Optimization of preparation conditions of epoxy-containing nanocapsules

2.1 Experimental materials

Epoxy resin (E-51) is supplied by San Mu Group Corporation of Jiangsu, Nanjing, Jiangsu, China. Urea, 37% formaldehyde, sodium dodecyl sulfate (SDBS), sodium dodecylbenzenesulfonate (DBS), and Gum Arabic were supplied by Guangzhou Chemical Reagent, Guangzhou, Guangdong, China. Triethanolamine, sodium chloride, hydrochloric acid, acetone, resorcinol and n-octanol were supplied by Damao Chemical Reagent of Tianjin, Tianjin, China. All commercial chemicals are used without further purification in this study.

2.2 Preparation of nanocapsule

1. E-51 was dissolved into the emulsifier solution in ultrasonic equipment and dispersed for 20 min to form O/W emulsion. This emulsion and urea (U), 37 wt.% formaldehyde (F) (weight ratio between urea and formaldehyde was 1:2), were added in a-250 ml three-neck, round-bottom flask and stirred at 60°C for 10 min. Then, resorcinol (0.5 g) and NaCl solution (4 wt.%) were added to the above solution and the pH value of the mixed solution was adjusted to about 3 by addition of hydrochloric acid solution. After the solution was stirred for 170 min, the microcapsules were rinsed by acetone and anhydrous ethanol, filtered, and dried.

2. Manufacture of epoxy specimen filled with nanocapsules: The resin mixture was prepared by E-51 and amine curing agent and the mass ratio between E-51 and amine curing agent was 2:1. The specimens were prepared by mixing 10% nanocapsules with the resin mixture described above. To obtain the sample, the composites were degassed, poured into a closed polyvinyl fluoride mold, and cured for 7 h at room temperature.

2.3 Determination of core materials content of nanocapsules (w(core)%)

The core materials content of nanocapsule was established by the method of extraction, and acetone was used as an extraction solvent. The samples were crushed and washed with acetone several times, and then dried at room temperature for 24 h.

w(core) %=(m1-m2)/m1×100%,

where m₁ and m₂ are the mass of nanocapsules and shell materials, respectively.

2.4 Performance test and structure characterization

The average diameter of nanocapsules was analyzed by nanoparticle size analyzer (ZS90). The surface topography of nanocapsules was analyzed by optical microscope (OM) and SEM. Before observation of SEM, samples were vacuum sputter coated with a thin layer of gold to provide electrical conduction. The mechanical properties of epoxy matrix filled with nanocapsules have been assessed by tensile test. The tensile strength was measured according to GB/T 1040-96 on a testing machine (DL-1000B). Samples with dimensions of 120 mm×10 mm×4 mm were used for all tensile tests, at a crosshead speed of 5 mm/min, with a force sensor of 500 N. DTA-TG (ZRY-2P) was used to discuss the thermal stability of nanocapsules. All experiments were carried out with a sample weight of about 8.0–10.0 mg, at a heating rate of 10°C/min, from 25°C to 600°C, under nitrogen atmosphere.

2.5 Analysis of finite element

In the process of finite element analysis, a simulating capsule is contained in specimen model, the crack propagation of nanocapsules with different shell thickness is studied and the stress distribution around the mode of crack propagation is predicted. In this paper, a 2D entity structure model is built. During the procedure of mesh division, Plane42 is used in bulk material and Plane82 is used in capsule shell. State analysis is applied to the model, in which the boundary layer between the bulk material and the capsule shell is assumed perfectly joined. By the finite element analysis, the crack of surrounding matrix and the sphere are subjected to random load, perpendicular to the crack plane. The stress distribution of the model is solved.

3.1 Design and results of orthographic experiment

On the basis of previous research [13, 14], core/shell mass ratio, different emulsifier, content of emulsifier, and agitation rate were investigated as the four major influencing factors, and three levels were designed for each factor. Through the method of orthographic factorial design, the most influencing factors on w(core) and average diameter are investigated. The design and results of orthographic experiment are shown in Tables 1–5, and OM images of nanocapsules prepared in conditions of orthogonal test design are shown in Figure 1.

Figure 1:

OM images of nanocapsule prepared in conditions of orthogonal test design.

Tables 1 and 2 show the orthographic factorial design of four factors and three levels for each factor. Tables 3–5 show the results and analysis of the effects of four factors at three levels on w(core) and average diameter of nanocapsules, respectively.

3.2 Effects of emulsifier on average diameter of nanocapsules

The results in Table 4 indicate that according to the analysis of variance, emulsifier was the most important influential factor on the average diameters of nanocapsules prepared by in situ technique. Not only the average diameter of nanocapsules will count, but also the dispersion and morphology of nanocapsules are important objects to study. Figure 1 shows the OM images of nanocapsules prepared in conditions of orthogonal test design, and the result indicates that sample 3 had uniform particle size and no adhesion occurred.

Hydrophilic-lipophilic balance (HLB) value is the characteristic index of emulsifier, which is a very important constant for helping to match the core and shell materials. In this section, HLB values of SDBS, DBS, and Arabic gum were 13.0, 10.6, and 8.0, respectively. The results in Figure 1 reveal that with the decrease in HLB, the dispersion of nanocapsules became better. The results in Table 4 and Figure 1 indicate that when Arabic gum was used as emulsifier, the average diameter of nanocapsules was 820 nm; moreover, the nanocapsules had uniform particle size and no adhesion occurred, which was the characteristic of the ideal products.

3.3 Effects of agitation rate on w(core) of nanocapsules

The results in Table 5 indicate that the most important factor on w(core) of nanocapsules is agitation rate. According to the results in Table 5, with the increase in agitation rate from 200, 300, and 400 r/min, the w(core) of nanocapsules first increased and then decreased subsequently, which changed from 57.3% to 75.0% to 69.2%.

The main reason was that with the increase in agitation rate, the core materials were better dispersed for shell materials to encapsulate and w(core) increased. With the increase in agitation rate to 400 r/min, many smaller droplets existed in emulsion due to the excessive dispersion, which resulted in the decrease in w(core). According to results in Table 5, the optimum agitation rate was 300 r/min based on the w(core).

3.4 Mechanical properties of epoxy matrix with different contents of nanocapsules

The results in Table 6 show the mechanical properties of epoxy matrix with different contents of nanocapsules.

The results listed in Table 6 reveal that when the content of nanocapsules was 5% and 10%, the tensile strength was increased. With the increase in the content of nanocapsules to 15% and 20%, the tensile strength was decreased. When the content of nanocapsules was 5% and 20%, the modulus of elasticity decreased, but when the content of nanocapsules was 10% and 15%, the modulus of elasticity was increased. From data in Table 6, it is observed that the epoxy sample containing 10% nanocapsules has the highest tensile strength (22.7 MPa) and modulus of elasticity (794 MPa). Thus, in terms of mechanical properties, the sample containing 10% nanocapsule seems more attractive for enhancing the bonding strength between the surface of the nanocapsules and resin matrix.

3.5 The thermal stability of nanocapsules

The thermal stability of the nanocapsules plays an important role in their applications in self-healing materials. Figure 2 demonstrates the thermogravimetric analysis (TG and DSC) curves of nanocapsules prepared in the optimum conditions. The temperature of initial mass loss of 32% (by weight) of nanocapsules is up to 100°C and 38% at 140°C due to the surface absorption of water and formaldehyde. A rapid decomposition process was observed between 140 and 460°C due to the fracturing of the shell material, resulting in rapid release of the core material, and the mass loss was about 84% by weight at 460°C. Further, the decomposition of the shell material led to weight loss between 460 and 600°C. According to the results above, nanocapsules prepared in the optimum conditions are suitable for storage.

Figure 2:

TG and DSC curves of nanocapsules.

3.6 Scanning electron microscope

The morphology of the nanocapsules was characterized by SEM, and the results are shown in Figure 3. Figure 3A and B display uniform spherical nanocapsules with rough surface, thus ensuring uniform dispersion of the nanocapsules in the matrix, to a certain extent, which was favorable for the interfacial joint between nanocapsule and matrix. There was a broken nanocapsule in Figure 3C, which indicated that the shell of the nanocapsule was thin (about 110 nm) and could coat more epoxy resin. The thin shell guaranteed that when the nanocapsules are ruptured with the immediate propagation of the microcracks, the curing agent could release easily, which was beneficial for improving the self-healing efficiency of the materials.

Figure 3:

SEM images of nanocapsules (A, B) and (C) crushed.

3.7 Analysis of crack propagates

The effect of shell thickness on the crack propagate approach was analyzed by finite element methods, and plan structure can be calculated using the commercial software ANSYS. The result is presented in Figure 4, which shows the path of an approaching crack. The nanocapsule size is defined as 820 nm and shell thickness is 78, 110, 138, and 183 nm in the model, in accordance with the characterization results of nanocapsules. The elastic module of epoxy resin and PUF shell materials is 3.4 GPa and 3.7 GPa, respectively. The Poisson’s ratios of the shell materials and matrix are equal (0.3). The stress distribution in the equatorial plane of the sphere is shown in Figure 4A–D, in the same loads; when the shell thickness of nanocapsules is 78 and 110 nm, the stress distributions near the nanocapsules and the surrounding matrix are almost similar, which will attract the crack propagating to the nanocapsule and then rupture it, which is a necessary condition of rupture for the healing process. At the same time, the thinner the shell thickness is, the larger is the stress distribution near the nanocapsules. When the shell thickness of nanocapsules is 138 and 183 nm, the stress distributions near the nanocapsules are lower than in the surrounding matrix, and there is the possibility that the crack propagates randomly and thus cannot achieve self-healing. Based on the above analysis, the shell thickness of nanocapsules should be <110 nm. We prepared the nanocapsules with an average of 110 nm shell thickness, as shown as in Figure 3C, which were suitable for self-healing materials.

Figure 4:

Stress distribution in the equatorial plane of the nanocapsule with shell thickness 78 nm (A), 110 nm (B), 138 nm (C), 183 nm (D).