WO2005121230A1

WO2005121230A1 - Process for decontamintaion of recycled polyester (pet) flakes

Info

Publication number: WO2005121230A1
Application number: PCT/BR2005/000117
Authority: WO
Inventors: Sati Manrich; Amélia Severino FERREIRA E SANTOS; José Augusto MARCONDES AGNELLI
Original assignee: Universidade Federal de Sao Carlos; Fundacao de Amparo a Pesquisa do Estado de Sao Paulo FAPESP
Current assignee: Universidade Federal de Sao Carlos; Fundacao de Amparo a Pesquisa do Estado de Sao Paulo FAPESP
Priority date: 2004-06-14
Filing date: 2005-06-14
Publication date: 2005-12-22
Anticipated expiration: 2006-12-14
Also published as: BRPI0402330B1; BRPI0402330A

Abstract

A process for the removal of contaminants from clean PET flakes utilizing an atmosphere of dry air at 120-200°C/3-30m3h/dew point between -20 to 60C) for at least 15 minutes and inert gas at 200-250 C/0.1-30m3/h for at least 10 minutes, followed by the conventional extrusion step in extruder (5), granulation in a granulator (6) and crystallization. The polymer is processed in reactor (4) for obtaining a recycled resin having a purity and physical chemical features that make it suitable to be applied in monolayer food packages made of 100°/0 recycled PET. The variable time, temperature and flow of air or gas are important for the viscosity control, optical characteristics and decontamination degree of the polymer. Purity standards set forth for the protection of the consumer as well as of the organoleptic properties of the bottled product meet the requirements of FDA, ILSI and ANVISA. The use of the granulated product is also described.

Description

PROCESS FOR DECONTAMINATION OF RECYCLED POLYESTER (PET) FLAKES

FIELD OF THE INVENTION The present invention relates to a process for decontamination during the recycling of poly (ethylene terephthalate - PET), such process making use of dry air and an inert gas for purifying and increasing the molar mass of the recyclate. BACKGROUND INFORMATION The drawbacks linked to the disposal of solid urban waste stem from the proper placement of said wastes, the physical space required by same and the development of diseases attaining nearby population making a living based on the trading of said waste products. In developed countries, chiefly in Europe, such problems are mostly related to the physical space for locating the wastes. The packaging sector is a target for solid waste management policies, since it can by itself represent nearly 30% by weight of the total plastics utilization according to market figures in the USA and Brazil, as cited in the articles by Duchin, F.; Lange, G. M. „Prospects for the recycling of plastics in the United States". Structural Change and Economic Dynamics, n.9, p. 307-331 , 1998, and Forlin, F. J.; Faria, J. A. F. "Consideracόes sobre a reciclagem de embalagens plasticas". Polimeros: Ciencia e Tecnologia, vol. 12, p. 1-10, 2002. In Europe, this sector represents nearly 20% by weight of the total mass of domestic solid residua as per the article by Onusseit, H. "Sustainable development during production and use of tape."/? 1^st AFERA Technical Seminar, 2002, Dϋsseldorf. Proceedings, the weight fraction of plastic packages being around 16%. In spite of the fact that the PET packages represent only nearly 3% of the overall plastics market, that is, 10% of the plastics market directed to the packaging sector, this is a highly specialized market, being basically applied to the sector of carbonated beverages, as already cited in the article by Duchin and Lange. In view of these conditions, European countries and the USA have provided legislation relative to the goals to be attained in terms of recovery and recycling of overall wastes. The expansion of the recycled plastic market through new technologies committed to market cost and application as well as flexibility and suitability to market fluctuations (production and demand) are paramount in the attainment of the plastics recycling goals. The plastics recycling aiming at the production of food-grade products has been globally forbidden until the nineties. Nowadays, due to the fact that this is a marketing domain to be exploited, this activity has become one of the main challenges of the sector. In the case of Brazil, the pressure exerted by international companies, the common Mercosul interest as well as the need to aid in the expansion of the recycling indices have made possible a breach in the Brazilian legislation directed to recycling industries that want to exert their activities in our country aiming at food-grade products. The opening of such a market in Brazil assures the value enhancement of the final product and therefore, a possibility of higher profits resulting from the recycling activities. Research work carried out in the National Center for Food Safety and Technology of the FDA according to an article by Bayer, F. L. „The threshold of regulation and its application to indirect food additive contaminants in recycled plastics". Food Additives and Contaminants, v. 14, n. 6-7, p. 661- 670, 1997, has shown that washing with caustic soda can remove only from 30% to 89% of the contaminants present in PET. This lack of efficiency is linked to limitations of the conventional aqueous medium in removing hydrophobic contaminants and compounds absorbed in the polymeric matrix, chiefly the non polar compounds, this being due to the polar nature of the cleaning medium. To this respect see the articles by Welle, F; Franz, R. .Analytical Quality Assurance for PET-Recycling". Maack Business Services, Session XII/3, 1-8, 1999 and Komolprasert, V.; Lawson, A. "Effects of aqueous-based washing on removal of hydrocarbons from recycled polyethylene terephthalate (PET)". In: Annual Technical Conference, 52., 1994. Proceedings, Society of Plastics Engineers, 1994, p. 2906- 2908. During the drying step, the removal of contaminants is restricted to volatile compounds, no matter the level and penetration depth of same, as cited by Pierce, D. E.; Pfeffer, R. L.; Sadler, G. D.- "Rutherford backscattering analysis of contaminants in PET". Nuclear Instruments and

Methods in Physics Research B, v. 124, p. 575-578, 1997. By combining washing and drying it is possible to remove approximately 99% of the organic contaminants. However, such removal level is considered insufficient for food-grade applications. In spite of the high temperatures involved, the extrusion step also involves limitations, caused by the closed structure of the extruder and the normally short residence time, as explained by Pierce, D. E.; King, D. B.; Sadler, G. D. Analysis of Contaminants in recycled Poly (ethylene terephthalate) by thermal Extraction Gas Chromatography - Mass Spectroscopy. In: American Chemical Society. Plastics, rubber, and paper recycling. 1995, cap. 37, p. 458-471. So, in order to make viable the use of recycled PET in the food market a number of other approaches, employing heat, vacuum, gas flow, solvents, surface chemical treatments and super critical fluid extraction (SFE), have been submitted to experimental tests and/or commercial practice. Processes encompassing the highest range of available technologies are decontamination processes involving solid state polymerization (SSP) and its various modifications. The need to adequate the intrinsic viscosity (IV) of the recycled pellets, at affordable costs, to conventional blow molding processes for sure adds to this tendency. Besides, these processes are considered "clean" technologies. Technology producers are large companies, as can be seen from the publications listed below. Thus, US patent 6,436,322 teaches a method for recycling PET flakes where the flakes are extruded and granulated under vacuum, after which the granulate is after-condensed in a solid phase under vacuum. Extrusion is preferably carried out in differentiated vent zones and after-condensation of the solid phase is dependent on temperature, vacuum and time spent in a tumble dryer. US patent 5,876,644 teaches a method of recycling post-consumer polyester to obtain recycled polyester of sufficiently high purity to meet food packaging requirements. The method includes cleaning comminuted pieces of post-consumer polyester to remove surface contaminants; melting the surface-cleaned post-consumer polyester pieces; extruding the post- consumer melt; blending the melt of post-consumer polyester with a melt of virgin polyester prepolymer; solidifying and pelletizing the blended melt while the virgin polyester prepolymer remains as prepolymer; and polymerizing the solid blended pellets in the solid state. Published JP 2002011719 teaches a method for recycling a PET (polyethylene terephthalate) bottle capable of manufacturing an excellent recycled PET bottle not substantially containing volatile impurities and solid foreign matter, the method comprising a cleaning step of the flake obtained by grinding the PET bottle by using an aqueous solution of a metal hydroxide, a solid state polymerization step of continuously solid state polymerizing the cleaned flake, a pelletizing step of melting and granulating the solid state polymerized flake by using an extruder having a devolatilizing means and a filtering means, and a bottle molding step for molding the PET bottle from the obtained pellets. U.S. patent 6,103,774 relates to a process for removing contaminants, which comprises removing contaminants from a contaminant-containing polyester material at a temperature of at least 150°C in the presence of an equilibrium amount of a polyester reversible side-product. U.S. patent 5,899,392 teaches the decontamination of RPET flakes through such steps as comminuting the RPET flakes to yield PET particles and removing contaminants from the PET particles through heating, passage of a gas on the particles or immersion in a solution. As can be seen, the distinction among these processes is based on the sequence of steps, equipment design, kind of atmosphere employed (vacuum or inert gas), particle size and on the admixture with virgin stock. The efficiency of the above-cited processes is situated around 99%. Aiming at being more competitive, usually those processes are limited to only the established limits of specific migration (10 ppb), taking profit of the good barrier properties shown by PET and attaining removal levels lower than 215 ppb. In this kind of process, the most challenging class of contaminants is considered as the non-volatile and polar (such as for example, benzophenone) compounds. Broadly, the "super-clean" processes are processes for extracting contaminants from the recycled PET. Thus, the knowledge of thermodynamic and transport properties between the phases is of paramount importance. PVT (pressure x volume x temperature) diagrams, as well as data for solubility, viscosity, diffusion coefficient and surface tension among others, are parameters that have been considered in developing research in this field. Presently developed processes attain the required purity levels, however, usually process durations are relatively long, chiefly if one considers the high initial contamination levels of the raw material. This is a hypothesis assumed to allow the recycling of food-grade PET, including the recycling of packages originally made for other uses. Fan et al. in Fan, G.; Maio, L.; Incarnato, L.; Scarfato, P.; Acierno, D.

"The relative significance of biaxial stretch ratio effects on the permeability of oriented PET film". Packaging Technology and Science, v. 13, p. 123- 132, 2000, state that the diffusion features of oxygen are similar to those of carbon dioxide. Due to its characteristics of interaction and diffusivity in PET this latter has been used in super-clean processes. According to Vieth, W. R. "Diffusion in and through polymers - principles and applications". New York: Oxford Univ. Press, 1991 , the co- diffusion between the contaminants and the oxygen from the atmospheric air on the polymer matrix makes possible to improve the transfer rate of contaminants, inclusive at higher rates than those observed under vacuum or other inert gas of lower molar mass. Differences in PET transport properties thermally treated in vacuum and in air have been previously identified in the literature by Ouyang e Shore "The mass transport in poly (ethylene terephthalate) and related induced-crystallization". Polymer, v. 40, p. 5401-5406, 1999. On the other hand, the utilization of atmospheric air in contact with PET at high temperatures is limited by the thermo-oxidative degradation and the resin yellowing. The presence of moisture causes hydrolysis and an accelerating character in the PET yellowing as stated by Santos et al. in Santos, A. S. F.; Medeiros, E. S.; Agnelli, J. A. M.; Manrich, S. "The role of relative air humidity in PET yellowing". In: World Polymer Congress, 2004, Paris, Proceedings Paris: IUPAC, 2004. However, by using humidity conditions and suitable temperatures it is possible to decontaminate PET under moderate temperatures in this atmosphere without causing losses in its structural and optical properties. Thus, in spite of the improvements in the state-of-the-art technique, there is still the need of a process for the decontamination of post-consumer poly (ethylene terephthalate) - PET flakes able to optimize the process duration and reduce operation costs and maintenance by using dry air under temperature, dew point and flow conditions suitable to attain per se the purity levels required by recycled PET to be used as a food-grade material, such process being described and claimed in the present application. Since the increase in the recyclate molar mass is usually also required, it can be obtained by including a second sequential step using inert gas under suitable temperature and flow conditions. Due to its extractive features, in case it is implemented, this step should become complementary to the overall decontamination process. SUMMARY OF THE INVENTION Broadly, the process for decontamination of PET post consumer flakes according to the invention comprises: a) in a decontamination reactor, submitting said clean PET post-consumer flakes to a flow of dry air, at a flow rate from 3 to 30 m³/h, under temperatures between 120°C and 200°C, at a dew point between -20°C to -60°C during at least 15 minutes, for crystallizing, dehydrating and decontaminating said flakes, while is kept as such the molar mass of same; b) in the same reactor, submitting the PET flakes to temperatures between 200°C and 250°C and inert gas flow between 0.1 to 30 m³/h, to effect solid state post condensation (SSP) of the flakes during at least 10 minutes, obtaining decontaminated flakes suitable for extrusion; c) directing to an extruder the so decontaminated PET flakes suitable for the manufacture of bottles; d) in that extruder, extruding under extrusion conditions, the PET flakes, obtaining an extrudate; e) directing the so-obtained extrudate towards a granulator and recovering the bottle-grade PET granules ready for molding. Alternatively, the PET flakes and its copolymers are decontaminated using only the flow of dry air of step b). Still alternatively, in the extrusion e) step a chain extending agent is employed in order to increase the molar mass of PET. Thus the invention provides a process for decontamination of previously cleaned PET flakes by contacting said flakes with dry air and an inert gas under moderate temperatures, followed by extrusion, so as to recover granules that are suitable for molding bottles using 100% recycled

PET. The invention provides still flakes, granulates or threads, useful in applications such as food-grade packaging, threads, pipes and high- strength plates, medical equipment and accessories, syringes and containers in general. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 attached is a flow sheet representative of the process according to the invention. FIGURE 2 attached is a graph showing the exponential decrease in the benzophenone concentration in PET at different atmospheres under identical temperature and nitrogen and dry air flow conditions. DETAILED DESCRIPTION OF THE PREFERRED MODES The PET flake useful for the invention is food-grade and non food- grade wastes. The post-consumer flake is cleaned using conventional cleaning processes, such as by contacting with an aqueous solution of non- ionic surface agent in a basic or neutral medium. The PET flake should contain a fraction of poly (vinyl chloride - PVC) lower than 5 ppm as cited by Scott, D.M. "A two-colour near-infrared sensor for sorting recycled plastic waste". Measurement Science Technology, v. 6, p.156-159, 1995. Under these conditions, the flake is submitted to a flow of dry air at a flow rate between 3 and 30 m³/h, preferably 10 m³/h, under temperatures between 120-200°C for at least 15 minutes. At this step the PET flakes are crystallized, dehydrated, decontaminated and have their molar mass preserved. As is well known by the experts, desiccants are used for drying atmospheric air for obtaining a dew point between -20°C and -60°C. Since the increment in the molar mass of PET is desirable so that the bottle injection is carried out at maximum output, the present invention suggests a further step of solid state polymerization in order to increase the molar mass of the PET flakes, since the intrinsic viscosity of the flakes is normally lower than (0.80+0.03) dL.g^"1. Thus, the following step of the inventive process, which involves the use of temperatures between 200°C and 250°C and flow of inert gas of the order of 0.1 to 30 m³/h (solid state polymerization conditions or SSP) makes possible to adequate the intrinsic viscosity of flakes according to the desired values within a minimum period of 10-15 minutes. Since the costs consequent to such step are relatively larger, its function is to recover the original resin properties and complete the contaminant removal of the dry airflow step. Any inert gas is adequate, such as for example, argon, nitrogen, etc. Therefore in the present invention the PET decontamination is carried out basically by using dry air under moderate temperatures, this being the time-determining process step. Both steps (with dry air and with inert gas) are carried out in the same device and under a fluidized bed, by altering the inlet ports for air or inert gas flow. As required or desired, other kind of devices can be used provided is secured the uniform contact of the gas or air with the flake surface. As an example, rotary drums, fixed-bed reactors or tumbled tanks can be used. In fixed-bed reactors, for example, considerations related to L/D ratio are important for avoiding preferential flow of gas or air against the walls. In order that the said device secures the proper processing of the flakes it should be coupled to an extruder, which is the next step for obtaining the bottle-grade resin useful for food-grade applications. In order to secure the retention of metal particles, upstream of the extruder feed hopper a metal scavenger (not represented) device should be installed. An efficient filtration zone (not represented) is also required for retaining possible residual solid contaminants. Later on the obtained granules are commercialized preferably already crystallized and for this another device heated at a temperature between 100°C and 150°C should be employed for at least 10 seconds. The connection with the extruder can be effected by controlled suction as a function of the residence time of the resin in the decontamination reactor or by other flow control devices. The extrusion equipment can be any, such as a single screw extruder having no degassing zone or a double, segmented-screw provided with a degassing zone, etc, the choice being directed towards the most convenient device. Also according to the invention and in order to adequate the intrinsic viscosity of the obtained resin to the levels required for bottle-grade PET (0.80±0.03) dL.g^"1, it is admitted to use a chain extending agent in the extrusion step. And still according to the invention, alternatively the d), e) and f) process steps related to the extrusion step are dispensed with, and causing no harm to the overall decontamination process. Specifically for the present invention, the determining variables of the desorption process are: temperature, dew point and outer medium concentration (flow of air/gas). In the inventive process, the gas or air distribution is uniform. Still, the inlet of previously heated dry air or gas is carried out by convection or through an independent conduction heating system of the flake. Contaminant features such as diffusivity, volatility and partition are also of paramount importance for the process adjustment, according to Pierce, D. E.; King, D. B.; Sadler, G. D. Analysis of Contaminants in recycled Poly (ethylene terephthalate) by thermal Extraction Gas Chromatography - Mass Spectroscopy. In: American Chemical Society. Plastics, rubber, and paper recycling. 1995, cap. 37, p. 458-471. A higher diffusivity of the penetrant was observed for the samples treated in air. A scheme representing the inventive process is shown in Figure 1. In this Figure are represented a source of dry air (1 ) and a source of inert gas (2), preferably nitrogen, both gases previously heated by any conventional heating system (not represented), the so-heated gases being directed to the decontamination reactor (4) via lines L1 and L2 respectively. It is equally possible to recycle the dry air to source (1 ) through line L1 '. The heating system (not represented) using resistances can be optional provided the temperature control and preservation are efficiently designed. The inlet of recycled material that has previously been cleaned with the aid of pump (3) through line L3 makes possible to operate the process in the continuous mode instead of in the batch mode. After the pre-determined residence time in each of the designed atmospheres, the decontaminated material having the adequate intrinsic viscosity is transferred to extruder (5), processed as granules and then directed via Line L4 to equipment (6) to be pre-crystallized. The obtained granules attend to the food-grade purity standards and can also be employed in applications of high mechanical specification. Alternatively, instead of granules, high-strength threads can be obtained. Figure 2 illustrates the higher efficiency in the removal of contaminants from the dry air atmosphere relative to the nitrogen atmosphere under similar flow and temperature conditions. The evaluation carried out under vacuum has also been carried out under the same temperature conditions. Such evaluation was based on symmetrical PET squares contaminated with 10%m/v toluene, 1% m/v benzophenone, 1% m/v eicosane and 1 % m/v trichlorethane. The tests carried on such previously contaminated flakes with toluene, benzophenone and eicosane have also indicated the ability of the present process in removing contaminants of the order of 1 ,000 ppm. Residual contents are of the order of ppm units, which are reduced to levels below 10 ppb in the food simulant, this meeting the limits for food plastic packages directed to consumer protection as required by the regulatory agencies. The present process constitutes a promising approach in terms of productivity, since it is able to profit from the diffusivity and atmospheric air interaction with PET. The oxygen contained in atmospheric air, approximately 20% by weight, has a certain interaction degree with PET and simultaneously a high diffusivity, making easier the removal of contaminants from the polymer matrix. It is therefore demonstrated that the process of the present invention is able to prepare recycled PET for mono layer packages containing 100% of recycled resin meeting the FDA, ILSI and Brazilian National Agency for Sanitary Control (ANVISA) requirements. Besides, the technical features of the injection-designed resin are also secured even if flakes of 0.60 dL.g^"1 initial intrinsic viscosity are used. The described technology is then innovative, competitive and environmentally friendly. The granulated or thread-like product obtained from the present process is useful for various applications such as food-grade packages, threads, pipes and high-strength plates, medical equipment and accessories, syringes and containers in general.

Claims

CLAIMS: 1. A process for decontamination of recycled polyester, wherein such process comprises the following steps: a) Providing post-consumer, clean PET flakes b) In a decontamination reactor (4), submitting said clean PET flakes to a dry air flow from (1 ) via Line 1 , at a flow rate from 3 to 30 m³/h, under temperatures between 120°C and 200°C, at a dew point between -20 to -60°C for at least 15 minutes, in order to crystallize, dehydrate and decontaminate such flakes, while maintaining as such the molar mass of same; c) In the same reactor (4), submitting said PET flakes at temperatures between 200°C and 250°C and flow of inert gas from (2) via line L2, at a flow rate of 0.1 to 30 m³/h, to effect post condensation of the flakes during at least 10 minutes, obtaining decontaminated flakes suitable for extrusion; d) directing to an extruder (5) the so-decontaminated PET flakes, suitable for the manufacturing of bottles; e) extruding in extruder (5), under extrusion conditions, the PET flakes, obtaining an extrudate; f) directing the so-obtained extrudate to a granulator (6) via Line L4 and recovering bottle-grade PET granules ready for molding.

2. A process according to claim 1 , wherein the PET flake is from food- grade PET.

3. A process according to claim 1 , wherein the PET flake is from a PET different from food-grade PET.

4. A process according to claim 1 , wherein alternatively the post- condensation step c) is omitted.

5. A process according to claims 1 , 2, 3 or 4, wherein in the extrusion step e) in case it is required to recover the molar mass of the recycled PET a chain extending agent is employed.

6. A process according to claims 1 , 2, 3 or 4, wherein alternatively, steps d), e) and f) are removed without causing harm to the overall decontamination process. 7. A process according to claims 1 , 2, 3, 4, 5 or 6, wherein said process is in the batch mode. 8. A process according to claims 1 , 2, 3, 4, 5 or 6, wherein said process is continuous, with the flakes being pumped towards reactor (4) with the aid of a pump (3) via line L3. 9. A process according to claims 1 , 2, 3, 5 or 6, wherein the inert gas is nitrogen.

10. A process according to claims 1 , 2, 3, 4, 5 or 6, wherein the decontamination reactor (4) operates with a fluidized bed, a fixed bed, rotating drums or tumbled tanks.

11. A process according to claims 1 , 2, 3, 4, 5 or 6, wherein the flake is previously cleaned with the aid of any known process.

12. A process according to claims 1 , 2, 3, 4, 5 or 6, wherein upstream of the extruder (5) hopper, a metal scavenger secures the retention of metal particles.

13. A process according to claim 12, wherein extruder (5) is equipped with a filtration zone designed to retain any residual solid contaminants.

14. A process according to claim 13, wherein extruder (5) is a single- screw extruder and without degassing zone, or any other up-to-date equipment comprising segmented double-screw including a degassing zone and the like.

15. A process according to claims 1 , 2, 3, 4 or 5, wherein additionally said granules are crystallized by heating between 100°C and 150°C for at least 10 seconds.

16. A process according to claims 1 , 2, 3, 4 or 5, wherein alternatively instead of granules, high-strength threads are obtained.

7. Use of the flakes, granules and threads obtained in the process according to one of the claims 1 , 2, 3, 4, 5 or 6 in mono layer food packages containing 100% of recycled resin, in medical equipment or accessories, syringes, threads, pipes, high-strength plates and containers in general.