Case study traceability of plastic materials for food contact

4.5.1 Types of food contact plastics and their components

Plastics for food contact can be rather difficult to trace, owing to the large variety of components and the various processes used to obtain the final product. Considering the finished product that comes in contact with food, a first classification can be made as follows:

1. Products composed of one type of plastic only. Examples are monolayer foils used, e.g., in packaging for bakery or in the form of bags for household application. It is worth noting that a mono-material film, consisting, e.g., of polypropylene, may contain more than one component. If the film is coloured, this is obtained by mixing a polypropylene matrix with a colourant carried by another polypropylene, normally of lower molecular weight.

2. Products composed of more than one type of plastic, either mono-component, such as multi-layer films or laminates, or multi-components, such as plastic bottles and caps.

3. Multi-layer products that can still be classified as 'plastic for food contact' as the food contact layer consists of plastic, but that contain other types of materials such as adhesives, aluminium foils, paper, etc. These materials are often referred as multi-material multilayers.

Plastic materials consist at the minimum of polymers (sometimes referred as 'resins') and one or more additives; the additives can be added to improve the processability of the polymer (e.g., antioxidants, slipping agents, etc.) or to impart specific physical and technological properties to the finished product

(such as colourants, plasticisers, antifog agents, etc.). The former are normally added by the producer of the polymer, whilst the latter are introduced in the formulation either by the manufacturer of the finished product, or by an intermediate operator (as in the case of coloured plastics) who is identified as 'compounder' or producer of a 'compound' or 'masterbatch'. It becomes evident, then, that even relatively simple finished products, such as those composed of only one type of plastic, may contain several components that make the tracing exercise rather complex.

4.5.2 Raw materials for production of food contact plastics

The raw materials used for manufacturing food contact plastic materials are:

• resins, most of the time purchased in the form of pellets and then submitted to various processing steps

• additives, added in-line during the production of the material or articles, or used off-line to produce a compound that is further processed for the manufacture of the material or articles

• plastic films or sheets, purchased as reels and then either coated and/or printed and laminated to another substrate

• primers, inks, varnishes and coatings, used in the printing process

• adhesives and tie-layer resins, used to laminate or bond together various layers

• non-plastic substrates such as paper, aluminium foil, etc.

Obviously, traceability can be attained only if the identification elements of each of the above raw materials are properly obtained by the relevant suppliers, but the most difficult part consists of the management of such identification elements in the various steps of storage, transformation and warehousing of finished goods.

4.5.3 Processing of plastics for manufacturing food contact products

Raw materials may undergo different types of processing that lead to the finished food contact article. The processing is selected on the basis of the type of finished product desired and its properties. Plastic films or sheets, either monolayer or multilayer, may be produced via extrusion. If a colourant or another additive is added, then the extrusion step is preceded by a compounding phase, where the matrix polymer is mixed with a coloured masterbatch (colour concentrate). The extruded film may undergo an orientation treatment in-line, where the film is drawn to obtain specific mechanical and physical properties such as mechanical strength, reduced gas permeability and ability to shrink. The semi-finished product consists of a reel of a width depending on the width of the extruder's head. Reels produced in extrusion are called 'master reels' and are too wide to be used in food packaging machines, therefore they undergo slitting and originate a larger number of reels. Each reel may undergo further processing, such as printing.

Although films or laminates represent a simple example of plastic product for food wrapping, one may see that the various steps require that the information to trace the material is transferred from one step to another, making the whole process rather difficult. This becomes more complex if other manufacturers are involved in intermediate steps, e.g., compounding, printing, etc. Most of the plastic food packaging materials commonly present in the market are composed of more than one layer, either co-extruded or glue laminated. From a traceability viewpoint these materials may be represented as a multiplication of the single steps needed for the production of a single layer. Schemes of production of a co-extruded and a laminated material are presented in Figs 4.5 and 4.6. Each of the different components

Customer packer-filler/retailer Fig. 4.5 Manufacturing of a co-extruded multilayer film.
Customer (packer-filler) Fig. 4.6 Production of a laminated film.

of the final co-extruded or laminated product will undergo a separate procedure that would eventually result in one single identification code when they are put together.

4.5.4 Limits of traceability systems in plastic processing

Bulk storage of resins

In many cases resins for the production of plastic food packaging materials are shipped in large quantities and stored in silos. These are normally refilled before they are empty and so they generally contain more than one batch of the resin. Although for physical reasons one may reasonably assume that batches are fed to the extruders in a first-in-first-out regime, mixing of batches to a certain extent is virtually unavoidable. This implies that a one-to-one correlation between production codes of the food contact product and resin batch cannot be fully guaranteed. of course this is not an issue for extruders fed from bags or single containers, but the use of silo storage by plastics processing companies is so widespread that discontinuing such practice is unrealistic and economically unjustified. Some measures may be adopted for reducing the uncertainty in the identification of the resin batch number versus product manufacturing. For example, recording the dates of feeding of silos and relevant batch codes would allow identification of which batches were present in the silos at the moment of production of a given material. Nevertheless extruders may store resins from more than one vender in the same silos. Therefore manufacturing companies may decide to adopt separate batch management.

on the other hand, one single silo may feed multiple extruders, thus making the whole materials flow rather complex and variable. If the finished material shows any delayed defect or is found non-compliant because of the resin stored in the silos, the manufacturer should then be ready to recall all materials produced from all batches that were present in the silos on the day of manufacturing the defective one. This implies a greater risk for the manufacturer, who is not in a position to identify with great precision the batch responsible for the defect or non-compliance, and will be required to recall more material than necessary to ensure that all products containing the defective resin batch are withdrawn. Normally the probability of lack of conformity or presence of defects, in particular leading to unsafe products, is so low that the risk for manufacturers is most of the times worth taking.

Reworked materials

Reworking of scrap materials is a common practice in plastic processing operations. Trims of films can be recycled in-line or stored in baskets and recycled in another production run. In the former case trims belong to the same finished product batch code, so no traceability problems would arise, while in the latter case several batches of trim stored in a basket can together be recycled into another film batch. Establishing a one-to-one correlation between the resin batches composing the trims and the final film batch would result in an extremely overcomplicated exercise. Also, defective batches of finished products may be reworked into the same type of product or even incorporated into other products with different composition. This is a practice that is adopted for maximising resin yield and reducing landfilling or other loss of raw materials. All these practices imply a high degree of complexity in the traceability systems, up to the point that manufacturers have to decide the extent of risk that they are willing to accept by limiting tracing back of raw materials, for example, to the scrap products that undergo reworking. Also in this case producers might be forced to recall more than necessary in case of defective or non-compliant product, but again frequency of damage is normally so low as to justify risk taking.

82 Chemical migration and food contact materials Printing inks

Inks used for printing food contact materials and articles normally have a rather complex composition. They may be purchased as ready-to-use colour series, where each single product of the same series differs from the others only in the pigment used. They can be directly loaded into either flexo- or rotographic machines, or as concentrated colours (in the form of viscous liquid or paste) that are added with chromic or technological varnishes, then solvent diluted, before use. Also, several additives may be used in ink preparation (antifoaming agents, adhesion promoters, antistatic additives, etc.). Alternatively, primers for surface treatment to improve sticking of colours may be used. In addition, protective overprint varnishes may be used for avoiding print deterioration and scratches. All of these components ultimately become integral parts of the finished product, and in principle all of them should undergo full traceability. Apart from possibly solvents, such components are left in only minimum quantities after evaporation at the end of the process, as they are not an intentional component of the finished product. In industrial practice all purchased batches are often stored in one single container, where they mix and, since they are liquid, lose reference to the original batch.

Different colours may also be mixed in different proportions to obtain special desired effects, as well as recovered when in excess and back flushed in the original container. Clearly, printing practices do not allow batch-to-batch correlation unless very sophisticated controls are put in place, the cost of which is hardly worth the risk. Transmission of colour to food is unlikely through plastic materials, as plastics are in general good barriers to large molecules such as organic colourants, but it might occur when porous paper is printed and laminated to thin low density plastic foils (such as LLDPE). For such reasons it is appropriate to establish recording systems that are able to provide indications not only as to the type of ink and ink components used (which is straightforward), but also to the batch or batches and the shift or shifts of production. An acceptable level of traceability may be ensured by recording the date of supplying of inks and ink components, relevant identification, type of storage system (e.g. identification of the container) and any production records that relate the respective storage system with the product and its relevant date of printing.

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