PETpla.net Insider 03 / 2015

PREFORM PRODUCTION 46 PET planet insider Vol. 16 No. 03/15 www.petpla.net Temperature therefore plays a more important role in AA generation. To keep temperature at a minimum, processors should follow these proce- dures:  Maximum heater adjustment should be 290°C (554 °F).  If there are percentage controlled heaters such as those com¬mon for injection nozzles, they should be set at a minimum and only raised for start-up.  All thermocouples need to be firmly inserted into the respec¬tive machine parts to avoid false (too low) readings.  Injection time should correspond to 10–12 g/s per cavity, i.e., a 15g preform should inject in 1.5–1.8 s to avoid shear stress.  Back pressure should be set to a minimum, again to avoid shear stress. The 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 (turn¬ing) during the automatic cycle. This time should be between 1 and 2 s. If it is longer, screw rpm can be reduced. Copolymers have a lower melt temperature than homopolymers (see Chapter 3.1) and should be exclu- sively used for the production of pre- forms for water bottles. Another resin feature should be low IV because high IV increases the melt viscosity of the resin, requiring higher injec¬tion force and thereby increasing shear stress. Typical IV values for water bottle pre- forms are 0.72–0.76. 3.6.2 AAinWater Bottles The process by which AA may be released from the bottle walls into the beverage is quite complex and both humidity and temperature play a role over time besides the initial AA level in the bottle. Taste tests have shown that con- sumers can detect a level of 20 to 40 parts per billion (ppb) of AA in their drinks. Starting with the lowest pos- sible AA level is paramount to delay this point in time as far out as possi- ble. Therefore, the initial level should be as small as possible. Experiments have shown that the 40 ppb threshold is reached after 350 days when the initial level was 2.5 ppm but as quickly as 120 days when that level was 3.5 ppm to begin with. Temperature also plays a role. The above numbers were obtained by keeping the product at 22°C (72 °F). Raising it to 29°F (84 °F) reduces the time to get to 40 ppb from 120 to 70 days while keeping the temperature as low as 15°C (59 °F) would extend it to 250 days. This effect can easily be expe- rienced by leaving a water bottle Figure 3.19