Issue 48

C.M.S. Vincente et alii, Frattura ed Integrità Strutturale, 48 (2019) 748-756; DOI: 10.3221/IGF-ESIS.48.68 753 Samples WG (%) P (%) K (g.cm 2 .min 1/2 ) LVL1 Uncoated 0.316 ± 0.023 0.315 ± 0.020 (1.23 ± 0.05) × 10 –4 Acrylic Coated 0.266 ± 0.006 0.264 ± 0.006 (1.04 ± 0.10) × 10 –4 PU Coated 0.208 ± 0.013 0.205 ± 0.013 (7.25 ± 0.08) × 10 –5 Uncoated 0.281 ± 0.016 0.277 ± 0.013 (3.26 ± 0.03) × 10 –4 LVL2 Acrylic Coated 0.236 ± 0.008 0.234 ± 0.006 (8.96 ± 0.08) × 10 –4 PU Coated 0.163 ± 0.011 0.161 ± 0.011 (5.34 ± 0.02) × 10 –5 Table 2 : Weight gain, open porosity and water absorption coefficient of samples LVL1 and LVL2. The effect of the surface roughness on wettability was described on the earlier work of Wenzel [31], concluding that the presence of roughness on a surface will contribute to enhance his natural hydrophilic or hydrophobic behaviour. PLA surfaces are described as intermediate surfaces between hydrophilic and hydrophobic, presenting a contact angle with water of approximately 73 o . The effect studied by Wenzel was demonstrated on a PLA surface in the work of Jordá–Vilaplana [32], showing that the contact angle with water of PLA sheets is reduced from 73.4 o to 27 o , when the root mean square roughness values vary from 12 to 55 nm. From these findings we can expect that the surface of sample LVL1 will be more hydrophilic than sample LVL2, promoting thus water absorption. Another important mechanism for water absorption in polymers is the diffusion of water molecules in the micro voids between polymeric chains [33]. It is also known that the processing temperature of the polymer is responsible for the creation of these micro–cracks. An increase in micro–cracks leads to an increase of water absorption [34]. This fact is linked to the porosity of the material. In our work, the extrusion temperature of 220 o C was responsible for the highest porosity, and consequently the highest amount of water absorbed, on sample LVL1. After verifying that the application of PU coating was efficient on protecting the PLA parts from water absorption, it’s now important to know which will be the impact of the PU coating on the mechanical properties of PLA parts. Effect of the PU coating on mechanical properties On Fig. 4 are displayed the mechanical properties of samples LVL1 and LVL2, estimated from tensile tests, before and after the application PU coating on specimens. Figure 4 : Mechanical properties of specimens LVL1 and LVL2, before and after the application PU coating. From the results presented on Fig. 4, we can observe that the application of PU coating improve the strength and ductility/toughness of samples. The best mechanical properties maximizing strength ( UTS =36.4 MPa, σ y =22.8 MPa and E =1.40 GPa), were obtained for PU coated LVL1 specimens, while the best values for ductility/toughness ( ε F =7.14 % and T =1.33 J/cm 2 ) were found on PU coated LVL2 specimens. In average the mechanical properties related with strength were improved by 17 % ( UTS ), 13% ( σ y ) and 8.0 % ( E ), while the mechanical properties related with ductility increases 24 % ( ε F ) and 16 % ( T ), with the application of the PU coating. The beneficial effect of the PU coating on the mechanical properties