Besides yellowing, vinyl is weakened by prolonged exposure to ultra-violet.

And it's Not vinyl.

In intense test chamber exposure equivalent to 2 years of Florida sunlight, no yellowing or degradation of the Endura flap material occurred.

At the same time we tested the Endura material, we tested vinyl flap samples from every leading pet door manufacturer. In each case, there was significant yellowing, crackling and degradation of the material.