Intumescence formation

Melamine and melamine salts (e.g., borates, phosphates) act in a similar manner promoting char formation and intumescence. 

Melamine and its salts are widely used in formulations of fire retardant additives, particularly of the intumescent type (4-71. The role played by melamine structures in these additives is however not yet understood. The thermal behaviour is of paramount importance in studies of the fire retardance mechanism. It is known that melamine undergoes progressive condensation on heating with elimination of ammonia and formation of polymeric products named "melam", "melem", "melon" (8.91. The following schematic reaction is reported in the literature (10-121 ... 

Linseed Mucilage is extracted from the seeds of the flax-plant, Limim Usitatissimum, by the agency of hot water. Alcohol causes the formation in it of white mucilaginous flocks. It is also precipitatad by protochloride of tin, and by acetate and subacetate of lead. It possesses an acid reaction, and is not changed by chlorine Or iodine. On evaporation, a Substance similar in appearance to gum-arabic is obtained, which, when digested in water, intumesces, and partially dissolves. 

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... 

Concurrently, a new halogen-free FR master batch for polyester has been developed in our laboratories which at only 5 wt % incorporation enables PET to obtain classification according to several standards such as the NF P 92 501 or NF P 92 503 (M classification), FMVSS 302 or BS 5852 (Crib 5).65 In this case, an intumescent behavior is observed but its mechanism of formation should... 

Tang et al.84,85 also examined the incorporation of MMT in intumescent PP with a compatibilizer (hexadecyltrimethylammonium bromide) which is usually used as surfactant for making OMMT. Evidence of making a nanocomposite is shown with and without the intumescent system. Cone calorimetry shows a large improvement in the flammability properties when using OMMT. The results are similar to what we showed above. They postulated a mechanism of action suggesting the formation of an aluminophosphate structure but no evidence was given. 

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. 

Camino, G., Costa, L., and Trossarelli, L. 1985. Study of the mechanism of intumescence in fire retardant polymers Part V—Mechanism of formation of gaseous products in the thermal degradation of ammonium polyphosphate. Polym. Deg. Stab. 12 203-211. 

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. 

Horrocks A.R. and Davies, P.J. 2000. Char formation in flame-retarded wool fibres. Part 1. Effect of intumescent on thermogravimetric behaviour. Fire Mater. 24 151-157. 

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. 

The melt-dripping behavior of various ethylene-acrylate formulations with chalk and silicone have also been addressed by exposing these formulations to a Bunsen burner. It has been demonstrated that viscosity plays a crucial role in this process by affecting the transport of volatile gases, eventual dripping, and the formation of the intumescent structure.32... 

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

The formation of borophosphate partially explains the good performance when APP and boric acid are mixed together in the epoxy resin. Indeed, in that case good mechanical resistance of the intumescent char is observed as borophosphate is a hard material, which also shows a good thermal stability. As a conclusion, the boron containing compounds provide the good structural properties of the char, whereas the phosphorus ensures the adhesion of the char to the steel. 

Fire-retarded materials functioning in the condensed phase, such as intumescent systems, form, on heating, foamed cellular charred layers on the surface, which protects the underlying material from the action of the heat flux or the flame. It is recognized that the formation of the effective char occurs via a... 

Whatever the formulations, curve of the rate of heat release versus time shows two peaks, the first before 200s 0,) and the second between 300s (REF) and 500s (OMMT), which is the typical behavior of intumescent systems. The first peak is attributed to the formation of the intumescent protective shield that leads to a decrease in heat and mass transfers between the flame and the material. When this shield is formed, the RHR decreases and a plateau is observed in some cases. The second peak corresponds to the destruction of the intumescent layer leading to a sharp emission of flammable gases, the higher the time for the second peak, the higher the thermal and mechanical stabilities of the intumescent shield. Then, a thermally stable residue is formed. When lamellar... 

XRD analysis could also provide interesting information regarding the inorganic materials formed during the carbonization process in complex formulations.28,36 As an example, in the case of intumescent coatings, the formation of titanium pyrophosphate resulting from the reaction between APP and titanium dioxide can be demonstrated (Figure 10.17). 

Kodolov, V.I., Shuklin, S.G., Kuznetsov, A.P., Makarova, L.G., Bystrov, S.G., Demicheva, O.V., and Rudakova, T.A. 2002. Formation and investigation of epoxy intumescent compositions modified by active additives. Journal of Applied Polymer Science 85(7) 1477-1483. 

---