In packaging printing and book printing industry, there are many types of papers with the character of high strength, but few one is still workable after being wet. In this chapter, we will introduce one type briefly to create general ideas about it.
Most of paper?¡¥s strength is the result of hydrogen bonding between hydroxyl and carboxyl groups on adjacent fibers. The removal of water during paper-making process generates there bonds, and process is reversible.
A wet-strength paper is a paper that remains strong when fully saturated with water. It is easy to define this in principle, but considerably more difficult in practice. We must first of all distinguish between wet-strength papers, waterproof papers and moisture or water-vapour-resistant papers. A water-vapour-resistant paper must contain a barrier, usually a plastic or metal film, to resist the penetration of water vapour. A waterproof paper is one that sheds liquid water easily. On the other hand, wet-strength papers will often be saturated with water, and under these conditions they must still retain strength. By definition, wet-strength paper should retain at least 30% of its dry strength when these two strengths are measured by means of the bursting strength test.
Paper that has been parchmentized is actually stronger wet than dry, principally because of the loss of individual fiber identity. However several more economic alternatives exits for generating wet-strength papers. In general, the chemicals used to augment the natural hydrogen binding are cross-liked during the manufacturing process. Chemical for producing wet-strength papers include urea, melamine, resorcinol and other phenolic or amino resins cross-linked with formaldehyde and condensation products of polyalkylene polyamines with dicarboxylic acids cross-linked with epichlorohydrin.
The condensation of these chemicals takes place in the drying section on the paper machine, and the reaction product (being highly water-insoluble) produces an effect on the paper sheet to increase its wet strength. Wet-strength papers are particularly useful for outside packaging as they are stable in adverse atmospheric conditions. They can remain sufficient wet strength for a long time under wet condition and are also suitable for the packaging printing of gods which themselves are wet. They are again frequently used for paper sacks. Paper Bag Printing