Sarah,
 
The answer comes from the chemistry of the "plastic" items. Although  
celluloid was available for a long time, the expense may have been too great for  
something as disposable as straws. I suspect that post war plastic development  
by chemical companies created the plastic straw. Quite some time ago, I learned 
 that you can have a plastic object dissolved by a chemist who can then 
recreated  the formula for that item. With the formula, you can then research 
patent dates  for the item. Plastic changed almost overnight and each forumula has 
its own  patent.
 
Ron May
Legacy 106, Inc.