Phosphorus polyols

Phosphorus polyols are the most important reactive flame retardants. A lot of research was done in the area of phosphorus polyols and after considering their structures, they were divided into the following groups  

Representative phosphorus polyols with ortho-phosphoric esteric structure are the products of a reaction of PO with polyphosphoric acids [4, 5, 30, 31] (reaction 18.8) or the products of PO addition to the condensates of phosphorus pentoxide with w-butanol (or other alcohols) (reaction 18.9) [5]. 

The phosphorus polyol (18.8) has an hydroxyl number of 300-310 mg KOH/g, a phosphorus content of 9.5-10% and a viscosity of 1,600-3,000 mPa-s at 5 °C. Of course due to some etherification reactions the number of PO units/hydroxyl group are higher than one, being in the range of 1-2 PO units. 

The resulting phosphorus diol has an hydroxyl number of 210-215 mg KOH/g and a phosphorus content of 11.2% [5]. 

These phosphorus polyols with ortho-phosphoric ester structure are not used at the moment because, the formulated polyols containing water as blowing agent are not resistant to hydrolysis. Due to hydrolysis, acidic groups appear which decrease markedly the reactivity of the formulated polyol. The resulting acidity inhibits the activity of the tertiary amine used as a catalyst in the foaming process. 

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Unfortunately, trialkyl phosphites are extremely susceptible to hydrolysis, being more rapidly hydrolysed than the ortho-phosphoric esters. Due to their susceptibility to hydrolysis, the phosphorus polyols with phosphite ester structure are not used at the moment. 

The best known phosphorus polyol of significant commercial importance is diethyl-N,N-bis (2-hydroxyethyl) aminomethyl phosphonate, which is obtained by a Mannich reaction between diethylphosphite, formaldehyde and diethanolamine (reaction 18.11) [5, 34] ... 

There are several possibilities for phosphonate polyol synthesis by Mannich reactions, Arbuzov reactions and by alkoxylation of phosphonic acids. Phosphonate polyols proved to be very efbcient flame retardants in practice. An important quality of these phosphorus polyols is the stability over time of formulated polyols containing phosphonate and water as reactive blowing agent, without a significant loss of their reactivity. 

An interesting structure of some phosphorus polyols used as reactive flame retardants is the phosphoramide. 

The same substances mentioned before are used as acid acceptors. The phosphorus polyol shown in 18.28 has an hydroxyl number of 490-495 mg KOH/g, a phosphorus content of 13-13.6% and a nitrogen content of 6.1-6.2%. The acidity of all phosphorus polyols presented in Section 18.2 is a maximum of 2 mg KOH/g. 

Vircol 82 Albright and Wilson Phosphorus polyol — — — Rigid PUR foam... 

Bayer markets a dimethylpropane phosphonate-based FR called Levagard VP SP 51009 to enable rigid PU foam to meet fire regulations. Clariant offers liquid phosphorus polyols for the protection of polyether flexible and slabstock moulded automotive foams. Flexible PU is also commonly flame retarded in Europe with chlorinated phosphate esters. Borates have not shown... 

Phosphorus-Containing Diois and Polyols. The commercial development of several phosphorus-containing diois occurred in response to the need to flame retard rigid urethane foam insulation used in transportation and construction. There are a large number of references to phosphorus polyols (112) but only a few of these have been used commercially. 

Wang, T. L., Cho, Y. L., and Kuo, P. L., Flame-retarding materials II. Synthesis and flame-retarding properties of phosphorus-on-pendent and phosphorus-on-skeleton polyols and the corresponding polyurethanes, J. Appl. Polym. Sci., 2001, 82, 343-357. 

With regard to reactive flame-retardants, two routes can be followed to improve thermal stability and fire behavior of PU foams use of brominated or phosphorus-containing polyol or, for rigid foams, the introduction inside polymer backbone of more thermally stable structure than urethane, mainly isocyanurate, but also uretidione rings or carbodiimide.19... 

Also phosphorus- and nitrogen-containing polyols are shown to be effective in flame retardancy of PU foams24 such as polyols based on phosphonic acid ester or obtained by partial or full substitution of methylol groups of tetrakis(hydroxymethyl)phosphonium chloride with amine several examples of such polyols were reported by Levchik and Weil.15 Rigid PU foam modified with these polyols showed improved oxygen index values moreover better results were achieved with higher functionality polyols. 

Sivriev, C. Zabski, L. Flame retarded rigid polyurethane foams by chemical modification with phosphorus-and nitrogen-containing polyols. Fur. Polym. J. 1994, 30, 509-514. 

Phosphorus-oxygen heterocycles in the stereoselective synthesis of 1,3-polyols 90S635. 

Reactive polyols which contain halogen groups, phosphorus, or both, are offered by a number of suppliers for flame-netardant urethane-foam applications. These materials can be used alone, or with other flame retardants as synergists. Although reactive flame retardants may appear to be more costly initially, in the long run they may be found to be less expensive than the additive types (31). 

The ingredients for polyurethane flexible foam are Freon 11, polyether polyol-triol, polyurethane and TDI. Polyurethane fire-resistant rigid foam is produced from Freon 11, polyether polyol-hexol, polyether polyol-phosphorus, polyisocyanates and polyurethane. MDI foam has advantages over TDI foam, particularly in that it is easier and safer to handle, and for this reason is widely used as a thermal insulation. Flexible foams account for over 60% of the consumption of polyurethanes31,32. 

Boutevin, B., Hervaud, Y., Mouledous, G., and Vera, R., Synthesis of polyols bearing phosphonate groups. Part 2. Epoxides used as starting materials. Phosphorus, Sulfur Silicon Relat. Elem., 161, 9,... 

Very recently Papa and Critchfield El9 7 described the prepara tion of foams based on a resole resin containing a high ortho-methylol content and a blend of quasi prepolymer of tolylene diiso cyanate, a phosphorus-containing polyol and polymeric isocyanate containing a minor portion of tris-(2-chloro-ethyl) phosphate. 

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