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7.3 Coal tar pitch
7.3.1 Introduction
The residue of distillation of coal tar is pitch. It can be the primary product in tar processing in which case the conditions of distillation including the maximum temperature can be controlled to give a pitch of desired properties. Pitches vary in nature from semi-solid at room temperature to hard and brittle.
Most start to melt at temperatures in the region of 100oC and their rheology is of some importance.
7.3.2 Viscosities
Clearly solid and liquid will often be in co-existence and the viscosity is a factor in any application.
This can be determined from the classical [6] penetrometer test. In the test the penetration of a needle into a 100 g sample of the subject pitch at 25oC is determined, contact with the needle have been of five seconds’ duration. From the results the dynamic viscosity can be calculated according to:
µ(poise) = 5.9 × 109/(penetration depth)1.93 where the penetration depth is in units decimillimetres:
1 decimillimetre = 10-4 m.
The penetration depth across a range of pitch samples was found in [6] to range from 1.0 to 195 decimillimetres. These figures are examined in the boxed area below.
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Considering first of all the penetration depth value at the high end of the range:
µ = 5.9 × 109/(195)1.93 poise = 2.2 × 105 poise Now 1 poise = 1 g cm-1 s-1 ≡ 0.1 kg m-1s-1, hence µ
= 2.2 × 104 kg m-1s-1.
Now the density of a coal pitch is roughly the same as that of water, so the kinematic viscosity can be estimated as:
υ = 2.2 × 104 kg m-1s-1/1000 kg m-3 = 22 m2s-1 or to one significant figure 2 × 107 cSt.
and this exceeds any value of a kinematic viscosity previously given in this book. For the penetration depth value of 1 decimillimetre the kinematic viscosity is 6 × 1011 cSt.
That values of the viscosity of coal tar pitch are exceedingly high is confirmed in much more recent work than that in [6]. Li and Li [7] present results for dynamic viscosities of pitch, including one of 16000 kg m-1s-1 at 200oC. Using the same density as previously, this gives for the kinematic viscosity:
υ = 16000 kg m-1s-1/1000 kg m-3 = 16 m2s-1≡ 1.6 × 107 cSt
This corresponds quite closely to the value at a much lower temperature for the pitch in [6] having given a penetration depth of 195 decimillimetre.
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Viscosities of fluids can be determined from the rate of exit of a sample fluid under gravity through an orifice. Time of the order of decades is required for a value of the viscosity of a coal tar pitch to be obtained in this way. At the University of Queensland in 1930 such an experiment was commenced [8], and it is still in progress! The first drop exited in December 1938 and seven further drops between then and November 2000. Fitting of the results so far has given a value for the dynamic viscosity:
µ = 2 × 108 kg m-1K-1
⇓ υ = 2 × 1011 cSt
which is of the same order of magnitude as one of the values in [6].
7.3.3 Vapour pressure
When it was first proposed to construct ‘dust-free roads’ by using tar instead of the inorganic materials used previously there was concern that the vapour from the tar would harm vegetation. Such fears were later shown to be without foundation provided that tars selected for such use did not in pre-use testing start to boil below 140oC [9], and there was the obvious health advantage that persons were no longer inhaling fine dust from the roads or experiencing eye irritation from it.
Reference [10] gives a generic value for the vapour pressure of coal tar pitch of < 0.01 kPa. In the boxed area below is an estimate of what the odour threshold of the materials in the pitch vapour would have to be if the vapour were to be detectable by the sense of smell.
Considering air at room temperature and 1 bar pressure:
PaV = naRT
where subscript ‘a’ denotes air. For the vapour, using subscript ‘v’:
PvV = nvRT
Combining the two: nv/na = Pv/Pa = (0.01/100) = 10-4 or 100 p.p.m.
Many pure organic substances have odour thresholds much lower than this; that for benzene is 12 p.p.m.
One would therefore expect that a coal tar pitch having the vapour pressure in the inequality in the previous paragraph would be detectable by the olfactory sense, and that inhalation in an unventilated area would be harmful. The vapour pressure for newly manufactured pitch will of course drop as the pitch loses its lightest constituents on standing.
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