Sodium polyacrylate, 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. Sodium polyacrylate is a chemical polymer made up of chains of acrylate compounds. It contains sodium, which gives it the ability to absorb large amounts of water. Sodium polyacrylate is also classified as an anionic polyelectrolyte.^[2] 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. It has been used as an additive for food products including bread, juice, and ice cream.

Some of the items listed above will be discussed in further detail in the next application sections. However, it is important to note that 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. Sodium polyacrylate is considered a thickening agent because it increases the viscosity of water-based compounds. In diapers, sodium polyacrylate absorbs water found in urine in order to increase the capacity to store liquid and to reduce rashes.

Coatings

Sodium polyacrylate can also be utilized as a coating for electrical wires in order to reduce the amount of moisture around wires. Water and moisture near wires can cause issues with transmitting electrical signals. This could cause potential fire hazards. Due to the effective absorption and swelling capacity of sodium polyacrylate, it can absorb water and prevent it from surrounding or infiltrating wires.

Agriculture

In the agricultural industry, sodium polyacrylate is used to help plants retain moisture in the soil. It can act as a water reservoir for plants and is commonly used by florists to keep flowers fresh. Furthermore, the use of sodium polyacrylate for growing domestic fruit and vegetables has been approved by the U.S. Department of Agriculture.

NASA Maximum Absorbency Garments (MAGs)

Sodium polyacrylate is used in the fabric of spacesuits designed by the National Aeronautics and Space Administration (NASA) to prevent rashes from developing during flight by absorbing various liquids. These garments are called Maximum Absorbency Garments or MAGs and sodium polyacrylate is used in the innermost layers of these spacesuits to aid in the absorption of liquid from the surface of the skin. Specifically, MAGs absorb liquid from urine and feces and can hold approximately 2 L of liquid.

Environmental Applications

Inhibition of Hydrogen Production from Waste-Diaper Material

Although sodium polyacrylate has beneficial environmental applications, in one study, sodium polyacrylate was found to have inhibitory effects on the bioH2 fermentation of cellulosic wastes. 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 only degradable 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. However, the salt removed from the soaking process can cause pollution including elevated total dissolved solids (TDS). A study was conducted to measure the effectiveness of instead using a low-salt skin preservation method with sodium polyacrylate which has a reduced amount of NaCl. The main goal was to retain the properties of commercial leather while reducing pollution. The results showed that sodium polyacrylate with low salt levels had an adequate curing efficiency with a significant reduction (>65%) of TDS. Around 40% NaCl is used in conventional curing processes but the process conducted with sodium polyacrylate used 15% NaCl and 5% sodium polyacrylate.

Removal of Metal Ions from the Environment

Studies have shown that sodium polyacrylate and other super-absorbent polymers or SAPs can be used to absorb and recover metal ions. 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. Sodium polyacrylate is able to do this because of its function group (-COO-) in its matrix which contributes to its effective adsorption capacity. Sodium polyacrylate has a very high adsorption capacity and one of the highest adsorption capacities for sodium polyacrylate was found with Cu(II) ions. Using a mild concentration of 0.01 M nitric acid, almost all of the copper could be recovered from the sodium polyacrylate matrix. The results of the study indicate the effectiveness of using sodium polyacrylate to rid the environment of toxic metals like copper. It is also a sustainable solution since sodium polyacrylate can be recycled and reused, therefore, reducing waste.

Drug Delivery Applications

Sodium polyacrylate can be used for microencapsulation to deliver substances like probiotics. 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. When combined with alginate (Alg), sodium polyacrylate was able to successfully encapsulate Lactobacillus plantarum MA2 and allowed better probiotic delivery compared to an Alg microcapsule. This result is true for both the small and large intestine. This research has shown that Alg-PAAS(1:2) could be a potentially effective microcapsule matrix in probiotic drug delivery. This capsule enhanced the survival of the probiotic when traveling both in-vitro and in-vivo.

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