SiO2 hybrid shell microcapsules synthesized by the

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Polysulfone/SiO2 hybrid shell microcapsules synthesized by the combination of Pickering emulsification and solvent evaporation technique and their application in self-lubricating composites Haiyan Li, Shuang Li, Zhike Li, Yanji Zhu, and Huaiyuan Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03370 • Publication Date (Web): 18 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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Polysulfone/SiO2 hybrid shell microcapsules synthesized by the combination of Pickering emulsification and solvent evaporation technique and their application in self-lubricating composites Haiyan Li*, Shuang Li, Zhike Li, Yanji Zhu, Huaiyuan Wang* Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, PR China Corresponding author：：Haiyan Li, Email: [email protected], Tel.: 86-459-6503985 Huaiyuan Wang, Email: [email protected], Tel.:86-459-6503083 Mailing address: No. 99, Xuefu street, Hi-tech Industrial Development Zone, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, Heilongjiang, PR China. Post code: 163318

Abstract: Lubricant oil-filled polysulfone/SiO2 (PSF/SiO2) hybrid shell microcapsules are prepared by the combination of Pickering emulsification and the solvent evaporation technique. Silica particles are used as stabilizers. The structure and properties of the microcapsules are influenced by the silica particle concentration, agitation speed, and encapsulation temperature. The formation of PSF/SiO2 hybrid microcapsules is confirmed by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA). The resulting microcapsules prepared at the optimum synthetic parameters show spherical, ideal structure with a rough outer surface, mean diameter of 5.0±0.6 µm, shell thickness of 0.83 µm, core content of 50.5wt.% and excellent thermal stability with an initial evaporating temperature of 250℃. The synthesized microcapsules are embedded into epoxy for application in self-lubricating composites. Investigated by friction and wear tests, the tribological properties of the self-lubricating microcapsules-incorporated epoxy composites obtain a significant improvement. 1

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Keywords: PSF/SiO2 hybrid shell, microcapsules, Pickering emulsification, solvent evaporation technique, self-lubricating composites Introductions The preparation of microcapsules has received significant attention due to their wide applications in polymers, coatings, medicine, food additives, catalysis, and so on.1-3 Many techniques have been devised to prepare microcapsules, such as in-situ polymerization, interfacial polymerization, microfluidic technique, layer-by-layer assembly, solvent evaporation, Pickering emulsion polymerization, and so forth.4-10 The solvent evaporation technique has been widely used to prepare core-shell microcapsules. This method can be easily performed with simple operation and a short experimental period. The colloidal stability and microcapsule size in this method depend strongly on the type of stabilizer or surfactant. Poly (vinyl alcohol), sodium dodecyl benzene sulfonate, gelatin, and relax 88A are frequently used surfactants.2, 7, 11-13

These surfactants are present in the final latex and may have detrimental effects

on the post application of the microcapsules. The removal of surfactants from the latexes is time-consuming and normally incomplete.14 The Pickering emulsification polymerization technique is widely used for microcapsule fabrications. An emulsion stabilized by solid particles instead of organic surfactants is called Pickering emulsion. Solid particles can greatly enhance the stability of the Pickering emulsions by the adsorption at the oil-water interface, which remarkably protects droplets from aggregating and guarantees a high encapsulation efficiency. Moreover, the solid particles can serve as a framework of microcapsules to form a more strengthened shell wall. Some inorganic particles, such as clay, 15TiO2,16 SiO2,17 graphene oxide18-19 and lignin,20 have been employed as stabilizers to prepare microcapsules. Polystyrene(PS) and poly(urea-formaldehyde) (PUF) are often used as the wall materials of 2


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microcapsules.18, 20-24 The PS wall needs to be synthesized by the polymerization of styrene. Nitrogen protection is required in the polymerization process, and the polymerization reaction time is relatively long (usually require 24 h). The PUF wall also needs to be prepared by the polymerization of urea and formaldehyde monomer under the appropriate pH and temperature conditions. Furthermore, the PUF wall is brittle, which limits the application of microcapsules as functional additives. Currently, there has been no report on combining the Pickering emulsification and solvent evaporation technique to prepare microcapsules, even though their combination could provide an easy to operate microencapsulate method that could produce microcapsules with excellent performance. Self-lubricating

polymer

composites

prepared

by

adding

lubricant-filled

microcapsules can obviously improve the anti-friction performance. Lubricant-filled microcapsules have been widely reported.25-31 PUF, poly(melamine-formaldehyde) (PMF), polyurea and polysulfone (PSF) have been used as the walls of microcapsules. Some of these wall materials, such as PUF and PMF, are brittle and can be easily broken in the fabrication process of composites. Alternatively, the operating temperature of these microcapsules is not more than 200℃, which limits their application in the preparation of self-lubricating composites. In this contribution, PSF/SiO2 hybrid shell microcapsules were prepared by the combination of Pickering emulsification and solvent evaporation technique using lubricant oil as a core material and silica particles as the stabilizer. This is a novel microencapsulation method. Silica particles provide colloidal stability of the colloidal system, and control the size of the microcapsules. The wall materials combine the excellent mechanical properties of PSF and the high thermal stability of SiO2. Synthetic lubricant oil-filled microcapsules are incorporated into epoxy to prepare 3


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self-lubricating composites. The self-lubricating properties and mechanism are evaluated by tribological tests. Experimental Materials Nano silica (SiO2, diameter: 15nm), the Pickering emulsion stabilizer, is obtained from

Nanjing

Hydratight

Nanomaterials

Co.

Ltd.

(China).

The

t-octylphenoxypolyethoxyethanol (Triton X-100), 3-iso-cyana-topropyltriethoxysilane (IPTS) and dibutyltin dilaurate (DBTDL) were purchased from Sinopharm Chemical Reagent Co. Ltd (China). Lubricant oil (70SN, density=0.82 g.cm-3) was supplied by Daqing Oil Field Co. Ltd (China). PSF (P-7304, η=0.45-0.65) was purchased from Dalian PSF Technology Co. Ltd (China). Dichloromethane (DCM, >99.5wt%) was purchased from Tianjin Damao Co.(China). Bisphenol-A epoxy resin (type E51) and tetraethylenepentamine (TEPA) curing agent were supplied by Heilongjiang Chemical Engineering Institute Co. Ltd. (China). Deionized water was used throughout the study. All materials were used as received without any further purification. Synthesis of PSF/SiO2 hybrid shell microcapsules containing lubricant oil Modiﬁed SiO2 particles were prepared according to the report of Chen.21 Firstly, 2.5 g of IPTS, 6.5 g of Triton X-100 and 0.03 g of DBTDL(catalyzer) were mixed in a 100 mL three-neck bottle, mechanically stirred at 50°C under nitrogen atmosphere, 20 h later the Triton X-100-IPTS (T-IPTS) was obtained. Then, 0.5 g of SiO2 was added to 100 mL of ethanol/deionized water mixture with a volume ratio of 3 to 1, ultrasonically dispersed 15 min. Subsequently, 0.025 g of T-IPTS was added to the dispersion and adjusted the pH value to 8 via dropping diluted ammonia solution at room temperature. Afterward, the temperature was heated up to 65°C, further mechanically stirred and reacted for 24 h. After reaction, the suspension was separated 4


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by ﬁltration and washed alternatively with ethanol and water for three times. Finally, the SiO2 particles were dried and stored for further experiments. A mixture of modified SiO2 particles with different contents was ultrasonically dispersed in 50 mL of distilled water which was defined as the water phase. While the oil phase was the mixture of 30 mL of DCM, 1.5 g of PSF and 2 g of lubricant. As the schematic plot in Figure 1 shows, the oil phase was added to the prepared water phase, the SiO2 stabilized O/W Pickering emulsion was formed after vigorously stirred at 12,000 rpm for 5 min under ice bath conditions. Then the above system was moved to a 250 mL three-neck flask, the DCM solvent evaporation was accomplished under different stirring speeds and temperatures. Finally, the PSF/SiO2 hybrid shell microcapsules containing lubricant oil were obtained by separating, washing and drying.

Figure 1 Schematic for the synthesis process of PSF/SiO2 hybrid shell microcapsules containing lubricant oil

Preparation of self-lubricating epoxy composites Self-lubricating epoxy composites were manufactured by adding 12wt% TEPA 5


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curing agent and 0-10wt% synthesized microcapsules into preheated (60℃) epoxy with mechanical stirring for 10min and ultrasonic agitation for 10 min. The bubbles were removed in a vacuum drying oven for 10 min. The mixture was cured at room temperature for 3 h and then 80°C for 6 h. The system was cooled to room temperature and the self-lubricating epoxy composites were achieved. Characterizations The chemical structure of microcapsules, PSF and lubricant oil were evaluated by Fourier transform infrared spectroscopy (FTIR) in the wavenumber range from 4000 to 500 cm-1 with a resolution of 4 cm-1. The thermal stability of microcapsules, PSF and lubricant oil were analyzed using thermogravimetric analysis (TGA). The samples were heated from 25 to 800℃ at a heating rate of 10℃/min under nitrogen atmosphere. The surface morphology, shell wall thickness of microcapsules, worn surface of self-lubricating composites were analyzed with scanning electron microscope (SEM) operating at 10 kV. The average diameter and size distribution were analyzed by laser particle size analyzer. The tribological performance of the self-lubricating composites was examined by friction and wear tests. A schematic diagram of the friction and wear tests with pin-on-disc configuration was presented in Figure 2. Before each test, the surfaces of samples and counterpart plate were polished with 1000-grit paper to an average roughness of 0.15–0.3 µm and then cleaned with anhydrous ethanol. The applied load was 1.0 MPa, the sliding velocities was 0.51 m/s, the friction duration was 30 min. The friction coefficient and wear rate were calculated by the equation given in our previous study.32 All tests were conducted under ambient laboratory conditions. Three measurements per sample were taken to get the mean and standard deviation.

6


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Figure 2 Schematic diagram of the friction and wear tests

Results and discussion Preparation of PSF/SiO2 hybrid shell microcapsules PSF/SiO2 hybrid shell microcapsules were fabricated with different SiO2 particle contents, encapsulation temperatures, and stirring speeds, as listed in Table 1. The mean diameter of the microcapsules was controlled by varying the SiO2 particle contents in the Pickering emulsions. As shown in Figures 3(a-e), the mean diameter of the microcapsules decreases with increasing SiO2 particle content, from 17.8±4.3 µm (0.25wt.%) to 0.5±0.13 µm (3wt.%), until reaching a “plateau”, as shown in Figure 3(f). When the SiO2 particle content was 0.25wt.%, there was little microcapsule agglomeration. Attempts at using SiO2 particle contents lower than 0.1wt.% were unsuccessful. The decrease in diameter with respect to silica content corresponds to increasing interfacial area. A SiO2 particle content lower than 0.1wt.% was insufficient for stabilizing the Pickering emulsions. Figure 3(g) shows the size distribution of microcapsules synthesized by sample 3. The mean diameter is 5.0±0.6 µm.

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Table 1 Parameters of PSF/SiO2 hybrid shell microcapsules preparation with different SiO2 particle contents (sample 1-5), encapsulation temperature (sample 6, 7, 3, 8) and stirring speed (sample 9, 10, 3, 11) Samples

SiO2 contents /wt.%

Temperature /

Stirring speed /rpm

1 2 3 4 5 6 7 8 9 10 11

0.25 0.5 1 2 3 1 1 1 1 1 1

30 30 30 30 30 20 25 35 30 30 30

600 600 600 600 600 600 600 600 200 400 800

8


Mean diameter /µm 17.8±4.3 8.2±2.8 5.0±0.6 1.1±0.6 0.5±0.1 6.2±1.2 5±0.7 4.7±0.6 5.1±0.7 4.6±0.8 4.7±1.1

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Figure 3 SEM morphologies of microcapsules with different SiO2 particles content (sample 1-5) (a) 0.25wt.% (b) 0.5wt.% (c) 1.0wt.% (d) 2.0wt.% (e) 3.0wt.% (f) the mean diameters of microcapsules as a function of SiO2 particles content (g) size distribution of microcapsules synthesized by sample 3

The encapsulation temperature is another important factor that affects the microcapsule properties. Temperature affects the velocity of the solvent evaporation. 9


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The boiling point of DCM is 39.8℃. The encapsulation temperature was adjusted from 20 to 35℃. The solvent evaporation method for fabricating core-shell microspheres is based on the spreading coefficient theory. The PSF polymer in the DCM solvent system tends to phase separate during the process of solvent removal above the critical concentration and after solvent evaporation, with PSF being precipitated to form core and shell layers, respectively.7 Figure 4 shows the morphologies of microcapsules in different encapsulation temperature conditions. A low encapsulation temperature (20℃) causes a slow evaporation velocity, resulting in a longer microencapsulation time. Upon increasing the temperature to 35℃, the fast evaporation of the DCM solvent causes incomplete phase separation, which results in the PSF polymers becoming trapped in a non-equilibrium configuration, as shown in Figure 4(d). It is appropriate to control the encapsulation temperature in the range of 25-30℃.

Figure 4 SEM morphologies of microcapsules prepared with different encapsulation temperature (sample 6, 7, 3, 8) (a) 20℃ (b) 25℃ (c) 30℃ (d) 35℃ The microcapsule size prepared by the solvent evaporation technique is affected by 10


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the stirring speed, which has been reported in many literature studies. However, in the combination method of Pickering emulsification and solvent evaporation technique, the microcapsule size has been decided in the high-speed emulsification stage and the subsequent stirring speed in the solvent evaporation process has no obvious effect on the size of the microcapsules. We can see from Figure 5 that the stirring speed is 200, 400, 600 and 800 rpm, respectively, with the microcapsule size barely changing. This also indicates that the SiO2 particles have excellent stabilization for the emulsion. We discover agglomeration when the stirring speed is 800 rpm, which is attributed to the velocity of the solvent evaporation. The faster the stirring speed, the faster the evaporation velocity, leading to incomplete phase separation of PSF, this is consistent with the effect of the encapsulation temperature.

Figure 5 SEM morphologies of microcapsules prepared with different stirring speed (sample 9, 10, 3, 11) (a) 200 rpm (b) 400 rpm (c) 600 rpm (d) 800 rpm Characterization of PSF/SiO2 hybrid shell microcapsules As the SEM morphologies displayed in Figure 6(a), the obtained microcapsules possess a spherical shape structure and rough outer surface, which are attributed to the 11


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SiO2 aggregates in the shell and are beneficial to the dispersibility of microcapsules in the epoxy matrix. Figure 6(b) shows a ruptured microcapsule with a wall thickness of 0.83 µm.

Figure 6 SEM morphologies of PSF/SiO2 hybrid shell microcapsules (sample 3). (a) signal magnifying spherical shaped microcapsules (b) ruptured microcapsule FTIR spectroscopy of the PSF/SiO2 hybrid shell microcapsules, PSF and lubricant oil are conducted to further reveal the chemical structure of the microcapsules, as shown in Figure 7. The microcapsule spectra show characteristic peaks of PSF at ≈1489, ≈1504, ≈1586 cm-1 (C=C aromatic rings stretching), ≈1323, ≈1295 cm-1(O=S=O asymmetric stretching), ≈1230 and ≈1000 cm-1 (C-O-C asymmetric stretching). The spectrum of the microcapsules also shows the characteristic peaks of lubricant oil at ≈2952 and ≈2857 cm-1 (=C-H asymmetric and symmetric stretching), in addition to the characteristic peaks of SiO2 ≈1095 cm-1 (Si-O-Si asymmetric stretching). FTIR curves confirm that the lubricant oil is efficiently encapsulated in the PSF/SiO2 hybrid shell microcapsules.

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Figure 7 FTIR spectra of PSF/SiO2 hybrid shell microcapsules, PSF and lubricant oil A TGA of the microcapsules, PSF and lubricant oil is performed to measure the thermal resistance of the microcapsules. As shown in Figure 8, the initial decomposition temperature of PSF is 500℃, and 32.0wt.% of the sample remains at 800℃, which is attributed to the formation of thermally stable carbonaceous materials, due to the presence of the aromatic structure in the PSF polymer backbone. The vaporization initial temperature of lubricant oil is 200℃, and vaporization finishes at 355℃, while the lubricant oil encapsulated in the microcapsules exhibited excellent thermal stability by elevating the initial vaporization temperature to 250℃. Simultaneously, the TGA cure slope of the microcapsules is smaller than that of the lubricant oil, which proved that the decomposition rate of the lubricant oil decreased. The above results indicate that the PSF/SiO2 hybrid shell could act as a protective thermal barrier and retard the decomposition of lubricant oil. The TGA curves also show that the lubricant oil is richly encapsulated by PSF/SiO2 hybrid shell microcapsules with the core content reaching 50.5wt.%.

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Figure 8 TGA cures of PSF/SiO2 hybrid shell microcapsules, PSF and lubricant oil Tribological properties of self-lubricating epoxy composites The measured frictional coefficient and specific wear rate as a function of microcapsule content are shown in Figure 9. The mean friction coefficient and specific wear rate of the pure epoxy under a sliding speed of 0.76 m/s and a normal load of 1.0 MPa are 0.58 and 3.95×10-13 m3/Nm, respectively. The friction coefficient and specific wear rate of the epoxy composite with 5wt.% microcapsules are significantly lower than that of the pure epoxy under the same conditions. Increased the microcapsule content to 10wt.% apparently further decreases the friction coefficient and specific wear rate to 0.32 and 0.36×10-13 m3/Nm, respectively. The obvious improvement in the tribological properties of the epoxy composites is attributed to the rupturing of microcapsules via the surface wear release of lubricant oil to lubricate the wear surfaces. Alternatively, a thin lubricant film is formed that lessens the direct contact between the composites and counterpart. When the microcapsule concentration is further raised to 25wt.%, the friction coefficient and 14


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specific wear rate are all slightly increased, which is attributed to the decreased mechanical properties with increasing microcapsule concentration. The mechanical properties of the material play an important role in controlling its wear resistance, as discussed in previous reports.13, 27

Figure 9 Frictional coefficient and specific wear rate of self-lubricant epoxy composites as a function of microcapsules content In order to better understand the tribological behaviors, the worn surface of the pure epoxy and microcapsules/epoxy composites are investigated by SEM, as shown in Figure 10. The worn surface of the pure epoxy is extremely rough, which is typical adhesive and abrasive wear. The composite with 10wt.% microcapsules exhibits a relatively smooth wore surface, which indicates that the lubricant oil stored in the microcapsules is released from the rupture microcapsules and acts as a lubricant to reduce friction and wear. Simultaneously, the magnifying SEM morphologies further reveal that the wear debris are trapped by the cavity of the rupture microcapsules, which also contribute to the improvement of the tribological properties. As the third-body, the reduction in the amount of wear debris due to the trapped by the cavity 15


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weakens the abrasive effect in the wear contact area.

Figure 10 SEM morphologies of the worn surface (a) pure epoxy (b) 10wt.% microcapsules filled epoxy composites (close view were shown in the top right corner) Conclusions Lubricant-filled microcapsules with a PSF/SiO2 hybrid shell have been successfully prepared, as verified by both morphological and thermogravimetric evidence, which show that the core/shell structure was formed. The optimum synthesis conditions of microcapsules are obtained. The size control of the microcapsules is conducted by varying the SiO2 particle contents. The encapsulated temperature and agitation speed affect the properties of the microcapsules. The ratio of the dispersed phase/continuous phase is 0.6:1, the mass ratio of core/shell is 1:0.75, the concentration of SiO2 emulsifier is 1.0wt%, the encapsulated temperature is 25-30℃ and the agitation rate is 400 rpm. The microcapsules synthesized by the above optimal preparation parameters have a compact spherical structure with a mean diameter of 5.0±0.6 µm and a thickness of 0.83 µm. Meanwhile, owing to the protection of the PSF/SiO2 hybrid shell wall, the microcapsules exhibits excellent thermal resistant properties. Friction and wear tests are performed to investigate the self-lubricating effect of epoxy/microcapsules composites. As a result, the tribological properties of composites 16


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embedded with microcapsules exhibit remarkable improvement. When the microcapsule concentration is 10wt.%, the frictional coefficient and specific wear rate are reduced by 44.8% and 90.9%, respectively, compared to the pure epoxy. Meanwhile, the SEM images revealed in the worn surface demonstrates that the lubricant oil, outflowed from microcapsules under wear process, form a lubricant film to lessen the contact of the composite sample and its counterpart. Alternatively, the debris captured by rupture microcapsules contributes to decreasing the frictional coefficient and specific wear rate. The synthesized microcapsules can be further applied in many polymer matrices to prepare self-lubricating polymer composites.

Acknowledgments: The research is financially supported by the National Young Top Talents Plan of China (2013042), National Science Foundation of China (21676052, 21606042), National Science Foundation of China (51175066), Program for New Century Excellent Talents in University (NCET-12-0704), the Science Foundation for Distinguished Young Scholars of Heilongjiang Province (JC201403), and Natural Science Foundation of Heilongjiang Province (E2015034).

References (1) White, S. R.; Sottos, N.; Geubelle, P.; Moore, J. Autonomic healing of polymer composites. Nature 2001, 409 (6822), 794. DOI: 10.1038/35057232. (2) Bandeira, P.; Monteiro, J.; Baptista, A. M.; Magalhães, F. D. Tribological performance of PTFE-based coating modified with microencapsulated [HMIM][NTf2] ionic liquid. Tribol. Lett. 2015, 59 (1), 13. DOI: 10.1007/s11249-015-0545-y. (3) Weiss, E.; Dutta, B.; Kirschning, A.; Abu-Reziq, R. BMIm-PF6@SiO2 microcapsules: particulated ionic liquid as a new material for the heterogenization of catalysts. Chem. Mater. 2014, 26 (16), 4781-4787. DOI: 10.1021/cm501840d. (4) Behzadnasab, M.; Esfandeh, M.; Mirabedini, S.; Zohuriaan-Mehr, M.; Farnood, R. Preparation and characterization of linseed oil-filled urea–formaldehyde microcapsules and their effect on mechanical properties of an epoxy-based coating. Colloids Surf. A 2014, 457, 16-26. DOI: 10.1016/j.colsurfa.2014.05.033. (5) Wei, J.; Ju, X. J.; Zou, X. Y.; Xie, R.; Wang, W.; Liu, Y. M.; Chu, L. Y. 17


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SiO2 hybrid shell microcapsules synthesized by the

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