Recycled poly(ethylene-terephthalate)/layered silicate nanocomposites: morphology and tensile mechanical properties

Various amounts (1, 3 and 5 wt%) of a non-modified natural montmorillonite clay (Cloisite Na.) or of an ion-exchanged clay modified with quaternary ammonium salt (Cloisite 25A) were dispersed in a recycled poly(ethylene terephthalate) matrix (rPET) by a melt intercalation process. Microphotographs of composite fracture surfaces bring evidence that particles of Cloisitew 25A are much better dispersed in the rPET matrix than those of Cloisite Na.. Moreover, WAXS measurements indicate that the lamellar periodicity of Cloisite 25A is increased in the composites, which evidences intercalation of rPET between silicate layers (lamellae) of the clay. In the case of Cloisite Na., a very small thickening of lamellae due to mixing with rPET indicates only minute intercalation. Uniaxial tensile tests show that both clays increase the modulus of the rPET composites; more effective Cloisitew 25A accounts for a 30% increase at loading of 5 wt%. Yield strength remains practically unaffected by the used fractions of the clays while tensile strength slightly decreases with the clay content; in parallel, strain at break dramatically drops. Tensile compliance of the composites is virtually independent of applied stress up to 26 MPa. Essential part of the compliance corresponds to the elastic time-independent component, while the viscoelastic component is low corresponding only to a few percent of the compliance even at relatively high stresses. The compliance of the composites is only slightly lower than that of the neat rPET, the reinforcing effect of Cloisitew 25A being somewhat stronger. Both clays have beneficial effect on the dimensional stability of the composites since—in contrast to the neat rPET—the creep rate does not rise at long creep periods.