Today’s industrialized nations have lots of cars, which means use lots of tires. According to the U.S. Environmental Protection Agency, for the United States alone, about 250 million scrap tires are generated annually. With this huge amount of scrap tires, tire recycling becomes a significant issue in our society.
Accumulation of scrap tires not only waste landfill space, they can damage the linings put in place to keep groundwater and surface water from mixing with landfill contaminants. Illegally discarded tires pose threats to public health and safety. Tire dumps becomes excellent breeding grounds for mosquitoes, thus the transportation of mosquito-transmitted diseases. Tire pile fires is an even bigger environmental problem because the fire can burn forever while sending up acrid black plume into the atmosphere and toxic oils into nearby water supplies. The toxic chemicals and pollutants contained in the plume and oil will lead to severe air and water pollution to the neighboring area and cause grave health issues to the surrounding living environment.
Fortunately, our society has taken a huge effort in tire recycling, and more than 80 percent of today’s scrap tires are pulled from the waste stream and reused in some way. For a complete list of today’s recyclable tire and rubber grades, please visit our section Current Recyclable Materials – Tire & Rubber.
The tires recycling process is quite complicated, however, it can also be simplified into the following five steps:
Step 1: Scrap tires are collected and grounded into small pieces.
Step 2: Sulfur in the rubber is broken down.
Step 3: Chemical bonds in the old rubber are arranged to create the bonds in new rubber.
Step 4: New rubber is ready to be filtered, cleaned, and dried.
Step 5: After the cleaning process, the rubber from the old tires is ready to make new products.
Following is more complex breakdown of the actual process, which includes crushing, pulverizing, devulcanizing (deodorizing), and cooling:
Because of its three-dimensional network structure restricting the material from melting, recycling of rubber is difficult through any simple heating procedure. In the conventional pan method, rubber is finely grounded into powders and mixed with oils and reagents. The mix is then heated with steam in a pressure vessel at a temperature of about 200 degrees Celsius for more than 5 hours. After the heating process, several more procedures of refining and straining are conducted before obtaining the final reclaimed rubber.
In the today’s continuous recycling process, devulcanization or chemical reaction breakdown of the rubber’s network structure is utilized. Moreover, parameter optimizations in the reactor such as shear stress, temperature, and internal pressure, helps to control the efficiency of devulcanization. Below is a demonstration of the continuous recycling process for cross-linked rubber waste in a modular screw type reactor: reactor as shown below:
[Courtesy of: Kenzo Fukumori and Mitsumasa Matsushita.]
Section I: Introduction
Section II: Present (YOU ARE CURRENTLY IN THIS SECTION)
Section III: Future
Complete Table of Contents
Reference