Process for polyesters with improved properties

The present invention relates to a process for producing polyesters displaying exceptionally low acetaldehyde concentrations and good clarity comprising the steps of: PA1 polycondensing in the melt phase, a polyester monomer/oligomer mixture under conditions sufficient to form a precursor having in intrinsic viscosity less than 75% of a possible maximum intrinsic viscosity; and PA1 solid stating said precursor under conditions sufficient to increase said intrinsic viscosity at least about 0.05 dl/g.

BACKGROUND OF THE INVENTION 
Although acetaldehyde occurs naturally in many foods, in the manufacture of 
polyesters, it is an undesirable product of degradation reactions. Because 
acetaldehyde is detectable at very low levels, even small amounts can 
adversely affect the taste and odor of food or beverages such as water. In 
the manufacture of polyester containers, therefore, it is desirable to 
reduce the amount of acetaldehyde in the container sidewall which in turn 
minimizes the amount of acetaldehyde that is absorbed into the contents of 
the container. We have discovered a process for producing polyester resins 
that yield satisfactorily reduced levels of acetaldehyde in container 
sidewalls with improved clarity. 
PRIOR ART 
Much of the art devoted to low acetaldehyde levels in PET focuses on 
treatment of the polymer after melt-phase polycondensation. A number of 
processes have been proposed which contact solid polyester pellets with 
water vapor or liquid to reduce acetaldehyde. According to U.S. Pat. No. 
4,591,629, the polyester should be treated prior to solid-state 
polycondensation, while in European Patent Application 222,714, gas 
containing water and carbon dioxide makes contact with the pellets during 
the crystallization step which precedes solid-state polycondensation. U.S. 
Pat. Nos. 5,270,444, 5,241,046, and 5,444,144 propose treating polyester 
with water following solid-stating polycondensation to reduce 
acetaldehyde. 
U.S. Pat. No. 5,362,844 describes a process in which PET is polymerized to 
an intrinsic viscosity of at least 0.60 dl/g (preferably at least 0.65 
dl/g) in a melt polycondensation reactor. Between the exit of the reactor 
and the exit of the pelletizer, the temperature of the PET is not 
increased and the residence time is sufficiently low so that the free 
acetaldehyde in the polymer is not increased by more than 30 ppm. The 
pellets are then hardened (a key step in the process), crystallized, and 
subjected to a drying (dealdehydization) step. Slight solid-state 
polycondensation of the PET occurs simultaneously with the 
dealdehydization. 
U.S. Pat. No. 4,340,721 describes a process to prepare low acetaldehyde PET 
in which the polymer is polycondensed in melt phase to an intrinsic 
viscosity of 0.55 to 0.70 dl/g. An essential feature of the process is the 
intrinsic viscosity achieved in melt phase is between 75 and 90% of the 
maximum intrinsic viscosity obtainable; this maximum is defined as a 
threshold beyond which viscosity can no longer be increased at given 
operating conditions because degradation reactions dominate the 
polycondensation reaction. 
DESCRIPTION OF INVENTION 
According to the present invention, there is provided a process comprising 
the steps of: 
polycondensing in the melt phase, a polyester monomer/oligomer mixture 
under conditions sufficient to form a precursor having an intrinsic 
viscosity less than 75% of a possible maximum intrinsic viscosity; and 
solid stating said precursor under conditions sufficient to increase said 
intrinsic viscosity at least about 0.05 dl/g. 
The process of the present invention provides polyesters having improved 
properties for packaging, particularly reduced levels of acetaldehyde and 
improved clarity. 
The process of the present invention further comprises a) esterification 
(or transesterification) of one or more dicarboxylic acids (or their 
dialkyl esters) to form a mixture of polyester monomer and oligomers; b) 
polycondensation to produce a low molecular weight precursor polymer 
having an intrinsic viscosity of less than 75% of a maximum attainable 
intrinsic viscosity; and c) crystallization and solid-state 
polycondensation to produce the desired product. 
The polyesters are any crystallizable polyester homopolymer or copolymer 
that are suitable for use in packaging, and particularly food packaging. 
Suitable polyesters are generally known in the art and may be formed from 
aromatic dicarboxylic acids, esters of dicarboxylic acids, anhydrides of 
dicarboxylic esters, glycols, and mixtures thereof. More preferably the 
polyesters are formed from terephthalic acid, dimethyl terephthalate, 
isophthalic acid, dimethyl isophthalate, 
dimethyl-2,6-naphthalenedicarboxylate, 2,6-naphthalenedicarboxylic acid, 
ethylene glycol, diethylene glycol, 1,4-cyclohexane-dimethanol, 
1,4-butanediol, and mixtures thereof. 
The dicarboxylic acid component of the polyester may optionally be modified 
with up to about 15 mole percent of one or more different dicarboxylic 
acids. Such additional dicarboxylic acids include aromatic dicarboxylic 
acids preferably having 8 to 14 carbon atoms, aliphatic dicarboxylic acids 
preferably having 4 to 12 carbon atoms, or cycloaliphatic dicarboxylic 
acids preferably having 8 to 12 carbon atoms. Examples of dicarboxylic 
acids to be included with terephthalic acid are: phthalic acid, 
isophthalic acid, naphthalene-2,6-dicarboxylic acid, 
cyclohexanedicarboxylic acid, cyclohexanediacetic acid, 
diphenyl-4,4'-dicarboxylic acid, succinic acid, glutaric acid, adipic 
acid, azelaic acid, sebacic acid, and the like. 
In addition, the glycol component may optionally be modified with up to 
about 15 mole percent, of one or more different diols other than ethylene 
glycol. Such additional diols include cycloaliphatic diols preferably 
having 6 to 20 carbon atoms, aliphatic diols preferably having 3 to 20 
carbon atoms and aromatic diols having 6 to 14 carbon atoms. Examples of 
such diols include: diethylene glycol, triethylene glycol, 
1,4-cyclohexanedimethanol, propane-1,3-diol, butane-1,4-diol, 
pentane-1,5-diol, hexane-1,6-diol, 3-methylpentanediol-(2,4), 
2-methylpentanediol(1,4), 2,2,4-trimethylpentane-diol-(1,3), 
2-ethylhexanediol-(1,3), 2,2-diethylpropane-diol-(1,3), hexanediol-(1,3), 
1,4-di-(hydroxyethoxy)-benzene, 2,2-bis-(4-hydroxycyclohexyl)-propane, 
2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane, 
2,2-bis-(3-hydroxyethoxyphenyl)-propane, and 
2,2-bis-(4-hydroxypropoxyphenyl)-propane. Polyesters may be prepared from 
two or more of the above diols. 
The resin may also contain small amounts of tri functional or 
tetrafunctional comonomers such as trimellitic anhydride, 
trimethylolpropane, pyromellitic dianhydride, pentaerythritol, and other 
polyester forming polyacids or polyols generally known in the art. 
Because acetaldehyde is formed from ethylene glycol esters, the polyesters 
of the present invention contain an amount of ethylene glycol from about 1 
to 100 mole % relative to the total glycols in the polyester and more 
preferably from about 85 to about 100 mole %. 
Prior to the polycondensation portion of the melt-phase process, a mixture 
of polyester monomer (diglycol esters of dicarboxylic acids) and oligomers 
are produced by conventional, well-known processes. One such process is 
the esterification of one or more dicarboxylic acids with one or more 
glycols; in another process, one or more dialkyl esters of dicarboxylic 
acids undergo transesterification with one or more glycols in the presence 
of a catalyst such as a salt of manganese, zinc, cobalt, titanium, 
calcium, magnesium or lithium. In either case, the monomer and oligomer 
mixture is typically produced continuously in a series of one or more 
reactors operating at elevated temperature and pressures at one atmosphere 
or greater. Alternately, the monomer and oligomer mixture could be 
produced in one or more batch reactors. Suitable conditions for 
esterification and transesterification include temperatures between about 
220.degree. C. to about 250.degree. C. and pressures of about 0 to about 
20 psig. 
Next, the mixture of polyester monomer and oligomers undergoes melt-phase 
polycondensation to produce a low molecular weight precursor polymer. The 
precursor is produced in a series of one or more reactors operating at 
elevated temperatures. To facilitate removal of excess glycols, water, 
alcohols, aldehydes, and other reaction products, the polycondensation 
reactors are run under a vacuum or purged with an inert gas. Inert gas is 
any gas which does not cause unwanted reaction. Suitable gases include, 
but are not limited to partially or fully dehumidified air, CO.sub.2, 
argon, helium and nitrogen. Catalysts for the polycondensation reaction 
include salts of antimony, germanium, tin, lead, or gallium, preferably 
antimony or germanium. Reactions conditions for polycondensation include a 
temperature less than about 290.degree. C., and preferably between about 
240.degree. C. and 290.degree. C. at a pressure sufficient to aid in 
removing undesirable reaction products such as ethylene glycol. The 
monomer and oligomer mixture is typically produced continuously in a 
series of one or more reactors operating at elevated temperature and 
pressures at one atmosphere or greater. Alternately, the monomer and 
oligomer mixture could be produced in one or more batch reactors. 
A key feature of the invention is maintaining the intrinsic viscosity of 
the precursor produced by melt-phase polycondensation at a level 
significantly lower than that which could be obtained at the final reactor 
operating conditions. High intrinsic viscosities, greater than about 75% 
of the maximum attainable intrinsic viscosity at the reactor conditions, 
require relatively long residence times in the reactor since the degree of 
polymerization increases more slowly as the intrinsic viscosity approaches 
its maximum attainable value. Unfortunately, long residence times at this 
stage of processing generate much of the acetaldehyde and acetaldehyde 
precursors found in the polyester. 
We have found that the intrinsic viscosity of the precursor produced in 
melt-phase polycondensation should be limited to less than 75% of the 
maximum intrinsic viscosity that could be attained at the operating 
conditions, preferably to less than 65% of the maximum attainable 
intrinsic viscosity, and more preferably to less than 55% of the maximum 
attainable intrinsic viscosity. The maximum intrinsic viscosity limitation 
was surprising as U.S. Pat. No. 4,340,721 states that if the precursor 
intrinsic viscosity is less than 75% of that attainable, the process is 
not a high-performance process. We find, however, that when producing 
precursor with lower intrinsic viscosities, higher production rates can be 
achieved, giving improved process performance, an advantage of the present 
invention. In addition, intrinsic viscosity of the polyester precursor 
should be less than 0.70 dl/g, preferably less than 0.65 dl/g, more 
preferably less than 0.60 dl/g. 
Intrinsic viscosity is measured using 0.25 g polyester in 50 ml of 60:40 
phenol/tetrachloroethane at 25.degree. C. 
Maximum attainable intrinsic viscosity for a particular catalyst system and 
finisher conditions (temperature and pressure) may be determined 
experimentally. About 25 g of polyester oligomer containing the desired 
catalyst system are melt polymerized at the temperature and pressure of 
interest in a flask. Several experiments were run at various 
polymerization times to find the maximum attainable intrinsic viscosity. 
Another key feature of the invention is that the precursor is crystallized 
and undergoes further polycondensation in the solid state by conventional, 
well-known processes, such as those disclosed in U.S. Pat. No. 4,064,112. 
Solid-state polycondensation can be conducted in the presence of an inert 
gas as defined above, or under vacuum conditions, and in a batch or 
continuous process. The polyester can be in the form of pellets, granules, 
chips, or powder. Temperature during the solid-state polycondensation 
process should be maintained less than 240.degree. C., preferably less 
than 230.degree. C. The increase in intrinsic viscosity during solid-state 
polycondensation is at least about 0.05 dl/g, preferably at least about 
0.10 dl/g. No hardening step is necessary prior to crystallization. 
It is well known in the art that vinyl ester ends, formed by degradation 
reactions during polycondensation, can later form acetaldehyde during 
molding of packaging from the polyester resin. We believe that at least 
part of the success of the current invention occurs because there are 
fewer vinyl ends in the product. 
By limiting the intrinsic viscosity of the precursor to below 75% of the 
maximum attainable intrinsic viscosity, residence time in melt-phase 
polycondensation is minimized, giving vinyl ends less time to form. In 
addition, at lower precursor intrinsic viscosities, the vinyl ends that 
are formed are more rapidly consumed by reactions with other end groups. 
Another advantage of maintaining lower precursor intrinsic viscosities is 
that more solid-state polycondensation is needed to attain the desired 
product intrinsic viscosity. We believe that a solid-state 
polycondensation step is necessary because the level of vinyl ends 
decreases during this part of the process. At the low temperatures 
characteristic of solid-state polycondensation, few vinyl ends are formed; 
however, many of the vinyl ends formed during melt-phase polycondensation 
can be consumed. 
Additives which are known to reduce AA may also be added. Such additives 
include polyamides selected from the group consisting of low molecular 
weight partially aromatic polyamides having a number average molecular 
weight of less than 15,000, low molecular weight aliphatic polyamides 
having a number average molecular weight of less than 7,000 and wholly 
aromatic polyamides and polyesteramides as disclosed in U.S. Ser. No. 
595,460. Suitable polyamides are disclosed in U.S. Ser. No. 548,162. 
Other ingredients may be added to the compositions of this invention as 
desired to enhance the performance properties of the polyesters. For 
example, surface lubricants, denesting agents, stabilizers, antioxidants, 
ultraviolet light absorbing agents, mold release agents, metal 
deactivators, colorants, nucleating agents, phosphorus-containing 
stabilizers, reheat rate (enhancers, zeolites, fillers and the like may be 
added. 
The following examples further illustrate the invention.

EXAMPLES 
Poly(ethylene terephthalate) modified with 1.5 wt % diethylene glycol, 1.1 
wt % 1,4-cyclohexanedimethanol, and an antimony-phosphorus catalyst system 
was produced as follows. Terephthalic acid was continuously esterified 
with ethylene glycol in a series of two reactors. The mixture of monomer 
and oligomer thus produced was then polycondensed in melt-phase to an 
intrinsic viscosity of 0.57 dl/g (which represents at most 53% of the 
maximum attainable intrinsic viscosity, which at the temperature, 
pressure, and catalyst levels employed was at least 1.08 dl/g). 
The precursor underwent crystallization, followed by solid-state 
polycondensation at a temperature of about 215.degree. C., reaching an 
intrinsic viscosity of 0.72 dl/g, making the increase in intrinsic 
viscosity during solid-state polycondensation 0.15 dl/g. The CDM color of 
the resin produced was L*=87.6, a*=1.7, and b*=0.9. Water bottle parisons 
were injected molded from this polyester resin. Acetaldehyde content of 
the parisons was 3.0 ppm (by weight) maximum, well within the 
specification of 4.0 ppm. 
COMATIVE EXAMPLE 
Poly(ethylene terephthalate) modified with 1.5 wt % diethylene glycol, 1.1 
wt % 1,4-cyclohexanedimethanol, and an antimony-phosphorus catalyst system 
was produced as follows. Terephthalic acid was continuously esterified 
with ethylene glycol in a series of two reactors. The mixture of monomer 
and oligomer thus produced was then polycondensed in melt-phase to an 
intrinsic viscosity of 0.66 dl/g under conditions where the maximum 
attainable Intrinsic viscosity was about 0.854 dl/g, or 77% of the maximum 
achievable intrinsic viscosity. 
The precursor underwent crystallization, followed by solid-state 
polycondensation at a temperature of about 215.degree. C., reaching an 
intrinsic viscosity of 0.80 dl/g, making the increase in intrinsic 
viscosity during solid-state polycondensation 0.14 dl/g. The CDM color of 
the resin produced was L*=82.5, a*=-1.7, and b*=1.0. Water bottle parisons 
were injected molded from this polyester resin. Acetaldehyde content of 
the parisons was 4.5 ppm (by weight) maximum, above the specification of 
4.0 ppm.