The most common - and the most valuable - of these are: Polyethylene Terephthalate (PET) - used in water bottles and plastic trays High Density Polyethylene (HDPE) - used for milk cartons and shampoo bottles Low Density Polyethylene (LDPE) - plastic carrier bags and bin liners. Polypropylene (PP) - margarine tubs and ready-meal trays These plastics have the greatest recycling demand and are .