What Caused the Viscosity Variations?

What Caused the Viscosity Variations?
Over time liquid encapsulant will thicken. On one occasion the viscosity that was too low. What was causing the viscosity too drop?
Mysteries of Science


Liquid encapsulate is designed to protect tiny electronic components often mounted onto small plug-in boards for computers. Once the liquid is applied it will harden by thermal polymerization.

Too thin and it will flow into areas causing connection problems. Too thick and it will not flow enough trapping air bubbles. Over time the material will thicken even without heat slowly curing at room temperature.

In earlier days the material was often shipped in dry ice that would last two or three days and stored in a freezer to extend shelf life. Because of the cooler storage conditions the viscosity of the encapsulates would normally increase, yet on one occasion the supplier was shocked when one customer reported a viscosity that was too low.

The temperature was ruled out as a factor and the viscometer was calibrated using standard materials. What was causing the material's viscosity to drop?

Here's the rest of the story.

The lab took a new sample from the freezer and measured the viscosity after each freeze-thaw cycle. Sure enough, the first cycle dropped the viscosity by 10%, the next caused a 7 or 8% drop, and the third one by only a percent or two. Whatever the mechanism, freeze-thaw cycles reduced viscosity.

Since the encapsulant is highly filled with powdered materials, perhaps the freezing cycle produced better wetting of the filler, or microscopic air bubbles were forced out, or some combination. But the mechanism still remains unknown.

The manufacturing process was changed to include several freeze-thaw cycles before shipment to bring the product to a higher level of stability. But the freeze-thaw trick was also used on other products where lower viscosity was a benefit.


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