Cobb Water Absorption

Testing Parameter

Gravimetric measurement requires conditioning test pieces at controlled temperature and relative humidity before clamping samples beneath a weighted metallic ring. Distilled water fills the reservoir for a precise duration, typically sixty seconds for standard packaging boards, before rapid blotting removes excess surface liquid. Analysts calculate absorption values in grams per square meter by weighing specimens immediately before exposure and directly after liquid contact.

Temperature fluctuations during conditioning alter cellulose swelling rates and skew final mass determinations. Operators must perform calibrations regularly to verify balance accuracy and maintain repeatability across production shifts.

Mill Limitation

Fiber morphology and internal sizing chemical concentrations dictate the boundaries where testing methods lose reliability. Highly porous unbleached kraft papers absorb liquids so rapidly that standard contact durations fail to capture initial capillary uptake phases. Surface treatments such as wax coatings or heavy polyethylene extrusions repel moisture completely and push absorption values down to zero, rendering short duration tests ineffective for differentiating barrier grades.

Laboratory technicians apply alternative contact durations when evaluating specialized liquid packaging boards to capture slower penetration kinetics across dense barrier structures. Production specifications balance sizing chemical costs against end use water resistance requirements to prevent unnecessary expense during stock preparation.