The moisture sensitivity of compression and tensile strength is compared for a range of packaging papers. It is shown that compression strength falls off more rapidly with increasing moisture content than tensile strength. This is especially true in the range up to 10% moisture content, where there is little effect of tensile strength. Results were obtained using the STFI short span compression test and tensile test carried out in silicone oil. Also included are Concora Medium Tests (CMT) for fluting. Concora Liner Tests (CLT) for liner and Ring Crush Tests (RCT) for compression.
This difference in moisture sensitivity is also very evident for papers which have been given different wet strengthening treatments. For example, after 60 min. of water immersion, such wet strength papers can retain a wet tensile strength which amounts to 30 – 40% of the 50% RH value. The corresponding wet compression strength retention is only 15% to 25%. It is also shown that the tensile stiffness is more moisture sensitive than the tensile strength.
The results are discussed with reference to the glass transition that cellulose and hemicelluloses at 20 0C pass through at a given moisture content corresponding to about 10% moisture content for a kraft paper. This transition particularly affects the moduli of the paper, while for tensile strength thermal softening apparently also has some positive effect, by reducing stress concentration.