PREFORM PRODUCTION PETplanet Insider Vol. 25 No. 04/24 www.petpla.net 21 end point of melting remain similar to the other resins. It is darker (lower L*colour), due to the very high Sb content (>300 ppm) and is yellower (higher b* colour) due to processing under dry air at 180-185 °C for long periods of time. Yellowness is an indication of thermo-oxidative degradation. It is assumed that the high Sb content is required to boost reactivity in both the CP and SSP, for making higher IV in the CP and compensating for lower temperatures in SSP, which indicates a lack of flexibility during processing. Generally, the level of contaminants tends to decrease with increasing IV across SSP. Both Resins 1 & 3 easily reach the industry standard for residual AA of <1 ppm indicating sufficient SSP treatment. Resins 2 & 4 have AA values >1 ppm, due to reduced SSP treatment. 2-Methyl-1,3-Dioxolane (MDO) is referred to as “hidden AA”, as it can be converted back to AA and ethylene glycol in the presence of water. The MDO level in the MTR resin is around 10 times higher than the other resins; this is thought to be caused by processing under dry air. Preforming 43 g preforms were made at PTI in Switzerland on an Arburg 370C mono-cavity moulding machine. Process parameters were kept the same for all resins: drying for 5-6 h at 160 °C to less than 10 ppm and injection moulding at barrel temperatures of 280 °C. The preform results in Table 3 can be used to compare the behaviour of the different resins, but the results cannot be compared directly to results from production scale preforming. All resins melted out without any issues during preforming. Neither the differing modification levels nor the level of crystallinity level influenced the melt out behaviour of the resins at 280 °C. No significant difference was observed in the melting behaviour of spherical and cylindrical pellets. An IV drop of around 0.02 dl/g was seen in all resins, regardless of solid-state treatment, which is typical during preforming as the resins were all well dried. This would suggest that moisture content, which was the same for all samples, had a more significant effect on IV drop than any difference in molecular weight distribution (polydispersity) between the samples. The influence of SSP processing can be seen when comparing the combined AA and MDO (mmol/ kg) loading of the preforms. Resins 1 & 3, with the longest SSP residence times and highest temperatures, have a much lower overall AA & MDO content, by 25-40%, compared to resins 2 (lowest SSP residence time) & 4 (lowest SSP temperature). A shift in Preform quality Unit Resin 1 2 3 4 IV (ASTM) [dl/g] 0.765 0.747 0.759 0.778 IV drop (ASTM) [dl/g] 0.019 0.023 0.019 0.021 Haze [%] 8.89 8.60 8.17 10.69 Colour L* [-] 82.03 81.56 84.28 76.10 Colour b* [-] 2.60 3.69 2.70 4.85 Residual AA [mg/kg] 6.2 9 6.2 6.7 Residual MDO [mg/kg] 5.9 8.4 8.1 9.9 Total MDO & AA [mmol/kg] 0.21 0.30 0.23 0.26 Table 3: Preform measurements - Note: Haze and colour measurements made directly on preform; glass preform used as zero reference for haze. Resin quality Unit Resin 1 2 3 4 Pellet crystallinity (av.) [wt%] 56.3 55.5 58.8 49.7 Colour L* [-] 88.0 87.9 90.6 83.7 Colour b* [-] -2.3 -2.73 -2.14 -0.31 Residual AA [mg/kg] 0.90 1.4 0.61 1.1 Residual MDO [mg/kg] 0.67 1.0 0.82 7.7 Antimony (Sb) content [ppm] 208 240 188 316 Table 2: Bottle grade resin measurements the MDO and AA balance can also be seen; this is due to equilibration. The colour of the preforms is directly influenced by bottle grade resin quality. The MTR resin, with a long residence time under air and very high Sb content, results in darker (lower L* colour) and yellower preforms (higher b*). It also has a significantly higher haze level than the other resins; this could be caused by either the high Sb content or, possibly, degradation reactions from the long residence time under air at high temperature. Summary SSP continues to prove its value in the PET processing chain. Better management of the IV split between CP and SSP can allow more process flexibility, such as lower Sb contents, and improve preform and bottle quality. Longer SSP times and higher SSP temperatures under N2 produce a cleaner resin, resulting in lower overall AA and MDO content in preforms and bottles while still maintaining good colour and low haze. SSP processing under dry air has limitations and leads to yellowness, an indication of oxidative degradation, as temperatures increase above 180 °C and residence times are extended to increase IV. www.polymetrix.com
RkJQdWJsaXNoZXIy NTY0MjI=