Degradation products and impurities

Other substances may be present in food contact plastics that were not originally intended to be present in the finished material or article, but arise from reactions during polymerisation or processing. Or they may be present as

Table 10.5 Substances commonly used in the manufacture of PET/PEN

Substance name

PM ref. no.


Restriction SML

Ethylene glycol



(T) 30 mg/kg

Diethylene glycol



(T) 30 mg/kg

Isophthalic acid



5 mg/kg

Terephthalic acid



7.5 mg/kg

2,6-naphthalene dicarboxylic acid,



0.05 mg/kg

dimethyl ester

2,6-naphthalene dicarboxylic acid



5 mg/kg




(T) 6 mg/kg

Adipic acid, bis(2-ethylhexyl)ester


Carrier for

18 mg/kg





0.05 mg/kg








Antimony trioxide



(T) = total migration of two or more moieties.

as Sb

(T) = total migration of two or more moieties.

impurities in one of the starting substances or additives. In Europe when a new additive or monomer has been developed for use in food contact applications, and it is not already positively listed or listed nationally, a dossier must be submitted to the European Food Safety Authority (EFSA) before it can be used. The dossier should contain sufficient information on the manufacture, uses and properties of the substance for EFSA to be able to make a safety evaluation (risk assessment). In particular, information on the migration of the substance into foods and/or food simulants needs to be provided. The level of migration will usually dictate the amount of toxicological testing required and as toxicological testing can be a costly exercise, migration testing is normally the first step.

When dossiers for new plastics' additives are submitted to EFSA for evaluation, it is a requirement for the petitioner to provide information on the technical effect of the additive in the polymer and any impurities that may be present and degradation products that arise in use (Note for Guidance, reference 6). The resulting SML may be a combination of those for the starting additive plus its degradation products. An example of this is 2,4,6-tris(tert-butyl)phenyl-2-butyl-2-ethyl-1,3-propanediol phosphite (PM ref. no. 95270) with an SML of 2 mg/kg that is the sum of the phosphite, phosphate and phenol individual migration values. Substances evaluated a long time ago may not have had such a thorough evaluation because degradation products and some impurities were not considered. It is also possible that a new substance will degrade partly or completely during a migration test and in this case a 'substitute test' may be more appropriate. In considering the migration of a new substance into food and food simulants, it is important to consider any impurities and degradation products.

Many of the reaction products derived from stabilisers can be predicted because they are added to react with free radicals or chemicals released during processing of the polymer. Many plastics will react with oxygen during processing and if additives are not present to reduce this autoxidation, the polymer will undergo 'ageing' reactions that have detrimental effects on their properties, such as discolouration, surface cracking and poorer physical properties such as impact strength. The most important primary antioxidants used in food contact polymers are usually based on hindered phenols. These react by interfering with the chain propagation reactions, where oxygen reacts with an alkyl free radical in the polymer chain (P") to form a peroxy radical (PO 2). These propagation reactions tend to occur when the polymer is at high temperature (for example during processing) or under stress and where a certain amount of oxygen is available.

A simple primary antioxidant is BHT (2,6-di-tert-butyl-p-cresol, PM ref. no. 46640). However, its use in food contact applications has declined somewhat recently as it has a low SML of 3 mg/kg and a relatively high migration into fatty food compared to some of its higher molecular weight rivals. Figure 10.5 illustrates the reaction of BHT with a free radical that in this example is designated PO 2, that is responsible for propagation reactions in the polymer giving oxidation products. Figure 10.6 illustrates the structure of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, a more commonly used hindered phenol antioxidant used in polyolefins and polystyrene. Secondary antioxidants decompose hydroperoxides and remove peroxide radicals as they are formed, without producing free radical intermediates, and prevent chain branching occurring. Phosphites such as tris(t-butylphenyl)phosphite (Fig. 10.7) are oxidised to form phosphates. Phosphites also react with water to form phenols. Therefore in tests carried out to assess the migration of a phosphite stabiliser, measurements of the corresponding phosphate and phenol should be included.

Fig. 10.5 BHT reaction with free radical PO'2 (P = polymer).

Fig. 10.6 Structure of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate.

When screening polyolefin food packaging for additives by GC/MS, it is common to see two peaks, one for tris(t-butylphenyl)phosphite, base ion m/e 441, and the other for the corresponding phosphate, base ion m/e 316. Some secondary antioxidants are a mixture of compounds, such as tetrakis(2,4-di-tert-butylphenyl)[1,1-biphenyl]-4,4'diylbisphosphonite (P-EPQ), PM ref. no. 83595, one of six reaction products7 of di-tert-butylphosphonite with biphenyl. Where there are a number of potential degradation products, although it is more convenient to concentrate on the major components, a minor component might be the most likely to migrate. Amongst reaction products of di-tert-butylphosphonite with biphenyl, the minor product 2,4-di-tertbutylphenol will have a higher migration rate into food because of its significantly lower molecular weight.

Another class of secondary antioxidants used in food contact plastics is thioether. The most common examples used in polypropylene, polystyrene and PVC are thiodipropanoic acid, didodecyl ester (DLTDP) and thiodipropanoic acid, dioctadecyl ester (DSTDP). Thioethers react with hydroperoxides to form sulphoxides as shown in Fig. 10.8.

Use of both primary and secondary antioxidants usually provides a synergistic effect, where the combined effect of two or more stabilisers is greater than the sum of the effects of the individual stabilisers. It is common practice to include both a phosphite, such as tris(t-butylphenyl)phosphite and a hindered phenol, such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate to provide improved heat stabilisation in polyolefin formulations.

Synergistic systems are also widely used in PVC formulations where additives are present to react with labile chlorine atoms in the polymer chain and scavenge hydrogen chloride that may be generated due to thermal degradation during processing. The resulting small concentrations of HCl, if left to remain would cause further HCl 'unzipping' degradation reactions of

Fig. 10.7 Oxidation of tris(t-butylphenyl)phosphite to tris(t-butylphenyl)phosphate.

the polymer (autocatalytic chain reactions). Epoxides are one group of chemicals that react rapidly with HCl to form chlorohydrins. Epoxidised soybean oil (ESBO) is a common plastics additive useful for this purpose. It is often present at levels of about 5% in PVC compounds. In some applications, such as 'Press to seal, twist to open' (PT™) closures for glass jars, ESBO is present at much higher levels (~ 35%) as it also acts as a plasticiser.

Calcium/zinc stearates are very effective stabilisers and can be used in conjunction with ESBO. They react with HCl to form calcium and zinc monochlorides Ca(OCOR)Cl with HCl. Other HCl scavengers used in PVC include tin stabilisers, although these are not used in plasticised PVC formulations. Typical tin stabilisers are based upon dioctyl- or dimethyl-tins and degrade to form a range of products with HCl. Most tin stabilisers have very low SMLs, as low as 0.006 mg/kg, expressed as total tin. Salts of 2-ethylhexanoic acid are also used as heat stabilisers in PVC compounds and can degrade to give 2-ethylhexanoic acid which has been shown to migrate from sealing compounds into fruit juices and baby food.8 Use of these stabilisers is being phased out owing to suspected undesirable toxicological properties of the substance.

Impurities in polymers generally originate from the starting substances used in synthesising monomers or additives. It is clear from the EU 'Practical Guide'9 that impurities in authorised substances themselves do not require specific authorisation, but they should comply with the general provisions of the EU Framework Regulation.1 In some cases impurities are specified with restrictions as it is possible that they accumulate in the polymer and do not get bound into the polymer structure. Therefore they may easily migrate, especially if they have a low molecular weight. For example, ethylbenzene, the precursor for manufacture of styrene is an impurity that often remains in finished polystyrene at levels around 10 mg/kg. Other impurities can be present in antioxidants and plasticisers and would probably arise from their precursors.

Other impurities may arise from additives that are not covered yet by the plastics legislation or are not evaluated by EFSA, such as polymerisation production aids and solvents. Although these substances are not intended to remain in the finished plastics, low levels may be detected when looking at sub-parts per million levels.

Oligomers is another class of substances that does not need to be separately authorised, if the monomers are already authorised. In the 'Note for Guidance', EFSA request that oligomers with a molecular weight below 1000 daltons be characterised, although identification may not be required. In most cases it is assumed that oligomers tend to be less toxic that the starting monomer(s). However, it is commonplace that concentrations in the polymer are correspondingly higher. This is certainly the case for PET, where the cyclic trimer can be present at 1-2% levels in the polymer.10 Indeed, styrene dimer and trimer are usually present at significantly higher levels than styrene in polystyrene materials. Some work has also been carried out on the measurement of caprolactam and laurolactam oligomers in nylons.1112

The migration of oligomers will decrease with increasing molecular weight, Fig. 10.9 illustrates the correlation of migrant molecular weight against migration into food from polypropylene at 40 °C for ten days. In this example it is assumed that the migrant is highly soluble in the food and that the initial concentration of the fictive substance in the polymer is 0.2%.

Catalysts are currently not covered specifically by EU directives on food contact plastics. They tend to decompose during the polymerisation process. Again, the degradation products are often predictable and may sometimes be found in the finished food contact material. However, catalysts are usually present at low levels and the degradation products are often volatile. For example, the common catalyst t-butyl perbenzoate may decompose to give benzene when used in some thermoset polymers.13 Tert-butyl peroxide is used as a catalyst in certain polymers and will decompose to give tert-butanol.

The blowing agent azodicarbonamide (PM ref. no. 36640) acts by releasing nitrogen gas during the blowing process. During the decomposition of azodicarbonamide, semicarbazide (SEM) can also be released and can migrate into foods. Highest concentrations (up to 25 ppb) were found in some baby foods packed in glass jars with PVC gaskets and, owing to the considerable unknowns relating to SEM's toxicological properties and exposure, it was judged by EFSA to be undesirable in baby food. Directive 2004/1/EC prohibited the use of azodicarbonamide in food contact materials from August 2005. It is also prohibited as a food additive in the EU following doubts about its several degradation products.

160 140

"g 100

J 60

1000 1500 2000

Molecular weight of migrant


Fig. 10.9 Correlation of migrant molecular weight and migration.

248 Chemical migration and food contact materials 10.5 Future trends

Future trends in food contact plastics are most likely to be directed at developing more environmentally friendly or sustainable materials, such as those that are biodegradeable and plant derived, to reduce the negative environmental impacts created by landfill and incineration of plastics.14 The driving force here is EU Directive 2004/12/EC, which amends Directive 94/62/EC on packaging and packaging waste, and approximately doubles packaging recycling targets and strengthens the target for recovery. There is also an ever increasing trend in the development and use of active and intelligent packaging with the associated benefits of increased food safety for the consumer. In particular, the availability of oxygen scavengers that can be incorporated into inner layers has been a key element in the design of new PET beer bottles.

In the field of antimicrobial food packaging, technology suppliers are developing systems to target bacteria on the surface of food.15 An antimicrobial chemical is intended to be immobilised in the packaging film and so not migrate into food (bread or cheese for example), but prevent any bacterial growth on food particles trapped in microscopic fissures in the surface contact layer. Only those antimicrobials on a positive biocides list for food contact will be permitted for use, provided also that they are effective in this particular type of application. Appropriate labelling of biocide-containing food packaging will also be required to communicate usage. EFSA will be assessing any risk of these biocides using the additional considerations outlined in 'Note for Guidance' - in particular evidence should be provided showing that:

• any migration into food is not intentional but only incidental

• its use does not exert any preservative effect on the food

• its use does not allow the selection of non-sensitive organisms on the food contact materials

• it does not allow the development of biocide resistance in sensitive microorganisms.

In addition, the petitioner should provide evidence that the substance is not used to replace the normal hygienic measures required in handling foodstuffs.

At present, nanotechnology is being applied in plastics packaging to a limited degree to improve the properties of materials and increase the efficiency of making packaging.16 Potential benefits include improved barrier properties (delivering longer shelf life or allowing material substitution) and better temperature performance using titanium, zinc, aluminium and iron oxides. Applications are also being developed in areas of active and antimicrobial food packaging. Nanotechnology is expected to be a major growth area in coming years in all food contact materials, and there is a major EU funded project in this area, 'SustainPack', with a budget of €36m of which €19m is being provided by the EU's Sixth Framework Research Programme. However, at present little is known about the toxicity or migration of nanoparticulates and substances used in the manufacture of nanocomposites.

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