Sodium polyacrylate (ACR, ASAP, or PAAS), also known as waterlock, is a sodium salt of polyacrylic acid with the chemical formula [−CH2−CH(CO2Na)−]n and has broad applications in consumer products. This super-absorbent polymer (SAP) has the ability to absorb 100 to 1000 times its mass in water. Sodium polyacrylate is an anionic polyelectrolyte with negatively charged carboxylic groups in the main chain. It is a polymer made up of chains of acrylate compounds. It contains sodium, which gives it the ability to absorb large amounts of water. When dissolved in water, it forms a thick and transparent solution due to the ionic interactions of the molecules. Sodium polyacrylate has many favorable mechanical properties. Some of these advantages include good mechanical stability, high heat resistance, and strong hydration. While sodium neutralized polyacrylic acids are the most common form used in industry, there are also other salts available including potassium, lithium and ammonium. The origins of super-absorbent polymer chemistry trace back to the early 1960s when the U.S. Department of Agriculture (USDA) developed the first super-absorbent polymer materials.
Background and history Super-absorbent polymers (SAPs) similar to sodium polyacrylate were developed in the 1960s by the U.S. Department of Agriculture. Before the development of these substances, the best water absorbing materials were cellulosic or fiber-based like tissue paper, sponge, cotton, or fluff pulp. These materials can only retain 20 times their weight in water, whereas sodium polyacrylate can retain hundreds of times its weight in water. The USDA was interested in developing this technology because they wanted to find materials that could improve water conservation in soil. Through extensive research, they found that the gels they created did not expel water as fiber-based materials would. Early adopters of this technology were Dow Chemical, Hercules, General Mills Chemical, and DuPont. Ultra-thin baby diapers were some of the first hygiene products to be developed which use only a fraction of the material compared to fluff pulp diapers. Super-absorbent technology is in high demand in the disposable hygiene industry for products like diapers and sanitary napkins. SAPs used in hygiene products are typically sodium neutralized whereas SAPs used in agricultural applications are potassium neutralized.
Fabrication methods
Overview Methods to fabricate sodium polyacrylate, like solution polymerization in water, inverse emulsion polymerization, inverse suspension polymerization, plasma polymerization, and pressure-induced polymerization have been employed to synthesize various polyacrylates. However, the process to obtain a solid-state product using these methods requires a lot of equipment and is very expensive. The products obtained from these methods also have defects like poor solubility and broad molecular weight distribution. Despite having drawbacks, the polymerization methods aforementioned are often used to form sodium polyacrylate and other SAPs. During solution polymerization, monomers are dissolved in a solvent that contains a catalyst to induce polymerization. Solution polymerization in water utilizes water as the solvent which means that the end product formed from the reaction is soluble in water. Inverse emulsion polymerization requires water, monomers, and a surfactant. Also, inverse emulsion polymerization is used to polymerize hydrophilic monomers. Hydrophobic monomers are emulsified through an aqueous phase. Free radicals are created in order to produce the polymer with either water or oil soluble initiators. Inverse suspension polymerization is carried out by using an aqueous solution of the monomer, cross-linking agent, and initiator which is then added to an organic phase which is stabilized by a surfactant. Plasma polymerization utilizes a range of technologies such as electron beams, ultraviolet radiation, or glow discharge in order to form polymers from a vapor made out of monomers. Gas discharge provided through this process initiates the polymerization of a group of monomers. Finally, pressure-induced polymerization applies pressure or compressive forces to solutions of monomers in order to create units which undergo polymerization and produce polymers. Another method tested in a study to produce sodium polyacrylate as an alternative to current methods began with Butyl acrylate-acrylic acid copolymer and poly (butyl acrylate). They were synthesized via suspension polymerization by using butyl acrylate as the main monomer and acrylic acid as a secondary monomer. Suspension polymerization uses physical and mechanical movement and agitation in order to mix monomers to form polymers. This process requires dispersing medium, monomers, stabilizing agents, and initiators. Next, the polymers were swollen in ethanol and hydrolyzed in an aqueous solution of sodium hydroxide. Finally, water-soluble sodium polyacrylates were obtained by washing and drying the hydrolyzed resultant. This is a different method compared to the manufacturing processes that have been previously utilized, but could be a potential method to specifically manufacture sodium polyacrylate. Overall, the various production methods of sodium polyacrylate will influence its swelling capability, absorbency, and other mechanical properties. It is also important to consider cost and feasibility when manufacturing polymers like sodium polyacrylate.
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