A more sustainable vehicle fleet is also vital to achieving a green transportation economy, according to Mr. Mukhopadhyay. India is a good example, with a plan that all new vehicles sold within its borders must be 100-percent electric by 2047, the 100th anniversary of the nation's independence, he said.
By 2030, 40 percent of new vehicle sales and all new vehicles for intra-city transport fleets must be pure EV, he said.
"As vehicles are one of the main sources of air pollution responsible for poor urban air quality, suitable measures should be taken by all concerned to build a green, sustainable future," he said.
The U.S. tire industry is well advanced in its efforts to achieve sustainability, according to John Sheerin, director of end-of-life tire programs for the U.S. Tire Manufacturers Association (USTMA).
Members of the trade group continue to prioritize research into the health and environmental impacts of the manufacturing and use of tires, Mr. Sheerin said, while at the same time striving to advance the safety and performance of tires and to manage scrap tires as valuable, reusable materials.
"We recognize the need to increase the use of sustainable materials," he said. "We are increasing the use of micronized rubber powders, which closes the loop in tire manufacturing."
USTMA members are also investigating the use of recycled carbon black from pyrolysis, orange and soybean oils instead of petroleum, bioisoprene instead of traditional synthetic rubbers and alternative sources of natural rubber such as guayule and dandelions, according to Sheerin.
The percentage of scrap tires that were recycled dropped from 2013 to 2017, to 81 from 96 percent of tires generated, Mr. Sheerin said. A slight drop in markets plus the sheer increase in scrap tires generated in those years was a major reason for this, he said.
Nevertheless, scrap tires represent one of the great success stories in recycling, the USTMA exec said.
"Only lead-acid batteries have a better recycling rate," he said. In one sense, scrap tires have a more impressive track record than lead-acid batteries, because scrap tires on their own have a negative economic value, he said.
Tire-derived fuel remains the stalwart of scrap tire recycling, accounting for 43 percent of the 3.4 million tons of tires recycled in 2017, according to Mr. Sheerin.
The Environmental Protection Agency has recognized the biogenic content in TDF, in both the greenhouse gas reporting rule from 2010 and the emissions from stationary sources rules from both 2011 and 2014, he said.
Ground rubber accounted for 25 percent of the scrap tire market in 2017, and civil engineering projects for another 8 percent, according to Mr. Sheerin. Some 16 percent of scrap tires were landfilled — the highest level in three or four years, because of a decline in the market — and the remaining 8 percent went to various markets, he said.
"2017 was not a boom year for rubber-modified asphalt," Sheerin said. "But we expect this year to be better, because of new road construction."
Reducing scrap tire stockpiles is also a major success story, according to Mr. Sheerin. Since the USTMA began its scrap tire program in 1990, the number of stockpiled tires has plummeted 94 percent, to about 60 million from an estimated 1 billion, he said.
But problem spots remain in several states, either because of bankruptcies among scrap tire processors or lapsed scrap tire abatement programs, Mr. Sheerin noted.
In South Carolina, for example, the state faces the cleanup of 800,000 stockpiled tires left by a processor that went bankrupt. The state has allocated $2.7 million for cleanup, Mr. Sheerin said.
Colorado has two or three massive tire monofills that the state is required to clean up by 2024, he said.
"Texas has 17 million stockpiled tires that they know of, and I know of no plan to clean them up," he said. "The state needs a new program."
Two states — Louisiana and West Virginia — claim to have no stockpiled scrap tires, according to Mr. Sheerin.
"Louisiana has done the work and the cleanups," he said. "The number of stockpiled tires is probably not zero, but the state is very environmentally aware.
"But West Virginia hasn't done the work," he said. "It hasn't even looked for stockpiles."