Sodium polyacrylate

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). A method called electrospinning is used to fabricate super-absorbent nanofibers (SANs) because of their advantageous properties like high surface area and porous structure. Electrospinning is a simple method that uses an electric field that collects filaments by forcing polymer melts or solutions. SANs have been successfully created by using sodium polyacrylate and poly(vinyl alcohol) (PVA) as a polymer matrix, which is a water-soluble polymer that is highly hydrophilic. As a result of this method of fabrication, SANs created in a study displayed high rates of absorption due to the capillary phenomenon shown by their highly porous structures. Also, the cross-linking structure improved the water absorption ability of the SANs. Adding PVA in this case gave structural stability to the SAN and prevented it from being dissolved in water. Overall, sodium polyacrylate can be combined with PVA in a nanofiber to produce a strong and effective structure. This technology could have many applications in various industrial fields because of the fast and high absorbency as well as the sustainable structure of the SANs which was produced through relatively easy and simple processing methods. The SANs were very effective when absorbing water since there was an increase in the absorption area. The swelling ratio also increased because of the cross-links and highly porous nature of the nanowebs. == Composites ==

Clay–polymer hydrogels Studies have been conducted which observe the effect of the mechanical properties of hydrogels based on the amount of clay combined with the polymer. When combining polymers with clay, the results are promising, showing an increase in the elastic modulus and the tensile strength of clay–polymer hydrogels. In general, combining inorganic substances with polymers can improve the electrical, mechanical, thermal, and gas barrier properties of materials like hydrogels. In order to obtain these results, ultra-high molecular mass polymers higher than a few millions are recommended to be used so that the mechanical properties can improve regardless of the type of polymer used. The mechanical properties for clay–polymer hydrogels have been studied including clay and polyethylene oxide (PEO) as well as clay and sodium polyacrylate (PAAS). Novel chitosan/sodium polyacrylate polyelectrolyte complex hydrogels (CPG) have been fabricated successfully in a study by cross-linking chitosan and sodium polyacrylate with epichlorohydrin (ECH) through the inhibiting protonation effect of chitosan in an alkali/urea aqueous solution. The CPG had a high swelling ratio because of sodium polyacrylate and acted differently in various pH solutions, physiological solutions, and salt solutions with different concentrations. As a result, CPG had smart responsive properties to different situations and exhibited high compressive strength, good biocompatibility and in vitro biodegradability. This fabrication process has shown success and has potential applications in the fields of agriculture, foods, tissue engineering, and drug delivery. ==Applications==

beads Overview Water-soluble polymers are used in many industries, especially polyacrylates. It is also used in ice packs to convert the water used as the cooling agent into a gel, in order to reduce spillage in case the ice pack leaks. Sodium polyacrylate has also been studied for utilization in many applications such as nanofiltration of water to absorb water and concentrate the liquid with microbes. Also, it is used for eco-engineering as a water-retaining agent in rocky slopes for increasing moisture availability in the soil. This can improve the water retention availability of the soil and infiltration capacity in sandy soil. Below is a table containing categories and lists of some products and applications that utilize sodium polyacrylate: Some of the items listed above will be discussed in further detail in the next application sections. However, the table provided above is not comprehensive and does not contain all of the possible or potential applications for using sodium polyacrylate. Sequestering agents Sodium polyacrylate is commonly used in detergents as a chelating agent. A chelating agent is used in detergents because it has the ability to neutralize heavy metals that can be found in dirt, water, and other substances that could be in clothes. The addition of sodium polyacrylate makes detergent more effective when cleaning clothes. Thickening agents Since sodium polyacrylate can absorb and retain water molecules, it is used often in diapers, hair gels, and soaps. Metalworking It's reportedly used as organic polymer quenching agent, along with polyalkylene glycol (PAG), polyvinyl pyrrolidone (PVP), polyethyl oxazoline (PEO). Sodium polyacrylate is commonly used in diapers to absorb liquids from urine and feces, but it has been found that waste disposable diapers (WDD) accumulate in landfills since sodium polyacrylate prevents and negatively affects H2 production from the dark fermentation of WDD. To be specific, WDD represents 7% of urban solid refuse and the current option is landfilling, which is degradable only during biological conditions. Such conditions include anaerobic degradation and composting. Considering the high amounts of cellulosic waste in WDD, in order to be more sustainable it has been recommended that sodium polyacrylate be replaced with special starches that can absorb significant amounts of water yet are still degradable by dark fermentation (DF). Overall, despite having many beneficial environmental applications, the usage of sodium polyacrylate in diapers can prevent waste from degrading properly over time. Low-salt animal skin preservation In the leather industry, salt-based preservation is typically used because it is versatile, cost-effective, and readily available. Heavy metals are very harmful pollutants and can have detrimental effects on aquatic environments and human beings because of high toxicity, bioaccumulation, and non-degradability. Activities like mining and petroleum refining can produce these heavy metals which necessitates a simple and environmentally sustainable process to absorb these harmful metals to prevent disastrous results. Sodium polyacrylate can absorb solutions quickly by swelling porous structure networks to reduce mass-transfer resistance. Also, sodium polyacrylate is a low-cost, non-toxic, and biocompatible option for water purification to recover metal ions. A study demonstrated that a sodium polyacrylate composite had high adsorption and desorption efficiency, implying that sodium polyacrylate can be recycled and reused as an effective absorbent for Cu(II) recovery. The delivery of probiotics to the digestive system can be difficult because the viability of probiotics decreases sharply throughout the gastrointestinal tract due to strong acid conditions. Although alginate (Alg) is the most extensively used native microcapsule matrix, combining Alg with sodium polyacrylate yields better results based on research comparing different encapsulation methods. Sodium polyacrylate is an oral safe food additive approved by the Food and Drug Administration (FDA) and has repeated carboxylate groups along its molecular chain. As a result, the acid buffering effect of sodium polyacrylate could be better than small molecular acid. Also, the binding capacity of sodium polyacrylate with calcium ions could be higher than Alg because of the high concentration of carboxylate groups and the increased flexible nature of the polymer chain. Sodium polyacrylate has been found useful in drug delivery applications. == Safety ==

Sodium polyacrylate itself does not irritate the skin. It is made up of large polymers that do not have the ability to infiltrate the skin. However, sometimes sodium polyacrylate is mixed with acrylic acid which is leftover from the manufacturing process. As a leftover of producing sodium polyacrylate, acrylic acid can cause a rash in contact with skin. It should be less than 300 PPM as the absorbent material in paper diapers. Also, if sodium polyacrylate is being used in a powder form it should not be inhaled. If spilled in an area with water, sodium polyacrylate could cause the ground to be very slippery. Finally, sodium polyacrylate can cause severe clogging if it enters sewers or drainage systems in large quantities. Otherwise, sodium polyacrylate is non-toxic and safe from any major risks. The data on its safety on environment is not adequate, however it is considered non-biodegradable and may cause salinization of soil when added in large quantities. == References ==