STRUCTURE AND MECHANICAL PROPERTIES OF THE BIAXIALLY STRETCHED POLY (ETHYLENE TEREPHTHALATE) FILMS PREPARED BY DRY-PROCESS

Abstract

This paper concerns with the changes of structure and some mechanical properties of the poly-(ethylene terephthalate) (PET) films in the biaxial stretching.Unoriented, low crystalline T-die extruded PET film with a thickness of 140μ were used. In a circulating air bath at 90°C, the films were stretched at a constant rate of 300%/min by various methods using a film-stretcher, (i.e., uniaxially stretched under free width or constant width, two-way succesively, and simultaneously biaxially stretched), and then heat setting was carried out at 110°C for 15min.Mechanical properties (tensile strength, elongation, Young's modulus and F5 value) of the variously stretched films were measured at 20°C, 65% R. H. using Autograph tensile tester. The crystalline orientation and degree of crystallization were investigated by X-ray diffraction and density measurement.The following results were obtained:(1) The stress-strain curves reflect the differences of orientation between machine and transverse directions quite distinctly.(2) Irrespective of stretch ratio the anisotropy of mechanical properties for the films uniaxially stretched under free width is greater than that stretched under constant width at the same stretch ratio of machine direction.The extent of the anisotropy of mechanical properties for the two-way successively biaxially stretched films is larger in the stretching direction (MD) than in the transverse direction (TD) for the TD-stretch ratio before “the balance point” of mechanical properties is attained. Then, the anisotropy is reversed after the balance point. The difference of the balance point between four mechanical properties in the stretched PET films is not so large as in polypropylene (PP) films.The mechanical properties of the simultaneously biaxially stretched films, irrespective of stretch ratio, are equal in both the machine and the transverse directions as seen for polyvinylchloride and PP films. That is, these properties are isotropic within the plane.(3) The degree of crystallinity for the uniaxially and simultaneously biaxially stretched films increases with increasing stretch ratio. For the two-way successively biaxially stretched films, however, it decreases with stretch ratio before the “balance point” is attained, and then it increases with stretch ratio.(4) The simultaneously biaxially stretched films (MD×TD=√14×√14) have higher tensile strength and lower Young's modulus, compared with the two-way successively biaxially stretched ones at the same stretch ratio (MD×TD=4.0×3.5) and with the balanced mechanical properties.(5) The difference between the uniaxial stretchings under free and constant width, can be observed not only in the mechanical properties but also in the changes of structure of transgauche rotational isomers.In the two-way successively biaxially stretched films at the stretch ratio near “balance point”, the change of orientation of the chain segments and crystallites occurs rather than the transformation of isomers from gauche to trans form.(6) In the simultaneously biaxially stretched films at higher stretch ratios, the trans contents and the F5 values are large, suggesting that the chain segments are extended and tautened in all directions within the plane.