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

routes of exposure for the general population include the dermal route (contact with flame- retarded textiles), inhalation and ingestion.

1.11.1 Environmental exposure [26, 29 - 30]

Environmental exposure may occur as a result of the manufacture, transport, use or waste disposal of flame retardants. Routes of environmental exposure are water, air and soil. Factors affecting exposure include the physical and chemical properties of the product, emission controls, disposal/recycling methods volume and biodegradability. Environmental monitoring helps to determine the extent of environmental exposure [31].

Most flame-retarded products eventually become waste. Municipal waste is generally disposed of via incineration or landfill. Incineration of flame retarded products can produce various toxic compounds, including halogenated dioxins and furans. The formation of such compounds and their subsequent release to the environment is a function of the operating conditions of the incineration plant and plant's emission controls [32].

There is a possibility of flame retardants leaching from products disposed of in landfills. However, potential risks arising from landfill processes are also dependent on local management of the whole landfill. Some products such as plastics containing flame retardants are suitable for recycling [33].

Polyurethane foam polymer

A Polyurethane commonly abbreviated PU is any polymer consisting of a chain of organic units joined by urethane links. Polyurethane foams can also be defined as plastic materials in which a proportion of solid phase is replaced by gas in the form of numerous small bubbles (cells) [34]. The gas may be in a continuous phase to give an open – cell material or it may be discontinuous to give non-communicating cells. Low density foams are dispersions of relatively large volumes of gas in relatively small volumes of solids having for example, a density less than 0.1 gcm-3. Medium foams are classified as having density of 0.1 to 0.4gcm-3. High density foams; therefore have a density higher than 0.4gcm-3 i.e. contain small volume of gas in the matrix [35]. Polyurethanes are based on the exothermic reaction of polyisocyanates and polyol molecules [36]. Many different kinds of polyurethane materials are produced from a few types of isocyanates and a range of polyols with different functionality and molecular weights.

History of polyurethane foam polymer

The pioneering work on polyurethane polymers was conducted by Otto Bayer and his co workers in 1937 at the laboratories of I.G Farben in Leverkusen Germany [37]. They recognized that using the polyaddition principle to produce polyurethanes from liquid diisocyanates and liquid polyether or polyester seemed to point to special opportunities especially when compared to already existing plastics that were made by polymerizing olefins or by poly condensation. The new monomer combination also circumvented existing patents obtained by Wallace Carothers on polyesters [24]. Initially, work focused on the production of fibers and flexible foams with development constrained by World War II (when PU's were applied on a limited scale as air crafting coating). It was not until 1952 that polyisocyanates became commercially available.

In 1954, commercial production of flexible polyurethane foam began based on toluene diisocyanate and polyester polyols. The first commercially available polyether polyol was introduced by Dufont in 1956 by polymerizing tetrahydrofuran. In 1960, more than 45,000 tons of flexible polyurethane foams were produced. As the decades progressed the availability of chlorofluoroalkane blowing agents, inexpensive polyether polyols and methylene diphenyl diisocyanate (MDI) heralded the development and use of polyurethane rigid foam as high performance insulation materials. Urethane modified polyisocyanurate rigid foams were introduced in 1967 offering even better stability and flammability resistance to low density insulation products. Also, during the 1960s, automotive interior safety components such as door panels were produced by back filling thermoplastic skins with semi-rigid foam.

In 1969, Bayer A.G exhibited an all plastic car in Dusseldorf, Germany. Parts of this car were manufactured using a new process called RIM (Reaction Injection Moulding) [36]. Polyurethane RIM evolved into a number of different products and processes. In 1980s, water blown micro cellular flexible foam was used to mould gaskets for panel and radial seal air filters in the automotive industry. Building on existing polyurethane spray coating technology, extensive development of two component polyurea spray elastomers took place in the 1990s.

During the same period, two new components polyurethane and hybrid polyurethane polyurea elastomer technology were used to enter the market place of spray- in­place load bed liners [38-39]. This technique creates a durable, abrasion resistant composite with the metal substrate and eliminates corrosion and brittleness associated with drop in thermoplastic bed liners. The use of polyols derived from vegetable oils to make polyurethane products began gaining attention beginning around 2004, partly due to rising cost of petrochemical feedstocks and partially due to an enhanced public desire for environmentally friendly green products [40].

Basic chemical of polyurethane foam [41]

Polyurethanes belong to the class of compounds called reaction polymers which include epoxies, unsaturated polyesters and phenolics [38-39]. A urethane linkage is produced by reacting an isocyanate group – N = C = O with a hydroxyl (alcohol group) – OH.

                                                                H O

R1 – N= C = O + R2 – O - H → R1 – N – C – O – R2

Although, polyurethane synthesis can be effected by reaction of chloroformic ester with diamines and of carbamic esters with diols.

RNH2 + ClCOOR' → ROH + ZOOCNHR1→(i)

RNHCOOR' - + HCl - ROOCNHR1 - + ZOH→(ii)

[ O (CH2)4 OOCNH (CH2)6 NH COO ]

The NCO group can react generally with compounds containing active hydrogen atoms i.e. according to the following:

RNCO + R'OH → RNHCOOR urethane---(iii)
RNCO + R'NH2 → RNHCONHR urea----(iv)
RNCO + R' COOH → RNHCOR'+CO2 Amide---(v)

       RNCO + H2O

        RNCO→ [RNHCOOH] → RNH2 + CO2

                          RNHCONHRUrea - - - (vi)

Thus, if the reagents are di or polyfunctional polymer, formation can take place while these reactions normally occur at different rates, they can be influenced appreciably and controlled by the use of catalysts. Reactions (v) and (vi) give rise to carbon (iv) oxide, a feature of value when forming foamed products but introducing difficulty if bubble - free castings and continuous surface coatings are required.

Linear products result if the reactants are bifunctional but higher functionality leads to the formation of branched chain or cross linked material. Chain branching or cross linking then occurs due to the formation of acylurea, biuret and allophanate links onto the main chain.

- RNCO + R'NHCOR' → R'NCOR' Acylurea

                                       CONHR -

- RNCO + R'NHCONHR' d   R' – N - CONHR – Biuret

                                                       CONHR –