The frictional behaviour of rubber is a topic of great interest and importance due to the invaluable uses of rubber in industry. The very particular behaviour of rubber also makes rubber friction a fascinating subject matter. Despite this it is still a topic not well understood. Previous studies have attempted to link the fracture mechanics of rubber crack propagation to the adhesive friction of rubber. The feasibility of such an approach to the adhesive friction of a rough rubber against a smooth surface, a configuration which can occur in various situations such as rubber seals or windscreen wipers, has been investigated. Rolling friction, described well by a fracture-like peeling process, is used to evaluate the viscoelastic dependence of sliding friction for various combinations of surfaces. A novel use of rubber is proposed as a material for particles to be used for jamming based soft robotics applications. This area of soft robotics is comparatively new and the materials that are being used at present are neither well established nor have been examined in great detail. Rubber would offer a material easily manufactured to desired shapes and dimensions with a wide range of moduli allowing modification to suit specific applications. The effect of jammed rubber particles on the response of a jammed packing to an externally applied load is examined. The evolution of inter-particle forces is studied using a rheometer configuration. Finite element techniques and modelling are employed to study the rubber in more detail.

Due to its large flexibility, low-price, large availability, and properties lignin is seen as an important compound with a wide range of applications. The increasing demand of fossil-based rubber materials is causing a serious threat to the environment and it is contributing to plastic- and marine pollution, ozone depletion and carbon dioxide emission (CO2) [1,2]. Numerous toxicological researches highlight that Carbon black may act as a universal carrier of wide variety of chemicals of varying toxicity to the human body [3,4]. Consequently, researcher endeavours in finding sustainable and eco-friendlier alternatives. The aim of this thesis was to further investigate the possibilities of replacing carbon black with modified lignin in rubber elastomeric materials- for the development of sustainable tyre technology. The research questions for this thesis were divided in four parts:   How does lignin (unmodified and modified) structure affect the mechanical properties of the rubber compound? How does lignin affect the cross-link and vulcanisation of the rubber compound? How does lignin affect the dispersion of the rubber compound? Which modification of lignin is more compatible with the rubber compound? Lignin is the second most abundant biopolymer on earth (after cellulose) and is mainly extracted from black liquor, which is obtained as a by-product from the pulp- and paper. In this study, pure lignin was obtained from Lignoboost process (Lignocity) and underwent an esterification process of aldehydes (1. Protonic, 2. Butyric, 3. Isobutyric 4. Methacrylic and 5. Crotonic). LignoCity 2.0 is a project focusing on the development of sustainable products and processes connected to lignin. The structure of the modified lignin was characterized using a FTIR-spectra. Furthermore, seven different rubber compounds were produced at Anva Poly Tech, which is a company that manufactures rubber materials in Sunne, Sweden. The mechanical testing involved: Tensile strength, IRHD, Hardness, Rebound Resilience and Rheometer curve. It was observable that the addition of lignin in rubber compounds did not significantly improve the mechanical properties compared to conventional carbon black. However, the rheometer curves of the lignin samples clearly indicate an increase in scorch time and that lignin takes part in the vulcanization process, thus the delay in crosslinking phase.  In addition, it was visible that the fully replacement of carbon black with lignin (unmodified and modified) increased the elongation at break. Furthermore, the FTIR spectra indicated a complete and successful modification of lignin. In addition, compared to unmodified lignin, it was visible that the modified lignin significantly improved the mechanical properties. Therefore, it was possible to conclude that the configuration and double bonds of the aldehydes had an impact on the vulcanization process. Butyric and isobutyric lignin were the better choices compared to the other lignin samples. / De rådande miljöproblemen som: plast- och gummiutsläpp i havet, växthusgasutsläppet och den ekologiska utarmningen i kombination med den ökande efterfrågan av fossilbaserade material har lett till en ökad satsning på att hitta mer hållbara och miljövänligare alternativ [1, 2]. Kimrök i gummimaterial utgör en del hälsorisker och samtidigt har negativ påverkan på miljön. Flertals studier visar att långtidsexponering av kimrök kan ge allvarliga lungproblem och även cancer [3,4]. På grund av dess stora tillgänglighet, låga kostnad och unika egenskaper anses lignin vara en möjlig och intressant framtidskandidat för ersättande av fossila produkter. Syftet med denna studie var att undersöka möjligheterna om att ersätta kimrök med modifierad lignin i gummimaterial för utvecklandet av ’grönare däckteknologi’. Frågeställningarna i detta arbete var uppställda i fyra i olika delar: Hur påverkar tillsättningen av lignin (omodifierad och modifierad) gummiblandningens mekaniska egenskaper? Hur påverkar tillsättningen av lignin tvärbindningarna och vulkningen i gummiblandningen? Hur påverkas tillsättningen av lignin gummiblandningars dispersion? Vilken modifikation av lignin är mest kompatibel med gummimaterialet? Lignin är en organisk biopolymer som är den näst mest (efter cellulosa) förekommande biomassan i naturen och produceras som en biprodukt från pappers- och massa industrin. Ren lignin erhålls genom extraktion från svartlut med diverse isolations metoder. I detta arbete erhölls lignin genom Lignoboost processen från Lignocity.  Lignocity 2.0 är ett projekt som syftar till att utveckla, kommersiella och effektivisera hållbara processer och produkter med fokus på lignin. I detta arbete modifierades ligninet genom en s.k. esterfierings process av fem olika aldehyder 1.Propionic, 2. Butyric, 3. Isobutyric, 4. Methacrylic och 5. Crotonic som sedan undersöktes i en FTIR-spektra. Sju olika gummiblandningar skapades (inklusive ett gummi som endast innehöll kimrök och ett gummi som ej innehöll kimrök eller lignin). Gummiblandningarnas mekaniska egenskaper undersöktes på följande sätt: Dragstyrka, IRHD (Hårdhet), Hårdhet, studselasticiteten och reometrisk karaktärisering De ligninbaserade gummiblandningarna gav ingen signifikant förbättring i de mekaniska egenskaperna. Dock visade den reometriska kurvan att tillsättning av lignin gav en ökning i bränntid samt att ligninet gav en förskjutning i tvärbindningsfasen. Vidare gav den reometriska kurvan en indikation på att ligninet deltog i vulkaniseringsprocessen. Isobutyric lignin hade den högsta bränntiden. Det var även bevisat att tillsättningen av lignin gav en ökning i töjning. Modifieringen av lignin gav en signifikant förbättring av de mekaniska egenskaperna jämfört med omodifierad lignin. FTIR-spektrumet av ligninproven indikerade på en lyckad modifiering och koppling av aldehydgrupperna. Trots att de ligninbaserade gummiblandningarna inte förbättrade de mekaniska egenskaperna så kunde intressanta kopplingar mellan aldehydens konfigurationer, dubbelbindningar och vulkaniserings processen göras. Butyric och isobutyric visade bäst resultat jämfört de andra ligninproven.

In this dissertation, four distinct but in some ways related topics, mostly related to experimental and theoretical investigations of friction and adhesion of elastomeric materials, are presented. First, an experimental and theoretical study of the interaction between elastic beams and granular media under compressive loading is performed. Buckling loads of beams with different dimensions and boundary conditions within granular media of different depths and grain sizes are measured, and theoretically approximated using the Ritz energy approach, based on the concept of beam on an elastic foundation. Several nondimensional parameters and a scaling law are derived to characterize different interaction regimes between the beams and granular support. The findings from this work is believed to be helpful for improved understanding of interactions between elastic beams and surrounding elastic foundation with applications to piles, oil pipelines, and robotic needle insertion into soft tissues. Second, the role of axial compliance on the friction of extensible strips is investigated. Significant changes were observed in the static and kinetic friction of strips, when the effective axial compliance was changed. The underlying causes of the changes in the frictional response are explained and quantitatively predicted using an extended shear lag model. We believe that this study provides insights into the effect of axial compliance on the frictional response of materials, paving the way for design and optimization of systems where the static and kinetic friction forces play an important role. Third, the effect of normal force and rate on the kinetic friction of two different elastomers, namely acrylic and silicone-based elastomers is evaluated. A custom-built pendulum test setup was used to perform the friction test in dynamic conditions. Two substantially different responses with respect to the change in normal force were observed and the role of different contributions to the frictional response of viscoelastic materials, i.e. bulk hysteresis friction, adhesion friction, and cohesion friction, are discussed. Different scenarios such as modifying the surface by using graphite powder, reducing test velocity, and also performing drop tests to characterize the surface hysteresis of the elastomers, were considered to further explore the origin of frictional responses of the elastomers. This study could improve insights gained from Dynamic Mechanical Analysis (DMA) data when obtaining and interpreting the effect of normal force on kinetic COF of elastomers with potential applications to tires, shoes, etc. where friction plays an important role. Last, a generalized scaling law, based on the classical fracture mechanics approach, is developed to predict the bond strength of adhesive systems. The proposed scaling law, which depends on the rate of change of bond area with compliance, is in apparent discrepancy with the previously reported scaling relationship that depends on the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when failure mechanism changes or the compliance of the load train is increased. Furthermore, the shear lag model is implemented to derive a closed-form relation for the system compliance and the conditions where the two models deviate from each other are discussed and demonstrated. The results obtained from this approach could lead to a better understanding of the relationship between the bond strength and the geometry and mechanical properties of adhesive systems, with applications to different types of adhesive joints such as bio-inspired adhesive, biomedical adhesive tapes, and structural adhesive joints. / Ph. D. / In this dissertation, four distinct but in some ways related topics, mostly related to experimental and theoretical investigations of friction and adhesion of elastomeric materials, are presented. The theoretical models are based on classic solutions for load transfer between two members through shearing an adhesive layer or frictional interface on extensible support layers. First, an experimental and theoretical study of buckling of elastic columns embedded in granular media is performed. In many engineering applications, it is desirable to insert and manipulate an elastic column like needle or drill rod within complex media, such as soft tissues or granular beds like sand and gravel. In these procedures the column is subjected to axial loading and it tends to buckle and lose stability due to a high length to thickness ratio. Burrowing a flexible structure through fragile media requires understanding the coupled interactions between a geometrically non-linear structure and its reconfigurable surroundings. Several nondimensional parameters and a scaling law are derived to characterize different interaction regimes between the columns and granular support in order to better understand the stability of elastic structures confined in a granular bed. Second, a comprehensive study that combines theory and experiments to investigate frictional responses of a system, i.e. static and kinetic friction, with change in system stiffness is presented. Friction plays an important role in many technologies such as tires, brakes, rubber seals, conveyer belts, and footwear. Understanding the role of system stiffness on the frictional properties of materials, from both experimental and theoretical points of view, has important implications for such technologies. Significant changes were observed in the static and kinetic friction of strips when the effective axial stiffness was changed. The underlying causes of the changes in the frictional response are explained and quantitatively predicted by a theoretical model. Furthermore, a permanent increase in kinetic friction of sufficiently soft extensible strip was found, with potential application to improved friction performance of materials where the kinetic friction plays a major role. Third, the effect of normal force and rate on the kinetic friction of two different elastomers, namely acrylic and silicone-based elastomers, is evaluated. A custom-built pendulum test setup was used to perform the friction test in dynamic conditions. Two substantially different responses with respect to the change in normal force were observed and the role of different frictional mechanisms is discussed. This study could improve insights gained from mechanical testing data at different temperatures and speed when obtaining and interpreting the effect of normal force on kinetic COF of elastomers, with potential applications to tires, shoes, etc., where friction plays an important role. Last, a theoretical model, to predict the bond strength of adhesive systems, is developed. The proposed model, which depends on the rate of change of bond area with compliance, is in apparent discrepancy with the previously reported scaling relationship thatdepends on the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when failure mechanism changes or the compliance of the load train is increased. The conditions where the two models deviate from each other are discussed and demonstrated. The developed model could help to better understand the role of system compliance on the bond strength of adhesive systems such as bio-inspired adhesive, biomedical adhesive, and structural adhesive, where the system stiffness changes significantly depending on the applications.

Cette étude concerne l’influence de la formulation sur les propriétés en fatigue des élastomères. Pour cela, douze matériaux ont été préalablement choisis de façon à ce que leur formulation soit simplifiée mais représentative de celles de matériaux industriels. La première partie de l’étude vise la description des mécanismes et des scénarios d’endommagement par fatigue, pour différents matériaux, niveaux de sollicitation et pourcentages de durée de vie. Une vaste campagne d’essais de fatigue interrompus a été menée permettant une analyse statistique de la population de défauts. Ces données ont permis d’une part de proposer des scénarios de ruine pour les différents matériaux étudiés, et d’autre part d’identifier deux types de mécanismes d’amorçage autour d’inclusions de différentes natures. La deuxième partie de l’étude vise à comprendre les mécanismes d’amorçage de fissure de fatigue. Un protocole expérimental permettant d’obtenir les champs d’énergie dissipée directement à partir des champs de température mesurés a été mis en place. Ce protocole a d’abord été développé à une échelle macroscopique et validé grâce à des simulations par éléments finis. Il a ensuite été appliqué avec succès à l’échelle des inclusions pour des cas 2D. Enfin, la troisième partie de cette étude propose un critère énergétique basé sur un protocole d’auto-échauffement permettant une détermination rapide des propriétés en fatigue. L’approche proposée utilise un critère à deux paramètres et permet de prédire la courbe de Wöhler déterministe avec une seule éprouvette, en une demi-journée d’essai et uniquement à partir de mesures thermiques. Ce critère a été validé sur une large gamme d’élastomères, et s’avère capable de rendre compte de l’influence de la gomme, du taux et du type de charges. / This study deals with the influence of the formulation on the fatigue properties of elastomeric materials. Twelve materials have been chosen so that their formulations are simplified but representative of these used in the industry. The first part of the study aims to describe the fatigue damage mechanisms and scenarios, for different materials, strain levels and fatigue life durations. A large experimental campaign of interrupted fatigue tests has been performed enabling a statistical analysis of the defects population. On one hand, these data led to propose damage scenarios for the studied materials, and on the other hand, to identify two classes of initiation mechanisms around different nature of inclusions. The second part of this study aims to better understand the fatigue crack initiation mechanisms. An experimental protocol enabling to obtain directly the dissipated energy fields from the measurement of the temperature fields has been set up. This protocol has been first developed at the macroscopic scale and validated thanks to numerical simulations. Then, it has been applied successfully at the scale of inclusions for 2D cases. Finally, the third part of this study suggests an energetic criterion based on a heat build-up protocol leading a fast determination of the fatigue properties. This approach uses a criterion with two parameters and is able to predict the deterministic Wöhler curve with one sample, in half a day and thanks to thermal measurements only. This criterion has been validated on a wide range of elastomeric materials and is able to reflect the influence of the gum, the type and the amount of carbon black.

Indiana University-Purdue University Indianapolis (IUPUI) / Fabrications of desirable fixed or removable dental prostheses depend upon accurate casts or dies. Recently, the most frequently used impression materials have been polyether (PE) and polyvinyl siloxane (PVS). However, both have their limitations: PVS is inherently hydrophobic, and PE is rigid. In order to take advantage of the desirable qualities of both PVS and PE impression materials, a new generation of impression material is being developed called vinyl polyether silicone (VPES, GC). The purpose of the present study was to compare the properties of hydrophilic PVS, PE, and VPES in regard to surface detail reproduction, contact angle, and tear strength. The hypotheses to be tested were: 1) VPES will show a significant superiority insurface detail reproduction compared with PVS and PE impression materials; 2) VPES will show a significant superiority in wettability compared with PVS and PE impression materials; 3) VPES will show a significant superiority in tear strength compared with PVS and PE impression materials. In order to test the surface detail reproduction, impressions were made of stainless steel dies with a parallel series of 15 different width lines on the surface and tested under dry and moist conditions. The wettability was assessed by contact angles of saturated CaSO4 aqueous solution drops on flat impression surfaces. A trouser tear test was employed to test the tear strength. The trouser-shaped specimens were prepared and tested in the Instron Universal Testing Machine. The data were analyzed by one-way ANOVA and Pearson‘s Chi square, (p < 0.05). All the materials showed better detail reproduction under the dry conditions than the moist conditions. There were no differences between the three materials in detail reproduction when impressing under either moist conditions or dry conditions. All the materials showed good wettability in the contact angle test. PVS rendered a contact angle as low as 34.19º. The contact angle of VPES was 44.84º, which was lower than 54.76º for PE. In the tear strength test, PE showed nearly two time higher tear strength than the other two impression materials. VPES showed slightly lower tear strength than PVS. The tear strength of the three materials tested in increasing order was VPES, PVS, PE. VPES showed comparable detail reproduction to PVS and PE and better wettability than PE, but showed the lowest tear strength compared with PE and PVS.