Sodium Polyacrylate Production

The process under analysis comprises three major sections: (1) Polymerization; (2) Particles size Reduction; and (3) Surface Cross-Linking and Packaging.

Polymerization. Acrylic acid is neutralized by a sodium hydroxide solution, resulting in a solution of sodium acrylate monomer. This solution is sent to another area, where it is continuously mixed with additional acrylic acid, a mix of chemicals that serves as polymerization initiator, and trimethylolpropane triacrylate, used as cross-linking agent. Such feed stream is then pumped onto a continuous, moving belt reactor, where the polymerization occurs, yielding a Sodium Polyacrylate gel.

Particles size reduction. The gel is passed through an extrusion unit, where the gel particles diameter is reduced to a range of 0.5–3 cm, in a way to increase the surface area and speed the drying step carried out in a circulating belt dryer, in which the gel water content is reduced to 5 – 10 wt.%. Residual acrylic acid and unreacted monomer also evaporate together with the water. The dried product is ground and classified. The particles are passed through an attrition mill and through roll crushers, yielding particles with grain size between 0.15 and 0.8 mm, which are then sieved with vibrating screens: overflow particles are returned to milling, while fines are partly recycled to the monomer solution, with the remainder sent to surface cross-linking.

Surface Cross-Linking and Packaging. Initially, a surface-crosslinking agent solution composed of 1,4-butanediol / propylene glycol / water / isopropanol is sprayed onto the dried polymer particles, in a vertical mixer. The coated polymer particles are then routed to a disk dryer, where they are dried thermally at 150 to 200° C for the surface cross-linking reaction occur. Afterwards, the Sodium Polyacrylate particles are packed and stored.