Abstract:
A method for devolatilization of a thermoplastic polymer containing at least one volatile component which includes the following three steps: heating the thermoplastic polymer so that the thermoplastic polymer is a heated liquid or molten thermoplastic polymer, flowing the heated liquid thermoplastic polymer through a packed bed liquid-gas contactor by centrifugal force, and flowing a stripping gas through the packed bed countercurrent to the flow of the heated liquid thermoplastic polymer so that the volatile component volatilizes into the stripping gas from the heated liquid thermoplastic polymer by gas-liquid contacting in the packed bed; and an apparatus therefor.

Description:
This application claims benefit of 60/089,059 filed Jun. 12, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The instant invention relates to the removal of volatile components from thermoplastic polymers and more specifically to methods and apparatus therefore that use centrifugal force. 
     Chisholm, U.S. Pat. No. 3,409,712, herein fully incorporated by reference, disclosed a method and apparatus for devolatilization of thermoplastic polymers by melting the polymer and flowing the melted polymer on a rotating disk surface. Chisholm, U.S. Pat. No. 3,424,832, herein fully incorporated by reference, disclosed a method and apparatus for pelletizing a thermoplastic polymer using a rotating chamber or hollow rotor. Hay, II et al., U.S. Pat. No. 4,940,472, herein fully incorporated by reference, disclosed a method and apparatus for devolatilization of thermoplastic polymers by melting the polymer and flowing the melted polymer on a rotating disk surface. Moore et al., U.S. Pat. No. 4,952,672, herein fully incorporated by reference, disclosed a method and apparatus for devolatilization of thermoplastic polymers by melting the polymer and flowing the melted polymer on a rotating disk surface. Baker Perkins Incorporated of Saginaw Michigan offered a centrifugal pelletizer for sale. Modern Plastics, December 1983, page 56. Haw, Master&#39;s Thesis, Case Western Reserve University, January 1995, entitled “Mass Transfer of Centrifugally Enhanced Polymer Devolatilization by using Foam Metal Packed Bed”, herein fully incorporated by reference, disclosed a method and apparatus for devolatilization of thermoplastic polymers by melting the polymer and flowing the melted polymer through a open-cell nickel metal foam gas-liquid contactor rotated within a stationary chamber. 
     The method and apparatus of Haw suffered from several problems. For example, the nickel metal foam tended to collapse under the centrifugal forces to which it was subjected. In addition, the mechanical seal used tended to contaminate the devolatilized polymer with the seal oil. Furthermore, the method and apparatus of Haw did not produce the devolatilized polymer in the form of pellets. 
     SUMMARY OF THE INVENTION 
     The instant invention is a method for devolatilization of a thermoplastic polymer containing at least one volatile component which comprises three steps. The first step is to heat the thermoplastic polymer so that the thermoplastic polymer is a heated liquid or molten thermoplastic polymer. The second step is to flow the heated liquid thermoplastic polymer through a packed bed liquid-gas contactor by centrifugal force. The third step is to flow a stripping gas through the packed bed countercurrent to the flow of the heated liquid thermoplastic polymer so that the volatile component volatilizes into the stripping gas from the heated liquid thermoplastic polymer by gas-liquid contacting in the packed bed. 
     The instant invention is also an apparatus for devolatilization of a thermoplastic polymer containing at least one volatile component, the apparatus comprising three elements. The first element is a rotatable chamber, the rotatable chamber containing a packing for gas-liquid contact processing. The second element is a polymer conduit, the polymer conduit extending into and terminating within the rotatable chamber so that molten thermoplastic polymer can be flowed into the chamber by way of the conduit while the chamber is being rotated so that the molten thermoplastic polymer then flows through the packing by centrifugal force so that the volatile component of the molten thermoplastic polymer volatilizes into the gas phase in the packed bed. The third element is a gas seal between the polymer conduit and the rotatable chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side cross sectional view of an apparatus embodiment of the instant invention; and 
     FIG. 2 shows an end view of the apparatus of FIG. 1 further including a pair of electromagnets. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to FIG. 1, therein is shown a side cross sectional view of an apparatus embodiment  10  of the instant invention. The apparatus  10 , includes a disk shaped chamber  12  which is rotated by a shaft  13  about the longitudinal axis of the shaft  13 . The shaft  13  is rotated by an electric motor (not shown). The chamber  12  contains an annular ring of nickel-chromium metal open cell foam packing  14  preferably cut from a larger piece of material by the electrode discharge machining process. The packing  14  is Celmet Brand #1 material available from Sumitomo Electric USA, NY, N.Y. A ring shaped support band  15  surrounds the packing  14 . The support band  15  is perforated by holes  16  therethrough. A ring shaped distributor band  17  is positioned on the inside of the packing  14 . The distributor band  17  is perforated by holes  18  therethrough. 
     A polymer conduit  19  extends into and terminates within the chamber  12 . The polymer conduit  19  includes flange  20  and seal mounting body  21 . A tubular chamber extension  22  terminates near the flange  20 . A gas seal  23  is mounted in the seal mounting body  21 . The details of the gas seal  23  are not shown in FIG. 1. A preferred gas seal  23  is available from the Durametallic Corporation of Kalamazoo Michigan as the GF 200 Dura Seal Brand gas barrier seal. Nitrogen at 0.2 MPa pressure is fed to the seal  23  by way of nitrogen port  24 . Gas barrier seals are known to chemical engineers. See, for example, Chemical Engineering Progress (1996), 92 (10), pages 58-63, herein fully incorporated by reference. 
     The term thermoplastic polymer is well understood in the art and includes, for example and without limitation, nylons, fluorocarbons, cellulose derivatives, acrylic resins, polystyrene, copolymers of styrene such as acrylonitrile/butadiene/styrene copolymers, ethylene/styrene interpolymers, polylactic acid, polyethylene and polyproyplene. Thermoplastic polymers soften and become molten when sufficiently heated. It is often desirable to reduce the concentration of volatile components in a thermoplastic polymer as discussed in U.S. Pat. Nos. 4,952,672, 4,940,472 and 3,409,712. For example, it is often desirable to reduce the concentration of residual styrene monomer in polystyrene. 
     The thermoplastic polymer is heated as discussed in U.S. Pat. Nos. 4,952,672 and 4,940,472 so that it can be flowed through the polymer conduit  19  and into the chamber  12 . The chamber  12  is rotated by way of the shaft  13  at, for example, 4,000 rpm for a 46 centimeter outside diameter chamber. The liquid molten heated thermoplastic polymer then is pooled by centrifugal force against the distributor band  17  aided by lip  17   a . The polymer then flows by centrifugal force through the holes  18 , through the packing  14 , through the holes  16  to pool at the peripheral edge of the chamber  12 . A series of apertures  25  are positioned in the periphery of the chamber  12 . Polymer flows by centrifugal force through the apertures  25  to form a strand of polymer. An endless band  26  of sharpened alloy tool steel forms a knife edge which cuts the strand of polymer into pellets. The band  26  travels on rollers  27  driven by an electric motor (not shown). The band  26  is preferably water cooled. 
     The packing  14  facilitates gas-liquid contacting. Gas-liquid contacting is a technique known to chemical engineers. See, for example, Section 18-19 to 18-48 of Perry&#39;s Chemical Engineers&#39; Handbook, fifth edition. When a partial vacuum (defined herein as a pressure more than 0 MPa but less than 0.1 MPa) is applied to vacuum port  28 , then volatile components of the polymer enter into the gas phase in the packing  14 , flow around the distributor band  17  by way of vent channel  29  in the chamber  12 , into the annulus between the conduit  19  and the extension  22 , and then through the port  28 . 
     Optionally, the chamber  12  contains a stripping gas conduit  30  so that nitrogen from the port  24  can be introduced into the chamber  12  by way of nozzle  31 . The stripping gas flows through the holes  16 , through the packing  14 , through the channel  29 , into the annulus between the conduit  19  and the extension  22 , and then through the port  28 . Any conventionally applicable stripping gas can be used such as carbon dioxide, methanol vapor, ethanol vapor, butane gas or other light hydrocarbons. For example, steam can be flowed through the conduit  30  as a stripping gas. Alternatively, the stripping gas conduit can extend through the shaft  13  into the chamber  12 . When the stripping gas is steam, then the preferred material of construction of the chamber  12  is a corrosion resistant steel. The use of a stripping gas is preferred in the instant invention when it is desirable to devolatilize more completely at the expense of handling the stripping gas. 
     Preferably, the chamber  12  is heated to facilitate the start up and operation of the apparatus  10 . The preferred means to heat the chamber  12  is shown in FIG.  2 . FIG. 2 is an end view of the apparatus  10  of FIG. 1 showing the chamber  12 , the shaft  13 , the band  26  and the apertures  25 . The chamber  12  is positioned between a first electromagnet  32  and a second electromagnet  33 . When the chamber  12  is made of a magnetic material, then when the chamber  12  is rotated, eddy electrical currents are generated in the chamber  12  which heat the chamber  12 . Preferably, the chamber  12  is made of a magnetic stainless steel (especially when steam is used as a stripping gas) such as the well known 17-4 PH type of stainless steel, see Section 6-38 of Marks&#39; Standard Handbook for Mechanical Engineers, eighth edition. 
     The packing of the instant invention must not collapse in use and thereby block the flow of polymer through the packing. Haw, supra, disclosed the use of open cell nickel metal foam as a packing in a centrifugal polymer devolatilizer. However, preferably the packing of the instant invention has a compressive strength at least twenty five percent greater than an equivalent packing made essentially of nickel. When the packing is an open cell metal foam, then preferably the average number of cells per centimeter of the foam ranges from about 2 to about 7. However, an open-cell metal foam is not required in the instant invention. For example, a packing comprised of a knitted metal wire, such as knitted stainless steel wire, can be used as well as any other packing known to the liquid-gas contacting art. Other examples of packing include macroreticular metal foam, wire screen, and wound woven metallic mesh When a loose packing is used, then a perforated band like the support band  15  can be used on the inner side of the packing. However, a rigid packing, such as open cell metal foam, is believed to be easier to balance. 
     FIG. 1 shows a chamber  12  having a single row of apertures  25 . However, it is preferred that the chamber  12  be longer and have many rows of apertures  25  to increase the productivity of the apparatus  10 . When a longer chamber  12  is used, then the end of the polymer conduit  19  can be closed and the polymer conduit adjacent the packing can then be perforated with holes to distribute molten polymer onto the packing  14 . 
     The operational parameters of the apparatus of the instant invention will depend, of course, on many factors such as the specific physical properties of the thermoplastic polymer being devolatilized. However, a good starting point is to follow the teachings of U.S. Pat. No. 4,952,672. When the thermoplastic polymer is general purpose polystyrene, then it is suggested that the polymer and chamber  12  be heated to about 240 degrees centigrade so that the polymer has a viscosity of about 300,000 centipoise.