Investigation Of Effects Of Two Flame Retardants On The Fire Characterisit Ics Of Flexible Poly Ether Foam

2.7.6.3    Water
Water acts as a chemical blowing agent and reacts with an isocyanate group resulting in a primary amine and carbon dioxide. Increasing the water content influences both the cell structure and the solid-state morphology of the foam. Higher water contents typically result in foams with lower density due to the increased blow reaction [58]. Also, since the hard segment content is also increased on reacting more water with the isocyanate, this increase s the stiffness of the polymer composing the foam struts. However, in general, it is observed that increasing the water content while maintaining other variables constant does not drastically affect the load bearing properties of the foam.
2.7.6.4    Physica l Blowing Agents
Although the carbon dioxide produced from the water    – isocyanate reaction acts as the principal source to blow the foam, some formulations also employ physical or auxiliary blowing agents. These are low boiling solvents, inert towards chemical reactions, and they are generally used to produce softer foams by reducing the foam density [58]. As the foaming reactions proc eed, the temperature reaches about 130ËšC, and this is high enough to vapourize the low boiling solvents and provide suppleme ntary gas to expand the foam. Addition of a physical blowing agent while maintaining the water/isocyanate content constant typically results in larger cells and a greater degree of cell openness, which results in a decreased foam density generally leading to an increase in foam softness.
However, foams can be produced with similar cellular structures and varying foam softness. This can be done by partially substituting the CO 2 produced from the water - isocyanate reaction with a physical blowing agent – the foam incorporating the physical blowing agent would be softer due to a comparatively lower hard segment content.
Until the early 1990’s the auxiliary blowing agent primarily used to produce soft low-density polyurethane was chlorofluorocarbon (CFCl 3) [63]. This blowing agent was, however, phased out in 1995 due to the environmental concern it caused regarding the depletion of the ozone layer especially over Antarctica [63]. Nevertheless, at that time, there existed a replacement, hydrochlorofluorocarbon H CFC
– 141b (CH3CFCl 2), which had performance and handling characteristics similar to that of CFC – 11, and was reported to have a depletion effect which was only 1 to 2% of that of CFC – 11. Although HCFC – 141b had much lower ozone depleting potential as hydrofluorocarbons (HFCs) could be developed for flexible polyurethane foam production. Also, amongst the currently used HCFCs, 141b has the highest ozone depleting potential, and is currently targeted to be phased out by December 2001, while other HCFCs h ave until 2010.
The above chain of events, has led companies which produce or utilize blowing agents to find suitable alternative measures. Technology for using methylene chloride as a blowing agent exists – however, appropriate adjustments in the catalyst package are required to overcome processing problems [57]. Other alternatives involving the use of acetone [64] and liquid carbon dioxide [65] have been suggested in the literature. Blowing agents such as pentane have been tried to replace CFCs although they are less satisfactory and also raise flammability concerns. Workers have also proposed the use of certain additives to achieve softer foams by partially disturbing the formation of the precipitating polyurea [66].
2.7.6.5    Catalysts
Since polyurethane foam production relies on two competing reactions, a balance between them is required to make foams with good open-celled structures and desired physical properties. While it is true that these reactions may proceed in the absence of catalysts, they gener ally proceed at rates too slow to be practical. This correct balance is required due to the possibility of foam collapse if the blow reaction proceeds relatively fast. On the other hand, if the gelation reaction overtakes the blow reaction, foams with clos ed cells might result and this might lead to foam shrinkage or ‘pruning’.    Catalyzing    a    polyurethane    foam,    therefore, involves choosing a catalyst package in such a way that the gas produced becomes sufficiently entrapped in the polymer. The reacting polymer, in turn, must have sufficient strength throughout the foaming process to maintain its structural integrity without collapse, shrinkage, or splitting.
The role of a catalyst in controlling the balance between the two reactions, as discussed above, is mor e conveniently represented by workers in terms of its selectivity [67]. Since the number of equivalents of water and alcohol present in the reacting mixture is different, yields of urea and urethane, which are representative of the blow and gelation reaction respectively, cannot be compared directly, but require to be normalized with respect to their limiting yields [67]. Therefore, the selectivity of a catalyst is defined in terms of ‘normalized’ blowing and gelation rates.
Normalized Blowing rate    =    % ur ea yield at time (t)/Limiting urea yield
 Normalized Gelling rate    =    % urethane yield at time (t)/Limiting urethane yield
Then to blow to gel selectivity can be defined as:
Blow to Gel Selectivity    =    Normalized Blowing Rate / Normalized Gelling Rat
Selectivity values greater than 1 are indicative of a strong preference towards blowing, while selectivities less than 0.4 are suggestive of a strong gelling catalyst [67]. Intermediate selectivity values indicate more balance cat alysts.
Polyurethane foam formulators generally choose catalysts from two major classes of compounds – tertiary amines and metal salts, primarily of tin [57,58,68]. Since catalysts differ both in activity and selectivity towards the polyurethane foaming reactions, the two kinds of combined not only to provide the desired balance of ‘blowing’ vs. ‘gelation’, but also to tune these reactions according to the needs of the production equipment.
In any chemical reaction, there are certain positions on reacting molecules which are more susceptible to attack by other added co-reactants. These positions are, therefore, more likely to undergo a given reaction. Catalysts characteristically function at these positions. In the formation of polyurethane foams, the cata lyst forms an activated complex with the reactants thus making it easier for the isocyanate moieties to chemically react with the active hydrogen containing compounds.
2.7.6.6    Tertiary Amine Catalysts
Tertiary amines, by definition, are compounds which con tain a nitrogen atom, having three substituent groups and a free pair    of    electrons.    Though    those    catalysts    are    generally thought of as blowing catalysts, they are known to catalyze the gelation reaction as well [57]. The cata lytic activity of the amine is determined by the availability of a free pair for complexation.    The        catalysis    mechanism        involves    the donation of these electrons by the tertiary nitrogen of the catalyst to the isocyanate group leading to the formation of an intermediate complex. The avail ability of the electrons is a function of both, the steric hindrance caused by the substituent groups, as well as the electron withdrawing or electron releasing nature of the substituent groups. Groups which tends to withdraw electron reduce the accessibil ity of the electrons and thus reduce the catalytic activity. N,N    – Dimethylcyclohexylamine is an example where the methyl groups have an electron releasing effect resulting in good catalytic activity [23].