Page 255 - Engineering Plastics Handbook

Polyphenylene Ether (PPE) Blends and Alloys 217

using conditions for polyamides will result in increased molded-in stress.
PPE/PA resins can be molded in most standard injection molding
machines. Reciprocating screw machines are suggested.
PPE/PA resins, like other engineered thermoplastics, should not be left
at elevated temperatures for prolonged periods without occasional purging.
Key molding parameters of PPE/PA resins include the following.

Drying. Like polyamides, PPE/PA alloys will absorb moisture but at
much lower levels. However, moisture still plays an important role in
material processing, part appearance, and part properties. Typically
PPE/PA resins have 0.1% moisture content when manufactured and
can increase to approximately 0.2% at the molder.
As a general guide, very good appearance in large parts can be obtained
with a moisture content ranging from 0.02 to 0.07%. Occasional issues can
occur at 0.08 to 0.10% moisture content. Splay and related appearance
issues are common in large parts at 0.10% and greater moisture. Small
parts or simple geometries typically exhibit little or no splay, even at mois-
ture levels of 0.10 to 0.12%.
Reducing moisture content below 0.02 to 0.07% can result in stiffer flow.
The residual moisture present in dried pellets will plasticize the molten
polyamide during melt processing. Too low a moisture content can also lead
to further polyamide polymerization during processing, and higher
polyamide molecular weight could contribute to stiffer flow.
There is virtually no benefit in reducing moisture below 0.02%. Drying
to below 0.02% generally does not improve appearance and could produce
material with stiffer flow. This can cause inconsistent processing.
Modern desiccant, dehumidifying dryers offer excellent drying capabil-
ities with many advanced features to assist the molder in obtaining a con-
sistent drying process. However, measuring the moisture content will
ensure the resin is in the 0.02 to 0.07% range. In addition the moisture
content should be measured periodically to ensure that it has not drifted
out of the 0.02 to 0.07% range.
Once moisture has equilibrated into the pellet, it typically hydrogen-
bonds to the amide groups on the polyamide. Removing the hydrogen-
bonded water molecule requires energy (time and temperature). High levels
of moisture generally require long times in dryers. Hence efforts need to
be made to keep the resin dry and minimize chances for moisture uptake.
Material handling procedures can affect the consistency of the starting
moisture content of the resin. Although the special barrier liners used in
PPE/PA resin bags and boxes greatly reduce the diffusion of water vapor
into the resin, they do not eliminate it. Working from box to box or bag to
bag will lead to a greater probability of varying initial moisture content
than working from a bulk truck or a railcar where the materials are gen-
erally blended through bulk handling systems.