Plastics Essay, Research Paper
INTRODUCTION In today s world, life without plastics is incomprehensible. Whether you are aware of it or not, plastics have become a familiar part of our lives, where these organic polymeric materials play a vital role in the modern society. Plastics versatility allows it to be used in everything from car parts to doll parts, and from soft drink bottles to the fridges that they are stored in. Such variations in its applications contribute to our health, safety and peace of mind. Though the growth of such synthetic plastics has virtually reshaped our environment since World War 2, there is a challenge to retain the benefits that the use of plastics can offer, while at the same time ensuring that they do not cause environmental harm. Despite the popularity and suitability of plastics, it has constantly been subjected to trenchant criticisms. Hence the role and the chemistry of plastics, and its impact on the society and the environment becomes an apparent issue. POLYMERS: THE BASICS: As defined by the Webster s Dictionary, plastic is any of various complex organic compounds produced by polymerisation, capable of being moulded, extruded, cast into various shapes and films, or drawn into filaments used as textile fibers. Plastics have a very unique chemical structure, as they are composed of long-chain molecules, which contain small groups of recurring atoms linked together. Chemically speaking, such giant molecule chains are called polymers, where these long polymer molecules become entangled with each other. The word, polymer, refers to something made of many units. Each link of this long chain is a basic unit or a monomer, encompassed with organic materials, such as the elements; carbon, hydrogen, nitrogen, chlorine and sulphur. In order to form the polymer chain, many monomers are hooked or polymerised together. This process of polymerisation is a chemical reaction usually in the presence of a catalyst, which combines single molecules into long-chain molecules. Though a considerable amount of force is required to disentangle the polymer chains, but when subjected to heat these chains move apart to permit one to slide over the other yet still retain their cohesiveness. Since polymers are very large and very complex molecules, the chemistry of their formation remains very complex. THE SYNTHESIS OF POLYMERS: The basic units of the long chain polymers are called monomers, which join, by two polymerisation processes for producing resins known as addition and condensation reactions. ADDITION POLYMERISATION- The monomers contain at least one double bond between a pair of carbon atoms. During this process, unsaturated or cyclic molecules add together without elimination of a portion of the monomer molecule. This form of polymerisation produces only specific polymer chain lengths, and a repeating unit derived from simple organic molecules (monomers) is apparently common throughout the chain. The average molecular weights for the addition polymers are generally by comparison larger than those of condensation polymers. For example, polymers involving the addition reactions include: Polyethylene (Polythene)- Ethylene, commonly produced by the cracking of ethane gas, forms the basis for the largest single class of plastics, the polyethylenes. Ethylene monomer has the chemical composition; CH2=CH2 (fig.1) As for the repeating unit of polyethylene, it has the chemical structure; (diag) This simple structure can be produced in linear or branched forms such as those illustrated in Figures 1 and 2. Branched versions are known as low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE); the linear versions are known as high-density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE). The overall chemical reaction: catalystnCH2 = CH2 CH2 CH2 CH2 CH2 CONDENSATION POLYMERISATION- The bonds between monomer molecules are formed with the elimination of a small molecule, such as water. This is due to one of the monomers losing a hydrogen atom, and another losing a hydroxyl group. Each monomer usually has two functional groups of condensation polymers, the polyesters, and the polyamides. The condensation reaction: (diag.) Now consider some examples of polyesters; Polyethylene terephthalate (PET)- PET is produced by the step-growth polymerisation of ethylene glycol and terephthalic acid. The presence of the large benzene rings in the repeating units, gives the polymer notable stiffness and strength, especially when the polymer chains are aligned with one another in an orderly arrangement by stretching. In this semicrystalline form, PET is made into a high-strength textile fibre, where its stiffness of makes them highly resistant to deformation, so that they have excellent resistance to wrinkling in fabrics.
Polycarbonate (PC)- PC is a special type of polyester used as an engineering plastic. It has exceptional stiffness, mainly by virtue of having more aromatic rings incorporated into the polyester chain: Polycarbonate is highly transparent, has an impact strength considerably higher than most plastics, and can be injection-moulded, blow-moulded, and extruded. These properties lead to its fabrication into large carboys for water, shatterproof windows, safety shields, and safety helmets. Also it is the favoured plastic for injection moulding into compact discs. THE CHARACTERISTICS OF POLYMERS: The characteristics of polymers enable plastics to be more versatile and economical then other possible sources of materials, where its role in the modern society is of major importance. The following distinct characteristics of polymers highlight the usefulness of plastics: 1 Polymers can be very resistant to chemicals. Since all cleaning fluids with warning labels describe what happens when the chemical comes in contact with skin or eyes are packaged in plastic, this emphasises the chemical resistance of these materials.2 Polymers can be thermal and electrical insulators. All electrical appliances and outlets are covered with polymeric materials. Pot and panhandles found in the kitchen and the foam core of refrigerators highlight thee concept of thermal resistance.3 In general, polymers are very light in weight with varying degrees of strength. Consider the range of applications, from toys to the frame structure of space stations, or from nylon fibre in hose or Kelvar, which is used in bulletproof vests.4 Polymers can be processed in various ways to produce thin fibres or very intricate parts. Plastics can be moulded into bottles or the bodies of a car, or also be mixed with solvents to become paint. Polymers have limitless range of characteristics and colours, where their properties can be easily modified through the means of adding a wide range of additives to broaden their applications. THERMOPLASTICS AND THERMOSETTING: Plastics are primarily defined on the basis of their chemical composition and their engineering behaviour. The fundamental distinctions between such polymers can be categorised into two specific groups: thermoplastics, and thermosets. THERMOPLASTICS- The term, thermoplastics means that once the polymer is formed, it can be heated and reformed over and over again. The molecules are held together by weak secondary bonding forces that causes the plastic to soften when exposed to light and return back to its original shape, when cooled back down to room temperature. The structure associated with thermoplastics is that of individual molecules that are separate from one another and flow past one another. The molecules may have low or extremely high molecular weight, and they may be branched or linear in structure, but the essential feature is that of separability and consequent mobility. It should also be noted that these substances also have little cross-linking. Since extrusion, moulding or pressing when exposed to heat can shape thermoplastics; it therefore offers versatility and a large range of applications. Due to this property of plastics allows for easy processing and facilitates recycling; the majority of polymers are therefore thermoplastics. Examples; Polyethylene- packaging, electrical insulation, milk and water bottles, packaging film, house wrap, agricultural film Polypropylene- carpet fibres, car bumpers, microwave containers Polyvinyl chloride (PVC)- floor and wall coverings, siding, credit cards, car instrument panels THERMOSETTING- A thermoset is a polymer that solidifies irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can t be softened again; the egg can t revert back to its original form. During the initial processing, thermosetting resins undergo a chemical reaction that results in an insoluble network. Essentially, the entire heated, finished article becomes one large molecule. For example, the polymer used in making a fibre-reinforced laminate for a golf club undergoes a cross-linking reaction when it is moulded at a high temperature. Any amount of heat does not soften the material to the point where it can be reworked and indeed may serve only to break it down. Hence, thermosetting substances must have sufficient cross-linking of their chains to prevent molecular motion on heating. Thermosets are therefore valued for their durability and strength and are primarily used in vehicles and construction. Examples; Polyurethanes- mattresses, insulation, toys, Unsaturated Polyesters- varnishes, furniture, boat hulls Epoxies- glues, coating for electrical circuits, helicopter blades THE INHERENT EFFECTS OF PLASTICS: Since the synthetic revolution, where plastics were first created artificially in laboratories during the 1860 s, it is apparent that the modern society is highly dependent on these synthetic materials which is capable of being formed into useful items by heating, milling, moulding or other similar processes. As statistical information has inferred, the global production of plastics now stands at a staggering 110 million tonnes, where this suggests that plastics have a enormous impact on both the society and the environment.
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