Polymers are big molecules made up of repeating units which are bonded covalently among the repeating units. Polymers are made up simple repeating units known as monomers. The word mono means one unit and as the molecule grow bigger the word poly is used which means many repeating units. The monomers within a polymer can either be the same monomer or combination of several monomers. Polymer actually refers to a large class of natural and synthetic materials with a wide variety of properties.
Polymers are known to have extraordinary properties which make it very useful in either industries as engineering polymers or in everyday life as commodity polymers. Polymers produced in the past have such low quality that they gain bad reputation. However researchers in the past continue to do research in order produce better polymers. And today polymer production had become one of the most important sectors and plenty of new high quality polymers had been produced. Polymers nowadays are made up of high quality material and slowly replacing metals in many applications. Polymers are used to produce products such as clothes, toys, furniture, machine components, and paints as additives, boats and automobile parts. In automobile industry, polymers are beginning to replaced metals because of its light weight which will reduce the overall car weight and also helps to prevent corrosion.
Polymers are not only produced synthetically but also produced naturally by living organisms such as plants. Cellulose is an example of polymer which is being produced by plants. Cellulose is a polymer which consists of polysaccharide as the monomer and being joined together by a glycosidic bond. Besides that natural polymeric material such as shellac, amber and natural rubber has been in use for centuries and plays a big role in human civilization. Proteins and DNA are also example of polymers which can be found within living things. To a certain extent, living things such as human being are also said to be polymers due to the fact that human consist of repeating units which are the cells.
Polymers can be divided into two types which are the addition polymers and the condensation polymers. Addition polymers are formed by a reaction in which the monomer units simple add to one another to form a long chain polymer which can be either branched or linear. The monomers for addition reaction usually contain C-C double bonds. Addition polymers include polyethylene, polypropylene, polystyrene, polyvinyl chloride (PVC), polytetrafluoroethylene (trademarked as Teflon), and poly methylmethacrylate that is also known as PMMA (Plexiglas), poly acrylonitrile (Orlon), Natural rubber and styrene butadiene rubber (SBR). Condensation polymers are formed by reaction of bifunctional group monomers. The polymerization process often produces by products such as water, hydrochloric acid and also salt such as sodium chloride. Often the productions of condensation polymers or also known as step growth polymers are more complicated compared to production of addition polymers. In production of step growth polymers, monomers react to form dimers, trimers, oligomer and lastly a long chain polymer. The difference between addition and step growth polymer is that during the initial state of step growth polymerization, reactant concentration decreases rapidly. The mechanism of step growth polymerization is also the same throughout the reaction compared to addition which consist of propagation, initiation and termination step. Step polymerization also requires no initiator to start the polymerization process. Besides that, step growth polymerization is not as exothermic as addition polymerization. However in order to produce polymer with high molecular weight, special polymerization reaction known as high yield reaction must be employed depending on the type of step growth polymer to be produced. It is important to use these high yield reactions because it enables the control over molecular weight control of a step growth polymer. In order for the formation of high molecular weight linear polymer from difunctional monomer to occur, a perfect stoichiometric balance of the two difunctional monomers must be introduced or alternatively a self-balancing reaction is necessary. Besides that high degree monomer purity is also required. The reaction that is responsible for the polymerization must be a very high yield reaction with the absence of side reaction. The most common high yield and cost feasible reactions include ester interchange, amidation by dehydration of ammonium salt, reaction of asocyanate with an alcohol or amine and Schotten Baumann reaction of an acid chloride with an amine or alcohol. Ester interchange reaction is used polyester while reaction of an asocyanate with an alcohol or amine is used to produce polyurethane. Amidation by dehydration of ammonium salt and Schotten Baumann reaction of an acid chloride with an amine or alcohol is used to produce polyamide nylon 6,6 and nylon 6,10 respectively. Step growth polymers include polyamide, polyester, polyurethane, phenol formaldehyde, cellulose acetate and silicones. Polyesters can further be divided into three main polymers that are poly ethylene terepthalate or trade marked as Dacron, Glyptal resin and casting resin.
Polyester is a category of polymer which contains ester functional group in its molecule. Even though polyester can be produced via reaction, polyester is also being produced naturally such as cutin and plants cuticles.Ester interchange technique is normally used to produce ·polyester with high molecular weight such as poly (ethylene terephthalate) which is trademarked as Terylene or Dacron. Direct esterification only produced polymer with the low molecular weight because of the difficulty to obtain the stoichiometric equilibrium.
Production of PET involves reaction of terepthalic acid with methanol to form dimethyl terephthalate ester which can easily be purified through distillation process.Reaction of dimethyl terepthalate with ethylene glycol at low temperature causes the formation of oligoester with the hydroxyl end group and finally ester interchange between oligoester occurs at 250°C produces polyester with high molecular weight. Polyester is a semi crystalline and amorphous polymer having melt temperature of about 265 to 271°C. PET is used extensively in the production of blow molded bottles due to its characteristic outstanding strength of oriented form. PET application in as fiber includes tire cords, rope, thread, cord, belts, filter cloths, monofilament, brushes, clothing, carpet and bristles. PET in the form of film or sheets are used in the production of photographic, x-ray films, biaxial oriented sheet for food packaging and transparencies.
Polyamide is polymers which contains amide functional group connected by a peptide bond. Polyamide is being produced naturally and artificially. Natural occurring polyamide is such as protein meanwhile artificially produced polyamide includes wool and silk. The most commonly produced polyamide is nylon. Polyamide is produced via Amidation by dehydration of ammonium salt and Schotten Baumann reaction of an acid chloride with an amine or alcohol. Direct amidation between diacid and diamine cannot produce high polymer because of the difficulty to achieve the stoichiometric equilibrium and high molecular weight polyamide. Polyamide is considered as the first engineering thermoplastic. Polyamide was invented in 1934 by Wallace Carothers from DuPont and the first commercially produced polyamide was nylon in 1938. Nylons are being described by a numbering system which indicates the numbers of carbon atoms in the monomer chains. Fiber polyamide applications includes tire cords that uses nylon 6 and nylon 6,6, rope, thread, cord, belts and filter cloths, brushes and bristles. Plastics applications of polyamide includes bearings, gears, cams, rollers, slides, door latches, thread guides, clothing, light tents, shower curtains and umbrella.
Polyamides divided can be divided into 2 types which is aliphatic polyamides and aromatic polyamides. Aliphatic polyamides can be divided into two more type which is the AA-BB polyamide and A-B polyamide. AA-BB polyamide include nylon 4,6, nylon 6,6, nylon 6,8, nylon 6,9, nylon 6,10 and nylon 6,12. These polyamides are normally produced via salt formation technique. The second type is A-B polyamides are nylon 4, nylon 6, nylon 7, nylon 11 and nylon 12. A-B polyamide is usually obtained through ring opening polymerization. Nylon 6 can be obtained from ring opening of caprolactam.
Polyamides are highly crystalline hydrogen-bonded linear polymers. The difference in the number of carbon atoms between the amide groups results in significant different on mechanical and physical properties. Lower number of carbon atoms will cause increase in specific gravity, melt temperature and mechanical properties. However lower number of carbon atoms will cause increase of moisture absorption in humid environment and also causes polyamide to have poor dimensional stability. For example nylon 6,6 has higher rigidity compared to nylon 6 which has high weather and thermal resistance and good barrier properties.
Aromatic Polyamides include Nomex fiber and Kevlar fiber. Nomex fiber is produced by reacting isophthaloyl chloride with m-phenylenediamine. Nomex fiber melt temperature is approximately 365oC and it is often use as fire retardant fabric in tire cord. Kevlar fiber is product via reaction between terephthaloyl chloride and p-phenylenediamine. Kevlar is often used in production of tires and ballistic vest because of its high strength. Kevlar has glass temperature of about 300oC and melt temperature of about 500oC. Kevlar has good dimensional stability and it does not creep or shrink at temperature as high as 200oC. Kevlar added with graphite fibers are used for aeroplane components.
Polyurethane is polymers consist of monomers which are connected by urethane or links. Polyurethane is commonly abbreviated as PU. Polyurethane polymers are formed through step-growth polymerization by reacting monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl groups in the presence of an acid or base catalyst. Polyurethanes should not be confused with the specific substance urethane that is also known as ethyl carbamate.
Phenol formaldehyde resin is any number of synthetic resins made by reacting phenol with formaldehyde. Phenol-formaldehyde resins were the first synthetic polymers to be commercialized. Bakelite which is a trademark for phenolic plastics revolutionizes the market for moulded and laminated parts. Phenolic plastics are very important industrial polymers. Their most common use in the industries is as adhesives for plywood and other structural wood products bonding. The chemical composition of phenol and formaldehyde and their combination into networks of permanently interlinked large molecules are explained briefly in the article aldehyde condensation polymer.
There are two basic methods used in the making of phenol formaldehyde polymer. For the first method, excess of formaldehyde is reacted with phenol in the presence of a base catalyst in water solution to yield a low-molecular-weight prepolymer known as resole. Resole, frequently in liquid form or solution, can be cured to a solid thermosetting network polymer by, for instance, sandwiching it between layers of wood veneer and then heating the assembly under pressure to form plywood.
The other method involves reacting formaldehyde with excess phenol using acid catalyst. This process produces Novolac which is a solid prepolymer. Novolac resembles the final polymer except that it has much lower molecular weight and is a thermoplastic. Novolac can be softened by reheating without undergoing chemical decomposition. Curing is accomplished by grinding the Novolac into powder form, mixing it with fillers such as wood flour, minerals and glass fibres and heating the mixture in a pressurized mould. Novolac requires addition of more formaldehyde during heating process in order for it to cure to a thermoset resin In order to be cured to a thermosetting resin.
Phenol formaldehyde resins make excellent wood adhesives for plywood and particle board because of their ability to form chemical bonds with the phenol-like lignin component of wood. They are desirable for exterior plywood, owing to their good moisture resistance. Phenolic resins are reinforced with fibres and flakes and moulded into insulating and heat resistant objects such as appliances handles, distributor cap and brake linings.