INTRODUCTION

Some of the most effective flame retardants are compounds that contain both phosphorus and nitrogen. Polyphosphazenes are a unique class of polymers with a backbone composed of alternating phosphorus and nitrogen atoms. The versatility of this polymer system is due mainly to the ease of modification of the precursor polymer poly(dichlorophosphazene). The highly reactive P-Cl bonds in this intermediate can be easily replaced by reaction with alcohols, amines, and organometallic reagents. Appropriate selection of the side groups allows control of the solubility, crystallinity, bioactivity, and surface characteristics. These properties have in turn resulted in a broad range of uses including non-burning fibers or foams and as flame retardants. The thermal behavior and decomposition of a variety of polyphosphazenes have been studied in some detail. At elevated temperatures, the thermal response depends on the types of side groups present. Polyphosphazenes, especially those that bear aryloxy side groups, possess a high temperature stability as well as excellent flame resistance. In the present work, we describe efforts to increase the flame resistance of polyphosphazenes while developing less expensive, more controlled methods for their synthesis. We have also incorporated phosphazenes into organic polymers in order to impart flame resistance to systems that are normally highly combustible.

POLYURETHANE / POLYPHOSPHAZENE BLENDS

Polyurethanes are a good example of a traditional organic polymer system that has useful structural and mechanical properties, but is limited by its low thermo-oxidative stability. Although many varieties of this polymer are widely used in aerospace applications and home construction, they are highly combustible. Polyurethanes could be made fire resistant if they were blended with a polymer that would modify the decomposition mechanism of the polyurethane, release noncombustible gases, and/or undergo reactions during heating to create a thermally insulating char to quench further combustion. However, the choice of suitable polymeric flame-retardants is restricted to species that allow retention of the advantageous mechanical properties of the polyurethane. In situ chemical reactions between two polymers in a mixture ensures intimate mixing and, given the appropriate chemistry, numerous graft and interpenetrating networks can be designed. We have synthesized composites of a structural polyurethane and poly[bis(4-carboxylatophenoxy)phosphazene] (1) as illustrated in Scheme 1. The thermal stability and flame resistance of these composites were analyzed.

Scheme 1

1. Flammability Tests

Polyurethanes, in the absence of flame retardants, are extremely combustible. In these tests, the pure polyurethane sample burned rapidly after exposure to the flame and was totally consumed within 40 sec, to leave a black char. A 20 wt% loading of phosphazene 1, as shown in Scheme 1, caused the samples to self-extinguish. Polymer 1 is a high char yielding material, and this char can coat the more combustible components and prevent further combustion. Also, previous work indicated that polymer 1 releases a large amount of carbon dioxide during pyrolysis, thus increasing the percentage of noncombustible gases present.

Oxygen index (OI) measurements were also performed on blends containing 20 wt% polyphosphazene 1 (2 wt % P). This showed an increased OI of 21.5 ± 0.2 to that measured for the pure polyurethane foam (OI = 20.0 ± 0.02). Although this is not a striking increase, and does not match the dramatic visual horizontal burn test observationt. We believe that the ~ 2 wt% total phosphorus in this polyurethane/polyphosphazene mixture represents the lower limit required to impart meaningful flame resistance.

2. Thermal Analysis

In general, poly[bis(p-R-phenoxy)phosphazenes], where R is a polar substituent, give high char yields of 50% or more at temperatures at or above 700 °C and also undergo a high degree of cross-linking to form a dense ultrastructure. Polymer 1 undergoes skeletal cleavage and cross-linking reactions when heated. The onset of cross-linking occurs at 200 °C, resulting in the small weight loss detected by TGA (Figure 1). Continued pyrolysis results in a further 35% weight loss at 320 °C, associated primarily with the loss of carbon dioxide. This is followed by the slow continuous weight loss of benzoic acid and a 55% char yield at 600°C.

The thermal weight loss (TGA) curve for the pure polyurethane in air is also shown in Figure 1. The onset of thermal degradation occurs at ~ 200°C, and the maximum rate of degradation for the two main decomposition steps occurs at ~ 300°C and 540°C. Significantly, above a 20 wt% loading of polymer 1 (where enhanced flame resistance in the horizontal burn test and OI measurements was detected) the TGA curves are essentially identical, except in the region between 550 - 600°C. Here the amount of char approximately parallels the amount of 1 initially present in the mixture. The overall char yield at 600°C is actually higher than expected based on the original composition of the samples. For example, the sample with 30 wt% 1 has a char yield of ~ 30% at 600°C. Based on the data obtained for the individual components, only ~ 20% char yield is expected. An explanation for this is that the high char of polymer 1 either entraps volatile molecules or limits the thermo-oxidative degradation of the polyurethane.

Figure 1. TGA curves for the polyurethane, polyphosphazene, and polyurethane / poly(organophosphazene) foams containing 5, 15, 20, and 30 wt% of polymer 1.

PHOSPHORYLATION OF PHOSPHAZENES

One of the reasons that phosphazenes have excellent flame retardant properties is the presence of both phosphorus and nitrogen in the backbone. In organic polymer systems, the addition of small-molecule phosphorus compounds (particularly phosphorus esters) is one of the most effective ways to decrease the flammability of the system. Extensive work has been carried out on the incorporation of phosphorus into organic polymers by way of small molecule or polymeric additives, copolymerizations, and chemical modification. The effect on combustion of the polymer depends on the chemical structure of the macromolecule, and is commonly attributed either to the generation of polyphosphoric acid and subsequent char formation, or to a modification of the decomposition mechanism of the polymer. Although polyphosphazenes already have a significant phosphorus content, improved fire resistance could result from the introduction of phosphate or phosphonate species into the side groups.

We have synthesized novel aryloxyphosphazenes, both cyclic and polymeric, that bear pendent phosphate groups. These groups were chosen to be analogues of commonly used flame retardant additives such as triethyl phosphate and triphenyl phosphate. Immobilization of the phosphate units on the phosphazene was expected to increase the thermal stability and flame resistance of the polymers without impairing the mechanical properties by plasticization. Phosphorylated cyclic trimers were synthesized as model compounds for the high polymers and as possible small-molecule additives. These trimers were blended with polystyrene and evaluated for their performance as oligomeric flame retardant additives. The thermal stability of the phosphorylated polymers has been studied and compared to those of non-phosphorylated phosphazenes.

1. Synthesis of Phosphorylated Phosphazenes

Phosphazenes bearing phenolic functionalities14 were induced to react with halogenated phosphate esters as shown in Scheme 2. The reactions involving diethyl chlorophosphate required longer reaction times than did the reactions with diphenyl chlorophosphate. At the high polymer level, full conversion of the hydroxyl groups to diethyl phenylphosphate species did not occur, and the resulting product was slightly crosslinked. Phosphorylation of 3 with diphenyl chlorophosphate was much easier, with full conversion obtained at room temperature. This polymer dissolved and precipitated very well, giving a light brown, fibrous material.

CONCLUSIONS

Phosphazenes, both cyclic and polymeric, have been examined for their application as flame-retardant materials and as flame-retardant additives to organic polymers. A blend of poly[bis(carboxylatophenoxy)phosphazene] with polyurethane precursors resulted in a urethane foam which exhibited increased thermal stability relative to the pure polyurethane. In addition, miscible blends of poly[alkoxyphosphazenes] with organic polymers have been studied. Phosphorylation of a functionalized phosphazene resulted both in a novel elastomer with excellent thermal stability and in a new class of oligomeric flame-retardant additives. Efforts have been made to control the architecture and reactivity, and thus the mixing properties, of phosphazenes using techniques such as the ?living? cationic polymerization of phosphoranimines. Block copolymers of phosphazenes and organic polymers are being developed for application as blending agents and flame-retardant materials.