what causes plastic to become brittle?

[crespo]

the blue plastic parts on my godaikin gardian has discolored and started to crack. when i attempted to put the head back on with delinger inside, the head joint snapped off with the slightest pressure. why does this happen with some plastics over time, but not with others? the white plastic found on the foot leg covers are still fine and flexible after over 20 years. is there a way to prevent this? is it a wise idea to try and glue the cracked parts? or will i just be adding more damage to the already brittle plastic?

[admin1 aka Ed]

I've heard discussions about "photo stabilizers" that weren't used in the early plastics industry. My understanding is that the blue plastic mix wasn't stable with light or heat.

These days the stabilizers are used to prolong plastic life.

Brittle Blue Popy plastic just degrades with any heat

Sorry man. My Guardian knees broke the same way.