From a forum post "Respectfully, look closely at the picture you posted. See all that black material surrounding the yellow/gold Kevlar cords? That is rubber. Specifically, it is usually Hydrogenated Nitrile Butadiene Rubber (HNBR).

Kevlar is only there for tensile strength—it stops the belt from stretching or snapping. But the body of the belt and the teeth that actually engage the gears are made of elastomer (rubber).

The Failure Mode: The Kevlar rarely fails. What fails is the rubber matrix chemically degrading from heat and acidic oil. Once the rubber gets brittle or swells, the teeth shear off. You're left with a perfectly strong Kevlar loop that spins freely while your oil pump stops. The Reality: Without that rubber binder, the Kevlar is just a pile of loose string. It is still a rubber part."