Orientation behavior of poly (ethylene terephthalate) / ethylene-vinyl alcohol (EVAL) copolymer laminates

The purpose of the project was to determine how the orientation behavior and properties of PET/EVAL copolymer laminates were related to the characteristics of the individual layers. Control films were extruded from poly(ethylene terephthalate) (PET) and ethylene-vinyl alcohol (EVAL) copolymer. Laminates of PET/EVAL/PET were coextruded having various levels of adhesion. Mechanical properties were obtained on the films as-extruded and after biaxial orientation, before and after heatsetting, in the machine and transverse directions. In addition, stretching properties were obtained on the films during orientation. The degree of orientation was estimated from birefringence measurements.

The equations of Schrenk and Alfrey, and Guillotte were used to predict the properties of the laminates from the properties of the substrate films. In the absence of a tie-layer, i.e. representing poor adhesion, most properties of the laminates were found to be additive; otherwise, prediction of properties depended upon the properties and volume percent of the tie-layer and the degree of adhesion existing. In the presence of good adhesion, the properties of the laminate could not be predicted by the methods used.

The orientability of the laminates was found to be controlled by the EVAL component and the degree of adhesion existing between the PET substrate and EVAL. EVAL was found to be crystalline, sensitive to moisture and processing conditions, and difficult to orient.

It was concluded that a tie-layer is not needed for films which will be oriented and can be detrimental to mechanical properties. Also, only the minimum amount of EVAL needed to impart good barrier properties should be used in PET/EVAL laminates.