The number of plastic materials is very large and ever-increasing. Plastics can be found almost everywhere in our modern world, from shopping bags to the cloth we wear. The reason why it seems like plastic can be used just about for everything is that many types of plastics exist with vastly different and adjustable properties. However, the majority of the plastic products produced worldwide are made from just six different plastic materials, namely polyethylene, polypropylene, polyvinylchloride, polystyrene, polyester, and polyurethane. All six are unbelievably useful and versatile. They account for more than 90 percent of the global plastics material’s consumption.
From 1990 to 2015 the consumption of plastic has more than tripled making it ever more important to reduce and recycle plastics. However, not all plastic is recycled, that is, the recycling effort varies vastly from country to country. Some like Germany recycle more than 99 percent of their plastic waste whereas the United Kingdom recovers less than 60 percent of its plastic1 and in many developing countries recycling is done on a much smaller scale due to missing regulations. To make recycling easier, the six most common types of plastics are often marked with the ASTM International Resin Identification Coding System.2 The symbols are printed or molded on the plastic products that identify the type of plastic resin out of which the product is made.
Isotactic Polypropylene (PP)
Polypropylene (PP) is the largest volume commodity thermoplastic in the world, greater than low-density, linear low-density, and high-density polyethylene individually but less than the three polyethylenes combined. It is one of the cheapest thermoplastics3 and accounts for about 23 percent of the global plastics materials consumption.1 Most commercial polypropylene is isotactic with an intermediate level of crystallinity. Isotactic polypropylene (i-PP) has high impact resistance, flexibility, and optical clarity and is relatively easy to mold. It competes in many areas with HDPE. It has a higher melting point, superior stress crack, and temperature resistance but lower impact resistance. It is often the better choice for more demanding applications unless a higher resistance against thermo-oxidative degradation is required. The packaging industry is by far the largest consumer of i-PP followed by the textile and automotive industry. Because of its low price, ease of processing, chemically inertness, and many other attractive properties, it has replaced metals, glass, wood, cellophane, and paper in countless applications.
Low-Density Polyethylene (LPDE, LLPDE)
Low-density polyethylene is the second largest volume commodity thermoplastic. It is the cheapest thermoplastic3 accounting for about 17 percent of the global plastics consumption.1 The two main grades are low density (LDPE) and linear low-density polyethylene (LLDPE). Both grades are very flexible and tough and inexpensive. Compared to HDPE, LDPE has a higher degree of short and long side-chain branching which results in lower density and crystallinity as well as in greater flexibility and toughness. However, LDPE has a significantly lower tensile strength, heat deflection temperature, and melting point than HDPE. LLDPE has similar strength as HDPE but is much more flexible. The polymer chains have a large number of short branches, which are shorter and more abundant than those of LDPE. It also has a narrower molecular weight distribution than LDPE. Both increase the degree of crystallinity which results in higher tensile and impact strength and greater puncture resistance compared to LDPE. However, it is less transparent and flexible than LDPE. Both LDPE and LLDPE are used for similar applications. Common uses include food packaging, trash and can liners, grocery bags, bubble packaging, overwrap and envelope films, industrial liners, shipping sacks, mattress bags, stretch- and shrink-wraps as well as agricultural and construction tarps.
Polyvinyl chloride (PVC)
Polyvinylchloride (PVC) is the third largest volume commodity thermoplastic. It is one of the cheapest thermoplastics3 and accounts for about 16 percent of the global plastics consumption.1 It is a clear and mostly amorphous polymer with good flame resistance. Chemically pure PVC is rigid and hard and has low impact strength. It is, however, compatible with many plasticizers which greatly improves its flexibility and toughness. A major drawback of PVC is its poor resistance to heat. It easily decomposes under moderate heat (dehydrochlorination), leading to the intense color formation and deterioration of the polymer properties. PVC finds extensive use is in the building and construction industry. This industry accounts for about 75 percent of all PVC consumption in the United States and for about 60 percent in the European Union. It is extensively used for water and sewage pipes, sidings, window frames, flooring, and wire and cable insulations.
High-Density Polyethylene (HDPE)
High-density polyethylene (HDPE) is one of the cheapest thermoplastics accounting for about 15 percent of the global plastics consumption.1 It has a much lower degree of branching than LDPE which results in a much higher degree of crystallinity typically in the range of 70 to 80 percent depending on its molecular weight and thermal history. The crystals are much larger and more uniform than those of LDPE resulting in much denser, more rigid and less permeable polyethylene which is much stronger and harder than LDPE. On the downside, it is less tough and flexible and has lower stress crack resistance. HDPE is typically opaque but can be easily colored to any requirement. The most common methods of processing HDPE are blow and injection molding. It is extensively used in the consumer goods industry for rigid containers such as buckets, pails, drums, lids, bottles as well as shopping and freezer bags. It competes with
Polystyrene (PS, EPS)
Polystyrene (PS, EPS) is a popular commodity thermoplastic accounting for about 7 percent of the global plastics consumption. This amorphous, nonpolar thermoplastic has excellent optical clarity due to the lack of crystallinity and good thermal and electrical properties for insulation purposes. However, polystyrene has several limitations; it is brittle, has poor oxygen and UV resistance, is readily attacked by hydrocarbon solvents and its upper service temperature is relatively low. Some of these limitations can be overcome by copolymerizing styrene with other monomers. The three most important copolymers are ABS, ASA and SAN which account for about 3 percent of the global plastic demand.
Styrene and its copolymers are molded into numerous consumer goods including dining utensils, food-packaging, plastic cups, housewares, foam cushioning, toys, CD cases, cosmetic containers, covers, and housings. It is also a popular material in the building and construction industry. Important products include insulation foam, siding, panels, bath and shower units and various fixtures.
Polyethylene terephthalate (PET, PETE)
Polyethylene terephthalate (PET) is the most important commercial polyester accounting for about 7 percent of the global plastics consumption. It is a transparent, amorphous thermoplastic when solidified by rapid cooling or a semi-crystalline plastic when cooled slowly or when cold-drawn. PET can be easily thermoformed or molded into almost any shape. Besides excellent processing characteristics, it has many other attractive properties like relatively high strength and toughness, good abrasion and heat resistance, low creep at elevated temperatures, and excellent dimensional stability, particularly when fiber-reinforced. PET finds many uses in the textile and packaging industries. The fibers have an excellent crease and wear resistance, low moisture absorption and are very durable. These characteristics make polyester fibers a popular choice for many textile applications. As a thermoplastic, PET is mainly used for the production of films (BOPET) and blow-molded bottles for soft drinks and water. Other uses of (filled) PET include handles and housings for appliances such as cookers, toasters and showerheads to name only a few applications.