Effect of color on scratch and mar visibility of polymers

Polymer scratch mechanics has been widely studied. However, the physics behind scratch visibility is still not well understood. Scratch deformation mechanisms certainly affect visibility but also other material properties, such as roughness, color or gloss. This work relates to methods to quantitatively evaluate the effect of sample color on polymer scratch and mar induced visibility, aiming at linking scratch behavior with the underlying physics. A custom-built black box was utilized for the analysis of samples possessing different colors and surface finishing. Automatic image analysis was performed through a commercial software to quantify the onset of visibility in terms of a critical load above which the scratch becomes visible. Trained surveyors visually quantified said critical load. White color was shown to delay the onset of visibility by reducing the contrast between the damaged and undamaged area. For instance, the critical load for the onset of visibility for a white polycarbonate copolymer was nearly twice the value reported for its black variant. Consistent results were obtained for all tested samples. Similar findings were reported for the mar phenomenon, highlighting how white color can also mask a different type of damage. The usefulness of the present work and some future perspectives are discussed

Modelling the interphase of 3D printed photo-cured polymers

3D printing, in particular the Polyjet technology, has been widely employed for the production of complex heterogeneous composites such as co-continuous architectures (the so-called Interpenetrating Phase Composites, IPCs). It is a manufacturing method in which discrete photopolymer droplets of different materials can be deposited on a build tray and cured by UV light lamp. Previous research already demonstrated how the characteristics of the interface between different photopolymers can vary if formed before or after UV curing process, with the formation of a narrow or broad interphase. In the present work, the dynamic-mechanical properties of multilayer bimaterial composites (made combining a glassy and a rubbery polymer) were investigated under tensile loading, which is relatively insensitive to the spatial arrangement of layers, as opposed to bending. The use of a simple parallel configuration allowed the development of an analytical model which incorporates the properties of the two photopolymers and their interphase, considering their effect on both elastic and dissipative behaviour of the composites; in particular, the interphase behaviour is quite close to that of the glassy polymer and even small quantities are sufficient to dramatically change the overall dissipative behaviour of the composite. It is demonstrated that reliable modelling of real co-continuous architectures as obtained by Polyjet printing (and similar techniques) should include the effect of the interphase

Properties and role of interfaces in multimaterial 3D printed composites

In polyjet printing photopolymer droplets are deposited on a build tray, leveled off by a roller and cured by UV light. This technique is attractive to fabricate heterogeneous architectures combining compliant and stiff constituents. Considering the layer-by-layer nature, interfaces between different photopolymers can be formed either before or after UV curing. We analyzed the properties of interfaces in 3D printed composites combining experiments with computer simulations. To investigate photopolymer blending, we characterized the mechanical properties of the so-called digital materials, obtained by mixing compliant and stiff voxels according to different volume fractions. We then used nanoindentation to measure the spatial variation in mechanical properties across bimaterial interfaces at the micrometer level. Finally, to characterize the impact of finite-size interfaces, we fabricated and tested composites having compliant and stiff layers alternating along different directions. We found that interfaces formed by deposition after curing were sharp whereas those formed before curing showed blending of the two materials over a length scale bigger than individual droplet size. We found structural and functional differences of the layered composites depending on the printing orientation and corresponding interface characteristics, which influenced deformation mechanisms. With the wide dissemination of 3D printing techniques, our results should be considered in the development of architectured materials with tailored interfaces between building blocks