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161
1.6 Furan Resins
CH
CHHC
HC
O
CHO
[H
2
]
Copper
Chromite
CH
CHHC
HC
O
CH
2
OH
The furan or furane resins mainly find use because of their
excellent chemical and heat resistance. In the past they have mainly been
used in applications peripheral to the plastics industry such as foundry
resins, for chemically resistant cements and for binders. Recent
developments have facilitated their use in laminates for chemical plant.
Preparation of intermedia tes
The two intermediates of commercial furan resins are furfural and
furfuryl alcohol. Furfural occurs in the free state in many plants but is
obtained commercially by degradation of hemicellulose constituents
present in these plants. There are a number of cheap sources of furfural,
and theoretical yields of over 20% (on a dry basis) may be obtained from
both corn cobs and oat husks. In practice yields of slightly more than half
these theoretical figures may be obtained. In the USA furfural is produced
in large quantities by digestion of corn cobs with steam and sulphuric acid.
The furfural is removed by steam distillation.
Furfural is a colourless liquid which darkens in air and has a
boiling point of 161.7°C at atmospheric pressure. Its principal uses are as a
selective solvent used in such operations as the purification of wood resin
and in the extraction of butadiene from other refinery gases. It is also used
in the manufacture of phenol-furfural resins and as a raw material for the
nylons. The material will resinify in the presence of acids but the product
has little commercial value.
Catalytic hydrogenation of furfural in the presence of copper
chromite leads to furfuryl alcohol, the major intermediate of the furan
resins:
The alcohol is a mobile liquid, light in colour, with a boiling point
of 170°C. It is very reactive and will resinify if exposed to high
temperatures, acidity, air or oxygen. Organic bases such as piperidine and
n-butylamine are useful inhibitors.
Resinification
Comparatively little is known of the chemistry of resinification of
either furfuryl alcohol or furfural.
162
It is suggested that the reaction occurs initially with furfuryl
O
CH
2
OH
2
H2O
O
CH
2
O
CH
2
OH
+Furfuryl Alcohol
- H
2
O
O
CH
2
O
CH
2
O
CH
2
OH
n
-H
2
O
O
CH
2
O
CH
2
O
CH
2
O
O
CH
2
O
O
CH
2
O
O
O
CH
2
O
CH
2
O
O
O
CH
2
alcohol:
The liberation of small amounts of formaldehyde has been detected
in the initial stage but it has been observed that this is used up during later
reaction. This does not necessarily indicate that formaldehyde is essential
to cross-linking, and it would appear that its absorption is due to some
minor side reaction.
Loss of unsaturation during cross-linking indicates that this
reaction is essentially a form of double bond polymerization:
This reaction, like the initial condensation, is favoured by acidic
conditions and peroxides are ineffective.
The polymerization of furfural is apparently more complex and less
understood.
For commercial use a partially condensed furan resin is normally
prepared which is in the form of a dark free-flowing liquid. Final cure is
carried out in situ.
The liquid resins are prepared either by batch or continuous process
by treating furfuryl alcohol with acid. Initially the reaction mixture is
heated but owing to the powerful exothermic an efficient cooling system is
necessary if cross-linking is to be avoided. Water of condensation is
removed under vacuum and the reaction stopped by adjusting the pH to the
point of neutrality. Great care is necessary to prevent the reaction getting
163
out of hand. This may involve, in addition to efficient cooling, a judicious
choice of catalyst concentration, the use of a mixture of furfuryl alcohol
and furfural which produces a slower reaction but gives a more brittle
product and, possibly, reaction in dilute aqueous solution.
The resins are hardened in situ by mixing with an acidic substance
just before application. A typical curing system would be four parts of
toluene-p-sulphonic acid per 100 parts resin. The curing may take place at
room temperature if the resin is in a bulk form but elevated temperature
cures will often be necessary when the material is being used in thin films
or coatings.
Properties of the cured resins
The resins are cross-linked and the molecular segments between
the cross-links are rigid and inflexible. As a consequence the resins have an
excellent heat resistance, as measured in terms of maintenance of rigidity
on heating, but are rather brittle.
Cured resins have excellent chemical resistance. This is probably
because, although the resins have some reactive groupings, most of the
reactions occurring do not result in the disintegration of the polymer
molecules. Therefore, whilst surface layers of molecules may have
undergone modification they effectively shield the molecules forming the
mass of the resin. The resins have very good resistance to water
penetration.
Compared with the phenolics and polyesters the resins have better
heat resistance, better chemical resistance, particularly to alkalis, greater
hardness and better water resistance. In these respects they are similar to,
and often slightly superior to, the epoxy resins. Unlike the epoxides they
have a poor adhesion to wood and metal, this being somewhat improved by
incorporating plasticizers such as poly(vinyl acetate) and poly(vinyl
formal) but with a consequent reduction in chemical resistance. The cured
resins are black in colour.
Applications
The principal applications for furan resins are in chemical plant.
Specific uses include the lining of tanks and vats and piping and for alkaliresistant tile cements. The property of moisture resistance is used when
paper honeycomb structures are treated with furan resins and subsequently
retain a good compression strength even after exposure to damp conditions.
Laminates have been prepared for the manufacture of chemical
164
plant. They have better heat and chemical resistance than the polyesterepoxide- phenolic- or aminoplastic-based laminates but because of the low
viscosity of the resins were not easy to handle. Because they were also
somewhat brittle, furan-based laminates have been limited in their
applications.
Furane resin-chopped strand mat laminates have tensile strengths
in excess of 140 MPa, a heat distortion temperature of about 218°C and
good fire resistance.
Not only does the material have excellent resistance to burning but
smoke emission values are reported to be much less than for fire-retardant
polyester resin. The laminates are being increasingly used in situations
where corrosion is associated with organic media, where corrosion is
encountered at temperatures above 100°C as in fume stacks and where both
fire retardance and corrosion resistance are desired as in fume ducts.
One other substantial development of the 1960s was the use of
ureaformaldehyde-furfuryl alcohol materials as foundry resins, particularly
for “hot-box” operations. The furfuryl alcohol component of the resin is
usually in the range 25-40%.
Furane resins are useful in impregnation applications. Furfural
alcohol resinified in situ with zinc chloride catalysts can be used to
impregnate carbon (including graphite) products and be cured at 93-150°C
to give products of greater density and strength and which have much
lower permeability to corrosive chemicals and gases.
The resins are also used for coating on to moulds to give a good
finish that is to be used for polyester hand-lay up operations.
Development work by Russian workers had led to interesting
products formed by reaction of furfuryl alcohol with acetone and with
aniline hydrochloride. The resins formed in each case have been found to
be useful in the manufacture of organic-mineral non-cement concretes with
good petrol, water and gas resistance. They also have the advantage of
requiring only a small amount of resin to act as a binder.
1.7 Organoelement Polymers
1.7.1 Silicones
To many polymer chemists one of the most fascinating
developments of the last 80 years has been the discovery, and the attendant
commercial development, of a range of semi-inorganic and wholly
inorganic polymers, including the silicone polymers. Because of their
165
general thermal stability, good electrical insulation characteristics,
C2H5O Si
C2H
5
C2H
5
O
C2H
5
H2O
Acid
O
Si
C
2H5
C2H
5
+ C2H5OH
constancy of properties over a wide temperature range, water-repellency
and anti-adhesive properties, the silicone polymers find use in a very wide
diversity of applications. Uses range from high-temperature insulation
materials and gaskets for jet engines to polish additives and water repellent
treatments for leather. The polymers are available in a number of forms
such as fluids, greases, rubbers and resins.
The possibility of the existence of organosilicone compounds was
first predicted by Dumas in 1840, and in 1857 Buff and Wohler found the
substance now known to be trichlorosilane by passing hydrochloric acid
gas over a heated mixture of silicone and carbon. In 1863 Friedel and
Crafts prepared tetraethylsilane by reacting zinc diethyl with silicon
tetrachloride.
2Zn(C2H5)2 + SiCl4 → Si(C2H5)4 + 2ZnCl
2
In 1872 Ladenburg produced the first silicone polymer, a very
viscous oil, by reacting diethoxydiethylsilane with water in the presence of
traces of acid.
Professor F.S.Kipping at the University College, Nottingham, between the
years 1899 and 1944. During this period Kipping published a series of 51
main papers and some supplementary studies, mainly in the Journal of the
Chemical Society. The work was initiated with the object of preparing
asymmetric tetrasubsituted silicon compounds for the study of optical
rotation. Kipping and his students were concerned primarily with the
preparation and study of new non-polymeric compounds and they were
troubled by oily and glue-like fractions that they were unable to crystallize.
the modern silicone industry. In 1904 he introduced the use of Grignard
reagents for the preparation of chlorosilanes and later discovered the
principle of the inter-molecular condensation of the silane diols, the basis
of current polymerization practice. The term silicone was also given by
Kipping to the hydrolysis products of the disubstituted silicon chlorides
because he at one time considered them as being analogous to the ketones.
The basis of modern silicone chemistry was, however, laid by
Nevertheless Kipping made a number of contributions of value to
In 1931 J.F.Hyde of the Corning Glass Works was given the task
166
of preparing polymers with properties intermediate between organic
polymers and inorganic glasses. The initial objective was a heat-resistant
resin to be used for impregnating glass fabric to give a flexible electrical
insulating medium. As a result silicone resins were produced. In 1943 the
Corning Glass Works and the Dow Chemical Company co-operated to
form the Dow Corning Corporation, which was to manufacture and
develop the organo-silicon compounds. In 1946 the General Electric
Company of Schenectady, NY also started production of silicone polymers
using the then new “Direct Process” of Rochow. The Union Carbide
Corporation started production of silicones in 1956.
There are at present about a dozen manufacturers outside the
Communist bloc. Amongst major producers, in addition to those already
mentioned, are Bayer, Rhone-Poulenc, Wacker-Chemie, Toshiba, Toray
and Shinetsu.
During the 1970s growth rates for the silicones were higher than
for many other commercial polymers, generally showing an annual rate of
growth of some 10-15%. In part this is due to the continual development of
new products, in part to the increasingly severe demands of modern
technology and in part because of favourable ecological and toxicological
aspects in the use of silicones. In the early 1980s world capacity excluding
the Eastern bloc was assessed at about 270000 tonnes per annum, being
dominated by the USA (41%) with Western Europe taking about 33% and
Japan 17%.
Nomenclature
Before discussing the chemistry and technology of silicone
polymers it is necessary to consider the methods of nomenclature of the
silicon compounds. The terminology used will be that adopted by the
International Union of Pure and Applied Chemistry.
The structure used as the basis of the nomenclature is silane SiH
4
corresponding to methane CH4. Silicon hydrides of the type
SiH3(SiH2)nSiH3 are referred to as disilane, trisilane, tetrasilane etc.,
according to the number of silicon atoms present.
Alkyl, aryl, alkoxy and halogen subsituted silanes are referred to
by prefixing “silane” by the specific group present. The following are
typical examples:
(CH3)2SiH2 dimethylsilane
CH3SiCl3 trichloromethysilane
(C6H5)3SiC2H5 ethyltriphenylsilane
167
Compounds having the formula SiH3(OSiH2)nOSiH3 are referred to
as disiloxane, trisiloxane etc., according to the number of silicon atoms.
Polymers in which the main chain consists of repeating –Si–О– groups
together with predominantly organic side groups are referred to as
polyorganosiloxanes or more loosely as silicones.
Hydroxy derivatives of silanes in which the hydroxyl groups are
attached to a silicon atom are named by adding the suffices -ol, -diol, -triol
etc., to the name of the parent compound. Examples are:
H3SiOH silanol
H2Si(OH)2 silanediol
(CH3)3SiOH trimethylsilanol
(C6H5)2(C2H5O)SiOH ethoxydiphenylsilanol
Nature of chemical bonds containing silicon
Silicon has an atomic number of 14 and an atomic weight of 28.06.
It is a hard, brittle substance crystallizing in a diamond lattice and has a
specific gravity of 2.42. The elemental material is prepared commercially
by the electrothermal reduction of silica.
Silicon is to be found in the fourth group and the second short
period of the Periodic Table. It thus has a maximum covalency of six
although it normally behaves as a tetravalent material. The silicon atom is
more electropositive than the atoms of carbon or hydrogen. The
electronegativity of silicon is 1.8, hydrogen 2.1, carbon 2.5 and oxygen 3.5.
It has a marked tendency to oxidize, the scarcity of naturally occurring
elemental silicon providing an excellent demonstration of this fact.
At one time it was felt that it would be possible to produce silicon
analogues of the multiplicity of carbon compounds which form the basis of
organic chemistry. Because of the valency difference and the
electropositive nature of the element this has long been known not to be
the case. It is not even possible to prepare silanes higher than hexasilane
because of the inherent instability of the silicon-silicon bond in the higher
silanes.
The view has also existed in the past that the carbon-silicon bond
should be similar in behaviour to the carbon-carbon bond and would have a
similar average bond energy. There is some measure of truth in the
assumption about average bond energy but because silicon is more
electropositive than carbon the С–Si bond will be polar and its properties
will be very dependent on the nature of groups attached to the carbon and
silicon groups. For example, the CH3–Si group is particularly resistant to
oxidation but C6H13–Si is not.
168
The polarity of the silicon-carbon bond will affect the manner in
which the reaction with ions and molecules takes place. For example, on
reaction with alkali, or in some conditions with water, it is to be expected
that the negative hydroxyl ion will attack the positive silicon atom rather
than the negative carbon atom to form, initially, Si–OH bonds. Reaction
with hydrogen chloride would lead similarly to silicon-chlorine and
carbon-hydrogen bonds.
It is important to realise that the character of substituents on either
the carbon or silicon atoms will greatly affect the reactivity of the carbonsilicon bond according to its effect on the polarity. Thus strongly negative
substituents, e.g. trichloromethyl groups, attached to the carbon atom, will
enhance the polarity of the bond and facilitate alkaline hydrolysis. A
benzene ring attached to the carbon atom will also cause an electron shift
towards the carbon atom and enhance polarity. Hydrogen chloride may
then effect acid cleavage of the ring structure from the silicon by the
electronegative chlorine attacking the silicon and the proton attacking the
carbon.
The foregoing facts of relevance to the preparation and properties
of silicone polymers may be summarized as follows:
(1) Silicon is usually tetravalent but can assume hexavalent
characteristics.
(2) Silicon is more electropositive than carbon and hence silicon-
carbon bonds are polar (12% ionic).
(3) The reactivity of the Si–С bond depends on the substituent
group attracted to the Si and С atoms.
(4) The reactivity also depends on the nature of the attacking
molecule.
Two further statements may also be made at this stage.
(5) Inclusion of silicon into a polymer does not ensure by any
means a good thermal stability.
(6) T he siloxane Si–О link has a number of interesting properties
which are relevant to the properties of the polyorganosiloxanes.
Preparation of intermedia tes
The polyorganosiloxanes are generally prepared by reacting
chlorosilanes with water to give hydroxyl compounds which then condense
to give the polymer structure, e.g.
169
Cl Si Cl
R
R
+ H2O
HO
Si
OH
R
R
1
1
(
Si
O
)
R
R
1
Similar reactions can also be written for the alkoxysilanes but in
CH3Cl + Mg
CH
3
MgCl
Ether
CH
3
MgCl + SiCl4
CH
3
SiCl
3
+ MgCl
2
CH
3
MgCl + CH3SiCl
3
(CH
3
)2SiCl + MgCl
2
CH3MgCl + (CH
3)2
SiCl
2
(CH3)
3
SiCl
+ MgCl
2
commercial practice the chlorosilanes are favoured. These materials may
be prepared by many routes, of which four appear to be of commercial
value, the Grignard process, the direct process, the olefin addition method
and the sodium condensation method.
The Grignard method
The use of the Grignard reagents of the type RMgX for the
production of alkyl- and aryl-chlorosilanes was pioneered by Kipping in
1904 and has been for a long time the favoured laboratory method for
producing these materials.
The reaction is carried out by first reacting the alkyl or aryl halide
with magnesium shavings in an ether suspension and then treating with
silicon tetrachloride (prepared by passing chlorine over heated silicon).
With methyl chloride the following sequence of reactions occur:
differences in the reactivities of the intermediates a high yield of
dichlorodimethylsilane is produced.
to remove the magnesium chloride, followed by distillation. It is then
necessary to distil fractionally the chlorosilanes produced. The fractional
distillation is a difficult stage in the process because of the closeness of the
boiling points of the chlorosilanes and some by-products (Table 15) and
80-100 theoretical plates are necessary to effect satisfactory separation.
production of silicone intermediates. Its great advantage is its extreme
flexibility since a wide range of organic groups may be attached to the
silicon in this method. Because of the need to use ether or other
inflammable solvents considerable production hazards arise. On economic
The reaction proceeds in a stepwise manner but because of the
The products are recovered from the reaction mixture by filtration
The Grignard method was the first route used commercially in the
170
grounds the main drawbacks of the process are the multiplicity of steps and
Compound
Boiling point, °C
(CH3)2SiCl2
70
the dependence on silicon tetrachloride, which contains only 16% Si and is
thus a rather inefficient source of this element.
Table 15 – Boiling point of some chlorosilanes and related compounds
CH3SiCl3
(CH
CH
SiCl
(CH
direct process. In 1945 Rochow found that a variety of alkyl and aryl
halides may be made to react with elementary silicon to produce the
corresponding organosilicon halides.
silicon is finely divided. The inclusion of certain solid catalysts in the
reactive mass may in some instances greatly facilitate the reaction. A
mixture of powdered silicon and copper in the ratio 90:10 is used in the
manufacture of alkyl chlorosilanes.
are passed through a heated mixture of the silicon and copper in a reaction
tube at a temperature favourable for obtaining the optimum yield of the
dichlorosilane, usually 250-280°C. The catalyst not only improves the
reactivity and yield but also makes the reaction more reproducible.
Presintering of the copper and silicon or alternatively deposition of copper
on to the silicon grains by reduction of copper (I) chloride is more effective
than using a simple mixture of the two elements. The copper appears to
function by forming unstable copper methyl, CuCH3, on reaction with the
methyl chloride. The copper methyl then decomposes into free methyl
radicals which react with the silicon.
is used the crude product from the reaction tube will be composed of about
73.5% dimethyldichlorosilane, 9% trichloromethylsilane and 6%
65.7
SiCl
3)3
SiHCl2
3
4
Si
3)4
57
41
57.6
26
The direct process
The bulk of the methylsilicones are today manufactured via the
Si + RX → RnSiX
(n = 0-4)
4-n
The hydrocarbon can be in either the liquid or vapour phase and the
In practice vapours of the hydrocarbon halide, e.g. methyl chloride,
Under the most favourable reaction conditions when methyl chloride