The action mechanism and research status of papermaking wet strength agent

Wet strength agents have been widely used in the paper industry, and their importance has been paid more and more attention. The mechanism of action of various wet strength agents including urea-formaldehyde resin, melamine-formaldehyde resin and polyamide epichlorohydrin And the research status is reviewed. And briefly introduced the shortcomings of existing wet strength agents, and made a prospect for the development of wet strength agents in China.

 

With the development of modern science and technology, the scope of use of paper is becoming wider and wider, and some of the papers are required to be applied under water or in water. Such as sea drawings, special map paper, banknote paper, advertising stickers, medicated cotton paper, tablecloth paper, industrial filter paper, photographic base paper, tea filter paper. This necessitates imparting a certain wet strength property to such paper, that is, adding a wet strength agent during papermaking or post-processing to impart a certain wet strength to the paper.

 

1. Several concepts related to wet strength

 

1.1 Wet strength

 

When the finished paper is saturated with water, it will lose most of its strength, and the remaining strength is called wet strength. For the wet strength of the paper, it is difficult to define an absolute value, and the wet strength of the paper is usually expressed as a wet 2 dry strength ratio, expressed as a percentage, where the strength generally refers to the tensile strength. The wet strength of the paper is divided into initial wet strength and rewetting strength. The initial wet strength refers to the strength of the finished paper from the paper machine after rewetting with water. Generally speaking, wet strength refers to rewet strength. Generally, non-wet papers can only retain about 2% to 10% of their dry strength after being saturated with water. After adding the wet enhancer, the wet strength of the paper can reach 20%~40% of the dry strength of the base paper. It is now possible to produce paper products with a wet strength of more than 50%.

 

1.2 wet strength agent and wet strength paper

 

Generally, an enhancer which can maintain the strength of the base paper by more than 15% after being sufficiently wetted with water is called a wet strength agent [3]. If the wet strength of the paper is above 15% of its dry strength, the paper is said to be wet strong paper. Wet strength paper is usually distinguished by the duration of wet strength, and non-wet strength paper loses its strength within a few seconds after being saturated with water. Some wet strength papers only shorten the time lost in strength. This type of paper is generally referred to as "temporary" wet strength paper, while some wet strength agents produce "long-lasting" wet strength.

 

2 wet strength production mechanism

 

The strength of the paper depends on the strength of the fiber itself and the strength of the bond between the fibers and the arrangement and distribution of the fibers in the paper, that is, the bond between the fibers in the paper and the fiber itself, the most important of which is the interfiber bond. This binding force is mainly the bonding force of hydrogen bonds. The ability of cellulose fibers to form hydrogen bonds lies in the presence of cellulose hydroxyl groups. The combination of hydrogen bonds between fibers allows the fibers in the paper sheets to be bonded to each other without binder. Give the paper a certain strength. D. Dunlop-Jone believes that in order to retain some of its initial dry strength when the paper is wet, there are generally one or several of the following methods: strengthening the original fiber bond; protecting the existing fiber bond; forming water insensitive New key; creates a network that physically coats the fibers. Humidification enhancer promotes the wet strength mechanism of paper sheets. It is generally believed that there are two kinds of mechanisms: one is called “protection” mechanism, which protects the existing interfiber bonding mechanism. The mechanism is that wet strength agents will produce an interlacing around the fibers. The chain network structure prevents the swelling and water absorption of the fibers to maintain the existing inter-fiber hydrogen bonds. This mechanism is considered to be a concentric cross-chain mechanism. The other is the "enhancement" mechanism, which produces new, water-resistant interfiber bonds and mechanisms that believe that there is a cross-linking structure between the fibers and the wet strength agent. This combination is destroyed by water in other naturally occurring bonds. After that still exists. The new water-resistant bond formed between the fiber and the wet strength agent, such as covalent bond and hydrogen bond, is used to crosslink the fiber. This mechanism is also called "reinforcing new bonds or intertwining" mechanism. . When the wet strength agent is added during the paper making process, the mixed slurry in contact between the wet strength agent and the fiber has both interlaced fibers and added wet strength agent molecules, which are dried to cause chemistry between them. The changes combine into a bond. When the paper sheet is wetted again, due to the physical interlacing effect and the hardening and non-swelling hardening effect after the wet strength agent is dried, the wet strength agent is fixed between the fibers to prevent water molecules from penetrating into the pores of the fibers. Avoid fiber swells due to water swell and break the fiber bond, resulting in wet strength.

 

3 Wet strength agents commonly used in the paper industry

 

The production method of early wet strength paper is mainly to add water-repellent agent to the pulp, so that the paper sheet has a water-resistant protective layer to adapt to its use, such as adding paraffin, polyvinylidene chloride and the like. It does not substantially increase the wet strength of the paper. Later, in the 1930s, it was found that some water-soluble synthetic resins were added to the papermaking pulp and cured on a paper machine to impart wet strength to the paper. Since then, the application of wet strength agents has developed rapidly. Wet strength agents generally have the following four characteristics [3]: they must be high polymers and have a certain mechanical strength to protect the fibers from swelling and damage; they must be cationic and can be adsorbed in negatively charged On the fiber, and to achieve rapid and complete retention; must be water-soluble or water-dispersive to ensure uniform distribution in the slurry; must be able to form a chemical network structure, reacting to a thermosetting type, making paper to water Swelling has a certain resistance. The selection of wet strength agents in the paper industry should be based on the end use of the paper, the price and characteristics of the wet strength agent.