Polymer nanocomposites char residue

Incorporation of modified clays into thermosetting resins, and particularly in epoxy35 or unsaturated polyester resins, in order to improve thermal stability or flame retardancy, has been reported.36 A thermogravimetric study of polyester-clay nanocomposites has shown that addition of nanoclays lowers the decomposition temperature and thermal stability of a standard resin up to 600°C. But, above this temperature, the trend is reversed in a region where a charring residue is formed. Char formation seems not as important as compared with other polymer-clay nanocomposite structures. Nazare et al.37 have studied the combination of APP and ammonium-modified MMT (Cloisite 10A, 15A, 25A, and 30B). The diluent used for polyester resin was methyl methacrylate (MMA). The... 

The flame-retardant characteristics of polymer nanocomposites are also found to be enhanced compared to the pristine system. This may be due to a significant reduction in the heat release rate and an increase of nonflammable char residue. The char creates a protective layer which impedes oxygen penetration and creates an insulating layer between the heat and the fuel. 

Generally, for condensed phase flame retardancy, the morphology of the char residue may help to clarify the combustion mechanism. Figure 11.7 presents TEM and SEM microphotographs of PP/1.0 wt% Ceo nanocomposites after cone calorimetry. Interestingly, compared with the pristine Ceo crystallites with a size of around 100 nm, after combustion the sizes were much smaller, 30-50 nm, though well proportioned. An explanation was that the combustion of polymers at elevated temperatures would destroy or disorder the stack state of Ceo crystals, making the size of crystals tend to be the same. 

The SEM image of the residue of the PP/Ceo sample showed that Ceo crystals only aggregated rather than forming a compact and continuous network. Thus, it was not the char residue that was responsible for the enhanced thermal stability and improved flame retardancy of PP/Ceo nanocomposites. Because the char residue could not confer flame retardancy on polymers, possible reasons may be concealed in the primary state or in the heating or combustion of polymer materials. 

With increase in bentonite content, T 5q% also increases dramatically. Char residue of the nanocomposites tends to increase compared with those of polymer matrix. However, an obvious charring effect of bentonite for PEA was not found. 

For more than a decade, numerous research studies have been carried out on the flame-retardant properties conferred by nanoparticles and mainly by organo-modified layered silicates (OMLS). Earlier work at Cornell University and National Institute of Standards and Technology in the United States showed that nanocomposites containing OMLS reduced polymer flammability and enhanced the formation of carbonaceous residue (char).14 Owing to a strong increase in polymer viscosity, impairing processability, and also due to the breakdown of ultimate mechanical properties, the acceptable rate of incorporation for nanoparticles to improve flame retardancy is generally restricted to less than 10 wt %. 

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... 

The comprehensive flame retardation of polymer-clay nanocomposite materials was reported by Dr. Jeff Gilman and others at NIST [7]. They disclosed that both delaminated and intercalated nanoclays improve the flammability properties of polymer-layered silicate (clay) nanocomposites. In the study of the flame retardant effect of the nanodispersed clays, XRD and TEM analysis identified a nanoreinforced protective silicate/carbon-like high-performance char from the combustion residue that provided a physical mechanism of flammability control. The report also disclosed that The nanocomposite structure of the char appears to enhance the performance of the char layer. This char may act as an insulation and mass transport barrier showing the escape of the volatile products generated as the polymer decomposes. Cone calorimetry was used to study the flame retardation. The HRRs (heat release rates) of thermoplastic and thermoset polymer-layered silicate nanocomposites are reduced by 40% to 60% in delaminated or intercalated nanocomposites containing a silicate mass fraction of only 2% to 6%. On the basis of their expertise and experience in plastic flammability, they concluded that polymer-clay nanocomposites are very promising new flame-retarding polymers. In addition, they predict that the addition... 

In Section 3.2.2, data were presented which show that char formation, possibly from a catalytic reaction between the polymer (PS, PA6, or a polymer compati-bilizer, PP-g-MA) and the clay, is often present when low peak HRRs (or mass loss rates) are observed. However, data were also presented which show that in the absence of any substantial charring there can still be a 50 to 60% reduction in peak HRRs if synthetic mica is used in PP/PP-g-MA nanocomposites. It appears that at least two mechanisms may be important to the function of nanocomposites one involving char formation and a second involving the inorganic residue alone. This dual mechanism may explain why the effectiveness of clay nanocomposites varies from polymer to polymer. 

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