PETpla.net Insider 04 / 2017

BOTTLE MAKING 30 PET planet Insider Vol. 18 No. 04/17 www.petpla.net .com 100815dmyL fied isophthalic acid), to replace part of the TPA. The co-polymers therefore have structures such as this: PET TETETE… PET-co-CHDM TETCTE… PET-co-IPA TEIETE… DEG, a by-product of the polymeri- zation reaction, is another comonomer which lowers the melt temperature but is not as effective at slowing down crystallization rates. DEG takes the place of EG in the chain. Several advantages are gained by using the co-polymer, especially in preform moulding applications: 1) Co-polymers crystallise more slowly than homopolymers, making it easier to fabricate clear preforms (see Crystallisation). 2) Co-polymers are easier to melt in the extruder as a result of the lower melting point and lower maximum degree of crystallinity. 3) Co-polymers impart better stress- crack resistance to the bottle (see Embrittlement and Stress Cracking). Some of the generalised effects of IV and co-polymer content are out- lined in table 2.1. Process/ Performance parameter Effect of increased IV Effect of increased co-polymer content Crystallisa- tion rate   Extruder motor load   Natural stretch ratio   Orientation   Side wall thickness   AA generat- ing potential   Aging rate   Stress crack resistance   Table 2.1 The generalised effects of IV and co-polymer content 2.2 Crystallisation of PET PET is a semi-crystalline resin. The word ‘crystalline’ refers to a region of ordered chain arrangement, as opposed to ‘amorphous’ where the polymer chains lack order (fig. 2.3). Melted PET, by definition, is amor- phous. Fig. 2.3 Rendering of a PET chain 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 semi-crystalline 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 perfect PET crystal is 1.455 g/cc. The density of a semi-crystalline sample with x fraction 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 Figure 2.2 An alcohol and an acid form the ester groups of PET that make it a polyester. 30 BOTTLE MAKING