixed in 1000 ml beaker. Under agitation 5 g urea, 0.5 g ammonium chloride and 0.5 g resorcinol were dissolved in solution. The pH was adjusted to approximately 3.5 by using 5 wt% solution of hydrochloric acid in deionised water. One to two drops of octanol was added as an antifoaming agent. 60 ml of linseed oil containing 0.7 wt% cobalt naphthenate and 2.5 wt% lead octoate driers was added slowly to form an emulsion and allowed to stabilize for 10 min under agitation. After stabilization, 12.67 g of 37 wt% aqueous solution of formaldehyde was added. The emulsion was covered and slowly heated and maintained at 55 ◦ C under stirring at 200 rpm for 4 h. Contents were cooled to ambient temperature. Microcapsules from the suspension were recovered by filtration under vacuum. These were rinsed with water, washed with xylene to remove suspended oil. The capsules were dried under vacuum.
Fig. 2. Particle size analysis of microcapsules.
Microcapsules, linseed oil and urea–formaldehyde resin were analyzed using thermo gravimetric analyzer (Auto TGA 2950HR, TA instruments) in nitrogen environment with a sample weight of about 3 mg. Heating rate was maintained at 20 ◦ C/min in the temperature range of 30–800 ◦ C. Similarly samples were also analyzed by using differential scanning calorimeter (SetsysTG-DSC 16, TA instruments) in oxygen environment with sample weight of about 3 mg at heating rate of 20 ◦ C/min, between 30 and 900 ◦ C. 2.4. Linseed oil content in microcapsules Amount of linseed oil present in microcapsules was determined by extracting oil in a soxhlet apparatus. A known weight of microcapsules (WC ) was crushed using pestle and mortar and transferred to a thimble of known weight (Wti ), pestle and mortar were rinsed with xylene and added to thimble. Extraction was carried out using xylene as a solvent for linseed oil. After 1 h of extraction, thimble was 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
Fig. 5. DSC curves of shell capsule and linseed oil.
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 × 100 WC
after every 10 min and the number of microcapsules broken were counted. Further stability was also studied at varying speeds in MTO and epoxy resin solution of viscosity 200 s (Ford cup no. B4 at 30 ◦ C) for 1 h.
2.5. Infrared spectroscopy Spectra of shell and core material extracted from soxhlet apparatus were recorded on Fourier Transform Infrared spectrophotometer (NICOLET 5700 Thermo Electron Corporation). The solid shell material collected after soxhlet extraction was mixed with KBr and palette was prepared for recording spectra. Infra red spectra of extract after soxhlating, neat linseed oil and urea–formaldehyde resin were also recorded. 2.6. Mechanical stability of microcapsules Mechanical stability was determined by subjecting microcapsules to stress generated during mechanical stirring at 200 rpm in mineral turpentine oil (MTO) and epoxy resin solutions of different viscosities. Samples were observed under optical microscope
Fig. 6. TGA curves of capsule ( ), linseed oil (....), and UF resin (
).
Fig. 7. Mechanical stability of microcapsules. (a) Effect of speed on mechanical stability. (b) Effect of viscosity on mechanical stability. (c) Effect of time on mechanical stability.
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
75
Fig. 8. Mechanical stability of microcapsules. (a) In MTO mixed @200 rpm for 1 h. (b) In MTO mixed @200 rpm for 4 h. (c) In epoxy resin mixed @200 rpm. (d) In epoxy resin mixed @500 rpm, (e) and (f) distribution of beads in clear films.
2.7. Preparation and performance evaluation in respect of corrosion 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 of xylene/butanol (4:1 ratio by volume). The solid content of epoxy solution was kept at 90 wt%. A cycloaliphatic polyamine hardner was used in stoichiometric amount for curing. Microcapsules were added under slow agitation to epoxy resin solution at ambient temperature. After mixing for 15 min the composition was mixed with the required amount of hardner. Clean mild steel panels, size 150 mm × 60 mm × 1 mm were coated on one side by brush to obtain an average dry film thickness of approximately 150 m. After 7 days of curing, cross-cut was made on panels and kept at ambient for 24 h. A composition without microcapsules was prepared as a control. Specimens coated with both compositions were exposed for a period 72 h in salt spray cabinet for evaluation of corrosion protection. 2.8. Evaluation of self-healing process The process of crack healing, on a coated surface, was carried out by incorporating microcapsules into a solvent free anticorrosive paint. This paint having solid content 98 wt%, density 1.4 g/cm3 was
used with the hardner described above. Colored linseed oil filled microcapsules were mixed in paint to produce contrast in the crack when 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 of curing a crack was generated in the coated panel by quick manual bending. Panel was immediately kept under objective lens of optical microscope to record healing process initiated due to release of linseed oil from ruptured microcapsules. 3. Results and discussion The formation of urea–formaldehyde capsules has been described by Park et al. [7]. The process comprises reaction of urea and formaldehyde to obtain methylol ureas, which further condenses under acidic conditions to form the shell material. Encapsulation of linseed oil in the capsules takes place simultaneously during formation of crosslinked urea–formaldehyde polymer. Reactants urea and formaldehyde are soluble in water. When the pH is changed to acidic and heated to 55 ◦ C, urea and 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 water compatible. The number of polar groups will gradually reduce as molecular weight of polymer increases. Finally after attaining
76
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
Fig. 9. Microscope photos of self-healing coating films.
certain molecular weight, hydrophylicity of urea–formaldehyde polymer molecule will reduce leading to separation from aqueous phase and get deposited on the already emulsified oil droplets (hydrophobic organic phase). This process continues and a thin shell is formed over oil droplets. The thickness of the shell has been optimized in such a way that the shell contains maximum amount of core material. A shell thickness of 0.2 m was obtained at rpm of 250 to contain 80% linseed oil. Fig. 1 shows the SEM micrographs of (a) capsule shell thickness and (b) microcapsule shell morphology. Fig. 2 shows the particle size distribution of microcapsules. Size ranges from 5 to 100 m. However, most of the particles fall in the size range around 50 m. This is quite satisfactory for using in paints. Microcapsules prepared from urea–formaldehyde resin (shell material) and filled with linseed oil (core material) were characterized using different instrumental techniques. Core and shell of the microcapsules were separated by soxhlet extraction. FTIR spectra were recorded. It is seen from the FTIR spectra of urea–formaldehyde resin and shell material (Fig. 3) that both are closely matching at charac-
teristic peaks of a N H stretching vibration at 1571 cm−1 , a C O stretching vibration at 1650 cm−1 , and a C H stretching vibration at 1460 cm−1 . C N stretching vibrations are shown at 1286 and 1142 cm−1 . The O H peak is shown as a broad absorption peak at 3500–3200 cm−1 . This spectrum confirms that shell material is made of urea–formaldehyde polymer. FTIR spectrum of urea–formaldehyde resin is also matching with spectrum reported in the literature [15]. Spectra of linseed oil and core material have also been found matching (Fig. 4) at characteristic peaks for C O and C C stretching vibrations. In view of above it is established that linseed oil has been successfully encapsulated in urea–formaldehyde shell. Thermal analyses of microcapsules as well as shell and core materials were carried out for further characterization. DSC analysis (Fig. 5) of shell material shows strong endothermic peak at 230 ◦ C, which is attributed to the degradation of UF resin as confirmed from TGA of shell material (Fig. 6). Exothermic peak observed at 155 ◦ C in the DSC curve of microcapsule corresponds to the curing of linseed oil as observed in DSC analysis of only oil (inserted figure in Fig. 5). Degradation of core (linseed oil) com-
C. Suryanarayana et al. / Progress in Organic Coatings 63 (2008) 72–78
77
Fig. 10. Salt spray performance of coatings at different exposure periods.
mences at 300 ◦ C as also shown in TGA curve. From these results it is further established that microcapsules contain both materials, i.e. linseed oil (as core) and UF resin (as a shell).
atmospheric oxygen and has formed continuous film in the crack. This is further confirmed from the superior corrosion resistance performance of healed films.
3.1. Mechanical stability of microcapsules
3.3. Corrosion resistance of self-healing coating films
Microcapsules should have sufficient mechanical strength to withstand shear stress generated during mixing and application of paint. Microcapsules should not break during mixing and application of paint. They should break and release healing material as and when crack is generated in the paint film. Viscosity of the paint, stirring speed and stirring time are the main parameters affecting the stability of microcapsules during mixing and application of paint. Effect of all the three parameters on the stability of microcapsules was studied by stirring microcapsules in epoxy resin solutions of varying viscosities at different speeds for varying time periods. Similar studies were also carried out by agitating microcapsules in MTO and epoxy solution at fixed speed of 200 rpm. It is seen from Fig. 7(a) that microcapsules were undamaged up to 200 rpm. About 75% of microcapsules were broken when stirred at 500 rpm in solvent (MTO). Microcapsule stability was affected due to high shear stress, when viscosity of the medium was increased. Only 10% microcapsules were found broken at a viscosity of 250 s (Fig. 7(b)). As the viscosity of commercial paints range between 85 and 150 s, the microcapsules are expected to withstand shear force during mixing and paint application. Mechanical stability of microcapsule is also found affected when they are stirred continuously for longer periods. From Fig. 7(c) it is seen that microcapsules are stable up to 120 min under stirring, and approximately 15% of microcapsules were broken after 4 h of continuous stirring at a speed of 200 rpm. However, during incorporation of microcapsules into paint composition it was observed that, microcapsules could be uniformly mixed without any damage into paint by stirring at around 150 rpm for 15 min. Fig. 8 presents optical photographs microcapsules dispersed in different medium under varying stirring conditions.
Corrosion of metallic substrate takes place when moisture and oxygen are transported through the cracks to the metal–coating interface. Healing of cracks, thus, provides an effective method to prevent corrosion. Performance of linseed oil as a healing material was assessed by exposing specimens coated with paint containing filled microcapsules to salt spray. Before exposure, coated surface was cross-cut up to the metal. Control specimens had the paint without microcapsules. Up to 72 h of exposure, specimens with paint containing capsules were found free from corrosion at the scribed lines (Fig. 10). Control panels, however, suffered from corrosion after 48 h of exposure. Superior corrosion resistance performance of healed films is due to the reason that, linseed oil released from ruptured microcapsules filled the crack and formed a film by oxidative polymerization with atmospheric oxygen which prevented the ingress of moisture and oxygen and thus prevented corrosion. 4. Conclusions Linseed oil along with driers has been successful encapsulated in to urea–formaldehyde shell. Microcapsules having sufficient strength to withstand shear generated during mixing in to paint and paint application. The rough morphology of microcapsule shell has provided good anchoring between microcapsule and paint matrix. Microcapsules in paint films released healing material, which during cracking healed cracks efficiently with satisfactory anticorrosive properties. Acknowledgement The authors sincerely thank to Dr. J. Narayana Das, Director, NMRL for his keen interest, constant encouragement and for giving permission to publish this paper.
3.2. Healing performance References For effective healing, the microcapsules incorporated in the paint film, should break immediately to release healing material when the cracks are generated in the paint film. It is observed that the shell surface of microcapsules is very rough (Fig. 1b), which will provide good bonding with the film matrix. This facilitates in breaking of microcapsules under stress due to cracking. The healing of crack in a paint film recorded under optical microscope at 100× magnification has been shown in Fig. 9. It is observed that the 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
[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 Organic Coatings 63 (2008) 72–78
´ ´ ´ R. Calafiore, T.M.S. Chang, P. de [10] G. Orive, R.M. Hernandez, A. Rodryguez Gascon, ´ J.L. Pedraz, Trends Biotechnol. 22 (February (2)) Vos, G. Hortelano, D.H.I. Lacyk, (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.
[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. Cec´ılio, M.G.S. Ferreira, Electrochem. Commun. 8 (3) (March 2006) 421–428. [15] C. Challener, JCT, January 2005, pp. 50–55.