I use cartridges because most of my writing is while mobile and I cannot carry a bottle of ink and a converter around. Plus, the pens I most commonly use are too small for a converter. When I insert a new cartridge I always wonder whether I can put the empty into the recycling or if its plastic is not recyclable. I use cartridges from Private Reserve, Kaweko, Diamene, and Pelikan. They all appear to be the same plastic but I don't know which it is. Does anyone know what plastic is used to make the standard international sized cartridges and whether most recycling systems can cope with it?