www.bankofengland.co.uk/banknotes/polymer/Pages/default.aspx
Why Polymer?
Polymer banknotes are manufactured from a transparent plastic film, specially coated with an ink layer that enables it to carry the printed design features of banknotes. The material allows the inclusion of ‘windows’ or clear portions in the design which enhance protection against counterfeits.
Polymer banknotes are:
Resistant to dirt and moisture so stay cleaner for longer than paper banknotes
More secure so will provide enhanced counterfeit resilience
More durable so will increase the quality of banknotes in circulation