At present, domestic and foreign industrial production methods of polyacrylamide are as follows.
1. Aqueous solution polymerization method
Acrylamide aqueous jujube method is the main method used in industrial production. The monomer solution in the formula must be purified by ion exchange. The reaction medium water should be deionized water, and the initiator mostly uses an oxidation-reduction initiator system composed of persulfate and sulfite to reduce the reaction initiation temperature. In addition, a chain transfer agent needs to be added, and isopropyl alcohol is commonly used. In order to eliminate the influence of possible metal ions, the integrator ethylenediaminetetraacetic acid (EDTA) is added if necessary. In order to easily control the reaction temperature, the monomer concentration is usually lower than 25%.
Since the heat of polymerization of acrylamide is as high as 82.8 kJ/mol, the heat of polymerization must be derived in time. If the monomer concentration is 25% to 30%, the polymerization is initiated even at 10C. If the polymerization heat is not exported, the solution temperature will automatically rise to 100C. A large amount of insoluble matter will be generated. Therefore, heat conduction becomes one of the key issues in production.
When producing low molecular weight products, it can be operated in the middle of the label reactor or continuous production with multiple kettles. The jacket is cooled to maintain the reaction temperature at 20~25C, and the conversion rate is 95%-99%. When producing high molecular weight products, because the products are jelly-like and cannot be stirred, in order to export the reaction heat in time, the industry adopts to mix the materials in the formula in the reactor and send them to the polyethylene bag immediately. Cooling reaction is carried out in the water tank of the polyethylene device equipped with the reaction materials. It should be noted that due to the obvious inhibitory effect of oxygen in the air, the preparation and addition must be carried out in N2. When using the persulfate-sulfite initiator system, the initiation temperature is usually 40 cents. If the production of ultra-high molecular weight products is required, the initiation temperature should be lower than 20C.
Since the monomers are not volatile and cannot be removed after the reaction, unreacted monomers will remain in the polyacrylamide, prolonging the reaction time, and increasing the reaction temperature can reduce the amount of residual monomers, but the production capacity decreases and the insoluble content will increase. In order to reduce the amount of residual monomers, some plants use a composite initiator system, which is composed of an oxidation-reduction initiator and a water-soluble azo initiator.
Under low temperature conditions, the redox initiator plays a role. When the temperature of the reaction material rises later, the azo initiator is decomposed to further play a role. The residual monomer content of polyacrylamide produced by this method can be as low as 0.02% ( Gas chromatography). Water-soluble azo initiators are 4,4'-azobis-4-cyanovaleric acid, 2, 2'-azobis-4-methylbutyronitrile sodium sulfate and 2, 2'-azobis- 2-Succinylpentane dihydrochloride, etc.
The method for determining the residual acrylamide is mainly used in the industry, but its sensitivity is poor. For extremely small amounts of monomers, flame ion harmonics or high-performance liquid color harmonics can be used for measurement. In order to produce polyacrylamides containing the fewest hair groups, an appropriate amount of sodium carbonate can be added to the polymerization formula to hydrolyze a small amount of amide groups into carboxyl groups, reducing the formation of insolubles. The polyacrylamide synthesized according to the above method is a high-viscosity fluid or a gel-like non-flowing substance. It can be used directly as a commodity, supplying monomers closer to the production plant. For long-distance transportation, it should be dried to produce powdery solids. The method of drying the colloidal substance can be the kneading drying method, but this method consumes a lot of energy and the product is seriously degraded. When the production scale is small, the method of granulation in the drying room after granulation by an extruder can be adopted, and then pulverized to obtain a powdery product. The larger and more advanced method is to granulate through an extruder and send it to a drum dryer, which is crushed to obtain a powdery commodity after drying.
2. Inverse emulsion polymerization
Acrylamide monomer is formulated into an aqueous solution with a concentration of 30% to 60% as a dispersed phase, which is added with a small amount of diethylamine tetraacetic acid, Na2SO, and oxidation-reduction initiator and an appropriate amount of water-soluble surfactant, the HLB value should be Lower. Aromatic hydrocarbons or saturated aliphatic hydrocarbons are used as the continuous phase, and oil-soluble surfactants are added, and the HLB value should be high, such as sorbitan oleate. Na2S0 has the effect of preventing the adhesion of milk particles. The ratio of dispersed phase to continuous phase is usually 3:7. The diameter of the dispersed phase latex particles obtained by polymerization is 0.1 ~ 10μ.m, which is related to the amount of surfactant. The reaction temperature is generally 40C, and the conversion rate can reach 98% in 6 hours. The advantage of this method is that the reaction heat is easy to export, the viscosity of the material system is low, it is easy to operate, and the product can be used directly without drying. The disadvantage is that the use of organic solvents is flammable, and the effective production capacity is lower than the solution polymerization method.
3. Suspension polymerization
The polymerization reaction in which the monomer aqueous solution is suspended in small organic beads in an organic solvent is called suspension polymerization, also called bead polymerization. Suspension polymer is mainly composed of monomer, initiator, organic solvent and suspending agent. The initiator is dissolved in the aqueous monomer solution. Dispersing agent is also called suspending agent. Its function is to make monomer aqueous solution into small droplets suspended in organic solvent under stirring to prevent the polymerized particles from sticking to each other during the polymerization process. The crystal grain size is generally 100 ~ 2000μm.
Unlike emulsion polymerization, the dispersion of the monomer solution is relatively low, generally the beads are suspended in the organic phase. Therefore, the organic carrier used is a large specific gravity solvent such as xylene, vinyl chloride, etc. The dispersant generally does not use emulsifier with high activity; the stirring intensity is also smaller than that during emulsion polymerization; the product obtained by suspension polymerization is in the form of particles, which is convenient to use.
In the suspension polymerization solution, the AM aqueous solution forms a stable solution in gasoline, xylene, and methane in the presence of a suspending agent (dispersion stabilizer) such as Span60, inorganic amide, Cn~C: fatty acid sodium, or acetic acid fiber. The suspension initiates polymerization. After the suspension polymerization is completed, azeotropic dehydration, separation and drying are carried out to obtain a bead or powder product. Adding inorganic salts NeCl, NaNO, or Na, CO during the polymerization process can adjust the surface tension of the system and increase the suspension stability, but has little effect on the polymerization process. However, the addition of a small amount of monobasic, dibasic or polybasic carboxylates usually increases the relative molecular mass of the product and reduces the polymerization rate.