wetstrength

Wet strength refers to the ability of a material to maintain mechanical integrity when exposed to moisture. In papermaking and related fiber products, it specifically describes how well paper retains tensile strength and toughness after immersion in water or in humid conditions.

In papermaking, wet-strength properties are achieved by adding wet-strength agents that chemically crosslink the cellulose fibers or otherwise stabilize the sheet. The most widely used classes include condensation-type resins such as urea–formaldehyde, melamine–formaldehyde, and glyoxal-based systems, as well as polyamide-epichlorohydrin (PAE) resins. These agents form covalent bonds between fibers, reducing swelling and maintaining structure when wet, thereby increasing durability in moist environments.

Wet strength can be permanent or temporary. Permanent wet strength remains after drying and rewetting, while temporary or less durable wet strength may diminish with time or following hydrolytic exposure. Testing typically involves measuring tensile strength after specified immersion times and expressing the result as wet-strength retention or a wet tensile index relative to the dry value.

Applications include tissues, paper towels, packaging boards, and labels that must withstand moisture or handling after wetting. Wet-strength chemistries are also used in some textiles and adhesives to improve performance under damp conditions.

Safety and environmental considerations are important, as older formulations can release residual formaldehyde. Regulations in many regions limit such emissions and exposure. Modern wet-strength chemistries strive to reduce hazardous components and improve recyclability and environmental compatibility.