PROCESS FOR PREPARING AQUEOUS POLYMER DISPERSIONS IN A TUBULAR REACTOR

Abstract

A process for preparing vinyl acetate-ethylene copolymers in the form of an aqueous dispersion. The process includes providing an aqueous dispersion of vinyl acetate-ethylene copolymers and subjecting the aqueous dispersion of vinyl acetate-ethylene copolymers to radically initiated emulsion polymerization in a continuously operated tubular-reactor. Where one or more dividing plates bearing liquid-permeable openings that are mounted within the tubular-reactor transverse to the flow direction of the reactor contents.

Claims

1-13. (canceled)

14. A process for preparing vinyl acetate-ethylene copolymers, comprising: providing an aqueous dispersion of vinyl acetate-ethylene copolymers; subjecting the aqueous dispersion of vinyl acetate-ethylene copolymers to radically initiated emulsion polymerization in a continuously operated tubular-reactor; and providing one or more dividing plates bearing liquid-permeable openings that are mounted in the tubular-reactor transverse to the flow direction of the reactor contents.

15. The process of claim 14, wherein the tubular-reactor contains 3 to 30 of the one or more dividing plates.

16. The process of claim 14, wherein there is no liquid-permeable gap between dividing plates and inner tubular-reactor wall.

17. The process of claim 14, wherein the one or more dividing plates are connected to one another by one or more bars.

18. The process of claim 17, wherein a construct comprises adjacent one or more dividing plates and one or more bars that are fastened on a floor of the tubular-reactor and/or on a cover of the tubular-reactor.

19. The process of claim 14, wherein the tubular-reactor contents traverse the one or more dividing plates exclusively through one or more liquid-permeable openings.

20. The process of claim 14, wherein the one or more dividing plates each have only one liquid-permeable opening.

21. The process of claim 14, wherein one or more guide plates are mounted at the edge of one or more liquid-permeable openings in the one or more dividing plates.

22. The process of claim 21, wherein a guide plate projects only into one or into both of the cells divided by a dividing plate.

23. The process of claim 21, wherein the one or more guide plates are mounted on the side of the dividing plate that lies downstream in the flow direction of the reactor contents.

24. The process of claim 21, wherein the one or more guide plates project by the length L into the two cells divided by a dividing plate and the ratio of the length L of a guide plate to a free diameter of a liquid-permeable opening is 0.25 and 10.

25. The process of claim 14, wherein the tubular-reactor comprises a stirring assembly which comprises a stirrer shaft on which one or more stirrer blades are mounted; and wherein the stirrer shaft is routed through a liquid-permeable opening in one or more dividing plates.

26. The process of claim 25, wherein the stirrer shaft does not completely fill a liquid-permeable opening in one or more dividing plates.

27. The process of claim 25, wherein the stirrer shaft extends from one longitudinal end to the other longitudinal end of a tubular-reactor.

28. The process of claim 14, wherein one or more baffles are mounted in the tubular-reactor axial to the flow direction of the reactor contents.

29. The process of claim 14, wherein the flow direction of the tubular-reactor contents is not reversed in the tubular-reactor.

Description

[0085] The examples which follow serve for further elucidation of the invention:

General Experimental Description:

[0086] The polymerization was carried out in a tubular reactor (1) having a length of 1600 mm and an internal diameter of 100 mm. The reactor volume was 12.5 liters. The reaction mixture was mixed transverse to the longitudinal axis by a stirrer (3) having multiple stirrer blades (4) with dimensions of 50 mm50 mm; the distance of the stirrer blades from the reactor wall was 25 mm and so contact with the reactor wall was avoided. Along the reactor axis there were an additional 5 further addition facilities (9a) to (9e) for initiator.

[0087] The tubular reactor (1) was fed continuously with the composition for polymerization from an upstream pressure vessel (6) having a volume of 16 liters. The upstream pressure vessel (6) was charged continuously with the corresponding compounds via pumps.

[0088] Following emergence from the tubular reactor (1), the product was transferred via a pressure-maintenance valve (7a) into an unpressurized container (8) having a volume of 1000 liters, where it was collected. At the end of the experiment, the product mixture was aftertreated and discharged.

Composition for Polymerization:

[0089] The following compounds were supplied continuously to an upstream pressure vessel (stirred tank) (6) and premixed:

[0090] 4.4 kg/h of water, 4.0 kg/h of a 20 wt % aqueous solution of a partially hydrolyzed polyvinyl alcohol having a degree of hydrolysis of 88 mol % and a Hppler viscosity of 4 mPas (determined according to DIN 53015 at 20 C. in 4 wt % aqueous solution), 10.4 kg/h of vinyl acetate, 1.15 kg/h of ethylene, 195 g/h of 5 wt % aqueous ascorbic acid solution, 1.5 g/h of formic acid and 4 g/h of 1 wt % aqueous iron ammonium sulfate solution.

[0091] The composition for polymerization was transferred into the tubular reactor (1) at a rate of 20 kg/h.

[0092] Potassium persulfate as initiator, in the form of a 3 wt % aqueous solution, was metered in at the metering points (9a) to (9e).

[0093] The finished product left the tubular reactor (1) with a conversion of 92% and was collected under reduced pressure in an unpressurized vessel (8).

[0094] After this, for removing excess ethylene, the dispersion was transferred to a further unpressurized vessel, in which a pressure of 0.7 bar was applied, and was postpolymerized therein by addition of 0.4 kg of a 10 wt % aqueous tert-butyl hydroperoxide solution and 0.8 kg of a 5 wt % aqueous ascorbic acid solution, based on 100 kg of dispersion, until the residual vinyl acetate value was <1000 ppm. The pH was adjusted to 4.5 by addition of sodium hydroxide solution (10 wt % aqueous solution). Lastly, the batch was dispensed from the unpressurized vessel via a 250 m sieve.

[0095] In the experiment, the mixture for polymerization was introduced at the bottom end of the tubular reactor (1) and the product was withdrawn at the top end.

[0096] The initiator metering rates were [0097] (9a) 0.11 kg/h [0098] (9b) 0.11 kg/h [0099] (9c) 0.21 kg/h [0100] (9d) 0.30 kg/h [0101] (9e) 0.40 kg/h

[0102] The transit rate was around 20 liters/h. The stirrer speed was 800 revolutions/minute. The pressure in the reactor (1) was established at bar via the transfer valve (7a).

Example 1, Comparative:

[0103] The emulsion polymerization was carried out in a plant according to FIG. 2. The tubular reactor (1) had no dividing plates and was operated with eight stirrer blades (4) in 4 planes.

[0104] After 30 h, the polymerization was ended and the free volume of the tubular reactor (1) was determined by making up with water and weighing the amount of water. With this approach, the reactor volume was ascertained at 8.75 liters. After 30 h, therefore, the reactor lost 3.75 liters of volume, synonymous with a corresponding buildup of wall deposits to an extent of 30% of the reactor volume.

[0105] During the experiment, samples were taken hourly. In this case the particle size distribution proved to be highly unstable; mono- and bimodal distributions were observed and the Beckmann Coulter Dw value fluctuated between 1000 and 6000 nm. Fluctuations were likewise observed in the solids content and conversion; the conversion was consistently <90%.

[0106] Overall it was not possible to achieve stable operating conditions or sufficient product quality. Because of the increasingly uneven temperature distributions in the reactor, as a consequence of the fouling which was already very heavy zonally, the experiment was discontinued after 30 h.

Example 2:

[0107] The emulsion polymerization was carried out in a plant according to FIG. 1. The tubular reactor (1) was divided by 9 dividing plates (10) into 10 cells having a height of 160 mm. The stirring assembly (3) was implemented with double-bladed stirrers (4) with dimensions of 5050 mm, so that a stirrer (4) was seated centrally in each cell. The reactor jacket, including the metering points, was left unaltered. The stirrer speed continued to be 800 rpm.

[0108] After 30 h, the polymerization was ended and the free volume of the tubular reactor (1) was determined by making up with water and weighing the amount of water. With this approach, the reactor volume was ascertained at 11.3 liters, meaning that after 30 h the tubular reactor (1) lost 1.2 liters of volume, synonymous with a corresponding buildup of wall deposits to an extent of <10% of the vessel volume.

[0109] During the experiment, samples were taken hourly. After a short onset time, the product quality proved to be very constant. The particle size distribution obtained was monomodal and was stable throughout the experiment. The residual monomer content after the tubular reactor (1) was 5%. An end product was obtained with properties as follows:

TABLE-US-00001 Solids content 58.5%, pH 4.5, Viscosity (Brookfield at 23 C. and 20 rpm) 1500 mPas, Particle size distribution Dw (Beckmann Coulter) 1300 nm, Glass transition temperature (DSC according to ISO 16 C. 11357)

Example 3:

[0110] The emulsion polymerization was carried out in the same plant and according to the same process as example 2, with the difference that guide plates (14) were mounted on both sides at the liquid-permeable openings (13) in the dividing plates (10), as shown in FIG. 3. The length L of the guide plates in the principal direction of flow was 1.5 times the free diameter of the liquid-permeable openings (13) in the dividing plates.

[0111] After 30 h, the polymerization was ended and the free volume of the tubular reactor (1) was determined by making up with water and weighing the amount of water. With this approach, the reactor volume was ascertained at 11.5 liters, meaning that after 30 h the tubular reactor (1) lost 1.0 liters of volume, synonymous with a corresponding buildup of wall deposits to an extent of <10% of the vessel volume.

[0112] During the experiment, samples were taken hourly. After a short onset time, the product quality proved to be very constant. The particle size distribution obtained was monomodal and was stable throughout the experiment. The mean particle size corresponded to the mean particle size of the batch product. The residual monomer content after the tubular reactor (1) was <5%. An end product was obtained with properties as follows:

TABLE-US-00002 Solids content 58.5%, pH 4.5, Viscosity (Brookfield at 23 C. and 20 rpm) 1500 mPas, Particle size distribution Dw (Beckmann Coulter) 1100 nm, Glass transition temperature (DSC according to ISO 16 C. 11357)