1

Selective dissolution and precipitation for mechanical recycling of post-use

plastics

Wenfa Ng

Novena, Singapore, Email Address: ngwenfa@alumni.nus.edu.sg

Abstract

Plastics is ubiquitous in almost all aspects of modern life – but characteristics such as good

durability and low mass-to-volume ratio that afford its use in myriad applications also present

significant challenges to its end-of-life disposal. In particular, poor biodegradability,

production from non-renewable resources, and, more important, the propensity of emitting

hazardous substances during incineration meant that plastics use incurs a significant

environmental footprint. Although various policy initiatives (ranging from reuse, reduce and

recycle) have been promulgated for tackling the problem, the relatively low take-up of such

initiatives around the world meant that significant challenges remain in solving the plastics

disposal problem. This essay aims to provide readers with a snapshot of the environmental

challenges surrounding plastic use and the various methods available for recycling post-

consumer plastic waste – with special emphasis on the selective dissolution and

reprecipitation (SDP) approach. More important, problems such as deterioration of polymers‟

properties due to leaching of additives and stabilizers during recycling would also be

delineated. Given an expansive literature, discussion of the various plastic recycling methods

(such as mechanical, chemical/feedstock, and thermal) would inherently not be exhaustive.

Nevertheless, the conceptual basis, advantages and disadvantages of each class of methods

would be highlighted. While recovered polymers are usually employed in lower value

applications relative to the plastics‟ original use (known as “down-cycling”), the essay would

also discuss recent trends and proof-of-concept methods in “up-cycling” plastics waste for

use as adsorbents in wastewater treatment, self-healing thermal polymers, or as carbon

nanotubes in supercapacitor applications. Finally, substantial coverage would be dedicated to

explaining the working principles and performance parameters (e.g., choice of solvents and

anti-solvents, temperature gradient etc.) governing the application of SDP in mechanical

recycling of waste polymers. Briefly, SDP separates individual polymer types from a mixture

by utilizing the differential dissolution temperatures of different polymers in a particular

solvent. Subsequent addition of an anti-solvent to the polymer-solvent mixture induces the

precipitation and recovery of the dissolved polymer as granular materials – a form-factor

well-suited for a variety of applications and polymer fabrication techniques. Collectively,

besides highlighting the environmental sustainability challenges surrounding plastic use, the

current exposition should also be useful as a general introduction to various plastic recycling

methods. Discussion of up-cycling approaches further illuminates an important trend in

environmental research: i.e., treading away from accepting redeployment of recycled

materials for original or low-value use to developing innovative approaches for converting

waste into high performance materials. Such an approach, if successful, would provide a

sizeable market for recycled plastics – thereby, helping anchor recycling into the fabric of life

2

by providing an economic incentive for an activity that hitherto relies on voluntary

participation.

Keywords: commingled plastic waste; polymer recycling; down-cycling; upcycling;

restabilization; environmental footprint; pyrolysis; polymer deconstruction; monomers;

selective dissolution and precipitation; waste repurposing;

Subject areas: environmental sciences; chemistry; physics; chemical engineering; polymer

science;

Conflicts of interest

The author declares no conflicts of interest.

Author’s contribution

Wenfa Ng reviewed the literature and wrote the manuscript.

Funding

No funding was used in this work.

3

Introduction

Plastics have found applications in wide-ranging areas such as insulation materials for

electrical cables, disposable carrier bags, beverages bottles and packaging materials - and is

almost a necessity in modern life. Although properties such as chemical inertness and

durability have contributed to plastic‟s widespread adoption, they are also partially

responsible for plastics‟ disposal problems. With increasing consumerism and use of

disposable products, the volume of plastic waste has increased over recent decades; for

example, according to the U.S. Environmental Protection Agency, the fraction of plastics in

municipal solid waste has increased from 0.5% to 12.4% during the period 1960 to 2010.23

Given the difficulty of disposing poorly biodegradable plastics, many initiatives have been

implemented (at least at the pilot or proof-of-concept stage) for reducing the amount of

plastic waste disposed either in landfill or via incineration. These efforts include: (i) reducing

use of plastic bags via imposing a surcharge on a per use basis; (ii) an outright ban (e.g., in

Delhi (India), state of Mahaeashtra (India), San Francisco (United States) and Rwanda29

), or

encouraging the use of alternatives (such as reusable and durable carriers based on materials

such as polyhydroxybutyrates). With greater awareness of the many toxic substances emitted

during plastic waste incineration and recycling operations, there are even calls for classifying

plastic waste as hazardous substances.33

Perhaps with the exception of poly(ethylene

terephthalate) (PET) – which is amenable to biodegradation by enzymes from many bacterial

species11

32

- most polymers such as polypropylene (PP),9 polyethylene (PE), polystyrene

(PS), and poly(vinyl chloride) (PVC) are poorly biodegradable, and thus, would remain in

their original state in landfills for many decades. Additionally, plastic waste takes up a large

volume for a given mass;34, 35

thus, making landfilling an unsustainable disposal option,35

a

significant problem particularly in land-scarce cities around the world. This, coupled with the

large amount of plastics waste generated36

- and the landfill‟s attendant short service life -

presents a challenging problem, the solution of which would have major implications on

environmental sustainability in general and the carbon and energy footprint of society in

particular.

Problems associated with poor biodegradability of plastics – such as paucity of

environmentally friendly final disposal options - are not confined to terrestrial systems. Small

millimetre sized plastic waste, for example, has been found on ocean surfaces, which presents

a threat to aquatic birds and marine animals as they may inadvertently ingest the micro-

plastic particles as food.20

Most commonly observed as large patches of floating plastics on

the ocean surface - such as that observed in the Pacific gyre – the propensity of plastics in

adsorbing recalcitrant pollutants such as polychlorinated biphenyls (PCBs) and polycyclic

aromatic hydrocarbons (PAHs), meant that microplastics could act as carriers for the toxins.

Thus, bioaccumulation of the chemicals up the food chain through the accidental ingestion of

microplastic particles by marine organisms is a real possibility – and its impacts on

ecosystem health and human safety should not be underestimated. Given differing surface

characteristics and hydrophobicity of different polymers, significant differences exist in the

4

types and relative amount of compounds attracted to the polymer surface (i.e., binding

affinity and uptake capacity, respectively). Indeed, a recent field experiment has

characterized the relative affinity of various common plastics for PCBs and PAHs:

specifically, amount of PCBs and PAHs sorbed onto HDPE, PP, and LDPE are consistently

higher than those on PET and PVC.57

Difficulty of finding environmentally acceptable and sustainable methods of

disposing plastic waste and producing native polymers from fossil fuels, has motivated the

search for alternative feedstocks (preferably renewable) for producing plastics. Specifically,

alternatives to petroleum-based plastics, known broadly as bioplastics, are derived from

biomass or microorganisms.35, 37, 38

Though often touted to be more environmentally friendly

than their fossil fuel-derived counterparts, careful accounting of materials and energy flows -

from biomass cultivation to harvesting and final bioplastic production – is needed to quantify

the true environmental impact (especially concerning greenhouse gas emissions) of

bioplastics production.35

Specifically, cradle-to-grave life cycle assessment (LCA), a popular

analytical framework for tracking the performance – from a holistic perspective - of a

particular material across its entire lifespan, suffers from inconsistencies arising from the

large variety of assumptions used in accounting for material and energy flows. Hence,

comparisons across studies for the same polymer is difficult.35

This short-coming reduces the

informational value and utility of such life cycle analyses – which are undertaken and

conducted with much effort and time. Additionally, the high production costs of most

bioplastics – particularly those requiring extraction from microbial production hosts – hamper

the widespread adoption of this class of environmentally friendly polymers; thereby,

restricting their use to niche applications with specific performance requirements usually

beyond those available in petroleum-derived commodity polymers.

Hence, given the poor cost competitiveness of bioplastics and resulting low market

access, policy instruments available for mitigating the environmental challenges of plastics

use are confined to the trios of reduce, reuse and recycle. Reuse of plastics in similar or

alternate applications, though best-in-class in terms of environmental protection, is

nevertheless limited by gradual material degradation, which will eventually results in final

disposal given unacceptable performance characteristics. On the other hand, recycling – for

example, deconstructing polymers into monomers, separating individual polymer types from

a plastic mixture, or converting plastic waste into fuels - is a proven approach (albeit at the

technical rather than socioeconomic level) for managing the burgeoning volume of plastic

waste, whose chemical inertness and low mass-to-volume ratio lead to significant challenges

in final disposal via landfilling. Nevertheless, a significant challenge exists in obtaining

recovered polymers with properties and at prices comparable to those of native polymers –

performance criteria critical for making recycling a viable business venture. Finally, while

incineration is the go-to option for reducing the volume of waste – particularly, in many land-

scarce cities around the world, possibilities of emitting toxic substances such as dioxins and

5

furans during incineration of plastics (and its associated additives and stabilizers) raises

concerns, amongst the general population and city planners, concerning the eco-sustainability

of this disposal approach.

Safety and health issues associated with plastic recycling

Though often presented as an environmental friendly option for tackling plastic waste,

recent studies have shown that there is an under-appreciation of the safety and health risks

associated with plastic recycling – particularly concerning worker exposure to toxic

substances through the inhalation and dermal exposure routes. Specifically, concentrations of

poly(brominated diphenyl) ether (PBDE) in soil, sediment and hair of workers at plastic

recycling facilities is significantly higher than background levels; thus, highlighting the

environmental and health concerns of labour-intensive plastic recycling operations. In fact,

the levels detected are similar to those observed in electronic waste recycling operations,17

which is known to pose some of the most significant health risks to workers. Even though a

large proportion of plastic recycling facilities in developed countries have relatively high

levels of automation, and generally adheres to stringent environmental safety and health

regulations, the situation in many developing countries where an increasingly fraction of

lower end plastic recycling is performed is diametrically opposite. Specifically, heavy

reliance on manual labour, and relatively lax environmental laws in many developing

countries, meant that workers‟ health in relation to the potential environmental benefits of

plastic recycling should be construed as a trade-off, where the lever of balance hinges on

society‟s views on environmental sustainability vis-à-vis human safety. Finally, in addition to

tackling problems associated with recycling plastic waste generated within developing

countries, there is an increasingly trend of shipping plastic waste from developed to

developing countries, where low wage cost helps improve the economics of recycling. But

the practice raises serious questions concerning intra-generational equity, safety, and transfer

of environmental costs between societies. Despite the aforementioned concerns, plastics

recycling remains environmentally preferable to other disposal schemes if the recycling

process is designed and conducted with best-available technologies and adheres to

environmental health and safety standards. Naturally, such a goal is easier to achieve in

developed economies relative to developing ones. But greater awareness of the environmental

impact and safety concerns of inter-boundary shipment of plastic waste as well as

international agreements limiting such waste transfer, together with offers of technical

assistance to developing countries for improving safety and health standards of plastic

recycling, would help reduce health and safety issues associated with plastic recycling

operations in less developed economies.

Requirement for pre-sorting; an Achilles’ heel of plastic recycling

Surveys and studies have indicated that recycling rates are gradually increasing across

major cities around the world, but the percentage of plastics recycled remains unsatisfactory;

6

for example, in 2010, only about 28% of HDPE bottles and 29% of PET bottles and jars are

recycled.9 Specifically, the key issue afflicting the plastic recycling industry is the low

fraction of pre-sorted plastic in the waste stream – an important requirement of downstream

recycling processes such as mechanical and chemical/feedstock recycling. Such a constraint

places great pressure on the economic viability of plastics recycling and, by extension, the

willingness of industry players to bring more capacity on-stream to tackle increasing volumes

of plastic waste given the high cost of separating commingled multi-component plastic waste.

In general, most of the post-consumer/ post-industrial plastic waste exists as a highly

compacted, multi-component and commingled mixture – thereby, presenting serious

challenges for many plastic recycling methods requiring pre-sorted plastics feedstock. Thus,

assuming that we are unable to increase the pre-sorting of plastics at source, the challenge is

in developing effective and economical processes for separating large volume of commingled

plastic waste into its constituent polymer types with purity levels sufficient for downstream

processing. By using near-infrared spectroscopy9 and machine-learning algorithms, robotic

systems have been demonstrated to be useful in identifying and sorting different types of

plastics.

Currently, many methods such as mechanical,34, 41-43

chemical/feedstock44-47

and

thermal recycling (i.e., using waste plastics as a source of fuel48

), exist for recycling post-

consumer/ post-industrial plastic waste and they each have their own advantages and

disadvantages.49, 50

In general, recycled plastics are less costly than their native counterparts if

the cost associated with the separation and recycling processes are well-controlled.

Additionally, many recycled plastics are only suitable for low-value applications.9 But, as

will be discussed later, efforts are underway in developing high value applications for

recycled polymers through up-cycling or repurposing post-use plastics.

Mechanical recycling

Mechanical recycling is perhaps the most visible and well-known approach of plastic

recycling. From the sorting of plastic waste at source to the use of various optical and

physical methods for separating commingled plastic waste, mechanical recycling is usually

the first step in the plastic recycling process, where the sorted plastics (relatively free of other

contaminants) would be channeled to other recycling methods, preferably for generating

higher value products. One approach of mechanical recycling is selective dissolution and re-

precipitation (SDP). Others include density flotation and optical separation etc. Nevertheless,

leaching of additives and stabilizers from polymers during mechanical recycling approaches

that require solvent dissolution (e.g., SDP) is an important problem precluding the application

of recovered polymers to high value – or sometimes, their original - applications without

reintroducing the additives necessary for conferring desired physical and chemical properties.

In particular, stabilizers are needed for protecting plastics against photo-oxidative

degradation, but they tend to leach out during polymer dissolution. Thus, in a process known

7

as restabilization, stabilizers are added to the recycled polymer for enhancing the processing

stability and service life of the recyclate.9 Of the various polymer types, PP is most affected

by performance degradation arising from leaching of additives, since presence of tertiary

carbon atoms in the polymer backbone render it vulnerable to thermo-oxidative degradation

during melt-processing or photo-oxidative degradation during use.9 Thus, restabilization is

critical to recycling PP for a variety of applications – particularly for use as crates and films,

where significant photo-oxidative damage is expected without the protection offered by

stabilizers and other additives.9

Chemical/feedstock recycling

Another approach for recycling plastics is chemical/feedstock recycling, which

includes the deconstruction of waste plastic into monomers, and the conversion (through

pyrolysis) of post-use plastic into useful materials or fuel oil for heating applications.

Comparing the two approaches, waste pyrolysis converts plastic waste into a mixture of small

molecules with high calorific value or useful properties via high temperature induced

degradation, while polymer deconstruction (whether by chemical or enzymatic means)

involves the breakdown of long-chain polymers into their constituent monomers. Similar to

other chemical processes, the objective is to optimize reaction conditions such that high

purity products are obtained together with small amounts of byproducts.

Polymer deconstruction for chemical/feedstock recycling necessarily requires good

chemical- or bio-degradability of the original polymers. For example, enzyme-mediated

degradation of biodegradable polymers into monomers suitable for future polymerization or

other applications has been demonstrated.11, 12

Moreover, given the substrate specificity of

most enzymes, tailored enzymatic cocktails could be formulated for degrading different

polymers in a plastic waste mixture. Although the aforementioned process could theoretically

be implemented as a one-pot approach (where all the required enzymes are brought into

contact with the plastic waste mixture at the same time), practical experience has shown that

the sequential use of polymer-specific depolymerizing enzymes would engender better

recovery of different monomers from mixed plastic waste through reducing the complexity of

the separation step necessary for differentiating between different monomers generated in the

one-pot approach.11, 12

Nevertheless, high cost of enzymes and their narrow operating

temperature window present obstacles to applying the approach to other classes of polymers

beyond the relatively more biodegradable polyesters.

In addition to enzyme-based methods for degrading (or deconstructing) polymers,

chemical methods of breaking down polymers either involve the use of chemicals or high

temperatures. Susceptibility to hydrolysis, for example, facilitates the recovery of polyols and

organic acids monomers from polyesters; thus, opening up the possibility of using chemical

8

(water or alkaline)-induced depolymerization for recycling plastics. On the other hand, high

temperatures have also been used - for example, in fast pyrolysis - for inducing the thermal

degradation of otherwise recalcitrant long-chain polymers into smaller molecules. The

obtained products possess properties different from the original polymers, or, as a mixture,

retain a significant fraction of the energy encapsulated within the polymer bonds. For heating

applications, the desired product of pyrolysis is usually liquid oil, which in addition to being

easier to transport, also affords a greater degree of control during combustion. Thus, at least

in the case of generating liquid fuel from plastic waste, the primary advantage of pyrolysis

lies in converting a high energy value mixture whose combustion is difficult to control, to a

substance (usually oil) with good calorific value and, importantly, affords ease of

combustion. One example of pyrolysis involves rapid heating of the plastic mixture to 400 –

500 oC (in the absence of oxygen), followed by rapid cooling, and generates a mixture

comprising methane and oil. The latter component can be further refined into diesel and other

fuels.28

In fact, a table-top pyrolysis device is shown to be effective in producing oil from

plastic waste.28

Nevertheless, the oil obtained needs to be refined before use – a net energy

requiring and greenhouse gas generating process if powered by fossil fuels. Thus, taking into

account the various energy (and sometimes, material) inputs required, liquid fuels derived

from pyrolysis may not be more environmentally sustainable than gasoline, unless the

pyrolysis process is powered by low-carbon renewable energy sources such as solar or wind

power.28

Finally, polyvinylchloride (PVC) plastic waste is not a suitable feedstock for

pyrolysis since the high temperatures required would produce toxic dioxin compounds.28

Thermal recycling

Finally, thermal recycling of plastics fits into the broader approach of recovering

residual material and energy value from waste. Specifically, dwindling supply of virgin

material from natural ores or fossil reserves, coupled with an expanding population with

higher material and energy needs, places enormous pressure on Earth‟s resources. Motivated

by the desire to satisfy increasing need for materials and energy, while partially alleviating

the pressure on Earth‟s finite biodiversity and ecosystem resources, the concept of waste

refinery - a holistic approach of mining the residual energy and material value from various

types of waste - has been advocated, and in some cases, demonstrated to be feasible at least at

the proof-of-concept stage.18

In particular, development of the waste-to-energy concept is

more advanced in developed countries such as U.S., Japan and Europe, and has engendered

the sustainable waste management movement.6 Perhaps the term thermal “recycling” is a

misnomer, since energy is the main product of the thermal route, while the small amount of

residual ash would be either deposited in landfills or used in road construction; however, the

term remains commonly used in the plastic recycling literature. Extraction of energy through

plastic combustion in waste-to-energy (or thermal recycling) facilities eliminates the difficult

(and usually costly) separation step necessary in many plastic recycling methods.

Nevertheless, the concept is not without problems. For example, though plastics have high

energy densities, possible emissions of toxic substances – in particular furans and dioxins –

during combustion presents a significant regulatory barrier to its adoption. Specifically, the

9

hazardous compounds typically arise both from the combustion of the polymer and the

multitude of additives present. Thus, the need of installing costly pollution control devices for

mitigating harmful emissions would mean that the waste-to-energy route for handling plastic

waste is likely to suffer from unfavourable process economics.

An emerging trend: up-cycling of recycled plastics

Besides the more common case of using recovered polymers in low-value

applications (i.e., down-cycling), there is an increasing emphasis, within the plastic recycling

field, at “up-cycling” plastic waste by converting them into higher value-added products such

as paramagnetic conducting carbon microspheres,51

carbon nanotubes,52

self-healing thermal

plastics, and biodegradable polyhydroxylalkanoate.53

More important, up-cycling falls under

the broader (and increasingly important) waste to high value product “remanufacturing”

approach. Even recovering the separated polymers as granular particles after precipitation

from polymer-solvent mixtures constitutes a form of up-cycling. Specifically, with high

surface area to volume ratio, hierarchical porous structure, and ease of packing within process

vessels, the recovered polymer, in its new lease of life as spherical particles, present myriad

application possibilities ranging from filler in impact-resistant materials to adsorbents for

wastewater treatment. Additionally, spherical polymeric particles are also more compatible

with existing polymer fabrication equipment such as those used in melt-extrusion or injection

molding.

Another example of up-cycling is the generation of microspheres through pyrolysis of

waste PET in supercritical carbon dioxide at 650 oC for 3 hours, followed by vacuum

annealing at 1500 oC for 0.5 hour.

58 Nevertheless, depending on process configurations and

operating strategies, the high temperature required meant that the approach‟s environmental

sustainability hinges on the process‟s overall energy utilization and carbon footprint. For

example, if fossil fuels are used for heating, the process may emit more greenhouse gases

than that arising from producing native PET from petroleum. Another approach uses catalytic

methods for carbonizing polypropylene, polystyrene and polyethylene into carbon nanotubes

suitable for fabricating into supercapacitors with good performance characteristics.13

Post-use

commercial high density polyethylene (HDPE) and regranulated HDPE (containing polyvinyl

chloride plasticizer) have also been converted, by a catalytic pyrolysis reforming process, into

hydrogen and carbon nanotubes.22

On the biological front, fermentation approaches are also

effective in using post-consumer polyethylene as substrates for microbial mediated

conversion into medium chain-length polyhydroxylalkanoate.14

In another study, L-Limonene

is first used as a solvent for precipitating polystyrene waste, and latter, as a monomer (with

polythiol as co-monomer) in ultraviolet light catalyzed thiol-ene polymerization reaction,

which yields a blended material comprising polystyrene dispersed within an elastomeric

poly(thioether) network. Characterization of the new material revealed enhanced mechanical

properties.4

10

Beyond “upcycling”, repurposing used plastics with functionalities embedded during

synthesis

Finally, there is an emerging trend towards making polymer-based products more

amenable for reuse in the same or different applications; for example, high performance

cross-linked polymers with different functional groups have been designed and shown to be

easily repurposed through temperature-induced reshaping.39

Such repurposing (without

chemical treatment) approaches should be differentiated from the upcycling methods

mentioned above since the application versatility of the polymer is designed and “built-in”

prior to its production. In comparison, up-cycling, though more desirable that downcycling,

aims to move recovered polymers to higher value applications through targeted treatment

steps, which may yield recovered polymers of differing quality and properties depending on

the extent of damage of post-use plastics and the presence of other contaminants. Similar to

up-cycling, there are many different ways of repurposing plastics that lies on the continuum

from simple direct re-deployment of used plastics to an available application (e.g., using post-

use plastics as adsorbents in wastewater treatment) to more sophisticated re-use applications

such as the thermal re-shaping example described above, that require specific functionalities

to be “encoded” within the polymer at the synthesis stage. In the case of repurposing used

plastic as adsorbents for removing pharmaceutical and personal care products (PPCPs) from

wastewater, the plastic adsorbent and the adsorbed PPCPs can be incinerated for energy

recovery upon exhaustion of sorption capacity.24

Though conceptually feasible and attractive

given the use of a waste material for removing potentially deleterious chemicals from the

environment, practical implementation of the proposal may require further research

examining possible emission products during incineration of the PPCP laden plastics

adsorbents.

Factors affecting the choice of recycling methods

In general, no method is applicable for all situations and localities and the choice of

the recycling method depends on the social and economic realities in each locality.54, 55

Additionally, the environmental performance of the plastic recycling process should be

carefully scrutinized56

using a life cycle assessment (LCA)40

approach, which, in essence, is

an environmental performance audit that takes into account all the material and energy flows

of various processing steps. The LCA method can also be integrated with other modeling

approaches such as input-output analysis or material-flow analysis to gain both a static and

dynamic picture of energy and material cycling within plastic processing operations. Wide

variation in the assumptions used in conducting the audit, however, has hampered efforts

aimed at comparing the environmental performance of different recycling approaches for the

same polymer across studies. Finally, the selection of the recycling method also depends on

the characteristics of the plastic waste such as its source, degree of compaction and extent of

contamination by other types of waste.

11

Commingled post-use plastic waste: A challenging waste stream

An analysis of the various sources of plastic waste in a typical city would reveal that

post-consumer commingled plastic waste is a particularly challenging waste stream given its

high degree of compaction and possible contamination with other types of waste material

such as paper, metals and yard waste. Studies have shown that the six principal types of

polymers found in municipal solid waste are PS, LDPE, HDPE, PP, PET and PVC,50, 59

which together accounts for up to 95% of plastic waste.60

Given the commingled nature of

municipal post-consumer plastic waste, the recycling method chosen should be able to

selectively separate various polymer types from a highly compacted waste source.

Additionally, the process should be amenable to cost-effective scale up for handling the large

volume of plastic waste typical of mid-sized cities with substantial commercial and industrial

activity. A candidate recycling method capable of tackling such a waste stream is selective

dissolution and precipitation (SDP).

Selective dissolution and precipitation for polymer recovery

Briefly, the SDP process entails the separation of a mixture of polymers, in a defined

sequence, through addition of different solvents, or progressive temperature variation. Such

an approach selectively dissolves specific polymer types from a plastics mixture, where a

polymer-solvent solution containing the dissolved polymer would be formed, and separated

from the remaining undissolved polymers through simple filtration. Specifically, the

mechanistic underpinnings of the approach lies in the existence of distinct dissolution

temperature for particular polymer-solvent pair.34

Although possibility exists of using

different solvents for dissolving specific polymers present in the waste mixture, the higher

process cost associated with utilizing different solvents (and associated sample workup) and

resulting increased risk of contamination meant that progressive temperature variation (either

increasing or decreasing) is the common approach used in SDP. Viewed from a different

perspective, using an energy separating agent such as temperature would generally lead to

less cross contamination (and thus, higher purity of recovered polymer) relative to using

different solvents as mass separating agents. As will be discussed later, a linearly increasing

temperature profile is generally preferable to a decreasing one since the total mass of plastic

waste cum solvent mixture requiring heating gradually decreases with dissolution of different

polymer; thereby, helping reduce energy use, greenhouse gas emissions and the

environmental footprint of the process. Thus, assuming an increasing temperature profile, the

temperature would progressively increase to the next set-point at which the next polymer in

line would dissolve in the solvent, leading to the formation of distinct polymer-solvent

solutions. Recovery of the dissolved polymer from the polymer-solvent solution occurs

through the addition of a suitable anti-solvent for inducing polymer precipitation – which

typically yields spherical granules as this is the geometrical form with the lowest surface

energy.34

Thus, moving down the dissolution sequence with temperature variation (or, less

common, solvent change) would help separate and recover the different polymer types in a

plastic waste mixture.

12

Selectivity of the SDP technique accrues to the existence of a unique temperature at

which a given polymer would dissolve in the chosen solvent. In general, high separation

selectivity could be achieved through a judicious choice of dissolution temperature and

solvent. Given the large variety of polymers present at varying relative abundances in typical

municipal and industrial plastic waste, “broad spectrum” solvents capable of dissolving a

variety of polymers at different dissolution temperatures would provide the necessary process

flexibility in tackling the anticipated batch-to-batch variation in plastic waste composition.

Doing so would also help reduce the number of solvents needed for separating a particular

waste mixture given the cost and process complexity associated with implementing mid-

sequence solvent change. Nevertheless, existence of unique dissolution temperatures enabling

the selective dissolution of individual polymer types in the chosen solvent will remain the key

criterion in solvent selection.

Based on studies in the literature, p-xylene and N-methyl pyrrolidone (NMP) are good

solvents for the SDP process. Similarly, 1-propanol is useful as an anti-solvent for

precipitating the dissolved polymer obtained in the first step of SDP.71

Specifically, many

studies in the literature document the utility and effectiveness of p-xylene and NMP in

dissolving a variety of native polymers such as PS,72

HDPE, LDPE,67

PET,61, 69

and PP49

in

single component recycling experiments. More recently, an interesting study demonstrates

the utility of using D-limonene as solvent for recovering polymers in SDP. Specifically, the

solvent is capable of dissolving polymers with high solubility and selectivity at ambient

conditions. More important, D-limonene can be recovered via vacuum distillation, and in

leaving a high purity polymer product behind, obviates the need of using an anti-solvent for

precipitating the dissolved polymer.

Advantages of SDP

Unlike in thermal or feedstock recycling where the plastic waste is converted to

energy and monomers, respectively, the ability to recover the original polymer is the key

advantage of SDP, which helps preserve the energy within the polymer‟s covalent bonds.61

Specifically, in removing the need for re-polymerization (such as in feedstock recycling),

SDP helps achieves significant savings in energy and material. Nevertheless, deconstructing

polymers into monomers (i.e., feedstock recycling) does offer greater application versatility,

particularly, in allowing the production of new polymer types (such as block copolymers)

from recovered monomers, which serve as modular building blocks. Additionally, compared

to most chemical/feedstock recycling methods, the SDP process is also capable of operating

at relatively low temperature and pressure,40, 45

which helps reduce the greenhouse gas

emissions and environmental footprint of plastics recycling. Depending on the solvent used,

SDP has also been shown to effect good dissolution of polymers from plastic waste at

ambient temperature and pressure. The ability to recover the solvent and anti-solvent via

distillation – and recycling them within the process - is another critical advantage of SDP,

13

which helps reduce the need for large volumes of make-up solvent and anti-solvent.34

More

important, an intrinsic advantage of SDP over other mechanical recycling methods is its

ability to handle commingled plastic waste without any presorting. Polymers with physical

and chemical properties comparable to native ones are also recovered by SDP as shown by

analysis of the recovered polymers via a variety of instrumental analysis methods such as

Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry and

viscosimetry. Additionally, FTIR spectroscopy also reveals that the characteristic peaks of

the recovered polymer are similar to those of native polymers, which indicates that the

polymers recovered by SDP are, in general, of good purity. Finally, SDP, together with

density flotation, can constitute a two-step separation method. Specifically, density flotation

could be used to first perform a coarse separation between groups of polymers with

significant density differences, while SDP could be subsequently deployed for performing a

fine-grained separation between polymers with smaller density differences. Although

theoretically feasible, the additional capital and operating costs associated with such a two-

step separation approach meant that single-step separation process – when feasible – would

always be preferable.

Disadvantages of SDP

As mentioned, possible leaching of additives from the polymers during the dissolution

step is one important drawback of SDP since additives help endow polymers with important

properties such as resistance to oxidation.34, 62-64

Nevertheless, since the recovered polymers

are unlikely to be used in the same applications as the original polymers, the loss of the

additives may even help facilitate the subsequent redeployment of the recovered polymers in

alternative applications where other additives are needed. On the other hand, there are also

cases where re-introduction of additives is not needed; for example, employing recovered

polymers as adsorbents for removing heavy metals and organic compounds from

wastewater.65, 66

Importance of dissolution sequence to SDP process design

Given that the dissolution sequence critically impacts on achievable purity and

percentage recovery of various polymers from the plastic waste mixture, its determination is

de rigueur in SDP process development. With various combinations of solvent, temperature

and their sequence of use existing within the operating envelope, determination of dissolution

sequence is also an area of creative process design. In general, the operating conditions (e.g.,

dissolution time, temperature, extent of mixing, type and concentration of solvent and anti-

solvent used, precipitation and filtration conditions) obtained from the single polymer

dissolution and precipitation studies would allow us to construct a dissolution sequence for

the progressive separation of the various polymer types in a plastic waste mixture.

Nevertheless, such an approach necessarily entails an important caveat: i.e., there would only

be small cross-interaction effects between different polymers (particularly those adjacent in a

14

sequence) on dissolution temperature and percentage recovery. Compared to few component

plastic waste mixture simulated using native polymers, determining the dissolution sequence

and other process parameters for actual post-consumer plastic waste is significantly more

difficult. Specifically, post-use plastics waste comprises other contaminants and the polymers

experienced varying levels of degradation during use, which, together will lead to

unpredictable effects on SDP selectivity and percentage recovery, and batch-to-batch

variation in process performance.

Criteria for evaluating SDP process efficiency

One of the key criteria for assessing the effectiveness of SDP in recovering waste

polymers is percentage recovery. In particular, percentage recovery is usually defined as the

percentage of polymer recovered from the original waste on a mass basis - assuming that

additives, fillers etc. only constitute a small fraction of the total mass and can be neglected.

Besides percentage recovery, selectivity is another important criterion for evaluating process

feasibility and efficiency. Universally applied in analyzing performance of separation

processes, selectivity is defined as the extent in which a sharp (or clean) separation exists

between two adjacent polymers in a dissolution sequence. Since SDP relies on selective

dissolution of individual polymer type for affording high polymer purity and percentage

recovery, determining the dissolution conditions for selective recovery of different polymers

is essential for process development. Nevertheless, clean separation between two polymers

with similar properties – though desired, is often difficult, and, in certain cases, unattainable,

given that co-dissolution of two or more types of polymers at a particular dissolution

condition might occur. Finally, though differences in dissolution temperatures between

polymer types offer opportunities for using a linear temperature profile to progressively

separate different polymers in a mixture, presence of additives and other impurities may

narrow or obviate the differences in dissolution temperatures; thereby, leading to the co-

dissolution of polymers which would otherwise be cleanly separated.

An increasing or decreasing temperature profile?

The SDP technique can be implemented either via an increasing or decreasing

temperature profile, but given the high specific heat capacity of most solvents and concerns

over the environmental footprint (i.e., greenhouse gas emissions) of heating large volumes of

solvent-cum plastic waste mixture, which profile is more environmentally friendly? The

answer depends, in large part, on the plastic waste composition: i.e., relative abundance of

high and low dissolution temperature polymers. Specifically, an increasing temperature

profile is more sustainable if a larger fraction of the plastic waste comprises high dissolution

temperature polymers, where progressive dissolution of different polymers meant that less

energy is required to heat the remaining polymers with higher dissolution temperatures.

Additionally, from a sustainability perspective, the requirement of heating the plastic waste

mixture to temperatures of more than 100 oC would, depending on the specific heat capacities

15

of the polymers and solvents, results in high energy consumption and a large carbon

footprint. Thus, practical application of the technique may involve the coupling of waste heat

generated during incineration of municipal solid waste to SDP.

What does the future portends for SDP?

Thus far, many studies have been reported on using SDP for recycling low density

polyethylene (LDPE),67

high density polyethylene (HDPE),68

polypropylene (PP),49

polyethylene teraphathlate (PET),61, 69

polystyrene (PS),70

and the separation of a mixture of

LDPE and PP.71

Nevertheless, the aforementioned studies are predominantly proof-of-

concept experiments utilizing native polymers (purchased from chemical companies) for

demonstrating the feasibility of using SDP in one-component polymer recycling. Hence, a

knowledge gap exists in examining the feasibility of using the SDP technique for recycling

various common polymers (e.g., PS, LDPE, HDPE, PP, PVC and PET) in actual plastic

waste. Currently, lab-scale plastic recycling experiments typically simulates a particular type

of polymer waste using plastic material with that specific polymer type dominating the

material composition. For example, PET mineral water bottles are commonly used to

simulate PET plastic waste. Nevertheless, differences exist between actual plastic waste

collected from municipal sources and simulated waste – particularly in the amount of

polymer degradation after use, polymer particle size, degree of contamination with other

waste etc. Thus, moving forward, validation of the SDP approach necessitates the

demonstration of the technique‟s utility and effectiveness in enabling good recovery of

polymer with high purity and desirable material characteristics from actual post-use plastic

waste mixture. Upon demonstration of both the feasibility and effectiveness (i.e., purity and

recovery yield) of SDP in recovering polymer from single component plastic waste, further

studies can be conducted to evaluate the effectiveness of the approach in recovering polymers

from multi-component plastic waste. Finally, although lab-scale studies help us gain a better

understanding of the various operating conditions (such as types and concentrations of

solvents, anti-solvents, and dissolution temperatures) important for employing SDP in

polymer recycling, a significant gap remains between bench-scale experiments and industrial-

scale implementation of SDP for recycling heterogeneous, multi-component and highly

compacted plastic waste obtained from myriad sources. Hence, larger studies (such as those

at the pilot-plant scale) are critical for validating observed effectiveness of laboratory scale

implementation of SDP, and provide a window into the potential economic viability of SDP

in municipal scale plastic recycling.

Conclusions

Society‟s reliance on plastics for a variety of applications has generated challenging

disposal and environmental sustainability issues – but is the way forward the total elimination

of plastics use? Considering the many practical benefits and conveniences that plastics has

afforded us – a visible example of which is the significant reduction in weight of cars, which

16

enables the production of more spacious vehicles with the same mass – and the tight

integration of the material in our daily life, eliminating the use of this highly beneficial

material is almost impossible. Despite much effort by municipalities around the world in

promoting or encouraging waste recycling, plastic recycling rates have remained

unsatisfactory (though with small incremental increases over the years) in cities and countries

around the world, due primarily to low take-up by the populace. While the socioeconomic

factors that hinders the more widespread adoption of recycling by society is beyond the scope

of this article, attempts at increasing recycling rates should focus on education as the

inculcation of the practice in the young would help embed the practice within the social

fabric. Naturally, media campaigns and other initiatives (such as making pre-sorting of plastic

waste easier) are positive in nudging the adult population towards incorporating the practice

of recycling into their daily life – and which may help inch recycling rates upwards. But more

can and should be done. From a technical perspective, the requirement of pre-sorting plastic

waste prior to downstream recovery processes such as chemical and feedstock recycling –

and the generally lukewarm response to large-scale city or region-wide initiatives aimed at

promoting the self-sorting of plastic waste – meant that, at present, the low fraction of pre-

sorted plastic in the waste stream remains a significant problem hampering plastic recycling.

While various plastic recycling approaches such as chemical/feedstock and thermal recycling

helps deconstruct used plastics into monomers and energy, respectively, mechanical recycling

is the only approach capable of recovering the polymer; thereby, expediting the re-use of the

polymer in the same or different applications. When coupled with recent advancement in up-

cycling approaches – where conversion of recovered polymers into higher value-added

products provide a new lease of life to an otherwise “waste” material, mechanical recycling

of polymers such as that effected by selective dissolution and reprecipitation (SDP) offers a

tantalizing opportunity for expediting the cycling of the material and energy loop associated

with plastics use. Being able to accept commingled (i.e., not pre-sorted) plastic waste as

feedstock, SDP also circumvents an important conundrum hampering the wider adoption of

many plastic recycling approaches. Although leaching of additives during dissolution results

in some material degradation of the recovered polymers, and use of non-optimal dissolution

conditions as well as solvent and anti-solvent choice introduces process inefficiencies, the

technical challenges are not insurmountable, given that methodologies for generating the

requisite solutions are available. Demonstrated to be effective in recovering polymers from

single or few-component plastic waste with high selectivity and recovery, the next logical

step for future research is the examination of the utility of SDP in recovering polymers –

suitable for high value-added applications – from complex, multi-component post-use plastic

waste mixture.

17

Further reading

(1) Law, K. L.; Morét-Ferguson, S. E.; Goodwin, D. S.; Zettler, E. R.; DeForce, E.;

Kukulka, T.; Proskurowski, G. Distribution of Surface Plastic Debris in the Eastern Pacific

Ocean from an 11-Year Data Set. Environ. Sci. Technol. 2014, 48 (9), 4732-4738.

(2) Rochman, C. M.; Hoh, E.; Kurobe, T.; Teh, S. J. Ingested plastic transfers hazardous

chemicals to fish and induces hepatic stress. Sci. Rep. 2013, doi:10.1038/srep03263

(3) Velzeboer, I.; Kwadijk, C. J. A. F.; Koelmans, A. A. Strong Sorption of PCBs to

Nanoplastics, Microplastics, Carbon Nanotubes, and Fullerenes. Environ. Sci. Technol. 2014,

48 (9), 4869-4876.

(4) Hearon, K.; Nash, L. D.; Rodriguez, J. N.; Lonnecker, A. T.; Raymond, J. E.; Wilson,

T. S.; Wooley, K. L.; Maitland, D. J. A High-Performance Recycling Solution for

Polystyrene Achieved by the Synthesis of Renewable Poly(thioether) Networks Derived from

d-Limonene. Adv. Mater. 2013, doi:10.1002/adma.201304370.

(5) Li, R.; Zhang, W.; Ji, S. Automated identification of cell-type-specific genes in the

mouse brain by image computing of expression patterns. BMC Bioinformatics 2014, 15 (1),

209.

(6) Castaldi, M. J. Perspectives on Sustainable Waste Management. Annual Review of

Chemical and Biomolecular Engineering 2014, 5 (1), 547-562.

(7) Wright, S. L.; Rowe, D.; Thompson, R. C.; Galloway, T. S. Microplastic ingestion

decreases energy reserves in marine worms. Current biology : CB 2013, 23 (23), R1031-

R1033.

(8) Rochman, C. M.; Manzano, C.; Hentschel, B. T.; Simonich, S. L. M.; Hoh, E.

Polystyrene Plastic: A Source and Sink for Polycyclic Aromatic Hydrocarbons in the Marine

Environment. Environ. Sci. Technol. 2013, 47 (24), 13976-13984.

(9) Xanthos, M. Recycling of the #5 Polymer. Science 2012, 337 (6095), 700-702.

(10) Lu, X.; Patel, S.; Zhang, M.; Woo Joo, S.; Qian, S.; Ogale, A.; Xuan, X. An

unexpected particle oscillation for electrophoresis in viscoelastic fluids through a

microchannel constrictiona). Biomicrofluidics 2014, 8 (2), -.

(11) Shah, A.; Kato, S.; Shintani, N.; Kamini, N.; Nakajima-Kambe, T. Microbial

degradation of aliphatic and aliphatic-aromatic co-polyesters. Appl. Microbiol. Biotechnol.

2014, 98 (8), 3437-3447.

18

(12) Nakajima-Kambe, T.; Ichihashi, F.; Matsuzoe, R.; Kato, S.; Shintani, N. Degradation

of aliphatic–aromatic copolyesters by bacteria that can degrade aliphatic polyesters. Polym.

Degrad. Stab. 2009, 94 (11), 1901-1905.

(13) Wen, X.; Chen, X.; Tian, N.; Gong, J.; Liu, J.; Rümmeli, M. H.; Chu, P. K.;

Mijiwska, E.; Tang, T. Nanosized Carbon Black Combined with Ni2O3 as “Universal”

Catalysts for Synergistically Catalyzing Carbonization of Polyolefin Wastes to Synthesize

Carbon Nanotubes and Application for Supercapacitors. Environ. Sci. Technol. 2014, 48 (7),

4048-4055.

(14) Guzik, M.; Kenny, S.; Duane, G.; Casey, E.; Woods, T.; Babu, R.; Nikodinovic-

Runic, J.; Murray, M.; O‟Connor, K. Conversion of post consumer polyethylene to the

biodegradable polymer polyhydroxyalkanoate. Appl. Microbiol. Biotechnol. 2014, 98 (9),

4223-4232.

(15) Foekema, E. M.; De Gruijter, C.; Mergia, M. T.; van Franeker, J. A.; Murk, A. J.;

Koelmans, A. A. Plastic in North Sea Fish. Environ. Sci. Technol. 2013, 47 (15), 8818-8824.

(16) Kaposi, K. L.; Mos, B.; Kelaher, B. P.; Dworjanyn, S. A. Ingestion of Microplastic

Has Limited Impact on a Marine Larva. Environ. Sci. Technol. 2013, 48 (3), 1638-1645.

(17) Tang, Z.; Huang, Q.; Cheng, J.; Yang, Y.; Yang, J.; Guo, W.; Nie, Z.; Zeng, N.; Jin,

L. Polybrominated Diphenyl Ethers in Soils, Sediments, and Human Hair in a Plastic Waste

Recycling Area: A Neglected Heavily Polluted Area. Environ. Sci. Technol. 2014, 48 (3),

1508-1516.

(18) Tonini, D.; Martinez-Sanchez, V.; Astrup, T. F. Material Resources, Energy, and

Nutrient Recovery from Waste: Are Waste Refineries the Solution for the Future? Environ.

Sci. Technol. 2013, 47 (15), 8962-8969.

(19) Nature Geoscience Editorial Message in a bottle. Nature Geosci 2013, 6 (4), 241-241.

(20) Law, K. L.; Morét-Ferguson, S.; Maximenko, N. A.; Proskurowski, G.; Peacock, E.

E.; Hafner, J.; Reddy, C. M. Plastic Accumulation in the North Atlantic Subtropical Gyre.

Science 2010, 329 (5996), 1185-1188.

(21) Zettler, E. R.; Mincer, T. J.; Amaral-Zettler, L. A. Life in the „Plastisphere‟: Microbial

communities on plastic marine debris. Environ. Sci. Technol. 2013, doi:10.1021/es401288x.

(22) Wu, C.; Nahil, M. A.; Miskolczi, N.; Huang, J.; Williams, P. T. Processing Real-

World Waste Plastics by Pyrolysis-Reforming for Hydrogen and High-Value Carbon

Nanotubes. Environ. Sci. Technol. 2013, 48 (1), 819-826.

19

(23) Rahman, A.; Linton, E.; Hatch, A.; Sims, R.; Miller, C. Secretion of

polyhydroxybutyrate in Escherichia coli using a synthetic biological engineering approach.

Journal of Biological Engineering 2013, 7 (1), 24.

(24) Klika, K. D. Waste Plastic and Pharmaceuticals, Could an Integrated Solution Help?

Environ. Sci. Technol. 2013, 47 (18), 10111-10112.

(25) Engler, R. E. The Complex Interaction between Marine Debris and Toxic Chemicals

in the Ocean. Environ. Sci. Technol. 2012, 46 (22), 12302-12315.

(26) Cole, M.; Lindeque, P.; Fileman, E.; Halsband, C.; Goodhead, R.; Moger, J.;

Galloway, T. S. Microplastic Ingestion by Zooplankton. Environ. Sci. Technol. 2013, 47 (12),

6646-6655.

(27) Tsui, A.; Wright, Z. C.; Frank, C. W. Biodegradable Polyesters from Renewable

Resources. Annual Review of Chemical and Biomolecular Engineering 2013, 4 (1), 143-170.

(28) Inman, M. Cooking up fuel. Nature Clim. Change 2012, 2 (4), 218-220.

(29) Vergara, S. E.; Tchobanoglous, G. Municipal Solid Waste and the Environment: A

Global Perspective. Annual Review of Environment and Resources 2012, 37 (1), 277-309.

(30) Young, E. W. K.; Berthier, E.; Beebe, D. J. Assessment of Enhanced

Autofluorescence and Impact on Cell Microscopy for Microfabricated Thermoplastic

Devices. Anal. Chem. 2012, 85 (1), 44-49.

(31) Koelmans, A. A.; Besseling, E.; Wegner, A.; Foekema, E. M. Plastic as a Carrier of

POPs to Aquatic Organisms: A Model Analysis. Environ. Sci. Technol. 2013, 47 (14), 7812-

7820.

(32) Wei, R.; Oeser, T.; Billig, S.; Zimmermann, W. A high-throughput assay for

enzymatic polyester hydrolysis activity by fluorimetric detection. Biotechnology Journal

2012, 7 (12), 1517-1521.

(33) Rochman, C. M.; Browne, M. A.; Halpern, B. S.; Hentschel, B. T.; Hoh, E.;

Karapanagioti, H. K.; Rios-Mendoza, L. M.; Takada, H.; Teh, S.; Thompson, R. C. Policy:

Classify plastic waste as hazardous. Nature 2013, 494 (7436), 169-171.

(34) Sperber, R. J.; Rosen, S. L. Recycling of thermoplastic waste: phase equilibrium in

polystyrene-PVC-polyolefin solvent systems. Polym. Eng. Sci. 1976, 16 (4), 246-251.

(35) Philp, J. C.; Ritchie, R. J.; Guy, K. Biobased plastics in a bioeconomy. Trends

Biotechnol. 2013, 31 (2), 65-67.

(36) Spokas, K. Plastics - still young, but having a mature impact. Waste Manage. 2008,

28 (3), 473-474.

20

(37) Matsumoto, K. i.; Taguchi, S. Enzyme and metabolic engineering for the production

of novel biopolymers: crossover of biological and chemical processes. Curr. Opin.

Biotechnol. 2013, (0).

(38) Han, J.; Hou, J.; Zhang, F.; Ai, G.; Li, M.; Cai, S.; Liu, H.; Wang, L.; Wang, Z.;

Zhang, S.; Cai, L.; Zhao, D.; Zhou, J.; Xiang, H. Multiple Propionyl Coenzyme A-Supplying

Pathways for Production of the Bioplastic Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) in

Haloferax mediterranei. Appl. Environ. Microbiol. 2013, 79 (9), 2922-2931.

(39) Yu, S.; Zhang, R.; Wu, Q.; Chen, T.; Sun, P. Bio-Inspired High-Performance and

Recyclable Cross-Linked Polymers. Adv. Mater. 2013, doi:10.1002/adma.201301513.

(40) Perugini, F.; Mastellone, M. L.; Arena, U. A life cycle assessment of mechanical and

feedstock recycling options for management of plastic packaging wastes. Environ. Prog.

2005, 24 (2), 137-154.

(41) Saito, T.; Satoh, I. Thermal strategy for the separation of a polymer mixture. Polym.

Eng. Sci. 2005, 45 (10), 1419-1425.

(42) Wei, J.; Realff, M. J. Design and optimization of free-fall electrostatic separators for

plastics recycling. AlChE J. 2003, 49 (12), 3138-3149.

(43) Shent, H.; Pugh, R. J.; Forssberg, E. A review of plastics waste recycling and the

flotation of plastics. Resour. Conserv. Recy. 1999, 25 (2), 85-109.

(44) Keane, M. A. Catalytic conversion of waste plastics: focus on waste PVC. J. Chem.

Technol. Biotechnol. 2007, 82 (9), 787-795.

(45) Sasse, F.; Emig, G. Chemical recycling of polymer materials. Chem. Eng. Technol.

1998, 21 (10), 777-789.

(46) Kamimura, A.; Yamamoto, S. An efficient method to depolymerize polyamide

plastics: a new use of ionic liquids. Org. Lett. 2007, 9 (13), 2533-2535.

(47) Hata, S.; Goto, H.; Yamada, E.; Oku, A. Chemical conversion of poly(carbonate) to

1,3-dimethyl-2-imidazolidinone (DMI) and bisphenol A: A practical approach to the

chemical recycling of plastic wastes. Polymer 2002, 43 (7), 2109-2116.

(48) Poller, R. C. Reclamation of waste plastics and rubber: recovery of materials and

energy. J. Chem. Technol. Biotechnol. 1980, 30 (1), 152-160.

(49) Poulakis, J. G.; Papaspyrides, C. D. Recycling of polypropylene by the

dissolution/reprecipitation technique: I. A model study. Resour. Conserv. Recy. 1997, 20 (1),

31-41.

21

(50) Simoneit, B. R. T.; Medeiros, P. M.; Didyk, B. M. Combustion products of plastics as

indicators for refuse burning in the atmosphere. Environ. Sci. Technol. 2005, 39 (18), 6961-

6970.

(51) Pol, V. G. Upcycling: Converting waste plastics into paramagnetic, conducting, solid,

pure carbon microspheres. Environ. Sci. Technol. 2010, 44 (12), 4753-4759.

(52) Pol, V. G.; Thiyagarajan, P. Remediating plastic waste into carbon nanotubes. J.

Environ. Monit. 2010, 12 (2), 455-459.

(53) Kenny, S. T.; Runic, J. N.; Kaminsky, W.; Woods, T.; Babu, R. P.; Keely, C. M.;

Blau, W.; O‟Connor, K. E. Up-cycling of PET (polyethylene terephthalate) to the

biodegradable plastic PHA (polyhydroxyalkanoate). Environ. Sci. Technol. 2008, 42 (20),

7696-7701.

(54) Drain, K. F.; Murphy, W. R.; Otterburn, M. S. Polymer waste - Resource recovery.

Conserv. Recy. 1981, 4 (4), 201-218.

(55) Kang, H. Y.; Schoenung, J. M. Electronic waste recycling: A review of U.S.

infrastructure and technology options. Resour. Conserv. Recy. 2005, 45 (4), 368-400.

(56) Patel, S. H.; Xanthos, M. Environmental issues in polymer processing: a review on

volatile emissions and material/energy recovery options. Adv. Polym. Tech. 2001, 20 (1), 22-

41.

(57) Rochman, C. M.; Hoh, E.; Hentschel, B. T.; Kaye, S. Long-Term Field Measurement

of Sorption of Organic Contaminants to Five Types of Plastic Pellets: Implications for Plastic

Marine Debris. Environ. Sci. Technol. 2012, 47 (3), 1646-1654.

(58) Hu, H.; Gao, L.; Chen, C.; Chen, Q. Low-Cost, Acid/Alkaline-Resistant, and

Fluorine-Free Superhydrophobic Fabric Coating from Onionlike Carbon Microspheres

Converted from Waste Polyethylene Terephthalate. Environ. Sci. Technol. 2014, 48 (5),

2928-2933.

(59) Huth-Fehre, T.; Feldhoff, R.; Kantimm, T.; Quick, L.; Winter, F.; Cammann, K.; van

den Broek, W.; Wienke, D.; Melssen, W.; Buydens, L. NIR - Remote sensing and artificial

neural networks for rapid identification of post consumer plastics. J. Mol. Struct. 1995, 348,

143-146.

(60) Braun, D. Recycling of PVC. Prog. Polym. Sci. 2002, 27 (10), 2171-2195.

(61) Vane, L. M.; Rodriguez, F. Dissolution and crystallization behavior of poly(ethylene

terephthalate)-diluent mixtures. J. Appl. Polym. Sci. 1993, 49 (5), 765-776.

(62) Drain, K. F.; Murphy, W. R.; Otterburn, M. S. A solvent technique for the recycling

of polypropylene. Degradation on recycling. Conserv. Recy. 1983, 6 (3), 123-137.

22

(63) Pospisil, J.; Sitek, F. A.; Pfaendner, R. Upgrading of recycled plastics by

restabilization -- An overview. Polym. Degrad. Stab. 1995, 48 (3), 351-358.

(64) Pfaendner, R. How will additives shape the future of plastics? Polym. Degrad. Stab.

2006, 91 (9), 2249-2256.

(65) Wang, Z.; Bai, R.; Ting, Y. P. Conversion of waste polystyrene into porous and

functionalized adsorbent and its application in humic acid removal. Ind. Eng. Chem. Res.

2008, 47 (6), 1861-1867.

(66) Wang, Z.; Bai, R. Preparing microgranules from waste polystyrene through a novel

temperature- and nonsolvent-induced phase separation method for potential adsorbent. Ind.

Eng. Chem. Res. 2005, 44 (4), 825-831.

(67) Papaspyrides, C. D.; Poulakis, J. G.; Varelides, P. C. A model recycling process for

low density polyethylene. Resour. Conserv. Recy. 1994, 12 (3-4), 177-184.

(68) Poulakis, J. G.; Papaspyrides, C. D. The dissolution/reprecipitation technique applied

on high-density polyethylene: I. model recycling experiments. Adv. Polym. Tech. 1995, 14

(3), 237-242.

(69) Poulakis, J. G.; Papaspyrides, C. D. Dissolution/reprecipitation: A model process for

PET bottle recycling. J. Appl. Polym. Sci. 2001, 81 (1), 91-95.

(70) Kampouris, E. M.; Papaspyrides, C. D.; Lekakou, C. N. Model process for the solvent

recycling of polystyrene. Polym. Eng. Sci. 1988, 28 (8), 534-537.

(71) Pappa, G.; Boukouvalas, C.; Giannaris, C.; Ntaras, N.; Zografos, V.; Magoulas, K.;

Lygeros, A.; Tassios, D. The selective dissolution/precipitation technique for polymer

recycling: A pilot unit application. Resour. Conserv. Recy. 2001, 34 (1), 33-44.

(72) Kampouris, E. M.; Papaspyrides, C. D.; Lekakou, C. N. A model recovery process for

scrap polystyrene foam by means of solvent systems. Conserv. Recy. 1987, 10 (4), 315-319.

(73) Kabamba, E. T.; Rodrigue, D. The effect of recycling on LDPE foamability:

Elongational rheology. Polym. Eng. Sci. 2008, 48 (1), 11-18.

(74) Miller-Chou, B. A.; Koenig, J. L. A review of polymer dissolution. Prog. Polym. Sci.

2003, 28 (8), 1223-1270.

(75) Starnes, W. H. Structural and mechanistic aspects of the thermal degradation of

poly(vinyl chloride). Prog. Polym. Sci. 2002, 27, 2133-2170.

(76) Arnold, J. C.; Maund, B. The properties of recycled PVC bottle compounds. 1:

mechanical performance. Polym. Eng. Sci. 1999, 39 (7), 1234-1241.

23

(77) Rostkowski, K. H.; Criddle, C. S.; Lepech, M. D. Cradle-to-gate life cycle assessment

for a cradle-to-cradle cycle: Biogas-to-bioplastic (and back). Environ. Sci. Technol. 2012, 46

(18), 9822-9829.

(78) Heinrich, D.; Andreessen, B.; Madkour, M. H.; Al-Ghamdi, M. A.; Shabbaj, I. I.;

Steinbüchel, A. From Waste to Plastic: Synthesis of Poly(3-Hydroxypropionate) in

Shimwellia blattae. Appl. Environ. Microbiol. 2013, 79 (12), 3582-3589.