Different types of consumer products and packaging are made from different types of plastics. The Society of the Plastics Industry (SPI) came up with a standardized coding system for the different types of plastics used numbered 1 through 7 that are stamped onto the plastic itself. These numbers help with identifying and separating different plastics to help make recycling faster and more efficient. The different codes are identified below.
1: Polyethylene tetraphthalate. This is commonly used for beverage bottles.
2: High Density Polyethylene. This is commonly used for milk cartons and shampoo bottles.
3: Polyvinyl Chloride (PVC). This is commonly used for plumbing pipes.
4: Low Density Polyethylene. This is commonly used for trash bags and sandwich type bags.
5: Polypropylene. This is used for more temperature durable plastics like drinking cups.
6: Polystyrene. This is Styrofoam and is used for cups and shipping packaging.
7: This is the catch all category for everything else, like polycarbonate for CD's.
Of all of the above materials, type 1 and 2 (like the milk cartons you referenced) are by far the most commonly recycled in standard household consumer materials.