April 24, 2001
Berkeley Lab Science Beat

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Often viewed as primarily a tool for basic research, the Lab's Advanced Light Source (ALS) nonetheless has its share of industrial users seeking solutions to their problems. Recently, the people who manufacture materials for disposable baby diapers set up shop at the ALS.

The global market for disposable diapers is $20 billion annually, but manufacturers face a twofold challenge. To keep the baby dry, the diaper must be able to take up a large quantity of liquid and then, as baby diaper scientists describe it, "hold it under load" as the baby moves around.

Parents don't make matters any easier.  These consumers demand thin diapers, and through their buying habits, have ruled out the extensive use of the bulky cellulose fluff that was once the stuff of disposable baby diapers.

Today, "superabsorbent polymers," materials with long chains of intertwining molecules that are able to soak up lots of liquid, now dominate the disposable diaper market.

By means of chemical reactions on the surfaces of submillimeter-sized superabsorbent polymer beads, manufacturers fashion thin shells of tightly "crosslinked" polymer in which the strands are connected at the cross links. The shell makes it more difficult for liquid to leak out, but actual performance depends on the microscopic details of the shell structure, such as variations in the crosslinking through and around the shell.

Examining this kind of structural detail has been difficult, but researchers now have found a way, using the synchrotron radiation produced at the ALS. 

Researchers from The Dow Chemical Company teamed with academic colleagues and used an x-ray microscopy technique to analyze a variety of shells formed in different ways. Researchers were able to map the variation in crosslink density of the different shells, providing a way to gauge the effect of shell-formation processes at the microscopic level and thereby, to improve the production process.

The experiments were such a success that Dow has been able to use the results to help develop the process technology now being designed for a new superabsorbent polymer manufacturing plant. Because of this and related work on x-ray spectromicroscopy of polymeric materials, Dow's Analytical Sciences division has conferred its highest internal award on researchers Gary Mitchell and Ed Rightor.

Superabsorbent polymers are used not only in baby diapers but in a wide range of products. They are an example of a polymer gel.

Occurring in both natural and synthetic forms, polymer gels exhibit an intriguing combination of the properties of both liquids and solids. One feature that makes gels useful is their ability to respond strongly to very weak external stimuli, such as minute changes in pH or temperature. For example, a polymer gel might first absorb a quantity of liquid and later release it as the external conditions change. Timed release of pharmaceuticals is one example among many in which a control stimulus determines the rate of release. Crosslinking is a key feature of the polymer microstructure that governs actual performance.

Dow sells superabsorbent polymers in the form of small (less than 1.0 mm in size) beads of sodium polyacrylate that are lightly crosslinked to form an insoluble, hydrophilic gel. The ability to soak up great quantities of fluid makes superabsorbent polymers attractive for use in diapers. But, like a sponge when compressed, some of the fluid is squeezed back out when the baby moves, negating part of the benefit. The strategy for preventing leakage under weight-bearing load is the formation of a thin shell of more tightly crosslinked polymer. The effectiveness of the shell depends in part on the density profile of the crosslinking through the shell, a distance of several microns. Several different methods have been developed to make surface crosslinked superabsorbent polymers gels, but up until now there has been no good way to visualize and assess the resulting core-shell structure and the crosslink density profile.

To obtain the desired information, the researchers turned to near-edge x-ray absorption fine structure spectromicroscopy, using a scanning transmission x-ray microscope at the Advanced Light Source.  They were able to make images of the polymers in the fully hydrated state (in excess water). Because the x-ray energy could be tuned to a value where the carbon in the polymer absorbs and the water is almost transparent, they could map the areas where crosslinking was higher by observing the increased carbon content in these regions.

Crosslinking was stimulated by treating the surface of  the superabsorbent polymer beads with varying solutions. Sectioned beads were then exposed to a saline solution to put them in the fully swollen state for imaging. Analysis of the images yielded two extreme cases for the crosslink profile through the shell. In one, the crosslink density decreased smoothly over a distance of 18 microns from a maximum at the outer surface. In the other, the density was uniform over a distance of five microns and then dropped abruptly.

These differences reflect a complicated interplay between the dynamics of the swelling of the bead in water, the diffusion rate of the crosslinker in the water phase, and the rate of the crosslinking reaction. And, this interplay can make or break a better baby diaper.

Dow was able to use this information to help guide the design of new superabsorbent polymer fabrication plant.

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