What causes the static build up on balloons?

Question

Just being curious

Answer 1

And in case you mean toy balloons rather than hot air balloons, rubber is one of those materials where it's fairly easy to strip off electrons by mechanical rubbing motion. Being an insulator otherwise, the balloon won't quickly redistribute the remaining charges, so you end up with a charged spot. If the air is too dry, charges won't be able to flow out of the air and onto the balloon, so it'll stay charged for longer.

Answer 2

Air 'rubbing' in a form of a wind against the surface of the balloon. That makes the charge accumulates with and electrically ungrounded balloon the charge has no place to go but to buildup.