Butadiene nitrile rubber: properties, production, application

Butadiene-nitrile rubber (BNK) is the main kind of raw material for the production of various types of rubber with optimum durability. This is a synthetic polymer material obtained by copolymerization of butadiene with acrylonitrile (NAC). It can be called nitrile, divinyl nitrile, butadiene-acrylonitrile rubbers or butacryl. In the international designation, this material is marked NBR (nitrile-butadienerubber), in the domestic design - SKN (synthetic nitrile rubber).

Where applicable

This type of rubber is most often used in those industries where the optimum resistance of rubber products to chemically aggressive environments is important. Of great importance are such properties of butadiene-nitrile rubber, as high elasticity and a small residual deformation. This material is widely used in the production of rubber elements that have direct contact with chemically active materials - it can be all kinds of seals, gaskets, rubber expansion joints, fuel and oil hoses, drive belts, auto fuel tanks, aviation and oil industries, printing offset plates and Other products.

Products made on the basis of this rubber, do not swell in oily liquids, antifreeze and water. Some types of such material are made of electrical wiring and rubber gloves, which have special strength and durability. It is used in the production of various adhesives, sealants and assembly foam. Rubber is the basis in the production of adhesives.

When and where did this rubber appear?

The production of butadiene-nitrile rubber was recorded in 1934 in Germany. At that time, German scientists created a unique material and patented it under the name Buna-N. During the Second World War, new material became extremely popular in the military industries.

Due to the lack of natural raw materials, the US top leadership launched a special program, involving the active development of production of butadiene-nitrile rubber and other synthetic raw materials for rubber goods. The material produced under this program was called GR-N. Today the BNK has become one of the most demanded rubbers for special purposes. It is manufactured 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 low temperatures. The main monomers for their production are butadiene-1,3 and acrylonitrile (NAC), mixed in a certain proportion. These substances do not depend on temperature. 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 when coagulating emulsifiers used for polymerization. In the produced rubbers, an insignificant amount of ash of mineral and volatile impurities (not more than 1%) is allowed. They can be filled with colorable or non-colorable antioxidants.

What is BNC?

In our country, rubbers of such types as butadiene-nitrile rubber-18 (SKN-18), butadiene-nitrile rubber-26 (SKN-26) and butadiene nitrile rubber-40 (SKN-40) are produced. The numeral index in grades shows the number of acrylonitrile units in polymers. They can contain, respectively, 18, 26 or 40% nitrile acrylic acid.

Changing the number of constituent ingredients, you can achieve different properties of the resulting material. Depending on the percentage of acrylonitrile, the properties of rubbers can vary in hardness, oil viscosity, and petrol resistance. The percentage of NAC influences the intermolecular effect of structural units. It is this factor that affects the use of butadiene nitrile rubber in certain areas of the national economy. Nevertheless, it is used as raw material for manufacturing a huge range of industrial rubber products.

Disadvantages of the material

Despite the fact that rubber products manufactured with the addition of BNK have a whole complex of excellent parameters (high strength and ductility in stretching, elongation, resistance to tearing and abrasion, excellent oil and petrol resistance), this material has some drawbacks.

The toughening of the operating conditions, connected with the increase in the speed of the mechanisms and the lack of cooling oil, leads to the fact that the rubber elements can only work at temperatures up to +150 degrees. As the operating temperature rises above this value, structuring and then destruction of rubbers created on the basis of the BNC occurs. In other words, the heated rubber becomes hard and brittle.

The influence of low temperatures also has a negative effect on rubber products, in the production of which butadiene-nitrile rubber was used. Optimum for them is considered working temperature not below -35 ° C.

Modern modifications of rubbers

To create rubber products with a unique complex of properties, more modern modifications of rubbers are used. One of the promising developments in the modification are hydrogenated butadiene-nitrile rubbers. They have excellent technological properties for various types of rubber production.

Rubber, made on the basis of rubber-modified rubbers, gives more stable indicators for weather resistance (up to -50 degrees) and extreme operating temperatures up to +160 degrees. It is much superior to products made on the basis of butadiene-nitrile rubbers, resistance to tearing and wear resistance. It has excellent resistance to the active effects of chemically aggressive media. However, this rubber is not so strong and elastic. Therefore, in order to improve the technological properties of the material, it is most often used in combination with conventional types of butadiene-nitrile rubbers.

Curing

The process of vulcanization of butadiene-nitrile rubbers is carried out with sulfur, as well as thiuram, organic peroxides, alkylphenol-formaldehyde resins and organochlorine compounds. The temperature can vary from 140˚ to 190˚ Celsius. When this process is carried out, a large vulcanization plateau appears. The increased content of NAC promotes an increase in the vulcanization rate. The quality of the rubbers obtained is evaluated by the inherent characteristics of vulcanizers.

Properties

The properties of BNCs are determined by the content of acrylonitrile. This type of rubbers is readily soluble in ketones, certain hydrocarbon solutions and esters. Aliphatic hydrocarbons and alcohol have virtually no effect on the dissolution of butadiene-nitrile rubbers. The increase in the composition of the material acrylonitrile promotes intermolecular action between polymer chains: the more NAK in the composition of the material, the higher the density and the temperature regime of vitrification increase. The increased content of NAC reduces the dielectric properties, reduces the degree of solubility in aromatic solvents and promotes an increase in swelling resistance in aliphatic hydrocarbons.

Depending on the flow of polymerization of rubber, it can be produced with different plastoelastic characteristics. They may be:

• Very hard (stiffness according to Defo 21.5 - 27.5 N). When marking such a rubber, the letter "T" is added to its name.
• Hard (stiffness according to Defo 17.5 - 21.5 N).
• Soft (stiffness according to Defo 7.5 - 11.5 N). When labeling such a rubber, the letter "M" is added to its name.

For BNK, manufactured with the participation of alkyl sulfonates as emulsifiers, the letter "C" is added to the marking. For example, SKN-26MS is a soft rubber in which 26% of the bound NAK is present and a biodegradable alkyl sulfonate emulsifier was used in the preparation.