Structure And Properties Of Nonwovens Containing Superabsorbent Fiber

Super absorbent fiber (SAF) is a new type of non-toxic and oil-repellent functional polymer material with excellent water absorbency, high absorption rate and good water retention capacity. Water absorbency of SAF is tens times higher than that of conventional synthetic fiber, or even higher. Currently, SAF has been widely used in some developed countries such as Japan, USA and some European countries in various fields including medical and health, building protection, petrochemicals, household chemicals, food packaging, environmental protection of communications, and so on. However, the production and application of SAF have only just got going in China. It is predicted that the development direction of SAF in near future mainly focuses on manufacturing of medical liquid absorption and retention materials such as personal health care products, medical dressings, surgical hole towels as well as industrial absorbent and filtration materials for alcohol, beverages, fuel, oil, gas, etc.In this study, SAF was blended with other fibers to overcome the shortcomings of poor mechanical properties and difficult subsequent processing, thus to develop a new type of superabsorbent nonwoven materials with both excellent water absorption capacity and good mechanical properties. The structure and properties of fabricated nonwovens were studied intensively by using variance analysis, image processing, numerical simulation and other technical means, aiming to provide some theoretical reference for the further application of SAF in areas such as medical protection, health care, dehydration of solutions, gas drying, oil-water separating, and also to provide some valuable theory and methods for further research on structure and properties of nonwoven fabrics. The main research topics and conclusions are as follows:1 Structure and properties of superabsorbent fiberThe fundamental characteristics of SAF such as morphology structure, breaking strength, breaking elongation and water absorption capacity were measured and analyzed through experimental and theoretical research. Results showed that:①SAF had strong water absorption and retention capacity and its absorbency in pure water reached up to 180. The water absorbencies of SAF in various aqueous solutions at different temperatures were in the range from 40 to 180. The water absorbency of SAF decreased acceleratedly with increasing water temperature, whereas it decreased deceleratedly with increasing Na+ concentration in external solution.②SAF had a circular cross-section. Under dry conditions, it was smooth and straight with almost no curl. After absorption, it swelled and cracked significantly with a fiber diameter increasement of over 80%.③The breaking strength and breaking elongation of SAF were both small and discrete. Its breaking strength was smaller than 0.7 cN·dtex-1 and its breaking elongation was lower than 2.4%, demonstrating poor mechanical property.④SAF can not withstand intense opening and carding processes, thus it has poor spinnability. Therefore, it can’t be used individually but needs to be blended with other fibers to develop superabsorbent materials.2 Preparation and statistical analysis of fundamental properties of nonwovens containing superabsorbent fiberConsidering that SAF has high absorption rate and changes into swollen gel and becomes sticky after absorption and that the fabricated nonwoven materials can be used as medical materials such as medical dressing, surgical hole towels, surgical drape, and absorbent and dehydrated materials for other occasions, a new type of three-layered superabsorbent nonwovens were prepared by hot calendaring process in this study. Both the top and bottom layers consisted of polypropylene fibers and bicomponent ES fibers, while the middle layer consisted of SAF, polypropylene fibers and bicomponent ES fibers. The fundamental properties of the three-layered superabsorbent nonwovens were analyzed statistically and the results showed that:①Water absorbency of the superabsorbent nonwovens were significantly influenced by the SAF content. The relationship between them was non-linear and the water absorbency increased first and then decreased with increasing SAF content. According to significant effect of factors obtained by variance analysis, it can be deduced with a 95% reliability that when the SAF content was 15%(i.e.,30% in the middle layer), water absorbency of the nonwoven material in pure water reached a maximum value ranging from 10.185 to 15.901, which was much higher than that of ordinary nonwoven materials.②During the swelling process, the absorption rate of the superabsorbent nonowoven material was very high at first and then lowered. The optimum SAF content for highest absorption rate ranged from 7.5% to 10%(i.e.15% to 20% in the middle layer), which was quite different from that for highest water absorbency.③Water absorbency and absorption rate of the superabsorbent nonwoven material were less affected by water temperature. However, at a higher SAF content, the rise of water temperature will hinder the absorption of water due to the inhibition of the exothermic swelling process of SAF.④Water retention capacity of the superabsorbent nonwoven material after being heated or being under pressure increased with increasing SAF content and about 60-70% of the initial water absorbency could be retained after being under pressure.3 Characterization and analysis of morphology structure of nonwovens containing superabsorbent fiberThe internal structure of the nonwoven superabsorbent nonwoven material were observed via Confocal Laser Scanning Microscopy (CLSM) and the morphology structure, especially the structure of the middle layer, including the fiber orientation, porosity, equivalent pore diameter, pore size distribution, etc. were investigated. Moreover, the effects of various factors such as processing technology, surface density of the nonwoven materials and the fiber fineness on the structure as well as the relationship between the structure and properties were discussed. Results showed that:①The porosity of the middle layer of the superabsorbent nonwoven material demonstrated a gradient structure, indicating that fluids flowed more, easily from outside to inside under the action of gradient pressure difference, thus the absorption capacity of the superabsorbent nonwoven material was enhanced.Moreover, the gradient structure also indicated that coarser SAF had a tendency to distribute to the surface during processing, causing more pores on the surface of the middle layer than inner. Pore size distribution curve of the superabsorbent nonwoven material showed a single peak, suggesting that SAF were blended uniformly with other fibers during processing. The fiber orientation degree within the nonwoven materials with different SAF content was remarkably consistent with each other, indicating that ways of carding and web-lapping are the key factors that determine the fiber orientation degree. Hot calendaring process of the nonwoven material had a significant,influence on the formation and distribution of bonding points. ①With SAF content increased, the probability of small pores in the nonwoven material decreased, whereas the average pore size and the difference between the maximum and minimum pore size increased. The shape of the measured pore size distribution curve was consistent with that of theoretical curve, but their magnitudes were variant. In general, the measured pore size was smaller than the theoretical value, indicating that the pore size of the nonwoven material was influenced by the hot calendaring processing.③It was obtained via analysis using Poisson Polyhedron theory that when material thickness and fiber density of the nonwoven material were constant, the probability of occurrence of small pores would be increased with decreasing fiber fineness and increasing surface density of the material. The surface density of the material had a greater effect on the pore size than fiber fineness. On the other side, when the surface density of the nonwoven material and fiber density were constant, the probability of occurrence of small pores would be decreased with increasing fiber fineness and material thickness. The probability of small pores of thin material was large and was nearly unaffected by fiber fineness, while the probability of small pores of material using finer fibers was also large and was nearly unaffected by material thickness.④It was found via analysis using Poisson Polyhedron theory, Hagen Poiseuille law and Darcy’s law that permeability coefficient of the superabsorbent nonwoven material increased with increasing porosity, i.e., the larger the porosity is, the higher the water absorption capacity will be. The rate of descent of permeability coefficient was greater than the shrinking rate of pore size owing to the swelling of SAF after absorption.Therefore, the water absorption capacity of the nonwoven material was also significantly influenced by the variation of SAF content dut to its effect on the porosity and pore size.4 Analysis of fluid diffusion and flowing process in nonwovens containing superabsorbent fiberA new method was proposed to characterize the diffusion absorption characteristics of fluids in nonwoven material by theoretical research. Based on the image processing technology, diffusion absorption test and numerical simulation of the flowing process, the diffusion absorption characteristics of aqueous solution in nonwoven material as well as the flow field state inside the nonwoven material were investigated. Results showed that: ①The diffusion absorption characteristics of fluids in nonwoven material can be evaluated effectively by using characteristic curve and unevenness curve of diffusion as well as the theoretically derived values of two feature points of slowdown and saturation, respectively. The lower the diffusion curve, the earlier the diffusion saturation and the more obvious diffusion unevenness, the weaker diffusion absorption capacity of aqueous solution in nonwoven material.②The diffusion absorption of water in superficial layer of superabsorbent nonwoven material showed an exponential growth characteristics that the diffusion area increased remarkably at earlier stage and tended to be stable at later stage. The diffusion absorption capacity of water was mainly derived from the diffusion and penetration effect of internal micropores and also influenced by SAF content to some extent. The diffusion absorption capacity of the nonwoven material first increased and then decreased with increasing SAF content, demonstrating an upward-convex curve shape. At the saturation stage of diffusion, the diffusion unevenness was remarkable, showing obvious direction selectivity, and thus the diffusion absorption capacity was lowered.③After the water flowing into the non-woven material, the formation of a region with a flow rate of approaching zero or even a cavitation region on the downstream face of coarser fibers (SAF) would be easier with a faster flow rate. This may lead to an inhibition of the continuous absorption of water. Therefore, use of finer SAF is beneficial for improving its absorption capacity when using in nonwoven materials.④Befor the water flowing into the fiber layer, fluctuation of flow rate was intense, whereas, after the water flowing into the fiber layer, fluctuation of flow rate was serious on layers with larger hole area or coarser fibers. As a result, under the fluctuation effect of running water, the structural damage of the nonwoven material may start from the surface layer or layers with larger holes or coarser fibers.