PLASTICS AND TEXTILES

Posted on Dec 31, 2020 in Other subjects

1. PLASTIC MATERIALS In general, plastic materials are a normal part of everyday life. They also play important roles in sectors such packaging’, telecommunications, transport, construction, medicine, agriculture and information technology. Plastics consist of long chains of molecules, mostly made of the element carbon. 1.1 THE ORIGIN OF PLASTICS Plastics can be divided into two types, according to their origin:  Natural plastics are obtained directly from vegetable or animal sources. For example, latex and cellulose are produced by plants, and casein is a protein that is found in milk.  Synthetic plastics are made with compounds that come from petroleum, natural gas and carbon. Most plastics that we use today are synthetic. 1.2 THE TRANSFORMATION OF PLASTICS The industrial production of plastic from raw materials is called polymerisation. There are two basic steps in this process:  During the manufacturing process, fillers may be added to reduce the cost of production and enhance certain properties of the raw materials. Some typical fillers include fibreglass, paper, silica, mineral powder and sawdust.  Chemical additives can also be used, such as plasticizers which increase the flexibility and resistance of the final product. Pigments may also be added to give the plastic a certain colour. 1.3 PROPERTIES OF PLASTICS Depending on their composition, certain plastics may be more rigid, more flexible or more elastic. In general, plastics are:  physically resistant to stretching, bending, twisting and compression.  good electrical, thermal and acoustic insulators.  ductile because they can be pulled to make threads.  malleable because they can be pressed into thin sheets.  light with a typical density of 0.9 to 1.3 grams per cubic centimetre.  impermeable (to gas and liquids).  recyclable in many ways, such as:  chemical recycling, which breaks plastic down into its chemical components.  mechanical recycling, which breaks plastic into small pieces that can be melted and made into tiny pellets.  energy recovery, which burns plastic to produce heat, electricity or energy for other industrial processes. Non-biodegradable plastic is made from petroleum products. Most of the plastics that we use today are non-biodegradable. Biodegradable plastic is becoming more common. some biodegradable plastics are broken down by bacteria and other biological agents. One example is biopol, which is used to make bottles and mouldings. lt can be broken down by microorganisms in soil. 2. THE CLASSIFICATION OF PLASTICS 2.1 CLASSIFICATION BY INTERNAL STRUCTURE  Thermoplastics are composed of polymer chains that are weaklyconnected to each other. When thermoplastics are heated, they soften and can be moulded into different shapes. The materials keep this new shape after they cool down. This heating and cooling process can be repeated as many times as needed.  Thermosetting plastics are composed of polymer chains that are strongly-connected. Thermosetting plastics can be heated and moulded, and they maintain that shape after they cool down. However, these materials can only be heated and shaped once. The process cannot be repeated.  Elastomers are composed of polymer chains that are laterally connected. They can be folded or rolled into a ball, like string. When we apply force to elastomers, they stretch because the polymer chains are very elastic. The production of elastomers includes a chemical process called vulcanisation, in which sulphur is added to rubber at 1 60’C. This gives elastomers more strength, resistance and durabiIity. However, it doesn’t change their natural elasticity. 2.2 INDUSTRIAL PLASTICS Many plastics are used for industrial purposes. These are classified into groups.3. PLASTIC FORMING TECHNIQUES Plastic products are typically made from powder, pellets or resins (thick and viscous liquids). Various techniques can be used to form plastic products, but the most important ones are extrusion, calendering, vacuum forming and moulding. 3.1 EXTRUSION The extrusion process typically includes the following steps: 1. Thermoplastic material, usually in the form of pellets, is added to an extrusion machine. These go into a heated cylinder where they melt and form a thick liquid. 2. Inside the cylinder, there is a screw that turns and pushes the plastic out through a die. The shape of the die gives the extruded plastic a particular form. 3. When the plastic leaves the machine, it is hardened in a bath of cool water. Uses: Tubes, water pipes, insulation for cables, plastic film and packaging. 3.2 CALENDERING In the calendering process, thermoplastic material is passed between rollers to make thin sheets. The last set of rollers ensures the plastic has the proper thickness. These rollers may also give the surface a shiny or matte finish. Uses: Kitchen countertops and cupboards. 3.3 VACUUM FORMING The vacuum forming process involves the following steps: 1. A sheet of thermoplastic material is placed over a mould. 2. The sheet of plastic is heated until it becomes soft. 3. The air under the sheet is sucked out to create a vacuum. This vacuum pulls the sheet onto the mould, which gives it the desired shape. 4. The sheet is cooled and removed from the mould. Uses: lce trays, plastic bathtubs, car dashboards, shop signs. 3.4 MOULDING There are various methods for using moulds to make prastic objects. Blow moulding 1. A tube of thermoplastic material is produced by an extrusion machine. Then this tube is introduced into a hollow mould. 2. The mould is filled with compressed ail which pushes the plastic against the sides of the mould. This gives the plastic the desired shape. 3. When the plastic cools, it is removed from the mould. Uses: Hollow objects, such as bottles for mineral water or cooking oil; some hollow toys, such as balls. lnjection moulding 1. Melted thermoplastic material is introduced into a mould. 2. After the material cools down and hardens. it is removed. Uses: Household containers, buckets, toys and certain parts for cars, planes and spacecraft. Compression moulding Compression moulding is done with a machine called a press. The main steps of compression moulding are as follows: 1. Thermosetting plastic materials, such as pellets or powder; are introduced into a cavity mould. 2. Heating elements soften the plastic while it is compressed by the other side. 3. The plastic is shaped by the moulds on both sides. 4. After the moulds cool. the plastic object is removed. Uses: Containers and outer casings for home appliances and other machines 4. TEXTILES 4.1 Natural fibres Natural fibres may come from animal, plant or mineral sources. In most cases, we clean, brush, stretch and dye the fibres, before spinning them into thread. Then we can weave the threads together to make cloth. 4.2 Synthetic fibres Synthetic fibres, such as nylon, polyester, rayon and Lycra, are plastic materials. They are long-lasting, resistant and impermeable. Nowadays, a mixture of natural and synthetic fibres is often used in the production of textiles.