Vulcanization enthalpy

Figure 7.32 Determination of vulcanization enthalpy of polyisoprene-sulfur mixture [3]...

It is possible to distinguish between SBR and butyl rubber (BR), NR and isoprene rubber (IR) in a vulcan-izate by enthalpy determination. In plastic-elastomer blends, the existence of high Tg and low Tg components eases the problems of experimental differentiation by different types of thermal methods. For a compatible blend, even though the component polymers have different Tg values, sometimes a single Tg is observed, which may be verified with the help of the following equation ... 

In the same vein, fig. 2.28 Illustrates the adsorption of long chain n-alkanes from n-heptane on different grades of graphite, to compare their surface properties. One of these adsorbents, Vulcan 3G, was a standard sample especially prepared under lUPAC auspices. The enthalpies were measured as displacement enthalpies in a flow calorimeter. All enthalpy and adsorption isotherms are S-shaped. In fig. 2.28a, right, is plotted as a function of for the... 

Flfure 2.28. Adsorption of long-chain alkanes from n-heptane on various grades of graphite, (a, left). Enthalpy isotherms for n-docosane (n-CjjH g) (a, right), correlation with the enthalpy of adsorption for a reference sample (b) adsorption Isotherms for four alkanes on Vulcan 3G. (Redrawn from H.E. Kem, G.H. Findenegg, J. Colloid Interface Set 75 (1980) 346.)... 

The good definition of these systems implies that the models of sec. 2.4 apply. Kern and Flndenegg ) fitted the experimental results for the adsorption of n-docosane from n-heptane on Vulcan 3G graphite at 25, 35 and 45°C with 12.4.34b] assuming 0 and r = 2.67. A good fit was obtained with a lateral Interaction parameter 2° of 1.16, 1.10 and 0.91 and equilibrium constants of 13.9, 10.6 and 8.2 for the three temperatures, respectively. The standard enthalpy of displacement of r moles of n-heptane by 1 mole of n-docosane was calculated from the temperature dependence of K. The value of -54.4 kJ mole obtained compared very well with the calorimetric result at monolayer coverage, -58.8 kJ mole L... 

The purpose o this work is to show that the values for temperatures o mold and storage bulb are important parameters in injection molding process o rubber. The study was performed by using two different thicknesses for rubber sheets and two values for the cure enthalpy. The problem was solved by applying an explicit numerical method with finite differences (6). Although rubber vulcanization consists of a complex series of reactions (8), the overall cure heat could be described by a first-order reaction with a single activation energy, as shown previously (6). 

Vulcanization Kinetics Studies. These studies were performed using a microcalorimeter (SETAJRAMrDSC111) working in isothermal conditions. About 100-150 mg of the compound encapsulated in the holder was introduced into the sensitive zone (1,6). The response of the DC is directly related to the rate of enthalpy change with the time. 

Several parameters of interest were considered in this work the one concerned with the operational conditions of the rubber in the storage bulb as time and temperature, the other with the working conditions in the mold as mold temperature, cure time, thickness of rubber sheets, cure enthalpy (or % vulcanizing agent). 

The nature of the vulcanizing agent and of the rubber (i.e., EPDM compounds with peroxide in this instance) is of prime importance to the kinetics of cure the activation energy and the preexponential factor play major roles, and the cure enthalpy is also of some importance in spite of its low value, whereas the order of the overall reaction is 1. Thus, because of the kinetics of cure, the time of cure varies considerably with the temperature, and thus there is a narrow temperature window through which meaningful data can be obtained. The cure enthalpy is rather low and thus does not intervene much in the process. Nevertheless, DSC experiments, which are based on measuring the enthalpy evolved, are also commonly used for determining the kinetics of cure [12]. 

The measurement of the degree of conversion by enthalpy measurement applies to elastomer cure also, since the vulcanization reaction is highly exothermic. Unlike for thermosets, Tg is not a measure of degree of cure for elastomers. Sircar discusses this and numerous other aspects of the characterization of elastomeric materials using thermal analysis in chapter 5 of Reference 5. 

FIG. 8 Cumulative enthalpy isotherms of displacement of water by V,V-dimethyldecyl-amine-V-oxide onto CPG silica glass (O) and Vulcan 3G graphitized carbon black ( ) at 298.15 K. 

Denoyel et al. (1993) proposed a method to assess the total area of microporous carbons by immersion calorimetry. It was based on the assumption of the existence of a direct relationship between the enthalpy of immersion and the total area of the solid accessible to the wetting molecules. They used a non-porous carbon black (Vulcan 3) as a reference material to obtain the area enthalpy of immersion, Afijmn, (J m ), of a carbonaceous surface into different liquids. In this way, and considering that the enthalpy of immersion is simply proportional to the surface area available to the immersion liquids, irrespective of its external or microporous character, and whatever the size and shape of the micropores, they obtained the surface areas available to the different liquids using Equation (4.11). 

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