Sodium polyacrylate is a synthetic polymer used in cosmetics and personal care products as a thickening agent, texture enhancer, film-forming agent, and emulsion stabilizer.
Sodium polyacrylate, also known as waterlock, is the sodium salt of polyacrylic acid, a high molecular weight polymer. A polymer is a substance which has a molecular structure built up from a large number of similar units (called monomers) bonded together. With sodium polyacrylate, the monomers are acrylic acid. Functionally, this structure is strong and flexible.
In its dry form, sodium polyacrylate exists as a white, fluffy powder. When it is added to water, however, it turns into a gel-like substance, absorbing many times its weight in water. This is why sodium polyacrylate is considered a “super absorbent polymer”. 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. In addition to use in cosmetics, super absorbent polymers have a wide array of applications, including candles, wound dressings, hot and cold therapy packs, diapers, potting soil, and many more.
In cosmetics and personal care products, sodium polyacrylate functions as a thickening agent, texture enhancer, film-forming agent, and emulsion stabilizer.
Sodium polyacrylate functions as a thickening agent based on its ability to absorb as much as 100 to 1000 times its mass in water. In the dry powder form, the positively charged sodium ions are bound to the polyacrylate, providing relatively little suspension and viscosity. In aqueous solutions, however, the sodium ions are free to move since they are replaced by positively charged hydrogen ions. Instead of an organized polymer chain, this leads to a swollen gel that can absorb a high amount of water, providing suspending and thickening properties to cosmetic formulations.
As a texture enhancer, sodium polyacrylate contributes to the smooth, glossy appearance of products. It keeps the texture of a product soft and supple while also conditioning the skin. By adding sodium polyacrylate to things like shampoos, conditioners, creams, and lotions, formulations will look and feel more rich, smooth, and creamy. Additionally, sodium polyacrylate can be used in exfoliating products to make strong scrubbing materials more gentle. It is also a film-forming agent. Upon application, sodium polyacrylate creates a thin, flexible film that gives a smooth feeling on the skin after it has been washed.
Sodium polyacrylate has a stabilizing effect on emulsions, which are formulations that contain both water and oil components. When water and oil are mixed together and vigorously shaken, a dispersion of oil droplets in water – and vice versa – is formed. When shaking stops, however, the two phases start to separate. An emulsifier like sodium polyacrylate can be added to the system to help the droplets remain dispersed. The hydrophobic portion of the polymer adsorbs at the oil-water interface, and the hydrophilic portion swells in the water, forming a gel network around the oil droplets to stabilize the emulsion. This improves the consistency of a product, which enables an even distribution of topical skin care benefits.
The safety of polymers that contain the acrylic acid monomer (i.e. sodium polyacrylate) has been assessed by the Cosmetic Ingredient Review (CIR) Expert Panel. The CIR Expert Panel evaluated the scientific data and concluded that sodium polyacrylate was safe for use in cosmetics and personal care products when formulated to avoid skin irritation.
Since sodium polyacrylate is a large compound, it is unable to penetrate the skin’s barrier. If a compound cannot penetrate through the skin, it cannot react with cells of the immune system. Therefore, sodium polyacrylate does not cause sensitivity reactions.
According to EWG, sodium polyacrylate is rated as a 3 on a scale of 1 to 10, with 1 being the lowest risk to health and 10 being the highest. Sodium polyacrylate is considered a moderate hazard ingredient, primarily due to contamination concerns. This ingredient may contain residual amounts of the monomers acrylic acid, methacrylic acid, or 2-ethylhexyl acrylate, which are known toxins. The CIR Expert Panel reports that while these monomers may produce adverse effects under some circumstances, the levels found in cosmetics and personal care products are not considered to present a safety risk.
References: Wikipedia, “Sodium Polyacrylate”, Wikipedia, “Polyacrylic Acid”, Wikipedia, “Superabsorbent Polymer”, Cosmetics Info, “Sodium Polyacrylate”, EWG’s Skin Deep Cosmetic Database, “Sodium Polyacrylate”.