 Sharan tested a wide range of materials and got good results from galvanised iron and aluminium sheets , but found that sheets of the special plastic developed by the OPUR just 400 micrometres ( 0 @.@ 016 in ) thick generally worked even better than the metal sheets and were less expensive . The plastic film , known as OPUR foil , is hydrophilic and is made from polyethylene mixed with titanium dioxide and barium sulphate . 
