Polypropylene is being used for bank notes for the same reason that is used to make dishwashers — it won’t be damaged by the washing process. Already a dozen countries have money printed on polypropylene, most notably Australia, where all bank notes are printed on polypropylene. The plastic bank notes are hard to counterfeit and cost less to produce, mainly because they last more than twice as long as paper bank notes, which cannot easily be washed. Now the United Kingdom is getting ready to give polypropylene bank notes a try, starting with a £5 note in 2016. The BBC News story:
Bank notes are just one of the newer uses of an increasing popular polymer. Forty years ago polypropylene was used mainly for nautical rope. Unlike any other common material of similar strength, it resisted water and would float. At that time, the high price of polypropylene ruled out most other uses, but now polypropylene has become the predominant plastic for food containers and a wide range of other uses.
Bankers warn of one unexpected quality of polypropylene when used to print money: it will shrink if exposed to heat, for example, in a clothes dryer. So even though polypropylene notes won’t be damaged in the washing machine, it is probably best to keep them out of the dryer.