Types of polyacrylamide

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The most important structural parameter of polyacrylamide is molecular weight, which can be divided into four categories according to the size of molecular weight: low molecular weight (<5 million), medium molecular weight (10 million ~ 15 million), high molecular weight (15 million ~ 22 million), and ultra-high molecular weight (>25 million). Polyacrylamides with different molecular weights have different applications: low molecular weight polyacrylamide is mainly used as a dispersant; medium molecular weight polyacrylamide is generally used as a paper strengthener; high molecular weight polyacrylamide has the widest range of applications, mainly used as a flocculant; ultra-high molecular weight polyacrylamide is mainly used in tertiary oil recovery in oilfields.

Another important structural parameter of polyacrylamide is its ionic nature. The ionic nature and degree of ionization of PAM are important structural factors affecting its performance and application. The introduction of ionic groups has many functions: improving the water solubility and dissolution rate of the polymer; increasing the viscosity of its aqueous solution; carrying different charges on the molecular chain to adsorb dispersed particles through electrostatic adsorption, playing a role in flocculation; introducing special groups to make the entire polymer have special properties, such as introducing sulfonic acid groups to improve the polymer’s resistance to calcium and magnesium ions.

PAM is divided into ionic and non-ionic (NPAM) types according to its ionization in water. Ionic types can be further divided into anionic (APAM), cationic (CPAM), and amphoteric PAM (AmPAM). At the same time, different ionic types of polyacrylamide can be further divided into more categories according to the different mole mass fractions of the ionic structural units in the total structural units of polyacrylamide (i.e., different degrees of ionization). Polyacrylamides with different ionization properties have different adsorption properties and performance, and their applications are more refined and specialized.

Non-ionic polyacrylamide

Non-ionic polyacrylamide does not carry ionizable anionic or cationic groups on its molecular chain. It does not ionize in water, but the amide groups on its molecular chain are easily hydrolyzed, and its aqueous solution will exhibit anionic electropositivity. It is generally prepared by homopolymerization of acrylamide and exhibits electrical neutrality, making the solution properties and adsorption performance of the polymer more tolerant to the salt concentration and pH value of the treatment environment and suitable for various wastewater treatment environments. The reaction formula is:

Non-ionic polyacrylamide
Non-ionic polyacrylamide reaction formula

Anionic polyacrylamide

Anionic polyacrylamide is mainly composed of carboxylic acid or sulfonic acid-based structural units and acrylamide copolymers. The anionic groups on their own chain structure have a very strong electrostatic repulsion, which makes the chains highly extended, resulting in a large hydrodynamic volume and therefore excellent flocculation ability and thickening properties. Anionic polyacrylamide is mainly used in low salt solutions and alkaline media. 

Anionic polyacrylamide (APAM) has many negatively charged groups on its macromolecular chain in addition to the amide groups introduced by acrylamide, which can ionize into poly anions and small cations in water. It is generally prepared by copolymerization of acrylamide and anionic monomer solutions or by hydrolysis of homopolymerized acrylamide. Common examples include copolymers of AM with anionic monomers such as sodium acrylate, maleic anhydride, and styrene sulfonate. The pH of the polymerization system affects the competitive polymerization rate of AM and anionic monomers and the structure of the polymer. As the pH value increases, the competitive polymerization rate of AM increases, and the polymerization rate decreases. When the pH is 5, a random copolymer is obtained. 

By adjusting the ratio of AM to acrylate and other monomers and reaction conditions in the reaction system, copolymers with different structures and molecular weights can be prepared for use as flocculants, paper additives, oil displacement agents in oil extraction, and scale inhibitors. The chelating action of the acrylic acid unit with polyvalent metal ions imparts a gel dispersion effect to the copolymer. The pH of the reaction system directly affects the copolymerization reactivity of acrylic acid monomers and AM monomers. 

APAM is generally prepared by copolymerizing AM with anionic monomer sodium acrylate in aqueous solution, and its reaction formula is:

Anionic polyacrylamide
Anionic polyacrylamide reaction formula

Cationic polyacrylamide

Cationic polyacrylamide (CPAM) is prepared by copolymerizing AM and cationic monomers, so that the molecular chain carries ionizable cationic groups. The positive charge groups carried by cationic polyacrylamide are provided by cationic monomers, and common quaternary ammonium salt cationic monomers used in industrial production include acryloxyethyl trimethylammonium chloride (DAC) and methacryloxyethyl trimethylammonium chloride (DMC). Tertiary amine monomers include acrylic acid-N, N-dimethylaminoethyl ester (DMAEA) and N, N-dimethylaminopropyl acrylamide (DMAPA).

In the 1980s, the Beijing Institute of Chemical Research prepared CPAM by copolymerizing AM and DMC. The mixed monomers accounted for 20% of the total system, with a mass ratio of AM to DMC of 3:1, and a composite initiator system was used for aqueous solution copolymerization at temperatures between 30 and 70°C. The introduction of cationic charges enhances the adsorption performance of polyacrylamide on negatively charged colloids, polymers, and solid surfaces, thereby increasing the effectiveness of polyacrylamide in adsorption, bonding, turbidity removal, decolorization, and flocculation. It is particularly suitable for the dewatering treatment of municipal sewage, sludge, papermaking sludge, and other industrial wastewater sludges.

The CPAM is prepared by copolymerizing AM and quaternary ammonium monomer acryloxyethyl trimethylammonium chloride (DAC) in aqueous solution. The reaction formula is:

Cationic polyacrylamide
Cationic polyacrylamide reaction formula

Amphoteric polyacrylamide

Amphoteric polyacrylamide (AMPAM) can ionize into poly anions and poly cations in water, with its molecular chain carrying both ionizable positive and negative charge groups. Preparation methods are as follows:

 ① Modify the amide groups on the molecular chain of the product obtained by copolymerizing acrylamide and anionic monomers, or use the product obtained by hydrolyzing the homopolymerized acrylamide polymer as the raw material. 

② Synthesize amphoteric polyacrylamide by graft copolymerization of natural polymers.

 ③ Copolymerize two or more vinyl monomers carrying anionic and cationic groups.

Industrially, it is common to copolymerize anionic, cationic monomers, and acrylamide to prepare amphoteric polyacrylamide. The reaction formula is:

Amphoteric polyacrylamide
Amphoteric polyacrylamide reaction formula

Amphoteric polyacrylamide has both positive and negative ionic charges on its molecular chain, combining the characteristics of both anionic and cationic polyacrylamides. It has a more pronounced anti-polyelectrolyte effect, a wider pH adaptability range, and lower requirements for the wastewater treatment environment.

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