Moat likely it was oxidized from exposure to direct sunlight.
Ever since the tree huggers decided that plastic was too long lasting, we've been subjected to a plethora of plastics, all with different formulas for faster biodegrading.
Unfortunately, they begin to degrade as soon as they are exposed to air and sunlight....real smart there, you government meddlers.

Trey is right, unless you KNOW exactly which material you are working with, it's nearly impossible to weld it.
ABS and PVC are the two easiest, but with literally dozens of types of plastics in use, it's near impossible to get the right weld material without a test lab or dumb luck.