Acrylamide products are solid or 50% aqueous solution, and oxygen in the air has obvious inhibitory effect on acrylamide in aqueous solution. Adding (25~3) x10-5 trivalent copper ions, ferric iron ions, nitroso ions, ethylenediaminetetraacetic acid, etc. can be used as a stabilizer.
Acrylamide molecules contain double active centers of double bonds and amide groups, which are prone to polymerization reaction and hydrolysis, complexation and addition reactions of amide groups. The important chemical reactions in the acrylamide industry are:
(1) Acrylamide hydrolysis reaction:
CH2 =CHCONH2 + H20→CH2 =CH-C00H + NH3
(2) Acrylamide and formaldehyde react under alkaline conditions to form N-methylolacrylamide:
(3) Two molecules of acrylamide and -molecular formaldehyde form N,N-methine bisacrylamide under the action of acidic catalyst (can be used as a crosslinking agent):
Acrylamide can also form complexes with transition metal ions. Acrylamide monomers can poison human nerves. Harm to both central nervous system and peripheral nervous system. Workers should prevent inhalation of acrylamide dust or its vapors through skin contact or respiratory tract. Persons who may come into contact with acrylamide should wear protective equipment to avoid contact.
Acrylamide is derived from the catalytic hydrolysis reaction of acrylonitrile:
In industrial production, skeleton copper is used as a catalyst. Propylene wax is mixed with an appropriate amount of water and sent from the bottom to the catalytic hydration tower equipped with skeleton copper. The reaction temperature is 70 ~ 105°C, the pressure is 0.29 ~ 0. 39MPa, the reaction material enters the flash After removing unreacted acrylonitrile in the evaporator, it is concentrated to make the acrylamide reach the required concentration, usually not more than 30%. During the concentration process, the acryl glue should be fully exposed to the air to prevent the polymerization reaction. Since the monomeric acrylamide F is a crystal at room temperature, it is easy to polymerize after melting, so it can only be refined by recrystallization.
2. Polymerization process
Acrylamide produces polyacrylamide through free radical polymerization under the action of free radical initiator:
Acrylamide in alcohol or pyridine solution, under the action of a strong catalyst such as sodium alkoxide, undergoes anionic polymerization to produce poly-β-propionamide.
In industrial production, free radical polymerization is used to produce polyacrylamide. There are many types of free radical initiators and initiator sources, including peroxides, persulfates, oxidation-reduction initiator systems, azo compounds, ultrasonic waves, ultraviolet rays, Ion gas, plasma, high energy irradiation, etc.
The polymerization methods used in industrial production are mainly solution polymerization and inverse emulsion polymerization, the former being the most widely used. In addition, there are related reports on the method of initiating solid-phase polymerization by γ-ray irradiation. When the aqueous solution of acrylamide is polymerized into an aqueous solution of polyacrylamide, the heat of polymerization is 82.8 kJ/mol. Relatively speaking, the heat released is very large, so how to timely derive the polymerization heat in the aqueous solution polymerization method has become an important technical issue. The second is how to reduce the residual monomer content. Because acrylamide is very toxic, in order to reduce its harmfulness, the residual monomer content is required to be less than 0.1% especially when used in water quality treatment. The third is how to convert the high viscosity fluid or polybrene obtained by the polymerization reaction into a solid substance, that is, the problem of drying and dehydration. Finally, how to freely control the relative molecular mass of the product. At 25°C, pH=1, the chain growth rate constant K and the chain termination rate constant K are (1.72±0.3) x104L/ (mol.g) and (16.3+0.7) x106L/ (mol. 8), K./K proportional to the kinetic chain length, =4.2+0.2, this value is very high, so when no chain transfer agent is present, the average molecular weight of polyacrylamide can exceed 2000 x104 The product. When acrylamide undergoes free radical polymerization in an aqueous solution, crosslinking may occur to form an insoluble polymer. This phenomenon is more severe when the polymerization temperature is too high. The theoretical explanation is that the polymer end groups generated by disproportionation have double bonds, which are caused by participating in the polymerization reaction or undergoing chain transfer to the polymer. In addition, the initiator persulfate and polyacrylamide also cause gel formation when heated.
Someone has studied the nitrogen content of the industrial product polyacrylamide and found that the nitrogen content is lower than the theoretical value. It is believed that this is due to the dehydration of NH in the molecule and the formation of imide groups:
Due to the presence of the imide group, the rigidity of the polymer chain is increased and the acid group is generated, which affects the miscibility with other molecules, so it exhibits insolubility. Others believe that imide groups are generated between molecules and are therefore insoluble or caused by crosslinking between macromolecules. Below 50C, the chain transfer constant to the polymer and water is very small, while the chain transfer to the initiator is more clear
Obviously, it is also easy to chain transfer to alcohol, especially isopropanol, so the industry often uses isopropanol as a chain transfer agent to control the relative molecular mass of the product. Trace metal ions such as Fe3+. Cu2+ in the aqueous solution can accelerate the reaction speed of the oxidation-reduction initiation system, but too much will cause adverse effects. As the chain of polyacrylamide grows, free radicals transfer to metal ions such as iron salts-an electron and a chain termination reaction occurs. Acrylamide is more stable when stored under 10~25°C core conditions. Generally, it is not necessary to add a polymerization inhibitor during use and transportation. However, molten acrylamide easily polymerizes violently and releases a lot of heat. Generally it becomes Insoluble polymer. The aqueous solution of acrylamide is very stable below 50C, and can be stored for a long time by adding a polymerization inhibitor. Polymerization inhibitors include sodium cyanide, t-butylhydroxyanisole, tetramethylthiuram-sulfur, and ketoferrin (N-nitrosohydroxylamine). The stability of the aqueous solution of acrylamide is influenced by the pH and the amount of dissolved air in a complicated manner. The chemical nature of acrylamide is very active, and its chemical reactions are mostly those characteristic reactions of amide groups and double bonds.
Acrylamide water-soluble monomers, can be decomposed in organic solvents containing low HLB value (3 ~ 6) emulsifier, and use peroxide (such as persulfate) as an initiator for inverse emulsion polymerization to obtain high relative Molecular mass water soluble polymers, they are excellent molecular flocculants. The series of polyacrylamide-based flocculants has the characteristics of high relative molecular weight (about 1000x104), complete varieties, and low price and availability, and is currently developing rapidly. Bead gels of polyacrylamide and its derivatives made by inverse emulsion polymerization have high-efficiency selective adsorption and separation functions similar to molecular sieves, and can provide powerful means for separating proteins, nucleic acids and other compounds. It is widely used in biomedical engineering such as molecular biology, biochemistry, microbiology and enzymology. The inverse emulsion polymerization requires high purity of acrylamide because it affects not only the relative molecular mass of the product, but also the solubility of the product. The maximum allowable content of impurities is shown in Table 2-8.
Table 2-8 The maximum allowable maximum t of acrylamide containing impurities
Out: ① There are still other impurities in the monomer, such as Yi Sheng, Liang Shi, etc., which has little effect on sudden cohesion.②Adopt radiation bulk polymerization method.