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Because this Handbook has a separate chapter on Intumescent Flame Retardants (see Chapter 4 of this book), in this chapter only a short overview of commercial intumescent FRs is given. Numerous academic publications on intumescent flame-retardant systems for polyolefins, elastomers and rubbers are out of the scope of this chapter. The broad subject of intumescent flame retardants was earlier discussed in a book [49], a number of reviews [50, 51] and a book chapter [52]. The mechanism of char formation in the pentaerythritol-ammonium polyphosphate (APP) systems was very extensively studied and described in great detail [53].

Water soluble monoammonium dihydrogen phosphate (MAP) and diammonium hydrogen phosphate (DAP) or short chain ammonium polyphosphate is the oldest class of flame retardants which are used on cellulosic materials such as wood, paper and cotton. A large volume of water-soluble ammonium polyphosphate is used in forest fire control, usually by aerial application, often in combination with ammonium sulfate. MAP and DAP can be applied to cotton or cotton-based blends by soaking, padding or spraying and then drying which will result in a FR finish non-durable for laundering. Non-durable finishes are most often used for disposable goods, for example medical gowns, party costumes, and sometimes wall covering. Ammonium phosphate finishes are resistant to dry-cleaning solvents but not to laundering or to leaching by water. However, some degree of durability of MAP can be achieved by combining with hydrophobic poly(methylhydrogen siloxane) or poly(dimethyl siloxane) [54]. Interestingly, cotton treated with MAP or DAP or water-soluble APP, urea and tauramine oxide and cured for 2 minutes at 170°C achieves durability of up to 15 washes [55].

Depending on fabric weight and density, 1-2% of phosphorus provides self-extinguishing performance and effectively prevents afterglow. Some organic co-additives can be added to the solution [56] to improve textile wetting and inhibit crystallization upon drying in order to avoid the formation of visible crystals of ammonium phosphates. Urea is used as a synergistic co-additive which helps to significantly increase the oxygen index and decrease char length as measured in ASTM D6413 flame test [57] and decrease the heat release rate [58]. Chitosan alone [59] or combined with sodium stannate [60] was also explored as a synergistic nitrogen source in combination with DAP. It was reported [61] that ammonium phosphates phosphorylate cellulose, which changes its mechanism of thermal decomposition suppressing the evolution of levoglucosan, a major fuel source, and increasing charring. It is also believed [61] that stannic oxide being a Lewis acid catalyzes the dehydration reaction of cellulose and increases char.

MAP and DAP combined with boric acid or other inorganic borates or sulfates are commonly used for flame retardant treatment of wood. Numerous formulations with water soluble phosphates for wood treatment can be found in the patent literature [62, 63]. Grexa and Utike [64] studied various inorganic flame retardants and their combination on particleboard using cone calorimeter and concluded that MAP and boric acid is the most efficient combination. In order to provide decay resistance to the wood treated with ammonium phosphates, apart from boric acid other antifungal additives can be applied [65]. Guanyl urea phosphate [66] or urea phosphate [67] in combination with boric acid are other systems for wood treatment which are somewhat less leachable than ammonium phosphate-based systems. Even more durable flame retardants applicable for outdoor use are achieved by the use of dicyandiamine and urea formaldehyde pre-condensates together with phosphoric acid and further polymerized after treatment [68]. DAP in combination with boric acid can be applied to paper which needs further treatment of the phenol-formaldehyde resole resin to preserve paper integrity [69].

Diguanidine hydrogen phosphate or monoguanidine dihydrogen phosphates are also used for non-durable cotton treatment [70]. These salts are particularly synergistic with 3-aminopropylethioxysilane [70]. Hexaammonium (nitrilotris(methylene))trisphosphonate (Formula 2.1) is another water-soluble ammonium salt which is used in nondurable automotive and aircraft upholstery to minimize the effect on “hand” (texture).

(2.1)

When MAP and DAP are heated under ammonia pressure and preferably in the presence of urea, relatively water-insoluble ammonium polyphosphate is produced [71]. At least five crystalline forms of APP have been reported in the literature [72] but only forms I, and II are commercially sold as flame retardants. It is believed that form I has a linear chain structure, relatively low molecular weight (from 30 to about 150 repeating units), relatively low thermal stability (onset of weight loss at about 240°C) and relatively high water solubility. It is mostly used in coatings. Form II is higher molecular weight (700-1000 repeating units), probably cross-linked [73], with onset of weight loss at ~270°C, and much more water-resistant. Form II is used in coatings, thermoset resins, PU foams, textile backcoatings and thermoplastics. APP is the principal component of many intumescent FR systems.

Finely divided ammonium polyphosphate is the major flame retardant for intumescent paints and mastics [74]. When the intumescent coating is exposed to a high temperature, APP yields polyphosphoric acid that then interacts with an organic component such as a pentaerythritol to form a carbonaceous char. This chemistry has been described in detail by Camino and Costa [75] and is covered in detail in this book in Chapter 4. A blowing (gas-generating) agent, typically melamine, is also added to foam the char, thus forming a fire-resistant insulating barrier to protect the substrate. In addition, the intumescent formulations typically contain resinous binders, pigments, plasticizers, and other additives. Mastics are related but more viscous formulations, intended to be applied in thick layers to girders, trusses, and decking for structural fire protection; these generally contain mineral fibers to increase coherence.

The intumescent systems concept originally developed for flame retardant coatings [76] was later adapted for thermoset low temperature processed polyolefins, elastomers and rubbers. The decomposing polymer can produce enough gaseous product for foaming and therefore use of melamine is unnecessary, but a charring agent is still needed. Although intumescent systems based on APP were intensively studied the main factors limiting their broad application are thermal stability and water solubility. Both thermal stability and water solubility can be improved by increasing the chain length (molecular weight) of the polyphosphate. Many varieties of APP form II with various coatings/encapsulations like a silane surface-reacted, melamine surface-reacted, melamine formaldehyde resin coated which further decrease the water solubility are commercially available [77]. These surface treatments allow decreasing the water solubility of form II from 0.5 to 0.01-0.1 g/liter. These coatings can also provide a synergistic effect to APP because they can work as charring agents to further enhance the activity of APP. Although cable manufacturers are trying to adopt APP or APP formulated systems in the cable jacketing [78] it seems to still have a very limited application due to water absorption issues. Silicone surface-treated ammonium polyphosphate in combination with pentaerythritol and methylmethoxysiloxane made by reacting trichloromethylsilane with methanol and water provide high LOI = 34 and UL-94 V-0 ratings in thermoplastic polyurethane (TPU) [79].

Over many years, APP producers and compounders tried to develop flame retardant compositions (formulated packages) which included along with APP the charring and foaming agents. Nitrogen-containing low molecular weight or polymeric products behave the best because they combined both charring and foaming functions [80]. For example, patents suggest that some commercial systems may contain tris(hydroxyethyl)isocyanurate [81], or poly(triazinyl piperazine) [82]. In more recent developments the condensation product of melamine, morpholine and piperazine was suggested as a charring agent and synergist with APP [83]. Reportedly, APP combined with such a condensate provides a V-0 rating in polypropylene at only 20 wt. % loading [84]. Interestingly, aliphatic polyamides which are considered not charrable polymers can also be used along with APP as charring agents. For example, a group of French researchers [85] developed a formulated system of APP/polyamide 6/poly(ethylene-co-vinyl acetate) (EVA), where EVA is used as a compatibilizer for polyethylenic polymers.

Alumina trihydrate (ATH) suppresses the intumescent performance of APP in polyolefins, elastomers and rubbers and therefore these two FRs are almost never used together in these polymers, but it seems not to be the case with unsaturated polyesters (UPE) [86]. In order to decrease the loading of ATH it can be partially or completely replaced with more efficient APP. For example, 15-25 parts APP and 50 parts ATH (per 100 parts resin) will provide a UL-94 V-0 rating in UPE [87]. One academic publication [88] shows that silane treated APP at 35 wt. % loading results in a decrease of the heat release rate by 70%, but more importantly total smoke released decreases by 50% as measured by a cone calorimeter test. Another publication suggests [89] use of diatomite/APP encapsulated in triphenyl phosphate as an effective flame retardant for UPE providing a V-0 rating at 20 wt. % loading. It was also shown that a combination of APP with expandable graphite is beneficial and probably shows a synergistic effect [90].

Although the polyurethane foam (PUF) industry prefers dealing with liquid FRs similar to other foam components, sometimes the use of solid FRs is a more economical way of achieving high flammability standards even if it requires the installation of special equipment. The fire-retardant effect of APP is more pronounced in dense foams, rather than in light foams [91]. For example, use of finely divided APP combined with ATH and a cyclic phosphonate allows achievement of class I in E-84 tunnel test [92] and a combination of APP, ATH and zinc borate allows passing the UL 790 roof assembly [93] test of spray foam roofing. Very efficient combinations of APP with expandable graphite were reported in the literature [94–96]. Guanidinium phosphate can be used instead of APP again in combination with expandable graphite or red phosphorus [97]. Use of lignin-modified polyol along with phenol-formaldehyde coated APP allows a significant improvement in charring of rigid PUF [98]. Usually, triethyl phosphate or some other low viscosity liquid FR is used in combination with solids to improve processability. Interestingly, one recent publication reports on the use of soluble APP in semi-rigid water blown foam [99].

It has been long recognized [100] that addition of a small amount (typically 2-3 wt. %) of multivalent metal salts or oxides provides a synergistic effect in APP based intumescent systems in polyolefins. Zinc borate [101] or some natural products like talc, zeolites [102] and clays show similar behavior. The synergistic effect is observed in a very narrow concentration range and it is believed to be due to the formation of cross-links in polyphosphoric acid involving multivalent metals [103]. Increasing the concentration of the synergist results in the formation of stochiometric crystalline phosphates which negatively affect intumescence and the effect switches from synergistic to antagonistic. In the academic literature there are numerous publications on the benefit of the addition of organically modified clays to the intumescent systems. Synergistic effects are often, perhaps erroneously, attributed to the physical effect of the clay reinforcing char, whereas it could be the same effect of chemical interaction with polyphosphoric acid and cross-linking.

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.

(2.2)

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].

(2.3)