http://www.washingtonpost.com/blogs/won ... cas-money/
Since American money was consolidated into a single system of currency in the late 1800s, U.S. dollars have been printed on a unique cotton blend paper. That paper has been supplied by a single company, Crane, for more than a century. And Crane relied on scraps of denim sold in bulk by the garment industry for its cotton. . . .
But something strange began to happen in the 1990s: Denim became tainted. . . .
Even a single fiber of spandex can ruin a batch of currency paper, degrading the strength of the material. But separating the spandex from the cotton would be a Herculean task, Rudd said. By the early 2000s, almost every pair of jeans contained at least a hint of stretch -- rendering them useless to Crane.