超吸水纤维非织造材料结构与性能研究
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摘要
超吸水纤维是一种新型的吸水倍率高、吸水速度快、保水能力强、无毒、拒油的功能性高分子材料,其吸水倍率比常规合成纤维大几十倍甚至更高。目前,在欧洲、美国、日本等发达国家,超吸水纤维已广泛用于医疗卫生、建筑防护、石油化工、日用化工、食品包装、通讯环保等多个领域,在国内,超吸水纤维生产与应用刚刚起步。预计未来用其加工医卫应用的卫生护理、医用敷料、手术洞巾等吸液保液材料,以及工业应用的酒精、饮料、燃油、燃气等吸水过滤材料,是超吸水纤维主要发展方向。
本课题将超吸水纤维与其它纤维混合使用,克服超吸水纤维力学性能差、后续难以加工的不足,开发了兼有优良吸水能力和较好力学性能的高吸水非织造材料,并采用方差分析、图像处理、数值模拟等技术手段,对超吸水纤维非织造材料的结构与性能进行了深入研究,旨在进一步拓展超吸水纤维在医用防护、卫生护理、溶液脱水、气体干燥、油水分离等领域的应用广度和深度,为继续开展非织造材料结构与性能的研究提供可以借鉴的理论与方法。本课题的主要研究内容与结论如下：
1超吸水纤维结构与性能研究
通过实验和理论研究,对超吸水纤维的结构形态、断裂强度、断裂伸长、吸水能力等基本性能进行了测试和理论分析,研究结果表明：
①超吸水纤维具有极强的吸水、保水能力,吸纯水倍率可高达180倍,采用不同温度、不同水溶液时,超吸水纤维吸水倍率从40到180不等。吸水倍率随水温升高以加速趋势逐渐降低,随溶液中Na+浓度增加以减速趋势逐渐降低。
②超吸水纤维截面呈圆形,干燥时表面光滑、平直,几乎无卷曲,吸水后具有显著的溶胀开裂特征,纤维直径会增加80%以上
③超吸水纤维断裂强度和断裂伸长均很小且离散性大,断裂强度<0.7cN·dtex-1,断裂伸长<2.4%,力学性能较差。
④超吸水纤维不能承受强烈的开松、梳理,可纺性不好,难以作为单一原料开发产品,需与其它纤维混合开发高吸水材料。
2超吸水纤维非织造材料制备及性能统计分析
考虑到超吸水纤维吸水速度快、吸水后膨胀、发粘的特点,以及考虑到用超吸水纤维制备的非织造材料可作为医用敷料、手术洞巾、手术铺巾等医用材料和其它场合的吸湿脱水材料,课题采用非织造热轧工艺,制备了上下层由丙纶纤维和双组分ES纤维组成,中间层由超吸水纤维、丙纶纤维和双组分ES纤维构成的三层结构超吸水纤维非织造材料,并对其性能进行了统计分析,结果表明：
①超吸水纤维含量始终以先增后减的非线性关系显著影响着非织造材料的吸水倍率。根据方差分析所得的因子显著性效应,可以95%的信度推断：当超吸水纤维含量15%(即芯层含30%)时,非织造材料对纯水的吸水倍率最高,可达10.185～15.901倍,远高于一般非织造材料的吸水倍率。
②吸水过程中,超吸水纤维非织造材料的吸水速度由快到慢变化。与最高吸水倍率不同的是超吸水纤维含量在7.5%-10%(即芯层含15%-20%)时超吸水纤维非织造材料具有较快的吸水速度。
③被吸的水温对超吸水纤维非织造材料的吸水倍率和吸水速度没有太大的影响,不过,当非织造材料中超吸水纤维含量较多时,水温升高会抑制其放热而不利于非织造材料吸水。
④超吸水纤维含量越高,非织造材料的耐温与耐压保水能力越强。含有超吸水纤维的非织造材料在受压状态下,稳定后的耐压保水率一般可达到最初吸水倍率的60%-70%。
3超吸水纤维非织造材料形态结构表征与分析
采用激光共聚焦显微镜对超吸水纤维非织造材料内部结构进行了观察,研究了超吸水纤维非织造材料的结构形态,特别是芯层结构特征,包括材料中纤维取向、孔隙率、当量孔径、孔径分布等,探讨了加工工艺、非织造材料面密度、纤维细度等因素对材料结构的影响以及结构对性能的影响,结果表明：
①超吸水纤维非织造材料的芯层孔隙率呈梯度结构,这种结构不仅表明流体在材料内外梯度压差作用下更容易由外向内流动,增强超吸水纤维非织造材料的吸液能力,而且也表明非织造材料加工过程中较粗的超吸水纤维有趋于表层的倾向,使得芯部表层孔隙多于内层；超吸水纤维非织造材料的孔径分布曲线呈单峰形态,说明加工中超吸水纤维与其它纤维混合均匀；超吸水纤维含量不同的非织造材料内部纤维取向度明显一致,表明梳理、铺网方式是决定纤维取向度的根本因素；热轧工艺对非织造材料粘结点的形成和分布影响明显。
②随着超吸水纤维含量增加,非织造材料小孔径概率降低,平均孔径以及最大最小孔径差异增大；实测孔径分布曲线与理论孔径分布形态基本一致,但幅度对比有差异。实测孔径总体小于理论孔径,说明非织造材料的热轧加工对孔径有影响
③由Poisson Polyhedron理论分析得到：在非织造材料厚度、纤维密度不变情况下：纤维细、材料面密度增大时,发生小孔径的概率增大。材料面密度对孔径的影响超过纤维细度的影响；在非织造材料面密度、纤维密度不变情况下：纤维粗、材料厚时,小孔径概率减小,孔径增大；材料薄,小孔径概率较大而且几乎不受纤维粗细影响；纤维细,小孔径概率较大而且几乎不受材料厚度影响。
④应用Poisson Polyhedron理论、Hagen Poiseuille定律和Darcy定律分析发现：超吸水纤维非织造材料的渗透系数随孔隙率增大而增大,即孔隙率提高,吸水能力增强；渗透系数衰减速度快于孔径因超吸水纤维吸水膨胀而缩小的速度。因此,在超吸水纤维含量变化带来孔隙率、孔径变化时,对非织造材料的吸水性也产生显著的影响。
4超吸水纤维非织造材料中液体扩散与流动过程分析
通过理论研究提出了一种表征非织造材料中液体扩散吸收特性的方法,基于图像处理技术、扩散吸水过程试验以及流动过程数值模拟技术,研究了水溶液在超吸水纤维非织造材料中的扩散吸收特征以及超吸水纤维非织造材料内部的流动状态。研究得到：
①液体在非织造材料中的扩散吸收特性可采用扩散特征曲线、扩散不匀曲线以及通过理论推导所得的降速和饱和两个特征点的数值给予有效评价。扩散曲线高度越低、扩散饱和时间越早、扩散不匀越明显,表明水溶液在超吸水纤维非织造材料中的扩散吸收能力越弱。
②超吸水纤维非织造材料的浅层扩散吸水具有前期扩散面积增长快、后期趋于稳定的指数函数形式增长特征。扩散吸水能力主要来自于内部多孔的扩散渗透吸收,超吸水纤维含量也有一定的影响。超吸水纤维含量与非织造材料扩散吸水能力之间的关系呈先增后减的上凸曲线关系。在扩散饱和阶段,扩散不匀明显,扩散带有明显的方向选择性,导致扩散吸收能力变弱。
③水流进入非织造材料后,水流速度越快越容易在较粗纤维(超吸水纤维)的背水面形成水流速度接近零甚至空化的区域,这种区域也许不利于超吸水纤维持续吸收流体中的水分。所以,提高超吸水纤维细度,有利于其在非织造材料中应用时更有效地吸水。
④水流进入纤维层前,流水速度脉动强烈,进入纤维层后,孔洞区域大的层面或粗纤维层面上流水速度脉动严重。因此,在流水脉动作用下,非织造材料结构破坏可能会先从表层、或存在较大孔洞层面、或粗纤维存在的层面开始。
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 fiber
The 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 fiber
Considering 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 fiber
The 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 fiber
A 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.
本课题将超吸水纤维与其它纤维混合使用,克服超吸水纤维力学性能差、后续难以加工的不足,开发了兼有优良吸水能力和较好力学性能的高吸水非织造材料,并采用方差分析、图像处理、数值模拟等技术手段,对超吸水纤维非织造材料的结构与性能进行了深入研究,旨在进一步拓展超吸水纤维在医用防护、卫生护理、溶液脱水、气体干燥、油水分离等领域的应用广度和深度,为继续开展非织造材料结构与性能的研究提供可以借鉴的理论与方法。本课题的主要研究内容与结论如下：
1超吸水纤维结构与性能研究
通过实验和理论研究,对超吸水纤维的结构形态、断裂强度、断裂伸长、吸水能力等基本性能进行了测试和理论分析,研究结果表明：
①超吸水纤维具有极强的吸水、保水能力,吸纯水倍率可高达180倍,采用不同温度、不同水溶液时,超吸水纤维吸水倍率从40到180不等。吸水倍率随水温升高以加速趋势逐渐降低,随溶液中Na+浓度增加以减速趋势逐渐降低。
②超吸水纤维截面呈圆形,干燥时表面光滑、平直,几乎无卷曲,吸水后具有显著的溶胀开裂特征,纤维直径会增加80%以上
③超吸水纤维断裂强度和断裂伸长均很小且离散性大,断裂强度<0.7cN·dtex-1,断裂伸长<2.4%,力学性能较差。
④超吸水纤维不能承受强烈的开松、梳理,可纺性不好,难以作为单一原料开发产品,需与其它纤维混合开发高吸水材料。
2超吸水纤维非织造材料制备及性能统计分析
考虑到超吸水纤维吸水速度快、吸水后膨胀、发粘的特点,以及考虑到用超吸水纤维制备的非织造材料可作为医用敷料、手术洞巾、手术铺巾等医用材料和其它场合的吸湿脱水材料,课题采用非织造热轧工艺,制备了上下层由丙纶纤维和双组分ES纤维组成,中间层由超吸水纤维、丙纶纤维和双组分ES纤维构成的三层结构超吸水纤维非织造材料,并对其性能进行了统计分析,结果表明：
①超吸水纤维含量始终以先增后减的非线性关系显著影响着非织造材料的吸水倍率。根据方差分析所得的因子显著性效应,可以95%的信度推断：当超吸水纤维含量15%(即芯层含30%)时,非织造材料对纯水的吸水倍率最高,可达10.185～15.901倍,远高于一般非织造材料的吸水倍率。
②吸水过程中,超吸水纤维非织造材料的吸水速度由快到慢变化。与最高吸水倍率不同的是超吸水纤维含量在7.5%-10%(即芯层含15%-20%)时超吸水纤维非织造材料具有较快的吸水速度。
③被吸的水温对超吸水纤维非织造材料的吸水倍率和吸水速度没有太大的影响,不过,当非织造材料中超吸水纤维含量较多时,水温升高会抑制其放热而不利于非织造材料吸水。
④超吸水纤维含量越高,非织造材料的耐温与耐压保水能力越强。含有超吸水纤维的非织造材料在受压状态下,稳定后的耐压保水率一般可达到最初吸水倍率的60%-70%。
3超吸水纤维非织造材料形态结构表征与分析
采用激光共聚焦显微镜对超吸水纤维非织造材料内部结构进行了观察,研究了超吸水纤维非织造材料的结构形态,特别是芯层结构特征,包括材料中纤维取向、孔隙率、当量孔径、孔径分布等,探讨了加工工艺、非织造材料面密度、纤维细度等因素对材料结构的影响以及结构对性能的影响,结果表明：
①超吸水纤维非织造材料的芯层孔隙率呈梯度结构,这种结构不仅表明流体在材料内外梯度压差作用下更容易由外向内流动,增强超吸水纤维非织造材料的吸液能力,而且也表明非织造材料加工过程中较粗的超吸水纤维有趋于表层的倾向,使得芯部表层孔隙多于内层；超吸水纤维非织造材料的孔径分布曲线呈单峰形态,说明加工中超吸水纤维与其它纤维混合均匀；超吸水纤维含量不同的非织造材料内部纤维取向度明显一致,表明梳理、铺网方式是决定纤维取向度的根本因素；热轧工艺对非织造材料粘结点的形成和分布影响明显。
②随着超吸水纤维含量增加,非织造材料小孔径概率降低,平均孔径以及最大最小孔径差异增大；实测孔径分布曲线与理论孔径分布形态基本一致,但幅度对比有差异。实测孔径总体小于理论孔径,说明非织造材料的热轧加工对孔径有影响
③由Poisson Polyhedron理论分析得到：在非织造材料厚度、纤维密度不变情况下：纤维细、材料面密度增大时,发生小孔径的概率增大。材料面密度对孔径的影响超过纤维细度的影响；在非织造材料面密度、纤维密度不变情况下：纤维粗、材料厚时,小孔径概率减小,孔径增大；材料薄,小孔径概率较大而且几乎不受纤维粗细影响；纤维细,小孔径概率较大而且几乎不受材料厚度影响。
④应用Poisson Polyhedron理论、Hagen Poiseuille定律和Darcy定律分析发现：超吸水纤维非织造材料的渗透系数随孔隙率增大而增大,即孔隙率提高,吸水能力增强；渗透系数衰减速度快于孔径因超吸水纤维吸水膨胀而缩小的速度。因此,在超吸水纤维含量变化带来孔隙率、孔径变化时,对非织造材料的吸水性也产生显著的影响。
4超吸水纤维非织造材料中液体扩散与流动过程分析
通过理论研究提出了一种表征非织造材料中液体扩散吸收特性的方法,基于图像处理技术、扩散吸水过程试验以及流动过程数值模拟技术,研究了水溶液在超吸水纤维非织造材料中的扩散吸收特征以及超吸水纤维非织造材料内部的流动状态。研究得到：
①液体在非织造材料中的扩散吸收特性可采用扩散特征曲线、扩散不匀曲线以及通过理论推导所得的降速和饱和两个特征点的数值给予有效评价。扩散曲线高度越低、扩散饱和时间越早、扩散不匀越明显,表明水溶液在超吸水纤维非织造材料中的扩散吸收能力越弱。
②超吸水纤维非织造材料的浅层扩散吸水具有前期扩散面积增长快、后期趋于稳定的指数函数形式增长特征。扩散吸水能力主要来自于内部多孔的扩散渗透吸收,超吸水纤维含量也有一定的影响。超吸水纤维含量与非织造材料扩散吸水能力之间的关系呈先增后减的上凸曲线关系。在扩散饱和阶段,扩散不匀明显,扩散带有明显的方向选择性,导致扩散吸收能力变弱。
③水流进入非织造材料后,水流速度越快越容易在较粗纤维(超吸水纤维)的背水面形成水流速度接近零甚至空化的区域,这种区域也许不利于超吸水纤维持续吸收流体中的水分。所以,提高超吸水纤维细度,有利于其在非织造材料中应用时更有效地吸水。
④水流进入纤维层前,流水速度脉动强烈,进入纤维层后,孔洞区域大的层面或粗纤维层面上流水速度脉动严重。因此,在流水脉动作用下,非织造材料结构破坏可能会先从表层、或存在较大孔洞层面、或粗纤维存在的层面开始。
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 fiber
The 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 fiber
Considering 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 fiber
The 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 fiber
A 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.
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