Microencapsulation of butyl stearate with melamine-formaldehyde resin: Effect of decreasing the pH value on the composition and thermal stability of microcapsules

The object of this study was to investigate how different decreasing of pH regimes during microencapsulation process with melamine-formaldehyde (MF) resin affects the composition, morphology and thermal stability of microcapsules containing a phase-change material (PCM). Technical butyl stearate was used as PCM. Microencapsulation was carried out at 70 degrees C. For all experiments the starting pH value was 6.0. After one hour of microencapsulation at the starting pH value, the pH value was lowered to final pH value (5.5; 5.0; 4.5) in a stepwise or linear way. The properties of microcapsules were monitored during and after the microencapsulation process. The results showed that pH value decreasing regime was critical for the morphology and stability of microcapsules. During microencapsulations with a stepwise decrease of pH value we observed faster increase of the amount of MF resin in the microencapsulation product compared to the microencapsulations with a linear pH value decrease. However, faster deposition in the case of microencapsulations with stepwise decrease of pH value did not result in thicker MF shells. The shell thickness increased much faster when the pH value was decreased in a linear way or in several smaller steps. It was shown that for the best thermal stability of microcapsules, the pH value during microencapsulation had to be lowered in a linear way or in smaller steps to 5.0 or lower.