Sorption Boundary
Equilibrium moisture content measured along a descending relative humidity gradient defines the upper boundary of a fibre’s moisture capacity. In textile laboratory practice, this desorption branch plots moisture regain as a saturated or damp specimen gives up water to progressively drier surrounding air at constant temperature. Natural fibres retain a higher percentage of water at every intermediate humidity level along this downward trajectory than they do during progressive wetting.
The curve originates at one hundred percent relative humidity and terminates at total oven-dry mass.
Internal Mechanics
Capillary condensation within the interior voids of the cell wall holds liquid water under reduced vapour pressure, delaying evaporation as ambient relative humidity falls. Hydrogen bonds established between water molecules and structural keratin or cellulose require higher energy inputs to rupture than was necessary during original formation. Water molecules become physically trapped within the narrowing spaces of the collapsing polymer network as the fibre shrinks.
This delay in moisture release causes the elevated regain profile observed along the desorption branch.
Commercial Valuation
Commercial invoice weights derived from samples dried down to standard testing conditions reflect higher mass than lots conditioned upward from a dry state. Mills purchasing bulk wool or raw cotton risk paying for uncredited water weight if test certificates fail to document the moisture path taken prior to final weighing. Contract rules specify that laboratory conditioning must begin from an artificially low moisture level to eliminate the higher regain inherent to the downward moisture sequence.
A half-percent weight inflation caused by descending balance protocols shifts transaction values against buyers.
Processing Impact
Fabric dimensions and internal yarn tensions behave differently depending on the sorption direction experienced during factory finishing. Fabrics entering heat setting along the desorption branch carry higher plasticizing moisture at identical ambient room conditions, altering thermal shrink rates and dye uptake. Dimensional instability occurs when garments cut from downward-conditioned piece goods encounter lower relative humidity during overseas transit.
Technical consistency requires monitoring ambient humidity history across cutting rooms to prevent pattern shrinkage.
