Studies on multilayer film coextrusion III. The rheology of blown film coextrusion

Multilayer blown film coextrusion was studied, both experimentally and theoretically. For the experimental study, an annular die with a feed-port system was designed and multilayer blown films were produced by rotating the inner mandrel with a one horsepower variable-speed drive at speeds from nearly 2 to 6 rpm, and by inflating the tubular molten film with air. The die has 16 feed slots and melt pressure transducers are mounted along the axial direction of the outer wall of the annular flow channel. The transducers were used to determine the pressure gradient in the annular flow channel, which then permitted determination of the reduction in pressure drop when different combinations of two polymer systems were coextruded. Polymers used for b own film coextrusion were: (1) low-density polyethylene with ethylene-vinyl acetate; (2) low-density polyethylene with high-density polyethylene; (3) low-density polyethylene with polypropylene; (4) high-density polyethylene with ethylene-vinyl acetate. For the theoretical study, stratified helical flow was analyzed using a power-law non-Newtonian model. A computational procedure was developed to predict the number of layers, layer thickness, and the volumetric flow rate as functions of certain processing variables (namely, the pressure drop in the die, and the angular speed of rotation of the inner mandrel of the die) and the rheological parameters of the individual polymers concerned. Comparison was made of the theoretical prediction of volumetric flow rate with experimental ones. Some representative results are presented of the theoretically predicted axial and angular velocity distributions, shear stress profiles, and shear rate profiles.     

Development of microlayer blown film technology by combining film die and layer multiplication concepts

Many polymers are extruded through blown film dies to produce both monolayer and multilayer films. A common type of die in use today to produce blown films is the spiral mandrel die. This type of die can be used effectively for many polymers in structures containing up to approximately ten layers. This paper will discuss the development of new technology using a feedblock and layer multipliers in combination with encapsulation technology and a unique film die to produce microlayer blown film structures with significantly larger numbers of layers than can be produced using conventional blown film technology. POLYM. ENG. SCI., 56:598–604, 2016. © 2016 Society of Plastics Engineers     

Evidences for flow‐assisted interfacial reaction in coextruded PA6/PP/PA6 films

We present an investigation of the progressive building of copolymer layers formed by chemical reaction at interfaces in multilayer coextruded polymer films. Analyzing the surface density of copolymers in the final films, we show that the interface always remains under‐saturated in copolymers, even for coextrusion parameters such that the time open to the interfacial chemical reaction is well above the time necessary to reach saturation without flow. Based on a numerical analysis of the flow all along the coextrusion line, we show that this under‐saturation of the interface, which may strongly affect the final adhesion between the different polymers, results from a competition between the interfacial chemical reaction and the dilution of yet formed copolymers in all the zones of the coextrusion line where surface of interface is increased, due to convergent, divergent or elongational flow. We also show that the coupling between flow and reaction kinetics needs be taken into account to control the final surface density of copolymers and precisely optimize the interfacial properties, through an optimization of the coextrusion parameters POLYM. ENG. SCI., 59:E44–E50, 2019. © 2018 Society of Plastics Engineers     

Morphology and properties of layered silicate‐polyethylene nanocomposite blown films

Film grade ethylene vinyl acetate (EVA), low density polyethylene (LDPE), and high density polyethylene (HDPE) were melt compounded with an organically modified montmorillonite, then blown into films. The morphology studies showed that all three types of film involve intercalated clay particles. The dependence of intercalation extent on the matrix as well as on the molecular weight of compatibilizers is discussed. The tensile testing data showed that the clay enhancing effects apply mainly to the modulus, instead of to the strength. The EVA‐based nanocomposite films exhibit the most significantly improved modulus while the HDPE‐based films have the least. Lower molecular weight compatibilizers could promote the clay enhancing effects while higher molecular weight compatibilizers could increase the matrix properties. Steady shear viscosities of an intercalated and an exfoliated system were also investigated. Comparing our data with that from the literature lead us to conclude that: 1) the zero‐shear viscosity of a nanocomposite is mainly determined by clay loading instead of by clay intercalation/exfoliation structures and the matrix viscosity; and 2) the clay orientation during a shear flow is highly dependent on the matrix flow behavior and to a lesser extent on the clay structural state. POLYM. ENG. SCI., 45:469–477, 2005. © 2005 Society of Plastics Engineers     

Three‐dimensional isothermal simulation of PP melt flow in laminating‐multiplying elements based on the finite element method

The laminating‐multiplying element (LME) with a high aspect ratio used in coextrusion process is highly desired since it has useful applications for the preparation of multilayer sheets or membranes. In this article, the flow behaviors of a polymeric melt through a high aspect ratio LME channel was simulated by the finite element method and verified by a coextrusion process. The results showed that the velocity Y distortion in LME lead to the interface deformation and the interface deformation can be improved by reducing the velocity Y gradient via decreasing the inlet flow rate or increasing the wall slip. POLYM. ENG. SCI., 59:973–981, 2019. © 2019 Society of Plastics Engineers     

Rheological properties,flow visualization and extrudate swell of NR compound by rotating‐die rheometer

An experimental apparatus coupled with a rotating die system was especially designed and manufactured to study the rheological properties, flow patterns and swelling behavior of natural rubber (NR) compound for different shear rates and die rotating speeds at a test temperature of 110°C, the results being compared with those by the static capillary die. It was found that NR compound used exhibited psuedoplastic non‐Newtonian behavior. The rotation of the capillary die could reduce the extrusion load. The wall shear stress for any given shear rates increased with increasing die rotating speed. The fluctuation of the entrance pressure drop increased with increasing die rotating speed. The flow pattern development in the rotating‐die rheometer was different from that observed in the static die. The flow patterns in the rotating die were clearly unstable and contained two flow components which included axial flow along the barrel and circumferential flow at the die entrance. The size and shape of the axial and circumferential flows were more dependent on the piston displacement. It was found that the swelling ratio of the NR compound decreased with increasing die rotating speed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Shear flow of molten polymer in melt blowing

Besides the air flow field, the flow field of molten polymer plays a key role in fiber formation in the melt‐blowing (MB) process. In this article, the flow field of molten polymer was discussed, and its effects on the fiber microstructures were studied through theory and experiments. First, this field was supposed to be a kind of shear flow field. Two equations were introduced and solved. Then, analyzing the solutions combining with the actual melt‐blown practice, we concluded that the distribution profile of this flow field was a series of inverse parabolic in the course of the polymer stream attenuating. Further inferring from this flow pattern, we could also assume that there could have been a novel cross‐sectional microstructures in the melt‐blown fibers. Finally, the comparison experiments concerning the MB and its fibers were designed and carried out. The results indicated that the shear flow field could be qualitatively described by the equations, and the assumptions about the microstructures are basically in agreement with the experimental results. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Molecular,rheological, and crystalline properties of low‐density polyethylene in blown film extrusion

The molecular weight and its distribution, degree of long chain branching and cooling rate strongly influence crystallinity during processing, which in turn determines the processability and the ultimate properties of the blown film. Generally a decrease in the number of branches and molecular weight of the polymer and the cooling rate results in an increase of the crystallinity. Length of the main chain and extent of branching in low‐density polyethylene (LDPE) are also factors that affect melt rheology and film crystallinity. Long chain branched polyethylene is suitable in the blown film process due to its better melt strength for bubble stability. The objective of this article is to describe the effect of molecular properties (e.g. molecular weight and its distribution, degree of long chain branching etc) of LDPE on film crystallinity at different cooling rates of blown film extrusion. Two different grades of LDPE were selected to investigate molecular characteristics, crystallinity, and rheology. The resins were processed in a blown film extrusion pilot plant using four different cooling rates. Molecular, rheological, and crystalline properties of the resins were key parameters considered in this study. POLYM. ENG. SCI., 47:1983–1991, 2007. © 2007 Society of Plastics Engineers     

The effect of processing conditions on the rheological properties of blends of ultra high molecular weight polyethylene with high‐density polyethylene

Blends of high‐density polyethylene (HDPE) with small amounts of ultra‐high molecular weight polyethylene (UHMWPE) were prepared by melt mixing in a twin‐screw microcompounder. Two types of UHMWPE differing in their states of chain entanglement were used. The blend composition, time of mixing, and rotation speed of the screws were varied. Rheological properties of the blends were studied in oscillatory shear and uniaxial elongational tests. Reduction in phase angle measured in dynamic shear rheology and increase in extensional strain hardening were found to be useful indicators for quantifying the extent of mixing of the two components. Although the disentangled UHMWPE showed reasonable mixing with HDPE during typical residence times of melt compounding operations, the entangled UHMWPE remained essentially undissolved. The extent of mixing increased with mixing time and screw speed. POLYM. ENG. SCI., 59:821–829, 2019. © 2018 Society of Plastics Engineers     

Fiber orientation in multilayer tubes processed by a conical extruder

A new extruder design has been developed for the coextrusion of two-layer annular sections. The extruder consists of a conical stator-rotor-stator assembly, which performs extrusion from each side of the rotor. Flow within this assembly is fully three-dimensional, with helicoidal streamlines in the vicinity of the rotor and the die entry region. Fiber orientation is created in a circumferential direction by these helicoidal streamlines; close to the inner and outer surfaces of the tube, the fibers are parallel to the main extrusion direction, whereas in the mid-thickness, they are oriented in the circumferential direction. It is demonstrated that the amount of orientation depends on rotor speed and die design. When using a short die and high rotor speed, an increased fraction of fibers are oriented in the circumferential direction. Polym. Compos. 25:331–341, 2004. © 2004 Society of Plastics Engineers.     

Thermal,mechanical, and barrier properties of polyethylene/surlyn/organoclay nanocomposites blown films prepared by different mixing methods

(Low‐density polyethylene) (LDPE)/clay nanocomposites were prepared by melt blending in a twin‐screw extruder by using different mixing methods. Zinc‐neutralized carboxylate ionomer was used as a compatibilizer. Blown films of the nanocomposites were then prepared. The effect of mixing method on the clay dispersion and properties of the nanocomposites was evaluated by wide‐angle X‐ray diffraction analysis, mechanical properties, thermal properties, and barrier properties. The structure and properties of nanocomposites containing different amounts of nanoclay prepared by selected mixing techniques were also investigated. It was found that melt compounding of Surlyn/clay masterbatch with pure LDPE and Surlyn (two‐step‐a method) results in better dispersion and intercalation of the nanofillers than melt mixing of LDPE/Surlyn/clay masterbatch with pure LDPE and surlyn (two‐step‐b method) and direct mixing of LDPE with clay. The films containing ionomer have good barrier properties. A wide‐angle X‐ray diffraction pattern indicates that intercalation of polymer chains into the clay galleries decreases by increasing the clay content. Barrier properties and tensile modulus of the films were improved by increasing the clay content. In addition, tensile strength increased in the machine direction, but it decreased in the transverse direction by increasing the clay content. DSC results showed that increasing the clay content does not show significant change in the melting and crystallization temperatures. The results of thermogravimetric analysis showed that the thermal stability of the nanocomposites decreased by increasing the clay content more than 1 wt%. J. VINYL ADDIT. TECHNOL., 21:60–69, 2015. © 2014 Society of Plastics Engineers     

Modeling viscoelastic secondary flows in three‐dimensional noncircular ducts

As process capabilities become more advanced, the need to predict flow phenomena at a smaller scale increases significantly. Viscoelastic secondary flows in square ducts were simulated using a finite volume approach. Single mode and multimode Giesekus and Phan‐Thien Tanner (PTT) models were implemented and were able to reproduce full three‐dimensional (3D) flow through a square duct. Results for low density polyethylene (LDPE), polystyrene, and polycarbonate are all in agreement with experiments [Dooley, Viscoelastic flow effects in multilayer polymer coextrusion, PhD Thesis, Technische Universiteit Eindhoven (2002)] as well as numerical results using a finite element method (FEM) and a meshless radial function method (RFM), [Lopez et al., SPE ANTEC Tech. Pap. (2010)]. The mathematical model presented here has shown the potential to model full 3D flow in more complex geometries. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Plastic deformation of low‐density polyethylene reinforced with biodegradable polylactide,Part 1: Microstructural analysis and tensile behavior at constant true strain‐rate

Blends of low‐density polyethylene (LDPE) and polylactide (PLA) were prepared by melt coextrusion. The plastic behavior of the LDPE/PLA blends was investigated at room temperature under uniaxial tension by means of a video‐controlled system. The constitutive behavior was analyzed in terms of the variations of true stress vs. true strain at constant true strain rate. With increasing concentrations of PLA, the blend show: (i) higher Young's modulus, (ii) stiffer viscoelastic response, (iii) increase of elastic limit stress, and (iv) earlier fracture. Particular attention was paid to the evolution of the volume strain with the applied strain. While dilatation begins very lately for the neat LDPE, the LDPE/PLA blends show increasing deformation damage as the PLA content is increased. Scanning electron microscopy of deformed specimens shows that cavitation occurs preferentially at the poles of the PLA particles whose adhesion to the LDPE matrix seems very weak despite the partial grafting of polyethylene chains with maleic anhydride. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.     

Thickness uniformity of HDPE blown film: Relation to rheological properties and density

Previous work has elucidated that the wall slip velocity and viscosity of polymer melts influence the thickness uniformity of blown film. The present study investigates the effects of the stress dependence of wall slip, the shear thinning and the density on the uniformity. We have prepared high‐density polyethylenes with a variety of molecular weight distributions, which have different rheological properties. Examination of the thickness uniformity of their blown film has shown that the uniformity is correlated with wall slip velocity, the stress dependence of the velocity, melt viscosity, shear thinning and density; the coefficient of the correlation is determined to be 0.990. The reason why the stress dependence of wall slip and the shear thinning affect the uniformity is explained in terms of polymer melt flow behavior in a die, while the effect of density is interpreted considering bubble fluctuation in the blow‐up process. Polym. Eng. Sci. 44:965–972, 2004. © 2004 Society of Plastics Engineers.     

Preparation and properties of gas barrier resin/rubber nanolaminated composites

An extremely high gas barrier resin was highly oriented and uniformly distributed in a rubber matrix layer by layer by the micro‐layer coextrusion technology and a special composite laminating device. The resulting composite had excellent gas barrier properties because of its alternating multilayer structure. Experimental tire samples were prepared and tested for high‐speed, endurance, and pressure performance. The results showed that because the barrier resin was evenly dispersed in the rubber matrix, the tire inner liner made of the composite showed excellent fatigue resistance and gas barrier performance. POLYM. ENG. SCI., 55:190–195, 2015. © 2014 Society of Plastics Engineers     

HDPE functionalization via high shear stress‐induced initiation and its effects on HDPE/GF composite

The functionalization reaction of high density polyethylene (HDPE) with maleic anhydride (MAH) or with MAH and γ‐methacryloxy‐propyltrimethoxysilane (MAS) performed in melt state through a high shear stress‐induced initiation by increasing the screw rotation speed of twin‐screw extruder and through a compounded initiation by adding some initiator and increasing the screw rotation speed was investigated in this article. The results show that by increasing the screw rotation speed during melt‐extruding process, the scission of HDPE chain bonds can be caused to form macroradicals, the functionalization reaction of HDPE with MAH or with MAH, and MAS can be realized. The percentage of grafting and the melt flow rate of the functionalized products depend on the screw rotation speed and reaction temperature. The crosslinking reaction during melt extrusion can be suppressed by increasing the screw rotation speed and the reaction of HDPE with MAH can also be promoted by adding a second grafting monomer MAS. The high shear stress‐induced reaction products have a higher reactivity with the coupling agents coated on the surface of glass fibers and can obviously increase the mechanical properties of HDPE/GF composite. The SEM experimental results indicate that an oriented crystal transition layer exists between the interface of glass fiber and the matrix, the interfacial bonding strength is the determining factor of the formation of the oriented crystal layer. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Three‐dimensional simulation on multilayer parison shape at pinch‐off stage in extrusion blow molding

We tried to predict the multilayer parison shape at pinch‐off stage in extrusion blow molding by nonisothermal and purely viscous non‐Newtonian flow simulation using the finite element method (FEM). We assumed the parison deformation as a flow problem. The Carreau model was used as the constitutive equation and FEM was used for calculation method. Multilayer parison used in this simulation was composed of high‐density polyethylene (HDPE) as inner and outer layers and low‐density polyethylene (LDPE) of which viscosity is five times lower than HDPE as a middle layer. We discussed multilayer parison shape in pinch‐off region. The results obtained are as follows; the parison shape of each layer was clearly visible in the pinch‐off during the mold closing. In addition, the distribution of parison thickness ratios for each layer was located for a large deformation near the pinch‐off region. The melt viscosity for each layer has an influence on the melt flow in the pinch‐off region. In a comparison with an experimental data of parison thickness ratios, the simulation results are larger than the experimental data. These simulation results obtained are in good agreement with the experimental data in consideration of the standard deviations. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

PANI–LDPE composites: Effect of blending conditions

A composite based on polyaniline (PANI) and low density polyethylene (LDPE) with electrical conductivity was developed. Polyaniline was polymerized by chemical oxidation and doped with dodecyl‐benzene‐sulfonic acid (DBSA). PANI–LDPE composites were prepared via melt blending and the films were obtained by compression molding. The influence of three variables of the blending (temperature, [PANI], rotor speed) on conductivity, microstructure and mechanical properties of the composites was studied by means of statistical tools and a 2³ experimental design. The results show that the PANI concentration is the most influential variable, which mainly affects the conductivity and the elongation at break of the composites. These changes are related to the microstructure of the composites. Statistically, the other variables don't show significant influence on conductivity and mechanical properties in the studied range. POLYM. COMPOS., 2009. © 2008 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     

LLDPE‐based mono‐ and multilayer blown films: Effect of processing parameters on properties

In this study we investigated the effect of processing parameters on the end‐use properties of mono‐ and five‐layer coextruded polyethylene (PE) blown films using three different linear low‐density PE (LLDPE) resins. The three investigated LLDPEs were a conventional Ziegler‐Natta gas phase ethylene‐butene copolymer, and two solution ethylene‐octene resins produced with Ziegler‐Natta and single‐site catalysts. The octene copolymers were produced using NOVA Chemicals Advanced SCLAIRTECH? process and catalyst technologies. It was found that within the investigated range of processing conditions, tear strength increased in the direction perpendicular to the highest orientation, impact and puncture strength increased with the overall orientation, and the effect of orientation due to shear stresses in the die was negligible because of rapid macromolecular relaxation before crystallization. Finally, it was also shown that due to changes in the size of the crystallites, haze increased with die gap and frost line height (FLH), and decreased as take‐up speed (TUS) increased. POLYM. ENG. SCI., 45:1214–1221, 2005. © 2005 Society of Plastics Engineers