SAMPE Journal - May/June 2021 - 43

Table 2. Cooling performance of the composite films.
Samples

Fitted line slope

Temperature reduction
(%)

R2

[5]

4%

0.81

19

0.98

5%

0.79

21

0.99

The UV shielding performances of 4%, 5%,
and 6% films are all over than 90% and the
shielding percentage increases monotonically as
the NP concentration increases. The same trend
is observed for Vis lights. In nIR range, however,
the 6% film had a smaller reflection than the 5%
film. This can be explained by the dispersion
of the NPs in the two films. In Figure 3 b2 and
c2, it can be seen that 6% film had a much more
uniform NP distribution than that of 5% film where
certain degree of NP agglomeration can clearly
be observed. This kind of agglomeration was not
able to change reflectance in short wavelength but
more influential for long wavelength range of solar
irradiation since it is a long-range phenomenon.
Obviously, the 6% film is more ideal since it reflects
more high energy solar irradiance and less low
energy Vis and nIR lights. Therefore, for this kind
of NP doped composite films, it is important to
have not only a reasonable NP concentration but
also a uniform NP dispersion to achieve the best
spectrum selective performance.
The temperature distributions of the tested
boxes are showed in the Figure 6 d. It indicates
that as the accumulation of solar energy input
into the test boxes, the temperature in the boxes
steadily increased. The gray dot line showed the air
temperature in the control box, and the red, green,
and blue dot lines were the air temperature in the
boxes covered with 4%, 5%, and 6% ZnO/LDPE
films, respectively. When the air temperature of
the reference box was 55ºC, the air temperatures
of other boxes lined by 4%, 5%, and 6% ZnO/LDPE
films were 2, 6, and 10ºC lower than that of the

6%

0.65

35

0.99

						

Figure 7. The stress-strain curves of the films doped with different
concentrations of pure LDPE, 4%, 5%, and 6% of ZnO NP in LDPE.

reference box, respectively. Regression analyses
were carried out to access the passive cooling
performance of the NP doped films. Table 2 shows
the slopes of the fitted lines to the data in Figure 6
d. In fact, the slopes represent the average portion
of temperature retention or 1 - temperature
reduction.
From Table 2, we are able to tell that the 4%
and 5% films had almost the same temperature
reduction amount (19 and 21%), while the 6% film
had a greatly increased temperature reduction
(35%). The reason for 6% film to have such a good
performance is because it has best shielding against
UV and high energy Vis but relatively low shielding

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SAMPE Journal - May/June 2021

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