Flame retardancy of poly(styrene-co-acrylonitrile) by the synergistic interaction between clay and phosphomolybdate hydrates

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Abstract

A combination of montmorillonite (MT) clay and catalysts was used to explore possible synergistic effects in reducing the flammability of poly(styrene-co-acrylonitrile) (SAN). Several catalysts, including ammonium phosphomolybdate hydrate (NHPMo), melamine phosphomolybdate hydrate (MEPMo), zinc phosphomolybdate hydrate (ZnPMo) and sodium phosphomolybdate hydrate (NaPMo), were used. The combination of MT (Cloisite 20A) with NHPMo showed synergistic enhancement in improving the char formation and reducing the peak mass loss rate of SAN40 as compared with SAN40 composites containing MT or NHPMo alone, while similar synergistic performance was not obtained between MT and NaPMo or ZnPMo or MEPMo. The better flame retardancy with this combination is probably due to the two primary aspects. One is the increased catalytic activity of the combination of Cloisite 20A and NHPMo probably due to the overlapping degradation temperature range of the combination of 20A/NHPMo and SAN40, which probably results in more char formation; the other is more NHPMo being around or on the clay stacks while less is in the galleries of the clay; this probably bridges the clay stacks and strengthens the clay network with char formed during the degradation process. High temperature rheological data confirmed the formation of a stronger network structure in SAN40/20/NHPMo; this benefits flame retardancy by allowing fewer cracks to form in the outer char layer on the polymer mass. The more continuous char layer suppresses bubbles transport of fuel vapors and heat transfer through the char layer, thereby reducing the mass loss rate.

Introduction

Montmorillonite (MT) clay has been extensively studied as a flame retardant in polymer resins because of its ability to reduce polymer flammability at relatively low add-on levels [1], [2], [3], [4], [5], [6]. The MT particles appear to retard heat transfer in the condensed phase and reduce the supply of evolved degradation products to the gas phase, thereby significantly reducing the peak release heat rate of the burning polymer. However, the total heat release is not significantly reduced. It has been reported that the flame retardancy of polymer nanocomposites with clay alone is not sufficient to achieve the requirements of industry, such as meeting requisite performance levels in the UL-94 standard [7], [8]. Further reduction in flammability of polymer/clay nanocomposites is needed for potential industrial applications [9], [10].

In general, the formation of char, which remains in the condensed phase during burning, reduces the supply of carbon to the flame and hence reduces heat release rate and total heat release. However, the addition of MT clay particles does not always enhance the formation of char [11]. The polymer/clay nanocomposites do burn more slowly, but most of the polymer is eventually consumed. It has been reported that some catalysts, such as Lewis acids, Friedel-Crafts alkylation reagents, etc., can improve the char yield of a polymer [12], [13], [14], [15], [16], [17], [18], [19]. So catalysts of this nature, used in polymer/clay nanocomposites, might be expected to improve the flame retardancy. Our previous research has indicated that a significant amount of char was obtained using zinc chloride in poly(acrylonitrile-co-styrene) (SAN). However, there was no desired synergism with MT clay on flame retardancy, presumably because the char was formed in the later stages of the pyrolysis [19].

Heteropoly acids (HPA) and their salts, polyoxometalates (POM), are an extensive class of polyoxoanions formed by group V and VII metals [20], [21]. The primary structure of HPA is the Keggin structure shown in Fig. 1 (a) [22]. Taking the [PMo12O40]3− anion for example, P is located in the center of a tetrahedral arrangement of oxygen atoms; the atoms of the transition metal Mo are bound to oxygen atoms, O; twelve oxygen–molybdenum octohedra, form a cage that surrounds the central atom, P. In HPA hydrates, hydronium ions such as H5O2+ bridge these anions (Fig. 1 (b)). POMs are formed by partial or complete substitution of hydronium ions. Both HPA and POMs are used widely as homogeneous and heterogeneous catalysts in organic synthesis due to their acid–base, multielectron redox properties and their ability to stabilize organic intermediates [23], [24], [25], but the use of HPA and POM in the flame retardant field was initiated in Ref. [26].

In this paper, phosphomolybdates (PMo) with different cations, including ammonium phosphomolybdate hydrate (NHPMo), melamine phosphomolybdate hydrate (MEPMo), zinc phosphomolybdate hydrate (ZnPMo) and sodium phosphomolybdate hydrate (NaPMo), were used as catalysts, combined with MT clay, to flame retard SAN. Some protons still remain in the PMo salts even if they are prepared stoichiometrically; these are responsible for its catalytic activity [21]. Moreover, both NHPMo and MEPMo could release (locally) non-reactive gases during their degradation in the condensed phase, which may make the char layer intumesce, thereby helping retard the transfer of heat. ZnPMo, combining both Lewis and Bronsted acids, was expected to produce more char. NaPMo was used as a reference. With this in mind, we examine whether there is a synergistic effect between the clay and each catalyst in improving the flame retardancy of SAN40, and we compare their relative effectiveness.

Section snippets

Materials

SAN40 (styrene-60: acrylonitrile-40) was provided from Asahi Kasei, Japan; Organoclay (Cloisite 20A)1 treated with dimethyl, dehydrogenated tallow, quaternary ammonium chloride surfactant, was provided by Southern Clay Products, Inc.; Tetrahydrofuran (THF) was purchased

Sample morphology

Fig. 2 provides the XRD patterns of the Cloisite 20A, 20A/PMo mixtures and SAN40 composites. For the Cloisite 20A, two strong characteristic diffraction peaks appear at 2θ = 3.5° and 2θ = 7.1°, corresponding to the d spacing of 25.5 Å and 12.4 Å, respectively. For the 20A/PMo mixtures, all the peaks shift slightly to lower 2θ values compared to that of 20A. The d001 spacings of 20A/NHPMo, 20A/MEPMo, 20A/ZnPMo and 20A/NaPMo are 28.8 Å, 28.3 Å, 29.2 Å and 29.0 Å, respectively, which are larger

Discussion

The above results show that the combination of Cloisite 20A and NHPMo not only enhances the char yield of SAN40, but also reduces the peak mass loss rate. It is of interest to determine why only the combination of 20A and NHPMo shows this synergism effect among the combinations used in the study.

The TGA and DTG curves of Cloisite 20A, 20A/PMo mixtures are shown in Fig. 8. The maximum degradation rate of Cloisite 20A is delayed by the addition of PMo salts. This is probably due to the

Conclusions

A synergistic enhancement between MMT clay and NHPMo in improving the flame retardancy of SAN40 is obtained. The combination of Cloisite 20A with NHPMo not only enhances the char yield of SAN40, but also reduces the peak mass loss rate of SAN40 at fire-level heat fluxes. The possible mechanism includes the following two primary aspects. One is the increased catalytic activity of the combination of Cloisite 20A and NHPMo probably due to the overlapping degradation temperature range of the

Acknowledgment

MF.L. acknowledges funding partially from the China State Scholarship and partially from the National Institute of Standards and Technology (NIST). X.Z. acknowledges the support of the MarylandNanoCenter and its NispLab. The NispLab is supported in part by the NSF as an MRSEC Shared Experimental Facility. T. K. acknowledges funding from the NIST under Grant 9H9184. We gratefully acknowledge Dr. Tom Ohlemiller at the NIST for helping with grammatical corrections.

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    This study was carried out by the National Institute of Standards and Technology (NIST), an agency of the US Government which, by statute, is not subject to copyright in the US.

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