Role of molecular architecture and temperature on extrusion melt flow instabilities of two industrial LLDPE and LDPE polyethylenes investigated by capillary rheology, high‐pressure sensitivity slit die and optical analysis

The characteristic time periodicity $\tau^{*}$ and the spatial characteristic wavelength ${\lambda}$ of extrusion flow instabilities of a linear and a branched commercial polyethylene (PE) are characterized via capillary rheology, optical analysis and modeled. The two investigated polyethylenes have the similar weight average molecular weight (Mw). The characteristic time periodicity $\tau^{*}$ is obtained and compared using three methods: (i) a highly sensitive pressure slit die, (ii) a new online optical analysis method based on the construction of a space–time diagrams, and (iii) an offline transmission polarization microscopy. In addition, the spatial characteristic wavelength ${\lambda}$ is quantified by offline transmission polarization microscopy. The characteristic time periodicity $\tau^{*}$ of the extrusion flow instabilities follows a power law behavior as a function of apparent shear rate to a power of −0.7 for both materials, $\tau$$^{*}{\propto^.\gamma}^{-0.7}_{app.}$. A qualitative model is used to predict the spatial characteristic wavelength of extrusion flow instabilities as well. It is found that the characteristic spatial wavelength ${\lambda}$ and the characteristic time periodicity $\tau^{*}$ have an Arrhenius temperature-dependent behavior

Role of molecular architecture and temperature on extrusion melt flow instabilities of two industrial LLDPE and LDPE polyethylenes investigated by capillary rheology, high-pressure sensitivity slit die and optical analysis

The characteristic time periodicity (Formula presented.) and the spatial characteristic wavelength (Formula presented.) of extrusion flow instabilities of a linear and a branched commercial polyethylene (PE) are characterized via capillary rheology, optical analysis and modeled. The two investigated polyethylenes have the similar weight average molecular weight (Mw). The characteristic time periodicity (Formula presented.) is obtained and compared using three methods: (i) a highly sensitive pressure slit die, (ii) a new online optical analysis method based on the construction of a space–time diagrams, and (iii) an offline transmission polarization microscopy. In addition, the spatial characteristic wavelength λ is quantified by offline transmission polarization microscopy. The characteristic time periodicity (Formula presented.) of the extrusion flow instabilities follows a power law behavior as a function of apparent shear rate to a power of −0.7 for both materials, (Formula presented.). A qualitative model is used to predict the spatial characteristic wavelength (Formula presented.) of extrusion flow instabilities as well. It is found that the characteristic spatial wavelength λ and the characteristic time periodicity (Formula presented.) have an Arrhenius temperature-dependent behavior

Application of the ramp test from a closed cavity rheometer to obtain the steady-state shear viscosity η ( γ̇ )

The steady-state shear viscosity η(˙γ) is required in controlling processing parameters for the extrusion processing of polymer melts. A new method, the so-called ramp test, is investigated in this study to obtain the steady-state shear viscosity with a closed cavity rheometer (CCR). To verify the method and the accuracy of the CCR data, three commercial polyolefin polymers, a low-density polyethylene (LDPE), a linear low-density polyethylene (LLDPE), and a polybutadiene (PBD), were used as model systems. Measurements of the magnitude of the complex viscosity $∣η^⁎(ω)∣$ were compared with the steady-state shear viscosity data obtained by capillary rheometer and CCR. Further, time–temperature superposition master curves of the magnitude of the complex viscosity and steady-state shear viscosity obtained by CCR were developed for LLDPE and PBD. The influence of the cavity sealing on the instrument’s accuracy to obtain the steady-state shear viscosity was investigated using the finite element method simulations. Thus, it was shown that the ramp test performed by CCR is a practical method to determine reliable and reproducible data of the steady-state shear viscosity within a wide range of temperatures (T = 50–180°C) for low and high viscous materials ( $∣η^⁎(ω)∣$ = 1.6–480 kPa s, M w = 144–375 kg mol$^{−1}$)

A new slit-radial die for simultaneously measuring steady state shear viscosity and first normal stress difference of viscoelastic liquids via capillary rheometry

A new slit-radial die capable of simultaneously obtaining steady state shear viscosity η(γ) and the average first normal stress difference coefficient via capillary rheometry has been developed. The steady state shear viscosity η(γ) and average first normal stress difference coefficient are calculated in the slit part and radial part of the die, respectively. The steady state shear viscosity η(γ) from the slit part of the slit-radial die is compared to shear viscosities η(γ) obtained from a capillary die and also the magnitude of the complex viscosity |η*(ω)| obtained from oscillatory shear experiment. The average value of first normal stress difference coefficient which is calculated in the radial part of the slit-radial die is compared to first normal stress difference coefficient Ψ$_{1}$(γ) obtained from transient shear experiment in a cone-plate geometry and the molecular stress function model predictions. The effect of variation of power law fitting parameters (consistency index, k and power law index, n) on average value of the first normal stress difference coefficient obtained from the radial part of the slit-radial die is discussed. As this die has the shape of the city map of Karlsruhe it is named as Karlsruhe die

Derivation of a Qualitative Model for the Spatial Characteristic Wavelength of Extrusion Flow Instabilities: Investigation of a Polybutadiene Rubber through Capillary, Slit and Complex Geometry Extrusion Dies

The extrusion flow instabilities of commercial polybutadiene (PBD) are investigated as a function of the different extrusion die geometries, such as round capillary, slit, and complex cross-section profile slit dies via capillary rheology. Qualitative models are used to fit the experimental data for the spatial characteristic wavelength (λ) of the appearing extrusion flow instabilities. A new qualitative model for the slit die geometry, rectangular cross-section, is derived based on the theoretical concept of the “two layers” extrudate and the force balance at the die exit region. The proposed qualitative model for the slit die geometry is used to predict the spatial characteristic wavelength (λ) for extrudates obtained by complex cross-section profile slit die geometries similar to industrial manufacturing. Correlation between the ratio of the extensional (Y$_s$) and shear (σ$_x$) stress at the die exit area and the characteristic dimension, height H for slit dies and diameter D for round capillary dies, is presented. Moreover, a geometry-dependent model is used to predict the spatial characteristic wavelength (λ) of the extrusion flow instabilities from a round capillary die to a slit die and vice versa

Mechano-Optical Characterization of Extrusion Flow Instabilities in Styrene-Butadiene Rubbers: Investigating the Influence of Molecular Properties and Die Geometry

The extrusion flow instabilities of two commercial styrene‐butadiene rubbers are investigated as they vary in isomer content (1,4‐cis, 1,4‐trans, and 1,2 conformation) of the butadiene monomer and the molecular architecture (linear, branched). The investigated samples have similar multimodal molecular weight distribution. Two geometries of extrusion dies, slit and round capillary, are compared in terms of the type and the spatial characteristics of the flow instabilities. The latter are quantified using three methods: a highly pressure sensitive slit die, online and offline optical analysis. The highly pressure‐sensitive slit die has three piezoelectric pressure transducers (Δt ≈ 10$^{-3}$ s and Δp ≈ 10$^{-5}$ bar) placed along the die length. The characteristic frequency (f$_{Char.}$) of the flow instabilities follows a power law behavior as a function of shear rate to a 0.5 power for both materials,  $_{ℎ.}$∝$^{˙0.5}$$_{app.}$. A qualitative model is used to predict the spatial characteristic wavelength (λ) of the flow instabilities from round capillary to slit dies and vice versa. Slip velocities (V$_{s}$) are used to quantify the slippage at slit and round capillary dies as well

Derivation of a qualitative model for the spatial characteristic wavelength of extrusion flow instabilities: Investigation of a polybutadiene rubber through capillary, slit and complex geometry extrusion dies

The extrusion flow instabilities of a commercial polybutadiene (PBD) are investigated as a function of different extrusion die geometries, such as round capillary, slit, and complex cross-section profile slit dies via capillary rheology. Qualitative models are used to fit the experimental data for the spatial characteristic wavelength (λ) of the appearing extrusion flow instabilities. A new qualitative model for the slit die geometry, rectangular cross-section, is derived based on the theoretical concept of the “two layers” extrudate and the force balance at the die exit region. The proposed qualitative model for the slit die geometry is used to predict the spatial characteristic wavelength (λ) for extrudates obtained by complex cross-section profile slit die geometries similar to industrial manufacturing. Correlation between the ratio of the extensional ( ) and shear ( ) stress at the die exit area and the characteristic dimension, height H for slit dies and diameter D for round capillary dies, is presented. Moreover, a geometry-dependent model is used to predict the spatial characteristic wavelength (λ) of the extrusion flow instabilities from a round capillary die to a slit die and vice versa

An Intriguing Array of Extrudate Patterns in Long‐Chain Branched Polymers During Extrusion

The present study highlights a range of surface and volume extrudate patterns that can be detected during the extrusion flow of long-chain branched polymers. Thus, four linear low-density polyethylenes (LDPEs) have been extruded using a single-screw extruder coupled to an inline optical imaging system. The selected LDPEs are selected to outline the influence of molecular weight and long-chain branching on the types of melt flow extrusion instabilities (MFEI). Through the inline imaging system, space–time diagrams are constructed and analyzed via Fourier-transformation using a custom moving window procedure. Based on the number of characteristic frequencies, peak broadness, and whether they are surface or volume distortions, three main MFEI types, distinct from those typically observed in linear and short-chain branched polymers, are identified. The higher molecular weight, low long-chain branching LDPEs exhibited all three instability types, including a special type volume instability. Independently of the molecular weight, higher long-chain branching appeared to have a stabilizing effect on the transition sequences by suppressing volume extrudate distortions or limiting surface patters to a form of weak intensity type

An Intriguing Array of Extrudate Patterns in Long‐Chain Branched Polymers During Extrusion

Abstract The present study highlights a range of surface and volume extrudate patterns that can be detected during the extrusion flow of long‐chain branched polymers. Thus, four linear low‐density polyethylenes (LDPEs) have been extruded using a single‐screw extruder coupled to an inline optical imaging system. The selected LDPEs are selected to outline the influence of molecular weight and long‐chain branching on the types of melt flow extrusion instabilities (MFEI). Through the inline imaging system, space–time diagrams are constructed and analyzed via Fourier‐transformation using a custom moving window procedure. Based on the number of characteristic frequencies, peak broadness, and whether they are surface or volume distortions, three main MFEI types, distinct from those typically observed in linear and short‐chain branched polymers, are identified. The higher molecular weight, low long‐chain branching LDPEs exhibited all three instability types, including a special type volume instability. Independently of the molecular weight, higher long‐chain branching appeared to have a stabilizing effect on the transition sequences by suppressing volume extrudate distortions or limiting surface patters to a form of weak intensity type

Modeling the spatial characteristics of extrusion flow instabilities for styrene-butadiene rubbers: Investigating the influence of molecular weight distribution, molecular architecture, and temperature

The extrusion flow instabilities of three commercial styrene-butadiene rubbers (SBR) are investigated as a function of molecular weight distribution (MWD); molecular architecture (linear, branched); and temperature. The samples have multimodal MWD, with the main component being SBR and a low amount, less than 10 wt. %, of low-molecular weight hydrocarbons. Deviation from the Cox-Merz rule at high angular frequencies/shear rates becomes intense as the amount of medium-molecular weight component increases. Optical analysis is used to identify and quantify spatial surface distortions, specifically wavelength (λ) and height (h), of the different types of extrusion flow instabilities. Qualitative constitutive models are reviewed and used to fit the experimental data for the spatial characteristics of extrusion flow instability. The fitting parameters as obtained by the models are correlated with molecular properties of the materials. It is found that the characteristic spatial wavelength (λ) increases as the extrusion temperature decreases. Hence, the influence of temperature on the spatial characteristic wavelength is investigated and an Arrhenius behavior is observed