31 Flame retardant and flame proof finishes

N Vasugi Raaja

Introduction

The desire for textiles having a reduced tendency to ignite and burn has been recognized for considerable time during man’s recorded history. The use of asbestos as a flame resistant material was recorded in Roman times. Wyld was issued apatent in 1735 describing a finishing treatment for cellulosic textiles based on alum, ferrous sulfate and borax. In 1882, Gay-Lussac published the first systematic study of the use of flame retardants. Before delving into the chemistry of flame retardant finishes, it is necessary to first understand that there is no single test that determines whether a fabric is flame retardant or not. The test conditions used to make this determination will reflect on the specific fabric and its intended end-use. For example, all apparel fabrics are expected to pass a 45 degree flame test; however, those destined to be used as children sleep-wear are expected to pass a vertical ignition test. Drapery, upholstery and carpets all have their own ignition tests designed to evaluate how the article performs in its environment. Flame retardant tests and fabric performance specifications are discussed in detail in other references, and the reader is urged to consult these for a more in-depth understanding

THEORY OF COMBUSTION

When solid materials are heated, physical and chemical changes occur at specific temperatures depending on the chemical make-up of the solid. Thermoplastic polymers soften at the glass transition temperature (Tg), and subsequently melt at Tm. At some higher temperature (Tp), both thermoplastic and non-thermoplastic solids will chemically decompose (pyrolyze) into lower molecular weight fragments.

Chemical changes begin at Tp and continue through the temperature at which combustion occurs (Tc). These four temperatures are very important when considering the flame resistance of fibers. Another important factor in combustion is the Limiting Oxygen Index (LOI). This is the amount of oxygen in the fuel mix needed to support combustion. The higher the number, the more difficult it is for combustion to occur.

Flammability Parameters for Fibers

2. Flame-retardant strategies

Figure 2.1 presents the combustion of any textile as a feedback mechanism in which fuel (from thermally degraded or pyrolysed fibres), heat (from ignition and combustion) and oxygen (from the air) feature as the main components. In order to interrupt the mechanism, five modes, (1) to

(5), are proposed and flame-retardants may exert one or more of these. Each stage with a relevant flame-retardant action is listed below:

1. Removal of heat: high heat of fusion and/or degradation and/or dehydration (for example, inorganic and organic phosphorus-containing agents, aluminum hydroxide or ‘alumina trihydrate’ in back-coatings);
2. Enhancement of decomposition temperature: not usually exploited by flame-retardants; more usual in inherently flame- and heat-resistant fibres (for example, aramids);
3. Decreased formation of flammable volatiles, increase in char: most phosphorus- andnitrogen-containing flame-retardants in cellulose and wool; heavy metal complexes in wool;
4. Decreased access to oxygen or flame dilution: hydrated and some char-promoting retardants release water; halogen-containing retardants release hydrogen halide;
5. Interference with flame chemistry and/or increase in fuel ignition temperature (Tc):halogen-containing flame-retardants, often in combination with antimony oxides.

Figure 2.1Combustion as a feedback mechanism

From the above, it can be seen that some generic flame-retardants function in more than one mode and this is true of the most effective examples. Some flame-retardant formulations, in addition, produce liquid phase intermediates, which wet the fibre surfaces thereby acting as both thermal and oxygen barriers – the well-established borate/boric acid mixtures act in this manner as well as promoting char. In order to simplify the clarification of different modes of chemical flame-retardant behaviour, the terms ‘condensed’ and ‘gas or vapour’ phase activities may be used to distinguish them. Both are composite terms and the former will include modes (1) to (3) above and the latter (4) and (5). Physical mechanisms often operate simultaneously, and these include exclusion of oxygen and/or heat by coating (mode (4)), increased heat capacity (mode (1)) and dilution or blanketing of the flame by non-flammable gases(mode (4)).

Thermoplasticity

Whether or not a fibre softens and/or melts (as defined by physical transitions in Table 6.3) determines whether it is thermoplastic or not. Thermo plasticity can influence considerably how a flame-retardant behaves because of the associated physical change. Conventional thermoplastic fibres like polyamide, polyester and polypropylene will shrink away from an ignition flame and avoid ignition – this can give the appearance of flame retardancy when infact, if the shrinkage was prevented, they would burn intensely. This so-called scaffolding effect is seen in cotton/polyester and similar blends where the molten polymer melts on to the non thermoplastic cotton and ignites. Similar effects are seen in composite textiles comprising thermoplastic and non-thermoplastic components.

Added to the above is the problem of molten and often flaming drips of polymer, which, while removing heat from a flame front and encouraging flame extinction (and hence achieving a ‘pass’ in vertical flame tests), can lead to burns or secondary ignition of lower surfaces (for example, carpets or human skin).

Most flame-retardants applied to conventional synthetic fibres during manufacture or as finishes function by increasing melt dripping and/or promoting extinction of flaming droplets. None to date decrease the fibre thermo plasticity and/or promote significant char formation as is the case in flame-retarded viscose fibres.

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 intothe 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 cross linking cellulose chains which further enhances char formation.

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 break down. Antimony and phosphorus enhances the efficiency of the halogenradicals. 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:

IV. FLAME RETARDANT CHEMICALSAND 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 productsneed 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.

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+3is oxidized to P+5 with hydrogen peroxide.

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.

RETARDANT SYSTEMS FOR SPECIFIC FIBERS

A. Rayon Additives

Flame retardant agents have been incorporated into rayon fibers during thefiber extrusion process. Listed below are several that have been used.

1.Thiophosphate (SANDOFLAM 5060)

2.Decabromodiphenyl Oxide (DBDPO) and Antimony Oxide

3.Phosphazines

B. Polyester

Polyester fabrics, being thermoplastic, will melt and shrink away from the heat source allowing some fabrics to pass particular tests without any treatment. Certain tests however, have a melt drip specification to meet so finishes will be needed. The flame retardants that work best for polyester are halogenated compounds that function in the vapor phase. One of the best products to serve this purpose was tris -(2,3 dibromopropyl) phosphonate. (TRIS). The product offered good flame retardancy, acceptable fabric hand and good durability to repeated laundering. It passed thechildren sleepwear requirements. It could be exhausted from a dye bath or applied via the pad-dry-thermosol method. However TRIS is banned from commercial use because it failed the Ames mutagenicity test. It is on the regulated suspect carcinogen list. This finding came after the product had been on the market several years causing several million yards of treated fabric to be recalled by the Consumer Protection Agency (CPSC). The fabric producer was stuck with this inventory.

C. Nylon

• Most nylon fabrics pass flammability standards because the polymer burns at very slow rate.
• However several finishes will enhance the fiber’s response to flammability tests.

    a. Thiourea-Urea-Melamine

This finish is applied by pad-dry-cure. It functions by lowering the melting point of nylon 40 0C.

 b. Halogenated Systems

Halogenated compounds such as DBDPO and chlorinated paraffins combined with Sb2O3 are effective.

FLAMMABILITY TESTS

Determining the flammability potential of a fabric requires an understanding of a number of different tests because flammability test and specifications vary with the end-use of the textile article. Some tests have been developed as research tools to quantify the retardancy value of finishes and fibers, LOI is one that is used often. Other tests have been developed to assess the flammability hazard of fabrics. These emulate actual in-service conditions that the textile is liable to encounter. Some of the variables are: 1. the way the heat source is presented as it is being ignited, i.e., vertical, horizontal or 45 degree angle, 2. temperature of the heat source, 3. Char length, after flame and after glow and melt drip are some of the specifications of the specific test. Horrocks (1,2) presents a thorough review of the tests.

A sampling of the more important fabric flammability tests are listed in table16. The reader is urged to consult the cited references for more in-depth information.

Some Important Fabric Flammability Tests

References

1. www.fibre2fashion.com
2. textilescommittee.nic.in
3. https://www.textilemates.com
4. www.technicaltextile.net/
5. Handbook of Fire Resistant Textiles, 1st Edition, F. Selcen Kilinc
6. Flame Retardants for Plastics and Textiles 2E: Practical Applications 2nd Edition, Edward D. Weil