We also found some statistical evidence for different flame-retardant contributions of fast char farmers and slow char farmers in a study of flame retardancy of styrenics Weil, b. We think that it will be found productive to add fast char farmers even to those engineering thermoplastics that provide, by themselves, a good char yield.
We find fast char farmers to include certain novolacs. Some other plausible examples are the special novolac and "cardo" molecules described by Polytechnic University researchers tin and Pearce, , 1a, b; Lin et al. Introduction of Reactive Cross-Linking Additives Even if the matrix polymer itself can char, the rate of polymer chain-polymer chain reactions leading to cross-linking may be relatively slow Arrhenius pre-exponential term may be small compared to the rate of reaction of some smaller molecule capable of causing cross- linking. Proof of concept was done years ago at SRI Brauman, b using xylylene dichloride plus latent Friedel-Crafts catalyst in a styrenic polymer a poorly charrable polymer that was thus induced to char.
Later industrial researchers used less toxic bifunctional benzylating agents Clubley et al. This approach may be somewhat polymer-specific. However, most or all of the high- performance engineering thermoplastics have aromatic groups in their backbone, and in many instances they have rather electron-rich aromatic groups bearing oxygen or nitrogen sub stituents. Thus, polyfunctional allylating agents or other types of polyfunctional electrophilic reagents should be able to cross-link these chains.
Finding suitable additives of this type is a challenge, because they must not be reactive at processing temperatures but must enter into reaction quickly under fire-exposure temperatures.
Flame Retardant Polymeric Materials: A Handbook
The delayed action could come from either the reagent or the catalyst. Some interesting hints and clues may be found in the literature pointing to possible polyfunctional allylating agents. It is tempting to consider that polybutadiene may have been allylating the styrenic component. By itself, under these same conditions, polybutadiene underwent reactions suggesting cationic mechanisms.
The efficacy of a noncombustible noncarbon barrier layer was shown some years ago Ellard, In this study of intumescent coatings, an advantageous protective effect was obtained when a refractory "char" component remained even if much of the carbon content from the polymer matrix was burned away. Glassy antimonate or phosphate barrier materials were shown to be effective.
TiO2 and mica were also effective. These refractory barriers also had high infrared reflectance. Although this study was directed to coatings, it should be applicable to plastics. GE has done extensive work in including siloxane units in polymers with resultant formation of improved fire-barrier material.
An excellent basic study by Kambour shows that this is a subtle effect with a distinct maximum of flame-retardant performance versus composition. In one study, the peak of flame retardancy coincided with a peals of char strength Kambour et al. It is significant that some recent research at NIST showed that in polycarbonate modified by silicone, while the rate of heat release was greatly reduced, the piloted ignition time was shortened and the flame-spread rate was increased. Apparently, the intumesced char did not form fast enough Kashiwagi et al. A recent example, which showed a very impressive reduction of rate of heat release by levels as low as 1 percent of a silicone additive in polystyrene, was presented by Dow Corning researchers Page et al.
Physical Reinforcement of the Barrier As char or any other barrier material forms, it can also build up stresses from shrinkage or stretching and from gases attempting to break out, and it can become cracked or porous. It has been shown Gibov et al. It has also been shown that some additives can aggravate this situation by causing perforations in the char; other additives can give more coherent and smooth char Scharf, To prevent this failure of the barrier, an approach known in the art of fire-retardant coatings is to use "bridging" additives Anderson et al.
We have found that in thermoplastics, small amounts of high-aspect materials such as mica or woliastonite can help, even in the range of a few percent. We have also seen evidence of synergism between certain high-aspect mineral additives, which we attribute to particularly effective bridging of the char.
We have also noted a small amount of evidence that coupling agents may aid these mineral additives in binding chars Well, , although some of the cited effects may have been from improved dispersion. There are even some indications that flame retardancy may peak. There is one Russian report that an elastomer containing an organophilicized clay gave the best flame retardancy and the optimum tensile strength at the same critical loading of surface-modifying agent Kireenkova and Zuev, Preventing Oxidative Destruction and Smoldering of the Barrier During or after the formation of a char barrier, it can also burn away.
The protection of char from smoldering is known to be possible using phosphorus or boron compounds, but this process has not been studied very systematically, nor under high thermal input conditions in regard to flame-retardant plastics.
As little as 0. A A- r If this effect is to be made use of, optimum boron and phosphorus additives should be sought.
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Boron and phosphorus compounds cover a wide range in volatility, and some chemical ingenuity may be needed to reduce the volatility losses. In our laboratory, we have studied phosphorus compounds from the standpoint of, on the one hand, release to the vapor phase, and on the other hand, retention in and on the char. We consider that especially interesting phosphorus compounds from the standpoint of nonvolatility are the thermally stable phospham, phosphorus nitrides, and oxynitrides Wei! Another approach to preventing the burning away of the char is to produce a ceramic or glassy protective layer.
The protection of char is perhaps the most logical use of a low-melting ceramic- or glass-forming component. This approach has been studied in connection with protection of carbon fibers and carbon-carbon composites. In principle, a very-Iow-melting glass could even offer protection at a pre-char stage.
This approach has a long history. British patent ! The use of low-melting glass-forming salts has been well demonstrated by work done at B.
Polymer Degradation and Stability
Goodrich with glass-forming compositions such as nickel sulfates and ammonium pentaborate Myers and Licursi, ; Myers et al. These low-melting glass formulations provided respectably good flame retardancy to a wide variety of plastics. However, these are water-soluble salts. It happens that the same factors that cause a glass to be low melting usually tend to cause it to be water-soluble.
A number of intriguing proprietary fire protective coatings claimed to work at very high temperatures are probably something of this sort-alkali silicates or berates which, after an organic matrix has burnt away, leave a good hire-protective barrier. A number of such. Well compositions, using fusible inorganic materials added to organic matrices, are described in the patent literature.
One recent British patent application Crompton, manages to name frits, basalt, sodium silicate, ceramic fibers, mullite, etc. The new idea claimed is to use two different fries with different melting ranges. A recent German patent application refers explicitly to reduced-heat-release and low- smoke aircraft cabin materials and attempts to cover oxides and oxygen acid salts of groups IlI-V of the periodic table as additives for all possible aromatic thermoplastics.
The examples seem limited to antimony oxide and zinc borate, but the claims are remarkably broad Buchert et al. Corning has devised a family of phosphate glasses whose melting range overlaps that of the higher-temperature engineering thermoplastics. They are marketing these glass-polymer blends for high-performance, dimensionally very stable. Although these glass- or ceramic-forming compositions can afford a high degree of fire protection under high heat load, they would have some inherent problems from the standpoint of aircraft uses.
They have rather high densities and perhaps greater rigidity than desired. Also, they may not work quickly enough. Nevertheless, it would seem that these glass-polymer compositions should be added to the fire-retardant compounders' Tool kit" anyway. To overcome the density problem and stiffness, the commercial glass-polymer blends can be considered as a masterbatch and let down to some much lower glass concentration, where the glass will not have to carry the full burden of being the flame-retardant component. Such an additive may very well cooperate with the other flame-retardant components and, in the best cases, may synergize.
At the very least, when the formulation has carbonized, the glass should remain to protect the char.
A further approach to low-melting glasses are materials such as amorphous phosphorus oxynitnde Well et al. They can be water resistant Bunker et al. Other clues have been found to low-melting glasses that might be useful in flame retardancy. Low-melting glasses based on modified zinc berates have been described as low-melting fluxes for porcelain glazes Jackson, , but seem not to have been explored in flame retardancy.
Smoke Considerations Through use of polymers that are inherently good char farmers along with additives to further improve the flame retardancy by methods other than flame-zone inhibition, smoke should.
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Combustion toxicity likewise is not expected to be a major problem for the same reason. Reducing the amount of material burnt seems to be the key to low smoke and toxic gas formation. A particular point of view we think should be kept in mind is that visible smoke, as measured by a photocell, may not represent the true hazard in regard to aircraft cabin safely.
Some vapors are perfectly transparent to the photocell, a typical example being acrolein formed from some organics , but they may prevent vision by being extreme eye irritants and thus inhibit exit from an aircraft cabin. A combination of theory, working hypotheses, and statistically planned expenmentacion seems to us to be the most cost-effective way to meet any advanced performance goal.
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It may be sufficient to have one layer that is extraordinarily fire-resistant, provided that the other layers do not defeat this effect.