The viscosities of eighteen tar products ranging from refined tar of 34° equiviscous temperature to electrode binder pitch of 102° softening point (Ring Ball) have been measured between wide temperature limits and within the viscosity range of 10 to 1010 centipoises. The applicability of three equation forms to these results has been assessed. A simple logarithmic relationship in terms of Fahrenheit temperature is satisfactory for most products at viscosities greater than 104 centipoises, but does not apply at lower viscosities or to the harder pitches. A double logarithmic viscosity/temperature relationship fits the results well, except those relating to the hard pitches, but it is insensitive and unreliable at viscosities of about 109 centipoises and greater. A third equation relating the logarithm of viscosity to the fifth power of absolute temperature has been shown to apply over the full range of materials, temperatures and viscosities examined.
The significance of the constants used in these equations is discussed with respect to their value in comparing the temperature susceptibility of tar products.
It is shown that the correct order of viscosity of a tar product, within the range of materials and viscosities investigated, can be predicted from its softening point or equiviscous temperature and its toluene‐insoluble content.
The rate of a photochemical reaction is generally defined as the decrease in the number of molecules of a particular species with time and can be expressed by ‒
dn
/
dt
=
γ
I
A
, where
γ
is the quantum efficiency and I
A
the intensity of the absorbed light in quanta per second. For thin layers of absorbing medium, i. e. layers in which the light intensity is not appreciably reduced, the value of
I
A
is given by Beer’s and Lambert’s laws,
I
A
=
α
clI
,
Discontinuities previously observed in the softening point and insoluble matter curves of tar‐bitumen systems have been investigated by carrying out a number of flow tests and viscosity measurements over a wide temperature range. The results provide evidence that stable mixtures comprising up to 60% bitumen are thixotropic at and below the softening temperature, due possibly to a marked association of certain tar components. With bitumen in greater excess the higher‐molecular‐weight hydrocarbons and oxygen‐containing compounds of the tar are pepsised by the bitumen. It is suggested that phase inversion occurs at a composition of about 50% by volume of each component.
Summary The properties of coal tar systems and the mechanical requirements of pipeline protections before and after installation of the pipelines are defined. Correct specification related to the particular service conditions and adequate inspection are of paramount importance. With correct selection of primer, grade of coal tar enamel and glass reinforcements coal tar based systems are providing the most reliable protection under almost all service conditions.
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