Recycling and Re-use of Waste Rubber

Martin J. Forrest

• Publisher: De Gruyter
• Year: 2019
• Print ISBN: 9783110644005
• eBook ISBN: 9783110644234
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Recycling of rubber materials is necessary from both an environmental and economic perspective. This book describes everything from the world market to the many novel technologies and processes developed for the re-use and recycling of our common rubber materials. Devulcanization, production of rubber crumbs, reprocessing and manufacture of new materials are thoroughly described and discussed.

1Introduction to recycling and the re-use of rubber

Since the birth of the rubber industry, technologists and chemists have worked to develop materials and products that are highly durable, resistant to temperature and other aggressive environments and, as a consequence, give high performance and have a long life. These advantages, from the consumer’s perspective, create problems when it comes to disposal at the end of a rubber product’s life. The same problems arise when dealing with the normal in-process waste produced in rubber factories. Processors of thermoplastics and thermoplastic rubbers are able to recycle their moulding flash and out-of-specification products, but this has not been possible for the processors of conventional rubber.

The pressure on the rubber industry and the many users of its products to recycle rubber has increased dramatically over the last 25 years due to a combination of economic, environmental, societal and legislative factors. The need to conserve natural resources, coupled with the publication of important legislation such as the European Union (EU) Landfill Directive (1999/31/EC), which banned the placing into landfill sites of any tyre-derived waste from 1st July 2006, gave fresh impetus to the search for technologies that could render waste rubber amenable to reprocessing, impart physical properties comparable to virgin rubber, and so achieve high added value. Although some methods of devulcanising and reclaiming waste rubber have been available for a many years (indeed the earliest techniques were developed in the 19th century), most either involved breaking down the rubber mechanically into crumb, so that it could be incorporated at low levels into new rubber compounds as a diluent filler, or attempting to break the crosslinks, resulting in some degradation of the rubber, leading to a relatively low value processing aid.

The worldwide interest in rubber recycling and the important role being played by industry is apparent in the number of presentations that are being given on the subject at major conferences. For example, in a paper presented to the Tire Technology Expo 2014 Conference, Mergell [1] discussed the future trends in truck tyres and highlighted how new recycling and retreading technologies will help to relax tight raw material markets in a cost-effective way. This presentation also mentioned some of the technical challenges with respect to resilience, mechanical strength and scorch, which recycled rubber still needs to fully address, and the efforts that Continental are making to closing the tyre production cycle to meet their 2025 material life cycle target.

Due to these various considerations, increased funding has become available from a number of sources, including national and regional governments, and this area of research and development is now an extremely active one with a large number of recycling and re- use options being explored. It is possible to divide different types of recycling activities and processes for waste rubber, and other polymeric products, particularly plastics, into four broad generic categories. These are:

–Primary: reprocessing into materials and products having properties that are the same, or at least comparable, properties to the original material or product.

–Secondary: where the recycled material is made into products that do not have (or need to have) properties that are the same or comparable to the original material or product.

–Tertiary: the reduction of the waste material or product into small chemical units (i.e., molecules) that can then be recycled into new materials and products by routes such as re-polymerisation.

–Quaternary: the recovery of the energy inherently present in the waste materials or products by methods such as incineration, or the burning of fuel products that are derived from pyrolysis processes.

As this book demonstrates, all of the above routes have been explored in the search for different ways to recycle and re-use waste rubber and end-of-life rubber products. Those that are at present most popular, and exploited to the highest degree, are the Secondary and the Quaternary routes. Of those processes that fall within the Secondary category, the generation of crumbed rubber, which is incorporated into a very wide range of new products (Chapter 7), is dominant at present, although devulcanisation to create a good quality product that can be added to virgin rubber at relatively high levels is increasing in popularity and the level of exploitation. With respect to the Quaternary route, its industrial importance is apparent from the very high proportion of waste tyres that are used for tyre- derived fuel (TDF), particularly in countries such as the USA.

The ‘Holy Grail’ of rubber recycling is to develop a ‘Primary category’ devulcanisation process. Such a process would be capable of recycling rubber back into new products, possessing equivalent properties in all respects to the original product, without having to use any virgin rubber in their manufacture. The attractiveness of such a process has meant that it has been one of the main goals of researchers in the field, and some good progress has been made towards achieving it, with devulcanisation processes being developed and optimised that are capable of regenerating a high quality product that has the capability to be used on its own to manufacture high-added-value products, or at very high levels of addition into virgin rubber compounds.

The challenge running alongside all this research and development (R&D) activity is to ensure that any process that achieves its technical targets will also be economically viable and so have good commercial potential. Increases in commodity prices tend to assist in achieving this commercial goal, although the costs associated with other aspects of a recycling process (e.g., energy and, if they are used, chemicals) can increase as well, reducing the overall net gain. A complete cost assessment exercise, taking into account every aspect and feature of a process (e.g., capital expenditure, material costs, labour costs, energy costs, storage, packaging and transportation costs, and so on) needs to be carried out to provide a complete and accurate evaluation of economic viability. Of course, where a product is being sold into the market-place, the final selling price, and a robust system to establish this and vary it over time as conditions change, are also vital in ensuring that a business is profitable and viable in the long term.

In addition to the above considerations that can be applied to a single process, there are a number of ways in which the various routes that are available for recycling rubber can be compared with the costs to produce new products and materials, and one is a comparison of the energetic balance of the different processes. Some generic indicative examples have been provided by Manuel and Dierkes [2] and are reproduced in Table 1.1.

Table 1.1: Energetic balance of the production of virgin materials and recycling of rubber products.

Process	Energy expenditure (kWh/kg product)
Tyre production in total	−20
Production of synthetic polymers	−13
Production of carbon black	−13
Re-treading of tyres	−6
Incineration of waste rubber	+5
Granulating of whole tyres	−0.2
Reclaiming of rubber	−0.7

Table 1.1 shows that producing reclaimed rubber consumes only a small percentage of the energy that it takes to produce new tyres, new polymers from monomers, or even virgin carbon black. It also shows the benefits of incinerating waste rubber to generate energy and illustrates why recovering energy from waste tyres is so attractive and carried out to such a large extent around the world (Chapter 8, Section 8.4).

With regard to overall environmental impact, in order to assess if a new recycling process, or recycling option, does compare well with those that are currently being used, a full life cycle assessment (LCA) can be carried out using the frame-work and principals of accepted standards (e.g., International Organization for Standardization standard ISO14044). A large number of such assessments have been carried out over the last 25 years, most of them comparing the environmental impacts of recovering and recycling waste rubber as opposed to incinerating it and/or placing it in landfill sites, because they have been commissioned with a view to assisting waste management strategies. Some of the more recent ones have compared a range of options for the re-use and recycling of the waste rubber. Examples of both of these types of recent LCA studies, all accessible in the public domain, are listed below:

–Comparative LCA of two options for scrap tyre treatment: material recycling versus TDF combustion [3]

–Economic advantages of using ultra-fine scrap tyre rubber in virgin and recycled rubber and plastic compounds; a comprehensive LCA [4]

–End-of-life tyre management LCA: a comprehensive analysis for Alberta Recycling Management Authority [5]

A large amount of information and data are presented in all three studies, but some of the salient points that are highlighted in their executive summaries are given below:

–The study published by Springer [3] concluded that, for the nine different environmental impact categories that were assessed, in all categories material recycling (e.g., in artificial turf products) as an end-of-life option for tyres provides a larger environmental benefit than co-incineration...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Acknowledgments
• Contents
• 1 Introduction to recycling and the re-use of rubber
• 2 Brief history of rubber recycling
• 3 Overview of the world rubber recycling market
• 4 Review of devulcanisation techniques
• 5 Characterisation of devulcanised rubber and products containing devulcanised rubber
• 6 Production of rubber crumb
• 7 Products made from rubber crumb and waste rubber
• 8 Other methods for recycling and re-using waste rubber
• 9 Conclusion
• Appendix 1 Brief overview of regulations applying to waste rubber in the European Union and the USA
• Abbreviations
• Index