We’re all familiar with how easily paper can soak up liquids. Corrugated “cardboard” boxes are no exception. The porosity of wood fibers, which is what corrugated boxes are made of, allows it to absorb and release moisture, a phenomenon that is known as hygroscopy.

We should do our best to keep boxes relatively dry because humidity can have a huge impact on their performance. And unfortunately, humidity is one factor that cannot always be controlled.

While having a certain amount of moisture in the air (around 6% – 7%) will ensure proper creasing, avoid cracks, maintain good bending and impact resistance, as well as cushioning and shock absorption, higher humidity levels can actually have a negative impact on those characteristics of corrugated board that we value most.

In this table below published in the Handbook of Package Engineering (c.1992), you can see that the greater the humidity the less stacking strength a box has. For example, at 50% relative humidity (RH) a corrugated box is at 80% of its potential stacking strength. In very humid environments, such as a warehouse with an uncontrolled climate in the summertime, stacking strength is greatly reduced. At 85% RH, a box has 50% of its potential stacking strength.

Humidity Factors Table

In a more recent publication, The Fundamentals of Packaging Technology (c.2013), an increase in RH from 40% to 90% “can result in a loss of about 50% of a corrugated container’s stack strength.” Therefore a corrugated shipping case headed for humid conditions will need to be much stronger in order to compensate for this loss.

When a corrugated board is produced, the liners and fluted mediums are held together with a starch adhesive, typically food grade corn starch. In its most basic formula, corn starch adhesive has very little moisture resistance, which would cause the composite layers of corrugated board to separate in high humidity conditions. Knowing this, corrugated board manufacturers add small amounts of resins to the adhesive to give it moisture resistance. The result is known as moisture resistant adhesive (MRA). The degree of moisture resistance varies from manufacturer to manufacturer and also depends on the conditions where the boxes will be used.

At FlexPAC, we have tools and experience to help determine the right corrugated board grade for a given situation. We also collaborate with our supply chain partners to develop the optimal packaging materials for your specific needs. If your boxes are crushing while stacked in your facility, or if you are planning to send product to areas where high humidity is prevalent, please contact us and allow FlexPAC to help you find the right solutions to your problems.

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