Quote:
Originally Posted by Cliff Bowman
High humidity causes the plastic covering to shrink in size but does not affect the cardboard part of the card under the plastic coating, this is what causes the cards to curl up if they’re not slabbed or in a rigid holder, and to crack if they are slabbed or in a rigid holder.
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I agree that the problem is caused by humidity, but it's probably a combination of paper and plastic. I went down a rabbit hole when looking this up on the web. Paper will change when it gets wet. Paper is usually rolled and dried under the pressure of rollers. The dried paper is essentially stretched leaving residual strain in the paper. When the paper gets exposed to higher humidity this allows the paper in the card to become "wet" and then when the paper redries without the stretching force being present the paper shrinks.
On the other hand, plastics are hydroscopic. They take up high amounts of water already in normal climate conditions (50% humidity in air). Direct contact with water results in even more water uptake. Hydrolysis resistance is low and as a result cracks are formed over time when exposed to water.
So it seems the paper causes the card to curl up and then the humidity causes the plastic to crack.