Engineering Materials
A polymer is a large molecule composed of repeating structural units. poly- The word polymer is derived from the Greek words meaning "many"; and - meros meaning "part". Plastic and rubber materials are common polymers. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (O,N, and Si). Some of the common and familiar polymers are polyethylene (PE), nylon, poly vinyl chloride (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber.
Properties These materials typically have low densities. They are not as stiff nor as strong as these other material types. Polymers are extremely ductile and pliable (i.e., plastic), which means they are easily formed into complex shapes. They are relatively inert chemically and un reactive in a large number of environments. One major drawback to the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some instances, limits their use. They have low electrical conductivities and are nonmagnetic.
Non-resistant to heat or temperature Sensitive to sunlight (radiations) Environmental hazards (Nonbiodegradability) Health hazards
Plasticizers used are mostly: Carbamates Pthalates Both of them are carcinogenics and hormones disrupters (e.g. estrogen hormone)
Non-biodegradable: Not decomposed by bacteria as other substances are decayed in nature. Biodegradable plastics are being developed e.g. PHBV (Polyhydroxy Butyrate Valarate) Glucose Bacteria PHBV
Some bacteria are cultured which can decompose polymers like Nylon. Flavo Bacteria and Pseudomonas are two microbes that secrete Nylolase enzyme nd are called Nylon Eaters, because they can decompose nylon. Similar are Oxo-biodegradable Plastics.
Naturally occurring polymers those derived from plants and animals have been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk. Other natural polymers such as proteins, enzymes, starches, and cellulose are important in biological and physiological processes in plants and animals.
It is the process by which monomers are linked together to generate long chains composed of repeat units. Two general classifications according to reaction mechanisms are Addition Polymerization Condensation Polymerization
Addition Polymerization (Chain Reaction) It is a process by which monomer units are attached one at a time in chain-like fashion to form a linear macromolecule. Three distinct stages initiation, propagation, and termination are involved in addition polymerization. During the initiation step, an active center capable of propagation is formed by a reaction between an initiator species and the monomer unit. Again, R represents the active initiator, and is an unpaired electron. Propagation involves the linear growth of the polymer chain by the sequential addition of monomer units to this active growing chain molecule
Propagation may end or terminate in different ways. First, the active ends of two propagating chains may link together to form one molecule The other termination possibility involves two growing molecules that react to form two dead chains thus terminating the growth of each chain.
Condensation (or step reaction) polymerization It is the formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species. There is usually a small molecular weight by product such as water that is eliminated (or condensed). No reactant species has the chemical formula of the repeat unit, and the intermolecular reaction occurs every time a repeat unit is formed. For example, consider the formation of the polyester, poly(ethylene terephthalate) (PET), from the reaction between ethylene glycol and terephthalic acid;
Thermo plastics (thermo softening plastic) Thermoplastics soften when heated (and eventually liquefy) and harden when cooled processes that are totally reversible and may be repeated. Thermoplastics are relatively soft. Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride). e. g. luncheon box
Above its glass transition temperature, T g, and below its melting point, T m, the physical properties of a thermoplastic change drastically without an associated phase change. Within this temperature range, most thermoplastics are rubbery due to alternating rigid crystalline and elastic amorphous regions, approximating random coils. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure. Brittleness can be lowered with the addition of plasticizers which interfere with crystallization to effectively lower T g.
Thermosetting They become permanently hard during their formation, and do not soften upon heating. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability. e.g. casing for domestic light switch. A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat (generally above 200 C (392 F)), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing. Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC). Once hardened a thermoset resin cannot be reheated and melted back to a liquid form. According to IUPAC recommendation: A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation, or both. A cured thermosetting polymer is called a thermoset.
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat.
They are materials that have low elastic moduli which show great extensibility and flexibility when stressed but which returns to their original dimensions, or almost so, when deformation stress is removed. In essence, the molecules act like a series of small helical spring. e.g. disposable surgical gloves. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber. Each of the monomers which link to form the polymer is usually made of carbon, hydrogen, oxygen and/or silicon. Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures, rubbers are thus relatively soft (E~3MPa) and deformable. Their primary uses are for seals, adhesives and molded flexible parts.
Polyethylene (PE) Polyethylene (abbreviated PE) or polythene is the most common plastic. The annual production is approximately 80 million metric tons. Its primary use is within packaging (plastic bag, plastic films). It is made by polymerization of ethene (ethylene).it has excellent chemical resistance, that it is not attacked by strong acids or strong bases, tough and flexible over wide range of temp. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin. The material continues burning on removal of the flame source.
Polyethylene terephthalate commonly abbreviated PET, OR PETE is a thermoplastic polymer resin and is used in synthetic fibre, beverage, food and other liquid containers. Its monomer can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct. PET consists of polymerized units of the monomer ethylene terephthalate. PET in its natural state is a colorless, semicrystalline resin. Based on how it is processed, PET can be semi-rigid to rigid, and very lightweight, good gas and fair moisture barrier, as well as a good barrier to alcohol and solvents. It is strong and impact-resistant.
Polyvinyl chloride, commonly abbreviated PVC, is the third-most widely-produced plastic. PVC is widely used in construction because it is durable, cheap, and easily worked. It can be made softer and more flexible by the addition of plasticizers. Pure polyvinyl chloride without any plasticizer is a white, brittle solid. It is insoluble in alcohol. Polyvinyl chloride is produced by polymerization of the monomer vinyl chloride. Used in production of pipes for industrial and municipal applications. PVC is commonly used as the insulation on electrical cables
Plasticizers: Added to decrease tensile strength of PVC, e.g. Phthalates Heat Stabilizers: To prevent thermal degradation during processing and help to extend the life of finished product. e.g. Sn, Pb, Ca, Zn. Lubricants: Aid the melt flow of PVC compound during processing. e.g. Waxes, Fillers: They are mainly added to lower the cost of PVC. About 80% of all the fillers used in PVC is calcium carbonate. Titanium oxide is second.
It is a polymer containing monomers of amides. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization examples being nylons. Polyamides are commonly used in textiles, automotives, carpet and sportswear due to their extreme durability and strength. The amide link is produced from the condensation reaction of an amino group and a carboxylic acid or acid chloride group. A small molecule, usually water, or hydrogen chloride, is eliminated.
Structure of polyamide
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate (PET). Fabric balls knitted from polyester thread or yarn are used extensively in apparel and home furnishings, from shirts and pants to jackets and hats, bed sheets, blankets, upholstered furniture and computer mouse mats. Industrial polyester fibers, yarns and ropes are used fabrics for conveyor belts, safety belts, coated fabrics etc.
Polyester fiber is used as cushioning and insulating material in pillows, comforters and upholstery padding. The majority of the world's PET production is for synthetic fibers (in excess of 60%) Polyester is a synthetic polymer made of purified terephthalic acid (PTA), dimethyl terephthalate (DMT) and monoethylene glycol (MEG).
Many times, however, it is necessary to modify the mechanical, chemical, and physical properties to a much greater degree than is possible by the simple alteration of molecular structure. Additives Foreign substances are intentionally introduced to enhance or modify many of these properties, and thus make a polymer more serviceable. Fillers Filler materials are most often added to polymers to improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties.
Materials used as particulate fillers include wood flour, silica flour and sand, glass, clay, talc, limestone. Plasticizers The flexibility, ductility, and toughness of polymers may be improved with the help of plasticizers. Their presence also produces reductions in hardness and stiffness. Plasticizers are generally liquids having low vapor pressures and low molecular weights. Heat Stabilizers Additives that counteract deteriorative processes are called stabilizers. One common form of deterioration results from exposure to light. Another important type of deterioration is oxidation. Sn, pb, ca,and zn.
Colorants Colorants impart a specific color to a polymer; they may be added in the form of dyes or pigments. Pigments are filler materials that do not dissolve, but remain as a separate phase; normally they have a small particle size. Dyes, Pigments, paints Flame Retardants The flammability of polymeric materials is a major concern, especially in the manufacture of textiles and children s toys. Most polymers are flammable in their pure form. The flammability resistance of the remaining combustible polymers may be enhanced by additives called flame retardants. Aluminum hydroxide, magnesium hydroxide.