PETpla.net Insider 04 / 2017

BOTTLE MAKING 31 PET planet Insider Vol. 18 Please order your copy at the PETplanet Insider book shop: www.petpla.net/book3 Stretch Blow Molding Second Edition by Ottmar Brandau € 130,00 320 pages © Copyright Elsevier 2012 molecules are long and entangled, however, the amorphous state can be ‘frozen in’ by rapidly cooling the PET melt. Crystallisation can occur at any temperature at which the poly- mer chains have sufficient mobility to rearrange themselves. The rate of crystallisation is a function of the temperature, the IV of the polymer, and any co-monomer content. Very rarely, polymers are able to crystallise completely. The temperature range for crystallisation is between the glass transition tem- perature (Tg) and the melt temperature (Tm). Below Tg, the resin is described as being “glassy,” because the mobility of the polymer chains is greatly reduced and is essentially locked in place irrespective of whether they are in an amorphous or crystalline state. Above Tm, the polymer chains have too much energy to form stable ordered structures, and the molten resin is amorphous. Between Tg and Tm, the polymer chains have enough energy to rearrange them- selves into the most ther- modynamically favoured structure, so the resin crys- tallises. Thermally induced crys- tals are arranged in struc- tures called “spherulites,” since they start from a point source (nucleation site) and grow, radiating outward, in a spherical pattern. This must be avoided since the crystal- lised regions cause haze, destroying the clarity of the preform, and prevent proper stretch-blow moulding. The mould cooling becomes essential and determines the quality of the preform at this stage. An increase in IV reduces the rate of crystal- lisation by lengthening the polymer chains, making it more difficult for a given chain to disentangle itself from other chains and form an ordered crystal. Co- polymer content changes the molecular structure of the chains, inhibiting crystallisation by essentially introducing a unit into the chain which may not easily form crystals due to structural differences, or disrupts the crystalline pattern.