Understanding polyacrylamide (1)

Polyacrylamide (PAM) is a water-soluble linear polymer polymerized by acrylamide (AM) monomer initiated by free radicals. It has good flocculation and is an indispensable flocculant for sewage treatment! This article introduces the difference and selection of polyacrylamide in detail!

Technical indicators of Polyacrylamide

The technical indicators of polyacrylamide generally include molecular weight, degree of hydrolysis, ionicity, viscosity, residual monomer content, etc., so the quality of PAM can also be judged from these indicators!

 1. Molecular weight

The molecular weight of polyacrylamide is very high, and it has been greatly improved in recent years. The molecular weight of PAM used in the 1970s was generally several million; after the 1980s, the molecular weight of most high-efficiency PAM was more than 15 million, and some reached 20 million. Each such PAM molecule is polymerized by more than 100,000 acrylamide or sodium acrylate molecules (the molecular weight of acrylamide is 71, and the molecular weight of PAM containing 100,000 monomers is 7.1 million). Generally, the high molecular weight PAM has better flocculation performance, the molecular weight of acrylamide is 71, and the molecular weight of PAM containing 100,000 monomers is 7.1 million. The molecular weight of polyacrylamide and its derivatives ranges from hundreds of thousands to more than 10 million. According to the molecular weight, it can be divided into low molecular weight (below 1 million), medium molecular weight (1 million to 10 million), and high molecular weight (10 million to 1500). Million), ultra-molecular weight (above 15 million).

The molecular weight of macromolecular organic matter is not completely uniform even in the same product, and the nominal molecular weight is its average value.

2. Degree of hydrolysis and ionicity

The ionicity of polyacrylamide has a great influence on its use effect, but its appropriate value depends on the type and nature of the material to be processed, and there will be different optimal values under different circumstances. If the ionic strength of the processed material is higher (containing more inorganic substances), the ionic strength of the PAM used should be higher, otherwise it should be lower. Generally, the degree of anion is called the degree of hydrolysis. The ionicity generally refers to cations.

Ionity=n/(m+n)*100% The PAM produced in the early days was polymerized by a monomer of polyacrylamide, which originally did not contain -COONa group. Before use, add NaOH and heat to hydrolyze part of -CONH2 groups to -COONa. The reaction formula is as follows:

-CONH2 + NaOH -→ -COONa + NH3↑ 

Ammonia gas is released during the hydrolysis process. The ratio of hydrolysis of amide groups in PAM is called the degree of hydrolysis of PAM, which is the degree of anion. This kind of PAM is inconvenient to use and has poor performance (heating and hydrolysis will obviously reduce the molecular weight and performance of PAM), and it has been rarely used since the 1980s.

Modern PAM has a variety of products with different anion degrees. Users can select appropriate varieties according to their needs and through actual tests. It does not need to be hydrolyzed and can be used after dissolution. However, due to habit, some people still call the process of dissolving flocculants as hydrolysis. It should be noted that the meaning of hydrolysis is hydrolysis, which is a chemical reaction. The hydrolysis of PAM will release ammonia gas; and the dissolution is only a physical effect and no chemical reaction. The two are essentially different and should not be confused.

3. Residual monomer content

The residual monomer content of polyacrylamide refers to the acrylamide monomer content that is not completely reacted and finally remains in the polyacrylamide product during the polymerization of acrylamide to polyacrylamide. It is an important parameter to measure whether it is suitable for the food industry. Polyacrylamide is non-toxic, but acrylamide has certain toxicity. In the industrial polyacrylamide, it is inevitable that trace amounts of unpolymerized acrylamide monomer remain. Therefore, the residual monomer content in PAM products must be strictly controlled. International regulations stipulate that the residual monomer content in PAM used in the drinking water and food industries shall not exceed 0.05%. This value of famous foreign products is less than 0.03%.

4. Viscosity

polyacrylamide solution is very viscous. The higher the molecular weight, the greater the solution viscosity of PAM. This is because PAM macromolecules are long and thin chains, which have great resistance to movement in solution. The essence of viscosity is to reflect the size of the friction in the solution, also known as the internal friction coefficient. The viscosity of the solutions of various high-molecular organic substances is relatively high, and increases with the increase of the molecular weight. One method to determine the molecular weight of polymer organics is to determine the viscosity of a solution of a certain concentration under certain conditions, and then calculate its molecular weight according to a certain formula, which is called "viscosity average molecular weight".


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