【作者】 黄益宾；

【导师】 柳和生；

【作者基本信息】 南昌大学 ， 材料加工工程， 2011， 博士

【摘要】 聚合物共挤成型是一种应用非常广泛的聚合物成型工艺,具有环保、成本低、制品性能优等特点,但也存在诸如挤出胀大、黏性包围及层间界面不稳定等问题,制约了共挤成型技术的进一步发展。多相熔体的黏弹特性差异以及非滑移黏着的剪切挤出机理是产生这些问题的根本原因。本文将共挤成型技术与气辅技术相结合,形成了全新的聚合物气辅共挤成型工艺,以期解决上述问题。通过实验和数值模拟相结合的方法对其进行了深入研究。首先,设计加工了一种采用缝隙进气的矩形截面气辅共挤口模,搭建了气辅共挤成型实验系统,并对形成稳定气辅共挤的条件、影响气垫膜层稳定性的因素以及气辅共挤出时各工艺参数对共挤出胀大、共挤界面黏性包围等的影响几个方面开展了实验研究,研究结果表明：(1)在气辅共挤出过程中气体的温度和压力是最为关键的两个因素,当进入气辅口模流道的气体温度和压力与熔体的温度和压力接近时才能形成稳定的气辅共挤出。(2)气辅共挤出制品的挤出胀大现象和黏性包围现象得到了极大改善,传统共挤出胀大率在35%以上,而气辅共挤出胀大率在3%以下,相比传统共挤,在口模出口处,气辅共挤界面的黏性包围程度减小30%以上；在螺杆转速较低的情况下(<10rpm),气辅共挤出产量增加约1.8%～3.8%,气辅共挤出和传统共挤出的制品外观质量无明显差别：(3)气辅共挤出胀大和黏性包围会随入口流率及流率比的增大而略微增大,但其增大的幅度远小于传统共挤。气辅共挤黏性包围程度受黏度比影响相对较大,随黏度比增大而增长,增长率与传统共挤基本相同。其次,讨论了建立聚合物完全滑移非黏着黏弹性口模共挤出数值模型的方法,分析了气辅共挤成型过程的边界条件,提出以壁面剪切应力为零来代替气垫膜层的作用,作为气辅共挤口模壁面的动力学边界条件。介绍了所采用的稳态有限元求解技术,给出了有限元计算的方法和技术路线。随后对矩形截面和L型截面气辅共挤口模内外两相聚合物熔体的成型流动过程做了有限元模拟,得到并分析了速度场、压力场、应力场及温度场等相关物理场量,明确了气辅共挤能消除挤出胀大,减小黏性包围程度并改善界面稳定性的根本机理。数值模拟并分析了工艺参数、材料参数等对气辅共挤出胀大、黏性包围和界面剪切应力的影响作用。主要结论有以下几点：(1)气辅共挤时两相熔体的速度场分布均匀一致,熔体呈柱塞状挤出,在口模出口处熔体表面的剪切速率、切向应力及法向应力均为零,气辅共挤流道内压降几乎为零,因而,气辅共挤能基本消除共挤出胀大,有效减小口模内的压力降,减小能量损耗,可大幅提高挤出速率,有效防止制品表面出现“鲨鱼皮”现象,从而全面提高共挤出制品的成型质量和生产效率。(2)气辅共挤出胀大不受熔体黏度值或黏度比值大小的影响,模外没有挤出胀大现象,气辅共挤出胀大率为零,这与实验结果基本相符。气辅共挤出胀大不受松弛时间影响,不受Giesekus本构参数α值影响。(3)气辅共挤界面稳定性优于传统共挤界面的主要原因为：a.气辅共挤层间界面上Y向和Z向速度值较传统共挤界面小,且其分布更为均匀,使得界面的推进和形成更为平缓；b.气辅共挤能有效改善界面上的剪切速率分布,减小剪切速率峰值；c.气辅共挤使得界面上大部分区域的剪切应力值趋近于零,在入口区域存在剪切应力,但其值远小于传统共挤；d.气辅共挤能有效减小界面上第一法向应力差,N1峰值减小可达27%。(4)气辅共挤出的黏性包围在口模入口汇料区域基本上即已完成,受到聚合物熔体黏度比、PTT本构参数ξ和流率比的影响较大,基本不受松弛时间和绝对流率的影响。气辅共挤界面剪切应力最大峰值在口模入口面上,随后迅速减小,在共挤流动大约10mm之后,τyz值趋于稳定,材料黏弹参数对τ、z峰值的影响也主要发生在口模入口区域,一般随黏度比增大而增大,随松弛时间增加而减小。无论是黏性包围还是界面剪切应力,对材料参数或工艺参数的敏感性均远小于传统共挤。(5)随着气体入口位置向口模入口方向推移,口模出口处的界面黏性包围程度逐渐减小,在口模入口和气体入口处,共挤界面上的剪切应力峰值也逐渐减小,也即随着气辅共挤区长度增加,界面稳定性也将得到提高。最后,提出了气辅共挤成型的工艺条件和气辅共挤出口模的设计准则。指出气体与熔体间压力差在0.1MPa以内,温度差在10℃之内时最利于形成稳定气垫膜层。降低流率值并使得两熔体流率保持一致,可减小气辅共挤出胀大率和黏性包围程度,增强界面稳定性。气辅共挤口模的进气缝隙厚度值取0.1mm～0.2mm,气辅流道长度设置为30～40mm较为合理。更多还原

【Abstract】 Polymer co-extrusion is a widely used molding technique due to environmental protection, low cost and high performance products. However, there are still some problems, such as co-extrusion swell, viscous encapsulation and interfacial instability. These problems seriously restrict the further development of co-extrusion. The primary causes are the viscoelasticity difference between multiphase melts and no-slip adhesive shearing extrusion mechanism. To solve these problems, the new gas-assisted co-extrusion technique is proposed in the paper by combining the co-extrusion and gas-assisted technique. The paper makes a thorough research on this new processing technique through experiments and numerical simulations.Firstly, the paper designs a rectangular profile gas-assisted co-extrusion die with gas inlet, and establishes the experimental system of gas-assisted co-extrusion. Then, the stable gas-assisted co-extrusion conditions, the factors affected the stability of the gas layer and the effects of processing parameters on co-extrusion swell and encapsulation in gas-assisted co-extrusion are investigated experimentally. The results show that:(1) In gas-assisted co-extrusion process, gas temperature and pressure are the two most critical parameters. Only when the gas temperature and gas pressure are equal to those of melts in the die can the stable gas-assisted co-extrusion be developed. (2) Extrusion swell and viscous encapsulation in gas-assisted co-extrusion are enormously decreased. The die swell ratio in traditional co-extrusion is over 35%, but in gas assisted co-extrusion, the die swell ratio is less than 3%. The degree of viscous encapsulation in gas-assisted co-extrusion is 30% less than that of traditional co-extrusion. On the same condition of low screw speed(less than 10rpm), the output of gas-assisted co-extrusion is 1.8%-3.8% more than that of the traditional co-extrusion. There is no obvious difference between gas-assisted co-extrusion and traditional co-extrusion in appearance quality. (3) In gas-assisted co-extrusion, the die swell ratio and the degree of viscous encapsulation increases slightly as the flow rate or its ratio increases. The viscosity ratio brings relatively great influence on the viscous encapsulation, the degree of encapsulation also increases with the viscosity ratio, and the growth rate is the same as traditional co-extrusion.Secondly, the method of building viscoelastic numerical model for full-slip flow in co-extrusion die is discussed. The boundary conditions are analyzed, and the shearing stress on the wall of co-extrusion die is set to be zero to replace the function of gas layer, which is used as the kinetic conditions of the wall. The key solution techniques of FEM adopted by the paper are studied, and the whole routine to solve the finite element analysis of 3D viscoelastic co-extrusion flow is presented. Then, the paper performs FEM simulations for the flow of polymer melts inside and outside the gas-assisted co-extrusion die with rectangle and L shaped section, obtains the physical fields such as velocity field, pressure field and stress field. Based on the penetrating analysis of the physical fields, the paper reveals the fundamental mechanism that explains why gas-assisted co-extrusion can eliminate die swell, reduce the degree of encapsulation and improve the stability of the interface. The influence of process parameters and material parameters on die swell, viscous encapsulation and shear stress of interface in gas assisted co-extrusion is simulated and analyzed. The main conclusions are presented as follows:(1) In gas-assisted co-extrusion, the distribution of two melts’velocity field is uniform and the melts are extruded like a rod; The shear stress, tangential stress and normal stress of melt surface in die outlet are all zero, the pressure drop in the die channel is almost zero, so gas-assisted co-extrusion can eliminate co-extrusion swell, reduce the pressure drop and energy loss in the die effectively. The extrusion speed can be improved greatly and the "sharkskin" on the surface of the polymer parts can be prevented effectively. Thus, gas-assisted co-extrusion can improve quality and efficiency of the co-extruded parts comprehensively. (2) In gas-assisted co-extrusion the die swell is not affected by the viscosity or viscosity ratio of melts. The swell ratio of gas-assisted co-extrusion is zero, and there are no extrusion swell outside of the die, which agrees with the experimental results. The co-extrusion swell is also not affected by relaxation time and parameter a of Giesekus constitutive equation. (3)The interface stability of gas-assisted co-extrusion is superior to that of traditional co-extrusion, the primary reasons are presented as follows:a. On the gas-assisted co-extrusion interface the velocity of in Y and Z axis direction is less than that of traditional co-extrusion, and is distributed uniformly, so that the co-extrusion interface advances and developes gently; b. In gas-assisted co-extrusion, the distribution of shear rate in interface can be improved effectively and the peak value of shear rate can be decreased; c. In the gas-assisted co-extrusion shear stress in most area of interface approaches to zero, although there is shear stress at the die entrance, the value is far less than that of traditional co-extrusion; d. The normal stress difference at the interface can be decreased effectively, peak value of N1 decreases by 27%. (4) In the gas-assisted co-extrusion the viscous encapsulation is almost completed at the entrance of co-extrusion die where two polymers converge. The viscous encapsulation is affected by polymer viscosity ratio,ξvalue of PTT constitutive equation parameter and the ratio of two melts’flow rates. It is almost not affected by relaxation time and flow rate. The peak of shear stress of the interface appears at the entrance surface, then, decreases rapidly. The value ofτyz tends to be stable when the melts flowed as far as about 10mm. The influence of material parameters on peak value ofτyz, which increases with viscosity ratio but decreases with relaxation time, occurs at the entrance. For viscous encapsulation or the shear stress, the sensitivity to material parameters or process parameters are both far less than that of traditional co-extrusion. (5)With the gas inlet moving towards die entrance, the viscous encapsulation degree at the die entrance decreases gradually. At the die entrance and the gas inlet, the peak of shear stress on the co-extrusion interface also decreases gradually. That means the stability of the interface improves as the length of the gas-assisted co-extrusion increases.Finally, the process conditions for gas-assisted co-extrusion and the principles for designing the gas-assisted co-extrusion die are proposed. It is pointed out that the stable gas layer can be propitiously formed when the pressure difference between gas and melts is within 0.1MPa and the temperature difference is within 10℃. Reducing the flow rate of melts and keeping them same can decrease the co-extrusion swell and viscous encapsulation and improve the stability of the interface. For gas-assisted co-extrusion die, the proper thickness of gas inlet is 0.1mm-0.2mm, the proper length of gas-assisted sect is 30mm-40mm.更多还原