If you work with diapers—whether you're running a production line, sourcing materials, or just trying to understand why one SAP costs more than another—you've probably seen a datasheet full of numbers like CRC, AUP, GBP, and pH. The problem is that most datasheets don't tell you which numbers matter, or how they interact.
This guide breaks down the key test indicators for diaper-grade Super Absorbent Polymer (SAP), based on standards including GB/T 22875-2018, GB/T 22905-2008, and GB/T 28004.2-2021. The goal isn't to turn you into a lab technician—it's to help you read a datasheet like someone who knows what they're looking at.
CRC (Centrifuge Retention Capacity): The Number Everyone Quotes, But Nobody Should Trust Alone
CRC—also called free absorption capacity or simply "absorption capacity"—measures how much saline solution a gram of SAP can hold after soaking and draining. It's the headline number on every datasheet, and it's the one buyers ask about first.
How it's tested: A weighed SAP sample goes into a permeable tea bag, which is sealed and submerged in 0.9% saline for 30 minutes. Then it's hung to drain for 10 minutes, and the swollen gel is weighed. Subtract the empty bag, divide by the dry SAP mass—that's your CRC.
What the numbers look like: Deionized water absorption can hit 50–100 g/g. Saline absorption, which is what actually matters for urine, typically lands between 40–60 g/g.
Here's the catch: a high CRC doesn't automatically mean a better diaper. SAP with very high free absorption usually has lower cross-linking density, which means weaker gel strength. Under a baby's weight, that gel can collapse and release liquid back to the surface. So when a supplier brags about a 55 g/g CRC, the first question should be: "What's the AUP?"
AUP (Absorbency Under Pressure): The Number That Predicts Real Performance
AUP measures how much saline SAP can absorb while under mechanical pressure—typically 0.3 psi or 0.7 psi. If CRC tells you what SAP can do in a perfect world, AUP tells you what it does when a baby is sitting on it.
How it's tested: A standard weight sits on top of the SAP sample, simulating body pressure. Saline is introduced, and the amount absorbed under that load is measured.
Typical values: Diaper-grade SAP usually requires AUP (0.3 psi) ≥20–30 g/g.
The real insight here is the AUP/CRC ratio. When that ratio is above 0.65, it tells you the surface cross-linking is well done and the gel holds its structure under pressure. This ratio is a far better predictor of leakage performance than CRC alone. A SAP with 45 g/g CRC and 30 g/g AUP will outperform one with 55 g/g CRC and 22 g/g AUP in almost every real-world scenario.
Absorption Rate: How Fast Does It Drink?
Absorption rate is the time, in seconds, for SAP to absorb a set amount of liquid. For diaper SAP, the saline absorption rate typically needs to be ≤30–40 seconds.
How it's tested: Two common methods. The first: 1.000 g of sample goes into a beaker, 5.0 mL of standard synthetic test liquid is poured in, the beaker is shaken to disperse the sample, and timing starts. Timing stops when the liquid's mirror surface disappears. The second—the "vortex method"—adds sample to a magnetic stir vortex and stops when the stir bar is completely covered by gel.
Faster absorption means urine spends less time on the surface, which means less skin contact and lower rash risk. But there's a trade-off: absorption speed is tied to particle size. Smaller particles have more surface area and absorb faster, but too many fines choke off permeability. That's why particle size distribution matters as much as the absorption rate number itself.
Retention Capacity (CRC) and Rewet: Can It Hold What It Drinks?
This is where things get confusing, because "CRC" gets used for two different things. In the context of retention, CRC means Centrifuge Retention Capacity—how much liquid SAP holds onto after being spun at 250 g for 3 minutes.
Typical values: Diaper-grade SAP usually requires centrifuge retention ≥30–40 g/g.
Retention capacity is about locking water in, not just taking it in. Higher retention means lower rewet—the amount of liquid that migrates back to the diaper surface under pressure. And rewet is what parents actually notice. A diaper that feels dry stays on longer and causes fewer complaints.
The relationship with cross-linking is straightforward: more cross-linking means a denser gel network, better retention, but lower free absorption. There's no free lunch here.
Rewet testing: Standard test liquid is injected into a diaper sample, allowed to absorb, then filter paper and a standard weight are applied. The paper is weighed for liquid that came back. Under GB/T 28004.2-2021, baby diapers must have rewet ≤15.0 g, adult diapers ≤20 g. Premium products often achieve 1–5 g.
Rewet is where SAP performance meets core design—layered diversion, suspended cores, and acquisition layers all play a role. A great SAP in a badly designed core will still rewet.
GBP (Gel Bed Permeability): The Forgotten Indicator That Causes Leaks
Permeability—measured in Darcy—describes how fast liquid moves through already-swollen SAP gel. This is the indicator that explains most "mystery leaks" in diapers that seem to have plenty of capacity left.
How it's tested: Pressure is applied to the SAP sample while liquid is continuously injected. Flow rate and pressure differential across the gel bed give you the permeability coefficient.
Why it matters: When SAP at the top of the core swells shut, liquid has to pass through that swollen layer to reach fresh SAP below. If permeability is poor—below 5 Darcy—urine pools on the surface and leaks out the sides. This is called gel blocking, and it's the reason a diaper can leak while still being 70% empty by weight.
Permeability depends on particle size distribution, particle shape (spherical beats irregular), and surface cross-linking. It's also why you can't just throw more SAP into a core and expect better performance.
Particle Size Distribution: The Invisible Variable
Particle size distribution is exactly what it sounds like—the percentage of SAP particles in each mesh range.
How it's tested: Standard sieves separate the sample, and each fraction is weighed.
Typical diaper-grade distribution: 20–60 mesh ≥70%, 60–80 mesh ≤20%, 80–100 mesh ≤5%, below 100 mesh <1%.
Small particles absorb fast but hurt permeability. Large particles flow well but absorb slowly. The right blend gives you both speed and flow. And fines below 100 mesh are a problem for a different reason: dust. Too many fines means a dirty production environment and potential safety issues.
pH: Small Number, Big Consequences
pH measures the acidity or alkalinity of the SAP hydrogel or extract. Diaper-grade SAP typically sits between 6.5 and 8.3.
How it's tested: A SAP gel or extract is prepared at a set concentration, and a calibrated pH meter does the rest.
Here's what changed recently: GB/T 28004.1-2021 tightened the pH range for finished diapers from 4.0–8.0 to 4.0–7.5, and switched the test solution from deionized water to saline. That last part matters—saline is closer to real urine, so the test is more representative of actual use.
Out-of-range pH can irritate infant skin. It's one of those indicators that doesn't get much attention until there's a complaint.
Residual Monomer: The Safety Number
Residual monomer is unreacted acrylic acid and other monomers left after polymerization. It's measured by HPLC or GC.
Typical limit: ≤500 ppm for diaper-grade SAP, though some standards allow up to 1000 ppm.
Residual monomer is a skin irritant and sensitizer. It's the main reason chemical safety is now part of diaper standards. GB/T 28004.1-2021 brought heavy metals, formaldehyde, and migratable fluorescent substances into the testing framework for the first time—a signal that raw material safety is no longer optional.
Moisture Content: Storage Matters
Moisture content is the water percentage in finished SAP, typically required to be ≤7%.
How it's tested: Dry the sample at 105°C ± 2°C to constant weight and calculate the loss.
Too much moisture and SAP clumps in storage, losing performance before it ever reaches the production line. It also raises microbial risk. This is a logistics and storage indicator as much as a quality one.
Extractables: Purity and Network Integrity
Extractables are water-soluble small molecules that migrate out of the gel network. The limit for diaper-grade SAP is typically ≤35 wt%.
How it's tested: The SAP gel is filtered, and the filtrate is evaporated, dried, and weighed.
High extractables mean insufficient cross-linking—a weak network that leaks small molecules. It's both a performance indicator and a purity/safety indicator. Suppliers who cut corners on cross-linking often show it here first.
Putting It Together: What to Look For
| Dimension | Key Indicators | What You're Actually Asking |
|---|---|---|
| Absorption | CRC, AUP | How much can it take, and how much under pressure? |
| Retention | Centrifuge Retention Capacity, Rewet | Does it hold what it takes? |
| Permeability | Absorption Rate, GBP | How fast, and can liquid keep moving? |
| Safety | pH, Residual Monomer, Extractables | Is it safe for skin? |
The best SAP isn't the one with the highest CRC. It's the one that balances absorption, retention, permeability, and safety. This is why surface cross-linking exists: it trades 5–15% of free absorption capacity for a 40–80% gain in absorbency under pressure. That trade-off is what separates a datasheet champion from a diaper that actually works.
Next time you're looking at a SAP datasheet, skip the CRC headline. Find the AUP/CRC ratio. That's the number that tells you what happens when a baby sits down.
