Zero-heel polyester process

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...

Reexamination Certificate

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C528S279000, C528S280000, C528S281000, C528S283000, C528S284000, C528S285000, C528S286000, C528S503000, C524S706000, C524S710000, C524S780000, C524S783000, C524S785000

Reexamination Certificate

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06528579

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for the preparation of polyesters of a glycol and a dicarboxylic acid which comprises the following steps:
a) introducing a feedstock comprising one or more glycols and one or more dicarboxylic acids or monoalcohol esters thereof into a reactor vessel,
b) heating the feedstock to an elevated temperature to cause the glycols and the acids or monoalcohol esters thereof to polycondense into a polyester,
c) removing all of the polyester from the reactor vessel,
wherein step b) is carried out in the absence of a preformed polyester (“zero heel” process).
BACKGROUND OF THE INVENTION
Polyesters of commercial importance include poly(ethylene terephthalate) (‘PET’), poly(butylene terephthalate) (‘PBT’), poly(trimethylene terephthalate) (‘PTT’) and poly(ethylene naphthalate) (‘PEN’). Currently, these and similar polyesters are prepared in batch and (semi-) continuous processes designed around a “heel” concept. The overall condensation polymerization reaction is comprised of an esterification step and a polycondensation step. In a process based on the heel concept, a portion of the esterification reaction product (that is liquid at the reaction temperatures) is left behind in the esterification reactor (the heel) while the balance is sent forward to polycondensation. The heel then acts as a solvent for the feedstock and also affords somewhat mass-averaged product quality, thus allowing uniform production.
Pengilly in U.S. Pat. No. 3,427,287 describes a method for preparing polyester resins comprising the continuous addition of terephthalic acid (‘TPA’) and monoethylene glycol (‘MEG’) in a molar ratio of 1:1.05 to 1:1.3 to a low molecular weight PET polyester having a degree of polymerization of from 3 to 20 while heating at a temperature above the melting temperature of the low MW polyester but below the distillation temperature of the glycol from said mixture to form a low MW polyester having a degree of polymerization of from 3 to 20 and continuously withdrawing an amount of the low MW polyester formed about equal to the amount of TPA and MEG added. A suitable polymerization apparatus is shown in a drawing, comprising a heated reaction vessel
1
that is filed about one-third full with low MW PET to which TPA is added from storage vessel
5
by means of a screw conveyor
6
through conduit
7
, and to which MEG is added from storage vessel
8
by means of control valve
9
through conduit
10
(numbers refer to the drawing in U.S. Pat. No. 3,427,287).
Rhinehart in U.S. Pat. No. 4,020,049 describes a nearly identical method, wherein a low MW polyester is produced having a degree of polymerization of from 1.4 to 10, at a pressure of from 20 to 1000 psig.
Another example of a process based on the heel concept may be found in U.S. Pat. No. 4,223,124 to Broughton et al. In this process a dicarboxylic acid is added to a heel solution, whereupon an esterification reaction is conducted by adding an initially deficient amount of glycol.
Problems that have in the past been associated (by those skilled in the art) with zero-heel processes concern the agglomeration of acid in the glycol due to low solubility, lack of uniform product, slow polymerization rates and the (increased) production of color body precursors and other impurities. Particularly undesired products are the polyether dimer and oligomers of the one or more glycol used in the preparation of the polyester. If, for instance, the glycol component is composed of or comprises 1,3-propanediol, then the resulting polyester suffers from increased amounts of dipropylene glycol which can reactively cleave to form acrolein. It is therefore common to avoid direct contact of the acid and glycol components (see also U.S. Pat. No. 3,442,868 and U.S. Pat. No. 3,849,379. Zero-heel processes are hence not popular.
Surprisingly a zero-heel process in respect of the preparation of polyesters has now been developed that does not suffer from the disadvantages mentioned above. We have found that a zero-heel process which has sufficient solubility of, for instance, terephthalic acid in 1,3-propanediol and produces polymer, for instance, polytrimethylene terephthalate, is possible. The preferred process increases the total through-put of the production facility via utilizing reactor space normally taken up by the heel and decreases the amount of dipropylene glycol produced (when the glycol is 1,3-propanediol). It has therefore a substantial impact of the economic feasibility of the polyester preparation.
SUMMARY OF THE INVENTION
Accordingly, there is provided a process for the preparation of polyesters of a glycol and a dicarboxylic acid which comprises the following steps:
a) introducing feedstock comprising one or more glycols and one or more dicarboxylic acids or monoalcohol esters thereof into a reactor vessel,
b) heating the feedstock to an elevated temperature to cause the glycols and the acids or monoalcohol esters thereof to polycondense into a polyester,
c) removing all of the polyester from the reactor vessel,
wherein step b) is carried out in the absence of a preformed polyester (“zero heel” process). It is highly preferred that one or more additives be added to the reaction vessel in step a) and/or b), wherein the additives comprise metal salts of strong or weak organic or inorganic bases. At least one catalyst which may be selected from condensation catalysts based on antimony, iron, titanium, zirconium, zinc, cobalt, lead, manganese, and niobium is added in step a) or b) or both.
DETAILED DESCRIPTION OF THE INVENTION
The zero-heel process can greatly simplify and reduce the cost of processing equipment. In a heel process, typically roughly 40%-60% of the esterified material is required to mix with incoming feed, while the balance moves forward to another reaction vessel for the polycondensation to occur. Therefore in a heel process two reactors are required. In contrast, the process described in this specification does not require the heel of material to start the reaction sequence, and hence allows the use of a single reactor (e.g., in a batch mode) for both processing steps, or in the case of a dual reactor design affords greater throughput per unit of reactor volume.
Albeit not essential, it is highly preferred in the zero-heel process of the present invention that the feed is intimately mixed, prior to the introduction thereof into the reactor vessel, e.g., in the form of a paste.
The ability to effectively react (or esterify) TPA with PDO without the use of a heel to produce oligomeric material effectively reduces the amount of time the polyester (polymer or oligomer) is subjected to thermal stress. The zero-heel process thus effectively reduces the risk of degradation reactions that lead to color body precursors. In the past, the heel process was thought to be the only possible way to achieve sufficient diacid solubility enabling effective reaction and acceptable polymer color.
The polyesters that may be made in accordance with the process of the present invention include such polyesters formed from dicarboxylic acids containing a total of from 2 to 16 carbon atoms, reacted with a glycol containing from 2 to 12 carbon atoms. The process of the present invention may also be used with monoalcohol diesters of said dicarboxylic acids, wherein the monoalcohol contains from 1 to 8 carbon atoms, such as methanol, ethanol, cyclohexanol, etc. However, the present invention is particularly suitable for the preparation of polyesters formed from dicarboxylic acids.
The dicarboxylic acids may be an alkyl-type containing a total of from 2 to 16 carbon atoms. Preferably, the acids are aryl or an alkyl substituted aryl-type containing from 8 to 16 carbon atoms. Specific examples of linear or alkyl dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and the like. Specific examples of an aryl acid include the various isomers of phthalic acid, such as paraphthalic acid (terephthalic acid) or isopht

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