DT Plastics

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Plastics

  • POLYMERS such as rubbers and plastics are organic structures.
  • They can be natural, modified or synthetic.
  • Synthetic polymers are produced from coal, gas and oil.
  • Most plastics have good chemical and electrical resistance.
  • They are light in weight, durable and have good strength to weight ratios.

PLASTICS ARE DIVIDED INTO TWO GROUPS

THERMOSETS

THERMOPLASTICS                                                                    (page 20&21 of AQA text book is a table of plastics)

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Thermosets

  • ONCE CURED, THEY CAN NOT BE SOFTENED. HAVE A HARDER SURFACE THAN THERMOPLASTICS SO ONLY A FEW ARE USED FOR CONSTRUCTION PURPOSES IN SCHOOLS.
  • Made up of molecules that are heavily cross-linked.
  • This results in a rigid molecular structure.
  • Although they soften the first time when heated, they then become permanantly stiff and CAN NOT BE RESHAPED.
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Thermoplastics

  • CAN BE SOFTENED BY HEATING AND HARDENED BY COOLING.
  • Each time a thermoplastic is reheated it will return to its original flat state.
  • Unless it has been damadged by over heating or over stretching.
  • They are made up of lines of long chain molecules.
  • Have very few cross linkages.
  • This allows them to soften when heated so they can bend in different shapes and become stiff again when cooled.
  • This process can be repeated many times.
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Example Thermoplastics

ACRYLIC:  (also known as perspex). Most common plastic in school workshops. Used for vacuum forming. Residstant to most acids and weather conditions.

POLYTHENE: Can be moulded into almost any form. Used for making bottles, bowls, toys etc. Available in large sheets. HIGH DENSITY- rigid and hard, used for machine parts . LOW DENSITY - tough and flexible, used for bottles.

POLYVINIL CHLORIDE: Better known as PVC. Tough material, can be purchased as a hard or flexible material. Can be welded or bonded with a glue. Used for water pipes, rain coats etc.

POLYTHENE: The most common plastic in everyday life. Used for plastic bottles, shopping bags, shampoo bottles and childrens toys.

POLYCARBONATE: Machines well and can be solvent bonded or welded. Tough and resistant to damadge, ideal for mobile phones. Used as an insulator, comes in a range of colours.

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More useful plastics

STYROFOAM: Lightweight dense foam. Easy to cut and shape with saws and files. Is ideal for model making.

EXPANDED POLYSTYRENE: Efficent insulator used in cavity walls and decorative tiles. Can also be used for model making.

NYLON : Used in engineering to make gears & bearings. It's oily nature means thst friction between two moving nylon parts is reduced.

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What forms are plastics available in?

  • Powders
  • Granules
  • Liquid
  • Foams
  • Rods
  • Tubes
  • Slabs
  • Sheets
  • Films
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Sectional drwing of a hot glue gun

(http://www.technologystudent.com/images5/htglue1.gif)

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