Polyesters production process

An atmospheric pressure process for the continuous production of polyester is disclosed wherein a melt of bis(3-hydroxy propyl) terephthalate, or its low molecular oligomers, obtained by esterifying terephthalic acid or transesterifying dimethyl terephthalate with propylene glycol, is intimately contacted with an inert gas to facilitate polymerization and removal of the reaction byproducts. The propylene glycol evolved and the inert gas are recycled.

FIELD OF THE INVENTION 
An improved process for the production of polyesters at atmospheric 
pressure is disclosed. 
TECHNICAL BACKGROUND 
Polyester production from terephthalic acid (TPA) or its esters, such as 
dimethyl terephthalate (DMT), and glycols is known. This has been 
accomplished by stage-wise melt polymerization of the dihydroxy ester of 
the bifunctional carboxylic acid, or low molecular weight oligomers 
thereof, under successively higher vacuum conditions. In order for the 
polymerization to continue to the degree needed for most commercial 
applications, the condensation by-products, especially the glycols, must 
be removed from the reaction system at vacuums as high as 1-3 mm Hg. Such 
processes require costly high vacuum equipment, multistage steam jets to 
create the vacuum, and N.sub.2 purged seals and flanges to minimize 
leakage of air into the system. Condensate from the steam jets and organic 
by-products from the system end up as a waste water stream that requires 
treatment and contributes to volatile organic emissions to the air. The 
present invention provides a less costly polymerization process that can 
be carried out at atmospheric pressure and in a closed loop configuration 
that eliminates volatile organic emissions and the waste water discharge. 
Atmospheric pressure processes to conduct polymerization, without employing 
vacuum and using an inert gas, have been disclosed in the prior art, but 
these have several drawbacks. 
U.S. Pat. No. 2,973,341 (Hippe) discloses a continuous process for the 
production of polyester condensate and an improved continuous process for 
making polyethylene terephthalate from dimethyl terephthalate and ethylene 
glycol. The process employs liquid dimethyl terephthalate and mixes with 
it ethylene glycol, in an excess molar ratio of 1.5:1, to form a liquid 
reaction mixture in a first stage below the transesterification 
temperature and then carrying the liquid reaction mixture through three 
separate temperature controlled stages. Transesterification occurs in the 
second stage at a temperature of not more than 197.degree. C.; vaporous 
reaction products are removed in the third stage at 197.degree. C. to 
230.degree. C. by passing an inert gas through the liquid reaction 
mixture; polycondensation occurs in the fourth stage at 230.degree. C. to 
255.degree. C. for a period of time sufficient to produce a filament 
forming polyethylene terephthalate condensate while again passing an inert 
gas through the liquid reaction mixture. Ethylene glycol by-product can be 
recovered from the fourth stage and recycled to the second stage of the 
reaction. 
U.S. Pat. No. 3,545,520 (Siclari et al.) discloses an apparatus for 
stripping substances and lightweight fractions from polymers including a 
means for introducing an inert gas counter current to the polymeric 
material with the consequent increase in viscosity of the polymers. The 
apparatus permits recycling a portion of the material removed from the 
vessel so that the material can be recycled into the reaction container. 
U.S. Pat. No. 3,469,618 (Siclari et al.) discloses a method for stripping 
off volatile fractions from polyamides and polyesters involving feeding 
material in the form of droplets or liquid threads though an inert gaseous 
atmosphere, while recirculating that atmosphere. 
U.S. Pat. No. 3,110,547 (Emmert) discloses a process for preparing a linear 
condensation polyester. In one embodiment of the invention, the polymer is 
extruded downwardly through a chamber while passing a current of inert 
gas, such as nitrogen, through the reaction vessel at a rate sufficient to 
keep the glycol partial pressure below 2 mm Hg while maintaining a 
temperature between 300.degree. C. and 400.degree. C. in order to rapidly 
finish the polymer by converting the polymer having a degree of 
polymerization of from about 15.degree. to 35.degree. to a finished 
polymer with a degree of polymerization of about 70.degree.. 
U.S. Pat. No. 3,390,135 (Seiner) discloses a continuous polyesterification 
process by direct esterification of dicarboxylic acids and polyhydric 
alcohols, and contacting the reaction product with a nonreactive gas to 
remove the water of esterification. 
U.S. Pat. No. 3,480,587 (Porter) discloses a polyester preparation process 
in which a lower molecular weight prepolymer is polymerized by conducting 
polycondensation in narrow tubes under conditions of turbannular flow 
achieved with an inert gas medium flowing cocurrently at high velocities. 
French A, 239,649 (Bayer) discloses a continuous process for preparing 
polybutylene terephthalate wherein monoesters or low viscosity 
polybutylene terephthalate is polymerized by cocurrently transporting it 
with a heated inert gas in the form of two phase annular flow through a 
long, narrow, helical tube of 3 to 100 nm (0.1 to 3.9 inch) diameter in 
which the inert gas flows in a velocity of 20-300 m/s (equal to 66 to 984 
ft/second). 
European Patent A, 0,182,351 (Mitsubishi) discloses a polyester process in 
which the ester or its oligomer is polymerized in the form of fine, 0.015 
to 0.5 mm particles sprayed into an inert gas stream. 
U.S. Pat. No. 5,064,935 (Jackson et al.) discloses a continuous process for 
preparing polybutylene terephthalate oligomer or prepolymer for feeding 
into a conventional polycondensation for PBT polymer. The prepolymer is 
prepared by feeding the reaction mass from a prior transesterification 
step into the top of a countercurrent column reactor through which a 
heated inert gas is passed upward by introducing it at the bottom. 
The processes disclosed above, however, suffer from one or more drawbacks 
such as (1) only a low molecular weight oligomer or a prepolymer is 
produced; (2) the quantity of inert gas used is very large to be 
economical; (3) the reactor size might be too large to be feasible for 
commercial scale operation; (4) the inert gas employed is not recycled in 
a closed loop to eliminate emissions; (5) a prepolymer of sufficiently 
high molecular weight is required to achieve high molecular weight 
polyester required for commercial application; (6) inert gas velocities 
employed are too high to be feasible for commercial scale production or a 
high pressure is required. Because of such drawbacks, the processes 
presently practiced for commercial production of polyester continue to be 
conducted under high vacuum as described above. 
The object of the present invention is to provide an improved atmospheric 
pressure process for continuous or batchwise production of polyesters, 
especially poly(alkylene terephthalates), particularly poly(propylene 
terephthalate), and poly(butylene terephthalate) of high molecular weight. 
SUMMARY OF THE INVENTION 
The invention relates to an atmospheric pressure method of polymerizing a 
dihydroxy ester of a bifunctional carboxylic acid, or of a low molecular 
weight polymerizable oligomer thereof, to a product with a high degree of 
polymerization (DP), preferably in the presence of a polyester 
polymerization catalyst, wherein by-products of the polymerization are 
removed from the system by means of an inert gas. This higher degree of 
polymerization is useful in fibers, films and other commercial 
applications. 
This process provides an improved method for producing linear aromatic 
polyesters, especially poly(propylene terephthalate) and poly(butylene 
terephthalate) referred to herein as PPT and PBT, respectively. The 
bifunctional acid in the production of PPT is terephthalic acid (TPA). The 
process involves the production of poly(propylene terephthalate) from 
terephthalic acid (TPA) and propylene glycol (PG), also known as 
1,3-propanediol, by esterification, or from dimethyl terephthalate (DMT) 
and propylene glycol by a transesterification stage, followed by 
polycondensation. The process is conducted at atmospheric pressure or 
above, thereby avoiding high vacuum equipment and eliminating possible air 
contamination that causes product decomposition and gel formation. The 
process comprises the following steps: 
(a) esterifying terephthalic acid (TPA) or transesterifying dimethyl 
terephthalate (DMT) with propylene glycol to produce dihydroxy propyl 
terephthalate or its low molecular oligomers, and 
(b) intimately contacting the dihydroxy propyl terephthalate or its low 
molecular weight oligomers in melt form with an inert gas flowing at a 
velocity of 0.2 to 3 ft/sec, such that the interfacial area between the 
melt and the gas phase is at least about 20 ft.sup.2 /ft.sup.3 of the 
melt, and removing the volatile reaction by-products with the inert gas 
wherein the polymerization is complete in less than about 5 hours, 
preferably less than 3 hours, of contact time while the reactants are kept 
at a suitable temperature to maintain them in the melt form so as to 
produce polypropylene terephthalate. 
The above processes are preferably conducted in the presence of a polyester 
polymerization catalyst. However, a catalyst is not needed for the 
esterification step (a) if the starting material is terephthalic acid. 
In a preferred embodiment of the invention, a single stream of inert gas is 
recycled through a polymer finishing stage, a polycondensation stage and a 
stage wherein propylene glycol is recovered for reuse in the process. 
The process for the production of PBT is similar to that described above 
for PPT except that the glycol employed is 1,4-butane diol (BD). For PBT 
production, it is preferred that the starting material is DMT. The acid 
end group present in the reaction mass, if TPA is used, catalyze 
cyclizationn of BD to tetrahydrofuran (THF) .

DETAILED DESCRIPTION OF THE INVENTION 
The polymerization step can be carried out in one vessel or more than one 
physically distinct vessel in series, wherein the reaction mass is 
polycondensed to some degree of polymerization in one vessel and then 
transferred to another vessel for further polymerization. This choice is 
based on mechanical considerations related to handling of the polymeric 
melt as its viscosity increases sharply as the degree of polyermizatlon 
increases, heat input requirements to volatilize the by-products of 
polycondensation and cost. 
The above processes may be carried out batchwise or continuously. Batchwise 
production may be preferred for preparing specialty polymers when the 
production required is not very large and strict quality control is 
required particularly with respect to additives. For large scale 
production for commodity applications, such as staple and yarn, it is more 
cost effective to carry out the above steps continuously wherein the 
reactants are fed substantially continuously into the processing vessels 
and the products are removed substantially continuously. The rates of feed 
and product removal are coordinated to maintain a substantially steady 
quantity of the reactants in the reaction vessels while the inert gas 
flows countercurrently to the flow of the melt. 
If two or more vessels are employed in series for conducting the 
polycondensation, it is preferred that a single stream of inert gas is 
employed that flows countercurrently to the flow of the melt in the 
process, i.e., the inert gas leaving a final stage of polymerization is 
led through the preceding stage and finally through a stage wherein the 
glycol is recovered for reuse and the inert gas is recycled back to the 
final stage of polymerization. 
Poly(propylene terephthalate) or (PPT) is manufactured in this process by 
first reacting terephthalic acid (TPA) or dimethyl terephthalate (DMT) 
with propylene glycol (PG). If DMT is the starting material, a suitable 
transesterification catalyst such as zinc acetate or titanium alkanoate is 
used for the reaction. Esterified DMT/TPA is polymerized as a melt at 
atmospheric pressure or above by intimately contacting the melt with a 
stream of inert gas (for example, but not limited to, N.sub.2 or CO.sub.2) 
to remove the condensation by-products, mainly, propylene glycol. 
Preferably, the inert gas is preheated to about polymerization temperature 
or above, prior to its introduction into the polymerization equipment. It 
is preferred that the inert gas velocity through the polymerization 
equipment be in the range of 0.2 to 3 ft/sec, most preferably 0.3 to 1.5 
ft/sec. The vapor leaving the polymerization (containing the propylene 
glycol removed) is treated to recover the propylene glycol for recycle to 
the esterification stage or for other uses. The inert gas stream is then 
cleaned up and recycled. Thus, the overall process operates as a closed 
loop system which avoids environmental pollution and integrates propylene 
glycol recovery and its recycle into the process. 
The quantity of inert gas flow should be sufficient to carry the propylene 
glycol to be removed at a partial pressure of propylene glycol below the 
equilibrium partial pressure of propylene glycol with the reaction mass at 
the operating temperature. The operating temperature during 
polycondensation is maintained sufficiently high so as to keep the 
reaction mass in a molten state. Preferably the temperature range is about 
230.degree. C. to 250.degree. C. to reach the high molecular weight 
polymer. The polymerization equipment is designed so that the interfacial 
area between the melt and the inert gas is at least 20 square feet, 
preferably at least about 30 square feet, per cubic foot of the melt and 
that this surface area is renewed frequently. Under these process 
conditions, the high degree of polymerization useful for fibers and films 
can be achieved in less than 5 hours of residence time, and preferably in 
less than 3 hours of residence time. 
To produce good quality product of the desired high degree of 
polymerization, the polymerization should be completed in a reasonably 
short period such as less than 5 hours, preferably less than about 3 
hours. The polymerization is considered completed when the degree of 
polymerization (DP) desired for a particular application is achieved. For 
most common applications, such as fibers, the DP should be at least 50, 
preferably at least 60, and most preferably at least 70. By "degree of 
polymerization" is meant the number average degree of polymerization. 
Exposure of the polymeric melt to high operating temperatures for 
prolonged period causes chain cleavage and decomposition reactions with 
the result that the product is discolored and a high degree of 
polymerization is not achieved. If the inert gas velocities are too low, 
polymerization takes longer. If the velocity is too high it can lead to 
entrainment of the reaction mass in the gas. In a continuous mode of 
operating, high inert gas velocities in a countercurrent direction can 
also hinder the flow of the melt through the equipment. Also, higher 
velocities may require larger quantities of gas flow without substantially 
increasing the effectiveness of polymerization. It has been found that the 
polymerization can be carried out effectively at 0.2 to 3 ft/sec 
velocities. 
The quantity of inert gas flow employed to remove the propylene glycol that 
evolves is sufficiently high so that the partial pressure of propylene 
glycol in the gas, at any point in the process, is well below the 
equilibrium partial pressure of propylene glycol with the melt at this 
point. Larger quantities of gas flow generally increase the rate of 
polymerization but the increase is not proportionately greater. Therefore, 
very large amounts of gas are not usually necessary or desirable as large 
quantities increase the size of recycling equipment and the cost. Very 
large quantities may also require larger size polymerization equipment in 
order to keep the gas velocity in the 0.2 to 3 ft/sec range. 
In the continuous embodiment of this invention, the inert gas flows 
countercurrently to the flow of the molten reaction mass at a velocity of 
at least 0.2 ft/sec, preferably at least about 0.3 ft/sec. The 
countercurrent flow reduces the quantity of inert gas required to achieve 
effective polymerization. The N.sub.2 flow, however, should be at least 
0.2 lbs/lb of polymer (equivalent to 1.5 moles of inert gas per mole of 
polymer repeat unit). Larger quantities of gas flow may however be needed 
to obtain the preferred gas velocities. 
In the process of this invention, the reactant is kept in a molten state, 
i.e., above its melting point which is about 225.degree. C. for the high 
molecular weight (high DP) polymer. The melting point is lower at the 
lower DP's and a lower temperature range such as 180.degree. to 
225.degree. may be used at those DP's. At temperatures much above 
260.degree. C., decomposition reactions cause product discoloration which 
interferes with the quality of the product. The reaction mass should 
preferably be maintained at about 180.degree. C. to about 260.degree. C., 
more preferably between 230.degree. to 250.degree. C. 
For the polycondensation to continue, propylene glycol generated must be 
removed from the reaction mass by the inert gas. This removal is 
facilitated if there is a high interfacial area between the melt and the 
gas phase. To complete the polymerization in a reasonably short period, 
the surface area should be at least about 20 ft.sup.2 /ft.sup.3 of the 
melt, preferably at least about 30 ft.sup.2 /ft.sup.3 of the melt. A 
higher surface area is preferred to increase the rate of polymerization. 
The reaction equipment for contacting the melt and the inert gas should 
also be designed to frequently renew the interfacial area and mix the 
polymer melt. This is particularly important as the degree of 
polymerization increases and the melt becomes very viscous. When the DP of 
the melt is low, it is quite fluid and the surface area requirements are 
easily met by simply passing the insert gas through the melt as bubbles, 
but at higher DP's, e.g., above 45 the melt is quite viscous and surface 
area generation and renewal is preferably achieved by mechanical means 
that make the melt spread into films, mix the melt films and regenerate 
such films. 
The rate of polymerization can be increased by using a suitable 
polymerization catalyst, particularly where a high interfacial area is 
provided for inert gas--melt contact. The increase in the overall rate, 
however, is not proportional to the concentration of catalyst as the 
removal of propylene glycol starts to limit the overall polymerization. 
The catalyst also increases the rates of decomposition reactions. An 
effective concentration of catalyst for a set of reaction conditions, such 
as temperature, gas flow, velocity and surface area, is such that it gives 
the most enhancement in the rate of polymerization without substantial 
decomposition. The optimum concentration of catalysts of various species 
can be determined by experimentation. It would generally be in the range 
of a few parts per million parts of the polymer, such as about 5-300 parts 
per million. 
Catalysts for facilitating the polymerization are any one or more polyester 
polymerization catalysts known in the prior art to catalyze such 
polymerization processes, such as, but not limited to, compounds of 
antimony, germanium, tin and titanium. Such metals which may be 
introduced, for convenience, as alkanoates solubilized in propylene 
glycol. Examples of such catalysts are found in U.S. Pat. No. 2,578,660, 
U.S. Pat. No. 2,647,885 and U.S. Pat. No. 2,789,772, which are 
incorporated herein by reference. 
Various dihydroxy esters of bifunctional carboxylic acids may be used in 
the processes described herein. These are monomeric compounds that can 
polymerize to a polymer. Examples of such compounds are 
bis(2-hydroxyethyl) terephthalate, bis (3-hydroxypropyl) terephthalate, 
bis (4-hydroxybutyl) terephthalate, bis (2-hydroxyethyl) 
naphthalenedioate, bis (2-hydroxyethyl) isophthalate, bis 
[2-(2-hydroxyethoxy) ethyl] terephthalate, bis [2-(2-hydroxyethoxy) ethyl] 
isophthalate, bis [(4-hydroxymethylcyclohexyl)methyl] terephthalate, bis 
[(4-hydroxymethylcyclohexyl)methyl] isophthalate, and a combination of 
such monomers and their oligomers. Mixtures of these monomers and 
oligomers may also be used to produce copolymers. 
By a "polymerizable oligomer" is meant any oligomeric material which can 
polymerize to a polyester. This oligomer may contain low molecular weight 
polyester, and varying amounts of monomer. For example, the reaction of 
dimethyl terephthalate or terephthalic acid with propylene glycol, when 
carried out to remove methyl ester or carboxylic groups usually yields a 
mixture of bis (3-hydroxypropyl) terephthalate, low molecular weight 
polymers (oligomers) of bis(3-hydroxypropyl) terephthalate and oligomers 
of mono(3-hydroxypropyl) terephthalate (which contains carbonyl groups). 
This type of material is referred to herein as "polymerizable oligomer". 
The process may be used to produce various polyesters such as poly(ethylene 
terephthalate), poly (1,3-propylene terephthalate) , poly (1,4-butylene 
terephthalate), poly(ethylene naphthalenedioate), poly(ethylene 
isophthalate), poly(3-oxa-1,5-pentadiyl terephthalate), poly 
(3-oxa-1,5-pentadiyl isophthalate), poly[1,4-bis (oxymethyl)cyclohexyl 
terephthalate] and poly [1,4-bis(oxymethyl)cyclohexyl isophthalate]. 
Poly(ethylene terephthalate), poly(propylene terephthalate) and 
poly(butylene terephthalate) are especially important commercial products. 
The process avoids high vacuum polymerization processes characteristic of 
the conventional art. Advantages of the process are a simpler flow 
pattern, lower operating costs and the avoidance of steam jets, hot wells 
and atmosphere emissions. The process also has environmental advantages 
due to the elimination of volatile organic emissions and waste water 
discharge. Furthermore, polymerization is conducted in an inert 
environment. Therefore, there is less decomposition and gel formation 
which results in better product quality. Propylene glycol and inert gas 
(e.g., N.sub.2 or CO.sub.2) are recycled continuously. The process is 
described in FIG. 1. 
FIG. 1 is a diagrammatic flow sheet for the continuous process of the 
invention. Reactant materials TPA or its dimethyl ester, DMT (1) and 
propylene glycol (22) are supplied continuously to an esterification 
reactor (2) for esterification (or transesterification) to dihydroxypropyl 
terephthalate and its low DP oligomers. The resulting esterified or 
transesterified product is an oligomer with a low degree of polymerization 
(DP). The resulting DP is from 1-3 if the starting material is DMT. If TPA 
is the starting material, the resulting oligomer usually has a higher DP. 
The molten reaction product formed in the esterification reactor (2) is 
conducted through transfer line (4) to a prepolymerization column (6) for 
polycondensation. A suitable polyester polymerization catalyst, such as a 
titanate, may be added at this point. Additives, such as TiO.sub.2, which 
is usually added to polyester as a delustrant for fibers, may also be 
added at this point. Other materials to optimize the polymerization rate 
are also introduced as this point. For example, if TPA is the starting 
material for esterification the oligomer from reactor 2 may contain too 
many carboxyl end groups; some propylene glycol (could be a portion of the 
propylene glycol recovered later in the process from the inert gas) may be 
added to balance the end groups to optimize the reaction rate and enable 
polymerization to the desired high degree. On the other hand, if DMT is 
the starting material for esterification, a small amount of TPA may be 
added to increase the overall rate of polymerization. The prepolymer, 
exiting the esterification column with a degree of polymerization from 
15-30, is conducted through transfer line (8) to finisher (10) in order to 
finish the polymer by raising the degree of polymerization to about 
50.degree. to about 150.degree., preferably about 60.degree. to about 
120.degree. and more preferably about 70.degree. to about 90.degree.. The 
finisher (10) is maintained at a temperature greater than about 
225.degree. C. but not too high to cause polymer decomposition. A 
temperature range of about 230.degree. C. to 250.degree. C. is preferred. 
The polymerization product is continuously removed from the finisher 
through line (30). An inert gas, preferably nitrogen, is heated in heater 
(12) at a temperature of from about 230.degree. C. to 260.degree. C. and 
is introduced through line (14) into the finisher to flow counter current 
to the direction of polymer flow in order to remove volatile reaction 
by-products, primarily propylene glycol. The inert gas flows through the 
finisher (10) and then through line (16) to prepolymerization column (6) 
removing volatile reaction by-products, which are mainly propylene glycol, 
in that reaction column. The hot inert gas stream containing organic 
vapors, which are mainly propylene glycol with minor amounts of methanol, 
water, and some thermal decomposition products, exits the 
prepolymerization column through line (18) and enters the glycol recovery 
system (20) where glycol is recovered from the stream by scrubbing it with 
the fresh propylene glycol feed (3) to the process, thereby also 
recovering the heat from the hot stream in line 18. The recovered glycol 
is recycled to the esterification reactor (2) through line (22). The inert 
gas stream containing the uncondensed volatile organics exits the glycol 
recovery system through line (24) and enters a clean up system (26). The 
clean up system may be an adsorption bed (26), such as an activated carbon 
bed, wherein the organic volatiles are adsorbed producing a clean nitrogen 
stream which can be heated and returned to the finisher (10). Thus, the 
nitrogen is employed in a closed loop and all processing equipment is 
operated at atmospheric pressure (or above, as is necessary to ensure the 
flow of nitrogen through the equipment in the loop). The inert gas flowing 
in the polymerization equipment (6) and (10) has a velocity of between 
about 0.2 to 3 ft/sec, preferably 0.3 to 1.5 ft/sec. The quantity of inert 
gas introduced into the system is sufficient so that the partial pressure 
of the by-products is maintained below the equilibrium pressure of the 
byproducts with the melt in order to provide for the continuous 
polymerization. The quantity of inert gas may be as small as about 0.5 
pounds for each pound of polyethylene terephalate produced. The adsorption 
bed (26) can be purged to remove the adsorbed products. The adsorbed 
products are transferred by line (28) to a combustion device, such as a 
boiler, (not shown) where they are converted to carbon dioxide and water 
by combustion thus completing an environmentally clean, emissions free 
process. The clean up system may also be a catalytic coverter that 
converts the organics to CO.sub.2. 
An embodiment of the present invention that further simplifies the 
continuous process described above comprises completing the 
polycondensation of the esterification oligomer to the final polyester 
product of high degree of polymerization in one vessel. The oligomer 
formed in the esterification reaction (2) is conducted directly to the 
finishing polymerizer (10) thereby the prepolymerization column (6) is 
eliminated. The inert gas leaving (10) is then fed to the glycol recovery 
column (20), is processed as described above and recycled. This embodiment 
is particularly suitable for polymerizing oligomers of about 5 DP or 
higher such as those generally obtained when TPA is used as the starting 
material for esterification. 
In one embodiment of the process of this invention, propylene glycol flow 
to the recovery scrubber (2) and other conditions may be adjusted such 
that the glycol leaving the scrubber is sufficiently hot so as to 
substantially glycolize any low molecular weight oligomers or entrained 
prepolymer that might enter the scrubber (2) in small amounts with the 
inert gas leaving the polymerization equipment. 
Alternatively, or in conjunction with the operation of scrubber (2) as 
described, the inert gas may be contacted with sufficient quantities of 
cold propylene glycol so that the inert gas leaving the glycol recovery 
step is cooled down to about the ambient temperature or colder. In this 
embodiment of the process, minor amounts of low boiling components such as 
methanol, water, or volatile from decomposition products, if any present 
in the inert gas, are also substantially removed with the cold propylene 
glycol stream, and it is not necessary to have the clean up system (26). 
In another embodiment of this invention, a fractionating column is used 
instead of the recovery scrubber (2). This allows recovery of the glycol 
as well as its refining prior to recycle without need for additional 
external heat. 
FIG. 2 illustrates one apparatus which is suitable for carrying out the 
polymerization of the invention particularly for use with the high 
viscosity material and degree of polymerization encountered in the 
finisher (10) of FIG. 1. It consists of a horizontal, agitated cylindrical 
vessel (32). The esterified DMT or TPA, or a low molecular weight oligomer 
thereof, is continuously introduced as stream (34) at one end of the 
vessel (32) and a preheated inert gas, such as nitrogen, is continuously 
introduced as stream (38) at the other end, so as to provide a counter 
current flow to the polymer flow. The nitrogen stream (38) carrying 
reaction by-product vapors, mostly propylene glycol, leaves as stream 
(40). The polymerized product, poly(propylene terephthalate), is removed 
as stream (36). The flow rates of streams (34) and (36) are coordinated to 
be equivalent to each other and controlled so as to provide the desired 
hold up of the melt in the finisher, usually about equivalent to 1 to 2 
hours times the flow rate, which is equivalent to a melt level at about 
1/4 to 1/3 the height of the vessel. The quantity of nitrogen introduced 
into the system is sufficient so that the partial pressure of the evolving 
reaction by-products is maintained at less than the equilibrium pressure 
of the by-products with the, for example, poly(propylene terephthalate) 
melt, so as to provide adequate driving force to remove propylene glycol 
from the melt into the gas stream. The diameter of the vessel is designed 
so that the superficial velocity of the inert gas stream is about 0.3 to 
1.5 ft/sec. 
The vessel is equipped with an agitator (42) which can be rotated at a 
controlled speed. The mechanical design of the agitator is such that 
(a) the walls of the vessel are wiped; 
(b) a large interfacial area of at least 20 ft.sup.2 /ft.sup.3 of the melt 
preferably greater then 30 ft.sup.2 /ft.sup.3 of the melt is created; 
(c) the surface area is renewed frequently; and 
(d) good radial mixing is provided. 
One design which achieves the above specified criteria, is a rotating disc 
and donut contactor comprising several alternating discs and donuts. The 
inert gas flows through the center of the donuts then flows radially 
towards the wall of the vessel in the space between the donut and the next 
disc, then around the disc towards the center of the next donut and so on. 
The discs and donuts are sized such that the velocity of gas through such 
passages does not exceed 5 ft/sec, and is preferably less than about 3 
ft/sec. For a continuous polymerizer it is preferred that the discs and 
donuts are spaced closer near the end where the oligomer or the prepolymer 
is fed, the spacing is increased gradually or incrementally along the 
length of the cylinder so as to accommodate free downward flow of the 
reaction melt from the agitator elements as its viscosity increases. The 
spacing may be as close as 1/4 inch near the feed end but preferably 1 
inch or greater near the product discharge end where the viscosity of the 
melt is the highest. 
The process of this invention may also be carried out for batchwise 
preparation of polyesters wherein a batch of low molecular weight oligomer 
(prepared in a separate vessel or in the polymerizer) is charged to the 
polymerization equipment and contacted with the inert gas as described 
until the desired high degree of polymerization is achieved. The oligomer 
is prepared by esterification as described except that it may also be 
prepared batchwise either in a separate vessel or in the polymerization 
vessel itself. The gas and melt contacting equipment may be similar to 
that described for the continuous embodiment of this invention. For the 
final stages of polymerization, equipment similar to that of FIG. 2 may be 
used except that the discs and donuts are spaced uniformly. For batchwise 
preparation it is advantageous to adjust the speed of the agitator as the 
viscosity of the melt increases. Initially, when the viscosity is low, the 
agitator may operate at as high as 100 rpm but toward the completion of 
polymerization a low speed of about 1 to 20 rpm, preferably about 2-12 rpm 
is desirable. Batchwise production is suitable for economic reasons when 
relatively small quantities of polyester are to be prepared or when a 
strict control of additives concentrations is required for product quality 
considerations. When the quantities to be prepared are very small, it may 
be more economical to not provide equipment for recycling the inert gas, 
or the propylene glycol, and discharge it to the atmosphere after 
rendering it harmless to the environment by known methods such as 
scrubbing it thoroughly with water and disposing off the water in an 
environmentally safe manner. 
The invention can also be conducted in a semibatch fashion wherein the 
polymerization equipment is fed intermittently, reaction mass is 
polymerized to a higher degree, and the product is discharged 
intermittently. 
EXAMPLE 1 
Polymerization of a low molecular weight oligomer of bis(3-hydroxy propyl) 
terephthalate, having a degree of polymerization (DP) of about 3, was 
conducted in a 15 mm diameter test tube heated to the polymerization 
temperature by placing it in a temperature controlled tube furnace. The 
test tube was equipped with means to introduce preheated N.sub.2 at a 
controlled rate near the bottom and an outlet was created near the top of 
the test tube to allow N.sub.2 to exit. A thermocouple was placed in the 
test tube to monitor and control temperature of the reaction mass. A 5 g 
sample of the oligomer was placed in the tube. The oligomer had been 
prepared by transesterifying DMT with propylene glycol and contained 24 
ppm titanium as a titanate catalyst. 
The temperature was ramped from 120.degree. to 245.degree. C. over a 20 to 
25 minute period while N.sub.2 was bubbled through the reaction mass as a 
superficial gas velocity of 1 to 1.2 ft/sec. This allowed the initial 
polymerization to occur at lower temperatures. No carry over of the low DP 
oligomers into the N.sub.2 stream was observed. N.sub.2 was introduced 
below the melt causing the melt to lift up and allowing it to fall along 
the tube walls to create interfacial area (estimated as &gt;30 ft.sup.2 
/ft.sup.3), and provide surface renewal and good mixing. During the 25 
minute period, the melt became quite viscous. The experiment was continued 
for another 10 minutes at 245.degree. C. (the actual temperature 
fluctuated between 235.degree. and 250.degree. C.) while the N.sub.2 
velocity was increased to about 2 ft/sec so as to push the more viscous 
melt up the walls of the tube to create surface area. The melt became very 
viscous indicating effective polymerization. 
The experiment was continued for another 10 minutes while the N.sub.2 
velocity was raised to 2.7 to 3 ft/sec. The melt had however become so 
viscous that effective bubbling, and surface area generation was not 
possible. The experiment was then stopped and the resulting polymer was 
analyzed for molecular weight distribution by GPC. The melt had a good 
polydispersity of 1.8 vs. the theoretical 2.0. The number average degree 
of polymerization (DP) calculated from the GPC data was 68.degree.. 
The experiment showed that initial polymerization can be easily achieved by 
bubbling the N.sub.2 through the reaction mass, but at higher DP's it 
would be more effective to generate surface area by mechanical means. 
EXAMPLE 2 
The same monomer was used in Example 1 was polymerized in a laboratory 
apparatus of the type shown in FIG. 2 which was constructed to provide 
surface area by mechanical means. 
The apparatus consisted of a 5- 1/4" glass tube with an inside diameter of 
about 1" which was held in a horizontal position. The tube was fitted with 
an agitator consisting of two 1/4 wide aluminum strips that substantially 
spanned the length of the tube and were held 180 degrees apart by two 
small strips attached at the two ends to form a rectangular shape frame of 
slightly smaller width than the diameter of the tube. The agitator was 
rotated by use of a motor with a variable speed gear reducer. The agitator 
provided mixing, surface renewal and wiping of the inside walls of the 
tube. A thermocouple was inserted into the tube from each of its two ends 
to monitor and control the temperature of the reaction mass. 
The tube was filled with 25 gms of the oligomer and heated with a heating 
tape wrapped around it. Also preheated N.sub.2 at about 125.degree. C. was 
passed through the tube at a velocity of 0.25 ft/sec. As the oligomer 
heated up and started to melt, the agitator was started at a slow speed. 
When the temperature reached 110.degree. to 125.degree. C. the reaction 
mass was completely molten and condensation products started to evolve and 
get carried by the N.sub.2. As the temperature increased further to 
150.degree. C., the evolution was faster. Also, it was observed that the 
evolution of condensation by-products was faster as the agitator speed was 
increased. The speed was increased to 45 rpm. After about 30 minutes, the 
melt was still very fluid and not being spread by the agitator onto the 
walls of the tube, indicating that the surface generation was not very 
effective. It is estimated that the surface area generated was only about 
15 to 20 ft.sup.2 /ft.sup.3 of the melt. The experiment was continued for 
about another 30 minutes during which the N.sub.2 inlet temperature had 
increased to about 173.degree. C. and the melt temperature was increased 
to about 182.degree. to 199.degree. C. and the agitator rpm was increased 
to 100. During this period the melt became viscous enough to be picked by 
the agitator and spread on the surface of the tube. The experiment was 
continued for another 30 minutes during which the N.sub.2 inlet 
temperature was raised to 220.degree.-240.degree. C., the melt temperature 
increased to 225.degree. to 250.degree. C. and N.sub.2 velocity was 
increased to about 1 ft/sec. The reaction mass started to get viscous and 
was spreading approximately 3/4 the way up the circumference of the tube. 
It is estimated that the surface being generated was about 50 ft.sup.2 
/ft.sup.3 of the melt. During this last 30 minute period the melt became 
quite viscous and the apparatus was shut down. The resulting product was 
analyzed by GPC as in Example 1. The polydispersity was 1.8 and the number 
average DP was 64. Higher DP's would have resulted if the experiment had 
been continued for a longer period.