The Hygroscopic Swelling Effect

Fanny Littmarck | August 19, 2014
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There have been many studies of how specific materials are affected by hygroscopic swelling. Let’s explore what hygroscopic swelling is and the effect it may have on engineering designs.

What Is Hygroscopic Swelling?

In simple terms, when something swells up because it gets wet, it’s experiencing hygroscopic swelling. Put a quarter cup of rice in a pot of water and you end up with half a cup of rice — hygroscopic swelling is at work. A leak upstairs prevents the door downstairs from shutting properly — the door frame underwent hygroscopic swelling. Leave a wad of paper in your pockets and run the laundry through the wash — hygroscopic swelling will make sure you wind up with a thicker (and, in this case, mushy) stack.

Hygroscopic swelling occurs when cooking rice.
“Rice p1160004″. Licensed under Creative Commons Attribution — Share Alike 3.0 via Wikimedia Commons.

Hygroscopic Materials

In other words, some materials are hygroscopic and have a tendency to absorb moisture from their surroundings and then store it. When that happens, the material changes by expanding in volume, becoming sticky, or otherwise altered. The list of hygroscopic materials is long. It includes paper, cotton, wood, nylon, polycarbonate, sugars, and many more. Not all materials are hygroscopic, however. Going back to our everyday examples, if you instead leave a coin in your laundry and run it through the wash, it will not expand in size.

 

Remember this childhood trick? A scrunched up straw wrapper expands when water is added to it, seemingly coming alive as a larva.

Hygroscopic Swelling Effects in Engineering

Let’s suppose you have a device or component made up of certain materials, some hygroscopic and others that are not. If the hygroscopic materials absorb moisture from the surrounding environment, they will swell up, leading to stresses and strains. Eventually, the hygroscopic swelling may cause the entire device or component to break or deform.

One paper analyzes the implications of hygroscopic swelling in Epoxy Molding Compounds (EMCs). EMCs are popular within the semiconductor industry as encapsulation materials to protect other components. The researchers studied sample beams comprised of copper and an EMC as they underwent moisture absorption. At first, the sample beams were warped concavely. As the EMC absorbed moisture, hygroscopic swelling caused the copper/EMC beam to warp in the opposite direction. The researchers then ran further tests to see whether or not the warpage could be reversed. They were unable to reverse all of the warpage. (Note: There’s more to this study. Read the full paper here.)

Another example also involves encapsulated microcircuits. Researchers studied five polymeric mold compounds and found that hygroscopic swelling can cause deformation as significant as that caused by a 90°C thermal expansion. Wow.

Additional Resources

Check out some additional research on hygroscopic swelling implications:

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