AU746982B2 - Process for preparing a polyvinyl alcohol gel and mechanically highly stable gel produced by this process - Google Patents

Abstract

A polyvinyl alcohol gel can be easily and quickly produced by a process having the following steps: (a) use of an aqueous polyvinyl alcohol solution with a degree of hydrolysis >/=98 % by moles; (b) admixture of an additive that is dissolved in the aqueous polyvinyl alcohol solution and forms a separate, finely distributed and aqueous phase after concentration of the solution; (c) drying of the aqueous solution to a residual water content of maximum 50 wt. % to cause the phases to separate and hence the polyvinyl alcohol to gel; (d) rehydrating the polyvinyl alcohol in an aqueous medium.

Description

WO 99/67320 PCT/DE99/00975 Process for producing a polyvinyl alcohol gel and a mechanically highly stable gel produced according to the process The invention refers to a process for producing a polyvinyl alcohol gel. The invention also refers to a mechanically highly stable gel produced according to the process.

It is well known that solutions containing polyvinyl alcohol (PVA) become more viscous when left to stand. It is also well known that PVA solutions can be converted into a gel if the solution is frozen and subsequently thawed (FR 2 107 711 However, gels formed in this manner have a relatively low strength.

It is also known through EP 0 107 055 B1 that the strength of PVA gels produced by freezing is increased by carrying out the freezing and thawing process at least once, but preferably repeating it two to five times. To this end, a PVA solution with a degree of saponification of> 95 mol but preferably of 98 mol is used. The upper temperature limit for freezing the solution is -3 degrees Celsius, the cooling rate can be between 0..1 to degrees Celsius per minute, the thawing rate between 0. 1 to 50 degrees Celsius per minute. The PVA that is used must have a degree of polymerization of at least 700. The concentration of PVA in the solution should be over 6 wt.% and is preferably between 6 and 25 The PVA gel produced by repeated freezing and thawing has a good mechanical strength and a high water content, which is maintained even under mechanical stress. The gel produced in this manner is highly elastic, non-toxic, and can be used in many applications, particularly in medicine.

Various substances and materials can be added to the gel to increase the strength, for instance glycol, glycerine, saccharose, glucose, agar, gelatine, methyl cellulose, etc. The admixture of agents such as heparin permits medical applications in which the active agent is continuously and evenly released from the gel over a period of time. In addition, microorganisms and enzymes can be mixed with the gel in order to create a biologically active system.

WO -99/67320 PCTIDE99/00975 2 It is known from US-PS 4 663 358 that adding organic solvents to the aqueous polyvinyl solution lowers the freezing point of the solution. This prevents the water from freezing at gelling temperatures below -10 degrees Celsius, preferably approximately -20 degrees Celsius, thus forming a more homogeneous and transparent gel. The low gelling temperature is used for creating compact grained gels with sufficient mechanical strength.

Producing PVA gels using the freezing method is complicated and time consuming.

DE 43 27 923 C2 disclosed a process by which PVA gels can be produced without using the freezing process. By using a PVA solution with a degree of hydrolysis of> 99 mol and adding a dissolved additive that has non-aqueous OH or NH 2 groups, a gelling of the PVA can be achieved at temperatures above 0 degrees Celsius. However, the gelling process takes several hours or may even require an additional storage period of many hours in order to ensure that it is cured sufficiently to provide full strength to the gel substance. Due the nature of this method, it is disadvantageous for producing large amounts of the gel substance.

Therefore, the purpose of the present invention is to be able to produce the PVA gel substances simply and quickly, as well as improving the quality of the gel substance as much as possible in this process.

Based on this objective, the process for producing a polyvinyl alcohol gel includes the following steps: a) Use of an aqueous polyvinyl alcohol solution with a degree of hydrolysis of> 98 mol b) The addition of an additive that is dissolved in the aqueous polyvinyl alcohol solution and forms a separate, finely distributed and aqueous phase after concentration of the solution.

c) Dehydration of the aqueous solution to a maximum residual water content of wt.% in order to cause the phases to separate and hence the polyvinyl alcohol to gel.

WO 99/67320 PCT/DE99/00975 3 d) Rehydration of the polyvinyl alcohol in an aqueous medium.

Surprisingly, the process according to the invention permits the gelling of the polyvinyl alcohol within several minutes at room temperature or even higher temperatures. The addition of the water-soluble additive and concentration by evaporation of the water results in a finely distributed phase separation, whereby gelling happens within a very short period of time during the PVA phase. A prerequisite is that the water-soluble additive creates a hydrous phase, so that within a very short period of time a respective proportion of water is removed through the phase separation of the PVA phase, whereby a gelling of the polyvinyl alcohol is achieved. It is advantageous if the water-soluble additive has an affinity to water at least comparable to that of the PVA.

The PVA phase, which is under-supplied with water during the gelling process, subsequently absorbs water during rehydration, whereby the elasticity and mechanical strength of the PVA gel is improved without reversing the gelling. It has been shown that a certain amount of electrolytes in the aqueous rehydration medium results in a higher stability of the PVA gel, so that the rehydration is advantageously carried out in tap water or, better yet, in a saline solution.

The process according to the invention has the advantage that it permits the production of PVA gel within a very short time without using complicated processes, in particular without a freezing process and without repeated dehydration processes, so that an extremely economical production of PVA gel is possible. The gel substances according to the invention are also characterized by a high elasticity and stability, in particular tensile strength, and in this regard are significantly superior to the PVA gel substances produced in the traditional way.

The stability and elasticity of the PVA gel substances are further enhanced by using a resaponified aqueous PVA solution in the production process.

WO 99/67320 PCT/DE99/00975 4 Polyethylene glycol is a preferred water-soluble additive which is added at a concentration of 4 to 30 preferably at 4 to 20 and more specifically at 6 to 16 wt.%.

Cellulose esters, cellulose ethers, starch esters, starch ethers, polyalkylene glycol ethers, polyalkylene glycols, long-chain alkanoles (C,H 2 ,+OH where n sugar esters and sugar ethers are other examples of possible additives.

A particularly advantageous area of application of the PVA gel substances is their production as biologically, physically or chemically active substances, i.e. incorporating biologically, physically or chemically active material in the PVA gel. Thus the PVA gel is superbly suited for the production of chemical or biological catalysts, for instance.

The dehydration of the aqueous solution for the purpose of phase separation and the associated gelling is carried out until a maximum residual water content of 50 wt.% is reached. A lower limit for the residual water content is 10 wt.% because the PVA gel produced should be completely rehydrateable, because the elasticity of the gel substance is lower if the residual water content drops below approximately 10 and because any incorporated biological materials can be damaged if the aforementioned residual water content is lower. An advantageous range for the residual water content is between 10 and wt.%.

Dehydration can conveniently be carried out in a short period of time by evaporating water at ambient air temperature if the aqueous solution is divided into small portions, especially portions in which there is only a small proportion of starch in the solution. In particular, it is advantageous to drip the solution onto a hard surface in such a way that the diameter of the drop is at least twice that of the height of the drop. This can similarly be achieved by pouring the solution into a form and/or coating a base material. A thin, or even film-like shape permits evaporation to the required residual water content within several minutes, for instance within 15 minutes. An acceleration of the dehydration process (and, therefore, the gelling process) can be achieved by carrying out dehydration in a drying oven at a higher temperature.

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t I WO 99/67320 PCT/DE99/00975 The saline solution used advantageously for rehydration preferably contains polyvalent anions.

The process according to the invention is particularly advantageous for immobilizing biologically active material in that it is extraordinarily gentle on the biological material so that the biological material has a significantly higher initial activity in comparison with other immobilization processes.

This can be enhanced if the aqueous medium in which the PVA gel is rehydrated is also a culture solution for the biologically active material.

The density of the PVA gel produced according to the invention can be modified by using suitable additives. For instance, the specific gravity can be increased by adding titanium oxide and lowered by adding hollow glass microballs.

As already mentioned, gelling according to the invention is possible at room temperature but can also be carried out at lower or higher temperatures. The biologically active material that is incorporated into the PVA gel can be enzymes, microorganisms, spores and cells.

The process according to the invention can be implemented in various embodiments. For instance, it is possible to carry out the dehydration of a drop for phase creation during the falling process in a drop tower, so that when the drop impacts the surface, the gelling has already occurred after the phase separation. This production process is particularly suited for producing PVA gel substances as chromatography material whose diameter is 10 to 100 [m for laboratory purposes, and which can otherwise be between 100 and 800 Pm. It is also possible to dehydrate an initial liquid that is set to a higher viscosity during extrusion of a strand and to carry out the gelling simultaneously.

Compared to previously known gel substances, the gel substance produced by the process t I V- c WO 99/67320 PCT/DE99/00975 6 according to the invention has superior mechanical stability, in particular with regard to abrasion resistance and tensile strength.

These superior mechanical characteristics permit, in particular, to produce the gel substance according to the invention in a lenticular form that is advantageous to the kinetics of the reaction and in which previously known gel substances had insufficient mechanical stability, in particular agitation stability. On the other hand, the gel substance according to the invention is stable and abrasion resistant for many months, even when subject to high-speed agitation. The lenticular form with a large diameter and low height has the effect that the chemically, physically and biologically active material is always arranged near the surface, resulting in a constellation that is advantageous for the kinetics of the reaction.

The present invention permits the addition of a magnetic additive to the polyvinyl alcohol substance by a very simple method in order to collect the gel substance out of a liquid by means of magnets.

It has been shown that the pore structure of the polyvinyl alcohol gel substance can be controlled by the molecular weight of the additive that effects the phase separation. By varying the molecular weight of the added polyethylene glycol, whose molecular weight is preferably in the range of 800 and 1350, the pore size of the polyvinyl alcohol gel substance can be adjusted to between 1 and 15 p.m.

Regarding the production of the aqueous solution ofpolyvinyl alcohol and the additive, it has been shown that the use of distilled water requires a higher degree of dehydration in order to achieve the same mechanical characteristics. The results are immediately better if normal tap water with a certain degree of hardness is used. Therefore, it can be assumed that a certain salt content in the water is advantageous for the process according to the invention.

1k ~K i WO 99/67320 PCT/DE99/00975 7 The invention is explained in more detail below by means of examples of embodiments.

Example 1: 16.8 g of water are added to 2 g of PVA and 1.2 g of polyethylene glycol (PEG 1000).

The solution is heated to 90 degrees Celsius until all the components have dissolved completely, so that a viscous, colourless solution is obtained. After cooling to 30 degrees Celsius, the polymer solution is dripped onto a polypropylene plate by means of a syringe and application of pressure. The dripping is carried out by touching the cannula onto the PP plate at a rate of approx. 1-2/second; the drop has a diameter of approx. 3 mm and a height of approx. 1 mm. After application of the drops, a white, wax-like film forms on the surface of the drops. After 89 wt.% of the water has evaporated at room temperature, the gel substances are rehydrated in water or a saline medium. The gel substances obtained have a diameter of 3-4 mm and a height of approx. 200-400 jpm.

Example 2: After the polymer suspension (composition: 2 g PVA, 1.2 g PEG 1000 and 15.8 g water) has cooled, 1 ml of a nitrogen-fixing mixed culture (Nitrosomonas europaea and Nitrobacter winogradsky) is added to 20 g of a polymer solution and dispersed, resulting in a dry biomass load of 0.06 The production of the gel substances is carried out according to Example 1. The gel substances obtained are rehydrated in a standard mineral salt medium for nitrogen-fixers. Compared to the same amount of free nitrogen fixers, after immobilization the immobilizates produced in this way have an initial activity of approx. 70% for Nitrosomonas spp. and 100% for Nitrobacter spp.

After occlusion of the nitrogen fixers in the PVA kyrogels at -20 degrees Celsius, the initial activity of Nitrosomonas spp. is approx. at -10 degrees Celsius approx. with decreasing stability of the PVA hydrogels.

The incubation of the immobilizates is carried out in the same medium at 30 degrees Celsius. After 19 days a maximum ammonium decomposition rate of between 7 and 8 WO 99/67320 PCT/DE99/00975 8 pmol NH4/ (gca,, x min) is achieved if 10 mg of gel substance is incubated in 30 ml of standard mineral salt medium.

Example 3: 1.6 g of polyethylene glycol (PEG 1000) is dissolved in 12.8 g H 2 0, followed by 1.6 g of PVA. The process is continued according to Example 1. After the polymer solution has cooled to 30 degrees Celsius, 4 ml of a culture of the strictly anaerobic bacteria Clostridium butyricum NRRL B-1024, which converts glycerine into 1.3-propane diol grown overnight in an oxygen-free atmosphere is dispersed in the solution (cell load of the polymer solution: 6 x 107 per ml). The gel substances are produced according to Example 1. After 70 wt.% of the water has evaporated at room temperature, the immobilizates are rehydrated in a mineral salt medium (20 times more than required). The incubation of the cell-loaded gel substance is carried out in the same medium (40 times more than required) at 30 degrees Celsius. The medium is changed several times during the growing phase in order to provide the immobilized biomass with an adequate nutrient supply.

If 0.25 g of the immobilized bio-catalyst thus obtained are placed in 40 ml of mineral salt medium together with 24.4 g L-1 glycerine, within 3.25 h the concentration of 1.3-PD increases by 2.8 g L-1. This corresponds to a catalyst activity of 0.14 g 1.3-PD per g cat per hour. After deducting the activity of fully developed cells, the result is a catalyst activity of 0.08 g 1.3-PD x h).

Example 4: 15.8 g of water is added to 2 g PVAL and 1.2 g polyethylene glycol (PEG 1000) and the process is continued according to Example 1.

After cooling the polymer suspension to approx. 30-37 degrees Celsius, 1 ml of a defined spore suspension of the fungus Aspergillus terreus is added to a polymer solution of 20 g and dispersed. The spore suspension is selected so that 5 d of fouling in the growth WO 99/67320 PCT/DE99/00975 9 medium results in a dry biomass load of 0.05 wt.%.

After 70 wt.% of the water has evaporated at room temperature, the immobilizates are rehydrated in a mineral salt medium for Aspergillus terreus (20 times more than required).

The incubation of the immobilizates is carried out in the growing medium. The growing medium is replaced with production medium for producing the itaconic acid.

Compared to the same amount of free fungus cells, the immobilizates thus produced have an initial activity of approx. 60% directly after immobilization. If 0.2 g of gel substance is incubated in 100 ml of production medium together with 60 g/1 glucose, a productivity of mg of itaconic acid (gca x h) is achieved after 7 d.

Example Larger amounts of gel substances are obtained by dripping the polymer solution (composition according to Example 1) through a system of multiple nozzles onto a conveyer belt. According to the principle of a belt dryer, the PVA drops are dehydrated in a tunnel drier to a defined residual moisture content and subsequently collected in a collection container by means of a scraper, where they are rehydrated and washed.

Example 6: In the production process according to Example 1, the polymer solution is not dripped, but poured into prefabricated semi-open forms of optional length with an interior diameter of 1-10 mm.

After rehydration in water, the strands can be stretched to 3-4 times their length without breaking. This stretching is irreversible. A strand produced in this way can be loaded with a weight of 500 g without breaking.

Example 7: WO 99/67320 PCT/DE99/00975 After a storage period of 14 days, the strands produced according to Example 6 are mechanically characterized in tap water. At that time, the strands are approximately 8 mm wide and approximately 1 mm high. The degree ofrehydration takes into account the strand's loss of weight after rehydration and 14 d storage in water in relation to the total mass of the polymer solution prior to the dehydration process. The strands have an elastic behaviour up to an elongation at tear of K WO 99/67320 PCT/DE99/00975 11 Mechanical characterization of the produced strands with various degrees of dehydration for the composition 10 wt.% PVA and 6 wt.% PEG 1000: Residual water content after dehydration 27 13 1 Degree of rehydration 76 74 68 65 63 57 Elongation at tear 455 420 410 390 380 360 E-modulus [N/mm 2 0.11 0.11 0.18 0.24 0.27 0.34 c 7 1 WO 99/67320 PCT/DE99/00975 12 Mechanical characterization of the strands at a degree of dehydration of 80 wt.% for the composition 10 wt.% PVA and 8 wt.% PEG for various types of PEG: Type of PEG 400 600 800 1000 1350 Degree of rehydration 57 66 82 84 92 Elongation at tear 410 290 360 420 370 E-modulus [N/mm 2 0.27 0.22 0.19 0.11 0.12 -Mechanical characteristics of the PVA hydrogel strands for various concentrations of PVA with the addition of 6 wt.% PEG 1000 at a degree of dehydration (amount of water evaporated during the dehydration process) of 80 wt.%;

PVAL

8 12 14 16 Elongation at tear 350 420 420 460 440 E-modulus [N/mm 2 0.09 0.11 0.17 0.19 0.25 K-4. 7~ W O 99/67320 PCT/DE99/00975 13 Mechanical characteristics of the strands at a degree of dehydration of 80 wt.% for the composition 10 wt.% PVA and 6 wt.% PEG 1000 for various rehydration media: Rehydration medium Elongation at tear E-modulus [N/mm 2 Tap water

K

2

HPO

4 (100 mmol/1)

K

2

SO

4 (120 mmol/1) CaCI 2 (120 mmol/1) KC1 (175 mmol/1) 420 410 530 360 370 0.11 0.17 0.15 0.10 0.15 Example 8: Gel substances are produced according to Example 1 and rehydrated in deionized water pS H2O). The degree of rehydration for various degrees of dehydration of the gel substances is defined directly after the rehydration process. With a degree of rehydration of 100 the weight of the gel substances prior to the dehydration process and after the rehydration process is the same, as shown in the attached drawing.

Claims (24)

1. Process for producing a biocatalyst with a biologically active material, introduced into a gel of polyvinyl alcohol, in the form of microorganisms, enzymes, spores and/or cells with the process steps of; a) dissolving an additive in aqueous polyvinyl alcohol solution having a degree of hydrolysis of 98 mol said additive form a separate, finely distributed and aqueous phase after concentration of the aqueous solution; b) adding the biologically active material; c) dehydrating the aqueous solution to a maximum residual water content of 50 wt in order to cause the phases to separate and hence the polyvinyl alcohol to gel; and d) rehydrating the polyvinyl alcohol in an aqueous medium.

2. Process according to Claim 1, in which the polyvinyl alcohol .solution has a concentration of 4 30 wt. preferably 6-16wt.%.

3. Process according to Claims 1 and 2, in which a water soluble additive is used which has an affinity to water at least similar to that of the polyvinyl alcohol.

4. Process according to Claims 1 to 3, in which the water-soluble additive is selected from the group cellulose esters, cellulose ethers, starch esters, starch ethers, polyalkylene glycol ethers, polyalkylene glycols, long-chain alkanoles (n sugar esters and sugar ethers.

Process according to Claim 4, in which polyethylene glycol is used as a water-soluble additive.

6. Process according to Claims 1 to 5, in which the water-soluble additive is used in a concentration of 4 20 preferably 6 10 wt.%.

7. Process according to Claims 1 to 6, in which the dehydration of the aqueous solution is carried out until a residual water content of at least 10 wt.% is reached.

8. Process according to Claim 7, in which the dehydration of the aqueous solution is carried out until a residual water content of 10 30 wt.% is reached.

9. Process according to Claims 1 to 8, in which the dehydration process is carried out after dripping the solution onto a hard surface.

Process according to Claims 1 to 9, in which the dehydration process is carried out after pouring the solution into a form. [Stamp:] Revised sheet IPEA/EP

11. Process according to Claims 9 and 10, in which the dripping or pouring is carried out in such a way that the gel substance is formed with a diameter at least double its height.

12. Process according to Claim 11, in which the dripping or pouring is carried out in such a way that the gel substance is formed with a diameter of 1 mm, preferably between 2 and 4 mm, and a height between 0.1 and 1 mm, preferably between 0.2 and 0.4 mm.

13. Process according to Claims 1 to 8, in which the dehydration process is carried out after pouring the solution to form a long strand.

14. Process according to Claims 1 to 13, in which the dehydration process is carried out after pouring the solution onto a base material.

Process according to Claims 1 to 14, in which the rehydration is carried out in tap water.

16. Process according to Claims 1 to 14, in which the rehydration is carried out in a saline solution.

17. Process according to Claim 16, in which a culture solution for the biologically active material is used as the saline solution.

18. Process according to Claims 16 and 17 using a saline solution containing polyvalent anions. [Stamp:] Revised sheet IPEA/EP

19. Process according to Claims 1 to 18, in which the dehydration process is completely carried out during the time the created drop falls in a drop tower.

Mechanically highly stable biocatalyst of polyvinyl alcohol, produced according to the process according to Claims 1 to 19.

21. Biocatalyst according to Claim 20, produced in a lenticular form in which the diameter is significantly greater than the height.

22. Biocatalyst according to Claims 20 and 21 with a magnetic additive.

23. Process for producing a product created by transformation with a biocatalyst according to Claims 20 to 22.

24. Process according to Claim 23 for producing 1.3-propane diol. Process according to Claim 24 for producing itaconic acid. [Stamp:] Revised sheet IPEA/EP A!