Combustion and fire retardancy of aliphatic nylons

Flame Retardancy of Nylon 6 Fibers: A Review

As synthetic fibers with superior performances, nylon 6 fibers are widely used in many fields. Due to the potential fire hazard caused by flammability, the study of the flame retardancy of nylon 6 fibers has been attracting more and more attention. The review has summarized the present research status of flame-retarded nylon 6 fibers from three aspects: intrinsic flame-retarded nylon 6, nylon 6 composites, and surface strategies of nylon 6 fibers/fabrics. The current main focus is still how to balance the application performances, flame retardancy, and production cost. Moreover, melt dripping during combustion remains a key challenge for nylon 6 fibers, and the further developing trend is to study novel flame retardants and new flame-retardancy technologies for nylon 6 fibers.

Thermal degradation and flame retardancy of nylon 6/aluminum methylmethoxy phosphonate composites

An aluminum methylmethoxyphosphonate (AlPo)-based flame retardant (FR) was synthesized. Thermal degradation and flame retardancy of nylon 6 (PA6)/AlPo composites were examined and compared with PA6/commercial aluminum diethylphosphinate (AlPi) composites. The PA6/AlPo composite achieved a V-0 rating at 20 wt% loading during the UL-94 test, and it exhibited the formation of a charred layer that protected the polymer from burning and reduced the release of gases during the combustion of PA6. AlPo demonstrated exceptional performance in gaseous and condensed phases in the PA6 matrix, whereas AlPi only worked in the gaseous phase. The differences between the thermal degradation mechanisms and flame retardancies of AlPi and AlPo were investigated via Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and cone calorimetry. A suitable degradation mechanism was proposed to aid the development of flame retardants in the future.

Synergistic Enhancement of Flame Retardancy Behavior of Glass-Fiber Reinforced Polylactide Composites through Using Phosphorus-Based Flame Retardants and Chain Modifiers

Flame retardancy properties of neat PLA can be improved with different phosphorus-based flame retardants (FRs), however, developing flame retardant PLA-based engineering composites with maintained mechanical performance is still a challenge. This study proposes symbiosis approaches to enhance the flame retardancy behavior of polylactide (PLA) composites with 20 wt% short glass fibers (GF). This was first implemented by exploring the effects of various phosphorus-based FRs up to 5 wt% in neat PLA samples. Among the used phosphorus-based FRs, the use of only 3 wt% of diphosphoric acid-based FR (P/N), melamine coated ammonium polyphosphate (APPcoated), and APP with melamine synergist (APP/Mel) resulted in achieving the V0 value in a vertical burning test in the neat PLA samples. In addition to their superior efficiency in improving the flame retardancy of neat PLA, P/N had the least negative effect on the final mechanical performance of PLA samples. When incorporated in PLA composites with 20 wt% GF, however, even with the use of 30 wt% P/N, the V0 value could not be obtained due to the candlewick effect. To resolve this issue, the synergistic effect of P/N and aromatic polycarbodiimide (PCDI) cross-linker or Joncryl epoxy-based chain-extender (CE) on the flame retardancy characteristics of composites was examined. Due to the further chain modification, which also enhances the melt strength of PLA, the dripping of composites in the vertical burning test terminated and the V0 value could be reached when using only 1 wt% PCDI or CE. According to the scanning electron microscopic analysis, the use of noted chain modifiers further homogenized the distribution and refined the particle size of P/N within the PLA matrix. Hence this could synergistically contribute to the enhancements of the fire resistance performance of the PLA composites. Such incorporation of P/N and chain modifiers further leads to the enhancement of the mechanical performance of PLA composites and hence the resultant product can be proposed as a promising durable bioplastic engineering product where fire risk exists.

Bio-Based Flame-Retardant Coatings Based on the Synergistic Combination of Tannic Acid and Phytic Acid for Nylon-Cotton Blends

Natural and synthetic polymeric fibers are used extensively in making fabrics for a variety of civilian and military applications. Due to the durability and comfort, nyco, a 50-50% blend of nylon 66 and cotton, is used as the material of choice in many applications including military uniforms. This fabric is flammable due to the presence of cotton and nylon but has good mechanical properties and is comfortable to wear. Here, we report a novel surface functionalization method that utilizes a synergistic combination of bio-based materials, tannic acid (TA) and phytic acid (PA), to impart flame-retardant (FR) properties to the nyco fabric. TA and PA were sequentially attached to nylon and cotton fibers through hydrogen bonding interactions and phosphorylation, respectively. The surface functionalization of the treated fabrics was confirmed using Fourier-transform infrared spectroscopy. Thermogravimetric analysis, microscale combustion calorimetry, cone calorimetry, and vertical flame testing were employed to study the effect of the functionalization on the thermal stability and flammability of the nyco fabric. Though reasonable durable functionalization is observed from elemental analysis, it is not enough to impart wash-durable FR treatment. These results indicate that flame retardancy is enabled through the enhanced char formation provided by the combination of TA and PA. The TA-PA system applied to nyco shows great promise as a bio-based FR system. This study for the first time also provides evidence for the selectivity of TA in imparting FR characteristics for nylon and PA in imparting FR properties for cotton. The combination of TA and PA provides promising FR characteristics to nyco.

The Effect of Halloysite Nanotubes on the Fire Retardancy Properties of Partially Biobased Polyamide 610

The main objective of the work reported here was the analysis and evaluation of halloysite nanotubes (HNTs) as natural flame retardancy filler in partially biobased polyamide 610 (PA610), with 63% of carbon from natural sources. HNTs are naturally occurring clays with a nanotube-like shape. PA610 compounds containing 10%, 20%, and 30% HNT were obtained in a twin-screw co-rotating extruder. The resulting blends were injection molded to create standard samples for fire testing. The incorporation of the HNTs in the PA610 matrix leads to a reduction both in the optical density and a significant reduction in the number of toxic gases emitted during combustion. This improvement in fire properties is relevant in applications where fire safety is required. With regard to calorimetric cone results, the incorporation of 30% HNTs achieved a significant reduction in terms of the peak values obtained of the heat released rate (HRR), changing from 743 kW/m2 to about 580 kW/m2 and directly modifying the shape of the characteristic curve. This improvement in the heat released has produced a delay in the mass transfer of the volatile decomposition products, which are entrapped inside the HNTs' lumen, making it difficult for the sample to burn. However, in relation to the ignition time of the samples (TTI), the incorporation of HNTs reduces the ignition start time about 20 s. The results indicate that it is possible to obtain polymer formulations with a high renewable content such as PA610, and a natural occurring inorganic filler in the form of a nanotube, i.e., HNTs, with good flame retardancy properties in terms of toxicity, optical density and UL94 test.

Effect of Thermal Conductive Fillers on the Flame Retardancy, Thermal Conductivity, and Thermal Behavior of Flame-Retardant and Thermal Conductive Polyamide 6

In this work, polyamide 6 (PA6) composites with improved flame retardancy and thermal conductivity were prepared with different thermal conductive fillers (TC fillers) such as aluminum nitride (AlN) and boron nitride (BN) in a PA6 matrix with aluminum diethylphosphinate (AlPi) as a fire retardant. The resultant halogen-free flame retardant (HFFR) and thermal conductive (TC) PA6 (HFFR-TC-PA6) were investigated in detail with a mechanical property test, a limiting oxygen index (LOI), the vertical burning test (UL-94), a cone calorimeter, a thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The morphology of the impact fracture surface and char residue of the composites were analyzed by scanning electron microscopy (SEM). It was found that the thermal conductivity of the HFFR-TC-PA6 composite increased with the amount of TC fillers. The TC fillers exerted a positive effect for flame retardant PA6. For example, the HFFR-TC-PA6 composites with the thickness of 1.6 mm successfully passed the UL-94 V-0 rating with an LOI of more than 29% when the loading amount of AlN-550RFS, BN-SW08 and BN-NW04 was 30 wt%. The morphological structures of the char residues revealed that TC fillers formed a highly integrated char layer surface (without holes) during the combustion process, as compared to that of flame retardant PA6/AlPi composites. In addition, the thermal stability and crystallization behavior of the composites were studied.

Effects of Zinc Borate on the Flame Retardancy Performance of Aluminum Diethylphosphinate in Polyamide-6 and its Composites

In this study, the flame retardancy contribution of zinc borate when used together with a traditional flame retardant (aluminum diethylphosphinate compound) was investigated for neat polyamide-6 and for 15 wt% short glass fiber reinforced composite. Melt mixing with twin-screw extrusion was the compounding method while injection and compression molding were the shaping methods of specimens. Three different flammability tests (limiting oxygen index, UL-94 vertical burning, mass loss calorimetry) indicated that many flame retardancy parameters could be improved significantly by replacing a certain amount of aluminum diethylphosphinate with zinc borate. For example, using aluminum diethylphosphinate alone resulted in only 32 % suppression in the value of the peak heat release rate for neat polyamide-6, while it was 82 % (more than two-fold) when used together with zinc borate. It was revealed by evolved gas analyses, char analyses, x-ray diffraction and thermogravimetry that the main contribution of zinc borate to aluminum diethylphosphinate was in terms of a barrier mechanism via formation of additional boron phosphate inorganic content in the barrier layer.

Synthesis of Phosphated K-Carrageenan and Its Application for Flame-Retardant Waterborne Epoxy

In this paper, phosphated K-carrageenan (P-KC) was obtained by reacting POCl₃ with the renewable source K-carrageenan (KC). P-KC and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were added into waterborne epoxy (EP) to improve its flame retardancy. The structure of P-KC was studied comprehensively using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermo-gravimetric analysis (TGA), showing the successful synthesis of P-KC. The flame retardancy of the EP was evaluated by the cone calorimeter test. The results showed that different mass ratios of DOPO and P-KC affected the flame retardancy of EP. When the mass ratio of DOPO and P-KC was 2:1, total heat release (THR) and total smoke production (TSP) decreased by 48.7% and 37.4%, respectively. The microstructures of residue char were observed by FTIR and scanning electron microscopy (SEM), indicating that the flame-retardant waterborne epoxy (FR-EP) system held a more cohesive and denser char structure. The char inhibited the diffusion of heat and oxygen, which played a key role in the flame retardancy.

Effects of α-ZrP on Crystallinity and Flame-Retardant Behaviors of PA6/MCA Composites

Novel flame-retardant Polyamide6/Melamine cyanurate-alpha-zirconium phosphate (PA6/MCA-α-ZrP) composites were prepared via the incorporation of the modified MCA (MCA-α-ZrP) into PA6. MCA-α-ZrP were synthesized through the self-assembly of α-ZrP, Melamine (ME), and cyanuric acid (CA) molecules. The results of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) showed that the incorporation of enough α-ZrP (30 wt% MCA) caused the increased crystallinity of PA6 and tended to form γ phase. The thermogravimetric analysis (TGA) and heat distortion temperature (HDT) test illustrated that the α-ZrP could increase the residue and HDT values of PA6/MCA and showed a synergistic effect with MCA. The combination of MCA and α-ZrP caused the enhancement of vertical burning test (UL-94) rating. Cone calorimeter test (CCT) gave clear evidences that PA6/MCA-10α-ZrP composites with low heat release rate (HRR), low total heat release (THR), and high amounts of char residues after combustion compared with PA6/MCA and PA6/MCA-30α-ZrP. What is more, excellent mechanical properties were kept even though MCA and α-ZrP were dispersed not as good as expected.

Aluminum hypophosphite microencapsulated to improve its safety and application to flame retardant polyamide 6

Aluminum hypophosphite (AHP) is an effective phosphorus-containing flame retardant. But AHP also has fire risk that it will decompose and release phosphine which is spontaneously flammable in air and even can form explosive mixtures with air in extreme cases. In this paper, AHP has been microencapsulated by melamine cyanurate (MCA) to prepare microencapsulated aluminum hypophosphite (MCAHP) with the aim of enhancing the fire safety in the procedure of production, storage and use. Meanwhile, MCA was a nitrogen-containing flame retardant that can work with AHP via the nitrogen-phosphorus synergistic effect to show improved flame-retardant property than other capsule materials. After microencapsulation, MCA presented as a protection layer inhibit the degradation of AHP and postpone the generation of phosphine. Furthermore, the phosphine concentration could be effectively diluted by inert decomposition products of MCA. These nonflammable decomposition products of MCA could separate phosphine from air delay the oxidizing reaction with oxygen and decrease the heat release rate, which imply that the fire safety of AHP has been improved. Furthermore, MCAHP was added into polyamide 6 to prepare flame retardant polyamide 6 composites (FR-PA6) which show good flame retardancy.

Bulk vs. surface flame retardancy of fully bio-based polyamide 10,10

PA 10,10 can be flame retarded either by melt-blending the polymer with intumescent formulations or by coating it with UV-curable mixtures.

Mapping the multimodal action of melamine-poly(aluminium phosphate) in the flame retardancy of polyamide 66

Multimodal action and synergism of melamine poly(aluminum phosphate) in the flame retardancy of polyamide 66.