Development of PET/kaolin nanocomposites with improved mechanical properties

Polyethylene terephthalate (PET) is a major polymer used in industries such as textile and packaging. Regarding the economic and environmental effect of polymeric packaging materials, there is a large demand for the development of reduced weight packaging systems. But on the other hand, low barrier and mechanical properties are the main drawbacks of such packaging systems. In spite of numerous studies conducted on the barrier and mechanical performance of PET-clay composites, little improvement has been reported. In this work, the influence of adding kaolin as a different type of clay material on the performance of PET will be investigated.;In the second part, a displacement method, in which one element is replaced by another in a compound, was applied; the precursor produced in the first part was blended with poly(ethylene oxide) (PEO) in an internal mixer to give a high concentrated PEO-Kaolin masterbatch, and this masterbatch was diluted with neat PET in a twin screw extruder to produce the final nanocomposites.;X-Ray diffraction results showed that the layered structure of kaolin was significantly altered due to the chemical treatment. Upon the intercalation of KAc molecules into the kaolin structure, the intensity of a kaolin characteristic peak was remarkably decreased and new peaks appeared at lower angles; these peaks eventually disappeared after blending with PET. SEM (scanning electron microscopy) images confirmed that the KAc treatment and blending with PEO had a significant effect on dispersion of the particles. From TEM (transmission electron microscopy) images, the particles in the nanocomposite were estimated to be 100-200 nm in length while their thickness was generally in the range of 10 to 50 nm. Compared to the as-received particles, the dispersion of the chemically treated kaolin in the PET matrix was remarkably improved. However, rheological studies showed that degradation of PET during the melt mixing step led to reduced melt viscosity and a considerable loss in molecular weight.;In the third part, the possibility of preparing calcined kaolin nanoparticles via wet-grinding and using them as a nanofiller in a PET matrix was investigated. SEM and TEM images showed that the ground particles with sizes between 50 and 250 nanometers were uniformly distributed in the PET matrix. It was shown that the modulus and barrier properties of PET were improved after incorporation of ground particles; however, the elongation at break was significantly smaller than that of neat PET. The presence of nanoparticles slightly impaired the optical properties of the samples as well.;The first part of this work focused on the chemical treatment of hydrous kaolin particles. Considering the specific chemical structure of kaolin, conventional modifiers such as alkyl aluminum which are widely used for smectite type clays are not applicable in case of kaolin. Since the final PET-kaolin nanocomposite was supposed to be in direct contact with foods, toxicity of the modifiers was another challenge to be considered. The modifier also needed to be thermally stable at processing temperature of PET. Eventually, potassium acetate (KAc) and dimethyl sulfoxide (DMSO) were chosen for chemical treatment of particles and their influences on the structure and physicochemical properties of the particles were investigated.;The last part was dedicated to studying the effect of secondary processes (such as hot-stretching) on the final properties of the PET/calcined-kaolin composites. Incorporation of calcined kaolin particles enhanced the final properties of PET matrix and this enhancement was shown to be more pronounced after uniaxial stretching above the glass transition temperature. However, the addition of particles was associated with haziness in the composites and the stretching process promoted this effect, which was assigned to crystallinity and debonding between the particles and the matrix.