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Synthesis and mechanical properties of large size silica shell microcapsules for self-healing cementitious materials
Colloids and Surfaces A 587 (2020) 124347
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and mechanical properties of large size silica shell microcapsules for self-healing cementitious materials
T
Yumin Rena, Nadeem Abbasa,b, Guangming Zhua,*, Jiaoning Tanga a
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silica shell microcapsule Nanoindentation Self-healing cementitious materials
Synthesis of inorganic shell microcapsules is very challenging in the field of self-healing cementitious materials. Here, we report successful fabrication of epoxy resins encapsulated into silica shell microcapsules with average particle size greater than 100 μm via interfacial polymerization. The silica shell microcapsules show good thermal stability. The nanoindentation test was used to determine the micromechanical properties of the microcapsules. The results show that silica shell microcapsules exhibit brittle rupture, high Young’s modulus and hardness of the shell, with a better deformation resistance. The observed micromechanical properties are attributed to the regular tetrahedral crystal structure of the silica shell materials.
1. Introduction Concrete is one of the most widely used man-made materials. However, it easily undergoes cracking defects due to its relatively low tensile strength. Imposed deformations and external loads could result in high tensile stresses [1]. Without immediate and proper treatment, the service life of concrete structures will be substantially reduced because cracks provide an easy path for the propagation of harmful substances [2]. Cracks can be repaired by deliberate external intervention once they are detected. However, it is very difficult to repair the small and deep cracks. Therefore, the promising concept of self-healing was adopted to the repair of cracks in cementitious materials, and it is especially useful to repair deep-micro cracks [3,4]. In literature, organic materials are usually used as the shell of microcapsules in self-healing concrete [5–11]. However, organic shell materials microcapsules have disadvantage of poor compactness and easy loss of core materials. Inorganic shell materials are more stable, compact and stronger than organic shell materials. The core-shell structure formed is more stable, which can greatly prolong the service life of the microcapsules. Beside stability, compactness and strength of the shell material, size of microcapsules is another important aspect need to be focused. The size of core, shell as well as microcapsule plays a key role for the application in cementitious materials. The microcapsules with large size (d > 100 μm) are highly demanded due to the porous structure of concrete. The microcapsules will not rupture under
⁎
the crack tip stress when their size is smaller than the pore size. Several efforts have been reported aiming to study the silica shell microcapsules, but the synthesis of microcapsule with size greater than 50 μm have not been reported so far [12,13]. As for the healing agent, it has been reported that the epoxy resin is an effective healing agent for concrete [9]. The healing agent used in this study is composed of bisphenol A epoxy resin E-51 and aliphatic epoxy resin A-1815D, epoxy resin E-51 is the main component. The suitable curing agent will be embedded into the concrete. When cracks occur in the concrete, the microcapsules will crack under the tip stress of cracks, releasing healing agent into the crack plane through capillary action. Polymerization of the healing agent is triggered by contact with the embedded curing agent, bonding the crack faces. In addition, Mechanical properties of microcapsules are of great importance in their application in cementitious materials. Mechanical properties of microcapsules mainly include rupture load, Young's modulus and hardness. Recently, nanoindentation has been widely used to measure the mechanical properties of microcapsules [14–16]. Young’s modules and hardness could be easily calculated by the load-displacement curves. Synthesis of silica shell microcapsules with large particle size is very challenging because the hydrolysis rate of silica precursor is difficult to control. The synthesis of large silica shell microcapsules requires the use of special molecular templates to induce hydrolysis of the silica precursor to the oil-water interface. During the hydrolysis of silica precursor, the radial growth of silica is inhibited so that it expands
Corresponding author. E-mail address: [email protected] (G. Zhu).
https://doi.org/10.1016/j.colsurfa.2019.124347 Received 26 October 2019; Received in revised form 13 December 2019; Accepted 14 December 2019 Available online 16 December 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 587 (2020) 124347
Y. Ren, et al.
Fig. 1. (a) Synthetic illustration of silica shell microcapsules via interface polymerization route; (b) OM image of O/W emulsion; (c) SEM image of silica shell microcapsules; (d) SEM image of a single silica shell microcapsule; (e) EDX elemental maps of Si, O, and C.
2. Experimental section
along the bottom plane. Aliphatic epoxy resin plays a key role in the above process. The aliphatic group changes the activity of the epoxy resin. The aliphatic epoxy resin can weakly interact with tetraethyl silicate (TEOS), but can not cause a curing reaction. The steric restrictions of the epoxy network can affect the silica structure growth. Therefore, aliphatic epoxy resin plays the role of molecular template in the synthesis of silica shell microcapsules with TEOS as the precursor [17–19]. In this study, large particle size silica shell microcapsules enclosing epoxy resins were successfully synthesized by interfacial polymerization methods. In addition, the mechanical properties of silica shell microcapsules were studied by nanoindentation technique.
2.1. Synthesis of silica shell microcapsules The silica shell microcapsules were synthesized via interfacial polymerization, which was based on the hydrolysis and condensation of tetraethyl orthosilicate catalysed by hydrochloric acid on the oil-water interface of emulsion. 10 g of epoxy E-51 (Suixin Chemical Co., Ltd), 5 g of epoxy A-1815D (Qingda-Qs Materials Co., Ltd), and 10 g of TEOS (Maclean Biochemical Technology Co., Ltd) were mixed into a homogeneous oil phase. The oil phase was added into aqueous solution of F127 (150 mL, 2 wt %) (Maclean Biochemical Technology Co., Ltd), and then emulsified at 300 rpm for 30 min to form a stable oil-in-water (O/W) emulsion. 2 mol/L HCl (Sinopharm Chemical Reagent Co, Ltd) 2
Colloids and Surfaces A 587 (2020) 124347
Y. Ren, et al.
Fig. 2. (a) EDX analysis of the surface of the microcapsules; (b) Particle size distribution of silica shell microcapsules; (c) FT-IR curves of shell materials, core materials and microcapsules; (d) TGA curves of shell materials, core materials and microcapsules.
was dropped into the emulsion to adjust the pH to 2.5. The emulsion was heated to 50 °C and stirred at 300 rpm for 24 h. Finally, the product was washed with acetone (Sinopharm Chemical Reagent Co, Ltd) and deionized water, and then naturally dried.
depth. β is a dimensionless correction factor which accounts for the deviation in stiffness due to the lack of axisymmetry of the indenter tip with β = 1.034 for Berkovich indenter. The Young’s modules can be computed from
2.2. Mechanical property test of silica shell microcapsules
1 − νi2 1 1 − ν2 = + Er E Ei
The nanoindenter with a depth control method in a quasi-static loading mode was used to determine the mechanical properties of silica shell microcapsules. The program of the depth control method was improved. The instrument would unload and export displacement-load curves when the indenter reached the set maximum displacement or the compressed material suddenly losed its force on the indenter (the compressed material was ruptured or collapsed) or the load on the indenter reached the set upper limit value. Berkovich indenter was used because silica is a brittle material and the deformation around the indenter during test is small and its influence on the overall deformation can be ignored [20]. The displacement and force resolutions were 0.01 nm and 50 nN respectively. The load velocity was 10 nm/s. Thermal drift and strain rate were 0.05 nm/s and 0.05 s−1 respectively. Young’s modules and hardness were calculated by the load-displacement curves using the Oliver-Pharr method [21]. With the definition, the hardness can be computed from
H=
Pmax A
3. Results and discussion 3.1. Synthesis and characterization of silica shell microcapsules Fig. 1(a) schematically illustrates various steps during the synthesis of silica shell microcapsules enclosing epoxy resins by interfacial polymerization method. Epoxy E-51, aliphatic epoxy A-1815D, and TEOS were mixed into a homogeneous oil phase. The O/W emulsion was stabilized by PEO-PPO-PEO (F127). The PPO section in the center is located on the hydrophobic interface, while the two lateral PEO sections extend into the hydrophilic phase to enhance the steric resistance of the emulsion droplets to coalesce upon collision. When H+ in the continuous phase collides with TEOS in the dispersed phase, the TEOS hydrolyzes and condenses at the oil/water interface, and the silica shell microcapsules are formed. Fig. 1(b) shows the optical image of the O/W emulsion droplets. Fig. 1(c) shows the SEM image of the silica shell microcapsules. It can be seen that the silica shell microcapsules are relatively uniform, with highly spherical and a relatively smooth surface. The broken microcapsule in Fig. 1(d) shows its good hollow structure, with the shell thickness of about 1 μm. It also indicates that liquid core has been successfully encapsulated by the silica shell. Fig. 1(e) shows the EDX mapping images of a complete microcapsule. The blue, green, and red correspond to Si, O, and C elements respectively. Since a silicon
(1)
where H is the hardness, Pmax is the peak indentation load, and A is the projected area of contact at peak load. The reduced modulus can be computed from
Er =
π S 2β A
(3)
where E and ν are the Young’s modules and Poisson’s ratio for the specimen, and Ei and νi are the same parameters for the indenter.
(2)
where Er is the reduced modulus, and S is the contact stiffness, which is defined as the slope of the unloading curve at the maximum indentation 3
Colloids and Surfaces A 587 (2020) 124347
Y. Ren, et al.
substrate was used during sample preparation, so for the EDX performance only on a part of the shell (rather than a complete shell) was selected to avoid silicon substrate. The atomic percentage of Si, O and C are 32.55 %, 63.73 %, and 3.72 %, respectively. It can be seen that the ratio of Si and O is approximately 1:2, and a small amount of C is likely to be from airborne contaminants. It indicates that the microcapsule shell material is silica. Moreover, the EDX spectrum in Fig. 2(a) also proves above conclusion. Fig. 2(b) shows the particle size distribution histogram of the silica shell microcapsules, and the average particle size is 110.7 μm. The synthesis of such large size silica shell microcapsules have not been reported till date. These large size silica shell microcapsules are desirable for self-healing concrete. FT-IR spectroscopy in Fig. 2(c) was used to characterize the chemical composition and structure of the silica shell microcapsules. In the spectrum of the core materials, the peaks at around 567, 833, and 920 cm−1 can be attributed to the epoxy group [22,23]. In the spectrum of the shell materials, the peaks at 458, 1058, 944 and 3428 cm−1 can be attributed to the bending vibration of SieO bonds, the symmetric and asymmetric bending vibrations of SieOSei bonds, and the bending and stretching vibrations of SieOH groups, respectively [24]. The characteristic peaks of both epoxy group and silica are shown in the spectrum of the microcapsules. The above results confirm that epoxy resins have been successfully encapsulated by the silica shell. The thermal stability of microcapsules is of great significance for their storage and self-healing cementitious materials application. The thermal performances of silica shell, epoxy resins and microcapsules were studied by TGA measurements. As shown in Fig. 2(d), the TGA curve of the shell materials depicts that the silica shell exhibits robust thermal stability ranging from room temperature to 750 °C. The weight loss of approximately 5 wt % below 100 °C is corresponded to the removal of absorbed water in the air. In the TGA curve of the core materials, there is no obvious weight losses below 250 °C. It indicates that the epoxy resins healing agent has a relatively high thermal stability. With an increase in temperature to 750 °C, the residual weight is about 10 wt %. The TGA curve of the microcapsules indicates that the silica shell microcapsules have a high thermal stability below 250 °C, and the approximately 5 wt % weight loss below 100 °C is corresponded to the removal of absorbed water in the air. In addition, with an increase in temperature to 750 °C, the residual weight is about 20 wt %. It can be demonstrated that the encapsulation loading of epoxy resins in the silica shell microcapsules is about 90 wt %, which is higher than that reported by Gilford [25]. The highest encapsulation loading of dicyclopentadiene (DCPD) in the urea formaldehyde (UF) shell microcapsules is 79.02 wt %. The high encapsulation loading is of great significance for the self-healing efficiency of microcapsules [26].
Table 1 (a) Rupture load of silica shell microcapsules; (b) The Young's modulus and hardness of silica shell materials. (a) Mechanical Property
Sample 1
Sample 2
Sample 3
Particle size (μm) Shell thickness (μm) Rupture load (mN)
109.82 1.19 673.48
117.74 1.13 667.53
125.65 1.04 658.64
(b) Mechanical Property
Sample 1
Sample 2
Sample 3
Young's modulus (GPa) Hardness (GPa)
7.53 0.38
6.70 0.34
5.88 0.29
load versus displacement curves for compressing a single silica shell microcapsule to rupture. There was an instant force drop when the microcapsule was ruptured. The silica shell microcapsules exhibited brittle rupture, which corresponded to the regular tetrahedral crystal structure of silica shell materials. Three silica shell microcapsules with different size and shell thickness were tested, and the results are shown in Table 1(a). The rupture load of the silica shell microcapsules decreased with the increase of size-thickness ratio, which is consistent with the thin-shelled hollow sphere destruction theory. The rupture load of microcapsules is not only related to the diameter-thickness ratio, but also depends on the microstructure of the shell materials. Organic polymers usually have a chain structure with low strength and poor resistance to deformation, while silica has a regular tetrahedral crystal structure with high strength and strong resistance to deformation. Therefore, the rupture load of silica shell microcapsules is much higher than the rupture load of microcapsules with organic shell materials, such as phenol formaldehyde, acrylate, melamine formaldehyde and urea formaldehyde [11,27,28]. The Young's modulus and hardness of shell materials are two important parameters in the mechanical properties of microcapsules. The Young's modulus indicates the stiffness of materials. The greater Young's modulus is, the less likely it is to deform. The indentation test was repeated three times for each sample to get more reliable results by removing random error. Fig. 3(b) shows the load-displacement curves. The unloading curves indicated that the silica shell exhibited evident brittleness. The Young's modulus and hardness of the silica shell materials were calculated by the load-displacement curves by using the Oliver-Pharr method [21] and corresponding results are shown in Table 1(b). The mean Young's modulus and hardness are 6.70 ± 0.83 GPa and 0.34 ± 0.05 GPa, respectively. The Young's modulus of the silica shell materials is much greater than that of urea-formaldehyde resin shell materials (E = 0.81 GPa) [15] and melamine formaldehyde resin shell materials (E = 1.70 GPa) [29]. In addition, the excellent
3.2. Mechanical properties of silica shell microcapsules The rupture load of microcapsules is of vital significance for their application in self-healing cementitious materials. Fig. 3(a) shows the
Fig. 3. Load-displacement curves of nanoindentation test for (a) silica shell microcapsules, and (b) silica shell materials. 4
Colloids and Surfaces A 587 (2020) 124347
Y. Ren, et al.
repair ability of the healing agent epoxy resins in cementitious materials has been proved [8,9]. Therefore, it can be concluded that these large size silica shell microcapsules enclosing epoxy resins have a better deformation resistance, and are of vital importance for the field of selfhealing cementitious materials.
[7]
[8]
4. Conclusions
[9]
In summary, large size silica shell microcapsules (d > 100 μm) enclosing epoxy resins used for self-healing cementitious materials were successfully synthesized by interfacial polymerization methods. These microcapsules exhibit good thermal stability. The nanoindentation test results show that the silica shell microcapsules exhibited brittle rupture and had a better deformation resistance, which all corresponded to the regular tetrahedral crystal structure of silica shell materials. The silica shell microcapsules are of great significance for self-healing cementitious materials.
[10]
[11] [12]
[13]
[14]
Author contributions section [15]
Yumin Ren and Guangming Zhu conceived and designed the study. Yumin Ren performed the experiments. Yumin Ren and Nadeem Abbas wrote the paper. Jiaoning Tang reviewed the manuscript. All authors read and approved the manuscript.
[16]
[17]
Declaration of Competing Interest
[18]
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
[19]
[20]
[21]
[22]
Acknowledgement The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (No. 51978410).
[23]
[24]
References [25] [1] H. Huang, G. Ye, Simulation of self-healing by further hydration in cementitious materials, Cem. Concr. Comp. 34 (4) (2012) 460–467. [2] K.V. Tittelboom, N.D. Belie, Self-healing in cementitious materials-a review, Materials 6 (6) (2013) 2182–2217. [3] M.D. Rooij, K.V. Tittelboom, N.D. Belie, et al., Self-healing phenomena in cementbased materials, RILEM State Art Rep. 11 (2013). [4] S.R. White, N.R. Sottos, P.H. Geubelle, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794–797. [5] B. Hilloulin, K.V. Tittelboom, E. Gruyaert, et al., Design of polymeric capsules for self-healing concrete, Cem. Concr. Comp. 55 (2015) 298–307. [6] L. Lv, Z. Yang, G. Chen, et al., Synthesis and characterization of a new polymeric
[26] [27] [28] [29]
5
microcapsule and feasibility investigation in self-healing cementitious materials, Constr. Build. Mater. 105 (2016) 487–495. F. Xing, Z. Ni, N.X. Han, et al., Self-healing mechanism of a novel cementitious composite using microcapsules, Advances in Concrete Structural Durability, Proceedings of Icdcs 2008, (2008), pp. 195–204. W. Li, Z. Jiang, Z. Yang, et al., Self-healing efficiency of cementitious materials containing microcapsules filled with healing adhesive: mechanical restoration and healing process monitored by water absorption, Plosone 8 (11) (2013). X. Wang, J. Zhang, W. Zhao, et al., Permeability and pore structure of microcapsulebased self-healing cementitious composite, Constr. Build. Mater. 165 (2018) 149–162. B. Dong, G. Fang, Y. Wang, et al., Performance recovery concerning the permeability of concrete by means of a microcapsule based self-healing system, Cem. Concr. Comp. 78 (2017) 84–96. L. Lv, E. Schlangen, Z. Yang, et al., Micromechanical properties of a new polymeric microcapsule for self-healing cementitious materials, Materials 9 (2016). H. Zhang, X. Wang, D. Wu, et al., Silica encapsulation of n-octadecane via sol-gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance, J. Colloid Interf. Sci. 343 (1) (2010) 246–255. M. Fujiwara, K. Shiokawa, Y. Tanaka, et al., Preparation and formation mechanism of silica microcapsules (hollow sphere) by water/oil/water interfacial reaction, Chem. Mater. 16 (25) (2004) 5420–5426. J. Su, X. Wang, H. Dong, Micromechanical properties of melamine-formaldehyde microcapsules by nanoindentation: effect of size and shell thickness, Mater. Lett. 89 (2012) 1–4. X. Wang, R. Han, T. Han, et al., Determination of elastic properties of urea-formaldehyde microcapsules through nanoindentation based on the contact model and the shell deformation theory, Mater. Chem. Phys. 215 (2018) 346–354. J. Lee, M. Zhang, D. Bhattacharyya, et al., Micromechanical behavior of self-healing epoxy and hardener-loaded microcapsules by nanoindentation, Mater. Lett. 76 (2012) 62–65. G. Rokicki, C. Wojciechowski, Epoxy resin modified by aliphatic cyclic carbonates, J. Appl. Polym. Sci. 41 (1990) 647–659. S. Ponyrko, L. Kobera, J. Brus, et al., Epoxy-silica hybrids by nonaqueous sol-gel process, Polymer 54 (2013) 6271–6282. G. Canosa, P.V. Alfieri, C.A. Giudice, et al., High-solids, one-coat paints based on aliphatic epoxy resin-siloxanes for steel protection, Prog. Org. Coat. 77 (2014) 1459–1464. J. Gong, H. Miao, Z. Peng, Analysis of the nanoindentation data measured with a Berkovich indenter for brittle materials: effect of the residual contact stress, Acta Mater. 52 (3) (2004) 785–793. W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564–1583. X. Tong, T. Zhang, M. Yang, et al., Preparation and characterization of novel melamine modified poly (urea-form-aldehyde) self-repairing microcapsules, Colloids Surf. A 371 (2010) 91–97. L. Yuan, G. Liang, J. Xie, et al., Preparation and characterization of poly (ureaformaldehyde) microcapsules filled with epoxy resins, Polymer 47 (2006) 5338–5349. R. Al-Oweini, H. El-Rassy, Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors, J. Mol. Struct. 919 (2009) 140–145. J. Gilford, Microencapsulation of Self-healing Concrete Properties, Master Thesis Louisiana State University, 2012. E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (4) (2002) 372–379. L. Souza, A. Altabbaa, Microfluidic fabrication of microcapsules tailored for selfhealing in cementitious materials, Constr. Build. Mater. 184 (2018) 713–722. G. Sun, Z. Zhang, Mechanical strength of microcapsules made of different wall materials, Int. J. Pharmaceut. 242 (2002) 307–311. M. Pretzl, M. Neubauer, M. Tekaat, et al., Formation and mechanical characterization of aminoplast core/shell microcapsules, ACS Appl. Mater. Inter. 4 (2012) 2940–2948.
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and mechanical properties of large size silica shell microcapsules for self-healing cementitious materials
T
Yumin Rena, Nadeem Abbasa,b, Guangming Zhua,*, Jiaoning Tanga a
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silica shell microcapsule Nanoindentation Self-healing cementitious materials
Synthesis of inorganic shell microcapsules is very challenging in the field of self-healing cementitious materials. Here, we report successful fabrication of epoxy resins encapsulated into silica shell microcapsules with average particle size greater than 100 μm via interfacial polymerization. The silica shell microcapsules show good thermal stability. The nanoindentation test was used to determine the micromechanical properties of the microcapsules. The results show that silica shell microcapsules exhibit brittle rupture, high Young’s modulus and hardness of the shell, with a better deformation resistance. The observed micromechanical properties are attributed to the regular tetrahedral crystal structure of the silica shell materials.
1. Introduction Concrete is one of the most widely used man-made materials. However, it easily undergoes cracking defects due to its relatively low tensile strength. Imposed deformations and external loads could result in high tensile stresses [1]. Without immediate and proper treatment, the service life of concrete structures will be substantially reduced because cracks provide an easy path for the propagation of harmful substances [2]. Cracks can be repaired by deliberate external intervention once they are detected. However, it is very difficult to repair the small and deep cracks. Therefore, the promising concept of self-healing was adopted to the repair of cracks in cementitious materials, and it is especially useful to repair deep-micro cracks [3,4]. In literature, organic materials are usually used as the shell of microcapsules in self-healing concrete [5–11]. However, organic shell materials microcapsules have disadvantage of poor compactness and easy loss of core materials. Inorganic shell materials are more stable, compact and stronger than organic shell materials. The core-shell structure formed is more stable, which can greatly prolong the service life of the microcapsules. Beside stability, compactness and strength of the shell material, size of microcapsules is another important aspect need to be focused. The size of core, shell as well as microcapsule plays a key role for the application in cementitious materials. The microcapsules with large size (d > 100 μm) are highly demanded due to the porous structure of concrete. The microcapsules will not rupture under
⁎
the crack tip stress when their size is smaller than the pore size. Several efforts have been reported aiming to study the silica shell microcapsules, but the synthesis of microcapsule with size greater than 50 μm have not been reported so far [12,13]. As for the healing agent, it has been reported that the epoxy resin is an effective healing agent for concrete [9]. The healing agent used in this study is composed of bisphenol A epoxy resin E-51 and aliphatic epoxy resin A-1815D, epoxy resin E-51 is the main component. The suitable curing agent will be embedded into the concrete. When cracks occur in the concrete, the microcapsules will crack under the tip stress of cracks, releasing healing agent into the crack plane through capillary action. Polymerization of the healing agent is triggered by contact with the embedded curing agent, bonding the crack faces. In addition, Mechanical properties of microcapsules are of great importance in their application in cementitious materials. Mechanical properties of microcapsules mainly include rupture load, Young's modulus and hardness. Recently, nanoindentation has been widely used to measure the mechanical properties of microcapsules [14–16]. Young’s modules and hardness could be easily calculated by the load-displacement curves. Synthesis of silica shell microcapsules with large particle size is very challenging because the hydrolysis rate of silica precursor is difficult to control. The synthesis of large silica shell microcapsules requires the use of special molecular templates to induce hydrolysis of the silica precursor to the oil-water interface. During the hydrolysis of silica precursor, the radial growth of silica is inhibited so that it expands
Corresponding author. E-mail address: [email protected] (G. Zhu).
https://doi.org/10.1016/j.colsurfa.2019.124347 Received 26 October 2019; Received in revised form 13 December 2019; Accepted 14 December 2019 Available online 16 December 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 587 (2020) 124347
Y. Ren, et al.
Fig. 1. (a) Synthetic illustration of silica shell microcapsules via interface polymerization route; (b) OM image of O/W emulsion; (c) SEM image of silica shell microcapsules; (d) SEM image of a single silica shell microcapsule; (e) EDX elemental maps of Si, O, and C.
2. Experimental section
along the bottom plane. Aliphatic epoxy resin plays a key role in the above process. The aliphatic group changes the activity of the epoxy resin. The aliphatic epoxy resin can weakly interact with tetraethyl silicate (TEOS), but can not cause a curing reaction. The steric restrictions of the epoxy network can affect the silica structure growth. Therefore, aliphatic epoxy resin plays the role of molecular template in the synthesis of silica shell microcapsules with TEOS as the precursor [17–19]. In this study, large particle size silica shell microcapsules enclosing epoxy resins were successfully synthesized by interfacial polymerization methods. In addition, the mechanical properties of silica shell microcapsules were studied by nanoindentation technique.
2.1. Synthesis of silica shell microcapsules The silica shell microcapsules were synthesized via interfacial polymerization, which was based on the hydrolysis and condensation of tetraethyl orthosilicate catalysed by hydrochloric acid on the oil-water interface of emulsion. 10 g of epoxy E-51 (Suixin Chemical Co., Ltd), 5 g of epoxy A-1815D (Qingda-Qs Materials Co., Ltd), and 10 g of TEOS (Maclean Biochemical Technology Co., Ltd) were mixed into a homogeneous oil phase. The oil phase was added into aqueous solution of F127 (150 mL, 2 wt %) (Maclean Biochemical Technology Co., Ltd), and then emulsified at 300 rpm for 30 min to form a stable oil-in-water (O/W) emulsion. 2 mol/L HCl (Sinopharm Chemical Reagent Co, Ltd) 2
Colloids and Surfaces A 587 (2020) 124347
Y. Ren, et al.
Fig. 2. (a) EDX analysis of the surface of the microcapsules; (b) Particle size distribution of silica shell microcapsules; (c) FT-IR curves of shell materials, core materials and microcapsules; (d) TGA curves of shell materials, core materials and microcapsules.
was dropped into the emulsion to adjust the pH to 2.5. The emulsion was heated to 50 °C and stirred at 300 rpm for 24 h. Finally, the product was washed with acetone (Sinopharm Chemical Reagent Co, Ltd) and deionized water, and then naturally dried.
depth. β is a dimensionless correction factor which accounts for the deviation in stiffness due to the lack of axisymmetry of the indenter tip with β = 1.034 for Berkovich indenter. The Young’s modules can be computed from
2.2. Mechanical property test of silica shell microcapsules
1 − νi2 1 1 − ν2 = + Er E Ei
The nanoindenter with a depth control method in a quasi-static loading mode was used to determine the mechanical properties of silica shell microcapsules. The program of the depth control method was improved. The instrument would unload and export displacement-load curves when the indenter reached the set maximum displacement or the compressed material suddenly losed its force on the indenter (the compressed material was ruptured or collapsed) or the load on the indenter reached the set upper limit value. Berkovich indenter was used because silica is a brittle material and the deformation around the indenter during test is small and its influence on the overall deformation can be ignored [20]. The displacement and force resolutions were 0.01 nm and 50 nN respectively. The load velocity was 10 nm/s. Thermal drift and strain rate were 0.05 nm/s and 0.05 s−1 respectively. Young’s modules and hardness were calculated by the load-displacement curves using the Oliver-Pharr method [21]. With the definition, the hardness can be computed from
H=
Pmax A
3. Results and discussion 3.1. Synthesis and characterization of silica shell microcapsules Fig. 1(a) schematically illustrates various steps during the synthesis of silica shell microcapsules enclosing epoxy resins by interfacial polymerization method. Epoxy E-51, aliphatic epoxy A-1815D, and TEOS were mixed into a homogeneous oil phase. The O/W emulsion was stabilized by PEO-PPO-PEO (F127). The PPO section in the center is located on the hydrophobic interface, while the two lateral PEO sections extend into the hydrophilic phase to enhance the steric resistance of the emulsion droplets to coalesce upon collision. When H+ in the continuous phase collides with TEOS in the dispersed phase, the TEOS hydrolyzes and condenses at the oil/water interface, and the silica shell microcapsules are formed. Fig. 1(b) shows the optical image of the O/W emulsion droplets. Fig. 1(c) shows the SEM image of the silica shell microcapsules. It can be seen that the silica shell microcapsules are relatively uniform, with highly spherical and a relatively smooth surface. The broken microcapsule in Fig. 1(d) shows its good hollow structure, with the shell thickness of about 1 μm. It also indicates that liquid core has been successfully encapsulated by the silica shell. Fig. 1(e) shows the EDX mapping images of a complete microcapsule. The blue, green, and red correspond to Si, O, and C elements respectively. Since a silicon
(1)
where H is the hardness, Pmax is the peak indentation load, and A is the projected area of contact at peak load. The reduced modulus can be computed from
Er =
π S 2β A
(3)
where E and ν are the Young’s modules and Poisson’s ratio for the specimen, and Ei and νi are the same parameters for the indenter.
(2)
where Er is the reduced modulus, and S is the contact stiffness, which is defined as the slope of the unloading curve at the maximum indentation 3
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substrate was used during sample preparation, so for the EDX performance only on a part of the shell (rather than a complete shell) was selected to avoid silicon substrate. The atomic percentage of Si, O and C are 32.55 %, 63.73 %, and 3.72 %, respectively. It can be seen that the ratio of Si and O is approximately 1:2, and a small amount of C is likely to be from airborne contaminants. It indicates that the microcapsule shell material is silica. Moreover, the EDX spectrum in Fig. 2(a) also proves above conclusion. Fig. 2(b) shows the particle size distribution histogram of the silica shell microcapsules, and the average particle size is 110.7 μm. The synthesis of such large size silica shell microcapsules have not been reported till date. These large size silica shell microcapsules are desirable for self-healing concrete. FT-IR spectroscopy in Fig. 2(c) was used to characterize the chemical composition and structure of the silica shell microcapsules. In the spectrum of the core materials, the peaks at around 567, 833, and 920 cm−1 can be attributed to the epoxy group [22,23]. In the spectrum of the shell materials, the peaks at 458, 1058, 944 and 3428 cm−1 can be attributed to the bending vibration of SieO bonds, the symmetric and asymmetric bending vibrations of SieOSei bonds, and the bending and stretching vibrations of SieOH groups, respectively [24]. The characteristic peaks of both epoxy group and silica are shown in the spectrum of the microcapsules. The above results confirm that epoxy resins have been successfully encapsulated by the silica shell. The thermal stability of microcapsules is of great significance for their storage and self-healing cementitious materials application. The thermal performances of silica shell, epoxy resins and microcapsules were studied by TGA measurements. As shown in Fig. 2(d), the TGA curve of the shell materials depicts that the silica shell exhibits robust thermal stability ranging from room temperature to 750 °C. The weight loss of approximately 5 wt % below 100 °C is corresponded to the removal of absorbed water in the air. In the TGA curve of the core materials, there is no obvious weight losses below 250 °C. It indicates that the epoxy resins healing agent has a relatively high thermal stability. With an increase in temperature to 750 °C, the residual weight is about 10 wt %. The TGA curve of the microcapsules indicates that the silica shell microcapsules have a high thermal stability below 250 °C, and the approximately 5 wt % weight loss below 100 °C is corresponded to the removal of absorbed water in the air. In addition, with an increase in temperature to 750 °C, the residual weight is about 20 wt %. It can be demonstrated that the encapsulation loading of epoxy resins in the silica shell microcapsules is about 90 wt %, which is higher than that reported by Gilford [25]. The highest encapsulation loading of dicyclopentadiene (DCPD) in the urea formaldehyde (UF) shell microcapsules is 79.02 wt %. The high encapsulation loading is of great significance for the self-healing efficiency of microcapsules [26].
Table 1 (a) Rupture load of silica shell microcapsules; (b) The Young's modulus and hardness of silica shell materials. (a) Mechanical Property
Sample 1
Sample 2
Sample 3
Particle size (μm) Shell thickness (μm) Rupture load (mN)
109.82 1.19 673.48
117.74 1.13 667.53
125.65 1.04 658.64
(b) Mechanical Property
Sample 1
Sample 2
Sample 3
Young's modulus (GPa) Hardness (GPa)
7.53 0.38
6.70 0.34
5.88 0.29
load versus displacement curves for compressing a single silica shell microcapsule to rupture. There was an instant force drop when the microcapsule was ruptured. The silica shell microcapsules exhibited brittle rupture, which corresponded to the regular tetrahedral crystal structure of silica shell materials. Three silica shell microcapsules with different size and shell thickness were tested, and the results are shown in Table 1(a). The rupture load of the silica shell microcapsules decreased with the increase of size-thickness ratio, which is consistent with the thin-shelled hollow sphere destruction theory. The rupture load of microcapsules is not only related to the diameter-thickness ratio, but also depends on the microstructure of the shell materials. Organic polymers usually have a chain structure with low strength and poor resistance to deformation, while silica has a regular tetrahedral crystal structure with high strength and strong resistance to deformation. Therefore, the rupture load of silica shell microcapsules is much higher than the rupture load of microcapsules with organic shell materials, such as phenol formaldehyde, acrylate, melamine formaldehyde and urea formaldehyde [11,27,28]. The Young's modulus and hardness of shell materials are two important parameters in the mechanical properties of microcapsules. The Young's modulus indicates the stiffness of materials. The greater Young's modulus is, the less likely it is to deform. The indentation test was repeated three times for each sample to get more reliable results by removing random error. Fig. 3(b) shows the load-displacement curves. The unloading curves indicated that the silica shell exhibited evident brittleness. The Young's modulus and hardness of the silica shell materials were calculated by the load-displacement curves by using the Oliver-Pharr method [21] and corresponding results are shown in Table 1(b). The mean Young's modulus and hardness are 6.70 ± 0.83 GPa and 0.34 ± 0.05 GPa, respectively. The Young's modulus of the silica shell materials is much greater than that of urea-formaldehyde resin shell materials (E = 0.81 GPa) [15] and melamine formaldehyde resin shell materials (E = 1.70 GPa) [29]. In addition, the excellent
3.2. Mechanical properties of silica shell microcapsules The rupture load of microcapsules is of vital significance for their application in self-healing cementitious materials. Fig. 3(a) shows the
Fig. 3. Load-displacement curves of nanoindentation test for (a) silica shell microcapsules, and (b) silica shell materials. 4
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repair ability of the healing agent epoxy resins in cementitious materials has been proved [8,9]. Therefore, it can be concluded that these large size silica shell microcapsules enclosing epoxy resins have a better deformation resistance, and are of vital importance for the field of selfhealing cementitious materials.
[7]
[8]
4. Conclusions
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In summary, large size silica shell microcapsules (d > 100 μm) enclosing epoxy resins used for self-healing cementitious materials were successfully synthesized by interfacial polymerization methods. These microcapsules exhibit good thermal stability. The nanoindentation test results show that the silica shell microcapsules exhibited brittle rupture and had a better deformation resistance, which all corresponded to the regular tetrahedral crystal structure of silica shell materials. The silica shell microcapsules are of great significance for self-healing cementitious materials.
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[11] [12]
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Author contributions section [15]
Yumin Ren and Guangming Zhu conceived and designed the study. Yumin Ren performed the experiments. Yumin Ren and Nadeem Abbas wrote the paper. Jiaoning Tang reviewed the manuscript. All authors read and approved the manuscript.
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Declaration of Competing Interest
[18]
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
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[22]
Acknowledgement The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (No. 51978410).
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[24]
References [25] [1] H. Huang, G. Ye, Simulation of self-healing by further hydration in cementitious materials, Cem. Concr. Comp. 34 (4) (2012) 460–467. [2] K.V. Tittelboom, N.D. Belie, Self-healing in cementitious materials-a review, Materials 6 (6) (2013) 2182–2217. [3] M.D. Rooij, K.V. Tittelboom, N.D. Belie, et al., Self-healing phenomena in cementbased materials, RILEM State Art Rep. 11 (2013). [4] S.R. White, N.R. Sottos, P.H. Geubelle, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794–797. [5] B. Hilloulin, K.V. Tittelboom, E. Gruyaert, et al., Design of polymeric capsules for self-healing concrete, Cem. Concr. Comp. 55 (2015) 298–307. [6] L. Lv, Z. Yang, G. Chen, et al., Synthesis and characterization of a new polymeric
[26] [27] [28] [29]
5
microcapsule and feasibility investigation in self-healing cementitious materials, Constr. Build. Mater. 105 (2016) 487–495. F. Xing, Z. Ni, N.X. Han, et al., Self-healing mechanism of a novel cementitious composite using microcapsules, Advances in Concrete Structural Durability, Proceedings of Icdcs 2008, (2008), pp. 195–204. W. Li, Z. Jiang, Z. Yang, et al., Self-healing efficiency of cementitious materials containing microcapsules filled with healing adhesive: mechanical restoration and healing process monitored by water absorption, Plosone 8 (11) (2013). X. Wang, J. Zhang, W. Zhao, et al., Permeability and pore structure of microcapsulebased self-healing cementitious composite, Constr. Build. Mater. 165 (2018) 149–162. B. Dong, G. Fang, Y. Wang, et al., Performance recovery concerning the permeability of concrete by means of a microcapsule based self-healing system, Cem. Concr. Comp. 78 (2017) 84–96. L. Lv, E. Schlangen, Z. Yang, et al., Micromechanical properties of a new polymeric microcapsule for self-healing cementitious materials, Materials 9 (2016). H. Zhang, X. Wang, D. Wu, et al., Silica encapsulation of n-octadecane via sol-gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance, J. Colloid Interf. Sci. 343 (1) (2010) 246–255. M. Fujiwara, K. Shiokawa, Y. Tanaka, et al., Preparation and formation mechanism of silica microcapsules (hollow sphere) by water/oil/water interfacial reaction, Chem. Mater. 16 (25) (2004) 5420–5426. J. Su, X. Wang, H. Dong, Micromechanical properties of melamine-formaldehyde microcapsules by nanoindentation: effect of size and shell thickness, Mater. Lett. 89 (2012) 1–4. X. Wang, R. Han, T. Han, et al., Determination of elastic properties of urea-formaldehyde microcapsules through nanoindentation based on the contact model and the shell deformation theory, Mater. Chem. Phys. 215 (2018) 346–354. J. Lee, M. Zhang, D. Bhattacharyya, et al., Micromechanical behavior of self-healing epoxy and hardener-loaded microcapsules by nanoindentation, Mater. Lett. 76 (2012) 62–65. G. Rokicki, C. Wojciechowski, Epoxy resin modified by aliphatic cyclic carbonates, J. Appl. Polym. Sci. 41 (1990) 647–659. S. Ponyrko, L. Kobera, J. Brus, et al., Epoxy-silica hybrids by nonaqueous sol-gel process, Polymer 54 (2013) 6271–6282. G. Canosa, P.V. Alfieri, C.A. Giudice, et al., High-solids, one-coat paints based on aliphatic epoxy resin-siloxanes for steel protection, Prog. Org. Coat. 77 (2014) 1459–1464. J. Gong, H. Miao, Z. Peng, Analysis of the nanoindentation data measured with a Berkovich indenter for brittle materials: effect of the residual contact stress, Acta Mater. 52 (3) (2004) 785–793. W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564–1583. X. Tong, T. Zhang, M. Yang, et al., Preparation and characterization of novel melamine modified poly (urea-form-aldehyde) self-repairing microcapsules, Colloids Surf. A 371 (2010) 91–97. L. Yuan, G. Liang, J. Xie, et al., Preparation and characterization of poly (ureaformaldehyde) microcapsules filled with epoxy resins, Polymer 47 (2006) 5338–5349. R. Al-Oweini, H. El-Rassy, Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors, J. Mol. Struct. 919 (2009) 140–145. J. Gilford, Microencapsulation of Self-healing Concrete Properties, Master Thesis Louisiana State University, 2012. E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (4) (2002) 372–379. L. Souza, A. Altabbaa, Microfluidic fabrication of microcapsules tailored for selfhealing in cementitious materials, Constr. Build. Mater. 184 (2018) 713–722. G. Sun, Z. Zhang, Mechanical strength of microcapsules made of different wall materials, Int. J. Pharmaceut. 242 (2002) 307–311. M. Pretzl, M. Neubauer, M. Tekaat, et al., Formation and mechanical characterization of aminoplast core/shell microcapsules, ACS Appl. Mater. Inter. 4 (2012) 2940–2948.