Preparation of gel-silica/ammonium polyphosphate core-shell flame retardant and properties of polyurethane composites

Towards the Reuse of Fire Retarded Polyamide 12 for Laser Sintering

The control of powder aging during Selective Laser Sintering (SLS) processing is one of the challenges to be overcome for the implementation of this technique in serial production. Aging phenomena, because of the elevated temperatures and long processing times, need to be considered when a fraction of the polymer powders present in the build chamber and not used to manufacture the parts are reused at various times. The aim of this study was to investigate the influence of successive reuse of blends of pure Polyamide 12 and its blends with two types of flame retardants (FR): ammonium polyphosphate (APP) and zinc borate (ZB). The composition of the blends was 70/30 (wt/wt) PA 12/FR. Four successive processing stages have been carried out by collecting the remaining powder blend each time. The powders were re-used using the same processing parameters after sieving. DSC measurements showed that the incorporation of FRs entailed a reduction in the processing window up to 4 °C; nevertheless, no further reduction was noted after aging. The TGA curves of aged blends of powders were also similar for pure PA 12 and PA 12 with FR. In addition, initial and reused powders presented a higher degree of crystallinity than the specimens processed from the powders. The heterogeneous character of the PA 12 after LS processing or reprocessing was shown through Pyrolysis Combustion Flow Calorimetry (PCFC) and cone calorimeter (CC) tests. FTIR analysis also showed that post-condensation reactions have occurred. The mode of action of the flame retardants was clearly seen on HRR curves at both tests. The first reuses of PA 12 powders entailed a significant reduction in time to ignition at the cone calorimeter (150 for the initial material to around 90 s for the reused material), indicating the formation of short polymer chains. Only in the case of zinc borate was it noticed that re-used powder was detrimental to the fire performance because of a strong increase in the value of pHRR (between 163 and 220 kW/m2 for reused material instead of 125 kW/m2 for the initial one).

Realizing balanced flame retardancy and electromagnetic interference shielding in hierarchical elastomer nanocomposites

The combination of electromagnetic interference (EMI) shielding performance and flame-retardant property is essential for applications in the field of electronics and electrics. To date, there have been few successful cases in achieving such portfolios, due to the different mechanisms and even mutual exclusivity of these two attributes. Herein, an ammonium polyphosphate@chitosan@carbon nanotube (APP@CS@MWCNT) core-multishell hybrid was synthesized by microencapsulation technology. Then, the hybrid was introduced into TPU matrix to fabricate TPU composites, acting as surface layer. Meanwhile, MXene film was used as intermediate layer to construct hierarchical TPU composites. The obtained results showed that after introduction of 1 wt% APP@CS@MWCNT hybrid, the peak of heat release rate (PHRR) and the peak of smoke produce rate (PSPR) of TPU composites decreased by 67.4% and 35.6%, respectively, compared with those of pure TPU. Owing to multiple reflection losses, interface polarization losses, and charge carrier movement-induced thermal dissipation, TPU/15AC@4M-SW exhibited the highest EMI shielding performance, and obtained shielding effectiveness values of 35.7 dB and 38.9 dB in X band and K band, respectively.

Synthesis of P-/N-Containing Bamboo-Activated Carbon toward Enhanced Thermal Stability and Flame Retardancy of Polylactic Acid

A P-/N-containing bamboo-activated carbon (BACm) was successfully synthesized by steam activation of bamboo charcoal and chemical grafting to as-prepared activated carbon using the reaction of phosphoric acid and urea. Characterizations of BACm presented a synergistic grafting of P and N elements to the BAC surface. The BACm was further loaded in a polylactic acid (PLA) matrix to prepare BACm/PLA composites. Mechanical strength study showed tensile strength dropped from 75.19 MPa to 61.30 MPa, and tensile modulus from 602.49 MPa to 375.56 MPa, suggesting a rigidity reduction and deformation resistance enhancement owing to the roughened surface of BACm that interlocked with the polymer. The thermogravimetric analysis showed that the carbon residue rate of BACm dramatically fell to 49.25 wt.% in contrast to 88.28% for the control BAC, and cone calorimeter measurements confirmed the enhancement of flame retardancy of the composites with BACm loading, and the carbon residue rate increased progressively with BACm loading in the composites, notably up to 8.60 wt.% for the BAC/PLA9 composite, which outweighed the theoretical residue rate by more than 50%. The elemental analysis also confirmed rich P/N levels of the dense carbon residue layer that could perform synergistically and effectively in fire suppression. The BACm tended to stimulate the earlier decomposition of the composites and formed a continuous residual carbon layer which functioned as an effective barrier hindering the mass and heat transfer between the combustion zone and the underlying matrix. Moreover, 9 wt.% of BACm loading could attain a V-0 rating (UL94) for the composite with an improved limiting oxygen index up to 31.7%. The biomass-based modified activated carbon in this work could be considered as an alternative flame retardant in polymer applications.

Flame Retardancy and Thermal Property of Environment-Friendly Poly(lactic acid) Composites Based on Banana Peel Powder

Banana peel powder (BPP) was used to prepare poly(lactic acid) (PLA) bio-based composites and the flame retardancy was enhanced by introducing silica-gel microencapsulated ammonium polyphosphate (MCAPP). The results showed that the limiting oxygen index (LOI) of PLA containing 15 wt % BPP was 22.1% and just passed the UL-94 V-2 rate. Moreover, with the introduction of 5 wt % MCAPP and 15 wt % BPP, the PLA composite had a higher LOI value of 31.5%, and reached the UL-94 V-0 rating, with self-extinguishing and anti-dripping abilities. The PLA/M5B15 also had a lower peak heat release rate (296.7 kW·m^-2), which was 16% lower than that of the PLA/B15 composite. Furthermore, the synergistic effects between MCAPP and BPP impart better thermal stability to PLA composites. According to the investigation of the char residue and pyrolysis gaseous products, MCAPP with BPP addition is beneficial to the formation of a higher quality char layer in the solid phase but also plays the flame retardant effect in the gas phase. This work provides a simple and efficient method to solve the high cost and flammability issues of PLA composites.

Thermomechanical and Fire Properties of Polyethylene-Composite-Filled Ammonium Polyphosphate and Inorganic Fillers: An Evaluation of Their Modification Efficiency

The development of new polymer compositions characterized by a reduced environmental impact while lowering the price for applications in large-scale production requires the search for solutions based on the reduction in the polymer content in composites' structure, as well as the use of fillers from sustainable sources. The study aimed to comprehensively evaluate introducing low-cost inorganic fillers, such as copper slag (CS), basalt powder (BP), and expanded vermiculite (VM), into the flame-retarded ammonium polyphosphate polyethylene composition (PE/APP). The addition of fillers (5-20 wt%) increased the stiffness and hardness of PE/APP, both at room and at elevated temperatures, which may increase the applicability range of the flame retardant polyethylene. The deterioration of composites' tensile strength and impact strength induced by the presence of inorganic fillers compared to the unmodified polymer is described in detail. The addition of BP, CS, and VM with the simultaneous participation of APP with a total share of 40 wt% caused only a 3.1, 4.6, and 3 MPa decrease in the tensile strength compared to the reference value of 23 MPa found for PE. In turn, the cone calorimeter measurements allowed for the observation of a synergistic effect between APP and VM, reducing the peak heat rate release (pHRR) by 60% compared to unmodified PE. Incorporating fillers with a similar thermal stability but differing particle size distribution and shape led to additional information on their effectiveness in changing the properties of polyethylene. Critical examinations of changes in the mechanical and thermomechanical properties related to the structure analysis enabled the definition of the potential application perspectives analyzed in terms of burning behavior in a cone calorimetry test. Adding inorganic fillers derived from waste significantly reduces the flammability of composites with a matrix of thermoplastic polymers while increasing their sustainability and lowering their price without considerably reducing their mechanical properties, which allows for assigning developed materials as a replacement for flame-retarded polyethylene in large-scale non-loaded parts.

Synthesis and Mechanical Characterization of a Ti(C,N)/Mo-Co-Ni/CaF2@Al2O3 Self-Lubricating Cermet

In this paper, an Al₂O₃ coated CaF₂ (CaF₂@Al₂O₃) nanocomposite powder is used as the additive phase of a Ti(C,N)-based self-lubricating cermet material. A novel self-lubricating ceramic material with a multilayer core–shell microstructure was prepared using a vacuum hot-pressing sintering process. The results show that the surface of the CaF₂ powder is coated with Al₂O₃, and when introduced into a Ti(C,N)–Mo–Co–Ni material system, it can utilize the high-temperature liquid phase diffusion mechanism of the metal Mo–Co–Ni phase in the sintering process. The CaF₂@Al₂O₃@Mo–Co–Ni multilayer core–shell microstructure is formed in the material. Compared with the direct addition of CaF₂ and Al₂O₃, the hardness and fracture toughness of the material are increased by 24.31% and 22.56%, reaching 23.93 GPa and 9.94 MPa·m^1/2, respectively. The formation of the multilayer core–shell microstructure is the main reason for the improvement of the mechanical properties of the material.

Modification of diatomite with melamine coated zeolitic imidazolate framework-8 as an effective flame retardant to enhance flame retardancy and smoke suppression of rigid polyurethane foam

In this work, the core-shell structure (ZIF-8@MA) was prepared first with melamine (MA) coated zeolitic imidazolate framework-8 (ZIF-8), and the ternary composite ZMD containing Si-N-Zn was successfully synthesized with the diatomite modified ZIF-8@MA. Subsequently, the prepared ZMD was added into rigid polyurethane foam (RPUF) to investigate its effect on fire safety of RPUF. The results of cone calorimeter and limiting oxygen index (LOI) tests indicated that ZMD effectively reduced the fire hazard of RPUF. This was because of the physical barrier effect of diatomite, the co-catalyzed char formation in the condensed phase of ZnO produced by the decomposition of ZIF-8 and silica produced by the decomposition of diatomite, and the gas phase effect of MA that enabled RPUF to achieve excellent flame retardancy and smoke suppression. The specific mechanism of flame retardancy and smoke suppression of ZMD for RPUF was also discussed in this study.

Zeolitic imidazolate framework-8 was coated with silica and investigated as a flame retardant to improve the flame retardancy and smoke suppression of epoxy resin

Synthesis of the core–shell structure of ZIF-8@SiO₂ and its effect on the flame retardancy of epoxy resin.

Studies on the thermal properties of poly(1,4-butylene terephthalate)/microencapsulated ammonium polyphosphate composites

This work aims to explore the potential of microencapsulated ammonium polyphosphate (MAPP) in poly(1,4-butylene terephthalate) (PBT). PBT/MAPP composites were prepared via melt blending method. The study of their thermal behaviors and thermal stability showed that the addition of MAPP with silica gel shell (MAPP(Si)) improved the relative crystallinity and thermal stability of PBT composites, in comparison with untreated ammonium polyphosphate and MAPP with polyurethane shell. Possible mechanisms for PBT/MAPP(Si) composites were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The results indicated that several PBT molecules may be grafted onto the surface of MAPP(Si), which improves the relative crystallinity and thermal stability of PBT/MAPP(Si) composites.

Preparation of lignin–silica hybrids and its application in intumescent flame-retardant poly(lactic acid) system

Lignin–silica hybrids (LSHs) were prepared by sol–gel method and characterized by Fourier transform infrared (FT-IR) spectra, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). LSH and ammonium polyphosphate (APP) were added into poly(lactic acid) (PLA) as a novel intumescent flame-retardant (IFR) system to improve the flame retardancy of PLA. The flame-retardant effect of APP and LSH in PLA was studied using limiting oxygen index (LOI), vertical burning (UL-94) tests and cone calorimeter. The thermal stability of PLA/APP/LSH composites was evaluated by thermogravimetric analysis (TGA). Additionally, the morphology and components of char residues of the IFR-PLA composites were investigated by SEM and XPS. With the addition of APP/LSH to PLA system, the morphology of the char residue has obviously changed. Compared with PLA/APP and PLA/APP/lignin, a continuous and dense intumescent charring layer with more phosphor in PLA composites is formed, which exhibits better flame retardancy. All the results show that the combination of APP and LSH can improve the flame-retardant property and increase the thermal stability of PLA composites greatly.