Study of multi-layer flow in coextrusion processes

Transcription

1 ТЕKA. COMMISSION OF MOTORIZATION AND ENERGETICS IN AGRICULTURE 2014, Vol. 14, No.1, Study of multi-layer flow in coextrusion processes Vitaliy Levanichev Volodymyr Dahl East-Ukrainian National University, Molodizhny bl., 20а, Lugansk, 91034, Ukraine, Received January : accepted February S u m m a r y : The article presents a research of the process of flow multilayer coextrusion. It analyzes the method of regulation of a multilayer structure by volumetric metering of each layer. It presents the data of the calculation of the flow based on slip on the channel wall. Key words: coextrusion, multilayer flow, slip on the wall, gravimetric dosing. INTRODUCTION The main task of coextrusion technology is the control of multi-layer structure. This means prediction of the distribution of layers depending on the given technological regimes. Modern gravimetric control systems provide the given percentage of layers by controlling the productivity of each extruder to the required level [4, 12]. Usually set proportional relationship between a given layer thickness and volumetric productivity of the conform extruder. Regulation is independent from the position of the layer a channel dies and / or design of the dies. This method contradicts the popular theoretical description of the multilayer flow, but gives high accuracy by controlling the thickness of each layer. According to the existing approaches for the calculation of multilayer flows on the border of two immiscible liquids, the following conditions must be met: the continuity of the tangential and normal components of the velocity (this means no slip at the interface), the continuity of the shear stress, balance difference of normal stresses at the surface to surface forces [7, 11, 15]. The boundary condition is also equal to zero velocity at the channel walls [5, 14, 16, 17, 19, 21]. Under such conditions, for example, a three-layer material LDPE-LDPE-LDPE at the percentage layers 33.3/33.3/33.3, the theoretical velocity profile shows that the productivity of the middle layer extruder has to be approximately 2 times higher than extruder external and internal layers. In fact, for equal thickness of three layers of extrusion speeds must be identical. Fig. 1. The theoretical velocity profile of the multilayer flow calculated on the basis of the power law

2 STUDY OF MULTI-LAYER FLOW IN COEXTRUSION PROCESSES 145 During the coextrusion, the layer thickness is directly proportional to the productivity of the respective extruders. The varying viscosity, the quantity of the filler, the flow index have no appreciable effect on the distribution layers. These experimental data contradict the theoretical results of modeling the multi-layer flow based on the power law. According to Figure 1, it requires increased productivity of extruders of the central layer ( the layer productivity Q2 2*Q1, Q2 2*Q3 with the layer thickness H2=H1, H2=H3), which is never observed in practice. Thus, you must have adequate models of the multilayer flow. Conduct a study of the characteristics of the multilayer polymer melt flow in the coextrusion processes. Identify the basic parameters and the regularities of the process to control the multilayer structure. Common description of the melt using the assumption that the velocity on the channel wall is zero. It is believed that on the channel wall, a layer of molecules fixed on which then flows the polymer melt. At the same time, the individual polymers LLDPE and PVDC this statement is not consistent with experimental studies [11]. Practical exploitation of the extrusion equipment resulting in actual production conditions shows that the rate on the channel wall is not zero for the polyamide (PA) and the polyethylene (LDPE), a mixture of plasticized polyvinyl alcohol (PVA) and polyamide-6 (PA6). During the processing of polyamide, when the line is stopped at the included heating for 1 hour and at the same time the screw speed is equal to zero - the degradation of the polymer takes place. But if you install the screw speed 1rev/min, no degradation is observed during 10 hours from the stop line. Therefore, a slight movement of the melt in the extruder ensures that there is no degradation of the polymer melt. On the other hand, a small stop time of the screw rotation leads to the degradation of materials. This discrepancy can not be explained with the help of melt flow models which have an assumption that the linear velocity of the polymer on the wall of the channel is zero [1, 6, 18]. Practical observations show a direct relationship between the time of operation of the head and washing time during the changeover. That is, the channel surface cleanliness and corrosion resistance affect the flushing time. OBJECTS AND RESEARCH The study was conducted by a coextrusion line equipped with a gravimetric system. The flow rate was measured with discreteness 0,01kg (for all extruders). The distribution of melt along the circumference in the dies is accomplished by a spiral mandrel. The melt temperature at the exit of the dies (control using a pyrometer) о С. The gravimetric based on the current value of the parameter α=q/n (the ratio of output to the speed of the rotation) calculates the necessary output of each extruder, and automatically sets it. Then every 0,5min the throughput of the extruder is measured and, if necessary, the screw speed is corrected. The set extruder output is in the range ±0,5%, according to the monitoring gravimetric control system of the coextrusion line. The main technological regimes are shown in Table 1. Table 1. Technological parameters of the process Characteristic/ Five-layer structure Screw diameter, mm Performance range, kg/h Drive power in kw The maximum rotational speed, r/min А (external) В С D Е (internal) , , Material PA EVA LDPE EVA PA Flow index (190 о С, 2,16kg) The basic material (80-100%) Masterbatches (0-20%) Flow index (230 о С, 2,16kg) The basic material 2,5-5,5 (80-100%) Masterbatches 8,0-22,0 (0-20%) 2,0-4,0 1,2-11,7 2,0-3,0 5,0-7,0 2,0-3,0 2,5-5,5 8,0-22,0

3 146 VITALIY LEVANICHEV Research of accuracy of control multilayer structure for different recipes Five-layer structure of the film material thickness of micron measure passed on the system of video microscope SVC. The absolute error of measurement of the layer thickness Δ=0,4 micron. The film thickness 40 microns, it is 100% of the thickness of the system for gravimetric dosing, the absolute error (in the analysis of the percentage of layers) is Δ=1%. We studied the effect of the following factors: MFI layer, set layer thickness on gravimetry, structure on the final thickness of the film. Range of MFI varied according to the Table 1. Main characteristics of recipes are shown in Table 2. The period of study 1 month. N - number of samples. Table 2. Main characteristics of recipes recipe number characteristics of some recipes Recipe 1 in a layer of "C" masterbatches 10% in the layer "C" masterbatches 3%, in the layer Recipe 2 "E" masterbatches 15% in the layer "C" masterbatches 3%, in the layer Recipe 3 "E" masterbatches 15% in the layer "C" masterbatches 20%, in layer Recipe 4 "E" masterbatches 3% Recipe 8 without masterbatches The data analysis of Table 3 shows that the final thickness of the layer does not influence the viscosity, the layer position in the dies. The main factor - the volume extruder output. There is a high standard deviation from the set thickness. It affects significant measurement error, since the film is very thin and difficult to determine the interphase boundary between the layers. Research of accuracy of control multilayer structure for a single recipe The following experiment was carried out as follows: to set different percentages in a multilayer structure (PA/EVA/LDPE/ EVA/PA), the total productivity of the plant has not changed. We measured the same crosssection. The error of measurement of the layer thickness (compared with the first study) was Δ=0,1%, because the analyzed primary tube thickness of 350 microns. We used the recipe 8 without masterbatches. Regression analysis of the data of Table 4 shows that the correlation coefficient is 0.95, the calculated value of the Fisher Fp=1411 much more than the table value Ft=7,04, the significance level α=0,01. The number of data points n=65, k - the number of factors studied, k=1, the degree of freedom v1=1, and v2=63, therefore there is a statistical relationship between the parameters of the volume extruder output and thickness of the layer. In the Figure 2, we see a good correlation parameters. The study of the values indicates that the middle layer is thinner by 0,5-3,0% from the given by gravimetry, at the same time, the outer and the inner layers are thicker on the 0,5-3,0%. Table 3. Summary table of the results of measurement Setting gravimetry, Characteristic Measurement of the layer thickness by SVC, Δ=1% Δ±0,5% material N Е D С В А Е D С В А Recipe ,4±2,0 3,3±0,7 31,2±3,2 3,1±0,8 42,0±3,1 Recipe ,4±2,0 4,2±1,0 33,6±2,6 4,2±0,3 37,6±2,4 Recipe ,5±1,2 4,0±1,3 19,7±1,9 3,1±0,9 62,6±2,4 Recipe ,7±2,1 3,4±0,9 47,3±3,2 3,3±0,7 24,3±2,2 Recipe ,5±1,6 3,6±1,0 32,7±2,0 3,7±1,0 39,6±2,3 Recipe ,2±2,0 3,3±0,3 30,8±2,9 3,2±0,7 43,6±4,3 Recipe ,1±1,7 2,8±0,7 31,8±4,5 2,7±1,0 43,6±4,0 Recipe ,0±2,7 3,6±1,1 33,0±3,9 3,6±0,8 38,9±2,9 Recipe ,6±1,9 3,7±1,2 31,2±3,0 3,2±1,4 41,3±1,4

4 STUDY OF MULTI-LAYER FLOW IN COEXTRUSION PROCESSES 147 Table 4. Comparative results of set and actual thickness Setting gravimetry, Δ±0,5% Measurement of the layer thickness by SVC, Δ=0,1% А В С D Е А В С D Е 1 18,5 2,5 36,6 2,8 39,6 15,8 3,75 32,7 2,41 45,3 2 18,5 2,5 36,6 2,8 39,6 15,8 3,75 32,7 2,41 45,3 3 18,5 2,5 36,6 2,8 39,6 19,2 2,82 32,1 3,38 42,5 4 18,5 2,5 36,6 2,8 39,6 19,2 2,78 31,1 3,33 43,6 5 17,6 2,4 34,9 2,7 42,4 18,0 4,0 36,6 4,57 36,9 6 17,6 2,4 34,9 2,7 42,4 20,1 2,83 41,1 2,83 33,1 7 17,5 2,4 30,3 2,7 47,1 17,6 2,85 24,7 2,3 52,6 8 39,9 2,6 30,3 2,9 24,2 47,1 1, ,45 18,8 9 20,0 2,5 38,5 2,5 36,0 23,8 2,58 35,4 2,58 35, ,0 2,5 30,0 2,5 20,0 45,2 1,69 25,7 1,69 25, ,6 2,8 36,7 2,5 18,5 43,2 5,09 31,9 2,95 16, ,1 2,7 30,3 2,4 17,5 51,6 2,02 23,9 2,27 20, ,9 2,5 30,5 2,8 24,2 46,2 1,36 29,9 1,63 20,9 average 27,6 2,52 34,1 2,68 33,1 29,4 2,86 31,4 2,68 33, layer thickness, % Gravimetry (volumetric dosing) SVC videomicroskope A C E N experiment Fig. 2. Comparison of gravimetric data control system and measuring the thickness of the layer with the using videomicroscope Data analysis gravimetric dosing system and the resulting distribution layers. For refinements on the conclusions the previous experiment, the research was conducted by distribution of each layer in the die. The main technological regimes are shown in Table 5. That is, for one section the thickness of the layers measured in a circle in different parts of the section. The primary tube thickness of 350 microns. Layers B, C and D were measured together because adhesive is more visually similar to polyethylene, and the border with the polyamide is more visible. The results of the analysis of the data of Tables 6, 7 show, as in the second experiment, that the thickness of the middle layer is 2,6% less than the set on gravity, accordingly on the outer layers it is much thicker. A detailed study of the cross sections using the video-microscope indicates, that everywhere there the boundary layer thickness is 0.5-5% which is formed because the slip of the melt on the surface of the channel dies. That is, the outer and inner layers have a double layer structure as though (Fig. 4).

5 148 VITALIY LEVANICHEV Table 5. Technological regimes of the primary molding in the manufacture of sample on prescription number 8 (colorless) Parameter A B C D E in total N, rpm/min Q, kg/h 19,9 1,1 17 1,1 11,1 50,2 Q/N kg/60rpm 0,450 0,056 0,150 0,058 0,251 Percent, % 36 2,5 39 2, Ρ, g/cm 3 1,14 0,9 0,9 0,9 1,14 I, А 27 2,4 15 2,1 8,1 Table 6. Summary data on the distribution of layers Recipe 8 layer parameter А В+С+D Е The structure given by gravity, % 36,0 44,0 20,0 Productivity, kg/h 19,9 19,2 11,1 The structure, measuring video microscope, the results of a 7-measurements on one section, % 37,3±0,78 41,4±4,38 21,3±4,16 Table 7. Summary data on the distribution of layers Recipe 1 layer parameter А В+С+D Е The structure given by gravity, % 45,0 25,0 30,0 Productivity, kg/h 12,7 5,6 8,4 The structure, measuring video microscope, the results of a 3-measurements on one section, % 46±1,35 22,4±1,68 31,6±0,5 0,035 0,03 0,025 E В+С+D А V, m/s 0,02 0,015 0,01 E B+C+D A 0, thickness layer, % Fig. 3. Multi-flow velocity profile obtained from the experimental data from Tables 6,7.

6 STUDY OF MULTI-LAYER FLOW IN COEXTRUSION PROCESSES 149 Fig. 4. The analytical description of a typical multilayer structure ( the touch of a ballpoint pen can be seen at the top) V*100, m/s W1 theoretical (power law) slip on the wall (thickness the wall layer 5,5%) 2 W W3 point of channel slip on the wall (thickness the wall layer 2,5%) Fig. 5. The velocity profile flow of the three- layer PE/PE/PE is installed equal to the productivity all extruders For the calculation of the velocity profile with the wall slip and the equality of the rates at the interface layers, as well as data gravimetric dosing system and the test results of the multilayer structure, a method allowing to predict the speed and thickness of boundary-layer of low viscosity was developed. The result of the calculation is given in Figure 5. "Corky" character of the flow in the channels of the extrusion equipment repeatedly confirmed [2, 8, 9, 20], but its theoretical description has been little studied, no calculations and models to help you analyze interaction near-wall low-viscosity layer W2 and the central layer W1, and the wall layer W3 which is directly contacted with the metal wall of the channel. Polymer extrusion processes are severely limited by flow instabilities and product distortions. At the same time, many problematic phenomena in extrusion associated with the near-wall flow, phenomena on the surface of the extrudate - is sharkskin (fragmentation of the extrudate surface) [3],

7 150 VITALIY LEVANICHEV die droop (adhesion of the melt, degradation products directly at the exit of the dies) [13]. The processes of near-wall flow influence on technological parameters of extrusion and coextrusion. Their analysis and control required when replacing recipe, during stopping and starting the extruder, for quick changeover equipment. This is important when implementing the principles of lean manufacturing. The nature of the boundary layer formation is also interesting. It is understood that there is its dependence on the coefficient of friction of the polymer melt by the metal and heat resistance polymer. Dies temperature affects in two ways, on the one hand the increase of the in temperature reduces the viscosity on the channel wall and boundary layer W2 becomes thinner and faster. On the other hand increasing the melt degradation and increasing thickness W3 on the walls. As the temperature decreases the reverse process, increase of the in viscosity takes place, boundary layer W2 increases, but the destruction of the melt is reduced it improves the stability of the boundary layer. With a significant reduction of temperature effects occur that are associated with defects in the surface of the melt, reaching the level of the critical shear stress. The model of polymer melt flow Consider the following mechanism (model) the flow of non-newtonian fluid. Since the polymer chains are in mutual meshing, steady state flow cork is always evolving the nature of the flow. Adhesion macromolecules block the development of a smooth velocity profile from zero at the wall of the channel to a maximum at the center, the diagram has a telescopic character [10]. In any case, it concerns the size of the channels of modern extrusion equipment and rheometers. We can distinguish the wall layer W3 whose molecules are compressed and practically no relax and central layer W1 (cork) which slides on the wall layer. The central layer is compressible and can have an elastic. Interaction occurs macroroughness and segments of molecules that protrude from the tangles and interact (showing adhesion) with the molecules of the wall layer. Area where there is an interaction, can be called lowviscosity layer W2. Macroroughness (molecular clumps), is characterized by the height H and the angle of inclination α. Microroughnesses - protruding segments of molecules, this model does not take into account, but it is assumed their presence and influence on the flow parameters. Flow scheme is presented in Figure 6. Fig. 6. The model of polymer melt flow

8 STUDY OF MULTI-LAYER FLOW IN COEXTRUSION PROCESSES 151 For interaction and is as follows 1. Newtonian region of high viscosity - speed Vt small and macroroughness segments and the central layer of molecules have a time to almost completely relax, cohesion is fully restored, so there is no noticeable decrease in viscosity/ 2. Non-Newtonian region where the relaxation rate Vr and flow velocity Vt comparable, that is macroroughness has no time to relax, but has moved to a new area, here it decreases rapidly μ - length of the line, where the central molecule in contact and wall layers, therefore a decrease in viscosity. 3. Newtonian region of low viscosity - speed increases so that the molecules do not have time to relax and increase adhesion. That is the interaction takes place on upper part macroroughness, thus the viscosity is stabilized. The structure of the melt surface, that is the geometry of the interacting macroroughness, may be visible at the output from the head, as rectangular velocity profile is saved when exiting the melt from the mold channel. The analysis of the movement of the central layer received the equation: H sin V t H, Vr 1 * tg (1) It should be noted that the transition from planar to volumetric flow model, the parameter μ can be regarded as the viscosity. At an angle α 0 modeled Newtonian flow, that is realized shear layers without relaxation. The model connects the stress relaxation in the polymer with the flow characteristics. The equation allows us to calculate the constant viscosity at low and high shear rate: H 0, sin H H, sin Table 8. The model parameters for the rheological characteristics of the LDPE at various temperatures Parameter α, deg. V r, m/s H, Pa*s µ о Pa*s (2) (3) μ Pa*s Model1 (170 о С) 82 0, Model2 (230 о С) 70 0, As a result of the periodic relaxation, macroroughness melt oscillations appear, and their frequency increases with increasing the flow velocity. It can be assumed, if they are equal with the frequency of temperature fluctuations of molecules, resonance occurs and the phenomenon of fragmentation surface of the extrudate is observed Viscosity, Pa*s ,0E-06 1,0E-04 1,0E-02 1,0E+00 1,0E+02 1,0E+04 Flow velocity, m/s LDPE, 170ºС Model 1 LDPE, 230ºС Model 2 Fig. 7. The rheological characteristics of LDPE at various temperatures and the approximation by equation (1) with the parameters according to Table 8

9 152 VITALIY LEVANICHEV 10000, ,000 Viscosity, Pa*s 100,000 10,000 1,000 0, ,0001 0,001 0,01 0, ,100 Flow velocity, m/s Model 1, 170 С Force of viscous friction, 170 С Model 2, 230 С Force of viscous friction, 230 С Fig. 8. Reducing the viscous friction forces in the field of non-newtonian flow The change is known in the effective area of interaction and rheological curve, possible to calculate the force of viscous friction. The analysis shows that in the region reduction viscosity decrease in the effective contact area occurs faster than increased shear stress so the force of viscous friction decreases (Fig. 8). The model requires further development. Parameters H and α characterize the strength of the interaction between the layers and depend on the intermolecular interaction, temperature, branching of the polymer, molecular weight, segment sizes of macromolecules. Therefore, it is required a search for the relationship model and molecular characteristics of the polymer. CONCLUSIONS 1. Modern coextrusion equipment together with gravimetric dosing systems allow us to study the flow properties of melts for a wide range of polymeric materials. 2. The study of multilayer structures and modes of their production indicates that there is almost "corky" nature of the flow. The deviation from the "cork" character indicates set and actual redistribution of the layer thickness. The thickness of the outer and inner layers is higher by 0.5-3% from the setting (control layer thickness proportional to the volumetric dosing). 3. Low viscous boundary layer W2 has enough stable thickness which is 0.5-5%. The tendency thickness increases the boundary layer with increasing thickness (productivity) corresponding to the outermost layer. 4. Improving coextrusion technology, especially for single and small batch production, requires the development of models of multi-layer flow with the wall slip and the formation of the wall region of the melt flow. 5. The first obtained full flow curve equation of non-newtonian liquid based on a physical model. REFERENCES 1. Altınkaynak M., Gupta M., Spalding S., Crabtree., 2011.: Melting in a Single Screw Extruder: Experiments and 3D Finite Element Simulations. Magazine International Polymer Processing XXVI (2), Basov N., Broy W., 1985.: Engineering plastics. M.: Chemistry (in Russian) 3. Brian Black W., 2000.: Wall slip and boundary effects in polymer shear flows, A dissertation at the University of Wisconsin Madison, [Electronic resource] wisc.edu / pub/ black_dissert.pdf 4. Dyadichev V., Tereshenko T., Dyadychev A., 2010.: Problems of specified quality polymer mixture preparation when utilizing waste in coextrusion equipment, TEKA Commission of motorization and power industry in agriculture, Vol. Xa, Dyadichev V., Levanichev V., Tereshtchenko T., 2003.: Method description rheology of the

10 STUDY OF MULTI-LAYER FLOW IN COEXTRUSION PROCESSES 153 polymer melt in a wide range of shear rates. Resource-saving technologies of production and fabrication of materials in mechanical engineering: Coll. Science. etc. Part 2. - Lugansk: publ EUNU. Dal, (in Russian) 6. Dyadichev V., Tereshenko T., Dyadycheva I., 2011.: Methods of choice of melt filtration system in the process of secondary polymer material extrusion, TEKA Commission of motorization and power industry in agriculture, Vol. XIa, Han C., 1979.: Rheology in polymer processing. tr. from Eng. ed. Vinogradov, G.V. Fridman, M.L. M.: Chemistry (in Russian) 8. Hatzikiriakos S., Dealy J., 1991.: Wall slip of molten high density polyethylene. [Electronic resource ] Department of Chemical Engineering, McGill University// Access mode kiriakos_dealy.pdf. 9. Kosarev A., 2003.: The unity of the dynamics and mechanisms of turbulence eddies and vortices Benard, [Electronic resource] / Agency for Science and Technology Information / / Access mode (in Russian) 10. Levanichev V., 2013.: Model of the polymer melt flow, Eastern-European journal of enterprise technologies, Vol 4, No 7(64), (in Russian) 11. Michaeli W., 1992.: Extrusion dies for plastics and rubber: design and engineering computations. Hanser Publishers. Munich, Rauwendaal C., 2008.: Рolymer extrusion. tr. from english. ed. A.J. Malkin - St. Petersburg. Profession (in Russian) 13. Rauwendaal C., 2008.: Identification and elimination of problems in extrusion. tr. from english. ed. Volodin, V. - St. Petersburg.: Profession, 328. (in Russian) 14. Tadmor Z., Gogos C., 1984.: Theoretical Foundations of polymer processing. tr. from english. M.: Chemistry (in Russian) 15. Tager A., 2007.: Physical chemistry of polymers. 4th edition. M.: Scientific World (in Russian) 16. Torner R., Akutin M., 1986.: Equipment for plastics processing plants. M.: Chemistry (in Russian) 17. Torner R., 1977.: Theoretical foundations of polymer processing (mechanical processes).. M.: Chemistry (in Russian) 18. Ulshin V., Levanichev V., Tereshenko T., 2011.: Research of process flow of the polymer melt in the channels coextrusion equipment, Lugansk: publ EUNU. Dal, No 3(157). (in Russian) 19. Vinogradov G., Malkin A., 1977.: Rheology of polymers. M.: Chemistry (in Russian) 20. Wei C., Yaqiang S., Chunqian L., Qian L., Changyu S., 2011.: Effect of micro-viscosity and wall slip on polymer melt rheology inside microchannel. [Electronic resource]/ Materials conference ANTEC Access mode e/ Yakovlev A., 1972.: The manufacturing of products from plastics. Leningrad, Chemistry, 344. (in Russian) ИССЛЕДОВАНИЕ МНОГОСЛОЙНОГО ТЕЧЕНИЯ В ПРОЦЕССАХ СОЭКСТРУЗИИ Виталий Леваничев Аннотация: В статье представлены результаты исследования многослойного течения в процессе соэкструзии. Проведен анализ метода регулирования многослойной структуры путем объемного дозирования каждого слоя. Приведены данные по расчету многослойного потока с учетом скольжения на стенке канала. Ключевые слова: соэкструзия, многослойное течение, скольжение на стенке, гравиметрическое дозирование.