Test methods for the evaluation of superabsorbent polymers

Because of their wide range of applications, the properties of SAPs have been evaluated by academic researchers and industrial analysts in many different ways. Some of the test methods are described below. 

Generally, when the terms of swelling or absorbency are used without specifying its conditions, they imply uptake of distilled water while the sample is freely swollen, that is, no load is put on the testing sample. The free absorbency capacity can be tested by the following methods. 

The vortex method is often employed in research and technical laboratories to offer a quick assessment of SAP swelling rate. Water or saline solution (50.0 g) is poured in a lOOmL beaker and its temperature is adjusted at 30°C. It is stirred at 600rpm using a magnetic stirrer. The SAP sample (mesh 50-60, Wq=0.50-2.0g) is added and a stopwatch is started. The time elapsing from the addition of the SAP into the fluid to the disappearance of vortex (tvd, s) is measured. The swelling rate (SR, g/g.s)=(50/Wo)/tvd. 

To achieve a comparative measure of sensitivity of the SAP materials to the composition of the aqueous fluid, a dimensionless swelling factor, f, is defined as f = 1 - (absorption in a given fluid/absorption in distilled water). A large f value means a high absorbency loss of the sample due to the presence of salt in the solution. Therefore, SAPs with lower f values are usually preferred. 

Absorption time is the time needed to absorb a certain amount of fluid, which is strongly correlated to the probability of leakage. In this test, a diaper is laid with the backsheet side on a polyurethane-foam base and covered with a cover plate. Weights are placed on the cover plate to simulate the weight of a baby lying on the diaper. Because of the foam base, pressure is distributed equally across the diaper. The cover plate has a hole which is placed over the area where the diaper typically absorbs urine. An application tube containing an electrode connected to a computer is mounted over that hole. A set amount of synthetic urine is pumped into the application tube, and with the help of the electrode the time is measured until all liquid in the tube is absorbed by the diaper. This is repeated three to four times to simulate the repeated urination of a baby. Results are recorded as absorption rates, that is, volume per time (mL/s). 

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