TO THE RESCUE: RECOVERED CARBON BLACKS

FEAT UR E
TO THE RESCUE:
RECOVERED CARBON BLACKS
Chris Norris, Analytical Services Manager at ARTIS, UK, explains work
on conducting a global recovered carbon black benchmarking
programme, assessing their composition and in-rubber performance
in relation to conventional carbon blacks.
D
uring RubberCon 2014, in Manchester,
Dr Akutagawa of Bridgestone
Corporation conveyed the importance
of sourcing raw ingredients from
alternative sources to satisfy the predicted
growth in tyre production. Recycling of tyres,
through processes such as pyrolysis, potentially
offers such alternative sources while assisting
with the environmental issues associated with
end-of-life tyres. The use of pyrolysis to recover
carbon black filler is nothing new, with a number
of ventures sporadically offering material to the
marketplace over a period of several decades.
It is clear that recovered carbon black (rCB)
has historically been treated and marketed
as conventional carbon black, which partly
explains the short-lived nature of a number
of these pyrolysis ventures. Such an approach
does not account for the unique properties of
rCB. To overcome this issue, and to challenge
negative market perceptions, UK-based materials
consultancy firm ARTIS recently conducted
a global benchmarking programme to better
understand the quality and properties of current
state-of-the-art rCB materials.
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MATER I A L S WORL D
Firstly, the structure level and surface area of
the rCB samples were considered, as these colloidal
properties are used as key indicators of the reinforcing
potential of a rubber filler. The data generated
demonstrates that the rCB samples fall in the colloidal
space between N550 and N330 carbon blacks
(carcass and tread grades) – a whole tyre feedstock
will contain a blend of carbon black grades. Based
on these measurements alone, rCB has historically
been marketed as a drop-in replacement for N550
or even N330 carbon black. Characterisation of
in-rubber performance, through use of a model
Styrene-Butadiene Rubber formulation, conclusively
demonstrated that simply regarding rCB as a dropin replacement for carbon blacks of similar colloidal
properties is not appropriate. Rheological, physical and
dynamic characteristics class rCB as a semi-reinforcing
filler, so it may find use in applications currently
occupied by N600 and N700 series carbon blacks.
Right: Overview of rCB
production.
Representation of the
spatial arrangement
of aggregates within
the rubber feedstock.
Silica
Representation
of the formation
of carbonaceous
residues formed
during pyrolysis.
Representation
of aggregate size
distribution in
final rCB.
Rebranding rCBs
Incorrect marketing has often led to rCB being used
within inappropriate formulations, which has been
a significant contributor to the lack of approvals
attained. The other major challenge to these recycled
materials revolves around dispersion within a rubber
matrix. For conventional carbon black, a number of
post-reactor processes are required to produce the
finished beaded product. Such refining steps have
often been overlooked in the pyrolysis industry,
with the resultant materials having very poor
dispersability. Poor dispersion negatively impacts on
processing, surface appearance, physical properties
and fatigue performance. Evaluation of current rCB
showed that post-reactor refinement is still being
overlooked by some, with others, such as carbon
clean tech (in Germany) and Reklaim (in the USA),
producing commercially available materials that
have dispersion ratings approaching those of the
conventional carbon blacks.
To better understand the issues with dispersability
and the disparity between colloidal and in-rubber
properties, the composition and surface activity of
rCB needs to be considered.
The deposition of the carbonaceous residue is the
dominating factor in controlling surface activity and
dispersion of rCB. The net result is to fuse a number
of primary aggregates together, which, without
sufficient post-reactor processing, produces a material
of very large rCB aggregate size and, consequently,
the poor dispersion of many of the unrefined
products. In addition, masking of the active sites of
the original carbon black has a significant impact
on filler-filler and filler-polymer interactions. This is
best demonstrated through assessment of the strain
dependency of rCB filled compounds, via dynamic
mechanical analysis.
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Right: Typical rCB
performance reference
N772.
Viscosity
Hysteresis,
tan δ max
Shore A
Elongation
to break
M300%
Tensile strength
Carbon blacks generated from pyrolysis processes are known to contain the
following:
• Original carbon black – The loadings and colloidal properties of the
carbon black contained within the feedstock will have an influence on the
performance of the rCB.
• Carbonaceous residue – Volatile species formed during the pyrolysis
process have a tendency to form condensate on the surfaces of carbon
black, which will carbonise as the process progresses, forming a layer of
carbonaceous residues.
• Inorganic materials – Ash content typically ranges from 10–25wt%, the
vast majority of which is accounted for by silica filler and zinc oxide. The
variances in ash content are a reflection of the feedstock used at each
location or process. For example, the use of a typical European tyre tread
compound as the feedstock will likely result in a higher rCB ash content
(due to increased use of silica filler) over that derived from a North
American tyre tread.
FEAT UR E
Challenges that need to be addressed to increase rCB utility:
• Classification – ARTIS is currently working with ASTM sub-committee
D24.67 (sustainability) to classify rCB and identify the correct parameters
that will differentiate between refined and poor quality recyclate.
Ultimately, this will allow potential users to make informed decisions
regarding their source(s) of carbon black.
• Characterisation – Combined evidence of several studies by ARTIS suggests
that both surface area and structure levels of rCB are being overestimated.
New or revised testing protocols may be necessary to better characterise rCB.
• Applications – Potential 100% replacement for N700–N500 series has been
demonstrated for some applications. The generation of application data is
essential, given the disparity between colloidal and in-rubber properties.
ARTIS has conducted a number of studies demonstrating the suitability of
refined rCB for applications such as tyre inner-liner and sidewalls.
refined rCB
unrefined rCB
conventional CB
Surface roughness
maps highlighting the
dispersability of rCB.
Furnace carbon black
Medium thermal N990
Austin black
Recovered carbon black
Above: Filler networking efficiency – this chart highlights the ∆E’ values of the rCB samples
to fall well below that predicted from their respective surface area measurements, confirming
reduced surface activity. Here, the carbonaceous residue is masking the graphitic crystallite
ends of the original carbon black, inhibiting filler-filler interactions. The large difference
between conventional and rCB may also suggest that the surface area predicted by statistical
thickness surface area measurements is overestimating that available to the polymer matrix.
When mixed in the rubber matrix, carbon black
aggregates have a tendency to associate with each
other to form agglomerates, owing to van der
Waals type attraction forces between particles. At
low dynamic strains, these filler-filler interactions
contribute to the compound stiffness. As the strain
increases, the carbon black network is progressively
disrupted, eventually leading to a plateau at high
dynamic strains where there is no contribution from
filler-filler interactions. This is commonly referred
to as the Payne effect. The difference between the
high and low strain elastic modulus (∆E’ = E’0-E’∞)
provides a measurement of the networking efficiency
of a given filler system. The commercial carbon blacks
generate an R2 value of 0.99, confirming the linear
relationship between networking efficiency and
surface area.
The correlation between networking efficiency
and surface area for the rCB samples (R2 = 0.95)
suggests the level of filler-filler interactions is not
influenced by the non-carbon species present, such as
silica. If it is accepted that the carbonaceous residues
formed on the carbon black surfaces are dictating
surface chemistry, then it is likely that such residues
will also form on the surfaces of the inorganic
components, imparting the same surface chemistry.
To some extent, this negates concerns over slight
variances in feedstock having a significant impact
on performance.
Although the data presented is far from exhaustive,
understanding the fundamental characteristics
will only help in recognising the potential of rCB
as an alternative, green source of raw materials.
Identification of suitable applications for refined
commercially available rCB products, such as those
offered by carbon clean technology and Reklaim,
will inevitably lead to greater product approvals.
For more information, email Chris Norris,
[email protected]
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