Page 115 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanoclay and polymer-based nanocomposites: Materials for energy efficiency 91

to the nanoclay surface [78]. Finally, and as already mentioned, due to net negative
charge that arises from isomorphous substitutions, there are charge-compensating
hydrated cations between the basal oxygen planes typically mono- or divalent (pri-
marily K, Na, Ca, and Mg) and surrounded by water molecules [79]. Those inter-
lamellar cations could be easily replaced by other cations or by organic molecules
according to the appropriate surface chemistry. Cation-exchange capacity presented
in mequiv/100 g (meq/100 g) indicates the level of potential isomorphic substitutions
and varies from layer to layer; hence, an average value on the complete nanoclay is
considered. To conclude, the modification of nanoclays is based on the six types of
active sites and their interaction with the chemical components [75]. In general, these
components can be attached to the nanoclay particles by different possible arrange-
ments: Firstly. organic molecules which may interspersed into the interspaces in
cation-exchange sites and adhere to the surface via electrostatic interactions; sec-
ondly, the organic molecules are physically adsorbed onto the external surfaces of
the particles to better coating the nanoclay particles; and finally, the organic mole-
cules are located within the interlayer spaces [61].

3.5.3 Nanoclays incorporation in PCM materials
3.5.3.1 Halloysite as supporting material for PCM

Mei et al. prepared a novel form-stable composite PCM based on capric acid (CA) and
halloysite nanotube (HNT). The composite PCM can contain as high as 60 wt% of CA
without any leakage within 50 melt-freeze cycles. The melting temperature and latent
heat of composite were determined as 29.34°C and 75.52 J/g. Furthermore, the ther-
mal storage and release rates were increased by 1.8 and 1.7 times, respectively. When
adding 5 wt% of graphite into the composite. A similar study was performed by Zhang
et al. that a novel form-stable composite PCM was prepared by absorbing paraffin
(P) into halloysite nanotube [80]. The paraffin could be 65 wt% without any leakage
after 50 thermal cycles. The melting temperature and latent heat of composite PCM
were determined as 57.16°C and 106.54 J/g, respectively. Moreover, to improve ther-
mal storage performance, graphite was added into the composite, and the melting and
freezing time of the composite were reduced by 60.78% and 71.52%.

3.5.3.2 Kaolinite as supporting material for PCM
Kaolinite is a natural industrial mineral, with porous structure, which has lower cost
[81]. Song et al. [81] developed novel form-stable composite PCM by impregnating
the lauric acid (LA) in the pores of the intercalated kaolinite (IKL). The maximum
weight percentage for LA in composite PCM was found as high as 48% without show-
ing any leakage problems of LA. The phase-change temperature and latent heat were
determined as 43.7°C and 72.5 J/g, respectively. Besides, the composite has a good
thermal stability, a high latent heat, and a high adsorption capacity; thus, the compos-
ite can be considered as potential PCM for thermal energy storage in buildings.