Non-isothermal Elongational Flow Research Articles

Elongational flow‐induced microstructure evolutions in polypropylene/layered double hydroxides nanocomposites

AbstractIn this work, the effect of the non‐isothermal elongational flow on the morphology and mechanical properties of polypropylene (PP)‐based nanocomposites containing Mg‐Al layered double hydroxides (LDHs) modified with stearate or oleate functional groups has been investigated. In particular, nanocomposites containing 5 and 10 wt% of LDHs prepared through melt compounding were subjected to uniaxial elongational flow at the exit of the extruder, leading to the formation of fibers characterized by different draw ratios (DRs). The mechanical characterization of the fibers demonstrated a progressive enhancement of the tensile strength as a function of the DR with increasing the content of nanofillers. Notably, for the fibers stretched at a DR of 200 and containing 10 wt% of LDHs modified with oleate groups, the tensile strength increased fourfold as compared to that of the unfilled matrix. These results have been attributed to a progressive enhancement of the extent of the dispersions of the embedded LDHs induced by the application of the elongational flow, as also confirmed by morphological analyses. In all, the obtained results demonstrated the beneficial effect of the elongational flow in promoting the achievement of superior mechanical properties in LDHs‐containing nanocomposites, hence widening the application field of these nanostructured materials.Highlights PP/organomodified LDH nanocomposites were obtained through melt compounding. Nonisothermal elongational flow was applied to the extrudates at the die exit. Anisotropic fibers showed progressively enhanced tensile strength values. Elongational flow promoted a gradual evolution of the material microstructure. A gradual improvement of the nanofiller dispersion was observed upon elongation.

Biocomposite PBAT/lignin blown films with enhanced photo-stability

Lignin can be obtained as a byproduct during cellulose-rich pulp fibers production and it is habitually treated as waste or intended for low-value destinations. However, due to UV absorption and mechanical properties, lignin can contribute to the fabrication of biodegradable blown films with superior performances. In this study, it was established the suitability of lignin for manufacturing biocomposite PBAT blown films with higher stiffness and photo-oxidation resistance. The effect of the filler concentration on the melt rheological behavior in non-isothermal elongational flow was investigated. The results allowed us to choose the correct filler concentration for producing films through a film blowing operation. The PBAT/lignin blown film composites displayed an increase of the elastic modulus if compared to neat PBAT films without affecting their elongation at break. Furthermore, the filler delayed the photo-oxidative degradation of PBAT hence potentially allowing open-air applications.

Bionanocomposite Blown Films: Insights on the Rheological and Mechanical Behavior.

In this work, bionanocomposites based on two different types of biopolymers belonging to the MaterBi® family and containing two kinds of modified nanoclays were compounded in a twin-screw extruder and then subjected to a film blowing process, aiming at obtaining sustainable films potentially suitable for packaging applications. The preliminary characterization of the extruded bionanocomposites allowed establishing some correlations between the obtained morphology and the material rheological and mechanical behavior. More specifically, the morphological analysis showed that, regardless of the type of biopolymeric matrix, a homogeneous nanofiller dispersion was achieved; furthermore, the established biopolymer/nanofiller interactions caused a restrain of the dynamics of the biopolymer chains, thus inducing a significant modification of the material rheological response, which involves the appearance of an apparent yield stress and the amplification of the elastic feature of the viscoelastic behavior. Besides, the rheological characterization under non-isothermal elongational flow revealed a marginal effect of the embedded nanofillers on the biopolymers behavior, thus indicating their suitability for film blowing processing. Additionally, the processing behavior of the bionanocomposites was evaluated and compared to that of similar systems based on a low-density polyethylene matrix: this way, it was possible to identify the most suitable materials for film blowing operations. Finally, the assessment of the mechanical properties of the produced blown films documented the potential exploitation of the selected materials for packaging applications, also at an industrial level.

Structure-Property Relationships in Bionanocomposites for Pipe Extrusion Applications.

In this work, bionanocomposites based on different biodegradable polymers and two types of nanofillers, namely a nanosized calcium carbonate and an organomodified nanoclay, were produced through melt extrusion, with the aim to evaluate the possible applications of these materials as a potential alternative to traditional fossil fuel-derived polyolefins, for the production of irrigation pipes. The rheological behavior of the formulated systems was thoroughly evaluated by exploiting different flow regimes, and the obtained results indicated a remarkable effect of the introduced nanofillers on the low-frequency rheological response, especially in nanoclay-based bionanocomposites. Conversely, the shear viscosity at a high shear rate was almost unaffected by the presence of both types of nanofillers, as well as the rheological response under nonisothermal elongational flow. In addition, the analysis of the mechanical properties of the formulated materials indicated that the embedded nanofillers increased the elastic modulus when compared to the unfilled counterparts, notwithstanding a slight decrease of the material ductility. Finally, the processing behavior of unfilled biopolymers and bionanocomposites was evaluated, allowing for selecting the most suitable material and thus fulfilling the processability requirements for pipe extrusion applications.

Compatibilization of Polypropylene/Polyamide 6 Blend Fibers Using Photo-Oxidized Polypropylene.

The use of polyamide/polyolefin blends has gained importance and concern for years, but they also show some issues to be adequately addressed, such as the incompatibility between the two components. This is usually overcome by using suitable compatibilizers, typically based on functionalized polyolefins. However, there is only little information about the use of a degraded polyolefins to induce compatibilization. This is even truer, as far as polyamide 6/polypropylene (PA6/PP) blends are concerned. In this work, compatibilization of PA6/PP blends by using small amounts of photo-oxidized PP was investigated; furthermore, the effects due to the presence of the photo-oxidized PP were studied also in relationship to the spinning operation, where the existence of the non-isothermal elongational flow can lead to significant, further morphological changes. It was found that isotropic samples showed significant enhancements of the tensile properties upon adding the photo-oxidized PP. Under non-isothermal elongational flow conditions, the presence of the photo-oxidized PP was particularly effective in improving the mechanical properties in comparison to the uncompatibilized blend fibers. Furthermore, an important result was found: The elongational-flow processing allowed obtaining anisotropic samples where the improvements of the properties, in comparison to the isotropic samples, were similar to those achieved by using a compatibilizer.

Effect of the elongational flow on morphology and properties of polypropylene/graphene nanoplatelets nanocomposites

In this work the effect of the presence of graphene nanoplatelets (GnP) on the morphology and mechanical properties of polypropylene/GnP nanocomposites has been investigated when these polymer systems are subjected to non-isothermal elongational flow in a melt spinning operation. The presence of the GnP magnifies the effect of the elongational flow. Indeed, the elastic modulus and the tensile strength of nanocomposites increases with the orientation more than that observed for the pure matrix. Moreover, the elongation at break increases at low values of the draw ratio for the nanocomposites, while, the matrix shows the expected decrease in deformability. These effects have been correlated with the decrease in the dimensions of the nanoplatelets caused by the elongational flow. The increase of contact surface between matrix and dispersed phase is responsible for these behaviours since the larger contact area improves the transfer of the stress from matrix to GnP. The best performance was observed for the nanocomposite with the lower GnP content and this phenomenon has been again correlated with the minor size of the GnP in this nanocomposite sample.

Rheological Behaviour, Mechanical Properties and Processability of Biodegradable Polymer Systems for Film Blowing

Films for agricultural or packaging applications are typically made of low density polyethylene (LDPE). They are produced through the film blowing process, which requires the use of polymers with suitable rheological properties. Furthermore, the short shelf-life which is often related to many packed products leads to huge amounts of plastic-based wastes. This suggests the use of biodegradable and/or compostable polymers in replacement for traditional ones. To this regard, only few data exist on the rheological properties of biodegradable polymers undergoing film blowing processing. In this work, a detailed investigation on the rheological, mechanical and processability behaviour of some biodegradable polymers (originating from MaterBi® and Bioflex® commercial groups) was carried out, in order to assess their suitability to industrial-scale film blowing by direct comparison with a traditional non-biodegradable polymer (LDPE). Rheological tests under shear and non-isothermal elongational flow allowed to find out the two most suitable polymer grades. The film blowing production operations at different draw and blow-up ratios (from 2.5 to 3.5) showed that the two samples are suitable for industrial scale operations. Results from tensile and impact tests indicated that the two selected polymers exhibit significantly different mechanical properties (up to 10–15%).

Characterization and Processability of Blends of Polylactide Acid with a New Biodegradable Medium-Chain-Length Polyhydroxyalkanoate

In this work the processability and the mechanical performance of polylactide acid (PLA) based blends with a sample of a new biodegradable medium-chain-length poly (hydroxyalkanoate) (PHA) were investigated. The two polymers are incompatible with scarce adhesion and with the dispersed PHA particles size increasing with the PHA content. Rheological tests in shear flow indicate that adding PHA improves the processability of the matrix by increasing the content of this second component in the blend, as observed also for the torque curves, due to the very low viscosity of this new sample of PHA. The processability is only slightly worsened in non-isothermal elongational flow as the melt strength decreases, but accompanied by an increase of the melt stretchability that improves the ability of the pure PLA matrix to produce thin films. Mechanical properties put in evidence a brittle–ductile transition adding PHA to PLA matrix. The elongation at break significantly increases when PHA is added, while the elastic modulus is significantly reduced only at higher contents of PHA. This has been interpreted in terms of both plasticization action of the PHA and also in terms of lubricant action of the PHA macromolecules that allows a better sliding of the PLA macromolecules subjected to solid deformation.

Processing and characterization of highly oriented fibres of biodegradable nanocomposites

Biodegradable polymeric materials are becoming day by day ever more important in packaging, agriculture, single-use cutleries and other large consumer applications. The major part of those materials is used under the form of film, i.e. subjected to elongational flow, but the main problem is that they often offer poor mechanical properties. Adding nanofillers, like Multi Walled Carbon Nanotubes (MWCNTs) may solve this problem but only if there is a full control of their orientation inside the material. Aim of this work is to investigate the processing-properties-morphology relationships for a system prepared under elongation flow of MaterBi and commercial MWCNTs. The materials were characterized both in shear and non-isothermal elongational flow and mechanical tests have been done on hot and cold drawn fibres. The morphology has been investigated by TEM.The shear viscosity increases dramatically in the presence of the MWCNTs, and the Cox–Merz relation does not hold.The mechanical properties of the hot stretched fibres strongly increase with the draw ratio but the increase is larger for the filled fibres because of the orientation of the MWCNTs. This is not true for the cold drawing because the very low mobility of the MWCNTs hinders their further orientation and the final properties seem driven by the matrix.

Compatibilization of polyethylene/polyamide 6 blend nanocomposite films

Polymer blends of incompatible components need to undergo compatibilization, in order to give rise to a blend with good physical properties. At the same way, polymer/clay nanocomposites show this problem because of different chemical nature of the polymer matrix and of the clay. Compatibilization is therefore more necessary if an incompatible polymer blend is filled with an organomodified clay in order to give a final material with good properties. In this work, a polyethylene/polyamide 6 blend filled with an organomodified clay has been compatibilized with a maleic anhydride grafted SEBS (styrene‐ethylene‐butylene‐styrene) copolymer and a glicidylmethacrylate‐ethylene copolymer. The results show that compatibilization improves the mechanical properties in terms of elongation at break, as well as the rheological properties under nonisothermal elongational flow; furthermore, an unexpected effect has been found, since going from the isotropic to the anisotropic material, a fragile‐ductile transition occurs, with a significant increase of the elongation values, while a small reduction of the elastic modulus was found, because of the lower crystallinity in comparison to isotropic samples. POLYM. COMPOS., 36:992–998, 2015. © 2015 Society of Plastics Engineers

Rheological Behavior Under Shear and Non-Isothermal Elongational Flow of Biodegradable Polymers for Foam Extrusion

The production of many items, in particular for food packaging applications, is based on foam extrusion and thermoforming. These operations require the use of polymers which can grant some specific rheological properties, both under shear and elongational flow. In this work, the behavior of some biodegradable polymers [Mater-Bi® and poly(lactic acid)] under shear and non-isothermal elongational flow was investigated and compared with a traditional, non-biodegradable polymer, in order to assess their suitability for industrial-scale foam extrusion and thermoforming. The rheological characterization evidenced the differences between the different biodegradable polymers and the reference polystyrene (PS), as well as the effect of humidity on their main rheological properties. This can be of great interest in helping to find an optimum solution in replacing PS for the production of food packaging items.

Effect of small amounts of poly(lactic acid) on the recycling of poly(ethylene terephthalate) bottles

Poly(ethylene terephthalate) (PET) is one of the most used commodity polymers, especially for food and beverage applications, and its recycling is of great importance because of the possible use in the textile and construction industries. On the other hand, the interest in biodegradable polymers has led, in recent years, to the use of materials such as poly(lactic acid) (PLA) also in the food and beverage industry. The presence of small amounts of PLA in the PET waste can significantly affect the post-consumer recycling process. In this work, the effect of the presence of small amounts of PLA on the recycling of PET bottles is investigated by rheological, mechanical, morphological and thermogravimetric analysis. The results indicate that this presence can significantly affect the rheological properties under non-isothermal elongational flow, while the mechanical properties were considerably affected only in some circumstances and the thermal stability was not significantly modified.

Effect of different matrices and nanofillers on the rheological behavior of polymer‐clay nanocomposites

AbstractIn this work, a comprehensive study of the rheological behavior under shear and isothermal and nonisothermal elongational flow of low density polyethylene (LDPE) and ethylene‐vinyl acetate copolymer (EVA) based nanocomposites was reported to evaluate their “filmability”, that is, the ability of these material to be processed for film forming applications. The influence of two different kinds of organoclay – namely Cloisite 15A and Cloisite 30B – and their concentration was evaluated. The presence of filler clearly affects the rheological behavior in oscillatory state of polyolefin‐based nanocomposites but the increase of complex viscosity and the shear thinning are not dramatic. A larger strain‐hardening effect in isothermal elongational flow is shown by the nanocomposites compared to that of the pure matrix, particularly for EVA based nanocomposites. The melt strength measured under nonisothermal elongational flow increases in the presence of the nanofiller, while the drawability is only slightly lower than that measured for the neat matrix. Moreover, the rheological behavior under nonisothermal elongational flow of EVA‐based nanocomposites is similar for both nanoclays used. Differently, LDPE‐based nanocomposites show a strong dependence on the type of organoclay. Finally, the mechanical properties of the materials were measured by tensile tests. They revealed that the presence of the filler provokes, in all the cases, an increase of the rigidity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 344–355, 2010

Lagrangian finite element method for 3D time-dependent non-isothermal flow of K-BKZ fluids

Abstract A new numerical approach for the simulation of non-isothermal three-dimensional time-dependent flow of viscoelastic fluids is presented. The viscoelastic fluids are of the K-BKZ integral type and the method is based on a Lagrangian kinematics description of the fluid flow. The K-BKZ fluid is assumed to be a thermorheological simple material using the extended Morland and Lee hypothesis by Crochet and Naghdi [M.J. Crochet, P.M. Naghdi, A class of non-isothermal viscoelastic fluids, International Journal of Engineering Science 10 (1972) 755–800], where the real time in the K-BKZ constitutive equation is replaced with a temperature dependent pseudo time. The spatial coordinate system attached to the particles is discretized by 10-node quadratic tetrahedral elements using Cartesian coordinates. The temperature and the pressure are discretized by 10-node quadratic and linear interpolation, respectively, in the tetrahedral particle elements. The spatial discretization of the governing equations follows a mixed Galerkin finite element method. This type of scheme ensures third order accuracy with respect to the discretization of spatial dimension. The temperature equation is solved in time utilizing an implicit variable step backwards differencing (BDF2) scheme, obtaining second order convergence of the temperature in time. A quadratic interpolation in time is applied to approximate the time integral in the K-BKZ equation. This type of scheme ensures third order accuracy with respect to the discretization of the time. The third order convergence in time and space is demonstrated on time-dependent non-isothermal elongational flow as a test case.

Nanostructural modifications of polyamide/MMT hybrids under isothermal and nonisothermal elongational flow

AbstractThe aim of this work is to investigate the effects of elongational flow on the nanoscale arrangement of the silicate inside polyamide‐based nanocomposites. Hybrids, at different loadings of a commercial organoclay, were produced by melt compounding using two polyamide matrices, a nylon‐6, and a copolyamide with similar molecular weight and rheological properties. The elongational flow characterization was performed under both isothermal and nonisothermal conditions by using, respectively, an elongational rheometer (SER) and a fiber‐spinning technique. The extensional rheological response of melt‐compounded nanocomposites, correlated to TEM and X‐ray analyses, was used to probe the nanostructural modifications developed during the uniaxial stretching. The results demonstrated that isothermal and nonisothermal elongational flow can modify the nanomorphology of the nanocomposite hybrids affecting the degree of silicate exfoliation as well as the extent of silicate orientation upon the stretching direction. The entity of structural modifications induced by the stretching were highly dependent on the initial nanomorphological state and on the polymer‐clay affinity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 981–993, 2009

Eva copolymer-based nanocomposites: Rheological behavior under shear and isothermal and non-isothermal elongational flow

Nanocomposites are a very promising new class of polymer composites that exhibit an interesting combination of chemical, physical and mechanical properties. Only small amounts of nanofiller are sufficient to generate great variation of many properties. This work focuses on the rheological properties of poly[ethylene- co-(vinyl acetate)] (EVA copolymer) based nanocomposites in shear and in isothermal and non-isothermal elongational flow. The nanocomposites, at two different filler contents, were prepared in two different extruders and intercalates were obtained for all the samples. The twin-screw compounder is more efficient. The change of rheological properties, both in shear and in elongational flow, with respect to the pure matrix seem to follow the change of interlayer distance of the silicate particles. The nanocomposites, however, present, in the usual processing conditions, rheological properties similar to those of the virgin polymer, making possible their use in processing operations where both shear and elongational flows are involved.

Eigenvalue Asymptotics in Non-isothermal Elongational Flow

We shall furnish an asymptotic description of the spectrum of a C0-semigroup generator arising in non-isothermal elongational flow. It will be shown that the spectrum coincides asymptotically with isolated eigenvalues lined up along a logarithmic curve. The strategy for this result is based on delicate asymptotic estimates and ideas developed for the isothermal regime. These estimates can be successfully employed only since the spectral mapping theorem is valid for the semigroup.

Shear and nonisothermal elongational characterization of a liquid crystalline polymer

AbstractA commercial liquid crystalline polymer (LCP) sample was characterized both in shear and in nonisothermal elongational flow over a wide range of temperatures (240–350°C). The LCP sample employed was a copolyesteramide of 2,6 hydroxynaphthoic acid, terephthalic acid, and aminophenol manufactured by Celanese and commercially known as Vectra B950. The viscosity decreases dramatically at temperatures above the crystal‐nematic transition of the polymer. At lower temperatures, the extrusion of the sample is mainly due to plastic flow. Die‐swell values are very small and contraction of the extrudate is also revealed, but the sample shows high elasticity. The polymer is not spinnable at very low temperatures, and the melt strength rapidly decreases with increasing temperature.

Molecular Weight Dependency of Rheological Characteristics of Linear Low Density Polyethylene

Five samples of linear low density polyethylene, polymerized with l-butene as comonomer and having different molecular weights but similar molecular weight distributions, have been characterized in shear and nonisothermal elongational flow. From the experimental data, generalized relationships have been found for the zero shear viscosity, the shift factors of the whole viscosity curve, the critical shear rate, the melt strength and the breaking-stretching ratio. It is then possible to predict with very good confidence values for the most important rheological characteristics of any LLDPE of similar polydispersity from the sole knowledge of the weight average molecular weight, in the range of temperature and flow conditions of industrial interest.

Nonisothermal Elongational Flow of Polymeric Fluids According to the Leonov Model

The time‐dependent nonisothermal elongational flows of polymeric fluids have been modeled using the Leonov constitutive theory. The stress growth at a constant elongational strain rate or constant pulling speed and stress relaxation after elongation have been considered under various thermal histories including constant cooling rate and a sudden jump or a drop in temperature. Following the concept of thermorheological simplicity, the nonisothermal effects have been conveniently incorporated into the theory via experimentally determined temperature shift factors together with the effect of nonzero cooling rates on glass transition temperature. The predicted results have been compared with the available experimental data of Bogue and co‐workers. In general, the agreement between theory and experiment is found to be good and comparable to the available results of other theories.