Synthesis of microcapsules from prepolyurethanes - Ha La Thi Thai

Journal of Science and Technology 55 (1B) (2017) 138–144 SYNTHESIS OF MICROCAPSULES FROM PREPOLYURETHANES Ha La Thi Thai*, Chi Luu Thien 1 Faculty of Materials Technology, HCMUT–VNUHCM 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam *Email: lathaiha67@yahoo.com Received: 30 December 2016; Accepted for publication: 3 March 2017 ABSTRACT Polyurethane (PU) microcapsules containing toluene diisocyanate (TDI) healing agent were synthesized by mixing PU with chain extender ethyleneglycol (EG) via interfacial polymerization of oil–in–water (gum arabic emulsifier). The morphology and size of the capsules greatly depend on a variety of factors including dispersion speed and emulsifier ratio. The preparation of PU prepolymer and microcapsulation of TDI are presented. The diameter of smooth spherical microcapsules ranged from 93, 160 and 239 µm are produced by varying the agitation rate from 800 rpm to 1200 rpm. The core content of microcapsules is influenced by the ratio of chlorobenzene (CB) solvent in oil phase. The microcapsules have about 40.5 wt% of core which are capable of application in self–healing coatings when using 20 wt% CB and 17.5 wt% emulsifer ratio. Keywords: prepolyurethane, microcapsule, interfacial polymerization. 1. INTRODUCTION Microencapsulation has been one of the most efficient techniques used in self–healing systems with various core materials such as diisocyanate, linseed oil [1], epoxy resins [2], amine [3], dyes, catalysts [4] and so on with different internal morphologies including monolithic, mononuclear, polynuclear, multi–walled or multi–core. Different types of microcapsules have been focused such as: encapsulation of dicyclopentadiene (DCPD) [4], a first generation self– healing system containing a liquid healing agent and Grubb’s catalyst particles based on ring opening metathesis polymerization (ROMP), microcapsules containing erythritol with interfacial polycondensation reaction by using the (W/O) emulsion [5], microcapsule for a self–repairing anticorrosion coating based on a polydimethylsiloxane (PDMS) [6] healing chemistry, microcapsule containing catalyst for self–healing polymeric material such stannous octoate or dibutyltindilaurate (DBTL) [7], etc. Recently, researchers have studied in terms of using polyurethane (PU) microcapsules to heal the microscopic cracks by the self–catalytic growth process with moisture of the isocyanate groups in core and achieved some results in self–healing properties and corrosion resistance such Ha La Thi Thai, Chi LuuThien 139 as monomer encapsulated with the self–healing property using isophoronediisocyanate (IPDI) encapsulated in polyurethane [8] and the coatings with polyurethane microcapsules containing hexamethylenediisocyanate (HDI) or toluene diisocyanate (TDI) as the core embedded in polymeric composites [9, 10]. In Vietnam, there is no research on function and application of self–healing coatings by applying microcapsules containing healing agent, in particular the PU microcapsules containing TDI with glycerol or other polyol. Because of the promise for outstanding applications of the PU microencapsulation in different fields such as surface coatings, corrosion protection, adhesive in construction, automobiles and medical supplies, in this paper we investigated the encapsulation of PU prepolymer with their diisocyanate TDI in a liquid–phase with solvent chlorobenzene (CB) via interfacial polymerization. 2. MATERIALS AND METHODS 2.1. Materials Toluene diisocyanate (TDI, Guangzhou, China), glycerol (99.5 %, Guangzhou, China), cyclohexanone (99.5 %, Aladdin, China), chlorobenzene (99.5 %, Nanjing, China), ethylene glycol (99.8 %, Shanghai, China) and gum arabic (Sigma – Aldric) were used without further purification. 2.2. Preparation of prepolymer PU prepolymer was prepared as a constituent material for microcapsule shell wall. The synthesis route is shown in Figure 1. The prepolymer solution was prepared from TDI and glycerol (mol ratio 6:1). TDI was dissolved into cyclohexanone solvent (15 wt% of monomer) in a 250 mL three neck flask. The mixture was then suspended in an 80 °C oil bath agitated using a magnetic stirrer. Glycerol was slowly added. The flack was purged with N2 and allowed to react for 10 h. The prepolymer solution was analyzed by FTIR and GPC. 2.3. Synthesis of microcapsules At room temperature, 57 ml of deionized water and 3 g of gum arabic surfactant were mixed in a 250 mL beaker glass. The beaker was suspended in water bath and temperature controlled by thermometer. The solution was agitated by digital mixer with stirring speed from 800 rpm to 1200 rpm in 90 min before beginning encapsulation. Meanwhile, prepolymer solution containing TDI core was dissolved into chlorobenzene (CB/Oil phase = 10–30 wt %). The mixture was then slowly poured into the gum arabic solution for 45 min. The water bath was heated to 50 °C, ethylene glycol (EG) as a chain extender was slowly added to the emulsion. After 2 hours of agitation, the mixer and hot plate were switched off. Polyurethane shell was formed at the interface between the water phase and oil phase (Figure 1). Once cooled to ambient temperature, the suspension of microcapsules was rinsed with deionized water 3 times and vacuum filtered. Microcapsules were air–dried for 24 h to constant mass before further analysis. Synthesis of microcapsules from prepolyurethanes 140 Figure 1. Synthesis of PU microcapsules using a prepolymer (TDI–glycerol) and a chain extender (EG). Ha La Thi Thai, Chi LuuThien 141 2.3. Analytical methods Fourier transform infrared (FTIR) spectroscopy was used for analysis of the structure of the prepolyurethane and microcapsules shell by using VERTEX80 (Bruker Optics) in Key Laboratory of Chemical Engineering and Petroleum Processing (CEPP), HCMUT–VNUHCM. Gel Permeation Chromatography (GPC) for obtaining information on molecular weight distribution of prepolyurethane which was dissolved in DMF solvent to a concentration 0.10 mg/L carried out by PL–GPC 50 Plus (Varian, Inc.) in Key National Laboratory of Polymer and Composite Materials (PCKLAB), HCMUT–VNUHCM. Thermogravimetric Analysis (TGA) was used as an analytical method to follow thermal properties of the filled microcapsules as well as pure TDI, CB and shell wall material by using LABSYS evo (TGA–DSC 1600 °C) in Ho Chi Minh City University of Education. For the TGA, 68.35 mg of the samples were heated at a rate of 10 °C/min in argon (Ar) environment. Dynamic Light Scattering (DLS) was used to determine the microcapsules size by using Laser HARIBA LA 950V2 in Key Laboratory of Chemical Engineering and Petroleum Processing of HCMUT. Scanning Electron Microscopy (SEM) was used to analyze how the microcapsules morphology changes with observations of the surface by using FE SEM S4800 HITACHI in Ho Chi Minh City Hi–Tech Park. Shoxlet Analysis (SA) was used to calculate the core content according to the formula: Core content = (mo – m1) / mo × 100%. Where mo is the mass of the original samples, m1 is the mass of the remaining solid of the samples after Shoxlet extraction. 3. RESULTS AND DISCUSSION 3.1 Synthesised PU prepolymer Figure 2. GPC spectrum of prepolymer (a), FTIR spectrum of: (b) prepolymer solution; (c) microcapsules and (d) shell of microcapsules. Synthesis of microcapsules from prepolyurethanes 142 The results in the Figure 2a and b illustrate that the prepolyurethane with the Mn at about 7913 and the PDI by 2.0070 has been successfully synthesized with specific peaks including 3304.96 cm–1, 1202.39 cm–1 and 1702.84 cm–1 for the linking of –NH–, –CO– and –C=O, respectively. Furthermore, the FTIR spectrum in Figure 2c of microcapsules shell ensures linkages of polyurethane and cores characterized by free isocyanate (–NCO) in 2262.70 cm–1 and peaks C–Cl aryl in 742.67 cm–1, stretching chlorides. Products microcapsules after Shoxlet (Shell) show no free isocyanate groups Figure 2d. 3.2. Effect of CB ratio Figure 3. Influences of (a) the solvent ratio; (b) emulsifier ratio to microcapsules; (c) TG–dTG of (1) CB; (2) TDI monomer; (3) microcapsules and (4) shell. When using 20 wt% CB in oil phase, the microcapsules have core contents 45.24 wt%, while the prepolymer concentration is decreased in 30 wt% CB, which is not enough to encapsulation therefore the core contents of microcapsules are reduced (Figure 3a). As shown in Figure 3c the contents of core materials in the microcapsules using 30 wt% CB in oil phase is by 32 wt% including about 3 wt% CB, 18 wt% TDI and 11 wt% unreacted prepolymer calculated according to the respective the mass loss stage (i) from 53 °C to 170 °C, the mass loss stage (ii) from 95 °C to 283 °C and the mass loss stage (iii) from 230 °C to 390 °C. Therefore, the TGA data shows that the Shoxlet analysis could be used to calculate the core content with the percentage error in the measuring is approximately 5%. 3.3. Effect of speed dispersion Stirring speed increases lead to the size of microcapsules significantly reduces. Agitation rate controls the equilibrium between shear forces and interfacial tension of the discrete oil droplets and the local velocity gradient the droplets experience. At low agitation rate, interfacial tension dominated and dispersed droplets remain large. Large droplets are broken up into small ones when strong shear forces are experienced under high agitation rate. When agitation rate increases from 800 to 1200 rpm, the particle sizes of microcapsules are descreased from 239 µm Ha La Thi Thai, Chi LuuThien 143 to 93 µm (Figure 4a, b, c). However, the results of SEM in Figure 4d, e show that at 1000 rpm the morphology of microcapsules is better than the other rate because of the effect of strong agitation at 1200 rpm lead to the broken up of microcapsules. Figure 4. Particle size at different stirring speeds: (a) 800 rpm; (b) 1000 rpm and (c) 1200 rpm. SEM images: (d) 1000 rpm and (e) 1200 rpm with 5 wt% emulsifier. 3.4. Effect of emulsifier ratio to microcapsule Figure 5. (a) particle size at 1000 rpm; (b,c) SEM images of microcapsules and (d,e) sell thickness with 17.5 wt% emulsifier. When emulsifier ratio is increased from 5.0 wt% to 17.5 wt%, the core content of microcapsules is increased from 27.84 wt% to 40.5 wt% (Figure 3b). With the emulsifier ratio Synthesis of microcapsules from prepolyurethanes 144 by 17.5 wt% the oil phase is so stabilizing that microcapsules have spherical shapes with uniform particle size about 87.5 μm in Figure 5a, smooth surface as Figure 5c, outer longer appear a few wrinkles on the surface but not agglomerated in Figure 5b. Meanwhile shell wall thickness in Figure 5d, e oscillates from 10 to 15 μm. However, the core content of microcapsules is decreased when the emulsifier ratio is continued to increase because of the poor disperse capacity of EG to the split–phase surface led to leak out of the core. 4. 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