Celluloid collars were a common men's fashion
accessory in the last half of the 19th century. Neck ties had come into fashion and they were traditionally worn with high stiff
collars. Collars absorb a lot of sweat and tend to get dirtier than the rest of the shirt ("ring around the collar"). They need a lot of washing where as the rest of the shirt doesn't. So attached collars tend to wear out long before the rest of the shirt.
Now that's all well and good when you can run down to
The Gap and buy a new shirt for $19.99 but in the
19th century not only were there no Gaps (indeed) but a nice
linen shirt was friggen expensive. To keep from having to throw out a perfectly good shirt with a worn collar, removable collars came into fashion. Initially removable collars were linen but tossing out linen collars proved to be expensive. Clothing makers soon developed much cheaper
paper collars. Unfortunately those didn't last long and they weren't very comfortable or flexible. In 1870, manufacturers came out with removable celluloid collars which were cost effective, lasted five times longer than paper collars, and proved to be much more flexible and comfortable than the paper kind.