Polymers for food packaging
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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