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The thermal stability and flame retardant properties of Polypropylene
Abstract
The results indicate that an intercalated structure is formed. Thermogravimetric analysis,limiting oxygen index (LOI) and UL-94 are used to characterize the thermal stability and the flame retardant properties.
It is found that there is a transformation of the char microstructure and an increment of LOI value in the PP systems with the OZrP, and the UL-94 level is improved, from failure, to a V-0 rating. The synergistic effect improves the flame retardant properties of PP remarkably.
The morphology and structure (HRTEM and selected area electron diffraction) of the char residue also indicate the presence of graphite sheets.
A possible mechanism for catalyzing carbonization is discussed.
Experimental
Results and discussion
Thermal stability and flame retardant properties of the PP/IFR/OZrP nanocomposites
They show that the intumescent flame retardant PP/OZrP nanocomposites decrease the initial degradation temperature compared to the intumescent flame retardant PP(PP1).
The probable reasons may be that the intimate contacts between the polymer molecules and the atoms of the inorganic crystalline layers in PP2 or PP3 are more extensive than that in PP1,and at the same time, there is a catalytic role played by the layered phosphate deriving from the Hoffman degradation of
C16, which may accelerate the charring process at the beginning of the degradatio.
The performance in the UL-94 burning test.
The V-0 grade is achieved when the amount of the OZrP is more than 2.5 wt% in IFR-PP systems.
The results clearly signify a synergistic effect between OZrP and IFR as well as an optimal content of the OZrP in this system corresponding to the best fire retardancy.
Structure analysis of the char residue
The results show that the intumescent charred residue of PP3 (Fig. b) is tighter and denser than that of PP1(Fig. a).
A lot of flaws can be seen on the surface of the char residue of PP1.
The microstructure of the char residue of PP3 (Fig. b) is more homogeneous than that of PP1 (Fig. a).
Moreover, high magnification SEM in Fig. d shows that some layers of the phosphate had not collapsed, serving as a barrier to oxygen and pyrol-ysis gases.
It is supposed that the nanocomposites can lead to the formation of ceramic-like materials with a homogeneous surface, which will protect the material throughout combustion, and also a mechanical reinforcement of the charred layer, reducing char opening.
Possible catalyzing carbonization mechanism
At the same time, there is a catalytic role played by the OZrP deriving from the Hoffman degradation of the modifier, C16.It is known that the thermal decomposition of alkyl ammonium in the OZrP will leave the hydrogen to form proton acid sites on the degraded OZrP layers , which also results in catalytic degradation of PP to yield macroradicals.
During combustion, PP evolves small molecules such as CO, CO2 , C2 H 4 and C3H6 in the presence of oxygen .these molecules volatilize quickly and burn, without conversion into graphite.
In the intumescent flame retardant PP systems, Zr4+partially replaces the ammonium ions in APP and produces inter-chain bridges that bring about a more homogeneous and more stable char , as
evidenced by the final residue and the observation of SEM.
Consequently, α -ZrP promotes the phosphorylation of petol and the OH groups to form on the PP by thermo-oxidation which is also accelerated, as evidenced by the increase in the initial weight loss and by the lowering of the initial degradation temperature.
The increase of LOI value and the improvement of UL-94 level are similar, as is the synergistic effect.
At higher concentrations of OZrP,several polyphosphate monomeric units and chains may participate simultaneously in the reaction of APP with the OZrP, and produce clusters of chains crosslinked by the salt bridges.
Highly crosslinked APP forms rigid unreactive gel-clumps, resulting in coarse, inhomogeneous char, which counteracts the catalysis. The LOI decreases.
(The R&D from State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230027 Anhui, China)
The catalyzing carbonization function of alpha-zirconium phosphate ( α-ZrP) based on an intumescent fire retardant (IFR) polypropylene (PP) system is reported.
The IFR system consists of ammonium polyphosphate and pentaerythritol.
The morphology of the PP/organophilic ZrP nano-composites is characterized by X-ray diffraction and high resolution transmission electron microscopy(HRTEM).
The IFR system consists of ammonium polyphosphate and pentaerythritol.
The morphology of the PP/organophilic ZrP nano-composites is characterized by X-ray diffraction and high resolution transmission electron microscopy(HRTEM).
The results indicate that an intercalated structure is formed. Thermogravimetric analysis,limiting oxygen index (LOI) and UL-94 are used to characterize the thermal stability and the flame retardant properties.
It is found that there is a transformation of the char microstructure and an increment of LOI value in the PP systems with the OZrP, and the UL-94 level is improved, from failure, to a V-0 rating. The synergistic effect improves the flame retardant properties of PP remarkably.
The morphology and structure (HRTEM and selected area electron diffraction) of the char residue also indicate the presence of graphite sheets.
A possible mechanism for catalyzing carbonization is discussed.
Experimental
α-ZrP was organically modified by a cationicsurfactant before melt intercalation with PP. The surfactant is an ammonium salt bearing long alkyl chains [hexadecyl trimethylammonium bromide (C16)].
The probable reasons may be that the intimate contacts between the polymer molecules and the atoms of the inorganic crystalline layers in PP2 or PP3 are more extensive than that in PP1,and at the same time, there is a catalytic role played by the layered phosphate deriving from the Hoffman degradation of
C16, which may accelerate the charring process at the beginning of the degradatio.
Indeed, The Pic indicates that the thermal stabilities of samples PP1 to PP3 are improved compared with that of PP0(pure PP).
As the OZrP content increases from 1 to 2.5 wt%, the amount of charred residue increases.
This means that OZrP promotes charring to form a carbonaceous material, and thus improve the flame retardant properties of PP.
As the OZrP content increases from 1 to 2.5 wt%, the amount of charred residue increases.
This means that OZrP promotes charring to form a carbonaceous material, and thus improve the flame retardant properties of PP.
Study on the synergistic effect between OZrP and IFR on the flame retardant PP is investigated.
Table 1 lists the compositions of the PP mixtures with the same total loading (OZrP/IFR) and the results of LOI measurements and UL-94 test.
The results show that the addition of OZrP into the PP matrix results in a significant increase in the LOI values.
The LOI values increase with the amount of OZrP at first until the maximum is reached.
The highest LOI value in this study is 37 with 2.5 wt% OZrP and 22.5 wt% IFR.
Then the LOI begins to decrease as the content of the OZrP increases, up to 5 wt%, perhaps because excess phosphate may restrain the expansion of IFR in the PP systems.
Table 1 lists the compositions of the PP mixtures with the same total loading (OZrP/IFR) and the results of LOI measurements and UL-94 test.
The results show that the addition of OZrP into the PP matrix results in a significant increase in the LOI values.
The LOI values increase with the amount of OZrP at first until the maximum is reached.
The highest LOI value in this study is 37 with 2.5 wt% OZrP and 22.5 wt% IFR.
Then the LOI begins to decrease as the content of the OZrP increases, up to 5 wt%, perhaps because excess phosphate may restrain the expansion of IFR in the PP systems.
The performance in the UL-94 burning test.
The V-0 grade is achieved when the amount of the OZrP is more than 2.5 wt% in IFR-PP systems.
The results clearly signify a synergistic effect between OZrP and IFR as well as an optimal content of the OZrP in this system corresponding to the best fire retardancy.
Structure analysis of the char residue
The SEM images of the microstructure of the charred residue after combustion of PP1 and PP3 (with 2.5 wt% OZrP) at 800℃ are shown in Fig.
The results show that the intumescent charred residue of PP3 (Fig. b) is tighter and denser than that of PP1(Fig. a).
A lot of flaws can be seen on the surface of the char residue of PP1.
The microstructure of the char residue of PP3 (Fig. b) is more homogeneous than that of PP1 (Fig. a).
Moreover, high magnification SEM in Fig. d shows that some layers of the phosphate had not collapsed, serving as a barrier to oxygen and pyrol-ysis gases.
It is supposed that the nanocomposites can lead to the formation of ceramic-like materials with a homogeneous surface, which will protect the material throughout combustion, and also a mechanical reinforcement of the charred layer, reducing char opening.
HRTEM was used to further characterize the graphitization of charred residue
Above the Fig. clearly shows that the charred residue is comprised of graphite sheets.
Above the Fig. clearly shows that the charred residue is comprised of graphite sheets.
Possible catalyzing carbonization mechanism
A possible reaction mechanism of the catalyzing carbonization and graphitization at the surface of the solid acid-type OZrP in the intumescent flame retardant PP/OZrP nanocomposites is presented.
OZrP can thermally loose H+, that is, proton acid sites on the degraded OZrP layers will attack molecular chains of PP to form cationic active sites,which result in catalytic degradation of PP and yield macroradicals.
Moreover, OZrP also possesses other Lewis acid sites, namely, Zr4+.these can capture macroradicals allowing recombination and leading to intermolecular cross-linking.
Moreover, OZrP also possesses other Lewis acid sites, namely, Zr4+.these can capture macroradicals allowing recombination and leading to intermolecular cross-linking.
At the same time, there is a catalytic role played by the OZrP deriving from the Hoffman degradation of the modifier, C16.It is known that the thermal decomposition of alkyl ammonium in the OZrP will leave the hydrogen to form proton acid sites on the degraded OZrP layers , which also results in catalytic degradation of PP to yield macroradicals.
During combustion, PP evolves small molecules such as CO, CO2 , C2 H 4 and C3H6 in the presence of oxygen .these molecules volatilize quickly and burn, without conversion into graphite.
As we add OZrP into PP matrix, a protective charred ceramic surface layer is formed after burning, which forms ‘‘a sealed autoclave microreactor’’.as supported by the SEM images, some layers of phosphate have still not collapsed after burning, serving as a barrier to the supply of oxygen and pyrolysis gases.
Thus, we presume that these pyrolytic products have more time to contact OZrP, and
are dehydrogenated and aromatized to form char.
At the same time, we hypothesize that Zr4+ partly reacts with carbon atoms from the degradation of PP to form ZrC, and the process may involve a dynamic thermo-dynamic equilibrium between ZrC and Zr, in which a conversion from carbon to graphite occurs.part of the driving force may come from the free energy of the transformation from carbon to the stable graphite phase.these graphitic materials are very stable at high temperature, which improves the fire retardancy of PP.
Thus, we presume that these pyrolytic products have more time to contact OZrP, and
are dehydrogenated and aromatized to form char.
At the same time, we hypothesize that Zr4+ partly reacts with carbon atoms from the degradation of PP to form ZrC, and the process may involve a dynamic thermo-dynamic equilibrium between ZrC and Zr, in which a conversion from carbon to graphite occurs.part of the driving force may come from the free energy of the transformation from carbon to the stable graphite phase.these graphitic materials are very stable at high temperature, which improves the fire retardancy of PP.
In the intumescent flame retardant PP systems, Zr4+partially replaces the ammonium ions in APP and produces inter-chain bridges that bring about a more homogeneous and more stable char , as
evidenced by the final residue and the observation of SEM.
Consequently, α -ZrP promotes the phosphorylation of petol and the OH groups to form on the PP by thermo-oxidation which is also accelerated, as evidenced by the increase in the initial weight loss and by the lowering of the initial degradation temperature.
The increase of LOI value and the improvement of UL-94 level are similar, as is the synergistic effect.
At higher concentrations of OZrP,several polyphosphate monomeric units and chains may participate simultaneously in the reaction of APP with the OZrP, and produce clusters of chains crosslinked by the salt bridges.
Highly crosslinked APP forms rigid unreactive gel-clumps, resulting in coarse, inhomogeneous char, which counteracts the catalysis. The LOI decreases.
The results of this work indicate possibilities for important future developments in the field of catalysis of intumescent and other IFR systems.
Solid acids catalysts might be developed in other applications.
(The R&D from State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230027 Anhui, China)
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