PROCESSES FOR THE PRODUCTION OF POLYACRYLAMIDE PARTICLES

Abstract: Processes are provided for production of substantially dry polyacrylamide powder in which levels of residual acrylamide monomer are significantly reduced. These processes are particularly suitable for use on an industrial or plant scale. One process comprises providing aqueous polyacrylamide gel particles contaminated with acrylamide monomer, applying amidase enzyme to the aqueous gel particles whilst they are at a temperature of from 50 to 95 DEG C and substantially immediately passing the aqueous gel particles to a drying stage and subjecting them in that stage to a temperature at least 60 DEG C. The final levels of residual acrylamide can be even further reduced with the introduction of a short holding stage of not more than 30 minutes at a temperature of 20 to 70 DEG C before passing the particles to drying.

Description: PROCESSES FOR THE PRODUCTION OF POLYACRYLAMIDE PARTICLES 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 polyacryla ides) are liable to 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 is 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 acrylamide 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 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 10 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 acrylamide in emulsion form with amidase so as to reduce residual acrylamide monomer. These include US 4,687,807, US 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 100 ppm) are only obtained after considerably longer maintenance or incubation periods of for instance at least 10 hours. US 4,925,797 describes an enzyme which is said to be suitable 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 (1993), 365-79, describes an unusual amidase enzyme produced by a strain of Methylophilus methylotrophus which is inhibited in the presence of 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 below detectable levels after 1 hour. These systems appear to be able to obtain reduction in free 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 substantially dry polyacrylamide particles having low levels of acrylamide monomer contamination. In particular it would be desirable to be 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. The 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. 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 for 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. It would be expected that without a maintenance period the amidase would have little chance to affect the acrylamide monomer levels 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 20°C. Generally it is below 10 and frequently below 5 and most preferably below 2 mmolar. Preferred enzymes have Km 0.5 to 2.5, most preferably 0.5 to 1.5 mmolar. The Km value can be below 0.5 mM, for instance 0.1 mM or 0.01 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 for 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 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 inhibited by ammonia. The amidase enzyme may be applied to the aqueous polyacrylamide gel particles 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 aqueous 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 least 70 wt%, from 0.1 to 8 mm. Particle size can be from 2 up to 4 or 6 mm, and can be 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 off the water and oil so as to provide hot particles typically having a temperature of 50 to 90°C, often around 80°C, and typically having a size 0.1 to 1 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 lubricant 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 is 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 100 ppm or even substantially zero, in particular because the manufacturer is given greater freedom in the first stage of 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 the amidase is applied, preferably by spraying the particles, during the comminution stage. When the particles are made 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 liquid 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 formed 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 10 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 10 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 DMAEMA (often as acid addition or 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 "substantially 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 90 wt% above 30μm, and often below lmm. They suitably have a size of 90 wt% between 70 and 700μm. 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 preferably 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. 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 10 ppm. In a second 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 at a temperature of from 20 to 70°C 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 60°C so as to produce substantially dry particles of polyacrylamide, the process being carried out such that the final content of acrylamide monomer in the substantially dry particles is below 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 10 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, preferably around 10 to 20 minutes. The aqueous gel particles to which amidase has been applied are held in the holding stage at a temperature of from 20 to 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 subjected 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 passing the particles to a drying stage in the hot zone of a fluid bed drier and subjecting them in that stage to a temperature of at least 60°C so as to produce substantially 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. The invention will now be illustrated with reference to the following examples. Example 1 A copolymer of 90% acrylamide and 10% sodium acrylate is formed by bulk gel polymerisation to give high IV and a content of around 1,000 ppm acrylamide free monomer. This is comminuted in conventional manner down to a final comminution stage which gives a particle size of around 0.5 m. In another process comminution gives a particle size of around 2 mm. During this final comminution stage, an aqueous suspension of cells of the type NCIMB 40756 is sprayed onto the particles at the rate of about 1 litre of the aqueous 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 conventional manner and they are optionally subsequently comminuted. Such particles can easily have a free acrylamide monomer content of below 100 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 40°C 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. Some of the products show visible signs of containing cellular debris and enzyme residues, but this is unimportant. Example 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 from 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 transferred 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 μm sieve and tested for viscosity, solubility and residual acrylamide content. For comparison, some samples were treated with sulphite. Results are given in Table 1 below. Table 1 Method of Residual Viscosity Solubility Residual Acrylamide Reduction Cp Acrylamide NONE-BLANK 56 Fair/Good 0.098% NONE-BLANK 52 Fair/Good 0.100% Sulphite Treatment 52 Fair/Good 0.061% Sulphite Treatment 53 Fair/Good 0.082% Sulphite Treatment 48 Fair/Good 0.073% Sulphite Treatment 52 Fair/Good 0.049% lϋ/g Amidase 52 Fair/Good 0.049% 2U/g Amidase 50 Fair/Good 0.033% 3U/g Amidase 55 Fair/Good 0.022% 5U/g Amidase 54 Fair/Good 0.021% lϋ/g Amidase 51 Good 0.032% 2U/g Amidase 53 Good 0.026% 3U/g Amidase 52 Fair/Good 0.017% 5U/g Amidase 52 Fair/Good 0.026% In a second series of tests 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. Table 2 Method of Residual Viscosity Solubility Residual Acrylamide Reduction Cp Acrylamide 5U/g 55 Fair/Good None Detected 15 Min Contact @ 40°C 50/g 52 Fair/Good None Detected 30 Min Contact @ 40°C 5U/g 52 Fair/Good None Detected 15 Min Contact @ 40°C 5U/g 53 Fair/Good None Detected 30 Min Contact @ 40 C C These tests show that acceptable levels of residual acrylamide can be obtained using a process in which an amidase of low Km is used in combination with passing treated polyacrylamide gel directly to the drying stage. In particular the residual acrylamide results obtained are better than those obtained with sulphite treatment, without 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 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.

Claims: CLAIMS 1. 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 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. 2. A process according to claim 1 which comprises passing the aqueous gel particles to the drying stage not more than 2 minutes after application of amidase enzyme is completed. 3. A process according to claim 1 or claim 2 in which the drying stage is carried out in a fluid bed drier and at a temperature of from 80 to 95°C. 4. A process according to any preceding claim carried out such that the final content of acrylamide monomer in the substantially dry particles is not more than 200 ppm, preferably not more than 100 ppm. 5. A process for the production of substantially dry particles of a polyacrylamide comprising providing aqueous polyacrylamide gel particles and contaminated with acrylamide monomer, applying amidase enzyme having a Km for acrylamide not more than 10 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 at a temperature of from 20 to 70°C 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 60°C to produce substantially dry particles, the process being carried out such that the final content of acrylamide monomer in the substantially dry particles is below measurable levels. 6. A process according to claim 5 in which the particles which have been treated with amidase are held in the holding stage for 10 to 20 minutes at 30 to 50°C. 7. A process according to claim 5 or claim 6 comprising cooling the aqueous polyacrylamide gel particles during application of amidase enzyme. 8. A process according to any of claims 5 to 7 comprising holding the aqueous gel particles to which amidase has been applied in the cold zone of a fluid bed drier during the holding stage and drying the particles in the hot zone of a fluid bed drier in the drying stage. 9. 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 passing the particles to a drying stage in the hot zone of a fluid bed drier and subjecting them in that stage to a temperature of at least 60°C so as to produce substantially dry particles of polyacrylamide. 10. A process according to any preceding claim in which amidase enzyme is applied to the aqueous gel particles by spraying. 11. A process according to any preceding claim in which the aqueous gel particles have size at least 50 wt% from 0.1 to 8 mm. 12. A process according to any preceding claim comprising providing the aqueous polyacrylamide gel particles by bulk gel polymerisation to form a rigid gel having 20 to 50 wt% polymer content and then comminuting the gel to a particle size at least 50 wt% from 0.1 to 8 mm. 13. A process according to claim 12 in which the amidase enzyme is applied to the particles by spraying during comminution. 14. A process according to any preceding claim in which the amidase enzyme has a Km for acrylamide of not more than 5 mM, preferably not more than 2 mM. 15. A process according to any preceding claim in which the amidase enzyme is one produced by culturing Rhodococcus rhodochrous strain NCIMB 40756 or a mutant thereof having the ability to produce an amidase.

Inventors: ARMITAGE YVONNE CHRISTINE, BROOKE ANTHONY PAUL, HUGHES JONATHAN, SYMES KENNETH CHARLES, ARMITAGE, YVONNE, CHRISTINE, BROOKE, ANTHONY, PAUL, HUGHES, JONATHAN, SYMES, KENNETH, CHARLES
Applicants: ALLIED COLLOIDS LTD, ARMITAGE YVONNE CHRISTINE, BROOKE ANTHONY PAUL, HUGHES JONATHAN, SYMES KENNETH CHARLES, ALLIED COLLOIDS LIMITED, ARMITAGE, YVONNE, CHRISTINE, BROOKE, ANTHONY, PAUL, HUGHES, JONATHAN, SYMES, KENNETH, CHARLES
Application year: 1997
Main classification: C08F6
IPC codes: C08F6, C08F20