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     

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 2 – Isothermal compressibility and wall slip

Abstract

Rheological characterisation of hydroxyapatite–high density polyethylene (HA–HDPE) composites has been performed in terms of isothermal compressibility and wall slip. Addition of HA to the polymer melt decreases the compressibility of the melt. The unfilled HDPE was found to exhibit wall slip at shear stresses as low as 0·10 MPa. The flow curves of the composites showed three distinct regions: a gradient at low shear rates; a plateau region; and a gradient at higher shear rate. An increase in rheometer pressure seems to suppress the slip in composites. The 40 vol.-% HA–HDPE composite exhibited two critical shear stresses, one corresponding to wall slip, which occurs in the lower shear rate region of the flow curve, and the other corresponding to a plateau, which is identified with the stick–slip behaviour of unfilled HDPE reported in the literature. The plateau shear stress increased with filler volume fraction and this effect is attributed to the decreased compressibility of the melt. A good correlation with a negative correlation coefficient was found to exist between compressibility and shear stress in the plateau region. The slip observed in unfilled HDPE and at low shear rates in the 40 vol.-% HA–HDPE systems has been explained in terms of a low molecular weight polymer layer formed at the melt/wall interface. The large interfacial slip observed in the plateau region is attributed to complete disentanglement of adsorbed chains from free chains at the melt/wall interface at and beyond the plateau region.     

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.     

A study of extrudate distortion in controlled-rheology polypropylenes

The melt fracture characteristics of controlled-rheology polypropylenes (CRPP) were studied by means of capillary rheometry experiments. CRPPs were produced through reactive extrusion of a commodity polypropylene resin using various peroxide concentrations. These materials exhibited lower molecular weights and narrower molecular weight distributions than those of the starting commodity resin. The CRPP materials studied were found to exhibit only gross melt fracture. At extremely high shear rates and relatively low temperatures, a sigmoidal flexure was observed in the flow curve of certain CRPPs. Generally, it was found that the severity of melt fracture decreased with increasing shear rate for a given material and temperature and in some cases, the extrudates exhibited completely smooth surfaces. Also, the severity of surface distortions was reduced when high L/D dies were employed at a given shear rate. The critical shear stress for the melt fracture onset was found to increase with decreasing molecular weight and polydispersity, and correlations have been developed between the critical stress values and the polymer polydispersity and shear compliance.     

Wall slip heating

When molten plastic is extruded, the upper limiting throughput is often dictated by fine irregular distortions of the extrudate surface. Called sharkskin melt fracture, plastics engineers spike plastics formulations with processing aids to suppress these distortions. Sharkskin melt fracture is not to be confused with gross melt fracture, a larger scale distortion arising at throughputs higher than the critical throughput for sharkskin melt fracture. Sharkskin melt fracture has been attributed to a breakdown of the no slip boundary condition in the extrusion die, that is, adhesive failure at the die walls, where the fluid moves with respect to the wall. In this article, we account for the frictional heating at the wall, which we call slip heating. We focus on slit flow, which is used in film casting, sheet extrusion, curtain coating, and when curvature can be neglected, slit flow is easily extended to pipe extrusion and film blowing. In slit flow, the magnitude of the heat flux from the slipping interface is the product of the shear stress and the slip speed. We present the solutions for the temperature rise in pressure‐driven slit flow and simple shearing flow, each subject to constant heat generation at the adhesive slip interface, with and without viscous dissipation in the bulk fluid. We solve the energy equation in Cartesian coordinates for the temperature rise, for steady temperature profiles. For this simplest relevant nonisothermal model, we neglect convective heat transfer in the melt and use a constant viscosity. We arrive at a necessary dimensionless condition for the accurate use of our results: Pé?1. We find that slip heating can raise the melt temperature significantly, as can viscous dissipation in the bulk. We conclude with two worked examples showing the relevance of slip heating in determining wall temperature rise, and we show how to correct wall slip data for this temperature rise. POLYM. ENG. SCI., 55:2042–2049, 2015. © 2014 Society of Plastics Engineers     

Rheological evaluation of metallocene polyethylenes with processing aids by multi‐wave oscillations

During the die flow of metallocene polyethylenes, flow instabilities may occur. Namely, wall slip, “sharkskin,” and stick‐slip (pressure oscillations) and gross fracture may be obtained depending on the volume flow rate and die geometry. It was reported that fluoroelastomers and boron nitride powders with hexagonal crystal structure can be used as suitable processing aids in melt extrusion processes. Fluoroelastomers at low concentrations act as die lubricants and may eliminate flow instabilities such as surface and stick‐slip melt fracture. On the other hand, specific boron nitride powders may not only eliminate surface and stick‐slip melt fracture, but also postpone gross melt fracture to higher volume flow rates. In this paper, a way for quantitative differentiation of the influence of polymer processing additives on rheological behavior is shown. Standard material functions show no clear‐cut differences. However, using multi‐wave oscillations with higher strain amplitudes make a quantitative assessment possible. Polym. Eng. Sci. 44:2047–2051, 2004. © 2004 Society of Plastics Engineers.     

A novel unified wall slip model for Poiseuille flow of polymer melt in the circular tube

In this study, to better reflect the slip effect of Poiseuille flow for polymer melt extruded through a circular tube, a novel unified wall slip model and flow equation based on two phase fluid system were proposed via a purely phenomenological approach. According to the different combinations of boundary conditions and flow parameters, the novel slip model was transformed into other models, such as adsorption–desorption model, entanglement–disentanglement model, lubrication layer model, Z–W model, and no‐slip model. The numerical simulation based on computed fluid dynamics was performed to verify the feasibility of the novel slip model. In the simulations, the radial flow velocity profile, shear rate, and viscosity distribution were obtained for six different models. Moreover, the effect of different slip coefficient combinations for the novel slip model on the radial flow velocity, slip velocity, volumetric flow rate error, and viscosity distribution of melt were also investigated and discussed. Results showed that the novel unified slip model not only incorporated the characteristics of other five models above mentioned, but also well interpreted the reason of simultaneously occurring the sharkskin surface defect and gross melt fracture phenomenon when flow rate of melt was extremely large. POLYM. ENG. SCI., 56:328–341, 2016. © 2015 Society of Plastics Engineers     

A study on the onset surface melt fracture of polypropylene materials with foaming additives

The melt fracture behaviors of linear and branched polypropylene resins with foaming additives were investigated. The effects of branching, processing temperature, additives, and blowing agent on the surface melt fracture of polypropylene materials were thoroughly studied. A CCD camera was installed at the die exit to precisely observe the onset of surface melt fracture of extruded foams. The critical wall shear stress was determined for various linear and branched polypropylene resins using a capillary die. It was found that the branching required to foam polypropylene resins also promotes melt fracture: the critical shear stress was decreased by 0.0175 MPa with an increase of 0.1 n/1000c in long‐chain branching. It was also observed that the dissolved blowing agent (butane) significantly suppressed the melt fracture of both linear and branched polypropylene resins. On the other hand, a noticeable increase in the critical shear stress of branched polypropylene materials was observed with the nucleating agent (talc) and the aging modifier (glycerol mono stearate), whereas almost negligible effect of the additives on the critical shear stress was observed for linear polypropylene materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Wall Slip of Linear Polymer Melts During Ultrasonic‐assisted Micro‐injection Molding

The wall slip of linear polymer melts under ultrasonic vibration is investigated by correcting the slip mechanism, and melt flow behaviors in ultrasonic‐assisted micro‐injection molding (UμIM) method are discussed. Based on the effect mechanism of ultrasonic vibration on the melt, theoretical models of the critical shear stresses for the onset of weak and strong wall slip during UμIM are established, and the change in rheological properties due to the onset of wall slip under ultrasonic vibration is experimental investigated by a built measurement system. The results show that the onset of weak and strong wall slip of the melt in micro cavity are promoted by ultrasonic vibration, which agree with the built theoretical models, and the melt filling capability in micro cavity is enhanced by reducing apparent viscosity and releasing shear stress of the polymer melt, which improves the molding quality of micro polymer parts via UμIM method. POLYM. ENG. SCI., 59:E7–E13, 2019. © 2018 Society of Plastics Engineers     

The synergistic effect of boron nitride particle and die on the capillary extrusion processability of mLDPE

The synergistic effects of boron nitride (BN) powder and die on the rheology and processability of metallocene‐catalyzed low density polyethylene (mLDPE) were investigated. The processability in the extrusion process is closely related to the interfacial properties between the polymer melts and the die wall. BN powder was added to mLDPE to reduce the friction coefficient and surface energy. Adding 0.5 wt% BN powder to mLDPE was very effective in improving the processability and the extrudate appearance. To study the effect of die surface property, three different dies were applied in capillary extrusion. One was conventional tungsten carbide (TC) die, and the others were hot‐pressed BN (hpBN) die and hot‐pressed BN composite (hpBNC) die. The applications of these BN dies were quite effective in delaying surface melt fracture (sharkskin) and postponing gross melt fracture to higher shear rate compared to the TC die. These improvements result from the fact that BN dies reduce the wall shear stress significantly and promote slip. The synergistic effect of processability could be obtained when both BN powder and hpBN die were used together.     

Effect of shear history on flow instability at capillary extrusion for long‐chain branched polyethylene

Effect of applied processing history on flow instability at capillary extrusion is studied using a commercially available low‐density polyethylene (LDPE) having long‐chain branches. It is found that processing history in an internal mixer in a molten state depresses long‐time relaxation mechanism associated with long‐chain branches, which is known as “shear modification.” Consequently, the onset of output rate for melt fracture increases greatly. Furthermore, it should be noted that the sample having intense shear history shows shark‐skin failure without volumetric distortion, although it has been believed that LDPE exhibits gross melt fracture at capillary extrusion. The reduction of elongational viscosity by the alignment of long‐chain branches along to the main chain is responsible for the anomalous rheological response. As a result, the sample shows shark‐skin failure like a linear polyethylene at a lower output rate than the critical one for gross melt fracture. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Capillary extrusion of composite materials

Investigations on the extrusion characteristics of composite systems were performed on the Sieglaff-McKelvey capillary rheometer, with particular emphasis on the characterization of flow instability and “melt fracture” phenomena. The mechanisms of melt fracture appear to be identical for both the filled and unfilled polymers (1. Polyethylene with glass beads; 2. Ethylene-propylene copolymer with graphitized carbon black). In all cases, the flow curves exhibit a plateau at some value of the shear stress. Above this shear stress plateau, melt fracture occurs. Although slip flow is the dominant mode of transport during melt fracture, the slippage in the tube may not be a necessary condition for the subsequent severe melt fracture.     

挤出过程中NR／CPB混炼胶壁滑移行为研究：Ⅱ.壁滑移速度

应用Ｍｏｎｓａｎｔｏ加工性能试验机，考察了天然胶／顺丁胶胶料在毛细管挤出过程中的壁滑移速度及其影响因素。发现，在较低的剪切速率下，试样的流动曲线出现不连续；壁滑移速度与壁面剪切应力之间大体上呈指数函数关系；发生壁滑移时的临界剪切应力随着温度的升高而下降。     

Experimental study and modeling of wall slip of polymethylmethacrylate considering different die surfaces

Wall slip of polymethylmethacrylate (PMMA) was studied on different flow channel surfaces using a rheological slit die and a high pressure capillary rheometer. As die surfaces polished steel, ground steel, and Si doped Diamond like carbon (DLC) were used. A new wall slip model is presented in this paper which assumes a lubricating film between the polymer melt and the die surface. The slip velocity has a power law dependency on wall shear stress. In the double logarithmic plot the wall slip curves are linear and can be parallel shifted to higher values with increasing temperature. The predicted dependencies of the wall slip velocity could be confirmed with experiments conducted with PMMA on polished steel. Furthermore, the die surface influences the flow behavior of PMMA. No wall slip was found on ground steel and on DLC. No complete film could be established by the lubricant on the ground steel die wall. The DLC‐coating exhibits a similar surface roughness and surface energy to polished steel, but the chemical composition is different. It is a metastable form of amorphous carbon containing sp² and sp³ bonds. As a consequence slip additives have a low ability to bond to this material. POLYM. ENG. SCI., 58:1391–1398, 2018. © 2017 Society of Plastics Engineers     

Processability of LLDPE/LDPE blends: Capillary extrusion studies

The processing behavior of a number of linear low‐density polyethylenes/low density polyethylene (LLDPE/ LDPE) blends with emphasis on the effects of long chain branches is presented. A Ziegler‐Natta linear low‐density polyethylene was blended with four low‐density polyethylene LDPE's having distinctly different molecular weights. The weight fractions of the LDPEs used in the blends were 1, 5, 10, 20, 50, and 75 wt%. Capillary extrusion reveals that the onset of sharkskin and gross melt fracture are slightly influenced with the addition of LDPE into LLDPE. However, the amplitude of the oscillations in the stick‐slip flow regime was found to scale well with the weight fraction of LDPE. Amounts as low as 1 wt% LDPE have a significant effect on the amplitude of pressure oscillations. These effects are clearly due to the presence of long chain branching (LCB); furthermore, it was observed that the onset of this flow regime was shifted to higher shear rates with increase of LDPE content. On the other hand, shear rheology is not sensitive to detect addition of small levels of LDPE up to 20 wt%. Extensional rheology can detect levels of LDPE as small as 1 wt% only at high Hencky strain rates (typically greater than 5s^?1) and only for certain blends, typically those that contain LDPE of high molecular weight. It is suggested that the magnitude of oscillations in the oscillating melt fracture flow regime is a sensitive method capable of detecting low levels of LCB. POLYM. ENG. SCI., 47:1317–1326, 2007. © 2007 Society of Plastics Engineers     

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.     

The effect of nanoclays on the processibility of polyolefins

The influence of a new processing additive (particles of organically modified nanoclays) on the processibility of polyolefins 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. Ziegler Natta and metallocene polyethylenes and one polypropylene were tested using these two pieces of equipment. It was found that the nanoclay additive had a significant effect on the extrudate appearance of polyethylene. It eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.05 to 0.5 wt%). To explain the possible mechanism for the effect of the additive on the processibility of the resins, shear and extensional rheological measurements were carried out for the pure resins as well those loaded with the nanoclay additives. It seems that the presence of nanoclays suppresses the development of extensional stresses to such high levels that can cause melt fracture phenomena. Finally, it was found that the combination of nanoclays with traditional processing aids such as fluoropolymers produce an enhanced processing aid that can increase the critical shear rates for the onset of melt fracture to levels much higher than the individual constituents when they are used independently. POLYM. ENG. SCI., 45:1098–1107, 2005. © 2005 Society of Plastics Engineers     

Sharkskin and oscillating melt fracture: Why in slit and capillary dies and not in annular dies?

The sharkskin and stick‐slip polymer extrusion instabilities are studied primarily as functions of the type of die geometry. Experimental observations concerning the flow curves, the critical wall shear stress for the onset of the instabilities, the pressure and flow rate oscillations, and the effects of geometry and operating conditions are presented for linear low‐density polyethylenes. It is found that sharkskin and stick‐slip instabilities are present in the capillary and slit extrusion. However, annular extrusion stick‐slip and sharkskin are absent at high ratios of the inside‐to‐outside diameter of the annular die. This observation also explains the absence of these phenomena in other polymer processing operations such as film blowing. These phenomena are explained in terms of the surface‐to‐volume ratio of the extrudates, that is, if this ratio is high, sharkskin and stick‐slip are absent. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

ELASTIC FRACTURE OF POLYMERIC FLUIDS

The elastic fracture of polystyrene solutions and melts was investigated using a plunger driven viscometer. The polymeric extrudate emerging from a circular capillary was photographed to determine the onset of fracture, and an effort was made to relate this information to the shear stress at the capillary wall, the recoverable shear strain, the entanglement density and the geometrical parameters characterizing the system. It was found that narrow molecular weight distribution polystyrenes dissolved in benzene clearly fractured as reported in the literature, but the onset of fracture could not be predicted by available criteria. As opposed to this, highly concentrated (polymer volume fraction up to 0.57) and elastic solutions of a wide molecular weight distribution polystyrene in benzene showed no fracture when judged using similar criteria. Nonetheless, this latter polymer fractured as a melt at a value of the wall shear stress less than that achieved in the solution runs. From an examination of the polymer rheology, it is concluded that the recoverable shear strain is the key quantity influencing the onset of elastic fracture. Also, the recoverable shear-shear rate behavior is different for the melt and the solution and it depends not only on the polymer molecular weight and its distribution but also on the solvent used. The surprising absence of elastic fracture for highly viscoelastic solutions can be understood if one realizes that a critical value of the recoverable shear strain is needed for fracture to occur.     

Wall slip and its implications in the design of single screw melt-fed extruders

The phenomenon of wall slip during the capillary flow of polymer melt is investigated for low and high density polyethylene. It is found that wall slip occurs in both cases, and that the effect is related to melt fracture. In addition, it is shown that a silicone fluid exhibits wall slip. The performance of the metering zone of a 38 mm diameter single-screw extruder is discussed in relation to wall slip. It is suggested that the power consumption of the extruder is reduced as a result of slip at the polymer/metal interface. Results based on experiments with the silicone fluid tend to support this hypothesis. A theoretical analysis of the effects of wall slip upon throughput rate and power consumption for a one-dimensional isothermal Newtonian case is included.