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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
The world is literally awash with plastics and this book practically provides a broad overview of plastic recycling procedures and waste management. With the huge amount of plastics floating in the oceans, fish and other sea creatures are directly suffering the consequences. On land, city leaders and planners are banning one-use plastics as well as plastic bags from grocery stores in an effort to stem the use. Many countries have made official announcements and warnings concerning the pollution caused from plastic wastes. These urgent developments have stimulated the author to study the problem and write Polymer Waste Management. Plastic recycling refers to a method that retrieves the original plastic material. However, there are many sophisticated methods available for the treatment and management of waste plastics such as basic primary recycling, where the materials are sorted and collected individually. In chemical recycling, the monomers and related compounds are processed by special chemical treatments. Other methods, such as pyrolysis, can produce fuels from waste plastics. These methods and others are treated comprehensively in the book. This groundbreaking book also discusses: * General aspects, such as amount of plastics production, types of waste plastics, analysis procedures for identification of waste plastic types, standards for waste treatment, contaminants in recycled plastics. * Environmental aspects, such as pollution in the marine environment and landfills. * The advantages of the use of bio-based plastics. * Recycling methods for individual plastic types and special catalysts.
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Contents
Cover
Title page
Copyright page
Preface
Chapter 1: General Aspects
1.1 History of the Literature
1.2 Amount of Wastes
1.3 Metal Content in Wastes
1.4 Analysis Procedures
1.5 Standards
1.6 Special Problems with Plastics
References
Chapter 2: Environmental Aspects
2.1 Pollution of the Marine Environment
2.2 Pollution of the Terrestrial Environment
References
Chapter 3: Recycling Methods
3.1 Alternative Plastic Materials
3.2 Mechanical Recycling
3.3 Primary Recycling
3.4 Renewable Polymer Synthesis
3.5 Preparation and Regeneration of Catalysts
3.6 Pyrolysis Methods
3.7 Metallized Plastics Waste
3.8 Mixed Plastics
3.9 Separation Processes
3.10 Triboelectrostatic Separation
3.11 Wet Gravity Separation
3.12 Supercritical Water
3.13 Solvent Treatment
References
Chapter 4: Recovery of Monomers
4.1 Process for Obtaining a Polymerizable Monomer
4.2 Pyrolysis in Carrier Gas
4.3 Fluidized Bed Method
4.4 Recovery of Monomers from Waste Gas Streams
4.5 Polyolefins
4.6 Poly(styrene)
4.7 Phenolic Resins
4.8 Poly(carbonate)
4.9 Poly(ethylene terephthalate)
4.10 Nylon
4.11 Poly(urethane)
4.12 Sequential Processes for Mixed Plastics
4.13 Waste Fiber Reinforced Plastics
References
Chapter 5: Recovery into Fuels
5.1 Poly(ethylene)
5.2 Thermal and Catalytic Processes
5.3 Mixed Waste Plastics
5.4 Hydrocarbon Fuels
5.5 High-Value Hydrocarbon Products
5.6 Purified Crude Oil
5.7 Lubricating Oil
5.8 Waxes and Grease Base Stocks
5.9 Co-pyrolysis of Landfill Recovered Plastic Wastes and Used Lubrication Oils
5.10 PVC Wastes
5.11 Iron Oxide Catalyst
5.12 Landfill
References
Chapter 6: Specific Materials
6.1 Catalysts for Recycling
6.2 Polyolefins
6.3 Poly(styrene)
6.4 Poly(carbonate)
6.5 Poly(ethylene terephthalate)
6.6 Poly(vinyl chloride)
6.7 Pyrolysis of Mixed Plastics
6.8 Technical Biopolymers
6.9 Co-processing of Waste Plastics and Petroleum Residue
6.10 Automotive Waste Plastics
6.11 Phthalates
6.12 Enzymatic Degradation
6.13 Electronic Waste
6.14 Fiberglass Reinforced Plastics
6.15 Usage in Concrete
6.16 Recycling of Floor Coverings
References
Index
Acronyms
Chemicals
General Index
End User License Agreement
Guide
Cover
Copyright
Table of Contents
Begin Reading
List of Illustrations
Chapter 1
Figure 1.1
Reflectance spectra of a black Poly(propylene) and a non-black Poly(propylene), reproduced from an open access article (21).
Figure 1.2
Flow diagram of a stock-based model (54).
Figure 1.3
Dichlorodiphenyltrichloroethane.
Chapter 2
Figure 2.1
Pathways of plastics into the marine environment (20).
Figure 2.2
Styrene oligomers.
Figure 2.3
Plastic additives.
Figure 2.4
Northern fulmars. Reprinted from Wikipedia, licence: GFDL/C-C-by-2.5 (64).
Chapter 3
Figure 3.1
Stages of mechanical recycling (12).
Figure 3.2
Change in density of waste EPS at different oven temperatures (26).
Figure 3.3
Solvents.
Figure 3.4
Electrospinning process.
Figure 3.5
Limonene concentration in the carbon dioxide exit stream (35).
Figure 3.6
Terpenes.
Figure 3.7
Transverse section of rotary reactors (61).
Figure 3.8
Materials separation process (74).
Figure 3.9
Strategies for the separation of plastics by froth flotation (78).
Figure 3.10
Activation for the Fenton reaction (92).
Figure 3.11
Device for using supercritical water (119).
Figure 3.12
Solvents for recovering a polyester polymer.
Figure 3.13
Turpentine liquid compounds.
Chapter 4
Figure 4.1
Dibenzoyl hexamethylene diamine.
Figure 4.2
N,N
-Dimethylaminopyridine.
Chapter 5
Figure 5.1
Schematic diagram of a device to obtain raw oil (21).
Figure 5.2
Apparatus for conducting the thermolysis of plastic waste with continuous waste plastics feeding (33).
Figure 5.3
System for recycling waste plastic (34).
Figure 5.4
Systems for conditioning synthetic crude oils (36).
Chapter 6
Figure 6.1
Device for catalytically cracking waste plastics (7).
Figure 6.2
SEM microgram of FCC, reprinted from (10) with permission from Elsevier.
Figure 6.3
SEM microgram of coked FCC, reprinted from (10) with permission from Elsevier.
Figure 6.4
TGA curve of poly(ethylene) (13).
Figure 6.5
Chain cracking in the case of thermal pyrolysis (13).
Figure 6.6
Chain cracking in the case of catalytic pyrolysis (13).
Figure 6.7
Reaction pathway of the lumping reaction (36).
Figure 6.8
TEM micrographs of calcined hierarchical ZSM-5 samples, reprinted from (37), with permission from Elsevier.
Figure 6.9
Scanning electron microscope images of (a) the Y-zeolite and (b) the 1 w% Co-Y-zeolite and (c) the 5% Co-Y-zeolite after pyrolysis-catalysis of the high density poly(ethylene); reprinted from (42) open access from Elsevier.
Figure 6.10
Bulk hydroprocessing process (55).
Figure 6.11
Schematic diagram of the method of recovering rare earth elements (56).
Figure 6.12
Schematic of a conical spouted bed reactor, reprinted from (57) with permission from Elsevier.
Figure 6.13
Apparatus for pyrolysis of hydrocarbonacetous materials (71).
Figure 6.14
Degradation conversion (75).
Figure 6.15
Correlation plot of iTOC-CRDS-measured C13 values versus AMS-measured renewable bio-source carbon content for PET films (103).
Figure 6.16
Grinding machine (142).
Figure 6.17
Samples of glass fibers as obtained after the pyrolysis treatment and after the further calcination treatment (143).
List of Tables
Chapter 1
Table 1.1
Plastics production in the world (6).
Table 1.2
The literature with
plastics recycling
in the title of the papers found in
Google Scholar
in March 2018.
Table 1.3
Quantities of recycled PVC in Europe (12).
Table 1.4
Plastics recycling in European countries (6).
Table 1.5
Metal content of poly(ethylene) samples (9).
Table 1.6
Plastics Industry (SPI) recycling codes (68).
Table 1.7
Studies of micro-plastics in seafood and food (88).
Table 1.8
Micro-plastics exported by rivers to seas (126).
Chapter 2
Table 2.1
Rankings of marine debris (18).
Table 2.2
Population density, concentrations of the styrene oligomers in seawater and in sea sand (37).
Table 2.3
Example of chemicals (39).
Table 2.4
Rate of ingestion (56).
Table 2.5
Percentages of fulmars with ingested plastics (69).
Table 2.6
Mean concentration values (71).
Chapter 3
Table 3.1
Solubility of EPS at 25°C in certain solvents (29).
Table 3.2
Solubility of PS at 50°C in cyclic monoterpenes (30).
Table 3.3
Time required for the dissolution (30).
Table 3.4
Terpene and terpenoid content (36).
Table 3.5
Main components of the pyrolysis liquids (% area in GC) (51).
Table 3.6
Results for end-of-life vehicle thermal and thermocatalytic pyrolysis (52).
Table 3.7
Plastic pyrolysis methods (53).
Table 3.8
Materials of municipal solid waste (67).
Table 3.9
Specific gravity and contact angles (80).
Table 3.10
Purity (86).
Table 3.11
Contact angle (94).
Table 3.12
Separation of PC/ABS mixtures at different temperatures (93).
Table 3.13
Changes of the contact angles of certain polymers due to ozonization (99).
Table 3.14
Polymer recovery by the dissolution/reprecipitation technique (104).
Table 3.15
Solvents for recovering a polyester polymer (122–124).
Table 3.16
Specific compounds in a turpentine liquid (125).
Chapter 4
Table 4.1
Properties of the used catalysts (11).
Table 4.2
Mass balance of the pyrolysis of PS in the temperature range of 450°C to 700°C (16).
Table 4.3
Product yields from the pyrolysis of PC (24).
Table 4.4
Yield of terepthalic acid with the variation of the amount of sodium hydroxide (29).
Table 4.5
Yield of terepthalic acid with the variation of the particle size of PET (29).
Table 4.6
Yield of terepthalic acid with the variation of the reaction temperature (29).
Chapter 5
Table 5.1
Properties of the catalysts (3).
Table 5.2
Effect of the catalyst on the product distribution (18).
Table 5.3
Yields from different processes (24).
Table 5.4
Properties of waxes (43).
Table 5.5
Properties of liquid fuel from plastic waste and petroleum diesel fuel (49).
Table 5.6
Plastic waste types in a landfill (51).
Chapter 6
Table 6.1
Gaseous products obtained from PE pyrolysis (3, 4).
Table 6.2
Product distributions from Zn-ZSM-11 catalyzed pyrolysis waste different ratios of plastic waste (15).
Table 6.3
Reaction rate constants (22).
Table 6.4
Product yields from different pyrolysis methods (24).
Table 6.5
Yields of the pyrolysis products (30).
Table 6.6
Calorific value data (31).
Table 6.7
Main products (C1–C8) obtained from HDPE pyrolysis (34).
Table 6.8
Catalysts used in the catalytic degradation of PE/PP polymer waste (36).
Table 6.9
Surface area and porosity of the catalysts (42).
Table 6.10
Product distribution (52).
Table 6.11
Overall product distributions of fresh, steamed and used FCC catalysts (52).
Table 6.12
Main products of post-consumer waste plastics (53).
Table 6.13
RON numbers (59).
Table 6.14
Products of pyrolysis (60).
Table 6.15
Performance of nickel- and iron-based catalysts (64).
Table 6.16
Pseudo first order degradation rate constants (70).
Table 6.17
Effect of degradation temperature (75).
Table 6.18
Effect of degradation temperature (75).
Table 6.19
Separation techniques for PVC waste plastics (84).
Table 6.20
Polyolefins (87).
Table 6.21
Product distribution as a function of temperature and hydrogen pressure using HDPE with 1% Pt/USY (89).
Table 6.22
Product distributions from an ECat-1 (93).
Table 6.23
Activation energy and coke content (94).
Table 6.24
Product distributions from FCC-R1 catalyzed pyrolysis waste at fluidizing nitrogen rates (99).
Table 6.25
Product distributions from FCC-R1 catalyzed pyrolysis waste at different ratios of plastic waste (99).
Table 6.26
Assemblies used in automotive parts (111).
Table 6.27
Major oxides in clay and automotive waste plastics ash (XRF analysis) (107).
Table 6.28
Elements in clay and automotive waste plastics ash (ICP analysis) (107).
Table 6.29
Phthalates (113).
Table 6.30
Categories of electrical and electronic waste (125).
Table 6.31
Compositions and typical densities of the main plastics in an electronic waste sample (131).
Table 6.32
Composition of metal elements extracted from FPCB and from conventionally recycled PCB (138).
Table 6.33
Properties of the Portland Pozzolona cement (155).
Table 6.34
Compressive strength test results (155).
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Polymer Waste Management
Johannes Karl Fink
This edition first published 2018 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2018 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com.
All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
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Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make This work is sold with the understanding that the publisher is not engaged in rendering professional services The advice and strategies contained herein may not be suitable for your situation You should consult with a specialist where appropriate Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read.
Library of Congress Cataloging-in-Publication Data ISBN 978-1-119-53608-6
Preface
The scientific literature with respect to plastic recycling increased dramatically after the mid-1970s and remains a growing field, since the production of polymers, and thus the problems concerning the disposal of these materials after their life cycle, are continuously growing.
Recently, in several countries, official announcements and warnings concerning the pollution caused by plastic wastes have been published.
For these reasons, the problems of plastic waste management have been collected from several recent scientific publications in this monograph.
Plastic recycling refers to a method that can regain the original plastic material. However, there are still more sophisticated methods available for the treatment and management of waste plastics.
These methods include the following:
Basic primary recycling, where the materials are sorted as such and collected individually. In chemical recycling, the monomers and related compounds are sampled by special chemical treatments. Other methods, such as pyrolysis can produce fuels from waste plastics, etc. These methods and others are treated in one of the chapters.
The book starts with general aspects, such as amount of plastics production, types of waste plastics, analysis procedures for identification of waste plastic types, standards for waste treatment and contaminants in recycled plastics.
Then, in another chapter, environmental aspects, such as pollution in the marine environment, such as ingestion of plastics by marine animals, and pollution in landfills are dealt with.
Furthermore, the recycling methods for plastics and then the methods for the recovery of monomers are reported in detail. Also, the advantages of the use of bio-based plastics are discussed.
Another chapter deals with the recovery into fuels, since this has also become an important aspect.
Finally, specific materials are detailed, including recycling methods for individual plastic types, and special catalysts. Here, special uses are also reported, such as the use of plastic fibers in concrete and others.
This textmay be of importance for scientists engaged in the problems of plastics waste management and also for the education of students that are interested in the current problems of plastics recycling.
The text focuses on the basic issues and also the literature of the past decade. The book provides a broad overview of plastic recycling procedures and waste management.
How to Use This Book
Utmost care has been taken to present reliable data. Because of the vast variety of material presented here, however, the text cannot be complete in all aspects, and it is recommended that the reader study the original literature for more complete information.
Index
There are three indices: an index of acronyms, an index of chemicals, and a general index. In the index of chemicals, compounds that occur extensively are not included at every occurrence, but rather when they appear in an important context. When a compound is found in a figure, the entry is marked in boldface letters in the chemical index.
Acknowledgements
I am indebted to our university librarians, Dr. Christian Hasenhüttl, Dr. Johann Delanoy, Franz Jurek, Margit Keshmiri, Dolores Knabl Steinhäufl, Friedrich Scheer, Christian Slamenik, Renate Tschabuschnig, and Elisabeth Groß for their support in literature acquisition. In addition, many thanks to the head of my department, ProfessorWolfgang Kern, for his interest and permission to prepare this text.
I also want to express my gratitude to all the scientists who have carefully published their results concerning the topics dealt with herein. This book could not have been otherwise compiled.
Last, but not least, I want to thank the publisher, Martin Scrivener, for his abiding interest and help in the preparation of the text. In addition, my thanks go to Jean Markovic, who made the final copyedit with utmost care.
Johannes Fink Leoben, 11th July 2018
Chapter 1General Aspects
Economic, ecological, and technical aspects of plastic waste handling have been summarized in monographs (1–4).
Plastics have become an indispensable ingredient of human life. They are non-biodegradable polymers mostly containing carbon, hydrogen, and a few other elements such as chlorine, nitrogen, etc. Rapid growth of the world population has led to increased demand for commodity plastics (5).
The total plastics production in the world is shown in Table 1.1.
Table 1.1 Plastics production in the world (6).
Year
1950
1.5
1977
50
1989
100
2002
200
2009
250
2011
280
2015
322
A list of acronyms and initials used in the waste management industry has been published (7).
1.1 History of the Literature
The issue of recycling of plastics was not important for scientists before the 1970s. The amount of literature concerning plastics recycling is collected in Table 1.2.
Table 1.2 The literature with plastics recycling in the title of the papers found in Google Scholar in March 2018.
Time range
1970–1975
1976–1980
1981–1985
1986–1990
1991–1995
1996–2000
2001–2005
2006–2010
2011–2015
2016–2018
As can be seen from Table 1.2, the boom started in the mid-1980s.
1.2 Amount of Wastes
The plastic wastes produced in the European Union in 2007 was about 52.5 Mt (8, 9). In 2008, 60 Mt were produced in Europe and the global production in 2008 was 245 Mt (10). In 2007 the amount of post-consumer plastic wastes obtained in the EU that year was 24.6 Mt, which is similar to that in 2008 (8, 10).
The total waste generated per year in 2010 in Pakistan was about 31 Mt per. In big Pakistani cities such as Karachi, about 7 to 8 Mt of solid waste is generated. It is estimated that about 6% to 8% of solid waste is post-consumer plastic waste, while only 10% of this amount is recycled (11).
The quantities of recycled poly(vinyl chloride) (PVC) in Europe are shown in Table 1.3.
Table 1.3 Quantities of recycled PVC in Europe (12).
Year
2003
2004
2005
2006
2007
2008
2013
2014
Also, the problems of plastics wastes in other countries have been highlighted, such as, in India (13) and Bangladesh (14, 15).
Consequently, there is a growing social concern related to the management of the plastic wastes, which should proceed according to a hierarchical approach in agreement with the following order: waste minimization, reuse, recycling, energy recovery and landfilling (16).
In 2014, nine countries in Europe reached a recovery ratio of more than 95% of the post-consumer plastic waste (6). The amounts are shown in Table 1.4.
Table 1.4 Plastics recycling in European countries (6).
Country
Switzerland
Austria
Netherlands
Germany
Sweden
Luxembourg
Denmark
Belgium
Norway
1.3 Metal Content in Wastes
1.3.1 Waste Poly(ethylene) and Pure High Density Poly(ethylene)
The metal content of both waste poly(ethylene) (PE) and pure high density poly(ethylene) (HDPE) used in a specific study (9) is shown in Table 1.5.
Table 1.5 Metal content of poly(ethylene) samples (9).
Metal
Al
Ca
Cr
Cu
Fe
Mg
Na
Pb
Ti
Zn
In pure HDPE, the total metal content is very low and accounts for less than 0.03%. In contrast, the metal content in waste PE is much higher and accounts for roughly 0.4%. The main metals present are Cu and Ti with a share of 0.162% and 0.151%, respectively (9).
1.4 Analysis Procedures
1.4.1 Fluorescence Labeling
The demand for polymers in combination with their high durability following rather short life phases ensures the flow of plastic waste into landfills (17). Therefore, plastic recycling has become indispensable. In order to produce economically attractive products based on recycled plastics, mono-fractional compositions of waste polymers are required.
However, existing measurement technologies, such as near infrared spectroscopy used in sorting facilities, show limitations with regard to the separation of complex mixtures of plastic flakes, especially when dark and black plastics are part of them. An innovative approach to overcome these obstacles and provide high sorting purities is to label different types of plastics with unique combinations of fluorescence markers, also known as tracers, which can be considered as optical fingerprints. They are incorporated into the virgin plastic resins at ppm levels during the production process and do not affect either the visual appearance nor the structural and mechanical integrity of the materials.
The goal is to realize the practical use of this concept in industrial processes. An industrial applicable spectroscopic measurement system has been designed and implemented that can identify polymer flakes with a size of a few millimeters transported on a conveyor belt in real time based on the emitted fluorescence of incorporated organic markers. In addition to the implementation of the opto-electrical measurement system, a multi-threading software application has been developed and realized which controls the hardware and collects the measured data and finally classifies the data (17).
In recent years, great effort has been expended in the development of the automated identification and sorting methods for post-consumer plastics in the waste streams that are reaching recycling processes (18). The final properties of the recycled materials largely depend on the purity of the plastic residue.
The use of fluorescence spectroscopy has been explored as a technique to identify certain waste polymers. In particular, the use of fluorescent markers for removing, for technical or safety related issues, selected HDPE containers from the waste stream has been studied. The results of this study indicate that identification by extrinsic fluorescence can be easily achieved even with a small proportion of markers of 10–3% without a significant change to the polymer structure.
The effect of thermal, hygrothermal and photochemical degradation on the fluorescence emission has been analyzed. Although the signal intensity decreases during the accelerated degradation, distinguishable fluorescent emission can be recorded even after sample exposure to aggressive conditions, thus enabling the correct identification of the marked plastics (18).
1.4.2 Time-Gated Fluorescence Spectroscopy
For the production of high-quality parts from recycled plastics, a very high purity of the plastic waste to be recycled is mandatory (19).
The incorporation of fluorescent tracers, i.e., markers, into plastics during the manufacturing process helps overcome the typical problems of non-tracer based optical classification methods.
Despite the unique emission spectra of fluorescent markers, the classification may become difficult when the host plastics show a strong autofluorescence that may spectrally overlap the fluorescence of the marker. Increasing the marker concentration is not a good option from an economic perspective and might also adversely affect the properties of the plastics.
A method that can suppress the autofluorescence in the needed signals is time-gated fluorescence spectroscopy. However, time-gated fluorescence spectroscopy is associated with a lower signal-to-noise ratio, which may result in larger classification errors.
In order to optimize the signal-to-noise ratio, the best time-gated fluorescence spectroscopy parameters were investigated and validated. A model for the fluorescence signal for plastics labeled with four specifically designed fluorescent markers was used. The implementation of time-gated fluorescence spectroscopy on a measurement and classification prototype system has also been demonstrated.
Mean values for a sensitivity of 99.93% and a precision of 99.80% could be achieved in this study. This shows that a highly reliable classification of plastics can be achieved in practice (19).
1.4.3 Content of Flame Retardants
The process of disassembling large plastic components from waste electrical and electronic equipment can increase the recovered value (20).
A higher quality and significantly higher mechanical properties can be achieved by the proposed process compared to post-shredder recycling. Today, the application of infrared spectroscopy and X-ray fluorescence in the sorting step enables the recycling of unrecovered plastics by the determination of their chemical structure and flame retardant content (20).
1.4.4 Identification of Black Plastics
Black polymers represent a much wider variety of materials than household plastic waste since they are mostly used for technical applications with special requirements (21). Various additives and filler materials, which are added in order to achieve specific properties of the plastics, complicate the identification, since spectra of the same kind of plastic can vary dramatically if different types or amounts of additives, e.g., flame retardants, fibers, or soot, are contained in the plastic parts.
Even lacquer films on the plastic part surface prevent any spectral identification and have to be removed before measurement. The importance of characterizing black polymers has led to a wide range of IR techniques, e.g., attenuated total reflection (22, 23), infrared transmission (24), emission spectroscopy (25), and photoacoustic spectroscopy (26).
Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy, particularly of light, on matter by means of acoustic detection (27).
Also, the use of reflectance measurements was demonstrated for characterizing soot filled polymers (28, 29).
1.4.4.1 Terahertz Spectroscopy
For a modern recycling cycle, a 100% mono-fraction sorting of plastic waste is needed. The final stage in most sorting machines is based on optical sensors like hyperspectral optical camera systems. These systems cannot detect black plastics because the reflectance is too low for stable detection.
THz systems offer the possibility of a spectroscopy analysis of shredded plastics (30, 31). From an economic viewpoint, full spectroscopy systems which cover a large area of the THz region are too expensive.
Test measurements have shown the possibility to separate plastics with electronic THz systems. The limitations in bandwidth can be compensated by external height sensors and sophisticated mathematic methods.
The system operates between 84 GHz and 96 GHz (31). Since the relevant plastics exhibit no specific absorption lines in this frequency range, a broadband approach is necessary to accumulate slight differences in dielectric properties. Using this technique, enough entropy can be gathered so that a machine learning algorithm can be trained to differentiate between different materials.
1.4.4.2 Middle Infrared Spectroscopy
The identification of black polymers which contain about 0.5% to 3% mass percent soot or black master batch is still a problem in recycling sorting processes (21).
Near infrared spectroscopy of non-black polymers offers reliable and fast identification, and is therefore suitable for industrial application. However, this method cannot be used for black polymers, because small amounts of carbon black or soot absorb all light in the near infrared spectral region.
However, a spectroscopy in the mid-infrared spectral region offers a possibility to identify black polymers. Mid-infrared spectral measurements can be carried out with Fourier transform infrared (FTIR) spectrometry, but the measurements are not fast enough to meet the economic requirements in sorting plants.
In contrast, spectrometer systems based on the photon up-conversion technique are fast and sensitive enough and can be applied to sort black polymer parts. Such systems are able to measure several thousand spectra per second. Hence, they are suitable for industrial applications (21).
In the middle infrared spectral region from 2.5 µm to about 16 µm wavelength, which corresponds to a wave number range from 4000 to about 600 cm–1, the different kinds of plastic material show additional vibrational modes, like deformation, rocking, and twisting modes, due to their molecular structure (21). In addition to the C–H group, other molecule groups, like O–H, N–H and O–C also contribute with their fundamental vibrations to the spectral features.
The various molecular groups with their different vibrational modes generate a unique spectrum of each polymer in the spectral range between 2500 cm–1 and 600 cm–1. This allows a definite identification of the polymer type. Therefore, this spectral range is called the fingerprint region, and middle infrared spectroscopy is the predominant analytical method for polymer characterization (21). Another important advantage of this spectral range is that reflectance spectra can be measured which allow the identification of black polymers. The reflectance spectra of a black poly(propylene) (PP) polymer part and a non-black part are compared in Figure 1.1.
Figure 1.1 Reflectance spectra of a black Poly(propylene) and a non-black Poly(propylene), reproduced from an open access article (21).
In the study, the results of the measurements were collected and analyzed by a principal component analysis method (21).
Principal component analysis is a statistical procedure that uses an orthogonal transformation to convert a set of observations of possibly correlated variables into a set of values of linearly uncorrelated variables called principal components (32, 33).
1.4.5 Raman Spectroscopy
Raman spectroscopy has potential for application in waste plastic recycling when large-scale, accurate sorting processes are required (34).
A high-accuracy rapid system for sorting a plurality of waste products by polymer type has been developed (35).
Raman spectroscopy and other complex identification techniques are used to identify and sort post-consumer plastics for recycling. The procedure reads information unique to the molecular structure of the materials to be sorted to identify their chemical compositions and uses rapid high-volume sorting techniques to sort them into product streams at commercially viable throughput rates.
The system uses a laser diode for irradiating the material sample, a spectrograph is used to determine the Raman spectrum of the material sample and a microprocessor-based controller is employed to identify the polymer type of the material sample (35).
In addition, a high-speed Raman identifier has been developed with a 3 ms measuring time (34). This identifier could be successfully integrated into an online sorting system in a shredded plastic recycling plant. A practical-scale (200–600 kg h–1) demonstration facility was constructed with 50 Raman apparatuses on a 30 cm wide conveyor with a speed of 100 mmin–1. This device also included preprocessing using specific gravity classification and putty removal.
The Raman identification system was used to control air jets to sort PP, poly(styrene) (PS), and an acrylonitrile-butadiene-styrene (ABS) copolymer with high accuracy from shredded plastics from post-consumer electrical appliances. The method of Raman plastic identification can also provide solutions to problems at recycling sites such as the detection of brominated flame retardants and the identification of black plastics (34).
1.4.6 Life Cycle Assessment
Life cycle assessment (LCA) is a technique to assess environmental impacts associated with all the stages of a product’s life from raw material extraction through materials processing, manufacture, distribution, use, repair and maintenance, and disposal or recycling (36, 37).
The basic idea of LCA is that all environmental burdens connected with a product or service should be assessed, back to the raw materials and down to the removal of waste (38). LCA is the only environmental assessment tool which avoids positive ratings for measurements which only consists of the shifting of burdens.
In the years from 1990 to 1993, the development of LCA was presented in a series of workshops at SETAC and SETAC-Europe, which culminated in the Code of Practice of 1993 (39). The basic structure which is now underlying the standardizing activities of ISO (38, 40) is
Goal definition and scoping,
Life cycle inventory analysis,
Impact assessment, and
Improvement assessment.
Also, the limitations of the LCA, the relationship between the LCA phases, and conditions for use of value choices and optional elements are described (40).
There is software available for support of LCA studies (41–43).
The basic issues of LCA have been described in several monographs (44–47).
For example, the Italian system of plastic packaging waste recycling, that collected and mechanically recycled the post-consumer PE and poly(ethylene terephthalate) (PET) liquid containers, has been investigated using this technique (48). The phases of collection, compaction, sorting, reprocessing and refuse disposal were individually analyzed and quantified in terms of energy and material consumptions as well as the emissions into the environment. The main goal of this study was the quantification of the real advantage of plastic container recycling and the definition of criteria, to be environmentally compatible and economically sustainable.
Also, the environmental impacts of lifetime extension versus energy efficiency for video projectors were investigated using LCA (49).
The results of the LCA study showed that the use stage dominates the life cycle impacts of the global warming potential and the primary energy demand. For the metal depletion potential, the production stage accounts for most of the total life cycle load. The highest shares in production emissions were identified for electronic components, i.e., printed wired boards and integrated circuits. Reconditioning and reuse of a secondary projector resulted in minor environmental impacts in comparison to the replacement and use of a primary projector with an energy efficiency increase of 5%. The saving potential of the primary energy demand is higher only in the case of a 10% more efficient device as compared to the secondary projector (49).
1.4.7 Analysis of Contaminated Mixed Waste Plastics
Mixed waste plastics, especially those obtained from municipalities, typically contain many different types of contaminants that must be removed or otherwise dealt with in any effective plastic reclamation process (50). Such contaminants can include, for example, non-melting fillers, pigments, wood, paper or metal, as well as a variety of plastics that may not be suitable for use as a feed material.
Various polymeric materials that may be present in mixed waste plastics may include PE, PP, PS, PET, ethylene-vinyl acetate, poly-(vinylidene chloride) (saran), ABS, and the like.
The ability to use a higher percentage of mixed waste plastics in the manufacture of new products, including composite wood and plastic building materials, is highly desirable.
Although many products have been manufactured successfully using scrap or recycled plastics of various types, the variability that exists in the composition and cleanliness of batches of mixed waste plastics obtained over time from either the same or different sources has previously caused serious problems with raw materials processing and manufacturing.
For example, the reclamation and reuse of a PE film is particularly problematic. In 2005, the U.S. Environmental Protection Agency reported that less than 3% of all PE film was recycled (51). Consequently, millions of tons per year of PE film is buried in landfills and never reused. Such films can include, for example, trash bags, shopping bags, bubble wrap, shrink wrap, meat packing wrap, blood bags, nursery films, and greenhouse films.
Various analytical methods have been used in the past to determine the types and properties of plastic present in mixed waste plastics, but with limited success (50).
For example, batches of mixed, reclaimed plastics have been analyzed by pressing a sample of the material between two hot plates at a suitable temperature to form a test plaque, which is then cut up and repressed several more times to make it more homogeneous. Sometimes the polymers present in such test plaques can be determined by visual inspection, although this method is highly inaccurate and only allows for gross distinctions to be made. At other times the samples may contain contaminant inclusions that are not representative of the entire batch and thus can skew the analysis. Other methods believed to have been tried to characterize mixed reclaimed plastics include, for example, melt filtration and solvent extraction.
More recently, the use of differential scanning calorimetry for various purposes has been described and explained. It has been found that the color contribution to the composite of the component plastic can be characterized by measuring a thoroughly homogenized sample of the plastic, and measuring its color parameters with a reflectance spectrophotometer. Furthermore, more useful information can be obtained by mixing small amounts of known pigments (black and white) with the material prior to homogenization, with subsequent homogenization, and color analysis. Measuring and correlating the results of such testing allow us to predict the effects of their raw material on the subsequent composite board color, which may be pigmented. It is believed that the use of prior known methods that did not thoroughly homogenize the samples would yield unforeseeable results due to small particles of highly pigmented plastic (50).
Such methods of analysis can be used to calculate the properties of materials recycled from a plurality of various batches that can be mixed together, i.e., reformulated, in the final stage (50).
Such methods allow the manufacturers to produce green products with a high percentage of reclaimed plastics without the need for separating the various components of the mixed waste plastics in the manner that has previously been required. Using these methods, manufacturers can now reformulate various batches of mixed waste plastics into feed materials for new products by blending together calculated amounts of various batches that, when combined, either alone or with some portion of virgin resin, yield a feed material having a set of physical properties falling within a desired, predetermined target window (50).
1.4.8 Construction and Household Plastic Waste
The recyclability of construction and household plastic waste collected from local landfills has been studied (52). Samples were processed from mixed plastic waste by injection molding. In addition, blends of pure plastics, PP and PE were processed as a reference set.
Reference samples with known plastic ratio were used as the calibration set for quantitative analysis of plastic fractions in recycled blends. The samples were tested for the tensile properties. Scanning electron microscope-energy-dispersive X-ray spectroscopy was used for elemental analysis of the blend surfaces and FTIR analysis was used for the quantification of the plastics contents (52).
1.4.9 Models for Forecasting the Composition of Waste Materials
Several methods to forecast the amount of waste that will emerge have been developed (53–57). These methods have also been applied to forecast the generation of electronic waste in several regional and national studies.
The material flow analysis (MFA) model can be used to describe, investigate, and evaluate the metabolism of anthropogenic systems (58). This model is based on the principle of mass conservation and can be used to quantify the flow of materials in a system defined by spatial and temporal boundaries. In an MFA model, the flows and the stocks interact with each other. The stocks increase when the inflows exceed the outflows of a system, and the stocks decrease when the outflows exceed the inflows. A flow diagram of a stock-based model is shown in Figure 1.2.
Figure 1.2 Flow diagram of a stock-based model (54).
The principle of the stock-based model can be described by the following equations:
(1.1)
(1.2)
Here, and are the product inflows entering society in year t and year t – k, respectively. is the outflow of obsolete products in year t. St and St–1 are the in-use stocks of product in year t and year t – 1, respectively. M is the maximum lifetime of the product and dk is the lifetime distribution density value (54).
Using this model, forecasts can be made based on information concerning the stock by (54):
Modeling the product lifetime distribution,
Extrapolating the stocks based on past information, and
Determining the initial year.
Substance Flow Analysis (STAN) is a free software that supports the performance of a material flow analysis (MFA) (59). The basic idea behind STAN is the combination of all necessary features of a MFA in one software product: Graphical modeling, data management, calculations and graphical presentation of the results. Application examples of this software have been detailed (59).
Also, an innovative model to forecast the composition of electronic waste materials has been presented (60, 61).
The methodology is based on the distribution delay forecasting method presented by Chancerel (62). A distribution delay forecasting method, also referred to as a market supply model, uses sales and average lifetime distribution data to forecast the amount of waste that will be discarded (60). The challenge to forecast emerging waste streams with a distribution delay method is to obtain detailed and reliable data on the historic numbers of products that were sold. In addition, the number of products that will be placed on the market in the near future should also be taken into account to make a correct forecast.
In order to demonstrate the applicability and of this proposed methodology, it was applied to forecast the evolution of plastic housing waste from flat panel displays and monitors, TVs, cathode ray tube TVs and cathode ray tube monitors. The results of the forecasts indicated that a wide variety of plastic types and additives, such as flame retardants, are found in the housings of similar products.
This case study demonstrates that the proposed methodology allows the identification of the trends in the evolution of the material composition of waste streams (60).
1.5 Standards
The standard ISO 15270:2008 provides guidance for the development of standards and specifications covering plastics waste recovery, including recycling (63). The standard establishes the different options for the recovery of plastics waste arising from pre-consumer and post-consumer sources. It also establishes the quality requirements that should be considered in all steps of the recovery process, and provides general recommendations for inclusion in material standards, test standards and product specifications.
Consequently, the process stages, requirements, recommendations and terminology presented in the standard are intended to be of general applicability (63).
1.5.1 Circular Economy Package
The European Commission recently introduced a circular economy package, setting ambitious recycling targets and identifying waste plastics as a key area where major improvements and focus is necessary (64).
The importance of plastics as a landmark case for the circular economy is denoted by the significant report on the new plastics economy released by the Ellen MacArthur Foundation. The multiple array of challenges facing used plastics has been vividly exemplified in a recent International Solid Waste Association (ISWA) report looking at the PP case.
The collection modalities were detailed. A crucial aspect affecting the quantity and quality of recycling was investigated, using recent empirical serial data from household dry recyclables collection in the United Kingdom, and specifically within the devolved administration of England (64).
In addition, the big challenges and big opportunities in the United Kingdom and other international locations were documented (65).
1.5.2 SPI Codes
The ASTM International Resin Identification Coding System, often abbreviated as the RIC, is a set of symbols appearing on plastic products that identify the plastic resin out of which the product is made (66). It was developed originally by the Society of the Plastics Industry, now the Plastics Industry Association, in 1988, but has been administered by ASTM International since 2008 (67).
The Plastics Industry (SPI) has given seven recycling codes for plastics. However, only two plastic types are commonly recycled with current methods: PET (SPI Code 1) and HPDE (SPI Code 2).
The codes are given in Table 1.6.
Table 1.6 Plastics Industry (SPI) recycling codes (68).
SPI Code
Usage
SPI Code
Usage
SPI Code
Usage
PET
HDPE
PVC
LDPE
PP
PS
Other
Subsequently, the codes are highlighted.
1.5.2.1 SPI Code 1: PET
A principal benefit of chemolysis for the breakdown of PET is that the complete reversion to the starting materials is possible. The depolymerization can be achieved with catalysts or with base and heating. Isolated monomers are well suited for repolymerization and include bishydroxyethylene terephthalate, dimethyl terephthalate, and terephthalic acid via the transesterification with ethylene glycol, methanol, water, or a hydroxide.
Discolored monomers produced with these processes cannot be used for a bottle-to-bottle process. However, transition-metal, Lewis-acid, and organic catalysts have been shown to facilitate transesterification to yield color-free monomers and are predicted to facilitate a closed-loop process for PET bottle recycling.
1.5.2.2 SPI Code 2 and 4: HDPE and LDPE
The thermal depolymerization of HDPE takes place above 400°C. Early studies on the volatiles found date back as early as 1954, when it was noted that decomposition proceeded through radical-based mechanisms, and little low molecular weight ethylene was recovered. Instead, a wax-like substance (so-called Arge wax) was formed.
A co-catalytic system based on a tandem dehydrogenation-metathesis sequence was applied to PE (69). The depolymerization relies on the selectively of one catalyst to dehydrogenate PE, forming internal double bonds, and an olefin metathesis catalyst to cleave the polymer into smaller segments at the double bonds (68).
1.5.2.3 SPI Code 3: PVC
One of the most problematic polymers for the environment, but also one of the most inexpensive and widely used, is PVC. Although PVC is useful for a large range of applications, it has a significant negative environmental impact when it breaks down. PVC releases phthalate plasticizers and chlorine-containing hydrocarbons, i.e., dioxins, during its environmental degradation in landfills or during thermal treatment. Thermal degradation products also include hydrochloric acid, tar, and a benzene-containing liquid fraction. In the presence of HCl, degradation of PVC is autocatalyzed and is thought to occur by radical-based reactions (68).
1.5.2.4 SPI Code 5: PP
The presence of an sp3-hybridized carbon in the PP backbone poises it for breakdown, especially in the presence of oxygen, from normal environmental exposure and in mechanical recycling processes. Therefore, stabilizers must be added before reprocessing. If they are absent, the molecular weight is severely diminished (and crystallinity is increased) even after the first melting and remolding steps. The molecular weight can drop by 20% in the first cycle and by over 60% by the third cycle if no stabilizer is present.
The state-of-the-art depolymerization techniques for PP involve pyrolysis over catalysts to produce products containing three to seven carbons with varying levels of saturation to be used for fuel. Catalysts to target selective and clean propylene regeneration have yet to be developed (68).
1.5.2.5 SPI Code 6: PS
As a result of sorting limitations in recycling plants, PS is not typically recycled in the U.S. Catalytic breakdown of PS has been reported in the presence of solid supported base or acid. Depending on the reaction conditions, low molecular weight dimers and oligomers and varying amounts of monomers can be isolated. In 2015, a study used mealworms to digest polystyrene. The products of isotopic labeling studies revealed that after mealworm digestion, 13CO2 was observed in addition to fecula and other 13C-labeled biomass (68).
1.5.3 Test Samples for Biodegradation
With the increasing use of plastics, their recovery and disposal have become a major issue (70). As a first priority, recovery should be promoted. Complete recovery of plastics, however, is difficult. For example, plastic litter, which comes mainly from consumers, is difficult to recover completely. Additional examples of plastics which are difficult to recover are fishing tackle, agricultural mulches and water-soluble polymers. These plastic materials tend to leak from closed waste management cycles into the environment. Biodegradable plastics are now emerging as one of the options available to solve such environmental problems.
Plastic materials, such as products or packaging, which are sent to composting facilities should be potentially biodegradable. Therefore, it is very important to determine the potential biodegradability of such materials and to obtain an indication of their biodegradability in natural environments (70).
The standard ISO 10210:2012 describes methods for the preparation of test samples used in the determination of the ultimate aerobic and anaerobic biodegradability of plastic materials in an aqueous medium, soil, controlled compost or anaerobic digesting sludge (71).
The methods described there are designed to provide dimensional consistency of test samples, resulting in improved reproducibility of test results during the determination of the ultimate biodegradability of the product. These methods apply to the following materials (71):
Natural and/or synthetic polymers, copolymers or mixtures of these,
Plastic materials that contain additives, such as plasticizers or colorants,
Plastic composite materials that contain organic or inorganic fillers, and
Products made from the above materials.
The standard ISO 13975:2012 specifies a method of evaluating the ultimate anaerobic biodegradability of plastic materials in a controlled anaerobic slurry digestion system with a solids concentration not exceeding 15%, which is often found for the treatment of sewage sludge, livestock feces or garbage. The test method is designed to yield a percentage and rate of conversion of the organic carbon in the test materials to carbon dioxide and methane produced as biogas.
The method applies to the following materials, provided they have a known carbon content (72):
Natural and/or synthetic polymers, copolymers or mixtures,
Plastic materials that contain additives such as plasticizers, colorants, or other compounds, and
Water-soluble polymers.
The standard does not apply to materials which exhibit inhibitory effects on the test microorganisms at the concentration chosen for the test (72).
The ultimate aerobic biodegradability of plastic materials in an aqueous medium can be measured by the oxygen demand in a closed respirometer (70).
The standard ISO 18830:2016 specifies a test method to determine the degree and rate of aerobic biodegradation of plastic materials when settled on marine sandy sediment at the interface between seawater and the seafloor, by measuring the oxygen demand in a closed respirometer (73). The method is a simulation under laboratory conditions of the habitat found in different seawater/sediment-areas in the sea, e.g., in a benthic zone where sunlight reaches the ocean floor, i.e., photic zone that, in marine science, is addressed as a sublittoral zone.
Similarly, the standard ISO 19679:2016 is a test method to determine the degree and rate of aerobic biodegradation of plastic materials when settled on marine sandy sediment at the interface between seawater and the seafloor, by measuring the evolved carbon dioxide (74).
Furthermore, the aerobic biodegradability of plastic materials in an aqueous medium can be determined by the analysis of the evolved carbon dioxide in the course of degradation (75–77).
1.5.4 Mixed Municipal Waste
The standard ISO 15985:2014 concerns a method for the evaluation of the ultimate anaerobic biodegradability of plastics based on organic compounds under high-solids anaerobic digestion conditions by the measurement of evolved biogas at the end of the test (78).
This method is particularly designed to simulate the typical anaerobic digestion conditions for the organic fraction of a mixed municipal solid waste. The test material is exposed in a laboratory test to a methanogenic inoculum derived from anaerobic digesters operating only on pretreated household waste. The anaerobic decomposition takes place under high-solids, i.e., more than 20% total solids and static non-mixed conditions. The test method is designed to yield the percentage of carbon in the test material and its rate of conversion to evolved carbon dioxide and methane, i.e., biogas (78).
1.5.5 Aerobic Composting
The standard ISO 16929:2013 can be used to determine the degree of disintegration of plastic materials in a pilot-scale aerobic composting test under defined conditions (79). It forms part of an overall scheme for the evaluation of the compostability of plastics as outlined in ISO 17088 (80). The test method explained in ISO 16929:2013 (79) can also be used to determine the influence of the test material on the composting process and the quality of the compost obtained. It cannot be used to determine the aerobic biodegradability of a test material (79).
The standard ISO 20200:2015 contains a method for determining the degree of disintegration of plastic materials when exposed to a laboratory-scale composting environment (81). The method is not applicable to the determination of the biodegradability of plastic materials under composting conditions. Further testing is necessary to be able to determine the compostability.
1.5.6 Contaminants in Recycled Plastics
Dirt, paper and mixtures of polymeric materials complicate the interpretation of data from procedures used to identify the contaminants in recycled plastics.
Existing ASTM and ISO methods have been collected along with currently practiced industrial techniques for the identification and classification of contaminants in recycled plastics flakes or pellets (82).
A procedure has been presented for separating recycled plastics based on their color and a procedure for washing dirty, ground plastic, which results in separation of light materials with a density of smaller than 1.00 g cm–3 (83). The method is not intended to represent generic washing procedures used in the plastics recycling industry. The procedures described herein are solely for the preparation of plastic samples for use in other analytical tests. The procedure includes a room temperature wash step to facilitate separation of paper, for example, labels, followed by washing at an elevated temperature.
1.6 Special Problems with Plastics
1.6.1 Stability of Plastics
The opportunities and risks with regard to polymers have been elucidated in a monograph (84).
One of the reasons for the great versatility of many synthetic polymers is their high resistance against environmental influences (85). However, this fact leads to extremely low degradation and long residence times for synthetic polymers once they enter the environment. The degradation of synthetic polymers can generally be classified as biotic or abiotic, following different mechanisms, depending on a variety of physical, chemical, or biological factors.
During the degradation process, polymers are converted into smaller molecular units, e.g., oligomers, monomers, or chemically modified versions. Also, some plastic types are possibly completely mineralized.
The most important processes for the degradation of synthetic polymers are (85):
Physical degradation due to abrasive forces, heating/cooling, freezing/thawing, wetting/drying,
Photodegradation, usually by UV light,
Chemical degradation, i.e., oxidation or hydrolysis, and
Biodegradation by organisms such as bacteria, fungi, or algae.
1.6.2 Additives
Plastics waste management is faced with challenges regarding the pollution caused by various chemical additives in plastic products used for enhancing polymer properties and prolonging their life (86).
Despite the usefulness of such additives in the functionality of polymer products, their potential to contaminate soil, air, water and food is problematic. These additives may migrate and undesirably lead to human exposure, e.g., via food contact with these materials. Also, the additives can be released from plastics during the various recycling and recovery processes and from the products produced from recyclates (86).
The additives used can be subdivided into the following categories (87):
Functional additives such as stabilizers, antistatic agents, flame retardants, plasticizers, lubricants, slip agents, curing agents, foaming agents, biocides, etc.,
Colorants such as pigments, soluble azocolorants,
Fillers such as mica, talc, kaolin, clay, calcium carbonate, barium sulfate, and
Reinforcing materials such as glass fibers and carbon fibers.
Subsequently, the properties and fields of application of the individual materials are explained (86).
Compatibilizers are substances that can be used to enable the creation of special resin blends. The individual components of the resins would be otherwise incompatible.
Plasticizers are used for improving the flexibility, durability and stretchability of polymeric films. At the same time, they reduce melt flow in extrusion processes.
Antioxidants are also used in polymers to delay the overall oxidative degradation of plastics when they are exposed to UV radiation.
Heat stabilizers can prevent thermal degradation of polymers when exposed to elevated temperatures, both in the course of use and during thermal processing.
Slip agents can significantly reduce the surface coefficient of friction of a polymer. In addition to providing lubrication of the film surface, they can be used to enhance the antistatic properties of a polymer. For example, this results in better mold release properties.
A lot of problems have been described and cited in other studies which deal with additives for polymers (86). The propensity, or ability, of plastics to sorb persistent organic pollutants, such as dioxins, is also known to potentially cause problems.
1.6.3 Plastics in Food
Methods for identification and quantification of micro-plastics in food, including seafood, have been reported in the literature (88). However, in some of the studies, quality assurance to avoid contamination from the air and equipment is not described, and it is not always clear how a particle is identified as being a plastic. The methods described for micro-plastics include one or more of the following steps (88):
Extraction and degradation of biogenic matter,
Detection and quantification (enumeration), and
Characterization of the plastic.
Some of the described methods for degradation of the biogenic matter have the drawback that some plastics are degraded to a certain degree. Enumeration is performed by examining the samples with the naked eye or with the aid of a microscope. In the literature, micro-plastics have been classified or named in several ways, including microfibers, film spherules, bead fragment, and films. Advanced techniques for the characterization and identification of the type of plastic are by FTIR and Raman spectrometry. Another technique to obtain structural information on the plastic is pyrolysis gas chromatography (GC)/mass spectroscopy (MS). Here, the identification is performed by the comparison with standard spectra or pyrograms of plastic (88).
The occurrence of micro-plastics in seafood and food has been reviewed (88). A short description of the experiments is summarized in Table 1.7.
Table 1.7 Studies of micro-plastics in seafood and food (88).
Food type
Reference
Epipelagic fish
(89)
Pelagic and demersal fish
(90)
Commercial fish
(91–93)
Brown shrimp
(94)
Mussels
(95–97)
Molluscs
(98)
Oysters
(92)
Honey
(99)
Beer
(100)
1.6.4 Seawater
The environmental consequences of plastic waste are visible in the increasing levels of global plastic pollution both on land and in the oceans (101).
