Corrosion Protection of Self-Healing Coating Contained Fiber/Spherical Shaped Capsules Formed on Substrate Metal
Authors: Tsuji Yuki , Yanagimoto Haruno , Kawamura Mitsuki , Hyono Atsushi , Chiba Makoto , Takahashi Hideaki 
Published: December 2020

Coating is one of the most useful techniques for corrosion protection of metallic materials, however, if coating is damaged, local corrosion will occur. It is important to repair the damaged area of coating soon, however it needs much cost. From these, our research group develop the coating with self-healing property, called by self-healing coating. In this coating capsules containing healing-agent of coating are dispersed. If the coating is damaged, capsules will be broken at the same time. The healing-agent containing in capsules will flow out. The healing-agent reacts with the water vapor in the air to form a healing-structure and cover the damaged area of coating. The capsules contained healing-agent are produced as follows. First, the prepolymer, this is precursor of shell of capsules, drip into the aqueous solution dissolved glycerol, it hardly dissolves in the aqueous solution since the prepolymer is the oil phase. A mixture of oil droplets dispersed in the aqueous solution is formed. When this mixture is agitated with high speed, the oil droplets become micro spheres and emulsion forms. Furthermore, the reaction between prepolymer and glycerol occurs only at this interface between oil phase and water phase, spherical shaped capsules contained a healing-agent in a polyurethane shell can be produced. From our previous study, the coating with these spherical capsules has a self-healing property, because healing-structure could be observed at the damaged area of coating, but this structure covered damaged area, incompletely. It is necessary to improve self-healing property of coating. From the self-healing mechanism of the coating, the self-healing property of the coating will be strongly depended on the amount of healing-agent flowing to the damaged area. Therefore, we start the development of the coating with capsules with other shape such as fiber shape are dispersed in order to develop an advanced self-healing coating with higher healing property. Considering the procedure of capsule formation, the shape of capsules can be controlled by the agitation speed and viscosity of prepolymer solution. The shape of capsules will be related to the amount of healing-agent flow from capsules to the damaged area of coating, when coating is damaged. e.g. more amount of healing-agent will flow from fiber shaped capsules than that from spherical shaped capsules, because fiber capsules will be arranged horizontally to the metal substrate due to the flow during the formation of coating, and the more capsules will be broken, when vertical damage was formed in the coating. In this paper, the shape of capsules produced from prepolymer solution with high viscosity and self-healing property of coating dispersed with these capsules were discussed.
The procedure synthesizing capsules is follows. The prepolymer prepared by reaction between TDI and glycerol in cyclohexanone as a solvent under 75 ℃, 600 rpm. In order to increase the prepolymer concentration and viscosity of solution, the heat treatment of prepolymer solution were performed under 140 ℃, 8 min. After heat treatment, IPDI as a healing-agent, and xylene as inhibiter solidification inside of capsules were added. First, this prepolymer solution drip into the sodium dodecyl sulfate and glycerol aqueous solution with low agitation speed (200 rpm). The prepolymer solution was not dissolved in the aqueous solution and small oil droplets were dispersed in the aqueous solution. The shape of some of droplets will be changed from spherical to long spheroidal or fiber by agitation of solution. Furthermore, the shell formation reaction take place only at the interface between oil and water phases, the spherical, spheroidal and fiber shaped capsules could be formed. For comparison, the capsules produced under high agitation speed (600 rpm) and those from prepolymer solution without heat treatment were also prepared. These capsules were observed by a scanning electron microscope SEM.
Fig. 1-a show the shape of capsules prepared from prepolymer solution without heat treatment under high agitation speed, 600 rpm. From this image, only spherical with 20 - 60 µm of diameter can be observed. The shape of capsules from same prepolymer under low agitation speed, 200 rpm, also only spherical however the size of capsules is bit larger, the maximum diameter is about 80 µm, as shown in Fig. 1-b. The shape of the capsule the produced from prepolymer with heat treatment with high agitation speed, shown in Fig. 1-c, are similar to that without heat treatment. In contrast to these, the capsules produced from prepolymer with heat treatment under low agitation speed have some kinds of shape, such as spherical, spheroidal and long fiber. The coating dispersed with these shaped capsules will be also discussed.

Figure 1