茂金属催化线型低密度聚乙烯的奇异流变性质—现象、机理及对策
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摘要
本课题为国家自然科学基金资助项目,研究焦点主要集中在探求聚合物熔体(主要为聚烯烃熔体)高速挤出时发生的极其复杂的不稳定流动现象的规律、机理和对策。实验采用恒速型双筒毛细管流变仪等仪器,研究对象为DOW化学公司生产的INSITETM工艺开发的牌号为ELITE5100G和ELITE5500G两种茂金属催化的线型低密度聚乙烯(m-LLDPE)。
研究结果表明,两种m-LLDPE熔体均为典型非牛顿流体,ELITE5500G黏流活化能较高,而ELITE5100G的入口压力降和挤出胀大比略大。随挤出速率提高,两种m-LLDPE的挤出物外观都经历了从有规畸变(鲨鱼皮畸变、黏-滑畸变)到无规畸变(挤出物整体扭曲、熔体破裂)的变化。在黏-滑畸变区,挤出压力出现振荡,流动曲线断裂。相同挤出温度下,ELITE 5500G的稳定挤出速率范围大,出现鲨鱼皮畸变的临界剪切速率(720s-1)高于ELITE5100G(174s-1)。压力振荡幅度大,流动曲线断裂后的斜率更小。尤为显著的是ELITE5500G在经历黏-滑转变后出现了第二光滑挤出区。第二光滑区对于高速、节能、安全生产具有重要的潜在意义。流动行为的差别与两种样品结构差异有关。
不同的畸变现象源自不同的扰动源和扰动机理。较低剪切速率下的有规鲨鱼皮畸变的空间起源在口模出口处,与出口处流动状态的突变有关。黏-滑畸变主要是由于熔体在口模壁处界面状态突变,造成熔体发生整体“时黏时滑”引起的。高剪切速率下的整体无规破裂起源于口模入口区的扰动。ELITE5500G出现第二光滑区与其黏-滑畸变特征及分子链含长链结构较多有关。
加入少量加工助剂(PPA)可明显改善m-LLDPE的流变行为。加入后,熔体剪切黏度明显下降,入口压力降和挤出胀大比减小,挤出压力降低。出现鲨鱼皮畸变的临界挤出速率明显推迟,压力振荡和黏-滑畸变现象减轻,第二光滑区变长。PPA含量足够大时,压力振荡消失,挤出物外观好转。表明PPA加入能显著改善熔体/口模壁的界面状态。但PPA对无规破裂没有明显的改善作用。加入少量碳纳米管(CNTs)可减小熔体的流变学弹性。减弱入口应力集中效应,使无规破裂区的挤出物整体畸变得到改善。同时还能减小熔体在毛细管内的应力集中效应,使鲨鱼皮畸变的肋脊高度减小,宽度增大。这与CNTs能诱导分子链取向以及具有内润滑作用有关。LDPE与ELITE5500G共混可明显改善挤出压力振荡现象,黏-滑畸变消失,但鲨鱼皮畸变更加严重。随LDPE含量增加,复合材料黏度降低,挤出胀大比和入口压力降增大,熔体弹性增大,应力集中效应向入口区迁移。m-LLDPE与LDPE共混相容性好,共混后不分相,产生共结晶现象。
This work was supported by the National Natural Science Funds. It emphasized on exploring the orderliness, mechanism and improving measures of the complicated insTab.ility phenomena occurred when the polymer melts, especially polyolefin melts, were extruded at high speed. In this work, rheological behaviors and distortion characteristics of metallocnen linear low density polyethylene (m-LLDPE) extruded at high speed were systematically studied by a double-capillary rheometer. ELITE5100G and ELITE5500G, which were produced by CGSC and INSITETM technology were studied as a object.
The results suggest that the molecular chain of ELITE5500G structure formal capability is better than ELITE5500G.The extrusion properties were compared, and indicate that the influence of temperature to melt viscosity of ELITE5500G is obvious and the appearance of extrudate is better, however, the influence of shear rate to melt viscosity of is stronger at lower temperature, which is due to the different molecule structures of them. The LCB could minish the entrance harass of linetype polymer. The pressure oscillation phenomenon occurs when ELITE5100G and ELITE5500G is extruded at high speed, which is due to the higher entanglement degree and narrow molecular weight distribution of it. And the grade of ELITE5500G is stronger than ELITE5100G..
The results show that, the sharkskin fracture of linear molecule is caused by the local interface insTab.ility at die exit, which is also a relative relationship with the extensional stress between the melt and die exit; While the extrusion pressure oscillation phenomenon is induced by the global interface insTab.ility of the melt/wall interphase, that is to say, the cause of extrusion pressure oscillation can be retrospected to a stick-slip transition on the interface between melts and die wall under high stress. The gross melt fracture occurred at high shear rate is due to the stress concentration of die entrance and the lasting slip between melt and die wall.
Because different phenomena has different disturbance headspring, we reformed them by vary ways. In the various efforts for the improvement of the melt fracture basis on the trouble properties, the sTab.le extrusion rate can be heightened by controlling the temperature; sharkskin melts fracture and pressure oscillation can be improved by increasing temperature. With a few LDPE interfused in ELITE5500G,stress convergence affect transfer to enterance . Sharkskin aberrance became more severity, and slip-stick transition on the interface and pressure surge phenomena disappeared. This way has no good for the gross melt fracture. Additionally, the application of fluoroelastomer(PPA) postpones or completely removes the occurrence of sharkskin fracture, decreasing the extrusion pressure and extrusion swelling ratio, and increasing the critical extrusion speed.As a result, broadening the available processing area and saving the energy source. The application of carbide nanotube (CNTs) can reduce shear viscosity and extrude inflation ratio when the content of CNTs reach to(0.8%~1.0%). Increasing the percent of CNTs, sharkskin can be improved, the height of sharkskin reduce while the width extended. Under higher shear rate, the extrude melt fill into break melt fracture, CNTs could improve both surface break fracture and whole break fracture.
研究结果表明,两种m-LLDPE熔体均为典型非牛顿流体,ELITE5500G黏流活化能较高,而ELITE5100G的入口压力降和挤出胀大比略大。随挤出速率提高,两种m-LLDPE的挤出物外观都经历了从有规畸变(鲨鱼皮畸变、黏-滑畸变)到无规畸变(挤出物整体扭曲、熔体破裂)的变化。在黏-滑畸变区,挤出压力出现振荡,流动曲线断裂。相同挤出温度下,ELITE 5500G的稳定挤出速率范围大,出现鲨鱼皮畸变的临界剪切速率(720s-1)高于ELITE5100G(174s-1)。压力振荡幅度大,流动曲线断裂后的斜率更小。尤为显著的是ELITE5500G在经历黏-滑转变后出现了第二光滑挤出区。第二光滑区对于高速、节能、安全生产具有重要的潜在意义。流动行为的差别与两种样品结构差异有关。
不同的畸变现象源自不同的扰动源和扰动机理。较低剪切速率下的有规鲨鱼皮畸变的空间起源在口模出口处,与出口处流动状态的突变有关。黏-滑畸变主要是由于熔体在口模壁处界面状态突变,造成熔体发生整体“时黏时滑”引起的。高剪切速率下的整体无规破裂起源于口模入口区的扰动。ELITE5500G出现第二光滑区与其黏-滑畸变特征及分子链含长链结构较多有关。
加入少量加工助剂(PPA)可明显改善m-LLDPE的流变行为。加入后,熔体剪切黏度明显下降,入口压力降和挤出胀大比减小,挤出压力降低。出现鲨鱼皮畸变的临界挤出速率明显推迟,压力振荡和黏-滑畸变现象减轻,第二光滑区变长。PPA含量足够大时,压力振荡消失,挤出物外观好转。表明PPA加入能显著改善熔体/口模壁的界面状态。但PPA对无规破裂没有明显的改善作用。加入少量碳纳米管(CNTs)可减小熔体的流变学弹性。减弱入口应力集中效应,使无规破裂区的挤出物整体畸变得到改善。同时还能减小熔体在毛细管内的应力集中效应,使鲨鱼皮畸变的肋脊高度减小,宽度增大。这与CNTs能诱导分子链取向以及具有内润滑作用有关。LDPE与ELITE5500G共混可明显改善挤出压力振荡现象,黏-滑畸变消失,但鲨鱼皮畸变更加严重。随LDPE含量增加,复合材料黏度降低,挤出胀大比和入口压力降增大,熔体弹性增大,应力集中效应向入口区迁移。m-LLDPE与LDPE共混相容性好,共混后不分相,产生共结晶现象。
This work was supported by the National Natural Science Funds. It emphasized on exploring the orderliness, mechanism and improving measures of the complicated insTab.ility phenomena occurred when the polymer melts, especially polyolefin melts, were extruded at high speed. In this work, rheological behaviors and distortion characteristics of metallocnen linear low density polyethylene (m-LLDPE) extruded at high speed were systematically studied by a double-capillary rheometer. ELITE5100G and ELITE5500G, which were produced by CGSC and INSITETM technology were studied as a object.
The results suggest that the molecular chain of ELITE5500G structure formal capability is better than ELITE5500G.The extrusion properties were compared, and indicate that the influence of temperature to melt viscosity of ELITE5500G is obvious and the appearance of extrudate is better, however, the influence of shear rate to melt viscosity of is stronger at lower temperature, which is due to the different molecule structures of them. The LCB could minish the entrance harass of linetype polymer. The pressure oscillation phenomenon occurs when ELITE5100G and ELITE5500G is extruded at high speed, which is due to the higher entanglement degree and narrow molecular weight distribution of it. And the grade of ELITE5500G is stronger than ELITE5100G..
The results show that, the sharkskin fracture of linear molecule is caused by the local interface insTab.ility at die exit, which is also a relative relationship with the extensional stress between the melt and die exit; While the extrusion pressure oscillation phenomenon is induced by the global interface insTab.ility of the melt/wall interphase, that is to say, the cause of extrusion pressure oscillation can be retrospected to a stick-slip transition on the interface between melts and die wall under high stress. The gross melt fracture occurred at high shear rate is due to the stress concentration of die entrance and the lasting slip between melt and die wall.
Because different phenomena has different disturbance headspring, we reformed them by vary ways. In the various efforts for the improvement of the melt fracture basis on the trouble properties, the sTab.le extrusion rate can be heightened by controlling the temperature; sharkskin melts fracture and pressure oscillation can be improved by increasing temperature. With a few LDPE interfused in ELITE5500G,stress convergence affect transfer to enterance . Sharkskin aberrance became more severity, and slip-stick transition on the interface and pressure surge phenomena disappeared. This way has no good for the gross melt fracture. Additionally, the application of fluoroelastomer(PPA) postpones or completely removes the occurrence of sharkskin fracture, decreasing the extrusion pressure and extrusion swelling ratio, and increasing the critical extrusion speed.As a result, broadening the available processing area and saving the energy source. The application of carbide nanotube (CNTs) can reduce shear viscosity and extrude inflation ratio when the content of CNTs reach to(0.8%~1.0%). Increasing the percent of CNTs, sharkskin can be improved, the height of sharkskin reduce while the width extended. Under higher shear rate, the extrude melt fill into break melt fracture, CNTs could improve both surface break fracture and whole break fracture.
引文
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