MXPA99009924A - Two-step method for dehydrating plastic dispersions - Google Patents

Abstract

The invention relates to a method for dehydrating a two-phase liquid mixture consisting of a thermoplastic synthetic melt phase and an acqueous phase, comprising the following steps:a) coagulation of the two-phase liquid mixture in a first extruder;b) dehydration of the coagulate in a twin screw extruder with a de-watering zone and screws working in an opposite direction;c) separation of constituents by degasifying. The inventive method is characterized in that a single screw extruder or a twin screw extruder with screws working in the same direction is used as a first extruder in step a).

Description

Two-step process for the dehydration of synthetic materials in dispersion The present invention relates to an improved process for the dehydration of liquefied synthetic materials containing water.

STATE OF THE ART European patent EP-A 0 534 235 describes a process for the preparation of modified thermoplastics at high impact strength, where a synthetic latex is coagulated in a first step with the addition of a coagulating additive (formate solution). of calcium) under 90 ° C. Next, this pre-coagulate with a solids content of about 60% is mechanically dehydrated in an extruder. The advantage of this method is, among others, the elimination of a part of the water contained in the rubber. In this way, a greater rubber performance is achieved and at the same time a decreased energy consumption. European patent EP-A 0 006 503 describes the preparation of coagulate from a graft polymer of a polybutyl acrylate latex by means of precipitation, adding a solution of calcium chloride at 95 ° C.

US Pat. No. 4,602,083 describes a process for the coagulation of water-containing polymeric latexes, by adding water-soluble and non-oxidizable salts, such as, for example, calcium hyphosphite or zinc methane sulfonate. When adding precipitating agents, there is usually a problem that this can lead to deterioration of the qualities of the plastic, since the agents can cause undesired side reactions, such as, for example, a yellowish coloration. European patent EP-A 0 683 028 describes a process for the dehydration of a two-phase liquid mixture of a thermoplastic liquefied synthetic material and an aqueous phase in a twin-screw extruder of opposite course. Here, the coagulation of the synthetic latex can be carried out directly in the extruder under the shear in the coagulation zone at a temperature within the thermoplastic range of the synthetic material. Alternatively, a pre-coagulate can also be used. The melt is conducted in partly filled helical threads and compressed in at least one of said helical thread passages under the formation of a locally very limited and steep pressure gradient to give a compact melt cake. In this way, the water, under the influence of the force of gravity, spills before reaching the melt cake downwards in such a way that the melt cake does not have contact with a continuous aqueous phase. Using this procedure, the water content of a polymer emulsion can be reduced, for example, from an initial water content of 55% by weight to 8% by weight of water. Subsequently, remaining quantities of volatile substances can be largely eliminated by means of a degassing extruder in a forward and reverse degassing zone. Finally, the granulate extracted from the granulator tube has a residual moisture of only 0.06% by weight. European patent EP-A 0 423 759 describes a process for the preparation of a particular polymer, wherein a polymer latex is mixed with a coagulation additive. The mixture is then reacted with an organic solvent in which the polymer is not soluble, for example, n-heptane. In this way granulated polymer particles are formed in the gravity separation of the polymer. This process is characterized in that at least one of the mixing steps must be carried out in a two-screw extruder.

Objective and solution European patent EP-A 0 683 028 provides a very good dehydration of liquid mixtures of two phases of synthetic melt material and an aqueous phase. Before going to the degassing pass in the extruder, water contents of, for example, only about 8% by weight can be obtained. Nevertheless, it has been found that temperatures of up to 350 ° C can be reached on the outer surface of the extruder barrel, due to the temperatures required for coagulation in the coagulation zone of the extruder, which can lead to an extremely high stress on the extruder. material. The energy must be supplied by conduction of heat via the surface of the cylinder. However, the density of the heat current is limited by the limited material strength, which must not be exceeded by the thermal stresses. The thermal tensions are created due to the temperature gradients of the surface external to the internal surface of the cylinder, necessary for the passage of heat. The above is even more problematic if the cylinder, to avoid corrosion, should be made of corrosion-resistant steel, since corrosion-resistant and highly alloyed materials are generally bad conductors for heat. Due to the poor ability to conduct temperatures, the limited strength of the materials and the limited surface for heat exchange, also the coagulation capacity and, therefore, also the total yield of the product are within a limited range. More than anything there is the problem that the wear of the equipment in the extruder of two spindles of opposite gear is very high. Therefore, the useful life of the cylinder, as well as the spindle mounted on the extruder is very short. Therefore, it has been proposed to develop an improved process for the dehydration of liquid mixtures of two phases of a synthetic thermoplastic phase and an aqueous phase, where the highest extreme stress of the material of the possible coagulation extruder is avoided. On the other hand, the total yield of liquefied synthetic material to be dehydrated should be as high as possible. The dehydration capacity must at least correspond to or even improve the performance achieved according to the procedure described in the European patent EP-A 683 028. In addition, the process must lead to the smallest possible remaining polymer parts in the waste water, since These are unwanted. At the same time, procedural steps should be avoided, such as the addition of precipitation additives to the coagulation of the latex. The objective was solved by means of a process for the dehydration of a liquid mixture of two phases of a synthetic thermoplastic phase and an aqueous phase, by means of a) the coagulation of the liquid mixture of two phases in a first extruder b) the dehydration of the coagulate in a two-screw extruder with spindles of opposite direction and with a dehydration zone c) the separation of volatile parts by means of degassing, characterized in that in step a) a single-screw extruder or extruder is used as an extruder of two spindles, where the extruder of two spindles is equipped with spindles of a single gear. Surprisingly, it has been found that the separation of steps a) and b) of the process which, in accordance with European patent EP-A 0683 028 are carried out in a single step, leads to a particularly effective effect if in step a) a coagulate is prepared in a single-screw extruder or in a single-run twin-screw extruder before dehydration and degassing itself in steps b) and e). Thus, the preparation of the coagulate is carried out in a very effective manner, since the energy supply necessary for the coagulation can be realized essentially by means of dissipation (shearing). In step b), the regulation of the temperature can be lower and, therefore, more easily and more easily handled, because the coagulation is already effective in step a). Preference is given to coagulation in an extruder in step a) of the process at a melting temperature of at least 30 ° C more, particularly preferably at a high number of revolutions of the spindles, that the subsequent dehydration in step b) of the procedure. This leads to a lower effort of the material, in particular in the cylinder of the dehydration extruder and at the same time allows a better control, ie a stable operation of the dehydration step b). Therefore, in steps a), b) and c) a more effective total dehydration of the liquid mixture of two phases of liquefied synthetic material and an aqueous phase is achieved than in the processes, according to the state of the art . In this way, at the end of step b) of the process, at least 94% of the aqueous phase can be extracted in liquid form.

FIELD OF APPLICATION OF THE INVENTION The process, according to the present invention, is generally suitable for the dehydration of liquid mixtures of two phases of a synthetic thermoplastic phase and an aqueous phase. It can be, for example, emulsion polymers for molding compounds of polymethylmethacrylate (see, for example, European patent EP-A 245 647) or latexes, such as, for example, high impact resistance modifiers. A corresponding ratio of two phase mixtures suitable for dehydration can be deduced from, for example, European patents EP-A 0 534 235 or EP-A 0 006 503. Latexes generally contain from 30 to 50% by weight. Weight of dispersed synthetic particles, whose average particle size can be, for example, 100 to 500 nm. Accordingly, the aqueous phase represents from 70 to 50% by weight; it generally contains dissolved emulsifiers or other additives and foreign substances. The latex particles consist of thermoplastic plastics that can be processed in their state of melt in an extruder. These include thermoplastic plastics with glass transition temperatures of 20 to 150 ° C, that is, a temperature range in which they are in a state of molten mass within which they are sufficiently resistant to decomposition. The melting temperature in the extruder for processing is generally between 50 and 250 ° C. The important classifications of thermoplastic plastics are copolymers based on styrene, butadiene and, in some cases, acrylic nitrile, as well as polyvinyl chloride, polyacrylate or polymethacrylate. An additional important classification is the latexes of thermoplastic multistage plastics containing latex particles with a thermoplastic solid phase and a viscous reticulated phase. These can be mixed, if necessary, during the process with a thermoplastic plastic which is supplied in solid or fused form for example in step c) of the process and which matches the plastic of the solid phase of the latex or which is compatible with the same Preferably, the plastic of the solid phase consists essentially of polymethylmethacrylate and the plastic of the viscous phase consists essentially of crosslinked polybutylacrylate which, for the purpose of adjusting the optical refractive index to the optical refractive index of the polymethylmethacrylate, copolymerized with styrene or acrylate of benzyl. Typical mixtures of this type contain, for example, from 4 to 50% by weight of the multistage latex plastic, wherein the part of the polybutylacrylate can represent from 2 to 80% by weight and the polymethylmethacrylate part from 20 to 98% by weight, as well as 2 to 60% by weight of the thermoplastic polymethylmethacrylate plastic. In the case that this is supplied in an unfused form, it is also possible that the latex of the multi-phase plastic is mixed with a polymethylmethacrylate latex and that the latex mixture is processed according to the process according to the present invention. .

Embodiment of the invention The extruder, used in step a) of the process, according to the present invention, contains either a single spindle (single-screw extruder) or two parallel mounted spindles, where both spindles are operated the same clockwise (extruder with two spindles of one gear). A typical single screw extruder for step a) can be characterized, for example, by the following data: spindle diameter D = 34 mm, length = 30 D. For production purposes, the extruder can, for example, present a Spindle diameter D from 50 to 250 mm and a spindle length from 20 to 40 times the spindle diameter (20 - 40 D). Preferably, the spindles of the extruder used for the dissipation of energy from the rotation of the spindle to the aqueous system of multiple phases, cause high shear precipitation. These can be achieved through the transportation or circulation of the product through narrow slots. Furthermore, it is advantageous if the spindles of the extruder are equipped with more than half the total length of the process with kneading or shearing devices. The kneading devices can be kneading blocks or kneading discs arranged in series. Also dispositions to the left and right of the disks can increase the effectiveness. The extruder can be operated, for example, within a temperature range of 150 to 300 ° C, preferably at a temperature above 200 ° C, in particular at temperatures of 240 to 260 ° C. Here, the revolutions of the spindles can increase up to 800 revolutions per minute (r / min), without significant wear and tear. Preference is given to 200 to 300 r / min. A preferred combination is a temperature of at least 240 ° C and at least 200 r / min. Next, the coagulate is introduced in a dehydration extruder, where step b) of the process is carried out. Extruders with corresponding dehydration zones can be deduced from, for example, US Pat. Nos. 4,110,843, US-PS 4,148,991 or US Pat. No. 5,232,649. However, preference is given to an extruder with spindles of opposite gear, in accordance with European patent EP-A 0 683 028, in which the melt is transported in helical, partially filled screw passages and compressed at minus one of the helical thread passes to give a compact cake of melt under formation a locally very limited and steep pressure gradient. In this way, a separation of the liquid phase and the continuous molten phase (melt cake) is achieved. Under the influence of the force of gravity, the water spills down and can be removed by means of suitable openings in the extruder cylinder. For dehydration in step b) of the process, the extruder can be operated, for example, within a temperature range of about 230 ° C, preferably not higher than 210 ° C, particularly preferably not higher than 200 ° C , at a spindle revolution of, for example, 80 r / min and under a pressure of approximately 40 bar. After extracting the aqueous phase, the liquefied synthetic material contains, in most cases, still 5 to 20% by weight of water in dissolved or liquid form enclosed. The separation of water-soluble secondary parts, such as emulsifiers or electrolytes from the melt, can be completed by adding in an additional mixing zone pure water or another volatile solvent, which dissolves the impurities but not the plastic, and if it is extracted from it as described above in an additional dehydration zone. In accordance with the present invention, at the end of step b) of the process at least 94% of the aqueous phase may have been separated in liquid form.

The remaining water and other volatile substances are largely separated in step c) of the process by means of degassing. The foregoing can then be carried out in a degassing zone of the dehydration extruder or also in a separate degassing extruder. The degassing is usually carried out under normal pressure and / or under a pressure of 0.01 to 0.99 bar or also, if necessary, in several steps with an increasing vacuum. The goal is a water content of less than 0.1% by weight, preferably from 0.03 to 0.06% by weight. After degassing, the melt can be brought to a suitable melt pressure for extrusion in a final pumping zone. During the separation of the phases, the necessary height and uniformity of the pressure can be safely maintained by physically separating the dewatering zone from the degassing zone. In this case, the functions of phase separation and degassing are distributed to two extruders, where only for the first function is required an extruder of two geared spindles. The pressure at the end of the dehydration zone, where a pumping zone is inserted backwards, can be precisely controlled for the melt extracted by means of a throttle valve. By a pipe, the melt can be introduced into a conventional degassing extruder. Here, if desired, after the degassing zone and in one or more mixing zones, another melt can be added as well as, if necessary, other additives, such as lubricants, stabilizers, antistatic agents, colorants, absorbents. of ultraviolet rays, among others. Then, in a further degassing zone, the last volatile parts of the melt can be removed under vacuum. At the end of the degassing extruder, the melt is extracted by means of a pumping zone in the form of a melt. This can be carried out by means of a granulator tube, from which several thin granules are extruded, cooled to a temperature below the softening temperature and cut into conventional moldable pellets. However, a plastic profile, for example a foil, can also be extruded directly by means of a suitable extrusion tube in a known manner.

Advantageous effects of the invention The method, according to the present invention, provides a series of advantages. Thus it provides a way to effect effective coagulation in step a) without adding any coagulation additive, which is usually always required. The physical separation of the coagulation step and the dehydration pass leads to less wear of the devices themselves. In this way, maintenance and resting times are reduced. Obtaining the polymer with the separated aqueous phase (remaining polymer content in waste water) is, surprisingly, considerably lower, also with high yield rates. Due to the overall improved dehydration, by means of the combined process, a considerably lower content of remaining emulsifiers in the polymer product is obtained at the same time. The above has an advantage, since traces of emulsifiers lead to a moisture absorption that represents the cause for an unwanted turbidity of the milk-white color ("hitenin"). Due to the lower content of remaining emulsifiers, the final product obtains a qualitative improvement.

EXAMPLES Example 1 (comparative example): A three-degree emulsion polymer with process with the following composition: Grade I: Methylmethacrylate / ethylacrylate / allylmethacrylate (parts: 95.7 / 4 / 0.3) Grade II: Butylacrylate / styrene / allylmethacrylate (82/17/1) Grade III: Methylmethacrylate / ethylacrylate (96/4) Relationship Grade I: II : III = 20/45/35 Polymer phase ratio / acousa phase = 45/55 The latex is pumped by means of a membrane dosing pump with a mass flow of 10 kg / h in an extruder cylinder of a two-extruder spindles geared of opposite gear. The diameters of the screws are 34mm. The spindles are three steps with an elevation of 30mm. The coagulation zone where the latex particles are compressed to give a melt cake has a length of 6 D and is maintained at a temperature of 230 ° C. The dehydration zone and the extraction zone have a total length of 20 D and are operated at a temperature of 210 ° C. The revolutions of the spindles are adjusted to 80 r / min.

In the area of the dewatering zone, the cylinder is opened by grooves 2 mm wide and 60 mm long, under which a collecting vessel for receiving the aqueous phase is mounted under pressure-proof pressure. Due to the overlap of the collection container with nitrogen, a pressure of 40 bar is set inside the container. By means of a valve, an amount of water of 5.27 kg / h is extracted from the collection container. The water extracted under pressure contains 0.5% by weight of organic substance (remaining polymer). The melt, largely released from the aqueous phase, still contains 8% by weight of water, which is removed to give 0.06% by weight of remaining moisture by means of a degassing extruder intercalated back and vacuum degassing . The obtained product is extracted in granulates and cut in a granulator device to give equigranulated granulates.

Example 2 (in accordance with the present invention): The equipment for latex dehydration consists, in accordance with the present invention, in three parts: an extruder with two geared spindles of one gear for the coagulation of the polymer latex, an extruder of dehydration for the separation of the aqueous phase from the melt phase, as well as a degassing extruder interspersed behind. The coagulation extruder has a spindle diameter of 34 mm and a process length of 30 D. The two-step extruder spindles have a length of 14 D per kneading blocks, such as for example 16 pieces of kneading blocks KB 5 -2-30R4 for energy dissipation. As a dehydration extruder, a twin-screw extruder of opposite operation is used, as described in example 1. The degassing extruder is identical to the device used in example 1. By means of a dosing pump 10 kg / h is pumped of the latex described in example 1 in the coagulation extruder. After the coagulation of the latex at 250 r / min of the spindles and a cylinder temperature of 250 ° C, a remaining polymer content of 0.35% by weight can be determined in the water extracted under pressure, after the separation of the phase watery The dehydration extruder was operated at a cylinder temperature of 190 ° C. After the dehydration step, the remaining water content was 6% by weight. The granulate cut after the extraction and cooling of the bars of molten mass appeared bright and without any color.

Example 3 (comparative example) According to the procedure described in example 1 an increased amount of latex of 15 kg / h is introduced. The adjusted spindle speed was increased to 100 r / min. The temperatures of the extruder of extraction by pressure was increased to 250 ° C to achieve a better coagulation. A temperature of 200 ° C was set in the area of the dehydration zone. After a 12 minute operation, it was found that the outlet duct of the water collection container was clogged. The test had to be canceled, since the low viscosity melt had spilled into the collection container.

Example 4 (comparative example): According to the procedure, described in example 1, an increased amount of latex of 20 kg / h was introduced. The number of revolutions of the spindles was increased to 120 r / min. The remaining polymer content in the water extracted under pressure was determined to give 2% by weight. The amount of water remaining to be degassed after dehydration was 11% by weight. The product extracted in the form of bars, then cooled and cut in a granulation device to give equigranulated granules was shown with a gray color, due to the abrasion of the metal in the extruder of two interlocking spindles of opposite course.

Example 5 (in accordance with the present invention) In the pressure extraction device, operated according to the procedure, in accordance with the present invention, the amount introduced was increased from 10 kg / h to 15 kg / h. The number of revolutions of the coagulation extruder was adjusted to 250 r / min, the number of revolutions of the dehydration extruder was adjusted to 80 r / min. The remaining polymer content in the water extracted by pressure was determined with 0.35% by weight. After dehydration, the remaining amount of water to be degassed was 6% by weight. The product extracted in the form of bars, then cooled and cut in a granulation device to give equigranulated granules was shown bright and without any color, nor did it show a gray tone, as in example 4.

Example 6 (in accordance with the present invention) In the pressure extraction device, operated according to the method, according to the present invention, the quantity introduced was increased from 10 kg / h to 20 kg / h. The number of revolutions of the coagulation extruder was adjusted to 250 r / min, the number of revolutions of the dehydration extruder was adjusted to 60 r / min. The cylinder temperature was adjusted to 250 ° C. After the

Claims (3)

The remaining water content could be a residual polymer content of 0.35% by weight in the water extracted by pressure. The dehydration extruder was operated at a cylinder temperature of 190 ° C. After dehydration, the amount of water remaining and to be degassed was 6% by weight. The product extracted in the form of bars, then cooled and cut in a granulation device to give equigranulated granules was bright and without any color, nor did it show a gray tone, as in example 4. Having described the invention as above, this is considered as a novelty, so it is claimed as property contained in the following: R E I V I N D I C A C I O N S

1. A process for the dehydration of a two-phase aqueous mixture consisting of a thermoplastic synthetic phase and an aqueous phase, by means of a) the coagulation of the two-phase liquid mixture in a first extruder b) the dehydration of the coagulate in a two-screw extruder with spindles of opposite direction and with a dehydration zone c) the separation of volatile parts by means of degassing, characterized in that in step a) a single-screw extruder or two-screw extruder is used as an extruder, where the two-screw extruder is equipped with one-way spindles.

2. A method according to claim 1, characterized in that step a) of the process is carried out at a melting temperature of at least 30 ° C more than step b) of the process.

3. A process according to claim 1 or 2, characterized in that at the end of step b) of the process at least 94% by weight of the water in liquid form has been separated.