Preparation and Properties of Self-healing Microcapsules Containing an UV-curable Oligomers of Silicone

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1 Preparation and Properties of Self-healing Containing an UV-curable Oligomers of Silicone Preparation and Properties of Self-healing Containing an UV-curable Oligomers of Silicone Rui-ying Xing 1, Qiu-yu Zhang 1 * and Jiu-li Sun 1 Key Laboratory of Applied Physics and Chemistry in Space of Ministry of Education, School of Science, Chang an Campus of Northwestern Polytechnical University, Xi an, Shaanxi , China Summary In this work a new series of microcapsules for self-healing coatings were successfully prepared by an in-situ polymerization route with poly (urea-formaldehyde) (PUF) as a shell material and a mixture of methacryloxypropyl terminated dimethyl silicone oil (2MA4000) and 1-hydroxycyclohexyl phenyl ketone (I-184) as core materials. The UV curing properties of the core materials were performed by the gel yield. The surface morphology and shell wall thickness, the particle size and distribution, and the chemical structure and properties of microcapsules were characterized by SEM, FTIR, laser particle size analyzer and thermogravimetric analysis (TGA). The results indicated that the gel yield of 2MA4000 with 3 wt.% Irgacure-184 was 63.35%. The spherical microcapsules (particle size of mostly μm) were produced with a relatively thin shell wall of nm, a filling content of 84 wt.%, and a high yield of 71.4 wt.%. The microcapsules exhibit a good heat resistance. The core of the microcapsule was under a good preservation state during the encapsulated process, and part of the core which was in the interface between oil and water participated in the reaction of urea and formaldehyde. It makes the microcapsule wall being the organosilicon modified PUF resins, which promotes the thermostability of the microcapsule wall and compatibility with organosilicon coating. 1. INTRODUCTION Microencapsulation is a technology that uses membranous materials to coat solids, liquids or gas into microparticles. Polymers and composites with microencapsulated healing agent offer tremendous potential for selfhealing without human intervention, thus extending the service life of materials 1-2. Self-healing polymer coatings, which can be considered as a special class of composite materials, is one of the most active fields in smart coatings. Self-healing coatings can automatically repair cracks when they are subjected to damage, scratches or cracking, which will prevent the infiltration of corrosive medium and be isolated with the corrosive environment, preventing further corrosion of the substrate protected; or can automatically repair *Corresponding author. address: qyzhang@nwpu.edu.cn Smithers Rapra Technology, 2012 scratches, to maintain the surface appearance and optical characteristic of a high decoration coating 3-4. The need for self-healing coatings has been clear in heavy-duty corrosionresistant coatings, construction plastic industry and composite materials fields especially wind power generator tower, blades, offshore drilling platforms, advanced houseboat, spaceflight apparatus and so on. Therefore, developing self-repairing coatings is becoming a research focus of experts at home and abroad. In organic coatings, silicone coatings is widely applied in various kinds of situations of high reliability requirements and difficult maintenance (such as aerospace) because of its excellent performance such as weather, heat and chemical resistance. At present, the research on selfhealing microcapsule is mostly around composites 5-13, and the core materials are mainly DCPD and epoxy resin, which cannot meet the special request for silicone coatings. According to the composition characteristics for silicone coatings, we propose a new kind of selfhealing microcapsule with a mixture of methacryloxypropyl terminated dimethyl silicone oil (2MA4000) of high boiling point and a proper quantity of photo-initiator as core materials and poly(urea-formaldehyde) (PUF) as a shell material. are dispersed throughout a silicone coating, which will be used in the territory of aerospace, and the self-healing mechanism is similar to that of polymeric composites. are ruptured by the propagation of mcirocrack when damage occurs because of atmosphere atomic oxygen and solar radiation, and the core materials as healing agents are released into the crack by capillary action. Because of the reaction activity Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2,

2 Rui-ying Xing, Qiu-yu Zhang and Jiu-li Sun of propenyl existed in the molecular chain of 2MA4000, the polymerization of core materials can be triggered by ultraviolet (UV) rays to perform self-healing. In this paper, the photocuring of 2MA4000 was investigated, and the structure and performance of self-healing microcapsules were characterized. 2. MATERIALS Methacryloxypropyl terminated dimethyl silicone oil (2MA4000) was purchased from Shanghai Sili Industry and Trade Co., Ltd., China. 1-hydroxycyclohexyl phenyl ketone (I- 184) and I-500 was obtained from Ciba. Benzophenone (BP) was purchased from Tianjin Yuanhang Chemicals Co., Ltd. Ethanolamine (TEA) and urea (U) were obtained from Tianjin Tianli Chemicals Co., Ltd. 37 wt.% formaldehyde (F), resorcinol, sodium hydroxide (NaOH) and hydrochloric acid (HCl) were purchased from Xi an Chemicals Regents Factory. Ammonium chloride (NH4Cl) was obtained from Tianjin Northern Tianyi chemical Regents Factory. Maleic anhydride modified polyvinyl alcohol (MPVA) was prepared in our laboratory. All solvents and substances except 2MA4000 used for preparation of MPVA solution, acid and base solutions and 1-octanol were of analytical grade. Figure 1 shows the chemical structure of 2MA4000, I-184, I-500 and BP. weighed. The mixtures were under stirring and ultrasonic dispersion for 15 min to form a homogeneous phase. Five to six drops of the mixture were squeezed onto a homemade sample platform, and were placed under UV irradiation using an UV spot light source for a specified amount of time. UV-light was transmitted trough an optical device (approximately 12 mm in diameter) to the sample. It was about 10 mm from the sample to the end of the device, and the light intensity is approximately 2620 mw/cm Preparation of were prepared by in situ polymerization in an oil-in-water emulsion. Under agitation, 1.25 g urea (U) and 0.125g resorcinol were dissolved in 30 g of 5 wt.% aqueous solution of MPVA in a 100 ml beaker, and the ph was adjusted to 2.5 by dropwise addition of NaOH and HCl. One to two drops of 1-octanol were added to eliminate surface bubbles. At 60, a slow stream of 7.5 g 2MA4000 with 3 wt.% I-184 were added to form an emulsion and allowed to stabilize for 15 min. After stabilization, 20 ml of 1.5 wt.% aqueous solution of NH4Cl and 3.04 g of 37 wt.% aqueous solution of formaldehyde (F) were added to the emulsion. The emulsion was covered and reacted for 3 h. The obtained suspension was cooled down to ambient temperature (20-25 ), rinsed with deionized water and acetone, filtered and air-dried for 24 h. Figure 2 represents the processing flow of microencapsulation. 4. CHARACTERIZATION 4.1 Photo-curing Property of 2MA4000 The irradiated products were wrapped with a piece of filtration fabric, and placed in a Soxhlet extractor using acetone as a medium for 12 h at 60. Then the gel yield can be calculated by formula 1: Gel yield = M M 0 100% (1) In the formula: M is the polymer weight after Soxhlet extracted; M 0 is the original sample weight. Figure 2. Microencapsulation of 2MA4000 utilizing in situ polymerization of urea with formaldehyde to form capsule wall 3. EXPERIMENTAL 3.1 Photo-curing of 2MA4000 2MA4000 and photo-initiators of different concentration were accurately Figure 1. Chemical structure of the materials used in this work 78 Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2, 2012

3 Preparation and Properties of Self-healing Containing an UV-curable Oligomers of Silicone 4.2 Characterization of Surface morphology and capsule shell thickness were observed using a scanning electronic microscope (SEM, JSM-6700F, JEOL). One to two drops of the microcapsules dispersion were placed on a piece of monocrystalline wafer and allowed to air dried overnight, and then the samples were sputtered with a thin layer (~10 nm) of gold-palladium to prevent charging under the electron beam. The morphology of microcapsules also was observed using metalloscope (MS, DMM-330C, Shanghai Caikang Optical Instrument Factory). Fourier-transform infrared (FTIR) spectra were obtained using a FTIR spectrometer (TENSOR27, BRUKER) to identify the chemical structure of the specimen, which was prepared by grinding the sample with potassium bromide (KBr) or by attaching sample to a KBr disc. The microcapsule size distribution was investigated using laser particle size analyzer (LS13320, Becman Coulter). The thermal stability of microcapsule was measured using the thermogravimetric analysis (TGA, HTG-1/2, Bejing Permanent Scientific Instrument Factory) at a heating rate of 10 /min in a nitrogen atmosphere. In the formula: M UF is the shell material weight in microcapsule; M 0 is the original microcapsule sample weight. 5. RESULTS AND DISCUSSION 5.1 Effect of Photo-initiators on the Photo-curing Property of 2MA4000 As the addition of photo-initiators is controlled of 3 wt.%, the effect of three different photo-initiators on the photo-curing property of 2MA4000 was examined. The results are shown in Figure 3. It can be seen from Figure 3, in the case of same dosage of photoinitiators, the polymerization rate is the fastest and the gel yield is also highest while using I-184 as the photo-initiator, and BP is the worst. It is because of decomposition of part of polymer gel caused by high-intensity UV-ray that the gel yield initially increase but then falls after 150 s. Therefore, I-184 was chosen as the photoinitiator, and the effect of its adding dosage on the photo-curing property of 2MA4000 is characterized. 5.2 Effect of the Adding Dosage of I-184 on the Photo-curing Property of 2MA4000 In the UV curing recipes, the concentration of the photoninitiator is lower relative to the oligomer, and it is generally about 1-5 wt.%. In this paper, the effect of adding dosage of I-184 on the photo-curing property of 2MA4000 was investigated, and the results are shown in Figure 4. It can be seen from Figure 4, it is not significant that the effect of the I-184 concentration on the gel rate. When the concentration of I-184 is increased up to 5 wt.%, the gel rate is lower because there are so many free radicals that the reaction of chain termination may be easier between themselves; when decreased low to 1wt.%, the gel rate is also lower because there are only a few radicals so that they may get outside interference easily. Therefore, in our study, the I-184 adding dosage is 3 wt.%. 5.3 Microcapsule Surface Morphology Figure 5 shows the SEM micrographs of self-healing microcapsules prepared. prepared are mostly spherical and depressions on some microcapsules are caused by thermal expansion and contraction; and the microcapsule size is mostly in a narrow range of μm (Figure 5a). The surface of microcapsule is a little rough and composed of PUF nanoparticles protruding from the surface (Figure 5b). Figure 5c shows the SEM micrographs of the broken wall shell. It can be proved that Figure 3. Effect of photoinitiators on the photo-curing property of 2MA4000 Determination of core content in microcapsule: The prepared microcapsule core content was determined by extracting method and using acetone as extracting solvent. were wrapped with pieces of filter paper and filtration fabric, cracked by a mortar and pestle, and placed in Soxhlet extractor for 24 h at 60. Then the core content of microcapsule can be calculated by formula 2. Core content in microcapsule = (1 M UF M 0 ) 100% (2) Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2,

4 Rui-ying Xing, Qiu-yu Zhang and Jiu-li Sun Figure 4. Effect of the adding dosage of I-184 on the photo-curing property of 2MA4000 shell material (Figure 6b) isolated by Soxhlet extracted with acetone from prepared microcapsules, microcapsules (Figure 6c) and a mixture of 2MA4000 and I-184. Figure 5. SEM photos of microcapsules: (a) a number of microcapsules; (b) single microcapsule; (c) damaged microcapsules (a) microcapsule has a single core structure from the graphs. The shell is thin and its thickness falls between 20 and 90 nm. In addition, the microcapsules yield is about 71.4 wt.% and its core content is approximately 84.1%. 5.4 Chemical Structure of Microcapsule Figure 6 shows the FTIR spectra of PUF (Figure 6a) prepared by the polymeration of U and F, microcapsule From Figure 6a and Figure 6b, we can see that the peak at 3320 cm -1, 1652 cm -1 and 1560 cm -1 are the characteristic absorption peaks of N-H, -C=O and- CN respectively. The three primary peaks indicated the formation of PUF material. From Figure 6b we can also see that the peak at cm -1, 1259 cm -1 and 806 cm -1 are the characteristic absorption peaks of Si-O-Si, Si-CH 3 and Si-C respectively, indicating that part of the core participated in the reaction of U and F, and the reaction mechanism may be Michael addition reaction. It makes the microcapsule wall being the organosilicon modified PUF resins, which promotes the thermostability of the microcapsule wall (proved in Figure 8) and compatibility with organosilicon coating. As the ph value of the reaction system was 2.5, and temperature 60, the reaction is so rapid that the wall shell could be formed in 10 min. Therefore, only part of the core material which was on the interface between oil and Figure 6. FTIR spectra: (a) PUF; (b) shell material; (c) microcapsules; (d) a mixture of 2MA4000 and I-184 (b) (c) 80 Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2, 2012

5 Preparation and Properties of Self-healing Containing an UV-curable Oligomers of Silicone water took part in the reaction of shell material. Combined with the SEM photos and the determination of core content, it is clear that there is sufficient reactive healing agent to meet the needs of follow-up repair work. Figure 7. Size distribution of microcapsules 5.5 Size Distribution of Figure 7 shows the size distribution of microcapsules. The microcapsule size is mostly in a range of μm, and the result is in harmony with that of SEM. 5.6 Thermal Stability of The thermal stability of microcapsules plays an important role in their applications in self-healing silicone coatings, which will be used in the territory of aerospace and may be prepared at a high temperature (200 for example). Figure 8 shows the TGA diagram of microcapsules. It indicates that the weight loss before 100 is mainly due to the removal of entrapped residual water and the elimination of small molecule compounds. The weight loss at temperature between 200 and 350 is mainly due to the decomposition of the microcapsule wall shell, and the weight loss at temperature between 400 and 500 is mainly due to the decomposition of organic silicon materials. Obviously, the shell material of microcapsule has a better heat stability than pure PUF, and the microcapsules are basically chemically stable below 200, indicating that the prepared microcapsules have a good thermal stability. Figure 8. TGA diagrams of PUF, shell material, microcapsule and core materials Figure 9. Surface morphology of microcapsules before and after heat treatment: (a) normal temperature; (b) 200 /2 h Figure 9 shows the OM photographs of microcapsules before and after heat treatment of 200 for 2 h. It can be seen from the photos, the surface morphology of microcapsules has a slight deformation after heat treatment, and the wall is so thin that part of the larger microcapsules are broken, but most of microcapsules are in good condition, and the weight loss is only (a) (b) Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2,

6 Rui-ying Xing, Qiu-yu Zhang and Jiu-li Sun about 2.1%, which is in harmony with the result of TGA. It once again illustrates that microcapsules prepared have a good thermal stability enough to meet the follow-up preparation requirements of self-healing silicone coatings. 6. CONCLUSION filled 2MA4000 of high boiling point and I-184 by in situ polymerization of urea-formaldehyde in an oil-in-water emulsion were successfully prepared to develop the self-healing silicone coatings. The core materials of microcapsules are of good UV curing properties. The spherical microcapsules (particle size of mostly μm) were produced with a relatively thin shell wall of nm, a fill content of 84 wt.%, and a high yield of 71.4 wt.%. The microcapsules exhibit a good seal and heat resistance. The core of the microcapsule was under a good preservation state during the encapsulated process, and part of the core which was on the interface between oil and water participated in the reaction of urea and formaldehyde. It makes the microcapsule wall being the organosilicon modified PUF resins, which promotes the thermostability of the microcapsule wall and compatibility with organosilicon coating. ACKNOWLEDGEMENTS This work was financially supported by the National Basic Research Program of China (Grant No. 2010CB635111), National Nature Science Foundation of China (Grant No ), and Basic Research of Northwestern Polytechnical University (Grant No. G9KY1020). REFERENCES 1. White S.R., Sottos N.R., Geubelle P.H., Moore J.S., Kessler M.R., Sriram S.R., Brown E.N. and Viswanathan S., Nature, 409 (2001) Brown E.N., Kessler M.R., Sottos N.R. and White S.R., J. Microencapsulation, 20 (2003) Suryanarayana C., Rao K.C. and Kumar D., Prog. Org. Coat., 63 (2008) Murphy E.B. and Wudl F., Prog. Polym. Sci., 35 (2010) Brown E.N., White S.R. and Sottos N.R., Compos. Sci. Technol., 65 (2005) Kesslera M.R., Sottos N.R. and White S.R., Composites Part A, 34 (2003), Wilson G.O., Moore J.S., White S.R., Sottos N.R. and Andersson H.M., Adv. Funct. Mater., 18 (2008) Yin T., Rong M.Z., Zhang M.Q. and Gui C.Y., Compos. Sci. Technol., 67 (2007) Y L., Liang G.Z., Xie J.Q., L L. and G J., Polymer, 47 (2007) Y L., Liang G.Z., Xie J.Q., L L. and G J., Polym. Degrad. Stab., 91 (2006) Y L., Gu A.j. and Liang G.Z., Mater. Chem. Phys., 110 (2008) B Blaiszik.J., Sottos N.R. and White S.R., Compos. Sci. Technol., 68 (2007) Rule J.D., Sottos N.R., and White S.R., Polymer, 48 (2007) Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2, 2012