GB2125801A - Freezing coagulation of polymer dispersions - Google Patents

Abstract

A process for the separation of a polymer from a polymer dispersion comprises freezing the coherent phase of the dispersion in a moving layer subjected to shearing forces, thawing the coherent phase and separating the phases.

Description

SPECIFICATION Freezing coagulation of polymer dispersions This invention relates to a gentle method of recovering polymers from polymer dispersions in a form in which they may be easily handled and transported by a particular form of freezing coagulation.

Many polymers are prepared by homo- or copolymerisation of suitable monomers in a liquid medium by suspension, emulsion or precipitation polymerisation. In these processes, the polymer is obtained in the form of a solids dispersion (latex), which is in most cases aqueous, from which the polymer must be separated if the latex is not to be used as such.

Separation of the polymer from the dispersion is normally carried out by coagulation. Various methods are known for this purpose (Houben Weyl, Methoden der organischen Chemie, 4th Edition, 1961, Volume XlV/1, pages 470 etseq).

Thus, from latices which have been prepared under alkaline conditions, for example, the polymer may be coagulated by acidification, for example using a mineral acid or an organic acid. In many cases, acidification alone is not sufficient for complete coagulation of the polymer and it is necessary to add strong electrolytes (salts containing polyvalent cations, such as Mg2+, Ca2+ or Al3+), as well as the acid.

One disadvantage of this method is that large quantities of acid or electrolyte are required for complete precipitation of the polymer. Apart from the economic disadvantages, there is the risk that relatively large quantities of precipitating agents may be left in the product and cause deterioration of important product properties. It therefore becomes necessary to wash the coagulated polymer with relatively large quantities of water, which in turn gives rise to economic and ecological problems.

Another disadvantage of coagulation by acid and electrolyte is that the polymer is partially obtained in the form of large lumps which contain in the interior thereof either latex which has not been precipitated or excess precipitating agent so that either complete coagulation or subsequent washing becomes difficult if not impossible. The opposite phenomenon also frequently occurs, namely that the coagulate is obtained in a very finely divided form so that subsequent isolation of the polymer by filtration or sedimentation becomes very difficult.

Another possible method involves coagulating polymer dispersions by the action of elevated temperatures, optionally at elevated pressures and by the additional action of electrolytes and shearing forces (DE-AS 1,570,185 and DE-OS 2,839,306).

The method has the disadvantage that the product is subjected to considerable thermal stress which has a damaging effect on the desired properties of the product. In addition, there are economic disadvantages in providing the energy required for producing the high temperatures employed.

In view of the disadvantages of the coagulation methods described above, it has been attempted to separate polymers from aqueous dispersion by freezing (Houben Weyl, Methoden der organischen Chemie, 4th Edition 1961, XIV/1, pages 472 et seq.).

This involves cooling the latex below the freezing point of the aqueous phase to freeze out the aqueous phase. When the frozen phase is subsequently thawed under suitable conditions, the polymer is obtained in the form of a coagulate and may be separated from the aqueous phase. In order to obtain technically acceptable speeds of coagulation, i.e. sufficiently high output rates, it has been attempted to freeze the latex in thin layers. Coagulation rollers which may be cooled from inside have been developed for this purpose. These rollers are immersed in the latex while rotating so that they carry a thin layer of latex with them as they rotate and freeze this layer on the surface thereof (U.S. Patent No.

2,187,146). The thin film of coagulate and ice is removed from the roller using a scraper and is carried away to be heated so that the ice melts and the polymer is left behind as a continuous, porous film which is then washed and processed. With all its advantages, this process has the disadvantage that it is only applicable to polymers which form a continuous, sufficiently solid film which is not brittle and yet porous. U.S. Patent No. 2,187,146 is therefore limited to the freezing coagulation of polychloroprene latices.

It was therefore an object of the present invention to develop a process for the separation of polymers from aqueous dispersions in which only a minimum of acid and optionally addition of electrolyte is required, the thermal stress to which the product is subjected is as slight as possible, the separated polymer is obtained in a form which is not too finely divided and yet with a large surface area so that it is easily washable and may be delivered to suitable machines for washing, dewatering and drying, which process should be applicable to a wide variety of polymer dispersions.

It has now surprisingly been found that this problem may be solved by carrying out the freezing coagulation in a moving layer of product subjected to shearing forces. The hitherto known method of freezing coagulation by means of a cooling roller was carried out in a stationary layer of product.

The present invention therefore relates to a process for the separation of polymers from polymer dispersions, comprising freezing the coherent phase, thawing the coherent phase and separating the phases, characterized in that freezing of the coherent phase is carried out in a moving layer of product subjected to shearing forces.

The process according to the present invention may be carried out in a variety of known apparatus.

Thus, for example, it is suitable to use thin layer evaporators developed for high viscosity techniques, comprising obliquely placed wiper blades to assist the transport of material through the vertical apparatus by gravity, thin layer contact driers which may be operated either vertically or horizontally, and horizontally arranged drying apparatus for pasty products liable to form crusts, equipped with heat able shearing and conveying elements which may be self-cleaning to varying extents by means of scrapers fixed to the housing and extending into the region of the shaft or by means of a scraper shaft engaging with the main shaft.

In the process according to the present invention, these pieces of apparatus are not employed for the originally envisaged purpose as evaporators or driers, that is to say they are not heated, but cooled.

Single shaft screw extruders designed to be cooled are also suitable for carrying out the process according to the present invention if means are provided to prevent the product adhering to the screw shaft and rotating with it. This may be achieved, for example, by providing shearing pins attached to the housing and engaging with the screwthread at a point where the latter has a localized interruption.

Other suitable screw extruders capable of being cooled include in particular those in which the threads are only designed for transporting the material. Intermeshing double screw extruders, especially whose in which the screw rotate in the same sense, are particularly suitable for this purpose.

In one preferred form of intermeshing double screws rotating in the same sense, the screw profile is interrupted over at least part of its total length by the introduction of grooves.

These grooves preferably have the same channel depth as the thread profile, and the number of grooves per shaft is equal to the number of threads of the threaded profile, the cross-section of the grooves is rectangular or trapezoidal and the grooves are helical with the same pitch as the screw thread, but twisted in the opposite direction. Such screws have been disclosed in DE-OS 3,038,973.

Cooling of the screw extruders to be used according to the present invention may be effected by way of the screw housing or by way of the screw shafts, which are made hollowforthis purpose, or both.

A wide variety of polymer dispersions may be coagulated according to the present invention; for example, polychloroprene-, SBR-, NBR-, BR- and natural rubber latices and ABS- or PVC-dispersions.

Mixtures of the latices or dispersions may also be easily coagulated by these means.

It is prefeable to use apparatus which requires little energy for transporting and size-reducing the material to be coagulated, since the energy gener ated must be removed from the system by additional cooling.

The housing and optionally the shafts of the apparatus mentioned aboveforthe process accord ing to the present invention are cooled by liquid heat carriers. The preferred temperature range for the cooling medium is from -50 to -10 C. Suitable cooling media include, for example, mixtures of water and glycol, or cooling sols, i.e. aqueous salts solutions or alcohols, such as methanol. One parti cularly effective method of cooling is achieved by means of liquids, such as ammonia or "Frigen" evaporating directly in the apparatus to be cooled.

The cooled product leaves the apparatus at a temperature of a few degrees below the freezing point, i.e. in the region of from -10 to 0 C.

The velocity gradient or shearing gradient be tween the internal walls of the housing and the rotors, wipers or screw shafts of the aforesaid apparatus sweeping over the said internal walls at a certain distance therefrom is taken as the criterion for the shearing stress to which the layer of product is subjected. This velocity gradient in terms of [1/s] is calculated from the circumferential velocity of that point of the rotor which is fqrthest removed from the shaft divided by the distance of this outermost point of the rotor from the internal wall of the housing. For the process according to the present invention, the velocity gradients of the above-mentioned apparatus are from 30 to 3,000 I/s.

The substance discharged from the abovedescribed apparatus for the freezing coagulation according to the present invention in a moving layer of product subjected to shearing stress is a mixture of the frozen aqueous phase of the latex and the coagulated polymer in a granular form. This "snowlike" mixture is subsequently continuously thawed, which may be achieved by various methods.

For example, the mixture may be thawed as a stationary layer on a conveyor belt apparatus designed to be heated. The heating means may act directly on the solidified mixture, e.g. when hot air or condensing steam is blown on the mixture, or the mixture may be sprayed with hot water or exposed to radiant heat. Indirect heating by contact is also possible, by heating the belt from underneath by means of a heat carrier.

Thawing of the solidified, snow-like mixture may also be carried out in a moving layer. In this case, a tubular stream may be used for pneumatic transport and the transport medium and energy carrier used in this case may be, for example, hot air or stream.

Conveyor apparatus in the form of screws or screw extruders may also operate with a moving layer of product. Such apparatus may act directly on the material by means of the media and methods described above with reference to the conveyor apparatus or they may thaw the mixture by heat from the walls obtained by contact heating. In the case of screw extruders, an additional source of energy for thawing is the shearing energy, which may be influenced by the geometrical form and speed of rotation of the screw.

If thawing by one of the above-mentioned methods is applied to the recovery of polymers which are particularly sticky or strongly adhering so that there is a tendency towards agglomeration into large lumps or the formation of obstinate deposits on the walls of the apparatus, this tendency may be counteracted by the addition of mould release agents before or during thawing or even at the stage of freezing coagulation.

The substance obtained after thawing is a separable mixture of aqueous phase and polymer particles, which may be almost completely separated by mechanical means. The major proportion of water may be separated, for example, on a shaker screen, a rotary filter or a band filter or it may be separated on a press band filter assisted by a second band.

Particularly suitable for the separation of rubbers is the known technique of expressing by means of press screws which may operate with filtration zones for lateral dewatering or without filtration zones, i.e.

with backward dewatering (H. Herrmann; Schneckenmaschinen in der Verfahrenstechnik, Springer Verlag 1972, pages 24-27 and pages 141-146). When employing such mechanical dewatering in press screws, the water content may be substantially lowered in the last section of the screw or in a separate, attached "expander screw" by heating by shearing forces, followed by evaporation by release of pressure. A final stage of drying by heat may also be carried out in a fluidized bed, flow drier, plate or band drier or some other known driers operating by convection or contact. Depending upon the type of polymer and the water content, it is suitable to use a combination of the simple separating apparatus initially mentioned and, for example, press screws attached thereto.

The process steps of thawing and subsequent separation of the phases may also influence each other by the apparatus employed. Thus, for example, thawing on a conveyor band apparatus may suitably be combined with the use of a band filter or press band filter for preliminary dewatering. Another example of a combination suitable for lower water contents and when press screws are used for the separation of water is the addition, upstream of the said screws, of a thawing section if a screw with moving layer of material is employed.

Example 1 A chloroprene rubber latex is prepared by a conventional method of emulsion polymerisation at 45"C from 94.6 kg of chloroprene, 5.4 kg of 2,3 dichloro-1,3-butadiene, 125kg of or water, 5.0 kg of the sodium salt of a disproportionated abietic acid, 0.65 kg of the sodium salt of a condensation product of formaldehyde and naphthalene sulphonic acid and 0.65 kg of sodium hydroxide (e.g. P.R. Johnson in Rubber Chem. Technol., 49, (1976), pages 665 et seq). Polymerisation is stopped at the stage of 65% conversion and the latex is freed from residual monomers by steam distillation.

Polychlorprene latex (31%, by weight, solids content) adjusted to pH 6 using acetic acid is fed at the rate of 8 kg per hour by means of a dosing pump into a double screw extruder having self-cleaning intermeshing screws rotating in the same sense. The external screw diameter is 32 mm, the core diameter is 17 mm, and the pitch of each two-thread screw is 20 mm. The two screw shafts, which are hollow, are accommodated in a coolable, closed housing 600 mm in length arranged downstream of the feed hopper. Methanol cooled to -32 C in a refrigerating machine flows as cooling medium both through the hollow shafts and through the housing. The latex entering the screw extruder at 20"C is continuously solidified as a moving product layer subjected to shearing stress.The screw extruder operated at a shaft speed of 100 revs per min discharges from its outlet the mixture of ice crystals and freeze coagulated rubber in the form ofstripsfrom 30 to 100 mm in length at a temperature of -18 C. When this mixture thaws, immediately after being discharged, complete coagulation of the polymer is observed.

Under the operating conditions mentioned above, the cooling medium flows from the screw shafts and housing at a high flow rate at -290C.

Example 2 A chloroprene rubber latex is prepared from 100 kg of chloroprene instead of from 94.6 kg of chloroprene and 5.4 kg of 2,3-dichloro-1,3-butadiene as described in paragraph 1 of Example 1. The latex (31%, by weight, rubber content) adjusted to pH 6 is subjected to freezing coagulation in the screw extruder described in Example 1, but the screw profile according to DE-OS 3,038,973 is repeatedly interrupted by the formation of grooves over a length of 560 mm. The grooves are cut into the screw thread as rectangular grooves 5 mm in width having the same pitch, depth of thread and thread number as the screw thread, but twisted in the opposite direction.

The housing and the screw shafts are cooled using methanol at -28 C. The cooling medium leaves the apparatus at -26 C. The screw extruder rotated at a speed of 230 revs. per min and fed with 12 kg of latex per hour at 20"C discharges the solidified mixture at a temperature of -8 C in the form of fine particles measuring up to 5 mm. The thawing process is found to be accompanied by complete coagulation of the polymer.

Example 3 The polychloroprene latex mentioned in Example 1 and pre-treated as described therein is subjected to freezing coagulation in a screw extruder similar to that mentioned in Example 2, but larger. A different cooling technique is employed, using "Frigen" directly evaporating in the zones of the screw which are to be cooled. This larger cooling screw has an external screw diameter of 51 mm, a core diameter of 28 mm, and a two-threaded screw pitch of 48 mm.

A coolable, closed housing 1060 mm in length is provided downstream of the intake aperture. The housing and screw shafts are cooled in the same manner by evaporating "Frigen". This larger double screw is also equipped, as described in detail in Example 2, with a thread groove 7 mm in width over a total length of 900 mm.

"Frigen" enters the screw housing at -24"C and leaves the housing at -23"C; it enters the screw shafts at -26 C and leaves the shafts at -7 C. The screw extruder operated at a speed of 150 revs. per min and fed with latex at 20"C at the rate of 66 kg per hour discharges the solidified mixture at a temperature of -1 C in the form of grains having a particle size up to 10 mm. The polymer is completely coagulated after thawing.

Example 4 A nitrile rubber latex is prepared by emulsion polymerisation at 17"C under conventional conditions (e.g. Houben Weyl, Methoden der organischen Chemie, 4th Edition 1961, Voluem 14/1, pages 703 et seq) from 25.7 kg of acrylonitrile, 74.3 kg of buta diene, 195kg of or water,2.0 of or the potassium salt of coconut fatty acid, 2.0 kg of the sodium salt of butylated naphthalene sulphonic acid and 2.0 kg of the sodium salt of a naphthaiene sulphonic acid/ formaldehyde condensation product. Polymerisa tion is stopped when conversion reaches 75%, and the latex is free from residual monomers by steam distillation.

This nitrile rubber latex (19% solids current) is subjected to freezing coagulation in the double screw extruder of Example 2. The pH of the latex is adjusted to 6 using acetic acid.

Methanol used as cooling medium enters the housing and shafts at -30"C and leaves at -27 C.

When the latex is fed in at the rate of 21 kg per hour at a temperature of 20"C and the screws are rotated at 230 revs. per min, the solidified mixture leaves the apparatus at -4 C in the form of particles measuring up to 3 mm and with complete coagulation of the polymer after thawing.

Example 5 A horizontally-disposed contact drier for pasty products, comprising a heatable housing which is "8-shaped" in cross-section and two heatable shafts equipped with shearing and conveying elements, i.e.

the main shaft and a cleaning shaft engaging with the main shaft ("AP 12 Conti" apparatus of List) is cooled by cooling of the housing and cooling of the shafts with a glycol-water mixture which has been cooled to -300C. The mixture leaves the apparatus at -28"C under the operating conditions mentioned above.

The apparatus is charged with a stream of 24 kg per hour of the latex mentioned in Example 2. The latex, which has been pre-treated as described therein, is fed at an inlet temperature of 20"C by means of a dosing pump. The apparatus operating with the main shaft rotating at 18 revs. per min and the cleaning shaft rotating at 72 revs. per min.

continuously solidifies the latex in a moving layer of product subjected to shearing stress, and at its discharge end the apparatus discharges a mixture of ice crystals and freeze coagulated rubber, the material being at a temperature of -3 C and the particles measuring from 5 to 40 mm. When this mixture is subsequently thawed, the polymer is found to have coagulated completely.

Example 6 A nitrile rubber latex is prepared according to Example 4, paragraph 1, from 34.9 kg of acrylonitrile and 65.1 kg of butadiene.

This latex (19% rubber content) adjusted to pH 6 as in Example 4 is coagulated in the apparatus described in Example 5. For this purpose, the housing and shafts are cooled with a glycol-water mixture which enters at -30 C and leaves the apparatus -28 C. The "AP 12 Conti" apparatus, operated at a speed of rotation of 7/28 revs. per min (main shaft/cleaning shaft), is continuously charged with latex at 20"C at the rate of 16 kg per hour and discharges the solidified mixture in the form of particles measuring up to 10 mm and at a temperature of -2"C. The polymer is completely coagulated after thawing.

Example 7 Methanol cooled to -29 C in a refrigerating machine flows as cooling medium through the housing of a double screw extruder having selfcleaning intermeshing screws rotating in the opposite sense. The external screw diameter is 32 mm, the core diameter is 24 mm, and the pitch of each four-thread screw is 60 mm. The two srew shafts are accommodated in the closed housing 400 mm in length arranged downstream of the feed hopper.

The methanol leaves the housing under operating conditions at -27 C.

The chloropene rubber latex of Example 2 has been pre-treated as described therein and is fed at the rate of 5 kgs per hour by means of a dosing pump into the extruder. The entering temperature of the latex is 20"C.

The apparatus operating with the shafts rotating at 35 revs. per min continuously solidifies the latex in a moving layer of product subjected to shearing stress, and at its discharge end the apparatus discharges a mixture of ice crystals and freeze coagulated rubber, the material being at a temperature of -1 C and in the form of strips from 60 mm in length. When this mixture is subsequently thawed, the polymer is found to have coagulated completely.

Example 8 Nitrile rubber latex of Example 6 has been pretreated as described therein and is fed at the rate of 4 kgs/h at 20"C to the apparatus of Example 7.

Methanol used as cooling medium enters the housing at -29 C and leaves at -28"C. The screws are rotated at 15 revs. per min, the mixture of ice and coagulated rubber leaves the apparatus at -9 C in the form of strips measuring 30 mm in length. After thawing the polymer is found to have coagulated completely.

Claims (11)

1. A process for the separation of a polymer from a polymer dispersion which comprises freezing the coherent phase of the dispersion in a moving layer subjected to shearing forces, thawing the coherent phase and separating the phases.

2. A process as claimed in claim 1 wherein the freezing of the coherent phase is carried out in a coolable thin layer apparatus.

3. A process as claimed in claim 1 wherein the freezing of the coherent phase is carried out in a coolable horizontal contact drier having coolable shearing and conveyor elements on a shaft which is substantially self-cleaning by means of scrapers attached to the housing and extending into the region of the shaft or by means of a cleaning shaft engaging with the main shaft.

4. A process as claimed in claim 1 wherein the freezing of the coherent phase is carried out in a cooiable screw extruder.

5. A process as claimed in claim 4 wherein the freezing of the coherent phase is carried out in a coolable, single shaft screw extruder having shearing projections attached to the housing and engaging with the screw thread which is locally interrupted.

6. A process as claimed in claim 4 wherein the freezing of the coherent phase is carried out in a coolable double screw extruder having intermeshing screws.

7. A process as claimed in claim 6 wherein the intermeshing screws rotate in the same sense.

8. A process as claimed in claim 6 or claim 7 wherein the screw profile is interrupted by the introduction of grooves over at least part of the total length thereof.

9. A process as claimed in any of claims 1 to 8 wherein the freezing is carried out using a cooling medium at a temperature of from -50 to -10"C giving a product having a temperature of from -10 to 0 C.

10. A process as claimed in any of claims 1 to 9 wherein the velocity gradient in the moving layer subjected to shearing forces is from 30 to 3,000 I/s.

11. A process as claimed in claim 1 substantially as herein described with particular reference to the Examples.