Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Process of treating scrap or waste product containing solid...
Reexamination Certificate
2000-09-01
2003-02-11
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
06518322
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an improved method for the conversion of contaminated polyethylene terephthalate (PET) scrap to decontaminated polybutylene terephthalate (PBT) giving ethanediol as a valuable by-product.
BACKGROUND OF THE INVENTION
PBT is one of the polyesters which is well used in industry as a stable strong thermoplastic with good blow moulding characteristics. There have been numerous developments in the industrial preparation of PBT. The initial problems revolved around the fact that 1,4-butanediol in the presence of acid will cyclise to form tetrahydrofuran which is a stable compound thus rendering the 1,4-butanediol unavailable for reaction, This side reaction affected the cost effectiveness of the process as 1,4-butanediol is an expensive reagent. This problem was initially solved with the discovery that PBT can be manufactured by transesterification of dimethyl terephthalate with 1,4-butanediol. The use of an ester as a source of the terephthalate component reduces the concentration of protons and consequently the quantity of 1,4-butanediol converted to tetrahydrofuran.
However, a significant amount of tetrahydrofuran is still produced and it is desirable to reduce this further.
The next series of developments focussed on fine tuning the transesterification of terephthalate esters with 1,4-butanediol.
In U.S. Pat. No. 3,859,257 a process is taught for producing a polyester polymer from 1,4-butanediol and a lower alkyl ester of a dicarboxylic acid which comprises the steps of:
(a) a catalytic precondensation of 1,4-butanediol and a lower alkyl ester of a dicarboxylic acid in a ratio of 1.05-1.25:1 at a temperature between 180-200° C. in the presence of an organotitanate catalyst while distilling off 1,4-butanediol and cleavage products; and
(b) polycondensation of the product of (a) at a temperature between 250-310° C. in a vacuum in the presence of a polycondensation catalyst (eg, zinc acetate dihydrate).
This was an attempt to overcome the disadvantages of the prior art including reducing the excess of 1,4-butanediol used, the amount of catalyst used and problems with discolouration and reducing the amount of industrially worthless polyesters with cross-linked portions.
In U.S. Pat. No. 4,499,261 a continuous multi-step process is taught for making PBT by transesterifying dimethyl terephthalate with an excess of 1,4-butanediol in the presence of a titanium catalyst. This patent addressed the problem of equipment becoming plugged by the freezing of esters of low solubility. It teaches the use of feeding recycled 1,4-butanediol into the condensers to dissolve the esters. It also discloses the use of mole ratios, temperatures, pressures, residence times and quantity of catalyst to reduce the production of tetrahydrofuran thus enabling the 1,4-butanediol to be recycled without any purification steps.
In U.S. Pat. No. 4,212,963, the continuous production under high pressure conditions of PBT having a narrow molecular weight distribution, a low carboxyl group content and an intrinsic viscosity of from 0.2 to 1.34 dl/g is taught. This is achieved by carrying out the polycondensation in a two-phase flow tube in which thin layers of the molten reaction mixture are produced by means of a circular stream so that high polycondensation velocities and hence short residence times can be achieved. The methods involve catalysts in both the transesterification and polycondensation steps according to the prior art. This patent further relates to the apparatus used for the production of PBT from dimethyl terephthalate or terephthalic acid and 1,4-butanediol.
On another front, due to the amount of polyesters produced these days and the problems with waste disposal, there is now a focus on methods of recycling or reusing these polyesters, especially PET which is most commonly seen as containers including soft drink bottles.
There has been extensive work in the area of recycling PET. It has been found that it is best to degrade the polymer into the original monomers and then react the monomers together to regenerate the original PET (for example British patent 601,135). It is also known from British patent 610,136 that non commercially used PET will transesterify with ethanediol but the process does not allow for removal of contaminants which are present in post-consumer PET.
The contaminants in post-consumer PET include paper, pigments, other plastics (for example, polyvinylchloride), colouring dyes, mineral sands and clay. Chemical contaminants include hydrocarbons, polyamides, proteins, pesticides, sugar, and citric acid. Further, the United States Food and Drug Administration requires that any process which may produce products which are to come into contact with food, must be able to remove the following contaminants: toluene, lindane, arsenic, chloroform and diazone.
U.S. Pat. No. 5,266,601 (corresponds to German patent 4220473) teaches that PET scrap can be (i) depolymerised with 1,4-butanediol and a catalyst, (ii) transesterified with 1,4-butanediol and a catalyst and then (iii) repolymerised with catalyst to form PBT. This process does not allow for decontamination of post-consumer PET and therefore has a very limited feedstock. The specification expressly states that the process is designed for colourless PET and that pigmented PET should only be used if the resultant PBT is itself to be pigmented in a dark colour. The effects of pigment on the reaction process is to be remedied merely by varying the amount of catalyst and/or residence time. Accordingly, this process is unsuitable for dealing with post-consumer PET.
U.S. Pat. No. 5,451,611 discloses a method for converting PET waste to a poly(alkylene)terephthalate, in particular PBT. This citation does discuss the need for the PET waste to be thoroughly washed with water, cleaned to remove all foreign matter and dried to a moisture content of less than 0.5% before being shredded and pulverised to a powder before it can be used. However, this method does not deal with removal of contaminants which are not water soluble and which cannot be removed by manual cleaning. The subsequent process involves heating the pulverised PET in the presence of 1,4-butanediol and catalyst at various pressures for quite a considerable time. The method in the citation takes a minimum of 4 hours although all of the examples take considerably longer, even with the use of catalysts. If the time for the washing, drying, shredding and pulverising is included then the time involved to recycle post-consumer PET via this method is not commercially viable.
Decontamination of post-consumer PET was developed by the current inventor as disclosed in PCT/AU93/00198 titled “Improved polyethylene terephthalate decontamination”. This invention teaches that PET scrap can be decontaminated by treating it with ethanediol at temperatures near 200° C. when the PET was uniquely found to become brittle due to partial transesterification. Foreign substances either dissolve in the hot solvent, react with the diol then dissolve or remain as discrete unreacted particles that survive subsequent crushing and are removed from the crushed PET by screening. This decontaminated crushed PET is called “crumb”. It was subsequently discovered by the inventor that this decontamination process could be affected by any alkanediol, including 1,4-butanediol and 1,6-hexanediol (South African patent 95/2933). This decontamination process has received the approval of the American Food and Drug Administration for use with products which will eventually be in contact with food.
The word “crumb” as used in this description and in the claims refers to the decontaminated product prepared by (a) transesterifying material containing PET with diol(s) at a temperature about the boiling point of ethanediol for a period of time sufficient to form a mixture containing embrittled PET; and (b) crushing the mixture and separating crushed material containing PET from uncrushed contaminants. It will be readily apparent to those skilled in the art that while this discussion refers
Cain Edward J.
Lee Katarzyna W.
Petrecycle Pty Ltd.
Ross PC Sheridan
LandOfFree
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