Consolidation Force
Surface tension forces in a wet fibrous web generate a powerful contracting pressure during the removal of water. As moisture evaporates, capillary pressure compaction brings individual fibers into closer proximity, leading to a denser sheet or fabric structure. This action occurs primarily in the final stages of drying when the liquid menisci between adjacent filaments curve, creating a negative pressure that draws the solid boundaries together.
Pore Reduction
Liquid bridges between adjacent fibers generate tension as they dry, reducing the total volume of the pore spaces. This capillary pressure compaction alters the permeability of nonwoven mats by permanently reducing the void ratio.
Critical Moisture
Fiber stiffness determines the threshold where this phenomenon ceases to compress the assembly. Below a certain moisture level, capillary pressure compaction stops because the remaining water does not form continuous bridges between the solid surfaces. If the fibers are rigid, such as coarse polyester, the material resists the compacting action, whereas flexible cotton or fine viscose fibers undergo substantial rearrangement.
Thickness Change
Thickness measurements of wet-laid webs demonstrate the permanent densification caused by the drying step. During industrial drying, capillary pressure compaction often reduces the loft of high-loft nonwovens, requiring carefully controlled temperatures to balance fiber bonding and bulk. If the drying rate is too high, the capillary pressure peaks rapidly, locking the fibers into a highly compressed state that reduces both insulating capacity and air permeability.
