Properties of biaxially oriented films

A major topic of interest relating to film structure is the effect of crystallinity on the deformation mechanism. The optical properties of biaxially oriented films were studied in 1957 by Stein [109] who determined the full set ofbirefrin-gences, by measuring the optical retardation as a function of the tilt of a PS film. Samuels [110] used complementary techniques of x-ray scattering, TEM of surface replicas and birefringence measurement in a study of the microstructure... 

The orientation and properties of polyethylene films produeed by the blown-film technique have been studied by many researehes in literature [2-6]. The correlation between orientation and morphology of oriented polyethylene (mostly blown film) has been studied thoroughly as well [7-10]. Mechanieal and viseoelastic properties of biaxially oriented films produeed by double bubble method were also investigated [11-13]. 

Table III. Properties of Biaxially Oriented PBZT Film...

PEN has excellent gas-barrier properties. The gas-permeation coefficients of unoriented cast film and biaxially oriented films for PEN and PET are given in Table 10.4. 

Kim, J. C Cakmak, M. and Zhou, X Effect of composition on crystalline orientation and optical properties of biaxially stretched PEN/PEI films, in Proceedings of the 55th SPE ANTEC 97 Conference, May 5-8, 1997, Toronto, ON, Canada, Society of Plastics Engineers, Brookfield, CT, 1997, Vol. 2, pp. 1588-1592. 

Mechanical properties of polymers are improved by orientation processes [22, 24, 29]. Because of the biaxial orientation, films from blown film lines exhibit, e.g., a high puncture and tear propagation resistance. Blown film lines can be designed to produce flexible PE-LD or PE-HD films of 5-200 pm. 

Monoaxially and biaxially oriented films of fluoropolymer are made by melt extrusion of the resin into flat webs or tubes. The main function of orientation is to enhance the mechanical properties of the film such as tensile break strength and tear resistance. The decision to orient is usually made according to the requirements of the end use for mechanical properties. All process surfaces that contact molten fluoropolymers must be corrosion resistant because of the formation of corrosive compounds such as HF and HCl from the high-temperature degradation of these plastics. 

Orientation of styrene-based copolymers is usually carried out at temperatures just above T. Biaxially oriented films and sheet are of particular interest. Such orientation increases tensile properties, flexibility, toughness, and shrinkability. PS produces particularly clear and sparkling film after being oriented biaxially for envelope windows, decoration tapes, etc. Oriented films and sheet of styrene-based polymers are made by the bubble process and by the flat-sheet or tentering process. Fibers and films can be produced by uniaxial orientation (237) (see Film AND SHEETING materials). 

Kim, J. C., Cakmak, M. and Zhou, X., Effect of composition on orientation, optical and mechanical properties of biaxially drawn PEN and PEN/PEI blend films. Polymer, 39, 4225-4234 (1998). 

The blown-film technique has several advantages the relative ease of changing film width and caliber by controlling the volume of air in the bubble and the speed of the screw the elimination of the end effects (e.g., edge bead trim and nonuniform temperature that result from flat film extrusion) and the capability of biaxial orientation (i.e., orientation both in the hoop direction and in the machine direction), which results in nearly equal physical properties in both directions, thereby giving a film of maximum toughness. 

The principal uses of biaxially oriented PET film are in capacitors, in slot liners for motors, and for magnetic tape. Although a polar polymer, its electrical insulation properties at room temperature are good (even at high frequencies) because at room temperature, which is well below Tg (69°C), dipole orientation is severely restricted. 

The biaxially oriented film has an excellent anti-curling property. The anti-curling property means that a film hardly remains curled when rolled once and then unrolled. Whitening has rarely been observed at a fold when the film is folded. This property is called delamination resistance and is an important index for the evaluation of both a base film for a magnetic recording medium and a base film for a photo film. 

Orientation Orientation improves the inherent barrier and mechanical properties of unoriented nylon film. After biaxial orientation, nylon film exhibits a significant improvement in oxygen and aroma-barrier properties. ... 

Oriented Film Film that has been oriented either monoaxially (in one direction), the direction of tear, or biaxially (in two directions), to achieve strength and stiffness. Monoaxially oriented tape exhibits stronger MD (machine direction) tensile properties than biaxially oriented tape. Materials that have been oriented bear an O before the acronym (e.g., OPS, oriented polystyrene). 

A detailed discussion of the many types of oriented plastic films, their properties and applications falls outside the scope of this chapter. Instead, we shall discuss the reasons why biaxially oriented films are used and their advantages and disadvantages in more general terms. 

Guastavino J, Mary D, Krause E, Laurent C, Mayoux C. On the electrical properties of poly(ethylene naphthalate 2,6-dicar-boxylate) biaxially-oriented films. Polym Int 1998 46(l) 72-6. 

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