Thin film mechanical characterization of UV-curing acrylate systems

This study presents the mechanical characterization of UV-curing acrylate systems. UV-curable polymers are commonly used in the stereolithography (SLA) technique to build multi-layered objects. Typically, the mechanical properties of the 3D-printed product are affected by the intrinsic material heterogeneity along the sample thickness. To understand what determines this heterogeneity, single layers of UV-curable polymer are characterized and the effect of process conditions on the mechanical properties is studied. Micro-compression experiments are carried out to determine the intrinsic mechanical properties which are representative of one single UV-cured layer. To determine the right conditions to generate maximally-cured micropillars, the evolution with irradiation time of monomer conversion, glass-transition temperature and yield stress has first been studied. Thereto, micrometer-sized pillars and dog-bone shaped samples have been prepared via UV-curing. Micro-compression measurements on maximally-cured micropillars are performed to study possible size effects. The results reveal that with decreasing pillar size, the yield stress decreases. Tensile measurements are performed on dog-bone shaped samples which have been processed in the same way as compared to the compression samples. These tensile tests show higher yield stress values when compared with compression tests. This size effect can be attributed to the rinsing with acetone during the sample preparation that leads to a removal of monomer from the crosslinked network. As a consequence, in the real 3D-printing process, the mechanical properties will depend on the feature size. In conclusion, a method is presented to determine the mechanical properties of one single layer of material used in the rapid-prototyping SLA process. The experimental procedure we adopted requires only a few millilitres of material and, therefore, is well suited for screening materials under real SLA process conditions.