PP / Polypropylene

Crucial to the development of polypropylene was the discovery of new catalysts. Polypropylene is produced from the monomer propene as part of the polymerisation process at low pressures or in the gas phase.

Depending on the arrangement of the methyl groups (CH3 atom groups) in the polymer chain, a distinction is made between isotactic and atactic polypropylenes. The different structure of the polymer chains in turn results in different properties of the polypropylene.

Isotactic PP has a high crystallinity and exhibits high stiffness / strength, while atactic PP is rather soft and tough and plays a subordinate role. The proportion of isotactic polypropylenes is over 90 %. Polypropylene is harder than PE-HD and has a higher softening range at low density. At -10 to -15 °C polypropylene reaches the so-called glass transition temperature and thus tends to become brittle at low temperatures, i.e. it becomes fragile at low temperatures. Hompolymerised polypropylenes can therefore in fact not be used below 0 °C. To eliminate this deficit, small amounts of ethylene are added by copolymerisation.

Polypropylene films are produced as unstretched, monoaxially stretched and biaxially stretched films. By far the most important use of polypropylene in the film sector is the production of biaxially oriented polypropylene films (PP-BO). These combine a number of excellent properties. The most outstanding are the very good mechanical properties such as tear, impact and puncture resistance combined with excellent optical properties such as gloss and transparency.

The films are insensitive to water and impermeable to water vapour. They have excellent heat and cold resistance as well as outstanding dimensional stability and scratch resistance. They have very good resistance to oils, fats and solvents and are odourless, tasteless and physiologically harmless.

PP-BO can be used in very thin film thicknesses and is free of plasticisers. It is used for bags and wraps or as very thin household or cling film for food. Another important application of polypropylene is non-stretched flat films produced by extrusion with further processing for lamination or coextrusion with other thermoplastics, for metallisation or as shrink film. Extrusion blow mouldings and injection moulded containers such as bottles, buckets, barrels and cans are also produced, primarily from isotactic or copolymer polypropylene. Another field of application is the production of sacks and big bags from homopolymer polypropylene. The use of injection-moulded parts such as cups, transport boxes, cassettes and lids is extensive.

Due to its high temperature resistance, polypropylene is suitable for food packaging, microwave heating and hot filling of liquids. These films are stretched biaxially, i.e. in both longitudinal and transverse directions. They have the most significant share in the production of polypropylene films with almost 90 %. They are often known as BOPP (biaxially oriented PP). According to newer conventions, however, the abbreviation PP-BO is used, as within new standards the abbreviation for the polymeric base material is always given first. Biaxial stretching achieves significant improvements in film properties with regard to transparency, stiffness, resistance to cold and impermeability to water vapour and gases.

Polypropylene films are not heat-sealable without further processing. In this state, they are used, for example, for the high-gloss lamination of cardboard boxes or as release films. PP-BO in particular is also used to produce laminated films in combination with aluminium foils, other plastic films and paper. However, an important prerequisite for the further processing of the films in the heat-sealing process, which is important for packaging technology, is the sealability of PP films. Various processes are used for this purpose:

Coextrusion: Due to significant advances in film technology in recent years, coextrusion is also the most widespread process for producing heat-sealable PP-BO films in the field of polypropylene films. For this purpose, smaller extruders are coupled to the stretching process. Copolymers of ethylene and a high proportion of propylene or of venyl acetate, ethylene and propylene serve as sealing layers. Since the sealing layers also pass through the stretching process, films with a very low film thickness are possible with this process.

Due to its wide range of variations and especially due to its significantly more economical production method, coextrusion is superior to the coating processes. Heat-sealable PP-BO films are used for food packaging, for example, of confectionery and bakery products, snack items, pasta and dried fruit or potato products. They are also widely used for packaging paper goods, textiles, cosmetic and medical articles. By adding additives, PP-BO can be made antistatic or particularly slippery and is suitable for use on all packaging machines.

Due to its specific properties and advantages, it has also prevailed as a packaging film in the very demanding cigarette industry against cellophane, which has been used for many decades. Due to the high competition of Cellophane and PP-BO in food and cigarette packaging from the beginning of the 1960's, the term "Cellopp market"(Cellophane and BOPP) is an important part in the history of film technology. These films are stretched on one side (mono) immediately after extrusion in the machine direction. This achieves significant improvements in the mechanical properties in the longitudinal direction of the films. This process is used, for example, in the production of adhesive rolls. Beyond that, it has rather a subordinate importance.