This research has concentrated on the development of methods for creating exfoliated clay nanocomposites with poly (ethylene terephthalate) (PET) for the purpose of improving barrier and other properties. For this purpose, extrusion blending and in situ polymerization were investigated. The melt extrusion was studied as a function of mobility of PET chain, affinity of clay modifier, and solid state polymerization (SSP). Three IVs of PET (0.48, 0.63, 0.74 dL/g) and three organic clays (Cloisite 10A, 15A, 30B) were melt blended with a twin screw extruder to evaluate variables on the properties. Addition of clay caused big molecular weight reduction after extrusion. Thermal stability experiments showed that the nanocomposites were sensitive to temperature. Fourier Transform Infrared (FTIR), however, indicated hydrolysis was the main reason for molecular weight reduction after extrusion. The SSP rate was decreased and crystallization rate became faster due to clay particles. There were basal spacing increases in PET/Cloisite 10A and PET/Cloisite 30B, but PET/Cloisite 15A did not show any change. After SSP reactions, PET/Cloisite 10A and PET/Cloisite 30B nanocomposites had a new peak at low angle in X-ray diffracton (XRD), indicating more expansion of basal spacing.
In situ polymerization was investigated in detail as a function of time and temperature of polymerization, mode of addition of the clay in esterification and in polycondensation, ethylene glycol/terephthalic acid ratio (E/T), diethylene glycol (DEG) suppressor, reactor pressure, antioxidant, and metal stabilizer. There was a limitation to reach 0.60 dL/g IV when the clay was added into the reactor at PET melt polymerization conditions. Foam generation made the melt polymerization of nanocomposites difficult. The concentrations of carboxyl and hydroxyl end groups showed big differences from normal values of PET, due to severe thermal degradation during melt polymerization. This thermal degradation caused drastic decreases in melting points and made the SSP rates slower than the observed in the nanocomposites produced by the melt intercalation method. X-ray diffraction (XRD) results showed that Cloisite 30B had the best affinity with PET due to hydroxyl groups in PET and the modifier of clays, while strong hydrophobicity of Cloisite 15A caused the worst affinity with PET. Nanocomposites formed by the in situ polymerization method had more exfoliated nanostructures than those produced by the melt intercalation method, even though they had a small amount of clay agglomerations.
Aluminium dish and reactor experiments implied the reason of foam generation and how to reduce the foam amount. Several additives were evaluated to improve the nanocomposite properties. Among them, melt polymerization of E/T=1.2 bishydroxy ethylene terephthalate (BHET) with Cloisite 30B at 270 °C was the best conditions for obtaining exfoliated nanostructures, considering all properties. The nanostructures analyzed by transmission electron microscopy (TEM) showed similar results with those of XRD, but TEM gave more detailed information compared with the results by XRD.
Tactoid and exfoliated nanocomposites were selected to evaluate the relationship between nanostructures and properties. Exfoliated nanocomposite had 11% improvement in tensile modulus and 29% improvement in oxygen permeability at 3 wt% of clay, while tactoid nanocomposite showed 2~7% improvement in tensile modulus and no improvement in oxygen permeability. According to the above results, it was found that the dispersion of clay platelets into single layers can have great impact on the properties even though the amount of clay was small. After stretching, improvements by the addition of clay particles were reduced, due to the structure change in the nanocomposites. Density, microscopy, and differential scanning calorimetry (DSC) results implied micro void generation during stretching.
The addition of 6 wt% clay in order to increase tortuous path length, was theoretically expected to produce 59% reduction in oxygen permeability, but only the permeability reduced by 37%, due to a small amount of intercalated nanostructure and more agglomeration of clay particles compared with that of 3 wt%. Low IV nanocomposite (0.39 dL/g), melt polymerized at 267 °C, improved the color of the nanocomposite. Cyclo hexane dimethanol (CHDM) was not a good monomer candidate for better nanocomposite, because color and oxygen permeability were worse than those of a control nanocomposite.