Contact mechanics of isotactic polypropylene

Mechanical properties of semi-crystalline polymers are strongly determined by the structure that develops upon partial crystallization of the material, and therefore a product's lifetime is directly related to its chemical and thermo-mechanical history. In this respect, changing the crystal phase, the molecular\u3cbr/\u3eweight, the cooling rate or the thermodynamic state can lead to completely different deformation kinetics and along with that, dierent failure modes. The effects of specic structural changes on intrinsic material properties are isolated and compared to observations in the scratch response of that same structure. This methodology clearly shows that the scratch resistance of a material can be qualitatively determined solely by its deformation kinetics; the surface penetration proves to be governed by visco-elasticity and the yield stress, whereas the friction force depends on the subtle interplay of penetration depth, shape of bowwave in front of the indenter tip (large strain behaviour) and in some cases crazing or cracking behind the indenter tip (localization). Likewise, the elastic recovery and therewith the final scratch depth gives important structural information which is related to intrinsic properties

Scratch Performance Of Filled Epoxy Systems

Polymer based materials are used in many applications nowadays. These light weight, strong materials are characterized in different groups, depending on their molecule structure. One of those groups are epoxies. These aromatic molecules are characterized by their strong adhesive character when irreversibly cured. High end industial glues and coatings are made out of epoxy resins. For many applications additives are used, e.g. to improve scratch resistance. Because fillers influence the intrinsic behavior of the epoxy materials, it is necessary to determine and predict this. In this way it will be possible to predict failure mechanisms. The main goal of coatings is to protect a product from contact damage. To understand the material resistance to scratches, different filled epoxy systems are tested. When particles with a higher stiffness than the matrix material are added, one expects the penetration depth to decrease, and therefore also the friction force. For additives with a lower stiffness the opposite behavior is expected. This report explains the preparation of smooth samples, needed for accurate scratch tests, as well as the experimental data for different types of fillers. Both 'hard' and 'soft' fillers are examined

Contact mechanics of isotactic polypropylene: effect of pre-stretch on the frictional response

Polymers are increasingly used in applications where relative moving parts are in contact. The dissipation of energy due to friction, i.e. heat production, reduces a product's lifetime significantly. Since in processing often an extrusion or injection moulding step is used in product formation, the induced anisotropic microstructure leads to a spatial variation of mechanical properties, for example frictional resistance. In this work we compare the scratch response of isotropic isotactic polypropylene (iPP) to the response of various oriented iPP systems. Subjected to single-asperity contact with a rigid diamond, the surface penetration and lateral force are measured. For various combinations of applied normal load and sliding velocity, the surface penetration and lateral force are measured. Optical profilometry measurements are used to explain the (large) differences in residual scratch profile between tests performed in the direction parallel and transverse to the orientation direction, respectively. As the anisotropy increases with the amount of orientation, both the maximum tensile stress and the strain hardening increase substantially. The penetration depth, for oriented systems governed by the transverse viscoelasticity and yield stress, is comparable for all loading angles and decreases with increasing amount of orientation. The direction of lowest frictional resistance is shown to be the direction parallel to the oriented crystals. The combination of decreasing global deformation and friction reduction as a result of pre-stretch decelerates strain localization, therewith delaying crack initiation which eventually leads to abrasive wear. Along with that, the substantial amount of elastic recovery after scratching in the transverse direction is related to the pre-tension of the perpendicular crystal network

Contact mechanics of isotactic polypropylene:Effect of pre-stretch on the frictional response

\u3cp\u3ePolymers are increasingly used in applications where relative moving parts are in contact. The dissipation of energy due to friction, i.e. heat production, reduces a product's lifetime significantly. Since in processing often an extrusion or injection moulding step is used in product formation, the induced anisotropic microstructure leads to a spatial variation of mechanical properties, for example frictional resistance. In this work we compare the scratch response of isotropic isotactic polypropylene (iPP) to the response of various oriented iPP systems. Subjected to single-asperity contact with a rigid diamond, the surface penetration and lateral force are measured. For various combinations of applied normal load and sliding velocity, the surface penetration and lateral force are measured. Optical profilometry measurements are used to explain the (large) differences in residual scratch profile between tests performed in the direction parallel and transverse to the orientation direction, respectively. As the anisotropy increases with the amount of orientation, both the maximum tensile stress and the strain hardening increase substantially. The penetration depth, for oriented systems governed by the transverse viscoelasticity and yield stress, is comparable for all loading angles and decreases with increasing amount of orientation. The direction of lowest frictional resistance is shown to be the direction parallel to the oriented crystals. The combination of decreasing global deformation and friction reduction as a result of pre-stretch decelerates strain localization, therewith delaying crack initiation which eventually leads to abrasive wear. Along with that, the substantial amount of elastic recovery after scratching in the transverse direction is related to the pre-tension of the perpendicular crystal network.\u3c/p\u3

Contact mechanics of polyolefins: effect of pre-stretch on the frictional response and the onset of wear

As a result of processing history, polymer products are, more often than not, anisotropic. In this work, single-asperity sliding friction experiments are used to characterize the tribology of semi-crystalline polymer films. We show the improved scratch resistance of oriented isotactic polypropylene and comment on the paradoxical results obtained for high-density polyethylene

Contact mechanics of high-density polyethylene: Effect of pre-stretch on the frictional response and the onset of wear

Nowadays, in many applications metal parts are replaced by light-weight polymer products. As a result of the processing history, these polymer fabricates are, more often than not, anisotropic, leading to a direction dependent mechanical performance. Recently we showed the frictional response of isotactic polypropylene is improved by pre-stretching the crystalline network. In the present work, the scratch response of isotropic high-density polyethylene (HDPE) is compared with that of several pre-stretched samples of the same material, subjected to a single-asperity contact with a rigid diamond indenter. The surface penetration and lateral force are measured in-situ for a range of applied loads and sliding velocities. In the direction perpendicular to the orientation, the observed response is comparable to that of isotactic polypropylene (iPP). Contrary, in the direction parallel to the oriented crystals abrasive wear is observed in HDPE already for relatively low applied loads. As the amount of anisotropy increases, the wear-rate also increases, leading to a decrease in global scratch resistance of these materials. The discrepancies between iPP and HDPE are explained by the intrinsic material behaviour; the lack of strain softening in HDPE prevents strain localization, hence the ever increasing local stress reaches its maximum value and brittle machining is observed

Temperature dependent two-body abrasive wear of polycarbonate surfaces

During the lifetime of polycarbonate surfaces, which for example are used as helmets or protective eye visors, friction and abrasive wear may result from scratching or sliding cycles. Previous research showed that it is essential to understand the intrinsic mechanical response of the polymer in order to further investigate its frictional and wear response. The Eindhoven Glassy Polymer (EGP) model is a 3D elasto-viscoplastic constitutive model, developed to describe the intrinsic mechanical response of polymer glasses. Temperature is a crucial player in the intrinsic response and also plays a pivotal role in the resulting frictional response as tested via a single-asperity scratch test. In the current study, a finite element model is used to investigate the effect of temperature on the frictional response of polycarbonate and detect the onset of crack formation and wear initiation. The results show that temperature has a strong effect on the intrinsic response of the polymer, i.e. drop in yield stress and altered strain-hardening and strain-softening response. However, it has a minute effect on its frictional response, the simulation model is able to capture this response quantitively. In addition, cracks are observed experimentally at elevated temperature. A critical positive hydrostatic stress value is selected as a criterion for crack formation. It has been shown that at elevated temperatures the value of the maximum positive hydrostatic stress increases due to the altered intrinsic response of the material on one hand, and the increased adhesion between the tip and the polymer on the other hand

Anomalous temperature dependence of isotactic polypropylene α-on-β cross-nucleation kinetics

A particular kind of heterogeneous nucleation, i.e., cross-nucleation, is sometimes observed in polymorphic substances, when a new crystal structure nucleates on the surface of a crystal of a different modification. Here we show a unique and apparently incongruous nucleation behavior in polymorphic isotactic polypropylene (i-PP). The rate of cross-nucleation of the monoclinic α-phase on the trigonal β-phase crystals increases with increasing temperature, in the vicinity of the α-crystals melting point. This behavior is contrary to that of the heterogeneous nucleation kinetics of the same crystal on various solid substrates, and also to the previously reported cases of cross-nucleation rate of other polymorphic systems, both exhibiting the expected decrease with temperature in the same range of undercoolings. i-PP α-on-β cross-nucleation apparently eludes the nucleation theory. The results are explained as a manifestation of a kinetic competition between α-on-β cross-nucleation and growth of β-crystalline seeds, and finally reconciled with the current understanding of nucleation. These new findings indicate that further theoretical efforts are needed to include the cross-nucleation phenomenon in the framework of a comprehensive understanding of polymorphic crystallization. Incidentally, this study highlights the intrinsic limits of the, industrially desirable, promotion of β-phase formation in polypropylene

Thermo-mechanically coupled modelling of a single-asperity scratch on an isotropic isotactic polypropylene surface

\u3cp\u3eIsotactic polypropylene (iPP) is a low cost semi-crystalline polymer that is easy to process, has a wide variety in properties and is, therefore, used in many applications. Many of these applications require enhanced wear-resistance to prolong the lifetime of the product. Essential is to first investigate the intrinsic response of the material in order to describe its friction and wear response. In this respect, a hybrid experimental-numerical approach is used to couple the intrinsic response to the single-asperity scratch response. The numerical model used is a 3D elasto-viscoplastic model based on the Eindhoven Glassy Polymer (EGP) model. For the first time a coupled thermo-mechanical EGP model is implemented in a Finite Element Method (FEM)-framework. The model is capable of accurately describing the intrinsic response of the material, which opens the door to qualitatively and quantitatively describe its frictional response and understand the damage formation mechanism (i.e. the initiation of wear). In this study, α- and β-phase iPP are studied. We show that the difference in the intrinsic response between the two phases has a significant influence on the friction and wear response. Moreover, a stick-slip phenomenon is proven to be the main responsible for the damage mechanism observed. The observed periodic “fish-scale” damage pattern results from periodic changes in resistance during the tip movement. A relation between the polymer intrinsic response and the damage formation mechanism is established. The influence of the applied load and scratch speed on damage formation is investigated as well.\u3c/p\u3