Lesson- 32 Recycling packaging materials

32.1 Introduction:

Recycling of packaging materials plays an increasing role in packaging, and numerous applications can already be found on the market. Ten or twenty years ago most post-consumer packaging waste was going into landfill sites or to incineration. Traditionally, only glass and paper/board were recycled into new applications. In the case of packaging plastics the situation is quite different. Only uncontaminated in-house production waste was collected, ground and recycled into the feedstream of the packaging production line without further decontamination. With increasing environmental demands, however, post-consumer plastics packaging materials have also been considered more and more for recycling into new packaging.

But recycling of packing plastics is also a question of recycling technology and collection of packaging waste. Today many countries have established collection systems for post-consumer packaging waste, like the green dot systems. Such country-wide collecting systems guarantee increasing recovery rates. Together with new developments of recycling systems and with increasing recycling capacity the way is open for some plastics for a high value recycling of packaging waste.

32.2 Recyclability of packaging plastics

It is generally known that food contact materials are not completely inert and can interact with the filled product. In particular, interactions between packaging plastics and organic chemicals deserve the highest interest in this context. Such interactions start with the time point of filling and continue during the regular usage phase of a package and even longer, in case a consumer ‘misuses’ the empty packaging by filling it with chemical formulations such as household cleaners, pesticide solutions, mineral oil or others. The inertness of common packaging polymers decreases in the following sequence:

Poly (ethylene naphthalate) (PEN)  -------- poly (ethylene terephthalate) (PET) --------- rigid poly (vinyl chloride) (PVC) ----- polystyrene (PS) ------ high density polyethylene (HDPE) ------ polypropylene (PP) ------ low density polyethylene (LDPE)

32.3 Improving the recyclability of plastic packaging

The source control is the first and most important step in closed-loop recycling of packaging plastics. There must be efficient recovery or sorting processes which are able to control the input fraction going into a closed-loop recycling process. The feed stream material should have a minimum polymer type purity of 99%. Some studies were undertaken to determine the impact of PET materials formerly used for non-food applications. Studies came to the same result that due to the low diffusivity of PET packages from non-food applications could also be used as input material for bottle-to-bottle recycling. This underlines the favourite position of PET bottles for a closed loop recycling.

32.4 Using recycled plastics in packaging

Technically, recycled plastics can, in principle, be applied in direct food contact applications or protected from direct food contact by a functional barrier. The use of recycled plastic materials in packaging applications has to comply with the relevant regulations and must not be at the expense of the public health, nor should it alternate the filling’s quality. In the following, practical examples of recycled plastics food packaging applications covered by a functional barrier as well as in direct food contact are described.

 32.4.1 Indirect contact applications applying functional barriers

In the most general understanding the concept of a functional barrier can be defined as follows: A functional barrier is a layer in the package which protects the food from external influences under the applied fill and storage conditions. In most cases the functional barrier is the food contact layer or, in complex multi-layer structures, one very close to it. This layer acts as a barrier against contamination from the packaging’s environment in general and, more specifically, from the recycled core layer or outer compartments of the package.

The functional barrier efficiency must not be confused with an absolute physical barrier such as glass or metal layers. It 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. These parameters are:

  • Manufacture conditions of the package

  • Thickness of the functional barrier layer

  • Type of functional barrier plastic

  • Molecular weight and chemical structure of penetrants (contaminants)

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

  • Time period 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

32.4.1.1 three-layer PP cups for dairy products

A study was presented34 where the safety in food contact use of symmetrically coextruded three-layered polypropylene (PP) cups with recycled post-consumer PP in the core layer (mass fraction 50%) and virgin food grade PP in the adjacent layers was investigated. The recycled PP, which contained about 95% PP and 5% PS, was completely under source control in the recollection system and had been used in its prior application for packaging yoghurt. The intended application for the recycled material was again packaging milk products such as yoghurt with storage for short times under refrigerated conditions.

32.4.1.2 Multi-layer PET bottles for soft drink applications

A study was published in which the effectiveness of a virgin PET layer in limiting chemical migration from recycled PET was investigated. For this purpose three-layer bottles were prepared with an inner core (buried layer) of PET which was deliberately contaminated. The model contaminants used were toluene, trichloroethane, chlorobenzene, phenyl decane, benzophenone, phenyl cyclohexane and copper (II) acetylacetonate. As a result no migration was detected through a barrier of virgin PET of 186 ± 39µm thickness into 3% acetic acid using general migration test conditions of 10 days at 40ºC and also after 6 months storage at room temperature. Also migration testing with 50% and 95% ethanol as severe contact media which are relatively aggressive to PET did not lead to measurable migration rates. Consideration of diffusion models using limonene as substance for which diffusion coefficients were available, gave estimates that for a 100µm thick PET layer a breakthrough of a substance with comparable molecular weight would take place after 7.5 years or 0.8 years at room temperature or 40ºC, respectively.

32.4.2 Direct contact applications

 Due to higher costs of manufacturing multi-layer bottles, the bottle manufacturing and recycling companies started the development of recycling processes without a functional barrier of virgin PET. The cleaning efficiencies of all the applied deep-cleansing recycling processes were investigated by challenge tests. Applications of direct contact recycled packaging material are in

 32.4.2.1 Mono-layer PET bottle for soft drink applications

PET is one of the most favoured candidates for closed loop recycling. Due to higher costs of manufacturing multi-layer bottles, the bottle manufacturing and recycling companies started the development of recycling processes without a functional barrier of virgin PET. One decade later several super-clean recycling processes were established on an industrial scale. The cleaning efficiencies of all the applied deep-cleansing recycling processes were investigated by challenge tests and the cleaning efficiencies are well known.

32.4.2.2 Mono-layer HDPE bottles for fresh milk

Milk bottles were recovered by a deposit system and were subjected to a bottle-to-bottle recycling process. Due to the recovery system the recycled HDPE was completely under source control and had been used in its prior application only for packaging fresh milk. The recovered material was recycled first by a conventional washing based recycling process and then further deep-cleansed using a super-clean process. Subsequently the recycled material was used with a content of 20 to 30% without a functional barrier. The intended application for the recycled material was again bottles for fresh milk with short time storage under refrigerated conditions.

Last modified: Saturday, 3 August 2013, 11:10 AM