Intumescents in Polymer Nanocomposites

Moreover, pyridinic nitrogen (it was reported that ammonia evolving from APP reacts with carbon-carbon double bonds to yield nitrogenated heterocycles) is observed at all temperatures in the case of zeolite and only up to 350°C in the case of a system without zeolite. It was proposed that the delay in condensation of the polyaromatic network allows the retention of pyridinic nitrogen to participate in improvement in the mechanical properties and therefore to improvement in the fireproofing properties of the material. 

Zeolite does not need to be a true catalyst, because it reacts (zeolite is a crucial reactant of the intumescent reaction) with the other ingredients of the intumescent formulation (reaction of phosphate with the aluminosilicate). The latter reaction permits the formation (in situ and in the conditions of fire) of species that stabilize the intumescent structure. So it can be expected that the use of species such as aluminosilicate, able to react with phosphate, should also provide enhanced flammability properties. 

The char formers commonly used in intumescent formulations for thermoplastics are polyols such as pentaerythritol, mannitol, and sorbitol. However, exudation and water solubility are problems associated with these additives. Moreover, these additives are often not compatible with the polymeric matrix, and the mechanical properties of the formulations are then very poor. We have developed intumescent polyolefin-based formulations using charring polymers [thermoplastic polyurethane (TPU) and polyamide-6 (PA6)] as carbonization agents. These formulated blends have improved mechanical properties compared with polymers loaded with classical flame retardants, and they avoid the problems 

FIGURE 6.9 LOI values versus APP content in the intumescent formulations EVA24-APP/PA6 and EVA24-APP/PA6nano. From PA6 clay nanocomposite hybrid as a char-forming agent in intumescent formulations. (From Ref. 32, copyright 2000, John Wiley Sons, Ltd., with permission.) 

PA6nano in the formulation allows a V-0 rating to be achieved at relatively low loading in APP (10 wt% in comparison with 13.5 wt%). This is a real advantage because it permits a decrease in the amount of APP in the formulation additional APP can sometimes lead to a blooming effect and to migration throughout the polymer. The lowered amount of APP also permits the preservation of mechanical properties. 

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