Char strength

PhenoHc and furfuryl alcohol resins have a high char strength and penetrate into the fibrous core of the fiber stmcture. The phenoHc resins are low viscosity resoles some have been neutralized and have the salt removed. An autoclave is used to apply the vacuum and pressure required for good impregnation and sufficient heat for a resin cure, eg, at 180°C. The slow pyrolysis of the part foUows temperatures of 730—1000°C are recommended for the best properties. On occasion, temperatures up to 1260°C are used and constant weight is possible even up to 2760°C (93). 

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. 

In that frame, the use of a plane-plane rheometer has been evaluated. The purpose of the experiment carried out in the rheometer is not to mimic fire testing (this is not possible since the heating rate [slow ramp vs. quenching], heating source [convection vs. radiation], sample size, and boundaries effect are different) but to develop a test method that will permit the characterization of the char strength when exposed to pressure (in that case compression force). 

FIGURE 10.13 Char strength measurement. (Reproduced from Jimenez, M. et al., Ind. Eng. Chem. Res., 45, 4500, 2006. With permission.)... 

Special calorimeter (UFA to assess combustion in under-ventilated conditions and evaluate tendency for dripping and char strength by performing experiments in vertical orientation)... 

Better flame retardancy. The addition of CNT, CNF, GNP, nanoclay or POSS will promote the char formation, increase the char strength. 

Figure 8.14 shows the HRR plots for the EVA/LDPE-based materials. As expected, there is a substantial reduction of the HRR with the addition of FRs. For FR-containing materials (FRCMs), the reduction in the HRR is much more substantial than that with nanoclay alone. Here, however, it is worthwhile to note that the reduction of the HRR by the FRCMs is also due partially to the reduced polymer content in the FRCMs. Partial replacement of FRs with nanoclay reduces the HRR/MLR further, probably because of an increase of the char strength in the presence of the nanoclay, which was also supported by the morphological and TGA results. 

Viscosity is a key parameter of intumescence because the intumescence coating must adhere to the substrate and not drip. At the same time, the expansion (swelling) of the char has to be high enough to limit heat transfer (formation of a low-density foamed char), and the char strength has to accommodate stresses coming from the flame and internal pressure. The addition of nanofiller increases the viscosity of the char, but the expansion and the char strength should be... 

Char strength is a crucial parameter because it has to accommodate internal pressure and external stresses to avoid the formation of cracks. Figure 6.16 describes the evolution of the force applied on the top of the intumescent char as a function of the gap between the rheometer plates (Figure 6.16a). EVA-APP/PA6 -OMMT exhibits better char resistance than the other formulations, those of EVA-APP/PA6-NPSi and EVA-APP/PA6 are significantly lower, and EVA -APP/PA6-OLDH exhibits intermediate behavior. This suggests that high char strength should be required to get the best performance associated with reasonable expansion. 

The expansion is related to the formation of an expanded foamed material with the role of limiting heat transfer, and the char strength permits cohesion of an insulative coating on the substrate and avoids the formation of cracks. The better flammability properties of formulations containing a nanofiUer might be explained on the basis of the particular chemistry of the system. Reaction between... 

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