Glycolysis process for recycling of post-consumer pet

Details Glycolysis process for recycling of post-consumer pet Glycolysis process for recycling of post-consumer pet

2000-01-05

2002-06-25

Cain, Edward J. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Process of treating scrap or waste product containing solid...

C521S048000

Reexamination Certificate

active

06410607

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a novel process for recycling post-consumer polyester. In particular, the process of the present invention involves depolymerization of the post-consumer polyester by way of glycolysis.
BACKGROUND
Polyesters are used to form a wide variety of articles. In order to conserve resources and reduce the amount of pollution resulting from discarded polyester articles, it has become important to recycle discarded polyester articles. Polyester may be recycled by depolymerizing high molecular weight polyester into monomers and oligomers that are then repolymerized to an appropriate molecular weight for formation into new articles. Since discarded polyester may be contaminated with foreign materials, early polyester recycling efforts were geared toward producing relatively low-grade recycled products that do not require high purity polyester. An example of such low-grade product commonly made from recycled polyester is the “fiber fill” used in comforters, coats, pillows, and such. Polyester recycling technology has recently developed to include depolymerization and purification of discarded polyester waste to produce recycled polyester having high purity. High purity recycled polyester is required for sensitive uses such as producing containers for food use.
Recyclable polyester is recovered from two sources, manufacturing waste and polyester articles which have been used and discarded. The latter category is termed “post-consumer waste” Post-consumer polyester waste may contain a variety of foreign materials in addition to materials typically present in a polyester through polycondensation. Examples of such foreign materials include polyesters with different compositions, catalyst metals, colorants, other polymers such as polyvinyl chloride and polyethylene, aluminum, sand, paper, glue, and chemicals or residues absorbed from anything stored in the container. If post-consumer polyester is to be recycled for sensitive applications such as food containers, beverage containers, medical devices, and the like, it is critical that such contaminants are removed from the polyester.
A method of recycling high molecular weight polyester, especially polyethylene terephthalate (“PET”), involves depolymerizing ground or crushed flakes of polyester via glycolysis. This process includes contacting the high molecular weight polyester with a glycol such as ethylene glycol to produce oligomers and/or monomers of the polyester. These materials are subsequently repolymerized as part of the preparation of new polyester articles. In the glycolysis of PET, the scrap PET is reacted with ethylene glycol, thus producing bis-(2-hydroxyethyl) terephthalate (“BHET”) and/or its oligomers. Glycolysis is an especially useful reaction for depolymerizing PET due to the fact that the BHET produced can be used as a raw material for both dimethyl terephthalate (“DMT”) based and terephthalic acid (“TPA”)-based PET production processes without major modification of the production facility. Glycolysis for depolymerizing polyester scrap recovered during various points in the manufacture of polyester articles is described in U.S. Pat. Nos. 3,884,850 and 4,609,680, which are incorporated herein by reference.
U.S. Pat. No. 5,223,544 discloses a process whereby the foreign material present in post-consumer PET is removed by a process of first depolymerizing the polyester in a reactor via glycolysis to provide a mixture of PET oligomers, monomers, and various immiscible contaminants. The reaction mixture is then fed to an unstirred separation device whereby the contaminants are allowed to migrate away from the polyester on the basis of density, thereby forming an upper layer of low density contaminants, a middle layer of polyester material, and a lower layer of high density contaminants. The middle polyester layer is thereafter separated from the contaminants by being removed from the separation device through a draw-off pipe.
Unfortunately, the process of U.S. Pat. No. 5,223,544 requires a bulky separation device. Such devices are undesirable. Additionally, the steps of feeding the reaction mixture to a separate separation vessel and allowing the unstirred mixture to separate requires an unacceptably long residence time. Such long residence time allows for the deleterious formation of an unacceptable amount of diethylene glycol. Diethylene glycol is undesirable since it promotes the production of unwanted copolymer in the recycled PET, and a lengthier process is inherently more inefficient and expensive.
Accordingly, there is a need for a method which will permit the expedient recovery of high quality polyester or its oligomers from waste and post-consumer materials. The present process overcomes these obstacles by providing a simpler and acceptable way for removing contaminants via glycolysis depolymerization of postconsumer polyester, and recovering high quality polyester monomers and/or oligomers acceptable for production of sensitive use articles.
SUMMARY OF THE INVENTION
The polyester depolymerization and purification process of the present invention includes the steps of contacting a contaminated polyester with an effective amount of a glycol to provide a molar ratio of greater than about 1 to about 5 total glycol units to total dicarboxylic acid units at a temperature between about 150 to about 300° C. and an absolute pressure of about 0.5 to about 3 bars. This process is conducted in an agitated reactor vessel for a time sufficient to produce, in the reactor, an upper layer having a relatively low density contaminant floating above a lower layer of a depolymerized oligomer of the polyester. The process further includes separating the layers from each other by removing the upper layer from the reactor in a first stream and the second layer from the reactor in a second stream.
DETAILED DESCRIPTION
The process of the present invention is a simpler and less expensive way of effectively removing contaminants from post-consumer polyester. Surprisingly, it was discovered that separation of contaminants from polyester could be achieved under agitated glycolysis conditions, and that the lower density contaminants form a distinct upper layer in the reactor vessel that can be readily separated from the lower layer containing the remainder of the glycolysis reaction mixture. It was further found that after separating the lower density contaminants from the remainder of the glycolysis reaction mixture, any immiscible contaminants which may have a relatively high density can be effectively removed by filtration or straining. This simple process requires less residence time in the vessel, fewer vessels and reduced costs, thereby producing less unwanted by-products.
The depolymerization and purification process of the present invention comprises contacting a contaminated polyester with an amount of a glycol to provide a molar ratio of greater than about 1 to about 5 total glycol units to total dicarboxylic acid units at a temperature between about 150 to about 300° C. and an absolute pressure of about 0.5 to about 3 bars in an agitated reactor vessel for a time sufficient to form, in the reactor, an upper layer of a relatively low density contaminant floating above a lower layer comprising a depolymerized oligomer of the polyester. As used herein, the term “oligomer” includes both monomers and oligomers of polyester. The present process further includes the step of separating the upper and lower layers by removing the upper layer from the reactor in a first stream and removing the lower layer from the reactor in a second stream. Advantageously, the entire reaction mixture does not need to be transferred to a dedicated separation vessel.
The process of the present invention is useful for recycling polyester. The present process is especially beneficial in recycling contaminated polyester which is used in packaging, preferably food packaging. Such polyesters are generally known in the art and may be formed from aromatic dicarboxylic acids, esters of aromatic dicarboxylic acids, gl

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