Rubber compounds were prepared and examined with respect to their modulus and hardness to see if there was any link between these two properties and cure time, cross linking agent concentration and filler concentration.
Aim:.
The object of the experiment is to prepare rubber samples and measure their physical properties. By varying cure time, concentration of curing agent and filler content, rubbers of widely different stiffness can be obtained. The hardness and tensile properties of the samples are to be measured and related to the polymer structure.
Introduction and Theory:.
Elastomers (rubbers) form an important class of materials and have uses ranging from large scale applications such as car tyres to specialty, high performance medical tubing. Elastomers are polymers which have been cross linked into a three dimensional network structure but in which the polymer strands are still mobile. .
Vulcanization, the process of cross linking elastomers, occurs by a chemical process initiated through some form of energy input. It occurs between two statistically favorable reactive sites. In practical vulcanization, the most widely used group of elastomers consists of those containing a diene site for cross linking, for example in this experiment cis-polyisoprene (natural rubber) was used. The structure of natural rubber is shown in Figure 1.
Figure 1: The structure of the monomer which polymerizes to form natural rubber.
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Natural rubber is cross linked by using sulfur as a vulcanizing agent. The basic ingredients for a sulfur cure are: .
1. Sulfur or sulfur donor (cross linker).
Sulfur: The original and still the most widely used curative, because of its versatility and cost, sulfur exists in the elemental state as an eight-membered ring (S8). The sulfur ring opening mechanism involves either a free radical or an ionic mechanism. The ionic mechanism is probably more logical.
2. Zinc oxide and fatty acid (activator).