MXPA97005960A - Lixiviation of waste polyester contaminants to be reuse in applications where contact with food exists - Google Patents

Abstract

A novel technique is described to treat polyester discarded by consumers, to reuse it in applications in which it comes into contact with food. The process comprises contacting polyester flakes discarded by contaminated consumers with at least one extraction solvent that is soluble in said pollutants, at a temperature sufficient to allow the rapid diffusion of the contaminants from the polyester flakes, and removal. of said extraction solvent containing said contaminants from polystyrene flakes

Description

LIXIVIATION OF POLLUTION POLLUTANTS POLLUTANTS TO BE REUSE IN APPLICATIONS WHERE IT EXISTS CONTACT WITH FOOD FIELD OF THE INVENTION The present invention relates to the field of the recycling of materials and, more particularly, refers to a process whereby contaminants are removed from packaging materials based on polyesters, for the reuse of said materials in applications that they come into contact with food.

Background of the Invention Concern in the environment has provided an impetus in the use of food containers that are at least partially composed of recycled waste materials. The glass and metal containers have been recycled for a long time, cleaning the surface of these materials almost impermeable. Plastics, on the other hand, could absorb contaminants, making it risky to rely on conventional cleaning techniques. The Food and Drug Administration (FDA) deals with the recycling of plastics in the brochure "Points to Consider for the Use of Recycled Plastics in Food Packaging: Chemistry Considerations" (Points to Consider in the Use of Recycled Plastics in Food Packaging : Chemical Considerations). An upper limit of 0.5 ppb of any contaminant in the daily diet is considered to be a negligible risk. An example is presented for a 20 ml polyethylene terephthalate (PET) vessel with very conservative considerations; The result is that a contaminant must not be present in the polymer in an amount greater than 0.217 ppm.

For polyesters, one technique for removing contaminants is the chemical treatment to enable the waste material to return to its monomeric components, which can be purified, subsequently re-polymerized to the original polyester that can be formed into a recycled container. This process, although it is strong in the removal of contaminants, can be more expensive than the production of virgin polyester. It is therefore desirable to develop a process that is not expensive to remove contaminants from plastic packaging used by the consumer. This invention discloses a process that meets FDA guidelines in an economical way. The Patents of the U.S.A. No. 5,049,647, "Method for the Reduction of Impurities in Polyester Resins, "and No. 5,073,203," Method for Recycling Bottles of Beverages, based on Polyethylene Terephthalate (PET) by Treatment with Carbon Dioxide, "describe a technique to extract contaminants from PET with supercritical carbon dioxide , a solvent that is well known for its excellent mass transfer properties, supercritical C02, however, is an expensive solvent that requires expensive equipment due to the high pressures that are required, and is unlikely to be viable US Pat. No. 4,680,060, "Process for Extraction of Contaminants from Plastics," describes a technique for removing contaminants, such as pesticides, from plastic containers by washing with propylene glycol. eliminate the pollutants absorbed on the surface of the plastic, the pollutants absorbed inside the plastic are not affected. This technique, therefore, can not satisfy current FDA guidelines. Various inventions (e.g., U.S. Patent No. 3,806,316, "Process for Extracting Textile Dyes," patent of the U.S. No. 4,003,880 and No. 4,118,187"Desorption of Fabric Dyes: Separation and Recovery of Polyester" describe, as part of their processes, steps to desorb dyes and / or polyester fiber finishes by contact with a solvent. The solvents used are chlorinated, particularly methylene chloride, or aromatic, therefore they would not be desirable in applications that come into contact with food. Therefore, there remains a need in the art for an extraction process that is capable of eliminating contaminants at minimum levels with solvents that are desirable in applications that come into contact with food, as well as being capable of being practiced on a commercial scale. .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT This invention describes a new process for removing contaminants from polyester-based packaging materials, discarded by the consumer; The material thus treated would be acceptable by the FDA for applications where said material comes into contact with food and meets the specifications required for further processing. The process comprises contacting polyester flakes discarded by the consumer and which are contaminated, with at least one extraction solvent that is soluble in said flake contaminated at a temperature sufficient to allow rapid diffusion of the contaminants from said polyester flake; and the removal of said extraction solvent containing said contaminants from said polyester flake. Polyesters are any polyesters that are suitable for use in packaging and, particularly, in food packaging. Suitable polyesters are generally known in the art and can be formed from aromatic dicarboxylic acids, esters of aromatic dicarboxylic acids, glycol and mixtures thereof. More preferably, the polyesters are formed from terephthalic acid, isophthalic acid, dimethylterephthalate, dimethylisophthalate, ethylene glycol, diethylene glycol, cyclohexanedimethanol, and mixtures thereof. In this process, the polyester flake discarded by the consumer is fed into a leaching system where it is placed in contact with an extraction solvent. It has been found that with an appropriate solvent and temperature, the contaminants rapidly diffuse unexpectedly through the polymer and reach equilibrium with the solvent. It is very important that the diffusivity of the contaminants be large, because the force of diffusion impulsion and concentration difference, becomes very small as the levels of contaminants required by the FDA are reached. Due to the rapid diffusion that occurs in the process described in this invention, contaminants can be eliminated on a commercially viable time scale. To eliminate contaminants to a permitted level, a system with multiple stages with flow of solvent to counter-current and transverse current, such as a multi-stage system, continuous and counter-current or a multi-contact system, batch already counter-current would be preferred. Preferred solvents or solvent mixtures have a number of characteristics. The solvent must be soluble in the polyester, resulting in swelling of the polymer, which facilitates rapid mass transfer; however, the solvent must not dissolve or significantly degrade the polyester or lead to color formation. Preferably, the solvent must eliminate a wide range of contaminants, both polar and non-polar. It is also desirable that the solvent be easily removed from the polyester after extraction. The solvent must be environmentally benign and must not have harmful effects on human health. Finally, it is preferable that the solvent be cheap. Preferred solvents include esters, ketones, alcohols, glycols and triglycerides of fatty acid, mixtures thereof and mixtures of the foregoing with water. More preferably the solvents are esters, ketones and triglycerides of fatty acid. More preferably they are acetone, butanone, ethyl acetate, n-propyl acetate, corn oil and My glycol 812, available from Huis America, Inc. Preferred temperatures are those at which there is sufficient free volume in the polyester / solvent system for allow the rapid diffusion of contaminant molecules; the presence of the preferred solvents plasticizes the polyester, greatly increasing the free volume relative to that of the solvent-free polymer. This has the effect of lowering the glass transition temperature. In this way, the temperature is preferably above the glass transition temperature for the solvent mixture of the polymer. For acetone and ethyl acetate, for example, the preferred temperature range for cleaning poly (ethylene terephthalate) is above 80 ° C, more preferably 80 to 125 ° C. Of course, if the vapor pressure of the solvent is above atmospheric pressure at the leaching temperature, a vessel operating above atmospheric pressure is required. The optimum extraction time per stage depends on the solvent and the temperature. For ethyl acetate at 120 ° C, equilibrium is reached in less than 30 minutes. After the leaching stage, it is important to restore the polyester to an appropriate product for re-use. This includes removal of the solvent from the flake and restoration of the polyester to an appropriate molecular weight and shape. After leaching, the flake is separated from the liquid solvent, then dried to remove most of any residual solvent. The contaminants can be removed from the recovered leaching solvent so that it can be reused. Next, the flake could be extruded and pelletized, preferably in a vented extruder so that the residual solvent or polluting vapors are easily removed. If necessary, the resulting pellets can then be crystallized and brought to the solid state, resulting in a product acceptable for use in applications where it comes in contact with food. Solid state polycondensation processes are well known in the art, as illustrated in U.S. Pat. No. 4,064,112. The polyester is heated to above the glass transition temperature but below the melting point of the glass; the byproducts of the polycondensation are removed by inert gas sweeping on the polyester or by applying vacuum. The extrusion, crystallization and solidification steps all result in the removal of any remaining solvent; In addition, if minor degradation occurs during leaching and extrusion, the molecular weight of the polymer is restored to an appropriate level. The product of this process is 100% recycled polyester and suitable for use in products that come into contact with food, such as bottles. Products that incorporate recycled polyester could contain up to 100% recycled content. Alternatively, after drying to remove the solvent, the flake could be fed into a molten phase polyester production line. Such processes are well known in the art, and include the esterification of dicarboxylic acid (s) or the transesterification of esters of dicarboxylic acid (s) with diol (s) followed by polycondensation under reduced pressure. The decontaminated polyester could be fed at any desired point in the process. Here, the leaflet would undergo partial depolymerization before growing to an adequate molecular weight. In this process, the recycled material could be mixed with raw and virgin materials to produce a material with any desired level of recycled content.

Comparative Example Polyester flakes contaminated with 388 ppm lindane (determined by Soxhiet extraction) were subjected to a conventional cleaning technique. Lindane is a non-polar and non-volatile solute that represents a contaminant of the type that is difficult to eliminate. An 8% suspension of the stained leaflet in a wash solution consisting of water with 1% Oakite RC3® detergent was stirred for 30 minutes at 82 ° C. After this washing, the leaflet was dissolved in trifluoroacetic acid. The contaminant was extracted from the acid with n-nonane, and analyzed by gas chromatography. It was determined that the level of lindane in the leaflet was 237 ppm. The level of residual lindane is 1000 times higher than that usually required by the FDA.

EXAMPLES The following examples show that the method of the present invention is capable of removing contaminants up to 500 times better than conventional cleaning techniques. Typically, polyester collected for recycling contains less than 2 ppm of any given contaminant and under any possible circumstances would not be likely to contain more than 20 ppm of any single contaminant such as lindane, toluene and methyl salicylate. The polyesters tested below contained 187-546 ppm of lindane, 29,500-49,100 ppm of toluene and 136,000 of methylene salicylate. These extraordinary concentrations of contaminants were required to show the degrees of removal that can be achieved with the process of the present invention because the detection limits in the analytical equipment used is 0.4 ppm.

Example 1 To 200 g of polyester bottle flakes contaminated with 546 ppm of lindane, 400 g of ethyl acetate was added in an autoclave. The bottle flakes in this Example and in Examples 2 and 4 were PET resin modified with 1.5 mol% cyclohexanedimethanol. The mixture was heated, with stirring, to 250 ° F (121 ° C) where it was kept for two hours. After cooling, the solvent was filtered from the flakes and the flakes were washed and dried overnight in a vacuum oven at 140 ° F (60 ° C). The dried leaflet was dissolved in trifluoroacetic acid. The contaminant was extracted from the acid with n-nonane and analyzed. The flake product contained 61 ppm of lindane, indicating that 89% of lindane was eliminated in only 2 hours in a single cycle. As shown by the following Examples, appropriate levels can be achieved by adding the appropriate number of extractions. Example 2 of analysis was also carried out using trifluoroacetic acid and n-nonane.

Example 2 Flakes from a polyester bottle (200 g) contaminated with 388 ppm lindane (analyzed by Soxhiet extraction) were mixed with 400 g of ethyl acetate in an autoclave at 250 ° F (121 ° C). After two hours, the level of lindane in the leaflets was 52 ppm. Longer extraction times did not eliminate additional lindane from the leaflets. The mixture was cooled and the contaminated solvent was filtered. The flakes were washed, dried and mixed with 400 g of fresh ethyl acetate solvent and placed in an autoclave at 250 ° F. After four hours, the lindane in the leaflets was reduced to 1.0 ppm. The solvent was filtered, the flakes were washed and 400 g of fresh ethyl acetate were added. The mixture was placed in an autoclave at 250 ° F for four hours. A sample of flakes was cryogenically milled into fine particles and mixed with a large excess of acetone. The mixture was kept overnight, then the acetone was analyzed to determine lindane. The level of lindane in the flakes was 0.43 ppm, which is almost 50 times better than that of the conventional cleaning technique described in the comparative example.

Example 3 A thermal desorption unit attached to a gas chromatograph was used to determine the amount of toluene present in the flake samples. Two hundred grams of polyester flakes were contaminated with 49, 100 ppm of toluene, a volatile, non-polar contaminant. The flakes were mixed with 400 g of ethyl acetate in an autoclave and heated to 250 ° F for one hour. After removing the liquid, 400 g of fresh ethyl acetate was added to the flakes and another extraction was made. The process was repeated until four extractions were completed. After the treatment, the flakes contained 1.57 ppm of toluene, a removal of 99.997% of the toluene. It is expected that the PET resin flakes discarded by the consumer would contain much lower levels of toluene than that used for this experiment, so that the removal of 99.997% would make the PET resin safe. Clearly more extractions could be made to obtain higher removal efficiencies if necessary.

Example 4 Two hundred grams of polyester flakes were contaminated with 136,000 ppm of methyl salicylate, a volatile, polar contaminant, which is often found in PET resin discarded by the consumer. The flakes were mixed with 400 g of ethyl acetate in an autoclave and heated to 250 ° F for one hour. After removing the liquid, 400 g of fresh ethyl acetate was added to the flakes and another extraction was made. The process was repeated until four extractions were completed. After treatment, the flakes contained 25.4 ppm of methyl salicylate, a 99.98% removal of methyl salicylate. This demonstrates that excellent removal efficiencies can be obtained with a polar compound. Clearly more extractions could be made to obtain higher removal efficiencies.

Example 5 Two hundred grams of polyester flakes made of PET resin modified with 3% isophthalic acid were contaminated with 187 ppm lindane. After being mixed with 400 g of ethyl acetate, it was heated at 250 ° F in an autoclave for one hour. The ethyl acetate was removed and the lindane was extracted from the flakes as described in Example 1. The polymer contained 20.5 ppm of lindane after only a single extraction. Thus, contaminants can be easily removed from the polyester containing isophthalic acid through the present invention.

Example 6 to a sample of 16 g of PET resin flakes containing 28,500 ppm of toluene were placed in 61 g of My glycol 812 oil and heated at 121 ° C for 2 hours. The oil was decanted from the leaflets and the leaflets were trampled softly and as dry as possible between filter paper. The flakes were placed in a Soxhiet apparatus to remove the oil, refluxed for 24 hours with ethyl acetate and the ethyl acetate was removed from the system. Methylene chloride was added and the sample was extracted for 24 hours, concentrated to 25 ml and analyzed by GC for toluene. The amount of toluene remaining in the flakes was determined to be 4037 ppm (86% removal) after a simple 2 hour extraction. Continuous or batch extractions could be performed to obtain acceptable levels.

Claims (15)

1. Novelty of the Invention 1. A process comprising: contacting polyester flakes discarded by a consumer, contaminated, with at least one extraction solvent that is soluble in said flakes contaminated at a temperature sufficient to allow the rapid diffusion of contaminants from said polyester flakes; and separating said extraction solvent containing said contaminants from said polyester flakes.
2. 2. The process of claim 1, wherein said temperature is above the glass transition temperature for the polymer solvent mixture.
3. 3. The process of claim 1, wherein said contact and removal steps are repeated until the amount of said residual contaminant in said polymer is less than 220 ppb.
4. 4. The process of claim 1, wherein said contacting step is carried out in a countercurrent or transverse current system.
5. 5. The process of claim 4, wherein said system is a multistage, countercurrent and continuous system, or a multistage, countercurrent and batch system.
6. 6. The process of claim 1, wherein said extraction solvent is selected from the group consisting of ketones, alcohols, glycols, fatty acid triglycerides, mixtures thereof and mixtures with water.
7. 7. The process of claim 6, wherein said extraction solvent is selected from the group consisting of acetone, butanone, ethyl acetate, n-propyl acetate, corn oil and Miglycol 812.
8. 8. The process of claim 1, wherein said polyester is polyethylene terephthalate and said temperature is greater than 80 ° C.
9. 9. The process of claim 1, further comprising extruding and pelletizing said decontaminated polyester flakes.
10. 10. The process of claim 9, further comprising crystallizing said decontaminated polyester pellets.
11. 11. The process of claim 9, further comprising solidifying said decontaminated polyester pellets to increase the molecular weight of said decontaminated polyester pellets.
12. 12. The process of claim 9, wherein said extrusion step is carried out in a vented extruder wherein the residual solvent and / or polycondensation byproducts are thereby removed by increasing the molecular weight of said polyester flakes to a desired level. .
13. 13. The process of claim 1, further comprising the step of feeding decontaminated polyester flakes to a melt phase production of polyester material.
14. 14. The process of claim 1, wherein said polyester is formed from components selected from the group consisting of aromatic dicarboxylic acids, esters of aromatic dicarboxylic acids, glycol, and mixtures thereof.
15. 15. The process of claim 13, wherein said polyester is selected from the group consisting of terephthalic acid, isophthalic acid, dimethyl terephthalate, dimethyl isophthalate, ethylene glycol, diethylene glycol, dimethanol cyclohexane and mixtures thereof.