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

The requirements for good catalytic activity are: (a) nucleophilic enough to attack the carbon of the isocy anate group, (b) ability to form an active hydrogen amine complex, and (c) solubility in water with the ability to form hydrogen bonds with water.
Since electron accessibility is also usually measured by the basicity, the catalytic activity is found to generally increase as the basicity increase. Thus, a plot of pH vs. catalytic activity usually yields linear behavior. Howe ver, exception exist, such as triethylenediamine in which the substituent groups on the nitrogen are pulled back, thereby reducing steric strain, and thus making the electron pair on the nitrogen atom easily accessible. Thus, the catalytic activity exhibited by this compound is greater than expected from its basicity.

A substantial amount of work has been done to study the functioning of catalysts and their complexation with the different foaming compon ents. However, much of this work has investigated the reaction mechanism of model compounds in dilute solutions at fixed temperatures [68]. Also, polar solvents are thought to increase the reactivity of the isocycnates by stabilizing the polarization of th e isocycnate group [68].
Besides the electronic effects discussed above, the role of a catalyst is also determined by other physical and chemical properties. For instance, catalysts which have low boiling points, such as triethylamine, are readily volati zed in the exothermic reactions taking place and are thus lost from the reacting mixture [23]. Once they volatilize, it leads to a decrease in the catalytic effect. Other catalysts, which contain hydroxyl groups, such as dimethylaminoethanol, chemically react with the growing polymer chain, and thus are no longer able to find their way to a reaction site [23]. This again leads to loss in the catalytic activity. Therefore, it is often helpful to use a mixture of catalysts, such that the reactions proceed at a reasonable rate at all times during the foaming as well as curing stages.
2.7.6.7    Organometallic Catalysts
While the amine catalysts discussed above exert some influence on the isocyanate -hydroxyl reaction, organomatallic salts favour this reaction almo st exclusively and are thus called gelation catalysts. The catalytic act is explained by three complimentary mechanisms [57]. The first mechanism describes the activation of the polyol into a tin alkoxide which then reacts with the isocyanate to yield a urethane linkage. The urethane linkage further reacts with a polyol – thereby propagating the polyol and regenerating the catalyst. In the second mechanism, the isocyanate molecules get activated and are in turn attacked by the polyol to again propagate the polymer and regenerate the catalyst [57]. The third mechanism involves the formation of a tin-amine complex which accepts a polyol molecule and further activates the complex [57]. This complex then reacts with an isocyanate group to yield a carbamate linka ge.
Unlike the tertiary amine catalysts, which usually volatilize during the foaming reactions, tin catalysts can remain in the foam permanently. However, they are also known to undergo certain chemical changes. Stannous salts are known to oxidize into their stannic form, which promotes the oxidative degradation of flexible foams [69].
2.7.6.8    Surfactants
Flexible polyurethane foam production relies greatly on the performance on non -ionic, silicone based surfactants which are added to realize a variety o f functions. In fact, the largest commercial application of silicone surfactants is in the area of polyurethane foams with worldwide production quantities of    30,000        metric        tons/year    for        the        polyurethane    foam industry [24]. Some of the main functions perfor med are reducing        surface            tension,        emulsifying        incompatible ingredients, promoting bubble nucleation during mixing, stabilization of the cell walls during foam expansion, and reducing the defoaming effect of any solid added. Of these functions, perhaps the most important is the stabilization of the cell walls, without which the foam would behave like viscous        boiling    liquid.    The        processing        window        for surfactants is generally in the range of 0.5 – 2.5 parts per hundred polyol (pphp), and the actual quantity of surfactant added is dependent on the type of surfactant used as well as on the other constituents of the foam formulation. Below a certain minimum concentration of surfactant, the foam can result    in            serious            imperfections    such    as    splitting, densification, or collapse. Addition of more than required quantities of surfactant has its own drawbacks. This usually results in an over -stabilization of the foam, resulting in closed cells, which result in a decreased airflow through the foam. Also, a high number of c losed cells in the foam leads to foam shrinkage on cooling, which is undesirable.
There are four main types of silicone surfactants that can be used in the production of polyurethane. These include silicone oil, ABA block copolymers, graft copolymers, and surfactants possessing a highly branched structure [25].
During the initial stages of foam formation, at liquid -liquid interface, surfactants have been    shown to promote the interfacial mixing of the water and the polyol [25]. At the latter stages, when u rea hard segments are generated, the surfactant has been shown to stabilize the precipitating urea phase. It has been shown by many studies that when a silicone surfactant was present, the urea phase separation does not lead to foam collapse, as it does wh en a surfactant is not present [70].
2.7.6.9    Cross – Linkage Agents
Cross-linkage agents in flexible polyurethane foams are usually low molecular weight species with hydroxyl and/or amine groups and have functionalities greater than or equal to 3. An example is diethanol amine (abbreviated as DEOA), a commercially utilized cross – linker, which is commonly used in moulded foam applications as it helps in a faster viscosity build-up and thus achieving shorter demold times.
H
HO – CH2 – CH2 – N – CH2 – CH2 - OH
Diethanolamine (DEOA)
Also, since moulded foam applications utilizes high molecular weight polyols and slightly higher catalyst does (as compared to slabstock formulation), using typical foam surfactants leads to an over – stabilization of the cell walls. Thus, lower potency surfactants utilized in mo ulded-foams, to counteract this over -stabilization effect. Since these surfactants are not potent enough to give dimensional stability to the foam, the addition of cross –linking agents helps achieve foam stability.
Addition of a cross-linking agent generally leads to a reduction in the stiffness of the foam. This is because the additional covalent linkage resulting from the cross -linking agent interferes with the phase separation behavior of the foam [71]. Thus it needs to be realized, that even though some components might be added in small concentrations, the role they play in influencing foam properties can be very significant and needs to be well understood.
2.7.7.0    Other Additives
Various additives are added to flexible polyurethane foam formulations depending on the required properties and the end use of the foam. Some additives are added for aesthetic reasons (e.g. colorants) where as others are added to improve product performance. Since polyurethane foams have a significant amount of aromatic content, UV stabilizers are added to retard the yellowing of foams on exposure to light [72,73]. Fillers are added to improve the physical strength and quality of foams [74-77]. Bacteriostats and flame retardants are also added in some formulations. Some other additives include the use of non -reactive plasticizers to reduce viscosity, cell -openers to prevent shrinkage of the foam on cooling, and compatibilizers to enhance the emulsification of the reactants [57]. The use of antistatic agents to minimize the build up of static electrical charges is important for foams used to package electronic devices.