Baby underwear can be regarded as an absorption system of multi-layer nonwovens. In the whole liquid absorption process, urine first contacts the surface layer and transports urine from the surface layer to the diversion layer. At this time, there is almost no liquid absorption on the surface layer. The urine transported to the guide layer will be quickly absorbed and can spread longitudinally along the baby's underwear because the fibers in the guide layer are fluffy and arranged longitudinally. After the urine is evenly distributed longitudinally, the urine continues to be transmitted downward to the absorption core layer, so that the absorption core layer can effectively absorb urine. The composite structure composed of sap and fluff pulp quickly absorbs and expands urine, locks urine and completes the whole liquid absorption process.
The first process of urine contact is the first one. Because the surface layer mostly adopts hydrophobic fibers, its water permeability principle is mainly through the capillary effect between fibers and water molecules passing through the surface gap under water pressure. The water permeability of the surface layer is affected by the fiber type, thickness and structural tightness of nonwovens.
The fibers in the diversion layer are mainly arranged vertically along the material, and the gap between the fibers is large, which can timely and effectively absorb the urine transported from the surface layer. At the same time, the diversion layer can temporarily store local urine and diffuse longitudinally until it is absorbed by the absorption core layer.
As a kind of super absorbent pulp, fluff pulp has a positive correlation between its absorption and the capillary effect of paper transverse fiber. Fluff slurry is essential in the absorption process in the core layer, which plays a role similar to the transmission pipeline. Urine usually flows into the absorption core layer in a certain area from the surface layer, which often leads to the saturation of the absorption core layer and affects the reabsorption of other sap that does not reach the absorption saturation state. Therefore, a certain amount of fluff pulp fiber needs to be added to the absorption core layer to diffuse urine, so that SAP can fully absorb urine. It can be seen that improving the liquid absorption capacity of the absorption core layer does not mean increasing the sap content. The performance of fluff pulp, the mixing ratio of SAP / fluff pulp and the gap between fibers will affect the absorption performance of baby underwear.
Sap in the absorption core is a cross-linked polymer polymerized by sodium polyacrylate. Carboxyl groups in macromolecular chains interact with alkane groups and water molecules. When SAP encounters water molecules, sodium ions will ionize and hydrolyze rapidly. With the increase of the number of internal positive charges, the electrostatic repulsion between macromolecular chains increases, the original three-dimensional network structure of SAP gradually expands, and water molecules enter the interior of SAP; In addition, due to the principle of charge conservation, the interior of SAP should remain electrically neutral, so the ionized Na + will not be too far away from the COO on the sap polymer chain. This situation leads to high ion concentration in SAP and high external osmotic pressure, making it easier for water molecules to enter sap; Finally, with the expansion of the grid, the elastic shrinkage force of SAP three-dimensional grid structure gradually increases, offsetting the resistance and electrostatic repulsion, making SAP reach the water absorption balance.
At present, the leak proof bottom film of baby underwear mostly adopts the breathable microporous film containing porogen CaCO3 particles. Through the great difference between the diameter of water droplets (& Ge; 20 & mu; m) and the diameter of water vapor molecules (about 0.4nm), the microporous membrane makes full use of this feature to make the microporous diameter between the diameter of water droplets and the diameter of water vapor molecules, which can not only effectively waterproof but also breathable.