Trade-Offs and Variables Such as Cost and Performance When Tires Are Manufactured With an Increase in Recycled Content
Ambient processing of ground tire rubber usually produces crumb rubber supplies of 10 to 40 mesh and is the least expensive recycled rubber on the market. Cryogenically ground rubber is available from 40 mesh in size and finer, but at increased cost compared to ambient grinding. Klingensmith and Barnawal [5] have suggested, however, that in large-scale production, prices may be comparable. However, given the recent rise in energy prices, it is unlikely that these costs are comparable even in large-scale production.
Major manufacturers generally indicate that they have successfully been able to incorporate approximately 5 percent by weight of crumb rubber into new tires, primarily for the passenger car, van, and light-truck tires.
Adding crumb rubber to a virgin compound is reported to have the effect of lowering the physical properties by approximately 10 to 15 percent. Once the initial reduction in these properties occurs, the physical properties are largely retained at that same level [45]. Ryan [45] concluded that there is an initial reduction of in modulus upon introduction of the recycled rubber particles, but the value then remains constant. However, the increase in modulus for the treated rubber manifested poor to unacceptable levels of process ability (reduced scorching time). Chandra and Pillai [46] concluded that in addition to the physical properties that impact the performance of tires with increased recycled content, the savings are “not significant enough” to merit the effort of introducing recycled materials into tire formulations in larger volumes.
The addition of recycled rubber to virgin rubber compounds generally lowers tensile strength and fatigue resistance, and it reduces air and moisture impermeability [13]. Air and moisture impermeability are critical safety considerations within the tire system. Air and moisture migration produce separations within the tire structure which can result in 1 9 unpredicted catastrophic tire failures during operation. Tables 2a–5b illustrate the tradeoff in mechanical properties, at the rubber compound level, associated with the use of recycled content.
While tensile strength and fatigue resistance are achieved at a compound level, their economic feasibility as part of a new tire was not discussed. Reports available reveal that the research that has been conducted is primarily evaluating the performance of rubber compounds with recycled content to those with virgin ingredients, and not as a complete tire system.
Since there is no published performance data for tires with treated or untreated rubber recycled, the findings are inconclusive in identifying the most suitable approach to incorporate recycled rubber into the tire compounds. Further research beyond a paper study is required.
It is reported by many researchers that the use of crumb rubber resulted in a higher curing rate, suggesting that it may play the role of process accelerator. This indicates a potential for the use of crumb rubber as a substitute for expensive curative ingredients, such as zinc oxide.
Tread compounds for tires require acceptable properties in abrasion resistance, wet and dry frictional values, cutting and cracking growth resistance, and low hysteresis to minimize internal heat generation and rolling resistance. Tire rolling resistance reduction is one of the factors considered in tire design that can also have a significant impact on the environment. Higher rolling resistance can produce poorer fuel economy (a 10 to 20 percent increase in some cases).
The tradeoff between fuel economy and vehicle design factors, such as comfort, noise suppression, and road adhesion, will require that these factors do not dominate the importance of the rolling resistance. The thrust of research activity to reduce energy loss due to rolling resistance, can be categorized as follows [47–50]:
• Tire material properties.
• Tire construction.
• Road and tire/vehicle interaction.
In general, the addition of crumb rubber into a virgin compound increases hysteresis. Increase in hysteresis is manifested as increased internal heat generation and thereby increased rolling resistance. Therefore, the use of crumb rubber is generally limited to areas of the tire that have reduced flexure. The internal resilience/hysteresis characteristics of typical tire rubber compounds, including the casing and materials themselves, generate heat. Due to the poor thermal conductivity of rubber, this heat causes temperatures within the structure to rise rapidly to levels that can lead to total disintegration of the tire. The failure can result in conditions such as bond failure, reduced tear strength of the tread rubber, and actual charring or
melting of the casing cords.
The control of heat generation within a tire of any type is essential as maximum speed or worst-case operating conditions are approached. As discussed previously, tires with recycled content manifest higher internal heat generation compared to the virgin compounds they replace. Therefore, the use of tires with recycled content will generally be limited to the low end of the performance requirements, including reduced life span, 2 0 until the recovery methods can produce recycled content comparable to the virgin ingredients.
Under Chapter 912, Statutes of 2001 (Sher, Senate Bill 1170) [51], the California Energy Commission is mandated to make recommendations on a California State Fuel-Efficient Tire Program. There is an inherent competition among the different programs sponsored by the State of California, where the coordination of efforts among the State agencies is vital for optimum outcome.
With the effort of so many companies and recycling firms trying to develop new uses for waste tires, the result has been many new and expanded uses. Therefore, if the costeffectiveness of alternate uses for waste tires continues to be more attractive, the economic incentive to continue the research required to increase the recycled content in new tires will remain limited.
Tire Manufacturers and Their Locations Worldwide and Manufacturers Who Have Used Recycled Content in Tires
The global distribution of tire manufacturers is included as Appendix A. The data was extracted from an article in Rubber and Plastic News, “Global Tire Report, 2001” [52]. The listing is divided into seven geographical regions: North America, comprising of the United States and Canada; Latin America, including Mexico, Central America, and South America; Europe, including Russia and most of the former Soviet Bloc nations; Asia, including India, Japan, the Pacific Rim, and former states of the Soviet Union located in Asia; Africa and the Middle East; and Australia and New Zealand. Within each region, tire makers are listed by country.
The major manufacturers in the U.S. indicate that they are making an ongoing effort to increase the recycled content of new tires. The general consensus in the industry is that up to 5 percent recycled content is accepted as reasonable. However, data associated with the number of tires produced with recycled content is considered proprietary. Some manufacturers indicated that they do not keep track of such data and/or do not compile it for public consumption. NATC primarily depended on published reports and its historic contacts that have been in the industry in establishing the summary of tire manufacturers’ activities with respect to recycled content. NATC approached RMA for information on recycled content practices by RMA’s member tire manufacturers. NATC received a response from RMA indicating that they do not provide names of contact personnel of its members. NATC then contacted individual tire manufacturer representatives affiliated with RMA and obtained response from limited contacts.
A subsequent request that directly originated from CIWMB has resulted in the following responses. The responses obtained from these contacts indicate that the activity towards increasing the recycled content is very limited. Letters and emails were sent to many of the major tire manufacturers in the United States. Each manufacturer was asked whether their tires contained recycled content and specified waste tires as the source of the recycled content. The following excerpts are taken from responses submitted to CIWMB:
Toyo Tires—“No, they are all new rubber tires. The only recycling we use in our tires are from parts that have not been distributed or used and are still in our factories. 2 1 However, we do have other rubber products that are not handled in this department that use recycled rubber in their construction.” Michelin of North America, Inc. (MNA) — “MNA does use some recycled rubber in the manufacture of some of its products. This includes rubber from used tires as well as rubber that is recouped during the manufacturing process. We don’t want to give you the impression that MNA is using old tires from scrap piles in making any of its new tires. We don’t. We do however regularly recycle the inner liner rubber from some postconsumer used tires.”
Goodyear Tire and Rubber Company—“Our tires do not contain any rubber from waste tires.”
Yokohama Tires—“Yes: Various Yokohama tires sold in the state contain recycled materials. The primary material is reprocessed rubber from used rubber products. The percentage by tire varies, in some cases 1%, in others slightly higher.” Bridgestone Firestone—“Globally, Bridgestone Corporation utilizes post consumer recycled tire material in various tire lines at varying percentages. The percentages used can vary region-to-region based on the availability and quality of supply of the recycled material. The tire type and its performance requirements also limit the potential percentage usage of recycled material.” “
Generally, the use of recycled tire material in new tires is challenged by the fact that unlike paper, metals, plastics and glass, it is not currently possible to obtain materials from tires that have properties adequately similar to the original materials used in manufacturing tires. Tire rubber materials are highly engineered, with specific qualities of hystereresis and other chemico-physical properties, designed to optimize wet and dry traction, long life, low rolling resistance, comfortable ride responsive handling and performance characteristics, at an affordable cost. Unfortunately, the products currently available from recycled tires do not provide performance-enhancing characteristics; rather they tend to degrade performance. For passenger tires, there are especially detrimental effects on tire wear life and rolling resistance (fuel consumption), therefore, the amount of post-consumer recycled material utilized must necessarily be very limited.”
Data from European countries show that the Netherlands, Sweden, and Germany have the highest used tire recovery rate (100 percent, 99 percent, and 96 percent, respectively) [6]. However, the data does not show how much of the recovery is directed to the increase of recycled content in new tires.
Comparison of the Use and Amount of Crumb Rubber From Waste Tires in Bias Ply Tires Versus the Use and Amount of Crumb Rubber From Waste Tires in Radial Tires
In general, the amount of crumb rubber that can be used in bias ply tires is higher than the amount that can be incorporated into radial tires. However, the volume of bias ply tires produced has shrunk to a point where no significant impact is realized as a result of incorporating crumb rubber into new bias ply tires. As a result, no data is available comparing the use and amount of crumb rubber in bias and radial tires.
The carcass and sidewall design of radial tires requires high strength and endurance properties (for example, tensile strength, shear strength, flex and abrasion and aging resistance). These requirements, to an extent, require the use of materials that have 2 2 precise properties, currently only found in virgin ingredients. Thus, relative to the reduction in the manufacture of bias tires, the increased use of radial ply tires, which have measurably improved longevity and improved fuel economy, has resulted in reduced recycled content per tire.
Table 6 shows the breakdown of the ground rubber market. Since the number of bias ply tires produced has shrunk substantially, most or all of the 50 million pounds of crumb rubber used for tire manufacturing was likely recycled into radial tires.
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