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The smart releasing behavior of a microcapsule based on chemical self-healing system caused by chemical trigger activation

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THE SMART RELEASING BEHAVIOR OF A MICROCAPSULE BASED

ON CHEMICAL SELF-HEALING SYSTEM CAUSED BY CHEMICAL

TRIGGER ACTIVATION

B. Dong1, Y. Wang1, N. Han1, F. Xing1

1 School of Civil Engineering, Guangdong Province Key Laboratory of Durability for Marine

Civil Engineering, Shenzhen University, Shenzhen, P.R.China. 518060 - e-mail: incise@szu.edu.cn; wang.ys1988@gmail.com; nxhan@szu.edu.cn; xingf@szu.edu.cn

Keywords: Microcapsule; Chemical self-healing; Smart releasing behavior; EDTA ABSTRACT

A novel chemical self-healing system based on microcapsule technology for cementitious composites is established in Guangdong Key Laboratory of Durability for Coastal Civil Engineering, Shenzhen University. The key issue of this system is how to release the healing material and how to activate the healing mechanism. In this paper, the study is focused on the releasing behavior. The smart releasing behavior of healing material in the microcapsule is characterized by EDTA (Ethylene Diamine Tetraacetic Acid) titration method. The experimental results show that releasing procedure of the corrosion inhibitor covered with PS is a function of the time, and is controlled by the wall thickness of the microcapsule. Moreover, the pH value affects the release rate of corrosion inhibitor. With the increasing of pH value, the releasing rate will increase greatly.

1. INTRODUCTION

Concrete is a basic material used for modern construction. Due to concrete creep, the humidity change and non-homogeneous settlement of buildings, concrete structures may generate a lot of cracks during service period. These cracks will affect the safety of the buildings and even cause severe accidents [1]. Self-healing technology then has been introduced to repair the cracks in a concrete structure automatically, resulting in an improvement of concrete structure performance. This technology does not require external monitoring because the cement matrix itself acts as a sensor. And practical requirements of self-healing concrete material are low cost, passive smart and easily to be distributed uniformly [2-3]. An acceptable work that initiates an engineering approach for self-healing cement composite with microcapsules is first developed in Shenzhen University, China. Even so, existing efforts are focusing on the crack repair and mechanical performance recovery. It is not an effective method to deal with the degradation of the concrete structure, which caused by the ions erosion (such as Cl-, SO42-, CO32- etc.). For this reason, a chemical trigger mechanism is

proposed and a chemical self-healing system with microcapsule based corrosion inhibitor is designed. In this study, a polystyrene resin (PS)/sodium monofluorophosphate (Na2PO3F) microcapsule is fabricated and studied. And its

releasing behavior is measured under a simulated concrete environment with improved EDTA that is designed for microcapsule mentioned above [4]. Moreover, an alterative pH value condition is design to characterize the smart releasing performance of the microcapsule. Further, micro-morphology of the microcapsule is studied to observe the damage pattern variation with pH value alteration.

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2. MATERIALS AND METHODS

For preparation of the microcapsule, sodium monofluorophosphate and microcrystalline cellulose are mixed into Polysorbate 80. Then the small spherical particles are molded by extrusion method. After that, the microcapsules are fabricated through the spray drying method. In the study, three types of microcapsules with different amount of capsule wall materials are fabricated, which can be seen in Table 1. The weight percentage of the microcapsule wall materials are 10%, 20% and 43% (Respectively, The samples are labeled as Na, Nb and Nc).

Table 1: The formulation used for microcapsule fabrication

Ingredients Capsule core materials Na Nb Nc

Na2PO3F 500g 0 0 0 Microcrystalline cellulose 500g 0 0 0 Polysorbate 80 50g 0 0 0 Talc powder 0 35g 70g 146g Polystyrene resin 0 50g 100g 150g Chloroform 0 500mL 1000mL 1500mL Pure water 50g 50g 50g 50g

Since microcapsule will be used in concrete structure for self-healing, understanding the releasing behavior of the microcapsules under the cementitious environment will be our best interests. In order to simulate the cementitious environment, Ca(OH)2

solutions with different pH levels are designed. Microcapsules are then added into Ca(OH)2 solutions for the measurement with different periods. The releasing behavior

is studied by EDTA titration method. And the chemical reaction involved is shown as the following chemical equation:

2

3 5 4 3 2 2

6Ca 3PO3F6OHCa PO( ) F CaF  3H O (1)

Since Ca2+ reacts with PO3- and F- that released from sodium monofluorophosphate inside the microcapsules to form Ca5(PO4)F and CaF2, and turns solution into cloudy,

the amount of Ca2+ consumption can be calculated and the amount of sodium monofluorophosphate is also deduced. The micro-morphology of the microcapsule samples is observed by Scanning Electronic Microscopy (SEM) (SU-70; Hitachi, Japan) and the particle size distribution of the microcapsule samples is measured by Laser Particle Analyzer (S3500; Microtrac, USA).

3. RESULTS AND DISCUSSION

The size of microcapsule samples is at the level of hundreds of micrometer. According the particle size distribution, the diameter of the capsules is mainly ranged from 400 to 1200m  . The mean diameter of the capsule core is 405.3m  and the m

mean diameter of the capsule with formula from Na to Nc is 694.9 , 720.1m  and m

772.0m respectively. Based on the size distribution results, the average wall thickness of the microcapsules with different formula can be calculated with the equation

2 0

D D

T   (where, T is the wall average thickness; D is the particle

diameter of capsule and D0 is the particle diameter of capsule core) [5].

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Na Nb Nc 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

the formula number for microcapsule PH=13 7Day 14Day 28Day 60Day 90Day the to tal r elea sing ma ss (g ) (a) 0 20 40 60 80 100 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 the to tal r elea sing ma ss (g ) Time (day) PH=7 PH=9 PH=11 PH=13 the formula Nb for microcapsule shown in Table. 2

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Figure 1: The total releasing mass of microcapsules with different conditions

Figure 2: The SEM images of microcapsules after soaked in the solutions with different pH values for the same soaking time

The correlation of the releasing amount of microcapsules changing with the releasing time in the simulated cementitious environment is shown in Figure 1(a). It can be found that the releasing amount with different capsule wall thicknesses are all increased with increasing of the releasing time when pH value is 13. Moreover, for three different microcapsules with different wall thicknesses, it is obvious that the releasing amount has the pattern as Nc< Nb <Na, which tells that the releasing amount

decreases with the increasing of the wall thickness of the microcapsules. This is the strong evidence proving that the releasing rate of core materials inside the microcapsules can be controlled effectively by controlling the wall thickness of the microcapsules. The influence of pH value on the releasing behavior of the microcapsule is shown in the Figure 1(b). It is obvious that the largest releasing amount happens at value of pH=7. With increasing the pH value, the releasing amount of the microcapsules decreases. It is well know that polystyrene resin can be swelled in the environment with water existing. The swelling could cause micro-cracks or pinholes on the surface of the microcapsules. On the other hand, microcrystalline cellulose has strong capability of absorbing the water while Na2PO3F can be easily

dissolved in the water. Moreover, there is a concentration gradient of core material existing between inside and outside of the microcapsules. As the result, the core material of microcapsule can be released from the micro-cracks or the pinholes when the water exists. At first beginning, the releasing rate should be slow since there is only little damage on surface of the microcapsules. With the time increasing, the number and size of cracks or pinholes increase. This directly results in a dramatically increasing amount of the core material releasing is obtained in experiment duration between 28 days to 60 days. Regarding to different pH values, the higher pH value stands the higher concentration of Ca2+ in the simulating solution. As mentioned in formula (1), Ca2+ can react with PO3- and F- to form Ca5(PO4)3F and CaF2 as the

precipitates. These precipitates can deposit on the surface of the microcapsules and

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fill the micro-cracks and pinholes, which eventually hinders the core material releasing. This is the reason that the core material releasing amount is larger when pH value is smaller. Under the neutral condition when pH value is 7, there is no Ca2+ existing and reaction of formula (1) can’t happen. As the result, the releasing amount is big at the first beginning. The results shown in Figure 1(b) indicate that the designed microcapsules in this experiment are the good candidates for working at alkaline environment to fulfill the smart releasing control, especially during the carbonation process of the concrete materials, the releasing amount of the microcapsules will increase with the reduced pH value.Further study is carried out by SEM observation to understand the surface damaging of microcapsules at different pH values. Figure 2 shows the SEM images of microcapsules after soaked in the solutions with different pH values for the same periods. For pH=13, it is found that there is no obvious surface damage for the soaked microcapsules. With the reduced pH value, the damage of the soaked surface is getting more and more obvious. When it reaches neutral environment with pH=7, the microcapsule surface has been totally destroyed after soaking. This further approves the result that different pH value has tremendous effect on microcapsule releasing behavior and can be used for smart controlling of the core material releasing.

4. CONCLUSION

The wall thickness of the microcapsules can be controlled by the weight percentage of the wall materials. Under the simulated cementitious environment, the releasing amount of the microcapsule core material increases with the increasing of the releasing time in disregard of the different wall thickness of the microcapsules. The core material releasing amount increases with the reduced pH values, which indicates that the designed microcapsules are the good candidates to be used under alkaline cementitious environment with smart releasing control. SEM images further approve that different pH values can be used to control the microcapsule releasing behavior. ACKNOWLEDGEMENTS

The authors would like to acknowledge financial support provided by Natural Science Foundation for the team project of Guangdong Province (No. 9351806001000001), National Key Basic Research Program funded by MOST (Project No. 2011CB013600; Issue No:2011CB013604) and National Natural Science Foundation of China (No. 50925829/51120185002/51272160)

REFERENCES

[1] S.J. Jaffer, C.M. Hansson. The influence of cracks on chloride-induced corrosion of steel in ordinary Portland cement and high performance concretes subjected to different loading conditions. Corrosion Science. 2008, 50(12): 3343-3355

[2] M. R. Kessler, N. R. Sottos and S. R. White. Self-healing structural composite materials. Applied Science and Manufacturing. 2003, 34(8): 743-753

[3] S.R.White, N.R.Sottos, P.H.Geubelle. Autonomic healing of Polymer Composites. Nature. 2001, 409: 794-797

[4] Zhou KH, Zhang LF and Zhang YM. Determination of calcium oxide in rare earth ore by EDTA titration method. Metallurgical Analysis. 2012,32(1): 64-67

[5] Joseph D. Rule, Nancy R. Sottos and Scott R. White. Effect of microcapsule size on the performance of self-healing polymers. Polymer. 2007, 48(12): 3520-3529

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