Controlling Static Electricity Charges in Thermoplastic Pellets

By R.J. Pierce & John Bozzelli

Related links: Thermoplastic Static Electricity

I. Introduction
Some operating conditions may generate levels of static electricity on thermoplastic pellets that can cause significant operating problems. These problems may manifest themselves as separation of lines, irregular product feeding or separation of blends of different materials, such as color concentrates in natural resins. These problems can be present in extrusion, injection molding or blow molding opera­tions, in other words, in any processing operation where granulated resins are handled enough to generate static charges.

There is a need for a surface additive that can be added centrally to reduce the effects of this buildup of static electricity to the point of eliminating these problems.

This paper presents work that demonstrates that polyethyleneglycol of the proper molecular weight can be used on polystyrene, SAN, and ABS products to reduce static without damaging the physical practices of the end product.

II. Discussion
Polyethyleneglycol 400 molecular weight (hereafter 400MW) was selected as a possible anti-static agent because it had demonstrated an ability to reduce static buildup when used for industrial applications. It also had demonstrated an ability to be mixed easily onto the surface of the pellets and not to cause splay during molding.

Samples of ABS resins were shipped to a testing laboratory where a continuous airway loop was available. Color blending equipment was used to make up blends of ABS high gloss and transparent ABS resins with several color concentrates. The samples were then dried in desiccated resin dryers for three hours at 180°F. The samples were then put into the airway loop, one sample at a time, and conveyed continuously for extended periods of time (see Fig.1 - Airway Test Loop).

At the end of the airway loop, the material dropped from a vacuum receiver through a clear plastic surge hopper and then into a fiber pack, where a vacuum wand would again pick up the material and send it back around the airway loop again. It was apparent, after a few minutes of operation, that there was a buildup of static electricity on the product as some resin, and a large quantity of color concentrate, would begin to buildup on the clear plastic surge hopper. 

At this point a measured amount of polyethyleneglycol 400 MW would be slowly added to the system.  The amount was base on 100 parts per million (ppm) of the total resin and color concentrate in the system. There was, in each case, an immediate elimination of the material clinging to the sides of the clear plastic surge hopper. The system would continue to run for several minutes. There was no recurrence of the buildup.

Samples were prepared of resin and color concentrate with and without 100ppm of polyethyleneglycol 400MW, for physical property testing. The actual results of these tests can be seen in Appendix A - Physical Properties Test Results.  There was no significant effect on physical properties.

Other molecular weight polyethyleneglycol products were tried for this purpose.  Even though lower molecular weight products will control static electricity, they have lower boiling points and can cause splay when used in SAN and ABS resins.

B. Polyethyleneglycol 400MW has also been used as an additive in salt-and-pepper blends of natural resins and color concentrate to reduce the separation of lines from the blend with good success.

C. Polyethyleneglycol with a molecular weight of 400MW is required for use with ABS and SAN to avoid surface splay. High molecular weights are significantly more viscous and are, therefore, more difficult to blend onto granules.

Figure 1 - Airway Test Loop

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Appendix A – Physical Properties Test Results

MAGNUM* 9010; 25:1 with 60% TiO2 Color Concentrate

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