Polyacrylonitrile

Polyacrylonitrile (PAN) is a synthetic, semicrystalline organic polymer resin with the linear formula (CH2CHCN)n. Nearly all PAN resins are copolymers where acrylonitrile serves as the primary monomer. This material goes into a wide range of products, such as ultrafiltration membranes, hollow fibers for reverse osmosis, textile fibers, and oxidized PAN fibers. PAN fibers also serve as the chemical precursor for high-quality carbon fiber, and PAN itself is a repeating unit in important copolymers like styrene-acrylonitrile (SAN) and acrylonitrile butadiene styrene (ABS) plastic.

History

The first synthesis of PAN occurred in 1930 by Hans Fikentscher and Claus Heuck at IG Farben’s Ludwigshafen plant. Research stalled because PAN does not melt and could not be dissolved in any industrial solvents available at the time. In 1931, Herbert Rein, head of polymer fiber chemistry at IG Farben’s Bitterfeld plant, obtained a PAN sample during a visit to Ludwigshafen. He discovered that pyridinium benzylchloride, an ionic liquid, could dissolve PAN.

By 1938, he had spun the first PAN fibers using aqueous solutions of quaternary ammonium sodium thiocyanate and aluminum perchlorate, and he also considered other solvents like DMF. Commercial production was delayed due to wartime infrastructure strains, the polymer’s inability to melt without degrading, and the lack of known solvents for solution processing. The first mass production of PAN fiber began in 1946 by the American company DuPont, using intellectual property taken from Germany during Operation Paperclip.

The product, branded Orlon, was based on a patent filed exactly seven days after a nearly identical German claim. In the German Democratic Republic, industrial PAN fiber production started in 1956 at VEB Film- und Chemiefaserwerk Agfa Wolfen, building on work by the “Wolcrylon” collective (including Max Duch and Herbert Lehnert). Raw material production had been set up earlier at Buna Werke Schkopau (for polyacrylonitrile) and Leuna works (for dimethylformamide). That same year, the collective received the GDR’s National Prize II Class for Science and Technology.

Physical properties

Though thermoplastic, PAN does not melt under normal conditions; it degrades before melting. It will melt above 300 °C if heated at rates of 50 degrees per minute or faster. Its glass transition temperature is around 95 °C, and its fusion temperature is 322 °C. PAN dissolves in polar solvents such as dimethylformamide, dimethylacetamide, ethylene and propylene carbonates, and in aqueous solutions of sodium thiocyanate, zinc chloride, or nitric acid.

Its solubility parameters at 25 °C are 25.6 to 31.5 J¹/² cm⁻³/², and its dielectric constants are 5.5 at 1 kHz and 4.2 at 1 MHz (both at 25 °C). The polymer can behave as either branched or linear. In the production of carbon fibers from a 600 tex (6k) PAN tow, the linear density of individual filaments is 0.12 tex, the filament diameter is 11.6 μm, the resulting carbon fiber has a filament strength of 417 kgf/mm², and the binder content is 38.6%.

Synthesis

Most commercial synthesis of PAN uses free radical polymerization of acrylonitrile. Typically, 1–10% of other vinyl comonomers are added, depending on the final application.

Common comonomers include acrylic acid, acrylamide, allyl compounds, and sulfonated styrene. Anionic polymerization can also be used. For textile applications, molecular weights range from 40,000 to 70,000; for carbon fiber production, higher molecular weights are preferred.

Applications

Homopolymers of PAN are used in hot gas filtration systems, outdoor awnings, yacht sails, and fiber-reinforced concrete. Copolymers containing PAN are often made into fibers for knitted clothing such as socks and sweaters, as well as outdoor items like tents. If a clothing label says “acrylic,” the material is a copolymer of polyacrylonitrile.

DuPont first spun PAN into fiber in 1942, marketing it as Orlon. Acrylonitrile is also a common comonomer with styrene in plastics like acrylonitrile, styrene, and acrylate blends. Labeling a garment as “acrylic” means the polymer contains at least 85% acrylonitrile monomer.

A typical comonomer is vinyl acetate, which allows the fibers to soften enough for dye penetration. Acrylic fibers are low-cost compared to natural fibers, offer better sunlight resistance, and resist moth damage. Halogen-containing comonomers produce modacrylics, which contain 35–85% PAN. The halogen groups improve flame resistance, making modacrylics suitable for sleepwear, tents, and blankets.

Some mattresses also use them to meet North American flame resistance requirements. However, modacrylics are costly and can shrink after drying. PAN also absorbs many metal ions, making it useful in absorption materials.