Bio-based plastics appear already in a broad array of consumption goods. Production of bio-based plastics currently comprises ca. 1% of total plastics production and this share is expected to rise [1
]. The Nova Institute has estimated this growth of overall production of bio-based plastics will increase by ca. 50% in 2021 (Figure 1
]. Hence the share of bio-based plastics would then increase towards ca. 1.5%, the exact figure depending on the growth of fossil-based plastics. The development and growth of bio-based plastics fit into the search for alternatives to crude oil as a feedstock of organic compounds. Crude oil is a finite feedstock, and today most of the products made from it end up as carbon dioxide in the atmosphere, contributing to global warming. While the production of bio-based compounds is not at all sustainable by definition, the primary raw material source has the potential to be renewable if sufficient care is taken in the development of harvesting and production processes.
There are many types of bio-based plastics, and further subdivisions can be made; for instance, they differ based on their degree of biodegradability, or on their molecular similarity with existing fossil-based plastics. For example, plastics like bio-based PET (polyethylene terephthalate) or bio-based PE (polyethylene) are essentially identical to their fossil-based counterparts PET or PE, and are called ‘drop-in’ bio-based plastics for this reason. The only difference is in the production processes of the building blocks of these plastics, as the primary raw materials are different. There are also bio-based plastics with building blocks of a particular basis that are much more easily derived from plant-based feedstocks and from which no fossil-based counterparts have been developed, for reasons of molecular chemistry. Examples of such plastics are PLA (polylactic acid), PHA (polyhydroxy alkanoate) and PEF (polyethylene furanoate), which really amount to being new materials with new properties. As such, they offer the opportunity to compete with fossil-based plastics based on performance and not simply on price alone. An example is the application of PEF for the packaging of carbonated beverages [3
]. It is for this reason that further growth of their market share is generally expected: Figure 1
shows the highest relative increases for bio-based PET and PHA, and in 2021 PEF is expected to be a newcomer to the market.
When bio-based plastics will be increasingly used for common applications like bottles, trays, packaging etc. they will also end up in waste streams and as such enter the established recycling processes for fossil-based plastics. As explained in the previous paragraph, a number of bio-based plastics have to be considered as new materials. Hence there may be risks that in some cases, from a certain minimum occurrence, they might prove to be incompatible with these processes, leading to a decreased quality of the recycled plastic stream in which the bio-based plastics have ended up [4
]. If this is the case, this would hamper the closure of material cycles in plastics recycling, which is particularly relevant given the current policy focus on the circular economy and on the recycling of plastics, as reflected in the recent launch of a strategy for plastics in the circular economy by the European Commission [5
In this paper, a review of the risks associated with the increased occurrence of bio-based plastics made from novel building blocks in existing recycling processes will be provided. The results of this analysis will, on the one hand, allow detection of knowledge gaps in this area. On the other hand, this analysis will serve policy developments in the field of the circular economy and, in particular, plastics recycling. In this way, the paper provides an outlook as to if and how policy makers should be prepared for the increased occurrence of bio-based plastics.
2. Materials and Methods
The premise for setting up the analysis is that the preferred end-of-life scenario for bio-based plastic is to collect it as plastic waste and send it for recycling. Hence, for those bio-based plastics that claim to be biodegradable, the possibilities for collection together with organic waste for composting or digestion purposes are not included. From the perspective of keeping material cycling in the economy, mixing these two types of plastics is less desirable. Also, it is logical to organise plastic recycling per product category to the extent that this is feasible.
In order to undertake the risk analysis, we started by outlining the currently applied recycling processes. In the first place, we have considered the recycling of PET and HDPE (high-density polyethylene) bottles. These two plastics display, for the moment, the best outlook in terms of the production of high-quality recyclates: selective collection is in place in many countries, efficient mechanical recycling processes have been developed, and there are examples of the high-grade application of recyclates [6
]. Next, we expand on the recycling of mixed household packaging waste, given the increased overall focus on plastics recycling as explained above.
In a next step, the impact of small amounts of bio-based plastics made from new building blocks was assessed. As we felt that this group of plastics is too heterogeneous in nature, we have chosen a case-by-case approach by considering subsequently the impact of three examples of such plastics, PLA, PHA and PEF, in order to develop a more general perspective on the risks. As preparation for these exercises, we first considered the impact of polyvinyl chloride (PVC) in PET recycling. PVC is known as an unwanted contaminant even in very low concentrations [8
], and gathering the available information on the case of PVC in PET recycling was considered instructive as a preparation for assessing the impact of the selected bio-based plastics. Given the very small amounts of these plastics appearing on the market, setting up separate collection is not viable and hence they will act as contaminants whose impact on recycling processes and products has to be analysed [9
]. The analysis starts by considering the physical, chemical and other, more practical, properties (for instance related to the particular application or appearance) of such contamination. Then, these are compared with the properties of PET and HDPE bottles or household packaging waste constituting the main flows. Eventually, this results in the identification of the possible pathways that particular bio-based plastics can follow and the possible impact that may arise from their presence in certain amounts.
The data at the base of this analysis was retrieved from research papers, policy documents, publications from sector organizations and websites. A deliberate and clear choice was made at the beginning of the research not to limit the consulted sources to peer-reviewed academic literature only. We realised that a significant part of this analysis pertains to unit operations interacting in a system, which is an area under focus by many other actors besides academic researchers. In fact, the reality that much of the work (including more conceptual aspects) in circular economy research is driven by non-academic actors has been reported before [10
]. In that way, one of the aims of this paper is also to stimulate and define areas for further knowledge building.
4. Discussion and Conclusions
The approach followed in the three cases above has shown that the introduction of bio-based plastics on the current operational range of recycling processes should be considered as the introduction of a number of types of novel plastics. In the first instance, every new introduction starts as a contamination, and with respect to further evolutions the following questions are most relevant:
Which incompatibilities may occur? From which amounts do they become noticeable?
How strong are the current recycling processes that have been developed?
By which amounts is the development of dedicated collection and recycling rewarding?
One aspect of assessing the impact of a contamination comes down to the probability of a novel plastic ending up in the final products of current plastics recycling. This is a consequence of the properties of the contamination compared to the main flow of plastics, and the organisation and technologies of sorting and recycling that deal with this main flow in a certain manner. Starting with the estimated, measured or known market penetration, it is possible to assess the pathway of a contamination and to obtain probable concentrations in the recycling processes and at the recyclate level, and comparing these with the lowest levels on which negative impacts have been observed or demonstrated—this is the second aspect of the impact of a contamination. This can be measured with tests; here the definition of what is being tested is crucial, as an incompatibility may be less noticeable in one application compared to another one. For instance the impact for an rPET bottle destined for beverages is different compared to the impact for an opaque rPET bottle destined to contain soap.
The latter two of the above questions pertain to the current recycling of PET: processes for high-grade rPET destined for bottle production have been established and sector organizations have been founded in particular to protect these activities, e.g., by promoting the design for recycling. This has resulted in published compatibility lists, e.g., discouraging particular materials for sleeves and add-ons, with the ability to produce high-grade rPET as the reference. Hence any negative impacts of contaminations have a higher chance of being discovered right in this chain, so it is not a coincidence that PET recycling features so prominently in the analyses above. To a certain extent it is beneficial that compatibility with the established recycling processes is being strived for, because it may lead to smoother introductions of novel plastics and the detection of possible issues well in advance. For instance, the market entrance of PEF has been anticipated in this way. On the other hand there is the risk that the mere power exerted by the incumbent actors in recycling inhibits the introduction of novel plastics for too long a time, even if such introductions would be beneficial for other reasons.
In this paper, a review has been carried out for three concrete bio-based plastics. For PLA, the evidence is clear that its presence, even in small amounts, is detrimental to the quality of rPET: contamination in the feed to mechanical recycling should be maintained well below 0.1% in order to protect rPET quality. For the bottle fraction, our estimations for now and coming years till 2021 have shown that state-of-the-art separation (equipped with NIR technology) might lead to a contamination in the feed to mechanical recycling of PET currently and in the near future not too far below this 0.1% threshold. In order to further elaborate this analysis, a next step could be to obtain better reference values by checking the numbers of the estimation, e.g., by sample measurements of bottle waste streams (also considering sleeves and add-ons). These actions could then be repeated to keep track of evolutions in PLA concentrations over time. Also, the current occurrence of any quality issues related to PLA in PET in the field could be checked, e.g., by interviewing companies. The outcome of these actions should allow an assessment of when the set of currently applied unit operations for separation and recycling would not be able to lead to rPET of sufficient quality in the longer term. If this were to be done, then further options to consider are investments in extra unit operations and/or developing adequate labeling of PLA bottles. The value of the latter option will also be dependent on any further developments in the direction of bringing value to a separate PLA stream for recycling PLA. It appears that there are several possibilities for additional technologies for PLA separation, like an extra separation of flakes using NIR technology, or technologies that are already known for PVC removal.
With respect to PLA contamination of mixed plastics, our estimations have shown that contamination may be in the range of several percent. Although any issues were not revealed in the current study, perhaps due to the application development in this area that is only emerging, with further sorting and separation technology and application developments the estimated higher concentration of PLA in mixed plastics might very well give rise to issues (e.g., due to its lower heat stability compared to many fossil-based plastics). Hence here also sample measurements and interviews with companies using mixed plastics as an input to their production could be helpful to establish reference values and to monitor evolution. In this respect, any plans to implement, extend or modify post-consumer plastics collection should be thoroughly evaluated with respect to the creation of possible contamination pathways of PLA into rPET.
For PHA, besides the different kinds of polymers considered, there is also a whole range of applications and only few data available, making analysis of the current situation difficult. Similar issues as encountered with PLA might occur, given the rather low heat stability of PHB. For the moment, no issues are known or expected as the main current applications of PHA do not seem to lead to end-of-life scenarios hampering the existing high-end mechanical recycling processes. There are no indications that this would change in the short term, but any trends in application of PHA are to be monitored given the expected steep increase in PHA production in coming years.
With respect to PEF, no issues are known with respect to the impact on mechanical recycling of common plastics. For the impact on rPET this has been effectively tested for contamination up to 2%. As PEF still has to be launched on the market, this allows us to conclude that in coming years no issues are to be expected. What could happen in the longer term is not clear; much is also dependent on to which extent a separate collection and recycling system for PEF would be operational and successful. Anyhow, this has gained the attention of both the (future) producer of PEF and the sector organization of PET recycling, hence it is very probable that any risk will be anticipated well. In fact the producer’s approach to assess in advance compatibility with the existing recycling landscape is to be encouraged as it is a clear demonstration of the necessary system thinking in this field.
Summarizing the three case studies, for PLA the facts are known so future risks can be assessed by measuring amounts. For PHA, it will be crucial to monitor future application development, and for PEF, a particular approach for contamination-related issues has been an element of project management. Hence the study did not reveal bottlenecks or negative impacts generally valid for all bio-based plastics. One of the next questions could then be what would be the next bio-based candidate material to appear in post-consumer plastics waste.
Overall, the challenge with respect to bio-based plastics is a matter of guiding well both their introduction together with developments in the recycling landscape, with a particular eye on their incompatibilities e.g., with process conditions or combinations with other plastics. The story in this paper is, therefore, fully written within the context of the current state of the art of applied recycling technologies and, in general, how post-consumer plastic collection and sorting have been organized. Hence, the findings are to be seen fully in the light of the current situation. With any future changes of the recycling landscape, the analysis has to be repeated. For instance, if there were to be initiatives in extended producer responsibility, maybe the waste streams obtained would become much more pure and some issues may simply disappear, if for example PLA bottles would then not end up with PET bottles any more.
With respect to developing policy advice, a number of suggestions for preparing next steps can be made. First, it is important that introduction of novel plastics is guided well, with a clear focus on the whole system; see as an example the way the introduction of PEF is being anticipated. Next, it is important that all plastic types occurring as contamination in current waste streams are considered in the context of any changes in the recycling landscape. On the other hand, with the supply of plastic types constantly changing and more abrupt changes to be expected in the (near) future, it has to be considered that from a certain moment the recycling landscape itself would need a reorganisation; such an operation requires a realignment of many actors and is, hence, complex, but it would avoid desirable developments in the production of plastics being blocked for an unnecessarily long time.
Finally, the current analysis did not aim to draw any conclusions about the mere desirability of bio-based plastics and/or future increases in these plastics. Such developments should, anyhow, not be steered too much by the concrete implications for the recycling landscape; as long as these plastics can be recycled well by themselves, the recycling landscape should be able to accommodate them over time, and here policy has the option to support or even guide this process by carefully managing the new entries temporarily as contaminants.