CH629508A5 - Process and apparatus for the preparation of polymer dispersions having a low monomer content - Google Patents

Description

The present invention further relates to a device for carrying out the method according to the invention, consisting of at least two series-connected, vertically standing, rotationally symmetrical containers, each with a gas discharge opening in the upper part and at least one liquid outlet in the lower part, which optionally means for tempering and / or stirring the contents of the container included, with each container in its upper part containing a dispersion device in the form of a hollow cone spraying against the container wall in such an arrangement that the axis of the hollow cone of sprayed material coincides approximately with the axis of the container and wherein the gas discharge opening is arranged above the spray device characterized in that each container below the spray nozzle (s) contains at least one, possibly covered, gas inflow opening, which is attached via a supply line from the outside of the container so that it does not not hit by the spray emerging from the spraying device and is not flooded by the dispersion collected in the lower part of the container.

The containers mentioned in the preceding paragraph are preferably the superimposed chambers of a vertically standing, rotationally symmetrical vessel which is divided into two or more superimposed chambers by one or more than one, preferably horizontal, partition. Each partition contains at least one opening, which is tightly connected to an ascending pipe, which ends open below the spraying device, but is optionally covered with a cap. At least one opening for discharging liquid is provided in the lower part of each chamber, this opening, with the exception of the opening in the lowermost chamber of the container, being connected to the spraying device of at least one other chamber of the device by means of a pump and optionally attachments for tempering the flowing liquid . This arrangement is particularly space-saving and has only relatively short gas transfer pipes from one chamber of the container to the next.

The vertical, rotationally symmetrical vessel advantageously contains 1 to 9, in particular 1 to 5, partitions, which divide the vessel into 2 to 10, in particular 2 to 6, chambers of approximately the same size.

The container jacket can be temperature-controlled over its entire surface or in partial areas, for example by means of a double jacket through which a liquid heating or cooling medium flows. In particular, it can be advantageous to heat or cool the container jacket where the sprayed dispersion hits inside.

At least one spray nozzle with a hollow cone-shaped spray jet (hollow cone nozzle) is used as the spray device for the dispersion, which is attached in the upper part of the container in such a way that the axis of the hollow cone made of sprayed material coincides approximately with the axis of the container. The spray jet of the nozzle is preferably directed downwards.

In addition to this hollow cone nozzle, other spray devices can be advantageous, for example the arrangement of a plurality of nozzles producing hollow cones on the container axis one above the other, the spray cones of the individual nozzles overlapping as little as possible. Furthermore, a spraying device can also be used, which consists of at least two, preferably three to twelve nozzles, which produce a flat jet in the shape of a segment of a circle and are arranged on a concentric ring around the axis of the container in such a way that opposite parts of the container wall which are as large as possible have the same size as the spray segment surfaces Cut angles from 15 to 90 °, preferably 40 to 60 °. Can be used as a spray device

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a rotating disc can also be used, the axis of rotation of which coincides with the axis of the container.

A single-component nozzle is used as the hollow cone nozzle, for example a spiral or eccentric nozzle (as described in Chemical Eng. Prog. 49, pp. 185 to 186), which produces a hollow cone that is as sharply delimited as possible.

It is expedient to provide the narrowest passage cross section of the nozzle approximately twice as large as the diameter of the large solid particles in the dispersion. In general, a passage cross section of at least 1 mm will be chosen for reasons of capacity, a passage cross section of 5 to 100 mm, in particular of 10 to 50 mm, is preferred.

The spray device should atomize the dispersion so that the largest droplets have a maximum diameter of 6 mm [measured according to Fraser-Eisenklam, Trans. Instn. Chem. Engs. 34 (1956), pp. 301-302, on a 20 ° C dispersion at normal pressure]. Particularly good results are achieved if the mean droplet size, measured according to Fraser and Eisenklam (loc. Cit.) On a 20 ° C dispersion at normal pressure, is between 0.05 and 3 mm in diameter.

The pressure required for spraying the dispersion can be generated using a pump or other suitable means; the dispersion which is under pressure from the polymerization can also be sprayed directly.

The scattering angle of the hollow cone, that is the angle between two opposite surface lines of the cone shell, which is the main spray direction of the nozzle, is advantageously between 30 and 180 °, so the cone shell can also be a horizontally lying disc. A scattering angle of 80 to 120 ° is advantageously chosen.

The exit velocity of the particles from the spraying device, measured according to H. Hege, Aufbereitungstechnik, 1969, pp. 142-143, is expediently between 5 and 250 m / s,

the exit speed is advantageously set between 10 and 100 m / s.

In general, the sprayed particles should leave at least 20 cm after leaving the spraying device until they hit the container wall. This distance is limited to approximately 400 cm by economic considerations regarding the container dimensions and by the size and exit velocity of the particles. A path length in the range from 40 to 200 cm is preferably selected.

Figures 1 and 2 show examples of devices for performing the method according to the invention.

Regarding Figure 1: Pumps (2), (2a) and (2b) pass the dispersion successively into three standing, rotationally symmetrical containers (1), (1 a) and (1 b), there by means of the hollow cone nozzles (3), (3a) and (3b) sprayed, collected in the lower part of the container and stirred with the stirring elements (4), (4a) and (4b). In each container below the spray nozzle (3), (3aj and (3b) there is a gas inflow opening (5), (5a) and (5b) with a cap (6), (6a) and (6b) for better distribution of the gas flow and a supply line (7), (7a) and (7b) from the outside of the container At the upper part of the container there is a gas discharge opening (8), (8a) and (8b), which in the first container (1> The withdrawn gaseous products can then be fed to a recovery system for the monomer or monomers and, if appropriate, to a gas cleaning system. The gas discharge openings (8a) and (8b) of the further containers ( la) and (lb) are connected to the gas inlet openings (5) and (5a) of the previous containers in the direction of dispersion flow via lines (7) and (7a).

Inert gas, preferably water vapor, is blown into the gas inlet opening (5b) of the last container (Ib) in the direction of dispersion flow. From the liquid discharge lines (9) / (9a) attached to the bottom of the container, the monomer-free dispersion is successively fed via pumps (2a) / (2b) into the subsequent container (la) or (lb) and finally via line (9b) discharged by means of a pump (2c) and, if necessary, further processed as usual after cooling. The containers (1), (la) and (lb)

are heated with a double jacket (10) or (10a) / (10b) through which a temperature control medium flows. The standing height of the dispersion (11) or (1 la) / (l lb) at the bottom of the container is such that it is not hit directly by the spray jet.

If appropriate, the containers can also be arranged differently from one another, for example at an angle above one another. Figure 2 shows a similar arrangement as Figure 1, with the difference that the three containers are formed by a single elongated, vertical container, which is divided into three separate chambers by horizontal partitions (12) and (12a). Otherwise, the numbers have the same meaning as in FIG. 1.

The process according to the invention is particularly suitable for removing monomers, in particular vinyl chloride, from aqueous vinyl chloride homo-, co- and graft polymer dispersions with at least 50% by weight, preferably 75% by weight and in particular 85% by weight, of polymerized vinyl chloride.

The vinyl chloride homopolymer, graft or copolymer dispersion to be treated according to the invention can be prepared by continuous or batch polymerization processes, with or without the use of a seed prepolymer. It can be in aqueous emulsion or suspension in the presence of 0.001 to 3% by weight, preferably 0.01 to 0.3% by weight, based on monomers of radical-forming catalysts, such as e.g. Diaryl, diacyl peroxides, such as diacetyl, acetylbenzoyl, dilauroyl, dibenzoyl, bis-2,4-dichlorobenzoyl, bis-2-methylbenzoyl peroxide; Dialkyl peroxides, such as di-tert-butyl peroxide, perester, Wietert.-butyl percarbonate; tert-butyl peracetate, tert-butyl peroctoate, tert-butyl perpivalate, dialkyl peroxide dicarbonates such as diisopropyl, diethylhexyl, dicyclohexyl diethylcyclohexylperoxide dicarbonates; mixed anhydrides of organic sulfoperic acids and organic acids, such as acetylcyclo-hexylsulfonyl peroxide; Azo compounds known as polymerization catalysts, such as azoisobutyronitrile, and also persulfates, such as potassium, sodium or ammonium persulfate, hydrogen peroxide, tert-butyl hydroperoxide or other water-soluble peroxides, and also mixtures of various catalysts are polymerized, with peroxidic catalysts also in the presence of 0.01 to 1 % By weight, based on monomers, of one or more reducing substances which are suitable for the construction of a redox catalyst system, such as, for example Sulfites, bisulfites, dithionites, thiosulfates, aldehyde sulfoxylates, e.g. Formaldehyde sulfoxide, can be used. If appropriate, the polymerization can be carried out in the presence of 0.05 to 10 ppm, based on metal per monomer, of soluble metal salts, for example copper, silver, iron or chromium.

Furthermore, the polymerization in the presence of 0.01 to

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5% by weight, based on monomers, of one or more emulsifiers, the emulsifiers also being able to be used in a mixture with the protective colloids mentioned above. Anionic, amphoteric, cationic and noniogenic can be used as emulsifiers. Suitable anionic emulsifiers are, for example, alkali metal, alkaline earth metal, ammonium salts of fatty acids, such as lauric, palmitic or stearic acid, acidic fatty alcohol sulfuric acid esters, paraffin sulfonic acids, alkylarylsulfonic acids, such as dodecylbenzene or dibutylnaphthalenesulfonic acid, and sulfosuccinic acid, and sulfosuccinic acid Alkali and ammonium salts of epoxy group-containing fatty acids, such as epoxystearic acid, of reaction products of peracids, for example Peracetic acid with unsaturated fatty acids such as oleic or linoleic acid or unsaturated oxy fatty acids such as ricinoleic acid. Examples of suitable amphoteric or cationic emulsifiers are: alkylbetaines, such as dodecylbetaine, and alkylpyridium salts, such as laurylpyridinium hydrochloride, and also alkylammonium salts, such as oxyethyldodecylammonium chloride. Suitable nonionic emulsifiers are, for example: partial fatty acid esters of polyhydric alcohols, such as glycerol monostearate, sorbitol monolaurate, oleate or palmitate, polyoxyethylene ether of fatty alcohols or aromatic hydroxy compounds; Polyoxyethylene esters of fatty acids and polypropylene oxide-polyethylene oxide condensation products.

Protective colloids can also be used for the polymerization, for example water-soluble cellulose derivatives such as methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, which may optionally still contain up to 40% acetyl groups, gelatin, and copolymers of maleic acid or its half-esters with styrenes.

In addition to catalysts, optionally protective colloids and / or emulsifiers, the polymerization can be carried out in the presence of buffer substances, for example alkali metal acetates, borax; Alkali phosphates, alkali carbonates, ammonia or ammonium salts of carboxylic acids, and of molecular size regulators, such as, for example, aliphatic aldehydes with 2 to 4 carbon atoms, chlorinated or bromocarbons, e.g. Di- and trichlorethylene, chloroform, bromo-form, methylene chloride, and mercaptans can be carried out.

Further suitable polymerization auxiliaries can be found in the book by H. Kainer “Polyvenyl chloride and vinyl chloride mixed polymers”, 1965, pages 12 to 59.

For example, one or more of the following monomers are suitable for copolymerization with vinyl chloride: olefins, such as ethylene, or propylene, vinyl esters of straight-chain or branched carboxylic acids having 2 to 20, preferably 2 to 4, carbon atoms, such as vinyl acetate, propionate, butyrate, -2- ethyl hexoate, vinyl isotridecanoic acid ester; Vinyl halides, such as vinyl fluoride, vinylidene fluoride, vinylidene chloride, vinyl ethers, vinyl pyridine, unsaturated acids, such as maleic, fumaric, acrylic, methacrylic acid and their mono- or diesters with mono- or dialcohols having 1 to 10 carbon atoms; Maleic anhydride, maleimide, and its N-substitution products with aromatic ", cycloaliphatic, and optionally branched, aliphatic substituents; acrylonitrile, styrene.

For example, elastomeric polymers obtained by polymerizing one or more of the following monomers can be used for the graft polymerization: dienes, such as butadiene, cyclopentadiene; Olefins. such as ethylene, propylene, styrene, unsaturated acids, such as acrylic or methacrylic acid, and their esters with mono- or dialcohols having 1 to 10 carbon atoms,

629508

tril, vinyl compounds such as vinyl esters of straight-chain or branched carboxylic acids having 2 to 20, preferably 2 to 4 carbon atoms, vinyl halides, such as vinyl chloride, vinylidene chloride.

After the polymerization, further substances for stabilizing or for improving their further processing properties can be added to the polymers obtained as an aqueous dispersion.

With the methods according to the invention, aqueous plastic dispersions can be brought to residual VC contents continuously without thermal damage in a short time with comparatively little use of inert gas for the treatment, which enable environmentally friendly further processing of the dispersion. For example, PVC dispersions produced by emulsion polymerisation can be brought to residual VC contents in a few minutes under the most gentle conditions (e.g. at 70 ° C), which, when sprayed subsequently, result in VC emission values that are well below 150 mg / cm3 exhaust air.

Even residual VC contents of 0.001% by weight (= 10 ppm; based on solids with 20% moisture) and below can be achieved in a comparatively short time.

Difficulties due to deposits or incrustations on the container walls practically do not occur in the process. When using easily condensable inert gases, especially water vapor, the recovery and reuse of the expelled residual monomers is not difficult and requires little effort. The introduction of these inert gases above the liquid level prevents the formation of unwanted foam.

Another advantage of the method according to the invention is that the more effective degassing means that comparatively smaller apparatus volumes can be used. Furthermore, due to the multi-stage procedure, the residence time spectrum of the polymer particles in the apparatus is considerably narrowed, which makes it possible to use higher treatment temperatures and thus a further reduction in the treatment time without thermal damage to the polymer particles.

By removing and recovering physiologically questionable substances immediately after the polymerization, harmful emissions to the ambient air are avoided at a very early stage in polymer production. The treated dispersion can be processed further by customary processes, for example by direct application for coatings or by spray drying, contact drying, and by separating off the majority of the aqueous phase, for example using a decanter, centrifuge or filter, and then stream drying. Special security measures that require conversions or additions are not necessary for this.

The following examples are intended to explain the invention in more detail. The measured values listed were determined as follows:

VC residual monomer content: gas chromatography according to the "head-space" method, magazine f. analytical chemistry 255 (1971), pp. 345 to 350,

K value: according to DIN 53 726.

Comparative experiment 1

The test is carried out in a device according to FIG. 3 of main patent no. 614962. This device consists of a standing cylindrical container with an opening for discharging liquid in the lower part, an opening for discharging gas in the upper part and a double jacket through which a liquid temperature control medium flows through two lines. In the upper part of the

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A spraying device is attached to the container, consisting of four flat jet nozzles, which together produce a hollow cone with a scattering angle of approximately 120 ° from sprayed material. The container is 1.4 m 0 and 2.8 m high.

Water of 75 ° C is used as the temperature control medium, the container contents are kept constant at 70 ° C.

In this device, 6300 kg of an aqueous dispersion produced by continuous emulsion polymerization with an amount of 45% by weight of polyvinyl chloride (K value 78) and 0.8% by weight of monomeric viral chloride are produced in one hour The gaseous "" iry'chloride is removed at the top, the treated dispersion at the bottom of the container. A dispersion filling of 1.26 m3 is kept constant in the container. The values determined are listed in the table below.

example 1

In a device according to Figure 1 of this document, consisting of 3 cylindrical containers of the same size, each 1.4 m 0 and 2.8 m high, each containing 1 hollow cone spray nozzle and a gas inflow opening, 6300 kg of the same dispersion as in Comparative test 1 sprayed. A dispersion filling of 0.42 m3 is kept constant in each container. The double jackets of the containers are fed with water at 75 ° C. No steam is introduced and not stirred. The escaping gaseous substances are conducted in countercurrent to the dispersion. The values determined are listed in the table.

The polyvinyl chloride powder obtained by drying from the dispersion treated according to the invention shows no differences in processing behavior when processed into a hard film from a calender compared to that from an untreated dispersion.

Form-escaping products are drawn off at the top, the treated dispersion at the bottom of the container. A dispersion filling of 5 m3 is kept constant in the container and stirred by the stirrer at a circumferential speed of 3 m / min. The values determined are plotted in the table.

Example 2

In the same device as described in Example 1, 10 m3 of the same dispersion as in Comparative Experiment 2 are sprayed in one hour, in each of the three containers a dispersion filling of 1.66 m3 is kept constant. Steam is blown into the inert gas feed line of the last container in the direction of dispersion flow, and the escaping gaseous products are conducted in countercurrent to the dispersion. The stirrers run at a peripheral speed of approx. 3 m / s. The double coats are empty. The values determined can be seen in the table below.

The polyvinyl chloride powder obtained from the dispersion treated according to the invention by drying shows no differences in processing behavior when processed into hard profiles on an extruder compared to a powder obtained from an untreated dispersion.

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Example 3

The procedure is analogous to Example 2, but at a higher temperature and pressure using an aqueous dispersion prepared by suspension polymerization with 33% by weight polyvinyl chloride (K = 68) and 1.35% by weight vinyl chloride, with a disper in each container -sion filling of 1.11m3 is kept constant. See table for values.

Comparative test 2 The test is carried out in a device according to FIG. 1 of main patent no. 614 962. This device consists of a standing cylindrical container with an opening for the discharge of liquids and a stirrer in the lower part, as well as an opening for the discharge of gas and a spray device with a spiral nozzle which produces a hollow cone in the upper part of the container. Steam can be introduced into the supply line to the spraying device. The container is 3 m 0 and 10 m high.

10 m 3 of an aqueous dispersion produced by suspension polymerization and containing 32% by weight of polyvinyl chloride (K = 70) and 1.5% by weight of monomeric vinyl chloride are sprayed into this device in one hour. Steam is introduced into the feed line to the spray nozzle, which condenses completely in the dispersion. The gas35

Example 4

It is carried out in a device analogous to that described in Example 1 and shown in FIG. 1, except that the dispersion is sprayed in succession into 6 containers of the same size and measuring 1.4 m in height and 2.8 m in height. Steam is blown into the last and third containers in the direction of the dispersion and the resulting gaseous products are conducted in countercurrent to the dispersion. A dispersion filling 45 of 0.83 m3 is kept constant in each container. An aqueous dispersion obtained by suspension polymerization with 30.5% by weight based on dispersion of a copolymer of 90% by weight vinyl chloride and 10% by weight vinyl acetate (% based on dry polymer) (K value = 60) is used . For values see table.

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Comparative experiment 1

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Comparative experiment 2

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Example 3

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1 sheet of drawings

Claims (12)

629508 PATENT CLAIMS 1. Process for the preparation of polymer dispersions by emulsion or suspension C, C double bond opening polymerization of the monomer or monomers in aqueous liquor under excess pressure, relaxation of the polymer batch and removal and condensation of the gaseous products which contain gaseous monomer, wherein the dispersion in a vertical, rotationally symmetrical container is sprayed radially, in the form of a hollow cone or a disk made of droplets of at most 6 mm in diameter, deposited there and collected in the lower part of the container and gaseous substances above the outlet of the dispersion be withdrawn from the spraying device, characterized in that that the dispersion is sprayed in at least two containers connected in series and the gaseous substances are guided in one direction and the dispersion in the opposite direction from container to container. 2. The method according to claim 1, characterized in that the dispersion is heated before spraying. 3. The method according to claim 1 or 2, characterized in that at least one inert gas is added to the gaseous substances to be removed. 4. The method according to claim 1, 2 or 3, characterized in that the gaseous substances are discharged only from the last container in the gas flow direction and are removed from the process sequence. 5. The method according to claim 3, characterized in that water vapor, e.g. superheated steam is used. 6. The method according to claim 3 or 5, characterized in that the gaseous substances are introduced into the container so that the gas flows through the hollow cone formed by the sprayed dispersion. 7. polymer dispersion prepared by the process according to claim 1. 8. An apparatus for performing the method according to claim 1, consisting of at least two series-connected, vertically standing, rotationally symmetrical containers, each with a gas discharge opening in the upper and at least one liquid outlet in the lower part, each container in its upper part a the dispersion in the form of a Contains the hollow cone spraying device spraying against the container wall in such an arrangement that the axis of the hollow cone made of sprayed material approximately coincides with the container axis, and wherein the gas discharge opening is arranged above the spray device, characterized in that each container below the spray nozzle (s) has at least one gas inflow opening contains, which is arranged via a feed line from outside the container so that it is not hit by the spray jet emerging from the spraying device and is not flooded by the dispersion collected in the lower part of the container. 9. The device according to claim 8, characterized in that the rotationally symmetrical containers contain means for tempering and / or stirring the contents of the container. 10. The device according to claim 8, characterized in that each container contains at least one covered gas inflow opening. 11. The device according to claim 8, characterized in that said containers are the superimposed chambers of a vertical, rotationally symmetrical vessel, which is divided by one or more, preferably horizontal, partition into two or more superposed chambers, that each partition contains at least one opening, the edge of which is tightly connected to an ascending, open at the top and bottom pipe, which ends below the spraying device of the upper chamber, possibly covered with a cap, and serves to draw gas from the lower chamber and into the upper chamber allow to flow in, and that the liquid outlets of the chambers, with the exception of that or that of the lowermost chamber, are connected to the spraying device of at least one chamber by means of a pump, possibly also by means of attachments for temperature control of the flowing dispersion. 12. The device according to claim 11, characterized in that the vessel is divided by 1 to 9, preferably 1 to 5, partitions into 2 to 10, preferably 2 to 6, approximately equal chambers. CH-PS 614 962 discloses a process for the preparation of polymer dispersions by emulsion or suspension polymerization of the monomer or monomers in an aqueous liquor, decompression of the polymer batch and removal and condensation of the gaseous products which contain gaseous monomer, the optionally heated dispersion being known in a vertical, preferably rotationally symmetrical container, radially, in the form of a hollow cone or a disc, of droplets of at most 6 mm in diameter, sprayed against the container wall, deposited there and collected in the lower part of the container, gaseous substances above the outlet of the dispersion be withdrawn from the spraying device. It has been found that the effectiveness of the process can be significantly increased if the dispersion is sprayed in at least 2 containers connected in series, the resulting gaseous substances, if appropriate with admixture of at least one inert gas, only in one direction and the dispersion in the opposite direction from the container leads to container. The dispersion is preferably sprayed in 2 to 10, in particular 2 to 6, containers connected in series. If there are more than 10 containers, the additional effect achieved with each additional container is generally not so great that the additional equipment is worthwhile. Several containers can also be combined in a larger container unit. The dispersion is introduced into each of these containers via a spray device, drawn off in the lower part and fed to the next container or discharged from the last container in the direction of flow, that is to say removed from the process sequence. Furthermore, gaseous substances are generally fed above the dispersion collected in the lower part of the container, drawn off above the spray nozzle and discharged or fed to the next container in countercurrent to the dispersion. The gaseous substances are advantageously discharged only from the last container in the flow direction. The line cross-sections should be selected taking into account the flow velocity of the medium flowing in them so that there is no backmixing of both the dispersion and the gaseous substances from container to container. To improve the removal of the gaseous substances, one or more inert gases can optionally be introduced in at least one container, advantageously in the last container in the direction of the dispersion flow. Suitable inert gases are, for example, nitrogen, air, carbon dioxide and, in particular, optionally superheated steam. In a preferred embodiment of the 2nd s 10th 15 20th 25th 30th 35 40 45 50 55 60 65 According to the process, the gaseous substances are introduced into the gas space of the next container in such a way that the gas flow flows through the hollow cone formed by the sprayed dispersion, advantageously through a gas inflow opening which is arranged not too far under the hollow cone nozzle so that it is neither from the spray jet hit from this nozzle is still flooded by the dispersion collected in the lower part of the container. This arrangement results in a particularly intensive mass transfer between the gas phase and the liquid phase. The dispersion in the containers can be stirred, expediently with stirring elements which are introduced into the container from the side or from below. If necessary, the stirring and spraying device can be arranged such that both are introduced into the container from above, for example if the spraying device consists of a plurality of segment spray nozzles which are arranged on a ring around the stirrer shaft. The level of the optionally stirred liquid in the containers is preferably regulated so that at most a small amount, for example less than 5% by weight, of the sprayed dispersion hits the liquid level directly. The containers can be half or more filled with dispersion and at the same time serve as collection and homogenization vessels. In general, you will not work with too high dispersion levels in the containers, because the sprayed dispersion is degassed as it descends on the walls of the container, and because at high dispersion levels, the risk of sprayed drops falling directly on the dispersion surface increases. A stand height of the dispersion in the containers is expediently chosen from approximately 1/10 to 6 ° of the total inner container height. To improve the degassing effect, the dispersion is expediently heated to temperatures of 50 to 120 ° C., in particular to 70 to 100 ° C., before spraying. This heating can take place indirectly via the tank wall, possibly additionally via tank internals or by direct introduction of water vapor into the dispersion. In the latter case, it is advantageous to dimension the steam supply in such a way that as much of the steam as possible condenses in the liquid. Avoid boiling the dispersion more intensely as this generally leads to foam disturbances. Weak evolution of gas from the dispersion will not pose any problems in most cases. The steam can be supplied both in the container and, advantageously, in the dispersion feed line to the spray nozzle. The pressure in the container is expediently kept higher than the saturation vapor pressure of the water vapor above the dispersion collected in the lower part of the container and should generally not be more than 200 torr, preferably not more than 100 torr, below the saturation vapor pressure of the water vapor of the sprayed-in dispersion. It can also be equal to the saturation vapor pressure of the water vapor of the sprayed-in dispersion or • be up to about 500 Torr. It should be avoided that larger amounts of water evaporate together with the monomer during the often necessary longer treatment time to remove the monomers, because this unnecessarily worsens the economy of the process and can lead to undesirably strong thickening of the dispersion and deterioration in the quality of the end product. In general, for example, with evaporated water quantities of 0.2 to 5 kg per 100 kg of polyvinyl chloride dispersion, favorable residual monomer contents of the dispersion are achieved. In individual cases, however, it can also be expedient to evaporate more than 5 kg of water per 100 kg of dispersion. 629508