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31
resistance of the laminates is inferior to those from unmodified resins.
CH CH
2
O
+
HSRSH
+
CH
2
CH
O
CHOHCH2S
R
S
CH
2
CH
OH
H2N(CH2)3NH(CH2)
3NH2
+ 3CH
2
CH
CN
CN(CH2)
3
NH(CH
2)3
N(CH2)2NH(CH2)2CN
(CH)
2
CN
Because of problems in handling, the polyamides have found only limited
use with epoxy resins, mainly for coating and adhesive applications.
The low molecular weight polysulphides have found somewhat
greater use. Of general structure HS–R–SH and with molecular weights of
approximately 1000 they will react with the epoxy group to cause chain
extension but not cross-linking. The normal hardeners must therefore be
employed in the usual amounts.
The polysulphides used are relatively mobile liquids with
viscosities of about 10 poise and are thus useful as reactive diluents. They
may be employed in any ratio with epoxide and products will range from
soft rubbers, where only polysulphides are employed, to hard resins using
only epoxide.
The more the polysulphide the higher will be the dielectric constant
and the lower the volume resistivity. There will be reduction in tensile
strength and heat distortion temperature but an increase in flexibility and
impact strength.
The polysulphides are frequently used in casting mixes and to a
less extent in coating, laminating and adhesive applications. Their value in
casting and encapsulation lies mainly with their low curing shrinkage and
flexibility in the cured state. Their tendency to corrode copper and the
somewhat inferior electric insulation properties of the blends does lead to
certain limitations.
Interesting amine flexibilizers have also been described. These
materials are made by cyanoethylation of amine hardeners such as
diethylenetriamine, to such an extent that only two reactive hydrogens
remain and the material is only difunctional, e.g. Figure 13.
Figure 13
The amine flexibilizers may be used in two ways:
(1) Where allowance is made for the reactivity of the hardener.
(2) Where the reactivity of the hardener is ignored.
32
Progressive replacement of amine hardener by a low-viscosity
CH
2
CH
O
CH
2
O
CH
2
CH
O
R
n
CH
2
CH
R
O
CH
2
CH CH
2
O
flexibilizer will reduce mix viscosity, increase pot life and reduce the heat
distortion temperature of the cured system. Higher impact strengths are
achieved using approximately equivalent amounts of hardener and
flexibilizer.
Using flexibilizers in addition to the usual amount of hardener,
very flexible products may be obtained.
Although in many respects they are similar to the liquid
polysulphides, the amine flexibilizers differ in three important respects:
(1) They reduce the reactivity of the system rather than increase it.
(2) They are compatible with a different range of room temperature
hardeners.
(3) They have a low level of odour.
Yet another approach to the production of flexible epoxy resinbased systems is to modify the epoxy resin itself. There are now available
polyglycol diepoxides of the general structure in Figure 14 where n is in the
range 2-7.
Figure 14
Used alone they give soft compositions and they are usually used in
blends with other epoxy resins. Compared with unmodified rigid resins the
blends have a greater toughness and elongation at break when cured whilst
the uncured resins have a lower viscosity. They have been used for
laminating safety glass to television tubes, in incapsulation applications
subjected to extensive thermo-cycling and in tooling.
Structure and properties of cured resins
Since the characteristic grouping of the resins largely disappears on
cross-linking it is difficult to make simple generalizations relating structure
to properties.
Being cross-linked, the resin will not dissolve without
decomposition but will be swollen by liquids of similar solubility parameter
to the cured resin. The chemical resistance is as much dependent on the
hardener as on the resin since these two will determine the nature of the
linkages formed.
The main skeleton of the resins themselves has generally good
chemical resistance
33
The thermal properties of the resin are dependent on the degree of
cross-linking, the flexibility of the resin molecule and the flexibility of the
hardener molecule. Consequently the rigid structures obtained by using
cycloaliphatic resins or hardeners such as pyromellitic dianhydride will
raise the heat distortion temperatur es.
The resins are somewhat polar and this is reflected in the
comparatively high dielectric constant and power factor for an insulating
material.
Applications
The epoxy resins are used in a large number of fields, including in
surface coatings, in adhesives, in potting and encapsulation of electronic
components, in tooling, for laminates in flooring and to a small extent in
moulding powders and in road surfacing.
The encapsulation of electrical components provides an interesting
extension to the use of plastics materials as insulators. Components of
electronic systems may be embedded in a single cast block of resin (the
process of encapsulation). Such integrated systems are less sensitive to
handling and humidity and in the event of failure the whole assembly may
be replaced using seldom more than a simple plugging-in operation.
Encapsulation of miniaturized components has proved invaluable,
particularly in spacecraft.
Systems based on the epoxy resins may be provided which are
closer to these requirements than can be obtained in other ways. The
polyester resins are very restricted because of their high shrinkage, the
corroding influence of polyester formulations on copper and on the
volatility of components. There is, however, some application of flexible
polyurethanes where good damping qualities are of importance. The low
shrinkage and simplicity of fabrication make epoxy resins admirably suited
for a number of tooling applications. Patterns, jigs, metal shaping moulds
and vacuum forming moulds are frequently made from these materials.
Since many of these products are quite large in bulk it is important that low
exotherm curing systems are used.
The choice of filler depends on the end use. Metal fillers will
improve machineability, hardness and thermal conductivity but may in
some cases inhibit cure.
Compared with the polyesters the epoxy resins generally have
better mechanical properties and, using appropriate hardeners, better heat
resistance and chemical resistan ce , in particul ar, res is tan ce to alkali s.
34
The laminates are employed mainly where an intermediate degree
of heat stability is required which does not justify the use of the more
expensive silicone and other laminates. They have additional advantages
over the silicones in their ease of forming by wet lay-up techniques and the
greater strength of the laminates.
Epoxy resin laminates are of particular importance in the aircraft
industry. It has been stated that the Boeing 757 and 767 ai rcraft use 1800
kg of carbon fibre/epoxy resin composites for structural purposes per
aeroplane. The resin has also been used with Aramid fibres for filamentwound rocket motors and pressure vessels.
Epoxy resins reinforced with carbon and Aramid fibres have been
used in small boats, where it is claimed that products of equal stiffness and
more useable space may be produced with a 40% saving in weight over
traditional polyester/glass fibre composites.
The properties of the laminates will depend on a number of factors,
of which the following are the most important:
(1) Resin used.
(2) Hardener used.
(3) Fillers and modifiers used.
(4) Type of reinforcement.
(5) Resin content of laminate.
(6) Curing conditions.
The electrical properties will also depend on the above factors as
well as on the test conditions, in particular tem pera ture, test frequen cy and
humidity.
Moulding powders based on epoxy resins have been available on a
small commercial scale for several years. Their particular advantages are
the very low shrinkage on cure and the high fluidity developed during the
moulding operation. This makes them particularly suitable for moulding
thin sections round relatively large metal inserts and for moulding around
delicate pins and inserts. Although some commercial grades are glass fibre
filled their low viscosity in the molten state allows them to be transfer
moulded without difficulty.
The finished mouldings have high dimensional stability, low water
absorption and good resistance to tracking. They also exhibit good heat
resistance and mouldings are said to have withstood temperatures of 200°C
without undue deterioration.
The application of the moulding powders is limited by their cost,
which is greater than that of general purpose phenolics. Main end uses have
35
been for electronic applications, where good electrical properties and heat
Property
Value
Flexural strength, MPa
Volume resistivity, Ωm
90-130
1016-10
resistance are required, particularly in mouldings containing inserts.
One limitation of epoxy moulding compositions is their short shelf
life (typically 1-3 months), which necessitates strict stock control. The
compounds may be compression, transfer or injection moulded, although
compression moulding is preferred for long-fibre grades.
Some properties of a typical grade of epoxy moulding powder are
given in Table 2.
Table 2 – Properties of a typical epoxide moulding composition
Mould shrinkage, cm/cm
After shrinkage 48 h at 105ºC, %
Water absorption, mg
The largest single end use of epoxy resins, accounting for over 50%
of production, is for surface coatings. They may be blended with other
resins such as alkyds, amino-resins and phenolics or they may be esterified
by heating with resin acids or fatty acids. They may be used in solution
form or more commonly as solventless coatings, either liquid resins or
powders, the latter being applied by fluid bed or electrostatic spray
technique. As a class the epoxides therefore have great versatility and this,
combined with excellent adhesion, good chemical resistance and flexibility,
has led to many industrial applications.
The excellent adhesion, high cohesion, low shrinkage on cure,
absence of volatile solvents and low creep of the resins have led to
important applications as adhesives, particularly for metal-to-metal and
metal-to-plastics bonding. As with the surface coating there is a diversity of
possible formulations available, selection being dependent on the
requirements of the end-product.
The resins have also found use in a number of other directions. The
use of the resins in floorings and road surfacings is somewhat spectacular.
In spite of the high initial cost, such floorings have excellent chemical
resistance and resistance to wear. The resins are claimed to be of particular
value at road junctions and roundabouts, where severe wear is experienced,
but where repairs and maintenance operations need to be kept to a
minimum because of the resultant disruption in the flow of traffic.
<0.002
Negligible
5-10
17
36
Epoxy resins are available in a powder form that contains a suitable
hardening system. The powder may be used for coating metals by fluidized
bed or by electrostatic spraying techniques. Unlike with nylon and
polyolefin powder coatings it is necessary to bake the coating in order to
cure the resin. The powder coating are particularly useful for application of
thick film to parts of a complicated or irregular shape and have good
chemical and electrical resistance. The coatings are much harder and adhere
more strongly to the substrate than the older more well-established
thermoplastic powders. The electrostatic spraying of epoxy powders to
form surface coatings presents an important challenge to the usual methods
using solutions.
1.2 Phenolic Resins
The phenolic resins may be considered to be the first polymeric
products produced commercially from simple compounds of low molecular
weight, i.e. they were the first truly synthetic resins to be exploited.
Although they are now approaching their centenary, phenolic resins
continue to be used for a wide variety of applications, such as moulding
powders, laminating resins, adhesives, binders, surface coatings and
impregnants. Until very recently the market has continued to grow but not
at the same rate as for plastics materials in general. For example, in 1957
production of phenolic resins was of the same order as for PVC and for
polyethylene and about twice that of polystyrene. Today it is less than a
tenth that of polyethylene and about one-third that of polystyrene. In the
early 1990s it was estimated that production in the USA was about
1200000 t.p.a., in Western Europe 580000 t.p.a. and in Japan 380000 t.p.a.
With most markets for phenolic resins being long-established but at the
same time subject to increased competition from high-performance
thermoplastics the overall situation had not greatly changed by the end of
the 1990s.
Phenolic moulding powders, which before World War II dominated
the plastics moulding materials market, only consumed about 10% of the
total phenolic resin production by the early 1990s.
In recent years there have been comparatively few developments in
phenolic resin technology apart from the so-called Friedel-Crafts polymers
introduced in the 1960s and the polybenzoxazines announced in 1998.
Phenolic resins are also widely known as phenol-formaldehyde
resins, PF resins and phenoplasts. The trade name Bakelite has in the past
37
been widely and erroneously used as a common noun and indeed is noted
+
H
2SO4
SO
3
H
+ H
2
O
SO
3
H
+
+
+
+
NaOH
SO
3
Na
H
2
O
H
2
O
SO
3
Na
NaOH
2
ONa
+
+
Na
2
SO
3
ONa
H2SO
4
+
2
OH
Na2SO
4
as such in many English dictionaries.
Raw materials
The phenolics are resinous materials produced by condensation of a
phenol, or mixture of phenols, with an aldehyde. Phenol itself and the
cresols are the most widely used phenols whilst formaldehyde and, to a
much less extent, furfural are almost exclusively used as the aldehydes.
Phenol
At one time the requirement for phenol (melting point 41°C), could
be met by distillation of coal tar and subsequent treatment of the middle oil
with caustic soda to extract the phenols. Such tar acid distillation products,
sometimes containing up to 20% o-cresol, are still used in resin
manufacture but the bulk of phenol available today is obtained synthetically
from benzene or other chemicals by such processes as the sulphonation
process, the Raschig process and the cumene process. Synthetic phenol is a
purer product and thus has the advantage of giving rise to less variability in
the condensation reactions.
In the sulphonation process vaporized benzene is forced through a
mist of sulphuric acid at 100-120°C and the benzene sulphonic acid formed
is neutralized with soda ash to produce benzene sodium sulphonate. This is
fused with a 25-30% excess of caustic soda at 300-400°C. The sodium
phenate obtained is treated with sulphuric acid and the phenol produced is
distilled with steam:
Today th e sulphonation route is somewhat uneconomic and largely
replaced by newer routes. Processes involving chlorination, such as the
Raschig process, are used on a large scale commercially. A vapour phase
38
reaction between benzene and hydrochloric acid is carried out in the
+
CH
2
CH
CH
3
CH
CH
3
CH
3
CH
CH
3
CH
3
[O]
CH
3
CH
3
OOH
C
+
OH
CH
3
CH
3
OOH
C
H
CH3COCH
3
presence of catalysts such as an aluminium hydroxide-copper salt complex.
Monochlorobenzene is formed and this is hydrolyzed to phenol with water
in the presence of catalysts at about 450°C, at the same time regenerating
the hydrochloric acid. The phenol formed is extracted with benzene,
separated from the latter by fractional distillation and purified by vacuum
distillation. In recent years developments in this process have reduced the
amount of by-product dichlorobenzene formed and also considerably
increased the output rates.
A third process, now the principal synthetic process in use in
Europe, is the cumene process.
In this process liquid propylene, containing some propane, is mixed
with benzene and passed through a reaction tower containing phosphoric
acid on kieselguhr as catalyst. The reaction is exothermic and the propane
present acts as a quench medium. A small quantity of water is injected into
the reactor to maintain catalyst activity. The effluent from the reactor is
then passed through distillation columns. The propane is partly recycled,
the unreacted benzene returned to feed and the cumene taken off (Figure
15). The cumene is then oxidized in the presence of alkali at about 130°C
(Figure 16). T he hydroperoxide formed is decomposed in a stirred vessel
by addition of dilute sulphuric acid. The mixture is passed to a separator
and the resulting organic layer fractionated (Figure 17). Some
benzophenone is also produced in a side reaction.
Figure 15
Figure 16
Figure 17
39
The economics of this process are to some extent dependent on the
OH
CH
3
OH
CH
3
OH
CH
3
value of the acetone which is formed with the phenol. The process is,
however, generally considered to be competitive with the modified Raschig
process in which there is no by-product of reaction. In all of the above
processes benzene is an essential starting ingredient. At one time this was
obtained exclusively by distillation of coal tar but today it is commonly
produced from petroleum.
A route to phenol has been developed starting from cyclohexane,
which is first oxidized to a mixture of cyclohexanol and cyclohexanone. In
one process the oxidation is carried out in the liquid phase using cobalt
naphthenate as catalyst. The cyclohexanone present may be converted to
cyclohexanol, in this case the desired intermediate, by catalytic
hydrogenation. The cyclohexanol is converted to phenol by a catalytic
process using selenium or with palladium on charcoal. The hydrogen
produced in this process may be used in the conversion of cyclohexanone
to cyclohexanol. It also may be used in the conversion of benzene to
cyclohexane in processes where benzene is used as the precursor of the
cyclohexane.
Phenol is supplied commercially either in the solid (crystalline)
state or as a “solution” in water (water content 8-20%). Where supplied as a
solid it is usually handled by heating the phenol, and the molten material is
pumped into the resin kettles or into a preblending tank. If the “solution” is
used care must be taken to avoid the phenol crystallizing out.
Other phenols
A number of other phenols obtained from coal tar distillates are
used in the manufacture of phenolic resins. Of these the cresols are the
most important:
o-Cresol p-Cresol m-Cresol
b.p. 191.0ºС b.р.201.9ºС b.p.202.2ºC
The cresols occur in cresylic acid, a mixture of the three cresols
together with some xylenols and neutral oils, obtained from coal tar
40
distillates. Only the m-cresol has the three reactive positions necessary to
OH
CH
3
CH
3
OH
CH
3
CH
3
OH
CH
3
CH
3
2,3-Xylenol
2,4- 2,5-
OH
CH
3
CH
3
OH
CH
3
CH
3
OH
CH
3
CH
3
3,4-
3,5-
2,6-
give cross-linked resins and so this is normally the desired material. The o-
isomer is easily removed by distillation but separation of the close-boiling
m- and p-isomers is difficult and so mixtures of these two isomers are used
in practice.
Xylenols, also obtained from coal tar, are sometimes used in oilsoluble resins. Of the six isomers only 3.5-xylenol has the three reactive
positions necessary for cross-linking and thus mixtures with a high
proportion of this isomer are generally used.
Other higher boiling phenolic bodies obtainable from coal tar
distillates are sometimes used in the manufacture of oil-soluble resins.
Mention may also be made of cashew nut shell liquid which contains
phenolic bodies and which is used in certain specialized applications.
A few synthetic substituted phenols are also used in the
manufacture of oil-soluble resins. They include p-tert-butylphenol, p-tert-
amylphenol, p-tert-octylphenol, p-phenylphenol and
dihydroxyphenylpropane (bis-phenol A).
Aldehydes
Formaldehyde (methanal) is by far the most commonly employed
aldehyde in the manufacture of phenolic resins. It is normally used as an
aqueous solution, known as formalin, containing about 37% by weight of
formaldehyde. From 0.5-10% of methanol may be present to stabilize the
solution and retard the formation of polymers. When the formalin is used
soon after manufacture, only low methanol contents are employed since the