Why do Photosensitive Resins turn yellow?To

Materials in classical stereolithography are currently suffering from two drawbacks. On one hand, cross-linked photopolymers have low toughness compared to most engineering polymers (e.g., thermoplastic materials such as polypropylene or ABS). 

On the other hand, the long-term properties of the utilized photopolymers are not sufficient. Photopolymers tend to yellow over time and, due to the tendency to take up water in a humid atmosphere, their mechanical properties degrade over longer periods of time. 

Yellowing first presents as a light tone all over, then progressively darkens until it reaches a dark brownish-yellow hue. Yellowing will be accelerated with exposure to UV light.