Biaxially oriented polypropylene BOPP processes

Orientation of polymers enhances many of their properties, particularly the mechanical, impact, barrier and optical properties. Biaxial orientation has the added advantage of allowing this enhancement in two directions, avoiding any weakness in the transverse direction. Biaxial orientation is particularly important in films, where it allows the production of thinner films having superior mechanical, optical and barrier properties and, if required, the ability to shrink when reheated. For example, shrink and barrier properties are particularly important in meat packaging, whereas good mechanical and optical properties and no shrinkage are desired in box overwrap applications of tape cassettes and CDs. 

BIAXIAL ORIENTATION PROCESSES FOR PP Tubular film extrusion 

The orientation can be done sequentially or simultaneously in the machine and transverse directions. However, clear differences have been observed between the two [3]. The properties (transparency, modulus and strength) were lower for the sequential process. In fact, relaxation processes in the material cause a reduction in properties, since the film is stretched first in the direction of extrusion and can therefore partially relax while it is being stretched in the other direction [3]. Thus, in order to obtain the same properties as for the simultaneous process, the first stretch ratio should be higher and the temperature lowered after this stretching to compensate for and minimize relaxation. Typical data for 

In all these processes and depending on the application of the film, as mentioned above for the shrink properties, an annealing stage can be included or not. 

Isotactic PP is polymorphic and may crystallize into more than one crystalline form. The most common is the monoclinic form, which is designated as a. The chains in a monoclinic PP have helical conformation and the crystallographic axis is parallel to the axis of the helix. Other crystalline forms have also been observed and depend mainly on cooling and deformation history [2]. 

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Biaxially oriented polypropylene (BOPP) films have higher stiffness than cast films and consequently can be used in much thinner gauges. Homopolvmers are used almost exclusively to provide maximum stiffness and water-vapor barrier. Oriented films are produced by the tenter frame and bubble processes. 

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