A Japanese research team found that the effect of humidity on cardboard differs depending on how it is dispersed.
Normal wet strength resins are: urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE).
Neutral sizing agents are alkylketene dimers (AKD) and alkenylsuccinic anhydride (ASA).
Sometimes wet strength can be obtained by curtain coating a polymer onto a paper or paperboard.
A wide range of paper materials are wet strengthened: Recycling of wet strengthened paper requires higher intensity reworking conditions in the re-pulping process to break the fibre web.