Zinc Oxide: This has been an important ingredient since the early days of rubber compounding. Originally used as an extender to reduce cost, it was subsequently found to have a reinforcing effect, and was later found to reduce vulcanization time.
Fatty Acid: in this experiment stearic acid fills this role.
3. Organic accelerator. .
Organic Accelerator: These all contain nitrogen and can perform as electron donors or acceptors. Here this role was filled by Dibenzothiazole disulfide (MBTS). Figure 1 shows the structure of MBTS. Note the sulfur and the nitrogen atoms present. .
Figure 2: Molecular structure of MBTS.
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The mechanisms appear to proceed in the following steps:.
1. The accelerator (Ac) reacts with sulfur (Sx) to give monomeric polysulfides of the type Ac-Sx-Ac, where Ac, is the organic radical derived from the accelerator.
2. The polysulfides can interact with the rubber to give polymeric polysulfides of the type rubber-Sx-Ac.
3. The rubber polysulfides then react, either directly or through a reactive intermediate, to give cross links or rubber polysulfides of the type rubber-Sx-rubber.
A typical rubber compound is a micro-composite with, on average, twelve components. It is these characteristics which make the modelling of both material behaviour and rubber processes such difficult tasks and hamper the diagnosis of processing problems.
In this experiment, natural rubber is cross linked by sulphur and sulphur-based curing agents which convert the pliable rubber gum into an elastic solid.
At first glance there seems to be not chemical difference between the structure of thermosetting rubbers and many plastics. However due to the cross linking process, rubbers retain their flexibility where as plastics become rigid.
According to the theory of rubber elasticity, the stress is related to the extension ratio as shown in Equation 1.
Equation 1.
Where £ = Strain.
= Concentration of network strands.