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151
uses because of their good performance at high temperature. Unfortunately,
C
C
O
O
O
C
C
O
O
O
+ H
3
N
X
NH
3
X
NH
C
HOOC
COOH
C
NH
X
NH
C
HOOC
COOH
C
O
NH
O
(I)
C
C
N
O
O
X
C
C
N
X
O
O
C
C
N
O
O
C
C
N
O
O
(II)
by their very nature, these polymers cannot be moulded by conventional
thermoplastics techniques and this led in the early 1970s to the availability
of modified polyimides such as the polyamide imides typified by Torlon
(Amoco Chemicals), the polyester imides (e.g. Icdal Ti40 by Dynamit
Nobel) and the polybismaleinimides such as Kinel (Rhone-Poulenc).
By the mid-1970s there were over 20 suppliers in the United States
and Western Europe alone although some companies have now withdrawn
from the market.
The general method of preparation for the original polyimides is
shown in Figure 42.
Figure 42
oxidation of durene (1,2,4,5-tetramethylbenzene), using a supported
vanadium oxide catalyst. A number of amines have been investigated and it
has been found that certain aromatic amines give polymers with a high
degree of oxidative and thermal stability. Such amines include m-
phenylenediamine, benzidine and di-(4-amino-phenyl) ether, the last of
these being employed in the manufacture of Kapton (Du Pont). The
structure of this material is shown in Figure 43.
The pyromellitic dianhydride is itself obtained by vapour phase
152
N
O
O
N
O
O
O
Figure 43
For convenience of application it is usual to utilize the two-stage
preparation shown above. Initially the soluble polymer (I) is formed which
is then converted into the insoluble thermally stable polyimide (II) (Figure
42). Suitable solvents for the high molecular weight prepolymer (I) include
dimethylformamide and dimethylacetamide.
In addition to the intramolecular condensation leading to the linear
polymer some intermolecular reaction may also occur which leads to crosslinking and hence greatly restricts mouldability.
In order to prevent premature gelation the reaction mixture should
be anhydrous, free from pyromellitic acid and reacted at temperatures not
exceeding 50°C.
Films may be made by casting (I) and heating to produce the
polyimide (II). Tough thin film may be obtained by heating for 1-2 hours at
150°C but thicker products tend to become brittle. A substantial
improvement can be obtained in some cases if a further baking of solventfree polymer is carried out at 300°C for a few minutes.
The first commercial applications of polypyromellitimides were as
wire enamels, as insulating varnishes and for coating glass-cloth (Pyre.ML ,
Du Pont). In film form (Kapton) many of the outstanding properties of the
polymer may be more fully utilized. These include excellent electrical
properties, solvent resistance, flame resistance, outstanding abrasion
resistance and exceptional heat resistance. After 1000 hours exposure to air
at 300°C the polymer retained 90% of its tensile strength.
The polymers also have excellent resistance to oxidative
degradation, most chemicals other than strong bases and high-energy
radiation. Exposure for 1500 hours to a radiation of about 10 rads at 175°C
led to embrittlement but the sample retained form stability.
Some typical properties of a fabricated solid grade (Vespel-Du
Pont) are given in Table 13 together with some data on a graphite-loaded
variety and a commercial polyamide-imide (Torlon 2000–Amoco).
153
Table 13 – Typical properties of fabricated unfilled and 15% graphite-
Property
Vespel,
Vespel, 15%
Torlon 2000,
Specific gravity
Water absorption (24 h), %
1.42
0.32
1.49
-
1.41
0.28
loaded polyimide polymers
Tensile strength
25°C, MPa
150°C, MPa
315°C, MPa
Elongation at break, %
Flexural modulus
23°C, MPa
260-300°C, MPa
Deflection temperature under
load (heat distortion
temperature), °C
conventional plastics form very difficult. Nevertheless the materials have
been used in the manufacture of seals, gaskets and piston rings (Vespel-Du
Pont) and also as the binder resin for diamond grinding wheels.
polyimide resins followed by subsequent pressing have found important
uses in the aircraft industry, particularly in connection with supersonic
airliners. Such laminates can be used continuously at temperatures up to
250°C and intermittently to 400°C.
in jet engines, for example in compressor seals. They are also being used in
data processing equipment for such purposes as pressure discs, sleeves,
bearings, sliding and guide rods and as friction elements. They are also
used as valve shafts in shut-off valves whilst their good heat stability and
deformation resistance leads to use in soldering and welding equipment.
One disadvantage of these materials is their limited resistance to hydrolysis
and they may crack in water or steam at temperatures above 100°C. For
this reason they have met recent competition from the
polyetheretherketones (PEEK), which are not only superior in this respect
but are also easier to mould and extrude.
unfilled
graphite
unfilled
filled
90
67
35
6-8
450000
3100
357
63
42
27
5
627000
4300
-
93
-
-
2.5
710000
4900
282
The limited tractability of the polymer makes processing in
Laminates produced by impregnation of glass and carbon fibre with
At the present time the principal applications of the polyimides are
154
1.5.4 Modified Polyimides
HOOC
O
O
O
+ H2N
R
NH
2
OC
N
O
O
R
NH
2H2O
O
O
O
HOOC
+
OCN
R
NCO
N
O
O
OC
R
NH
+ 2CO
2
+
O
O
O
HOOC
R,OOC
NH
R
NH
COOR
,
NH
R
N
O
O
OC
+ 2CO2 + 2R,OH
The successful introduction of the polyimides stimulated attempts
to produce somewhat more tractable materials without too serious a loss of
heat resistance. This led to the availability of a polyamide-imides,
polyester-imides and the polybismaleinimides, and in 1982 the polyetherimides.
If trimellitic anhydride is used instead of pyromellitic dianhydride
in the reaction illustrated in Figure 18.35 then a polya mi d e -imide is formed
(Figure 43). The Torlon materials produced by Amoco Chemicals are of
this type.
Figure 43
Both the polyimide and polyamide-imide reactions described above
require starting materials of high purity and the use of capped amines (in
fact diisocyanates or diurethanes) has been suggested (Figure 44). It is
understood that one of these reactions has been used by Rhone-Poulenc to
produce their Kermel fibres. Closely related is the Upjohn process
involving the self-condensation of the isocyanate of trimellitic acid,
although in this case the product is a true polyimide rather than a
polyamide-imide (Figure 45). Whereas the polyimides are modified
polyimides described above are produced by condensation reactions the
polybismaleinimides may be produced by rearrangement polymerisation.
This avoids the production of volatile low molecular mass by-products.
Figure 44
155
OCN
O
O
O
2
+
H
2
N
R
NH
2
HC
HC
CO
NH
R
NH CO
CH
CH
HOOC
COOH
COOH
HOOC
CH
CH
CONH
R
NH
CO
HC
HC
-H2O
N
O
O
R
N
O
O
N
O
O
R
N
O
O
N
O
O
R
N
O
O
N
O
O
R
N
O
O
+ H2N
R
,
NH
2
N
NH
O
O
R
N
O
O
NH
R
,
+
HS
R
,
SH
R
,
N
O
O
S
R
N
S
O
O
+
HCN CH
R
,
CH NOH
CO
N
O
O
NH
R
N
O
O
R
,
CO
NH
O
-CO
2
O
O
N
O
N
O
O
O
Figure 45
The key starting materials in this case are the bismaleimides, which
are synthesised by the reaction of maleic anhydride with diamines:
A variety of bifunctional compounds react with the bismaleimides
to form polymers by rearrangement reactions. These include amines,
sulphides and aldoximes:
of amine the polymer will have terminal double bonds which allow a cure
site to give a thermosetting polymer via a double bond polymerization
mechanism. This approach was developed by Ciba-Geigy with their
product P13N:
If the bismaleimide-amine reaction is carried out with a deficiency
156
N
O
O
CH
2
N
O
O
O
N
O
O
O
H
2
C
N
O
O
The polybismaleinimides, typified by the Rhone-Poulenc material
N
O
O
O
O
R
,
O
O
N
O
O
R
,
Kinel, may be processed like conventional thermosetting plastics. The
original polymers have double bonds at the ends of the chains and
polymerization occurs through them during the moulding process to bring
about cross-linking, in this case without the formation of any volatile byproducts. The properties of the cured polymers are broadly similar to those
of the polyimides and polyamide-imides. Moulding temperatures vary from
type to type but are usually in the range 200-260°C followed by post-curing
for about 8 h at 250°C.
Unfilled polybismaleinimides are used for making laminates,
impregnating glass and carbon fibre fabrics, for making printed circuit
boards and for filament winding. Grades are also available filled with a
diversity of materials such as glass fibre, asbestos, carbon fibre,
molybdenum sulphide, graphite and PTFE. They find use in aircraft and
spacecraft construction, and in rocket and weapons technology. Specific
uses include brake equipment, rings, gear wheels, friction bearings and cam
discs.
The polyester-imides form yet another class of modified polyimide.
These are typified by the structure shown in Figure 46.
Figure 46
Polyimides and related materials have also been used in a number
of specialist applications. Polyimide foams (Skybond by Monsanto) have
been used for the sound deadening of jet engines. Polyimide fibres have
been produced by Rhone-Poulenc (Kermel) and by Upjohn.
Polyamide-imides
The polyamide-imide Torlon was marketed in the early 1970s as a
compression moulding material and from the mid-1970s an injection
moulding grade has been available. In solution form in N-methyl-
157
pyrrolidone it has been used as a wire enamel, as a decorative finish for
N
O
O
O
C O
CH
3
CH
3
N
O
O
n
kitchen equipment and implements and as an adhesive and laminating resin
in spacecraft. The compression moulding grade, Torlon 2000, can accept
high proportions of filler without serious detriment to many properties.
Polymers of this type have exceptional good values of strength,
stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a
tensile load of 70 MPa the creep modulus drops only from 4200 to 3000
MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500
and 2500 MPa respectively. If the test temperature is raised to 150°C the
creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa
in 100 h.
Three months immersion in water leads to a 5% w/w absorption of
water which at this level leads to a reduction in the heat distortion
temperature (ISO) of 100°C.
Torlon-type polymers are unaffected by aliphatic, aromatic,
chlorinated and fluorinated hydrocarbons, dilute acids, aldehydes, ketones,
ethers and esters. Resistance to alkalis is poor. They have excellent
resistance to radiation.
Uses of the pol yamide-imides include pumps, valves, gear wheels,
accessories for refrigeration plant and electronic components. Interesting
materials may be made by blending the polymer with graphite and PTFE.
This reduces the coefficient of friction from the already low figure of 0.2
(to steel) to as little as 0.02-0.08.
Polyamide-imides may also be produced by reacting a diacid
chloride with an excess of diamine to produce a low molecular mass
polyamide with amine end groups. This may then be chain extended by
reaction with pyromellitic dianhydride to produce imide linkages.
Alternatively the dianhydride, diamine and diacid chloride may be reacted
all together.
Polyetherimides
In 1982 General Electric introduced Ultem, a polyetherimide with
the following structure:
158
The presence of the either linkages is sufficient to allow the
material to be melt processed, whilst the polymer retains many of the
desirable characteristics of polyimides. As a consequence the material has
gained rapid acceptance as a high-temperature engineering thermoplastics
material competitive with the polysulphones, poly(phenylene sulphides)
and polyketones. They exhibit the following key characteristics:
(1) Very high tensile strength without the use of reinforcement.
(2) A glass transition temperature of 215°C, a deflection
temperature of 200°C and a Vicat softening point of 219°C.
(3) A high UL Temperature Index of 170°C (for mechanical with
impact).
(4) Fla me resistance (LOI of 47 and UL94 V-0 rating at 0.41 mm
thickness).
(5) Very low smoke emission, superior even to polyethersulphone.
(6) Excellent hydrolytic stability (a weakness of many polyimides).
Although the polymer has a regular structure, it is amorphous, the
natural polymer being transparent and orange in colour.
The polyetherimides are competitive not only with other highperformance polymers such as the polysulphones and polyketones but also
with polyphenylene sulphides, polyarylates, polyamide-imides and the
polycarbonates.
Because of its high stability, the processing “window” (range of
processing conditions) is wider than for many other thermoplastics. The
main points to bear in mind are:
(1) The need to use dry granules.
(2) The need to use high melt temperatures (340-425°C).
(3) The low moulding shrinkage of 0.005-0.007 cm/cm
(typical of an amorphous material).
(4) The high melt strength, facilitating thermoforming and blow
moulding techniques.
The markets for polyetherimides arise to an extent from stricter
regulations concerning flammability and smoke evolution coupled with
such features as high strength, toughness and heat resistance. Application
areas include car under-the-bonnet uses, microwave equipment, printed
circuit boards and aerospace (including carbon-fibre-reinforced laminated
materials). The polymer is also of interest in flim, fibre and wire insulation
form.
General Electric now also offer polyetherimide-polycarbonate
blends. Although these materials are not transparent and have a lower
159
specification than the basic polyetherimide, they are less expensive and
find use in microwave oven trays and automotive reflectors.
1.5.5 Elastomeric Polyamides
Although some of the polyamides are somewhat rubbery, they have
never achieved importance as rubbers. On the other hand, the past decade
and a half has seen interest aroused in thermoplastic elastomers of the
polyamide type which may be considered as polyamide analogues of the
somewhat older and more fully established thermoplastic polyester rubbers.
Most of the commercial polymers consist of polyether blocks
separated by polyamide blocks. The polyether blocks may be based on
polyethylene glycol, polypropylene glycol or, more commonly,
polytetramethylene ether glycol. The polyamides are usually based on
nylon 11 but may be based on nylons 6 or 66 even a copolymer, e.g. 6/11.
A wide range of polyether-polyamide block copolymers were first
offered by Atochem in 1981 under the trade name Pebax. These are made
by first producing a low molecular weight polyamide using an excess of
dicarboxylic acid at a temperature above 230°C and under a pressure of up
to 25 bar. This is then combined with a polyether by reaction at 230-280°C
under vacuum in the presence of a suitable catalyst such as Ti(OR)4.
Products varying widely in their properties can be produced by
variation of:
(1) The nature of the polyamide block.
(2) The nature of the polyether block.
(3) The lengths of the two blocks.
(4) The relative amounts of the two blocks present.
Variation in the polyamide block nature and length is a prime
influence causing variations in Tm, specific gravity and chemical
resistance.
Variation in the polyether block is the prime influence causing
variations in Tg, hydrophilic properties and antistatic properties.
Further variation in properties is obtained by incorporating such
additives as antistatic agents, ultraviolet stabilizers and antioxidants.
As a result of this flexibility in formulation, the range of physical
properties possible is somewhat greater than normally achieved with
thermoplastic polyesters or thermoplastic polyurethane rubbers. For
example, hardness can range from Shore A60 (a fairly soft rubber) to Shore
D63, which is commonly rated as a moderately hard plastics material.
Typical properties of five basic materials in the Pebax range and one
160
hydrophilic grade material (Grade 4011) are given in Table 14 in order to
Grade №
6333
5533
4033
3533
2533
4011
Specific gravity
1.01
1.01
1.01
1.01
1.01
1.10
illustrate the range of properties av ailable.
Table 14 – Selected properties of polyether-polyamide block copolymers
of the Pebax type (After Deleens, 1987)
Properties
Hardness (Shore D)
Moisture absorption
(24 h in water), %
T
, °C
m
Tensile strength,
MPa
Elongation at break,
%
down to about -40°C and only Grade 6333 breaks in an Izod test at this
temperature (using specimens of thickness 3.2 mm). The materials
generally show excellent resistance to crack growth from a notch during
flexure; some grades are reported to have withstood 36x106 cycles in a de
Mattia flexing test. Softer grades are generally more transparent than hard
ones as a result of the lower amount of crystalline polyamide block
material.
standard equipment used for thermoplastics. Typical melt temperatures
range from about 230°C for the harder grades down to about 200°C for the
softer polymers. Mould temperatures are about 25-30°C.
conveyor and drive belts, ski and soccer shoe soles, computer keyboard
pads, silent gears in audio and video recorders and cameras, and thin film
for medical applications.
have been Dow (following on work by Upjohn) and Akzo, whose initial
development grades have been trade marked Arnetal.
sports footwear, loudspeaker gaskets and, in the case of filled grades, watch
straps.
63
1.2
173
51
380
55
1.2
168
44
455
40
1.2
168
36
485
35
1.2
152
34
710
25
1.2
148
29
715
40
120
195
-
-
Due to the polyether blocks, these polymers retain their flexibility
These polymers may be extruded and injection moulded on
The thermoplastic elastomer polyamides have found use in
Other companies interested in thermoplastic polyamide rubbers
Applications of the elastomeric polyamides include keyboard pads,