Rheological Behaviour and Characterisation of Pitch-Based Carbon Precursors
Pitch material is an important precursor in the production of carbon bre, carbon composites and synthetic graphite. It has a complex transformation during pyrolysis which incorporates the separation of a liquid crystalline phase, known as mesophase. This thesis attempts to enhance the understand...
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| Language: | en |
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2015
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| Online Access: | http://hdl.handle.net/2263/46244 Ramjee, S 2015, Rheological Behaviour and Characterisation of Pitch-Based Carbon Precursors, PhD Thesis, University of Pretoria, Pretoria, viewed yymmdd <http://hdl.handle.net/2263/46244> |
| Summary: | Pitch material is an important precursor in the production of carbon bre, carbon composites
and synthetic graphite. It has a complex transformation during pyrolysis which
incorporates the separation of a liquid crystalline phase, known as mesophase. This thesis
attempts to enhance the understanding of this change in composition, structure and its
resultant behaviour.
In this study, two pitches, a coal-tar pitch (MP110) and a (SASOL) Lurgi-gasi er
pitch, are pyrolised to produce material at di erent stages of mesophase development.
These pitches produce mesophase of di erent mosaic type and therefore also resultant
coke.
The MP110 was thermally treated up to a temperature of 437 and produced
anisotropic pitch (which still contains signi cant particulate matter). The nucleated
mesophase spheres did not coalesce to produce domains; this behaviour being attributed
to the particulate material. The SASOL pitch produced a di erent type of mesophase
material. The mesophase produced was of ne mosaic domains; a sample with continuous
mesophase regions was also produced with a mesophase content of approximately 60%
(by volume).
The aromatic starting material of MP110 produced higher quinolone and toluene
insoluble (QI and TI) compounds after pyrolysis. This was also observed in the increase
of C/H (molar ratio of carbon to hydrogen). The more aliphatic SASOL starting pitch
showed similar trends to its MP110 counterparts with respect to QI (quinoline insolubles),
TI (toluene insolubles) and C/H. The glass transition and associated temperatures of
the pitches were analysed via thermal mechanical analysis (TMA), dynamic scanning
calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). The techniques
showed consistency between instruments, with TMA providing the likeliest re
ection of
the true thermodynamic glass transition temperature. The loss of volatile components
was accompanied by an increase in glass transition temperature (observed in conjunction with C/H and mesophase content). For anisotropic MP110 pitches, two relaxations were
observed, one for the isotropic fraction, the other for the mesophase. No such behaviour
was easily observed for the SASOL pitches.
Rheological measurements were obtained to understand the behaviour of the pitches.
Measurements were limited to a speci c viscosity range. The measurements of the samples
were therefore made at di erent temperatures. The relation of the measurement
temperature to the glass transition temperature is thus of extreme importance. The
temperature governs the state of the structure; whether it be suspension, emulsion or gel.
Oscillatory shear experiments were undertaken for the pitch material. Predominantly
isotropic material showed transition from viscoelastic solid to viscoelastic liquid as previously
observed in pitch material.
The anisotropic MP110 pitches did not allow for the production of mastercurves due
to non-linear viscoelastic e ects caused by the softening of mesophase. This being the
transition from a suspension of hard spheres to an emulsion of deformable droplets (depending
on temperature).
For the higher mesophase content anisotropic SASOL pitches, mastercurves were produced;
it had a similar shape to the isotropic pitches (at temperatures closer to the
glass transition), but signi cantly increased elasticity was observed at higher temperatures.
This phenomenon supported the hypothesis of a strong interaction between the
components and phases of the pitch, and thus the possibility of gelled systems.
Rotational shear-rheometry was also utilised and showed that isotropic pitch material
behaves as a predominantly isoviscous
uid.
The anisotropic MP110 pitches of approximately 30% mesophase showed high- and
low-shear viscosity plateau
uid behaviour. This being caused by the breakup of agglomerated
mesophase spheres. This was tested by the implementation of the Krieger-
Dougherty suspension model. The possibility of droplet deformation was investigated
for these samples by utilising a Krieger-Douherty based emulsion analogue, which con-
rmed suspension like behaviour (at the measured temperatures). MP110 samples with
more mesophase were measured at higher temperatures. Their behaviour is more akin to
Power-law shear-thinning behaviour. Being further away from the continuous isotropic
phase glass transition temperature, the behaviour observed is similar to that of emulsions.
SASOL anisotropic pitches showed signi cant yielding upon shear, which is attributed
to structure breakdown. This behaviour is appropriately described by a yield stress, shearthinning
model such as Herschel-Bulkley. Measurements of viscosity for these samples
were made at temperatures signi cantly further from the glass transition temperature as
compared to that of the MP110 pitches. This corroborates strong interaction between
its components. The observed shapes of the curve, at temperatures of measurement,
support the notion of a gel structure. This behaviour is rst proposed via the complex
structure observed (clusters of ne mosaic mesophase domains) and supported by strong interaction of the components inferred from obtained rheological properties. === Thesis (PhD)--University of Pretoria, 2015. === tm2015 === Chemical Engineering === PhD === Unrestricted |
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