End-of-Life Tyres - Exploiting their Value

Rapra Industry Analysis Report Series

End-of-Life Tyres – Exploiting their Value

P.W. Dufton

Europe’s leading plastics and rubber consultancy with over 80 years of experience providing industry with technology, information and products

End-of-Life Tyres – Exploiting their Value

A Rapra Industry Analysis Report by

Dr. Peter Dufton

January 2001

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Tel: +44 (0)1939 250383

Fax: +44 (0)1939 251118

http://www.rapra.net

The right of Peter Dufton to be identified as the author of this work has been asserted by him in accordance with Sections 77 and 78 of the Copyright, Designs and Patents Act 1988.

© 2001, Rapra Technology Limited ISBN: 1-85957-241-3 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—without the prior permission of the publisher, Rapra Technology Limited, Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK.

End-of-Life Tyres–Exploiting their Value

Contents 1 INTRODUCTION............................................................................................................ 1 1.1 Background ........................................................................................................ 1 1.2 The Report ......................................................................................................... 1 1.3 METHODOLOGY ............................................................................................... 2 2 EXECUTIVE SUMMARY................................................................................................ 3 2.1 Conclusions........................................................................................................ 3 2.2 Quantities of tyres involved ................................................................................ 4 2.3 Options for Reducing and Recycling Waste........................................................ 5 2.4 Options for Second Use ..................................................................................... 6 2.4.1 Retreading ............................................................................................. 6 2.4.2 Crumb and Reclaim................................................................................ 6 2.4.3 Pyrolysis................................................................................................. 7 2.4.4 Energy Recovery.................................................................................... 8 2.4.5 Physical Use of Whole or Part Tyres ...................................................... 8 2.5 Collection and Dispersal..................................................................................... 8 3 THE TYRE, ITS DESIGN AND MANUFACTURE ......................................................... 11 3.1 Tyre Construction ............................................................................................. 11 3.2 Tyre Design...................................................................................................... 14 3.3 Tyre Manufacture ............................................................................................. 19 4 TYRE RETREADING ................................................................................................... 25 4.1 Industry Structure and Organisation ................................................................. 25 4.2 The Retreading Process................................................................................... 27 4.3 Types of Retreaded Tyres ................................................................................ 27 4.4 UK and EU Standards and Specifications ........................................................ 28 4.5 Retreading in the UK ........................................................................................ 30 4.6 Retreading in the USA...................................................................................... 31 4.7 Future Prospects for Retreading....................................................................... 31 5 RECYCLING AND REUSE ALTERNATIVES FOR SCRAP TYRES ............................. 33 5.1 Introduction ...................................................................................................... 33 5.2 Materials Present in Tyres................................................................................ 33 5.3 Materials Recovery from Tyres......................................................................... 34 5.3.1 Mechanical Methods of Recovery......................................................... 34 5.3.1.1 Mechanical Breakdown Only .................................................... 35 5.3.1.2 Swelling Followed by Mechanical Breakdown .......................... 35 5.3.1.3 Freezing Followed by Mechanical Breakdown.......................... 35 5.3.2 Thermal Methods of Recovery.............................................................. 36 5.3.2.1 Combustion.............................................................................. 36 5.3.2.2 Pyrolysis................................................................................... 36 5.3.2 Chemical Methods of Recovery............................................................ 37 5.4 Disposal of Whole Tyres................................................................................... 37

End-of-Life Tyres–Exploiting their Value 5.5 Tyre Shredding .................................................................................................38 6 TYRES AS A SOURCE OF MATERIALS......................................................................39 6.1 Introduction.......................................................................................................39 6.2 Crumb Production and Use...............................................................................39 6.2.1 Introduction...........................................................................................39 6.2.2 Equipment and Production Techniques.................................................40 6.2.2.1 Machinery Developments .........................................................41 6.2.2.2 Process Developments .............................................................47 6.2.3 Cryogenic Developments......................................................................48 6.2.3.1 Limitations of Current Technology.............................................51 6.2.3.2 New Concepts ..........................................................................51 6.2.4 Production Experience in the UK ..........................................................55 6.2.5 Production and Experience in Other Countries .....................................58 6.2.6 General Observations on Crumb and the Market Place ........................61 6.2.7 Developments in Crumb Materials ........................................................62 6.3 Applications of Rubber Crumb ..........................................................................71 6.3.1 Sports, Leisure and Safety Surfaces.....................................................71 6.3.2 Sheet and Coating Products .................................................................80 6.3.3 Compounding Ingredient.......................................................................81 6.3.4 Road Surfaces ......................................................................................84 6.3.5 Miscellaneous Products ........................................................................91 6.4 Pyrolysis ...........................................................................................................93 6.4.1 Introduction...........................................................................................93 6.4.2 Developments in North America ...........................................................94 6.4.3 Developments in the UK .......................................................................97 6.4.4 Developments in Other Countries .......................................................101 6.4.5 Products and Overview.......................................................................104 6.5 Reclaim...........................................................................................................107 6.5.1 Reclaim and Devulcanisation..............................................................108 6.5.2 Products from Reclaim Rubber ...........................................................111 6.6 Miscellaneous Conversion Techniques ...........................................................112 6.6.1 Gasification and Liquefaction ..............................................................112 6.6.2 Miscellaneous Chemical and Biological Treatments............................115 6.6.3 Microwave Devulcanisation.................................................................117 7 TYRES AS A SOURCE OF ENERGY.........................................................................119 7.1 Introduction.....................................................................................................119 7.2 Combustion of Tyres.......................................................................................120 7.3 Methods of Incineration...................................................................................120 7.3.1 Cyclonic Furnaces ..............................................................................121 7.3.2 Direct Kiln Burning ..............................................................................121 7.3.3 Fluidised Bed Combustion ..................................................................121 7.3.4 Starved Air Incineration.......................................................................121 7.3.5 Water-tube Chain Gate Boilers ...........................................................122

End-of-Life Tyres–Exploiting their Value 7.4 Incineration in the UK ..................................................................................... 122 7.5 Incineration in Continental Europe.................................................................. 124 7.6 Incineration in the USA................................................................................... 125 7.7 Cement Kilns.................................................................................................. 126 7.7.1 General .............................................................................................. 126 7.7.2 UK...................................................................................................... 128 7.7.2 Other Countries.................................................................................. 129 8 THE USE OF TYRES IN WHOLE OR PART FORM .................................................. 133 8.1 Introduction .................................................................................................... 133 8.2 The Marine Environment—Reefs and Erosion Control ................................... 133 8.3 Barriers and Walls .......................................................................................... 135 8.4 Constructive Landfill ....................................................................................... 136 8.5 Other Uses for Tyres ...................................................................................... 140 9 THE MANAGEMENT OF USED TYRES—NATIONAL DEVELOPMENTS & REGULATORY ACTIVITY.................................................................... 145 9.1 UK Disposal Policies ...................................................................................... 145 9.2 UK Tyre Industry Activity ................................................................................ 148 9.3 EU Waste Initiatives ....................................................................................... 150 9.4 Regulations and Practice in the European Union............................................ 153 9.4.1 The Netherlands................................................................................. 153 9.4.2 Germany ............................................................................................ 153 9.4.3 Belgium .............................................................................................. 154 9.4.4 France................................................................................................ 155 9.4.5 Italy .................................................................................................... 156 9.4.6 Spain.................................................................................................. 156 9.4.7 Finland ............................................................................................... 156 9.4.8 Norway............................................................................................... 157 9.4.9 Sweden .............................................................................................. 157 9.5 Regulations and Practice Outside the European Union .................................. 158 9.5.1 USA ................................................................................................... 158 9.5.2 Canada .............................................................................................. 160 9.5.3 Japan ................................................................................................. 161 9.5.4 Mexico................................................................................................ 162 9.5.5 Poland................................................................................................ 162 9.5.6 Australia ............................................................................................. 162 10 THE SCALE OF THE INDUSTRY—AN ANALYSIS AND DISCUSSION .................. 165 10.1 Tyre Arisings in the UK................................................................................. 165 10.2 The UK Tyre Market ..................................................................................... 168 10.3 Tyre Construction and Weight ...................................................................... 170 10.4 Retreading.................................................................................................... 171 10.5 Crumb .......................................................................................................... 172 10.6 Reclaim ........................................................................................................ 172 10.7 Destination of Used Tyres ............................................................................ 172

End-of-Life Tyres–Exploiting their Value 10.8 Comparison with Other Countries .................................................................175 10.8.1 Europe ..............................................................................................175 10.8.2 USA ..................................................................................................179 10.8.3 Canada .............................................................................................181 10.8.4 Japan................................................................................................182 10.9 Options .........................................................................................................183 10.9.1 Retreading ........................................................................................183 10.9.2 Crumb...............................................................................................185 10.9.3 Pyrolysis ...........................................................................................187 10.9.4 Incineration .......................................................................................188 10.9.5 Physical Uses ...................................................................................188 10.10 Recycling and Disposal Philosophies and the Future..................................189 10.10.1 The Future in the UK.......................................................................190 10.10.2 The Future in Europe ......................................................................192 APPENDIX A—GLOSSARY ..........................................................................................193 APPENDIX B—SUPPLIERS OF SHREDDERS, GRANULATING EQUIPMENT AND INCINERATION PLANT........................................................................195 APPENDIX C—METHODOLOGY OF ESTIMATING USED TYRE ARlSlNGS...............199

End-of-Life Tyres–Exploiting their Value

1 INTRODUCTION

1.1 Background The previous Rapra Industry Analysis Report on waste tyres in 1995 reviewed the efforts within the tyre industry to reverse the situation whereby used tyres were being added to illegal dumps and landfill. Since those days, tyres have become the subject of much debate, despite being a mere fraction of post-consumer or industrial waste. The groundwork laid by the EU Working Group in the early 1990s treating tyres as a priority waste led to a range of government/industry bodies being set up in many countries, to co-ordinate activity and recommend actions within their national domains. A pan-European organisation is now the forum for commercial debate on the many options and programmes arising in the industrial sector. This organisation, the European Tyre Recycling Association (ETRA), is also developing strong links at the political level to ensure that EU Directives that have a direct bearing on the business of tyre retrieval and treatment can be implemented in a positive and sustainable way. Much activity is now taking place to provide an easier transition through the time period that these Directives become operative. The time is appropriate, therefore, for an update of that previous Rapra Industry Analysis Report, with new data, up-to-date information and a review of the initiatives, both commercial and regulatory, that are or soon will be in place.

1.2 The Report This study is the fifth undertaken by Rapra since the 1970s when a report was undertaken for the then Secretary of State for Industry in the UK government. Each one has become increasingly international and of much wider interest, with this edition drawing information, once again, from countries within the European Union, from North America, and even further afield. Much of the detailed calculations and data, however, refer to the United Kingdom. The report follows a tyre through its life and considers the various options for its subsequent management, recovery and recycling. Following this short introduction, the executive summary (Section 2) discusses the main conclusions and recommendations resulting from the reports. A brief description of a tyre’s construction and design is given in Section 3, accompanied by a discussion of trends in tyre manufacturing and how these may affect subsequent recycling. The retread industry and its relevance to the recycling issues is discussed in detail in Section 4. The different routes that a non-retreadable tyre may possibly take are considered in Section 5. These cover use as a material, use as a source of energy, non-rubber physical uses and other, soon to become illegal, disposal options.

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End-of-Life Tyres–Exploiting their Value Technologies and processes that provide a secondary material or materials are covered in Section 6. Individual sections deal with rubber crumb, pyrolysis, and reclaim rubber and other chemical and/or thermal processes which yield a selection of end products. The recovery of energy from end-of-life tyres by incineration and the techniques involved are described in Section 7. This is followed by a brief review of the use of end-of-life tyres in some physical inert form for breakwaters, barriers and other civil engineering applications in Section 8. Information on the regulatory initiatives and legislative pressures that are likely to affect the management of end-of-life tyres are presented in Section 9. This includes a review of progress in Europe, North America and Japan. The quantities involved in the production and use of tyres are discussed in Section 10. The facts and figures obtained during the study are analysed, with comparisons between the activities in different countries presented, the options for recovery discussed and the emerging trends in the management of end-of-life tyres reviewed. Many of the recommendations made in previous Rapra Industry Analysis Reports on endof-life tyres have now become incorporated into much of the activity of the UK Used Tyre Working Group. This is all to the common good, and vindicates the efforts made in the past and the activities of the leading authorities today towards the common goal that is the exploitation of the residual value that lies within each and every end-of-life tyre. The report also includes three Appendices: a glossary (Appendix A), a non-exclusive address list of equipment suppliers (Appendix B) and the methodology for calculating used tyre arisings (Appendix C).

1.3 METHODOLOGY This report is based on extensive desk research, discussions with leading authorities and correspondence with others. It is believed that the methodology follows a logical progression down the supply chain from a new tyre to a used tyre, around the retread or reuse loop, before becoming an endof-life tyre. Quantitative information was gathered along this progression which, when analysed, provides a consistent and fairly accurate picture of tyre arisings and their subsequent destinations for the UK and other countries. The various options available for recycling and disposal are examined, with relevant points highlighted.

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End-of-Life Tyres–Exploiting their Value

2 EXECUTIVE SUMMARY

2.1 Conclusions This study on used tyre arisings, their further uses and ultimate destinations has shown that with the developments and business climate over the last five years, around 27%– 32% of the annual arisings in the UK remain unrecovered, a range similar to that first achieved in the mid-1990s. This amounts to around 125,000–150,000 tonnes a year. Unfortunately, the figure will rise slightly in absolute terms before it improves dramatically in two to three year’s time. The total annual arisings, oscillating since the late 1990s around 440,000–450,000 tonnes, will continue at this level until 2003 after which they are expected to rise again to 460,000 in 2005. The current estimates for Europe as a whole are in the region of 2.5 million tonnes, increasing to 3 million tonnes by 2008. Much time, energy and resource is now expended on progressing profitable recovery and recycling options to allow the EU to reduce the quantities of tyres annually being added to the stockpile, underground or otherwise. Reduction can only be achieved by fewer tyres becoming unfit for their original purpose at any one time. This is a responsibility on tyre manufacturers to prolong the life of their products, and on all vehicle drivers to avoid damage to tyres or bad practices when using those vehicles. Perturbation in the number of tyre arisings in recent years has perhaps more correlation with the economic cycle than with driver habits, but the inherent longevity of tyres has improved enormously over the last twenty years. Retreading, the only option to be regarded as truly reuse for original purpose, must continue to have first claim on used tyres wherever possible. It has to be accepted, however, that the level of sales for retreaded passenger tyres may never return to former heights. The replacement market has changed too much for that to happen. It remains important, though, to continue striving to improve the image of retreaded tyres and the education (or is it enlightenment?) of key points in the supply chain. All tyres, however, whether retreaded or not, eventually become unsuitable for their role on vehicles, which leads to the recycling and recovery options for useful purposes. Recycling processes that provide materials appear still to offer an uncertain way forward in the UK. However, the tonnage of tyres consumed to generate crumb rubber has been on the increase. Incorporation in road surfaces seems as far away as ever, so further facilities are unlikely to come on stream in the near future. One pyrolysis technology appears very promising and is being encouraged within the UK. The output materials have been tested and confirmed for their commercial value. Once markets are established several small-scale operations could be set up close to cities and towns as the source of a reliable quality of arisings. Recycling as fuel for the recovery of the energy content is an identifiable market. In the UK, this has taken the form of the SITA (former Elm) plant and consumption by the cement companies. The uncertain future of the SITA operation has shifted energy generation from tyres to the cement industry that does plan to increase its consumption. 3

End-of-Life Tyres–Exploiting their Value Estimates for a few years ahead already have this sector taking over 40% of total arisings, which includes a contribution from a proposed tyres-to-electricity via pyrolysis project. There would, therefore, be no room for any further large-scale project. Further projects, therefore, should be limited to those which would consume no more than 10% of annual arisings at a time (in the range of 20,000–40,000 tonnes). There would still remain sufficient annual arisings that could find use in various miscellaneous ventures, as well as the existing dumps. Already an increase is being seen in tyre usage for landfill engineering, and there is an increased awareness of the civil engineering possibilities, beyond crumb in roads. These should be encouraged as environmentally positive, helping to protect waterways and coastal erosion among many others, as well as thereby reducing the excavation of earthworks and quarries necessary for such civil projects. With an increase in recovery activities between now and the year 2004, by which time annual arisings in the UK are estimated to reach 480,000 tonnes, a scenario is envisaged (see Section 10.10) whereby the EU recommendations of 65% recovery should be easily passed, although retreading will level out at around 17%–18%. The unaccounted fraction should drop to around zero. The adoption of techniques which consume significantly large quantities of scrap tyres has so far only been fulfilled via the energy route, whether in dedicated steam and power generation or in the manufacture of cement. So far, the goals of large-scale consumption in roads or civil works continue to remain elusive. The pyrolysis and feedstock routes are likewise some way from commercial quantities, although much more is anticipated for the pyrolysis techniques now being pursued. It would be wrong to attempt to prevent any particular venture if it takes a percentage of the scrap arisings or stockpile and converts them to useful energy or materials. The economics of any particular venture would, however, be affected by the influence that venture may have on existing schemes and the availability of arisings as a raw material for the proposed operation. A detailed hierarchy for recycling and/or recovery may become necessary when any country or community is approaching saturation with scrap disposal schemes. This may happen during the current decade as Europe comes to terms with the various Directives that impinge on the management of used tyres. The last two or three years have shown the benefit for waste tyre management of cooperation between all parties in the supply chain and the co-ordination of efforts within the UK. Although practicalities on the ground remain a national concern, the lead taken by ETRA in the wider debate has been of huge benefit to the industry. ETRA has the respect of the industry and the ear of policy makers and so will continue to be the pre-eminent forum for this subject in the years ahead.

2.2 Quantities of tyres involved Total used tyre arisings during 1999 in the UK are estimated as •

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32.5 million car tyres (211,000 tonnes)

–

an increase of 3% since 1994.

End-of-Life Tyres–Exploiting their Value •

just under 4.6 million truck tyres (239,000 tonnes) –

an increase of over 50% since 1994.

Used tyre arisings during 2004 are estimated as •

33.0 million car tyres (215,000 tonnes)

–

an increase of 2% on the 1999 figure.

•

just over 4.6 million truck tyres (244,000 tonnes)

–

an increase of 2% on the 1999 figure.

These figures emphasise the need to operate schemes which will take large quantities of scrap tyres to alleviate the size of problem. The disposal pattern in the UK for 1999/2000 is estimated as shown in Table 2.1 (extracted from Section 10.7): Table 2.1 Summary of Destinations for Used Tyres in UK 1999 2000 Destination Tonnes % Tonnes Net Export 9,900 2.2 2,100 Retread 76,000 16.9 77,000 Part worns 30,500 6.8 30,400 Incineration 71,000 15.8 65,000 Other uses 134,000 29.7 130,000 Recovery 321,500 71.4 304,500 Vehicle shredder residue 45,500 10.1 45,600 Landfill or dumps 83,500 18.5 94,600 Total arisings 450,400 444,700

% 0.5 17.3 6.8 14.6 29.2 68.5 10.2 21.3

Source: Rapra estimates

In comparison with other countries, the UK emerges as a middle ranking recycler (see Section 10.8), with just over one-third of arisings still unproductively stored or landfilled. Across Western Europe a comparable figure is around 40%, although this is an average of low figures for northern countries and very high figures for Mediterranean nations.

2.3 Options for Reducing and Recycling Waste There are various approaches by which it is possible to gainfully deploy the quantities of tyres arising after use on a vehicle. These contribute to an increase in resource recovery (be it as energy or materials) and form part of the logical progression that forms the basis for waste management now being taken by the European Union for the handling of all waste: Reduce—Reuse—Recycle—Recover. The hierarchy provides a framework to encourage the reduction of waste generation, reuse in original form, recycling to remake the original form and recovery of resources for a useful purpose. Any products remaining have been sent for disposal, usually in a landfill site. Such a route has become increasingly expensive, and will eventually cease to be a legal alternative, by 2006. Much effort is being put into tyre development and design to make a lighter, more consistent and uniform product, and to reduce the rolling resistance of tyres. Such measures will have the effect of making energy consumption (vehicle fuel) more efficient during the life of the tyre. 5

End-of-Life Tyres–Exploiting their Value The reuse option for used tyres can be achieved through the practice of improving appropriate used tyre casings by retreading. All tyres, however, whether retreaded or not, eventually reach the end of their useful life, which leads to the third and fourth elements—of recycling and recovery for a useful purpose. Arguably, recycling is not possible for a complex product as a tyre, but the word tends to be used interchangeably with recovery. Potentially, though, some recovery routes could provide ingredients that can be utilised in the manufacture of new tyres. Recovery methods are divided into three main approaches: materials recovery, energy recovery and reuse in whole or part form for miscellaneous physical purposes. Recycling for the retrieval of materials takes on two forms: the production of particulate matter from the whole tyre that may or may not be modified, and the more destructive breakdown by thermal and/or chemical processes to more basic hydrocarbons, carbon, and, of course, steel. The carbon would be the material stream that could find its way into new tyres.

2.4 Options for Second Use

2.4.1 Retreading Retreading should remain the first option for the recycling of all used tyres. Taking the UK as an example, only about 15% of car tyres and 50%–70% of truck tyres are suitable for retreading (Section 4.5). Yet only about 9% of used car tyres and 24% of used truck tyres are in fact retreaded. This shows a decrease of almost 50% for car tyres and 33% for truck tyres since the last report. To utilise all these tyres would require a two-fold increase in the size of the retreading industry in the UK. This is not going to happen, if only because the replacement market has changed so much and there continues to be a ‘credibility gap’ that would require a radical change in attitudes towards retreaded and remoulded car tyres. Retreaded truck tyres form an integral part of the operation of haulage fleets although exhortation to increase their use for large vans and small trucks, usually run by owner/operators, would be appropriate. Roadside checks across the UK continue to show that a high percentage (14%) of cars have at least one illegal tyre. This does not bode well for an increase in casings suitable for retreading. Tyre maintenance also needs to be improved, even for fleet operators. These checks have shown to a strong degree that many tyres are under- and indeed over-inflated in comparison to the makers’ recommendations. This also dramatically reduces a tyre’s life and its efficiency under braking.

2.4.2 Crumb and Reclaim There is no traditional reclaim activity in the UK and very little elsewhere, although Vredestein in the Netherlands does offer a service. The quantity of crumb from retread buffings has declined along with the market for retreaded tyres. Since most material comes from a truck tyre the lesser decline in truck tyre retreading is reflected in the figures. Around 13,000 tonnes of crumb come from retreading activity. Direct crumbing 6

End-of-Life Tyres–Exploiting their Value has increased greatly in recent years. Although several crumb operations have closed or ‘mothballed’, others have opened in the last 2–3 years (Section 6.2). Most provide a ‘raw’ material directly for specific end uses, in contrast to the past when some operations were related to waste management activities and shredding for disposal. Quantities are difficult to estimate. Based on DTI figures, it is estimated that direct crumb output in the UK on an annual basis is around 50,000–60,000 tonnes. Most continues to be consumed in resilient surfaces although an increase is anticipated in highway engineering, even if not in road surfaces as such. A change has been noted in attitudes towards cryogenic grinding. Whereas this approach was considered unprofitable in Europe, it is in practical use in North America and the available techniques are being re-examined. These minimise the use of the cryogenic medium and are more thoroughly engineered than ever before. Proprietary chemical reclaim or modification processes continue to be announced. None has become an ‘industry standard’. De-Vulc, which was announced at the time of the last report is still available although its optimistic claims have subsided. The latest processes emerging from Asia via North America appear scientifically sound but it is too early to measure the degree of commercial acceptance. Trials and pilot operations for activated modified rubber (AMR) or RU-Rubber are under way; the outcome will provide a better view on the viability of these latest techniques (Section 10.9.2). Crumb continues to be regarded as a useful recyclate raw material and not as a major part of solving the used tyre problem. This would change if the efforts of the proponents of particulate rubber in asphalt road paving bring forward the consumption of significant quantities in European roads. Portugal is currently in the vanguard of European activity, although quantities have been utilised in France, Belgium and other locations for a number of years.

2.4.3 Pyrolysis Interest and activity in pyrolitic techniques has grown enormously over the last five years. Developments have continued on small batch plants such as the Beven Recycling retort units. These can be used singly for batch operations or as a larger ‘continuous’ operation utilising four retorts. A modular microwave-based system (from AMAT) has come to fruition after many years gestation, and provides good quality outputs that have been tested and found totally acceptable in the marketplace. More capital-intensive systems from a range of countries have incorporated top-class plant engineering expertise, unlike some ventures proposed in the past. None, however, is yet understood to be operating fully on a commercial basis. All claim to offer quality material that would command a fair price in the market. Pyrolytic distillation for subsequent clean incineration as a fuel in combined cycle plants is proposed within some of the schemes available. These are attractive in areas of high population in preference to direct incineration where concerns over potential emissions may exist.

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End-of-Life Tyres–Exploiting their Value

2.4.4 Energy Recovery The utilisation of tyres as a fuel has been the largest destination for used tyres within the European Community for a number of years. In the UK, this originated with the opening of a tyres-to-energy operation. The design capacity has only been achieved in one year and problems have reduced the share of arisings that the plant has taken since 1997. Under new ownership, there is doubt as to whether it will reach full capacity on reopening after a re-fit. Tyres can and have been successfully consumed in cement kilns (Section 7.7). There has been a resurgence in plans to consume them in the UK during the period under review in this report. Various trials and applications are currently coming to fruition with strong growth expected in the near future. The cement industries in Germany, France and Italy among others have been consuming tyres for many years. The setting of new emission levels for particular materials will cause a re-think of the strategy of tyres as a fuel in some older kilns. The subject of emissions from plants consuming tyres is generating some uncertainty about various incineration routes. This could be to the advantage of a current proposal in the UK to use an initial pyrolysis process followed by a secondary combined-cycle power generation system. This is claimed to be virtually emissions-free.

2.4.5 Physical Use of Whole or Part Tyres The potential for the use of tyres, whether whole or as large pieces, in what can be described loosely as civil engineering works, has now become more widely recognised within Europe. No problems have been found to be associated with use above or below the water table, but projects involving use below the water table have to be examined on an individual basis. From a wider ecological standpoint, the use of tyres also reduces the requirement for earth and spoil which are often excavated specifically for new roads, embankments, etc. The programme within Europe to prepare standards for tyre-derived materials and the applications to which they are put should provide case studies and codes of practice for civil engineering applications. In turn these will instil a greater confidence to utilise tyres within a specification driven industry.

2.5 Collection and Dispersal Efficient 100% collection remains the goal for all used tyres. This will allow the maximum opportunity for the better tyres to be passed for retreading and the others to pass into the various downstream activities to exploit the value residing within them. Market forces dictate the supply and demand of used tyres, as they have always done. Their source and destination changes as the price of transport and tipping varies, with ‘spot’ prices differing from those set under contracts. Larger waste management companies have become much more involved, with traditional tyre collection by the retreading sector moving into the hands of specialist tyre collection firms, some of which started life as casing dealers. All must operate under duty of care provisions and must be licenced as carriers of waste in the UK. Such organisations have now become the clearing

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End-of-Life Tyres–Exploiting their Value houses at which tyres should be checked for retread potential before passing irrevocably into the other routes now available for legal recovery. Since the previous report, it is understood that all EU countries now have a formal system of collection and/or an approved body that monitors the progression of used tyres into recycling and recovery. The principles of producer responsibility are also in action with all sectors of the tyre industry becoming involved in national activities, which in some countries is even led by tyre manufacturers. Although the voluntary system of a disposal charge is still operated by some retailers in the UK, the customer has no way of being sure the monies are indeed directed to fund responsible recovery. It is now felt that a mandatory scheme should be introduced whereby a dedicated, audited fund supports the collection and recovery sectors. This works well in many other countries, yet the UK’s fiscal policies and revenue structures would make this a difficult fund to administer. A proportion of the UK landfill tax is available for projects to investigate better options for waste management through the landfill operators. Unfortunately, tyres are but one of many waste streams, and the benefits from any project would take time to become commercial realities.

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End-of-Life Tyres–Exploiting their Value

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End-of-Life Tyres–Exploiting their Value

3 THE TYRE, ITS DESIGN AND MANUFACTURE An understanding of the nature and construction of a tyre and the materials involved is a useful prerequisite to the later discussion of the recycling and reuse of tyres. The outline of the subject given in this section provides some information on trends in construction and manufacture. Specific details should be sought from textbooks and standard reference journals. The major changes taking place in the world of tyre manufacture are, as always, aimed at efficiency. Production methods have been changing to incorporate more flexible manufacturing systems. These allow for short runs of any one design before switching to a different one, and perhaps back again. Lower rolling resistance, better grip and longer life, the often incompatible goals for a tyre, have become reconciled somewhat with the advent of the silica-containing tread compounds of the last few years.

3.1 Tyre Construction Until the 1970s in Europe and the 1980s elsewhere, all tyres were made as a cross-ply construction, known also in the USA as bias-ply, named after the configuration of the reinforcing textile material as seen along the direction of tyre rotation. A cut-away view of a cross-ply tyre is shown in Figure 3.1 giving the different components included in a tyre. There is always an even number of layers, or plies, of reinforcement to keep the load distribution in the structure symmetrical. Although a 4-ply construction is shown in Figure 3.1, car tyres will probably have only 2 plies at the smaller sizes, whereas truck tyres will have 6, 8 or more.

Figure 3.1 Diagonal Ply Tyre (Four-Ply Construction Tubing), Tubed The cross-ply tyre has almost disappeared from Western Europe in both car and truck tyre ranges and is slowly decreasing in market share elsewhere. The radial-ply tyre (Figure 3.2) has become the replacement and has greater flexibility, lower rolling resistance and 11

End-of-Life Tyres–Exploiting their Value greater mileage through reduced tread wear. In these tyres, there is only one carcass reinforcement layer, placed radially, or at 90° to the direction of rotation. In addition, there are also tread reinforcement, or bracing, layers called belts, which run circumferentially under the tread.

Figure 3.2 Radial Ply Tyre (Two-Ply Construction), Tubeless In all tyres there are three types of components: the reinforcement layer(s), the various rubber parts and the steel bead wires that locate a tyre to the wheel of a vehicle. The reinforcement, which will usually be a textile or steel, is made up into a fabric. Textile tyre cord, which can be rayon, nylon or polyester, is woven into a fabric with a very strong, close warp and a widely spaced weft. This is then rubberised (impregnated with a sticky rubber compound), which effectively breaks the weft strands and leaves a closely laid unidirectional series of cords held together with rubber. In a steel fabric, standard steel cords are placed in a parallel, close-packed formation and rubberised directly to form a 'ply' of reinforcement. Steel fabric provides greater strength and also greater stiffness, and is used for carcasses in larger tyres and for belts for most tyres. Rayon appears now to be used only within Europe, but has had a resurgence through its suitability for high-performance tyres that can reach high temperatures in use. It has good resistance to thermal degradation. Goodyear revealed in 1998 that it had been developing an ultra-tensile wire for the manufacture of all steel car tyres, in conjunction with Bekaert, the leading independent tyre wire producers. Goodyear has subsequently changed to rayon reinforcement for the Eagle Aquasteel run-flat tyre since the all-steel carcass prevented the electronic signals used by a pressure-sensing device from reaching receivers in the vehicle chassis. Goodyear said at the time that the introduction of such tyres would be gradual and the company would continue to use rayon and polyester. Goodyear has pushed its run-flat concepts towards the general OE (original equipment) car market and if and as this approach bears fruit, then rayon would be in demand for these tyres also.

12

End-of-Life Tyres–Exploiting their Value However, rayon substitution will continue in all types of tyre with its eventual elimination for standard S and T tyres in 10 to 15 years time (speed categories for tyres (as given in BS AU 144f) are listed in Table 3.1). Table 3.1 Speed Category Speed symbol Speed km/h Mile/h approx. F 80 50 G 90 56 J 100 62 K 110 68 L 120 75 M 130 81 N 140 87 P 150 93 Q 160 100 R 170 106 S 180 113 T 190 119 U 200 125 H 210 130 V 240 150 W 270 170 Y 300 190 Table 2 from BS AU 144f: 1998 is reproduced with the permission of BSI under licence number 2000SK/0614. Complete standards can be obtained from BSI Customer Services, 389 Chiswick High Road, London W4 4AL.

Dunlop has been continuing its longer-term developments with aramid fibres as announced in the early 1990s. Originally used as a replacement for the steel bead wire to reduce the overall tyre weight, further efforts have examined the concept of aramid for belts and other sections of the carcass. The ULW tyre is an ultra lightweight concept with two plies of trimmed aramid for the belts and a rayon radial carcass, and aramid reinforced in the bead apex region. The resultant tyres are 25% lighter than comparable steel-belted tyres made today. A 195/65 R 15 Dunlop SP Sport 200 ULW weighs only 7.3 kg compared with 8.9 kg for a conventional tyre. ULW tyres contribute towards improved active driving safety and comfort, help to reduce fuel consumption and have less tendency to cause vibration. The rolling resistance of the ULW tyres is lower than a conventional tyre over the entire speed range on the Audi A3, the primary development vehicle for the project. The concept is now being pursued for run-flat versions that, again, should benefit from the weight reduction compared with other types. ULW tyres using the aramid technology could have positive effects on the disposal of such tyres. The shredding of used ULW tyres would consume less energy, and there are no metal constituents once the beads are removed. This would make the preparation of ground rubber more economical and, obviously, the product is metal-free.

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End-of-Life Tyres–Exploiting their Value Apart from various packing strips and an inner lining material, the major rubber compound components are the sidewalls, made from a flexible formulation, and the abrasionresistant tyre tread which carries the pattern and provides contact with the road surface. With all types of tyres, steel bead wires are included at the extremes of the reinforcing plies to ensure a complete seal to the rim. Many turns of small diameter wire are wrapped in a rubber-impregnated tape to form a flexible hoop of high strength. In traditional factories, these various components are built together on the building drum of a tyre-making machine. Although many of the separate processes have become automated in more recent times, much of the quality of a tyre depends on the skill and care of the person operating the machine. One major requirement is to prevent any air entrapment between the various layers. This would be a potential source of ply separation in later service. The builder also applies various shaped strips at appropriate points to provide a gradual transition from one major component to the next and to give smooth interior surfaces. Each of these duties may involve a different rubber compound, selected to give the most desirable properties. There are many variables and different manufacturers will have evolved their own combination of materials, so it is impossible to accurately catalogue the ingredients involved. The 'average tyre', therefore, is a rather ambiguous concept.

3.2 Tyre Design The major requirements of a tyre in service demand the designer to take account of various, often conflicting, factors. These, such as comfort, wet grip, tread wear, rolling resistance and stability, have not changed, and neither has the general concept of the rubber-plus-reinforcement approach to the tyre. Radialisation continues apace and, apart from some specialised requirements and the needs of the ‘classic’ and ‘vintage’ fraternity, replacement of cross-ply tyres by radial tyres is complete in Europe. With the radialisation trend, car tyres have become wider and have lower aspect ratios. In 1980, 75 and 80 series tyres took about 90% of the market, 70 series tyres about 8% with 60 and lower series tyres on 2%. Now, 70 and 65 series tyres take about 50% and the 60 and lower series tyres, about 20%. Lower aspect ratio tyres provide superior vehicle steering response and are designed for the higher speed ratings. Any stiffening and hardening of the ride is taken as an indication of a firm positive vehicle and is accepted by the owners of high-performance vehicles. There would appear to be no real change in the amount of materials used to construct these tyres although the higher rated tyres, H, V or Z, are understood to contain a greater quantity of rubber, over 20% in some cases, than the S- or T-rated 80 series tyre. Thus there will be a gradual increase in the rubber content of end-of-life tyres in the future. Virtually all passenger tyres consist of textile casings with steel belts, although it is believed that some of the ultra low aspect ratio tyres may use a steel reinforced casing. The life of low aspect ratio tyres has now become established; they do seem to be changed more frequently than standard series tyres and the industry is using figures of somewhat over two years. Depending on the retreadability of high-performance tyres, this will be a factor in any future increase of end-of-life tyres.

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End-of-Life Tyres–Exploiting their Value The trend of the car manufacturers to increase the models for which they specify H, V or Z tyres means that the demand for these tyres will increase over the next decade, with a consequence that the demand for S- and T- rated 80 series radial tyres will decrease. It is now also known that HVZ tyres with an ultra-low aspect ratio, with bigger rims to house more efficient braking systems, have improved rolling resistance. It is important to point out that driver style and an efficient attitude will do more to save fuel than any amount of technology in the tyre. According to Goodyear, fuel consumption may be reduced by 2% by: •

a 100 kg reduction in car weight,

•

a 0.6 bar increase in tyre pressure,

•

a 2 km/h speed decrease at 120 km/h,

•

having no full stops from 90 km/h each 30 min, or

•

a 10% reduction in rolling resistance by construction changes.

Rolling resistance has been of concern ever since the oil crises of the 1970s. Introduction of new polymers over the years has provided a slow reduction in this parameter. One method to help in the reduction of this parameter has been the use of silica reinforcement in place of carbon black in tread compounds, but this is not the only way. Leading tyre companies are now talking of ‘low hysteresis compounding’ for all rubber compounds in the different parts of a tyre. Trial-and-error techniques have given way to the computer and the analysis of pastaccumulated data. Data retrieval is now so important. Finite element modelling and analysis are now used for design and optimising a range of properties. Goodyear believes that FEA optimisations will help create a reduction in rolling resistance of up to 30% by the early 2000s (this would give a fuel reduction of about 5%). Although the tread dissipates most energy, all parts of a tyre do so (see Table 3.2). Table 3.2 Contribution of Tyre Parts to Rolling Resistance Area Contribution Proportion of whole (%) tyre volume (%) Tread region 33 28 Apex 15 8 Sidewall 12 15 Liner 7 6 Base 6 11 Other 28 32 Hysteresis cannot be completely eliminated from the tread region, because most tyres rely on some degree of hysteresis for the creation of wet grip. Work continues on all elements within a tyre. Note that the apex dissipates most energy per unit volume. Bridgestone, the Japanese tyre manufacturer, has developed a range of tyres based on theories evolved and refined for the optimisation of various parameters. Finite element 15

End-of-Life Tyres–Exploiting their Value work and thousands of computer iterations are performed in ‘sensitivity analysis’ to predict the tyre contours and profile which would generate the ‘best’ combination of parameters under load and in use. It was established in the early 1990s that, for a constant tyre diameter, rolling resistance is constant as a tyre aspect ratio is lowered, provided the tyre width stays constant, i.e., the rim gets bigger. When both tyre and rim diameter are kept constant, the tyre width has to increase to generate the low aspect ratio and so the rolling resistance increases. This is mainly due to the fact that the tread area is enlarged and this contributes more to the rolling resistance. Today’s requirements have sprung from a need to combine: •

long tyre life (high wear resistance),

•

high security (high wet grip), and

•

low fuel consumption (low rolling loss).

A change in the SBR types used for the treads has allowed more silica to be used. Poor abrasion properties and high cost associated with emulsion SBR (E-SBR) have given way to new compounding concepts. With high vinyl solution SBR (S-SBR), a better compromise between hysteresis properties and wet grip without wear has become possible. The compound trend is for: •

more NR,

•

more BR,

•

more S-SBR,

in place of E-SBR. All these improve compound elasticity and hence resilience. The tyre industry is now prepared to pay for high performance. This may be an opportunity for special modified and up-graded carbon blacks, the price and development of which the tyre industry has hitherto been unprepared to pay for. Over recent years, the tyre makers have introduced low rolling resistance, ‘green tyres’ by replacing much of the carbon black with silica. But grip, handling performance and wear are compromised by the relative absence of carbon black. The requirement, therefore, is for a reinforcement using both silica and carbon black. Pirelli’s answer to this dilemma is the P3000 range launched in Brazil in 1998, covering the ‘T’ rating sector, that accounts for some 65% of total world tyre demand. The same year, Michelin also introduced a ‘hybrid’ compound for high-performance tyres. This is used in the tread compound for good wet grip and dry handling, with an under-tread compound containing only carbon black to increase the stiffness of the tread area. These Pilot Sport tyres include a new lightweight high-strength steel for the belts. Despite the increased use of silica in tyres, consumption only amounts to around 2%–3% of the carbon black used in tyres. The majority of the silica is used to improve the cut resistance of truck treads; new uses are for good ice grip and rolling resistance.

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End-of-Life Tyres–Exploiting their Value Pirelli has said that silica would be a filler grade for combining with carbon black. Industry consensus seems to be that silica would be a primary reinforcing filler for winter tyre treads and some high-performance car tyres. It would only be a secondary reinforcement for general-purpose treads. Another area of activity is that of run-flat tyres. Early concepts from the 1970s and 1980s have fallen by the wayside, but new generations of tyres have come from the major companies. The Michelin TRX, the Dunlop Denovo, Pirelli’s DIP tyre and the Continental CTS design all foundered on the need for special rims, special servicing or the plain fear of losing the fifth wheel. Goodyear offered its extended mobility tyres (EMTs) to both the OE sector and the aftermarket in the USA during 1998. A key factor in Goodyear’s decision to offer EMTs was the outrage felt when a driver was attacked and killed while changing a flat in a violent part of one of the nation’s cities; the run-flat capability had taken on a different connotation entirely. Michelin introduced its PAV (pneu accrochage vertical, or vertically anchored) tyre in 1997. This is now known as the PAX system. Although seen as an advanced run-flat system, it does offer more. This extra does seem to be required, otherwise Michelin’s ZP (zero pressure) self-supporting tyre, which has already proved successful in the USA, would be sufficient. The PAX package consists of a tyre with short, strong sidewalls which is mechanically locked onto the rim, a flexible support ring that supports the weight of the vehicle when the tyre deflates, a flat-base profile wheel that allows for easy mounting of the ring and a pressure monitoring/warning system. As with other systems before it, PAX will be accepted if its merits are felt to outweigh its disadvantages, in today’s technological climate. Two major factors count against it: the industry-wide ramifications that would arise from the introduction of an entirely new tyre/rim system and the unsprung weight penalty that arises from the presence of an insert to support the tyre when deflated. The benefits of PAX are claimed to be numerous: •

run-flat capability—up to 200 km at 80 km/h,

•

elimination of the spare wheel, which saves weight and frees interior space,

•

bead locking to prevent tyre detachment in a blow-out,

•

reduced rolling resistance—10% improvement over conventional tyres,

•

improved steering response due to shorter, stiffer sidewalls,

•

smaller tyre diameter for a given load and speed capability, which facilitates improved vehicle packaging, and

•

increased space within the wheel, which allows bigger brakes to be packed within a given tyre diameter.

Although Michelin and Conti (with its own system called the Conti Wheel System) believe that a step change in tyre/rim design should be beneficial, others have had a different view. 17

End-of-Life Tyres–Exploiting their Value Goodyear says that it is working on ways to make run-flat tyres and still reduce weight, improve the run-flat capability and give a better rolling resistance, all with a conventional rim. The approvals gained initially by Goodyear were for armoured vehicle programmes where safety and mobility were of prime concern. A second phase is seen to focus on saving vehicle weight and freeing space by the elimination of the spare wheel. The company feels that all this can be achieved with the existing rim system. Slight changes in weight and comfort should be countered during the vehicle development programme and a change in suspension settings. In mid-2000, Michelin and Goodyear announced that they had joined forces in the quest to establish something approaching a standard run-flat technology. They have set up a technical joint venture to exchange patented information on their respective systems, the Michelin PAX system and Goodyear’s EMT. A legal entity, Global Run-Flat Systems Research, Development & Technology BV, will be based in the Netherlands, but all the real work will continue at the owners’ respective R&D facilities. Pirelli has a licence agreement with Michelin and has been producing tyres. Sumitomo supports the joint venture and has made moves to obtain a licence for the Dunlop brand that it controls in Asia, while Goodyear expects to market PAX under both the Goodyear and Dunlop brands in North America and Europe. The different brands of PAX tyre will all be compatible with ‘standard’ PAX rims. Michelin apparently has no particular plans to market any EMT tyres. Bridgestone has made no comment and Continental will not be seeking a licence. Suddenly there is now a strong prospect of a ‘standard system’ emerging for the tyre and automotive industry. Continental, in fact, has a two-system strategy for run-flats, one of which is an add-on to the conventional tyre and rim. The Conti Wheel System (CWS) will be introduced via the car manufacturers and the Conti Safety Ring, CSR, is aimed at the replacement market. The basic features of the CWS are a new tyre-to-rim junction with flexible bead and filler rings, a supporting ring for run-flat capability and a warning system. The CSR is being promoted as the only run-flat system that can use the conventional tyre and rims designs at a reasonable price. The system consists of two ring halves made of a sinus-shaped metal ring on a flexible rubber support. The halves are currently fixed by screws. Run-flat, or self-supporting, tyres give such a good ride that a driver does not notice if a tyre loses virtually all its air. This has serious implications for cornering and braking performance. As a result, several companies have gone on record saying that they would not sell such tyres unless the vehicles to which they are fitted are equipped with tyre pressure monitoring systems. Dunlop Tyres introduced a ‘smart tyre’ containing a microchip on which can be recorded pressure, distance travelled, maximum temperature reached each day and other data. This can be read each day by a sensor when the vehicle fitted with such tyres passes through a special reading station at the depot or garage for trucks, buses or other commercial vehicles.

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End-of-Life Tyres–Exploiting their Value

3.3 Tyre Manufacture As an ever-wider variety of models are developed by the motor industry, the tyre industry has been responding with new production methods. It is essential to match the trend towards fragmentation in car markets, which means greater flexibility in the tyre factories. Tyre designers have wished to remove the need for splicing. Most of the new production technologies are believed to achieve this to some degree or another. Advantages obtained include perfect symmetry and uniformity, quality consistency, lower production scrap rates, less probability of premature failure and increased durability. Next generation systems are likely to incorporate radical changes: •

filament or ribbon winding of belts, which would be made of short fibre composites for shear stiffness,

•

special weaving machines, able to produce jointless toroidal carcass structures,

•

direct extrusion of treads, sidewalls or other rubber components onto the building drum,

•

injection moulding of ring-shaped parts, such as beads or bead-filler combinations, or complete belt structures, and

•

use of organic fibres in place of steel wire to create jointless belts without cut edges.

Energy consumption should also see a decline. Reducing energy consumption will take on added significance as a way to reduce overall production costs. Continuous mixing technology is expected to play a more significant role, as is the increased use of masterbatches and pre-mixed ingredients. The goal of these new technologies is to eliminate as many steps involving semi-finished components as possible. These include the production of treads, sidewalls, plies and other components at the assembly site. Reducing the number of semi-finished components and moving to a truer tyre-building system instead of an assembly process will lower the number of heating and cooling cycles, again saving energy. Toroidal carcass building and direct extrusion of components are understood to be already in use as part of the C3M technology at Michelin. The C3M process is understood to be a flow-line system that makes components in their final net-shape. Space required is drastically reduced by elimination of the requirement for calendars, bias cutters, large extruders, mixers, and batch-off lines. Flexibility is generated not so much by the remaining machines themselves, but that for the capital outlay ‘freed’ by the new approach a relatively large number of building machines can be introduced. Each building machine is focused on a metallic torus (rather than the collapsible cylinders of conventional drum machines). This is fixed in size and proportion. Flexibility for any one machine is in the capability of varying the construction within a fixed rim size and width. Mixers/extruders would be programmed to prepare materials for the types of tyre scheduled to be built. This eliminates the need to provide for re-programming of a multiplex extrusion process, the preparation of reinforcement plies or the calendering of 19

End-of-Life Tyres–Exploiting their Value new sidewall or tread components. This allows for flexibility of raw materials usage and more rapid stock turn-round, typically only a few hours from raw material delivery to shipping of the new tyre in which it has been incorporated. By comparison, the conventional factory turn-round would be around 2 weeks. Energy savings can be large with only a single heating/cooling cycle in build before final cure, rather than several. Mixing is crucial with a low temperature regime of less than 90°C, and very high levels of dispersion. Michelin uses either raw rubber or black masterbatch; it is not known which. Conventional mixes do not work on the mixer/extruders. Although pre-prepared components are used by most companies for conventional tyres, Michelin is already believed to extrude some parts directly onto the building drum. It has thus extended this dramatically for the C3M method, and uses continuous mixer/extruders to build all rubber parts direct onto the torus. These include: •

breaker edge packing,

•

sidewall, and

•

2 apexes around the beads.

Early treads produced using this system were believed to be also strip wound, similar to Orbitread, but later it was understood that a change to a ‘ring-tread’ pre-cure system had taken place. This overcomes the problem faced early-on whereby the delicate structure of the breakers (belts) was disrupted by the pressures exerted while the tread pattern was formed. The ply is knitted all around the tyre, with the looped ends passing around the bead wires, to maintain the structural integrity of the tyre. The beads themselves are wound directly onto the torus as bright wire rather than hoops of pre-wound wire covered with a tape and some rubber compound. The reinforcement in the breakers is also wound continuously from a creel directly onto the building drum. It is believed that this knitting process is now the time determinant for the building cycle. Heavy curing presses are not needed. The conventional press would have tread pattern segments for imprinting the pattern into the rubber accompanied by high steam and/or hot water under pressure to inflate the tyre to the final shape via an internal bladder and to provide the heat for the chemical vulcanisation to take place. In C3M, the tread package may already be semi pre-cured. The torus can be heated during build to cure the inner surface so that the shape is already defined by the time building is complete. One source reports the use of autoclaves for curing, but the patents talk of using electrical radiant heating. To summarise, the advantages for the C3M system include: •

one-tenth of usual manning levels,

•

one-half of usual capital costs,

•

one-tenth of usual factory floor space requirement,

•

factory building time about 3 years (compared to the usual 5–6 years),

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End-of-Life Tyres–Exploiting their Value •

stock turn round increased ten-fold,

•

lower energy consumption,

•

electric cure—quicker cure,

•

eliminates tackifiers and anti-tack agents,

•

no ageing of rubber content prior to cure, and

•

no cold component joints.

The capital cost has been estimated at less than US$14 million for a 500,000 units/year car tyre plant. This is equivalent to the cost of a single heavy calender. Michelin set up its first C3M plants outside France in 1997, one in Sweden and a second in the USA. This brought the number of C3M factories in production to six. The company has always refused to divulge any further information about the process, other than that it occupied less factory space and that it is said to be able to switch between different tyres efficiently, an important attribute for flexibility in meeting market demands. Several other companies have announced new production methods in recent times: Goodyear’s Impact process, Pirelli’s Flexi system and MIRS, with Continental using its own variation of flexible manufacturing. In early 1998, Goodyear announced Impact, Integrated Manufacturing Precision Assembly Cellular Technology, a process that would increase productivity by 135%, halve the number of manufacturing steps, reduce labour by 35% and materials consumption by about 15%. No patents have been sought, so very little information will become available about the process. The developments were to be implemented over a period of five years as cells at various existing factories are duplicated. The first cells were understood to be in both Europe and the USA. There appeared to a long lead-time for the requisite equipment, but a new plant in Brazil was to carry some of the Impact features. This would make 750,000 units per year after coming on stream in 1999. Flexibility is a key aspect with the ability to turn to less popular, but higher added-value, sizes a distinct advantage. Impact is understood to assemble, inject and compound differently from a conventional factory. It also eliminates splices and improves quality and reduces cost. The ability to install Impact cells within existing factories is believed to be a strong differentiator from other systems, and allows for a smoother introduction and integration of such new technology into the company’s operations. In 1997, Pirelli introduced the Flexi system that produces large, high-performance tyres (in the US$300+ range) in batches as small as 100. A more comprehensive scheme was not revealed until mid-2000. This produces mainstream tyres with a similar flexibility for runs of a few hundred, at reduced times and costs. Continental’s future productivity depends on the new systems for manufacturing that it has developed. These include MMP (Modular Manufacturing Process), ESA (single-stage building), SAV2000 (truck tyre building) and the C+K automated tyre building equipment.

21

End-of-Life Tyres–Exploiting their Value The C+K equipment was developed with Krupp Elastomertechnik. It is claimed to give a high level of quality, a greater degree of standardisation, low process costs and up to 60% reduction in investment costs. These make the project a key element in the Conti drive for cheaper manufacturing. The machine is first being used in the United States. The MMP will enable Conti to produce short series at low cost and to respond quickly to fluctuations in demand. Owing to the comparatively low expenditure involved and the flexibility of the process, entry costs to new markets will be much lower. It was estimated that a new car-tyre project would cost just one-tenth of US$200–300 million that a conventional factory would cost. After more than five years of development, Pirelli announced its Modular Integrated Robotised System (MIRS) in July 2000. The first unit is installed at Bicocca, near Milan. MIRS is expected to produce better quality product, to lower production costs, to improve performance and thus give better value from just 350 m2 of plant. Special software enables high-performance tyres to be made in such a compact unit. The computer-controlled robotic handling system will be installed at all Pirelli factories by the end of 2003. A second unit will be accommodated at Bicocca by the end of 2000, to be followed by the first non-Italian subsidiary at Burton-on-Trent, UK, in early 2001. MIRS will allow a reduction of inventory as it can absorb sudden upward fluctuation demands for different sizes of tyre and it will reduce lead times from six days to 72 minutes. The company comments that in conventional tyre production, only 12% of material is processed at a given time with the other 88% in storage waiting to enter the manufacturing sequence. MIRS is so flexible that a tyre can be produced every three minutes. MIRS has been developed with the goal of promoting efficient yet localised production, which may be installed in well over 60 global sites. First production is planned for performance tyres, yet the system is capable of making run-flat tyres and the new Cyber concept tyre, which is an intelligent model capable of monitoring pressure and other parameters continually updating the driver about possible problems. It is understood that the system is aimed at a wider range of tyres than car fitments, with trucks potentially being of more interest in the Far East than the present target of various passenger types. A fast turnaround and the potential of one tyre per size being made are the major value points for the system. The robots utilised within the system are capable of handling flexible items (rather than the rigid components usually associated with robotics) and high speeds in excess of those current within the industry. They handle the entire operation through all steps from compounding to complete cured tyres without intermediate storage. The system uses a minimum of energy and very low wastage. An intriguing point is the complete lack of smell, making these small units seem totally unlike traditional tyre factories. The software that integrates all aspects of product and process design with the manufacturing environment is the secret that provides total flexibility of MIRS. It controls the movement of eight robots, the production specification, the automatic provision of materials, the tyre size and the selection of the appropriate building drum. Lower costs are achieved through energy savings, very low material wastage and the reduced number and time of the stages involved; mould change time is reduced from 400 minutes to just three minutes and the different stages for manufacture from 14 to, again, three! 22

End-of-Life Tyres–Exploiting their Value In essence, the traditional batch production of tyres has been transformed into a much more continuous activity with the consequent opportunity to greatly improve quality and consistency of the product. Tyres are produced continuously by the eight robots around the construction drums. These drums are passed from one ‘mechanical hand’ to another at high speeds without stopping or human intervention. On each drum, extruders gradually apply a strip reinforced with steel cords with a circumferential and axial element to their mode of deposition. The robotic movements allow the extruders to follow the complete profile of the desired tyre. The final assembly robot ‘hands’ the drum to the next machine that in turn feeds it to a curing press. The curing press consists of a carousel with six moulds revolving around an axis and is synchronised with the building stages to maintain a continuous cycle. Once the vulcanisation is complete, the same robot puts the empty drum back into the production sequence and the cured tyre is passed to a final stage for laser-controlled quality checks. If there is a call for a change of tyre size, the same machines are fitted with drums for the new dimensions. The flexibility of the announced system to make different size tyres without long, laborious mould changes should allow Pirelli to cut time and costs in a low margin business.

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End-of-Life Tyres–Exploiting their Value

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End-of-Life Tyres–Exploiting their Value

4 TYRE RETREADING Retreading, in one form or another, has been used to extend the life of a tyre ever since the rubber tyre was introduced to provide an extra suspension component on wheeled vehicles during the nineteenth century. Tread wear during the life of a new tyre represents a loss of only 10%–20% of total tyre weight and around 30%–40% of the actual tread compound. For suitable worn tyres, free from damage (which would affect their integrity and therefore safety), a fresh patterned tread may be fixed to the casing, thus prolonging the useful life of a valuable resource. To prepare any tyre for retreading, a further 20%–30% of the original tread rubber volume is removed by a grinding or ‘buffing’ process. This provides a suitable uncontaminated surface for bonding to the new tread material. The material that is removed during the buffing operation is a reusable commodity, which goes by various names such as buffings, raspings, and sometimes crumb. This material is sold by the retreaders to rubber moulders, crumb producers or intermediate operations.

4.1 Industry Structure and Organisation Retreading companies range in size from the small independent concern with an output of about 10,000 tyres per year to plants run by the OE tyre manufacturers which produce as many as 300,000 retreaded tyres per year. There are approximately 50 independent retreaders in the UK with a further four or five plants under the control of the tyre manufacturers. The recognised trade association for retreading in the UK is the Retread Manufacturers Association (RMA) to which about 85% of retread companies belong. However, some of the bigger, independent companies did leave the RMA to join the British Rubber Manufacturers Association (BRMA) in the mid-1990s. Since those days, the sector has been trimmed by the liquidation of some companies and the decline of others. There are now only three or four independent car tyre retreaders remaining in the UK. An important organisation in the retreading field is BIPAVER (Bureau International des Associations de Vendeurs et Rechapeurs des Pneumatiques), the International Federation of National Associations of Tyre Specialists and Retreaders. The secretariat moved from Switzerland to the UK in the early 1990s. It now has offices adjacent to the National Tyre Dealers Association (NTDA). Members are national trade associations representing tyre retailers and the retread sector. Trade associations in non-European countries, principally the United States and South Africa, may hold associate membership. At past meetings topics such as deeper minimum tread depth regulations and regrooving have been discussed. Regrooving is the practice of grooving a new pattern into the base tread rubber that remains after the pattern has been worn by use. In practice this is carried out primarily on truck and bus tyres, since regrooving of car tyres is illegal in the UK. Michelin is a long time proponent of regrooving and designs its truck tyres accordingly. However, not all truck tyres have been so designed, with a result that many inappropriate and possibly dangerous regrooved tyres have been put onto the road.

25

End-of-Life Tyres–Exploiting their Value In the early part of 2000, BIPAVER split its administration into two, with the RMA taking over responsibility for the retreading section. One of the RMA’s current projects, in conjunction with the BRMA, is to consider various amendments to the ECE 108 and 109 Regulations. It is hoped that an agreed position between BIPAVER and BLIC (Bureau de Liaison des Industries du Caoutchouc) can be found so that a common position accepted by all sectors of the tyre industry can be formally submitted to the EU by the end of 2000. Bandag has commented on the level of retreaded truck tyres in the replacement market around the world. It is 60% in the USA with 50% in Europe generally, but with Scandinavia higher than average at 65%. It is only 1% in Africa and the Middle East. The retread activity is still to be regarded as an integral part of the tyre industry as seen in Figure 4.1. This shows that the retreading process promotes the recycling of tyres as tyres, before the casing eventually becomes downgraded through use and enters the scrap cycle. Raw materials are converted into new tyres. These are then distributed to the vehicle manufacturers (as original equipment) or to tyre dealers and distributors (for replacement sales) whence they reach the vehicle user.

Figure 4.1 The Tyre Industry

26

End-of-Life Tyres–Exploiting their Value After a first life, which may end from worn tread or damage to the casing, the tyre may take one of four routes: •

to a tyre dealer in exchange for a new or retreaded tyre (this is the major route for passenger tyres),

•

to a retreader (mainly commercial vehicle and off-road tyres),

•

to a casing collector (likewise), or

•

with the vehicle for total disposal (mainly passenger car tyres).

Whether they pass through the retreading loop or not, all tyres eventually become scrap and, apart from a very few which may become raw material again (as reclaim or crumb), they will be lost from the industry. Thus, good casings for entry to the retreading process come from tyre dealers, casing merchants and vehicle users. The latter are mainly fleet operators of trucks, buses and, to a far lesser extent, passenger cars. TARRC, UK, has developed an electromagnetic method for the detection of rust in tread belts to decide whether a casing is good enough for retreading. Ongoing work involves an abrasion tester that gives the life prediction of a retread and can help assess the retreadability of new tyres.

4.2 The Retreading Process Tyre retreading consists of taking a sound used tyre, whose tread pattern depth has reached or nearly reached the minimum legal limit, and applying new rubber which incorporates a full depth of tread. Sometimes, mainly for off-the-road and some truck casings, breaker reinforcement may be added. The process can be considered in 6 stages: •

input of worn casings and new rubber materials,

•

examination and selection of tyres, inspection of other materials,

•

preparation of casings and components,

•

application of new materials,

•

vulcanisation, and

•

inspection and quality control.

4.3 Types of Retreaded Tyres The word retreading is used in a general sense and covers several ways of providing a new life to a worn casing. The processor may renew the tread area only or may retread the complete outer surface of the casing from bead to bead. When only the tread is renewed, the process is termed ‘top capping’ when only the worn part is replaced, or ‘recapping’ when the new material is extended over the shoulders at the edge of the tread. ‘Bead to bead’ remoulding applies a thin veneer to the tyre sidewalls, providing all fresh material on the tyre exterior. 27

End-of-Life Tyres–Exploiting their Value The ‘capping’ processes are generally applied to large tyres (truck and off-the-road) whereas passenger car tyres are more usually given the full ‘bead to bead’ treatment. There are various ways in which the retreading of a tyre may be undertaken. These can be divided into ‘hot’ methods, where unvulcanised material is used with curing temperatures of 150–180°C; and ‘cold’ methods, where prevulcanised tread material is applied and a temperature of around 100°C is used. Another approach would be to consider whether vulcanising took place with or without moulds or in an autoclave. •

Retreading using Unvulcanised Extrusions

The retreading compound is in the form of an extruded section. It may be supplied at a width to cover the whole area to be retreaded, when it is called Camel-Back, or in narrower strip for winding. The extruded material is then vulcanised to the casing in a patterned mould. •

Retreading by Direct Extrusion

An extruder, equipped with a reciprocating die, forms the tread pattern directly onto the compound as it is applied to the casing. The casing is subsequently vulcanised in an autoclave. •

Retreading by Smooth Tread Extrusion and Pattern Cutting

A layer of retreaded rubber is formed on the casing and consolidated by pressure. The tread pattern is cut out of the applied rubber either in the same operation, or subsequently. Again, vulcanisation takes place in an autoclave. •

Prevulcanised Tread Replacement

A pre-cured tread with pattern is prepared by moulding in a previous separate operation. A thin strip of unvulcanised cushion rubber compound is inserted between the casing and the tread rubber to form the bonding agent between the new and the old components. The tread of appropriate length is then applied to the casing and consolidated by pressure. The cushion rubber is then cold-vulcanised.

4.4 UK and EU Standards and Specifications In the UK, tyres are subject to British Standards, the requirements for which are contained in the following documents: BS AU 50

—

Specification for tyres and wheels.

BS AU 144f:1988

—

Specification for retreaded car and commercial vehicle tyres. This latest revision is now mandatory.

BS AU 159(d):1990

—

Specification for repairs to tyres for motor vehicles.

BS 2M47:1990

—

Specification for retreaded tyres for aircraft.

To comply with the Motor Vehicle Tyres (Safety) Regulations, all retreaded tyres produced and sold in the UK must now be manufactured and marked according to BS AU 144f. Indeed, the RMA requires that its members work to very high standards given in the

28

End-of-Life Tyres–Exploiting their Value Association's manual “Operational Standards for Tyre Retreading”. This is a recognised world authority on retreading techniques. In BS AU 144f, the minimum speeds of 70 mph (112 kph) for passenger car tyres and 60 mph (96 kph) for commercial vehicle tyres have been deleted. These are replaced by the requirement that retreaded tyres are marked with load indices (LI) and ply rating (PR) where applicable, and speed symbols, and that the tyres comply with the associated loads and speeds. Performance verification by rolling drum tests has now been included for the first time as part of BS AU 144f. The tests are technically identical to those of ECE Regulation 30 for passenger tyres, “Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers”. The shortened 6-hour drum test for commercial vehicle tyres is technically equivalent to the 47-hour test of the truck version, ECE Regulation 54, which is also allowed as an acceptable alternative. Criteria are included for the inspection and repair of tyres during retreading, and for inspection after the process. The evolution of European regulations has taken many years and these are aimed at enhancing the quality of retread products by formalising procedures and deterring the less scrupulous operators. The approach has been towards a type approval system as is used for vehicle and other components. Unlike type approval for new tyres, the new regulations will concentrate on approval of the retreading process. Manufacturers will, however, still be given an individual ‘E’ number that must appear on the sidewall of the retreads. In the UK, type approval is given by a government agency known as the Vehicle Certification Agency (VCA), and the most important requirement is to have in place an acceptable quality management system. The VCA may also visit the manufacturer’s factory to view the process at first hand. Once type approval has been granted, it is followed up each year. This is known as conformity of production, and involves repeat testing with a further plant inspection. The initial focus was on developing a directive for retreads through EU DG III, but evolved to support a UN ECE initiative to draft two new retread regulations (ECE 108 and ECE 109) which could serve as a basis for such a future EU directive. In the UK, the RMA welcomes these new regulations, which it believes will raise the standard of retread manufacture within Europe and may help with the poor public image of the retreaded tyre. Much of the ECE Regulations are based on BS AU 144f. Many UK retreaders already work to higher standards than the British Standard. These changes are good for the industry and should enhance its image in the eyes of potential customers. Although some EU countries are adopting the ECE regulations into their national legislation, BLIC hopes to speed the process by continuing to lobby the EU to adopt these regulations, possibly as two separate directives. Retread companies now have to be granted approvals at a national level and there has been a rapid increase in activity to obtain the quality mark across Europe. BLIC will be monitoring progress and making suggestions on how the effectiveness of the regulations may be improved. It is expected that by the end of 2002 the majority of European retreaders will have obtained the mark. After that date it is likely to become mandatory.

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End-of-Life Tyres–Exploiting their Value

4.5 Retreading in the UK Reduced sales in the UK market have resulted from: •

strength of sterling impacting on the competitiveness in export markets,

•

pressure from budget and part-worn tyres,

•

lack of public demand combined with poor perception of retreads, including within some segments of the tyre industry itself, and

•

the reduced profitability of selling retreads compared with other tyres.

A whole range of tyre sizes and tread patterns has to be catered for and retailers now hold enough stock for 3 weeks, compared to 8 weeks in the past. Retread prices to the trade are in the range of £10–20. There is less communication within the industry since wholesalers have entered the sector and some manufacturers cannot talk directly to the retailers. The closure of Sabre in the last 12 months (on top of the departure of Monarch and Technic) is another example of the declining retread market. The days of 30% of casings available being accepted for retreading of car casings are long gone, with the figure now being as low as 15%. The figure is believed to be still around 50%–70% for truck tyres. Operations are now about managed scrap collection instead of casing collection. Ten years ago, 70% of casings went through the retread system with just 30% taken by casing dealers. In 1998, it was estimated that 12% of the car tyre replacement market was still held by retreads (at some 16 million units), compared to 20% of the market in the 1970s and 1980s and 14% in 1994. The share dropped to only 8% in 1999 and is still declining. For truck tyres, 40% to 50% of casings are suitable for retreading. However, ‘Customer’s own casings’ (COC), which are retreaded for a second or even third time, would not be recorded as a replacement sale. It is understood that as many as 35% of the truck tyre casings which are retreaded belong to the COC category, where the tyres remain the property of the fleet or transport operator which has a contract for COC business with a particular retreader. Unsuitable casings are still the property of the operator and may become a further financial burden when disposal via a licenced contractor has to be arranged. The downturn in the passenger car retread market has meant that there is too little demand to support the industry capacity and this has led to closures. Technic, one of the largest independent passenger retread companies in the world before its closure during 1999, has now reopened under new ownership as Technic Tyre plc. Production is at a much-reduced level than previously. Some fast-fit retail chains have been involved with the promotion of retreads in selected outlets with some limited success. There may be some further scope to address the poor image of retreads. However, this issue does not stand alone and is influenced by a range of other factors. The quality of tyre retreads has continually improved and yet sales in the UK of car retreads have dropped by a half to two-thirds from a total of over 4 million tyres in 1995.

30

End-of-Life Tyres–Exploiting their Value Market developments, such as the emergence of many budget tyres, have had a major impact. Efforts to highlight the virtues of a retread must be accompanied by another strand of ‘market persuasion’. Indeed, efforts by the Used Tyre Working Group to encourage Government departments to use retreads on its vehicles have been acknowledged as progressing very slowly. The Department of Trade and Industry (DTI) has also discussed the scope for highlighting the benefits of retreaded tyres in minimising the impact of motoring costs. The DTI is also optimistic that these benefits would be mirrored in publications such as the Department of Environment, Transport and the Regions’ (DETR) Green Transport documents and the successor programme to the Cleaner Vehicle Task Force.

4.6 Retreading in the USA The USA, the largest market for tyres in the world, has the greatest volume of scrap tyres and the largest retreading industry. However, this industry has contracted perhaps even more than in Europe. The long-term decline in the passenger car sector has accelerated since the mid-1980s, but production of commercial vehicle retreads has increased slightly since that date and has maintained a similar share of the replacement market over the last decade. The differential in price between a retread and a new car tyre has continued to be low enough to depress retreader activity in this sector. Production has declined from a peak of 40 million retreads in the early 1970s to around 18 million in 1986, to 6 million in the mid1990s, and to under 2 million in 1999. The sector would appear to be in terminal decline, providing a derisory 1% of the replacement market. In the truck sector, where 22 to 23 million retreads a year were made in 1999, there was a gradual increase from the mid-1980s to the mid-1990s as the change from cross-ply to radial construction progressed far enough to provide a sufficient number of worn casings to supply retreaded radials to the market. Previously this short fall had been made up by imports. Light truck tyre retreading is declining but medium and heavy truck tyre retreading is increasing. However, new tyre sales are growing faster still, so that the overall retread share of the truck tyre replacement market is down from 38% five years ago, to 33%. The retread share of the light truck tyre market and the medium/heavy truck tyre market is 16% and 54%, respectively.

4.7 Future Prospects for Retreading Various organisations have expressed concern about the implications of ECE Regulations 108 and 109 in practical terms. The regulations are based on new tyre production which is very different from retreading. New tyres are made in long runs of the same type and size whereas rereads are much more individual, with different brands of used casing all with differing measurements. As a result, retreads are not suited to type approval as based on new tyre standards. It is also becoming clear that the proposals are not likely to be implemented in a consistent way across Europe. Some national authorities are requiring companies to attain full ISO 9002 accreditation whilst others have indicated that the standards would be lower than that. 31

End-of-Life Tyres–Exploiting their Value There is also concern over the lack of testing facilities. Almost all independent tyre test facilities are to be found in Germany, with limited capacity in the UK, France and Spain. There are no independent facilities in the rest of the EU. The implication of the regulations is that a capacity for the testing of 4000–5000 tyres is needed, whereas existing accredited test houses test less than 2000. In addition, the sample required for testing, 0.01%, is too small to be meaningful, especially when the individual nature of each retread is considered. A further concern is the nature of the drum tests that are now seen as bearing little resemblance to real-life experience and which do not provide a test for the retreading process itself. The test was devised years ago for cross-ply tyres and bears little relation to the outcome on the road. Work at TARRC found that the tests can generate temperatures of 100°C whereas the actuality on the road is only about half that level. This throws serious doubt onto the relevance of these tests. One major, independent continental retreader has carried out tests on retreads made from buffed-down new tyres with no road history whatsoever. The failure pattern for these is the same as for conventional retreads, and significantly, for new tyres as well. Harmonisation of the speeds required for different truck tyres in different countries should also be addressed. For instance, tyres tested in the UK are tested to 100 km/h. In France and Germany, the standard is 130 km/h for tractor units, and in Germany trailer tyres need only be tested to 80 km/h, despite the fact that trailer and tractor usually travel together at the same speed! Since the traditional price advantage enjoyed by retreads has vanished as the budget tyre prices have dropped, it is unwise to count on its return. The solution is seen by many to be a concentration on niche markets. Indeed many now believe that the retreading of 80 and 70 series passenger car tyres is a lost cause. One such niche is the environmentally aware consumer who has ecology above finance in his or her order of priorities. Such awareness is expected to increase in the future. The green argument could be carried to fleet buyers, especially companies who themselves make or market environmentally friendly products. Another niche market is that of high-performance tyres, where younger people especially are looking for a cheaper tyre, so that they can spend more on items such as alloy wheels, etc., that they perceive as enhancing their image. Other targets are the 4x4 and light vans markets and also winter tyres. The latter has traditionally been a good market for retreaded tyres. To combat the fall in outlets stocking retreaded tyres, the sector needs to respond wherever possible by improving its image in the retail trade from which should spring a more positive image to present to the consumer. This can only be done by the training of retailers’ staff, by improving point-of-sale material and by offering guarantees of longevity. After all, a quality retreaded tyre has endured several inspections and has been tested to the same standard as new tyres.

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End-of-Life Tyres–Exploiting their Value

5 RECYCLING AND REUSE ALTERNATIVES FOR SCRAP TYRES

5.1 Introduction As seen in Section 4, out of the 36 million tyres arising each year in the United Kingdom, only approximately 4 million tyres are accepted for retreading. What options are there for the remaining 32 million tyres and which of those options are currently utilised? The tyres may be reused for non-tyre purposes in such enterprises as breakwaters, barriers for prevention of soil movement, weights for silage retention on farms. This option uses the physical entity of the tyre and the very property that creates the waste problem, its non-degradability. (This option will be discussed in Section 8). Various methods of recovering the energy and materials that have gone into the manufacture of the original tyre represent several further options. The techniques available can be broadly classified under the headings of mechanical, thermal and chemical recovery. The final option is the least satisfactory and yet has remained prevalent as the estimated figures show. This is the dumping of tyres as refuse, either below or above ground. As discussed in Section 9, regulations shall forbid this disposal route within the next six years or so.

5.2 Materials Present in Tyres Even with the move to radial tyres and the increasing use of steel in the constructions of both car and truck tyres, the greatest weight in tyres is the rubber compound. This is between 80% and 85% for most types of tyre, except car cross-ply tyres in which it is typically around 75%. An indicative list of ingredients found in a typical rubber compound is given in Table 5.1. Although the polymer, as a scrap arising, may be a combination of natural rubber and different synthetic rubber, the original composition is of very little consequence. The different polymers would never be separated in later recovery operations.

Rubber hydrocarbon Carbon black Oil Zinc oxide Sulphur Other chemicals* Total

Table 5.1 A Typical Tyre Compound Parts 100 50–60 15–20 3–4 1–2 2

% by Weight 56 30 10 2 1 1 100

* includes organic vulcanisation activators and accelerators, processing aids and inorganic fillers.

After rubber, the largest component is carbon black. This is a carefully prepared material that has closely defined physical and chemical properties. Tyre tread compounds require 33

End-of-Life Tyres–Exploiting their Value a highly reinforcing black; a furnace black produced by the combustion of a highly aromatic oil, and having a surface area of about 100 m2/g, is usually used. This type of black accounts for about two-thirds of the total amount of black used in tyres. Most of the remaining third will be semi-reinforcing blacks used mainly in casing compounds. These blacks are made by the combustion of oil or gas and typically have surface areas of about 20 m2/g. Steel wire used in beads and reinforcing cords is normally coated with brass plate. This gives rise to a further source of zinc, to add to the zinc oxide compounding ingredient. The textile components may be rayon, nylon or polyester fibres. Some special tyres do incorporate polyaramid materials, as described in Section 3.

5.3 Materials Recovery from Tyres High-energy processes are used in the mixing of these ingredients, so that even before vulcanisation the material is homogeneous to the naked eye. Vulcanisation is achieved when the mix is heated under pressure. Vulcanisation changes the chemical nature of the ingredients and builds the resultant products together in a complex manner; strong chemical bonds are formed. Macroscopically, the vulcanisate is homogeneous; it cannot be separated into its component parts by mechanical action. Thus, although mechanical processes offer a relatively simple method of breaking down a tyre, the greatest subdivision that can be achieved is into bead wire, textile and steel tyre reinforcing cord and rubber compound. Solvent extraction might be used to recover the process oil from the rubber compound but any further breakdown of the rubber compound can only be achieved by thermal methods, such as pyrolysis and incineration or chemical and thermal methods of a ‘reclaim’ nature. These may partially unravel the crosslinked hydrocarbon chains, which would still retain much of the carbon black and other compounding chemicals, or, as the severity of the process conditions increased, achieve a breakdown of all the large chain molecules into simpler chemical species.

5.3.1 Mechanical Methods of Recovery Three variations of mechanical means of breakdown have been utilised: •

mechanical breakdown only,

•

swelling followed by mechanical breakdown, and

•

freezing followed by mechanical breakdown.

Although all three methods are feasible, only the first is commonly used. The second has the disadvantage that solvents must be recovered or safely discarded. The third possibility became a consideration once large quantities of freezing media became available. Freezing usually involves liquid nitrogen which has great freezing capacity but is very expensive.

34

End-of-Life Tyres–Exploiting their Value

5.3.1.1 Mechanical Breakdown Only This method employs a sequence of shredding and cracking operations, particle separation and further grinding to provide a mixture of steel, textile and rubber. The rubber may, or may not, pass through further comminution stages to provide crumb particles of finer and finer particle size. A variety of magnetic, sieving and air flotation equipment is used to separate the various component particles. The increasing proportion of steel in tyres and the virtual elimination of cross-ply tyres have created the need for stronger equipment and the consequent consumption of greater amounts of electrical power to run the processes. These techniques are discussed in Section 6.2.

5.3.1.2 Swelling Followed by Mechanical Breakdown Both the strength and resilience of a rubber vulcanisate are reduced when that vulcanisate is swollen with a rubber solvent. The application of swelling techniques to assist the breakdown of tyres was the subject of considerable interest in the past but has seen little development recently. As expected, the use of swelling techniques does facilitate the mechanical breakdown of tyre waste (for example, swollen tyre waste can be broken down in a ball mill) but it is very doubtful whether whole waste tyres can be handled in this fashion economically. Suitable solvents include aromatic or chlorinated hydrocarbons. Any reappraisal of these techniques must make a careful study of the balance between energy savings and the costs of the (oil-based) solvents needed. Patent applications for such processes are occasionally seen, even though the use of volatile solvents is strictly controlled and discouraged in many applications. Another promising area for further investigation has been the use of waste mineral oils such as engine oil for similar processes, but none would appear to have a promising future. The potential fire hazard associated with the mechanical handling of any swollen scrap should not be overlooked.

5.3.1.3 Freezing Followed by Mechanical Breakdown Cooling a rubber to below its glass transition temperature reduces the rubber's resilience and consequently reduces the amount of energy required to break it down mechanically. The promise of cryogenics was that the technique would render all the components of a tyre more brittle. Various pilot plants have shown the technical viability of cryogenics, although none are used for tyres at the present time in the UK. Operations that successfully employ these methods are active in North America. The techniques now available would seem to have addressed the ‘problem’ of the high cost of liquid nitrogen, and of the need for a regular throughput of tyres to minimise the consumption of this expensive item. Systems on the market today are receiving a mixed reception, but the technique can no longer be discounted. More detail of such systems may be found in Section 6.2.3. 35

End-of-Life Tyres–Exploiting their Value

5.3.2 Thermal Methods of Recovery 5.3.2.1 Combustion The thermal decomposition of tyres does have the advantage over mechanical methods of breakdown that the compounding ingredients of the rubber become temporarily accessible for recovery. Materials that are potentially reclaimable by scrap tyre incineration include zinc oxide and carbon black, together with the steel of bead and cord wires. Recovery of the carbon depends on the temperature of operation of the incinerator. A cyclone incinerator designed to give an off-take temperature of up to 1000°C will produce an ash that contains about 30% carbon. However, little interest has been shown in the recovery of carbon by this method. More attention has been paid to achieving complete combustion of this carbon in order to improve the thermal efficiency of the furnace. Most operations that incinerate tyres do so as an efficient, or at least very convenient, method of process steam raising. A variety of furnace designs exist for the combustion of tyres and are covered in Section 7.3. Specific end uses for the combustion gases and their heat are also covered in Section 7.

5.3.2.2 Pyrolysis Pyrolysis is the non-oxidative thermal decomposition of material and has been studied extensively in various countries, including the USA, France and Germany, as well as in the UK. In practice, pyrolysis covers all the processes where the combustion process is incomplete, giving rise to the production of various combustible products ranging from carbon to gases, plus quantities of steel and ash. The thermal destruction of waste tyres should be economically viable because: •

the products obtained (liquid hydrocarbons, coke residue or carbon black and gas) have properties that would allow their easy reuse as a raw material and fuel,

•

the process can be energetically self-sufficient as a proportion of the gases, and sometimes, the liquid products serve as fuel,

•

compared with incinerators, the investments are lower as the process uses lower temperatures, operates without air and does not produce large quantities of exhaust gases requiring very complicated purification,

•

the liquid and solid products obtained can be easily transported or stored and therefore need not be used immediately,

•

the process conducted at over 350°C (although usually much higher in practice) produces low-molecular hydrocarbons which can be used for the production of many final products, and

•

it is not necessary to grind used tyres very small, hence saving energy and labour, and all types and size of tyres can be recycled.

36

End-of-Life Tyres–Exploiting their Value It has been found that the nature of the starting material (e.g., car or truck tyres) or its degree of comminution (whole, quartered, shredded, powdered) has little effect, but that pyrolysis temperature has a major effect on the composition of the reaction products: the higher the temperature, the greater the percentage of gases (and in some cases solid residue), while the liquid component would decrease. The liquids would be available as a quite complex set of distillates containing different amounts of aromatic and olefinic fractions. Economics and perceived end uses would normally determine whether multiple distillations would be viable. Pyrolysis is considered further in Section 6.4.

5.3.2 Chemical Methods of Recovery Chemical reaction with the unsaturation of the rubber hydrocarbon has been used to facilitate breakdown of the tyre. These reactions require comminution of the tyre to achieve adequate contact between the reactants. Recovery of a material resembling unvulcanised compound is known as reclamation. There are many processes involving the action of chemicals, with heat and mechanical work, to attack and break down the vulcanised structure. The attack is neither complete, nor selective and the resulting product is a sticky, partially devulcanised consistency. Most of the processes and the equipment on which they were carried out could not cope with the advent of large numbers of steel reinforced tyres. The steel in both casings and belts would not break up in a consistent fashion on machinery designed for textile reinforcements. The additional process stages that would have to be added made many plants uneconomical, with a result that many closed in the late 1970s and early 1980s. However, new approaches to this problem have been developed over the last few years. Some of these are discussed in more detail in Sections 6.5 and 6.6. Other processes are really chemical treatments for mechanically prepared crumb, for example, the Tirecycle process, Symar-D from Canada, Vistamer technology from the USA and AMR from China, which are described in Section 6.2. Various institutes, mainly in the USA and within the countries of the former Soviet Union, have been investigating reactions with oils, ozone and also high pressure water. The specialist and sometimes hazardous nature of the reactants are likely to prevent the emergence of any commercial venture based on these techniques. The ventures addressing the ‘feedstock’ approach for gasification methods presented in the last edition of this book have not come to fruition. Just as they were likened to the chemical refinery approach for handling volumes of plastics waste, their non-emergence parallels the demise of many of the plastics ventures by some of the petrochemical majors.

5.4 Disposal of Whole Tyres The disposal of tyres as refuse is the last option and one which will be given due consideration in Section 9. The due handling of used tyres is arguably both the start and end of a tyre's journey as a post-consumer article. The route of dumping tyres is one that creates the largest problem, both environmentally and commercially. In the absence of 37

End-of-Life Tyres–Exploiting their Value regular outlets for used tyres into the previous recovery modes, the tyre dealer, retreader, local authority and individual citizen has only this course of action. In an ideal world where the materials in a tyre would be wholly reusable, dumping would indeed be the last resort. Too often it has been the action of first resort. This has been changing under the pressure of environmental views and concerns about the squandering of useful materials. Most of the more developed countries now have strategies for handling waste and some are developing suitable techniques and markets. This has culminated within the EU with a range of legislative Directives and other regulatory instruments. Guidelines and mandatory procedures are laid down in the UK under the umbrella of first the Control of Pollution Act and then the Environmental Protection Act. These formally control the handling of tyres and provide the framework within which Local Authorities operate. This framework includes duty of care provisions and the licensing of carriers for waste tyres. The tyre industry still is very concerned about instances of mysterious tyre mountains appearing in unacceptable places and causing problems that detract from the efforts of the majority. The implementation of a Landfill Tax in the UK has concentrated minds even further. The existence of Landfill Tax Credits that can be used to exploit and develop recycling techniques and strategies has also moved the industry forward.

5.5 Tyre Shredding The shredding of tyres is most likely regarded as a prerequisite for many of the routes considered in the preceding sections. Shredding into lumps would greatly facilitate many of the operations that make up the recovery of materials and energy. Indeed some, such as the production of crumb, require a first stage shredding anyway. Fuel for furnaces and feed for pyrolysis plants should be shredded, and transport to landfill sites becomes more efficient with shredded material. Landfill sites in many countries now only accept shredded tyres. The consideration of treating tyres in the vicinity of their first disposal in small operations has to be set against the transport requirements to carry large enough volumes to the site of bigger operations such as power stations, kilns or chemical works. The concept of a network of shredding stations, as discussed in previous reports, has to some extent come about with the emergence of large waste management companies as major players in the used tyre business. The opportunity to use these shredded tyres rather than take them to landfill has to be encouraged. An awareness of potential sources of such material and techniques for their use has to be broadcast in the market place. A non-exhaustive list of companies who can offer shredding and granulating equipment is given in Appendix B.

38

End-of-Life Tyres–Exploiting their Value

6 TYRES AS A SOURCE OF MATERIALS

6.1 Introduction The use of tyres as a source of materials will be examined in this section, with discussion of the techniques employed to extract the materials and the applications for the resultant products. Great changes in thinking have taken place in the tyre and rubber industry over the last twenty years. Until the mid-1970s it was still possible to achieve the recovery of the ‘constituents’ of a tyre, primarily for reuse in the tyre industry, i.e., a reasonably true recycling for original purpose. Therefore, reclaim rubber and tread crumb were given due prominence. Since then, the high consumption of textile reinforcement has been reduced in favour of steel leading to the well understood difficulties for the recycling industry. Higher standards and specifications for materials and tyres themselves are the result of continuous design and manufacturing development. These factors, combined with wider economic issues, had pushed the emphasis in other directions (not recycling). However, environmental concerns that arose during the 1990s have demanded a reappraisal of the need to reuse previously discarded sources of raw materials in some form. Various routes being studied and implemented are described in the sections that follow. Various processes are involved which utilise heat, pressure, mechanical work, freezing, chemical action and differing combinations of these. The materials recovered include solids, liquids and gaseous products that may or may not be reused within the tyre and rubber industries.

6.2 Crumb Production and Use

6.2.1 Introduction The term ‘crumb’ is generally used by technologists to cover any rubber compound that has been reduced to a powder. In the context of this report, however, it refers more specifically to a granular material produced from waste rubber, especially tyres. There are three main types of crumb: •

Buffed crumb is a product of part-worn (not used) tyres. When a tyre is submitted for retreading, rubber compound is first removed from the tread in the form of ‘buffings’ or ‘raspings’. These buffings, which would otherwise be waste, may be ground and sieved to a series of particle sizes. Since it is removed from the tread, buffed crumb is free from tyre reinforcement fabric or steel and comprises a high-quality, abrasion-resistant rubber compound. Buffings remained the traditional source of crumb in the UK, and other countries, until production by direct tyre crumbing started to increase again in the last few years. 39

End-of-Life Tyres–Exploiting their Value Shortages have sometimes occurred as buffings are a by-product of the retreading process and thus availability is limited by the amount of retreading which is undertaken. •

Whole tyre crumb produced by room temperature (ambient) methods is manufactured directly from scrap tyres. Typically, tyres are cut into large pieces or shredded to about 3–5 cm in size. The chips are passed through a coarse cracker mill or cutter, and then through fine grinding mills and screens. Commercial roll-mills have also been widely used. Such methods changed little over the years until the more recent advent of purpose-built machines and systems designed to handle reinforced rubber. The shredding of steel-containing tyres is energy-intensive, with rapid wear and tear of the equipment. Many operations were equipped to handle textile-reinforced tyres and found it impractical to process the steel-braced radials that formed the major proportion of the UK's supplies of used tyres by the late 1970s. The production of whole tyre crumb by ambient grinding methods was a preliminary step in reclaimed rubber production in the UK; the advent of steel-braced tyres contributed to the demise of the Dunlop and Uniroyal reclaim plants. Since those days, the problems posed by steel reinforcement have been overcome. Several continental plants utilise ambient grinding techniques for steel-braced tyres. In the UK, various companies handle tyres as a feedstock for crumb production, although commercial and fiscal pressures have led to some operations being closed, or at least mothballed. Others that were expected to be coming on-stream have gone silent, and the prospective proprietors probably have moved onto more lucrative ventures.

•

Whole tyre crumb produced by cryogenic methods is also made directly from used tyres. Cryogenic processes had become attractive because of the ability to readily handle steel-reinforced casings and also because very fine powders down to 100 mesh can be produced. The main drawback has been the high cost of liquid nitrogen; operational problems have also hindered the development of the process. Production plants were set up in several countries but have often been short-lived. Cryogenic techniques are based on the principle that vulcanised rubber becomes brittle at low temperatures. After the cooling of tyre fragments by, usually, liquid nitrogen, the glass-hard rubber is expelled into a hammer mill or fragmenter where it is reduced to powder. At this point the reinforcement is separated from the rubber. Further comminution is carried out if required. Major producers of industrial gases (like BOC, Air Products and Praxair) have been instrumental in the development of cryogenic grinding processes for scrap rubber in several countries.

6.2.2 Equipment and Production Techniques There are now a large number of companies that offer size reduction equipment for the waste and recycling industries. Much of the equipment is not suitable for tyres and so plant has to be selected with caution. Experience has shown that many processors need to make custom alterations to some or even all stages of equipment they buy. This would appear to be especially the case where units reasonably suitable for car tyres are pushed 40

End-of-Life Tyres–Exploiting their Value into service for truck tyres for the majority of the time. Good engineering skills are a prerequisite in tyre crumbing operations. Indeed, some processing companies have specifically set out to capitalise on these skills by establishing a machinery operation to offer their expertise to third parties. As all processors know, removal of steel wire and (textile) fibre is very important and many are finding that their customers demand fibre-free product. The first step in a large operation is a magnetic separation to remove loose steel. Then the rubber is cut into 50 mm ‘high steel' chips (some operations may stay at 100 mm shreds) which are eventually reduced to 6–8 mm chips that are 99% steel-free. Initial reduction is by shredders, maulers and hammer mills. Shredders are designed for processing tyres and probably are the best method, despite sometimes having problems with the volume of tyres that can be handled. Maulers on the other hand can handle large volumes but suffer from malfunctions because they were not designed for tyres. Hammer mills are best suited for processing the 50 mm and 8 mm chip sizes in the secondary and granulation reduction stages, despite using a large amount of energy and needing frequent maintenance. A few comments on some equipment and process developments are given in this section but, necessarily, the discussion is not exhaustive.

6.2.2.1 Machinery Developments Columbus McKinnon Corporation (CM) has for many years refined its range of tyre shredding equipment. CM is regarded as a world leader in the primary reduction equipment for turning whole tyres into 5 x 5 cm and 2.5 x 2.5 cm chips. Chips of these dimensions have been found the ideal size for tire-derived fuel (TDF) and a variety of civil engineering applications. These are also the usual feedstock for a granulation operation. CM is at the forefront of the continual improvement of technology in terms of long-term reliability and increasing cost-effectiveness. CM systems are believed to process over 100 million tyres a year across the world. The bead, especially in truck tyres, is difficult to remove from the surrounding rubber. One of the most difficult elements in processing scrap tyres are the steel reinforcements (bead and wire). A typical plant may require two, or even three, debeaders to keep up with the shredder stage of the equipment, the result being non-too-clean steel, which has a lower scrap metal value. The removal of loose steel, whether from bead or reinforcement, is accomplished by the use of magnetic separators. All traces must be removed prior to a crumbing stage otherwise serious disruption can occur as cutters and knives are subsequently damaged by its presence. However, the removal of the steel also sees the loss, in the worst cases, of as much as 20%–30% of material as rubber remains adhered to the steel fragments. To counter this, CM introduced its Zero Waste process, see Figure 6.1, which is capable of separating clean liberated bead and tread steel from the contaminated steel fragments with rubber still attached to them. The CM Wire Screen accepts material directly from the magnetic separator, eliminating the need for any additional conveyor systems. It screens out all loose steel fragments and recirculates all steel fragments with rubber attached back 41

End-of-Life Tyres–Exploiting their Value to the shredder for additional reduction, causing liberation of the steel and rubber fragments. All steel removed by the screen is clean enough to be sold to the steel industry, while 100% of the rubber from the tyres can become TDF or feedstock for granulate.

Figure 6.1 Columbus McKinnon Corporation’s Zero Waste Processing System Reproduced with permission from Tire Technology International, 1996, 185. So, at this stage, 100% of the tyre is recycled. A double benefit is achieved by the processor; the expense of landfill for the useless contaminated steel is avoided and income from the sale of clean steel is received. The further bonus claimed by CM is the elimination of any debeading requirement. It could be argued that with such a system the costs of shredding would rise. Calculations have shown that there is a very marginal increase, mainly through the purchase of a few extra sets of shredding knives per year. The savings from the sale of the steel and the elimination of the requirement to buy and operate vastly outweighs this. CM believes that this new advance in primary processing will prove itself in the years ahead and be the most cost-effective way to remove bead wire and tread belt steel from rubber tyre chips. The company’s rotary shear shredder design is based on the patented Holman technology (the patent covers the proprietary method in which the knives are clamped to the rotor). The CM chipper, born of a customer requirement to regrind primary chips in high volumes

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End-of-Life Tyres–Exploiting their Value and to specific sizes (yet not so small as to become crumb), uses the same basic design and cutting technology. Customer feedback has led to the latest design of CM shredder that can cope with super singles and other large truck tyres to provide a reliable machine for cleanup activities by local authorities and to serve the recent growth in civil engineering demand for chips. An adaption for one particular customer can provide chips in the 10 cm size range with no fines. The new design features a new knife configuration that will allow operators to get more use from any one knife. As the blades wear, operators can move them to other positions within the machine to remain effective for the shredding operation as a whole. CM also has an agreement with another US shredding equipment manufacturer to exclusively offer its rotary shredder within Europe. Barclay Roto-Shred Inc., California, has signed up to sell the Barclay 6 3ULPDU\ 6KUHGGHU WKDW FDQ KDQGOH RYHU  WRQQHV RI tyres (about one million car tyres) before any blade resharpening is required. The patented side knife design allows the knives to be resharpened, giving up to five uses or 50,000 tonnes per knife set. The Barclay machine can process super singles and larger truck tyres with a modest 50 horse power (HP) (37 kW) motor. 

Eurectec, California, offers granulation and product manufacturing plant based on a mix of European and American technology. The Tiremaster System can process 6000 t/y into crumb with sizes 5 mesh to 100 mesh. A Shredmaster 125 is placed in line with a C6000 Granulator. Downstream, a PressMaster 200 unit would take the crumb and prepare mats, tiles, etc. The machine can prepare mat-like items up to 1.4 x 4 m and up to 20 cm thickness. The travelling bed of the press can be subdivided to make products of smaller dimensions. In 1998, the EGS System was introduced. Tyres are fed to a Shredmaster 62-40T, truck tyres after prior debeading in the Beadmaster, which shreds all the tyres into nominal 5 cm pieces at around 5 tonnes per hour. A Grizzly shredder then reduces these pieces further, to shred of dimensions of about 6 mm, 18 mm or 25 mm, depending on the size of machine and the settings chosen. Magnetic separation of the free metal, pneumatic separation of the textile fluff and a further removal of residual stones and grit now take place. The small shred, now free of most of the steel and textile, passes into the Granmaster granulators. The shredders, with 5 t/h throughput, are capable of processing a sufficient volume of material to feed two granulator lines, each with a capability of 2–3 t/h. The first granulator in a line reduces the shred to 2–4 mm crumb, and also removes the final remnants of steel and textile. The second Granmaster reduces the crumb to 0.5–2 mm. Finally, when necessary, a third unit reduces the feed to 500 micron and finer. To give a very high purity and even finer particle sizes, a unit called the SuperCollider may be used as a fourth stage. The Impact 500 SuperCollider reduces the crumb to powder (80 to 100 mesh (0.17–0.10 mm)). To further enhance the value of the crumb another machine restores some of the former elastic properties of the rubber. Rotary Screenmaster screens separate the crumb from the various granulators into a range of mesh sizes (typically 5, 10 and 35 mesh) for bagging and shipping. Hermann Berstorff offers a co-rotating twin-screw extruder equipped with so-called kneading or grinding blocks to convert pellets of 2–4 mm in size into powder of 400 μm or less. A plant outline consists of a conventional shredding plant followed by a grinding mill 43

End-of-Life Tyres–Exploiting their Value station equipped with grooved or toothed rolls and with a high degree of friction between the pair of rolls. The extruder permits a two-stage grinding operation whereby the particles are removed from the machine and passed through a cooling loop before being fed back into the second grinding stage. Further cooling is needed after this last grind to avoid coking or spontaneous ignition. The shredder stage can process approximately 3 tonnes per hour of tyres and the pelletiser is designed to produce about 1500 kg of crumb particles per hour. An extensive fibre removal operation is then carried out before the rubber is fed, by gravimetric loss-inweight conveyors, into the extruder. The extruder can be run at a maximum of 450 kg/h (although some data given appears to indicate a maximum of only 300 kg/h). The Neatworld ST100E shredder is a 32-tonne machine that is powered by two 75 kW electric motors to drive heavy duty shredding knives that make it possible to process all sizes of tyre casings. The machine has a capacity of 10 tonnes per hour. Garb-Oil and Power Co., Salt Lake City, is a subsidiary of the Garbalizer Corp., of America and was set up nearly 30 years ago to exploit and sell processes developed and researched by Garbalizer. These processes consist of patented off-the-road tyre processes, crumb rubber manufacturing plants, pyrolysis, waste-to-energy, co-generation, and other power generation techniques. The company sells the appropriate equipment and collects a royalty from the operators. It uses patented technology owned by its sister company, Garbalizer Machinery Corp., in its crumb rubber operations (both ambient and cryogenic), and has been developing projects in Spain, Italy and the USA. It uses various engineering firms on a development and construction basis. A Russian engineering concern, JSC Tushino Engineering Works, now offers turnkey plant for the production of crumb from scrap tyres. After a shredder reduces the tyres to 20 x 40 mm pieces, a hammer mill reduces the chips to 10 x 20 mm particles. The third stage consists of a feed-screw extruder for the preparation of 0.5 mm crumb (about 40 mesh). Link Pty Ltd., near Brisbane, Queensland, has being making and supplying dedicated and custom-designed tyre reduction machinery for some time now. It offers a range of turnkey plant but also complete recycling plants. It manufactures shredders, grinders, granulators, de-beaders and cutters and is best known for its innovative one-pass system that produces steel-free 20 mm chips. The PS-series shredders can take whole tyres down to uniform shreds of around 50 mm in diameter. An ambient grind system has been especially designed to make 30 mesh crumb. The company has agents in Europe and North America and has installed complete tyre recycling plants in Australia, the USA and Spain. Many organisations are looking for suitable baling machines to compact tyres for transportation and for use as a civil engineering ‘raw material’. This requirement can be satisfied by a large machine developed and made in the USA. Encore Systems, Inc., Minnesota, offers a portable baler that converts 100 car or light truck tyres into an approximately one-tonne block. The WTB-58P baler weighs 5770 kg and measures 7.2 m in length, 3 m high and 2.5 m wide. The blocks made are 0.75 x 1.25

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End-of-Life Tyres–Exploiting their Value x 1.5 m, and have been used for lightweight fill for construction of low-lying roads and for erosion control. Benefits include thermal insulation, increased drainage and cost savings. A static unit from Autotreads & Accessories, UK, can turn 20–25 car or light van tyres into a bale only 70 cm wide. The TB-20 compresses the tyres by a sidewall to sidewall action into a cylindrical bale for easy handling. An optional ‘mobile package’ allows the baler to be easily transported to site. Konings Rubber Technology, the Netherlands, offers turnkey recycling plants for ambient grinding of steel-belted tyres. Capacity of 1.5 tonnes to 5 tonnes per hour produces crumb from 16 mm after a first stage down to 80 mesh or smaller after a second grinding stage. The company also offers the additional features necessary for surface activation, the preparation of rubber compounds and incorporation into plastics formulations. Processing machinery for the downstream manufacture of rubber products such as mats sheets and tiles is also available. Svedala Industri AB, Sweden, is a well-known maker of industrial processing systems and has increasingly become involved in waste processing and recycling activities. It is best known in the waste tyre field for its pyrolysis developments in the USA (see Section 6.4.2). The company does, however, offer size reduction equipment that can be utilised to shred tyres and other solid wastes. It also has experience in waste combustion systems. MeWa Recycling Anlagen, Germany, offers tyre recycling machinery ranging from firststage shredding or cutting, through plant for the preparation of coarse granulate for thermal recovery, to plant for crumb and powdered rubber manufacture. NIMBY Srl, Italy, offers shredding and granulation plant to produce rubber powder and granulate from 500 μm up to 3 mm in size. The shredder produces 100 mm pieces suitable for cement kiln fuel. These pieces are also the feedstock for a granulating mill that produces 5–15 mm crumb. A third stage constitutes a pulverising centrifugal mill to generate the final particles. Préciméca, France, makes a wide range of waste reduction equipment. For the tyre sector, it makes a series of twin-rotor rotary shears, all with hydraulic drive. Power input ranges from 55 kW up to more than 200 kW. Small machines can handle 2 to 5 tonnes per hour and a mobile version can handle up to 20 t/h. The largest machine can shred around 50t/h. The shears can be equipped with two types of rotor; either fitted with monolithic blades containing one or more teeth, or blades with 8–14 replaceable tips per blade. It only takes a few minutes to replace worn or broken tips. Shaft bearings are separate from the machine body, so offer the advantages of a longer, protected life and the rapid replacement of shafts when necessary. MAC/Saturn Shredders bought Granutech Environmental Systems in the mid-1990s, which was run as a separate company until the beginning of 1999. The increasing call for integrated systems rather than individual pieces of plant brought about a consolidation into Granutech-Saturn Systems, which now offers one of the most comprehensive product lines for tyre reduction. The product portfolio covers all stages from a basic Saturn HT shredder to an M-series machine with power ratings from 20 HP to 400 HP to the Granutech Grizzly, a G-3 granulator and cracker mills.

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End-of-Life Tyres–Exploiting their Value Most customers just take a shredder, to aid disposal of tyre wastes, but the modular approach does allow the addition of granulation equipment at a later time. After an initial shred, the Grizzly takes the shred down to less than 15 mm with the removal of over 50% of the wire content. The third stage involves feeding those 15 mm chips into the 2000 HP G-3 granulator that reduces the chips to 4 mm, 6 mm or 10 mm depending on the screen installed. Fibre separation occurs after this stage. Stage 4, granulation, occurs through high output ‘powderisers’ that will reduce the crumb to less than 2 mm and completely separate any remaining wire and fibre. For high value markets when 40 mesh or finer is demanded, cracker mills are installed as a final, fifth stage. The French company, CIMP, has a demonstration line at Perrone. The level of output required determines the number of stages and size-reduction machines. Two types of granulator are available and different shredders. Ten thousand t/y throughput of tyres would call for 4 or 5 machines dependant on whether truck tyres had already been partly shredded beforehand. Such a unit would produce 6000 t/y of 1–4 mm granulate. To reduce the output particles to a finer mesh would require extra mechanical mills or cryogenic systems, according to the company. Eldan, Denmark, has been associated with rubber recycling for many years. It offers a modular approach, with units to cover various stages of size reduction, to provide a combination to suit the particular customer. The range includes the Super Chopper SC 1412T, the Eldan Heavy Rasper HR 122T, and the Eldan Granulator FG 952. Various classification and separation modules are available to remove the steel and textile and to sort the granulate as required. Tire Resource Systems, Inc., USA, makes a range of primary reduction and tyre handling plant. This covers debeaders, balers and portable cutters that can handle off-the-road and huge mining tyres up to 1.3 m wide and over 3 m in diameter. The cutters use a two-blade scissor action and are available in either stationary or portable form. Metpro, Wales, has designed and developed a range of heavy-duty machinery to shred tyres and also makes a complete system to shred and granulate to produce rubber crumb. A large rotary mono-shear can process 6 tonnes an hour of truck tyres to 20 cm pieces. A further pass, or a secondary shear, will reduce those pieces to the 5–8 cm size acceptable for TDF. The machinery blades wear well and need changing every 4–5 months. SSI Shredding Systems in Oregon has developed a purpose-built machine for shredding tyres to join the long line of machines that the company have made for nearly 20 years. The 3000-ESP is available in 200 or 250 HP direct drive and includes various patented features. They come as standard with oversized components, extra large shafts and special bearing protection. Proprietary cutter hard facings offer much longer cutter life than is typical for such machines. Suppliers of cryogenic plants for the granulation of tyre rubber are covered in Section 6.2.3.

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End-of-Life Tyres–Exploiting their Value

6.2.2.2 Process Developments Thermal Flux Corp., USA, has commercialised a method of recycling solid tyres and their metal rims that does not damage the rubber or leave minute pieces of rubber fouling the metal. Using a new magnetic heating treatment, Thermal Flux delaminates rubber and polymers from the metal rims without distortion of either the metal bands or the rubber. The tyres are sorted by size and the rubber is cut in two places. The prepared tyres are placed on magnetic tables, called Flux Furnaces, which use alternating magnetic fields to heat the steel until it expands slightly, breaking the adhesive bond and pushing the rubber off. The metal bands are recycled for use in the manufacture of after-market tyres and for scrap metal. The rubber is sold-on to be reclaimed as crumb rubber for use in various applications. Another route has been patented by a joint venture established by Huron Recovery, USA, and Solid Tire Recovery Inc., Canada. This is claimed to use no heat, no acids and to produce no waste or dangerous by-products. A processor, called the ‘Stripper’, generates clean and consistent rubber and the high-grade steel from the rim is acceptable to speciality steel mills. A solid tyre manufacturer, Maine Tire Inc., is also involved with the process development. The Berstorff SSSE (solid state shear extrusion) process (first described in the last edition of this report) has continued to be developed at the Polymer Reclamation Center at Northwestern University, IL. A commercial version of the original co-rotating twin-screw extruder has been built by Berstorff Maschinenbau GmbH, Germany. The ZE-90 (diameter of 90 mm) is a scale-up of the prototype 40 mm version. The new tyre recycling line is designed as a 3-step process. The first step is a tire cutter and shredder that removes the bead and cuts the casing into pieces approximately 85 x 50 mm each. The second step is a crushing calendar which further reduces tyre pieces to chips of about 6 mm while steel wire and textile are separated. The third and final step is the pulveriser from which powdered rubber emerges to be classified and bagged. The SSSE process can produce a controlled particle size powder of unusual morphology by varying screw configuration and processing parameters such as feed rate and screw revolution speed. This mechano-chemical process causes chemical bonds to rupture which leads to partial devulcanisation. The morphology of the particles was mostly an open, cauliflower-like one with particle size typically 100 micron in diameter with a large surface area. This is expected to be advantageous in any subsequent processing for the rubber as an ingredient in co-mixing with waste plastics or virgin rubber. The development team believed that co-pulverisation with post-consumer plastics produced a better blend than the subsequent mixing of separately made powders. Future effort was to concentrate on this aspect of recycling of rubber granulate with polyolefin scrap. Theoretical aspects of the process have also been the subject of a development programme at the Illinois Institute of Technology in Chicago, based on experiments with a single-screw laboratory extruder. The high stress induced in the ‘thin films’ of polymer does cause partial devulcanisation and irregular shapes of the resultant particles. Both can aid the later reformulation of the materials into further compounds. 47

End-of-Life Tyres–Exploiting their Value Heat transfer is a critical factor when scale up is considered for a commercial system. The stored energy during compressive shear is converted into heat when microcracks are formed in the material. In addition, some energy is consumed during the partial devulcanisation and the creation of fresh surfaces within the elastomers. During the pulverisation, the temperature can rise to vulcanisation temperatures and so cause agglomeration of rubber particles by the recombination of the freshly devulcanised surfaces. Therefore, a very efficient heat removal system is needed to ensure that production of fine particles takes place. Reprocessing of material by further passes through the SSSE extruder has the effect of causing the breakage of big particles and the agglomeration of fine ones, thus making the particle size distribution narrower. The mean particle diameter also reduces, by up to over 25% after four passes. Particles in the range of 40 μm to 1700 μm are produced, with more than 70% having a diameter less than 500 μm. Based on crosslink density and the gel fraction measurements for particles of different sizes, there may be an optimum particle size produced by the SSSE with sufficient devulcanisation with minimum degradation of the rubber powder process for a specific application. A further process, based on very high shear, has been proposed by a small organisation based at the University of Kent, UK, and is under development by Watson Brown HSM Ltd., the firm set up to exploit the original ideas. This technique has already been found to cause at least partial devulcanisation of the materials so far tested. Both a batch mixer and a screw-based continuous mixer are envisaged to generate the high shear required to break a crosslink, but not the polymer chain segments themselves. The process relies on getting sufficient elongational shear to the central areas of the polymer chains to break some of the crosslinks present, whilst excluding oxygen or other radical-acceptors that may terminate the radicals generated on the polymer segments by those ruptures. The initial batch mixer has managed to solubilise a wide range of vulcanisates back into a raw stock based on the polymer from which the vulcanisate was originally made. Tyre materials tested have already been in a crumb form (size unreported). Moving part nips are required to be only a fraction of a millimetre. The cooling required has been found to be an order of magnitude greater than that of a conventional internal mixer. The time taken to achieve some solubilisation has been generally less than 10 minutes.

6.2.3 Cryogenic Developments A cryogenic method for the grinding of scrap tyres has been developed, in the Ukraine, that is claimed to reduce the energy required for the process by a third. A careful consideration of the low-temperature physics of gases and the generation of those low temperatures has enabled this process to become an engineering reality. The energy required to grind 1 kg of tyre at cryo-temperatures is given as 7 kJ and the energy to cool the rubber down as 321 kJ. However, a 95% heat exchanger efficiency within the system reduces the latter figure to 16.5 kJ. The total figure is therefore 23.5 kJ, but, taking the cryogenerator efficiency as only 10%, the total energy consumption on 48

End-of-Life Tyres–Exploiting their Value cooling and grinding, assuming leakage is zero, will be 235 kJ/kg. Figures for existing cryogenic grinding systems are at least 864 kJ per kg of rubber. Other work from the Ukraine is described as cryogenic electrohydraulic crushing as a result of electropulse impact wave generation in liquid nitrogen. A three-stage process has been designed to grind tyres without the need for preliminary shredding. Pulsed ultra-high hydrostatic pressures are generated in a liquid, here the liquid nitrogen, which then acts as the source of impact on the solid material within the liquid. The grinding forces are generated by the impact wave, by the high-speed flow of the liquid, by ultrasound vibrations and by cavitation effects. Such crushers always work more effectively on small particles so that at ambient temperatures preliminary cutting to a size of 5 cm is required. Since the arising impact wave is effectively repulsed from the rubber, the grinding of large rubber artefacts is impossible. However, work at the Iyal Institute of the Ukrainian Academy of Sciences showed that impact waves generated by pulsed electrical discharges can breakdown the solid tyre material. Ten pulses of 15 μs duration each releasing 20 kJ discharged at 40 kV were found to ‘disintegrate’ a quarter tyre into rubber particles of 1 mm or less. Since whole tyres could not be disaggregated by this one step alone, a three-step crusher process was proposed. This takes place in one vessel that can be operated continuously. A tyre is fed into the top of the vessel and cooled by the liquid nitrogen to embrittlement temperature at which it is shattered by a punch. ‘Lumps’ of the broken tyre drop under gravity into the lower portion of the vessel where they undergo the electrohydraulic crushing described in the previous paragraph. The size of the particles produced is controlled by the rate of the ascending flow of cryogenic liquid in the crusher and the graded holes in a classifier. A suspension of small particles is pumped through a separator and the liquid returned to the crusher. The energy necessary for the grinding of a 32 kg tyre is no more than 250 kJ. This amount of energy can be produced with an impact pulse of 3 Hz frequency and an energy of 21 kJ acting for 4 s. This time is sufficient to allow the punch to break a tyre into pieces. The productivity of the crusher is claimed to be 900 tyres per hour. This is an order of magnitude higher than that of corresponding mechanical crushers. An extension of this work, at the Ukrainian Institute of Nuclear Research in Kiev, has utilised the equivalent ‘photohydraulic’ effect to generate very high-speed pulses to initiate brittle fracture at room temperature. This eliminates the necessity for cryogenic equipment and the costs of liquid nitrogen. It, again, is claimed to work on whole tyres. A laser generates a high-energy beam that enters a liquid medium that, under specific conditions, can cause the emergence of strong hydraulic waves with very high-pressure amplitude. The main feature of this effect is the duration of the impact pulse and the length of the wave front. These are both much shorter than in other methods for exciting hydraulic waves. Whereas for mechanical or electrohydraulic impact the pulse length is at least 10-2–10-3 s, the photohydraulic pulse is down in the 10-6–10-9 s range. With such low loading times the failure of the rubber will be brittle in nature, since the manifestation of elastic properties requires a force to be applied for at least 10-4 s. However, to enhance the occurrence of brittle cracking, the tyre is stressed within the machine by pulling the beads apart along the rotational axis of the tyre (in effect, returning the tyre towards the shape it possessed during building and prior to inflation and cure).

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End-of-Life Tyres–Exploiting their Value Such deformation has the effect of inducing forces that tend to delaminate the casing cords. The laser is in the optical or infrared range with a power capability of up to 109 W, and is located below the reaction vessel into which the tyre is loaded vertically from above. The pressure medium is water. The pulse frequency depends on the rate of collapse of the cavitation void created within the liquid, and can reach 10 kHz. Once the tyre has been lowered into the chamber and placed under load, the laser delivers several pulses with sufficient power density for the creation of cavitation in the water. The pulse duration, between 1 ns and 1 ms, and the number of pulses is determined on the basis of ensuring complete separation of the cords from the rubber. Comminution of the rubber remaining after the withdrawal of the ensuing wire tangle can be continued via the same technique under controlled conditions, for the production of particles in different size ranges. For the comminution of tyres in the brittle state, a few impacts with an overall duration of no more than 7 x 10-6 s are sufficient, since a crack will move at the speed of sound and so be able to travel 1 cm in this time. Whereas 20 kJ is released within 15 x 10-3 s for electrohydraulic impact, for photohydraulic impact (with equal power) 10 kJ of energy will be released within 7 x 10 -6 s. Even taking into account the fact that laser efficiency, at 2%, is much lower than the 90% of the electrohydraulic impact route, laser comminution of the rubber is believed to be considerably more economic, since the impact duration for the method is three orders of magnitude shorter than for any other impact approach so far announced for the mechanical processing of tyres. Energy input calculated for the electrohydraulic cryogenic method described previously is 235 kJ/kg, but the laser route amounts to just 12 kJ/kg of tyre. The processing time of one tyre at an impact frequency of 50 Hz is less than half a second. This method, if proven on a wider scale, would considerably reduce energy consumption for the recycling of tyres, and can be tailored to tyres of any size or weight. Recovery Technologies Inc. (RTI), Canada, has gained wide experience over recent years in the installation of cryogenic size-reduction systems for waste materials that are hard to process. Tyres are one of the product categories that have been treated by RTI, whose expertise has been the development of appropriate technology for complete turnkey cryogenic recycling systems. Much of its work in the early 1990s is described in the previous edition of this Rapra report. The systems start with whole tyres and end with granules of a few millimetres in size and the ability to reduce further to material finer than 100 mesh (0.15 mm) has been achieved in conjunction with Praxair Inc., the US gases company. The RTI cryogenic Reclaprocessor Size Reduction System makes efficient use of liquid nitrogen, which reduces the electrical power requirement and is claimed to generate a cost advantage when compared to alternative methods. RTI has obtained the exclusive worldwide rights to manufacture and market the Praxair VFGS and UFGS systems (see Section 6.2.3.2) either alone or as part of its own systems. Since it is accepted in the rubber industry that finer particles generally offer superior properties when compounded or processed further, then such particles would be deemed 50

End-of-Life Tyres–Exploiting their Value of greater value to an end-user. This in turn could make cryogenically ground rubber more cost effective. Techniques to produce finer meshes of cryogenically prepared rubber crumb include changes in specific temperature and time parameters, such as •

lowering the feed entrance and exit temperatures, and

•

increasing the hammer mill speed,

and also changes in equipment geometry that are determined by experience: •

changing the openings of the mill retaining screen, and

•

changing the tolerances and geometry of hammers, breaker plates and mill screens.

6.2.3.1 Limitations of Current Technology Feed and product mill exit temperatures are limited by liquid nitrogen to its boiling point of -186°C. Impact energy increases by the square of the mill velocity but drag increases by at least the cube of the velocity. So, changing a mill’s internal geometry can help but gives diminishing results. Increased conventional mill speeds also create a domino effect of problems. An increase in speed produces an increase in drag which increases the power requirement. Increasing power requirements means that there is an increase in the temperature of the air (or nitrogen) in the mill. Increased heating requires an additional injection of nitrogen for cooling. The increased injection of nitrogen brings two further problems in itself. These are the additional costs and the additional power requirements for impacting the rubber that is saturated with nitrogen. The additional power causes increased overall heating and we find ourselves in a “Catch-22” situation. Hitting the embrittled rubber harder (i.e., with more energy) with the same number of impacts does not work either. As the hit becomes harder, more energy is absorbed by the particle, thereby increasing its temperature. The faster the temperature rises, the fewer additional impacts are permissible in order to stay below the glass transition temperature. Recooling and refeeding the larger mesh sizes back into the mill is not cost-effective because feed material smaller than 20–30 mesh (0.6–0.85 mm) tends to ‘bounce’ at conventional speeds and additional second stage grinding is slow, resulting in increased nitrogen costs per kilogram of fines. The ‘bounce’ problem also exists when feeding the ‘oversizes’ into a second cryogenic mill with a fine retaining screen. In addition, feeding rubber into a mill with a fine retaining screen significantly reduces throughput and, therefore, increases nitrogen consumption.

6.2.3.2 New Concepts To overcome some of these problems, Praxair introduced its Very Fine Grinding System (VFGS) concept. The VFGS, called V-mill for short, begins with a conventional mill to which a range of proprietary modifications has been made. Starting with 4-mesh crumb,

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End-of-Life Tyres–Exploiting their Value the V-mill performance can produce in excess of 60% finer than 80 mesh at a rate of more than 1800 kg/h and a nitrogen consumption of about 1.5 kg/kg of rubber feed. The V-mill technology is claimed to maximise the quality of fines produced while minimising the production of course rubber, which may be more difficult to sell. Another major claim is the reduction in nitrogen cost per weight of fines by over 50% in comparison to conventional cryogenic methods. Praxair believes that its VFGS system represents the limits of existing ‘conventional’ cryogenic grinding technology. The company maintains that, in the USA at least, cryogenic grinding is almost always more cost effective when the crumb rubber to be produced below 20–30 mesh (0.6–0.85 mm), but that ambient grinding is best for above 4 mesh (4.5 mm). The ‘next generation’ of technology is found in the UFGS concept, the Ultra Fine Grinding System. This new system uses extremely high impact velocities to eliminate the ‘bounce’ problem, achieves a reduction in drag and also minimises the number of impacts. Starting with the usual frozen, brittle materials, the U-mill concept allows operators to increase the production of fines by operating at impact velocities around 3 times those of conventional mills, which results in impact energies of almost 10 times those found in conventional mills. The technology also employs a vacuum environment that reduces the drag. The philosophy adopted is that of ‘hit as hard and as little as possible’. Starting, again, with clean 4 mesh crumb, the U-mill concept is understood to be capable of producing more than 900 kg/h of at least 80% finer than 80 mesh and as much as 90% finer than 100 mesh (0.15 mm) while using only 0.75 kg of nitrogen per kg of feed. The mill is available as a self-contained unit that can be retrofitted to existing cryogenic systems. This system, which is covered by US Patent No. 5,597,123 (28 January 1997), produces greater particle size reduction ratios, uses less nitrogen and has a greater production output of very fine particles than either the V-mill or conventional cryogenic grinding systems. The benefits for the U-mill are listed as: •

a decrease in the percentage of ‘oversizes’ to as little as 5%, or even less,

•

a reduction in the nitrogen cost per weight of very fine crumb by as much as 80%, and

•

an increase in production rate of fines by up to 300%.

The U-mill system can also be justified for coarser particle production by decreasing velocity and increasing the throughput capability, while maintaining unit nitrogen consumption at a constant or slightly lower level. The approach of Crumb Rubber Technology (CRT) Inc., USA, (cryogenic grinding without liquid nitrogen or other gases) was described briefly in the last edition of this report. The CRT process has been redesigned to have more capacity and to cool the rubber faster than in the first generation system. When the company introduced the process (in the mid-1990s), the market requirements in terms of crumb quality were very ambivalent. Today the end-product manufacturing sector 52

End-of-Life Tyres–Exploiting their Value that uses crumb is much larger and these manufacturers specify exactly the type and quality of crumb rubber they need. The main technological advance has been a smaller, vertical, cooling chamber that maintains a constant -100°C cooling temperature. There is no pre-cooling station as in the previous (Mk1) version as a so-called push button start up saves time and cost. Metal contamination has been virtually eliminated by a patented 2-stage separation process that uses opposing magnetic fields. The CRT process takes whole car tyres that are shredded to 5 cm particles in a Tryco Tire Grinder. This shredder is an integral step in the system and can produce 2.5 cm or 5 cm chips at a rate of 1800 kg/h (over double the throughput of the Mk1). The shredded tyre chips pass through the cooling chamber via an auger before entering a Williams hammermill. The pulverised chips are then separated from fibre in an air classifier and from steel in the patented magnetic system before a series of vibrating screens classifies the resultant crumb products. The processing equipment is versatile, generating a range of particle sizes, and can handle other materials which become brittle at low temperatures. Tirex, Canada, has a patent on its TCS-1 scrap tyre disintegration system. By mid-1998, Tirex had reportedly sold 11 complete systems, nine of which were purchased by companies affiliated to Oceans Tire, USA. Unlike previous cryogenic tyre technology, the TCS-1 system is claimed to process up to five tyres a minute. It separates the thread and sidewall sections, cuts them into 30 cm sections and feeds them into a chamber, which blows compressed cold air cooled to 150°C. After freezing, the TCS-1 disintegrator fractures the sections into crumb pieces and fibres of steel and textile. The absence of grinding allows the system to produce an extremely pure grade of crumb, finer than 40 mesh, which is in strong demand by recycled product makers. Tirex moved to Montreal in 1996 to take advantage of government assistance to manufacturers of new recycling technologies. This bore fruit in 2000 as the company was given tax credits by the Government after successful demonstrations of the TCS-1 technology to Scientific Representative, the Customs and Revenue agency. The credits cover around a one-third share of the R&D spent since 1998 and also allow Tirex to apply for more funding from Quebec’s Scientific R&D Programme. The firm will now move ahead with its plans to commercialise the process and initiate worldwide marketing efforts. At the end of 1998, KTI Inc., USA, acquired Recovery Technology Inc (RTI). KTI Inc., is very strong across the USA in a wide range of waste handling and downstream processing. However, this is the first rubber-related business for KTI. The company announced immediate plans for a new cryogenic facility in Chicago during 1999. The Chicago site was purchased by KTI when the firm acquired a tyres-to-energy project. The 20 MW plant was to be reorganised and renamed as the New Heights Recovery & Power LLC. KTI planned to install the crumbing unit on this site as the first stage in expanding the facilities to also include its other (non-rubber) recycling businesses. KTI has invested in Oakhurst Co., whose new subsidiary Oakhurst Technology Inc., (OTI) will have the responsibility of refurbishing the generation plant. OTI would also operate any cryogenic crumb plant under licence and other future recycling projects on the site. 53

End-of-Life Tyres–Exploiting their Value Although KTI has now become part of Casella Waste Systems, both the operations are now in production and RTI continues to run cryogenic plants in Canada and has involvement in several overseas projects. One of these, in Portugal, is now operational, producing crumb for the Portuguese asphalt rubber programme. The business philosophy is to own operations it launches in North America and to sell plant overseas, perhaps taking a 10% equity stake as in Portugal. RTI calls its line of crumb materials Reclaprene Crumb Tire Rubber (or CTR), and offers: •

4–10 mesh, coarse ground, Reclaprene CTR for use as an extender or to replace more expensive base materials in compression moulded products, roll or sheet goods, and sports surfaces.

•

20–30 mesh for use in rubber/plastic extrusions, foam rubber, rubberised asphalt and crack sealant.

•

Finer, 40–60 mesh for inclusion in formulations that demand UV inhibitors, crosslinking, copolymers and traditional elastomeric performance. This grade range would be used for further surface activation or devulcanising stages prior to final moulding.

Wirtech, a process engineering company in Switzerland, has set up two rubber granulate lines in Japan to provide 40–60 mesh crumb. It makes 10 tonnes from 800 car tyres. It is essential to know the likely end use for the crumb in downstream products since this factor would affect the choice of plant. A shredding stage reduces tyres to 5 cm fragments, which are then dropped into the freezing chamber. After cooling to below -100°C, the brittle pieces are fed into hammer mills. The fragments of rubber, textile and metal are passed through a series of separators and sorted into size, ranging from 3 mesh down to less than 80 mesh. The knives are a critical item and have to be changed after 800–1200 hours of use. Sparks may ignite fibrous residue so great care must be taken. Micronis, France, makes milling and size-reduction equipment specialising in cryogenic systems, and is a partner of Wirtech in a cryogenic development to accompany ambient granulation systems to provide a complete and appropriate package for recycling of rubber from tyres. Tyres are a small but growing part of the Micronis portfolio. It has, for example, erected 10–15 plants for PVC granulation, but then moved into cryogenic applications after joining forces with AGA, Sweden. Liquid nitrogen is expensive when used within a screw cooler, so the company designed a 2-stage cooling hopper in which material can reside for 30–60 minutes rather than the few minutes in a screw system. Placed after an ambient stage 100 mesh crumb is made from 5–6 mm granulate and consumes around 1 kg N 2 per 1 kg of granulate. It has collaborated with ATI, Wirtech, Eldan and CIMP and developed partnerships in the US and Far East for end-use products. Messer Greisheim, Germany, has been conducting trials for the reduction of tyres on equipment at Mülsener Recycling u. Handelsgesellschaft (MRH), a company that carries out grinding services for many material wastes. MRH has operated ambient rubber 54

End-of-Life Tyres–Exploiting their Value grinding for a number of years, to the tune of 10,000 tonnes of truck tyres a year, producing granulate form 0.4 to 7 mm in size. It joined with Messer to develop a so-called reduced energy operation. Output is increased over two-fold from 500–750 kg/h up to 1200–1650 kg/h with the cryogenic system. Particle size distribution is 90% less than 500 μm, at best, in its ambient system. This acts as the feedstock to achieve a distribution with 50% below 200 μm in the cryogenic system: •

Using a pin mill after an auger-like pre-cooler particle size is classified: 0–160 μm, 160–400 μm, and greater than 400 μm.

•

Material is passed into the rotating mill at cryogenic temperatures through which the particles proceed towards the exit that remains at ambient temperature.

•

This decoupling of the cooling from the final grinding allows for a reduction in energy consumption and a high proportion of fines.

The pin mill takes 1000 kg/h and consumes 0.75 kg of liquid nitrogen per kg of rubber. KamEcoTech, located in Tartarstan, has developed a unique mechanical and cryogenic technology based upon the use of atmospheric air cooled to cryogenic temperatures. This offers the potential for lower energy consumption, a high degree of separation and higher quality than other systems. Air-turbo refrigeration that does not employ fluorocarbons or ammonia provides temperatures in the range of -80 to -110°C. A unit could consume 13,000 t/y at a rate of 1000 kg/h. This will generate just under 7800 t/y of crumb in sizes from 5 mm down to 0.6 mm. The operation requires 520 kWh and would utilise 6 people per shift on 24 hour working.

6.2.4 Production Experience in the UK Since the previous report, there have been changes in the crumb-producing sector within the UK. Several companies who were operating crumb facilities have closed and no new facilities dedicated to tyre granulation have opened, as far as can be ascertained. The then-existing producers of crumb from retread buffings are still trading, although the drop in retread activity in recent years has impacted on these firms as well. Gates Rubber Company, Dumfries, continues to produce crumb for in-house use, with little available for the open market. Gates produces 30 and 40 mesh crumb from tyre buffings rather than whole tyres, at rates of between 0.25 tonne and 1 tonne per hour. The company can generate around 1000–2000 tonnes a year. Other companies have been active in the stripping of truck tyre tread for later crumbing either in-house or by other processors, including Alruba, Ashbourne, Wellington Rubber, Leeds, and J. Allcock, Manchester. Duralay, Rossendale, has in-house processing facilities for several thousand tonnes a year for its carpet underlay business and takes suitable material from Colway Tyres, a large car tyre retreader in Co. Durham. Colway shreds the nearly 3 million scrap tyres that it finds are unsuitable for retreading. It is believed to remain the only direct granulator who

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End-of-Life Tyres–Exploiting their Value handles passenger tyres. Both companies are part of the Environmental Waste Services group. A longstanding crumb grinding processor is Rushden Granulating Co. in Northamptonshire. The company operates an Eldan granulating system specifically to handle tyres that are first shredded. Further in-house modifications and developments have been introduced into the plant. Output is around 3,000 tonnes taken from roughly 6,000 tonnes of truck tyres. Most of this output goes to contractors for leisure surfaces in sizes of 1–3 mm for surface layers and 2–6 mm for shockpads and bulk layers. Another longstanding supplier of crumb for surfaces and other purposes is Riverside Rubber, Northampton. It does not, however, take in tyres, but buys tyre shred and other rubbers for granulation. It supplies material for processing into rubber sheeting as well as crumb laid by its own contractors for surfaces. The Four Ashes site of Leigh Rubber Processing (now part of the Onyx group) was closed for business in 1998. Leigh Rubber Processing had capacity capable of processing more than 40,000 scrap tyres per week. The plant, that used to generate around 4,500 tonnes of granulate per year, is understood to remain ‘intact’. Another granulating company, General Rubber, Havant, has closed since the last report. Highway Tyre and Rubber Recycling, Blaydon, processes all manner of tyres from car to small earthmover types, solid tyres and conveyor belt and other miscellaneous scrap rubber goods. They are converted into usable commercial products. The heavy duty shredder that the company has installed now shreds steel-belted radial tyres to the optimum size required by customers to provide fuel for cement kilns and power stations. Although not specifically set up to handle tyres, a UK company founded in 1997 uses a cryogenic process to recycle rubber materials for customers who can then re-deploy the material into more products. Rubber Recovery Technologies Ltd., Swansea, provides a number of different services and products that can help manufacturers maximise their use of in-house rubber rejects and offcuts and significantly reduce the firm’s landfill costs. The company uses ambient grinding to produce powder down to 400 micron (~ 40 mesh) but, for ultra-fine powders, it recommends its cryogenic processing to reduce the rubber down to 50 micron (300 mesh) or less. RIAB Recycling Ltd., Mid Glamorgan, was reported as handling about 2500 tyres a month (30 kt a year) in 1996. The company, however, ceased trading and the plant was closed by mid-1998. Eldan, who had supplied the plant for the RIAB operation, reported in 1999 that Vredestein Jakobs had bought one of the two units, with the other going to South Africa. Charles Lawrence Recycling, the rubber crumb preparation division of the Charles Lawrence group, was set up in 1991 to supply in-house material from truck tyres. Tyre peelings from the retread sector were the original source of material, but as retread technology moved towards finer buffings the company chose to move into the processing of recycled materials for themselves. This was due to the fact that fine retread buffings were deemed too thin for the production of sports surfaces.

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End-of-Life Tyres–Exploiting their Value The equipment first purchased has been substantially altered and rebuilt to achieve the essential requirements as the company see them. These are minimum downtime and close control and tolerances for the size of granulate produced. The company takes in between 15,000 and 20,000 tonnes of tyres a year which are broken into 30 mm pieces, at which stage it is possible to remove 90% of the steel bead wire and reinforcements by magnet. At an average of 60% rubber recovery the output of crumb is 8000 to 12000 tonnes per year. To help keep overall downtime to the least possible, the process is not one in-line series of machines but separate stages so that back-up machines can be brought into service when a particular element is affected without stopping material throughput. The plant is in 4 stages: •

A single-shaft hydraulically driven shredder produces 150 mm pieces.

•

A machine generates 80 mm pieces and removes a lot of steel.

•

A machine reduces the particles to pass a 30 mm sieve; two streams emerge. Drum magnets remove much of the fine steel fragments at this point.

•

A high-speed granulator.

The particles are then passed to a classifier to produce the size ranges required by the customers. Sometimes stage 2 is missed if higher throughput is required. Charles Lawrence Recycling produces 5 size ranges of crumb for in-house use and outside sale. These are 0.5–1.5 mm, 1–3 mm, 1–4 mm, 2–6 mm and 2–8 mm. In addition, it has developed a unique way of producing clean 30 mm fragments that can make a good base layer to a finer surface. These larger fragments require less binder (which can cost up to eight times the price of rubber) so bringing good cost savings. The total depth of a safety surface is 50–150 mm depending on the impact characteristics desired, with playgrounds at the higher end and sports tracks at the lower end of the scale. A pitch requires about 50 tonnes but a play area uses 10–20 tonnes. Material is exported to destinations as diverse as Japan, United Arab Emirates, France and Italy but currently most work is for the UK. The company has developed an automatic continuous mix and pour system that provides a consistent, level surface compared with the traditional batch mixer approach and operates as three divisions: •

Granulation of 15,000 to 20,000 tonnes of commercial vehicle tyres annually.

•

A contracting operation that lays tracks and surfaces.

•

An engineering unit that makes granulation machinery for sale.

The company has become more involved in the development of applications for the crumb it makes. Trials on industrial flooring and non-slip decking for offshore rigs. Chips have been prepared for use in highway drainage. Within the offshore connection, the company has become involved in the use of rubber in concrete in producing rubber inlays for concrete blocks utilised as fixings for subsea pipes.

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End-of-Life Tyres–Exploiting their Value The company has to send to landfill around 20 tonnes per day of scrapped wire at a cost of, currently, £28/t. Charles Lawrence Recycling is in partnership with the Steel Construction Institute, and other partners including Rapra Technology, to initiate a project to examine ways of producing clean wire for reuse and the incorporation of rubber into further construction industry products.

6.2.5 Production and Experience in Other Countries In the USA, much entrepreneurial activity continues to generate innovative uses for crumb, as fully described in Section 6.3, for which new sources of crumb have been required. Within Europe, several crumb producers are operating: Gummiwerk Kraiburg, Heldt KG and Gummi-Mayer in Germany, Vredestein Rubber Recycling and Rumal in the Netherlands, and Gezolan (owned by Kraiburg) in Switzerland. The Rumal operation handles around 200,000 truck tyres a year. This yields about 6000 tonnes of crumb, 2000 tonnes of clean steel and around 3000 tonnes of scrap, which includes the beads. The crumb is made in three or four stages. After debeading and shredding, the final step for most crumb is the main grinder. For some grades of crumb, Rumal uses a fourth stage which is a cryogenic grinder. This machine takes medium crumb from the third stage, cools it to -50°C with liquid nitrogen, and can grind the rubber to any size. The crumb produced is regular and uniform in size with smooth sides. The normal grinder produces rough, irregular crumbs. 3Rnv, Belgium, operates a rubber recycling plant that produces a range of particle sizes completely free from steel and textile fibres. Ikarus-Recycling GmbH has developed a full range of capabilities to take waste tyre materials through to new products. It makes rubber powder and crumb, through a cryogenic plant, which are then used to make mats, flooring, and solid wheels, among other items. The technology is called Tyre GmbH and the name Relaston is used for the materials and related products. The company also has some proprietary devulcanising technology that works, without depolymerisation, below 80°C. Cryogenic technology developed by Crumb Rubber Technology Inc., has been used in a joint venture. Green-Man Technologies, based in Boston, which make products from recycled rubbers and plastics, has invested in a cryogenic rubber crumbing facility in conjunction with BFI Tire Recyclers of Georgia, a subsidiary of Browning Ferris Industries (BFI), the giant US waste firm. The facility is based next to BFI’s tyre shredding unit from which it receives 1 cm chips as feedstock, and has a capacity to convert a minimum of 30,000 tonnes of shred a year. A variety of particle sizes will be made in the chemical-free, single-pass, four-phase recycling system in which the tyre chips are cooled to -130°C using natural air. The larger mesh sizes will be used for low-end applications including industrial mats and truck beds. The finer particles will be blended with various plastics for higher value uses. In late 1997, Environmental Processing Systems (EPS) Inc., announced its first facility to exploit its patented cryogenic crumbing process described in the previous edition of this 58

End-of-Life Tyres–Exploiting their Value report. The operation in South Carolina was to be named the Santee River Rubber Co., with an annual production capacity of 46,000 tonnes of crumb from 63,000 tonnes of waste rubber (tyres are not specifically mentioned). The plant was set to use EPS’s patented EPS 100 system with cryogenic technology based on a closed-loop nitrogen regeneration system. This is claimed to produce costeffective fine-mesh crumb suitable for a wide range of products. Bateman Engineering Inc., that has technical expertise in material processing and cryogenics, was to build the plant and become a partner with EPS for ventures outside the USA. The South Carolina facility eventually came on stream in early 2000. It includes both a cryogenic unit as described above but also a high-volume, high-efficiency ambient processing facility to generate the feedstock for the cryogenic division or for outside sale. Most of the 25 mm tyre chips go into the cryogenic fine grind processing system. Both tyres and other elastomeric waste can be handled. Sold as PolyDyne, the fine ‘powders’ are derived from whole tyre rubber, SBR, IIR, EPDM, CR, NBR, natural rubber and other compounds. The plant has a capacity for 7 million waste tyres a year and has an output capacity for 68,000 tonnes of fine rubber powder, making it one of the largest individual facilities in the USA and probably in the world. A process control system called Impact II has been developed by EPS that continually monitors the operation and systematically controls the production process. Material and process variability are eliminated so that consistency and quality can be guaranteed to the customer. Continental Tire has a three year contract with EPS to supply fine powders in a -80/-140 mesh blend for use in the manufacture of new tyres. Continental are working with the automotive industry and the state of North Carolina to examine the feasibility of using up to 25% recycled rubber in new tyres. Currently around 6% to 8% of recycled rubber is used. EPS plans to expand both within the USA and internationally. It has already set up a dedicated downstream division to sell and promote its family of powder polymers for use in tyres and retreading, automotive parts, thermoplastics blended goods, and construction materials such as roofing, asphalt, coating sealants and similar developments. Another cryogenic grinding operation in the USA, set up as Cryopolymers Inc., was originally a subsidiary of Messer Greisheim GmbH, Germany, a global supplier of industrial gases. GreenMan Technologies took over the firm in 1997 to enlarge its inhouse source of finely ground crumb for utilisation in rubber and plastics products. GreenMan’s main product is the proprietary GEM-stock material made of recycled tyre rubber and plastic. The compound used for the company’s first commercial product (a 132-litre refuse container) comprised 76% plastic (20% PE, 56% PP), 20% crumb from tyres, 2% special chemical reagent, colour and a scent. Messer Greisheim will provide GreenMan with access to its advanced R&D in the field of cryogenics, through its US arm, MG Industries. GreenMan also acquired two recycling plants from BFI. Another firm with which GreenMan is involved is Crumb Rubber Technology, Inc. GreenMan is also pursuing other recycling opportunities through a joint venture with Cambridge Recycling Technologies Inc.; GreenMan will supply land and scrap tyre 59

End-of-Life Tyres–Exploiting their Value feedstocks, and Cambridge Recycling will add its technology and design expertise to as many as five plants forecast to be built. The aim for both parties is to create sustainable and profitable businesses, rather than just the recycling of wastes. These new plants will extract carbon black and steel in a proprietary process consuming more than 1.5 million tyres annually. GreenMan closed the GEM-stock operation in January 1998, saying that when appropriate, the company would use third-party injection moulding capacity to re-establish its product lines. The ex-Cryopolymers facility proved a reliable source of crumb rubber but was closed at the end of 1998 after a bad fire damaged the operation. The company now has four facilities: one in Minnesota, two in Georgia and one in South Carolina operating crumb plants and a highly successful TDF business. Tirex, Canada, started making mat products in 1999 which are sold to a merchandising company, which, in turn, distributes them to major retail outlets and other distributors of floor matting products. Continuous work on the product manufacturing side has led to the formulation of RuCrTherm composites that combine Rutex recycled crumb rubber with other materials. Products made from RuCrTherm can be moulded or extruded with compression-moulded goods taking less time to cure than those that use sulphur or polyurethane. In addition, the products have greater tear resistance, flexibility and stress stability. Quantum Group subsidiary, Eurectec Inc., has supplied equipment for a recycling plant to Poseidon Products GmbH, Germany. The company is a partnership between Quantum and StEG (Strukturentwicklungsgesellschaft) Ueckermunde mbH and is operating in Penkun on a 40,000 m2 site. When complete, Poseidon Products will make high quality crumb rubber and a range of value-added products from the materials. Quantum Group has been building a facility in a prison in San Diego, California, to produce crumb via its SuperCollider cryogenic grinding system and to fabricate a range of products using its Eco-press technology. The company’s Revulcon process is also expected to be installed during 2000. PolyTek Rubber and Recycling, Inc., Phoenix, specialises in crumb rubber and rubber road materials. It is one of the largest in the USA and has been involved in major civil engineering projects throughout the Southern and West Coast states. The company turns shred into crumb using both cryogenic and ambient systems. It also prepares shred for TDF in Tennessee. It has two ambient and two cryogenic units. At Queen Creek, Arizona it consumes 25 kt/y of tyres mainly for production of 30 mesh crumb used in hoses and mud flaps. By the end of 2000, capacity will reach 75 kt (two-thirds in the ambient units). Eighty percent of crumb made in Arizona goes into road asphalt. A machine takes 40 mm chips down to around 20 mm that becomes feedstock for the cracker mills that is steelfree but contains fibre. A screening and roller system removes fibre and stones, with the set gap providing the size and grading parameters. A proprietary electrostatic system is used to separate fine crumb from fine powder. The 20 mesh crumb is used for road making. The cryogenic system uses hammer mills and generates less waste. The 80 mesh crumb is targeted at the tyre makers for new compounds. This method consumes less power, but

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End-of-Life Tyres–Exploiting their Value a liquid nitrogen consumption of 0.5 kg per kg of rubber adds to the costs at around 14 cents/kg (6 cents/lb). Steel is regarded as waste because it requires further cleaning. PolyTek is considering a cooling tunnel in addition to a pass through a hammer mill to remove the last vestiges of adhering rubber. It is now investigating a chemical way to use only half of the machinery that is presently required. The firm claims that 80 mesh crumb can only be made by the cryogenic system, that the tyre companies are only concerned to receive a fine particle size and are not interested in the nature of the surface.

6.2.6 General Observations on Crumb and the Market Place Although crumb production in the UK probably shrank during 1997/1998, there is still a latent demand. This has resulted in a renewed growth in the sector. Commercial and fiscal factors have been playing a disproportionately strong role and have forced several closures, as reported in Section 6.2.4. In other countries there would seem to be much activity, but not much specifically reported. This is believed to be due to the entry of crumb operations into the mainstream of the rubber industry, especially in the USA. As discussed elsewhere in this report, the rubber recycling industry is no longer seen as a segment of the waste disposal industry, ‘divorced’ from the mainstream rubber sector. It is a part of industrial activity that competes with other rubber businesses to supply products that end users are prepared to buy on their merits. It was reported in mid-1998 that there were 5 crumbing operations in the UK, making 40,000 tonnes of output. This is thought to now have risen to between 50,000 and 60,000 tonnes. In addition, there is still the contribution from retread buffings; these are calculated at around 14,000 tonnes (see Section 10.4). This implies that between 80,000 and 90,000 tonnes of end-of-life tyres have been absorbed by this sector. The leisure and safety surface sector consumes about 20,000 to 25,000 tonnes, mainly from directly granulated tyres. The road sector is now beginning to take quantities of crumb for a range of products, estimated at around 2000 tonnes. The use of buffings and crumb in the manufacture of solid tyres is estimated to consume about 4000 tonnes a year. Another large user of crumb is the carpet underlay sector, which can use up to 80% of rubber crumb in its product compositions. A figure of 10,000 tonnes is estimated for this use. Utilisation in general rubber mouldings and other products, such as tyre compounds, is thought to be growing after years of static consumption; with the acceptance of crumb for new applications a figure of around 8000 to 10,000 tonnes is estimated. The forgoing figures suggest a total of between 45,000 and 50,000 tonnes per year for crumb consumption allowing for any underestimate for the uncertainty and range of unidentified uses. It is understood that there is very little export of UK production. On the contrary, imports have been coming in especially from South Africa and some EU countries such as Germany and the Netherlands. 61

End-of-Life Tyres–Exploiting their Value The barriers to the development of wider crumb usage are quite high. There are technical problems to face, such as long development times. For instance, highway acceptance requires a 2-year trial with up to 1-year for plant building. Commercial interests will point out that initial costs would be £4–8/t for aggregate as opposed to £180–200/t for crumb. Total lifetime costs have to be carefully evaluated and made clear to the potential customer/client. Some in the industry believe that markets will never manage to emerge independently in the UK, primarily due to the high risks as perceived by venture capitalists. The hope for Government assistance for, e.g., retreading or civil projects, to bridge the gap is unlikely, unfortunately, to be accomplished. The gate fee for truck tyres went from £20/t during the early 1990s, to zero and then up to £55–60 during 1997–1999. The 40 mesh (2–6 mm) crumb tends to be used for running tracks, loose-fill material for playgrounds and equestrian uses, although larger particles are utilised for under-layers and some outdoor arena applications. The 10–40 mesh (0.5–2 mm) crumb is used for the production of compression moulded rubber products, including mats, truck flaps, truck bed liners and patio tiles. Crumb of 100 to 200 mesh is needed for new markets where the rubber is used as an active filler rather than an inert one. A loading of 10% can be added to rubber compounds without any effect on tensile strength, elongation at break or cure properties. Loadings of 20% and 30% give results that show only minor variances for those parameters.

6.2.7 Developments in Crumb Materials Composite Particles Inc., USA, now owns the rights and equipment associated with a special technology that had its origins as an invention and development at Air Products. The technology involves surface modification of the outer few molecular layers on polymer particles through treatment with reactive gas atmospheres. This enables polymers to be combined with other materials to form unique composites. Fine rubber crumb is treated with a mixture of fluorine and one of oxygen, sulphur, halogens or carbon monoxide. The fluorine reacts with the surface of the rubber to create free radicals which then react with other components of the gas to form reactive groups such as hydroxy (OH) and carboxylic acid (COOH). This results in a permanent change in the structure and properties of the surface. This change facilitates the mixing of these particles with other polymers and results in tenacious bonding within the new composition. Treatment enables these particles to be used in high-performance polymer systems such as polyurethanes, epoxies and polysulphides. Developments resulted in two product lines initially: the Vistomer R Series based on surface-modified rubber particles derived mainly from tyres and the Vistomer UH Series based on surface-modified polyethylene particles. Examples of end products made with Vistomer include cast polyurethane wheels, casters, rollers and enclosures, coatings based on polyurethanes, epoxies, and polysulphides, polyurethane foam products, rubber shoe soles, microcellular, non-pneumatic PU tyres for bicycles, wheel chairs and industrial trolleys, caulks, adhesives, sealants and polymer concrete. The technology is claimed to be noteworthy for two reasons: 62

End-of-Life Tyres–Exploiting their Value •

By surface treating the scrap tyre rubber, the value of this material is increased substantially—often exceeding its original cost.

•

Utilisation of this technology saves much energy.

Urethane Technologies Inc. (UTI), USA, also uses a surface modification approach to activate ground rubber. UTI incorporates a solid material that absorbs the moisture that is invariably present in the crumb rubber and prevents the rubber from bonding to a PU binder. In this way, UTI uses its treated rubber as a cost-effective filler in some polyurethane products. UTI is a custom manufacturer of PU systems. The treated crumb rubber from Composite Particles also can be used in the same way. The benefit of UTI’s approach is a lowering of raw materials costs for a PU product that retains its original physical properties. The treated rubber adheres well to PU and is sold under the trademark Stermic. UTI has found that Stermic is cost effective in PU tyres and industrial wheels, as well as shoe soles and other PU products. NRI Industries Inc., Canada, has developed a treated crumb called Symar-D. Unlike DeVulc, Symar-D is produced using a high energy process that modifies the surface of the rubber particles. Currently all output is used in the company’s own manufacturing processes. Coarse crumb (mesh size between 4 and 20) is subjected to thermo-mechanical treatment within a mixer equipped with a high velocity rotor. Multiple collisions between rubber particles and with the chamber walls cause dramatic changes to the particle morphology and chemical structure. It has been found that this modification process can be best controlled by monitoring not only the total amount of energy transferred per unit mass of processed material but also the energy consumption profiles during the ‘mixing’ cycle. Although batch weight changes affect the levels of the power demand curve, the total or process energy estimates can be used to control the batch independently of such variations. Discharge, being the end of the process, needs to be controlled to guarantee the consistency of the Symar-D process. It was found that the process energy rather than the total energy is the best parameter to control batch termination as it produces the least variation in the final product. It has also been shown that a significant parameter is the minimum torque on a disc rheometer which appears to determine the extent of ‘re-activation’, by correlating closely with both process energy and total energy introduced into the batch process technique. US Patent, no. 5,883,140, issued in 1999, covers the process. The presence of nonhydrocarbon ingredients, such as ZnO, protective agents and fillers, increases the value of Symar-D and provides positive savings that may not be apparent when only the rubber itself is considered. More cost-effective compound reformulations can be achieved by using a higher percentage of Symar-D loadings relative to the incorporation of inert ‘standard’ crumb. Reduced accelerator levels to obtain practical cure rates in Symar-D compounds provide important cost savings. Symar-D has low inherent green elasticity compared to new elastomer and its use can result in savings in compounding power costs. Symar-D can also incorporate faster into a compound during internal mixer operations, reducing batch mixing time and so increasing the Banbury capacity. Symar-D can be used effectively as an ‘active filler’ up to 50 phr in SBR compounds. The adjustment of the curing package when using increasing amounts of recycled rubber is an 63

End-of-Life Tyres–Exploiting their Value important step in the compound optimisation process. The addition of adjusted amounts of sulphur and accelerators is especially important to guarantee the highest levels of properties such as 100% modulus and hardness. The material is combined with PE to produce a mouldable and extrudable elastomeric material. The devulcanisation part of the technique has been patented. The matrix is a linear low-density film grade of polyethylene with the combined material processing at LDPE conditions and production rates. The company uses the Symar-D product in hundreds of products for the automotive and other markets. Another material produced by NRI is Micron, made using proprietary technology. This grinds tyre crumb into extremely fine particles. The characteristics of Micron allow the manufacture of sophisticated compounds for the tyre industry. Granulite is a high quality, tyre-derived material used in applications such as asphalt paving and crack sealants. The company has also developed a tyre-derived TPE, Symar-T, which has been used in a range of parts to be found on several current vehicles. As well as winning a prestigious award for innovation in the automotive business, NRI has been accredited as a ‘Full Service Supplier’ to Ford Motor Co. Production of Symar-T currently consumes the rubber from 150,000 scrap tyres annually. A proprietary process is used to alloy the ‘revulcanisable’ rubber and proprietary reagents with a PE/PP mix to form the TPE material. DaimlerChrysler has been using the material in a radiator-to-fascia seal on its Jeep Grand Cherokee utility vehicle. NRI comments that the process allows production of an alloy with recycled rubber content at least 10 times higher than previous systems without TPE property compromises. Degussa-Hüls produces Vestenamer 8012, a granular trans-polyoctenamer (TOR) that is said to open the door to easier processing of recycled tyre crumb rubber while improving the properties of end products. Vestenamer 8012 acts as a binder and crosslinking agent, coating the ground rubber to be mixed using simple blenders, internal mixers or continuous processing equipment. With a melting point of 55°C prior to crosslinking, the Vestenamer TOR also functions as a plasticiser at processing temperatures, ensuring complete coating of the crumb rubber and reducing the batch viscosity for easier mixing, according to the manufacturers. As a crosslinkable processing aid, the additive is used in difficult-to-compound rubbers, in rubber and plastic masterbatches, as well as in calendering, hose production, extrusion and injection moulding. Just 3% of the Vestenamer TOR can aid the processing of crumb, providing 30% better binding properties. Vestenamer TOR can be used as a binder to improve the filler properties of crumb rubber when used in virgin formulations or, with additional curatives, can be used directly with fine rubber crumb as the base material for moulded rubber goods. It can be turned from a granular additive into a liquid dispersion by making a 20% blend with warm mineral oil for use at around 80°C for coating ground rubber in a powder mixer. Comparisons can be made with crumb bound with polyurethane. PU has the advantage of ambient or relatively low temperature curing regime and the resultant articles are often cheaper. One disadvantage of these PU-bound products is reduced weathering resistance. Vestenamer TOR covered rubber components are resistant to weathering and UV light.

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End-of-Life Tyres–Exploiting their Value A process combining physical softening and chemical surface activation has been developed by Flow Polymers Inc., for enhancing the properties of ground rubber crumb. The treatment from the Cleveland-based company is called GRA (Ground Rubber Additive); the composition remains a proprietary formulation. One part of the additive is blended with four parts of crumb by mastication in a Banburytype internal mixer at 90–100°C for five minutes. The resulting crumb is dropped onto a tight-nip mill where it bands in two to three minutes. The band is sheeted off as a soft slab and can then be added to tyre compounds at levels up to 10%. The GRA treatment has resulted in a partial recovery of a range of properties when compared to those achieved by the addition of a similar amount of untreated 40 mesh crumb to the same compound formulations. Tirecycle, a technology to modify recycled crumb rubber, first appeared in the 1980s. Rubber Research Elastomerics (RRE) Inc., and various associated operations have had a somewhat ‘chequered’ business history since then. During this time the verities of the original technological concept have remained. Early work gave the impression of the technique becoming a route to ‘solving’ the used tyre problem. In reality, it is just one of many solutions for improving the properties of a recycled material, rubber, after disposal for subsequent use as a ‘secondary raw material’. Over the years the processing technology has evolved to cover a broad spectrum of products produced by more than one technique. The Tirecycle technology involves the surface treatment of particles of vulcanised rubber with various low molecular weight polymer compounds. By treating particles of ground cured rubber the material becomes a reactive ingredient in later formulations. Unsaturated bonds within the particle of ground rubber can be activated to promote crosslinking or chemical bonding with other particles of treated rubber and/or virgin rubber compounds. The resultant thermoset Tirecycle compound can be made compatible with thermoplastic rubbers or plastics by applying a further secondary treatment. Having started with tyre-derived crumb rubber, Tirecycle has become the generic name for a range of treated scrap rubber stock, covering most of the major elastomer families. Compared to the accepted level of around 10% of recycled crumb rubber as the limit for incorporation into practicable compounds, Tirecycle compounds can be incorporated at concentrations as high as 85% of the finished compounds (i.e., 1200 parts of Tirecycle to 100 parts of virgin polymer). Much work has been carried out on speciality rubber types, covering EPDM, NBR, IIR, etc., but this discussion will be confined to the tyre-derived rubber materials. RRE has found that it is sensible to split the activities into the three components: •

preparation of the particles,

•

preparation of the liquid polymer for the surface treatment, and

•

the co-ordination of the particles and treatment with the final overall material formulation.

RRE demands that as clean a ground rubber as possible is obtained which, in the case of tyres, means the total removal of both textile fibre and metal wires.

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End-of-Life Tyres–Exploiting their Value The company now tends to perform as a practical recycling support organisation that can solve a customer’s problem by allowing the ‘re-work’ of process scrap back into products. This is practicable for the non-tyre goods, which tend to be made with the wider array of elastomer types. In the tyre arena, the quoted examples are for pre-cured tread compounds for retreaders. The nature of the Tirecycle approach would suggest that for the approach to have any meaningful impact on used tyres, the manufacturing source of arisings would have to be known and the tyre ‘dismantled’ so that individual components could be treated and perhaps incorporated back into new compounds at the tyre factory. This would seem to be a costly exercise for both recyclers and tyre manufacturers. RRE would need to operate rather like Vredestein in Europe, offering a service to recycle a customer’s dedicated scrap and selling treated material of known parameters, which a prospective purchaser would use as a formulation ingredient after some development work, preferably in conjunction with RRE. As with the various alternatives to ‘traditional’ reclaiming discussed in this section, Vredestein worked on a way of (re)activating the surface of rubber crumb particles to make them more reactive and so compatible with virgin compound in higher proportions than is accepted for traditional fine crumb materials. The first of the Surcrum family of materials became available in the early 1990s. Since then the programme has grown to encompass widely available source materials from used tyres and a range of developments whereby customers’ scrap is processed in closed loops, for reuse in the factories of origin. The Surcrum alternative starts, as do many others, with ground rubber crumb. Ambient grinding produces a highly structured and so relatively active surface on the particles. The second step is a chemical activation based on the addition of a crosslinkable polymer and a curing system. Both additives are adjusted to suit the parent polymer of the feedstock and the eventual application of the resulting Surcrum. For an application as a partial replacement of a virgin compound, special slow-curing types of Surcrum have been developed. These types do not interfere with the curing behaviour of the compound. For other applications, faster curing types of Surcrum are available. The surface activation results in a higher crosslink activity compared to untreated crumb. As a result, Surcrum has less of a (potentially) deleterious influence on the properties of a compound than untreated powdered rubber. The main processing advantage is its positive influence on air venting properties during curing. When Surcrum is used in tyre applications, it is generally used at levels less than 10 percent. The addition of Surcrum to a compound does not require an adjustment of the compound recipe, because it carried its own curing system in its coating; however, it does result in an increase of overall compound viscosity. Allowing for this, the compound can be processed as normal. Work in Poland at the Stomil Rubber Research Institute has examined a range of low molecular weight, low viscosity polymers as surface coatings for rubber crumb. Such a process is believed to be the basis of a number of the ‘crumb treatment’ or ‘modified crumb’ systems that are in existence at the present time. Polymers examined include NBR, carboxylated NBR, NR and polyoctenamer. The most interesting results were obtained using a 30% solution of polyoctenamer in highly aromatic 66

End-of-Life Tyres–Exploiting their Value oil and white mineral oil. After mixing the crumb in the solution, a Sulphur/CBS/TMTD cure system was added plus zinc oxide and stearic acid. The resultant mix was vulcanised. The coated crumb can also be added into other rubber compounds. Rumber Materials Inc., (RMI) has perfected a technique for the co-mixing of rubber crumb with PE or PP. It offers Rumber Composite to polymer processors who can add it to its own compounds to act as an impact modifier and to resist tearing or cracking. RMI uses a patented means of mixing granulated whole tyre scrap with a thermoplastic material; the choice of materials is dependant on the requirements desired for the end product. The composite can be used in both extrusion and moulding processes with a product list as varied as buckets in either rubber or plastic, pallets, rubbish bins and highway signs. The company also makes lumber that can be used for decking and trailer flooring. The patented process provides a good cushioned surface that reduces physiological stress on a horse’s joints and feet during transportation in horseboxes. The planking can also incorporate a new traction surface that helps to prevent the animals from slipping. The Commonwealth Scientific and Industrial Research Association (CSIRO), Australia, has patented a method for surface modifying rubber crumb to allow better chemical compatibility with rubbers or plastics. Most of its work appears to have been carried out on plastics with treated crumb as a dispersed second phase within a continuous-phase plastic matrix. Both LDPE and ABS have been tried as the matrix in collaboration with industry and government organisations. The CSIRO technology is claimed to represent a new approach to materials engineering. It has the advantage of facilitating rubber recycling in value-added applications; only the outermost surface layer of the crumb rubber is modified without affecting its desirable bulk properties. Another key point is that the technique may potentially save more energy than other tyre-reuse methods and the manufacturing process for virgin resins. Some of the performance advantages for crumb containing composites that have been independently corroborated include: •

increased impact strength,

•

increased crack resistance at low temperature,

•

increased modulus,

•

reduced density,

•

improved barrier properties,

•

increased coefficient of friction for traction applications,

•

better wear resistance,

•

improved compression set,

•

higher heat-distortion temperature, and

•

better long-term weatherability due to UV screening by carbon black from rubber.

Suggested applications for the new composite materials cover a range of the ‘usual suspects’, including shoe soles, automotive components, tyres, solid tyres and wheels, 67

End-of-Life Tyres–Exploiting their Value roofing materials, insulation, window gaskets, sealants, containers for hazardous waste, industrial closures and conveyor belts. CSIRO claimed overwhelming interest from around the world in its process that incorporates a ‘bridging’ molecule as well as the surface treatment. Early efforts have produced an ABS/rubber pipe that is thought could potentially replace PVC under certain conditions. Other work has concentrated on polyolefins and PS. Another process for treating crumb rubber has emerged from Malaysia: MR-TC or Malaysian Rubber Tire Crumb. Quantum Polymers, formerly known as B.G. Polymers Sdn Bhd, announced plans to build a crumbing plant and to install downstream equipment to add unspecified reagents and additives to the crumb and allowing the material to react under controlled temperature conditions to form the MR-TC. According to Quantum, the rejuvenated crumb can be used in concentrations of up to 30 phr in tread compounds for tyre retreading. The company will not operate a complete tyre recycling operation but will source 25 mm clean tyre chips as a feedstock. Quantum Polymers USA has now set up its first operation in the former Baker Rubber Co. factory in South Bend, Indiana, that used to be one of the largest crumb operations in North America. 30 mesh and 40 mesh crumb is produced and then subjected to the proprietary ‘rejuvenating’ process. The resultant MR-TC can be mixed into standard compounds or masterbatch for onward sale. The company has found that the process, developed originally for reject latex gloves and then for the NR-rich tyre rubber in Malaysia, has not been quite so successful on the predominantly SBR/BR-based crumb obtained within the USA. Coupled with low prices for virgin NR and SBR, it has not been economically worthwhile to focus purely on tyre crumb rejuvenation. Attention has now moved towards non-tyre rubbers such as EPDM. A retired oil engineer in Northern Michigan has spent the last four years developing and refining a process for combining waste plastic, usually PP or HDPE, with crumb rubber from car tyres to make a thermoplastic elastomer compound. It is produced as pellets of a highly mouldable material by a process that does not incorporate any bonding agents or additives and that the inventor will never patent. XyleX is being registered as a trademark for the material by the tiny company formed by the inventor, World Tech Management Inc., and whose shareholders include a wide range of GM and Delphi Automotive employees. Corporate sponsorship has been declined, as have been inducements to sell the process for large sums of money. World Tech Management Inc., is now evolving towards a joint venture with a private conglomerate that can underwrite the next stage of a bigger rubber refinement plant and tyre rubber processing equipment, and also a facility to clean and dry the scrap plastics stream. It is surmised that the effects are achieved by the linking of free radical molecules formed after a very highly intensive shearing process in a twin-screw extruder system. It also may have similarities to the SSSE process. XyleX pellets containing 30, 40, 50 and 65 percent of rubber have processed without problems and no scrap at several top quality moulding and test operations with 30% to 40% lower cycle times and plasticising temperatures 30% lower than for virgin plastic resins. XyleX is thought to cool faster than pure plastic because the rubber in the pellets does not get hot enough to ‘melt’, and therefore absorbs heats allowing the plastic to cool much faster and so shorten the cycle time per mould. 68

End-of-Life Tyres–Exploiting their Value The new rubber refining facilities were expected to be sited near a tyre or rubber source within the state of Michigan. A second facility would clean and dry recycled plastic and prepare and pelletise up to 22,000 tonnes of product a year. One rubber plant is envisaged to supply three of the pellet operations. Serious interest is understood to be shown by the automotive sector. The recreational goods market is also to be explored. However, the keenest interest has come, so far, from the artificial lumber industry, because adding an elastomeric component provides impact resistance and increased flexibility and skid resistance. In 1999, World Tech Management Inc., were selling the pellets at 35 cents a pound (77 c/kg) and making a profit. Yet another development for a modified crumb has been quietly maturing in North America since the technology was sourced in China some years ago. Activated modified rubber (AMR) is a rubber devulcanisation and reactivation technology that chemically treats crumb rubber to break down the sulphur bonds created during vulcanisation. The technique was developed at Guangzhou Research Institute for the Utilisation of Reclaimed Resources. The invention that provides the reaction mechanism to break sulphur crosslinking without damage the polymer chains is a series of chemical additives. This allows for up to 50 parts of AMR to be blended into retread compounds and up to 40 parts into new tyre formulations. Synthesis of three kinds of chemicals is the essence of this technology: •

An activating agent named LR-101 utilising prepolymers having reactive end or side groups and conjugated double bonds. Special nucleophilic groups are introduced into these prepolymers. The synthesis of the activating agent is a simple mixing operation of chemicals in a tub at 120 rpm at 140°C for 4 hours. This activator breaks sulphur bonds selectively according to the free radical chain reaction mechanism.

•

A modifying agent called LB-13. This is a mixture of polymers with reactive end or side groups, conjugated double bonds and poly-rings. The modifying agent helps form crossover penetrating networks with raw rubber in compounding and enhances physical properties of the resultant vulcanised rubber products. This is made in a similar fashion to the activating agent, but at below 95°C for only 3 hours.

•

Catalysts: LA-16 for use with NR-based powders and LA-78 for synthetic rubbers.

The production of AMR powder is carried out under normal temperatures and pressure. It takes only 5 minutes to devulcanise around 40 mesh powder mixed with the tailor-made additives in a high-speed mixer. Original properties are restored and rubber compound mixed with AMR powder has better processing and dynamic properties than that mixed without it. A demonstration plant has been constructed in Dayton, Ohio to serve as a test site for AMR. Landstar, Inc., Canada, acquired the exclusive North American rights when it bought Rubber Rebound Corp., in early 1999. The Ohio facility has an initial capacity of 10 tons per day. The worldwide rights remain with Rubber Rebound, now a subsidiary of Landstar. The company strongly believes in the approach of ‘compelling economics’ to embrace productivity, standards and a viable end use. As such the requirement is for resource preservation to replace any lingering ‘disposal mentality’. To this end it has moved to

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End-of-Life Tyres–Exploiting their Value search for sustainable end uses for crumb as the best way of managing a reusable source of materials. The company has found that the Link equipment from Australia is of good standing to provide the 30–40 mesh crumb as the feedstock for the AMR. Properties have been found acceptable, so there is a large amount of interest at present. A firm called Pinnacle Products has become involved in these evaluations. An advanced variant of AMR is RU-Rubber that has been devulcanised by the Chinese method from 4–8 mesh particles. This makes the process even more attractive, since the expense of making quite fine crumb is avoided. The process gives about 80% of original properties in a truck tread compound, for instance, with just the elongation somewhat reduced. This is achieved with RU-Rubber added at levels as high as 70 parts and in some cases even higher. The Japanese Tyre Association is currently investigating RURubber in detail for use in car tyre compounds. From a different part of the world comes the Levgum process. This new technology incorporates a specially prepared modifier, called Isramod which, when used with waste rubber, acts as an effective agent for rubber devulcanisation. Levgum Ltd., Israel, has a patent pending on the process that is available under licence. The modifier is mechanically mixed at room temperature with pre-stretched rubber waste to produce a mechanico-chemical reaction resulting in a devulcanised product with allegedly unique properties. Isramod is comprised of easily available chemicals. The dry process does not release any reagents to atmosphere, nor produces any liquid effluent, and so can be regarded as environmentally friendly. The process does not need special equipment and can be carried out on conventional process plant. The resultant high quality rubbers, Isregene (sheet form) and Isracrumb (granules), are cost-effective solutions for a wide range of applications as a substitute for virgin rubber. It is not clear, for tyres, what level of shredding or crumbing is required before the application of this technology to create the devulcanised material. An adjustment of the formulation of the Isramod used in generating the sheet or crumb can be used to alter the final properties of different articles made from them. Currently licencees can only obtain Isramod from Levgum Ltd. The Isramod material is said to be cheaper than De-Link at no more than US$2/kg. Another material from the same source is designated as LPCRD (Liquid Product of Controlled Rubber Destruction). It is described as a carbon black containing low molecular weight suspension. It has been used in paints and costs around US$250/t. It appears that chips of tyre material and a simple solution of the chemicals is all that is required, in contrast to the De-Link system that contains 11–12 components. Sulphursulphur bonds are broken first and then sulphur-carbon bonds. The resultant materials are very homogeneous and processable downstream. Application examples include a flexible coating with a rubber/olefin mix and sound insulation from fibre, crumb and a special binder. Isramod acts as a ‘prolonged action chemical knife’ on larger pieces of rubber that do not need to be in small particulate form. The materials are put through a friction mill with a 0.1

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End-of-Life Tyres–Exploiting their Value mm nip and friction ratio (relative speed of the two rolls) of 1:1.2 for 10–15 minutes at a temperature of less than 60°C. Isregene, the resultant devulcanised rubber, is understood to have been used in tiles, mats, playgrounds etc. Independent tests have been conducted by Vredestein, the Netherlands, and Recycled Rubber & Recovery, UK, with positive comments emerging. When compounded with fresh rubber, viscosity needs adjusting but the tensile strength, modulus, elongation to break and hardness of resultant blends are all satisfactory. Tyres do have to be reduced down to rubber only before the Levgum process can be utilised.

6.3 Applications of Rubber Crumb This section contains brief discussions on a range of end uses and applications for products that incorporate crumb rubber. It also refers to many entrepreneurial activities, mainly in the USA, which have found a market for such goods. The prevalence of American references is taken as an indication that perhaps the more open market which exists in the USA provides an outlet for products, which, although consuming a recycled material, also fulfil a need with consistent quality and at a competitive cost.

6.3.1 Sports, Leisure and Safety Surfaces Sports surfaces are an important and attractive market for crumb. Rubber granules are used either in the supporting structure for the playing field or mixed with the material that comprises the surface. Rubber granules make the playing surface and the running track more resilient, and less rigid, while allowing the surface to maintain traction and shape. Many are made from rubber crumb bound with one- or two-part polyurethane binders. The size and shape of the rubber particulate is often determined by the eventual uses for that particular surface. Equally important, however, are the correct selection of binder and the ratio of binder to rubber. Polymeric MDI prepolymers are one class of material especially designed for use in various surface applications. Supplied as 100% solids, low volatility resins, these cure when activated by atmospheric moisture. This curing at ambient temperatures is affected by relative humidity (curing is retarded at low humidity levels and accelerated by high humidity) and by temperature (curing is accelerated by a rise in temperature, although systems will normally cure sufficiently (albeit slowly) at temperatures as low as 4°C). Under most conditions, curing is such that the surface of a typical mix may be touch-dry in eight hours and can tolerate foot traffic within 24 hours. Rubber crumb/PU surfaces can consist of factory-made slabs or tiles, or alternatively, applied in situ using the ‘wet lay’ method, whereby the rubber and PU are blended and poured directly onto the substrate. •

Factory-made slab or engineered tiles.

Rubber crumb bonded with PU was first exploited as a factory process. A normal blend of 20%/80% by weight PU/rubber is cured in low pressure, hot air (70 to 160°C) curing 71

End-of-Life Tyres–Exploiting their Value presses. Depending on the heat supply, curing time is between 4 and 15 minutes. Similar processes are employed in the production of moulded components and shock pads. Engineered tiles can also be made in a similar fashion, although these would incorporate energy absorbing profiles below the playing surface. Both types of tile can be mechanically interlocked but still require fixing with an appropriate adhesive to the prepared substrate. Whilst engineered or slab tiles are declining in terms of overall market share when compared to the in-situ systems, their main advantage is that the manufacturing processes, and thus quality, can be more easily controlled. •

In-situ cast systems

In the ‘wet lay’ process, the PU prepolymer and rubber crumb are mixed on site. The uncured mixture is then spread and compacted onto the prepared substrate, and cures to form a tough, seamless surface. This has obvious advantages when laying large or irregularly shaped areas. The characteristics of in-situ systems can be varied by the correct choice of rubber granule and binder, the thickness of the layer and the proportions of binder used. To obtain the desired combination of hardness, durability, permeability, etc., two or more layers of crumb mix may be applied. The lower layer may consist of an energy absorbing underlay with a reduced polymer content, whilst the top layer provides a close knit, abrasion resistant surface. A PU primer may sometimes be necessary to prepare and seal a substrate. For applications over small areas, the rubber and PU can be adequately mixed using an electric drill and paddle. For large areas a mechanical mixer is recommended. Half the prepolymer is added to half the rubber crumb in the mixing vessel and mixed. The remaining rubber is then added, followed by the remaining prepolymer. The mix is thoroughly blended for 15 minutes and then poured into place, spread and levelled. Elastomeric granules used in bound surfaces are generally larger than those used in traditional rubber compounding. This is because the granules are not dispersed at low levels into a solid, dense elastomeric structure, such as a rubber formulation, but rather the granules form the basis for a relatively open three-dimensional network. This network is held together by the binder which essentially only coats the surface of the granules and completes the open cell structure. Consistency of granule size and size distribution are very important so that binder to surface area ratio, product performance properties and product appearance can be maintained. Granulate for the wearing (top) course of playground surfaces would tend to be made from 1–3 mm and 1–4 mm particle sizes. The top course could be from 15 to 25 mm thick. Sometimes a coloured surface is required which would not use tyre rubber but PVC or EPDM, the latter from regrind. The bulk layer for playground surfaces or other sports uses, or in-situ shockpads for artificial grass surfaces, would tend to be made up with crumb from 6 mm up to 15 mm, with 6%–8% of polyurethane binder. Such layers could be anything from 30 to 100 mm in depth.

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End-of-Life Tyres–Exploiting their Value For athletics, tracks with an impervious solid top layer would contain 1–4 mm size granulate as a filler in the solid polyurethane. The bulk layer uses 1–3 mm and 1–4 mm particle sizes with 20% polyurethane binder. Granules can be produced from a number of different feedstocks, usually by grinding or chopping at ambient or cryogenic operations. Feedstocks include whole tyre chips, tyre tread peels, tyre buffings from retreading operations, moulded co-products such as flash pads, mould runners or scrap sheets, and rubber formulations mixed and cured on purpose in sheet form. Feedstock type may determine granule form, especially in larger products. For instance, 1–3 mm granules produced from tyre buffing feed stock will be long, thin strands while granules produced from tread peels will be essentially spherical. Products produced from these granules will exhibit different appearances and, potentially, performance characteristics. Other important granule properties include colour, chemical composition, physical properties (such as hardness and chemical resistance), moisture content, cleanliness, availability and pricing. The binder performs two main functions in bound granule composites. The first is to adhere the granules together to complete formation of a stable three-dimensional matrix. The second is the formation of the top wear layer, potentially of a colour different from the granules, which forms the initial basis for composite appearance and wear characteristics. American Tire Recyclers, Inc., (ATR), Jacksonville, Florida, has expanded its operations in recent times. It now focuses on a range of products for a variety of equestrian and sports arenas, paths and walkways, some of which are made under licence. These include Rebound, for soil and turf preparation, De-Vulc, using technology from STI-K Polymers, Equestri-Foot, SportsTurf RubberStuff, PermaPark and PermaPath. The company operate a cryogenic system utilising Praxair’s technology for VFGS as described in Section 6.2.3.2. This process makes 80 to 100 mesh material. The company offers 40 mesh crumb for asphalt programmes and is now equipped with laboratories and development facilities to provide QA testing and compounding support to customers in the moulded products sector. As business has grown, the company has developed marketing strategies to bring it alongside the decision makers (landscape architects, builders of athletics tracks and similar specifiers) in the final user sectors. This now takes them a step nearer to the final customer rather than only selling to materials suppliers in the merchant business. Indeed, RubberStuff is now sold through Toys R’ Us in several states for use in backyard playgrounds. ReVived Rubber is the name under which ATR now makes and sells material devulcanised using the De-Link reagent under licence from STI-K Polymers. The crumb produced by the VFGS process from Praxair is also available in 80 mesh, sold as SuperFine Crumb. Wal-Mart has commenced selling RubberStuff from ATR in its stores throughout the USA. The agreement with Wal-Mart will make RubberStuff convenient for residential purchases and readily available at around US$7 per bag. The product will not decay, float or blow away. It drains so well that even after a hard rain it is ready for play in less than half an hour. Wal-Mart is keen about the protection of the environment and realises the importance of being able to recycle discarded tyres. Each bag represents four recycled tyres that have found a useful ‘after life’. Customer demand has driven the ATR activity over the last 5 years or so and as sales to schools, local authorities and now Wal-Mart grow, more production capacity is to be installed. 73

End-of-Life Tyres–Exploiting their Value Larger crumb, or perhaps small chips, in the 5–10 mm size range, has found an enormous market in the US equestrian world. Perma-flex rubber footing material is accepted as the standard for equestrian arenas and riding farms throughout the country. TIREC has developed this specialised market through assiduous interaction with the potential customers and their specific needs. For example, the crumb particles must have no wire in them and be no smaller than 5 mm in diameter. The market for the material must be within a relatively small distribution circle otherwise transport costs become too great. The solution has been a coalition of manufacturers who licenced the tradename and must adhere to agreed product quality specifications. Each partner can sell within the defined group of States, usually about 5 or 6, allotted to them. Safe Sports Surfaces, South Carolina, is one of many companies that have become involved in the generation of a business based on recycled rubber-based surfacing products. These products include playground surfacing, running tracks, jogging trails, landscaping, turf systems and speciality products for golf course applications. For playgrounds, goods such as CushFil and CushPlay can be installed either as a loose fill rubber in granular or strip form or as an enhanced pour-in-place system. A two-layer system gives a greater cushioning effect and allows for a colouring scheme on the top layer to the customer’s own choice. The company has laid many tracks and trails including reputedly the largest in the USA at Seneca, South Carolina. Landscaping products include CushWalk decorative walkways and CushStep stepping stones. The company is a dealer for the Rebound range of turf enhancement products. These include soil amendment for enhanced grass growth, top dressing for improved survival of in-place grass and sub-surfaces for artificial turf to improve playability. Ace Surfaces North America has been providing cushioned recreational sports surfaces for over 30 years. Rebound Ace is a multi-layer system with a foundation layer made from recycled rubber as a mat. This has been manufactured by AV Syntec, Australia. Together, these companies have pioneered the use of such surfaces on tennis and basketball courts. The highest profile installation is on the courts in Melbourne that hold the Australian Open Tennis Championships every year. Nike has chosen the recycled rubber cushioning system for its Reuse-A-Shoe outdoor surfacing programme. This Nike programme utilises a percentage of old tennis shoe regrind and it is formulated into Syntec’s recycled cushioning mat. SynTenniCo has been making NovaTee synthetic golf tees for over ten years and these have been installed all over the world. The products are a patented mixture of tyre crumb rubber and silica sand with vermiculite. The cushioning effect within the product does not compact over time and use, which led the firm to consider using the tee construction on a larger surface area. This has resulted in Field Turf that may be used as a substitute for grass, replicating its feel and texture as much as possible, yet it cannot be destroyed by over-use and weather damage. It uses long synthetic fibres on the top ‘grassy’ layer and by duplicating earth through a synthetic method it has very good porosity. The crumb rubber keeps the sand from compacting resulting in a good rate of flow of surface water through the Field Turf. Its first uses were, however, for indoor surfaces where the cushioning properties minimised injuries compared to the traditional hard surfaces. It now has strong exposure

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End-of-Life Tyres–Exploiting their Value for outdoor arenas through its introduction of 10% Nike Grind (from the Reuse-a-Shoe scheme) in the infill for Field Turf fields and the association with the Nike name. One of the largest outlets for PU-bound recycled rubber is artificial safety surfaces for playgrounds. These surfaces provide a tough, durable and safe covering which can reduce injuries to children. They can also be made more attractive by using different colours and incorporating patterns into the surface. The binder and shredded rubber is usually mixed on-site and put down in layers. The first, or base, layer can vary in depth from 20 to 100 mm, while the surface layer is usually only 18 to 20 mm thick. If a highly coloured surface is required, another rubber, EPDM, either new or possibly recycled, is used for the top layer. The system is best laid within a specified range of temperature and humidity. A UK company, Rosehill Polymers, has developed the expertise for the manufacture of such surfaces and other products incorporating crumb rubber from recycled tyres and other rubber waste. The company has been involved in the preparation of PU prepolymers for recycling post-production waste for well over 25 years. The company was a partner, during the mid-1990s, in one of the Eureka European recycling projects, EU446, whereby scrap rubber from tyres and also plastic bottles could be combined with a PU binder to generate a range of marketable end products. The choice of PU as a binder was a natural one, as no other material can match its versatility and flexibility, combined with its resistance to abrasion, weathering and chemical attack. Even so, the unique PU prepolymer process took four years to develop. It is based on the reaction of a blend of polyols, such as the Caradol range from Shell and MDI. The choice of grades is critical to the end product, with each application having its own demands for physical properties and environmental resistance. Rosehill produces unique PU binders which are one-part and solvent-free, relying on atmospheric moisture for the final cure, and which are made in a continuous process. The one-part binder is seen as the ultimate in simplicity and consistency for the end user, as it removes the uncertainties associated with multi-component mixing by operators unfamiliar with the complexities of urethane chemistry. Although the ratio of binder to recycled rubber (and also rubber to plastics) can be balanced to give the desired cost/performance criteria, a typical binder content would be in the region of only 13%. The resultant material can either be laid directly as a complete surface or moulded into tiles or other solid products. Traditionally, an operation like this would involve purchase of raw materials and converting them into finished products for sale to the end user. Rosehill, however, sells a concept whereby the customer purchases a processing machine, moulds, if required, and a supply of binder, but sources the largest component, the scrap rubber, locally. As well as the now familiar items such as playground surfaces, traffic cones and solid tyres, Rosehill has engineered solutions for replacements to a variety of items originally made from natural materials. The latter tend to be attacked by the elements and so have a limit to their useful life coupled to unpredictable and deteriorating properties. An example of this can be seen in the lumber operations of North America where the traditional method of transportation is to float felled logs downstream as part of a huge raft made from chained logs. The myriad of aquatic life that feasts off the main raft timbers 75

End-of-Life Tyres–Exploiting their Value causes them to be destroyed after a few trips. Intriguingly, Rosehill came up with an artificial log made from moulded tyre rubber bound with PU. The material has a similar density to timber and so floats but is impervious to termites and other water-borne bugs. A similar system has been devised to replace wooden rail crossing sleepers. The usual timber sleepers tend to crack and distort making for a noisy and bumpy passage, and are constantly in need of realignment. Vulcanised rubber could be used but the costs are high. A sophisticated sports surface has been developed for use on basketball courts in North America by Seamless Attenuating Technologies, Inc. (Sartec). Sartec has been pioneering the use of a mix made from recycled tyre granulate and athletic shoes. The SmartCells brand of surface consists of 40% recycled material and is structured as two platforms separated by a series of columns that act as springs. Testing to compare biomechanical data with those recorded for traditional wooden floors has shown highly promising results. The firm, based in Washington state, uses other local firms to prepare the materials and to mould the pads from compounded material. In addition to considering the new product for other sports flooring uses in volleyball and football, the company believes its greatest market could be in safety flooring in the healthcare industry. Use of Smartcells for hospitals and care centres may prevent the thousands of leg injuries sustained in those premises per year in the USA. The company has involved the Clean Washington Center (CWC) to help publicise its efforts and influential coaches in the National Basketball Association have become enthusiastic about the surfacing. Further court surfaces were laid in 1998 to provide more testing opportunities to professional and amateur athletes. Costs are likely to be greater than for hardwood flooring, but the Smartcells surfaces are estimated to last at least twice as long as a traditional court and to incur fewer maintenance costs. United Rubber Recyclers Inc., has rapidly become a large firm in this industry in the USA. It began as Waste Tire Management Inc., in 1995 and was sold by its founder to Republic Industries the following year. Under the new title, the firm is a subsidiary of United Waste Services Inc., a Republic company, and processes between 15,000 and 20,000 tyres per day. United Rubber Recyclers has established a good reputation as a prime supplier of landscaping materials, fencing products and paving for playgrounds, plus jogging and hiking trails. It also makes imitation mulches, which resemble tree bark such as cedar and redwood, for gardens, and matted materials, such as tree rings, for the horticultural market. A major achievement has been acceptance of the company’s spray-in-place imitation mulch for large highway projects. It is made from a combination of different sized chips, with a binder. The product is sprayed near highway fencing and when it hardens, it looks like bark or wood chips and helps to keep weeds down. United also makes a pourin-place paving material consisting of chips in a binder. The soft, pliable surface is ideal for hiking and jogging trails. One project was on trails in Tallulah Gorge State Park in northern Georgia. The company is based in Lawrenceville, Georgia with another facility in South Carolina. Most of the product is purchased by municipalities and state highway departments, although the company has started to move into the small retail market. A landscaping company in Ohio, USA, has started a subsidiary firm to provide safe materials for the ever-growing playground market. The special feature that Recycled Rubber Resources (RRR) brings to its material is that it is coloured. The complete spectrum from blue to red is available. RRR buy in shredded tyre chips and treat them 76

End-of-Life Tyres–Exploiting their Value with a non-toxic coating system invented in Texas. The material is sold as Boing! which now has patent protection and meets ASTM and US Consumer Product Safety Commission safety guidelines. The coloured coating contains an antibacterial, an antifungicide, and a fire retardant additive providing standard colours of green, purple, blue, red and yellow. Since RRR insists on a totally metal-free product, it has purchased a specialised rare-earth magnetic separator through which the purchased shred is passed since shredders would only guarantee product up to 99% metal-free. The particle size has been ‘standardised’ to three-eighths of an inch (~10 mm) to meet the requirement that small children would not choke if a particle was ingested. The material has also been used in horse arenas, golf courses and athletics facilities, mainly as a soil amendment medium. American Surface Technologies, Inc., (AST) starts with high quality, crumb rubber and sells pre-mixed, pre-packaged pour-in-place surfacing materials. The plant in Alberta, Canada uses 1000 tyres-a-day equivalent of crumb rubber to prepare SafetyPlay, a product that has resulted from extensive consultations in the marketplace. The company found that many potential customers were apprehensive about the need for on-site crews and plant to prepare and mix a ratio of binder chemicals into the basic rubber particles. Also, the traditional approach is usually very uneconomic to pave small areas. For its manufacturing process, AST takes its feedstock rubber from an approved supplier of crumb. The shape of the material is an important factor, the company has found, in selecting materials. Uniformity in geometrical shape is used for SafetyPlay because it imparts high tensile strength as well as resiliency and long-term durability. Binders and additives are mixed and dispersed through a computerised blend system that can automatically change the recipe for different products. AST has found with its control systems that it saves 25% of binders compared with traditional on-site pour-in-place methods and so reduce costs to the customer. There are two types of SafetyPlay. The S1 base course or shock pad is primarily intended for an underlay over compacted gravel, concrete or asphalt. It uses large particles, 4–6 mm, and is usually black. The W2 surface is made from SBR with 1–3 mm particles using coloured PU and pigmented crumb rubber, used as a substitute for EPDM top surface. A red and a green colour were first offered by AST during 1999. The products are supplied in 55-gallon (210 litres) drums, 5-gallon (20 litres) tins and also 50 lb (22 kg) bags. Berleburger Schaumstoffwerk GmbH (BSW) is a recognised global leader in the manufacture of products that utilise a range of recycled materials, including tyre rubber. The products are used for membrane protection, vibration insulation and sound damping. Among its rubber-based products, Regupol is used in a broad range of applications for athletics tracks, playgrounds, gymnasia and multi-purpose flooring, sports mats, sound deadening sheets, safety elements, etc., both separately or combined with bitumen sheets, membranes, PS, PU and mineral wool. No more than 10% of crumb goes into surfaces in its business. In the UK, around 10,000 tonnes a year goes into 80–100 sports pitches plus a further 8– 10,000 tonnes for the safety surface under play equipment. So far, the material has not been used in flat playgrounds where there is no equipment. This is at present an unexploited market.

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End-of-Life Tyres–Exploiting their Value Sports pitches last around 10–12 years and the shock pad is usually replaced at the same time. For gymnasia the shock pad may remain for 25 years where the top surface of PU or PVC based material can be replaced. SARCO is a British company that is exploiting the civil engineering properties of shock pads, made by a patented process, akin to modified asphalt but, in essence, utilising the dry method. This incorporates rubber crumb into the aggregate, not into the bitumen. A pug mill is used within which the aggregate is heated to 220°C. This is then agitated with the rubber for 50–60 seconds and allowed to cool to 180°C at which point bitumen is added and again agitated for 50–60 seconds. The mix is dropped from the mixing mill at a temperature of around 145–155°C, although 140°C can be used. The balance of rubber and bitumen is critical: 5% rubber to 10% bitumen. The process can accommodate up to 30% rubber overall. Changes in percentage rubber content give a change in properties. The resultant material is not a safety surface but a replacement for the asphalt within, say, schoolyards. The shock pads manufactured are to be regarded as elastomeric impact absorbing materials. The range of applications for which they are suitable tends to be covered by various civil engineering standards, however roads, as such, are not a target for the company’s efforts. The pads have been widely used as base layers for sports fields, and are now being considered for rail applications such as infill for level crossings and bases for rails where the electrical insulation properties of rubber bitumen can be exploited. Sureflex, the SARCO patented product, is regarded as impact absorbing asphalt and has first been used as a sports surface underlayer. The first pitch was laid in 1997 in Manchester, meeting BS 7044. A typical composition is: Aggregate Crumb Binder

62 wt% 29 wt% 9 wt%

29 vol% 34 vol% 11 vol%, also modified with SBS

Air voids are around 26 vol%. A finer rubber is used for tennis court compositions. There has been a 50% drop in injury incidence in a school playground trial laid with the Sureflex mix and a bonus of noise reduction. The 5% rubber addition mentioned above gives elasticity. Work at Nottingham University on products made by the SARCO method is being sponsored by the Highways Agency. There are two projects: for sports/safety materials and for road material to translate the US experience to UK conditions. The work will be considering sports medicine as well as civil engineering applications. Areas to be examined are uses for tennis courts and as a shock layer for Astroturf, bridge decks and cycle paths. It is known that 85% of safety surface injuries occur on impact, which is the time of peak deceleration. There is a test programme to evaluate components and a surface and determine the correlation with models developed from theory. This uses a head-form impact drop test to gather data. Peak deceleration is measured for sports surfaces and from the data a Severity Index is devised as a measure of safety. 78

End-of-Life Tyres–Exploiting their Value Crumb or ground rubber can be used for soil conditioning if it is first weathered to prevent any toxic effect from some of the active ingredients. It improves the moisture retaining properties of sandy soils and the drainage of clay soils. Desert soils have been rendered more fertile by the use of a rubber crumb based additive; this is believed to aid water retention and also to provide stabilisation for the growing roots of plants and grasses. The Rebound System has become recognised worldwide and is used in many applications. Rebound is a product incorporated into soil to produce a top quality turf with pleasing aesthetics, reduced soil compaction and healthy root development. JaiTire Industries, Denver, Colorado, was granted the worldwide licensing rights to market and sell the technology by International Soils Systems (ISS) who developed it. Rebound combines recycled crumb rubber from used tyres with organically composted materials. Used on athletic fields of all types, the playing surface becomes more resilient, with quicker divot recovery and a reduction in ground-related player injuries. ISS has patented its know-how after spending more than six years on the development. The system typically requires 30% less water and fertiliser because soil aeration is improved while soil compaction is reduced. Test data collected during a monitoring programme has confirmed better water infiltration, less compaction, improved seed germination and root growth, and also warmer soil temperatures in the test field. ATR, Jacksonville, Florida, has patented a process that mixes crumb rubber and compost to create surfaces for sports fields and parks. JaiTire Industries also licences a patent from Michigan State University for using crumb rubber as a “turf top-dressing” for golf courses and athletics fields. The material is sold under the name Crown III. In 1997, around 1200 tons were sold. Crown III has been applied at over 1500 sites throughout the USA, including courses designed by Jack Nicklaus and the Cotton Bowl in Dallas. JaiTire has been disappointed at the very slow take-up by schools and municipalities for which the company felt its product was ideally suited. The material was applied as a top dressing for an acre of new turf in a heavily utilised part of a city park in Pittsburgh. Point State Park is the finish post for marathon races and charity walks and the crumb was introduced on re-turfing the finish line area. The rubber protects the crowns of the grass and can absorb some of the shock and weight of pedestrian traffic. Use as a top dressing has been shown to reduce the amount of returfing required, thus lowering maintenance costs. Also less watering is needed and a longer grass-growing season has been observed. After the USA, JaiTire made its first export sales into the United Kingdom. Thirty-five of the top 100 golf courses in the UK and Ireland now use the material, including three of the five courses at St. Andrews, Scotland. Carnoustie, Royal Lytham and Royal Birkdale are other famous courses that now use Crown III to reduce wear on heavily trafficked parts of their courses. On football pitches, the material provides drainage, stabilises the surface and reduces frost problems. It has found favour with an increasing number of local authorities after a record number of games was played on the first pitch treated during 1999 due to far less damage and downtime than previously experienced.

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End-of-Life Tyres–Exploiting their Value Crown III is now being marketed in other European countries; Germany, Austria and Switzerland are being covered by Eximlink International, and Tyre Recycling Product LTDD, a subsidiary of AD International, is the distributor for France, Spain and Portugal. Another interesting end use for larger crumb, or rubber chips, is as a potting medium for orchids. A Colorado company, Fantasy Orchids, has a patent on its product, named EpiGro, which can be used straight from the bag like any other compost or compound. EpiGro is available in sizes similar to the traditional bark medium, between 25 and 60mm in length, from JaiTire Industries. The tyre chips provide the same growing environment for orchids as the traditional forest bark, but without the deterioration or other disadvantages of bark, such as water absorption, insects or mould. Orchids grow in air, not in soil, so the rubber just holds the plants upright. With bark, growers have to repot and remove soggy, clinging material. Since the rubber does not deteriorate, growers do not have to repot and the orchids can grow undisturbed. Major sales are now occurring in Hawaii, California and Florida, the big growing areas. One medium-sized grower used to buy 30 truckloads of bark a year, at 20 t per load—a big market that can be exploited. Waste Tire Management, Georgia, now markets its proprietary Perma-Mulch (made by its Encore Rubber Products division) through retail outlets like Ace Hardware and Wal-Mart. The loose crumb rubber, coloured to resemble wood mulch, and mats will be sold for landscaping to commercial customers and government entities. The matting has been laid on a well-travelled path in the Tallulah Gorge park in the state.

6.3.2 Sheet and Coating Products Carpet underlay is a long-established application for crumb in the UK. Carpet underlay is taken to mean a material which is separate from the carpet and laid underneath it, as distinct from carpet backing, which is integral with the carpet. Crumb-based underlays are essentially crumb bound by an appropriate binder. Even in the UK, however, they have only a small share of the market in comparison with integral carpet backings of latex foam, etc., and sponge underlays. Companies active in this field are Duralay and Lintafoam. Rosehill Polymers, UK, has long been involved with the techniques to convert crumb rubber into usable products, making use of the company’s expertise in PU chemistry. In 1996, the company started to promote safety tiles made from crumb rubber under its own name. The tiles are permeable, keeping non-skid and anti-slip properties intact because surface water drains away. Various profiles are on offer, ranging in thickness from 25 mm up to 85 mm and covering critical drop heights from under a metre up to 2.5 metres for the thickest version. In addition to playground locations, the tiles have been applied in gymnasiums, training rooms, walkways at golf clubs and entrances to shopping malls. Special moisture curing PU adhesives are supplied by Rosehill to bond the tiles. Another operation that concentrated on applications of crumb within end products has produced dividends over the years. Enviro-Flex, USA, makes rubber roofing and coating products. The developments have been the end results of a search for opportunities in which rubber crumb is used either as a filler or aggregate in latex products.

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End-of-Life Tyres–Exploiting their Value E-Flex 2 is a finish coat made with 30 mesh crumb and E-Flex 5, certified by Underwriter’s Laboratories, is a finish coat consisting of neoprene asphalt emulsion and 20 mesh crumb. A further stage of development, to make application more user-friendly, has resulted in a successful spray method. Another applicator from the same source is for the laying of resilient surfaces for playgrounds, running tracks, etc. It is called the Mini Paver and is a self-contained trailer unit that lays 1.2 m or 1.8 m wide rubberised paths in thickness ranging from 3–25 mm. Moving in one pass, the Mini Paver can install a resilient rubber surfacing, based on 5 mm crumb rubber and 20% urethane binder, at a rate of 333 m2 per hour. As well as fine rubber crumb as an arena surfacing material, the equestrian world uses a variety of rubber-based products including mats for walkways and stalls. Another application is the use of ‘blocks’ or ‘bricks’ rather than matting in some situations. A Canadian show jumping venue has installed rubber bricks for walkways and at the entrances to its arenas. Interlocking bricks are made by an Alberta company, Champagne Edition, Inc., taking advantage of the waste tyres collected under the province’s Tire Recycling Management Association activities. The rubber bricks provide a better grip for the horses, reducing the risk of slips or falls. They also cushion the horses as they step, which keeps legs fresh for the competition. Horses wear attachments called calks to compete which, under most conditions, make walking on asphalt treacherous. The bricks ensure that the horses remain healthy and competitive. The company exports the bricks to the rest of North America and to Europe. Each brick contains around 1 kg of crumb and are available in red, green or black, selling at US$2.25 a brick, which comes to around US$75 per square metre. A German company, Ikarus-Recycling GmbH, makes a range of products from its own production of rubber powder: mats, flooring and animal stall liners. The firm uses its knowhow to also make a range of porous irrigation hose and other irrigation products for arid regions where plant growth is being encouraged. The fine powders are also being utilised in protective coating systems to reduce corrosion on a wide range of surfaces.

6.3.3 Compounding Ingredient Crumb has always been used by the rubber compounder as a general compounding ingredient. Crumb used to be formulated in some tyre compounds as well as general rubber goods. However, it is very seldom used in road vehicle tyres today. Typical incorporation levels are of the order of 2% to 4% though higher levels are not uncommon. As a compounding ingredient in such small proportions, crumb is used both as a processing aid and as an extender. It improves mould release, thus increasing the sharpness of mouldings, and stiffens the compound, making for better control in extrusion and reduction of die-swell. In general, incorporation of crumb facilitates processing by assisting the release of gases from a compound and therefore saving energy. It has long been recognised that the extent to which 30 or 40 mesh tyre crumb can be added to a high quality rubber compound is limited. The reduction of properties, such as tensile strength, elongation, flex cracking, tear strength and abrasion resistance, in the final product increases as the percentage added rises. 81

End-of-Life Tyres–Exploiting their Value Much technical work has shown that very fine, micron size, crumb can be usable in the most stringent compound applications. There is only a minimal loss in properties of the finished product even with substitution levels up to 25 phr. However, the costs to make such fine crumb has made this approach uneconomical and is probably only worthwhile for specialist requirements and perhaps best suited to higher value elastomer wastes. The Holy Grail for crumb rubber has been the reuse of large quantities in new tyre compounds. A small percentage has always been incorporated into tread and other component compounds with an ease of processing among the advantages. In more recent times various developments have come to fruition with the incorporation of up to 10% of recycled content in the tyre rubber. In the USA, however, new CAFE (corporate average fuel economy) requirements for light trucks, vans and sports utility vehicles are likely to mean a need for more energy efficient OE tyres. This translates into lower rolling resistance. It also confronts the dilemma that increased recycled rubber content in a tyre tends to result in a greater heat build up and so increased rolling resistance. These factors work adversely on the need for lower fuel consumptions and, perhaps, also on the life of the tyre. Companies such as Michelin have on-going programmes to assess recycled rubber content in tyre components and the dynamic behaviour of tyres made from these components. A whole range of mechanical and physical properties have been shown to remain essentially the same as recycled content up to 10% of rubber weight is introduced into the compounds. The formulations of some of the compounds do, however, have to be adjusted for best effects. Properties of tyres made with these component compounds have been shown to have equivalent properties to a reference tyre for rolling resistance, tread life, traction, ride and handling, air retention and machine durability. A well accepted rule of thumb is that when more than 5% of a recyclate is added to a virgin compound the processing and physical properties can be adversely affected. Ideas and developments are continually being made to improve this figure. Many of the surface treatments given to particulate rubbers have this goal. Apart from the chemically aggressive use of treatment with radicals, most modification involves some form of coating with a compatible polymeric additive. This is believed to be the basis for Tirecycle, first introduced in the late 1980s, and the De-Link process announced in the mid-1990s. A driver to improve the quoted percentage that compounds can usefully contain has been the moves in recent times by major consumers of rubber products to incorporate a much higher percentage of recycling into their goods. Ford and Michelin, in the USA, announced a requirement for 10% in OE tyres in the ‘near future’ during 1997. The automotive sector, in general, has been encouraging suppliers to provide products with 25% recycled content. This higher figure may continue to be unrealistic across many components, but a level of 10% is a reachable target in other cases. Formulations to meet these targets are under investigation and development in many organisations in the rubber industry. The addition of processing aids based on fatty acid derivatives is found to improve properties of compounds that contain either treated or untreated crumb, but not always back to those exhibited by a virgin equivalent. Ford continues, with its RAT (Recycle Action Team) programme, to maintain, and increase, the recycled content of all its vehicles. Recycled-content tyres have been on a high proportion of the firm’s F-Series pick-up trucks and more recently its Windstar minivans. Brake pedals and splashguards are now commonly provided with a strong 82

End-of-Life Tyres–Exploiting their Value recycled-content, as are floor mats and many plastics-based items such as front grilles, fans and fan shrouds, luggage racks and roof liners. Ford has been examining the ability of De-Vulc material to be revulcanised as a proportion of a blend of virgin and recycled rubber material. Early work on natural rubber compounds for tyre treads and related applications, and on other formulations for vehicle mounts and bushings, has shown that the effect of De-Vulc on cure properties depends on the type of cure system, and possibly on other components in the compound. Thus, no generalisations can be made and each compound will have to be examined and optimised separately. The physical properties of tensile strength, elongation and tear strength were generally reduced by the presence of De-Vulc. However, compound optimisation may provide alleviation in these important properties. It should be pointed out that the physical properties actually obtained with De-Vulc present still represent a relatively high quality compound. The modulus and heat ageing characteristics are essentially unchanged by the presence of De-Vulc. The use of De-Vulc would appear to be a promising compounding tool. Crumb and treated rubber particles may also be used as a component in mixtures with thermoplastics such as PE, PS and PU. They are sometimes added in large quantities to extend or modify the properties of polymeric materials and converted into injection moulded parts and extruded sheet goods and hoses. Some of the reported activities in this field are presented here, but they represent only a glimpse of what has been developed. Renewed Materials Industries, Inc., USA, makes and sells Rumber Board from recycled crumb and plastics; it is used in the construction of utility trailers, flooring, low-boys, tiltdecks, fixed decks, and backhoes. Rubber particles treated by Composite Particles, Inc., have been considered for the toughening of epoxy polymer systems. Treated particles of 75 μm size, designated as Vistamer R-4200, are not as effective as a reactive liquid rubber, CTBN, a carboxylated random NBR; however, when used together in a blend there is a synergistic toughening. The plastic zone is increased in size ahead of any crack tip and so enhances the toughening effect. The best effect with a constant 10% presence of toughening agents is achieved with a 75 CTBN:25 Vistamer blend. This is a more than three-fold increase on the toughness of neat epoxy. Vistamer particles have already proved an advantage within footwear compounds. Their use adds wet grip to PU formulations and, with 10% to 25% of the modified rubber, increases the wet coefficient of friction to be comparable with pure rubber. Vistamer can be added in higher fractions to other polymers. A loading of 60 phr can be added to nitrile formulations for work boots and 30 phr in EVA foam soles. These are all under commercial development. Reebok reported that it is now using surface treated rubber scrap in the production of some ranges of its footwear. It did not reveal the name of its collaborator who carries out the chemical treatment. Marking compounds consisting of 75% recycled material have been approved for use and show excellent properties. Non-marking compounds are limited to 50%. This is a surprisingly high level and equates to a presence of 15% carbon black which ordinarily would create much marking. However, the interaction of the 83

End-of-Life Tyres–Exploiting their Value chemical treatment with the recyclate enables an unusually high level of carbon black to be present without showing any significant marking. The presence of a high fraction of recyclate has brought significant cost reductions on the order of 30%.

6.3.4 Road Surfaces The use of crumb rubber in road structures is discussed in this section and asphalt modification is examined. The use of granulate and crumb rubber as an additive to asphalt has long been regarded as a major route for the disposal of great quantities of rubber from scrap tyres. However, experience with the use of crumb in road construction has been evolving in various countries for nearly fifty years, with very little use materialising. Proponents believe that asphalt rubber is more flexible and thus reduces cracking. It creates a surface that resists aquaplaning and icing. Vehicles can stop on treated roads in a shorter distance and the surface substantially reduces traffic noise. Vehicle traffic or road miles travelled per vehicle is nearly doubling each year according to Federal Highway Administration statistics. This amount of traffic puts undue stress on roads to the point that new road construction and maintenance cannot keep pace under present funding. In order for road engineers and technologists to meet the new demands being placed upon highways, new hot mix asphalt designs and asphalt binder specifications need to be developed. The binder qualities of conventional asphalts (asphalt cements) now produced by refineries need to be enhanced by adding chemical modifiers which improve performance under many types of loads and climatic stresses. An asphalt binder is defined as the substance used to bind the aggregate particles together or to a substrate. Hot mix asphalt concrete is a designed aggregate and asphalt cement mixture produced in a hot mix plant where the aggregates are dried, heated, mixed with (liquid) asphalt cement and then transported, placed and compacted while still at an elevated temperature of 125°C or 135°C to give a durable, deformation and fatigue resistant pavement course. These definitions appear to present an ideal picture of how a road should perform. However, with the tremendous pounding to the surface and internal structure of the road by increased vehicular traffic, many of the designs incorporated into today's pavements cannot withstand the increased loads. This causes deterioration such as stress cracking, rutting and rippling. Environmental conditions, such as extreme temperature variations and moisture, also play an important role in the service life of roads. It is well known that roads need relaxation time for recovery. It is known that during the off-peak traffic hours, the small stress cracks within an asphalt road will tend to heal or close during resting hours. However, increased traffic loads around the clock prevent this healing, thus accelerating failure. It is felt by some experts that rubber, when properly reacted in the asphalt cement, provides memory to the binder matrix and thereby accelerates healing without loss of physical properties in the road. The occurrence of road failures and the trend to improve the properties of asphalt binders began with the rubber industry in the early 1940s. Rubber reclaimers realised that scrap rubber had many inherent properties that could improve the asphalt binder's performance. The components in a tyre include carbon black for ultraviolet resistance and strength, antioxidants for oxygen resistance, antiozonants for ozone resistance, sulphur for improved strength, and the rubber polymers, such as natural rubber, for improved tack or adhesion, and the styrene-butadiene and polybutadiene rubber for improved cohesion of 84

End-of-Life Tyres–Exploiting their Value the asphalt matrix. These ingredients found in scrap tyres transfers the enhancing properties to asphalt binders. The proponents of rubber in asphalt are usually found as members of the Rubber Pavements Association, a group of crumb rubber producers and asphalt rubber paving contractors. This association is a successor of the Asphalt Rubber Producers Group that originally represented organisations that had patented the use of crumb rubber in asphalt paving mixes and those using such techniques under licence from the patent holder. The original system was developed in the early 1960s by Charlie McDonald, then a city engineer in Phoenix, Arizona. He had ‘dissolved’ rubber crumb particles into asphalt for the city streets and also for a runway at the airport and subsequently discovered a greatly extended life for the surfaces, with reduced maintenance costs, improved traction and reduced highway noise. This was the birth of the so-called ‘wet’ process that is still used in the vast majority of rubber asphalt projects. There is, however, a ‘dry’ process whereby crumb rubber is substituted for a portion of the aggregate used within the asphalt mix. In some instances, used tyre crumb rubber is used both in the aggregate and the asphalt mix on the same surfacing projects. In Phoenix, the use of asphalt rubber proved a technical success from the start, despite wide variations in mixing and laying procedures. However, it was expensive due to both licencing fees and the limited quantities required/available. The patents have now expired for these early processes. Another factor back in the 1960s and 1970s was opposition from the Federal Highway Administration (FHWA), whose then director had close ties with the National Asphalt Pavement Association (NAPA). NAPA did not take kindly to any process or technique that would extend the life of pavement surfacing and reduce the need for its members’ activities, although at the time other factors were raised to support rubber’s rejection. FHWA and NAPA questioned the recyclability of asphalt rubber pavement and the possibility that heating asphalt containing rubber crumb particles would pollute the air, although they retreated on that point when tests showed heating standard asphalt raised pollution issues. In addition, the FHWA found many reasons to delay tests that would eventually disprove these assertions. It is now widely accepted that proponents of asphalt rubber made a strategic error in pursuing the mandate for its use in federally funded highway projects as part of the 1991 ISTEA highway bill. The asphalt rubber association had opposed this mandate, correctly anticipating that it would set off a storm of protest from state, county and local highway engineers who objected to being told what to do. Though the mandate was later repealed, the damage had been done and the lobbying office in Washington was closed as the asphalt rubber proponents retreated to regroup. This left, in 1996, only Arizona, Florida and California as significant consumers of asphalt rubber. Since then, use has been growing rapidly across several states. California’s Department of Transportation (Caltrans) has experimented extensively with asphalt rubber under a variety of conditions and, in most cases, it has performed extremely well. Tests in the state have even indicated that asphalt rubber surfaces need only be as much as 50% as thick as traditional asphalt, thus making it cost-effective even without considering that it lasts as much as three times longer.

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End-of-Life Tyres–Exploiting their Value In southern California and the San Francisco area, rubberised asphalt is being used for roadways because of its cost-effectiveness and durability. According to the County of Los Angeles Rubberized Asphalt Concrete Technology Center (RACTC), based in Alhambra, other benefits include resistance to reflective cracking, rutting and shoving, a skid resistance surface, reduced tyre noise, and colour contrast for striping and marking. According to the Center, temperature is critical in the rubberised asphalt concrete process. The initial rolling must be done prior to the temperature of the fresh surface dipping below 144°C to ensure a specified density of 95% relative compaction. The RACTC was set up as a joint effort by Los Angeles County and the California Integrated Waste Management Board as an information and service hub. New Mexico and Tennessee have also embarked on serious programmes. In Texas, where semi-autonomous district engineers have not always looked with favour on the rubber option are now increasing their use of asphalt rubber. Nebraska State Recycling Association has acted as the catalyst for interested parties to examine and promote projects for both urban and rural roads. One county engineer from Ontario has successfully cleaned up all waste tyres in his county and continues to use them to improve the roads under his charge. He had overcome scepticism, ridicule, political opposition and financial obstacles to demonstrate how asphalt rubber road surfacing is a practical solution to the waste tyre problem. Before the federal mandate fiasco 45 states had tried some form of modified asphalt containing crumb rubber. Many will now eventually revisit this potential solution to the waste tyre problem within their jurisdiction. Work in Canada at the Ontario Ministry of Transportation has investigated the effect of crumb particle size and partially devulcanised tyre rubber on the properties of crumb modified asphalt. The aim has been to improve both high temperature and low temperature performance of rubber modified asphalt mixes. Cryogenically ground 30 mesh and ambient ground 80 mesh rubbers were employed. Partially devulcanised 30 mesh material was also used in the trials. Various tests appropriate for road materials indicated that a finer crumb should provide greater resistance to rutting, a high temperature phenomenon, and also slightly better thermal cracking resistance. A thermo-mechanical process of high-shear mixing whereby the 30 mesh crumb is incorporated into asphalt under high shear and temperature was found to result in a mean particle size of only 0.4 microns. This compares to the 590 microns of 30 mesh materials, a significant size reduction. It was noted that quite large improvements could be effected in high temperatures performance by these changes in particles size, but much smaller gains were achieved at the winter temperatures for cracking phenomena. Since the patents on the use of crumb rubber in asphalt mix expired in 1992, Arizona, home of the original work in this field, has seen independent contractors use crumb from well over a million tyres with hot asphalt for city streets. FNF Construction has used an estimated 50,000 tonnes in over a half million tonnes of hot mix. In addition to the state Department of Transportation, cities such as Phoenix and Glendale are using the product. As with the original work by McDonald in the 1960s, today’s engineers are not claiming to be recyclers. The material improves the engineered benefits of asphalt in both the hot desert climate and cold mountain conditions, both extremes found within the state, and the recycling aspect is an added bonus. Since 1992, Arizona Department of 86

End-of-Life Tyres–Exploiting their Value Transportation specifications of rubber have tripled, with roughly 15% of the state’s highways having some rubber mix in them. A US patent, 5936015, has been awarded to Creanova for an improved rubber-modified asphalt paving binder that incorporates a quantity of TOR. The binder can contain from about 20% to about 80% of asphalt cement, from about 0.5% to about 20% of crumb rubber and from about 0.01% to about 10% of the polyoctenamer. The process provides the ability to use a low-cost raw material (10 mesh crumb instead of finer crumb) and allows a thinner application of the asphalt/aggregate concrete (37 mm instead of 75 mm). Processing costs may also be reduced as there is no separate mixing time as in a wet process. The simple process also only needs a conveyor belt or screw in addition to the normal mixing equipment for traditional asphalt concrete. Arizona uses a mobile wet mixer for its asphalt rubber (AR) and the high viscosity of the hot mixes can cause pick-up on the rollers of the paving equipment. TOR can help by reducing the viscosity and providing an even spread. Surfaces of TOR with AR showed no cracks in a trial in Grey County, Ontario, whereas with AR only, pick up on the rollers gave imperfections on the road surface and subsequent collection of water that froze in winter. Creanova believes it can offer lower raw material and processing costs yet provide high performance. An international expert from Portugal is leading much of the European work in this field. His company, Consulpav, also has a presence in California working with the Rubber Pavement Association (RPA) and has devised a range of tests that have become industry standards. Three point bend flex fatigue tests are used to measure lifetime to 50% of modulus. AR gives 10 times the life for a given loading regime. Although laid at half the thickness of standard surfaces, which induces twice the strain, the AR versions still give longer lifetimes. In a thin layer a good reflective fatigue is needed. AR provides this as the presence of the crumb particles prevents reflective cracking. The basic nomenclature and the differences between the US and EU must be remembered in all debates on rubber material in road construction. There are also many other materials besides crumb rubber that can be introduced at various stages in the preparation of road surfacing. These two points are summarised in Tables 6.1 and 6.2, respectively. Table 6.1 Nomenclatures used for Rubber Material in Road Construction Region Binder Mix Europe Rubber Bitumen Asphalt USA Asphalt Rubber Asphalt Concrete

Route Wet Wet Dry

Table 6.2 Materials Utilised in Road Surfacing Method Materials Modified bitumen Elastomer, plastomer, latex, crumb rubber Bitumen with additives Stone also being added Added to main aggregate Polymers, recycled plastics, fibres, rubber aggregates

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End-of-Life Tyres–Exploiting their Value To put the ‘rubber pavement’ discussion into the context of world roadmaking, a survey by the PIARC (the world Road Association) covering 23 countries gave the level of modification utilised globally as shown in Figure 6.2:

Figure 6.2 Level of Modification of Road Materials

There are two ways of looking at these figures! The pessimists would say that given the tiny proportion of programmes that take crumb why should we expend all the effort? However, the optimist realises that there is a huge potential market out there from which only a few percentage points increase for crumb rubber would solve many fears and concerns about the recycling of used tyres. The survey also covered the scale of usage by the various countries (Table 6.3) and provided a summary of the reasons for not using modification materials. Table 6.3 Level of Rubber Consumption by the Countries that use Modified Bitumens Rubber consumption Large usage Medium usage Small usage None Italy Ireland Denmark, Norway, Sweden, Russia < 0.5% Spain, France Belgium Hungary, the Netherlands 2–6% Germany, UK Austria, Poland, Portugal Very high (33%–70%) Austraila Canada Malaysia Source: PIARC

General reasons given in the survey for not using modifiers included: •

unnecessary for traffic and/or climate conditions,

•

absence of specifications,

•

problems in hot recycling (when the bitumen surface is remelted after being dug/scraped off when resurfacing takes place) later, and

•

binder too costly,

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End-of-Life Tyres–Exploiting their Value together with the following reasons specific to rubber •

resistance from polymer and bitumen producers,

•

storage instability,

•

most expensive option, and

•

no crumb or mixer machinery available.

Starting with Arizona, the new attitudes have been taken up by many states, the leading ones being California, Florida and Texas. The number of tyres consumed for roads in those states in 1998 and 1999 is given in Table 6.4. Table 6.4 Tyre Consumption in Asphalt Rubber in Some US States (000s) Arizona California Texas Florida 1998 2,050 360 1,110 1,600 1999 1,800 1,500 433 2,955 Source: ADOT presentation, ETRA meeting, Brussels, March 2000

Colas France is a major road construction company and has developed a rubberised surfacing material known as Colsoft. Road contact noise is the key, so that a reduction caused by a change in the nature of the wearing course is to be encouraged. Standard NF S31-119 applies to vehicles at 90 km/h and NF S31-085 to all sounds. Colas use the dry route with a hot mix at 165–175°C containing 2% to 4% of 2 mm rubber used in layers of 25–30 mm thickness. Some sound measurements were recorded on stretches of road before and after Colsoft was laid. One, at Daumesnil, Paris, gave the following results: dB(A) Before After

day 71.7 67.1

night (22.00–6.00) 66.1 60.3

showing a reduction of 4.6 dB(A) and 5.8 dB(A), respectively. Using S31-119, the figures were 80 dB(A) and 72.6 dB(A)—a drop of 7.4 dB(A). The company has found that no matter the measuring technique used for such sound testing, or the location, there is a reduction in the range of 4–7 dB(A). The ambient temperature must be above 5°C when laying the material. Traffic and pedestrian access must be delayed until the mix/surface temperature has dropped below 100°C. A gap-graded material is used but in a dense mix. The surface also offers improved skid resistance. In 1994, around 11,000 m3 of Colsoft was laid, rising to 220,000 m3 during 1998 and 400,000 m3 during 1999. So far this has been entirely in France, but sister companies in Ireland and the UK plan to use Colsoft where and when appropriate. Portugal has now espoused the use of rubber crumb within its road pavement programme. It is using the Arizona wet method and has paved 30 km of roads in the north.

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End-of-Life Tyres–Exploiting their Value It has been found that the viscosity of the mix can vary with the choice of crumb source. As a result, the time and temperature of mixing has to be monitored. The conditions are determined by laboratory checks on the bitumen and crumb to be used for a particular job. A crumb plant in the south of Portugal will supply only for asphalt rubber production. The average content seems to be 18% rubber, taken from car tyres, with no extender oil. The crumb size must be less than 16 mesh. The Portuguese have found that skid resistance is, on average, improved by 25%. Shredded tyres have been used for drainage backfill (but only a total of 500 tonnes over the last five years) on 35 sites in the UK. The top 200 mm of vulnerable trenches are so treated. The forecast is for around 100 t/y. There are 10,000 km of trunk road covering 50,000 acres in the UK, 8% of which are concrete. There are 200 x 106 m2 of asphalt surface with a life in excess of 10 years. This means that over 20 x 106 m2 are resurfaced every year. A thin surfacing technique is used for this purpose. If this resurfacing quantity was accomplished as 20 schemes in a year, it would mean around one million square metres for each one. At approximately ½ tyre per m2, this means 500,000 tyres per year for each job. This equates to around 3,500 tonnes per job. As far as the Highways Agency (HA) is concerned, asphalt with rubber must perform as well as the traditional medium in way of skid, noise, etc. It must also be competitive against £8/t for aggregate and £28/t for glass cullet. The surfaces must be maintainable and be recyclable when the wearing course is stripped off another 10–15 years down the line. Existing surfaces are cold-planed prior to renewal. Comments were made that the US experience of recycling has been variable. A development by Colas, UK, is Safedrain, a rubber and bitumen top layer for roadside drains. The company had taken over the rights to the concept from Roadtex, which had developed a method of using granulated tyre material to solve the problem of stray gravel on motorways and other main roads. Roadside ‘French Drains’ consist of a pipe at the bottom of a narrow ditch alongside the carriageway, at a depth of about a metre, covered with gravel and aggregate. Vehicles that stop in an emergency may cause damage to other vehicles as gravel is sprayed outwards. The top 150 mm or so can be made from coarse rubber granules bonded with bitumen, yet remain porous. Safedrain has similar permeability to conventional granular materials. Developed in conjunction with the Transport Research Laboratory, several trial surfaces were laid on major roads in the UK. Now fully accepted, the Safedrain concept is taking about 100 tonnes of granulate a year in various Highway Agency projects. Another application for edge material could be to prevent the breaking up of the shoulder of the road on country lanes. Heavy vehicles cause great damage to tarmac edging and this disrupts the drainage pattern and so weathering creates more damage. The rubber asphalt mix can support the road edge, while carrying surface water away and withstanding overrunning vehicles. In general, there has been little interest in the use of crumb in Western European road surfaces. Local authorities tend to point to the adequate and inexpensive road surfacing materials available in the form of concrete, bitumen, asphalt, etc., and are reluctant to 90

End-of-Life Tyres–Exploiting their Value make higher initial investments in road construction against the possibility of lower repair costs or other hypothetical advantages. It also has to be borne in mind that European roads use different constructions from those in the USA, and the roads in many European countries are thought to be less prone to cracking. Possibly crumb will be used in special situations such as at road junctions, corners, airport surfaces and bridges, to improve grip and wear resistance, and also outside hospitals for quietness. In summary it would still appear that regulation would be required as a driver for the dramatic increase in rubber uptake as a road material for which so many proponents are waiting.

6.3.5 Miscellaneous Products A range of high-quality soundproofing panels is available from the Royal Mat Company, Canada. Used primarily as a subfloor in construction and, especially, renovation, the Neutra-Phone panels are a result of collaboration with sound specialists and the National Research Council. The panels are made by a thermocompression process that gives the finished panels a special configuration of grooves, the dimensions and angles of which were calculated to optimise the absorption of vibrations and airborne sound. The panels perform well in ASTM standard tests for both airborne vibrations and impact noise and are a patented item. Parking curb guards are manufactured by GNR Technologies, Canada. Park-It curbs are being installed by contractors, paving manufacturers and property managers to protect walls, vehicles and other valuables in garages and surface parking facilities. A German company, New Environmental Technologies, has developed some very life-like roof tile products from scrap plastic and rubber. The company has been expanding capacity for its patented Roweflex products to 900,000 m 2 per year with over 70% committed to long-term contracts that stretch to the end of 2003. The lightweight tiles are easy to install by conventional methods and have been found to provide improved insulation from heat, cold and noise in both Europe and Asia. A spread of companies showed a range of products for use in highway traffic control and road safety applications at the annual expo of the American Traffic Services Safety Association. This confirms that one of the major areas for the use of recycled rubberbased goods is for road furniture and traffic control scenarios. Panels, glare screens, guideposts, cone bases, etc., can be made profitably from recycled materials. RL Bates Company, USA, and its recycling subsidiary Enviroform, have devised a special binder formula for use in mouldings that are made by both cold and hot compression processes. Interesting applications are for wheeled sign bases that are greatly used by supermarkets, both outside and within the aisles of the stores. The unique castor design allows the signs to be easily moved. Other products are rumble strips, ramps for access over kerbs, a pipe protector/holder to allow trucks and other vehicles in construction areas without damaging pipes or other services lines.

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End-of-Life Tyres–Exploiting their Value The American subsidiary of Kraiburg offers a product for use as a walkway and rail crossing material that is slip-resistant. PedeStrail is part of the Kraiburg Hi-Rail system and features modular, rubber panels that interlock between the rails for a smooth watertight fit. The high-density rubber design uses 100% recycled tyre rubber and allows the panels to flex in changing weather (temperature) conditions. The product also meets the requirements of the American Disabilities Act that stipulates that any crossing surfaces be flush and level with the rail top. Other products are Hi-Rail full depth rubber grade crossing pads and Hi-Rail RS rail seals that provide a durable rubber buffer to improve asphalt grade crossings. The Hi-Rail RS is a full depth rubber strip that fits snugly against the rails and fills the gap between the metal rails and the end of the asphalt-covered roadway. It can flex with the passing traffic and its contoured design provides a positive seal to block out moisture and protect both the roadbed and the rail fastening system. The flangeway grip also eliminates gaps, which can trap debris. The design allows for easy snap-in placement and removal for refurbishment of either the road or the rail system. Rediweld Rubber and Plastics Ltd., UK, offers a range of road furniture and traffic calming products, some of which use recycled rubber. The Traficop range includes speed cushions, flexible rubber curbing, ramp mats and similar products made from compression-moulded rubber and injection moulded plastics. The company also makes Takpave rubber tiles for road crossings and pedestrian walkways. Rediweld uses about 20 tonnes of recycled rubber a week. The rubber is raspings from the truck retread sector that are screened and cleaned before entering the production process. Italian researchers at the University of Bologna are exploring possibilities for the use of recycled rubber ‘powders’ in polymer mortar. They are studying the effects of replacing some of the sand filler in unsaturated polyester mortar recipes and have found that only around 3 volume-% should be replaced (just over 1 weight-%); any higher level of replacement of the inorganic aggregate leads to a marked reduction in mechanical properties and an unworkable, stiff, mixture. The use of a silane coupling agent helps to reduce some of the slight porosity that is introduced into the mortar by the organic content. Compressive and flexural strengths of the resin composites are reduced somewhat by the presence of the organic filler, yet are still greater than those for a traditional Portland Cement mortar. Environmental Products Co., USA, turns crumb rubber into burial vaults which contain the casket. The ‘En Viro Vault’ uses around 225 kg of crumb and measures 212 x 80 x 80 cm. The rubber versions are as strong as concrete but more water-resistant as well as being half the weight. Roll-Gom, France, which is one of the largest rubber recyclers in the world, opened a new plant in the USA in 1996 to turn tyres into castors. The plant near Charlotte, North Carolina, has capacity for 3500 tonnes. The company turns over 3 million tyres into over 15,000 tonnes of rubber in its French operations. The granulate so generated is pulverised to a fine powder, and the metal and other impurities are removed. About 60% of the tyre weight is usable rubber. Additives like sulphur are incorporated, and the powder is revulcanised at 180°C and moulded into strips or rings between 150 mm and 500 mm in diameter. One million a month are made for use as castors for items as diverse as dustbins, trolleys, scaffolding or cement mixers. Recycled rubber has been found suitable for a middle weight range where the castors have to carry 200–400 kg. Heavier loads need the properties of virgin rubbers while the lighter ones are catered for by plastics against which the recycled rubber cannot compete on price. 92

End-of-Life Tyres–Exploiting their Value The company exports about 60% of its products and now has a subsidiary called Ecogom which prepares and lays crumb rubber for safety surfaces. The US operation had a secure customer base when it started manufacturing wheels for garbage containers and refuse carts, in the shape of the US operations of its European clientele. In 1998, the company used almost one million tyres, from both cars and trucks, in making three million wheels. These range from 15 to 40 cm in diameter and can carry weights from 110–270 kg per wheel.

6.4 Pyrolysis

6.4.1 Introduction For the discussion within this section, combustion is defined as thermal decomposition in an oxidising atmosphere and pyrolysis is defined as thermal decomposition in an inert atmosphere. With combustion methods of tyre disposal the end products are reasonably constant. The combustible materials form water vapour, carbon oxides and sulphur oxides while the non-combustibles form ash or clinker. The heat generated by the reaction can be used for steam raising or in other thermal heat exchange processes. Pyrolysis has no such clear-cut results. Depending on the conditions under which it is performed, a wide range of gaseous and liquid hydrocarbon mixtures can be obtained, together with varying amounts of residual char. A great surge in development of pyrolitic methods for recovering oils from hydrocarboncontaining wastes took place in the years following the oil crises of the 1970s. As a result, there have been numerous attempts to introduce economically viable techniques of tyre pyrolysis over the last 25 years. Commercial-scale pyrolysis plants have tended to fail because of the low value of the end products in relation to the capital and operational costs. This was exacerbated when the cost of oil dropped again in relative terms from the mid-1980s, and is still the case today. A detailed review was published in 1983 by E.G. and G. Idaho Inc., for the US Department of Energy, entitled “Scrap Tires: A Resource and Technology Evaluation of Tire Pyrolysis and some other Technologies”. This and much published work in Europe from the University of Hamburg and the Université Libre de Bruxelles are most frequently cited by workers in the field. Professor Fontana and his team at the latter institution are particularly active in monitoring pyrolysis developments. Various well-publicised operations in the USA, the UK, Germany and other countries have all closed. A strong conclusion from this, especially for large plants, has been that pyrolysis alone is unlikely to produce commercial products other than secondary raw materials. It is significant, therefore, that many of the more recent ventures incorporate second stages. These include immediate cracking of the oils and/or of gases, downstream treatment of the carbon chars or the direct use of the hot by-products for steam or power generation.

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End-of-Life Tyres–Exploiting their Value These all appear to use one of four process classifications, based primarily on the type of vessel in which the pyrolysis is carried out: vertical shaft, rotary kiln, fluidised bed, small batch retorts. Notwithstanding past failures, further projects continue to be reported from various countries. Some of the current ones are described in the following sections.

6.4.2 Developments in North America In 1997, the STMC stated “Pyrolysis is inherently sound, but there are no commercially viable markets for the by-products. It is non-competitive on a price or quality basis. There is no commercially viable operation in this country (USA), in this hemisphere or in Europe.” Proponents vigorously disagree and believe that their version of the process will be profitable, once a plant is up and running. The Pyro Division of Svedala Industries Inc., has been pursuing a development of the pyrolysis method for tyre disposal that places emphasis on the quality of by-products generated rather than the rate of throughput of the waste input stream. The Process Research and Test Center (PRTC) of the company has evolved a proprietary manufacturing process to maximise revenue-earning ‘pyro-products’. They have recognised the failure of so many pyrolysis proponents to obtain outputs that can be marketed on a sustainable basis. The most potentially valuable element of pyro-products has always been the solid char (or carbon char) fraction. It is also well known that the ratio of yields for gas, oils and solids varies with the pyrolysis temperature. Much effort was devoted by Svedala to maximising carbon char yield from a continuous process and then improving the quality of the resultant pyro-char, an area found extremely difficult in the past. Carbon blacks made from pyrolysis residues, no matter how fine and so, potentially, more reinforcing, have been found to be less successful in rubber reinforcement than their nominally similar virgin counterparts such as N-990 or N-770. The Svedala PRTC process generates a black filler (pyrolysis carbon black, CBp) that performs the same as, or slightly more reinforcing, than SRF (N-770 type) and approaching the performance of GPF (N-660 type) carbon blacks. A potential use is as an additive to road asphalt, in a similar way to the use of ground rubber. Batch process dynamics were refined to give a yield in the 35%–40% range, an increase from the 25% achieved by other, earlier, continuous systems. These parameters were then used after a continuous rotary kiln had been built. Proprietary elements were developed for: •

the continuous feed of the shredded tyre chips into the reactor through air locks, preventing any oxygen ingress,

•

the shape of the internal shell and vapour extraction system to keep carbon in the char,

•

the separation of wire and char on discharge from the reactor, and

•

the use of the pyro-gas for process heat.

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End-of-Life Tyres–Exploiting their Value In addition, the company has patented a technology, as part of the PRTC process integration, for upgrading the char by particle grinding and other techniques. A CBp product called P-Black 079 was found to have high potential for use in industrial rubber goods. The company plans to offer 30 to 100 tonnes per day plants, costing between US$4 million and US$10 million. It would also provide worldwide support to market the refined CBp. A typical turnkey plant to process 50 tonnes a day (or ~5000 tyres) with an investment of US$4 million is projected to realise revenues of US$6000 per day from CBp sales (at 33 c/kg) or US$1.6 million per year. Such a plant would generate around 5000 tonnes of upgraded pyro-char which is claimed can compete in a market that has a demand for 50,000 tonnes per year in the US industrial rubber goods sector. Svedala had two plants running in the USA in early 1999. Based on 100 tonne per day units, the rotating kilns generate 52% of hydrocarbons, 37%–42% of char and 11% of steel. Feedstock is 50–70 mm tyre chips with the main product being a CBp that can be used at up to 100% replacement in mainly non-tyre products; some good grade oil is also produced. The CBp is a match for N-774 and sells for 44 c/kg. Four more projects were in the pipeline at that time. It was noted that 28–30 similar plants would fulfil all non-tyre carbon black requirements. Work at Laval University, Quebec, Canada, supported by Pyrovac, a Canadian pyrolysis company, found roughly similar improvements from the use of CBp instead of commercial carbon blacks in road pavement asphalt binders. More detailed testing of CBp in bitumen and asphalt concrete confirmed that the pyrolitic carbon material provided enhanced properties to the modified bitumen, over and above that generated by conventional carbon black. A very strong absorption of bitumen onto the CBp particles provides this extra ‘stability’. In addition, the positive impact of the pyrolitic carbon black on the concrete prepared with CBp-modified bitumen was also evident. Tests indicated that the compacted modified concrete showed a higher resistance to rutting, high temperature, water and ageing effects. The mixtures also exhibited a significant ability to withstand severe thermal stress when subjected to gradual contraction and freezing. It is believed that the lower-cost CBp could find significant and sustainable markets as a bitumen reinforcement in road-pavement asphalt mixes. Another partnership that has plans to generate carbon black, steel and oil from tyres is that between Environmental Waste Management Corp., (EWMC) International Inc., Canada, and a subsidiary of Continental General Tire Inc., CG Tire Inc., USA. It has been exploring EWMC’s Rubber Reverse Polymerisation Processor that uses a patented microwave technology to breakdown tyres into the solid and fluid components. Yet another organisation to surface in this fraught sector is Titan Technologies. Air Group Inc., has acquired exclusive worldwide marketing rights and a 50% interest in Titan’s proprietary hydrocarbon recycling technology. The first American facility was scheduled for Broussard, Louisiana, and would be modelled on two facilities built in South Korea by Titan. This facility is a joint venture between Air Group and Environmental Remediation Holding Corp., which specialises in oil 95

End-of-Life Tyres–Exploiting their Value remediation and hazardous waste clean up. The resultant oils, carbon black and other byproducts were budgeted to earn US$400 million a year from the Louisiana plant. A plant was built in Austria in 1998 in which small chips of tyre grindings are fed into a decomposition chamber, where heat and catalysts break them down. Seven stainless steel cylinders are employed from which the gases are drawn off to water-cooled condensers and the steel and carbon black are removed from the bottom. The first US operation is believed to be based at Lincoln County, West Virginia, and not in Louisiana as originally announced. The Jerrold Company claims to have designed and patented a machine that turns discarded rubber tyres back into the basic tyre chemicals. All the energy required for this recycling process comes from the light waste gas produced during the process. It claims to be pollution-free except for the release of some carbon dioxide and some water vapour. This invention is said to turn waste tyres into a source of energy and raw materials. It is essentially a destructive distillation of whole tyres under pressure. For even heating, the tyres are sorted by approximate size and compacted into a perforated metal canister. The canister is placed in a pressure vessel with a heating element at its centre. The vessel is heated gently and a vacuum pump removes the air and water vapour. It is then sealed and reheated. As in any distillation process, light gases are given off first. The pressure rises to 350 psi at which point the venting starts. In early 1999, Thermex Technologies opened a pyrolysis operation in Quebec province. The firm planned to process more than a million tyres each year through its twin processing units. Whole tyres would be converted into oil, steel and carbon black. Development aid has been forthcoming from both local and national sources in Canada. The company claims not to need to stockpile tyres but processes all that are delivered to the plant. Sales of the technology were expected to result in up to six processing units being installed during that year. The Pyrovac Group is based in Quebec, Canada. It comprises a group of companies that are involved in the development, engineering and exploitation of environmental technologies. Pyro Systemes is one of those units which has investigated the vacuum pyrolysis route and runs a commercial operation in Jonquiere. The development was carried out at the Pyrovac Institut in collaboration with Laval University and the resultant technology is protected by patent and trademarked as Pyrocycling. Use of a vacuum shortens the residence time of primary fragments in the reactor and removes the need for any secondary action. A high-grade oil and good quality carbon black result from the continuous 3.5 tonnes per hour system. Molten salt is used as a heat-exchange medium. This lies under a heating plate along which the material moves. A demonstration plant was set up first for biomass, tree bark, in which the feedstock resides for 7 minutes before becoming vapour. The capacity of this unit is 160,000 t/y for bark and costs Can$40 million. The tyre unit operates at about 500°C and 6–8 kPa pressure. The output is about 45% pyrolitic carbon black (CBp) and 35% oil. One good feature is the ability to remove the zinc oxide by demineralisation. Coke-like deposits are found on CBp that is formed at high pressure and low temperature. A process of part pressure and high temperature with a low residence time produces CBp that is accepted as equivalent to the N-700 series. 96

End-of-Life Tyres–Exploiting their Value The various output streams have been valorised for a range of end uses: •

A naphtha fraction is a source of limonene and benzene, toluene and xylene, worth US$2–9/litre. Pyrovac has also worked with Exxon to provide an unleaded medium oil for the chemical industries.

•

Heavy oil for coke feed (taken by SNCF, Toulouse).

•

The bitumen left over.

•

CBp has found use in road surfaces for temperate climates in rubber as N-774 or N330 when treated and in plastics as a pigment and UV stabiliser.

A 10,000 t/y unit would cost US$6.5–10 million, with a 38,000 t/y plant, for US$11–16 million. Cyntech Industries, based in Houston, Texas, announced plans in mid-1999 to build a hydrocarbon plant to process scrap tyres into methanol at 40% less than the prevailing costs. Cyntech will use its patent-pending Thermal Reduction Technology process which is a catalyst-free, closed-loop gasification system that breaks down rubber and plastics into methanol, gas oil and process gas. The process is understood to use heat and pressure in a pyrolitic process to make a gas oil, liquefied petroleum gas (LPG) and carbon black. The company would convert the carbon black into synthesis gas as feedstock for Air Products methanol process. The LPG and gas oils would be sold as a mixed output stream. An 18 million gallon (~70,000 t) plant would process 9 million waste tyres per year. Toups Technology Licencing owns a patented electromagnetic recycling technology (EMR) that it claimed in 1999 had been licenced to an unnamed group that intend to build and operate up to 20 tyre recycling plants throughout the USA. The EMR process is designed to produce gas, oil carbon black and steel.

6.4.3 Developments in the UK Beven Recycling, Oxford, has been developing a batch process. The technology is the culmination of refinements of an early method (from Herbert Beven & Co. Ltd.) designed originally to take organically rich waste. A joint venture with AEA Technology at Harwell followed in the early 1990s which concentrated on using only tyres as the raw material input, and which developed the equipment further. Beven Recycling was set up in 1993 after the privatisation of the AEA; AEA’s shares in the joint venture were bought by Beven Recycling. The following details are taken with the permission of Beven Recycling from the company's literature. The plant is made up of retorts that can be sized to suit a particular market, e.g., an 8retort plant can process about 1600 tyres (10.5 tonnes) in 24 hours or around 500,000 tyres per year. These generate about 4.2 tonnes of carbon, 3 tonnes of oil, 1.7 tonnes of gas and 1.6 tonne of steel per day. Tyres are loaded whole (sometimes baled) up to a maximum of just over a tonne (about 150–175 car tyres) into retorts which are suspended over furnaces. The system is purged with nitrogen and the burners started. As the tyres heat, the vapour produced passes from 97

End-of-Life Tyres–Exploiting their Value the retort into water cooled condensation vessels where the liquid phase, which is oil, condenses out. The remaining gases pass through a small scrubber before being piped to the gas burners to self-fuel the process. When the process is completed the retorts and contents are cooled and purged with nitrogen. The oil is pumped to storage. The next cycle commences almost at once with a second set of furnaces and retorts. When the content of the first retorts is cool, it is removed and the steel separated from the carbon before the retorts are refilled with tyres for the next cycle. The plant has been designed to be fully automatic and does not require highly skilled operators. The emissions have been fully monitored and analysed and found to be well within current UK guidelines. The demonstration plant has full Integrated Pollution Control Authorisation to operate (from the Environment Agency) as well as planning permission. The stack emissions are monitored at regular intervals throughout the run. The Beven Technology has Entrust approval, project number 151275.233. This means that landfill tax credit finance can be used to help fund the setting up of installations. The batch pyroliser consists of a furnace, kiln, charge vessel, pre condenser, main condenser, gas scrubber and dehumidifier. Exhaustive testing has been carried out both at AEA at Harwell and at the Universities of Leeds and Loughborough on the by-products and their chemical and physical analysis. Typical figures are given for the ratio of material output (resulting from 1000 kg tyres) in Table 6.4. Table 6.4 Material Output from the Beven Batch Pyrolysis Process Material Weight (kg) Oil 230–275 Carbon 400–420 Steel 130–160 Gas 190–220 Source: Beven Recycling

Analyses of the carbon and oil fractions are given in Tables 6.5 and 6.6. The oil approximates to a diesel oil, being similar in viscosity and calorific value but with a higher aromatic content. Table 6.5 Analysis of Carbon Fraction Material Weight (%) Carbon 87.5 Ash at 815°C 10.1 Zinc oxide in ash 44.1 Silica in ash 15.7 Sulphur 2.5 Chlorine 0.1 Source: Beven Recycling

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End-of-Life Tyres–Exploiting their Value Table 6.6 Analysis of Oil Fraction Viscosity @ 60°C 2.38 cSt Calorific value 41 MJ/kg Sulphur 1.5% Carbon residue 1.3% Water