Nitrile-Butadiene rubber (Buna-N, NBR rubber) is a synthetic polymer, the product of radical copolymerization of butadiene with acrylonitrile (ACN) in the aqueous emulsion at 30 °C (high temperature) and at 5 °C (low temperature).
Low-temperature NBR has better technological properties than high-temperature ones, and their different types have good physical and mechanical properties. NBR molecules consist of statistically alternating units of butadiene and ACN:
– [- CH2-CH = CH-CH2-] n – [-CH2-CH (CN) -] m-
The content of 1,2-butadiene units does not exceed 10% and decreases with the increase in the amount of the ACN added.
There are NBR rubbers with low (17-20%), medium (27-30%), high (36-40%) and very high (50%) ACN amount.
The physical properties of NBR depend significantly on ACN amount. Nitrile butadiene rubbers readily dissolve in ketones, aromatic and chlorinated hydrocarbons, esters and very poorly in aliphatic hydrocarbons and alcohols. As the amount of the bounded ACN in the polymer increases, intermolecular interaction between the polymer chains and density increases, glass transition temperature increases, dielectric properties decrease, solubility in aromatic solvents decreases, and the swelling resistance in aliphatic hydrocarbons increases.
Depending on the control conditions of the polymerization process, NBR is produced with different properties:
- Very rigid (hard) – stiffness 21,5 -27,5 N or Mooney viscosity higher than 120;
- Rigid – stiffness 17,5-21,5 N or Mooney viscosity 90-120;
- Soft – stiffness 7,5-11,5 N or Mooney viscosity 50-70.
NBR processing is difficult due to the high rigidity caused by large intermolecular interaction. The processing of rubbers of different grades depends on their initial viscosity, as well as on the content of nitrile groups. For all hard-type rubbers, pre-plasticization is necessary, with the most effective mechanical plasticizing on rollers at a temperature of 30-40 °C.
Thermo-oxidative destruction of NBR is ineffective and does not find practical application. Significant difficulties arise in the manufacture of rubber compounds based on NBR in rubber mixers, because in this case, due to increased heat generation, high temperatures lead to increased rigidity of the compounds due to scorch or thermal rubber structuring.
Usually, multi-stage mixing modes are used with cooling and maturing of the masterbatches between the stages. NBR rubber compounds have little confections adhesiveness. Forming of NBR rubber compounds is difficult due to high rigidity and great elastic recovery.
Rubbers obtained by low-temperature polymerization have better technological properties compared to hot polymerization rubbers.
Nitrile-butadiene rubbers can be vulcanized by sulfur in the presence of sulfur vulcanization accelerators, as well as thiuram, organic peroxides, alkylphenol-formaldehyde resins, organochlorines. Vulcanization is carried out at temperatures of 140-190 °C. With the increase of ACN, the vulcanization rate increases.
Since NBR rubber does not crystallize during deformation, unfilled NBR rubber products have low strength characteristics and have no practical significance.
The rubber with the ingredients of a compound is mixed on mixing rolls at a roll temperature of 30-40 °C. The total mixing time is 41 minutes, including plasticizing the rubber for 15 minutes. Curing duration of rubber compounds mixed according to a standard recipe at 142 ± 1 ° C is 50-60 minutes.
The main properties of NBR rubber depend on ACN content. As the ACN content increases, strength properties, hardness, wear resistance, swelling resistance in aliphatic hydrocarbons and resistance to thermal aging increase. At the same time, elasticity and frost resistance are significantly reduced, heat formation increases with multiple deformations. NBR rubber is highly resistant to swelling in aliphatic hydrocarbons, fats, and vegetable oils, but strongly swells in polar, aromatic and chlorine-containing organic compounds.
One of the significant advantages of NBR rubber in comparison with NR rubber is their higher resistance to thermal aging, which is explained by the formation of oxidation intermediates during oxidation. NBR rubbers have good adhesion to brass-coated metal as NR rubbers. The strength of their fastening to aluminum and its alloys, steel, cast iron, brass, bronze, zinc, magnesium is higher than the strength of the rubber itself.
NBR can be used in combination with natural, isoprene, butadiene and butadiene-styrene rubbers, which are added to improve the technological properties of rubber compounds and increase the frost resistance of rubber. Combining them with ethylene-propylene and chloroprene rubbers improves ozone resistance and resistance to heat aging, and combination with thiokols, polyvinyl chloride, fluoroelastomers, and phenol-formaldehyde resins improves oil, petrol and ozone resistance.
The main area of application of NBR rubber is the production of various oil and petrol resistant rubber products – hoses, gaskets, seals, gasoline containers, various rolls, etc., used in automotive, oil, printing and other industries.
Rubbers are used for the manufacture of heat-resistant rubber products used in water, oils, solvents and certain other environments at temperatures up to 150 °C. NBR rubbers are also used in acid-and alkali-resisting rubber coatings, especially if they are simultaneously demanded non-polar hydrocarbon resistance.
Nitrile-butadiene rubbers in combination with acetylene black can be used for making conductive rubbers.
NBR used for making heat and oil resistant ebonites with good mechanical properties. NBR rubber compounds with adding of polyvinyl chloride are used for the production of fireproof and resistant to aggressive environments coatings.
Owing to their high resistance to oils and other aggressive agents, NBR has found wide application for the manufacture of various oil-and-petrol resistant rubber technical products – gaskets, sleeves, rings, cuffs, oil seals, OPR plates, gasoline containers.
Rubbers are used for the production of insulating and electrically conductive rubbers, heels and soles of shoes, glues and ebonites, protective coatings, resistant to aggressive environments.