SOLVENT EXTRACTION OF LOW MOLECULAR WEIGHT COMPONENTS FROM SOLID POLYMERS

Abstract: A method for the removal of low molecular weight components from solid polymer resins and articles is provided. According to the method, low molecular weight components are solvent extracted from polymers, preferably in a continuous process, optionally facilitated by ultrasonification, heat, or both, wherein the polymers are comprised generally of ethylenic backbones having pendant groups selected from alkyl acrylate groups, cyclic olefinic groups, and/or benzylic groups. The solid polymer resins so treated can meet FDA requirements for food packaging applications.

Description: SOLVENT EXTRACTION OF LOW MOLECULAR WEIGHT COMPONENTS FROM SOLID POLYMERS TECHNICAL FIELD OF THE INVENTION This invention relates generally to the removal of low molecular weight components from solid polymer resins and articles, and, more particularly, the invention relates to improved methods for solvent extraction of low molecular weight components from polymers comprised of ethylenic backbones having pendant groups selected from alkyl acrylate groups, cyclic olefinic groups and/or benzylic groups. BACKGROUND OF THE INVENTION Reactive extrusion is a convenient technique for the post-polymerization modification of polymers. Typically, a polymer, reactive agents, and a catalyst are introduced into the reactive extruder under heating sufficient to promote a melt reaction between reactive agents and the molten polymer and to produce a new modified polymer. The polymer melt can then be extruded into a form useful for further storage or processing. One polymer that can be produced by reactive extrusion is ethylene/methyl acrylate/cyclohexenylmethyl acrylate (EMCM). However, in addition to the desired polymer, reactive extrusion processes commonly generate traces of oligomers with molecular weight less than about 2000, as well as low molecular weight components, e.g. residual reactive agents and reaction by-products. The presence of oligomers and low molecular weight components in the polymer is generally not preferred. For example, if the polymer is intended for a food packaging application, traces of the low molecular weight components may migrate into the packaged food, which may give rise to a malodor or an off-taste or may lead to further study to meet requirements set by regulatory agencies, such as the U.S. Food and Drag Administration (FDA). Although reactive extrusion processing of the polymers can frequently substantially eliminate malodor or off-taste due to low molecular weight components (such as by vacuum devolatilization or nitrogen/steam stripping), and reduce migration of the volatile low molecular weight components to very low levels (<50 ppb in the edible dietary intake (EDI)) which are beneath regulatory thresholds, it is desirable in many instances to have low cost and effective alternative processes to reduce any non- volatile low molecular weight components or further remove these low molecular weight components from EMCM and other polymers, particularly when the polymers are to be used for packaging food items. Traditional cleaning of solid polymers to remove non-volatile low molecular weight by-products has involved, for example, solvent extraction of finely divided resin particles, and, with increased cost and mass transport problems, precipitation from a dilute polymer solution from a solvent into a non- solvent. Lewellen et al., U.S. Patent No. 6,010,391, teaches the polishing of soft acrylic articles, hi one step, the articles are treated with a mineral spirits fraction, whereafter residual mineral spirits can be removed by ultrasonication using a solvent, such as an aqueous solution of 2- butoxyethanol and detergents. Flynn et al., U.S. Patent No. 6,008,179, teaches a composition comprising a methyl ether of a perfluorobutyl compound and an organic solvent. The composition can be used in a method of ultrasonic cleaning. However, there remains a need for efficient and cost-effective alternative methods for extraction or cleaning of solid polymer particles with large surface to volume ratios such as powders, pellets, fibers, strands, thin sheets and films. The present invention provides such an efficient and cost-effective method. SUMMARY OF THE INVENTION According to one aspect of the present invention, a method is provided for treating a solid polymer to remove low molecular weight components, e.g., components with a molecular weight less than about 2000, less than about 1000, or less than about 500, contained therein. A solid polymer is contacted with one or more solvents under conditions effective for extracting the low molecular weight components from the solid polymer. The solid polymer treated in accordance with the invention can be in the form of powder, pellets, fibers, strands, thin sheets, films, and the like, and will generally comprises an ethylenic backbone containing alkyl acrylate pendant groups, cyclic olefinic pendant groups and/or benzylic pendant groups. The solvent with which the solid polymer is contacted is typically selected from the group consisting of - alcohols, C 3 -C 6 ketones, C 3 -C 8 acetates, and C 3 -C 8 ethers. According to another aspect of the present invention, a method is provided for treating a solid polymer, in which a poly(ethylene-methyl acrylate) copolymer, a poly(cyclohexene- methyl acrylate) homopolymer (CHAA), a poly(ethylene/cyclohexene-methyl acrylate) copolymer, a poly(ethylene/vinyl cyclohexene) copolymer (ENCH), or a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) (EMCM) terpolymer is contacted with one or more solvents selected from the group consisting of Ci-C-i alcohols and acetone under conditions effective for extracting low molecular weight components having molecular weights less than about 2000 from the solid polymer. hi a related embodiment, the solid polymer and the solvent can further be heated to a temperature below the melting point or the glass transition temperature of the polymer. According to another aspect of the invention, an ultrasound-assisted solvent extraction method is provided in which a solid polymer is contacted with one or more solvents to form a polymer/solvent mixture. The polymer solvent mixture is subjected to an ultrasonication treatment under conditions and for a duration effective for extracting the desired low molecular weight components from the polymer. The solid polymer generally comprises an ethylenic backbone containing alkyl acrylate pendant groups, cyclic olefinic pendant groups, benzylic pendant groups, or a combination thereof. The solvent with which the solid polymer is contacted is selected from the group consisting of C C 4 alcohols, C 3 -C 6 ketones, C -C 8 acetates, and C 3 -C 8 ethers. Preferably, in this aspect of the invention, the polymer is selected from a polyethylene- methyl acrylate) copolymer, a poly(cyclohexene-methyl acrylate) homopolymer (CHAA), a poly(ethylene/cyclohexene-methyl acrylate) copolymer, a poly(ethylene/vinyl cyclohexene) copolymer (ENCH), or a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) terpolymer; and the one or more solvents are selected from Ci-C alcohols and acetone. In yet another embodiment of this invention, an ultrasound- and heat-assisted solvent extraction method is provided in which a solid polymer is contacted with one or more solvents to form a polymer/solvent mixture. The polymer solvent mixture is subjected to an ultrasonication treatment under conditions and for a duration effective for extracting the desired low molecular weight components from the polymer, wherein the conditions comprise heating the mixture to a temperature below the melting point or the glass transition temperature of the polymer. By practice of the present invention, greater than about 75%, preferably greater than about 85%), more preferably greater than about 95% of components having molecular weights less than about 2000 are extracted from the solid polymer. Moreover, the methods of the invention provide good extraction selectivity such that removal of low molecular weight components is maximized while extraction of higher molecular weight polymeric components is minimized. Thus, greater than about 90%>, preferably greater than about 98% of the extractives removed according to the present invention will have molecular weights less than about 2000, and preferably less than about 1000. BRIEF DESCRIPTION OF THE DRAWINGS The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: Figures 1 illustrates gel permeation chromatography traces of EMAC and low molecular weight components extracted from the EMAC with various solvents; Figure 2 illustrates gel permeation chromatography traces of repelletized EMAC and low molecular weight components extracted from the repelletized EMAC with various solvents; Figure 3 illustrates normalized gel permeation chromatography traces of repelletized EMAC and low molecular weight components extracted from the repelletized EMAC with various solvents; Figure 4 is a representation of one illustrative implementation of a solvent extraction procedure according to the present invention; and Figure 5 is a representation of one illustrative implementation of an ultrasound-assisted solvent extraction procedure according to the present invention. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Illustrative embodiments of the invention are described below, the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-, environmental-, and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The present invention provides methods for the removal of low molecular weight components, such as low molecular weight monomers, oligomers, additives, odorous and other by-products, and the like, from a solid polymer. The solid polymer from which the low molecular weight components are removed may be in any of a variety of forms when treated according to this invention. By way of example, the polymer may be in the form of pellets, powders, fibers, strands, thin sheets, films, etc. The solid polymer will generally have an ethylenic backbone and will further comprise pendant groups linked to the ethylenic backbone, the pendant groups being selected from alkyl acrylate pendant groups, cyclic olefinic pendant groups, benzylic pendant groups, or a combination thereof. By "ethylenic backbone," it is meant that the backbone comprises a chain structure or backbone of saturated carbon atoms which, generally, is created during a polymerization process. For example, homopolymerization of ethylene provides an ethylenic backbone. Copolymerization of ethylene and acrylic acid, methacryhc acid, alkyl acrylate, or alkyl methacrylate also results in an ethylenic backbone with pendant acid or ester moieties. In general, the number of carbon atoms in the backbone is a number between 2 and about 30,000. In general, the polymer will contain between about one and about 100 mole percent of the pendant benzylic, alkyl acrylate and/or cyclic olefinic moieties. Preferably, the composition contains between about 5 and 50 mole percent of the pendant moieties, more preferably between about 10 and 25 mole percent. However, the skilled individual will appreciate that such levels are illustrative only and can be varied to suit the needs of a particular application of interest. In one illustrative embodiment, the polymer treated in accordance with this invention will comprise an ethylenic backbone having alkyl acrylate pendant groups linked to the ethylenic backbone. Such polymers are also commonly referred to as ethylene-alkyl acrylate copolymers, and are well known in the art to include copolymers of ethylene and acrylic or methacryhc esters of linear, branched or cyclic alkanols having, for example, 1-28 carbon atoms. One illustrative ethylene-alkyl acrylate copolymer of particular interest comprises an ethylene-methyl acrylate copolymer. The specific ethylene-alkyl acrylate employed in the process of the present invention is not critical and can include copolymers containing high weight percentages of alkyl acrylate or high weight percentages of ethylene. Such polymers are commercially available. For example, a suitable copolymer containing about 76 wt.% ethylene and 24 wt.% methyl acrylate is available as EMAC 2260 from Chevron Chemical Company (San Francisco, Ca.) Methods for the preparation of these polymers are well known. One example of the preparation of ethylene-methyl acrylate copolymers is disclosed in U.S. Pat. No. 3,350,372 which is incorporated herein by reference in its entirety. Additional preparation methods are disclosed, for example, in U.S. Pat. Nos. 5,631,325 and 5,543,477, the disclosures of which are incorporated herein by reference in their entireties. In another embodiment of this invention, the solid polymer comprises an ethylenic backbone having cyclic olefinic pendant groups, such as those described in International Application No. WO/99/48963, from Chevron Chemical Company, and pending U.S. Patent Application Serial No. 09/127,316, the disclosure of which is incorporated herein by reference in its entirety. Many illustrative cyclic olefinic pendant groups will conform with the structure below: where q ls q 2 , q , q 4 , and r are selected from the group consisting of -H, -CH 3 , and -C H 5 ; and where m is -(CH ) n - with n being an integer in the range from 0 to 4; and wherein, when r is -H, at least one of q l5 q 2 , q and q 4 is -H. Illustratively, but not by way of limitation, the cyclic olefinic pendant groups can be selected from the group consisting of cyclohexene-4-methylene radical, 1 -methyl cyclohexene- 4-methylene radical, 2-methyl cyclohexene-4-methylene radical, 5-methyl cyclohexene-4- methylene radical, 1,2-dimethyl cyclohexene-4-methylene radical, 1,5-dimethyl cyclohexene-4- methylene radical, 2,5-dimethyl cyclohexene-4-methylene radical, 1,2,5-trimethyl cyclohexene- 4-methylene radical, cyclohexene-4-ethylene radical, 1 -methyl cyclohexene-4-ethylene radical, 2-methyl cyclohexene-4-ethylene radical, 5-methyl cyclohexene-4-ethylene radical, 1,2-dimethyl cyclohexene-4-ethylene radical, 1,5-dimethyl cyclohexene-4-ethylene radical, 2,5-dimethyl cyclohexene-4-ethylene radical, 1,2,5-trimethyl cyclohexene-4-ethylene radical, cyclohexene-4-propylene radical, 1 -methyl cyclohexene-4-propylene radical, 2-methyl cyclohexene-4-propylene radical, 5-methyl cyclohexene-4-propylene radical, 1,2-dimethyl cyclohexene-4-propylene radical, 1,5-dimethyl cyclohexene-4-propylene radical, 2,5-dimethyl cyclohexene-4-propylene radical, 1,2,5-trimethyl cyclohexene-4-propylene radical, cyclopentene-4-methylene radical, 1 -methyl cyclopentene-4-methylene radical, 3 -methyl cyclopentene-4-methylene radical, 1,2-dimethyl cyclopentene-4-methylene radical, 3,5-dimethyl cyclopentene-4-methylene radical, 1,3-dimethyl cyclopentene-4-methylene radical, 2,3-dimethyl cyclopentene-4-methylene radical, 1,2,3-trimethyl cyclopentene-4- methylene radical, 1,2,3,5-tetramethyl cyclopentene-4-methylene radical, cyclopentene-4- ethylene radical, 1 -methyl cyclopentene-4-ethylene radical, 3-methyl cyclopentene-4-ethylene radical, 1,2-dimethyl cyclopentene-4-ethylene radical, 3,5-dimethyl cyclopentene-4-ethylene radical, 1,3-dimethyl cyclopentene-4-ethylene radical, 2,3-dimethyl cyclopentene-4-ethylene radical, 1,2,3-trimethyl cyclopentene-4-ethylene radical, 1,2,3,5-tetramethyl cyclopentene-4- ethylene radical, cyclopentene-4-propylene radical, 1 -methyl cyclopentene-4-propylene radical, 3-methyl cyclopentene-4-propylene radical, 1,2-dimethyl cyclopentene-4-propylene radical, 3,5-dimethyl cyclopentene-4-propylene radical, 1,3-dimethyl cyclopentene-4-propylene radical, 2,3-dimethyl cyclopentene-4-propylene radical, 1,2,3-trimethyl cyclopentene-4-propylene radical, and 1,2,3,5-tetramethyl cyclopentene-4-propylene radical. The cyclic olefinic pendant groups described above are generally linked with the ethylenic backbone of the polymer by means of one or more linking groups. Such linking groups are known within the art, and, most typically, will be selected from the following: -O-(CHR),- -(C=O)-O-(CHR) n - -NH-(CHR) n - -O-(00)-(CHR) n - -(C=O)-NH-(-CHR) n - and -(C-O)-O-CHOH-CH 2 -O- wherein R is hydrogen or an alkyl group selected from the group consisting of methyl, ethyl, propyl and butyl groups and where n is an integer in the range from about 1 to 12. However, it should be noted that a linking group is not required. Preferred polymers include a poly(ethylene-methyl acrylate) copolymer, a poly(cyclohexene-methyl acrylate) homopolymer (CHAA), a poly(ethylene/cyclohexene- methyl acrylate) copolymer, a poly(ethylene/vinyl cyclohexene) copolymer (EVCH), or a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) (EMCM) terpolymer. hi another illustrative embodiment, the polymer treated in accordance with the present invention comprises an ethylenic backbone having a combination of alkyl acrylate pendant groups and cyclic olefinic pendant groups. Examples of such polymers can be found, for example, in international Application No. WO/99/48963, from Chevron Chemical Company, and pending U.S. Patent Application Serial No. 09/127,316, the disclosure of which is incorporated herein by reference in its entirety. One particularly preferred polymer according to this aspect of the invention is exemplified by the terpolymer, ethylene-alkyl acrylate- cyclohexene methyl acrylate, also referred to herein as EMCM. In another illustrative embodiment of the invention, the solid polymer used in the disclosed process comprises an ethylenic backbone having benzylic pendant groups, such as those described in U.S. Patent No. 5,859,145, assigned to Chevron Chemical Company, the disclosure of which is incorporated herein by reference in its entirety. Thus, the solid polymer can comprise an ethylenic backbone and moieties which contain a benzyl radical and which are pendant or terminal to the ethylenic backbone. A pendant moiety which contains a benzyl radical, as that term is used herein, is any group which is a side-chain or branch or is terminal to the ethylenic backbone and which contains a benzyl radical. The benzyl radical can comprise a phenyl radical directly bonded to a methylene radical. The methylene radical may be joined to other alkyl or alkylene, alkenyl, alkynyl, aryl, or heteroatom-containing substituents that, together with the benzyl radical, form the unsubstituted moiety that is pendant to the ethylenic backbone. These radicals may be substituted with a hydrocarbyl radical or a heteroatom or heteroatom-containing radical or may be unsubstituted. A substituted phenyl radical has at least one radical substituted in place of at least one hydrogen atom of the phenyl radical. An unsubstituted methylene radical generally consists of one carbon atom and two or three hydrogen atoms. A substituted methylene radical generally consists of one carbon atom, one hydrogen atom, and at least one radical substituted in place of one of the hydrogen atoms. A benzyl radical maybe bonded to the remainder of its pendant moiety through its phenyl radical, hi this case, its methylene radical may be a methyl radical or a substituted methyl radical. Preferably, the benzyl radical is a component of a cyclic benzylic side chain, by which is meant a side chain in which at least two of the carbons of the benzyl group are members of a cyclic alkyl, alkenyl, or alkynyl group that is not coextensive with the benzyl group. Preferred cyclic benzyl radicals include cyclic benzyl ether groups, cyclic benzyl amine groups, or cyclic benzyl amide groups. A heteroatom-containing radical is any radical which contains an element other than carbon and hydrogen. The heteroatom-containing radical generally improves the S oxygen- scavenging abilities of the composition. A heteroatom having pi bonds to adjacent carbon atoms is preferred. When present, the heteroatom-containing radical is preferably bonded directly to the benzyl radical with no moieties present between the heteroatom-containing radical and the benzyl radical. The heteroatom-containing radical may be bonded to the benzyl radical in any combination of three possible ways. For example, the heteroatom-containing radical may be bonded to the methylene radical. It may also be substituted onto the methylene radical in place of one of the hydrogen atoms, in which case the methylene radical is attached directly to the backbone or the moiety attached to the backbone or to another heteroatom- containing moiety. Or, the heteroatom-containing radical may be substituted in place of one of the hydrogen atoms of the phenyl radical. Examples of heteroatom-containing radicals include amine, ether, sulfide, and ketone radicals, and preferred radicals are esters and amides. Radicals which may be substituted or joined onto the benzyl radical include alkyl radicals containing from 1 to 18 carbon atoms, alkoxy radicals having from 1 to 16 carbon atoms, alkenyl or alkynyl radicals containing from 2 to 18 carbon atoms, alkenoxy or alkynoxy radicals having from 2 to 18 carbon atoms, amine radicals having from 1 to 6 carbon atoms, aryl radicals or substituted aryl radicals having 6 to 24 carbon atoms, aryl ether radicals or substituted aryl ether radicals having from 6 to 24 carbon atoms, and ester and amide radicals of acids having from 1 to 16 carbon atoms. Aryl and aryl ether radicals can be substituted in the same manner as the methylene and the phenyl radicals, subject to the limitation that the aryl and aryl ether radicals, after substitution, have 6 to 24 carbon atoms total. Preferably, the radicals which are substituted onto the benzyl radical are selected from the group consisting of alkyl radicals containing from 1 to 6 carbon atoms, alkoxy radicals having from 1 to 6 carbon atoms, amine radicals having from 1 to 6 carbon atoms, aryl radicals and substituted aryl radicals having 6 to 15 carbon atoms, aryl ether radicals and substituted aryl ether radicals having from 6 to 15 carbon atoms, and ester and amide radicals of acids having from 1 to 6 carbon atoms. Preferred radicals which provide higher oxygen scavenging rates are alkyl, alkoxy, and amine radicals. Preferably, the benzyl moieties which are pendant to the ethylenic backbone comprise benzyl thioester, more preferably benzyl amide, and most preferably benzyl ester moieties. Preferably, the amide or ester is bonded directly to the ethylenic or polyethylenic backbone. Other preferable pendant moieties contain benzyl ether groups, benzyl amine groups, and -CH 2 -aryl containing groups where the aryl group includes more than one ring, such as 1,3- dihydroisoindole, anthracene, phenanthrene, naphthalene and the like. hi one preferred embodiment, a polymeric composition of the present invention contains between about one and 20 mole percent benzyl radicals. More preferably, the composition contains between about two and 15 percent, and more preferably still, between about 5 and 12 mole percent benzyl radicals. Preferably, the benzyl radicals are bonded directly to a heteroatom-containing group. The exact amount of benzyl radicals and heteroatom- containing radicals as well as the amount of transition-metal salt are normally determined by the application in which the composition is going to be employed. According to the present invention, one or more of the solid polymers described above is contacted with a solvent in order to extract undesirable low molecular weight components from the polymer. By low molecular weight components, it is meant the components present in the polymer after it is produced that have molecular weights less than about 2000, less than about 1000, or less than about 500, depending on the particular polymer being produced and/or the particular components that are desired to be removed. The solvent with which the solid polymer is contacted is selected from C C 4 alcohols, C 3 -C 6 ketones, C -C 8 acetates, and C 3 -C 8 ethers, or a mixture thereof. In one illustrative embodiment of the invention, the solvent is selected from ethanol, isopropanol, acetone, ethyl acetate, or a mixture thereof. The solvents of the invention have been found to advantageously extract the low molecular weight components from the solid polymer matrix, while typically not extracting higher molecular weight polymeric material that is desired to remain in the solid polymer following extraction. This selectivity of the solvents identified herein is particularly advantageous for extracting components from the solid polymer having molecular weights less than about 2000, preferably less than about 1000, while causing only minimal extraction of polymeric material having higher molecular weights. One or more of the solid polymers described above are contacted with the solvent or solvent mixture under conditions effective for extracting these low molecular weight components from the polymer. This will typically involve contacting the solid polymer with the solvent for a duration in the range of about 0.5 hr. to about 2 days. Most typically, the contact time between the polymer and the solvent will range from about 2 to 20 hrs. The duration of contact between the polymer and the solvent can, of course, be varied depending on a number of factors, e.g., the temperature at which the extraction is performed, the particular solvent used, the solid polymer being treated, the nature and quantity of the low molecular weight components to be removed from the polymer, the particle size of the polymer, polymer- solvent interactions (such as swelling), the intensity of ultrasonic energy that may also applied to the mixture, etc. The temperature of the extraction solvent can be varied as a matter of operational convenience provided the temperature does not exceed the melting temperature or the glass transition temperature of the polymer being treated. Generally, the extraction temperature will be in the range of about 15 °C to about 65 °C. Most typically, the extraction temperature will be between about 35 °C and about 55 °C. hi addition, physical agitation or circulation can be used to maximize the efficiency to remove oligomer and odorous compounds. The skilled individual in the art will recognize that the solvent extraction approach described herein can be employed in any of a variety of settings and can be readily adapted for use in a large scale polymer production facility if desired. For example, Figure 4 illustrates one possible approach for the implementation of the disclosed process. The pellet extraction system shown in Figure 4 centers around the solvent extractor 10. Operating temperatures and flow rates for the solvent extractor 10 and other pieces of equipment depend on the nature of the polymer and the extraction solvent. Polymer, in pellet form, passes along with fresh and recycled extraction solvent through the solvent extractor 10, a device that allows for intimate contacting of polymer pellets and solvent. Residence time in this extractor can be on the order of minutes to several hours, depending on the polymer-solvent system requirements and the equipment used. The extraction rate in this equipment can be enhanced by the use of ultra-sound, thereby reducing the required residence time, as described elsewhere in this document. Pellets leaving the solvent extractor undergo separation of the solvent from the polymer by filtration, and then the pellets are passed to a pellet dryer 12. The purpose of this step is not only to remove solvent from the surface of the polymer, but also to allow for removal of solvent absorbed by the polymer. Evaporating solvent from the pellets without first extracting a large proportion of the solvent by filtration or similar technique may lead to the retention of non-volatile oligomers in the pellet. Warm, dry nitrogen gas, or other dry inert gas, is passed through the pellets. The nitrogen stream leaving the dryer 12 is rich in solvent vapor. This stream passes to a condenser where the solvent vapor is condensed and recycled back to the extractor. The nitrogen is recycled back to the dryers. Spent solvent leaving the solvent extractor 10 is sent to one or more distillation columns 14 for removal of dissolved impurities (i.e. lower molecular weight compounds and comonomers) from the solvent stream. Recovered solvent is recycled back to the solvent extractor 10 and impurities are removed. hi another aspect of the present invention, a method for ultrasound-assisted solvent extraction is provided. It has been found that ultrasonication of the solid polymer while in contact with the disclosed solvents significantly facilitates the removal efficiency of the low molecular weight components from the solid polymer, possibly by increasing the mobility of the low molecular weight components. Illustratively, six hours of solvent extraction used in conjunction with ultrasonication removed a greater quantity of low molecular weight components than were removed after 24 hours of extraction with the same solvent without ultrasonication. Therefore, according to this aspect of the present invention, a method is provided in which solid polymer is contacted with solvent to form a polymer/solvent mixture and ultrasonic energy is applied to the polymer/solvent mixture under conditions for improving the extraction efficiency of low molecular components from the polymer. This use of ultrasonication effectively reduces the contact time between the polymer and the extraction solvent that is necessary to extract a given quantity of low molecular weight components. For example, the extraction efficiency, as measured by extractives removed per unit time, can be increased by greater than about 2 to 10 times, preferably greater than about 5 times, using ultrasound- assisted solvent extraction compared with extraction with the same solvent(s) in the absence of ultrasonication. The ultrasonic energy may be applied by any of a variety of known techniques. Illustratively, the ultrasound can be applied from outside an extraction vessel, or, alternatively, from inside an extraction vessel, for example using a probe design. The latter may be a preferred approach for use in a large scale operation. For certain applications, the vessel in which extraction is performed can contain a fluidized bed or a current counterflow of solid versus solvent to achieve optimal extraction efficiency. The ultrasonic energy level that is applied to the solid polymer while in contact with solvent can be varied to best suit the needs of a given implementation of this invention. The energy level employed may vary, for example, depending on the design of the system, e.g., external versus internal design. Typically, but not by way of limitation, the energy level of the ultrasound applied to a polymer/solvent mixture will be in the range of about 50-10000 watts. For lab scale applications, the energy level will be in the range of about 50-500 watts. The conditions under which the ultrasonic energy is applied, e.g., the energy level, contact time, etc., are selected so as to effectively remove the desired quantity of low molecular weight components from the solid polymer of interest, one illustrative embodiment, the treatment conditions are selected such that the levels of low molecular weight components in the solid polymer are reduced by at least about 85%, preferably by at least about 90%, following one or more solvent extractions described herein. In another illustrative embodiment, the levels of the low molecular weight components are reduced to below about lOOppm, preferably below about 30ppm, for each low molecular weight component present in the solid polymer. The process of this invention can be readily adapted for use on essentially any scale from small, laboratory scale solvent extractions to large, commercial scale operations. Figure 5 illustrates one possible implementation of this aspect of the invention in which, fresh resin to be cleaned is to be fed continuously from a feeder 20. The resin pellets travel through a tube/pipe 22 with assistance from screw pressure (not shown), while the fresh solvent is injected into the system near the outlet of the cleaned pellets at solvent inlet 24. The resin pellets are cleaned as they travel through the tube/pipe 22 equipped with at least one ultrasonicator 26. The tube/pipe 22 maybe heated to increase the cleaning efficiency. The dirty solvent is taken out at waste outlet 28 near the inlet of the fresh resin and sent to a solvent recycling unit 30, where the solvent is purified by distillation. The purified solvent is fed back to the system through the solvent inlet 24. The cleaned pellets are sent out to a dryer (not shown), which removes further solvent left in the pellets. EXAMPLES The following examples are provided to demonstrate certain illustrative embodiments of this invention. It should be appreciated by those skilled in the art that the techniques disclosed in the examples which follow represent those found by the inventors to function in the practice of the invention and thus can be considered to constitute examples of illustrative modes for its practice. However, those skilled in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. EXAMPLE 1 250 grams of EMAC or EMCM pellets and 400-450 grams of solvent were added to a one-litter bottle. The pellets and solvent were agitated by tumbling the bottle on a roller for 22- 24 hours at room temperature. The pellets and extract were filtered with #2 filter paper and rinsed with 3x25 mL solvent. The polymer pellets were dried in a vacuum oven at 50-60 °C for 24-48 hours. The solvent was removed from the polymer extractives using a rotary evaporator at 50-70 °C. The weight percentage of low molecular weight component was calculated based on the weights of the total extractives and the starting polymer. Various solvents were tested for their efficiency in removing low molecular weight components from EMAC and EMCM. These components, because of their low molecular weights (-200-2000 MW), can migrate from inside of the polymer pellets to the solvents. The results of these experiments are shown below in Table 1. Table 1. Effect of Solvent on Oligomer Extraction from EMAC The EMAC/solvent mixtures were agitated by tumbling on a roller at room temperature for 22-24 hrs. hi C2380-E, 200 g of THF was used to swell the EMAC for 2 hrs before acetone (260 g) was added. Figure 1 illustrates the GPC traces of EMAC-2260 and low molecular weight components extracted from it with various solvents. The low molecular weight components extracted from various solvents have different molecular weight distributions which vary depending on the particular solvent used. It can be seen that solvents like THF and ethyl acetate effectively extract lower molecular weight components from the solid polymer. However, some higher molecular weight polymeric material was extracted as well by these solvents. Consequently, solvents having high solubility with the high and low molecular weight components of the solid polymer may not be the best choice for oligomer extraction and EMAC clean up where it is desired to optimize the extraction of low molecular weight components, e.g., less than about 2000 MW, while minimizing the extraction of higher molecular weight polymer. For example, if it is desired to remove components with molecular weights less than about 1000, acetone, isopropyl alcohol and ethanol remove almost as much as THF and ethyl acetate, but remove very little of the higher molecular weight polymer, hi addition, strong solvents may cause excessive swelling of EMAC (especially at elevated temperatures). This helps to remove the oligomers, but may cause problems in large production scale. These problems include reduced effective capacity, and difficulty in transferring polymer and in removing the solvent after extraction, h view of the above, the tested disclosed solvents are effective for removing the low molecular weight components from EMAC, but for many applications, acetone, ethanol and/or isopropyl alcohol will be preferred. EXAMPLE 2 250 grams of EMAC or EMCM pellets and 400-450 grams solvent were added to a one- litter bottle. The bottle was ultrasonicated in an ultrasound bath having an average sonic power of 45-150 w for 45 minutes to 20 hours. The polymer was filtered with #2 filter paper and rinsed with 3x25 mL solvent. The ultrasonication, filtration and rinsing were repeated one or several times. In some instances, the polymer/solvent mixtures were agitated by tumbling on a roller before filtration and rinsing. The polymer was dried in a vacuum oven at 50-60 °C for 24-48 hours. The solvent was removed from the polymer extractives using a rotary evaporator at 50-70 °C. Ultrasonication dramatically increased oligomer mobility and improved the efficiency of the solvent extraction process. As shown in Table 2, solvent extraction performed for 1 hour in combination with ultrasound at 50 °C removed more than one-third of the oligomer removed at room temperature during a 24 hour extraction without ultrasound. Six hours of extraction performed in combination with ultrasound removed more oligomers than was removed after 24 hours of extraction without ultrasound. Table 2. Oligomer Extraction from EMAC, Roller vs. Ultrasonic Bath Solvent: ethyl acetate. EXAMPLE 3 We also tested the efficacy of using solvent mixtures for removal of the low molecular weight components from EMAC. 250 grams of EMAC pellets were swelled with 250-350 grams of chloroform or THF in a 1 -liter bottle. Weak solvent (300-450 grams of isopropyl alcohol, acetone, etc.) was added to the bottle. The mixture was agitated by tumbling on a roller for 4-10 hours at room temperature. The mixture was washed with solvents one or more times to extract the oligomers. When a solvent mixture was used, a small amount of relatively strong solvent, such as THF and chloroform, was first mixed EMAC to swell the polymer. We typically added equal weights of the strong solvent and polymer. Excess amount of solvent dissolved the polymer and made the polymer pellets stick together. After all the solvent was absorbed by the pellets, a weaker solvent, such as isopropyl alcohol or ethanol was used for the extraction of low molecular weight components. Using a strong solvent such as chloroform extracts more higher molecular weight oligomer from the EMAC pellets, but it does not help significantly in removing lower molecular weight components having molecular weights below about 1000. EXAMPLE 4 Multiple solvent washes also improve the efficiency of oligomer removal as shown in Table 3. In large production scale, a continuous process with fresh solvent and fresh polymer feeding in opposite directions is expected to further improve the process. Table 3. Oligomer Removal from EMAC via Solvent Extraction Single Wash vs. Triple Wash, Commercial vs. Repelletized Extraction was done with ethyl acetate in an ultrasonic bath at about 50 °C. Repelletized EMAC was produced as small spheres with pellets per gram being about 165, and having diameters about 1/5 the size of commercial EMAC pellets (which are somewhat elliptical or disk shaped with pellets per gram of about 33). As shown in Table 3, about 30%) more oligomers were removed from repelletized EMAC than from the commercial EMAC under the same extraction condition. However, the extractives from repelletized EMAC contained more polymers than extracted from the commercial pellets, but similar quantities of oligomers with molecular weight of less than 500. EXAMPLE 5 We also tested the effectiveness of the solvent wash process using 10 day, 40 °C, 95% ethanol extraction conditions and using cleaned EMAC monolayer film. Monolayer film (1.5-2 mils thick) was made from cleaned EMAC or EMCM using a Randcastle extruder. About 100 grams of the monolayer film were soaked in 1 gallon of 95% ethanol in a one-gallon bottle. The film/ethanol mixture was put in a 40°C oven. The film was removed after 10 days at 40°C. The ethanol was removed from the ethanol extract using a rotary evaporator at about 50°C. When the solution volume was reduced to about 30mL, the concentrated mixture was transferred to a 50mL flask. Ethanol was then completely removed by further rotary evaporation before weighing the film extractives. Using a typical commercial EMAC without solvent extraction clean up, about 0.5%> low molecular weight components were extracted from the monolayer film. As shown in Table 5, less than 0.04% low molecular weight components were extracted from film made from triple solvent washed EMAC. This represents about a 93 % reduction in low molecular weight components from the EMAC. Table 5. 10 Day 40 °C Ethanol Extraction on Solvent Extracted EMAC Monolayer Film Film extraction was done by soaking the monolayer EMAC film in 1 gallon of 95% ethanol at 40 °C for 10 days, hi C2380-05, the actual time and temperature were 75 hrs at 50 °C and 165 hrs at 40 °C). The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. More specifically, it will be apparent that certain agents which are chemically related may be substituted for the agents described herein while the same or similar results would be achieved. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Claims: What is Claimed is: 1. A method for treating a solid polymer, comprising: contacting a solid polymer with one or more solvents under conditions effective for extracting low molecular weight components from the solid polymer; wherein the solid polymer comprises an ethylenic backbone containing alkyl acrylate pendant groups, cyclic olefinic pendant groups, benzylic pendant groups, or a combination thereof; and wherein the solvent is selected from the group consisting of Cι-C alcohols, C 3 -C 6 ketones, C 3 -C 8 acetates, and C 3 -C 8 ethers. 2. The method of claim 1, wherein the cyclic olefinic pendant groups are linked to the ethylenic backbone with linking groups selected from the group consisting of: -O-(CHR) n -; -(C=O)-O-(CHR)„-; -NH-(CHR) n -; -O-(C=O)-(CHR) n -; -(C=O)-NH-(-CHR) n -; -(C=O)-O-CHOH-CH 2 -O-; and no linking group; wherein R is hydrogen or an alkyl group selected from the group consisting of methyl, ethyl, propyl and butyl groups and where n is an integer in the range from 1 to 12. 3. The method of claim 1 , the cyclic olefinic pendant groups have the structure: where q l5 q 2 , q 3 , q 4 , and r are selected from the group consisting of -H, -CH , and -C 2 H 5 ; and where m is -(CH 2 ) n - with n being an integer in the range from 0 to 4; and wherein, when r is -H, at least one of q ls q , q 3 and q 4 is -H. 4. The method of claim 1, wherein the solid polymers comprise copolymers of ethylene and acrylic esters of liner, branched or cyclic Ci-C 28 alkanols. 5. The method of claim 1, wherein the solid polymers comprise copolymers of ethylene and methacryhc esters of liner, branched or cyclic Ci-C 28 alkanols. 6. The method of claim 1, wherein the alkyl acrylate pendant groups is a methyl acrylate pendant group. 7. The method of claim 1 , wherein the benzyl pendant groups are selected from benzyl thioester groups, benzyl amide groups, benzyl ester groups, benzyl ether groups, cyclic benzyl ether groups, cyclic benzyl amine groups, cyclic benzyl amide groups, or benzyl amine groups. 8. The method of claim 1, wherein the solid polymer is a poly(ethylene-metlιyl acrylate) copolymer, a poly(ethylene/cyclohexene-methyl acrylate) copolymer, a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) or a poly(ethylene/vinyl cyclohexene) copolymer. 9. The method of claim 1 , wherein the solid polymer is a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) terpolymer. 10. The method of claim 1, wherein the solvent is selected from a C 2 -C 3 alcohol. 11. The method of claim 1 , wherein the solvent is selected from the group consisting of acetone, isopropanol and ethanol. 12. The method of claim 1, wherein the solid polymer is in the form of powder, pellets, fibers, strands, thin sheets and films. 13. The method of claim 1, wherein greater than about 75% of components having molecular weights less than about 1000 are extracted from the solid polymer . 14. The method of claim 1, wherein greater than about 90 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 2000. 15. The method of claim 1, wherein greater than about 95 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 1000. 16. The method of claim 1, wherein greater than about 98 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 500. 17. A method for treating a solid polymer, comprising: contacting a solid polymer with one or more solvents under conditions effective for extracting low molecular weight components having molecular weights less than about 2000 from the solid polymer; wherein the solid polymer comprises a poly(ethylene-methyl acrylate) copolymer, a poly(ethylene/cyclohexene-methyl) copolymer, a poly(ethylene/vinyl cyclohexene) copolymer, or a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) terpolymer; and wherein the one or more solvents is selected from the group consisting of C 2 -C 3 alcohols and acetone. 18. The method of claim 17, wherein the solvent is selected from isopropanol or ethanol. 19. The method of claim 17, wherein the solid polymer is in the form of powder, pellets, fibers, strands, thin sheets and films. 20. The method of claim 17, wherein at least about 75% of the low molecular components are extracted from the solid polymer. 21. The method of claim 17, wherein greater than about 95 %> of the low molecular weight components extracted from the solid polymer have molecular weights less than about 1000. 22. The method of claim 17, wherein greater than about 98 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 500. 23. A method for treating a solid polymer, comprising: contacting a polymer with one or more solvents to form a polymer/solvent mixture; applying ultrasonic energy to the polymer/solvent mixture under conditions effective for extracting low molecular components from the polymer; wherein the solid polymer comprises an ethylenic backbone containing alkyl acrylate pendant groups, cyclic olefinic pendant groups, benzylic pendant groups, or a combination thereof; and wherein the solvent is selected from the group consisting of - alcohols, C 3 -C 6 ketones, C 3 -C 8 acetates, and C 3 -C 8 ethers. 24. The method of claim 23, wherein the cyclic olefinic pendant groups are linked to the ethylenic backbone with linking groups selected from the group consisting of: -O-(CHR) n -; -(C=O)-O-(CHR) n -; -NH-(CHR) n -; -O-(C=O)-(CHR) n -; -(C=O)-NH-(-CHR) n -; -(C-O)-O-CHOH-CH 2 -O-; and no linking group; wherein R is hydrogen or an alkyl group selected from the group consisting of methyl, ethyl, propyl and butyl groups and where n is an integer in the range from 1 to 12. 25. The method of claim 23, wherein the cyclic olefinic pendant groups have the structure: where q l3 q 2 , q 3 , q 4 , and r are selected from the group consisting of -H, -CH , and -C 2 H 5 ; and where m is -(CH ) n - with n being an integer in the range from 0 to 4; and wherein, when r is -H, at least one of q l5 q 2 , q 3 and q 4 is -H. 26. The method of claim 23, wherein the solid polymers comprise copolymers of ethylene and acrylic esters of liner, branched or cyclic C C 28 alkanols. 27. The method of claim 23, wherein the solid polymers comprise copolymers of ethylene and methacryhc esters of liner, branched or cyclic Cι-C 8 alkanols. 28. The method of claim 23, wherein the alkyl acrylate pendant groups is a methyl acrylate pendant group. 29. The method of claim 23, wherein the benzyl pendant groups are selected from benzyl thioester groups, benzyl amide groups, benzyl ester groups, benzyl ether groups, cyclic benzyl ether groups, cyclic benzyl amine groups, cyclic benzyl amide groups, or benzyl amine groups. 30. The method of claim 23, wherein the solid polymer is a poly(ethylene-methyl acrylate) copolymer. 31. The method of claim 23 , wherein the solid polymer is a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) terpolymer. 32. The method of claim 23, wherein the solvent is selected from a C 2 -C alcohol. 33. The method of claim 23, wherein the solvent is selected from the group consisting of acetone, isopropanol and ethanol. 34. The method of claim 23, wherein the solid polymer is in the form of powder, pellets, fibers, strands, thin sheets and films. 35. The method of claim 23, wherein greater than about 75% of components having molecular weights less than about 1000 are extracted from the solid polymer. 36. The method of claim 23, wherein greater than about 90 %> of the low molecular weight components extracted from the solid polymer have molecular weights less than about 2000. 37. The method of claim 23, wherein greater than about 95 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 1000. 38. The method of claim 23, wherein greater than about 98 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 500. 39. A method for treating a solid polymer, comprising: contacting a polymer with one or more solvents to form a polymer/solvent mixture; applying ultrasonic energy to the polymer/solvent mixture under conditions effective for extracting low molecular components having molecular weights less than about 2000 from the polymer; wherein the solid polymer comprises a poly(ethylene-methyl acrylate) copolymer, a poly(ethylene/cyclohexene-methyl) copolymer, a poly(ethylene/vinyl cyclohexene) copolymer, or a poly(ethylene/methyl acrylate/cyclohexene-methyl acrylate) terpolymer; and wherein the one or more solvents is selected from the group consisting of C 2 -C 3 alcohols and acetone. 40. The method of claim 39, wherein the solvent is selected from the group consisting of isopropanol and ethanol. 41. The method of claim 39, wherein the solid polymer is in the form of powder, pellets, fibers, strands, thin sheets and films. 42. The method of claim 39, wherein greater than about 15% of components having molecular weights less than about 1000 are extracted from the solid polymer. 43. The method of claim 39, wherein the method is continuous. 44. The method of claim 39, wherein the contacting and applying occur in a fluidized bed process. 45. The method of claim 39, wherein the contacting occurs in a solvent-polymer counterflow process. 46. The method of claim 39, wherein greater than about 95 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 1000. 47. The method of claim 39, wherein greater than about 98 % of the low molecular weight components extracted from the solid polymer have molecular weights less than about 500. 48. The method of claim 39, wherein less than about 30 ppm of the low molecular weight components remain in the product after extraction.

Inventors: CAI GANGFENG, CHING TA YEN, YANG HU, CAI, GANGFENG, CHING, TA, YEN, YANG, HU
Applicants: CHEVRON PHILLIPS CHEMICAL CO, CHEVRON PHILLIPS CHEMICAL COMPANY LP
Application year: 2001
Main classification: C08F6
IPC codes: C08F6