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| "Gel blocking" is the phenomenon that
when particles of the AGM are wetted and swollen, liquid does not
permeate through the swollen AGM layer or the permeation is slow.
Gel blocking can be a particularly acute problem if the particles
of AGM do not have adequate gel strength and deform or spread under
stress once the particles swell with absorbed fluid (see the illustration
below). |
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| In general, increasing gel strength of AGM can
contribute to decrease gel blocking. Gel strength relates to the tendency
of the AGM to deform or "flow" under external stresses.
High gel strength usually can be obtained by crosslinking. However,
crosslinking has a deep impact on the absorbent capacity of AGM. In
general, absorbent capacity or "gel volume" has inverse
power-law dependence on the level of crosslinking. That is, high crosslinking
level results in high gel strength but low gel volume. In order to
obtain an AGM with high gel strength (less gel blocking) while maintaining
gel volume as high as possible, chemical reactions on the surfaces
of the AGM particles are often used to form thin shells of so-called
surface crosslinking (see the illustration below). |
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| A performing AGM in disposable products
need to meet following requirements: |
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- High absorption capacity especially
under pressure
- High absorption speed
- High fluid permeability
- No gel blocking at wet state
- Safe
- Stable
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| To evaluate if an AGM can meet these
requirements mentioned above, following test methods are often used: |
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- ，Gel Volume (Centrifuge Retention Capacity)
A measure of how many grams of urine can be absorbed by one gram
of AGM under defined centrifuge force. A performance measure.
- Free Swell Capacity
Also a measure of AGM's absorption capacity but is under free
swelling (no pressure applied) condition. A performance measure.
- Performance Under Pressure (PUP) (also called Absorbency Under
Load, AUL)
An evaluation of AGM's capacity to absorb urine under pressure,
usually under 0.7 psi (pounds per square inch). A performance
measure.
- Saline Flow Conductivity (SFC)
A measure of AGM's permeability towards liquid at swollen state.
A performance measure.
- Free Swelling Rate
A measure of AGM's free swelling absorption speed in synthetic
urine. A performance measure.
- Flow Rate
A measure of AGM's flow property. A runnability measure.
- Density
A runnability measure
- Particle size distribution
A measure of runnability, performance and safety.
- Residual monomer
Residual acrylic acid (ppm). Safety measurement
- Moisture content
The percentage of moisture in AGM. A runnability measurement.
- Extractable
A measure of soluble un-crosslinked acrylic acid oligomers. Safety
and performance measurement.
- pH
pH of an AGM is related to the neutralization degree of polyacrylic
acid. Normally it is at the range of 5.5 - 6.8. A safety measurement.
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| (Absorption properties tested in synthetic
urine) |
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| Centrifuge Gel Volume (g/g) |
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| Performance Under Pressure (g/g) |
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Saline Flow Conductivity (SFC)
(10-7 cm3 sec/g) |
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| Particle size distribution (micron) |
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| The base component of AGM, sodium polyacrylate
is considered to be non-toxic, but inhalation of airborne particles
of the powder or contact with eyes can have some adverse reactions.
It is recommended to wear masks and safety
glasses when handling AGM especially when shaking the AGM powder
from a disposable diaper to obtain the powder for analysis, etc. |
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Synthetic urine formulation
In AGM evaluation, synthetic urine is often used. Formulation of the
widely used Jayco synthetic urine is summarized in the table below: |
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Jayco Synthetic Urine
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* Jayco synthetic urine is available from Jayco Pharmaceuticals
Company of Camp Hill, Pennsylvania.
All of the chemicals are of reagent grade. The pH of the synthetic
urine is in the range of 6.0 to 6.4.
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(Will introduce AGM suppliers in next month. Please come back
to check latest information)
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