Rubber From Trees or From Oil? Choosing Robot Wheel Treads

Grip for the Competition Field

1The robotics team at Northgate Middle School is getting ready for a regional competition. Their 4 kg robot has to push game pieces across a field of foam tiles, and in last year's matches its hard plastic wheels kept slipping. Team members Ximena Ruiz and Arash Tehrani found that wheel treads are sold in several kinds of rubber. Before choosing, their coach, Mr. Paulo Salave'a, asked them to find out what each kind is made from.

2Natural rubber starts as latex, a milky sap that farmers collect by cutting thin grooves in the bark of rubber trees. Most of the world's natural rubber comes from farms in warm, wet parts of Southeast Asia. The latex is dried into sheets. To make it tough enough for wheels and tires, factories mix it with sulfur and heat it, and a chemical reaction links its long molecules to each other. In some places, rainforest has been cleared to plant more rubber trees.

3Synthetic rubbers are new substances that people make through chemical processes. The most common one for tires, called SBR, usually starts as crude oil. At a chemical plant, small molecules taken from the oil react and link into very long chains. Polyurethane, another tread material, is also made mostly from chemicals that come from crude oil or natural gas. Silicone rubber is different: silicon, which is made from quartz sand, reacts with chemicals made from natural gas, and the products link into long chains.

4Synthetic rubber changed history. During World War II, the United States lost access to most of the natural rubber from Southeast Asia, which it needed for tires, boots and other equipment. The country quickly built factories to make synthetic rubber, and within a few years they were producing most of the rubber it used. Today, about half of the rubber used in the world is synthetic. Tires of every kind wear down as they roll, though, and the tiny worn-off bits from car tires can wash off roads and into streams.

5To compare treads, the team put each kind on the same robot with the same 4 kg load and drove it on the same foam tiles. To measure grip, they locked the wheels and pulled the robot sideways with a spring scale, recording the force when it began to slide. To measure wear, they drove the robot for 2 hours with each tread, weighed the treads before and after, and worked out the mass lost per hour. A set of treads is worn out once it has lost 6 g. The team set three criteria: a grip of at least 38 N, a set that lasts at least 15 hours of driving, and a price of no more than $25 per set.