There are mainly partial hydrolysis products of polyacrylic acid, acrylic acrylamide copolymer and polyacrylamide. Although there are other carboxyl-containing monomers, they cannot be polymerized to a sufficiently high molecular weight (such as maleic anhydride).
The hydrolysis of polyacrylamide is usually carried out with Na2CO3 at 20 ~ 95°C, the reaction is as follows:
During the reaction, the viscosity of the system increases, and the reaction rate starts quickly, and then gradually slows down. When the degree of hydrolysis is close to 50%, the reaction is difficult to proceed due to the neighboring group effect.
The copolymerization rate of acrylic acid and acrylamide depends on the pH value, and the reactivity ratio of acrylic acid decreases with increasing pH value.
The products obtained by the two methods of hydrolysis and copolymerization show that the distribution of carboxyl groups on the chain is irregular, and the performance in use (such as flocculation) also proves to be indistinguishable.
Both methods can produce high molecular weight products (up to 1000x10*). However, if the proportion of acrylic acid in the copolymerization method is too large, it is difficult to obtain a high relative molecular product. The content of carboxylic acid groups in general products ranges from 2% to 50%; the most used is 30%. It is understood that the content of this carboxyl group makes the macromolecular chains have the greatest stretch in the water, and at the same time the amide groups retained are sufficient for adsorption, so as to achieve the best flocculation effect.
The carboxylic acid group is a weak acid, and its dissociation is affected by the pH value. Generally, the pH is greater than 5 and the dissociation is anionic. It should be neutral to alkaline during use. The specific degree of dissociation is also related to the density (ionicity) of carboxyl groups on the macromolecules. Figure 3-1 shows the relationship between dissociation rate and pH. The solid line in Figure 3-1 is a 30% carboxyl group, the dotted line on the left is a low-level carboxyl group, and the dotted line on the right is a polyacrylic acid.
The 2-acrylamido 2-methyl propane sulfonic acid (AMPS) proposed by Lubrizal Corp.
The amide is copolymerized, and the resulting product has a relatively high molecular weight, which is used for flocculation. Its preparation is as follows:
The use of polymer sulfonation to prepare such flocculants has commercial value. The polyacrylamide sulfomethylation reaction is as follows:
The sulfonic acid polymer is strongly anionic. The sulfonic acid group has a greater ionization degree and stronger acidity than the carboxylic acid group, and can maintain its dissociation at a lower pH value to exert the flocculation effect.
Sulfonic acid type flocculant is not as large in application as carboxylic acid type, and it is common. Mainly used in "red mud" treatment in Bayer process aluminum smelting, pulp waste liquid treatment, clay industry waste liquid treatment, drilling fluid treatment agent, etc.