Evaluation of activation energy conformity derived from model-free non-isothermal predictions and Arrhenius isothermal results

Organically modified montmorillonite (OMMT) was used as synergist to enhance the flame-retardant and mechanical properties of poly(butylene succinate)/intumescent flame retardant (PBS/IFR) composites. The flame-retardant, thermal degradation and combustion properties of PBS and its flame-retardant composites were characterized by limiting oxygen index (LOI) test, vertical burning (UL-94) test, thermogravimetric analysis, cone calorimeter and scanning electron microscopy, respectively. The results indicate that PBS/IFR composites exhibit excellent flame retardance when OMMT is at an appropriate content. PBS/IFR composite with 20 wt% IFR and 1.5 wt% OMMT has an LOI of 40.1% and can pass the UL-94 V0 rating. The synergistic effect between OMMT and IFR on the flame-retardant properties of PBS depends on the content of OMMT, and excessive OMMT diminish this synergistic effect. The possible flame-retardant mechanism of OMMT on PBS/IFR composite is proposed. The results of mechanical test also indicate that OMMT can effectively increase the notched impact strength of PBS/IFR composites.     

Metal‐organic framework MIL‐53 (Fe)@C/graphite carbon nitride hybrids with enhanced thermal stability,flame retardancy,and smoke suppression for unsaturated polyester resin

Metal‐organic framework MIL‐53 (Fe)@C/graphite carbon nitride hybrid (MFeCN), a novel flame retardant, was synthesized by hydrothermal reaction and subsequently added into unsaturated polyester resin (UPR). The structure, morphology, and thermal stability of MFeCN were characterized by Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy‐dispersive X‐ray spectroscopy (EDS), and thermogravimetric analysis (TG). The thermal stability and flammability of the UPR composites were characterized by TG and cone calorimeter tests (CCT). The results of CCT demonstrated that the peak heat release rate (pHRR), total heat release (THR), peak smoke production rate (pSPR), and total smoke production (TSP) of UPR/MFeCN‐4 were reduced by 39.8%, 10.2%, 33.3%, and 14.5%, respectively, comparing with UPR. The results of TG and CCT indicated that MFeCN could improve the thermal stability, flame retardancy, and smoke suppression properties of the UPR composites. The residues after CCT were then characterized by laser Raman spectroscopy (LRS), XPS, and SEM. Finally, based on the above experimental results and analysis, the flame retardancy mechanism of MFeCN was proposed.     

Organic‐montmorillonite modified shape memory epoxy composite

A series of organic‐montmorillonite (OMMT) modified shape memory epoxy (SMEP) composites were prepared for the purpose of application on space deployable structures. Tensile test, dynamic mechanical analysis (DMA), X‐ray diffraction (XRD), scanning electron microscope (SEM), and fold‐deploy shape memory test methods were used to characterize the mechanical, structure, and shape memory properties of these materials. The results showed addition of OMMT could improve the composites' toughness, tensile strength, transition temperature, and shape recovery speed, while shape recovery ratio was unaffected. Composite with 3wt%. OMMT had the optimum combination property. It could fully recover its original shape in about 2 min at 185°C under the maximum bending angle of 180°. Its elongation at break and tensile strength were increased by 835 and 17.4%, respectively, compared to that of neat SMEP. The transition temperature also slightly increased. Copyright © 2010 John Wiley & Sons, Ltd.     

Organically modified montmorillonite as a synergist for intumescent flame retardant against the flammable polypropylene

Low flame retardant efficiency is a key bottleneck for currently available retardants against the flammable polypropylene (PP). Herein, the organically modified montmorillonite (OMMT) was utilized as a synergist for our previously reported intumescent flame retardant (IFR) that was constructed from ammonium polyphosphate (APP) and hyperbranched charring foaming agent (HCFA) to further enhance the retardant efficiency against PP. The resultant's combustion behavior was thoroughly investigated by cone calorimetry, limiting oxygen index (LOI), vertical burning test (UL‐94), and scanning electron microscopy (SEM). The results showed that 20% addition of IFR with OMMT showed a positive effect and improved the flame retardancy of the PP systems. Especially, addition of 2 wt% OMMT obviously increased the LOI values of PP systems with 20% total loading flame retardants from 29% to 31.5% and the samples meet V‐0 rating as well as the reduction of the heat release rate (HRR), total heat release (THR), CO₂, and CO production occurred. On the other hand, the SEM images were also revealed that OMMT initiated a dense and strong char on the surface of the material, which resulted in efficient flame retardancy of PP matrix during combustion. In addition, thermal degradation behavior discussed by thermogravimetric analysis (TGA) indicated that OMMT could improve the thermal stability of PP systems under high temperature, and promoted char residues of PP/IFR systems. Copyright © 2016 John Wiley & Sons, Ltd.     

Synergistic flame‐retardant effect of DOPO‐based derivative and organo‐montmorillonite on glass‐fiber‐reinforced polyamide 6 T

Herein, a bridged 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) derivative (PN‐DOPO) in combination with organ‐montmorillonite (OMMT) was used to improve the flame retardancy and mechanical properties of glass‐fiber‐reinforced polyamide 6 T (GFPA6T). The flame retardancy and thermal stabilities of the cured GFPA6T composites were investigated using limiting oxygen index, vertical burning (UL‐94) test, cone calorimeter test, and thermogravimetric analysis (TGA). The morphological analysis and chemical composition of the char residues after cone calorimeter tests were characterized via scanning electron microscopy and energy dispersive spectrometry. The results indicate that 2 wt% OMMT combined with 13 wt% PN‐DOPO in GFPA6T achieved a V‐0 rating in UL‐94 test. The peak heat release rate and total smoke release remarkably decreased with the incorporation of OMMT as compared to those of GFPA6T/15 wt% PN‐DOPO. The TGA results show that the thermal stability and residual mass of the samples effectively increased with the increase in OMMT content. The morphological analysis and composition structure of the residues demonstrate that a small amount of OMMT could help form a more thermally stable and compact char layer during combustion. Also, with the incorporation of OMMT, the layers consisted of more carbon‐silicon and aluminum phosphate char in the condensed phase. Furthermore, GFPA6T/PN‐DOPO/OMMT composites exhibited excellent mechanical properties in terms of flexural modulus, flexural strength, and impact strength than the GFPA6T/PN‐DOPO system. The combination of PN‐DOPO and OMMT has improved the flame retardancy and smoke suppression of GFPA6T without compromising the mechanical properties.     

Improvement of thermoplastic polyurethane's flame retardancy and thermal conductivity by leaf‐shaped cobalt‐zeolitic imidazolate framework–modified graphene and intumescent flame retardant

In this work, a new type of leaf‐shaped cobalt‐zeolitic imidazolate framework–modified graphene (Co‐ZIF‐L@RGO) hybrid was successfully prepared and blended with an intumescent flame retardant (IFR). It was added into thermoplastic polyurethane (TPU) to study the effect of its combination with IFR on the thermal conductivity and flame retardant performance of TPU. The morphology and structure of the Co‐ZIF‐L@RGO hybrid were characterized by scanning electron microscope (SEM), Fourier transform infrared and X‐ray diffraction (XRD). The results showed that Co‐ZIF‐L were uniformly loaded on the surface of graphene. Furthermore, compared with pure TPU, the limiting oxygen index values of the composite material with 3 wt% Co‐ZIF‐L and 27 wt% IFR increased to 32.6%. Their UL‐94 rating reached V‐0 rating. Their peak heat release rate, total heat release, peak smoke production rate and total smoke production were also greatly reduced by 84.4%, 70.1%, 60.3% and 62.5%, respectively. The thermogravimetric‐infrared test results showed that the amount of toxic gas emissions was effectively suppressed. The residual carbon was analyzed by SEM, laser Raman spectroscopy and XRD, and flame retardant mechanism was further investigated. Besides, the addition of this hybrid improved the thermal conductivity of TPU.     

Synergistic effects of 4A zeolite on the flame retardant properties and thermal stability of a novel halogen‐free PP/IFR composite

The synergistic effects of 4A zeolite (4A) on the thermal degradation, flame retardancy and char formation of a novel halogen‐free intumescent flame retardant polypropylene composites (PP/IFR) were investigated by the means of limiting oxygen index (LOI), vertical burning test (UL‐94), digital photos, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), cone calorimeter test (CCT), laser Raman spectroscopy (LRS) and X‐ray photoelectron spectroscopy (XPS). It was found that a small amount of 4A could dramatically enhance the LOI value of the PP/IFR systems and the materials could pass the UL‐94 V‐0 rating test. Also, it could enhance the fire retardant performance with a great reduction in combustion parameters of PP/IFR system from CCT test. The morphological structures observed by digital and SEM photos revealed that 4A could promote PP/IFR to form more continuous and compact intumescent char layer. The LRS measurement, XPS and TGA analysis demonstrated that the compactness and strength of the outer char surface of the PP/IFR/4A system was enhanced, and more graphite structure was formed to remain more char residue and increase the crosslinking degree. Copyright © 2013 John Wiley & Sons, Ltd.     

Synergistic effect of cocondensed nanosilica in intumescent flame‐retardant polypropylene

Amino‐functionalized nanosilica (SiO₂‐NH₂) was prepared through cocondensation method using aminopropyltriethoxysilane as comonomer to hydrolyze and cocondense with tetraethylorthosilicate. The synergistic effect of combination of ammonium polyphosphate and pentaerythritol with SiO₂‐NH₂ on the thermal and flame‐retardant properties of intumescent flame‐retardant (IFR) polypropylene (PP) has been investigated by thermogravimetric analysis (TGA), scanning electron microscopy, Raman spectra, X‐ray diffraction (XRD), limiting oxygen index (LOI), and UL 94 tests. When 1.0 wt.% SiO₂‐NH₂ was added, the LOI value of the PP/IFR composite with 25 wt.% of IFR increased from 26.6% to 31.7%, while the UL 94 rating raised from not classified to V‐0. The TGA data demonstrated that the SiO₂‐NH₂ nanoparticles increased the charred residue of the PP/IFR composites. The morphological structures and the orderliness of the charred residue proved that SiO₂‐NH₂ promoted the formation of compact intumescent charred layer, which effectively protected the underlying polymer from burning. The XRD patterns of the charred residue indicated that nanosilica reacted with APP to form SiP₂O₇ crystal structure during combustion, which was beneficial to the formation of compact charred layers. In comparison with the inorganic SiO₂‐cal nanoparticles, the amino‐functionalized nanosilica revealed much more efficient synergistic flame‐retardant effect due to the difference of surface properties.     

Morphology and fracture behavior of toughening‐modified poly(vinyl chloride)/organophilic montmorillonite composites

Toughening‐modified poly(vinyl chloride) (PVC)/organophilic montmorillonite (OMMT) composites with an impact‐modifier resin (Blendex 338) were prepared by melt intercalation, and their microstructures were investigated with wide‐angle X‐ray diffraction, transmission electron microscopy, and scanning electron microscopy. The mechanical properties of the PVC composites were examined in terms of the content of Blendex and OMMT, and the fracture toughness was analyzed with a modified essential work of fracture model. Intercalated structures were found in the PVC/OMMT composites with or without Blendex. Either Blendex or OMMT could improve the elongation at break and notched impact strength of PVC at proper contents. With the addition of 30 phr or more of Blendex, supertough behavior was observed for PVC/Blendex blends, and their notched impact strength was increased more than 3319% compared with that of pristine PVC. Furthermore, the addition of OMMT greatly improved both the toughness and strength of PVC/Blendex blends, and the toughening effect of OMMT on PVC/Blendex blends was much larger than that on pristine PVC. Blendex and OMMT synergistically improved the mechanical properties of PVC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 286–295, 2004     

Flame retardant properties and mechanism of an efficient intumescent flame retardant PLA composites

The charring agent (CNCA‐DA) containing triazine and benzene rings was combined with ammonium polyphosphate (APP) to form intumescent flame retardant (IFR), and it was occupied to modify polylactide (PLA). The flame retardant properties and mechanism of flame retardant PLA composites were investigated by the limited oxygen index (LOI), vertical burning test (UL‐94), thermogravimetric analysis, microscale combustion calorimetry, scanning electron microscopy, laser Raman spectroscopy analysis and X‐ray photoelectron spectroscopy. The analysis from LOI and UL‐94 presented that the IFR was very effective in flame retardancy of PLA. When the weight ratio of APP to CNCA‐DA was 3:1, and the IFR loading was 30%, the IFR showed the best effect, and the LOI value reached 45.6%. It was found that when 20 wt% IFR was loaded, the flame retardancy of PLA/IFR still passed UL‐94 V‐0 rating, and its LOI value reached 32.8%. The microscale combustion calorimetry results showed that PLA/IFR had lower heat release rate, total heat release, and heat release capacity than other composites, and there was an obvious synergistic effect between APP and CNCA‐DA for PLA. IFR containing APP/CNCA‐DA had good thermal stability and char‐forming ability with the char residue 29.3% at 800°C under N₂ atmosphere. Scanning electron microscopy observation further indicated that IFR could promote forming continuous and compact intumescent char layer. The laser Raman spectroscopy analysis and X‐ray photoelectron spectroscopy analysis results indicated that an appropriate graphitization degree of the residue char was formed, and more O and N were remained to form more cross‐linking structure. Copyright © 2016 John Wiley & Sons, Ltd.     

Imidazolium‐modified clay‐based ABS nanocomposites: a comparison between melt‐blending and solution‐sonication processes

Acrylonitrile–butadiene–styrene (ABS) nanocomposites containing imidazolium‐modified montmorillonite have been prepared by melt‐blending (MB) and solution‐sonication in order to study the effects of processing on the morphology and properties of the polymer/clay composites. The structure‐property relationships of the prepared composites have been studied by means of X‐ray diffraction (XRD), transmission electron microscopy (TEM), mechanical testing, dynamic‐mechanical analyses (DMA), thermal gravimetrical analyses (TGA), fluorescence probe confocal microscopy, and fluorescence spectroscopy (FS). X‐Ray and TEM show that both nanocomposites have a mixed intercalated/exfoliated structure. Fluorescence probe confocal microscopy reveals that the sonicated sample has a more homogeneous dispersion: this result is confirmed by the values of elongation at break and flexural elastic modulus measured for the composites. Fluorescence spectroscopy has also been used to investigate the distribution of clay in the composites and results indicate that clay layers in ABS are preferentially located in the styrene‐acrylonitrile (SAN) phase, independent of the dispersion process used. Published in 2008 by John Wiley & Sons, Ltd.     

Influence of Fe‐MMT on the fire retarding behavior and mechanical property of (ethylene‐vinyl acetate copolymer/magnesium hydroxide) composite

In this work, Fe‐montmorillonite (Fe‐MMT) is synthesized and used as a synergistic agent in ethylene vinyl acetate/magnesium hydroxide (EVA/MH) flame retardant formulations. The synergistic effect of Fe‐MMT with magnesium hydroxide (MH) as the halogen‐free flame retardant for ethylene vinyl acetate (EVA) is studied by thermogravimetric analysis (TGA), limiting the oxygen index (LOI), UL‐94, and cone calorimetry test. Compared with that of Na‐MMT, it indicates that the synergistic effects of Fe‐MMT enhance the LOI value of EVA/MH polymer and improve the thermal stability and reduce the heat release rate (HRR). The structure and morphology of nanocomposites are studied by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical properties of the EVA composites have also been studied here, indicating that the use of Fe‐MMT reduces the amount of inorganic fillers. MH hence enhances the mechanical properties of the EVA composite while keeping the UL‐94 V‐0 rating. Copyright © 2008 John Wiley & Sons, Ltd.     

本质阻燃聚苯乙烯/蒙脱土纳米复合物的制备及性能

通过原位聚合法制备了本质阻燃聚苯乙烯[P(St-co-AEPPA)]/有机改性蒙脱土(OMMT)纳米复合物[P(St-co-AEPPA)/OMMT], 并用普通聚苯乙烯/有机改性蒙脱土(PS/OMMT)复合物作为对比实验, 研究了含磷、氮单体丙烯酸羟乙基-苯氧基-二乙基磷酰胺(AEPPA)和OMMT等添加物对本质阻燃聚苯乙烯性能的影响.用X射线衍射仪(XRD)和透射电子显微镜(TEM)分析了复合材料的结构与形貌, 并对OMMT在基体中的分散机理进行了讨论.用差示扫描量热仪(DSC)、热重分析(TGA)和微型量热仪(MCC)研究了材料的热性能和燃烧性能.结果表明, AEPPA和OMMT能够显著提高基体的热稳定性和阻燃性.     

The synergistic flame‐retardant effect of O‐MMT on the intumescent flame‐retardant PP/CA/APP systems

The flame retardancy of a novel intumescent flame‐retardant polypropylene (IFR‐PP) system, which was composed of a charring agent (CA), ammonium polyphosphate (APP), and polypropylene (PP), could be enhanced significantly by adding a small amount (1.0 wt%) of an organic montmorillonite (O‐MMT). The synergistic flame‐retardant effect was studied systematically. The thermal stability and combustion behavior of the flame‐retarded PP were also investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL‐94), scanning electronic microscopy (SEM), and cone calorimeter test (CCT). TGA results demonstrated that the onset decomposition temperatures of IFR‐PP samples, with or without O‐MMT, were higher than that of neat PP. Compared with IFR‐PP, the LOI value of IFR‐PP containing 1.0 wt% O‐MMT was increased from 30.8 to 33.0, and the UL‐94 rating was also enhanced to V‐0 from V‐1 when the total loading of flame retardant was the same. The cone calorimeter results showed that the IFR‐PP with 1.0 wt% of O‐MMT had the lowest heat release rate (HRR), total heat release (THR), total smoke production (TSP), CO production (COP), CO₂ production (CO₂P), and mass loss (ML) of all the studied IFR‐PP samples, with or without O‐MMT. All these results indicated that O‐MMT had a significantly synergistic effect on the flame‐retardancy of IFR‐PP at a low content of O‐MMT. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and Characterization of Core‐Shell Selenium/Carbon Colloids and Hollow Carbon Capsules

A novel Se/C nanocomposite with core‐shell structures has been prepared through a facile one‐pot microwave‐induced hydrothermal process. The new material consists of a trigonal‐Se (t‐Se) core and an amorphous‐C (a‐C) shell. The Se/C composite can be converted to hollow carbon capsules by thermal treatment. These products were characterized by transmission electron microscopy (TEM), powder X‐ray diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED), energy‐dispersive X‐ray (EDX) spectroscopy, and X‐ray photoelectron spectroscopy (XPS).     

Flame retardancy mechanisms of poly(1,4‐butylene terephthalate) containing microencapsulated ammonium polyphosphate and melamine cyanurate

The flame retardancy mechanisms of poly(1,4‐butylene terephthalate) (PBT) containing microencapsulated ammonium polyphosphate (MAPP) and melamine cyanurate (MC) were investigated via pyrolysis analysis (thermogravimetric analysis (TGA), real‐time Fourier transform infrared (FTIR), TG‐IR), cone calorimeter test, combustion tests (limited oxygen index (LOI), UL‐94), and residue analysis (scanning electron microscopy (SEM)). A loading of 20 wt% MC to PBT gave the PBT composites an LOI of 26%, V‐2 classification in UL‐94 test and a high peak heat release rate (HRR) in cone calorimeter test. Adding APP to PBT/MC composites did not improve their flame retardancy. In comparison with the addition of ammonium polyphosphate (APP) to PBT, MAPP with silica gel shell and MAPP with polyurethane shell both promoted the intumescent char‐forming and improved the flame retardancy of PBT through different mechanisms in the presence of MC. These two halogen‐free PBT composites with V‐0 classification according to UL‐94 test were obtained; their LOI were 32 and 33%, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Study on the Preparation of Cr‐based Catalysts Doped by Zn using Sol–Gel Auto‐Combustion Method and Its Application for Synthesis of 1‐Chloro‐2,3,3,4,4,5,5‐heptafluorocyclopentene

High‐surface‐area chromium‐based catalysts in the presence of a small amount of zinc were prepared via a sol–gel auto‐combustion method using chromic nitrate, zinc nitrate, and citric acid. First, the auto‐combustion behavior of the dried gel was investigated by derivative thermogravimetry and (DTG)‐TG and infrared (IR) techniques. The results revealed that the dried gel exhibited self‐propagating combustion properties. Second, the as‐burnt powders were characterized by IR , X‐ray diffraction (XRD) , Brunauer–Emmett–Teller analysis (BET) , and scanning electron microscopy (SEM) . The findings showed that the gels were directly converted into CrZn ‐O nanoparticles with high surface area during the auto‐combustion process. Third, the pre‐fluorination Cr‐Zn catalysts were characterized by XRD , BET , SEM , X‐ray photoelectron spectroscopy (XPS) , and Fourier transform (FT)‐IR spectroscopy of pyridine adsorption techniques. It was found that the presence of zinc led to significant structural changes in the catalyst, the particle size was smaller, the surface area became larger, and more active sites appeared. Finally, the catalytic activities of the samples were tested for the fluorination of 1,2‐dichlorohexafluorocyclopentene (1,2‐F6 ) with anhydrous hydrogen fluoride. The obtained results indicated that the pre‐fluorination activated Cr‐Zn catalysts prepared by this sol–gel auto‐combustion method exhibited high efficiency in the synthesis of cyclic hydrofluorocarbons.     

Flame Retardant and Smoke Suppressant of Fe‐Organophilic Montmorillonite in Polyvinyl Chloride Nanocomposites

Two kinds of polyvinyl chloride (PVC)/organophilic montmorillonite (OMT) nanocomposites are prepared by a melt intercalation method. This study has been designed to determine if the presence of iron and zinc ions in the structure of montmorillonite (MMT) lattice can affect thermal, flame retardant and smoke suppressant properties. The information about the morphological structure of PVC/OMT nanocomposites was obtained using X-ray diffraction and transmission electron microscopy. The thermal and flame retardant properties of the nanocomposites were characterized by thermogravimetric analysis, limiting oxygen index and smoke density. The nanocomposites based on Fe-OMT exhibit better thermal, flame retardant properties and lower degradation degree than those of pure PVC. The degradation mechanism was studied by pyrolysis, gas chromatography and mass spectrometry (Py-GC-MS).     

Template‐free Fabrication of Hierarchical Macro‐/mesoporous N‐doped TiO2/graphene Oxide Composites with Enhanced Visible‐light Photocatalytic Activity

Hierarchical macro‐/mesoporous N‐doped TiO₂/graphene oxide (N‐TiO₂/GO) composites were prepared without using templates by the simple dropwise addition mixed solution of tetrabutyl titanate and ethanol containg graphene oxide (GO) to the ammonia solution, and then calcined at 350 °C. The as‐prepared samples were characterized by scanning electron microscopy (SEM), Brunauer‐Emmett‐Teller (BET) surface area, X‐ray diffraction (XRD), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and UV‐Vis absorption spectroscopy. The photocatalytic activity was evaluated by the photocatalytic degradation of methyl orange in an aqueous solution under visible‐light irradiation. The results show that N‐TiO₂/GO composites exhibited enhanced photocatalytic activity. GO content exhibited an obvious influence on photocatalytic performance, and the optimal GO addition content was 1 wt%. The enhanced photocatalytic activity could be attributed to the synergetic effects of three factors including the improved visible light absorption, the hierarchical macro‐mesoporous structure, and the efficient charge separation by GO.     

Nano‐TiO2 as an Efficient Catalyst for Tandem Knoevenagel–Michael‐Cyclocondensation Reaction of Dimedone with Aromatic Aldehydes and Ammonium Acetate or Aromatic Amines under Solvent‐free Conditions

TiO₂ nanoparticles in anatase and rutile forms was characterized and studied by several techniques including X‐ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and successfully applied as an efficient and heterogeneous catalyst in the synthesis of 1,8‐dioxo‐decahydroacridines via the one‐pot multi‐component condensation reaction of dimedone with aromatic aldehydes and ammonium acetate or aromatic amines under mild and solvent‐free conditions.