Particle Size: The Revolution in Fluoropolymer Process Aids

Particle Size: The Revolution in Fluoropolymer Process Aids Steven R. Oriani, DuPont Performance Elastomers L.L.C., Elkton, MD

Mathurin G. Meillon and Dr. David Bigio, University of Maryland, College Park, MD Abstract Fluoropolymer process aids have traditionally been assumed to function most efficiently when present as highly dispersed particles distributed within a polyolefin resin. The current work, however, conclusively demonstrates the superiority of large-particle process aids using in-situ measurements, and demonstrates that fluoropolymer accumulates at the die entrance and flows towards the exit. Practical implications of the findings are discussed, including optimization of fluoroelastomer viscosity and the use of interfacial agents to stabilize the fluoropolymer dispersion. Introduction In 1961, DuPont discovered that blending tiny amounts of fluoropolymer into high viscosity polyolefin resins could dramatically increase the critical shear rate for the onset of melt fracture1. Several decades later, investigators showed that a lubricating layer of fluoropolymer accumulates at the die surface, leading to enhancement of wall slip and shear stress reduction in the polyolefin host resin2-4. More recently, direct observations of the die coating process indicate that a wave front of fluoropolymer propagates from die entrance to exit, followed by rearrangement of the coating into streaks5-7. The arrival of a fluoropolymer wave front at the die exit corresponds with the elimination of melt fracture in that localized area of the extrudate7. The preceding description of the die coating process indicates that, from a practical standpoint, successful use of a polymer process aid (PPA) hinges on deposition of fluoropolymer at the die entrance, followed by flow of the fluoropolymer along the die towards the exit. Seemingly mundane, these notions have in fact reversed deeply embedded ideas about how fluoropolymer coatings develop on internal die surfaces. Before 2002, conventional wisdom held that the fluoropolymer droplets (i.e., particles) within the polyolefin matrix continually migrate towards the die wall, contacting and wetting the entire surface “like rain on a sidewalk”8. This view has largely been abandoned in favor of a boundary layer model6-7, 9-11, in which fluoropolymer droplets do not cross streamlines of the flowing host resin. Instead, the particles contact the die surface due to streamline compression at the die entrance, after which they lose all “particle” character and merge to form a flowing layer of fluoropolymer. One of the most surprising outcomes of the boundary layer model is the prediction that small fluoropolymer particles, i.e., those traditionally thought to migrate fastest towards the die surface12, in fact contribute less to the formation of the coating than do larger particles13. Over the past few years, industrial process aid users have been quick to take advantage of the lower dosing and more reliable performance of the large-particle Z Technology™ process aids, which rely on interfacial agents and higher viscosity fluoroelastomers than traditional process aids to control the morphology of the polyethylene – fluoropolymer blend entering the die. By independently examining the effects of particle size, fluoroelastomer viscosity, and shear rate in the die, the present article bridges a gap between this modern industrial practice and previous laboratory work11, which investigated a single fluoroelastomer – polyethylene viscosity ratio. In addition, high magnification video images of the coating process confirm the validity of the boundary layer model under a wide range of conditions. Experimental Equipment The present work uses the same apparatus as described by Kharchenko et al.6 for in-situ measurement of the fluoropolymer coating thickness, using an imaging technique known as Frustrated Total Internal Reflection (Frus-TIR). Briefly, the equipment consists of a capillary rheometer, where the pre-dispersed fluoropolymer – polyethylene blends are melted and extruded at constant shear rate through a sapphire die. The transparent die permits examination of the die-polymer interface using laser light, and the pattern of laser light scattering can be

analyzed to quantify the thickness of a fluoropolymer layer interposed between the die surface and the polyethylene melt. The sapphire die has a diameter and length of 1.6mm and 38.2mm, respectively. Frus-TIR measurements to determine coating thickness were taken 2mm from the die exit, over a circumferential region corresponding to about 1.26% of the total die circumference. All coating experiments were conducted at 180ºC, using apparent shear rates ranging from 112 to 215 sec-1. Dispersions of fluoroelastomers in polyethylene were prepared through various compounding schemes using both twin screw and single screw extruders, depending on the type of fluoroelastomer and the desired particle size. In most cases, the initial fluoroelastomer – PE blend was produced using a 28mm co-rotating twin screw using 3-lobe, fully intermeshing screws. Further dilutions of the blends were sometimes carried out using a small 18mm Haake® twin screw extruder, or a 19.1mm single screw extruder. The latter employed a screw with 15 feed flights followed by 5 transition and 5 metering flights, and a compression ratio of 3:1. Materials The polyethylene used as the host resin for the fluoroelastomer dispersions was LL1001.09, an ethylene-butene LLDPE produced by Exxon-Mobil Corporation. The LLDPE has a density of 0.918 g/cc, a melt index of 1.0 (190ºC, 2160g), a weight average molecular weight Mw of about 80 kg/mol, and contains about 200ppm of a phenolic anti-oxidant as the sole additive. Three different fluoroelastomers, composed of vinylidene fluoride and hexafluoropropylene in a 3:2 weight ratio, were dispersed in the LLDPE. The fluoroelastomers differ widely in viscosity, and are denoted FE-L, FE-M, and FE-H to indicate low, medium, and high viscosity. Elastomer viscosities are commonly measured in terms of Mooney units, a figure based on the torque needed to turn a rotor encapsulated by the polymer under test. Table I lists the Mooney viscosities of the three fluoroelastomers used in the study. Table I

more detailed look at the rheology of the polymers is given in Figures 1 and 2, where the shear and extensional

d

igure 1 Shear viscosity of polymers at 180ºC Figure 2 Extensional viscosity of polymers at 180ºC

ccurate at the

Mooney ViscosityFluoroelastomer ML1+10, 121 C, ASTM D1646

FE-L 27.7FE-M 55.0FE-H 73.0

Aviscosities of the fluoroelastomers and the LLDPE are compared at the temperature and shear rates used in the coating experiments. These data were acquired using a Rosand® RH7-2 two-bore capillary rheometer, equippewith 1.5mm diameter flat entry capillary dies having L/D of 15:1 and 0:1. F

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Figure 1 indicates that the shear viscosities of FE-L, FE-M, and FE-H bracket that of the LLDPE, respectively yielding an approximate fluoroelastomer to LLDPE shear viscosity ratio of 1:2, 1:1, and 2:1. All the fluoroelastomers exhibit greater shear thinning than the LLDPE, causing these ratios to become most a215 sec-1 shear rate. Extensional viscosity, shown in Figure 2, does not follow the same pattern as shear viscosity.

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FE-M and FE-H both exhibit much greater extensional viscosity than the LLDPE, while FE-L more closely matchesLLDPE. In addition, the fluoroelastomer extensional viscosity decreases with increasing shear rate, while the LLDPE moves in the opposite direction. The observation that the ratio of FE-L to LLDPE extensional viscosity ranges from about 1.8 to 0.76 over shear rates of 112 to 215 sec

able II shows results of additional characterization of FE-L and FE-M according to the technique of Schroff and

able II

ample Preparation

nlike an extruder, the piston-driven capillary rheometer used for the Frus-TIR measurements has no distributive e

th

able III

esults and Discussion

-1 becomes significant in later discussion. TMavridis14, 15, using solution viscometry in MEK and dynamic mechanical analysis at 265ºC to determine the zero shear viscosity and long chain branching index (LCBI). For comparison, a commercial fluoroelastomer used in process aid applications for over two decades is also included in the Table. Consistent with the extensional viscosity results, FE-M does show evidence of long chain branching, whereas FE-L does not. Neither FE-L nor FE-M is highly branched, however, unlike the commercial process aid. T

S Umixing capability. To ensure that a steady rate of fluoroelastomer passes through the die, the fluoroelastomers werfirst diluted to 1000ppm in LLDPE prior to Frus-TIR testing. Table III summarizes the compounding techniques employed to generate uniform dispersions of small, medium, or large sized particles in LLDPE using the three fluoroelastomer types. Compounding conditions were adjusted to compensate for fluoroelastomer viscosity, withe aim of producing dispersions having weight average particle sizes of approximately 2.5μm, 3.5μm, and 5.5μm. In the case of FE-L, however, it was not possible to achieve particle sizes in the “large” range, because the large FE-L particles were too fragile to withstand even the gentlest mixing. T

R

ique provides a powerful tool to analyze the evolution of the fluoroelastomer coatings inside xtrusion dies, without introducing artifacts caused by the need the disassemble dies to access the interior. Figure 3

, while the

at

FE-L FE-M FE-H FreeFlow™ 10Zero Shear Viscosity, Pa-s (265 C) 324 4863 no data 14954Long Chain Branching Index (LCBI) 0 0.13 no data 0.39

Weight Average FE

Fluoroelastomer Blend

particle diameter

(microns)Initial blend on

28mm twin screw Dilution to 1000ppmFE-L small 2.5 1%, 200 C, 150 rpm Single screw, 20 rpm, 200 C

medium 3.8 10%, 200 C, 150 rpm Single screw, 4 rpm, 160 CFE-M small 2.3 1%, 200 C, 200 rpm Haake twin screw, 70 rpm, 180 C

medium 3.4 5%, 200 C, 200 rpm Haake twin screw, 13 rpm, 190 Clarge 5.6 none Haake twin screw, 3 rpm, 200 C

FE-H small 2.8 1%, 200 C, 150 rpm 28mm twin screw, 150 rpm, 240 Cmedium 3.8 1%, 200 C, 150 rpm Single screw, 20 rpm, 200 C

large 5.4 10%, 200 C, 150 rpm Single screw, 20 rpm, 200 C

Frus-TIR Results The Frus-TIR techneshows how the coating of fluoroelastomer FE-M grows inside the die as a function of the amount of fluoroelastomer/LLDPE blend that has passed through the die, the particle size of the blend, and the shear rate. For simplicity, only the blends containing the largest and smallest particles (5.6μm and 2.3μm) are shownmiddle-sized 3.8μm blend has been omitted. In agreement with the boundary layer model, the 5.6μm particle blend yields dramatically faster growth of the fluoroelastomer coating than the 2.3μm blend, as well as a thicker coatingequilibrium. In practical applications, therefore, a process aid that delivers large fluoroelastomer particles to an extrusion die can be dosed at lower levels while providing equal or better functionality than a traditional process aid

designed for easy dispersion13. Two further aspects of the results in Figure 3 deserve mention. First, note that thequilibrium coating thickness is not a steady value, but varies over time. The coating exists in a dynamic equilibrium, as it is fed in a discontinuous fashion (particle by particle) at the die entrance and then dragged downstream by shear stress from the flowing LLDPE. The thickness variability increases along with particand coating thickness, probably due to waves in the coating visible in images presented later. Secondly, the rate coating build up for either particle size appears nearly independent of shear rate, subject only to the amount of fluoroelastomer/LLDPE blend processed through the die. The rate of coating build-up relates to a commercially important feature of process aids, i.e., the elapsed time needed to eliminate fracture after introducing a PPA. Fi3 illustrates this aspect by indicating the blend volume at which fracture becomes fully eliminated. Regardless of shear rate, all three of the large particle FE-M tests eliminate fracture after extruding 40cm

e

le size of

gure

t

particle size and shear rate. Blends contain 1000ppm FE-M in LLDPE.

o compare the results of the Frus-TIR coating measurements for all three fluoroelastomer viscosities and particle zes, Figure 4 plots the average equilibrium coating thicknesses, grouped by particle size. The general trend of

PPA coating

3 blend volume, at which point the coating thickness is in the 140 to 190nm range. Similarly, the two small particle FE-M tests clear fractureafter extruding 160cm3 of the blend, yielding a coating thickness of 150nm. Thus, the coating thickness at the point of fracture elimination is largely independent of particle size, even though the extrusion time needed to grow the coating to the critical thickness for fracture elimination depends strongly on this parameter. As will be discussed later, the independence between shear rate and blend volume needed to clear fracture for a given particle size is nouniversal, but applies only to the 1:1 viscosity ratio blends. Figure 3 Fluoroelastomer coating thickness as a function of

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Tsithese data strongly support the conclusion that for a given fluoroelastomer viscosity, equilibrium coating thickness correlates directly with fluoroelastomer particle size entering the die. From a practical perspective these results unequivocally demonstrate that process aids, regardless of shear rate or fluoroelastomer viscosity, coat die surfaces most efficiently when dispersed relatively coarsely. Figure 4 also shows that with the possible exception of the small particle FE-L results, the equilibrium coating thickness is independent of shear rate. As with the coating thickness – particle size correlation described above, this result also stands in agreement with the mass balance onthe boundary layer proposed by Kharchenko et al.6, given by: Where d = equilibrium coating thickness, C = PPA concentration Vs = polyethylene slip velocity S = PPA particle size ρ = PPA density = shear rate in the

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2/1⎞⎛ 2 ScVd s ⎟⎜=

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B b pr nal to LLDPE shear rateecause oth and Vs are approximately oporti ate effect on these arameters cancels out, leaving equilibrium coating thickness relatively unchanged. Finally, Figure 4 shows that the

ticle

ing

y

r

the sapphire the

f

osity, and fluoroelastomer particle size (weight average particle diameter given in μm). Blends

elt Fra

nother perspective on the coating thickness data can be gained by examining the volume of each blend passed through the die before melt fracture became eliminated. Figure 5 presents these results. In a general sense, Figure 5

PPAγ 6, the shear r& oplow viscosity fluoroelastomer FE-L tends to generate thinner coatings than either FE-M or FE-H at a given parsize and shear rate. Because virtually no fluoroelastomer – polyethylene chain entanglement occurs at the interface between these polymers5, the shear stress on the fluoroelastomer die coating is independent of fluoroelastomer molecular weight. In addition, at any given shear stress, the low viscosity fluoroelastomer is more deformable and flows faster (i.e., greater in the mass balance equation). These factors result in thinner die coatings when usFE-L. Surprisingly, the same process does not lead FE-H to generate thicker coatings than FE-M. In fact, FE-H and FE-M yield nearly identical coating thicknesses at the medium and large particle sizes, while at the small particle size FE-H unexpectedly yields thinner coatings than FE-M. A possible explanation for these results centers on veracity of an implicit assumption: that all fluoroelastomer particles contacting the die surface stick, flow, and become part of the coating. As viscosity rises, fluoroelastomer particles exhibit more solid-like behavior, possiblcausing an ever-increasing fraction to simply tumble along the die surface without wetting. A small particle sizeaggravates this phenomenon because surface tension raises the internal pressure of the particle in inverse proportionto the radius, making a small particle even “harder” and less deformable. While no data exist to directly confirm odeny the hypothesis that some fluoroelastomer particles (particularly small ones from high viscosity fluoroelastomers) may contact the die surface without wetting it, an observation supporting the notion can be described as follows. Several tests were run using a steel capillary die having dimensions identical todie, using the same materials and conditions. Although the steel die does not permit Frus-TIR measurements, blend volume needed to eliminate fracture was recorded. Significantly, fracture was eliminated after passing less oa given blend through the steel die, as compared to the sapphire die. This observation suggests that, compared to the sapphire die, a greater percentage of fluoroelastomer particles contacting the steel surface adhere and become part of the coating, and thus by extension some particles that contact the sapphire die surface must not stick. Die surface composition has been linked to variations in the morphology and amount of the deposited fluoroelastomer coating16, which further implies that the probability of a die-particle collision resulting in coating formation is lessthan 100%. Given the strong evidence that die surface influences this probability, fluoroelastomer characteristics such as viscosity, particle size, monomer composition, etc., are likely to be equally important. Figure 4 Average equilibrium coating thickness as a function of shear rate, fluoroelastomer visc

PPAγ&

contain 1000ppm fluoroelastomer in LLDPE.

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appears as a mirror image of Figure 4, i.e., large particle blends generate thick fluoroelastomer coatings and liminate fracture quickly, whereas small particle blends generate thin coatings and require large extrusion volumes

end

ly

ly

al er

n in μm). Blends contain 1000ppm fluoroelastomer in LLDPE.

long with the Frus-TIR measurements, high magnification video allows the study of both the unsteady state oating process and steady state extrusion with the fully coated sapphire die. Figure 6 shows the sequence of events

zed by video, illustrated using the FE-H, large particle, high shear rate run. tarting with a clean die and pure LLDPE flowing, the video shows occasional dirt particles, moving at various

me

eto do the same. Beyond validating the large-particle technology for process aids, Figure 5 illustrates interesting trends with respect to effects of shear rate and particle size on the ability of the various blends to fully coat the die exit and eliminate fracture. Using a limited data set, Figure 3 previously showed that the volume of FE-M blneeded to clear fracture is independent of shear rate. Comparing all the available data, Figure 5 confirms this conclusion, i.e., the blend volume required for FE-M to completely coat the die exit depends solely on the FE-M particle size. Results using FE-L and FE-H in the small particle size range, however, unexpectedly show not only that shear rate does alter the blend volume needed to clear fracture, but that shear rate trends for FE-L and FE-H move in opposite directions. The reason FE-L tends to become less effective with increasing shear rate probabstems from ratio of the extensional viscosities between FE-L and LLDPE (see Figure 2). At 112 sec-1, the ratio ofFE-L to LLDPE extensional viscosity is greater than one, suggesting that during the extensional deformation in the contraction at the die entrance, the LLDPE deforms more than the FE-L. As a result, at low shear rates the roughspherical shape of the FE-L droplets are preserved, and the boundary layer thickness in fact remains close to the diameter of the particles measured under quiescent conditions. At 215 sec-1, however, this extensional viscosity ratio is reversed, predicting that the FE-L droplets become elongated in the flow direction. The stretching causes the particles to become thin in the direction perpendicular to the die surface, even as they become larger in the flow direction. The net result is that at high shear rates the FE-L particles become functionally smaller than the nomin2.5μm measured under static conditions, reducing boundary layer thickness and hence coating ability. On the othhand, the small particle FE-H blend probably suffers from the opposite problem, i.e., the inability to stick to the die surface and spread from the die entrance towards the exit as discussed earlier. At low shear rates, both of these processes are likely to be disadvantaged - the particles impinge on the die surface more gently, and less shear stress exists to spread the particles if they do stick. As a result, the small particle FE-H blend functions more effectively asshear rate increases. Figure 5 Volume of LLDPE-fluoroelastomer blend passed through the die to eliminate melt fracture as a function of shear rate, fluoroelastomer viscosity, and fluoroelastomer particle size (weight average particle diameter give

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Video Images of the Coating Process Acobserved for all of the eleven runs analySspeeds depending on their distance from the die wall. When PPA is introduced, large numbers of particles becovisible, most of them out of focus and traveling quickly. Critically, even though an abundance of fluoroelastomer

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particles are flowing past the die surface, only rarely does one stop and stick to the wall. Instead, the vast majority of coating occurs when particles contact the die at the entrance, where they coalesce to generate a “wave” of fluoroelastomer that propagates downstream. Once the fluoroelastomer waves pass the field of view, the melt fracture in that localized area of the extruded strand soon disappears. In Figure 6, image D shows the emergence ofthese waves, where they have progressed about halfway across the field of view. Some time after the first waves have passed and fracture has been eliminated (at least locally), ripples or streaks appear to form in the fluoroelastomer coating. In Figure 6, the first streak appears almost simultaneously with the waves, apparentlybeing gouged by a particle in the flowing LLDPE. In general, however, no clear-cut timing nor mechanism for the appearance of streaks can be determined, except to note that they mostly appear after melt fracture is completely eliminated. Figure 6 Sequence of events leading to the formation of a die coating (FE-H, 5.4μm particle size, 215 sec

B – 1000ppm FE-H introduced (100% melt fracture) C – First thin line of fluoroelastomer coating appears (a few non-fractured streaks)

aks present (no melt fracture)

-1) A - Flowing LLDPE, no fluoroelastomer (100% melt fracture) D – Waves of fluoroelastomer appear, followed shortly by streaks or ripples in the coating surface (fracture rapidly diminishing) E – Steady state coating with waves, flow direction stre

100ìm

E

die machining marks

die flaws

dirt particlePPA particles(out of focus)

Particle near wall (in focus)

First PPAcoating PPA waves

Onset of streaks

100μm 100μm

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Flow Direction

Figure 6, continued

100μm

ERipples orstreaks incoating

Practical Implications: Optimizing Fluoroelastomer Viscosity and the Role of Interfacial Agents The work described above details an extremely strong argument for a coating process that, to culminate in the elimination of melt fracture, involves the following sequence of events: Event Controlling Parameters 1. Fluoroelastomer particle contact at die entrance Particle size (accounting for entrance deformation) 2. Particle adhesion to die surface Die surface chemistry, fluoroelastomer “stickiness”, shear rate 3. Flow of fluoroelastomer coating downstream Fluoroelastomer rheology, interface shear stress, coating thickness Considering only the first step for the moment, delivering large fluoroelastomer particles to the die entrance so as to maximize particle-die contact can readily be accomplished by using a fluoroelastomer having viscosity much greater than the host polyethylene17. Such a viscosity mismatch limits the dispersion that can occur in every process step leading up to the die, including masterbatching, pelletizing, and the film extruder itself. At some point, however, further increasing fluoroelastomer viscosity only serves to reduce the fluoroelastomer adhesion and flow, therefore inhibiting formation of a complete coating at the die exit. The above coating process, therefore, predicts the existence of an optimal fluoroelastomer viscosity range, one that balances all the factors. Before considering such an optimal viscosity range, however, the role interfacial agents in modern process aid formulations must be discussed. An interfacial agent functions to reduce dispersion of fluoroelastomer particles, thereby limiting the degree of the fluoroelastomer-polyethylene viscosity mismatch needed to deliver suitably large particles to the die. In other words, the interfacial agent makes possible the use of fluoroelastomers having viscosity low enough to remain sticky and deformable, yet still survive (in large particle form) the typical extrusion processes upstream of the die. Low molecular weight and incompatible with the polyethylene, interfacial agents accomplish this feat by encapsulating the fluoroelastomer particles within the polyethylene matrix, thereby providing a slippery surface that reduces shear stress on the particles18. Polyethylene glycol has traditionally been used for this purpose, but its low thermal stability and migratory behavior that can inhibit printability has led to the development of polycaprolactone as an improved interfacial agent. To demonstrate the existence of an optimal fluoroelastomer viscosity in practical applications, Figure 7 shows results of blown film evaluations using fluoroelastomers having a wide range of Mooney viscosities (ML 1+10, 121ºC), in the presence of a polycaprolactone interfacial agent. In these tests, the die was initially clean and free of fluoroelastomer, and melt fracture covered the entire film surface. Process aid was then introduced, and the time needed to fully clear fracture was recorded. The results in Figure 7 show that starting at a low fluoroelastomer viscosity such as FE-L, the time needed to completely coat the film die exit so as to eliminate fracture decreases

rapidly as the fluoroelastomer viscosity increases, up to Mooney viscosity of 60. Over the 25 to 60 Mooney viscosity range, the ever-increasing fluoroelastomer particle size entering the film die dominates the coating process, outweighing any drawbacks from the higher fluoroelastomer viscosity in terms of sticking and flowing on the die surface. Within the 60 to 80 Mooney range, the various factors outlined above are essentially balanced, e.g., increases in fluoroelastomer viscosity result in larger particles entering the die, at the expense of slightly decreased sticking and wetting ability. As a result, the time to clear fracture remains relatively constant within this viscosity range. Above 80 Mooney, however, further viscosity increases begin to shift the rate limiting steps for die coating towards particle adhesion and flow of the fluoroelastomer on the die surface, leading to a slow down in the coating rate. From a practical standpoint, the die coating rate has enormous importance. Relative to polyolefins, fluoropolymers are at least an order of magnitude more expensive. Achieving and maintaining a die coating with the minimum process aid level, therefore, reduces cost in the highly competitive market for blown films. Numerous laboratory tests as well as full scale manufacturing evaluations at polyethylene producers and film converters have documented the finding that, for a given process aid level, rapid die coating equates to reduced process aid levels in use 9, 13, 18. Figure 7 Blown film melt fracture tests: all runs contain 100ppm fluoroelastomer, 200ppm PCL, dosed to the hopper of the film extruder using 2% masterbatches. Host resin: 1.0 MI LLDPE. Melt temperature: 230ºC. Extrusion equipment: 63.5mm extruder, barrier Screw with Maddock tip, 101.6mm diameter die with 0.76 mm gap, 45.5 Kg/hr output.

onclusions

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hen polyethylene containing fluoroelastomer particles flows through an extrusion die, a fluoroelastomer coating

. 5-

well as e

he

h

Wdevelops on inner die surfaces caused by particles contacting the wall at the die entrance where they adhere, coalesce, and flow downstream in waves. Melt fracture is eliminated when the coating reaches the die exit. Thegrowth of the fluoroelastomer layer depends critically on the particle size entering the die, with large particles (ca6μm) generating much faster coating growth than small particles (ca. 2-3μm). Equilibrium coating thickness also correlates positively with fluoroelastomer particle size, but is largely independent of polyethylene shear rate and tends to decrease with fluoroelastomer viscosity only at the lowest viscosity tested. Over a wide range of fluoroelastomer viscosities, particle sizes, and polyethylene shear rates, coating thickness measurements asthe observations from video imaging of the coating process qualitatively agree with the boundary layer mass balancproposed by Kharchenko et al6. In general, the volume of polyethylene-fluoroelastomer blend required to pass through the extrusion die to develop sufficient coating at the die exit to eliminate fracture varies inversely with tequilibrium coating thickness and particle size. In practical applications where the fluoroelastomer particles are created “in-situ” as the polyethylene-fluoroelastomer blend is mixed and processed, the proposed coating model suggests that an optimal fluoroelastomer viscosity range exists. In these situations, the fluoroelastomer viscosity must be chosen to balance the benefits of increasing viscosity to limit dispersion so as to create large particles, wit

the disadvantages of decreased die wall adhesion and flowability. Blown film testing indicates that the optimum fluoroelastomer viscosity for fastest melt fracture elimination (in the presence of interfacial agent) lies within a range of 60 to 80 Mooney (ML 1+10, 121ºC). Acknowledgements

ank Kalman Migler for the use of the Frus-TIR apparatus and for valuable experimental

eferences

The authors wish to thguidance, and David Morgan for particle size analysis. R

. Blatz, P.S., US 3,125,547, (filed 1961, issued 1964). lends: Simultaneous Slippage and Entrance Pressure

3.

4. 993). n of

1

6. nko, S.B., McGuiggan, P.M., and Migler, K.B., “Flow Induced coating of Fluoropolymer Additives:

7. e Fluoroelastomer

8. ids (PPA)”, in Plastics

9. s of Melt Fracture Elimination using Fluoropolymer Process

10. nd Oriani, S., “Eliminating Surface Melt

11.

12. f Processing Aids in the Extrusion of

13. Fluoropolymer Particle Size”, J. Plastic

14. les, 32, 8454-8464 (1999).

ylene 2005, Maack Business Services

17. rp, J.J., “Fundamentals of Morphology Formation in Polymer Blending” in Two-Phase Polymer

18. Large Particles in Fluoroelastomer Process Aids”,

12. Kanu, R.C., and Shaw, M.T., “Rheology of Polymer B

Loss in the Ethylene-Propylene-Diene (EPDM)/Viton® System”, Polymer Eng. Sci. 22(8), 507-511 (1982). Rudin, A., Blacklock, J.E., Nam, S., and Worm, A.T., “Improvements in Polyolefin Processing with a Fluorocarbon Elastomer Processing Aid”, SPE ANTEC Tech. papers 32, 1154-1157 (1986). Hatzikiriakos, S.G., Stewart, C.W., and Dealy, J.M., Intern. Polymer Processing, VIII, 30 (1

5. Migler, K.B., Lavallee, C., Dillon, M.P., Woods, S.S., and Gettinger, C.L., “Visualizing the EliminatioSharkskin through Fluoropolymer Additives: Coating and Polymer-Polymer Slippage”, J. Rheol. 45, 565-58(2001). KharcheDevelopment of frustrated total internal reflection imaging”, J. Rheol., 47(6) (2003). Meillon, M.G., Morgan, D., Bigio, D., Migler, K., and Oriani, S., “A Description of thCoating Evolution as a Polymer Processing Aid”, SPE ANTEC Tech. papers 51 (2005) Amos, S., Giacoletto, G., Horns, J., Lavallee, C., and Woods, S., “Polymer Processing AAdditives (Hanser, New York, 2001), 553-584. Oriani, S.R., and Chapman, G.R., “FundamentalAids”, TAPPI Polym., Laminations and Coatings conf., 2002. Meillon M., Morgan, D., Bigio, D., Kharchenko, S., Migler, K., aFracture using PPA: the Role of PPA Domain Size”, SPE ANTEC Tech Papers 50, 96-100 (2004). Meillon M., D. Morgan, D. Bigio, S. Kharchenko, K. Migler, Zhou, H., Macosko, C., Oriani, S., “Coating Kinetics of Fluoropolymer Processing Aids for Sharkskin Elimination: The Role of Droplet Size”, J. Non-Newtonian Fluid Mechanics, Vol. 131, November 2005, pp 22-31. Achilleos, E.C., Georgiou, G., and Hatzikiriakos, S.G., “The Role oMolten Polymers”, J. Vinyl Additive Tech. 8, 7-24 (2002). Oriani, S.R., “Optimizing Process Aid Performance by Controlling Film and Sheeting 21, July 2005, pp 179-198. Schroff, R.N., and Mavridis, H., Macromolecu

15. Schroff, R.N., and Mavridis, H., Macromolecules, 34, 7362-7367 (2001). 16. Lavallee, C., “Advances in Polymer Processing Additives (PPA)”, Polyeth

(2005). ElmendoSystems, (Oxford University Press, NY 1991), 167-183. Oriani, S.R., and King, W.J., “The Mounting Support forAddcon World 2005 (Rapra, 2005).

1

2006 PLACE Conference

September 17-21 Cincinnati, Ohio

Particle Size -The Revolution in

Fluoropolymer Process Aids

Presented by: Steven OrianiScientist

What do particles have to do with Process Aids?

Droplets or “particles” of fluoroelastomer (FE) in LLDPE

As this blend flows through the die, FE layer accumulatesInitiates slip at PE – FE interface

How does the FE layer form ?

Traditional model

Boundary Layer model

Particles migrate towards dieDie surface becomes covered“like rain on a sidewalk”1

Small particles favored –migrate faster than large2

1. Amos et. al, “Polymer Processing Aids (PPA)”, in Plastics Additives (Hanser, New York, 2001), 553-584.2. Achilleos et. al, “The Role of Processing Aids in the Extrusion of Molten Polymers”, J. Vinyl Additive Tech. 8, 7-24 (2002). 3. Kharchenko et. al, “Flow Induced coating of Fluoropolymer Additives: Development of frustrated total internal reflection imaging”, J. Rheol., 47(6) (2003).

Particles follow streamlinesContact at die entrance3

Stick and flow towards exit

Large particles favored –inc. boundary layer thickness

2

Video CameraPressure sensor

Apparatus

8 different FE-PE blends, weight avg. particle sizes of ~2, 3, and 5μm3 fluoroelastomers, single LLDPE, 3 shear ratesMeasure: FE accumulation, steady state thickness, pressureVideo images of the entire process

Kharchenko, McGuiggan, Migler (2003)

Materials

400

600

800

1000

1200

1400

1600

1800


Shea

r Vis

cosi

ty (P

a-s)

112 1/sec215 1/sec

0

10000

20000

30000

40000

50000

60000

70000

80000

90000


Exte

nsio

nal V

isco

sity

(Pa-

s)

112 1/sec215 1/sec

Shear viscosity ratiosFE - PE approximately1:2, 1:1 and 2:1

Extensional viscositiesof FE-M and FE-L > > PE

FE-L and PE similar, buttrend opposite with shear rate

FE-L: low viscosityFE-M: medium viscosityFE-H: high viscosity

Examples of FE Accumulation on Die Surface

0

50

100

150

200

250

300

350

0 50 100 150 200 250Volume of Blend Extruded ( )

Coa

ting

Thic

knes

s (n

m)


2

cm3

215 sec-1

155 sec-1

112 sec-1

112 sec-1215 sec-1

no melt fracture

melt fracture

no melt fracture

melt fracture

0

50

100

150

200

250

300

350

0 50 100 150 200 250Volume of Blend Extruded ( )

Coa

ting

Thic

knes

s (n

m)


2

cm3

215 sec-1

155 sec-1

112 sec-1

112 sec-1215 sec-1

no melt fracture

melt fracture

no melt fracture

melt fracture

2.3 micronparticle size

medium viscosity (FE-M), two particle sizes, 3 shear rates

FE layer grows faster and thicker with large particles

Melt fracture is eliminated more efficiently (lower blend volume)

3

50

100

150

200

250

300

350


Ave

rage

Coa

ting

Thic

knes

s (n

m)

112155215

5.4μm 5.6μm

3.8μm 3.8μm3.4μm

2.8μm 2.3μm 2.5μm


Shear rate (sec-1)

50

100

150

200

250

300

350


Ave

rage

Coa

ting

Thic

knes

s (n

m)

112155215

5.4μm 5.6μm

3.8μm 3.8μm3.4μm

2.8μm 2.3μm 2.5μm


Shear rate (sec-1)

FE coating thickness at steady state

Large FE particles always result in thicker coatingsCoating thickness generally shear rate independentSmall FE-L and FE-H particles gave particularly thin coatings

0

50

100

150

200

250

300

350


Ble

nd v

olum

e to

0%

frac

ture

(

)

112155215


cm3

5.4μm 5.6μm

3.8μm 3.8μm3.4μm

2.8μm 2.3μm 2.5μm

Shear rate (sec-1)

0

50

100

150

200

250

300

350


Ble

nd v

olum

e to

0%

frac

ture

(

)

112155215


cm3

5.4μm 5.6μm

3.8μm 3.8μm3.4μm

2.8μm 2.3μm 2.5μm

Shear rate (sec-1)

Melt fracture elimination

Large FE particles always clear fracture fasterFE-L particle may flatten out at high shear ratesSmall FE-H particles: may not stick & spread on die surface

die machining marks

die flaws

dirt particlePPA particles(out of focus)

Particle near wall (in focus)

First PPAcoating PPA waves

Onset of streaks

100μm 100μm

100μm100μm

A B

C D

Video of Coating Process

4

Event Controlling ParametersParticle contact at die entrance Particle sizeParticle adhesion to die surface Die surface, FE “stickiness”,

Shear rate

Flow of FE coating downstream FE rheology, shear stress,coating thickness

Analysis of Coating Process

In industrial use, the FE particle size is affected by• Rheology of FE and PE• Mixing• FE concentration

FE viscosity >> PE desirable for large particles, but …compromises sticking & spreading

Coating analysis therefore predicts an optimal FE viscosity range

0

20

40

60

80

100

120

20 40 60 80 100 120 140


Tim

e to

0%

Fra

ctur

e (m

in)

º


0

20

40

60

80

100

120

20 40 60 80 100 120 140


Tim

e to

0%

Fra

ctur

e (m

in)

º


Interfacial AgentsPolyethylene glycol, PolycaprolactoneCoats FE particles, reduce dispersionHelps create particles that are large AND sticky + spreadable

Influence of FE viscosity on fracture elimination in blown film (with interfacial agent)

100ppm FE200ppm polycaprolactone

FE dispersestoo easily – smallparticle size

FE particles large, but do not stick and spread efficiently

The information set forth herein is furnished free of charge and is based on technical data that DuPont Performance Elastomers believes to be reliable. It is intended for use by persons having technical skill, at their own discretion and risk. Handling precaution information is given with the understanding that those using it will satisfy themselves that their particular conditions of use present no health or safety hazards. Since conditions of product use and disposal are outside our control, we make no warranties, express or implied, and assume no liability in connection with any use of this information. As with any material, evaluation of any compound under end-use conditions prior to specification is essential. Nothing herein is to be taken as a license to operate or a recommendation to infringe on patents. While the information presented here is accurate at the time of publication, specifications can change. Check www.dupontelastomers.com for the most up-to-date information.

Caution: Do not use in medical applications involving permanent implantation in the human body. For other medical applications, discuss with your DuPont Performance Elastomers customer service representative and read Medical Caution Statement H-69237.

DuPont™ is a trademark of DuPont and its affiliates.

Viton® is a registered trademark of DuPont Performance Elastomers.

Copyright © 2006 DuPont Performance Elastomers. All Rights Reserved.

ConclusionsBoundary layer model consistent with all observations

• Coating thickness measurements• Video imaging

Large FE particle size • Greater mass flux of FE contacting at die entrance• More efficient process aids

FE rheology affects• Particle size• Particle ability to stick and spread

5

Thank YouPRESENTED BY

Steven OrianiScientistDuPont Performance Elastomers L.L.C.

Please remember to turn in your evaluation sheet...

Particle Size: The Revolution in Fluoropolymer Process Aids

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