Rubber compounding vulcanizing agents

Fig. 107.—Tensile strengths of natural rubber plotted against the degree of cross-linking with bis-azo vulcanizing agent (O), expressed as equivalent percent (pXlOO). Upper curve ( ) sample prepared using one equivalent percent of bis-azo compound plus monoreactive ethyl azodi-carboxylate for the total degrees of modification of the units indicated on the ordinate scale. (Flory, Rabjohn, and Shaffer.

The substances listed in Table 28 correspond to the basic structure of elastomeric closures. The other components in rubber formulations are curing or vulcanizing agents, accelerators, activators, antidegradants, plasticizers, fillers, and pigments. The most common additives used to compound rubber for the pharmaceutical industry are listed in Table 29. The amount of each component may vary from rubber to rubber, and, depending on the component, the amount can reach more than 50% of the total mass of a formulation. While accelerators are used in amounts of around 1%, fillers may make up more than 50% of the formulation mass. 

The basic compounding formulation specifies the minimum requirement of fillers, vulcanizing agents, and other substances that must be added to the rubber compound to achieve the desired properties. After the rubber, cure system and reinforcing filers have been selected it will be necessary to make several adjustments before all requirements are satisfied. It is generally sensible to start with the simplest mix formula for meeting the requirements. The recipe or the formula is usually written on the basis of hundred parts of rubber. For example if 5 parts of zinc oxide is to be added it is denoted as 5 phr (five parts per hundred rubber). Elementary compounding... 

The hypothesis has been advanced that during the vulcanization of natural rubber by means of polynitro compounds, a partial reduction of polynitro- to nitroso compounds takes place, the latter being known as vulcanizing agents. 

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