Intumescent stability

These coatings provide the most effective fire-resistant system available but originally were deficient in paint color properties. Since, historically, the intumescence producing chemicals were quite water-soluble, coatings based thereon did not meet the shipping can stability, ease of application, environmental resistance, or aesthetic appeal required of a good protective coating. 

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... 

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. 

An extensive study was conducted on the effect of chemical and structural aspects of zeolites on the fire performance of the intumescent system, ammonium polyphosphate-pentaerythritol (APP-PER), where a marked improvement of the fire-retardant properties within different polymeric matrices has been observed.56 58 The synergistic mechanism of zeolite 4A with the intumescent materials was investigated using solid-state NMR. Chemical analysis combined with cross-polarization dipolar-decoupled magic-angle spinning NMR revealed that the materials resulting from the thermal treatment of the APP-PER and APP-PER/4A systems were formed by carbonaceous and phosphocarbonaceous species, and that the zeolite enhances the stability of the phosphocarbonaceous species. 

Boric acid in conjunction with APP was reported in epoxy intumescent coating.30-31 Boric acid and its derivatives were used in phenolics to impart thermal stability and tire retardancy. For example, Nisshin steel claims the use of boric acid and aluminum trihydroxide (ATH) in phenolics for sandwich panel.32 It was also reported that the small amounts of boric acid (around 0.25% by weight) in polyether imide (PEI) and glass-filled and PEI can reduce peak HRR by almost 50% in the OSU Heat Release test for the aircraft industry.33 In applications where high modulus and high strengths are needed, boric acid can be added without the softening effects of other additives such as siloxanes. 

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. 

Moreover, in the stabilization phase of the intumescent structure, the change in the viscosity of the charred material under stress may explain the loss of the protective character of the intumescent shield. Indeed, if the shield becomes too hard, the creation and the propagation of cracks leading to a rapid degradation of the material occurs.33... 

So, the viscoelastic properties of intumescent materials in the range of temperature corresponding to the development, the stabilization, and the destruction of the protective shield are, as a consequence, an important task of investigation. The way to investigate those parameters is detailed in Section 10.4.1. 

At 360°C-370°C, an important increase in the apparent initial viscosity begins that corresponds to the complete degradation of the initial material and to the carbonization of the system. The foamed material appears to be constituted by solid particles only. From 450°C-460°C, the viscosity value is more stable and increases slightly. At this temperature, a char oxidation/degradation probably starts. Complementarity, the viscosity can be analyzed versus temperature and versus time (Figure 10.10) in order to better visualize the mechanical and thermal stabilities of the protective intumescent layer and to have a better understanding of the carbonization process. 

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... 

Vannier, A., Duquesne, S., Bourbigot, S., Castrovinci, A., Camino, G., and Delobel, R. 2008. The use of POSS as synergist in intumescent recycled poly(ethylene terephthalate). Polymer Degradation and Stability 93(4) 818-826. 

Duquesne, S., Le Bras, M., Jama, C., Weil, E.D., and Gengembre, L. 2002. X-ray photoelectron spectroscopy investigation of fire retarded polymeric materials Application to the study of an intumescent system. Polymer Degradation and Stability 77(2) 203-211. 

Anna, P., Marosi, Gy., Csontos, I., Bourbigot, S., Le Bras, M., and Delobel, R. 2001. Influence of modified rheology on the efficiency of intumescent flame retardant systems. Polymer Degradation and Stability 74(3) 423M-26. 

Duquesne, S., Magnet, S., Jama, C., and Delobel, R. 2005. Thermoplastic resins for thin film intumes-cent coatings—Towards a better understanding of their effect on intumescence efficiency. Polymer Degradation and Stability 88(l) 63-69. 

F. Laoutid, L. Ferry, E. Leroy, and J.M. Lopez-Cuesta, Intumescent mineral fire retardant systems in EVA copolymer Effect of silica particles on char cohesion, Polym. Degrad. Stabil., 2006, 91 2140-2145. 

Wang, D. Y Liu, Y Ge, X. G Wang, Y. Z Stec, A., Biswas, B Hull, T. R and Price, D. 2008. Effect of metal chelates on the ignition and early flaming behaviour of intumescent fire-retarded polyethylene systems. Polymer Degradation and Stability 93 1024—30. 

Kandola, B. K., Akonda, M. H., and Horrocks, A. R. Use of high-performance fibres and intumescents as char promoters in glass-reinforced polyester composites, Polym. Degrad. Stabil. 2005, 88, 123-129. 

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.]...

Hercules PE. [Hercules] Pentaeryth-ritols used in prod, of alkyd resins, rosin esters, oil-modified urethane resins, drying oils, synthetic lubricants, plasticizers, intumescent paints, plastics, stabilizers for plastics, explosives. 

R D efforts in halogen-fiee FRs are often aimed at designing a protective closed barrier on the burning polymer surface to reduce heat and mass transfer to the combustion zone. In some polymer applications this can be achieved by the use of intumescent systems. However, these are not always suitable in other apphcations for reasons of water uptake, thermal stability or actual FR performance. Therefore, it is of interest to the material developer to have options available to control the structure of the burning polymer surface layer. In this layer no cracks should appear which could allow for the escape of volatile, ignitable gases and so sustain the combustion process. 

EVA and Exolit AP 750 at 30% addition levels develops an intumescent coating between 200 and 460 °C to give better thermal stability and good fire retardancy. Above 460 °C, however, the protection decreases. 

Work in China has shown the synergistie effect of silicotungistic acid (SiW ) on pol q)ropylene flame retarded by an intumescent FR (NP28 phosphorus-nitrogen compound). The tungsten compound increased the thermal stability of the PP formulation at temperatures above 500 °C. The SiWi2 could efficiently promote the formation of compact intumescent charred layers with phosphocarbonaceous structures. 

Intumescent flame retardants (IFR) that contains phosphorus are also used in halogen-free flame-retardant systems. Most reported IFRs are mixtures of the three ingredients, an acid source, a polyol, and a nitrogen-containing compound (Halpem et al. 1984). Since processing of ABS resin requires that the additives withstand temperatures in excess of 200 °C, the commonly used intumescent system, ammonium polyphosphate, pentaerythritol, and melamine, which do not have sufficient thermal stability, cannot be incorporated into ABS resin under normal processing conditions they are usually used in polyolefins. 

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