Ionic superabsorbent polymers with enhanced swelling characteristics

#The present invention relates to superabsorbent polymers having enhanced swelling characteristics. More particularly, the present invention relates to an absorbent composition comprising an ionic superabsorbent polymer material and an absorbency-enhancing additive having a dipole moment greater than water which significantly increases the swelling characteristics of ionic superabsorbent polymers in the presence of salt-containing solutions such as urine, menses, blood and other body exudates.

#The use of superabsorbent polymers such as ionic superabsorbent polymers is standard throughout the absorbent products industry. The absorbent core of many absorbent articles, such as diapers, sanitary napkins, training pants, and incontinence products, is typically comprised of one or more layers of cellulose fluff pulp, intermixed with superabsorbent polymers. Other layers are often used in combination with the absorbent core depending upon the intended application of the product.

#Ionic superabsorbent polymers, also commonly referred to as ionic hydrogels or ionic hydrocolloids, are typically cross-linked ionic polymers that are able to absorb an amount of pure water equal to at least ten times their dry weight and retain the pure water under a moderate external pressure. Ionic superabsorbent polymers can be anionic in nature (e.g., acrylate based or sulfonate based), or can be cationic in nature (e.g., a partly neutralized polyamine), and as such can either have positive or negative charges along the backbone of the polymer structure. Ionic superabsorbent polymers are electrically charged in solution because various groups attached to the polymer chain easily become ionic. Examples of groups which can become electrically charged in ionic superabsorbent polymers include carboxylate groups and amine groups.

#FIG. 1 is a graph showing the effect of glycine on the swelling of an anionic superabsorbent polymer in the presence of saline solution.

#FIG. 2 is a graph showing the absorbent capacity of glycinated and non-glycinated ionic superabsorbent polymers under various loads.

#FIGS. 3 and 4 are graphs showing the effect of glycine on the swelling of an anionic superabsorbent polymer in the presence of porcine blood.

#In accordance with the present invention, the swelling capacity of ionic superabsorbent polymers in the presence of salt-containing solutions is significantly enhanced by an absorbency-enhancing additive. More specifically, it has been discovered that the absorption capacity of ionic superabsorbent polymer materials when contacted with salt-containing solutions is markedly improved by the presence of a sufficient amount of absorbency-enhancing additive having a dipole moment greater than water and capable of forming a zwitterion over a wide pH range. It is believed that such absorbency-enhancing additives tend to accumulate in the interior of the ionic superabsorbent polymer and counteract the decrease in osmotic activity typically experienced by ionic superabsorbent polymers when used to absorb salt-containing solutions such as urine and other biological liquids.

#The absorbent composition of the present invention comprises an ionic superabsorbent polymer material (i.e., superabsorbent polymers containing backbones that are easily ionized) and the absorbency-enhancing additive. Preferably, the absorbency-enhancing additive is combined with the ionic superabsorbent polymer. The ionic superabsorbent polymer materials suitable for use in combination with the absorbency-enhancing additives described herein include both cationic and anionic superabsorbent polymers. As used herein, the term “ionic superabsorbent polymer” is meant to include cross-linked ionic polymers which are able to absorb an amount of pure water equal to at least about ten times their dry weight. Ionic polymers are also referred to in the art as ionic hydrogels or ionic hydrocolloids. The ionic superabsorbent polymers of the present invention are preferably cross-linked polymers as cross-linking prevents the polymer chains from significantly dissolving into the surrounding aqueous medium and allows the polymer to be a separate phase apart from the aqueous medium. Examples of anionic superabsorbent polymers include, for example acrylate based or sulfonate based polymers. Particularly preferred anionic superabsorbent polymers include polyacrylates, with poly(acrylic acid) being particularly preferred. An example of a cationic superabsorbent polymer is a partly neutralized polyamine such as a polyallyl amine. The counterions of the ionic superabsorbent polymers of the present invention are not critical and can include, for example, sodium, potassium, and lithium for anionic superabsorbent polymers, and chloride or bromide counterions for cationic superabsorbent polymers.

#In accordance with the present invention, it is preferred that the ionic superabsorbent polymers used in combination with the absorbency-enhancing additives described herein have a degree of ionization in aqueous salt solutions of at least about 10% to aid in the accumulation of the absorbency-enhancing additive in the interior of the ionic superabsorbent polymer as discussed below. Preferably, the degree of ionization of the ionic superabsorbent polymers is at least about 20%, more preferably at least about 40%, and still more preferably from at least about 50% to 80% or more. As discussed more fully below, the electric field of the ionized superabsorbent polymer plays a significant role in the collection and partition of the absorbency-enhancing additives described herein in the interior of the polymer which results in increased osmotic activity of the ionic superabsorbent polymer in the presence of salt solutions.