Degradation of laser sintered polyamide 12 parts due to accelerated exposure to ultraviolet radiation

Laser Sintering is an additive manufacturing technology capable of producing robust, geometrically complex polymer parts, with polyamide 12, also known as nylon-12, the most common and well understood feedstock powder. While Laser Sintering has drawn increased interest from industry for producing end-use components directly, little information is currently available regarding the effects of in-service conditions on mechanical performance. When considering applications where parts spend significant periods of time outdoors, such as the automotive sector, the effect of exposure to natural UV light is an obvious concern.

To assess their suitability for long-term outdoor applications, Laser-sintered polyamide 12 parts were subjected to up to 2 months of constant UVB radiation. UV exposure was found to cause significant yellowing of the parts, evidenced by their reflectance of blue light decreasing continually as exposure time was increased beyond 24 h. Results from UV/visible spectroscopy demonstrated an increase in UV absorbance with exposure time, attributed to increasing carbonyl concentration. Similarly, X-ray photo-electron spectroscopy showed increases in surface oxygen concentration proportional to UV exposure, with changes to the O 1s and C 1s environments after 2 months of exposure consistent with the generation of new carboxylic acid groups. Photo-degradation led to substantial decreases in both Ultimate Tensile Strength and Elongation at Break, with the deterioration first becoming appreciable between one and two weeks of exposure. Mono-energetic Positron Spectroscopy, a non-destructive surface analysis technique, demonstrated a correlation between these mechanical properties and free volume at the parts’ surfaces. Decreases in free volume were concluded to be due to changes in the nature of the polymer crystal structure and an increase in overall crystallinity at the part surfaces after UV exposure, identified using DSC. Overall, it was concluded that UV exposure causes significant deterioration of Laser-sintered polyamide 12 parts and this should be mitigated against prior to use in long-term outdoor applications.