NBR is a family of unsaturated copolymers of 2-propenenitrile and various monomers of butadiene (1,2-butadiene and 1,3-butadiene). Choice among the various NBR polymers depends on the acrylonitrile (ACN) content which determines the main properties of these types of polymer: the higher the ACN content, the better the resistance to fuels and oils; the lower the content, the better the resistance to low temperatures. The main problem with this rubber is finding the optimal compromise between resistance to oil and fuel on one hand and elasticity and resistance to the cold on the other.
In addition to these properties, nitrile rubber demonstrates:
The most commonly used NBR polymers are those with a 33% to 38% ACN content.
HNBR is widely known for its physical strength and retention of properties after long-term exposure to heat, oils and chemicals. This type of rubber has a high tensile strength ranging from 21 to 31 Mpa.
HNBR can be used with a wide range of temperatures (-40°C to 165°C) with minimal degradation of the compound. To obtain HNBR able to resist low temperatures, it is necessary to select polymers with low ACN content; conversely, to obtain HNBR able to resist high temperatures, polymers with high ACN content must be chosen.
As elastomers, HNBR polymers have excellent resistance to fluids used in the automotive industry (motor oils, fuel, coolants etc.) and chemicals used in the industry.
An improved version of NBR and XNBR. In these polymers, carboxylic groups (R-COO-) are found on the double bond of the butadiene part, in addition to the sulfur bridges. These groups establish chemical bonds (ionic) with zinc (Zn2+), thereby improving the physical properties of the rubber. Normal types of this rubber contain about 7% of carboxyls; however types with 2% also exist.
XNBR is nitrile rubber that cross-links with all the vulcanization systems of normal NBR: in addition, the presence of -COOH groups makes it cross-link with metal oxides such as ZnO or Pb3O4.3O4.
These free groups are introduced in order to increase the degree of cross-linking of the vulcanization systems and improve abrasion resistance and elastic resilience.
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