Lignin-based salt-tolerant water-retaining agent and preparation method thereof

Abstract

The present application belongs to the technical field of water-retaining agent, and more particularly relates to a lignin-based salt-tolerant water-retaining agent and a preparation method thereof. By grading lignin with different molecular weights and polydispersity coefficients through an organic solvent, each fraction has different molecular structures and physicochemical properties, which have different effects on the performance of the finally prepared water-retaining agent product, and the water-retaining performance and water-retaining and water-absorbing stability of the lignin-based salt-tolerant water-retaining agent provided by the present application are optimized. By adding initiators in stages, the initiators are added for the first time in the hydrophilic monomer, and for the second time after blending of the lignin and the hydrophilic monomer, which successfully overcomes the influence of the steric hindrance effect during graft copolymerization of lignin. The reaction conditions of the preparation method of the present application are mild, the required equipment is conventional equipment, and the industrial scale production is convenient, and the obtained lignin-based salt-tolerant water-retaining agent product has good stability, and has good water-retaining performance, biocompatibility and degradability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of water-retaining agents, and more particularly relates to a lignin-based salt-tolerant water-retaining agent and a preparation method thereof. BACKGROUND

[0002] In recent years, land desertification and salinization in China have led to a significant decline in land productivity and accelerated land desertification, which poses a serious challenge to the long-term development of agriculture and forestry. This phenomenon is mainly caused by multiple factors such as drought climate, soil parent material rich in salt ions, unreasonable irrigation, and overdevelopment. Saline-alkali soils are widely distributed in the northwest, north, northeast, and coastal areas of China, with the most serious situation in the northwest, accounting for 69.03% of the total amount of saline-alkali soil in China, and showing an increasing trend year by year.

[0003] To address this challenge, water-saving fine agriculture has become a new trend in global agricultural development. Water-retaining agents, as a kind of functional polymer materials, have shown great potential in agricultural water conservation, water storage, and fine development due to their unique water absorption, water retention, and fertilizer saving characteristics. Water-retaining agents can effectively absorb and store a large amount of water through their internal strong hydrophilic groups such as -OH, -COOH, and -CONH2, while slowing down soil water evaporation, improving soil humidity and fertility, and having a positive effect on improving soil structure and promoting plant growth.

[0004] However, there are many types of water-retaining agents on the market, which can be divided into three categories according to their structure and source: natural, semi-synthetic, and artificial synthesis. Although each type of water-retaining agent has its own advantages, it still faces many problems in practical application, such as the difficulty of balancing degradation and environmental friendliness, high cost, and others. Artificial polymer water-retaining agents are difficult to completely degrade in soil, which can easily lead to secondary soil pollution. Although natural polymer materials such as humic acid and alginic acid are concerned for their biodegradability, they are difficult to be widely applied due to the limitation of material source and raw material requirements. The water-retaining agents on the market are generally expensive, which limits their large-scale application in agricultural production, and the potential of water-saving and efficiency improvement has not been fully realized.

[0005] Therefore, it is an urgent need to develop an environmentally friendly, low-cost, energy-efficient water-retaining agent for agriculture and forestry.

[0006] In addition to moss and fungi, all plants contain lignin. Lignin is a three-dimensional macromolecular network compound with complex structure composed of structural units of phenylpropane type with aromatic characteristics. Due to the presence of active groups such as aromatic group, phenolic hydroxyl group, alcoholic hydroxyl group, carbonyl group, methoxyl group, carboxyl group, conjugated double bond and the like in the molecular structure of lignin, many chemical reactions such as oxidation, reduction, hydrolysis, alcoholysis, acidolysis, photolysis, acylation, sulfonation, alkylation, halogenation, nitration, polycondensation or graft copolymerization can be carried out, and lignin can be biodegraded. Due to its unique chemical structure and biodegradability, lignin is considered as one of the most potential raw materials for producing water-retaining agents. However, the hydrophilicity and water absorption and retention capacity of lignin itself are relatively weak, and it needs to be modified by chemical or physical methods to enhance its performance. How to accurately control the reaction conditions in the modification process to avoid excessive modification leading to performance decline while maintaining the biodegradability of lignin has become a big challenge for lignin used in water-retaining agents. In practical application, water-retaining agents need to have good stability and durability to ensure long-term effective water-retaining effect. However, due to the complex molecular structure of lignin, it is easily affected by environmental factors and undergoes degradation or performance change. How to improve its stability and durability in the actual soil environment is a problem to be solved. SUMMARY

[0007] In view of the above technical problems, the present application provides a lignin-based salt-tolerant water-retaining agent and a preparation method thereof, so as to prepare the water-retaining agent by using staged lignin and hydrophilic monomers for stage-by-stage graft copolymerization, so that it has good water-retaining property, biocompatibility and degradability.

[0008] To achieve the above-mentioned purpose, in a first aspect, the present application provides a preparation method of a lignin-based salt-tolerant water-retaining agent, comprising the following steps:

[0009] Preparation of staged lignin: fully stir lignin and a first organic solvent, collect a first upper layer solution, evaporate and recover the first organic solvent, to obtain a first fraction lignin and a first lower layer insoluble substance, fully stir the first lower layer insoluble substance and a second organic solvent, collect a second upper layer solution, evaporate and recover the second organic solvent, to obtain a second fraction lignin and a second lower layer insoluble substance;

[0010] Graft copolymerization: mix acrylamide, 2-acrylamide-2-methylpropanesulfonic acid and deionized water, deoxidize, heat to a first temperature, add a first metered amount of initiator, after reacting for a first time period, add one of the staged lignin including the first fraction lignin and the second fraction lignin, add a crosslinking agent, add a second metered amount of initiator, after reacting for a second time period, to obtain a polymerization product;

[0011] The polymerization product is added into an alkaline aqueous solution, heated to a second temperature, soaked, dried and crushed to obtain the lignin-based salt-tolerant water-retaining agent.

[0012] The lignin used in the application is selected from one or a mixture of several of corn cob, Chinese pine, bamboo cane, wheat straw, birch, bagasse, curved willow, giant bamboo grass, eucalyptus, reed, poplar, meadow grass, and rice straw. The lignin has a more complex three-dimensional structure than cellulose, chitosan and other types of polymers, and its application in the synthesis of water-retaining agents will greatly improve the overall utilization value of lignocellulosic raw materials. In the application, the fractionated lignin is divided according to different molecular weights and polydispersity coefficients. In addition to the first fraction lignin and the second fraction lignin, the lignin can be prepared to contain more fractions, such as the third fraction lignin, the fourth fraction lignin, and even more. Due to differences in raw material sources, pulping processes, separation methods, etc., lignin has inhomogeneity, which has different effects on subsequent applications. Therefore, the lignin can be improved by fractionation. After the lignin is fractionated by different organic solvents, each fraction has different molecular structures and physical and chemical properties, which will have different effects on the performance of the finally prepared water-retaining agent product. The use of organic solvents to fractionate lignin not only achieves the purpose of regulating the structure and properties of lignin, but also plays a role in adjusting the performance of the water-retaining agent product. Fractionating and separating the lignin into lignin fractions with different molecular weights and polydispersity coefficients makes the molecular structure of each fraction more uniform, thereby improving the stability of the water-retaining agent. Compared with high molecular weight lignin, low molecular weight lignin often has smaller steric hindrance and can better perform graft copolymerization, thereby improving the liquid absorption performance of the water-retaining agent.

[0013] According to some preferred embodiments, the organic solvent is selected from one or several of ethyl acetate, methanol, acetone, and dioxane. The first organic solvent and the second organic solvent are any of the above-listed organic solvents, wherein the second organic solvent is different from the first organic solvent. Preferably, if the fractionated lignin contains the third fraction lignin and the fourth fraction lignin in addition to the first fraction lignin and the second fraction lignin, the corresponding third organic solvent and the fourth organic solvent are also different from each other and from the first organic solvent and the second organic solvent. In the above preparation method, the organic solvents used to prepare the fractionated lignin can be recycled, which can greatly save industrial costs.

[0014] According to some preferred embodiments, the raw material used in the preparation method contains, by mass percentage:

[0015] acrylamide 5-15%;

[0016] 2-acrylamido-2-methylpropane sulfonic acid 2.5-5%;

[0017] Fractionated lignin 5-10%;

[0018] Initiator 0.2-0.6%;

[0019] Crosslinking agent 0.01-0.025%;

[0020] Deionized water 70-85%.

[0021] According to some preferred embodiments, the initiator is a complex initiator system of hydrogen peroxide and persulfate salt, the persulfate salt being selected from one or several of potassium persulfate, sodium persulfate and ammonium persulfate.

[0022] According to some preferred embodiments, the first metered portion is 1 / 3 of the total amount of initiator, and the second metered portion is 2 / 3 of the total amount of initiator.

[0023] According to some preferred embodiments, the crosslinking agent is selected from N,N-methylene bisacrylamide and / or 1,4-bis(4'-vinylphenoxy)butane.

[0024] According to some preferred embodiments, the first temperature and the second temperature are both 60-90℃.

[0025] According to some preferred embodiments, the first time period is 15-30min, and the second time period is 2-3h.

[0026] According to some preferred embodiments, the first fractionated lignin has a weight average molecular weight of 1250-1850 and a number average molecular weight of 1050-1550; and the second fractionated lignin has a weight average molecular weight of 2550-3150 and a number average molecular weight of 1650-2150.

[0027] In a second aspect, the present application provides a lignin-based salt-tolerant water-retaining agent prepared by the preparation method of the first aspect of the present application. The lignin-based salt-tolerant water-retaining agent has good liquid absorption performance and strong salt tolerance, and can be applied to saline-alkali soil, arid soil, weathered soil, gray desert soil and other soils, and has good water-retaining effect. The lignin-based salt-tolerant water-retaining agent product has good stability, is non-toxic, is not limited by season and region in use, and is convenient to store and transport.

[0028] Differing from the prior art, the technical scheme has the effects that the lignin is classified by organic solvents according to different molecular weights and polydispersity coefficients, each fraction has different molecular structures and physical and chemical properties, and the water-retaining performance and water-retaining and water-absorbing stability of the prepared lignin-based salt-tolerant water-retaining agent are optimized. Due to the complex macromolecular structure of lignin, low molecular activity, poor grafting property and poor compatibility with the matrix, it is difficult to introduce a hydrophilic structure into lignin, and the preparation of a lignin composite hydrogel material is difficult, the traditional initiation method has low initiation efficiency, and the prepared water-retaining agent has poor liquid absorption performance or cannot be well formed into a gel. In the present application, the initiator is added dropwise in stages, a small amount of initiator is first added dropwise in the hydrophilic monomer solution, and when the appearance is viscous, the remaining initiator is added dropwise after blending with lignin, so that the influence of the steric hindrance effect of lignin grafting copolymerization is greatly weakened, and the polymer product has a relatively stable three-dimensional network structure after forming, and the hydrophilic groups in the polymer product greatly improve the liquid absorption performance of the water-retaining agent. At the same time, after grafting copolymerization with lignin, lignin also plays a good role as a skeleton. Compared with the water-retaining agent prepared by the traditional initiation method, the water-retaining agent prepared by the present application has superior liquid absorption performance when the lignin content is the same, and has good liquid absorption performance when the lignin content reaches 30%.

[0029] The reaction conditions for preparing the lignin-based salt-tolerant water-retaining agent are mild, the required equipment is conventional equipment, industrial-scale production is convenient, the product stability of the lignin-based salt-tolerant water-retaining agent is good, the lignin-based salt-tolerant water-retaining agent has good water-retaining performance, biocompatibility and degradability, is non-toxic and non-polluting, is not limited by seasons and regions, is convenient to store and transport, and can be well applied to a larger range of agriculture and forestry.

[0030] The above invention content is only a summary of the technical scheme of the present application. In order to enable those skilled in the art to more clearly understand the technical scheme of the present application, and then implement the content recorded in the specification and drawings, and in order to enable the above-mentioned purposes and other purposes, characteristics and advantages of the present application to be more easily understood, the following describes the specific embodiments of the present application and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0031] The accompanying drawings are only used to show the principles, implementation manners, applications, characteristics and effects of the specific embodiments of the present application and other related contents, and cannot be considered as limitations of the present application.

[0032] In the drawings of the specification:

[0033] Figure 1 The figure is a process flow diagram of the lignin-based salt-tolerant water-retaining agent preparation method of the present application. DETAILED DESCRIPTION

[0034] In order to explain the possible application scenarios, technical principles, specific implementation schemes, and the purposes and effects that can be achieved, the following will be described in detail in combination with the specific embodiments listed and the accompanying drawings. The embodiments described herein are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0035] In this document, the term "embodiment" means that the specific features, structures or characteristics described in combination with the embodiment can be included in at least one embodiment of the present application. The term "embodiment" appearing at various places in the specification does not necessarily refer to the same embodiment, and does not particularly limit the independence or association between other embodiments. In principle, in the present application, as long as there is no technical contradiction or conflict, each technical feature mentioned in each embodiment can be combined in any way to form a corresponding implementable technical solution.

[0036] Unless otherwise defined, the meanings of the technical terms used herein are the same as those commonly understood by those skilled in the art to which the present application belongs; the use of related terms herein is only for the purpose of describing specific embodiments, and is not intended to limit the present application.

[0037] In the description of the present application, the word "and / or" is a description of the logical relationship between the objects, which means that there can be three relationships, for example, A and / or B, which means that there are three cases: A exists, B exists, and A and B exist at the same time. In addition, the character " / " in this document generally represents that the associated objects before and after are a "or" logical relationship.

[0038] In the present application, the terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual quantity, primary and secondary or order relationship between the entities or operations.

[0039] In the present application, without more limitation, the "includes", "contains", "has" or other similar expressions used in the sentence are intended to cover non-exclusive inclusion, and these expressions do not exclude the presence of other elements in the process, method or product including the described elements, so that the process, method or product including a series of elements can not only include those limited elements, but also include other elements not explicitly listed, or also include the elements inherent to such process, method or product.

[0040] In the present application, "greater than", "less than", "exceed" and the like are understood as not including the number; "above", "below", "within" and the like are understood as including the number. In addition, in the description of the embodiments of the present application, the meaning of "multiple" is two or more (including two), and similar expressions related to "multiple" are also understood in this way, for example, "multiple groups", "multiple times" and the like, unless otherwise explicitly specified.

[0041] In the description of the embodiments of the present application, the spatially related expressions used, such as "center", "longitudinal", "transverse", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "vertical", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like, indicate the orientation or positional relationship based on the orientation or positional relationship shown in the specific embodiments or the drawings, and are only for the convenience of describing the specific embodiments of the present application or for the reader to understand, and do not indicate or imply that the indicated device or component must have a particular position, a particular orientation, or be constructed or operated in a particular orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0042] Unless otherwise explicitly specified or limited, in the description of the embodiments of the present application, the terms "mount", "connect", "connect", "fix", "set" and the like should be broadly understood. For example, the "connection" can be fixed connection, or detachable connection, or integrated setting; it can be mechanical connection, or electrical connection, or communication connection; it can be directly connected, or indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art to which the present application belongs, the specific meaning of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0043] There are many types of water-retaining agents on the market, but they are mainly concentrated in the field of artificial high molecular materials. Since artificial high molecular materials are difficult to completely degrade in soil, they can easily cause secondary accumulation pollution of soil. Although many researchers have applied natural high molecular materials to water-retaining agents, such as humic acid, alginic acid and amino acid, the sources of these materials are limited, and the use of raw materials has strict requirements, making it difficult to be widely promoted and applied. In addition, the water-retaining agents on the market are expensive and difficult to be widely promoted and applied in agricultural production.

[0044] Therefore, it is an important research direction to prepare an environment-friendly, low-cost, energy-efficient water-retaining agent for agriculture and forestry, and to develop fine agriculture, water retention and storage. Lignin is the only high-polymerization-degree aromatic organic raw material obtained from renewable resources in nature, and is widely available and low in price. Therefore, it is a good alternative to traditional water-retaining agents to prepare soil water-retaining agents from fractionated lignin.

[0045] In the present application, if not specially stated, the reagents used are all commercially available in the art.

[0046] In the present application, if not specially stated, the test methods and devices are all conventional test methods and devices used in the art.

[0047] Please refer to Figure 1 The process flow diagram of the preparation method of the lignin-based salt-tolerant water-retaining agent is shown in the figure, and the main steps and preparation process of the lignin-based salt-tolerant water-retaining agent prepared by the present application are all carried out according to the process flow.

[0048] Preparation of fractionated lignin: the lignin and the first organic solvent are fully stirred, the first upper layer solution is collected, the first organic solvent is evaporated and recovered, the first fractionated lignin and the first lower layer undissolved substance are obtained, the first lower layer undissolved substance and the second organic solvent are fully stirred, the second upper layer solution is collected, the second organic solvent is evaporated and recovered, the second fractionated lignin and the second lower layer undissolved substance are obtained;

[0049] Graft copolymerization: acrylamide, 2-acrylamide-2-methylpropanesulfonic acid and deionized water are mixed and stirred, oxygen is removed, the temperature is raised to a first temperature, a first metered amount of initiator is added, after reacting for a first time period, fractionated lignin including the first fractionated lignin and the second fractionated lignin is added, a crosslinking agent is added, a second metered amount of initiator is added, and after reacting for a second time period, a polymerization product is obtained;

[0050] The polymerization product is added to an alkaline aqueous solution, the temperature is raised to a second temperature, soaked, dried and crushed to obtain the lignin-based salt-tolerant water-retaining agent.

[0051] Specifically, lignin and an organic solvent are added to a reactor, the upper layer solution is collected after stirring for 1 h, and the first fractionated lignin is obtained after evaporating and recovering the organic solvent, and the undissolved lignin in the lower layer is sequentially dissolved and fractionated by repeating the above steps with other organic solvents to obtain the second fractionated lignin, the third fractionated lignin and the fourth fractionated lignin, respectively.

[0052] In the reactor, acrylamide, 2-acrylamide-2-methylpropanesulfonic acid and deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 60-90℃, a small amount of persulfate and hydrogen peroxide was added dropwise, after reaction for 15-30 min, the lignin fraction product was added, and after adding the crosslinking agent, the persulfate and hydrogen peroxide were continuously added dropwise, and after reaction for 2-3 h, the polymerization product was obtained. The polymerization product was added to a 5% NaOH aqueous solution, heated to 60-90℃ and soaked for 0.5-1 h, then dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product. The product is black powder, and after the liquid absorption reaches the swelling equilibrium, it presents a gel state. The raw material components and their mass fractions used in the above preparation process are as follows: acrylamide 5-15%; 2-acrylamide-2-methylpropanesulfonic acid 2.5-5%; lignin fraction product 5-10%; hydrogen peroxide 0.1-0.3%; persulfate 0.1-0.3%; crosslinking agent 0.01-0.025%; deionized water 70-85%.

[0053] Based on the above preparation process, the lignin is selected from one or more of corn cob, Masson pine, bamboo cane, wheat straw, birch, bagasse, curved willow, emperor grass, eucalyptus, reed, poplar, anemarrhena, and rice straw.

[0054] Based on the above preparation process, the lignin fraction product is a different lignin fraction with different molecular weights and polydispersity coefficients separated by organic solvent fractionation. Preferably, the organic solvent is selected from one or more of ethyl acetate, methanol, acetone, and dioxane.

[0055] Based on the above preparation process, the initiation system is a hydrogen peroxide / persulfate composite initiation system, and the persulfate is selected from one or more of potassium persulfate, sodium persulfate, and ammonium persulfate.

[0056] Based on the above preparation process, the first dropwise addition of a small amount of initiator is 1 / 3 of the total amount of initiator, and the second dropwise addition of initiator is 2 / 3 of the total amount.

[0057] Based on the above preparation process, the crosslinking agent is selected from one or both of N,N-methylenebisacrylamide and 1,4-bis(4'-vinylphenoxy)butane.

[0058] Based on the above preparation process, the organic solvents used for fractionation of lignin can be recycled and used, saving industrial costs.

[0059] Example 1

[0060] Please refer to Figure 1 The preparation process flow chart of the lignin-based salt-tolerant water-retaining agent is shown in the figure, and the present example provides a preparation process of a lignin-based salt-tolerant water-retaining agent, which includes the following steps:

[0061] The corn cob lignin and ethyl acetate were added into a reactor, and after stirring for 1 h, the upper solution was collected, and after evaporation to recover the ethyl acetate, the first fraction lignin was obtained. The lower undissolved lignin was repeatedly dissolved and fractionated by using methanol, acetone, and dioxane in sequence, respectively, to obtain the second fraction lignin, the third fraction lignin, and the fourth fraction lignin, respectively. The molecular weight of each fraction lignin obtained in Example 1 and the molecular weight and the polydispersity coefficient of the un-fractionated lignin of Comparative Example 1 are shown in Table 1.

[0062] In a reactor, 77 kg of acrylamide, 27 kg of 2-acrylamide-2-methylpropanesulfonic acid, and 827.5 kg of deionized water were added, stirred, and deoxygenated by nitrogen for 20 min. The temperature of the reaction system was raised to 70°C, 0.4 kg of ammonium persulfate and 0.4 kg of hydrogen peroxide were added dropwise, 66 kg of the first fraction lignin was added after 20 min of reaction, and the remaining 0.8 kg of ammonium persulfate and 0.8 kg of hydrogen peroxide were added dropwise after adding 0.1 kg of N, N-methylene bisacrylamide. After 2.5 h of reaction, a polymerization product was obtained. The polymerization product was added to a sufficient amount of 5% NaOH aqueous solution, soaked at 80°C for 1 h, dried, and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0063] Table 1 Test results of the molecular weight and the polydispersity coefficient of each fraction lignin obtained in Example 1 and the un-fractionated lignin of Comparative Example 1

[0064]

[0065] Example 2

[0066] The present example provides a preparation process of a lignin-based salt-tolerant water-retaining agent, including the following steps:

[0067] The pine wood lignin and ethyl acetate were added into a reactor, and after stirring for 1 h, the upper solution was collected, and after evaporation to recover the ethyl acetate, the first fraction lignin was obtained. The lower undissolved lignin was repeatedly dissolved and fractionated by using methanol, acetone, and dioxane in sequence, respectively, to obtain the second fraction lignin, the third fraction lignin, and the fourth fraction lignin.

[0068] In a reactor, 67 kg of acrylamide, 33 kg of 2-acrylamido-2-methylpropanesulfonic acid and 847.9 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 60°C, 0.33 kg of ammonium persulfate and 0.33 kg of hydrogen peroxide were added dropwise, 50 kg of the first fraction of lignin was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.67 kg of hydrogen peroxide were added dropwise after the addition of 0.1 kg of N, N-methylene bisacrylamide, and the reaction was carried out for 3 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 70°C for 1 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0069] Example 3

[0070] The present example provides a preparation process of a lignin-based salt-tolerant water-retaining agent, comprising the following steps:

[0071] In a reactor, 67 kg of acrylamide, 33 kg of 2-acrylamido-2-methylpropanesulfonic acid and 847.9 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 60°C, 0.33 kg of ammonium persulfate and 0.33 kg of hydrogen peroxide were added dropwise, 50 kg of the first fraction of lignin was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.67 kg of hydrogen peroxide were added dropwise after the addition of 0.1 kg of N, N-methylene bisacrylamide, and the reaction was carried out for 3 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 70°C for 1 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0072] In a reactor, 67 kg of acrylamide, 33 kg of 2-acrylamido-2-methylpropanesulfonic acid and 847.9 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 60°C, 0.33 kg of ammonium persulfate and 0.33 kg of hydrogen peroxide were added dropwise, 50 kg of the first fraction of lignin was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.67 kg of hydrogen peroxide were added dropwise after the addition of 0.1 kg of N, N-methylene bisacrylamide, and the reaction was carried out for 3 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 70°C for 1 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0073] Example 4

[0074] The present example provides a preparation process of a lignin-based salt-tolerant water-retaining agent, comprising the following steps:

[0075] In a reactor, 67 kg of acrylamide, 33 kg of 2-acrylamido-2-methylpropanesulfonic acid and 847.9 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 60°C, 0.33 kg of ammonium persulfate and 0.33 kg of hydrogen peroxide were added dropwise, 50 kg of the first fraction of lignin was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.67 kg of hydrogen peroxide were added dropwise after the addition of 0.1 kg of N, N-methylene bisacrylamide, and the reaction was carried out for 3 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 70°C for 1 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0076] In a reactor, 68 kg of acrylamide, 28 kg of 2-acrylamido-2-methylpropanesulfonic acid and 851.35 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 70°C, 0.33 kg of ammonium persulfate and 0.16 kg of hydrogen peroxide were added dropwise, 51 kg of the third lignin fraction was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.34 kg of hydrogen peroxide were added dropwise after adding 0.15 kg of N,N-methylenebisacrylamide, and the reaction was carried out for 3 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 70°C for 0.5 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0077] Example 5

[0078] The present example provides a preparation process of a lignin-based salt-tolerant water-retaining agent, comprising the following steps:

[0079] Poplar lignin and ethyl acetate were added to a reactor, the upper solution was collected after stirring for 1 h, and the first lignin fraction was obtained after evaporation and recovery of ethyl acetate. The unsolved lignin in the lower layer was dissolved and fractionated by repeating the above steps with methanol, acetone and dioxane in turn to obtain the second, third and fourth lignin fractions, respectively.

[0080] In a reactor, 68 kg of acrylamide, 28 kg of 2-acrylamido-2-methylpropanesulfonic acid and 851.35 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 70°C, 0.33 kg of ammonium persulfate and 0.16 kg of hydrogen peroxide were added dropwise, 51 kg of the third lignin fraction was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.34 kg of hydrogen peroxide were added dropwise after adding 0.15 kg of N,N-methylenebisacrylamide, and the reaction was carried out for 3 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 70°C for 0.5 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0081] Example 6

[0082] The present example provides a preparation process of a lignin-based salt-tolerant water-retaining agent, comprising the following steps:

[0083] Poplar lignin and ethyl acetate were added to a reactor, the upper solution was collected after stirring for 1 h, and the first lignin fraction was obtained after evaporation and recovery of ethyl acetate. The unsolved lignin in the lower layer was dissolved and fractionated by repeating the above steps with methanol, acetone and dioxane in turn to obtain the second, third and fourth lignin fractions, respectively.

[0084] 76 kg of acrylamide, 25 kg of 2-acrylamido-2-methylpropanesulfonic acid, and 845.26 kg of deionized water were added to a reactor. The mixture was stirred and nitrogen gas was introduced for deoxygenation for 20 min. The system temperature was raised to 90 °C. 0.83 kg of ammonium persulfate and 0.33 kg of hydrogen peroxide were added dropwise. After reacting for 20 min, 50 kg of secondary lignin fraction was added. Then, 0.24 kg of N,N-methylenebisacrylamide was added, followed by the addition of the remaining 1.67 kg of ammonium persulfate and 0.67 kg of hydrogen peroxide. The reaction was carried out for 2.5 h to obtain the polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, heated to 70 °C, soaked for 1 h, dried, and pulverized to obtain a lignin-based salt-tolerant soil water-retaining agent.

[0085] Example 7

[0086] This example provides a preparation process for a lignin-based salt-resistant water-retaining agent, including the following steps:

[0087] Add Napier grass lignin and ethyl acetate to the reactor, stir for 1 hour, collect the upper layer solution, evaporate and recover ethyl acetate to obtain the first-stage lignin, and repeat the above steps to dissolve and classify the undissolved lignin in the lower layer with other methanol, acetone and dioxane in sequence to obtain the second-stage lignin, third-stage lignin and fourth-stage lignin respectively.

[0088] 88 kg of acrylamide, 44 kg of 2-acrylamido-2-methylpropanesulfonic acid, and 813.3 kg of deionized water were added to a reactor. The mixture was stirred and nitrogen gas was introduced for deoxygenation for 20 min. The system temperature was raised to 75 °C. 1 kg of ammonium persulfate and 0.5 kg of hydrogen peroxide were added dropwise. After reacting for 20 min, 50 kg of secondary lignin fraction was added. Then, 0.2 kg of N,N-methylenebisacrylamide was added, followed by the remaining 2 kg of ammonium persulfate and 1 kg of hydrogen peroxide. The reaction was carried out for 3 h to obtain the polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, heated to 65 °C, soaked for 1 h, dried, and pulverized to obtain a lignin-based salt-tolerant soil water-retaining agent.

[0089] Example 8

[0090] This example provides a preparation process for a lignin-based salt-resistant water-retaining agent, including the following steps:

[0091] Sugarcane bagasse lignin and ethyl acetate were added to the reactor and stirred for 1 hour. The upper layer solution was collected, and the ethyl acetate was evaporated to recover the first-stage lignin. The undissolved lignin in the lower layer was then dissolved and graded by repeating the above steps with other solvents such as methanol, acetone, and dioxane in sequence to obtain the second-stage, third-stage, and fourth-stage lignin, respectively.

[0092] In a reactor, 76 kg of acrylamide, 33 kg of 2-acrylamido-2-methylpropanesulfonic acid and 823.1 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 70°C, 0.33 kg of ammonium persulfate and 0.26 kg of hydrogen peroxide were added dropwise, 66 kg of the third fraction of lignin was added after 20 min of reaction, and the remaining 0.67 kg of ammonium persulfate and 0.54 kg of hydrogen peroxide were added dropwise after the addition of 0.1 kg of N, N-methylenebisacrylamide, and the reaction was carried out for 2 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 90°C for 0.5 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0093] Comparative Example 1

[0094] The present example provides a preparation process of a water-retaining agent, comprising the following steps:

[0095] In a reactor, 77 kg of acrylamide, 27 kg of 2-acrylamido-2-methylpropanesulfonic acid and 827.5 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 70°C, 0.4 kg of ammonium persulfate and 0.4 kg of hydrogen peroxide were added dropwise, 66 kg of corncob lignin was added after 20 min of reaction, and the remaining 0.8 kg of ammonium persulfate and 0.8 kg of hydrogen peroxide were added dropwise after the addition of 0.1 kg of N, N-methylenebisacrylamide, and the reaction was carried out for 2.5 h to obtain a polymerization product. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 80°C for 1 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0096] Comparative Example 1 and Example 1 differ in that the raw material for preparing the lignin-based salt-tolerant soil water-retaining agent product is corncob lignin that has not been fractionated by an organic solvent. The molecular weight and polydispersity coefficient of the un-fractionated lignin are shown in Table 1.

[0097] Comparative Example 2

[0098] The present example provides a preparation process of a water-retaining agent, comprising the following steps:

[0099] In a reactor, corncob lignin and ethyl acetate were added, stirred for 1 h, and the upper solution was collected. After evaporation to recover the ethyl acetate, the first fraction of lignin was obtained. The lower insoluble lignin was then dissolved and fractionated by repeating the above steps with other solvents such as methanol, acetone and dioxane to obtain the second fraction of lignin, the third fraction of lignin and the fourth fraction of lignin, respectively.

[0100] In the reactor, 77 kg of acrylamide, 27 kg of 2-acrylamide-2-methylpropanesulfonic acid and 827.5 kg of deionized water were added, stirred and deoxygenated by nitrogen for 20 min, the system temperature was raised to 70 ℃, 66 kg of the first fraction of lignin was added, and after adding 0.1 kg of N, N-methylene bisacrylamide, the remaining 1.2 kg of ammonium persulfate, 1.2 kg of hydrogen peroxide was added dropwise, and the polymerization product was obtained after 2.5 h of reaction. The polymerization product was added to a 5% NaOH aqueous solution, soaked at 80 ℃ for 1 h, dried and crushed to obtain a lignin-based salt-tolerant soil water-retaining agent product.

[0101] The difference between Comparative Example 2 and Example 1 is that the initiator is not added in two stages, but is added in a conventional initiation manner after the lignin is added.

[0102] The water-retaining agent performance of Examples 1-8 and Comparative Examples 1-2 described above was tested as follows

[0103] 1. Water absorption performance

[0104] 0.1 g of the dried and crushed water-retaining agent was weighed into a beaker containing deionized water and a 0.9 wt% NaCl solution, soaked at room temperature for a period of time, filtered with a stainless steel mesh after reaching liquid absorption equilibrium, and the remaining water was removed by standing. The gel mass was weighed. The liquid absorption rate of the water-retaining agent was calculated as follows: liquid absorption rate = (gel mass - dry product mass) / dry product mass.

[0105] As shown by the water absorption performance test results in Table 2, the above lignin-based environmentally friendly water-retaining agent has good water absorption and salt water absorption rates, strong water absorption and salt tolerance, and can meet the market demand for the product. The water-retaining agent products prepared in Examples 1-8 show that the liquid absorption performance is closely related to the amount of initiator, the amount of crosslinking agent, the ratio of hydrophilic monomers, the polymerization temperature and time, etc. Among them, the increase in the amount of 2-acrylamide-2-methylpropanesulfonic acid is beneficial to the improvement of the salt tolerance of the water-retaining agent product. At the same time, it can be shown that the water absorption rates of the water-retaining agent products prepared from different lignin fractions separated by organic solvent fractionation have certain differences.

[0106] Table 2 Water-retaining agent water absorption performance test results of Examples 1-8 and Comparative Examples 1-2

[0107]

[0108] The results of the molecular weight and polydispersity index of the lignin fractions obtained from Example 1 and the lignin of Comparative Example 1 shown in Table 2 show that, in the case of a small polydispersity index, the lignin fraction with a smaller molecular weight has a better water absorption performance. Comparative Example 1 shows that the water absorption performance of the water retention agent prepared from the lignin without organic solvent fractionation is worse than that of the water retention agent prepared from the lignin fraction of Examples 1-8. Therefore, the lignin fractionated by the organic solvent can adjust the performance of the water retention agent.

[0109] 2. Water retention performance

[0110] A certain amount of the material fully absorbed with water is placed in a culture dish in a closed environment at 30°C, and the change in mass over time is measured at intervals to evaluate the water retention capacity. The water retention rate of the water retention agent is calculated according to the following formula: water retention rate = (mass of the product after being kept at a constant temperature for a certain time / mass of the initial fully water-absorbed product) x 100%.

[0111] The results of the water retention rates of the water retention agents of Examples 1-8 and Comparative Examples 1-2 shown in Table 3 show that the lignin-based water retention agent has a good water retention value after 6 days, has a strong water retention performance, and can meet the market demand of the product. Examples 1-8 show that the water retention performance of the water retention agent prepared from the different lignin fractions fractionated by the organic solvent has certain differences, and the water retention performance of the water retention agent prepared from the first fraction lignin and the second fraction lignin is better than that of the water retention agent prepared from the third fraction lignin and the fourth fraction lignin and the water retention agent prepared from the lignin without organic solvent fractionation. In a certain range, the increase in the amount of the lignin fraction product is beneficial to the improvement of the water retention performance of the water retention agent.

[0112] Table 3 Water retention rate test results of the water retention agents of Examples 1-8 and Comparative Examples 1-2

[0113]

[0114] 3. Product stability test

[0115] The water retention agent products of Examples 1-8 are kept in a closed constant-temperature and constant-humidity environment at 40°C for 30 days, and the high-temperature stability is tested.

[0116] The results of the high-temperature stability test of the water retention agent shown in Table 4 show that the stability of the lignin-based water retention agent meets the requirements of the conventional agricultural and forestry water retention agent, and can meet the market promotion and application demand of the product.

[0117] Table 4 High-temperature stability test results of the products of Examples 1-8

[0118]

[0119] 4. Product degradability test

[0120] The water-retaining agent products prepared in Examples 1-8 were completely buried in natural soil with water retention capacity and humidity of 50%, taken out after being placed in a closed constant temperature and humidity environment for 60 days, washed, dried and weighed, and the degradability was tested. The degradation rate of the water-retaining agent was calculated according to the following formula: degradation rate = (product mass before burying in soil - product mass after burying in soil) / product mass before burying in soil x 100%.

[0121] As shown in Table 5, the water-retaining agent degradability test results show that the above lignin-based water-retaining agent has strong degradability, which can meet the market promotion and application requirements of the product.

[0122] Table 5: Product degradability test results of Examples 1-8

[0123]

[0124] In summary, the present application uses organic solvents to fractionate lignin, which not only achieves the purpose of regulating the structure and properties of lignin, but also plays a role in adjusting the performance of the water-retaining agent product. Compared with traditional lignin-based water-retaining agents, the water-retaining agent product prepared by the present application not only has superior liquid absorption and water retention performance, but also has strong salt resistance, which can be applied to saline-alkali soil, drought soil, weathered soil, gray desert soil and other soils, and has good water retention effect. At the same time, the present application still has good liquid absorption performance when the lignin addition amount reaches 30% of the solid content, greatly reducing the product cost, which is conducive to large-scale application in agricultural and forestry production, and fully plays its potential in water saving and efficiency improvement. In addition, the water-retaining agent product prepared by the present application is stable, green and environmentally friendly, and is not limited by season and region, and is convenient to store and transport.

[0125] Finally, it should be noted that although the above embodiments have been described in the specification and drawings of the present application, this does not limit the patent protection scope of the present application. Any equivalent structure or equivalent flow replacement or modification based on the essential concept of the present application, using the content described in the specification and drawings, as well as direct or indirect implementation of the technical solutions of the above embodiments in other related technical fields, are all included in the patent protection scope of the present application.

Claims

1. A method for preparing a lignin-based salt-tolerant water-retaining agent, characterized by, The method comprises the following steps: Preparation of fractionated lignin: mixing lignin and a first organic solvent, collecting a first upper layer solution, evaporating and recovering the first organic solvent, obtaining a first fractionated lignin and a first lower layer insoluble, mixing the first lower layer insoluble and a second organic solvent, collecting a second upper layer solution, evaporating and recovering the second organic solvent, obtaining a second fractionated lignin and a second lower layer insoluble; Graft copolymerization: mixing acrylamide, 2-acrylamide-2-methylpropanesulfonic acid and deionized water, deoxidizing, heating to a first temperature, adding a first metered amount of initiator, after reacting for a first time period, adding one of the fractionated lignins comprising the first fractionated lignin and the second fractionated lignin, adding a crosslinking agent, adding a second metered amount of initiator, after reacting for a second time period, obtaining a polymerization product; Adding the polymerization product to an alkaline aqueous solution, heating to a second temperature, soaking, drying and crushing, obtaining the lignin-based salt-tolerant water-retaining agent.

2. The production method according to claim 1, characterized by, The organic solvent is selected from one or more of ethyl acetate, methanol, acetone and dioxane.

3. The production method according to claim 1, characterized by, According to mass percentage, it comprises: acrylamide 5-15%; 2-acrylamide-2-methylpropanesulfonic acid 2.5-5%; fractionated lignin 5-10%; initiator 0.2-0.6%; crosslinking agent 0.01-0.025%; deionized water 70-85%.

4. The method of claim 1, wherein, The initiator is a hydrogen peroxide / persulfate composite initiation system, and the persulfate is selected from one or more of potassium persulfate, sodium persulfate and ammonium persulfate.

5. The preparation method according to claim 1, characterized in that, The first metered amount is 1 / 3 of the total amount of initiator, and the second metered amount is 2 / 3 of the total amount of initiator.

6. The method of claim 1, wherein, The crosslinking agent is selected from N, N-methylenebisacrylamide and / or 1,4-bis(4'-vinylphenoxy)butane.

7. The preparation method according to claim 1, characterized in that, The first temperature and the second temperature are both 60-90℃.

8. The preparation method according to claim 7, characterized in that, The first time period is 15-30 min, and the second time period is 2-3 h.

9. The method of claim 1, wherein, The weight average molecular weight of the first fractionated lignin is 1250-1850, and the number average molecular weight is 1050-1550; the weight average molecular weight of the second fractionated lignin is 2550-3150, and the number average molecular weight is 1650-2150.

10. A lignin-based salt-tolerant water-retaining agent, characterized by, Prepared by the preparation method of any one of claims 1-9.

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