What Is Wet Strength?

Generally, when a hygroscopic material absorbs a certain amount of water, its physical strength will decrease accordingly. Paper is particularly noticeable in this regard. Taking kraft paper as an example, the strength under normal conditions (20 ° C, relative humidity 65%) is 100 (initial strength). After immersion in water at a certain temperature for a certain time, its residual strength is generally only about o-50% of the initial strength (Depends on soaking time and temperature). The strength measured under high temperature or soaked condition is called wet strength. ^[1]

(I) Wet tensile strength

Tested by standard conditions (temperature, solution and time)

Paper and cardboard are sometimes used under wet conditions. For example, wallpaper should be attached to the wall in wet conditions: fish paper, and fish should be packed when taken out of the water: rice paper, which must be broken when rendered with water and ink: banknote paper, when people do not wash clothes Pay attention to soaking, even rubbing, it is best not to break: or due to unavoidable factors such as the carton wet with rain, it still requires a certain strength so as not to cause serious damage. Here is the concept of wet strength.

The reason why the strength of the paper or paperboard decreases after being soaked with water is that the hydroxyl groups of cellulose on the surface of the fiber are saturated with water, causing hydrogen bonds to break, and the fibers are bonded by water bridges, so the strength of the paper decreases.

Wet strength refers to the tensile strength after immersion in water for a specified period of time. It can also be expressed as the retention of tensile strength after immersion in water for a specified period of time.

The measurement method is as follows:

According to the tensile strength measurement method, 10 pieces of each sample in the longitudinal and transverse directions were cut. If the retention of tensile strength after immersion in water is measured, the measurement of the tensile strength of paper F also requires 10 strips in each of the longitudinal and lateral samples.

The sample to be immersed in water is immersed in water at a temperature of (23 ± 1) ° C. The immersion time is determined according to the requirements of the product standard, which is generally 1h or 2h. If it is a very absorbent paper, just wet the center and keep both ends

Since the wet strength is the strength that a paper sheet can retain after being re-wetted, the factors that determine the strength of the wet strength are mainly the degree to which the paper sheet is prevented from being re-wetted and the hydrogen bonds are broken. Sizing will affect the wet strength of the paper to a certain extent; however, to achieve higher wet strength is mainly the role of wet strength agents. Wet strength agents either form a cross-linked network on the fiber surface to reduce the fiber's water absorption and swelling, or produce an insoluble adhesive effect between the fibers, or a covalent bond between the fibers to obtain the wet strength of the paper. Therefore, the nature and amount of wet strength agent are the main factors that determine the wet strength of paper.

In the early production of wet-strength paper, the fiber was gelatinized with concentrated sulfuric acid, or rayon slime such as cellulose acetate, nitrocellulose, hydroxyethyl cellulose, etc. was added to the fiber to precipitate it on the fiber to improve the resistance. Water performance to obtain wet strength, these methods may cause fiber decomposition due to complicated processes or high temperature and high acidity conditions, the paper is brittle, the humidification effect is poor, and has not been widely used. In the 1930s, it was discovered that the addition of certain water-soluble synthetic resins to paper stock and maturation by a paper machine could impart wet strength to the paper. Since then, the application of wet strength agents has developed rapidly.

Wet strength agents generally have the following basic characteristics: can increase the mechanical strength of paper, protect the fiber-fiber combination, prevent swelling and damage; must be cationic, and can attract and stay with negatively charged fibers Must be water-soluble or dispersible in water to ensure its uniform distribution throughout the paper; Must be able to form a chemical network structure (generally heat-cured) so that the paper can resist water swelling; The raw materials for the production of wet strength agents are relatively cheap and easily available materials, and the production equipment and process are simple.

Wet strength agents used in the paper industry are generally divided into two categories, namely formaldehyde resins (such as urea-formaldehyde resins and melamine-formaldehyde resins) and polyamide polyamine-epichlorohydrin resins. The former is acid-cured thermosetting wet-strength resins. The latter type is alkali-cured thermosetting wet-strength resin. And polyethyleneimine, dialdehyde starch, polyacrylamide with glyoxal substituent and other substances are also used in special cases. '

The use of thermosetting resin wet strength agent can achieve satisfactory humidification strength. The use of thermosetting resin wet strength agent and humidification strength process are divided into three stages: monomer synthesis, condensation and aging. According to the different use conditions, the polymer is condensed under acidic conditions or the wet-strength resin used under acidic conditions is called acid-cured thermosetting wet-strength resin, and the resin condensed or used under medium and alkaline conditions is called alkali-cured thermosetting wet-strength resin. ^[3]