Use of functional barriers

Instead of using 'super-clean' recycling technologies which remove, or reduce substantially, the amount of post-consumer compounds and contaminants in the polymer down to similar levels to those in virgin polymers, so-called functional barrier packaging systems can also be efficiently applied to achieve the same effect. No or only negligibly low migration levels of any unwanted foreign compounds can be achieved.

A functional barrier can be generally defined as a package construction that limits the extent of migration of a component from the package to food or a food simulant in amounts below a threshold value.19 This value is usually established by regulatory institutions and is generally derived from toxicological evaluations. In the case of unknown contaminants as potential migrants from recycled materials US FDA's Threshold of Regulation concept may be applied where there is some certainty that the legal authorities will accept the application of the principle. It applies a general dietary concentration of 0.5 ppb (mg kg1) as the threshold which is derived from toxicological data on oral feeding studies.20-22 According to this concept the migration of any non-carcinogenic compound leading to dietary concentrations equal or lower than the threshold is not considered a significant health risk.

From recent investigations on the nature and quantities of typical postconsumer contaminants5'7'8'11'23 the presence or influence of carcinogenic principles can be largely excluded. Therefore, a package construction containing recycled plastics material which is usually achieved by a multi-layer structure (e.g. a sandwich structure where the food contact layer consists of virgin material) is compliant with US FDA's food packaging regulations as long as the threshold is not exceeded. Lacking such a concept in Europe, this approach is very useful to demonstrate or justify compliance with the general requirements of Article 3 of the European Framework Regulation 1935/ 2004/EC24 according to which the food contact materials and articles should not endanger human health.

It is generally known25 that only a very limited number of packaging materials such as glass or metal provide absolute protection properties concerning the penetration of chemical compounds from layers behind or from the environment. In the case of multi-layers with plastics materials as functional barriers there occurs, in most cases to a certain extent, an unavoidable mass transfer from the plastics layers into the product. This must be understood as a functional quantity which, however, must comply with food regulations. Therefore it is necessary firstly to understand functional barrier characteristics and mechanisms and, secondly, to define the functional barrier efficiency in relation to food safety and to establish appropriate test methods. This is especially important with those food packaging applications where recycled plastics are covered by plastics functional barriers.

Over the last decade numerous publications have dealt with the functional barrier concept from different points of view and with different scientific and pragmatic intentions.1926-44 Many of these papers have discussed and proposed theoretical treatments and approaches to this issue and developed diffusion theory based mathematical models to describe the mass transfer processes and the developing concentration profiles in the packaging system. In the first theoretical treatments of the functional barrier one important point was not, or not sufficiently, taken into account. It was assumed that the functional barrier layer made from virgin materials was free of contaminants just after manufacture of the package. However, since multi-layer plastic structures are mostly manufactured under coextrusion conditions, where extreme temperatures far above the melting point of the plastic are applied, a significant inter-diffusion between the in-situ formed polymer layers occurs in reality.

Taking co-extrusion temperatures up to 280 °C into account, it can be estimated, in relation to the polymer type and thickness, that middle layer contaminants are penetrating the functional barrier layer partially or completely within a time range of seconds down to fractions of one second. As a consequence, more or less significant contamination of a 'virgin' functional barrier layer is likely to occur during manufacture. This compromises the originally designed functional barrier efficiency to a reduced efficiency and could even result in the possibility of complete penetration, with the consequence of having direct food contact with the contaminants originating from the middle layer at the start of migration, i.e., after the time when the foodstuff is filled into the packaging. Other papers have investigated this question experimentally and taken the in-situ contamination into account for their modelling approach. Any mathematical model which does not consider this physical process of in-situ contamination would underestimate the actual amount of migration. It should be noted that the same effect, i.e., a reduction of the functional barrier efficiency, could occur when multi-layer packaging sheets are stored for long periods before they are used to pack food.

As a consequence, three principally different situations for a functional barrier packaging system can be assumed as depicted in Fig. 9.2. The corresponding kinetic migration characteristics are outlined in Fig. 9.3 which categorises the kinetic migration behaviour possibilities for a migrant at time t = 0 (for instance the time of package fill) into three typical cases:

1. Clean functional barrier: full lag time of the functional barrier takes effect.

2. Contamination functional barrier: depending on the degree of contamination of the functional barrier, only reduced lag time available.

3. Fully contaminated functional barrier: direct contact and no lag time.

As opposed to absolute barriers such as an aluminium layer of at least 6 to 7 mm, the effectiveness of functional barrier systems is related to a 'functional' quantity in terms of mass transfer, which is dependent on the technological and application-related parameters of the respective food-package system.45 These parameters are:

• manufacture conditions of the package (e.g. high temperatures applied)

• type of functional barrier plastic

Status of FB at time

Fig. 9.2 Possible levels of contamination of functional barrier packaging structures at time of package fill (t = 0).

Status of FB at time






° o


o ° <

0 ,

O 0


Fig. 9.2 Possible levels of contamination of functional barrier packaging structures at time of package fill (t = 0).

Lag time (clean FB)

Lag time (clean FB)

Lag time (contamination FB)

Fig. 9.3 Possible migration behaviour characteristics of migrants from functional barrier packaging structures dependent on status of FB at time of package fill (t = 0).

• thickness of the functional barrier layer

• molecular weight and chemical structure of contaminants

• concentration and mobility of contaminants in the matrix behind the functional barrier

• time between manufacture of package and filling

• type of foodstuff, i.e., fat content, polarity etc.

• filling conditions and storage (time, temperature) of the packed foodstuffs.

Within a European project FAIR-CT98-4318 'Recyclability'5,33 a comprehensive work programme has been carried out to systematically study determinants in most of these functional barriers. One major aim of this project was to develop screening procedures of barrier properties of polymers to evaluate whether they are likely to behave as functional barriers and to elaborate testing procedures for functional barriers and multilayer materials. Another aim was to provide tools based on predictive, validated approaches, thus allowing optimisation of the functional barrier packaging structures and reduction of the need for testing to a minimised extent. To establish the necessary experimental data sets with respect to diffusivities of numerous packaging plastics from storage up to extrusion conditions, suitable chemical compounds have been used as surrogates to test general functional barrier behaviour.

The results of these comprehensive studies have been recently published in two papers.43'44 To study functional barrier contamination effects taking place during manufacture when multi-layers are produced at high temperatures, and when the molten polymer layers are put in contact together, methods were elaborated to determine diffusion coefficients in molten polymers. It was found that diffusion in the melt of (at ambient temperature) glassy polymers is much slower than in (at ambient temperature) rubbery polymers. Intrinsic diffusion coefficients in normal storage and service conditions were measured using Moisan type tests46 for which a particular three-layer test with solid-solid plastics contact was designed. It was found that interface effects, (e.g. associated to a poor solubility of a migrant in the (liquid) food simulant) can strongly influence both the lag time and migration. For migration into aqueous media, an increase in the hydrophobic character of migrants results in a decrease of migration at equilibrium and an increase of the apparent lag time.

Finally, for the simulation of migration, a numerical model was developed. This model takes into account a stepwise migration, first during processing at high temperatures (programme 'multitemp'), then during storage of the empty package or after filling (programme 'multiwise'). A database of diffusion coefficients was proposed for a broad range of polymers. Since other parameters such as partition coefficients K (understood as the ratio at equilibrium of the migrants concentration in food simulant and the concentration in polymer) and the mass transfer coefficient at the food-polymer interface H needed for the calculation are not always available, so it is recommended to work with default parameters (K infinite, H infinite) because this leads to an overestimated predicted value, which favours consumer protection. Based on determined activation energies for diffusion other quick tools for functional barrier efficiency testing, such as accelerated tests with time-temperature correlated acceleration factors (manufacture process and storage), have also been proposed.

Another study on the migration behaviour in flexible thin multi-layer structures has recently been carried out and results have been presented.47 One of the essential findings was that testing of thin multi-layer films with current recognised and accepted alternative fat simulating liquids such as 95% ethanol and iso-octane, lead in many cases to too strong interactions which can cause swelling effects within the packaging structures and lead to increased diffusion. One example is shown in Fig. 9.4 where the migration kinetics of an organic migrant from a PET-PA-PE film is shown for three different temperatures.

The migrant was initially used as an additive in the adhesive between the PET and PA layer and the migration test was carried such that the PE layer was in contact with the food simulant, 95% ethanol. In this case, the efficiency of the PA layer as a functional barrier was investigated. This situation is totally analogous to the hypothetical case that PCR PET was used and the test migrant was a contaminant in the PCR PET. Therefore the investigations on this multi-layer can be considered as very instructive in relation to the question, 'how should functional barrier efficiency tests on recycled plastics packaging applications be conducted?'. In Fig. 9.4 it can be recognised that at room temperature the PA layer acts as a very efficient FB whereas at 40 °C a lag time phase is observed with reduced functional barrier efficiency and


Fig. 9.4 Kinetic migration behaviour of an organic substance (initially used in the adhesive between the PET and PA layer) from a PET-PA-PE multi-layer structure into 95% ethanol in contact with the PE layer at different temperatures (20 °C, 40 °C and 60 °C).

at 60 °C the functional barrier effect is completely lost. The reason for this accelerated and exaggerated migration behaviour at 60 °C is due to interactions between the food simulant and the PA layer. With regard to the practical use for functional barrier efficiency testing it can be stated that with known relationships between the migration kinetics at the three applied test temperatures one can derive a very quick test at exaggerated conditions, for instance, at 60 °C or at 40 °C, to conclude whether or not the functional barrier efficiency is adequate under package storage and service conditions and in relation to a certain acceptable specific migration limit relevant for the migrant of interest.

As another test approach to meet any delays in migration due to functional barrier effects, the artificial ageing of a multi-layer structure was considered. The motivation for this idea was to replicate potential migration processes which would take place after the manufacture of a functional barrier package until the time of filling. Following ageing, the functional barrier structure could then be tested using the usually applied conventional migration test conditions according to relevant EU directives. To investigate this test strategy, in the study mentioned, the same test film as described above and shown in Fig. 9.4 was stored for 21 months at room temperature and tested again. In addition, this test film was finally artificially aged by a one week 60 °C conditioning. In all cases the migration behaviour at 40 °C was measured to compare if at all, and how, the effects of the 'dry' storage and hot treatment of the test film would behave. The results are shown in Fig. 9.5. It can be recognised that all tests provided the same kinetic lag time behaviour. This can be explained only by the understanding that the test film was already, after the manufacturing process (where high temperatures had been applied), in the thermodynamically preferred equilibrium situation which did not change

Fig. 9.5 Influence of 21 months room temperature storage (B) and artificial ageing for one week at 60 °C (C) on the kinetic migration behaviour of a PET-PA-PE multilayer film as depicted in Fig. 9.4 at 40 °C (A).

Fig. 9.5 Influence of 21 months room temperature storage (B) and artificial ageing for one week at 60 °C (C) on the kinetic migration behaviour of a PET-PA-PE multilayer film as depicted in Fig. 9.4 at 40 °C (A).

again after long-term storage or even short treatment at elevated temperature (60 °C). This and other results that were obtained showed that the partitioning effects that occur during manufacture under coextrusion conditions overwhelm any sequential effects by a later ageing of the test film. It was concluded that, at least for the usual thin flexible multi-layer structures, ageing appears to be unnecessary.

Finally, the current available elaborated knowledge and the proposed test methodologies are suitable to serve for national or federal authorities and industry as a basis for safety evaluation, establishing criteria and guidelines for the appropriate functional barrier protection design of recycled plastics for food packaging.

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