Contacts adhésifs sur surfaces texturées d'élastomères : effet d'échelle

Les contacts adhésifs dont une des surfaces est micro-texturée à l'aide de motifs a soulevé une attention considérable ces deux dernières décennies. La compréhension fondamentale de ces contacts est primordiale car elle permettrait de contrôler les propriétés d’adhésion des matériaux par texturation plutôt que par fonctionnalisation chimique. Néanmoins le rôle précis des paramètres majeurs qui gouvernent ce type de contacts reste à identifier clairement. Nous allons discuter les conditions de formation d’un contact entre des hémisphères élastiques en PDMS et des surfaces micro-texturés avec des piliers hexagonaux souples, également en PDMS. Nous avons mis en œuvre le test « JKR » qui relie l’énergie d’adhésion à l'aire de la surface de contact entre l'hémisphère et l'échantillon plan. Toutefois, ce dispositif expérimental permet, outre une utilisation 'quasi-statique' classique, d'explorer en fréquence le comportement du contact : un mode dynamique a été développé à cet effet qui travaille en chargement cyclique. Ceci permet d'analyser l’effet de la sollicitation fréquentielle en fonction des paramètres des surfaces texturées. Dans un contexte de tests statiques, nous décrivons le rôle structural des piliers et discutons un phénomène d’hystérèse de contact après un chargement et un déchargement, exclusivement due au contact entre les plots et sans influence sur le comportement du contact sur plots. Dans un deuxième temps, nous montrons que la variation du rapport d'aspect des motifs de texturation de surface conduit à un comportement invariant lors de la formation d’un contact entre un hémisphère et un substrat souple élastique : l'homothétie géométrique à l'échelle étudiée conduit à des comportements au contact similaires en mode statique ainsi que dynamique

Corrélation entre la formulation et les propriétés adhésives de films issus de la tex acrylate

L'étude a consisté à comparer les mécanismes de filmification de deux dispersions acrylate industrielles (DI et D2) et à comprendre l'incidence de la formulation sur les performances des films adhésifs sensibles à la pression. La structure des films, tant interne que surfacique, se différencie nettement pour les deux latex. La fragmentation des membranes particulaires atteint un stade très avancé pour DI tandis que D2 conserve une structure essentiellement cellulaire. La formulation à l'aide d'un tensioactif de type sulfosuccinate conduit à des distributions différentes du tensioactif dans le film pour les deux latex considérés. En présence de D2, le tensioactif agit comme un plastifiant des chaînes polymères, il permet la fragmentation de la membrane interparticulaire. L'effet couplé de molécules fortement exsudées en surface du film et d'une plus grande interpénétration aux interfaces entre les particules entraîne une diminution rapide et forte du niveau adhésif avec le taux de tensioactif utilisé. L'effet existe également pour Dl mais est beaucoup plus progressif. L'ajout d'un agent réticulant de type isocyanate a également des conséquences très différentes selon le latex considéré. Dans le cas de DI, la réticulation semble minoritaire par rapport aux phénomènes d'interdiffusion entre particules. L'hypothèse d'une auto-réticulation du latex est vraisemblable. A l'inverse, la dispersion D2 réticule effectivement. L'isocyanate se répartit de manière homogène en zone interparticulaire, conduisant à une augmentation des pontages entre particules. La chute d'adhérence est ainsi beaucoup plus significative.The study deals with the comparison of the filmification mechanisms of two industrial acrylate dispersions (DI and D2) and with understanding the influence of formulation on the performance of resulting pressure adhesive films. The structure of the films is clearly different according to latex formulation. The fragmentation of membranes between particles is significant for the film obtained from DI whereas film resulting from D2 keeps a cellular structure.Formulation with a sulfosuccinate surfactant leads also to differentiated structures : with dispersion D2, the surfactant acts as a plasticizer allowing fragmentation of the membranes. This effect combined with a strong exudation of molecules at the surface of the film leads to a quick and consequent decrease in adhesion properties with the level of surfactant. The effect of a crosslinking agent also depends on the latex. For the film resulting from dispersion DI, crosslinking is much smaller than interdiffusion between particles. Hypothesis of a self-crosslinking mechanism seems reasonnable. On the contra, for film based on D2, the isocyanate crosslinker is distributed more homogeneously through interparticular region, leading to an increase in bridgings between particles. The decrease of adherence is thus much more significant.MULHOUSE-SCD Sciences (682242102) / SudocSudocFranceF

Etude des mécanismes d'ahésion mastics polyuréthanes / supports PVC

L'objectif est de mettre en évidence les mécanismes d'adhésion polyuréthanes (PU)/polychlorure de vinyle (PVC), d'améliorer les formulations et les propriétés des assemblages. Le collage de matériaux non modifiés induit des ruptures interfaciales. Nous avons montré que l'absence d'agent de démoulage dans le PVC entraîne une rugosité plus importante et une fonctionnalité différente. Les vieillissements accélérés sous UV permettent de créer des fonctions réactives en surface. L'utilisation d'un primaire modifie l'interface. La rupture peut alors devenir cohésive. Le retrait de l'agent de couplage du PU diminue la cohésion du matériau, la rupture des assemblages est mixte. L'utilisation d'un prépolymère 100% diol fait chuter la cohésion et une rupture cohésive apparaît. L'adhérence de PU sur PVC peut donc être améliorée significativement en modifiant les propriétés de surface des supports et/ou la cohésion des mastics.We wanted to point up adhesion mechanisms between PU sealants and PVC substrates, to suggest new ways to improve assemblies. Usually, interfacial failures are observed when bonded PU/PVC are tested. The results of PVC modification are that:- taking release agent off leads to higher roughness, difference in functionally and hence cohesive failure.- creating reactive functions on surface by ageing makes adherence better.- applying adhesion promoter permits to obtain higher polarity and strength of assembly. PU formulation has been studied too. To sum up the main points we can say that:- without coupling agent, the cohesion goes down and we get dual cohesive/interfacial failure. - an addition of solvent leads to an increase in cohesion and no evolution of the type of failure. - using a 100 % diol prepolymer permits a strong fall in cohesion and a cohesive failure. Consequently, PU/PVC adhesion can be significantly improved by modifying either substrate surfaces or sealant formulations.MULHOUSE-SCD Sciences (682242102) / SudocSudocFranceF

Etude de l'interface entre deux polymères différents (Echelle microscopique et comportement macrascopique)

Le but de l'étude est de mettre en évidence la contribution des mécanismes responsables de la résistance d'un assemblage de deux polymères : un élastomère copolymère butadiène acrylonitrile hydrogéné et une polyoléfine : polyéthylène-co-octène. Ces mécanismes sont l'interdiffusion et la co-réticulation. La co-réticulation est liée à l'interface moléculaire créée lors de l'étape d'interdiffusion. La réticulation par faisceau d'électrons est utilisée pour conserver l'état d'interdiffusion. Les réseaux des deux polymères sont analysés et la formation de ponts de réticulation par irradiation est mise en évidence. La formation de ponts de réticulation interfaciaux dans les assemblages peut être proposée malgré la faible profondeur de l'interface. Afin d'augmenter la quantité d'interface, ce travail s'est orienté vers l'étude de mélanges de ces deux polymères incompatibles. La formation de ponts de réticulation peut être envisagée pour interpréter le comportement mécanique de certains mélanges.The aim of this study is to point out the contribution of the mechanisms responsable for the interfacial strength of a joint of polymers : hydrogenated nitrile butadiene rubber and a polyolefin, the copolymer of ethylene and octene. These mechanisms are the interdiffusion and co-crosslinking phenomena. The co-crosslinking is closely related to the molecular interface created during the interdiffusion step. A technique of crosslinking by electron beam is used to avoid the interdiffusion state. The network of both polymers are studied and the crosslink formation by radiation is shown. The formation of interfacial crosslinks in plane joints obtained can be proposed although the interface size is nil. In order to increase the interface quantity, the work is oriented towards the study of polymer blends. In spite of the incompatibility of the two polymers, the formation of interfacial crosslinks can be considered to interpret the mechanical behaviour of some blends.MULHOUSE-SCD Sciences (682242102) / SudocSudocFranceF

Influence of Plasma Chamber Set-Up on the Surface Modification of Non-Vulcanized and Pure SBR Rubber Treated at Radio-Frequencies Air Plasma

International audienceNon-vulcanized styrene-butadiene rubber thin films were exposed to Radio-Frequency low-pressure air plasma. Two different configurations corresponding to direct and downstream RF plasmas were compared. Optical emission spectroscopy provided information about plasma composition and temperature. The plasma treated SBR surfaces were characterized by contact angles and XPS spectroscopy. Two different phenomenological mechanisms are proposed to describe the observed differences according to the plasma configuration used and account for the impact of the activated gas phase on the structure of plasma-treated non-vulcanized SBR surfac

Stability of Plasma Treated Non-vulcanized Polybutadiene Surfaces: Role of Plasma Parameters and Influence of Additives

International audienceSurface modification studies of non-vulcanized BR elastomers (butadiene rubber) by low-pressure air plasma treatment and the effect on ageing and adhesion performances are presented in this paper. In particular, the influence of discharge power and distance from the glow discharge, and impact of antioxidant molecules in the BR formulation were examined. To characterize the changes to the BR surface, XPS spectroscopy, contact angle measurements, AFM nanoindentation experiments and tack measurements were utilized. Oxidation and crosslinking were the main mechanisms observed on the polymer chains regardless of the plasma conditions used. Beyond a certain threshold of plasma energy (in our case, discharge power of similar to 60 W and exposure time of similar to 30 s), a steady state was reached irrespective of the distance from the glow discharge. The presence of antioxidant molecules considerably reduced crosslinking phenomena while maintaining oxidation processes on polymer chains and increasing the nitrogen content in the near surface region. The mechanisms responsible for these differences have been identified. Interestingly, the COOH/C=O ratio changed according to the balance between oxidation and crosslinking. The hydrophobic recovery rate was mainly driven by temperature-dependent dynamics and varied according to the degree of crosslinking in the surface region. It was found to be lower in air atmosphere in the presence of antioxidant molecules. Finally, the presence of antioxidant molecules in the BR formulation allowed the adhesion performances after plasma exposure to significantly increase

Consolidation by Spark Plasma Sintering of Polyimide and Polyetheretherketone

This article presents two high- temperature thermoplastic powders which were sintered by spark plasma sintering in order to get homogeneous mechanical properties. Dense polyimide (PI) and polyetheretherketone (PEEK) specimens were obtained at temperatures as low as 320 degrees C for PI and 200 degrees C for PEEK, respectively. Relative densities higher than 99% were reached for both materials. In order to characterize their properties, in situ measurements with compression and hardness tests were carried out on sintered samples. This method allowed to obtain polymeric materials with improved mechanical properties. (C) 2014 Wiley Periodicals, Inc

New 2-in-1 Polyelectrolyte Step-by-Step Film Buildup without Solution Alternation: From PEDOT-PSS to Polyelectrolyte Complexes

Although never emphasized and increasingly used in organic electronics, PEDOT-PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrene sulfonate)) layer-by-layer (lbl) film construction violates the alternation of polyanion and polycation rule stated as a prerequisit for a step-by-step film buildup. To demonstrate that this alternation is not always necessary, we studied the step-by-step construction of films using a single solution containing polycation/polyanion complexes. We investigated four different systems: PEDOT-PSS, bPEI-PSS (branched poly­(ethylene imine)-poly­(sodium 4-styrene sulfonate)), PDADMA-PSS (poly­(diallyl dimethyl ammonium)-PSS), and PAH-PSS (poly­(allylamine hydrochloride)-PSS). The film buildup obtained by spin-coating or dipping-and-drying process was monitored by ellipsometry, UV–vis-NIR spectrophotometry, and quartz-crystal microbalance. The surface morphology of the films was characterized by atomic force microscopy in tapping mode. After an initial transient regime, the different films have a linear buildup with the number of deposition steps. It appears that, when the particles composed of polyanion-polycation complex and complex aggregates in solution are more or less liquid (case of PEDOT-PSS and bPEI-PSS), our method leads to smooth films (roughness on the order of 1–2 nm). On the other hand, when these complexes are more or less solid particles (case of PDADMA-PSS and PAH-PSS), the resulting films are much rougher (typically 10 nm). Polycation/polyanion molar ratios in monomer unit of the liquid, rinsing, and drying steps are key parameters governing the film buildup process with an optimal polycation/polyanion molar ratio leading to the fastest film growth. This new and general lbl method, designated as <i>2-in-1 method</i>, allows obtaining regular and controlled film buildup with a single liquid containing polyelectrolyte complexes and opens a new route for surface functionalization with polyelectrolytes

Optimization of the Spark Plasma Sintering Processing Parameters Affecting the Properties of Polyimide

The present study deals with the optimization of polyimide (PI) mechanical properties, obtained by Spark Plasma Sintering (SPS), by using a method combining Design of Experiments (DOE) with physical, structural, and mechanical characterizations. The effects of SPS parameters such as temperature, pressure, dwell time, and cooling rate on the density, mechanical properties, and structure of PI were investigated. The experimental results revealed that the mechanical properties of the material were optimized by raising the sintering temperature up to 350 degrees C. The optimized SPS processing parameters were a temperature of 350 degrees C, a pressure of 40 MPa, and a dwell time of 5 min. Under these conditions, a relative density of 99.6% was reached within only a few minutes. The corresponding mechanical properties consisted of Young's modulus of 3.43 GPa, a Shore D hardness of 87.3, and a compressive strength of 738 MPa for a maximum compressive strain of 61.8%. Moreover, when working at 320 degrees C and at 100 MPa, an increase in the dwell time was necessary to enhance the properties. Contrary to the other parameters, the cooling rate appeared to be a non-significant parameter. Finally, correlations between the PI structure and the mechanical properties were made to demonstrate the densification mechanisms. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41542