Additives production and processing additives

20.9.1 Catalysts

PET cannot be successfully produced commercially without the use of a catalyst. For ester interchange reactions (DMT process) the major catalyst used is a cobalt organic acid salt, or less prevalent are the manganese, calcium and zinc variants. For direct esterfication alkoxides of antimony, germanium and titanium are used. Currently there is no specific regulation covering the use of catalysts; however, in the US they fall under what is known as the basic polymer doctrine.8 The FDA stated many years ago that a 'basic polymer' is the product that results when a polymerisation process has been carried to commercial completion. Substances such as catalysts, chain regulators, chain transfer agents, and other materials used at low levels (generally 1 % or below) and required to produce the resin are considered part of the basic polymer and are not subject to independent regulatory consideration. This principle is often referred to as the 'basic polymer doctrine'. This basic polymer doctrine reflects the practical reality that the US FDA cannot write generic regulations for food packaging materials that specifically clear every substance that might be a trace component or contaminant of packaging materials. Where a substance is used during polymerisation in small quantities and either becomes a part of the resin or is otherwise removed from the resin at the conclusion of polymerisation its potential for migration is minimal. As there is no reasonable expectation of migration the substance is not considered a food additive.

In Europe aids to polymerisation are not yet regulated on a European wide basis and as such are covered by the Framework Regulation 1935/2004, by legislation in some member states and by guidance documents (i.e. the German Bundesinstitut für Risikobewertung - Empfehlung XVII -Polyterephthalsäurediolester). If there is no member state legislation for a proposed new catalyst then an individual risk assessment on that substance should be carried out. As previously suggested in this chapter, difficult regulatory situations such as this should be discussed with experts in the field. One PET catalyst is regulated at European level and that is antimony trioxide. As antimony performs a useful function in the final polymer, enhancing infra-red reheating of preforms, it is both an additive and a catalyst, and therefore subject to the existing EU legislation on food contact plastics.

Antimony trioxide is widely used in the PET industry and is subject to other EU regulations. To ensure that antimony trioxide could be used in food packaging without hindrance an industry consortium collected the data required and submitted a petition to the EU Scientific Committee on Food (SCF9*), which was replaced by the European Food Safety Agency (EFSA10t) in 2002. The SCF evaluated the data supplied and gave antimony trioxide an SML of 10 ppb with an analytical tolerance of 10 ppb. This was a disappointing result for the industry consortium who then asked the World Health Organisation (WHO) drinking water group to evaluate the data. The WHO evaluated the data and proposed a tolerable daily intake of 360 ppb of antimony trioxide and a drinking water level of 18 ppb. With this new information the industry consortium approached the newly established EFSA and asked for a reevaluation. This resulted in a raised SML of 40 ppb for antimony trioxide; the Directive 2005/79/EC of 18/11/2005 confirmed this increase in SML. At the time of writing there is also an EU risk assessment taking place on antimony trioxide and a lot of environmental and human health data is being collected and will soon become available.

* Answers scientific and technical questions concerning consumer health and food safety associated with the consumption of food products and in particular questions relating to toxicology and hygiene in the entire food production chain, nutrition, and applications of agrifood technologies, as well as those relating to materials coming into contact with foodstuffs, such as packaging.

fSet up provisionally in Brussels in 2002, EFSA provides independent scientific advice on all matters linked to food and feed safety - including animal health and welfare and plant protection - and provides scientific advice on nutrition in relation to Community legislation. The Authority communicates with the public in an open and transparent way on all matters within its remit. EFSA's risk assessments provide risk managers (consisting of EU institutions with political accountability, i.e., European Commission, European Parliament and Council) with a sound scientific basis for defining policy-driven legislative or regulatory measures required to ensure a high level of consumer protection with regards to food safety.

440 Chemical migration and food contact materials 20.9.2 Processing aids

DEG control

Very small amounts (low ppm) of alkali, alkaline earth metal salts or quaternary ammonium compounds are added to the reactors to control the amounts of DEG that are generated in the polymerisation reaction. The use of these materials in the manufacture of polymers for food contact packaging is covered under the basic polymer doctrine for use in the US, by EU legislation and legislation in some member states.

Polycondensation degradation

Melt stabilisers such as phosphoric acid and its salts and esters are added to the polymerisation process to reduce thermal degradation and colour formation (yellow). They also have the added effect of stabilising the polymer when it is being later processed into food packaging. Many of the phosphorus based stabilisers are listed in 2002/72/EC for use in food contact applications and are covered by the basic polymer doctrine of the US FDA.

Colour management

The colour of PET is measured instrumentally on a three-dimensional scale (see Fig. 20.6). The vertical axis is known as the L scale and is generally recorded as Hunter units (Lh) in the US and CIE Lab (L*) in Europe. This scale measures black/whiteness with the higher the number the 'whiter' the polymer. The two horizontal axes measure red/green (ah and a*) with the higher the number the redder the polymer, and blue/yellow (bh and b*) with the higher the number the yellower the polymer. In an ideal world a PET polymer would have a highish L*/Lh (~80), a middle ah/a* (~0) and a slightly negative bh/b* (-1.5),

The manufacturing process for PET can take many hours and as a result the polymer is subjected to high temperatures for extended periods of time.


100 L





0 L Black

Fig. 20.6 The three dimensions of colour.

It is therefore not surprising that there is degradation in the final polymer. This degradation takes the form of a yellowing of the polymer or an increase in the bh/b* number. To control the yellowness of the polymer low levels of cobalt acetate (blue) or even lower levels of synthetic dyes (blue/red) are added to the polymer. In the US the use of cobalt acetate falls within the US basic polymer doctrine, and in Europe it is covered by a risk assessment using the framework legislation 2004/1935/EC, national legislation and a very low SML which is less than 0.05 mg/kg. The dyes are authorised for use by current US FDA regulations on colorants and by some member states (e.g. the 'French positive list') approvals in Europe. One other aspect of colour management is the use of re-heat substances in the polymer and these are discussed later.

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