HANOVER, Germany—A key engineering executive at Continental A.G. is predicting car tires could be made of 100 percent renewable raw materials by 2020, but at the same time he cautioned that finding acceptable substitutes for materials used widely today is a "rocky road."

Boris Mergell, head of material and process engineering for Continental, said the non-oil-based materials content of modern-day tires already is at about 45 percent, but increasing that to 100 percent is not a straight-forward process.

"Not all raw materials in tires can simply be replaced by renewable materials," he said.

"In many cases, such materials have a negative impact on braking performance or rolling resistance—and we will not accept any compromise here," he said.

"Also, the widespread replacement of fossil materials with renewable raw materials is not always a solution since it requires acreage that might already be used in food production."

Among the changes chemists at Conti are working on include replacing fossil oils with rapeseed oils and polyester with rayon for reinforcing materials. In addition, the tire maker is increasing the quantities of recycled rubber from old tires to replace virgin synthetic and natural rubber.

Other alternative materials research is going into producing rubber from dandelion latex and replacing carbon black with silica (silicic acid) in the compound.

These changes could shift 20 percent of the weight of a tire to "natural" materials, he said.

"We still need to conduct numerous tests on materials and in our process engineering to make significant progress," he said, warning that the development of true "green tires" could take another five years.