Polymers - Natural and man made
Polymers (Greek poly - many; mers - units or parts) are macro molecules (giant molecules containing large number of carbon atoms of high molecular masses) built up by the linking together of a large number of small molecules.
For example polyethylene is a polymer formed by the combination of small ethene molecules.
These small molecules are called monomers. Monomer comes from a greek words, mono meaning single and mers meaning parts. Monomer means single part. The monomers in a polymer may be identical, like the links on a metal watch or the monomers may be different, like the beads in a multi colored necklace.
Polymer is a generic name given to a vast number of materials of high molecular weight. These materials can exist in different forms and can have different chemical structures, physical properties etc. Polymers can be classified into natural and synthetic (man made) polymers. Natural polymers are polymers isolated from natural materials. The most well known natural polymers are proteins, nucleic acids and the polymers of glucose (starch, glycogen, cellulose). Synthetic polymers were originally derived from these natural polymers. Synthetic polymers are polymers synthesized from low molecular weight compounds. The first commercially successful synthetic polymer was cellulose nitrate (Celluloid, 1869) which was first practically molded as a substitute for ivory in billiard balls, cellulose nitrate is also an explosive and its use in motion picture film and in billiard balls occasionally produced spectacularly inflammable incidents. Cellulose acetate, replaced it.
You have learnt that macromolecules are formed by a process known as polymerization, in which large compounds are built by joining smaller ones together.
Polymerization process is of two types. They are addition polymerization and condensation polymerization. Addition polymerization is a chemical reaction in which simple molecules (monomers) are added to each other to form long-chain molecules (polymers) without by-products. For example you might have heard about the polymer polyethene. This is an addition polymer of the monomer type ethane. Condensation polymerization is a process in which water or some other simple substance separates from two or more of the polymer molecules upon their combination. For example you heard about Nylon. This is a condensation polymer.
You have a large number of polymers that are synthesized in the laboratory. Based on their properties these are called by different names in the commercial sector. Some of them are resins, plastics and fibers.
Resins
The term resin is applied to naturally occurring amorphous solids. Resin is a natural or synthetic compound which begins in a highly viscous state and hardens with treatment. This hardening is what makes them useful to mankind, as resin is hard enough to be used in varnishes, paints, lacquers, sealants and adhesives. Resin is soluble in alcohol, but not in water. Resin had been produced naturally by many plants. Natural resins have been used in various civilization for thousands of years: one notable example of an ancient use of resin can be seen in Egypt countrt, where resin was used in the preparation of mummified bodies. Natural resins are continue to be used today for a wide range of applications which stretch from perfume to the treatment of the bows for instruments such as violins and cellos. These can be easily molded and casted in different shapes. These can be used for surface coatings.
Now you can synthesize a large number of polymers called synthetic resins. Synthetic resin is a man-made polymeric compound that physically resembles natural resin. For example, Phenol - formaldehyde resin known as Bakelite, was developed in the United States by the Belgian-born chemist Leo Baekeland in 1909. Heating these resins under pressure gave soft solids which could be molded and then hardened; they were both safe and economical. Resins have good strength, heat stability and high resistance to chemical corrosion and moisture penetration. It is for these reasons resins are widely used in the preparation of electrical components, laminated glues, adhesives etc. These are thermosetting polymers.
Epoxy resins are made by polymerization of ethelene oxide. Silicon resins are prepared from alkylated silicon compounds. Carbo wax is a resin.
Plastics
Plastics are polymeric substances and these are classified into thermosetting polymers and thermo plastics based on properties of heating polymers.
Some polymers soften on heating and can be converted into any shape that they can retain on cooling. This type of process can be repeated several times. Such as polymers, that soften on heating and stiffen on cooling are termed as thermo plastics. For example Polyethylene, poly styrene, Poly vinyl chloride etc.
Thermosetting polymers are those which on heating sets into a mass and once set cannot be reshaped. For example the above mentioned resin known as Bakelite, egg yolk etc. Now you can able to prepare polymers which are strong as steel. Some of human organ replacements like heart valves and artificial limbs are being manufactured from polymeric substances.
Fibres
Chemical substances, natural or synthetic which can be drawn into fibers(threads) from the melt or from their solution are known as fibre producing substances. Fibers were originally made from naturally occurring materials such as cotton, jute, wool and silk. But fibers are now extensively made from synthetic polymeric materials. Hence fibers are classified as synthetic and natural. But fibers are derived from polymeric substances only. These are semi synthetic fibers which are obtained from partially modified natural materials such as cellulose.
Some important and synthetic fibres are
Rayons.: These are artificial silks. These are derived from cellulose.
Nylon 66: This is a polyamide. It is suitably used for parachute fabric sewing thread
Terylene or Dacron: This is a polyester
Orion: This is a poly acrylonitrile fibre.
Rubber
Natural rubber is a polymer found in the sap of the rubber tree. Chemically, this is a polymer of isoprene. Natural rubber has long been known, but became valuable only with the development of vulcanizing (heating in the presence of sulfur) which gives a much more rubbery and coherent substance.
Significant forms of synthetic rubber used today are the Buna S, Buna N. Other types (Thiokol, Neoprene) have somewhat different properties and specialty uses. Buna-S, by far the most significant, has been made since 1933 in Germany. Both Buna-S type and Buna-N rubber can be vulcanized.
True synthetic rubber, a polymer of isoprene, was achieved almost simultaneously in 1955 by several major rubber manufacturers using different catalysts. The process is a polymerization of isoprene itself and the product is virtually identical to the natural product. The feed stocks for the synthetic rubber industry are products of the petrochemical industry.
Rubber, either natural or synthetic, is normally vulcanized, most often with sulfur, although for some specialty uses other agents are used. The majority of rubber used is used for automobile tires. For this and similar applications a filler or reinforcing agent must be added, of which carbon black is by far the most common.
