GB2459943A - Removal of aqueous impurities from siloxane based polymers - Google Patents

Abstract

A process for reducing residual content of aqueous impurities from siloxane based fluids is described. The process involves the step of contacting the siloxane based fluid with a material comprising superabsorbent fibres. The process is particularly suitable as a means of extracting aqueous impurities such as residual hydrochloric acid and/or water during the manufacture of organohydrogensiloxanes or diorganosiloxanes involving hydrolysis. Preferred superabsorbent fibres are an open structured non-woven material made from sodium polyacrylic acid. A method for preparing organohydrogensiloxanes or diorganosiloxanes involving hydrolysis of an organohydrogenchlorosilane or a diorganochlorosilane respectively and an optionally end-blocking triorganochlorosilane also comprising aqueous impurity removal by contact with superabsorbent fibres is also outlined. The process may be used after the siloxane based fluid has already undergone one or more standard techniques for separating water and or chlorides from siloxanes, such techniques including stripping, gravity separation, centrifugation, coalescence and/or membrane separation.

Description

REMOVAL OF AQUEOUS IMPURITIES FROM SILOXANE BASED POLYMERS

[0001] This invention relates to a method of maximizing the removal of aqueous impurities such as water and/or chlorine species (generally ionic chloride e.g. in the form of aqueous hydrochloric acid) from siloxanes which are sensitive to residual aqueous content and/or residual chloride content subsequent to polymerisation.

[0002] The manufacture of polydiorganosiloxane and/or organohydrogenpolysiloxane polymers is a multi-step process. The hydrolysis of chlorosilane based precursors is well known in the art and yields a mixture cyclic and linear silanol-blocked oligomers generally referred to in the industry as "hydrolysate".

[0003] In typical processes for preparing organohydrogensiloxanes or diorganosiloxanes a first step involves the hydrolysis of an organohydrogendichlorosilane or diorganodichlorosilane and optionally an end-blocking triorganochlorosilane resulting in the preparation of hydrogen chloride gas and/or hydrochloric acid as by-products. These need to be substantially completely removed from the end-product to meet industrial requirements and to avoid problems such as gelling due to interaction with residual water.

The removal of residual hydrochloric acid and water from these end products are particularly important due to the sensitivity of the product store action with water and/or hydrochloric acid.

[0004] Numerous procedures have been reported in the art for preparing linear and/or cyclic organohydrogensiloxanes. Examples include US 2758124 in which the production of trimethylsilyl-terminated methylhydrogenpolysiloxanes is described by reacting methyldichlorosilane and trimethylchlorosilane with a large excess of water.

[0005] U56143912 describes methylhydrogenpolysiloxanes produced via a similar process which in a first step only requires at most 0.5 mol of water per mole of hydrolysable chlorine but requires a second step wherein the resulting partial hydrolysate is treated with additional water to remove chlorine from remaining Si-Cl groups resulting in the formation of hydrochloric acid.

[0006] US5395956 describes a process for preparing cyclic organohydrogensiloxanes. In which an organohydrogendichlorosilane is mixed with about a stoichiometric equivalent of water to form a hydrolysate. The hydrolysate is diluted in an inert solvent and contacted with an acidic rearrangement catalyst to effect formation of cyclic organohydrogensiloxanes.

[0007] US5189193 describes a process for preparation of cyclic organohydrogensiloxanes by contacting an organohydrogendihalosilane with a substantially stoichiometric quantity of water to form a mixture containing cyclic and short-chain linear organohydrogensiloxanes.

The resulting mixture is distilled to separate the cyclic and linear organohydrogensiloxanes from other components of the mixture. The separated cyclic and linear organohydrogensiloxanes are then contacted with an additional quantity of water to effect hydrolysis and condensation of the linear organohydrogensiloxanes to higher molecular weight linear organohydrogensiloxanes which are easily separated from the cyclic organohydrogensiloxanes by a second distillation step.

[0008] Similarly polydimethylsiloxanes are produced by the hydrolysis of dimethyldichlorosilane which inevitably results in the problem of having to remove the resulting by-products hydrochloric acid and hydrogen chloride gas. A large number of variations of this process have been devised, such as for example in US3627805, US3983148, US4382145, EP 0515082, US5476916 and WO 2006/041561. Several of these proposals specifically deal with the reduction in the level of aqueous by-products, typically hydrochloric acid and/or water. Whilst in general the problem of removal of these by-products from polydimethylsiloxanes is significantly easier than for its removal from organohydrogensiloxanes because of the relative stability of the siloxane polymers involved there is still a need to remove the very low residual levels of water and/or hydrochloric acid which can remain within the polymers after standard purification steps have been completed.

[0009] The acidic nature of the resulting aqueous by product may be neutralised by any suitable process e.g. passage through sodium carbonate (US3983148). Typically aqueous by-products resulting from the hydrolysis of dimethyldichlorosilane can be largely removed by washing with water and using standard separation techniques such as for example stripping, gravity separation (which can be enhanced by mechanical baffles to shorten the mean flow path of the phases to be separated), centrifugation, coalescence and/or membrane separation. In EP0592149 a method for the removal of ionic chloride from hydroxyl-terminated siloxanes is proposed. This involves contacting the siloxanes with a molecular sieve effective in removing residual water from the siloxanes. This process has a major disadvantage however in that the molecular sieve has a negative effect on organohydrogensiloxane cyclic products and is known to decrease the usable amount of end product.

[0010] EP0446831 describes a method for the removal of ionic chloride from polysiloxanes involving contacting the polysiloxane hydrolysate containing residual chloride with weakly basic alkaline metal compounds at a temperature of less than 10000 and then separating the polysiloxane fluid from the solid alkaline metal compounds by filtration means such as filter presses, bag filters and/or cartridge filters.

[0011] Whilst the bulk of the aqueous by-products (water and hydrochloric acid) of the hydrolysis reactions described above can be successfully removed from the polymer product, there typically remains very low levels of water and/or hydrochloric acid within the polymer product. The ability to remove these residual low levels of water and/or hydrochloric acid from the polymer products has been a long term problem within the industry, particularly with respect to the production of organohydrogensiloxanes in which the presence of residual aqueous by-products can lead to problems with gelling of the resulting products due to secondary reactions of the residual water with the Si-H groups.

[0012] Absorbent materials are known to be used to extract water from bulk chemicals.

WO 0 1/093977 describes the use of such materials to separate water from lipophilic fluids such as for example cyclopentasiloxane. A particularly preferred use in WO 0 1/093977 is to expose emulsions to the absorbent materials to enable water to be absorbed out of the emulsion in order to facilitate recovery of the lipophilic fluid. U52007/0062855 describes a method for removing water and/or water based compounds from organic liquids/fluids using a fibrous media that contains, is pregnated, or is formed from at least one superabsorbent compound. The fibrous media are preferably nanofibers. U56101818 describes a process involving contacting a chemical mixture comprising water and at least one, halogenated hydrocarbon with a drying agent comprising a water-soluble polymer such as a polyacrylic acid or a salt of polyacrylic acid e.g. sodium polyacrylate.

[0013] The inventors have now identified a new purification process for removing residual low levels of aqueous impurities from siloxane polymers.

[00141 In accordance with the present invention there is provided a process for reducing residual content of aqueous impurities from siloxane based fluids comprising the step of contacting said siloxane based fluid with a material comprising superabsorbent fibres.

[0015] It is to be understood that for the sake of this invention the term aqueous impurities means water, hydrochloric acid and any other aqueous impurity removed from the polymer through absorption into the superabsorbent fibres in accordance with the method of the present invention.

[0016] The present method can be conducted in standard reactors for contacting a liquid with a solid. The present method can be conducted as a batch, semi-continuous or continuous process. The present method can be conducted, for example, in a fixed-bed reactor. It is preferred that the present method be conducted as a continuous process.

[0017] Whilst the present invention may be utilised to remove by-products, when present in the siloxane based fluids in any amount, the inventors have found that it is particularly effective for removing residual water remaining in the siloxanes after standard separation techniques for separating water from siloxanes, for example, stripping, gravity separation (which can be enhanced by mechanical baffles to shorten the mean flow path of the phases to be separated), centrifugation, coalescence and/or membrane separation Therefore, it is preferred that the water content of the mixture be less than 1.0 weight %, more preferably less than 0.5 weight % and most preferably less than 0.1 weight % prior to exposure to the material comprising superabsorbent fibres.

[0018] The present invention may be utilised to remove residual water and chloride content from any appropriate siloxane based fluid prepared by the hydrolysis of chlorosilanes as discussed above. The siloxane based fluid may be prepared from any suitable chlorosilane having the following formula: R2SiCI2 wherein each R may be the same or different and can be hydrogen or a hydrocarbon radical such as an alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group containing 1-20 carbon atoms. The hydrocarbon radical can be a group such as methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, tert-butyl, sec-butyl, pentyl, hexyl, cyclohexyl, dodecyl, phenyl, tolyl, benzyl, beta-phenylethyl and naphthyl.

[0019] Some examples of suitable diorganochlorosilanes include compounds such as dimethyldichlorosilane (CH3)2SiCl2, diethyldichiorosilane (C2H5)2SiCl2, di-n- propyldichlorosilane (n-C3H7)2SiCl2, di-i-propyldichlorosilane (i-C3H7)2SiCl2, di-n- butyldichlorosilane (n-C4H9)2SiCl2, di-i-butyldichlorosilane (i-C4H9)2SiCl2), di-t-butyldichlorosilane (t-C4H9)2SiCl2), n-butylmethyldichlorosilane CH3(n-C4H9)SiCl2, Octadecylmethyldichiorosilane CH3(C18H37)SiCI2, diphenyldichlorosilane (C6H5)2SiCl2, phenylmethyldichlorosilane CH3(C6H5)SiCI2 and dicyclohexyldichlorosilane (C6H11)2SiCl2.

[0020] Some examples of suitable organohydrogenchlorosilanes i.e. compounds where one R is hydrogen and the second is an organic group include compounds such as methyldichlorosilane CH3SiCI2 and any of the silanes listed in the preceding paragraph in which one of the alkyl substituents is replaced by hydrogen. If desired trichlorosilanes R3SiCI can also be used, where R is the same as defined above. A preferred trichlorosilane for example is trimethylchlorosilane (CH3)3SiCI [0021] The resulting siloxane based fluids may have linear, branched or cyclic structures or may be a mixture thereof. The present invention is particularly directed to the purification of cyclic siloxane based fluids having from 3 to 12 (RHSiO) groups or 3 to 12 (R2SiO) silicon atoms in the ring, where R is as hereinbefore described.

[0022] The siloxane based fluid mixture also contains a residual amount of chlorine species, largely present as a component in ionic form (chloride) within the water e.g. hydrochloric acid and/or bound in the polymer matrix. Extraction of the water remaining in accordance with the present invention is also seen to remove such residual chlorine species.

The ionic chloride content removed can be easily determined by measuring the chloride content by any suitable technique prior to and after contact with the SAP fibres of the present invention. By "ionic chloride", it is meant all sources of chloride in the water which are titratable with a base. The present method is particularly effective for reducing the ionic chloride content of the siloxanes to less than 500 parts per billion (ppb).

[0023] For the sake of this application it is to be understood that the term superabsorbent is a reference to acidic superabsorbent hydrogels having a pH less than or equal to 5.7, i.e., hydrogels based on polyacrylic acid whose degree of neutralization is preferably less than or equal to 60 mol %, processes. Such superabsorbents are swellable hydrogel forming addition polymers, known as superabsorbent polymers (SAPs). They are networks of flexible hydrophilic addition polymers, which can be both ionic and nonionic in nature. They are capable of absorbing and binding aqueous fluids by forming a hydrogel. They are particularly used in particle form in personal hygiene products such as tampons, diapers, sanitary napkins, incontinence articles, training pants for children, insoles and other hygiene articles for the absorption of body fluids.

[0024] Superabsorbents are also used in other fields of technology where fluids, especially water or aqueous solutions, are absorbed. These fields include for example packaging e.g. SAP-polystyrene co-extrudates for example for food packs such as meat, fish, poultry, fruit and vegetables); cosmetics, medicine (wound plasters, water-absorbent material for burn dressings or for other weeping wounds, e.g. as described in GB2377177), as a carrier material for pharmaceuticals and medicaments, rheumatic plasters and in ultrasound gel; in textiles; in chemical process industry applications (e.g. as catalyst for organic reactions, immobilization of large functional molecules (enzymes), adhesive for agglomerations, heat storage media, filtration aids, hydrophilic component in polymer laminates, dispersants, and liquefiers); building and construction; installation; water treatment; waste treatment; water removal such as in W02006/083279; cleaning; agriculture industry; fire protection; and co-extrusion agent in thermoplastic polymers (hydrophilicization of multilayer films).

[0025] The inventors have identified that the use of fibrous SAPs is suitable for the present invention rather than the more commonly used particulate form. This is because conventional particulate SAPs greatly swell at the surface on wetting with liquid, so that transportation of liquid into the particle interior is substantially compromised or completely prevented. This trait of SAPs is known as gel blocking. Furthermore, for the application of the present invention it was identified that the use of particulate SAPs swiftly leads to the formation of a barrier sayer to both the absorption of subsequent aqueous impurities into the particulate SAPs. Furthermore, as the aqueous impurities are absorbed into the particulate SAPs other problems occur, for example, the swelling of particulate SAPs effectively blocks passage of the siloxane fluid over and/or through the swelled SAP particles. Furthermore as the SAP particles continue to swell, resulting pressure caused due to the blockages formed, can result in damage to pipework and/or containers holding the siloxane fluids being purified.

[00261 SAP fibres suitable for the present invention are in particular polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, cross-linked cellulose or starch ethers, cross-linked carboxymethylcellulose, partially cross-linked polyalkylene oxide or natural products that swell in aqueous fluids, for example guar derivatives, alginates and carrageenans. Specific examples of suitable SAP fibres for the present invention are described in GB2352178.

[0027] Suitable grafting bases can be of natural or synthetic origin. Examples are starch, cellulose or cellulose derivatives and also other polysaccharides and oligosaccharides, polyvinyl alcohol, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, polyamines, polyamides and also hydrophilic polyesters.

[0028] Preferred hydrogel-forming polymers are cross-linked polymers having acid groups which are predominantly in the form of their salts, generally alkali metal or ammonium salts.

Such polymers swell particularly strongly on contact with aqueous fluids to form gels.

Preference is given to polymers which are obtained by crosslinking polymerization or copolymerization of acid-functional monoethylenically unsaturated monomers or salts thereof. It is further possible to copolymerize these monomers without crosslinker and to crosslink them subsequently. Examples of such monomers bearing acid groups are monoethylenically unsaturated 03-to C25-carboxylic acids or anhydrides such as acrylic acid, (e.g. sodium polyacrylic acid), methacrylic acid, ethacrylic acid, [alpha]-chloroacrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid and fumaric acid. It is also possible to use monoethylenically unsaturated sulphonic or phosphonic acids, for example vinylsulphonic acid, allylsulphonic acid, sulphoethyl acrylate, sulphomethacrylate, sulphopropyl acrylate, su Iphopropyl methacrylate, 2-hyd roxy-3-acryloyloxypropylsu phonic acid, 2-hyd roxy-3-methacryloyloxypropylsulphonic acid, vinylphosphonic acid, allylphosphonic acid, styrenesulphonic acid and 2-acrylamido-2-methylpropanesulphonic acid. Preferably the Superabsorbent fibres are made from a sodium polyacrylic Acid.

[0029] Preferably the SAP fibres, used in accordance with the present invention, are provided in or on a suitable material. The material is effectively a support for the SAP fibres and may be a woven or non-woven material although the latter is preferred. Most preferably the material is an open structured non-woven material. Such a material was found to avoid gel blockage and aid passage of the siloxane fluid through and/or past and/or over the material comprising SAP fibres. The material may consist solely of the SAP fibres.

However, preferably the SAP fibres form part of the material or are attached to the support material in some appropriate manner which will not inhibit their water absorbent functionality.

SAP fibres may be attached to the material by, for example, adhesion, by being bound (immobilised) chemically or physically or by being encapsulated or any other suitable means.

The material preferably additionally comprises at least one non-SAP ingredient i.e. at least one other type of compatible fibres or the like. Preferably the chosen material should be stable at room temperature i.e. it does not decompose or self react when stored at room temperature and is a non-irritant. It should be insoluble or substantially insoluble in the ingredients of the siloxane material with which it is brought into contact. Preferably the material is environmentally friendly and will biodegrade or is recyclable after usage or will be easily disposable. Examples include polyamides, polyesters, polyester copolymers and olefin copolymers, suitable fibres compatible therewith and mixtures and/or bi-component products thereof.

[0030] Preferably the material is in the form of an open structured non-woven material consisting of Sodium Polyacrylic Acid (SAP) fibres and polyester or a polyester based bi-component fibre. Preferably the material has a grammage of from about 800 to 8000 g/m2 more preferably from 1000 to 3000 g/m2, most preferably from 1200 to 2500 g/m2. The material may be of any suitable thickness for the application but is typically between 10mm and 50mm thick and can be rolled and/or folded if greater thicknesses are required.

[0031] It will be appreciated that this invention could be utilised in any other chemical process requiring the removal of low levels of residual water, particularly for continuous processes providing there is no negative chemical interaction between the product from which water is to be removed and the SAP fibres and or the support material.

Example

[0032] The following example is provided merely as an example and is therefore not intended to limit the scope of the invention in any other manner.

[0033] Subsequent to formation by hydrolysis a sample of a cyclic methylhydrogensiloxane was water-washed to remove the bulk of the chloride content from the polymer product. This washing step resulted in the formation of a dilute hydrochloric acid in the methyl hydrogen siloxane. The bulk of the hydrochloric acid was separated using standard neutralisation and separation methods. The resulting methylhydrogen siloxane fluid concentrate, however still contained residual chloride contaminants (in ionic and/or non-ionic form) as well as low levels (less than 1% by weight) of water. The material containing the SAP fibres in accordance with the present invention utilised sodium polyacrylic acid (NaPAA) fibres as the SAP fibres in/on a polyester based support material and was inserted into a filter bag prior to contact with the siloxane concentrate to be purified. The concentrate was brought into contact with the superabsorbent fibres in a suitable filter bag at ambient temperature to evaluate possible issues with materials of construction of the structured fibre. The concentrate was passed through the filter bag for a period of several days and the methylhydrogensiloxane concentrate was analysed for chloride content prior to and subsequent to passage through the filter bag using ion chromatography (IC) analysis in accordance with ASTM ref. E 180 pam. 2.4 and as is shown in Fig. 1 below in every instance after passage through the filter the chloride levels were reduced.

[0034] Subsequent to use the material containing the SAP fibres was visually analyzed and it was noted that no detrimental effects were observed. No unwanted effects such as increased pressure drop were observed during passage of the siloxane concentrate through the filter bag containing material containing the SAP fibres.

Claims (12)

1. CLAIMS1. A process for reducing residual content of aqueous impurities from a siloxane based fluid comprising the step of contacting said siloxane based fluid with a material comprising superabsorbent fibres.
2. 2. A process in accordance with claim 1 characterised in that the siloxane based fluid is selected from the group of an organohydrogensiloxane and a polydiorganosiloxane.
3. 3. A process in accordance with any preceding claim characterised in that the superabsorbent fibres are made from a sodium polyacrylic acid.
4. 4. A process in accordance with any preceding claim characterised in that the material comprising superabsorbent fibres is an open structured non-woven material.
5. 5. A process in accordance with any preceding claim characterised in that the siloxane based fluid has undergone one or more standard separation techniques for separating water and or chlorides from siloxanes selected from the group of stripping, gravity separation, centrifugation, coalescence and/or membrane separation prior to the step of contacting said siloxane based fluid with a material comprising superabsorbent fibres.
6. 6. A process in accordance with any of claims 1 to 5 characterised in that the water content of the siloxane based fluid mixture is less than one weight percent.
7. 7. A process in accordance with any preceding claim characterised in that the process is a continuous process.
8. 8. Use of superabsorbent fibres for reducing residual content of aqueous impurities from a siloxane based fluid.
9. 9. Use in accordance with claim 8 characterised in that the superabsorbent fibres are sodium polyacrylic acid fibres.
10. 10. A method for the manufacture of organohydrogensiloxanes involving hydrolysis of an organohydrogendichiorosilane and optionally an end-blocking triorganochiorosilane characterized in that residual aqueous impurities such as hydrochloric acid and/or water in the resulting organohydrogensiloxane product are removed, at least partially, by contacting said organohydrogensiloxanes with a material comprising superabsorbent fibres in accordance with any one of claims 1 to 7.
11. 11. A method for the manufacture of diorganosiloxanes involving hydrolysis of an diorganodichiorosilane and optionally an end-blocking triorganochlorosilane characterized in that residual aqueous impurities such as hydrochloric acid and/or water in the resulting diorganosiloxane product are removed, at least partially, by contacting said diorganosiloxanes with a material comprising superabsorbent fibres in accordance with any one of claims 1 to 7.
12. 12. Use of a process in accordance with any one of claims 1 to 7 as a means of removing residual hydrochloric acid and/or water in the manufacture of organohydrogensiloxanes or diorganosiloxanes