Note: Descriptions are shown in the official language in which they were submitted.
1:~A,5'1 80
This invention relates to a process for producing alpha,omega-
diacyloxysiloxanes. This application is a division of copending applica-
tion Serial No. 289,796 filed October 28, 1977.
It is known that linear alpha,omega-siloxanediols, hereinafter
called siloxanediols, may be prepared by the hydrolysis of dimethyldi-
chlorosilane. Depending on temperature, amo~mt of water, the nature of
solvent, if any, and catalysts, siloxanediols of a wide range of chain
length may be obtained. However, it is difficult to obtain siloxanediols
of very short chain lengths by this process, because of their tendency to
condense to longer chains under the influence of the by-product acid.
Normally hydrolysis produces an average chain length of over 30 dimethyl-
siloxane units, corresponding to a hydroxyl content of 1.5 percent or less.
The process also produces a large amount, up to 50 percent or more, of
cyclic siloxanes.
A somewhat better process involves equilibration of linear or
cyclic siloxane with dimethyldîchlorosilane and an acid catalyst, to pro-
duce an alpha,omega-dichlorosiloxane, followed by hydrolysis to give the
siloxanedîol. By this process, the hydroxyl content may be increased to
2.~ percent, corresponding to a chain of 18 dimethylsiloxane units. If a
solvent is used, the hydroxyl content may be increased to 3 percent (15
dimethylsiloxane units?. The problem lies in the rapid condensation of the
intermed;ate alpha-chloro-omega-siloxanol during the hydrolysis step; the
shorter the chain, the more rapîd the condensation.
Therefore it is an ob~ect of one broad aspect of this invention
to provide a process for the controlled preparation of alpha,omega-diacyl-
oxysiloxanes.
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By a broad aspect of this invention, a process is provided
for making alpha, omega-diacyloxysiloxanes of the formula
R'COO(R2SiO)nOCR', in which at least 50 percent of the R groups are
methyl and the remaining R groups are selected from the class con-
sisting of vinyl and phenyl groups, R' is a hydrocarbon radical
having from 1 to 3 carbon atoms, and n has an average value of from
3 to 25, which comprises: heating a siloxane selected from the
class consisting of cyclic siloxanes, linear siloxanes and mixtures
thereof with an acid anhydride having from 4 to 8 carbon atoms and
a carboxylic acid having from 2 to 4 carbon atoms in the presence
of an acid clay for at least 1 hour until equilibration occurs at
a temperature of from 120C to 150C.
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By a variant thereof, the alpha,omega-diacyloxysiloxane is
alpha,omega-diacetoxysiloxane.
Cyclic siloxanes used in the above-recited process generally
have the formula (R2SiO) , in which x is a number of from 3 to 10, pre-
ferably from 3 to 6, and R is as defined above.
Linear siloxanes used in the above-recited process generally
have the formul~
HO~R2SiO) H or R'COO(R2SiO) OCR'
in which y has an average value of from 2 to 300 and preferably from 2
to 50.
The siloxanes may contain a minor amount of a silane of the
formula
R2Si)OH)2 or R2Si(OCOR )2
Examples of suitable cyclic siloxanes include cyclic dimethyl-
siloxanes of the formula [(CH3)2SiO] where x is from 3 to 6; cyclic
methyl vinyl siloxanes, e.g., heptamethylvinylcyclotetrasiloxane and
tetramethyltetravinylcyclotetrasiloxane; and cyclic methyl phenyl
siloxanes.
Examples of suitable linear siloxanes include the hydrolysis
products and cohydrolysis products of dimethyldichlorosilane, methylvinyl-
dichlorosilane, methylphenyldichlorosilane, and diphenyldichlorosilane.
Examples of suitable silanes include dimethyldiacetoxysilane,
methylvinyldiacetoxysilane, methylphenyldiacetoxysilane, diphenyldiacetoxy-
silane and diphenylsilanediol.
The carboxylic acids used in providing the alpha,omega-siloane-
diols of aspects of this inventio~ are preferably the water-soluble unsub-
stituted monobasic acids having from 2 to 4 carbon atoms. Likewise the
7~
acid anhydrides are preferably those with from 4 to 8 carbon atoms.
Examples of suitable acids and anhydrides include acetic, propionic,
butyric, acrylic and crotonic.
When the siloxane, i.e., a cyclic and/or linear siloxane
is equilibrated with, for example, acetic anhydride, the chain
length of the resulting diacetoxysiloxane is determined mainly by
the mole ratio of acetic anhydride to siloxane units, the reaction
being essentially:
n(R2SiO) + ~CH3C0)20 ~ CH3COO(R2SiO)nCOCH3
where n is the same as above, and the mole ratio of R2SiO to acetic
anhydride may be varied between 3:1 and 25:1.
The carboxylic acid acts as a solvent and cocatalyst, but has
little effect on the equilibrium chain length. The amount used is not
critical, but it should be between 2 and 20 percent of the total reaction
n~xture. In many cases one mole of acid per mole of anhydride is satis-
factory.
The principal catalyst is an acid clay prepared by treatingclay with sulfuric acid. Suitable grades include FILTRGL 13 (fine)
and FILTROL 24 (coarse), FILTROL being the Trade Mark of acid clays
of Filtrol Corporation. The amount required is not critical, but
good results are obtained with from 0,5 to 2 percent of the total
reaction mixture.
If the initial silane or siloxane has a high hydroxyl content
some changes must be made in the ratio of reactaats. Bearing in mind that
two hydroxyl groups generate one molecule of ~Jater~ which destroys one
molecule of acid anhydride, the amount of the latter must be increased
accordingly. In other words, for every mole of water formed by condensation,
or which may be present as an impurity, one additional mole of acid
anhydride must be employed. In some cases enough carboxylic acid is
generated so that none need be added.
The equilibration times and temperatures are inversely related.
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~o to five hours at reflux temperature, approxim~tely 140C., is generally
sufficient. ~len FILTROL 13 is used, ten (10) hours is required at 130C.
and twellty (20) hours at 120C. Slightly longer times are required with
FILTR01 24. Shorter times are sufficient is the equilibration is carried
out at temperatures up to 150C. under slight pressure, or if the amount
of catalyst is increased. The reaction may be carried out at any tempera-
ture between 100 and 200C., but the preferred range is from 120C. to
150C.
The equilibrated material is coolèd to room temperature and the
unreacted carboxylic acid is removed by washing several times with water.
If desired, sodium chloride or other salt may be added to facilitate phase
separation by increasing the density of the water layer. Normally the
acid clay is wetted by the water and is removed with the water. Alter-
natively, it may be removed first by filtration.
~ le washed material still contains most of the acyloxy end
groups, as well as some free carboxylic acid. At this stage, it is some-
what unstable, as any prematurely formed hydroxyl groups tend to condense
with residual acyloxy groups, thus producing longer-chain siloxanes. The
rate is somewhat variable, but in general there is only slight loss of end
groups in one hour and very considerable loss in 24 hours. Thus the
washing step should be completed with deliberate speed.
In order to produce a stable siloxanediol as provided by the
parent applic~tion, further hydrolysis is necessary. A siloxanediol of
adequate stability is reached when the total of acyloxy groups and free
carboxylic acid is reduced to less than 0.25 percent by weight of the
siloxanediol.
Although hydrolys;s of acyloxy groups is slow in neutral or acid
~ solution,-it proceeds-more rapidly as the pH is increased, especially if
the temperature is also raised. Very rapid hydrolysis takes place in
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strongly alkaline solutions of pH 13 or more. Such a pH also causes con-
densation however, and severely reduces the final hydroxyl content. On
the other hand, saturated sodium bicarbonate, which has a pH of 8.3, drops
oater to ~ pH of 7.5 as the acid is neutralized, and gives incomplete
neutralization even after 24 hours at room temperature. Somewhat better
results are obtained with sodium carbonate and potassium carbonate solu-
tions at room temperature, but even so, as the pH is raised beyond 10.5
some condensation occurs. In general the pH should be in the range of 8 to
11, and preferably in the range of 8.5 to 10.5.
Best results are obtained by heating with an aqueous solution of
a weak base to a temperature between 30 and 105C. Suitable weak bases
include sodium bicarbonate, sodium carbonate, sodium sesquicarbonate,
potassium bicarbonate, potassium carbonate and ammonium hydroxide.
The desired pH may be achieved with sodium bicarbonate by boiling
the mixture. This causes evolution of carbon dioxide and the gradual con-
version of bicarbonate to carbonate. Two hours of boiling with 20 percent
sodium bicarbonate is sufficîent to achieve substantially complete hydroly-
sis and neutralization with little condensation. There is one slight
disadvantage to this process in that there is no control over the final pH.
It tends to keep rising and eventually goes beyond 10.5, with resulting
loss of hydroxyl groups.
Another way to achieve the desired pH is to heat with bicarbonate
at a somewhat lower temperature, preferably 50 to 90C., add a little
carbonate to bring the pH to the desired level and heat a short time
longer, e.g., for a half hour.
The process of aspects of the invention provided by the parent
application produces a homologous mixture of silo~anediols. If desired,
~ the various siloxanediols, e.g., hexamethyltrisiloxanediol, octamethyl-
tetrasiloxanediol and decamethylpentasiloxanediol can be separated from
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the mixtule by fractional distillation.
~ le siloxanediols of aspects of the invention provided by the
parent~application are particularly useful as antistructure agents in
silicone rubber. They are useful as treating agents for inorganic sur-
faces to make them hydrophobic. They also are useful as chemical inter-
mediates in the formation of other siloxanes.
In the following examples all parts are by weight unless other-
wise specified.
Example 1
The following materials were mixed together: 740 parts of octa-
methylcyclotetrasiloxane (D4), 204 parts of acetic anhydride, 120 parts of
acetic acid, and 40 parts of acid clay (FILTROL 13). The mixture was
heated to reflux temperature and kept under reflux at 137.5 - 139C. for
4 hours. It was then cooled, filtered, and analyzed by gel permeation
chromatography~ which showed that 80 percent of the D4 had been converted
to a mixture of short, lînear siloxanes averaging 6 to 7 siloxane units.
Similar results were obtained from a sample taken after 2.5 hours. This
material was washed with 1000 parts of 10 percent aqueous sodium chloride,
then with a slurry of 900 parts of water and 100 parts of sodium bicar-
bonate, and finally with 1000 parts of aqueous sodium bicarbonate, at
which point a strong odor of acetic acid was still present.
Seven portions of 100 parts each were hydrolyzed in 336 parts
of water with added sodium carbonate or sodium bicarbonate. The conditions
and results are summarized in the following table:
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~ ~ ~ St7 ~ ~
Na2CO' Temp. Time, Final OH, OAc,
Example parts C. Hours pH % %
lA 26 30 6 9.7 5.55 4.70
lB 39 30 6 9.9 6.83 2.60
lC 52 30 6 10.2 6.52 2.07
lD 36 60 4 9.87 6.24 0.09
lE 31 60 4 9.75 6.20 0.13
lF 26 70 5 9.56 6.34 0.04
lG 22* 70 5 8.66 6.70 0.21
It can be seeD that hydrolysis was incomplete at 30C., even
when a large excess of sodium carbonate was used and the final pH of the
water solution was above 10 (Example lC). The high acetoxy (OAc) level
results in a somewhat unstable material. Hydrolysis carried out at 60C.
and 70C. was much more nearly complete and gave satisfactory products,
even when the final pH was as low as 8.66 ~Example lG).
Example 2
In similar fashion 444 parts of D4, 56 parts of acetic anhydride,
30 parts of acetic acid, and 10 parts of FILTROL 13 were heated at reflux,
144~C.~for 1 hour. A sample analyzed by gas chromatography showed the
following (Ac=CH3CO; D=(CH3)2SiO):
AcOH 19.04%
D3 1.26%
D4 14.20%
AcOD3Ae 0.08%
D5 8.44%
AcOD4Ae 1.87%.
_ D6 2.84~
AeOD5Ae 2.69%
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D7 0.67%
AcOD6Ac 3.25%
` . ~ D8 0.44%
~` ~ AcOD7Ac 3.77%
Dg 0.44%
AcOD8Ac 3.84%
D1o 0.16%
AcODgAc 3.96%
Dll O.09%
AcODloAc 4.11%
Other linear diacetoxysiloxanes were observed in the rar.ge of 1 to 4 per-
cent up to AcODlgAc. Similar results were obtained on a sample taken after
2 hours. In this case equilibration was essentially complete in 1 hour.
Example 3
A mixture of 250 parts of D4, 17 parts of a methylvinylsiloxane
(MeViSiO) , consisting of linear and cyclic siloxanes in a ratio of 6:4,
and having a viscosity of 100 cP; 50 parts of acetic anhydride; 23 parts
of glacial acetic acid; and 5 parts of FILTROL 13 LM (low-moisture grade)
was heated to 145C. in a closed vessel for 5 hours with continuous
agitation. The mixture was then cooled and washed three times with
aqueous sodium bicarbonate at 40~C., whereupon most of the acid clay was
found to have been removed with the water. The washed fluid was then
heated to 80C. with a slurry of lOO parts of sodium bicarbonate in 460
parts of water. After 3 hours, lO parts of sodium carbonate was added
and heating continued for another 2 hours. The aqueous phase was drawn
off while still hot, and found to have a pH of 9.8. The final siloxandiol
was analyzed, with the following results:
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Hydroxyl content 4.51 percent
Vinyl content 2.14 percent
, pH ~ 7.87
~` ~ Acetoxy (calculated as
acetic acid) 122 ppm
Viscosity 32.6 cSt
Specific gravity 0.979
This siloxanediol was tested in a silica-filled silicone rubber and found
to be an effective antistructure agent. On a weight basis it is much more
effective than other siloxanediols having a hydroxyl content in the range
of 2 - 3 percent.
