Study on the effect of the concentration of organically modified nanoclay on the microstructure and physical (mechanically, WVP and color) properties of Poly Ethylene Terephthalate (PET)- based nanocomposites

Nanocomposites are prepared by introduction of dispersed nanoscale particles into the polymer matrix based on four template synthesis (sol-gel preparation); intercalation of polymer; and in situ intercalative polymerization and last one is melt blending, the most favorable and practical method due to its simplest, economical and environmentally friendly technic. This method involves annealing, statically or under shear, a mixture of the polymer and organically modified clay at the softening point of the polymer based on usual compounding devices, such as, extruders or mixers (Papaspyridesb 2008).
PET is a semicrystalline thermoplastic polyester which has been extensively used in all sizes as a packaging material in direct contact with food, beverages and as an alternative packaging to polyvinyl chloride for edible oils (Kirwan et al. 2011).
As polymeric nanocomposites are mainly used as structural materials, the layered silicate clay are preferred. The crystal lattice of 2:1 layered silicates, consists of two-dimensional layers where a central octahedral sheet of alumina is fused to two external silica tetrahedral by the tip. Montmorillonite (MMT) belongs to type 2:1 layered smectite clay which in the basic structure, the trivalent Al-cation in the octahedral layer is partially substituted by the divalent Mg-cation (Pavlidoua and Papaspyridesb 2008). As evident, MMT is greatly hydrophilic in the interlayer and incompatible with organic polymer such as PET, thus to increase compatibility of clay with polymer, inorganic inter-layer cations (Na, K or Ca2) exchanged by the cationic surfactants (e.g., quaternary ammonium salt). Modified MMT or organoclay interacts better with polymer due to its increased gallery space (Utracki et al. 2007; Parvinzadeh et al.2010).
Three different types polymer/clay nanocomposites can be obtained depending on the preparation method and the nature of the components used, including polymer matrix, layered silicate and organic cation. Tactoid nanocomposites formed when stacks of modified layered silicates are retained after introduction into the polymer. Subsequently, interaction between the nanolayers and polymer is not only unsuccessful but reduces mechanical properties of composite as well.
Our main objective of this research was to study the effect of the nanoclay addition on mechanical, colorimetric and transparency properties of poly (ethylene terephthalate) (PET) nanocmposite films.
Bottle-grade poly (ethylene terephthalate) granules with intrinsic viscosity of (IV) = 0.82 dl g-1 were provided by the Iranian Tondgooyan Petrochemical company. The organically modified montmorillonite, Cloisite 15A, was supplied by Southern Clay Products Inc. Standard of TPA was supplied from Fluka Chemical, trademarked Sigma-Aldrich Corp., Switzerland. High-pressure liquid chromatography (HPLC) grade water, aceto-nitrile, acetic acid and methanol (HPLC grade) were purchased from Merck (Darmstadt, Germany). TPA was dissolved into methanol with a slight increase in temperature. Working standard solutions were prepared on the day of use at concentrations of 0.4, 40, 100 and 1,000 ppb and calibration graphs were plotted using these concentrations of standard solutions.
The PET granules and nanoclay particles were dried in an oven for 24 h at 110 and 80C before extrusion, respectively. Melt blending technique was used for preparing nanocomposite films in a co-rotating twin screw extruder ZSK 25 .The temperature profile (throat to die) was as follows: 250, 270, 275, 270, 270 and 265C with a screw speed of 250 rpm. PET granules were dry mixed with 1, 3, and 5% wt of Cloisite 15A. The total weight of material per batch was 300 g. The resulting nano-composite strand was cooled in a water bath, granulated and dried overnight in oven at 110C. A laboratory press with a temperature plates of 280C under a pressure of 5 MPa for 10 min was applied to compress specimens. Then cooled them in water and ice bath to achieve transparent films.
The influence of different amount of nanoclay addition on resultant nanocmposites was studied by Fourier transform infrared spectroscopy (FT-IR) and mechanical test. Also, influence of nanoclay presence on water vapor permeability (WVP), color and transparency of the nanocomposites were investigated.
The results showed that nanoclay addition improved the mechanical properties (Young’s modulus, elongation at break and tensile strength) and WVP up to 3% (wt). However, nanoclay addition reduced the transparency of resultant nanocomposites films but it prevented wave transmission at three UV region which leads to better protective effect of film as a food packaging materials. It seems that introduction of Cloisite 15A into the PET matrix reinforced the mechanical properties of resultant nanocomposites. The Young’s modulus of the nanocompo-sites significantly increased compared with the neat PET, indicating that PET/Cloisite 15A nanocomposites were stiffer than PET. The maximum Young’s modulus was observed for PET/C15A containing 3% wt with an increment about 8 MPa. This increase in modulus may be attributed to uniform dispersion and alignment of nanoclay along with compatibil-ity with PET matrix as confirmed by XRD, DSC and SEM. The Young’s modulus enhancement is consistent with that of other research (KIMet al. 2007; Scaffaroet al. 2011; Ghanbari et al. 2013a,b]. Tensile strength like elongation at break shows same trend, increases on increasing nanoclay content except for nanocomposite containing 5% which indicates brittle behavior compared to PET. This can be explained consider-ing that higher aspect ratio of nanoclay tends to aggregate and forms tactoids (as shown in SEM and XRD) and conse-quently indicates poor mechanical properties.