Flame retarding mechanism of Polyamide 6 with phosphorus-nitrogen flame retardant and DOPO derivatives

The Impact of β-Radiation Crosslinking on Flammability Properties of PA6 Modified by Commercially Available Flame-Retardant Additives

A comparative study was conducted investigating the influence of β-radiation crosslinking (β-RC) on the fire behavior of flame retardant-modified polyamide 6 (PA6). In order to provide a comprehensive overview, a variety of commercially available flame-retardant additives were investigated, exhibiting different flame retarding actions such as delusion, char formation, intumescence and flame poisoning. This study focused on the identification of differences in the influence of β-RC on fire behavior. Coupled thermal gravimetrical analysis (TGA) and Fourier transformation infrared spectroscopy (FTIR) were used to conduct changes within the decomposition processes. Dynamic thermal analysis (DTA) was used to identify structural stability limits and fire testing was conducted using the limiting oxygen index (LOI), vertical UL-94 and cone calorimeter testing. Crosslinking was found to substantially change the fire behavior observed, whereas the observed phenomena were exclusively physical for the given formulations studied: warpage, char residue destruction and anti-dripping. Despite these phenomena being observed for all β-RC formulations, the impact on fire resistivity properties were found to be very different. However, the overall fire protection properties measured in UL-94 fire tests were found to deteriorate for β-RC formulations. Only β-RC formulations based on PA6/EG were found to achieve a UL-94 V0 classification.

Flame Inhibition and Charring Effect of Aromatic Polyimide and Aluminum Diethylphosphinate in Polyamide 6

An aromatic macromolecular polyimide (API) was synthesized and characterized, and used as a synergistic charring flame retardant in glass fiber reinforced polyamide 6 (GF/PA6). API and aluminum diethylphosphinate (ADP) exhibited better flame inhibition behavior and synergistic charring flame retardant behavior compared with ADP alone. The 5%API/7%ADP/GF/PA6 sample achieved the lower peak value of the heat release rate (pk-HRR) at 497 kW/m² and produced higher residue yields of 36.1 wt.%, verifying that API and ADP have an outstanding synergistic effect on the barrier effect. The API/ADP system facilitated the formation of a carbonaceous, phosphorus and aluminum-containing compact char layer with increased barrier effect. FTIR spectra of the residue and real-time TGA-FTIR analysis on the evolved gases from PA6 composites revealed that API interacted with ADP/PA6 and locked in more P⁻O⁻C and P⁻O⁻Ar content, which is the main mechanism for improving flame inhibition and charring ability. In addition, the API/ADP system improved the mechanical properties and corrosion resistance of GF/PA6 composites compared to ADP alone.

Effects of Tung Oil-Based Polyols on the Thermal Stability, Flame Retardancy, and Mechanical Properties of Rigid Polyurethane Foam

A phosphorus-containing tung oil-based polyol (PTOP) and a silicon-containing tung oil-based polyol (PTOSi) were each efficiently prepared by attaching 9,10-dihydro-9-oxa-10-phosphaphenanthrene (DOPO) and dihydroxydiphenylsilane (DPSD) directly, respectively, to the epoxidized monoglyceride of tung oil (EGTO) through a ring-opening reaction. The two new polyols were used in the formation of rigid polyurethane foam (RPUF), which displayed great thermal stability and excellent flame retardancy performance. The limiting oxygen index (LOI) value of RPUF containing 80 wt % PTOP and 80 wt % PTOSi was 24.0% and 23.4%, respectively. Fourier transfer infrared (FTIR), Nuclear Magnetic Resonance (NMR) and thermogravimetric (TG) analysis revealed that DOPO and DPSD are linked to EGTO by a covalent bond. Interestingly, PTOP and PTOSi had opposite effects on Tg and the compressive strength of RPUF, where, with the appropriate loading, the compressive strengths were 0.82 MPa and 0.25 MPa, respectively. At a higher loading of PTOP and PTOSi, the thermal conductivity of RPUF increased while the RPUF density decreased. The scanning electron microscope (SEM) micrographs showed that the size and closed areas of the RPUF cells were regular. SEM micrographs of the char after combustion showed that the char layer was compact and dense. The enhanced flame retardancy of RPUF resulted from the barrier effect of the char layer, which was covered with incombustible substance.