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

In uncatalyzed systems the above reaction is known to be insignificant    [58].    Also,    this    reaction    is    generally    not favourable under the catalytic conditions used for flexible foam production.
2.7.5    Blow Reaction
The first step in blow reaction involves the reaction of an isocyanate group with water to yield a thermally unstable carbamic acid which decomposes to give an amine functionality, carbon dioxide and heat.

In the second step of blow reaction, the newly formed amine group reacts with another isocyanate group to give a disubstituted urea and additional heat is generated. The total heat generated from the blow reaction is approximately 47 kcal per mole of water reacted [57] along with the carbon dioxide released in the first step and serves as the principal source for ‘blowing’ the foam mixture; in addition to the blowing agent which is usually added.

Also, since the typical isocyanates util ized in foam production are difunctional, the second part of the blow reaction serves as a means of chain extend the aromatic groups of the typically used isocyanate molecules to form linear hard segments.
However, it should be noted that this reaction sch eme can also produce covalent cross – linking points when molecules with functionality greater than two, such as di aethanol amine, are added to the formulation [57].
There are other secondar y reactions, involving the form ation of biuret and allophanate lin kages which could lead to the formation of covalent cross – linkage points. In the formation of biuret, a hydrogen atom fro m the disubstituted urea reacts

with an isocyanate group to form a biuret linkage. The allophanate forming reaction has been discussed earlier.   It is important to note that both reactions scheme described above occur simultaneously, and therefore it is critical to control the relative rates of these reactions in order to obtain a foam with stable cellular structure and good physical properties. If the blow reaction takes plac e too fast in comparison to the gelation reaction, it would result in the cells opening before there is sufficient viscosity build -up to provide the foam structure with enough strength to uphold the foam, leading to the collapse of the foam. On the other hand, if the gelation reaction is faster than the blow reaction it may result in a foam with closed cells, which is no desirable.
2.7.6    Basic Components of Flexible Polyurethane Foam
There are many different components needed to synthesize a flexible foam. The s even major ones are isocyanate, polyol, water, physical blowing agents, catalyst, surfactants, and cross – linkage agents as shown in Table 2 [57]. The desired end properties of the foam dictate the choice of specific components along with their required quantities.