Urea formaldehyde resin-coated epoxy resin microcapsules were prepared by two-step in situ polymerization. The effects of five factors on the yield, coverage rate, repair rate, and morphology of the microcapsules were investigated by five factors and four levels of orthogonal test. These five factors were the mass ratio of the core to the wall material (
A finishing process is an important means to adjust surface characteristics, but its process is complex, and it is easy to produce many fine defects in the process of finishing and subsequent production. In recent years, by imitating the self-repair function of the organism and the participation of external environmental factors, internal and external damage can be automatically repaired in the matrix material and the useful life of the material can be prolonged [
In the preparation of self-repair microcapsules, the selection of core and wall materials for a microcapsule system is particularly important. A microencapsulated self-repairing system mainly includes the dicyclopentadiene-Grubbs curing agent system, epoxy resin core microcapsule self-repairing system, isocyanate microcapsule self-repairing system, dry oil microcapsule self-repairing system, polar solvent core self-repairing system, siloxane self-repairing system, and silicone oil self-repairing system. However, it is still difficult for these self-repairing systems to be widely used in the field of coatings. Based on research results, the microcapsule preparation method selected was in situ polymerization, which is easy to control and easily realized [
Urea (
Preparation of urea formaldehyde-coated epoxy resin microcapsules was composed of three processes [
The mixture of urea and formaldehyde solution was added to the beaker at the mass ratio of 1.00 : 1.85. The mixture was mixed well, and the pH of the solution was slowly adjusted to 8.0–9.0 by adding triethanolamine. The mixture was continuously stirred for 1 h in a constant temperature water bath at 70°C, and the wall material solution was prepared and cooled at room temperature.
The epoxy resin and benzyl alcohol (as diluent) were added to the beaker to be mixed completely. The mass ratio of benzyl alcohol to epoxy resin was fixed at 0.15 : 1. Certain amounts of deionized water and sodium dodecyl benzene sulfonate (emulsifier) were placed in another beaker for the emulsifier solution. Then the diluted epoxy resin solution was poured into the emulsifier solution and mixed, and the beaker was put into the water bath. In the water bath, the temperature was adjusted to 60°C, then the agitator was set for 30 min, and the core material emulsion was obtained by dropping in octanol for defoaming.
At a certain speed, the wall urea formaldehyde resin was slowly added to the core material and the citric acid monohydrate was added to adjust the solution pH to 2.5–3.0. Then the water bath was slowly heated to 70°C, and stirring continued for 3 h. The product was allowed to stand for 0–48 h, then washed with deionized water and anhydrous ethanol, and filtered. The remaining product was heated to 40°C for 48 h, and the white powder obtained was used for the microcapsules [
A L16 (45) orthogonal experiment with 5 factors and 4 levels was used to optimize the preparation of microcapsules. The 5 factors were the mass ratio of the core material to the wall material (
Influencing factors and levels.
Level | Stirring rate (r/min) | Deposition time (h) | |||
---|---|---|---|---|---|
1 | 0.6 : 1 | 1 : 100 | 400 | 0 | 8 : 1 |
2 | 0.8 : 1 | 3 : 100 | 600 | 16 | 9 : 1 |
3 | 1.0 : 1 | 5 : 100 | 800 | 32 | 10 : 1 |
4 | 1.2 : 1 | 7 : 100 | 1000 | 48 | 11 : 1 |
Experimental arrangements.
Sample | Stirring rate (r/min) | Deposition time (h) | |||
---|---|---|---|---|---|
1 | 0.6 : 1 | 1 : 100 | 400 | 0 | 8 : 1 |
2 | 0.6 : 1 | 3 : 100 | 600 | 16 | 9 : 1 |
3 | 0.6 : 1 | 5 : 100 | 800 | 32 | 10 : 1 |
4 | 0.6 : 1 | 7 : 100 | 1000 | 48 | 11 : 1 |
5 | 0.8 : 1 | 1 : 100 | 600 | 32 | 11 : 1 |
6 | 0.8 : 1 | 3 : 100 | 400 | 48 | 10 : 1 |
7 | 0.8 : 1 | 5 : 100 | 1000 | 0 | 9 : 1 |
8 | 0.8 : 1 | 7 : 100 | 800 | 16 | 8 : 1 |
9 | 1.0 : 1 | 1 : 100 | 800 | 48 | 9 : 1 |
10 | 1.0 : 1 | 3 : 100 | 400 | 32 | 8 : 1 |
11 | 1.0 : 1 | 5 : 100 | 1000 | 16 | 11 : 1 |
12 | 1.0 : 1 | 7 : 100 | 600 | 0 | 10 : 1 |
13 | 1.2 : 1 | 1 : 100 | 1000 | 16 | 10 : 1 |
14 | 1.2 : 1 | 3 : 100 | 800 | 0 | 11 : 1 |
15 | 1.2 : 1 | 5 : 100 | 600 | 48 | 8 : 1 |
16 | 1.2 : 1 | 7 : 100 | 400 | 32 | 9 : 1 |
17 | 0.8 : 1 | 1 : 100 | 600 | 32 | 8 : 1 |
Detailed list of amounts of materials.
Sample | Epoxy resin (g) | Benzyl alcohol (g) | Sodium dodecyl benzene sulfonate (g) | Deionized water (g) |
---|---|---|---|---|
1 | 18.00 | 2.70 | 0.18 | 143.82 |
2 | 18.00 | 2.70 | 0.54 | 161.46 |
3 | 18.00 | 2.70 | 0.90 | 179.10 |
4 | 18.00 | 2.70 | 1.26 | 196.74 |
5 | 24.00 | 3.60 | 0.72 | 263.76 |
6 | 24.00 | 3.60 | 1.20 | 239.28 |
7 | 24.00 | 3.60 | 1.68 | 214.80 |
8 | 24.00 | 3.60 | 2.16 | 190.32 |
9 | 30.00 | 4.50 | 0.30 | 269.70 |
10 | 30.00 | 4.50 | 0.90 | 239.10 |
11 | 30.00 | 4.50 | 1.50 | 328.50 |
12 | 30.00 | 4.50 | 2.10 | 297.90 |
13 | 36.00 | 5.40 | 0.36 | 359.64 |
14 | 36.00 | 5.40 | 1.08 | 394.92 |
15 | 36.00 | 5.40 | 1.80 | 286.20 |
16 | 36.00 | 5.40 | 2.52 | 321.48 |
17 | 24.00 | 3.60 | 0.24 | 191.76 |
The amounts of 20.00 g urea and 50.00 g 37% formaldehyde solution were fixed for all samples, and 30.00 g of urea formaldehyde (the wall material) was obtained according to equation (
Epoxy resin (10.00 g) and 1.50 g benzyl alcohol were mixed until becoming smooth, then 0.53 g, 1.11 g, 1.76 g, or 2.50 g microcapsules were added to make 5.0%, 10.0%, 15.0%, and 20.0% microcapsule concentrations of the mixture, respectively, and then the mixture was poured into the standard polytetrafluoroethylene dye. After vacuum drying for defoaming, the mixture was cured at room temperature for 12 h and then dried at 40°C for 10 h and at 80°C for 2 h and the mixture was demolded as microcapsule specimens for the repair rate test.
The tensile strength of the material was analyzed by a microcomputer-controlled electronic universal testing machine, CMT6104, MTS Systems Corporation. The microstructure of the microcapsules was analyzed using a Quanta 200 environment scanning electron microscope (SEM), FEI Company, and L2800 Biomicroscope, Guangzhou Liss Optical Instrument Co. Ltd. The components of the microcapsules were analyzed using a Nicolet iS5 infrared spectrometer, Thermo Nicolet Corporation. The microcapsule powder was directly weighed after drying to test the yield of the microcapsules. Then 1.0 g microcapsules were put into a funnel and soaked in ethyl acetate, the ethyl acetate being replaced every 24 h. The residual was washed with deionized water after 72 h soaking in ethyl acetate and dried, and the material obtained was the wall material. The coverage rate was calculated by the following:
In formula (
In formula (
The morphologies of A–P shown in Figure
Surface morphology of samples 1-16 in Table
SEM of the uncoated microcapsules and encapsulated intact microcapsules was performed, and samples 8 and 10 were photographed, as shown in Figure
SEM of samples 8 and 10.
Figure
Infrared spectra of microcapsules of sample 6 (
Assignment of peaks.
Peak (cm-1) | Assignment |
---|---|
3400 | N-H stretching vibration |
2970 | C-H stretching vibration |
1615 | C=O stretching vibration |
1516 | C-N stretching vibration |
1249, 916 | Expansion vibration of C-O in epoxy resin |
The yield, coverage rate, and repair rate of samples 1–16 in Table
Orthogonal experiment design results.
Sample | Yield (g) | Coverage rate (%) | Repair rate (%) |
---|---|---|---|
1 | 26.17 | 74.00 | 105.50 |
2 | 23.69 | 48.00 | 91.20 |
3 | 31.01 | 48.00 | 100.40 |
4 | 13.10 | 35.00 | 77.80 |
5 | 29.01 | 65.00 | 107.60 |
6 | 25.43 | 78.00 | 105.60 |
7 | 8.49 | 63.00 | 95.03 |
8 | 27.81 | 58.00 | 98.20 |
9 | 36.12 | 71.00 | 74.20 |
10 | 40.82 | 78.00 | 106.20 |
11 | 6.48 | 45.00 | 74.37 |
12 | 22.34 | 80.00 | 98.30 |
13 | 11.56 | 60 | 72.07 |
14 | 5.94 | 25 | 68.72 |
15 | 23.76 | 77 | 94.50 |
16 | 39.68 | 85 | 93.30 |
The yield of microcapsules is an important basis for judging whether microcapsules can be used in the industry. Using the smallest amount of materials to prepare more microcapsules is desirable. Therefore, the range analysis and remarkable analysis of the yield are very important. According to the yield calculation results of the orthogonal experimental microcapsules [
Range of microcapsule yields.
Mean value | Stirring rate | Deposition time | |||
---|---|---|---|---|---|
Mean value 1 | 23.492 | 25.715 | 24.440 | 15.735 | 29.640 |
Mean value 2 | 22.685 | 23.970 | 24.700 | 17.385 | 26.995 |
Mean value 3 | 26.440 | 17.435 | 25.220 | 35.130 | 22.585 |
Mean value 4 | 20.235 | 25.733 | 18.492 | 24.603 | 13.633 |
Range | 6.205 | 8.298 | 6.728 | 19.395 | 16.007 |
Remarkable analysis of microcapsule yields.
Factor | Sum of squares of deviations | Freedom | Saliency | ||
---|---|---|---|---|---|
78.556 | 3 | 1.000 | 9.280 | ||
186.265 | 3 | 2.371 | 9.280 | ||
Stirring rate | 120.111 | 3 | 1.529 | 9.280 | |
Deposition time | 935.327 | 3 | 11.906 | 9.280 | |
591.161 | 3 | 7.525 | 9.280 | ||
Error | 78.56 | 3 |
The coverage rate of microcapsules is closely related to the content of the microcapsule core and the repair rate of microcapsules. The range analysis and remarkable analysis of the coverage rate were calculated as shown in Tables
Range of coverage rates of microcapsules.
Mean value | Stirring rate | Deposition time | |||
---|---|---|---|---|---|
Mean value 1 | 51.250 | 67.500 | 70.500 | 60.500 | 71.750 |
Mean value 2 | 66.000 | 57.250 | 67.500 | 52.750 | 66.750 |
Mean value 3 | 68.500 | 58.250 | 50.500 | 69.000 | 66.500 |
Mean value 4 | 61.750 | 64.500 | 59.000 | 65.250 | 42.500 |
Range | 17.250 | 10.250 | 20.000 | 16.250 | 29.250 |
Remarkable analysis of coverage rates of microcapsules.
Factor | Sum of squares of deviations | Freedom | Saliency | ||
---|---|---|---|---|---|
695.250 | 3 | 2.379 | 9.280 | ||
292.250 | 3 | 1.000 | 9.280 | ||
Stirring rate | 974.750 | 3 | 3.335 | 9.280 | |
Deposition time | 589.250 | 3 | 2.016 | 9.280 | |
2072.250 | 3 | 7.091 | 9.280 | ||
Error | 292.250 | 3 |
The repair rate of microcapsules is an indication of the repair power of microcapsules for epoxy resin. The higher the repair rate, the better the repair ability of microcapsules in materials. Therefore, range analysis and remarkable analysis of the repair rate are very important. According to the calculations of the repair rate of the orthogonal experimental microcapsules, the main sequence of the conditions affecting the repair rate of microcapsules was shown by Tables
Range of repair rates of microcapsules.
Mean value | Stirring rate | Deposition time | |||
---|---|---|---|---|---|
Mean value 1 | 93.725 | 89.843 | 94.693 | 91.887 | 101.100 |
Mean value 2 | 101.608 | 92.930 | 97.900 | 83.960 | 88.433 |
Mean value 3 | 88.267 | 91.075 | 85.380 | 101.875 | 94.093 |
Mean value 4 | 82.147 | 91.900 | 87.775 | 88.025 | 82.123 |
Range | 19.461 | 3.087 | 12.520 | 17.915 | 18.977 |
Remarkable analysis of repair rates of microcapsules.
Factor | Sum of squares of deviations | Freedom | Saliency | ||
---|---|---|---|---|---|
820.058 | 3 | 40.065 | 9.280 | ||
20.468 | 3 | 1.000 | 9.280 | ||
Stirring rate | 409.865 | 3 | 20.025 | 9.280 | |
Deposition time | 706.808 | 3 | 34.532 | 9.280 | |
784.849 | 3 | 38.345 | 9.280 | ||
Error | 20.47 | 3 |
Figure
Range of effects of factors on yield, coverage rate, and repair rate.
Figure
SEM image of the best microcapsule (sample 17).
The particle size of the microcapsules was measured using ImageJ software [
Particle size distribution of the best microcapsule.
Table
Comprehensive properties of microcapsules prepared by optimization.
Sample | Yield (g) | Coverage rate (%) | Repair rate (%) | ||
---|---|---|---|---|---|
17 | 45.63 | 75 | 30.64 | 34.59 | 113.04 |
In order to further verify the effect of the different concentrations of microcapsules on self-repair properties, the optimized microcapsules were added to the epoxy resin at 5.0%, 10.0%, 15.0%, and 20.0% [
Repair rate of samples with different microcapsule concentrations.
SEM of epoxy resin with 10.0% and 15.0% microcapsule concentrations.
The surface morphology and chemical composition of microcapsules prepared by an orthogonal experiment were characterized by scanning electron microscopy and infrared spectroscopy. The results showed that microcapsules with a high coverage rate, high repair rate, and high yield were successfully prepared. Through the orthogonal experimental five factors and four levels, the morphology of the microcapsules and the range and remarkable analysis on the yield, the optimal repair rate and the coverage rate of microcapsules were elucidated. The optimum process parameters for preparing urea formaldehyde-coated epoxy resin microcapsules were obtained by A2B1C1D3E1. When
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
The authors declare no competing financial interest.
This project is supported by the Natural Science Foundation of Jiangsu Province (BK20150887), Youth Science and Technology Innovation Fund of Nanjing Forestry University (CX2016018), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).