PETpla.net Insider 05 / 2014

24 PET planet insider Vol. 15 No. 05/14 www.petpla.net MATERIAL / RECYCLING to the type of reaction where many monomer units are chemically linked to form polymers (“polys” many). A resin with only one type of monomer is called a homopolymer. Copolymer resins are the result of modifying the homopolymer chain with varying amounts of a second mono- mer (or comonomer) to change some of the performance properties of the resin. Manufacture of PET There are a few chemical routes to manufacture PET, but basically a compound with two acids, such as terephthalic acid (TPA), is esterified with a compound with two alcohols, ethylene glycol (EG). Because 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 esterifi- cation reaction is reversible, and this is the key to understand 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 (IV), as explained subse- quently, is about 0.6). The melt is pelletised and can be used for some applications such as in fibre at this point. To continue the polymerisation, a process called “solid stating” is needed. Solid stating produces high molecular weight PET needed for fab- ricating bottles. 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 the amount of diethylene glycol (DEG) or acetaldehyde (AA), or to improve the colour. Because the catalyst residues remain in the PET, they are still present during drying and pro- cessing. Therefore, different grades of PET from different manufacturers react differently if not processed at optimum conditions. For example, the dimethyl terephthalate (DMT) process (used chiefly by Eastman) requires an additional catalyst, which may result in a greater tendency of the resin to oxidise or “yellow” when overdried. PET- a linear condensation polymer PET does not branch: each mo- lecule is a long “linear” chain. In addi- tion, because it is formed by a rever- sible condensation reaction, it has a very simple distribution of molecular weights or chain lengths. The result as far as end users are concerned is that the chemical structure of a grade of PET can be described quite completely by only two measures: IV, which is a measure of molecular weight, and the copolymer 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 on the aver- age molecular weight or the average number of repeat units of the polymer chains. This is usually determined by measurement of the intrinsic viscosity, or IV, as explained later. The relation- ship between molecular 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 proper- ties in the final product but also affect the processing in predictable ways. The range of IVs used for PET bottle manufacturing is from about 0.73 to 0.86. Copolymer content PET copolymers are made by replacing a few per cent of one of the starting components with a different monomer. Eastman uses cyclohexane dimethanol (CHDM) to replace part of the EG. Most other resin manufactur- ers use IPA (isophthalic acid), which is also called purified isophthalic acid, to replace part of the TPA. DEG, a by-product of the polymeri- sation reaction, is another comonomer that lowers the melt temperature but is not as effective at slowing down crys- tallisation rates. DEG takes the place of EG in the chain. Several advantages are gained by using the copolymer especially in preform moulding applications:  Copolymers crystallise more slowly than homopolymers, making it easier to fabricate clear preforms (see Chapter 2.2.4).  Copolymers are easier to melt in the extruder as a result of the lower melting point and lower maximum degree of crystallinity.  Copolymers impart better stress- crack resistance to the bottle. (Some of the generalised effects of IV and copolymer content are out- lined in the book.) Crystallisation of PET PET is a semicrystalline resin. The word “crystalline” refers to a region of ordered chain arrangement, as opposed to “amorphous,” where the polymer chains lack order. Melted PET, by definition, is amorphous. When polymers are in an amor- phous state, the molecular chains can be compared to a tangled web of spa- ghetti or springs. The analogy to tan- gled, stretched springs is particularly suitable for semicrystalline polymers because under certain conditions the polymer chains tend to coil into ordered structures, forming crystal- line regions. The repeating units of the homopolymer chain fit together neatly, forming a close-packed array, which has a higher density than the amor- phous state. Density measurement is com- monly used to determine the degree of crystallinity. At room temperature, amorphous PET has a density of 1.335g/cc. The calculated density of a perfect PET crystal is 1.455g/cc. The density of a semicrystalline sample with x fraction of crystallinity is: 1 x (1-x) p = 1.455 + 1.335 The crystal structure has a lower energy state than the amorphous arrangement, so it is the favoured arrangement. Because polymer 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

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