Arthur Richard Horrocks

This study has investigated the impact of fire retardants in carbon fibre-reinforced epoxy composites (CFRC) on physico-mechanical and oxidative properties of carbon fibres after exposure of CFRCs to high temperatures and fire. Three fire retardants were chosen based on their activity in condensed phase (ammonium polyphosphate, APP) and/or vapour phase (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, DOPO, and resorcinol bis-(diphenyl phosphate), RDP). The composites were subjected to high heat fluxes (75–116 kWm-2) and fire using a cone calorimeter and propane burner. Post-exposure, the carbon fibres extracted from different plies were analysed for surface oxidation, mass loss, diameter reduction, and changes in tensile and electrical properties. Carbon fibres exhibited differing degrees of oxidation across the plies, with surface ply fibres showing greater oxidation and diameter reductions, while underlying plies experienced limited oxidation due to restricted oxygen access. The charred residues from fire-retarded samples (residue levels: APP > RDP > DOPO > control) adhered to the fibres, reducing oxidation and preserving tensile properties. However, the charred residues increased the electrical conductivity of the carbon fibres. This analysis has enabled the evaluation of each retardant's effectiveness.

The thermal stability of melt-spun hydroxypropyl–modified lignin/polyamide (PA1010) 50:50 wt% blended precursor fibres, crucial for the thermal stabilisation stage in carbon fibre production, was enhanced by pre-treating the fibres with a graphene oxide (GO) suspension, synthesized via the modified Hummers method. This pre-treatment allowed the fibres to be subsequently thermally stabilised at a faster heating rate of 20 °C/min, compared to the typical 0.1–0.25 °C/min used for lignin-based fibres, thereby reducing overall thermal stabilisation time from 29 h to 2.5 h. The stabilised filaments were successfully carbonised at 950 °C, yielding coherent, void-free carbon fibres without inter-filament fusion. The tensile modulus of GO-treated filaments improved from 1.3 GPa to 2.3 GPa after thermal stabilisation. However, derived carbon fibres were brittle in nature. Various characterisation techniques, including DSC, TGA, FTIR, SEM-EDX, AFM, XPS, and tensile testing, were used to analyze the physico-chemical changes. DSC showed that GO improved the polycrystallinity of the precursor filaments and contributed to the formation of a three-dimensional cross-linked network during heat stabilisation, suppressing the PA melt endotherm. TGA confirmed that GO-treated filaments had higher char yields (∼40 %) than untreated fibres (∼30 %), further supporting GO-induced crosslinking reactions. FTIR, SEM-EDX, and AFM confirmed an even GO coating. A study of GO pre-treatment variables suggested that a reduction in GO concentration is required to reduce resulting carbon fibre brittleness at the expense of increased thermal stabilisation time.

Carbon fibres have been produced from hydroxypropyl-modified lignin (TcC)/bio-based polyamide 1010 (PA1010) blended filaments. Two grades of PA1010, with different molecular weights and rheological properties, were used for blending with TcC. An oxidative thermal stabilisation step was used prior to carbonisation in an inert atmosphere to prevent the fusion of the filaments during the latter step. Thermal stabilisation was not possible using a one-step stabilisation process reported in the literature for lignin and other lignin/synthetic polymer blends. As a consequence, a cyclic process involving an additional isothermal phase at a lower temperature than the precursor filaments' melting point, was introduced to increase the cross-linking reactions between the lignin and polyamide. Thermally stabilised filaments were characterised by DSC, TGA, TGA-FTIR, ATR, and SEM techniques. Polymer rheology and heating rate used during thermal stabilisation influenced the thermal stabilisation process and mechanical properties of the derived filaments. Thermally stabilised filaments using optimised conditions (heating in the air atmosphere at 0.25 degree celsius/min to 180 degree celsius; isothermal for 1 h, cooling back down to ambient at 5 degrees C/min; heating to 250 degree celsius at 0.25 degree celsius/min, isothermal for 2 h) could be successfully carbonised. Carbon fibres produced had void-free morphologies and mechanical properties comparable to similarly thermally stabilised and carbonised polyacrylonitrile (PAN) filaments. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ )

This study examines the effects of heat and fire on the physical, mechanical and electrical properties of carbon fibre and those in carbon fibre reinforced composites (CFRCs). Carbon fibres were exposed to controlled heating (thermogravimetric analysis (TGA) and a tube furnace) in inert and air (oxygenated) environments and simulated fire (cone calorimetry at 35–75 kW m−2 and jet fire (propane burner) of 116 kW m−2) atmospheres. In inert atmospheres there was a minimal effect on the properties of carbon fibres, but in an oxygenated environment, significant oxidation began at temperatures ≥550 °C, resulting in a reduction in fibre diameter, which reduced further with increasing temperature and exposure duration. Tensile strength and electrical conductivity of carbon fibre decreased with reduction in fibre diameter. CFRCs exposed to 75 kW m−2 in a cone calorimeter and direct flame in a propane burner (116 kW m−2) showed varying degrees of oxidation in CFRC plies, with surface ply fibres experiencing more oxidation and consequent reductions in fibre diameter and tensile properties compared to fibres in underlying plies, where oxidation was limited due to restricted oxygen availability. Fibres exposed to the propane burner exhibited notable damage, including pitting and internal oxidation. Despite this, the overall electrical properties of residual carbon fibres did not significantly decrease, indicating that they still pose an electrical hazard if exposed during a high heat or fire event.

Following application of three commercially available, flame retardants (FR) to polyester and exposure to an atmospheric plasma in the presence or absence of simultaneous UV 308 nm excimer laser radiation, the retentions to a 30 min 40 degrees C water-soak for the mono- and dimeric cyclophosphonate (PE-CONC and PCO 900 respectively) and a 30 min methanol-soak treatment for the resorcinol bis(diphenyl phosphate) (RDP) agents were generally enhanced with respect to unexposed controls. These results suggested that the formation of PETFR bonding had occurred as a consequence of plasma/UV exposure. The effect of plasma/UV process sequence (pre-exposure and/or post exposure with respect to FR impregnation) as well as plasma gas type, showed that maximum post-soaking FR retentions occurred when both pre- and post-exposure conditions operated. Plasma/UV exposed fabrics both before and after soaking treatments were examined for morphological changes by SEM and surface phosphorus retention by EDX, FTIR-ATR, XPS as well as TGA/DTG/DTA, each of which showed evidence on the surface of changes in surface topography, increased phosphorus levels and modified thermal behaviour. When subjected to vertical fabric strip flammability testing, these changes were insufficient to generate significant changes in the burning rate apart from in post-methanol soaked, RDPcontaining fabrics where almost all plasma/UV process sequences reduced specimen burning rates. During all experiments, the presence or absence of the UV laser radiation appeared to have no obvious effect on post-soaking FR retentions, suggesting that its presence is not required since plasma activation alone appears to be the key to improving PET -FR bonding.

Thermal and thermo-oxidative decompositions of polyamide 6.6 (PA66) in the presence and absence of zinc (ZnSt), calcium (CaSt) and copper stannates (CuSt), and in the presence of antimony trioxide (ATO), have been studied by TGA-FTIR and pyrolysis GC/MS. It is shown that whilst ATO has a negligible effect on the rate of decomposition and products of pyrolysis under both slow heating on a TGA and rapid heating on a pyrolysis/GC/ MS apparatus, the stannates have a catalytic effect in the order ZnSt ≈ CaSt > CuSt, evident through release of volatiles at a lower temperature and in a different product distribution following rapid high temperature pyrolysis. In particular, rapid pyrolysis at high temperature in the presence of the stannates promotes formation amongst the pyrolysis products of, for example, 6-aminohexanenitrile. 1,6-hexanediamine, 1-methyl-3-formylindole and 1,2,3,7-tetramethylindole, whilst yields of others are reduced, e.g., hexanedinitrile, caprolactam, diaminomethylidene(2-hydroxypropyl)azanium, prop‑2-enenitrile (acrylonitrile) and azacyclodecan-5-ol. We suggest that these effects arise from complexation of the electropositive metal in the stannate (Zn, Ca or Cu) with the C=O groups of PA66, thus weakening the C(O)-C and C-N bonds adjacent to the C=O groups. The fact that ZnSt has the most pronounced effect on the pyrolysis product distribution and CuSt the least, we explain in terms of the order of electropositivity of the metal (Zn>Ca>Cu).

This work reports the use of cross-linkers in bio-based blends from hydroxypropyl-modified lignin (TcC) and a bio-based polyamide (PA1010) for possible use as carbon fibre precursors, which, while minimising their effects on melt processing into filaments, assist in cross-linking components during the subsequent thermal stabilisation stage. Cross-linkers included a highly sterically hindered aliphatic hydrocarbon (Perkadox 30, PdX), a mono-functional organic peroxide (Triganox 311, TnX), and two different hydroxyalkylamides (Primid® XL-552 (PmD 552) and Primid® QM-1260 (PmD 1260)). The characterisation of melt-compounded samples of TcC/PA1010 containing PdX and TnX indicated considerable cross-linking via FTIR, DSC, DMA and rheology measurements. While both Primids showed some evidence of cross-linking, it was less than with PdX and TnX. This was corroborated via melt spinning of the melt-compounded chips or pellet-coated TcC/PA1010, each with cross-linker via a continuous, sub-pilot scale, melt-spinning process, where both Primids showed better processability. With the latter technique, while filaments could be produced, they were very brittle. To overcome this, melt-spun TcC/PA1010 filaments were immersed in aqueous solutions of PmD 552 and PmD 1260 at 80 °C. The resultant filaments could be easily thermally stabilised and showed evidence of cross-linking, producing higher char residues than the control filaments in the TGA experiments.

Application of a combined atmospheric plasma/UV laser to cotton fabrics impregnated with selected non-durable flame retardants (FRs) has shown evidence of covalent grafting of the latter species on to cotton fibre surfaces. As a result, an increase in their durability to water-soaking for 30 min at 40 °C has been recorded. Based on previous research plasma gases comprising Ar80%/CO220% or N280%/O220% were used to pre-expose cotton fabric prior to or after FR impregnation to promote the formation of radical species and increased –COOH groups on surface cellulosic chains, which would encourage formation of FR-cellulose bonds. Analysis by scanning electron microscopy (SEM/EDX), X-ray photoelectron spectroscopy (XPS) and thermal analysis (TGA) suggested that organophosphorus- and nitrogen- containing flame retarding species in the presence of the silicon-containing molecules such as 3-aminopropyltriethoxy silane (APTS) resulted in formation of FR-S-O-cellulose links, which gave rise to post-water-soaking FR retentions > 10%. Similarly, the organophosphorus FR, diethyl N, N bis (2-hydroxyethyl) aminomethylphosphonate (DBAP), after plasma/UV exposure produced similar percentage retention values possibly via (PO).O.cellulose bond formation, While none of the plasmas/UV-treated, FR-impregnated fabrics showed self-extinction behaviour, although burning rates reduced and significant char formation was evident, it has been shown that FR durability may be increased using plasma/UV treatments.

The kinetics of pyrolysis of organosolv (TcA) and hydroxypropyl-modified (TcC) lignins have been investigated using thermogravimetric analysis (TGA). Three isothermal models (single first order, Guggenheim and Avrami-Erofeev) and one non-isothermal model (Kissinger) were used to analyse the mass-loss data. Sensible derived kinetic parameters, i.e., activation energy and pre-exponential factor, were obtained only for the initial stages of pyrolysis where the kinetics were approximately first order. Models that analysed TGA data beyond the initial stage gave inconsistent results, indicating the complexity of subsequent decomposition steps occurring at higher temperatures and/or longer times. The kinetics of the initial stage are important for designing routes to lignin's valorisation into useful products, such as carbon fibres, activated carbons, polymer additives, etc. TcC had a higher activation energy (41.5 kJ/mol) for initial decomposition than TcA (39 kJ/mol), consistent with its greater thermal stability observed previously during conversion of lignin-based fibres into carbon fibres.

Atmospheric plasma treatment can modify fabric surfaces without affecting their bulk properties. One recently developed, novel variant combines both plasma and UV laser energy sources as a means of energising fibre surfaces. Using this system, the two most commonly used fibres, cotton and polyester, have been studied to assess how respective fabric surfaces were influenced by plasma power dosage, atmosphere composition and the effects of the presence or absence of UV laser (308 nm XeCl) energy. Plasma/UV exposures caused physical and chemical changes on both fabric surfaces, which were characterised using a number of techniques including scanning electron microscopy (SEM), radical scavenging (using 2,2-diphenyl-1-picrylhydrazyl (DPPH)), thermal analysis (TGA/DTG, DSC and DMA), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Other properties studied included wettability and dye uptake. Intermediate radical formation, influenced by plasma power and presence or absence of UV, was key in determining surface changes, especially in the presence of low concentrations of oxygen or carbon dioxide (20%) mixed with either nitrogen or argon. Increased dyeability with methylene blue indicated the formation of carboxyl groups in both exposed cotton and polyester fabrics. In the case of polyester, thermal analysis suggested increased cross-linking had occurred under all conditions.