Super Absorbent (Super)absorbentresin， SAR stands for Super Absorbent Resin. It is a high molecular weight polymer containing strong water-based groups such as carboxyl and hydroxyl groups, and exhibiting a three-dimensional cross-linked network structure. It can quickly absorb hundreds to thousands of times its own mass of water, and has strong water retention performance without stimulating biological tissues.
There are various types of superabsorbent agents, including starch, cellulose, and polyacrylic acid. Among them, starch graft copolymer resin has become a research focus in this field due to its unique advantages in raw materials and resin properties.
Starch is a natural polymer composed of two parts: amylose and amylopectin. amylopectin is an amorphous structure that only exists in microcrystals. Compared to amylose, water molecules can easily penetrate and gelatinize. The content of the two components varies depending on the starch variety, therefore, the selection of starch variety has a significant impact on the product properties. Due to the high pollution and impurities in the production and processing of ordinary starch, its synthesized products can be applied in agriculture, chemical industry, environmental protection, and other fields. But with the increasing application of SAR in the pharmaceutical field, the requirements for starch quality are also becoming higher and higher. Medicinal starch, as a common medicinal excipient, has high purity and is based on the requirements of Part II of the 2005 edition of the Chinese Pharmacopoeia. It is characterized by high added value and strong market competitiveness, and has significant advantages such as high water absorption and strong disintegration. It has broad development prospects.
In recent years, SAR has been widely used in medical and health fields, including artificial organs, medical test specimens, absorption of exudate, and the production of wound bandages to prevent infection. In addition, the application of SAR in slow release technology for medicine has also attracted people's attention. By adjusting its structure to suit the characteristics and moisture content of organisms, the goal of controlling drug release rate can be achieved.
1. Research status of superabsorbent resin
Before the 1950s, the water absorbing materials used by humans were mainly natural products and inorganic substances, such as natural fibers, polysaccharides, as well as silica gel, calcium oxide, phosphoric acid, sulfuric acid, etc. In the 1950s, Flory established the water absorption theory of water absorbing polymers through extensive experimental research, known as the Flory water absorption theory, laying a theoretical foundation for the development of water absorbing polymers.
In 1966, Fanta et al. of the North Research Institute of the U.S. Department of Agriculture first published a paper stating that "the water absorbent resin of starch derivatives has superior water absorption capacity. The swelling gel formed after water absorption has strong water retention property. Even if there is pressure, it will not separate from water, and even has moisture absorption and desorption properties. The water absorption properties of these materials are better than those of previous polymer materials". The absorbent resin was initially industrialized by Henkl company under the trade name SGP, and by 1981 had reached an annual production capacity of several thousand tons.
In 1978, many Japanese companies used water-soluble polyacrylic acid and adopted different crosslinking methods to produce high-performance superabsorbent resins, such as AquakeepIOSH water absorbent has a water absorption capacity of 800—1000MI／g。 At the same time, Dow Chemical Company in the United States copolymerized acrylic acid and ethyl acrylate to obtain a polymer aqueous solution, which was then mixed with epichlorohydrin to obtain a film like superabsorbent resin, greatly improving the properties of the polymer and promoting the development of synthetic water absorbent resins.
In the 1980s, people began to use other natural compounds such as alginates, proteins, chitosan and its derivatives to prepare superabsorbent resins. These new methods have opened up new avenues for the development and research of novel water absorbing agents. At the same time, water absorbing composite materials have also emerged, which have developed rapidly due to their ability to improve the salt resistance, water absorption rate, and other properties of superabsorbent resins.
The large-scale commercialization of superabsorbent resin began in the early 1980s when Sanyo Chemical Industry Co., Ltd. in Japan developed a synthesis method of starch acrylic crosslinking monomer graft copolymerization and used its products in physiological and sanitary materials such as diapers. Subsequently, the demand for superabsorbent resin sharply increased, and its production capacity increased from 5000 tons in 1980 to 207000 tons in 1990, and then surged to 1.292 million tons in 1999. Among them, the consumption of hygiene products is the largest, accounting for 80.85% of the total production of water absorbent resin.
China's research on superabsorbent polymers started relatively late, only in the early 1980s. Although more than 20 units across the country have carried out development work in the past 20 years, and some have already entered the pilot stage, most are still in the laboratory stage with little industrialization. China is a populous country, and the amount of high water absorbent resin consumed is astonishing. In 2000, the domestic demand was 18000 tons, and it is expected that by 2005, the market demand for high water absorbent resin in China will reach 2.5 tons&30000 tons, accounting for 2.5% -3.0% of the world's total consumption. At present, China mainly relies on imports to meet its needs, so it is necessary to increase research on superabsorbent resins and industrialize them as soon as possible.
The research on superabsorbent materials is still in the exploratory stage, mainly exploring appropriate process conditions, monomer types, ratios, etc. to synthesize products with good comprehensive performance. Due to the fact that the synthesis of this type of product is a multiphase system, there are numerous factors involved in the synthesis process, and the impact situation is complex. Moreover, there may be multiple cross effects between various influencing factors, which makes many products still in the research stage. Each researcher's synthesis method is different, and there are still many problems in the industrialization process.
2. Classification of superabsorbent resins
The development of superabsorbent resin is rapid, with a wide variety of types and various classification methods.
2.1. Classification by Raw Material Source
Classify them according to their raw material sources, mainly including:
1. Starch series
The research on superabsorbent resin originated from starch based superabsorbent resin and has developed rapidly since its emergence. Mainly including starch grafting, carboxymethyl starch, phosphorylated starch, starch sulfonate, etc.
2. Cellulose system
Using cellulose and its derivatives as the main raw materials, it is prepared by etherification, esterification, cross-linking, grafting and other methods. Including fiber grafting, carboxymethyl cellulose, hydroxypropylated cellulose, xanthated cellulose, etc.
3. Synthetic polymer system
The monomers commonly used in the preparation of synthetic superabsorbent resins are acrylic acid, acrylamide, vinyl acetate, ethylene oxide, etc. At present, common ones include polyacrylic acid salts, polyacrylamide, polyvinyl alcohol, polyoxoane hydrocarbons, etc.
4. Protein system
Including soy protein, silk protein, gluten, etc.
5. Other natural substances and their derivatives
Including pectin, alginate, chitosan, heparin, etc.
6. Blended and composite systems
It includes the blending of super absorbent resin, the composite of super absorbent resin and inorganic gel, and the composite of super absorbent resin and organic matter.
2.2 Classification based on crosslinking methods
According to the classification of crosslinking methods, there are mainly: using crosslinking agents for network reaction; Self networking reaction; Radiation irradiation network reaction. Table 2.1 below:
2.3 Classification based on hydrophilic methods
According to the classification of hydrophilic methods, there are mainly: hydrophilic monomer polymerization; Hydrophobic polymers are hydrophilic and undergo methylation, sulfonation, etc; Graft copolymerization of hydrophobic polymers and hydrophilic monomers; Hydrolysis of polymers containing nitrile and lipid groups. See Table 2-2 below.
3. The influence of different factors on the water absorption performance of superabsorbent agents.
The experiments and analysis referenced in the following text mainly use medicinal starch and acrylic acid as the main raw materials, partially neutralize acrylic acid, and then graft copolymerization to synthesize superabsorbent agents. The influence of different factors on the water absorption performance of superabsorbent agents is discussed in detail.
3.1 Instruments and reagents
3.1.1 Instruments Rotary evaporator, Shanghai Jiapeng Technology Co., Ltd; Circulating water multi-purpose vacuum pump, Zhengzhou Changcheng Technology Industry and Trade Co., Ltd; Electric constant temperature water bath, Beijing Changfeng Instrument and Meter Company; Glass instrument airflow dryer, Yingyu Yuhua Instrument Factory, Gongyi City; Electronic balance, Beijing Dolis Instrument System Co., Ltd; Electric suction device, Shanghai Medical Equipment Industry Corporation Medical Suction Device Factory; High speed universal crusher, Tianjin Test Instrument Co., Ltd; Electric vacuum drying oven, Beijing Huabolian Medical Equipment Co., Ltd;
A21004401133LP Infrared Spectrometer, Shimadzu Corporation, Japan.
3.1.2 Trial drug Medicinal starch, analytical purity, Shandong Liaocheng Luxi Medicinal Excipients Co., Ltd; Acrylic acid (AA), analytical grade, BASF Chemical Co., Ltd., Tianjin; 1,4-hydroquinone (polymerization inhibitor), Shuanglin Chemical Reagent Factory, Hangzhou, Zhejiang; Sodium hydroxide, analytical grade, fine chemical plant in Laiyang Economic and Technological Development Zone; Potassium persulfate (KPS), analytical grade, Tianjin Dongfang Chemical Plant ≯ N, N-methylenebisacrylamide (NMBA), analytical grade, Tianjin Kemio Chemical Reagent Development Center.
3.2 Methods and Results
3.2.1Pre experimental processing Acrylic acid is subjected to vacuum distillation to remove the polymerization inhibitor, stored in a refrigerator for later use, and prepared with a mass concentration of 300G/L NaOH
3.2.2 Synthesis Take a certain amount of medicinal starch and deionized water in a small beaker, stir, and heat it in a water bath at 80 ℃ to gelatinize it for about 30 minutes; After complete gelatinization, continue stirring and cooling to facilitate graft copolymerization reaction. In addition, under the protection of an ice water bath, neutralize a certain amount of acrylic acid with a 30% NaOH solution to neutral or weakly acidic, add an appropriate amount of initiator and crosslinking agent, and stir to dissolve. Then pour it into a beaker containing gelatinized starch, stir evenly, and pour it into a test tube. Heat it in a water bath for about 2.5 minutesh， The white gel like solid is obtained, cut into blocks, dried at 110 ℃, and crushed to obtain the product.
3.2.3Determination of water absorption rate
Weigh a certain amount of dry resin into a beaker, add sufficient deionized water, stir, stand for water absorption for a period of time, filter it with a self-made mesh bag, stand until no water flows out, and weigh the quality of gel after water absorption. Calculate the water absorption rate according to the following formula:
Q=(W1-W0)／W0
In the formula, Q represents the water absorption rate (g)·g-)，W。 Represents the mass of dry resin (g), W represents the mass of resin after water absorption (g).
3.2.4 The influence of various factors on the water absorption rate of the product A pharmaceutical starch grafted acrylic superabsorbent was synthesized using a method of pre polymerization followed by high-temperature crosslinking. The relationship between the raw material ratio, initiator mass fraction, crosslinking agent mass fraction, acrylic neutralization degree, temperature, and resin water absorption performance was studied, and the resin was characterized by infrared spectroscopy.
3.2.4.1 The influence of raw material ratio on product water absorption rate The influence of raw material ratio on the water absorption of resin is significant. Fix the dosage of medicinal starch, keep other conditions unchanged, and change the dosage of acrylic acid. The measured results are shown in Figure 1. As shown in Figure 1, the maximum water absorption rate of the product is achieved when the mass ratio of acrylic acid to starch is 6, and the water absorption rate decreases when it is greater or less than 6. This is because when the mass ratio of acrylic acid to starch is less than 6, as the amount of acrylic acid increases, the grafting efficiency improves and the hydrophilic groups in the graft copolymer increase, resulting in an increase in water absorption rate. When the mass ratio of acrylic acid to starch is greater than 6, the self crosslinking of acrylic acid increases, the corresponding steric hindrance increases, the hydrophilicity decreases, and the water absorption rate decreases.
3.2.4.2 Effect of initiator mass fraction on product water absorption rate Keeping other conditions unchanged. Changing the mass fraction of initiator (percentage of total raw material), the measured results are shown in the figure
As shown in Figure 2. As shown in Figure 2, initially with the increase of initiator mass fraction, the water absorption rate also increases significantly. When the initiator mass fraction reaches 1.3%, the water absorption rate of the product is the highest, and then it shows a downward trend. This is because at the beginning, as the mass fraction of the initiator increases, the number of free groups in the system increases, thereby accelerating the progress of the graft copolymerization reaction,
The grafting rate increases, resulting in an increase in water absorption rate; But when the mass fraction of the initiator is greater than 1.3%, due to the²¯ Excessive and excessive amount of 1208²¯ The termination reaction with the active chain is unfavorable for the growth of the docking branch chain, resulting in most of the chains grafted on starch being short chains. The relative molecular weight of the polymer is small, and the water solubility increases, thereby reducing the water absorption rate.
3.2.4.3 The influence of crosslinking agent mass fraction on product water absorption rate The amount of crosslinking agent determines the size of the resin spatial network, which has a significant impact on the resin's water absorption rate.In this experiment, N, N-methylenebisacrylamide was used as the crosslinking agent. Other conditions were kept constant, and the mass fraction of crosslinking agent (percentage of total raw material) was changed. The measured results are shown in Figure 3.
According to Figure 3, when the mass fraction of crosslinking agent is 0.5%, the water absorption rate of the product is the highest; Greater than or less than 0.50A, the water absorption rate is not high. This is because the mass fraction of crosslinking agent is too small, with few crosslinking points, which cannot form a good three-dimensional network structure. Increase the solubility of the resin. Decreased water absorption rate; And the mass fraction of crosslinking agent is too high. The cross-linking points in the resin are too dense, and the average relative molecular weight of the chain segments between the cross-linking points in the network decreases, resulting in network shrinkage. Restricted swelling during water absorption, resulting in a decrease in water absorption rate.
3.2.4.4 The effect of neutralization degree on product water absorption rate changes the neutralization degree of acrylic acid. All other conditions remain unchanged.The measured results are shown in Figure 4.
As shown in Figure 4, the product has the highest water absorption rate when the monomer neutralization degree is 75%. This is because when the neutralization degree is low, the ion concentration on the resin network structure is small, the osmotic pressure inside and outside the network decreases, and the water absorption rate is low; When the neutralization is too high, the ion concentration in the network structure is high, and there are many strong hydrogen bonds between water molecules and ions. However, due to the directional nature of hydrogen bonds,
Water molecules bound by hydrogen bonds have a certain orientation in space, adjacent hydrogen bonds interfere with each other, and adjacent charged carboxyl groups also repel each other, limiting the movement of the chain and preventing the micropores of the polymer from fully exerting their water storage capacity, resulting in a decrease in water absorption rate. In addition, when the neutralization is too high, the water solubility of the polymer also increases.
3.2.4.5The effect of polymerization temperature on the water absorption rate of the product changes the polymerization temperature while keeping other conditions unchanged. The measured results are shown in Figure 5.
As shown in Figure 5, the product has the highest water absorption rate at a polymerization temperature of 60 ℃. This is because when the polymerization temperature is too low, the polymerization reaction rate is slow, and the degree of self crosslinking of the resin decreases, preventing the formation of a network like structure with high water solubility and low water absorption rate; An increase in temperature is beneficial for the decomposition of initiators, and both chain initiation and grafting reactions are accelerated. Therefore, raising the temperature within a certain range is beneficial for monomer polymerization and an increase in grafting degree; But when the polymerization temperature is too high, the polymerization reaction rate is too fast, which can easily lead to burst polymerization and sticking, and the self crosslinking of the resin increases. The rate of chain transfer and chain termination reaction accelerates, resulting in a decrease in grafting rate. The water absorption rate decreases.
3.2.4.6 The infrared characterization of the resin is shown in Figure 6.
From Figure 6, it can be seen that medicinal starch isone hundred and fifty-sevenThere is a stretching vibration peak of C-0 in CHOH at cm-1, located at 1018There is a stretching vibration peak of C-0 in CH2OH at cm-1, while in
one thousand and eightyThere is a stretching vibration peak of ether bond C-0-C at cm-1, while the grafted starch exhibits a stretching vibration peak at 1seven hundred and sevenA stretching vibration peak of C-O was generated at cm-1. onefive hundred and sixty-eightCm-1 and 1four hundred and sixAt cm-1, asymmetric and symmetric stretching vibration peaks of C () 0 in C () were generated, and at 1four hundred and fifty-eightA C-H stretching vibration absorption peak of methylene group appeared at cm-1, indicating that acrylic acid was grafted onto medicinal starch.
3.3 Discussion
So far, the relationship between the liquid absorption mechanism, structure, and liquid absorption performance of superabsorbent agents is not very clear, and the more commonly used theory is the ion network theory. This theory suggests that water molecules interact with the hydrophilic groups of the resin through hydrogen bonds. Ionic hydrophilic groups begin to dissociate upon contact with water, anions are fixed on the polymer chain, and cations are mobile ions that dissociate further with the hydrophilic groups. The increase in the number of anions leads to an increase in the electrostatic repulsion between ions, causing the resin network to expand; At the same time, in order to maintain electrical neutrality, cations cannot diffuse to external solvents, resulting in an increase in the concentration of mobile cations in the resin network, and the osmotic pressure inside and outside the network increases accordingly, allowing water molecules to further infiltrate. As the water absorption increases, the concentration difference of ions inside and outside the network gradually decreases, and the osmotic pressure difference tends to zero. At the same time, as the network expands, its elastic contraction force also increases, gradually offsetting the electrostatic repulsion of anions, and ultimately reaching water absorption equilibrium.
