Monomers the basic building blocks

20.6.1 1,4 benzene dicarboxylic acid - terephthalic acid - TA (PTA)

Terephthalic acid (TA) (diacid) is now the major monomer used in the production of PET. It is produced on a world-wide scale and is available in a number of purities. These can be generally referred to as PTA (pure TA), MTA (medium purity TA) or CTA (crude TA). These descriptions are subjective and used in different ways by manufacturers. The purity of TA is the self-limiting process in PET manufacture. The major impurity in any TA grade is 4-carboxybenzaldehyde and it is a chain growth inhibitor. As the level of impurity increases less reaction can take place. From a food contact point of view the purity or impurities in the TA is important. Both US and EU law state that monomers should be of good technical quality but do not define what that technical quality should be. If no other information is available the

Polyethylene terephthalate (PET)

—O—CH—CH2—Of-C——C—O—CH2—CH2—O —

Fig. 20.5 The PET molecule.

product steward will need to give an opinion on the purity of the raw material and at the same time understand the business need to optimise costs. In attempting to lower the purity of the TA a business might decide to submit a petition to the appropriate legal authority, for example using the US FDA premarket notification for food contact substances process.4

Most petitions require a dossier of information on the substance and details of how to prepare these dossiers are given in ref. 5 for submission to the US authorities and in ref. 6 for Europe. One issue for both European and US authorities to consider is that although they require similar information to be submitted to the authorities, the information requirement is not the same and this can result in extra and unnecessary costs for the petitioner.

In Europe PTA has been given a specific migration limit (SML) of 7.5 mg/kg. This SML was established on the basis of a temporary tolerable daily intake (tTDI) as at the time of the drawing up of a European wide directive PTA was in common use in the manufacture of PET for food contact applications but did not have a fully prepared dossier and certain information on its safety was missing. During 2000 the European Commission (DG Sanco) told industry that all tTDIs would be removed and that the use of starting materials with tTDIs would be disallowed. Industry has since submitted the missing data to the European Food Safety Authority (EFSA) and has been requested to provide further information. At the time of writing the extra data requested is being generated by industry.

20.6.2 Dimethyl-1,4-benzenedicarboxylate - dimethyl terephthalate - DMT

Dimethylterephthalate (DMT) (ester) is the methyl salt of TA and is the other major monomer used to produce PET. DMT is not as efficient in its use in the production of PET, as at the start of the process an extra ester interchange reaction is required. This extra reaction uses a cobalt or manganese based catalyst and it generates methyl alcohol as a by-product. Methyl alcohol is highly flammable and consequently needs a specially adapted plant to handle this by-product. There are at least two occasions when DMT polymer plants have been destroyed due to accidents with methyl alcohol handling. DMT based polymer is generally more susceptible to yellowing in its downstream processing and has slightly different properties in use. DMT has been available for many years and as a chemical has been fully evaluated for its safety over that time period. There is much data available on DMT demonstrating its safety in use in food contact materials, and because of this and despite its processing difficulties DMT has been fully approved for use in the US and in Europe with its consequent tacit approval for use in food contact applications in the rest of the world.

20.6.3 Ethane-1,2-diol - monoethylene glycol - MEG

As explained above MEG (diol) is an organic base and the other major component of PET along with TA. MEG has been available for use in PET production for food contact for very many years. MEG is listed in the original US FDA regulation for PET (21CFR 177.1630) and has been approved for use in Europe for plastics intended for food contact applications since the first directive 90/128/EC. In Europe it is listed with a specific migration limit of 30 mg/kg; however, this SML must be measured as MEG plus diethylene glycol (DEG) and the total migration of the two materials must not exceed the 30 mg/kg limit.

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