Aqueous solution polymerization
Since most of the monomers of SAP are water-soluble, SAP can be produced by solution polymerization using water as the solvent. More than 90% of the world’s production capacity uses this method.
The reason why gamma rays and microwaves are not commonly used is that they require a special container, and the radiation generating device has many inconveniences in operation. Although gamma rays are reported to have a high rate of initiation, they do not have the advantage of being popular in the field of civilian production due to expensive equipment and dangerous radiation leakage. Thermally induced products generally turn yellow, have a high residual monomer rate, and are not uniformly polymerized, so they are not suitable for manufacturing high-performance products.
The aqueous solution polymerization method has the advantages of low cost, low equipment requirements, low investment, simple process, high production efficiency, safe operation, pure system, uniform cross-linking structure, etc.
Increase drying, pulverizing and screening processes, long product production lines, uneven particle size distribution after product crushing, etc.
Initiator initiation can easily control the initiation rate and adjust the water absorption performance, so it is widely used. Using water as a solvent, acrylic acid and mixed monomers of acrylic acid and sodium acrylate partially neutralized by alkali are prepared by cross-linking polymerization, drying and crushing in the presence of a cross-linking agent and an initiator.
The production method does not produce toxic and harmful substances, and the whole process can be carried out in an environment-friendly atmosphere.
Inverse Suspension Polymerization
The synthesis of SAP by reversed-phase suspension polymerization is different from the synthesis of SAP from aqueous solution.
Inverse suspension polymerization SAP products have fast water absorption rate, high water absorption capacity and uniform particle size distribution.
The processing is simple, the product can be obtained only by drying, and the beaded product can be directly obtained without crushing and screening, which solves the problems of heat transfer and difficulty in stirring in the aqueous solution polymerization method.
The disadvantage of this method is that the material requirements of the main equipment are high, the equipment investment is large, and since the organic solvent is used in the production process, a solvent recovery device is required, which is prone to pollution.
In addition, the reversed-phase suspension polymerization method can only carry out intermittent production, the equipment utilization rate is low, and the production efficiency is low.
The synthesis process is that organic solvent is used as dispersion medium as oil phase, and the mixed aqueous solution of sodium acrylate and acrylic acid partially neutralized by alkali is used as water phase, which is dispersed in oil phase medium in the form of droplets, and formed under the action of suspending dispersant and stirring. A stable dispersion of water-in-oil droplets, i.e., a water-in-oil suspension, where the initiator and crosslinking agent are dissolved in the water-phase droplets to carry out the polymerization process.
After the polymerization reaction, the organic solvent in the oil phase needs to be removed by distillation under reduced pressure, and then the gel is dried to obtain the SAP product.