Heat resistance of NBR

Although, the heat resistance of NBR is directly related to the increase in acrylonitrile content (ACN) of the elastomer, the presence of double bond in the polymer backbone makes it susceptible to heat, ozone, and light. Therefore, several strategies have been adopted to modify the nitrile rubber by physical and chemical methods in order to improve its properties and degradation behavior. The physical modification involves the mechanical blending of NBR with other polymers or chemical ingredients to achieve the desired set of properties. The chemical modifications, on the other hand, include chemical reactions, which impart structural changes in the polymer chain. 

The heat resistance of NBR can further be enhanced hy the use of low sulfur or sulfur donor curing systems. Plasticizers of low volatihty should he chosen to produce heat stable compounds. 

The opportunities for improving the heat resistance of NBR vulcanisates through the use of improved polymers and optimised compounds have been described in Sections 4.6 and 6.3. The progress in this field has been sufficient to ensure that NBR will retain many automotive and general engineering applications in which improved heat resistance is necessary. 

Hydrogenated nitrile rubber (HNBR) can be blended with NR to improve its oil and ageing resistance without drastically alfecting the dynamic properties. HNBR is widely used to make vibration dampers, timing belts, power transmission belts and bearings because of its ideal balance of properties like excellent heat and oil resistance coupled with good mechanical properties. It combines the oil and fuel resistance of NBR with the heat and oxidation resistance of EPDM rubber. The polarity dilference between the polymers could be reduced by the incorporation of small quantities of dichlorocarbene modified NR (DCNR), which is formed during the alkaline hydrolysis of chloroform in the presence of NR. Some studies in this respect have been reported in the literature. ... 

The oil, fuel, and heat resistance of nitrile rubber, or NBR, have made this elastomer very important to the automotive non-tire and industrial rubber business. NBR is considered to be the major oil, fuel, and heat resistant elastomer in the world. 

While HNBR and NBR can be compounded with many of the same plasticizers and softeners, care must be taken with HNBR to assure that the plasticizer used is not so volatile that it detracts from the overall heat resistance of the finished compound. Common plasticizers used at a 20 phr level are summarized in Table 3.10 [15]. While plasticizers such as Dioctyl phthalate and dibutoxy ethoxy ethyl adipate are effective in reducing hardness and viscosity, they are also volatile at high temperatures and can give finished part shrinkage in prolonged heat aging. For an overall balance of low-temperature flexibility improvement, viscosity reduction, and stabUity to heat aging, the trioctyl trimellitate and the triisononyl trimellitate plasticizers are excellent choices. 

Acrylonitrile-butadiene rubber (also called nitrile or nitrile butadiene rubber) was commercially available in 1936 under the name Buna-N. It was obtained by emulsion polymerization of acrylonitrile and butadiene. During World War II, NBR was used to replace natural rubber. After World War II, NBR was still used due to its excellent properties, such as high oil and plasticizer resistance, excellent heat resistance, good adhesion to metallic substrates, and good compatibility with several compounding ingredients. 

Based on these considerations, Croft prepared six formulations containing various combinations of NBR and NBR/PVC with CR and SBR and measured their oil, heat and ozone resistance, physical properties, and adhesion characteristics. Whereas the physicals are satisfactory for aU compounds, formulations based on NBR, NBR/PVC with CR performed better on heat and oil aging than the compounds containing SBR as shown in Tables 11.6 and 11.7. However, the adhesion is better with the latter compounds. It has been suggested that cuprous sulfide formed on the wire surface interacts with the double bond in SBR to provide the improvement in adhesion. 

Nitrile rubber, also known as nitrile-butadiene rubber (NBR), is a copolymer of acrylonitrile and butadiene (Fig. 1). As a base polymer for Rubber-based adhesives, it provides a number of specialized properties, which supplement those summarized in the article Rubber-based adhesives typical characteristics. NBR adhesives comprise a range of materials that may differ in proportion of comonomer or may be compounded with other resins. NBR adhesives are characterized by high oil and plasticizer resistance, excellent heat resistance and high adhesion to metallic substrates. 

The physical and mechanical properties of the nitrile rubbers are very similar to those of natural rubber. Buna-N does not have exceptional heat resistance. It has a maximum operating temperature of 200°F/93°C and has a tendency to harden at elevated temperatures. The nitrile rubbers will support combustion and burn. NBR has good abrasion resistance and tensile strength. 

NBR is commonly selected where good oil resistance is needed but high-temperature service conditions are not required. If high protection against heat is not needed, then the price of NBR is usually quite reasonable. 

A sulfur cure is used in many applications where heat resistance and low compression set are not major factors. Sulfur cures are also best for dynamic applications at moderate temperatures. An important thing to remember with sulfur in NBR is that it is more difficult to disperse so it should be added at the beginning of the mixing... 

Elastomers containing polyepichlorohydrin, also known as ECO, CO, or GECO according to ASTM, offer an excellent balance of properties, combining certain desired dynamic properties of namral mbber (NR), with much of the fuel, oil, and chemical resistance of other specialty elastomers such as nitrile (NBR), polyacrylate (ACM), and neoprene (CR) mbbers. The combination of the basic properties of oil, fuel, heat, low-temperature flexibility, and ozone resistance imparted by the saturated main chain and the chlorine groups, coupled with low permeability, makes polyepichlorohydrin a very useful elastomer for automotive applications. Specific applications include fuel hoses, emission tubing, air ducts, seals, and diaphragms. 

The decrease in modulus at high temperatures noted previously for Nitrile compounds shown in Table 16A.1 is to be expected. As exposure at elevated temperatures of NBR elastomers progresses with time, the modulus then increases and the part will become stiff and brittle. Dumbell test pieces for this test were preheated 10 min inside the environmental chamber before testing. The HNBR compound on the other hand is much more heat resistant and did not show as great an effect of high-temperature exposure. 

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