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

Surfactants [45]

Surfactants are added to the foam formulation to decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic medium. They are used to modify the characteristics of both foam and non foam polyurethane polymers. In foams, they also aid in nucleation, stabilization and regulation of the cell structure. The choice of surfactants depends upon the type of foam preparation.

Both ionic and non ionic surface active agents have been employed. Anionic surfactants have been used for the preparation of polyester and polyether prepolymer foams. Non­ionic surfactants are used in polyester and polyether urethanes. Examples of surfactants are block or graft copolymers, polymethylsiloxanes, polyalkylene oxides etc.

Chain extenders and cross linkers

Chain extenders (f=2) and cross linkers (f=3 or greater) are low molecular weight hydroxyl and amine terminated compounds that play an important role in the polymer morphology of polyurethane fibers, elastomers, adhesives and certain integral skin and micro cellular foams. The elastomeric properties of these materials are derived from the phase separation of the hard and soft copolymers segments of the polymer, such that the urethane hard segment domains serve as cross links between the amorphous polyether (or polyester) soft segment domains. This phase separation occurs because the mainly non-polar, low melting soft segments are incompatible with the polar, high melting hard segments.

The soft segments, which are formed from high molecular weight polyols are mobile and are normally present in coiled formation, while the hard segments which are formed from the isocyanate and chain extenders are stiff and immobile [46].

The choice of chain extender determines flexural, heat and chemical resistance properties. The most important chain extenders are ethylene glycol, 1, 4-butanediol (1, 4 - BDO or BDO) 1, 6 - hexanediol, hydroquinone bis (2-hydroxy ether) ether (HQEE). All of these glycols form polyurethanes that phase separate well and form well defined hard segment domains and are melt processable. They are all suitable for thermoplastic polyurethanes with the exception of ethylene glycol since its derived bis – phenyl urethane undergoes unfavourable degradation at high hard segment levels.

Catalysts

The catalyst most widely used commercially in polyurethane processes are tertiary amines and organotin compounds, catalysts can be classified as to their specificity, balance and relative power on efficiency. Traditional amine catalysts have been tertiary amines such as triethylenediamine (TEDA also known as 1, 4-diazobicyclo [2.2.2] octane or DABCO) and dimethylethanolamine (DMEA).

Tertiary amine catalysts are selected based on whether they drive the urethane (polyol + isocyanate) or gel reaction, the urea (water + isocyanate or blow) reaction or the isocyanate trimerization reaction. Since most tertiary amine catalysts will drive all three reaction to some extent, they are also selected based on how much they favour one reaction over another. Molecular structure gives some clue to the strength and selectivity of the catalyst. The requirement to fill large, complex tooling with increasing production rates has led to the use of blocked catalysts to delay front end reactivity while maintaining back end cure. Increasing aesthetic and environmental awareness has led to the use of non-fumigitive catalyst for vehicle interior and furnishing applications in order to reduce odour [47].

Organometallic compounds based on mercury, lead, tin (dibutyltin dilaurate) and zinc are used as polyurethane catalysts. Mercury carboxylates such as phenylmercuric neodeoconate are particularly effective catalysts for polyurethane elastomer, coating and sealants: lead catalysts are used in highly reactive rigid spray foam insulation applications. Since the 1990s, bismuth and zinc carboxylates have been used as alternatives to lead and mercury because of the toxicity but they have short comings of their own.

Physical properties of polyurethane foams

Generally, the physical properties of polyurethane foams depend on the method by which they are prepared. For example, the windows may or may not be ruptured in the final stage of expansion, depending on the relative rate of molecular growth (gelation) and gas reaction, giving rise to flexible or rigid foams [48].

In polyurethane foam preparation, the variety in choice of simple molecules is great and consequently, the properties of the product are wide. Choice of the polyol has a major effect on the foam properties especially on its rigidity and flexibility. The crosslink density of the urethane polymer determines whether the foam will be flexible (low cross-link density) or rigid (high cross-link density). Rigid foams are prepared from highly branched resins of low molecular weight while flexible foams are prepared from polyols of moderately high molecular weight and low degree of branching.

Mechanical properties of polyurethane foam

The mechanical properties of polyurethane foam are highly dependent on the proportion of the allophanate linkage which increases the reaction time and temperature for toluene diisocyanate based urethane. They are influenced by the functionality and molecular shape.

Chemical properties of polyurethane foam polymer

The chemical properties of polyurethane foams are also a function of the preparation process. For example, solvent resistance of polyurethane structure increases at higher cross­link densities, appears to be unaffected by the type of aromatic diisocyanate and is reduced with the use of a large excess of isocyanate. The aliphatic and cycloaliphatic isocyanate can produce a polymer with an outstanding resistance to sunlight as aliphatic are normally less photosensitive than their aromatic counterpart [49-50].