Char Formation Research Articles

State-of-the-Art on Advancements in Carbon-Phenolic and Carbon-Elastomeric Ablatives.

Ablative composites serve as sacrificial materials, protecting underlying materials from high-temperature environments by endothermic reactions. These materials undergo various phenomena, including thermal degradation, pyrolysis, gas generation, char formation, erosion, gas flow, and different modes of heat transfer (such as conduction, convection, and radiation), all stemming from these endothermic reactions. These phenomena synergize to form a protective layer over the underlying materials. Carbon, with its superb mechanical properties and various available forms, is highlighted, alongside phenolics known for good adhesion and fabric ability and elastomers valued for flexibility and resilience. This study focuses on recent advancements in carbon-and-phenolic and carbon-and-elastomeric composites, considering factors such as erosion speed; high-temperature resistance; tensile, bending, and compressive strength; fiber-matrix interaction; and char formation. Various authors' calculations regarding the percentage reduction in linear ablation rate (LAR) and mass ablation rate (MAR) are discussed. These analyses inform potential advancements in the field of carbon/phenolic and carbon/elastomeric ablative composites.

Effects of steam feeding on microwave heating of cellulose with magnetite used as susceptor

Microwave heating (MWH) of lignocellulosic biomass with susceptors has attracted attention for H2 production. In this study, H2 is produced from cellulose via MWH at 800 °C in a steam atmosphere with magnetite used as the susceptor. The effects of steam feeding and MWH on the product yields are experimentally investigated in comparison with conventional heating (CH). The H2 and CO2 yields are increased by steam feeding for both MWH and CH because the water–gas shift reaction occurs. Without steam feeding, MWH enhances H2 and CO production, thereby increasing the maximum energy efficiency. With steam feeding, the steam gasification of C is more predominant for CH than for MWH as the reduction in the C content in the char of CH is higher than that of MWH. MWH reduces the formation of secondary char, and it is therefore expected to overcome issues such as the clogging or blockage of gas lines.

Hydrophobic micro-crosslinked sulfonated polystyrene flame retardant for simultaneous fire safety, transparency and high performance of polycarbonate

Sulfonate is considered to be the best flame retardant, which can endow polycarbonate (PC) excellent flame retardancy with low addition. Unfortunately, the water absorption and solubility of sulfonate will increase the cost of transportation and storage. In addition, sulfonate flame retardant is easy to migrate and precipitate from PC composites resulting in a negative impact on the sustainable use of PC materials. To ameliorate this issue, a sulfonate flame retardant (CSPS) with unparalleled hydrophobicity and thermal stability was synthesized through the incorporation of sodium p-styrene sulfonate as the main chain of crosslinked polystyrene. The PC/CSPS composites could pass UL 94 test V-0 rating with 0.3 wt.% CSPS addition, and the LOI is increased from 24.5 % to 31.4 %. Simultaneously, the PC/CSPS composites preserve their initial transparency. Moreover, the combustion test results indicated that the peak of heat release rate (PHRR) and the peak of smoke production rate (PSPR) of PC/CSPS-0.3 composites decreased by 40 % and 41 % respectively, and the time to reach PHRR and PSPR increased by 35.8 % and 56.5 % respectively. These results were attributed to the catalytic char formation and gas phase limitation of CSPS, which made PC composites with higher fire safety. Importantly, the PC/CSPS-0.3 composites can maintain tensile strength similar to that of PC. This research proposes a simplified and efficient methodology for fabricating PC composites with outstanding water resistance, flame retardancy, transparency, and mechanical fortitude, which is of considerable importance for the application of PC composites in emerging fields.

B/P/N flame retardant based on diboraspiro rings groups for improving the flame retardancy, char formation properties and thermal stability of cotton fabrics

Developing flame retardant cotton fabrics (CF) is crucial for minimizing the harm caused by fires to people. To improve the flame retardancy of CF, this paper has synthesized a novel flame retardant called diboraspiro tetra phosphonate ammonium salt (N-PDBDN). The structure of N-PDBDN has been analyzed using FT-IR and NMR. Treating CF with N-PDBDN can increase the limiting oxygen index (LOI) to 36.2 % with a weight gain of 10.1 %. Moreover, even after undergoing 50 laundering cycles (LCs), the LOI remains at 27.1 %, indicating good flame retardancy and durability. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) show the presence of P and N elements on N-PDBDN treated CF, suggesting successful bonding between N-PDBDN and cellulose. Thermogravimetric analysis (TGA) results demonstrate that the addition of N-PDBDN significantly enhances the thermal stability and carbon formation ability of CF. Furthermore, cone calorimetry tests reveal reduced heat release rates (HRR), prolonged time to ignition (TTI), and 38 % lower total heat release (THR) in CF treated with N-PDBDN compared with pure cotton. Finally, a potential flame retardant mechanism involving N-PDBDN is proposed. These findings indicate that incorporating an ammonium phosphate group into CF can effectively improve the flame retardancy and durability.

Core-shell flame retardant/APP-PEI@MXene@ZIF-67: A nanomaterials self-assembly strategy towards reducing fire hazard of thermoplastic polyurethane

Ammonium polyphosphate (APP) is the most commercially valuable phosphorus-based flame retardant, but poor interaction with the polymer interface limits the flame-retardant efficiency. In this work, a new type of multi-layered nanomaterial coating APP (APP-PEI@MXene@ZIF-67) was prepared and introduced into thermoplastic polyurethane (TPU) polymers by a simple and green assembly strategy. The effects of different loading shells of modified APP on the flame retardancy of TPU composites were investigated, as well as their gas/condensed phase thermal degradation products and their flame-retardant mechanisms. The results showed that APP-PEI@MXene@ZIF-67 was more favorable for catalytic char formation than APP, and the residual char formed by its composites was denser and more continuous. Compared with the pure TPU, the pHRR, THR, pSPR, and TSP of TPU/APP-PEI@MXene@ZIF-67–2BL composites were decreased by 72.75 %, 87.25 %, 59.58 %, and 85.97 %, respectively, which significantly reduced the heat and smoke release during the combustion process. The gas-phase and condensed-phase flame retardant mechanisms synergized to construct a possible approach to address the fire risk and heat/smoke hazards of TPUs and promote the extensive application of TPU polymers.

Effect of CO2 Gasification on Nitrogen Oxide Emission in Oxy-Fuel Combustion.

Due to the cumulative effect of the recycled flue gas in oxy-fuel combustion, the reduction of NO has become the focus of research in which the role of CO2 gasification has a nonnegligible effect. In this article, the heterogeneous gasification mechanisms of CO2 on coal char during oxy-fuel combustion were studied by density functional theory and transition-state theory. The zigzag char model and char(N) model were selected to investigate the CO2 adsorption and gratification reactions at the molecular level. By comparison, the CO2 gasification reaction is preferred to occur on the zigzag char model with a reduction in the energy barrier and an increase in the reaction rate. When considering char nitrogen conversion, the gasification reaction is more prone to releasing NO first, leading to the contraction of the aromatic ring and the generation of CO at high temperatures. Therefore, the study further highlights the promoting role of CO2 gasification on the homogeneous and heterogeneous reduction of NO, which is mainly reflected in the reduction effect of CO. Moreover, it also accelerates the consumption of coal char and NO formation, which is favorable for the reduction reactions to a certain extent.

Chemical insight into thermal degradation, chemical kinetics, and thermodynamics behavior of sewage sludge through thermogravimetry and Fourier transform infrared analysis

Pyrolysis emerges as a promising path for sewage sludge (SS) management and value creation by producing compounds with enhanced utility. This study focuses on a comprehensive analysis of the thermal decomposition of SS using thermogravimetric analysis (TGA) and Fourier transform infrared spectrometry (FTIR). The TGA and FTIR were employed to evaluate the pyrolysis behavior, kinetics, and characteristics during pyrolysis of SS. The process revealed three distinct stages: 1st stage ranged between 30–185 °C with the removal of moisture, 2nd stage saw devolatilization of SS in the range of 185–595 °C, and the 3rd stage was mainly the formation of char in the temperature range of 595–900 °C. Pyrolysis kinetics, reaction mechanisms, and thermodynamic properties were extensively investigated at three different heating rates of 10, 20, and 30 °C /min. It was observed that with the temperature rise, the decomposition rate also increased rapidly, notably for the heating rate of 30 °C/min. Using Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Tang methods, activation energies for SS pyrolysis were estimated for α in the range of 0.2–0.85, and the average value for that range was found to be 84.72, 92.29, 91.99, and 91.76 kJ/mol, respectively for different iso-conversional methods. The chemical kinetics analysis through Coats-Redfern methods identified that 3-D diffusion-reaction mechanisms have the highest coefficient of determination among the other models. Criado z-master plot emphasizes the prevalence of Avrami-Erofeev (A4) as the bestsuited reaction model in combination with α, providing a detailed examination that illuminates the complex processes and mechanisms inherent in the pyrolysis of sewage sludge.

A novel approach utilising phosphorylated casein for fire-retardant and formaldehyde-free medium density wood fibreboards

Casein has been employed in composite materials as a flame retardant (FR). However, in the current research, casein has not been applied to a medium density fibreboard (MDF) material system, especially in a formaldehyde-free MDF material system, to investigate its impact on mechanical and fire-resistant performance. Polyamidoamine-epichlorohydrin (PAE) resin has been utilised as a binder to manufacture formaldehyde-free MDF samples in this research. Phosphorylation was conducted to enhance the flame-retardant ability of casein. This chemical treatment resulted in the significant improvement on the char-forming performance of the casein, leading to the dramatic reduction of the peak heat release rate (PHRR) by ⁓45% compared with non-treated casein. The char formation of the phosphorylated casein in the MDF sample also decreased its PHRR more effectively by ∼18% with the drop of the THR value (∼40 MJ/m2) than that of the casein. Besides, the addition of the phosphorylated casein to the MDF based on the PAE binder did not diminish the binder`s bonding performance, resulting in the improvement of the flexural modulus by ∼23% and flexural stress by ∼16%, while the IB strength retained compared with that of the control MDF sample. The current study clearly demonstrates that the jointly use of phosphorylated casein and PAE is a promising avenue to overcome one of the ongoing challenges in the MDF material system with regard to mitigating the reduction in mechanical performance associated with the introduction of flame-retardant materials.

Synergistic effect of phytic acid and eggshell bio-fillers on the dual-phase fire-retardancy of intumescent coatings applied on cellulosic substrates

Cellulosic substrates, including wood and thatch, have become icons for sustainable architecture and construction, however, they suffer from high flammability because of their inherent cellulosic composition. Current control measures for such hazards include applying intumescent fire-retardant (IFR) coatings that swell and form a char layer upon ignition, protecting the underlying substrate from burning. Typically, conventional IFR coatings are opaque and are made of halogenated compounds that release toxic fumes when ignited, compromising the roofing's aesthetic value and sustainability. In this work, phytic acid, a naturally occurring phosphorus source extracted from rice bran, was used to synthesize phytic acid-based fire-retardants (PFR) via esterification under reflux, along with powdered chicken eggshells (CES) as calcium carbonate (CaCO3) bio-filler. These components were incorporated into melamine formaldehyde resin to produce the transparent IFR coating. It was revealed that the developed IFR coatings achieved the highest fire protection rating based on UL94 flammability standards compared to the control. The coatings also yielded increased LOI values, indicative of self-extinguishing properties. A 17 °C elevation of the IFR coating's melting temperature and a significant ∼172% increase in enthalpy change from the control were observed, indicating enhanced fire-retardancy. The thermal stability of the coatings was improved, denoted by reduced mass losses, and increased residual masses after thermal degradation. As validated by microscopy and spectroscopy, the abundance of phosphorus and carbon groups in the coatings' condensed phase after combustion indicates enhanced char formation. In the gas phase, TG-FTIR showed the evolution of non-flammable CO2, and fire-retardant PO and P–O–C. Mechanical property testing confirmed no reduction in the adhesion strength of the IFR coating. With these results, the developed IFR coating exhibited enhanced fire-retardancy whilst remaining optically transparent, suggestive of a dual-phase IFR protective mechanism involving the release of gaseous combustion diluents and the formation of a thermally insulating char layer.

Ethanolysis of enzymatic hydrolysis lignin with Ni catalysts on different supports: The roles of catalytic sites

Catalytic lignin solvolysis (CLS) holds promise for efficient lignin utilization, yielding small molecules with minimal or no char formation. However, the role of different active sites of catalyst in CLS are rarely discussed. Here, Ni catalysts on different supports, i.e., SiO2, Al2O3, MgO, and ZrO2, were prepared with the aim of manipulating the relative importance of metal and acid-base functionalities to investigate the role of different active sites in the enzymatic hydrolysis of lignin (EHL) ethanolysis. The main ether linkage, i.e., β-O-4, in EHL is cleaved without the participation of a catalyst. Metal sites suppress repolymerization through hydrogenating active intermediates, while acid and base sites facilitate the conversion of phenolic monomers into complex alkylated and etherified products and also promote repolymerization reactions. Among the catalysts, Ni/SiO2 demonstrated the highest hydrogenation activity and yielded the most monomers (24.7 wt %) at 280 °C for 6 h under 2 MPa H2 in ethanol. These findings shed light on the catalytic mechanisms in CLS, offering valuable insights for future catalyst design.

Quantification of degree of interactions during co-pyrolysis of nine typical carbonaceous wastes

Organic municipal waste is a mixture of carbonaceous feedstock and cross-interaction of volatiles in their co-pyrolysis has been well documented, but quantifying their extent of interactions in co-pyrolysis is difficult. In this study, the ratio of char yield from co-pyrolysis to calculated average of pyrolysis of single feedstock was used as an indicator to probe the interaction in co-pyrolysis of every two typical carbonaceous wastes (sawdust, rice, spinach, chicken manure, office paper, corrugated paper, plastic bottles/films, fiber towel) at 550 °C. The results indicated that the co-pyrolysis of sawdust and corrugated paper had the highest degree of interaction (0.44). Sawdust and rice with ash-rich feedstock (spinach, chicken manure and office paper) resulted in dominance of gasification reactions to reduce char production in co-pyrolysis process. In comparison, the organics from the degradation of spinach and corrugated/office paper could react with the derivatives (mostly radicals) of plastic bottle (PET), plastic film (PE) or fiber towel (polyesters) via secondary condensation to generate more char. The chicken manure with high ash content hardly forms char of enhanced yield during its co-pyrolysis with other feedstocks. The enhancing char yield through secondary condensation generally resulted in additional oxygen-containing deposit on char and reduced thermal stability, which was the opposite to the formation of char with dominance of gasification. Co-pyrolysis of varied feedstock also led to change of morphologies of char via filling mechanism or formation of organics.

Intrinsic flame-retardant epoxy resin composite containing schiff base structure with satisfied flame retardancy and mechanical properties

To solve the non-renewable and flammability of traditional epoxy composites, a new type of intrinsic flame-retardant epoxy (VH/GF) composite was fabricated from vanillin-based epoxy resin (VH) and glass fibre (GF). The T5% of VH/GF was higher than that of DGEBA/GF, and CY800 of VH/GF reached above 84 wt%. VH/GF obtained a limiting oxygen index as high as above 90 % and passed UL-94 V-0 rating. Compared with DGEBA/GF, the total heat release, total smoke production, maximal smoke density of VH/GF reduced by more than 72 %, 91 % and 81 %, respectively. The physical barrier of glass fibre and excellent char formation ability of VH endowed VH/GF composite with good condensed phase flame-retardant effect. When the mass ratio of matrix and glass fibre was 5:5, the storage modulus and glass transition temperature of VH/GF were 6.34 GPa and 205 °C, and its tensile strength and elongation at break were 348.36 MPa and 4.01 %. This work provides new research insights for bio-based epoxy resin in high-performance fire safety composites.

Revealing the anion-dependent effects on potassium-assisted biomass pyrolysis

Metal additives could significantly improve the product quality of biomass pyrolysis. However, the effect of anions added with metal cations has not been fully understood. To shed light on the anion effect, biomass pyrolysis assisted with different potassium agents was launched at parallel conditions, and the product and element distributions were investigated. Compared to the weak influence of neutral agents (i.e., KCl), alkali agents (i.e., KOH) strongly promote the formation of char (20.0–31.3 wt%), H2 (15–92 cm3/g), and phenols (50–65 area %), but inhibit CO, carbohydrates, and furans. Oxidizing agents (i.e., KMnO4) perform similar effects with alkali agents and raise the yields of CO2 and H2O. Alkali agents facilitate carbon and hydrogen migration into biochar and gas, respectively, where higher basicity is related to enhancing the above tendency. Furthermore, proton affinities were calculated, and the correlation between dehydrogenation and additive properties including proton affinity and anion charge was first established. Based on the correlation results, the shielding effects of anions were disclosed, where the dehydrogenation effects of K+ were shielded by anions endowing low proton affinity. The competition effect of borate and phosphates anions hinders the K+ effect by introducing intermediate cross-linking structures. Besides, the self-decomposition of acetate, dehydrogenation catalyzed by iron and manganese, and dehydration catalyzed by chloride were recognized as parallel effects that do not severely influence the potassium effect. This study provides novel insight into the effect of additives in pyrolysis and will help reinvent the in-situ product-upgrading approach of biomass pyrolysis.

Three-Dimensional Cross-Linking Network Coating for the Flame Retardant of Bio-Based Polyamide 56 Fabric by Weak Bonds.

Weak bonds usually make macromolecules stronger; therefore, they are often used to enhance the mechanical strength of polymers. Not enough studies have been reported on the use of weak bonds in flame retardants. A water-soluble polyelectrolyte complex composed of polyethyleneimine (PEI), sodium tripolyphosphate (STPP) and melamine (MEL) was designed and utilized to treat bio-based polyamide 56 (PA56) by a simple three-step process. It was found that weak bonds cross-linked the three compounds to a 3D network structure with MEL on the surface of the coating under mild conditions. The thermal stability and flame retardancy of PA56 fabrics were improved by the controlled coating without losing their mechanical properties. After washing 50 times, PA56 still kept good flame retardancy. The cross-linking network structure of the flame retardant enhanced both the thermal stability and durability of the fabric. STPP acted as a catalyst for the breakage of the PA56 molecular chain, PEI facilitated the char formation and MEL released non-combustible gases. The synergistic effect of all compounds was exploited by using weak bonds. This simple method of developing structures with 3D cross-linking using weak bonds provides a new strategy for the preparation of low-cost and environmentally friendly flame retardants.

A multifunctional integrated coating reinforced with exfoliated boron nitride towards superior decorative, antibacterial, durable, and flame-retardant performances

To solve the problems of traditional transparent fireproof coatings with a single-function nature and susceptibility to environmental aging, a novel functionalized exfoliated boron nitride (h-BN) with organophosphate (PPB) was synthesized and applied in an amino resin, preparing an integrated coating with decorative, fire-resistant, antibacterial, and weathering functions. The results indicate that the introduction of h-BN not only maintains the coating's high transparency but also enhances its hardness and adhesion capabilities. Furthermore, the presence of h-BN positively influences the flame retardance and smoke suppression of MBPPB coatings. Notably, when the mass ratio of PPB to h-BN is 98:2, the resulting MBPPB4 coating reduces the flame spread rate (FSR) by 45.7% and the smoke density rating by 62.6% compared to MBPPB0 coating without h-BN. Thermogravimetric analyses indicate that coatings containing h-BN exhibit superior char formation and thermal stability, where the MBPPB3 coating, with a PPB to h-BN ratio of 98.5:1.5, shows a 49.8% higher char yield at 800 °C than MBPPB0. The accelerated aging test demonstrates that the incorporation of h-BN also reduces the incidence of coatings blistering and cracking, thereby exhibiting outstanding weatherability. Additionally, h-BN significantly enhances the coating's antibacterial efficacy, with an E. coli inhibition rate of up to 99.0% for MBPPB3 coating.

A rare‐earth‐based nano‐flame retardant with 2D MXene for improving fire safety and mechanical property of thermoplastic polyurethane composites

AbstractA kind of rare earth flame retardant, two‐dimensional lamellar compounds (MXene) loading with cerium phosphate nanorod, MXene@CePO4 (CMX) was prepared by hydrothermal and electrostatic adsorption methods. It was then applied in thermoplastic polyurethane (TPU). With the 3 wt% addition of CMX improving the mass rate of residual char from 4.42 wt% to 11.33 wt% compared to pristine TPU, there were reductions of 53.8%, 12.3%, and 24.7% in peak heat release rate (pHRR), total heat release (THR), and smoke density maximum (Ds max), respectively. Moreover, the TPU/CMX composites were shown to have superior mechanical properties compared with pristine TPU. For TPU/CMX‐3.0, there was a visible improvement in tensile strength (TS) and elongation at break (EB) of 12.5% and 54.1% respectively, compared to pure TPU. The analysis of the flame retardancy mechanism of TPU/CMX shows that CMX plays an effective role in quenching free radicals in the gas phase and promotes char formation of TPU in the condense phase. Therefore, this paper offers a new solution for designing multifunctional rare earth flame retardants that are applied in TPU, thereby extending its application in industries and daily life.

Impact of surface delignification on fire retardancy of wood treated with polyelectrolyte complexes.

Wood is a natural composite widely employed as a residential building interior finishing. Although wood is readily available and offers benefits to the occupants, such as enhanced well-being, it is rarely employed in commercial construction due, amongst others, to the potential hazard offire propagation. The application of flame retardant (FR)treatments leads to a reduction of wood flammability andsupports wood as interior finishing. Polyelectrolyte complexes (PECs) deposition is an innovative surface treatment that has already proven its efficiency for fabrics. Forwood, recent studies have highlighted that the weight gain impacted the fire-retardancy, and a minimum of 2 wt.-%was set to obtain fire protection. This study explored the potential of surface delignification to activate the woodsurface and facilitate the PEC impregnation. Yellow birch (Betula alleghaniensis, Britt.) was surface delignified (0.3 mm) using sodium chlorite. The treatment impact on wood was evaluated by spectroscopy analysis (FTIR, Raman), and the increase in wood wettability was demonstrated (contact angle decreases from 50° to 35° after the surface delignification). Then, PECs consisting of polyethyleneimine and sodium phytate were surface impregnated in wood anddelignified wood. The flame retardancy was evaluated using a cone calorimeter. Despite the increase in weight gain(1.5 wt.-%±0.3 wt.-% to 4.3 wt.-%±2.5 wt.-%), fire performance was not improved. This study demonstrates that lignin strongly affects char formation, even in the presence of PECs.

The Effects of Nitrogen‐Containing Monomers on the Thermal Degradation and Combustion Attributes of Polystyrenes Chemically Modified With Phosphonate Groups

AbstractIn the present study, polystyrene (PS) is chemically modified with diethyl(acryloyloxymethyl)phosphonate (DEAMP) and an N‐containing monomer, selected from different classes of compounds, via a ter‐polymerization route; thus, exploring possible P–N synergistic effects on fire retardance of the base polymer. The successful incorporation of P and N monomeric units is confirmed by Fourier Transform Infrared (FT‐IR), 1H and 31P Nuclear Magnetic Resonance (NMR) spectroscopies. The thermal degradation and combustion attributes of modified polymeric materials are measured using standard techniques, including Thermo‐Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), “bomb” calorimetry, and Pyrolysis Combustion Flow Calorimetry (PCFC). The thermal and combustion studies demonstrate that the thermal stability and combustion characteristics of styrenic polymers are significantly altered by the presence of even nominal amounts of P‐ and N‐containing groups, and in certain cases, synergistic interactions of these groups are also evident. For instance, as revealed by TGA, the extent of char formation, under the oxidative atmosphere, in the prepared ter‐polymers, is enhanced by 16–44%, when compared to the unmodified PS. The heat release rates and heat release capacities of ter‐polymers, measured using the PCFC technique, are reduced by 18–50%, in comparison to the same parameters obtained for the unmodified counterpart.

Insights into pyrolysis process of coconut shell waste hydrochar: In-situ structural evolution and reaction kinetics

Coconut shell (CS) can be efficiently utilized by coupling hydrothermal carbonization (HTC) pretreatment with pyrolysis to meet the goal of global green development. In-situ Raman spectroscopy and thermogravimetric analyzer (TGA) were adopted to investigate the structural evolutions and reaction kinetics characteristics of hydrochar during the pyrolysis process. According to the results, the intensity of peak D and peak G in Raman spectrum decreased with increasing hydrothermal degree. Mild HTC treatment (≤200 °C) led to an increase in ID/IG, indicating the formation of large aromatic rings through small ring condensation. In contrast, severe HTC treatment (≥220 °C) reduced the ID/IG ratio, indicating an increased polycondensation of aromatic compounds and a higher degree of aromatization, resulting in larger molecules sizes. The spectral area below 400 °C reduced with increasing pyrolysis temperature due to the reduction of oxygen-containing functional groups and thermal radiation. Above 400 °C, the total area remains stable as the Raman spectrum scattering ability increases due to char formation and aromatization reactions. HTC-200 exhibited an increase in ID/IG from 0.54 to 0.72 and ID/I(Me + Ke + GL) from 0.58 to 0.68 with increasing pyrolysis temperatures. The activation energy during pyrolysis ranged from 194.50 to 238.34 kJ/mol for CS and 217.51–259.51 kJ/mol for HTC-200. Hydrothermal carbonization had a significant effect on pyrolysis behavior of hydrochar, as it was found that the mild HTC hydrochar followed D-type diffusion models when conversion rate was less than 0.65, and transitioned to higher reaction order in the late-stage of pyrolysis.

Utilization of spent coffee grounds as charring agent to prepare flame retardant poly(lactic acid) composites with improved toughness

The objective was to utilize spent coffee grounds (SCG) as charring agent to combine with ammonium polyphosphate (APP) to prepare flame retardant poly(lactic acid) (PLA) composites with improved toughness. PLA/APP-SCG and PLA/APP-SCG/KH560 composites were prepared, and silane coupling agent KH560 was applied to improve particle-matrix interfacial compatibility. The particle-matrix interface, char formation, flame retardancy, mechanical properties and fracture morphology of PLA composites were studied. Results showed that PLA/APP-SCG5% and PLA/APP-SCG20% passed UL-94 V-0 rating, and increase in charred residues was favorable for improving flame retardancy. Improved toughness was also obtained compared to PLA, attributed to debonding of APP from matrix under external force as well as plasticization effect of coffee oil contained in SCG. PLA/APP-SCG5%/KH560 and PLA/APP-SCG20%/KH560 showed smaller elongation at break and impact strength compared to PLA/APP-SCG5% and PLA/APP-SCG20%, respectively. The improved interfacial compatibility was unfavorable for debonding of APP from matrix, and both APP and SCG played the role of enhancing strength, thus decreasing toughness. PLA/APP-SCG/KH560 counterparts were actually set as parallel samples to prove that PLA/APP-SCG composites showed improved toughness with weak interfacial compatibility. This study has provided a practical approach to utilize bio-derived wastes as charring agent to prepare flame retardant PLA composites with enhanced toughness.