聚合物挤出过程中的数值模拟技术

介绍了计算流体力学中常用的数值计算方法,并从螺杆挤出过程、口模的设计以及挤出胀大等方面综述了数值模拟技术在聚合物挤出过程中的应用,最后针对聚合物挤出过程数值模拟发展的方向作了简要论述。     

高分子熔体和浓溶液流变性能的研究

基于多重缠结网络结构模型和高分子链上缠结点在流动中可进行动态解缠和再缠结的多重蠕动机理,用统计力学和动力学相结合的方法,分别计算出了缠结链组的末端距分布函数;处于缠结状态下高分子链构象统计分布函数;受力下聚合物熔体粘弹性形变自由能和解除外力下高分子挤出体可回复性粘弹性形变自由能,提出了高分子挤出体可回复形变的粘弹性分子理论。推导出的高分子熔体的回忆函数、简单剪切流下的本构方程和物料函数,并采用一种新的方法测定出物料的四种参数： η0、 GN0、 n′和 a。对于高分子挤出体,可回复性粘弹性形变由快速弹性形变和慢速粘弹性形变两者组成,当把两种形变量的复合结构参数－分子链的反式构象分数引入两种形变自由能表达式后,就从理论上得到了可回复形变量同挤出胀大比间的定量表达式,从而建立起一个具有分子链结构参数的新的挤出胀大比方程,可回复形变量同挤出条件（温度、挤出速率和量以及口模长径比不同的挤出机）和树脂结构特征（分子量及分布）的关系式以及在特殊情形下的简化表达式,并用几种高分子熔融体系的挤出胀大比和可回复性形变量的实验数据对理论进行验证,理论方程同实验数据较好的符合。     

芯棒与口模反向旋转挤出聚乙烯管结构与性能的研究

采用自行研制的新型旋转挤出装置,通过芯棒与口模同时反向旋转挤出制备聚乙烯(PE)管.结果表明,在口模与芯棒反向旋转挤出过程中,管道内外壁除受到轴向应力作用外,还受到芯棒和口模旋转所施加的环向应力,其合力方向不再是沿管道轴向而是与其有一定的角度.因此,分子链的取向方向和以此链为初级成核点形成的串晶偏离轴向,增强了管道抵抗...     

一个新的挤出胀大方程

本文对粘弹性流体在应力消除后的自由弹性回复进行了分析,提出一个新的挤出胀大方程,把挤出胀大比和可回复剪切应变联系起来,并与Graessley等和Tanner的理论以及实验数据作了比较。     

线型与支化聚烯烃熔体高速挤出时的不稳定扰动源

采用恒速型双毛细管流变仪对比研究线型与支化聚烯烃熔体在高速流场中的流动曲线、挤出畸变、挤出压力变化及粘弹性的特征,分析讨论了引发熔体不稳定流动的扰动源位置及扰动性质.结果表明,高速流场中的扰动源有:口模入口区的扰动、口模壁处的扰动、口模出口区的扰动.支化聚合物易受入口区的扰动干扰,造成挤出物无规破裂;线型聚合物易受口模壁处的扰动干扰,造成挤出压力振荡和挤出物有规畸变;鲨鱼皮畸变主要由于口模出口区的振荡扰动造成.     

聚合物口模挤出流动分析方法

综述挤出口模中聚合物熔体流动分析方法。着重讨论了包括有限差分法、有限元法和边界元法等在内的数值方法。并且对挤出口模流道几何形状简化和数学模型简化进行了评价     

聚四氟乙烯纤维加工技术北大核心CSCD

汇总了膜裂、乳液纺丝、糊料挤出等三种聚四氟乙烯(PTFE)纤维成型方法。研发了强剪切挤出口模等六项技术控制裂膜纤维的细度、均匀性、力学性能、热收缩率,形成工业化裂膜长丝和短纤加工技术;提出PTFE/PVA(聚乙烯醇)共混、利用硼酸与PVA的络合特性制备凝胶纺丝液并纺制PTFE短纤的思路,研究了烧结和牵伸对纤维的影响,形成凝胶法PTFE短纤加工技术;根据PTFE树脂剪切作用下原纤化特征,设计了导入口、导入段锥度和毛细孔长径比控制初生纤维强度的糊料挤出口模,筛选了润滑剂,形成糊料挤出PTFE长丝加工技术。     

丙烯-乙烯共聚物/乙烯-1-辛烯共聚物层状复合材料的力学性能研究

利用自主设计的一套可进行微层共挤出的口模,分别制备了2层、16层、32层和64层丙烯-乙烯共聚物(PPE)/乙烯-1-辛烯共聚物(POE)交替层状复合材料。研究了制得的层状复合材料的应力-应变行为,利用等效盒子模型(equivalent box model)描述了层状复合材料与相应的常规PPE/POE共混材料力学行为的区别.通过对共挤出材料和共混材料的拉伸数据进行分析后发现,具有层状结构特别是多层结构的共挤出材料具有比共混材料更为优异的屈服和断裂伸长性能.     

浆态床反应器中甲醇合成反应的数学模拟

为了对低温液相甲醇合成反应的模试及进一步的工业应用提供一定的预测和参考,用搅拌釜中得出的低温液相甲醇合成反应的动力学方程,经过合理的反应器模型假设,对低温甲醇合成鼓泡浆态床反应器进行了数学模拟,其中的非线性常微分方程组,采用变步长四阶Runge-Kutta法进行数值计算,结果表明,模试操作条件下对CO转化率和H2转化率的模拟计算结果与实际的模模试值的误差分别为13％和4％,模拟结果与模试实验结果比较相符;对不同条件下的反应速率和转化率进行了模拟预测。     

不同管径聚乙烯管的旋转挤出

通过口模旋转挤出制备3种管径聚乙烯(PE)管,系统研究其结构与性能。结果表明,相较于传统挤出PE管内串晶平行与轴向,旋转挤出过程中聚合物熔体的流动是轴向牵引流动和环向拖曳流动的叠加,其方向偏离轴向,可诱导串晶偏离轴向排列,从而提高PE管的环向取向度,实现PE管的环向增强,抑制裂纹在PE管内沿轴向扩展。随PE管管径的增加,在相同旋转角速度下环向流动线速度增大,串晶偏离轴向的夹角增加,环向取向度更高,因而旋转挤出制备的大口径PE管具有更优的性能。     

Dynamic Theory of Die Swell for Entangled Polymeric Liquids in Tube Extrusion: Correlations of Total and Ultimate Extrudate Swell Effects to Growth Time,Shear Stress and Aspect Ratio Under the Free States

The dynamic theory of die swell deduced in a previous paper was extensively applied to study the xtrudate swelling behaviors of two entangled polymeric liquids (HDPE and PBD) in a simple shear flow at steady shear stress. The mechanism and dynamics for the recoils and the recoveries of viscoelastic strains in the extrudate were investigated under the free recovery and dynamic states. It was found that in the course of recovery the free recoil and the growth of die swell in the extrudate may be divided into two recovery regions (instantaneous and delayed regions) and three growth stages (instantaneous, delayed, and ultimate extrudate swelling stages). The free recoil and the extrudate swelling behaviors may be expressed as a function of shear stress. The correlations of instantaneous, delayed, total and ultimate extrudate swell effects to the molecular parameters and the operational variables in the simple shear flow at steady shear stress were derived from the dynamic theory of die swell. Also, two sets of new universal equations on the total extrudate swelling effect (TESE) and ultimate extrudate swelling effect (UESE) were deduced. The first is the universal equation of the logarithmic correlation between the TESE and the growth time under the free and dynamic states; the second is the universal equation of the logarithmic correlation between the UESE and the operational variables under the free and equilibrium states. The first equation was verified by experimental data of PBD with different molecular weights at different operational variables. The second equation was verified by experimental data of HDPE at two temperatures and different operational variables. An excellent agreement result was obtained. The excellent agreement shows that the two universal equations can be used directly to predict the correlations of the TESE and UESE to the growth time, the molecular parameters, and the operational variables under the dynamic and equilibrium states.     

Extrudate swell and flow analysis of polystyrene melt flowing in an electro‐magnetized die in a single screw extruder

An electro‐magnetized capillary die via a parallel co‐extrusion technique was used to study the changes in the overall and radial extrudate swell ratio of polystyrene (PS) melt flowing in a single screw extruder. The effects of magnetic flux density, wall shear rate (screw rotating speed) and die temperature were studied. The results suggested that, in the case of non‐magnetic die the average overall swell ratio of the melt ranged from 1.25 to 1.55. The swelling ratio increased with increasing wall shear rate up to 8.5 sec^?1 and then decreased at 17.1 sec^?1. Increasing die temperature caused a reduction of extrudate swell ratio. The changes in extrudate swell ratio can be explained using the simultaneously measured velocity profiles during the flow in the die, and the swell ratio decreased with increasing radial position. Melt contraction of the melt layer near the die wall was observed. The die temperature was found to have no effect on the change of the radial extrudate swell profiles. When an electro‐magnetized die was used, the average overall swell ratio was found to increase with increasing magnetic flux density to a maximum value and then decreased at higher flux densities. The magnetic flux density of the maximum swell was changed by the wall shear rate. It was associated with a balance of elastic and magnetic energies during the flow. The magnetic energy was thought to have a pronounced effect on the swell ratio at low shear rate and low die temperature. Considering the radial position, the highest swell ratio occurred at the duct center, in the range 2.4–3.3. There was no extrudate contraction of the melt layer near the die wall. Copyright © 2005 John Wiley & Sons, Ltd.     

Extrudate swell behavior of PS and LLDPE melts in a dual die with mixed circular/slit flow channels in an extrusion rheometer

Extrudate swell behaviors of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts in a dual channel die, having mixed circular/slit flow channels, in a constant shear rate rheometer were examined. The extrudate swell ratio for PS melt was observed to be higher than that for LLDPE melt for all cases, this being associated with the differences in molecular structures that could be described in terms of power law indexes and secondary flows near the die entrance. In single channel die, the extrudate swell of both PS and LLDPE melts in circular flow channel die was greater than that in slit flow channel, whereas, in dual channel die the slit channel exhibited a higher extrudate swell ratio, the results being explained by revealing the flow patterns of the melt in the barrel and die of the rheometer. It was found that the dimensionless size of the vortex flows near the entrance, and the extent of disentanglement of molecular chains on entering the die were the important factors for the differences in the extrudate swell ratios of the melts at the die exit influenced by the die designs used. Copyright © 2003 John Wiley & Sons, Ltd.     

Computer‐Aided Optimization of the Extrusion Process of Automobile Rubber Seal

In this work, the extrusion process of a kind of π‐shape automobile rubber seal was considered using computer‐aided simulation technology. The extrusion flow was assumed to be isothermal and steady, and the finite element method was used to analyze the extrusion process. It was found that the velocity profile was quite uneven near the die exit when a straight die, developed strictly according to the desirable product dimension, was used and the local distortion of the extrudate was fairly prominent when compared with the die geometry. We adjusted the structure of the die and predicted the extrudate's swell by the computer simulation in advance. Simulation results confirmed that the distortion of the seal was greatly suppressed if a short enlarged inflow part was added to the upstream of the original straight die. Then, the extrusion experiment, using the newly designed die, was performed under practical conditions and the swell of the rubber seal was reduced to be within the range of acceptable tolerance. The profiles of the acceptable rubber seals were also well predicted by the computer simulation. Finally, the influences of the take‐up imposed on the end of extrudate and flow rate on the final seal shape were also investigated numerically.     

Dynamic Theory of Die Swell for Entangled Polymeric Liquids in Tube Extrusions: New Set of Equations for the Growth and Ultimate Extrudate Swelling Ratios under the Free States

A new dynamic theory of die swell for entangled polymeric liquids in a steady simple shear °ow is proposed which can be used to predict the correlation of the time-dependent and time-independent extrudate swelling behaviors to the molecular parameters of polymers and the operational variables. The theory is based on the O-W-F constitutive equation and the free recovery from Poioeuille flow with different ratios. Experimentsshow that the magnitudes of the simple shear in the steady simple shear flow may be resolved into the free recoil resulting from the recoverable elastic strains and the viscous heating resulting from the unrecoverable viscoelastic strains. For distinguishing the recoil from the viscous heating, a partition function and twoexponential fractions of conformation for the recoil and the viscous heating were defined. Thus the instantaneous, delayed and ultimate recoverable strain, and recoil in the free recovery were correlated to the partition function, the fraction of recoverable conformation, the molecular parameters, and the operational variables. Also the dynamics of the growth equation on the delayed viscoelastic strain and the delayed recoil in freestate were deduced. After introducing the condition of uniform two dimensional extensions, the definition ofswell ratio and the operational variables into the above correlation expressions and growth equations, then the correlations of the delayed extrudate swelling effect and the ultimate extrudate swelling effects to the molecular parameters and the operational variables were derived. Finally, two new sets of equations on the growth variables and ultimate extrudate swelling ratios under the dynamic and equilibrium states were also deduced from this dynamic theory of die swell. The first set of equations on the ultimate extrudate swelling ratio under the free and equilibrium states was verified by HDPE experimental data at two temperatures and different operational variables. The second set of equations on the growth extrudate swelling ratios under free and dynamic states was verified by PBD experimental data with different molecular weights and different operational variables. An excellent agreement is obtained, which shows that the two sets of equations for the growth and ultimate extrudate swelling ratios can be used directly to predict the correlation of extrudate swelling ratios to the molecular parameters and the operational variables.     

Melt extrudate swell behavior of graphene nano-platelets filled-polypropylene composites

The extrudate swell ratios of polypropylene (PP) composite melts filled with graphene nano-platelets (GNPs) were measured using a capillary rheometer within a temperature range of 180–230 °C and apparent shear rate varying from 100 to 4000 s⁻¹ in order to identify the effects of the filler content and test conditions on the melt die-swell behavior. It was found that the values of the extrudate swell ratio of the composites increased with increasing apparent shear rate, with the correlation between them obeying a power law relationship, while the values of the extrudate swell ratio decreased almost linearly with rise in temperature. The values of the melt extrudate swell ratio increased approximately linearly with increasing shear stress, and decreased roughly linearly with an increase of the GNP weight fraction. In addition, the extrudate swell mechanisms are discussed from the observation of the fracture surface of the extrudate using scanning electronic microscopy. This study provides a basis for further development of graphene reinforced polymer composites with desirable mechanical performance and good damage resistance.     

经不同程度降解聚丙烯的熔体流变性质

<正> 聚丙烯(PP)的热降解和化学控制降解法七十年代开始应用于聚合物工业加工过程。聚丙烯在高温下热降解,一般认为服从无规断链机理。随降解过程进行,分子量变小,分子量分布变窄,由定向聚合反应产生的特征性高分子量尾端降解最为显著。该过程往往可显著改善PP熔体加工性,因此研究降解过程对PP熔体流变性质的影响殊属     

Rheological characterization of the die swell phenomenon of rubber compounds

The die swell phenomenon of rubber compounds in capillary experiments with various ratios of length to diameter of capillaries is investigated. This knowledge is important for the design of injection heads for the extrusion of rubber profiles. The die swell of viscoelastic rubber compounds depends on the geometry of the capillary dies, on the melt temperature and on the shear strain rate. One empirical relationship will consider all these dependencies. Usage of this equation and identification of only one new material parameter enables the comparison and assessment of the die swell of different materials, independently of the corresponding geometry of the capillary die used. Furthermore, the influence of melt temperature, molecular structure and extrusion process on the die swell can be identified. The investigation was performed with various rubber compounds as well as rubber blends used in industry, mainly EPDM and carbon black in different compositions.     

Flow behaviour of gas-containing LDPE/i-PP melts

An experimental study was conducted to investigate the rheological behaviour and extrudate swell of polyolefin blends based on two grades of low-density polyethylene (LDPE) and an isotactic polypropylene (i-PP). Blending was carried out on a twin-screw extruder “Brabender” at different composition ratios in the temperature range from 140 to 190°C. The LDPE/i-PP blends mixed with 0.5 wt.% blowing agent were extruded by means of “Brabender” extrusiograph at melt temperature of 200°C and different extrusion rates. The influence of composition content on the viscosity and extrudate swell was considered. The foam structure and morphology are discussed in terms of shear rate, molecular characteristics and composition content. The presence of layered structure was observed: an outer smectic layer and an inner partially crystalline layer. The thickness of smectic layer and size of spherulites were determined.