Nitrile rubber, also known as Buna-N, Perbunan, acrylonitrile butadiene rubber, and NBR, is
a synthetic rubber copolymer of acrylonitrile and butadiene. Trade names include Nipol,
Krynac and Europrene. Nitrile butadiene rubber is a family of unsaturated
copolymers of 2-propenenitrile and various butadiene monomers. Although its physical
and chemical properties vary depending on the polymer’s composition of nitrile, this
form of synthetic rubber is unusual in being generally resistant to oil, fuel, and other
chemicals. It is used in the automotive and aeronautical
industry to make fuel and oil handling hoses, seals, and grommets, since ordinary rubbers
cannot be used. It is used in the nuclear industry to make protective gloves. NBR's
ability to withstand a range of temperatures from −40 to 108 °C makes it an ideal material
for aeronautical applications. Nitrile butadiene is also used to create moulded goods, footwear,
adhesives, sealants, sponges, expanded foams, and floor mats.
Its resilience makes NBR a useful material for disposable lab, cleaning, and examination
gloves. Nitrile rubber is more resistant than natural rubber to oils and acids, but has
inferior strength and flexibility. Nitrile gloves are nonetheless three times more puncture-resistant
than natural rubber gloves. Nitrile rubber is generally resistant to aliphatic
hydrocarbons. Nitrile, like natural rubber, can be attacked by ozone, ketones, esters
and aldehydes.
Production
Emulsifier, 2-propenenitrile, various butadiene monomers, radical generating activators, and
a catalyst are added to polymerization vessels in the production of hot NBR. Water serves
as the reaction medium within the vessel. The tanks are heated to 30–40 °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. 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 → nitrile
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 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 nitrate, aluminium sulfate, and other coagulating agents in an aluminium 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 5–15 °C instead of 30–40 °C. Under lower temperature conditions,
less branching will form on polymers. Properties
The raw material is yellow in color, though it can be orange or red tinted, depending
on the manufacturer. Its elongation after fracture is ≥ 300% and possesses a tensile
strength of ≥ 10 N/mm². NBR has good resistance to mineral oils, vegetable oils, benzene/petrol,
ordinary diluted acids and alkalines. An important factor in the properties of NBR
is the ratio of acrylonitrile groups to butadiene groups in the polymer backbone, referred to
as the ACN content. The lower the ACN content, the lower the glass transition temperature;
however, the higher the ACN content, the better resistance the polymer will have to nonpolar
solvents as mentioned above. Most applications requiring both solvent resistance and low
temperature flexibility require an ACN content of 33%.
Applications
The uses of nitrile rubber include disposable non-latex gloves, automotive transmission
belts, hoses, O rings, gaskets, oil seals, V belts, synthetic leather, printer's roller,
and as cable jacketing; NBR latex can also be used in the preparation of adhesives and
as a pigment binder. Unlike polymers meant for ingestion, where
small inconsistencies in chemical composition/structure can have a pronounced effect on the body,
the general properties of NBR are not altered by minor structural/compositional differences.
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 relatively
cheap. Among the highest producers of NBR are mainland China and Taiwan.
A hydrogenated version of nitrile rubber, HNBR, also known as HSN is commonly used to
manufacture o-rings for automotive air-conditioning systems.
In January 2008 the European Commission imposed fines totaling €34,230,000 on the Bayer
and Zeon groups for fixing prices for nitrile butadiene rubber, in violation of the EU ban
on cartels and restrictive business practices. Criminals have also been known to wear these
gloves during the commission of their crimes. These gloves are often chosen because of their
tight, thin fit that allows the hands to remain dexterous. Ironically, because of the thinness
of these gloves, fingerprints may actually pass through the material as glove prints,
thus transferring the wearer's prints onto whatever surface is touched or handled.
Dangers Use of these gloves when working with nitric
acid or other strong oxidizers can be hazardous; the sample may spontaneously oxidize the gloves
on contact, resulting in thermal and chemical burns from both the heat of oxidation and
the caustic sample. HNBR
Hydrogenated Nitrile Butadiene Rubber is widely known for its physical strength and retention
of properties after long-term exposure to heat, oil, and chemicals. Trade names include:
Mill-Right ES, THERBAN, and ZETPOL. Depending on filler selection and loading,
HNBR compounds typically have tensile strengths of 20 – 31 MPa when measured at 23°C. Compounding
techniques allow for HNBR to be used over a broad temperature range, -40° to 165°C,
with minimal degradation over long periods of time. For low-temperature performance,
low ACN grades should be used; high-temperature performance can be obtained by using highly
saturated HNBR grades with white fillers. As a group, HNBR elastomers have excellent
resistance to common automotive fluids and many industrial chemicals. Like NBR, fluid
and chemical resistance improves as the ACN content is increased.
The unique properties and higher temperature rating attributed to HNBR when compared to
NBR has resulted in wide adoption of HNBR in automotive, industrial, and assorted, performance-demanding
applications. On a volume basis, the automotive market is the largest consumer, using HNBR
for a host of dynamic and static seals, hoses, and belts. HNBR has also been widely employed
in industrial sealing for oil field exploration and processing, as well as rolls for steel
and paper mills. XNBR
An improved version of Nitrile Butadiene Rubber is Carboxylated Nitrile Butadiene Rubber.
In this execution there are beside the sulfur bridges also carboxyl groups R-COO- on the
double bond of the butadiene part. These groups will make ionic cross links with zinc Zn++
to give improved physical properties as compared to a non-carboxylated Nitrile rubber. These
ionic crosslinks are formed along with sulfur links. The carboxyl groups which are needed
for these extra links are distributed randomly and are present at levels of 10% or less.
See also Medical gloves
Ozone cracking References
External links Manufacturing capabilities in Nitrile Rubber
MERL - Rubber Selection Guide Nitrile Rubber, AAA Acme Rubber Co
Risk Assessment of Butadiene, CAS No. 106-99-0, EINECS No.203-450-8, June 2001
Existing Substances Evaluation: Butadiene, The Green Lane, Environment Canada
Antitrust: Commission fines synthetic rubber producers € 34.2 million for price fixing
cartel, EUROPA Portal Quality Standards for Nitrile Gloves
Information & Resource Site on Nitrile Gloves http:www.klozure.com/ViewSolutionsPage?page=materials
http:www.iisrp.com03HNBR-11Dec2001.pdf