PETpla.net Insider 04 / 2017

BOTTLE MAKING 29 PET planet Insider Vol. 18 No. 04/17 www.petpla.net Stretch Blow Molding, part 2 Material Basics by Ottmar Brandau Ottmar Brandau has newly revised his book “Stretch 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 suc- cessive chapters in a series of articles. weights or chain lengths. As far as end users are concerned, the result is that the chemical structure of a grade of PET can be described by only two measures: IV (intrinsic viscos- ity), which is a measure of molecular weight, and the co-polymer content. In contrast, a polymer such as poly- ethylene can have unique molecular weight distributions and widely vary- ing degrees and types of branching, which affect processing and perfor- mance profoundly. Intrinsic Viscosity The properties of the PET polymer are largely dependent upon the aver- age molecular weight or the average number of repeat units of the polymer chains. This is usually determined by the measurement of intrinsic viscosity (IV). The relationship between molec- ular weight and IV is fairly linear. High IV PET has a higher molecu- lar weight than low IV PET. The longer chains give the resin better properties in the final product and also affect the processing in predictable ways. IVs used for PET bottle manufacturing are in the range of about 0.73–0.86. Co-polymer Content PET co-polymers are made by replacing a few percent of one of the starting components with a differ- ent monomer. Eastman uses CHDM (cyclohexane dimethanol) to replace part of the DEG. Most other resin manufacturers use IPA (Isophthalic Acid), which is also called PIA (puri- Manufacture of PET There are a few chemical routes to manufacturing PET. A compound with two acids, such as terephthalic acid (TPA), is esterified with a compound with two alcohols, such as ethylene glycol (EG) (see fig. 2.2). Since there are two functional groups on each component, they can continue to link up to form long chains. Water is a by-product of this process. This esterification reaction is reversible, and this is the key to under- standing much of the behaviour of PET. Commercially, the polymerisation is done in two stages. Melt-phase con- densation results in molten polymer with about 100 repeat units (intrinsic viscosity, as explained below, is about 0.6). The melt is pelletised and can be used for some applications such as fibre production at this point. To continue the polymerisation, a process called “solid stating” is used. Solid stating produces high-molecular- weight PET needed for fabricating bot- tles. Catalysts Different catalysts are required for the two main chemical routes to manufacture PET. Special catalyst combinations can be used to influ- ence the side reactions, to reduce he amount of diethylene glycol or allyl alcohol, or to improve the colour. Since the catalyst residues remain in the PET, they are still present during drying and processing. Therefore, different grades of PET from differ- ent manufacturers react differently if not processed at optimum condi- tions. For example, the DMT process (used chiefly by Eastman) requires an additional catalyst, which may result in a greater tendency for the resin to oxidise or ‘yellow’ when over-dried. PET is a Linear Condensation Polymer PET does not branch: each mol- ecule is a long “linear” chain. In addi- tion, because it is formed by a revers- ible condensation reaction, it has a very simple distribution of molecular