Synthesis of Cyanate Ester Microcapsules via Solvent Evaporation

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Synthesis of cyanate ester microcapsules via solvent evaporation technique and its application in epoxy resins as a healing agent Junwei Gu, Xutong Yang, Chunmei Li, and Kaichang Kou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03093 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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Industrial & Engineering Chemistry Research

Synthesis of cyanate ester microcapsules via solvent evaporation technique and its application in epoxy resins as a healing agent Junwei Gu*, Xutong Yang+, Chunmei Li, Kaichang Kou Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaan Xi, 710072, P. R. China

Abstract: Bisphenol A cyanate ester/polyglycidyl methyacrylate (BADCy/PGMA) microcapsules were successfully fabricated via solvent evaporation technique, herein, BADCy as a core material, synthesized PGMA as a shell material. Optimal BADCy/PGMA microcapsules with dense core-shell structures were implanted in the Bisphenol A epoxy resin (E-51) matrix, to fabricate the corresponding (BADCy/PGMA)/E-51 self-healing composites. Results revealed that the optimal BADCy/PGMA microcapsules presented a spherical shape and rough surface, with mean diameter of 31.5 µm, wall thickness of 2.2 µm. The core material of BADCy still kept its reactivity after being encapsulated by the shell of PGMA. The BADCy/PGMA microcapsules also presented relatively good thermal stability and proper mechanical stability. Moreover, the fabricated (BADCy/PGMA)/E-51 composite with 8 wt% BADCy/PGMA microcapsules possessed relatively good self-healing performance.

Keywords: Bisphenol A cyanate ester (BADCy); Polyglycidyl methyacrylate (PGMA); Self-healing microcapsules; Bisphenol A epoxy resin; Solvent evaporation.

*

Corresponding author to J.W. Gu, E-mail address: [email protected] & [email protected], Tel: +86-29-88431621.The author Xutong Yang+ contributed equally to this work and should be considered co-first author. 1

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1. Introduction Self-healing polymer materials can timely repair local damage and micro-cracks during the process and usage, finally to eliminate the hidden trouble and prolong the service life of the polymer materials 1-3. To our knowledge, self-healing polymer materials can be classified into extrinsic self-healing and intrinsic self-healing. Extrinsic self-healing includes microcapsules 4, hollow-fibers 5, nano-particles 6, micro-vessels 7 and carbon nanotubes 8, etc. Intrinsic self-healing includes reversible covalent bond (acylhydrazone bond 9, disulfide bond 10

, N-O bond

11

and ‘Diels-Alder’ reaction

12

), and reversible non-covalent bond

(hydrogen bond 13, hydrophobic effect 14, electrostatic interaction 15, ionic interaction 16

, macromolecular diffusion interaction 17 and metal-ligand interaction 18, etc).

The microcapsules were firstly applied in the self-healing composites by White 19-20 in 2001, relying on the released healing agent from ruptured microcapsules. The microcapsules and the catalyst were simultaneously implanted into the polymer matrix. When the polymer matrix produced micro-cracks, the microcapsules were then ruptured and the embedded healing agents were then released into the crack plane. Healing is accomplished by the polymerization between embedded healing agent and catalyst, which could form a highly crosslinked polymer and bond to the cracks' surface, finally to achieve the self-healing

21

. The healing method of the

polymerization reaction caused by the micro-cracks could realize the effect of positioning healing 22. Cyanate resins possess excellent mechanical properties, high thermal stability, low water absorption, good weathering resistance, unique dielectric properties and an epoxy-like processability

23-26

, etc., and have presented a great potential in the fields

of aerospace, electronics, insulations and adhesives, etc 27-28. 2


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In our present work, to further extend the service life of epoxy resin matrix

29

,

polyglycidyl methyacrylate (PGMA) was firstly synthesized via dispersion polymerization, and then the corresponding Bisphenol A cyanate ester/PGMA (BADCy/PGMA) microcapsules were successfully fabricated by solvent evaporation technique, herein, BADCy as a core material, synthesized PGMA as a shell material. Then obtained BADCy/PGMA microcapsules were then implanted in the Bisphenol A epoxy resin (E-51) matrix, to fabricate the corresponding (BADCy/PGMA)/E-51 self-healing composites. The size, morphology, chemical structure, core content, reactivity and thermal stability of the BADCy/PGMA microcapsules were also analyzed and investigated by scanning electron microscopy (SEM), laser particle size analyzer, Fourier transform infrared (FTIR), thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC). And the corresponding self-healing performance of the (BADCy/PGMA)/E-51 self-healing composites was also preliminarily investigated. 2. Materials and Methods 2.1 Materials Glycidyl methyacrylate (GMA) was obtained from Aladdin Reagent Co., Ltd., China. Bisphenol A cyanate ester (BADCy) was received from Jiangsu Wuqiao Resin Factory Co., Ltd., China. Benzoyl peroxide (BPO) and polyvinyl alcohol (PVA) were purchased from Kermel Chemical Reagent Co., Ltd., China. Poly vinyl pyrrolidone (PVP-30) and sodium dodecyl sulfate (SDS) were supplied by Alfa Aesar Chemical Co., Ltd., China. Dichloromethane was obtained from Jinhuada Chemical Reagent Co., Ltd., China. Bisphenol-A epoxy resin (E-51) was received from Wuxi Resin Factory Co., Ltd., China. Absolute alcohol (EtOH) was purchased from Fuyu Fine Chemical Industry Co., Ltd., China. 3



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2.2 Synthesis of PGMA microspheres PGMA microspheres were synthesized via dispersion polymerization according to our previous work [21]. GMA (20g) was dissolved in EtOH by ultrasonic vibration, followed by the addition of BPO (0.4 g) and PVP-30 (1.0 g). Then the mixture above was kept reacting for 24h under 75oC with stirring at 200 r/min. The obtained PGMA microspheres were then centrifuged and washed by distilled water for three times, followed by freeze-dried for 24h. 2.3 Synthesis of BADCy/PGMA microcapsules BADCy and PGMA were dissolved in 40 mL CH2Cl2 to form oil phase. PVA (0.25 g) and SDS (0.025 g) were dissolved in 50 mL distilled water to form aqueous phase. Then the aqueous phase was added into the oil phase above, emulsifying for 30 min under 1000 r/min to form stable oil-in-water emulsion, stirred for another 3h at 40oC to volatilize CH2Cl2 completely. The obtained suspension liquid was then centrifuged and washed by distilled water for three times, followed by dried in vacuum for 24h to obtain the corresponding BADCy/PGMA microcapsules. Scheme 1 illustrated the schematic diagram of the formation process for BADCy/PGMA microcapsules by solvent evaporation technique. And the synthetic formula of BADCy/PGMA microcapsules with different content of core material (BADCy) and shell material (PGMA) were also listed in Table 1. 2.4 Fabrication of (BADCy/PGMA)/E-51 self-healing composites E-51 matrix, curing agent of 2-ethyl-4-methyl imidazole (2-ethyl-4-methyl imidazole/ E-51 matrix=5/100, wt/wt) and BADCy/PGMA microcapsules were stirred uniformly, degassed in a vacuum oven to remove inner air bubbles, and then poured into the preheated mold. The mixtures above were then cured at 70oC for 1h followed by at 150oC for another 1h, finally to obtain the corresponding (BADCy/PGMA)/E-51 4


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self-healing composites. The scratches on the (BADCy/PGMA)/E-51 self-healing composite with 8 wt% BADCy/PGMA microcapsules were created artificially. 2.5 Characterization Scanning electron microscope (SEM) morphologies of the samples were analyzed by VEGA3-LMH (TESCAN Corporation, Czech Republic); The diameters and size distributions of the samples were obtained from a LS13320 laser particle size analyzer (Beckman Coulter Corporation, America); Fourier transform infrared (FTIR) spectra of the samples were obtained on Bruker Tensor 27 equipment (Bruker Corporation, Germany); Differential scanning calorimetry (DSC) analyses of the samples were carried out at 10oC/min (nitrogen atmosphere), over the whole range of temperature (25-350oC) by DSC1 (Mettler-Toledo Corporation, Switzerland); Thermal gravimetric (TG) analyses of the samples were carried out at 10oC/min (argon atmosphere), over the whole range of temperature (25-800oC) by STA 449F3 (NETZSCH Corporation, Germany); The crack morphologies of the samples were obtained on DMM-300C upright metallurgical microscope (Shanghai Caikang Optical Instrument Corporation, China). 3. Results and Discussion 3.1. PGMA microspheres Figure 1 showed the SEM image and particle size distribution of PGMA microspheres. It could be seen that PGMA microspheres presented regular spherical and compact structures. The surface of PGMA microspheres appeared smooth, and there was little adhesion for PGMA microspheres each other (Figure 1a). As also seen from Figure 1b, the mean diameter of the PGMA microspheres was 1.75 µm and the corresponding C.V. value was 16.0%. 3.2. BADCy/PGMA microcapsules 5



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Figure 2 presented the SEM images of the BADCy/PGMA microcapsules with different mass ratio of raw materials. Besides Figure 2(d), the other four obtained products presented different degrees of agglomeration, hardly to form the integrated microcapsules' structure. With a small amount addition of PGMA microspheres (Figure 2a～～c), PGMA deposited completely on the surface of the core (BADCy), and the shell (PGMA) was relatively thin, resulting in the incomplete encapsulation. Therefore the obtained microcapsules were easy to rupture under the shearing force. With the increasing addition of PGMA microspheres (Figure 2d), the deposited rate of PGMA on the surface of the core (BADCy) was more moderate, easy to contact with BADCy, finally to form compact microcapsules. However, with the excessive addition of PGMA microspheres (Figure 2e), the deposited rate of PGMA was very fast, difficult to deposit on the surface of the core (BADCy). It could be deduced that the corresponding BADCy/PGMA microcapsules with optimal comprehensive performance could be obtained, when the proportion of BADCy/PGMA/CH2Cl2 was 0.5/2/40 (wt/wt/wt). The corresponding SEM images and size distribution of the optimal BADCy/PGMA microcapsules were also shown in Figure 3 and Figure 4, respectively. It could be seen that the optimal BADCy/PGMA microcapsules with dense core-shell structures presented spherical shape and rough surface. The thickness of the outer shell of PGMA was about 2.2 µm, which could protect the inner core of BADCy. Meantime, the corresponding mean diameter of the BADCy/PGMA microcapsules was 31.5 µm. FTIR spectra of PGMA, BADCy and the BADCy/PGMA microcapsules were shown in Figure 5. The band at 910 cm-1 was ascribed to the absorption vibration peak of epoxy group. The bands at 1150 cm-1, 1260 cm-1 and 1730 cm-1 could be assigned to the vibration absorption peaks of C-O-C, C-O and -C=O, respectively. The bands at 6


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2950 cm-1 and 3000 cm-1 could also be ascribed to the symmetrical and asymmetrical stretching vibration peaks of -CH3. Meantime, the bands at 830 cm-1 and 1500 cm-1 could be assigned to the stretching vibration peaks of benzene ring and aromatic hydrocarbon, respectively. In addition, strong characteristic vibration peaks of -OCN were also appeared near 2280 cm-1 and 2238 cm-1. It demonstrated that the PGMA and BADCy co-existed in the microcapsules. DSC curves of PGMA, BADCy and BADCy/PGMA microcapsules were presented in Figure 6. It was indicated that no exotherm peak was observed for the shell material of PGMA. As we know, the BADCy matrix could be cured only needing the action of heat, no need of the additional curing agent. An exotherm peak was presented in the curve of pure BADCy, ascribed to the curing reaction of pure BADCy at high temperatures. Compared that of pure BADCy, the BADCy/PGMA microcapsules also showed a similar exotherm peak, which revealed that the core material of BADCy could also be cured at high temperatures after being encapsulated by the shell of PGMA. Therefore, we think that the BADCy possesses its reactivity after being encapsulated 30. Figure 7 presented the TGA curves of PGMA, BADCy and BADCy/PGMA microcapsules. It could be seen that the weight loss of the BADCy/PGMA microcapsules was below 1 percent over the range of 0-220oC, which was mostly due to the loss of absorbed water and other small molecule volatilization. Then the BADCy/PGMA microcapsules began to decompose above 220oC, and the weight loss reached 84.5 percent at 800oC. It could be mainly attributed to the fuses, chars and decomposes of the PGMA and BADCy at relatively higher temperatures. The corresponding heat-resistance index (THRI) of 167.2oC (Table 2) also revealed that the BADCy/PGMA microcapsules possessed relatively good thermal stability. And the 7



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corresponding calculated content of BADCy in the BADCy/PGMA microcapsules was about 72.2 wt%. 3.3. (BADCy/PGMA)/E-51 composites SEM images of impact fractures for pure E-51 and (BADCy/PGMA)/E-51 composite with 8 wt% BADCy/PGMA microcapsules were shown in Figure 8. Both the pure E-51

and

(BADCy/PGMA)/E-51

composite

with

8

wt%

BADCy/PGMA

microcapsules presented typical brittle fracture morphologies. Compared with that of pure E-51 matrix, a small quantity of BADCy/PGMA microcapsules was randomly appeared on the surface of impact fractures, which revealed that the BADCy/PGMA microcapsules were successfully implanted in the E-51 matrix. Meantime, the preferable integrity of the BADCy/PGMA microcapsules also revealed proper mechanical stability that they could resist the mechanical stirring and curing process.. Figure 9 showed the cracks at two different locations of the (BADCy/PGMA)/E-51 composite with 8 wt% BADCy/PGMA microcapsules before and after self-healing. There were obvious changes in the cracks at two different locations after the treatment at 200oC for 6h. The corresponding macro-cracks were become shallow, narrow & small, and the micro-cracks were disappeared after the treatment at 200oC for 6h. The reason was that, the cured E-51 (thermosetting resin) couldn’t melt or flow under the action of heat, therefore, it couldn’t play a role in healing the produced cracks. Under the circumstances, the BADCy/PGMA microcapsules implanted in the E-51 matrix were ruptured under the action of scratches, and the embedded healing agent of BADCy could release into the crack plane, and the healing was accomplished by the curing reaction of BADCy under the action of heat. Herein, the BADCy could be cured only needing the action of heat, no need of the additional curing agent. 4. Conclusion 8


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PGMA microspheres possessed regular spherical and compact structure. When the mass ratio of BADCy/PGMA/CH2Cl2 was 0.5g/2g/40g, the optimal BADCy/PGMA microcapsules with dense core-shell structures were successfully fabricated via solvent evaporation technique. SEM images revealed that the optimal BADCy/PGMA microcapsules presented a spherical shape and rough surface, with mean diameter of 31.5 µm, wall thickness of 2.2 µm. DSC analyses revealed that the core material of BADCy still kept its reactivity after being encapsulated by the shell of PGMA. TG analyses revealed that BADCy/PGMA microcapsules presented good thermal stability, and the content of core for BADCy was 72.2 wt%. SEM analyses revealed that BADCy/PGMA microcapsules presented proper mechanical stability. Moreover, the fabricated

(BADCy/PGMA)/E-51

composite

with

8

wt%

BADCy/PGMA

microcapsules possessed good self-healing performance. Acknowledgements The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (No. 51403175); Shaanxi Natural Science Foundation of Shaanxi Province (No. 2015JM5153); Fundamental Research Funds for the Central Universities (Nos. 3102015ZY066 and 3102015BJ(II)JGZ020); The Innovation Foundation for Doctorate Dissertation of Northwestern Polytechnical University (No. CX201523). References (1)Hillewaere, X.K.D.; Teixeira, R.F.A.; Nguyen, L.T.; Ramos, J.A.; Rahier, H.; Prez, F.E.D. Autonomous self-healing of epoxy thermosets with thiol-isocyanate chemistry. Adv. Funct. Mater. 2014, 24, 5575. (2)Hillewaere X.K.D.; Prez F.E.Du. Fifteen chemistries for autonomous external self-healing polymers and composites. Prog. Polym. Sci. 2015, 49-50, 121. 9



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Ideal

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Figure and Table Captions Scheme 1 Schematic diagram of the formation process for BADCy/PGMA microcapsules by solvent evaporation technique Figure 1 SEM image and particle size distribution of PGMA microspheres. (a)SEM image; (b)Particle size distribution. Figure 2 SEM images of the BADCy/PGMA microcapsules with different mass ratio of raw materials. Proportion of BADCy/PGMA/CH2Cl2: (a)2g/8g/40g; (b)1g/4g/40g; (c)1g/2g/40g; (d)0.5g/2g/40g; (e)0.5g/4g/40g. Figure 3 SEM images of the optimal BADCy/PGMA microcapsules. (a)Several integrated microcapsules; (b)An integrated microcapsule; (c)A ruptured microcapsule. Figure 4 Size distribution of the optimal BADCy/PGMA microcapsules Figure 5 FTIR spectra of PGMA, BADCy and the BADCy/PGMA microcapsules Figure 6 DSC curves of PGMA, BADCy and BADCy/PGMA microcapsules Figure 7 TGA curves of PGMA, BADCy and BADCy/PGMA microcapsules Figure 8 SEM images of impact fractures for pure E-51 and (BADCy/PGMA)/E-51 composite with 8 wt% BADCy/PGMA microcapsules. (a)Pure E-51; (b)(BADCy/PGMA)/E-51 composite. Figure 9 Cracks at two different locations of the (BADCy/PGMA)/E-51 composite with 8 wt% BADCy/PGMA microcapsules before and after self-healing. (a)Location 1; (b)Location 2. Table 1 Formula of BADCy/PGMA microcapsules with different content of core material (BADCy) and shell material (PGMA) Table 2 Characteristic thermal data of the BADCy/PGMA microcapsules

14


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Scheme 1 Schematic diagram of the formation process for BADCy/PGMA microcapsules by solvent evaporation technique

15



25

(a)

(b) 20

Volume (%)

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15

10

5

0 0.5

1.0

1.5

2.0

2.5

3.0

Diameter (µm) Figure 1 SEM image and particle size distribution of PGMA microspheres. (a)SEM image; (b)Particle size distribution.

16


3.5

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Figure 2 SEM images of the BADCy/PGMA microcapsules with different mass ratio of raw materials. Proportion of BADCy/PGMA/CH2Cl2: (a)2g/8g/40g; (b)1g/4g/40g; (c)1g/2g/40g; (d)0.5g/2g/40g; (e)0.5g/4g/40g. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Figure 3 SEM images of the optimal BADCy/PGMA microcapsules. (a)Several integrated microcapsules; (b)An integrated microcapsule; (c)A ruptured microcapsule.

18


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8 7 6

Volume / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


5 4 3 2 1 0 0.1

0.5 1

2

5

10

20

30 40 50 60 70

Diameter / µm Figure 4 Size distribution of the optimal BADCy/PGMA microcapsules

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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PGMA

BADCy

BADCy/PGMA

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm

)

Figure 5 FTIR spectra of PGMA, BADCy and the BADCy/PGMA microcapsules

20


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PGMA

Heat flow / (w/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


BADCY

BADCY/PGMA Exo 0

50

100

150

200

250

300

350

o

Temperature / C Figure 6 DSC curves of PGMA, BADCy and BADCy/PGMA microcapsules

21



100

80

Mass loss/ %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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BADCy/PGMA

BADCy 60

40

PGMA

20

0

100

200

300

400

500

600

700

800

o

Temperature / C Figure 7 TGA curves of PGMA, BADCy and BADCy/PGMA microcapsules

22


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Figure 8 SEM images of impact fractures for pure E-51 and (BADCy/PGMA)/E-51 composite with 8 wt% BADCy/PGMA microcapsules. (a)Pure E-51; (b)(BADCy/PGMA)/E-51 composite.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Figure 9 Cracks at two different locations of the (BADCy/PGMA)/E-51 composite before and after self-healing. (a)Location 1; (b)Location 2.

24


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 1 Formula of BADCy/PGMA microcapsules with different content of core material (BADCy) and shell material (PGMA) Water phase

Oil phase

Samples Water(g)

PVA(g)

SDS(g)

CH2Cl2(mL)

BADCy(g)

PGMA(g)

1

50

0.25

0.025

40

2

8

2

50

0.25

0.025

40

1

4

3

50

0.25

0.025

40

1

2

4

50

0.25

0.025

40

0.5

2

5

50

0.25

0.025

40

0.5

4

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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Table 2 Characteristic thermal data of the BADCy/PGMA microcapsules Weight loss temperature / oC THeat-resistance index* / oC

Samples

BADCy/PGMA microcapsules

5%

30%

50%

262.2

393.9

420.7

*The sample’s heat-resistance index is calculated by Equation 1 31-32 THeat-resistance index=0.49*[T5+0.6*(T30-T5)] (Equation 1) T5 and T30 is corresponding decomposition temperature of 5% and 30% weight loss, respectively.

26


167.2

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


For Table of Contents Only

27


Synthesis of Cyanate Ester Microcapsules via Solvent Evaporation

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