Applications of Processed Waste Tires
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
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
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:
The major consideration of this application is related to cost since the selling price of virgin rubber and plastic resin is relatively low.
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.
Reclaimed rubber is made from crumb rubber. The most common rubber reclaiming process is described as follows:
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.
Construction and industrial applications:
Among the wide variety of commercial applications, the following applications have exhibited a growing market potential:
For the above applications, the initial capital cost is not a major problem. Several studies have shown that even if the initial cost were increased by using crumb rubber, the improved function and life of the product would significantly reduce maintenance cost, and consequently the total cost of the product.
The following reports discuss these applications in detail.