Intumescent formulations

Stilbert and Cummings (10) describe very ably the status of fire-retardant coatings known to industry at the present time. Dow Latex 744B plasticized with Santicizer B-16 was added to a typical intumescent formulation to produce a decided improvement in the scrub resistance of the coatings, if less than 15% latex was used. 

Ammonium polyphosphates, on the other hand, are relatively water insoluble, nonmelting solids with very high phosphorus contents (up to about 30%). There are several crystalline forms and the commercial products differ in molecular weights, particle sizes, solubilities, and so on. They are also widely used as components of intumescent paints and mastics where they function as the acid catalyst (i.e., by producing phosphoric acid upon decomposition). They are used in paints with pentaerythritol (or with a derivative of pentaerythritol) as the carbonific component and melamine as the spumific compound.22 In addition, the intumescent formulations typically contain resinous binders, pigments, and other fillers. These systems are highly efficient in flame-retarding hydroxy-lated polymers. 

In short, an intumescent formulation has to be optimized in terms of physical (char strength, expansion, viscosity,. ..) and chemical (thermal stability, reactivity) properties in order to form an effective protective char that will be able to protect its host polymer (reaction to fire) or a substrate like steel or wood (resistance to fire).16... 

Typical acid source (dehydrating agent) used in numerous intumescent formulations. 

The use of polyols such as pentaerythritol, mannitol, or sorbitol as classical char formers in intumescent formulations for thermoplastics is associated with migration and water solubility problems. Moreover, these additives are often not compatible with the polymeric matrix and the mechanical properties of the formulations are then very poor. Those problems can be solved (at least partially) by the synthesis of additives that concentrate the three intumescent FR elements in one material, as suggested by the pioneering work of Halpern.29 b-MAP (4) (melamine salt of 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]-undecane-3,9-dioxide) and Melabis (5) (melamine salt of bis(l-oxo-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octan-4-ylmethanol)phosphate) were synthesized from pentaerythritol (2), melamine (3), and phosphoryl trichloride (1) (Figure 6.4). They were found to be more effective to fire retard PP than standard halogen-antimony FR. 

As the polyol-based char formers needs to be substituted, Li and Xu40 reported the synthesis of a novel char former for intumescent system based on triazines and their derivatives. It is a macromo-lecular triazine derivative containing hydroxyethylamino, triazine rings and ethylenediamino groups (Figure 6.7). They showed that the new char former in an intumescent formulation containing APP... 

Within the area of natural fibers, wool has the highest inherent nonflammability. It exhibits a relatively high LOI of about 25 vol % and low flame temperature of about 680°C.25 The inherent FR activity of the fiber can be associated with char-forming reactions which may be enhanced by a number of flame retardants. Based on their fundamental work to enhance char formation, Horrocks and Davies offer intumescent formulations based on MP to flame-retarded wool.61 From TGA and SEM characterization, they proposed a comprehensive model on the mechanism of protection via an intumescent process, which involves the formation of cross-linked char by P-N and P-0 bonds resistant to oxidation. More recently, they used spirocyclic pentaerythritol phosphoryl chloride (SPDPC) phosphorylated wool to achieve intumescent wool which exhibits large char expansion and good flame retardancy.62... 

Note that the time to ignition is significantly shortened (45 s vs. 75 s). In this particular example, we can note that the incorporation of nanoparticles in intumescent formulations seems to act as char reinforcer and char expander. ... 

Industrial furnace tests, according to UL-1709 standard, have been carried out in a 1.5 m3 furnace (Figure 6.20) for further comparison with the heat radiator test. Different intumescent formulations have been examined The first type comprised three basic intumescent (ingredients include APP, PER, and melamine) epoxy resins (IF1, IF2, and IF3) whose performance is compared to a reference commercial intumescent epoxy resin (IF4). 

FIGURE 6.21 (a) Industrial furnace and (b) heat radiator test on five intumescent formulations. 

The quick overview of the mechanisms of action reveals that the formation of an expanded charred insulative layer acting as thermal shield is involved. The mechanism of action is not completely elucidated, especially the role of the synergist. Reaction may take place between the nano-filler and some ingredients of the intumescent formulation (e.g., the phosphate) in order to thermally stabilize the charred structure. Only physical interactions are observed (e.g., action of POSS with phosphinate), and these interactions permit the reinforcement of the char strength and avoid the formation of cracks. The development rate and the quality of this layer are therefore of the primary importance and research work should be focused on this. 

Le Bras, M., Bourbigot, S., Delporte, C., Siat C., and Le Tallec, Y. 1996. New intumescent formulations of fire retardant polypropylene Discussion about the free radicals mechanism of the formation of the carbonaceous protective material during the thermo-oxidative treatment of the additives. Fire Mater. 20 191-203. 

Bourbigot, S., Le Bras, M., Delobel, R., Breant, P., and Tremillon, J.-M. 1996. 4A Zeolite synergistic agent in new flame retardant intumescent formulations of polyethylenic polymers—Study of the constituent monomers. Polym. Deg. Stab. 54 275-283. 

At an optimum addition level of only 1.5 w t %, nano-size magnesium-aluminum LDHs have been shown to enhance char formation and fire-resisting properties in flame-retarding coatings, based on an intumescent formulation of ammonium polyphosphate, pentaerythritol, and melamine.89 The coating material comprised a mixture of acrylate resin, melamine formaldehyde resin, and silicone resin with titanium dioxide and solvent. It was reported that the nano-LDH could catalyze the esterification reaction between ammonium polyphosphate and pentaerythritol greatly increasing carbon content and char cross-link density. 

Similarly, numerous different nanoparticles, including organomodified clays,3 nanoparticles of silica,4 layered double hydroxides (LDH),5 or polyhedral silsesquioxanes (POSS),6 have been combined with intumescent formulations in polymeric materials to create large synergistic effects (see Chapter 12 for more details) the nanoparticles acting as char reinforcer or char expander that result in differences in terms of FR properties. 

In that way, the viscoelastic behavior of a PP-based intumescent formulation including polyurethane (PU) as carbon source in association with APP has been evaluated.30 The use of PU/APP mixture in PP brings a decrease in the peak heat release from 1700 kW/m2, in the case of the virgin matrix, to 300kW/m2, in the case of the intumescent formulation. 

Fire Resistance and Intrinsic Properties of Epoxy-Based Intumescent Formulations... 

FIGURE 10.14 Compression force versus gap of EVA/PA6/APP-based intumescent formulations measured at400°C without (REF) and with nanoparticles (Si02, silica OMMT, organomodified montmorillonite LDH, lamellar double hydroxide). 

Bourbigot, S., LeBras, M., Dabrowski, F., Gilman, J., and Kashiwagi, T. 2000. PA-6 clay nanocomposite hybrid as char forming agent in intumescent formulation. Fire and Materials 24 201-208. 

S. Bourbigot, M. Le Bras, F. Dabrowski, J.W. Gilman, and T. Kashiwagi, PA6 clay nanocomposite hybrid as char forming agent in intumescent formulations, Fire Mater., 2000, 24 201-208. 

Dabrowski, F. Le Bras, M. Cartier, L. Bourbigot, S. The use of clay in an EVA-based intumescent formulation. Comparison with the intumescent formulation using polyamide-6 clay... 

Figure 5.31. Weight difference of intumescent formulation based on LDPE vs. temperature. [Adapted, by permission, from Le Bras M, Bourbigot, Le Tallec Y, Laureyns J., Polym. Degradat. Stabil., 56, 1997, 11-21.]...

Le Bras and co-workers [50] developed flame retardant intumescent formulations using the association of APP as the acid source and PA-6 as the carbonisation agent in an EVA (8%) copolymer matrix. 

A number of minerals have S5mergistic action with organic halogenated and phosphorated flame-retardants and synergistic processes have also been observed in thermosets, between borates and other hydrated minerals. Ineorporation of minerals in intumescent formulations makes it possible to manage better the morphology of the expanded structure that develops on exposure to flame. 

Phosphorus oxynitride (PON) is the name for a family of closely related materials that have been around for over 100 years. They are not well known but are thermally very stable and may be considered as future flame retardants and for intumescent formulations since PON is analogous to ammonium polyphosphate. It can be made from inexpensive starting materials such as ammonium phosphate, melamine phosphate or urea phosphate just by prolonged and intensive heating. 

Pol5mrethane-phosphate combinations ate known to form flame retardant intumescent systems. However, the intumescent formulation caimot be permanent beeause of the water solubility of the phosphate. This problem could be solved by the teehnique of mieroeneapsulation. Microcapsules of diammoniiun hydrogen phosphate (DAHP) with a PU shell are synthesised. Cotton fabrics can be coated with PU formulations that enclose the microeapsules and show a signifieant FR effect. 

Flame retardant intumescent formulations have been developed using charring polymers PA6, thermoplastic polyurethanes (TPUs), and hybrid clay-PA6 nanocomposites as carbonisation agents. The advantage of the eoncept is to obtain FR polymers with improved mechanical properties and to avoid the problem of migration and solubility of the additives. 

Other amide containing formulations can provide useful improvements to the meehanieal as well as fire properties of EVA compounds. These new intumescent formulations use PA6 and a PA6 elay nanocomposite hybrid as carbonisation agents. Work in both Franee and the US has shown that the clay allows the thermal stabilisation of a phosphoro-carbonaceous stmcture in the intumescent ehar which increase the efficiency of the shield and, in addition, the formation of a ceramie that can act as a protective layer. 

Another application of compatibilisers is in intumescent formulations for PP compositions used in vehicles, where better flame retardancy is being sought for various reasons. The intumescent mixtiue sometimes incorporates a polyamide as the carbonisation polymer, together with ammonium polyphosphate (APP) to improve the fire performance. PA and APP have limited compatibility, and EVA can overcome this. Attempts have been made to demonstrate that such mixtmes can count towards the 80% of recyclable vehicle weight demanded by the EU Directive relating to end-of-vehicle life issues. 

Keywords intumescent coating, intumescent formulations, fire retard-ance, fire protection, char formation, heat flux, thermal decomposition, carbonization, pyrolisis, antagonistic effect, UL-94, LOI, spumification. 

Burbigot and co-workers have shown improved performance when replacing part of the ammonium polyphosphate in an intumescent formulation for PP [47]. 

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