WO2013113879A1

WO2013113879A1 - Polyoxymethylene copolymers

Info

Publication number: WO2013113879A1
Application number: PCT/EP2013/052041
Authority: WO
Inventors: Bernd-Steffen Von Bernstorff; Laurence POTTIÉ
Original assignee: Basf Se
Priority date: 2012-02-02
Filing date: 2013-02-01
Publication date: 2013-08-08

Abstract

What are described are specific polyoxymethylene copolymers having a weight-average molecular weight (M_W) in the range from 5000 to 15 000 g/mol, and processes for preparation thereof by polymerization of trioxane and optionally comonomers in the presence of at least one cationic initiator and of at least one di(C_1-6-alkyl) acetal as a chain transfer agent.

Description

polyoxymethylene

The invention relates to low molecular weight polyoxymethylene copolymers, processes for their preparation and their use.

Polyoxymethylene homopolymers or copolymers, also referred to as polyacetal or polyformaldehyde or POM, are generally high molecular weight thermoplastics which exhibit high stiffness, low coefficients of friction and excellent dimensional stability and thermal stability. Therefore, they are used in particular for the production of precision parts.

In particular, the high strength, hardness and rigidity in a wider temperature range make them advantageous for molding applications. The further processing takes place, for example, by injection molding at temperatures in the range of 180 to 230 ° C, as well as by extrusion. The preparation of polyoxymethylene is carried out, for example, by direct polymerization of formaldehyde or by cationic or transition metal-centered cationic polymerization of trioxane. For stabilization, in order to prevent depolymerization under the influence of acid or thermal stress, the end groups are frequently protected by etherification or esterification.

Another possibility for stabilization against acid influence and thermal stress is the preparation of copolymers, for example by copolymerization of trioxane with 1, 4-dioxane. Here, the unstable end groups are degraded by hydrolysis to formaldehyde for stabilization. Typical copolymers ^® for example, under the brand names Hostaform ^® from Ticona / Celanese and Ultra form available from BASF SE.

The homopolymer typically has a melting point of about 178 ° C, the copolymer of about 166 ° C.

Processes for the preparation of polyoxymethylene homo- or copolymers are described, for example, in WO 2007/023187 and WO 2009/077415.

US 6,388,049 relates to low molecular weight polyoxymethylene polymers and compositions containing them. Preparation Examples 14 to 16 give trioxane-based and butanediol-formal copolymers in which methylal was used as regulator. The added amount of comonomer is in each case 1, 46 mol%, corresponding to about 4.4 wt .-% butanediol formal. Number average molecular weights of 1100, 5500 and 35000 g / mol are obtained. Polyoxymethylene is also used as a binder for powder injection molding. In this case, filled with inorganic powders, in particular metal powders or ceramic powders, POM molding compounds are processed by injection molding into moldings and subsequently debinded and sintered. Since the high loading of the POM with the inorganic powders impairs the flowability, it is necessary to use well-flowing POM materials in order to keep the pressures required during the injection molding to a reasonable level.

To improve the flowability of the POM, the molecular weight could be lowered or flow improvers added. A flow improver should have a very good miscibility with POM and show a rapid degradation under an acid-gas atmosphere in order to avoid defects in the desired moldings.

It is an object of the present invention to provide reduced molecular weight polyoxymethylene copolymers which can be used as a viscosity modifying additive for higher molecular weight polyoxymethylene homopolymers or copolymers.

The object is achieved by a polyoxymethylene copolymer having a weight average molecular weight (M _w ) in the range of 5000 to 15,000, preferably 5000 to 10,000 g / mol or 6000 to 13,000, preferably 6000 to 9000 g / mol or 6500 to 1 1000, especially preferably 6500 to 8000 g / mol, in particular 7000 to 7500 g / mol, characterized in that it is based on the polymer, at least 90 wt .-% of trioxane and butanediol as monomers abgelei tet, with a proportion of butanediol , based on the polymer, in the range of 0.5 to 4 wt .-%, preferably 1 to 4 wt .-%, preferably 2 to 3.5 wt .-%, in particular 2.5 to 3 wt .-%.

Alternatively and preferably, the number average molecular weight (M _n ) is preferably 3000 to 6000 g / mol, particularly preferably 3200 to 5000 g / mol, in particular 3500 to 4100 g / mol. In this molecular weight range, a particularly advantageous flow improvement is achieved for polyoxymethylene homopolymers or copolymers having a higher molecular weight. The use of a polyoxymethylene copolymer according to the present invention, which has a proportion of butanediol formal, based on the polymer, in the range of 1 to 4 % By weight, shows a higher crystallinity and a higher hardness in spite of the low molecular weight due to the compared to US 6,388,049 reduced comonomer. In spite of good viscosity-reducing properties for polyoxymethylene homopolymers or copolymers having a higher molecular weight, this results in an advantageous hardness of these polymer blends and thus advantageous mechanical properties for use.

The molecular weight determination can be carried out as described in the examples. The determination of the molecular weights is usually carried out by gel permeation chromatography (GPC) or SEC (Size Exclusion Chromatography). The number average molecular weight is generally determined by GPC-SEC.

The object is also achieved according to the invention by a process for preparing polyoxymethylene copolymers obtained by polymerization of trioxane, optionally comonomers, in the presence of at least one cationic initiator, and at least one di (Ci _-6 alkyl) than as a controller and the obtainable by this process polymers ,

The object is also achieved by using the above polyoxymethylene encopolymers as a viscosity-modifying additive for polyoxymethylene homopolymers or copolymers having a weight-average molecular weight of at least 50,000 g / mol.

Preferably, the ratio between weight average molecular weight (M _w ) and number average molecular weight (M _n ), also referred to as polydispersity or M _w / M _n , is in the range of 1.5 to 3.0, preferably 1.5 to 2, 45th

Polyoxymethylene copolymers (POM) according to the invention generally have at least 50 mol% of repeating units - CH ₂ O - in the main polymer chain. Polyoxymethylene copolymers are preferred which in addition to the recurring units -CH ₂ 0- still 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

, said R ¹ to R ⁴ independently represent a hydrogen atom, a Crbis C ₄ alkyl group or a halogen-substituted alkyl group having 1 to 4 carbon atoms and R ⁵ is a -CH ₂ -, -CH ₂ 0 -, a C to C ₄ Alkyl or C 1 to C ₄ haloalkyl-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-dioxolane and 1, 3-dioxepane (= butanediol, BUFO) as cyclic Ethers and linear oligo- or polyformals such as polydioxolane or polydioxepan called comonomers.

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

wherein Z is a chemical bond, -O-, -ORO- (R = Cr to C ₈ -alkylene or C ₃ - to C ₈ -cycloalkylene) are 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.

End-group-stabilized polyoxymethylene polymers which have predominantly C-C or -O-CH ₃ bonds at the chain ends are particularly preferred. The polymers according to the invention are derived, based on the polymer, from at least 90% by weight of trioxane and butanediol formal as monomers.

The polyoxymethylene copolymers are derived, preferably exclusively, from trioxane and butanediol formal as monomers, with a content of butanediol formal, based on the polymer or on the monomers, in the range of 0.5 to 4 wt .-%, preferably 1 to 4 wt .-%, preferably 2 to 3.5 wt .-%, in particular 2.5 to 3 wt .-%. 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 which act as chain transfer agents and whose presence can generally never be completely avoided.

Preferably, the inventive polymer at the chain ends, based on the polymer, 3 to 6 wt .-% of radicals of the general formula -OR, wherein R is Ci-6-alkyl, preferably Ci _-4 alkyl.

Preferably, in the preparation of the polyoxymethylene copolymer of the invention, based on the polymer or the sum of monomers and regulators, 3.75 to 4.25 wt .-%, preferably 3.8 to 4.2 wt .-%, in particular 3 , 9 to 4.1 wt .-% methylal or an equimolar amount of another di (Ci _-6- alkyl) acetal concomitantly used as a regulator.

Particularly preferred are methylal, z. B. on a laboratory scale, or ButylaI (n-butyl), z. B. industrially, also used as a controller. More preferably, butylal (n-butylal) is used as a regulator which has the advantage of being non-toxic, while Methylal is classified as toxic. The use of butylal as a regulator is a further advantage over the polyoxymethylene copolymers known from US Pat. No. 6,388,049. Preference is given to using butylalI as regulator in the preparation of the polymer. Butylal, based on the polymer, in an amount of 0.5 to 4 wt .-%, particularly preferably 1 to 3.5 wt .-%, in particular 1, 5 to 2.5 wt .-%, is preferably used. Together with the specific amount of comonomer and the specific molecular weight, polyoxymethylene copolymers with particularly suitable mecha- These properties make them useful as a viscosity-modifying additive for higher molecular weight polyoxymethylene homo- or copolymers without significantly affecting mechanical properties, particularly hardness.

Polyoxymethylene copolymers with a proportion of butanediol formal, based on the polymer, in the range from 0.5 to 4 wt.%, Preferably 1 to 4 wt.%, Preferably 2 to 3.5 wt. , in particular 2.5 to 3 wt .-%, in their preparation Butylal as a regulator in an amount, based on the polymer, from 0.5 to 4 wt .-%, particularly preferably 1 to 3.5 wt .-%, in particular 1, 5 to 2.5% by weight, is used. The number average of the polyoxymethylene copolymer is more preferably 3000 to 6000 g / mol, more preferably 3200 to 5000 g / mol, especially 3500 to 4100 g / mol. This special combination of molecular weight, comonomer content, comonomer selection, regulator fraction and regulator selection leads to particularly suitable mechanical properties which allow the advantageous use as a viscosity-modifying additive for relatively high molecular weight polyoxymethylene homopolymers or copolymers. Initiators (also referred to as catalysts) are the cationic initiators customary in the trioxane polymerization. Proton acids are suitable, such as fluorinated or chlorinated alkyl- and arylsulfonic acids, for example perchloric acid, trifluoromethanesulfonic acid or Lewis acids, for example tin tetrachloride, arsenic pentafluoride, phosphoric pentafluoride and boron trifluoride, and their complex compounds and salt-like compounds, for example 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 are inert compounds such as aliphatic, cycloaliphatic hydrocarbons such as cyclohexane, halogenated aliphatic hydrocarbons, glycol ethers, etc. can be used. Particular preference is given to triglyme (triethylene glycol dimethyl ether) as solvent and 1,4-dioxane. Particularly preferred according to the invention as cationic initiators Bronsted acids in an amount in the range of 0.01 to 1 ppm (preferably 0.02 to 0.2 ppm, in particular 0.04 to 0.1 ppm), based on the sum of monomers and regulators, used. In particular, HCI0 _{4 is used} as a cationic initiator or initiator. In addition to the initiators cocatalysts can be included. These are alcohols of any kind, for example aliphatic alcohols having 2 to 20 C atoms, such as t-amyl alcohol, methanol, ethanol, propanol, butanol, 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 type, in particular diethylene glycol 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-hexanediol, 1, 4-cyclohexanediol, 1, 4-cyclohexanedimethanol and neopentyl glycol.

Monomers, initiators, cocatalyst and, if appropriate, regulators may be premixed in any way or may also be added to the polymerization reactor separately from one another.

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

The preparation of polyoxymethylene copolymers according to the invention is carried out by polymerization of trioxane, butanediol and optionally ren further comonomers, in the presence of at least one cationic initiator and at least one di (CI_ ₆ alkyl) acetal as regulator.

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 (terminating agents) to the polymerization melt. Suitable deactivators are, for example, ammonia and primary, secondary or tertiary, aliphatic and aromatic amines, for example 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. The deactivators are usually added to the polymers in amounts of preferably 0.01 ppmw (parts per million by weight) up to 2 wt .-%. Furthermore, alkali metal or alkaline earth metal alkyls are preferred as deactivators which have 2 to 30 C atoms in the alkyl radical. Particularly preferred metals are Li, Mg and Na, with n-butyllithium being particularly preferred. According to one embodiment of the invention, from 3 to 30 ppm, preferably from 5 to 20 ppm, in particular from 8 to 15 ppm, based on the sum of monomers and regulator, of a chain terminating agent can be used. In particular, sodium methylate is used as a chain terminator. POMs from trioxane and butanediol formal are generally obtained by bulk polymerization, using any reactors with high mixing efficiency can. The reaction can be carried out homogeneously, for example in a melt, or heterogeneously, for example as polymerization to a solid or solid granules. Suitable examples are shell reactors, ploughshare mixers, tube reactors, Listreaktoren, kneaders (eg Busskneter), extruders with, for example, one or two screws, and stirred reactors, the reactors may have static or dynamic mixer.

The trioxane polymerization can be thought of as three reaction steps, the initiation, the propagation and transfer reactions. In the transfer reactions, chain transfer to the polymer, to a protic species such as water, or to a transfer agent such as butylal can occur. The transfer reactions to other polymer chains allow the random distribution of the comonomer units along the polymer chains. These reactions occur between the carbonium of one active chain and the oxygen of another polymer chain as long as active carbonium ions are present in the reaction mixture.

Transfer reactions to protic species such as water reduce both the molecular weight and the thermal stability of the polymer as unstable hydroxyl end groups are formed. Therefore, the polymerization is carried out under dry conditions.

Transfer reactions to aprotic species such as low molecular weight acetals reduce the molecular weight and produce stable ether end groups, thus increasing the thermal stability of the polymer. Therefore, chain transfer agents or regulators such as methylal or butylal are preferably used, which are added in the desired amount to the monomer mixture. In the POM used in conventional catamold compositions, the butylal content is usually about 0.35 wt% and the weight average molecular weight of the POM is about 97,000 g / mol, with a ratio M _w / M _n of about 4 ; 2.

The POM polymerization has no termination step. The living polymer is in equilibrium with formaldehyde monomer until a comonomer end group is achieved which is a stable end group. A process for stabilizing the polymer ends is therefore the depolymerization of the unstable chain ends until only stable comonomer end groups remain. This process is used in the shell-and-roll process in which the polymers obtained have a majority of methylal or butylal derived end groups (eg, -O- (CH ₂ ) 4 -OH). The chain ends can also be deactivated by adding an alkaline compound. This procedure is used in particular in continuous processes in which living end groups are typically inactivated with sodium methoxide. The resulting polymer has a plurality of -CH ₂ -O-CH _{3 end} groups. In a bulk polymerization, for example in an extruder, the melted polymer produces a so-called melt seal, whereby volatile constituents remain in the extruder. The above monomers are metered into 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 1400C. The monomers (trioxane) are preferably also metered in a molten state, for example at 60 to 120.degree. Due to the exothermic nature of the process, the polymer usually only has to be melted in the extruder at the start of the process; Subsequently, the amount of heat released is sufficient to melt the molten POM polymer or to keep it molten.

The melt polymerization is generally carried out at 1, 5 to 500 bar and 130 to 300 ° C, and the residence time of the polymerization mixture in the reactor is usually 0.1 to 20, preferably 0.4 to 5 min. Preferably, the polymerization is carried out to a conversion above 30%, e.g. 60 to 90%.

A crude POM is frequently obtained which, as mentioned, contains considerable proportions, for example up to 40%, of unreacted residual monomers, in particular trioxane and formaldeyde. 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 trioxane. In addition, other oligomers of formaldehyde may also be present, e.g. the tetrameric tetroxane. This crude POM is preferably degassed in one or more stages in known degassing devices, for example in degassing pots (flash pots), vented extruders with one or more screws, thin film evaporators, spray dryers or other conventional degassing devices. Particularly preferred are degassing pots (flash pots).

Preferably, the degassing of the raw POM is operated in such a way that is degassed in a first flash to below 6 bar absolute, to obtain a gaseous stream and a liquid stream, which is fed to a second flash, which is operated at below 2 bar absolute , obtaining a vapor stream, which is recycled into the monomer plant.

For example, in a two-stage degassing, the pressure in the first stage is preferably 2 to 18, in particular 2 to 15 and particularly preferably 2 to 10 bar, and in the second stage preferably 1, 05 to 4, in particular 1, 05 to 3.05 and especially preferably 1.05 to 3 bar amount. The teilentgaste Polyoxymethylenhomo- or copolymers can then be fed to an extruder or kneader and therein with conventional additives and processing aids (additives), provided in the usual amounts for these substances. Such additives are, for example, lubricants or mold release agents, colorants such as pigments or dyes, flame retardants, antioxidants, light stabilizers, formaldehyde scavengers, polyamides, nucleating agents, fibrous and pulverulent fillers or reinforcing agents or antistatics and other additives or mixtures thereof. From the extruder or kneader, the desired product POM is obtained as a melt.

The preferred shell-batch process batch process involves the following steps: In the first step, a non-closed reaction vessel ("shell") is filled with the liquid monomer / comonomer mixture Initiator is pumped by, for example, an HPLC pump. at a temperature in the range of preferably 60 to 100 ° C., more preferably 70 to 90 ° C., in particular 75 to 85 ° C. A solvent may be used, the boiling point of which is more than 100 ° C., and is miscible with the monomers.

In the second step, the initiator, preferably aqueous HCI0 ₄ in a solvent, is mixed with the monomers. After an induction time, the polymerization and crystallization take place simultaneously in the third step, at the end of which, after the homogeneous reaction, there is a solid polymer block. The induction time is often less than 120 seconds, for example 20 to 60 seconds. In the fourth step, the solid crude POM is removed from the shell, mechanically reduced, and further processed in an extruder, for example, by depolymerization to reach stable end groups (degassing). In addition, stabilizers and other ingredients can be added. As a standard mixture of stabilizers, a mixture of antioxidant, acid scavenger and nucleating agent can be considered.

After emptying the reaction container this can be filled again with liquid monomer to start a new cycle. In contrast to the process according to the invention, the preparation of the POM copolymers according to US Pat. No. 6,388,049 in completely molten state takes place in tubular reactors. The blend production takes place in two series-connected reactors. For example, the resulting polymer may be ground into a coarse powder, sprayed with a buffer solution, and then fed to the extruder. The buffer serves to neutralize acids remaining in the melt.

To successfully complete the shell circle process, the synthesis should be fast; d. H. have a short induction period. In addition, the resulting oligomers should be rapidly and completely solidified in the polymerization and form a polymer block which does not adhere too strongly to the container wall.

The preparation of the low molecular weight POM is particularly advantageously possible by using a small amount of initiator, a high amount of regulator and capping the chain ends. The low molecular weight POM thus obtained is both thermally stable and chemical resistant and has a viscosity which may be lower by a factor of 1000 compared to a conventional high molecular weight POM as heretofore used in catamold compositions ,

When using the low molecular weight POM as a viscosity modifying additive for POM having a weight average molecular weight of at least 50,000 g / mol, preferably at least 80,000 g / mol, the addition to a thermally and chemically stable POM system will increase its viscosity by a factor of at least 10 can be reduced without appreciably affecting the mechanical strength of the high molecular weight POM.

The invention is explained in more detail by the following examples.

Examples

Preparation of POM Oligomers Laboratory scale polymerization was performed in a process that reflects the shell circle process. The monomers and the controller were heated to 80 ° C in open iron or aluminum reactors with magnetic stirring. The mixture was a transparent liquid. At an instant t = 0, an initiator solution consisting of HClO ₄ in triglyme was injected, with a proton concentration of typically 5 ppm relative to the monomers, or correspondingly lower for the low molecular weight POM. In a successful polymerization The mixture became cloudy in a short time (induction period typically in the range of a few seconds to one minute) and the polymer precipitated.

Aftercare and determination of weight loss

The obtained polymer block was then ground to a powder and refluxed for one hour in an extraction solution of methanol, water and sodium carbonate. After cooling, the polymer was filtered off and washed with a washing solution of aqueous sodium carbonate. The powder was then dried and the weight loss was determined. This procedure gives an indication of the polymerization yield since remaining monomers or lowest molecular weight oligomers are extracted in this step. The living centers of the crude polymer chains as well as residual acid sites are partially extracted or neutralized. All cations should be neutralized to obtain a polymer that is stable enough for further study or further processing. Acid residues would otherwise shift the balance towards formaldehyde and affect thermal stability.

Investigation of thermal stability

A few grams of the extracted and dried polymer were heated to 220 ° C under a nitrogen atmosphere. After four hours, the weight loss of the polymer was determined. The result indicates how many unstable end groups are contained in the polymer, influenced by the amount and distribution of the comonomers along the polymer chain, as well as by the amount of regulators.

thermal stability

GV N ₂ : The weight loss (GV) in percent of a sample of 1, 2 g of granules with heating to 222 ° C for 2 hours under nitrogen.

At the beginning of the determination of GV the used balance was adjusted. The sample was weighed out to an accuracy of 0.1 mg in a jacketed vessel consisting of two test tubes (normal test tube, 100 × 10 mm, custom-made, thick-walled test tube 100 × 12.5 mm).

An approximately 400 mm long thin copper wire was attached to the upper bead of the outer glass. With this, the double-walled vessels were suspended in a special apparatus (see FIG. 9 and the related description of the figures in WO 2006/074997). In the case of GM determination under nitrogen, this was done for adaptation. Solution of the special atmosphere for 15 min in the upper half of the apparatus, ie without increasing the temperature. Subsequently, the test tubes were lowered to the bottom and left there for 2 h at 222 ° C. The nitrogen flow was 15 l / h and was controlled on each test tube by means of a Rotas.

After 2 h, the double-walled vessels were removed from the apparatus by means of the copper wire and cooled in air for 20 to 25 minutes. Then the balance was weighed back and GV calculated according to GV [%] = (loss x100 / initial weight).

Molar Mass

The molecular weight determination of the polymers was carried out by size exclusion chromatography in a SEC apparatus. This SEC apparatus consisted of the following separation column combination: a guard column of length 5 cm and diameter 7.5 mm, a second linear column of length 30 cm and diameter 7.5 mm. Release material in both columns was PL-HFIP gel from Polymer Laboratories. The detector used was a differential refractometer from Agilent G1362 A. A mixture of hexafluoroisopropanol with 0.05% trifluoroacetic acid potassium salt served as eluent. The flow rate was 0.5 ml / min at a column temperature of 40 ° C. 60 microliters of a solution of the concentration 1, 5 g of sample per 1 liter of eluent were injected. This sample solution was previously filtered through Millipor Millex GF (pore size 0.2 microns). Calibration was carried out using narrowly distributed PMMA standards from PSS (Mainz, DE) with molecular weights of M = 505 to 2,740,000 g / mol. The tensile modulus (modulus of elasticity) was determined according to ISO 527 (23 ° C, 1 mm / min).

Yield stress, yield strain, breaking stress and elongation at break were determined to ISO 527 (23 ° C, 50 mm / min). The Charpy notched impact strength was determined to ISO 179 1 eA (F) (23 ° C, 2.9 m / s).

The Charpy impact strength (unnotched) was determined to ISO 179 1 eA (U) (23 ° C, 2.9 m / s). Reactions with Butylal as regulator

The commercially available POMs produced by the Shell ^{Circle process and} marketed by BASF SE under the brand Ultraform® are listed in Table 1 below.

Table 1

The POM used for the initially described Catamold method corresponds to the Ultraform ^® Z2320, which is prepared with a butyral content of 0.35 wt .-%.

To reduce the molecular weight was subsequently increased the proportion of Butylal. The proportion of butanediol-comonomer was in each case unchanged 2.7% by weight, based on the polymer. The initiator concentration was 0.2 ppm based on the monomers.

The results are summarized in Table 2 below. Table 2

Example Butylal Induction time Mn Mw Mw / Mn Extraction Thermal Weight [g / mol] [g / mol] [-] [%] Stability

% Rod. [%]

1 3 8 6080 15800 2.4 7.23 4.5

2 4 7 5350 12700 2.4 3.56 4.3

3 4.5 9 4700 1 1000 2.4 1 .8 3.8

4 5.00 15 3940 8160 2.2 7.62 1 .9

5 5.50 20 3640 7580 2.1 9.65 5.0 In order to reduce the molecular weight, the proportion of butylal was subsequently kept at 5.5% by weight. The proportion of butanediol comonomer was changed. The initiator concentration was 0.05 ppm based on the monomers. Table 3

Bufo: butanediol-formal reaction with methylal as regulator

The above procedure was repeated using methylal as a regulator. The best results were achieved with an amount of about 4.0% by weight of methylal, which is close to the molar equivalent of 5.5% by weight of butylal. However, unlike Butylal, the induction time was significantly higher. The results are summarized in Table 4 below. The butanediol comonomer content was again 2.7% by weight. The initiator concentration was 0.05 ppm based on the monomers. Table 4

Rheology studies

Rheology investigations were carried out on the standard Ultraform products and the POM according to the invention according to Example 2. For this, a plate-plate rheometer at 190 ° C was used and the dynamic viscosity as a function of Seher rate was determined. The results are summarized in Table 5 below. Table 5

The oligomers of the invention have a very low melt viscosity.

Extrusion of POM Blends Blends of different Ultraform ^® POM polymers with different proportions of low molecular POM oligomers of Examples 1 to 8 were extruded for two minutes at 190 ° C in a midi-extruder. The resulting blend properties, determined from GPC measurements, are summarized in Table 6 below.

Table 6

The blends were also subjected to the above rheology measurements. The results are summarized in Table 7 below. Table 7

Thus, the low molecular weight POM according to the invention can be used particularly advantageously as a viscosity-modifying additive for polyoxymethylene homopolymers or copolymers having a weight-average molecular weight of at least 50,000 g / mol.

The low molecular weight POM according to the invention are chemically and mechanically stable and reduce neither the overall strength nor the overall mechanics when mixed with high molecular weight POM. In this case, the viscosity of the high molecular weight POM can be greatly reduced, the effect being retained in several melt passes. No formaldehyde evaporates, the POM blend remains a solid, and therefore the classic Catamold manufacturing process can also be performed with the POM blends. These advantages are not achieved when using even lower molecular weight compounds, such as POM dimethyl ether such as Me-0- (CH ₂ 0) 4-Me. Only the specific use of the POM according to the invention leads to the advantages mentioned. In another experiment, blends of 12 Ultraform ^® Z2320 were with low-POM-oligomer of Example 4 in the weight ratio 50 in Example 50 were mixed in a mini-extruder. Three samples were mixed for one minute, two minutes and five minutes respectively. Subsequently, a molecular weight profile for the mixed blend was recorded by size exclusion chromatography.

The attached FIG. 1 shows the dependence of the size exclusion chromatography (SEC) detector signal in arbitrary units, plotted against the molecular weight in g / mol. The solid line shows the molecular weight distribution for mixing for one minute, the triangles show the molecular weight distribution after mixing for 2 minutes, and the circles show the molecular weight distribution for a mixing time of 5 minutes. It was found that the molecular weight distribution remains the same at the three mixing times, which indicates a stability of the polymer blend. Furthermore, the bimodal molecular weight distribution resulting from the blend polymers is retained. It does not come to a molecular weight balance by transacetalization. In a further experiment complex shear viscosities for blends of Ultraform ^® Z2320 with POM-oligomer of Example 4 in a weight ratio of 50 were: investigated 50th The investigation was carried out by rotational rheology at a shear rate of 10 rad / s. While the Ultraform ^® Z2320 showed a complex shear viscosity of about 100 Pa.s, was found for the inventive blend with the POM-oligomer of Example 4, a complex shear viscosity of about 17 Pa.s. Thus, the blends with the low molecular weight POM show a preferred low shear viscosity.

As a result, it can be stated that in the case of the POM blends, the bimodal molecular weight distribution is retained even after thermal stress, and the shear viscosity decreases greatly, so that the processing properties improve markedly. Due to the improved flowability, especially in injection molding, long flow paths and thin wall thicknesses can be tolerated without the result deteriorating. In contrast to other flow improvers, the addition of the POM oligomers does not lead to phase separation under shear and thus also not to an Exudation of the flow improver, which in turn would result in tool coatings.

Due to the combination of low molecular weight POM with high molecular weight POM, the molding compositions are not brittle, but still have a high strength.

By the stability in the melt, the POM blends are suitable in an advantageous manner for the metal or ceramic powder-injection molding after the Catamold ^® process. In this process, the POM molding materials are subjected to a threefold melting and shearing: when mixing the two polymer components, when the metal or ceramic powder is introduced, and finally during injection molding. In a recycling and reuse of parts of the injection-molded body, such as the sprues, there is a further thermal stress. This shows the advantages of the POM systems according to the invention.

Under the brand Catamold ^® polymer particles are distributed, the inorganic material powder, particularly metal powders or ceramic powders contain. These powders are typically first coated with a thin layer of polyethylene and then compounded in a polyoxymethylene binder. This Catamold granulate is then processed by injection molding into a green body, transferred by debinding in a brown body and then sintered to a sintered shaped body. The process is known as Metal Injection Molding (MIM) and allows the production of metal or ceramic moldings with complex shapes.

The Catamold granules have a content of inorganic fillers of about 90 wt .-%.

The green bodies produced using polyoxymethylene homo- or copolymers have very good mechanical properties, in particular dimensional stability.

The debinding is often carried out by the action of an acidic atmosphere, for example HN0 ₃ atmosphere at 1 10 to 140 ° C with degradation of the POM binder. In the brown body obtained, the inorganic particles are connected to one another via their thin polyethylene coating. Due to the acidic depolymerization of the POM, the binder can be removed without residue.

The sintering of the brown body preferably follows in a sintering furnace at temperatures in the range of about 1100 to 1500 ° C to obtain the desired metal or ceramic shaped body. The better the flowability of the filled Polyoxymethylenhomo- or -copolymermassen, the finer structures can be formed in the molding. On the other hand, the metal or ceramic particles must be transported homogeneously with the molding compound. A suitable property spectrum of flowability and creep compliance is often achieved by POM with a weight average molecular weight above about 85,000 g / mol.

In addition to the observed changes in viscosity, the addition of a low molecular weight POM component to the high molecular weight Ultraform ^® also increases the material rigidity. These changes are shown in Table 8 below.

Table 8

Interestingly, in some cases, the increase in stiffness is accompanied by an increase in impact strength. This demonstrates the added benefit of using low molecular weight POM as an additive to improve different mechanical properties in addition to hardness.

Claims

claims

1 . Polyoxymethylene copolymer having a weight average molecular weight (Mw) in the range of 5000 to 15,000 g / mol, characterized in that it is based on the polymer, at least 90 wt .-% of trioxane and Bu- tandiolformal derived as monomers, with a Proportion of butanediol formal, based on the polymer, in the range of 0.5 to 4 wt .-%, preferably 2 to 3.5 wt .-%, in particular 2.5 to 3 wt .-%.

Polymer according to Claim 1, characterized in that the weight-average molecular weight (M _w ) is 6000 to 9000 g / mol, preferably 6500 to 8000 g / mol, in particular 7000 to 7500 g / mol.

Polymer according to claim 1, characterized in that the number-average molecular weight (M _n ) is 3000 to 6000 g / mol, preferably 3200 to 5000 g / mol, in particular 3500 to 4100 g / mol.

Polymer according to one of claims 1 to 3, characterized in that the ratio M _w / M _n in the range of 1, 5 to 3.0, preferably 1, 5 to 2.45.

Polymer according to one of claims 1 to 4, characterized in that it is derived exclusively from trioxane and butanediol as monomers.

Polymer according to one of claims 1 to 5, characterized in that it has at the chain ends, based on the polymer, 3 to 6 wt .-% radicals of the formula -OR with R d _-6 alkyl.

Polymer according to Claim 6, characterized in that, in the preparation of the polymer, based on the polymer, 3.75 to 4.25% by weight, preferably 3,

8 to 4.2% by weight, in particular 3,

9 to 4.1 wt .-% methylal or an equimolar amount of another di (Ci _-6 alkyl) acetal also be used as a controller.

Polymer according to claim 6, characterized in that in the preparation of the polymer butylal as regulator, preferably in an amount of, based on the polymer, 0.5 to 4 wt .-%, particularly preferably 1 to 3.5 wt .-% , in particular 1, 5 to 2.5 wt .-%, is used.

Process for the preparation of polyoxymethylene copolymers according to one of Claims 1 to 8 by polymerization of trioxane and optionally como monomers in the presence of at least one cationic initiator and at least one di (Ci _-6- alkyl) acetal as a regulator.

10. The method according to claim 9, characterized in that, based on the sum of monomers and regulators, 3.75 to 4.25 wt .-%, preferably 3.8 to 4.2 wt .-%, in particular 3.9 to 4.1% by weight of methylal or from 0.4 to 4% by weight, preferably from 1 to 3.5% by weight, in particular from 1.5 to 2.5% by weight, of butyl or an equimolar amount another di (Ci _-6- alkyl) acetal are used as a regulator.

1 1. Method according to one of claims 9 or 10, characterized in that as a cationic starter a Bronsted acid in an amount in the range of 0.01 to 1 ppm, preferably 0.02 to 0.2 ppm, in particular 0.04 to 0, 1 ppm, based on the sum of monomers and regulator, is used.

12. The method according to any one of claims 9 to 1 1, characterized in that methylal and / or butylal be used as a regulator.

13. The method according to any one of claims 9 to 12, characterized in that 3 to 30 ppm, preferably 5 to 20 ppm, in particular 8 to 15 ppm, based on the sum of monomers and regulator, a chain stopper be used.

Polyoxymethylene copolymers obtainable by a process according to any one of claims 9 to 13.

Use of polyoxymethylene copolymers according to any one of claims 1 to 8 or 14 as a viscosity modifying additive for polyoxymethylene homopolymers or copolymers having a weight average molecular weight of at least 50,000 g / mol.