The most common types of plastics are the following. PET, HDPE, PVC, LDPE, PP and PS. All of these plastics can be found in different colors, therefore they are very hard to identify. During production a so-called Resin Identification Code is usually placed on the product to help identify the type of plastic used, making identification and recycling easier. The identification code is a triangle made up by arrows with a number of or the abbreviation of the plastics inside or below. In case there is no code to be found on the product, on could refer to the manufacturer for more information.
Polyethylene is the most widely used plastic in the world. Annual production is close to 90 million metric tons. It is a crystalline thermoplastic polyolefin. Packaging industry is the main user of polyethylenes (plastic bags, foils, films, bottles). The two most common types are the low density LDPE, and the high density HDPE. Plastic bags and films are typically made out of LDPE. Packaging industry uses LDPE in the highest volume. Its density is 0,91-0,94 g/cm3. HDPE has an excellent chemical resistance and so it is used for containers of chemicals (shampoo, detergent, motoroil, acids, etc.), hot and cold water pipes and protection tubes. Its density varies typically between 0,93-0,97 g/cm3. Resin identification codes are: HDPE: 2, LDPE: 4.
There are other types of polyethylene, but these usually aren't thermoplastics. One type is crosslinked polyethylene, XPE (PE-X or XLPE), which is typically made by crosslinking LDPE using crosslinking agents during production or after production with different techniques. Because it is not a thermoplastic, its recycling is problematic and requires special solutions. UHMWPE (ultra high molecular weight polyethylene) is an other kind of polyethylene with a high molecular weight. It has outstanding abrasion resistance, and is being used for almost 35 years for hip transplants, but for synthetic ice too.
Polyproplyene (PP) is the second most produced plastic with 56 million metric tons per year. It is also a thermoplastic, crystalline polyolefin, with a density of 0,85-0,95 g/cm3. Its resin identification code is: PP: 5. It has great mechanical properties and chemical resistance. Because it has many different types, it is used in many applications. It is found in office, household and garden appliances, but also in vehicle industry. Flower pots, plastic garden furniture, plastic buckets, plastic strings, plastic toys, plastic crates, plastic boxes, and syringes are typically made out of PP. A special type is BOPP (biaxially oriented polypropylene), which is a transparent film, typically used to cover chocolate boxes and cigarette boxes.
Polystyrene (PS), similar to polyethylene and polypropylene is a commercial plastic, with annual productions of 21 million metric tons. Its density is 1,04-1,09 g/cm3, so contrary to PP and HD/LDPE, it does not float in water. It is an amorphous thermoplastic. Its resin identification code is: PS: 6. As a homopolymer it has a weak impact resistance and thus mainly used as a copolymer (eg. HIPS, ABS, SAN, SB), but these variants are usually not compatible with each other, and so they need to be handled separately during recycling. The amorphous structure enables easy thermoforming and therefore yogurt cups, single-use kitchenware and boxes are made out of PS. Other products are CD and DVD covers, handles of "disposable" razors, etc. Expanded polystyrene (EPS) is a foam-like material made out of PS. It is used in order to protect delicate products when transporting, or nowadays as food containers.