ADDITIVES AND CHEMICAL STRUCTURES USED IN SEL F-HEAL ING TECHNOL OGY
Polymers are involved in the self-healing processes either as materials of capsules or materials that are released from various storage containers to heal the damage.
The microcapsules are prepared by an in situ polymerization.!’ The urea-formalde-
hyde resin is the most frequently used wall material! It is produced by water-based polymerization, using the following steps:
dissolve 1.25 g of ethylene–maleic anhydride copolymer surfactant in 50 mL of
deionized water in a warm bath to obtain a 2.5% aqueous surfactant solution
• place the wall-forming materials of the microcapsule (5 g of urea, 0.5 g of ammonium chloride, and 0.5 g of resorcinol) in a 500-mL beaker at room temperature
• add 200 mL of deionized water and the previously prepared surfactant solution
• adjust pH to 2.6 to 3.5 by the addition of a NaOH solution
• add 90 mL of ethyl phenylacetate (solvent, healing agent) to form an oil–water emulsion
• agitate with an emulsification isotropic machine
• place beaker in a temperature-controlled water bath and agitate with a digital mixer
• add 12.67 g of formaldehyde solution
• heat contents to 55o C at a rate of 18oC/min
• hold mixture for 4 h at this temperature
• rinse microcapsules with deionized water
• filter and air-dry microcapsules for several days.
It is clear from the description that the process of formation of microcapsules is simple.
Figure 13.1 shows the effect of polymerization time on the morphology of formed microcapsules.
After 45 min mixing, stable droplets are formed.
At around 60 min, theliquid surrounding droplets becomes opaque and cloudy because the molecular weight ofpolymer increases and the solubility of the urea-formaldehyde decreases.
After 75 min,the surface morphology of microcapsules changes, and the smooth surface of the droplets begins to show some bristle-like forms; this indicates the deposition of the urea-formalde
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