De-Yi Wang

Functionalized layered double hydroxide-based epoxy nanocomposites with improved flame retardancy and mechanical properties

Functionalized layered double hydroxides (LDHs) based on a multi-modifier system composed of hydroxypropyl-sulfobutyl-beta-cyclodextrin sodium (sCD), dodecylbenzenesulfonate (DBS) and taurine (T) have been designed and fabricated in this paper, aiming at developing high performance fire retardant epoxy nanocomposites. In this multi-modifier system, sCD was utilized to improve the char yield, DBS was used to enlarge the inter-layer distance of LDH and T was used to enhance the interaction between the epoxy matrix and LDH layers. Based on these functionalized LDHs, bisphenol A epoxy resin and diamino diphenyl sulfone (DDS), a series of functionalized LDH/epoxy nanocomposites have been developed. The structural morphologies of the LDH/epoxy nanocomposites were investigated by transmission electron microscopy (TEM) and wide-angle X-ray scattering (WAXS), revealing that sCD-DBS-T-LDH/EP shows a much better dispersion state than the epoxy composites containing unmodified LDHs or single modifier modified LDHs. Importantly, with only 6 wt% functionalized LDH, the sCD-DBS-T-LDH/EP nanocomposite reached V0 rating in the UL-94 vertical burning test. Furthermore, the incorporation of sCD-DBS-T-LDH into epoxy resins led to a significant reduction in peak heat release rate, total heat release and total smoke production, which exhibited superior fire resistance over its counterparts at the equivalent filler loading. The notably improved fire resistance could be attributed to the formation of the consolidated and compact char layers during combustion which significantly suppressed heat and mass transfer between the polymer matrix and flame zone. Additionally, sCD-DBS-T-LDH/EP showed the best impact and tensile strength compared to those of other LDH/epoxy composites. This work offers a new approach to develop high performance fire retardant polymer/LDH nanocomposites.

Few layered Co(OH)2 ultrathin nanosheet-based polyurethane nanocomposites with reduced fire hazard: from eco-friendly flame retardance to sustainable recycling

Huge consumption of thermoplastic polyurethane (TPU) results in two serious challenges for our society: fire hazards and environment pollution. To address these challenges, on the one hand, ultrathin β-Co(OH)2 nanosheets were devised and synthesized by a surfactant self-assembly technique and their application in TPU reduced the fire hazard; on the other hand, a green method was developed to sustainably recycle TPU nanocomposites into high value-added carbon materials. The structural characterizations demonstrate that the ultrathin β-Co(OH)2 nanosheets showed a typical two-dimensional lamellar morphology, possessing a lateral dimension of ca. 2 μm and a low thickness of ca. 2.6 nm, corresponding to the two layers of β-Co(OH)2. The incorporation of ultrathin β-Co(OH)2 nanosheets into the TPU matrix effectively reduced the heat release and restrained the toxicity of the volatiles, which was attributed to the ultrathin β-Co(OH)2 nanosheets in the TPU matrix offering an enhanced barrier effect and catalytic charring capability and Co3O4 decomposed from β-Co(OH)2 possessing superior catalytic oxidation of CO. Moreover, a green autocatalytic process in a sealed autoclave was developed to convert TPU/Co(OH)2 nanocomposites into high value-added Co/CNTs nanocomposites with more than 85% yield. This innovative idea may be expanded to other polymer systems and opens a new door to developing high performance polymer nanocomposites via green approaches and in particular, the sustainable recycling of the polymer-based materials.

A novel biobased epoxy resin with high mechanical stiffness and low flammability: synthesis, characterization and properties

Exploring renewable biobased epoxy resins possessing intrinsic fire retardancy and high mechanical and thermal properties will greatly advance their potential to satisfy sustainability demands. Herein we develop a biobased route to synthesize a novel eugenol-based difunctional epoxy resin (TPEU-EP) with a full aromatic ester backbone. With 3,3′-diaminodiphenyl sulfone (33DDS) as the curing agent, TPEU-EP is compared with a standard bisphenol A epoxy resin (DGEBA) regarding their cure reactions and ultimate properties. The results show that TPEU-EP/33DDS expresses a higher reaction activation energy and a slower curing rate than DGEBA/33DDS. The isothermal cure reaction of TPEU-EP/33DDS is found to be autocatalytic. We accurately model the curing kinetics and elaborate on the related mechanisms based on the isoconversional analysis. The structure–property study reveals that TPEU-EP/33DDS manifests a 27%, 20% and 17% higher storage modulus (30 °C), Youngs modulus and hardness than DGEBA/33DDS, respectively. TPEU-EP/33DDS displays a high glass temperature (168.4 °C) and thermal stability (up to 300 °C), and shows a much higher damping than DGEBA/33DDS in the glassy state. Moreover, compared with DGEBA/33DDS, TPEU-EP/33DDS shows a 130% and 3.3 increase in char yield (in N2) and limiting oxygen index and a 68% and 40% decrease in the heat release rate and total heat release (microscale combustion test), respectively. Impressively, TPEU-EP/33DDS can self-extinguish in a vertical burning test, and the cone calorimeter test further confirms that TPEU-EP/33DDS has a much improved flame retardancy with a notably lowered smoke production. In brief, TPEU-EP possesses good intrinsic flame retardancy, low smoke production, and excellent mechanical properties, showing high promise for application. Our contribution will open a new avenue to develop sustainable high-performance flame-retardant epoxy resins.

Multifunctional intercalation in layered double hydroxide: toward multifunctional nanohybrids for epoxy resin

Multifunctional intercalation in layered double hydroxide (LDH) has been developed via designed multi-modifiers with varied functions in order to transfer these functions to epoxy materials by using nanocarriers. The functions of the multi-modifier system include: (i) functionalized hydroxypropyl-sulfobutyl-beta-cyclodextrin (sCD) and phytic acid (Ph) aiming at enhancing flame retardancy; (ii) sodium dodecylbenzenesulfonate (SDBS) enlarges the interlayer space of LDHs for facilitating LDHs dispersion in the epoxy matrix; (iii) chalcone imparts the anti-UV property to the epoxy matrix. In contrast to conventional LDH-based epoxy composites, the functionalized LDH-(fLDH) based epoxy nanocomposites show a significant improvement in both flame retardancy, such as passing the UL94 V0 rating and reduction of the peak heat release rate over 70%, and anti-UV properties in terms of well maintenance on the impact, flexural and micro-mechanical properties after 100, 200, 300, and 400 h of UV irradiation, respectively. Such multifunctional nanohybrids may be used to develop varied functional and sustainable epoxy-based materials for some advanced applications.

Intumescent multilayer hybrid coating for flame retardant cotton fabrics based on layer-by-layer assembly and sol–gel process

An intumescent coating composed of a nitrogen-modified silane hybrid (SiN) and phytic acid (PA) was deposited on cotton fabric through layer-by-layer assembly in order to reduce flammability. SiN was synthesized via a sol–gel process and characterized by 29Si-nuclear magnetic resonance. This intumescent coating system lowered the thermal stability of the cotton due to the catalyzed effect on degradation, but significantly improved the char formation. In a vertical flame test, fabrics coated with 15 bilayers (BLs) of SiN–PA extinguished the flame immediately upon removing the ignition source, while untreated cotton was completely burned out. Cone calorimeter data revealed that 15BL-coated cotton resulted in a 31% and 38% reduction in peak heat release rate and total heat release, respectively, relative to those of the uncoated control. This superior fire retardant performance is believed to be attributed to the formation of intumescent char layer on fibers that could effectively inhibit the oxygen and heat permeation when burning. In addition, as evidenced by thermogravimetric analysis-Fourier transform infrared spectroscopy results, the increased amount of inflammable gases and the decreased amount of flammable gases during the degradation of coated cotton fabrics was another important factor to improve the flame resistance. These results demonstrate that the combination of layer-by-layer assembly and sol–gel method will provide an effective alternative to current flame retardant treatments.

Synthesis and characterization of functional eugenol derivative based layered double hydroxide and its use as a nanoflame-retardant in epoxy resin

Aiming to develop a multi-functional flame retardant for epoxy resins, a novel bio-based eugenol derivative containing silicon and phosphorus [((1,1,3,3-tetramethyldisiloxane-1,3-diyl)bis(propane-3,1-diyl))bis(2-methoxy-4,1-phenylene)bis(phenylphosphonochloridate), SIEPDP] was synthesized, and was further used to modify Mg–Al layered double hydroxide (SIEPDP-LDH). This modified SIEPDP-LDH was used as a novel nanoflame-retardant for bisphenol epoxy resins and compared with unmodified pristine LDH. X-ray diffraction (XRD) analysis justified that intercalation of SIEPDP into LDH increased the interlayer distance to 2.95 nm. Morphological analysis (XRD and TEM) revealed that the SIEPDP-LDH was dispersed well in the epoxy matrix in a partially exfoliated manner. Results from the cone calorimeter tests showed that even a low loading of SIEPDP-LDH into epoxy resin led to a significant decrease in heat release rate and total heat release compared to unmodified LDH/epoxy composites. More interestingly, SIEPDP-LDH/epoxys UL-94 classification passed V-0 with only 8 wt% loading. Moreover, the addition of SIEPDP-LDH enabled the increase in the impact strength and modulus of the cured epoxy. These data indicated that SIEPDP-LDH could serve as not only a nanoflame retardant but a good reinforcing agent as well.

An in situ polymerization approach for functionalized MoS2/nylon-6 nanocomposites with enhanced mechanical properties and thermal stability

The enhancement in the thermal and mechanical properties of polymer/inorganic nanosheet composites depends strongly on their interfacial interaction. Herein, we exfoliated bulk MoS2 into nanosheets which was subsequently functionalized using lipoic acid. Nylon-6 (PA6) nanocomposites were prepared through in situ ring-opening polymerization of e-caprolactam initiated by lipoic acid functionalized MoS2 (f-MoS2). MoS2/PA6 composites were prepared by melt blending of pristine MoS2 powder with PA6 in parallel with f-MoS2/PA6 composites. The morphology of MoS2/PA6 and f-MoS2/PA6 composites observed using scanning electron microscopy and transmission electron microscopy demonstrated that f-MoS2 exhibited a better dispersion state than pristine MoS2 within a PA6 matrix. The incorporation of f-MoS2 induced significant thermal stabilization in the PA6 matrix: with 1 wt% loading of f-MoS2, the T−5% and T−50% of the resultant PA6 nanocomposite were increased by 36 and 23 °C, respectively, relative to those of neat PA6. As demonstrated by the increase in the storage modulus and the glass transition temperature determined from dynamic mechanical analysis, f-MoS2 was more effective than MoS2 in terms of reinforcing mechanical properties of PA6. At very low fractions of f-MoS2 nanosheets (≤1% by weight), PA6 nanocomposites exhibited up to 80% improvement in tensile properties compared to neat PA6 because of strong covalent polymer–filler interfacial interactions. These findings corresponded well with the nanoconfinement effect including a nanoscale interfacial layer that can support the superior reinforcements of f-MoS2 over pristine MoS2. The in situ polymerization approach proposed herein will open a new and exciting avenue for the development of transition metal dichalcogenide reinforced polymer nanocomposites.

Impact of halogen-free flame retardant with varied phosphorus chemical surrounding on the properties of diglycidyl ether of bisphenol-A type epoxy resin: synthesis, fire behaviour, flame-retardant mechanism and mechanical properties

This work aimed to investigate the effect of two types of phosphorus-containing flame retardants (P-FRs) with different chemical surroundings (phenylphosphonate-based (PO–Ph) and phenylphosphoric-based (PO–OPh)) on the flame-retardant efficiency for diglycidyl ester of bisphenol-A type epoxy (EP) resin. The two series of P-FRs which were named as FPx and FPOx (x = 1, 2 and 3), respectively, showed reactivity with epoxy group that was examined by differential scanning calorimetry (DSC) and variable temperature FTIR spectroscopy (VT-FTIR). A comparative study between the FPx and FPOx (x = 1, 2 and 3) containing flame-retardant epoxy was carried out via investigating their flammability, thermal stability and mechanical properties. The most significant difference in flame retardancy between them was that FPx (x = 1, 2 and 3) endowed EP with a V-0 rating in UL 94 test at 5 wt% loading, while FPOx (x = 1, 2 and 3) showed no rating at such loading. Importantly, it is found that there was almost 10 times difference in the flame-retardant efficiency for EP between FPx and FPOx, though they had similar chemically molecular structures. Moreover, TGA-FTIR and TGA-MS coupling techniques (TGA, thermogravimetric analysis; MS, mass spectroscopy) were employed to study the thermal decomposition of FP1 and FPO1; the impacts of FP1 and FPO1 on the thermal decomposition of EP were studied by TGA-FTIR as well. Furthermore, an online temperature detection experiment was designed to collect the temperatures by thermocouples and infrared thermometers in the UL 94 test. Based on the above results, the flame-retardant mechanisms of FP1 and FPO1 in EP are discussed. In addition, the impact of P-FRs on mechanical properties of EP was studied by means of tensile test and dynamic mechanical analysis.

Comparative study on synergistic effect of LDH and zirconium phosphate with aluminum trihydroxide on flame retardancy of EVA composites

Flame-retardant ethylene vinyl acetate (EVA) composite based on aluminum trihydroxide (ATH), layered double hydroxide (LDH) and organo-modified zirconium phosphate (mZrP) were prepared by melt-compounding method. The synergistic effect of LDH and mZrP with ATH on the fire behavior and thermal stability of EVA composites was studied by limiting oxygen index, UL-94 test, cone calorimeter and thermogravimetric analysis. EVA composite with ATH and LDH passed the V-0 rating while EVA composite with ATH and mZrP exhibited relatively low peak heat release rate. EVA/ATH composite with 10xa0mass% LDH exhibited a char yield of 34xa0% at 700xa0°C, while its counterpart with 10xa0mass% mZrP showed 29xa0%, indicating LDH possessed superior flame-retardant synergistic efficiency with ATH over mZrP in terms of promoting char formation. Regarding the heat release rate (HRR), EVA/ATH composite with 10xa0mass% mZrP displayed a 73xa0% reduction in PHRR, whereas its counterpart with the equivalent loading of LDH showed a lower flame-retardant synergistic efficiency (a 58xa0% reduction in peak HRR). The results above demonstrated that LDH mainly functioned as catalyst in char formation, while mZrP was beneficial to restraining heat release.

Bioinspired polydopamine-induced assembly of ultrafine Fe(OH)3 nanoparticles on halloysite toward highly efficient fire retardancy of epoxy resin via an action of interfacial catalysis

Inspired by the core–sheath-dot structure of corncobs, halloysite nanotubes (HNT) were sequentially functionalized with a biomimetic polydopamine (PDA) nanocoating and ultrafine Fe(OH)3 nanoparticles to prepare hierarchical HNT@PDA@Fe(OH)3, with the aim of endowing epoxy resin (EP) with improved fire retardancy, thermal stability and mechanical properties. The target product was characterized via Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). As a result, in a study of thermal degradation a nanocomposite of EP with HNT@PDA@Fe(OH)3 generated a notably higher yield of char and exhibited a lower maximum degradation rate than its counterparts. An investigation of fire retardancy revealed that EP/5HNT@PDA@Fe(OH)3 possessed an LOI value of 33.9% and a UL-94 vertical burning rating of V-1, which represent significant enhancements in comparison with neat EP (LOI = 24.1%, no rating). In a cone calorimeter test (CCT) at 50 kW m−2, EP/5HNT@PDA@Fe(OH)3 gave rise to a 41% reduction in peak heat release rate (pHRR) relative to that of EP/5HNT. A TG-FTIR test disclosed that HNT@PDA@Fe(OH)3 notably decreased the evolution of volatiles (CO, aliphatic compounds, aromatic compounds, and carbonyl compounds), which resulted in less flammable gases. Variable-temperature FTIR, Raman spectra and SEM observations revealed that char with a more compact and continuous structure was obtained with HNT@PDA@Fe(OH)3. In addition, the tensile strength and modulus were remarkably enhanced, accompanied by an increase in the dynamic storage modulus (E′). Finally, a probable mechanism was proposed to account for the improved fire retardancy, which involved catalytic charring behavior at the interface (determined by TG-GC-MS) and intensive protection by char.