Types, Development and Synthesis of Wet Enhancers (1)

The wet strength of paper refers to the strength of a paper sheet in water or in a humid environment. Under normal circumstances, the strength of the paper is only 5%-10% of its strength when it is wetted without additives. After adding the wet enhancer, the paper can maintain 20%-40% of the original dry strength. . Wet-strength agents are functional additives, most of which are synthetic resins that are resistant to hydrolysis. Adding them to pulp before paper is formed not only improves wet strength, but also creates favorable conditions for the operation of high-speed paper machines.

First, the type and development of wet strength agent

Very long ago, people used sulphuric acid to treat the base paper to make it sheep skin, coat it on the paper surface with waterproof paint, plastic film or metal foil, or sizing paper with gelatin or animal glue, and then make glue paper. Contact with bauxite, formaldehyde or glyoxal at high temperatures forms a protective layer. These methods are quite expensive and the production efficiency is also low, and this method itself can only play the role of water resistance and does not substantially increase the wet strength of the paper.

In 1935, people first discovered that urea-formaldehyde resin (abbreviated as UF resin) was used as a surface sizing agent. After heat treatment, the paper could have a certain wet strength. UF resin that was directly added to the slurry was then developed. However, because it has a negative charge, it is not easily combined with a load-bearing fiber. The retention rate is low and the wet strength effect is poor. In 1942, melamine formaldehyde resin (MF resin) was developed. , It can be directly added into the pulp, and the paper can obtain good wet strength after heating and drying. In 1946, the cationic modified UF resin was successfully tested. The resin retention rate and wet strength paper efficiency are similar to those of the MF, but the price is low, which creates the conditions for mass production of wet strength paper. The use of urea-formaldehyde resin and melamine resin as wet strength agents not only simplifies the processing but also achieves wet strengths up to 50%. While improving the wet strength, it can also make certain dry strength indicators of the paper (such as crack length, bursting degree and folding resistance, etc.) and sizing degree correspondingly improve.

At the same time, people have also developed other compounds that can make paper and board have a certain wet strength. Such as phenolic resin, polyethyleneimine (PEI), synthetic latex (styrene butadiene latex, nitrile latex, etc.), dialdehyde starch (referred to as DAS), but the phenolic resin color is very deep, and the need for high temperature (150 °C) Mature, synthetic latexes are expensive and need to be made into latex. Polyethylene rubbers are generally only suitable for the production of unstretched wet strength papers; dialdehyde starches do not change the paper absorbency and wetness. The strength is independent of the drying temperature, but it only gives the paper a temporary wet strength. Therefore, these wet strength agents are rarely used except for certain special purposes.

In 1960, we developed a mid-maturing wet strength polyamide polyamine epichlorohydrin resin (PPE or PAE resin). PPE resins have the advantage that many MF resins and UF resins cannot be designed. It is not only a highly efficient moisturizer that matures under moderate conditions, but also does not lose the softness and absorbency of the paper while improving the wet strength. It is especially suitable for medical hygiene paper. And in the production process, the foam is small, the whiteness of the paper is reduced to yellow, and there is good re-slurry performance.

Researchers have developed wet strength agents such as PPA, CPAM, and PA on the basis of PPE resin. Polyamide polyamine resin (PAA) is a cationic water-soluble polymer, which is mainly used for cardboard packaging, paper packaging, liquid packaging materials such as milk and fruit juice, etc. The effect is better than UF resin and MF resin. Cationic polyacrylamide (CPAM) is an enhancer with a dry and wet reinforcing effect, and can make the paper have a higher whiteness. It is easy to disperse into a slurry after recovery of the broke. The curing time is short and the price is moderate. It is suitable for pH. The range is 4.5-7.5. The main drawback is that the wet strength of the reinforced paper is lost by about 40% after it is wetted, and therefore it is not suitable for papers that require long-lasting wet strength. The American paper mill uses it to produce handkerchiefs, facial tissues and other fine papers. Paper species. Polyamine (PA) wet strength agents are among the lowest cost among the three water soluble polymers. This kind of product has good storage stability, and has both dry and enhanced properties. It has stronger wet endurance than CPAM but not as good as PPE resin. The United States uses it to produce toilet paper and corrugated paper.

It can be seen that the wet strength agent has been developed so far and is mainly composed of two major types of products, namely, acidic curing resins represented by MF resins and UF resins and alkaline mature resins represented by PPE resins. Alkaline curing resin is more advantageous than acidic curing resin and will become the development trend of paper industry agent.