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Expert overviews covering the science and technology of rubber
and plastics
ISSN: 0889-3144
Volume 15, Number 7, 2004
V. L. Shulman
Tyre RecyclingR
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RAPRA REVIEW REPORTS
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Item 1Macromolecules33, No.6, 21st March 2000, p.2171-83EFFECT
OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC
POLYURETHANESPil Joong Yoon; Chang Dae HanAkron,University
The effect of thermal history on the rheological behaviour of
ester- and ether-based commercial thermoplastic PUs (Estane 5701,
5707 and 5714 from B.F.Goodrich) was investigated. It was found
that the injection moulding temp. used for specimen preparation had
a marked effect on the variations of dynamic storage and loss
moduli of specimens with time observed during isothermal annealing.
Analysis of FTIR spectra indicated that variations in hydrogen
bonding with time during isothermal annealing very much resembled
variations of dynamic storage modulus with time during isothermal
annealing. Isochronal dynamic temp. sweep experiments indicated
that the thermoplastic PUs exhibited a hysteresis effect in the
heating and cooling processes. It was concluded that the microphase
separation transition or order-disorder transition in thermoplastic
PUs could not be determined from the isochronal dynamic temp. sweep
experiment. The plots of log dynamic storage modulus versus log
loss modulus varied with temp. over the entire range of temps.
(110-190C) investigated. 57 refs.
GOODRICH B.F.USA
Accession no.771897
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Previous Titles Still AvailableVolume 1Report 1 Conductive
Polymers, W.J. Feast
Report 2 Medical, Surgical and Pharmaceutical Applications of
Polymers, D.F. Williams
Report 3 Advanced Composites, D.K. Thomas, RAE, Farnborough.
Report 4 Liquid Crystal Polymers, M.K. Cox, ICI, Wilton.
Report 5 CAD/CAM in the Polymer Industry, N.W. Sandland and M.J.
Sebborn, Cambridge Applied Technology.
Report 8 Engineering Thermoplastics, I.T. Barrie,
Consultant.
Report 10 Reinforced Reaction Injection Moulding, P.D. Armitage,
P.D. Coates and A.F. Johnson
Report 11 Communications Applications of Polymers, R. Spratling,
British Telecom.
Report 12 Process Control in the Plastics Industry, R.F. Evans,
Engelmann & Buckham Ancillaries.
Volume 2Report 13 Injection Moulding of Engineering
Thermoplastics,
A.F. Whelan, London School of Polymer Technology.
Report 14 Polymers and Their Uses in the Sports and Leisure
Industries, A.L. Cox and R.P. Brown, Rapra Technology Ltd.
Report 15 Polyurethane, Materials, Processing and Applications,
G. Woods, Consultant.
Report 16 Polyetheretherketone, D.J. Kemmish, ICI, Wilton.
Report 17 Extrusion, G.M. Gale, Rapra Technology Ltd.
Report 18 Agricultural and Horticultural Applications of
Polymers, J.C. Garnaud, International Committee for Plastics in
Agriculture.
Report 19 Recycling and Disposal of Plastics Packaging, R.C.
Fox, Plas/Tech Ltd.
Report 20 Pultrusion, L. Hollaway, University of Surrey.
Report 21 Materials Handling in the Polymer Industry, H. Hardy,
Chronos Richardson Ltd.
Report 22 Electronics Applications of Polymers, M.T.Goosey,
Plessey Research (Caswell) Ltd.
Report 23 Offshore Applications of Polymers, J.W.Brockbank, Avon
Industrial Polymers Ltd.
Report 24 Recent Developments in Materials for Food Packaging,
R.A. Roberts, Pira Packaging Division.
Volume 3Report 25 Foams and Blowing Agents, J.M. Methven,
Cellcom
Technology Associates.
Report 26 Polymers and Structural Composites in Civil
Engineering, L. Hollaway, University of Surrey.
Report 27 Injection Moulding of Rubber, M.A. Wheelans,
Consultant.
Report 28 Adhesives for Structural and Engineering Applications,
C. O’Reilly, Loctite (Ireland) Ltd.
Report 29 Polymers in Marine Applications, C.F.Britton,
Corrosion Monitoring Consultancy.
Report 30 Non-destructive Testing of Polymers, W.N. Reynolds,
National NDT Centre, Harwell.
Report 31 Silicone Rubbers, B.R. Trego and H.W.Winnan, Dow
Corning Ltd.
Report 32 Fluoroelastomers - Properties and Applications, D.
Cook and M. Lynn, 3M United Kingdom Plc and 3M Belgium SA.
Report 33 Polyamides, R.S. Williams and T. Daniels, T & N
Technology Ltd. and BIP Chemicals Ltd.
Report 34 Extrusion of Rubber, J.G.A. Lovegrove, Nova
Petrochemicals Inc.
Report 35 Polymers in Household Electrical Goods, D.Alvey,
Hotpoint Ltd.
Report 36 Developments in Additives to Meet Health and
Environmental Concerns, M.J. Forrest, Rapra Technology Ltd.
Volume 4Report 37 Polymers in Aerospace Applications, W.W.
Wright,
University of Surrey.
Report 38 Epoxy Resins, K.A. Hodd
Report 39 Polymers in Chemically Resistant Applications, D.
Cattell, Cattell Consultancy Services.
Report 40 Internal Mixing of Rubber, J.C. Lupton
Report 41 Failure of Plastics, S. Turner, Queen Mary
College.
Report 42 Polycarbonates, R. Pakull, U. Grigo, D. Freitag, Bayer
AG.
Report 43 Polymeric Materials from Renewable Resources, J.M.
Methven, UMIST.
Report 44 Flammability and Flame Retardants in Plastics, J.
Green, FMC Corp.
Report 45 Composites - Tooling and Component Processing, N.G.
Brain, Tooltex.
Report 46 Quality Today in Polymer Processing, S.H. Coulson,
J.A. Cousans, Exxon Chemical International Marketing.
Report 47 Chemical Analysis of Polymers, G. Lawson, Leicester
Polytechnic.
Report 48 Plastics in Building, C.M.A. Johansson
Volume 5Report 49 Blends and Alloys of Engineering
Thermoplastics, H.T.
van de Grampel, General Electric Plastics BV.
Report 50 Automotive Applications of Polymers II, A.N.A.
Elliott, Consultant.
Report 51 Biomedical Applications of Polymers, C.G. Gebelein,
Youngstown State University / Florida Atlantic University.
Report 52 Polymer Supported Chemical Reactions, P. Hodge,
University of Manchester.
Report 53 Weathering of Polymers, S.M. Halliwell, Building
Research Establishment.
Report 54 Health and Safety in the Rubber Industry, A.R. Nutt,
Arnold Nutt & Co. and J. Wade.
Report 55 Computer Modelling of Polymer Processing, E.
Andreassen, Å. Larsen and E.L. Hinrichsen, Senter for
Industriforskning, Norway.
Report 56 Plastics in High Temperature Applications, J. Maxwell,
Consultant.
Report 57 Joining of Plastics, K.W. Allen, City University.
Report 58 Physical Testing of Rubber, R.P. Brown, Rapra
Technology Ltd.
Report 59 Polyimides - Materials, Processing and Applications,
A.J. Kirby, Du Pont (U.K.) Ltd.
Report 60 Physical Testing of Thermoplastics, S.W. Hawley, Rapra
Technology Ltd.
Volume 6Report 61 Food Contact Polymeric Materials, J.A.
Sidwell,
Rapra Technology Ltd.
Report 62 Coextrusion, D. Djordjevic, Klöckner ER-WE-PA
GmbH.
Report 63 Conductive Polymers II, R.H. Friend, University of
Cambridge, Cavendish Laboratory.
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Report 64 Designing with Plastics, P.R. Lewis, The Open
University.
Report 65 Decorating and Coating of Plastics, P.J. Robinson,
International Automotive Design.
Report 66 Reinforced Thermoplastics - Composition, Processing
and Applications, P.G. Kelleher, New Jersey Polymer Extension
Center at Stevens Institute of Technology.
Report 67 Plastics in Thermal and Acoustic Building Insulation,
V.L. Kefford, MRM Engineering Consultancy.
Report 68 Cure Assessment by Physical and Chemical Techniques,
B.G. Willoughby, Rapra Technology Ltd.
Report 69 Toxicity of Plastics and Rubber in Fire, P.J. Fardell,
Building Research Establishment, Fire Research Station.
Report 70 Acrylonitrile-Butadiene-Styrene Polymers, M.E. Adams,
D.J. Buckley, R.E. Colborn, W.P. England and D.N. Schissel, General
Electric Corporate Research and Development Center.
Report 71 Rotational Moulding, R.J. Crawford, The Queen’s
University of Belfast.
Report 72 Advances in Injection Moulding, C.A. Maier, Econology
Ltd.
Volume 7Report 73 Reactive Processing of Polymers, M.W.R.
Brown,
P.D. Coates and A.F. Johnson, IRC in Polymer Science and
Technology, University of Bradford.
Report 74 Speciality Rubbers, J.A. Brydson.
Report 75 Plastics and the Environment, I. Boustead, Boustead
Consulting Ltd.
Report 76 Polymeric Precursors for Ceramic Materials, R.C.P.
Cubbon.
Report 77 Advances in Tyre Mechanics, R.A. Ridha, M. Theves,
Goodyear Technical Center.
Report 78 PVC - Compounds, Processing and Applications,
J.Leadbitter, J.A. Day, J.L. Ryan, Hydro Polymers Ltd.
Report 79 Rubber Compounding Ingredients - Need, Theory and
Innovation, Part I: Vulcanising Systems, Antidegradants and
Particulate Fillers for General Purpose Rubbers, C. Hepburn,
University of Ulster.
Report 80 Anti-Corrosion Polymers: PEEK, PEKK and Other
Polyaryls, G. Pritchard, Kingston University.
Report 81 Thermoplastic Elastomers - Properties and
Applications, J.A. Brydson.
Report 82 Advances in Blow Moulding Process Optimization, Andres
Garcia-Rejon,Industrial Materials Institute, National Research
Council Canada.
Report 83 Molecular Weight Characterisation of Synthetic
Polymers, S.R. Holding and E. Meehan, Rapra Technology Ltd. and
Polymer Laboratories Ltd.
Report 84 Rheology and its Role in Plastics Processing, P.
Prentice, The Nottingham Trent University.
Volume 8Report 85 Ring Opening Polymerisation, N. Spassky,
Université
Pierre et Marie Curie.
Report 86 High Performance Engineering Plastics, D.J. Kemmish,
Victrex Ltd.
Report 87 Rubber to Metal Bonding, B.G. Crowther, Rapra
Technology Ltd.
Report 88 Plasticisers - Selection, Applications and
Implications, A.S. Wilson.
Report 89 Polymer Membranes - Materials, Structures and
Separation Performance, T. deV. Naylor, The Smart Chemical
Company.
Report 90 Rubber Mixing, P.R. Wood.
Report 91 Recent Developments in Epoxy Resins, I. Hamerton,
University of Surrey.
Report 92 Continuous Vulcanisation of Elastomer Pro les, A.
Hill, Meteor Gummiwerke.
Report 93 Advances in Thermoforming, J.L. Throne, Sherwood
Technologies Inc.
Report 94 Compressive Behaviour of Composites, C. Soutis,
Imperial College of Science, Technology and Medicine.
Report 95 Thermal Analysis of Polymers, M. P. Sepe, Dickten
& Masch Manufacturing Co.
Report 96 Polymeric Seals and Sealing Technology, J.A. Hickman,
St Clair (Polymers) Ltd.
Volume 9Report 97 Rubber Compounding Ingredients - Need,
Theory
and Innovation, Part II: Processing, Bonding, Fire Retardants,
C. Hepburn, University of Ulster.
Report 98 Advances in Biodegradable Polymers, G.F. Moore &
S.M. Saunders, Rapra Technology Ltd.
Report 99 Recycling of Rubber, H.J. Manuel and W. Dierkes,
Vredestein Rubber Recycling B.V.
Report 100 Photoinitiated Polymerisation - Theory and
Applications, J.P. Fouassier, Ecole Nationale Supérieure de Chimie,
Mulhouse.
Report 101 Solvent-Free Adhesives, T.E. Rolando, H.B. Fuller
Company.
Report 102 Plastics in Pressure Pipes, T. Stafford, Rapra
Technology Ltd.
Report 103 Gas Assisted Moulding, T.C. Pearson, Gas Injection
Ltd.
Report 104 Plastics Pro le Extrusion, R.J. Kent, Tangram
Technology Ltd.
Report 105 Rubber Extrusion Theory and Development,B.G.
Crowther.
Report 106 Properties and Applications of Elastomeric Polysul
des, T.C.P. Lee, Oxford Brookes University.
Report 107 High Performance Polymer Fibres, P.R. Lewis, The Open
University.
Report 108 Chemical Characterisation of Polyurethanes,M.J.
Forrest, Rapra Technology Ltd.
Volume 10Report 109 Rubber Injection Moulding - A Practical
Guide,
J.A. Lindsay.
Report 110 Long-Term and Accelerated Ageing Tests on Rubbers,
R.P. Brown, M.J. Forrest and G. Soulagnet, Rapra Technology
Ltd.
Report 111 Polymer Product Failure, P.R. Lewis, The Open
University.
Report 112 Polystyrene - Synthesis, Production and Applications,
J.R. Wünsch, BASF AG.
Report 113 Rubber-Modi ed Thermoplastics, H. Keskkula,
University of Texas at Austin.
Report 114 Developments in Polyacetylene - Nanopolyacetylene,
V.M. Kobryanskii, Russian Academy of Sciences.
Report 115 Metallocene-Catalysed Polymerisation, W. Kaminsky,
University of Hamburg.
Report 116 Compounding in Co-rotating Twin-Screw Extruders, Y.
Wang, Tunghai University.
Report 117 Rapid Prototyping, Tooling and Manufacturing,
R.J.M.
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Report 118 Liquid Crystal Polymers - Synthesis, Properties and
Applications, D. Coates, CRL Ltd.
Report 119 Rubbers in Contact with Food, M.J. Forrest and J.A.
Sidwell, Rapra Technology Ltd.
Report 120 Electronics Applications of Polymers II, M.T. Goosey,
Shipley Ronal.
Volume 11
Report 121 Polyamides as Engineering Thermoplastic Materials,
I.B. Page, BIP Ltd.
Report 122 Flexible Packaging - Adhesives, Coatings and
Processes, T.E. Rolando, H.B. Fuller Company.
Report 123 Polymer Blends, L.A. Utracki, National Research
Council Canada.
Report 124 Sorting of Waste Plastics for Recycling, R.D. Pascoe,
University of Exeter.
Report 125 Structural Studies of Polymers by Solution NMR, H.N.
Cheng, Hercules Incorporated.
Report 126 Composites for Automotive Applications, C.D.
Rudd,University of Nottingham.
Report 127 Polymers in Medical Applications, B.J. Lambert and
F.-W. Tang, Guidant Corp., and W.J. Rogers, Consultant.
Report 128 Solid State NMR of Polymers, P.A. Mirau, Lucent
Technologies.
Report 129 Failure of Polymer Products Due to Photo-oxidation,
D.C. Wright.
Report 130 Failure of Polymer Products Due to Chemical Attack,
D.C. Wright.
Report 131 Failure of Polymer Products Due to Thermo-oxidation,
D.C. Wright.
Report 132 Stabilisers for Polyole ns, C. Kröhnke and F. Werner,
Clariant Huningue SA.
Volume 12Report 133 Advances in Automation for Plastics
Injection
Moulding, J. Mallon, Yushin Inc.
Report 134 Infrared and Raman Spectroscopy of Polymers, J.L.
Koenig, Case Western Reserve University.
Report 135 Polymers in Sport and Leisure, R.P. Brown.
Report 136 Radiation Curing, R.S. Davidson, DavRad Services.
Report 137 Silicone Elastomers, P. Jerschow, Wacker-Chemie
GmbH.
Report 138 Health and Safety in the Rubber Industry, N. Chaiear,
Khon Kaen University.
Report 139 Rubber Analysis - Polymers, Compounds and Products,
M.J. Forrest, Rapra Technology Ltd.
Report 140 Tyre Compounding for Improved Performance, M.S.
Evans, Kumho European Technical Centre.
Report 141 Particulate Fillers for Polymers, Professor R.N.
Rothon, Rothon Consultants and Manchester Metropolitan
University.
Report 142 Blowing Agents for Polyurethane Foams, S.N. Singh,
Huntsman Polyurethanes.
Report 143 Adhesion and Bonding to Polyole ns, D.M. Brewis and
I. Mathieson, Institute of Surface Science & Technology,
Loughborough University.
Report 144 Rubber Curing Systems, R.N. Datta, Flexsys BV.
Volume 13Report 145 Multi-Material Injection Moulding, V.
Goodship and
J.C. Love, The University of Warwick.
Report 146 In-Mould Decoration of Plastics, J.C. Love and V.
Goodship, The University of Warwick.
Report 147 Rubber Product Failure, Roger P. Brown.
Report 148 Plastics Waste – Feedstock Recycling, Chemical
Recycling and Incineration, A. Tukker, TNO.
Report 149 Analysis of Plastics, Martin J. Forrest, Rapra
Technology Ltd.
Report 150 Mould Sticking, Fouling and Cleaning, D.E. Packham,
Materials Research Centre, University of Bath.
Report 151 Rigid Plastics Packaging - Materials, Processes and
Applications, F. Hannay, Nampak Group Research &
Development.
Report 152 Natural and Wood Fibre Reinforcement in Polymers,
A.K. Bledzki, V.E. Sperber and O. Faruk, University of Kassel.
Report 153 Polymers in Telecommunication Devices, G.H. Cross,
University of Durham.
Report 154 Polymers in Building and Construction, S.M.
Halliwell, BRE.
Report 155 Styrenic Copolymers, Andreas Chrisochoou and Daniel
Dufour, Bayer AG.
Report 156 Life Cycle Assessment and Environmental Impact of
Polymeric Products, T.J. O’Neill, Polymeron Consultancy
Network.
Volume 14Report 157 Developments in Colorants for Plastics,
Ian N. Christensen.
Report 158 Geosynthetics, David I. Cook.
Report 159 Biopolymers, R.M. Johnson, L.Y. Mwaikambo and N.
Tucker, Warwick Manufacturing Group.
Report 160 Emulsion Polymerisation and Applications of Latex,
Christopher D. Anderson and Eric S. Daniels, Emulsion Polymers
Institute.
Report 161 Emissions from Plastics, C. Henneuse-Boxus and T.
Pacary, Certech.
Report 162 Analysis of Thermoset Materials, Precursors and
Products, Martin J. Forrest, Rapra Technology Ltd.
Report 163 Polymer/Layered Silicate Nanocomposites, Masami
Okamoto, Toyota Technological Institute.
Report 164 Cure Monitoring for Composites and Adhesives, David
R. Mulligan, NPL.
Report 165 Polymer Enhancement of Technical Textiles, Roy W.
Buckley.
Report 166 Developments in Thermoplastic Elastomers, K.E.
Kear
Report 167 Polyole n Foams, N.J. Mills, Metallurgy and
Materials, University of Birmingham.
Report 168 Plastic Flame Retardants: Technology and Current
Developments, J. Innes and A. Innes, Flame Retardants Associates
Inc.
Volume 15Report 169 Engineering and Structural Adhesives, David
J. Dunn,
FLD Enterprises Inc.
Report 170 Polymers in Agriculture and Horticulture, Roger P.
Brown.
Report 171 PVC Compounds and Processing, Stuart Patrick.
Report 172 Troubleshooting Injection Moulding, Vanessa Goodship,
Warwick Manufacturing Group.
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Report 173 Regulation of Food Packaging in Europe and the USA,
Derek J. Knight and Lesley A. Creighton, Safepharm Laboratories
Ltd.
Report 174 Pharmaceutical Applications of Polymers for Drug
Delivery, David Jones, Queen's University, Belfast.
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ISBN 1-85957-489-0
Tyre Recycling
Valerie L. Shulman(European Tyre Recycling Association
(ETRA))
-
Tyre Recycling
1
Contents
1 Scope
..........................................................................................................................................................3
2 Introduction
..............................................................................................................................................3
2.1 Sustainable Development: The Context for Recycling
....................................................................3
2.2 The Size of the Problem
...................................................................................................................5
3 The Tyre: The Raw Material for Recycling
...........................................................................................6
3.1 The Structure of the Tyre
..................................................................................................................6
3.2 Tyre Composition
.............................................................................................................................7
3.3 Tyre Wear and Use
...........................................................................................................................8
4 Material Valorisation of Post-Consumer Tyres
.....................................................................................9
4.1 Preparation for Recycling
...............................................................................................................10
4.2 Recycling Treatments and Technologies
........................................................................................10
4.2.1 Level 1 Treatments: Destruction of the Structure of the
Tyre ............................................11 4.2.2 Level 2
Treatments: Liberation and Separation of the Elements of the Tyre
.....................12 4.2.3 Level 3 Treatments: Multi-Treatment
Technologies
..........................................................13
4.3 Material Outputs
.............................................................................................................................14
5 Traditional and Evolving Markets
........................................................................................................16
5.1 Material Production
........................................................................................................................16
5.2 Applications and Products
..............................................................................................................18
5.2.1 Whole Tyres
.......................................................................................................................19
5.2.2 Shred and Chips
.................................................................................................................20
5.2.3 Granulate
............................................................................................................................21
5.2.4. Powders and Speciality Powders
.......................................................................................23
5.3 Energy Recovery
............................................................................................................................25
5.3.1 Use in Cement Kilns
..........................................................................................................25
5.3.2 Use for Electricity and Steam Generation
.........................................................................26
6 The Future
..............................................................................................................................................26
Additional References
....................................................................................................................................30
Abbreviations and Acronyms
........................................................................................................................31
Abstracts from the Polymer Library Database
...........................................................................................33
Subject Index
................................................................................................................................................119
Company Index
............................................................................................................................................129
-
Tyre Recycling
2
The views and opinions expressed by authors in Rapra Review
Reports do not necessarily re ect those of Rapra Technology Limited
or the editor. The series is published on the basis that no
responsibility or liability of any nature shall attach to Rapra
Technology Limited arising out of or in connection with any
utilisation in any form of any material contained therein.
-
Tyre Recycling
3
1 Scope
This review summarises current tyre recycling practices and the
factors that have contributed to their growth and ef cacy as
viable, economically and environmentally sound alternatives for
treating post-consumer tyres. While it relies on the European
model, it draws upon experiences and expertise from around the
world, which have often precipitated action in the European Union.
The introduction will summarise the current context for recycling,
the extent of the post-consumer tyre problem, and the
characteristics and composition of the tyre that is the raw
material for recycling. The report will review the progress and
current status of:
A. Recycling treatments and some of the advances that have
facilitated the development of more diversi ed and ef cient
treatments and processes.
B. Ways in which the traditional markets for post-consumer tyre
materials have expanded and multiplied.
C. Industry initiatives that have contributed to the evolution
of a more ‘level playing eld’ for post-consumer tyre materials.
D. Issues and actions for the future.
Material recycling appears to be one of the most signi cant
future routes for sustainable development in the tyre related
industries. The treatments, technologies, materials and
applications presented in this Review are not exhaustive, but
provide a snapshot of how the industry has evolved to date. The nal
section will explore some of the issues that remain to be addressed
and resolved in future.
2 Introduction
Recycling is not a new concept. Prior to World War II, recycling
was a relatively common industrial practice for a variety of
materials and products - including tyres. However, once synthetic
rubber became readily available, recycling was, for the most part
abandoned (a.1).
More than half a century later, recycling is again becoming an
accepted industrial activity. However, as it is interpreted today,
the concept of recycling is inextricably linked to the production
and management of waste and by extension, to its prevention and
minimisation. Recycling has evolved into one of the four pillars
which support improved resource management through the prevention
of waste and the reuse, recycling and recovery of the wastes that
do occur in order to achieve sustainable development goals.
2.1 Sustainable Development: The Context for Recycling
During the nal years of the 20th Century, it became apparent
that the unbridled economic growth of the past could not be
sustained in future without irreparable damage to the environment.
Discussions initiated during the 1960s culminated in a proposal for
change, at the global level.
The Stockholm meeting of the United Nations Conference on
Environment and Development (UNCED) in 1972 is often marked as the
turning point in the move towards more sustainable growth practices
(a.2, a.3). It signalled a break from the past and the beginning of
a new era.
The goals of the conference were limited. They were rst, to
introduce the concepts and practices inherent in
sustainability and second, to provoke suf cient concern and
interest for world leaders to make a commitment to de-link economic
growth from negative environmental impacts.
Simply stated, sustainability requires policies and actions
which foster economic and social growth, which meet current needs
without detriment to the environment. The aim is to not compromise
the ability of future generations to meet their own needs.
‘Environment’ was de ned in the broadest sense to include all of
the conditions, circumstances and influences affecting development.
The speci c issue was the improved management of natural resources,
concentrating on the prevention and control of pollution and
waste.
Delegates adopted the principle and accepted the challenge of
implementing the sustainable model of development for the 21st
Century. For the next twenty years they undertook an exhaustive
awareness campaign to draw the support of national and local
governments, non-government organisations (NGOs), industry and the
public at large.
The global economic and social nature of the plan led to the
involvement of other organisations within the United Nations
infrastructure. Described in Figure 1, these bodies provide the
international framework within
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Tyre Recycling
4
which intra-and-inter-national trade occur, including the
movement of wastes.
The Basel Convention was created in 1989 under UNEP to ll the
gap between existing mandates which facilitate and monitor world
trade on the one hand, and those which are concerned with sound
environmental practices, on the other.
The mission of the Basel Convention is to monitor the
trans-boundary movements and management of wastes to ensure their
environmentally sound treatment and disposal and, to provide
support to governments by assisting them to carry out national
sustainable objectives (a.4, a.5).
By the 1992 UNCED meeting in Rio de Janeiro, much of the
groundwork had been completed. The goal of the conference was to
propose alternative strategies and actions that could be undertaken
in the short, medium and long-term in order to ensure that
consideration and respect for the environment would be integrated
into every aspect of the development process.
The Basel Convention provided the common framework for the
classi cation, management and treatment of waste. Brie y, waste was
de ned as:
‘..substances or objects which are disposed of or intended to be
disposed of or are required to be disposed of by the provisions
under national law.’
Both the Basel Convention and the OECD independently prepared
catalogues of the substances, objects, materials,
etc., that are de ned as waste and separated out those de ned as
hazardous or dangerous. A nal list contains those wastes that are
not perceived to pose a risk to the environment or human health.
However, it is important to note that the lists are not mutually
exclusive and that under certain conditions, a ‘waste’ can and
often does appear on more than one list. Virtually every
conceivable material, product or residue is listed - those that are
not speci cally named fall under the rubric ‘other’. Tyres, tyre
related wastes and other rubber wastes were identi ed as:
• B3140 Waste pneumatic tyres, excluding those destined for
Annex IVA operations (recovery)
• B3080 Waste parings and scrap of rubber.
The definition and annexes served as a guide for transboundary
movements of waste, principally for environmentally sound
management. Examples of recovery and disposal operations were
appended. Environmentally sound management was broadly de ned
as:
'..taking all practicable steps to ensure that waste is managed
in a manner that will protect human health and the environment
against adverse effects which may result from such waste.'
Within the context of the de nitions of waste and its
environmentally sound recovery and disposal, the OECD laid down the
provisions for its transboundary movement and acceptance, within
and outside of the member countries. Each country was invited to
prepare
Figure 1 International bodies concerned with waste
United Nations Conference on Environment and Development (UNCED)
formulates strategies and actions to stop and reverse the effects
of environmental degradation and promote sustainable,
environmentally sound development in all countries.
United Nations Conference on Trade and Development (UNCTAD)
promotes trade between countries with different social and economic
systems and provides a centre for harmonising the trade and
development policies of governments and economic groupings.
Organisation for Economic Cooperation and Development (OECD) is
a permanent body under the UNCTAD. It was created to assist in
removing restrictions and facilitating trade between and among
member and non-member countries, ensuring that the substances,
materials and products, etc., involved do not pose a threat to the
environment or humanity in the receiving country.
Basel Convention, under the UN Environment Programme (UNEP), is
speci cally concerned with the control of trans-boundary movements
of hazardous and other wastes and their disposal, from OECD
countries to non-OECD countries. Further, it is concerned with the
identi cation of those products and materials which could cause
damage to the receiving country(ies).
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Tyre Recycling
5
a list of those wastes that it would no longer accept for either
recovery or disposal, due to lack of appropriate treatment
facilities or risks to human health, among other reasons. Thus,
procedures were also set out for the non-acceptance of wastes and
their return, should they be delivered in error.
Once the framework was established, various tools were examined
to assess their capacity for targeting potential environmental
impacts. Life-cycle analysis (LCA) was selected as the most
appropriate and effective tool for determining the points at which
the greatest environmental impacts occur, thus making possible the
suggestion and selection of less damaging options (61). For
example, the approach permitted the evaluation of industrial
outputs from the production or extraction of raw materials through
the design and manufacture of materials and products, as well as
during product use.
The de nitions, annexes and provisions were accepted by the
delegates and also adapted by many countries to comply with
national policy and priorities.
The most hazardous wastes and the most prevalent sources of
pollution were targeted for immediate attention. Five priority
waste streams were also distinguished. In addition to the more
general category of 'household waste', post-consumer tyres (at
present, subsequent to discussion and debate, post-consumer tyres
are not de ned as hazardous waste and do not appear on any list as
a dangerous or hazardous waste), demolition waste, used cars,
halogenated solvents and hospital waste were earmarked for
action.
2.2 The Size of the Problem
The priority waste streams were not necessarily hazardous or
large. However, each did pose some degree of dif culty related to
its management. The basic
reasons included: the inability to accurately calculate the
quantity of arisings, the lack of effectiveness of the treatment or
disposal practices at that point in time, and/or, the potential
threat to human health.
Post-consumer tyres were not classi ed as hazardous or
dangerous. However, accurate data on annual accumulations were not
readily available. At the time, the principal methods of managing
them were by domestic reuse, retreading, and the use of limited
quantities for material recycling or as a secondary fuel. The
preponderance was sent to land lls. A large percentage of those
that were not landfilled were stored in warehouses or derelict
buildings, on farms, or scattered around the countryside, in rivers
and streams. In addition to being unsightly, they were found to be
a breeding-ground for vermin and insects. Large quantities were
also exported to developing countries with less well-de ned
environmental regulations or the means to deal with them.
The overall market for either the raw material or the nished
product is not particularly large compared to
other wastes. Table 1 illustrates the relative consumption of
key material streams in the EU. It is evident that the overall
quantity of rubber used is relatively small, however, the
production units are comparatively large, and the units of waste
conspicuous and unattractive.
World production of natural and synthetic rubber is estimated to
be approximately 20,000,000 tonnes per year. About 20%, or
approximately 4,000,000 tonnes are consumed in the European Union
each year. Indications are that an additional 1,000,000 tonnes are
imported annually from outside of the EU as nished goods, including
tyres (a.6). Comparable amounts are utilised in North America and a
growing percentage is consumed in Asia.
About 75% of the combined rubber resources worldwide, are used
in various sectors of the automotive
Table 1 Examples of other waste streams in tonnesProduct
Consumption SubsetPaper ±79,000,000Plastics ±37,000,000
Packaging ±19,980,000Glass ±15,000,000Aluminium ±8,860,000
Automotive/construction ±5,316,000Packaging ±1,594,800
Rubber ±5,000,000Tyres ±3,000,000
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Tyre Recycling
6
industry. The bulk, close to 60%, is consumed in the production
of tyres for two principal markets, passenger cars (including
utility vehicles) and trucks, as well as for smaller diverse
categories grouped as ‘other’ (e.g., agricultural, aeroplane,
bicycle, motorcycle, civil engineering, industrial, mining).
Hundreds of non-tyre automotive products, i.e., appearance items,
belts, hoses, housings, mouldings, rings, and seals, among others,
utilise the other 15% of the rubber.
The remaining 25% of natural and synthetic rubbers are consumed
by a broad cross-section of other industrial sectors to manufacture
thousands of general rubber products. More than 20 categories are
represented including such diverse products as footwear, bladders,
residential and commercial construction supplies, marine products,
ooring and roo ng components, non-automotive equipment, consumer
products such as pads and tool handles, seals and expansion joints,
civil engineering and road materials, etc.
Since the 1990s world tyre production has been reported to be
approximately 1,000,000,000 units per year. In units sold, which
are somewhat less than those produced, passenger car tyres account
for slightly more than 90%, while truck and ‘other’ categories,
together constitute about 10% (a.7, a.8, a.9).
Once a tyre in any category is permanently removed from a
vehicle without the possibility of being re-mounted for continued
on-road-use, it is de ned as waste. It is generally accepted that
for each tyre sold, whether newly manufactured, retreaded or
part-worn, one tyre has become waste. In the 15 Member States of
the EU alone, post-consumer tyres amounted to more than 2,600,000
tonnes of waste in 2003. Projections for 2004 indicate that the
expanded Union of 25 states will account for annual arisings of
approximately 2,850,000 tonnes.
3 The Tyre: The Raw Material for Recycling
A pneumatic tyre is often described as an engineering marvel. It
is basically a large, round, black, hollow shell lled with
compressed air that can support more than
50 times its own weight. It is meticulously constructed of over
thirty different component parts to meet diverse performance
standards in order to provide maximum comfort and safety on dry or
wet, slippery or rutted surfaces, at high or low speeds. It is
often pushed beyond its limits or abused by careless behaviour.
During its on-road life it is ignored until it needs repair or
is grudgingly replaced. Once a tyre is permanently removed from a
vehicle that it has diligently served over thousands of kilometres,
it is a waste and follows another route.
The external features of the tyre have not changed perceptibly
since the radial tyre was introduced more than fty years ago.
However, internally, changes have been made and others are planned
which will continue to improve performance and durability as well
as environmental quality. Some of these changes will also impact
upon the ways in which the tyres are valorised at the end of their
on-road life.
3.1 The Structure of the Tyre
Figure 2 illustrates seven critical parts of the tyre structure,
each of which serves a speci c function and impacts upon potential
recycling actions. The letters in the gure correspond to the
component descriptions which follow. The tread and sidewalls are
observable exterior elements, while the belts, casing, beads, apex,
and inner liner are interior components.
A. The tread is the part of the tyre that comes directly in
contact with the road to maintain traction when the vehicle moves
forward, back, turns or stops, in wet or dry weather. Rubber
compounds with a high concentration of natural rubber and llers,
which vary according to tyre category and local conditions, are
moulded into a design in which the solid parts of the tread clear
away the water while the channels allow the water to ow outwards
enabling the tread to maintain contact with the surface.
Figure 2 Tyre cross-section
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Tyre Recycling
7
B. The belts provide structural support to the tread helping to
maintain the tyre shape. Made from layers of rubber sheets
containing brass coated high carbon steel wires, they are placed on
the bias at alternate angles under the tread. This helps to control
road contact, provide a smoother ride and can reduce uneven wear.
Some manufacturers have introduced aramid ‘bandages’ to replace
steel belting (215).
C. The sidewalls on either side are attached to the casing. This
contributes to structural integrity by reinforcing the interface
between the tyre and the wheel rim and setting the inner dimension.
Side-walls are designed to ex up and down over road irregularities
while staying relatively rigid horizontally to respond to driving
actions such as steering, braking, etc. Because they are exposed to
abrasion damage as well as to UV and ozone degradation, the
compounds used in these parts contain many ingredients to
counteract these actions, e.g., anti-oxidants, as well as the newer
anti-ozonants.
D. The casing provides the shape and internal structure of the
tyre, and bears stress. It is traditionally made of twisted metal,
natural rayon, nylon or polyester cords that are then coated with a
natural rubber substance. As a rule, truck tyres contain a
proportionately greater ratio of metals to textiles than do
passenger tyres. Since the 1980s, a family of special aramids has
been introduced into some products, primarily to reduce tyre
weight.
E. The beads are structural components that frame the edge of
the casing to anchor the tyre to the metal wheel rim so that it
does not shift or become free during driving actions. They are made
from coils of zinc or bronze coated single lament high strength
steel wire that are coated with rubber and add appreciably to the
weight of the tyre. Innovative non-metal materials are being
introduced to reduce tyre weight.
F. The apex, at the end of the bead, is used to gradually shape
the tyre making the transition from the almost in exible bead to
the mid-point of the more pliable sidewalls. It is moulded from
ller and reinforcing resins.
G. The inner liner is an integral part of the tyre, providing a
lining for the casing in order to contain the air and maintain
consistent pressure, which contributes to improved rolling
resistance and energy savings. Like the inner tube that it has
replaced in a majority of tyres, it is most often produced from
butyl rubber.
While the external features of the tyre have changed little
during the past fty years, the ingredients and production processes
have changed considerably. Many of the newer ingredients which have
improved longevity, resistance to abrasion, etc. also contribute to
the durability and effectiveness of post-consumer tyre materials
downstream, when they are recycled and used in products and
applications.
3.2 Tyre Composition
The material composition of a tyre varies by category, i.e.,
passenger car, utility vehicle, truck, other, etc. However, all
categories of tyres include four fundamental groups of materials:
rubbers, carbon blacks/silicas, reinforcing materials and
facilitators. All but the reinforcing materials are ingredients in
the tyre compounds.
Table 2 provides a generic profile of the material composition
of pneumatic car and truck tyres produced for the European
market.
The rst group of materials which account for ±40-45% by weight
are natural and synthetic rubbers, the former tapped from the Hevea
tree, the latter generally derived from petroleum based products.
The ratio of natural to synthetic rubber is approximately two to
one in truck tyres, four to three in car tyres. Different polymers
and additives are used in each part of the tyre.
The second most prevalent materials are carbon blacks and/or
silica, which amount to ±23-27% of the tyre
Table 2 Composition by weight of car and truck tyres
Material Car/utility %
Truck/Lorry %
Rubber/Elastomersa ±43% ±45Carbon black and silicab ±27%
±20Metals ±11% ±22Textiles ±5% ±1Vulcanisation aidsc ±3 ±3Additives
±3 ±3Aromatic oils ±8 ±8
a Rubber content: truck ±30% naturalb Different varieties of
carbon black are used for
different purposes and may appear in other categories of
material
c Sulfur, stearic acid, zinc oxide
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Tyre Recycling
8
weight. A range of carbon blacks of varying shapes, sizes and
structures are used in different parts of the tyre. Larger sizes
can be used in the inner liner, while smaller particles can be used
in the casing or tread. During the past twenty years, attempts have
been made to replace some carbon blacks with silicas in selected
applications, such as the Green tyre. More recently, several modi
ed carbon blacks have come onto the market for use in tyres.
The third group are reinforcing materials, comprised primarily
of metals or textiles. Metals in the beads, belts and casing can
add ±25% to the weight of a truck tyre while in car tyres, which
utilise a larger portion of textiles in the casing, metals are
±11-13% of the total weight. Natural rayon, nylon and polyester
used in the casing cords amount to ±5% of the weight of a car and
±1% of a truck tyre. Manufacturers have experimented with a variety
of materials to partially replace the metal content in order to
reduce the weight of tyres and have had limited success with a
class of aramids, which would increase the bre and reduce the metal
content.
The fourth group of materials are used as facilitators during
the various stages of tyre production. Small amounts of extender
oils, waxes, anti-oxidants, the newer anti-ozonants and other
ingredients are added to the tyre compound to enhance performance,
or to facilitate curing and manufacturing ef ciency. Several
varieties of carbon black; titanium dioxide; zinc oxide and sulfur
are used to facilitate the vulcanisation process and are evenly
distributed throughout the polymer matrix. Calcium and aluminium
are used in small amounts as are trace amounts of magnesium,
phosphorous, potassium, sodium, chloride and silica.
The compounding process modifies the hardness, strength and/or
toughness of the rubber and increases its resistance to abrasion,
oil, oxygen, chemical solvents and heat. Different ingredients are
used to produce speci c qualities.
At times ten different rubber compounds are used in a tyre, each
of which is a mix which contains a number
of ingredients that modify and improve the physical properties
of the rubber. However, while each tyre manufacturer has its own
special formula to provide unique characteristics, tyre compounds
in general share many similarities and contain all of the
ingredients necessary to provide quality on-road service.
Once all of the ingredients have been compounded and the
structure has been assembled, the tyre is vulcanised. Vulcanisation
is a curing process which transforms the rubber into a strong,
elastic and rubbery hard state. Heat causes the vulcanisation
agents to combine with the rubber to create chemical links between
the rubber molecules. The crosslinking between the molecules makes
the rubber stronger and more durable and contributes to improved
wear and durability. At the same time, the sulfur also creates a
bond between the rubber and the copper that is in the brass coating
of the wires. The nal structure is an integrated whole.
Vulcanisation is generally considered irreversible. In other
words, after it has been altered, the once long, convoluted rubber
molecule cannot return to its original form.
Table 3 presents the average weights of three categories of new
tyres produced for the European market.
3.3 Tyre Wear and Use
The average on-road life of a tyre varies by category. Truck
tyre life is extended in some countries by re-grooving, i.e.,
re-cutting the tread grooves, or by retreading. In recent years,
truck tyre manufacturers have begun to offer multiple retreadings
as part of the tyre sale. The package is sold as a strategic
maintenance programme which ensures tyre safety and access to
replacement tyres. Similar packages are not economically viable for
passenger car tyres as they can only be retreaded once. Further,
the availability of budget tyres has removed the nancial
incentive.
Table 3 Weight ranges of new tyresCategory Range Av.wt
Units/tonnePassenger car tyres ±7-9 kg 8 kg 125Vans and light
utility vehicles ±8-11 kg 9 kg 111Trucks1 (load index of 121) 40-75
kg 56 kg 18Other (bicycle, motor cycle, agriculture, air plane,
construction, mining)
0.5-2.5-1000 kg NA2
1 the average of the category ‘trucks’ re ects the preponderance
of smaller truck tyres2 NA: Information not available
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Tyre Recycling
9
Longevity is also affected by driver care as well as by
maintenance and driving conditions, e.g., climatic extremes, speed
and road surfaces. Table 4 approximates the on-road life of
different categories of tyres produced for the European market.
Estimates are that the average for the North American market can be
up to ±50,000 miles (±80,000 kilometres) due to differences in road
surface, speed limits and climate conditions.
While a tyre can reach the end of its on-road life at almost any
point after construction, the most common reasons for replacing
them are accident or wear. Tyre wear is most evident on the tread,
although sidewall damage is also common primarily due to driver
behaviour or road conditions. It is generally accepted that tyres
lose approximately 20% of their weight, principally from the tread,
during their on-road life.
The required tread depth for on-road use in OECD countries is a
minimum of 1.6 mm in the most worn groove. However, many non-OECD
countries, particularly in Asia, Africa, parts of Latin America,
among others, do not have the same restrictions. Table 5
shows the approximate tread depth for three categories of new
tyres in the EU. To ensure tyre safety, a number of manufacturers
mould tread wear indicators into the tread as bands or cushions
which become apparent as the tread wears to the de ned legal limit
for continued use.
Once a tyre has been permanently removed from a vehicle without
the possibility of being returned to the road, it is waste (126,
258). Figure 3 brie y explains the four Rs of sustainable waste
management, which through their use, minimise waste and reduce
reliance on natural resources.
Material recycling and energy recovery offer alternative and
complementary means of gaining the greatest sustainable bene t from
natural resources and their wastes and thereby reducing the
consumption of virgin resources. Material recycling is emerging as
a commercially, technologically and economically viable option for
the future.
4 Material Valorisation of Post-Consumer Tyres
The recent life-cycle assessment of ‘an average passenger car
tyre’ conducted by BLIC, the European tyre manufacturers’
association confirmed the importance and ef cacy of tyre recycling
as a principal means of valorising post-consumer tyres in Europe.
In comparison with reuse, retreading and incineration for energy
recovery and in cement kilns, material recycling is the only one
which resulted in a positive impact on the environment.
The BLIC report concludes that the net environmental effect from
processing post-consumer tyres is negative.
Table 4 New tyre wear expectationsTyre category Estimated
kilometresPassenger car ±35,000 - 45,000 kmUtility vehicle/light
truck ±60,000 - 70,000 kmLong haul truck ±180,000 - 200,000 km
Figure 3 Sustainable waste management options
Reuse: Includes the sale of part-worn tyres for domestic on-road
and other uses as well as for export to countries with less
restrictive road-use requirements.
Retreading: re-manufactures a tyre using as the core a carefully
selected, undamaged casing, which reduces production energy as well
as virgin resources.
Recycling: transforms a waste into a raw material that can be
reintegrated into the economic stream as a resource to substitute
the use of virgin resources.
Recovery: transforms a waste into energy or fuel, which can be
reintegrated into the economic stream as a resource to substitute
the use of other energy sources.
Table 5 Examples of new tyre tread depth in the EU
Tyre category Average tread depthPassenger car 7-8 mmUtility
vehicle/light truck 10-12 mmLong haul truck 18-21 mm
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Tyre Recycling
10
In other words, the bene ts accrued from recycling outweigh the
environmental impacts that result from processing. One contributing
factor appears to be the production of useful products from the
recycled materials. Conversely, both forms of incineration for
energy recovery presented produce a balanced impact - neither
negative nor positive effects on the environment (a.10).
Although in considerably greater detail than earlier analyses,
it corroborates the findings from studies undertaken in Germany
(52), the USA (225), the UK (126), and Russia (356). It also draws
parallel conclusions to those reported in the recent mass balance
study in the UK (a.11), which examined the economics of recycling
and projected several scenarios for the future.
Together, these studies re ect the vast improvements that have
occurred within the industry during the past fteen years in terms
of the effectiveness of the treatments, the quality and consistency
of the material outputs and the range of applications and products
in which the materials are used. Improved overall ef ciency has
also had the effect of lowering production costs and increasing
competivity.
4.1 Preparation for Recycling
The starting point for material recycling is the same as other
industries - the sourcing of a continuous ow of raw material. In
most EU States post-consumer tyres destined for recycling are
collected by specialised tyre collection companies. Tyres are
usually collected under commercial contract from regular sources
which include garages, retail outlets, depots and vehicle
dismantlers, among others. In some regions, they are also removed
from long-standing stockpiles or clean-up sites.
The tyres are sorted by category, i.e., car, utility, truck, and
then by size, often prior to delivery to the treatment facility.
Road-worthy part-worn tyres are removed for domestic reuse or
export under OECD regulations. Retreadable casings are diverted to
appropriate facilities. Recently, there is even an active
competition between recycling and energy recovery facilities for
the available tyres.
Post-consumer tyres are a waste and, therefore, must be shipped
in compliance with Basel Convention and OECD regulations. However,
as they are non-hazardous wastes destined for recovery, the
documentation and information required is limited to general
information.
Once delivered to the treatment facility, the tyres come under
the jurisdiction of the local authority(ies) which regulate the
quantity and location where the tyres may be stored on the
premises. Appropriate zoning and land-use permission, as well as
the necessary permits and licenses must be obtained. Under certain
circumstances, waste management permits may also be required before
the tyres can be processed or the materials used (59).
Prior to processing, on site or at a facility, the tyres must be
cleaned of debris such as glass, stones, or miscellaneous items as
well as from partially burned tyre fragments. Tyres acquired from
stockpiles or other long-term storage areas are often washed prior
to use.
4.2 Recycling Treatments and Technologies
Under the European Commission’s waste legislation, tyre
recycling is a recovery operation that encompasses two distinct but
interrelated functions:
• transformation of post-consumer tyres with the use of diverse
treatments, e.g., size reduction, pyrolysis and technologies
(physical, chemical, thermal or biological) in order to produce a
broad range of materials which will be reintegrated into the
economic stream as a resource to substitute the use of virgin
resources, and
• use of the materials in myriad consumer and industrial
products as well as construction and civil engineering
applications.
A distinction is often made between treatments and technologies.
The terms are de ned as follows:
• A treatment is a specialised method of processing a material
or substance to achieve a speci ed result, for example, size
reduction is a treatment designed to reduce a tyre into smaller
pieces or particles for which one of several different technologies
can be used.
• A technology is the specialised tool, i.e., a type of
equipment or process which is used to achieve a treatment's end
result. Thus an ambient granulator is a speci c type of equipment
that can be used to reduce a tyre into granulate or powder.
The treatments used to recycle post-consumer tyres range from
the simplest mechanical cutting devices to sophisticated and
complex multiphase chemical, mechanochemical and/or thermal
processes. They
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Tyre Recycling
11
appear to have overcome many of the principal obstacles inherent
in the recycling of thermoset rubbers (413). Speci cally, the
treatments and technologies that have evolved do not attempt to
dissolve or melt the rubber into the virgin compound. Rather, they
attempt to exploit and enhance the properties of the tyre
compound.
There are four basic levels of treatment. Each can be described
in terms of its functions, which become increasingly complex as
they progress through successive levels. The capabilities can be
expanded by linking two or more different technologies to operate
in tandem in order to produce the desired result.
Level 1 Destruction of the structure of the tyre - Primarily
simple mechanical means which destroy one or more of the physical
attributes of the tyre, e.g., shape, weight bearing capacity,
rigidity, among others. The most common methods include bead
removal, compression or cutting.
Level 2 Liberation and separation of the elements of the tyre -
Treatments which process the tyre to segregate the principal
components, e.g., the rubber, metals, textiles. The most common are
ambient and cryogenic size reduction technologies. Level 1 outputs
are often used as feedstock.
Level 3 Multiphase treatments and technologies - Rubber
materials liberated during Level 2 provide
feedstock for treatments and technologies which modify one or
more characteristics of the material. Devulcanisation, reclaim,
surface modi cation and pyrolysis, are among the most
prominent.
Level 4 Material upgrading treatments - Materials modi ed during
Level 3 provide the feedstock for treatments that further refine
and upgrade them. Technologies are used that enhance selected
properties or characteristics. The preparation of thermoplastic
elastomers, upgraded carbon products and improved reclaim are the
most representative examples.
Figure 4 illustrates the progressive, often inclusive nature of
the treatments from the ' rst cut', by mechanical means, through to
'higher level' specialised treatments and technologies which add
distinctive characteristics or properties to the material
outputs.
4.2.1 Level 1 Treatments: Destruction of the Structure of the
Tyre
This is de ned as simple mechanical means which destroy one or
more of the physical attributes of the tyre, e.g., shape, weight
bearing capacity, rigidity, among others. The most common means
include bead removal, compression or cutting. The majority of the
outputs are used directly in civil engineering or construction
applications. The remainder are used as feedstock for further
treatment.
Figure 4 Schematic of the four levels of treatment
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Tyre Recycling
12
Bead removal is used on car and truck tyres as a pre-treatment
for later recycling treatments. It is a mechanical procedure that
removes the rubber coated steel coil wires by force (pulling) or by
cutting or tearing the connecting points that anchor the bead to
the casing. The carcass can be directly used for civil engineering
applications or as feedstock for later recycling treatments.
Sidewall removal is used primarily on car tyres as a
pre-treatment for later recycling treatments or the sidewall can be
used directly in civil engineering applications. It is a mechanical
cutting procedure that destroys the structure of the tyre by
removing the support on either side.
Tread removal is used on car or truck tyres as a pre-treatment
for later recycling treatments or the tread can be used directly in
civil engineering applications. It is a mechanical cutting
procedure which frees the strips of tread from the tyre carcass.
The tread can be used directly to produce simple products or as a
feedstock for later recycling treatments.
Compression is used on car or truck tyres. It is a mechanical
procedure that destroys the structure of a tyre by placing it under
the force of controlled pressure to permanently deform it. The
number of tyres and pressure used is dependent upon the desired nal
result. The units can be directly used for a number of civil
engineering applications.
Baling is used on car and truck tyres. It is a mechanical
procedure that destroys the structure of a speci ed number of tyres
by placing them in a chamber under high pressure (±65 tonnes) to
permanently deform them into a cube or rectangular solid, which is
then secured with a stipulated number of straps at speci ed points.
The number of tyres used is dependent upon the desired nal
dimensions of the unit. The units can be used directly for a number
of civil engineering applications.
Cutting is used on car and truck tyres as a pre-treatment for
later recycling treatments or for direct use in civil engineering
applications. It is a mechanical procedure that guillotines,
scissors or slices the tyre through the middle of the tread
producing two equal halves with the sidewall attached, or into
halves or quarters across the diameter of the tyre.
Equipment can be stationary or mobile, dependent upon how the
material will ultimately be used. Generally, neither the bead wires
nor the belts are removed prior to or during processing. The
material can be used directly
in civil engineering applications, as a secondary fuel or as
feedstock for further processing.
4.2.2 Level 2 Treatments: Liberation and Separation of the
Elements of the Tyre
Level 2 treatments separate out the principle components of the
material, e.g., the rubber, metals, textiles. The most common
technologies are ambient and cryogenic size reduction as well as
some new technologies which are designed to reduce the material to
pieces of ±0 to ±15 mm. Whole tyres and Level 1 outputs are
generally used as feedstock. The outputs can be used directly in
applications or products, or as feedstock for Level 3
processing.
Shredding and chipping are used on whole car or truck tyres.
Shredding is a treatment that uses different technologies to
fragment the tyre. Usually, a set of knives is used to produce
material ±50 mm to ±300 mm that is irregularly shaped or
equidimensional. Neither the bead wires nor belts are removed prior
to, during or after processing unless it is accomplished as the rst
step in size reduction processing.
Chipping is generally a second processing of shred which results
in material ±10 mm to ±50 mm that is either irregularly shaped or
equidimensional.
Ambient grinding uses whole or pre-treated car or truck tyres in
the form of shred or chips, or sidewalls or treads. Ambient
grinding is a multistep technology. Processing takes place at or
above normal room temperature. The rubbers, metals and textiles are
sequentially separated out. First, the material is sheared with a
system of knives. If the reinforcing and bead wires are not removed
prior to processing, the metals are magnetically separated out
during the granulation process. The material may continue through
one or more sequential granulators to further reduce it in size.
The material passes through a series of screens and sifting
stations to remove the nal vestiges of impurities and ensure
consistency of size (126). During the nal phase, the textile
residues are removed by air separators.
Cryogenic processing generally uses pre-treated car or truck
tyres as feedstock, most often in the form of chips or ambiently
produced granulate. Processing takes place at very low temperature
using liquid nitrogen or commercial refrigerants to embrittle the
rubber. It can be a four-phase system which includes initial size
reduction, cooling, separation, and milling. The material enters a
freezing chamber where liquid nitrogen is used to cool it from –80
to –120 °C, below the point where
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rubber ceases to behave as a exible material. The cooling
process embrittles the rubber and allows it to be fractured to the
desired size resulting in a smooth and regular shape (398, 419,
448). Because of its brittle state, bres and metal are easily
separated out in a hammer mill. The granulate then passes through a
series of magnetic screens and sifting stations to remove the last
vestiges of impurities.
Both ambient and cryogenic processing can be repeated to produce
ner particles. Increasingly, the two with their attendant
technologies, are combined into one continuous system in order to
bene t from the advantages and characteristics of each and to
reduce overall costs. The ambient system is generally used for the
initial size reduction phases. The cryogenic system is used to
further reduce the material in size and then to remove the metals
and textiles. The outputs from either or both systems can be used
directly or as feedstock for further processing.
4.2.3 Level 3 Treatments: Multi-Treatment Technologies
Level 3 treatments and technologies further process the material
to modify one or more characteristics by means of mechanical,
thermal, chemical, mechanochemical or multitreatment procedures.
The outputs of Level 2 are most often used as feedstock. Reclaim,
surface activation, devulcanisation and pyrolysis are
representative examples of the range of treatments used. The
outputs can be used directly in applications or products, or as
feedstock for Level 4 processing.
Rubber reclaim uses ambiently size reduced granulate as
feedstock. It is a two-phase thermomechanical shearing process
(120, 387) that changes the characteristics of the input material.
During the rst phase the granulate is plasticised. During the
second phase the plasticised material is processed by thermal and
mechanical treatments that break down the vulcanised structure in
order to restore some of the original characteristics of virgin
rubber, i.e., reducing some of the crosslinks in the granulate. The
resulting material has a maximum particle diameter of 0.425 mm and
an average diameter of 0.360 mm. Other reclaiming processes utilise
chemical treatments in order to transform the elastomeric
properties (212, 375).
Surface modification/activation uses buffings or peelings from
retreading, or ambiently size reduced granulate produced from truck
tyre treads as the
principal feedstock. Surface modi cation is a three-phase
treatment. In the rst phase, the feedstock is ambiently reduced to
a ne powder of >0.04 mm from which all metals and textiles are
removed. In the second phase, the powder is activated by coating it
with high molecular weight unsaturated polymers in aqueous
dispersion with an appropriate curing system. The third phase
occurs during vulcanisation on a double belt press that rolls the
material at very high pressure. During the curing process
(vulcanisation), bonds are formed between the polymer chains of the
coating and the polymer to which the post-consumer tyre powders are
added. As a result, the rubber particles activated by the coating
are bound to the new three-dimensional network. The original rubber
properties are retained (330, 427).
Devulcanisation uses ambiently or cryogenically size-reduced
granulate or powder as a feedstock. It is a two-phase treatment in
which the rst phase is size reduction, which generally takes place
in a different facility. The second phase ‘reactivates’ the
material by reducing the number of crosslinks between the rubber
molecules that occurred during vulcanisation so that the resulting
material can be revulcanised (177). The feedstock is mechanically
(85), chemically (178) or thermally broken down to restore some of
the original characteristics of the rubber. Chemical activation
agents can change some of the physical and/or chemical properties
of the resulting material while mechanical technologies retain the
same properties of the feedstock. Recent research has led to
partial ‘devulcanisation’ systems, which utilise ultrasound or even
microbes (84, 216, 330). These devulcanisates can be revulcanised
by using traditional methods without further additives.
Pyrolysis uses pre-treated car or truck tyre chips as the
principal feedstock. It is a two-phase treatment which uses thermal
decomposition to heat the rubber in the absence of oxygen to break
it into its constituent parts, e.g., oil, gas and carbon (71, 97).
Cracking and post-cracking take place progressively as the material
is heated to 450-500 °C and above. De-polymerisation and oil and
gas production take place progressively, the balance of product
shifting to gas as temperatures increase. A clean oil free char can
be produced. Pyrolytic char is a coarse powder with a particle size
ranging from 0.4 m to over 1,000 m in diameter. The char can be
used in low value production processes as a colorant or ller. The
output can continue on for further processing during Level 4 to
produce economically more interesting carbon products, which can
act as replacements for certain types of carbon black used in
rubber compounding.
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4.2.4 Level 4 Treatments: Material Upgrading
Level 4 treatments re ne, upgrade, modify or generate specific
characteristics or properties in materials produced by Level 3
treatment, which most often provide the feedstock. Upgraded
reclaim, reactivated/surface modi ed/devulcanised materials,
upgraded char (carbon products) and new compounds such as TPEs are
among the most representative.
Post-treatments of pyrolytic char issued from pyrolysis are
mechanical separation, physical or chemical treatments that can
upgrade the char by reducing it in size and separating out
impurities. Post-treatments generate materials that have similar
characteristics to a variety of different grades of carbon black
currently utilised for the production of a broad range of
commercial products and are valuable for technical rubber goods.
Resonance disintegration is an innovative example of particle
fragmentation, using resonance forces to vibrate particles apart
(a.12). Resonance disintegration can take pyrolytic char from a
maximum particle size of 600 m to below 30 m with 50% below 1 m
after a single processing with a surface area, structure and
dispersion in rubber compound very similar to standard carbon
blacks.
Thermoplastic elastomer production uses granulate produced from
car or truck tyres as the feedstock. The treatment requires two
feedstocks, i.e., granulate and a thermoplastic (polypropylene). It
is a two-phase reactivation and mixing process, which combines the
material qualities of the former with the processing behaviour of
the latter. The reactivation and mixing processes occur under high
shear forces in a conventional internal mixer. The granulate acts
as an elastomeric compound during the rubber phase in crosslinked
thermoplastics creating a new compound (not a blend). Chemical or
mechanochemical activation of both the dispersed elastomeric domain
and the matrix phases result in a linkage between granulate
particles, acting as a dispersed elastomeric domain within the
thermoplastic matrix (54, 93). Special crosslinking systems,
compatibilisers and additives allow the properties of the new
material to be varied in function for the intended use.
Table 6 shows the increasing percentage of the total weight of
the tyre that is lost during processing. The principal loss is due
to the removal of the metals. Generally, smaller materials retain
fewer impurities from metal or textile. As speciality treatments
most often use smaller granulate or powders as feedstock, there is
usually no further loss during processing. One of the few
exceptions could be the upgrading of pyrolytic char.
For many years, all metals and textiles were disposed of in land
lls. As recycling has become increasingly ef cient, the waste
materials have become cleaner. Subsequently, arrangements have been
made with metal recycling facilities for further treatment (307).
New uses for textile residues are also being developed. Several
companies have devised equipment which forms the uff into
briquettes which can be used for energy
recovery. Thus, the largest source of recycling residues is also
being handled in an environmentally sound manner, reducing the
overall impact of post-consumer tyres on the environment.
4.3 Material Outputs
The materials outputs from the four levels of treatments are
broadly classi ed into six categories: cuts, shred, chips,
granulate, powders and ne powders. Figure 5 illustrates the
continuum of material that result from recycling treatments.
Each category is comprised of one or more subcategories with
different parameters, creating a continuum from less than ±500 m to
>300 mm. The apparent overlap between the larger granulate and
the smaller chips is a function of processing speci cations. Speci
cally, granulate is characterised by multistep processing in which
metals and textiles are removed, while chips are characterised by
processing which merely fragments the tyre leaving the metals and
textiles intact.
The six basic categories of materials are described as
follows.
Table 6 Percentage of processed material per tonneOutput ± %
product ± % lossShred and chips (unseparated) 95% ±5%Large
granulate ±7 to 12 mm 70% ±30%Granulate or powder truck 60%
±40%Granulate or powder car 40% ±60%
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15
• Cuts: irregularly shaped fragments >300 mm;
• Shred: irregularly shaped fragments of ±50 mm to ±300 mm in
any dimension;
• Chips: irregularly shaped fragments of ±10 mm to ±50 mm
• Granulate: finely dispersed particles between ±1 mm to ±10 mm.
Subcategories could include ranges of, for example, ±0.5-2 mm, ±2-7
mm, and ±7-15 mm, with variations according to purchaser
specifications. Ambiently produced granulate is characterised by
irregularly shaped surfaces. Cryogenically produced granulate is
characterised by smooth regular surfaces.
• Powder: finely dispersed particles of less than 1 mm.
Subcategories could include ranges of, for example, ±0-0.5, ±0-1.5,
±0.750-1.6 mm. Surface characteristics depend upon the treatment or
technology. Ambiently produced powder is characterised by
irregularly shaped surfaces. Cryogenically produced powder is
characterised by smooth regular surfaces. Specialised powders: nely
dispersed powders that exhibit unique characteristics dependent
upon the treatment or technology used.
• Devulcanisates are powders characterised by reduced
crosslinks.
• Reclaim particles are characterised by reduced crosslinks with
a diameter of 0.300-0.420 mm.
• Surface modi ed powders are characterised by activated
surfaces with high crosslink density with nely dispersed particles
of less than 1 mm.
• Thermoplastic elastomers are powders which constitute a new
compound and which exhibit the same shore hardness and polymer base
of traditional thermoplastics.
• Fine powder: finely dispersed particles of
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Tyre Recycling
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particles. As an example, a sample of 1 mm powder contains
particles ranging from less than 0.30 mm to more than 1 mm. The
distribution in the sample should show that more than 90% of the
particles are within the 0.30 to 1 mm range.
Particle distribution is determined with a series of screens.
Dependent upon the size of the material, a set number of screens is
placed in descending order. The material ows through the tier
within a speci ed time. The material that remains on each screen is
weighed and expressed as a percentage of the sample.
While the size and particle distribution are often critical to
the selection of a material, the chemical and physical properties
provide information about the content of the material and how it
will react physically in response to certain conditions (81).
Table 7 provides a comparison of the different material outputs
in terms of:
1. Size: each material is described as a range of particle
sizes, e.g., granulate can be described as a range from ±0.5 mm to
±15 mm, while devulcanisates are described as less than 1 mm. Whole
tyres are discussed as a unit, and bales by the number of tyres
required to produce a unit.
2. Key characteristics: describe some of the principal
characteristics of the particular material which can distinguish it
from other materials of the same size, e.g., granulate produced
ambiently and cryogenically present very different
characteristics.
3. Traditional materials: lists some of the traditional
materials for which particular post-consumer tyre materials could
be used as substitute for virgin resources.
5 Traditional and Evolving Markets
It is evident that during the past fteen years tyre recycling
has made great strides towards meeting its rst goal, i.e., the
production of materials that can be used to substitute for virgin
resources. Signi cant quantities of material are produced annually
with indications that capacity will continue to grow, at least
within the near future (64). Treatments and technologies have
evolved to new levels of sophistication and ef ciency which allow
the production of ner and cleaner materials, at more competitive
prices.
However, the issues surrounding the second part of the equation,
i.e., the use of the materials in applications and products, could
become more dif cult to address as material production continues to
increase. Three traditional large markets coupled with smaller
niche markets, consistently consume almost 75% of the material
produced. Newer niche markets, many of which have the capacity to
use increasingly sophisticated materials, are beginning to evolve,
albeit very slowly. Until now, production and use have maintained a
relatively even pace. In general, there has been some expansion
into new realms, and there are strong indications that this pattern
could continue (72).
5.1 Material Production
In 2003, European recyclers processed slightly more than 650,000
tonnes of post-consumer tyres, 25% of total annual arisings in the
15 Member States. This represents a six-fold increase over 1992
when only ±5% of tyres was recycled. Figure 6 illustrates the
steady expansion of recycling capability in the EU during the past
decade (the data are collected for the ETRA Annual Report to the
Waste Topic Centre from each of the EU Member States).
Figure 7 illustrates the percentage of the total treated for
each category of material. Whole tyres accounted for slightly more
than 65,000 tonnes. The quantity of whole tyres used for recycling
has increased nominally each of the past four years, with a total
increase for the period of almost 2% (a.6). It is important to note
that in the EU, whole tyres, shred or chips destined for energy
recovery are not included in calculations for recycling as they are
most frequently delivered directly to the recovery facility and
treated on site.
Approximately 78,000 tonnes of the total were used to produce
shred and chips, of which 74,000 tonnes were available for use in a
variety of applications. The quantity of shred and chips produced
has increased marginally during each of the past four years, with a
total increase for the period of almost 2%.
By far, the largest quantities of tyres are used for the
production of granulate. The quantity has remained relatively
stable during the past 4 years, although the ratio to other
materials has changed. The ±410,000 tonnes of tyres input resulted
in ±200,000 tonnes of clean material containing less than 5% of
impurities, textiles or metals.
Powders, including speciality powders, used ±84,500 tonnes of
tyres of which somewhat over 40,000 tonnes of
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Table 7 Summary of material characteristicsMaterial Size Key
characteristics Substitutes for traditional
materialsWhole tyres unit Lightweight, low compacted density,
high
void ratio, good compressibility, water permeability, thermal
insulation
Concrete block, clay, quarried aggregate, gravel lled drums
Construction bale
100-125 tyres
Lightweight, low compacted density, good thermal insulation,
limited de ection, exceeds speci cations for speci c gravity,
compressibility, deformation, creep, hydraulic conductivity
Construction block, stone riprap, gravel, packed earth, crushed
rock in wire cages or other containers
Shred
Chips
±50-±300 mm±10-50 mm
Lightweight, low compacted density ±0.5 tonne/m3, high void
ratio with good water permeability between 10-1 and 10-3 m/s. Good
thermal insulation and compressibility, low earth pressure and high
friction road characteristics
Crushed rock or gravel, large grain sand, lightweight clay or
light expanded clay aggregate, y ash from coal burning
Size reduced Dependent upon useAmbient ±0.50-
15 mmCrosslinked macro structure retains same characteristics as
the tyre; irregular shape; some thermal degradation temperature
stressed. May exhibit a nominal degree of reduced crosslinking
Product: Virgin rubbers, EPDM, clay, sand, gravel, y ash from
coal burning polyurethane
Cryogenic ±0.5-50 mm
Clean surfaces, regular particle size and shape, glossy smooth
surface, no surface decomposition or thermal stress
Powder
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clean material