Mass loss rate polymer-clay nanocomposites

Recently, new approaches on flame retardancy deal often with nanofillers and in this section some examples of improvements of fire behavior of polymeric foams obtained by use of nanoclays or nanofibers will be shown. Much more details on flame retardancy of polymeric nanocomposite may be found elsewhere as for example in the book edited by A. B. Morgan and C. A. Wilkie105 or in scientific review.106 Polymer nanocomposites have enhanced char formation and showed significant decrease of PHRR and peak of mass loss rate (PMLR). In most cases the carbonaceous char yield was limited to few weight %, due to the low level of clays addition, and consequently the total HRR was not affected significantly. Hence, for polymer nanocomposites alone, where no additional flame-retardant is used, once the nanocomposite ignites, it burns slowly but does not self-extinguish... 

To clarify the mechanisms of the clay-reinforced carbonaceous char formation, which may be responsible for the reduced mass loss rates, and hence the lower flammability of the polymer matrices, a number of thermo-physical characteristics of the PE/MMT nanocomposites have been measured in comparison with those of the pristine PE (which, by itself is not a char former) in both inert and oxidizing atmospheres. The evolution of the thermal and thermal-oxidative degradation processes in these systems was followed dynamically with the aid of TGA and FTIR methods. Proper attention was paid also to the effect of oxygen on the thermal-oxidative stability of PE nanocomposites in their solid state, in both the absence as well as in the presence of an antioxidant. Several sets of experimentally acquired TGA data have provided a basis for accomplishing thorough model-based kinetic analyses of thermal and thermal-oxidative degradation of both pristine PE and PE/MMT nanocomposites prepared in this work. 

Combustion of polymeric materials involves a complex process, where both condensed and vapor-phase reactions occur at exposed surfaces that are sources of flame and/or thermal radiation of the most common parameters measuring the flammability of polymeric materials are heat release rate (HRR) and mass loss rate (MLR) from cone calorimetry. Recently, nanocomposites containing nanoparticles have been of great interest in the composite industries. In particular, polymer blends containing clays have not been comprehensively studied for their flammability, in spite of the fact that most plastic products are made out of blends of more than two polymer. Furthermore, because the dispersion of nanoparticles is a key factor in determining the HRR and MLR of nanocomposites [23-26], we investigated correlations between flammability and dispersion in air and under nitrogen, especially for polymer blends. 

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. 

Polymer-clay nanocomposites reduce flammability by slowing the mass loss rate of fuel to the flame, thus keeping the heat release rate (HRR) low (Chapter 3). However, the material will eventually bum completely, leaving only a small amount of noncombusted carbon, with very little reduction in... 

A trend that has already begun to arise is the use of multiple types of nanofillers in the same polymer to yield a multicomponent nanocomposite. Some workers have found that some types of nanofillers cannot bring all of the desired properties to the final material, so clays have been combined with multiwall carbon nanotubes to bring enhanced properties.The observation for most polymer additives is that they cannot be used for all applications in all polymers, and the same observation will surely be made about nanocomposites. A clay may be used to enhance the flammability performance, bnt it could also be combined with a conductive nanoflller to impart antistatic aspects or electrical conductivity in the final system. One potential way to look at the use of multiple nanoparticles is that each nanoparticle plays a complementary role in flammability reduction. For example, one could choose a clay for mass loss rate or fuel release reduction, but then use a colloidal particle to flu in the gaps between clay plates as the nanocomposite thermally decomposes. Perhaps even more useful, the colloidal particle could have catalytic or flame retardant properties that encourage... 

The second significant independent variable that layered silicates provide to increase thermal stability of the polymer in polymer-clay nanocomposites is an increase of the melt viscosity. If thermal degradation of the polymer is diffusion controlled, an increase in viscosity of the polymer melt will slow the mass loss associated with gas escaping from the composite during TGA evaluations. The increase in viscosity of dispersions is a function of the surface area of the dispersed phase. For example, water-based dispersions will increase in viscosity as the particle size of the dispersed phase decreases at constant total volume of the dispersed phase. This is the result of an increase in total surface area of the dispersed phase. Particle-particle interaction has increased as a function of increased total particle surface area. The surface area [17] of fully exfoliated montmorillonite is approximately 750 m /g. This enormous number results in a significant increase in polymer-montmorillonite melt viscosity at low concentration of montmorillonite and low shear rates [18]... 

Cone calorimetric evaluations of polymer-clay nanocomposites indicate that PHRR and mass loss rate (MLR) can be significantly reduced when compared to the pure polymer. However, in many cases, the ignition temperature is lower, the total heat released (THR) has not changed, and the total mass loss (TML) has not changed for the polymer-clay nanocomposites when compared to the pure polymer. An examination of the flame-retardant behavior of polymer-clay nanocomposites indicates that the presence of the clay delays the decomposition of polymer in the cone calorimeter test and does not prevent the decomposition. These observations in relation to the definitions listed above for flame retardants excludes clay from being considered to be a flame retardant in the same category as commercially available flame retardants. Because of these inadequacies, considerable effort has been made to identify synergies that may exist between commercial flame retardants and clay in polymer. 

The production of a char barrier must serve to retain some of the polymer and thus both the energy released and the mass loss rate decrease. The amount of smoke evolved, and specific extinction area, also decreases with the formation of the nanocomposite. There is some variability in the smoke production but apparently the formation of the nanocomposite gives a reduction in smoke however, the presence of additional clay does not decrease smoke. 

Koo and co-workers [78] attempted to develop polyamides 11 and 12 with enhanced flame retardancy and thermal and mechanical properties by the incorporation of montmorillonite clays, silica and carbon fibre-polymer nanocomposites. Flammability properties of the nanocomposites were compared with those of the virgin polyamides, using cone calorimetry with an external heat flux of 50 kW/m. Cone calorimetry was also used in an evaluation of polyamide 6 - anion modified Mg/Al interlayer formulation [79]. The data from the cone calorimeter shows that the heat production rate (HPR) and mass loss weight of the sample with 5 wt% MgAl(H-DS) decrease considerably to 664 kW/mVs and 0.161 g/mVs from 1064 kW/mVs and 0.252 g/mVs... 

The mechanism of flammability reduction for polymer nanotube and nanofiber nanocomposites (Chapter 10) is similar to that for clays a nanofiUer-rich surface or barrier forms, which slows the rate of mass loss and therefore the rate of heat release. There is little reduction in the total heat release, indicating that the carbon nanoflbers and nanotubes only reduce the flammability of the... 

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