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Novel Polyurea Microcapsules Using Dendritic Functional Monomer: Synthesis, Characterization and Its Use in Self-Healing and Anticorrosive Polyurethane Coatings. Pyus Dilipkumar Tatiya, Rahul Kishore Hedaoo, Pramod P. Mahulikar, and Vikas Vitthal Gite Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie301813a • Publication Date (Web): 18 Dec 2012 Downloaded from http://pubs.acs.org on January 9, 2013

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Industrial & Engineering Chemistry Research

Novel Polyurea Microcapsules Using Dendritic Functional Monomer: Synthesis, Characterization and Its Use in Self-Healing and Anticorrosive Polyurethane Coatings.

Pyus. D Tatiya, Rahul K. Hedaoo, Pramod. P. Mahulikar, Vikas V. Gite* Department of Polymer Chemistry, School of Chemical Sciences, North Maharashtra University, Jalgaon, Maharashtra, India- 425 001 Email: [email protected] Phone No: + 91 257 2257431

Fax No: + 91 257 2258403

1 ACS Paragon Plus Environment


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Abstract Polyamidoamine (PAMAM) dendrimer of zero generation was synthesized and characterized by Fourier Transform Infrared Spectra (FTIR), nuclear magnetic resonance (NMR) spectroscopic techniques. A novel chemistry has been developed to synthesize polyurea microcapsules containing solvent and linseed oil as the active healing agent by interfacial polymerization of commercial methylene diphenyl diisocyanate (MDI) and dendritic 0.0 G PAMAM capable of cross linking to form a shell material. Spherical with some irregular shape microcapsules were observed with average diameter from 20 to 270 µm at different agitation rates (3000-8000 rpm). Interfacial interaction between polyurea microcapsules and polyurethane coating were studied by (FTIR) and it showed that chemical bonds were formed by the reaction between isocyanates and the amine groups present on wall of microcapsules. Thermal stability of microcapsule showed that prepared microcapsules experienced the excellent stability up to 380

0

C. The

anticorrosive performance of PU coating loaded with different percentage of microcapsules was carried out in 5% NaCl aqueous solution. The results showed that composite provides

satisfactory anticorrosive

property at 5 % capsule loading under an accelerated corrosion process. The idea and approach presented in this work have potential to fabricate microcapsules which could provide better anticorrosive and mechanical properties to coating composite.

Keywords: Microencapsulation, PAMAM dendrimer, self healing coatings, anti corrosive, polyurea.


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Introduction For last three decades, highly branched dendritic polymers such as hyperbranched or dendrimers have been paid great attention because of their unique molecular architecture.

1-4

Linear polymers,

synthesised by the classical polymerization process are usually random in nature and produces molecules of different size having some smaller or longer branches. In contrast to traditional linear polymers, dendritic polymers exhibit significantly improved physical and chemical properties. As a result of controlled size, more functional end groups, globular and non-entangled structures, dendritic polymers show many useful properties like interior cavity, high reactivity, decreased melt and solution viscosity, good solubility, etc. as 5

compared to their linear analogs of similar molecular weight. They can also be tuned with respect to functionality and polarity to adjust the properties for certain applications.

6

The special architecture and behavior make dendritic polymers suitable for various applications in different fields like adhesives, drug deliveries

12

7

coatings,

8

catalysts,

9

chemical sensors,

10

light harvesting materials,

11

etc. Use of highly-branched polymers (hyperbranched and dendrimers) enhanced the

curing rate of epoxy resin

13 a, b

improved the flow and performance characteristics of reduced VOC

14

coatings , lead to excellent drying with low viscosity in the field of alkyd, rapid curing in polyurethanes(PU) and dramatically increased toughening in amine cured epoxies.

15

Dendrimer (0 G PAMAM) has been

recently reported as a crosslinking agent and induced better gas separation performance for polyimide 16

film.

Che et al have demonstrated the effect of single-walled carbon nanotubes functionalized with

PAMAM dendrimer in epoxy composite.

17

The result revealed that the grafted from PAMAM acts as a

covalent matrix binding agent and also enhance the dispersion of CNT which tend to increase mechanical properties of composite. In the recent years, considerable advances have been made in using the unique architecture of a dendrimer in host-guest system to protect and carry different materials.

18

19

Microcapsulation , in several fields including agriculture, food, pharmaceutical, cosmetic, printing, textile and protective/smart coatings has been used to encapsulate many different active materials like pesticides, flavours, drugs, enzymes, ink, dyes and healing agents respectively.

20, 21

Microcapsules have

been prepared from a variety of material such as PU, polyurea, urea-formaldehyde (UF), phenol formaldehyde (PF), melamine formaldehyde (MF), etc.

22

Although polyurethane is a group of versatile

polymeric material, having excellent physical, chemical and mechanical properties and demanded in the field of industrial coatings,

23

the research on its performance enhancement for anticorrosive nature is in

progress. In the field of polymeric composites, microcapsules containing self-healing agents have been received more attention to have protective/smart coatings. M. Huang et al synthesised the PU microcapsules, containing HDI as a core material and embedded in polymeric composites to achieve the 24

self-healing properties and improved corrosion resistance of the coatings. The main features required in preparation of self healing composites are the microcapsules which act as self healing reservoir must possess sufficient strength in order to withstand the stress of application, impermeable shell wall to prevent leakages and diffusion of liquid healing agent along with high bond capacity to host polymer.

25

Interfacial

interaction between microcapsules and polymer matrix also plays an important role to decide the good



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mechanical properties and self healing function of coating composite. Li et al demonstrated that PF microcapsules modified with silane coupling agent improves the interfacial performance of PF microcapsule-epoxy composite.

26

A striking contribution by Gunter Klug and Jurgen Weisser

27

reported

the possibility of using a guanidine compounds and amine as crosslinkers for fabrication of robust polyurea microcapsules which motivated us to use of dendritic poly (amidoamine), due to the similarity in its chemical functionality to guanidine compounds. Therefore we decided to use low generation PAMAM dendrimer for the preparation of polyurea microcapsules which can serves as a functional monomer as well as crosslinking means. The present work dealing with a new approach to form polyurea microcapsules via interfacial polymerization of aromatic diisocyanate (MDI) and zero generation dendritic PAMAM with several amino groups capable to form crosslinked polyurea shell wall. The main objective set in using dendritic polymer as a one of the reactant for preparation of microcapsules is to encapsulate self healing agent with shell having amine functional groups that will provide good strength and better interfacial interaction through cross linked structure and chemical bonding with polymer binder (Figure 1). To our knowledge this is first report of preparation of microcapsules to encapsulate healing agent by interfacial polymerization between diisocyanate and multifunctional PAMAM dendrimer. In service of this objective, hydrophobic linseed oil (as self healing agent) and suitable solvent was utilized as a core material. The performance of formed microcapsules for corrosion protection in a PU coating by healing of cracks was evaluated by immersion studies.

Figure 1: Interaction between microcapsule and PU matrix

Material and Methods Materials The materials used in experiments include methylene diphenyl diisocyanate (Kishore Polyurethanes Pvt. Ltd., Nasik, India) was of commercial grade and used as such. Ethylenediamine (EDA) and methacrylate was purchased from s. 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


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Brand, Pune, India. Acrylic polyol SETALUX 1196 VV-60YA and Desmodur W were used as received from Nuplex resin, USA and Bayer Material Science.

Experimental Synthesis of Microcapsules Synthesis of 0.0 G PAMAM Dendrimer. Synthesis of PAMAM dendrimer was carried out in two-step process. First step was Michael addition reaction between ethylenediamine (EDA) as an initiator core and methacrylate resulting to tetra ester and second step was amidation of the resulting esters with large excess of ethylenediamine.

28 a, b

The first step involved in synthesis of half generation (- 0.5 G) PAMAM by attaching four acrylate moieties on each amino group of ethylenediamine (EDA) (Scheme 1). To a 150 mL, two neck flask equipped with condenser, stirrer and thermometer 24.1 g of methacrylate (0.28 moles) was taken and to it 4.0 g of ethylene diamine dissolved in methanol added at room temperature with stirring. The mixture was then allowed to stand for 48 h at room temperature and at the end excess 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 one liter reaction flask containing 65.42 g of ethylenediamine dissolved in methanol was added the - 0.5 G PAMAM (10 g) dissolved in methanol. The mixture was then allowed to stand under stirring for 55 h at room temperature. After completion of reaction excess EDA was removed by vacuum distillation. The compound obtained was referred as “0.0 G PAMAM”.

O

O

OMe

MeO 48 hrs NH2 +

H2N

4

Ethylenediamine(EDA)

CH3OH MeO

Methacrylate

O

H2N NH2 4 moles H N

O

O

H2N N H2N

N H

H N

N O

O

N H

N

N

COOCH3

-0.5 G

OMe O

55 hrs CH3OH

NH2 NH2

0.0 G PAMAM

Scheme 1: Synthesis of PAMAM dendrimer up to zero generation



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Preparation of Microcapsules. Microcapsules were prepared by interfacial polymerization of an oil in-water emulsion technique.

29

About 50 mL of aqueous solution containing 1.0 wt % of PVA as a protective colloid was added into a 100 mL flask. Then 1.5 g of MDI and 2 g of linseed oil were dissolved into 10 mL of xylene in order to obtain organic phase. Subsequently, both organic and aqueous phases were mixed and subjected to high speed 0

disperser at defined rpm (3000, 5000 and 8000 rpm) and temperature of 25 C for 5 mins 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 % of PVA. Then emulsion was stirred at 300 rpm in a three neck round bottom flask and PAMAM solution was added drop wise. Addition was continued for 5 mins under agitation. The 0

reaction was maintained at ambient temperature around 30 C under stirring for next 30 minutes and then 0

for next 1.5 h at temperature around 45 – 50 C. Scheme 2 represents the shell wall forming reaction. Formation of microcapsules was checked by observing reaction mixture time to time under optical microscope. After 1.5 h when stable microcapsules were observed, reaction mixture was allowed to cool at room temperature under stirring. Then, microcapsules from the suspension were recovered by filtration under vacuum, rinsed with water, washed with xylene to remove suspended oil and dried under vacuum.

H N

O

O

H2N N H

NH2

N

N H2N

H N

O

N H N H

H N

H O N

O

O

H N

N H2 C

N N

H N O

NCO

MDI

0.0 G PAMAM

H2 C

H2 C

OCN

NH2

N H

O

+

N H

O N H

N

H N

H2 C

N O

O

N H

H N

N O

H2 C

N H

Polyurea

Scheme 2: Illustrate possible reaction mechanism for the formation of polyurea shell of microcapsule.


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Preparation of PU Coating Loaded With Microcapsule. Polyurea/PU coatings were prepared by dispersing polyurea microcapsules (from 0 to 5 % ) in 40 wt % solution of acrylic polyol (Setalux 1196 VV-60YA) in xylene with Desmodur W (H12MDI) (20 w % of polyol) and catalyst dibutyltin dilaurate. The coatings were applied by brush on all sides of steel panels of dimension (150 mm X 70 mm X 1 mm) and allow to cure at room temperature for 24 h.

Characterization. Spectra of the samples were recorded on FTIR spectrophotometer (Shimadzu 8400, Japan) by using KBr pellets. Preliminary observations and morphological study of microcapsules was completed on optical microscope (Labomed Sigma, 2124001, Texas) on 40-100 X resolutions. Microcapsules were observed for their morphological study under scanning electron microscopy (JEOL JSM 6360 and JEOL JSM 5400, Japan). Particle size of microcapsules was analyzed by using laser particle size analyzer (Mastersizer 2000, M41100167, Malvern, UK). Thermal decomposition of microcapsules was carried out using thermo gravimetric analyzer (Shimadzu TGA 50, Japan) by heating the sample from room 0

0

temperature to 800 C at the heating rate of 10 C /min in an inert nitrogen environment.

Determination of Core Content. The amount of active material in microcapsules was determined by xylene extraction of core material using Soxhlet apparatus. Firstly, a certain amount of dried microcapsules (Wm) was sealed in a filter paper bag. Then the sample bag (Ws) was placed in a Soxhlet apparatus. The weight of round bottom flask was noted (Wo). Extraction was carried out with xylene as a solvent for linseed oil. After 6 h of extraction, the sample bag was carefully taken out from Soxhlet apparatus and after completely wearing the solvent; it was dried in oven for 12 h. Meanwhile the core material collected in the RB was subjected to vacuum distillation to remove xylene and final weight of RB containing core material noted (W ov). The final weight of the dried sample bag (W sd) was also noted. Thus percentage of core content (αc), shell material (αs) and solvent entrapped (αsv) were determined with the help of following formulas.

Wov − Wo   ×100  Wm 

(1)

= 

Ws − Wsd   × 100  Wm 

(2)

= 100 − (αc + αs )

(3)

a) Core content (α c, %)

= 

b) Shell material (α s, %)

c) Solvent entrapped (α sv , %)



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Immersion Study. The performance of formed microcapsules for corrosion protection in a PU coating composite by healing of cracks was demonstrated through immersion studies. The steel panels coated with PU composite containing microcapsules loaded in different concentration were compared against steel sample coated without loading microcapsules. Induced crack was developed by manual hand scribing with a razor blade in order to rupture the microcapsules and after the scribing, the samples were allowed to heal at room temperature for 48 h. Immersion studies of these coatings panels were carried out in salt solution (NaCl, 5 wt. %). The corrosion of the damaged area was monitored at different intervals periods by visual inspection using digital camera (Canon A 3100). The samples 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 introduction of microcapsules, cross hatch adhesion test was carried out for above prepared four coating samples. Adhesion test was done with cross hatch cutter (Elcometer, 107) according to EN ISO 2409.

Results and Discussion Spectroscopic Techniques. FTIR spectra of -0.5 G and 0.0G PAMAM dendrimers (supporting information) contain a -1

characteristic band at 1735 cm , which is assigned to the ester group; the absence of absorption band for >N–H stretching at 3300 - 3500 cm

-1

indicates all –NH2 groups present in EDA are consumed during

formation of – 0.5 G PAMAM. Spectrum (B) shows absorption bands at 3475 cm

-1

which is assigned to

-1

>N–H stretching of primary amine, 3288 cm absorption band correspond to >N–H stretching of secondary -1

amine. The absorption bands at 2935, 2819 cm corresponds to aliphatic >C–H stretching. The frequency -1

-1

at 1641 cm observed due to >C=O stretch of amide. The absorption bands at 1567, 1483, 1327 cm are -1

due to >N–H bending of N substituted amide; at 1117 cm due to –C–C– bending indicates the formation of zero generation of polyamidoamine (PAMAM). The structure of PAMAM also confirmed by NMR spectroscopy performed on Varian Mercury YH-300 MHz spectrometer using DMSO-d6 as a solvent and TMS as a standard; d 1.87(-NH2), d 2.18(-CH2C=O); d 2.99(-CH2NH2); d 2.44(-CH2-NH-C=O); d 7.99(-NHC=O); d 2.6(>N- CH2); d 2.53(-CH2-CH2–NH2) (Scheme 1).


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Figure 2: Illustrates the FT-IR spectra of (A) Linseed oil (B) Polyurea microcapsules containing core material and (C) Extracted polyurea shell

Figure 2 shows the FTIR spectra of the shell, linseed oil and microcapsules. For linseed oil, -1

spectrum (A) revealed stretching vibration peak of ester >C=O at 1745 cm , aliphatic –C=C– stretching -1

-1

vibration at 1693, 1546 cm , C–H bend of methylene group at 1462 cm and –C–O stretching vibration -1

peak at 1190 cm . -1

The spectrum of polyurea shell (C) showed the strong band at 3315 cm , which is assigned to N– H stretching vibration. The C=O stretching frequencies for –CONH– presents in polyurea microcapsules -1

-1

observed at 1658 cm . The absorption peak at 1600 cm corresponds to –C=C– present in aromatic ring. Absorption band at 2280 cm

–1

is assigned to –NCO group from unreacted MDI (wall-forming monomer).

Furthermore, absorption band for –N–H bending of N-substituted amide in polyurea can be observed at -1

1517 cm . The spectrum of microcapsules (B) showed the characteristics absorption bands for >C=O (1745 -1

cm ), aliphatic

-1

-1

-1

–C=C– (1693 cm ), C–H (1462 cm ) and –C–O– (1190 cm ) present in spectrum of

linseed oil along with the entire vibration bands observed in polyurea shell spectrum. Hence, from FTIR data of linseed oil, polyurea shell and microcapsules, it can be concluded that linseed oil is present in polyurea microcapsules.



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Core Materials in Microcapsules. Viability of the encapsulated linseed oil dissolved in xylene was initially assessed by a simple visual inspection under optical microscope and video recording (supporting information) by digital camera. A small amount of dry capsule powder was placed on glass slide after ﬁltration and drying. Under the optical microscopic observations the one specific selected microcapsule was pricked with the help of pointed capillary. It was observed and recorded that encapsulated liquid was emerged from the burst capsule, confirmed encapsulation of linseed oil. From the Soxhlet extraction it was concluded that about 27 % core material and 28 % solvent encapsulated within the microcapsule and remaining 42 % mass was from polyurea shell wall material.

Microcapsule Size and Size Distribution. Microcapsule size and size distribution are affected by number of factors including emulsifier concentration, temperature, agitation rate, geometry of mixing equipment, etc.

24

In the present

communication we studied the effect of agitation rate on microcapsule size and size distribution by keeping other factors constant. Figure 3 represents the polyurea microcapsules prepared at different agitation rate from 3000 – 8000 rpm. Microcapsules primarily observed under optical microscope from 40 X to 100 X resolution. The result showed that nearly spherical shape microcapsules were obtained in all the cases. Further, it was observed that the microcapsules size decreases with increasing agitation rate applied during emulsion step. This may be because at high agitation rate, larger droplets are experienced strong shear force which result breaking of large droplets into smaller one, while dominating interfacial tension at 30

low agitation rate led to larger size microcapsules.

Figure 3: PU microcapsules obtained at various agitation rates (a) 3000 rpm (Magnification: 10 x) (b) 5000 rpm (Magnification: 10 x) and (c) 8000 rpm (Magnification: 40 x)


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Scanning Electron Microscopy. The surface morphology of microcapsules synthesized under various agitation rates was also observed using SEM micrographs and the results are presented in Figure 4. From the SEM images it was cleared that the spherical shape microcapsules with non porous, compact shell wall were obtained at all agitation rates with average diameter from 20 to 270 µm. Non porous shell wall ensures the impermeability of prepared capsules towards leakages and diffusion of liquid healing agent. Further we realized that at low agitation rates, surface of microcapsules were comparatively smooth with some contraction than microcapsules at higher agitation rate [Figure (i) to (iv)]. While at higher agitation rate (8000 rpm), microcapsule showed wrinkled, damaged walls and intense contraction [Figure (v) and (vi)]. It is well known form the literature that wrinkles observed on microcapsules may be due to the interaction of fluid induced shear forces, shell determined elastic forces, inhomogeneous reaction kinetics and compression forces acting on the membrane.

31

At higher agitation rate, more surface area are available for interacting

the above mentioned forces and due to this reason intense contraction and wrinkles may formed on microcapsules.

Figure 4: SEM Micrographs of polyurea microcapsules obtained at 3000 rpm (‘i’ and ‘ii’), 5000 rpm (‘iii’ and ‘iv’) and 8000 rpm (‘v’ and’ vi’)



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Particle Size Analysis. Figure 5 shows the mean particle sizes and particle size distribution of the microcapsules filled with linseed oil as core material prepared at different agitation rates at emulsion stage. The mean particle sizes of the prepared microcapsules were 266, 147 and 24 µm at 3000, 5000 and 8000 rpm, respectively. In present study it has been observed from the graph that the particle size distribution became narrower and the average particle size became smaller with increasing agitation rate. This finding is in favor with the observations reported previously.

32

Higher agitation rate tends to form finer oil droplets in emulsion

systems and also favored homogenization of the emulsion which results into narrow uniform size distribution of capsules. Therefore, it can be deduced that the microcapsule of a desirable particle size could be prepared through the optimization of agitation rate. From the SEM and particle size analysis observation we conclude that 5000 rpm could be proper agitation speed for manufacturing microcapsules. Moreover, the particle size distribution would be improved further by optimizing the surfactant concentration along with agitation speed.

Figure 5: Particle size histogram of PU microcapsules at different agitation rate a) 8000 rpm b) 5000 rpm and c) 3000 rpm

Thermogravimetric Analysis. The TGA themograms of extracted shell and microcapsules loaded with linseed oil are shown in 0

Figure 6. TGA of microcapsules showed that there is less than 3% weight loss observed up to 120 C that may be due to absorbed moisture. Further, microcapsules experienced three stages of weight loss. Initial 0

stage weight loss observed from 125 – 270 C presumably due to escaping entrapped xylene from the 0

microcapsules. Second stage weight loss from 270 -375 C attributed to degradation of linseed oil. The


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0

third stage weight loss observed above 375 C was associated with degradation of shell wall material which is in concord with the results for extracted shell material. Extracted shell wall decomposed 0

0

substantially above 380 C with about 24 % of the original mass remaining at 800 C which exhibit the advanced thermal stability of the polyurea shell wall prepared from PAMAM as compared to polyurethane, 33 polyurea, PF and UF shell walls.

30, 32, 33, 34

Therefore present work revealed that newly fabricated

polyurea microcapsules have better ability to preserve the core material from surrounding environment. The compositional analysis of microcapsules in terms of percent weight loss was also done through TGA data and calculation of derivative weight loss. It was found that total content of core was about 55 % of which 28 % was contributed by entrapped solvent and 27 % through linseed oil and remaining about 42 % was of shell material, which are in good agreement with the results obtained from Soxhlet extraction method.

Figure 6: Thermogravimetric analysis of microcapsule and shell



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Interfacial Interaction Study. It was demonstrated that some coating adhesion will always be sacrificed by addition of microcapsules.

35

To reveal the change in adhesion induced by the introduction of microcapsules adhesion

test was performed and the results are shown in Figure 7. PU samples without microcapsules and with 1 % microcapsules showed better adhesion. Adhesion decreased for 3 and 5 % microcapsules containing PU coatings. In this study although we observed declined adhesion of coating with rising microcapsule percentage, it was still less than 15 % for maximum microcapsule loading (5 %). The possible explanation for better adhesion is high polarity exerted on the surface of microcapsule due to the presence of free amino groups on the wall of microcapsule results into increase in interaction between shell wall, PU and substrate that would have conserved adhesion between matrix and substrate

Figure 7: Illustrates the adhesion rating for coating with different loading of microcapsules

The interfacial interaction as seen from adhesion test can also be concord by IR analysis and the -1

resulting spectrum is shown in Figure 8. Stronger intensity of the vibration at 1635 cm (C=O stretching frequencies for –CONH) in PU matrix embedded with microcapsule compared with pristine PU matrix confirms additional urea character formed by reaction of free amine groups of shell wall and diisocyanate. Further intensity of unreacted –NCO stretching frequency from MDI observed in PU matrix was weaken in coating composite illustrate the consumption of –NCO groups due to reaction with amine group. From the Figure it was illustrated that broad absorption peak for –N-H stretching in PU matrix shifted to higher wave number in microcapsule loaded coating. Probably it may be due to introduction of microcapsule within PU matrix disturbed intermolecular hydrogen bonds formed in PU matrix, which affect the distinct construction formed in pristine PU matrix.


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Figure 8: Illustrates the FTIR spectra of (A) Microcapsule (B) Microcapsule + PU matrix and (C) PU matrix Immersion Study. The results from accelerated corrosion immersion test (Figure 9) in salt solution clearly demonstrated that compared with control sample, specimens with increasing microcapsules content from 2 to 5 % (Figure 9 j, k and l) revealed decreasing order of corrosion and blister at the scribed lines after 120 h of immersion studies. In contrast, rapid corrosion was seen in the control specimen within 24 h (Figure 9 a) and exhibited severe corrosion after 120 h, most prevalently within the groove of the scribed area and also extending rusting across the substrate surface (Figure 9i). From the images of the coated steel panels, it can also be illustrated that the scratched area of the steel panel coated with PU coating with 5 % microcapsules showed practically least corrosion after 120 h of immersion in salt solution (Figure 9 l). Corrosion resistance performance of the coatings is may be due to filling of cracks by newly formed film through oxidative polymerization of linseed oil released from the ruptured microcapsules. The healed crack in this way restricts the diffusion of salt ions and thus protects the substrate from the corrosion process even after 120 h, while severe corrosion was observed in and around the crack of the control specimen at the beginning. Therefore, it could be concluded that polyurea microcapsule containing linseed oil (healant) offered better anticorrosion property at 5 % capsules loading to the PU coating on steel panels tested for accelerated corrosion process.



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Figure 9: Corrosion Test results of PU coatings loaded with and without microcapsules after 24 h, 72 h and 120 Conclusion 1 0.0 G PAMAM dendrimer was synthesized and characterized by FTIR and H NMR spectroscopy. Linseed oil filled polyurea microcapsules were synthesized by interfacial polymerization of 0.0 G PAMAM dendrimer and MDI. Encapsulation of linseed oil within polyurea shell wall confirmed by FTIR study of microcapsules, core and shell material. Spherical microcapsules with mean average diameter in the range of 20-300 µm were prepared by adjusting agitation rate over the range of 3000-8000 rpm. Polyurea microcapsules prepared by using PAMAM as one of the reactant shows grater thermal stability up to 380 0

C. Core content of resultant microcapsule was about 55 % as derived from derivative TGA. Microcapsules

having amino functional wall shows improved adhesion performance with both polymer matrix and substrate through chemical bonding which is confirmed by FTIR and adhesion test. PU coatings on steel substrate embedded with the polyurea microcapsule containing linseed oil showed escalating corrosion protection capability with increasing microcapsules loading from 2 to 5 % and proved better anticorrosive property at 5 % capsules loading under accelerated corrosion testing.


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Acknowledgement The authors are very grateful to Department of Science and Technology (DST), Govt. of India for providing INSPIRE fellowship. We also acknowledge Kishore Polyurethanes Pvt. Ltd. Nasik, India for providing the methylene diphenyl diisocyanate, Indofil Industries Ltd. Thane, India for helping in determination of particle size.

Associated Content Supporting Information

IR 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 at http://pubs.acs.org/.



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