The steadily increasing production of plastic causes severe environmental problems, which include high energy demand during production, consumption of fossil fuels and accumulation of plastic waste in landfills and natural environments. In the context of packaging, approaches to reduce or slow-down the demand for virgin plastic have been developed and are already applied to different extents. These strategies include the reduction of packaging weight and/or volume, the reuse of packaging and the recycling of certain polymers (3R).
1. Recycling steps
Identification and sorting
In 1988, the Society of the Plastics Industry (SPI) devised the resin identification code (RIC) aiming at the efficient identification and separation of different plastics (Table 1). In 2013, ASTM International issued the replacement of the three “chasing arrows”, which are often associated with recycling, by a solid equilateral triangle symbol to focus only on resin identification, not on recyclability. Manual or automated sorting systems exist at an industrial scale separating plastics intended for recycling from other waste. Usually, presorting efficiently segregates glass, metal and paper from the waste stream. Sorting of plastics might be supported by analytical techniques (e.g. near-infrared and Fourier-transform spectroscopy, optical color recognition systems) , although efficient separation remains a challenge due to different shapes of the plastics, entrapped air, coatings and paints that slow-down or even impede the analysis.
Primary mechanical recycling of plastics describes the conversion of thermoplastic polymers into products with equivalent properties [1, 2]. Such closed-loop processes can only be applied for plastics which have not been used at all or which have been thoroughly cleaned and separated from contaminating plastics before recycling. Secondary mechanical recycling generally leads to products of lower mechanical properties. Thermoplastics composed of only one polymer (e.g. PE, PP, PS, PET, and PVC) may be mechanically recycled after use, but a reduction of the polymer’s molecular weight and changes in physico-chemical properties often have to be taken into account . Tertiary chemical recycling describes processes in which polymers are chemically or biologically degraded into smaller molecules that can serve as new building blocks for further chemical syntheses [1, 3]. Quaternary recycling processes recover energy from plastics through incineration.
2. Recycled materials
Recycled polyethylene terephthalate (PET)
PET has become the most used packaging material for water and soft-drinks worldwide [4, 5]. Public concerns about environmental impacts of PET disposal, as well as the recyclability and availability of collected PET bottles promoted the development of PET recycling processes. In the beginning, PET recyclates were mainly used in the production of polyester fibers. Bottle-to-bottle recycling processes were then developed together with appropriate regulatory frameworks allowing the use of recycled PET in food contact materials (FCMs) [6, 7]. So-called “challenge tests” were established to measure whether a recycling process can reduce any chemical contamination below a set limit and thus comply with the legal requirements . Contaminations might originate from PET containers previously filled with non-food liquids, from non-food-contact grade PET or from other types of polymers entering the recycling stream (Figure 1). Further PET degradation products, process chemicals or sorbed food components can result in unwanted impurities.
A typical PET recycling process starts with the collection, separation and sorting of post-consumer PET bottles in materials recovery facilities. In recycling facilities post-consumer PET is washed to remove dirt, glue and food leftovers and then ground into flakes. Additional cleaning steps necessary to obtain the purity required for food packaging include high temperature treatment, vacuum or inert gas treatment and surface treatment with non-hazardous chemicals to achieve so-called super-clean PET .
Besides PET, processes for the recycling of other packaging materials composed of e.g. polyolefins, multilayer materials and (nano-)composites have been developed. Polyolefins are susceptible to oxidation and it is still a challenge to maintain the material’s quality during a recycling process. However, additives and stabilizers may help to achieve recycled polyolefins of sufficient quality for reuse . The recycling of plastic from multilayer materials is currently not economically feasible; instead recycling only focusses on the fiber-based layers. The production of blends composed of plastic and fibers or nanomaterials is an alternative way to increase the stability of recycled plastic, but the further recyclability of these composite materials has not been routinely assessed so far.
3. Market and recycling data
The prices for post-consumer resins increased over the last 25 years, but they were highly volatile . In general, transparent recycled materials are more expensive than colored ones and pellets are more expensive than flakes. Currently, post-consumer HDPE is more expensive than PET. In 2012, 1’640, 779, 582 and 37 kilotons of PET were collected for recycling in the EU, the U.S., Japan and Switzerland, respectively [12-15]. The recycling rates were the highest in Japan and Switzerland, followed by the EU and the U.S.
In the EU, recycled plastic materials and articles intended to come into contact with foods were regulated under Commission Regulation (EC) No 282/2008, commonly referred to as Recycling Regulation. Article 4 of the regulation specifies that all articles and plastic materials used for recycling must have been produced in accordance with Community legislation on plastic FCMs and articles. Currently, 127 recycling processes, which can be found using the keyword RECYC, have been registered and partially evaluated by the European Food Safety Authority (EFSA), but none of the evaluated recycling processes has been authorized by the European Commission to this point (December 1, 2014). More than 80% of these processes describe the recycling of PET.
In the U.S., the use of recycled plastic in the manufacturing of food contact articles is evaluated on a case-by-case basis by the U.S. Food and Drug Administration (FDA). Between 1990 and 2014, 176 processes for producing post-consumer recycled plastic were judged suitable. Three-quarter of the registered processes specify the recycling of PET.
In 1997, the Japanese Ministry of the Environment enforced the Container and Packaging Recycling Law with the aim to reduce the waste of glass, PET and paper
5. Safety issues
The possible sources of contamination during recycling processes are diverse and often unknown (Figure 1). In different studies, phthalates, heavy metals and brominated flame retardants have been identified in recycled plastics used as FCMs [16-19]. In some of these studies, unwanted contaminants were present, but the legal limits were not exceeded. When black plastic items used in food contact were contaminated with brominated flame retardants, the chemicals were not expected at all, because they are generally not allowed to be used in FCMs . They probably originated from waste electric and electronic equipment that was fed into the recycling stream.
Environmental and health issues
Plastic recycling reduces the amount of plastic waste, but it can generate emissions that have an impact on the direct environment of the factory. Studies showed that especially in developing countries, occupational health and safety may not be sufficiently ensured in the plastic recycling industry . This fact is of special concern due to the high amounts of plastic send for recycling to these countries.
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