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Polymers for food packaging

Polymers for food packaging. An overview. Polymers. What are polymers? Types of polymer Polymerisation Polymer properties. What are polymers. Polymers are macromolecules They are based on a repeating unit derived from a small molecule

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Polymers for food packaging

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  1. Polymers for food packaging An overview

  2. Polymers • What are polymers? • Types of polymer • Polymerisation • Polymer properties

  3. What are polymers • Polymers are macromolecules • They are based on a repeating unit derived from a small molecule • They may be natural - e.g. polysaccharides - or synthetic • They possess a variety of properties useful to food packaging

  4. Polyethylene LDPE HDPE Polypropylene Polystyrene Polyvinyl chloride (PVC) Polyethylene terephthallate (PET) Polycarbonate Polyamide (Nylon) Cellulose Cellulose acetate Cellophane Examples of polymers

  5. Classification of polymers • By properties • Thermoplastic; melts on heating • Thermosetting; decomposes on heating • By Polymerisation process • Addition • Condensation

  6. Addition polymerisation • E.g. polyethylene

  7. Condensation polymerisation • E.g. polyester Teraphthallic acid + ethylene glycol --> PET

  8. Mechanical Properties • Stress and Strain; • Stress = Force/Area • Strain = change in length/original length

  9. Mechanical Properties (2) • Yield strength • Ultimate tensile strength • Elongation at break • Burst Strength • Tear strength • Stiffness • Crease or flex resistance • Coefficient of friction

  10. Other properties • Permeability • Gases can permeate through polymers Gas diffused/Area = Permeability/thickness x pressure difference

  11. Polymer degradation • Polymers can be degraded by • Oxygen • Heat • Light • uv • These tend to embrittle and weaken the polymer.

  12. Polymer processing • Extrusion • Blow Moulding • Injection Moulding • Thermoforming • Polymer foams

  13. Extrusion • Molten Polymer is forced through a die by means of a screw ram.

  14. Injection Moulding • Molten polymer is forced into a mould

  15. Vacuum forming • Polymer sheet is heated using ir heaters • A vacuum is applied below the sheet, drawing the sheet into the mould • Related processes are pressure forming and mould forming

  16. Other processes • Film extrusion • polymer is extruded into a thin, cylindrical film which is wound up as film • Blow moulding • extruded polymer is fed into a mould where it is blown into shape. This is used to make bottles.

  17. Polymer foams • Polymer is injection moulded with the addition of foaming agents so that a foam is formed in the mould. • Typically used for soft drink cups, using polystyrene where the thermal insulation property is useful.

  18. Other polymer products • These include various film products • Laminated films where a number of polymers or polymer and paper products are layered together • Shrink wrap • Cling film

  19. In conclusion • An understanding of the relation between the chemical structure and the physical properties enables the creation of “designer” polymers. • As a result the range of polymer products is very large and growing rapidly • This session is therefore only a very brief look at polymers.

  20. The End

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