Transcript 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
 They may be natural - e.g. polysaccharides or synthetic
 They possess a variety of properties useful
to food packaging
Examples of polymers

Polyethylene


LDPE
HDPE
Polypropylene
 Polystyrene
 Polyvinyl chloride
(PVC)

Polyethylene
terephthallate (PET)
 Polycarbonate
 Polyamide (Nylon)
 Cellulose



Cellulose acetate
Cellophane
Classification of polymers
 By
properties
Thermoplastic; melts on heating
 Thermosetting; decomposes on heating

 By
Polymerisation process
Addition
 Condensation

Addition polymerisation
 E.g.
polyethylene
Condensation polymerisation
 E.g.
polyester
Teraphthallic acid + ethylene glycol --> PET
Mechanical Properties
 Stress
and Strain;
Stress = Force/Area
 Strain = change in length/original length

F
l
 , 
A
l
Mechanical Properties (2)
Yield strength
 Ultimate tensile strength
 Elongation at break
 Burst Strength
 Tear strength
 Stiffness
 Crease or flex resistance
 Coefficient of friction

Other properties
 Permeability

Gases can permeate through polymers
N 
 p
A x
Gas diffused/Area =
Permeability/thickness x pressure difference
Polymer degradation
 Polymers
can be degraded by
Oxygen
 Heat
 Light
 uv

 These
tend to embrittle and weaken the
polymer.
Polymer processing
 Extrusion
 Blow
Moulding
 Injection Moulding
 Thermoforming
 Polymer foams
Extrusion
 Molten
Polymer is forced through a die by
means of a screw ram.
Injection Moulding
 Molten
polymer is forced into a mould
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
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.
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.
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

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.
The End