.3. Characterization of urea–formaldehyde microcapsules
Particle size distribution was performed based on our former
statistical methods. Microcapsule size analysis was performed with
an optical microscope (LV100POL, Nikon) and image analysis soft-ware (Motic image plus 2.0). Mean diameter was determined from
data sets of at least 800 measurements.
Surface morphology and wall thickness of the microcapsules
were observed using a scanning electron microscope (SEM, JSM-6360, FEI). Prior to being observed under the SEM, samples were
covered with a gold layer.
Thermogravimetric analysis (TGA) was carried out on TA SDT-Q600, scanning from room temperature to 623.15 K with a scanning
rate of 10 K/min under constant N2
flow.
Yield (Y) of the microcapsules was defined as the proportion of
the mass of the microcapsules obtained to the total mass of ingredi-ents used. Before weighing, the microcapsules were washed several
times with deionized water and air-dried.
To study the barrier properties of microcapsules prepared under
different conditions in the atmosphere, 1.0 g (W) of washed and
dried microcapsules were put into a tare bottle. Then the lid was
opened and the bottle was put swiftly into an oven with a temper-ature setting of 50
◦
C. The samples were cured for the first setting
time of 6 h to remove the absorbed water. The bottle was taken out,
weighed and the net weight of bottle was subtracted when it cools
down and this was recorded as W1
. The bottle without lid was put
into the oven again and then the bottle was taken out at a time
period of 1 h and weighed. The net weight of the bottle was sub-tracted and this was recorded as W2
.This procedure was repeated several times until Wn
was obtained. The residual weight of the
samples was calculated as
RW % =
Wn
W0
× 100%
where Wn
(g) is the net weight of the microcapsule powder after
being cycled for n times and W0
(g) is the original net weight of the
microcapsule powder.