Intumescing barrier

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Intumescent additives. React with the polymer substrate to produce a char layer which forms an effective barrier between heat source and oxygen and... 

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). 

Intumescent layers of such thicknesses are not difficult to achieve, but let us consider some limitations to this approach as well as some clues to improving intumescent char or char-like barriers. 

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. 

Few recent scientific papers are devoted to resistance to fire using intumescent coating, but a growing market for this application exists. A rapid survey of the literature shows that the formulations used are always based on the well-known trio APP/PER-Melamine. Nevertheless a lot of work remains to be done to understand and to quantify the mechanisms of intumescence acting as protective heat barrier. 

Several micron-sized layered silicates, such as talcs, can improve the fire retarding behavior of EVA by partial substitution of metal hydroxides. Clerc et al.63 have shown that better fire performance was achieved using higher values of the lamellarity index and specific surface area for four different types of talcs in MH/EVA blends. Expanded mineral and charred layers were formed, similar to intumescent compositions with APP, proving the barrier effect on mass transfer, even at the micron scale for the mineral filler. 

Kandola, B.K. and Horrocks, A.R. 2000. Complex char formation in flame-retarded fibre intumescent combinations—IV. Mass loss calorimetric and thermal barrier properties. Fire Mater., 24 265-275. 

The effectiveness of coke as a heat barrier retarding heat transfer to the undecomposed polymer depends primarily on the thermophysical properties of coke. Semiempirical models of intumescent coatings > -i ) allow the coke effectiveness to... 

Intumescent paints or sealants offer the finest fine of fire retardant and fire barrier products in the market today. 

Melamine and melamine derivatives, such as melamine cyanurate or melamine polyphosphate, are representatives of nitrogen-containing flame retardants (Fig. 11.20). Melamine is an inert diluent and removes heat through decomposition melamine polyphosphate provides an isolating barrier through an intumescent mechanism. 

Physically covering the polymer with a reflecting intumescent coating as a heat shield may provide an adequate barrier to heat transfer and thus, prevent the initial low-temperature degradation of the polymer leading to ignition [16]. 

Another solution to improve the fire-retardant (FR) properties of polymers is the use of intumescent additives [2, 3]. Intumescent technology [4, 5] has found a place in polymer science as a method of imparting flame retardancy to polymeric materials. On heating, FR intumescent materials form a foamed cellular charred layer on their surfaces [6, 7], which protects the underlying materials from the action of heat flux and flame. The proposed mechanism [8] is based on the charred layer acting as a physical barrier, which retards heat and mass transfer between the gas and the condensed phase. 

In general, coatings for thermal protection fall into two groups those which undergo a chemical change in the presence of heat (e.g. ablative and intumescent) and those which provide a fixed insulation barrier (including ceramic materials). 

It is difficult to achieve a fire resistance rating with just composite protective systems. However, fire resistance and reduced smoke and toxicity can be achieved by the use of fire barriers which include ceramic fabric, ceramic coatings, intumescent coatings, or... 

R D efforts in halogen-fiee FRs are often aimed at designing a protective closed barrier on the burning polymer surface to reduce heat and mass transfer to the combustion zone. In some polymer applications this can be achieved by the use of intumescent systems. However, these are not always suitable in other apphcations for reasons of water uptake, thermal stability or actual FR performance. Therefore, it is of interest to the material developer to have options available to control the structure of the burning polymer surface layer. In this layer no cracks should appear which could allow for the escape of volatile, ignitable gases and so sustain the combustion process. 

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