Plastics Essay Research Paper INTRODUCTION

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 pl