Phosphorus-based condensed-phase

The mode of action of phosphorus-based flame retardants is believed to take place in either the condensed or the vapor phase (refs. 1,2) depending on the type of phosphorus compound and the chemical composition of the polymer. Phosphorus has been reported to be 3 to 8 times more effective than bromine depending on the polymer type (ref. 3). 

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

Viscose or rayon A well-known inherently FR viscose fiber is Viscose FR, marketed by Lenzing. The fiber is produced by adding Sandoz 5060 (Clariant 5060)-bis(2-thio-5,5-dimethyl-l,3,2-dioxa-phosphorinyl)oxide in the spinning dope before extrusion. As this additive is phosphorus based, it is similar to other phosphorus-based FRs in terms of mode of action (condensed phase). 

Condensed phase. In condensed-phase modification, the flame retardant alters the decomposition chemistry so that the transformation of the polymer to a char residue is favored. This result could be achieved with additives that catalyze char rather than flammable product formation or by designing polymer structures that favor char formation. Carbonization, which occurs at the cost of flammable product formation, also shields the residual substrate by interfering with the access of heat and oxygen. Phosphorus-based additives are typical examples of flame retardants that could act by a condensed phase mechanism. 

The ideal approach to reducing volatile yield is to use an inherently stable matrix polymer and reference has already been made to the chemical structures that confer maximum thermal stability. Alternatively, there are additives which function in the condensed phase and modify the degradation pathway. Phosphorus-based additives, usually working through a phosphoric acid intermediate, are popular additives, particularly for polyurethanes. They act predominantly in the condensed phase, increasing char yield and reducing volatile production. 

Chemically acting flame retardants work best (bromo- and chloro-organic systems in the gas phase, phosphorus and nitrogen-containing systems in the condensed phase). Physically acting inorganic flame retardants based on metal hydroxides and salts have a weaker elFect. 

Fyrolflex BDP from Akzo Nobel Chemicals has been shown to exhibit higher thermal properties and hydrolytic stability than other aryl phosphates, and to provide similar or better fire retardant performance than RDP. It is a bisphenol A bis(diphenyl phosphate) compound that provides good physical properties in formulations based on PC/ABS, HIPS and polyphenylene oxide/HIPS blends. Upon thermal decomposition of the flame-retarded polymers, phosphorus tends to accumulate in the solid residue, a result which indicates that the primary FR action of BDP is most likely to be in the condensed phase. 

Flame retardancy of m-PPO is improved by incorporating thermally stable phosphorus based additives, such as red phosphorus and organic phosphoms compounds like triphenyl phosphate, triphenyl phosphine and triphenyl phosphine oxide. The mechanism would appear to be gas phase activity rather than reactions in the condensed phase for all except red phosphoms where both are seen. 

Our results indicate that the high flame resistance for the graphite cloth laminates based on resin E is due to both the gas-phase and condensed-phase reactions of phosphorus. 

Unlike halogenated additives silica gel /K2CO3 does not significantly affect the specific heat of combustion when added to PP [227]. Furthermore, since the CO yield and the soot (mean specific extinction area) are not significantly increased by these additives, the use of this system may represent an improvement over halogenated and some phosphorus based additives, which commonly tend to increase CO yield and soot [227]. These data indicate that these additives most likely act primarily in the condensed phase not in the gas phase. The cone calorimeter results for PP are summarized in Table 5.5 along with data for the other polymers examined [227]. 

If conditions are right, phosphorus-based molecules can voIatiHze and are oxidized, producing active radicals in the flame. On the other hand, phosphorus flame retardants tend to react with the polymer or to oxidize to phosphoric acid in the condensed phase. This favors mostly condensed-phase mechanisms. It is challenging to design a phosphorus-based flame retardant that will volatilize into the flame at relatively low temperatures but wiU not be lost dnring polymer processing. 

Upon heating, melamine-based salts dissociate, and re-formed melamine volatilizes in a manner similar to pure melamine. However, in the case of melamine salts, a larger portion of melamine undergoes progressive condensation than does pure melamine therefore, the condensed-phase contribution of the salts is larger. If the anion contains phosphorus, the phosphoric acid released will phos-phorylate many polymers and produce a flame retardant effect similar to that of other typical phosphorus-based additives (see above). Melamine condensates and phosphoric acid react further at temperatures above 600° C, where triazine rings are opened and cross-linked. A (PON) type of structure known as phosphorus oxynitride is formed. Phosphorus oxynitride is very thermally stable and in some polymers can contribute to condensed-phase mechanisms. ... 

Solid (Condensed) Phase Sohd phase fire retardant(s) alter the physical burning characteristics by either forming an insulative fire barrier or by changing the surface morphology to interfere with the release or generation of combustible gases. This mechanism is commonly seen with phosphorus-based flame retardants. 

First of all, it is important to evaluate how the phosphorus is allocated between the condensed and vapor phases during the thermal degradation of the polymer, even if the volatilization of phosphorus does not show that it acts in the vapor phase. Vahabi et al. have proposed an original approach to study the efficiency of phosphorus in both condensed and vapor phases. Two indices (called respectively charring efficiency and efficiency in the vapor phase) were calculated to evaluate the influence of phosphorus to improve the charring and to decrease the EHC (measured from TGA and PCFC). These calculations are based on the measurement by elemental... 

---