WO2006044154A1

WO2006044154A1 - Method for treating recycled polyethylene terephthalate for rapid decontamination and solid-stating

Info

Publication number: WO2006044154A1
Application number: PCT/US2005/035031
Authority: WO
Inventors: Donald W. Hayward
Original assignee: Phoenix Technologies International, Llc
Priority date: 2004-10-18
Filing date: 2005-09-30
Publication date: 2006-04-27

Abstract

A process for treating recycled polyethylene terephthalate comprises providing a quantity of RPET flakes containing contaminants, melting the RPET flakes, and extruding the RPET flakes, to form fibers, into an atmosphere that causes the outward diffusion of the contaminants from the RPET fibers and causes an increase of the intrinsic viscosity of the RPET.

Description

TITLE

METHOD FOR TREATING RECYCLED POLYETHYLENE TEREPHTHALATE FOR RAPH) DECONTAMINATION AND

SOLΠ)-STATΓNG

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This Application claims the benefit of US Provisional Patent Application Serial No. 60/619,859, filed on October 18, 2004.

FIELD OF THE INVENTION

[0002] This invention relates generally to a process for treating recycled polyethylene terephthalate (RPET) for rapid decontamination and solid-stating. More particularly, the invention is directed to a process for treating contaminated RPET₅ to cause the rapid outward diffusion of trapped contaminants and an increase the intrinsic viscosity.

BACKGROUND OF THE INVENTION

[0003] Polyethylene terephthalate resin is widely used for containers, such as carbonated soft drink (CSD) bottles and other packages. Post-consumer recycled polyethylene terephthalate is widely processed into useful products. This recycling process begins when used PET containers, such as carbonated beverage bottles, are collected, sorted, washed, and separated to yield a clean, mostly pure source of RPET. This RPET is most frequently in "ground" or flake form, and is either melt processed by an end user in that form, or is further converted by palletizing and/or solid-stating.

[0004] Regardless of the specific method or end use, the RPET material is always subjected to a grinding operation in order to make the material easier to handle and process. Conventional grinding machinery for RPET reduces the RPET to 3/8 inch flakes. RPET grinding equipment ensures that a consistent flake size will be produced by employing a grate or screen through which the ground material must pass in order to leave the grinder. This screen has openings of a specific dimension, e.g., 3/8 inch, and thus acts to produce RPET flake with a predominate dimension equal to the size of the screen. Screens having 3/8 inch apertures are most commonly employed in the RPET industry, although some processors use screen dimensions as large as ¹A inch, or as small as ¹A inch. [0005] All equipment made for processing RPET, including grinders, material handling and conveying equipment, dryers, crystallizers, and blowers, are designed to be used with RPET flake of about 3/8 inch size. In addition, most extruders and extruder screws are designed to work best with flakes of this dimension. The bulk density of 3/8 inch flake RPET is typically 22 to 35 pounds per cubic foot.

[0006] Similarly, RPET and virgin PET in pellet form are almost always of a standard physical size, resulting in a bulk density for the resin of 50 to 58 pounds per cubic foot; which equates to less than 48 pellets per gram, or a spherical pellet of about 0.120 inch diameter. Essentially all PET processing equipment which employ pellets as the feedstock are designed for pellets of these dimensions. [0007] A critical aspect of processing RPET, and key to achieving a consistently high-quality end product, is the comprehensive decontamination of the RPET flakes. Although much decontamination occurs during the washing and sorting processes, it is known that "clean" RPET flakes can still contain residual contaminants in concentrations as high as 4%. These contaminants are predominately label and basecup glues, polyolefms, PVC, paper, glass, moisture, and metals; all of which negatively affect the quality and performance of the finished product. Of recent concern are the infrequent toxic contaminates which may be introduced into the recycle stream. Examples of these contaminates are pesticides, solvents, herbicides, and chlorinated hydrocarbons, which could contaminate RPET through incidental contact, or by the recycling of a PET container which was used by the consumer to hold a toxic substance for some period of time. These sources of contamination, although rare, are nevertheless appropriately of great concern to those who would incorporate RPET into containers for food-contact use. With regard to this possibility, the FDA has set protocols for the levels of such contamination in food-contact applications, and has established surrogates and concentration limits to establish the effectiveness of the decontamination methods. [0008] PET packaging, and CSD bottles in particular, are very sensitive to the IV (intrinsic viscosity) of the PET. The IV of CSD bottle-grade PET must be of a certain value, or the physical properties of the bottles will suffer. A typical IV range for bottle-grade PET is from about 0.72 to about 0.84 dg/L. [0009] In order to achieve the proper IV for virgin PET resin, the manufacturer will solid-state the resin. Solid-stating is a process by which PET resin in pellet form (the solid state, as opposed to the melted state) is subjected to a high temperature in the absence of oxygen. Because PET is a poly-condensation polymer, this solid-stating will, over time, build the mean molecular weight of the resin though tranesterification, resulting in an increase in the measured IV. This solid-stating process can be carried out on any virgin or recycled PET resin. Solid-stating is essential in the manufacture of PET resins for many applications, including packaging, engineering resins, strapping, sheet manufacture, and others. [0010] The drawback to these processes are that both decontamination and solid-stating are very time-consuming, and therefore the costs to manufacture resin employing these processes, either separately or concurrently, are high. [0011] It would be desirable to treat RPET, in a manner that would effect rapid decontamination and increase its intrinsic viscosity.

SUMMARY OF THE INVENTION

[0012] Accordant with the present invention, a process for treating RPET for its rapid decontamination and solid-stating, has surprisingly been discovered. The process comprises providing a quantity of RPET flakes containing contaminants, melting the RPET flakes, and extruding the RPET flakes, to form fibers, into an atmosphere that causes the outward diffusion of the contaminants from the RPET fibers and causes an increase of the intrinsic viscosity of the RPET. [0013] The inventive process is particularly useful for treating RPET for subsequent use in the preparation of food-grade and other containers.

DETAILED DESCRIPTION OF THE PREFERRED

EMBODIMENT

[0014] The present invention is directed to a process for treating RPET, comprising providing a quantity of RPET flakes containing contaminants, melting the RPET flakes, and extruding the RPET flakes, to form fibers, into an atmosphere that causes the outward diffusion of the contaminants from the RPET fibers and causes an increase of the intrinsic viscosity of the RPET. [0015] By the term "RPET flakes" as it is used herein is meant generally the commercially available recycled polyethylene terephthalate materials produced by conventional PET recycling methods, usually in flake form, but which may additionally be in the form of chunks, spheres, pellets, and the like, and which are generally made available in bulk in a substantially uniform particle size from about VA inch to about ¹A inch.

[0016] According to the present invention, a quantity of RPET flakes containing contaminants is provided for further processing. The quantity of RPET flakes provided in the initial step of the inventive process may easily be determined by a routineer in the art of polymer processing, depending upon the quantity of polymer ultimately desired for further processing. [0017] The RPET flakes are melted and extruded into fibers by conventional methods that are well-known in the polymer processing art. The fibers are extruded into a chamber containing an atmosphere capable of causing the outward diffusion of the contaminants, from the interiors of the RPET fibers to the surfaces of the RPET fibers; thence into the surrounding atmosphere. [0018] Simultaneously, the intrinsic viscosity of the RPET fibers will rise, due to the atmosphere and physical characteristics of the RPET fibers. The atmosphere will be described generally hereinafter as an "inert atmosphere." [0019] The fibers are placed in the inert atmosphere at a temperature ranging from about 130 degrees to about 205 degrees C₅ preferably continuously and immediately subsequent to the extrusion process. The surface to volume ratio of the individual RPET strand cross-sections are several hundred to several thousand times greater than the surface to volume ratio of conventional RPET pellets or flakes, thereby rapidly decontaminating the RPET and reducing the time required to solid-state the resin to a desired endpoint.

[0020] Intrinsic viscosity build during solid-stating is limited by the diffusivity of water and other volatiles from the solid resin. Because this diffusivity is determined in large part by the surface to volume ratio of the resin physical configuration, the rate of the reaction can be increased by increasing the surface to volume ration of the RPET. Since diffusion will take place much more rapidly at high surface to volume rations, the resultant intrinsic viscosity building will be correspondingly more rapid.

[0021] As will be readily apparent to one ordinarily skilled in the art, the diameters of the extruded RPET fibers may vary over wide limits, generally dictated by the desired efficacy of the decontamination and solid-stating processes. Preferably, the mean strand diameter of the RPET fibers is less than or equal to about 300 microns.

[0022] The inert atmosphere into which the PRET fibers are extruded may be any atmosphere conducive to both the decontamination and solid-stating of the RPET fibers. The atmosphere may comprise a gas such as nitrogen or argon, at a temperature of about 150 degrees C or greater. Alternatively, the atmosphere may comprise air at a temperature of about 135 degrees C or greater. Yet another alternative atmosphere may comprise a vacuum at a temperature of about 150 degrees C or greater.

[0023] The resultant decontaminated, solid-stated RPET is beneficial for use alone, or for combining with other polymer materials for subsequent melt processing and forming operations. For example, the addition of the RPET produced by the inventive process to a quantity of virgin polyethylene terephthalate would extend the volume of the virgin PET. [0024] If the original contaminated RPET flakes were used for subsequent processing, the resulting melt would be considered unusable or of low quality. The inventive process, however, converts this erstwhile useless RPET material into a higher grade of RPET, which may even be useful for the manufacture of food-grade containers.

[0025] The inventive process for treating RPET containing contaminants described hereinabove is generally disclosed in terms of its broadest application to the practice of the present invention. Occasionally, the process conditions as described may not be precisely applicable to each RPET/contaminant/IV combination included within the disclosed scope. Those instances where this occurs, however, will be readily recognized by those ordinarily skilled in the art. In all such cases, the process may be successfully performed by conventional modifications to the disclosed method.

[0026] The invention is more easily comprehended by reference to specific embodiments recited hereinabove which are representative of the invention. It must be understood, however, that the specific embodiments are provided only for the purpose of illustration, and that the invention may be practiced otherwise than as specifically illustrated without departing from its spirit and scope.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A process for treating RPET, comprising: providing a quantity of RPET flakes containing contaminants; melting the RPET flakes; extruding the RPET flakes into an atmosphere, to form fibers; and causing the outward diffusion of the contaminants from the RPET fibers and an increase of the intrinsic viscosity of the RPET.

2. The process for treating RPET according to Claim 1, wherein the RPET flakes are melted at a temperature ranging from about 130 degrees C. to about 205 degrees C.

3. The process for treating RPET according to Claim 1, wherein the RPET flakes are extruded at a temperature ranging from about 130 degrees C. to about 205 degrees C.

4. The process for treating RPET according to Claim 1, wherein the mean strand diameter of the extruded RPET fibers is less than or equal to about 300 microns.

5. The process for treating RPET according to Claim 1, wherein the atmosphere comprises nitrogen or argon.

6. The process for treating RPET according to Claim 5, wherein the temperature of the atmosphere is greater than or equal to about 150 degrees C.

7. The process for treating RPET according to Claim 1, wherein the atmosphere comprises air.

8. The process for treating RPET according to Claim 7, wherein the temperature of the atmosphere is greater than or equal to about 135 degrees C.

9. The process for treating RPET according to Claim I₅ wherein the atmosphere comprises a vacuum.

10. The process for treating RPET according to Claim 9, wherein the temperature of the atmosphere is greater than or equal to about 150 degrees C.

11. A process for treating RPET, comprising: providing a quantity of RPET flakes containing contaminants; melting the RPET flakes at a temperature ranging from about 130 degrees C. to about 205 degrees C; extruding the RPET flakes at a temperature ranging from about 130 degrees C. to about 205 degrees C. into an atmosphere comprising nitrogen, argon, air, or a vacuum having a temperature greater than or equal to about 135 degrees C, to form fibers; and causing the outward diffusion of the contaminants from the RPET fibers and an increase of the intrinsic viscosity of the RPET.

12. The process for treating RPET according to Claim 11 , wherein the mean strand diameter of the extruded RPET fibers is less than or equal to about 300 microns.

13. The process for treating RPET according to Claim 11, wherein the temperature of the atmosphere is greater than or equal to about 150 degrees C.