Process for the recovery of methyl polysiloxanes in the form of cyclosiloxanes

A process for the recovery of methyl polysiloxane in the form of methyl cyclosiloxane of the general formula (R.sub.2 SiO).sub.x, wherein R is selected from alkyl, alkenyl, aryl and alkaryl groups and "x" is an integer selected from 3-6, in which: PA1 a) liquifying silicone feedstock as herein defined is liquefied in a solvent selected from alcohol or siloxane in presence of a catalyst at a temperature of between 110.degree. C.-180.degree. C. to obtain a liquefied mass consisting of a mixture of methyl polysiloxane, solvent and filler; PA1 b) adding a metal hydroxide to the liquefied mass so as to convent the fillers to their corresponding silicates, the said silicates thus obtained are removed and the liquid recovered; and PA1 c) cyclyzing methyl polysiloxane in the liquid medium thus obtained in the presence of a cracking catalyst in the temperature range of 115-160.degree. C. so as to crack the liquid methyl polysiloxane to a mixture of volatile methyl cyclosiloxane. The process may be used to recover methyl cyclosiloxane from silicone containing scrap material.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a process for the recovery of methyl
 cyclosiloxane from silicone scrap material, which includes elastomeric
 scrap, parings, mold flashes, mold component, extruded profile, RTV mold
 and off-specification discarded material of silicone polymers hereinafter
 referred to as `silicone feedstock`. More particularly the present
 invention relates to a process in which almost pure cyclics are recovered
 from silicone feedstock without generating further secondary waste. Methyl
 cyclosiloxanes is used in the manufacture of silicone polymers.
 The silicone feedstock that is available can be in different forms e.g.,
 cured, partially cured or uncured silicone in the form of heat cured
 rubber (HCR); room temperature vulcanising, (both RTV-1 & RTV-2) and
 liquid silicone rubber (LSR). They may be condensation or addition cured
 type, filled with reinforcing and/or semi-reinforcing fillers, having no
 or little commercial utility in their commonly available form or quality.
 The difficulty faced by the industry was to recover Methyl cyclosiloxane
 from filler filled silicone feedstock. Almost all such silicone feedstock
 contain fumed silica, also known as silicic acid, colloidal silica,
 millimicron silica etc., as reinforcing fillers. No process is known for
 recovering cyclics from such filler filled silicone feedstock
 successfully. In case of filled silicone feedstock, with mainly fumed
 silica, corresponding metal silicates are produced. Metal silicates find
 multifarious use in industrial process and can be disposed of.
 First the silicone feedstock is liquified in presence of anionic
 surfactants/amines and/or mineral acids as catalyst where a volatile
 solvent is used as media. When fumed silica is present in the silicone
 feedstock, it is removed as a silicate from the liquid mixture by
 decantation or mechanical separation. After separation of such filler, the
 volatile solvent is removed by stripping at low pressure and Methyl
 polysiloxane is obtained as a residue. Methyl polysiloxane can be
 converted to a mixture of cyclics by cracking with alkali metal hydroxide
 or mineral acid at high yields which is as high as over 90%.
 2. Desription of the Related Art
 The use of silicone and silicone containing materials are increasing
 world-wide and a significant quantity of such silicone feedstock are
 generated during manufacture and processing of such silicone products. The
 known process for the utilization of silicone feedstock lacks versatility
 and is applicable to a particular type of silicone feedstock. In some
 cases, recovery was very low.
 U.S. Pat. No. 4,111,973 issued to Bluestein relates to an improved process
 for obtaining better yield and purity of fluoroalkyl cyclotrisiloxane in a
 cracking reaction of diorgano polysiloxane using an effective amount of
 aliphatic alcohol as a stabilising agent with addition an effective amount
 of cracking catalyst.
 This process is specifically directed to the recovery of pure fluoroalkyl
 cyclosiloxane from a mixture of diorganopolysiloxanes. The text of this
 patent is silent as to the effects on the gel mass when small amount of
 trifunctional groups, is present as impurity. Therefore, this process is
 only applicable to pure difunctional alkyl polysiloxane.
 U.S. Pat. No. 4,764,631 issued to Halm et al provides a method for
 preparing a product cyclodiorganopolysiloxalle via the vapour phase
 re-arrangement of other cyclopolydiorganosiloxane or mixtures thereof.
 This process is also applies to pure difunctional
 cyclopolydiorganosiloxane and the process is applicable to silicone
 feedstock which are in the form of volatile cyclics only.
 U.S. Pat. No. 2,860,152 issued to Fletcher teaches a method of producing
 cyclic diorganosiloxanes having a composition different from starting
 organo polysiloxane. In this process the diorgano polysiloxane and an
 inert solvent having b.p. more than 250.degree. C. were used. At least 20%
 by wt. of the siloxane was used as solvent. Temperature and pressure in
 reaction zone were maintained in a manner such that in presence of alkali
 catalyst, only cyclics were available from the reaction mixture while the
 solvent remained non-volatile under those conditions. The inert solvent
 shifts the polymer/cyclic equilibrium of the reaction more towards the
 cyclics and with more cyclics in the reactor, lesser the tendency of the
 reaction mass to gel. Therefore, at least 20% solvent is required to delay
 the gelation of the reaction mass. However, at the end of run, when the
 reaction mass gets gelled, the entire reaction mass is discharged as the
 siloxane/solvent cannot be separated.
 Thus, the process does not completely resolve the problem of gelation and
 therefore recovery is poor.
 U.S. Pat. No. 5,420,325 issued to John S Razzano teaches a method of
 producing cyclics where an effective amount of high boiling alcohol is
 used in the liquid siloxane hydrolysate. The high boiling alcohol allows
 the removal of trifunctional species in an efficient manner and completely
 eliminates formation of a gel. The siloxanes and the alcohol in the
 residue can be recovered and reused. Thus, Razzano has resolved the
 problem of gelation by cracking a diorganopolysiloxane containing a
 portion of trifunctional group to a mixed cyclosiloxane. Razzano has not,
 however, taught as to how to recover the remaining siloxane from alcohol.
 In the aforesaid literature the focus has been on the cracking of a liquid
 organopolysiloxane containing a portion of trifunctional group but there
 is no teaching with regard to silicone feedstock filled with fillers,
 specially fumed silica. These processes can function only in case of
 liquid methyl polysiloxane which is free of fillers.
 U.S. Pat. No. 5,110,972 issued to Tremco Incorporated teaches a method
 where silicone scrap is dissolved in high boiling solvent (b.p. greater
 than cyclics) and sulphuric acid. The sulphuric acid is then neutralized
 with KOH and with additional of KOH methyl polysiloxane is cracked to
 cyclics at 115.degree. C. under reduced pressure. Again, the said process
 can only be applied to unfilled feedstock.
 There is no teaching in this prior art as to how the residual silicone is
 separated from filler and solvent rich residue. Thus this process also
 does not aim at solving the problem of recovery of methyl polysiloxane
 from filler filled scrap.
 Chinese Patent No. CN 1086518A describes a method of manufacturing
 organocyclic silicone compounds by pyrolysis of silicone rubber under
 atmospheric pressure. This patent uses silicone rubber as a raw material.
 After washing and breaking into small pieces, the rubber is mixed with
 organo cyclosiloxane compounds and reacted in presence of KOH as a
 catalyst under normal atmospheric pressure and at 200-500.degree. C. This
 process produces unfavourable results at 200-500.degree. C. in presence of
 KOH because demethylation of dimethylpolysiloxane occurs rapidly, leading
 to an explosion.
 German Patent No. DE 4126319 A1 relates to a process for the recovery of
 silicone cyclic from silicone rubber. In this invention, silicone rubber
 is pyrolyzed at 550-600.degree. C. under vacuum and/or inert atmosphere.
 Major component in the outlet is Hexamethylcyclotrisiloxane (D3). In this
 paper the process and apparatus used are not clear. Even if practicable,
 the capital cost of such process using rotating pipe oven will be
 extremely high because of (a) non-feasibility of high temperature rotary
 seal; (b) inert gas blanketing of seal face against accidental leak; (c)
 complex anti-fouling condensing of D3, a major condensate which sublimes
 at 64.degree. C.; (d) hazardous in combination with oxygen at 600.degree.
 C. as this causes rapid oxidation; (e) non-continuous operation due to
 intermittent cleaning of reactor walls of complex crust formed; (f)
 progress of pyrolysis hindered by crust formed around the rubber; (g) long
 retention time for complete pyrolysis. With these disadvantages this
 process has many practical drawbacks. None of the prior art therefore
 teaches recovery of siloxane from a non-specific silicone feedstock in
 such a way that process will be free from waste generation and will be
 environment friendly.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a process for the
 recovery of methylcyclosiloxane from silicone feedstock where the effluent
 is environment friendly and free from pollutants.
 It is a further object of the present invention to provide any efficient
 and economical process for the recovery of methyl cyclosiloxanes from
 fumed silica filled silicone feedstock.
 DETAILED DESCRIPTION OF THE INVENTION
 Accordingly, the present invention relates to a process for producing a
 mixture of methyl cyclosiloxanes of general formula (R.sub.2 SiO).sub.x,
 where R is selected from alkyl, alkenyl, aryl and alkaryl groups and "x"
 is an integer selected from 3-6.
 According to the present invention there is provided a process for the
 recovery of methyl polysiloxane in the form of methyl cyclosiloxane of the
 general formula, (R.sub.2 SiO).sub.x wherein R is selected from alkyl,
 alkenyl, aryl and alkaryl groups and "x" is an integer selected from 3-6,
 in which:
 a) liquifying silicone feedstock as herein defined is liquefied in a
 solvent selected from alcohol or siloxane in presence of a catalyst at a
 temperature of between 110.degree. C.-180.degree. C. to obtain a liquified
 mass consisting of a mixture of methyl polysiloxane, solvent and filler;
 b) adding a metal hydroxide to the liquified mass so as to convert the
 silica fillers to their corresponding silicates, the said silicates thus
 obtained are removed and the liquid recovered; and
 c) cyclyzing methyl polysiloxane in the liquid medium thus obtained in the
 presence of a cracking catalyst in the temperature range of 115to
 160.degree. C. so as to crack the liquid methyl olysiloxane to a mixture
 of volatile methyl cyclosiloxane.
 When the solvent used in the process is a non-silicone compound the process
 further comprises stripping the liquid resulting from step (b) to separate
 the solvent from the mixture.
 The starting material employed in the process of the invention is silicone
 feedstock. Silicone feedstock contains organosiloxane polymers in which
 the substituting groups are methyl, phenyl, vinyl and/or other hydrocarbon
 radicals. Such polymers may or may not contain inorganic fillers, silanes,
 organic additives and catalyst. Such silicone feedstock may be in the form
 of solid, semi-solid, paste, gel and thixotropic compounds.
 A: Liquification
 In this step solvent is added to the silicone feedstock. The solvent is
 selected from alcohol from C.sub.5 to C.sub.12 and/or silicone cyclics.
 Most preferred solvent is Octyl alcohol, 2-Ethyl Hexanol and/or mixed
 methyl cyclosiloxane. In the liquification of filler filled silicone
 waste, an anionic surfactant is used as catalyst for linearization.
 Mineral acids/tertiary amines can also be used. Linear alkyl benzene
 sulphonic acid is the most preferred liquification catalyst. For
 liquification, a pressure vessel is used with an agitator. A temperature
 in the range of 110-180.degree. C. is employed for liquification. The
 preferred temperature of liquification is about 140.degree. C. Pressure in
 the range of 2 to 8 Kg/cm.sup.2 is required to aid the liquification. The
 preferred pressure for liquification is about 4 Kg/cm.sup.2 When tertiary
 amines are used in the step of liquification the amines come out of the
 reaction mass as an unreacted (ready for reuse) immiscible layer
 in-between the siloxane and silicate. After completion of the reaction the
 metal silicates may be removed from the reaction mass in a known manner.
 B. Separation of Filler
 In the step of separation the silica fillers containing free OH groups are
 reacted with metal hydroxide and separated out as metal silicates. The
 removal is effected by converting the fumed silica in the fillers to
 Na-silicate, Ca-Silicate and Al-Silicate or mixture thereof, by the use of
 their corresponding hydroxides. In the step of separation an effective
 amount of Ca(OH), NaOH or A1(OH).sub.3 or mixture thereof is added to the
 liquified mixture with a small amount of water for formation of the
 corresponding silicates. A temperature in the range of 110 to 220.degree.
 C. is required for the separation. The pressured temperature is about
 170.degree. C. The pressure of 2 to 10 Kg/cm.sup.2 and preferably about 4
 Kg/cm.sup.2 is used.
 C. Separation of Solvent
 After separation of fillers, the liquid contains a mixture of liquid methyl
 polysiloxane and solvent. The step of separation of solvent is optional.
 The step of separation of solvent is not required when silicone cyclics
 are use as solvent. When the solvent is separated it is effected by
 stripping under heat and reduced pressure. Recovered solvent is recycled
 to the liquification stage.
 D. Cyclization of Linear Methyl Polysiloxane
 The liquid methyl polysiloxane after removal of filler and solvent (if
 necessary) is cracked to a mixture of volatile methyl cyclosiloxanes in
 presence of an alkali metal hydroxide or other known catalyst (mineral
 acids, etc.). Potassium hydroxide is used as catalyst for cracking the
 liquid methyl polysiloxane. A temperature in the range of
 115.degree.-160.degree. C. is needed for cracking of under reduced
 pressure. The optimum temperature for cyclization is 140.degree. C. under
 reduced pressure. The process is continuous, but only a small amount of
 water is required to be charged at regular intervals for cyclic formation.
 Initially, a 50% water solution of KOH is used. Cracked cyclics are found
 to essentially contain a mixture of one or more of
 Hexamethylcyclotrisiloxane (D3), Octamethylcyclotetrasiloxane (D4),
 Decanethylcyclopentasiloxane (D5), and Dodecamethylcyclohexasiloxane (D6).