Deswelling stresses and reduced swelling of superabsorbent polymer in composites of fiber and superabsorbent polymers

Abstract

ABSTRACT:The desired performance characteristics of absorbent products such as diapers include a low number of leaks, adequate liquid absorption capacity, and high skin dryness. By measuring the absorption behavior of mixtures of cellulose fluff pulp and superabsorbent polymers in nonimmersed conditions, we identify four major parameters that affect the swelling of the polymer in a composite: the Donnan ion-exclusion effects of limiting the amount of liquid in contact with the superabsorbent polymer, the restriction of swelling by the physical constraints of the fibrous network that surrounds the polymer particles, the restriction of swelling by the capillary tension provided by the pores between the cellulose fibers, and the osmotic pressure of extracted polyelectrolytes. Swelling of the superabsorbent polymer in composites is always smaller than the value measured by immersion of the polymer followed by centrifugation. Donnan exclusion of ions from the gel phase results in about 16% less swelling than when the polymers are swollen in excess liquid. The swelling restriction imposed by the fiber network, which surrounds the granules, reduces swelling a further 10%. In addition, the presence of small pores between the fibers exerts capillary tension on the polymer and lowers the swelling by another 10%. The external compression of the composite pads does not reduce the extent of swelling of the superabsorbent polymer because the stress is supported by the fibers.

Swelling and shrinking kinetics of a lamellar gel phase

“…In everyday life and many industrial processes, materials swell by absorption of solvent, e.g., washing powder, 1 foodstuff, 2 diapers, 3 eyeballs, 4 and clay. 5 Conversely, if the solvent flow is reversed materials shrink, as for a hypertonic cell with a lower solute concentration than its environment.…”

Enhanced Absorbent Products Incorporating Cellulose and Its Derivatives: A Review

“…Fluff pulp, which is usually provided as bleached softwood kraft fibers, is typically a main component of highly absorbent products (Jerkeman 1970;Parham and Hergert 1980;Nagorski 1994;Buchholz et al 2005;Lund et al 2012). Although, as noted earlier, the fibers themselves can accommodate some liquid (Zohuriaan-Mehr and Kabiri 2008), most of the absorption capacity of such products can be attributed to the inclusion of SAPs (Aliouche et al 2001).…”

“…According to Buchholz et al (2005) a network of cellulosic fibers intertwined through a loose matrix of SAPs in an absorbent product can be expected to restrict the swelling of the SAPs. A constraining pressure of about 2 kPa was estimated in the cited work.…”

“…This section will consider osmotic pressure and various studies showing how osmotic pressure can account for such swelling, especially when a cellulosic material or a hydrogel is placed in water having few or no salt ions (Grignon and Scallan 1980;Vitta et al 1986;Oppermann 1995;Sannino et al 2003;Buchholz et al 2005;Ganji et al 2010). The same principles also apply to cellulosic fibers, though usually to a lesser extent.…”

Quantitative measurements of capillary absorption in thin porous media by the Automatic Scanning Absorptometer

“…On the other hand, understanding liquid transport in porous media enables one to positively influence the performance of a wide variety of applications, such as inkjet printing (Lundberg, 2011;Lundberg et al, 2006;Svanholm, 2007;Yang, 2003), heat storage (Nagel et al, 2013) or even diapers (Buchholz et al, 2005). On the other hand, knowledge of liquid transport in porous media can be vital to prevent damage as a result of salt intrusion in building materials (Pel et al, 2003).…”