METHOD FOR QUENCHING OF CATALYST IN
THE PRODUCTION OF LOW MOLECULAR
WEIGHT POLYCARBONATES
This application relates to the manufacture of polycarbonates via a melt
polycondensation reaction, and in particular to an improved method for finishing
such compositions through the addition of a quenching agent to neutralize the
alkaline condensation catalyst.
Aromatic polycarbonates are useful in a great many applications because of
their desirable physical properties, including strength and optical clarity. There
are three processes known for the production of aromatic polycarbonates,
which are illustrated in Fig. 1. The conventional interfacial process and the
phosgene-based melt process start with the reaction of phosgene with carbon
monoxide. The "no phosgene" melt process was developed to eliminate the
use of highly toxic phosgene in the reaction process.
Both types of melt processes make use of a diarylcarbonate such as
diphenylcarbonate (PPG) as an intermediate, which is polymerized with a
dihydric phenol such as bisphenol A (BRA) in the presence of an alkaline
catalyst to form a polycarbonate in accordance with the general reaction shown
in Fig. 2. This polycarbonate may be extruded or otherwise processed, and
may be combined with additives such as dyes and UV stabilizers. In many
cases, however, the presence of residual catalyst has a detrimental effect on
the quality of the product, leading to poor color, molecular weight or rheological
properties. Residual catalyst may also interact with additives, detracting from
their efficacy. Thus, it is desirable to reduce the levels of residual catalyst to
minimize these interactions. Such reduction is referred to as "quenching."
In the production of optical quality polycarbonates, several reactors are used in
sequence to prepare the final product. The fourth and final reactor in this
sequence subjects the reaction mixture to both high temperature and high
vacuum. This treatment assists in the removal of unreacted monomer and
short oligomers, and improves the overall quality of the final product. For
optical quality (OQ) products, this is also the phase of the reaction at which
quencher is added. Because of the very small amount of actual quencher
which is required, the quencher is conventionally added in a solvent. For
example, the assignee company has made use of toluene as a solvent for the
delivery of n-butyl tosylate as a quencher. Unfortunately, this system
negatively impacts on the process for the continuous production of OQ
polycarbonates, because decomposition occurs in the nozzle, which results in
fluctuating flow rates.
It would therefore be desirable to have an alternative system for quenching
polycarbonate reactions that occur at high temperature and vacuum, which
does not suffer from this drawback. It is an object of the present invention to
provide such a system.
SUMMARY OF THE INVENTION
The present invention provides a method for quenching alkaline catalyst
present in the product of a melt polycondensation reaction, comprising adding
to the product a sulfonic acid ester quencher in a carrier. The carrier comprises
a first carrier component effective to solubilize thie quencher and having a lower
boiling point than the quencher, and a second carrier component soluble in the
first carrier component and having a higher boiling point than the quencher. An
exemplary carrier composition contains equal parts of diphenylcarbonate and
toluene.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows three alternative processes for production of polycarbonate; and
Fig. 2 shows the reaction carried out in a base-catalyzed melt
polycondensation reaction.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an improved methodology for the addition of
quencher to the product of a melt condensation reaction under conditions of
high temperature and high vacuum. It will be appreciated by persons skilled in
the art that variation in temperature and vacuum levels occurs depending on
the nature of the apparatus employed and the product desired. In general,
however, the term "high temperature" as used herein refers to temperatures
greater than 260X, and preferably in the range of about 270 to 310 "C; the
term "high vacuum" refers to pressures of less than about 0.8 torr, and
preferably in the range of about 0.2 to 0.6 torr.
In accordance with the invention, finished polycarbonates are made by adding
a quencher composition to the product of a melt polycondensation reaction.
The quencher compositions comprises a sulfonic acid ester (e.g. alkyl tosylate)
quencher in a carrier comprising first and second carrier components. The
amount of quencher in the quencher composition can be varied over a
substantial range, since the carrier acts primarily as a diluent to facilitate
uniform addition of small amounts of quencher to the product of the melt
polycondensation reaction. Quencher compositions with higher concentrations
of quencher can be used in smaller amounts, while quencher compositions with
lower quencher concentrations are used in larger amounts to arrive at the same
final levels of quencher. In general, the level of quencher in the quencher
composition will be from about 0.1 to 10 % by volume, preferably around 1 %.
The two carrier components together provide a carrier which effectively delivers
the quencher to the product of the melt polycondensation reaction without the
aforementioned processing difficulties. The first carrier component is a low
boiling point solvent (lower boiling point than the quencher), which is capable of
solubilizing the sulfonic acid ester quencher but which is essentially unreactive
with the polycondensation product under conditions of high temperature and
high vacuum. Examples of specific materials suitable for use as the first carrier
component include xylene, toluene, benzene, chlorobenzene, anisole and
ethylbenzene.
The second component is selected to have a higher boiling point than the
sulfonic acid quencher, and to be soluble in the first component. This second
component will frequently be a component of the melt condensation reaction,
thus avoiding the introduction of impurities. Thus, suitable materials for use as
the second component include diphenylcarbonate, phenol, bisphenol A (BPA),
polycarbonate oligomers, BPA derivatives and propylene carbonate.
The first and second components are combined in the quencher compositions
used in the invention Is ratios of from about 4:1 to 1:4, preferably around 1:1.
The resulting quencher composition is injected into a stream of melt
polycondensation reaction product and processed (for example by extrusion) to
disperse the quencher composition throughout the melt. The resulting products
are high quality polycarbonate, and processing proceeds without the
instabilities in flow rate observed when only the low-boiling solvent Is utilized.
COMPARATIVE EXAMPLE
A liquid solution at room temperature containing 1 part n-butyl tosylate and 99
parts toluene was injected at a rate of 2.65 kg/hr into a pipeline in the upper
stream of a melt polycarbonate continuous reactor operating at 285''C and 0.4
torr. Polycarbonate throughput was 4000 kg/hr, and the level of alkaline
catalyst (NaOH) was 1 X 10"^ mole/ mole BPA. Thus, the quencher was added
in a 6-fold excess relative to NaOH. After one day of operation, instabilities in
flow rate were observed due to the deposition of coal tar-like decomposition
products of the quencher at the feeding nozzle.
EXAMPLE
A liquid solution at room temperature containing 1 part n-butyl tosylate, 50 parts
diphenylcarbonate and 50 parts toluene was injected at a rate of 2.65 kg/hr into
a pipeline in the upper stream of a melt polycarbonate continuous reactor
operating at 285X and 0.4 torr. Polycarbonate throughput was 4000 kg/hr,
and the level of alkaline catalyst (NaOH) was 1 X 10"* mole/ mole BPA. Thus,
the quencher was added in a 6-fold excess relative to NaOH. After several
days of operation, no instabilities in flow rate were observed. The vacuum level
in the reaction remained unchanged.
What is claimed is:
1. A method for quenching alkaline catalyst present in the product of a melt
polycondensation reaction, comprising adding to the product a sulfonic acid
ester quencher in a carrier comprising a first carrier component effective to
solubilize the quencher and having a lower boiling point than the quencher, and
a second carrier component soluble in the first carrier component and having a
higher boiling point than the quencher.
2. The method of claim 1, wherein the sulfonic acid ester quencher is n-
butyl tosylate.
3. The method of claim 1, wherein the second carrier component is
selected from the group consisting of diphenylcarbonate, phenol, bisphenol A
(BPA), polycarbonate oligomers, BPA derivatives and propylene carbonate.
4. The method of claim 1, wherein the first carrier component is selected
from the group consisting of xylene, toluene, benzene, chlorobenzene, anisole
and ethylbenzene.
5. The method of claim 4, wherein the second carrier component is
selected from the group consisting of diphenylcarbonate, phenol, bisphenol A
(BPA), polycarbonate oligomers, BPA derivatives and propylene carbonate.
6. The method of claim 5, wherein the sulfonic acid ester quencher is n-
butyl tosylate.
7. The method of claim 6, wherein the first carrier component is toluene.
8. The method of claim 6, wherein the second carrier component is
diphenylcarbonate.
9. The method of claim 8, wherein the first carrier component is toluene.
10. The method of claim 1, wherein the first and second carrier components
are present in a ratio of 4:1 to 1:4, by volume.
11. The method of claim 10, wherein the first and second carrier
components are present in a ratio of about 1:1, by volume.
12. The method of claim 10, wherein the sulfonic acid ester quencher is
combined with the carrier at a level of from 0.1 to 10 %, by volume.
13. The method of claim 12, wherein the sulfonic acid quencher is n-butyl
tosylate.
14. The method of claim 12, wherein the second carrier component is
selected from the group consisting of diphenylcarbonate, phenol, bisphenol A
(BPA), polycarbonate oligomers, BPA derivatives and propylene carbonate.
15. The method of claim 12, wherein the first carrier component is selected
from the group consisting of xylene, toluene, benzene, chlorobenzene, anisole
and ethylbenzene.
16. The method of claim 15, wherein the second carrier component is
selected from the group consisting of diphenylcarbonate, phenol, bisphenol A
(BPA), polycarbonate oligomers, BPA derivatives and propylene carbonate.
17. The method of claim 16, wherein the sulfonic acid quencher is n-butyl
tosylate.
18. The method of claim 17, wherein the first carrier component is toluene.
19. The method of claim 17, wherein the second carrier component is
diphenylcarbonate.
20. The method of claim 19, wherein the first carrier component is toluene.
21. The method of claim 1, wherein the quencher and carrier are added to
the melt polycondensation product under conditions of high temperature and
high vacuum.

Improved performance in high temperature, high vacuum reactors is obtained
by quenching alkaline catalyst present in the product a melt polycondensation
reaction using a sulfonic acid ester quencher in a carrier. The carrier if formed
from a first carrier component effective to solubilize the quencher and having a
lower boiling point than the quencher, and a second carrier component soluble
in the first carrier component and having a higher boiling point than the
quencher. An exemplary carrier composition contains equal parts of
diphenylcarbonate and toluene.