Green synthesis of bio-based flame retardant/natural rubber inorganic-organic hybrid and its flame retarding and toughening effect for polylactic acid.

Polylactic acid (PLA) has garnered significant interest as a bio-based polymer due to its favorable thermal and processing characteristics, as well as its notable economic and environmental benefits. However, the drawbacks such as flammability and poor toughness of PLA severely constrained its applications in more fields. Here, based on the outstanding flame-retardant properties of core-shell flame retardant (CSFR) and the toughening potential of natural rubber (NR), we synthesized inorganic-organic hybrid of CSFR-NR using an aqueous synthesis to synchronous optimization of the comprehensive performance of PLA. The as-prepared CSFR-NR with "hard core and soft shell" possess the ability to promote char formation and facilitate uniform dispersion in the PLA matrix. Consequently, the PLA/CSFR-NR showed an excellent flame retardancy with the limiting oxygen index (LOI) value of 31.5 % and UL-94 V-0 rating and synergistic toughening effect with absolutely improvement in elongation at break and notched izod impact strength, achieving a balance between the fire safety and mechanical performance. Moreover, the degradation rate of PLA has also been substantially promoted by CSFR-NR in simulated seawater. Hence, this study offers a straightforward, efficient, and environmentally friendly strategy for creating high-performance flame retardant and toughened bioplastics.

Flame-retardant poly(L-lactic acid) with enhanced UV protection and well-preserved mechanical properties by a furan-containing polyphosphoramide.

Despite good biodegradability and mechanical strength, the intrinsic flammability of poly(L-lactic acid) (PLA) impede its practical application. Introducing phosphoramide is an effective method to enhance the flame retardancy of PLA. However, most of the reported phosphoramides derive from petroleum resources, and their addition tends to deteriorate the mechanical properties, especially toughness, of PLA. Herein, a bio-based, furan-containing polyphosphoramide (DFDP) with high flame-retardant efficiency was synthesized for PLA. Our study found that 2 wt% DFDP enabled PLA to pass a UL-94 V-0 rating, and 4 wt% DFDP increased the limiting oxygen index (LOI) to 30.8 %. DFDP effectively maintained the mechanical strength and toughness of PLA. The tensile strength of PLA with 2 wt% DFDP reached 59.9 MPa, and its elongation at break and impact strength were increased by 15.8 % and 34.3 %, respectively, relative to those of virgin PLA. The UV protection of PLA was significantly enhanced by introducing DFDP. Hence, this work provides a sustainable and comprehensive strategy for the creation of flame-retardant biomaterials with improved UV protection and well-preserved mechanical properties, which possess a broad prospect in industrial application.

Large-scale preparation of a versatile bioinspired sponge with physic-mechanochemical robustness for multitasking separation.

Developing novel biomaterials integrating robustness and multitasking separation performance are of importance. However, those were limited in application due to the expensive, time-consuming and complex fabrication process. In this work, with the inspiration from high porosity and surface area of natural materials, the porous superhydrophobic melamine sponges (SMS) coated hydrophobic TiO2 and epoxy copolymer were fabricated via a facile, inexpensive, eco-friendly and large-scale strategy. The SMS showed excellent superhydrophobic property, and could well resist the harsh mechanical damage, chemical corrosion, extreme temperature, and irradiation of UV without losing antiwetting ability. Besides, it displayed selective oil absorbing ability, recyclability, and self-cleaning ability. Moreover, the SMS displayed superior multitasking performance for continuous oil/water separation, surfactant-stabilized O/W emulsions separation (separation efficiency above 99%), and bacterial/fungus containing filtration (filtration efficiency over 60% for S. aureus, 90% for E. coli and C. albicans). With the multifaceted features, the SMS is a promising sponge material for treatment of industry oily or bacterial/fungus-containing wastewater in practical application.

Interfacial silicon­nitrogen aerogel raise flame retardancy of bamboo fiber reinforced polylactic acid composites.

A triazine derivative containing nitrogen and silicon (SiN) was synthesized and the SiN hybrid aerogel was covered on the surface of bamboo fiber (BF). The modified BF was identified as MBF. The MBF and ammonium polyphosphate (APP) were used to regulate the flame retardancy and mechanical properties of polylactic acid (PLA). The PLA/BF composites were investigated using limiting oxygen index (LOI), UL-94 vertical combustion, cone calorimetry, thermogravimetric analysis linked with infrared spectra (TG-IR) etc. The char residue of MBF at 800 °C is as high as 43.5 % which is 200 % more than that of BF. Incorporating 9 wt% APP generates a PLA9 which displays the UL-94 V2 rating and a LOI value of 28.0 vol%. PLA9/MBF composites display the UL-94 V0 rating and increased LOI values while PLA9/BF composites obtain the UL-94 V2 rating and decreased LOI. The MBF reduces the release of flammable gases during combustion, enhances charring ability and decreases the thermal conductivity of composites. Besides, the tensile and impact strength of PLA9/20MBF is 20 % and 37 % more than that of PLA9/20BF due to stronger interfacial adhesion. This work provides a good method to regulate the flame retardancy and mechanical properties of PLA/BF composites.

Roles of phosphoramide derivatives in flame retardancy, thermal degradation and crystallization behaviors of polylactic acid.

Two kinds of phosphoramide derivatives containing one (POFA) or two (BPOFA) phosphoramide bonds were synthesized using diphenylphosphinic chloride, phenylphosphonic dichloride and furfurylamine, respectively. The flame retardancy, thermal degradation and crystallization behaviors of polylactic acid (PLA)/POFA and PLA/BPOFA blends were investigated using the limiting oxygen index (LOI), UL-94 vertical burning tests, cone calorimeter tests, thermogravimetric analysis and differential scanning calorimetry. A small amount of 1.0 wt% BPOFA increases the LOI of PLA from 20.4 vol% to 28.0 vol% and achieves the UL-94 V0 rating, while 7.0 wt% POFA is needed to achieve the UL-94 V0 rating indicating that the PLA/BPOFA has better flame retardancy than that of PLA/POFA. Both POFA and BPOFA decrease the amount of flammable gaseous compounds and play positive roles in transesterification of PLA. More importantly, BPOFA is easier to catalyze the degradation of PLA and improve UL-94 rating by melting-away mode. Moreover, the BPOFA accelerates the crystallization rate and thus improves the crystallinity of PLA, while POFA does not show a positive effect on crystallization behaviors of PLA.

Synergistic effect of poly(ionic liquid) and phosphoramide on flame retardancy and crystallization of poly(lactic acid).

Crystallinity and flame retardancy are two key properties for poly(lactic acid)(PLA) in applications. In this paper, a quaternary phosphonium salt poly(ionic liquid) (PIL) and a phosphamide (POFA) were prepared. The PIL, POFA and their blend were used to regulate the flame retardancy and crystallization behaviors of PLA using the limiting oxygen index, UL-94 vertical burning, and thermogravimetric analysis, and differential scanning calorimetry etc. The results showed that a synergistic effect exists between PIL and POFA on flame retardancy. When 6 wt% PIL/POFA (2/1) was added into PLA, its LOI value is 28.0 vol%, and achieves the UL-94 V-0 rating while the PLA composites containing 6 wt% PIL or POFA just achieve the UL-94 V2. The PIL/POFA improves the flame retardancy of PLA by melting-away mode. In addition, the crystallization rate of PLA containing PIL/POFA is faster than that of PLA/PIL and PLA/POFA. The degradation of PLA induced by PIL/POFA produces some small molecular oligomers, which enhances the molecular chain mobility and rearrangement, thus contributes to better flame retardancy and faster crystallization.

Governing effects of melt viscosity on fire performances of polylactide and its fire-retardant systems.

Extreme flammability of polylactide (PLA) has restricted its real-world applications. Traditional research only focuses on developing new effective fire retardants for PLA without considering the effect of melt viscosity on its fire performances. To fill the knowledge gap, a series of PLA matrices of varied melt flow index (MFI) with and without fire retardants are chosen to examine how melt viscosity affects its fire performances. Our results show that the MFI has a governing impact on fire performances of pure PLA and its fire-retardant systems if the samples are placed vertically during fire testing. PLA with higher MFI values achieves higher limiting oxygen index (LOI) values, and a lower loading level of fire retardants is required for PLA to pass a UL-94 V-0 rating. This work unveils the correlation between melt viscosity and their fire performance and offers a practical guidance for creating flame retardant PLA to extend its applications.

Fabrication and Mechanism Study of Cerium-Based P, N-Containing Complexes for Reducing Fire Hazards of Polycarbonate with Superior Thermostability and Toughness.

A superior comprehensive performance is essential for the extensive utilization of polymers. Current flame-retardant strategies for polycarbonates (PCs) usually realize satisfied fire resistance at the cost of thermostability, toughness, and/or mechanical robustness. Thus, we report a rare-earth-based P, N-containing complex with a lamellar aggregated structure [Ce(DPA)3] by a coordination reaction between a tailored ligand and cerium(III) nitrate. The results indicate that incorporating 3 wt % Ce(DPA)3 enables the resultant PC composite to achieve UL-94 V-0 rating, with a 55% reduction in the peak heat release rate. Besides, the initial (T5) and maximum (Tmax1 and Tmax2) decomposition temperatures are significantly increased by 21, 19, and 27 °C, respectively, in an air atmosphere. Moreover, the impact strength and elongation at break of the PC composite containing 3 wt % Ce(DPA)3 are greatly increased by 20 and 59%, respectively, relative to pristine PC, while its tensile strength (57 MPa) is still close to that of bulk PC (60 MPa). Notably, this work provides a novel methodology for revealing the evolution mechanisms of chemical structures of vapor and residual products during thermal decomposition, which is conducive to guiding fire and heat resistance modification of PC in the future.

A Novel Synergistic Flame Retardant of Hexaphenoxycyclotriphosphazene for Epoxy Resin.

Hexaphenoxycyclotriphosphazene (HPCP) is a common flame retardant for epoxy resin (EP). To improve the thermostability and fire safety of HPCP-containing EP, we combined UiO66-NH2 (a kind of metal-organic frame, MOF) with halloysite nanotubes (HNTs) by hydrothermal reaction to create a novel synergistic flame retardant (H-U) of HPCP for EP. For the EP containing HPCP and H-U, the initial decomposition temperature (T5%) and the temperature of maximum decomposition rate (Tmax) increased by 11 and 17 °C under nitrogen atmosphere compared with those of the EP containing only HPCP. Meanwhile, the EP containing HPCP and H-U exhibited better tensile and flexural properties due to the addition of rigid nanoparticles. Notably, the EP containing HPCP and H-U reached a V-0 rating in UL-94 test and a limited oxygen index (LOI) of 35.2%. However, with the introduction of H-U, the flame retardant performances of EP composites were weakened in the cone calorimeter test, which was probably due to the decreased height of intumescent residual char.

The preparation of layered double hydroxide wrapped carbon nanotubes and their application as a flame retardant for polypropylene.

Carbon nanotubes (CNTs) wrapped with layered double hydroxide (LDH-w-CNTs) were facilely obtained through in situ introduction of CNTs into the hydrothermal reaction system of LDH, with the goal of combining their unique physical and chemical characteristics to meet new advanced applications. Morphological observations indicated that LDH lamellae enwrapped the surface of CNTs and the wrapping degree was dependent on the functionalization of CNTs. Zeta-potential measurements showed that the interaction between the positive charge of LDH and the negative charge of CNTs was the main driving force of the wrapping process. Both hybrids led to a reduction in the peak heat release rate (PHRR) of polypropylene, indicating that they could confer better flame retardancy on polypropylene with respect to LDH and CNTs.

Bioinspired Design of Strong, Tough, and Thermally Stable Polymeric Materials via Nanoconfinement.

The combination of high strength, great toughness, and high heat resistance for polymeric materials is a vital factor for their practical applications. Unfortunately, until now it has remained a major challenge to achieve this performance portfolio because the mechanisms of strength and toughness are mutually exclusive. In the natural world, spider silk features the combination of high strength, great toughness, and excellent thermal stability, which are governed by the nanoconfinement of hydrogen-bonded ß-sheets. Here, we report a facile bioinspired methodology for fabricating advanced polymer composite films with a high tensile strength of 152.8 MPa, a high stiffness of 4.35 GPa, and a tensile toughness of 30.3 MJ/m3 in addition to high thermal stability (69 °C higher than that of the polymer matrix) only by adding 2.0 wt % of artificial ß-sheets. The mechanical and thermostable performance portfolio is superior to that of its counterparts developed to date because of the nanoconfinement and hydrogen-bond cross-linking effects of artificial ß-sheets. Our study offers a facile biomimetic strategy for the design of integrated mechanically robust and thermostable polymer materials, which hold promise for many applications in electrical devices and tissue engineering fields.

Mechanical and water-resistant properties of rice straw fiberboard bonded with chemically-modified soy protein adhesive.

In this work, rice straw and soy protein were used to make fiberboard which may replace wood fiberboard. Soy protein isolates (SPI) were modified by epoxidized oleic acid to improve the soy protein adhesive properties such as adhesion strength and water resistance. The effects of NaOH content, the addition of modified-SPI adhesives and fiberboard density on the mechanical and water-resistant properties of the rice straw fiberboards were investigated. FTIR and XRD results of modified SPI indicated the epoxidized oleic acid and soy protein reacted with each other. FTIR and SEM images of rice straw fibers showed that NaOH solution removed the wax layer through chemical etching. The results of investigating mechanical properties and water absorption illustrate that when the soy protein-based adhesives content and density and the hot pressing temperature and pressure of fiberboard are 12%, 0.8 g cm-3, 140 °C and 6 MPa, respectively, the panels have optimal mechanical and water-resistant performances. Moreover, the panels meet the requirements of chinese medium density fiberboard (MDF) Standard of GB/T 11718-2009. Since biological raw materials are recyclable and biomass, the fiberboard bonded with modified soy protein adhesive has no toxicity and is easily biodegradable. In addition, the rice straw burned to produce haze has been preferably utilized.

Construction of flame retardant nanocoating on ramie fabric via layer-by-layer assembly of carbon nanotube and ammonium polyphosphate.

A new flame retardant nanocoating has been constructed by the alternate adsorption of polyelectrolyte amino-functionalized multiwall carbon nanotube (MWNT-NH2) and ammonium polyphosphate (APP) onto flexible and porous ramie fabric. Scanning electron microscopy indicates that the adsorbed carbon nanotube coating is a randomly oriented and overlapped network structure, which is a promising candidate for flame retardancy applications. Attenuated total reflection Fourier transform infrared spectroscopy and energy-dispersive X-ray analysis confirm that the APP is successfully incorporated into the multilayers sequentially. Assessment of the thermal and flammability properties for the pristine and nanocoated ramie fabrics shows that the thermal stability, flame retardancy and residual char are enhanced as the concentration of MWNT-NH2 suspension and number of deposition cycles increases. The enhancements are mostly attributed to the barrier effect of intumescent network structure, which is composed of MWNT-NH2 and the absorbed APP.

Percolation-dominated superhydrophobicity and conductivity for nanocomposite coatings from the mixtures of a commercial aqueous silica sol and functionalized carbon nanotubes.

Superhydrophobic conductive nanocomposite coatings are prepared for the first time from the simple mixture of a commercial aqueous silica sol and functionalized multiwalled carbon nanotubes (MWNTs) by air-spraying at ambient conditions followed by fluorosilane treatment. The relationship between MWNT content and the structure and properties of the nanocomposite coatings is investigated systematically. An ultra-low threshold (<5 vol.%) for superhydrophobicity is observed, which suggests that MWNTs are superior to any other spherical fillers for the construction of superhydrophobic nanocomposite coatings. When the content of nanotubes is below the threshold, the surface roughness mainly caused by the silica nanoparticles is not enough for creating superhydrophobic surfaces. Only above the threshold, the multiscale hierarchical structure is enough for both high water contact angles (>165°) and extremely low sliding angles (<2°). The conductivity is also percolation dominated, while the threshold for conductivity is much higher than that for superhydrophobicity, which can be ascribed to the encapsulated structure and the agglomeration of nanotubes in the composite coatings during air-spraying. Moreover, the aqueous silica sols hold merits of great film-forming capability at relatively low calcination temperatures, and being free of organic solvents.

Surface-initiated graft polymerization on multiwalled carbon nanotubes pretreated by corona discharge at atmospheric pressure.

Surface-initiated graft polymerization on multi-walled carbon nanotubes pretreated with a corona discharge at atmospheric pressure was explored. The mechanism of the corona-discharge-induced graft polymerization is discussed. The results indicate that MWCNTs were encapsulated by poly(glycidyl methacrylate) (PGMA), demonstrating the formation of PGMA-grafted MWCNTs (PGMA-g-MWCNTs), with a grafting ratio of about 22 wt%. The solubility of PGMA-g-MWCNTs in ethanol was dramatically improved compared to pristine MWCNTs, which could contribute to fabricating high-performance polymer/MWCNTs nanocomposites in the future. Compared with most plasma processes, which operate at low pressures, corona discharge has the merit of working at atmospheric pressure.

Effects of reactive compatibilization on the morphological, thermal, mechanical, and rheological properties of intumescent flame-retardant polypropylene.

Flame-retardant polypropylene (PP) samples were in situ compatibilized with maleic anhydride grafted PP. Compatibilization reaction was verified by an IR spectrum and gel content measurements. Electron microscopy images showed that compatibilization could considerably reduce the size of the flame-retardant domains, control the phase morphology, and improve the interfacial adhesion between PP and intumescent flame retardant (IFR) with different IFR loading levels. The limiting oxygen index (LOI) of flame-retardant PP increased to different extents after compatibilization, indicating an improvement in the flame retardancy. Compatibilization enhanced the thermal stability to some extent and remarkably delayed thermal oxidative degradation of flame-retardant PP. For PP containing 20 wt % flame retardant, the temperature at which the maximum weight loss rate occurred was enhanced by about 99 degrees C after compatibilization. The storage modulus and glass transition temperatures were elevated to different extents. Tensile strengths of samples reduced in the presence of flame retardant alone but in the additional presence of compatibilizer were restored to levels similar to those of pure PP. Elongation-at-break values, however, showed IFR concentration-dependent reductions that were less for compatibilized samples. Furthermore, the complex viscosity of a compatibilized PP melt turned slightly smaller, which is favorable to melt processing.

Fabrication of fullerene-decorated carbon nanotubes and their application in flame-retarding polypropylene.

Multi-walled carbon nanotubes were decorated with fullerene (C(60)) via a three-step chemical functionalization, with the goal of combining their unique physical and chemical characteristics and simultaneously improving the solubility of CNTs in organic solvents. C(60) molecules, about 0.67% by molecule, were homogeneously bonded onto the surface of the CNTs. Electron microscopy clearly shows that C(60) molecules are introduced onto the surface of the CNTs, and this is also evidenced by their UV-VIS absorption spectra. Cone calorimetry measurements showed that compared with pristine CNTs, fullerene-decorated CNTs further reduced the flammability of polypropylene, which may be due to the free-radical-trapping effect of C(60) and the barrier effect of the CNT network.

C(60) reduces the flammability of polypropylene nanocomposites by in situ forming a gelled-ball network.

The thermal and flame retardancy properties of polypropylene/fullerene (PP/C(60)) nanocomposites were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and cone calorimetry with the C(60) loading varied from 0.5 to 2% by weight. Dispersion of C(60) in the PP matrix was characterized by transmission electron microscopy (TEM) and optical microscopy (OM). TGA and DSC results showed that the presence of C(60) could remarkably enhance the thermal property and cone calorimeter measurements suggested that C(60) could to some extent reduce the flammability of PP, with a significant reduction in peak heat release rate and a much longer time to ignition. Furthermore, the larger the loading level of C(60), the better the flame retardancy property of PP/C(60) nanocomposites. The flame retardation mechanism and corresponding model were proposed with the help of rheological measurements, TEM and x-ray diffraction. C(60) reduced the flammability of PP by trapping free radicals in the gas phase and in situ forming a gelled-ball crosslink network to improve the flame retardancy of PP in the condensed phase. Finally, this suggested mechanism was supported by the results of advanced rheological extended systems (ARES), gel content, infrared spectrum, OM, and atomic force microscopy (AFM) measurements.