Hygroscopic resins

Polymer Swelling-type Hygroscopic resin Conductivity change depending 20-90<9 r.h. <1 min. Humidity measurement [46]... 

Dew con- Resin dispersion- Hygroscopic resin + Conductivity change depending 94-100<9 r.h. <10 s Dew detector or VTR [56]... 

PET, nylon, ABS, and PC come under the classification of hygroscopic resins. These types of polymer resin collect moisture inside the peUet itself. Removal of this moisture requires dry air as weU as heat. These resins therefore demand proper design and carefnl machine selection for each application. Desiccant dryers are the dominant technology for these resins. 

IVpical nonhygroscopic resins are PE, PP, polystyrene (PS), and PVC. Fillers such as talc, calcium carbonate, carbon black, and wood flour, when added to nonhygroscopic resins like PE or PP in sufficient quantity, can make the resin behave like hygroscopic resins. 

In order to properly dry hygroscopic resins, the drying system must provide the following parameters ... 

In the past barrier screws were used only with PET pellets because pellets are not susceptible to air entrapment. Powders, however, are susceptible to air entrapment and do not process well on conventional barrier screws. The internally vented barrier screw developed by Eastman achieves higher throughputs of PET powder. This screw design can also be used for other hygroscopic resins like ABS. 

Resin material can affect fiber/resin delamination, one of the prerequisites for conductive anodic filament growth. Meashng occurs at about 260°C for FR-4, but may occur at lower temperatures for boards with more hygroscopic resins. 

In oven dryers the plastic is spread on thin layers Qess than 1/2 in.) in trays and exposed to hot air circulating in a closed system through a dehumidifier. The drying time of hygroscopic material depends on the resin system but is usually 2 - 4 h at the recommended temperature (see Table 4-1). The oven or hopper should be capable of maintaining a temperature of at least 250° 5°F. It may also be necessary to dry non-hygroscopic resins to eliminate surface moisture which has condensed onto the pellet surfaces. If moisture exceeds a permissible amount, it will be converted to steam in the injection cylinder and cause blisters and specks on the surface as well as internal voids. 

Formulations requiring both a dry polymer plus blending with other ingredients can lead to special handling requirements. Once moisture-sensitive resin is dried, it will pick up moisture when exposed to the atmosphere. Additives or other components added to formulations containing hygroscopic resins need to be moisture free. If the additives cannot be dried with the resin, special handling procedures or individual feeders are required to mix the dry resins and other additives or components at the extruder feed throat. 

Figure 15-21. (a) Effect of moisture on hygroscopic resin pellet, (b) Effect of moisture on nonhy-groscopic resin pellet. 

The experiment used acrylic, a hygroscopic resin which was dried for 5 hours prior to molding. After drying, the moisture level was analyzed. No moisture was detected. 

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