PP films, biaxially streched / PP-BO These films are biaxially stretched both lengthwise and in a lateral direction. At almost 90%, they are the most significant type used in polypropylene film production and are frequently known as BOPP (biaxially oriented PP). In line with recent conventions however, the abbreviation PP-BO is used – this is because in new standards the code for the polymer base material is always given first. Biaxial stretching improves film properties with regard to transparency, rigidity, resistance to cold and impermeability to water vapour and gases. Polypropylene films are not heat sealable without further processing. In this state they can be used for high-gloss lamination of boxes or as separating films. PP-BO in particular is utilized in manufacturing compound films together with aluminium foils, other plastic films and paper. The sealability of PP films is a key prerequisite for processing films further using the heat sealing method that is so important in packaging. Various methods come into play here: Coextrusion:
Due to significant advances in film technology in recent years, coextrusion has become the most widespread method of producing heat sealable PP-BO films when it comes to polypropylene films too. For this purpose, smaller extruders are incorporated into the stretching process. The sealing layers are copolymers made of ethylene and a high proportion of propylene or made of vinyl acetate, ethylene and propylene. As the sealing layers also undergo the stretching process, this method enables very thin films to be produced. Thanks to its high variation range and cost-effectiveness, coextrusion is superior to the varnishing method. Heat sealable PP-BO foils are used for food packaging such as for confectionary and baked goods, snack products, pasta and dried fruit or potato products. In addition, they are widely used for packaging paper goods, textiles, cosmetics and medical items. Additives can give PP-BO anti static or exceptionally low friction properties and it is suitable for use in all packaging machines. Due to its specific properties and benefits, it has increasingly replaced cellophane in the demanding cigarette industry where cellophane was used as packaging material for decades. Intense competition between cellophane and PP-BO in food and cigarette packaging led to the term “cellopp market” (cellophane and BOPP) emerging in the early 1960s and playing an important part in the history of film technology.
