Char silicate surface layer

Polyurethane/clay-based nanocomposites are already being used for automobile seats and it also exhibit superior flame retardancy. Phenolic resin impregnated with montmorillonite clay was already identified as the resin for manufacturing rocket ablative material with MMT. The nanolevel dispersion of clay platelets leads to a uniform char layer that enhances the ablative performance. The formation of this char was slightly influenced by the type of organic modification on the silicate surface of specific interactions between the polymer and the silicate platelets surface, such as... 

Specific aspects of barrier formation were discussed above. A silicate or sihcate-char surface layer acting as a barrier for heat and mass transport is probably the main general fire retardancy mechanism of all layered-silicate nanocomposites. Most sources claim that this mechanism is responsible for the strongly improved performance in a cone calorimeter test. In particular, the strong reduction in PHRR is used to propose that layered silicates are the most promising approach for fire retardancy of polymers. However, the barrier effects and their influences on cone calorimeter results are not described in detail, so that the specific characteristics of these mechanisms are unclear. 

Several micron-sized layered silicates, such as talcs, can improve the fire retarding behavior of EVA by partial substitution of metal hydroxides. Clerc et al.63 have shown that better fire performance was achieved using higher values of the lamellarity index and specific surface area for four different types of talcs in MH/EVA blends. Expanded mineral and charred layers were formed, similar to intumescent compositions with APP, proving the barrier effect on mass transfer, even at the micron scale for the mineral filler. 

During the ablation experiment, temperature within the char layer exceeds 1000°C and approach 2000-2500°C at the surface. At these temperatures, any carbonaceous residue from the pol3oner will contain graphite. Additionally, mica-type layered silicates, such as montmorillonite, irreversibly transform into other aluminosilicate phases. Between 600 and 1000 C, montmorillonite dehydroxylates and has been observed to initially transform into spinel, cristobolite, mullite and/or pyroxenes (enstatite) (24). At temperatures greater than 1300 C, mullite, cristobolite and cordierite form and subsequently melt at temperatures in excess of 1500 C (mullite 1850 C, pure cristobolite 1728°C and cordierite --ISSO C) (25). The presence of an inorganic that transforms into a high viscosity melt on the surface of the char will improve ablation resistance by flowing to self-heal surface flaws. This is known to occur in silica-filled ablatives (26). 

This silicate morphology may act as an cfBcient barrier to oxygen diffusion towards the bulk of the polymer. Surface polymer molecules trapped within the silicate are thus brought to a close contact with oxygen to produce the thermally and oxidative stable charred material providing a new char-layered silicate nanocomposite acting as an effective surface shield... 

The improvement in thermal stability of the nanocomposites compared to the neat EVA/natural rubber is due to the barrier effect and insulating properties of organoclay. The well dispersed plate-like silicate layers form a tortuous path in the polymer matrix which gives a barrier effect and inhibits the diffusion of volatile degradation product from the inside of the polymer matrix. Moreover the well-dispersed silicate layers restrict the movement of polymeric chains, hence reducing the free volume for diffusion of volatile degradation products. Other researchers also confirm that organoclay tends to form a compact char-like residue on the surface of the nanocomposites when it is burnt. This char-like structure is incombustible and acts as an insulator which inhibits heat transfer to the inside of the nanocomposites. At 8 phr... 

A typical heat release rate curve for a neat epoxy system and the respective layered silicate nanocomposite, is shown in Fig. 2.12. Both peak and average heat release rate, as well as mass loss rates, are all significantly improved through the incorporation of the nanopartieles. In addition, no increase in specific extinction area (soot), CO yields or heat of combustion is noticeable. However, the mechanism of improved flame retardation is still not clear and no general agreement exists as to whether the intercalated or exfoliated structure leads to a better outcome. The reduced mass loss rate occurs only after the sample surface is partially covered with char. The major benefits of the use of layered silicates as a flame retardation additive is that the filler is more environmentally-friendly compared to the commonly used flame retardants and often improves other properties of the material at the same time. However, whilst the layered silicate strategy is not sufficient to meet the strict requirements for most of its application in the electrical and transportation industry, the use of layered silicates for improved flammability performance may allow the removal of a significant portion of conventional flame retardants. 

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