【作者】 韩万里；

【导师】 王新厚；

【作者基本信息】 东华大学 ， 纺织工程， 2014， 博士

【摘要】 近年来,随着工业的飞速发展及对环境保护的加强,熔喷非织造布市场需求不断增长,熔喷非织造技术得到了迅速发展。熔喷纤维细度一般为2-5个微米,在医用材料、精细过滤材料、吸油材料、保暖材料、电池隔板等各个领域得到了广泛应用。目前熔喷技术发展的两个最重要趋势是熔喷设备的宽幅化和熔喷纤维的纳米化。本文针对这两方面进行了相关研究,其主要内容包括熔喷衣架型模头的多目标优化、宽幅模头的设计、熔喷纳米纤维的制备、非织造布性能的测试、纳米纤维成形机理的研究等。本论文首先对熔喷衣架型模头内的聚合物熔体流动进行三维数值模拟,通过分析出口速度和滞留时间的分布特点,指出两者将会显著影响熔喷非织造最终产品的性能。在此基础上,以获得最小出口速度CV值和最短滞留时间为目标,对单衣架型模头的几何参数进行了正交设计优化,找出影响出口速度CV值和滞留时间的显著性因素,得到了优化的衣架型模头；基于此结果,采用遗传算法,在参数变量的全局定义域内进行了优化搜素,进一步对衣架型模头进行了多目标优化。为实现模头的宽幅化,我们提出了两种宽幅衣架型模头设计方法,第一种方法是双衣架型模头的拼接方法,采用解析法求出双衣架型模头歧管的拼接形状方程,并在成型面区内引入第二椭圆歧管,获得了幅宽为3.4m,出口速度CV值小于1%的双衣架型模头。第二种方法是设计了幅宽为4m的多歧管模头,同时分析和比较了多种聚合物在该宽幅模头内的分配状况,结果显示该模头具有耗能小,对多种聚合物适应性强等优点。另外,本文在熔喷工业生产条件下,采用多种工艺成功制备了微米纤维和纳米纤维,测试了微、纳米非织造纤网性能并对两者进行了比较,指出纳米熔喷纤网在孔径大小,透气率和静水压力等性能方面具有独特优点；最后,本文对熔喷纳米纤维制备过程中存在的Rayleigh不稳定现象建立了数学建模,并设计熔喷实验对其进行了验证,阐明了该模型的正确性。最后通过对比熔喷纳米纤维工艺和静电纳米纤维工艺的相同和不同点,初步探究了熔喷纤维纳米化存在的障碍和极限问题。全文共分六章。第1章对国内外关于熔喷技术领域的相关理论和实验研究方面的文献进行了综述,主要涉及衣架型模头优化和设计、熔喷气流场、纤维拉伸模型以及纤维制备工艺的研究。第2章对衣架型模头内聚合物流场进行了数学建模,以聚合物熔体出口速度CV值和滞留时间为目标函数,采用数值模拟方法对工业用衣架型模头的几何参数进行了多目标优化。通过正交设计方法,选取歧管角度、成型面高度和狭缝区宽度三个因子对目标函数的影响进行了研究。结果显示选取的三个因子对流体的出口速度影响显著,而只有歧管角度和狭缝区宽度两因子对滞留时间有影响,但影响作用较小;同时只有歧管角度和成型面高度之间的交互作用对出口速度有一定的影响,其显著性也较小。通过正交设计实验,确定了影响目标函数的显著性因子,得到了优化的衣架型模头几何参数,该优化模头的最终出口速度CV值为7.7%,滞留时间为292s。随后采用多目标遗传算法在变量参数全局定义域内进行优化搜索,进一步改善了模头中成型面高度和狭缝区宽度两因子在目标函数优化过程中的制约关系。通过并列遗传算法和目标规划统一函数法求得了目标函数的最优解,获得了衣架型模头的最优几何参数,使衣架型模头的出口速度CV值降低为5.1%,滞留时间下降为169s。第3章设计了宽幅模头,根据解析方程求得双衣架型模头拼接位置处的歧管形状曲线,通过插入第二歧管和多歧管模头设计等方法,获得了出品速度均匀的宽幅模头。在第2章单衣架型模头优化过程中发现,歧管对聚合物流体在出口横向均匀分配方面起着至关重要的作用。本章通过对歧管内聚合物流体进行数学建模,求解出双衣架型模头拼接处歧管的形状解析方程。模拟结果显示该拼接方法有效的解决了简单拼接模头中间位置处存在的凸状高速区域,获得了更均匀的出口速度,使幅宽为3.4m的双衣架型模头出口速度CV值降至10%以下。基于双衣架设计基础,在成型面里引入第二椭圆歧管,使其对聚合物进行了第二次微调分配,进一步降低了出口速度的CV值。另外经过第二歧管的调节,模头内的流体压力降减小,可以降低能耗。另外根据歧管的分配作用,提出了一种多歧管宽幅模头的设计方法。通过多级歧管对聚合物流体层层分配,有效地减小了出口处速度CV值和模头内的压力降；同时该模头避免了双衣架型模头在生产过程中需要两个计量泵为其供料的弊端,而且对多种物料流体具有较好的适应性,实现了熔喷非织造分配流道的宽幅化设计。第4章主要是在工业生产条件下,制备了微米和纳米熔喷纤维,并对其纤维直径及其分布,纤网性能特征进行了测试和比较。首先在熔喷工业生产条件下,采用多种工艺参数和不同喷丝板制备了微米和纳米熔喷非织造布,详细讨论了工艺参数对熔喷纤维直径及其分布的影响。结果显示热空气压力、热空气温度的增加有利于纤维牵伸细化；工业生产条件下采用多孔喷丝板制备的熔喷纤维直径分布为正态分布,不同于实验室条件下采用单孔制备的熔喷纤维直径的指数正态分布。纳米喷丝板制备的熔喷纤维平均直径可达600nm-800nm。其次,采用不同的实验仪器,对纤网的密度、孔径分布、空气透气率、静水压和断裂伸长率等性能进行了测试和比较。通过对比微米熔喷纤网和纳米熔喷纤网,发现纳米纤网密度较小,但其透气率和断裂伸长率与微米纤网相近。另外,两种熔喷纤网的平均孔径都随着纤维直径的减小而显著降低,分布也更均匀,但纳米纤维表现更突出。第5章主要是对在工业生产条件下,采用多孔喷丝板制备纳米熔喷过程中出现的Rayleigh不稳定现象进行了理论分析和实验验证,同时比较了熔喷纳米纤维和静电纳米纤维制备过程中的异同现象,初步探讨了熔喷纤维牵伸细化过程中存在的障碍和极限问题。本章对熔喷纺丝过程中纤维细度牵伸到纳米级后出现的Rayleigh不稳定现象进行了数学建模和分析,指出纤维直径,空气温度和聚合物粘度对此现象影响显著。当纤维直径牵伸至纳米级后,其表面张力激增,Rayleigh不稳定现象加剧而导致纤维断裂,从而形成圆形颗粒;通过熔喷实验,通过不同粘度的聚合物,不同生产工艺条件制备了纳米纤维,结果验证了理论模型的正确性。另外通过高速摄影仪抓拍纤维在熔喷和静电生产过程中的运动轨迹,指出熔喷纤维运动路径远小于静电纤维的运动路径,同时熔喷纤维所处的气流场比静电纤维所处的电场均匀性差。熔喷纤维所受的气流牵伸力衰减速度要比静电纤维所受的电场力衰减速度快,基于以上原因,在同样的接收距离条件下,熔喷纳米纤维最终直径要比静电纺纤维最终直径大,目前在工业生产条件下制备的熔喷纤维最终平均直径难以突破500nm。第6章是全文的结论和展望。对本文研究工作中所取得的成果进行了总结,对所存在的不足进行了阐述,并建设性地指出了下一步的研究方向。更多还原

【Abstract】 In this thesis, multi-objective optimization of the coat-hanger die, the design of wide coat-hanger die, fabrication of melt blowing nanofiber, properties of melt blown web structure and mechanism of nanofiber formation in melt blowing were investigated. First, the polymeric fluid flow in coat-hanger die was simulated using the three-dimensional finite element method. The outlet velocity and the residence time distribution were analyzed, which could affect the quality of melt blown nonwoven fabric. A method combining the orthogonal array design and the numerical simulation is used to optimize the geometry parameters of the single coat-hanger die with uniform outlet velocity and minimal residence time. The significant factors were obtained and they were used for optimization of coat-hanger die with the multi-objective genetic algorithm method in the global domain. Then, two different methods were proposed for the design of wide coat-hanger die. A simple analytic method was developed for the double coat-hanger die and an ellipse cavity was inserted in the double coat-hanger die slot. The result showed that the outlet velocity CV value was less than1%for3.4meter width of double coat-hanger die. Meanwhile, multi-manifold width distribution was designed for different polymer melts. The uniform outlet velocity and samll pressure drop were obtained.Melt blowing micronfibers and nanofibers have been produced using different multi-holes dies from available polymers under commercial process conditions. The different properties between microfiber webs and nanofiber webs were compared.The results showed that the smaller pore size, the higher air permeability and the larger hydrohead were achieved for nanofiber webs, which fiber average diameter was less than one micron. Finally, the Rayleigh instability theory of melt blown nanofiber was established, which illustrated the reason of fiber breakup.Both theory and experimental evidenced that surface tension influenced the fiber breakup. Besides, by comparing similarities and differences of melt blowing and electrospinning, this research try to provide a general understanding of limit on barrier of nanofiber from melt blowing technology.This thesis contained6chapters.In chapter1, the research work refering to the theory and experimental of melt blowing technology at home and abroad was reviewed.They are mainly focused on design and optimization of coat-hanger die, airflow field of melt blowing, fiber drawing model of melt blowing and the research of melt blowing process.In chapter2, the multi-objective methods based on the numerical simulation of the polymer flow was proposed to optimize the geometry parameters of the coat-hanger die with uniform velocity and minimal residence time.A method combining the orthogonal array design and the numerical simulation was used and the effects of the manifold angle, the land height and the slot gap on the outlet velocity and the residence time were investigated.The results showed that the effects of all the three parameters were significant for the outlet velocity while manifold angle and slot gap were significant for the residence time.Besides, the interaction between manifold angle and slot gap influenced the outlet velocity. The significant factors were determined and the optimal geometry parameters of coat-hanger die were obtained using orthogonal array design. The CV value of outlet velocity and the residence time decreased to7.7%and292s respectively. Then, the vector evaluated GA (genetic algorithm) method was further used to find the parameter values for uniform outlet velocity and minimal residence time in global domain.The relationship between land height and slot gap was improved.The optimal geometry of coat-hanger die was obtained using goal programming function. The CV value of outlet velocity and the residence time of optimal coat-hanger die were5.1%and169s respectively.In chapter3,the wide coat-hanger die was designed based on the modification of manifold connected,the inserted second manifold and multi-manifold.The uniform outlet velocity was achieved. Based on the previous study, the simple connected manifold for the wide coat-hanger die was not able to satisfy the requirements of melt blowing practical production.In this chapter, a numerical approach was developed for optimal design of double coat-hanger die. The flow convection zone at the center of simple connected double coat-hanger die was improved. And then, the second manifold was inserted in the double coat-hanger die slot for the melt flow distribution. The result of outlet velocity CV value was under1%for3.4meter width of double coat-hanger die and the melt pressure drop was also decreased. In addition, a multi-manifold design method was proposed for the wide coat-hanger die. Both the CV value of outlet velocity and pressure dropt were decreased with the multi-manifold’s distribution. At the same time, the multi-manifold coat-hanger die could reduce drawbacks which might be generated by two metering pump for feeding polymer melt. A variety of polymer distributions were adapted in the multi-manifold wide coat-hanger die.In chapter4, melt blown microfibers and nanofibers have been produced using different multi-holes dies under commercially processing conditions. Mean fiber diameter along with fiber diameters distribution were studied. The properties of melt blown web structure were tested and compared between microfiber webs and nanofiber webs.The results showed that the fiber diameter decreased with the air pressure, air temperature increasing. The fiber diameter distribution which produced by multi-hole dies was normal distribution, which was different from the one observed in a single hole melt blowing process. The average of melt blowing fiber diameter was from600to800nm. The properties of melt blown webs were tested and investigated using instruments, such as density of webs, pore distribution, air permeability, hydrohead and elongation at break. Nanofiber web’s density was lower but the air permeability and elongation at break was similar with microfiber web’s. The web’s pore size decreased and became more uniform with the fiber diameter decreasing, especially for nanofiber webs. Meanwhile, more uniform nanofiber diameter was obtained using the new designed melt blowing die.In chapter5, the nanofiber breakup was observed which was produced by multi-holes dies with different commercially processing conditions.The Rayleigh instability theory of melt blowing was established and illustrated the reason of fiber breakup. The experimental results revealed that polymer viscosity, fiber diameter and the melt blowing process conditions, such as air pressure, air temperature, significantly influenced the fiber breakup.Besides, a high-speed camera was used to capture the fiber path below a single-hole melt blowing slot die and electrospinning process.The air flow field and electric field for fiber drawing were simulated and analyzed.The results showed that melt blowing fiber whipping amplitude was smaller than electrospinning’s.The electric field for electrospining fiber was more uniform while the drawing force for melt blowing attenuated quickly in the same collect distance.Comparing with melt blowing, process and electrospinning process, a general understanding of limit on the nanofiber for commercial melt blowing was explored and the results showed that the fiber diameter of melt blowing was hard to reach500nm under the commercial melt blowing conditions.In chapter6, conclusions and outlooks were presented.Main research findings and insufficiencies of this thesis were summarized. Meanwhile, the further research points involved in this field was described.更多还原