WO2006134121A1

WO2006134121A1 - Method for producing polyoxymethylenes from purified monomers

Info

Publication number: WO2006134121A1
Application number: PCT/EP2006/063177
Authority: WO
Inventors: Jens Assmann; Michael Triller; Claudius Schwittay; Knut ZÖLLNER
Original assignee: Basf Aktiengesellschaft
Priority date: 2005-06-15
Filing date: 2006-06-14
Publication date: 2006-12-21
Also published as: DE102005027802A1

Abstract

The invention relates to a method for producing polyoxymethylene copolymers (POM) or hornopolymers by polymerising suitable monomers. The invention is characterised in that at least one partial amount of the monomers is brought into contact with a Lewis acid compound prior to polymerisation, whereby molecular sieves, having a pore diameter of less than 0,6 nm (6 A), are excluded as Lewis-acid compounds.

Description

Process for the preparation of polyoxymethylenes from purified monomers

description

The invention relates to a process for the preparation of Poiyoxymethylenhomo- or - copolymers (POM) by polymerization of suitable monomers, characterized in that at least a portion of the monomers is brought into contact before polymerization with a Lewis acidic compound, wherein molecular sieves having a pore diameter below 0.6 nm (6 Å) as a Lewis acidic compound.

In addition, the invention relates to the use of Lewis acidic compounds for the purification of monomers which are used for the preparation of Poiyoxymethylenhomo- or co-polymers, wherein molecular sieves having a pore diameter below 0.6 nm (6 Ä) s Lewis acid Connection are excluded.

Furthermore, the invention relates to a process for the purification of monomers which are used for the preparation of Polyoxymethyienhomo- or copoiymeren, by contacting with Lewis acidic compounds, wherein molecular sieves having a pore diameter below 0.6 nm (6 Å) as a Lewis acid compound with exception of.

Finally, the invention relates to the polyoxyethylene homopolymers and copolymers obtainable by the first-mentioned process.

Polyoxymethylene polymers (POM, also called polyacetals) are obtained by homo- or copolymerization of 1, 3,5-trioxane (trioxane in short), formaldehyde or other Formaldehydquelie. The polymerization is usually carried out cationically, with strong protic acids, for example perchloric acid, or Lewis acids, such as tin tetrachloride, being metered into the reactor as initiators. Subsequently, the reaction is usually stopped by adding ammonia, amines or other basic deactivators.

The conversion in the polymerization is usually not complete, but the POM crude still contains up to 40% unreacted monomers. Such residual monomers are, for example, trioxane and formaldehyde, as well as optionally used comonomers such as 1, 3-dioxolane, 1, 3-butanediol formal or ethylene oxide. The residual monomers are separated in a degassing device. It would be economically advantageous to recycle it directly into the polymerization.

However, the separated residual monomers are often contaminated with the deactivators, and recycling of these deactivator-containing residual monomers into the recycled Actor degrades the product properties and slows the polymerization or brings them completely to a standstill. For example, DE 195 81 289 T1 on page 5 in the middle and page 8 above describes the problem that amine compounds inactivate the polymerization catalyst and thus prevent the polymerization.

Therefore, the deactivators and other interfering impurities must be removed from the monomers. This purification of monomers or residual monomers used for POM preparation is described in the following documents:

According to DE 32 31 797 A1, unreacted trioxane is withdrawn from oxymethylene polymers in several steps A to D and, after addition of a polymerisation inhibitor, i.a. Ammonia or higher amines, condensed. In a purification zone E, the inhibitor which is undesirable in the polymerization is removed again, in the case of ammonia or amines by ion exchange, extraction or distillation, before the trioxane is recycled into the polymerization (bridge paragraph, page 7/8).

The unpublished German patent application Az. 102004025366.8 of 19.05.04 describes the removal of residual monomers from POM by degassing and condensation. Prior to recycling to the POM preparation, the separated residual monomers can be purified, including - without further details - distillation, rectification, pervaporation, sublimation, crystallization, adsorption, absorption, chemisorption, thermal diffusion, thickening, concentration, evaporation, drying Freeze-drying, freezing, condensation, melting, electrophoresis, ion exchange, chromatography, complexation or chelation.

Published patent application DE 1 237 583 discloses the purification of molten trioxane with weakly acidic ion exchangers which contain carboxyl groups and optionally sulfonic acid groups (ie both Brönsted acids).

The patent specification GB 1 254 344 describes the purification of crude trioxane from water and formic acid by distillation and subsequent treatment with a molecular sieve. The pore diameter of the molecular sieve is 3 to 5 angstroms (A, 1 A s 0.1 nm).

Patent FR 1 422 094 teaches the purification of cyclic aldehyde oligomers, eg trioxane, of water, formic acid, acetic acid or other impurities by contacting them with molecular sieves composed of microporous silico-aluminates of the alkaline or alkaline-earth metals. In the examples, a molecular sieve with an effective pore diameter of 4 Å (0.4 nm) and a BET surface area of 450 or 300 m ² / g is used. The cleaning effect of the known cleaning methods is not satisfactory in all cases. In particular, basic impurities, for example amines, are not always sufficiently removed or the technical effort required for their adequate removal is high,

The object was to remedy the disadvantages described, and to provide a process for POM preparation in which the monomers are easily freed from impurities. The method should above all the mentioned basic impurities, e.g. Ammonia or amines, reliable separation.

Ideally, the process should lower the concentration of impurities to such an extent that the polymerization reaction is not disturbed or slowed down.

Accordingly, the method defined above for POM production was found. In addition, the said use of the Lewis acidic compounds, as well as said process for monomer purification, was found. Finally, the said polyoxymethyl homo- and copolymers were found.

Polyoxy methyl homo- and copolymers

The polyoxyethylene homo- or copolymers (POM) are known as such and are commercially available. The homopolymers are prepared by polymerization of formaldehyde or, preferably, trioxane; Comonomers are also used in the preparation of the copolymers. The monomers are preferably selected from formaldehyde, trioxane and other cyclic or linear formates or other formaldehyde cues.

In general, such POM polymers have at least 50 mole percent of recurring units -CH ₂ O- in the polymer backbone. Polyoxymethytencopoiymere are preferred, in particular those in addition to the recurring units

-CH 2 O- even up to 50, preferably 0.01 to 20, in particular 0.1 to 10 mol% and very particularly preferably 0.5 to 6 mol% of recurring units

in which R ¹ to R ⁴ independently of one another are a hydrogen atom, a C 1 - to C ₄ -alkyl group or a halogen-substituted alkyl group having 1 to 4 C atoms and R ^{5 is} a -CH ₂ -, -CH ₂ O-, a to C ₄ alkyl or Ci to C ₄ -Haloafkyl substituted methylene group or a corresponding oxymethylene group and n is a Value in the range of 0 to 3 has. Advantageously, these groups can be introduced into the copolymers by ring opening of cyclic ethers. Preferred cyclic ethers are those of the formula

wherein R ¹ to R ⁵ and n have the abovementioned meaning. For example, ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxane and 1,3-dioxepane (= butanediol formal, BUFO) are cyclopean Ethers mentioned as well as linear oligo- or polyformals such as polydioxotane or Poiydioxepan called comonomers.

Also suitable are Oxymethylenterpolymerisate, for example, by reacting trioxane, one of the above-described cyciischen ethers with a third monomer, preferably bifunctional compounds of the formula

and or

wherein Z is a chemical bond, -O-, -ORO- (R is d- to Cβ-alkylene or C 3- to Cβ-cycloalkylene) are prepared.

Preferred monomers of this type are ethylene diglycidyl, digiycidyl ether and diether from glycidylene and formaldehyde, dioxane or trioxane in the molar ratio 2: 1 and diether from 2 mol glycidyl compound and 1 mol of an aliphatic diol having 2 to 8 carbon atoms such as the DsgiycidySether of ethylene glycol, 1 , 4-butanediol, 1, 3-butanediol, cyclobutan-1, 3-diol, 1, 2-propanediol and cyclohexane-1,4-diol, to name just a few examples.

End-group stabilized Polyoxymethyjenpolymerisate having at the chain ends predominantly CC or ™ O-CH3 bonds are particularly preferred. The preferred polyoxymethylene copolymers have melting points of at least 15O ⁰ C and molecular weights (weight average) M _w in the range of 5,000 to 300,000, preferably from 7000 to 250,000. Particular preference is given to POM copolymers with a non-uniformity (MJM _n ) of from 2 to 15, preferably from 2.5 to 12, particularly preferably 3 to 9. The measurements are generally carried out by gel permeation chromatography (GPC) / SEC (size excusion The M _n value (number average molecular weight) is generally determined to mean GPC / SEC.

If appropriate, the molecular weights of the polymer can be adjusted to the desired values by the rule used in the trioxane polymerization, and by the reaction temperature and residence time. The regulators include acetals or formals of monohydric alcohols, the alcohols themselves and the small amounts of water acting as chain transfer agents, the presence of which as a rule can never be completely avoided. The regulators are used in amounts of from 10 to 10,000 ppm, preferably from 20 to 5,000 ppm.

In the case of formaldehyde as monomer, the polymerization can be anionic or cationic, in the case of trsoxane aSs monomer, it can be initiated cationically. Preferably, the polymerization is initiated cationically.

Initiators (also referred to as catalysts) are the cationic initiators customary in the trioxane polymerization. Proton acids such as fluorinated or chlorinated alkyl and aryl sulfonic acids, e.g. Perchloric acid, trifluoromethanesulfonic acid or Lewis acids, e.g. Tin tetrachloride, arsenic pentafluoride, phosphoric acid pentafluoride and boron trifluoride, and their complex compounds and salt-like compounds, e.g. Boron trifluoride etherates and triphenylmethylene hexafluorophosphate. The initiators (catalysts) are used in amounts of about 0.01 to 1000 ppm, preferably 0.01 to 500 ppm and in particular from 0.01 to 200 ppm. In general, it is advisable to add the initiator in dilute form, preferably in concentrations of 0.005 to 5 wt .-%. Suitable solvents for this purpose may be inert compounds such as aliphatic or cycloaliphatic hydrocarbons, e.g. Cyclohexane, hydrogenated aliphatic hydrocarbons, glycol ethers, cyküsche carbonates, lactones, etc. can be used. Particularly preferred solvents are triglyme (triethylene glycol dimethyl ether), 1,4-dioxane, propylene carbonate or gamma-butyrolactone.

In addition to the initiators cocatalysts can be included. These are alcohols of any type, for example aliphatic alcohols having 2 to 20 carbon atoms, such as t-amyl alcohol, methanol, ethanoi, propanol, butanoϊ, pentanol, hexanol; aromatic alcohols having 2 to 30 C atoms, such as hydroquinone; halogenated alcohols having 2 to 20 C atoms, such as hexafluoroisopropanol; very particular preference is given to glycols of any kind, in particular diethylengiykoi and triethylene glycol; and aliphatic dihydroxy compounds, in particular diols having 2 to 6 carbon atoms, such as 1, 2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,4-hexanedol, 1,4-cyclohexanediol, 1, 4-cyclohexanedimethanol and neopentyl glycol.

Monomers, initiators, cocatalyst and, if appropriate, regulators can be premixed in any desired manner or else added separately to the polymerization reactor.

Furthermore, the stabilization components may contain sterically hindered phenols as described in EP-A 129369 or EP-A 128739,

The polymerization mixture is preferably deactivated directly after the polymerization, preferably without a phase change taking place. The deactivation of the initiator residues (catalyst residues) is generally carried out by adding deactivators {demolition agents) to the polymer melt. Suitable deactivators are e.g. Ammonia and primary, secondary or tertiary, aliphatic and aromatic amines, e.g. Trialkylamines such as triethylamine, or triacetonediamine. Also suitable are basic-reacting salts, such as soda and borax, furthermore the carbonates and hydroxides of the alkali metals and alkaline earth metals, and also alkoxides, such as sodium ethanolate. Furthermore, alkali or Erdalkalialkyle are preferred as deactivators, which have 2 to 30 carbon atoms Sm Alkyirest. Particularly preferred metals are Li, Mg and Na, with n-butyl lithium being particularly preferred.

The deactivators are usually added to the polymers in amounts of preferably 0.01 ppmw (parts per million by weight) up to 2% by weight, either as such or dissolved or suspended in water, methanol, other alcohols or other organic solvents, or in molten POM polymer.

Formaldehyde POM can be prepared in the usual way by polymerization in the gas phase, in solution, by precipitation polymerization or in bulk (substance). Trioxane POMs are typically obtained by bulk polymerization using any reactors with high mixing efficiency. The reaction can be carried out homogeneously, e.g. in a melt, or heterogeneous, e.g. ais polymerization to a solid or solid granules. Suitable examples are tray reactors, plowshare mixers, tubular reactors, list reactors, kneaders (e.g., Buss kneaders), extruders with, for example, one or two screws, and stirred reactors, which reactors may comprise static or dynamic mixers.

In the case of mass polymerization, for example in an extruder, a so-called melting seal for the extruder feed can be produced by means of molten polymer, as a result of which volatile constituents remain in the extruder. You dose the above Monomers in the polymer melt present in the extruder, together or separately from the initiators (catalysts), at a preferred temperature of the reaction mixture of 62 to 114 ⁰ C. Preferably, the monomers (trioxane) in the molten state are dosed, for example at 60 to 12O ⁰ C.

The melt polymerization is generally carried out at 1, 5 to 500 bar and 130 to 300 ⁰ C, and the residence time of the polymerization mixture in the reactor is usually 0.1 to 20, preferably 0.4 to 5 min. The polymerization is preferably carried out to a conversion of more than 30%, for example 60 to 90%.

In each case, a crude POM is obtained which, as mentioned, contains considerable proportions, for example up to 40%, of unconverted residual monomers, in particular trioxane and formaldehyde. Formaldehyde can also be present in the crude POM if only trioxane was used as the monomer, since it can be formed as a degradation product of the Tn oxans. In addition, other oligomers of formaldehyde may also be present, e.g. the tetrameric tetroxane.

Trioxane is preferably used as the monomer for the preparation of the POM, which is why the residual monomers also contain trioxane, moreover usually 0.5 to 10% by weight of tetroxane and 0.1 to 75% by weight of formaldehyde.

The crude POM is usually degassed in a degassing device. Degassing devices (Fiash pots), degassing extruders with one or more screws, film extruders, thin film evaporators, spray dryers, strand degassers and other conventional degassing devices are suitable as degassing devices. Degassing extruders or degassing pots are preferably used. The latter are particularly preferred.

The degassing can be carried out in one stage (in a single degassing device). Likewise, it can be multi-stage - for example, in two stages - done in several degassing. In the multi-stage degassing the degassing devices can be the same or different in type and size. Preference is given to using two different Entgasungstöpfe one behind the other, wherein the second pot has a smaller volume.

In a single-stage degassing, the pressure in the degassing device is usually 0.1 mbar to 10 bar, preferably 1 mbar to 2 bar and more preferably 5 mbar to 800 mbar, and the temperature is usually 100 to 260, preferably 115 to 230 and in particular 150 to 210 ^° C. In the case of a two-stage degassing, the pressure in the first stage is preferably 0.1 mbar to 10 bar, in particular

0.5 mbar to 8 bar and particularly preferably 1 mbar to 7 bar, and in the second stage preferably 0.1 mbar to 5 bar, in particular 0.5 mbar to 2 bar and particularly preferably added 1 mbar to 1, 5 bar. The temperature in a two-stage degassing usually does not differ significantly from the temperatures mentioned for the one-stage degassing.

The temperature control of the polymer in the degassing erfoigt in a conventional manner by heat exchangers, double jacket, tempered static mixer, internal heat exchanger or other suitable devices. The adjustment of the degassing pressure is also carried out in a manner known per se, e.g. Mitteis pressure control valves. The polymer may be molten or solid in the degasser.

The residence time of the polymer in the degassing device is generally 0.1 sec to 30 min, preferably 0.1 sec to 20 min. In a multi-stage degassing, these times refer to a single stage.

The residual monomers liberated during degassing are separated off as a vapor stream. Regardless of the design of the degassing (one or more stages, Entgasungstöpfe or extruder, etc.), the residual monomers are usually selected from trioxane, formaldehyde, tetroxane, 1, 3-dioxolane, 1, 3-dioxepan, ethylene oxide and oligomers of formaldehyde.

The separated residual monomers {vapor stream) are withdrawn in a conventional manner. They can be condensed and recycled to the polymerization. The quantitative ratio of trioxane and formaldehyde in the vapor stream can be varied by adjusting appropriate pressures and temperatures.

Lewis acidic compounds

According to the invention, at least a subset of the monomers is contacted with a Lewis acidic compound prior to polymerization. Disclaimed except for the Lewis acidic compounds are molecular sieves with a pore diameter below 0.6 nm (6 Å). Suitable Lewis acidic compounds are all electron-pair acceptor compounds (compounds capable of accepting an electron pair provided by a Lewis base).

The Lewis acidic compounds are preferably selected from silica gels, molecular sieves with a pore diameter of at least 0.6 nm (6 Å), aluminum silicates, aluminum phosphates, silicon aluminum phosphates and aluminum oxides.

It is understood that the Lewis acidic compound should be resistant to the monomer to be purified under the conditions encountered in the contacting. A ⁽ s) silica colloidal, shaped or unshaped silicic acid of elastic to (preferably) solid consistency and pore structure with narrow, medium or wide pores is usually suitable as highly condensed polysilicic acid with a surface-rich leaf structure Bulk densities of 0.7 to 0.8 g / ml on; suitable wide-pore Kiesefgele have as Xerogel bulk densities of 0.4 to 0.5 g / ml.

The silica gel can be used as a water-rich gel, as a xerogel (obtainable by drying the water-rich gel to 95% by weight solids content), as a powder of different particle size or applied to support materials such as glass plates, metal foils or oxidic support materials. Naturally, these support materials should also be inert to the monomers contacted with them.

Preferred silica gels are xerogels, in particular those in powder or bead form, as in the trade, for example, as Sylobead® from Grace Davison, as silica gel 560 and silica gel 490 from Zeochem, as Sorbead® from Engelhard or as D1-1. 10 are available from BASF.

Suitable molecular sieves are all compounds which have defined pores with a uniform pore diameter of at least 0.6 ntn (6 Å). The pore diameter is preferably at least 0.7 nm (7 Å), in particular at least 0.8 nm (8 Å) and particularly preferably at least 0.9 nm (9 Å).

Zeolites are particularly suitable as molecular sieves. Most molecular sieves or zeolites also have a large internal surface area. Zeolites are according to RÖmpp Online Chemie-Lexikon, Thieme Verlag Stuttgart 2005, keyword "zeolites" (document identifier RD-26-00214), hydrous alkali or Erdaika- fi-alumosilicate understood, especially those of the general formula 1

M ₂ / zO ^• Al ₂ O ₃ * x SiO ₂ • y H ₂ O (1)

With

M = mono- or polyvalent metal (in particular alkali metal or alkaline earth metal), z = valence of the cation (for example 1 or 2) x = for example about 1.8 to about 12 y = for example 0 to about 8,

the other molecules with a smaller cross-section than the pore openings can accommodate reversibly in the cavities of the zeolite grid. Particular preference is given to zeolites of the following structural types: MOR, MFI, MEL, BEA, GME, LTL, MAZ ₁ MWW, DON, VFI and FAU, in particular in their alkali metal or alkaline earth metal form.

Suitable zeolites are known and commercially available. They occur naturally or are preferably obtained synthetically, for example by reaction of SiO 2 -containing (eg water glasses, silicas, silica sols) and Al ₂ O ₃ -hitigen (eg aluminum hydroxides, aluminates, kaolins) compounds with, for example, alkali metal or alkaline earth metal hydroxides.

For example, the type 13X molecular sieve commercially available e.g. as Zeosorb® 13X from Tricat Zeolites GmbH, Zeochem® Z10-01 from Zeochem AG, and 13X Zeolite Molecular Sieve from Dalian Absorbent Co. Ltd,

Aisonium aluminate (aluminosilicates) come i.a. Zeolites, feldspars and feldspar agents. ZeoSithe have already been described. Suitable feldspars are, for example, leucite, nepheline, sodalifh, nosean, hauyn and lasurite.

Suitable aluminosilicates are also the trimorphic group Al ₂ SiO ₅ (Al ₂ O ₃ • SiO ₂ ) with the minerals andalusite, sillimanite and kyanite, furthermore mullite, pyrophyllite and many clay minerals.

Suitable aluminum phosphates (aiumophosphates, AIPO for short) are derived from the above-mentioned zeolites of the formula 1 in that Si ^{4+ is} replaced by P ⁵⁺ in the formula. The structures thus consist of linked AI04 and PO ^ tetrahedral assemblies, the formally neutral aiumophosphates of the sum forms! AlPO ₄ result. Defects result in excess negative charges in the lattice, which are compensated by counter cations (eg, alkali, alkaline earth). The nomenclature of the Aiumophosphate is analogous to the classical Zeolithnomenkiatur. Well suited are eg AlPO-5 (AFI), AiPO-11 (AEL) and AlPO-41 (AFO).

Suitable silicon-aluminum phosphates (siicoalumophosphates, SAPO for short) are also derived from zeolites. In these compounds, AIO4, SJO4 and Pθ4 tetrahedral assemblies coexist, resulting in a net negative charge of the lattice which is counterbalanced by counter cations (e.g., alkali, alkaline earth). The nomenclature of the Siiicoalumophospahte corresponds to the Zeolithnomenkiatur. For example, SAPO-11 (AEL), SAPO-5 (AFI) and SAPO-40 (AFR) are well suited.

Suitable Aiuminiumoxide suitable are in particular gamma, delta, theta or eta-Abθ3, and the hydrated precursors of these phases. The hydrated precursors are preferably boehmite, pseudoboehmite or hydrargillite. Gamma Al 2 O 3 is particularly preferred. If necessary, the basicity of the Aiuminiumoxids can be increased by doping with impurities. The dopants used are usually zinc, alkali metal, alkaline earth or lanthanoid compounds, in particular the hydroxides or oxides, in amounts of from 100 ppmw to 1% by weight, based on the aluminum oxide.

Well suitable aluminas are e.g. D10-10 from BASF, Selexsorb® CD and Selexsorb® CDX from Almatis, and Alumina Spheres 1.0 / 160 from Sasol.

Of the Lewis acidic compounds mentioned, the molecular sieves having a pore diameter of at least 0.6 nm, the aluminum silicates and the aluminum oxides are preferred. Particularly preferred are the molecular sieves and the aluminas.

In the process according to the invention, preference is given to using no Bronsted acidic compounds, in particular no Bronsted acidic ion exchangers or adsorbers, for purifying the monomers.

In a preferred embodiment, the Lewis acidic compounds have a specific surface area of at least 50 m ² / g, determined by gas adsorption (according to BET = Brunauer, Emmet and Teller) according to DiN 66131. Preferably, the specific surface area is at least 100, in particular at least 200 and more preferably at least 300 m ² / g.

In another, likewise preferred embodiment, the Lewis acidic compounds have a specific total pore volume of at least 0.1 ml / g, determined by mercury intrusion according to DIN 66133 (June 1993). Preferably, the specific total pore volume is at least 0.3 ml / g, in particular at least 0.4 ml / g.

The Lewis acidic compound is preferably used in solid form and is usually in the form of Kugein (diameter eg 2 to 5 mm), strands or rods (diameter eg 1, 5 to 7 mm, length eg 3 to 15 mm), Wageπradextrudaten (Diameter eg 3 to 10 mm) or chippings (longest dimension eg 0.5 to 2 mm). Possibly. you can set by grinding ("splitters") a suitable grain size.

It is surprising that the separation of the basic impurities with certain compounds, namely the Lewis acidic compounds according to the invention, works well. It would have been expected that basic impurities could be removed or neutralized with any acidic adsorber or catalyst. This is not the case. Design of the incontact treatment

In the process according to the invention, the Lewis acidic compounds are preferably in the form of a solid, agitated or fluidized bed, in particular as a fixed bed. The contact to be contacted, i. Monomers to be purified may be solid, liquid or gaseous, preferably open.

The contact time of the monomers with the Lewis acidic compound is preferably 0.05 sec to 20 min, in particular 0.1 sec to 2 min and particularly preferably 0.5 to 30 sec. The contact time is thus lower than in many processes of the prior art Technology.

Also preferably, the pressure when contacting the monomers with the Lewis acidic compound is 1 to 200 bar, in particular 2 to 100 bar and particularly preferably 5 to 50 bar.

Likewise preferably, the temperature when contacting the monomers with the Lewis acidic compound 30 to 200 ⁰ C _1, in particular 50 to 160 ⁰ C and preferably 70 to 140 ° C.

The process according to the invention for POM preparation can be carried out batchwise or, preferably, continuously. The apparatus Ausgestaitung of the bed is the usual; For example, you can arrange the bed in an absorber or other suitable container, which is flowed through by the monomer to be purified. Preferably used fixed bed absorber. The absorber or other container is preferably filled to 10 to 70 vol .-% of its volume with the Lewis acidic compounds.

The required amount of the Lewis acidic compound or the dimensions of the bed depend in the usual way u.a. according to the type and concentration of the impurities to be removed and the amount of monomer or the monomer flow rate.

A particularly simple embodiment consists of a tube which is filled with the Lewis acidic compound and is mounted in the monomer feed of the polymerization reactor. The Lewis acidic compound can be fixed in the tube by means of filter elements located in front of and behind it.

Prior to first use, the Lewis acidic compounds are usually activated to remove adsorbed substances such as water or carbon dioxide during storage. The activation is generally carried out by heating to 200 to 600, preferably 200 to 400 ⁰ C under inert gas (eg nitrogen or argon), while- mbar spietsweise in an inert gas stream, or by vacuum treatment at 200 to 400 ⁰ C and an absolute pressure of 20 to 500th

After a certain period of operation (service life), the Lewis acidic compounds are loaded with impurities, eg higher-molecular carbonaceous compounds, and the cleaning performance decreases. Regeneration can remove the adsorbed compounds. For this purpose, the Lewis acidic compounds are first treated usually for 1 second to 2 hours, preferably for 60 seconds to 1 hour at 25 to 450, preferably 150 to 450 ⁰ C with inert gas, for example as an inert gas stream. Subsequently, it is usually treated for 1 second to 2 hours, preferably for 60 seconds to 1 hour at 100 to 700, preferably 150 to 45O ⁰ C with an oxygen-containing gas, for example, also as a gas stream. As the oxygen-containing gas, it is possible to use, for example, a mixture of nitrogen or argon with oxygen or air, wherein the oxygen content in the gas is usually 0.1 and 21% by weight.

Surprisingly, no polymerization occurs during the contacting of the monomers with the Lewis acidic compound. Such a polymerization would be undesirable because the polymer formed could render the Lewis acidic compound unusable or clog the tubing or absorber vessel. Microfilters mounted in front of and behind the bed of the Lewis acidic compound, as well as the Lewis acidic compound itself, do not show any deposits of formed polymer.

The size of the absorber, the type and amount of the Lewis acidic compounds and the flow rate of the feed stream or the residence time in the absorber depend on the type and amount of impurities, the required cleaning performance (reproducible concentration of impurities in the purified monomer) and the desired regeneration cycles from.

According to the invention, at least a partial amount of the monomers is brought into contact with the Lewis acidic compound. This partial amount is preferably from 10 to 100, in particular from 20 to 100, and particularly preferably from 50 to 100,% by weight of the total monomer amount.

Preferably, the monomers contacted with the Lewis acidic compound are all (Embodiment S)) or partially (Embodiment W)) of those monomers which have been separated from the POM as unreacted residual monomers. In the remaining in embodiment iϊ) remaining other part of the contacted with the Lewis acidic association monomers may, for example, hand to see Frese see monomers. Accordingly, the process of the invention is preferably characterized in that at least a portion of the monomers contacted with the Lewis acidic compound are those monomers which have been separated from the POM as unreacted residual monomers.

From the method description it follows that the method according to the invention in the embodiment i) is preferably characterized by the following steps:

a) suitable monomers are polymerized to POM, b) unreacted residual monomers are separated from the POM, c) the separated residual monomers are brought into contact with the Lewis acidic compound, d) the resulting residual monomers are introduced into the polymerization (step a)) recycled.

In the embodiment ii), the method according to the invention is preferably characterized by the following steps:

a ^* } suitable monomers are polymerized to POM, b ^* ) unreacted residual monomers are separated from the POM, c ^* ) the separated residual monomers are mixed with fresh monomers to form a mixture, d *) the mixture is reacted with the Lewis acidic compound in Contacted, e ^* ) the resulting mixture is recycled to the polymerization (step a)).

The separated residual monomers or the mixture can be brought into contact with the Lewis acidic compound (step c) or d *)) and then collected in a buffer tank or another device and / or temporarily stored before being recycled to the polymerization (Step d) or e *)). Alternatively, the contacting with the Lewis acidic compound and the intermediate storage in the buffer tank can be spatially and temporally combined by introducing the Lewis acidic compound into the buffer tank, so that the monomers are treated during the intermediate storage with the Lewis acid compound.

Preferably, however, the residual monomers are first brought into contact with the Lewis acidic compound and immediately recycled into the polymerization. Therefore, the process is preferably characterized in that the residual monomers or the mixture after contacting with the Lewis acidic compound (step c) or d ^* )) are recycled directly and without further intermediate storage into the polymerization (step d). or e ^* )). Additives and blending of the PQM

The polyoxymethylene homo- and copolymers which are obtainable by the process according to the invention can be provided with customary additives. Such additives are, for example

Talc,

Polyamides, especially mixed polyamides, alkaline earth silicates and alkaline earth glycerophosphates, esters or amides of saturated aliphatic carboxylic acids, ethers derived from alcohols and ethylene oxide, non-polar polypropylene waxes,

- Nucleating agents, fillers, - Schiagzäh modifying polymers, in particular those based on Ethyien- PropySen (EPM) - or ethylene-propylene-diene (EPDM) rubbers, flame retardants,

Plasticizer,

- adhesion promoters, - dyes and pigments,

Formaldehyde scavengers, in particular amine-substituted triazine compounds, zeolites or polyethyleneimines

- Antioxidants, especially those with phenolic structure, Benzophenonde- derivatives, BenzotriazoJderivate, acrylates, benzoates, oxanilides and hindered amines (HALS = hindered amine light stabilizers).

These additives are known and described, for example, in Gächter / Müller, Plastics Additives Handbook, Hanser Verlag Munich, 4th edition, 1993, Reprint 1996,

The amount of additives depends on the additive used and the effect desired. The person skilled in the usual amounts are known. If used, the additives are added in the customary manner, for example individually or together, as such, as a solution or suspension or preferably as a masterbatch.

The final POM molding composition can be prepared in a single step, e.g. mixing the POM and the additives in an extruder, kneader, mixer or other suitable mixing device with melting of the POM, discharges the mixture and then usually granulated.

However, it has been found to be advantageous to premix some or all of the components "cold" in a dry mill or other mixing apparatus, and homogenize the resulting mixture in a second step with melting of the POM - optionally with the addition of further components - in an extruder or other mixing device. In particular, it may be advantageous to premix at least the POM and the antioxidant (if used).

The extruder or the mixing device can be provided with degassing devices, for example, to remove residual monomers or other volatile constituents in a simple manner. The homogenized mixture is discharged as usual and preferably granulated,

The polyoxymethylene homopolymers and copolymers obtainable by the process according to the invention for POM preparation are likewise provided by the invention.

A further subject of the invention is the use of Lewis acidic compounds for the purification of monomers used for the preparation of polyoxymethylene homopolymers or copolymers, with the exception of molecular sieves having a pore diameter of less than 0.6 nm (6 Å) as the Lewis acidic compound are. Preferably, this use is characterized in that the Lewis acidic compounds mentioned in claim 5 are used, ie the Lewis acidic compounds are selected from silica gels, molecular sieves with a pore diameter of at least 0.6 nm (6 Å), aluminum silicates, aluminum phosphates, Silicon aluminum phosphates and aluminum oxides.

The subject of the invention is also a process for the purification of monomers which are used for the preparation of polyoxymethyl homo- or copolymers by contacting with Lewis acidic compounds, wherein molecular sieves with a pore diameter below 0.6 nm (6 Å) as the Lewis acidic compound excluded are. This cleaning method is preferably characterized by at least one feature mentioned in this description, in particular by at least one of the features mentioned in claims 1 to 15.

The process for POM production and the purification process are characterized by an optimized, low contact time of the monomers with the Lewis acidic compound. In particular, basic impurities are effectively separated from the monomers by contacting with the Lewis acidic compounds. The process is particularly suitable for removing the amines used in the POM preparation as deactivator from the separated residual monomers which are to be recycled into the polymerization. The deactivator concentration in the purified, recycled monomers according to the invention is so low that the polymerization reaction is not impaired. Accordingly, the POM preparation process according to the invention allows the polymerization of deactivator-containing residual monomers, in particular of trioxane or formaldehyde containing ammonia or of trioxane, in a simple and problem-free manner. As a result, the POM production is less sensitive to poor trioxane or formaldehyde quanity

The purification process according to the invention allows the purification of the monomers in an equally simple manner.

Examples:

The use of a monomer mixture which was recycled from a residual monomer removal and deactivator-containing mixture was simulated by using for the polymerization a monomer mixture to which a deactivator (triacetonediamine) had been added. This was done as follows;

A monomer mixture consisting of 94.99 wt .-% of liquid trioxane, 5 wt .-% dioxolan and 0.01 wt .-% ButySal was mixed with 0.5 ppmw triacetonediamine, heated to 80 ⁰ C and 1.5 kg / h pumped through a fixed bed absorber. The absorber was a 10 cm long tube with 9 mm inner diameter, which was filled with a solid, present as granules Lewis acidic compound (see table) and was heated to 120 ⁰ C. The Lewis acidic compound was fixed in the tube by means of filter discs placed in front of and behind it. The amount of the Lewis acid compound was 3.5 g; the grain size was 1, 0 to 1, 6 mm and was possibly obtained by previously milling ("splitting") of the Lewis acidic association.

In comparative examples 4V to 6V, instead of the Lewis acid compounds according to the invention, other compounds were used or the absorber tube was omitted, see table.

The absorber tube leaving monomer mixture, to which ppmw as initiator 2 perchloric acid (than 0.01 wt .-% solution in 1, 4-dioxane) was mixed in a vessel equipped with static mixers tubular reactor at 175 ⁰ C and poSymerisiert 26 bar into the monomer has been. After a polymerization time (residence time) of 6 minutes, triacetonediamine (as a 0.1% by weight solution in 1,3-dioxolane) was metered in and added as deactivator so that the deactivator was present in 10-fold molar excess to the initiator. The residence time in the deactivation zone was 9 min.

The polymer melt was discharged in a conventional manner, cooled and granulated. The yield was determined. The table summarizes the results. A yield of zero means that no granulatable product was obtained.

Table (V means for comparison)

The examples show that good yields were achieved when using the Lewis acidic compounds of the invention. If, instead, a Bronsted acid ion exchanger (Example 4V) or an excessively porous molecular sieve with a pore diameter below 0.6 nm (6A, Example 5V) were used, no product was obtained. If the adsorber tube completely precipitated, no product was also obtained.

Claims

claims

1. A process for the preparation of Potyoxymethylenhomo- or copolymers

(POM) by polymerization of suitable monomers, characterized in that at least a subset of the monomers is brought into contact with a Lewis acidic compound prior to polymerization, wherein molecular sieves having a pore diameter below 0.6 nm (6 Å) as Lewis -sour compound are excluded.

2, Method according to claim 1, characterized in that the polymerization is initiated cationically.

3. Process according to claims 1 to 2, characterized in that the monomers are selected from formaldehyde, trioxane and other cyclic or linear formals.

4. The method according to claims 1 to 3, characterized in that the partial amount is 10 to 100 wt .-% of the total monomer.

5. Process according to claims 1 to 4, characterized in that the Lewis acidic compounds are selected from silica gels, molecular sieves having a pore diameter of at least 0.6 nm (6 Å) ₁ aluminum silicates, aluminum phosphates, silicon aluminum phosphates and aluminum oxides.

6. The method according to claims 1 to 5, characterized in that the Lewis acidic compounds have a specific surface area of at least 50 m ² / g, determined by gas adsorption (BET) according to DIN 66131.

7. Process according to claims 1 to 6, characterized in that the Lewis acidic compounds have a specific total pore volume of at least 0.1 ml / g, determined by mercury intrusion according to DIN 66133 (June 1993).

8. Process according to claims 1 to 7, characterized in that the Lewis acidic compounds are present as a solid, agitated or fluidized bed.

9. Process according to claims 1 to 8, characterized in that the contact time of the monomers with the Lewis acidic compound is 0.05 sec to 20 min.

10, Process according to claims 1 to 9, characterized in that the pressure when Inkontaktbrängen of the monomers with the Lewis acidic compound is 2 to 100 bar.

1 1. A method according to claims 1 to 10, characterized in that the

Temperature when contacting the monomers with the Lewis acidic compound is 30 to 200 ° C.

12. Process according to claims 1 to 11, characterized in that at least a part of the monomers brought into contact with the Lewis acidic compound are monomers which have been separated from the POM as unreacted residual monomers,

13. Process according to claims 1 to 12, characterized by the following steps:

14. Process according to claims 1 to 12, characterized by the following steps:

a ^* ) suitable monomers are polymerized to POM, b ^* ) unreacted residual monomers are separated from the POM, c ^* } the separated residual monomers are mixed with fresh monomers to form a mixture, d *) the mixture is reacted with the Lewis acidic compound in Contacted, e ^* ) the resulting mixture is recycled to the polymerization (step a)).

15. The method according to claims 1 to 14, characterized in that the residual monomers or the mixture after contacting with the Lewis acidic compound (step c) or d ^* )} are recycled directly and without further intermediate storage in the polymerization ( Step d) or e *)).

16. Use of Lewis acidic compounds for the purification of monomers used for the preparation of Polyoxymethylenhomo- or Copoiymeren with exclusion of molecular sieves with a pore diameter below 0.6 nm (6 Å) as the Lewis acidic compound.

17. Use according to claim 16, characterized in that the Lewis acidic compounds mentioned in claim 5 are used.

18. A process for the purification of monomers used for the preparation of pofyoxymethylene homo- or copolymers by contacting them with Lewis acidic compounds, with molecular sieves having a pore diameter of less than 0.6 nm (6 Å) as Lewis acid. are excluded acid compound.

19. The method according to claim 18, characterized by at least one of the features mentioned in claims 1 to 15.

20. Poiyoxymethylenhomo- and copolymers obtainable by the process according to claims 1 to 15.