Poly (ethylene terephthalate) (PET) is a semicrystalline thermoplastic polyester widely used in the packaging industry. One of the main markets for PET is plastic bottles for carbonated drinks (CSD), water, juice and oxygen sensitive products. In order for PET to be utilized in packaging applications requiring even lower gas permeability, coating of PET bottles and melt blending with higher barrier polyamides have been studied. For example, a blend component with PET is poly (m-xylylene adipamide), MXD6, which itself shows excellent barrier properties and good transparency. In recent decades, the polymer/montmorillonite (MMT) nanocomposite route indicates other possibilities to enhance properties of conventional PET, and reveals better mechanical and barrier properties than traditional polymers.
In this research, a modified melt blending method was developed to prepare PET/Na-MMT, MXD6/Na-MMT, and PET/MXD6-Na-MMT nanocomposites with Na-MMT loading from 0.5% to 5% (wt), and with the purpose of preparing exfoliated nanostructure in the absence of organic modifiers. The hydrophilic nature of Na-MMT allows it to be exfoliated in water to form clay dispersion. The melt blending of PET with a Na-MMT slurry, and MXD6 with a Na-MMT slurry of different clay weight percentages were performed in a twin screw extruder. The thermal degradation and hydrolytic degradation in the presence of water resulted in the reduction of molecular weight (M.W.) of the nanocomposites. All the nanocomposites processed by solid state polymerization reached specific M.W. values (0.8 dL/g) which were similar to each other.
In order to study the effects of pristine nanoclay on the thermal properties of PET and MXD6, all nanocomposite samples were subjected to thermal analysis by differential scanning calorimetry (DSC) and the results are discussed. The introduction of Na-MMT hinders the mobility of polymer chains, which results in slight increases of the glass transition temperatures. Additionally, the presence of Na-MMT in the PET or MXD6 matrix acts as a nucleating agent, which accelerates the rate of crystallization. Among the nancomposites with different concentrations of Na-MMT, there are no significant differences in the crystallization behaviors. This indicates that the exfoliation morphological structure achieved for the polymer nanocomposite shows the similar thermal behavior.
Wide angle X-ray diffraction (WXRD) and transmission electron microscopy (TEM) are used as indicators of the extents of the dispersion state of clay platelets within the polymer matrices. The micrographs of PET/Na-MMT nanocomposites (0.5 wt%, 1 wt%, 2 wt%, 3 wt%) as well as MXD6/Na-MMT nanocomposites (2 wt% and 3 wt%), reveal a well-exfoliated structure. The 5 wt% MXD6/Na-MMT nanocomposites and 5 wt% PET/Na-MMT nanocomposites show partial intercalation and agglomeration. The absence of diffraction peak indicates the possibilities of formation of exfoliation morphology while TEM reveals the agglomerates at high clay concentration. The results give strong evidence that the WXRD needs to be combined with TEM in the study of morphology of polymer nanocomposites.
Tensile tests show that the maximum improvement of Young’s modulus is around 24% at 2 wt% clay addition in PET and 21% for the 2 wt% MXD6 nanocomposite. The polymer becomes brittle due to the presence of Na-MMT. Dynamic mechanical analysis (DMA) showed that the storage modulus doubled at 5 wt% clay addition in the case of PET/Na-MMT nanocomposites, and is 1.5 times larger than recorded for neat MXD6. Moreover, it is observed that PET nanocomposites had lower permeability values than the pure PET. When introducing 2 wt% Na-MMT into a PET matrix, the value of permeability dropped to 4.2 cc*mil/(100*in2*day*atm), which shows a 52% enhancement of the oxygen barrier. For MXD6/Na-MMT nanocomposites, optimum oxygen barrier was given by 3 wt% clay loading with this nanocoposite, showing 70% barrier enhancement. It was, therefore, concluded that nanocomposites prepared by using modified melt blending method showed better oxygen barrier properties compared to neat PET and MXD6.
It also has been found that orientation can strongly influence the morphology of layered silicate nanocomposites and thus have an influence on their mechanical, and barrier properties. During uniaxial orientation, in the presence of Na-MMT in PET, strain hardening occurs sooner than in pure PET. In order to describe the stress-strain behavior of PET nanocomposites during uniaxial stretching, a computational model was successfully developed to enable the prediction the effect of clay loading and varies extension ratios on the mechanical properties. For biaxially stretched blow molded PET bottles, strain-induced crystallization causes enhanced oxygen barrier properties for PET. The presence of microvoids in the case of blow molded nanocomposite bottles, however, results in less improvement between PET and PET nanocomposite bottles.
When exfoliated MXD6 nanocomposite is transferred into the PET matrix, it is found that the majority of clay platelet stays inside the MXD6 domain. It is found 37% and 87% improvements were achieved for Young’s modulus and oxygen barrier properties of PET/MXD6-MMT. The obtained 5 wt% MXD6 nanocomposite blended with PET, shows a high degree of exfoliation, indicating that the morphology structure plays a significant role in property enhancement.