HK1118070A1 - Method for producing homopolymers and copolymers of polyoxymethylene and corresponding device - Google Patents

Abstract

The invention relates to a device for producing homopolymers and copolymers of polyoxymethylene. Said device comprises the following elements: A) a reactor (1) having a polymerization zone (2) and a directly adjacent deactivation zone (2) for the polymerization and deactivation of homopolymers and copolymers of polyoxymethylene in a homogeneous phase in a manner known per se, B) a relief unit (4) which optionally comprises a metering device (5) for additives to be added to the polymer, C) a granulator (6), D) an extraction device (7), and E) optionally a drying device (8). The inventive device and the polymerization method carried out therewith allow to achieve especially low residual monomer contents in a simple and power-saving manner.

Description

Method for producing polyoxymethylene homo-and copolymers and device suitable for this purpose

Technical Field

The invention relates to an improved method for the homogeneous preparation of polyoxymethylene homo-and copolymers and also to a device which is particularly suitable for this purpose.

Background

The preparation of paraformaldehyde is known per se. The polymerization can be carried out in bulk or in solution, and can likewise be carried out at normal pressure or under pressure.

Various methods for producing oxymethylene homopolymers or copolymers are known. A number of publications describe the continuous polymerization of the monomers on an industrial scale, for example U.S. Pat. No. 3, 3,027,352, U.S. Pat. No. 3, 3,803,094, DE-A-1,161,421, DE-A-1,495,228, DE-A-1,720,358 and DE-A-3,018,898. Kneaders, extruders, rolls or belt polymerization reactors are described in particular.

A common feature of these processes is that a phase transition from gaseous or liquid monomer to semicrystalline solid polymer occurs during the polymerization reaction. This creates problems of dissipation of the heat released during the polymerization and crystallization and thus causes a loss of conversion.

There are also previously described processes in which the polymer is prepared in a homogeneous phase. EP-A-080,656 describes ｃA continuous bulk polymerization process of trioxane in ｃA homogeneous liquid phase at temperatures above 135 ℃. The advantages of the process are, in particular, the simple operation of the process, the very low energy consumption and the stable product quality of the polymer.

EP-A-638,599 and DE-A-4423617 describe improvements in homogeneous polymerization processes by simpler operation of the process. Here, the polymerization is transferred from the polymerization reactor without a separator simply into the deactivation reactor. Between the polymerization zone and the deactivation zone is an uninterrupted transfer, which is determined only by the addition of the deactivator. Another improvement consists in removing the unstable chain ends in the presence of residual monomers. Here it is possible to reduce the content of unstable chain ends to up to about 0.1% by weight. However, when the product is processed by a degassing apparatus, impurities remain in the product produced by the polymerization process.

EP-A-699,695 proposes an improvement to the above-mentioned polymerization process. Here, in the presence of residual monomers, unstable chain ends are removed to a level of 0.01-1%, then the product is freed of most of the residual monomers at the reactor inlet by reducing the pressure in the granulator, and the remaining residual monomers, together with impurities dissolved in the product, are removed by extraction with a solvent, and the product is granulated after drying and stabilization.

WO-A-01/58,974 describes another homogeneous polymerization mode. Wherein, if appropriate, the product melt is deactivated and discharged, cooled and granulated under high pressure and in the presence of a liquid solvent. The method reduces the susceptibility to foaming during degassing and minimizes dust generation during granulation.

To address the cost pressures encountered in various respects, there remains a need for more advantageous polymerization processes and process methods.

It is an object of the present invention to provide a simple, energy-efficient process for homogeneously preparing oxymethylene homopolymers or oxymethylene copolymers, which process is effective for reducing the residual monomer content.

It is a further object of the present invention to provide a process for the homogeneous preparation of oxymethylene homopolymers or copolymers which makes it possible to prepare stable polyoxymethylene mixtures in an energetically favorable manner by simple means.

Surprisingly, it has now been found that combining the individual steps of the process in the aforementioned order leads to a particularly advantageous energy operation of the process. Firstly, the temperature of the polymer melt is used here to reduce the residual monomer content and secondly to remove the residual impurities effectively in the downstream extraction stages.

It has also been found that in one apparatus it is possible not only to reduce the residual monomer content but also to introduce stabilizers into the polymer.

The invention provides a process for preparing polyoxymethylene homopolymers and copolymers, comprising the following steps:

i) at least one-CH-forming polymer is homogeneously polymerized in the polymerization zone in a manner known per se₂Monomers of-O-units and, if appropriate, one or more comonomers and at least one initiator,

ii) removing the polymer prepared in step i) at a temperature above its melting point and removing its unstable chain ends and/or terminating its end groups and deactivating the initiator in a manner known per se by adding a deactivator in a deactivation zone immediately downstream of the polymerization zone,

iii) transferring the polymer melt to a reduced pressure zone,

iv) removing residual monomer from the polymer melt by applying reduced pressure to the reduced pressure zone,

v) granulating the polymer,

vi) extracting remaining residual monomers and/or oligomers and other impurities from the polymer,

vii) if appropriate, drying the polymer.

Step i) comprises a homogeneous polymerization reaction known per se.

For this purpose, -CH will be formed at a temperature above the melting point of the homopolymer or copolymer obtained, at a pressure of up to 2000 bar₂The monomers or mixtures of different monomers of the-O-unit are (co) polymerized together with customary initiators and, if appropriate, regulators.

For preparing polyoxymethylene homopolymers or copolymers, formation of-CH₂The monomers or mixtures of different monomers of the-O-unit are reacted in the manner described above.

Polyoxymethylene homo-or copolymers generally comprise unbranched, chain-like polymers which generally comprise at least 50 mol%, preferably at least 80 mol%, in particular at least 90 mol%, of oxymethylene units (-CH)₂-O-). Small amounts of branching agents may be used if desired. The amount of branching agent is generally not more than 1% by weight, preferably not more than 0.3% by weight, based on the total amount of monomers used to prepare the polyoxymethylene homopolymer or copolymer.

The molecular weights of these polymers can vary widely. These polymers generally have the general formula (-CH)₂-O-) x, wherein x is in the range of 10-10000, preferably 300-3000.

The expression paraformaldehyde groups here includes not only groups derived from homopolymers of formaldehyde or from cyclic oligomers thereof, for example trioxane or tetraoxane, but also groups derived from copolymeric components.

The copolyformals can be derived from formaldehyde or its cyclic oligomers, in particular trioxane, and cyclic ethers, aldehydes such as glyoxylates, cyclic acetals which, if appropriate, can be substituted, and/or from linear oligo-or polyacetals.

Preferred cyclic ethers or acetals are those of the formula

Wherein x is 0 or 1, R²Is C₂-C₄Alkylene, if appropriate, which is substituted by one or more C₁-C₄Alkyl radical, C₁-C₄-alkoxy groups, and/or substitution by halogen atoms, preferably by chlorine atoms.

By way of example only, mention may be made of ethylene oxide, 1, 2-propylene oxide, 1, 2-butylene oxide, 1, 3-dioxane, 1, 3-dioxolane, 1, 3-dioxepan, and 1, 3, 6-trioxacyclooctane as cyclic ethers, and also chain oligo-or polyoxymethylenes, such as polydioxolane or polydioxepane, as comonomers.

The material may also include a block copolymer that includes not only polyoxymethylene blocks but also structural units derived from a hydroxyl terminated polymer. Preferred block copolymers are derived from polyoxymethylene homopolymer blocks or from polyoxymethylene copolymer blocks, and from polyalkylene glycol blocks or from hydroxyl-terminated polybutadiene blocks.

These copolymers are illustrated in EP-A-1,418,190.

It is preferable to prepare an oxymethylene copolymer comprising polyoxymethylene groups having a formula (-CH) of 99.9 to 90 mol%₂-O-) x, preferably derived from trioxane, and containing from 0.1 to 10 mol% of recurring structural units derived from one of the above-mentioned comonomers.

It is particularly preferable to prepare an oxymethylene copolymer comprising polyoxymethylene blocks containing 99.9 to 90 mol% of a copolymer of the formula (-CH)₂-O-) x, preferably derived from trioxane, and containing from 0.1 to 10 mol% of recurring structural units of the general formula

-(CH₂-CH₂-O-)z

Wherein z is an integer of at least 1.

Other oxymethylene copolymers which may be prepared are polymers having repeating structural units obtained, for example, by the reaction of the above-mentioned trioxane and one of the cyclic ethers or acetals, and with a third monomer, preferably a bifunctional dioxolane, a bifunctional dioxane or a bifunctional epoxide. Examples of difunctional epoxides are compounds of the formula

Wherein Z is a bond, -O-or-O-R¹-O-(R¹＝C₂-C₈Alkylene or C₂-C₈Cycloalkylene).

Some examples of preferred monomers of this type which may be mentioned are ethylene glycol diglycidyl ether, and diethers composed of glycidyl compounds and formaldehyde in a molar ratio of 2: 1, and also diethers composed of 2 moles of glycidyl compounds and 1 mole of an aliphatic diol having from 2 to 8 carbon atoms, such as the diglycidyl ether of ethylene glycol, 1, 4-butanediol, 1, 3-cyclobutanediol, 1, 2-propanediol, 1, 4-cyclohexanediol, and also diglycerol glyoxal.

Methods for preparing the above-mentioned POM homopolymers and copolymers are known to the person skilled in the art and have been described in the literature.

During homogeneous polymerization, the polymerization mixture is in liquid form, or in the state for a period of time during which polymerization takes place.

The molecular weight of the (co) polymers obtained can, if appropriate, be adjusted by using regulators known per se for the preparation of polyoxymethylenes.

Examples of regulators are acetals, such as methylal, acetal or butyraldehyde, or of the formula HO-R¹Other dihydric alcohols of-OH, in which R¹Is a divalent aliphatic radical, and very small amountsAnd (3) water. These may act as chain transfer agents. The amounts of regulator used are generally up to 50000ppm, preferably 100-3000 ppm.

As the initiator, there can be used a cationic initiator which is generally used for preparing an oxymethylene homopolymer or an oxymethylene copolymer. Examples of such initiators are protic acids, such as fluorinated or chlorinated alkyl and aryl sulfonic acids, for example trifluoromethanesulfonic acid, trifluoromethanesulfonic anhydride, or heteropolyacids, such as hexatungstated or hexamolybdated phosphoric acid, or Lewis acids, for example tin tetrachloride, arsenic pentafluoride, phosphorus pentafluoride and boron trifluoride, and also their complexes and salt-like compounds, for example boron trifluoride etherate and triphenylmethyl hexafluorophosphate.

The amount of initiator generally used is from 0.01 to 1000ppm, preferably from 0.03 to 100ppm, based on the monomer (mixture).

According to the invention, the pressure and temperature in the polymerization zone are chosen such that the monomers and the polymer are present in a homogeneous distribution, preferably completely dissolved in one another. The reaction pressure and reaction temperature are suitably chosen to ensure that this state is reached.

Typical polymerization temperatures range from 130 ℃ to 170 ℃.

Typical deactivation temperatures range from 150 ℃ to 250 ℃, preferably from 170 ℃ to 200 ℃.

Typical polymerization pressures and deactivation pressures range from 10 to 2000 bar, preferably from 15 to 200 bar.

The polymerization and deactivation can be carried out in reactors known for the preparation of POM homopolymers or copolymers. Those which are generally used are kneaders, extruders or, preferably, tube reactors with static mixers, which are temperature-controlled and designed to be pressure-closed.

Steps i) and ii) are preferably carried out at elevated temperature and pressure.

The steps i) and ii) are particularly preferably carried out in one reactor, in particular a tubular reactor with a static mixer.

The polymerization times are very wide and generally vary from 0.1 to 20 minutes. The polymerization time is preferably from 0.4 to 5 minutes.

After the polymerization, the thermal polymer is deactivated in a manner known per se. This is achieved by adding an initiator-deactivating additive directly to the polymer melt after the polymerization. This step may be carried out as described in EP-A-699,695 or EP-A-638,599. Basic compounds are known to be useful as deactivators, for example sodium carbonate, disodium hydrogen phosphate or tertiary amines.

The polymerization and deactivation are preferably carried out in one reactor, for example a tubular reactor, with uninterrupted transfer between the polymerization zone and the deactivation zone, which is determined solely by the addition of the deactivator. However, the two steps of the method may also take place in different devices.

The stabilization of the crude (co) polymer obtained is carried out simultaneously with the deactivation of the initiator. In the case of homopolymers, this can be brought about by end-capping of the end groups, for example by etherification or esterification with suitable end-capping agents, for example with acetic acid derivatives, such as acetic anhydride; in the case of copolymers, this can be done by controlling the degradation of the polymer chains produced until stable monomer units are reached. These measures are known per se.

After deactivation and stabilization of the polymer melt, it is transferred to a reduced pressure zone to remove residual monomer and the residual monomer is removed by applying reduced pressure.

The reduced pressure zone is formed by the space filled by the hot polymer melt. The temperature of the polymer melt is used to remove most of the residual monomers from the polymer melt by applying a pressure below atmospheric pressure, preferably a pressure of less than 500 mbar, in particular a pressure of less than 200 mbar. The process steps can be carried out in the isolated part of the tubular reactor and are preferably carried out in an extruder. However, other devices, such as flash chambers, may also be used.

For this purpose, it is preferred to transfer the polymer after step ii) under pressure to an extruder in which the residual monomers are removed by depressurization and suction.

The use of a twin-screw extruder is particularly preferred.

If appropriate, stabilizers and processing aids (also referred to below as additives) can be added to the polymer before it leaves the decompression zone. These additives are chosen such that they are not removed from the polymer by the subsequent extraction stage.

In a preferred mode of the process of the invention, after the residual monomers have been removed, the mixture of additives is fed into the extruder and mixed into the hot polyoxymethylene homopolymer or copolymer.

In this embodiment of the process of the invention, the additive mixture is chosen such that it comprises only components that are resistant to the subsequent extraction stage, these components preferably being insoluble in hot water.

For the purposes of this specification, the expression insoluble in hot water means that the solubility of the compound in hot water is so low that at least 95% by weight thereof remains in the mixture under the selected extraction conditions.

The components which can be used in the additive mixture are compounds which are customarily used for stabilizing and/or modifying polyoxymethylenes.

These include, for example, antioxidants, acid scavengers, formaldehyde scavengers, uv stabilizers or heat stabilizers. The additive compounds may also include processing aids, such as coupling agents, lubricants, nucleating agents, mold release agents, fillers, reinforcing materials or antistatic agents, and also additives which impart desired characteristics to the molding composition, such as dyes and/or pigments, and/or impact modifiers, and/or additives which impart electrical conductivity; and compounds of these additives, but are not limited to the ranges of these examples mentioned.

Once the residual monomer is removed from the reduced pressure zone and any additives are added, the polymer is pelletized and the remaining residual monomer and/or oligomers and/or other impurities are removed from the polymer in an extraction stage.

The granulation and extraction can be carried out in apparatuses known per se.

Examples of pelletizers are strand pelletizers, water-cooled strand pelletizers, and underwater pelletizers.

One example of an extraction apparatus is a counter current scrubber.

Downstream of the extraction stage there is preferably a drying process to separate the particles from the adhering residual extractant.

Then, if appropriate, additives are added to the polymer in a manner known per se. In this stage of the process, it is also possible to add additives which will dissolve out of the polymer correspondingly in the extraction stage.

The extraction process may be carried out with any extractant required to remove oligomers and/or residual monomers. Preferably with hot water.

The hot water treatment is preferably carried out as a countercurrent washing, in particular under pressure and at a temperature of more than 100 ℃.

Typical treatment temperatures range from 100-170 deg.C, preferably from 110-150 deg.C, and particularly preferably from 120-135 deg.C.

At the chosen extraction temperature, the process pressure is generally 1 to 5 bar above the vapor pressure of the extraction medium.

In another preferred form of the process according to the invention, the polyoxymethylene homopolymer or copolymer melt is solidified and pelletized after leaving the decompression zone, without adding a mixture of additives to the polymer in the decompression zone and then treated by a hot water extraction process.

In this embodiment, after the hot water treatment, the particles and the additive mixture for the polyoxymethylene homopolymer or copolymer are mixed, preferably by melting the particles in an extruder and introducing the additive mixture into the extruder by mixing. The extrudate is then granulated again.

In this embodiment of the process of the invention, an additive mixture containing components dissolved in hot water may be used.

The polyoxymethylene homo-and copolymers prepared according to the invention can be further processed by molding processes, such as blow molding, injection molding or extrusion, in a manner known per se to give moldings.

The invention also provides equipment for implementing the method. The apparatus comprises the following components in the following order:

A) a reactor (1) comprising a polymerization zone (2) and, immediately downstream thereof, a deactivation zone (3) for the homogeneous polymerization and deactivation of polyoxymethylene homopolymers and copolymers in a manner known per se,

B) a pressure-relief assembly (4) having, if appropriate, metering means (5) for the stabilizer of the polymer,

C) a granulator (6),

D) an extraction device (7) for extracting the liquid,

E) if appropriate, a drying apparatus (8), and

F) if appropriate, means (17) for introducing a stabilizer for the polymer.

The reactor (1) preferably comprises a tubular reactor equipped with a static mixer.

The pressure-reducing assembly (4) preferably comprises an extruder, preferably a twin-screw extruder.

The extraction device (7) preferably comprises a counter-current scrubber.

The apparatus (17) preferably comprises an extruder, particularly preferably a twin-screw extruder.

Two embodiments of the inventive apparatus and the inventive method are described in the following figures by way of example.

Fig. 1 is a diagrammatic representation of the apparatus of the present invention, which comprises: the reactor (1), the decompression assembly formed by the extruder (4), has a metering device (5) for the additives for the polymer, a granulator (6), an extrusion device (7), and a drying apparatus (8).

The reactor (1) is composed of a polymerization zone (2) and, immediately downstream thereof, a deactivation zone (3), at which point the feeding of the deactivator is started through a line (17). The reactants are introduced into the reactor (1) through line (9). The polymer melt leaving the reactor (1) via line (10) is likewise freed from residual monomers by suction in the extruder (4) via line (11), and the additives extracted with hot water are introduced into the polymer via metering device (5) and mixed into the polymer in the extruder (4). The extrudate is then granulated in a granulator (6) and introduced via line (12) into an extraction apparatus (7) where it is subjected to a counter-current hot water treatment (13, 14). The extraction-treated particles leave the extraction apparatus (7) through line (15) and are dried in a drying apparatus (8), where they leave through line (16).

Fig. 2 is another illustration of the apparatus of the present invention, the apparatus comprising: a reactor (1), a pressure-reducing assembly formed by an extruder (4), and a granulator (6), an extrusion device (7), a drying apparatus (8), and another extruder (17).

The reactor (1) is composed of a polymerization zone (2) and, immediately downstream thereof, a deactivation zone (3), the deactivator being fed at the beginning of this point via line (20). The reactants are introduced into the reactor (1) through line (9). The polymer melt leaving the reactor (1) via line (10) is likewise freed from residual monomers by suction in the extruder (4) via line (11). The extrudate is then granulated in a granulator (6) and introduced via line (12) into an extraction apparatus (7) where it is subjected to a counter-current hot water treatment (13, 14). The extraction-treated granules leave the extraction apparatus (7) via line (15) and are dried in the drying apparatus (8) and are introduced into the extruder (17) via line (16). If a metering device (18) is used, the additives are introduced into the polymer and mixed into the polymer in an extruder (17). The polymer mixed with additives leaves the extruder (17) through line (19).

Claims (18)

1. A process for the preparation of polyoxymethylene homopolymers and copolymers comprising the steps of:

i) at least one-CH-forming polymer is homogeneously polymerized in the polymerization zone in a manner known per se₂Monomers of-O-units and, if appropriate, one or more comonomers and at least one initiator,

ii) removing the polymer prepared in step i) at a temperature above its melting point and removing its unstable chain ends and/or terminating its end groups and deactivating the initiator in a manner known per se by adding a deactivator in a deactivation zone immediately downstream of the polymerization zone,

iii) transferring the polymer melt to a reduced pressure zone,

iv) removing residual monomer from the polymer melt by applying reduced pressure to the reduced pressure zone,

v) granulating the polymer,

vi) extracting residual monomers and/or oligomers and other impurities from the polymer, and

vii) if appropriate, drying the polymer.

2. The process of claim 1, wherein steps i) and ii) are carried out in a tubular reactor equipped with a static mixer.

3. The process of claim 1, wherein the pressure reduction is carried out in an extruder.

4. The method of claim 3, wherein a twin screw extruder is used as the extruder.

5. The process of claim 4, wherein after removal of residual monomers, an additive mixture is added to the extruder and mixed into the hot polymer in the extruder, the additive mixture being selected such that it is not extracted in step vii).

6. The method of claim 5, wherein the additive mixture includes only hot water insoluble components.

7. The process of claim 1, wherein the extraction process is a hot water treatment to remove water soluble oligomers and/or residual monomers.

8. The process of claim 7, wherein the hot water treatment is carried out as a countercurrent wash under pressure and at a temperature above 100 ℃.

9. The process of claim 1, wherein after extraction and drying, the granules are mixed with an additive mixture for polyoxymethylene homo-or copolymer, and the extrudate is then pelletized therein.

10. The method of claim 9, wherein the pellets are mixed by melting the pellets in an extruder and by mixing the additive mixture into the extruder.

11. The method of claim 9, wherein the additive mixture comprises a hot water soluble component.

12. An apparatus for the preparation of polyoxymethylene homopolymers and copolymers, comprising the following components in the following order:

A) a reactor (1) comprising a polymerization zone (2) and, immediately downstream thereof, a deactivation zone (3) for the homogeneous polymerization and deactivation of polyoxymethylene homopolymers and copolymers in a manner known per se,

B) a pressure-relief assembly (4) having, if appropriate, metering means (5) for the stabilizer of the polymer,

C) a granulator (6),

D) an extraction device (7) for extracting the liquid,

E) if appropriate, a drying apparatus (8), and

F) if appropriate, means (17) for introducing a stabilizer for the polymer.

13. The apparatus of claim 12, wherein the reactor (1) is a tubular reactor equipped with a static mixer.

14. The apparatus of claim 12, wherein the pressure-reducing assembly (4) is an extruder.

15. The apparatus of claim 14, wherein the extruder is a twin screw extruder.

16. The apparatus of claim 12, wherein the extraction apparatus (7) is a counter current scrubber.

17. The apparatus of claim 12, wherein the means (17) for introducing a stabilizer for the polymer is an extruder.

18. The apparatus of claim 17, wherein the extruder is a twin screw extruder.