Backcoating is a very common and cost efficient method of flame retarding cotton or synthetic textiles or their blends. The phosphorus-based backcoatings are more limited to cellulosics because their efficiency relies mostly on charring. The durability of backcoating in laundering depends on the binder and the hydrolytic stability of the flame retardant. Horrocks et al. [104] studied a wide range of phosphate salts and some phosphate esters and concluded that ammonium polyphosphate is the most efficient FR for cotton and cotton polyester blends because APP decomposes to polyphosphoric acid and involves cotton in charring [105–107]. In textile backcoatings coated ammonium polyphosphate are more preferred over untreated APP because of better water resistance. There are numerous patents [108] on the use of APP in cellulose based barrier fabric for mattresses in the USA which need to pass the severe Consumer Product Safety Commission (CPSC) 1633 open flame test.
Melamine phosphate also has been originally developed for intumescent coatings but found some use in polyolefins. Melamine phosphate is converted to the pyrophosphate and further to the polyphosphate with the loss of water on heating. The pyrophosphate is reported to be only soluble in water to the extent of 0. 09 g/liter water, whereas melamine orthophosphate is soluble to 0.35 g/liter. More thermally stable melamine pyrophosphate and melamine polyphosphate ensured safe processing even in polyamides and polyesters. Different applications of melamine phosphate and pyrophosphate were reviewed by Weil and McSwigan [109, 110]. A detailed study of the thermal decomposition of melamine phosphates has been published [111].
In intumescent thermoplastic formulations, melamine phosphates have been shown to have an advantage over ammonium polyphosphate by causing less mold deposition and having better water resistance [112]. Further encapsulation of a melamine polyphosphate/pentaerythritol system with thermoplastic polyurethane improves compatibility, water resistance and flame-retardant performance in polyethylene [113]. Melamine phosphates are typically less efficient than APP, because they are more thermally stable and have a lower phosphorus content. However interestingly, melamine pyrophosphate combined with a triazine based charring agent made from cyanuric chloride, ethanolamine, and diethylenetriamine provides a V-0 rating in polypropylene at only 25 wt. % FR loading [114, 115]. Encapsulation of melamine polyphosphate with 4,4’-oxydianiline-formaldehyde also helps to boost the oxygen index of polyurethane composites [116].
A further improvement of the thermal stability of melamine polyphosphate was done by the partial replacement of some melamine groups with Al, Zn or Mg [117, 118]. These show enhanced performance because of increased fire residues, notably in polyamides and epoxies [119]. The same group of inventors [120] also synthesized melamine mixed trimethylene amine phosphonate salts (Formula 2.2), but their commercial status is unknown.
The ethylenediamine salt of phosphoric acid (1:1) (EDAP) having a phosphorus content of 63 wt. % was introduced to the market in the early 90s [121, 122]. In contrast to APP and melamine salts, EDAP shows a self-intumescent behavior because it melts at about 250°C, right around where its thermal decomposition starts and because it contains aliphatic carbons which undergo charring. EDAP is very efficient because it quickly activates as an intumescent FR once it reaches this temperature. EDAP is more soluble in water compared to the form II of APP and is less thermally stable which limits its applications to polyolefins. A flame retardant made from diethylene triamine and polyphosphoric acid by heating at 200°C, has a higher thermal stability with the beginning of decomposition at about 300°C [123].
In order to improve thermal stability and decrease water solubility, EDAP has often been sold as a mixture with melamine or melamine phosphates. Some of these mixtures are also synergistic because the temperature of thermal decomposition of EDAP and melamine phosphates are different, and the extended temperature interval better matches the thermal decomposition of the host polymer. Some synergists, such as phase transfer catalysts (quaternary ammonium salts) or spirobisamines may further enhance the action of EDAP and melamine pyrophosphate or APP combinations [124, 125]. Coating of EDAP with amine cured epoxy allows for a decrease in water sensitivity [126].
Intumescent systems based on the mixed salts of melamine and piperazine phosphates were first developed in Italy [127] and marketed for wire and cable applications [128]. Later, an improved method of synthesis of polymeric piperazine pyrophosphate (Formula 2.3), which results in a product with superior thermal stability [129] was developed in Japan. It is more resistant to water than coated ammonium polyphosphate. Another patent [130] shows the milling of piperazine pyrophosphate together with melamine pyrophosphate and the addition of some polymethylsiloxane oil for decreasing dusting and improving processability. This intumescent flame retardant is said to be effective in polypropylene at about 25 wt. %, and in low density polyethylene (LDPE), high density polyethylene (HDPE) or EVA at about 30 wt. % [131]. It is stable enough to permit extrusion and molding at 220-240°C and it is effective in cable jackets [132]. Recently many patents were filed on TPU formulations for cable application based on piperazine pyrophosphate and melamine pyrophosphates combined with bisphosphates [133] and stabilized with epoxidized novolac [134]. Addition of a small amount of silica improves dispersion and boosts flame retardant performance [135]. With a stabilizing amount of hydrotalcite or zinc oxide [136] or calcium glyceorate [137] or zinc cyanurate or calcium cyanurate [138] or boehmite [139] it is a particularly effective intumescent flame retardant for unreinforced and glass-reinforced polypropylene [140]. A recent publication showed synergism of piperazine pyrophosphate and aluminum hypophosphite in glass-filled polyamide 6 [141]. A mixed salt of piperazine and aluminum diphosphate was also found to be efficient in polypropylene [142].
2.5 Metal Hypophosphites, Phosphites and Dialkyl Phosphinates
The requirements for flame retardants in polyesters and polyamides are stringent because of high processing temperatures and sensitivity to hydrolytic degradation catalyzed by possible acids or catalytic decomposition assisted by some metals. Since the most common use of flame retardant polyesters and polyamides is in connectors, there is a requirement for long-term dimensional stability, which means minimal water absorption which is especially difficult to maintain with polyamides. Because polyesters and polyamides are semicrystalline and a flame retardant can be accommodated only in the amorphous regions, there is an issue of exudation (“blooming”) of low molecular weight flame retardants. For many years brominated flame retardants dominated in this market sector and many phosphorus-based flame retardants were not considered for polyesters and polyamides.
However, some time ago it was discovered that calcium hypophosphite (Ca-Hypo, Ca(HPO2)2) combined with melamine cyanurate provides V-0 in glass-filled poly(butylene terephthalate) (PBT) at 25 wt. % of total loading [143]. Ca-Hypo was also found efficient in the polycarbonate (PC) blends PC/ABS [144] and PC/poly(butylene terephthalate) (PBT) [145]. Later it was found that aluminum hypophosphite (Al-Hypo, Al(HPO2)3) alone or in combination with melamine cyanurate [146, 147] or melamine polyphosphate [148] is a more effective flame retardant because it requires only a 15-20 wt. % total loading for achieving V-0 rating in glass filled PBT. Al-Hypo starts to decompose (disproportionate) at about 300°C with the evolution of phosphine