what is biaxial film orientation? how does that increase the tensile strength of plastic film?

Answer 1

Bi-axially drawn PET films have many superior properties to non-oriented films, such as their mechanical properties, barrier functionality, and better profile. This can be explained by the orientation process, which is performed when a polymer is stretched above the glass transition temperature.

Preferably, the film has an orientation imbalance, with a higher degree of transverse orientation than machine orientation. Skin layers such as heat seal layers can be provided. Preferably, the film contains one or more layers of a casting promoter material employed to promote casting of a high gauge HDPE sheet subsequently high biaxially oriented to provide the film. The film can also be cavitated, or otherwise modified (e.g., corona-treated, coated, metallized, etc.) to provide films suitable for special applications. The film of the invention has superior properties, including lower water vapor transmission rate, better dead-fold, higher tensile strength, and consistent gauge profile. Also provided is a method for making the biaxially oriented film.

Answer 2

The previous answer had the right idea, but let me simplify it. All film is made of polymer chains. When a majority of these chains cross each other in two directions like stitching, that creates biaxial orientation. If you had to pull one of these chains apart, it might be easy. But try tugging 100, 1000, or 10000 of these chains, and see how much strength it takes to break them apart. Therefore a biaxial oriented film is that much tougher to break in two directions, although they are easy to rip apart at any other angle.