According to the Casio Timepiece Repair Guide:
"All of the plastic Casio bands are made of polyurethane. Polyurethane, like most plastics, is pretty dense and fairly waterproof --- but no plastic is acid proof. The same acids and proteins in body sweat which corrodes jewelry and leather will (in time) take out all of the moisture in the polyurethane plastic; leaving it shiny and brittle. Rough handling of the brittle plastic can break it easily. This is normal. Periodic rinsing of the band with warm water will help remove bacteria & fungus without significantly shortening the band's life. Popular replacement watch bands are often made of polyvinyl plastic, which is very soft (and smells like your swimming pool)."
The replacement bands Casio mentions at the end of this answer refer to the bands found in stores NOT the replacement bands they sell through authorized ......s. It is also worth keeping in mind that polyurethane can take a large variety of forms and not all bands are created equal; you will find that some models have bands which last a lot longer than others. And remember that conditions vary from location to location and person to person so while one enthusiast may find that his DW-6900 band lasts for ten years another may have to buy a new band in three years (for example). The only sure thing is that resin bands WILL break and have to be replaced at some point.