MELTING PHENOMENA AND MECHANISM in CO-ROTATING TWIN SCREW EXTRUDER

MELTING PHENOMENA AND MECHANISM in CO-ROTATING TWIN SCREW EXTRUDER Myung Ho Kim LG Chemical Ltd. / Research Park, Science Town, Yusung, Korea C.G. Gogos Polymer Processing Institute, Newark, NewJersey, U.S.A. Abstract The heating and melting phenomena in co-rotating twin screw extruders is quite complex and awfully difficult to analyze it. The main difficulties are not only complexity in the rotor geometry but also the ariation of operating conditions. It has been obsered the ariation of both screw configuration and the operating conditions gae rise to the different melting phenomena and the processing alues such as percent torque and melt temperature. In the recent years, some attention has been paid to the research of polymer melting in co-rotating twin-screw extrusion in systematic way(1,2,3). The ehicle to understand and analyze these complex phenomena was inented. In this study, based on the preious experimental results(1,2,4,5,6) and the systematic experiments to illuminate each distinct heat generation terms (1,2,3) were used to elucidate the complex polymer melting progressing in co-rotating twin-screw extrusion. Introduction Melting of polymer pellet and powder particulate systems in co-rotating twin screw extruders has been studied many researchers (7,8). The effort has been deoted to explain the complex physical phenomena with the conentional Energy Balance Equation. An examination of the terms of the Energy Equation (1) reeals the following DT ρ Cp Dt = q τ : γ& + S (1) where ρ is density of polymer, C p is the heat capacity of polymer, τ is shear stress tensor, γ&is shear rate tensor and &S is heat source term. But in recent years, it was founded that the heat generation during the solid deformation is not negligible in iscoelastic materials(1,2,9). Also frictional heat generation and VED in solid-melt mixture were also important factors to get fully molten state in the deice using the dissipatie mix melting mode. Kim reconsidered the melting phenomena of each piece of polymer processing equipment (2)and re-think what factors are omitted. A new form of energy equation inoling three new terms is as follows: s s 1 γ + γ ρc p DT Dt = q η γ + npff υ + η n + e ( t) s s (&: γ& ) [ τ : γ ] ( ε& : ε& ). + S t s γ (2) The detailed meaning of each terms can be found in the other references (1,2,3). Only a short description will be reealed here. The left-hand side is of course the same as in Equation 1. The right hand side, on the other hand, contains three more terms both of which are olume heat source terms : a. The first inoles the mechanical energy dissipated during the plastic (irreersible) deformation of particulates, which we termed Plastic Energy Dissipation (PED) b. The second term inoles the dry frictional heating as np contact points per unit olume slide past each other with an aerage relatie elocity, which we termed Frictional Energy Dissipation (FED). c. The third term inoles the iscous heat generation in extensional flow field, which occurs in solid-melt mixture. We termed Viscous Energy Dissipation in Extensional flow (VEDex). Fig. 1 depicts the conceptual difference of the melting in the dissipatie mix melters such as co-rotating twin screw extruders in the energy point of iew for the conentional approach(7,8) and this approach(1,2,3). The main effect occurs in the energy of solid portions. Unlike Fig. 1a the temperature of the solid portion reaches around its transition temperatures by PED and FED as well as the conduction from the melt portion. On the melt portion, the heat dynamo can be a combination of VED, VEDex and conduction not just by VED. Present research explains how these terms can be used in each state of melting classified by Table 1 (1,2). Summary of Relationship between Melting Mechanisms and Structural State of Melting

In Table 1, the seen structural states of melting are well summarized, which can be obsered in melting region of co-rotating twin screw extruders. Once its releant to the melting mechanisms are clearly state whateer the screw configuration is complex and the operating conditions are different, the complex melting phenomena can be subscribed by these classification. First of all, let us explain how PED mechanism related with structural state. In many structural state, the deformation of solid occurs this PED inoled to heat up solid particulate. Especially for the higher portion of solid state material inoled the role of PED becomes important such state as indiidual particles, clustered structure, melt bound slid particles and solid rich suspensions. The eolution of state from indiidual particles to solid rich suspensions implys that the amount of molten portions increase. The sources of the origin of melt are crucial to explain these eolution. In the early stage of melting section, there are four possible mode of melt formation: 1) Seerely deformed regions that can generate heat in the particulates to reach its transition temperature by PED itself. 2) High frictional heating between two particles which can generate enough heat for the boundary to reach its transition temperature by FED. 3) A small particle between two large particles with high surface to olume ratio being heated by FED which is easily deformed and finally reaches its transition temperature. 4) Drag melt remoal in the hot barrel side. Fig. 2 depicts the aboe four sub-mode of melt formations in schematically. To understand these eolution, it should be noted that the net effect reeals by the combination of two more mechanisms dependent on the operating conditions and material properties such as amount of PED and iscosities. Let s turn to the effect of FED. The term frictional heating implies that this heating mechanism only occurs in solid state. Once the melt occupied between the solid particulates this term take oer it s role to iscous dragging. As follows the effect of FED affects only in the initial stage of heat for large surface area such that powder like material. The second term of right hand side in Equation 2 is iscous energy dissipation. Conentionally this term is only applied in a shear field, but in this work this term is expanded to extensional flow field also. As shown in Fig. 3, the solid particles approach each other due to the compression action of kneading disks and the melt between the adjacent particles induces extensional flow. The extensional iscosity of polymers usually shows strain hardening and the extensional iscosity become higher as the strain rate become higher. For a simple Newtonian Fluid the heat generation by elongational flow VEDex is ery large as soon as melt films/regions can be generated between adjacent solid regions in dissipatie mix melting. In our opinion, dramatic rates of melting in dissipatie mix melting are due to PED, FED and VEDex in solid rich mixtures inside elongational flowing melt regions. Here dissipatie mix melting is summarized in a sequential way, in terms of propagation of heating and melting: 1)Initially solid particulates are coneyed from upstream or are charged in the olume of the mixing/melting chamber. 2) Solid particulates are deformed and shear each other in locally fully filled regions such as the intermeshing region and pushing flight region; at the same time drag melt remoal melting occurs between the hot barrel surface and locally compacted solid agglomerates or solid particulates where particulate size is large enough to be captured in such regions. 3) A melt-bound solid structure is formed which deforms in a bulk way, where the binding melt coming from the following four sources: a. Melt coming from local drag melt remoal in hot barrel surface b. Large deformed particle regions by PED c. Large deformed small particles by PED d. Interface between two adjacent particles by FED 4)When these melt bound solid structures enter pressurized coneying regions, their deformations become seere by the confining geometry which comprise the barrel wall, rotors surfaces, forwarding material from the upstream, and backward pressure of downstream. In this region the localized actie melting by PED and FED occurs rigorously. 6)When the thickness between two adjacent particles is small enough, the compression of these bulk structures squeezes the molten phase, then deforming the solid particles later. In the squeezing flow of the molten material, VEDex is generated, which helps the melting of the surfaces of each of the indiidual particles. 6)When the thickness between two adjacent particles become larger, the VEDex by elongation flows becomes smaller and less significant. The continuum suspension iscosity of melt rich mixture which may be much higher than matrix iscosity is then used. The VED of shear flows with this matrix iscosity finally plays a main role in dissipatie mix melting. This final state is fairly well described by Rauwendaal (10). But to be used, the temperature of indiidual solid particles in the suspension structure should not be assume at the feed temperature or a temperature rise by conduction only. During passes through preious states, much deformation along with frictional heating increases the temperature of indiidual particles.

Summary of Melting Mechanisms in Corotating Intermeshing Twin Screw Extruders In particular, the melting in intermeshing co-rotating twin screw extruder is quite important in industrial applications. In this machine the expansion-contraction action of kneading blocks plays an important role in destroying solid beds into melt mix structures. The sequence of screw configuration for melting section is quite specific for each application. It is almost impossible to make generalizations about all these screw configurations. Howeer, based on different research surrounding melting and twin screw work, three types of melting configurations are summarized and their typical configurations are shown in Fig 4. It is well known that the melting of polymers is completed within seeral L/D, and this melting task performs in melting section which is composed of seeral kneading blocks sequence followed by pressure consumption element such as reerse screw, reerse kneading discs, neutral kneading discs, or special mixing elements. Only a detailed summary is shown for the screw configuration I, but the same kind of summary can be obtain in other reference (2). Configuration I: Relatiely lengthy kneading discs sequence Pressurized coneying start within the kneading discs section Free coneying in screw section: Negligible effect of conduction heating, PED and local drag remoal melting so that solid temperature can be assumed at the feeding temperature. Free coneying in kneading disc section: Before solid particles reach the pressurized coneying region, deformation of indiidual particles occurs in the fourth pocket of kneading disc action. The barrel temperature is high enough to induce localized drag melt remoal occurring for particles dragged by the pushing side of kneading disc. The molten material can be used as glue of the solid particles; it creates melt bound solid particulate structure earlier. This sizable melt bound solid particulate structure can be easily caught by the fourth pocket and the locally filled channel in compression. Pressurized coneying in kneading disc section: To oercome the pressure of the restriction element, material should be backed up some length to generate pressure. The first structural state is melt bound solid particulate structure deliered from upstream in the free coneying region. Once indiidual deformed particles or small melt bound structures fill together in one channel of kneading discs, compression action of kneading disc pair induces bulk deformation of this structure. The small particles or seerely deformed particles (internal temperature is higher than less deformed solid particles) deform more easily, then their internal temperature increases. Also the sliding between the adjacent particles also creates molten material as indicated in Fig. 2b. At the barrel surface, melt is also generated by drag melt remoal, Fig. 2d. If the amount of melt is large enough, the bulk deformation can not sustain any more. Instead of bulk deformation, the compression action of each kneading disc pair generates squeezing in the molten phase which incorporates VED in elongational flow. This VEDex plays an important role in melting of the solid material. Finally if the distance between adjacent particles becomes enough large, the contribution of the VED in elongation flow becomes smaller, but the bulk suspension iscosity takes oer to melt the rest of the material by VED in shear flow. The final structural state of melting is melt rich suspension, which is in a flowable state, but includes unmelted particles (so called escapee ). Once these escapees generate in the melting section, it is ery difficult to remoe them down stream. Then it is highly recommended that further research for remoing these escapees should be conducted. Configuration II: Relatiely short kneading discs sequence Pressurized coneying starting from upstream of coneying element Configuration III: Using long solid coneying section Unlike the aboe two configurations before introducing melting section conduction heating, PED and local drag remoal melting occurred in solid coneying section. Conclusion The melting phenomena in intermeshing co-rotating twin screw extruders is ery complex and also is affected by screw configuration and the operating conditions such as screw speed and barrel temperature settings. It is ery difficult to generalize such complex phenomena at this point. Howeer, from the preious generalization of dissipatie mix melting and experimental obseration (1,2,4,5,6), a kind of generalization has been suggested. It considers the degree of fill, mode of coneying, and the seen structural eolution of state as well as the melting mechanisms taking place alone the length of the meting section in Co-TSE. With similar application of this methodology, melting in a more realistic screw

configuration was interpreted as the same sequence of melting eolution. Acknowledgments The authors acknowledge the financial support of the industrial sponsors of the Polymer Mixing Study and Drs. Jong-Ki Yeo and Jin-Young Yu for their support to write this paper. References 1. C.G. Gogos, Z. Tadmor and M.-H. Kim, Ad. In Polym. Tech., 17, 4, p. 1, 1998 2. Myung-Ho Kim, Melting Phenomena and Mechanism in Polymer Processing Equipment, Ph.D. Dissertation, Steens Institute of Technology, 1999 3. C.G. Gogos and M.-H. Kim, SPE ANTEC Tech. Papers, 58, p. 134, 2000 4. M. Esseghir, D.W. Yu, C.G. Gogos, D.B. Todd and Z. Tadmor, SPE ANTEC Tech. Papers, 55, p. 3864, 1997 5. M.Esseghir, C.G. Gogos, Z. Tadmor, D.B. Todd and D.W. Yu, Proceedings of the 11 th Semiannual Meeting, Chap 3, Polymer Mixing Study, Polymer Processing Institute, May, 1996 6. M.-H. Kim, C.G. Gogos and Z. Tadmor, Proceedings of the 15 th Semiannual Meeting, Chap. 1, Polymer Mixing Study, Polymer Processing Institute, Oct., 1997 7. S. Bawiskar and J.L. White, Intern. Polym. Process., X, 2, p. 105, 1995 8. H. Potente and U. Melisch, Intern. Polym. Process., XI, 2, p. 101, 1996 9. M.-H. Kim and C.G. Gogos, SPE ANTEC Tech. Papers, 58, p. 139 (2000) 10. C. Rauwendaal, SPE ANTEC Tech. Papers, 51, p. 2232, 1993 VED VED VEDex T exit T exit T m T g T melt T ag T solid Passie Solid Bed Conduction only T m T g T melt T ag T solid Actie Solid Bed PED FED Conduction (a) (b) (c) force heat V Conentional Approach Using Conentional Energy Balance Equation (7,8) Systematic Approach Using Modified Energy Balance Equation (1,2,3) Fig. 1. Comparison of energy representation along the extruder axis between the conentional and the systematic approach using modified energy balance equation. Fig. 2. Four possible melt sources for the early stage of melting state eolution. (a) Large deformed region (b) Interface between two particle (c) Large deformed small particle (d) drag melt remoal at the barrel side (d)

Table 1. Descriptio and discusion of the three modes of characteristic states Characteristic Aspect Degree of fill State Nomenclature Fully Filled Partially Filled Characteristics Definition Flow olume is filled with material, nomater whether it is solid or any other structure. Flow olume is partially filled with material. Mode of Coneying Free Coneying Particles are freely coneyed. Pressurized Coneying Generating pressure with fully filled region and partially filled region that backed up with localized and linked with fully filled region. Major structural changes occurs in this region. Structural State Indiidual Particles Clustered Structure No matter how each particle is deformed, it has it s own boundaries; no clustered or agglomerated particles. A bunch of particles forms clustered structures bound with melt, which proided with four different melt formation Melt Film This structure is obsered only in partially filled regions of free coneying region. In fully filled region melt formed by drag remoal melting mode is immediately commingled with other structure. Melt bound Solid Particulates Indiidual solid boundaries are clearly shown, but a thin melt binder bonds the particles tightly to each other. Solid Rich Suspension Melt Coered Structure The solid particle boundaries become ague, melt between the solid particles gets thicker, (a coherent and mixed solid bed). At the barrel side a thick melt coers the solids rich suspension. The source of melt is due to the combination of PED and VED brought about by the ceaseless, relentless deformation and flow of the solids-rich suspension and by drag remoal melting from the preious pair of kneading elements rather than by drag remoal in that pair. The melt spurting from the kneading discs in front also become another important source of this structure. Melt Rich Suspension Small portion of solid particles floating inside melt. Fig. 4. Typical screw configuration for the melting section of intermeshing co-rotating twin screw extruders (a) Configuration I: relatiely long kneading disc section, (b) Configuration II: relatiely short kneading disc section, and (c) Configuration III: long solid coneying section. 0 Compression of solid rich melt-solid mixture. i 0 F N j Extensional flow generated by approaching of two solid particulate VEDex Normal force (F N ) deforms solid PED Fig. 3. Schematic diagram of squeezing of high olume fraction solids in a melt-solid mixture, which generates localized extensional flow.