Spherical pellets exiting an SSP reactor PREFORM PRODUCTION PETplanet Insider Vol. 25 No. 04/24 www.petpla.net 20 Influence of PET processing on preforms and bottles based on an article by Brent Culbert, Senior Process Engineer, Polymetrix AG Solid State Polycondensation (SSP) has been an integral part in the PET processing chain since the 1970s and is today considered a given in the manufacturing of PET bottles. It complements the melt phase by increasing the molecular weight of PET in the solid state above values that cannot be comfortably reached in continuous polymerisation, eliminates difficulties associated with the stirring of the viscous melt, limits degradation and removes byproducts such as acetaldehyde (AA) and oligomers. Process conditions, such as temperature and residence time, can vary considerably during SSP. This can be due to differences in the starting IV after continuous polymerisation (CP) or the final IV, which depends on the bottle grade targeted - mineral water or CSD for example, or equipment constraints and throughput variation. To illustrate the influence of SSP on preforms and bottles, four bottle grade resins from four different commercial production facilities were compared in a standard preform trial at Packaging Technologies & Inspection (PTI) European headquarters in Switzerland. The resins, shown as 1 to 4 in Table 1, all target the mineral water market and do not contain a reheating agent. Their IVs range from 0.77 to 0.80 dl/g (ASTM); level modification varies from 3.9 to 4.6mol% (DEG+IPA). These differences do not significantly influence the preform results shown. Resins 1, 2 & 4 are all spherical pellets, crystallised directly from the melt; resin 3 is cylindrical and cooled below the glass transition (Tg) before storage and SSP processing. Generally, the longer residence time and the higher the temperature during SSP, the greater the IV increase. Resins 1, 2 & 3 were all processed under nitrogen N2 for between 9 h and 22 h at temperatures over 200 °C. Resin 4, from a Melt to Resin plant (MTR), has undergone a solidstate treatment (“conditioning”), which involves processing under dry air for an estimated 15-18 h at 180-185 °C, to remove AA while SSP also occurs. In Table 2, resin 4, the MTR resin, is clearly shown to differentiate from the other resins in several respects. It has a lower crystallinity, because of lower SSP temperatures, which is typified by a lower beginning of melting, although its melting point and PREFORMS Process parameters Unit Resin 1 2 3 4* Intrinsic Viscosity (IV) by ASTM** [dl/g] 0.785 0.768 0.779 0.800 Total modification (DEG+IPA) [mol%] 4.65 4.19 3.88 4.57 SSP processing Medium [-] N2 N2 N2 dry air SSP residence time [h] 22 9 17 15-18 SSP temperature [°C] 203 207 208 180-185 Pellet shape [-] spherical spherical cylindrical spherical Table 1: SSP processing conditions of various bottle grade resins *SSP conditions not known and therefore estimated **ASTM: American Society for Testing and Materials
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