ABOUT PLASTICS A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs. Monomers of plastic are either natural or synthetic organic compounds. The word plastic is derived from the Greek πλαστικός (plastikos) meaning capable of being shaped or molded, from πλαστός (plastos) meaning molded.12 It refers to their malleability, or plasticity during manufacture, that allows them to be cast, pressed, or extruded into a variety of shapes—such as films, fibers, plates, tubes, bottles, boxes, and much more. The common word plastic should not be confused with the technical adjective plastic, which is applied to any material which undergoes a permanent change of shape (plastic deformation) when strained beyond a certain point. Aluminum which is stamped or forged, for instance, exhibits plasticity in this sense, but is not plastic in the common sense; in contrast, in their finished forms, some plastics will break before deforming and therefore are not plastic in the technical sense. There are two types of plastics: thermoplastics and thermosetting polymers. Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be moulded again and again; examples are polyethylene, polypropylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene (PTFE).3 Thermosets can melt and take shape once; after they have solidified, they stay solid.The raw materials needed to make most plastics come from petroleum and natural gas.4
HISTORY OF PLASTIC BAGS Parks, In Birmingham, UK in 1856.11 It was unveiled at the 1862 Great International Exhibition in London.12 The development of plastics has come from the use of natural plastic The first human-made plastic, called parkesine, was patented by Alexander materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., rubber, nitrocellulose, collagen, galalite) and finally to completely synthetic molecules (e.g., bakelite, epoxy, polyvinyl chloride, polyethylene). In 1866, Parkes formed the Parkesine Company to mass produce the material. The company, however, failed due to poor product quality as Parkes tried to reduce costs. Parkesines successors were Xylonite, produced by Daniel Spill (an associate of Parkes), and Celluloid from John Wesley Hyatt. Parkesine was made from cellulose treated with nitric acid and a solvent. The generic name of Parkesine is pyroxylin, or Celluloid. Parkesine is often synthetic ivory. The Parkesine company ceased trading in 1868. Pictures of Parkesine are held by the Plastics Historical Society of London. There is a plaque on the wall of the site of the Parkesine Works.
CHEMICAL STRUCTURE Common thermoplastics range from 20,000 to 500,000 amu, while thermosets are assumed tohave infinite molecular weight. These chains are made up of many repeating molecularunits, known as repeat units, derived from monomers; each polymer chain will have severalthousand repeating units. The vast majority of plastics are composed of polymers of carbon andhydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some ofcommercial interests are silicon based.) The backbone is that part of the chain on the main"path" linking a large number of repeat units together. To customize the properties of aplastic, different molecular groups "hang" from the backbone (usually they are "hung" as part ofthe monomers before linking monomers together to form the polymer chain). This fine tuning ofthe properties of the polymer by repeating units molecular structure has allowed plastics tobecome an indispensable part of twenty first-century world. Some plastics are partially crystalline and partially amorphous in molecular structure, givingthem both a melting point (the temperature at which the attractive intermolecular forces areovercome) and one or more glass transitions (temperatures above which the extent of localizedmolecular flexibility is substantially increased). The so-called semi-crystalline plastics includepolyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and somepolyurethanes. Many plastics are completely amorphous, such as polystyrene and itscopolymers, poly (methyl methacrylate), and all thermosets.