This article is about the manufacturing process of super absorbent polymer.
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Superabsorbent polymers are now commonly made from the polymerization of acrylic acid blended with sodium hydroxide in the presence of an initiator to form a poly-acrylic acid sodium salt (sometimes referred to as sodium polyacrylate). This polymer is the most common type of SAP made in the world today.
Sodium polyacrylate is a light white crystalline particle under normal conditions. It is odorless, non-toxic, and light in texture. It is the lightest material per unit mass among general-purpose resin materials, and has excellent water absorption and water retention properties.
Other materials are also used to make a superabsorbent polymer, such as polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile to name a few. The latter is one of the oldest SAP forms created.
The principle of water absorption of polyacrylic resin is quite different from that of other desiccants. It absorbs water hundreds of times heavier than itself to form a gel. The gel structure is determined by the cross-linking properties of polyacrylic resin. To achieve, the water can not be squeezed and flow out within a certain pressure limit range.
Therefore, polyacrylic acid polymers can be used as materials for synthesizing superabsorbent polymers. Their water absorption properties are not only related to materials, but also have a lot to do with the process of synthesizing superabsorbent polymers.
The industrial preparation of superabsorbent resin adopts the method of chemical polymerization. The raw materials are industrial-grade acrylic acid, industrial-grade sodium hydroxide, the initiator is sodium persulfate, and the cross-linking agent is divinylbenzene.
Industrial-grade polypropylene is manufactured to prevent storage and transportation from deteriorating due to polymerization, and a polymerization inhibitor is added to affect its polymerization effect. Therefore, before preparing polyacrylic acid resin, polypropylene needs to be distilled and separated.
However, the structure and chemical properties of polypropylene do not have high temperature resistance, so the air in the distillation device needs to be evacuated so that the distillation purification step can be carried out without reaching the boiling point of polypropylene.
This step is to remove impurities contained in industrial grade sodium hydroxide, so as not to affect the polymerization effect of polypropylene.
After dissolving sodium hydroxide in distilled water, the remaining solution after filtering out impurities is the lye required for bulk polymerization.
Slowly add the distilled polypropylene into the sodium hydroxide lye, stir and neutralize for use.
The neutralization temperature should not be too high, and the test data show that the polyacrylic acid polymer resin obtained when the neutralization temperature is in the range of 10-50 ℃ has the best water absorption and water retention performance. At this time, polypropylene exchanges ions with sodium hydroxide in lye to generate polypropylene salt and water.
During the neutralization reaction, the reagents should be dosed to ensure that the polypropylene and the lye are completely reacted and consumed.
After the neutralization reaction, an appropriate amount of sodium persulfate and divinylbenzene are added to complete the polymerization reaction of the polypropylene salt under the joint action of the initiator and the crosslinking agent.
It should be noted that the polymerization reaction needs to be carried out under ambient conditions with a temperature below 60 °C, and the polymerization time is about 2 h. Then raise the temperature to 70 °C and keep it at a constant temperature for more than 3 h to obtain the polymerized gel substance of polypropylene salt.
Collect the gel and put it into an oven, dry the water at 70~80°C to obtain the solid polyacrylic acid resin, then crush the polyacrylic acid solid and grind it into powder particles, which is the polyacrylic acid resin material commonly used in industry.
The picture below briefly explains the general production process of sodium polyacrylate.
Today superabsorbent polymers are made using one of 4 primary methods: direct polymerization, gel polymerization, suspension polymerization, and solution polymerization. Each of the processes has its respective advantages and yields different qualities of the product.
Direct polymerization relies only on the ion exchange of polypropylene monomer and lye, under the joint action of initiator and crosslinking agent, the polymerization reaction is carried out in the environment of light and heat.
It is the preparation process closest to the basic flow.
Solution polymers offer the absorbency of a granular polymer supplied in solution form. Solutions can be diluted with water prior to application and can coat most substrates or be used to saturate them. After drying at a specific temperature for a specific time, the result is a coated substrate with super absorbency. For example, this chemistry can be applied directly onto wires and cables, though it is specially optimized for use on components such as rolled goods or sheeted substrates.
Solution-based polymerization is commonly used today for the SAP manufacture of co-polymers, particularly those with the toxic acrylamide monomer. This process is efficient and generally has a lower capital cost base. The solution process uses a water-based monomer solution to produce a mass of reactant polymerized gel. The polymerization’s own exothermic reaction energy is used to drive much of the process, helping reduce manufacturing costs. The reactant polymer gel is then chopped, dried, and ground to its final granule size. Any treatments to enhance the performance characteristics of the SAP are usually accomplished after the final granule size is created.
The inverse emulsion polymerization method is to prepare the raw material polypropylene into a solvent form, use a non-polar solvent as a dissolving agent for polypropylene, and dissolve it in an oily active agent to make an oily solvent when adding an initiator and a crosslinking agent .
In the preparation of raw materials, the method of “polypropylene monomer + oily solvent of initiator and crosslinking agent + lye” is used, and the solution of insoluble polyacrylate is used as the medium.
A mixture of frozen acrylic acid, water, cross-linking agents, and UV initiator chemicals are blended and placed either on a moving belt or in large tubs. The liquid mixture then goes into a “reactor” which is a long chamber with a series of strong UV lights. UV radiation drives the polymerization and cross-linking reactions. The resulting “logs” are sticky gels containing 60-70% water.
The logs are shredded or ground and placed in various sorts of driers. The additional cross-linking agent may be sprayed on the particles’ surface; this “surface cross-linking” increases the product’s ability to swell under pressure—a property measured as Absorbency Under Load (AUL) or Absorbency Against Pressure (AAP). The dried polymer particles are then screened for proper particle size distribution and packaging.
The gel polymerization (GP) method is currently the most popular method for making the sodium polyacrylate superabsorbent polymers now used in baby diapers and other disposable hygienic articles.
In the polymerization step, the non-polar solvent in which polypropylene is dissolved is mixed with the oily solvent in which initiator and crosslinking agent are dissolved to form an emulsion, so that the polyacrylate has the conditions for polymerization to form on the outer layer of the polypropylene solvent of the emulsion. The “oil-in-water” structure completes the polymerization process of polyacrylate.
Because the “oil-in-water” structure closes and isolates the free performance of polyacrylate, so that it only performs a single polymerization function, which speeds up the reaction rate of polyacrylate, and the preparation rate of inverse emulsion polymerization is 5 times faster than that of Solution polymerization.
Due to the appearance of the dispersion medium, not only the function of heat transfer and temperature control is realized, but also the polymerization of the superabsorbent resin material can be carried out under low temperature conditions. The inverse emulsion polymerization method breaks the limitation of polyacrylic resin materials due to temperature conditions.
In addition, the oil phase of the inverse emulsion polymerization method can be reused many times. Although the saving effect of the bulk polymerization method is not achieved, it still solves the problem of cost waste of excessive initiator and excessive crosslinking agent.
Suspension polymerization is similar in principle to the above-mentioned Gel polymerization.
What these two methods have in common is that the dispersant is used to realize the transfer of heat, variable temperature control and acceleration of the polymerization reaction rate.
However, the difference from Gel polymerization is that Suspension polymerization uses the water phase as the separated phase and the oil phase as the continuous phase. It suspends the dispersant dissolved in polypropylene on the surface of the oil phase in the form of droplets, and the polymerization reaction occurs in the suspended droplets.
Suspension polymerization, like Gel polymerization, is easy to transfer the heat of the reaction site due to the thermal conductivity of the dispersant, so that the polymerization reaction is not limited by temperature conditions. When polymerization occurs, the viscosity of alkaline substances and polypropylene is low, and it is not easy to retain unreacted impurities.
Compared with Gel polymerization, the advantage of Suspension polymerization is that its solvent can be easily recovered after distillation, and the oil phase can be recycled for multiple times with little harm to the environment.
This process suspends the water-based reactant in a hydrocarbon-based solvent. The net result is that the suspension polymerization creates the primary polymer particle in the reactor rather than mechanically in the post-reaction stages. Performance enhancements can also be made during, or just after, the reaction stage.
Note: The suspension process is practiced by only a few companies because it requires a higher degree of production control and product engineering during the polymerization step.
In summary, superabsorbent polymer is a reliable polymer material with the function of water absorption and water retention.
Its preparation process principle is to generate polypropylene salt by reacting polypropylene with alkali, then complete the polymerization reaction under the joint catalysis of initiator and crosslinking agent, and combine into polymer substances.
In industrial preparation, it is divided into four types according to the process difference. The four types of preparation processes have their own advantages and disadvantages in terms of preparation cost, preparation quality, process and waste treatment. It depends on the specific application of polyacrylic acid water-absorbing resin to comprehensively consider the most appropriate preparation process.
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