Carbonaceous char barriers

Carbonaceous char barriers may be formed by the normal mode of polymer burning, and besides representing a reduction in the amount of material burned, the char may act as a fire barrier. The relationship of char yield, structure, and flame resistance was quantified by Van Krevelen (5) some years ago. For polymers with low char-forming tendencies, such as polyolefins, one approach to obtain adequate char is to add a char-forming additive. Such additives generally bear a resemblance to intumescent coating ingredients (6, 7). 

However, other types of barriers besides carbonaceous char have been shown to function in flame retardancy. In brief, these include the following ... 

A wide range of intumescent epoxy coatings are available. These can be described as a mix of thermally reactive chemicals in a specific epoxy matrix formulated for fireproofing applications. Under fire conditions, they react to emit gases, which cool the surface while a low density carbonaceous char is formed. The char then serves as a thermal barrier. 

The PLA/ramie hybrid alone presents no efficient charring to protect matrix, however, the addition of APP in PLA/ramie hybrid enhances the formation of coherence of carbonaceous charring layer as protective shield and thermal barrier (as shown in Fig. 4.37). The TGA test demonstrates the increase of char. When cooperating with APP, ramie fiber being rich in polyhydric compound acts as a charring agent to form an intumescent flame retardant system. This intumescent mechanism has been discussed in the literature. 

ID CNTs inserted in the 2D clay platelet network is believed to be responsible for this, as this unique nanostructure provides larger tortuosity and obstacle for the heat transport. The large improvement in thermal stability of chitosan may arise from following two reasons (1) good heat barrier properties of CNTs and clay for polymer matrix during formation of chars and (2) formation of carbonaceous layer. 

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