Moisture in Polymers
The reason you dry certain plastics is to get the moisture out. But why does the moisture have to be taken out before processing? Even though some chemistry is involved, we are going to explain this with the analogy of tiny magnets. Everybody understands magnets. Some plastics pull moisture out of the air. 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. 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. 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. If moisture, also a polar substance, is present, you get two mini-magnets that attract each other. It is this “magnetic attraction” that pulls the moisture out of the air and holds it to the polymer. To get rid of it you have to dry these polymers at sufficient temperature to 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.
Some moderately hygroscopic resins like ABS and acrylic can be processed wet, but the moisture will turn to steam and cause splay. The moisture does not break up the backbone of these polymers.
With other resins like PET, PBT, PC, nylon, and TPU exposure to moisture at processing temperature causes a chemical reaction (hydrolysis), which breaks long chains into shorter fragments, reducing strength and other properties. The “broken” polymer also flows easier, so if the polymer is not uniformly dried, your process must contend with variation in viscosity of the incoming resin at the feed throat. There may be no splay visible at low levels of moisture in the resin. The part will look fine and may even pass tests at room temperature, but will fail prematurely at low or high temperatures
Again, why do you dry hygroscopic resins? To get the moisture out, but more importantly to ensure maximum polymer performance.
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