Advances in Polymer Processing Additives (PPA) Claude Lavallée Dyneon L.L.C. 3M Canada Company London, Ontario, Canada Presented at the Polyethylene 2005, Maack Conference, Zurich, Feb 1-3, 2005 ABSTRACT In previous work 1,2, flow visualization was used to understand how polymer processing additives (PPA) eliminate sharkskin in linear low-density polyethylene (LLDPE). The PPA migrates to and coats the die wall, induces slippage, and eliminates sharkskin. In this work, the interface between the PPA and LLDPE was further characterized using reflection light microscopy, scanning electron microscopy, energy dispersive X-ray, and profilometry. The coating is characterized by long stripes in the flow direction. The coating density obtained from different technologies was characterized on gold and nickel. The coating thickness was also measured. INTRODUCTION Over the last 20 years, a large amount of work has been done to understand how to best reap the benefits of polymer processing additives (PPA). As an example, several papers were written on the topic of additive interactions 3,4, and cover a broad range of additives such as acid neutralizers, antiblocks 5-7, pigments 8, and anti-oxidants 9. A good amount of work was also done to develop new benefits and applications. For example, although PPA were mainly used in Linear Low Density Polyethylene (LLDPE) for melt fracture elimination, they have shown benefits in eliminating die lip buildup 10, gels 11-14, die swell 15, as well as controlling degradation by allowing lower processing temperature. In many cases, PPA could be considered as process stabilizers. A good review of this type of study was recently published 16. A large amount of work was also done to understand the slip velocity induced by the use of a PPA. Studies have been carried out using plate, slit, and capillary rheometers 17,18. Recent work showed a detailed study of the effect of the PPA at the micro-scale 1. In this case, the study showed release of built-up material from the extruder (cleaning effect), elimination of sharkskin and the formation of a coating on the die wall. In addition, this paper showed the increase of slip velocity and the change in flow profile induced by the PPA. Finally, those measurements were used to confirm that the slippage occurs at the PPA/polyethylene interface and that sharkskin is related to the elongational stretch at the die exit and not to a critical shear rate, shear stress, or a slip-stick phenomenon inside the die. A complementary study was later published 2. It was a preliminary evaluation of the coating formation on the wall. A second and more extensive study was later published 19. The present work is an extension of these two previous studies. The coating formation is one aspect of PPA technology that has been studied only to limited extent. The accepted model for the coating process is a dynamic process by which the coating is constantly formed and removed, resulting in an equilibrium layer on the wall. If this is the case, the coating characteristics such as the thickness or coated area should depend on the process conditions such as concentration, shear rate or temperature. Examples of the effect of the shear rate on the coating process were published, and showed a direct correlation between coating time and shear rate 2021. The appearance of the coating is also of interest, since it can help understand the coating process. Some work was published on the subject, but conflicting results are reported 22-24. It should be noted that these studies were all done under conditions that do not reflect real life (high concentration, treated surface). For this reason, we attempted to examine the coating as it is formed under typical processing conditions.
Under flow conditions, it is well known that one of the components of a binary blend can encapsulate the second one. This has been linked to viscosity, elasticity and normal stress differences 25,26. Based on encapsulation, the equilibrium PPA layer should be 0.4 µm in a capillary die (R=0.8 mm) when taking into account the geometry and concentration (1000 ppm). Similarly, it should be about 0.25 µm in a 20 mil (508 µm) slit die. This assumes complete migration of the additive to encapsulate the LLDPE and no accumulation of the PPA on the surface over time, i.e. the average concentration in the die being equal to the bulk of the feed composition. If the PPA does not migrate completely, the thickness will be lower than calculated, whereas if it does accumulate the thickness will be higher. Most likely, both will occur to a limited extent and the order of magnitude of the coating thickness should still be a fraction of a micron. Our previous work 1,2 has shown that the PPA coating may not be a uniform layer. The interface between the PPA and PE is characterized by lines in the flow direction. Those were believed to be PPA strands on the die surface. However, it was not clear at the time if the strands were an artifact or not nor their origin was clear. The following work is an attempt to characterize the coating obtained with two commercial PPAs using several analytical techniques. EXPERIMENTAL Equipment For this work, a Haake counter rotating, intermeshing, conical twin-screw extruder or an Instron Barrel Rheometer were used to supply the molten polymer to the die. The melt temperature of the extrudate was approximately 200ºC. The die consisted of a stack of metal block and three removable shims. The middle shim was used to set the die gap at 0.5 mm. The two outer shims formed the surface of the die, and were removed for analysis. Materials The polyethylene used for this study was a well-stabilized butene film grade LLDPE (ExxonMobil LL-1001.32, available from ExxonMobil) with a melt index of 1.0 a density of 0.918. This base resin material was selected for its overall low level of additives, and the absence of PPA in its formulation. Three processing additives were used in this study. PPA-1 is a commercially available copolymer of hexafluoropropylene and vinylidene fluoride (Dynamar PPA FX 9613). PPA-2 is a commercially available proprietary synergistic PPA blend (Dynamar PPA FX 5920A). PPA-3 is a commercially available terpolymer of hexafluoropropylene, vinylidene fluoride and tetrafluoroethylene (Dynamar PPA FX 5911). The PPA was added via a 2% masterbatch prepared in the base resin. In each case, the master batch was tumble blended with base resin to achieve a mass fraction of PPA of 0.1%. Before the test, the equipment was purged using a commercially available purge compound (HM-10, Heritage Plastics) comprising a mass fraction of 70% CaCO 3 in a 10 MI LDPE. The metal shims were also cleaned with butanone in a sonic bath. Procedure Before each test, the equipment was purged and cleaned. The base resin was then added and extruded until constant conditions were obtained. The shear rate was typically 300 s -1. The PPA was then added and extruded until the pressure reached equilibrium. At this point, the extruder was stopped, the die was removed, dismantled, and the shims were collected. This process is usually done in less than 1 minute and there is very little effect from the die removal and dismantling on the coating appearance. The LLDPE was then peeled from the die shim surface. There was no significant difference obtained when peeling the LLDPE while the die was warm or cold. The shims were then examined using an Olympus BX60 reflection light microscope equipped for Nomarski interference contrast at 1000X magnification.
Some of the samples were also examined by Scanning Electron Microscopy (SEM) on a Hitachi S-4500 field emission scanning electron microscope (FESEM). A 10 kv accelerating voltage was used with a working distance of 15 mm. The sample was tilted 30 degrees to improve surface topography. The energy dispersive X-ray (EDX) analysis was carried out using an EDAX light element detector, which is attached to the SEM. Finally, profilometry data was collected with a P-10 Tencore Surface Profiler. The scan length was 200 µm, at a scan speed of 20 µm/s. The sampling rate was 100 Hz and 100 traces were collected at a spacing interval of 2 µm. The stylus force was 1 mg. RESULTS AND DISCUSSION Coating structure Previously published work showed that a pattern is formed on the die surface when using a PPA 2. This was observed directly while processing in a sapphire die (Figure 1 A), as an imprint on the extrudate from that die (Figure 1 B), and also as a coating left on a stainless steel die after extrusion (Figure 1 C). However, the nature of the pattern and its origin was not clearly understood. In this work, we attempt to clarify the structure of the coating and its origin. In order to obtain better quality pictures, the metal surface of the die was highly polished and plated with nickel or gold. This does not only improve the image quality, but also is more relevant to the production equipment dies, that are often nickel or chrome plated. An example of the type of images obtained from this improvement is given as Figure 2. In this case, individual strands of PPA, a few microns wide can be observed on the die surface. Those strands are aligned in the flow direction. Close examination also reveals rows of PPA droplets approximately 1 µm in size. It is important to point out here that although the coating is discontinuous, the PPA effect is fully observed. This implies that the uniformity of the coating is more important than its continuity, in order to induce slippage. The results obtained by optical microscopy show a definite pattern on the die surface. However, since this technique being easily subject to artifacts, we decided to confirm the results by a few different techniques. For example, the optical microscopy used here can easily confuse height and refractive index variations, or it can exaggerate slight variations in height. Furthermore, a discontinuous coating or a height variation in a continuous coating would give similar images. SEM, coupled with EDX was used to confirm the existence of the pattern and determine if the coating is discontinuous or if the ridges observed by light microscopy correspond to thickness variation in a continuous coating. The three pictures (Figure 3) show clearly that the coating is discontinuous. On the SEM picture, the ridges of PPA are visible as four lines marked by their shadow. The edge of a fifth line is visible at the top of the picture. The EDX maps corresponding to this area show a good match of the pattern for the SEM, the carbon map, and the fluorine map. The alignment between the SEM image and the EDX is not perfect, but the pictures can be overlaid to show a perfect match. We verified this at several locations on the die under a range of magnification. From these pictures, the ridges of PPA-2 are 2 µm wide and 6-7 µm apart. Since the coating is discontinuous, it is possible to measure the height of the ridges and obtain the coating thickness. This was accomplished using a profilometer. The data from the instrument was collected as X, Y, Z coordinates and plotted as 2D or 3D, charts and maps. An example of this is given as Figure 4, where a topographic map of PPA-1 on a nickel surface is plotted. Again, the ridges in the flow direction (horizontal) are clearly visible. From this chart, PPA-1 gives ridges that are approximately 5 µm wide and 5 µm apart. This difference between PPA-1 and PPA-2 was obtained several times and was confirmed by light microscopy. To confirm that the pattern is indeed lines in the flow direction and not just random blotches, we looked at individual scan lines and average scan lines in both the flow direction and across the flow direction. This is plotted as Figure 5, where a flat profile is observed along the flow direction, whereas the ridges can be seen across the flow direction. It should be noted that the Y scale greatly exaggerates the height of the ridges. It should also be noted that the metal surface position was not corrected and is therefore above zero. The Figure 5 C shows that once the
noise visible in Figure 5 A is averaged, the surface is virtually flat. Figure 5 D shows that the ridges visible in Figure 5 B are persistent along the flow direction. It should be noted that the average broadens the shape of the ridges on the chart. From this data, the ridges are a few microns wide and a few microns apart as stated earlier. They are also a fraction of a micron high, approximately 0.2 µm according to Figure 5. Metal surface The importance of the metal surface for the formation of a PPA coating has been described elsewhere 27. The PPA is believed to interact with the oxides and hydroxides present on the metal surface. This interaction is believed to be H-bonding. If this is indeed part of the mechanism, there should be a visible difference between a gold and a nickel surface. This difference is clearly visible in Figure 6. The two pictures on the top row of Figure 6 (gold) clearly show a lower coating density than the bottom row (nickel). This is directly related to the fact that the nickel surface is covered by oxides and hydroxides while the gold surface is not. The plot shown as Figure 7 confirms the role of the oxides on the metal surface. Here, a profilometer scan line for PPA-2 is shown on both nickel and gold. Again, a larger number of ridges can be observed on the nickel than the gold. It should be pointed out that the coating is still formed on a gold surface. This is an indication that although the oxides and hydroxides help the coating formation, they are not essential to the coating process. Effect of the Synergist The two pictures on the left column of Figure 6 show that the coating pattern of PPA-1 is much less regular than the pattern of PPA-2 (right column). PPA-2 contains a synergist which significantly improves the coating formation. Although both show similar efficacy in a base resin with a low level of additives, the coating density and regularity differ significantly. This is an indication that the coating uniformity in itself is not a key to the performance. However, the synergist in PPA-2 helps laying the coating in a regular pattern. This allows using a lower level of fluoropolymer to cover the same surface area. A regular pattern allows obtaining equivalent efficacy at a lower fluoropolymer level. Interactions Several factors can actually affect the coating formation. For example, as mentioned earlier, additives interactions play a crucial role in the coating formation. A simple example of the effect of interactions is given on Figure 8. In this figure the coating obtained with PPA-1 alone is shown (Figure 8A). The coating obtained from formulations containing 3000 ppm of Synthetic silica and containing 3000 ppm of talc are shown on Figure 8B and Figure 8C respectively. The abrasive effect of the antiblocks present in the resin is clearly visible on these pictures. Similar work was done to show the effect of Hindered Amine Light Stabilizers 27 on the PPA. Process Conditions Shear Rate The process conditions can also have a significant impact on the PPA coating formation. This was clearly demonstrated in previously published work 20, where the shear rate was shown to be one of the controlling factors for the coating formation. This is summarized in Figure 9 where an excellent correlation between the shear rate and the time to coat a blown film line die is plotted. As the shear rate increases, the shear stress increases as well. This increases the shear gradient across the die and pushes the PPA more efficiently toward the wall. It must be pointed out that the correlation is broken at higher shear rate. This is related to the onset of cyclic melt fracture. At this
critical point in the flow behavior, the pressure and throughput fluctuations disturb the coating process, decreasing the efficacy of the coating formation. This phenomenon is further exemplified in Figure 10 where the coating obtained at 190ºC for four different shear rates is shown. At the lower shear rate (200 s -1 ), the coating is sparse, individual droplets are visible and only partial strands are formed. At the next shear rate (300 s -1 ), the stronger flow field allowed the formation of strands on the die surface. At even higher shear rates (400 s -1 ), the higher flow field induces a higher strand density. At the highest shear rate (600 s -1 ), the pressure fluctuations related to the onset of cyclic melt fracture disturb the coating process and the coating is very scattered and uneven. Temperature The temperature effect is related also to the flow field. For a given shear rate, as temperature is increased, viscosity, shear stress and the flow field gradient are decreased. This will affect the PPA efficacy, as previously reported and as shown on Figure 11, where the reduction in pressure obtained from the PPA coating was used as a measure of efficacy. At low temperature (150ºC), the onset of Cyclic Melt Fracture (CMF) is reached at 400 s -1 and the coating efficacy is lower. This leads to a lower pressure reduction. As the temperature is increased to 190ºC, the flow conditions are optimum (away from the CMF onset) and the pressure reduction is maximized. If the temperature is increased further (250ºC), the shear stress and the flow field are significantly decreased and the coating formation is more difficult. This implies that one can use both temperature and shear rate to optimize the flow conditions and obtain a good coating formation. As an example, on Figure 12, the temperature and shear rate were changed simultaneously to keep the shear stress constant. An increase in 20ºC allows doubling the shear rate while keeping the shear stress approximately constant. The three conditions shown here are approximately equivalent for shear stress and flow gradient, and the coating density obtained in each case is equivalent. Concentration It was described earlier that the coating process is under equilibrium. PPA droplets are constantly deposited and removed from the die wall. If this is the case, any factor affecting this deposition removal process should have an impact on the coating process. The most obvious factor is likely the concentration. As concentration is increased, the deposition rate will increase. Typically, at the normal usage level, in a well formulated resin, the die will be coated in about one hour. For the purpose of this study, the conditions were selected to extend this time to five hours in order to gain accuracy on the coating time measurement. An example of this is given on Figure 13. On this chart, the melt fracture elimination curve (an indirect measurement of the die coating rate) for a resin containing a fixed level of 1000 ppm of PPA-1 is plotted with round symbols. The melt fracture elimination obtained for the extrusion under identical conditions, but with increasing levels of PPA from 500 to 1400 ppm, is plotted with square symbols. If the concentration controls the deposition rate, then the increasing level curve can be mathematically normalized to a fixed level of 1000 ppm. The normalized curve is plotted with triangles. One can easily see on Figure 13 that the normalized curve overlaps perfectly the fixed level curve, confirming the proportionality between coating formation time and concentration. This concentration effect is broadly used commercially. Many film producers will use a higher concentration of PPA for a few minutes to pre-coat the die and then use only a maintenance level to replenish the coating. PPA Grade Selection A large number of publications have been devoted to the various aspects of PPA formulation and usage. However, there is much less information on grade selection. The PPA grade is usually selected for its compatibility with other additives or its compatibility with the process conditions. Another factor that should be taken into consideration when selecting a PPA is the target benefit desired.
Several benefits can be obtained from using a PPA. The best known is melt fracture elimination, but pressure reduction, die buildup elimination, gels reduction are other typical benefits. In order to optimize PPA performance, the source of each of these problems and the way the PPA will affect it should be taken into account. Sharkskin melt fracture has been extensively studied and several mechanisms have been used to explain the phenomenon. The most commonly presently accepted mechanism is related to the stretching of the extrudate outer layers due to the elastic recovery at the die exit 28. Since this is a die exit phenomenon, if the die lip is coated with a PPA, the melt fracture is eliminated 29. Coating the die further upstream will provide additional benefits such as pressure reduction, but will have limited impact on the melt fracture phenomenon. In this case, the rate at which the die lip is coated is what is important. This rate is usually proportional to the required level in use. From the die coating kinetic curves (melt fracture elimination curve or pressure reduction rate curve), a rate constant can be extracted. Pressure reduction is also one of the important benefits provided by a PPA. The back pressure in the extruder is largely related to the pressure at the die entrance which is directly related to the die geometry. Larger die length/die gap ratios lead to higher pressure. Since the pressure is a function of the die land length, the length of the coating upstream will significantly impact pressure. The percent area coverage will also affect the pressure, as this will affect the slip velocity at the die wall. Die buildup and the formation of cross-linked gels are in part related to the polymer (polyolefin) metal contact. The polymer in contact with the hot metal can degrade and form radicals that will eventually lead to localized crosslinking. This cross-linked material will either buildup (internal die buildup) or will be released from the metal surface and show in the finished good as a gel. By minimizing the polyolefin metal contacts and the contact time, the amount of buildup and gels will be decreased. A PPA can provide this protection, by forming a barrier between the polyolefin and the metal. It also minimizes the contact time by increasing the melt speed at the die wall (wall slip). For these extrusion problems, covering as much of the metal as possible will be beneficial. Three PPA technologies were compared for their ability to coat the entrance or the exit of the die as well as the percent coverage of the die they provided. The pressure reduction provided by each technology was also recorded. The technologies are based on either of, an elastomer (PPA-1), a thermoplastic terpolymer (PPA-3), or a synergist containing formulation (PPA-2). For each of these formulations, the coating rate constant was calculated and is reported in Table I. In Table I, the PPA-2 technology exhibits the fastest coating rate, while PPA-3 shows the slowest rate of the three. However, the ranking for pressure reduction is reversed with PPA-3 providing the best pressure reduction followed by PPA-1 and then by PPA-2. This can be correlated with the coating patterns shown in Table II. PPA-3 gives a very fine and very uniform structure that covers the whole die, from the entrance to the exit. This additive provides high surface area coverage and is very efficient at coating far upstream from the die exit. For this reason, it provides high pressure reduction. This material would then be optimal for applications where pressure reduction, gels, and die buildup are the main concerns. This material is slower at coating the die lip and is not as optimal for melt fracture elimination as PPA-2. Although PPA-2 is efficient at coating from the entrance to the die exit, it gives a very discontinuous coating. This structure provides lower pressure reduction than PPA-3. However, the die lip is coated very quickly as shown by the rate constant in Table I. For this reason, this PPA is preferred for melt fracture elimination. Because of the partial die coverage, PPA-2 is not as optimal for pressure reduction, gels, and die buildup as PPA-3. It should be noted that higher level of PPA-2 will eventually provide a more uniform coating. However, this will provide only little extra benefits for melt fracture elimination. For a fixed PPA level, the discontinuous coating is a way to spread the PPA more efficiently over a larger surface area. PPA-1 provides intermediate benefits. The die exit is evenly coated, but the die entrance coating is weaker. This provides intermediate pressure reduction between PPA-2 and PPA-3. PPA-1 does not contain a synergist; consequently, its coating kinetics is comparatively slower. PPA-1 will then be the best candidate for applications where melt fracture elimination is required but where the conditions are too harsh for PPA-2, such as high temperatures, or chemically reactive interfering additives.
CONCLUSIONS Several techniques were used to analyze the PPA coating formed during extrusion on the die metal surfaces. The coating was found to be discontinuous and strand like. The strands are only a few microns in diameters and a few microns apart, but stretch in the flow direction over at least a few 100 microns. Their thickness was measured using a profilometer and is about 0.2 µm. The formation of similar structures was observed in polymer blends 30,31. The role of metal oxides in the adhesion of the PPA to the die wall was confirmed by comparing a nickel die and a gold die. The nickel die exhibited a higher coating density than the gold die. We believe this is related to the ability of the PPA to form H-bonding with the oxides and hydroxides which are present on the nickel die, but absent from the gold die surface. A clear difference between the three PPA technologies investigated was also observed. PPA-2 exhibited a regular but discontinuous pattern, whereas PPA-1 presented a more random and regular pattern on the die surface. PPA 3 exhibits a very fine and continuous coating. The three PPAs exhibit different abilities to coat the die entrance, the die exit, and to reduce pressure. This indicates that along with process conditions and additive interferences, the target application and the target benefit should also be considered when selecting a PPA. ACKNOWLEDGEMENTS The author gratefully acknowledges the work of Jason Dockendorff, Julie Reale, Heng-Yong Nie, and Brad Kobe. REFERENCES 1 Migler, K.B.; Lavallée, C.; Dillon, M.P.; Woods, S.S.; Gettinger, C.L., J. Rheol., 45 (2), 565-581, 2001 2 Migler, K.B.; Lavallée, C.; Dillon, M.P.; Woods, S.S.; Gettinger, C.L., SPE ANTEC, 59, 1132-1136, 2001 3 Johnson, B.V.; Blong, T.J.; Kunde, J.M.; Duchesne, D., TAPPI Laminations & Coat. Conf., 1988, 249, 1988 4 Blong, T.J., Fronek, K., Johnson, B.V., Klein, D., and Kunde, J., SPE Polyolefins VII International Conference Proceedings, Houston, Feb. 1991 5 Blong, T.J., Duchesne, D., SPE ANTEC, 47, 1336, 1989 6 Blong, T.J.; Duchesne, D., J. Plast. Film Sheeting, 5(4), 308-320, 1989 7 Blong, T.J.; Duchesne, D., Plast. Compd., 13(1), 50-2, 56-57, 1990 8 Duchesne, D.; Schreiber, H.P.; Johnson, B.V.; Blong, T.J., Polym. Eng. Sci., 30(16), 950-956, 1990 9 Blong, T.J., Focquet, K., and Lavallée, C., SPE ANTEC, 55, 3011-3018, 1997 10 Van den Bossche, L. ; Georjon, O.; Donders, T.; Focquet, K.; Dewitte, G.; Briers, J., Maack PolyEthylene 97, Milano, Italy, 1997 11 Butler, T.I., Pirtle, S.E., TAPPI Laminations & Coat. Conf., 1996, 601-607, 1996 12 Woods, S.S., Amos, S.E., TAPPI Laminations & Coat. Conf., 1998, 675-685, 1998 13 Slusarz, K.R.; Christiano, J.P.; Amos, S.E., SPE ANTEC, 58, 144-148, 2000 14 Slusarz, K.R.; Amos, S.E., TAPPI Laminations & Coat. Conf., 2001 15 Amos, S., SPE RETEC Tech. Papers, 133-143, 1997 16 Amos, S.E.; Giacoletto, G.M.; Horns, J.H.; Lavallée, C.; Woods, S.S., Polymer Processing Aids (PPA). in Plastic Additives Handbook, 5 th edition, Edited by Dr. Hans Zweifel, Hanser Gardner Publications, Inc., Cincinnati, pp. 553-584, 2000
17 Hatzikiriakos, S.G.; Dealy, J.M., J. Rheol., 35 (4), 497-523, 1991 18 Hatzikiriakos, S.G.; Dealy, J.M., SPE ANTEC, 49, 2311-2314, 1991 19 Lavallée, C.; TAPPI Polym. Laminations Coat. Conf., 2003, Paper 28-2, 2003 20 Lavallée, C.; Woods, S.S., SPE ANTEC, 58, 2857-2861, 2000 21 Neumann, P., TAPPI Polym. Laminations Coat. Conf., 2005, Paper 6-1, 2005 22 Priester, D.E.; Stewart, C.W., SPE ANTEC, 50, 2024-2028, 1992 23 Chapman, G.R.; McMinn, R.S.; Priester, D.E.; Phillips, W.L., US Patent, 5,089,200, 1992 24 Lo, H.H.-K.; Chan, C.-M.; Zhu, S.-H., Polym. Eng. Sci., 39 (4), 721-732, 1999 25 Zryd, J.L.; Dooley, J., TAPPI Polym. Laminations Coat. Conf., 1998, 823-830, 1998 26 Dooley, J.; Hyun, K.S.; Hughes, K., Polym. Eng. Sci., 38 (7), 1060-1071, 1998 27 Woods, S.S.; King III, R.E.; Lavallée, C., TAPPI Laminations & Coat. Conf., 2000, 1115-1124, 2000 28 Cogswell, F.N.; J.Non-Newtonian Fluid Mech.. 2, 37-47, 1977 29 Kharchenko, S.B.; Migler,K.B.; McGuiggan, P.M.; SPE ANTEC, 61, 2689-2693, 2003 30 Migler, K.B.; Hobbie, E.K.; Qiao, F., Polym. Eng. Sci., 39 (11), 2282-2291, 1999 31 Migler, K.B., Phys. Rev. Lett., 86 (6), 1023-1026, 2001
TABLES Table I: Pressure Reduction and Coating Rate Constant for Three PPA Technologies PPA % Pressure Reduction Coating Rate K (1/min) PPA-3 62 0.149 PPA-1 44 0.243 PPA-2 39 0.331 Comments Best Pressure Reduction Fastest Coating Rate Table II: Photomicrographs of the PPA Coating for Three Technologies (10 µm/div) PPA Entrance Exit Comments PPA-3 Even & fine structure coating up to die entrance PPA-1 Good coating of exit, poorer coating of die entrance PPA-2 Discontinuous coating, but present at both entrance and exit
FIGURES A) B) C) 35 µm 35 µm 100 µm Figure 1: PPA Patterns A) Coated on a Sapphire Die Surface (500X), B) Imprinted on a Polyethylene Extrudate (500X), C) Coated on a Stainless Steel Surface (200X). 30 µm Figure 2: Photomicrograph of a PPA-2 Coating on a Gold Surface (1000X). SEM Carbon Fluorine Figure 3: SEM and EDX Analysis of PPA-2 on Gold.
Figure 4: Topographic Map Obtained by Profilometer Analysis of PPA-1 on a Nickel Surface. A) B) C) D) Figure 5: Profilometer Analysis of PPA-1 on Nickel. A) Profile Along the Flow Direction, B) Profile Across the Flow Direction, C) Average Profile Along the Flow Direction, D) Average Profile Across the Flow Direction.
A) B) C) D) 30 µm Figure 6: Photomicrograph of a PPA Coating Obtained at 300 s -1 on a Metal Surface (1000X). A) PPA-1 Coating on Gold, B) PPA-2 Coating on Gold, C) PPA-1 Coating on Nickel, D) PPA-2 Coating on Nickel Figure 7: Profilometer Analysis, Profile Across the Flow Direction of PPA-2 on Nickel and Gold.
A) B) C) Figure 8: Photomicrograph of a PPA-1 Coating Obtained at 300 s -1 on Nickel Surfaces (500X, 10 µm/div). A) No AB, B) 3000 ppm SiO2, C) 3000 ppm Talc, 100 y = 6.40E+08x -2.86 0.6mm 0.9mm t 1/2 (min) 10 1 10 100 Shear Rate (1/s) Figure 9: Half Time for the Coating Formation as a Function of Shear Rate with Two Different Die Gaps. (C8, 1MI, 0.920, 220ºC)
200 s -1 300 s -1 400 s -1 600 s -1 Figure 10: Photomicrograph of a PPA-2 Coating Obtained on Gold Surfaces (1000X, 10 µm/div) Over a Range of Shear Rates. At 600 S -1, the Extrusion Conditions Are at the Onset of CMF. 40 Pressure Reduction (%) 30 20 10 0 150 170 190 210 230 250 Temperature ( o C) Figure 11: Pressure Reduction for a 1MI LLDPE with 1000 ppm PPA-1 at 400 s -1 (0.5 mm Diameter, L/D = 40)
180ºC 150 s -1 200ºC 300 s -1 220ºC 600 s -1 10μm/div Figure 12: Photomicrograph of a PPA-1 Coating Obtained at 300 s -1 on Nickel Surfaces (500X, 10 µm/div). 120 500 ppm 700 ppm 1000 ppm 1200 ppm 1400 ppm Melt Fracture (%) 100 80 60 1000 ppm FX PPA-1 9613 Incremental levels of PPA-1 FX 9613 Hourly Equated to 1000ppm Blown Film Data Effect of Mass Throughput 0.5 MI, C-4, LLDPE 5000 ppm Talc 40 20 0 0 1 2 3 4 5 6 Time (hours) Figure 13: Effect of Concentration on Melt Fracture Elimination Rate.
2005 PLACE Conference September 27-29 Las Vegas, Nevada Advances in Polymer Processing Additives Presented by: Claude Lavallée Senior Research Specialist Dyneon a 3M Company 3M Canada Company Synopsis Introduction Flow Visualization PPA Coating Effect of Process on the Coating PPA Type selection Slide 2 Polymer Processing Additive Mechanism PPA exists as immiscible droplets in polyolefin matrix PPA has high affinity for metal die wall Forms low surface energy, dynamic coating Allows melt to slip through Extruder Polymer Flow PPA Die Wall Slide 3 1
Sharkskin Formation Slide 4 One of the proposed mechanisms: Upon die exit, the outer layer of the melt is stretched by the elastic recovery of the flow profile. When the die is coated with a PPA, there is slip at the die wall, giving a blunt flow profile Capillary Rheo-optics Photograph Slide 5 Materials Base Resin Resin-1: POP, C8, MI 1, ρ=0.870 (Sapphire die) Resin-2: LLDPE, C4, MI 1, ρ=0.918 PPA (1000 ppm) PPA-1: Dynamar PPA FX 9613 PPA-2: Dynamar PPA FX 5920A Synergistic blend PPA-3: Dynamar PPA FX 5911 Slide 6 2
Velocimetry with PPA Velocity (mm/s) Slide 7 70 60 50 40 30 20 10 0 n = 0.8 Resin-1, PPA1 20 RPM - Output 4g/min 260 s -1 With PPA No PPA n = 0.56 Slip 0 0.2 0.4 0.6 0.8 1 Depth (mm) Flow Visualization Slide 8 Resin-1, PPA-1 250X Frame Width: 500mm Micrographs of extrusion with PPA Inside die at the Wall Polymer Interface Extrudate at the Air-Polymer Interface Slide 9 3
Coating Details 1000x 10μm/div Slide 10 PPA-2 on Gold - Resin 2 Is the Pattern Real or Is It an Artifact? Carbon Fluorine Gold Nickel Iron Slide 11 PPA-2, S.E.M., 1000X Mag., EDX Analysis Profilometry Y Average PPA-1 on Nickel Metal Surface Slide 12 4
Difference Between PPA-1 and PPA-2 Coating PPA-1 PPA-2 Gold 1000x 10μm/div Die Coating at 300 1 s -1 Nickel Slide 13 Shear Rate Effect 200 s -1 300 s -1 1000x 10μm/div Gold Shim 400 s -1 600 s -1 PPA-2 190 o C Slide 14 Effect of AB - 200 o C, 300 s -1 1000 ppm PPA-1 10 μm/div No AB 3000 ppm SiO 2 3000 ppm Talc (synthetic) Slide 15 5
Summary (Part 1) Coating is discontinuous Strand like (PPA-2) (super-strings) More strands on Nickel than Gold (polarity) Spacing: 5-10μm Width: about 1-2 μm Thickness: about 0.1-0.2μm Affected by Process Conditions & Interactions Slide 16 Selecting the Right PPA Slide 17 Selecting the right PPA for the application can depend on several factors Additive Package Chemical Interactions Physical Interactions Resin Selected Density Already Published Melt Index Process Conditions Temperature Target Benefit Melt fracture elimination Pressure reduction Die Buildup elimination Target Benefit Slide 18 Melt Fracture Elimination Melt fracture is a die exit phenomenon Need to coat the die lip Indicated by coating kinetics Pressure Reduction Pressure is related to die L/D Need to coat further upstream on die % coverage matters Die Buildup Elimination Need to coat as much metal upstream as possible Indicated by coating upstream on die 6
Test Conditions Instron Capillary Rheometer testing Resin -2, 190C, 300 s -1 Slit die 10 mil gap, 0.15 W, 60 L/Gap Coated to equilibrium (2 barrels) Measurement: Pressure reduction Coating rate Area coated Slide 19 Standard Extrusion Results PPA PPA-3 % Pressure Reduction 62 Coating Rate K (1/min) 0.149 Comments Best Pressure Reduction PPA-1 44 0.243 PPA-2 39 0.331 Fastest Coating Rate Slide 20 Coated Areas Slide 21 PPA Entrance Exit PPA-3 PPA-1 PPA-2 Comments Even & fine structure coating up to die entrance Better coating of exit, poorer coating of die entrance Discontinuous coating, but present at both entrance and exit 7
PPA Selection Guidelines FX 5920A FX 5922 Melt fracture elimination Resins containing inorganic additives such as antiblock, TiO2 Fast Acting Synergist FX 9613 FX 9614 Melt fracture elimination Potentially chemically interactive additives High Temperatures More Fluoropolymer Slide 22 FX 5911 Die build-up reduction Extrusion pressure reduction Higher output with high MW resins PP HDPE Better coating upstream More Fluoropolymer FX 5912 Wire coating applications Die build-up reduction Extrusion pressure reduction More Fluoropolymer Acknowledgement K. Migler - NIST Jason Dockendorff Lily Jiang Julie Reale Surface Science Western Heng-Yong Nie Brad Kobe Slide 23 Thank You PRESENTED BY Claude Lavallée Senior Research Specialist Dyneon LLC a 3M Company 3M Canada Company Please remember to turn in your evaluation sheet... 8