Polymer Melt Extrusion Research Articles

Niestabilnosci przeplywu stopionych tworzyw polimerowych w trakcie wytlaczania. Cz. I. Podzial i charakterystyka niestabilnosci przeplywu

Niestabilnosci przeplywu stopionych tworzyw polimerowych w trakcie wytlaczania. Cz. I. Podzial i charakterystyka niestabilnosci przeplywu

Model of the behavior of viscoelastic media at high strain rates

A new model was proposed to describe the behavior of melts and concentrated solutions of polymers at high strain rates. This model is a lattice of elastic elements, which, under deformation, transform into ellipsoids, are oriented, and elastically interact with each other. The calculation results give a qualitatively correct insight into the onset of structural instability and self-organization, as it is really observed in experiments in rotational flows and extrusion of polymer melts from capillaries.

A New High Sensitivity System to Detect Instabilities During the Extrusion of Polymer Melts

AbstractThis publication is concerned with the development of a novel high sensitivity instability detection die for a laboratory size extruder. The high sensitivity system consists of piezoelectric transducers placed along the die length to monitor local pressure fluctuations, having temporal and pressure resolution of up to Δt ≈ 10−3 s and Δp ≈ 10−5 bar, at a nominal pressure of 500 bar. The system is tested on short chain branched polyethylenes, namely on the detection and characterization of the sharkskin instability. It is shown that the determined instabilities are related to characteristic peaks in the corresponding in situ pressure Fourier‐transform spectra and that other peaks are caused by different, but defined, sources. The in situ results obtained are validated through optical image analysis of the instabilities.

Topological structure of microporous oriented polypropylene films

According to the data of scanning electron microscopy, gravimetry, and permeability measurements, the porous structure of membrane samples obtained in a multistage process including extension of annealed polypropylene films formed at the stage of polymer melt extrusion has been studied. It has been shown that, as the cooperativity of lamella ordering (self-assembly) increases at the stage of film extension (pore formation), the topological model of the porous phase of membranes changes from model (1), i.e., a random network of channels, to model (2), i.e., oriented through flow channels. The transition between two models is controlled by the melt spin draw ratio.

Interconnected, microporous hollow fibers for tissue engineering: Commercially relevant, industry standard scale‐up manufacturing

AbstractSignificant progress has been achieved in the field of tissue engineering to create functional tissue using biomimetic three‐dimensional scaffolds that support cell growth, proliferation, and extracellular matrix production. However, many of these constructs are severely limited by poor nutrient diffusion throughout the tissue‐engineered construct, resulting in cell death and tissue necrosis at the core. Nutrient transport can be improved by creation and use of scaffolds with hollow and microporous fibers, significantly improving permeability and nutrient diffusion. The purpose of this review is to highlight current technological advances in the fabrication of hollow fibers with interconnected pores throughout the fiber walls, with specific emphasis on developing hollow porous nonwoven fabrics for use as tissue engineering constructs via industry standard processing technologies: Spunbond processing and polymer melt extrusion. We outline current methodologies to create hollow and microporous scaffolds with the aim of translating that knowledge to the production of such fibers into nonwoven tissue engineering scaffolds via spunbond technology, a commercially relevant and viable melt extrusion manufacturing approach that allows for facile scale‐up. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 3311–3323, 2014.

Interconnected, Microporous Hollow Fibers for Tissue Engineering: Commercially Relevant, Industry Standard Scale-up Manufacturing

Significant progress has been achieved in the field of tissue engineering to create functional tissue using biomimetic three-dimensional scaffolds that support cell growth, proliferation, and extracellular matrix production. However, many of these constructs are severely limited by poor nutrient diffusion throughout the tissue-engineered construct, resulting in cell death and tissue necrosis at the core. Nutrient transport can be improved by creation and use of scaffolds with hollow and microporous fibers, significantly improving permeability and nutrient diffusion. The purpose of this review is to highlight current technological advances in the fabrication of hollow fibers with interconnected pores throughout the fiber walls, with specific emphasis on developing hollow porous nonwoven fabrics for use as tissue engineering constructs via industry standard processing technologies: Spunbond processing and polymer melt extrusion. We outline current methodologies to create hollow and microporous scaffolds with the aim of translating that knowledge to the production of such fibers into nonwoven tissue engineering scaffolds via spunbond technology, a commercially relevant and viable melt extrusion manufacturing approach that allows for facile scale-up.

A Combined NMR Relaxometry and Surface Instability Detection System for Polymer Melt Extrusion

A combined system for solidification and instability detection during the forced extrusion of polymers is constructed and first investigations are presented. The system comprises a home built Halbach‐type NMR magnet, monitoring the NMR relaxation at different positions along the extrudate length, and a highly sensitive transducer in the die recording mechanical pressure fluctuations. The system is complemented by an optical extrudate visualization setup. First results are shown for low density and high density polyethylene (LDPE and HDPE), which have different branching structures. Three relevant processing parameters were varied during the investigations, i.e., extruder rotation speed, draw ratio and extrusion temperature. Overall, the characteristics of NMR relaxation show a clear dependence on the magnet position along the extrudate length, which can be related to the cooling, onset and propagation of crystallization, and solidification of the polymer melt. The coupling of a low‐field NMR with a high sensitivity instability detection system shows the potential of creating a versatile tool for a unique online control method in polymer processing. In the future, this combination could provide insight into the influence of branching structures on polymer processing of polyethylene, for example.

Superlattices of lamellae in microporous oriented polyolefine films

The structure and mechanism of the formation of superlattices lamellae in microporous polyolefine (polyethylene and polypropylene) films obtained by polymer melt extrusion followed by annealing, uniaxial extension, and thermal fixation stages have been studied by scanning electron and atomic-force microscopy. It has been shown that oriented anisometric particles, i.e., lamellae aggregates, are formed in films as the spin draw ratio λf increases. At the stage of uniaxial extension (pore formation) of annealed films, a particle ensemble transforms to spatial superlattices of lamellae. Numerical processing of electron microscopy images of the film surface show that the nonmonotonic dependences of the correlation length of density fluctuations and the ratios of the alternation period of particles along extension to their thickness on the parameter λf correspond to a unified mechanism of lamellae ordering.

Extrusion Uniformity Optimization for Shaping Channel Design in Large-Scale Die-Plate of Underwater Pelletizing

As an important component of large-scale extruder system for polymer material, Die-plate can be regarded as the final determinant for pelletization uniformity. Polymer melt was shaped in shaping channels and extruded from die-holes from Die-plate and then be diced. The design of shaping channels directly influences the uniformity for polymer-melt extrusion pressure and extrusion velocity of different die holes. In this paper, a three-dimensional flow model for polymer flow in Die-plate channel was presented, and then finite element simulation was used to analyze the distribution of extrusion velocity in each die-hole. Based on simulation result, a BP neural network model was applied to analyze the relationship between geometry factors of Die-plate, such as length of shaping channels, die hole extrusion velocity distribution, and extrusion uniformity. This optimization method can be used in the design of Die-plate to improve effect and quality of actual polymer production and processing.

Experimental investigation of flow induced molecular weight fractionation during extrusion of HDPE polymer melts

In this work, two different HDPEs with virtually identical number, M n , and weight, M w , average molecular weights were investigated from rheological as well as die drool phenomenon point of view. It has been revealed that long-chain branching, low polymer melt elasticity and shear viscosity significantly reduce die drool phenomenon at the die exit region. It has been concluded that die drool phenomenon of HDPE polymer melts can be explained by the flow induced molecular weight fractionation.

Prediction of elongation viscosity of polymer melts

Melt extension flow is a common flow pattern during polymer processing, such as entrance converging flow in die extrusion or runner injection of polymer melts from an extruder barrel, blow molding, blowing film and melt spinning. Extensional viscosity is one of the important characterizations of the flow characteristics for polymer fluids. A new extension viscosity equation was established based on White-Metzner model, Vinogradov-Malkin viscosity equation and a new relaxation time equation in the present paper. The melt elongation viscosities of metallocene linear low-density polyethylene (mLLDPE) and polyvinyl butyral (PVB) resins at 130°C were estimated applying this viscosity equation, and the predictions were compared with the measured data of mLLDPE and PVB resins at 130°C reported from reference. The results showed that calculations were close to the experimental data. The parameters in this equation were easy to be determined and the equation was convenient to use for estimating the extension viscosity of polymer melts. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Effect of Poly(2-ethyl-2-oxazoline) on Multi-Walled Carbon Nanotubes Reinforced Poly(vinyl alcohol) Composites

The effect of poly(2-ethyl-2-oxazoline) (PEOX) as a surface modifier for multi-walled carbon nanotubes (MWCNTs) in a poly(vinyl alcohol) (PVA) composite was studied through a simple polymer wrapping and melt extrusion processes. MWCNTs were carboxylated in a reaction with HNO3, followed by mixing with 10 wt.% PEOX in ethanol to produce a MWCNT-PEOX complex. High resolution transmission electron microscopy identified a PEOX layer existing on the surface of MWCNTs. Compared with pristine MWCNTs, MWCNT-PEOX showed an improved dispersion in hydrophilic solvents such as water and ethanol. The PVA composite reinforced with 0.5 wt.% MWCNT-PEOX exhibited ~54% higher Young's modulus than pure PVA.

Prediction of extrudate swell in polymer melt extrusion using an Arbitrary Lagrangian Eulerian (ALE) based finite element method

Accurate prediction of extrudate (die) swell in polymer melt extrusion is important as this helps in appropriate die design for profile extrusion applications. Extrudate swell prediction has shown significant difficulties due to two key reasons. The first is the appropriate representation of the constitutive behavior of the polymer melt. The second is regarding the simulation of the free surface, which requires special techniques in the traditionally used Eulerian framework. In this paper we propose a method for simulation of extrudate swell using an Arbitrary Lagrangian Eulerian (ALE) technique based finite element formulation. The ALE technique provides advantages of both Lagrangian and Eulerian frameworks by allowing the computational mesh to move in an arbitrary manner, independent of the material motion. In the present method, a fractional-step ALE technique is employed in which the Lagrangian phase of material motion and convection arising out of mesh motion are decoupled. In the first step, the relevant flow and constitutive equations are solved in Lagrangian framework. The simpler representation of polymer constitutive equations in a Lagrangian framework avoids the difficulties associated with convective terms thereby resulting in a robust numerical formulation besides allowing for natural evolution of the free surface with the flow. In the second step, mesh is moved in ALE mode and the associated convection of the variables due to relative motion of the mesh is performed using a Godunov type scheme. While the mesh is fixed in space in the die region, the nodal points of the mesh on the extrudate free surface are allowed to move normal to flow direction with special rules to facilitate the simulation of swell. A differential exponential Phan Thien Tanner (PTT) model is used to represent the constitutive behavior of the melt. Using this method we simulate extrudate swell in planar and axisymmetric extrusion with abrupt contraction ahead of the die exit. This geometry allows the extrudate to have significant memory for shorter die lengths and acts as a good test for swell predictions. We demonstrate that our predictions of extrudate swell match well with reported experimental and numerical simulations.

Planar Entry Converging Flow During Extrusion of Polymer Melts

In this article, a planar entry converging flow during die extrusion of polymeric melts was analyzed and a differential equation of entry converging boundary streamline equation with rheological parameters and channel geometry is established. By applying limit theory, the entry converging boundary streamline equation in the case with different non-Newtonian index (n) was discussed, and the corresponding expressions of entry natural convergent half angle and convergent region length were derived. The entrant flow pattern might be described with the half angle of entry natural convergence (α0) and the convergent region length (L e ) of the melts, and α0 and L e were mainly a function of the entry elastic storage deformation energy (e) and n. The values of α0 and L e were calculated by means of these simplified expressions. It was found that the estimations of α0 decrease nonlinearly white L e increases linearly with an addition of e. Finally, a preliminary verification of the natural converging half angle equation was made. The results showed that the estimations of 2α0 based on the experiments of a low-density-polyethylene (LDPE) and a high-density-polyethylene (HDPE) were close to the data reported in reference.

Computational analytical approach to shear-extensional flow in fiber spinning of liquid crystalline polymer melt

A continuum theory of constitutive equation of co-rotational derivative type was developed for anisotropic viscoelastic fluid—liquid crystalline(LC) polymer. The theory was specialized to a constitutive equation of co-rotational type LCP-H model and LCP-Qs model. Using the constitutive equation of LCP-Qs model, the shear-extensional flow was studied for the extrusion process near the die exit of the fiber spinning of liquid crystalline polymer melt. Bifurcation of extensional viscosity was observed in the case without orientation and the two cases when the directors were parallel to flow and vertical to flow for the LC polymer melts. The extensional viscosity increases when the director rotates from the flow direction to that vertical to flow. Based on the LCP-Qs model a computational analytical theory was developed for extrusion process of LC polymer melt, contraction of the extrudate was predicted by the theory. The computational symbolic manipulation such as computer software Maple was used for the problem solution. An important conclusion can be drawn that the director tumbling has remarkable influence on extensional viscosity but no principal one on LC polymer melt extrusion.

The effect of temperature gradients on the sharkskin surface instability in polymer extrusion through a slit die

The sharkskin surface instability is commonly observed in the extrusion of polymer melts. We present a series of experiments in which a specifically designed rectangular slit die with insulated and independently heated sides and is used to induce precise temperature gradients across a flowing polyethylene melt. Our previous experiments demonstrated that the character of the surface distortions produced by the sharkskin instability was a function of the die wall temperature and therefore the extrudate had viscoelastic properties at the surface. In this paper, we explore the role of temperature and viscoelastic property gradients near the capillary wall. The amplitude of the sharkskin instability is quantified and plotted against apparent shear and extension rates. Analysis of the data demonstrates that the amplitude and frequency of the instability is independent of bulk temperature and temperature gradient and is dependent only on wall temperature. The data are normalized using a dimensionless Weissenberg number based on the extension rate to collapse the data collected over all temperatures and gradients onto a single master curve. We conclude with an example of a rectangular extrudate exhibiting varying surface roughness due to differential die heating and discuss the implications of our observations on the sharkskin surface instability mechanism and on commercial applications.

The melt processing of polymer microcapillary film (MCF)

This paper reports experimental results and initial numerical modelling for a novel extrusion process developed to continuously manufacture thermoplastic microcapillary films (MCFs) that contain embedded hollow capillaries. Using a linear low-density polyethylene (LLDPE), initial polymer melt extrusion experiments were carried out within a slit die with a gas-entrainment assembly that had a single hollow needle. By suitable control of flow and downstream haul-off conditions, a stable regime was found in which hollow microcapillaries were formed within the extrudate. Two multiple injector assemblies were also tested, a prototype 10-injector assembly and a 17-injector assembly. The final external shape of the film produced from these injectors, together with the dimensions and sections of the internal capillaries was shown to depend on the injector geometry, the melt drawing length, and the draw ratio. Draw resonance was noted for high draw ratios where the melt drawing length was greater than two-third the width of the die exit. Both simple and more advanced numerical modelling using POLYFLOW ® provides insight into the factors controlling the stable production of the microcapillary film.

Dynamics of molecules adsorbed on a die wall during polymer melt extrusion

A new, quantitative model to describe the dynamics of polymer molecules grafted on a solid wall is presented. This model is based on the bond vector probability distribution function (BVPDF) which contains the necessary information about the spatial conformations of the grafted chains. All macroscopic quantities of practical interest such as wall shear stress are shown to follow from second moments of the BVPDF. The derived equation of motion for the BVPDF takes into account all important mechanisms on the grafted chain such as retraction, convection, and (convective) constraint release. The proposed model can further be used to derive the quantitative stick–slip law given the molecular and wall surface parameters.

Control of the sharkskin instability in the extrusion of polymer melts using induced temperature gradients

The extrusion of polymer melts is often rate limited by the onset of an elastic surface instability known as sharkskin. Appearance of these surface distortions is generally unacceptable for commercial applications. The desire to forestall the onset of sharkskin to higher output rates has motivated a considerable amount of research to characterize the nature of the instability. In this manuscript, we will present a series of detailed experiments using a custom fabricated extruder and die. By incorporating thermal breaks and precise localized temperature control of the die and barrel, predetermined temperature gradients could be induced across the extrudate. Polymers are typically very poor thermal conductors, and therefore the effects of heating or cooling from a boundary can be designed to only affect the properties of the extrudate very close to the die wall. We will present data correlating the amplitude and frequency of the sharkskin instability to the bulk and die surface temperature as well as the shear rates. The result is a quantitative processing map that characterizes the instability and demonstrates that by modifying the rheology of the polymeric fluid very near the die exit corner, it is possible to suppress or control the sharkskin instability through isolated die heating or cooling. By reformulating our data into Weissenberg and Deborah numbers using the relaxation time evaluated at the wall temperature, we demonstrate that the sharkskin surface instability is dependent only on flow kinematics and viscometric properties of the fluid very near the die wall, a result of the stress singularity present at the die exit, and independent of bulk fluid properties. This technique could conceivably increase the profitability of extrusion processes and be extended to develop precisely-controlled sharkskin for designing specific functionality into extruded surfaces.

In-process vibrational spectroscopy and ultrasound measurements in polymer melt extrusion

Spectroscopic techniques have the potential to provide powerful, molecular-specific, non-invasive measurements on polymers during melt processing operations. An exploration is reported of the application and assessment of sensitivity of in-process vibrational spectroscopy—on-line mid-infrared (MIR), on-line near-infrared (NIR), in-line NIR and in-line Raman—for monitoring of single screw extrusion of high-density polyethylene and polypropylene blends. These vibrational spectroscopic techniques are compared with novel in-line ultrasound velocity measurements, which were acquired simultaneously, to assess the sensitivity of each method to changes in blend composition and to explore the suitability for their use in real time process monitoring and control.