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Raw materials and their production method
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Biochar can be produced by pyrolysed any biomass at a very high temperature (>500 °C)82
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CNTs is produced by separating it from graphite by either:83–85 catalytic chemical vapor disposition (CCVD), laser ablation, arc discharge |
Graphene is produced by separating it from graphite by either: micromechanical exfoliation, liquid exfoliation, chemical vapor deposition, flame synthesis, pulsed laser deposition |
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Advantages of filler as a reinforcement material
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Biochar is a renewable material and the use of it to replace non-renewable carbon materials would reduce the ubiquitous dependency on fossil fuels, minimise wastes and promote sustainability. The addition of biochar in the composites may improve their thermal stability. E.g. the addition of 20 and 30 wt% biochar to epoxy composites have increased the Td10% by 7–20%. Overall, the thermal degradation of the epoxy/biochar composites was delayed.86 Biochar exhibits aliphatic functional groups on its surface, which make it hydrophobic. Due to this, when biochar is incorporated in wood and polymer composites, it may lower the resulting moisture absorption of the entire composite.66
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CNTs possess excellent adsorption ability, owing to its ability in creating a strong interaction with other molecules. E.g. CNTs has been used as an adsorbent for various heavy metal ions such as copper, nickel, cobalt, vanadium, silver, cadmium and other earth elements.87
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Graphene recorded the highest thermal conductivity than biochar and CNTs which is about 5300 W mK−1.72E.g. the incorporation of rGO into n-eicosane/silica microcapsules increased the thermal conductivity by 83–193% and decreased the latent heat by 6–15%.88
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Disadvantages of filler as a reinforcement material
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The properties of biochar are mainly dependent on the properties of biomass (raw material) and thermal conditions during pyrolysis.9 Modification is needed to increase the properties of biochar, i.e. by increasing the specific surface area and pore fraction, forming functional groups, etc.89 The different feedstock used to produce biochar will produce biochar with different properties. E.g. the percentage of biochar loading at which best mechanical properties obtained was inconsistent for epoxy composites with three different biochars; plastic waste char, wood shavings char, and pine cone char.86
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Homogenous dispersion of CNTs especially at high loading is difficult to achieve. Modification is needed to reduce the aggregation and improve the dispersion of CNTs caused by the inactive surface of CNTs.70 The high cost of CNTs does not compensate for the enhancement of properties on numerous occasions, unless for premium end-products.87
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Graphene may cause a reduction in the mechanical strength of composites, mainly due to poor interface with matrices. Hence, it needs to be modified to GO or rGO. E.g. the mechanical strength and deformation at break of epoxy/graphene composites are much lower than the neat epoxy resin due to the poor interface between the nanofiller and matrix.90 Similar to CNTs, graphene is expensive and considering the cost, it can only be used for premium end-products.87 The cohesive energy between graphene layers is around 2 eV nm−2 which is considered very high, and this causes graphene to irreversibly agglomerate or restack when compounded with molten polymer or when solvents evaporate from graphene dispersion.91
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