Hello,
It is plastic!, Dacron is used for the sails on racing boats, can also be used on scale or semi scale boats, but to my eyes it does not look right.
I think ordinary Cotton Sheeting, from the local Market looks better, the Sails on my Schooner are made from Cotton.
If you make your sails from Cotton, shrink the cloth FIRST, soak the cloth, hang it out to dry, do this about 3 times, then cut and make the sails, OR your sails will shrink on the model !!!!.
John.