Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
1999-01-29
2001-06-12
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S282000, C528S308600, C524S779000, C526S065000, C526S066000, C526S067000, C526S068000, C526S071000
Reexamination Certificate
active
06245879
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the purification of a carbonyl-containing 1,3-propanediol composition. In a specific aspect, the invention relates to treatment of a 1,3-propanediol-containing stream from the manufacture of polytrimethylene terephthalate so as to permit recycle of the 1,3-propanediol to the polymerization process.
Polytrimethylene terephthalate is a polyester useful in making fibers for carpets and textiles. In the preparation of polytrimethylene terephthalate by the esterification/condensation polymerization of 1,3-propanediol and terephthalic acid, the 1,3-propanediol is initially present in the esterification reaction mixture in a molar excess with respect to the acid. In order to achieve high polymer molecular weight, excess 1,3-propanediol is removed under vacuum and condensed as the polycondensation reaction proceeds. This condensed 1,3-propanediol stream typically also contains carbonyl-containing by-products of the polymerization process.
Although it would be desirable to recycle this excess 1,3-propanediol to the polymerization process, the addition of the by-products along with the 1,3-propanediol could result in the accumulation of carbonyl-containing byproducts in the system and interfere with the production of high-quality polyester.
It is known to purify such byproduct streams by adding a substantial amount of water, acidifying the solution, and then distilling the 1,3-propanediol under basic conditions. It would be desirable to recover purified 1,3-propanediol from the byproduct stream without the necessity of adding large quantities of water to the system.
It is therefore an object of the invention to provide a process for preparing polytrimethylene terephthalate in which relatively pure 1,3-propanediol is recovered from the distillate stream and recycled to the polymerization process.
BRIEF SUMMARY OF THE INVENTION
According to the invention, polytrimethylene terephthalate is prepared in a process comprising the steps of:
(a) contacting, in a polymerization reaction mixture at a temperature within the range of about 200 to about 300° C. under less than atmospheric pressure, terephthalic acid (or ester thereof) and a molar excess of 1,3-propanediol, for a time sufficient to produce a reaction product mixture comprising polytrimethylene terephthalate and a distillate comprising unreacted 1,3-propanediol, by-product carbonyl compounds and a minor amount of water;
(b) adding a sufficient amount of a base to impart to the distillate a pH higher than 7;
(c) heating the base-containing distillate to a temperature sufficient to distill a major portion of the 1,3-propanediol from the mixed condensate; and
(d) passing at least a portion of the distilled 1,3-propanediol to the polymerization reaction mixture as a recycle stream.
DETAILED DESCRIPTION OF THE INVENTION
The invention process involves adding a base to a carbonyl-containing 1,3-propanediol composition. As used herein, “carbonyl” refers to a compound, whether containing the C═O group or not, detected by a method in which total carbonyls are determined by conversion to 2,4-nitrophenylhydrazone derivatives and measured calorimetrically, such as ASTM E411-70. The source of such carbonyl species can be acetals, aldehydes or ketones.
The carbonyl-containing 1,3-propanediol composition is a distillate stream from the preparation of polytrimethylene terephthalate. As used herein, “1,3-propanediol-based polyester” refers to a polyester prepared by reacting at least one diol with at least one aromatic diacid (or alkyl ester thereof) in which at least 80 mole percent of the diol is 1,3-propanediol. “Polytrimethylene terephthalate” refers to such a polyester in which at least 80 mole percent of the diacid is terephthalic acid (or an alkyl ester thereof). Other diols may include, for example, ethylene glycol, diethylene glycol, 1,4-cyclohexanone dimethanol, bis(3-hydroxypropyl)ether and 1,4-butanediol, and other diacids may include, for example, isophthalic acid and 2,6-naphthalene dicarboxylic acid. The condensation polymerization of polytrimethylene terephthalate usually generates as much as about 4 mole percent of bis(3-hydroxypropyl)ether which, in effect, becomes a comonomer and is incorporated into the polyester chain.
Polytrimethylene terephthalate is prepared by the polycondensation reaction of the 1,3-propanediol with terephthalic acid (or an alkyl ester thereof) under vacuum and at a temperature within the range of about 200 to about 300° C. The diol(s) are added to the polymerization reaction mixture in a molar excess of 5 to 50% or more with respect to the acid. As the polycondensation reaction proceeds, the excess diol is removed under vacuum and condensed. The condensed stream typically contains, in addition to about 70 wt % or more diol(s), polymerization byproducts such as allyl alcohol, acrolein and other carbonyl compounds, acetals, alcohols, glycol ethers, diacids, polyester oligomers and adducts, and about 1-20 wt % water. Additional 1,3-propanediol can be added if needed to adjust the water concentration to this range. The carbonyl content (as C═O) of the distillate stream can typically range from 500 to 2500 ppm, depending upon the purity of the starting 1,3-propanediol and the polymerization reaction conditions. The pH of the stream is typically within the range of about 3 to about 6. The diol product of the invention treatment process preferably contains less than about 300 ppm carbonyl, most preferably less than about 200 ppm, qualifying it for use as a recycle stream in high-quality polyester manufacture.
A base is added to the 1,3-propanediol-containing composition in an amount sufficient to raise the pH thereof to a value greater than 7, preferably about 7.5 to about 14, most preferably within the range of about 8 to about 10. If desired, the base can be added in increments over the course of the purification process. The amount of base added is nominally the amount needed to neutralize acidic species present in the condensed stream plus any amount needed to reach the target pH. However, additional base may be needed, particularly if the stream contains soluble or suspended polyester adducts or oligomers, since reaction of the base with these oligomers or other impurities may consume base during the purification process. If the stream contains solid polymerization by-products, it is preferable to remove such solids by, for example, filtration prior to addition of the base to the liquid mixture.
Suitable bases for the purification treatment are typically inorganic bases such as the alkali and alkaline earth hydroxides, carbonates and bicarbonates. Preferred bases are sodium and potassium hydroxides. Organic bases such as amines are to be avoided.
The base-containing condensed stream is optionally further treated with a borohydride represented by the formula MBH
x
Y
y
, in which M is an alkali metal cation or a tetraalkylammonium cation, Y is a ligand, x is at least 1, and x+y=4, with x=4 and y=0 most preferred. Examples of such borohydrides include the metal tetrahydridoborates such as lithium borohydride, potassium borohydride, sodium borohydride and rubidium borohydride; metal organoborohydrides such as lithium trimethylborohydride, lithium triethylborohydride, lithium thexylborohydride, potassium tri-iso-amylborohydride, potassium tri-sec-butylborohydride, potassium triethylborohydride, potassium triphenylborohydride, sodium triacetoxyborohydride, sodium tri-iso-amylborohydride, sodium tri-sec-butylborohydride, sodium triethylborohydride, sodium trimethyloxyborohydride, and sodium triphenylborohydride; tetraalkylammonium borohydrides such as tetramethylammonium borohydride, tetraethylammonium borohydride and tetrabutylammonium borohydride. Potassium borohydride, sodium borohydride and tetraalkylammonium borohydrides are preferred. If desired, the borohydride can be used in supported form on basic alumina, silica gel and the like.
The amount of borohydride added to the condensed stream will generally be that amount which
Kelsey Donald Ross
Scardino Betty Marrou
Zronek Steven Charles
Acquah Samuel A.
Shell Oil Company
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