WO2001090240A1

WO2001090240A1 - Process for the production of finely divided polymers

Info

Publication number: WO2001090240A1
Application number: PCT/US2001/014732
Authority: WO
Inventors: Evan Koslow
Original assignee: Koslow Technologies Corp.
Priority date: 2000-05-23
Filing date: 2001-05-08
Publication date: 2001-11-29

Abstract

A multi-step process is described for the production of finely divided polymers which comprises: mechanically grinding the polymer into a powder, followed by dispersion of this ground powder into a mixture of water and surfactant at elevated temperature, pressuer, and shear. the powdered polymer placed into such an environment, rapidly equilibrates to the elevated temperature, melts, and disperses into an even finer series of droplets that can be chilled to form very fine particles. The resulting dispersion can be used as a latex, or the solid polymer particles can be separated from the fluid by conventional washing and drying steps.

Description

PROCESS FOR THE PRODUCTION OF FINELY DIVIDED POLYMERS

Field of the Invention

This invention relates to polymer dispersions useful for binders and thermoplastic applications. More particularly, it relates to the process of producing polymer dispersions by mixing polymer powder with water and surfactant to form a slurry and pumping the slurry into a dispersion reactor to form finely divided polymers.

Background of the Invention

Finely divided polymeric particles are useful for a wide range of applications, including as binders in composite carbon block filter elements, and in other thermoplastic applications. Moreover, thermoplastic polymers in finely divided form have found use in a number of other commercial applications where it is either impossible or inconvenient to utilize the conventional cube or pellet forms. For example, powdered themoplastic polymers in dry form have been used to coat articles by dip coating in either a static or fluidized bed, by powder coating wherein the powder is applied by spraying or dusting, and by flame spraying. In dispersed form, thermoplastic powders have been applied as coatings by roller coating, spray coating, slush coating, and dip coating to substrates such as metal, paper, paper board, and the like. Finely divided polymers have also been widely employed in conventional powder molding techniques. Other important applications of these polymer powders include paper pulp additives, mold release agents for rubber, additives to waxes, paints, polishes, binder for woven fabrics, and the like.

Conventional processes for making finely divided thermoplastic Polymers generally employ the cubes or pellets, which are obtained directly from the synthesis process. These processes are of three main types, i.e., mechanical grinding, solution and dispersion.

In the past, extremely fine dispersions of thermoplastic polymers have been manufactured by extruding polymers into a high-shear environment containing a mixture of water and surfactant held at elevated temperature and pressure. Under these process conditions, the polymer forms a dispersion of fine droplets. This dispersion can be cooled to convert these polymeric droplets into solid particles, which are separated from the water- surfactant mixture through conventional washing and drying steps.

For example, Peoples et al. disclose in U.S. Patent No. 3,432,483 a method of continuously producing finely divided polymers. The process consists of introducing the polymer and water-surfactant mixture into a dispersion zone by means of a conventional extruder and dispersing the polymer into a water-surfactant mixture at elevated temperature and pressure, (i.e., at a temperature above the melting point of the polymer and at a pressure sufficient to maintain the water in an aqueous state), agitating the dispersion to achieve a reduction in the size of polymer droplets within the mixing zone, and subsequent cooling of the dispersion to form solid particles of finely divided polymer. These particles are separated from the water-surfactant solution by filtration washing, and drying. The typical method for the introduction of the polymer into the process consists of direct extrusion of liquid polymer.

Warner et al. in U.S. Patent No. 4,123,403 describe a similar process consisting of producing a mixture of water and surfactant and dispersing a continuous heat-plasticated polymer phase into this solution. Warner et al. apply shear to the mixture while sustaining laminar flow through the use of a special reactor and rotor. In this system, the applied shear and the addition of water produces a discontinuous polymer phase, which can be cooled to produce solid polymer particles that are highly uniform and of small size. Broehng et al. in U.S. Patent No.4,252,969 describe a refinement of U.S. Patent No.3, 432,483 (Peoples), wherein a particle-size controlling amount of alkanolamide surface active agent is used to provide improved control and range of particle size.

Heimberg et al. in U.S. Patent No.5, 246, 779 describe the production of microfine polypropylene particles. In this case, the polypropylene is a vis broken polypropylene polymer with a melt flow greater than 1. In most cases, the polymer melt flow is increased through prior reaction with organic peroxides in a conventional plastics extruder.

Ondrus et al. in U.S. Patent Nos. 5,338,609 and 5,336,731, and Heimberg et al. in U.S. Patent No.5,209,977 describe partially crosslinked olefin copolymer microfine powders with low (less than 1) melt flow rates. The dispersion is produced by the process described in U.S. Patent No. 3,432,483 (Peoples), but with olefins functionalized to contain unsaturated alkoxysilanes and through the addition of a catalyst that promotes crosslinking of the polymer during the dispersion process. Although the Peoples process operates best with high melt flow materials, the in-situ crosslinking of the polymer allows the final product to have fractional melt flow.

All of the aforementioned involve extruding polymers into a high shear environment at elevated temperatures and pressures. The problems associated with these conventional extruding processes are high capital costs and low productivity. The extrusion of the polymer requires large and expensive extruders that often limit the production capacity of the process. For example, to manufacture 20,000,000 pounds/year of material requires an extruder with nearly 3,000 pounds/hour production capacity. Such a machine requires enormous capital and specialized expertise to operate. In addition, the extruder injects the molten polymer into a bath of water and surfactant at elevated pressure, temperature, and shear in order to get the polymer to disperse. However, even with high temperatures and shear, the molten polymer is not readily dispersed and the reactor volume must be large in order for the residence time to be long enough to allow the dispersion of the bulk material.

Therefore, it is the purpose of the current invention to provide a means for significantly reducing the cost of the equipment used to produce finely-divided polymers, provide a simplified process, and enhance yields and product quality.

That is, the unique process according to the present invention reduces cost over conventional processes by eliminating the capital- intensive extruder equipment. In addition, the present invention readily disperses the polymer in the dispersion reactor resulting in the reduced reactor times, and higher yields and product quality of the finely divided polymers. Finally, the present invention greatly simplifies the polymer dispersion process because unlike the extruder process, the equipment of the present invention requires no special expertise to operate.

The present invention also provides many additional advantages which shall become apparent as described below.

Summary of the Invention

The present invention relates to the production of finely divided polymers. Raw polymer is either purchased as reactor-grade powder or ground into a coarse powder. This coarse powder is mixed with water and a surfactant to form a slurry mixture. The slurry mixture may be pre-heated in a heat exchanger and introduced into the reactor using a high-pressure pump. The polymer powder and water-surfactant slurry mixture is rapidly heated in the reactor while undergoing shear. The polymer powder rapidly disperses into fine droplets under these process conditions. Finally, the emulsion is cooled to produce fine particles in the water-surfactant mixture. The finely divided polymer can thereafter be substantially separated from the water-surfactant mixture and dried.

Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawings, wherein like parts have been given like numbers.

Brief Description of the Drawings

Figure 1 is a schematic diagram of the polymer dispersion process according to the present invention.

Detailed Description of the Invention

The polymer dispersion process of the present invention is illustrated in Fig. 1. Bulk polymer is obtained as either pellets, reactor-grade powder, or in other conventional physical forms and placed in raw polymer tank 4. If required, the polymer is subjected to grinding such as in a conventional polymer disk grinding mill 6 to reduce the size of the polymer particles to less than 20 mesh, preferably to less than 50 mesh. The polymer is then stored in coarse polymer powder holding tank 8. This coarse polymer powder is then fed through feed control valve 10 to eductor 12 where it is mixed with a water-surfactant mixture supplied from water-surfactant holding tank 14. The resulting polymer slurry from eductor 12 is then pumped through a high-pressure slurry pump 16 to heat exchanger 18. The slurry, which may be preheated by heat exchanger 18, is then injected into dispersion reactor 20. Dispersion reactor 20 is held at elevated temperatures above the melting point of the polymer, and at elevated pressures so that the water in the reactor is in the aqueous state.

Once within dispersion reactor 20, the polymer is rapidly heated to a temperature above the melting point of the polymer or maintained at the elevated temperature provided by heat exchanger 18. This temperature is usually 20-75°C above the melting point of the polymer. Under suitable conditions, the powder is liquefied, and under the influence of shear, droplets of polymer are reduced in size. The final size of the polymer droplets is related to the temperature within the reactor, the melting point of the polymer, the melt index of the polymer, the selecting of surfactant and surfactant concentration, and applied rate of shear.

Following the conversion of the polymer to fine droplets, those droplets are solidified into solid particles by cooling the slurry or discharging the contents of dispersion reactor 20 into a cooled heat exchanger 18. Once the slurry has cooled significantly below the melting point of the polymer, the polymer powder becomes stable and will no longer coalesce or agglomerate into larger particles.

This cooled dispersion is vented to the atmosphere through let down valve 22 and may, optional, be pumped into an elutriator (not shown) to separate any remaining large particles from the desired fine particles. Large particles can be recycled to the polymer injection section of the process. The fine particles are then subjected to conventional solids-liquid separation on vacuum belt filter unit 24. Solids-liquid separation belt filter unit 24 has a drain subunit 24a which drains excess water and surfactant, followed by wash subunit 24b executing one or more wash cycles, and thereafter followed by vacuum dry subunit 24c which removes the bulk of the residual wash fluid.

The water-surfactant mixture collected from the drain 24a of the solids-liquid separation belt filter unit 24 is directed back to the polymer slurry process through recycle pump 26 to surfactant-water holding tank 14. The wash fluid from wash subunit 24b is accumulated in wash water holding tank 28 and reprocessed using processes such as ultrafiltration or reverse osmosis unit 32. Once the wash fluid surfactant has been concentrated back to the original target concentration specified for the slurry process, the mixture is returned through recycle pump 26 to surfactant-water holding tank 14 for use in the slurry process. The water separated by these membrane processes may be recycled for future wash applications or vented to a drain. When two wash cycles are applied with separate membrane separations and recovery of surfactant, essentially no surfactant discharge is experienced and losses of both water and surfactant from the process are minimized.

It should be noted there are alternative solid-liquid separation technologies that may be used to accomplish the separation of the water- surfactant mixture from the ultrafine particles such as centrifuges or other vacuum or pressure filters.

The moist polymer powder cake is ejected from the belt filter, sometimes with the assistance of air jets projecting from compressor 54 disposed below the belt. Fused silica from fused-silica tank 50 and fused silica feeder 52 may be mixed with the damp polymer powder to improve the free flow properties of the dry powder. Typically, the amount of fused silica is about 0.1 to 2.0 wt. % of the dried polymer powder. The airborne damp polymer powder is then subjected to drying using a high-velocity air drier cyclone 34 or other suitable drying equipment (flash drier). The powder is then transferred to bag house 36 and product package unit 38 for packaging.

Because coarse powder requires only a small series of steps to disperse into finer droplets, the yield of finely dispersed particles is exceptionally high. In some cases, the small number of large particles, whose size is limited by the size of the original coarse powder injected into the process, is acceptable in the final product and elutriation is not required.

Grinding of the polymer is also not always required if the polymer can be obtained directly as reactor-grade powder. Such reactor-grade powders are produced by certain slurry-reactor processes used for the production of polyethylene and are sold under, for example, the trade name of Escorene™ by EXXON Chemical Company. Such materials are usually linear low-density polyethylene (LLDPE) or similar polymers. These reactor-grade powders can be further enhanced by grinding. For example, reactor-grade powders are often sold as powders averaging 300 micrometers in diameter. Passing these powders through disc grinder 6 can reduce the average particle size to 140 micrometers without significant cost and at a high throughput. These 140 micrometer particles can be easily reduced to an average particle size of 10 to 20 micrometers in a subsequent slurry dispersion process outlined above. The presence of up to 5% by weight of particles greater than 140 micrometer in diameter in the final product is generally not considered unacceptable for most applications.

In general, the polymers suitable for this invention include any polymer whose decomposition temperature is somewhat higher then its melting point temperature and less than the critical temperature of water. Preferred polymers which are suitable for this invention are polyolefins, vinyls, olefin-vinyl copolymers, olefin-allyl copolymers, polyamides, acrylics, polystyrene, polyesters, flurocarbons, and the like.

The surfactants used in this invention are typically water soluble, non-ionic surfactants such as block polymers of ethylene oxide and propylene oxide. Other suitable surfactants are disclosed in U.S. Patent 3,432,483 which is fully incorporated herein by reference.

Dispersion reactor 20 is equipped with means to impart shear along the length of dispersion reactor 20. Shear is applied along the length of dispersion reactor 20 by use of an elongated shaft having a small clearance with the reactor wall or using a series of individual high-shear elements in dispersion reactor 20 which is driven by motor 48. The design of dispersion reactor 20 limits axial dispersion. This is accomplished either through the use of a narrow gap between a central rotor and surrounding reactor wall or through the use of devices along the length of dispersion reactor 20 such as perforated plates and baffles to prevent rapid bulk mixing in the axial direction. Dispersion reactor 20 can also be broken into one or more sections comprising heating and dispersion sections and cooling sections, each operating under different shear conditions. That portion of the rotor operating within the heated portion of dispersion reactor 20 can be supplied with heated fluid, while that portion of the rotor operating within the cooled section of the reactor can be supplied with a cooling fluid.

Heat may be applied to dispersion reactor 20 by oil from hot oil heating unit 42 through oil pump 44. It should be noted that heat may be added to dispersion reactor 20 using heating bands or other suitable heat transfer fluids. Cooling can be accomplished using any conventional heat exchanger method, including the use of a cooling coil located on the surface or within the wall of dispersion reactor 20.

Dispersion reactor 20 may be fitted with an upright dead-ended section or stubber, (not shown), that is filled with a gas bubble. This stubber serves to prevent pressure pulses and allows accurate control of reactor pressure by providing a volume of compressible fluid within the system. In some cases, the stubber includes a pressurization tank having one portion filled with reactor fluid and another section filled with gas. A thick rubber septum can be used to separate these two materials to provide a transmission of pressure forces. The reactor is equipped with instrumentation suitable to measure temperature, pressure, and other critical parameters.

While I have shown and described several embodiments in accordance with my invention, it is to be clearly understood that the same are susceptible to numerous changes apparent to one skilled in the art.

Therefore, I do not wish to be limited to the details shown and described but intend to show all changes and modifications which come within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process for the production of a finely divided polymer dispersion comprising the steps of:

mixing a powdered polymer with water and a surfactant to form a polymer slurry; and

introducing said polymer slurry into a dispersion reactor at a temperature above the melting point of said powdered polymer, and at a pressure sufficient to maintain said water in an aqueous state, thereby forming said finely divided polymer dispersion.

2. The process of claim 1 wherein said polymer is a reactor-grade polymer.

3. The process of claim 1 wherein said polymer is a coarse polymer powder having an average particle size less than 20 mesh.

4. The process of claim 3 wherein said coarse polymer powder has an average particle sizes less than 50 mesh.

5. The process of claim 1 wherein said slurry is introduced into said dispersion reactor by means of a high-pressure pump.

6. The process of claim 1 further comprising the step of heating said polymer slurry via a heat exchanger prior to introducing said polymer slurry into said dispersion reactor.

7. The process of claim 1 wherein said polymer is selected from the group consisting of: polyolefins, vinyls, olefin-vinyl copolymers, olefin allyl copolymers, polyamides, acrylics, polystyrene, polyesters, and flurocarbons.

8. The process of claim 1 further comprising the steps of:

mixing said polymer slurry in said dispersion reactor; separating a portion of said finely divided polymer dispersion from said dispersion reactor;

cooling the separated finely divided polymer dispersion to a temperature below the melting point of the separated finely divided polymer dispersion;

reducing the pressure of the cooled finely divided polymer dispersion;

separating the cooled finely divided polymer dispersion from said water and surfactant;

washing the water and surfactant depleted finely divided polymer dispersion; and

drying the washed finely divided polymer dispersion.