PETpla.net Insider 08 / 2017

BOTTLE MAKING 33 PET planet Insider Vol. 18 No. 08/17 www.petpla.net Figure 2.19 AA level vs. temperature. This relationship is exponential. Dia- gram courtesy of Shell. Temperature, therefore, plays a more important role in AA generation. To keep temperature at a mini- mum, processors should follow these procedures:  Maximum heater adjustment should be 290°C (554°F)  If there are percentage-controlled heaters such as is common for injection nozzles, they should be set at a minimum and raised only for start-up  All thermocouples need to be firmly inserted into the respective machine parts to avoid false (too low) readings  Injection time should correspond to 10–12 g/s/cavity, i.e., a 15g preform should inject in 1.5–1.8s to avoid shear stress  Back pressure should be set to a minimum, again to avoid shear stress – typical range is 15–20 bar (200–300 psi)  Screw speed should be set to a minimum. This can be done by measuring the time that the screw is not recovering (turning) during automatic cycle. This time should be between 1 and 2s. If it is longer, screw rpm can be reduced. Co-polymers have a lower melt temperature than homopolymers and should be exclusively used for the production of preforms for water bottles. Another resin feature should be low IV because high IV increases the melt viscosity of the resin requir- ing higher injection force, and thereby increasing shear stress. Typical IV values for water bottle preforms are 0.72–0.76. AA in water bottles The process by which AA may be released from the bottle walls into then beverage is quite complex and both humidity and temperature play a role over time, besides the initial AA level in the bottle (Fig. 2.20). Figure 2.20 AA migration over time Figure 2.21 AA migration vs. storage temperature Taste tests have shown that consumers can detect a level of 20–40ppb of AA in their drinks. Start- ing with the lowest possible AA level, it is paramount to delay this point in time as far out as possible. Tempera- ture also plays a role (see Fig. 2.21). This effect can easily be expe- rienced by leav- ing a water bottle on the dashboard of a car for a few weeks during the summer months. The water in this bottle will taste stale and have a light, unpleas- ant sweetness to it. Water bottlers have decreased the amount of AA they accept in their bot- tles. While 10 years ago 6 ppm was widely accepted, some companies now demand levels as low as 1.5 ppm. Machine manufac- turers have responded by fine-tuning the screw and hot runner designs and are able to deliver preforms at hose levels. There are two ways to measure AA level. The most common one is the ground-parison method. Preforms, bottles, or resin are cut into small pieces and ground in to particles smaller than 1mm. In order to avoid creating AA during grinding, the speci- men is cooled down with liquid nitro- gen before entering the grinder. The ground material is then placed in a closed glass vial and heated to 150°C (302°F) for 30 min. The prepared sample now contains head space with AA in it and it is this gas that is being measured in a gas chromatograph. The other method uses blown bot- tles. These are purged with nitrogen and stored for 24 hours. Then a fixed gas volume is extracted with a syringe and measured in the gas chromato- graph. It is apparent that both proce- dures are cumbersome and time-con- suming. With the development of 144, 192, and even 216-cavity preform