Rubber Recycling

Sadhan K. De, Avraam Isayev, Klementina Khait, Sadhan K. De, Avraam Isayev, Klementina Khait

• Publisher: CRC Press
• Year: 2005
• Print ISBN: 9780849315275
• eBook ISBN: 9781135504786
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

The safe disposal and reuse of industrial and consumer rubber waste continues to pose a serious threat to environmental safety and health, despite the fact that the technology now exits for its effective recycling and reuse. Mountains of used tires confirm the belief that chemically crosslinked rubber is one of the most difficult materials to recyc

1
Manufacturing Practices for the Development of Crumb Rubber Materials from Whole Tires

Michael W.Rouse

1.1 Abstract

The processing or reduction of whole tires into useful particle sizes has been a challenge to the recycler since Charles Goodyear first discovered vulcanization. This chapter discusses the development of converting tires into a useful particle form, for reuse in many applications. Materials made from tires are called tire-derived materials (TDMs) and include a higher refined portion, crumb rubber materials (CRMs), which can be reused in the manufacture of tire compounds and for many other applications.

The tire is probably one of the best-engineered, longest-lasting products developed by man. It is designed for performance, aesthetics and comfort, control, and safety. Scrap tires are a valuable and viable national resource; the challenge today is to find the best way to utilize them in a wide range of applications.

1.2 Background

The tire is nearly indestructible to normal mechanical fracturing mechanisms. However, if the tire can be broken down into its separate major components—fiber, steel, and rubber—it can be made into very viable compounding ingredients for new tires. This rubber material or TDM can also be used in manufactured products, as an asphalt modifier for enhancing plastics, and for energy conversion and soil amendments. The TDM can be made into many gradations of crumb rubber material, or CRM. With the use of CRM, the potential for a wider variety of applications becomes even more viable. These higher and better uses include industrial, commercial, governmental, agricultural, and sports applications. Depending on the final particle size and gradation of the CRM derived from TDM using a particular tire type, the CRM can be an excellent raw material for many other applications. TDM can be used as a supplemental fuel, with or without the inherent wire reinforcements, or for civil engineering applications in road construction.

In general, CRM cannot be produced in a one-step process, such as punching a casing to make a part for a child’s swing or a punched part for a muffler hanger, and cannot be used as one-step shredding for civil engineering applications. The production of CRM requires a number of key processing steps. This chapter discusses the methods and methodology to produce high-grade CRMs for today’s markets. The production of TDM and CRM for specific uses encompasses many options and variables. The availability and cost of appropriate equipment, plus the expertise needed to operate the equipment for desired end products, present a challenge for the processor. Only the highlights can be covered in this chapter. It is hoped that the ideas presented herein can be a guide to initiate or improve the successful development of TDM in a safe and environmentally sound manner.¹

Approximately 290 million tires² are produced and 23 million passenger, truck/bus, and off-the-road tires retreaded annually in the U.S.³ Eventually, the tires reach the end of their useful life and are introduced into the scrap tire stream. This production value represents what is entering our environment.⁴ Today, less than 10% of the tires removed from vehicles are recovered for recapping or resale as used tires. The remainder of this scrap tire stream is increasingly finding new markets or higher economic value other than disposal. To recycle means to put or pass through a cycle again, as for further treatment or different application. Recycling is the major focus of processing to convert TDM to its highest use.

The use of TDM as a fuel is not a form of recycling, but rather, a method of disposal. What is intended is for the processor to take advantage of the tires’ inherent properties and develop unique markets for the processor’s efforts. However, if the production of tire-derived fuels leads eventually to higher and better uses of the scrap tire ingredients, then this stepped approach is a major move in the right direction of utilizing the scrap tire to its fullest potential.

Tires are an oil derivative; its components of construction, such as the steel and cord fibers, depend on energy for production. With the world price of crude oils continually escalating, the value of TDM will only increase. This price escalation will spur on the necessary processing developments to utilize the scrap tire to its maximum value.

The polymer composition of a tire makes it very unique and valuable for other applications. However, scrap tires from a landfill have no economic value and present health, safety, environmental, and handling problems. Tires do not biochemically degrade sufficiently when buried. They may resurface in landfills, providing an excellent breeding ground for vermin and mosquitoes. The exception to this case is tire monofills that have been properly operated for future recovery of whole tires or some shredded form of them.

The purpose of this chapter is to discuss various alternatives to disposal of waste tires, so they can become a resource. Worn tires can be sold as “used tires” or recapped for continued use as tires, but eventually tires lose their total recyclability and must find another use. It is true that a small portion of the scrap tires entering the environment can find a recyclability life (use over many more applications without loss of integrity of the tire rubber compound or its components). Eventually, the useful life as a tire will end, but the rubber polymers can be successfully recovered and used again in a new tire compound as a filler or extender. Tire compound applications do require very stringent CRM quality requirements if CRM is to be used for this purpose. Worn tires can be used as artificial reefs, highway crash barriers, bank reinforcements, children’s swings, and industrial and commercial parts to luggage items, just to mention a few uses. Whole or shredded tires can be burned directly (pyrolyzed) or gasified for energy recovery. If oil or carbon char is produced from tires as a building block of a new raw material ingredient, it could be argued that this activity is recycling, although the major composition of the tire has been altered due to depolymerization.

The focus herein is the reduction or commutation techniques for reducing whole tires of all types into a tire-derived material form, TDM. This includes the manufacture of tire-derived fuel (TDF). TDM can be further refined into CRM. CRM is not used in TDF due to cost restrictions. An equivalent cost competitiveness of CRM can be made to a basic fossil fuel such as gas, oil, or coal to wood wastes; refuse-derived fuels (RDF); and other energy sources. To illustrate this point, assume that No. 5 fuel-grade oil is selling for $30 (US) per barrel and is being used in a multi-fuel furnace for power generation at the same conversion efficiency. This would equate to $148 (US) per ton or roughly 7.4 cents per pound if a minus 2-inch wire free TDF was used. If the equivalent fuel price for both fuels were used, this would leave no room for return on investment to the energy user additional metering equipment and initial testing by the time the CRM was produced and shipped to the energy user. The economics under these conditions is not viable. Therefore, if a 30% discount price for TDF was used or $103 (US) per ton compared to oil, this may be a starting point for the energy user to consider using TDF as a supplemental fuel to justify the costs of testing, metering equipment installation, TDF handling, and the possibility of new monitoring controls for stack emissions.

Table 1.1 compares the various fuel values of TDF and TCF to other fuels on an “as received” basis. Normally, a range of high and low value range reflects fuel values. Care should be taken to identify the particular fuel value being used. This table can assist in obtaining a quick idea of a particular fuel’s equivalent value.

Figure 1.1, showing the tire flow diagram, depicts the life or ecosystem of the tire from its initial manufacture, to its useful life as a tire, to the capture of its qualities for other uses when it is removed from the transport mode. ...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• About the Editors
• Contributors
• 1. Manufacturing Practices for the Development of Crumb Rubber Materials from Whole Tires
• 2. Quality Performance Factors for Tire-Derived Materials
• 3. Untreated and Treated Rubber Powder
• 4. Tire Rubber Recycling By Mechanochemical Processing
• 5. Recycling Cross-Linked Networks Via High-Pressure, High-Temperature Sintering
• 6. Powdered Rubber Waste In Rubber Compounds
• 7. Rubber Recycling By Blending With Plastics
• 8. Strategies for Reuse of Rubber Tires
• 9. Ultrasonic Devulcanization of Used Tires and Waste Rubbers
• 10. Devulcanization By Chemical and Thermomechanical Means
• 11. Conversion of Used Tires to Carbon Black and Oil By Pyrolysis
• 12. Markets for Scrap Tires and Recycled Rubber