Accepted Manuscript Title: Polyurethane-based microcapsules containing reactive isocyanate compounds: study on preparation procedure and solvent replacement Authors: F. Alizadegan, S. Pazokifard, S.M. Mirabedini, M. Danaei, R. Farnood PII: DOI: Reference:
S0927-7757(17)30620-9 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.058 COLSUA 21745
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
1-5-2017 20-6-2017 21-6-2017
Please cite this article as: F.Alizadegan, S.Pazokifard, S.M.Mirabedini, M.Danaei, R.Farnood, Polyurethane-based microcapsules containing reactive isocyanate compounds: study on preparation procedure and solvent replacement, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyurethane-based microcapsules containing reactive isocyanate compounds: study on preparation procedure and solvent replacement F. Alizadegana, S. Pazokifarda, S.M. Mirabedini*a, M. Danaeia, R. Farnoodb a b
Iran Polymer and Petrochemical Institute, P.O. Box 14965-115, Tehran, Iran
Department of Chemical Engineering and Applied Chemistry, University of Toronto, Canada
GRAPHICAL ABSTRACT
a)
b)
c)
d) SEM micrographs of microcapsules a) before, b) after 24 h, c) after 48 h immersion in 10 wt% NaCl solution, and d) possible reactions during 48 h immersion in 10 wt% NaCl solution
Highlights
Synthesis of PU-based microcapsules containing reactive isocyanates were studied. Various techniques were used for characterization of pre-polymer and microcapsules. The spherical microcapsules prepared in this study had a diameter of 50-200 µm. Toxic solvents were replaced with a benign once in the microcapsules preparation. Synthesized microcapsules have a smart prospect for use in self-healing coatings.
Abstract In this study, preparation and characterization of isophorone diisocyanate (IPDI) filled polyurethane (PU) microcapsules were studied. At first, a pre-polymer was synthesized through reaction of toluene 2, 4- diisocyanate (TDI) with 1, 4-butanediol using either cyclohexanone or n-butyl acetate solvent. The synthesized pre-polymer was characterized
using Fourier transform infrared spectroscopy, gel permeation chromatography, and free isocyanate content. Polyurethane-based microcapsules containing reactive isocyanates were synthesized using TDI-based pre-polymer and IPDI as the shell and core materials, respectively. Shape and morphology of prepared microcapsules were studied using optical microscopy and scanning electron microscopy (SEM). The spherical microcapsules prepared in this study had a diameter of 50-200 µm and shell thickness of 2-20 µm. The synthesis procedure developed in this work, replaces toxic solvents commonly used in the preparation of PU microcapsules, namely cyclohexanone and chlorobenzene, with a more benign solvent, namely n-butyl acetate. SEM microscopy of the scratched epoxy specimens showed appropriate healing performance using both type of microcapsules. Keywords: Microencapsulation; Polyurethane; Solvent Replacement; Butyl acetate; Selfhealing *Corresponding author: Tel: +98 21 4866 2401; Fax: +98 21 4866 2054; Email address:
[email protected] (S.M. Mirabedini) 1. Introduction Polymer coatings are usually exposed to physical and chemical stress during their preparation, application and service life, which may create defects and lead to loss of functionality [1]. Defects in polymeric coatings can be caused by reduced mechanical stiffness, loss of corrosion protection, and decreased durability [2]. Over the years, various materials [3-4] and methods [5-8] have been introduced to address this issue, among which self-repairing coatings has been shown to be a promising and cost - effective method [7-8]. Self-repairing technology is particularly useful in situations where timely repair of the damaged protective coating is impossible. In 2001, White et al. [9], introduced the first practical application of self-healing materials based on dicyclopentadiene (DCPD) encapsulated in urea–formaldehyde. These microcapsules were initially used in an epoxy-based composite in the presence of a Grubbs catalyst [10]. However, this self-healing system suffered from high cost of Grubbs’
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catalysts, the need for microencapsulation of both healing agent and catalysts, and possibility of deactivation of catalysts at elevated temperature [11–13]. To address these shortcomings, many materials and methods have been proposed to replace DCPD including drying oils [14-16], monomer [17-18] and bulky [19-20] isocyanates. These systems are typically activated by oxygen and moisture from the surrounding environment. Oil filled systems have been found tremendous interest owing to their non-toxic nature, low cost, availability, and facile processing. In spite of various benefits, there are two main shortcomings for this system including; low Mw healing materials which causes to low curing rate and decreased mechanical properties [16] of the healed area and poor interactions between poly(urea-formaldehyde) (PUF) shell material and polymeric matrix. To overcome these limitations, a catalyst free system containing microencapsulated isophorone diisocyanate (IPDI) in polyurethane (PU) was introduced by Yang et al. [17]. They used cyclohexanone solvent in pre-polymer synthesis and chlorobenzene in microcapsules preparation stage. In another study, Huang et al. [18] reported the microencapsulation of hexamethylene diisocyanate (HDI) in PU shell prepared from methylene diphenyl diisocyanate (MDI) pre-polymer. Such a system allows for self-healing in an aqueous media or in high humid conditions [19, 20]. Recently, PU-based microcapsules containing monomeric and multi-functional IPDIbased polyurethane pre-polymer core as healing agent (BIH) was reported by Haghayegh et al. [19, 20]. They reported that using multi-functional IPDI as the healing agent, enhances crack filling quality compared with its counterpart containing monomeric IPDI via reaction with atmospheric moisture. In another study, the effect of chemical structure of shell material on the performance of this system was evaluated by Di Credico et al. [21]. They
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synthesized microencapsulated IPDI in either PU or PUF single layer shells, and PU-PUF dual-layer shell. The dual or multi layer microencapsulation preparation methods propose a flexible way to make functional microcapsules with reactive core content and protective and inert shell material [22]. The above studies rely on the use of chlorobenzene and cyclohexanone as solvents in the preparation of PU-based microcapsules. However, chlorobenzene and cyclohexanone are highly toxic and are suspected human carcinogen [23]. In the current study these toxic solvents, are replaced with n-butyl acetate. Butyl acetate is known to have a considerably lower toxicity than chlorobenzene and cyclohexanone with oral acute toxicity of 10,000– 17,000 ppm in mouse and rats [24]. 2. Experimental 2.1. Materials Toluene 2,4-diisocyanate (TDI), isophorone diisocyanate (IPDI), 1,4-butanediol (BD), gum arabic (GA), cyclohexanone (CH), chlorobenzene (CB), and butyl acetate (BA) were purchased from Merck. As IPDI compound is sensitive to the moisture, it was purified via vacuum distillation before usage. CH and CB solvents were also firstly pre-dried using calcium chloride (Merck) and then fully dried using phosphorus pentoxide (Dae Jung Co.) under low pressure conditions [19]. Epoxy resin, KER 828, based on diglycidyl ether of bisphenol A (DGEBA) and its amine containing hardener (CRAYAMID 140C) were purchased from HEXION Specialty Chemicals and Cray Valley, respectively. All chemicals were analytical grade and used as-received without further purification. 2.2. Synthesis of isocyanate based pre-polymer TDI-based pre-polymer was synthesized according to the following procedure [17]. TDI (21.86 g) was dissolved in pre-dried, either CH (141.66 g) or BA (141.66 g) solvent,
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under mechanical stirring for 5 min. The solution was then added into a 2-necked glass vessel under N2 gas purging. The temperature was set at 80 °C using a temperaturecontrolled oil bath set-up. BD (4.12 g) was gently added to the solution under magnetic stirring. N2 gas was purged to the vessel for 20 min and it was sealed with cork and the suspension was stirred for further 24-36 h until the NCO value of product reached to the theoretical value (ca. 20-22 %.) [17]. Thereafter, the reaction products using either CH (PPUCH) or BA (PPUBA) solvent were distilled at 100 °C for 4-5 h to remove the solvent, water, and excess TDI from the synthesized pre-polymer and collected to the receiving trap, leaving a yellowish, viscous product in the vessel. The solid content was measured as about 15.5 wt % for both pre-polymers. The products were kept in a fridge until subsequent usage. 2.3. Measurements NCO Content of the synthesized pre-polymer was determined via titration method according to the procedure explained in ASTM D 2572 using: NCO
[( B V ) N 0.0420] 100 W
(1)
where, B (mL) and V (mL) are volume of HCl for titration of the blank and the specimen, respectively. N shows normality of HCl, and 0.0420 is milli-equivalent weight of the NCO group, and W (g) represents the weight of specimen [19]. The functional groups of pre-polymer were verified using a Bruker EQUINOX 55 Fourier transform infrared (FTIR) spectrophotometer (Ettlingen, Germany) collecting 16 scans in the 600–4000 cm−1 range with 4 cm−1 resolution, using KBr disc technique. The molecular weight and poly-dispersity of the pre-polymer were determined by gel permeation chromatography (GPC) technique using an Agilent GPC 1100 equipped with IR detector. Tetrahydrofuran THF solvent was used as the solvent. The test was
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performed according to standard method ASTM D6579-11. The solvent was removed from the synthesized pre-polymer by the aid of a low-pressure oven for 3 h at 60 °C. Prepolymer (20 mg) was dissolved in pre-dried THF (20 mL) and the solution was then passed through a 3 μm filter. The resultant solution was injected to the chromatography column with a flow rate of 1 mL.min-1. The molecular weights were calibrated using polystyrene standards. 2.4. Microcapsules preparation PU-based microcapsules were prepared via in situ polymerization in an oil-in-water emulsion [19-20]. 4.5 g GA as surfactant was dissolved in 30 mL deionized water under magnetic stirring for 3 h at ambient temperature. In a separate pot, pre-polymer (2.9 g) was dissolved in 4 g pre-dried solvent, either CB or BA, under magnetic stirring, and when pre-polymer was completely dissolved, IPDI (6 g) was added to the mixture under mechanical mixing. The mixture was then added slowly to the prepared GA solution. The temperature was increased up to 70 °C at a rate of 7 °C.min-1 under various mechanical mixing speeds of 500, 700, 900, 1200 and 1500 rpm with an axial three-bladed impeller stirrer (Heidolph RZR 2102, Germany). When the mixture reached temperature of 50 °C, BD (3.1 g) as a chain extender was slowly added to the emulsion and reaction was continued under defined mixing speed for another 60 min. The microcapsule suspension was then cooled down to ambient temperature, vacuum filtered (MUNKTELL, Germany, Filter Discs Grade: 389), rinsed with de-ionized water several times and left to dry at room temperature for 48 h. Solvent content of microcapsules (SC) was calculated about 53.5-57.9 % for both types using Eq. 2: SC
WCB
(WCB WPr ePolymer )
100
(2)
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where, WCB and WPre-Polymer stand for weight of CB (g) and solid pre-polymer (g), respectively. 2.5. Characterization of microcapsules Surface morphology and shape of microcapsules were evaluated using optical microscopy (OLYMPUS CX21FS1) and Scanning Electron Microscopy (SEM) (Inca 250, Oxford). The vacuum freeze-dried PU-based microcapsules were mounted on a conductive SEM sample holder and sputter coated with a thin layer (~10 nm) of gold for 60 s under Ar atmosphere using EMTECH, K450X device and evaluated using SEM operated in the secondary electron mode at 20 kV. Particle size and size distribution of microcapsules were determined via at least 200 measurements in SEM micrographs and by analyzing the data in Image J software. The presence of isocyanate as the core material of the microcapsules was analyzed using FTIR spectroscopy. To this end, few milligrams of microcapsules was crushed in a mortar in the presence of liquid nitrogen. Resultant materials were mixed with dry KBr powder and pressed into the tablet. FTIR spectrum was then taken collecting 16 scans in the 600–4000 cm−1 range with 4 cm−1 resolution. Thermal behavior of the microcapsules was studied using a thermogravimetric analyzer TGA – PL-1500 (Polymer Laboratories) from ambient temperature to 650 C at a heating rate of 10 C.min−1 under N2 atmosphere. 2.6. Self-healing properties of microcapsules embedded epoxy resin The quality of healed crack area on the microcapsule-embedded epoxy coatings was evaluated using SEM. Based on previous founding [16, 19, 20], characteristic properties of synthesized microcapsules and the coating thickness limitation, PPUCH900 and
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PPUBA900 microcapsules with 5 wt % loading level were selected as the appropriate conditions. To this end, 5 wt % of microcapsules was dispersed in a defined amount of KER 828 epoxy resin under magnetic stirring for 10 min. The dispersion was placed in a low pressure oven for 30 min to remove trapped air. A stoichiometric amount of the curing agent, with a resin:hardener wt. ratio of 2:1, was added to the dispersion. Neat epoxy coating was also considered as a control sample. The coating samples were applied on the degreased glass plate with a wet film thickness of 400 ± 20 µm using film applicator (Zehntner ZUA2000 universal applicator 0–3000 µm). The specimens were left for about 30 min at room temperature, and then post cured for 40 min at 80 C. Dry film thickness of the sample was measured as 240 ± 25 µm using Elcometer 645 (FeNFe). A crack with 5 cm length and approximate 200 µm depth was then created on the coating surface the specimens were exposed to the humidity cabin for 48 h at 100 % RH according to ASTM D2247 test practice. The core content release within the crack, from ruptured microcapsules was evaluated using a scanning electron microscope. 3. Results and discussion 3.1. Pre-polymer characterization Fig.1 shows variation of free NCO content (%) during the synthesis of pre-polymer as a reaction product of TDI and BD. As it can be seen in this figure, with progressing reaction time, the number of reactive NCO groups decreased remarkably due to the consumption of reactive groups during the polymerization process. For PPUCH (Fig. 1a), the initial and final (after 24 h of reaction) values of NCO % of TDI monomer were determined as 48.3 and 22.3 %, respectively, which are in agreement with Yang et al. [17]. However, free NCO content for prepared pre-polymer using BA solvent was determined as 27.32 and 19.28 % (Fig. 1b) after 24 h and 36 h, respectively. The
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difference in NCO % content of PPUCH and PPUBA may be explained based on the solubility difference of BD and TDI monomers in CH and BA. The Hildebrand solubility parameters of TDI, BD, CH and BA were reported as 23.7, 24.7, 21.3 and 17.8, respectively [19]. The solubility parameter of CH is almost similar to the solubility parameters of TDI and BD compounds; therefore higher solubility of these ingredients in CH solvent is expected. Higher solubility leads to more interactions between the TDI and BD in CH solvent, and hence the free NCO content would be lower after similar reaction time. The lower free NCO content reveals higher progressive reaction between diole (BD) and diisocyanate (TDI), resulting in higher Mw of the prepared pre-polymer. The higher Mw of pre-polymer is resulted in the higher Mw and higher stiffness of the microcapsule shell. Figure 1
FTIR spectra of TDI, BD and two types of pre-polymer are shown in Fig. 2. The sharp and intense absorption peak in 2270 cm-1 is related to the presence of NCO functional groups [17]. Stretching absorption peak for C-H vibration bond was considered to correspond to the NCO groups in pre-polymers. Hence, the ratio of the area under the stretching absorption peak NCO (near to 2270 cm-1 wave number) to the area under the stretching absorption peak bond CH (near the wave number of 2950 cm-1) was used to compare the two types of NCO pre-polymer synthesized from PPUCH and PPUBA. Using this approach, the ratios of 6.188 and 6.988 were obtained for PPUBA and PPUCH, respectively, confirming the results of NCO content obtained from the titration method. Therefore, microcapsule shells prepared from PPUBA were expected to have a higher stiffness than those prepared from PPUCH. This result confirms validity of calculated NCO
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content and formation of pre-polymer compounds. The possible reaction between 2,4toluene diisocyanate and 1,4-butane diole is shown in Fig. 3. Figure 2 Figure 3 GPC spectra of PPUCH and PPUBA are shown in Fig. 4. According to the results, the numerical and weighted average Mw were 962.24 and 1232.9 g.mole-1 for PPUCH and 697.6 and 816.3 g.mole-1 for PPUBA, respectively. For comparison, the numerical and weighted average Mw of the pre-polymer were reported as 1270 and 1690 g.mole-1 [17]. Higher average Mw of pre-polymer leads to higher mechanical strength of final polymer and hence stronger microcapsule shell. Figure 4 According to the reaction shown in Fig. 3 and the values of average Mw obtained from GPC, the number of repeated units (n) can be calculated for each pre-polymer as 1.986 and 0.98 for PPUCH and PPUBA, respectively. In comparison, the value of n for PPUCH prepared by Yang et al. was calculated as 3.152 [17]. These results are in agreement with the titration results for determination of free NCO and they confirm a reduction in ‘n’ by replacing CH with BA in the pre-polymerization stage. The overall results showed a lower Mw of PPUBA compared to PPUCH as reported in the literature [17, 25]. Hence, it is expected that PPUBA microcapsules to have a lower stiffness compared to those of PPUCH. However, CH persists in the soil and atmosphere and its biodegradation is hindered due to its toxicity [26]. Therefore, in spite of higher Mw and lower free NCO % of CH, because of the higher risk of working with a more toxic solvent, the application of BA in the microcapsule preparation is more desirable. Besides, lower Mw of pre-polymer leads to a better solubility of PPUBA in BA and facilitates
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microcapsules
preparation
process.
Furthermore, lower stiffness
of PPUBA
microcapsules can be addressed by increasing the microcapsule shell thickness to avoid premature microcapsule rupture during the blending process [26]. 3.2 Microcapsules characterizations Optical and SEM micrographs of the microcapsules synthesized at different mixing speeds and using two different pre-polymers are shown in Fig.s 5 and 6. The micrographs of PPUCH-based microcapsules reveal that the microcapsules are poly-dispersed spherical particles ranging in size from about 10 to 500 µm and had no inter-capsule bonding. Spherical microcapsules and smooth outer surface with some wrinkles were obtained in all mixing speeds apart from those capsules achieved at low agitation speed. This result is in agreement with those reported in the literature for IPDI-filled PU microcapsules [15, 27]. Fluid-induced shear forces, in homogeneous reaction kinetics, and shell-determined elastic forces were considered as main determining factors for observed morphology of microcapsules [15, 16]. The micrographs reveal that with increasing mixing speeds, microcapsule size decreased, probably owing to increase in the shear forces experienced by the droplets during the microcapsule formation process. No visible air bubbles within the microcapsules was observed, indicating that the IPDI was well-surrounded by the pre-polymer materials [19]. These microcapsules had a relatively smooth surface. The surface morphology of microcapsules depends on the core material properties (e.g. viscosity, surface tension, miscibility with shell material, etc), core: shell ratio and the microencapsulating process [19]. Optical and SEM micrographs of PPUBA-based microcapsules with about 57.9 wt % solvent (Fig. 6) show that the surface of these microcapsules was rougher than PPUCH microcapsules. As it can be seen from the micrographs, microcapsules prepared using
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PPUCH compared with PPUBA microcapsules at same solvent content were more spherical and had a smoother surface. The differences in shape and size of these two types of microcapsules is probably due to differences in the hydrophobic properties and solubility in water for CH and BA (0.049 g/100mL and 0.68 g/100mL, respectively, at 20 °C) as well as the lower Mw of PPUBA. The results also revealed that with increasing agitation speed from 500 to 1500 rpm, the average particle size of PPUBA microcapsules decreased from ca. 250 μm to 20 μm. Under the same preparation conditions and agitation speed, the average particle size of microcapsules prepared using PUUCH pre-polymer was higher than PPUBA. This is likely due to higher solvent content, lower viscosity and lower Mw of pre-polymer for PPUBA microcapsules. The morphology of internal surface of both types of microcapsules (PPUCH900 and PPUBA900) was similar, as shown in SEM micrographs (Fig. 7). This is because, for both types of microcapsules core material is IPDI compound with similar physicochemical properties. Figure 5 Figure 6 Figure 7 The mechanical properties and appearance of a self-healing coating are affected by microcapsule size, which can be controlled by mixing speed during the microcapsules preparation process.
In the self-healing coatings, the amount of healing materials
available for release at the crack site depends on the size and loading of microcapsules. Larger microcapsules contain a higher amount of healing agent. The right combination of appropriate self-healing property, mechanical properties and proper appearance can be achieved with using optimized particle size and wt% of microcapsules [26, 28]. Rule et al. [29] suggested that ideally, the size of the microcapsules should be less than the
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thickness of the coating film. It is also recommended that in order to minimize the decline in physical and mechanical properties, the maximum size of microcapsules must be less than one third of the final thickness of the coating layer. However, reducing microcapsule size was shown to lower the self-healing efficiency of the coating layer, thus resulting in a higher concentration of microcapsules to achieve the desirable self-healing performance [16, 30]. Furthermore, a higher microcapsule loading was shown to decrease the tensile property of the coating layer [2]. On the other hand, others have reported that larger microcapsules – exhibited a better self-healing property [15]. To achieve a balance between self-healing properties and mechanical properties, size and quantity of loaded microcapsules in the coating should be optimized. The overall yield of microencapsulation was calculated as 72.82±9.39 and 71.23±5.08 for PPUCH and PPUBA microcapsules, respectively, at various mixing speeds. According to these results, the solvent type had no significant effect on the yield. In contrast, Huang et al. [18] found that the microencapsulation reaction yield was a function of the mixing speed, such that by increasing the mixing speed from 300 to 2000 rpm, the reaction yield decreased from 74 to 54 %. Lower yields at elevated mixing speed in their case could be due to microcapsule break up, and/or fine microcapsules escape through the filter paper during the washing stage. Influence of agitation speed on microcapsules size and shell thickness By increasing the mixing speed during the microcapsule preparation, the core to shell ratio remained constant, but the solvent content increased and the healing-agent amount decreased [30]. As discussed earlier, with increasing agitation speed, the microcapsules size decreased. However, since core to shell ratio remained unchanged, the shell thickness reduced.
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Reduction in shell thickness leads to decrease in strength and hence premature rupture of microcapsules. Particle size and size distribution were calculated via examination of SEM micrographs and analyzing the data in Image J software. The size and shell thickness of microcapsules for different agitation speeds are provided in Table 1. As it can be seen, by increasing the agitation speed from 500 rpm to 1500 rpm, the average size and shell thickness of microcapsules decreased which is consistent with earlier results from literature [15]. Table 1 Component analysis and thermal properties In self-healing coatings, the amount of healing agent that can be released in the crack area is dependent on the microcapsules size and core content [15]. TGA technique was used for determining the wt% of encapsulated IPDI in the PPUCH-based microcapsules prepared at different agitation speeds. TGA thermographs of microcapsules prepared at different agitation speeds, as well as those for IPDI monomer and CB are depicted in Fig. 8. Microcapsules prepared at different mixing speeds had similar thermal behavior. The weight loss for all samples occurred in three temperature zones (less than 200, 200-250 and 250-500 C). Below 200 °C, the weight loss is due to the solvent evaporation (b.p. 131 °C) that was trapped inside the microcapsules. In the second temperature zone (200-250 °C), the weight loss was mainly due to IPDI evaporation as the evaporation temperature of neat IPDI is about 158 °C. Rapid weight loss starting at around 200 °C indicates the evaporation of encapsulated IPDI [17]. The weight loss in the third temperature range (250-500 °C), is mostly due thermal deterioration of microcapsules’ shell materials. Summary of TGA test results for all samples are depicted in Table 2.
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Figure 8 According to the results, with increasing agitation speed, the amount of CH solvent increased and IPDI trapped in microcapsules decreased, while the core to shell ratio remained nearly constant. The weight loss in the third temperature zone (250-500 °C), is mainly due the thermal deterioration of microcapsules’ shell materials. Table 2 Stability of microcapsules The effect of aging times (5 and 8 months in closed container at ambient temperature) on IPDI wt% and reactivity was evaluated using FTIR spectroscopy and TGA technique. FTIR spectroscopy results of crashed microcapsule samples and neat IPDI are shown in Fig. 9. Reactivity of core material was estimated by comparing the absorption peak area at around 2270 cm-1 (assigned to NCO groups) to the absorption peak area related to CH at 2950 cm-1. Table 3 shows that with increased storage time, the reactivity of IPDI decreased to about 63% after eight months storage, likely due to the penetration of moisture from the atmosphere in to the microcapsule and reaction with IPDI monomers. Figure 9 Table 3
TGA thermographs of fresh and aged microcapsules, neat IPDI and CB are shown in Fig. 10. As microcapsules were progressively aged, the initial drop in TGA plot that was caused by CB evaporation gradually disappeared. Results summarized in Table 4 show a significant decrease in the core content of microcapsules (CB+IPDI) with aging from 31.61 % to 12.44 %. This could be simply due to the evaporation of CB solvent. Figure 10 Table 4
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After 5 and 8 months aging, more than half of IPDI in the microcapsules was reacted with humidity. However, based on the percentage of active IPDI, relative reduction in the microcapsules’ core content to fresh microcapsules was calculated, as 53.67 and 56.54 wt %, after 5 and 8 months aging, respectively. The percentage of shell content increased simply due to lower amount of core content. For comparison, similar trends were reported for microcapsules prepared at 900 rpm mixing speed where reduction of 7.9 and 8.6 wt % of active IPDI in the environmental conditions in closed containers were observed over 3 and 6 months of aging, respectively. [17].
Microcapsules stability For investigation of microcapsules stability in wet conditions, microcapsules were immersed for 24 and 48 h in 10 wt % NaCl solution. Fig. 11 (a-c) shows SEM micrographs of these microcapsules, representing a change in the physical state of core materials from fluid to solid state. The possible reason of this phenomenon is probable penetration of water from microcapsules wall. Fig. 11d illustrates possible solidification reactions of core materials during 48 h immersion in 10 wt % NaCl solution [21]. Isocyanate groups from core materials react with water to form an unstable carbamic acid compound which is then decomposed to amine and carbon dioxide compounds. In the next step, created amine groups react with isocyanate groups to form a solid polyurea compound [21]. Figure 11
3.3. Crack filling performance of microcapsules embedded specimens
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Fig. 12 shows SEM micrographs of crack area of a neat epoxy sample and samples containing 5 wt % of PPUCH900 and PPUBA900 microcapsules. These SEM micrographs Fig.s 12 b and c clearly illustrate that the crack was filled with the released healing agent. As the healing agent microcapsule content were the same for both microcapsule-embedded samples and the vapor pressure of both solvents is almost equal, there was no significant difference in the appearance of the healed areas in Fig.s 12 b and c. However, crack width was slightly larger in Fig.s 12 b and c compared to that of Fig. 11 a. This could be explained based on the release of CO2 produced by the reaction of IPDI released during the self-healing process [20]. Figure 12
3. Conclusions Spherical polyurethane microcapsules based on TDI pre-polymer and containing IPDI as healing agent were prepared at 500-1500 rpm mixing speeds using interfacial polymerization by cyclohexanone, chlorobenzene and butyl acetate solvents. The average yield of microencapsulation of IPDI was estimated 72.82± 9.39 wt%. Microcapsules with different size and shell thickness were synthesized by varying the mixing speeds during the synthesis process. The best agitation speed was selected to be 900 rpm and resultant microcapsules were 40-100 µm in size and 4.87± 1.8 µm in shell thickness. Scratch tests performed on microcapsule-containing epoxy coatings confirm that these microcapsules exhibited self-healing properties. Furthermore, aging studies indicated that the shell to core ratio of microcapsules increased and IPDI content decreased over time likely due to the interaction with ambient moisture. The microcapsules synthesized in this work offer an interesting prospect for using in self-healing coatings. However, additional work is required
17
to better optimize microcapsule size and content and to evaluate the effect of other environmental conditions such as pH on the performance of these microcapsules.
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[8] E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Experimental Mechanics 42 (2002) 372–379. [9] S.R. White, N. Sottos, P. Geubelle, J. Moore, M.R. Kessler, S. Sriram, E. Brown, S. Viswanathan, Autonomic healing of polymer composites, Nature 409 (2001)794–797. [10] D.Y. Wu, S. Meure, D. Solomon, Self-healing polymeric materials: a review of recent developments, Prog. Polym. Sci. 33 (2008) 479–522. [11] A. Kumar, L. Stephenson, J. Murray, Self-healing coatings for steel, Prog. Org.Coat. 55 (2006) 244–253. [12] M. Kessler, N. Sottos, S. White, Self-healing structural composite materials, Compos. Part A: Appl. Sci. Manufact. 34 (2003) 743–753. [13] J.D. Rule, E.N. Brown, N.R. Sottos, S.R. White, J.S. Moore, Wax protected catalyst microspheres for efficient self-healing materials, Adv. Mater. 17 (2005)205–208. [14] C. Suryanarayana, K.C. Rao, D. Kumar, Preparation and characterization ofmicrocapsules containing linseed oil and its use in self-healing coatings, Prog.Org. Coat. 63 (2008) 72–78. [15] S.M. Mirabedini, I. Dutil, R.R. Farnood, Preparation and characterization of ethyl cellulose-based core–shell microcapsules containing plant oils, Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 74– 84. [16] Behzadnasab M., Esfandeh M., Mirabedini S. M., Zohuriaan-Mehr M. J., Farnood R. R., “Preparation and characterization of linseed oil-filled urea–formaldehyde microcapsules and their effect on mechanical properties of an epoxy-based coating”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Elsevier B.V., 457, 16–26, 2014.
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[17] Yang J., Keller M. W., Moore J. S., White S. R., Sottos N. R., “Microencapsulation of Isocyanates for Self-Healing Polymers” , Macromolecules, 41, November, 9650–9655, 2008. [18] Huang, M.; Yang, J. Facile microencapsulation of HDI for self-healing anticorrosion coatings. J. Mater. Chem 2011, 21 (30), 11123-11130. [19] M. Haghayegh, S.M. Mirabedini, H. Yeganeh, Microcapsules containing multifunctional reactive isocyanate-terminated polyurethane pre-polymer as a healing agent. Part 1: synthesis and optimization of reaction conditions, J Mater Sci (2016) 51:3056–3068. [20] M. Haghayegh, S.M. Mirabedini, H. Yeganeh, Microcapsules containing multifunctional reactive isocyanate-terminated polyurethane pre-polymer as a healing agent. Part 1: synthesis and optimization of reaction conditions, J Mater Sci (2016) 51:3056–3068. [21] B. Di Credico, M. Levi and S. Turri, An efficient method for the output of new selfrepairing materials through a reactive isocyanate encapsulation, Eur. Polym. J., 2013, 49, 2467–2476. [22] Y. Yang, H. Liu, M. Han, B. Sun, J. Li, Multilayer Microcapsules for FRET Analysis and Two-Photon-Activated Photodynamic Therapy, Angew. Chem. Int. Ed., 2016, 55, 1–7 [23] U.S. Department of Health and Human Services Public Health Service, Reports on Carcinogens, 12th ed., 2011. [24] W.F. Von Oettingen, The aliphatic acids and their esters: toxicity and potential dangers. II Acetic acid and esters, A.M.A. Archives of Industrial Health 21 (1960) 28– 65.
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Figure Caption
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Figr-1FIGURES CAPTION Figure 1. Variation of NCO Content % during pre-polymer synthesize using; a) Chlorobenzene, and b) Butyl acetate solvents Figure 2. FTIR spectra of TDI, BD and two different pre-polymers synthesized using CH and BA solvents Figure 3. The purposed reaction of prepared polyurethane pre-polymer of 4,2- and 4,1toluene diisocyanate butane dial [17] Figure 4. GPC spectra for; a) PPUCH and, b) PPUBA Figure 5. Optical and SEM micrographs of the PPUCH microcapsules synthesized at different mixing speeds using cyclohexanone as solvent Figure 6. Optical and SEM micrographs of the PPUBA microcapsules synthesized at different mixing speeds using n-butyl acetate as solvent Figure 7. SEM micrographs of internal surface of; a) PPUCH900 and, b) PPUBA900 microcapsules Figure 8. TGA thermograms of synthesized microcapsules at various mixing speeds along with CB,IPDI and microcapsule’s shell Figure 9. FTIR spectra of IPDI monomer, fresh microcapsules, 5 and 8 months aging microcapsules prepared at 900 rpm agitation rate Figure 10. TGA thermographs of neat IPDI, CB and fresh, 5 and 8 months aged microcapsules prepared at 900 rpm mixing speed Figure 11. SEM micrographs of microcapsules a) before, b) after 24 h, c) after 48 h immersion in 10 wt% NaCl solution, and d) possible reactions during 48 h immersion in 10 wt% NaCl solution Figure 12. SEM micrographs (top view) of crack area of (a) neat epoxy sample, (b) sample with 5 wt % of PPUCH900 microcapsules, and (c) samples with 5 wt% PPUBA900 microcapsules
22
50 45
NCO content (%)
40 35 30 25
a) PPUCH
b) PPUBA
20
22.27 % 19.28 %
15 10 0
6
12
18 24 30 Reaction time (h)
Figure 1
23
36
42
48
Figure 2
24
+ Figure 3
25
Figure 4
26
Mixing speed (rpm) 500
Optical image
700
900
1200
1500
Figure 5
27
SEM micrograph
Mixing speed (rpm) 500
Optical image
700
900
1200
1500
Figure 6
28
SEM micrograph
a)
b)
Figure 7
29
100
PPUCH500
80
PPUCH700 PPUCH1100
Weight loss (%)
PPUCH900
60
40
Shell
20 IPDI
CB CB
0 0
100
200
300 400 Temperature (C)
Figure 8
30
500
600
2.1 8 months aging 2
5 months aging
Transmittance (%)
1.9 1.8 1.7
Fresh Microcapsules
1.6 1.5
IPDI monomer
1.4 1.3 1.2 1.1
4000 3600 3200 2800 2400 2000 1600 1200
Wavenumber (cm-1) Figure 9
31
800
400
100 PPUCH900 (8 months aging) PPUCH900 (5 months aging)
80
Weight loss (%)
PPUCH900
60
40
Shell
20
IPDI
CB
0 0
100
200
300 400 Temperature (C)
Figure 10
32
500
600
a)
b)
d)
Figure 11
33
c)
Figure 12
34
TABLES
Table 1. Average microcapsules size and shell thickness (± one standard deviation) for different mixing speeds using PPUCH and PPUBA pre-polymers Mixing speed
Size distribution
Average size
(rpm)
(μm)
(μm)
500
700
900
1200
1500
Shell thickness (μm)
PPUCH
100-500
215±10
19.26±2.50
PPUBA
155-550
230±15
21.35±2.30
PPUCH
50-250
108±5
7.65±1.40
PPUBA
65-270
117±10
8.15±1.10
PPUCH
40-100
62±3
7.87±1.80
PPUBA
45-110
65±3
8.0±0.80
PPUCH
20-80
33±2
3.07±0.97
PPUBA
25-85
35±2
3.0±0.55
PPUCH
10-50
18±2
2.16±0.40
PPUBA
12-50
20±1
1.85±0.60
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Table 2. Composition of microcapsules synthesized at various mixing speeds as determined by TGA analysis. Sample
Core content (%)
Shell
CB
IPDI
(%)
PPUCH500
28.51
25.73
44.91
PPUCH700
28.55
24.32
46.32
PPUCH900
31.61
19.49
48.12
PPUCH1100
33.16
16.74
49.59
36
Table 3. The ratio of FTIR absorption peak areas of NCO to C-H (ANCO:AC-H) and % relative reactivity of PPUCH900 for various storage times. FTIR data for IPDI is provided for comparison. Sample
Storage time
ANCO:AC-H
Relative reactivity %
0
2.757
100
5
1.926
69.86
8
1.717
62.96
-
8.215
-
(month) PPUCH900
IPDI
37
Table 4. Effect of aging on the composition of microcapsules prepared at 900 rpm mixing speed (CB: chlorobenzene, IPDI: isophorone diisocyanate). Sample
Shell (%)
Core (%) CB
IPDI
Fresh PPUCH900
31.61
19.49
48.9
5 months PPUCH900
18.94
9.03
72.03
8 months PPUCH900
12.44
8.47
79.09
38
