'Plastic', derived from the Greek word 'Plastikos', denotes any flexible or malleable material found in nature or produced synthetically. Nowadays, plastics are taken so common that it seems quite inconceivable that there was ever a time when they were not quite such familiar objects in our daily life. While natural plastics like pine resins, pitch, tar, amber, beeswax, tallow, copal, shell lacca, gutta-percha, natural keratin were long utilized in human history, it was only about a few hundred years ago that the story of man-made type actually began.
Development of Different Plastics
In 1862, at the Great Crystal Palace Exhibition in England, the English Inventor Alexander Parkes displayed a material derived from cellulose nitrate that was dubbed Parkesine and was expected to be used for a myriad purposes, one of these being as a cheaper option to rubber. However, in fact, it proved too expensive and inconvenient for commercial production and did not catch on. Around the same time, James Hyatt of Albany, New York, seeking to win a contest for finding an alternative for ivory(used then to make Billiard balls), developed the first commercially viable form of cellulose plastic by blending Camphor with Collodion. He called this Celluloid and, despite the severe drawbacks of its inflammable nature, it was a success, revolutionizing in particular the photography and film industry.
In 1894, the two English Scientists, Cross and Bevan, took out patents for Cellulose Acetate, which was similar in character to Celluloid but safer. When dissolved in an acetone solution, it could be used to water-proof the cloth covered wooden-frames of the early airplanes and therefore was in great demand during the First World War. Artificial silks, utensil handles, transparent sheets, and other practical articles also began to be made from Cellulose Acetate. However, the quality of these objects being rather poor, further research was carried on. In 1907, came Leo Hendrik Baekeland's 'Bakelite', a liquid resin that on hardening had the property of retaining its shape and form permanently. This was the first thermosetting plastic and proved so indispensable that it is still in use today. Rayon, invented by Louis Marie Hilaire Bernigaut, the Count of Chardonnet, in 1891, and Cellophane, by the Swiss Engineer Dr. Jacques Edwin Brandenberger in 1900, proved to be equally beneficial. Next came Nylon, which was produced first in the famous DuPont Lab and its varied potential tapped by the chemist Wallace Hume Carothers. Another important step was the development of Polyvinyl Chloride (PVC) or Vinyl by the B.F. Goodrich organic chemist, Waldo Semon. Saran, the plastic that modernized food packaging, was discovered in the Dow Chemical Lab by Ralph Wiley in 1933, and the DuPont chemist Roy Plunkett discovered Teflon in 1938. Teflon today is widely used in kitchenware. The tough and transparent Polymethylmethacrylate or Acrylic, popularly known as Perspex and obtained from Methyl Methacrylate, began to be produced commercially in 1932 for air-craft canopies. Two researchers from the Imperial Chemical Industries Ltd. (I.C.I) in England, E. W. Fawcett and R.O. Gibson, stumbled upon Polythene in 1933 while testing the effects of highly pressurized conditions on, amongst other chemicals, ethylene gas. However, it was to be quite sometime before the white, waxy traces they discovered could be produced in sufficient enough quantities for analysis and subsequent use. Polythene came into extensive use only during the Second World War and had a critical role in the Allied Military effort, being put to use as light-weight radar insulation. Later its possibilities for such items as bottles, containers, and bags were discovered, and today it is the most commonly used type in the world. After the war, in 1949, an engineer named James Wright invented plastic putty that was stretchable and non-decaying. Marketed under the name 'Silly Putty', it became very popular as a children's play item. Another popular and important material, Velcro, came in 1957, invented by the Swiss engineer, George de Maestral.
The Plastic Industry has come a long way since then. Nowadays, apart from being used in packaging, textiles, toys, credit cards, televisions, cars, computers, and computer peripherals, plastics are regularly and successfully utilized by doctors to replace worn-out body parts, enabling people to live more productive and longer lives. They are being genetically engineered to possibly reduce the environmental and financial costs of the large-scale oil-fed petrochemicals-to-polymer refineries. U.S. scientists have created the first electricity conducting plastics, though more research is required before these can be put to practical use. In 2000, the Nobel Prize for Chemistry was awarded to researchers who originally demonstrated that they can conduct electricity.
The Plastic materials needed to be 'processed' - molded or shaped, that is - into functional items, for which specific machinery was required. It was only after these were developed that extensive processing became possible. Two processing methods - Extrusion and Injection Molding - came into wide use.
Extrusion, which consists of passing heated material through requisite perforations, originated about 1880 in the Italian Spaghetti and Macaroni Industry. In about 1850, wire was insulated with rubber and Gutta-Percha using an extrusion process. Some Celluloid was extruded towards the end of the nineteenth century, but commercial extrusion of plastics began only in 1930. And it was only after 1931, upon the designing by Horst Heidrich of an electrically heated extruder fed with cold granulated plastic material, that specialized machinery sped up the process.
Injection Molding, which means forcing just sufficient plastic material into a mold to fill it, originated from type-metal, die-casting machines. A successful, manually operated molding machine was developed in Germany in 1919. Ten years later power-operated machines were being designed for Cellulose Acetate and by the Second World War some degree of automatic control had been developed.
Plastics are compounds made with long chains of carbon atoms. When two or more atoms are joined together we call it a molecule. Now let us take for example Polythene, a tough, solid material made from Ethylene, which is a gas. It requires the linking together of many thousands of ethylene molecules to form one molecule of Polythene, and many lines of linked ethylene molecules in an extremely tangled form to make one piece of Polythene. The small linkable molecules are called Monomers, the linking process is Polymerization, and the large molecules thus formed are called Polymers. Sometimes (as with Ethylene) certain chemicals known as Catalysts or Initiators are required to bring about the linking process. Since all synthetic plastics are made by joining together small molecules to make a very big one, it is important to know the best conditions for polymerization. With the exact knowledge of the long, tangled molecular structure that gives its special characteristics, it is now possible to alter or improve the properties of a particular material.
The raw materials of the Plastics Industry are mainly Oil, Air, Salt and Water, but it takes many different and complicated steps to convert these into plastics. Various components are separated from extracted crude oil by distillation. In this process the oil is heated to boil out one after another the various components or fractions that have different boiling temperatures. The vapors formed on boiling are condensed back into liquids, and each separately collected from the distillation column. The light fractions (the ones with the lower boiling points) are used for making petrol and similar fuels. The Heavy fractions (ones with the higher boiling points) are cracked or converted into simpler chemicals. To a large extent the cracking can be controlled to produce some materials in greater quantities than others. In this way the chemicals used as raw materials (feed-stocks) by the Plastics Industry are obtained.
One of these chemicals, Ethylene, can not only be converted directly into Polythene, but also into other chemicals used in plastic-making. Ethylene, Propylene, and Butadiene (used for making some synthetic rubbers) are known as Petrochemicals. In the manufacture of Vinyl Chloride, chemicals from other sources are used - Chlorine, obtained from common salt. Plastics such as Nylon are produced using Ammonia. Nitrogen is obtained from the air and used to make Ammonia.
PVC is made by a polymerization process in which the monomer in the form of bubbles or droplets is kept suspended in water by stirring during polymerization and is separated by the addition of a small amount of an Emulsifying Agent (a chemical like soap or detergent).
The process is carried out in a large vessel called Autoclave which can withstand high pressures. This is partly filled with water mixed with little amounts of Catalyst and Emulsifying Agent. An amount of Vinyl Chloride, about the same as the water quantity, is added and stirred until it is broken into small droplets suspended in the water. The autoclave and its contents are then heated to about 50 degrees centigrade and the Vinyl Chloride droplets polymerize to form solid particles of PVC. These remain suspended in the water, forming a milky emulsion, or Latex. The emulsion is cooled and then run out of the Autoclave. Sometimes it is used as an emulsion, but more often it is dried and a fine powdery form of PVC obtained.
Perspex is made by polymerizing Methyl Methacrylate in a cell constructed from two sheets of glass separated by a rubber ring. Actually the Methyl Methacrylate is partly polymerized, becoming thick and syrupy, before being put into the cell. The polymerization is completed by heating the filled cell in an oven. On completing polymerization, the cell is cooled and the glass plates removed, leaving a sheet of clear, solid Polymethyl Methacrylate - Perspex.
Polythene is manufactured by compressing the monomer, Ethylene gas, to an enormous pressure, with a catalyst, in a special, high, pressure reactor. In the reactor, at a temperature between 150-250 degrees centigrade, the compressed Ethylene polymerizes into Polythene, which is forced out as a molten stream through a special valve at the bottom of the reactor. It is then cooled and cut into small granules.
Nylon Polymers are made by heating the chemicals together so that the long chain molecules are built from bits of each reactant alternately.
These are the two broad categories of Plastics. Thermoplastics - such as Polythene, Polyvinyl Acrylics, PVC, Polystyrene - can be softened by heating and harden again on cooling. These two processes can be repeated over and over again. This is because chemically in Thermoplastics the long molecules of the chemicals remain separate from each other. Thermoplastics are processed into useful articles in three stages. The material is first heated enough to soften it, then it is forced into the desired shape and lastly it is left to cool while it is still held in its new shape.
Thermosetting Materials - such as Polysters, Phenolics, Urea, Formaldehyde, Epoxides, Bakelite Plastic in utensil handles and the White of an egg - on the other hand, first soften on heating and then, with further heating, set hard and afterwards cannot be softened again by heat. This is due to the strong chemical links formed between the long molecules by heating. This firmly joining together is called cross-linking. For example, an egg. Things are made from Thermosetting materials by forcing the material into the desired shape by heat and pressure and then keeping it hot until it has set or cured. A Hydraulic Press is often used to provide the pressure, with molds heated by steam or electricity.
Some plastic articles are made by heating and shaping a piece of Thermoplastic sheet which has previously been made by extrusion, or by calendaring (when softened material is squeezed between hot metal rollers), or has been polymerized as a sheet.
Polymers are essentially molecular materials which consist of atoms combined into molecules by covalent bond and the molecules are held together by secondary bonds such as van der Waal's bonds or hydrogen bonds. The unique characteristic of Polymer molecules that distinguishes them from organic molecules is their size. The name Polymer is derived from the Greek 'Poly' for many and 'Meros' for parts. A Polymer molecule consists of a repetition of the unit called a 'mer'. 'Mers' are derived from starting molecules called 'monomers'. Thus, Mer is a repetition unit of a small molecule which may combine with others to form a large molecule called a Macromolecule.
Today organic Polymers constitute a principal category of materials of construction. The unique characteristics of Polymers, such as light weight, resistant to decay and chemical attack, poor conduction of heat and hence electrical conductivity, easy shaping and fabrication have made them competitors of materials traditionally used. Polymers are used in a variety of forms, including molded products such as radio cabinets, telephone sets, adhesives and paints, wrapping fibers etc. A material suitable for repairing a heart can be produced, or one that is suitable for lining a space craft so that it can withstand high temperatures caused by friction during re-entry into the earth's atmosphere. Other recent developments include adhesives that are so strong that they eliminate the use of nails in the construction of wooden buildings.
Plastics have lately received much bad publicity as an environmental hazard. However, in most cases, this is really more because of the negligent way in which they are used rather than the plastics themselves.