SEMI-EFFICIENT

Benzothiazole sulphenamide accelerators are suitable for semi-efficient and efficient vulcanisation systems. 

The chemical microstructures of cis-polyisoprene (HR) vulcanised with sulfur and N-t-butyl-2-benzothiazole sulfenamide (TBBS) accelerator were studied as a function of extent of cure and accelerator to sulfur ratio in the formulations by solid-state 13C NMR spectroscopy at 75.5 MHz [29]. Conventional (TBBS/Sulfur=0.75/2.38), semi-efficient (SEV=1.50/1.50) and efficient (EV=3.00/1.08) vulcanisation formulations were prepared, which were cured to different cure states according to the magnitude of increase in rheometer torque. The order and types of the sulfurisation products formed are constant in all the formulation systems with different accelerator to sulfur ratios. However, the amount of sulfurisation has been found to vary directly with the concentration of elemental sulfur. 

As sulfur content increased, the change in sulfur bond ratios exhibit a transition from a semi-efficient , or lower sulfur/accelerator ratio, to a conventional ,or higher sulfur/accelerator ratio, cure system (4). Overall, the... 

As discussed, pressure to move new medicines to the market faster has compressed many aspects of the research and development cycle. Process research initiates their search for scalable routes early in the discovery programs in the hope of having an efficient or semi-efficient route to prepare up to 100kg of API as a program nears the final preclinical stages and prepares for phase I studies in humans [79],... 

In fact, the physical properties obtained depend on the types of cross-hnk formed and the extent of main-chain modification by side reactions. This is usually being largely determined by the vulcanization system, although cure time and temperature also have an important effect. Generally, there are three types of typical sulfur vulcanization systems, namely, conventional vulcanization, efficient vulcanization and semi-efficient vulcanization. 

For semi-efficient vulcanization systems, intermediate sulfur level of 1-2 phr and 2.5-1 phr of accelerator are often used. The vulcanizates have physical properties intermediate between those of conventional vulcanization and efficient vul-canizationvulcanizates. hi fact, they give some improvements in reversion, ageing resistance and compression set compared with conventional vulcanizationvulca-nizates, but resistance to fatigue and low temperature crystallization is impaired. However, they have higher scorch safely, particularly when sulfenamide accelerators are used in the system. 

As a result of these reactions, the vulcanizate will exhibit reduced resistance to compression set and creep. It may be noted that although vulcanizates with predominantly monosulphide cross-links show better heat resistance, they may not have the excellent overall physical properties of predominantly polysulphide cross-linked networks. For this reason, a compromise is usually aimed for in practice and so-called semi-efficient vulcanizing systems are used. 

There are basically six types of crosslinking mechanisms for NBR elastomers, very high sulfur for a special ebonite, normal sulfur, semi-EV (semi-efficient vulcanization or low sulfur systems), EV (efficient vulcanization with sulfur donors alone), peroxide and zinc oxide/peroxide for carboxylated nitrile. The innumerable combinations that can and are used are as numerous as there are rubber chemists in the world, but with care they all work in any given application. 

Krejsa and Koenig studied the behavior of the sulfur crosslinked carbons using A(-r-butyl-2-benzothiazole (TBBS) with conventional, semi-efficient, and efficient formulations [67], C NMR was used to follow the nature of the crosslinked carbon structures with increasing cure time. At low cure states, polysulfide crosslinked peaks at 38 and 51 ppm appeared. As these resonances decreased in intensity, new resonances appeared at 44,57,64,17, 14, and 12 ppm. The majority of the later peaks are... 

Conventional Vulcanisation, Semi-Efficient Vulcanisation and Efficient Vulcanisation... 

NR (ISNR 5 grade) was modified with 10 phr each of phosphorylated cashew nut shell liquid(PCNSL) prepolymer and an aromatic oil plasticiser (spindle oil) in a typical semi-efficient vulcanisation system. Despite the lower chemical crosslink density, the PCNSL modified NR vulcanisates showed higher TS, EB, thermal stability and resistances to fatigue failure and thermooxidative decomposition, as compared with the vulcanisate containing the same dosage of spindle. 12 refs. 

CURVE EQUATIONS FOR DESCRIBING FATIGUE LIFE OF NR/BR BLENDS CURED WITH CONVENTIONAL, SEMI-EFFICIENT OR EFFICIENT CURING SYSTEM... 

Optimised NR bushing compounds were developed using semi-efficient cure systems in which N-tert-butyl-2-benzothiazole was used as the primary accelerator, zinc dibutyldithiophosphate as the secondary accelerator, N-cyclohexyl thiophthalimide as retarder, andhexamethylene bisthiosulphate disodium dihydrate as a stabilising hybrid crosslinker. These cure systems gave compounds with improved stability of mechanical and dynamic mechanical properties and shortened mould cycle times. 

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