Many studies have demonstrated that charge density is an important factor affecting the behavior of cationic retention aids. In the presence of anionic solutes or colloids, the flocculation efficiency increases with the increase of charge density of cationic polyacrylamide. Additional studies have shown that anionic solutes or gels obtained from thermomechanical slurries have less functional interference with cationic polymers with a wide range of density variations.
Many studies have shown that the flocculation efficiency of cationic polyacrylamide increases with the increase of molecular weight. High molecular weight is expected to induce more efficient bridging capacity. It is speculated that this effect is due to the creation of more extended polymer chains and rings and the reduced mobility of larger polymers on the cellulose surface and at fiber burial sites. And some studies have found that the flocculation effect of cationic retention aids with large molecular weight is not obvious in the presence of anionic dissolved substances or colloidal substances.
The principle of adding time of cationic polyacrylamide: when the retention aid is added to the paper machine at the same time as the industrial production, the maximum retention efficiency can be achieved. The optimal action time of the polymer retention aid and the solid filler surface in the suspension is only a few seconds, especially under the action of the powerful hydraulic shear force of the modern new paper machine, the molecular mass will continue to decrease, and the larger molecules will redistribute into a flatter conformation. For different grades of paper, it is very important to ensure the initial efficiency of the additives and the uniformity of the finished paper. For a stable retention effect, it is also particularly important to minimize the amount of retention aid used, so it is also meaningful to use a screen to add retention aid.
Some researchers have tried adding retention aids in reverse order. Relevant studies have found that adding in the opposite order can improve the retention of latex particles and also contribute to the retention of fillers. The retention effect will be maximized when the time interval between the addition of the retention aid and the addition of the filler is the shortest. Even though many studies have warranted the addition method in the reverse order, when applied to papermaking, such benefits are not sufficient to overcome the problems in practical situations. Because with the passage of time of action in the stirring suspension with the fiber, the retention aid will reduce its effective performance. Each time the retention aid is added ahead of the other additives, it increases the time it is subjected to hydraulic shear in solution. If the main additive is added to the papermaking system late, it will cause uneven distribution of the finished paper in the width direction. Relevant studies have also concluded that the traditional method of adding retention aids has great relative advantages.
Generally speaking, the dosage of cationic polyacrylamide is between 0.01% and 1.00%. If the dosage is too much, the increase of fine fiber filler in the paper will lead to the decrease of paper strength. However, when cationic polyacrylamide is used alone as a retention aid, it is easy to have the disadvantages of poor uniformity and strength. So people consider binary retention aid, drainage aid system and cationic polyacrylamide particle system to enhance the effect. In the anionic and cationic polyacrylamide binary retention system, the cationic polyacrylamide of low molecular weight and high charge density is used in combination with the anionic polymer of high molecular weight and low charge density. The mechanism of this system is to use polymers to entangle the fine components with each other to generate large fiber flocs, increase the porosity of the paper sheet, allow water and air to pass through the paper web more freely, and promote the water to be removed from the flocs. However, there are some disadvantages in the system, and the water inside the floc cannot be removed.
The performance tests and indicators of cationic polyacrylamide include intrinsic viscosity, cationic degree, infrared spectrum analysis and product flocculation, which are related to the relative molecular mass of the product, the amount of initiator and the concentration of monomers.