Biaxially stretched film

1 General

The abbreviation code of bi axially oriented plastics film is BOPF. In the production process of BOPF, by changing the process conditions, the films with basically the same physical and mechanical properties in both vertical and horizontal directions can be prepared, which are usually called balance films (i.e. isotropic); It is also possible to prepare anisotropic films with higher mechanical strength in one direction than in the other, which are commonly referred to as strengthened films or semi strengthened films. In general, the mechanical strength in the longitudinal direction is always greater than that in the transverse direction. The biaxial stretching of films is to stretch the untensioned films or sheets in both vertical and horizontal directions below the melting point and above the glass transition temperature, so that the molecular chains or specific crystal surfaces are parallel to the film surface, and then conduct heat treatment under the condition of tension. The tensile film improves mechanical strength, such as tensile strength, impact strength, tensile elastic modulus, tear strength, fatigue bending property and surface roughness, and improves heat resistance, plug resistance, transparency, luster, permeability, moisture resistance, electrical insulation, thickness uniformity and dimensional stability

When the biaxial stretch film is heated, the molecular orientation relaxes and the film shrinks. Therefore, it is also called heat shrinkable film 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., which are mainly the first four.

There are two kinds of production methods of biaxial stretch film, plane method and tube film method

The process flow of the plane casting method is basically the same as that of the production of the flowing film, except that a widening (i.e. transverse stretching) process is added after the longitudinal stretching.

The planar biaxial stretching method can be divided into three types: simultaneous biaxial stretching method (i.e. one time stretching method), vertical horizontal stretching method (i.e. twice stretching method) and vertical horizontal vertical stretching method (i.e. three times stretching method).