1.4 Nanofillers for Bionanocomposites
13
Table 1.2 Typical properties of HNTs [25]
Type of property
Description
Ref.
Chemical formula
Al 2 Si 2 O 5 (OH) 4 ·nH 2 O
[45, 85]
Aluminium (%)
20.9
[84]
Silicon (%)
21.76
[84]
Hydrogen (%)
1.56
[84]
Length
100–2000 nm
[87]
Internal diameter
30–50 nm
[87]
External diameter
1–30 nm
[87]
Aspect ratio
10–50
[45]
Elastic modulus (theoretical value)
140 GPa (230–340 GPa)
[88, 89]
Mean particle size in aqueous solution
143 nm
[90]
Particle size range in aqueous solution
50–400 nm
[90]
BET surface area
22.1–81.6 m 2 g −1
[91]
Lumen space
11–39%
[45]
Density
2.14–2.59 g cm −3
[45]
Structure water release temperature
400–600 °C
[45]
low numbers of hydroxyl groups on HNT surfaces is to minimise the tube–tube
interaction, and thus to promote their miscibility with polymers or other solvents
[92–95]. The main advantages of HNTs include their wide availability [45], costeffectiveness, environmental friendliness [96], high aspect ratio [97] and high heat
resistance in addition to their good mechanical properties, as shown in Table 1.2.
HNTs are widely used for nanoreactors [87], sorbent for contaminants and drug
delivery [85]. Most raw halloysite contains impurities such as illite, feldspar, perlite
and metal ions to be removed before its use. However, previous studies [16, 98]
have shown that pure HNTs can be obtained after stirring 10 wt% HNTs in distilled
water with gradually improved HNT dispersion up to 60 °C. This step is followed
by centrifugation and washing three times in distilled water before being dried in air
at 60 °C for 12 h. Sodium hexametaphosphate [98] and Tween 80 [16] can be added
to the suspension in order to improve the stability of HNTs within the suspension. In
nanocomposite systems, HNTs are considered as promising nanoreinforcements due
to their outstanding material features, demonstrating overall performance improvement of thermoplastics or their blends such as polyamide, PP/PCL and polybutylene terephthalate (PBT) via melt compounding without any modification [45, 87].
However, the surface functionalisation of HNTs is required to increase their compatibility with polymers in order to warrant maximum stress transfer from polymer
matrices to HNTs in nanocomposites [45, 87]. Two approaches exist for HNT modification, namely covalent modification and non-covalent modification [45]. Among
them, covalent modification uses the reaction between a covalent agent and hydroxyl
groups on HNT surfaces to increase their interfacial adhesion.
13
Table 1.2 Typical properties of HNTs [25]
Type of property
Description
Ref.
Chemical formula
Al 2 Si 2 O 5 (OH) 4 ·nH 2 O
[45, 85]
Aluminium (%)
20.9
[84]
Silicon (%)
21.76
[84]
Hydrogen (%)
1.56
[84]
Length
100–2000 nm
[87]
Internal diameter
30–50 nm
[87]
External diameter
1–30 nm
[87]
Aspect ratio
10–50
[45]
Elastic modulus (theoretical value)
140 GPa (230–340 GPa)
[88, 89]
Mean particle size in aqueous solution
143 nm
[90]
Particle size range in aqueous solution
50–400 nm
[90]
BET surface area
22.1–81.6 m 2 g −1
[91]
Lumen space
11–39%
[45]
Density
2.14–2.59 g cm −3
[45]
Structure water release temperature
400–600 °C
[45]
low numbers of hydroxyl groups on HNT surfaces is to minimise the tube–tube
interaction, and thus to promote their miscibility with polymers or other solvents
[92–95]. The main advantages of HNTs include their wide availability [45], costeffectiveness, environmental friendliness [96], high aspect ratio [97] and high heat
resistance in addition to their good mechanical properties, as shown in Table 1.2.
HNTs are widely used for nanoreactors [87], sorbent for contaminants and drug
delivery [85]. Most raw halloysite contains impurities such as illite, feldspar, perlite
and metal ions to be removed before its use. However, previous studies [16, 98]
have shown that pure HNTs can be obtained after stirring 10 wt% HNTs in distilled
water with gradually improved HNT dispersion up to 60 °C. This step is followed
by centrifugation and washing three times in distilled water before being dried in air
at 60 °C for 12 h. Sodium hexametaphosphate [98] and Tween 80 [16] can be added
to the suspension in order to improve the stability of HNTs within the suspension. In
nanocomposite systems, HNTs are considered as promising nanoreinforcements due
to their outstanding material features, demonstrating overall performance improvement of thermoplastics or their blends such as polyamide, PP/PCL and polybutylene terephthalate (PBT) via melt compounding without any modification [45, 87].
However, the surface functionalisation of HNTs is required to increase their compatibility with polymers in order to warrant maximum stress transfer from polymer
matrices to HNTs in nanocomposites [45, 87]. Two approaches exist for HNT modification, namely covalent modification and non-covalent modification [45]. Among
them, covalent modification uses the reaction between a covalent agent and hydroxyl
groups on HNT surfaces to increase their interfacial adhesion.