Method of making hydroxy end-terminated linear siloxanes

Details Method of making hydroxy end-terminated linear siloxanes Method of making hydroxy end-terminated linear siloxanes

2001-02-20

2001-11-13

Shaver, Paul F. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Silicon containing

Reexamination Certificate

active

06316655

ABSTRACT:

FIELD OF THE INVENTION
This invention is a process for making hydroxy end-terminated linear siloxanes. In particular, the process involves hydrolyzing a solution of chlorosiloxanes in volatile methyl siloxanes and separating the resulting hydroxy end-terminated linear siloxane products.
BACKGROUND OF THE INVENTION
Hydroxy end-terminated linear siloxanes are an important class of silicone additives useful in many commercial silicone compositions. In particular, short chain hydroxy end-terminated linear siloxanes, that is linear siloxanes having a degree of polymerization of less than 10, perform an essential role in most silicone rubber compositions as a plasticizer because of their high ratio of hydroxy groups to dimethylsiloxy units.
However, short chain hydroxy end-terminated linear siloxanes are not easily manufactured because of their inherent propensity to further polymerize by the additional condensation of the terminal hydroxy groups. Thus, short chain hydroxy end-terminated linear siloxanes are difficult, if not impossible, to make by conventional methods such as via the hydrolysis of dichlorodimethylsilane. Such techniques generally produce linear polydimethylsiloxanes with terminal hydroxy groups having a chain length (degree of polymerization) of 10-12, or a SiOH content of 3-4%. A linear polydimethylsiloxane with a SiOH content of 3-4% often does not function optimally as a plasticizer in silicone rubber compositions, where the effectiveness is proportional to the silanol content. Thus, alternative methods of producing short chain short chain hydroxy end-terminated linear siloxanes, having chain lengths of 2-9 and SiOH contents of 8-10% have been described in the art.
U.S. Pat. No. 5,488,125 to Omura et al. discloses a method for manufacturing organosilanes having silanol groups, useful as dispersing agents in silicone rubber compositions. The method disclosed in the '125 patent involves hydrolyzing an organoalkoxy silane, and in particular dimethyl dimethoxy silane, having at most a 20 ppm content of chlorine, with water having an electroconductivity of at least 10
10
M&OHgr;, then further adding a macro-porous cation exchange resin. Similarly, U.S. Pat. No. 5,576,408 teaches a two-stage hydrolysis of organoalkoxysilanes using a macro-porous cation exchange resin and controlling the amount of water in each stage.
U.S. Pat. No. 4,395,563 to Hayes teaches the hydrolysis of alkoxysilanes, and a process for controlling the average chain length of silanol-stopped polysiloxanes. The Hayes patent in particular discloses the preparation of silanol-stopped polydimethylsiloxanes having 2 to 8 siloxane units.
While the prior art methods advantageously control the rate of reaction to provide specific chains of polysiloxanes, these methods also produces substantial quantities of a water/alcoho/siloxane stream that is often treated as a waste-product stream in commercial processes.
The present inventors have discovered a process for preparing hydroxy end-terminated linear siloxanes based on the hydrolysis of chlorosiloxanes. In particular, the present inventors have unexpectedly found a method that avoids or minimizes the further polymerization of the short chain hydroxy end-terminated linear siloxanes, especially with un-reacted chloride terminated polysiloxane reactants. Furthermore, the present invention does not produce high quantities of waste stream products, as does the method of producing short chain hydroxy end-terminated linear siloxanes based on the hydrolysis of the alkoxysilanes.
The object of the present invention is to provide a process for producing hydroxy end-terminated linear siloxanes.
SUMMARY OF THE INVENTION
The present invention is a process for producing hydroxy end-terminated linear siloxanes comprising the steps of,
a) mixing chlorosiloxanes having the formula
ClR
2
SiO(R
1
2
SiO)
x
SiR
2
Cl
 where x is an integer having a value from 1-10, each R and R
1
is an independently selected monovalent hydrocarbon group,
with volatile methyl siloxanes to form a solution of chlorosiloxanes in volatile methyl siloxanes;
b) hydrolyzing the solution of chlorosiloxanes in volatile methyl siloxanes to form a hydrolysis mixture of reaction products;
c) separating the hydrolysis mixture of reaction products into an aqueous phase and a siloxane phase comprising hydroxy end-terminated linear siloxanes and volatile methyl siloxanes; and
d) recovering the hydroxy end-terminated linear siloxanes from the siloxane phase.
DETAILED DESCRIPTION OF THE INVENTION
The first step in the process of the present invention is to mix chlorosiloxanes having the formula,
ClR
2
SiO(R
1
2
SiO)
x
SiR
2
Cl  (I)
where x has value from 1-10,
R and R
1
are alkyl, alkenyl, or aryl groups
with volatile methyl siloxanes to form a solution of chlorosiloxanes in volatile methyl siloxanes.
Chlorosiloxanes that can be used in the first step of the present invention are described by formula (I), as shown above. The chlorosiloxanes may be a single species or a mixture of such siloxanes. The value of x in formula (I) can be from 1 to 10. Preferably x has a value from 1 to 4 and more preferably is 1 or 2. Even more preferred is when the chlorine end-terminated linear siloxane is a mixture of siloxanes of formula (I) where x=1 and x=2. Yet even more preferred is when the ratio of siloxanes where x=1 to siloxanes where x=2 is 1.5:1.
R and R
1
is an independently selected monovalent hydrocarbon group. The R and R
1
substituents in formula (I) can independently be an alkyl, alkenyl, or aryl group. Examples of alkyl groups as R and R
1
substituents can include methyl, ethyl, propyl, isopropyl, or butyl. Examples of alkenyl groups as R and R
1
substituents can include vinyl or allyl groups. Preferably R and R
1
is an alkyl group, and most preferably R and R
1
is methyl.
The chlorosiloxanes are well known in the art and can be prepared by any of the methods known in the art. In particular, the chlorosiloxanes can be prepared by the catalyzed redistribution of polyorganosiloxanes as taught in U.S. Pat. No. 5,068,383, which is hereby incorporated by reference. Using the processes taught in the '383 patent, chloride end-terminated polyorganosiloxanes are redistributed over an alumina, silica-alumina, activated carbon, zeolites, or acid clay catalyst to produce the chloride end-terminated linear siloxane having the desired chain length.
The volatile methyl siloxanes used in the present invention can be cyclic methyl siloxanes, linear methyl siloxanes, or mixtures thereof. Representative linear volatile methyl siloxanes are hexamethyldisiloxane with a boiling point of 100° C., viscosity of 0.65 mm
2
/s, and formula Me
3
SiOSiMe
3
; octamethyltrisiloxane with a boiling point of 152° C., viscosity of 1.04 mm
2
/s, and formula Me
3
SiOMe
2
SiOSiMe
3
; decamethyltetrasiloxane with a boiling point of 194° C., viscosity of 1.53 mm
2
/s, and formula Me
3
SiO(Me
2
SiO)
2
SiMe
3
; dodecamethylpentasiloxane with a boiling point of 229° C., viscosity of 2.06 mm
2
/s, and formula Me
3
SiO(Me
2
SiO)
3
SiMe
3
; tetradecamethylhexasiloxane with a boiling point of 245° C., viscosity of 2.63 mm
2
/s, and formula Me
3
SiO(Me
2
SiO)
4
SiMe
3
; and hexadecamethylheptasiloxane with a boiling point of 270° C., viscosity of 3.24 mm
2
/s, and formula Me
3
SiO(Me
2
SiO)
5
SiMe
3
. The volatile methyl siloxanes can also be a mixture of linear methyl siloxanes described above.
The cyclic methyl siloxanes have the general formula [(CH
3
)
2
SiO]
z
. The value of z can be 3-8, but is preferably 4-6. The cyclic methyl siloxanes useful in the present invention have a boiling point less than 250° C. and a viscosity of 0.65-5.0 centistokes (mm
2
/s). Some representative cyclic methyl siloxanes are hexamethylcyclotrisiloxane [(Me
2
)SiO]
3
; octamethylcyclotetrasiloxane [(Me
2
)SiO]
4
; decamethylcyclopentasiloxane [(Me
2
)SiO]
5
; and dodecamethylcyclohexasiloxane [(Me
2
)SiO]
6
. The volatile methyl siloxanes can also be a mixture of the c

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