Composites: Part B 49 (2013) 6–15

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Composites: Part B

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Effects of processing conditions of poly(methylmethacrylate) encapsulatedliquid curing agent on the properties of self-healing composites

Qi Li a, Ananta Kumar Mishra a, Nam Hoon Kim b, Tapas Kuila c, Kin-tak Lau c,d,e, Joong Hee Lee a,b,c,⇑a BIN Fusion Research Centre, Department of Polymer and Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Koreab Department of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Koreac WCU Programme, Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Koread Centre of Excellence in Engineered Fibre Composites, Faculty of Engineering and Surveying, University of Southern Queensland, Toowoomba, Australiae Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Special Administrative Region

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 September 2012Accepted 15 January 2013Available online 31 January 2013

Keywords:A. Thermosetting resinB. Mechanical propertiesD. Thermal analysisE. Cure

1359-8368/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compositesb.2013.01.011

⇑ Corresponding author at: BIN Fusion Research Ceand Nano Engineering, Chonbuk National UniversitRepublic of Korea. Tel.: +82 63 270 2342; fax: +82 63

E-mail address: jhl@jbnu.ac.kr (J.H. Lee).

A series of microcapsules were prepared by solvent evaporation technique using liquid curing agent,polyetheramine as the core material and poly(methylmethacrylate) as the shell material. The desiredmorphology, shell wall thickness, curing agent content, and size distribution of microcapsules have beenobtained by fine tuning the processing conditions such as reaction temperature, core–shell weight ratio,agitation speed, and emulsifier concentration in the medium. The resulting microcapsules exhibit excel-lent thermal and curing agent storage stability. Maximum healing efficiency of 93.50% has been obtainedwith 15 wt.% epoxy containing microcapsules. The microcapsules containing liquid curing agent can effi-ciently be utilized for the fabrication of epoxy-based self-healing composites.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The development of self-healing materials is an area of greatinterest due to their self-healing capability in case of a damageevent [1–5]. The main strategy behind self-healing techniques isto encapsulate the healing agents by fragile-walled capsules andembed them in a matrix. As soon as cracks destroy the capsules,the healing agent would be released into the path of a propagatingcrack and bind them [6]. Such an outstanding capability makesthem promising materials in a wide range of applications.

Literature report suggests the encapsulation of several self-healing agents such as dicyclopentadiene, polydimethylsiloxaneand epoxy to prepare the self-healing thermoset materials [6–14]. However, encapsulations of curing agents for epoxy have notyet been successfully applied in self-healing systems [15–17]. Thisis mainly due to the difficulty in microencapsulation of suitablehardeners for curing epoxy [18]. The traditional amine-type curingagents for curing epoxy at room temperature are highly active, andhence, difficult to encapsulate in water or organic solvents [19–23].Due to the complexity associated with the encapsulation of curingagents, a scant amount of research is reported in the literature. Afew researchers have used the in situ technique to prepare self-

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ntre, Department of Polymery, Jeonju, Jeonbuk 561-756,270 2341.

healing materials, where the curing agent and the monomers aredispersed in water and agitated to prepare the encapsulated curingagents [18,24–26]. However, difficulty in maintaining the pH of thesystem and incomplete polymerization of the monomers used toprepare the shell materials limit the final applications.

This has motivated us to prepare microcapsules containing aliquid curing agent which can be easily released during any dam-age event. Polyetheramine (used as the hardener for epoxy) hasbeen chosen as the curing agent for microencapsulation due toits flowability and low temperature curability. Considering thatpolyetheramine is very active and not many polymers are suitablefor its encapsulation, poly(methylmethacrylate) (PMMA) was cho-sen as the shell wall material. PMMA also has the merit of highchemical stability, biocompatibility, and better compatibility withepoxy [27,28]. PMMA microcapsules have been prepared by a con-trolled phase separation process within droplets of an oil-in-water(O/W) emulsion. The solvent evaporation technique was employedto form a polymer shell around the reactive amine in this work.

The shell wall thickness, curing agent content, size distribution,and storage stability of the microcapsules play vital roles in theself-healing efficiency of the resulting composites. Hence, the ef-fect of processing parameters such as temperature, feeding weightratio of core–shell, agitation speed, and emulsifier concentrationon the aforementioned properties and morphologies of the micro-capsules have been explored. The effect of the microcapsules (pre-pared at different processing parameters) on the mechanical andself-healing properties of epoxy have also been investigated.

Q. Li et al. / Composites: Part B 49 (2013) 6–15 7

PMMA microcapsules containing polyetheramine have been char-acterized by fourier transform infrared (FTIR) spectroscopy, scan-ning electronic microscopy (SEM), optical microscopy (OM) andthermogravimetric analysis (TGA) to investigate their chemicalstructure, surface morphology, size distribution, and thermal sta-bility, respectively.

2. Experimental

2.1. Materials

Polyetheramine (D-230) (density: 0.92 g/ml) was purchasedfrom Kukdo Chemicals, South Korea. PMMA (average Mw:996,000; Tg: 105 �C) used for shell wall material was supplied bySigma–Aldrich, Germany. Dichloromethane (DCM) and polyvinylalcohol (PVA) were also obtained from Sigma–Aldrich, Germany.An SPI-Pon-Araldite epoxy embedding kit purchased from SPI-Chem chemicals, USA, was used to embed the microcapsules. Allreagents used in this work were of analytical-grade and used with-out further purification.

2.2. Preparation of PMMA microcapsules containing the curing agent

Calculated amounts of curing agent and PMMA were dissolvedin DCM (30 ml), which act as the dispersed phase. The above solu-tion was added to the continuous phase (50 ml of aqueous PVAsolution) under high-speed agitation at room temperature to pro-duce an O/W emulsion. The resulting mixture was then pouredinto a 150 ml aqueous solution with the calculated amounts ofPVA with continuous energetic agitation. DCM was allowed toevaporate completely to produce PMMA microcapsules containingthe core material (curing agent). The microcapsules were washedseveral times with deionized water and then dried. Table 1 repre-sents the designations of the microcapsules prepared by varyingthe processing parameters. The synthesized microcapsules are des-ignated as cPa-b-c-d, where, cP, a, b, c, and d represent the encap-sulated curing agent, reaction temperature, core–shell weightratio, agitation speed, and emulsifier concentration, respectively.

2.3. Preparation of self-healing epoxy specimens

In order to prepare fully cured epoxy matrix, the stoichiometricratio of the epoxy and the curing agent required is 90/30. Based onthis assumption, the excess wt.% of epoxy would be achieved byadding extra wt.% of epoxy beyond this ratio (for example, A-10-epoxy indicates additional 10 wt.% epoxy involved system). Thus,the A-10-epoxy composite was prepared by mixing 103 parts ofepoxy with 30 parts of curing agent. In order to test self-healingefficiency, a tapered double cantilever beam (TDCB) specimenwas used. The specimens were designed in an open silicone rubbermold. Self-healing epoxy-based TDCB specimens with differentwt.% of microcapsules were molded using a silicone rubber moldand cured for 24 h at room temperature.

Table 1Weight loss (%) of the microcapsules prepared at 20 �C with various processing conditions

Temperature (�C) Core–shell weight ratio (core/shell) Agita

1 2 4 6 200

60 0.12 0.17 0.15 0.35 0.1580 0.33 0.33 0.43 0.77 0.41

100 0.81 0.88 1.02 1.74 0.94120 2.01 2.20 2.67 4.30 2.59

2.4. Characterization

The surface morphology and shell thickness of the PMMAmicrocapsules were analyzed with SEM (S-3000N, HITACHI, Japan).The microcapsules were embedded in an SPI-Pon-Araldite epoxy,which was cured at 60 �C for 6 h and cut by a razor blade at roomtemperature. The shell thickness of microcapsules was measuredby observing the cross-sections of the embedded sample. The sizedistribution of the microcapsules was investigated using an opticalmicroscope (OM, IMS-M-345, Sometech instrument, Korea). Morethan 250 samples were measured to obtain the size distributionof microcapsules. FTIR spectroscopic study was performed on aFTIR spectrophotometer (JASCO FTIR 4100, Japan) to identify thefunctional groups present on the specimen. The samples wereground to fine powder and made into disc shaped palettes withpotassium bromide powder for FTIR measurements.

The core content of the prepared microcapsules was deter-mined by an extraction method using methanol as the extractingsolvent [10,26]. Microcapsules were ground with a mortar and pes-tle at 125 �C. The crushed microcapsules were collected andwashed with methanol several times, then dried at room temper-ature. Knowing the initial weight of intact microcapsules (Wi)and the weight of residual shell wall (Ws) of microcapsules, theshell wall contents (Wshell) and core contents (Wcore) of the micro-capsules were calculated as:

Wshell ¼Ws

Wi� 100% ð1Þ

Wcore ¼ 1�Ws

Wi

� �� 100% ð2Þ

The amount of curing agent in the microcapsules was alsodetermined by TGA (TA Instrument, Q50, USA) under nitrogenatmosphere at a heating rate of 10 �C/min.

Curing agent storage stabilities of the microcapsules were mea-sured by calculating their weight loss at periodic intervals uponexposing the microcapsules to room temperature and upon heattreatment at different temperatures.

The notched impact strengths of the resulting composites andvirgin epoxy were measured using a Zwick/Roell impact tester(HIT25P, Germany) according to ASTM D-256 standards whilekeeping the sample dimensions at 63.5 � 12.7 � 6.35 mm3 with anotched depth of 1.66 mm. The tensile samples were preparedaccording to ASTM D-638. The tensile properties were measuredon an MTS system (Landmark 370, USA) with a speed of 1 mm/min. Five specimens of each composition were tested and an aver-age value was obtained.

The self-healing efficiency of the microcapsule embeddedepoxy was tested in short-groove TDCB specimens (as seen inFig. 1). A sharp pre-crack was created by gently tapping a razorblade into the molded starter notch in the short-groove TDCBspecimen. The fracture specimen was tested under the displace-ment controlled process at a cross head speed of 10 lm/s. Sam-ples were tested to failure, measuring compliance and peak load.

for 10 h storage.

tion speed (rpm) Emulsifier concentration (wt.%)

300 400 500 0.5 1 2

0.15 0.16 0.25 0.18 0.15 0.150.43 0.55 0.69 0.46 0.43 0.401.02 1.28 1.51 1.32 1.02 1.022.67 3.05 3.71 3.17 2.67 2.65

Fig. 1. Schematic of the specimens used for self-healing test.

8 Q. Li et al. / Composites: Part B 49 (2013) 6–15

The cracked ample was unloaded, allowing the crack faces tocome back into contact, and healed in this state for 24 h at roomtemperature.

3. Results and discussion

3.1. Microencapsulation of curing agent

The effect of different processing parameters (core–shell weightratio, temperature, agitation speed, and emulsifier concentration)on the microencapsulation of curing agents in PMMA shell, suchas curing agent content and size distribution, have been investi-gated using FTIR, SEM, OM and TGA.

3.1.1. Effects of core–shell weight ratioThe effects of different initial weight ratios of core–shell mate-

rials (varied from 1:1 to 6:1) on the morphology and size of micro-capsules are shown in Fig. 2. In this case, the reaction temperature,agitation speed and emulsifier concentration were kept constant at40 �C, 300 rpm and 1 wt.%, respectively. Spherical microcapsuleswith smooth and compact outer surfaces are obtained at allcore–shell weight ratios (Fig. 2a–d). However, few microcapsules

Fig. 2. SEM images of microcapsules with various core–sh

are found to be ruptured and shrunk at a core–shell weight ratioof 6 (Fig. 2d). Wide ranges of size distribution (varying from 5 to125 lm) can be seen in all the cases, which is independent of thecore–shell weight ratio. Hence, the core–shell weight ratio is anirrelevant factor for the size and size distribution of the microcap-sules. This is because, the size of the core droplet in emulsion is notaffected in fluid flow due to the strong shear force by the propellerwhen other processing parameters are kept constant [29,30].

3.1.2. Effects of processing temperatureThe rate of solvent evaporation during microcapsules prepara-

tion increases with increasing processing temperature and directlyinfluences the morphology of microcapsules. Fig. 3a–c shows theSEM images of the microcapsules cP20-4-300-1, cP60-4-300-1and cP80-4-300-1 prepared at three different temperatures: 20,60 and 80 �C. Surface morphologies of these microcapsules are dif-ferent according to the processing temperature. The microcapsulesprepared at 20 �C (Fig. 3a) have relatively rough surfaces compar-ing to the microcapsules prepared at 40 �C (Fig. 2c). The rough sur-face results from the interaction of fluid-induced shear forces andshell-determined elastic forces [8,31]. Fig. 3b and c shows the por-ous structure on the surfaces of the microcapsule while preparingthe samples at 60 and 80 �C. The amount of holes on the surfaces ofthe microcapsules seems to increase with increasing processingtemperature. This is attributed to the faster evaporation of lowboiling solvent (DCM) through the shell of the microcapsules at60 and 80 �C. The microcapsules involving these holes cannot beused to prepare self-healing materials due to their inability to storethe liquid curing agents. However, relatively smooth outer surfacecan be observed with the microcapsules prepared at 40 �C, which isimportant for uniform microcapsule thickness.

3.1.3. Effects of agitation speedThe size distributions of the microcapsules were measured by

changing the agitation speeds while keeping all other factors

ell weight ratios; (a) 1:1, (b) 2:1, (c) 4:1, and (d) 6:1.

Fig. 3. SEM images of microcapsules prepared at various temperatures of (a) 20, (b)60, and (c) 80 �C.

Fig. 4. Size distributions of microcapsules prepared at various agitation speeds.

Fig. 5. Effect of agitation speed on the mean diameters of microcapsules preparedat various temperatures.

Q. Li et al. / Composites: Part B 49 (2013) 6–15 9

(core–shell weight ratio of 4, temperature of 40 �C and emulsifierconcentration of 1 wt.%) constant. With increasing the agitationspeed from 200 to 500 rpm, the mean diameter of the microcap-sules decreases noticeably with narrow size distribution (Fig. 4).This is because agitation speed controls the equilibrium betweenthe shear and interfacial tensile forces of the discrete oil droplets[7,8,10]. The interfacial tensile force would dominate at low agita-tion speed, and hence form comparatively large droplets. Strongshear force under high agitation speed forms smaller droplets.

Fig. 5 shows various mean diameters of the microcapsules pre-pared at different agitation speeds in the range of 200–500 rpm.The mean diameters of the microcapsules can be obtained in therange of 33–110, 36–123 and 49–276 lm by adjusting the agita-tion speeds in the range of 200–500 rpm at 20, 40 and 60 �C,respectively. The relationship between the mean diameter and agi-tation speed is linear in log–log scale as shown in the inset of Fig. 5.This is in accordance with the results reported by other researchers[7,10]. The mean diameters of microcapsules increase with in-crease in processing temperature due to lower viscosity causinglow shear force.

3.1.4. Effects of emulsifier concentrationStabilization of an emulsion plays a vital role in microcapsule

synthesis. PVA can provide the necessary structural stability toprevent aggregation of the emulsified droplets in solution [32].The influence of emulsifier concentration on the size distributionis shown in Fig. 6. It is seen that the average diameter of the micro-capsules decreases and the size distribution is narrowing downwith increasing the concentration of emulsifier. This is due to theadsorption of the emulsifier (PVA) at the interfaces of the oildroplets reducing the surface tension of water [33]. The size ofthe oil droplets in the emulsion becomes smaller with the increasein PVA concentration. Meanwhile, the shear force increases with

Fig. 6. Size distributions in volume percentage of microcapsules for variousemulsifier concentrations (prepared at 40 �C and 300 rpm).

Fig. 7. FT-IR spectra of (a) the curing agent, (b) the PMMA shell material, and (c) themicrocapsules with curing agent.

Fig. 8. SEM images of fractured surface of microcapsules embedded in epoxy withvarious core–shell weight ratios of (a) 1:1, (b) 2:1, and (c) 4:1.

10 Q. Li et al. / Composites: Part B 49 (2013) 6–15

the increase in viscosity of the emulsion, which is contributingtowards the reduction in the size and narrowing of the sizedistribution.

3.2. Characterization of the microcapsules

The microcapsules containing curing agents were characterizedby FTIR, SEM and TGA. The core contents were calculated byextracting the core material with methanol.

3.2.1. Chemical structure of the microcapsulesFig. 7 shows the FTIR spectra of curing agent, PMMA microcap-

sules shell wall (without curing agent) and cP40-4-300-1. The FTIRspectrum of curing agent displays absorption peaks for NAH groupat 830, 1592 and 3370 cm�1. The appearance of the FTIR band at1110 cm�1 corresponds to the CAOACstr vibration (Fig. 7a). PMMAmicrocapsule shell exhibits a peak at 1730 cm�1 corresponding toC@Ostr vibration. The CAH asymmetric and symmetric stretchingvibrations appear at 2998 and 2845 cm�1, respectively. The bandat 1200 cm�1 represents the OACH3 stretching vibration as shownin Fig. 7b. Characteristic peaks of both PMMA shell and thecuring agent can be seen in the FTIR spectrum of the broken

microcapsules (Fig. 7c), which indicate the successful encapsula-tion of the curing agent.

3.2.2. Measurement of the thickness of the shell wallIn order to investigate the thicknesses of the shell walls, the

microcapsules were embedded in an SPI-Pon-Araldite epoxyembedding Kit and cut with a sharp razor blade to facilitate mem-brane thickness measurement before performing SEM study. Theshell thicknesses of the capsules can govern the mechanicalstrength and the permeability of the shell wall, which plays vitalroles in capsule-based controlled release applications. Changes inthe processing parameters (core–shell weight ratio, temperature,

Q. Li et al. / Composites: Part B 49 (2013) 6–15 11

agitation speed, and emulsifier concentration) influence the thick-ness of the microcapsules to a marginal extent. However, a signif-icant change in the thickness can be seen by varying the core–shellweight ratio.

Fig. 8 shows the SEM images of the broken microcapsules. It canbe seen that the thicknesses of the shell walls vary from 11.2 to37.2 lm by changing the core–shell weight ratio, with a maximumthickness observed at low core–shell weight ratio (<2). This can beascribed to the availability of greater amounts of PMMA shells toencapsulate smaller amounts of the core material [25]. The

Fig. 9. Core contents of microcapsules prepared at various processing temperatureswith different (a) core–shell weight ratios, (b) agitation speeds, and (c) emulsifiercontents.

thicknesses of the shell walls are more uniform and much thinnerat higher core–shell weight ratios compared to that of lower core–shell weight ratios.

In case of any microcracks in the microcapsules’ embeddedmatrix, shell walls that are too thick cannot be fractured to releasethe curing agent into the crack plane. This results in the loss of self-healing ability of the composite. Hence, in the present study, themicrocapsules shown in Fig. 8b and c can be considered as poten-tial materials compared to the microcapsules shown in Fig. 8a.

3.2.3. Calculation of the core contentThe amount of the healing agent which is encapsulated plays an

important role to determine the efficiency of microcapsules. Thecore contents of the microcapsules have been calculated usingthe extraction method as shown in Fig. 9. All the processing param-eters studied here can influence the core content. However, the ef-fects of core–shell weight ratio and the processing temperature aremore significant to determine the core content of the microcap-sules. It is well known that the microcapsules with thin shell wallscan fill more core materials considering the microcapsules of sim-ilar size [25].

High core–shell weight ratio has been noted to rupture the shellwall (Section 3.1.1, Fig. 1d) due to excessive core content with verythin shell walls. Hence, the ideal core–shell weight ratio is found tobe 2–4 as observed in Fig. 9a. In these cases, the microcapsules pos-sess thin shell walls with enough interior space for storing the cur-ing agents, which guarantees high healing efficiencies upon usage.The highest core content of 21.1 wt.% is achieved at a core–shellweight ratio of 4, keeping the emulsifier concentration, agitationspeed and temperature constant at 1 wt.%, 2300 rpm and 40 �C,respectively.

The core contents of the microcapsules remain fairly similar at20 and 40 �C. However, as the temperature increases to 60 �C, thecore content slightly decreases. This is attributed to the increasedporosity of the microcapsules which cannot encapsulate the liquidcuring agent completely. This is in good agreement with our earlierspeculations (Section 3.1.2). The microcapsules prepared at 20 �Chave rough outer surfaces, providing enough space for the smallmicrocapsules to attach on their surfaces. The aggregated micro-capsules cannot be dispersed uniformly in the matrix. This caninduce poor mechanical properties and low self-healing efficiencyof the composite. The rough outer surface can also lead to a varia-tion in the shell thickness, leaving behind weak portions on theshell wall. Hence, 40 �C will be suitable for the preparation of effec-tive self-healing microcapsules.

Fig. 10. TGA curves of (a) the curing agent, (b) PMMA shell material, and (c)microcapsules (cP40-4-300-1).

Table 2Weight loss (%) of the microcapsules prepared at 40 �C with various processing conditions for 10 h storage.

Temperature (�C) Core–shell weight ratio (core/shell) Agitation speed (rpm) Emulsifier concentration (wt.%)

1 2 4 6 200 300 400 500 0.5 1 2

60 0.14 0.15 0.15 0.38 0.15 0.15 0.16 0.30 0.18 0.15 0.1580 0.40 0.43 0.48 0.79 0.48 0.48 0.62 0.72 0.51 0.48 0.42

100 0.80 0.97 1.00 1.91 0.98 1.01 1.41 1.68 1.37 1.01 1.00120 2.47 2.85 2.77 4.17 2.76 2.77 3.12 3.73 3.18 2.77 2.87

Table 3Weight loss (%) of the microcapsules prepared at 60 �C with various processing conditions for 10 h storage.

Temperature (�C) Core–shell weight ratio (core/shell) Agitation speed (rpm) Emulsifier concentration (wt.%)

1 2 4 6 200 300 400 500 0.5 1 2

60 0.16 0.18 0.25 0.53 0.24 0.25 0.29 0.45 0.18 0.25 0.2480 0.66 0.73 0.94 1.37 0.83 0.94 1.03 1.25 0.46 0.94 0.94

100 1.66 1.69 1.71 2.42 1.71 1.71 1.99 1.50 2.22 1.71 1.71120 3.52 3.83 4.15 5.20 3.65 4.15 3.71 3.71 4.34 4.15 4.14

Fig. 11. Weight losses of microcapsules (cP40-4-300-1) as a function of storagetemperature and time.

12 Q. Li et al. / Composites: Part B 49 (2013) 6–15

The effect of agitation speeds on the core contents of the micro-capsules are shown in Fig. 9b. The core content is quite low(17.1 wt.%) at an agitation speed, core–shell weight ratio, temper-ature and emulsifier concentration of 200 rpm, 4, 40 �C and 1 wt.%,respectively. The core content increases to 20.9 wt.% with increas-ing agitation speed from 200 to 300 rpm, maintaining otherparameters (core–shell weight ratio of 4, temperature of 40 �Cand emulsifier concentration of 1 wt.%) constant. However, furtherincrease in the agitation speed does not produce any significantimprovement in the core content of microcapsules.

The core content increases sharply with increasing the emulsi-fier concentration from 0.5 to 1 wt.% and then continues to in-crease slightly for further increasing the concentration up to2 wt.% as shown in Fig. 9c. The highest core contents of 21.1 and21.2 wt.% for 20 and 40 �C were achieved respectively.

3.2.4. Thermal stability of the microcapsulesThe embedded microcapsules in self-healing composites expe-

rience heat impact upon thermal curing process. Hence, thermalstabilities of the microcapsules are important for their end applica-tions as healing agents. TGA curves of the curing agent, PMMA shellwall and cP40-4-300-1 microcapsules are shown in Fig. 10. TheTGA curve of curing agent shows only one step weight loss startingat 90 �C and continues up to 290 �C (Fig. 10a). The thermal decom-position of the PMMA shell occurs in the temperature range of280–435 �C (Fig. 10b) consisting of two stages of degradations[34]. However, microcapsules possess three stages of degradations.The primary weight loss starts at 140 �C near the melting point ofPMMA (Fig. 10c) due to the weight loss of the curing agent fromthe broken PMMA microcapsules. The degradation temperatureof the curing agent has been shifted slightly higher because of itsencapsulation within the PMMA microcapsules. The second andthird steps weight losses starting at 280 and 325 �C correspondto the thermal decomposition of the PMMA shell. This providessufficient evidence of the presence of curing agent inside themicrocapsules. Curing agent content of the microcapsules has beencalculated by considering the weight loss in microcapsules from140 �C to 280 �C. The core content of microcapsules is noted tobe approximately 20 wt.%. This is in good agreement with our ear-lier observation in Fig. 9.

3.2.5. Storage stability of the microcapsulesIn addition to the thermal stability, the microcapsules should

have long-term storage stability. The storage stabilities of the

microcapsules have been calculated by exposing the microcapsulesat different temperatures. The weight loss of the microcapsules ex-posed to room temperature for 6 months and 1 year are only about0.021 and 0.1 wt.%, respectively. An OM study does not find anyleakage of the curing agents from the microcapsules, and appear-ances of the microcapsules also remain unchanged after long stor-age time (pictures not shown here). When the storage temperatureis increased, weight loss of the microcapsules becomes more sig-nificant. Keeping the microcapsules at high temperature for longtime would lead to substantial weight loss. The storage stabilitiesof the microcapsules prepared by varying the processing parame-ters are presented in Tables 1–3.

It can be seen that the weight loss is highly dependent on core–shell weight ratio (Tables 1–3). The percent weight loss of themicrocapsules become higher by increasing the core–shell weightratio from 1 to 4. Tables 1–3 also demonstrate that the storage sta-bility of the microcapsules decreases with increasing agitationspeed. Significant improvements in the storage stabilities of themicrocapsules is observed with increasing the concentration ofemulsifier from 0.5 to 1 wt.%. However, further increase in theemulsifier concentration to 2 wt.% cannot change the storage sta-bility significantly. The reduced storage stabilities of the microcap-sules prepared under different processing conditions is mainly due

Q. Li et al. / Composites: Part B 49 (2013) 6–15 13

to the formation of thin shell wall, which imparts high permeabil-ity at elevated temperatures.

The microcapsules prepared at 60 �C (Table 3) possess fast deg-radation than those prepared at 20 and 40 �C (Tables 1 and 2). Thepoor storage stability of the former may be due to the porous struc-ture of the shell wall.

Fig. 11 shows the effects of temperature on the weight loss (%)of the microcapsules cP40-4-300-1. The negligible weight loss upto 100 �C may be due to the loss of the adsorbed water. Almostnegligible amounts of weight loss at 60 �C suggest that the micro-capsules can be exposed to 60 �C for a long time. The microcapsulespossess high storage stability and heat resistance below the melt-ing temperature of the PMMA shell. Hence, it can be used as a po-tential healing agent for the preparation of epoxy-based self-healing composites.

3.2.6. Mechanical property of the microcapsulesMechanical properties of the self healing composites are crucial

for their end applications. In order to establish the suitability of themicrocapsules for healing epoxy, it was added to the epoxy. The ef-fect of the microcapsule’s size on the impact and tensile strength(TS) was studied with the samples containing equal wt.% of differ-ent microcapsules. The size of the microcapsules were varied from36 to 132 lm by changing the agitation speed from 200 to500 rpm, respectively, while maintaining other parameters (core–shell weight ratio of 4, temperature of 40 �C and the emulsifierconcentration of 1 wt.%) constant.

Fig. 12 shows the influence of different sizes of the microcap-sules (prepared by varying the agitation speed) on the impactstrengths of the epoxy-based composites. An increasing trend inthe impact strength can be observed with decreasing mean sizeof the microcapsules. An adverse effect on the impact strength ofthe epoxy-based composites is noticeable with the microcapsuleshaving mean size of 132 lm (prepared at 200 rpm). However, inother cases, the impact strengths of the composites are similar orhigher than the neat epoxy. The small size microcapsules (pre-pared at 500 rpm) improve the impact strengths of the compositesby 20% compared to the pure epoxy. This can be ascribed to the ini-tiation of a number of cracks during fracture. In case of the damageevent, cracks can rapidly propagate through the PMMA microcap-sules and failure occurs expending plenty of energy. The smallersize of capsules can adsorb more energy than that of its larger sizecounterpart while maintaining similar microcapsule content in the

Fig. 12. Impact strength of the epoxy containing different mean diameters ofmicrocapsules.

epoxy [35]. This proves that the liquid curing agent encapsulated inmicrocapsules can be a potential candidate for the preparation ofepoxy-based composites.

The typical stress–strain curves of the microcapsules embeddedepoxy are shown in Fig. 13. The TS of the pure epoxy has beennoted to be 73.51 MPa. TS of the resulting composites decreaseto a smaller extent with the addition of microcapsules with differ-ent sizes. The epoxy-based composites containing 5 wt.% micro-capsules with average diameters of 132, 72, 45 and 36 lmimpart the TS of 67.5, 55.6, 51.1 and 48.5 MPa, respectively (Table4). However, the elongations at break of the composites increase inthe composites containing small microcapsules (prepared to500 rpm). This is possibly due to the plasticizing action of themicrocapsules. Small size microcapsules possess high surface areawhich is helpful in increasing the plasticizing action, therebyreducing the TS.

3.2.7. Self-healing behavior of microcapsules embedded epoxyConsidering the broad applications of the microcapsules in the

future self-healing composites, the effects of the microcapsulesprepared at different processing conditions on the healing effi-ciency seem to be much more significant. Crack healing efficiency‘g’ is defined as the ability of a healed sample to recover fracturetoughness

g ¼ Khealed=Kvirgin

where Kvirgin is the critical load of the virgin specimen and Khealed isthe critical load of the healed specimen [6,14].

Self-healing efficiencies observed for specimens containingmicrocapsules prepared at different agitation speed are shownin Table 4. An increase in the self-healing efficiencies is observedwith the increase in agitation speed (small microcapsules). Max-imum healing efficiency of 93.83% has been achieved with theepoxy containing 15 wt.% microcapsules prepared at 500 rpm.This is due to the high core content of the microcapsules preparedat high agitation speed. This is also in good agreement with ourresults earlier (Section 3.2.3). The contents of healing agent playan important role to determine the self-healing efficiency ofmicrocapsules.

In order to determine the healing efficiency of the epoxy withvarying microcapsule content, cP40-4-300-1 was chosen. This isbecause the microcapsule, cP40-4-300-1 possesses a balanced TS,

Fig. 13. Stress–strain curves of the epoxy samples containing microcapsulesprepared at various agitation speeds.

Fig. 14. (a) Typical load–displacement curves obtained from TDCB tests for cP40-4-300-1, and (b) healing efficiency of A-10 epoxy specimens with different contents ofmicrocapsules (cP40-4-300-1) (error bars are based on five samples).

14 Q. Li et al. / Composites: Part B 49 (2013) 6–15

core content and healing efficiency (Table 4). Fracture testing withshort-groove TDCB specimens with 10 wt.% excess epoxy and15 wt.% of microcapsules (cP40-4-300-1) demonstrated goodhealing efficiency (Fig. 14a). Fig. 14b shows that the healingefficiency increases with cP40-4-300-1 content and the maximumvalue (93.50%) is obtained for the 15 wt.% of microcapsule(cP40-4-300-1) filled epoxy. However, no significant change inhealing efficiency is observed with further increasing microcapsulecontent. The high recovery of fracture toughness is due to therelease of sufficient amount of curing agent to the crack plane fromthe ruptured microcapsules. This can adequately cure the residualepoxy groups to form the new thermoset material in the originalmatrix interface [36]. It may be due to the optimum curing ofthe self-healing specimen containing A-10 epoxy filled with15 wt.% microcapsules. Considering the curing agent content ofmicrocapsules to be 20 wt.% (Section 3.2.3), A-10 epoxy theoreti-cally require �16 wt.% of microcapsules (cP40-4-300-1). Furtherincrease in A-10 epoxy content has no influence in improving thefracture toughness. This is because there is no curing agent leftout to react with the epoxy. Hence, this is in good agreement withour calculation of curing agent content of the microcapsules. Thehigh healing efficiencies of the microcapsule embedded epoxyinfers the potential application of the in house synthesizedmicrocapsules to prepare self-healing epoxy materials.

4. Conclusions

PMMA microcapsules containing highly active polyetheramine(liquid curing agent) were successfully prepared in oil-in-wateremulsion. The key factors, such as core–shell weight ratio, agita-tion speed, emulsifier concentration and temperature were foundto influence the morphology, size distribution, shell wall thick-ness, core content and storage stability of the microcapsules.The ideal reaction temperature to achieve curing agent encapsu-lated microcapsules with smooth outer surfaces is found to be40 �C. Low and high temperature render the formation of roughand porous microcapsules, respectively. SEM study shows theshell thickness is strongly dependent on the core–shell weight ra-tio with a marginal change in the thickness by varying otherparameters. The appropriate core–shell weight ratios to achievea perfect microcapsule is observed to be 2.00–4.00. Highercore–shell weight ratio is noted to collapse and shrink the micro-capsules. Size and size distribution of the microcapsules weremainly determined by agitation speed and emulsifier concentra-tion. The microcapsules containing polyetheramine have highthermal and storage stability. The impact strength of the epoxycontaining 5 wt.% microcapsules is enhanced by 20% comparedto that of pure epoxy. The TS of microcapsules embedded epoxydecrease with decrease in mean diameter of microcapsules, how-ever, the increasing trends in the elongation at break can be ob-served under similar conditions. Maximum healing efficiency of93.50% is obtained with 15 wt.% microcapsule content. Hence,the microcapsules prepared in this work can be potentially usedin epoxy-based self-healing composites. The liquid curing agent(polyetheramine) encapsulated in the PMMA microcapsules canbe efficiently released at room temperature to cure propagatingcracks in case of a damage event.

Acknowledgements

This study was supported by the Converging Research CenterProgram (2012K001428), the Human Resource Training Projectfor Regional Innovation, and the World Class University(WCU) Pro-gram (R31-20029) funded by the Ministry of Education, Scienceand Technology (MEST) and National Research Foundation (NRF)of Korea.

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