Acceleration modulus

The first term again represents drag in steady motion at the instantaneous velocity, with Cd an empirical function of Re as in Chapter 5. The other terms represent contributions from added mass and history, with empirical coefficients, Aa and Ah, to account for differences from creeping flow. From measurements of the drag on a sphere executing simple harmonic motion in a liquid, Aa and Ah appeared to depend only on the acceleration modulus according to ... 

Morton number, =gii Ap/p a acceleration modulus, = d/U ) dU /dt) displacement modulus, =xjd Mach number, = characteristie velocity/c... 

The optimum modulus occurs at about a 2 1 weight ratio of OTOS to OBTS. Similar optimums have been observed with other accelerator combinations. The examples shown in Figure 4 are calculated from regression equations developed from designed experiments in a black-filled natural mbber compound. On a molar basis, the synergistic accelerator complex appears to consist of two dithiocarbamate ligands and one mercaptobenzothiazole moiety, as shown in stmcture (15) (14). 

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

The ash content of furnace blacks is normally a few tenths of a percent but in some products may be as high as one percent. The chief sources of ash are the water used to quench the hot black from the reactors during manufacture and for wet pelletizing the black. The hardness of the water, and the amount used determines the ash content of the products. The ash consists principally of the salts and oxides of calcium, magnesium, and sodium and accounts for the basic pH (8—10) commonly found in furnace blacks. In some products potassium, in small amounts, is present in the ash content. Potassium salts are used in most carbon black manufacture to control stmcture and mbber vulcanizate modulus (22). The basic mineral salts and oxides have a slight accelerating effect on the vulcanization reaction in mbber. 

These cures, characterized by their abiHty to proceed at low temperatures, are accelerated by moisture and develop high modulus. 

Accelerators may be added to improve the physical properties of the polymer when needed. Where high modulus or low oil swell is required, thiocarbanihde is the preferred accelerator, with a cure time of 60 min at 100°C. Tetraethyl thiuram disulfide and sodium dibutyl dithiocarbamate are preferred for high tensile strength and cured at 121°C. 

The difference in degree of cure of the blends by different curatives has also been explained on the basis of changes in curative distribution with accelerator types and the effect of cure temperature. The tensile properties of the blend cured by S/ZDEC at 170°C were significantly lower and modulus was higher than those cured by S/MET and S/DPG as shown in Table 11.17. Lowering of cure temperature by 20°C significantly improved these properties. However, the standard deviation in the results increased limiting the potential for any solid conclusion. 

Functionally, accelerators are classified as primary or secondary. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. Secondary accelerators, on the other hand, are usually scorchy and provide very fast cure. There are a wide variety of accelerators available to the compounder including accelerator blends this number well over 100. In order to rationalize the extensive range of materials it is useful to classify them in terms of their generic chemical structure listed below and shown in Figure 14.4. 

The comparisons between accelerator classes are shown in Figure 14.5. Figures 14.6 and 14.7 show comparisons of primary accelerators CBS, TBBS, MBS, and MBTS in NR and SBR, respectively. Major differences in scorch safety, cure rate, and modulus development are observed. 

In the transition zone, EHL is still important, but as more water is removed, EHL at the microscale (MEHL) becomes more important, and when the water layer is reduced to molecular levels, another mechanism, BL takes over. Since BL is the main mechanism by which friction is generated in the overall skidding process, any material properties which increase the proportion of BL in the transition zone relative to EHL, i.e., accelerate the transition from EHL to BL, will have an impact on overall skid performance. As discussed above, modulus is an important factor in determining the rate of water removal in EHL. Eor MEHL, it is the modulus on the microscale at the worn surface of the tread that is critical. There is evidence that after a certain amount of normal wear, a significant part of the surface of silica-filled compounds is bare silica, whereas in black-filled compounds, the surface is fully covered by rubber.The difference in modulus between rubber and silica is very large, so even if only part of the worn surface is bare silica, it would make a significant impact on the... 

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