What do you know about Plastics?

七月 26, 2017

What is Plastic?

Plastics are the most versatile materials ever invented. Indeed, the word "plastic," which derives from the Greek word plastikos, meaning to mold or form, has come to be used as a general description for anything particularly adaptable or flexible. Since the first plastic, celluloid, was developed as a replacement for elephant ivory in the 1860s, many different types of plastics, including nylon, polyethylene, and Teflon®have revolutionized the manufacture of commercial goods as diverse as nylon stockings and car-body parts. Although the use of plastic continues to grow and revolutionary new plastics are constantly being developed, concerns have been raised about the environmental effects of using and disposing of so much plastic material, prompting the invention of bioplastics.

Science

Plastics are synthetic chemicals extracted mainly from petroleum and composed of hydrocarbons (compounds made from chains of hydrogen and carbon atoms). Most plastics are polymers, long molecules made up of many repetitions of a basic molecule called a monomer; in effect, the monomers are like identical railroad cars coupled together to form a very long train. Thus, as many as 50,000 molecules of ethylene (which has two carbon atoms bonded to four hydrogen atoms) can be joined end to end into a familiar polymer called polyethylene (or polythene). The process of building polymers by adding together monomers is called additive polymerization. Another process called condensation polymerization (or polycondensation) builds up polymers by removing some atoms from each monomer so they can join together in a different way. Polyesters such as Dacron® and Terylene (two different brand names for similar materials) are made by polycondensation. Whichever process is used, the chemical properties of the monomer normally govern those of the polymer that is eventually formed.

Polymerization produces two different kinds of plastics. Sometimes, polymers form very long straight or branched chains. These are present in so-called thermoplastics, which always soften when heated and harden when cooled down. Examples include polyethylene and polystyrene. Polymers can also form more complex three-dimensional structures, which give plastics very different physical properties. Thermosetting plastics, as these are called, harden the first time they are heated when cross-links form between different plastic molecules. Thermosetting plastics never soften again no matter how many times they are heated and this makes them particularly suitable for objects that need to operate in hot environments. Epoxy resins and bakelite are examples of thermosetting plastics.