POLYMERS- From morning till night

Polymers from morning till night---

Scientific Principles--

The field of polymers is so vast and the applications so varied, that it is important to understand how polymers are made and used. Since there are over 60,000 different plastics vying for a place in the market, knowledge of this important field can truly enrich our appreciation of this wonder material. Companies manufacture over 30 million tons of plastics each year, and spend large sums on research, development, and more efficient recycling methods. Below we learn some of the scientific principles involved in the production and processing of these fossil fuel derived materials known as polymers. Polymerization Reactions The chemical reaction in which high molecular mass molecules are formed from monomers is known as polymerization. There are two basic types of polymerization, chain-reaction (or addition) and step-reaction (or condensation) polymerization. Chain-Reaction Polymerization One of the most common types of polymer reactions is chain-reaction (addition) polymerization. This type of polymerization is a three step process involving two chemical entities. The first, known simply as a monomer, can be regarded as one link in a polymer chain. It initially exists as simple units. In nearly all cases, the monomers have at least one carbon-carbon double bond. Ethylene is one example of a monomer used to make a common polymer. The other chemical reactant is a catalyst. In chain-reaction polymerization, the catalyst can be a free-radical peroxide added in relatively low concentrations. A free-radical is a chemical component that contains a free electron that forms a covalent bond with an electron on another molecule. The formation of a free radical from an organic peroxide is shown below: In this chemical reaction, two free radicals have been formed from the one molecule of R2O2. Now that all the chemical components have been identified, we can begin to look at the polymerization process.

Step 1: Initiation The first step in the chain-reaction polymerization process, initiation, occurs when the free-radical catalyst reacts with a double bonded carbon monomer, beginning the polymer chain. The double carbon bond breaks apart, the monomer bonds to the free radical, and the free electron is transferred to the outside carbon atom in this reaction. Step 2: Propagation The next step in the process, propagation, is a repetitive operation in which the physical chain of the polymer is formed. The double bond of successive monomers is opened up when the monomer is reacted to the reactive polymer chain. The free electron is successively passed down the line of the chain to the outside carbon atom. This reaction is able to occur continuously because the energy in the chemical system is lowered as the chain grows. Thermodynamically speaking, the sum of the energies of the polymer is less than the sum of the energies of the individual monomers. Simply put, the single bounds in the polymeric chain are more stable than the double bonds of the monomer. Step 3: Termination Termination occurs when another free radical (R-O.), left over from the original splitting of the organic peroxide, meets the end of the growing chain. This free-radical terminates the chain by linking with the last CH2. component of the polymer chain. This reaction produces a complete polymer chain. Termination can also occur when two unfinished chains bond together. Both termination types are diagrammed below. Other types of termination are also possible. This exothermic reaction occurs extremely fast, forming individual chains of polyethylene often in less than 0.1 second. The polymers created have relatively high molecular weights. It is not unusual for branches or cross-links with other chains to occur along the main chain. Step-Reaction Polymerization Step-reaction (condensation) polymerization is another common type of polymerization.

SYNTHETIC RUBBER --

Any artificially produced substance that resembles natural rubber in essential chemical and physical properties can be called synthetic rubber. Such substances are produced by chemical reactions, known as condensation or polymerization, of certain unsaturated hydrocarbons. The basic units of synthetic rubber are monomers, which are compounds of relatively low molecular weight that form the building units of huge molecules called polymers (see Polymer). After fabrication, the synthetic rubber is cured by vulcanizationAny artificially produced substance that resembles natural rubber in essential chemical and physical properties can be called synthetic rubber. Such substances are produced by chemical reactions, known as condensation or polymerization, of certain unsaturated hydrocarbons. The basic units of synthetic rubber are monomers, which are compounds of relatively low molecular weight that form the building units of huge molecules called polymers (see Polymer). After fabrication, the synthetic rubber is cured by vulcanization.development : The origin of synthetic-rubber technology can be traced to 1860, when the British chemist Charles Hanson Greville Williams determined that natural rubber was a polymer of the monomer isoprene, which has the chemical formula CH2:C(CH 3)CH:CH2. Many efforts were made during the next 70 years to synthesize rubber in the laboratory by using isoprene as the monomer. Other monomers also were investigated, and during World War I (1914-1918) German chemists polymerized dimethylbutadiene (formula CH2:C(CH3)C(CH 3):CH2) producing a synthetic rubber called methyl rubber, which was of limited usefulness. A breakthrough in synthetic-rubber research did not occur, however, until about 1930, when the American chemist Wallace Hume Carothers and the German scientist Hermann Staudinger did scientific work that contributed greatly to present-day knowledge that polymers are huge, chainlike molecules made of large numbers of monomers.

SYNTHETIC FIBERS--

Synthetic fibers derived from natural cellulose were first developed at the end of the 19th century and became known as rayons. In a typical rayon-making process, natural cellulose made from wood pulp is treated with chemicals to form a thick liquid. This liquid is then extruded as filaments into a weak acid bath that converts the filaments back into pure cellulose. Rayons are not, therefore, completely synthetic but are actually regenerated fibers. Acetates and triacetates, which are true synthetic fibers, were developed shortly after rayon. They are derived from cellulose acetate in a process similar to that used for making rayon. Most synthetic fibers are now derived from organic polymers, materials consisting of large organic molecules. Most of them are thermoplastic—that is, they are softened by heat. The first commercially successful organic synthetic fiber, nylon (polyamide), dates from 1938. Since then many other fibers, including acrylic (polyacrylonitrile), aramid (aromatic polyamide), olefins (polyethylene and polypropylene), polyester, and spandex (polyurethane), have been developed. In a typical fiber-spinning process, a molten polymer or polymer solution is extruded through tiny holes in a spinneret into an environment that causes the filaments to solidify. The fiber's properties depend on the base polymer, the spinning process, and the post-spinning treatment of the fiber, which can include drawing, annealing, applying a finish, and coating. Fiber properties such as weight, abrasion resistance, heat resistance, chemical resistance, moisture resistance, strength, stiffness, elasticity, and ease of dyeing and coloring can be optimized by such treatments. Carbon and graphite fibers are high-strength materials that are used as reinforcing agents in composites. Carbon fibers are produced by using heat to chemically change rayon or acrylic fibers. Carbonization occurs at temperatures of 1000°C to 2500°C(1832°to 4532°F) in an inert atmosphere .

Use of polymers in cement---

Cement-based materials are widely used in the civil infrastructure. Polymers as admixtures can improve the properties, particularly in relation to water absorption reduction, toughness enhancement, vibration damping and increase of the bond strength of cement to reinforcements. Polymeric admixtures include particles, short fibers and organic liquids. Latex in the form of an aqueous particle dispersion is most common. Other than being used as admixtures, polymers are used as partial replacement of fine aggregate, for coating, sealing and repairing concrete and for coating steel reinforcing bars for corrosion protection

USES OF DIFFERENT POLYMERS--

Polyethylene Terephthalate (PET or PETE) :Soft drink bottles, peanut butter jars, salad dressing bottles, mouth wash jars Liquid soap bottles, strapping, fiberfill for winter coats, surfboards, paint brushes, fuzz on tennis balls, soft drink bottles, film High density Polyethylene (HDPE) :Milk, water, and juice containers, grocery bags, toys, liquid detergent bottles ,Soft drink based cups, flower pots, drain pipes, signs, stadium seats, trash cans, re-cycling bins, traffic barrier cones, golf bag liners, toys Polyvinyl Chloride or Vinyl (PVC-V):Clear food packaging, shampoo bottles ,Floor mats, pipes, hoses, mud flaps . Low density Polyethylene (LDPE):Bread bags, frozen food bags, grocery bags,Garbage can liners, grocery bags, multi purpose bags Polypropylene (PP):Ketchup bottles, yogurt containers, margarine, tubs, medicine bottles ,Manhole steps, paint buckets, videocassette storage cases, ice scrapers, fast food trays, lawn mower wheels, automobile battery parts. Polystyrene (PS):Video cassette cases, compact disk jackets, coffee cups, cutlery, cafeteria trays, grocery store meat trays, fast-food sandwich container,License plate holders, golf course and septic tank drainage systems, desk top accessories, hanging files, food service trays, flower pots, trash cans