Investigation Of Effects Of Alum And Potassium Sesquicarbonate On The Fire Characteristics Of Flexible Polyurethane Foam

New technologies , new processes and new applications introduce new fire hazards (e.g. new ignition sources such as welding sparks and short circuits) [4]. Modern fire fighting techniques and equipments have reduced the destruction due to fires. However, a high fuel load in either a residential or a commercial building can offset even the best of building construction [5]. Wood, paper, textiles and synthetic textiles all burn under the right conditions, many burn rigorously and ignite readily. The ability to control or reduce flammability of materials have engaged the mind of scientists. Fire hazards may be reduced by either retarding the fire or initiating a chemical reaction that stops the fire. It has been observed that some of the fire retardant chemicals have adverse effects on the properties of materials on which they are imparted [6]. The choice of suitable polymeric flame retardants is restricted to

species that allow the retention of advantageous properties of the polyurethane.

LITERATURE REVIEW

Flame retardants

Flame retardants are materials that resist or inhibit the spread of fire. They are chemicals added to polymeric materials, both natural and synthetic to enhance flame retardant properties [7]. A fire retardant is a material that is used as a coating on or incorporated into a combustible product to raise the ignition or to reduce the rate of burning of product [8].

Chemicals used as flame retardants can be inorganic, organic, mineral, halogen or phosphorus-containing compounds. In general, fire retardants reduce the flammability of materials by either blocking the fire physically or by initiating a chemical reaction that stops the fire. Flame retardant systems used in synthetic or organic polymers act in five basic ways [7].

Gas dilution:- This involves using additives that produce large volumes of non-combustible gases on decomposition. These gases dilute the oxygen supply to the flame or dilute the fuel concentration below the flammability limit. Examples are metal salts, metal hydroxides and some nitrogen compounds.

Thermal quenching:- This is the result of endothermic decomposition of the flame retardant. Metal hydroxides and metal salts act to decrease the surface temperature and rate of burning.

Protective coating:- Some flame retardants form a protective liquid or char barrier which limits the amount of polymer available to the flame front and also act as an insulating layer to reduce the heat transfer from the flame to the polymer. This includes phosphorus compounds.

Physical dilution:- Inert fillers (glass fibres) and minerals act as thermal sinks to increase the heat capacity of the polymer or reduce its fuel content.

Chemical interaction:- Some flame retardants such as halogens and phosphorus compounds dissociate into radicals species that compete with chain propagating steps in the combustion process.

Flame retardants have faced renewed attention in recent years, aside from various conventional alternatives such as antimony or phosphorus based retardants which have toxicological problems of their own, nanoadditive flame retardants such as carbon nano tubes, nanographites, layered double hydroxides (LDH) have been shown to enhance a number of polymer properties, thermal stability, strength, oxidation resistance, processing, rheology and flammability in polyurethane foams [9].

History of flame retardants [10]

In 450BC, alum was used to reduce the flammability of wood by the Egyptians while the Romans used a mixture of vinegar and alum on wood in about 200BC. In 1638, a mixture of clay and gypsum was used to reduce the flammability of theatre curtains. Alum was also used to reduce the flammability of balloons in 1783.

Gay Lussac reported a mixture of ammonium phosphate, ammonium chloride and borax to be effective on linen and hemp. In 1821 and 1912, Perkins described a flame retardant treatment for cotton using a mixture of sodium stannate and ammonium sulphate [6]. The advent of synthetic polymers earlier this century was of special significance, since the water soluble inorganic salts used up to that time were of little or no utility in hydrophobic materials. Modern developments were therefore concentrated on the development of polymer- compatible flame retardants.

By the out break of the Second World War, flame proof canvas for outdoor use by the military was produced by a treatment with chlorinated paraffins and an insoluble metal oxide, mostly antimony oxide as a glow inhibitor together with a binder resin [11].

After the war, non-cellulosic thermoplastic polymers became more and more important as the basic fibres used for flame retardant applications. In 1971, cotton supplied 78% of the fibres used to produce children's sleepwear whereas in 1973, it supplied less than 10% in the U.S.A [12].

With the increasing use of thermoplastics and thermosets on a large scale for applications in building, transportation, electrical engineering and electronics, new flame retardant systems were developed. They mainly consist of inorganic and organic compounds based on bromine, chlorine, phosphorus, nitrogen, metallic oxides and hydroxides.

Today, these flame retardant systems fulfill the multiple flammability requirements developed for the above mentioned applications.

Types of flame retardants

A distinction is made between reactive and additive flame retardants. Reactive flame retardant are reactive components chemically built into a polymer molecule while additive flame retardants are incorporated into the polymer during polymerization [4, 7].

Reactive - type of flame retardants is preferable because they produce stable and more uniform products, such flame retardants are incorporated into the polymer structure of some plastics. Additive -type of flame retardants, on the other hand, are more versatile and economical. They can be applied as a coating to woods, woven fabrics, and composites or as dispersed additives in bulk materials such as plastics and fibres.

There are three main families of flame-retardant chemicals; [12, 13].

Inorganic flame retardants

Metal hydroxides form the largest class of all flame retardants used commercially today and are employed alone or in combination with other flame retardants to achieve necessary improvements in flame retardancy. Antimony compounds are used as synergistic co-additives in combination with halogen compounds. To a limited extent, compounds of other metals also act as synergists with halogen compounds. They may be used alone but are most commonly used with antimony trioxide to enhance other characteristics such as smoke reduction. Ionic compounds are used as flame retardants for wool or cellulose based products. Inorganic phosphorus compounds are primarily used in polyamides and phenolic resins or as components in intumescent formulations.

Metal hydroxides function in both the condensed and gas phases of a fire by absorbing heat and decomposing to release their water of hydration. This process cools both the polymer and dilutes the flammable gas mixture. The very high concentrations (50 - 80% ) required to impart flame retardancy often adversely affect the mechanical properties of the polymer into which they are incorporated.

Antimony trioxide is used as a synergist. It is utilized in plastics, rubbers, textiles, papers typically, 2 - 10% by weight with organochlorine and organobromine compounds to diminish the flammability of a wide range of plastics and textiles. Antimony oxides and antimonates must be converted to volatile species. This is usually accomplished by release of halogen acids at fire temperatures. The halogen acids react with the antimony containing materials to form trihalides or halide oxides. These materials act both in the substrate (condensed phase) and in the flame to suppress flame propagation. Other antimony compounds include antimony pentoxide available primarily as a stable colloid or as redispersible powder.

Sb2O 3 + 6HCl →      2SbCl3 + 3H2O

Sb2O3 + 2HCl → 2SbOCl + H2O

Within the class of boron compounds by far the most widely used is boric acid. Boric acid (H3BO3) and sodium borate (Na2B4O7. 10H2O) are the two flame retardants with the longest history and are used primarily with cellulose material e.g. cotton and paper. Both products are effective but their use is limited to products for which non durable flame retardancy is accepted since both are very water soluble.

Zinc borate is water insoluble and is mostly used in plastics and rubber products. It is used either as a complete or partial replacement for antimony oxide in PVC, nylon etc., for example,

Sb2O5 + 6NH4BF3 → 6NH3 + 6BF3 + 2SbF3 + 3H2O

Red phosphorus and ammonium polyphosphate (APP) are used in various plastics. Red phosphorus was first investigated in polyurethane foams and found to be very effective as a flame retardant. It is now used particularly for polyamides and phenolic applications. The flame retarding effect is due to the oxidation of elemental phosphorus during the combustion process to phosphoric acid or phosphorus pentoxide [12-13].

Ammonium polyphosphate is mainly applied in intumescent coatings and paints. Intumescent systems puff up to produce foams. Because of these characteristics, they are used to protect materials such as wood and plastics that are combustible and those like steel that lose their strength when exposed to high temperatures.

Halogenated organic flame retardants [14]

These can be divided into three classes; aromatic, aliphatic and cycloaliphatic. Bromine and chlorine compounds are the only halogen compounds having commercial significance as flame retardant chemicals. Fluorine compounds are expensive and are ineffective because the C - F bond is too strong. Iodine compounds although effective are expensive and too unstable to be useful.

Halogenated flame retardants vary in their thermal stability. In general, aromatic brominated flame retardants are more thermally stable than chlorinated aliphatics, which are more thermally stable than brominated aliphatics.

(a) Bromine-based flame retardants are highly brominated organic compound which usually contain 50 - 85% by weight of bromine. The highest volume brominated flame retardant in use today is tetrabromobis – phenol A(TBBPA) followed by decabromodiphenyl ether(DeBDE). Both of these flame retardants are aromatic compounds. TBBPA is used as a reactive intermediate in the production of flame retarded epoxy resins used in printed circuit boards. It is also used as an additive flame retardant in ABS systems. DeBDE is solely used as an additive [15].

(b) Chlorinated paraffins are by far the most widely used aliphatic chlorine-containing flame retardants. They have applications in plastics, fabrics, paints and coatings. Aromatic chlorinated flame retardants are not used for flame retarding polymers.

Organophosphorus flame retardants

One of the principal classes of flame retardant used in plastics and textiles is that of phosphorus, phosphorus - nitrogen and phosphorus - halogen compounds. Phosphate esters with or without halogen are the predominant phosphorus - based flame retardants in use.

Although, many phosphorus derivatives have flame retardant properties, the number of these with commercial importance is limited. Some are additive and some reactive. The major groups of additive organophosphorus compounds are phosphate esters, polyols, phosphonates, etc. The flame retardancy of cellulosic products can be improved through the application of phosphonium salt. The flame retardant treatments attained by phosphorylation of cellulose in the presence of a nitrogen compound are also of importance.

Halogenated phosphorus flame retardants combine the flame retardant properties of both the halogen and the phosphorus group [13]. In addition the halogens reduce the vapour pressure and water solubility of the flame retardant, thereby contributing to the retention of the flame retardant in the polymer. One of the largest selling members of this group, tris (1-chloro-2-propyl) phosphate (TCPP) is used in polyurethane foam.

(a) Nitrogen-based compounds can be employed in flame- retardant systems or form part of intumescent flame retardant formulations [16]. Nitrogen based flame retardants are used primarily in nitrogen-containing polymers such as polyurethanes and polyamides. They are also utilized in PVC and polyolefins and in the formulation of intumescent paint systems.

Melamine, melamine cyanurate, other melamine salts and guanidine compounds are currently the most used group of nitrogen-containing flame retardants. Melamine is used as a flame retardant additive for polypropylene and polyethylene. Melamine cyanurate is used in polyamides and terepthalates.

1.4 Mechanism of action of flame retardants

To understand flame retardants; it is necessary to understand fire. Fire is a gas-phase reaction. Thus, in order for a substance to burn, it must become a gas.

Natural and synthetic polymers can ignite on exposure to heat. Ignition occurs either spontaneously or results from an external source such as a spark or flame. If the heat evolved by the flame is sufficient to keep the decomposition rate of the polymer above that required to maintain the evolved combustibles within the flammability limits, then a self sustaining combustion cycle will be established [17-19].