Absorbent articles for personal hygiene of the type indicated above are designed to absorb and contain body exudates, in particular large quantity of urine. These absorbent articles comprise several layers providing different functions, for example a topsheet, a backsheet and in-between an absorbent core, among other layers. The function of the absorbent core is typically to absorb and retain the exudates for a prolonged amount of time, minimize re-wet to keep the wearer dry and avoid soiling of clothes or bed sheets.
The majority of currently marketed absorbent articles comprise as absorbent material a blend of comminuted wood pulp with superabsorbent polymers (SAP) in particulate form, also called absorbent gelling materials (AGM), see for example U.S. Pat. No. 5,151,092 (Buell). Absorbent articles having a core consisting essentially of SAP as absorbent material (so called “airfelt-free” cores) have also been proposed (see e.g. WO2008/155699 (Hundorf), WO95/11652 (Tanzer), WO2012/052172 (Van Malderen)). Absorbent cores with slits or grooves have also been proposed, typically to increase the fluid acquisition properties of the core or to act as a folding guide.
WO2012/170778 (Rosati et al., see also WO2012/170779, WO2012/170781 and WO2012/170808) discloses absorbent structures that comprise superabsorbent polymers, optionally a cellulosic material, and at least a pair of substantially longitudinally extending channels. The core wrap can be adhesively bonded through the channels to form a channel bond. The channel bonds may be permanent, so that their integrity is at least partially maintained both in dry and wet state. As the absorbent structure absorbs liquid and swells, the absorbent structure takes a three-dimensional shape with the channels becoming visible. The channels are indicated to provide improved fit and/or better liquid acquisition/transportation, and/or improved performance throughout the use of the absorbent structure. Any superabsorbent polymer particles known from the superabsorbent literature are indicated to be suitable.
The properties of superabsorbent polymers have been characterized in various ways. The absorbent capacity (CRC) in grams of liquid per gram of superabsorbent particles has been used, as well as their absorption speed as measured by the Free Swell Rate (FSR) and their permeability as measured by the Urine Permeability Measurement (UPM) test.
International patent application WO2012/174,026A1 discloses the K(t) method which can be used to determine the time dependent effective permeability (K(t)) and the uptake kinetics (T20) of a gel layer formed from hydrogel-forming superabsorbent polymer particles under a confining pressure. The application indicates that these SAP can be used to reduce leakage, especially at the first gush, i.e. when the article starts to be wetted.
It has now been found that although the absorption properties of conventional SAP may not be negatively impacted at first gush when used in a core with channels, the liquid absorption of the SAP can be significantly reduced in the following gushes after the fluid has been already absorbed in these cores comprising channels compared to cores without channels. Without wishing to be bound by theory, the inventors believe that the three-dimensional channels which are formed as the SAP absorbs a fluid can create a resistance to swelling for the superabsorbent polymers and reduce their swelling kinetics. As the channels otherwise facilitate the distribution of the fluid along the core, it was on contrary expected that any conventional SAP could be used in these cores. Accordingly the inventors have found that for absorbent cores comprising such channels it can be advantageous to use these SAP having a T20 of below 240 s to maintain sufficient speed of absorption beyond first gush.