Note: Descriptions are shown in the official language in which they were submitted.
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PROCES8ES FOR THB PRODUCTION OF
PQLYACRYLA~IDE PARTICLE8
This invention relates to the production of
substantially dry polyacrylamide particles having reduced
levels of acrylamide monomer contamination.
It is known that polymers made from acrylamide (ie
polyacrylamides) are liable ~o be contaminated by residual
acrylamide monomer and that this is undesirable. One way
of minimising the monomer contamination is to subject the
polymer to treatment with sulphite or other reagent which
will react with the monomer in the polymer. Sulphite
treatment i5 described in for instance JP-A-61/115909.
Unfortunately these chemical reagents have the side effect
of degrading the polymer, thereby reducing its molecular
weight.
For this reason there have been various disclosures of
methods for reducing contamination by acrylamide monomer
with the use of an amidase enzyme, which converts
acry~amide to acrylic acid but without the side effect of
degradation of the polymer.
For instance, in EP-A-329,325 we describe the
production of particulate polyacrylamides having low
acrylamide monomer content involving mixing amidase with
aqueous gel particles containing the polymer. That
application indicated that various conventional amidases
could be used for this purpose.
The mechanism by which that invention operated
successfully was not explained in EP-A-329,325 but others
have postulated that the mechanism necessarily involved
migration of the amidase into the aqueous gel particles of
the polymer.
The amounts of monomer remaining in the products made
in accordance with the examples in EP-A-329,325 are quoted
as 400 and 500 ppm (based on dry polymer) although it is
stated in the description that much lower values can be
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obtained when the amount of amidase and the materials that
are used are appropriate.
The process described in EP-A-329,325 can be very
effective in reducing acrylamide monomer content, but
requires a maintenance period of at least lO minutes,
usually at least 30 minutes, between contacting the aqueous
gel particles with the amidase and drying the aqueous gel
particles.
Such a process can be difficult to incorporate into an
industrial scale process. In an industrial scale process
for production of dry polyacrylamide particles, hot gel
particles are produced, either direct from polymerisation
or from a comminution stage, and pass almost immediately to
a drying stage at high temperature. It can be difficult to
incorporate an extended maintenance period between these
two points.
other processes are known for reduction of acrylamide
monomer in polyacrylamides, using amidase enzyme.
Likewise, all of these require a significant residence time
after application of amidase before the polyacrylamide is
passed to a drying stage. In fact, it has generally been
thought that this maintenance period is essential to enable
the amidase to be effective, in the belief that it would be
denatured and become ineffective at the high temperatures
which normally prevail during drying stages.
There have been publications of methods of treating
acry~amide in emulsion form with amidase so as to reduce
residual acrylamide monomer. These include US 4,687,807,
~S 4,786,679 and US 4,742,114. All of these exemplify
processes in which the amidase is contacted with the
polyacrylamide emulsion for significant periods, normally
at least 2 hours, before adequate reduction of monomeric
acrylamide is obtained. Very low levels of residual
acrylamide (for instance below lO0 ppm) are only obtained
3~ after considerably longer maintenance or incubation periods
of for instance at least lO hours.
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US 4,925,797 describes an enzyme which is said to be
suita~le for treating all forms of polyacrylamide. This
enzyme is heat treated before use, which is said to improve
its activity. Heat treatment in the exemplified processes
is of the order of 1 or 2 hours. Only treatment of polymer
latex is demonstrated. In all the systems exemplified the
amidase is contacted with the polyacrylamide latex for at
least 30 minutes, and in some cases at least 2 or 3 hours.
A review article by Carver and Jones, Microb. Growth
Cl Compd,, [Int. Symp.] 7th (19933, 365-79, describes an
unusual amidase enzyme produced by a strain of
Meth~lophilus methylotroPhus which is inhibited in the
presence o~ ammonia and requires heat-treatment to
reactivate it. After heat-treatment it can be used to
reduce levels of acrylamide monomer in a polyacrylamide
emulsion. These levels are said to become ~elow detectable
levels after 1 hour.
These systems appear to be able to obtain reduction in
~ree acrylamide but only with use of a relatively long
maintenance period in which the amidase is kept in contact
with the polyacrylamide before further treatment. Specific
processes are demonstrated only for aqueous suspensions of
polyacrylamide.
The present invention is concerned with providing
s~stantially dry polyacrylamide particles having low
levels of acrylamide monomer contamination. In particular
it would be desirable to ~e able to do this in the context
of a standard industrial process for forming dry
polyacrylamide powder from aqueous gel without the
necessity to introduce significant amounts of new equipment
or to modify significantly the plant on which the polymer
particles are produced.
ThP invention provides in a first aspect a process for
the production of substantially dry particles of a
polyacrylamide comprising
providing aqueous polyacrylamide gel particles
contaminated with acrylamide monomer,
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applying amidase enzyme to the aqueous gel particles
whilst they are at a temperature of from 50 to 95~C, and
substantially immediately passing the aqueous gel
particles to a drying stage and subjecting them in that
stage to a temperature of at least 60~C so as to produce
substantially dry particles of polyacrylamide.
This process has the significant advantage that it may
be carried out on a plant already adapted for the
production of substantially dry polyacrylamide particles.
There is no need ~or a residence time of half an hour or
more before the particles pass to the drying stage. We
have also found unexpectedly that this process can lead to
the production of acceptably low levels of residual
acrylamide monomer contamination in very convenient manner.
~5 ~t would be expected that without a maintenance period the
amidase would have little chance to affect the acrylamide
monomer ~evels in the aqueous polyacrylamide gel particles
before being denatured in the high temperature drying
stage. We have found surprisingly that this is not the
case.
In the process of the invention the amidase enzyme
preferably has a very low value of Km for acrylamide. This
is normally measured at pH 7 and 20OC. Generally it is
below lO and frequently below 5 and most preferably below
2 mmolar. Preferred enzymes have Km 0.5 to 2.5, most
preferably 0.5 to l.5 mmolar. The Km value can be below
0.5 ~M, for instance O.l mM or O.Ol mM.
Mixtures of different amidases may be used in the
process. Preferably all amidases have the low Km value for
acrylamide discussed above.
Some of the amidases which have been proposed in the
literature ~or treating polyacrylamides do not have a
sufficiently low Km value to give the particular preferred
results achievable with the invention but others do. A
particularly preferred amidase is the amidase produced by
the microorganism which has been deposited under the
deposit number NCIMB 40756. This and other suitable
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amidases are described in more detail in our copending
International Application No. PCT/GB96/01951.
Preferably the amidase is not inhibited by materials
normally present in a process of producing substantially
dry polymer particles from aqueous gel or in the growth
medium for the microorganism which produces the amidase.
For instance, the amidase is preferably not inhi~ited by
ammonia.
The amidase enzyme may be applied to the aqueous
polyacrylamide gel partic~es in any suitable form.
Normally it is applied in the form of a liquid suspension
or solution. For instance it may be applied in the form of
an aqueous solution of amidase or as a reverse phase
emulsion of amidase. It may be applied in the form of an
agueous suspension.
In one process, the amidase is be present in the
suspension in the pure (molecular) form.
In another process it is present in the form of
bacterial cells and/or cell debris which contain the
amidase enzyme, for instance whole cell form.
Amidase may be applied in any suitable manner. One
suitable method of application is spraying the particles of
aqueous gel with the liquid suspension or solution of
amidase.
Dosage of amidase may be at any suitable level, for
instance from 0.1 to 10 U/g (activity units per gram dry
solids content of polymer), preferably 0.5 to 7 U/g, for
instance 1 to 3 U/g.
The particles of aqueous polyacrylamide gel preferably
have size at least 50 wt~, often at le~5t 70 wt%, from 0.1
to 8 mm. Particle size can be from 2 up to 4 or 6 mm, and
can ~e for instance 0.2 to 1 or 2 mm.
In the process of the invention it is preferred to
form relatively coarse, substantially rigid, gel particles
in the normal way and then to treat these particles.
One way of making the aqueous gel particles is by
reverse phase bead polymerisation followed by distilling
,
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off the water and oil so as to provide hot particles
typically having a temperature o~ 50 to 90oC, often around
80OC, and typically having a size O.l to l mm.
Another, preferred, way of making the aqueous gel
particles is by bulk gel polymerisation to form a rigid gel
having a polymer solids content normally of 20 to 50%,
often 30 to 45% by weight, followed by comminution to a
particle size at least 50%, and usually at least 70~, from
0.1 to 6 or 8 mm. Preferably the comminution leads to an
average particle size in the range 0.2 to 4 mm, often
around 2 mm.
Standard comminution systems can be used, including
ubricant if desired.
These aqueous gel particles, however they are formed,
typically are contaminated with significant amounts of
acrylamide monomer, often in the range 500 to 2,000 ppm.
The processes of the invention are particularly
suitable for treating aqueous gel particles contaminated
with levels of acrylamide monomer above 400 or 500 ppm. It
20 i5 advantageous to be able to provide a process which can
reduce levels of acrylamide monomer from these levels to
acceptably low levels, for instance below 200 or lO0 ppm or
even substantially zero, in particular because the
manufacturer is given greater freedom in the first stage of
2S the process. That is, it is possible to carry out the
initial polymerisation stage to form the aqueous gel and to
allow levels of acrylamide monomer in this gel to be 400 or
500 ppm or greater, in the knowledge that these levels will
be reduced in the amidase application stage of the process.
The need to control polymerisation conditions very
carefully so as to ensure that the lowest possible
acrylamide monomer levels are present in the aqueous gel is
removed .
In preferred processes the aqueous gel particles are
produced by bulk gel polymerisation and comminution and tha
amidase is applied, preferably by spraying the particles,
during the comminution stage. When the particles are made
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by reverse phase bead polymerisation, application of
amidase, preferably by spraying the particles, can be as
the particles are carried away from the distillation
apparatus after the polymerisation.
In a preferred process, in which the particles are
made by comminution of gel, it is preferred to spray the
particles during or near the end of the comminution stage
so as to achieve intimate contact between the li~uid which
contains the amidase and the particles.
In the processes of this aspect of the invention the
aqueous gel particles have a temperature in the range 50 to
95~C when amidase is applied, preferably 70 to 90~C.
The process of the invention is carried out on
particles of a polyacrylamide, ie a polymer ~ormed from
monomers comprising acrylamide. The polyacrylamide may be
a homopolymer of acrylamide. Alternatively it may be a
copolymer with other monomers, which may be anionic, non-
ionic or cationic.
The polyacrylamide, when anionic or non-ionic,
preferably has such high molecular weight that its
intrinsic viscosity is at least 6 or lO dl/g and frequently
at least 15, 20 or even 30 dl/g. It is usually not above
50 dl/g. When the polymer is cationic, intrinsic viscosity
is generally above 8 and usually above lO or 12 dl/g,
typically above 14 dl/g. Generally it is not above 20 or
25 dl/g.
The polymer may be a non-ionic homopolymer of
acrylamide or it may be a copolymer of acrylamide with
anionic or cationic monomers, usually in an amount of 3 to
90% by weight, often below 70% and preferably below 50%, by
weight based on the total weight of monomers. Any of the
typical anionic monomers may be used such as ethylenically
unsaturated carboxylic or sulphonic monomers, especially
acrylic acid (including water-soluble salts thereof~. Any
of the conventional cationic monomers may be used such as
diallylammonium monomers for instance DADMAC or cationic
esters such as DMAEA or DMA~MA (often as acid addition or
_
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quaternary ammonium salts) or cationic amides such as
DMAPMA.
The polymer may be linear or cross-linked, in
conventional manner.
In the process the aqueous gel particles are carried
to the drying stage substantially immediately after
application of amidase is complete. By Usu~stantially
immediately~, we mean a time which is as short as is
convenient in the context of an in-line production process,
typically below 5 minutes, often 10 seconds to 2 minutes,
often around 1 minute or less.
The drying stage may be any standard drying system,
for instance a fluid bed drier or other drying oven. In
the drying stage the particles are subjected to
temperatures of 60~C and greater, for instance 70 to 100~C,
preferably 75 or 80 to 95~C.
The particles are normally produced in a continuous
process, although they can be produced batchwise. In the
process any given aqueous gel particle will tend to spend
not more than 1 hour, often not more than 30 minutes, in
the drying stage.
The particles are subjected to the drying stage so
that they are substantially dry. That is, they have a
final water content of below 20%, usually below 10% by
weight. They generally have a final size at least so wt%
above 30~m, and often below lmm. They suitably have a size
of 90 wt~ between 70 and 700~m.
By the technique of the invention it is easily
possible to achieve reduced monomer contents. Generally
content of acrylamide monomer in the substantially dry
polymer particles is below 300 ppm calculated on the basis
of the dry weight of polymer. Preferably it is below 200
ppm and most pre~erably below 100 ppm. Best products have
values below 50 ppm and can show values of 0 ppm, ie an
amount below measurable levels, up to 20 ppm. ~Lowest
measurable levels~ in this specification are those
measurable by standard techniques used in the industry.
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The precise level can vary with the type of polymer. We
find that the lowest measurable level of acrylamide for the
polymers used in the invention is normally below 5 or lO
ppm.
5In a second aspect of the invention we provide a
A process for the production of substantially dry particles
of a polyacrylamide comprising
providing aqueous polyacrylamide gel particles
contaminated with acrylamide monomer,
~oapplying amidase enzyme which has a Km for acrylamide
of not more than lO mM, preferably not more than 5 mM, to
the agueous gel particles whilst they are at a temperature
of 50 to 95~C,
holding the aqueous gel particles to which amidase has
~een applied in a holding stage at a temperature of from 20
to 70OC for not more than 30 minutes, and then
passing the particles to a drying stage and subjecting
them in that stage to a temperature of at least 60OC so as
to produce substantially dry particles of polyacrylamide,
20the process being carried out such that the final
content of acrylamide monomer in the substantially dry
particles is ~elow measurable levels.
Such a process is particularly suitable for producing
acrylamide polymer particles which are substantially dry
and which have extremely low levels of residual acrylamide
monomer. This is achieved in this aspect of the invention
by the choice of a combination of an amidase enzyme having
very low Km for acrylamide and a moderate holding period
after treatment. This moderate holding period can also be
incorporated into an industrial process without the very
long maintenance periods, for instance more than lO hours,
which are required in certain other prior art processes.
The duration of the holding stage is not more than 30
minutes. It may be for instance at least 2 minutes,
prefera~ly around lO to 20 minutes.
The aqueous gel particles to which amidase has been
applied are held in the holding stage at a temperature of
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from 20 ~o 70~C, often around 30 to 50~C, for instance
about 40~C.
In one type of process this may be achieved by cooling
the particles during the latter stages of application of
amidase to the aqueous gel particles (for instance by
spraying) and then holding the treated particles at the
required temperature for the duration of the holding stage
(for instance 10 to 15 minutes). For instance, the holding
stage may be in the cold zone of a fluid bed drier, after
which the particles are then fed into the hot zone of the
fluid bed drier and sub~ected to conventional drying in the
drying stage.
This aspect of the invention leads to extremely low
levels of monomer content in the final product. They are
below measurable levels of acrylamide monomer.
In this aspect of the invention it is possible to use
any of the amidases described for the first aspect of the
invention in any of the forms described above. Methods of
producing the aqueous gel particles and of applying the
amidase described above for the first aspect of the
invention may also be used. The polymer type may also be
any of those discussed above for the first aspect of the
invention.
In a third aspect of the invention we provide a
process for the production of substantially dry particles
of a polyacrylamide comprising
providing aqueous polyacrylamide gel particles
contaminated with acrylamide monomer,
applying amidase enzyme which has a Km for acrylamide
of not more than 10 mM, preferably not more than 5 mM, to
the aqueous gel particles whilst they are at a temperature
of 50 to 95~C,
holding the aqueous gel particles to which amidase has
been applied in a holding stage in the cold zone of a fluid
bed drier at a temperature of from 20 to 70~C for not more
than 30 minutes, and then
. ~
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11
passing the particles to a drying stage in the hot
zone of a fluid bed drier and sub~ecting them in that stage
to a temperature of at least 60OC so as to produce
su~stantially dry particles of polyacrylamide.
In the third aspect of the invention any of the
additional process features described above for the first
and second aspects of the invention may be used.
~ he invention will now be illustrated with reference
to the following examples.
Example l
A copolymer of 90% acrylamide and 10% sodium acrylate
is formed by bulk gel polymerisation to give high IV and a
content of around l,000 ppm acrylamide free monomer. This
is comminuted in conventional manner down to a final
cs ;nution stage which gives a particle size of around 0.5
mm. In another process comminution gives a particle size
of around 2 mm. During this final co inution stage, an
aqueous suspension of cells of the type NCIMB 40756 is
sprayed onto the particles at the rate of abou~ l litre of
the agueous suspension to 40 kg of the aqueous particles.
At this stage the particles have a temperature of about
80~C.
In a first process the particles are immediately
passed into a fluid bed drier where they are dried in a
conventiona} manner and they are optionally subsequently
co~inuted. Such particles can easily have a free
acrylamide monomer content of below lO0 ppm and often below
50 ppm.
In a second process the particles are either cooled
before spraying or are cooled immediately after spraying
and are held for about 15 minutes at a temperature of
around 40OC before being fed into the fluid bed drier and
then dried. Such particles, optionally after comminution,
typically have a free acrylamide monomer content which is
so low as to be not measurable, ie 0 ppm acrylamide.
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12
Some of the products show visible signs of containing
cellular debris and enzyme residues, but this is
unimportant.
Exam~le 2
The following test was a laboratory simulation of
plant conditions.
Polyacrylamide gel was formed by bulk solution
polymerisation from monomer mixture comprising 30 wt%
sodium acrylate and 70 wt% acrylamide. The gel had a
polymer solids content of 32%. The gel was heated to 80~C
and then subjected to a lubricated comminution stage in a
Waring blender.
Amidase ~rom the microorganism deposited under number
NCIMB 40756 having Km l.l mM in the form of a 39% solids
suspension in water was applied to the gel during the
comminution stage. Amidase was added at various levels
given in U/g below. Immediately comminution was finished
the comminuted treated gel was trans~erred to a laboratory
fluid bed drier in which the temperature had been
stabilised at 80~C.
When dry the polymer samples were ground and passed
through an 850 ~m sieve and tested for viscosity,
solu~ility and residual acrylamide content.
For comparison, some samples were treated with
sulphite.
Results are given in Table l below.
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13
T~ble 1
Method of Re~idual Vi~co~ity So~ubilLty ResLdual
Acrylamide Reduction Cp Acrylamide
5NONE-8LANR 56Fair/Good 0.098
NONE-8LANK 52~air/Good 0.100
Sulphite Treatment 52Fair/Good 0.061~
Sulphite Treatment 53Fair/Good 0.082%
Sulphite Treatment 48Fair/Good 0.073%
10sulphite Treatment 52Fair/Good 0.049~
lU/g Amidase 52Fair/Good 0.049%
2U/q Amida~e 50Fair/Good 0.033%
3U/g Amidase 55Fair/Good 0.022%
5U/g Amida$e 54Fair/Good 0.021
15lU/g Amidase 51 Good O.032
2U/9 Amidaqe 53 Good 0.026
3U/q Amida~e 52Fair/Good 0.017
SU/q Amida~e 52Fair/Good 0.026~
In a second series of kests a short holding period was
introduced before passing to the fluid bed drier. Doses of
amidase and contact times are given in Table 2 below.
~able Z
Method of Re~idual Visco~ity Solubility Re~idual
25Acrylamide Reduction Cp Acrylamide
5U/g 55 Fair/Good None Detected
15 Min Contact @ 40~C
5U/g 52 Fair/Good None Detected
30 Min Contact @ ~0~C
5U/g 52 Fair/Good None Detected
15 Min Contact @ 40~C
5U/g 53 Fair/Good None Detected
30 Mln Contact @ 40~C
~ 35
These tests show that acceptable levels of residual
acrylamide can be obtained using a process in which an
Jamidase of low Km is used in combination with passing
treated polyacrylamide gel directly to the drying stage.
40 In particular the residual acrylamide results obtained are
better than those obtained with sulphite treatment, without
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~4
the risk of degrading the polymer, which is normally
present with the use of sulphite.
In the second series of tests it can be seen that
residual acrylamide levels can be reduced to zero with the
introduction of a short holding period.
These results are surprising in that they are even
better than would be expected in view of the Km of the
enzyme used.
It would not be expected that the amidase itself could
lo migrate through the polymer gel because of the high
molecular weight of the amidase (generally above 200,000)
and the high polymer content of the aqueous gel particles
and the high molecular weight of the polymer. Indeed, we
believe the amidase does not migrate through the gel to any
significant extent. Further, it would not be expected that
an amidase which had Km values such as those indicated
above, for instance around 1 mmolar, would be capable of
reducing the free acrylamide monomer content to the low
levels which are obtained in the invention.
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