The reasons for these unusual structural effects are not obvious from the calorimetric data alone; but these results have been interpreted in terms of an interaction between the phosphorus and nitrogen-containing groups within the same molecule. This happens prior to interaction with the cellulose so that the species which reacts with the cellulose is most probably a phosphoramide rather than a phosphonate.

4. CONCLUSIONS

It has been shown that considerable insight into the mechanism of flame retardant action on textiles can be obtained by a combination of oxygen index testing using two or more oxidants, thermal analysis, and static oxygen bomb calorimetry. The first two methods, when used together, allow a determination of the site of activity of the flame retardants. The calorimetry gives a more detailed picture of the type of interaction involved and allows, in many cases, a quantitative estimation of the efficiency of this interaction.

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NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 411, Fire Safety Research, Proceedings of a Symposium Held at NBS, Gaithersburg, Md., August 22, 1973, (Issued November 1974)

ADDITIONAL STUDIES OF THE TRANSFER OF FLAME

RETARDANT EFFECTS WITH CELLULOSIC FABRICS

Bernard Miller

Textile Research Institute, Princeton, New Jersey

Burning rate measurements on double layers of the same fabric
when one layer has been treated with a flame retardant have indi-
cated that certain effects of the retardant can be transferred to
the untreated layer. To learn more about the mode and chemistry
of this phenomenon, a study of non-flaming combustion of cellulosics
has been carried out on mixed systems using thermogravimetric analy-
sis. By arranging to have untreated cotton physically separated
from the flame retardant material during heating it was possible
to determine that the transfer depends on a chemical process and is
most likely the effect of a volatile product generated during
heating. Data are presented also showing that rayon containing an
alkoxy-phosphazene flame retardant does not transfer its flamma-
bility properties to untreated rayon.

Key words: Cellulosics; cotton; DAP; fabric flammability; flame
retardants; flammability; rayon; thermogravimetric analysis.

The customs of usage of textile materials make it highly probable that many fabrics will be used, either deliberately or inadvertently, as components of multilayer assemblies. When such structures have layers made up of different materials, it is necessary for us to know whether or not the flammability behavior of the combinations would be predictable from knowledge of the behavior of the individual fabrics involved. A large amount of experimental data is needed to answer this question; much of it is still to be obtained. We have been studying one aspect of the problem: the behavior of double layers of a single material when one of the layers has been treated with a flame retardant. In a previous publication [1] it was shown that the effect of a flame retardant, in terms of altering the burning rate, could be transferred to an adjacent untreated fabric. This was found to be the case for cotton treated with diammonium phosphate (DAP), for nylon treated with thiourea, and for polyester treated with tris (2,3-dibromopropyl) phosphate (T23P). With cotton plus DAP, transfer was most evident when the treated layer was placed underneath the untreated cotton (during horizontal burning). This has led to the conjecture that the transfer effect is the result of the crossover of a volatile product formed during the heating of the treated fabric (that is, physical contact between the layers may not be a necessary condition for this phenomenon).

It was thought that useful information relevant to this and other points might be obtained if potential transfer situations could be studied without the occurrence of flaming combustion. Consequently, an experimental program was devised to study the pyrolysis behavior of such systems with controlled degrees of contact between components.

2. PYROLYSIS WEIGHT LOSS STUDIES

Weight loss studies on heating in air were carried out with the aid of a thermogravimetric analyzer (TGA) which used a platinum cup as a macro sample holder (fig. 1). This holder, considerably larger and deeper than the sample holders of most commercial TGA units, allowed the stacking of materials one above the other. One material could be placed in the bottom of the cup below a removable metal mesh screen which served to support a piece of fabric as the upper component. It was thus possible to obtain weight loss data under programmed heating for a variety of stacked combinations.

This technique has been extremely useful in studying many types of flame retardants on cellulosic substrates. It is, however, not limited to such substrates. Triphenylphosphine oxide has been studied as a flame retardant for polyester. [5] The results, as shown in figure 11, indicate that there is considerable vapor phase activity in this system. Differential thermal analysis and thermogravimetric analysis show no differences in the decomposition of the polymer when the phosphine oxide is present (fig. 12). It is therefore reasonable to assume that there is no condensed phase activity exerted by triphenylphosphine oxide on polyester.

Triphenylphosphine oxide has also been investigated as a flame retardant for nylon-6. [6] The oxygen index results shown in figure 13 indicate that the triphenylphosphine oxide acts in the vapor phase in this system also. Similar studies of samples containing varying amounts of triphenylphosphine, triphenylphosphite and triphenylphosphate are shown in figures 14-16. cases the dependence of the flame retardant efficiency on the chemical nature of the oxidant indicates considerable vapor phase activity. As with the polyesters, thermal analysis of the treated nylons shows no significant alteration in the decomposition pattern when the flame retardants are present. On this basis it can be concluded that these types of thermally stable phosphorus compounds act almost completely in the vapor phase. This is, of course, of considerable importance in these systems since it indicates that they will produce no alteration in the melt-drip characteristics of the thermoplastics. It also has significance in terms of smoke and carbon monoxide evolution since all of these vapor phase flame retardants act to inhibit oxidation and should, therefore, increase the amount of incomplete combustion products present in the off-gases from the burning fabrics.