Biaxially Oriented Plastic Film (BOPF)

In the production of Biaxially Oriented Plastic Film (BOPF), adjustments to process conditions can yield films with nearly identical physical and mechanical properties in both the longitudinal and transverse directions, referred to as balanced films (isotropic). Alternatively, films with higher mechanical strength in one direction than the other can be produced, termed tensilized films or semi-tensilized films (anisotropic). Typically, the longitudinal direction exhibits greater mechanical strength. The biaxial stretching process involves stretching an unstretched film or sheet in both longitudinal and transverse directions within the temperature range between the glass transition temperature and the melting point. This aligns molecular chains or specific crystalline planes parallel to the film surface, followed by heat treatment under tension. Stretched films exhibit enhanced mechanical properties, including tensile strength, impact resistance, elastic modulus, tear resistance, fatigue bending resistance, and surface gloss, as well as improved thermal stability, cold resistance, transparency, gas barrier properties, moisture resistance, electrical insulation, thickness uniformity, and dimensional stability.
When heated, the molecular orientation relaxes, causing the film to shrink. Thus, BOPF is also known as heat-shrinkable film in the packaging industry.
Industrialized BOPF variants include: BOPP (biaxially oriented polypropylene), BOPET (biaxially oriented polyethylene terephthalate), BOPA (biaxially oriented polyamide), BOPS (biaxially oriented polystyrene), BOPVC (biaxially oriented polyvinyl chloride), BOCIPE (radiation-crosslinked polyethylene), BOPVDC (biaxially oriented polyvinylidene chloride copolymer), and BOPVA (biaxially oriented polyvinyl alcohol), with the first four being the most prevalent.
BOPF production methods fall into two categories: flat film (planar) processes and tubular (bubble) processes. The planar casting method follows a workflow similar to cast film production but incorporates an additional transverse stretching (broadening) step after longitudinal stretching.
Planar biaxial stretching techniques include:

Simultaneous biaxial stretching (one-step stretching).


Sequential longitudinal-transverse stretching (two-step stretching).


Longitudinal-transverse-longitudinal stretching (three-step stretching).

For example, the longitudinal-transverse stretching method is commonly used in BOPP (biaxially oriented polypropylene) film production.