Sodium polyacrylate

Sodium polyacrylate, also known as waterlock, is a sodium salt of polyacrylic acid with the chemical formula _n and has broad applications in consumer products. This super-absorbent polymer 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 developed the first super-absorbent polymer materials.

Background and history

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. 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.

Super-absorbent nanofibers (SANs)

Super-absorbent polymers are an innovative class of hydrogel products that can be used in many applications including hygiene products, drug delivery systems, agriculture, biomedicine, and wastewater treatment. A method called electrospinning is used to fabricate super-absorbent nanofibers 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 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 as well as clay and sodium polyacrylate. A study compared laponite/PEO and laponite/PAAS blend hydrogels. Laponite is a synthetic clay that has the ability to swell when placed in water. The results showed that both hydrogels have a similar elastic modulus. However, the tensile strength of laponite/PAAS is much stronger than laponite/PEO blend hydrogels. The reason for this difference is based on the clay–polymer interaction strength in each hydrogel blend. In laponite/PAAS, the interaction is much stronger compared to the laponite/PEO blend.

Metal ions

Experiments and studies have shown that the incorporation of 0.3 wt% sodium polyacrylate in collagen fibers can improve the mechanical properties and thermal stability of the composite films. Sodium polyacrylate can form films and composites with different cationic polymers, proteins, and other substances, all of which can benefit the properties of the resulting film. Furthermore, sodium polyacrylate has the potential to combine with metal ions because of its characteristic polyanionic property which would allow for more reinforcing of the material. When collagen and sodium polyacrylate blend films were combined with Ca²⁺, Fe³⁺, and Ag⁺ ranging from 0.001 to 0.004 mol/g, the surface of the composites became coarser and the internal structure became more stratified as more metal ions were added. When the ions were added, tensile strength increased. The optimal amounts for each ion are as follows: Ca²⁺, Fe³⁺, and Ag⁺. The composite films also had better thermal stability.
Overall, the study showed that metal ions added to Co-PAAS blend composite films can be used as an alternative to reinforce collagenous composite materials. These three ions were combined with the Co-PAAS film because of their relevant biological applications. Ca²⁺ is one of the major elements in animal tissues including bone and teeth and has a strong interaction with collagen. Next, Fe³⁺ is an important trace element in the human body and participates in protein chelation. Finally, Ag⁺ has antibacterial properties and can improve the stability and transparency of the Co-PAAS film.