Surface tearing and wall slip phenomena in extrusion of highly filled HDPE/wood flour composites

The extrudate surface tearing of highly filled high‐density polyethylene (HDPE)/wood flour composites has been investigated in relation to the rheological properties and the wall slip phenomenon in capillary dies. Rotational and capillary rheometers were employed to measure the rheological properties. Mooney analysis was used for determination of wall slip velocity. The results showed considerable increase of storage modulus, dynamic and shear viscosity with increasing wood flour loading. It was also found that all wood filled composites did not obey the Cox–Merz rule. The wall slip velocity depends on wood filler content and shear rate. Generally, with increasing shear rate the slip velocity sharply increases leading to plug‐like flow. It was observed that the surface of the extrudates becomes smoother with increase in shear rate and wood flour content. POLYM. ENG. SCI., 46:1204–1214, 2006. © 2006 Society of Plastics Engineers     

Rheology of wood plastics melt. Part 1. Capillary rheometry of HDPE filled with maple

Shear and extensional flow properties of the melts of high‐density polyethylene (HDPE)‐maple composites were studied with capillary rheometry to understand the effects of the wood content, particle size, and maleated polyethylene (MAPE). The viscosity data were compared with the values for neat matrix resin for reference. The effects of commercial wood particle size grades were examined at 60% by weight of wood loading. It was found that both shear and extensional viscosities increase with wood content but the filler content dependence is not as significant as for suspensions of inorganic fillers at similar filler loadings. Commercial wood particle size grades were found to result in less change in viscosity than wood content. The Mooney analyses conducted on the lower branch of the capillary shear flow data revealed a significant contribution of wall slip and confirmed the presence of a yield stress at higher filler contents. The internal lubrication role of MAPE was also illustrated in detail through the changes in both shear and extensional flow. POLYM. ENG. SCI., 45:549–559, 2005. © 2005 Society of Plastics Engineers     

Effects of stick–slip transition on polymer melt apparent shear viscosity measurement

A twin-bore capillary rheometer is used for the apparent shear viscosity measurement of commercial polyolefin melts based on Ostwald-de Waele model. The effects of stick–slip transition of linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) are investigated. The maximum error of apparent shear viscosity calculated by corrected shear rate is 23% when the stick–slip transition occurs. Based on the entanglement‑disentanglement theory, a schematic diagram for shear stress curve containing stick–slip transition is presented to illustrate polymer melt flow in capillary. In this study, the critical stress at the beginning of stick–slip transition at 220 °C is 23.01 kPa higher than that at 190 °C, and why it increases with increasing temperature is discussed with a molecular mechanism in combination with entropy elasticity and entanglement‑disentanglement theory. Through the analysis of ULDPE, PS, EVA, and K-Resin, it can be found that short branches or side groups are helpful to avoid the stick–slip transition. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48230.     

Rheological behavior of uncured styrene‐butadiene rubber at low temperatures,pure and filled with carbon black

The flow behavior of an uncured styrene‐butadiene rubber (SBR) has been studied by using a specific preshearing capillary rheometer in the range of temperatures encountered in extrusion, i.e. between 40°C and 90°C. A pure SBR and various SBR compounds filled with different amounts of carbon black (from 17 to 33 wt%) have been characterized. It was observed, for all tested materials, that the flow curve could be divided in different parts: at low shear rate, the material exhibits a classical behavior, where stress increases regularly with the shear rate. Above a certain critical stress, flow features changed, characterized by the simultaneous onset of wall slip and upstream instabilities. This critical stress is independent of temperature but increases linearly with carbon black amount. Flow curves at different filler contents were superimposed, using a shift factor that varies with filler content. Two theories for time/filler content superposition were proposed. Finally, a general viscosity law for uncured SBR compounds was introduced. This law is based on a Carreau‐Yasuda equation, where zero‐shear viscosity and characteristic time depend on both temperature and filler content, through Arrhenius and Krieger‐Dougherty expressions, respectively. POLYM. ENG. SCI., 55:2156–2162, 2015. © 2015 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.     

Measurements of slip velocity and frictional heating in the capillary extrusion of linear‐low‐density polyethylene with a fluoropolymer processing aid

The slip velocity and frictional or slip heating of linear‐low density polyethylene with a fluoropolymer processing aid in capillary flow were measured by rheo‐particle image velocimetry and thermal imaging. The pure polymer did not show slip before the stick‐slip regime but exhibited strong slip when blended with the processing additive. However, for shear stresses beyond the stick‐slip regime, the pure polymer and the blend exhibited the same flow behavior with slip. The slip velocity increased with the shear stress at two different rates before and after the stick‐slip and the contribution of slip to the total flow rate exhibited a minimum. Significant rises in temperature were measured under slip and no slip conditions, being these much higher than the values predicted by the adiabatic flow assumption. Clear difference was made between viscous and frictional heating before the stick‐slip regime, even though they could not be distinguished from one another at higher stresses. Overall, in the presence of slip, frictional and viscous heating act synergistically producing higher temperature rises in the melt. Finally, in contrast to predictions by numerical simulations of viscous heating, measured velocity profiles did not evidence the heating effects in the shear stress range analyzed in this work. POLYM. ENG. SCI., 56:837–845, 2016. © 2016 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     

Effect of wood fibers on the rheological properties of i‐PP/wood fiber composites

The effects of wood fibers on the melt rheological behavior of isotactic poly(propylene) (i‐PP)/wood fiber (WF) composites have been studied at WF concentrations of 0–32.2 vol % at 493 K. Shear stress–shear rate variations obeyed a power law equation, and the composites exhibited shear thinning, which increased with filler content. At a low shear rate, the apparent melt viscosity increased, while melt elasticity, after an initial decrease, also increased with WF concentration. At a higher shear rate, after an initial decrease, the melt viscosity showed an increase, as did melt elasticity, with increase in filler content. A titanate coupling agent, LICA 38, used to modify the wood fiber surface, modified these rheological parameters by functioning as a plasticizer/lubricant. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 644–650, 2004     

Slip velocity and slip layer thickness in flow of concentrated suspensions

The rheological characterization of highly filled suspensions consisting of a Newtonian matrix (hydroxyl-terminated polybutadiene), mixed with two different sizes of aluminum powder (30% and above by volume) and two different sizes of glass beads (50% and above by volume), was performed using a parallel disk rheometer with emphasis on the wall slip phenomenon. The effects of the solid content, particle size, type of solid particle material, and temperature on slip velocity and slip layer thickness were investigated. Suspensions of small particles of aluminum (mean diameter of 5.03 μm) did not show slip at any concentration up to the maximum packing fraction. However, suspensions of the other particles exhibited slip at the wall, at concentrations close to their maximum packing fraction. In these suspensions, the slip velocity increased linearly with the shear stress, and at constant shear stress, the slip velocity increased with increasing temperature. The slip layer thickness increased proportionally with increasing size of the particles for the glass beads. Up to a certain value of (filler content/maximum packing fraction), ϕ/ϕ_m, the slip layer thickness divided by the particle diameter, δ/D_P, was 0, but it suddenly increased and reached a value that was independent of ϕ/ϕ_m and the temperature. On average, the ratio of δ/D_P was 0.071 for aluminum and 0.037 for glass beads. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 515–522, 1998     

Evaluation of dynamic elastic properties of wood‐filled polypropylene composites

Dynamic modulus of elasticity (MoE) and shear modulus of wood‐filled polypropylene composite at various filler contents ranging from 10% to 50% was determined from the vibration frequencies of disc‐shaped specimens. Wood filler was used in both fiber form (pulp) and powder form (wood flour). A novel compatibilizer, m‐isopropenyl‐α,α‐dimethylbenzyl‐isocyanate(m‐TMI) grafted polypropylene with isocyanate functional group was used to prepare the composites. A linear increase in dynamic MoE, shear modulus, and density of the composite was observed with the increasing filler content. Between the two fillers, wood fiber filled composites exhibited slightly better properties. At 50% filler loading, dynamic MoE of the wood fiber filled composite was 97% higher than that of unfilled polypropylene. Halpin‐Tsai model equation was used to describe the changes in the composite modulus with the increasing filler content. The continuous improvement in elastic properties of the composites with the increasing wood filler is attributed to the effective reinforcement of low‐modulus polypropylene matrix with the high‐modulus wood filler. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1706–1711, 2006     

Rheological properties of glass bead-filled low-density polyethylene composite melts in capillary extrusion

The melt flow of glass bead-filled low-density polyethylene composites in extrusion have been observed by using a capillary rheometer to investigate the effects of temperature, shear rate, and filler content on the rheological properties of the melts. The results show that the melt shear flow obeys a power law, and the dependence of the apparent shear viscosity, η_app, on temperature is in accord with an Arrhenius equation. At the same temperature and shear rate, η_app increases slightly with increasing the volume fraction of glass beads, but the flow behavior index decreases with increasing filler content. In addition, the first normal stress difference of the melts linearly increases with increasing wall shear stress. Good agreement is shown with the N₁ calculated with the equation presented in this article and the pressured data from the sample melts. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1451–1456, 1999     

Enhanced apparent payne effect in a gum vulcanizate at low temperature approaching glass transition

For a vuleanized elastomer compound containing no filler, variation of dynamic moduli with strain was examined in the transition zone approaching glass transition at 0.1, 1, 10, and 15 Hz. In the complete absence of filler‐filler/filler‐polymer interactions, the nonlinearity in dynamic moduli becomes enhanced upon approaching glass transition regardless of testing frequency. Under the test protocol employed, the extraordinary nonlinearity at 10 Hz and 15 Hz is shown to be associated with a spectacular shear‐heating event. At 10 Hz and ?19°C, a pronounced yielding‐like peak in the in‐phase stress component emerges at small strain (< 2%) during testing of increasing strain. However, the corresponding temperature rise here is only about 0.4°C. Interestingly, at or above 0°C, no such peak in the in‐phase stress component emerges for compounds filled with reinforcing filler above the percolation threshold. Discussion is also made relating the observations to the sliding friction of rubber materials.     

Rheological properties of PDMS filled with CaCo3: The effect of filler particle size and concentration

The effects of filler particle size and concentration on the rheological properties of hydroxyl terminated polydimethylsiloxane (HO‐PDMS) filled with calcium carbonate (CaCO₃) were investigated by an advanced rheometric expansion system (ARES). The Casson model was used to describe the relationship between shear stress and shear rate for steady‐state measurement. Micron‐CaCO₃ could not afford the CaCO₃/HO‐PDMS suspensions obvious shear thinning behavior and a yield stress high enough, whereas nano‐CaCO₃ could provide the suspensions with remarkable shear thinning behavior and high yield stress. Incorporation of nano‐CaCO₃ into HO‐PDMS resulted in the transformation of HO‐PDMS from a mainly viscous material to a mainly elastic material. With increasing nano‐CaCO₃ content, shear thinning behavior of nano‐CaCO₃/HO‐PDMS suspensions became more obvious. Remarkable yield stress was observed in nano‐CaCO₃/HO‐PDMS suspensions with high filler content, and increased with increasing nano‐CaCO₃ content. The degree of thixotropy was quantitatively determined using a thixotropic loop method. It was found that nano‐CaCO₃ favored more the buildup of filler network structure in the suspensions than micron‐CaCO₃ at the same weight fraction. Furthermore, increasing nano‐CaCO₃ content accelerated the establishment of filler network structure in the nano‐CaCO₃/HO‐PDMS suspensions. An overshoot phenomenon was observed in the nano‐CaCO₃/HO‐PDMS suspensions at high shear rates. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3395–3401, 2006     

高密度聚乙烯的壁面滑移行为

用双筒毛细管流变仪和旋转流变仪研究了两种牌号的高密度聚乙烯(HDPE)熔体的壁滑行为,考察了发生壁滑的临界剪切应力。通过剪切速率扫描的方法发现两种牌号的HDPE熔体在较高的剪切速率下均发生粘滑转变,在150～230℃温度范围内,HDPE发生粘滑转变的临界剪切应力位于0.20～0.38 MPa范围内,与文献报道值一致;而且发生粘滑转变的临界剪切应力随温度线性增加,这与B rochard和de Gennes提出壁滑的解缠机理一致。用壁滑外推长度表征壁滑程度,发现两种牌号的HDPE熔体的壁滑外推长度均处于0.05～0.09 mm范围内。     

Melt fracture behavior of polypropylene‐type resins with narrow molecular weight distribution. II. Suppression of sharkskin by addition of adhesive resins

Standing on a hypothesis that the sharkskin of a polymer with a narrow molecular weight distribution at extrusion processing originates from a stick‐slip of the polymer at the die wall, the suppression of the sharkskin was tried by means of suppressing the slip by the addition of adhesives. To polypropylene (PP)‐type resins with narrow molecular weight distributions such as a PP‐type thermoplastic elastomer, PER and a controlled rheology PP were added small amounts of adhesives such as maleated PP, maleated PER, reactive polyolefin oligomers, ethylene/ethylacrylate/maleic anhydride (MAH) copolymer, ethylene/vinyl acetate copolymer, and styrene/MAH copolymer, and their melt fracture behaviors at capillary extrusion were observed. It was found that the sharkskin of the PP‐type resins with narrow molecular weight distributions was suppressed by the addition of the adhesive resins with good adhesion to metal. The suppressive effect of the sharkskin was generally the more remarkable by the higher loading of the adhesives with the higher MAH content. This is the direction of increasing adhesion. From this fact, it was assumed that the sharkskin of the PP‐type resins with narrow molecular weight distribution does not originate from a periodic growth and relaxation of tensile stress at the extrudate surface but from a stick‐slip at the die wall. Based on this mechanism, it may be said that the sharkskin can be suppressed by both ways of directions of promoting and suppressing the slip at the die wall. The former way is the previously known method, and the latter way is the method proposed in the present study. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2120–2127, 2002     

Slip flow of coal water slurries in pipelines

Experiments were carried out on a pilot scale slurry transport apparatus to investigate slip flow behavior of coal water slurries (CWSs) in pipes with various diameters (25, 32, 40 and 50 mm). The effects of volume concentration, particle size and slurry temperature on wall slip behavior (wall slip velocity, critical wall shear stress and slippage contribution) were investigated. A numerical technique based on Tikhonov regularization was applied to determine the wall slip behavior. As the slurry temperature or the particle size increased, the critical wall shear stress was observed to decrease and the wall slip velocity was observed to increase significantly, while when the solid concentration was very close to the maximum packing fraction, a slight increase in volume concentration would lead to a rapid decrease in wall slip velocity and a sharp increase in critical wall shear stress. The temperature influence on critical wall shear stress and yield stress increased as volume concentration increased. At low wall shear stress, the slippage contribution was mainly dependent on the difference between yield stress and critical shear stress. While at high wall shear stress, it was dependent on both of the shear viscosity of the bulk slurries and the wall slip velocity.     

Wall slip of molten polymers

There is considerable experimental evidence that the classical no-slip boundary condition of fluid mechanics is not always a valid assumption for the flow of high molecular weight molten polymers. In fact, molten polymers slip macroscopically at solid surfaces when the wall shear stress exceeds a critical value. Moreover, for linear polymers there exists a second critical wall shear stress value at which a transition from a weak to a strong slip occurs. These two modes of slip (weak and strong) are due to flow-induced chain detachment/desorption at the polymer/wall interface and to chain disentanglement of the polymer chains in the bulk from a monolayer of polymer chains adsorbed at the interface. In this review, the two physical mechanisms of slip are discussed and validated on the basis of published experimental data. The slip velocity of molten polymers is a complex function and has been reported to depend on wall shear and normal stresses, temperature, and molecular characteristics of polymers (molecular weight and its distribution). Proposed slip models, static and dynamic, are also reviewed and their significance on the rheology and flow simulations of molten polymers is discussed.     

Wall slip velocity measurement of molten polypropylene in capillary flow based on length‐corrected Mooney technique

In this study, to measure wall slip velocity of molten polypropylene (PP) by using different length‐to‐diameter (L/D) ratios of capillary dies with fixed diameter, a length‐corrected Mooney technique was proposed. Moreover, the effects of pressure, temperature, and L/D ratio were considered to better represent wall slip mechanism. To verify the feasibility of the length‐corrected Mooney technique, a series of capillary rheological experiments for molten PP were carried out. Meanwhile, the power‐law quantitative equations of slip velocity were established by shear stress. Moreover, the effects of L/D ratio and temperature on rheological properties of PP were investigated. In addition, numerical simulations for slip velocity and rheological properties of PP were performed. Numerical results validated that the length‐corrected Mooney technique, and the power‐law quantitative equations of slip velocity were available. Results showed that wall slip velocity of molten PP decreased with the capillary die's L/D ratio, but increased with the temperature and shear rate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44589.     

Effects of filler–filler and polymer–filler interactions on rheological and mechanical properties of HDPE–wood composites

High‐density polyethylene (HDPE)–wood composite samples were prepared using a twin‐screw extruder. Improved filler–filler interaction was achieved by increasing the wood content, whereas improved polymer–filler interaction was obtained by adding the compatibilizer and increasing the melt index of HDPE, respectively. Then, effects of filler–filler and polymer–filler interactions on dynamic rheological and mechanical properties of the composites were investigated. The results demonstrated that enhanced filler–filler interaction induced the agglomeration of wood particles, which increased the storage modulus and complex viscosity of composites and decreased their tensile strength, elongation at break, and notched impact strength because of the stress concentration. Stronger polymer–filler interaction resulted in higher storage modulus and complex viscosity and increased the tensile and impact strengths due to good stress transfer. The main reasons for the results were analyzed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Viscosity corrections for concentrated suspension in capillary flow with wall slip

Corrections for viscosity measurements of concentrated suspension with capillary rheometer experiments were investigated. These corrections include end effects, Rabinowitsch effect, and wall slip. The effects of temperature, particle concentration, and contraction ratio on the end effects were studied and their effects were accounted for using an entrance and exit losses model. The non‐Newtonian effect and the nonlinearity of slip velocity against wall shear stress were described using a slip model. The true viscosity of a concentrated suspension with glass powder suspended in a non‐Newtonian binder system was calculated as a function of shear rate and effective particle concentration, taking into consideration particle migration, which is calculated by a diffusive numerical model. Particle size was found to affect significantly the viscosity of the suspension with viscosity decreasing with increasing particle size, which can be reflected by a decrease in the value of the power‐law index in the Krieger model. © 2009 American Institute of Chemical Engineers AIChE J, 2010