Hygroscopic plastics Drying

These absorb moisture, which then has to be carefully removed before the plastics can be fabricated into acceptable products (2,3). Low concentrations, as specified by the plastic supplier, can be achieved through efficient drying systems and properly handling the dried plastic prior to and during molding, extrusion, etc. (Figs. 7-24 and 7-25). When desired processor can have these hygroscopic plastics properly dried and shipped in sealed containers. 

Drying hygroscopic plastics should not be taken casually. The simple tray dryers or mechanical convection hot-air dryers that are... 

PA 66 and 6 highly hygroscopic. Too dry an atmosphere m es them brittle but an excess of moisture causes swelling, plasticization and a reversible decrease of the mechanical and insulating properties. Saturation swelling can reach 10%. 

The drying operation for non-hygroscopic plastics is different. They collect moisture only on the surface. Drying this surface moisture can... 

The non-hygroscopic plastics collect moisture only on the surface. Drying this surface moisture can be accomplished by simply passing warm air over the material. Moisture leaves the plastic in favor of the warm air resulting in dry air. The amount of water is limited or processing can be destructive. 

Moisture retention in and on plastics has always been a problem for all processors. Surface moisture or moisture absorbed within the plastic can cause splay, an unsightly surface defect of the molded part, and reduce mechanical properties. The increased use of hygroscopic plastics (see Chapter 9) also requires care and the assurance of proper drying of material via the usual technique, using dryers and/or vented barrels (Fig. 2-11). There are advantages of using vented barrels as opposed to the more familiar dryers (100) ... 

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