Monoaxially oriented films

There are other differences between cast, monoaxially oriented, and balanced biaxially oriented film. Typical figures illustrating these effects are given in Table 11.5. 

Table 11.5 Comparison of cast, monoaxially oriented and biaxially oriented polypropylene film ...

The clarity of a translucent crystalline polymer, such as polypropylene (PP), may be improved by biaxial orientation. Monoaxial orientation of a crystalline film produces an anisotropic birefringent film. 

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. 

Table 6.9. Effect of Monoaxial Orientation on Electrical Properties of PVDF Films ...

Monoaxially oriented polypropylene film tapes have been widely used for carpet backing and for woven sacks (replacing those made from jute). Combining strength and lightness, oriented PP straps have gained rapid and widespread acceptance for packaging. 

Dartek UF-410. A monoaxially oriented nylon 66 film with good slip characteristics. 

OxyShield OBS. Oriented barrier, monoaxially oriented coextruded nylon 6/EVOH/nylon 6 clear film with slip additive, having excellent gas barrier properties through a broad range of relative humidity.fi ... 

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). 

This effect can be understood by the following simplified reasoning (see Fig. 2). The film is transported by the feeding rolls at a rate ki and stretched by the drawing rolls at a rate F2 > Vi. The stretching ratio, A, equals K2/K1. At the boundary of the film (e.g. the points on the curve AC) the material is only subjected to a tension in the direction of the tangent to the boundary. The stress is here monoaxial so that the material contracts freely both in the y-direction (transverse) and the z-direction (perpendicular to the film). This will result in monoaxial orientation. 

A.F. Wilkie, Biaxially and monoaxially oriented polypropylene cold seal release film, US Patent 5 489 473, assigned to Borden, Inc. (Columbus, OH), February 6,1996. 

An important application for polypropylene is film tape. This is made by slitting unoriented film (cast or blown) into tapes 2 or 3.5 mm wide and stretching under heat about seven-fold. With cast film the orientation is more completely monoaxial and there is a tendency for the film to split along its length (fibrillate). Tubular film does not self-split so easily and also has a somewhat softer feel. Such tapes may be woven into sacks and these have... 

This stretching, similar to the drawing of fibres, which promotes orientation and crystallite formation, is called biaxial orientation. It gives the film added strength and gas-barrier properties. In some processes, monoaxial (uniaxial) drawing is employed, e.g., polypropylene, which is then slit into thin strips and fabricated into heavy duty sacks, carpet backing, etc. The stenter process is used to make biaxial oriented poly(vinylidene dichloride) ("ding" film), polyester, polyamide and polypropylene films. 

The films and fibers are then both processed into the previously discussed semifinished products by means of conventional textile processing methods. Filament yams, woven fabrics and unidirectional fabrics, but also endless fiber fleeces and staple fiber fleeces, can be made from the fibers [40]. The films can be employed as semifinished products directly after being mono-, bi- or (rarely) multiaxially stretched. For the most part, the fibers are tailored into tapes, monoaxially stretched and woven into oriented textile products in order to produce the essentially woven fabrics. 

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