Phosphazene fluoroelastomers

Abstract In this paper the synthesis, properties and applications of poly(organophos-phazenes) have been highlighted. Five different classes of macromolecules have been described, i.e. phosphazene fluoroelastomers, aryloxy-substituted polymeric flame-retardants, alkoxy-substituted phosphazene electric conductors, biomaterials and photo-inert and/or photo-active phosphazene derivatives. Perspectives of future developments in this field are briefly discussed. 

The first phenomenon observed is the improved resistance of these materials to combustion, in a way that they may be classified as intrinsically self-extinguishing substrates. For instance, the LOI value for PTFEP is reported to be 48 [452], which is much higher than reported for classical organic plastics [283], while phosphazene fluoroelastomers have been considered as fire-retardant materials since the very beginning of their preparation and utilization [562]. Similarly to aryloxy- and arylamino- substituted POPs [389,390] (vide infra),it may be expected that the flame-resistance properties of phosphazene fluoroelastomers could be successively exported to stabihze organic macromolecules when blended with these materials. 

Similarly to PTFEP, phosphazene fluoroelastomers proved to be rather stable to thermal treatments up to 175 °C [560,561], while for temperatures above this value degradation phenomena start to be relevant. 

In the case of poly(alkoxyphosphazenes) (IV) or poly(aryloxyphos-phazenes) (V) a dramatic change in properties can arise by employing combinations of substituents. Polymers such as (NP CHjCF ) and (NP CgH,).) are semicrystalline thermoplastics (Table I). With the introduction of two or more substituents of sufficiently different size, elastomers are obtained (Figure 4). Another requirement for elastomeric behavior is that the substituents be randomly distributed along the P-N backbone. This principle was first demonstrated by Rose (9), and subsequent work in several industrial laboratories has led to the development of phosphazene elastomers of commercial interest. A phosphazene fluoroelastomer and a phosphazene elastomer with mixed aryloxy side chains are showing promise for military and commercial applications. These elastomers are the subject of another paper in this symposium (10). 

Other Applications. Thus far the phosphazene fluoroelastomers (PNF) and aryloxyphosphazene elastomers (APN) have moved to the commercial stage. In addition to elastomers, phosphazenes are being investigated as fluids, resins and plastics. Other areas which hold promise include fire resistant paints (55), fiber blends and additives, agrichemicals and herbicides, drug release agents and electrically conducting polymers (6). 

Figure 6. Phosphazene fluoroelastomers (PNF) were used to prepare various rubber end items, some of which are commercially available.

Phosphazene Fluoroelastomers. The work of Rose (19) along with subsequent efforts at several laboratories has led to the commer-... 

The phosphazene fluoroelastomers can be compounded and processed to give an excellent balance of properties (Table 1) which make them suitable for a variety of applications which require the combination of fuel and oil resistance with a wide service temperature range ( ). Initial development work focused on demanding applications in aerospace, military, petrochemical and gas pipeline areas. The phosphazene fluoroelastomers are currently being used or evaluated in both military and commercial applications, and their continuing demand in these areas is now established. Some examples are shown in Figure 5 (21). 

Figure A. Chemical structure and properties of phosphazene fluoroelastomer, PNF gum rubber.

Figure 5. Phosphazene fluoroelastomer (PNF) compounds were used to prepare molded rubber parts for diverse applications. Shown here Include 0-rlngs fuel hoses, shock absorption and vibration damping mounts. Photograph courtesy of the Firestone Tire and Rubber Co.

Eypel F. [Ethyl] Polyfiuoroalkoxy-phosphazene ctunpdL fluoroelastomer fiu o-ring, othor sealing applies. 

Applications of poly[(fluoroalkoxy)phosphazenes] in dentistry have also been reported (Dootz et al, 1992 Kawano et al, 1994). The use of a polyphosphazene fluoroelastomer as denture liner for the covering of dental prosthesis has been described. This material is known under the name of Novus (Hygenic Corp. Akron, USA). 

Poly((2-trifluoroethoxy)( 1 -difluoro-2,3,4-hexafluoropentoxy) phosphazene) Phosphonitrilic fluoroelastomers... 

Aside from the commercial fluoroelastomer mentioned above, many applications for polyphosphazenes have been proposed. In particular, recent smdies have looked at modified polyphosphazenes as potential biomedical materials [20,21]. Phosphazenes with hydrophilic side chains in water exhibit a lower critical solution temperature (LCST) (Section 3.3) and their hydrogels have been considered for controlled drug release and other biomedical applications [22-24]. However, no large-scale use has yet emerged. 

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