Recent Developments in Organophosphorus Flame Retardants Containing P-C Bond and Their Applications

Abstract

Organophosphorus compounds containing P-C bonds are increasingly developed as flame retardant additives due to their excellent thermal and hydrolytic stability and ease of synthesis. The latest development (since 2010) in organophosphorus flame retardants containing P-C bonds summarized in this review. In this review, we have broadly classified such phosphorus compounds based on the carbon unit linked to the phosphorus atom i.e., could be a part of either an aliphatic or an aromatic unit. We have only considered those published literature where a P-C bond was created as a part of synthetic strategy to make either an intermediate or a final organophosphorus compound with an aim to use it as a flame retardant. General synthetic strategies to create P-C bonds are briefly discussed. Most popular synthetic strategies used for developing P-C containing phosphorus based flame retardants include Michael addition, Michaelis-Arbuzov, Friedels-Crafts and Grignard reactions. In general, most flame retardant derivatives discussed in this review have been prepared via a one- to two-step synthetic strategy with relatively high yields greater than 80%. Specific examples of P-C containing flame retardants synthesized via suitable synthetic strategy and their applications on various polymer systems are described in detail. Aliphatic phosphorus compounds being liquids or low melting solids are generally applied in polymers via coatings (cellulose) or are incorporated in the bulk of the polymers (epoxy, polyurethanes) during their polymerization as reactive or non-reactive additives. Substituents on the P atoms and the chemistry of the polymer matrix greatly influence the flame retardant behavior of these compounds (condensed phase vs. the gas phase). Recently, aromatic DOPO based phosphinate flame retardants have been developed with relatively higher thermal stabilities (>250 °C). Such compounds have potential as flame retardants for high temperature processable polymers such as polyesters and polyamides. A vast variety of P-C bond containing efficient flame retardants are being developed; however, further work in terms of their economical synthetic methods, detailed impact on mechanical properties and processability, long term durability and their toxicity and environmental impact is much needed for their potential commercial exploitations.

Keywords: Cone calorimetry; Michael addition; Michaelis-Arbuzov reaction; P-C bond; TGA; UL 94; addition; flame retardant; organophosphorus compounds; substitution; transformation.

Figure 1

Chemical structures of 1a and 1b and their counter ion ⁻OTs.

Figure 2

Chemical structures of 2, 3 and 4.

Figure 3

Chemical structures aliphatic phosphonates 5a and 5b.

Figure 4

Chemical structures of phosphonium derivatives: 6, 6a and 6b.

Figure 5

Chemical structure of 7a and 7b.

Figure 6

Phosphinic acid derivatives 7c and 7d.

Figure 7

Chemical structure of 8.

Figure 8

Cyclic tert-butyl phosphonate 9.

Figure 9

DOPO derivatives of 10a and 10b structures.

Scheme 1

Chemical synthesis of compound 11b.

Scheme 2

Synthetic route of compound 14.

Figure 10

Flame retardant compounds 15, 16 and 17.

Figure 11

Various DOPO derivatives.

Scheme 3

Synthetic route for oligomer 30.

Scheme 4

Schematic route for compound 32b synthesis.

Figure 12

Monomer 33 and its corresponding polymers.

Figure 13

Flame retardant 34a–34d structures.

Figure 14

Chemical structures of 35 and 36.

Figure 15

Phosphine oxide derivative 38 and its precursor 37.

Figure 16

Structure of DOP-Cl 39.

Figure 17

Chemical structure of bisphosphonate 40.

Scheme 5

Oxidation of 41a to 41b.

Figure 18

DOPO analogues.

Scheme 6

Synthetic route for phosphine oxides DPPO (42d) and DPPA (42e).

Figure 19

Chemical structure of compound 43.

Figure 20

Phosphine oxide derivative 44 and phosphonate derivative 45.

Figure 21

Triazine derivatives 46 and 47.

Scheme 7

Synthesis of DOPO based phosphinate derivative 48b from 48a.

Figure 22

DOPO based phosphinate derivatives 49 and 50.

Figure 23

DOPO-BQ (48a) functionalized DGEBA resin 51.

Figure 24

Phosphine oxide derivative 52.

Scheme 8

Synthetic route for DOPO based phosphinate 53b.

Figure 25

Chemical structures of compounds 54 and 55.

Figure 26

Phosphonate derivatives 56a and 56b.

Figure 27

Chemical structures of compounds 56c to 56e.

Figure 28

Phosphonate derivatives of polyphenols 57a and 57b.

Scheme 9

Synthesis of the phosphorus-containing vinyl monomer 58b from 58a.

Figure 29

Chemical structure of compounds 59 and 60.

Scheme 10

Two-step amination of 61a to yield 61b.