Monday 8 June 2009

Chemistry – Materials of the modern world

(Crude oil and plastic) (table unavilable here) There’re various kinds of materials around as, such as paper and wood. All products are made by raw materials (extract from nature), processed into product, and their fate at last is becoming wastes. Materials can be further classify into natural material (such as wood and silk) and man-made material (such as plastic and alloy), Different material have their own properties and purpose. Crude oil The raw material of plastics mostly come form crude oil (petroleum). They come from Earth’s crust and sometimes trapped in the rock under the sea bed. It’s dark, thick, smelly and viscous. It formed from the remains of dead sea plants and animals buried under thick layers of rock millions of years ago. They slowly changed to crude oil under high pressure, heat and the effect of bacteria. They distributed in many places in the world. We use oil rig to get the crude oil if it’s found under sea bed. The oil is then carried by pipelines or vehicles to the oil refinery. Crude oil is a mixture of hydrocarbons with only contains carbon and hydrogen; common hydrocarbons include methane (CH4), ethane (C2H6), butane (C4H10) and hexane (C6H14) , which methane is the simplest hydrocarbons molecule. Generally larger hydrocarbons have higher boiling and melting point. Fractional distillation is used to separate different hydrocarbons with different boiling point, and separate them into different fractions. In the fractionating tower, heated crude oil is in gas form until the temperature is low enough for it to condense, them it’s collected. In fact, fractions are still mixtures of hydrocarbons, but they have similar size and properties. Those fraction maybe be further transported to other factory can process into useful product. Naphtha is useful as a maw material for plastics, detergents, paints and drugs. It’s so useful so that it’s also called “black gold”. Plastics Plastics and made from chemicals derived from naphtha. Naphtha can be broken into small molecules (e.g. ethane C2H4). Through breaking down the double bond between carbons, they can from a macromolecule with the help of heat pressure or chemicals. The processes of plastic production include mixing raw materials, applying heat and pressure, adding other chemicals, moulding into shapes and colouring. Epoxy resin comes from the chemical reaction between the epoxy resin (large molecule) and the hardener (small molecule). Those hardener joins the epoxy resin together and forms a macromolecule and the macromolecule makes up the strong and rigid epoxy resin glue. Polystyrene comes from the styrene, with help of some catalysts. During the reaction it may release heat. The properties of plastics include easily moulded into different shapes, easily to be coloured, elastic, resistant to corrosion by acids and alkalis, insulators of heat and electricity, and it’s light. Although there’re many advantage of using plastics, the overuse of plastics will waste natural resources, and produces lots of wastes aw well because they do not decompose naturally. Those plastic wastes may spoil the scenery, block the drains and lead to the accumulation of foul waters, causing bad smells and provide a breeding place fir mosquitoes and pests. Birds and fish caught by plastic wastes in water won’t be able to find food and may die due to suffocation if they swallow the wastes. The two ways to treat with those wastes is to put them into landfill sites or by incineration. However, burning plastics gives out lots of toxic substances and carcinogens, so that most plastic is put into landfill sites in Ta Kwu Ling, Tuen Mun and Tseung Kwan O. But still, plastic will not be decomposed and usually are bulky, once they’re buried; they stay nearly forever and take up lot of space. It’s expected that all landfill sites will be used up in 2020. There’re several possible solutions to the environmental problem of the overuse of plastics: 1) The 4Rs – Recycle, reuse, reduce and replace. When plastic is put into recycling bin and is collected, it’ll be sorted into different kind of plastic referring to the plastic code, and transport to the corresponding recycling plant. After removing the dirt and impurities, it’s grinded into powders, melted and moulded into new products. The above process is the recycling period of plastics. However, recycling industry still faces lots of problem like the high cost of land and man power, as well as the difficulties to sort different plastic. Thus some plastic is transported to mainland and other countries to finish the recycling process. 2) Degradable plastics – Some plastics can be decomposed under sunlight (photodegradable plastic) and by micro-organisms (biodegradable). Note that photodegradable plastics will become debris under sunlight and can be decomposed by micro-organisms. It’ll finally becomes CO2 and water.

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