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Categories: Book printing >> Support Center
Wet-Strength Papers in Packaging Printing and Book Printing
Origin:Image Printing  Data: 3/3/2011  Keywords: Wet-Strength Papers printing

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.

Wet strength of paper is a measure of how well the fibreweb is holding together upon a force of rupture. Wet strength is routinely expressed as the ratio of wet to dry tensile force at break. The cellulose fibreweb of paper is mainly held together by hydrogen bonds. These are dependent on physical contact between the fibres and can be broken by wetting of the fibres. The residual strength of a wetted paper is less than 10%. Various techniques, such as refining of the pulp and wet pressing on the paper machine can be used to mechanically somewhat reduce the strength loss of the paper upon wetting.

 To improve the wet strength it is common to use chemicals. The use of chemicals can retain as much as 10 - 30 % of the original dry strength of the paper. The wet strength chemicals improves the dry strength as well. Wet strength chemicals improve the tensile properties of the paper both in wet and dry state by crosslinking the cellulose fibres with covalent bonds that do not break upon wetting. Different wet strength chemicals are in use, like wet strength resins or neutral sizing agents. Normal wet strength resins are: urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PEA). Neutral sizing agents are alkyl ketene dimer (AKD) and alkyl succinic anhydride (ASA).

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.

 
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As a professional book printing factory in China, Image Printing Packaging Ltd focus on book printing and packaging printing mainly for oversea and domestic customers. We are located in Shenzhen city, the book printing and packaging printing industrial center of China and even worldwide. Please be kindly note we offer a wide range of professional customed printing service with aggressive prices
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