###### Abstract  
The rheometer curing curves of 50/50 blends of natural rubber (NR) and two different halogenated rubbers with a combination of conventional accelerated sulfur (CV) and 3 phr of a bismaleimide (MF\({}_{3}\)) at 170 \({}^{\circ}\)C indicates that a co-curing reaction has been taken place between NR and the halogenated rubbers via Diels-Alder reaction. To further confirm whether the co-curing reaction has taken place in the early stage of curing, a complex test methodology was applied with the help of a rubber process analyzer. In this test, the blends with CV and with CVMF\({}_{3}\) were subjected to cure at 170 \({}^{\circ}\)C for a predetermined time so that both the CV and CVMF\({}_{3}\) cured blends will have the same magnitude of curing torque. It is then cooled down to 40 \({}^{\circ}\)C and the storage modulus (G\({}^{\prime}\)) was evaluated as a function of strain from 0.5% to 100% at a constant frequency of 1 Hz. The results reveal that the blends cured with CVMF\({}_{3}\) exhibit a higher G\({}^{\prime}\) due to the enhanced network strength because of the formation of bismaleimide crosslinks than the same cured with only the CV system. The swelling resistance and the mechanical properties of the blends cured with CVMF\({}_{3}\) were significantly higher than those cured with only the CV system. rubber; curing; strain sweep; rheometer; rubber process analyzer +
Footnote †: journal:  
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0000-0001-8800-7800][PERSON]  
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0000-0001-8800-7800]R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R.R  
quantity of the compounding ingredients makes it a widely accepted production control instrument in the rubber industry [1; 2]. Nowadays, the rheometers are modified in such a way that they can be utilized to test the curing behavior as well as the viscoelastic properties on the same sample to be tested. RPA is one such version of a modified rheometer that can be used to test the curing behavior as well as the viscoelastic properties of the sample after curing. It uses a rotorless biconical die design. The lower die of RPA can oscillate from 0.05\({}^{\circ}\)of arc to 90\({}^{\circ}\) of arc at an oscillation frequency of 0.1 to 2000 cycles per minute (0.33 Hz to 33 Hz). The temperature can be programmed to change upward or downward between 40 \({}^{\circ}\)C and 230 \({}^{\circ}\)C. Because of the possibilities of applying a wide range of strains and frequencies to the test sample, RPA can be employed to evaluate the viscoelastic properties of the rubber compounds after the curing has been completed [2; 3; 4]  
It is well-known that natural rubber (NR) is a non-polar, highly unsaturated elastomer. Because of its non-polar nature, NR exhibits poor resistance to hydrocarbon solvents, oils and greases [5]. On contact with these substances, the NR-based compounds undergo failure due to swelling. Therefore, NR is not considered a material of choice for the development of oil seals or gaskets. Moreover, the unsaturated chemical structure of NR makes it vulnerable to weather elements such as oxidative ageing and ozone attack [6]. To overcome the above-mentioned limitations, NR is frequently blended with polar elastomers such as chloroprene rubber (CR) [7; 8; 9; 10] or relatively less polar and less unsaturated elastomers such as bromobutyl rubber (BIIR), etc., [11] and curing the same with proper curing agent. It is well-known that the most appropriate curing system for the NR is the accelerated-sulfur system. The accelerated sulfur system is a package which comprises sulfur, accelerator, activator (Znic oxide, ZnO) and a fatty acid (stearic acid). The accelerated sulfur system produces sulfidic crosslinks in the cured network. Generally, three types of sulfidic crosslinks such as monosulfide (C-S-C), disulfide (C-S-S-C), and polysulfide (C-Sx-C) have been identified in the cured network of NR after curing with the accelerator sulfur system.