[Show abstract][Hide abstract] ABSTRACT: Keywords: A. Polymer–matrix composites (PMCs) B. Mechanical properties B. Interface D. Scanning electron microscopy (SEM) D. Differential scanning calorimetry (DSC) a b s t r a c t The aim of this investigation was to study a new family of wood polymer composites with thermoplastic elastomer matrix (pebax Ò copolymers) instead of commonly used WPC matrices. These copolymers are polyether-b-amide thermoplastic elastomers which present an important elongation at break and a melt-ing point below 200 °C to prevent wood fibers degradation during processing. Moreover these polymers are synthesized from renewable resources and they present a hydrophilic character which allow them to interact with wood fibers. We have used two pebax Ò grade with different hardness and three types of wood fibers, so the influence of the matrix and wood fibers characteristics were evaluated. Composites were produced using a laboratory-size twin screw extruder to obtain composite pellets prior to injection moulding into tensile test samples. We have evaluated fibers/matrix interaction by differential scanning calorimetry (DSC), infrared spectroscopy (IRTF) and scanning electron microscopy (SEM). Then, the mechanical properties, through tensile test, were assessed. We also observed fibers dispersion into the matrix by tomography X. DSC, IRTF and SEM measurements confirmed the presence of strong interface interactions between polymer and wood. These interactions lead to good mechanical properties of the composites with a reinforcement effect of wood fibers due also to a good dispersion of fibers into the matrix without agglomerate.
Full-text · Article · Aug 2012 · Composites Science and Technology
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t We have studied the thermal stability of a new family of wood polymer composites (WPC) which use a thermoplastic elastomer matrix (pebax Ò copolymers). These copolymers are poly(ether-b-amide) thermoplastic elastomers which show a significant elongation at break and a melting point below 200 C which helps prevent degradation of wood fibres upon processing. Moreover these polymers present a hydrophilic character able to interact with wood fibres. Another important feature is that these polymers are synthesized from renewable sources. We have been using two types of pebax Ò matrices and two species of wood flour as fillers. Composites were made by using a laboratory-size twin-screw extruder to obtain homogeneous composite pellets prior to injection moulding into tensile test samples. The thermal stability of the matrix, wood fibres and composites was investigated using thermogra-vimetric analysis under air and nitrogen atmosphere. In our study, we have shown a spectacular improvement of thermal stability of the composites under air atmosphere, as opposed to measurement performed under nitrogen. The presence of wood in pebax Ò hinders the thermo-oxidation in air by the formation of char residue in the earlier stage of degradation. We have also determined an optimal range of wood content in which we observe the protective synergism.
Full-text · Article · Feb 2012 · Polymer Degradation and Stability
[Show abstract][Hide abstract] ABSTRACT: This work deals with the rheological behavior and adherence properties of pressure sensitive adhesive formulations dedicated to medical applications. We have developed a specific viscoelastic substrate which mimics adhesion on human skin to measure the adherence properties of PSAs when they are stuck on the human skin. By comparing peeling results of PSAs, dedicated to medical applications, stuck on human skin and on this viscoelastic substrate we show that this substrate, based on a blend of natural proteins, presents a better representation of the interactions occurring at the skin/adhesive interface than conventional substrates used for peel test (i.e. glass and steel).
Full-text · Article · Nov 2008 · International Journal of Pharmaceutics
[Show abstract][Hide abstract] ABSTRACT: This work deals with the study of the viscoelastic and adherence properties of pressure-sensitive adhesive (PSA) formulations dedicated to medical applications. We have developed a specific viscoelastic substrate to measure the adherence properties of PSAs that mimics adhesion on human skin. In the present article, we describe several experiments dedicated to a better understanding of adhesion on viscoelastic substrates without discussing specifically the case of human skin. In this way, we have studied different model adhesive formulations based on real medical formulations, and we have related the rheological behavior to the adherence properties obtained on different substrates to study the various specific effects due to the viscoelasticity of soft substrates. We propose from this study a failure criterion that allows one to derive a reasonable estimate of the peeling transition rate from cohesive to interfacial or stick–slip failure.
Full-text · Article · Apr 2007 · The Journal of Adhesion
[Show abstract][Hide abstract] ABSTRACT: In this Note, we focused on the sharkskin defect, classically observed in linear polymer extrusion. The aim of this work is to link the sharkskin onset to the rheological properties of polymer melt and more particularly to the shape of the molecular weight distribution. Using the model of Brochard–de Gennes, we show that it is possible to eliminate the sharkskin if Mn≈10Me. To cite this article: A. Allal et al., C. R. Physique 8 (2007).
No preview · Article · Jan 2007 · Comptes Rendus Physique
[Show abstract][Hide abstract] ABSTRACT: We have studied the relationship between rheological and peeling properties for hot-melt pressure-sensitive adhesives based on homopolymers or copolymers blended with tackifying resins. In this article, we particularly try to demonstrate that it is possible to define a quantitative link between rheology and adherence when the model formulations are deposited on substrates with strong (thermodynamic) adhesion. We describe the experimental results obtained on these model formulations and discuss the quantitative relationships obtained. In the case of “adhesion modulation” (derived from different treatments of the substrates), we show that the relationships become much more complicated, even with the same model adhesives. At the end, we discuss on the competition between adhesion and dissipation in the case of poor adhesion.
Full-text · Article · May 2006 · The Journal of Adhesion
[Show abstract][Hide abstract] ABSTRACT: This chapter contains sections titled: AbstractIntroductionMain Features of the Viscoelastic Behavior of the Pure Components, Blends, and Full Adhesive Formulations Rheological ExperimentsRheological Behavior of the Pure Components: [SI], [SIS], and Pure BlendsRheological Behavior of the Full Adhesive FormulationsA Model of the Rheological Behavior A Model for the Pure CopolymersA Model for the Blends [SIS-SI]A Model for the Full Adhesive Formulations [SIS-SI-Resin]Discussion Molecular DesignOn the Variation of the Secondary Elastic Plateau ModulusConclusions
[Show abstract][Hide abstract] ABSTRACT: This work is focused on the sharkskin defect encountered during the extrusion of linear polymers. This defect is characterized by the development of surface cracks, perpendicular to the flow direction. Our purpose is to establish relationships between sharkskin onset and rheological and molecular properties of the extruded polymer. Starting from the elasticity theory of Griffith, we show that the period of the defect is proportional to its amplitude, and that the critical stress for the onset of sharkskin is a function of the plateau modulus, the weight average molecular weight and the molecular mass between entanglements. On the other hand, we show that the cracks propagation velocity is controlled by the extrudate velocity and the Rouse time. We explain that the critical shear rate for the onset of sharkskin depends on the longest time of the distribution of relaxation times, i.e. the tube renewal. Finally, the present approach allows to clarify the ambiguity of simultaneous apparition of sharkskin defect and wall slip, as reported in the literature.
No preview · Article · Mar 2006 · Journal of Non-Newtonian Fluid Mechanics
[Show abstract][Hide abstract] ABSTRACT: Dans ce travail, nous nous sommes focalisés sur le défaut de peau de requin, rencontré fréquemment en extrusion de polymères linéaires. Notre objectif est de relier ce défaut aux propriétés rhéologiques du polymère et à sa structure moléculaire. En nous appuyant sur la théorie de l'élasticité de Griffith, nous montrons d'une part que la période des fissures est proportionnelle à leur amplitude et que la contrainte seuil d'apparition de la peau de requin est fonction du module du plateau caoutchoutique, de la masse moyenne en masse et de la masse entre enchevêtrement. D'autre part, nous montrons que la vitesse de propagation de la fissure dans le massif polymère est contrôlée par la vitesse de sortie de l'extrudat et par le temps de Rouse. Enfin, notre travail sur les paramètres d'apparition de la peau de requin nous a permis de montrer que la contrainte de cisaillement critique d'apparition du défaut est du même type que celle de la transition glissement faible-glissement fort du modèle de Brochard. Cette contrainte induit un étirement des chaînes macromoléculaire près de la paroi, qui stockent ainsi une énergie élastique suffisante pour fissurer l'extrudat en sortie de filière. Cette approche permet d'expliquer l'ambiguïté sur l'apparition simultanée du défaut de peau de requin et du glissement à la paroi, rapportée dans la littérature.
[Show abstract][Hide abstract] ABSTRACT: “Molecular rheology” is the missing link between the macromolecular structure of polymeric materials and their viscoelastic properties in the melt state. It complements the engineering or continuum mechanics aspects of rheology, which generally ignores the molecular details of the objects under study. The pioneering vision of the diffusion and relaxation processes of flexible macromolecular chains initiated by P.-G. de Gennes has lead to very effective and predictive models of viscoelasticity of polymer melts, which go far beyond academic interest.We present, in this paper, two very different examples of application of molecular rheology: molecular design of block copolymers corresponding to expected end-user properties (which are directly linked to linear viscoelastic properties) and formulation of blends of linear polymers in order to get strain-hardening effects in non-linear viscoelasticity usually obtained with long-chain branched (LCB) materials.
Full-text · Article · Jun 2005 · Journal of Non-Newtonian Fluid Mechanics
[Show abstract][Hide abstract] ABSTRACT: The processing and application properties of hot-melt pressure-sensitive adhesives (HMPSA) are governed, to a large extent, by their rheological properties. Coating of the HMPSA is performed at high temperatures in the molten state. At room temperature, the adhesive satisfies the Dahlquist criterion and, consequently, has permanent tack. We have particularly studied the full formulations based on triblock and diblock copolymers, and also those based on newly designed molecules, such as tetrablock or radial copolymers. We have demonstrated in the previous articles of this series that, for these systems, the volume fraction of the free polyisoprene is the most important parameter that drives the tack performances by controlling the level of the secondary elastic plateau modulus observed in the low-frequency range. To improve the end-user properties, we have increased the diblock content in the blends. We describe here the dynamic mechanical properties, at room temperature, of the pure copolymer blends (i.e., without addition of a tackifying resin) and the full HMPSA formulations. We focus particularly in this article on blends that contain a high diblock content. The effect of the morphology of the diblock copolymer on the rheological behavior of the adhesive is discussed in detail. Finally, we propose a model, based on molecular dynamics concepts, which describes the rheological behavior in a very wide range of frequencies for all copolymers and full formulations of the study.
Full-text · Article · Jun 2005 · The Journal of Adhesion
[Show abstract][Hide abstract] ABSTRACT: The aim of this work is to use a recently developed statistical model of dispersions with nonhydrodynamic interactions to describe the linear viscoelastic properties of emulsions of Newtonian liquids. None of the existing models can describe the rheological behavior of such systems, particularly the elastic properties, in the linear regime. We first present the results of numerical simulations of our model applied to emulsions. We show that taking nonhydrodynamic interactions into account allows to predict that emulsions of two purely viscous liquids have a complex viscoelastic behavior. We then compare the model to experimental results on oil/water emulsions, stabilized with ionic and nonionic surfactants. We find out that our statistical mechanical approach gives a much better description of the viscoelastic behavior of these samples than purely hydrodynamic models do. However, the elasticity observed is underestimated by our model. We indicate further theoretical developments which could improve the description of the viscoelastic properties of emulsions.
No preview · Article · Mar 2005 · Journal of Colloid and Interface Science
[Show abstract][Hide abstract] ABSTRACT: The viscoelastic properties of hot melt pressure-sensitive adhesives (HMPSA) based on formulations of block copolymers and tackifying resins have been studied in detail, through the variation of the complex shear modulus, G*, as a function of frequency, y . In this first article, we analyze the individual behavior of the components of HMPSA blends: (1) the two copolymers, styrene-isoprene (SI) diblock copolymer and styrene-isoprene-styrene (SIS) triblock copolymer and (2) two tackifying resins. The viscoelastic behavior of the overall formulation is also presented. We have mainly studied the effects of (1) the molecular characteristics of the SI and SIS copolymers and (2) the composition of the blends (mainly the effect of SI content, S content in SIS and SI, resin content) on the viscoelastic properties. A theoretical approach based on concepts of molecular dynamics leads to a model which describes reasonably well the linear viscoelastic properties of individual components and their formulations. Our systematic study can be used to design new copolymer molecules which can mimic the rheological behavior and end-user properties of regular formulations at room temperature.
No preview · Article · Aug 2003 · The Journal of Adhesion
[Show abstract][Hide abstract] ABSTRACT: The viscoelastic properties of hot melt pressure-sensitive adhesives (HMPSA) based on formulations of block copolymers and tackifying resins have been studied in detail, through the variation of the complex shear modulus, G*, as a function of frequency, omega. In this first article, we analyze the individual behavior of the components of HMPSA blends: (1) the two copolymers, styrene-isoprene (SI) diblock copolymer and styrene-isoprene-styrene (SIS) triblock copolymer and (2) two tackifying resins. The viscoelastic behavior of the overall formulation is also presented. We have mainly studied the effects of (1) the molecular characteristics of the SI and SIS copolymers and (2) the composition of the blends (mainly the effect of SI content, S content in SIS and SI, resin content) on the viscoelastic properties. A theoretical approach based on concepts of molecular dynamics leads to a model which describes reasonably well the linear viscoelastic properties of individual components and their formulations. Our systematic study can be used to design new copolymer molecules which can mimic the rheological behavior and end-user properties of regular formulations at room temperature.
No preview · Article · Aug 2003 · The Journal of Adhesion
[Show abstract][Hide abstract] ABSTRACT: We extend the previous work by Benallal et al. on the relationship between structure and rheological properties of linear polymer melts. The aim of this paper is to quantify the effect of the chemical structure on the viscoelastic properties. We show that these properties are governed by the monomer dimensions and the interaction energy. To cite this article: A. Allal et al., C. R. Physique 3 (2002) 1451–1458.
No preview · Article · Dec 2002 · Comptes Rendus Physique
[Show abstract][Hide abstract] ABSTRACT: The aim of this work is to use a recently developed statistical model of dispersions with non-hydrodynamic interactions (Dagréou et al., 2002) to describe the linear viscoelastic properties of suspensions of rigid hairy particles in a polymeric matrix. We first present numerical simulations of our model applied to this system; we demonstrate that taking physical interactions into account allows one to predict the complex relaxation behaviour of filled polymers. We then compare the statistical model to experimental results on suspensions of grafted silica particles in a polystyrene matrix and show that they are in reasonable agreement up to volume fractions close to percolation.
L'objectif de ce travail est d'utiliser un modèle statistique (Dagréou et al., 2002) pour décrire les propriétés viscoélastiques linéaires de polymères chargés. Nous présentons d'abord des simulations numériques du modèle appliqué à ce système; nous montrons que la prise en compte des interactions non hydrodynamiques permet de prévoir un comportement complexe en élasticité. Nous comparons ensuite le modèle avec des résultats expérimentaux obtenus pour des suspensions de particules de silice greffée dans une matrice de polystyrène; il existe un assez bon accord entre théorie et expérience, jusqu'à des fractions volumiques proches de la percolation.
No preview · Article · Dec 2002 · The Canadian Journal of Chemical Engineering
[Show abstract][Hide abstract] ABSTRACT: Rheological models based on molecular dynamics (as opposite to empirical relationships) are now preferred to link the molecular weight distribution (MWD) of linear polymers to their rheological properties. These models incorporate the double reptation concept, which represents the relaxation modulus as an integral over the molecular weight distribution. We propose a method that incorporates a detailed modeling of all the relevant relaxation processes, including Rouse fast and longitudinal modes and glassy relaxation. In addition, we take into account the effect of polydispersity on the relaxation times for reptation, i.e., “tube renewal.” In order to demonstrate the importance of these features of our technique, we compare it with one involving the direct inversion of the double reptation integral without accounting for tube renewal and additional relaxation processes. To invert the relaxation modulus in terms of the molecular weight distribution, one must either solve the ill-posed problem using an efficient numerical algorithm or postulate a function to describe the MWD. The second approach is more robust and less sensitive to noisy data, but one must assume the form of the MWD a priori. We present a procedure for selecting this function and use it to compare the two approaches to inverting a G(t) model. Data for several binary blends and commercial polymers are analyzed using both approaches. We conclude that the more detailed technique is necessary when the MWD is broad or when there is a significant amount of low-molecular weight material present.
No preview · Article · Jan 2002 · Journal of Rheology