Generally speaking, a fabric is deemed to be flame retardantif it does not Ignite and create a self-sustaining flame when subjected to aheat source. Therefore a pile fabric made from rayon fibers is considered to behighly flammable while a tightly woven fabric made from high twist yarns easilypasses the 45 degree ignition test. Neither however pass the vertical ignitiontest. Thermoplastic fibers present another anomaly - certain fabrics do notignite when tested by the vertical test, the fabric melts and shrinks away fromthe heat source. Other fabrics made from the same fiber ignite and fail thetest because the fabric construction prevents the rapid withdrawal of the meltfrom the flame. And finally, there are some fibers which will not ignite atall, they will however char.

I.       Theory ofCombustion

When solid materials are heated, physical and chemicalchanges occur at specific temperatures depending on the chemical make-up of thesolid. Thermoplastic polymers soften at the glass transition temperature (Tg),and subsequently melt at Tm. At some higher temperature (Tp), boththermoplastic and non-thermoplastic solids will chemically decompose (pyrolyze)into lower molecular weight fragments. Chemical changes begin at Tp andcontinue through the temperature at which combustion occurs (Tc). These fourtemperatures are very important when considering the flame resistance offibers. Another important factor in combustion is the Limiting Oxygen Index(LOI). This is the amount of oxygen in the fuel mix needed to supportcombustion. The higher the number, the more difficult it is for combustion tooccur

II.      Combustion Of Cellulose

When cellulose fibers are heated, three classes of volatilechemicals are generated at pyrolysis temperature, 3500 C, 1. Flammable volatiles,e.g., alcohols, aldehydes and alkanes, 2. flammable gases, e.g., carbonmonoxide, ethylene and methane and 3. non-flammable gases, e.g., carbon dioxideand water vapor. If oxygen is present when the pyrolysis products reach orexceed the combustion temperature, oxidation takes place (burning) and thevolatiles are converted to carbon dioxide and water as shown in figure 64.

III. Flame Retardancy

Feedback Mechanism

Figure 66 shows the combustion process as a feedback system which may be interrupted at various points to create flame retardancy. Thus to be effective, a flame retardant must interfere with the feedback mechanism in one or more of the following ways:

(a). Removal of heat.

(b). Increase decomposition temperature Tp at which significant volatiles form.

(c). Decrease the amount of combustible gases and promote char formation.

This should happen at reduced temperatures so ignition will not occur.

(d). Prevent the access of oxygen to the flame or dilute the fuel gases in the flame to concentration below which they will not support combustion.

(e). Increase the combustion temperature, Tc, of the fuels and/or interfere with their flame chemistry.

Rarely do flame retardants function by a single mode. Today it is more common to refer to their retardant activity as either functioning in the condensed phase (modes (a), (b) and (c)), the vapor phase (modes (d) and (e), or both. Water, either from a fire hose or water from a hydrated salt will extinguish a flame by mode (a). Nomex and Kevlar function by mode (b). The structures of these polymers are such that Tp has been significantly increased. Most flame retardants for cellulose fibers function by mode (c), promoting the formation of char and reducing the amount of levoglucosan produced by pyrolysis. Some phosphorous and borate flame retardant are thought to form glassy polymers on the surface of the fibers, insulating the polymer from heat (modes (a) and (d)).

Some of the variables that must be identified in order to characterize the burning behavior of a textile material are listed in table 24. These factors are influenced by the nature of the fiber, yarn and fabric structure, density and dimensions, presence of moisture, dyes, finishes and impurities, environmental factors, e.g., temperature, humidity, oxygen availability and air velocity. Test procedures usually standardize environmental factors so the other factors may be evaluated.

Parameters that Characterize Burning Textiles

Char Formation

Since char is a major factor in retarding the burning of cellulosic fibers, it is beneficial for the reader to briefly look at the mechanism of its formation in order to appreciate and understand the significance of the materials that work well as flame retardants. Overall, the formation proceeds by the total dehydration of cellulose into carbon and water.

How Certain Elements Work

There are many compounds reported in the literature that function as flame retardant finishes for specific fibers. Most all of these compounds have a few elements in common that provide the necessary protection - namely boron, phosphorous, nitrogen and halogens. Before delving into the specific flame retardant compounds, it would be instructive to discuss how these elements work.

1. Boron

Boric acid (H3BO3) and borax (Na2B4O7) are often used as non-durable flame retardants in applications such as cellulose batting and shredded newspaper for insulation. Boron functions in the condensed phase as a lewis acid and as mentioned earlier, coats the fiber with a glassy polymer to insulate the polymer.

2. Phosphorus and Nitrogen

Phosphorus and nitrogen also work in the condensed phase. Phosphorus compounds react with the C (6) hydroxyl of the anhydroglucose unit blocking the formation of levoglucosan. This reduces the amount of fuel to the flame. Additionally, phosphorous promotes char formation. The acidity associated with certain phosphorous analogues and its electrophilic nature lowers the activation energy for dehydrating cellulose. Additionally there is the possibility of crosslinking cellulose chains which further enhances char formation.

Nitrogen alone is not an effective flame retardant, however it acts synergistically with phosphorous. It is thought that nitrogen reacts with phosphorous to form polymeric species containing P-N bonds. Nitrogen enhances the electrophilicity of phosphorous thereby making it a stronger Lewis acid and also promoting the phosphorylation reaction with the C (6) hydroxyl of the anhydroglucose ring. This mechanism may be written as follows:

3. Halogens

Chlorine and bromine operate in the vapor phase by forming free radicals that scavenge hydrogen and hydroxyl free radicals. Combustion occurs by a free radical, chain reaction mechanism of which hydrogen and hydroxyl radicals are major reaction species. The halogen radicals deactivate them, causing the chain reaction to brea k down. Antimony and phosphorus enhances the efficiency of the halogen radicals. Phosphorus effect is additive while antimony is synergistic. The optimum ratio of Sb:X is 1:3. This suggests that SbX3 is an important intermediate in this process. The important gas phase reactions in combustion are:

Species that remove H and or HO will slow the combustion reaction. Halogen do this in the following manner:

Flame Retardant Chemicals and Processes For Cellulose

Durable and non-durable finishes may be used to render cotton, rayon or other cellulosic fibers flame retardant. There are many applications where non-durable flame retardants are adequate, for example, on drapery and upholstery fabrics that will not be laundered. Should the products need cleaning, the finish can be reapplied afterwards. However, there are applications where durability is important, e.g. firefighter suits, foundry worker clothing, children sleepwear.

A. Non-Durable

1. Boric Acid/Borax

A mixture of boric acid/borax (sodium borate) is a commonly used non-durable flame retardant finish for cellulosic fibers. It is the safest with regard to carbon monoxide and smoke production during burning.

2. Diammonium Phosphate and Phosphoric Acid

Phosphorus based flame retardants function in the condensed phase. Nondurable, semi durable and durable treatments can be obtained with phosphorus based compounds. The presence of calcium ions negates the activity of phosphorous compounds. Whereas the ammonium salts decompose thermally into phosphoric acid by the loss of ammonia, the calcium salts do not. Presumably the calcium salts are not volatile and buffer the acidity of phosphoric acid so the generation of char is diminished.

3. Sulfamic Acid and Ammonium Sulfamate

Combinations of these compounds also function as non-durable flame retardants.

B. Durable

1. Tetrakis(hydroxymethyl)Phosphonium Derivatives

The bulk of today's durable flame retardant for cellulose centers around the use of derivatives of tetrakis(hydroxymethyl)- phosphonium salts (THP). These derivatives can be applied by padding, drying, curing and oxidizing to yield serviceable flame retardant fabrics. Add-ons are high and the handle of the fabric is stiffer so the finish is normally used for protective clothing applications.

a. Tetrakis (hydroxymethyl) phosphonium Chloride (THPC)

THPC is the most important commercial derivative and is prepared from phosphine, formaldehyde and hydrochloric acid at room temperature. It contains 11.5 % phosphorous and is applied by a pad-dry-cure -> oxidize -> scour process.

The compound is highly reducing in character and the methylol groups condense with amines to form insoluble polymers. It is applied with urea, dried and cured. Control of pH and the oxidation state of the phosphorus is important in determining the flame retardant properties and the durability of the finish. The release of HC1 may cause the fabric to tender during curing unless pH is controlled. The final step in finishing requires oxidation of P+3 to P+5 with hydrogen peroxide. This step too must be controlled to prevent excessive tendering of the fabric. An alternative to the THPC is THPS. Sulfuric acid is used instead of HC1 and the corresponding phosphine sulfate is formed in place of the phosphine chloride.

b. THPC-Urea Precondensate

The Proban process (Albright and Wilson) replaces heat curing with an ammonia gas curing at ambient temperature. This minimizes fabric tendering associated with heat and acids. A Precondensate of THPC with urea (1: 1 mole ratio) is applied, dried and the fabric passed through an ammonia gas reactor. An exothermic reaction creates a polymeric structure within the voids of the cotton fiber. The ammonia cure gives a P:N ratio of 12. Weight percentages of the respective elements should be P,N > 2%. To enhance durability and light fastness of dyes, P+3 is oxidized to P+5 with hydrogen peroxide.

c. Tetrakis (hydroxymethyl) phosphonium Hydroxide (THPOH)

From the forgoing discussion, THPC is usually partly neutralized with amines, amides and/or alkali. Complete neutralization of THPC with sodium hydroxide yields a compound referred to as THPOH. The distinction between THPC used in a partially neutralized condition and THPOH is difficult to define. If the curing agent is basic as is ammonia, the distinction become meaningless.

THPOH-ammonia has received a great deal of commercial attention. The major advantage over THPC is reduced fabric tendering and reduced stiffness. Fabrics padded with THPOH give off formaldehyde during drying.

2. N-Methyloldimethyl Phosphonopropioamide (PYROVATEX CP)

Pyrovatex CP provides a method of attaching phosphorus to cellulose making use of N-methylol reactivity with cellulose. It is applied with a methylolated melamine resin using a phosphoric acid catalyst by a pad-dry-cure process. The high nitrogen content of melamine provides synergistic activity to the phosphorus of the flame retardant.

Fabric stiffening occurs when sufficient chemical is applied to give 2-3% phosphorus on weight of fabric. Also the acid may cause high strength loss if left in the fabric after curing; therefore, it is desirable to wash the fabric following the curing step. The finish tends to produce smoke in the curing oven. The smoke is composed of volatile fragments of the finish which condense in the cooler reaches of the oven. The condensate may drip back onto the fabric causing unsightly spots.

3. Fyrol 76

Fyrol 76 is an oligermeric phosphonate containing vinyl groups. The finish is applied with N-methylol acrylamide with a free radical initiator (potassium persulfate), dried and cured.

This product exhibits better abrasion resistance than THPOH-NH3.

4. Phosphonic and Phosphoric Acid Derivatives

The literature is rich with references showing many imaginative ways of introducing phosphorus and nitrogen into cellulose fibers. Many products have been offered by chemical companies which have not succeeded as commercial ventures. It is beyond the scope of this book to completely review the full range of flame retardants, the reader is urged to consult other literature readings for a more thorough understanding (1, 2, 4). Cellulose phosphorylates with phosphoric and phosphonic acids. Urea, dicyandiamide and cyanamide are used to buffer the tendering action of the acids.

Whenever levels of phosphorus attached are high enough, flame retardancy protection is good. Cellulose phosphate esters are hydrolytically unstable so durability to laundering is poor. The phosphonate esters are more durable however. The phosphates tend to chelate calcium ions when laundered in hard water. This reduces the flame retardancy of the finish as discussed earlier in the chapter. The phosphonates are less prone to do this.

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