Expert's Opinion

How to Check For SAP Content in a Diaper Without Probes.

By Carlos Richer, Contributor | February 16, 2010

A simple method for estimating SAP content.

Here I introduce a simple method to estimate the amount of SAP and Cellulose in the pad mix of any diaper.You only need to know the original weight of the pad, its total free swell capacity, and its retentive centrifugal capacity. You also need to know the same data (free swell and retentive capacity) for the SAP and for the Cellulose at 100%.

Extracting a small sample of SAP with little or no fluff is a bit hard but not impossible, you only need enough samples and a good amount of patience. A simple alternative is to assume that your competitors are using the same kind of SAP that you have and use the properties of your own SAP to calculate the quantity of SAP of your competitors; typically the error is not large unless you are using a very different kind of SAP (for example when using patent free SAP versus regular SAP).

Fluff is basically a commodity, for the same reason you can use your own fluff and assume it will have the same property as in other diapers.
For best results keep the temperature of the saline water and the Relative Humidity in the lab at the same level while doing the SAP extraction and the experiments.

This is the equation you need to use:
Y= ((E-A*G)/(B-A)+(F-C*G)/(D-C))/2
A= Free swell capacity of Fluff.
B= Free swell capacity of SAP.
C= Centrifugal retentive capacity of Fluff.
D= Centrifugal retentive capacity of SAP.
E= Total free swell capacity for the pad you want to test.
G= Centrifugal retentive capacity for the pad you want to test.
G= Weight of the Pad without the ADL and the top and bottom sheets.
(*) Note that A, B, C and D are constants when you use the same raw materials so you do not need to measure every time, unless you are not sure if the raw material has changed.

In this equation Y is the amount of SAP in the mix that you want to measure.
Here Step by Step:
1.Using a tea bag” made of hydrophilic nonwoven and sealed with heat, measure the total capacity (B) and centrifugal capacity (D) of 1 gram of SAP.Note: You can also use a larger tea bag with 5 grams of SAP and calculate an average.Make sure you provide enough additional volume to the SAP tea bag in order to allow it to expand and make sure the tea bag is strong enough so it will not open during centrifugal test. Now you know constants B and D.

2.Using a ¨tea bag¨ made of hydrophilic nonwoven and bonded with heat, measure the total capacity (a) and centrifugal capacity (c) of 10 grams of fluff and then calculate the average result for 1 gram of fluff. An alternative way is to make a special diaper in the diaper machine with an exact amount of fluff and no SAP and take the average for 1 gram. Now you have constants A and C.
Take a sample diaper with an unknown mix of SAP and cellulose (the sample you want to measure) and cut around the pad.Using another diaper sample (or several), extract the ADL and measure its weight.Estimate the amount of nonwoven and back sheet considering the area of the pad. Now you can calculate the weight of the original mix (G)= weight of the pad – weight of ADL – weight of Nonwoven – weight of back sheet.
Now you measure the total free swell capacity (E) and the centrifugal retention (F) for a pad with an unknown quantity of SAP and fluff.Now you know E and F,

Once you know the amount of SAP it is easy to know the amount of Fluff because you already know the total weight of the mix.

An example using a typical baby diaper of Tier 3 from the USA:
From step 1 let us assume we found in the laboratory that:
The free swell capacity of 100% fluff is 13.0 grams of saline per gram of fluff (A)
The centrifugal retention of 100% fluff is 1.7 grams of saline per gram of fluff (C)

From step 2 let us assume we found in the laboratory that:
The free swell capacity of 100% SAP is 32 grams of saline per gram of SAP (B)
The centrifugal retention of 100% SAP is 21 grams of saline per gram of SAP (D)

From step 3 let us assume we found that (weight of unknown mix):
Weight of the absorbent pad – grams ADL – Nonwoven and Film = 29 grams (G)

From step 4 let us assume we found that:
The free swell capacity of the unknown pad mix is 495 grams of saline (E)
The centrifugal retention of the unknown pad mix is 247 grams of saline= (F)

Then now we are able to calculate:
Amount of SAP= ((495-13*29)/(32-13) + (247-1.7*29)/(21-1.7) )/2
Amount of SAP = (118/19) + (197.7/19.3) /2 = 8.21 grams of SAP
Amount of Fluff= G- Y= 29 – 8.21 = 20.79 grams of Fluff
Observation: It is possible to use different RPM speeds for the centrifugal retention test or different times of the immersion free swell capacity test as long as you are consistent using exactly the same test when you test the individual components and when you test the pad mix with the unknown quantity of SAP.


After 21 years with Mexican diaper manufacturer Absormex where he served as general director, Carlos Richer retired in 2005 to start his own company dedicated to providing consulting services to the disposable diaper industry. His website,, contains useful links related to the diaper industry, as well as links to websites of diaper manufacturers and raw material suppliers from all over the world.

Related Application:

Related Other:

  • NWI Celebrates New Investment

    May 25, 2017
    Advanced spunbond line opening attracts hundreds of industry members

  • PHS Group Turns Dirty Diapers Into Energy

    PHS Group Turns Dirty Diapers Into Energy

    Tara Olivo, associate editor||April 13, 2017
    LifeCycle process is changing the way diapers, sanitary pads, tampons and incontinence products are disposed of in the U.K.

  • P&G CFO Discusses Strategy

    P&G CFO Discusses Strategy

    Karen McIntyre, editor||March 2, 2017
    Moeller updates analysts on brand restructuring, efforts in incontinence and baby diapers at CAGR