what is polyacrylamide

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Polyacrylamide Overview

Polyacrylamide (PAM) is a collective term for homopolymers and copolymers derived from acrylamide (AM). In industrial production, any substance containing more than 1/2 acrylamide monomer units can be called polyacrylamide. Its structural formula is shown in Figure 1-1.

Structural formula of polyacrylamide
Figure 1-1 Structural formula of polyacrylamide

The degree of polymerization, denoted as n, varies widely, ranging from 10² to 10⁵, with corresponding molecular weight distributions also broad, from a few thousand to tens of millions. These are divided into four categories: low molecular weight (up to 1 million), medium molecular weight (1 million to 10 million), high molecular weight (10 million to 15 million), and ultra-high molecular weight (greater than 17 million). The properties and applications of PAM vary greatly depending on the molecular weight range.

Properties of polyacrylamide

Physical properties of polyacrylamide

Polyacrylamide is a linear, water-soluble, high molecular weight polymer system that imparts a certain degree of viscoelasticity to aqueous solutions. Colloids and dry powders are the two main forms used in domestic production. Polyacrylamide is an important polymer with wide-ranging applications across various industries.

The polyacrylamide molecular chain has a normal head-to-tail structure with a predominantly irregular conformation. Dry solid PAM at room temperature is a hard, glassy polymer. The product obtained by cooling and drying is a white, loose, amorphous solid; the product obtained by precipitation and drying from solution is a glassy, semi-transparent solid; and the product obtained by casting and drying on a glass plate is a semi-transparent, brittle film. Commercial polyacrylamide is a powdered product after drying and has good hygroscopicity, with a moisture absorption rate of 30% to 80%. The actual moisture absorption rate is directly proportional to the ionic degree of PAM. Water is the best solvent for polyacrylamide, and its solubility is related to factors such as product form, structure, and molecular weight. Its aqueous solution is a clear, uniform, high-viscosity liquid, and its viscosity varies with temperature, time, shear rate, temperature difference, and pH value. Polyacrylamide is insoluble in most organic solvents, such as benzene, methanol, ethanol, and ketones, but can dissolve in a few organic solvents. Its solubility depends on factors such as molecular conformation, physical appearance, dissolution method, dissolution time, and temperature.

Polyacrylamide aqueous solutions have good tolerance in electrolyte-containing solutions and are insensitive to most salts, such as ammonium chloride and sodium sulfate (strong electrolytes). They are compatible with surfactants and exhibit non-Newtonian fluid characteristics under stress. Polyacrylamide is resistant to fungal attack but not to other microbial attacks. It has flocculating properties and can neutralize suspended substances to achieve flocculation. It also has adhesive properties and can increase viscosity through mechanical and physicochemical effects, achieving better application results. Polyacrylamide has drag-reducing properties, reducing environmental friction losses, and demonstrating extremely high drag reduction effects. It has thickening properties, exhibiting strong thickening characteristics under neutral conditions and in acidic solutions, with the best thickening effects achieved when in a semi-network structure. Therefore, in the petroleum extraction field, the physical properties of PAM are utilized flexibly. The physical properties of solid PAM are shown in Table 2-1.

Table 2-1 Physical properties of solid PAM
Characteristic Indicators
Appearance White powder or translucent
Scent Odorless
Density(23℃)/g/cm³ 1.302
Boundary surface tension/(mN/m) 35~40
Vitrification temperature/℃ 188
Thermal weight loss/°C About 290℃,initial weight loss;about 430℃,weight loss 70%;about 555℃,weight loss 98%
Pyrolysis gas Less than 300℃:NH₃;More than 300℃:H₂、C〇、NH₃
Solvent Water, acetic acid, acrylic acid, dimethylformamide
Non-solvent Hydrocarbons, alcohols, lipids

Although some polymers have shown good performance in laboratory research and demonstrated good oil displacement effects, there is still a need for more comprehensive technical indicators in industrial production to measure the advantages and disadvantages of polyacrylamide, in order to use it better. The technical requirements for powdered oil displacement polyacrylamide are shown in Table 2-2.

Table 2-2 Technical requirements of polyacrylamide for oil repelling in powder form
Indicators Viscosity-averaged molecular weight M/10⁶
9.5≤M<12 12≤M<16 16≤M<19 19≤M≤22
Appearance White powder
Viscosity-average molecular weight/10⁶ ≥9.5 ≥12 ≥16 ≥19
Characteristic viscosity/(dL/g) ≥15 ≥17.5 ≥21.2 ≥23.7
Solid content/% ≥88
Hydrolysis degree (molar fraction)/% 23~27
Filtering factor ≤1.5 -
Screen coefficient ≥15 ≥20 ≥24 ≥28
Water insoluble matter/% ≤0.2
Viscosity(1000mg/L)/(mPa-s) ≥31 ≥40 ≥45 ≥50
Dissolution rate/h ≤2
Residual monomer/% ≤0.05 ≤0.05 ≤0.1 ≤0.1
Particle size/% ≥1.0mm ≤5
≤0.2mm ≤5

Chemical properties of polyacrylamide

The formation of polyacrylamide (PAM) mainly involves the polymerization and copolymerization of acrylamide (AM) monomers under the initiation of a reaction by an initiator. The molecular structure of PAM contains an active side group: the amide group (—CONH₂), which can react with other substances to form various acrylamide polymers. The following are several typical reactions that PAM can undergo.

Hydrolysis reaction

PAM can undergo hydrolysis through its amide side group to form products containing carboxyl groups. We call this partially hydrolyzed polyacrylamide, and the hydrolysis reaction equation is shown in formula .

Due to the effect of neighboring groups, the hydrolysis reaction cannot be complete. Therefore, the actual degree of hydrolysis of partially hydrolyzed polyacrylamide can only reach 70%. Generally, when it is less than 70%, the degree of hydrolysis depends on the alkalinity of the added base and the ambient temperature. Alkaline hydrolysis can result in products with higher molecular weight and viscosity. Of course, the choice of base also affects the result, and the most common bases are Na2CO₃ and NaOH.

Due to the current synthesis technology and industrial production limitations, HPAM remains the primary polymer used for oil recovery. Given its widespread application in oil fields, the production of PAM generally refers to the production of HPAM. Considering its importance, the preparation process includes homopolymerization and post-hydrolysis techniques, as well as copolymerization for hydrolysis degrees above 70%. Currently, post-hydrolysis homopolymerization is the most commonly used technique.

Hydroxymethylation reaction

The hydroxymethylation reaction refers to a rapid reaction that occurs between polyacrylamide and formaldehyde under weakly alkaline conditions at 40-60℃. The reaction equation can be seen in Equation (2-5) to Equation (2-6).

2 5
2 6

In fact, the principle of the reaction is that the oxygen atom of formaldehyde introduces the hydrogen atom from the amide group in the molecular structure of polyacrylamide, thereby forming a new carbon-hydrogen bond. Of course, the reaction can also occur when acid is added under heating conditions, forming a gel with a cross-linked structure.

To prepare hydroxymethylated polyacrylamide, it is generally necessary to adjust the pH value to 10.2, add formic acid, stir for 2 hours at (32±2)℃, and then adjust the pH value to 7.5. The resulting polymer can be added to a drum dryer and heated at 165℃ for 15 minutes. The obtained polymer can be used as a surface adhesive. This type of reaction is often used to plug the caves or cracks in oil well water production.

Sulfomethylation reaction

The sulfo-methylation reaction refers to a chemical reaction in which polyacrylamide reacts with NaHSO₃ and HCHO under alkaline conditions to produce anionic derivatives. Adding NaHSO₃ to hydroxymethylated polyacrylamide solution can also obtain sulfo-methylated polyacrylamide. The reaction equation can be seen in Equation (2-7).

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The reaction rate of sulfo-methylation is related to temperature and pH value. Generally, a pH value in the range of 10 to 13 and a temperature in the range of 50 to 68℃ are preferred. As sulfo-methylation progresses, the concentration of anions on the polyacrylamide molecular chain gradually increases, and the electrostatic force and steric hindrance increase, which slows down the reaction rate. Therefore, sulfo-methylation cannot be carried out completely, and usually only reaches about 50%.

Due to the sulfonic acid group of sulfo-methylated polyacrylamide being insensitive to salt, it is also used as a thickener.

Hoffman degradation reaction

Under alkaline conditions, polyacrylamide reacts with sodium hypochlorite or sodium hypobromite to undergo the Hofmann degradation reaction, yielding acrylic acid-aminoethyl polymer containing aminoethylene structural units. The reaction equation can be seen in Equation (2-8).

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Furthermore, the introduction of amino groups can increase the adsorption capacity of clay and enhance the thermal stability of the slurry, which is widely used in the paper industry.

Sulfoxylation reaction

Under the action of potassium hydroxide solution, polyacrylamide and ethylene oxide can be heated to generate alkoxy polyacrylamide. Sulfonated alkoxy polyacrylamide can be obtained by reacting it with sulfuric acid. This is represented by Equation (2-9).

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Co-polymerization reaction

In principle, the copolymerization of polyacrylamide with other monomers is carried out under certain conditions using special methods, and the copolymer produced is still referred to as polyacrylamide.

Amine methylation reaction

Polyacrylamide reacts with formaldehyde and dimethylamine, and then reacts with chloromethane to produce a copolymer of acrylamide containing anionic side groups. This reaction can accelerate the clarification speed when treating wastewater and is commonly used in wastewater treatment technology.

Polyacrylamide: Relationship between Properties and Performance

Polyacrylamide production can be controlled through the selection of different initiation systems, co-monomers, and polymerization reaction parameters to achieve a product structure with diverse physicochemical properties and application performance. The relationship between the physicochemical properties and application performance of polyacrylamide is shown in Table 3-1.

Table 3-1 Physicochemical properties and application properties of polyacrylamide
Physical and chemical properties Structural factors Application Performance Applications Industrial field
Absorbency Acylamino Dispersion Dispersion aids, surface coating Paper, textile medicine
Acylamino, ionic groups Adherence Increase paper strength, drilling mud, building material adhesion Paper geology, petroleum construction
Acylamino Adherence Soil and water conservation Agriculture
Linear long chain, acylamino ionic groups Flocculation Solids recycling, wastewater treatment, water purification, retention and filtration aids Mining, mineral processing environmental utilities, farming paper, mineral processing
High viscosity Linear long chains, ionic groups Rheology control Reducing resistance, thickening Fire-fighting, chemical, naval resistance reduction three oil recovery
Cross-linkability Cross-linking groups Gel Thickening, dissection Tertiary oil recovery
Ionic groups Gel Increasing the wet strength of paper, fixing soil, and preserving wall surface coating Paper agriculture, forestation, desert transformation, construction
Cross-linking groups, acylamino groups, ionic groups High water absorption Water retention, liquid retention, moisturizing diapers Agricultural plant protection, medical auxiliary materials
Acylamino Biological inertness and biocompatibility Intracorporeal implantation of filled controlled release drugs Medicine

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