Available online at ScienceDirect. Materials Today: Proceedings 3S (2016 ) S88 S95

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1 Available online at ScienceDirect Materials Today: Proceedings 3S (2016 ) S88 S95 5th International Conference on Functional Materials & Devices (ICFMD 2015) Microcapsules of poly(urea-formaldehyde) (PUF) containing alkyd from palm oil Nurshafiza Shahabudin, Rosiyah Yahya, Seng Neon Gan* Chemistry Department, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia. Abstract This paper describes the encapsulation procedure of an alkyd resin derived from palm oil. The palm oil-based alkyd having carboxylic and hydroxyl moieties can be tailor-made to suit the intended application. Microencapsulation process was done using poly(urea-formaldehyde) (PUF) resin as shell material. The microcapsules obtained were free-flowing, with % core encapsulated, and are intended to be used in self-healing material development. The functional groups of alkyd and shell material were observed using ATR-FTIR. DSC analysis shows melting peaks and glass transition of PUF and alkyd in the microcapsules. TGA analysis indicated that the shell of the microcapsules was thermally stable up to 250 C. FESEM examination showed that the microcapsules had rough outer surface The Authors. Published by Elsevier Ltd The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference on Functional ( Selection Materials and & Devices Peer-review (ICFMD under responsibility 2015). This is of an Conference open access Committee article under Members the CC of 5th BY-NC-ND International license Conference on Functional Materials ( & Devices (ICFMD 2015). Keywords: Microcapsules; Alkyd; Palm oil; Biorenewable; Poly(urea-formaldehyde) 1. Introduction Recently, self-healing process utilizing microcapsules as carriers of healing agent has been an interesting topic that has attracted many researchers and the number of related publications has been growing fast. Many healing mechanisms have been studied and the healing agents encapsulated, such as dicyclopentadiene (DCPD), 5- * Corresponding author. Tel.: ; fax: address: sngan@um.edu.my The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Selection and Peer-review under responsibility of Conference Committee Members of 5th International Conference on Functional Materials & Devices (ICFMD 2015). doi: /j.matpr

2 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S88 S95 89 ethylidene-2-norbonene (ENB), siloxane-based, epoxy, glycidylmethacrylate (GMA) and unsaturated polyester just to name a few. The latest, extensive reviews have been provided by Hillewaere and Du-Prez, 2015 [1] and Zhu et al., 2015 [2] on the self-healing designs, reactions and preparations. Yang et al., 2015 [3] has provided a review on the chemical and physical aspect of self-healing materials. It is of interest to study the potential role of a selected palm oil-based alkyd which contains a certain controlled amount of hydroxyl and carboxylic groups capable of reacting with epoxy groups [4]. This alkyd provides a usage of natural material as a self-healing agent. Therefore, in this work, we presented an encapsulation method of the alkyd using poly(urea-formaldehyde) (PUF) resin. This is an attempt to produce alkyd-containing microcapsules to be used in this attractive and rapid-growing field of self-healing materials. 2. Materials and method 2.1. Materials Urea, and 1-octanol were purchased from ACS Sigma-Aldrich while formaldehyde (37 % aqueous) was from Systerm. Ammonium chloride was from Fluka. 1, 3-dihydroxybenzol (resorcinol) and ethylene maleic anhydride (EMA) copolymer (Mw ) were from Riedel de-haen. Refined, bleached and deodorized palm kernel oil (PKO) and glycerol were obtained from Emery Oleochemicals (M) Sdn. Bhd. Phthalic anhydride (PA) was from Hanwha Chemical, Korea and lithium hydroxide was from JT Baker, US. All materials were used as received Core synthesis and preparation of microcapsules The selected alkyd has an oil-length of 65 %, and an acid number of 17 mg KOH/ g. Details of its synthesis and characterization were described elsewhere [4]. An aqueous solution of EMA was prepared by dissolving g of EMA in 125 ml of water in a 500 ml beaker at ambient temperature. Designated amount of alkyd was poured into the aqueous solution to form emulsion by stirring at rpm g urea was added to the emulsion, followed by 0.25 g ammonium chloride and 0.25 g resorcinol. The emulsion was adjusted to ph 3.5 by adding a few drops of 10 % sodium hydroxide solution. Generally about 3-6 drops (< 0.3 ml) were used and had not caused significant change in the total volume g of formaldehyde solution (37 %) was added into the mixture. Temperature of the water bath was raised to 55 C. After about 4 h of reaction, a white slurry has formed. The stirring was reduced to 200 rpm and the slurry was allowed to cool down to room temperature overnight. The product was washed with distilled water to remove unreacted chemicals and finally rinsed with ethanol. It was filtered under suction and dried under the fan for 1 h. The microcapsules were separated according to different sizes using sieves (Endecotts, certified acc. to BS410, ISO 3310) with mesh sizes of 50, 150, 200, 300 and 500 microns. Microcapsules of a size larger than 500 microns were discarded Characterization of microcapsules The yield of the microcapsules was calculated from the weight of the dried product over the total weight of capsules-forming raw materials. The amount of alkyd in microcapsules was determined by solvent extraction method. The microcapsules were ruptured with a pestle in a mortar. A mixture of acetone and ethanol was used to extract and dissolve the core. The insoluble shell was filtered and thoroughly washed and dried at 70 C for 24 h in a vacuum oven. The microcapsules core content was calculated using the equation 1, where W s refers to the weight of sample and W m refers to the weight of the insoluble shell: E core = (W s W m )/ W s x 100 % (1) Infrared spectra of the samples were collected from an attenuated total reflectance-fourier transform infrared spectrometer (ATR-FTIR) (Nicolet 6700 FTIR, Thermo Scientific). Spectra were recorded for the range of cm -1 at 4 cm -1 resolution with 4 scans. The spectra of intact microcapsules, the extracted shell and core materials

3 90 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S 88 S95 were compared with neat alkyd. Differential scanning calorimetry (DSC) was conducted on Mettler-Toledo DSC822e, calibrated with indium standard. An intercooler (HAAKE EK/90 MT) was used for sub-ambient temperature. Measurement of the samples were carried out from -60 C to 300 C at a heating rate 10 C min -1, under a nitrogen flow of 20 ml min -1. Thermogravimetric analysis (TGA) was carried out using a Perkin-Elmer TGA 6, in a nitrogen environment of 20 ml min -1 flow rate. Samples were heated from 50 C to 900 C at 20 C min Microscopic and FESEM analysis The morphology and shapes of microcapsules were examined using a handheld digital microscope (AnMo Electronics, Taiwan) at 50x and 200x magnifications. The average diameter of microcapsules was determined on data sets of more than 250 particles as recommended by Yuan and Liang [5]. To examine the external morphology of the shell of microcapsules, field-emission scanning electron microscopy (FESEM) was used (FEI Quanta FEG 450, USA). The capsules were mounted on a single-stub sample holder and the analysis was carried out under vacuum using an electron acceleration voltage of 5.0 kv. 3. Results and discussion The reaction mechanisms involved in the formation of the PUF shell had been reported by many other researchers. Typically in the preparation, a low molecular weight pre-polymer was formed from the condensation of urea and formaldehyde at the initial stage. Subsequently, the prepolymer deposits onto the surface of the dispersed core material and polymerize to form the shell [6,7] Preparation of microcapsules: Effects of agitation rate and concentration of emulsifier Effect of agitation rate Agitation rate has affected the size distribution of microcapsules. High agitation rate has resulted in smaller core droplets [7,8]. However, very high agitation rate above 500 rpm would lead to frequent collisions causing deposition of the PUF and alkyd on the stirrer and reactor s wall, leading to poor yield. In this study, the agitation rate was maintained between 300 rpm to 500 rpm. As shown in Table 1, the mean diameter of the microcapsules changed from 412 µm to 360 µm as the agitation rate was increased from 300 to 500 rpm. Fig.1 shows size distribution of microcapsules which was from 300 microns to 600 microns. Microcapsules produced were poly-dispersed and sieved to separate the residue particles before being used in further processes. Sample A1 prepared at 500 rpm of agitation rate has produced microcapsules at 17 % yield. These microcapsules have relatively thin wall and contain up to 90 % core, whereas, sample A2 which was made with agitation rate of 400 rpm has produced a higher yield (40 %) with more stable microcapsules, which were more free-flowing. The faster stirring has dispersed the alkyd into smaller droplets. However extremely high stirring rate can lead to collisions between the droplets leading to agglomeration of the microcapsules thus lowering the yield [9]. On the other hand, sample A3 prepared with lower agitation rate (300 rpm) has produced microcapsules with a slightly thicker shell (core content was 81 %) and lower yield to 15 %. Lowering the agitation rate has resulted in bigger alkyd droplets and the urea and formaldehyde would be in excess, which presumably had polymerized to form the agglomerated residues. Effect of concentration of emulsifier (1.0 %, 2.5 %, 5.0 %) Emulsifier concentration has a crucial role during the in-situ polymerization. Too low, the droplets will tend to agglomerate into bigger sizes; an increase in concentration, will maintain the sizes of droplets [10,11]. Fig. 2 shows the appearances of samples A4, A2 and A5 which were synthesized in 1.0, 2.5 and 5 % EMA respectively. Sample A4 has produced agglomerated mass with no distinctive microcapsule as seen in Fig. 2a. Whereas, in Fig. 2c, the microcapsules (A5) have thin shells with surfaces sticking to each other; they were fragile and could not be separated as free flowing microcapsules. Only sample A2 (Fig 2b) synthesized in 2.5 % EMA could successfully be

4 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S88 S95 91 isolated as stable free flowing microcapsules. Presumably the higher concentration of EMA leads to higher solution viscosity. Thus, insufficient PUF prepolymer was able to be deposited onto the alkyd droplet, and consequently, thin shell was formed. Fig. 1. Size distribution (left) and digital microscopic images of microcapsules (50x magnification) (right), prepared at different agitation rates (rpm): (a) 500; (b) 400; (c) 300.

5 92 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S 88 S95 Table 1. Characterization data of microcapsules. Code [EMA] (Wt. %) Agitation rate (rpm) Core (g) Yield (%) Core content (%) Mean diameter (micron) Capsules descriptions A Free-flowing, thin shell. A Free-flowing. A Free-flowing with some agglomerated particles. A No individual microcapsules. Big lumps of white particles. A Fragile, thin wall, sticky and not free flowing. Fig. 2. Digital microscopic images of microcapsules (200x magnification) synthesized at different EMA concentration (wt. %): (a) 1.0; (b) 2.5; (c) Spectroscopic analysis using ATR-FTIR Fig.3 shows the infrared red spectra of A2 microcapsules, which has all the characteristic peaks of PUF and alkyd. The alkyd s characteristic peak at 1730 cm -1 corresponds to the carbonyl group. The characteristic peaks of PUF are seen at 2900 & 2800 cm -1, 1600 cm -1 and 1500 cm -1 which correspond to C-H, NH and C-N respectively. The O-H of alkyd and N-H of amine appear at 3200 to 3500 cm -1. A1 and A3 also show characteristic peaks as exhibited by A2.

6 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S88 S Thermal analysis using DSC and TGA Fig. 3. Infrared peaks of PUF/alkyd microcapsules with alkyd core and PUF shell. DSC thermograms of A1-A3 shows glass transition (T g ) and melting (T m ) peak which correspond to the T g and T m of the encapsulated alkyd, and the PUF shell (Fig. 4). The T g of A2 is at C, which is closely resembling that of the alkyd. At higher temperature range, a sharp melting peak is seen at 148 C that corresponds to the melting of PUF. A similar observation was found in A1 and A3 microcapsules, which show the T g of alkyd at -10 C and -13 C respectively. A1 and A3 show melting peaks at 155 C and 132 C respectively, which were attributed to the melting of PUF shell. Fig. 4. DSC thermogram of A1, A2 and A3 microcapsules with extracted shell and neat core.

7 94 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S 88 S95 Fig.5 shows the thermal degradations of A2 (microcapsules), the alkyd and the PUF shell. A2 was thermally stable up to 250 C and subsequently decomposed completely within the range of C. Degradation of PUF occurs around C, while the alkyd has started to break down around 250 C. The thermal degradations of the core and shell have occurred in overlapping temperature ranges, consequently TGA could not be used to determine the amount of core and shell accurately. The TGA data of the other samples were summarized in Table 2. A1 and A3 capsules were thermally stable up to 250 C and 230 C respectively and the similar trend of degradation as of A2 capsules was observed. 50% weight loss occur at around 350 C for all microcapsules. Fig. 5. TGA thermograms of A2 microcapsules, alkyd core and PUF shell. Table 2. TGA data of microcapsules, core and shell. Sample Onset degradation temperature ( C) Temperature at 50 wt. % ( C) A A A Core Shell Surface morphology of microcapsules As shown in Fig. 6a, the free-flowing microcapsules were spherical in shape, and have rough outer surface. Fig. 6b shows the multi-layered PUF formed from PUF nanoparticles embedded in the layers of the shell (Fig.6c).

8 Nurshafi za Shahabudin et al. / Materials Today: Proceedings 3S ( 2016 ) S88 S Conclusion Fig. 6. FESEM micrographs of PUF/alkyd microcapsules (a) 800x; (b) 2500x; (c) 12000x. Scale bar (μm): (a) 100 (b) 40 (c) 10. Instrument setting HV at 5.00 kv at low vacuum. The selected palm oil-based alkyd has been encapsulated by the PUF resin. Using a suitable agitation rate and emulsifier concentration, free-flowing microcapsules with up to 40 % yield could be obtained. The microcapsules were thermally stable up to 250 C and had a rough outer surface. The microcapsules have encapsulated % alkyd by weight. This work provides an approach for making the alkyd microcapsules which could be used as the healing agent for epoxy matrix. Acknowledgements This work is supported by the Ministry of Science, Technology and Innovation, Malaysia (MOSTI) and University of Malaya under SF0874 and PG A grants. N.S. acknowledges the Ph.D scholarship by the Ministry of Higher Education, Malaysia (MOHE). References [1] Hillewaere XKD, Du Prez FE. Fifteen chemistries for autonomous external self-healing polymers and composites. Prog Polym Sci. 2015; 6: [2] Zhu DY, Rong MZ, Zhang MQ. Self-healing polymeric materials based on microencapsulated healing agents:from design to preparation. Prog Polym Sci doi: /j.progpolymsci [3] Yang Y, Ding X, Urban MW. Chemical and Physical Aspects of Self-Healing Materials. Prog Polym Sci. 2015:1 26. doi: /j.progpolymsci [4] Lee SY, Gan SN, Hassan A, et al. Reactions between epoxidized natural rubber and palm oil-based alkyds at ambient temperature. J Appl Polym Sci. 2011;120: [5] Yuan L, Liang G, Xie J, Li L, Guo J. Preparation and characterization of poly(urea-formaldehyde) microcapsules filled with epoxy resins. Polymer (Guildf). 2006;47(15): [6] White SR, Sottos NR, Geubelle PH, et al. Autonomic healing of polymer composites. Nature. 2001;409: [7] Brown EN. In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene. J Microencapsul. 2003;20(6): [8] Blaiszik BJ, Caruso MM, McIlroy D a., Moore JS, White SR, Sottos NR. Microcapsules filled with reactive solutions for self-healing materials. Polymer (Guildf). 2009;50(4): [9] Chen M, Liu J, Liu Y, Guo C, Yang Z, Wu H. Preparation and characterization of alginate N-2-hydroxypropyl trimethyl ammonium chloride chitosan microcapsules loaded with patchouli oil. RSC Adv. 2015;5: [10] Guo H, Zhao X. Preparation of microcapsules with narrow-size distribution by complex coacervation: effect of sodium dodecyl sulphate concentration and agitation rate. J Microencapsul. 2008;25(4): [11] Tiarks F, Landfester K, Antonietti M. Preparation of polymeric nanocapsules by miniemulsion polymerization. Langmuir. 2001;17(3):