Acrylonitrile Copolymers (Nitriles)

Properties

Acrylonitrile polymers (Nitriles) are copolymers of acrylonitrile and various other monomers. The three most important comonomers are styrene, butadiene, and methyl acrylate. About 30 to 85 percent by weight of the copolymer is acrylonitrile and about 15 percent or more is rubber (butadiene) or methyl acrylate.

The most important nitrile products are acrylic fibers with about 85 percent nitrile, and the three thermoplastics acrylonitrile-styrene (SAN), acrylic-styrene-acrylonitrile (ASA), and acrylonitrile-butadiene-styrene (ABS).

Acrylonitrile is a very polar monomer. Its glass transition temperature is about 385 ± 15 K (112 ± 15°C). When copolymerized or blended with other monomers, it increases the glass transition temperature, mechanical strength, heat and chemical resistance. It also improves the miscibility with polar solvents and monomers which is an important characteristic when used in formulated products such as coatings and adhesives.

The only other nitrile monomer of commercial importance is methacrylonitrile. It is used in the preparation of copolymers (elastomers) similar to acrylonitrile but on a much smaller scale. It is also an important chemical intermediate in the preparation of acids, amides, amines, esters, and other nitriles.

Most acrylonitrile copolymers have excellent chemical resistance against oils, greases, dilute acids and alkalis, but are attacked by halogenated and aromatic hydrocarbons.

 

Poly(styrene-co-acrylonitrile) (SAN)

One of the most important acrylonitrile copolymers is poly(styrene-co-acrylonitrile) (PSAN). The relative composition is typically between 60 and 80% by weight styrene and 20 to 40% acrylonitrile. The two monomers have a tendency for alternation in the backbone. The copolymer combines the clarity and rigidity of polystyrene with the hardness of acrylonitrile. Compared to styrene polymers, it has much improved chemical resistance, better heat stability, improved mechanical properties and a slightly higher heat-distortion temperature. The copolymer also has much improved creep resistance and weatherability over polystyrene. However, these copolymers often yield yellow products.

 

Poly(styrene-co-acrylonitrile-co-butadiene) (ABS)

Of even greater importance are terpolymers of styrene, acrylonitrile and butadiene (ABS). These opaque thermoplastics have high stress and impact resistance and higher tensile strength than pure PS and acrylonitrile. ABS resins can be polymerized in a wide variety of grades. The proportion of the three monomers can vary greatly. The butadiene content is usually between 15 to 85 percent and the styrene-to-acrylonitrile ratio is between 80:20 and 60:40. The butadiene portion provides flexibility and high impact resistance whereas the SAN portion provides strength, good dimensional stability, and creep resistance.

 

Acrylic Fibers

Acrylic fibers are synthetic fibers made from polyacrylonitrile with an average molecular weight of about 100,000. The acrylonitrile content is at least 85 percent. Typical co-monomers are vinyl acetate and methyl acrylate. Both monomers improve the flexibility, toughness, and resilience of the rather brittle acrylonitrile. Acrylic yarns are often perceived as "cheap" because they are usually priced lower than its natural-fiber counterparts and fabrics made from acrylic fiber feel less soft and warm then wool and cotton.

 

Many other acrylonitrile copolymers have been synthesized. Among those that are commercially produced are acrylic-styrene-acrylonitrile (ASA), acrylonitrile-butadiene rubber (NBR), methyl methacrylate-acrylonitrile-butadien-styrene (MABS), and acrylonitrile-acrylate (ANA).

Acrylic-styrene-acrylonitrile (ASA) copolymers have excelent resistance to UV irradiation, moisture, heat and cracking. It probably has the best weathering resistance of all acrylonitrile elastomers. It also has good chemical resistance, and high impact strength. Its mechanical properties are very similar to acrylonitrile butadiene styrene (ABS) elastomers.

Acrylonitrile butadiene rubber (NBR), usually shortened to nitrile, is an important elastomer. The acylonitrile (AN) content usually varies between with 15 and 45 percent. NBR grades with high AN content have better oil and abrasion resistance, whereas grades with low AN content have better low temperature flexibility and resilience. In general, nitrile rubbers have only moderate physical properties.
Amine, carboxy, and epoxy functionalized butadiene-acrylonitrile rubbers are important tougheners for epoxy formulations such as coatings and adhesives.

 

Commercial Acrylonitrile Polymers (Nitriles)

Polyacylonitrile copolymers (ABS, SAN, ASA, MABS, NBR, and ABS blends) are important thermoplastics. They are produced on a large scale and sold under various tradenames. Important manufacturers include Sabic, Ineos, LG Chem, Trinseo, and BASF among many others.

 

Applications

The acrylonitrile market is very broad. ABS is the most important acrylonitrile copolymer followed by SAN, ABS blends, ASA, and MABS.

ABS is a low cost engineering and commodity plastic that is easy to mold and tailor. ABS is widely used for apllications in kitchen and household appliances (ovens, washing machines, dryers, toaster, refrigerators, vacuum cleaners etc.).It is also extensively used for toys including Lego and Kre-O bricks. Other important industries for ABS include automotive, construction, and electronics.

Due to its similar mechanical properties, ASA copolymers are used for similar applications as ABS. However, ASA has superior weathering resistance which makes it more suitable for outdoor applications than ABS. One of the most important markets for ASAs are automotive body parts such as mirror housings and radiator grills. ASA thermoplastics are also extensively used in the building & construction, appliance, electrical/electronics, and sports goods industries.

Nitrile rubbers (NBR) are mainly used for disposable non-latex gloves and for elastomeric parts such as transmission belts, O-rings, gaskets, hoses, and oil seals.

SAN is one of the most important acrylonitrile copolymers. Like ABS, it is widely used for apllications in appliances (refrigerators, coffee machines, kitchen utensils, etc.). Other important applications include housings for scales, batteries, computer, and other consumer products.

Acrylic fibers are sometimes used in the apparel industry for sweaters, socks, and tracksuits. Other important textiles made with acrylics include blankets, area rugs, upholstery, luggage suitcases, awning, and outdoor furniture.

 

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