PP / PolypropylenE The discovery of new catalysts was crucial in developing polypropylene. Polypropylene is produced during the polymerization process at low pressure or in the gas phase from the monomer propene. A distinction is made between isotactic and atactic polypropylenes depending on the structure of the methyl groups (CH3 atom groups) in the polymer chain. In turn, the different structure of the polymer chains affects the properties of the substance. Isotactic PP has a high level of crystallinity and exhibits high rigidity / toughness, while atactic PP is soft and ductile and plays a less important role. Over 90% of polypropylenes are isotactic. Polypropylene is harder than PE-HD and has a higher softening range combined with a lower density. At -10 to -15°C polypropylene reaches the so-called glass transition temperature where it tends to become brittle and breakable at low temperatures. Homopolymerized polypropylenes cannot therefore really be used below 0°C. To resolve this shortcoming, low quantities of ethylene are added by means of copolymerization. Films made of polypropylene are produced as unexpanded, monoaxially expanded and biaxially expanded films.
By far the most important use of polypropylene in the plastic film segment is the production of biaxially oriented polypropylene films (PP-BO) , which offer a range of favourable properties. Top features include very good mechanical properties like resistance to tearing, shock and piercing combined with excellent optical properties such as gloss and transparency. The films are not susceptible to water and are impermeable to water vapour. They possess outstanding resistance to heat, cold and scratches, plus superb dimensional stability. They exhibit very good resistance to oils, fats and solvents, have no taste or odour and are physiologically harmless. PP-BO can be used in very thin film thicknesses and is free of plasticizers. It is utilized in pouches and wrappers or as very thin household and cling film for foodstuffs. Another important use of polypropylenes are in non-expanded flat films produced by way of extrusion for further processing – lamination or coextrusion with other thermoplastics, metallization – or for shrink film. Extrusion blown and injection-moulded hollowware such as bottles, buckets, drums and cans are also produced, primarily using isotactic or copolymer polypropylene. Other deployment scenarios are bags and big bags made of homopolymer polypropylene. It is also used extensively for die cast items such as cups, transport crates, cassettes and lids. Thanks to its high upper temperature limit, propylene is suitable for food packaging, heating in microwave ovens and for hot filling with liquids.
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