Retention Capacity: Why We Use SAP 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. Whether in diapers, adult incontinence products, or period care, consumers demand reliable performance that delivers comfort, dryness, and security. At the center of this functionality are superabsorbent polymers (SAPs), which greatly increase the amount of fluid that can be absorbed and retained over time.

Understand why capacity matters in SAPs and how it helps shapeproduct design and performance in the hygiene market.

Considering Absorption vs. Retention

Superabsorbent polymers absorb and retain a lot of water on a weight basis. Good SAPs can absorb and retain 300 to 500 grams of distilled water for each gram of SAP. The presence of salt in water, however, reduces this retention capacity to a still substantial 30 to 45 grams per gram of SAP.

This change is due to the impact that dissolved salts have on the charges within the swollen polymer network which drive the swelling or absorption of water. This is important as urine, the primary fluid absorbed by hygiene articles, contains a large amount of salts.

The range of absorption for SAP depends on factors like:

• Number of cross links in the polymer network
• Specific chemistry of the superabsorbent polymer
• Surface chemical treatments modifying other properties

To design a hygiene product, the choice of superabsorbent polymer may involve comparing the absorption of different SAPs in the market. Knowing how a chosen SAP performs helps determine how much is needed for a hygiene product to meet consumer performance requirements in the market.

It’s also important to understand that the SAP with the highest capacity may not be the best choice for every absorbent hygiene product design.

How SAP Capacity Value Is Measured

SAP suppliers provide a capacity value measured by the absorption of saline, a convenient proxy for human urine, to allow for differentiation of various SAP offerings. This measurement is made using an industry standard test method that uses a small amount of SAP sealed into a teabag weighed dry.

The teabag is submerged in 0.9% saline for 30 minutes, which allows the SAP to fully absorb as much saline as possible. Then the teabag is removed and allowed to hang so that saline that is not absorbed will drip off and not impact the test result. The bag is weighed and the change in weight is called the Freeswell Absorption Capacity or Free Swell Capacity (FSC). This value is typically between 45 and 75 grams of saline per gram of SAP.

Since a SAP is usually distributed in an absorbent article where the particles are separate, discrete particles, the test continues by using a centrifuge to remove saline from between the particles in a manner similar to the spin cycle of a washing machine. When centrifugation is complete, the bag is weighed and the change in weight from the initial dry measurement is the Centrifuge Retention Capacity (CRC). This value is usually between 30 to 45 grams of saline per gram of SAP.

Retention is one of the few metrics reported on SAPs that directly translates into values for a hygiene product.

If the hygiene product has a specification for its retention capacity of 600 grams of saline, then the amount of SAP required for that product can be roughly estimated by knowing the SAP retention capacity. For a SAP with 30 grams per gram retention, about 20 grams of SAP are needed. If the SAP has a higher retention value, say 40 grams per gram, then only 15 grams of SAP are needed. It’s not a perfect correlation, but it is a useful guide.

Other SAP properties may be needed to make a successful hygiene product, leading to the selection of a SAP that may have lower capacity. Follow the Lygos blog for more discussions on the SAP characteristics that improve product performance.

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 has high absorbency and retention capacity comparable to leading petro-based SAPs, and is based on a natural material,

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

Retention capacity a practical and predictive measure of SAP performance in hygiene product design. By understanding this metric, 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 Soltellus™ can support your hygiene product development.