2. What is a Plastic?
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A plastic is defined as an organic material that has the ability to
flow into a desired shape when heat and pressure are applied and to
retain that shape when they are withdrawn.
Composed primarily of a binder with-
1. Plasticizers
2. Fillers
3. Pigments
4. Other additives.
3. TYPES OF PLASTIC:
There are 2 types of plastic that is:
a) Thermoplastics
- A polymer that softens and melts gradually
- when heated and it can be reshaped when still
warm.
b) Thermosets
- A polymer that can’t be reshaped after manufacture
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4. Thermoplastics:
ex:polyethylene, polypropylene, polystyrene
& polyvinyl chloride (PVC) most common
thermoplastics.
Any object made from thermoplastic can be
remolded into a new shape.
Can be used for light structural properties.
5. Thermosets:
Most popular thermosets are unsaturated
polyester, epoxides, phenol-formaldehyde
and polyurethane.
Polyester used in manufacture of fiberglass
product and composite materials.
Phenol-formaldehyde used in lavatory seats,
electrical fittings and equipment decorative
laminates.
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6. Chemical type evaluation:
1)Gas permeation: covers both oxygen and co2.
2)Water vapours permeability(g/m2/day):BS3177.
3)Total immersion test : weight change in 7 days.
4)Water absorption.
5)Resistance to fat and oils.
6)Resistance to strong acid/strong alkali.
7)Environmental stress cracking.
7. Permeability of plastic to gases and
organic substance:
All plastics are some degree permeable to
atmospheric gases i.e:nitrogen, oxygen and
co2.
Fick’s first law: states that the amount of gas
passing perpendicular to unit surface against
unit time is propational to the concentration
gradient
Q= -Ddc
dx.
8. Henry law:
Which states that the amount of gas dissolved in
given mass (of plastic )is directly propational to the
partial pressure applied by gas.
C=sp
where; c –conc.
p –partial pressure.
s –solublity constant of the gas in polymer.
From these two law it can be deduced that:
P=DS
Where,p-permeability coefficient.
9. Fabrication and moulding process:
Extrusion:
Using this processing it is possible to produce a wide range of different form
of plastic, such as tubes, sheets and films, structural parts, etc.
1º Step: Plastic pellets are placed in a feed hopper which feeds into the system.
2º Step. A turning screw pushes the plastic into the barrel where heaters
increase the temperature and a melted polymer is obtained.
3º Step. The melted plastic is forced through a shaping die. Depending on the
particular shape of this element, a continuous shape is formed and pulled out of
the extrusion machine.
4º Step. Solidification by cooling .
10. injection moulding:
Plastics pellets flows, due to gravity, from the feed hopper onto a turning
screw. It is converted into a melted plastic by the action of heaters situated
along the barrel. The screw moves the molten plastic forward, forcing the
plastic through a gate into the cooled mould. The mould is opened once the
plastic has solidified and the piece is pushed from the mould by automatic
ejector pins. After we get the manufactured piece, the mould is closed and
clamped and the process begins again.
11. Compression molding:
This is a similar process to extrusion. In compression moulding,
plastics pellets, sometimes called moulding powder, are placed
in the feed hopper and pushed to the gate by the action of the
turning screw. It is heated and compressed while it passes
through the barrel. After the gate, the molten charge is quickly
transferred to a press where it is moulded while still hot. The
part is removed after sufficient cooling.
12. Sterilization of Plastics:
Objective: sterilization is to prevent the introduction into the
body of pathogenic organisms not normally present.
Introduction:
Sterilization can be defined as ‘the removal or destruction of all
living organisms, including resistant forms such as bacterial or
fungal spores’.
Disinfection is a lower grade of sterilization and involves only the
destruction of pathogenic organisms in the vegetative (or non-
sporing) state; it does not involve the destruction of spores.
Sterilization is the only acceptable standard for surgical purposes
although disinfection may well be suitable for other purposes.
13. Sterilization Methods:
Sterilization can be achieved through a variety of methods and these will be
considered individually with particular emphasis on the applicability of the
method to the sterilization of plastics devices.
No matter which sterilization method is used, the objective is to reduce the
bioburden (the number of microorganisms present) to a safe level.
[Production in a ‘clean room’ (of any standard) does not make a device
sterile; it simply reduces the initial bioburden and concentration of foreign
particles to make sterilization more effective.]
14. Dry Heat:
Dry heat is not generally regarded as being suitable for plastics
due to the low thermal transmission properties of plastics and the
difficulty of insuring that all parts of the product have been exposed to
the required temperature for an adequate time .
Gamma Rays:
Gamma rays are produced from a Co60 source and have a high
penetrating power (up to 50 cm).
This allows a high packing density in the sterilization chamber.
This can also mean that products at the outer edges of the packing
can be subjected to much higher radiation doses than those at the
center of the pack.
Materials to be gamma sterilized need a margin of error in their
resistance to radiation to insure that there is no excessive
degradation if items are at the outer edges.
15. Gaseous Chemicals (EtO)
Ethylene oxide is a powerful alkylating agent and is regarded
by the EPA as a toxic and possibly carcinogenic gas
(exposure to EtO is regulated by the EPA and OSHA).
When mixed with air, EtO is not only flammable but can also
be explosive.
The effectiveness of EtO sterilization depends on many
variables such as time, gas concentration, temperature and
relative humidity (necessary to moisten bacteria to insure
effective destruction).
16. EtO sterilization requires evacuation of the sterilization
chamber, the introduction of moisture, the introduction of the
EtO gas (either in the pure state or as a 10 to 15% mixture with
an inert gas), and keeping the internal pressure of the chamber
lower than one atmosphere3 to prevent leakage of the EtO to the
atmosphere.
After the specified exposure time, the EtO is purged and the
chamber is flooded with filtered sterile air to remove any
residual EtO.
This complex process and subsequent monitoring takes longer
than radiation sterilization but recent technology advances have
greatly reduced the cycle time for EtO sterilization.
17. In Summary
• Plastics
– Lighter …..…………………………….
– Cheaper …………...
– More Durable ………………………
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