This document is the accepted manuscript version of the following article: Nazir, R., & Gaan, S. (2019). Recent developments in P(O/S)–N containing flame retardants. Journal of Applied Polymer Science, 47910 (27 pp.). https://doi.org/10.1002/app.47910 Recent developments in P(O/S)-N containing flame retardants Rashid Nazir, Sabyasachi Gaan Additives and Chemistry Group, Advanced Fibers, Empa Swiss Federal Laboratories for Materials Science and Technology, St Gallen, Switzerland Abstract Synthesis of organophosphorus compounds containing nitrogen as heteroatom and its application as a flame retardant have attracted much attention in the academic and industrial communities over the past decade. Such compounds are relatively easy to synthesize and offer advantages such as high thermal stability, which is use- ful for high temperature processing, improved char stability, density and yield during the thermal decomposition of polymer, and release phosphorus species active in the flame inhibition process. Though a variety of P-N compounds can be found in the lit- erature, this review mostly summarizes the recent (since 2013) development in phosphorus (O)-nitrogen containing flame-retardants which have been published in peer-reviewed journals. General strategies of synthesizing P(O)-N compounds as flame retardants from various phosphorus-based starting materials are highlighted in this review. Some of the most common classes of researched P(O)-N containing compounds as flame retardants include the phosphinamides, phosphonamides, phosphoramides, phosphoramidates, phosphorodiamidates, phosphonamidatesand their thio counterparts which are usually obtained via a one or two-step synthetic strategy. Incorporation of these compounds as flame retardants in various polymer systems such as polyurethane, epoxy resins, polyamides, cellulose, polylactide, polybutylene terephthalate, polycarbonates, and acrylonitrile-butadiene-styrene are discussed in detail in this review. Special emphasis on the various fire and thermal performances of the new materials are also summarized. The mechanical perfor- mance of new materials and the influence of these additives on polymer processing are also briefly discussed. 1 Corresponding Address: Dr. Sabyasachi Gaan Group Leader (Additives and Chemistry) Advanced Fibers, Empa Swiss Federal Laboratories for Materials Science and Technology Lerchenfeldstrasse 5 9014 St. Gallen Switzerland Tel +41 58 765 611 Fax +41 58 765 862 [email protected] www.empa.ch 1. Introduction In a modern society, polymeric materials are increasingly used in diverse sectors such as construction, transportation, electronics, packaging, industrial machinery, aerospace, etc. Increased use of organic polymers in such materials heighten the risk of fire, as most of them ignite easily. Thus the development of effective flame retard- ant (FR) strategies, which guarantee the public health and safety, motivates academ- ia and industry. In addition, the development of such technologies helps lower the danger of fire accidents and loss of property. Some flame retardants (FRs) based on halogens which were extensively used in the past but have been proven to be toxic and thus banned from use today.1 New phos- phorus based FRs have been considered as an alternative to halogenated FRs, and their usage in various applications have been explored extensively.2-4 In addition, the development of sustainable halogenated FRs has also risen dramatically.1 The ver- satile nature of phosphorus, i.e., high efficacy (low loadings in polymers and coat- ings), chemical diversity and ability to exhibit multiple mode of action, makes it a key element for a new FR development. The mode of action of a FR additive is crucial when developing the right formulations of polymer materials to meet specific fire tests. Mode of action of the FR strongly de- pends on its relationship with the polymer matrix. Generally, the mode of action of FRs are classified into two phases, i.e., the condensed phase and the gas phase, 2 with phosphorus based flame retardants (P-FRs) exhibiting both mechanisms.5, 6 In the condensed phase, P-FRs facilitates the formation of char via crosslinking, facili- tating cyclization, aromatization or graphitization by dehydration of the polymeric ma- terial. Additionally, some P-FRs act via intumescence, where the residue acts as a protecting sheet, reduce the heat transfer speed to the underlying polymeric materi- al.7-9 It is also claimed that for some P-FRs, gas-phase flame inhibition mechanism and condensed-phase mode of action occur in parallel during the thermal decompo- sition of the material.10 All compounds discussed in this review are either incorporated into the polymer matrix as complete polymer chains or as a part of the copolymer chain as a co- monomer. Their incorporation in the material can also be done via surface modifica- tion or blending or by mixing of unreactive additives. Thermosets contain functional groups such as alcohols, amines, epoxy, halogens, etc. which allow for the incorporation of reactive additives during curing.3 Usually reactive FRs have an advantage since they are covalently bonded to the polymer matrix and therefore have a minor effect on the physical properties of the material. In contrast, addition of nonreactive FR additives results in a decrease in the glass transition temperature and the possibility of leaching is obvious. However, non-reactive additives dominate the market because of their lower price and easy processability. In addition, reactive FRs requires careful formulation of various ingredients, whereas non-reactive FR ad- ditives could be used in various polymer matrices without much modification.2, 11 P-FRs containing heteroatoms, for instance, silicon, nitrogen, boron, and sulfur offer a vast choice of specific beneficial interactions compared to the phosphorus com- pounds made only of pure hydrocarbons.12 Such interactions lead to the reduction of the load of FRs in the material and improved FR efficacy. 13-18 More recently DOPO based phosphonamidate (EDA-DOPO) has been shown to be non-toxic and success- fully up-scaled in the industry.19, 20 The use of phosphorus-nitrogen (P-N) compounds as FRs are extensively studied, and their FR effect in many cases is attributed to P-N synergism. It is believed that P-N synergism encourages the cross-linking of polymer- ic chains during the thermal decomposition process. It boosts the retention of phos- phorus in the condensed phase and produces thermally stable char. Additionally, the density of the residue formed at elevated temperatures increases and the matrix is more likely to be retained during the combustion process, consequently contributing 21 to condensed-phase action and giving higher char yields. Cone calorimetry and 3 thermal experiments of polyurethane foams containing P(O)-N compounds have indicated their possible gas phase flame inhibition effect.22, 23 Recently, gas phase species such as phosphorus nitride (PN) and other PN species have been detected during the thermal decomposition of dimethyl phosphoramidate. The flame inhibition effect of such species is unknown.24 2. Nomenclature of P-N containing phosphorus compounds A wide variety of P-N containing organophosphorus compounds are known in the lit- erature. The general structure of various P-N bond containing trivalent and pentava- lent organophosphorus compounds are summarized in Figure 1. Trivalent phosphorus R P NR R N P NR R P NR2 R P NR2 R2N P NR2 R NR2 NR2 phosphine(amin) phosphine(diamin) phosphinetriamin RO P NR RO P NR2 RO P NR2 R P NR2 OR NR2 OR phosphoramidite phosphorodiamidite phosphonamidite Pentavalent phosphorus O O O O R P NR2 R P NR2 R2N P NR2 RO P NR2 R NR2 NR2 OR phosphinamide phosphonamide phosphoramide phosphoramidates O O S S RO P NR R P NR RO P NR2 R P NR2 RO P NR2 R P NR2 NR2 OR OR OR phosphorodiamidate phosphonamidate phosphoramidothioates phosphonamidothioic acid S S S S R N P NR R2N P NR2 R P NR2 R2N P NR2 R2N P NR2 OR R R NR2 phosphorodiamidothioic acid phosphinothioic amide phosphonothioic diamide phosphorothioic triamide Figure 1: Nomenclature of PN containing organophosphorus compounds 4 3. Synthesis of P(O)-N compounds 3.1 General Synthesis Methods A straightforward and common way to synthesize P(O)N and P-N compounds is by nucleophilic substitution reactions of P(O)-Cl and P-Cl compounds respectively.25, 26 Thiophosphorylated aminoheterocycles have been synthesized using this methodol- ogy.27 The P(O)-N containing compounds are also commonly synthesized from the P-H (phosphonates and phosphonates) using carbon tetrachloride(CCl4) and an amine via an Atherton-Todd reaction where it is believed that a P-Cl bond is formed 28 insitu as an intermediate. However, regardless of the easy handling of this reaction, it has found no commercial successes as CCl4 is a carcinogen and depletes the ozone.29, 30 More recently one pot two-step approach has been employed to convert P-H bonds to P(O)-N bonds. In these cases, P(O)-H bond has been transformed to P-Cl via the use of several chlorinating agents such as tert-butyl hypochlorite (t- 31 32-34 35 BuOCl), chlorine gas (Cl2), Copper(II) chloride (CuCl2), sulfuryl chloride 36 22, 37 23, (SO2Cl2), n-chlorosuccinimide (NCS), and trichloroisocyanuric acid (TCCA). 36, 38-40 and P(O)-Br via N-bromosuccinimide (NBS).41 Furthermore, P(O)-Cl bond formation has also been investigated via Michaelis-Arbuzov rearrangement of trialkyl 42 phosphite with Cl2 . Moreover, synthesis of P (O)-N bonds can also be achieved via 43, 44 45, 46 an oxidative coupling method using iodine (I2) or copper (Cu) as catalysts. However, upscaling of these synthetic procedures are challenging because of their exothermic nature, expensive raw material as well as side products and moderate reactions yields. As a possible alternative, the Staudinger-phosphite reaction has al- so been proposed to achieve P(O)-N bond.47-50 This synthesis procedure involves the reaction of alkyl azides with trialkyl phosphite to produce phosphorimidates, followed by rearrangement to their corresponding phosphoramidates.