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Applications of Processed Waste Tires
(Crumb Rubber)

In general, processed waste tires refer to applications of chipped tires or crumb rubber.  Crumb rubber, also referred to as ground rubber, is a wire-free fine rubber particle made by size reduction from scrap tires. Various size reduction techniques can be used to achieve a wide range of particle sizes down to 600 microns or less.  Chipped tires also result in wire-free shredded tire particles of relatively large particle size compared to crumb rubber. In this discussion, 'crumb rubber' will refer to both crumb rubber and chipped tires for convenience.
Compared to the applications of whole tires, crumb rubber has larger and broader potential markets.

Main Markets for Crumb Rubber
Rubber modified asphalt will be discussed in a separate chapter due to its large market potential and technical specifications.

For crumb rubber (excluding civil engineering applications) approximately 120 million pounds were sold on the open market in 1992. This increased to 440 million pounds in 1996. Civil engineering applications also constitute a rapid growing market (an increase of almost 350 % from 1990 to 1998)1).

Manufacture of Crumb Rubber:
Crumb rubber is made by a combination or application of several size reduction technologies. These technologies may be divided into two major processing categories, mechanical grinding and cryogenic reduction.

Mechanical Grinding: Mechanical grinding is the most commonly used  process. The method consists of mechanically breaking down the rubber into small particles using a variety of grinding techniques, such as cracker mills, granulators, etc. The steel components are removed by a magnetic separator (sieve shakers and conventional separators, such as centrifugal, air classification, density, etc. are also used). The fiber components are separated by air classifiers or other separation equipment. These systems are well established and can produce crumb rubber (varying particle size, grades, quality, etc. ) at relatively low cost. The system is easy to maintain and requires few people to operate and service. Replacement parts are generally easy to obtain and install.
The other important advantage of mechanical grinding relates to the shape and physical properties of the crumb rubber particles. The shape and surface texture of the crumb rubber particles are relatively round and smooth, and are able to form molecular cross-links with virigin rubber material. The rubber particles are broken down under high shear stress. Since the tire compound consists of a carbon-sulfur cross-linked matrix (see Anatomy of a Tire), the grinding process causes 'de-linking' of the material. The resulting 'de-linked' material is more viscous compared to virgin rubber and is a unique characteristic of mechanically ground crumb rubber. For applications involving compounding with virgin rubber or plastic, crumb rubber provides some advantageous attributes to the viscoelastic compound. The crumb rubber particles do not cause a deterioration of tensile strength at low to moderate loading levels.
The main disadvantage relates to cost.

Cryogenics: The cryogenic process consists of freezing the shredded rubber at an extremely low temperature ( far below the glass transition temperature of the compound). The frozen rubber compound is then easily shattered into small particles. The fiber and steel are removed in the same fashion as in mechanical grinding.
The advantages of the system are cleaner and faster operation resulting in the production of fine mesh size.
The most significant disadvantage is the slightly higher cost due to the added cost of cooling (liquid nitrogen, etc.).

Applications of Crumb Rubber
Crumb rubber incorporated into rubber or plastic products:

  • Crumb rubber has been incorporated into rubber and plastic materials, such as floor mats, vehicle mud guards, carpet padding, adhesives, etc. (See SUMMARY OF EXISTING AND POTENTIAL USE OF RECYCLED CRUMB RUBBER for detail.). There are three major advantages for utilizing crumb rubber in the manufacture of rubber and plastic products.
  • Use as a filler for reducing cost.
  • Adding functionality or modifying properties of the end products.
  • Environmentally beneficial product as a consequence of recycling and waste minimization.
  • The major consideration of this application is related to cost since the selling price of virgin rubber and plastic resin is relatively low.

    Reclaimed rubber:
    Reclaimed rubber is made from crumb rubber. The most common rubber reclaiming process is described as follows:

    1. Crumb rubber is mixed with water, oil, and chemicals which are expected to devulcanize the rubber.
    2. The mixture is heated under pressure.
    3. The resulting partially-devulcanized rubber may be formed into slabs or bales and shipped to manufacturers who process and vulcanize the material with virgin rubber or plastic resin.
    Compared to crumb rubber, reclaimed rubber loses its elastic properties during the reclaim process. The material is capable of forming molecular cross-links with the matrix material.  Although, the material does not behave identically to virgin rubber, reclaimed rubber is expected to have more extensive usage than crumb rubber, especially for rubber products.
    The greatest obstacle for the reclaiming industry is the extensive use of synthetic polymers in the manufacture of new tires. Modern tire compounds contain approximately five different kinds of synthetic rubber and other various chemicals for different cross-linking methods. The traditional reclaiming technique based on the thermal  decomposition of carbon-carbon and carbon-sulfur linkages is not always effective with these newer synthetic materials.
    In addition, the tire materials are being required to achieve higher performance under extreme conditions. The amount of reclaimed rubber used by the tire industry is approximately 1-2 % of the raw materials.
    Because of these reasons, the reclaim rubber industry has declined, and fine crumb rubber has replaced the material as a cheaper supplemental filler.
    Some studies and trials for developing a new type of reclaimed rubber have been made. The basic concept is more closely related to surface modification of the crumb rubber. The finely-ground rubber (mechanical grinding preferred) is treated with a liquid polymer in order to form cross-links with the base matrix. It is reported that reclaimed rubber has superior bonding properties and imparts extended usage to rubber products. This reclaimed rubber could penetrate the markets of crumb rubber, if there was a clear cost advantage.

    Crumb rubber for civil engineering applications
    Because of the scale and required material properties, civil engineering applications have been considered as suitable for the use of crumb rubber. The strength and physical properties of crumb rubber make this material attractive for these types of applications.
    As shown in the list, in most cases, crumb rubber is used as a raw material to improve the required properties of the product. Crumb rubber is not an inexpensive filler in this application, but has been chosen for its properties. Also, some applications do not require fine rubber particles. Since cost is strongly related to particle size, this situation may contribute a significant cost  benefit as well.

    See Also Rubber modified asphalt
    Construction and industrial applications:
    Among the wide variety of commercial applications, the following applications have exhibited a growing market potential:
    Mesh: The number of openings per linear inch counting from the center of any wire to a point exactly 25.4 mm (1 inch) distant, or by an opening specified in inches or millimeters, which is understood to be the clear opening or space between the wires.


    1. Michael Blumenthal. What's new with ground rubber?  BioCycle March 1998 v39 n3 p40(3)
    2. U.S.Environmental Protection Agency et al, Scrap tire Technology and Markets Noyes Data Corporation, NJ 1993
    3. James E. Mark, Burak Erman, Frederick R. Eirich. "Science and Technology of Rubber" 1994 Academic Pres Inc.
    4. G. Alliger, I. J. Sjothun. "Vulcanization of Elastomers" 1963 Reinhold Publishing Co.
    5. K. Oliphant, W.E. Baker. The use of cryogenically ground rubber tires as a filler in polyolefin blends. Polymer Engineering and Science, Feb 15, 1993 v33 n3 p166(9)

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