This also explains why once you dry a polymer you generally have to process it within 30 minutes. The magnetic attraction is still strong enough to hold the water to the polymer. This also explains why you can’t dry a resin properly by leaving it in the dryer overnight at a lower temperature. This will drive the two attracting magnets far enough apart that the magnetic attraction is minimized, and the circulating air from the dryer can extract the moisture. To get rid of it you have to dry these polymers at temperatures above the boiling point of water, 212 F. It is this magnetic attraction that pulls the moisture out of the air and holds it to the polymer. If moisture, also a polar substance, is present, you get two mini-magnets that attract each other. Hygroscopic resins contain carbon and hydrogen but, also have a “polar” segment of atoms that behave like a small magnet with north and south poles-i.e., positive and negative charges. Third: Why are some resins hygroscopic in the first place? Polycarbonate, nylon, PBT, PET, ABS, acrylic, urethanes, and many other hygroscopic resins are long-chain polymers, as are polyethylene, polystyrene, etc. So, it is “wet” when you open up the bag, gaylord, hopper truck, or railcar. Bottom line: The resin makers make it dry but it picks up moisture while it is in the packaging. Some resins are so sensitive to moisture that they are packaged and shipped in vacuum-sealed barrier containers. Stick the wrapped onion in the refrigerator and watch, as time passes, how the onion shrivels and dries out. Wrap an onion in the clear (polyethylene) plastic wrap found in your kitchen. Polyethylene by itself is not a true “barrier” resin it is “porous” to moisture vapor (humidity). Why? Because polyethylene-based bags do not block the moisture in the air from getting to the pellets. They are “hygroscopic.” Even if your material supplier dries the pellets and immediately packs them in plastic bags, the resin will still contain moisture when you get them. Second: Some plastics pull moisture out of the air. ![]() Creating a Capable Process Using Process Capability
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |