EP4587483A2

EP4587483A2 - The method of obtaining polyacrylamide hydrogels through continuous frontal polymerization

Info

Publication number: EP4587483A2
Application number: EP23834307.3A
Authority: EP
Inventors: Anait Oganesovna TONOYAN; Sevan Paruyrovich DAVTYAN; David Sevanovich DAVTYAN
Original assignee: Individual
Current assignee: Individual
Priority date: 2022-09-13
Filing date: 2023-11-03
Publication date: 2025-07-23
Also published as: WO2024058688A2; CA3267787A1; WO2024058688A3

Abstract

The invention relates to the field of polymer chemistry and pertains to an industrial technology for producing superabsorbents, in particular, polyacrylamide hydrogel (PAH), which can be used in agriculture, medicine, cosmetology, for cleaning oil pipelines, and for creating hygiene products. The technical result achieved in the implementation of the claimed invention is a reduction in the labor intensity of the polyacrylamide hydrogel production process, an increase in the reliability of the gel production process through achieving process stability, and an enhancement in the quality of the obtained polyacrylamide hydrogel by substantially improving the homogeneity of the gel, enabling stable and precise production of pores of specified sizes and density, and eliminating toxicity of the final product. The method of obtaining polyacrylamide hydrogels by continuous frontal polymerization includes the supply of acrylamide or its derivatives into a vertically oriented continuous-action reactor from bottom to top and conducting polymerization using the frontal polymerization process with continuous forced extraction of the resulting polyacrylamide hydrogel by pulling it out from the upper part of the reactor, wherein the supply of acrylamide or its derivatives into the reactor is carried out at atmospheric pressure.

Description

THE METHOD OF OBTAINING POLYACRYLAMIDE HYDROGELS THROUGH CONTINUOUS FRONTAL POLYMERIZATION

The invention relates to the field of polymer chemistry and pertains to an industrial technology for producing superabsorbents, in particular, polyacrylamide hydrogel (PAH), which can be used in agriculture, medicine, cosmetology, for cleaning oil pipelines, and for creating hygiene products.

Polyacrylamide hydrogel is of particular interest due to its ability to absorb water and active substances, as well as its exceptional physico-mechanical properties, which enable a wide range of applications.

In recent decades, polyacrylamide hydrogels have been a highly relevant and sought-after subject of research in various scientific disciplines, including chemistry, physics, biology, medicine, and more. These hydrogels are cross-linked polymer structures with the ability to swell by absorbing large amounts of moisture and active substances without losing their original shape. A notable advantage of these hydrogels is their ability to release the absorbed substances back into the surrounding environment. The wide spectrum of properties of hydrogels opens up opportunities for their use in various areas of human activity, including agriculture, bioengineering, pharmacology, medicine, cosmetics, plastic surgery, the production of hygiene products, and even the manufacture of diapers, among others. They are produced worldwide, with particularly active suppliers in the USA, China, Ukraine, and other countries. However, recently, the USA, China, and many European countries have discontinued the use of polyacrylamide hydrogels in agriculture and all areas directly affecting humans, including in diapers, baby products, the food industry, and invasive medical procedures. This is due to the fact that PAH, while inherently safe, is derived from a highly toxic monomer, acrylamide. The toxicity of the resulting product is due to the traditional synthesis method, which takes several hours, during which the resulting hydrogel captures a certain amount of acrylamide, from which it is formed. Success in using PAH in human-related fields, especially in medicine, depends on the properties of the materials used. These materials must undoubtedly be biocompatible, stable, and non-toxic.

Traditional methods of synthesizing polyacrylamide hydrogels involve a two-stage process with a subsequent washing step to remove impurities and residual toxic monomer, acrylamide. The first and second stages of the process involve the synthesis of a linear polymer followed by the cross-linking of the obtained linear polymer to create a three-dimensional polymer network, responsible for absorbency. Significant drawbacks of traditional methods for producing polyacrylamide hydrogels include their complexity, the multi-stage nature of the process, and high energy consumption. Additionally, these traditional methods are characterized by low environmental friendliness, and the resulting product is of low quality, mainly due to the toxicity of the resulting product, as mentioned earlier. Moreover, difficulties in controlling the size and density of pores (responsible for the absorption and release of various active substances and compounds) during the synthesis process can also be considered as drawbacks.

These limitations are partly due to the fact that polymerization processes are primarily exothermic reactions. Therefore, when conducted in batch reactors where polymerization occurs throughout the entire reaction mass, it is necessary not only to dissipate the heat generated to prevent a thermal reactor explosion but also to reduce the concentration of reactants. Cooling devices are typically employed to address the first issue, while the second issue is addressed by adding solvents to the reacting mixture, significantly increasing the complexity of the polyacrylamide hydrogel production process. Furthermore, a substantial amount of energy is expended for each batch production cycle to cool the reactor vessel and maintain a constant temperature in the reaction medium. At the end of the process, the solvent is removed from the reactor, either discarded or subjected to an additional cycle of solvent removal and purification for subsequent loading into the batch reactor. Moreover, at the end of the process, the resulting polymer is removed from the reactor, the reactor is cleaned and prepared for the next loading. It's worth noting that the prolonged duration of the polymerization process throughout the reactor volume creates conditions for additional unwanted reactions, such as reactions with the reactor walls, oxidation reactions in the presence of air, and more. The extended duration of the polymerization process throughout the reaction mass contributes to strong adhesion of the reaction mixture and the resulting product to the reactor walls, leading to structural disruptions in the polyacrylamide hydrogel when it is removed from the reactor. Undesirable reactions involving the capture of the starting monomer during the synthesis of polyacrylamide superabsorbents, synthesized from acrylamide, which is toxic to the human body, lead to a significant deterioration in the quality of the final product. Despite the exceptional advantages of polyacrylamide superabsorbents compared to those documented in literature and production, the presence of residual toxic acrylamide in the end product has significantly reduced the value of this superabsorbent gel. In some countries (such as Russia, China, and Europe), its sale and usage have been prohibited.

The closest analogue to the present invention is a method for obtaining polyacrylamide hydrogels through continuous frontal polymerization, with the following composition of the starting mixture: a mixture of acrylamide and sodium acrylate - from 0.1% to 50%; N,N’- methylene bisacrylamide - from 0.5% to 8%; a potassium persulfate initiator - from 0 to 1%; nanoscale bentonite - from 0.1% to 7%; water - the rest [patent RF N22681212, 09.07.2018],

The drawback of the closest analogue is the insufficient quality of the obtained hydrogel due to the heterogeneity of the resulting hydrogel, which prevents precise control of the pore sizes and density in the obtained hydrogel. This is because the frontal polymerization of liquid monomers in continuous tubular reactors is typically accompanied by the adhesion of the highly viscous polymer mass to the reactor walls, leading to the formation of jet streams, a flow rate gradient across the radius of the reactor, and, as a result, the formation of a highly elongated stream of monomeric liquid that, upon reaching the end of the reactor, exits without complete reaction. Therefore, it is challenging to guarantee the production of a hydrogel with a uniform structure and the desired pore size using this technology. In addition, the adhesion of the highly viscous polymer mass to the reactor walls destabilizes the polymerization front, which may lead to the spontaneous polymerization process dying out.

The technical problem addressed by the claimed invention is to eliminate the drawbacks of known solutions and develop a high-performance, environmentally friendly, energy-efficient technology for obtaining non-toxic high-quality polyacrylamide hydrogel. This technology aims to ensure the production of a product with precisely specified structural characteristics for regulating both sorption and desorption properties of the hydrogel (properties that facilitate the absorption and release of liquids).

The technical result achieved in the implementation of the claimed invention is a reduction in the labor intensity of the polyacrylamide hydrogel production process, an increase in the reliability of the gel production process through achieving process stability, and an enhancement in the quality of the obtained polyacrylamide hydrogel by substantially improving the homogeneity of the gel, enabling stable and precise production of pores of specified sizes and density, and eliminating toxicity of the final product.

The technical result is achieved in the method for obtaining polyacrylamide hydrogels through continuous frontal polymerization, which includes the delivery of acrylamide or its derivatives into a vertical continuous-action reactor from the bottom upwards and carrying out polymerization using the frontal polymerization process with continuous forced extraction of the resulting polyacrylamide hydrogel by pulling it from the upper part of the reactor. The delivery of acrylamide or its derivatives to the reactor is done at atmospheric pressure.

Polyacrylamide hydrogels, superabsorbents (PAH) obtained by this method, are crosslinked polymer structures with the ability to swell by absorbing a significant amount of water and active substances without losing their original shape. A notable advantage of these obtained hydrogels is their ability to release the absorbed substances back into the surrounding environment. To produce these hydrogels, polyacrylamide is used. Polyacrylamide is the general term for a group of polymers and copolymers based on acrylamide and its derivatives.

In addition to acrylamide or its derivatives (monomers), mixtures thereof with crosslinking agents and/or polymerization initiators and other additives that provide an additional means of regulating the absorption and release of liquids in the final product may be used as the starting reagents for obtaining polyacrylamide hydrogels.

Cross-linking agents, for example, can include N,N' -methylene bisacrylamide, and as the polymerization initiator, potassium persulfate can be used. Additives that enable the regulation of liquid absorption and release can include a colloidal solution of bentonite.

The delivery of the monomer (acrylamide or its derivatives) from the feed tank to the reactor is carried out at atmospheric pressure, meaning no artificially created pressure is applied to move the liquid from the feed tank to the reactor. This can be achieved, for instance, using the principle of communicating vessels, where the feed tank and the reactor are positioned appropriately (in parallel).

The optimal feed rate of the monomer or its mixture with the initiator and/or crosslinking agent into the reaction zone, where the frontal polymerization wave is localized, is in the range of 1-20 cm/min and depends on the burning rate of the monomer or the mentioned mixture. This, in turn, depends on the chemical nature of the supplied monomer or components of the mentioned mixture.

The continuous forced withdrawal of the synthesized polyacrylamide hydrogel from the reactor can be accomplished by pulling the polyacrylamide hydrogel out of the reactor using a cable. The polyacrylamide hydrogel formed in the reactor's neck is wound onto spools. The withdrawal speed of the polyacrylamide hydrogel from the reactor can be equal to the burning rate of acrylamide or its derivatives in the reactor during frontal polymerization.

The initiation of the frontal polymerization process can be achieved through a thermal impulse (local initiation, e.g., spot heating) in the mixture section. Furthermore, the reaction temperature is maintained in the reactor using any known method to ensure an adiabatic process.

For carrying out the polymerization process, a vertical reactor of a cylindrical type is considered optimal. The internal walls of the reactor can be coated with a polytetrafluoroethylene layer or a silicone organic rubber layer to reduce wetting with the monomer and the resulting polymer and to prevent adhesion.

Implementing the disclosed method for obtaining polyacrylamide hydrogels through continuous frontal polymerization in a vertical continuous-action reactor, with the constant supply of initial reagents into the reactor from bottom to top at atmospheric pressure, and the continuous forced removal of the obtained PAH through the upper part of the reactor, allows for the production of a product of the highest quality, which is due to the following.

From a technological point of view, frontal polymerization has incomparable advantages compared to traditional polymerization methods. The traditional process, as mentioned earlier, is carried out in batch reactors in two stages. In the first stage, a linear polymer is obtained, and in the second stage, the polymer is cross-linked to create three-dimensional polymer networks. The process, starting from the first stage, occurs throughout the entire volume and lasts for several hours (unlike frontal polymerization where polymerization spreads from one end of the reactor to the other in an auto-wave mode). This process generates heat, and to prevent a thermal explosion during polymerization, a cooling system is installed, leading to additional energy costs for cooling the reactor. Additionally, the reaction rate needs to be reduced, and thus the heat generation which is achieved by adding solvents to the initial reacting mixture. When the process is completed, these solvents are released into the environment, since other disposal methods greatly affect the cost of the final product. Hence, this leads to cooling costs and environmental concerns due to the release of solvents into the environment. The main drawback, especially in the case of polyacrylamide hydrogel, is the presence of some residual toxic acrylamide in the product.

On the other hand, the process of frontal polymerization (FP) is conducted in a single step due to the nature of FP, which occurs in the auto-wave mode, propagating the polymerization reaction from one end of the reactor to the other. All the undesirable phenomena associated with traditional methods are eliminated under the continuous conditions of auto-wave FP. This is because during FP, the entire process occurs rapidly with simultaneous polymerization and cross-linking, resulting in the production of a high-quality product, notably free from residual acrylamide. Since the process proceeds rapidly, the reaction wave expels all impurities. Therefore, conducting the process under auto-wave FP conditions allows for the production of a safe product with no traces of toxic acrylamide, enabling the use of the resulting hydrogel in medicine, agriculture, and hygiene products without health risks.

Furthermore, due to the technology's reliance on maintaining the heat generated during the reaction to prevent a loss of reaction speed and the damping of the polymerization wave, there is no need to cool the reactor or reduce the reaction rate using solvent additives. This results in enhanced process efficiency and energy savings due to the elimination of the initial heating and subsequent cooling steps. The high efficiency of the process is also attributed to implementing FP in continuous-action reactors with a continuous feed of the reaction mixture and the constant withdrawal of the produced PAG. During the research, it was found that, under certain conditions (such as the occurrence of adhesion of the reacting mass to the reactor walls), a convex polymerization line (burning front) could form. This leads to the development of jet flows and a gradient of flow rates across the reactor's radius, ultimately producing a highly elongated stream of monomer liquid that exits the reactor without completing the reaction.

Consequently, polymerization occurs non-uniformly, and areas of unreacted monomer remain in the product, significantly reducing the homogeneity of the resulting PAG and preventing the stable and precise production of products with specified pore sizes and densities.

Also, because of the formation of the convex front, non-uniform residence times of the product across the reactor's cross-section and the creation of significantly non-uniform temperature fields are observed. These factors result in the non-uniformity of the properties of the produced polymer.

It has been established that the forced supply of the reaction mixture to the reactor from a feed tank under artificially created pressure (e.g., by a compressor or pump) contributes to the formation of a convex polymerization line (see Fig. 2). In this case, implementing the claimed method of supplying the reaction mixture to a vertical reactor from bottom to top in the absence of pressure (at atmospheric pressure) (see Fig. 3) and continuously extracting the resulting polymer upward (from the upper part of the reactor) allows for a significant reduction in the adhesion of the reacting mass to the reactor walls and the elimination of the convex front formation, which results in the generation of jet flows and a gradient of flow rates across the reactor's radius. It has been noted that the polymerization line's geometry is approximated to a plane, and the front is stabilized. Consequently, during the entire process, the monomer polymerizes uniformly, leading to increased PAG quality by consistently obtaining a polymer with a high degree of uniformity and structured porosity of specified sizes and densities. This also results in good reproducibility of the product's structure.

The possibility of implementing FP in a single stage significantly reduces the complexity of the proposed method and leads to an acceleration of the process of obtaining the final product.

The use of a reactor with its internal walls coated with a layer of polytetrafluoroethylene or silicon-organic rubber also reduces the interaction between the reacting mass and the finished product with the reactor walls by reducing the wettability of the reactor walls by the reacting mass and the finished polymer. This provides an additional increase in the quality of the resulting polyacrylamide hydrogel by enhancing the degree of structural homogeneity of the hydrogel.

The implementation of this method in the mode of frontal polymerization in a vertical continuous-action reactor, in combination with a continuous feed of the starting reagents from bottom to top at atmospheric pressure (without external pressure) and the continuous forced withdrawal of the resulting PAG by pulling it from the upper part of the reactor, also achieves increased reliability of the frontal polymerization process. This is due to the stabilization of the polymerization front (burning front) and, consequently, the exclusion of its sudden extinguishment and process stoppage.

The possibility of stabilizing the front is a result of the continuous feed of starting reagents into the vertical reactor from the bottom to the top at atmospheric pressure, which allows for the elimination of the adhesion of the viscous reacting mass to the reactor walls. This, in turn, prevents the periodic elongation of the front with the formation of jet flows that lead to the instability of the burning front and results in a consistently maintained polymerization line that approximates the geometry to a plane. In this geometric configuration of the polymerization line, the process proceeds uniformly and stably without sudden bursts caused by jet flows, which can lead to the extinguishment of the burning front and process stoppage.

During the research, it was found that in the presence of a convex front, in which jet flows are formed, polymerization occurs non -uniformly. In other words, jet flows can contribute to the ejection of monomer onto the reactor walls in the region of the synthesized PAG, leading to the adhesion of monomer to the reactor walls and making the extraction of PAG difficult.

Thus, obtaining a consistently maintained polymerization line that approximates the geometry to a plane, the absence of a convex front in which monomer jet flows form, contributes to the uniform polymerization of monomer across the entire front. This leads to the prevention of monomer adhesion to the reactor walls, which facilitates the extraction of the synthesized PAG. Consequently, the PAG extraction proceeds smoothly while preserving the burning front's intensity to ensure a continuous polymerization process, excluding the extinguishment of the front.

Thus, the implementation of the method for obtaining polyacrylamide hydrogel by feeding the reaction mixture into a vertical reactor from bottom to top without the application of pressure and conducting the frontal polymerization process with continuous pulling of the polymer through the reactor throat significantly improves the quality of the resulting hydrogel by ensuring the stable production of a product with precisely specified structure. As a result, it allows for the regulation of both sorption and desorption properties of the hydrogel.

The essence of the invention is illustrated by Figures 1-7.

Figure 1 depicts a schematic representation of obtaining polyacrylamide hydrogel using the claimed method. Figure 2 illustrates the convex front line of polymerization during the implementation of frontal polymerization with the supply of the reaction mixture under pressure.

Figure 3 illustrates the polymerization front line with a geometry close to a plane.

Figure 4 shows Raman spectra of pure acrylamide.

Figure 5 displays Raman spectra of crystalline polyacrylamide.

Figure 6 presents Raman spectra of PAG.

Figure 7 illustrates data obtained using gel chromatography.

In the figures, positions 1-15 are marked as follows:

1 - feeding vessel,

2 - reaction mixture,

3 - filling valve,

4 - vertical reactor,

5 - thermocouples,

6 - lower part of the reactor,

7 - upper part of the reactor,

8 - obtained polyacrylamide hydrogel,

9 - guiding rollers,

10 - pulling rope of the obtained polyacrylamide hydrogel,

11 - motor with control,

12 - attachment element of the rope to the obtained polyacrylamide hydrogel,

13 - convex line of polymerization,

14 - jet flows,

15 - polymerization line with geometry close to a plane.

The disclosed invention is implemented as follows.

To carry out the polymerization process in the FP mode, the feeding vessel 1 is connected to the lower part 6 of the vertical reactor 4. The feeding vessel 1 is positioned parallel to the reactor 4. Next, the feeding vessel 1 is filled with the reaction mixture 2 by opening the filling valve 3, and a continuous feed of the reaction mixture 2 into the reactor 4 is initiated. The flow of the reaction mixture into the reactor occurs by gravity until the level of the mixture in the reactor 4 reaches the level of the liquid mixture in the feeding vessel 1 (as the feeding vessel and the reactor are interconnected vessels). The rate of feeding the reaction mixture 2 into the reaction zone, where the polymerization wave is localized, is 10 cm/min. To maintain the level of the reaction mixture in the reactor, a continuous supply of the reaction mixture into the feeding vessel is carried out at the same speed. The delivery of the reaction mixture 2 to the reactor 4 is made at the bottom of the reactor 6. The initiation of the frontal polymerization process is carried out through a thermal impulse (point heating) in the section of the mixture. Throughout the process, the reactor maintains an adiabatic temperature. The temperature in the reactor is monitored using thermocouples 5. In the frontal polymerization mode, both polymerization and the "stitching" of the formed polymers occur simultaneously at high speed, resulting in the formation of three-dimensional polymer networks responsible for the sorption properties of the final product. Frontal polymerization occurs in the regime of auto-wave propagation of the polymerization reaction. The delivery of the reaction mixture 2 into the reactor 4 from the bottom to the top occurs by gravity without the application of pressure generated by a compressor or pump, with the continuous forced extraction of the PAG 8 from the reactor 4 using the pulling rope 10, which contributes to minimizing the adherence of the reaction mixture to the reactor's walls and the formation of a polymerization front line 15 with geometry close to a plane. Under these conditions, bending of the polymerization front line 13 with the emergence of jet flows 14 is eliminated. As a result, during the entire process of obtaining PAG 8, there is no formation of highly elongated streams of monomeric liquid that, reaching the end of the reactor, exit without reacting, contributing to an increase in the homogeneity of the obtained PAG. The formed PAG 8 is continuously removed through the upper part of the reactor 7 (throat) using the pulling rope 10, attached to the end of the PAG 8 by means of a rope attachment element 12. When the motor 11 is turned on, PAG 8 is pulled out of the reactor and wound on a spool. Guiding rollers 9 ensure reliable extraction of PAG 8 from the reactor 4.

The results of the structural characteristics of the samples of polyacrylamide hydrogels obtained using the developed method (samples 1-6) are presented in the table. The results of the research on the structural characteristics of the samples of PAG obtained by the developed method (samples 1-6) are presented in the table. Samples PAG 7-10 were obtained using an alternative method (the synthesis conditions differ from the claimed method by the parameters specified in the table, and the rest of the synthesis stages were carried out according to the claimed method described above) and are provided for comparison.

Table

Therefore, the research results confirm that the developed method allows achieving the production of polyacrylamide hydrogels (samples 1-6) with improved uniformity and excellent reproducibility of the structure in parallel processes. Throughout the entire process, no extinguishing of the polymerization front was observed, and the process proceeded steadily. Furthermore, the research results of the samples obtained using the claimed method, specifically through scanning spectroscopy and gel chromatography (Fig. 4-7), confirmed the absence of even traces of toxic acrylamide in the obtained samples.

In contrast, when obtaining samples of PAG (samples 7-10) using an alternative method, the polymerization process was interrupted from 2 to 5 times due to the extinguishing of the polymerization front, requiring the reinitiation of the process. Besides non-uniformity (in terms of pore distribution), different-sized regions of unreacted monomer were found in the volume of each of the obtained PAG samples.

Thus, the advantages of the developed method for producing PAG have been experimentally confirmed. The developed method can be used for synthesizing polyacrylamide hydrogels under conditions that surpass the currently existing methods of synthesis (including traditional ones) of these gels, both in terms of process efficiency and environmental safety, as well as the properties of the resulting polyacrylamide hydrogel superabsorbent.

Claims

A method for producing polyacrylamide hydrogels through continuous frontal polymerization, comprising the introduction of acrylamide or its derivatives into a vertical continuous reactor from bottom to top and conducting polymerization using the frontal polymerization process with continuous forced removal of the resulting polyacrylamide hydrogel by stretching it from the upper part of the reactor, wherein the introduction of acrylamide or its derivatives into the reactor is carried out at atmospheric pressure.

SUBSTITUTE SHEET (RULE 26)