Continuous process for producing a silicone polymer

Details Continuous process for producing a silicone polymer Continuous process for producing a silicone polymer

1999-08-26

2001-04-24

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From silicon reactant having at least one...

C528S021000, C528S033000, C528S037000, C524S860000

Reexamination Certificate

active

06221993

ABSTRACT:

The present invention relates to a continuous process for producing a silicone polymer, in particular a continuous process for producing a silicone polymer by polymerisation of siloxanes in the presence of water and the presence or absence of a filler catalysed by a phosphazene base.
Cyclosiloxanes are critical intermediates in the silicone industry, primarily as starting materials for polymerisation reactions. Several general routes are known for the preparation of cyclosiloxanes. Together with hydroxy-endblocked linear polydiorganosiloxanes, they are formed as a product of hydrolysis of corresponding diorganodihalosilanes.
Various catalysts are known for the polymerisation of cyclosiloxanes. Examples are alkali metal hydroxides, alkali metal alkoxides or complexes of alkali metal hydroxides and an alcohol, alkali metal silanolates, and phosphonitrile halides (sometimes referred to as acidic phosphazenes). Such polymerisation reactions can be carried out substantially in the absence of solvent, in solvents (such as non-polar or polar organic solvents) or in emulsion. An endblocking agent may be used to regulate the molecular weight of the polymer and/or to add functionality, for example to add vinyl functional end groups. Polymerisation may be terminated by using a neutralising agent which reacts with the catalyst to render it non-active. In most cases catalyst residues and salts following neutralisation remain in the polymer product which may cause some re-equilibration of silicone polymer back to the siloxane starting materials. These residues and salts are desirably removed, such as by filtration. Volatile materials, including cyclosiloxanes, are removed from the silicone polymer by stripping, typically at temperatures of from 120 to 200° C. and under reduced pressure of 200 to 20000 Pa, to afford a silicone polymer having a volatile material content of typically from 1.0 to 5% by weight.
Another known process for producing a silicone polymer by polymerisation of siloxanes is condensation polymerisation of silanol or other hydrolysable group containing linear siloxanes. For example, in GB 2311994 there is described a method of effecting polycondensing which comprises contacting at a temperature of from 0 to 200° C. and a pressure up to 4.67×10
−4
Nm
−2
, a silanol-containing organosiloxane with an amount of a peralkylated phosphazene base which is effective for polycondensation of said organosiloxane. The preferred peralkylated phosphazene base has the formula R
#
N═P−{N═P(NR*
2
)
3
}
n
{R
#
2
N}
3-n
wherein R
#
is a C
1-4
alkyl radical, R* is a C
1-10
alkyl radical and n is 2 or 3.
Phosphazene bases are known to be extremely strong bases. Numerous phosphazene bases and routes for their synthesis have been described in the literature, for example in Schwesinger et al, Liebigs Ann. 1996, 1055-1081. The use of a phosphazene base catalyst for the ring-opening polymerisation of a cyclosiloxane on a laboratory scale has been described in Molenberg and Möller, Macromol Rapid Commun. 16, 449-453 (1995). Octamethylcyclotetrasiloxane (D4, where D denotes an —Si(CH
3
)
2
O— unit) was polymerised in toluene solution in the presence of methanol and the phosphazene base I described hereinbelow, used as a 1 molar solution in hexane. All the components were carefully dried before the reaction, which was carried out under an argon atmosphere containing less than 1 ppm O
2
and H
2
O. The methanol was deprotonated by the phosphazene base to form methoxide ions which initiate the reaction. The phosphazene base catalyst was used in an amount of at least 871 ppm based on the weight of D4. A similar reaction system has been used by Van Dyke and Clarson in Poly Prep ACS Div Polym Chem 1996, 37, 668. In this case, tetraphenyltetramethyl-cyclotetrasiloxane, the phenylmethyl analog of D4, was polymerised. The catalyst system was the same as in Molenberg and Möller, but was used at concentrations which were higher based on the weight of D4, and again all the reaction components were carefully dried beforehand.
The present inventors have found that addition of this hexane/methanol activated catalyst gives erratic polymerisation behaviour. They therefore sought a catalyst medium that gives reproducible polymerisation, preferably without the need for solvent, and surprisingly found that it is possible to carry out polymerisation of siloxanes with a phosphazene base catalyst in the presence of water. To ensure the presence of water it is sufficient to avoid totally anhydrous conditions. Very small amounts of water, e.g. a few molecules, have been found to suffice to allow the polymerisation to take place. Furthermore, the present inventors found that it is not essential to form a methoxide ion, e.g. by using methanol, in contrast to the prior art teaching. Surprisingly, even lower levels of phosphazene base catalyst can be used where water is present, than were used in the prior art, whilst maintaining or improving the polymerisation efficiency.
Polymerisation of siloxanes may occur in the presence of a filler. Silicone polymer-filler mixtures are known for use as bases for various silicone rubber compositions, silicone compounds and greases, etc. Conventional mixtures are generally produced by first polymerising silicone oligomer into a silicone polymer with the desired viscosity and then mechanically mixing the resulting silicone polymer with the selected filler. However, such methods involve two different types of processes, necessitating a separate polymerisation step and mixing step.
As a result, the process is complicated and disadvantageous on a cost basis. In addition, it is difficult in such methods to mix and disperse filler into high-viscosity silicone polymers and large amounts of energy are consumed. This problem becomes particularly significant when the molecular weight of the silicone polymer is as high as that of a so-called gum.
Attempts have been made to overcome these problems by carrying out the polymerisation in the presence of the filler. U.S. Pat. No. 4,448,927 discloses a method of polymerising a hydroxy-endblocked polydiorganosiloxane and/or a polydiorganocyclosiloxane in the presence of an acidic or neutral reinforcing filler and catalysed by trifluoromethane sulfonic acid. EP-A-0 019 816 discloses a method of bulk polymerisation of a hydroxy-endblocked polydiorganosiloxane and/or a polydiorganocyclosiloxane in the presence of an acidic or neutral reinforcing filler and catalysed by sulfuric acid or a sulfonic acid. EP-A-0 019 093 discloses a method of polymerising a hydroxy-endblocked polydiorganosiloxane in the presence of an inorganic reinforcing or extending filler and a basic diorganosilanolate catalyst. U.S. Pat. No. 4,431,771 discloses the polymerisation of a hydroxy-endblocked polydiorganosiloxane in the presence of an acidic or neutral reinforcing filler and a catalyst selected from sulfuric acid, sulfonic acids, perfluorinated alkane sulfonic acid, and a combination of quaternary ammonium carboxylate and carboxylic acid. While these processes have been successful with linear starting materials, they have been less successful with cyclosiloxanes, as the rates of polymerisation have generally been regarded as too slow.
Thus, the present inventors have found that phosphazene base catalysts are well suited for polymerisation of cyclosiloxanes in the presence or absence of fillers.
The present inventors have found that silicone polymers made by polymerisation of siloxanes catalysed by phosphazene bases have enhanced thermal stability over silicone polymers made using conventional catalysts. This enhanced thermal stability is attributed to the very low levels of catalyst residues remaining in the product after catalyst neutralisation. Thus, stripping of silicone polymers having enhanced thermal stability may be effected at correspondingly higher temperatures than those used for stripping silicone polymers made using conventional catalysts, which results in correspondingly lower volatile materials cont

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