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From the second world war and early on, the polymer commercial industry—long-chain synthetic molecules—“plastics” is a common misnomer—has grown rapidly. In 2015, more than 320 million tons of polymers were produced worldwide, excluding fiber.
In the past five years, designers of polymer products have initially not considered what happens after the end of their useful life. But this situation is beginning to change and will receive more attention in the next few years.
Plastic industry
"Plastic" has become a misleading way of describing polymers. They usually come from long-chain molecules of oil or natural gas, with hundreds or thousands of links in each chain. Long chains convey important physical properties such as strength and toughness that short molecules cannot match at all.
"Plastic" is actually an abbreviation for "thermoplastic", a term that describes polymeric materials that can be shaped and reshaped using heat.
The modern polymer industry was actually founded by Wallace Karos in the 1930s. His arduous study of polyamides led to the commercialization of nylon, as the shortage of silk during war forced women to look for other materials to make stockings.
When other materials became scarce during the Second World War, researchers looked for synthetic polymers to fill the gap. For example, Japan's conquest of Southeast Asia cut off the supply of natural rubber for automotive tires, leading to the emergence of a synthetic polymer substitute.
Breakthroughs in chemistry have driven the further development of synthetic polymers, including polypropylene and high density polyethylene, which are now widely used. Some polymers were discovered by chance, such as Teflon.
Ultimately, the combination of demand, scientific advancement and accidental discovery forms a complete set of polymer products that you can now easily identify as “plastic”. These polymers are quickly commercialized as they seek to reduce the weight of their products and provide inexpensive alternatives to natural materials such as cellulose or cotton.
Plastic type
The synthetic polymers produced globally are mainly polyolefin-polyethylene and polypropylene. There are two types of polyethylene: "high density" and "low density." On a molecular scale, high-density polyethylene looks like a comb with regularly spaced short teeth. In contrast, the low-density version looks like a long-toothed comb with irregular spacing; if viewed from a height, it looks a bit like a river and its tributaries. Although they are all polyethylene, the difference in shape allows these materials to behave differently when molded into films or other products.
There are several reasons for the advantages of polyolefins. First, they can be produced using relatively inexpensive natural gas. Second, they are the lightest synthetic polymers produced on a large scale; their density is so low that they can float. Third, polyolefins are resistant to damage from water, air, grease, and detergents—all of the problems these polymers can encounter when used. Finally, they are easily molded into products that are strong enough that the boxes made of them are not deformed in transport trucks that are exposed to the sun all day.
However, these materials have serious drawbacks. Their degradation rate is very slow, which means that polyolefins exist in the natural environment for decades to centuries. At the same time, waves and wind erosion mechanically wear them, creating particles that can be swallowed by fish and animals that move toward us along the food chain.
Recycling polyolefins is not as straightforward as people hope because of collection and cleaning problems. During reprocessing, oxygen and heat can cause long chain damage, while food and other materials can contaminate the polyolefin. Continued advances in chemistry have created new grades of polyolefins that increase strength and durability, but they cannot always be mixed with other grades during recycling. More importantly, polyolefins are often combined with other materials in multi-layer packaging; although these multilayers work well, they are not recyclable.
Due to the growing shortage of oil and natural gas, the production of polymers has been criticized. However, the proportion of natural gas or petroleum used to produce polymers is very low; less than 5% of the oil or natural gas produced each year is used to produce plastics. In addition, ethylene can be extracted from sugar cane ethanol, as Brazil's Braskem does commercially.