Structural integrity evaluation of polymers used in additive manufacturing under UV light and humidity exposure

The mechanical properties of polymers change over time when they are exposed to UV light and moisture. This work presents the results of continuously exposing a nylon-based composite used in additive manufacturing (AM) to UV light and humidity for 24-, 48-, 96-, 168-, 336- and 504-hour periods. Sample coupons were printed in a Markforged Two ® composite printer using Onyx ®, which is a nylon matrix composite reinforced with short carbon fibers. For UV exposure, the samples were exposed to commercial 253 nm UV lamps, whereas for humidity, an ACE UV-260 humidity chamber was used at 50% relative humidity and 22°C with bi-distilled water. The effects of said variables were measured using the Charpy impact energy (per ASTM D6110), water absorption, and Shore hardness D (per ‎ASTM D2240). It was found that nylon indeed presents 1.03% ±0.28 water absorption over as little as 24 hr of exposure and about 5.6% ±0.48 water absorption for 504 hrs. Regarding the Charpy impact energy, the absorbed energy decreases from 450 kJ/m^2 ±15.96 at 24 hr to 254 kJ/m^2 ±33.9 at 504 hr of humidity exposure. The Shore hardness D varies from 59.1±0.82 for zero exposure to 59.7 ±1.5 at 24 hr and 66.8±2.5 after 504 hr of UV exposure. We can conclude that water absorption makes nylon a more fragile material, whereas UV exposure hardens the material. Future results could include using tensile axial tests and infrared spectroscopy to assess water absorption.