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AN INTRODUCTORY REVIEW MECHANICAL RECYCLING OF POLYMERS FOR DUMMIES

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Increase of notched impact strength combined with a decreasing flexural modulus for recycled abs from weee and elv with (blue x) no impact modifier, (orange x) 5 wt%, (grey x) 10 wt% and (yellow x) 20 wt% impact modifier sebs-g-ma (altered figure with permission from [77]) for mixed waste streams by adding an appropriate modifier, often refer to as compatibilisers [39], [78]-[81]. Compatibilisers can mainly be divided into three groups (Figure 11):  Block copolymers  Copolymers with functional groups  Graft copolymers They all attempt to minimize the interfacial distance and interconnect both phases by either using the same polymers as the matrix and the dispersed phase polymer or by interaction or reaction between the compatibilisers' moieties and the disperse phase and the compatible backbone polymers with the matrix. Grafted compatibilisers exist out of a grafted monomers and a backbone polymer. The latter can be preferentially the matrix polymer or an elastomer, the graft monomers are often maleic anhydride, glycidyl methacrylate, methacrylic acid, etc. [72], [74], [79], [80], [82]-[84]. The effectiveness of the compatibiliser strongly depends on: architecture of the block and/or copolymers, the reactivity of the moieties (when used) and the diffusion possibility of the compatibiliser across the interface. Condensation polymers offer the grand advantage of end-groups which have the possibility to react with the functional groups of the compatibiliser. Most important is that the interfacial tension of the compatibiliser is in between the one of both dispersed and matrix phase wherefore it will preferentially move towards the interphase. There are plenty of comparisons in literature of different types of blends, with the optimal amount and which compatibiliser one should select.
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PLASTICS TO RESOURCE
AN INTRODUCTORY REVIEW
MECHANICAL RECYCLING OF POLYMERS FOR DUMMIES
Laurens Delva1 | Karen Van Kets1 | Maja Kuzmanović1 | Ruben Demets1,2 | Sara Hubo1 | Nicolas Mys1,2 | Steven De Meester2 |
Kim Ragaert1,*
1Centre for Polymer and Material Technologies,
Department of Materials, Textiles and Chemical
Engineering, Ghent University, Belgium
²Department of Green Chemistry and
Technology, Ghent University, Belgium
Correspondence
Laurens.Delva@ugent.be (L.D.)
Karen.VanKets@ugent.be (K.V.K.)
Maja.Kuzmanovic@ugent.be (M.K.)
Ruben.Demets@ugent.be (R.D.)
Sara.Hubo@ugent.be (S.H.)
Nicolas.Mys@ugent.be (N.M.)
Steven.DeMeester@ugent.be (S.D.M.)
* Kim.Ragaert@ugent.be (K.R.)
Tel.: + 32 (0)9 3310392
Authors’ introduction
It’s a bit unusual perhaps, to name this introductory review ‘Mechanical Recycling
for Dummies
’. The text provides an accessible introduction to the technical aspects
of mechanical recycling; it is aimed at (polymer) engineering students, needing a
general introduction. Or at polymer scientists, wanting to explore the world of
recycling and likewise, at recyclers wanting an insight into why polymers are so
damn complex to recycle. None of these target groups are dummies. But all of them
are out of their comfort zone with at least one section of this review. Recognize
yourself in this profile? Then go ahead, read. Recycling (mechanical or otherwise)
of polymers is a complex matter, which really does require a multidisciplinary
understanding to do well. We have chosen to go broad rather than deep in terms of
content. We have selected to just share instead of publishing in a journal, so that
anyone can access the text. If you would like to this use educationally, share it
within your company or network, feel free. But please, do remember to make the
proper references to us as authors. Proper courtesy will keep us encouraged to
provide you with more of these open papers.
Abstract
This article will give you a comprehensive overview of the recycling of polymers with a strong focus on mechanical recycling. Starting with
an overview of basic waste management hierarchy, the manuscript continues with the principles and scientific challenges associated with
mechanical recycling. Furthermore, different industrial technologies are highlighted focusing on post-consumer polyester (PET) bottle-to-
bottle recycling, solid plastic waste (SPW) from post-consumer packaging waste and SPW from waste from electrical and electronic (WEEE)
equipment. In addition, various additives commonly found in or added to recycled polymers are discussed.
Keywords
Recycling; Mechanical Recycling; Polymers; Solid Plastic Waste
1 INTRODUCTION
Plastics combine some inherent qualities like low density,
durability, low cost, good processing capabilities and corrosion
resistance making them usable in a wide application range [1], [2].
The production of plastics has risen from 1.5 million tonnes in 1950
to a staggering 348 million tonnes in the year 2017
(64.4 MT for Europe), leading to a total world production of 9
billion tonnes of plastics in the last 65 years. After China, the
biggest annual producers of plastics (29.4% of total production of
thermoplastics and polyurethanes), are Europe and NAFTA, both
good for respectively 18.5 and 17.7% of the global amount in 2017
[3]. The plastic challenges associated with these number involve
mainly climate change measured in the carbon footprint and the
plastic pollution causing enormous plastic leakage into the
oceans [4].
Plastics possess a large potential to improve their circularity
because of low recycling rates, therefore CO2 gains could be
substantial. Mechanical recycling i.e. cleaning, re-melting and
upgrading of plastic waste produces less than 20% of the CO2
emissions associated with making new plastics [5]. In the
European Union, the potential for recycling plastic waste remains
largely unexploited . Reuse and recycling of end-of-life plastics
remains very low, particularly in comparison with other materials
such as paper, glass or metals [4].
This article gives a comprehensive overview of the mechanical
recycling of polymers. It commences with an overview of the
hierarchy within waste management, followed by the principles
and scientific challenges associated with mechanical recycling.
Furthermore, different industrial mechanical recycling
technologies are highlighted focusing on post-consumer
poly(ethylene terephthalate) (PET) bottle-to-bottle recycling,
recycling of solid plastic waste (SPW) from post-consumer
packaging waste and recycling of SPW from waste from electrical
and electronic equipment (WEEE). In addition, various additives
commonly found in or added to recycled polymers are discussed.
2 SOLID PLASTIC WASTE (SPW)
2.1 Origins of plastic waste
The overall plastics market is dominated by the ‘plastic big 5’,
namely polypropylene (PP), polyethylene (PE), polyvinyl chloride
(PVC), polystyrene (PS) and PET, who together make up around
70% of all manufactured plastics worldwide. In Europe, the
dominating markets for these plastics are packaging (40%),
building and construction (20%), automotive (10%), electronic and
electronical (6%) and agriculture (3%). All other markets
combined (consumer and household goods, furniture, sport,
health and safety) make up the remaining large of 20% [3]. An
overview of these markets and their typically used polymers is
shown in Figure 1.
Since competition for natural resources increases globally,
markets such as plastics production are becoming more
vulnerable to access and prices [6].
The European economy is currently experiencing a strong drive
towards a focus on sustainability, both in production and
consumption of products. European countries have evolved over
the last two decades from a focus on disposal methods to a
greater focus on prevention and recycling. Recycling is considered
a major waste reduction strategy, as part of the 4Rs strategy:
reduce, reuse, recycle (materials), recover (energy) [7]. Recent
pivotal documents include the European Commission’s Circular
Economy Package [8] and the New Plastics Economy report by the
Ellen Mac Arthur Foundation [9].
Most of the plastic waste is generated by households and (food)
packaging waste in turns dominates this waste stream. In 2010,
the average European generated 513 kg municipal solid waste
(MSW). Generation of municipal waste per capita has declined
slightly from 2004 to 2012, but it is clearly better managed now
than ten years ago. Countries that have developed efficient
municipal-waste management systems generally perform better
in overall waste management. The number of countries recycling
and composting more than 30% of municipal waste increased
from 11 to 17 out of 35, and those landfilling more than 75% of their
municipal waste declined from 11 to 8 [6].
Recycling rates are steadily increasing. In 2016, 27.1 million tonnes
of post-consumer plastics waste ended up in the official waste
streams. 72.7% was recovered through recycling and energy
recovery processes while 27.3% still went to landfill. This trend of
increasing recycling and recovery rates and decreasing landfill is
shown in Figure 2 [3].
There is a strong legislative drive to continuously increase
recycling rates for plastics, especially given the ambitious targets
set in the recent Circular Economy Package [9] and its subsequent
action plans, which foresee a common EU recycling target of 65%
of all packaging waste by 2025 (and 75% by 2030), including a
55% recycling target specifically for plastics packaging.
P a g e | 3
FIGURE 1. Plastics demand by polymer and market segment (2017) [3]
FIGURE 2. 2006-2016 waste treatment evolution in Europe; green: recycling; blue: energy recovery; red: landfilling [3]
P a g e | 4
2.2 Waste Management Hierarchy
Like other waste streams, plastic waste has to be coped with in
the most sustainable way. The waste management hierarchy in
Figure 3 presents a priority list of practices to process waste
streams. The list prioritizes the different waste handling methods
from least environmentally friendly to most sustainable [10].
Prevention can be found in the uppermost position of this waste
management hierarchy. ‘Prevention is better than cure’ is the
primordial objective in waste management, because no collection
and processing of the waste materials are necessary. The second
most desirable option is re-use, in which the waste materials or
products do not undergo structural changes, and are used again
for the original purpose.
However, it is inevitable that extensive amounts of plastic waste
will still come into existence. Therefore, the focus has to be in the
recycling of these waste streams with a view to closing loops and
grow further into a ‘circular economy’ way of thinking. The
recycling pathway, either mechanical or chemical, leads to
secondary raw materials which can be used for their original or
other purposes. When the previous methods cannot be applied,
energy recuperation through incineration is the preferred option.
Landfilling is the last resort and has to be avoided at all times.
FIGURE 3. Adapted figure from [11]: The Waste Management Hierarchy
2.2.1 Prevention
Prevention is defined [12] as the measures taken before a
substance, material or product has become waste, that reduce
either (i) the total quantitative amount of waste generate, (ii) the
negative impacts of the generated waste on the environment and
human health or (iii) the content of harmful substances in
materials and products.
Different measures can be undertaken to prevent new waste.
Extension of the lifetime of a product can be seen as prevention
effort. Through material quality, better and/or modular product
design in which spare parts are foreseen, product lifespan can be
prolonged. Better product design, involving a reduction (e.g.
thinner product or optimised volume/weight ratio) in material
use, also reduces the total plastic waste generated. Furthermore,
the substitution of plastics by other materials (paper, wood, glass,
metal, etc.) can lead to a smaller amount of plastic waste,
however, life-cycle assessment studies should be performed to be
able to choose the most sustainable material. Finally, the design
for recyclability principle will contribute to the prevention of
plastic waste materials. Besides lowering the amount of plastic
used, the focus also has to be put on reducing hazardous
compounds in the plastic production [13].
In order to develop a product with the lowest possible impact on
the environment, all stages in the product’s life cycle have to be
considered. An optimal eco-design of plastic products will have a
positive influence on its recyclability (at end-of-life) and the
degree to which they can incorporate recycled materials (at start-
of-life) [14]. The European Union strongly focuses on growing
towards a Circular Economy and sets out directives for its
countries. The Eco-design Directive (2009/125/EC) strongly
advices the Design for Recycling principle, in which products are
being developed regarding a straightforward recycling. Efficient
material use and easy dismantling of the diverse materials are the
key measures of this principle. However, in this approach, the
Design for Recycling strategy only concentrates on the product’s
end-of-life. Hence, it is also essential to aim attention at the
design at the product’s start-of-life. Therefore, Design from
Recycling [15], [16] is a complementary counterpart of the Design
for Recycling principle. Recycled polymer material are matched
with potential products based on their strengths and weaknesses.
2.2.2 Re-use
Re-use involves products which have been designed and produced
to fulfil a minimal amount of rotations over their life span. Mostly
these products are selectively collected, reconditioned and
reprocessed for the same purpose. The best known example is the
multi-trip plastic crates and pallets. Other examples include
refillable drink bottles and reusable plastic containers. Products
5 | P a g e
sold through second-hand stores (clothing, toys, electronics, etc.)
are categorized under re-use.
In order to let prevention and re-use succeed, the awareness of
the consumer related to the exhaustibility of earth’s resources,
has to be (further) elevated [14]. For example, consumers have to
recognize that the consummation of single-use grocery bags is
unacceptable.
2.2.3 Recycling
a. Mechanical recycling
Mechanical recycling involves only mechanical processes
(grinding, washing, separating, drying, re-granulating and
compounding) [14]. The obtained recyclates can replace virgin
plastics in the fabrication of new plastic products. Common
processing techniques after re-melting are injection moulding,
extrusion, rotational moulding and heat pressing [17] [18].
This technique is only applicable on thermoplastic materials, as
thermosets will not re-melt. Examples of mechanical recycling of
post-consumer plastics waste:
Collection and grinding of sorted, clean PP crates and blending
of the regrind with virgin polymer to mould new crates;
Collection of low density polyethylene (LDPE) films used in
agriculture and industrial packaging, pre-washing, grinding,
washing, separating, drying and melt-filtration/re-
granulation and processing into refuse bags;
Collection and sorting of PET bottles used for drinks
packaging, grinding, washing, separating, drying and
processing into polyester fibres, sheets or containers [19].
As mechanical recycling is the most ubiquitous in today’s industry,
this will form the focus of the remainder of this review.
b. Chemical recycling
Also called thermochemical or feedstock recycling, chemical
recycling involves mechanisms in which the collected plastic
waste is chemically degraded into its monomers or other basic
chemicals. The processes involved are amongst others hydrolysis,
pyrolysis, hydrocracking and gasification [14] [18] [22]. The output
may be reused for polymerisation into new plastics for the
production of other chemicals or as an alternative fuel [23].
Although various techniques have been successfully established
in the promising field of feedstock recycling, the industry suffers
from high investment levels, high energy consumption and high
input levels, making only very large plants economically viable at
the current time [24].
c. Energy recovery
Incineration of polymers releases the caloric value of the
polymers and is also considered as a recycling process. It is a
common and highly practiced method for waste reduction and
energy recovery [19], often used upon highly contaminated or
complex polymers waste streams, such as medical waste and
hazardous-goods packaging. While energy recovery unavoidably
terminates the lifecycle of a polymer product, it remains
preferable to landfill.
3 MECHANICAL RECYCLING OF POLYMERS
3.1 Introduction
Plastic waste can be recycled in different ways depending on
types of polymers, product and packaging design, if the products
consist of the single polymer or mixed polymers [7]. Mechanical
recycling is one of the most common methods for recycling of
thermoplastic polymers such as PP, PE and PET [15, 25]. This
process implies collection, sorting, washing and grinding of the
material [14], [18]. A schematic overview is presented in Figure 4.
Collection and sorting are discussed separately below. Further,
washing of products is a mandatory step for removal of food
residues, pulp fibres or adhesives. There are various techniques to
remove residues, e.g. via wet by water or dry cleaning of the
surfaces through friction without using water [7]. Afterwards, the
size reduction from products to flakes via grinding is the last step
in mechanical recycling. The compounding and pelletizing can be
the optional reprocessing of the flakes into granulate due to
easier work for converters [14].
3.2 Collection
As the first step in the value chain of mechanical recycling,
collection systems play a crucial role in the conversion of waste
raw materials into new plastic products. Collection schemes
should be optimised, on the grounds that they determine the
composition of the waste streams and accordingly the
downstream procedures, like the pre-treatment, separation and
recovery operations [20]. These collection operations should be
cost-effective and, approved and supported by waste owners in
order to counteract landfilling. There are four main collection
methods for plastic packaging waste: kerbside, drop-off, buy-back
and deposit/refund programs [21]:
Kerbside collection is the most widely accessible collection
method. For civilians, it is the most convenient procedure to
participate in, what results in high recovery rates. Residents
are requested to separate potential valuable recyclables
(plastic, paper and cardboard, metals) from their household
waste, commingled or not, into special receptacles or bags.
In drop-off recycling, different containers for designated
materials to be recycled, are placed at central community
places. Such collection programs often suffer from low or
unpredictable throughput.
Buy-back centres, mostly run by private companies, purchase
recyclables from consumers. These centres impose
specifications to the recyclable waste materials, which
makes the contamination level low.
Deposit/refund programs imply the refunding of a deposit
when plastic containers are returned to the appropriate
redemption centre, or to the original seller.
Besides the municipal plastic waste fraction, proper collection
schemes must be supported/designed for the acquiring of
valuable raw materials in electrical and electronic appliances,
end-of-life vehicles and plastic agricultural films. Furthermore, it
will be important to harmonize the different existing collection
systems. In addition to the plastic waste collection for individuals,
organized sorting and collection of plastic waste from companies
is extremely important. Good waste management of plastic
industrial waste greatly improves the uptake rates of potentially
reusable and recyclable materials. Often companies are rewarded
with incentives.
3.3 Sorting
The incoming waste streams consists of mixed plastics of
unknown composition and is likely contaminated by organic
fractions (such as food residues) and non-plastic inorganic
fractions (metals, wood, paper, …) [22]. Cleaning, regrinding and
sorting of the waste stream will be necessities in the light of
qualitative products through mechanical recycling. An optimal
sorting plant knows four different stages to separate the
incoming plastic waste streams [23]. First of all, the non-plastic
contaminations (metals, wood, paper, etc.) must be removed. The
plastic fraction has to be separated in rigid and non-rigid (e.g.
foils) components. In order to obtain a good recycled product, the
plastic waste materials preferably have to be divided into
coloured and non-coloured (transparent) fractions. At last, the
different plastic types have to be sorted out.
In the first place, metals have to be removed from the waste
stream, as these can damage the recycling plant’s machinery.
FIGURE 4. Scheme of the basic principal steps in a mechanical recycling process
7 | P a g e
Ferrous metals are simply removed by utilizing magnets. The
removal of non-ferrous metals (mostly aluminium, but also zinc,
copper, lead) is based on the induction of eddy currents. An eddy
current separator consists of a conveyor belt and a high speed,
independent magnetic rotor, which is capable of generating a
strong, very rapidly alternating magnetic field [24]. Once the
conducting particles pass the rotor, eddy currents are induced and
the non-ferrous particles are repelled by the magnetic field.
To sort out the non-rigid (foils, bags) from the rigid plastics, wind
sifters are often applied. These devices blow or suck out the non-
rigid plastic waste with one ventilator based on differences in
specific weight (surface to mass ratio) [25]. Wind sifters are also
able to separate paper contaminations (like etiquettes) from the
rigid plastic fraction. Ballistic separators, which consist of a
shaking screen, are as well capable of separating non-rigid and
rigid plastics from each other.
A broad array of colours are contained in the plastic waste
streams. Besides colour, plastic particles can vary in terms of
opacity. Separation based on colour, using optical colour
recognition sensors, can remarkably enhance the value of these
recyclables. Likewise or light coloured, separated plastic particles
can easily be recoloured, so they can match new design
specifications. Sometimes, different plastic types can be
separated based on their colours [26].
Contamination of one polymer in the matrix of another polymer
material, usually results in a decrease of (mechanical) properties
and reprocessing problems. A proper example of such a polymer
pair that certainly must be separated is PET contaminated with
PVC. At the processing temperature of PET, PVC will degrade and
form the high corrosive hydrogen chloride gas. PET, on the
contrary, will not melt at PVC processing parameters. For that
reason, accurate and cost-effective sorting methods should be
used and further developed. Automated separation methods can
be subdivided into two categories, namely direct and indirect
sorting techniques [27].
FIGURE 5. Density range of the most common polymers [14]. abbreviations: HDPE (high density polyethylene), LLDPE (linear low d ensity
polyethylene), PP (polypropylene), HIPS (high impact polystyrene), C-PVC (chlorinated pvc), U-PVC (unplasticized pvc), P-PVC (plasticized pvc),
PBT (polybutylene terephthalate), C-PET (crystalline pet), A-PET (amorphous pet), ABS (acrylonitrile butadiene styrene), PC (polycarbonate),
PMMA (polymethyl methacrylate)
Direct sorting methods are based on material properties like
density and electrostatic characteristics. Most straightforward
technique is the density-based sink-float method (Figure 6). By
using water as flotation medium, polymers with densities below 1
g/cm³ (unfilled PP and PE) will float and are separated from the
heavier polymers (PET, PS, PVC, ABS, …), which will sink. To further
separate the different polymers, density modifiers such as salts
can be added to the water to create denser flotation media (heavy
density separation). However, most polymer types have a density
range, and these ranges often overlap as shown in Figure 5. For
this reason, it is difficult to sort out polymers into mono-streams
only based on density differences. To enhance the performance of
density separation, density media separators like centrifuges and
hydrocyclones can be applied. The machines improve the material
wettability and also sort out plastic particles based on size and
shape [27].
To separate polymer types with similar density ranges, more
advanced techniques are needed. Selective flotation or froth
flotation is based on the differences of hydrophobicity of polymer
surfaces [28]. Through conditioning with wetting agents or
physical treatments (flame, corona) the wettability of polymers
can be improved. Air bubbles will attach to hydrophobic surfaces
and will therefore induce flotation of the specific material. On the
contrary, hydrophilic particles will remain completely wetted and
stay in the liquid medium. Another promising technique is
triboelectric separation, which is grounded on the surface charge
transfer phenomenon [27]. Plastic particles are rubbed against
each other and become oppositely charged. Subsequently, the
particles are separated by different deflection in an electrostatic
field.
Indirect separating techniques, on the other hand, make use of
sensors capable of fast detecting and locating different types of
polymers. FT-NIR (Fourier Transform Near Infrared) is by far the
most applied technique for the sorting of plastic waste, as
illustrated in Figure 7. However, this technique knows a few
limitations; false readings are not excluded because of possible
contaminations (paper, dirt) and this method has detections
issues with black, dark or multilayer materials [14]. Besides FT-
NIR, X-Ray detection (separation PVC containers) and laser sorting
are upcoming sensor-based techniques for fast automated
sorting of mixed plastic waste [7].
FIGURE 7. Principle of a spectroscopic NIR sorting technique [29]
FIGURE 6. Principle of sink-float separation [25]
9 | P a g e
4 CHALLENGES FOR MECHANICAL RECYCLING
When recycling mono- and mixed plastic products, different issues
and challenges will occur [14], [27]. During the reprocessing of
polymers, thermo-mechanical degradation can occur, as a
combination of heat and mechanical shear. The other types of
degradation can occur during lifetime, such as the exposure of
plastic products to heat, oxygen, light or moisture [15].
Furthermore, the heterogeneous nature of the plastic waste
makes it difficult due to immiscibility of the polymers and
separation of the phases. The third main limitation is
contamination of the mixture by different additives, fillers or even
other polymers that are hard to recycle. During the reprocessing
of these mixtures, often the processing temperature is set at
temperature of the highest melting component, which can lead to
overheating and degradation of polymers with lower melting
point, and affect the final properties of recycled product.
4.1 Polymer incompatibility
The mechanical recycling of mixed polymers will inevitably lead
to the formation of polymer blends [14]. The most common
polymers are essentially immiscible in the melt phase. This can
cause low mechanical properties of the final products, due to the
low adhesion and phase separation between the polymer phases.
The simplest way to represent the miscibility of polymer blends is
by using the following equation of free Gibbs energy [30], where
the Gibbs free energy of the polymer mixture 𝛥GAB has to be lower
than the summation of the Gibbs free energy of the different
polymeric constituents A and B:
∆𝐺𝑚𝑖𝑥 = ∆𝐺𝐴𝐵 (𝐺𝐴+ 𝐺𝐵)≤ 0
(1)
∆𝐺𝑚𝑖𝑥 = ∆𝐻𝑚𝑖𝑥 − 𝑇∆𝑆𝑚𝑖𝑥 ≤ 0
(2)
Equation 2 consists of two terms, concerning the enthalpy of
mixing (∆𝐻𝑚𝑖𝑥) and the entropy of mixing (∆𝑆𝑚𝑖𝑥). Blends can
be miscible, partially miscible or immiscible depending on these
thermodynamics. A polymer blend will be miscible and
homogeneous if the value of the free energy of mixing is negative
(∆𝐺𝑚𝑖𝑥 < 0) and if the criterion for Equation 3 is positive [30]:
𝜕2∆𝐺𝑚𝑖𝑥
𝜕φ2> 0
(3)
Where φ is the volume fraction of polymer B. When the Gibbs free
energy has a positive value (∆𝐺𝑚𝑖𝑥 > 0), the blend will be
immiscible.
The basic theory for the calculation of the Gibbs free energy was
done by Flory and Huggins[31]:
∆𝑆𝑚𝑖𝑥 = −𝑘𝐵𝑁(φ𝐴ln𝐴)+ φ𝐵ln𝐵)
(4)
∆𝐻𝑚𝑖𝑥 = 𝑘𝐵𝑇𝑁χ𝐴𝐵φ𝐴φ𝐴𝐵
(5)
𝑁 = n𝐴+ n𝐵
(6)
The free Gibbs energy of mixing can then be given by the following
equation:
∆𝐺𝑚𝑖𝑥 = 𝑘𝐵𝑇𝑁𝐴𝐵φ𝐴 φ𝐵+ φ𝐴ln(φ𝐴)
+ φ𝐵ln(φ𝐵))
(7)
where kB is the Boltzmann constant, T is the temperature, N is the
total number of mole, φ𝐴 and φ𝐵 are the volume fraction of
component A and B, and χ𝐴𝐵 the Flory-Huggins parameter which
has to be negative that the spontaneous mixing happens.
The dimensionless Flory-Huggins parameter χ𝐴𝐵 is considered as
a measure of interaction energy between the polymer A and
polymer B in the blend. In reality however, this parameter is
strongly dependent on the temperature, pressure and
concentration. In that case, χ𝐴𝐵 can be calculated by making use
of the solubility parameter δ , using the following equation:
χ𝐴𝐵 = 𝑉
𝑚𝑅𝑇𝐴+ δ𝐵)2
(8)
Where 𝑉
𝑚 is the mixing volume, R the gas constant, T the
temperature and δ𝐴 and δ𝐵 the respective solubility parameters
of polymer A and polymer B. The solubility parameter concept can
allow a predictive capability of assessing the potential of
miscibility of liquids. The solubility of a polymers is considered as
the possible interactions between two polymers, divided into
dispersive or Van der Waals forces (δ𝑖𝑑), polar forces (δ𝑖𝑝 ) and
hydrogen bonding (δ𝑖ℎ). Hence, the solubility parameter can be
written as [32]:
χ𝐴𝐵 = δ𝑖𝑑2+ δ𝑖𝑝2+ δ𝑖ℎ2
(9)
Completely compatible mixtures are called homogeneous blends
and show a one-phase morphology. On the other hand, immiscible
blends can have different kinds of morphologies [40]: spherical
particles, cylinders, fibres, sheets or co-continuous phases, as
shown in Figure 8.
FIGURE 8. Morphologies of immiscible polymer blends: (a) droplets (b) cylinder
(c) laminar and (d) co-continuous [15]
It is known that immiscible blends causing distinct phase
morphologies have inferior mechanical properties compared with
the virgin polymers. Additionally, coalescence of the dispersed
phase might occur in an immiscible blend [33]. The coalescence
phenomenon is considered as the merging of two or more
particles into one new larger “daughter” particle. Such
morphologies can strongly affect the final product performance,
as properties such as impact strength and elongation at break are
very sensitive to the dispersion and distribution of the second
phase [33].
In a recent study, Hubo et al. [22] have compared mechanical
properties of different recycled mixed polyolefin (MPO) blends.
For mixtures based on recycled polypropylene (PP) and
polyethylene (PE), they obtained low impact strengths, due to
incompatibility of PP and PE in the melt phase, which
consequently leads to the phase separation. Further, they have
noted large variations in the values for tensile modulus,
explaining that the impurities such as wood or non-PO in the
mixtures will have a pronounced weakening effect during the
tensile loading, while during bending test this effect was less
pronounced. The phase separation that occurs during melting
leads to an inferior transfer of stresses and strains within the
material, resulting in lower modulus for the blends [34].
4.2 Polymer degradation
Environmental factors can cause changes in polymer, physical or
chemical nature, resulting in bond scission and subsequent
chemical transformations. Those transformations in polymers
structure are categorized as a polymer degradation. It reflects in
changes of material properties such as mechanical, electrical
characteristics, in cracking, crazing, discoloration, phase
separation or delamination [35]. There are several types of
degradation: photo-degradation, thermal-mechanical or
biological degradation.
Photo-degradation is related to the sensitivity of polymers to
absorb the harmful part of the tropospheric solar radiation. Most
of the polymers tend to absorb high levels of energy radiation in
the UV part of the spectrum, which causes the activation of their
electrons to higher reactivity and oxidation, cleavage, and other
degradation types [35].
Thermal-mechanical degradation can occur during the thermal
reprocessing of plastic waste as a result of overheating, both in
terms of time and temperature [36]. There is a series of chemical
reactions involved in thermal degradation which lead to physical
and optical changes of properties, compared to the initially
polymer properties [35]. During reprocessing, chain scission can
occur, as well as chain branching. Depending on the initial
molecular weight (Mw) of the polymer, one or the other
mechanism will initiate, which will cause a reduction in Mw or an
increase in Mw, respectively [14]. These mechanisms are shown in
Figure 9.
FIGURE 9. Random chain scission (a) and crosslinking (b) [14]
Thermal-mechanical degradation in most of the cases begins with
a homolytic scission of a C-C covalent bond in the polymer
backbone, causing generation of free radicals. These free radicals
may undergo some chemical reactions such as disproportionation,
causing chain scission, or crosslinking also known as branching
[37].
11 | P a g e
Both degradation and immiscibility will strongly influence the
rheological behaviour of the melt during processing. The
rheological behaviour of polymer blends and recycled materials is
often quite complex. The important factors are the chemical
nature of the polymers in the mixtures, processing temperature,
viscosity ratio, concentrations and processing [38] conditions
[48].
5 ADDITIVES IN/FOR RECYCLED POLYMERS
Additives are universally incorporated in plastics. Finding plastics
without additives is very unlikely, even more for their recycled
counterpart. End-of-life plastics have a certain amount of
degradation whereof the degree of deterioration of the properties
depends on the lifetime of the plastics. To overcome the effect of
degradation, or rather improve the mechanical properties of the
recycled plastic (blends), additives are added to the recycled
waste streams. First the remaining additives in the plastics such
as: fire retarders, anti-oxidants and thermal stabilizers are
discussed. Other additives which are mixed with the recycled
polymers during reprocessing are: impact modifiers,
compatibilisers, coupling-agents, glass and wood fibres, talc,
chain extenders, etc. This incomplete list can be extended by many
other additives and depends on the final requirements of the end-
product and its applications.
5.1 Anti-oxidants, UV-and thermal stabilizers and fire
retardant fillers
The properties of the recycled plastics are rarely the same as
those of its virgin counterpart. Typically, they will be worse. Even
polymers with short lifetimes are susceptible to degradation. This
is mostly related to the influence of UV-light and/or oxidation
[39][41]. During the first processing step, anti-oxidants are
intermixed to prevent thermal oxidation during processing. Some
plastics (such as PP with its tertiary carbon atoms and PET with
its chromophoric groups) are also prone to photo-oxidation
caused by a combination of UV-light and oxygen [42], [43].
Consequently, (re)processing polymers requires extra
stabilisation. Common used stabilizers are hindered amines light
stabilizers (HALS), which are anti-oxidants used as light
scavengers. Other UV-stabilizers are nickel complex and carbon
black. As for thermal oxidation, this is countered by hindered
phenols (primary stabilizer) as radical scavengers. They provide
longer product stability and preserve the melt viscosity.
Synergistically, phosphites and sulphites are used to counter the
formation of hydroperoxides. They are also called secondary
stabilizers and are used to stabilize during processing conditions
[44][46]. After reprocessing, some of these additives can still be
active/functional to stabilize the recycled end-product. As a means
of determining the remaining amount of active thermal anti-
oxidants, the oxidation induction time (OIT) can be used [39], [47].
Some polymers, like PVC, tend to degrade during processing
solemnly due to thermal (and mechanical) influences. The
degradation consists of dehydrochlorination and discolouration.
Hence thermal stabilizers will be added, which react with the
released hydrogen radicals to prevent further degradation of the
processed polymer [48], [49].
Other waste streams, such of construction materials,
transportation ELV, WEEE can contain fire retardants for safety
reasons. However, in the past few years the regulations towards
fire retarders has been altered. Older waste products mostly
contain brominated or chlorinated flame retardants such as
polybrominated biphenyls and polybrominated diphenyl ethers
which can lead to the release of dioxins [50][52]. The use of
these fire retarders are prohibited nowadays as they have been
specified as hazardous. Although as it is established by REACH,
these former approved fire retarders are “unintended impurities
present in the final product and can be present in the starting
materials or is a result of reactions during the lifetime or during
the production process”. It is advisable to separate that waste
stream since they are not suitable for mechanical recycling. By X-
ray fluorescence (XRF), the contaminated WEEE-products can be
separated from the non-hazardous waste products [52][54]. The
big variety of fire retarders on the market does not improve the
recyclability of the waste. Very recently, the recyclability by
reprocessing of different flame retardants in SPW was reviewed
by Delva et al. [55]. Some examples of fire retardant fillers are:
phosphorus, silicon, boron, nitrogen containing flame retardants
and miscellaneous elements. Which fire retardant filler is used is
strongly dependant on the polymer system, their mode of action,
combustibility of the polymers, smoke release or toxic fume
production. Often a mix of synergistic flame retarders is used to
anticipate different types of combusting polymers. The molecules
can influence the reprocessing in terms of rheology [50], [51].
5.2 Impact modifiers and chain extenders
Recycled post-consumer waste streams have one common trend:
all of them show lower elongation at break and reduced impact
resistance. By adding an elastomer to the recycled polymer, the
impact resistance can be increased by preventing major crazing
and ending the propagation of cracks, due to the dissipation of
stress and the excellent elasticity of elastomers [56][58].
Studies for toughness improvement usually concern [59][68]:
The content, shape, size and distribution of the elastomer
filler
The molecular weight of the components
The interfacial adhesion with its interparticle distance and its
effects on the mechanical properties in relation to the
structure of the blend
The influences of processing
The stress-strain rate/temperature conditions.
To reduce the ecological fingerprint even more, the elastomers to
optimize the recycled polymers’ properties, can be recycled
polymers themselves. Some examples are: poly(vinyl butyral)
(PVB) from wind shields for polyamide (PA) toughening or EPDM
from shoe soles or roofing applications, styrene butadiene rubber
(SBR) and natural rubber (NR)/SBR scraps from automotive tires
for PP [68][70]. However, the addition of impact modifiers have
also a counter side: the rubber toughening of polymers usually
results in a severe reduction in the tensile modulus and tensile
strength [71][76]. Figure 10 shows, 20 wt% of the impact modifier
SEBS-g-MA improved the impact resistance by almost a double
with the trade-off the decreasing flexural modulus by only a
fourth.
5.3 Compatibilisers
The great challenge of mechanical recycling is finding the
cheapest method and at the same time the optimal separation
technique. Post-consumer pure mono-stream polymer wastes are
rarely found. As previously discussed, a solution to a 100% purified
waste stream of commingled plastics has not been found yet. As
many plastic waste stream consists of one or more impurities, the
mechanical properties (originated from the low miscibility) will
be feeble. Nevertheless most thermoplastics may be reprocessed
with small amounts of impurities (< 5%) without excessive
deterioration of mechanical properties. Except for the impact
properties, which may only be improved
FIGURE 10. Increase of notched impact strength combined with a decreasing
flexural modulus for recycled abs from weee and elv with (blue x) no impact
modifier, (orange x) 5 wt%, (grey x) 10 wt% and (yellow x) 20 wt% impact
modifier sebs-g-ma (altered figure with permission from [77])
for mixed waste streams by adding an appropriate modifier, often
refer to as compatibilisers [39], [78][81]. Compatibilisers can
mainly be divided into three groups (Figure 11):
Block copolymers
Copolymers with functional groups
Graft copolymers
They all attempt to minimize the interfacial distance and
interconnect both phases by either using the same polymers as
the matrix and the dispersed phase polymer or by interaction or
reaction between the compatibilisers’ moieties and the disperse
phase and the compatible backbone polymers with the matrix.
Grafted compatibilisers exist out of a grafted monomers and a
backbone polymer. The latter can be preferentially the matrix
polymer or an elastomer, the graft monomers are often maleic
anhydride, glycidyl methacrylate, methacrylic acid, etc. [72], [74],
[79], [80], [82][84].
The effectiveness of the compatibiliser strongly depends on:
architecture of the block and/or copolymers, the reactivity of the
moieties (when used) and the diffusion possibility of the
compatibiliser across the interface. Condensation polymers offer
the grand advantage of end-groups which have the possibility to
react with the functional groups of the compatibiliser. Most
important is that the interfacial tension of the compatibiliser is in
between the one of both dispersed and matrix phase wherefore it
will preferentially move towards the interphase. There are plenty
of comparisons in literature of different types of blends, with the
optimal amount and which compatibiliser one should select.
However, they often lack a cost/benefit analysis and there are still
no final guidelines on the best approach since every blend is
P a g e | 13
FIGURE 11. (Left) block copolymers, (middle) copolymers with functional monomers and (right) graft copolymers
different [62], [70], [73], [75], [81], [84][90]. A few examples of
compatibilizing strategies available for post-consumer packaging
waste is presented later in this manuscript.
5.4 Talc, glass and wood fibres
Just like virgin polymers, recycled waste streams can be filled
with cheap yet effective particles, to improve their stiffness. This
can be done by adding talc, glass or wood fibres, nanoclays or
other micro- or nanosize particles which should have higher
stiffness than the matrix polymer. The fillers of commingled
plastics from their previous lifetime might cause difficult
processing and undesirable changes in mechanical properties in
the other components. This will eventually lead to a deterioration
of the entire product. PA and PP are often mineral filled or glass
fibre reinforced in the automotive industry to ensure the parts will
endure high strength at elevated temperatures [70], [91][93]. As
for most of the fillers, they are not compatible with the matrix. A
suitable modifier (coupling agent) should be selected in order to
improve interfacial adhesion. This modifier preferentially has
polar groups in its structure, due to the polar groups or ions
present in the fillers. The addition of the modifier will
consequently change the morphology of the blend to a better
dispersion and usually also better processing conditions in terms
of a more continuous flow. As for the optimal amount of
reinforcing filler, which is intermixed during (re)processing, this
depends on its final application of the product and typically
amounts between 20% and 60% [68], [94][96].
6 EXAMPLES OF MECHANICAL RECYCLING
6.1 Post-consumer PET (bottle-to-bottle) recycling
Poly(ethylene terephthalate) (PET) is a semi-crystalline
thermoplastic condensation polyester [97]. The PET demand
accounts for 7,1% of the total European plastic demand
corresponding to a total demand of 3,5 MT (PET fibers not
included) [3]. It is mainly used in packaging applications such as
bottles, containers, trays and foils. Good mechanical strength, low
permeability towards moisture and oxygen, inertness, low specific
weight and high clarity are some of the key properties which
contribute to the applicability of PET in packaging applications
[98]. Next to the packaging applications, a huge end-market for
PET is the use as fibers in textile industry (approximately 1 MT in
2011 in Europe [99]).
Petcore, the European PET association, estimated the overall
collection rate in Europe for 2014 for PET bottles and containers
at 57% by comparing the collected waste with the PET placed in
the market [100]. The overall collection rate is often confused
with the overall recycling rate, which is of course more valuable
for the European policy but also more difficult to calculate [101].
As previously mentioned, separation post-consumer waste in their
mono-polymers is essential to reach high-quality recycling. PET
bottles can quite easily be separated from other polymers using
automated or hand sorting techniques. It should definitely be
separated from PVC, as previously explained, degradation
products of PVC are known to facilitate PET degradation by
catalyzing chain scission reactions during reprocessing [102].
Normally the PVC content should be lower than 0.25% [100]. It is
typically separated (depending on the country) in clear, blue and
green PET.
Different recycling processes are industrially being used to
mechanically recycle the sorted post-consumer PET bottles.
Although these recycling systems all are very specific, typically
the following process steps are found [103], the order of which
can vary:
Conventional preparation (grinding, washing, removal of
caps and labels)
Re-extrusion (melt reprocessing)
Solid-state polycondensation (removal of various
contaminants and/or increasing of intrinsic viscosity)
The first conventional treatment steps consist of grinding,
washing and removing caps and labels from the PET bottles. The
washing aims at removing all surface contaminants and is usually
done in a NaOH water solution. Remaining polyolefin
contaminations can also be removed in this step by float-sink
separation.
The re-extrusion step is done by extruding the flakes at
temperatures above the melting temperature of the PET
(>260 °C), mostly combined with the use of vacuum to remove
low-molecular contaminants.
Solid-state polycondensation (SSP) is typically performed to
restore the intrinsic viscosity of post-consumer PET flakes.
Typically post-consumer PET displays lower intrinsic viscosities
(due to reduction in molecular weight, presence of moisture,
hydrolysis, etc.) compared with virgin PET grades. SSP involves
heating of the PET at a temperature between the glass transition
temperature and the melting temperature in a reactor.
Condensation reactions occur between the chains terminal groups
in the amorphous phase of the polymer, in a temperature range
of 180240 °C. The reaction proceeds under vacuum to remove by-
products. Additionally, at these elevated temperatures, post-
consumer contaminants from inside the flakes will be able to
diffuse to the surface to be removed by vacuum-extraction (if
residence time is sufficient).
Alternative processes to increase intrinsic viscosity is by adding
chain extenders (during solid-state polycondensation or during
reactive extrusion) which can act as cross linkers between the
individual PET chains leading to increased molecular weight and
intrinsic viscosity [104].
FIGURE 12. Scheme of pet super-clean recycling process based on pellets.
adapted from [103]
An example of an industrially used recycling process is the “super-
clean PET recycling process based on pellets. A scheme of this
process is shown in Figure 12. This process combines all of the
above mentioned process steps. Typically, the reclaimed PET
pellets (mostly diluted with virgin PET - depending on cost and
properties) can then be used to produce new bottles, containers,
fibers or thermoforms. Nowadays, up to 50% of a PET bottle can
be made from post-consumer PET bottles [103]. In thermoform
applications on the other hand this percentage can easily be
higher.
rPET is often re-used in new food contact applications, however,
for safety considerations, each recycling process has to follow a
safety assessment procedure governed by the European Food and
Safety Agency (EFSA). The evaluation principle of EFSA is based on
the cleaning efficiency of the recycling process. This efficiency is
determined by so-called challenge tests, where PET is deliberately
(over)contaminated with (model) contaminants [105]. These
evaluations are published on-line, e.g. the EFSA evaluation of the
EREMA process, EREMA is a machine manufacturer for plastic
recyclers [106].
Nowadays, clear and light-colored (mainly blue and green) PET
bottles are easily collected and separated for secondary recycling
in Europe. However, opaque bottles and thermoforms are
currently not recycled. Several countries in Europe are currently
evaluating the possibility to additionally recycle PET thermoforms
and other difficult to recycle PET waste streams (e.g. in France
15 | P a g e
[107] and in Belgium [108]). The main difficulties until today
include:
Complex compositions (multilayers, inks, glues, absorbent
pads, etc.)
High rPET content in thermoforms
PET flakes with lower intrinsic viscosity
Equipment of recycling plants (more fine dusts, brittleness of
thermoforms, etc.)
Food contact regulations
6.2 SPW from post-consumer packaging waste
The diverse applications for plastic packaging materials lead to a
large variety of compositions of packaging which oftentimes
consists of multiple layers in order to benefit from the different
properties each polymer brings to the table. These packaging
materials generally have a short lifespan and end up fairly soon
and abundantly in the waste. Plastic packaging products are
mainly produced in LDPE, PP, PET, HDPE, PVC and PS [3], so these
polymers will also dominate the composition of the post-
consumer plastic packaging waste.
The collection systems for plastic packaging waste in Europe
differ from country to country. Countries like Germany and the
Netherlands use collection systems which allow all plastic
packaging waste to be collected together. Belgium has a similar
system which collects drinking cartons, metal packaging and
plastic bottles, i.e. the so called PMD-system. In order to anticipate
on the Eropean Circular Economy Action plan (EU Commission,
closing the loop, [8]), an extension of the Belgian PMD system is
taking place in a phased manner. In short, this entails the
extention of the plastic fraction of the PMD allowing other plastic
packaging materials such as foils [109].
This extension of the plastic waste stream implies a more complex
waste stream as the composition changes substantially. These
changes in composition in turn demand other and better sorting
and separation processes to ensure a pure plastic waste end
stream for recycling.
The PET fraction mainly originates from PET bottles and
thermoformed trays. The bottles are initially (either by consumer
or recycling plant) separated from the other products and are
send to the bottle-to-bottle recycling plants (see above). The
other fractions eventually end up in sink-float sedimentation
tanks to produce a light fraction (i.e. float) and a heavy fraction
(i.e. sink) [110]. The light fraction mainly consists of polyolefins (PE
and PP) and is therefore used to produce different qualities of
mixed polyolefin (MPO) streams, either ‘hard’ MPO’s (rigid
packaging) or ‘soft’ MPO’s (flexible packaging) [111]. The heavy
sink fraction is currently not being recycled and is prepared (e.g.
lowering of the chlorine content) and send to energy recovery.
Despite the fact that the PE and PP in these MPO’s are quite
similar (i.e. they all consist of carbon and hydrogen atoms), even
these mixtures are often not miscible due to differences in
molecular structure and form heterogeneous blends which in turn
have quite low properties compared to virgin polymers due to the
formation of weak interfaces [22]. Therefore, these MPO waste
streams are nowadays mechanically recycled mainly into ‘low-
quality’ products such as garden furniture, outdoor flooring and
traffic signalization elements by industry [112].
As depicted before, different strategies can be used to upgrade
the properties of mixed polymers. Likewise for MPO’s the addition
of stabilizers, compatibilisers and/or impact modifiers can be
required to level the properties compared to virgin grades.
Typically copolymers (e.g. ethylene propylene diene monomer
(EPDM) [113], ethylene-propylene rubber (EPR) [114] or block
copolymers (e.g. SEBS [115]) are used to compatibilise polyolefin
blends. Recently, Eagon et al. [116] have very carefully synthesized
well-defined di-block and tetra block copolymers which increased
the toughness of PE/iPP blends significantly even at very low
percentages (1 wt%) as can be seen in Figure 13 (a). They point out
the importance of molecular weight, which must ensure an
increase in number of entanglements between the blocks and the
homopolymer chains in the melt state. A sketch of the interface
between the PP and PE is shown in Figure 13 (b).
P a g e | 16
(a)
(b)
FIGURE 13. Tensile properties of PE/iPP blends with and without addition of long di -block or tetra-block copolymer [116] and (b) Schematic interpretation of
interfacial strength between iPP and PE with and without addition of long di-block or tetra-block copolymer [117]
6.3 SPW from WEEE
The production of electrical and electronic equipment or (EEE) is
growing at an increasing pace due to our rapid economic growth
and growing demand for consumer goods [118]. This vast
production rate is accompanied by a large amount of waste
electronic and electrical equipment (WEEE). This sector, next to
the transportation and appliances area, is expected to grow even
more. The rapid development of the EEE technology combined
with the lower life cycle of most present-day products poses a
significant issue as far as their disposal is concerned. In view of
this environmental problem concerning the management of these
WEEE a legislation was drafted to improve reuse, recovery and
recycling of the latter. Thanks to the European Directive
(2000/53/EC) and the WEEE directive (2002/96/EC) at least 70-
80% of materials of end-of-life vehicles (ELV) and WEEE have to
be recovered in the form of energy and/or materials [119]. This
means that a large amount of polymers reenters the market for
reprocessing. A study by Achilias et al. [119] demonstrates that a
typical WEEE fraction contains 20-30% plastics. The general
composition of the plastic fraction itself is depicted in Figure 14.
As can be seen from this graph the main constituents of the WEEE
plastic fraction are ABS, high impact polystyrene (HIPS),
polycarbonate (PC), PC/ABS and PP.
To better understand and anticipate the performance of these
recycled products further investigation regarding the changes in
properties during recycling still has to be done. The main problem
with these materials is the variability in the product composition
due to the presence of polymer mixtures, additives or
contaminations. In addition, it is well known that polymers are
subject to degradation, both by daily use as during processing of
the materials [120]. In its everyday use, polymers are susceptible
to photo-degradation, oxidation, leaching of various additives
(e.g. plasticizers) and other weathering phenomena. These effects
are furthermore exalted during melt
FIGURE 14. Typical WEEE composition according to Achilias et al. [119]
17 | P a g e
reprocessing operations causing the properties of the recycled
materials usually to be worse than that of the virgin material. As
EEE products typically have a long lifespan compared to other
polymer applications (e.g. packaging), degradation will play a
more prominent role in influencing the final properties of the
recycled material. Here, during its lifetime, stabilizers are slowely
depleted/degraded until oxidation and degradation of the
polymer chains can occure reducing the quality of the recycled
product.
Possible ways of upgrading these recycled plastics include
blending them in small portions with their virgin grade [121]. Often
however, depending on their lifetime and amount of weathering
the plastic has undergone, these blends suffer from more or less
limited miscibility requiring the use of suitable compatibilisers for
stabilization of the system [121], [122].
Another way of improving the polymer properties is by blending
the recyclates with suitable additives. Pospíšil et al. [122] made an
extensive overview on the stabilization of recycled plastics by
upgrading them in a melt blending process with suitable
additives. They stress the importance of the history of the plastic
as the application determines the amount of weathering the
material has undergone. Structural changes take place by
reprocessing, heat ageing or weathering that differentiate the
WEEE plastic from its original virgin form. The gravity of the
changes is dependent on the service time of the plastic and the
aggressiveness of the environment. Careful selection and analysis
of the plastic waste is incumbent for reuse into new applications.
Frequently stabilizers are re-added to the system because they
have been consumed during their lifetime and processing. In this
case heat stabilizers such as hindered phenols or hindered amines
are usually combined with phosphorous or sulfuric type secondary
stabilizers [44], [46], [123]. UV-stabilizers prolong the lifetime of
the plastics by absorbing harmful radiation and deactivate the
latter by radiationless processes [124]. Metals, fillers and
pigments can be added to the recyclate as well, yet one should be
careful of residual components that could catalyze the
decomposition of peroxides forming radicals that accelerate
degradation. In this case metal ion deactivators, antacids and
deactivators for residual fillers can be added.
7 CONCLUSIONS
This paper has reviewed the current state-of-the-art in the field
of mechanical recycling of SPW. Mechanical recycling is the current
industrially ubiquitous technique for the recovery of waste
polymers. Different technological aspects such as collection and
sorting have been discussed, as well as the materials science
behind the main challenges associated with an efficient
mechanical recycling economy such as contamination,
degradation and the mixing of different plastics types in waste.
Furthermore, several industrial technologies were highlighted
focusing on post-consumer PET bottle-to-bottle recycling, SPW
from post-consumer packaging waste and SPW from waste from
WEEE. In addition, various additives commonly found in or added
to recycled polymers were discussed.
READY TO LEARN MORE?
This introductory review has been broad, rather than deep. If you would like to get acquainted in more detail with our work, you could check
our publications (https://biblio.ugent.be/person/802000178474) or have a look at our website (www.cpmt.eu).
Some of our key publications:
Waste Management: Ragaert, Kim; Delva, Laurens; Van Geem, Kevin. Mechanical and chemical recycling of solid plastic waste
Waste Management: Brouwer, Marieke T.; Thoden van Velzen, Eggo U.; Augustinus, Antje; Soethoudt, Han; De Meester, Steven; Ragaert,
Kim. Predictive model for the Dutch post-consumer plastic packaging recycling system and implications for the circular economy
Waste Management: Delva, Laurens; Hubo, Sara; Cardon, Ludwig; Ragaert, Kim: On the role of flame retardants in mechanical recycling of
solid plastic waste
Journal of Cleaner Production: Huysveld, Sophie; Hubo, Sara; Ragaert, Kim; Dewulf, Jo. Advancing circular economy benefit indicators and
application on open-loop recycling of mixed and contaminated plastic waste fractions
Polymer Engineering and Science: Ragaert, Kim; Hubo, Sara; Delva, Laurens; Veelaert, Lore; Dubois, Els. Upcycling of Contaminated Post-
Industrial Polypropylene Waste: a Design from Recycling Case Study
Polymer Engineering and Science: Peelman, Nanou; Ragaert, Peter; Ragaert, Kim; Erkoc, Mustafa; Van brempt, Willem; Faelens, Femke;
Devlieghere, Frank; De Meulenaer, Bruno; Cardon, Ludwig. Heat resistance of biobased materials, evaluation and effect of processing
techniques and additives
Polymers special issue: Fiorio, Rudinei; D’hooge, Dagmar R.; Ragaert, Kim; Cardon, Ludwig. A statistical analysis on the effect of
antioxidants on the thermal-oxidative stability of commercial mass- and emulsion-polymerization ABS
THE AUTHORS
KIM
RAGAERT
PROFESSOR SUSTAINABLE USE
AND RECYCLING
OF POLYMERS AND COMPOSITES
Prof. dr. Kim Ragaert obtained her PhD in Polymer Engineering in 2011. She lectures materials
science and polymer processing at Ghent University’s Faculty of Engineering and Architecture,
where she holds a tenure track position in the domain of ‘Sustainable Use and Recycling of
Polymers and Composites’ at the Department of Materials Textiles and Chemical Engineering.
Her dynamic research team is made up of a dozen researchers from the disciplines of
mechanical engineering, chemistry and materials science. They work to develop the necessary
scientific tools to enable the improved mechanical recycling of polymer-based materials.
Specific research topics include upcycling of mixed solid plastic waste, modeling of polymer
degradation, Design for and from Recycling, micro-fibrillar composites, WEEE plastics and
recycling of multilayer packaging materials. She leads and participates in several (inter)national
recycling projects, alongside a variety of industrial partners, policy makers and other academia.
Prof. Ragaert is the chair of the Plastics to Resource pipeline within CAPTURE, Ghent University’s
overarching Resource Recovery platform, wherein she creates synergies with colleagues active
in thermochemical recycling, polymer design for recyclability, food packaging,
sorting/decontamination and LCA.
19 | P a g e
STEVEN
DE MEESTER
PROFESSOR SUSTAINABLE DESIGN OF
CHEMICAL PRODUCTION CHAINS
Prof. dr. ir. Steven De Meester has been associate professor since 2016 at the Department of
Green Chemistry and Technology at Ghent University. After graduating in bioscience engineering,
he started working at the EnVOC research group. He worked there as a sustainability consultant
on the feasibility study for FISCH together with essenscia (Flanders), and on the ERDF project
eco2chem.
During his research within the FP7 project PROSUITE (Prospective Sustainability Assessment of
Technologies) he obtained his doctorate. He was also active as a project manager at OWS NV and
at Capture, an interdisciplinary research initiative in which scientists, together with the sector,
look for innovation and valorization in the recovery of raw materials. He teaches on waste
processing, chemical technology, thermal operations, environmental management systems and
environmental impact analysis, among other things.
From his background in chemical technology and sustainability assessment, he conducts
research into the sustainable design of chemical production chains, and in particular the
purification of components from complex flows such as waste plastics. The complex raw
material flow is then analyzed, after which a suitable purification chain is developed. This is
scaled up and optimized for the best attainable economic and environmental performance.
LAURENS
DELVA
POSTDOCTORAL RESEARCHER
Laurens Delva is a Postdoctoral Researcher at the Centre for Polymer & Material Technologies
at Ghent University, Belgium. In 2015, he obtained his PhD in Engineering from Ghent University,
on the topic of (re)processing nanoclay filled polypropylene. His current research focuses on the
recycling of mixed polymer waste streams and the drawing of microfibrillar composites. He also
lectures to engineering students on polymer materials and material characterization.
KAREN
VAN KETS
DOCTORAL RESEARCHER
Karen Van Kets is a doctoral fellow at the Centre for Polymer & Material Technologies at Ghent
University, Belgium. In July 2015 she obtained her Master of Science in Chemical Engineering
Technology from Ghent University. She was involved in the research on the introduction in
circular economy by the concepts of Design from Recycling. Her current research focuses on the
mechanical re-recycling of the impact modifier SEBSgMAH in a PP-PET blend.
MAJA
KUZMANOVIC
DOCTORAL RESEARCHER
Maja Kuzmanovic is a Doctoral Researcher at the Centre for Polymer & Material Technologies, at
Ghent University, Belgium. She obtained her first Master in Materials Engineering with focus on
biomaterials, at Faculty of Technology and Metallurgy, University of Belgrade, Serbia in July 2013.
The second Master in Materials Science and Engineering, focused on nanotechnology, she
obtained in July 2014 at University Carlos III of Madrid, Spain. Her current PhD research focuses
on the drawing of microfibrillar composites from recycled polymers.
RUBEN
DEMETS
DOCTORAL RESEARCHER
Ruben Demets works as a Doctoral Researcher at Ghent University, Belgium, at the Centre for
Polymer & Material Technolgies and at the Department of Green Chemistry and Technology. He
obtained his Master of Science in Chemical Engineering Technology (Polymers) at Ghent
University, in 2016. He currently works on the MIP-ICON project PROFIT, which focuses on the
valorization of plastic materials from complex waste streams. The project involves research on
separation techniques and the quality of mixed plastic waste
SARA
HUBO
SENIOR RESEARCHER
Sara Hubo is a senior researcher at the Centre for Polymer & Material Technologies at Ghent
University. She graduated at Ghent University in 2002 as a Master in Biochemistry and has been
involved in various research projects on polymer processing, more specifically defining the
structure-property relationships of extruded and injection moulded (recycled) polymers.
21 | P a g e
NICOLAS
MYS
POSTDOCTORAL RESEARCHER
Nicolas Mys is a postdoctoral researcher at the Centre for Polymer & Material Technologies, at
Ghent University. He obtained his PhD on the processing and characterization of novel polymeric
materials to spherical powders for fusion based additive manufacturing under supervision of
prof. Ludwig Cardon in 2017. He currently works on the MATTER project, a Catalisti-ICON project
involving the recycling of mixed (post-consumer) plastic waste streams and to use the
generated data to develop a decision supporting framework for mechanical and
thermochemical recycling.
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... Recycled HDPE is used to produce plastic wood (timber), recycled plastic furniture, lawn and garden products, buckets, crates, office products, and automobile parts [9]. Plastic recycling mostly carried out using mechanical recycling or secondary recycling, which is currently the most ubiquitous, economic, and best technology in today's industry [10][11][12]. With proper control over processing Proceedings of the 13th AUN/SEED-NET Regional Conference on Materials (RCM 2020) conditions, many polymers can be subjected to several primary and secondary mechanical recycling cycles without concern for performance loss [13]. ...
... One of the stages in the secondary recycling process is the washing process. This process is one of the most crucial steps, and it is mandatory in the entire recycling process for the removal of food residues, pulp fibers, or adhesives because the value of the recycled plastic relies significantly on its purity [10,11]. In this study, the HDPE washing process was carried out and its cleaning efficiency was studied by identifying pH, conductivity, and alkalinity of the water used for rinsing. ...
Conference Paper
A study of the washing process for recycling HDPE (High-Density Polyethylene) materials was reported. The materials used are taken from one source and the same type, namely white, used packaging bottles. The study begins with washing the HDPE waste through a series of washing processes, using NaOH 48% solution and followed by water rinsing. In this study, samples of used HDPE bottles with the same color were washed at two different temperatures, at 27°C (room temperature) and 50°C, for 30 minutes. Characterization of the washing water was carried out to measure pH, conductivity, and alkalinity. The results show that the difference in washing water temperature does not significantly affect the resulting pH, while the conductivity of washing water at room temperature was found to reach lower conductivity with fewer rinsing process than washing water with a temperature of 50°C. The 0% alkalinity is found in the second and fifth rinse for washing water at the temperature of 27°C and 50°C, respectively. Before and after the washing procedure, the samples were subjected to a mechanical test using the Leeb Hardness Tester to see whether the washing process affected the sample's mechanical properties. The result for Leeb Hardness Tester shows an increasing value from 540.4 HL for the sample before treatment to 577.8 HL and 586.8 HL after treatment with a temperature range of 27°C and 50°C, respectively. Thermal analysis was carried out by DSC (Differential Scanning Calorimetry) and TGA (Thermogravimetric Analysis). The curve of DSC results showed no significant difference in material properties after five times the washing process. Characterization of the sample was also carried out by FTIR (Fourier Transform Infrared Spectroscopy) in the range from 4000 to 550 cm⁻¹ to identify the effect of heat on the polymer chains. Both the treated and untreated samples showed the same results.
... Figure 1. Scheme of the basic principal steps in a mechanical recycling process [12]. ...
Article
Full-text available
Several mixed recycled plastics, namely, mixed bilayer polypropylene/poly (ethylene terephthalate) (PP/PET) film, mixed polyolefins (MPO) and talc-filled PP were selected for this study and used as matrices for the preparation of microfibrillar composites (MFCs) with PET as reinforcement fibres. MFCs with recycled matrices were successfully prepared by a three-step processing (extrusion—cold drawing—injection moulding), although significant difficulties in processing were observed. Contrary to previous results with virgin PP, no outstanding mechanical properties were achieved; they showed little or almost no improvement compared to the properties of unreinforced recycled plastics. SEM characterisation showed a high level of PET fibre coalescence present in the MFC made from recycled PP/PET film, while in the other MFCs, a large heterogeneity of the microstructure was identified. Despite these disappointing results, the MFC concept remains an interesting approach for the upcycling of mixed polymer waste. However, the current study shows that the approach requires further in-depth investigations which consider various factors such as viscosity, heterogeneity, the presence of different additives and levels of degradation.
... For the mechanical recycling of contaminated plastics the washing step is required (Delva et al., 2019). The great challenges in MSW recycling without washing are aesthetical issue, mechanical strength reduction and poor structural integrity due to the existence of impurities, etc. . ...
Article
The excessive consumption of plastic films in many applications due to their lightness and versatility and the low recycling rate of this type of material is a very significant matter that increases the problem of plastic film pollution. Plastic recycling has been a popular topic in conferences and technical journals during the past few years, but studies on the washing process are rarely published. Washing is an essential step in the mechanical recycling of these materials. This work provides an assessment on the feasibility of the washing procedure to clean post-consumer polyethylene film, presented in municipal solid waste, which had not been collected selectively, to increase mechanical recycling of post-consumer plastic films. Particularly, the study analyses (1) the characteristics of the washing water after cleaning procedure at room temperature, at 60 °C and at 60 °C with addition of NaOH chemical; (2) benefits of a drying stage before washing on the cleaning efficiency and characteristics of residual water and (3) benefits of physical or physical-chemical treatment of water by using a single step of settling or a two-step process that includes flocculation-coagulation and later settling, in the possibility of wastewater recycling for its use again in the washing process. Results showed very low differences between washing procedures at room temperature and at 60 °C. However, with the addition of NaOH chemical best cleaning was achieved although a more difficult wastewater treatment was found, due to high COD, BOD5, chlorides, nitrogen and phosphorous content. Results also showed that drying before washing significantly improved decontamination of post-consumer polyethylene film decreasing the consumption of fresh water and the requirement of depuration of it. Finally, the stage of physical-chemical treatment of wastewater by means of the use of coagulants and flocculants showed the possibility of increasing the reuse of water in the process for cleaning of plastic with relatively low cost.
Book
Recycling and Reusing of Engineering Materials: Recycling for Sustainable Developments covers the latest research and developments in recycling and reusing processes, including new fundamental concepts, techniques, methods and process flows. The book provides applications of these novel technologies to recycling processes and analyzes new and modern ways of recycling techniques. It provides a comprehensive literature review on fundamental aspects of recycling processes, recycling goals, characterization of waste streams, legislative policies and evaluation, electronic recycling, aircraft recycling, recycling processes, energy savings and issues, environmental issues, societal issues, recycled materials, market development for recycling, processing facilities, and awareness and importance of recycling safety. The book is an indispensable reference for researchers in academia and industry. Scientists can use this book for literature reviews and experimental details, and the industry can use its comprehensive detail for literature reviews and to upgrade their processes and systems.
Chapter
Plastics are lightweight, easy to mold, and comparatively less costly materials with various uses in different industries. Additionally, their resistance to corrosion, relative strength and toughness, chemical inertness, electrical inertness, and potentiality for reuse make them ideal polymeric materials for commercial uses. In today's world, plastics have suppressed the use of other traditional materials to become the most widely preferred ones. However, there are still many challenges for the plastic recycling and reusing despite many engineering practices being developed and used. Plastic accumulation quickly turns into a global problem with significant environmental, economic, social, and technological concerns. This chapter examines the latest advances in mechanical, thermal, and chemical recycling, and examining current and past literature on recycling methods. The chapter also focuses on catalytic-enhanced chemical recycling of polymers and plastics with the case preference of the widely applied polymer types such as polyethylene terephthalate, polypropylene, polyvinyl chloride, and polyethylene, since they are the most common thermoplastic polymers available. The applications of recycled polymers are given, and recommendations for future research on plastic and polymer recycling are provided in the last section of the chapter.
Thesis
Full-text available
This doctoral dissertation has focused on the structure-properties-processing relationships in microfibrillar composites (MFCs) prepared from polypropylene (PP) and poly(ethylene terephthalate) (PET). The MFC concept consists of three processing steps – melt blending via twin-screw extrusion, fibrillation via cold drawing, and isotropization by injection moulding or extrusion, resulting in binary composites of the higher melting temperature component being dispersed as microfibres in a matrix i.e. the lower melting temperature component. The microfibrillar structure of the reinforcement may contribute to improved mechanical properties – both stiffness and toughness. MFCs are considered an interesting class of environment-friendly fibrereinforced composites produced with conventional processing equipment and, as such, they have a large possibility to find their place in numerous applications. Although various experimental studies have been done within the field of MFCs, still, some shortcomings may be found. The development of a microfibrillar structure of the reinforcement and retaining this structure during the final melt processing step was depicted as one of the crucial factors for achieving high-quality composites, but it was not yet investigated in detail. Moreover, the influence of the fibrillation step on certain properties such as degradation and dynamic mechanical properties was not widely reported. Further on, the literature review has shown that additives and compatibilizers may play an important role in the development of the microstructure of MFCs, especially pointing out the importance of the step of production at which they are added. Overall, carefully linking the processing, their structure, with their resulting properties is essential. The research questions of this doctoral research are, therefore, as follows: i) How do the properties of MFCs change upon application of different cold draw ratios?; ii) What are the optimal processing parameters used in MFC production?; iii) What is the effect of the addition of elastomers and elastomerbased compatibilizers at different steps of the MFC manufacturing on the morphology and mechanical properties of MFCs?; and iv) Can the MFC concept be applied to novel applications such as thermoforming and the upcycling of immiscible recycled PP/PET blends? The polymers used in this research were limited to PP as matrix and PET as reinforcement, while a polyolefin based elastomer (POE) and a POE grafted maleic anhydride (POE-g-MA) were used as additives. The first part of the research focused on evaluating the influence of the fibrillation and isotropization processing stages on the development of the MFC microstructure. Fibrillation was found to be a crucial step in the production of the MFCs; therefore, different draw ratios were applied during the experimental part to define an optimal aspect ratio of the microfibres, and the results are discussed in Chapter 4. The optimal draw ratio was found to be equal to eight. Overall, the morphological characterisation proved the existence of an immiscible PP-PET blend after melt blending, and the highly oriented fibrillar state of the reinforcement component after cold drawing. Moreover, after the final isotropization step, the microscopic images showed the PET fibres were preserved and they were well dispersed within the PP matrix. Physical-chemical characterisation has shown a significant influence of the microfibrillar structure on the crystallisation and degradation behaviours of the MFCs. The crystallisation temperature of the PP component has been increased, as the PET fibres act as strong heterogeneous nucleating agents for the PP spherulites, forcing the PP lamellae to orient perpendicular to the PET fibres surface, by forming a transcrystalline layer. The positive influence of this high nucleating effect was noticed in the MFCs’ dynamic mechanical properties, as they have increased with the increment of the fibre aspect ratio. Beside the fibrillation step, in Chapter 5, the effect of different injection moulding temperatures on various properties of the MFCs was studied too. Processing the MFCs at the lowest injection moulding temperature (210 ºC) resulted in the best MFC morphologies and performances. During the experimental work of this PhD, the optimal weight ratio of the polymer components for the PP/PET blend was found to be 80/20, respectively, while 6 wt% was found as the optimum quantity for the additives. Further research was focused on the addition of elastomer-based additives into the MFCs in different stages of processing and their effect on the crystalline microstructure and mechanical properties. Fillers and additives are widely used in polymer blending, hence, Chapter 6 of this dissertation was devoted to study their influence. The main objective of this part was to evaluate the potential of adding a compatibilizer during the isotropization step and its influence on the fibrillar morphology. However, postponing the compatibilizer addition did not improve the distribution of long fibres, neither created the best mechanical properties for a compatibilized PP-PET MFC. With the addition of POE-g-MA, the fibre aspect ratio has been reduced, but the nucleating effect of the fibres was still present. Small-angle light scattering indicated a decrease in the PP spherulite size in MFC samples due to the presence of long PET microfibres, as well as in compatibilized samples, although the fibre length was affected. In addition, it was proven that the presence of POE and POE-g-MA also may act as nucleators for the PP matrix; therefore, in these MFC samples, the nucleating effect was more pronounced. Mechanical characterisation confirmed the reinforcing effect of the PET microfibres in the MFCs. Particularly, the increase in tensile modulus was noticed in non-compatibilized MFC due to the large interfacial area between the microfibres and the matrix; therefore, it was considered that some interfacial contact between the components exists, which made the stress transfer more effective. Higher values for yield strength and strain at break, as well as for impact strength, were observed for the MFC containing POEg- MA added during extrusion. Outstanding mechanical properties for this sample were achieved due to the presence of the elastomeric compatibilizer, which has enhanced the interfacial adhesion between PP and PET in the final composite, resulting in better stress transfer under the applied load. The second part of this PhD research was focused on examining novel applications for MFCs. Injection moulding as the final processing step was, for the first time, replaced by extrusion and thermoforming. Applying these two techniques has brought new insight for the production of MFCs, and opens up the possibility of using the MFC concept for producing packaging trays, eventually suitable for food packaging applications. This investigation was presented in Chapter 7 and showed that producing MFCs in the form of sheets for thermoforming could be an innovative processing method. Structural analysis has shown high aspect ratio microfibres dispersed within the matrix and possessing an excellent level of orientation. Yield strength and strain at break increased with the addition of the compatibilizer, thus, facilitating the thermoforming process. However, a real optimisation of the thermoforming process itself was not the focus of this study and remains open for further research. In the end, the feasibility of using the MFC concept to upcycle recycled materials was explored in Chapter 8. Several recycled materials such as mixed bilayer PP/PET films, polyolefins (MPO), and talc-filled PP were selected for this study and used as matrices for MFCs. Recycled MFCs were successfully prepared by the 3-step processing, although these mixtures presented significant difficulties in processability. Unfortunately, outstanding mechanical properties were not achieved for these series of MFCs; they showed little or almost no improvement compared to their recycled matrices. SEM characterisation showed a high level of PET fibre coalescence present in the MFC made out of recycled bilayer film, while in other MFCs, a large heterogeneity of the microstructure was identified. In spite of the disappointing results of these recycled MFCs, the MFC concept can still be considered an interesting approach for upcycling of mixed polymer waste and to improve the original properties of recycled materials. However, this requires further in-depth investigations, taking into account various factors such as viscosity, heterogeneity, presence of different additives, levels of degradation, etc. Overall, this doctoral dissertation presented a deep and extensive study on the MFCs concept, focusing on the relationship between structure, properties, and processing, stressing the importance of each processing parameter, as well as polymer components and additives selected for the final composition. The identified gaps within the MFC field have been covered by the experimental work and reported results. A particular contribution to the scientific world has been made by introducing a novel method for the production of MFCs, i.e., extrusion and thermoforming, and trying to upcycle different recycled mixtures. Despite the current limited commercial use of MFCs, the introduction of new environmental regulations, imposing the utilisation of certain quantity of recycled materials, as well as the recyclability of the manufactured products, may provide a boost for MFCs. The investigations presented within this dissertation have opened some new research questions and proposals for future work, which are presented at the end of this doctoral thesis.
Article
The effectiveness of an advanced treatment of wastewater generated by non-hazardous plastic solid waste (PSW) washing, based on the Sequencing Batch Biofilter Granular Reactor (SBBGR), was assessed in terms of gross parameters, removal efficiencies and sludge production. The proposed treatment was also compared with the conventional treatment, which was based on primary and secondary treatments, using the activated sludge process, performed by Recuperi Pugliesi, a leading company in the plastic recycling industry located in Bari, Italy. The company produces low-density polyethylene (LDPE) regenerated granules from PSW used in agricultural and floricultural greenhouse activities and industrial packaging after a washing stage in the aqueous phase. The latter generates large volumes of wastewater, the conventional treatment of which is characterised by large quantities of sludge and the associated disposal problems. Under steady-state conditions, the SBBGR provided impressive removal efficiencies regarding the main gross parameters (over 90% for COD and TKN, over 99% for BOD5, TSS, VSS and NH3, and over 80% for TN) with a statistically better effluent quality than that of the conventional treatment. The SBBGR effluent quality was modelled in terms of washing water characteristics by using generalized additive models (GAMs). The SBBGR treatment was characterised by a specific sludge production five times lower than that of the conventional treatment (0.21 kg TSS vs. 1.0 kg TSS per m³ of wastewater treated). Compared with the conventional treatment, the proposed process showed a five-fold reduction in the cost of sludge disposal, which saved 50% of the operating cost.
Article
Full-text available
Imagine a waste-less world seems to be impossible but, we can make it happen throughout having the circular economy where we use resources repeatedly and recycle materials endlessly. Solid waste management became more challenging during the last decades because the 3-R (reduce-reuse-recycle) traditional management method does not fulϐil the needs anymore and new strategies are shifting toward energy recovery approaches. While the percentage of solid waste constitutions and the moisture content differ between regions hence, the heat value produced from them varies from one country to another. In this study two sets of municipal solid waste (MSW) from Tehran city have been investigated in comparison with one sample of US MSW, and their heat values had been calculated based on modiϐied Dulong equation considering different constitutions and further splitting them into molecules and atoms which they are consist of. Municipal solid waste includes organic, inorganic, combustible, and recyclable materials. Results showed the higher the moisture content the lower the heat value. Keywords: Municipal solid waste, Combustible component, Heat value, Modiϐied Dulong Equation.
Article
Full-text available
Flame retardants are used in a wide range of plastics to extend the time-of-escape from fires. By definition, they are designed to perform this task only in case of a fire, which is then automatically the end of the plastic's lifetime. However, not all flame retardant plastic products are eventually set on fire, which is why they are abundant in plastic waste, potentially interfering with the mechanical recycling systems in place. To date, there has been little information on the influence of flame retardant additives during the mechanical recycling of solid (thermo)plastic waste. This contribution provides a comprehensive overview of the state of the art concerning the mechanical recycling of flame retardants containing polymers and plastics. In a first part, this review discusses the effect of mechanical melt reprocessing on the flame retardant properties of different recycled thermoplastic polymers, addressing questions whether the flame retardant additives are still present and effective after recycling and whether they interfere with the mechanical recycling itself. Special attention is paid to Waste from Electrical and Electronic Equipment containing flame retardants. A second part of the review lists several upgrading strategies for common polymeric waste streams that consist of adding virgin flame retardants to recycled plastics with the purpose of bringing an additional value to the compound.
Article
Full-text available
This review presents a comprehensive description of the current pathways for recycling of polymers, via both mechanical and chemical recycling. The principles of these recycling pathways are framed against current-day industrial reality, by discussing predominant industrial technologies, design strategies and recycling examples of specific waste streams. Starting with an overview on types of solid plastic waste (SPW) and their origins, the manuscript continues with a discussion on the different valorisation options for SPW. The section on mechanical recycling contains an overview of current sorting technologies, specific challenges for mechanical recycling such as thermo-mechanical or lifetime degradation and the immiscibility of polymer blends. It also includes some industrial examples such as polyethylene terephthalate (PET) recycling, and SPW from post-consumer packaging, end-of-life vehicles or electr(on)ic devices. A separate section is dedicated to the relationship between design and recycling, emphasizing the role of concepts such as Design from Recycling. The section on chemical recycling collects a state-of-the-art on techniques such as chemolysis, pyrolysis, fluid catalytic cracking, hydrogen techniques and gasification. Additionally, this review discusses the main challenges (and some potential remedies) to these recycling strategies and ground them in the relevant polymer science, thus providing an academic angle as well as an applied one.
Chapter
Full-text available
It is very important to understand the interaction between plastics and environment in ambient conditions. The plastics degrade because of this interaction and often their surface properties change resulting in the creation of new functional groups. The plastics after this change continue to interact with the environment and biota. It is a dynamic situation with continuous changing parameters. Polyethylene, polypropylene, and polyethylene terephthalate (PET) degrade through the mechanisms of photo-, thermal, and biodegradation. The three polymers degrade with different rates and different pathways. Under normal conditions, photo- and thermal degradation are similar. For polyethylene, photo-degradation results in sharper peaks in the bands which represent ketones, esters, acids, etc. on their infrared spectrum. The same is true for poly propylene but this polymer is more resistant to photo-degradation. The photo-oxidation of PET involves the formation of hydroperoxide species through oxidation of the CH2 groups adjacent to the ester linkages and the hydroperoxides species involving the formation of photoproducts through several pathways. For the three polymers, interaction with microbes and formation of biofilms are different. Generally, biodegradation results in the decrease of carbonyl indices if the sample has already been photo-degraded by exposure to UV. Studies with environmental samples agree with these findings but the degradation of plastics is very subjective to the local environmental conditions that are usually a combination of those simulated in laboratory conditions. For example, some studies suggested that fragmentation of plastic sheet by solar radiation can occur within months to a couple of years on beaches, whereas PET bottles stay intact over 15 years on sea bottoms.
Article
Full-text available
Within this research the effect of injection molding temperature on polypropylene (PP)/poly(ethylene terephthalate) (PET) blends and microfibrillar composites was investigated. Injection molding blends (IMBs) and microfibrillar composites (MFCs) of PP/PET have been prepared in a weight ratio 70/30. The samples were processed at three different injection molding temperatures (T im) (210, 230, 280 • C) and subjected to extensive characterization. The observations from the fracture surfaces of MFCs showed that PET fibers can be achieved by three step processing. The results indicated that T im has a big influence on morphology of IMBs and MFCs. With increasing the T im , distinctive variations in particle and fiber diameters were noticed. The differences in mechanical performances were obtained by flexural and impact tests. Establishing relationships between the processing parameters, properties, and morphology of composites is of key importance for the valorization of MFC polymers.
Article
The additive packages routinely used today usually consist of a phenolic antioxidant and a phosphorus or sulfur containing secondary stabilizer. Several years ago some questions were raised about the health and environmental hazard of these additives and industry has not offered an alternatively solution yet. Nature produces a large number of antioxidants, which play a key role in radical reactions taking place in the human body. The substances containing these antioxidants are used in natural medicine for ages and they are applied in increasing quantities also in the food industry. The application of natural antioxidants for the protection of polymers is in its infancy, the information available is limited and often contradictory. This review paper summarizes published results, analyzes them and points out the advantages and drawbacks of the approach. Although a wide variety of compounds have been added to polymers to improve their stability, the most promising candidates are the carotenoids, the flavonoids, other natural phenols and phenolic polymers including lignin. Available results indicate that flavonoids are much more efficient stabilizers than the hindered phenols used in industrial practice. On the other hand, most of the natural antioxidants discolor the polymer and their solubility is limited. Nevertheless, natural antioxidants can be efficiently used in specific applications, but further research is needed to explore all their advantages and include them into additive packages used in practice.
Article
Polyethylene (PE) and isotactic polypropylene (iPP) constitute nearly two-thirds of the world's plastic. Despite their similar hydrocarbon makeup, the polymers are immiscible with one another. Thus, common grades of PE and iPP do not adhere or blend, creating challenges for recycling these materials.We synthesized PE/iPP multiblock copolymers using an isoselective alkene polymerization initiator. These polymers can weld common grades of commercial PE and iPP together, depending on the molecular weights and architecture of the block copolymers. Interfacial compatibilization of phase-separated PE and iPP with tetrablock copolymers enables morphological control, transforming brittle materials into mechanically tough blends.