Improvement of cure characteristics and physical properties of EPDM by incorporating skim rubber

Abstract

Ethylene-propylene-diene rubber (EPDM) consisting of ethylene, propylene and an unsaturated diene is one of the synthetic rubbers for which the demand is rising fast. Owing to its non-polar and saturated backbone structure, EPDM exhibits good resistance to heat, oxidation and to polar solvents. They are good insulators for electricity as well.  Such properties have extended its application to rubber components used in automotive industry, building and construction industry and in wire and cable industries. 
It is reported that   modification of polyisoprene with a small amount of petroleum-processing products containing constituents similar to those of the albuminous fragments in NR will change the curing characteristics and physical properties of synthetic polyisoprene to the level of NR. Furthermore, skim rubber is used for increasing the rate of vulcanization of SBR due to the presence of higher amounts of non-rubber ingredients in them, which are capable of acting as a co-accelerator and an activator for vulcanization. 
This study clearly shows that ingredients such as decomposed products of proteins and natural fatty acids present in skim rubber function as co-activators for the vulcanization of EPDM. In this experiment the two accelerator systems namely tetramethylthiurandisulfide (TMTD)/2-mercaptobenzothiozole (MBT)  and TMTD/N-tert-butyl-2-benzothiazylsulfenamide (TBBS) were used to cure EPDM compounds   incorporated with varying amounts of skim rubber, ranging from 5 phr to 15 phr as an additive in the formulation.
The improvement of cure charateristics in the presence of skim rubber as an additive was studied using Oscillating Die Rheometer, in the presence of both accelerator systems. The significant improvement of optimum cure time was observed when the skim rubber quantity was gradually increased in the formulation. However scorch time reduced while increasing the quantity of skim rubber. Therefore, it can be concluded that the non-rubber ingredients present in skim rubber can act as co-activators for the vulcanization of EPDM. These co-activators function by a mechanism similar to the mechanism of action of stearic acid in conventional vulcanization systems containing zinc oxide.
Mechanical properties such as tensile strength, 100% and 300% modulus, hardness and rebound resilience were studied for the prepared compounds. It can be concluded that the optimum amount of skim rubber needed to be incorporated for the improvement of cure characteristics and mechanical properties of EPDM is around 5 phr. Results obtained further revealed that the optimized action of these co-activators present in skim rubber is achievable with the TMTD/TBBS accelerator system than with the TMTD/MBT accelerator system.