Rheol Acta DOI 10.1007/s00397-007-0171-1
ORIGINAL CONTRIBUTION
Silanols cured by borates as lubricants in extrusion of LLDPE. Impact of elasticity of the lubricant on sliding friction Oleg Kulikov & Klaus Hornung & Manfred Wagner
Received: 18 May 2006 / Accepted: 18 January 2007 # Springer-Verlag 2007
Abstract Addition of a viscoelastic material based on silanols cured by boron oxide was used to delay sharkskin and stick–slip instabilities in extrusion of linear low-density polyethylene (LLDPE). Delay of flow instabilities to rates of extrusion 25–35 times higher than without additive and about 40% less extrusion pressure at the same throughput are achieved by the use of this material as an additive (∼0.1%) to LLDPE or as a coating of the extrusion die. Mechanical properties of the lubricant were changed by small variations of composition to investigate the impact of elasticity on lubrication and sharkskin delay. Both lubrication and sharkskin delay were considerably improved when more elastic lubricants were used while the chemical composition of the lubricants was nearly the same. Filling the lubricants with powders of metal oxides or especially particulates having plate-like particles (kaolin, mica, BN) helped to delay the flow instabilities further to even higher throughputs. Together with experimental results, we present a tentative explanation for the importance of elasticity of polymer processing aids in the delay of sharkskin and the stabilization of slip.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. O. Kulikov (*) : K. Hornung Universität der Bundeswehr München, LRT-7.1, 85577 Neubiberg, Germany e-mail:
[email protected] M. Wagner Fachgebiet Polymertechnik/Polymerphysik, TU Berlin, Fasanenstr. 90, 10623 Berlin, Germany
Keywords Sharkskin . Stick-slip . Melt fracture . Polymer processing additives . Elasticity . Shear cracks
Introduction The use of thermoplastic polyolefin resins in industry continues to increase because of competitive costs and good recyclability in comparison to other plastics. The most important processing operations for polyolefin resins are extrusion and injection molding. In the processing of highly viscous polymeric melts two major problems appear. Firstly, the molten polymer near the die wall degrades because of long exposure to high temperatures. Secondly, at higher flow rates, melt flow instabilities like sharkskin, stick–slip and gross melt fracture occur. Apparent slip and sharp drop in the friction losses during extrusion of molten polyethylene at high flow rates was first observed by Bagley et al. (1958). The phenomenon of slip of polymer along a rigid wall attracted much attention of researchers because of its importance for practical application in sharkskin reduction, see, e.g., El Kissi and Piau (1990), Leonov and Prokunin (1994), Adewale and Leonov (1997), Yarin and Graham (1998), and Denn (2001). The slip of polymer melts occurs not only along a rigid wall, but also between immiscible polymers. In general, it appears that systems, which are more incompatible, show larger viscosity deviations, see, e.g., Zhaoa and Macosko (2002). In extrusion of polymers, frictional losses can be diminished by the use of processing additives which are immiscible in the matrix polymer. The existing theories of slip do not consider true interfacial slip between immiscible polymers or between the molten polymer and a rigid components to the boundaries (Joseph 1997; Joseph et al. 1997; Hatzikiriakos and Migler 2004), and thus postulate a low-viscosity region
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at least at the micro- or nanoscale near interfaces and boundaries. An alternative point of view may be to consider frictional sliding along an interface between two rapidly deforming solids, which is a basic problem of mechanics that arises in a variety of contexts including moving machinery surface interactions, machining of materials (e.g., cutting), the failure of the reinforced composites (e.g., fiber pullout), and earthquake dynamics (fault rupture; Persson 2000; Granick et al. 2003; Coker et al. 2005). The problem of steady sliding under classical Coulomb friction is unstable, and for situations of rapidly varying normal stress it leads to self-healing pulses of slip propagation (Adams 1998; Caroli 2000; Brener et al. 2005). It is widely accepted that self-healing pulses of slip occur during earthquakes (Heaton 1990; Nielsen and Madariaga 2003). Weertman (1980) theoretically concluded that a self-healing pulse of slip propagates along the frictional interface between dissimilar elastic solids. In a numerical study, Andrews and Ben-Zion (1997) and Shi Zh and Ben-Zion (2006) examined wrinkle-like propagation of slip pulses on a fault between dissimilar elastic solids. The calculations show that propagation of slip pulses occurs in only one direction (referred to below as the positive direction) that is the direction of slip in the more compliant medium. Motion is analogous to a propagating wrinkle in a carpet. Displacement is larger in the softer medium, which is displaced away from the fault during the passage of the slip pulse. Lambros and Rosakis (1995a,b) and Xia et al. (2005) recently performed sliding experiments along a bimaterial interface for several loading configurations and obtained asymmetric bilateral ruptures. In all cases, the rupture fronts in the positive direction had stable properties. Polymer melts show rubber-like behavior when deformed rapidly, and we can expect to find similarities in slip of polymer melts to the slip of rubbers along a solid boundary. Schallamach (1971) and others have noticed that the lateral force required to drag a glass lens across a rubber slab is reduced when detachment waves are formed. The Schallamach waves detach the rubber from the hard surface, and relative motion between the two surfaces occurs only in the regions where contact has temporarily been lost. The effect is similar to a caterpillar moving over a leaf. A review of the subject of rubber friction was presented by Roberts (1992). Recent experimental observations of Rubio and Galeano (1994) and of Baumberger et al. (2002, 2003) on the frictional motion of sheared gels sliding along a glass surface also indicate the existence of inhomogeneous modes of sliding. Comninou and Dundurs (1978a,b) as well as Gerde and Marder (2001) showed theoretically that a dynamic wave involving separation can propagate along an interface when two elastic media or an
elastic body in contact with a rigid surface are compressed and simultaneously sheared. Separation between the sliding objects is actually very small and can be in the range of a few atomic scales. Uenishi and Rossmanith (2003) recently detected by optical means a macroscopic interface separation between similar materials during propagation of a Rayleigh pulse. A combination of both models allows the propagating fault rupture to occur by opening in certain regions and slipping in others. Thus, the dominating modeling ideas for apparent slip of molten polymers do not imply true slip along the interfaces but rather a continuous velocity profile at least at the nanoscale, whereas theoretical models of slip of elastic solids including soft gels postulate a true velocity discontinuity in the slip area. As we will show here, our experimental results make us think that the physical mechanism of slip of molten LLDPE may be closer to the slip of an elastic solid than it was previously accepted. To elucidate the effect of elasticity of polymer processing aids on the sliding friction of LLDPE, we investigated the effects of a large number of additives with different elasticity. In particular, high molecular weight silicone fluids (siloxanes) have been employed for many years as lubricants in extrusion and release agents in injection molding. A method to obtain an elastic but yet plastic product by treatment of liquid dimethyl siloxane (silanol) with boric oxide was first invented in 1943 by McGregor and Warrick (1943). James Wright (1944) independently described a material called “bouncing putty” based on an organosiloxane–boron compound with additives of zinc hydroxide. Bouncing putty was renamed later to “Silly Putty®” because of its main ingredient, silicone. “Silly Putty®” is a viscoelastic fluid (Wilkinson 1960). If rolled into a ball and dropped, the material bounces like rubber. However, upon longer inspection, the material is seen to sag under its own weight; although, the putty does not flow indefinitely on a flat surface. It flows only above some threshold shear stress, i.e., it behaves like a Bingham fluid. In addition, if a shock or impulsive load is applied to the putty, it will shatter like a solid body (see http://www. campoly.com/notes/sillyputty.pdf). Silly Putty is known as a Dow Corning 3179 Dilatant Compound, but in the range of load frequencies from 0.1 to 40 Hz it shows shearthinning behavior (drop of viscosity with increased shear rate). The viscosity of Silly Putty also depends on the time of shearing, and it undergoes a decrease in viscosity with time of kneading, i.e., it shows tixotropic behavior. Mechanical properties of silanols cured by boric acid depend on the molecular weight of silanols and the amount of boric acid used for curing. We used low and high molecular weight silanols cured by boric acid as die coating to investigate the impact of viscous and elastic properties of the die coating on slip and sharkskin delay of LLDPE while
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the chemical composition of the coatings differed only marginally.
Experimental Materials and methods All experiments were performed by use of LLDPE “LL1201 XV” from ExxonMobil Chemicals. This material was selected for its clarity, its overall low level of additives, and the absence of processing additives in its formulation. It is characterized by density (0.925 g/cm3), melting point (123 °C), and melt index (0.7). The following materials were employed in experiments to suppress sharkskin: (1) Q1-3563 Fluid from DOW CORNING-hydroxylterminated dimethyl siloxane, viscosity—75 mPa·s, (2) 4-2737 PA FLUID from DOW CORNING hydroxylterminated siloxane, silanol content—3.8%, viscosity— 41 mPa·s, (3) 3-0133 POLYMER from DOW CORNING hydroxylterminated dimethyl siloxane, silanol content— 1,530 ppm, viscosity—2,000 mPa·s, (4) Polyester-polyurethane-based thermoplastic elastomer Pearlstick® 46–73/32 from Merquinsa, (5) Polyester-polyurethane-based thermoplastic elastomer Elastollan® S 50 A from Elastogran, (6) Siloxane-diisocyanate-based thermoplastic elastomer TPSE 160 from Wacker Chemie, (7) E41 silicon rubber RTV 1 from Wacker Chemie, (8) SilGel® 612 from Wacker Chemie is a pourable, addition-curing, two-component silicone rubber RTV 2 that cures at room temperature to a very soft, gel-like vulcanizate.
complex modulus G* divided by 2πf. The elasticity factor G1/G2 and the complex viscosity of LLDPE “LL1201 XV” as a function of frequency are presented in Fig. 1. A ram extruder from Loomis with a barrel of 60× 200 mm (diameter·length) and a hydraulically driven piston was used to extrude molten PE through a die. The piston velocity was controlled by a computer. Values of pressure and of piston position were digitized during extrusion and transmitted to the computer for records. Die and extrudate were illuminated by a stroboscope and video-recorded by a camcorder at 25 frames/second. The stroboscope was synchronized with the camcorder so that the video records were triggered simultaneously with the data records to get precise correspondence between them. In addition to the ram extruder we also used a screw extruder “POLYTEST20T” from Schwabenthan-Maschinen equipped with an annular die of 40 mm diameter to produce blown film.
Results and discussion Extrusion of virgin LLDPE through a clean die made from steel The “Flow curve,” which is a plot of apparent shear stress versus apparent shear rate, is commonly used for rheological characterization of molten polymers. However, we discuss in this article the origin of slip and sharkskin instabilities during flow of LLDPE, phenomena, which may include friction and fracture. The use of reduced quantities like apparent shear stress and apparent shear rate in the characterization of friction and fracture would be confusing. Therefore, we present characteristic curves of the directly measured quantities, i.e., the pressure (P) at the die entrance versus the averaged extrudate velocity (V) to characterize the flow in a circular die. The average extrudate velocity (V) is derived from the volumetric flow rate Q by
Instrumentation A rotational rheometer “RHEOTEST RT-20” from HAAKE-Thermo was used to measure the dynamic– mechanical behavior of viscoelastic materials at temperatures between 165 and 205 °C. We used a parallel plate geometry with a diameter of 20 mm and a gap size of 1 mm. Frequency sweeps were carried out at frequencies f between 0.06 and 40 Hz at controlled stress of 63.6 Pa. A viscoelastic material can be characterized by its storage modulus G1 and loss modulus G2. We use an “elasticity factor” G1/G2 as a criterion of elasticity at a defined frequency f instead of the inverse ratio G2/G1, which is the loss tangent δ. The use of the ratio G1/G2 instead of tan (δ) is more convenient to present data of complex viscosity and elasticity in one plot. The complex viscosity is defined as the
V ¼
4Q pd 2
ð1Þ
where d is the diameter of the die. If needed, the apparent shear stress Cτa can be calculated from the reported pressure P by ta ¼
4L d
ð2Þ
where L is the length of the die, and the apparent shear rate : g a is given by :
ga ¼
32Q 8V ¼ pd 3 d
ð3Þ
Characteristic curves are presented in Fig. 1 for a steel die of dimension 6×32 mm by a solid line, for a steel die
Rheol Acta Fig. 1 a The elasticity factor G1/G2 and the complex viscosity of LLDPE “LL1201 XV” as a function of frequency. b Characteristic curves for LLDPE “LL1201 XV”, i.e., plots of the extrusion pressure versus average extrusion velocity for tubular dies 6×12 and 6×32 mm as well as for a sharp diaphragm (orifice) of 6 mm diameter at 165 °C. Critical extrusion velocities where onset of melt fracture is observed are indicated in the legend (bottom right corner) and marked by crosses in circles at characteristic curves. It is interesting to note that a straight line (dash–dotted line) can connect three crosses
6×12 mm by a dashed line, and for a sharp diaphragm of 6 mm diameter by a dotted line. Extrusion was done at a temperature of 165 °C. The diaphragm was made from a steel disk (2 mm thickness) having a conical entrance angle of 90°. The onsets of sharkskin are marked in the plots by crosses in circles (7.4; 6.3; 4.7 mm/s). Extrudate appearance is presented in Fig. 2 for extrusion through the die 6× 32 mm. Sharkskin appears at the extrudate surface at velocities from about 7 to 80 mm/s (see Fig. 2b,c, and d). At velocities of 88 mm/s and above for the die 6×32 mm
(above 100 mm/s for the die 6·12 mm), periodic “stick– slip” transitions are observed resulting in a “bamboo-like” appearance of the extrudate when smooth areas and areas of sharkskin alternate (see Fig. 2e). In the case of the diaphragm, we do not observe any “stick–slip” transitions. At velocities above 150 mm/s for the die 6·32 mm (140 mm/s for the die 6·12 mm) we see “super-flow” extrusion where the extrudate has a smooth surface (see Fig. 2f ). At velocities above 230 mm/s (220 mm/s for the die 6×12 mm) craters of cavitation appear (see Fig. 2g),
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Fig. 2 Appearance of extrudate produced with a steel die 6×32 mm
and at velocities above 450 mm/s, the extrudate shows irregular wavy distortions and severe surface fracture, i.e., “gross melt fracture” (see Fig. 2h). Extrusion of virgin LLDPE through a clean die made from glass The use of a glass die 6×32 mm gives us more information about the peculiarities of sharkskin as we observe both the interior and the exterior of the die near its end. The glass die with a sharp rim was produced in the following way: We made a notch at a glass tube having an outer diameter of 8 mm and bent it against a sharp knife so that a crack cut the tube across. In this way, we cut a piece of the glass tube of proper length, then heated one edge by an open flame and reamed it to a funnel to fix the die in the bottom plug of the extruder barrel. We charged the barrel with LLDPE, extruded molten polymer as described above, and made video records of the extrudate and of the die. These photos are presented in Fig. 3. Figure 3a shows a “wavy” stage of sharkskin. Figure 3b,c, and d shows an “eruptive” stage of sharkskin. Figure 4 presents the “eruptive” stage of sharkskin under larger magnification. The eruptive sharkskin itself has two modes, one is at velocities below 19 mm/s, and another is at velocities above 28 mm/s. The high-velocity mode is
characterized by deeper variations of relief in comparison to the low-velocity mode, i.e., the ridges are higher and the valleys between them are wider. For the high-velocity mode, cracks inside the glass die correspond to every ridge of the sharkskin (see Fig. 4c). We do not observe any crack inside the die for the low-velocity mode (see Fig. 4a). It is especially easy to see the difference between the two modes in Figs. 3c and in 4b, where the two modes alternate and cracks inside the die correspond to every second ridge (see Fig. 4b). The effective frequency of the sharkskin instability is about 20 Hz, see Fig. 4a and c. The LLDPE is an elastic fluid at this frequency (G1/G2=1.4 at 20 Hz). The sharkskin relief has reproducible elements of ridges and valleys. As the ridges of the sharkskin structure are not parallel to the die exit, we can derive their kinetics from only one picture without the need of a high-speed camera. Suppose we lay a piece of black paper with a narrow vertical slit to the right edge of Fig. 4a and move it slowly to the left. As the slit moves, we can see in the opening how molten LLDPE accumulates near the die exit. At some point, the accumulated material detaches from the die face and a newly-formed ridge drifts downstream with acceleration, which is caused by tensile stress in the surface layer near the die exit. Then a new portion of PE accumulates at the die exit while the valley grows and separates the newlyformed ridge from the die exit. The valley between the
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Fig. 3 Appearance of extrudate produced by a glass die. a Onset of “wavy” sharkskin. b Eruptive sharkskin in a low-velocity mode caused by adhesion failure at the die face. d Eruptive sharkskin in a
high-velocity mode caused by adhesion failure both at the die face and die land. c two modes of eruptive sharkskin together. Difference in surface roughness produced by every mode is visible
ridges has a very rough irregular surface. We see in Fig. 4c that cracks produce the valleys, and it is logical to assume that also the rough surface of the valley in Fig. 4a is produced by a crack. We do not observe any crack in Fig. 4a, and considering the geometry of our pictures, this is only possible if the cracks propagate only in a radial direction, i.e., along the die face. A sketch presented in Fig. 5a illustrates this situation in the cross-section of the flow and shows the crack of adhesion failure in a moment when it ruptures the extrudate near the die rim and produces a seed crack. At higher velocities, the crack propagates not only along the die face, but also inside the die as shown in Fig. 5c. We see such a crack in Fig. 4c as a dark area upstream from the die exit. In the middle of Fig. 5, the upper frame (marked by number 1) shows schematically the moment of adhesion failure. The second frame shows the moment when the crack penetrates into the extrudate and produces a seed crack. The bottom frame shows how the molten PE flows upstream of the seed crack to the die face and accumulates
there while the seed crack develops to the valley between the ridges. Therefore, sharkskin in its eruptive stage is caused by cracks, which deviate from the die wall toward the extrudate. Sharkskin could be suppressed by the use of coatings of the die, which prevent deviations of the cracks from the boundary and notching of the extrudate. In Fig. 6a, a ridge is peeling from the die face without a visible crack and fracture of the extrudate. We see in the right part of this frame that another ridge is peeling from the die face so that a valley with very rough relief is produced. There were no voids detected inside the transparent die up to 200 mm/s. At the rate of extrusion above 220 mm/s, we see macroscopic voids of surface cavitation inside the die as well as their prints at the surface of the extrudate. An example is presented in Fig. 6b. At the rate of extrusion above 340 mm/s, we detect voids of surface as well as of volume cavitation inside the die. The voids of volume cavitation and craters at the surface of the extrudate are presented in Fig. 6c for an extrusion rate of 380 mm/s.
Fig. 4 Appearance of extrudate in eruptive sharkskin mode produced with a glass die. a Eruptive sharkskin in the low-velocity mode caused by adhesion failure at the die face. c Eruptive sharkskin in the high-
velocity mode caused by adhesion failure both at the die face and in die land. b Two modes of the eruptive sharkskin alternate. Difference in the surface relief produced by every mode is visible
Rheol Acta Fig. 5 Eruptive sharkskin modes in cross section (schematic). Frame A corresponds the mode shown in Fig. 3b and Fig. 4a. Frame C corresponds to the mode shown in Fig. 3d and Fig. 4c. The kinetics of the highvelocity mode as it is observed in Fig. 4 is presented in the middle frame
Extrusion of virgin LLDPE through a coated steel die We used thermoplastic elastomers or silicon rubber to coat the inside of a steel die. We measured the Elasticity Factor G1/G2 of the elastomers at a frequency of 20 Hz because the specific frequency of sharkskin is close to 20 Hz. We also determined the complex viscosity, i.e., the complex modulus G* divided by 2πf, at a frequency f of 0.1 Hz to characterize the coatings. The complex viscosities of the coatings were then normalized to the complex viscosity of molten LLDPE at f=0.1 Hz (15.1 kPa·s). The viscosity ratios thus obtained are termed “relative viscosity” in tables and figures. The characteristic curve by use of thermoplastic elastomer 46–73/32 for coating is presented in Fig. 7 by a dashed line. This viscoelastic material shows less elasticity (G1/G2= 1.06) as LLDPE (G1/G2=1.4) at a frequency of 20 Hz. We observed no lubrication at velocities below 60 mm/s. Sharkskin defects appeared at velocities above 73 mm/s in narrow striations. The characteristic curve shows “stick– slip” transitions above 200 mm/s and a “super flow” extrusion mode above 230 m/s with prints at the surface similar to those we attributed to surface cavitation as seen Fig. 6 a A ridge is peeling from the die exit without a crack during stick-slip transition. b corresponds to onset of surface cavitation during “super flow”, i.e. slip inside the die. Surface cavitation produces specific prints at the extrudate surface. c corresponds to onset of volume cavitation inside the die. Volume cavitation produces deep irregular craters at the extrudate surface. Depth and number of the craters grow as rate of extrusion is increasing
in the case of the glass die. Above 300 mm/s irregular craters similar to those we attributed to volume cavitation were dominating. The characteristic curve by use of thermoplastic elastomer S50A for coating is presented in Fig. 7 by the dotted line. This viscoelastic material shows an enhanced elasticity factor at a frequency of 20 Hz (G1/G2=1.86) in comparison to LLDPE. We observe considerable lubrication at velocities above 8 mm/s. Surface defects appeared at velocities above 58 mm/s first as dullness and above 80 mm/s as irregular craters and tearing of the surface layer. The characteristic curve by use of thermoplastic elastomer TPSE 160 for coating is presented in Fig. 7 by the short-dashed line. This material shows a much higher elasticity factor at a frequency of 20 Hz (G1/G2=2.52) in comparison with molten LLDPE. We observed good lubrication at velocities above 1 mm/s. Surface defects appeared at velocities above 142 mm/s first as dullness and above 170 mm/s as irregular craters. The characteristic curve by use of silicon rubber E41 is presented in Fig. 7 by dash–dotted line. This rubber is very elastic (G1/G2>10) at a frequency of 20 Hz. We observed high lubrication at velocities above 1 mm/s and especially
Rheol Acta Fig. 7 Characteristic curves for extrusion through a die coated by thermoplastic elastomers. Solid line, reference characteristic curve for the corresponding clean die; dashed line, the die is coated by TPUE 46–73/32; dotted line: TPUE S50A. Short-dashed line: TPSE 160. Dash-dotted line, E41 silicon rubber. Elasticity factors and relative viscosity values are presented in the legend (left upper corner). The complex viscosity values are normalized to the complex viscosity of LLDPE at 0.1 Hz (15.1 kPa·s)
at high velocities. Surface defects appeared at velocities above 292 mm/s as irregular craters similar to those we attributed to volume cavitations. The elastomers used for coating, their characteristic mechanical properties, and the extrusion velocities at the onset of melt fracture are summarized in Table 1. The use of siloxanes cured by boric acid as die coating We cured DOW 4-2737, DOW 3-0133, or their blends by boric acid under heating, and measured elasticity of the resulting product at a frequency of 20 Hz as well as the complex viscosity at a frequency of 0.1 Hz. Sharp changes in the elasticity factor G1/G2 and the complex viscosity of the cured silanols occurred when the molar ratio of boric acid to silanol functional radicals is above 1.15, i.e., above about 2 wt% of boric acid for DOW 4-2737 and above 0.1 wt% of boric acid for DOW 3-0133. The cured silanols we used for coating the inside of a steel die with Table 1 Melt fracture onset when elastomers are used to coat steel die (elasticity factor G1/G2 at f=20 Hz and relative viscosity at f=0.1 Hz) Number
1 2 3 4
Coating elastomer 46–73/32 S50A TPSE 160 RTV 1, E41
Properties of the material G1/G2
Rel. viscosity
1.06 1.86 2.52 >10
0.86 11.3 0.02 –
Melt fracture onset, mm/s 83 58 142 292
dimensions 6·32 mm. Composition and mechanical properties of the cured silanols are summarized in Table 2 together with data of melt fracture onset for comparison. Characteristic curves are presented in Figs. 8 and 9, and the onsets of sharkskin are marked at the curves by crosses in circles. DOW 4-2737 (63 wt%) cured with boric acid (37 wt%) shows essentially viscous behavior at a frequency of 20 Hz in comparison to LLDPE (G1/G2=0.17). Onset of sharkskin occurs at about 15 mm/s. At extrusion rates from about 50 to 90 mm/s, surface defects get smaller. At extrusion rates above 195 mm/s, extrusion is going into the so-called “super flow” mode without sharkskin and with less friction losses. At extrusion rates above 235 mm/s, some irregular craters appear at the extrudate surface similar to those we attributed to volume cavitation. The characteristic curve is presented in Fig. 8 by the dashed line. When DOW 4-2737 (90 wt%) was cured by boric acid (10 wt%) and used for coating of the die, surface defects appeared at about 60 mm/s in very narrow striations. From about 185 mm/s up to 245 mm/s extrusion shows “stick– slip” transitions, and enters in the “super flow” mode above 245 mm/s. In the range of extrusion rates from 205 to 345 mm/s, the extrudate shows defects similar to those we attributed to surface cavitation. Above 345 mm/s, irregular craters resulting from volume cavitation appear. The characteristic curve is presented in Fig. 8 by the dotted curve. When DOW 4-2737 (81 wt%) was blended with DOW 3-0133 (9 wt%) and cured by boric acid (10 wt%), and then used for coating of the die, surface defects appeared at
Rheol Acta Table 2 Melt fracture onset when silanols cured with boric acid are used to coat steel die (elasticity factor G1/G2 at f=20 Hz and relative viscosity at f=0.1 Hz)
Number
Coating composition 3-0133
1 2 3 4 5 6 7 8
– – 9 18 90 99 90 99.9
4-2737 63 90 81 72 – – 9 of RTV 2 –
about 129 mm/s in very narrow striations. The extrudate showed irregular craters above 200 mm/s and the severity of the defects grow, as the rate of extrusion was increased. The characteristic curve is presented in Fig. 8 by the shortdashed curve. This coating produced pronounced slip above 10 mm/s but no “super flow” at high rate of extrusion. When DOW 4-2737 (72 wt%) was mixed with DOW 3-0133 (18 wt%) and cured by boric acid (10 wt%), and then used for coating of the die, surface defects appeared at about 193 mm/s in narrow striations. The characteristic curve is presented in Fig. 8 by a dash–dotted line. This coating resulted in pronounced slip above 7 mm/s, but no “super flow” at high rates of extrusion. At extrusion rates above 210 mm/s, the extrudate showed defects similar to those we attributed to volume cavitation, and the severity of the defects grows as the rate of extrusion increases.
Fig. 8 Characteristic curves for extrusion with the die coated by silanols cured by boric acid. Solid line, reference characteristic curve for the corresponding clean die; dashed line, the die is coated by 4-2737 (63 wt%) cured with boric acid (37 wt%); dotted line, 4-2737 (90 wt%) cured with boric acid (10 wt%); short-dashed line, 4-2737 (81 wt%) blended with 3-0133 (9 wt%) and cured with boric acid (10 wt%); dash–dotted line: 4-2737 (72 wt%) blended with 3-0133 (18 wt%) and cured with boric acid (10 wt%)
Properties of the material B(OH)3
G1/G2
Rel. viscosity
37 10 10 10 10 1 1 0.1
0.15 0.83 0.99 1.21 1.46 2.36 2.92 3.04
0.17 0.21 0.26 0.29 0.09 0.22 0.34 0.0008
Melt fracture onset, mm/s 15 60 129 193 199 192 164 176
When the viscous silanol fluid DOW 3-0133 (90 wt%) was cured with boric acid (10 wt%) and used as coating, the extrudate showed some dullness above 185 mm/s and irregular tearing of the surface in the form of craters above 199 mm/s. The characteristic curve is presented in Fig. 9 by the dashed curve. When DOW 3-0133 (99 wt%) was cured with boric acid (1 wt%) and used as coating, the extrudate showed a dull surface above 150 mm/s and irregular tearing of the surface in the form of craters above 192 mm/s. The characteristic curve is presented in Fig. 9 by the dotted curve. When DOW 3-0133 (90 wt%) was blended with SilGel® 612 from Wacker Chemie (9 wt%) and cured with boric acid (1 wt%), and then used for coating, the extrudate showed surface defects at relatively low rate of extrusion (above 165 mm/s). The characteristic curve is presented in
Rheol Acta Fig. 9 Characteristic curves for extrusion with the die coated by silanols cured by boric acid. Solid line, reference characteristic curve for the corresponding clean die; dashed line, 3-0133 (90 wt%) cured with boric acid (10 wt%); dotted line, 3-0133 (99 wt%) cured with boric acid (1 wt%); short-dashed line, 3-0133 (90 wt%) blended with RTV 2 silicon rubber (9 wt%) and cured with boric acid (1 wt %); Dash–dotted line, 3-0133 (99.9 wt%) cured with boric acid (0.1 wt%)
Fig. 9 by the short-dashed curve. This coating shows good lubrication at low velocities. When DOW 3-0133 (99.9 wt%) was cured with boric acid (0.1 wt%) and used as coating, the extrudate showed a dull surface above 150 mm/s and irregular tearing of the surface in the form of craters above 176 mm/s. The characteristic curve is presented in Fig. 9 by the dash– dotted curve. It shows good lubrication at low velocities. In summary, we do not see the “super flow” mode of extrusion at velocities below 400 mm/s when using coatings made from silanols cured by boaric acid in cases where the coating is elastic (G1/G2>1) at a frequency of 20 Hz. It is interesting to note that with the use of elastic coatings, the onset of surface defects corresponds to the extrusion rate when the respective characteristic curve is crossing the characteristic curve of the reference. We see better lubrication at low velocities with the use of more elastic materials for coating (compare Figs. 8 and 9). Impact of inorganic fillers in the composition of coating materials Powder of inorganic fillers was added to the blend of DOW 4-2737 (77.5%) and DOW 3-0133 (19.5%) under stirring. The blend was cured by boric acid (3%). The following inorganic fillers were used: natural kaolin, kaolin calcinated at 600 °C, bentonite, mica, BN, silica, aluminum hydrate, and borax Na2B4O7. The use of borax and silica as well as of plate-like fillers like kaolin, BN, and mica results in a
high efficiency to delay melt fracture onset. The data on melt fracture onsets for the case of filled coatings are summarized in Table 3. A remarkable difference showed up between the use of calcinated kaolin and the use of untreated natural kaolin as filler (+19%) in the coatings. The characteristic curve for calcinated kaolin is presented in Fig. 10 by the dotted line, and the onset of melt fracture was detected at 156 mm/s. The characteristic curve for the case of natural kaolin is presented by the dash–dotted line with the onset of melt fracture at 176 mm/s. In the case of calcinated kaolin, Table 3 Melt fracture onset when a blend of cured silanols DOW 42737 and DOW 3-0133 (4:1 weight ratio) with fillers is used to coat steel die No.
Curing agent
Filler
Melt fracture onset, mm/s
1 2 3 4 5 6 7 8 9 10 11 12
3% B(OH)3 3% B(OH)3 3% B(OH)3 3% B(OH)3 3% B(OH)3 3% B(OH)3 3% B(OH)3 3% B(OH)3 3% B(OH)3 6% B(OH)3 16% Borax 14% Borax
Bentonite (+19%) Al(OH)3 (+19%) Borax (+6%) Calcinated Kaolin (+19%) Kaolin (+19%) BN (+19%) Quartz (+19%) Kaolin (+33%) Mica (+9%) Borax (+19%)
132 140 140 156 176 180 196 198 198 201 201 68
Graphite (+41%)
Rheol Acta Fig. 10 Characteristic curves for extrusion with the die coated by DOW 4-2737 (82.2%) treated by phosphoric acid (3.8%) and cured by borax (14%) having additives of graphite (+41%), by the blend of the DOW 4-2737 (77.5%) and DOW 3-0133 (19.5%) cured by boric acid (3%) and having additives of kaolin or calcinated kaolin (+19%)
lubrication in the range from 4 to 40 mm/s as well as the efficiency to delay surface melt fracture was lower than in the case of natural kaolin. With larger amounts of kaolin (+33% of the weight of the blend “silanols + boric acid”),
the onset of melt fracture was detected at 198 mm/s. We also used borax as filler (6%) in DOW Q1-3563 (91%) cured by boric acid (3%). The onset of melt fracture was detected at about 140 mm/s. At larger content of borax
Fig. 11 Appearance of the extrudate produced with a steel die 6×32 mm coated by a blend of DOW 4-2737 (62%) with DOW 3-0133 (17%) cured by boric acid (5%) with additives of borax (16%)
Rheol Acta
(16%) in the blend of DOW 4-2737 (62%) with DOW 3-0133 (17%) cured by boric acid (5%), the melt fracture onset was detected at 203 mm/s. The appearance of the extrudate is presented in Fig. 11. Thus, the use of inorganic fillers based on oxides helps to delay the onset of melt fracture. Graphite powder was mixed with DOW 4-2737, and the blend was cured under heating by phosphoric acid (3.7%) and borax (14%). The characteristic curve is presented in Fig. 10 by the dashed line. The use of graphite as filler shows no lubrication at velocities below 8 mm/s. It also leads to a low efficiency in delaying the onset of sharkskin (63 mm/s). A tentative explanation may be the low adhesion of silanols to graphite. Investigation of the potential of cured silanols for industrial use Silanols cured by borates were blended with matrix polymer (LLDPE) in amounts of about 0.1 wt% and used as polymer processing additives (PPA). The additive is immiscible with the matrix polymer. During extrusion, it deposits on the surface of barrel and die and replaces molten polyolefin at the metal surface. To enlarge adhesion of the PPA to metal, we used silanols cured by borax and phosphoric acid. Cured silanols can be blended with powder of polyethylene in a mixer with quickly rotating blades for fabrication of a master batch or blended directly with polyethylene granules. In our experiment of blown film extrusion, it took about 30 min (“induction time”) to suppress sharkskin at an extrusion temperature of about 190 °C, and even a shorter time at a temperature of about 230 °C. Meanwhile, pressure at the extrusion die dropped below 60% of the value without addition of PPA at the same throughput. Silanols are cheap (∼4 Euro/kg) and nontoxic. Borates are known as chemicals that are very effective in controlling and eliminating insects and fungi. Although they are not harmful to mammals, they are toxic for insects. Boric acid is recognized for its application as a moderate antiseptic agent and emulsifier. Phosphoric acid is a component of “Coca-Cola” drinks. The use of “Silly Putty®” as a toy for children since 1950s can be a good argument that silanols cured by phosphoric acid, boric acid, and borax are not harmful to humans and may be used in the production of polymer films for food packaging as Food and Drug Administration compliant additives. The benefits from the use of cured silanols as novel PPAs include those from the use of the fluorinated polymers currently in use in the industry plus lower price, higher efficiency, and better stability at high temperature. Inclusions of the cured silanols trapped in the produced polymer film are soft and elastic. They do not scratch the film surface in the contact
area of two film layers and may work as antiblock agent. We made experiments with LLDPE, but the results may also be useful for processing of other polymers by extrusion and by injection molding to reduce pressure and/or temperature in extruders as well as to prevent gelling of the molten polymer in the extruder. The dispersion of cured silanols in brittle polymers may improve their toughness. Tentative explanation of the experimental results The origin of sharkskin is still debated. One widely accepted explanation of sharkskin proposed by Cogswell (1977) focuses on rapid acceleration and stretching of the extrudate surface after the extrusion die. In accordance with ideas of Hill et al. (1990) based on the apparent relation between adhesive failure and melt fracture and Howells and Benbow (1962), we believe that the sharkskin instability is caused by the swelling of the extrudate, flowing of the molten polymer around the die rim, stretching of its surface layer outside the die, and periodical failures in adhesion of the molten PE at the die rim. Due to stretching, the surface layer accumulates elastic energy and releases it during the act of adhesion failure. The adhesion failure propagates as a crack. At the die rim, the crack deviates from the die wall and produces a seed crack, i.e., a notch. The seed crack grows, ruptures the surface layer and creates a valley. Upstream from the crack, the molten PE decelerates, flows along the die rim, and forms another ridge that will detach from the die in the next act of adhesion failure. From our experimental data, we can distinguish two stages of sharkskin instability. The first one has a smooth “wavy” appearance. Dhori et al. (1997) explained this in terms of “common line motion” and “dewetting–rewetting.” The other one is an “eruptive” stage, which manifests itself in narrow ridges with sharp slopes and wide valleys between the ridges. It shows lower time frequencies than the wavy sharkskin. For the dies with a rounded rim, the eruptive sharkskin smoothly replaces the wavy one. With the die having a sharp rim, we detect two modes of the eruptive sharkskin. One corresponds to detachment only from the die face, while the other is caused by detachment also from the inner die surface near the die exit. The latter shows deeper fracturing of the extrudate. We observed surface cavitation during extrusion of molten LLDPE (Kulikov and Hornung 2004) and quickly fluctuating voids during extrusion of a clay paste with slip inside a transparent die (Kulikov and Hornung 2002). From this, we might consider local detachment of material from the rigid wall (the voids of detachment) or self-healing shear cracks as a plausible mechanism of the slip. In this model, every crack in its propagation separates an elastic body from a rigid wall and results in a small relative displacement of the elastic body relative to the rigid wall.
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The propagation of many cracks manifests itself in continuous slip. A flux of elastic energy is going to the tip of every crack to break the chemical bonds between the bodies. At its tail, the banks of the crack collapse and the bonds recover, at least partially. The self-healing shear crack converts elastic energy to heat. Low adhesion (weak chemical bonding) between the contacting bodies promotes slip. In the referred models of slip, the fault or a self-healing slip pulse propagates in the direction of bulk motion of an elastic body along the rigid surface. There is a compressive stress near the tip of the crack, and the cracks do not deviate into the inside of the elastic body. The die exit is a natural source of cracks, as it is a point where the boundary conditions of polymer flow suddenly change. The propagation of cracks along the rigid wall of the die upstream is unstable to deviations into the inside of the extrudate, whereas it is stable in the opposite direction. We know from the literature (see, e.g., Hatzikiriakos and Migler 2004) and from our experiments that sharkskin can appear at relatively low rates of extrusion in the condition of partial slip between the melt and a die wall having low surface energy, e.g., in the case of a die made from Teflon. In the same die, we see no sharkskin when the melt slips at higher rates of extrusion during super-flow. This discrepancy can be explained by assuming that slip at low velocities inside the Teflon die is caused by shear cracks that propagate upstream (unstable), whereas during superflow, the shear cracks propagate downstream (stable). The direction of stable propagation of the shear cracks can be reversed from downstream to upstream if there is an elastic coating at the die surface. With the use of the elastic coating, slip at low extrusion velocities would be stable whereas there will be no stable super-flow at high rates of extrusion. We observe in our extrusion experiments that there is lubrication at low rates of extrusion, but super-flow is suppressed at extrusion velocity below 400 mm/s if an elastic material coats the die. The use of relatively tough but very elastic silicon rubber resulted in the best lubrication and in the most efficient delay of melt fracture onset in comparison to any other materials we used for coating of the die. The use of very elastic and soft materials for coating of the die resulted in good lubrication at low extrusion velocities.
Conclusions This research presents empirical results to demonstrate the importance of elasticity to diminish friction losses and to suppress sharkskin instability in extrusion of LLDPE. Together with our experimental results, we present a tentative explanation for the delay of sharkskin onset and the occurrence of slip by the use of elastic coatings of
extrusion dies. We demonstrated here that elasticity of the coating is an important parameter for the reduction in friction in polymer processing. Molten polyethylene slips along elastic coatings already at very low rates of extrusion, the slip is stable up to very high extrusion velocities, and stick–slip instability is postponed to very high extrusion rate. We think that our experimental results may find their use not only in polymer processing but also in other industrial applications. Further systematic experimental and analytical investigations are required to elucidate the true nature of slip friction and sharkskin instability. It is interesting to note that Mother Nature uses elasticity to reduce friction in joints of mammal bones (Zhu and Granick 2003) and in sliding of snails along a rigid surface (McKinley 2006). Acknowledgement We are grateful to many people for technical and financial supports: Dr. Steffen Krause, Daniela Hertel, Heinz Jesper, Ludwig Kassecker, Dieter Maehler, Petra Moertter, Hans Sprau, and to the following companies for contributing polymers and chemicals: Dow Corning, Wacker Chemie, Merquinsa, Elastogran, and ExxonMobil Chemicals. Financial support by the German Science Foundation (Deutsche Forschungsgemeinschaft) is gratefully acknowledged.
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