Nitrile butadiene rubber (BNK) is the main raw material for the production of various types of rubber with optimal durability. This is a synthetic polymer material obtained by copolymerization of butadiene with acrylonitrile (NAC). It may be called nitrile, divinyl nitrile, butadiene-acrylonitrile rubbers or butacryl. In the international designation, this material is marked NBR (nitrile-butadienerubber), in domestic - SKN (synthetic rubber nitrile).
Where applicable
This type of rubber is most often used in those industries where the optimal resistance of rubber products to chemically aggressive environments is important. Of great importance are the properties of nitrile butadiene rubber, such as high elasticity and slight residual deformation. This material is widely used in the manufacture of rubber elements that have direct contact with chemically active materials - it can be all kinds of gaskets, oil seals, rubber expansion joints, fuel and oil hoses, drive belts, fuel tanks in the auto, aviation and oil industries, printing offset plates and other products.
Products made on the basis of this rubber do not swell in oily liquids, antifreezes and water. Of some types of such material, a sheath of electrical wiring and rubber gloves are provided, which have special strength and wear resistance. It is used in the manufacture of various adhesives, sealants and foam. Rubber is the basis in the production of adhesives.
When and where did this rubber appear?
The production of nitrile butadiene rubber was recorded in 1934 in Germany. At that time, German scientists created a material unique in its properties and patented it under the name Buna-N. During World War II, new material became extremely popular in military industries.
Due to the shortage of natural raw materials, US senior management has launched a special program involving the active development of production of nitrile butadiene rubber and other types of synthetic raw materials for rubber goods. The material produced under this program was called GR-N. Today, BNK has become one of the most popular special-purpose rubbers. It is made in more than 20 countries.
BNK production
This type of material is obtained by structural polymerization in an aqueous emulsion. The process is carried out both at high and at low temperatures. The main monomers for their production are butadiene-1,3 and nitrile of acrylic acid (NAC), mixed in a certain proportion. These substances are temperature independent. Taking into account the laws of statistical copolymerization, it should be noted that this tandem of monomers should have the properties of an azeotropic composition containing about 40% acrylonitrile in a mixture of monomers.
In the production of this type of rubber, there is a need for more complete purification during the coagulation of emulsifiers used for polymerization. A small amount of ash of mineral and volatile impurities is allowed in manufactured rubbers (not more than 1%). They can be seasoned with tintable or non-tintable antioxidants.
What is BNK?
In our country, rubbers of such types as nitrile butadiene rubber-18 (SKN-18), nitrile butadiene rubber-26 (SKN-26) and nitrile butadiene rubber-40 (SKN-40) are manufactured. The numeric index in the grades shows the number of acrylonitrile units in the polymers. They may contain, respectively, 18, 26 or 40% nitrile of acrylic acid.
By varying the number of constituent ingredients, various properties of the resulting material can be achieved. Depending on the percentage of acrylonitrile, the properties of rubbers can vary in hardness, oil viscosity, and gas resistance. The percentage of NAC affects the intermolecular effect of structural units. It is this factor that affects the use of nitrile butadiene rubber in certain areas of the national economy. Nevertheless, it is used as raw material for the manufacture of a huge range of industrial rubber products.
Material flaws
Despite the fact that rubber products manufactured with the addition of BNK have a whole range of excellent indicators (high strength and ductility in tension, elongation, tensile and abrasion resistance, excellent oil and gas resistance), this material has some disadvantages.
The toughening of operating conditions associated with an increase in the speeds of mechanisms and a lack of cooling oil leads to the fact that rubber elements can only work at temperatures up to +150 degrees. When the operating temperature rises above this value, structuring and then destruction of the rubbers created on the basis of BNK occur. In other words, heated rubber becomes hard and brittle.
The impact of low temperatures also negatively affects rubber products in the production of which nitrile butadiene rubber was used. The optimum temperature for them is considered to be at least -35 Β° C.
Modern rubber modifications
To create rubber products with a unique set of properties, more modern modifications of rubbers are used. One of the promising developments in the modification are hydrogenated nitrile butadiene rubbers. They have excellent technological properties in various types of rubber production.
Rubber made on the basis of rubbers modified with polyvinyl chloride gives more stable indicators for weather wear resistance (up to -50 degrees) and extreme working temperature up to +160 degrees. It significantly surpasses products made on the basis of nitrile butadiene rubbers in tear resistance and wear resistance. It has excellent resistance to the active effects of chemically aggressive environments. However, this rubber is not so strong and flexible. Therefore, in order to improve the technological properties of the material, it is most often used in combination with conventional types of nitrile butadiene rubbers.
Vulcanization
The curing process of nitrile butadiene rubbers is carried out using sulfur, as well as thiuram, organic peroxides, alkyl phenol formaldehyde resins and organochlorine compounds. The temperature can vary from 140Λ to 190Λ Celsius. During this process, the appearance of a large plateau of vulcanization. The increased content of NAC contributes to an increase in the rate of vulcanization. The quality of the rubbers obtained is evaluated by the intrinsic characteristics of the vulcanizers.
The properties
The properties of BNK are determined by the content of acrylonitrile. This type of rubber is highly soluble in ketones, some hydrocarbon solutions and esters. Aliphatic hydrocarbons and alcohol have almost no effect on the dissolution of nitrile butadiene rubbers. An increase in the composition of the material of acrylonitrile contributes to the intermolecular effect between the polymer chains: the more NAC in the composition of the material, the higher the density and temperature regime of glass transition. The increased content of NAC reduces the dielectric properties, reduces the degree of solubility in aromatic solvents and contributes to an increase in resistance to swelling in aliphatic hydrocarbons.
Depending on the course of rubber polymerization, it can be produced with various plastoelastic properties. They may be:
- Very hard (Defoe hardness 21.5 - 27.5 N). When marking such rubber, the letter βTβ is added to its name.
- Solid (stiffness according to Defoe 17.5 - 21.5 N).
- Soft (Defoe hardness 7.5 - 11.5 N). When marking such rubber, the letter βMβ is added to its name.
For BNKs manufactured with the participation of alkyl sulfonates as emulsifiers, the letter βCβ is added to the marking. For example, SKN-26MS is a soft rubber in which 26% of the bound NAC is present, and a biodegradable alkyl sulfonate emulsifier was used in the preparation.