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Monday, November 30, 2015

Effects of Moisture Content on Mechanical Properties

The moisture content of a sheet of paper is dependent on the relative humidity (RH) of its surrounding atmosphere because paper is hygroscopic (increase in RH causes and an increase in moisture content & decrease in RH causes a decrease in moisture content).  Besides swelling, the uptake of water molecules produces an increase in fiber flexibility, which is important for many mechanical properties of paper. Moisture pickup due to water vapor in the air has a weakening effect on fiber-to-fiber bonds and influences mechanical properties in this way.

Tensile Strength
Tensile strength increases slightly to a maximum at 30-35% RH and then decreases quite rapidly at higher relative humidity. This decrease has been attributed to a weakening of fiber-to-fiber bonding.

Tearing Resistance
Tearing resistance increases over the entire range of RH, however, at some point above 80% the curve will fall off rapidly due to the disruption of interfiber bonding.

Stiffness
Stiffness continuously decreases with increasing moisture content primarily as a result of increased fiber flexibility.

Bursting Strength
Bursting strength is thought to be primarily a function of tensile strength and stretch. From 20-35% RH, both tensile and stretch increase, producing an increasing burst; from 35-55% stretch continues to increase and tensile falls off at an increasing rate, but stretch continues to rise although at a decreasing rate. Finally, above 55% RH, the decrease in tensile strength is greater than the increase in stretch, and bursting strength decreases continuously.

Folding Endurance
Folding endurance is strongly influenced by moisture content, increasing noticeably up to 65-75% RH and then falling off beyond that. Folding endurance is a complex function of various factors and this response is not easily analyzed.

Considering the variation in these properties due to variations in moisture, it is important to understand what conditions measurements are made under.  If a sample is quickly moved from the machine to the lab for testing, the additional moisture from the machine can have a significant effect on the results. Conversely, paper will be most brittle when moisture-free. Using standard laboratory conditions and conditioning procedures will result in more consistent data that can be compared and used to make reasonable adjustments to achieve the desired results.

Reference: Scott, William & Stanley Tosset, "Properties of Paper: An Introduction", p. 91-93, 1989.