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171
chlorotrimethylsilane together with small amounts of other silanes, silicon
SiO2 + 2C
Si
+ 2CO
(1)
Si + 2Cl
2
SiCl
4
(2)
CH
3
OH + HCl
CH
3
Cl
+ H2O
(3)
MgCl
2
Mg
+ Cl
2
(4)
Formula SiO
2
+ 2C + 2CH
3
OH + 2Cl
2
+ 2Mg
(CH3)2SiO + 2MgCl
2
+ H2O + 2CO
Mol.Wt. 60 24 64 142 48,6
74
190,6 18 56
tetrachloride and high boiling residues.
The reaction products must then be fractionated as in the Grignard
process.
The direct process is less flexible than the Grignard process and is
restricted primarily to the production of the, nevertheless all-important,
methyl- and phenyl-chlorosilanes. The main reason for this is that higher
alkyl halides than methyl chloride decompose at the reaction temperature
and give poor yields of the desired products and also the fact that the copper
catalyst is only really effective with methyl chloride.
In the case of phenylchlorosilanes some modifications are made to
the process. Chlorobenzene is passed through the reaction tube, which
contains a mixture of powdered silicon and silver (10% Ag), the latter as
catalyst. Reaction temperatures of 375-425°C are significantly higher than
for the chloromethylsilanes. An excess of chlorobenzene is used which
sweeps out the high boiling chlorophenylsilanes, of which the
dichlorosilanes are predominant. The unused chlorobenzene is fractionated
and recycled.
The direct process involves significantly fewer steps than the
Grignard process and is more economical in the use of raw materials. This
may be seen by considering the production of chlorosilanes by both
processes starting from the basic raw materials. For the Grignard process the
basic materials will normally be sand, coke, chlorine and methane and the
following steps will be necessary before the actual Grignard reaction:
Rochow has summed the entire Grignard process from basic raw
material to polymer as:
On the other hand only the additional steps (1) and (3) will be
required in the direct process which gives the summarized equation:
172
Formula SiO2 + 2C + 2CH
3
OH
(CH3)2SiO + H
2
Mol.Wt. 60 24 64
74
18 56
The olefin addition method
The basis of this method is to react a compound containing Si–H
groups with unsaturated organic compounds. For example, ethylene may be
reacted with trichlorosilane
CH2=CH2 + SiHCl3 → CH3CH2SiCl3
The method may also be used for the introduction of vinyl groups
CH≡CH + SiHCl3 → CH2=CHSiCl3
The trichlorosilane may be obtained by reacting hydrogen chloride
with silicon in yields of 70% and thus is obtainable at moderate cost. As
the olefins are also low-cost materials this method provides a relatively
cheap route to the intermediates. It is, of course, not possible to produce
chloromethylsilanes by this method.
Sodium condensation method
This method depends on the reaction of an organic chloride with
silicon tetrachloride in the presence of sodium, lithium or potassium.
4RCl + SiCl4 + 8Na → SiR4 + 8NaCl
This reaction, based on the Wurtz reaction, tends to go to
completion and the yield of technically useful chlorosilane is low.
The commercial value of this method is also limited by the hazards
associated with the handling of sodium.
Rearrangement of organochlorosilanes
Several techniques have been devised which provide convenient
methods of converting by-product chlorosilanes into more useful
intermediates. A typical example, valuable in technical-scale work, is the
redistribution of chlorotrimethylsilane and trichloromethylsilane to the
dichlorosilane by reacting at 200-400°C in the presence of aluminium
chloride.
(CH3)3SiCl + CH3SiCl3 ↔ (CH3)2SiCl2
General methods of preparation and properties of silicones
A variety of silicone polymers has been prepared ranging from
low-viscosity fluids to rigid cross-linked resins. The bulk of such materials
are based on chloromethysilanes and the gross differences in physical
173
states depend largely on the functionality of the intermediate.
2(CH
3
)
3
SiCl +
2H
2
O
(CH
3
)
3
SiOSi(CH
3
)
3
+ H
2
O
2(CH
3
)
3
SiOH
Si
CH
3
Cl
Cl
CH
3
H
2
O
Si
O
CH
3
CH
3
Si
CH
3
Cl
Cl
CH
3
HO
Si
OH
CH
3
OH
Si
CH
3
O
O
(CH
3
)3Si
O
is designated M (for monofunctiona
CH
3
Si
CH
3
O
is designated D (for difunctional)
Si
O
CH
3
O
is designated T (for trifunctional)
O
Si
O
O
O
Reaction of chlorotrimethylsilane with water will produce a
monohydroxy compound which condenses spontaneously to form
hexamethyldisiloxane.
Hydrolysis of dimethyldichlorsilane will yield a linear polymer.
Hydrolysis of trichloromethylsilane yields a network structure.
For convenience a shorthand nomenclature is frequently used in
silicone literature where
derivative of silicon tetrachloride, is designated Q.
The tetrafunctional of silica, which may be considered the
174
Thus hexamethylsilane may be referred to as M-M or M2. A linear silicone
Si O
Si
CH
3
O
Si
O
Si
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
(CH
3
)
3
Si
O
Si
(CH
3)3
R/Si 3:1
CH
3
Si
CH
3
CH
3
O
Si
O
CH
3
CH
3
Si
CH
3
CH
3
CH
3
3
R/Si 2.2:1
CH
3
Si
O
O
R/Si 1.5:1
Si
O
CH
3
O Si
CH
3
CH
3
O Si
CH
3
O
O
Si
O
CH
3
CH
3
R/Si 1.5:1
polymer with a degree of polymerisation of n would be referred to as MD
M. The compound would be referred to as TM3.
2
n-
Difficulties arise in characterising commercial branched and
network structures in this way because of their heterogeneity. In these cases
the R/Si ratio (or specifically the CH3/Si ratio in methylsilicones) is a
useful parameter. On this basis the R/Si ratios of four types are given in
Figure 47
Since both Si−О and Si−CH3 bonds are thermally stable it is
predictable that the polydimethylsiloxanes (dimethylsilicones) will have
good thermal stability and this is found to be the case. On the other hand
since the Si−О bond is partially ionic (51%) it is relatively easily broken
by concentrated acids and alkalis at room temperature.
Figure 47
175
The bond angle of the silicone-oxygen-silicon linkage is large
(believed to be about 140-160°) while the siloxane link is very flexible.
There is evidence to indicate that intermolecular forces between
silicone chains are very low. This includes the low boiling points of
organosilicon polymers, the low tensile strength of high molecular weight
polymers even when lightly cross-linked to produce elastomers, the
solubility data, which indicate a low cohesive energy density, and lowtemperature coefficient of viscosity.
1.7.2 Silicone Fluids
The silicone fluids form a range of colourless liquids with
viscosities from 1 to 1000000 centistokes. High molecular weight
materials also exist but these may be more conveniently considered as
gums and rubbers. It is conveinient to consider the fluids in two classes:
(1) Dimethylsilicone fluids.
(2) Other fluids. These other fluids are used only for specialized
purposes and will be considered only in the section on applications.
Preparation
The conversion of the chlorosilane intermediates into polymers is
accomplished by hydrolysis with water followed by spontaneous
condensation. In practice there are three important stages:
(1) Hydrolysis, condensation and neutralization by either a
batchwise or continuous process.
(2) Catalytic equilibration.
(3) Devolatilization.
When batch hydrolysis is being employed a weighed excess
amount of water is placed in a glass-lined jacketed reactor.
Dichlorodimethylsilane is run in through a subsurface dispersion nozzle
and the contents are vigorously agitated. The reaction is carried out under
reflux to prevent loss of volatile components. Although the hydrolysis
reaction itself is endothermic the absorption of the HC1 evolved on
hydrolysis generates enough heat to render the overall reaction exothermic
and it is necessary to control the reaction temperature by circulating a
coolant through the jacket of the reactor. When hydrolysis is complete the
agitation is stopped and the oily polymer layer is allowed to separate from
the dilute acid phase which is then drawn off. The oil is then neutralized in
a separate operation by washing with sodium carbonate solution,
176
decantation and filtration. The condensate at this stage consists of a
mixture of cyclic and linear polymers, and careful control of reactant ratios,
acid concentration, reaction temperature and oil-acid contact time should
be maintained since these will affect the composition of the product, which
should be as constant as possible for further processing. The batch process
has the advantage that these variables are controlled without undue
difficulty.
In the continuous process the chlorosilane and the water are ran
into the suction side of a centrifugal pump. The reacting mixture is then
passed through a loop of borosilicate glass pipe where the hydrolysis is
completed and from there back to the pump. The mixture then passes to a
decanter to allow separation of the two ingredients. The decanting stage is
critical and care must be taken in order to avoid low yields and difficulties
in the neutralization stage which is carried out as in the batch process.
The products of the hydrolysis reaction under normal conditions
will consist of an approximately equal mixture of cyclic compounds,
mainly the tetramer, and linear polymer. In order to achieve a more linear
polymer, but with a random molecular weight distribution, and also to
stabilize the viscosity it is common practice to equilibrate the fluid by
heating with a catalyst such as dilute sulphuric acid. This starts a series of
reactions which would lead to the formation of higher molecular weight
polymer except that controlled amounts of the monofunctional
chlorotrimethylsilane or more usually the dimer, hexamethyldisiloxane
(Me3SiOSiMe3), are added as a “chain stopper” to control molecular
weight, the latter functioning by a trans-etherification mechanism. The
more the chain stopper is added, the lower becomes the average molecular
weight of the equilibrated product. When assessing the amount of chain
stopper to add it is necessary to calculate the amount of trifunctional
material present as an impurity in the fluid before equilibration.
In practice, for fluids of viscosities below 1000 centistokes, the
equilibration reaction will take a number of hours at 100-150°C. Residual
esters and siliconates which may occur during the reaction are hydrolyzed
by addition of water and the oil is separated from the aqueous acid layer
and neutralized as before.
For some applications it is desirable that the fluids be free from the
volatile low molecular products that result from the randomising
equilibration reaction. This operation may be carried out either batchwise
or continuously using a vacuum still. Commercial “non-volatile” fluids
have a weight loss of less than 0.5% after 24 hours at 150°C.
177
General properties
1.1n1.0log +=η
As a class dimethylsilicone fluids are colourless, odourless, of low
volatility and non-toxic. They have a high order of thermal stability and a
fair constancy of physical properties over a wide range of temperature (70°C to 200°C). Although fluids have prolonged stability at 150°C they will
oxidize at 250°C with an increase in viscosity and eventual gelling. The
oxidation rate may, however, be retarded by conventional antioxidants.
The fluids have reasonably good chemical resistance but are
attacked by concentrated mineral acids and alkalis. They are soluble in
aliphatic, aromatic and chlorinated hydrocarbons, which is to be expected
from the low solubility parameter of 14.9MPa
1/2
. They are insoluble in
solvents of higher solubility parameter such as acetone, ethylene glycol and
water. They are themselves very poor solvents.
It has been shown that for linear dimethylsilicones the viscosity (η)
in centistokes at 25°C and the number (n) of dimethylsiloxy groups are
connected by the surprisingly simple relationship
Branched polymers have lower melting points and viscosities than
linear polymers of the same molecular weight. The viscosity of the silicone
fluids is much less affected by temperature than with the corresponding
paraffins.
Applications
Silicone fluids find a very wide variety of applications mainly
because of their water-repellency, anti-stick properties, low surface tension
and thermal properties.
Polish additives
A well-known application of the dimethylsilicone fluids, to the
general public, is as a polish additive. The polishes contain normally 2-4%
of silicone together with a wax which has been formulated either into an
aqueous emulsion or a solution in a volatile solvent. The value of the
silicone fluid is not due to such factors as water-repellency or anti-stick
properties but due to its ability to lubricate, without softening, the
microcrystalline wax plates and enable them to slide past each other, this
being the basis of the polishing process. The effort in polishing a car with a
polish containing silicone fluid is claimed to be less than half that required
with a conventional wax polish. The protective action is at least as good if
not slightly superior.
178
Release agents
Si
CH
3
H
O
O
OH
CH
3
Si Si
CH
3
O
O
Si
O
CH
3
Dilute solutions or emulsions containing 0.5-1% of a silicone fliuid
have been extensively used as a release agent for rubber moulding, having
replaced the older traditional materials such as soap. Similar fluids have
also been found to be of value in the die-casting of metals. Silicones have
not found extensive application in the moulding of thermosetting materials
since the common use of plated moulds and of internal lubricants in the
moulding power obviate the need. Their use has also been restricted with
thermoplastics because of the tendency of the fluids to cause stress
cracking in many polymers
Silicone greases do, however, have uses in extrusion for coating
dies etc., to facilitate stripping down. Greases have also found uses in the
laboratory for lubricating stop cocks and for high-vacuum work.
Water-repellent applications
The silicones have established their value as water-repellent
finishes for a range of natural and synthetic textiles. A number of
techniques have been devised which result in the pick-up of 1-3% of
silicone resin on the cloth. The polymer may be added as a solution, an
emulsion or by spraying a fine mist; alternatively, intermediates may be
added which either polymerize in situ or attach themselves to the fibre
molecules.
In one variation of the process the textile fabric is treated with
either a solution or emulsion of a polymer containing active hydrogen
groups, such as the polymer of dichloromethylsilane. If the impregnated
fabric is heated in the presence of a catalyst such as the zinc salt of an
organic acid or an organotin compound for about five minutes at 100150°C the hydrogen atoms are replaced by hydroxyl groups which then
condense so that individual molecules cross-link to form a flexible waterrepellent shell round each of the fibres:
179
Leather may similarly be made water repellent by treatment with
solutions or emulsions of silicone fluids. A variety of techniques is
available, the method chosen depending to some extent on the type of
leather to be treated. The water repellency may be obtained without
appreciably affecting the ability of a leather to transpire.
Silicone fluids containing Si–H groups are also used for paper
treatment. The paper is immersed in a solution or dilutes emulsion of the
polymer containing either a zinc salt or organo-tin compound. The paper is
then air-dried and heated for two minutes at 80°C to cure the resin. The
treated paper has a measure of water repellency and in addition some antiadhesive properties.
Lubricants and greases
Silicone fluids and greases have proved of use as lubricants for
high-temperature operation for applications depending on rolling friction.
Their use as boundary lubricants, particularly between steel surfaces, is,
however, somewhat limited although improvement may be obtained by
incorporating halogenated phenyl groups in the polymer. Higher working
temperatures are possible if phenyl-methylsilicones are used.
Greases may be made by blending the polymer with an inert filler
such as a fine silica, carbon black or metallic soap. The silicone-silica
greases are used primarily as electrical greases for such applications as
aircraft and car ignition systems.
The fluids are also used in shock absorbers, hydraulic fluids,
dashpots and other damping systems designed for high-temperature
operation.
Miscellaneous
Dimethylsilicone fluids are used extensively as antifoams although
the concentration used in any one system is normally only a few parts per
million. They are useful in many chemical and food production operations
and in sewage disposal.
The use of small amounts of the material in paints and surface
coatings is claimed to help in eliminating faults such as “silking” in
dipping applications and “orange peel” in stoved finishes.
Inter esting graft polymers based on silicone polymers are finding
use in the manufacture of polyurethane foams, particularly, of the
polyether type, because of their value as cell structure modifiers.
Another use in conjunction with other polymers is as a flow
promoter for thermoplastics such as polystyrene.
180
The columns in vapour phase chromatographic apparatus usually
incorporate high molecular weight dimethylsilicone fluids as the stationary
phase.
The fluids have also found a number of uses in medicine. Barrier
creams based on silicone fluids have been found to be particularly useful
against the cutting oils in metal machinery processes which are common
industrial irritants.
1.7.3 Silicone Resins
Preparation
On the commercial scale silicone resins are prepared batchwise by
hydrolysis of a blend of chlorosilanes. In order that the final product shall
be cross-linked, a quantity of trichlorosilanes must be incorporated into the
blend. A measure of the functionality of the blend is given by the R/Si
ratio. Whereas a linear polymer will have an R/Si ratio of just over 2:1, the
ratio when using trichlorosilane alone will be 1:1. Since these latter
materials are brittle, ratios in the range 1.2 to 1.6:1 are used in commercial
practice. Since chlorophenylsilanes are also often used, the CH3/C6H5 ratio
is a further convenient parameter of use in classifying the resins.
The chlorosilanes are dissolved in a suitable solvent system and
then blended with the water which may contain additives to control the
reaction. In the case of methylsilicone resin the overall reaction is highly
exothermic and care must be taken to avoid overheating which can lead to
gelation. When substantial quantities of chlorophenylsilanes are present,
however, it is often necessary to raise the temperature to 70-75°C to effect
a satisfactory degree of hydrolysis.
At the end of the reaction the polymer-solvent layer is separated
from the aqueous acid layer and neutralized. A portion of the solvent is
then distilled off un t il th e c o rre ct s ol id s co n te nt is reached.
The resin at this stage consists of a mixture of cyclic, linear,
branched and cross-linked polymers rich in hydroxyl end-groups, but of a
low average molecular weight. This is increased somewhat through
“bodying” the solution by heating with a catalyst such as zinc octoate at
100°C until the viscosity, a measure of molecular weight at constant solids
content, reaches the desired value.
The resins are then cooled and stored in containers which do not
catalyze further condensation of the resins.