Char formers

Borates, through their ability to act as glass network formers, can act as excellent char formers and drip suppressants in fire retardant applications. In many cases this involves processing into polymeric materials, leading to specific requirements for thermal stability and particle size. Most common borate materials, however, exhibit relatively low dehydration temperatures and may be unsuitable for use in many polymer systems. Zinc borates are often used because they have unusually high dehydration onset temperatures and can be produced as small particle size powders. 

The use of polyols such as pentaerythritol, mannitol, or sorbitol as classical char formers in intumescent formulations for thermoplastics is associated with migration and water solubility problems. Moreover, these additives are often not compatible with the polymeric matrix and the mechanical properties of the formulations are then very poor. Those problems can be solved (at least partially) by the synthesis of additives that concentrate the three intumescent FR elements in one material, as suggested by the pioneering work of Halpern.29 b-MAP (4) (melamine salt of 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]-undecane-3,9-dioxide) and Melabis (5) (melamine salt of bis(l-oxo-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octan-4-ylmethanol)phosphate) were synthesized from pentaerythritol (2), melamine (3), and phosphoryl trichloride (1) (Figure 6.4). They were found to be more effective to fire retard PP than standard halogen-antimony FR. 

As the polyol-based char formers needs to be substituted, Li and Xu40 reported the synthesis of a novel char former for intumescent system based on triazines and their derivatives. It is a macromo-lecular triazine derivative containing hydroxyethylamino, triazine rings and ethylenediamino groups (Figure 6.7). They showed that the new char former in an intumescent formulation containing APP... 

FIGURE 6.7 Synthesis of macromolecular triazines derivatives as char former for intumescent systems. (From Li, B. and Xu, M., Polym. Deg. Stab., 91, 1380, 2006.)... 

FIGURE 6.8 Synthesis of a phosphorus-containing char former. (From Xie, F. et al., Macromol. Mater. Eng., 291, 247, 2006.)... 

Poly vinyl chloride Synthetic-char formers >180 >180 450 37-39... 

Char formers Usually phosphorus compounds, which remove the carbon fuel source and provide an insulation layer against the fire s heat... 

These mechanisms, however, are not as effective in limiting the rate-of-mass loss, energy release or total mass consumption as is the formation of a monolithic, insulating char barrier. Those organophosphorus compounds most commonly used today, the chloroalkyl phosphates and the aryl phosphates, or even the inorganic ammonium polyphosphates and elemental red phosphorus are not very effective char formers in these polymer systems. 

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. 

By acting as char formers, as phosphorous flame retardants do. They are also subdivided into nonhalogenated organophosphate esters, ammonium polyphosphate, and others. When heated, they produce a solid form of phosphoric acid that in turn chars the material and shields it from releasing of flammable gases feeding flames. Phosphorous flame retardants account for about 20% of flame retardants in the industry (mainly not with polyolefins). Boron compounds also work as char formers [2]. 

Metal chelates lower decomposition temperature (decrease thermal stability of material), increase char yield and limiting oxygen index, and improve smoke density rating. They are useful S5mergistic additives for Al(OH)3 and Mg(OH)2. Their two major functions are catalysts of dehydrochlorination and char formers. 

Uses Hame retardant, smoke suppressant in polymers, esp. PVC-based wire and cable, rigid film and coatings, and halogenated polyesters char-former in non-halogenated PE/EVA and epoxy sterns Properties Wh. to ott-wh. powd. 1.5-2.0 mean particle size 99% < 25 sp.gr. 3.30 decomp. temp. > 375 F < 0.9% free moisture Charmax MO [R.J. Marshal]... 

Alumina frihydrafe, by itselfor in combination with antimony trioxide, is an effective fire retardant additive (69,70). Antimony trioxide by itself has been shown to effectively reduce the smoke generated in a flexible formulation of PVC (71), as has zinc borate (72) and barium metaborate (73). Phosphate plasticizers, described earlier, are also effective as char formers (71). Smoke suppressants, as a special class of materials, are designed to reduce the... 

For melamine phosphate-pentaerythritol-polypropylene, the char former/blowing agent ratio was shown to have a significant effect on flame retardancy [37]. 

It is thought that pol5mieric char formers such as PVA, starch, glucose derivatives and polyfimetional aleohols may present a new trend in the global search for ecologically safe flame retardant systems. 

When red phosphorus acts in the solid phase, it is oxidised to polyphosphoric acid, a char-former. In the vapom phase, red phosphorus contributes to flame retardancy by a different mechanism, generating free radicals such as PO that neutralise the free radicals promoting combustion. 

The chemistry of flame-retardant additives is highly varied and is optimised not only for specific polymer chemistries, but also to address flammability effects such as flame spread, dripping, smoke release and so on. Flame-retardant chemistry includes classes of compounds such as halogenated organics, char formers, crosslinking compounds, mineral fillers, intumescent packages, phosphorus compounds, nitrogen-based compounds and even certain metal and boron compounds. 

The structures of the aromatics listed in Table 4.1 are shown in Figs. 4.2 (coke formers) and 4.3 (char formers). Some of the most important aromatics are the following... 

Graphitization of Aromatics. As mentioned above, not ail aromatic hydrocarbons form coke. Some, such as phenanthrene and biphenyl, do not graphitize and are considered char formers. These compounds are branched aromatics (as opposed to the linear structure of the coke-former aromatics) with a preferred axisof growth asshown in Fig. 4.3. Thischaracteristic prevents the formation of extensive graphitic planes and of a liquid mesophase.l l... 

---