Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-10-02
2004-04-13
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C560S078000, C562S480000, C562S485000, C562S486000, C562S487000, C568S858000
Reexamination Certificate
active
06720448
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of recovering chemical species by depolymerization of poly(ethylene terephthalate) from a material containing poly(ethylene terephthalate). In particular, the present invention relates to a method of obtaining an aqueous solution comprising chemical species from depolymerization of poly(ethylene terephthalate), for instance in the form of bottles or other manufactured articles coming from a differentiated salvage dump.
In addition, the present invention relates to use of said chemical species from depolymerization of poly(ethylene terephthalate) in the processes for producing regenerated poly(ethylene terephthalate).
2. Prior Art
Known in the art is the existence of some chemical methods involving recovery of terephthalic acid or fragments of polymers of the poly(ethylene terephthalate) type starting from materials containing poly(ethylene terephthalate).
Poly(ethylene terephthalate) also identified by the abbreviation (PET) is a saturated polyester resin obtained from terephthalic acid (TPA) and ethylene glycol (EG). PET is a material widely used in the textile field and in the food-industry packaging sector and in particular in the manufacture of bottles for soft drinks.
Due to its large use, an increasingly growing interest in poly(ethylene terephthalate) (PET) recycling has been developed in the most recent years.
Substantially chemical methods of recovering useful substances for preparing poly(ethylene terephthalate), such as the terephthalic acid or fragments of polymers of the poly(ethylene terephthalate) (PET) type are distinguishable from each other due to the mechanism used which can belong to one of the following main categories: alcoholysis, glycolysis, acid hydrolysis, neutral hydrolysis and alkaline hydrolysis.
Obviously, each individual mechanism can find application through various techniques differing from each other due, for example, to a different number of phases, different temperatures, pressures, involved reagents, reaction solvents.
The chemical method utilizing alcoholysis brings to formation of bis or poly(alkyl)terephthalates, whereas glycolysis produces bis or poly(hydroxyethyl)terephthalates that are chemical intermediates in the production of poly(ethylene terephthalate) by known processes of trans-esterification.
The chemical method utilizing acid hydrolysis is carried out by making PET react with a large excess of a strong acid in solution, sulfuric acid for example.
The sulfuric acid acts in a very short period of time, say some minutes, at a temperature included between room temperature and 95° C., by dissolving the starting PET with formation of terephthalic acid (TPA). The chemical method utilizing acid hydrolysis is scarcely applied in the industrial field, mainly due to the high corrosiveness of the reaction system and also due to the huge amount of salt solution produced for neutralizing the employed acid.
The chemical method utilizing neutral hydrolysis is conducted by treating PET with water or steam, under pressure at a temperature included between 200 and 300° C., in the presence of appropriate catalysts. This method too has some drawbacks. The main disadvantages of this technology are represented by high energy consumptions and the impossibility of eliminating all mechanical impurities from the terephthalic acid (TPA) obtained by precipitation, such as undissolved particles and insoluble polymers originally present in the starting material.
Finally, the chemical method utilizing alkaline hydrolysis is almost always carried out by use of alkaline hydroxides or ammonium hydroxides.
Use of these bases leads to formation of aqueous solutions of the corresponding salt of the terephthalic acid (TPA). These solutions can be easily cleared from mechanical impurities by filtering, flocculating or settling processes. In addition, terephthalic acid (TPA) is recovered from said aqueous solutions by precipitation in an acid medium.
Of all the above mentioned methods, the method utilizing alkaline hydrolysis has recently found many applications. Some of them are reproduced hereinafter:
a first application contemplates treatment of PET with a concentrated solution of an alkaline hydroxide, under pressure and at temperatures close to or higher than 250° C. The PET/alkaline solution ratio is greater than 20.
a second application contemplates treatment of PET with a stoichiometric amount of an alkaline hydroxide in ethylene glycol (EG) at a temperature included between 100 and 200° C. If an ammonium hydroxide is used as the base, the method is carried out under pressure. In both cases, at all events, the obtained reaction mixture is dissolved in water.
Finally, a third application provides for PET to be extruded in the presence of hydroxide at temperatures higher than 250° C. Subsequently, the obtained salt is dissolved in an aqueous solution.
Generally problems resulting from use of alkaline or alkaline-earth hydroxides or from use of concentrated solutions of such hydroxides are well known. By alkaline hydroxides it is meant a sodium hydroxide for example, by alkaline-earth hydroxide it is for example meant a calcium hydroxide.
The main disadvantages are connected with difficulties in manipulating these types of very aggressive reagents by the operators. In addition, modifications in the plants are required to be adopted together with a series of technical expedients in plant construction due to the big problems connected with corrosion of these reagents, above all if used in solution.
In the above first application, disadvantages consist in being obliged to heat, filter and recover great amounts of solution. In addition, in this application use of great amounts of hydroxides and adoption of high pressures is provided.
In the above second and third applications, the main disadvantage resides in the fact that it is impossible to obtain terephthalic acid (TPA) free from undesirable yellow-pink colorations. In fact, in the absence of water the hydroxides employed at temperatures higher than 100° C. react with the ethylene glycol (EG) resulting from the hydrolysis reaction. The reaction between hydroxides and ethylene glycol brings to formation of strongly red-colored and water-soluble products. Formation of these colored compounds inhibits precipitation of white TPA or of uncontaminated chemical species from depolymerization of the poly(ethylene terephthalate). Therefore, the terephthalic acid (TPA) containing colored impurities must be bleached. The methods reproduced in literature for TPA bleaching for example are long and expensive. For instance, some bleaching methods involve extraction of impurities by use of water-insoluble higher alcohols.
Therefore there is a need for a method of recovering substances useful in the production of regenerated poly(ethylene terephthalate) from a material containing poly(ethylene terephthalate) coming for example from a differentiated salvage dump. These substances can be terephthalic acid or chemical species from depolymerization of poly(ethylene terephthalate) having a reduced molecular weight as compared with the poly(ethylene terephthalate) polymer present in the starting material.
In particular, a method is required which enables recovery of uncontaminated terephthalic acid or chemical species in the form of polymeric fragments of the poly(ethylene terephthalate) type, free from colored contaminations and impurities.
Still more particularly, there is a need for a method of recovering terephthalic acid or polymeric fragments of poly(ethylene terephthalate) having a reduced molecular weight by a process that does not involve use of alkaline hydroxides, alkaline-earth hydroxides and ammonium hydroxide or concentrated solutions of these hydroxides.
In addition, there is a requirement for a method of recovering terephthalic acid or polymeric fragments of poly(ethylene terephthalate) having a reduced molecular weight as compared with the poly(ethylene terephthalate) polymer present in the starting material, which is cheap
Barnes & Thornburg
Conte James B.
Oh Taylor Victor
Rotman Alan L.
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