Absorbency Under Load: Why We Use SAPs in Absorbent Hygiene Products

Absorbent hygiene products have the ability to hold liquid even though they are thin and form fitting because they include SAP in the product. Consumers consistently demand reliable performance from products across various categories, including diapers, adult incontinence products, and period care and prioritize comfort, dryness, and security in these essential items.

At the center of this functionality are superabsorbent polymers (SAPs), which must absorb fluid without becoming a continuous gel that blocks fluid flow. Historically, a key measure of avoidance of continuous gel formation is absorbency under load (AUL), also known as absorption against pressure (AAP) or absorption under pressure (AUP).

Considering Absorption Under Load

Superabsorbent polymers absorb and retain a lot of water on a weight basis. Good SAPs can absorb a lot of distilled water for each gram of SAP. The presence of salt in water, however, reduces this absorption to a still substantial 30 to 45 grams per gram of SAP.

In the 1980s and ’90s, early in the history of SAP development, the focus was on making the SAP absorb as much liquid as possible above any other attribute. The result of this work was that highly absorbent polymers had one major drawback: they formed gels that actually blocked fluid flow. Under certain conditions, this blocking prevented fluid from reaching SAP in a hygiene product that had not yet been exposed to fluid and led to fluid not being absorbed. This phenomenon was termed “GEL BLOCKING” 

Through research, it was found that chemically modifying the surface of the polymer particles, a process known as surface cross linking, allowed adjacent polymer particles to remain independent and not form a continuous gel. The consequence of this treatment, though, did reduce the fluid retention capacity of the polymer some; however, the effective performance of the polymer in an absorbent hygiene product was maximized.

Work in the mid-to-late 1990s found that, for the product used in the research, the higher the AUL, the lower the leakage would be for the hygiene product using that SAP (as verified by consumer use tests). Unfortunately, this bias has remained in SAP development since then, despite the fact that product designs and materials have advanced considerably over the last 30 years.

The argument for AUL is that the polymer must be able to absorb under the pressure of a wearer who is either sitting or lying in the hygiene product. The reality, though, is that fluid follows the path of least resistance and goes into a hygiene product where the pressure is low.

The true need for AUL, then, is to have just enough AUL to prevent continuous gel formation. For polyacrylate SAPs, this value needs to be at or above about 12 g/g at 0.7 psi of applied pressure. This value does not apply to other types of SAPs, such as polyaspartic acid-based SAPs. Each chemistry must be evaluated to find the value of AUL that prevents gel blocking.

How AUL Is Measured

SAP suppliers provide an AUL value measured by the absorption under load of saline (a convenient proxy for human urine) to allow for differentiation of their various SAP offerings. This measurement is made using an industry standard test method that uses a small amount of SAP in a cylinder with a weight on the sample. The bottom of the cylinder is a mesh screen to both hold the SAP and allow fluid to enter the cylinder.

The weight used in the cylinder varies to provide different applied pressures (0.01, 0.29, 0.57, 0.90, or 0.70 psi), depending on what the SAP supplier has determined is the best way to measure AUL for their SAP offerings.

The combination of sample, cylinder, and weight are placed on a porous glass disc that is submerged in saline so that the saline can enter the cylinder from the bottom. The height of the saline is adjusted to be even with the top of the glass disc so that fluid is pulled into the test apparatus rather than being forced into the device.

After 30 minutes, the cylinder with the swollen SAP is removed and weighed to determine the amount of fluid that has been absorbed. This value is usually between 15 to 25 grams of saline per gram of SAP at 0.7 psi for polyacrylate SAPs.

For SAPs made from aspartic acid, it has been found that gel blocking is avoided at values as low as 7 g/g of saline at 0.7 psi. This surprising result reminds us not to be biased by our experience from polyacrylate.

AUL Impacts Hygiene Product Performance in a Limited Way

The direct link between higher AUL and lower leakage rates for hygiene products that was developed in the 1990s only applies to the specific design that was used to produce the data for the patent covering this metric. In the intervening 30 years, designs have changed dramatically, with the products performing better while becoming thinner and lighter. Further, specific design changes, like the addition of channels to the hygiene product where fluid can flow relatively freely, impact how fluid gets into the product and moves within the product.

All of these advancements have allowed the use of SAPs with minimal AUL values to be used successfully in the market. We are now at the point in hygiene product development where a SAP with just enough AUL to prevent gel blocking meets today’s product requirements and consumer experience. Accepting this fact opens the doors to using new SAP materials based on new chemistry, such as aspartic acid, that have unique performance attributes and can impact the sustainability footprint of hygiene products in a very positive way.

Expanding the Future of SAP Performance

While today’s superabsorbent polymers have helped the industry make enormous advances in hygiene products, their petroleum-based origins and long-term persistence present challenges in sustainability. Lygos is addressing these challenges with an aspartic acid-based polymer that can serve as a next-generation SAP platform. It performs comparably to leading petro-based SAPs, and is based on a natural material.

Advances in polymer chemistry are driving the next generation of hygiene materials, with polyaspartic vs. polyacrylic SAPs offering different pathways to performance and sustainability.

By understanding the true impact of AUL, manufacturers can optimize material usage, control costs, and deliver consistent performance to consumers. At the same time, forward-looking alternatives like aspartic acid based superabsorbents offer a way to maintain high standards while preparing for the future of sustainable hygiene solutions.

Contact us today to learn more about SAP performance, testing methods, and how Lygos can support your hygiene product development.

Gel Blocking needs to be emphasized as it is a unique term within the industry.