Nitrile rubber

  Nitrile rubber, or Buna-N,is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.

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Acrylonitrile butadiene rubber (NBR) is a family of unsaturated copolymers of 2-propenenitrile and various butadiene monomers (1,2-butadiene and 1,3-butadiene). Although its physical and chemical properties vary depending on the polymer’s composition of acrylonitrile (the more acrylonitrile within the polymer, the higher the resistance to oils but the lower the flexibility of the material), this form of synthetic rubber is generally resistant to oil, fuel, and other chemicals. Its resilience makes NBR the perfect material for disposable lab, cleaning, and examination gloves. In the automotive industry, it is used to make fuel and oil handling hoses, seals and grommets. NBR’s ability to withstand a range of temperatures from −40°C to +120°C makes it an ideal material for extreme automotive applications. Acrylonitrile butadiene is also used to create moulded goods, footwear, adhesives, sealants, sponge,expanded foams and floor mats. Compared to natural rubber, nitrile rubber is more resistant to oils and acids, but has inferior strength and flexibility. Nitrile rubber is generally resistant to aliphatic hydrocarbons. However (like natural rubber), it can be attacked by ozone, aromatic hydrocarbons, ketones, esters and aldehydes.

Production Process

In the production of hot NBR, emulsifier (soap), 2-propenenitrile (acrylonitrile), various butadiene monomers (including 1,3-butadiene, 1,2-butadiene), radical generating activators, and a catalyst are added to polymerization vessels. Within the vessel, water serves as the reaction medium. The tanks are heated to 30°C-40°C to facilitate the polymerization reaction and to promote branch formation in the polymer. Because several monomers capable of propagating the reaction are involved in the production of nitrile rubber, the composition of each polymer can vary (depending on the concentrations of each monomer added to the polymerization tank and the conditions within the tank). One repeating unit found throughout the entire polymer may not exist. For this reason, there is also no IUPAC name for the general polymer. The reaction for one possible portion of the polymer is shown below:

1,3-butadiene + 1,3-butadiene + 2-propenenitrile + 1,3-butadiene + 1,2-butadiene → acrylonitrile butadiene rubber

Monomers are usually permitted to react for 5 to 12 hours. Polymerization is allowed to proceed to ~70% conversion before a “shortstop” agent (such as dimethyldithioarbamate and diethyl hydroxylamine) is added to react with the remaining free radicals. Once the resultant latex has “shortstopped”, the unreacted monomers are removed through a steam in a slurry stripper. Recovery of unreacted monomers is close to 100%. After monomer recovery, latex is sent through a series of filters to remove unwanted solids and then sent to the blending tanks where it is stabilized with an antioxidant. The yielded polymer latex is coagulated using calcium chloride, aluminum sulfate, and other coagulating agents in an aluminum tank. The coagulated substance is then washed and dried into crumb rubber.

The process for the production of cold NBR is very similar to that of hot NBR. Polymerization tanks are heated to 5 °C–15 °C instead of 30 °C–40 °C. Under lower temperature conditions, less branching will form on polymers (the amount of branching distinguished cold NBR from hot NBR).

Environmental Concerns

Acrylonitrile is considered an environmentally hazardous substance that cannot be freely ejected into the environment (it must be disposed through a rotary kiln, fluidized bed, liquid injection incineration, or underground injection). Because the compound is quite volatile and readily soluble in water, its release to the environment from waste sites is a concern. 1,3–butadiene is very reactive in the presence of hydroxyl radicals, and therefore possess high ozone creation potential. As mentioned previously, however, unreacted monomer recovery is nearly 100% in the process outlined above. Since unused monomers can easily be recycled, its disposal is not an unbearable concern. Thus, despite more stringent disposal restrictions, acrylonitrile butadiene rubber is produced throughout North America.

Misc

Because it is not crucial that every acrylonitrile butadiene polymer has one specific chemical structure (unlike polymers meant for ingestion, where small inconsistencies in chemical composition/structure have a pronounced effect on the body, the general properties of NBR are not altered by minor structural/compositional differences) quality control on its production is relatively lax. The production process itself is not overly complex; the polymerization, monomer recovery, and coagulation processes require some additives and equipment, but they are typical of the production of most rubbers. The necessary apparatus is simple and easy to obtain. For these reasons the substance is widely produced in poorer countries where labor is cheaper. Among the highest producers of NBR are Taiwan and China.