WO2007036476A1

WO2007036476A1 - Method for producing polyoxymethelenes in a tubular reactor with dynamic mixing zones

Info

Publication number: WO2007036476A1
Application number: PCT/EP2006/066571
Authority: WO
Inventors: Knut ZÖLLNER; Jens Assmann; Claudius Schwittay
Original assignee: Basf Aktiengesellschaft
Priority date: 2005-09-28
Filing date: 2006-09-21
Publication date: 2007-04-05
Also published as: DE102005046593A1

Abstract

A method for producing polyoxymethylene homo- or co-polymers (POM) in a tubular reactor is disclosed, characterized in that the tubular reactor comprises the following zones: I) an initial mixing zone (I), where suitable monomers are mixed with a polymerisation initiator, II) a polymerisation zone (II), where the obtained mixture is reacted to POM, III) a deactivator mixing zone (III), where a deactivator is admixed to the reaction mixture and IV) a deactivation zone (IV), where the reaction mixture is deactivated by the deactivator, wherein a) at least one of the zones, the initial mixing zone (I) or deactivator mixing zone (III) comprise actuated (dynamic) components, and b) the maximum diameter of the tubular reactor in the initial mixing zone I is not greater than 90 % of the maximum diameter in the polymerisation zone (II).

Description

Process for the preparation of polyoxymethylenes in a tubular reactor with dynamic mixing zones

description

The invention relates to a process for the preparation of polyoxymethylene homopolymers or copolymers (POM) in a tubular reactor, characterized in that the tubular reactor has the following zones:

I) an initiator mixture zone I in which suitable monomers are mixed with a polymerization initiator, II) a polymerisation zone II in which the resulting mixture is converted to POM,

IM) a deactivator mixing zone IM in which a deactivator is mixed into the reaction mixture and

IV) a deactivation zone IV, wherein the reaction mixture is deactivated by the deactivator,

in which

a) at least one of the zones initiator mixing zone I and the deactivator mixing zone IM has moving (dynamic) internals, and b) the maximum diameter of the tubular reactor in the initiator mixing zone I is at most 90% of the maximum diameter in the polymerisation zone II.

The invention further relates to the polyoxymethylene homopolymers or copolymers obtainable by this process, and to the use of said tubular reactor for the preparation of polyoxymethylene homo- or copolymers.

Polyoxymethylene polymers (POM, also called polyacetals) are obtained by polymerization of 1,3,5-trioxane (trioxane for short) or another formaldehyde source, comonomers such as 1,3-dioxolane, 1,3-butanediol formal or ethylene oxide being used to prepare copolymers be used. The polymerization is usually carried out cationically; For this purpose, strong protic acids, for example perchloric acid, or Lewis acids, such as tin tetrachloride or boron trifluoride, are metered into the reactor as initiators (catalysts). Subsequently, the reaction is usually stopped by metering in ammonia, amines, alkali metal alcoholates or other basic deactivators.

The melt polymerization of POM in a tubular reactor is known. A particular problem in melt polymerization is the addition of the terminating agent. In this case, very small amounts of a low molecular weight component (deactivating agent) must be present. tor) are mixed effectively with a high-viscosity polymer melt. If the termination is too early, the desired molecular weight can not be established; if terminated too late, side reactions that degrade the polymer chains and form undesirable end groups, see Shieh et al., J. Pol. Res., 2003, Vol. 10, pages 151-160.

EP 638 357 A2 discloses the preparation of polyacetals in a flow tube reactor in which the process zones (mixing of monomer and initiator, polymerization, deactivation, and chain end degradation or end-capping) merge into one another. The reactor is composed of a mixing zone for monomer and initiator, a polymerization zone, a deactivator mixing zone, and a deactivation and stabilization zone and equipped with static mixing elements along the entire length. The static mixing elements and the reactor diameter in the zones are to be dimensioned such that certain retention times are met. Further details on the reactor diameter are not made.

EP 638 599 A2 teaches the preparation of POM in a tubular reactor with static mixing elements. According to page 3 line 19-21, such a reactor is preferable to other aggregates such as kneaders, extruders or stirred kettles, ie units with moving internals. Also in this reactor, the zones are flowing into each other and there are certain residence times to be met. Details on the reactor diameter are missing.

EP 80 656 A1 describes the preparation of oxymethylene polymers in an apparatus without flowing transitions, namely a polymerization reactor (extruder, kneader, stirred tank, gear pump, flow tube with or without static mixing elements) and a deactivation reactor, which from the polymerization reactor by built-in bottlenecks, which the flow rate locally increase the polymer melt, is spatially separated. The deactivation reactor may contain special kneading elements. Different diameters of the two reactors are not mentioned; rather, the example teaches identical diameters.

The unpublished German patent application number 102004057867.2 from 30.1 1.04 discloses the POM production in a tubular reactor with static

Mixing elements, composed of a mixing zone, a polymerization zone and a deactivation zone, wherein the reactor diameter in the mixing zone is <90% of the diameter in the polymerization zone. Dynamic (moving) mixing elements are not mentioned. The object was to provide an improved process for the preparation of polyoxymethylene homo- and copolymers. In particular, the addition of the polymerization initiator or the deactivator should be optimized.

In addition, it should be possible with the process to produce a POM which has a high viscosity (low melt volume rate, MVR).

Accordingly, the method defined at the outset, the polymers mentioned in the introduction and the use mentioned at the outset, have been found. Preferred embodiments of the invention can be found in the subclaims. All mentioned pressures are absolute pressures.

polyoxymethylene

The polyoxymethylene homopolymers or copolymers (POM) are known as such and are commercially available. They are usually prepared by polymerization of formaldehyde or, preferably, trioxane; Comonomers are also used in the preparation of copolymers. The monomers are preferably selected from trioxane and other cyclic or linear formals or other sources of formaldehyde.

In general, such POM polymers have at least 50 mol% of repeating units -CH 2 O- in the main polymer chain. Suitable polyoxymethylene copolymers are, in particular, those which, in addition to the repeating units -CH 2 O-, also contain 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 C 1 to C ₄ AIkVl- or C 1 to C 4 haloalkyl substituted methylene group or a corresponding oxymethylene group and n has a value in the range of 0 to 3. 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-dioxolane and 1, 3-dioxepane (= butanediol, BUFO) may be mentioned as cyclic ethers and linear oligo- or polyformals such as polydioxolane or polydioxepan called comonomer. 1, 3-dioxolane and 1, 3-dioxepane are particularly preferred comonomers.

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

CH ₂ - Z - CH ₂

and or

O _v ^ O 0 ^ 0

wherein Z is a chemical bond, -O-, -ORO- (R is d- to Cs-alkylene or C3 to Cs-cycloalkylene) can be prepared.

Preferred monomers of this type are ethylene diglycide, diglycidyl 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 diglycidyl ethers of ethylene glycol, 1 , 4-butanediol, 1, 3-butanediol, cyclobutane-1, 3-diol, 1, 2-propanediol and cyclohexane-1, 4-diol, to name just a few examples.

Regulators and initiators

If appropriate, the molecular weights of the polymer can be adjusted to the desired values by the regulators customary in the trioxane polymerization, and by the reaction temperature and residence time. Suitable regulators are acetals or formals of monohydric alcohols, the alcohols themselves, and the small amounts of water acting as chain transfer agents, the presence of which is generally never complete. constantly avoiding, in question. The regulators are used in amounts of 10 to 10,000 ppmw (parts per million by weight), preferably from 20 to 5,000 ppmw. Methylal and butylal are preferred regulators.

Preferably, the polymerization is initiated cationically. Polymerization 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. Proton acids are preferably used as the polymerization initiator. Particularly preferred is perchloric acid.

The initiators (catalysts) are used in amounts of about 0.01 to 1000 ppmw, preferably 0.01 to 500 ppmw and in particular from 0.01 to 200 ppmw, based on the monomers used. In general, it is advisable to add the initiator in dilute form in order to be able to precisely meter and homogeneously distribute said small amounts of initiator. Suitable are e.g. Dilutions with an initiator content of 0.005 to 5 wt .-%. Suitable diluents or solvents are u.a. aliphatic or cycloaliphatic hydrocarbons e.g. Cyclohexane, 1,4-dioxane; halogenated aliphatic hydrocarbons; Glycol ethers such as triglyme (triethylene glycol dimethyl ether); cyclic carbonates such as propylene carbonate; or lactones such as gamma-butyrolactone.

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

deactivators

Subsequent to the polymerization, the polymerization mixture is deactivated, preferably without a phase change takes place. The deactivation of the initiator residues (catalyst residues) takes place by adding a deactivator (terminating agent) to the reaction mixture.

Suitable deactivators are e.g. Ammonia, aliphatic and aromatic amines, basic salts such as soda and borax, and organic compounds of alkali metals or alkaline earth metals. These are usually added to the polymers in amounts of preferably up to 1% by weight.

The organic compounds of the (alkaline) alkali metals, preferably of the sodium, include the corresponding salts of (cyclo) aliphatic, araliphatic or aromatic matic carboxylic acids having preferably up to 30 carbon atoms and preferably one to four carboxyl groups. Examples of these are: alkali metal salts of formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, caprylic acid, stearic acid, cyclohexanecarboxylic acid, succinic acid, adipic acid, suberic acid, 1,10-decanedicarboxylic acid, M-cyclohexanedicarboxylic acid, terephthalic acid, 1,2,3-propanetricarboxylic acid. acid, 1,3,5-cyclohexane tricarboxylic acid, trimellitic acid, 1,2,3,4-cyclopentanetetracarboxylic acid, pyromellitic acid, benzoic acid, substituted benzoic acids, dimer acid and trimer acid, and neutral and partially neutral montan wax salts or montan wax ester salts (montanates). Salts with different acid radicals, such as, for example, alkali metal paraffin, alkali metal olefin and alkali arylsulfonates or also phenolates and also alcoholates, such as, for example, methanolates, ethanolates, glycolates, can be used according to the invention.

Preference is given to sodium salts of mono- and polycarboxylic acids, in particular the aliphatic mono- and polycarboxylic acids, preferably those having 2 to 18 carbon atoms.

Atoms, in particular having 2 to 6 carbon atoms and up to four, preferably up to two carboxyl groups, and sodium alcoholates having preferably 2 to 15, in particular 2 to 8 carbon atoms used. Examples of particularly preferred representatives are sodium acetate, sodium propionate, sodium butyrate, sodium oxalate, sodium malonate, sodium succinate, sodium methoxide, sodium ethoxide, Natirumglykonat. Very particular preference is given to sodium methoxide, which is used particularly advantageously in an amount of one to 20 times equimolar to the initiator. It is also possible to use mixtures of various (earth) alkali metal compounds, wherein hydroxides can also be used.

Furthermore, alkaline earth metal alkyls are preferred as deactivators which have 2 to 30 C atoms in the alkyl radical. As particularly preferred metals called its Li, Mg and Na, with n-butyllithium being particularly preferred.

Preferred deactivators are those of the formula 1

in which: R ¹ , R ³ and R ⁴ independently of one another denote hydrogen or a C ¹ -C 10 -alkyl group,

R ^{2 is} hydrogen or a C ¹ -C ¹⁰ -alkyl group or -OR ⁵ ,

R ^{5 is} hydrogen or Ci-io-alkyl. Preferred radicals R ¹ to R ⁵ are independently hydrogen or a C ¹ -C ⁴ -alkyl group, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, t-butyl.

Particularly preferred deactivators of formula 1 are substituted N-containing heteroaromatic cycles, in particular derivatives of piperidine, with triacetonediamine (4-amino-2,2,6,6-tetramethylpiperidine) being particularly preferred.

The deactivator is, based on the throughput of trioxane in amounts of 0.001 to 25 ppmw, preferably 0.01 to 5 ppmw, in particular 0.05 to 2 ppmw added. The deactivator is preferably present diluted in one of the following carriers or solvents. The concentration of the deactivator in the vehicle or in the solvent is 0.001 to 10, preferably 0.01 to 5, in particular 0.05 to 2, very particularly preferably 0.1 to 1 wt .-%.

The deactivator is preferably added in an aprotic, non-aromatic solvent, for example the abovementioned monomers and comonomers, such as dioxolane, trioxane, butanediol formal, ethylene oxide or oligomeric to polymeric polyacetals.

In a particularly preferred embodiment of the process according to the invention, the deactivator is added to the polymerization mixture in a carrier substance having ether structural units. Preferably, carrier substances which have the same structural units which are present in the particular POM polymer to be produced are suitable. These include, in particular, the monomers listed above as well as oligomeric to polymeric polyoxymethylene or polyacetals.

The deactivator is preferably added in liquid form at temperatures of 140 to 220 ° C; in oligomeric or polymeric POM as a carrier substance, for example in the form of a polymer melt.

Design of the reactor

According to the invention, the process is carried out in a tubular reactor which has several zones and in which certain zones are provided with internals. For the internals see below.

According to the invention, the tube reactor, viewed in the flow direction, contains an initiator mixing zone I, a polymerization zone II, a deactivator mixing zone IM and a deactivating zone IV. In one possible embodiment and in contrast to the mentioned EP 80 656 A1, the deactivation is not separated by built-in bottlenecks, which increase the flow rate locally, or by blocking elements of the polymerization. In another likewise possible embodiment, such bottlenecks or damming elements are present in order to separate the deactivation, for example in the form of a connecting tube between two zones.

The pressure, residence time and shear in a zone can vary widely depending on whether the zone contains internals or not (see below). The following ranges of pressure, residence time or shear include zones with and without internals. The lengths or length / inner diameter ratios of the individual zones depend in the usual way on the throughput of the reactor and the desired residence time.

The shear in a static mixer is calculated according to:

γ = k-v / D

The viscosity is calculated according to:

η = (Δp-D ² ) / (NeRe-Lv)

where: y: shear [1 / s] k: manufacturer constant for the static mixer v: flow rate in the mixer

D: diameter of the mixer η: viscosity

Δp: Pressure drop across mixer NeRe: Static mixer statistic = Newton Reynolds number

L: length of the mixer

In the initiator mixture zone I, the monomers are mixed with the polymerization initiator and - if used - the regulator. Monomers, initiators and, if appropriate, regulators may be premixed in any desired manner or may also be added to the polymerization reactor separately from one another.

The monomers, for example trioxane, are preferably metered in the molten state, for example at 60 to 180, in particular 62 to 170 and particularly preferably 120 to 160 ° C. (temperature of the reaction mixture during metering). The pressure is usually 2 to 100, preferably 20 to 80 bar. The residence time in zone I is usually 1 to 300, preferably 5 to 60 s. The shear is usually at 5 to 1000, preferably 10 to 750 and in particular 20 to 500 1 / s. The viscosity of the reactor contents in zone I is usually 0.1 mPa.s to 100 Pa.s, preferably 0.1 mPa.s to 20 Pa.s.

In order to minimize the proportion of unstable end groups in the POM, it has proved to be advantageous to dissolve the initiator in the regulator before it is added to the monomers. In addition, it may be advantageous to dose the initiator at different points of the tubular reactor. The preferred minimum distance is 1 D, where D is the diameter of the reactor at the given location.

In the polymerization zone II, the mixture obtained in the initiator mixture zone I is converted to the polyoxymethylene. The polymerization is preferably carried out to a conversion of at least 30, in particular at least 60%. The polymerization is carried out at the temperatures customary for POM preparation, for example 130 to 200, preferably 130 to 170 ° C. The pressure is usually 5 to 200, preferably 10 to 100 bar.

The residence time in zone II is usually 0.1 to 40, preferably 1 to 20 min. The shear is usually from 1 to 300, preferably from 2 to 100 and in particular from 3 to 50 1 / s. The viscosity of the reactor contents in zone II is usually 1 to 1000, preferably 10 to 500 and in particular 100 to 400 Pa-s.

The deactivator mixing zone IM serves to mix in the deactivator in the zone II leaving reaction mixture containing polymer and unreacted monomers. Zone IM is usually operated at 130 to 230, preferably 150 to 220 ° C. The pressure is usually 1 to 100, preferably 5 to 50 bar.

The residence time in zone IM is usually 1 to 600, preferably 5 to 150 s. The residence time in zone III is preferably at most 75%, in particular at most 50% and particularly preferably at most 25% of the residence time in deactivation zone IV.

The shear in zone IM is usually at 5 to 100, preferably 10 to 50 1 / s. The viscosity of the reactor contents in zone IM is usually 10 to 1500 Pa · s, preferably 20 to 800 Pa · s.

In the deactivation zone IV, the reaction mixture is deactivated by the deactivator. Preferably, the deactivation takes place without a phase change. Zone IV is usually operated at 150 to 220 ° C. The pressure is usually 5 to 100, preferably 5 to 50 bar. The residence time in zone IV is usually 0.5 to 20, preferably 1 to 10 min. The shear is usually at 0.1 to 500, preferably 1 to 100 and in particular 3 to 75 1 / s. The viscosity of the reactor contents in zone IV is usually 1 to 1000, preferably 100 to 800 and in particular 150 to 600 Pa-s.

Also according to the invention ^'

a) has at least one of zones initiator mixing zone I and deactivator mixing zone IM moving (dynamic) internals.

In a preferred embodiment, the deactivator mixing zone IM, but not the initiator mixing zone I has moving internals.

In another, likewise preferred embodiment, the initiator mixture zone I, but not the deactivator mixing zone IM, has moving internals.

In particular, the process is characterized in that the initiator mixing zone I or the deactivator mixing zone IM or both zones I and IM contain a device which is selected from mixing pumps, gear pumps, kneaders, extruders, rotor and stator equipped inline mixers, cone mixers or stirred kettles.

Preference is given to using a mixing pump or a gear pump. Such pumps are described, for example, in DE 19614894 A1, US 5005982 A, JP 63077526 A, JP 87050671 B, JP 82041291, SU 901628, DT 2355230, JP 09271654 A and DT 2550069, and others. available from the companies Zimmer, Witte or Maag. Preferred are u.a. Pumps with holes with teeth.

Kneaders or extruders are u.a. Two-shaft machines, for example the counter-run models ORP (LIST) or Reavisc® or Reasol® (Buss-SMS), or the Coläufer models CKR or CRP (LIST) or Reaflow® or Reacom® ( Fa. Buss-SMS). Also suitable are single-shaft machines, for example DTB (Discotherm® B) or Reactotherm® from Krauss Maffei or Buss-SMS. Such devices are also available in continuous operation.

Suitable inline mixers, cone mixers or stirred kettles are likewise commercially available and known to the person skilled in the art. Suitable kneaders or mixers are also described in Saechtling (ed.): Kunststoff-Handbuch, 27th Edition, Hanser-Verlag, Munich, 1998, on pages 202-213.

The dimensioning of the moving internals and their speed of movement, for example, the size and speed of the mixing or gear pump, the size and rotation or stroke number of the kneader, the diameter and speed of the extruder screws, the size and speed of the rotor-stator system in the inline mixer or the cone in the cone mixer, or the shape and speed of the stirrer in the stirred tank, are more common For example, on the viscosities of the substances to be mixed and the amounts (flows) of these substances. In the initiator mixture zone I, these substances to be mixed are the monomers, the initiator and optionally the regulator; in the deactivator mixing zone IM, it is the reaction mixture containing polymer and (residual) monomers, and the deactivator.

If both zones I and IM have moving internals, the fixtures or devices may be the same or different. For example, a mixing or gear pump can be used in zone I and a kneader or extruder in zone IM.

In addition to the moving (dynamic) internals, the tube reactor can also contain unmoved (static) internals. Particularly preferably, at least one of the zones of initiator mixture zone I, polymerization zone II and deactivation zone IV has stationary (static) internals.

A zone may contain stationary and moving installations. However, this is not preferred, i. the zone I or IM preferably contains either moving or stationary installations.

Static mixers, for example superimposed, twisted sheet metal elements (Kenics mixers from Chemineer Ine), plates with inclined bores, for example, interfacial surface generator (ISG) and low pressure drop mixer (both from Ross Engineering Ine) are preferably suitable as immobile internals. ; Metal fabric packages (for example SMV mixer from Sulzer) or mixers of the type SMV, SMX, SMXL or SMR (from Sulzer Koch-Glitsch); Inliner series 45 (from Fa. Lightnin Inc.); and CSE mixer (from Fa. Fluitec Georg AG). The said mixers and manufacturers are to be understood as examples; Other products can be used.

If the initiator-mixing zone I and the polymerization zone II have stationary installations, the number of bars, sheet-metal elements, perforated plates or other mixing elements (hereinafter referred to collectively as bars) in zone I is preferably 0 to 500, in particular 0 to 300 and preferably 0 to 100% higher than in zone M.

If the deactivation zone IV and the polymerization zone II have stationary internals, the number of bridges in zone IV is preferably from 0 to 500, in particular from 0 to 300 and preferably from 50 to 200%, higher than in zone M. The zones I to IV may have different diameters. This is preferably the case. In addition, the zones I to IV can each be divided into sections of different diameters.

The initiator mixture zone I usually consists of one to four sections, preferably of one section. According to the invention

b) the maximum diameter of the tube reactor in the initiator mixture zone I is at most 90% of the maximum diameter in the polymerization zone II.

The maximum diameter in zone I is preferably from 10 to 90, in particular from 20 to 90, particularly preferably from 30 to 90 and preferably from 50 to 85% of the maximum diameter in zone II.

The polymerization zone II contains in particular two to ten sections, preferably two to five sections. The diameter of the smallest section in zone II is preferably 20 to 100%, in particular 30 to 90% and particularly preferably 45 to 75% of the diameter of the largest section of zone II.

The deactivator blending zone IM usually contains a section.

The deactivation zone IV usually has one to five, preferably two sections. The diameter of the smallest section in zone IV is generally 20 to 100%, preferably 30 to 90% and in particular 50 to 90% of the diameter of the largest section of zone IV.

In the particularly preferred embodiment of the deactivation zone IV in two sections, the first section contains a 20 to 500, preferably 50 to 300 and in particular 50 to 200% higher number of webs than the polymerization zone II and the second section a -100 to +500, - 100 to +300 and preferably -100 to +200% higher number of bridges than the polymerization zone II.

In the preferred embodiment of zone IV in multiple sections, the shear in the smallest diameter section, preferably the first section following the polymerization zone, is 10 to 200, preferably 15 to 100 and most preferably 20 to 75 1 / s. In the following sections, the shears usually correspond to the above values.

In a preferred embodiment i), the maximum diameter of the tubular reactor in the polymerization zone II is at most 95%, in particular 20 to 95, particularly preferably 30 to 90 and in particular 50 to 85% of the maximum diameter in the deactivation zone IV, ie the largest section in zone II is smaller than the largest section in zone IV.

In another preferred embodiment ii) the maximum diameter of the tubular reactor in the deactivation zone IV is at most 95%, in particular 20 to 95, more preferably 30 to 90 and in particular 50 to 85% of the maximum diameter in the polymerization zone II, i. the largest section in zone II is larger than the largest section in zone IV.

In a further preferred embodiment iii) the maximum diameter of the tubular reactor in the polymerization zone II and in the deactivation zone IV are identical.

Whether embodiment i), ii) or iii) is more advantageous depends i.a. from the residence time and the temperature profile in the tubular reactor.

Separation of the POM and addition of additives

The reaction mixture leaving the deactivation zone IV usually also contains unreacted monomers, so-called residual monomers, in addition to the polymer (POM). Preferably, after the deactivation zone IV, the POM is separated from the reaction mixture. This is done in a manner known per se, e.g. by separating the residual monomers from the POM in a residual monomer removal.

Suitable degassing devices are for this purpose, for example strand degassers, degassing extruders, degassing pots (flash pots) or combinations of such degassing devices. They are for example operated at 150 to 250, preferably 160 to 220 ° C and usually at reduced pressure, e.g. at 0.1 to 900, in particular 3 to 500 mbar.

In the deactivation zone IV or after, e.g. During the residual monomer removal or degassing, the polymer can be provided with conventional additives. The addition of additive can be carried out with the concomitant use of solvents or without solvents. Suitable additives are, for example

Talc,

Polyamides, in particular mixed polyamides, alkaline earth metal 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, impact-modifying polymers, in particular those based on ethylene-propylene (EPM) or ethylene-propylene-diene (EPDM) rubbers, flame retardants, plasticizers, adhesion promoters, dyes and pigments,

Formaldehyde scavengers, in particular amine-substituted triazine compounds, zeolites or polyethyleneimines - antioxidants, in particular those having a phenolic structure, benzophenone derivatives, benzotriazole derivatives, acrylates, benzoates, oxanilides and sterically 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 proven to be advantageous to premix some or all of the components first in a dry mixer or another mixing apparatus "cold" and the resulting mixture in a second step with melting of the POM - optionally with the addition of other components - in an extruder or other 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.

Obtained polyoxymethylenes

The polyoxymethylene homopolymers or copolymers obtainable by the process according to the invention are likewise provided by the invention. End-group stabilized polyoxymethylene polymers having predominantly CC or -O-CHa bonds at the chain ends are particularly preferred.

The preferred polyoxymethylene polymers have melting points of at least 150 ° C and weight average molecular weights M _w in the range of 5,000 to 300,000, preferably 7,000 to 250,000. Particular preference is given to POM polymers having a nonuniformity (weight average M _w / number average M _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) or SEC (size exclusion chromatography), the M _n value (number average molecular weight) is generally determined by GPC / SEC.

Typical conditions for molecular weight determination by GPC are, for example: eluent hexafluoroisopropanol containing 0.05% by weight trifluoroacetic acid potassium salt; Column temperature 40 ° C; Flow rate 0.5 ml / min; Detector differential refractometer (e.g., Agilent G1362A); Calibration with narrowly distributed polymethyl methacrylate standards (for example those from the company PSS with molecular weights from M = 505 to M = 2,740,000), elution ranges lying outside this interval are estimated by extrapolation.

For the polyoxymethylene homopolymers and copolymers it is possible to determine dso, dio and dgo values of the weight-average molecular weight M _w . The dso value is _w have the Mw value at which 50% of the polymer having a smaller M _w corresponding to 50% and a larger M. The dio value is the M _w value at which 10% of the polymer has a smaller M _w and 90% has a larger M _w . The dgo value is the M _w value at which 90% of the polymer _w a smaller M and 10% have a larger Mw.

In the case of the polyoxymethylenes according to the invention, the quotient dso / d-io is preferably 2.25 to 5.5, preferably 2.75 to 5 and in particular 3.2 to 4.5. The quotient dgo / dso is preferably from 1.25 to 3.25, preferably from 1.75 to 2.75 and in particular from 2 to 2.5. All D values or quotients refer to the M _w .

The POM polymers have very low proportions of low molecular weight fractions and preferably an asymmetric, unimodal distribution curve, the difference of the above-mentioned. Ratios expressed as dso / dio minus dgo / dso, at least 0.25, preferably 1 to 3 and especially 1, 0 to 2.3.

The polyoxymethylenes obtainable by the process according to the invention preferably have a residual formaldehyde content in accordance with regulation VDA 275 in the granules of not more than 500, preferably not more than 100, preferably not more than 10 ppmw. Another subject of the invention is the use of a tubular reactor described above for the preparation of Polyoxymethylenhomo- or copolymers.

The inventive method is characterized by a rapid and uniform mixing of the initiator or the deactivator in the reaction mixture. This reduces the required amount of deactivator. The residence time distribution narrows in the deactivator mixing zone IM, and the distribution of polymer chain lengths becomes more uniform.

With the method, a POM is obtained which has an advantageously high viscosity (low melt volume rate, MVR).

The embodiment of the invention with moving internals in zone IM has the further advantage that the initiator mixing in zone I and the polymerization in zone II can be carried out at lower pressure. Thus, the zones I and II can be designed for lower pressures; In particular, one can use weaker, cheaper pumps and reactor components with lower pressure resistance. This improves the economy of the process.

Examples

Inventive Example 1:

A liquid mixture of 98 parts by weight of trioxane, 2 parts by weight of 1, 3-dioxolane as a comonomer and 40 ppmw methylal as a regulator was fed at 5 kg / h a tubular reactor, wherein the methylal premixed with the comonomer.

The initiator used was a 0.1% strength by weight solution of 70% strength by weight perchloric acid in 1,4-dioxane. Triacetonediamine in the form of a 0.1% by weight solution in 1,3-dioxolane was used as the deactivator.

The construction of the tube reactor was as follows, where D inner diameter and L / D denote the ratio length / inner diameter:

I) Initiator mixture zone I:

D = 12 mm and L / D = 10, contained static mixers as stationary fixtures; Adding the initiator solution in an amount of 0.1 ppmw, calculated as initiator; the pressure in zone I was X bar, see Table M) Polymerization zone M: D = 27 mm, L / D = 15, without internals

IM) deactivator mixing zone IM:

Gear mixing pump as moving internals, type 4,7-1 from Witte, modifi- graced, with perforated teeth; The desactivator solution was metered into the inlet flange of the pump; the required amount of deactivator Y is given in the table

IV) Deactivation zone IV: D = 27 mm, L / D = 12, contained static mixers as immobile internals.

The polymer leaving the tube reactor was freed from the residual monomers in a flash degasser. The product was granulated. On the granules obtained, the melt volume rate MVR was determined according to ISO 1 133 at 190 ° C melt temperature and 2.16 kg nominal load.

Comparative Example 2V:

The procedure was as described in Example 1, but no deactivator mixing zone IM was present. Instead, the deactivator solution was metered with a piston pump directly into the flange, which connected the zones Il and IV.

The table summarizes the results.

table

In the process according to the invention (pump as a deactivator mixing zone), zone I could be operated at a considerably lower pressure, and the required amount of deactivator was only about half, compared to the noninventive method without deactivator mixing zone.

The POM obtained according to the invention had a significantly lower melt volume rate - ie a significantly higher viscosity - than the POM not according to the invention.

Claims

claims

1. A process for the preparation of polyoxymethylene homopolymers or copolymers (POM) in a tubular reactor, characterized in that the tubular reactor has the following zones:

I) an initiator mixture zone I in which suitable monomers are mixed with a polymerization initiator,

II) a polymerization zone II, in which the resulting mixture is converted to POM,

in which

a) at least one of the zones of initiator mixing zone I and of the deactivator mixing zone M1 has moved (dynamic) internals, and b) the maximum diameter of the tubular reactor in the initiator-mixing zone

I maximum 90% of the maximum diameter in the polymerization zone

II is.

2. The method according to claim 1, characterized in that the Desaktivatormi- shear zone IM, but not the initiator mixing zone I has moving internals.

3. The method according to claim 1, characterized in that the initiator mixing zone I, but not the deactivator mixing zone IM has moving built-in structures.

4. Process according to claims 1 to 3, characterized in that the initiator mixing zone I or the deactivator mixing zone IM or both zones I and IM contain a device which is selected from mixing pumps, gear pumps, kneaders, extruders, provided with rotor and stator Internal mixers, cone mixers or stirred kettles.

5. Process according to claims 1 to 4, characterized in that at least one of the zones initiator mixing zone I, polymerization zone II and deactivating zone IV has stationary (static) installations.

6. Process according to claims 1 to 5, characterized in that the maximum diameter of the tubular reactor in the polymerization zone II is at most 95% of the maximum diameter in the deactivation zone IV.

7. Process according to claims 1 to 5, characterized in that the maximum diameter of the tubular reactor in the deactivation zone IV is at most 95% of the maximum diameter in the polymerization zone II.

8. The method according to claims 1 to 7, characterized in that after the deactivation zone IV, the POM is separated from the reaction mixture.

9. Polyoxymethylene Homo- or copolymers, obtainable by the process according to claims 1 to 8.

10. Use of a tubular reactor as defined in claims 1 to 8 for the preparation of Polyoxymethylenhomo- or copolymers.