Method for recovering residual monomers in the preparation of vinyl ester-ethylene copolymers

Abstract

The invention relates to a process for preparing vinyl ester-ethylene copolymers by free-radical polymerization of vinyl ester, ethylene and optionally other olefinically unsaturated monomers in an aqueous medium at a pressure of 5 to 120 bar absolute, characterized in that a) the polymerization mixture is depressurized to a pressure of 1 to 15 bar absolute, resulting in a gas phase containing ethylene and an aqueous phase containing vinyl ester and vinyl ester-ethylene copolymer, b) the gas phase containing ethylene from stage a) is separated and then absorbed into vinyl ester and the mixture thus obtained is used for the free-radical polymerization of vinyl ester, ethylene and optionally other olefinically unsaturated monomers, and c) the aqueous phase from stage a) is depressurized to a pressure of 0.1 to 0.5 bar absolute, forming a gas phase containing vinyl ester and water, which is separated, then condensed and then used for the free-radical polymerization of vinyl ester, ethylene and optionally other olefinically unsaturated monomers.

Description

[0001] The present invention relates to a process for preparing vinyl ester-ethylene copolymers by free-radically initiated polymerization of vinyl ester, ethylene and optionally other ethylenically unsaturated monomers in an aqueous medium at an absolute pressure of 5 to 120 bar with recovery of unreacted monomers.

[0002] Polymers based on vinyl esters, ethylene and optionally other monomers such as vinyl chloride or (meth)acrylates are used in a variety of applications, for example in coating agents or adhesives for a wide variety of substrates, especially in the form of aqueous dispersions or water-redispersible polymer powders. Such polymers are usually stabilized by protective colloids such as polyvinyl alcohol or low molecular weight surface-active compounds.

[0003] High conversions are the state of the art in industrial-scale polymerizations. For example, under polymerization conditions, the polymerization of liquid monomers such as vinyl acetate or vinyl chloride is usually completed to a residual monomer content of <0.1% by weight, preferably <0.05% by weight, and in the case of vinyl chloride <0.01% by weight. Ethylene is partly present in gaseous form during the polymerization at 5 to 120 bar absolute, with the result that such high degrees of ethylene conversion cannot usually be achieved under the polymerization conditions customary on an industrial scale. This is because firstly ethylene polymerizes more slowly than, for example, vinyl acetate, and secondly ethylene is mainly present in the gas phase and cannot participate in the polymerization there under the customary conditions of emulsion polymerization or suspension polymerization, since in the case of such polymerization processes the polymerization reaction takes place only in the participating liquid phase with a certain proportion of ethylene dissolved in water, monomers and particles.

[0004] For economic reasons, industrial-scale polymerization should be completed in the shortest possible time period, but this inevitably means that the ethylene used cannot be fully polymerized to completion. Usually, the polymerization is interrupted at a residual ethylene gas content of ≤10 wt %, preferably ≤5 wt %, and the reaction mixture is decompressed. The decompression procedure usually includes transferring the reaction mixture (polymer dispersion+residual gas) from a pressure reactor to a non-pressure reactor while removing residual ethylene. The obtained latex can then be further stripped of monomers in a known manner. Excess ethylene is usually handled by burning.

[0005] Due to the poor monomer utilization and the high disposal costs of the residual monomers, this method corresponding to the prior art is not sustainable. The reuse of the ethylene obtained is inhibited by the fact that the residual gas must first be recompressed back to high pressure (>80 bar). For economic reasons, this energy-intensive method prevents recycling, since the residual gas must also be purified in a complex manner before recompression, for example to avoid pressure fluctuations during compression.

[0006] From the prior art known various methods for reclaiming residual ethylene.For example, in DE10253043, after the emulsion polymerization of vinyl ester and ethylene in one step is finished, the reaction mixture is decompressed to a pressure of 0.1 to 5 bar absolute, and the gas phase with the residual gas containing ethylene is compressed to a pressure of 2 to 20 bar absolute, and finally returned to the reactor for emulsion polymerization.For ethylene recovery, WO2007 / 074075 teaches the complicated multi-stage fractionation cryogenic condensation of ethylene from the residual gas.EP3321292 describes the high pressure polymerization method at least 1500 bar for the preparation of ethylene-vinyl copolymer.In order to reclaim ethylene and vinyl comonomer, decompression is carried out in multiple stages, first decompressed to 200 to 300 bar, then decompressed to 40 to 60 bar, finally decompressed to 0.1 to 5 bar, each stage removes the monomer mixture contained in the gas phase.

[0007] The object of the present invention is to provide a process for preparing vinyl ester-ethylene copolymers which makes it possible to reuse as large a proportion of the residual gas obtained as possible for the free-radically initiated polymerization of vinyl ester and ethylene in an economically viable manner and preferably to increase the space-time yield.

[0008] The present invention provides a process for preparing vinyl ester-ethylene copolymers by free-radically initiated polymerization of vinyl esters, ethylene and optionally other ethylenically unsaturated monomers in an aqueous medium at a pressure of 5 to 120 bar absolute, characterized in that

[0009] a) depressurizing the polymerization mixture to a pressure of 1 to 15 bara to produce a gas phase comprising ethylene and an aqueous phase comprising the vinyl ester and the vinyl ester-ethylene copolymer,

[0010] b) separating the ethylene-containing gas phase from stage a), then absorbing it into vinyl ester and using the mixture thus obtained for the free-radically initiated polymerization of vinyl ester, ethylene and optionally further ethylenically unsaturated monomers, and

[0011] c) decompressing the aqueous phase from stage a) to a pressure of 0.1 to 0.5 bar abs. to form a gas phase comprising vinyl ester and water, which is separated and then condensed and used for the free-radically initiated polymerization of vinyl ester, ethylene and optionally further ethylenically unsaturated monomers.

[0012] The polymerization mixture from stage a) is generally an aqueous dispersion produced by free-radically initiated polymerization of vinyl esters, ethylene and optionally further ethylenically unsaturated monomers in an aqueous medium at a pressure of from 5 to 120 bar absolute.

[0013] In the polymerization mixture of stage a), the monomers, in particular the monomers which are liquid under the polymerization conditions, are converted to an extent of preferably 85 to 99% by weight, more preferably 87 to 98% by weight, particularly preferably 90 to 96% by weight. The conversion of the monomers is generally the quotient of the weight of the vinyl ester-ethylene copolymer contained in the polymerization mixture of stage a) and the total weight of the monomers and the vinyl ester / ethylene copolymer contained in the polymerization mixture of stage a).

[0014] In stage a), the polymerization mixture is depressurized to an absolute pressure of 1 to 15 bar, preferably 2 to 10 bar, particularly preferably 2 to 5 bar. For this purpose, the polymerization mixture is generally transferred to a low-pressure vessel or phase separation device under appropriate pressure. A gas phase containing ethylene and an aqueous phase containing vinyl esters and vinyl ester-ethylene copolymers are generally formed.

[0015] The ethylene-containing gas phase of stage a) preferably contains ethylene to an extent of ≥75% by weight, particularly preferably ≥85% by weight and most preferably ≥95% by weight, based on the total weight of ethylene contained in the polymerization mixture of stage a) or based on the total weight of ethylene contained in the gas phase and the aqueous phase of stage a).

[0016] The ethylene-containing gas phase of stage a) preferably contains ethylene to an extent of 50 to 95% by weight, particularly preferably 70 to 90% by weight, most preferably 75 to 90% by weight, based on the total weight of the ethylene-containing gas phase of stage a).

[0017] In addition, the ethylene-containing gas phase may also contain other components, such as vinyl esters, other monomers, water or inert substances, for example nitrogen, argon or saturated hydrocarbons such as ethane. The proportion of other components, based on the total weight of the ethylene-containing gas phase of stage a), is preferably 5% to 50% by weight, particularly preferably 10% to 30% by weight, most preferably 10% to 25% by weight.

[0018] The gas phase of stage a) preferably contains ≤20% by weight, particularly preferably ≤10% by weight, of vinyl ester, based on the total weight of vinyl ester in the gas phase and the aqueous phase of stage a). The gas phase of stage a) preferably contains ≤2% by weight, particularly preferably ≤1% by weight, of water, based on the total weight of water in the gas phase of stage a) and the aqueous phase.

[0019] The aqueous phase of stage a) preferably contains 35% to 65% by weight, particularly preferably 40% to 60% by weight, of vinyl ester-ethylene copolymers. The aqueous phase of stage a) preferably contains 0.5% to 5% by weight, particularly preferably 1% to 3% by weight, of monomers, in particular vinyl esters such as vinyl acetate. The aqueous phase of stage a) preferably contains 34.5% to 64.5% by weight, particularly preferably 39% to 59% by weight, of water. The figures in % by weight are based on the total weight of the aqueous phase of stage a).

[0020] The decompression in stage a) is preferably carried out adiabatically. Before the decompression in stage a), the polymerization mixture has a temperature of preferably 75° C. to 120° C., particularly preferably 80° C. to 110° C. After the decompression in stage a), the polymerization mixture preferably has a temperature of 75° C. to 120° C., particularly preferably 80° C. to 110° C.

[0021] In stage b), the ethylene-comprising gas phase of stage a) and the aqueous phase comprising the vinyl ester and the vinyl ester-ethylene copolymer can be separated in a manner conventional per se, for example using a phase separator.

[0022] The ethylene-containing gas phase b) is usually absorbed into the vinyl ester, i.e. usually the starting material for the polymerization. This can be achieved, for example, in a mixing device, such as a static mixer, a stirrer, a mixing tube or in particular an absorption device. Preferred absorption devices are designed in the form of a column, in particular a column with random packing or structured packing. Preferably, inert substances such as nitrogen, argon or saturated hydrocarbons are removed from the mixing device, in particular at the top of the mixing device, for example by a pressure holding device, and discharged from the process. The temperature of the vinyl ester is preferably adjusted to 5° C. to 20° C. before entering the mixing device. In a preferred embodiment, the vinyl ester is led to the mixing device in a countercurrent manner to the ethylene-containing gas phase b); the ethylene-containing gas phase b) is absorbed into the vinyl ester here. Any other substances contained in the ethylene-containing gas phase b), in particular the vinyl ester that entered the ethylene-containing gas phase in stage a), are preferably condensed in the mixing device and preferably leave the mixing device together with the ethylene absorbed into the vinyl ester.

[0023] The mixture thus obtained is generally fed into a reactor for free radical initiated polymerization of vinyl esters, ethylene and optionally other ethylenically unsaturated monomers. The mixture can be compressed to the reactor pressure, for example by means of a pump, preferably after leaving the mixing device and / or before being introduced into the reactor.

[0024] The mixture obtained in stage b) preferably contains 0.5% to 5% by weight of ethylene, based on the amount of vinyl ester.

[0025] In the case of vinyl ester-ethylene copolymers with a particularly high ethylene content, or in the case of feedstocks, in particular ethylene, which are particularly highly contaminated with inert substances, it may be advantageous to compress the ethylene-containing gas phase from stage a) before stage b). This may be advantageous for increasing the ethylene recovery. In this case, a compression ratio of 1.5 to 3 is preferably selected. The compression ratio is the ratio of the compressor outlet pressure to the compressor inlet pressure. Particularly preferably, the ethylene-containing gas phase from stage a) does not need to be compressed.

[0026] In stage c), the aqueous phase from stage a) is decompressed to a pressure of 0.1 to 0.5 bar absolute, preferably 0.15 to 0.4 bar absolute, particularly preferably 0.2 to 0.3 bar absolute, to form a gas phase containing vinyl ester and water and an aqueous phase containing vinyl ester-ethylene copolymer. The gas phase containing vinyl ester and water of stage c) is usually separated, then condensed and then used for free radical polymerization of vinyl ester, ethylene and optionally other olefinically unsaturated monomers. In stage c), any ethylene remaining in the aqueous phase in stage a) is converted, preferably almost completely, into the gas phase of stage c), and preferably completely or mostly dissolved in the condensate of stage c), and preferably used for free radical polymerization. Any ethylene not condensed in stage c) or not dissolved in the condensate of stage c) is preferably discharged via a vacuum pump, in particular together with uncondensed water and vinyl ester and any inert substances.

[0027] The decompression in stage c) is preferably carried out adiabatically. Before the decompression in stage c), the aqueous phase has a temperature of preferably 75° C. to 120° C., particularly preferably 80° C. to 110° C. During the decompression in stage c), it is preferably cooled to 20° C. to 50° C., in particular 20° C. to 40° C. The gas phase formed in stage c) and containing vinyl ester and water has a temperature of preferably 50° C. to below 80° C., in particular to below 75° C. The decompression in stage c) can be carried out, for example, in a conventional phase separator.

[0028] Alternatively, heat can also be provided in stage c), for example by heating or preferably by steam.Particularly preferably, the steam and aqueous phase are conducted countercurrently through a separation apparatus, for example a column with random or structured packing.

[0029] The condensation in stage c) is preferably carried out at a temperature of 0°C to 15°C, particularly preferably 5°C to 10°C.

[0030] In an alternative embodiment, the condensation in stage c) is carried out in two stages. The first stage is preferably carried out at a temperature of 15° C. to 40° C., particularly preferably at a temperature of 20° C. to 35° C. The second stage is preferably carried out at a temperature of 0° C. to 15° C., particularly preferably at a temperature of 5° C. to 10° C. In this way, water and vinyl ester can be condensed out successively. This process is characterized by a particular energy efficiency. The advantage of the two-stage condensation is also that the first stage produces mainly water as condensate and the second stage produces mainly vinyl ester as condensate. This enables separate processing. For example, in order to discharge water-soluble or vinyl ester-soluble impurities, recycling can only be carried out in part. Preferably, the condensate is completely recycled into the free-radical initiated polymerization.

[0031] Conventional condensers can be used. The condenser is preferably connected on the gas side to the phase separator of stage c).

[0032] The condensate from stage c) preferably contains 25% to 75% by weight, particularly preferably 40% to 60% by weight, of vinyl esters. The condensate from stage c) preferably contains 25% to 75% by weight, particularly preferably 40% to 60% by weight, of water. These figures in % by weight are all based on the total weight of the condensate from stage c).

[0033] The condensate from stage c) preferably contains 25 to 75% by weight, particularly preferably 35 to 65% by weight, of vinyl ester, based on the total weight of the vinyl ester contained in the aqueous phase of stage a) containing vinyl ester and vinyl ester-ethylene copolymer. If energy is additionally supplied to stage c), for example in the form of heating power or steam, the condensate preferably contains 50 to 100% by weight, particularly preferably 90 to 100% by weight, of vinyl ester, based on the total weight of the vinyl ester contained in the aqueous phase of stage a) containing vinyl ester and vinyl ester-ethylene copolymer.

[0034] The condensate from stage c) is reused for the free-radically initiated polymerization of vinyl ester and ethylene. Preferably, the condensate, optionally after temperature adjustment, is introduced directly or immediately into the reactor for the free-radically initiated polymerization of vinyl ester and ethylene, for example using a pump.

[0035] After separation of the gas phase containing vinyl ester and water, what remains in stage c) is usually an aqueous phase containing vinyl ester-ethylene copolymer (aqueous phase from stage c)). The aqueous phase preferably contains ≤2% by weight, particularly preferably 0% to 1% by weight, of vinyl ester, based on the total weight of the aqueous phase containing vinyl ester-ethylene copolymer. The aqueous phase formed in step c) preferably contains ≤10 ppm, particularly preferably 0-5 ppm, of ethylene.

[0036] The residual monomer content of the polymer dispersion remaining after stage c) is preferably from 1 to 10 000 ppm, particularly preferably from 500 to 5000 ppm. If energy is additionally supplied to stage c), for example in the form of heating power or steam, the residual monomer content is preferably from 1 to 1000 ppm, particularly preferably from 10 to 100 ppm.

[0037] The vinyl ester content of the polymer dispersion after stage c) is, for example, 50 to 80% by weight lower than at the reactor outlet. If additional energy is supplied to stage c), for example in the form of heating power or steam, the vinyl ester content is ≥99% by weight lower than at the reactor outlet.

[0038] The aqueous phase formed in stage c) preferably has a temperature of 50°C to below 80°C.

[0039] After stage c), the aqueous phase from stage c) can be subjected to an afterpolymerization using known methods, usually by redox-catalyzed afterpolymerization, for the purpose of further removing residual monomers. Volatile residual monomers can also be removed by distillation, preferably under reduced pressure and optionally while passing an inert entraining gas such as air, nitrogen or water vapor through or through the mixture (stripping).

[0040] As an alternative or in addition, the aqueous phase from stage a) can be post-polymerized or stripped.

[0041] The aqueous phase from stage a) and the aqueous phase from stage c) are preferably subjected to postpolymerization or stripping. Particularly advantageously, the two-stage decompression in steps a) and c) can be connected to a two-stage postpolymerization, because the aqueous phase from stage a) (first postpolymerization) and the aqueous phase from stage c) (second postpolymerization) are both postpolymerized. The first postpolymerization results in the polymerization of preferably 25% to 90% by weight, particularly preferably 50% to 75% by weight of the vinyl ester contained in the aqueous phase of stage a) to completion. Of the vinyl ester remaining after the first postpolymerization, preferably 25% to 75% by weight, particularly preferably 50% to 75% by weight, is converted into the gas phase in stage c). The vinyl ester still remaining in the aqueous phase from stage c) can then be polymerized to completion in the second postpolymerization. This method variant makes it particularly advantageous to achieve a low residual monomer content of preferably ≤100ppm, particularly preferably ≤50ppm.

[0042] In the process according to the invention, postpolymerization and stripping can be omitted or can be carried out in a shorter period of time than in conventional processes, since the residual monomer content has already been reduced by stages a) to c) of the process according to the invention.

[0043] The process according to the invention is generally applicable to batch or semi-batch processes and is particularly advantageous for continuous processes.

[0044] The aqueous polymer dispersions thus obtained have a solids content of 30% to 75% by weight, preferably 50% to 60% by weight.

[0045] Suitable vinyl esters are those of carboxylic acids having 1 to 18 carbon atoms. Preference is given to vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of α-branched monocarboxylic acids having 9 to 13 carbon atoms, for example VeoVa 9 or VeoVa 10 (Trade name of Shell). Vinyl acetate is particularly preferred.

[0046] Examples of suitable monomers copolymerizable with vinyl esters and ethylene are acrylates or methacrylates of unbranched or branched alcohols having 1 to 18 carbon atoms. Preferred methacrylates or acrylates are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate and 2-ethylhexyl acrylate. Particularly preferred are methyl acrylate, methyl methacrylate, n-butyl acrylate, tert-butyl acrylate and 2-ethylhexyl acrylate. Also suitable are vinyl halides such as vinyl chloride.

[0047] Based on the total weight of the monomer mixture, 0 to 50% by weight of auxiliary monomers can optionally also be copolymerized. Preferably, 0.1 to 15% by weight of auxiliary monomers are used. Examples of auxiliary monomers are ethylenically unsaturated monocarboxylic acids and dicarboxylic acids; ethylenically unsaturated carboxamides and nitriles; ethylenically unsaturated sulfonic acids or their salts. Other examples are pre-crosslinking comonomers, such as polyethylenically unsaturated comonomers or post-crosslinking comonomers, for example N-hydroxymethyl acrylamide (NMA). Also suitable are epoxy-functional comonomers such as glycidyl methacrylate and silicon-functional comonomers.

[0048] Preference is given to using mixtures of vinyl acetate and ethylene; and mixtures of vinyl acetate and other vinyl esters (such as vinyl laurate or vinyl esters of α-branched monocarboxylic acids having 9 to 13 carbon atoms) and ethylene; and mixtures of vinyl chloride, ethylene and vinyl esters (for example vinyl laurate).

[0049] The weight proportions of monomers and comonomers are selected so as to generally give a glass transition temperature Tg of -50°C to +50°C, preferably -20°C to +20°C. The glass transition temperature Tg of the polymer can be determined in a known manner by differential scanning calorimetry (DSC). Tg can also be calculated approximately in advance by the Fox equation. According to Fox TG, Bull. Am. Physics Soc. 1, 3, page 123 (1956): 1 / Tg=x1 / Tg1+x2 / Tg2+...+x n / Tg n , where x n is the mass fraction of monomer n (wt% / 100) and Tg n is the glass transition temperature (in Kelvin) of the homopolymer of monomer n. Tg values ​​for homopolymers are listed in Polymer Handbook 2nd Edition, J. Wiley & Sons, New York (1975).

[0050] The polymers are prepared by free radical initiated polymerization in an aqueous medium, preferably by the suspension polymerization process, in particular by the emulsion polymerization process, preferably in the presence of protective colloids and / or emulsifiers. Such processes are known per se. The polymerization temperature is generally between 40° C. and 100° C., preferably between 60° C. and 90° C. The pressure used during the polymerization is generally between 5 and 120 bar absolute.

[0051] The polymerization is usually initiated with a water-soluble or monomer-soluble initiator or a redox initiator combination that is usually used for emulsion polymerization or suspension polymerization. Examples of water-soluble initiators are sodium, potassium and ammonium salts of peroxodisulfuric acid, hydrogen peroxide, tert-butyl peroxide, tert-butyl hydroperoxide, potassium peroxydiphosphate, tert-butyl peroxypivalate, cumene hydroperoxide, isopropylbenzene monohydroperoxide, azobisisobutyronitrile. Examples of monomer-soluble initiators are dicetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, dibenzoyl peroxide. The mentioned initiators are usually used in an amount of 0.01% to 0.5% by weight based on the total weight of the monomers. The redox initiator used is usually a combination of the mentioned initiator and a reducing agent. Examples of suitable reducing agents are alkali metal and ammonium sulfites and bisulfites, such as sodium sulfite; derivatives of hyposulfurous acid, such as zinc or alkali metal formaldehyde sulfoxylates, such as sodium hydroxymethanesulfinate and ascorbic acid. The amount of the reducing agent is preferably 0.01% to 0.5% by weight, based on the total weight of the monomers.

[0052] During the polymerization, a substance acting as a chain transfer agent can be used to control the molecular weight. If a chain transfer agent is used, they are usually used in an amount of 0.01% to 5.0% by weight based on the monomer to be polymerized. Chain transfer agents can usually be metered in separately, or have been premixed with the reaction components. Examples of such substances are n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol and acetaldehyde. Preferably, a substance acting as a chain transfer agent is not used.

[0053] Examples of suitable protective colloids are partially hydrolyzed polyvinyl alcohol; polyvinyl pyrrolidone; polyvinyl acetal; water-soluble forms of polysaccharides such as starch (amylose and amylopectin), cellulose and its carboxymethyl, methyl, hydroxyethyl and hydroxypropyl derivatives; proteins such as casein or caseinates, soy protein, gelatin; lignin sulfonates; synthetic polymers such as poly(meth)acrylic acid, copolymers of (meth)acrylates with carboxyl-functional comonomer units, poly(meth)acrylamide, polyvinylsulfonic acid and water-soluble copolymers thereof; melamine-formaldehyde sulfonates, naphthalene-formaldehyde sulfonates, styrene-maleic acid copolymers and vinyl ether-maleic acid copolymers. Partially or completely hydrolyzed polyvinyl alcohol is preferred.

[0054] The protective colloids are generally added to the polymerization in a total amount of 1% to 20% by weight, based on the total weight of the monomers. This portion of the protective colloid can, for example, be included completely in the initial charge or partly in the initial charge and partly metered in.

[0055] Emulsifiers suitable for polymerization are anionic, cationic or other nonionic emulsifiers. Examples of anionic surfactants are alkyl sulfates with a chain length of 8 to 18 carbon atoms, alkyl ether sulfates or alkyl aryl ether sulfates with 8 to 18 carbon atoms in the hydrophobic group and up to 40 ethylene oxide units or propylene oxide units, alkyl sulfonates or alkyl aryl sulfonates with 8 to 18 carbon atoms, esters and monoesters of sulfosuccinic acid with monohydric alcohols or alkylphenols. Examples of nonionic surfactants are alkyl polyglycol ethers or alkyl aryl polyglycol ethers with 8 to 40 ethylene oxide units. Typically, emulsifiers are used in an amount of 0.1% to 5% by weight based on the amount of monomers.

[0056] By means of the process according to the invention, polymer dispersions having a low residual monomer content can be obtained in an advantageous manner. This is also particularly important if the polymer dispersion is subsequently dried in a spray dryer to form a powder, since in this case it is necessary to remove the residual monomers from the dryer offgas in a complex manner in order to comply with emission limits.

[0057] The process according to the invention makes it possible to separate the residual vinyl ester and ethylene monomers in a technically simple, efficient, energy-saving and therefore economical manner and to reuse them for the polymerization. In this case, a recompression step with a compressor, in particular a multistage compressor, or a temperature regulation of the vinyl ester and ethylene can be omitted. Further steps for purifying the residual gas can be omitted. In this way, the residual vinyl ester and ethylene monomers can be almost completely recycled for the polymerization, as a result of which the handling of the residual gas is significantly simplified.

[0058] The polymer dispersion is usually concentrated in step c) of the process according to the invention, for example, by 1 to 20% by weight, in particular by 2 to 8% by weight. As a result, dispersions with a relatively high solids content can be obtained in a simple and effective manner. On the other hand, the polymerization can be carried out at a lower solids content to prepare polymer dispersions with a conventional solids content. This reduces fouling during the polymerization and accelerates the removal of heat from the polymerization reactor, which allows an increase in the space-time yield and reduces reactor downtime for defouling.

[0059] The following examples serve to illustrate the present invention in more detail and should in no way be considered limiting.

[0060] Embodiment 1:

[0061] The following polymer dispersions were removed from a reaction for the emulsion polymerization of vinyl acetate and ethylene:

[0062] At a pressure of 50 bar and a temperature of 95° C., 200 kg / h of an aqueous polymer dispersion having a residual monomer content of 3% by weight of vinyl acetate monomers and 0.3% by weight of ethylene (each based on the total weight of the polymer dispersion).

[0063] Comparative Example 1a:

[0064] Single-stage decompression of the polymer dispersion to 1.0 bar:

[0065] With the help of a control valve, the mass flow of the polymer dispersion was depressurized to a pressure of 1.0 bar absolute. As a result, 0.293% by weight of ethylene entered the gas phase and 70 ppm of ethylene remained in the dispersion. A total offgas stream of 2.5 kg / h was released. This was compressed to 3 bar using a cooled liquid ring compressor with an electrical power of 2 kW according to the method described in DE 10253043 to recover >90% of the ethylene.

[0066] The polymer dispersion was then postpolymerized for 1 hour with addition of initiator.

[0067] The polymer dispersion thus obtained contained 100 ppm of vinyl acetate and 35 ppm of ethylene.

[0068] Comparative Example 1b:

[0069] Single-stage decompression of the polymer dispersion to 0.2 bar:

[0070] With the help of a control valve, the mass flow of the polymer dispersion was depressurized to a pressure of 0.2 bar absolute. As a result, 0.2998 wt.-% of ethylene entered the gas phase and 2 ppm of ethylene remained in the dispersion. A total offgas stream of 13 kg / h was released. This was compressed to 3 bar using a cooled liquid ring compressor with an electrical power of 50 kW according to the method described in DE 10253043 to recover >90% of the ethylene.

[0071] The polymer dispersion was then postpolymerized for 1 hour with addition of the same amount of initiator as in Comparative Example 1a.

[0072] The polymer dispersion thus obtained contained 30 ppm of vinyl acetate and 1 ppm of ethylene.

[0073] Example 1c:

[0074] Two-stage decompression of polymer dispersion at 3 bar and 0.2 bar:

[0075] With the help of a control valve, the mass flow of the polymer dispersion was first decompressed to a pressure of 3 bar absolute. As a result, 0.22% by weight of ethylene entered the gas phase and 700 ppm of ethylene remained in the dispersion. The ethylene thus discharged was dissolved into the vinyl acetate feed without further compression, which was then introduced into the polymerization reactor.

[0076] In the second step, the remaining dispersion is depressurized to 0.2 bar. A further 698 ppm of ethylene enter the gas phase, and only 2 ppm of ethylene remain in the dispersion here. Of the 700 ppm, 650 ppm are dissolved again in the condensate of VAM, with the result that only 50 ppm are discharged into the exhaust gas.

[0077] The polymer dispersion was then postpolymerized for 1 hour with addition of the same amount of initiator as in Comparative Example 1a.

[0078] The polymer dispersion thus obtained contained 30 ppm of vinyl acetate and 1 ppm of ethylene.

[0079] Table: (Comparative) Summary of Examples 1a-c:

[0080]

Claims

1. A process for preparing vinyl ester-ethylene copolymers by free-radically initiated polymerization of vinyl esters, ethylene and optionally other ethylenically unsaturated monomers in an aqueous medium at an absolute pressure of 5 to 120 bar, characterized in that a) depressurizing the polymerization mixture to an absolute pressure of 1 to 15 bar to produce a gas phase comprising ethylene and an aqueous phase comprising the vinyl ester and the vinyl ester-ethylene copolymer, b) separating off the ethylene-containing gas phase from stage a) and then absorbing it into vinyl ester and using the mixture thus obtained for the free-radically initiated polymerization of vinyl ester, ethylene and optionally further ethylenically unsaturated monomers, and c) the aqueous phase from stage a) is depressurized to an absolute pressure of from 0.1 to 0.5 bar, a gas phase comprising the vinyl ester and water is formed, the gas phase having a temperature of from 50 to below 80° C. and is separated off and then condensed and then used for the free-radically initiated polymerization of the vinyl ester, ethylene and optionally other ethylenically unsaturated monomers.

2. The method for preparing a vinyl ester-ethylene copolymer according to claim 1, characterized in that: The ethylene-comprising gas phase of stage a) comprises ≥ 75% by weight of ethylene, based on the total weight of ethylene contained in the polymerization mixture of stage a).

3. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: The gas phase of stage a) contains ≤ 20% by weight of vinyl ester, based on the total weight of vinyl ester contained in the polymerization mixture of stage a).

4. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that The decompression in stage a) and / or stage c) is carried out adiabatically.

5. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: The gas phase which is formed in stage c) and contains the vinyl ester and water has a temperature of 50°C to less than 75°C.

6. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: The condensation in stage c) is carried out at a temperature of from 0°C to 15°C.

7. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that The condensation in stage c) is carried out in two stages, and The first stage is carried out at a temperature of 15°C to 40°C, and the second stage is carried out at a temperature of 0°C to below 15°C.

8. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: The condensate from stage c) contains from 25 to 75% by weight of vinyl ester, based on the total weight of the condensate from stage c).

9. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: The condensate from stage c) contains from 25 to 75% by weight of vinyl ester, based on the total weight of vinyl ester contained in the aqueous phase from stage a) containing vinyl ester and vinyl ester-ethylene copolymer.

10. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: In stage c), energy is supplied to the aqueous phase from stage a), and the condensate of stage c) contains 50 to 100% by weight of vinyl esters, based on the total weight of vinyl esters contained in the aqueous phase containing vinyl esters and vinyl ester-ethylene copolymers of stage a).

11. The method for preparing a vinyl ester-ethylene copolymer according to claim 1 or 2, characterized in that: The aqueous phase from stage a) and / or the aqueous phase from stage c) is subjected to afterpolymerization or stripping.

Images