PETpla.net Insider 09 / 2019

MATERIALS / RECYCLING PET planet Insider Vol. 19 No. 09/19 www.petpla.net 28 PET - A review of material developments A desire for a plastics-free world has been identified by market research company Euromonitor as one of the top ten global consumer trends of 2019. In relation to PET however, the facts tell a different story: around 80% of the PET resin market still goes into more than 500 billion bottles worldwide annually, and PET remains the beverage packaging material with the strongest growth rates and prospects. And this is with good reason. Alongside advantages like rupture safety, resealability and recyclability, the life cycle assessment of PET bottles can be impressive. Following our reviews of the achievements in stretch blow moulding and filling (issue 3/2019), recycling (4/2019) and bottle making (issue 6/2019), we will now take a closer look at developments on the materials side. Like most polyesters, PET is manufactured in a multi-stage proce- dure. The first step is an esterifica- tion based on the monomers purified terephthalic acid (PTA) (70%) and monoethylene glycol (MEG) (30%) at 220-260 °C. The components can also be substituted with bio-based variants, such as from sugar cane. The precondensate this creates is reconstructed through polycondensa- tion into long-chain PET. The result is a polyester that can be used to make films and fibres. To obtain bottle grade PET, this polyester subsequently undergoes solid state polycondensation (SSP) to achieve higher molar masses. The long chains, indicated by this high molar mass, are reflected in the intrinsic velocity (IV) values, which are around +/- 0.80 for bottle grade PET. Because of the pressure, carbonated soft drinks require a high viscosity (0.80-0.88) but for still water, a lower IV value is sufficient (0.70-0.80). Alternatively, MTR technology (Melt to Resin) eliminates the step of solid- stating; during melt polycondesation, by means of a special high-viscosity finishing reactor, high-molecular PET material with almost the required IV can be obtained. Co-monomers are required as catalysts for these processes and these can constitute up to 10% of the weight. Their purpose is to optimise the bottle grade PET with regard to properties like colour, melting temper- ature or crystallinity. 1941 to today PET can be traced back to an invention by John Rex Whinfield and James Tennant Dickson. In 1941 they applied for a patent for the “highly polymeric substance” they had developed for the textile indus- try in the laboratories of the English company Calico Printers Associa- tion as a cheap alternative to silk or nylon. The chemical company ICI bought the world manufacturing rights in 1947 and began full-scale commercial production in 1955 under the name “Terylene”. The PET yarn was found to be strong and moisture-resistant, and there- fore became a popular fibre for use in manufacturing shirts. It took until the end of the 1970s (see PETplanet 06/2019) until its modification and use in bottle production. Since then, the material has continually been adapted to suit the requirements of the sector. There can be great differences depending on the type of product to be bottled and the processes used to do this. Still waters for example require a high level of transparency and a low proportion of acetaldehyde; sensitive beverages require an effective UV barrier; car- bonated drinks require an increased gas barrier. The properties can be adapted precisely to suit the require- ments profile by modifying the mate- rial or dosing tailored additives. The type of further processing must also be considered: the lower the degree of crystallinity, the less heat is necessary for melting the resin, while the higher the degree of crystallinity, the more heat is required. A reduction of crystallisation speed (achievable through co-monomers or additives) is of little significance in a single-stage process but is of much greater significance in a two-stage process where the preforms must be cooled quickly so that no crystallisa- tion takes place. Enjoy our overview of 20 years of achievements in bottle grade PET - as reported in PETplanet Insider! MATERIALS by Gabriele Kosmehl