Rubber compound yielding

Although direct analysis of rubber compounds yields simultaneously information about polymer and additives, a quantitative determination of the additives is difficult. This requires separation of these components from the polymer and fillers by means of extraction. Following the standard procedure at Akron Rubber (cfr Chp. 2.2 of ref. [3a]), Coz et al. [25] have examined four unknown cured rubber compounds (radial passenger tyre tread, radiator hose, oil pan seal and engine gasket). Tables 6.7 and 6.8 compare the reconstructed formulations and actual recipes for the tyre and radiator hose. 

Effect of Antirad Agents on the Scission Yield of a Natural Rubber Compound... 

Thermogravimetric analysis can yield a considerable amount of information about the composition of an elastomer system and is a common means of testing of EPDM rubber compounds [51]. The DTG curve may serve as an identifier of the type of elastomer in a compounded formulation. 

These feedstocks were tested in a number of carbon black furnace reactors employing different geometries and different operating conditions. The experiments were designed to develop data for correlation between feedstock composition, yield, production rate, and properties of carbon black produced. The physical properties of the carbon blacks were measured as were the properties of uncured rubber compounds and vulcanizates in standard test recipes. 

Extensive studies on different rubber compounds (see, for example, Table 1 in [105]) yield Ec 0.05 to 0.15 eV per filler-filler bond [105,106], i.e., typical values for physical (van der Waals like) bonds. Similar values were obtained within an approach which assumes a hypothetical analogy between the structure of a statistical carbon black network and that of a Gaussian elastomeric (unfilled) polymer network [107]. As in the Kraus approach, the carbon black network scission process is assumed to be thermally activated. 

Wan and Isayev (1996) examined a hybrid approach of control-volume finite-element and finite-difference modelling of injection moulding of rubber compounds. The effect of vulcanization on viscosity and yield stress during cavity filling is reported. On comparing two versions of the modified Cross viscosity models - with and without the effect of cure - the use of a viscosity model that accounts for the cure was found to improve the accuracy of the cavity-pressure-prediction models. When the modified Cross model was further extended to include the yield stress and was implemented in the simulation program a significant improvement in the prediction of cavity pressure was obtained in the case of low injection speed. 

Equistar Chemicals Flexathene TP 38KC01. A polypropylene-ethylene copolymer formulated for film applications requiring medium stiffness, excellent impact strength, and high heat resistance. Produced in a proprietary gas-phase process, TP 38KC01 yields a product with greater physical properties and more uniform rubber dispersion than melt-blended rubber compounds. 

Generally it was found that 0.15 volume fraction small particles seem to be required to produce yield values. The yield value Y and viscosity level increase with both increasing particle loading (Figs. 5[a], [b]) and decreasing particle size (Fig. 5[c]). Note Y is largest for the ISAF compounds and lower for the FEE It is lowest for the SRF compounds. This is the same order as the particle size (ISAF has the smallest particles). Larger yield and viscosities are also found for calcium carbonate compounds [S25], For rubber compounds the yield value is of order 50 to 100 kPa and the power law exponent n about 0.2. In Table II, we present values Y,A,B,K, and n of Eqs. (56) and (57). 

The most recent study of this type, that of Montes and White [M35], compares its predictions with the rheological behavior of rubber compounds and we cite it here. Montes and White write the yield function [compare Eq. (49c)]... 

Rubber compounds have yield values, so the actual behavior in screw extruders must be more complex than indicated by the preceding analysis. If the rubber compound is a plastic material, then Eqs. (119a) and (119b) may be replaced by... 

It has long been reported in literature [18, 19] that (carbon black) filled compounds are yield stress materials, i.e., when plotted versus the shear stress, the shear viscosity appears bounded by a critical shear stress Oc so that below it, no flow occurs (in other words, the viscosity goes to infinity as the shear stress decreases towards Oc). The right graph in Fig. 4 shows indeed that the shear viscosity q(o) increases, as the shear stress decreases, but one would hardly derive a bounding critical shear stress from such data. In other terms, that filled rubber compounds are essentially nonlinear viscoelastic materials is experimentally well demonstrated but that they are yield stress materials might be considered as a controversial subject. 

Polymer composites have attracted a great deal of interest in recent years. In most cases, fillers are used as additives for improAdng the mechanical behavior of the host polymeric matrix. The reinforcement of elastomers by mineral fillers is essential to the rubber industry, because it yields an improvement in the service life of rubber compounds. The state of filler dispersion and orientation in the matrix, the size and aspect ratio of the particles as well as the interfacial interactions between the organic and inorganic phases haw been shown to be crucial parameters in the extent of property improvement [1,2]. 

Continuum mechanics theories of various types have been developed through the years to represent the flow of suspensions. The earliest efforts of this type were one-dimensional, but from the 1940s there have been three-dimensional efforts, which we will emphasize here. Scott [11] in a 1931 study, one of the earliest studies of the rheology of rubber compounds for these materials, suggested combining a yield value, Y, and a power law... 

The viscosity levels of silica-filled compounds are higher at the same volume loading and particle size [102] than carbon black. The viscosity levels are also higher than larger polar particles, such as calcium carbonate, talc, and zinc oxide [102]. Sihca-rubber compounds appear to exhibit yield values, higher than other particulates at the same volume fraction. This seems associated with small particle size coupled with strong particle-particle interaction. 

Such a behavior might also exist with filled high molecular weight polymer systems, for instance rubber compounds, but it has so far never been observed (to the author s knowledge). Consequently, even if the yield stress exists only because the shear rate/shear stress range of observation does not allow the above behavior to be observed, simple yield-stress-containing equations for viscosity properties are useful, at least over a limited range of stress/rate. 

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