Molecular sieve abstract
In a process for the manufacture of shaped polyethylene terephthalate
structures, such as film or fiber, a crystalline sodium aluminosilicate
molecular sieve is used as the catalyst for the polymerization of
di(2-hydroxyethyl phthalate) or higher oligomer intermediate. In
film, especially for audio and video tapes, the molecular sieve
serves both as the catalyst and as a slip additive. In addition
to the fact that this process eliminates antimony trioxide, heretofore
used as the polymerization catalyst, the polyethylene terephthalate
film made by the process of this invention has considerably fewer
defects such as microgel inclusions, black specks, and pimples.
Further, the amount of diethylene glycol formed in the process is
reduced. Finally, polyethylene terephthalate degradation, contributing
to the exudation of lower molecular weight materials, also is reduced.
Molecular sieve claims
I claim:
1. In a process for manufacturing a shaped polyethylene terephthalate
structure, which process comprises the steps of
causing ethylene glycol to react with an esterification partner
selected from the group consisting of terephthalic acid and dimethyl
intermediate,
converting the resulting intermediate to polyethylene terephthalate
by heating it at a reduced pressure in the presence of a polymerization
catalyst, and
forming the resulting polyethylene terephthalate in the melt into
a shaped structure,
the improvement comprising using crystalline sodium aluminosilicate
molecular sieve having an average particle size of about 0.5-2.0
micrometers and a pore size such that it adsorbs molecules having
an effective diameter of up to about 10 A, in an amount of about
900-2700 ppm, based on the final shaped article, as the polymerization
catalyst for the intermediate, no antimony compound catalyst being
used.
2. The process of Claim 1 wherein the shaped structure is a film,
and the amount of crystalline sodium aluminosilicate molecular sieve
is 100-1550 ppm, based on the film.
3. The process of Claim 1 wherein the shaped structure is a fiber,
and the amount of crystalline sodium aluminosilicate molecular sieve
is 000-1800 ppm, based on the fiber.
4. The process of Claim 1 wherein the intermediate is made by transesterification
of dimethyl terephthalate, and the transesterification step is carried
out in the presence of catalytic amounts of zinc acetate and of
a lithium compound.
5. The process of Claim 4 wherein the lithium compound is lithium
glycolate.
6. The process of Claim 1 which is a continuous dimethyl terephthalate
transesterificaton process.
7. The process of Claim 1 wherein the shaped structure is a film,
and the molecular sieve used as polymerization catalyst is retained
therein in sufficient amount to serve as slip additive.
8. The process of Claim 1 wherein the intermediate is an oligomer
having predominantly carboxyl end groups having up to ten repeating
units, which is made without a catalyst.
9. The process of Claim 8 wherein the intermediate is isolated
and fed to a separate polymerization reactor.
10. The process of Claim 1 wherein the inermediate is subjected
to polymerization without being isolated.
Molecular sieve description
BACKGROUND OF THE INVENTION
Shaped structures of polyethylene terephthalate (sometimes abbreviated
herein to PET), such as, for example, film and fibers, are well
established commercial products. The film finds numerous applications,
e.g., in packaging, photographic film base, and audio and video
tape. PET films sometimes are difficult to handle, and especially
to wind, because of the tendency of adjoining film layers to cling
to one another. This problem can be solved, however, by adding to
the film-forming material, prior to the extrusion of the film, certain
fillers or slip additives, including certain crystalline sodium
aluminosilicates, for example, as described in U.S. Pat. Nos. 3876608
to Anderson et al.
PET is commercially produced by a two-step process which comprises
as the first step either ester interchange (or transesterification)
between ethylene glycol and dimethyl terephthalate, preferably catalyzed
by zinc acetate in the presence of a lithium compound such as, e.g.,
lithium glycolate, to produce di(2-hydroxyethyl) terephthalate (sometimes
abbreviated herein as DHET), or direct esterification of terephthalic
acid with ethylene glycol, which in the first step produces an oligomer
having up to about ten repeating units. In the second step, either
DHET or the oligomer, as the case may be, is polymerized to polyethylene
terephthalate at an elevated temperature and at a reduced pressure.
The polymerization step requires a catalyst, and a common catalyst
for this step is antimony trioxide, as disclosed in U.S. Pat. No.
2647885 to Billica.
While antimony trioxide is an excellent catalyst, it nevertheless
has various drawbacks, such as producing in the film defects such
as black specks, large area polarization defects, and pimple rejects
caused by polymer inclusions. Further, in other applications, for
example, in making fibers, antimony trioxide, which often contains
impurities, imparts undesirable color to the product. It also appears
that a reaction between antimony trioxide and ethylene glycol occurs,
producing a fairly volatile compound, which has a tendency to form
deposits in the extruding equipment as well as in the spinnerets
through which fibers are spun, so that they may affect the shape
or the size of the fibers. Other, nonvolatile antimony compounds
precipitate from the process stream and cause fouling of reaction
vessels.
Finally, it is expected that with increasing concern about environmental
problems, use of antimony trioxide may be limited or restricted
in the future.
It, therefore, would be highly desirable to be able to produce
the polymer in the presence of a catalyst which would not have all
the above shortcomings.
SUMMARY OF THE INVENTION
According to the present invention, there is provided
in a process for manufacturing a shaped polyethylene terephthalate
structure, which process comprises the steps of
causing ethylene glycol to react with an esterification partner
selected from the group consisting of terephthalic acid and dimethyl
terephthalate, to form a low molecular weight intermediate,
converting the resulting intermediate to polyethylene terephthalate
by heating it at a reduced pressure in the presence of a polymerization
catalyst, and
forming the resulting polyethylene terephthalate in the melt into
a shaped structure,
the improvement comprising using crystalline sodium aluminosilicate
molecular sieve having an average particle size of about 0.5-2.0
micrometers and a pore size such that it adsorbs molecules having
an effective diameter of up to about 10 A, in an amount of about
900-2700 ppm, based on the final shaped article, as the polymerization
catalyst for the intermediate, no antimony compound catalyst being
used.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention can be conducted essentially
in the same manner as the processes heretofore employed in the industrial
practice, except that the antimony trioxide catalyst is eliminated
from the polymerization step.
Crystalline sodium aluminosilicate molecular sieve is a commercial,
synthetic zeolite. Typical suitable zeolites include those available
from Union Carbide Co. under the names Linde Molecular Sieve.RTM.
Type 13X and Type 4A. The former can be represented by the following
formula:
where the value of X depends on the degree of hydration of the
material. The activated material can adsorb about one third of its
weight of water, but the commercial material contains less than
1.5 to less than 2.5% of water, depending on its particle size.
It has a pore size such that it can adsorb molecules having an
effective diameter of up to about 10 A, and especially those that
are adsorbed on molecular sieves of types 3A, 4A, and 5A.
The second zeolite has the formula:
Generally speaking, the usual commercial crystalline sodium aluminosilicates
which can be ground to the desired particle size are suitable in
the process of this invention. This catalyst is retained in the
PET and in the final fabricated, shaped structure, but it is not
considered harmful; and, in fact, it is desired in the film as a
slip additive, as discussed above.
The amount of sodium aluminosilicate should be at least about 900
ppm. For PET film, the preferred amount is about 1100-1550 ppm,
based on the final PET film, since at this level it provides the
optimum slip properties. For fiber manufacture as well as for most
other uses, the preferred amount of sodium aluminosilicate is about
1000-1800 ppm, based on the final shaped structure because at this
level it produces PET of sufficient molecular weight to be useful
for its intended purposes, without adversely affecting the appearance
or the performance of the final shaped structure. Since the catalyst
is retained in the PET product, it is recommended to use in such
other applicatons the lowest amount of catalyst which produces PET
of sufficient molecular weight for the intended purpose; however,
a larger amount of catalyst will not impair the quality of the product,
although it is likely to adversely affect its appearance. The commercial
material, which may have an average particle size of about 2-10
micrometers (5.0 micrometers for Linde.RTM. Type 13X), is finely
ground to the desired average particle size. For example, for the
video tape, the average particle size is about 0.5 micrometer, while
for other applications such as, e.g., audio tape, it can be larger,
within the specified range. For use in fibers and shaped structures
in which the molecular sieve does not have a separate function,
such as improving slip, the fine size of about 0.5-1 micrometer
is satisfactory.
In the preferred amounts, this zeolite, in addition to being an
excellent polymerization catalyst, also provides a good degree of
slip for the PET film. Within the range of 900-1000 ppm, winding
of the film may be more difficult, while above 2700 ppm, the zeolite
may cause undesirable surface characteristics such as, e.g., excessive
roughness or loss of clarity. In general, the amount of sodium aluminosilicate
molecular sieve used in this application depends more on the surface
quality and customer acceptance than on its catalytic activity.
In a preferred embodiment of this invention, the initial esterification
reaction will be conducted in a continuous manner, although a batch
process also can be employed. For each mole of esterification partner,
two moles of ethylene glycol are fed - in the case of transesterifcation
of dimethyl terephthalate, together with the transesterification
catalyst, which preferably is zinc acetate together with lithium
glycolate--to a reactor maintained at a temperature of about 235.degree.
C. and a pressure of about 36 kPa at a rate such that the residence
time is about 70-80 min. The amount of zinc, as zinc acetate, is
about 50 ppm, and that of lithium, as lithium glycolate, is about
20 ppm, both based on the final film. Lithium glycolate is used
both as a diethylene glycol formation inhibitor and as an ester
interchange catalyst. In the case of direct esterification, no catalyst
is necessary; see, e.g., U.S. Pat. No. 4340550 to Ho.
Methyl alcohol formed in the transesterification reaction is continuously
removed from the reactor and is recycled to the esterification of
terephthalic acid. Sodium aluminosilicate molecular sieve is added
to DHET formed in the transesterification step and is continuously
preheated to about 260.degree. C. at a pressure gradually decreasing
along its path from about 13.3 kPa to 4 kPa, to remove excess ethylene
glycol.
The so concentrated material then is subjected to a still higher
temperature of about 285-290.degree. C. and a lower pressure of
about 0.67 kPa, to complete the polymerization reaction. Ethylene
glycol formed in this step is removed at the reduced pressure and
recycled.
In the case of direct esterification, the solid oligomer intermediate
may be isolated and then introduced into a separate polymerization
reactor but usually is piped to the polymerization vessel without
isolation.
PET resin obtained in this process is then fabricated into the
final product. It thus can be extruded into a film, which advantageously
is stretched in a conventional manner in the machine direction and,
optionally, in the transverse direction; or it can be melt-spun
into fibers in a manner known to the art.
In comparative plant runs of film-manufacturing process, employing
sodium aluminosilicate molecular sieve at the level of 1325 ppm,
PET film production rate was maintained at the same level in the
absence and in the presence of 400 ppm of antimony trioxide catalyst
at the DHET polymerization stage, this being the only difference
between those runs.
In addition to the surprising fact that the elimination of antimony
trioxide from the usual commercial process has not reduced the efficiency
of the polymerization reaction, an additional benefit has been realized
in that the PET film produced by the improved process of this invention
has a lower incidence of defects such as large area polarization
(LAP) defects, black specks, and pimples. Further, the film-extruding
equipment can be operated with longer intervals between cleanups
because of a lower exudation rate.
Exudation is primarily due to cyclic ethylene terephthalate oligomer
(principally trimer), which is always formed in equilibrium reactions
through the entire polymerization process and is believed to be
unavoidable, accompanied by smaller amounts of low molecular weight
PET degradation products. These materials collect especially around
the extruder's die. The improvement caused by the elimination of
antimony trioxide appears to reduce the amount of low molecular
weight PET degradation products.
Film made according to this process in plant-size equipment, which
had a thickness of 14 micrometers and had a degree of stretch of
approximately 3.5 times in the machine direction and 4.0 times in
the transverse direction, was examined according to the standards
adopted for commercial PET film. In the LAP test, using a piece
of polarized plastic, about 25.times.37.5 cm, and examining the
film under polarized light, microgel inclusions and very small black
particles, which tend to shine under these conditions, are counted.
Three fields of view (total area of about 2 m.sup.2) are examined,
and the total number of defects found in those three fields is added.
When operating according to the earlier process, in which 400 ppm
of antimony trioxide had been used, the number of such LAP defects
was about 2-6/m.sup.2. After eliminating antimony trioxide from
the process, this FIGURE gradually decreased within 48 hours to
0/m.sup.2 and remained at that level.
Black specks (larger black particles) and pimple defects also were
virtually completely eliminated from the PET film made by the process
of the present invention. Pimples are polymer inclusions or air
borne contamination. Pimples caused by polymer inclusions, black
specks, and LAP defects all are primarily wall scale that forms
on transfer line piping and sloughs off, causing defects in the
film. About 90% of such defects had been found to contain antimony.
The elimination of antimony trioxide also had a desirable side
effect of reducing the amount of diethylene glycol from 1.98 to
1.61%. Diethylene glycol normally is formed from ethylene glycol
during polymerization and is incorporated into the polymer chain.
At high levels, it adversely affects such film properties as. e.g.,
its tensile strength and high temperature resistance. Antimony trioxide
is known to enhance diethylene glycol formation from glycol.
Further laboratory experiments were carried out as follows.
PET was prepared in the laboratory by heating the indicated materials
in glass tubes in a resistance-heated metal block. The starting
material was in all cases an oligomer prepared by the direct esterification
of terephthalic acid with ethylene glycol. It had an average molecular
weight of about 1350 and had an average number of seven repeating
units. Its freezing point was 245.degree. C. The carboxyl group
level was about 750 microequivalents per gram. It is noted that
the number of free carboxyl groups is not necessarily related to
the molecular weight of oligomer but distinguishes this oligomer
from the intermediate obtained from dimethyl terephthalate, which
intermediate has almost exclusively hydroxyl end groups. Surprisingly,
molecular sieves very effectively catalyze the polymerization of
both types of intermediates.
Antimony trioxide catalyst was used as a 1% solution in ethylene
glycol. Molecular sieve catalyst was Linde Molecular Sieve.RTM.
13X, type 9356 which had a nominal pore size of 10 A, obtained
from Union Carbide Corporation. It was added as a 10% suspension
in ethylene glycol. The suspension was repeatedly milled and filtered
to remove large particles. |