The abbreviation for Biaxially Oriented Plastics Film is BOPF. In the production process of BOPF, by changing the process conditions, thin films with basically the same physical and mechanical properties in the vertical and horizontal directions can be produced, which is usually called a balance film (i.e. isotropic); It is also possible to produce anisotropic films with higher mechanical strength in one direction than in the other, which is commonly referred to as tensioned or semi strengthened films. In general, the longitudinal mechanical strength is always greater than the transverse strength. Biaxial stretching of thin films is the process of stretching an unstretched film or sheet in both longitudinal and transverse directions within a temperature range below the melting point and above the glass transition temperature, so that the molecular chains or specific crystalline planes are oriented parallel to the surface of the film, and then subjected to heat treatment under tension conditions. Stretching films improve mechanical strength, such as tensile strength, impact strength, tensile modulus of elasticity, tear strength, fatigue bending and surface roughness, as well as heat resistance, plug resistance, transparency, gloss, breathability, moisture resistance, electrical insulation, thickness uniformity, and dimensional stability
Biaxially stretched films undergo molecular orientation relaxation and shrinkage during heating, hence they are also known as heat shrink films in the packaging industry
At present, the varieties of BOPF that have achieved industrial production include BOPP, BOPET, BOPA, BOPS, BOPVC, BOCIPE (radiation cross-linked polyethylene), BOPVDC (polyvinylidene chloride copolymer), BOPVA (polyvinyl alcohol), etc., mainly consisting of the first four types.
There are two main types of production methods for biaxially stretched films: flat method and tube film method
The process flow of the flat casting method is basically the same as that of the production of flow induced thin films, except that an expansion (i.e. transverse stretching) process is added after longitudinal stretching.
The plane biaxial stretching method can be divided into three types: simultaneous biaxial stretching method (i.e. single stretching method), longitudinal transverse stretching method (i.e. secondary stretching method), and longitudinal transverse longitudinal stretching method (i.e. tertiary stretching method).
