by Academician Alexander BERLIN, Director of the Semenov Institute of Chemical Physics, RAS
Fires have long caused immense economic damage claiming a great toll of life. Broadly used organic polymers serve more often as a fuel in such situations, especially in modern cities. That is why search is underway for ways to reduce their combustibility in many countries.
Fire hazard of materials has several characteristics. They include combustibility, or the ability to be inflamed, with the process subsequently spreading and developing accompanied by smoke and flame, toxic pyrolysis of products (decomposition of substance under the effect of high temperatures) and, finally, fire-resistance presupposing the preservation of strength, hardness and other functional properties of the product. Combustibility, in its turn, is made up of inflammation and self-inflammation temperatures, burnout rate and the flame's spread along the surface, limiting parameters ensuring this self-supporting process (for instance, the air composition).
The complicated distribution of temperature in space, concentrations of initial and intermediate substances, and also the presence of a vast number of various destruction products make it extremely difficult to carry out experiments in the process of research and produce strict quantitative polymer combustion theories that would take into account chemical and other special features of concrete systems. Nevertheless, general qualitative regularities exist and we offer their brief outline below.
Combustion as such is usually divided into its gaseous and heterogeneous (smoldering) forms. In the former case most of the heat, responsible for the process of
Diagram of combustion of polymer materials.
chemical transformation, is released as a result of the oxidation of gaseous destruction products. And the area of maximum rate of its isolation (gas flame) is usually several millimeters or more above the surface of the construction or the product. And it should be noted that the surface turns out to be significantly cooler than the gas flame (400 - 650° С as against 1,100 - 1,200°C and over). While in the process of smoldering heat is largely released from the surface layer where maximum temperatures are observed (800 - 900°C).
In such processes air oxygen serves as an oxidant, and as a fuel serves hydrogen- and carbon-containing gaseous destruction products transformed into water and carbon dioxide or-in the event of incomplete oxidation-carbon monoxide. If, for any reason, the flame temperature drops, it leads to a jumping of combustion regime - to a very slow oxidation process. The difference in their rates is enormous. That is why we can speak about the fact of critical (limit) conditions determining the borders of a possible combustion and depending on the geometry of samples and flame, gaseous medium temperature and also on the material, although they do not serve as its absolute characteristics.
These conditions were applied in the experimental method of the evaluation of polymer combustibility, first proposed by F. Martin, a British researcher, in the 1960s. The sample in the form of long bars or cylinders of about 10 mm in diameter is placed in a vertical tube, with oxygen and nitrogen supplied from below in certain proportions. The material is set aflame from the top with the help of a special gas burner, and after removing the gas burner, the sample either completely burns out or is rapidly extinguished. Such experiments are staged with various composition of the gas atmosphere. The critical oxygen composition in the mixture (in volume percentage), above which self-maintained combustion is possible, and it is impossible if it is less, is described as an oxygen index (01) and it characterizes given material's
Dependence of the level of combustibility (01) for polyoxymethylene (1.3) and polyethylene (2.4) on the concentration of Al(OH)3 (1.2) and Al2 O3 (3.4). Dotted line 5 corresponds to the so-called "self-extinguishing" material (OI = 27).
combustibility. And in physical terms it means that with a lower O2 concentration more heat is consumed for heating inert nitrogen gas, the flame temperature drops and the combustion process reaches its critical stage. This method is currently broadly used for experiments throughout the world.
In most cases it is simply impossible to save an organic polymer in an intensive fire. However, more often fires break out from low-calorie sources such as cigarettes, matches, candles, short circuit. That is why it is highly important to ensure more stringent conditions for the start of the process (high temperatures, energy flow, and so on).
What should be done to reach it? Methods currently applied are based on the following four principles: changing in the flame's heat balance at the expense of greater heat losses; reduction of heat flow from the flame to the polymer with the help of protective layers (let us say, of the formed coke); a slower process of the material's gasification; changing in the ratio of combustible to incombustible products of its decomposition in the latter's favor. The easiest method to increase heat losses would be to glue the polymer to the surface of a heat-conducting article, for instance, metal one: the thinner is the layer of the former, the greater will be the heat loss to the base and the worse conditions for self-maintained combustion.
Inert fillers introduced in the material contribute to a lower degree of combustibility. The fillers can be divided into two groups. First, mineral substances heat-resistant up to 1,000°C may be used for the purpose: metal oxides, calcium and lithium fluorides, silicates, technical carbon, inorganic glass, metallic powder, and so on. (Regrettably, if great amounts of any of them are added to the polymer it becomes too brittle and its physical and mechanical properties do not meet the necessary requirements.) The other group includes fillers decomposed at temperatures below 400 - 500°C with consumption of heat and emission of carbon dioxide and/or water and ammonium vapors: hydroxides, carbonates, hydrocarbonates of metals, ammonium phosphates, and so on. Classical example of this is an aluminum hydroxide, with water spilled out from it. In this case heat is consumed for heating and decomposition of aluminum hydroxide and for heating of formed water to the flame temperature.
Photos of samples of hardened epoxide resin containing (a) and not containing (b) an intumescent composition based on phosphate polyammonium.
Coke formation under the flame's effect on the polymer is an important factor having an impact on all stages of polymer combustion. As a result, the output of gaseous combustion products drops, with the flow of combustible gases to the flame reduced. And in reality the fact is that carbon remaining in condensed phase could get into the flame and oxidize to CO2 , with a great amount of heat released. In the event of intensive fire that will be the final result and, consequently, coke formation will be of no use. However, in this case weak combustion sources are the subject of discussion. That is why the effect of coke formation is so important. Of course you remember that coal serves as good fuel for a stove, but, first, you need to kindle coal in it with the help of a chip.
Consequently, another method to reduce the level of polymer combustibility is to modify the process of its destruction by increasing the amount of coke. Cellulose-based polymers serve as the most typical and studied example in this case. So we may indicate two ways of cellulose destruction: first one is with the formation of water and carbon residue and, second one is of gaseous levoglucosan (chemical compound produced as a result of cellulose pyrolysis). In the latter version we will have gaseous carbon finally oxidized down to CO2 (with a lot of heat released), while in the former one it remains in condensed form, with incombustible gaseous water alone released. The introduction into cellulose of compounds contributing to its dehydration (i.e., shifting the decomposition reaction in the first direction) allows to reduce the level of combustibility for cellulose-based materials. Substances of this type include, for instance, phosphorous compounds transformed into phosphoric acids in the pyrolysis process.
Another example of destruction is associated with the study of combustion of chlorparaffins. If a substance in this range is evaporated before its decomposition in the process of heating, with all other transformations made in gaseous form, the chlorine effect is insignificant and leads to low dilution of combustible gases by a small amount of hydrochloride. 01 remains on a low level - just 17 - 19. But if decomposition occurs in condensed form, 01 becomes rather high - 40 - 45, with substantial change in the composition of enflamed gases. For low-molecule chlorparaffins the amount of carbon and hydrogen in their composition is the same as in the initial substance. However, in the event of decomposition of their high-molecule condensed counterparts, a great amount of coke is formed that is not enveloped by the gas flame. Carbon is removed from it (and it is precisely carbon that serves as fuel). As a result, the ratio of combustible gases to inert hydrochloride undergoes a substantial change. In that case the effect of dilution, but this time of a small amount of combustible gases in a great amount of hydrochloride, is quite significant.
Thus, change in the direction of destruction proves to be fundamental, and this produces change in the ratio of gaseous combustible to incombustible substances.
But it would be wrong to conclude that the entire effect of coke formation is reduced to nothing but reduction of fuel output. The coke "cap" on the polymer surface protecting it from the flame serves as a screen from the heat flow by changing the ratio of the heat consumed by the process of polymer decomposition to the loss of heat in favor of the latter. The following tendency is typical of many hydrocarbon polymers: the greater amount of coke remains during their pyroly-
Values of the oxygen index of the hardened epoxide composition with CCl4 content in free (1) and microcapsulated (2) form.
sis, the less combustible they are. Chemical structure itself determines in many respects the direction of destruction: the more condensed aromatic or heteroaromatic groupings the initial material contains, the greater the coke output is. And the indicator of this output for polymer pyrolysis may be estimated if you know its chemical composition. This approach allows to forecast to a certain degree the level of combustibility of new materials and synthesize with a certain end in view.
Phosphorus and its compounds are among the best inhibitors of the processes of combustion and smoldering of various polymers. So not only low-molecule but also polymer phosphorous antipyrenes are now used for the purpose. These admixtures are marked by a high level of resistance to various external effects and are effective with relatively low phosphorus content.
Of interest is creation of fire-resistant polymer compositions containing ordinary epoxide, polyether and other resins by reinforcing them with "fireproof phosphorous chemical fibers, with their physical and mechanical properties improved and the combustion level reduced owing to coke formation on the surface of the burning polymer.
Oxides and hydroxides of various metals, salts of organic and inorganic acids, chelate complexes* have been broadly used as reliable antipyrenes in the past few years. Their substantial advantage lies in the possibility of their use in lesser concentrations than phosphorus and halogen-containing compounds.
The idea of a coke "cap" to be used as protection for material on its surface from fire was brought to its logical consummation after the introduction of the so-called intumescent coatings. In the process of inflammation they form porous cokefoam whose thickness grows tens of times. It has low heat conductivity and can withstand the heat flow for some time. These coatings represent complicated compositions consisting of binding polymers and a whole range of admixtures for foam formation, required viscosity and rapid carbonization in the process of heating. The development of such compositions requires fine adjustment of many processes both in the sphere of temperature indicators and intensity. And the solution of such a multi-factor problem belongs to the sphere of art and cooking rather than strictly to the field of science.
At the same time a new trend is developing-introduction of antipyrene admixtures in the form of microcapsules into polymer materials. Their coatings are made of polymer, for instance, gelatine, polyvinyl alcohol, and so on, with tens and hundreds of microns in size. Antipyrenes used for these purposes are divided into two groups-with high- and low-boiling temperatures. The
* Chelate complexes-compounds, in which the ligand is attached to the metal's central atom by means of two or a greater number of connections. Used in chemical industry, for instance, for separation of metals close in their properties. - Ed.
former (with the boiling temperature higher than that for microcapsules to burst) includes, for instance, trichlorethylphosphate and trisdibrompropylphosphate. The mechanism of their action and effectiveness is similar to the version where they have been introduced as ordinary admixtures into polymer and intensify the coke-formation process. And the basic effect of micro-capsulation consists in the improvement of antipyrene's compatibility with the polymer, hampering its removal in the form of "sweating" from the material in the process of protracted operation.
A perfectly new method has been discovered for the second group-antipyrenes whose boiling temperature is considerably lower than that required for the capsules to open. The group includes carbon tetrachloride, tetraflu-ordibromethane and other freons-haloidhydrocarbons. In the process of microcapsulation they reduce the level of combustibility of the polymer composition much more effectively than if introduced by the usual procedure. The fact is that the liquid inside the microcapsules 40 to 160 mem in size has been overheated by the time of opening (by 100 - 200° С above the boiling temperature). This stable (metastable) condition of the liquid is attributable to the absence of boiling nucleus. As soon as we have the required temperature, the microcapsule coating starts decomposing, with defects formed on its surface, and these defects act as nucleus of gaseous form. If by that time the liquid has been overheated, the pressure is sharply increased and the microcapsule blows up. And the more overheated it is, the greater is the effect.
A series of such "blows" leads to the dispersion of the polymer matrix: particles of the material separate from the basic mass and are driven away from the flame zone. That is, the organic polymer, which in ordinary conditions is pyrolized under the effect of the flame forming combustible gas products, is driven away as a result of dispersion in the form of solid particles enveloped by a gaseous antipyrene cloud. Moreover, the material containing microcapsulated effective antipyrene, such as, for instance, tetrafluordibromethane, may be not only incombustible but also have a fire-extinguishing effect.
Finally, if we wish to obtain an absolutely incombustible material, we should switch over from organic polymers to inorganic ones, for instance, polyoxides. True, in that case we shall be confronted with formidable problems, such as high temperatures of processing most of the respective substances, brittle and insufficiently plastic inorganic polymers.
Take, for instance, phosphate glass*. Standard methods are used for processing it separately or as mixed with organic polymers-that makes the material less combustible. Next, take boron oxide, used as a basis for inorganic and organic-cuminorganic materials. Its vitrification temperature is comparatively low, and it may be both reduced and increased through modification by various compounds.
Such compositions are used, in particular, as binding agents for reinforced plastics, manufacturing in this way incombustible coal plastics that are heat resistant at temperatures from 175°C to 300°C and are close in strength to their epoxide counterparts. Both boron oxide and boric acid may serve as an initial compound in this case. It is important that the said group of binding agents is marked by excellent adhesion to fibers-on a par with epoxide resins.
Modification of inorganic polymers by linear organic polymers has good prospects for the future, for they are similar to the structure of bones, teeth, tortoise shell and other natural constructions. This sphere, we believe, will prove very fruitful in the nearest future.
In conclusion, let's point out an important aspect. Many methods of inhibiting combustion processes are based on admixtures (antipyrenes) introduced in the material, with chlorine or bromine atoms in their composition, or on the chemical modification of polymers with the admixture of these elements. At the same time, currently it has been established that it is their emission that contributes to the destruction of the Earth's protective ozone layer. That is why the development of haloid-free methods to reduce the level of combustion is among the key problems confronting polymer material science today.
* See: A. Berlin, "Man and Nature", Science in Russia, No. 2, 2005. - Ed.