Sorption Divergence
Equilibrium moisture content variations measured across identical ambient relative humidity conditions distinguish the desorption path from the absorption path in hydrophilic fibres. Designated across textile physics as moisture regain hysteresis, this divergence means a fibre drying down from a saturated state retains more water than a bone-dry fibre absorbing moisture at the exact same temperature and humidity. The physical effect applies to all porous and moisture-absorbing materials, including cotton, wool, viscose and nylon, but ceases to apply in non-hygroscopic synthetics such as polypropylene.
Capillary Condensation
Molecular sorption theory attributes this path divergence to the structural rearrangement of polar sorption sites and capillary condensation inside internal pores. During absorption, water molecules must overcome internal chain cohesion to break intermolecular hydrogen bonds and open new sorption sites. During desorption, the swollen polymer matrix maintains fully exposed hydroxyl or amide groups, allowing capillary menisci within microvoids to retain liquid water at lower relative pressures.
Ink-bottle pore geometry reinforces moisture regain hysteresis, because narrow pore necks prevent entrapped moisture from evaporating until humidity drops well below the initial filling point.
Commercial Mass
Financial transactions for commercial yarn and fabric shipments rely on standard commercial regain percentages to correct gross invoice weights. If bulk cotton bales or wool tops reach moisture equilibrium through desorption rather than absorption, the actual water mass per container increases significantly while ambient relative humidity remains compliant. Commercial buyers adjust moisture regain hysteresis offsets during trading settlement to avoid paying standard fiber prices for excess water weight.
Conditioning Equilibrium
Quality laboratories condition physical test specimens strictly along the absorption path starting from an initial pre-dried state below ten percent relative humidity. International testing standards mandate this pre-conditioning protocol to guarantee that strength, elongation and yarn count measurements remain fully comparable across testing facilities.
