Polymers for food packaging

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Polymers for food packaging
An overview
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Polymers

• What are polymers?
• Types of polymer
• Polymerisation
• Polymer properties

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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

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Examples of polymers

• Polyethylene
• LDPE
• HDPE
• Polypropylene
• Polystyrene
• Polyvinyl chloride (PVC)

• Polyethylene terephthallate (PET)
• Polycarbonate
• Polyamide (Nylon)
• Cellulose
• Cellulose acetate
• Cellophane

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Classification of polymers

• By properties
• Thermoplastic melts on heating
• Thermosetting decomposes on heating
• By Polymerisation process
• Addition
• Condensation

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Addition polymerisation

• E.g. polyethylene

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Condensation polymerisation

• E.g. polyester

Teraphthallic acid ethylene glycol --gt PET
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Mechanical Properties

• Stress and Strain
• Stress Force/Area
• Strain change in length/original length

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Mechanical Properties (2)

• Yield strength
• Ultimate tensile strength
• Elongation at break
• Burst Strength
• Tear strength
• Stiffness
• Crease or flex resistance
• Coefficient of friction

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Other properties

• Permeability
• Gases can permeate through polymers

Gas diffused/Area Permeability/thickness x
pressure difference
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Polymer degradation

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

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Polymer processing

• Extrusion
• Blow Moulding
• Injection Moulding
• Thermoforming
• Polymer foams

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Extrusion

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

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Injection Moulding

• Molten polymer is forced into a mould

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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

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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.

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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.

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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

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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.

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The End