Issue 48

C.M.S. Vincente et alii, Frattura ed Integrità Strutturale, 48 (2019) 748-756; DOI: 10.3221/IGF-ESIS.48.68 751 Water absorption tests Water absorption was analysed according to the ASTM D570 – 98 standard with both uncoated and coated specimens. The specimens were weighted before water immersion and his dry mass was recorded. The number of tests repetitions for each pair sample/coating was three. The samples immersion was done by covering half of the specimen’s height with water on a glass container. To guarantee that water reaches the bottom part of the samples, an absorbing filter paper was placed on the bottom of the specimens. In each defined time interval, the cubes were taken from the container, the excessive water was removed with paper and were posteriorly weighted. The weighing of the cubes was done in intervals of 30 minutes, for the first 4 hours, in intervals of 1 hour, for the next 4 hours, and in intervals of 24 hours, for the next 4 days. In total, the cubes spent 104 hours inside water. Some properties can be obtained from this type of test, namely: weight gain ( WG ) and open porosity ( P ), calculated by Eq. (1) and Eq. (2): 100%    sat dry dry m m WG m (1)   2 / 100%     sat dry H O m m P V (2) where m sat is the saturated mass (the mass at the end of the experiment), m dry is the dry mass (the mass at the beginning of the experiment), ρ H2O is the density of water (1 g.cm – 3 ) and V is the volume of the cube (1 cm 3 ). The absorption coefficient ( K ) was calculated graphically as the initial slope of the graph where x axis is the square root of the immersion time, and the y axis is the weight gain per unit of area of the cube. The weight gain per area is calculated by Eq. (3):    m m wet WG dry area A m dry (3) where m wet is the mass of the cube in a certain instant of time, and A is the area of one of the faces of the cube. Morphological characterization The external surface surfaces of PLA specimens were inspected by scanning electron microscopy (SEM). The SEM images were obtained with a field emission type microscope (Hitachi ® – S2400) operating at 20.0 kV, in a secondary electron emission mode. The surface of specimens was fixed on the SEM sample holder with a conductive adhesive and the sample surface was cleaned with an air flux before coating process on a sputtering system (Quorum–Q150TES). The conductive coating was performed with Au – Pt alloy using a current of 40 mA and evaporation time of 60 s. Figure 2 : Mechanical properties of samples LVL1 and LVL2.