PETpla.net Insider 05 / 2014

25 PET planet insider Vol. 15 No. 05/14 www.petpla.net MATERIAL / RECYCLING to rearrange themselves. The rate of crystallisation is a function of the temperature, the IV of the polymer, and any comonomer content. Poly- mers are very rarely able to crystallise completely. The temperature range for crystallisation is between the glass transition temperature (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 they are essentially locked in place regardless of whether they are in the 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 thermodynami- cally favoured structure, so the resin crystallises. Thermally induced crystals are arranged in structures called “spheru- lites” because they start from a point source (nucleation site) and grow, radiating outward, in a spherical pat- tern. This must be avoided because the crystallised regions cause haze, destroying the clarity of the preform, and prevent proper stretch blow moulding. Mould cooling becomes essential and determines the quality of the preform at this stage. An increase in IV reduces the rate of crystallisation by lengthening the polymer chains, making it more dif- ficult for a given chain to disentangle itself from other chains and form an ordered crystal. Copolymer content changes the molecular structure of the chains, inhibiting crystallisation by essentially introducing a unit into the chain that may not easily form crystals due to structural differences or disrupt the crystalline pattern. “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 direc- tion of stress, orienting the chains and imposing a linear ordered structure throughout the area the stress is Please order your copy at the PETplanet insider book shop: https://www.petpla.net/books Bottles, Preforms and Closures A Design Guide for PET Packaging Second Edition by Ottmar Brandau € 115,00 180 pages © Copyright Elsevier 2012 applied. This “extended chain” or stress-induced crystallinity is neces- sary in the blow-moulded container for mechanical strength. Summary PET occurs in three different states: Amorphous, non-oriented, and clear, such as preforms and melted plastic resin; thermally (by means of temperature) crystallised, such as resin pellets: strain-induced crystals, such as bottle sidewalls. PET is trans- formed 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. Therefore, PET appears white.  During the injection process, these crystals are melted, resulting in an amorphous melt, which is injected into the preform mould cavities. The preform is rapidly cooled down to avoid recrystallisation. Preforms therefore do not have a crystal structure. This state is called amor- phous. In the amorphous state the molecular chains show no orienta- tion and no crystallinity, and their appearance has been compared with a bowl of spaghetti. There is nothing to reflect light and therefore the PET is clear. It also has little strength or barrier properties.  In the reheat stretch blow machine, the material is forced by the stretch rod and blow air to orient in the axial and hoop direction, form- ing small, strain-induced crystals. 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 sidewall given the correct preform design and process condi- tions (higher crystallinity levels are achieved in the heat-set process). The finished bottle will have amorphous portions in the neck and gate area where the bottle was not stretched, oriented portions in the sidewalls, and sometimes thermally crystallised portions around the gate, a common preform defect that cannot be corrected during blow moulding. * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 2.2. Copyright Elsevier 2012