Polyaspartic acid (PASP)’s Synthesis Process and Modification technology

1.Introduction

At present, there are two main process technologies for the synthesis of PASP: the first is the L-aspartic acid method using L-aspartic acid monomer as raw material, which refers to the product PASP obtained through high-temperature polycondensation, hydrolysis under alkaline conditions, and subsequent refining steps such as neutralization under certain conditions. The second is the maleic anhydride method using maleic anhydride, maleic acid or fumaric acid and other nitrogen-containing compounds that can produce ammonia as raw materials. This method first prepares the intermediate polysuccinimide from the raw material, then the polysuccinimide undergoes a hydrolysis reaction to prepare PASP salt, then the PASP hydrochloride is acidified to prepare PASP, and finally the PASP and its salt are separated and purified to obtain the product PASP.

Although the scale inhibition, dispersion, chelation and other properties of PASP prepared by the above two process technologies are equivalent, when using L-aspartic acid as the raw material for solid phase thermal shrinkage polymerization production process, although it has the advantages of wide source of aspartic acid, simple production process, high conversion rate and no pollution, solid L-aspartic acid is in an ionic crystal state, large particle size, high crystallinity, and dense internal structure, which leads to slow heat transfer, uneven reaction, and high energy consumption in the solid phase thermal shrinkage polymerization reaction, which ultimately affects the scale inhibition performance of PASP. In addition, the raw material L-aspartic acid is relatively expensive, resulting in high production costs, and therefore, there are disadvantages such as poor market competitiveness

Although the molecular weight of the product synthesized by the maleic anhydride method is low, it still has good applications in scale inhibitors, corrosion inhibitors, etc., and it has good economic efficiency, so it has become a hot spot for current research and development. At present, PASP is mostly synthesized by maleic anhydride process using maleic anhydride as raw material.

2 Research progress

At present, the research and development of PASP focuses on the synthesis process and modification technology.

2.1 Synthesis process technology

The process of synthesizing PASP using L-aspartic acid. The method is to heat-condense L-aspartic acid in the presence or absence of a catalyst to obtain a polysuccinimide solid powder, and then hydrolyze it under alkaline conditions to obtain PASP. The entire production process of this process does not produce any "three wastes" and by-products that pollute the environment. It has the advantages of simple production process and easy control, but there are shortcomings such as narrow molecular weight distribution and light color of the obtained product.

A method for synthesizing PASP using ionic liquids. The method is to react maleic anhydride, nitrogen-containing compounds and water in a mass ratio of 1: (1-2): (1.5-2.5) at normal pressure and 60-90°C for 1-3 hours to obtain ammonium maleate; 1%-5% of acidic ionic liquid is added to the ammonium maleate solution as a catalyst, and a polymerization reaction is carried out at normal pressure and 160-200°C to generate polysuccinimide; polysuccinimide is hydrolyzed at pH 10-12 and 20-40°C to obtain the product. The method is not only green and environmentally friendly, but also the ionic liquid catalyst can be reused, which can reduce production costs.

A method for synthesizing PASP using maleic anhydride and ammonia. The method is to react molten maleic anhydride with ammonia in a closed reactor, absorb the remaining gas after the reaction, add NaOH solution, and neutralize to obtain a mixture of sodium maleamate and ammonium maleamate; then directly heat the mixture in a closed reactor and perform polycondensation under stirring, evaporate the water after the polycondensation reaction to obtain a polysuccinimide solid; add water to another reactor and start stirring, add the polysuccinimide solid, slowly add NaOH solution, heat and perform hydrolysis to obtain a PASP solution. The method simplifies the production process of PASP, and the obtained PASP can meet the molecular weight requirements of different application fields, and the production process is environmentally friendly and has a low production cost.

PASP is synthesized by a microwave semi-solvent method. First, maleic anhydride or fumaric acid and ammonia or ammonium salt are used as raw materials, a small amount of solvent is added, and the microwave frequency is (915±50) MHz or (2 450±50) MHz, and the power is 200-20 000 W. The low molecular weight polysuccinimide (PSI-I) is synthesized, and the solvent is completely recovered at this time; then PSI-I is used as the raw material, and the microwave frequency is the same, and the power is 400-50 000 W. The condition is irradiated for 1-30 minutes without solvent, and the high molecular weight polysuccinimide (PSI-II) is obtained; PSI-II is then hydrolyzed to obtain PASP with a higher molecular weight. This method is not only simple in process, does not require solvent separation, has a fast reaction speed, high yield, energy saving, and less pollution, but can also improve the molecular weight and scale inhibition performance of the product, but the requirements for equipment are relatively high.

A new method for synthesizing PASP has been developed. The method is to add maleic anhydride to the reactor, then add deionized water to the reactor, and then pass ammonia at a rate of 25-35 mL/min, wherein the mass ratio of maleic anhydride to deionized water is 1: (0.1-1), and the molar ratio of maleic anhydride to ammonia is 1: (0.1-3); after passing ammonia, heat to 60-100 ° C for reaction for 20-40 min, and then continue to heat to 140-240 ° C for polymerization reaction for 1-10 h; after the reaction, pour the reddish-brown liquid into a separatory funnel and add anhydrous ethanol, extract the separated liquid to obtain polyaspartic acid viscous material, and obtain the product PASP after drying. This method can realize the one-step synthesis of PASP, has a simple process, can greatly improve production efficiency, and reduce costs, and can realize large-scale production.

PASP was synthesized using liquid ammonia and maleic anhydride as raw materials. The effects of the molar ratio of maleic anhydride to liquid ammonia, the synthesis temperature and time of ammonium maleate, the polymerization temperature and time, and the hydrolysis pH on the polysuccinimide yield and the molecular weight of PASP were studied. The results showed that the optimal reaction conditions were (maleic anhydride) ∶ (liquid ammonia) = 1 ∶ 1.2, the synthesis temperature of ammonium maleate was 85 ℃, the reaction time was 2 h, the polymerization temperature was 210 ℃, the polymerization time was 4 h, and the hydrolysis pH was 9. Under the condition of no catalyst, the polysuccinimide yield can be as high as more than 95%.

A method for synthesizing PASP in one step using a solid acid catalyst. The method comprises the following steps: placing maleic anhydride in a reactor, adding deionized water, cooling with cooling water, then dripping ammonia water, and discharging water after the reaction is completed; then adding an acid-modified montmorillonite catalyst and liquid paraffin to the reactor, raising the temperature to the polymerization temperature for reaction, cooling with cooling water after the reaction, and discharging the reaction liquid; pouring out the liquid paraffin in the reaction liquid for recovery, obtaining a reddish brown viscous liquid, and placing it in a reactor, adding deionized water to the reactor and filtering it, precipitating the filtrate with a precipitant, separating the liquids, and drying with filter paper to obtain PASP. Because the method uses a new solid acid catalyst, the molecular weight of PASP can be increased. In addition, by adding a slightly excessive amount of ammonia water, the polymerization and hydrolysis reactions are carried out in one system, which can simplify the synthesis process and facilitate large-scale production.

2.2 Modification technology

Due to the single type of functional groups in the PASP molecule, its performance is single and its application is limited, so PASP modification has become the mainstream direction of research.

A method for preparing PASP modified with a dicarbonyl compound. The method uses water as solvent and reacts polysuccinimide with a dicarbonyl compound under the catalysis of NaOH to obtain a dicarbonyl compound modified PASP. The method can obtain an ecological modified PASP with high scale inhibition efficiency and a stable and suitable viscosity average molecular weight.

A method for synthesizing a PASP modified with a Schiff base structure. The method comprises the following steps: adding maleic anhydride to a reactor filled with distilled water, stirring in a water bath at 30-60°C, then dripping ammonia water into the reactor, reacting at 60-100°C for 0.5-3 h; heating the oil bath to 180-240°C under Ar conditions, reacting for 20-40 min, then adding dimethylformamide to dissolve the viscous substance, and obtaining the product polysuccinimide; dissolving polysuccinimide and thiocarbazide (CD) in a reactor filled with distilled water, heating and stirring for 6-12 h to obtain the product PASP/CD; finally, heating and reflux the aqueous solution of PASP/CD and the ethanol solution of p-chlorobenzaldehyde to obtain the modified PASP with Schiff base structure. The main chain of the modified PASP corrosion inhibitor with Schiff base structure prepared by the method belongs to a biodegradable polymer, and the corrosion inhibition rate can reach more than 90%.

In order to enhance the scale inhibition performance of PASP, a modification study was conducted on it. The results showed that: 29.4 g of maleic anhydride was used to obtain the precursor polysuccinimide by polymerization, and 1/2 of the amount of polysuccinimide was used as the matrix, and thiourea was used to open the ring. When the amount of thiourea was 1 g, the modified polycondensation temperature was 100 °C, and the reaction time was 2.5 h, the modified PASP with the best scale inhibition effect was obtained. Infrared spectroscopy analysis confirmed that an amino group of thiourea condensed with the carboxyl group on the original side chain to form an amide bond, and a new functional group was added to the side chain of PASP to improve its scale inhibition rate.

A PASP graft copolymer and a synthesis method thereof. The method is to use maleic anhydride and urea as raw materials, add maleic anhydride, distilled water, urea and 1:1 phosphorus-sulfuric acid mixture into a four-necked flask under oil bath conditions, mechanically stir for 1 to 2 hours, move the product to a solid phase reaction device until the viscosity average molecular weight is 5,000 to 10,000, cool, add distilled water for washing, vacuum filter and dry to obtain polysuccinimide; take a certain amount of polysuccinimide and water to mix to form a suspension, mix tryptamine with an appropriate amount of water, slowly add it to the suspension, and react for 24 hours; after adjusting the pH, drop the solution into anhydrous ethanol, remove the supernatant, and vacuum dry for 24 hours to obtain a PASP/tryptamine graft copolymer. The green, efficient, non-toxic, phosphorus-free, biodegradable and fluorescent water treatment agent prepared by the method is used in industrial circulating cooling water treatment to reduce secondary pollution of industrial water and eutrophication of water bodies.

A PASP corrosion inhibitor with a side group containing an imidazole ring and a long-chain alkane and a preparation method thereof. The method is that polysuccinimide reacts with alkylamine compounds and 2-aminoimidazole in a solvent, and finally adjusts the pH to 8-10 to hydrolyze the remaining diimide group to obtain the desired corrosion inhibitor, wherein the alkylamine compound is an alkane compound of C6-C18. The PASP corrosion inhibitor prepared by the method has a hydrophilic main chain and a hydrophobic side chain. The hydrophilic main chain and the short-chain side imidazole are stably attached to the metal surface to form a film through hydrophilic and hydrophobic effects or coordination effects, isolating the corrosion factor from direct contact with the metal; the hydrophobic side chain intervenes in the water medium to repel the corrosion factor, playing a second protective role. In addition, the raw materials for the preparation of the PAPS corrosion inhibitor are widely available, the process is simple and environmentally friendly, and it is suitable for industrial production.

A method for preparing a PSAP derivative containing carboxylic acid groups and sulfonic acid groups. The method uses aspartic acid as a raw material and adopts a solid phase thermal polycondensation method to obtain polysuccinimide. After the polysuccinimide is dissolved in water, 2-aminoethanesulfonate and glutamate aqueous solutions are added dropwise at the same time, and the solution pH is adjusted to 8-10, the reaction temperature is 10-30 °C, and the reaction time is 18-26 h to obtain a red-brown transparent solution; the solid is precipitated with anhydrous ethanol, filtered under reduced pressure, and washed with acetone to obtain a yellow-brown solid product, and a PAPS derivative containing sulfonic acid and carboxylic acid groups is obtained after vacuum drying. This method improves the comprehensive scale inhibition and dispersion performance of PASP by introducing carboxylic acid and sulfonic acid groups into the structure, has good scale inhibition and corrosion inhibition effects, and has good biodegradability.