METHOD FOR QUENCHING OF POLYCARBONATE AND
COMPOSITIONS PREPARED THEREBY
CROSS REFERENCE TO RELATED PATENTS
None
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
This application relates to the finishing of polycarbonate using a melt process,
and in particular to a method for quenching residual catalyst used in the
polycarbonate-forming reaction, and the products formed by this reaction.
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 diary Icarbonate such as
diphenylcarbonate (DPC) as an intermediate, which is polymerized with a dihydric
phenol such as bisphenol A (BPA) 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 Theological 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."
Commonly assigned US Patent No. 5,606,007, which is incorporated herein by
reference, discloses the use of acidic compounds to quench residual alkalinity. The
acid compounds tested are shown to produce polycarbonates with improved heat and
water resistance, and low yellowness indices. An important consequence of residual
alkaline catalyst which is not directly addressed by this patent is the base-catalyzed
coupling of UV absorbers to the polycarbonate backbone. Normally, amoimts of
liquid quencher of less than 4 ppm are utilized. Although this amount is small, it
corresponds to approximately 4 times the theoretical amount of quencher which
should be necessary to neutralize all of the catalyst used. Nevertheless, despite the
excess of quencher, reaction between UV absorber and polycarbonate backbone still
occurs, impairing the efficiency of UV protection. Furthermore, it does not appear
that the addition of more quencher has any significant effect on the levels of reaction
between the UV absorber and the polycarbonate once a threshold has been reached.
Thus, there remains a need for a method for quenching residual alkaline catalyst
which more effectively reduces the interaction of UV absorbers with polycarbonate.
It would be desirable to provide such a method.
It would further be desirable to provide finished polycarbonate compositions
with high UV retention values and good color stability.

SUMMARY OF THE INVENTION
These and other objects of the invention are provided by a method for
finishing polycarbonate produced by melt condensation of a diaryl carbonate and a
dihydric phenol in the presence of a basic catalyst to produce an intermediate
polycarbonate composition, comprising the steps of;
(a) combining the intermediate polycarbonate composition with an alkyl
tosylate quencher and phosphorous acid; and
(b) processing the combination of the intermediate polycarbonate
composition and the quenchers composition to blend the combination and quench
residual basic catalyst present in the intermediate polycarbonate composition. The
efficiency of quenching can be measured by UV retention. Surprisingly, although
phosphorous acid alone has no impact on the UV retention, the combination of alkyl
tosylate quenchers and phosphorous acid provides a significant improvement over the
use of the alkyl tosylate quencher alone. Thus, the method of the invention permits a
reduction in the amount of alkyl tosylate quencher used, and can achieve superior
results which cannot be achieved even by increasing the alkyl tosylate levels.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows three methods for manufacturing polycarbonates; and
Fig. 2 shows the reaction of a diarylcarbonate and a dihydric phenol to
produce a polycarbonate.
DETAILED DESCRIPTION OF THE INVENTION
In the method of the present invention, residual alkaline catalyst present in a
polycarbonate composition formed from the reaction of a diaryl carbonate and a
dihydric phenol is quenched using an alkyl tosylate and phosphorous acid to provide a
polycarbonate composition with improved properties. The method of the invention
can be employed as a finishing step in the preparation of polycarbonates where an
intermediate polycarbonate composition is formed from the reaction of a diaryl
carbonate and a dihydric phenol in the presence of a basic catalyst in a melt.
Preparation of polycarbonate compositions using this basic technique are known in
the art, for example from US Patents Nos. 5,717,057, 5,606,007 and 5,319,066 which
are incorporated herein by reference.
While a preferred diaryl carbonate for use in the method of the invention is
diphenylcarbonate, other diaryl carbonates may be used to make specialty
polycarbonates. Various methods for synthesis of diaryl carbonates are known, for
example fix)m US Patents Nos. 5,210,268, 5,834,615 and 5,713,453 which are
incorporated herein by reference.
A preferred dihydric alcohol for use in the method of the present invention is
bisphenol A. Other dihydric alcohols, including those listed in US Patent No.
5,717,057 may also be used.
Catalysts used in the method of the present invention to form the intermediate
composition are basic catalysts such as alkali metal or alkaline earth metal compounds
or nitrogen-containing basic compounds which are effective to catalyze the production
of polycarbonates by melt condensation of the diaryl carbonate and the dihydric
phenol. Any of the known catalysts useful for this purpose may be employed.
The method of the invention provides a finishing step to reduce or eliminate
residual catalyst in the polycarbonate product in order to minimize detrimental effects
on the properties of the finished product. In accordance with the invention, this
finishing step is accomplished by combining the intermediate polycarbonate
composition with an alkyl tosylate and phosphorous acid; and processing the
combination of the intermediate polycarbonate composition and the alkyl tosylate and
phosphorous acid to blend the compositions and quench residual basic catalyst present
in the intermediate polycarbonate composition.
The alkyl tosylate and the phosphorous acid may be introduced directly into
the polycarbonate product of a base-catalyzed polycondensation. Alternatively, the
intermediate polycarbonate product may be pelletized, and then remelted, for example
in an extruder, for compounding with the quenchers. The amount of alkyl tosylate
added will depend on the amount of alkaline catalyst used in the original reaction. For
common coirmiercial levels of catalyst, a suitable level of n-butyl tosylate quencher is
between 1 and 7 ppm.
The alkyl tosylate quencher may be combined with the polycarbonate
condensation product in any of several ways. In a first embodiment, the alkyl tosylate
is combined with a polycarbonate powder, optionally containing additional finishing
additives, and introduced through a side feed of the extruder. In a second
embodiment, the alkyl tosylate is combined with a liquid carrier, for example
propylene carbonate, and injected into the molten polycarbonate within the extruder.
In a third embodiment, the alkyl tosylate is combined with polycarbonate in pellets
and introduced through a side feed of the extruder. The pellets may be formed by pre-
compounding the alkyl tosylate with polycarbonate; by compacting a mixture of
polycarbonate powder and alkyl tosylate; or by coating alkyl tosylate as a layer on the
exterior of polycarbonate pellets. Introduction of alkyl tosylate quenchers using each
of these approaches is described in US Patent Application Serial No. 09/273,379,
which is incorporated herein by reference.
The introduction of phosphorous acid can be accomplished as part of a powder
masterbatch, optionally including other additives. This masterbatch is introduced via
a side feed of the extruder. The phosphorous acid can also be introduced by liquid
injection in water solution via a nozzle mounted in the extruder. The amoimt of
phosphorous acid added will depend on the amount of alkaline catalyst used in the
original reaction. For common commercial levels of catalyst, a suitable level of
phosphorous acid quencher is between 1 and 6 ppm.
As described in the examples below, test runs were made comparing
polycarbonate finished with quenching compositions in accordance with the invention
to polycarbonate finished without quencher or using an alkyl tosylate quencher or
phosphorous acid quencher alone. To analytical tests were used to evaluate the
efficiency of quenching.
In the first test, the reactivity between polycarbonate and a UV absorber
(Cyasorb 5411''''^, supplied by Cytec) was measured. Since this UV absorber has a
reactive OH group, it can react in the presence of residual catalyst with the
polycarbonate backbone. The amount of this reaction which occurs can be determined
by measuring the UV absorbance due to the UV absorber, and is directly proportional
to the amount of residual catalyst. Quantitatively, the amount of reaction can be
expressed as
The higher the UV retention is, the better. Surprisingly, despite the fact that
phosphorous acid alone was ineffective to improve UV retention, the UV retention
observed for the combination of the n-butyl tosylate and phosphorous acid was higher
than that obtained for n-butyl tosylate, even at higher levels.
The second test performed is a color stability test. In this test, the yellowness
indices of two chips of polycarbonate molded at 360°C, one for a standard cycle time
and one for a prolonged cycle time of 15 minutes are compared. The greater the
difference between the yellowness index of the two chips, the less color stability the
composition exhibits. Thus, it is desirable to have a low value for the color stability
rating. The best results were obtained using the combination of n-butyl tosylate and
phosphorous acid.
The invention will now be further described with reference to the following,
non-limiting examples.
EXAMPLE 1
Experiments to evaluate the efficiency of quenchers and quencher
combinations in accordance with the invention were performed on a large scale JSW
co-rotative twinscrew extruder 160 mm, at 300°C, with a screw speed of 200 rpm and
a throughput of 5300 kg/hr. An intermediate polycarbonate composition was finished
by introduction of various additives. For all samples, water at a level of 100 ppm and
pentaerythritol tetrastearate (PETS) were injected into the extruder via nozzles and a
polycarbonate powder masterbatch containing 0.1 wt % heat stabilizer and 0.3 wt %
UV absorber (Cyasorb 5411) was added via a side feeder. Phosphorous acid, when
used, was also introduced with this masterbatch. Samples finished with n-butyl
tosylate were treated by injecting a solution containing 4% n-butyl tosylate in
propylene czirbonate at a rate of 0.5 to 1.0 kg/hr.
A total of six experiments were conducted as summarized in Table 1. The
resulting products were evaluated for % UV retention and color stability, as reflected
in the increase in yellowness index (YI) following 15 minutes at 360°C. As shown in
Table 1, sample 6 which was treated with both n-butyl tosylate and phosphorous acid
exhibited surprisingly superior results.
EXAMPLE 2
Solid polycarbonate pellets containing alkyl tosylate quencher which can be
used in the method of the present invention can be prepared in the three ways.
Precompounded quencher masterbatch pellets were prepared by dry blending
of mixture of 100 parts polycarbonate powder and 0.3 parts n-butyl tosylate in a
Henschel blender for a few minutes. The dry blended mixture was then compounded
in a Leitritz co-rotative twin screw extruder 34 mm at 270°C, 250 rpm and 15 kg/hr.
No vacuum was applied during the compounding to avoid volatilization of the
quencher After compounding, dry, transparent and natural pellets were obtained, and
no processing issues were experienced. Analysis of the pellets by HPLC determined
that they contained 1950 ppm n-butyl tosylate and 400 ppm of p-toluene sulfonic acid
(also active as a quencher) which was formed from hydrolysis and thermal
decomposition of the n-butyl ester.
Compacted pellets were prepared by dry blending a mixture of 98 parts
polycarbonate powder, 2 parts pentaerythritol tetrastearate (PETS) and 0.3 parts of n-
butyl tosylate. The dry blended mixture was compacted in a UMT compactor having
a die diameter of 3 mm and a length of 12 mm at 15 kg/hr. The knife was adjusted to
obtain regular white cylinders with an average length of 12 mm. PETS is used as a
lubricant to minimize heat generation by friction. Formulations containing 1.5 parts
PETS were difficult to compact but acceptable. Formulations containing less than 1
part PETS could not be compacted in this apparatus. Analysis of n-butyl tosylate in
the compacted pellets was done by HPLC, and indicated levels of 2990 ppm with
excellent consistency (Std. Deviation = 46).
Coated pellets were prepared in a 150 liter Nauta-Hosokawa conic blender. 90
kg of polycarbonate pellets were introduced into the blender at room temperature and
pre-blended for 5 minutes at 300 rpm. While maintaining the blending, 270 gr of n-
butyl tosylate (0.3 wt % of the total PC weight) were then air sprayed into the pellets
over a period of 2 minutes, creating a mist in the space over the top of the pellets.
Blending continued for 10 minutes, during which time the quencher was absorbed by
the pellets and the mist disappeared. The resulting pellets were dry, slightly hazy, and
have the characteristic odor of the butyl tosylate. HPLC analysis indicated an average
amount of quencher as 2500 ppm, suggesting some quencher accumulation on the
wall of the blender.
What is claimed is:
1. A method for finishing polycarbonate produced by reaction in a melt of a
diaryl carbonate and a dihydric phenol in the presence of a basic catalyst to produce
an intermediate polycarbonate composition, comprising the steps of
(a) combining the intermediate polycarbonate composition with an alkyl tosylate
and phosphorous acid; and
(b) processing the combination of the intermediate polycarbonate composition, the
aUcyl tosylate and the phosphorous acid to blend the combination and quench residual
basic catalyst present in the intermediate polycarbonate composition.
2. The method of claim 1, wherein the aUcyl tosylate is n-butyl tosylate.
3. The method of claim 2, wherein the amount of n-butyl tosylate is from 1 to 7
ppm.
4. The method of claim 3, wherein the amount of phosphorous acid is from 1 to 6
ppm.
5. The method of claim of claim 1, wherein the alkyl tosylate is combined with
the intermediate polycarbonate composition in a liquid carrier.
6. The method of claim 3, wherein the n-butyl tosylate is processed in a
propylene carbonate liquid carrier.
7. Tlie method of claim 3, wherein the amount of n-butyl tosylate is from 2 to 5
ppm.
8. The method of claim 7, wherein the amount of phosphorous acid is from 1 to 6
ppm.
9. An aromatic polycarbonate composition comprising
(a) an aromatic polycarbonate obtained by reacting a diary] carbonate and a
dihydric phenol in the presence of a basic catalyst in a melt;
(b) an alky] tosylate; and
(c) phosphorous acid.
10. The composition of claim 9, wherein the alkyl tosylate is n-butyl tosylate.
11. The composition according to claim 10, wherein the n-butyl tosylate is present
in an amount of from 1 to 7 ppm.
12. The composition according to claim 10, wherein the phosphorous acid is
present in an amotint of from 1 to 6 ppm.
13. The composition according to claim 11, wherein the phosphorous acid is
present in an amount of from 1 to 6 ppm.

Polycarbonate produced by melt condensation of a diaryl carbonate
and a dihydric phenol in the presence of a basic catalyst to produce an intermediate
polycarbonate composition is finished by quenching with both an alkyl tosylate
quencher and phosphorous acid. The efficiency of quenching can be measured by UV
retention. Surprisingly, although phosphorous acid alone has no impact on the UV
retention, the combination of alkyl tosylate quenchers and phosphorous acid provides
a significant improvement over the use of the alkyl tosylate quencher alone. Thus, the
method of the invention permits a reduction in the amount of alkyl tosylate quencher
used, and can achieve superior results which cannot be achieved even by increasing
the alkyl tosylate levels.