Development of Self-Healing Coat Using a Micro-Capsules for Corrosion Protection of Metal
Authors: Chiba Makoto , Anetai Kazuki , Yamada Chinami , Pletincx Sven , Verbruggen Hilke , Hyono Atsushi , De Graeve Iris , Herman Terryn , Takahashi Hideaki 
Published: February 2020

Introduction
Many kinds of metals are used as structural material in the building or other constructions. It is important to protect the metal from corrosion, because the corrosion of the metal may cause a significant damage of the building and the constructions. One way of the corrosion protection of the metal is to cover with organic or inorganic coatings, isolating the metal substrate from water or oxygen. When some parts of the coating is removed mechanically or chemically, the corrosion of the material may occur at the damaged areas. Therefore, it is necessary to keep the paint layer protective by a periodic painting that costs much labor and expense. From the point of view of time- and money-saving, we started to develop a “self-healing coating” on the metals for corrosion protection. When the organic coating is damaged by mechanical impulsions, the coating at the damaged areas is healed automatically without painting again, heating or any other external treatments.The above concept is accomplished by the following process. Micro capsules that contain reactive liquid agents are dispersed in coatings (Fig. 1-a). The reactive liquid agent in the capsule is easy to react with water or oxygen in the air to form an organic compound with properties similar to those of the coatings. When the coating is damaged to form flaws, the capsule is also destructed (Fig. 1-b). Then, the reactive liquid agent included in the capsule flows out, and the flaw is filled with the organic compound after reacting with water or oxygen in the air (Fig. 1-c). In the present investigation, capsules that contain a prepolymer agent were dispersed in polyurethane coatings to examine the self-healing ability of the coating. The effect of glycerol on the formation of the capsule was also examined.
Experimental
At First, 7.7 g of tolylene-2,4-diisocyanate (TDI) and 1.3 g of glycerol were added to 18 g of cyclohexanone, and mixed by stirring with a magnetic stirrer at 600 rpm and 75 ˚C for 24 h. In the period, a prepolymer is formed in cyclohexanone by the reaction shown in Fig. 2. Then, 9.5 g of isophorone diisocyanate and 17 g of chlorobenzene were added to 11 g of the prepolymer/cyclohexanone to obtain a prepolymer solution. Micro droplets of the prepolymer solution were dropped into an aqueous solution containing 3 g of sodium dodecyl sulfate (SDS) and 0.10 – 0.50 g of glycerol. The droplet of the prepolymer solution reacts with SDS/glycerol solution at the interface between two phases to form polyurethane capsules as shown in Fig. 3, since the prepolymer solution doesn’t dissolve in the toSDS/glycerol solution, As a result of the reaction, capsules, which have a polyurethane shell and contain high reactive liquid, were obtained. The capsules collected on a filter were observed by scanning electron microscopy (SEM).The polyurethane dispersed with the microcapsules was coated on Al substrate by spreading 1.5 g of the prepolymer solution containing 10 mg of the capsules after mixing with ethylene glycol ultrasonically, and then kept at room temperature for 48 h to allow the polymerization of the prepolymer and the evaporation of ethylene glycol. Self-healing ability of the coating was evaluated by SEM after damaging the coating with a sharp blade at the pressure of 6.1 MPa.
Results and Discussions
Fig. 4 shows SEM images of micro capsules synthesized with a) 0.5 and b) 0.1 g of glycerol. All the capsules are spherical, and some of them are coalescent, forming clusters. Fig. 5 shows distribution of the diameter of the micro capsules synthesized with different amounts of glycerol. The most frequent diameter of capsules synthesized with 0.5 g and 0.1 g of glycerol is 4.0 and 3.2 µm, suggesting the coalescence of capsules is easier with larger amounts of glycrol. All the coatings with capsules showed self-healing after damaging with the blade, and the healing effect was more remarkable with smaller amounts of glycerol.

Figure 1