Combining boron nitride with a fluoroelastomer: An enhanced polymer processing additive

The effect of a new processing additive (boron nitride powder in combination with a fluoroelastomer) on the rheology and processability of molten polymers is studied. The equipment used includes an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross‐head) and a parallel‐plate rheometer. Metallocene polyethylenes with and without boron nitride (BN) and fluoroelastomer are tested in extrusion. First, it is demonstrated that BN is a superior processing aid compared to conventional fluoropolymer ones. Second, it is found that the combination of BN powders with a small amount of a fluorelastomer improves even further the processability of molten polymers (melt fracture performance).     

Boron nitride as a processing aid for the extrusion of polyolefins and fluoropolymers

The influence of a new processing additive (fine particles of boron nitride) on the processability of polyolefins and fluoropolymers in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and an extruder. Two metallocene polyethylenes and several Teflon® fluoropolymers were tested using these two pieces of equipment. The additive had a significant effect on the extrudate appearance of polyethylene and fluoropolymer particularly in the crosshead dies. It was found to eliminate surface melt fracture and to postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.005% to 0.5%). To explain the possible mechanism for the effect of the additive on the processability of the resins, rheological measurements using both parallel‐plate and sliding‐plate rheometers were carried out. The rheology of the resins did not seem to change significantly with the addition of boron nitride except for the low‐shear‐rate (low‐frequency) range, where the behavior of the filled resin was found to be similar to that of a crosslinked polymer or a phase‐separated entangled blend. Practical wire coating and tubing extrusion studies for these resins were also carried out.     

The effect of teflon™ coatings in polyethylene capillary extrusion

Two LLDPE resins were used in this work to determine the critical conditions for the occurrence of wall slip and melt fracture in capillary extrusion. It was found that the polymer-metal interface fails at a critical value of the wall shear stress of about 0.1 MPa and, as a result, slip occurs. At values of wall shear strees of about 0.18 MPa the extrudate surface appears to be matte, while small amplitude periodic distortions (sharkskin) appear on the surface of extrudates at wall shear stresses above 0.25 MPa. Using a special slit die, the polymer–wall interface was coated with Teflon? in order to examine the effect of this coating on the processability of polyethylenes. It was found that use of Teflon? promotes slip, thus reducing the power requirement in extrusion and, most importantly, eliminates sharkskin at high extrusion rates. © 1995 John Wiley & Sons, Inc.     

Effect of combining boron nitride with fluoroelastomer on the melt fracture of HDPE in extrusion blow molding

Boron nitride (BN) is a new polymer processing aid which not only eliminates surface melt fracture in the extrusion of molten polymers, but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values that depend on resin type and additive concentration. In this work, the influence of BN as a polymer processing additive is first examined in the extrusion blow molding of high‐density polyethylene (HDPE) resins in order to evaluate its usefulness and performance in operations other than continuous extrusion. The equipment used includes both a Battenfeld/Fisher 50‐mm extrusion blow molding machine and a parallel‐plate rheometer. Two types of HDPE, which are blended with boron nitride at various concentration levels, are tested accordingly. It is found that the degree of BN dispersion, characteristics of the HDPE resins, extrusion temperature, and induction time play an important role in eliminating melt fracture. Finally, the influence of combining BN with fluoroelastomer, as an enhanced and potentially better processing aid on the melt fracture of a third HDPE is examined. It is found that such a combination is a superior processing aid that allows extrusion blow molding at very high shear rates.     

Effect of boron nitride on melt fracture phenomena and processability of lldpe during extrusion

Melt fractures related to processing instabilities limit processing rates in many commercially important polymer processing operations, such as fiber spinning, film blowing, extrusion, and various coating flows. Therefore, melt fracture is responsible for deteriorating the quality and the mechanical properties of final products for rates greater than a critical processing one at which melt fracture occurs. In this study, a commercial linear low-density polyethylene (LLDPE) was modified by adding a small amount of boron nitride (BN) during extrusion in order to improve processability. Capillary rheometry was used to assess processability at various temperatures, levels of applied shear rate, and the length-to-diameter (L/D) ratio for both the pure resin and resins containing boron nitride. Also, parallel-plate rheometry was used to evaluate the dynamic rheological properties of these resins. The relationship between the characteristic relaxation time and the critical shear rate for the onset of melt fracture and slip is discussed.     

Ultrasonic improvement of rheological and processing behaviour of LLDPE during extrusion

The effects of ultrasonic oscillations on die pressure, productivity of extrusion, melt viscosity and melt fracture of linear low density polyethylene (LLDPE) as well as their mechanism of action were studied in a special ultrasonic oscillation extrusion system developed in our Laboratory. The experimental results showed that, in the presence of ultrasonic oscillations, the melt fracture or surface distortion of LLDPE extrudate is inhibited or disappears. The surface appearance of the LLDPE extrudate was greatly improved. The productivity of LLDPE extrudate was increased in the presence of ultrasonic oscillations. The die pressure, melt viscosity and flow activation energy of LLDPE decreased with the rise in ultrasonic intensity. The shear sensitivity of LLDPE melt viscosity decreased due to the increase of its power law index in the presence of ultrasonic oscillations. Inducing ultrasonic oscillations into LLDPE melt greatly improved its processability. A possible mechanism for the improved processibility is proposed. © 2003 Society of Chemical Industry     

Study on the melt fracture of metallocene poly(ethylene‐octene) in capillary flow

Shear viscosity and melt fracture of a metallocene poly(ethylene‐octene) were studied using a capillary rheometer and dies with different lengths. The true wall shear stresses determined at zero die length showed a dip at high shear rates. The shear viscosity was derived from the true wall shear stress. With increasing shear rates, the extrudate staged from smooth to three types of melt fracture with regular patterns, and then turned into irregular shapes. Three types of regular melt fractures—sharkskin, helix, and spiral (in sequence)—were observed with an increase of the shear rates. The wavelength of the regular melt fracture was measured from extrudates, and the corresponding frequency was calculated. The frequency increased at elevated melt temperatures. Both shear viscosity and frequency at different temperatures correlated well by using the time–temperature Williams–Landel–Ferry (WLF) superposition. Additionally, it was found that the frequency decreased slightly for a longer die but it increased when the shear rate went up. Three frequency functions were associated with three melt fracture patterns, respectively. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 903–911, 2005     

Gross melt fracture elimination: The role of surface energy of boron nitride powders

Boron nitride (BN) is an effective processing aid for the extrusion of polyethylenes. It postpones the onset of gross melt fracture to significantly high shear rates not previously attained with conventional fluoropolymers. However, BN particles containing relatively high amounts of boron oxide (B₂O₃) do not perform well as processing aids. A reliable procedure has been developed for measurement of surface energy of powders using the capillary rise technique through the use of Washburn's equation. It is based on finding the contact angle from liquid penetration experiments with polar and non‐polar liquids. Both the dispersive and non‐dispersive components of surface energy are determined. With this technique, the surface energy of a number of different powders has been assessed. These results of the surface energy of BN powders have been found to correlate well with the critical shear rate for the onset of melt fracture, indicating the important role that surface energy plays in gross melt fracture elimination.     

Melt fracture,melt viscosities,and die swell of polypropylene resin in capillary flow

The melt fracture, shear viscosity, extensional viscosity, and die swell of a polypropylene resin were studied using a capillary rheometer and dies with a 0.05‐cm diameter and length/radius ratios of 10, 40, and 60. A temperature of 190°C and shear rates between 1 and 5000 s^?1 were used. A modified Bagley plot was used with consideration of pressure effects on both the melt viscosity and end effect. The shear viscosity was calculated from the true wall shear stress. When the true wall shear stress increased, the end effect increased and showed critical stresses at around 0.1 and 0.17 MPa. The extensional viscosity was calculated from the end effect and it showed a decreasing trend when the strain rate increased. Both the shear and extensional viscosities correlated well with another polypropylene reported previously. The die swell was higher for shorter dies and increased when shear stress increased. When the shear rates increased, the extrudate changed from smooth to gross melt fracture with regular patterns (spurt) and then turned into an irregular shape. In the regular stage the wavelength of the extrudates increased when the shear rate increased. The frequency of melt fracture was almost independent of the shear rate, but it decreased slightly when the die length increased. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1587–1594, 2003     

Ultrasonic oscillations effect on rheological and processing properties of metallocene‐catalyzed linear low density polyethylene

The effects of ultrasonic oscillations and die materials on die pressure, productivity of extrusion, melt viscosity of metallocene‐catalyzed linear low density polyethylene (mLLDPE), as well as their mechanism were studied in a special ultrasonic oscillations extrusion system developed in our lab. Die materials used in our experiment included steel, brass, and polytetrafluoroethylene (PTFE)_. The experimental results showed that ultrasonic oscillations as well as die materials have great influence on the rheological and processing behavior of mLLDPE. Ultrasonic oscillations can greatly increase the productivity of mLLDPE melt extruded through different dies, and can decrease the die pressure and the melt viscosity of mLLDPE. Compared with steel or brass die, mLLDPE melt extruded through PTFE die is more sensitive to ultrasonic oscillations. A possible mechanism for the improved processability of mLLDPE is proposed in this article. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1873–1878, 2003     

The effct of asymmetries of die exit geometry on extrudate swell and melt fracture

Experimental investigations were performed to see how the die exit geometry and the extrusion velocity influence on extrudate swell and melt fracture for several polymer melts [low-density polyethylene, styrene-butadiene rubber (SBR) and SBR/HAF (carbon black) compound]. Four different types of die exit geometry were considered; 0° (symmetric. usual capillary die), and 30°, 45° and 60° (asymmetric dies) were chosen for the die exit angle. Extrudate diameters were measured without draw-down under isothermal condition. Polymer melts were extruded into an oil that has the same density and temperature as those of the extrudate. Extrudate swells from dies with different diameters were correlated with volumetric flow rates. It was observed that the extrudate swell increases with increasing volumetric flow rate and exhibits through a minimum value at about 45° die exit angle. As to the fracture phenomena, it was observed that the critical shear for the onset of melt fracture increases with the increasing die exit angle up to 45°. However, for 60° die exit angle, the onset of melt fracture is again similar to that of 0° exit angle.     

Flaring dies to suppress die drool

Die lip build‐up is the unwanted material accumulation on extrusion die lips. Here, flared dies are shown experimentally to suppress die lip build‐up. A semiempirical method for flared die design is also provided. Nonlinear viscoelastic constitutive equations are used to calculate the wall shear stress and first normal stress difference in flared dies. By incorporating melt memory, a promising design method for die flaring is presented. The stress history upstream of the die exit governs the die design. The upstream gap is selected to maximize undershoot of the first normal stress difference N₁ at the die wall caused by flaring. The flare length, on the other hand, is selected to ensure a steady N₁ at the die lips.     

Improvement in processability of metallocene polyethylene by ultrasound and binary processing aid

The effect of ultrasonic vibration and binary processing aid in improving the processability of metallocene linear low‐density polyethylene (mLLDPE) was investigated. During extrusion, ultrasonic vibration clearly reduced the die pressure and apparent viscosity of mLLDPE but had only a slight effect on its melt fracture. The effect of diatomite/PEG binary processing aid (BPA) was excellent in reducing the viscosity and eliminating the sharkskin fracture of mLLDPE. The effect of ultrasonic vibration and binary processing aid in improving the processability of mLLDPE was synergetic. With a combination of ultrasonic vibration and a small amount of processing aid, the flowability of mLLDPE was further improved, and the critical shear rate for the onset of sharkskin fracture was increased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1927–1935, 2007     

Melt fracture of two broad molecular weight distribution high‐density polyethylenes

The melt fracture instabilities of two broad molecular weight distribution (MWD) high‐density polyethylenes (one Ziegler–Natta and one metallocene HDPEs) are studied as functions of the temperature and geometrical details and type of die (cylindrical, slit, and annular). It is found that sharkskin and other melt fracture phenomena are distinctly different for these resins, despite their almost identical rheology. It is also found that the critical conditions for the onset of various melt fracture phenomena depend significantly on the type of die used for their study. For example, sharkskin melt fracture in slit and capillary extrusion was obtained at much small critical shear stress values compared with those found in annular extrusion. Moreover, the metallocene HDPE shows significant slip at the die wall in the sharkskin flow regime. On the other hand, the Ziegler–Natta HDPE has shown no sign of slip. These differences are discussed on the basis of differences in their MWDs that influence their melt elasticity. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Die design for rigid PVC—the effect of die land length on extrudate swell

PVC profile extrusion compounds have a unique morphology. While other polymers gradually decrease in extrusion die swell with increasing length/thickness (L/D) ratio, PVC profile extrusion compounds have a low die swell, quite independent of the die's L/D ratio in the range of 5 to 20. The fact that the die land length can be changed without changing the extrudate swell is an important consideration, which makes die design and balancing dies simpler and easier for PVC profile extrusion compounds. While other polymers substantially increase extrudate swell with increased shear rate, the swell of the PVC profile compounds is not much affected by shear or extrusion rate. This unique behavior allows wider processing latitude in profile extrusion and faster extrusion rates than with other polymers. Another unique factor in the rheology of PVC profile extrusion compounds is that extrusion die swell increases with increasing melt temperature, while other polymers have decreasing die swell with increasing melt temperature. The unusual rheology of PVC profile extrusion compounds is attributed to its unique melt morphology, where the melt flow units are 1 um bundles and molecules that have low surface to surface interaction and entanglement at low processing temperatures but increased melting and increased entanglement at higher processing temperatures. Other polymers, unlike PVC, have melt flow at the molecular level.     

Influence of the die configuration on the gross melt fracture of linear polyethylene

In this study, capillary extrusion experiments and rupture visualization experiments were carried out with a combined die configuration where two different dies were attached in a capillary rheometer. We observed that the gross melt fracture (GMF) and rupture of the melt occurred simultaneously when the diameter of the die located at the upstream position was larger than that of the die located downstream. However, when the location of the dies were interchanged, that is, the diameter of the upstream die was smaller than that of the downstream die, the upstream rupture did not accompany GMF up to a certain extent of shear rate. From these observations, we present a new theory on the origin of GMF. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Modeling of three‐dimensional flow and heat transfer in polystyrene foam extrusion dies

A three‐dimensional mathematical model was developed to investigate the nonisothermal, non‐Newtonian polymer flow through the dies used in the polystyrene foam extrusion process. The model, based on the computational fluid dynamics (CFD) code, Polyflow, allowed for the shear rate and temperature dependence of the shear viscosity of the blowing agent laden polystyrene melt. The model also accounted for viscous heating. The shear viscosity of the polystyrene‐blowing agent mixture was measured experimentally at several temperatures. The model was used to calculate pressure, flow, and temperature distributions in two different dies used for industrial‐scale extrusion of polystyrene foams. The article presents a selection of computed results to illustrate the effect of die design on uniformity of flow at the die exit, the overall pressure drop in the die, relative magnitudes of pressure drop in the land section versus the rest of the die, and temperature distribution in the die. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Influence of poly(ethylene glycol)‐containing additives on extrusion of ultrahigh molecular weight polyethylene/polypropylene blend

The influence of poly(ethylene glycol) (PEG)‐containing additives on the extrusion behavior of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blend was studied. It was found that the addition of small amounts of PEG to UHMWPE/PP blend resulted in significant reduction of die pressure and melt viscosity, and obvious increase of the flow rate at a given die pressure, while PEG/diatomite binary additives enhanced the improvement in the processability of UHMWPE/PP blend. When pure HDPE was extruded with the die through which UHMWPE/PP/PEG blend was previously extruded, the extrusion pressure of HDPE increased with the extrusion time gradually. This meant that PEG might migrate to the die wall surface and coat it in the extrusion of UHMWPE/PP/PEG blend. FTIR spectra and SEM micrographs of the UHMWPE/PP/PEG extrudates indicated that PEG located not only at the surface but also in the interior of the extrudates. So, the external lubrication at the die wall, combined with the internal lubrication to induce interphase slippage of the blend, was proposed to be responsible for the reduction of die pressure and viscosity. In addition, an ultrahigh molecular weight polysiloxane and a fluoropolymer processing aid were used as processing aids in the extrusion of UHMWPE/PP as control, and the results showed that only minor reduction effects in die pressure and melt viscosity were achieved at their suggested loading level. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1282–1288, 2006     

Functionalizing polymer surfaces by field‐induced migration of copolymer additives—role of shear fields

Flow‐induced migration polyethylene‐co‐methacrylic acid (PE‐co‐MA) and polystyrene‐b‐polydimethylsiloxane (PS‐b‐PD MS) copolymer additives in commercial long‐chain branch polyethylene (PE) and narrow‐molecular distribution polystyrene (PS) hosts was investigated in a capillary flow device. Attenuated Total Reflection Fourier Transform Infrared (ATR‐FTIR) spectroscopy and Dynamic Contact Angle (DCA) measurements were used to characterize surface composition of polymer specimen following extrusion through metallic dies with various length‐to‐diameter (L/D) ratios, (1100 ? L/D ? 3000). Results from experiments covering a broad range of shear rates and polymer residence times in the dies are reported. Provided that the polymer residence time in the die is sufficiently long, shear is found to increase the concentrations of low molecular weight copolymer additives on the host polymer's surface. The surface composition of copolymer additive is found to vary strongly with the wall shear rate and die L/D ratio. Decreasing the die diameter at fixed flow rate is found, for example, to be a more effective method for enhancing transport of additive to a polymer's surface than increasing shear rate at fixed diameter. A mechanism based on shear‐induced diffusion is proposed to explain the observed migration.     

A study of “worms” melt fracture in polyethylene extrusion blow molding process using capillary rheometry

During the polyethylene (PE) blow molding process of large size drums, string‐like defects, which are referred to here as worm melt fracture, can sometimes be observed on the extrudate surface. Such string‐like defects, in various shapes and sizes, are also observed in the capillary extrusion at high shear rates after the slip‐stick transition. The PE resin with broader molecular weight distribution (MWD) exhibits a greater degree of worm melt fracture while narrow MWD PE resin, which has higher slip velocity and a uniform slip layer, shows a lesser degree of worm melt fracture. It is hypothesized that the worm melt fracture is related to the die build‐up. Based on the mechanism of the fast die build‐up, it is proposed that the cohesive slip layer, which is a failure within the polymer melts at an internal surface, could emerge out from the die as these string‐like materials attached on the extrudates. The broader MWD resin, which has more small polymer chains and a lower plateau modulus, is postulated to have a weaker polymer melt, which then makes it easier to have such an internal failure and consequently have more string‐like defects at high shear rates. POLYM. ENG. SCI., 56:650–656, 2016. © 2016 Society of Plastics Engineers