The effects of industrial processing and chemical variation on the near-surface properties of poly(ethylene terephthalate) films and bottles

Abstract

The sub-micron surface region of poly(ethylene terephthalate) (PET) films and bottles has been investigated. Uniaxially drawn and annealed PET films were characterised by bulk and grazing incidence X-ray diffraction (GIXD). Surface-specific diffraction patterns collected below the critical angle (ɑc) for total external reflection reveal a radial shift in diffraction peaks, whereas bulk signals collected above this critical angle occur at the predicted locations. It is calculated that this shift can be described by a ~5 % reduction of the a, b and c unit cell imrameters for PET crystals within the sub-micron surface region. This shift is less apparant in the un-oriented specimens. The crystallite orientation is consistently higher within the surface region of the drawn PET films than the bulk. There is a good agreement between the X-ray diffraction and FTIR orientation measurement over the range of films analysed. Polarised FTIR-ATR spectroscopy was used to map the molecular orientation along the inner and outer surfaces of stretch-blown PET bottles. This work reveals complex orientation trends. For 2-litre bottles, the inner surface shows consistently higher orientation levels than the outer surface. There are also two orientation switches at the shoulder and base regions on the outer surface, but only one switch on the inner wall. In all cases, the maximum orientation occurs on the inner surface at 4-6 cm from the base. This analysis was repeated for various bottles, including examples produced from various polyester co-polymers and a series of bottles stretch-blown after different preform heating times. Heating for 5 seconds longer than the recommended time prior to the stretch seriously reduces the orientation in the finished bottle. Lowering the polymer molecular weight has a similar effect. The use of the popular 1340 cm"' PET IR band for crystallinity evaluation was questioned. It is concluded that the use of this band for crystallinity calculation is erroneous if the specimen possesses molecular orientation. It is shown that the 1340 cm"' band height reveals the orientation trends in highly drawn specimens of low crystallinity. Simple measurements reveal that the 1340 cm ' transition moment for this band lies approximately in the chain direction.