BOTTLE MAKING 22 PETplanet Insider Vol. 22 No. 11/21 www.petpla.net Keeping the AA level low by Dr Eng Christina Haerter, Head of Application Engineering at Netstal and Christian Tilsner, Technical Director at MHT Mold & Hotrunner Technology Acetaldehyde (AA) is a small molecule formed by the body during the degradation of ethanol (alcohol) and the cause of a hangover after a long evening’s drinking. It also occurs in a variety of foods such as yoghurt, beer, green apples and coffee and contributes to the taste. It produces a fruity, fresh taste very similar to that of apple. When PET is processed into preforms, AA is formed as a degradation by-product of PET through a reaction of OH-end groups of PET with water, glycol and oxygen. AA is a colourless and highly volatile liquid. Individual molecules can migrate through the PET of a bottle wall and enter the packaged liquid, as AA is miscible in water. A higher concentration of the substance is undesirable, particularly in water packaged in PET bottles, as a change in taste occurs after a certain storage time, the threshold of which is 0.01-0.025 ppm. The beverage is then in breach of European Framework Regulation No. 1935/2004 or the US Federal Food, Drug and Cosmetic Act. Both regulations ensure that the organoleptic quality of food remains the same. This is why some water bottlers have set limits for AA in preforms and bottles. In preforms, the limit is often 4 ppm. In Germany, the rule is much stricter at 2 ppm. AA blockers are used to achieve these low values. In addition, the International Agency for Research on Cancer (IARC) has classified AA as a possible carcinogen because it readily forms bonds with human DNA and is thought to have mutagenic and carcinogenic effects, particularly in combination with other substances. However, the limits of 6 ppm that apply in Switzerland and the EU are not exceeded during the production of PET bottles, from which we can be confident that the bottles do not pose any health hazards. Netstal launched its new PET line with sideways removal in September 2020. It is compatible with industry standards and thus offers the user a high level of flexibility. The main focus in the design of the machinery was to keep AA levels low because if the AA blocker is used to prevent migration of AA from the bottle wall into water, it is often a 2-aminobenzamide with an SML (specific migration limit) of up to 50 μg/l. This blocker is thought to contain hormone-like substances and should therefore be avoided. Furthermore, the use of an additional additive results in high costs. Particular attention therefore had to be paid to factors that affect the AA value. The formation of AA can be influenced by drying temperature, the amount of drying air, processing temperature, residence time in the dryer, plasticising or the individual mould. Tests carried out by Netstal have shown that residence time has by far the greatest impact on the AA level. If the plasticising process is too great in relation to the throughput, degradation and therefore the formation of AA are enhanced. For this reason, it is important to consider the scale of the plasticisation procedure, particularly for small, lightweight water preforms. Netstal has developed a completely new screw for its latest PET line whose design guides the shear into the melt in a controlled way and is also optimised in terms of residence time in the molten state. Material and thermal homogenisation takes place again at the tip of the screw. The melt then enters the shotpot with a uniform, thermal cross-section. By using the screw, which has extremely robust process properties, PET can be processed at temperatures around 20 K lower than before. In combination with the low shear, this means low AA values. The new screw design on its own has made it possible to reduce the use of AA blockers through a partner company that has already tested its effectiveness. The choice of the correct hot runner size or model can also affect the AA value. As with the plasticising process mentioned above, the residence time of the melt, shear and temperature control are also influencing factors in the hot runner’s AA value. The most important drivers that contribute to the formation of AA are the temperature and residence time of the melt in areas that have an elevated temperature. The primary task of the hot runner manifold is to distribute the melt flow simultaneously to all cavities at the same flow rate. When designing the balancing system, the volume, channel diameter, stagnation zones, together with deviations and temperature control, have an effect on preform quality or AA content. The balance between residence time, pressure loss and associated shear is critical in hot runner design. The residence time of the PET in the hot runner is a function of the preform weight and the volume of material in the manifold, including nozzle and cycle time. Preforms with low weights or long cycle times cause the melt to remain longer in the manifold, resulting in an increase in AA content. The hot runner manufacturer MHT selects the channels or volumes to suit a specific application as required. If the volume of the hot runner manifold is to be kept as low as possible, the Legacy model can demonstrate its full potential through short flow pathways or fewer manifold levels. However, flow channels cannot be reduced indefinitely as the reduced flow cross-section leads to increased pressure loss and thus to higher shear loads.
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