Isosorbide as a Platform for the Generation of New Biobased Organophosphorus Flame Retardants

Starch is available, in abundance, annually from a variety of seed grains [1,2]. Starch may be hydrolyzed to provide glucose which, in turn, can be reduced and dehydrated to afford isosorbide, a reactive diol [3,4]. Isosorbide may serve as a base for the preparation of both polymers and polymer additives, particularly plasticizers and flame retardants [5,6]. It may also be used for the generation of therapeutic agents and surfactants. The preparation of biobased phosphorus flame retardants has become increasingly important as threats posed to the environment and human health from the use of traditional organohalogen flame retardants has become widely recognized [7-18].

Isosorbide is a diether diol of cupped structure with hydroxyl groups of different reactivity, one endo to the skeletal structure and the other exo. However, both are sufficiently reactive that bis adducts may be formed readily. This has been exploited for the generation of several classes of biobased flame retardants. Isosorbide may be directly converted to phosphorus esters by treatment with an appropriate phosphoryl chloride or with a phosphite in the presence of carbon tetrachloride (Atherton-Todd procedure) [19]. These esters display outstanding flame retarding characteristics when incorporated, at relatively low levels, into DGEBA epoxy resin [20, 21]. Alternatively, isosorbide may be converted to the bis-acrylate. Michael addition of phosphite affords flame-retarding compounds with the phosphorus moiety attached via a P-C bond. These compounds display superior thermal stability and are suitable for use with polymers that process at high temperatures. Oligomeric flame retardants may also be generated from isosorbide [22]. More novel flame retardants may be produced by first treating isosorbide with 10-undecenoic acid, a biomaterial from castor oil, to generate a bis-ester containing terminal unsaturation. This compound smoothly undergoes thiol-ene reaction with 2-hydroxyethanethiol to provide a derivative with terminal hydroxyl groups. Esterification with multiple phosphorus agents generates a series of compounds with good flame-retarding properties [23,24]. These compounds contain sulfur as sulfide. This may readily be oxidized to sulfoxide or sulfone to permit an evaluation of the oxidation level of sulfur on flame retardancy [25].

Reactive vinyl monomers containing phosphorus may also be generated from isosorbide. Treatment of isosorbide with diphenylchlorophosphate may be used to generate a mixture of isomeric diphenylphosphato alcohols. These alcohols may be separated using column chromatography and treated with acryloyl chloride to provide the corresponding diphenylphosphato acrylates [26]. These acrylates behave as typical vinyl monomers and may be incorporated into vinyl polymers to provide flame retardance. Since these materials are covalently bound into the polymer the possibility of loss due to volatilization or leaching is precluded. The level of comonomer required is low enough that the properties of the copolymer are not markedly different from those of the polymer containing no comonomer. Copolymers of styrene containing exo-5-(diphenylphosphato)isosorbide-5-endo-acrylate at a level to provide materials containing 1 and 2% phosphorus have been prepared (Figure 1).

The presence of the comonomer at this level dramatically reduces the peak heat release rate (PHRR) for combustion of the polymer (Figure 2). The presence of 1% phosphorus brings about a 50% reduction in PHRR compared to that for poly(styrene) containing no comonomer.

Isosorbide represents an excellent platform for the generation of biobased phosphorus flame retardants of both the additive and reactive type. Numerous bis-phosphorus derivatives display excellent flame retardant properties as polymer additives. Alternatively, isosorbide may be converted to phosphatoacrylates which function well as reactive flame retardants for incorporation into vinyl polymers.

Support for this work from Chemtura Corporation (now Lanxess) is gratefully acknowledged. Diphenylchlorophosphate was provided by ICL-IP America; the diglycidal ether of bisphenol A (DGEBA) by the Dow Chemical Company.

Authors declare that there are no conflicts of interest.

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