Effect of chemical structure and cross-link density on the heat resistance of phenolic resin

Boosting Molecular Cross-Linking in a Phenolic Resin for Spherical Hard Carbon with Enriched Closed Pores toward Enhanced Sodium Storage Ability

Phenolic resin (PF) is considered a promising precursor of hard carbon (HC) for advanced-performance anodes in sodium-ion batteries (SIBs) because of its facile designability and high residual carbon yield. However, understanding how the structure of PF precursors influences sodium storage in their derived HC remains a significant challenge. Herein, the microstructure of HC is controlled by the degree of cross-linking of resorcinol-benzaldehyde (RB) resin. We reveal that robust molecular cross-linking in RB resin induced by hydrothermal treatment promotes closed-pore formation in the derived HC. The mechanism is devised for the decomposition of a highly cross-linked RB three-dimensional network into randomly stacked short-range graphitic microcrystals during high-temperature carbonization, contributing to the abundant closed pores in the derived HC. In addition, the high cross-linking degree of RB resin endows its derived HC with a small-sized spherical morphology and large interlayer spacing, which improves the rate performance of HC. Consequently, the optimized hydrothermal treatment HC anode shows a higher specific capacity of 372.7 mAh g-1 and better rate performance than the HC anode without hydrothermal treatment (276.0 mAh g-1). This strategy can provide feasible molecular cross-linking engineering for the development of closed pores in PF-based HC toward enhanced sodium storage.

Average and Local Effect of Thermal Fatigue on the Coefficients of the Thermal Expansion of a Complex Continuous Composite Fibre Used for Car Clutch Facing: A Multi-Technique Study

The clutch facing is a complex organic matrix composite in dry clutch systems. When the clutch engages, there is a sliding contact between the clutch facing and the mating surfaces, resulting in temperature increases of up to 300 °C. These thermal cycles activate several mechanisms that can have consequences on such material: cracking and, more generally, the ageing of the polymer. The thermomechanical properties of the material therefore evolve according to the number of thermal cycles. This study focused on investigating the local and average evolution of the coefficients of thermal expansion (CTE) of clutch facing as a function of thermal cycles. Several techniques were employed, including image stereocorrelation for determining the CTE, Dynamic Mechanical Analysis (DMA) tests for monitoring the ageing of the material and acoustic emission for highlighting the damage. The results showed that the average CTE decreased as a function of the temperature and the number of loading cycles, while locally, it increased in some areas and decreased in others. These differences appear to be the result of material heterogeneity (actual yarn tracing, etc.) and interaction between cracking and ageing mechanisms in the polymer matrix. Indeed, thermal cycling led to cracking and additional crosslinking, which is influenced by ageing conditions.

Effect of elastomeric urethane monomer on physicochemical properties and shrinkage stress of resin composites

This study aimed to evaluate the effect of an elastomeric urethane monomer (Exothane-24) in different concentrations on physicochemical properties, gap formation, and polymerization shrinkage stress of experimental resin composites. All experimental composites were prepared with 50 wt.% of Bis-GMA and 50 wt.% of TEGDMA, to which 0 wt.% (control), 10 wt.%, 20 wt.%, 30 wt.%, and 40 wt.% of Exothane-24 were added. Filler particles (65 wt.%) were then added to these resin matrixes. Ultimate tensile strength (UTS: n = 10), flexural strength (FS: n = 10), flexural modulus (FM: n = 10), hardness (H: n = 10), hardness reduction (HR: n = 10), degree of conversion (DC: n = 5), gap width (GW: n = 10), and polymerization shrinkage stress in Class I (SS-I: n = 10) and Class II (SS-II: n = 10) simulated configuration. All test data were analyzed using one-way ANOVA and Tukey's test (α = 0.05;  = 0.2). Exothane-24 in all concentrations decreased the H, HR, DC, GW, SS-I, and SS-II (p < 0.05) without affecting the UTS, and FS (p > 0.05). Reduction in FM was observed only in the Exothane 40% and 30% groups compared to the control (p < 0.05). Exothane-24 at concentrations 20% and 30% seems suitable since it reduced GW and polymerization SS without affecting the properties of the composite resins tested, except for H.

Closed-Loop Recyclable High-Performance Polyimine Aerogels Derived from Bio-Based Resources

Organic aerogels are an intriguing class of highly porous and ultralight materials which have found widespread applications in thermal insulation, energy storage, and chemical absorption. These fully cross-linked polymeric networks, however, pose environmental concerns as they are typically made from fossil-based feedstock and the recycling back to their original monomers is virtually impossible. In addition, organic aerogels suffer from low thermal stability and potential fire hazard. To overcome these obstacles and create next-generation organic aerogels, a set of polyimine aerogels containing reversible chemical bonds which can selectively be cleaved on demand is prepared. As precursors, different primary amines and cyclophosphazene derivatives made from bio-based reagents (vanillin and 4-hydroxybenzaldehyde) to elevate the thermal stability and reduce the environmental impact are used. The resulting polyimine aerogels exhibit low shrinkage, high porosity, large surface area, as well as pronounced thermal stability and flame resistance. More importantly, the aerogels show excellent recyclability under acidic conditions with high monomer recovery yields and purities. This approach allows for preparation of fresh aerogels from the retrieved building blocks, thus demonstrating efficient closed-loop recycling. These high-performance, recyclable, and bio-based polyimine aerogels pave the way for advanced and sustainable superinsulating materials.

Influence of dodecyl surfactants on the cross-linking, plasticization and damping behavior of epoxy novolac resins

A series of modified epoxy novolac resins (ENRs) were prepared by incorporating dodecyl chain surfactants with polar groups such as amine, carboxylic acid, phenol, resorcinol and benzene sulfonic acid. Except for the case of benzene sulfonic acid, no macrophase separation is seen in any of the modified ENRs. The addition of these dodecyl surfactants to ENR hinders the cross-linking reaction as revealed from their dynamic and isothermal DSC curing measurements and reduced glass transition temperatures (T_g). The cross-link density evaluated from the storage modulus (E') in the rubbery region using DMTA measurements is found to be low with the addition of surfactants in agreement with their plasticization behavior. The stiffness of the materials obtained at low temperatures showed a moderate increase for ENRs with carboxylic acid and phenol surfactants. Upon heating, their storage modulus drops at low temperatures compared to ENR supporting the mechanism of plasticization in them. This high value of E' for the plasticized material in the glassy phase is different from the generally known behavior. Such unusually high storage modulus together with increased 'd' spacing from the XRD results seems to indicate 'partial segmental confinement' of epoxy chains. It is believed that high damping obtained in these two materials is due to 'partial segmental confinement' of epoxy chains because of interaction with lauric acid and phenolic surfactants and the associated internal and interfacial friction in them. Dielectric relaxation measurements support the plasticization process based on the high dielectric loss and ionic conductivity in the surfactant modified ENR, as compared to the pristine ones.

Tailoring a Phenolic Resin Precursor by Facile Pre-oxidation Tactics to Realize a High-Initial-Coulombic-Efficiency Hard Carbon Anode for Sodium-Ion Batteries

As the leading anode material for sodium-ion batteries (SIBs), hard carbon (HC) still faces the puzzle of low initial Coulombic efficiency (ICE) in achieving commercialization. From the perspective of precursors, the low ICE has been attributed to the large specific surface area and porosity produced by the rapid decomposition of polymers during the carbonization. Therefore, increasing the cross-linking degree of precursors will be an effective shortcut to improve the ICE. Herein, a facile pre-oxidation tactic was successfully employed to tailor the cross-linking degree of phenolic resin precursors to precisely control the specific surface area of the obtained HC. As the pre-oxidation time is increased, the optimal HC with the lowest specific surface area shows an ICE elevated by 22.2% (from 62.5 to 84.7%) compared to the original pre-oxidation HC and delivers a high reversible capacity of 334.3 mAh g^-1 at 20 mA g^-1. Besides, the pre-oxidation also introduces abundant carbonyl groups, which increase the disorder degree of HC and supply abundant adsorption sites of Na⁺, thus enhancing the rate performance. When matched with a layered O3-NaNi_1/3Fe_1/3Mn_1/3O₂ cathode, the full cell achieves an energy density of ca. 256.2 Wh kg^-1 with superior rate performance. This work sheds light on the positive effect of pre-oxidation in elevating the ICE of HC and provides effective guidance to achieve a high ICE for other HC materials.

Diffusion of water and protein drug in 1,4-butanediol diglycidyl ether crosslinked galactomannan hydrogels and its correlation with the physicochemical properties

The aim of the present study was to obtain a better and safer galactomannan-based material for drug release applications. A novel epoxy-crosslinked galactomannan hydrogel (EGH) was prepared from guar gum using 1,4-butanediol diglycidyl ether as a crosslinking agent. The diffusion rate constant of water molecules in freeze-dried EGH positively correlated with water uptake/equilibrium swelling rate (WU/ESR), and the water molecules participated in Fickian diffusion. The ESR, WU/ESR, and bovine serum albumin (BSA) loading capacity of a customized EGH with a crosslinking density of 48.9% were 48.7 ± 0.15 g/g, 95.3%, and 56.4 mg/g, respectively. The release of BSA from freeze-dried EGH was affected by the WU/ESR and the pH; the release equilibrium time was ~40 h at pH 1.2, decreasing to ~24 h at pH 7.4. Furthermore, the cumulative release rate increased from 63.5% to 80.7% and the t₅₀ decreased from 59 to 41 min upon changing from the acidic to basic pH. The release process conformed to the Ritger-Peppas and Hixson-Crowell models, and represented Fickian diffusion and chain relaxation. The EGH showed no cytotoxicity toward HeLa cells. Together, these results demonstrate the properties of a novel galactomannan-based hydrogel that can potentially be employed as a vehicle for drug delivery.

Preparation and electrochemical performance of activated carbon microspheres from recycled novolak phenol formaldehyde

In this study, an attempt is made to obtain porous activated carbon microspheres (ACMs) as supercapacitor electrodes by recycling waste novolak phenol formaldehyde (NPF) resins. These NPF-ACMs were prepared by a three-step procedure of hydrothermal synthesis, carbonization, and activation in turn. The effects of temperature, time, and sodium dodecyl sulfonate (SDS) addition on NPF-based microspheres were studied by the orthogonal method. The optimal preparation process of NPF-based microspheres was the following: 230 °C, 4 h, and a mass ratio of SDS: NPF of 24:1 by hydrothermal synthesis. Based on the above optimal conditions, NPF-ACMs were made, the yield of the microspheres after carbonization and chemical activation are 54% and 38%, and their electrochemical properties were analyzed. The NPF-ACMs had uniform size, a high surface area of 2528 m² g^-1, good dispersion, a low impedance of 0.46 Ω, highest specific capacitance of 118.6 F g^-1 at 0.5 A g^-1, good rate capability with 79% retention from 0.1 to 10 A g^-1. Moreover, it showed high capacitance retention of 99.5% after 1000 cycles at a scan rate of 5 mV s^-1. The results showed that waste NPF can be used as promising ACMs of the electrode material to increase its utilization value.

Synthesis of Silicon Hybrid Phenolic Resins with High Si-Content and Nanoscale Phase Separation Structure

In this paper, a set of silicon hybrid phenolic resins (SPF) with high Si-content were prepared by mixing phenolic resins with self-synthesized silicon resins. In order to obtain the nanoscale phase structure, condensation degree and the amount of Si-OH groups in silicon resins were controlled by the amount of inhibitor ethanol in the hydrolytic condensation polymerization of siloxane. Increasing the amount of ethanol resulted in more silanol groups and a lower degree of condensation for silicon resins, which then led to more formation of Si-O-Ph bonds in hybrid resin and improved compatibility between silicon resin and phenolic resin. When 400% ethanol by weight of siloxane was used in the sample SPF-4, nanoscale phase separation resulted. The residual weight of the cured SPF-4 at 1000 °C (R1000) significantly increased compared to pure phenolic resins. The result of the oxyacetylene flame ablation and the Cone Calorimeter test confirmed the improved ablative property and flammability after the modification. The performance improvement of the cured SPF-4 was attributed to the nanoscale phase structure and high silicon content, which promoted the formation of dense silica protective layers during pyrolysis.

Facile surfactant-free synthesis of polybenzoxazine-based polymer and nitrogen-doped carbon nanospheres

Facile surfactant-free synthesis of monodisperse polymer and nitrogen-doped carbon nanospheres with controllable sizes less than 200 nm based on the benzoxazine chemistry.

Super flame-retardant lightweight rime-like carbon-phenolic nanofoam

The desire for lightweight nanoporous materials with high-performance thermal insulation and efficient anti-ablation resistance for energy conservation and thermal protection/insulation has greatly motivated research and development recently. The main challenge to synthesize such lightweight materials is how to balance the relationship of low thermal conductivity and flame retardancy. Herein, we propose a new concept of lightweight “rime-like” structured carbon-phenolic nanocomposites to solve this problem, where the 3D chopped network-structured carbon fiber (NCF) monoliths are incorporated with nanoporous phenolic aerogel to retain structural and functional integrity. The nanometer-scaled porous phenolic (NP) was synthesized through polymerization-induced phase separation and ambient pressure drying using phenolic resin (PR) solution as reaction source, ethylene glycol (EG) as solvent and hexamethylenetetramine (HMTA) as catalyst. We demonstrate that the as-prepared NCF-NP nanocomposite exhibits with a low density of 0.25–0.35 g/cm³, low thermal conductivity of 0.125 Wm⁻¹K⁻¹ and outstanding flame retardancy exceeding 2000 °C under arc-jet wind tunnel simulation environment. Our results show that the synthesis strategy is a promising approach for producing nanocomposites with excellent high-temperature heat blocking property.

Microstructure, Mechanical and Thermal Properties of Three-dimensional Braided Glass Fiber Reinforced Phenolic Cryogel Composites

Phenolic cryogel plates (PCPs) and three-dimensional braided glass fiber reinforced phenolic cryogel composite plates (PCCPs) were successfully fabricated by microemulsion-templated sol gel polymerization and freeze-drying methods. Then, pore morphology, mechanical and thermal properties of the composites were investigated. The experimental results showed that the aerogels made by freeze-drying method had the best microstructure. In addition, it was discovered that the compressive and tensile strength of the cryogel plates had equally increased for approximately 8 and 30 times after compounding with glass fiber. Simultaneously, the mechanical properties of the 20 wt% and 25% solid content (PCPs) achieved the optimal value compared with PCPs of other contents. Moreover, it was found that the cryogels had excellent thermostability, and their thermal conductivity decreased with the reducing of the solid contents, what's more, the joining of glass fiber had the increased the composites’ thermal conductivity to some degree. Finally, considering the requirement of low density, good mechanical and thermal performance, the 20 wt% solid content PCCP had the best comprehensive performance compared with others.

The Effect of Introducing B and N on Pyrolysis Process of High Ortho Novolac Resin

In this contribution, high ortho novolac resins modified with phenylboronic acid were synthesized. The thermal stability of novolac resins cured with hexamethylenetetramine (HMTA) and chemical states of B and N via a pyrolysis process were studied. For the cured o-novolac modified with phenylboronic acid, the temperature with maximum decomposition rate increased by 43.5 °C, and the char yield increased by 5.3% at 800 °C compared with cured o-novolac. Density functional theory (DFT) calculations show the existence of hydrogen bonding between N of HMTA and H of phenol in modified resin. Thus, N could still be found at high temperature and C=N structure could be formed via a pyrolysis process. B₂O₃ was obtained at 400 °C by the cleavage of B–O–C and B–C bonds and it reduces the oxygen loss which may take part in the formation of carbon oxides in the system. The melting B₂O₃ on the surface of the resin will prevent small molecules and carbon oxides from releasing. Moreover, introducing B into the system helps to decrease the interlayer distance and improve graphite structures via a pyrolysis process.

Preparation, characterization, and properties of graphene-based composite aerogels via in situ polymerization and three-dimensional self-assembly from graphene oxide solution

In this work, hierarchical porous graphene-based composite aerogels are synthesized by a simple and facile one-pot polymerization-induced phase separation.

Thermal stability of phenolic resin: new insights based on bond dissociation energy and reactivity of functional groups

Based on bisphenol-F-like model molecules, the bond dissociation energies and Fukui function were calculated to interpret the relationship between the atomistic structure and thermal properties of the phenolic resin.