PETpla.net Insider 06 / 2017

BOTTLE MAKING 25 PET planet Insider Vol. 18 No. 06/17 www.petpla.net Stretch Blow Molding, part 3 Material Basics / Drying of PET by Ottmar Brandau Ottmar Brandau has newly revised his book “Strech Blow Molding”, first published by hbmedia / PETplanet Publisher, and now re-issued in a second edition under the Elsevier imprint. PETplanet Insider is publishing extracts from successive chapters in a series of articles.  During the injection process, these crystals are melted, result- ing in an amorphous melt which is injected into preform mould cavities. The preform is rapidly cooled down to avoid re-crystal- lisation. Preforms, therefore, do not have a crystal structure – this state is called amorphous state. In the amorphous state, the molecular chains show no ori- entation and no crystallinity, and their appearance has been com- pared with a bowl of spaghetti. There is nothing to reflect light and therefore the PET is clear. It also has minimal strength and barrier properties.  In the reheat stretch-blow machine (RSBM), the material is forced by the stretch rod and blow air to orient in the axial and hoop direction, forming small, strain-induced crys- tals. These crystals do not reflect light and the bottle appears clear. It also has higher strength and barrier properties. Crystallisation levels of up to 25% can be achieved in the bottle side wall, given the correct “Extended chain” or “oriented” crystallisation During stretch-blow moulding, the amorphous chains in the preform are stretched and oriented, and a differ- ent form of crystallinity is developed. The chains are aligned in the direction of stress, orienting themselves and imposing a linear ordered structure throughout the area of applied stress. This ‘extended chain’ or stress- induced crystallinity is necessary in the blow-moulded container for mechanical strength. Summary PET occurs in three different states:  Amorphous, nonoriented, and clear, such as preforms and melted plastic resin  Thermally (by means of tempera- ture) crystallised, such as resin pellets  Strain-induced crystallized, such as bottle side walls PET is transformed several times as it goes from pellet to preform to bottle.  As resin pellets, PET is thermally crystallised to a level of 50–70%. Thermally induced crystals are arranged in large structures called spherulites, which reflect light, making PET appear white.