Introduction:

With the development of modern science and technology, the uses of paper is becoming increasingly broad, some of which require the application and processing under the action of water. Such as marime tharl paper, special map paper, currency paper, advertising poster paper, absorbent cotton paper, tablecloth paper, industrial filter paper, photographic base paper, tea filter paper, etc.

It is necessary to endow these kind of paper wet strength performance, that is, adding wet strength agents during the papermaking process or finishing process to give the paper a certain wet strength.

What is a wet strength agent?

Fibers are hydrophilic. After wetting the paper, water immerses between cellulose macromolecules, cutting off the original hydrogen bonds of the paper and connecting them through water bridges, thereby greatly reducing the strength of the paper.

Usually, paper that is completely wetted by water can only retain 4% to 10% of its original dry strength.

At this point, the form of the paper page will be damaged under slight external force.

Generally speaking, a reinforcing agent that can maintain a dry strength of over 15% of the original paper, after fully soaking the paper with water, is called a wet paper reinforcing agent, also known as a wet strength agent.

How does wet strength agent work?

There are currently two mechanisms of wet strength agents: namely, the theory of protection and the theory of enhancement.

According to the theory of protection, after the addition of wet strength agent, the chemical crosslinking between resins will form a network structure, which is wrapped around hemicellulose. This chemical cross-linking will not be hydrolyzed, thus preventing the water absorption and swelling of hemicellulose and alleviating the reduction the reduction of paper strength under wet conditions.

According to the theory of enhancement, the addition of wet strength agent forms a chemical bond with the fiber, and at the same time strengthens the internal hydrogen bond. Thermosetting resins contain highly active functional groups, which form covalent bond with hydroxyl groups in fibers. The covalent bonds won’t be broken by the wetting of the paper.

The above two theories are not contradictory. For many wet strength agents, the two theories often work at the same time.

What are the commonly used enhancers?

At present, the reinforcing agents used in the paper industry are mainly polyamide epichlorohydrin resin (hereinafter referred to as PAE resin), melamine formaldehyde resin (hereinafter referred to as MF resin), and urea formaldehyde resin (hereinafter referred to as UF resin).

Polyamide epichlorohydrin (PAE): PAE resin is thermosetting and can be polymerized into water-soluble by heating. Therefore, the storage temperature should be below 30 ℃. PAE must be stored under acidic conditions to prevent the formation of epoxy groups, and alkali should be added during use.

The epoxy and fiber hydroxyl form a cross-linked structure to produce the required wet strength. When the pH value of the solution is greater than 5.0, gel occurs in the dilute solution. In order to maintain the stability of the resin, PAE resin should be acidified to a pH of 3.5~6.0 at the end of preparation.

PAE resin can choose an anionic reinforcing agent to share with it, which can easily produce a reinforcing effect. The effective anionic resins are APAM and carboxymethyl cellulose or anionic latex.

Urea formaldehyde resin (UF): Urea formaldehyde resin is a resinous substance formed by the polycondensation of CO (NH2) 2 and formaldehyde (CH2O). During the polycondensation process, the molecular weight increases, the viscosity of the reaction medium continuously increases, and the reaction continues. The resin becomes insoluble in water. Therefore, the polycondensation reaction can obtain water-soluble and organic solvent soluble products according to controlled conditions.

Urea formaldehyde and melamine formaldehyde resins are widely used in industry. In addition to imparting strength to paper, they can also increase its wear resistance, folding resistance, elongation, wet paper tear strength, and web shrinkage stability.

Experiments have shown that when using melamine resin in the tank, the shorter the curing time of the resin, the higher the wet strength. The difference in wet strength increases with the increase of resin addition.

When using the resin in the tank, the ratio of amino group to formaldehyde has an influence on the efficiency of wet strength. The higher the content of formaldehyde, the higher the efficiency of wet strength.

Melamine resin (MF): Melamine resin refers to a resinous substance formed by the polycondensation of melamine C3N3 (NH2) 3 and formaldehyde (CH2O).

Its properties are similar to those of urea formaldehyde resin, both of which belong to the amino resin category.

It can react with six formaldehyde molecules to form hexamethylm elamine.

Applying water-soluble melamine resin to the pulp in the beater can improve the wet strength of the paper.

Due to the different charges between melamine resin and pulp fibers, the resin adheres to the surface of the fibers because of electrostatic attraction. Thus, it increases the binding force between the fibers and reduces the deformation of the paper.

In addition, the pulp contains melamine resin, which reduces the swelling of the fibers. This is the basic reason for the reduced deformation of the paper during dry and wet periods. The decrease in swelling deformation means an increase in wet strength.

Polyethyleneimine (PEI): The original method of manufacturing polyethyleneimine monomer is to react ethanolamine with sulfuric acid to generate aminoethylsulfuric acid. Then it will be realized by adding alkali to convert it into ethyleneimine, controlling the polymerization reaction, and using various acidic catalysts. The toxicity of monomer is very strong, but the polymer is a safe substance.

The mechanism of action of polyethyleneimine is similar to that of anionic UF and MF resins, but the resulting wet strength level is less than thermosetting resins.