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synthesized using the in situ polymerization method, and the other is NPG-silica
microcapsule fabricated by the interfacial hydrolysis method.
3.1 Paraffin-Polymer Microcapsules (Solid-Liquid PCM)
Paraffins, the n-alkanes (C
n
H
2n
รพ
2
) with different numbers of carbon atoms in their
chain, are common solid-liquid PCMs used as the core material of microcapsules
because of their appropriate phase transition temperature, large latent heat of
fusion, chemical stability, and capability of being microencapsulated [
9
,
23
-
27
].
3.2 Synthesis of Paraffin-Polymer Microcapsules
In situ polymerization method has been widely used to synthesize microcapsules
comprising PCM paraffin encapsulated in polymer shells [
28
]. This method gen-
erally involves two immiscible liquids, e.g., paraffin and water, in which poly-
merization reaction occurs at the interface between the PCM particles and the
continuous phase water. Melamine-formaldehyde (M/F) and urea-formaldehyde
are often used as the shell materials due to their good chemical stability and
mechanical strength. Figure
1
shows the fabrication process of microcapsules
comprising solid-liquid paraffin core and melamine-formaldehyde shells. In the
process, melamine and formaldehyde were first added into alkaline water solution
with a certain weight ratio [
28
]. The mixture was heated to make a transparent
aqueous solution of melamine-formaldehyde prepolymer. In the meantime, an oil-
in-water emulsion of paraffin oil-in-acidic water was made with a homogenizer
and heated up to 60 C. A certain amount of the hot melamine-formaldehyde
prepolymer solution was added into the paraffin oil-in-water emulsion, and a
polymerization was processed for hours to form shells of melamine-formaldehyde
resin at the oil-water interface.
Two reaction steps are involved in the formation of the microcapsule resin shell.
The first step is melamine-formaldehyde precondensation in basic environment.
NH
2
N
NH
2
NH
2
N
NH
2
OH
O
O
H
H
+
+
+
H
H
N
N
N
N
H
H
OH
NH
2
NH
2
N
NH
2
N
OH
OH
-
O
+
N
N
H
H
HN
O
H
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