The Effect of Die Geometry on Interfacial Instability in Multi-Layer Streamlined Coextrusion Die (SCD): Part II

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1 The Effect of Die Geometry on Interfacial Instability in Multi-Layer Streamlined Coextrusion Die (SCD): Part II Karen Xiao, Brampton Engineering Inc., Brampton, Ontario, Canada L6T 3V1 Bill Wybenga, Wybenga Technical Advisors, Brampton, Ontario, Canada L6T 3V1 ABSTRACT Due to both technological advancement in equipment and resin design, multi-layer coextrusion has become an increasingly viable option of obtaining different structures. One often encountered, and yet not well understood, issue is interfacial instability. In the first part of this study, interfacial instability between PE/tie interface was examined using three different die geometries [1-2]. In this paper, barrier materials such as nylon 6 and nylon 6,66 will be examined at the nylon/tie interface using the same three geometries presented in previous studies. A Brampton Engineering 7-layer Streamlined Coextrusion Die (SCD) was used in the study. These experimental results were simulated in the hope of finding a parameter which would enable us to predict the issue numerically. INTRODUCTION Multi-layer coextrusion is a rapidly growing area of polymer processing. It is a process in which two or more layers are extruded through a single die. Technological advancement in both machinery and resin design has made blown film coextrusion a viable method of obtaining many different structures. The greatest advantage of coextrusion comes from the ability to combine the important properties of different materials into one structure. However, one of the most common critical problems encountered in industry, that limits the desirable operating range in coextrusion, is the occurrence of interfacial instability. Interfacial instability is a phenomenon in which the flow of viscoelastic polymeric materials gives rise to unstable interfaces; it affects the product quality significantly. So far, it is widely understood that there are two types of interfacial instabilities, zig-zag and wave and they have been widely studied both experimentally and theoretically via modeling. Many researchers have found that both elastic characteristics as well as shear characteristics must be taken into consideration when analyzing a coextrusion sytem [3-11]. Ramanathan et. al. [3] basically classified interfacial instability into two types, which they termed zig-zag and wave. They pointed out that zig-zag instability appears as a series of chevrons pointing in the flow directions, and that it is characterized by a critical interfacial shear stress above which the instability occurs. They defined wave instability as a train of parabolas oriented in the flow direction, and like zigzag, the wave instability is initiated at an internal interface. Perdikoulias and Tzoganakis [4] studied interfacial instability between two layers of the same material. They found that broad MWD materials have a greater tendency to exhibit interfacial instability and that these interfacial instability are more due to layer ratio than processing conditions or die geometries. Whereas materials with narrow MWDs tend to exhibit an interfacial instability related to interfacial stress, and hence can be affected by whatever affects the interfacial stress. Zatloukal et. al. [7-11] developed a novel Total Normal Stress Difference, TNSD, criterion for detecting the onset of wave interfacial instabilities. They concluded using numerical simulations that the die geometry influencing merging of the layers has a crucial effect on interfacial instability. In particular, the geometry, which permits a high pre-acceleration of the minor flow prior to the merging point has the highest stabilizing influence. From the optimal material point of view, they have also concluded that coextruded materials with extensional thinning and no strain hardening behaviour should be preferred. Martyn et. al. [12] pointed out that there are two locations where instability could arise. One location is where the interface is created, i.e., when the melt streams combine; the other location is a point just prior to the die exit region where the interface usually experiences a maximum shear stress. The authors found

2 through stress birefringence and velocity fields in a flat film coextrusion die that wave type instability is caused by disturbances in the area where the two streams converge. This paper is a continuation of a previous work [1-2], in which the interfacial instability at PE/tie interface was examined. This paper investigates the interfacial instability phenomenon at the PA/tie interface. Similar to the previous publications [1-2], the onset of interfacial instability was determined experimentally using several commonly used resin combinations, and two die geometries to determine: 1) The effect of die geometries on interfacial instability 2) A parameter, which will give us a good indication of the onset of interfacial instability. The parameters studied include average velocity ratio, shear rate ratio, and elongational rate. EXPERIMENTAL A Brampton Engineering seven-layer Streamlined Coextrusion Die (SCD) with two die geometries was used in the study. Table 1 lists the structure and different resin combinations investigated. Table 2 lists the relative viscosity ratio of the different nylons, i.e., the relative viscosity, with PA6,66 A having the lowest formic acid viscosity (FAV) number, is defined as: Relative Viscosity Ratio = FAV FAV PA PA666 A It can be seen that in all structures, layers A-E had the same material, LDPE1, for simplification purposes; layer F had the same adhesive resin while the material in layer G was varied. The resins were carefully chosen to purposely create interfacial instability. The layer ratio of G was varied from 8-20% while keeping the outputs of the other layers constant. Layer ratios higher than 20% were not tested since, in practice, it is highly unlikely that the percent nylon on the outside layer will be higher than 20%. In each case, the onset of interfacial instability was recorded. These experiments were performed in both Geometries 1 and 2. Table 1: Structures investigated in the experiment. Structure A-E (inside) F G (outside) 1 LDPE1 Tie 1 PA6 A 2 LDPE1 Tie 1 PA6 B 3 LDPE1 Tie 1 PA6 C 4 LDPE1 Tie 1 5 LDPE 1 Tie 1 PA666 A 6 LDPE 1 Tie 1 PA666 B Table 2: Relative viscosity ratio of the polyamides investigated: Material Name Relative Viscosity Ratio PA6 A 1.53 PA6 B 1.42 PA6 C PA6,66 A 1.0 PA6,66 B 1.04

3 RESULTS AND DISCUSSION By examining the relative viscosity ratio of the materials, the viscosity trend seems to be: PA6 A> PA6 B> > PA6 C> PA6,66 B> PA6,66 A To more accurately measure the viscosities of these materials, a Kayness Galaxy V capillary rheometer was used. The viscosities for all the materials were measured at their respective processing temperatures, i.e., 270C for PA6s while 250C for PA666s. Figure 1 compares the shear viscosities of the PA6 materials investigated while Figure 2 compares the PA666 materials. It can be seen that in the shear rate range for a conventional blown film process, i.e., between /sec, the shear viscosities of PA6s show the following trend: PA6 A> PA6 B> PA6 C> From Figure 2, it can be seen that both PA666s show higher shear viscosities at their processing temperatures than with PA666 A having a higher viscosity than PA666 B, i.e., PA6,66 A> PA6,66 B> Comparing Figures 1 and 2 to Table 2, it is evident that it is of paramount importance to know the viscosities of the different materials at their respective processing temperatures. The FAVs, similar to MFIs for polyolefins, can be misleading when used to predict the actual performance of an material; true viscosity curves must be used PA6 A PA6 B PA6 C Shear Viscosity (Pa-sec) Shear Rate (sec -1 ) Figure 1: Shear viscosity is plotted against shear rate for the four types of PA6s investigated.

4 10000 PA666 A PA666 B Shear Viscosity (Pa-sec) Shear Rate (sec -1 ) Figure 2: Shear viscosity as a function of shear rate is plotted for PA666, and is compared to. Tables 3 and 4 show the onset of interfacial instability in Geometry 1 and Geometry 2, respectively, for the structures mentioned in Table 1. Examining the onset of interfacial instabilities for these structures, it seems that at each layer percentage, the degree of interfacial instability also varies. The observations, therefore, were numbered from 1-5, with 1 indicating the absence of interfacial instability and 5 indicating severe interfacial instability similar to the picture shown in Figure 3. The occurrence of interfacial instability for PA materials was more of a gradual process, which worsens with decreases in layer percentages; by comparison, for polyolefin materials, the appearance of interfacial instability was more rapid at onset. Table 3: The onset of interfacial instability for Geometry 1. Ranking the severity of interfacial instability from 1-5, with 1 = no interfacial instability, and 5 = severe interfacial instability Material in G Layer Layer % PA6 A PA6 B PA6 C PA6,66 A PA6,66 B

5 Table 4: The onset of interfacial instability for Geometry 2.. Ranking the severity of interfacial instability from 1-3, with 1 = no interfacial instability, and 5 = severe interfacial instability Material in G Layer Layer % PA6 A PA6 B PA6 C PA6,66 A PA666 B Figure 3: The severe type of interfacial instability with nylon structures. Examining Tables 3 and 4, several observations can be made in terms of the occurrence of interfacial instability for nylon materials: 1) PA6,66s are more prone to interfacial instability than PA6s. 2) For both PA6s and PA6,66s, the lower the viscosities of a material, the easier it is for the material to exhibit interfacial instability. 3) In general, it is easier to generate interfacial instability with Geometry 2 than it is with Geometry 1. 4) Both the material type and the geometry of the die at the nylon/tie interface affect interfacial instabilities. To summarize from the above observations, the order of materials in terms of its ease to obtain interfacial instability can be seen to have the following trend: PA6,66 B > PA6,66 A >> PA6 C > PA6 A> PA6 B

6 Simulations of experimental observations In an attempt to quantify the onset of interfacial instability, simulations were done using the Flow 2000 TM simulation program. Simulations were done for the sets of experiments concentrating on the F and G layers to calculate: 1) The average velocity ratio, R v, which is the average velocity of the incoming stream over the average velocity of the stream after the merge area, i.e., V 1 /V 2 as shown in Figure 4. 2) The shear rate ratio, R s, which is the shear rate of the stream after the merge area over the shear rate of the incoming stream, i.e. S 1 /S 2 as shown in Figure 4. 3) The maximum elongational rate, E r, of the incoming stream as indicated in Figure 5. 4) The shear stress at the interface where the elongational viscosity is also the highest. This is also indicated in Figure 5. These simulation results are summarized in Tables 5-8, respectively. Stream 1 Stream 2 Figure 4: A schematic of the merge area of the two streams. Figure 5: An example of the merge area showing elongational rate along the flow path for 20% PA6 A in G layer.

7 Table 5: Velocity ratio around the merge area for different structures. The shaded area indicates where interfacial instability occurs. Parameter Material Layer Ratio Geometry 1 Geometry 2 R v PA6 A PA6 B PA6 C PA6,66 A PA6,66 B Table 6: Shear rate ratio around the merge area for different structures. Parameter Material Layer Ratio Geometry 1 Geometry 2 R s PA6 A PA6 B PA6 C PA6,66 A PA6,66 B

8 Table 7: Elongational rate around the merge area for different structures. Parameter Material Layer Ratio Geometry 1 Geometry 2 E r PA6 A PA6 B PA6 C PA666 A PA666 B Table 8: Shear stress at the interface. The shaded area indicates where interfacial instability occurs. Parameter Material Layer Ratio Geometry 1 Geometry 2 E r PA6 A PA6 B PA6 C PA666 A PA666 B

9 It seems that for all the parameters studied, all the parameters showed higher values for geometry 2 than those for geometry 1. This indicates that the design for geometry 2, especially around the merge region, was not desirable. This was also supported by the experimental results in that materials exhibited interfacial instability more readily in geometry 2 than in geometry 1. By carefully examining Tables 5-8, the following observations were made: 1) For PA6s, at the onset of interfacial instability, the velocity ratio exhibits a value of greater than 3. Higher viscosity PA6s are more forgiving at higher velocity ratios, i.e., not as prone to interfacial instability. 2) It is easier for PA666s to have interfacial instability. The velocity ratio at the onset of instability had a value of 2.0 and greater. 3) Similar to what was observed for the polyolefin materials, shear rate ratio is a poor indicator of onset of interfacial instability. 4) A high elongational rate is not desirable for the onset of interfacial instability, although it is not totally consistent here in Table 7. This could be due to the lack of accurate elongational viscosity data for nylon. In this work, due to the more Newtonian nature of these materials, it was assumed that the Trouton ratio for elongational viscosity was valid, i.e., η = 3η E. This may not be entirely accurate. 5) Generally speaking, high shear stress values were also observed at the onset of interfacial instability, however, there doesn t seem to be a critical shear stress value at which interfacial instability occurs. γ CONCLUSIONS Interfacial instability is more readily observed with lower viscosity PA6s and PA666s. No reliable correlations were found between the onset of interfacial instability and shear stress ratio, shear rate ratio and elongational rates. The difficulty in measuring the elongational viscosities of PA materials may have contributed to this issue. Velocity ratio seems to be the best indicator of predicting the onset of interfacial instability in this study. The occurrence of interfacial instability in PA6s and PA666s are affected by both material properties such as viscosity, as well as geometries. ACKNOWLEDGEMENT We would like to thank Mr. Edgard Chow and Mr. Robert Armstrong of the Eval Company of America for the use of their facility for the experiments. We would also like to acknowledge Dr. Steve Tanny and Mr. Steve Gawronski of DuPont for donation of Bynel TM adhesive resins. We would like to thank Mr. Eric Noon from BASF for both material donation as well as useful technical discussions about nylon processing. REFERENCES 1. Xiao, K., Eliminating Interfacial Instability at PE/Tie Interface Using a Multi-layer Stacked Coextrusion Die, SPE ANTEC CD-ROM Proceedings (2004) 2. Xiao, K., The Effect of Die Geometry on Interfacial Instability in Multi-layer Streamlined Coextrusion Die (SCD), TAPPI CD-ROM Proceedings (2003) 3. Ramanathan, R., Shanker, R., Rehg, T., Jons, S., Headley, D.L., and Schrenk, W.J., Wave Pattern Instability in Multi-layer Coextrusion An Experimental Investigation, SPE ANTEC Proceedings, 224 (1996) 4. Perdikoulias, J., and Tzoganakis, C., Interfacial Instability Phenomena in Blown Film Coextrusion of Polyethylene Resins, TAPPI Proceedings, 97 (1995)

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