Processibility and Vulcanization Tests

Wise R.W, Processing and Vulcanization Tests, Rubber Technology by Mourice Morton, 1973, Van Nostrand Reinhold Company. 

In most cases the pigments used are also required to be migration-resistant. The suitability test is also carried out with 5 different pigment concentrations. To test the fastness to bleeding, the unvulcanized colorations are brought into defined contact with a white milled sheet of specific composition and vulcanized wet for 20 minutes in open steam at 140°C. During this process, half the coloration is often covered with a wet cotton cloth to determine whether the cloth, the rubber or both are stained by bleeding. 

Basic operations such as mastication, mixing, calendering and extrusion are similar for ebonite and soft rubber. The problems, especially in the compounding and vulcanization process which arise in the manufacture of ebonites, are quite different from those with soft rubbers and different tests are used for control of manufacture and for the assessment of the quality of the product. The outstanding differences between ebonites and soft rubbers are detailed below. 

The addition of a wide variety of compounding ingredients to facilitate processing, effect vulcanization, and modify or improve vulcanizate properties, calling for tests for... 

Most testing of rubber and rubber compounds is conducted to determine processing characteristics or to measure physical properties after vulcanization. Processability of a rubber compound is dependent on the compound s viscosity and elasticity. Generally, the physical properties of vulcanized rubber compounds are measured by static and dynamic mechanical tests designed to simulate the mechanical conditions of finished rubber products. Following are the test procedures and instruments most widely used to measure and evaluate processability and finished properties. 

Elastomeric systems are one of the most important classes of materials in industry because of their specific dynamic properties. Elastomers are typically crosslinked to a greater or less degree in order to improve their properties. The crosslinking process termed vulcanization prevents permanent deformation under load and ensures elastic recovery on removal of the load. The introduction of crosslinking, however, limits the ability to characterize those systems because of the insolubility of the resulting network. Additionally, the extremely low concentration of the chemically modified structures induced by the vulcanization challenge many of the traditional analytical techniques. Many approaches including physical tests and chemical analysis have... 

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

Measuring Vulcanization. The formation of a three-dimensional structure during vulcanization increases the stiffness (modulus) of the compound. Therefore, following the modulus increase versus cure time provides a continuous picture of the vulcanization process. Oscillating disk rheometers provide a useful method to do this (17). In this test, a preweighed sample of uncured mbber is placed into a preheated cavity containing a conical rotor. The cavity is closed and the rotor is set to oscillate within the mbber sample. As vulcanization proceeds, the compound s resistance to rotor movement increases and this resistance is followed as a function of time, thereby generating a continuous profile of cure behavior. These cure curves,... 

Plasticity test determines the deformation or flow of a rubber compound having little elastic recovery. It should be remembered that elasticity is present even in an un-vulcanized rubber and the energy will be stored in the compound during deformation and then released when the force is removed. Rubber before vulcanization is in between a plastic and an elastic state when warmed up it becomes more plastic and less elastic. From commercial points of view, the plasticity tests are of little significance, but are important because their results assist in determining the processibility of the material and are used frequently for process control purposes. 

The cooling period is of practical importance for the vulcanized compound, when this material has been extracted from the mold and let cool in the surrounding atmosphere. On the other hand, the effect of the assumptions made on the heating process in the mold will be tested when there is no curing agent in the rubber. The effect of these assumptions will be considered in Chapter 4. These calculafions could also be considered on rubbers perfectly cured, reheated in the mold after cooUng to room temperature. 

The presence of sulfur in polysulfides, polysulfones, and in sulfur-vulcanized rubber can be demonstrated by the following somewhat uncertain test. The sample is heated in dry air (pyrolysis) and the gases formed during this process are bubbled through a dilute barium chloride solution. The presence of sulfur is indicated by a white precipitate of barium sulfate. 

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