Semianalytical models to predict the crystallization kinetics of thermoplastic fibrous composites

The growing interest for continuous fiber‐reinforced polymer composites leads to the development of new processes such as resin transfer molding for thermoplastics (RTM‐TP) or tape placement. In the aim of optimization, their simulations are required and have to include all involved physical phenomena and the associated couplings. During the consolidation step, the crystallization of the semicrystalline matrix occurs between the fibers of the multiscale reinforcement. A tricky task is to provide a realistic model able to describe the crystallization kinetics, which includes the effect of fibers on the polymer phase change and avoiding large computation time. In 2004, Haudin and Chenot proposed a generalization of the Avrami model, written in a differential form to compute the evolution of the crystallization of a neat thermoplastic in an infinite volume. In the present article, new extensions are proposed to predict the crystallization in long‐fiber thermoplastic composites, without or in the presence of transcrystallinity on fiber surfaces. In both cases, they are compared to three‐dimensional numerical simulations using a previously validated numerical method. All the numerical and analytical results are consistent. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44508.     

Thermal stress–strain of self‐compacting concrete in compression

Self‐compacting concrete or self‐consolidation concrete (SCC) is being used in underground and other industrial structures that may be subjected to high temperatures during operation or in case of an accidental fire. The proper understanding of the effects of elevated temperatures on the stress–strain relationship of SCC is necessary in the assessment of structural safety. This paper presents the high temperature behavior from an experimental study carried out on SCC subjected to high temperatures. The effects of temperature, strength grade, and polypropylene (PP) fibers on the initial elastic modulus, strain at peak stress, and stress–strain curves of SCC are studied, which offered a test basis for estimating the deformation of SCC under high temperature. An empirical constitutive formula for the thermal stress–strain of SCC is developed on the basis of the deformation characteristics of PP fiber‐modified SCC. Copyright © 2012 John Wiley & Sons, Ltd.     

Linear viscoelasticity characterization of egg albumen/glycerol blends with applications in material moulding processes

The effect of composition, storage conditions and temperature over the rheological properties of dough-like materials prepared within a mixing-rheometer was studied using different egg albumen protein/glycerol ratios: 50/50, 60/40 and 65/35. As protein/glycerol ratio increases from 50/50 to 65/35, the mechanical spectra display a dramatic increase (i.e. 15,000-fold in the storage modulus at 1 rad/s), as a greater degree of structuration is achieved. An evolution from a predominantly viscous behaviour into a predominantly elastic one is also observed (i.e. the loss tangent shifts from 1.9 to 0.3 at 1 rad/s).When the viscoelastic properties were studied along ageing, a relaxation related minimum was found at relatively short times, which could define the optimal condition for bioplastic processing (e.g. injection moulding). Freezing also plays an important role by inhibiting ageing and strongly retarding the evolution of G′ and G″ along ageing time after thawing. When samples are submitted to a thermal treatment up to 90 °C, a minimum is always found both in G′ and G″ that takes place at lower temperature as the protein/glycerol ratio increases (from 62 to 49 °C).     

Ruthenium‐Catalyzed One‐Pot Tandem Isomerization–Transfer Hydrogenation Reactions of γ‐Trifluoromethylated Allylic Alcohols and β‐Trifluoromethylated Enones

The ruthenium–2‐propanol combination was found to transform γ‐trifluoromethylated allylic alcohols and β‐trifluoromethylated enones into the corresponding saturated alcohols in excellent yields via a one‐pot tandem process involving isomerization and transfer hydrogenation(s). High stereospecificity was demonstrated and evidence for two mechanistic pathways is provided. The method was applied to a rapid synthesis of trifluoromethylated citronellol.     

Modeling the Porosity Evolution of a Powder under Uniaxial Compression

The purpose of this work is to present a new model describing the evolution of powder porosity versus applied pressure during uniaxial compression. This model is based on quasi‐chemical treatments and population balances. It is generally admitted that compression is composed of four steps: a first reorganization of the initial particles, a fragmentation of some initial particles, a second rearrangement of the initial and secondary particle mixture and a plastic deformation stage. So the model is constructed around these four steps. Experimental results on eighteen different powders show a fair agreement with theoretical results and material physical constant values. The use of this model can allow classifying the reorganization, fragmentation and plastic deformation abilities of the products and it will be useful in order to optimize the elaboration process.     

Global insight into microwave stoneware firing: Macro and microstructural changes

Industrial competition and environmental concerns lead to the exploration of alternative and energy-efficient technologies for ceramic materials processing. The main objective of this work was to present microwave heating as a viable option for stoneware processing. Stoneware functional properties are presented and discussed, with emphasis on impact strength, water absorption, porosity, and color. Microstructure analyses show that microwave- and gas-fired samples have higher densifications than electrically fired samples. A relevant finding for processing conditions is that microwave firing requires temperatures approximately 100°C lower than those required by conventional firing. Microwave-fired samples’ rupture energies are approximately twice (0.57 ± 0.06 (J)) those of the reference samples (0.26 ± 0.03 (J)), and their water absorptions are approximately one-half (1.5% at 1170°C and 0.8% at 1190°C) of those of the reference samples (2.0%), whereas the water absorption of electrically fired samples at 1180°C has been estimated to be 7.5%. Color analysis also evidences a shift to lower microwave firing temperatures, what is attributed to the enhanced transformations promoted by microwave heating when comparing with the transformations promoted by conventional (gas or electric) heating.     

Melt processing of nanocomposites of cellulose nanocrystals with biobased thermoplastic polyurethane

Nanocomposites of thermoplastic polyurethane (TPU) with cellulose nanocrystals (CNC) without and with surface treatment are obtained by melt processing. Nanocomposites are obtained with nanofiller weight content near of the theoretical percolation threshold (3.9 wt%). Visual observation of CNC agglomerates is sufficient to prove the inefficiency of the mixing in systems with untreated CNC. The crystallization kinetics of the TPU changes with the addition of CNC and this is confirmed by differential scanning calorimetry analysis. Thermogravimetric analysis prove that the addition of CNC increases the thermal stability of the TPU. From the rheological analysis it is possible to verify the absence of percolation and an intermediate state of sol–gel transition in the nanocomposites. CNC/TPU nanocomposites with 5 wt% of treated CNC present better mechanical performance than de neat TPU and the other processed nanocomposites and display around 130% increase in Young's modulus while retaining significant values of toughness, tensile strength and elongation at break.     

The Effect of Implanted Al and/or Y on the Scale Formation on Low-Al Fe20Cr2Al Alloy

The very early stages of the oxidation of an Fe20Cr2Al alloy, unmodified and ion-implanted by aluminium, yttrium and a combination of both elements, Al and Y, were studied at 1100 °C in oxygen using two-stage-oxidation exposures with ¹⁸O₂ as a tracer and subsequent characterisation of the scales using SIMS analyses of distribution of oxygen isotopes and oxide-related negative ion clusters, SEM observations of the surface morphology and photoluminescence spectroscopy analysis of the phase composition. The scales formed in all cases, except for the Al-implanted alloy, exhibited layered structures, with the outer part comprising Fe- and Cr-rich oxide, and the inner part being Al₂O₃, which grew due to a mixed outward–inward mechanism . The alumina sub-layers contained the transient oxides and α-Al₂O₃. Implanted Al significantly affected the mechanism of the scale growth, providing that the scale consisted essentially of α-Al₂O₃, and grew via a mixed inward-outward mechanism typical for scales on alumina formers.