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Karina This article was published on 11 June 2021.
Polypropylene (PP) is a thermoplastic from the polyolefins group. The methyl group improves mechanical properties and thermal resistance, although the chemical resistance decreases. The chemical formula for PET is (C3H6)n. PP was discovered in 1951.
Production (in theory)
PP is a linear hydrocarbon polymer. The monomer is propylene, the second most produced building block in the petrochemical industry. Propylene is made by cracking naphtha or natural gas. It is gaseous at room temperature. Propylene is transformed into polymers through catalyst aided chain-growth polymerisation. The polymer can be used in different processing technologies such as injection molding, blow molding, sheet extrusion, oriented film extrusion (to produce Orientated Polypropylene (OPP) film), injection-stretch blow molding and thermoforming.
Oxidation usually occurs at the tertiary carbon centers leading to chain breaking via reaction with oxygen. Polymer stabilizers (UV-absorbing additives and anti-oxidants such as phosphites and hindered phenols) can be added to prevent polymer degradation.
Production (in practice)
The main commercial technologies for PP production are Spheripol by LyondellBasell (market share of around 39%), UNIPOL PP (16%), Novolen (11%), Innovene PP (7%), Hypol (4%), Borstar PP (3%), Chisso (3%) and Sumitomo (3%). The top 10 producers account for more than 50% of the total production of PP globally. These include Lyondell Basell, Sinopec Group, PetroChina Group, Braskem Group, Borealis/Borouge, SABIC, Exxon Mobil, Reliance industries, Total PC and Formosa Plastics Corp. About 50% of PP is produced in Asia (including China).
Different recycling methods are available for PP: mechanical recycling, solvent-based recycling, pyrolysis, feedstock recycling (including biological decomposition). Recycling PP without a loss of quality is an energy intensive process that results in a lot of GHG emissions. Hence PP is often downcycled into lower quality products like park benches or flowerpots. In 2020 The Recycling Partnership has launched the Polypropylene Recycling Coalition - an industry collaboration to improve PP recycling, and to improve recovery of PP and develop the market for high-quality recycled PP in the USA.