Bob A. Howell

The availability of polymeric materials has dramatically impacted societal development. However, in the main, these materials are highly flammable and for most applications must be flame‐retarded. This usually requires the presence of an additive to, by one means or another, reduce flammability to acceptable limits. While many substances have been used as flame retardants, the most effective have been organohalogen, particularly brominated aromatics, and organophosphorus compounds. With recognition of the hazards associated with the use of organohalogen compounds, organophosphorus flame retardants have become prominent. Both the toxicity and effectiveness of these materials are strongly dependent on structure. In particular, both seem to be largely dictated by the level of oxidation at phosphorus. Compounds with a high oxidation level at phosphorus (e.g., phosphates) undergo decomposition at relatively low temperature in a thermally degrading polymer matrix to form acidic substances which promote cationic crosslinking and char formation at the surface of the degrading polymer. Those with a low oxidation level at phosphorus (e.g., phosphonates) decompose at higher temperatures to generate PO radicals. Some of these escape reaction with components of the degrading polymer matrix and are evolved to the gas phase. There they efficiently scavenge combustion propagating radicals.

Polymeric materials have been a great boon to the development and wellbeing of mankind. However, in the main, these materials are flammable and must be flame retarded for most applications. Many substances have been utilized to impart a measure of flame retardancy. The most widely used and most effective have been organic: organohalogen and organophosphorus compounds. Organohalogen compounds have been popular, low-cost, very effective flame retardants for polymeric materials. However, with the recognition that these compounds readily migrate from a polymer matrix into which they have been incorporated, persist in the environment and pose serious risks to human health, the use of organophosphorus compounds has become prominent. In particular, organophosphorus compounds of appropriate structure derived from readily-available, renewable, low-cost, non-toxic biobased precursors are attractive. Avoidance of the issues of environmental persistence and toxicity associated with organohalogen compounds is possible with these materials. Migration from a polymer matrix may be removed as a deficiency through the use of reactive compounds, i.e., compounds that may be incorporated directly into the polymer structure either by copolymerization or grafting, or oligomeric compounds. Oligomeric materials of branched structure display characteristics of broad compatibility, high effectiveness and lack of migration.

The rapidly increasing development of new organophosphorus flame retardants presents a need for toxicity assessment.

The deficiencies of popular phthalate plasticizers (ready migration from a polymer matrix into which they have been incorporated, flammability, environmental pollution, human health risks) have stimulated efforts to develop new effective, nonmigrating, low-cost, nontoxic replacements. In the main, these have been based on readily-available, nontoxic biobased precursors. Some, including those prepared from plant oils, have been generated from biomaterials themselves. However, the more numerous and generally more effective have been generated from discrete compounds produced from various biomaterials. Several structural features of effective plasticizers have been recognized. Polar functionality is required to assure compatibility with a wide range of polymeric materials, including poly(vinyl chloride) (PVC), the most heavily plasticized polymer. A branched structure greatly enhances the effectiveness of compounds used as plasticizers. An oligomeric structure may strongly limit or prevent migration from a polymer matrix. Hyperbranched oligomers of defined structure derived from the readily-available, inexpensive, nontoxic biomonomers, glycerol and adipic acid contain all these features and are excellent plasticizers. They contain ester functionality, are highly branched, and display a large number of end groups, all of which contribute to their effectiveness as plasticizers.

With the recognition that traditional phthalate plasticizers readily migrate from a polymer matrix into which they have been incorporated, have become widespread environmental pollutants and pose risks to human health, the development of new, effective, nontoxic, nonmigrating plasticizers has gained urgency. A focus has been the generation of plasticizers from renewable, inexpensive, nontoxic biobased precursors. Many small molecule plasticizers have been prepared from readily-available bioprecursors. However, the most promising are branched oligomeric materials. Fully compatible oligomeric plasticizers do not migrate from a polymer matrix. Highly branched materials are effective in increasing free volume and display good plasticizing impact. Using new technology that permits the generation of hyperbranched poly(ester) without gelation and with control of molecular weight and endgroup identity, oligomeric materials have been prepared from the nontoxic biomonomers, glycerol and adipic acid. The monomers are readily available at modest cost. The oligomers may be obtained in a simple one-step process and function as very effective plasticizers for polymeric materials.

As the development and use of organophosphorus flame retardants have become more prevalent, concerns about potential toxicity are being raised. To date, most studies on potential toxicity have focused on simple phosphate esters and often include both alkyl and aryl esters or, in some cases, even halogenated phosphate esters (which should more properly be included among organohalogen flame retardants). The inclusion of several kinds of esters into a single study leads to muddled conclusions. Each class of phosphate esters should be subjected to toxicity assessment separately. Furthermore, the potential toxicity of the more effective (lower level of oxygenation at phosphorus: phosphonates, phosphinates, phosphine oxides) organophosphorus flame retardants has been little explored. The few results available suggest that these compounds exhibit no or minimal toxicity (lower than that of phosphates?). This represents an area that should be the focus of careful detailed studies to establish any potential toxicity.

The development of new organophosphorus flame retardants for polymeric materials is spurred by relatively low toxicity, effectiveness, and demand for replacement of more traditional materials. To function, these compounds must decompose in a degrading polymer matrix to form species which promote modification of the solid phase or generate active radical moieties that escape to the gas phase and interrupt combustion propagating reactions. An understanding of the decomposition process for these compounds may provide insight into the nature of flame retardant action which they may offer and suggest parameters for the synthesis of effective new organophosphorus flame retardants. The thermal degradation of a series of organophosphorus esters varying in the level of oxygenation at phosphorus—alkyl phosphate, aryl phosphate, phosphonate, phosphinate—has been examined. Initial degradation in all cases corresponds to elimination of a phosphorus acid. However, the facility with which this occurs is strongly dependent on the level of oxygenation at phosphorus. For alkyl phosphates elimination occurs rapidly at relatively low temperature. The same process occurs at somewhat higher temperature for aryl phosphates. Elimination of a phosphorus acid from phosphonate or phosphinate occurs more slowly and at much higher temperature. Further, the acids formed from elimination rapidly degrade further to evolve volatile species.