What is polyacrylamide?

Polyacrylamide (PAM) is a linear polymer with the chemical formula (C3H5NO)n.

It is a hard, glassy solid at room temperature, with products such as gum liquid, latex and white powder, translucent beads and flakes.

Good thermal stability. It can be dissolved in water in any proportion, and the aqueous solution is a uniform and transparent liquid.

The viscosity of the solution will decrease after long-term storage due to the slow degradation of the polymer, especially when the storage and transportation conditions are poor.

Polyacrylamide is widely used as lubricant, suspending agent, clay stabilizer, oil repellent, water loss reduction agent and thickening agent in drilling, acidizing, fracturing, water plugging, cementing and secondary and tertiary oil recovery, and is an extremely important oilfield chemical.

1. Introduction of polyacrylamide

Polyacrylamide (PAM) is a generic term for polymers obtained by homopolymerization of acrylamide or copolymerization with other monomers, and is one of the most widely used species of water-soluble polymers.

As polyacrylamide structural unit contains amide group, easy to form hydrogen bond, so it has good water solubility and high chemical activity, easy to graft or cross-link to get branched chain or mesh structure of a variety of modifiers,

in oil extraction, water treatment, textile, paper, mineral processing, medicine, agriculture and other industries have a wide range of applications, “a hundred industry auxiliaries “known as.

The main application areas abroad are water treatment, papermaking, mining, metallurgy, etc.;

the largest domestic use is in the field of oil extraction, the fastest growing use is in the field of water treatment and papermaking.

2.Polyacrylamide physical and chemical properties

Polyacrylamide is a water-soluble linear polymer made from acrylamide (AM) monomer by free radical-initiated polymerization, which has good flocculation properties and can reduce the frictional resistance between liquids,

and can be divided into four types: non-ionic, anionic, cationic and amphoteric according to ionic properties.

Polyacrylamide (PAM) is insoluble in most organic solvents, such as methanol, ethanol, acetone, ether, aliphatic and aromatic hydrocarbons,

with the exception of a few polar organic solvents, such as acetic acid, acrylic acid, chloroacetic acid, ethylene glycol, glycerol, molten urea and formamide.

However, these organic solvents have limited solubility and often need to be heated, otherwise they have little application value.

It can be dissolved in water in any proportion, and the aqueous solution is a homogeneous and transparent liquid. The size of molecular weight has little effect on solubility,

but when the concentration of the solution is higher than 10%, for high molecular weight polymers can show a gel-like structure due to the bonding of intermolecular hydrogen atoms.

High molecular weight solutions are pseudoplastic fluids

At a suitable low concentration, polyacrylamide solution can be regarded as a net-like structure, with mechanical inter-chain entanglement and hydrogen bonding together forming net-like nodes; at higher concentrations, the solution contains many chain-a-link contacts, making the PAM solution gel-like.

aqueous PAM solution has good compatibility with many organic substances that can be miscible with water, good compatibility with electrolytes, and is insensitive to ammonium chloride, calcium sulfate,

copper sulfate, Potassium hydroxide, sodium carbonate, sodium borate, sodium nitrate, sodium phosphate, sodium sulfate, zinc chloride, boric acid and phosphoric acid are not sensitive.

Polyacrylamide is white powder or small granular material, density is 1.302g/cm3 (23 ℃), glass transition temperature is 153 ℃, softening temperature 210 ℃, the general method of drying contains a small amount of water,

dry and will quickly draw water from the environment, the homopolymer separated by freeze-drying method is white loose non-crystalline solid, but when precipitated from the solution and dried, it is a glassy part transparent solid,

completely dried polyacrylamide PAM is brittle white solid, commercial polyacrylamide is usually dried under moderate conditions, generally with a water content of 5% to 15%,

and polymer films prepared by casting on glass plates are transparent, hard, and friable solids.
Under anoxic conditions, weight loss due to water loss when heated to 210°C; continued heating to 210-300°C when the amide group decomposes to produce ammonia and water;

when the temperature rises to 500°C it forms a black flake of only 40% of the original weight.

3. Polyacrylamide preparation method

There are two steps in the production of polyacrylamide.

Monomer production technology.

The production of acrylamide monomer is based on acrylonitrile as raw material, which is hydrated under the action of catalyst to produce the crude product of acrylamide monomer,

which is flash-distilled and refined to obtain the fine acrylamide monomer, which is the raw material for the production of polyacrylamide.

Acrylonitrile + (water catalyst/water) → combined → acrylamide crude product → flash → refined → refined acrylamide.

By the history of catalyst development, monomer technology has gone through three generations.

The first generation is sulfuric acid catalyzed hydration technology, the disadvantage of this technology is low conversion of acrylonitrile, low yield of acrylamide products, low by-products,

which brings a great burden to refining, in addition to the high cost of equipment due to the strong corrosive nature of the catalyst sulfuric acid, which increases the production cost;

the second generation is binary or ternary skeleton copper catalyzed production technology, the disadvantage of this technology is the introduction of metal copper ions in the final product that affects polymerization, which The third generation is microbial nitrile hydrate enzyme catalytic production technology,

this technology has mild reaction conditions, at room temperature and pressure, with high selectivity, high yield and high activity, the conversion of acrylonitrile can reach 100%, the reaction is complete, no by-products and impurities.

In addition, gas chromatography analysis shows that the acrylamide product contains almost no free acrylonitrile and has high purity,

which is especially suitable for the preparation of ultra-high relative molecular mass polyacrylamide and non-toxic polyacrylamide required by food industry.
The technology of microbial-catalyzed acrylamide monomer production was firstly established by Japan in 1985 with 6000t/a acrylamide plant, then Russia also mastered this technology, and in the 1990s, Japan and Russia established 10,000-ton microbial-catalyzed acrylamide plants one after another.

After Japan and Russia, China is the third country in the world to have this technology. The microbial catalyst activity is 2857 international biochemical units,

which has reached the international level. The microbial catalytic acrylamide monomer production technology in China was developed by Shanghai Pesticide Institute after three five-year plans, including the Seventh Five-Year Plan, the Eighth Five-Year Plan and the Ninth Five-Year Plan. This technology has been put into production in Rugao, Jiangsu,

Nanchang, Jiangxi, Shengli Oilfield and Wanquan, Hebei, with excellent quality and reaching the quality index of producing super high relative molecular mass polyacrylamide.

The quality index of super high relative molecular quality polyacrylamide has been achieved. It marks that our microbial catalytic acrylamide technology has reached the international advanced level.

Polymerization technology.

Polyacrylamide production is based on acrylamide aqueous solution as raw material, under the action of initiator, polymerization reaction is carried out,

after the reaction is completed the generated polyacrylamide gel block is cut and cut, granulated, dried and crushed, and finally polyacrylamide products are made.

The key process is the polymerization reaction. In the subsequent processing, attention should be paid to mechanical cooling,

thermal degradation and cross-linking, so as to ensure the relative molecular mass and water solubility of polyacrylamide.

Acrylamide + water (initiator/polymerization) → polyacrylamide gel → granulation → drying → crushing → polyacrylamide product

China’s polyacrylamide production technology has probably gone through 3 stages:

The first stage is the earliest use of disk polymerization, that is, the mixed polymerization reaction solution in stainless steel plates, and then these stainless steel plates pushed to the insulation baking room,

polymerization for a few hours, from the baking room out of the guillotine polyacrylamide cut into strips, into the meat grinder granulation, baking room drying, crushed to make finished products.

This process is completely manual workshop type.

The second stage is the use of kneading machine, that is, mixed polymerization reaction solution in the kneading machine heating, polymerization began, the beginning of the kneading machine, while polymerization while kneading, after polymerization,

granulation is also basically completed, pour out the material by drying, crushing finished products.
The third stage is, the late 1980s, the development of the conical kettle polymerization process, by the Ministry of Nuclear Industry five in Jiangsu Jiangdu Chemical Plant trial success.

The process in the lower part of the conical kettle with material-making rotary knife, polymer in the press out at the same time,

that is, into a granular, dry by the drum dryer, crushed to get the product.

In order to avoid the polyacrylamide block sticking to the wall of the polymerization kettle, some technologies use fluorine or silicon polymer compound coated on the inner wall of the polymerization kettle,

but this coating layer is easy to fall off during the production process and contaminate the polyacrylamide products.
There are also rotatable conical kettle, after the polymerization reaction is completed, the kettle is reversed to pour out the polyacrylamide gel,

granulation method (there are mechanical granulation, cutting granulation, there are also wet granulation, i.e. granulation in dispersion), drying method (there is the use of rotary drying through the flow,

but also the use of vibration fluidized bed drying) and crushing method. Some of these differences are differences in the quality of equipment,

and some are differences in the specific way of oil used, but in general, polymerization technology tends to be fixed conical kettle polymerization, vibration fluidized bed drying technology.

Polyacrylamide production technology in addition to the above unit operation, there are more obvious differences in the process formula, triggered by the former addition of alkali co-hydrolysis process and after the addition of alkali post-hydrolysis process,

the two methods have their advantages and disadvantages, the former addition of alkali co-hydrolysis process is simple, but there is a hydrolysis heat transfer susceptible to cross-linking and relative molecular mass loss, after the addition of alkali post-hydrolysis although the process increased,

but the hydrolysis is not easy to produce cross-linking, the loss of relative molecular mass of the product is also not large.

The initiators used for polyacrylamide polymerization in China are inorganic initiator, organic initiator and inorganic-organic mixed system in 3 types.

(1) Peroxides
Peroxides are broadly divided into inorganic peroxides and organic peroxides.

Inorganic peroxides such as potassium persulfate, ammonium persulfate, sodium perbromide and hydrogen peroxide.

Organic peroxides such as benzoyl peroxide, lauryl peroxide and tert-butyl hydroperoxide, etc..

They are used with reducing agents such as ferrous sulfate, ferrous chloride, sodium metabisulfite and sodium thiosulfate.
(2) Azo compounds

Such as azo diisobutyronitrile, azo bis(dimethylvaleronitrile), azo bis(sodium cyanovalerate) and azo amidine salt series developed in the 1980s, such as azo N-substituted amidine propane hydrochloride is a class of competing products,

their incorporation concentration of 0.005-1 parts per million, high catalytic efficiency, help to produce high relative molecular mass of the product, and soluble in water, easy to use.Reverse suspension polymerization method: Polyacrylamide is one of the most important organic polymer flocculants in industry.

In industry, aqueous solution method, reverse suspension polymerization method is usually used to produce polyacrylamide.

The following is to introduce the process of reverse suspension polymerization method to produce polyacrylamide.

Reverse suspension polymerization method is the most widely used and relatively mature method for making polyacrylamide (PAM) microspheres today.

Strong stirring is used to disperse the monomer or monomer mixture in the medium (medium is organic solvent), which becomes fine particles and then polymerization of monomer, initiator, organic solvent and dispersing stabilizer.

When the polymerization is completed, after boiling and dehydration, separation and drying can be obtained in the form of particulate products.

Reverse suspension polymerization method to obtain the product, solid mass fraction > 90%, polymerization rate > 95%, monomer residue < 0.5%, product particle size between 10-500 microns, the product has good water solubility.

This method is easy to realize industrialization because of simple process, convenient operation control, easy removal of polymerization heat, easy separation, washing and drying of polymer, pure, uniform and stable product.

But the reverse suspension polymerization method in industrial production also has problems, first of all, by the stirring speed is very influential, easy to agglomerate, gel, azeotropic system instability,long time out of water and other shortcomings.

In addition, the wide particle size distribution, the use of a large number of organic solvents, the safety of production operations, and the high cost of polymerization are a series of reasons why the reverse suspension polymerization method is rarely used in the production of polyacrylamide in China.