BACKGROUND OF THE INVENTION The process of the present invention relates to methods of producing syndiotactic vinyl aromatic polymers. In the production of syndiotactic vinyl aromatic polymers such as syndiotactic polystyrene (SPS), a devolatilization step is typically used to remove residual monomers, process solvents, and other volatile components from the SPS polymer. This process is complicated by the fact that residual vinyl aromatic and other monomers can autopolymerize upon heating to form atactic vinyl aromatic and other polymers, for example, atactic polystyrene, which are unwanted contaminants in SPS polymers. Atactic vinyl aromatic polymers degrade the SPS polymer properties such as heat distortion temperature and reduce the crystallization rate of SPS homopolymer and copolymer resins.

In order to prevent discoloration of the SPS polymer, the devolatilization process is typically preceded by a deashing step to extract active catalyst residues. Deashing requires treatment of the polymer with a deashing agent such as hydrochloric acid, potassium hydroxide and the like. Alternatively, at low catalyst levels, active catalyst residues can simply be deactivated prior to devolatilization, thereby remaining in the final resin. Deactivation is typically achieved by intimate mixing of the polymer with an active nucleophilic agent, preferably a protic solvent such as methanol.

Several methods of devolatilization are known in the art, including melt devolatilization wherein the polymer is first melted and then devolatilized in the fluid state; and solid state devolatilization, wherein solid polymer is heated and devolatilized at a temperature between the glass transition temperature and the melting point of the polymer.

JP 03056504 by Yamamoto discloses a melt devolatilization process, wherein SPS powder containing volatiles is fed to a twin screw extruder where it is melted and devolatilized. Although volatile residues are reduced, a catalyst deactivation or deashing step is needed to prevent discoloration due to the presence of active catalyst residues.

JP 03064303 by Yamamoto discloses a two step solid state devolatilization process, wherein SPS powder containing volatiles is first fed to a dryer where it is heated to a temperature between the glass transition temperature and the melting point of SPS and further devolatilized by melt devolatilization in a vacuum vented twin screw extruder as described in JP 03056504, above.

However, this method is very time consuming, taking 9 or 10 hours to complete, and a catalyst deactivation or deashing step is also needed.

Several methods of deashing and deactivation of active catalyst residues from SPS polymers are known. U. S. Patent No. 5,321, 122 issued to Kuramoto et al. discloses a process of purifying a styrene polymer by deashing with an alcoholic alkaline solution and washing with an alcohol. U. S. Patent No. 5,426, 176 issued to Teshima et al. discloses a process for purifying a styrene polymer by deashing with a deashing agent, for example HC1, KOH, at a temperature which is greater than or equal to the glass transition temperature of the polymer. U. S. Patent No.

5,449, 746 issued to Teshima discloses a method of purifying a styrene polymer by treating with a swelling agent, for example ethylbenzene, and a deactivating agent, for example methanol or ethanol. U. S. Patent No. 5,612, 452 issued to Teshima and Yamasaki discloses a process for simultaneously deactivating and deashing crystalline styrene polymers by treating the polymers with a poor solvent containing 15 to 10,000 ppm water. However, these methods are additional finishing steps which increase the manufacturing complexity and cost of the SPS polymer.

In U. S. Patent No. 5, 877, 271 a method of rapidly heating the syndiotactic polymer in order to devolatilize it with low color body formation is disclosed. In U. S. Patent No. 6,031, 070, steam was used in order to both deactivate the residual catalyst and reduce residual monomer in the resulting polymer. A multiple stage process involving twin dryers, the first using steam and the second using nitrogen, followed by melt extrusion of the final product was exemplified.

Despite the advance in the art occasioned by the foregoing processes, a procedure that economically and consistently produces low residual monomer content syndiotactic vinylaromatic polymers is still desired, especially for end uses involving food contact. Examples include food packaging and food handling equipment as well as direct contact applications such as baking trays and food storage and/or reheating containers. For such applications polymers having residual monomer contents of less than 500 ppm, preferably less than 100 ppm and even more preferably less than 50 ppm are highly desired. Therefore, there remains a need for a process of devolatilizing syndiotactic vinyl aromatic polymers that produces polymers having reduced volatiles, especially reduced residual monomer content.

SUMMARY OF THE INVENTION The present invention is a multi-step polymer devolatilization process comprising: a) heating a feed mixture comprising solid particulated syndiotactic vinylaromatic polymer; residual vinyl aromatic monomer (s) and, optionally, residual process solvents; and active catalyst residues in the presence of steam in a first contacting apparatus and separating the polymer product; b) contacting the polymer resulting from step a) with an inert fluid under devolatilizing conditions, and separating the fluid and polymer, and c) melting, extruding and pelletizing the resulting polymer product.

This improved devolatilization process removes volatiles, especially residual vinyl aromatic monomer (s), from syndiotactic vinyl aromatic polymer solids while simultaneously deactivating the active catalyst residues, resulting in a product suitable for food contact application.

In addition, a separate deashing and/or catalyst deactivation step is not needed. Surprisingly, polymers having low contents of residual monomers and other volatile components, low content of atactic polystyrene, reduced discoloration, and improved whiteness are obtained using the improved devolatilization process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION All reference to the Periodic Table of the Elements herein shall refer to the Periodic Table of the Elements, published and copyrighted by CRC Press, Inc., 2001. Also, any reference to a Group or Groups shall be to the Group or Groups as reflected in this Periodic Table of the Elements using the IUPAC system for numbering groups. For purposes of United States patent practice, the contents of any patent, patent application or publication referenced herein is hereby incorporated by reference in its entirety, especially with respect to the disclosure of analytical or synthetic techniques and general knowledge in the art.

The term"comprising"and derivatives thereof is not intended to exclude the presence of any additional component, step or procedure, whether or not the same is disclosed herein. In order to avoid any doubt, all compositions claimed herein through use of the term"comprising"may include any additional additive, adjuvant, or compound whether polymeric or otherwise, unless <BR> <BR> stated to the contrary. In contrast, the term, "consisting essentially of'excludes from the scope of any succeeding recitation any other component, step or procedure, excepting those that are not essential to operability. The term"consisting of'excludes any component, step or procedure not

specifically delineated or listed. The term"or", unless stated otherwise, refers to the listed members individually as well as in any combination.

The term"polymer", as used herein, includes both homopolymers, that is, polymers prepared from a single reactive compound, and copolymers, that is, polymers prepared by reaction of at least two polymer forming reactive, monomeric compounds. The term"crystalline"refers to a polymer that exhibits an X-ray diffraction pattern at 25°C and possesses a first order transition or crystalline melting point (Tm). The term may be used interchangeably with the term "semicrystalline". The term"syndiotactic"refers to polymers having a stereoregular structure of greater than 90 percent syndiotactic, preferably greater than 95 percent syndiotactic, of a racemic triad as determined by 13C nuclear magnetic resonance spectroscopy.

Syndiotactic vinyl aromatic polymers are homopolymers and copolymers of vinyl aromatic monomers, that is, monomers whose chemical structure possess both an unsaturated moiety and an aromatic moiety. The preferred vinyl aromatic monomers have the formula: H2C=CR-Ar ; (I) wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms, and Ar is an aromatic radical of from 6 to 10 carbon atoms, including alkyl or halo ring substituted aromatic radicals. Examples of such vinyl aromatic monomers are styrene, alpha-methylstyrene, ortho- methylstyrene, meta-methylstyrene, para-methylstyrene, vinyl toluene, para-t-butylstyrene, vinyl naphthalene, divinylbenzene, chlorostyrene, bromostyrene, and the like. Syndiotactic polystyrene is the currently preferred syndiotactic vinyl aromatic polymer. Typical polymerization processes and coordination catalyst systems for producing syndiotactic vinyl aromatic polymers are well known in the art and are described in U. S. Patents 4,680, 353,5, 066,741, 5,206, 197 and 5,294, 685, and elsewhere.

During polymerization of the vinyl aromatic monomer, the polymerization reaction is not typically carried to completion and a mixture of syndiotactic vinyl aromatic polymer and volatiles, such as residual monomers and process solvents, is produced. This mixture typically contains from about 2 to about 99 percent solid, non-volatile, high molecular weight polymer, preferably from about 30 to about 95 percent, more preferably from about 40 to about 95 percent, and most preferably from about 70 to about 90 percent by weight based on the total weight of the mixture.

The bulk density of the feed is typically less than 400 kg/m3, preferably less than 350 kg/m3. The average particle size (du,,) is generally less than 500, um, preferably less than 400 pm. The polymer can then be recovered from this mixture using a finishing process such as devolatilization to produce resins which are useful for forming injection molded articles, films, fibers, etc. The

process of the present invention is an improved process for devolatilizing the syndiotactic vinyl aromatic polymer/volatile mixture, hereafter referred to as"feed"mixture.

The feed mixture is typically discharged from a polymerization reactor or polymer recovery system at a temperature below 100°C, typically from about 10 to about 90°C. This mixture is then devolatilized in the presence of steam and optionally one or more other catalyst deactivating agents at a temperature between the glass transition temperature (typically around 100 °C) and the melting point of the devolatilized syndiotactic vinyl aromatic polymer (typically from 200 to 320°C). To reduce the time necessary to achieve the desired level of volatiles removal, the feed mixture may be heated to a temperature of at least 110 °C, more preferably to at least 125 °C prior to or simultaneously with contacting with steam. In one embodiment of the invention, the steam itself is used as the heating medium to attain the foregoing temperature limits.

Any means of heating the feed mixture can be used in the process of the present invention.

Examples of such heating means include but are not limited to indirect dryers, where the feed material is in contact with a metal surface heated by an appropriate heat transfer fluid, for example, disc, drum, low and high speed paddle-type, rotary, and screw conveyor dryers; kinetic energy heaters using a plow-type mixer/dryer augmented by high speed choppers, pneumatically conveyed hammer mills or batchwise operated mixer/homogenizers utilizing high speed agitators; direct dryers, which utilize a hot gas stream for heating, for example, flash dryers; all types of fluid bed dryers, conveyor-type, tray, and direct heated rotary dryers; conventional dryer/heater devices augmented with auxiliary heating technology, such as radiant infrared, microwave heating or similar technology, and combinations thereof. Additional suitable equipment includes insulated, gas purged, mass flow hoppers, and storage silos.

The first stage of the devolatilization process is performed in the presence of steam which deactivates residues of the active catalyst contained within the feed mixture thereby rendering it inactive for further polymerization reactions and assists in removal of residual volatile components (purging). Preferably the steam is generated in a separate process and heated to a temperature above the boiling point of water at the pressure of the devolatilization (superheated steam).

Alternatively, liquid water can be injected directly into the devolatilization apparatus or admixed with the incoming feed mixture, and thereby vaporized by heating the entire mixture. The configuration of the feed mixture entrance, purge gas (steam) entrance, and liquid injection ports should be arranged in the devolatilization apparatus so as to maximize the contact time between the feed mixture and the steam. It is desirable that the steam rapidly diffuse into the polymer mass and

react with active residual catalyst components, thereby deactivating them, such that prolonged exposure to elevated temperatures and polymer discoloration or degradation is avoided.

Highly desirably, the steam is contacted with the feed mixture in a first, devolatilizer wherein the steam is injected in a counter-current flow with respect to the flow of feed, at a temperature from 130 to 210°C, preferably from 150 to 210°C, in a quantity so as to provide a steam: volatiles mass ratio from 0.5 to 10, preferably from 1 to 5.

In addition to steam, other catalyst deactivating agents may be utilized in step a) as well.

Typically, the vaporized form of any active nucleophilic compound which is capable of deactivating residual active catalyst components may be used in addition to steam. Such compounds include a wide variety of polar organic and inorganic compounds, such as those represented by the general chemical formula: C, HjOkS, NmX (II), wherein X is fluorine, chlorine, bromine or iodine, i is an integer from 0 to 6, j is an integer from 0 to 14, k is an integer from 0 to 3, 1 and m are integers from 0 to 2, and n is an integer from 0 to 6, such that all appropriate valencies are fulfilled.

Typically, a suitable catalyst deactivating agent, if employed, is characterized by a molecular weight below around 100 Dalton, limited solubility in the polymer produced, and compatibility with steam or water vapor. This prevents the need for removal in a subsequent step and degradation by reaction with the steam employed in the deactivation and devolatilization process. In order to facilitate reuse of monomers and other volatile components recovered from the devolatilization process and to prevent potential polymer discoloration, it is preferable that any secondary catalyst deactivating agent also be unreactive with the vinyl aromatic monomers and process solvents at the conditions employed in the devolatilization process. Typical secondary deactivation agents employed according to the present invention include carbon dioxide, carbon monoxide, hydrogen sulfide, sulfur dioxide, ammonia, polar organic compounds such as alcohols, aldehydes, ketones and the like, and combinations thereof.

The quantity of steam or mixture thereof with other catalyst deactivating agent (s) necessary to achieve the desired deactivation is dependent upon the residual level of all active catalyst components in the feed mixture, however, considerable excess is typically used to ensure complete deactivation. The mass flow rate of steam/catalyst deactivating gas mixture used is typically in the range of 0.1 to 80 percent of the feed mixture flow rate. Highly desirably, the steam or steam mixture is passed through the apparatus in a countercurrent manner with respect to the feed mixture. Steam temperatures from 150 to 270°C, preferably from 150 to 210°C are desirably employed.

Other inert gases, which have no effect upon active catalyst residues or evolved volatile components and which are not appreciably absorbed into the polymer, may also be present in addition to steam or the mixture of steam and other catalyst deactivating compound. Typical inert gases include nitrogen, noble gases such as argon and helium, alkanes such as methane and ethane, hydrogen, and combinations thereof. These components act as diluents and can assist in conveying volatiles out of the devolatilization apparatus and reducing the residual volatiles content in the dried product. In order to best achieve the desired level of deactivation, the molar ratio of inert gases to steam or mixture of steam and other catalyst deactivating compound should typically not exceed 99/1.

The first stage of the improved devolatilization process of the present invention can be performed at a variety of operating pressures within the devolatilization apparatus, provided that the steam is maintained in the vapor state at the temperature and pressure employed in the devolatilization process. The devolatilization process can be carried out at or near atmospheric pressure or, with appropriate design of the devolatilization apparatus, at elevated or reduced pressures. Operation at sub-atmospheric pressures is possible by application of vacuum to evacuate evolved volatiles and excess steam or other deactivation agent from the devolatilization apparatus.

Highly desirably the first devolatilization step is conducted at or near-atmospheric pressure using super heated steam, especially steam at from 130 to 210°C.

As a result of heating the feed mixture, volatile components including residual vinyl aromatic monomers, are released from the polymer, vaporized, and conveyed out of the apparatus along with a flowing discharge stream. The residence time of the polymer in the devolatilization apparatus should be sufficient to reduce the residual vinyl aromatic monomer content in the devolatilized polymer from the initial value in the feed mixture, typically 5 to 60 percent, to less than 3 percent, preferably less than 1 percent based on the weight of the devolatilized polymer. The residence time needed in the devolatilization apparatus to achieve such a reduced volatiles level is dependent upon the original volatiles content of the feed mixture, the temperature in the devolatilization apparatus, the total flow rate of catalyst deactivating and inert gases, the absolute pressure in the devolatilization apparatus, and the physical characteristics of the feed mixture.

Generally, the devolatilization is conducted under conditions such that the residence time needed to achieve the residual vinyl aromatic monomer content recited above is 24 hours or less, typically 12 hours or less, preferably 4 hours or less, more preferably 1 hour or less and most preferably 30 minutes or less.

Desirably, the feed mixture is rapidly heated to a temperature between about 110°C and the melting temperature of the syndiotactic vinyl aromatic polymer. Preferably, the mixture is. heated to a temperature which is approximately 20°C to 100°C below the melting point of the fully dried polymer. Rapid heating can generally be performed in an apparatus capable of increasing the temperature of the feed mixture at an average rate of at least 10°C/minute, typically at least 10 to 1000°C/minute, preferably at least 20°C/minute, more preferably at least 30°C/minute, and most preferably at least 40°C/minute. By heating at a faster rate, the residual monomer is more likely to volatilize rather than polymerize, thereby resulting in production of reduced quantities of atactic vinyl aromatic polymer.

Following step a), the polymer product is further devolatilized by contacting a second time with counter current flow of a devolatilizing agent in a second devolatilization device such as those previously disclosed with respect to step a). Suitable devolatilizing agents include steam, the secondary devolatilizing agents and inert gases previously described for use in step a), and mixtures thereof. An especially desired devolatilizing agent used in step b) is steam or a mixture of steam and a secondary devolatilizing agent. Preferred examples of suitable devolatilization devices for use in step b) include direct and indirect heated dryers. Most desirably the apparatus for step b) is a high temperature, rotary dryer equipped with counterflow injection of devolatilizing agent and gas removal port. In. the second contacting step, the devolatilizing agent employed is desirably at the same or at a higher temperature than the steam or steam mixture use in the first devolatilizing step.

Desirable temperatures for the devolatilization agent in step b) are from 150 to 270°C, preferably from 170 to 230°C.

In order to obtain reduced discoloration in the polymer produced according to the process of the present invention, it is important that the feed stream is not contacted with air or oxygen until at least step a) and preferably both step a) and step b) are completed. Therefore, it is important that the feed stream and the polymer produced remain in contact with a catalyst deactivating gas or an inert gas as defined above until the desired volatiles level is achieved. In addition, the steam or steam mixture should be filtered prior to injection into the devolatilization apparatus to remove contaminants and particles that may cause color body formation.

The final stage of polymer devolatilization is a melting, extruding and pelletizing step, desirably utilizing a vented extruder or an extruder operating under reduced pressure. Examples of such melt extrusion devices employed in step c) include vented or vacuum equipped single and twin screw extruders, equipped with water bath or similar coolers and choppers or similar pelletizing apparatus. Filtration of the molten devolatilized product may be employed if desired as well.

These units can also be used in producing formulated products by mixing additives such as antioxidants, processing aids, impact modifiers, flame retardants, fillers, for example, fiberglass, minerals, or other polymeric materials with the polymer produced to form blends or alloys as well.

By incorporating a pelletizer, such as a water bath cooler and chopper, the resulting devolatilized extrudate can be formed into uniformly shaped pellets in a highly efficient manufacturing process.

Highly desirably the extruded product attains a sufficient degree of crystallization prior to cooling and pelletizing to form opaque, solid pellets that resist blocking and agglomeration.

Desirably, the feed mixture is retained at an elevated temperature between step a) and step b) as well as between step b) and step c). Typical product temperatures are from 120 to 200°C between steps a) and b) and from 140 to 200°C between steps b) and c). Preferably the melting and extrusion step is conducted directly following steps a) and b) in a continuous manner. The melt extrusion step is conducted at increasing temperatures up to the melting point of the polymer and preferably at least 10°C greater than the polymer's crystalline melting point. Maximum extrusion temperatures of the polymer melt are desirably less than 50°C greater than the crystalline melting point of the polymer. For syndiotactic polystyrene the final stage of the melt extruder is desirably from 240 to 320°C. A vacuum is preferably employed during the melting and extrusion to further reduce volatile components of the melt. Generally, pressures from 1000 to 5000 Pa, preferably from 1000 to 1500 Pa are employed during the extrusion.

For crystallized, opaque pellets produced using the devolatilization process of the present invention, the reduced discoloration of the syndiotactic vinyl aromatic polymer can be measure according to ASTM E313 which measures a Yellowness Index or YIE. Typically, the polymer produced by the process of the present invention obtains a YIE of less than 10. Alternatively, ASTM D1925 which compares the Yellowness Index of nearly transparent extruded films of equal thickness using a light transmission technique can be used.

Residual vinyl aromatic monomer content can be determined using headspace gas chromatography with an appropriate solvent, for example, orthodichlorobenzene, by reference to samples of known composition. Atactic polymer content can be determined by Soxhlet extraction using methyl ethyl ketone, which is a solvent for atactic vinyl aromatic polymers, and a non-solvent for crystalline, syndiotactic vinyl aromatic homopolymers and copolymers.

Typically, the syndiotactic vinyl aromatic polymers produced in accordance with the present invention have a weight average molecular weight (Mw) of at least 15,000, preferably at least 50,000, and most preferably from 150,000 to 500,000. The polymer crystalline melting point, Tm, is desirably greater than 240 °C, preferably greater than 245 °C. Additionally, the polymer

prior to devolatilization desirably has a bulk density of at least 175 kg/m3, preferably at least 200 kg/m3 up to 400 kg/m3, preferably less than 350 kg/m3 ; and an average particle size, dp50, of at least 75 D, m, preferably at least 100 lem, up to 450 jum preferably up to 400 pm.

The improved devolatilization process of the present invention produces syndiotactic vinyl aromatic polymers having reduced levels of residual vinyl aromatic monomer and other volatile components, preferably levels of vinyl aromatic monomer less than 500 ppm, preferably less than 400 ppm, and reduced color as compared to polymers produced using conventional catalyst deactivation technology. Additionally, the steam used as the catalyst deactivating and devolatilizing agent can be condensed and reused, thereby reducing the amount of potential emissions to the environment.

The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted.

Amounts are in weight parts or weight percentages unless otherwise indicated.

EXAMPLE 1 A feed of syndiotactic polystyrene homopolymer containing 20 percent volatile components, less than 1 percent atactic polymer, and active metallocene catalyst residues is fed from a polymerization reactor system inertly, without contacting air, at a rate of 1800 kg/hr to a finishing process consisting of two heated, rotary dryers equipped with countercurrent steam injection ports followed by an extruder. The first dryer is a model SJS 48-30 dryer and the second is a model CRJS 60-23 dryer, both made by Hosokawa Bepex Corp. The extruder is a Berstorff ZE 130A twin screw extruder, with L/D = 25, equipped with one vacuum (1500 Pa) vent. The first dryer is operated with a feed zone jacket temperature of 185°C, a discharge zone jacket temperature of 210°C, and a rotor speed of 100 rpm. Steam at approximately atmospheric pressure is filtered to remove contaminants, preheated to 210°C, and fed to the first dryer counter-currently to the solids flow at a rate of 800 kg/hr, giving a steam/volatiles mass ratio of 2.22 : 1. The powder product discharge temperature is 196°C.

The second dryer is supplied with steam similarly preheated to 210 °C at a feed rate of 310 <BR> <BR> kg/hr. , giving a steam/polymer mass ratio of 0.251. The secondary dryer is operated with a uniform jacket temperature of 210°C. Powder leaving the dryer has a temperature of 143°C and is charged directly to the feed port of the extruder. The extruder barrel set point temperatures ranged from 150 to 275°C and the extruder screw speed is 150 rprn. The devolatilized extrudate is formed into strands, cooled in a water bath, and cut into pellets.

The residual styrene content of the resulting polymer pellets is 350 ppm as measured by headspace gas chromatography. The content of atactic polystyrene in the polymer is 0.52 percent.

Residual organic decomposition products of the polymerization catalyst are less than 12 ppb, the limit of detection by gas chromatograph-mass spectroscopy (GC-MS).

EXAMPLE 2 A syndiotactic styrene/p-methylstyrene copolymer comprising 92.3 percent styrene and 7.7 percent p-methylstyrene, containing 20 percent volatile components, less than 1 percent atactic polymer, and active metallocene catalyst residues is fed directly from a polymerization reactor system to the same finishing process as was employed in Example 1 at a rate of 1350 kg/hr. The feed zone jacket temperature of the first dryer is maintained at a temperature of 120°C, the discharge zone jacket temperature is maintained at 130 °C, and the rotor speed is 100 rpm. A countercurrent of steam at approximately atmospheric pressure and 175 °C is charged to the first dryer at a feed rate of 600 kg/hr. , giving a mass ratio (steam: volatiles) of 2.222. Devolatilized powder exits the dryer at a temperature of 129°C and is fed to the secondary dryer where it is again contacted continuously with the same steam source at a flow rate of 300 kg/hr, giving a mass ratio (steam: powder) of 0. 278. The second dryer is operated at a uniform jacket temperature of 175°C.

Hot powder leaving the secondary dryer has a temperature of 146°C and is charged to the feed port of the extruder, extruded and pelletized in a water cooled pelletizer. The extruder barrel set points are 150 to 260°C. Final monomer content of the pellets is 377 ppm. The content of atactic polystyrene in the polymer is 1.1 percent. Residual organic decomposition products of the polymerization catalyst are less than 12 ppb, the limit of detection by gas chromatograph-mass spectroscopy (GC-MS).