Preparation and properties of poly (lactic acid) halloysite nanotube nanocomposites
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Date
2015-09-01
Authors
Tham Wei Ling
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Abstract
In this research, halloysite nanotube (HNT) was used to improve the properties of
poly(lactic acid) (PLA). The PLA/HNT nanocomposites were prepared using melt
compounding followed by compression molding. Three types of impact modifiers
i.e., maleic anhydride grafted styrene-ethylene/butylene-styrene copolymer (SEBS-g-
MA), N,N’-ethylenebis(stearamide) (EBS), and epoxidized natural rubber (ENR),
were used to toughen the PLA nanocomposites. The properties of PLA/HNT
nanocomposites were characterized by mechanical tests (i.e., tensile, flexural, and
impact tests), thermal analysis (i.e., differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA)), morphological analysis (i.e., field emission
scanning electron microscopy (FESEM) and transmission electron microscopy
(TEM)), and Fourier transform infrared spectroscopy (FTIR). The impact strength of
PLA/HNT nanocomposites was increased significantly to approximately 300% by
the addition of 15 wt% ENR. A finely dispersed HNT could eventually increase its
nucleation effect and assist in the crystallization process of PLA whilst in the
presence of EBS and ENR. Nevertheless, the presence of SEBS-g-MA and HNT
retard the formation of large crystallites of PLA. The kinetics of water absorption of
PLA/HNT nanocomposite fitted Fickian diffusion behaviour at immersion
temperatures of 30oC and 40oC. For the samples exposed to 50oC, the water uptake
followed Fick's law during the initial stages, but at longer time periods, deviations
and weight loss were observed. This may have been due to the hydrolysis of PLA at
higher immersion temperatures, which was confirmed by the pH change, reduction of
molecular weight, increase in carbonyl index (CI) values, and thermal property
changes. From the TGA results, it was found that single decomposition took place in
the nitrogen atmosphere, while a double-stage decomposition process occurred in the
oxygen atmosphere. The incorporation of EBS and ENR could improve the thermal
stability of PLA/HNT under oxygen atmosphere. The PLA/HNT6/ENR5
nanocomposites exhibited high thermo-oxidative stability under oxygen atmosphere.
Furthermore, the addition of 5 wt% of ENR in PLA/HNT6 nanocomposites increased
the oxidation onset temperature (OOT) (from 239.5oC to 296.5oC). The
PLA/HNT6/ENR5 showed the lowest oxygen permeability coefficient ( O2 P ) value of
approximately 1.567x10-4 cm3.m.m-2.day-1.kPa-1. However, the incorporation of
SEBS-g-MA could not improve the oxygen gas barrier properties of PLA/HNT
nanocomposites. From the UV-Vis spectroscopy analysis, the incorporation of
impact modifiers in PLA/HNT nanocomposites decreases light transmittance and
nearly no UV light can be transmitted through thin film at 300 nm. Nevertheless, all
PLA/HNT nanocomposites remain transparent from visual view. In summary, the
addition of ENR in PLA/HNT nanocomposites can effectively improve the
properties of polymer nanocomposites in term of impact strength, activation energy
of water diffusion (Ea), OOT, and oxygen gas barrier properties among the others
impact modifiers.