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Natural-fiber-reinforced polypropylene (PP) composites with a series of advantages including light weight, chemical durability, renewable resources, low in cost, etc., are being widely used in many fields such as the automotive industry, packaging, and construction. However, the flammability of plant fiber and the PP matrix restricts the application range, security, and use of these composites. Therefore, it is of great significance to study the flame retardants of such composites. In this paper, sisal-fiber-reinforced polypropylene (PP/SF) flame-retardant composites were prepared using the two-step melt blending method. The flame retardant used was an intumescent flame retardant (IFR) composed of silane-coated ammonium polyphosphate (Si-APP) and pentaerythritol (PER). The influence of different blending processes on the flammability and mechanical properties of the composites was analyzed. The findings suggested that PP/SF flame-retardant composites prepared via different blending processes showed different flame-retardant properties. The (PP/SF)/IFR composite prepared by PP/SF secondary blending with IFR showed excellent flame-retardant performance, with a limited oxygen index of about 28.3% and passing the UL-94 V-0 rating (3.2 mm) in the vertical combustion test. Compared with the (PP/IFR) /SF composite prepared by a matrix primarily blended with IFR and then secondly blended with SF, the peak heat release rate (pk HRR) and total heat release (THR) of the (PP/SF)/IFR composite decreased by 11.3% and 13.7%, respectively. In contrast, the tensile strength of the (PP/SF)/IFR system was 5.3% lower than that of the (PP/IFR)/SF system; however, the overall mechanical (tensile, flexural, and notched impact) properties of the composites prepared using three different mixing processes were similar. [ABSTRACT FROM AUTHOR]

In this work, three kinds of APP, coated with melamine (MF‐APP), silane (GW‐APP), epoxy (MC‐APP) were employed to compound with novolac resin (Novolac) and melamine (ME), aimed to study the effect of the modified APP on the flame‐retardant performance, mechanical properties, and thermal stability of polyformaldehyde (POM). The results showed that composites with modified APP exhibited better flame retardant and mechanical performance than that with unmodified APP. In contrast, GW‐APP had the best synergistic effect with Novolac and ME, and POM/GW‐APP composites reached UL‐94 V‐0 rating and its limit oxygen index (LOI) value was up to 34.0%. The morphology of the carbon layer showed that the silane coating material can promote the charring process in condensed phase better than epoxy and melamine coating materials in flame retardant POM system, leading to the formation of integrated char layers with more quantity and higher quality, which effectively delayed the mass and heat transfer during combustion. [ABSTRACT FROM AUTHOR]

Benzoxazine monomers of traditional bisphenol‐A benzoxazine (BA‐a) and bisphenol‐A benzoxazine containing trialkoxysilane (BA‐a‐Si) were synthesized and incorporated into polyoxymethylene/ammonium polyphosphate/melamine (POM/APP/ME) to improve the fire retardancy of POM. The flame retardation, thermal, and mechanical properties of POM composites were evaluated by cone calorimeter, limiting oxygen index (LOI), and the Underwriters Laboratories‐94 (UL‐94) vertical burning tests, as well as thermogravimetric analysis and mechanical tests. Addition of BA‐a‐Si (2 wt%) into POM could simultaneously improve its flame retardancy (UL‐94 vertical burning rating to V‐0 rating and LOI value reached 52.1%) and notched impact strength (increased by 33.3% and 100.0% compared with that of POM/APP/ME and POM/APP/ME/BA‐a composites). Notably, compared with POM/APP/ME composite, the smoke productions of both the composites with benzoxazines were significantly suppressed. In contrast, the peak heat release rate and total heat release of composite with 2 wt% BA‐a‐Si were 33.3% and 32.9% lower than those of composite with 2 wt% BA‐a, which were closely related to the formation of high quality and quantity of charred residue. In summary, benzoxazine, especially siloxane‐containing benzoxazine, plays an effective role in flame retarding POM acting as charring agent and synergistic flame retardant. [ABSTRACT FROM AUTHOR]

A type of trialkoxysilane‐containing naphtholoxazine compound (Naph‐boz) was successfully synthesized and combined with ammonium polyphosphate/melamine (APP/ME) as an intumescent flame retardant (IFR) to improve the flame‐retardant efficiency of polyoxymethylene (POM). The Underwriters Laboratories 94 (UL94) vertical burning test, limiting oxygen index (LOI), cone calorimeter, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and Raman spectral analysis were used to study the flame‐retardant properties and related mechanism. The results showed that the formulation with 20 wt.% of APP, 6 wt.% of ME, and 4 wt.% of Naph‐boz passed UL94 V‐1 rating, and the LOI value was improved to 40.3%. Compared with pure POM, the IFR with Naph‐boz had greater reduction in peak heat release rate (lower 74.9%) and total heat release (lower 40.2%). SEM images showed that compact and reinforcing charred layer was formed during the POM/IFR/4Naph‐boz samples combustion, which was beneficial at reducing and maintaining low combustion parameters throughout the cone calorimeter test. The synergistic flame‐retardant effect between Naph‐boz and APP/ME was considered as the reason for the improvement in flame retardancy POM. Furthermore, because of the Naph‐boz was conducive to the compatibility between the flame retardants and matrix, the notched Izod impact strength of POM/IFR/4Naph‐boz composite was higher than that of POM/IFR system. [ABSTRACT FROM AUTHOR]