Key Engineering Materials Vols. 554-557

Abstract: Crystallization from polymer melt is one of the most fundamental phenomena of material phase transformations. The possibility of controlling crystallization kinetics is essential to achieve the proper polymer microstructure and consequently obtain desired material properties, reducing undesired effects such excessive anisotropy of shrinkage, warping and insufficient dimensional stability. Due to the high transformation rate, the simulation of crystallization is fundamental to mimic this important physical phenomenon under several testing and processing conditions by using commercial software. User subroutines were developed and implemented into finite element-based model to simulate crystal growth in semicrystalline polymers with various crystal morphologies. These subroutines allowed the commercial program Abaqus to be customized for solving the Kolmogoroff-Avrami-Evans equation with Hoffmann-Lauritzen model in order to simulate the variation of the polymer crystallization degree. The micro-structural evolution in non-isothermal conditions and with different cooling rates was considered. The study was performed on isotactic PP (SABIC PP 505) for its simplicity to the measure polymer crystals. A tensile test specimen, produced by injection molding, was chosen as case study to evaluate the crystallization evolution. The paper reports the numerical and experimental results.

Abstract: Poly(lactic acid) (PLA) is a biodegradable thermoplastic polyester derived from renewable resources which may replace conventional polymers for some applications. To overcome some of its limitations such as poor gas barrier properties and low elongation at break, one method is to blend PLA with small amounts of other bio-based polymers. In this study, two processes, eg classical twin screw extrusion and a multilayer co-extrusion process have been used to combine PLA and poly(3-hydroxybutyrate-co-3-valerate) PHBV to obtain films with different blend morphologies. The effect of the morphology on the crystallinity has been studied and has hightlightned new behavior of PHBV. The addition of a nucleating agent in the PHBV to modify its crystallinity, has also been studied.

Abstract: For several years, modeling hyperelasticity has been focused on and leaded to a large choice of strain energy potential forms. Since then, many advances have been made in constitutive modeling of rubber like materials. These models are nowadays widely used in many applications like constitutive modeling of soft tissues in biomechanics problems or plastic thermoforming simulation. In this work, constitutive modeling of TPO sheets for thermoforming application is considered. Experimental measurements have shown that the material is transversely isotropic. To take into account this anisotropy, we implemented some new transversally isotropic hyperelastic constitutive models in Abaqus software with the help of user subroutines. Furthermore, different particular forms of the strain energy potential are investigated and their hyperelatic constants are fitted to the measurement data from tensile tests performed in different directions. Based on the results of these investigations, a transversely isotropic form of the energy potential derived from the Yeoh constitutive model is adopted and several tests are analyzed for validation purpose. The chosen model is a good compromise that achieves accurate predictions with limited amount of tests and limited identification efforts. Another key finding of this work is the influence of the anisotropy on the thermoformed parts.

Abstract: This paper is concerned with understanding the behaviour of Polyethylene Terephthalate (PET) in the injection stretch blow moulding (ISBM) process where it is typically bi-axially stretched to form bottles for the packaging industry. Preforms which have been pre sprayed with a pattern and heated in an oil bath have been stretched and blown in free air using a lab scale ISBM machine whilst being monitored via high speed video. The images have subsequently been analysed using a digital image correlation system (VIC 3D). The results have been used to validate appropriate simulations of the free-blow process using ABAQUS®/Explicit FEA with a suitable viscoelastic material model, along with experimental process data obtained using an instrumented stretch rod.

Abstract: This work aims to highlight the importance of interphase triggered from interdiffusion at neighboring layers on controlling the interfacial flow instability of multilayer coextrusion based on a compatible bilayer system consist of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) melt streams. A fundamental rheological measurement on the bilayer structures provides a good strategy to probe the mutual diffusion process occurred at neighboring layers and to quantify the rheology and thickness of the interphase generated thereof. By implementing steady shear measurements on the multilayer’s, subtle interfacial slippage can be observed at a condition of short welding time and rather high shear rate due to the disentanglement of chains at the interphase. Pre-shear at an early stage on the multilayer was found to greatly promote the homogenizing process by inducing branched structures and hence increasing interfacial area. In coextrusion, some key classical decisive parameters concerning the interfacial instability phenomena such as viscosity ratio, thickness ratio and elasticity ratio, etc. were highlighted. These key factors that are significant in controlling the interfacial stability of coextrusion in an incompatible system seem not that important in a compatible system. In comparison to the severe flow instability observed in the coextrusion of PMMA/PE incompatible bilayer, the coextrusion of PMMA/PVDF compatible bilayer appears to be smooth without apparent interfacial flow instability due to the presence of the interphase. Interdiffusion can reduce (even eliminate) the interfacial flow instability of coextrusion despite of the very high viscosity ratio of PVDF versus PMMA at low temperatures. Indeed, in the coextrusion process, on one hand, the interdiffusion should be studied by taking into account of the effect of polymer chain orientation which was demonstrated to decelerate the diffusion coefficient. On the other hand, the interfacial shear stress was able to promote mixing and homogenizing process at the interface, which favours the development of the interphase and guarantees the stable interfacial flow. The degree of the interphase is related to a lot of parameters like contact time, processing temperature, interfacial shear stress and compatibility of the polymers, etc. Therefore, apart from the classical mechanical parameters, the interphase created from the interdiffusion should be taken into consideration as an important factor on determining the interfacial instability phenomena. References [1] H. Zhang, K. Lamnawar, A. Maazouz, Rheological modeling of the diffusion process and the interphase of symmetrical bilayers based on PVDF and PMMA with varying molecular weights. Rheol. Acta 51 (2012) 691-711 [2] H. Zhang, K. Lamnawar, A. Maazouz, Rheological modeling of the mutual diffusion and the interphase development for an asymmetrical bilayer based on PMMA and PVDF model compatible polymers, Macromolecules (2012), Doi: http://dx.doi.org/10.1021/ma301620a [3] H. Zhang, K. Lamnawar, A. Maazouz, Role of the interphase in the interfacial flow stability of multilayer coextrusion based on PMMA and PVDF compatible polymers, to be submitted. [4] K. Lamnawar, A. Maazouz, Role of the interphase in the flow stability of reactive coextruded multilayer polymers, Polymer Engineering & Science, 49, (2009), 727 - 739 [5] K. Lamnawar, H. Zhang, A. Maazouz, one chapter” State of the art in co-extrusion of multilayer polymers: experimental and fundamental approaches” in Encyclopedia of Polymer Science and Technology (wiley library) (feature article)

Abstract: The poly (lactic acid) (PLA), through its organic origin and its biodegradation properties, can be a good alternative to petroleum-based polymers. To this end, the forming process as well blown extrusion and foaming of PLA was investigated in this study as an alternative for the production of food packaging. Through this work, we present some promising routes to enhance its processing ability which presents several challenges mainly due to the poor shear and elongation properties of this biopolymer. To our knowledge, there is no paper dedicated to the investigation of foaming and/or blown extrusion of PLA that involves structural, rheological and thermo-mechanical properties. To achieve this objective, various formulations of PLA with multifunctionalized epoxy, nucleants and plasticizer were prepared and characterized on the basis of their linear viscoelasticity and extensional properties. The balance of chain extension and branching has been also investigated using solution viscosimetry, Steric exclusion chromatography (SEC) and rheology (relaxation spectrum, Van Gurp Palmen curves….). We pushed further by characterizing both the structure and thermo-mechanical properties of PLA formulations. On one hand, a batch foaming assisted with supercritical CO2 was achieved following a full characterization in physicochemical, rheological and thermal domain, The influence of the foaming parameters, the extent of chain modification as well as the contribution of crystallization on cell morphology was evaluated. Based on these parameters, structures ranging from micro to macro-cellular-cell were obtained. On the other hand, the stability maps of blown processing for neat and modified PLA were established at different die temperatures. We have achieved a great enhancement of the blown processing windows of PLA with high BUR (Blow Up Ratio) and TUR (Take Up Ratio) attained. We were able to demonstrate that a higher kinetic of crystallization can also be reached for chain-extended and branched PLA formulated with adequate amounts of nucleants and plasticizers. Induced crystallization during process was also demonstrated. Through this work, blown films with interesting thermo-mechanical and mechanical properties have been produced using an optimal formulation for PLA. References [1] A. Maazouz, K. Lamnawar, B. Mallet, Patent: C08L67/00; C08J5/10. FR2941702 (A1). (2010) [2] Y.-M. Corre, A. Maazouz, J. Duchet, J. Reignier, Batch foaming of chain extended PLA with supercritical CO2: Influence of the rheological properties and the process parameters on the cellular structure. J. of Supercritical Fluids,58 (2011) 177-188 [3] B. Mallet, K. Lamnawar, A. Maazouz, Compounding and processing of biodegradable materials based on PLA for packaging applications: In greening the 21st century material’s world, Frontiers in Science and Engineering, 1-2(2011) 1-44 [4] B. Mallet, K. Lamnawar, A. Maazouz, Improvement of blown extrusion processing of PLA: structure-processing-properties relashionships. Polymer engineering and Science (To appear in 2013).

Abstract: Abstract. Upon processing, polymeric products feature a complex microstructure. Besides evolving over the molded component, a through-the-thickness variation is also developed. This is the result of the thermo-mechanical environment (combined thermal and mechanical fields) applied during processing, which varies with the molding technique, the selected molding conditions and polymer properties (rheological, thermal, constitution). This complex microstructure makes rather intricate the establishment of structure-properties relationships in processed polymers. In fact, the basic identification of most relevant morphological parameters determining the behavior of the moldings is been revealed a difficult endeavor, further complicated by the multi-scale morphology presented by polymeric materials. This work follows an inductive approach for establishing the relationships between the structure and the properties (mechanical and barrier) of molded poly(ethylene terephthalate), PET. These relationships are investigated for specimens prepared by different methods, from “simple” to more “complex” stretching modes. Initially, PET specimens were prepared by stretching thin films at different high temperatures and strain rates, followed by quick cooling in a universal testing machine equipped with a thermal camera (uniaxial stretched specimens). More closely to processing, PET injection molded preforms were free blown without a mold with distinct conditions (free blown specimens). Finally, PET bottles were produced from the preforms also under different blown conditions. The morphology of all specimens was assessed by bi- and tri-refringence and DSC. The mechanical properties were evaluated by tensile tests at room temperature. Also, the oxygen transmission rate, OTR, was assessed for the PET bottles. For this low crystallinity and slowly crystallizable polymer, the initial modulus is mainly related to the amorphous phase (i.e., molecular mobility and orientation level). The yield stress appears to be determined by the degree of crystallinity and level of molecular orientation. In the case of free blown specimens (bi-axially stretched) the anisotropy of the initial modulus depends upon the induced anisotropy of the molecular orientation. OTR is influenced by the molecular orientation and the degree of crystallinity of the polymer. An attempt to interpret these types of relationships by molecular dynamics simulations is made.

Abstract: The experimental processing parameters, such as applied pressure and forming temperature have been analysed during polymer hot embossing of micro-cavities. The viscoelastic characteristics of polymer above the glass transition temperature have been investigated with the classical viscoelastic models. Generalized Maxwell Model has been used to describe polymer behaviours in the glass transition temperature range. The parameters include relaxation time, storage modulus and loss modulus of the Generalized Maxwell Model that have been introduced. The identification of polymer characteristics has been carried out through Dynamic Mechanical Analysis (DMA). The storage modulus, the loss modulus and the damping factor of the selected polymer have been obtained with different imposed frequencies. The master curve of complex modulus has been obtained by applying the time temperature superposition principle. The experimental data has been identified with optimized fitting parameters of Generalized Maxwell Model. A proper agreement between the experimental measurement and the identification of viscoelastic model is observed. The resulting constitutive equations have been implemented in finite element software in order to achieve the numerical simulation of the hot embossing process.

Abstract: In the tube hydroforming (THF) process, a pressurized fluid is used to expand a thin walled tube inside a closed die in order to fill the die cavity. THF has many advantages that render this process interesting for different industries such as automotive and aerospace. In this work, to investigate the effect of different process parameters, such as the friction condition, tube thickness and end-feeding on the final product, THF experiments were performed on stainless steel 321 (SS 321) tubes using a round-to-square die. Experimental loading paths were obtained via the data acquisition system of the hydroforming press, which is fully instrumented. An automated deformation measurement system, Argus®, was used to measure the strains on the hydroformed tubes. The THF process was simulated using Ls-Dyna software. The variation in the strain and thickness measured from the experiments were compared to the simulation results at critical sections. Comparison of the results from the finite element (FE) simulations and experiments showed good agreement, indicating that the approach can be used for predicting the final shape and thickness variations of the hydroformed parts for more complex shapes in aerospace applications.