Preparation and Properties of Polyacrylonitrile

Monomers and Other Basic Raw Materials

1. Monomer

The main monomer for preparing polyacrylonitrile is acrylonitrile (AN), which can be produced from petroleum, natural gas, coal, calcium carbide, etc., and has a variety of process routes. Currently the propylene ammoxidation method is widely used.

Product List

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CAS NO.	Product Name
25014-41-9	Polyacrylonitrile, average Mw 150,000
25014-41-9	Polyacrylonitrile,mw 150000
26658-88-8	Polyacrylonitrile,co-polymer with 6% methyl acrylate,mw 80,000
25014-41-9	2-Propenenitrile,homopolymer, mw 150000
4635-82-9	Acrylonitrile-2-d1
53807-26-4	Acrylonitrile(d3)
55757-47-6	Acrylonitrile-3-13c

Among all the methods of propylene ammoxidation, the Sochio method is the most important. This method involves the reaction of propylene in the presence of ammonia, air, and water, using bismuth molybdate and uranium oxyantimonate as catalysts, on a fluidized bed at a temperature of 450 °C and a pressure of 150 kPa. The reaction proceeds as follows:

In addition to the above-mentioned main reactions, a series of side reactions occurred. Polyacrylonitrile is a colorless liquid at room temperature and pressure. Its main physical properties are as follows:

• Density: 0.806 g/cm³
• Boiling point: 77.3 ~ 77.4 °C
• Latent heat of vaporization: 32.5 J/mol
• Polymeric hot: 72.4 J/mol
• Freezing point: -83.6 °C
• Refractive index (n²⁰d): 1.3888
• Flash point: 2.5 °C
• Explosion limit: 3.05% ~ 17.5% (volume fraction)
• Acrylonitrile is slightly soluble in water and can dissolve with most organic solvents to form an azeotrope system.

Due to the poor elasticity of fibers made of acrylonitrile homopolymer, most of the commonly used fiber-forming polyacrylonitrile are binary copolymers. The function of the second monomer is to reduce the force between macromolecules and improve fiber elasticity. Vinyl monomers containing ester groups, such as methyl acrylate, methyl methacrylate and vinyl acetate, are usually used in an amount of 5% ~ 10%.

The purpose of adding the third monomer is to improve the dyeability and hydrophilicity of the fiber. Generally, ionizable vinyl monomers are selected, which can be divided into two categories: one is monomers that have affinity for cationic dyes and contain carboxyl or sulfonic groups, such as sodium propylene sulfonate, sodium methpropylene sulfonate, itaconic acid, etc. The other is affinity for acid dyes, containing amino, acylamino, pyridyl and other monomers, such as vinylpyridine, 2-methyl-5-ethyl pyridine, dimethyl aminoethyl methacrylate, etc., with an addition of 0.5% ~ 3%.

2. Initiator

The initiators used in the polymerization of acrylonitrile are mainly of the following types.

• Azo initiators: such as azobisisobutyronitrile, azobisisoheptanitrile, etc.;
• Organic peroxides: such as octanoyl peroxide, diisopropyl peroxydicarbonate, etc.;
• Redox system: oxidizing agents such as persulfate, hydrogen peroxide, and oxygen salts; reducing agents such as sulfite, sodium bisulfite, copper oxide, etc.

The polymerization process routes of acrylonitrile are different, and the initiators used are also different. For example, the sodium thiocyanate (NaSCN) solvent route and the dimethyl sulfoxide (DMSO) solvent route mostly use azobisisobutyronitrile as the initiator, and the aqueous phase polymerization route uses an oxidation-reduction initiator system.

3. Solvent

The polymerization of acrylonitrile is often carried out by solution polymerization. Commonly used solvents include sodium thiocyanate aqueous solution and zinc chloride aqueous solution, nitric acid, dimethyl sulfoxide, dimethylformamide, dimethylacetamide (DMAc), etc.

4. Other additives

In order to control the relative molecular weight, isopropyl alcohol (IPA) is often added during the polymerization of acrylonitrile.

In addition, in order to prevent the polymer from coloring, a small amount of reducing agent or other additives, such as thiourea dioxide (TUD), stannous oxide, etc., need to be added during the polymerization process to improve the whiteness of the fiber.

Polymerization of Acrylonitrile

1. Homogeneous solution polymerization

The so-called homogeneous solution polymerization means that the solvent used can dissolve both monomers and polymerization products. After the reaction, the polymer solution can be directly used for spinning, so this method is also called one-step method. For example, this method is used for the polymerization of acrylonitrile in concentrated aqueous solution of sodium thiocyanate, zinc chloride, nitric acid, dimethylformamide, dimethyl sulfoxide, and so on.

Taking the one-step homogeneous solution polymerization of sodium thiocyanate as an example, the technological process is shown in figure 1.

Fig. 1 flow chart of one-step homogeneous solution polymerization of sodium thiocyanate

The raw material acrylonitrile, the second monomer methyl acrylate (MA), isopropanol and 48.8% sodium thiocyanate aqueous solution are respectively measured by the metering tank and put into the batching bucket. The initiator azodiisobutyronitrile (AIBN) and the light color agent thiourea dioxide (TUD) were weighed and added to the batching bucket through a cyclone sealing hopper. Itaconic acid (ITA) is adjusted to a certain concentration of aqueous solution through the metering bucket and added to the batching bucket. After adjustment, the reagent mixing bucket is continuously injected with a stable flow rate, and then fully mixed with the unreacted monomers removed from the polymerization slurry and adjusted the temperature, the polymerization is continuously fed into the polymerization kettle with a metering pump.

After the polymerization reaction is completed, the feed liquid enters the monomer removal tower, and the unreacted monomer is separated and extracted to the monomer condenser. The reactant mixture is condensed and brought back to the reagent mixing barrel. For low conversion rate polymerization reactions, the discharge mixture contains 40% ~ 45% non-reactive monomers, while for medium conversion rate reactions, the content is about 30%. The monomer content in the feed liquid should not exceed 0.3%.

After the monomer is removed, the polymer solution can be sent to spinning after deaeration, humidity control and filtration.

2. Heterogeneous polymerization

The heterogeneous polymerization of acrylonitrile generally adopts the aqueous precipitation polymerization method using water as the medium. Aqueous phase precipitation polymerization refers to using water as the medium, and the monomer has a certain solubility in water. When the water-soluble initiator initiates polymerization, the polymerization product is insoluble in water and continuously precipitates out of the water phase. Aqueous precipitation polymerization has the following advantages.

(1) The aqueous phase polymerization usually adopts the water-soluble redox initiation system, the decomposition activation energy of the initiator is low, and the polymerization can be carried out at 30 °C ~ 50 °C or even lower, and the color of the product is white.

(2) The reaction heat of aqueous phase polymerization is easy to control, and the relative molecular weight distribution of polymerization products is narrow.

(3) The polymerization speed is faster, the particle size of the product is uniform and the moisture content is lower, the conversion rate of polymerization is higher, the slurry material is easy to be treated, and the recovery process is relatively simple.

Figure 2 shows the process flow chart of continuous aqueous precipitation polymerization. From the diagram, it can be seen that monomers, initiators, and water are injected into the polymerization kettle through a metering pump to control a certain amount of pH, and the reaction materials stay in the kettle for a certain time to react. After reaching the specified conversion rate, the polymer slurry containing monomers flows to the alkali termination kettle, and the pH of the system is adjusted with NaOH aqueous solution to terminate the reaction. Then the slurry containing monomer is sent to the monomer removal tower, and the clean acrylonitrile copolymer is obtained after the polymer slurry is dehydrated and washed by centrifuge. Most of the polyacrylonitrile fiber factories abroad adopt two-step polymerization, which accounts for more than 70% of the total output of polyacrylonitrile fiber.

Fig. 2 schematic diagram of continuous aqueous precipitation polymerization process

1. AN+MA metering pressure regulator tank; 2. NaClO₃-Na₂SO₃, aqueous solution metering pressure regulator tank; 3. HNO₃+third monomer metering regulator tank; 4. metering pump; 5. agitator and motor; 6. polymerization kettle; 7. alkali termination kettle; 8. transfer pump; 9. stripping unit tower; 10. centrifugal dehydrator

Properties of Polyacrylonitrile

The appearance of polyacrylonitrile is white powder and its density is 1.14 ~ 1.15 g/cm³. It softens and decomposes when heated to 220 °C ~ 230 °C.

Due to the role of the side cyano groups, the main chain of the polyacrylonitrile macromolecule assumes a helical spatial conformation. After the second monomer and third monomer are introduced into the acrylonitrile homopolymer, the side groups of the macromolecules change greatly, increasing the irregularity of its structure and conformation.

It is generally believed that acrylonitrile homopolymer has two glass transition temperatures, which are 80 °C ~ 100 °C in the low-order region and 140 °C ~ 150 °C in the high-order region. The two glass transition temperatures of acrylonitrile terpolymer are relatively close, ranging from 75 °C ~ 100 °C.

The chemical stability of polyacrylonitrile is much lower than that of polyvinyl chloride. Under the action of acid or alkali, the cyano group of polyacrylonitrile will be converted into an amido group, and the amido group can be further hydrolyzed to form a carboxyl group and release NH₃. The higher the temperature, the more violent the reaction. The NH₃ released during alkaline hydrolysis reacts with the cyano group of unhydrolyzed polyacrylonitrile to form an amidine group, causing the polymer and fiber to turn yellow.

In the cyano group of polyacrylonitrile, the trivalent bond between carbon and nitrogen atoms (one σ bond and two π bonds) can absorb strong energy such as photons of ultraviolet light and convert it into heat, preventing the polymer from degrading. As a result, the final polyacrylonitrile fiber has very excellent light resistance.

Polyacrylonitrile has good thermal stability. Generally, polyacrylonitrile used in fiber making does not change when heated to 170 °C ~ 180 °C. If there are impurities in polyacrylonitrile, it can accelerate the thermal decomposition of polyacrylonitrile and cause its color to change. When the polyacrylonitrile solution is heated at 100 °C for a long time, the ring formation of the molecular chain will occur. When polyacrylonitrile is heated for a long time in the presence of air or oxygen, the color of the polymer will darken, first to yellow and finally to brown. The polymer loses its solubility properties at this point. If polyacrylonitrile is heated to 250 °C ~ 300 °C, thermal cracking will occur and hydrogen cyanide, ammonia, nitriles, amines, and unsaturated compounds will be decomposed.

Polyacrylonitrile is stable to various alcohols, organic acids (except formic acid), hydrocarbons, ketones, esters, and other substances, but it is soluble in concentrated sulfuric acid, amides, and sulfoxide solvents.