Plastics Materials 7 Episode 12 potx

Cellulose Esters
627
for a variety of purposes. Thin sheet is useful for high-quality display boxes
whilst thicker sheet is used for spectacle frames.
Triacetate film is used in the graphic arts, for greetings cards, and for
specialised electrical applications such
as
non-conducting separators.
The use of cellulose acetate for moulding and extrusion is now becoming
small owing largely to the competition of the styrene polymers and polyolefins.
The major outlets at the present time are in the fancy goods trade as
toothbrushes, combs, hair slides etc. Processing provides
no
major problem
provided care is taken to avoid overheating and the granules are dry. The
temperatures and pressure used vary, from
160
to 250°C and 7 to 15 ton/in2
respectively, according to grade. The best injection mouldings are obtained
using a warm mould.
Secondary cellulose acetate has also been used for fibres and lacquers whilst
cellulose triacetate fibre
has
been extensively marketed in Great Britain under the
trade name Trice].
Biodegradable Cellulose Acetate
Compounds
As
a
result of development work between the Battelle Institute in Frankfurt and
a German candle-making company, Aetema, biodegradable cellulose acetate

compounds have been available since
1991
from the Rh6ne-Poulenc subsidiary
Tubize Plastics. They are marketed under the trade names Bioceta and Biocellat.
The system is centred round the use of an additive which acts both as a plasticiser
and
a
biodegrading agent, causing the cellulose ester to decompose within 6-24
months.
The initial use was
as
a blow moulded vessel for vegetable oil candles.
However, because of its biodegradability it is of interest for applications where
paper and plastics materials are used together
and
which can, after use, be sent
into a standard paper recycling process. Instances include blister packaging (the
compound is transparent up to 3mm in thickness), envelopes with transparent
windows and clothes point-of-sale packaging.
Compared with more common plastics used
as
packaging materials, the
compound does have some disadvantages, such as a high water vapour
permeability and limited heat resistance, losing dimensional stability at about
70°C. It is also substantially more expensive than the high-tonnage polyolefins.
Last but not least its biodegradability means that it must be used in applications
that will have completed their function within
a
few months of the manufacture
of the polymer compound.

22.2.3
Other Cellulose Esters
Homologues of acetic acid have been employed to make other cellulose esters
and
of
these cellulose propionate, cellulose acetate-propionate and cellulose
acetate-butyrate are produced
on
a commercial scale. These materials have
larger side chains than cellulose acetate and with equal degrees of esterification,
molecular weights and incorporated plasticiser, they are slightly softer, of lower
density, have slightly lower heat distortion temperatures and flow a little more
easily. The somewhat greater hydrocarbon nature of the polymer results in
slightly lower water-absorption values (see
Table
22.2).
It should, however, be realised that some grades of cellulose acetate may be
softer, be easier to process and have lower softening points than some grades of
628
Cellulose Plastics
cellulose acetate-butyrate, cellulose acetate-propionate and cellulose propionate
since the properties of all four materials may be considerably modified by chain
length, degree of substitution and in particular the type and amount of
plasticiser.
Cellulose acetate-butyrate
(CAB) has been manufactured for a number of
years in the United States (Tenite Butyrate-Kodak) and in Germany (Cellidor
B-Bayer)
.
In a typical process for manufacture on a commercial scale bleached wood

pulp or cotton linters are pretreated for 12 hours with 40-50% sulphuric acid and
then, after drying, with acetic acid. Esterification of the treated cellulose is then
carried out using a mixture of butyric acid and acetic anhydride, with a trace of
sulphuric acid as catalyst. Commercial products vary extensively in the acetate/
butyrate ratios employed.
The lower water absorption, better flow properties and lower density of CAB
compared with cellulose acetate are not in themselves clear justification for their
continued use. There are other completely synthetic thermoplastics which have
an even greater superiority at a lower price and do not emit the slight odour of
butyric acid as does CAB. Its principal virtues which enable it to compete with
other materials are its toughness, excellent appearance and comparative ease of
mouldability (providing the granules are dry). The material also lends itself to use
in fluidised bed dip-coating techniques, giving a coating with a hard glossy finish
which can be matched only with more expensive alternatives. CAB is easy to
vacuum form.
A
number of injection mouldings have been prepared from CAB with about
19% combined acetic acid and 44% combined butyric acid. Their principal end
products have been for tabulator keys, automobile parts, toys and tool handles. In
the United States CAB has been used for telephone housings. Extruded CAB
piping has been extensively used in America for conveying water, oil and natural
gas, while CAB sheet has been able to offer some competition to acrylic sheet for
outdoor display signs.
In the mid- 1950s
cellulose propionate
became commercially available
(Forticel-Celanese). This material is very similar in both cost and properties to
CAB. Like CAB it may take on an excellent finish, provided a suitable mould is
used,
it

is less hygroscopic than cellulose acetate, and is easily moulded.
As with the other esters a number of grades are available differing in the
degree of esterification and in type and amount of plasticiser. Thus the
differences in properties between the grades are generally greater than any
differences between 'medium' grades of cellulose propionate and CAB. Whereas
a soft grade of the propionate may have a tensile strength of 20001bf/in2
(14MPa) and a heat distortion temperature of
51"C,
a hard grade may have
tensile strength as high as
6000
lbf/in2 (42 MPa) and a heat distortion temperature
of
70°C.
Cellulose acetate-propionate
(Tenite Propionate-Kodak) is similar to cellu-
lose propionate. With the shorter side chains, cellulose propionate and cellulose
acetate propionate tend to be harder, stiffer and of higher tensile strength than
CAB. Like CAB they are easy to vacuum form and also tend to be used for
similar applications such as steering wheels, tool handles, safety goggles and
blister packs.
Many other cellulose esters have been prepared in the laboratory and some
have reached pilot plant status. Of these the only one believed to be of current
importance is
cellulose caprate
(decoate). According to the literature, degraded
Cellulose Ethers
629
wood pulp is activated by treating with chloroacetic acid and the product is
esterified by treating with capric anhydride, capric acid and perchloric acid. The

material is said to be useful as optical cement.’
22.3 CELLULOSE ETHERS
By use of a modification of the well-known Williamson synthesis it is possible
to prepare a number of cellulose ethers. Of these materials ethyl cellulose has
found a small limited application as a moulding material and somewhat greater
use for surface coatings. The now obsolete benzyl cellulose was used prior to
World War I1 as a moulding material whilst methyl cellulose, hyroxyethyl
cellulose and sodium carboxymethyl cellulose are useful water-soluble
polymers.
With each of these materials the first step is the manufacture of alkali cellulose
(soda cellulose). This is made by treating cellulose (either bleached wood pulp or
cotton linters) with concentrated aqueous sodium hydroxide in a nickel vessel at
elevated temperature. After reaction excess alkali is pressed out, and the resultant
‘cake’ is then broken up and vacuum dried until the moisture content is in the
range 10-25%. The moisture and combined alkali contents must be carefully
controlled as variations in them will lead to variations in the properties of the
resultant ethers.
22.3.1
Ethyl Cellulose
Ethyl cellulose is prepared by agitating the alkali cellulose with ethyl chloride in
the presence of alkali at about 60°C for several hours. Towards the end of the
reaction the temperature is raised to about 130-140°C. The total reaction time is
approximately
12
hours. The reaction is carried out under pressure.
If the etherification were taken to completion the product would be the
compound shown in
Figure
22.5.
CH,OC,H,

0
-,
C H
I

C
c
I
0.
C,H,
I
H
Figure
22.5
It is essential that there be sufficient alkali present, either combined with the
cellulose, or free, to neutralise the acid formed by both the main reaction and in
a side reaction which involves the hydrolysis of ethyl chloride.
630
Cellulose Plastics
Ethyl ether and ethyl alcohol which are formed as by-products are removed by
distillation and the ethyl cellulose is precipitated by hot water. The polymer is
then carefully washed to remove sodium hydroxide and sodium chloride and
dried.
The properties of the ethyl cellulose will depend
on:
(1) The molecular weight.
(2)
The degree of substitution.
(3) Molecular uniformity.
The molecular weight may be regulated by controlled degradation

of
the alkali
cellulose in the presence of air. This can be done either before or during
etherification. The molecular weight
of
commercial grades is usually expressed
indirectly as viscosity of a 5% solution in an
80:20
toluene-ethanol mixture.
The completely etherified material with a degree
of
substitution of
3
has an
ethoxyl content of 54.88%. This material has little strength and flexibility, is not
thermoplastic, has limited compatibility and solubility and is
of
no
commercial
value.
A
range of commercial products are, however, available with
a
degree of
substitution between 2.15 and 2.60, corresponding to a range
of
ethoxyl contents
from 43 to 50%.
The ethoxyl content is controlled by the ratio of reactants and to a lesser degree
by the reaction temperature.

Whereas mechanical properties are largely determined by chain length, the
softening point, hardness, water absorption and solubility are rather more
determined by the degree of substitution (see
Figure
22.6).
ETHOXYL CONTENT
OF
ETHYL CELLULOSE IN
'/D
90"
Figure
22.6.
Influence
of
the ethoxyl content
of
ethyl cellulose on softening point moisture absorption
and
hardness. (Hercules
Powder
Co.
literature)
Typical physical properties of ethyl cellulose are compared with those of the
cellulose ethers
in
Table
22.2.
The solubility of ethyl cellulose depends
on
the degree

of
substitution. At low
degrees of substitution
(0.8-1.3)
the replacement of some
of
the hydroxyl groups
by ethoxyl groups reduces the hydrogen bonding across the cellulosic chains
to
such an extent that the material is soluble in water. Further replacement
of
hydroxyl groups by the less polar and more hydrocarbon ethoxyl groups
Cellulose
Ethers
63
1
increases the water resistance. Fully etherified ethyl cellulose
is
soluble only in
non-polar solvents.
The relationship between degree of substitution and solubility characteristics is
predictable from theory and is summarised in
Table
22.5.
Table
22.5
Solubility
of
ethyl
cellulose

Average number
of
ethoxyl
groups per glucose unit
Solubility
-0.5
0.8-1.3
1.4-1
.E
1.8-2.2
2.2-2.4
2.4-2.5
2.5-2.8
soluble in
4-8%
sodium hydroxide
soluble in water
swelling in polar-non-polar solvent mixtures
increasing solubility in above mixtures
increasing solubility in alcohol and
less
polar
solvents
widest range
of
solubilities
soluble only in non-polar solvents
Ethyl cellulose is subject to oxidative degradation when exposed to sunlight
and elevated temperatures. It is therefore necessary to stabilise the material
against degrading influences during processing or service.

In
practice three types
of stabiliser are incorporated, an antioxidant such as the phenolic compound
2,2’-methylenebis-(4-methyl-6-tert-butylphenol),
an acid acceptor such as
an
epoxy resin for use where plasticisers may give rise to acidic degradation
products and an ultraviolet absorber such as
2,4-dihydroxybenzophenone
for
outdoor use. Plasticisers such
as
tritolyl phosphate and diamylphenol have a
beneficial stabilising effect.
Ethyl cellulose has never become well known in Europe and apart from one or
two specific applications has not been able to capture any significant proportion
of
the market held by the cellulose esters. Although it has the greatest water
resistance and the best electrical insulating properties amongst the cellulosics this
is of little significance since when these properties are important there are many
superior non-cellulosic alternatives. The principal uses for ethyl is cellulose
injection mouldings
are
in those applications where good impact strength at low
temperatures is required, such as refrigerator bases and flip lids and ice-crusher
parts.
Ethyl cellulose is often employed in the form of a ‘hot melt’ for strippable
coatings. Such strippable coatings first became prominent during World War
I1
for packaging military equipment. Since then they have been extensively used for

protecting metal parts against corrosion and mamng during shipment and
storage.
A
typical composition consists of
25%
ethyl cellulose,
60%
mineral oil,
10%
resins and the rest stabilisers and waxes. Coating is performed by dipping
the cleaned metal part into the molten compound. The metal part is withdrawn
and
an
adhering layer of the composition is allowed to harden by cooling. Hot
melts have also been used for casting and paper coating.
The ether is also used in paint, varnish and lacquer formulations.
A
recent
development is the use of ethyl cellulose gel lacquers. These are permanent
coatings applied in a similar way to the strippable coatings. They have been used
in the United States for coating tool handles, door knobs and bowling pins.
632
Cellulose Plastics
22.3.2
Miscellaneous Ethers
Only one other cellulose ether has been marketed for moulding and extrusion
applications,
benzyl cellulose.
This material provides a rare example of a polymer
which although available in the past is no longer commercially marketed. The

material had a low softening point and was unstable to both heat and light and has
thus been unable to compete with the many alternative materials now
available.
A
number of water-soluble cellulose ethers are marketed!
Methyl cellulose
is
prepared by a method similar to that used for ethyl cellulose.
A
degree of
substitution of 1.6-1.8 is usual since the resultant ether is soluble in cold water
but not in hot. It is used as a thickening agent and emulsifier in cosmetics, as a
paper size, in pharmaceuticals, in ceramics and in leather tanning operations.
Hydroxyethyl cellulose,
produced by reacting alkali cellulose with ethylene
oxide, is employed for similar purposes.
Hydroxypropyl cellulose,
like methyl cellulose,
is
soluble in cold water but not
in hot, precipitating above 38°C. It was introduced by Hercules in 1968 (Klucel)
for such uses as adhesive thickeners, binders, cosmetics and as protective
colloids for suspension polymerisation. The Dow company market the related
hydroxypropylmethyl cellulose
(Methocel) and also produce in small quantities a
hydroxyethylmethyl cellulose.
Reaction of alkali cellulose with the sodium salt
of
chloracetic acid yields
sodium carboxmethyl cellulose,

(SCMC). Commercial grades usually have a
degree
of
substitution between
0.50
and 0.85. The material, which appears to be
physiologically inert, is very widely used. Its principal application is as a soil-
suspending agent in synthetic detergents.
It
is also the basis of a well-known
proprietary wallpaper adhesive. Miscellaneous uses include fabric sizing and as
a surface active agent and viscosity modifier in emulsions and suspensions.
Purified grades of SCMC are employed in ice cream to provide a smooth texture
and in a number of pharmaceutical and cosmetic products.
Schematic equations for the production of fully substituted varieties of the
above three ethers are given below
(R
represents the cellulose skeleton).
R(ONa),,
+
CH,Cl
R(ONH,),,
Methyl
Cellulose
R(ONa),,
+
CH,
CH,
-
R(OCH,CH,OH),,

Hydroxyethyl
Cellulose
R(ONa),,
+
ClCH,.COONa
-
R(OCH,COONa>,,
+
NaCl
Sodium Carboxymethyl Cellulose
22.4 REGENERATED CELLULOSE
Because of high interchain bonding, cellulose is insoluble in solvents and is
incapable of flow on heating, the degradation temperature being reached before
the material starts to flow. It is thus somewhat intractable in its native form.
Cellulose, however, may be chemically treated
so
that the modified products may
Regenerated
Cellulose
633
be dissolved and the solution may then either be cast into film or spun into fibre.
By treatment of the film or fibre the cellulose derivative may be converted back
(regenerated) into cellulose although the processing involves reduction in
molecular weight.
In
the case of fibres three techniques have been employed:
(1)
Dissolution of the cellulose in cuprammonium solution followed by acid
coagulation of extruded fibre (‘cuprammonium rayon’-no longer of
commercial importance).

In
this case the acid converts the cuprammonium
complex back into cellulose.
(2) Formation of cellulose acetate, spinning into fibre and subsequent hydrolysis
into cellulose.
(3) Reaction of alkali cellulose with carbon disulphide to produce a cellulose
xanthate which forms a lyophilic sol with caustic soda. This may be extruded
into a coagulating bath containing sulphate ions which hydrolyses the
xanthate back to cellulose. This process is known as the viscose process and
is that used in the manufacture of rayon.
By modification of the viscose process a regenerated cellulose foil may be
produced which is known under the familiar trade name Cellophane.
The first step in the manufacture of the foil involves the production of alkali
cellulose. This is then shredded and allowed to age in order that oxidation will
degrade the polymer to the desired extent. The alkali cellulose is then treated
with carbon disulphide in xanthating chums at 20-28°C for about three
hours.
The xanthated cellulose contains about one xanthate group per two glucose
units. The reaction may be indicated schematically as
R.ONa
+
CS, R.O.C.SNa
II
S
The resultant yellow sodium cellulose xanthate is dispersed in an aqueous
caustic soda solution, where some hydrolysis occurs. This process is referred to
as ‘ripening’ and the solution as ‘viscose’. When the hydrolysis has proceeded
sufficiently the solution it transferred to a hopper from which it emerges
through a small slit on to a roller immersed in a tank of 10-15% sulphuric acid
and 10-20% sodium sulphate at 35-40°C. The viscose is coagulated and by

completion of the hydrolysis the cellulose is regenerated. The foil is
subsequently washed, bleached, plasticised with ethylene glycol or glycerol
and then dried.
The product at this stage is ‘plain’ foil and has a high moisture vapour
transmission rate. Foil which is more moisture proof may be obtained by coating
with pyroxylin (cellulose nitrate solution) containing dibutyl phthalate as
plasticiser or with vinylidene chloride-acrylonitrile copolymers. A range of foils
are available differing largely in their moisture impermeability and in heat
sealing characteristics.
Regenerated cellulose foil has been extensively and successfully used as a
wrapping material, particularly in the food and tobacco industries. Like other
cellulose materials it is now having to face the challenge
of
the completely
synthetic polymers. Although the foil has been able to compete in the past, the
634
Cellulose
Plastics
advent
of
the polypropylene film in the early
1960s
produced a serious
competitor which led to
a
marked reduction in the use
of
the cellulosic
materials.
Regenerated cellulose does, however, have the advantage that it biodegrades

well aerobically in composting (rather more slowly anaerobically).
22.5
VULCANISED FIBRE
This material has been known for many years, being used originally in the
making of electric lamp filaments. In principle vulcanised fibre
is
produced by
the action of zinc chloride on absorbent paper. The zinc chloride causes the
cellulosic fibres to swell and be covered with a gelatinous layer. Separate layers
of
paper may be plied together and the zinc chloride subsequently removed to
leave a regenerated cellulose laminate.
The removal
of
zinc chloride involves an extremely lengthy procedure. The
plied sheets are passed through a series of progressively more dilute zinc chloride
solutions and finally pure water in order to leach out the gelatinising agent. This
may take several months. The sheets are then dried and consolidated under light
pressure.
The sheets may be formed
to
some extent by first softening in hot water or
steam and then pressing in moulds at pressures of
200-500
Ibf/in2
(1.5-3SMPa). Machining, using high-speed tools, may be camed out on
conventional metal-working machinery.
A
number
of

grades have been available according to the desired end use. The
principal applications of vulcanised fibre are in electrical insulation, luggage,
protective guards and various types of materials-handling equipment. The major
limitations are dimensional instability caused by changes in humidity, lack of
flexibility and the long processing times necessary to extract the zinc chloride.
References
1.
PAIST,
w.
D.,
Cellulosics, Reinhold,
New
York
(1958)
2.
STANNETT,
v.,
Cellulose Acetate Plastics,
Temple
Press,
London
(1950)
3.
FORDYCE,
c.
K.,
and
MEYER,
L.
w.

A.,
Ind. Eng. Chem.,
33,
597
(1940)
4.
DAVIDSON,
K.
L.,
and
sirric,
M.,
Water Soluble Resins,
Reinhold,
New
York
(1962)
Bibliography
DAVIDSON,
R.
L.,
and
SI~IG,
M.,
Water Soluble Resins, Reinhold,
New
York
(1962)
MILES,
F.

D.,
Cellulose Nitrate,
Oliver
and
Boyd,
London
(1955)
OTT,
G.,
SPURLIN,
H.
M.,
and
GKAFFLIN,
M.
w.,
Cellulose and its Derivatives
(3
vols),
Interscience,
New
PAIST,
w.
D.,
Cellulosics,
Reinhold,
New
York
(1958)
KOWELL,

R.
M.
and YOUNG,
K.
A.
(Eds.),
Modified Cellulosics,
Academic
Press,
New
York-San
SxwNETr,
v.,
Cellulose
Acetate Plastics,
Temple
Press,
London
(1950)
YAKSLEY,
v.
E.,
FLAVELL,
w.,
ADAMSON,
P.
s.,
and
PEKKINS,
N.

G.,
Cellulosic Plastics,
Iliffe,
London
York,
2nd
Edn
(1954)
Francisco-London
(1978)
(
1964)
23
Phenolic Resins
23.1 INTRODUCTION
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. Their early development
has been dealt with briefly in Chapter 1 and more fully elsewhere.'
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
e
same order

as for
PVC and for polyethylene and about twice that of polystyren
's,
Today it is
less than a tenth that of polyethylene and about one-third that of polysthene. In the
early 1990s it was estimated that production in the USA was about 1
20@000
t.p.a.,
in Western Europe 580
000
t.p.a. and in Japan
380
000
t.p.a. With most markets for
phenolic resins being long-established but at the same time subjeFo 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 I1 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 which are discussed
briefly at the end of the chapter.
Phenolic resins are also widely known
as
phenol-formaldehyde resins,
PF
resins and phenoplasts. The trade name Bakelite has in the past been widely and
erroneously used

as
a
common noun and indeed is noted
as
such in many English
dictionaries.
23.2
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.
635
636
Phenolic Resins
23.2.1
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% a-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 vaporised benzene
is

forced through a mist of
sulphuric acid at 100-120°C and the benzene sulphonic acid formed is
neutralised 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
(Figure
23.1).
SO,H
+
NaOH
4
@
+
H,O
+
2NaOH
+
moNa
+
Na$O,
+
H,O
+
H,SO,
-
2aoH
+
NqSO,
S0,Na

Figure
23.1
Today the 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 reaction between benzene
and hydrocholoric acid is carried out in the presence of catalysts such as an
aluminium hydroxide-copper salt complex. Monochlorobenzene is formed and
this is hydrolysed 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
Raw Materials
637
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 23.2).
The cumene is then
oxidised in the presence
of
alkali at about
130°C
(Figure 27.3).
The
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 23.4).
Some benzophenone is also produced in a side
reaction.
CHI
Figure 23.2
Figure 23.3
Figure 23.4
The economics of this process
are
to some extent dependent on the 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
oxidised 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.
Other routes for the preparation of phenol are under development and include
the Dow process based on toluene. In this process a mixture
of
toluene, air and
catalyst are reacted at moderate temperature and pressure to give benzoic acid.
This is then purified and decarboxylated, in the presence of air, to phenol
(Figure
23.5).
Pure phenol crystallises in long colourless needles which melt at
41°C.
It
causes severe burns on the skin and care should be taken in handling the material.
638
Phenolic Resins
Figure
23.5

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 crystallising out.
23.2.2
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
(Figure
23.6).
OH OH OH
0-Cresol
p-Cresol
rn-Cresol
b.p.
191.0T
b.p.
201.9T
b.p.
202.2T

Figure
23.6
The cresols occur in cresylic acid, a mixture
of
the three cresols together with
some xylenols and neutral oils, obtained from coal tar distillates. Only the
m-cresol has the three reactive positions necessary to 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 oil-soluble resins.
Of the six isomers
(Figure
23
7)
only 3,5-xylenol has the three reactive positions
2,3-Xylenol
CH,
2,4-
2,5-
OH
I

OH OH
@CH, CH,
&CH3
cH3
&cHJ
CH,
3,4-
35
2,6-
Figure
23.7
Chemical Aspects 639
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 specialised 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).
23.2.3 Aldehydes
Formaldehyde (methanal) is by far the most commonly employed aldehyde in the
manufacture of phenolic resins. Its preparation has been described in Chapter 19.
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
stabilise the solution and retard the formation of polymers. When the formalin is
used
soon
after manufacture, only low methanol contents are employed since the

formalin has a higher reactivity. Where
a
greater storage life is required the
formalin employed has a higher methanol content, but the resulting increasing
stability is at the expense of reduced reactivity.
Furfural (see Chapter 28) is occasionally used to produce resins with good
flow properties for use in moulding powders.
23.3 CHEMICAL ASPECTS
Although phenolic resins have been known and widely utilised for over 60 years
their detailed chemical structure remains to be established. It is now known that
the resins are very complex and that the various structures present will depend on
the ratio of phenol to formaldehyde employed, the pH of the reaction mixture and
the temperature of the reaction. Phenolic resin chemistry has been discussed in
detail and will be discussed only briefly here.
Reaction of phenol with formaldehyde involves a condensation reaction which
leads, under appropriate conditions, to a cross-linked polymer structure. For
commercial application it is necessary first to produce a tractable fusible low
molecular weight polymer which may, when desired, be transformed into the
cross-linked polymer. For example, in the manufacture of a phenolic (phenol-
formaldehyde, P-F) moulding a low molecular weight resin is made by
condensation of phenol and formaldehyde. This resin is then compounded with
other ingredients, the mixture ground to a powder and the product heated under
pressure in a mould. On heating, the resin melts and under pressure flows in the
mould. At the same time further chemical reaction occurs, leading to cross-
linking. It
is
obviously desirable
to
process under such conditions that the
required amount of flow has occurred before the resin hardens.

The initial phenol-formaldehyde reaction products may be of two types,
novolaks and
resols.
23.3.1
Novolaks
The novolaks are prepared by reacting phenol with formaldehyde in
a
molar ratio
of approximately
1
:0.8
under acidic conditions. Under these conditions there is
640
Phenolic
Resins
CH,OH
Figure
23.8
a slow reaction of the two reactants to form the
o-
and p-hydroxymethylphenols
(Figure 23.8).
These then condensate rapidly to form products of the bis(hydroxypheny1)-
methane (HPM) type (e.g.
Figure 23.9).
Figure 23.9
There are three possible isomers and the proportions in which they are formed
will depend
on
the pH of the reaction medium. Under the acid conditions

normally employed in novolak manufacture the 2,4’- and 4,4’-HPM compounds
are the main products
(Figure 23.10).
OH OH OH
I
I I
2,2’HPM 2,4’HPM
4.4’HPM
Figure 23.10
These materials will then slowly react with further formaldehyde to form their
own methylol derivatives which in
turn
rapidly react with further phenol to
produce higher polynuclear phenols. Because of the excess of phenol there is a
limit to the molecular weight of the product produced, but
on
average there are
5-6 benzene rings per molecule.
A
typical example of the many possible
structures is shown in
Figure
23.11.
The novolak resins themselves contain
no
reactive methylol groups and do not
form cross-linked structures
on
heating.
If,

however, they are mixed with
compounds capable of forming methylene bridges, e.g. hexamethylenetetramine
or
paraformaldehyde, they cross-link
on
heating to form infusible, ‘thermoset’
structures.
Chemical Aspects
641
OH
OH
Figure
23.11
In general it is considered essential that the bulk of the phenol used initially
should not be substituted, Le. should be reactive, at the
o-
and p-positions and is
thus trifunctional with respect to the reaction with formaldehyde.
23.3.2 Resols
A
resol is produced by reacting a phenol with
an
excess of aldehyde under basic
conditions.
In this case the formation of phenol-alcohols is rapid but their subsequent
condensation is slow. Thus there is a tendency for polyalcohols, as well as
monoalcohols, to be formed. The resulting polynuclear polyalcohols are of low
molecular weight. Liquid resols have an average of less than two benzene rings
per molecule, while a solid resol may have only three to four.
A

typical resol
would have the structure shown in Figure
23.12.
OH OH
I
CH,OH
Figure
23.12
Heating
of
these resins will result in cross-linking via the uncondensed
methylol groups or by more complex mechanisms. The resols are sometimes
referred to as one-stage resins since cross-linked products may be made from the
initial reaction mixture solely by adjusting the pH. On the other hand the
novolaks are sometimes referred to as two-stage resins as here it is necessary to
add some agent which will enable additional methylene bridges to be formed.
23.3.3 Hardening
The novolaks and resols are soluble and fusible low molecular weight products.
They were referred to by Baekeland as A-stage resins. On hardening, these resins
pass through a rubbery stage in which they are swollen, but not dissolved, by a
variety of solvents. This is referred to as the B-stage. Further reaction leads to
rigid, insoluble, infusible, hard products known as C-stage resins. When prepared
from resols the B-stage resin is sometimes known as a resitol and the C-stage
642
Phenolic Resins
product a
resit.
The terms A-,
B-
and C-stage resins are also sometimes used to

describe analogous states in other thermosetting resins.
The mechanism of the hardening processes has been investigated by Zinke in
Austria, von Euler in Sweden and Hultzsch in Germany using blocked methylol
phenols
so
that only small isolable products would be obtained.
In
general their work indicates that at temperatures below
160°C
cross-linking
occurs by phenol methylol-phenol methylol and phenol methylol-phenol
condensations, viz
Figure
23.13.
CH,OH
+-
CH,OH
CH,.
0
.
CH,
Figure
23.13
As these condensation reactions can occur at the two
ortho
and the
para
positions in phenol, m-cresol and 3,5-xylenol, cross-linked structures will be
formed. It has been pointed out by Megson' that because of steric hindrance the
amount

of
cross-linking that can take place is much less than would involve the
three reactive groups of all the phenolic molecules. It is now generally
considered that the amount of cross-linking that actually takes place is less than
was at one time believed to be the case.
Above 160°C it is believed that additional cross-linking reactions take place
involving the formation and reaction of quinone methides by condensation of the
ether linkages with the phenolic hydroxyl groups
(Figure
23.14).
Figure
23.14
These quinone methide structures
are
capable
of
polymerisation and of other
chemical reactions.
It
is likely that the quinone methide and related structures formed at these
temperatures account for the dark colour of phenolic compression mouldings. It
is to be noted that cast phenol-formaldehyde resins, which are hardened at much
Resin Manufacture 643
lower temperatures, are water-white in colour. If, however, these castings are
heated to about 180°C they darken considerably.
In addition to the above possible mechanisms the possibility of reaction at
m-positions should not be excluded. For example, it has been shown by Koebner
that
0-
and p-cresols, ostensibly difunctional, can, under certain conditions, react

with formaldehyde to give insoluble and infusible resins. Furthermore, Megson
has shown that 2,4,6-trimethylphenol, in which the two ortho- and the one
para-
positions are blocked, can condense with formaldehyde under strongly acidic
conditions. It is of interest to note that Redfarn produced an infusible resin from
3,4,5,-trimethylphenol under alkaline conditions. Here the two
m-
and the
p-positions were blocked and this experimental observation provides supplemen-
tary evidence that additional functionalities are developed during reaction, for
example in the formation of quinone methides.
PI{
Figure
23.15.
Effect
of
pH
on the gel time
of
a
P-F
cast
resin. (After Apley')
The importance of the nature
of
the catalyst on the hardening reaction must
also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films
at room temperature, but as the pH rises there will be a reduction in activity
which passes through a minimum at about pH 7. Under alkaline conditions the
rate of reaction is related to the type

of
catalyst and
to
its concentration. The
effect of pH value
on
the gelling time of a casting resin (phenol-formaldehyde
ratio
-1
:2.25)
is shown in Figure
23.15.
23.4 RESIN MANUFACTURE
Both novolaks and resols are prepared in similar equipment, shown dia-
grammatically in Figure
23.16.
The resin kettle may be constructed from copper,
nickel
or
stainless steel where novolaks are being manufactured. Stainless steel
may also be used for resols but where colour formation is unimportant the
cheaper mild steel may be used.
In
the manufacture of novolaks,
I
mole of phenol is reacted with about
0.8
mole of formaldehyde (added as 37% w/w formalin) in the presence of some acid
as catalyst. A typical charge ratio would be:
Phenol 100 parts by weight

Formalin (37% w/w) 70 parts by weight
Oxalic acid
1.5
parts by weight
644
Phenolic
Resins
t
Figure
23.16.
Diagrammatic representation
of
resin kettle
and
associated equipment
used
for the
preparation
of
phenolic resins. (After Whitehouse, Pritchett and Bamett2)
The reaction mixture is heated and allowed to reflux, under atmospheric
pressure at about
100°C.
At this stage valve A is open and valve
B
is closed.
Because the reaction is strongly exothermic initially it may be necessary to use
cooling water in the jacket at this stage. The condensation reaction will take a
number of hours, e.g.
2-4

hours, since under the acidic conditions the formation
of phenol-alcohols is rather slow. When the resin separates from the aqueous
phase and the resin reaches the requisite degree of condensation, as indicated by
refractive index measurements, the valves are changed over (Le. valve A
is
closed and valve
B
opened) and water present is distilled off.
In
the case of novolak resins the distillation is normally carried out without the
application of vacuum. Thus, as the reaction proceeds and the water is driven
off,
there is a rise in the temperature of the resin which may reach as high as
160°C
at the end of the reaction. At these temperatures the fluid is less viscous and more
easily stirred.
In
cases where it is important to remove the volatiles present, a
vacuum may be employed after the reaction has been completed, but for fast-
curing systems some of the volatile matter (mainly low molecular weight
phenolic bodies) may be retained.
The end point may be checked by noting the extent of flow of
a
heated pellet
down a given slope or by melting point measurements. Other control tests
include alcohol solubility, free phenol content and gelation time with
10%
hexa.
Moulding
Powders

645
In
the manufacture of resols a molar excess of formaldehyde (1.5-2.0: 1) is
reacted with the phenol in alkaline conditions.
In
these conditions the
formation of the phenol alcohols is quite rapid and the condensation to a resol
may take less than an hour.
A
typical charge for a laboratory-scale preparation
would be:
Phenol
94
g
(1 mole)
Formalin
(40%)
0.88
ammonia
4
cm3
112 cm3
(1.5
moles formaldehyde)
The mixture is refluxed until the reaction has proceeded sufficiently. It may
then be neutralised and the water formed distilled
off,
usually under reduced
pressure to prevent heat-hardening of the resin. Because of the presence of
hydroxymethyl groups the resol has a greater water-tolerance than the

novolak.
The reaction may be followed by such tests as melting point, acetone or
alcohol solubility, free phenol content or loss in weight
on
stoving at 135°C.
Two classes of resol are generally distinguished, water-soluble resins prepared
using caustic soda as catalyst, and spirit-soluble resins which are catalysed by
addition
of
ammonia. The water-soluble resins are usually only partially
dehydrated during manufacture to give an aqueous resin solution with a solids
content of about
70%.
The solution viscosity can critically affect the success in
a given application. Water-soluble resols are used mainly for mechanical grade
paper and cloth laminates and in decorative laminates.
In
contrast
to
the caustic soda-catalysed resols the spirit-soluble resins have
good electrical insulation properties.
In
order to obtain superior insulation
characteristics a cresol-based resol is generally used.
In
a typical reaction the
refluxing time is about 30 minutes followed by dehydration under vacuum for
periods up
to
4

hours.
23.5
MOULDING
POWDERS
Novolaks are most commonly used in the manufacture of moulding powders
although resols may be used for special purposes such as in minimum odour
grades and for improved alkali resistance. The resins are generally based
on
phenol since they give products with the greatest mechanical strength and speed
of cure, but cresols may be used in acid-resisting compounds and phenol-cresol
mixtures in cheaper compositions. Xylenols are occasionally used for improved
alkali resistance.
The resols may be hardened by heating and/or by addition of catalysts.
Hardening of the novolaks may be brought about by addition of hexamethy-
lenetetramine (hexa, hexamine). Because
of
the exothermic reaction
on
hardening (cure) and the accompanying shrinkage, it is necessary to incorpo-
rate inert materials (fillers) to reduce the resin content. Fillers are thus
generally necessary to produce useful mouldings and are not incorporated
simply to reduce cost. Fillers may give additional benefits such as improving
the shock resistance.
Other ingredients may
be
added to prevent sticking to moulds (lubricants), to
promote the curing reaction (accelerators), to improve the flow properties
(plasticisers) and to colour the product (pigments).
646
Phenolic Resins

/N\
23.5.1
Compounding Ingredients
It is thus seen that a phenol-formaldehyde moulding powder will contain the
following ingredients:
(1)
Resin.
(2)
Hardener (with Novolaks).
(3)
Accelerator.
(4)
Filler.
(5)
Lubricant.
(6)
Pigment.
(7)
Plasticiser (not always used).
In addition to the selection of phenol used and the choice between novolak and
resol there is a number of further variations possible in the resin used. For
example, in the manufacture of a novolak resin slight adjustment of phenol/
formaldehyde ratio will affect the size of novolak molecule produced. Higher
molecular weight novolaks give a stiff-flow moulding powder but the resin being
of lower reactivity, the powders have a longer cure time.
A
second variable is the
residual volatile content. The greater the residual volatiles (phenolic bodies) the
faster the cure. Thus a fast-curing, stiff-flow resin may be obtained by using a
phenol/formaldehyde ratio leading to larger molecules and leaving some of the

low molecular weight constituents in the reaction mixture. Yet another
modification may be achieved by changing the catalyst used. Thus whereas in the
normal processes, using oxalic acid catalysts, the initial products are
p-p-
and
o-p-diphenylmethanes, under other conditions it is possible to achieve products
which have reacted more commonly in the ortho-position. Such resins thus have
the p-position free and, since this
is
very reactive to hexa, a fast-curing resin is
obtained.
Hexa is used almost universally as the hardener. It is made by passing a slight
excess of ammonia through a lightly stabilised aqueous solution of formalde-
hyde, concentrating the liquor formed and crystallising out the hexa
(Figure
23.17).
Between
10
and
15
parts of hexa are used in typical moulding compositions.
The mechanism by which it cross-links novolak resins is not fully understood but
it appears capable of supplying the requisite methylene bridges required for
cross-linking. It also functions as a promoter for the hardening reaction.
6CH,O
+
4NH,
Figure
23.17
Moulding

Powders
641
Basic materials such as lime or magnesium oxide increase the hardening rate
of novolak-hexa compositions and are sometimes referred to as accelerators.
They also function as neutralising agents for free phenols and other acidic
bodies which cause sticking to, and staining of, moulds and compounding
equipment. Such basic substances also act as hardeners for resol-based
compositions.
Woodflour, a fine sawdust preferably obtained from softwoods such as pine,
spruce and poplar, is the most commonly used filler. Somewhat fibrous in nature,
it is not only an effective diluent for the resin to reduce exothei-m and shrinkage,
but it is also cheap and improves the impact strength of the mouldings. There is
a good adhesion between phenol-formaldehyde resin and the woodflour and it is
possible that some chemical bonding may occur.
Another commonly employed low-cost organic filler is coconut shell flour.
This can be incorporated into the moulding composition in large quantities and
this results in cheaper mixes than when woodflour is used. The mouldings also
have a good finish. However, coconut shell flour-filled mouldings have poor
mechanical properties and hence the filler is generally used in conjunction with
woodflour.
For better impact strength cotton flock, chopped fabric or even twisted cord
and strings may be incorporated. The cotton flock-filled compounds have the
greatest mouldability but the lowest shock resistance whilst the twisted cords and
strings have the opposite effect. Nylon fibres and fabrics are sometimes used to
confer strength and flexibility and glass fibres may be used for strength and
rigidity.
Asbestos may be used for improved heat and chemical resistance and silica,
mica and china clay for low water absorption grades. Iron-free mica powder is
particularly useful where the best possible electrical insulation characteristics are
required but because of the poor adhesion of resin to the mica it is usually used

in conjunction with a fibrous material such as asbestos. Organic fillers are
commonly used in a weight ratio of
1
:
1
with the resin and mineral fillers in the
ratio
1.5:
1.
In some countries the extensive use of asbestos as a filler is somewhat
discouraged because of the hazards associated with its use. In other parts of the
world moulding compositions of enhanced heat resistance have been developed
by the use
of
especially heat-resisting polymers used in conjunction with asbestos
and other mineral fillers.
Stearic acid and metal stearates such as calcium stearate are generally used as
lubricants at a rate of about
1-3%
on the total compound. Waxes such as
carnauba and ceresin or oils such as castor oil may also be used for this
purpose.
In order that the rate of cure of phenolic moulding compositions is sufficiently
rapid to be economically attractive, curing is carried out at a temperature which
leads to the formation of quinone methides and their derivatives which impart a
dark colour to the resin. Thus the range of pigments available is limited to blacks,
browns and relatively dark blues, greens, reds and oranges.
In some moulding compositions other special purpose ingredients may be
incorporated. For example, naphthalene, furfural and dibutyl phthalate are
occasionally used as plasticisers or more strictly as flow promoters. They are

particularly useful where powders with a low moulding shrinkage are required.
In such formulations a highly condensed resin is used
so
that there will be less
reaction, and hence less shrinkage, during cure. The plasticiser is incorporated to
648
Phenolic Resins
Table
23.1
Novolak resin
Hexa
Magnesium oxide
Magnesium stearate
Nigrosine
dye
Woodflour
Mica
Cotton flock
Textile shreds
Asbestos
GP
grade
IO0
12.5
3
2
4
100
-
-

-
-
Electrical
grade
100
14
2
2
3
120
-
-
-
40
Medium shock-
resisting grade
100
12.5
High shock-
resisting grade
100
17
2
3.3
3
-
-
-
150
-

the extent of about
1%
to give these somewhat intractable materials adequate
flow properties.
Some typical formulations are given in
Table
23.1.
23.5.2
Compounding
of
Phenol-Formaldehyde Moulding Compositions
Although there are many variants in the process used for manufacturing
moulding powders, they may conveniently be classified into dry processes and
wet processes.
In
a typical
dry
process, finely ground resin is mixed with the other ingredients
for about 15 minutes in a powder blender. This blend is then fed
on
to a heated
two-roll mill. The resin melts and the powdery mix is fluxed into a leathery hide
which bands round the front roll. The temperatures chosen are such that the front
roll is sufficiently hot to make the resin tacky and the real roll somewhat hotter
so
that the resin will melt and be less tacky. Typical temperatures are 70-100°C
for the front roll and 100-120°C for the back.
As
some further reaction takes
place

on
the mill, resulting in a change of melting characteristics, the roll
temperatures should be carefully selected for the resin used. In some processes
two mills may be used in series with different roll temperatures to allow greater
flexibility in operation.
To
achieve consistency in the end-product a fixed mixing
schedule must be closely followed. Milling times vary from 10 minutes down to
a straight pass through the mill.
The hide from the mill is then cooled, pulverised with a hammer-mill and the
resulting granules are sieved.
In
a typical general purpose composition the
granules should pass a 14
X
26
sieve.
For
powders to be used in automatic
moulding plant fine particles are undesirable and
so
particles passing a
100
X
41
sieve (in a typical process) are removed.
In
addition to being more suitable for
automatic moulding machines these powders
are

also more dust-free and thus
more pleasant
to
use.
For
ease of pelleting, however, a proportion
of
‘fines’ is
valuable.
For the manufacture of medium-shock-resisting grades the preblend of resin,
filler and other ingredients does not readily form a hide
on
the mill rolls.
In
this
case the composition is preblended in an internal mixer before passing
on
to the
mills.
Moulding Powders
649
Extrusion compounders such as the Buss KO-Kneader have been used for
mixing phenolic resins. It is claimed that they produce in some respects a better
product and are more economical to use than mill-mixers.
High-shock grades cannot be processed on mills or other intensive mixers
without destroying the essential fibrous structure of the filler.
In
these cases a wet
process is used in which the resin
is

dissolved in a suitable solvent, such as
industrial methylated spirits, and blended with the filler and other ingredients in
a dough mixer. The resulting wet mix is then laid out
on
trays and dried in an
oven.
23.5.3
Processing Characteristics
As
it is a thermosetting material, the bulk of phenol-formaldehyde moulding
compositions has in the past been largely processed on compression and transfer
moulding plant, with a very small amount being extruded. The injection
moulding process as modified for thermosetting plastic is now being used
significantly but still
on
a smaller scale than the traditional processes.
Moulding compositions are available
in
a number of forms, largely determined
by the nature
of
the fillers used. Thus mineral-filled and woodflour-filled grades
are generally powders whilst fibre-filled grades may be of a soft-lumpy texture.
Fabric-filled grades are sold in the form of shredded impregnated ‘rag’. The
powder grades are available in differing granulations. Very fine grades are
preferred where there is a limited flow in moulds and where a high-gloss finish
is required. Fine powders are, however, dusty and a compromise may be sought.
For mouldings in which extensive flow will occur, comparatively coarse (and
thus dust-free) powders can be used and a reasonable finish still obtained. For the
best pelleting properties it would appear that some ‘fines’ are desirable for good

packing whilst ‘fines’ are generally undesirable in powders employed in
automatic compression moulding.
Since the resins cure with evolution of volatiles, compression moulding is
carried out using moulding pressures of 1-2 ton/in2 (15-30MPa) at 155- 170°C.
In
the case of transfer moulding, moulding pressures are usually somewhat
higher, at 2-8 todin’ (30-120 MPa).
As
with other thermosetting materials an
increase in temperature has two effects. Firstly, it reduces the viscosity of the
molten resin and, secondly, it increases the rate
of
cure.
As
a result of these two
effects it is found that in a graph of extent of flow plotted against temperature
there is a temperature of maximum flow
(Figure
23.18).
c
ic
w
c
x
Y
EEB
100
I20
140
160

I80
TEMPERATURE
IN
‘C
Figure
23.18.
Dependence of the extent
of
flow on temperature for a general purpose phenolic resin.
Curves of this type may be obtained from measurements made on widely different pieces of
equipment, e.g. the Rossi-Peakes flow tester and the flow disc. Thus no scale has been given for the
vertical axis
650
Phenolic Resins
There is
no
entirely satisfactory way of measuring flow.
In
the
BS
2782 flow
cup test
an
amount of moulding powder is added to the mould to provide between
2
and 2.5g
of
flash. The press is closed at a fixed initial rate and at a fixed
temperature and pressure. The time between the onset of recorded pressure and
the cessation

of
flash (Le. the time at which the mould has closed)
is
noted. This
time is thus the time required to move a given mass of material a fixed distance
and is thus
a
measure of viscosity. It is not a measure of the time available for
flow. This property, or rather the more important ‘length of flow’ or extent of
flow, must be measured by some other device such as the flow disc or by the
Rossi-Peakes flow test, neither of which are entirely satisfactory. Cup flow times
are normally
of
the order of 10-25 seconds if measured by the
BS
specification.
Moulding powders are frequently classified as being of ‘stiff flow’ if the cup
flow time exceeds 20 seconds, ‘medium flow’ for times of
13-19
seconds and
‘soft flow’ or ‘free flow’ if under
12
seconds.
The bulk factor (i.e. ratio of the density of the moulding to the apparent
powder density) of powder is usually about 2-3 but the high-shock grades may
have bulk factors of 10-14 when loose, and still as high as 4-6 when packed in
the mould. Powder grades are quite easy to pellet, but this is difficult with the
fabric-filled grades.
Phenol-formaldehyde moulding compositions may be preheated by high-
frequency methods without difficulty. Preheating, by this or other techniques,

will reduce cure time, shrinkage and required moulding pressures. Furthermore,
preheating will enhance the ease of flow, with consequent reduction in mould
wear and danger of damage to inserts.
Moulding shrinkage
of
general purpose grades is in the order of 0.005-0.08 in/
in. Highly loaded mineral-filled grades have a lower shrinkage whilst certain
grades based
on
modified resins, e.g. acid-resistant and minimum odour grades,
may have somewhat higher shrinkage values.
Cure times will depend on the type of moulding powder used, the moulding
temperature, the degree
of
preheating employed and, most important,
on
the end-
use envisaged for the moulding. The time required to give the best electrical
insulation properties may not coincide with the time required, say, for greatest
hardness. However, one useful comparative test is the minimum time required to
mould a blister-free flow cup under the
BS
771 test conditions. For general
purpose material this is normally about 60 seconds but may be over twice this
time with special purpose grades.
One
of
the disadvantages of thermosetting plastics which existed for many
years was that whilst the common moulding processes for thermoplastics were
easily automated this was much more difficult with thermoset compression

moulding. With the development
of
the reciprocating single-screw injection
moulding machines, equipment became available which facilitated the adoption
of injection moulding to thermosets. In this adapted process the thermosetting
granules are carefully heated in the barrel
so
that they soften but do not cross-link
before entering the mould cavity. The moulds are, however, heated to curing
temperatures
so
that once the mould is filled cure is as fast
as
possible consistent
with obtaining the best balance of properties in the end-product.
As
a result
of
these considerations, typical injection moulding conditions
are:
Melt temperature 110-140°C
Cylinder temperature 65-90°C
Nozzle temperature 85-1 20°C
Moulding
Powders
65
1
Mould temperature 165-195°C
Injection pressure 85-250 MPa
Screw back pressure

Screw speed 65-85 rev/min
<7
(typically 1)MPa
Curing time 15-80
s
In order to obtain
a
good control of cylinder temperature, a fluid heat transfer
system is desirable. Such fluid may be heated in an adjacent temperature
controller or perhaps more commonly be circulated in channels which are built
in between electrical heaters and the barrel chamber. Special temperature-
controlled nozzles are employed to avoid setting
up
either by cooling or cross-
linking whilst moulds are usually electrically heated. Many machines are now
available which may be changed from thermoplastics to thermosetting moulding
and vice versa by a change of the nozzle end-cap and change of screw. For
thermosetting plastics screws often have a low compression ratio and are water
cooled.
There is a slowly resolving but intensive controversy over the relative merits
of compression, transfer and injection moulding. Compared with compression
methods both injection and transfer moulding are advantageous in that they are
more easily automated, mouldings are flash free and have a good surface finish,
it is easier to mould thick and/or void free sections and it is possible to increase
cure rates by frictional heat. It is probably also true that in all these instances
injection moulding has a slight advantage over transfer. Injection moulding can
be very fast and claim has been made that sometimes cycles may be reduced to
one-sixth of the compression moulding time. Pelleting and preheating are also
unnecessary. Yet another advantage is that the thermoplastics moulder may, by
small machine changes, be able to handle a range of materials without the

purchase of compression presses. The increased versatility of the machines can
also give greater flexibility in planning and potentially increase the loading factor
of the equipment.
There are, however, disadvantages to the injection moulding process. Injection
moulding machines are very much more expensive than compression presses and
with the larger sizes injection machines may be several times the price of
compression machines of similar mould size capacities. There may also be
possible technical disadvantages. If not moulded carefully the mouldings may
exhibit inferior and anisotropic mechanical properties, particularly with thin-
walled mouldings. The dimensional stability on heating may be worse and the
shrinkage more variable than occurs with compression moulding. The selection
between compression and injection moulding must therefore be made with care,
with perhaps
a
tendency for injection moulding to be preferred with fairly small,
thick-section long-run mouldings.
Injection moulding compositions have a number of requirements with regard
to granule flow and cure characteristics not always met by conventional
formulations. For example, granules should be free-flowing (i.e. of a narrow
particle size distribution and not too irregular in shape). There are also certain
requirements in terms of viscosity.
The viscosity should quickly reach a suitable value on heating in the barrel. It
should not be too high since it may be difficult
to
fill the mould. At the same time it
should not be
so
low that little heat is generated by friction. At the injection melt
temperature of 100-130°C the compound should have a good stability but should
cure rapidly at the high curing temperatures

as
exist within the mould.