JP2001520703A - Recycling of polyester for food - Google Patents

Abstract

  (57) [Summary] A method for recycling used polyester is disclosed which recycles polyester to obtain a sufficiently pure recycled polyester that meets food packaging requirements. The method comprises the steps of: cleaning the crushed strips of used polyester to remove contaminants from the surface; melting the strips of used polyester whose surface has been cleaned; extruding the melted used melt; Mixing the melt with a melt of virgin polyester prepolymer; solidifying the mixed melt with the virgin polyester prepolymer remaining as a prepolymer; and pelletizing; It consists of polymerizing in the solid state.

Description

Description: FIELD OF THE INVENTION The present invention relates to the recycling of used polymer products, and more particularly, to the recycling of used polyester products into suitable products for packaging food products. Recycling to obtain recycled polyester products of sufficient purity to meet specifications. BACKGROUND OF THE INVENTION The widespread use of polymeric products ("plastics") as a consumable has led to an interest in the practical recycling or recycling of such products by as much as possible in a suitable manner. Is increasing. Interest in such recycling is governed by several considerations. They are environmental factors, in principle occupation by landfill waste and other disposal issues; limited resource conservation, because polymers are part of the production of petrochemicals from crude oil And economic issues, because the price of used plastic is much lower than the price of raw materials, and instead the price of physically collecting and transporting used plastic material to factories for proper recycling Because it depends more on In addition, public policy is expected to continue to emphasize environmental issues, and legal regulations for recycling have already taken effect in some jurisdictions and are expected to take effect in other areas in the future. Is done. The interest in recycling used products is increasingly focused on polyesters (ie, polyethylene terephthalate, "PET", and related compounds). More specifically, for decades, polyester has found its widest use as a synthetic material in textiles. The task of recycling fibrous material is quite different from the recycling of molded polyester products because polyester is generally mixed and woven when introduced into yarns and fabrics. More recently, however, polyester chemistry and techniques for forming polyester have evolved in that polyester is widely used as a material in "disposable" soft drink bottles. Thus, once these polyester bottles are abandoned, there is a significant opportunity for economical and efficient recycling if the appropriate technology is developed. In this regard, the term "recycle" often tends to be used in a relatively broad sense, but in fact also covers many different ways to reuse any given material. Thus, in one sense, if a polyester bottle is used to contain non-food liquids rather than being discarded directly, it will be "recycled," but the word "recycle" Is rarely applied to the pursuit of such empty containers. Rather, recycling is generally recognized as falling into one of three general categories, which are also recognized by the U.S. Environmental Protection Agency (EPA) and the U.S. Food and Drug Administration (FDA). . The first is primary recycling, which involves the use of industrial waste and waste collection before consumption. The second is a secondary recycle, which is a physical rework (eg, grinding and re-extrusion) of the polymer. The third is a third recycle, in which the polymers are reprocessed to isolate the chemical components and reused to make new products. Since the invention deals with the recycling of used ones, primary recycling is not relevant here. As used herein, the term “spent” refers to polyester articles (eg, food containers or bottles) that have been discarded after their first use. Since such containers or bottles are generally disposed of in a household trash or may be reused in non-food compositions, they are subject to various contaminants or contamination when recycled. May contain residues. With respect to polyester, the third recycle generally involves some chemical conversion of the polyester (typically polyethylene terephthalate) back into its constituent monomers (terephthalic acid and ethylene glycol) in some types of depolymerization steps. This is one of the processes. Typical depolymerization steps include, for example, methanolysis performed by Eastman Kodak Company and Hoechst. Shell is implementing another method called "glycolysis", which mixes spent polyester with glycol (and potentially other compounds) and decomposes it back into its component precursors Is what you do. Another recycling technique provides a method for removing associated substances often used in polyesters of consumer goods. In this regard, U.S. Pat. No. 5,523,329 to Moore et al., Entitled "Recovering Polyester from Used Film," assigned to the same assignee as the pending application, Methods for removing polyvinylidene (PDVC) from polyester supports and frequently used in combination, for example, photographic film supports and emulsions thereof, are disclosed. To be used as a packaged food product, any polymer, whether unused or recycled, must meet the proper standards for contacting food. As used herein, the term "virgin" refers to polymers made directly from chemical precursors, and their purity is generally considered to be acceptably high. Therefore, if a food grade polyester is to be produced from used polyester, the recycling process not only produces a reconstituted polyester with the appropriate physical and chemical properties, but also ensures that it meets food quality standards. Must be pure enough. In the United States, the FDA generally states that the level of recycling for contaminants present in food contact articles is less than 0.1 for food exposure to those substances. It is in the position of less than 5 ppb. According to FDA calculations, the residual concentration of possible contaminants in food packaging materials is about 215 ppb or less. At those levels, if the contaminant was present in a polyester container made from recycled material, and if the contaminant had migrated to the entire food (100%), 1 Based on a 5% PET consumption factor (ie, the FDA assumes that 5% of the food is packaged with polyester), less than 0.5 ppb in the desired order, based on a 5% PET consumption factor It is said that there is. At levels greater than about 215 ppb, migration studies can also be used to indicate that the levels contained in a daily diet will still be less than 0.5 ppb. As noted above, if a polymer such as a polyester is made from pure precursor components such as terephthalic acid and ethylene glycol, one will generally be able to make the desired purity range. Also, and as would be expected, if the used polyester is re-reacted to form monomers and then re-polymerized to form a polymer, it is equivalent to the pure raw material by glycolysis or methanolysis. If resolved to a similar degree, a similar level of purity would be achieved. However, the drawback of such processes is that they are more expensive and less efficient than processes for producing polyester from virgin precursors, due to the vast nature of their extensive chemical processing. That is. Because one purpose of recycling is to promote its economic appeal, recycling methods tend to be used sparingly and in some cases, rather than for their market interests May be deemed to address the legal conditions that have been committed. Therefore, it is an object of the present invention to provide a method for recycling used polyester, and the resulting recycled polyester product, wherein the problems and depolymerization or similar chemical reactions are reduced. It minimizes underutilization and instead provides an economically beneficial way of recycling polyester. The present invention is a method for recycling polyester to obtain a sufficiently pure recycled polyester that meets food packaging conditions, the method comprising the steps of cleaning used polyester crushed strips and removing contaminants from the surface. Removing; surface-cleaning melting the used polyester strip; extruding the used melt; mixing the melt of the used polyester and the melt of the unused polyester prepolymer; using the unused polyester prepolymer as the prepolymer This object is achieved by a method characterized by solidifying and pelletizing the mixed melt while leaving; and polymerizing the solid mixed pellets in the solid state. In another embodiment of the present invention, both the spent polyester and the unused polyester are first pelletized and then the mixture of the respective pellets is polymerized in the solid state. In yet another embodiment, the intrinsic viscosity (IV) of the used polyester is reduced somewhat before solid state polymerization or mixing with the unused polyester precursor. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flowchart of a first embodiment of the recycling method of the present invention; and FIG. 2 is a schematic flowchart of a second embodiment of the recycling method of the present invention. DETAILED DESCRIPTION The present invention is a method for recycling polyester to obtain a sufficiently high purity recycled polyester that meets food packaging requirements. FIG. 1 shows a first embodiment of the present invention. Spent polyester bottles, indicated broadly at 10, are typically transported from a consumer recycle depot to a crushing plant, indicated schematically at wheel 11, where the bottles are typically turned into strips of used polyester. Is crushed into flakes 12. In this regard, since the bottles 10 are discarded in other consumer trash bins or in other similar environments, they generally have different amounts of surface contaminants to be removed in the rinsing step 13, That is, it is dirty. To wash the flakes, any suitable combination of solvents, surfactants, water, agitation, and other ingredients and techniques can be used, and in a preferred embodiment, the flakes are mixed with a basic solution (eg, Wash with a combination of sodium hydroxide, NaOH) and a detergent at a temperature between about 20 ° and 75 °. FIG. 1 also illustrates one of the advantages of the invention, wherein the washed flakes may be transported as flakes to different locations for further processing. Thus, the grinding and washing portions of the process can be performed at separate locations if desired or convenient. The invention can also, of course, be performed entirely in a single location. In the next step, the flakes are dried in a dryer 14 at a temperature sufficient to volatilize organics and water and do not alter the chemical properties of the crushed polyester flakes. Since the melting point of the polyester is generally about 250 ° C., the drying step is performed at a somewhat lower temperature, eg, 130 ° -200 ° C., to achieve drying while avoiding any chemical changes. Should. However, somewhat higher temperatures can be employed in a nitrogen (N ₂₎ in an inert atmosphere such as. The surface-cleaned spent polyester strip is then melted in a suitable heating device 15 and extruded using an extruder 16 into a molten phase. If desired, the step of extruding the melt further applies the melt in a vacuum to remove any additional volatile organics and water remaining after the drying step. Can consist of: The melt can also be filtered through a screen filter 17, for example, before or after extrusion. Extrusion is preferably carried out through a two-stage single screw extruder, which provides a simple and efficient extrusion process. Such extruders are well known to those skilled in the polymer and polyester arts and need not be discussed at length here. Cascade extrusion can also be used, where the polyester is melted in the first part of the extruder and then dropped into a volatile column, where it is blown with gas to further purify it, and then blown and melted The extruded polyester is extruded. In addition, extruders of a design other than a two-stage single screw extruder can be introduced if desired or necessary. In this regard, the extrusion process slightly reduces the molecular weight of the used polyester, i.e., from an intrinsic viscosity (IV) of between about 0.7 and 0.8 to a value of between about 0.6 and 0.7. Reduce to IV. Preferably, the IV of the spent melt is within about 0.1 of the IV of the virgin polyester prepolymer mixed in the next step. Although various special techniques are used to measure IV, it is understood that the present invention is not limited to any one of the special techniques as long as the measurements are made in a manner consistent with the others. Will be. In the next step of the invention, the melt of spent polyester is a relatively low IV (0.5-0.7, preferably 0.58-0.64) polyester generally used for subsequent solid state polymerization. Is mixed with a melt of the unused polyester precursor resin 20. In the mixing step, a static mixer is generally used and may contain fillers, both of which are conventional in the art and will not be discussed in detail here. Similarly, suitable formulations and techniques for producing virgin polyester are widely known in their industry and will not be discussed in detail here. The basic recipe is, of course, the condensation reaction between terephthalic acid and ethylene glycol, which is then polymerized to the desired molecular weight. The mixed melt is then solidified and pelletized to produce a plurality of mixed pellets 22, each pellet containing a mixture of virgin polyester and used polyester. In a preferred embodiment, a larger amount of virgin polymer and a smaller amount of recycled polymer are mixed, with a ratio of about 75% virgin polymer to 25% spent polymer being particularly preferred. The pellets 22 are then polymerized in the solid state ("solid phase") using a suitable solid state polymerization apparatus, as depicted more broadly at 23 in FIG. A variety of equipment and techniques can be used to perform solid state polymerization, which are generally well known in the art. Illustrative techniques include, for example, those described in U.S. Pat. Nos. 4,064,112, 4,154,920, 4,165,420, and 4,238,593. Can be As described in the specific examples below, the solid state polymerization process facilitates the further diffusion of volatile organic impurities from the pellets and conforms to polyester pellets that meet FDA guidelines for use in food packaging. Is obtained. FIG. 2 shows another embodiment of the present invention. Here, the steps and apparatus shown in FIG. 2 are the same as those in FIG. 1 unless otherwise described, and the same reference numerals in FIG. 2 are used in FIG. FIG. 2 again shows that the used bottles 10 are collected and transported to a crushing plant 11, where they are generally formed into flakes and washed. The flakes are again dried, melted, filtered, and extruded into pellets 25. The pellets 25 of FIG. 2, however, are similar in that the pellets 22 of FIG. 1 are each a mixture of unused polyester and used polyester, whereas the pellets 25 of FIG. 2 are all formed from only used polyester. , Is different from that of FIG. FIG. 2 shows that the steps of collecting, grinding, washing, drying, melting, extruding, and pelletizing all take place in a first factory, and then the spent pellets are transported to a second factory. In the embodiment of FIG. 2, spent pellets 25 are mixed with pellets 26 formed only from virgin polyester. This produces a mixture of pellets, each individual pellet being all used polyester or all unused polyester. This mixture of pellets is then subjected to a solid state polymerization reaction with the same specifications as described above to produce fully polymerized polyester pellets that also meet FDA food contact requirements. As in the first embodiment, a larger amount of virgin polyester pellets (eg, 75%) is mixed with preferably somewhat smaller (eg, 25%) used pellets and fed to the solid state polymerization reaction. A mixture is formed. Tables 1-4 illustrate the advantages of the present invention and its beneficial cumulative effect for removing contaminants and contaminant residues from used bottles. For example, Table 1 shows the decrease in the amount of toluene in untreated spent polyester by solid state polymerization. As shown in Table 1, if the flaked materials contain 1000 ppm toluene before washing, the washing step will not remove almost all of them. In the drying step, however, the initial toluene content of the sample is reduced from 1840 ppm to a 1.6 ppm toluene content, a reduction ratio of over 1000. Following the extrusion step, solid state polymerization (for different uses) reduced the amount of toluene, from a toluene amount of 5.85 ppm, 0.215 ppm by extrusion, and then by solid state polymerization, a reduction ratio of 27. The final amount of toluene obtained is 0.032 ppm, ie 32 ppb. Table 2 shows that phenyldecane can be greatly reduced by simple washing and drying steps, and Table 3 shows similar results from washing and drying calcium monomethyl arsenate. Table 4 shows the reduction of benzophenone using the present invention, and Table 5 shows the reduction of chloroform using the present invention. In each case, the reduction is cumulative from step to step, resulting in a high-purity end product. In another embodiment, the invention provides a method for cleaning the surface and reducing the intrinsic viscosity of the milled spent polyester to about 0% by adding a component selected from the group consisting of water, dihydric alcohols, and polyhydric alcohols. .2 to 0.3 IV units; melt surface purification used polyester strip; melt extrude used melt; melt used polyester with melt of unused polyester precursor Mixing; solidifying the mixed melt and pelletizing: and recycling the used polyester by polymerizing the solid mixed pellets in the solid state. The components are added to the melt of the used polyester or added to the crushed strips of the used polyester prior to the step of melting the used polyester. Indeed, under appropriate ambient conditions, the step of adding the components is sufficient to reduce the surface cleaned ground spent polyester to an IV between about 0.02-0.3 IV units under ambient humidity conditions. After storage for an appropriate period of time. In another embodiment, the step of adding the components comprises adding ethylene glycol, another suitable dihydric alcohol, or a polyhydric alcohol, as described above. Those polyhydric alcohols that are suitable for such use are described in Title 21 of the Code of Federal Regulations, especially in subchapter B, "Food for Human Consumption" and its chapters. Part 177, "Indirect Food Additives, Polymers; Substances for Use as Basic Components of Single and Repeated Use." Food Contact Surfaces "). For polyesters, suitable materials are described in Section 177.1630," Polyethylene phthalate polymers ". In another embodiment, the invention provides a method for cleaning the surface and reducing the intrinsic viscosity of the milled spent polyester to about 0% by adding a component selected from the group consisting of water, dihydric alcohols, and polyhydric alcohols. From 0.2 to 0.3 IV units; melt extruding the surface-cleaned spent polyester strip; and solidifying, pelletizing, and polymerizing the spent polyester in the solid state. Become. In both of their later embodiments, the reduction in IV helps to match the IV of the corresponding polyester precursor, together with the used polyester that can be mixed to improve product uniformity, And (whether mixed or not) provides an additional location in the solid state polymerization reaction step, which, as described above, contributes to further driving out any remaining organic material from the spent polyester. As with the previously described embodiments, the methods of these latter embodiments include a milling step, filtration of the used melt prior to its extrusion, washing of the milling strips to remove contaminants, washed milling. It also includes the steps of drying the strips and applying a vacuum to the extruded melt to help remove volatile organics and water. In the drawings and specification, while the preferred embodiments of the invention have been disclosed generally and certain terms have been used, they are used only in a generic and non-limiting sense and not for purposes of limitation, The scope of the invention is set forth in the following claims.

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Claims (1)

[Claims]   1. Recycled polyester for high enough purity to meet food packaging requirements A method of obtaining a recycled polyester, wherein the method comprises:   Extruding the melt of cleaned spent polyester;   Solidifying and pelletizing the spent melt;   Used polyester pellets unused polyester prepolymer pellets Mix with; and   A method comprising polymerizing a mixture of pellets in a solid state.   2. Further, grinding the used polyester material into strips; and   The process of cleaning the ground polyester strip to remove contaminants from the surface is common. In addition, it is performed before the step of melting the used polyester strip that has been cleaned The method according to claim 1, wherein:   3. About 75% by weight of unused polyester pellets and about 25% by weight of used 2. The method according to claim 1, wherein polyester pellets are mixed. Cycle method.   4. The step of mixing the pellets involves the use of unused and unused pellets. Use a pellet whose intrinsic viscosity is lower than the intrinsic viscosity of used pellets. 2. The recycling method according to claim 1, wherein the mixing is performed by mixing.   5. The difference between the intrinsic viscosity of unused pellets and the intrinsic viscosity of used pellets is about 0 . The recycling method according to claim 4, wherein the number is 1 or less.   6. Recycled polyester for high enough purity to meet food packaging requirements A method of obtaining a recycled polyester, wherein the method comprises:   Clean the melt of used polyester with unused polyester prepolymer Mixed with the melt of-   Solidifying and pelletizing the mixed melt; and   A method comprising polymerizing a mixed melt in a solid state.   7. Further, the used polyester material is crushed into small pieces;   Cleaning the crushed polyester strip to remove contaminants from the surface; and   Melting the cleaned used polyester strip; and   The step of extruding the molten used polyester, All used, cleaned polyester melt and unused polyester pre-po The method according to claim 6, wherein the step is performed before the step of mixing the immersion melt. How to recycle.   8. The process of cleaning the crushed strips involves cleaning the crushed strips and contaminating the surface Remove the debris and remove the washed crushed strips to remove volatiles and organic matter. Dry at a temperature that does not change the chemical properties of the crushed strips The recycling method according to claim 2 or 7, comprising:   9. The step of mixing the pellets involves the use of unused and unused pellets. Use a pellet whose intrinsic viscosity is lower than the intrinsic viscosity of used pellets. 7. The recycling method according to claim 6, wherein the mixing is carried out.   10. The difference between the intrinsic viscosity of unused pellets and the intrinsic viscosity of used pellets is approximately 10. The recycling method according to claim 9, which is 0.1 or less.   11. Recycled polyester to make it sufficiently pure to meet food packaging requirements A method of obtaining a degree of recycled polyester, the method comprising:   Clean the surface and reduce the intrinsic viscosity of the crushed used polyester to water, divalent By adding a component selected from the group consisting of coal and polyhydric alcohol , Reduced between about 0.02-0.3 IV units;   Melt extruding the surface-cleaned used polyester strip; and   Solidify, pelletize, and polymerize the used polyester in the solid state A method comprising:   12. Further, the step of filtering the used melt is performed before the step of extruding the melt. The method according to claim 2, 7, or 11, wherein: Icle way.   13. The process of extruding the used polyester melt is a two-stage single screw -Claim 1 characterized in that the polyester is extruded through an extruder, Item 12. The recycling method according to Item 7 or 11.   14． In addition, the melt of the cleaned used polyester is Solidifying, pelletizing, and solid phase mixing The method according to claim 11, wherein the method is performed before the step of polymerizing.   15. About 75% by weight of unused polyester melt and about 25% by weight of used The polyester melt according to claim 6 or 11, wherein the polyester melt is mixed. The recycling method described in the section.   16. The step of adding the component comprises adding the component to the melt of the used polyester. 12. The method of claim 11, comprising:   17． The process of adding the component is a process of cleaning the surface of the used polyester It is performed before the step of adding to the pieces and melting the used polyester The method according to claim 11, wherein:   18. In the process of adding the components, the surface clean crushed used polyester is Sufficient to reduce its IV between about 0.02-0.3 IV units under conditions 18. A method as claimed in claim 17 comprising adding water after storage for an extended period of time. .   19. The step of adding the ingredient comprises adding ethylene glycol. 12. The method of claim 11 wherein the method comprises:   20. Further, the used polyester material is crushed into small pieces;   Washing the ground polyester strips to remove surface contaminants; and   The washed pulverized strip was sufficiently pulverized to remove organic matter and water and pulverized. Drying at a temperature that does not change the chemical properties of the strip; and   Filtering the melt of used polyester, 12. The method according to claim 11, wherein the step is performed before the step of extruding the melt. Recycling method.   21. Furthermore, following the solid-state polymerization process, a food package is formed from polyester. 12. The recycling method according to claim 1, comprising the step of forming Method.   22. Furthermore, dicarboxylic acids, dicarboxylic esters, glycols and polyfunctional Forming an unused polyester prepolymer from the group consisting of Claims consisting of performing before the step of mixing spent and unused melts. Item 15. The recycling method according to Item 1, 6 or 14.   23. The step of extruding the used polyester melt is further extruded. Claims comprising vacuuming the melt to remove volatile organic substances and water. Item 12. The recycling method according to Item 1, 7 or 11.   24. In addition, terephthalic acid and ethylene glycol The step of forming the repolymer is the step of mixing the used melt with the unused melt. 15. The recycling method according to claim 1, 6 or 14, which is performed before.   25. The method according to claim 1, 6, 11 or 14. Formed polyester.   26. The recycling according to claim 1, 6, 11, or 14. Polyester soft drink bottles formed by the polyester method.