Study On Melt Elasticity Effect Of Lldpe And Its Blending Systems And Filling Systems

Polymer materials are typical of their non-linear viscoelasticity，of which meltelasticity effect can be displayed in many aspects. Especially study on extensionalviscosity and molecular mechanism of strain-hardening property has been the hottopics of researchers’ discussion, but clear and systematical theory is still not formed.Constant pressure capillary rheometer and constant rate double-capillary rheometerwere used in this study to research the melt elastcity effect of LLDPE and its blendsand composites with fillers. Development laws of melt extrusion flow instability andextensional viscosity were emphasized. The main results and innovation points are asfollows.Extrudate morphology of LLDPE and its blends and composites with differentfillers in constant pressure capillary rheometer was systematically studied. And theresults indicated that LLDPE and its composites actually experienced the similar flowprocess compared with that in constant rate capillary rheometer, i.e., melt stick-sliptransition or integral slippage. So the stick-slip distortions or the second gloosy zone ofthe extrudates were observed by LLDPE composites filled with graphene，MWNTsand carbon black fillers.The effect of length-diameter ratio of capillaries on melt spurt phenomenon wasrevealed. It showed that the bigger the length-diameter ratio was，the smaller thecritical shear stress of melt spurt phenomenon became. And the melt spurtphenomenon almost disappeared when the length-diameter ratio of capillaries was5/1.Variation laws of rheological curves after melt spurt phenomenon were expalined, i.e.,they depended on development trends of both capillary pressure gradient and slipperyrates.The entrance pressure fluctuation of LLDPE, LDPE and their composites wasdiscovered in constant pressure capillary rheometer by Bagley correction method. Andthe critical shear rate of entrance pressure drop fluctuation was just that of sharkskin distortion or integral distortion of different extrudates. Uhland model was applied toexplain the cause of entrance pressure fluctuation successfully.In constant rate double-capillary rheometer, the influence of temperature onstick-slip transition zone was explained. The critical shear rate of stick-slip transitionbecame bigger and the ranges of stick-slip transition zone became wider as thetemperature increased.Two and even more strain-hardening zones of LLDPE and its composites werediscovered and corelation of variation laws of entrance pressure drop and extensionalviscosity was revealed. Molecular entanglements networks theory proposed by deGennes was applied to develop new flow models during melt extenisonal precess andthen to explain the new phenomona discovered in this study. The results showed thatprogressive disentanglements of molecules led to different strain-hardening zones atdifferent extensional rates.Based on molecular entanglements networks theory proposed by de Gennes,effects of one-dimensional, two-dimensional and three-dimensional fillers on extrusiondistortion(superficial distortion and integral distortion) and extenisonalstrain-hardening were analysed in detail. Addition of graphene and MWNTs might leadto slippage at wall and strain-hardening at lower shear stress, and the latter mightenable the degree of strain-hardening to be more serious. As temperature increased,critical shear rates of both stick-slip transition and extensioal strain-hardening becamebigger, but specific mechanisms were different.The research results of this study contribute to the perfection and development ofscientific theories of polymer melt elasticity effect and extrudate distortion. In addition,they can help to understand the differences between shear and extensional rheology.And what is important to the industry processing is that they can benefit steady andhigh speed production of different polymer materials.