1.1 Introduction
3
surface areas of BCs are typically in range of 250–390 m
2 /g as opposed to 10 m
2 /g
for wood charcoals [29, 30]. Due to aforementioned remarkable characteristics, the
use of BCs has led to widespread applications such as negative ion supply [31],
humidity regulators [32], water purification [33], oxidation prevention, antibacterial
[34] and antifungal features, as well as breathability [30]. The ability of BCs to
regulate humidity directly benefits the hindrance to the growth of bacteria and fungi
with well-maintained freshness of food [34], which is a key requirement for food
packaging industries in order to increase the shelf life of products. Additionally, BCs
are also capable of absorbing significant levels of infrared energies from the environment, which can be then emitted to support cell activation for efficient human
blood circulation [35]. Elemental inclusions within BCs such as calcium, potassium,
sodium and iron have also been utilised for the purposes of food cooking, baking
and storage [36, 37].
Apart from BCs, several types of clays have been used in nanocomposite systems
like kaolin [38], mica [39], sepiolite [40] and MMT [12]. Among them, MMT is
most popular clay fillers owing to their layered silicate structures with exchangeable
cations and reactive –OH groups on MMT layer surfaces [25], as well as high strength
and stiffness, large aspect ratio with natural abundance and availability in large
quantities [25, 41]. MMT is capable of forming stable suspensions in water while its
hydrophilic characteristic also promotes the dispersion of these inorganic crystalline
layers in water-soluble polymers like PVA [12]. Moreover, layered silicates in their
pristine state has high ion-exchange capacity, which allows for the modification of
interlayer spacing in order to achieve better compatibility with base polymers [25].
Modified MMTs such as Cloisite 30B clays, containing 90 meq/100 g of a methyl,
tallow, bis-2-hydroxyethyl ammonium chloride surfactant [42], has been used in
many studies for the preparation of bionanocomposites. For instance, Cloisite 30B
clays contain –OH groups to be more compatible with base polymers when compared
with Na
+ MMTs and Cloisite 15A counterparts [43]. Moreover, Cloisite 30B clays
have much larger d-spacing value as opposed to those of other modified clays like
Cloisite 15A, Cloisite 20A and Cloisite 25A clays [44].
HNTs, as the alternative aluminosilicate clays [Al 2 Si 2 O 5 (OH) 4 ·nH 2 O], has
received great attention owing to their novel 2D natural hollow nanotubular structures, resembling that of multi-walled CNTs (MWNTs) [45]. In addition, HNTs have
high aspect ratios, good functionality and biocompatibility, as well as high mechanical strength [45]. These aforementioned material characteristics warrant the wide
use of HNTs in biological/medical applications such as bioreactors, drug delivery
and nanocoating [45]. Moreover, HNT surfaces contain alumina and silica groups
to facilitate hydrogen bonding interactions between HNTs and PVA matrices, which
makes HNTs an ideal nanofiller candidate for PVA bionanocomposite films [45].
3
surface areas of BCs are typically in range of 250–390 m
2 /g as opposed to 10 m
2 /g
for wood charcoals [29, 30]. Due to aforementioned remarkable characteristics, the
use of BCs has led to widespread applications such as negative ion supply [31],
humidity regulators [32], water purification [33], oxidation prevention, antibacterial
[34] and antifungal features, as well as breathability [30]. The ability of BCs to
regulate humidity directly benefits the hindrance to the growth of bacteria and fungi
with well-maintained freshness of food [34], which is a key requirement for food
packaging industries in order to increase the shelf life of products. Additionally, BCs
are also capable of absorbing significant levels of infrared energies from the environment, which can be then emitted to support cell activation for efficient human
blood circulation [35]. Elemental inclusions within BCs such as calcium, potassium,
sodium and iron have also been utilised for the purposes of food cooking, baking
and storage [36, 37].
Apart from BCs, several types of clays have been used in nanocomposite systems
like kaolin [38], mica [39], sepiolite [40] and MMT [12]. Among them, MMT is
most popular clay fillers owing to their layered silicate structures with exchangeable
cations and reactive –OH groups on MMT layer surfaces [25], as well as high strength
and stiffness, large aspect ratio with natural abundance and availability in large
quantities [25, 41]. MMT is capable of forming stable suspensions in water while its
hydrophilic characteristic also promotes the dispersion of these inorganic crystalline
layers in water-soluble polymers like PVA [12]. Moreover, layered silicates in their
pristine state has high ion-exchange capacity, which allows for the modification of
interlayer spacing in order to achieve better compatibility with base polymers [25].
Modified MMTs such as Cloisite 30B clays, containing 90 meq/100 g of a methyl,
tallow, bis-2-hydroxyethyl ammonium chloride surfactant [42], has been used in
many studies for the preparation of bionanocomposites. For instance, Cloisite 30B
clays contain –OH groups to be more compatible with base polymers when compared
with Na
+ MMTs and Cloisite 15A counterparts [43]. Moreover, Cloisite 30B clays
have much larger d-spacing value as opposed to those of other modified clays like
Cloisite 15A, Cloisite 20A and Cloisite 25A clays [44].
HNTs, as the alternative aluminosilicate clays [Al 2 Si 2 O 5 (OH) 4 ·nH 2 O], has
received great attention owing to their novel 2D natural hollow nanotubular structures, resembling that of multi-walled CNTs (MWNTs) [45]. In addition, HNTs have
high aspect ratios, good functionality and biocompatibility, as well as high mechanical strength [45]. These aforementioned material characteristics warrant the wide
use of HNTs in biological/medical applications such as bioreactors, drug delivery
and nanocoating [45]. Moreover, HNT surfaces contain alumina and silica groups
to facilitate hydrogen bonding interactions between HNTs and PVA matrices, which
makes HNTs an ideal nanofiller candidate for PVA bionanocomposite films [45].