SPECIALTY PLASTICS
Polyimide (PI)
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Polyimide (PI)
O
C
N
C
O
Imide group
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General Description
Most important group of highly heat
resistant plastics, often referred to as
"exotics".
–the polyamide imides,
–the polyesters and
–the polyetherimides as well as
–imides with heterocyclic groups
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Manufacture by polycondensation or
polyaddition.
Polycondensates are usable as thermosets
or thermoplastics. However this does not
mean that they are moldable as a melt.
Polyaddition products are useful only as
thermosets.
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The Du Pont polyimides, such as Vespel
(polycondensation polymer), Pyroline
laminates and Kapton films have melting
points which are far above the
decomposition temperature.
Torlon (Amoco) and Polyimid 2080
(Upjohn) are thermoplastic polyimides.
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Some polyimides demand special processing
techniques, so they are available only as semifinished products.
The semi-finished products are manufactured
by the powder sinter technique.
There are also polyimide grades which can be
processed by compression molding but at very
high temperatures and further annealing is
required for best results.
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Polyimide (PI)
Some polyimide grades can be injection
molded.
The melt temperature is 350 °C and
the injection pressures are very high.
The cycle times are long because of
the large temperature difference during
cooling.
Injection moldable polyimides can also be
extruded.
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Polyimide (PI)
Multiple laminates are manufactured in heated presses,
using the vacuum bag method, or in low-pressure
autoclaves.
Filament winding is also common.
This method involves a heat-, cure- and cool-program. For
condensation resins, pauses must be interspersed for
escape of the volatile by-products so as to achieve a
product essentially free of voids.
[The volatile side-products of polycondensation reactions
are aromatic acids, aromatic nitrogen compounds and
solvent vapor.]
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Polyimide (PI)
• The polyimides can also be applied by
spraying.
• Polyimide on aluminum gives glossy,
heat-resistant coatings which are
resistant to solvents and prevent stain
formation.
• There are many adhesives for bonding
polyimide molded articles to each other
or to other materials.
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Polyimide (PI)
General Description
Often the advantages of polyimides cannot be
exploited as only a limited selection of moldings &
semi-finisheds can be manufactured.
Up to now it has not been usual to process
molding compounds or prepregs in plastics
processing plant.
The raw material manufacturers prefer to carry
out the complicated processing themselves and
produce moldings to clients' specifications. Some
polymer manufacturers thus also offer finished
parts commercially
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Polyimide (PI)
Structure and General Properties
As the aromatic or heterocyclic ring
compounds in the chains of the
macromolecule are closely pressed
together, the properties are
- high strength over a wide temperature
range
(- 240 to + 370°C),
- high stiffness and hardness,
- high thermal stability,
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Polyimide (PI)
Structure and General Properties
- favorable friction and abrasion
performance,
- good electrical properties,
- high resistance to high energy radiation,
- highly flame resistant,
- low outgassing under high vacuum,
- good resistance to chemicals and water.
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Polyimide (PI)
Additives
Functional additives and reinforcements are of
importance:
• Molybdenum disulfide (MoS2), graphite and
PTFE for bearing material.
• Glass-, Carbon- and aramide-fibers are used
as reinforcements.
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Polyimide (PI)
Availability
A wide variety of imide-based thermosets
and thermoplastics with functional additives
and reinforcements are commercially
available:
• As resins and compounds in the form of dry
powders or dissolved in solvents
• Compressed and machined moldings
• Prepregs in the B-state, honeycombs, films,
laminates, foams, coating materials, wire
enamel and adhesives.
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Polyimide (PI)
Availability
The polyimides - Vespel moldings and
Kapton films are products of the firm Du
Pont de Nemours (US) and can only be
obtained as finished precision moldings and
films from Du Pont.
There is a limited availability of semifinisheds in the form of rods, tubes,
profiles, sheets, rings and disks.
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Polyimide (PI)
Resistance to Chemicals
• Polyimide is resistant to solvents, fuels,
grease, oils, detergents, dilute acids and
alkalis
• But not resistant to strong acids and alkalis
or oxidants.
• It is not resistant to long-term exposure to
boiling water and steam although the
decrease in strength is reversible.
• It is resistant to stress crack formation
except in alkaline media.
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Polyimide (PI)
Weathering Resistance
• Molded articles lose strength and
elongation at break after long exposure
outdoors and should therefore not be
used. However they resist mold.
Resistance to High Energy Radiation
• Molded items of PI are resistant up to an
absorbed dose of 107 J kg-l, higher than
most other plastics.
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Polyimide (PI)
Water Absorption
Moisture content of polyimide at equilibrium
under normal environmental conditions (23°C
of, 50% RH) is 1.25% - the dimensional changes
caused by alteration of the moisture content
of the surroundings is small
After 1000 hrs - 0.11 % in MD & 0.06% in TD
After 10000 hours - 0.28 in MD & 0.16% in TD
Only very narrow tolerances and extreme
conditions preclude the use of PI.
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Polyimide (PI)
Machining
Machining is performed with standard
carbide-tipped steel metalworking tools.
The materials must remain cool enough to
be touched with the bare hand.
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Polyimide (PI)
Joining
• The preferred method of joining is by
adhesion.
• Polyimide molded items can be bonded to
each other and to other materials.
• Choice of adhesive is made according to
the operating temperature.
• For service temperatures below 150°C
Epoxide resins are suitable. Phenolic resin
adhesives are somewhat more heat
resistant.
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Polyimide (PI)
Applications
In the form of moldings and machined items, PI
are used for piston rings, valve seats, bearings,
seals, welding-torch handle grips, jet engine
accessories, electronic connectors, coil cores,
covers for meters and automobile accessories.
PIs are also used as valve stems in valves. High
thermal stability and heat resistance are the
reasons for application in connection with
soldering and welding.
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Polyimide (PI)
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Polyimide (PI)
Applications
The largest and most important area of application
for PI items up to now-is in the engines, for example
in the Boeing 747 for seals in the different
compressor stages.
More recently because of the favorable friction and
wear characteristics and lubricant-free operation,
polyimide molded articles have been used in office
machinery and computers as bearings, rollers, slide
and guide rails.
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Polyimide (PI)
• Kapton films are successfully used in a wide
range of applications from -269°C to
+400°C. These films are flame resistant,
do not melt, are resistant to solvents and
to high-energy radiation.
• PI films are used for insulating washers,
cable and wire sheathing as well as printed
circuit substrates.
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Polyimide (PI)
• Films coated on one or
both sides with FEP can
be heat sealed and act
as a moisture barrier.
Films coated by
vaporizing aluminium
protect space ships
against the rays of the
sun and the effect of
heat on the power
plant. Space suits are
also insulated with such
films.
•Polyimide films serve as
sound insulation at high
operating temperatures.
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Polyimide (PI)
MANUFACTURERS
Vespel - Solvay Advanced Polymers.
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Polyimide (PI)
Further Reading
1. BRYDSON J.A, Plastics Material, Butterworth
Heinemann, oxford, New Delhi (2005)
2. DOMININGHAUS H., Plastics for Engineering,
Hanser publishers, Munich, New York (1998)
3. CHARLESS A . HARPER, Modern Plastics Hand
Book McGraw –Hill, New York (1999)
4. MARGOLIS J. M., Engineering Thermoplastics,
Marcel-Dekker, New York (1985)
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Table- I Guide values of properties of PI
Properties
Units
PI
PI (Glass Filled)
Density
g/cm3
1.43
1.75
Water absorption (23°C, 24h)
%
25- 50
Mechanical
N/mm2
75-100
Impact strength (Izod)
J/m
3.8-7.6
Rockwell hardness
scale
48-58 E
Tensile strength
Thermal
Service temperature
in air without mechanical loading
short-term
400
400
long-term
260
260
Glass transition temperature
°C
Heat deflection temperature
°C
HDT, ISO
Method A
Coefficient of linear expansion between 0 and 50 °C
360
280-360
°C
mm/mm/°C
18 – 38
0.35 – 0.41
50-63
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Table- I Guide values of properties of PI
Properties
Units
PI
Thermal conductivity
W/mK
0.29-0.35
Volume resistivity
Wcm
1016-1017
Surface resistance
W
1015- 1016
PI (Glass Filled)
Electrical
Dielectric constant
Dissipation factor, tan d
3.41
50 Hz
0.0011
Dielectric strength
kV/mm
22
Tracking resistance
class C
>300
>250
mg
0.32
0.41
Water absorption (24h)
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