Functionalization of α-Zirconium Phosphate and Its Effect on the Flame Retardance of an Intumescent Flame Retardant Polypropylene System

Abstract

A kind of core–shell flame retardant, functionalized α-zirconium phosphate (FZ) was prepared by wrapping α-zirconium phosphate (α-ZrP) with a kind of polyphosphazene to shield the solid acid sites on the surface of α-ZrP.

The synergistic effect between polyphosphazene and α-ZrP was studied. The effects of FZ on the thermal properties and flame retardance of intumescent flame retardant polypropylene (IFR/PP) composites were investigated.

It was found that the presence of FZ improved the thermal stability of the matrix and increased the amount of char yield at 800 °C significantly. For IFR/PP composites, the LOI of the composites was only 26.4% and the V-1 rating was obtained by UL-94 vertical testing. With an increase of FZ content to 3 wt%, the composites could achieve a LOI value of 32.8% and a UL-94 V-0 rating, whereas, without a coating layer of polyphosphazene, the LOI of the composite decreased to 27.5% and a V-1 rating was obtained.

A synergistic effect between FZ and IFR was found. During the primary stage of combustion, the functional layer was decomposed and could quench free radicals from PP chain scission, and in the later stage, could also promote char formation due to the presence of solid acid sites on the surface of uncovered α-ZrP, thus a continuous and compact intumescent char was produced on the substrate.

Preparation of flame-retardant PP composites

Results and discussion

Characterization of the prepared flame-retardant FZ

Thermogravimetric analysis of PP/FZ composites

Flame retardance of the prepared PP/FZ composites

Analysis of the char residues

Morphologies and SEM-EDXS analysis of the residual char

SEM images of char residue of PP0 (A1: inner surface, A2: outer surface), PP1 (B1: inner surface, B2: outer surface), PP2 (C1: inner surface, C2: outer surface), PP3 (D1: inner surface, D2: outer surface), PP4 (E1: inner surface, E2: outer surface), and PP5 (F1: inner surface, F2: outer surface).

SEM graphs of the char residues, (a) PP4 outer surface and (b) EDXS spectra of the outer surface, (c) PP4 inner surface and (d) EDXS spectra of the inner surface, (e) amplified region of the PP4 inner surface and (f) EDXS spectra from spot scanning.

Flame-retardant mechanism
It can be found that MPP first degrades into polyphosphoric acid, which is called the acid source, and melamine, which is called the gas source (chemical reaction 1.1).

The majority of the polyphosphoric acid then crosslinks with PER to yield char and further degrades with phosphorus-containing compound radicals (mainly P–O˙ radicals) including phosphate ester radicals and pyrophosphate radicals, which could inhibit the free radicals produced by the PP composites (chemical reaction 1.2).

And the polyphosphazene on the surface of α-ZrP decomposes with BPS free radicals and linear polyphosphazene, which would produce amorphous char and ammonia (the gas source), and metaphosphoric acid (the acid source) for further degradation; meanwhile, the substantial number of solid acid sites on the surface of the lamellar α-ZrP core could act as reaction sites to accelerate the degradation of the PP composites, which could enhance the char yield during combustion, and also could improve the mechanical strength of the carbon layer to segregate the heat and gas transfer (chemical reaction 1.3).
With the development of the combustion process, the phosphorus-containing compounds and two sources (the acid source and carbon source) act together to form the precursor to intumescent char via cyclization and carbon yielding reactions, which is then further cross-linked and foamed by the nonflammable gas (gas source).

In this case, an intumescent char is formed and strengthened by the α-ZrP core of FZ and the matrix is covered by this char. As a result, it can be seen that the char layer is continuous, compact and thermally stable. The combustible gas could be effectively diluted by the incombustible gas and inhibited by the char layer. The heat is segregated as well. The phenomenon of intumescent and phosphorus-containing char, also with α-ZrP to catalyze degradation and carbonization, and its residues to strengthen the char layer, show that there is a comprehensive flame-retardant mechanism, including a gas phase mechanism, a condensed phase mechanism, a phosphorus–nitrogen synergistic mechanism, and cooperation between and efficient flame retardant and inorganic solid acid during combustion of the PP/IFR/FZ system.

Schematic illustration of the flame retardant mechanism of the PP/IFR/FZ composites

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