What Are the Factors Affecting British Standard Flame Retardancy in Polyurethane Foam?

Chemical change is the process of producing new substances after the molecular groups of various reactants interact with each other. Many properties of substances are determined by their molecular structures, and understanding the molecular and group structures in polyurethane reactants is instructive for production.

The main indicators of British standard flame retardancy are generally threefold: thermal weight loss (the mass lost when the specified size of sponge is heated at a specified temperature for a specified time, with smaller values indicating better thermal stability); smoke density (the amount of smoke generated when the foam burns, indicating the ease of light passing through the smoke, with smaller amounts of smoke being better); and ease of combustion (the more difficult it is to ignite, with further subdivisions based on ignition time and burning rate).

TDI (toluene diisocyanate) has one benzene ring, MDI (diphenylmethane diisocyanate) has two benzene rings, and crude MDI has multiple benzene rings. Benzene rings are very stable substances, requiring a large amount of energy (bond dissociation energy) to break. As the number of benzene rings increases, the thermal stability of the foam increases (crude MDI > MDI > TDI), making it less likely to decompose when heated. With more benzene rings, there are more carbon atoms in the molecule, resulting in more smoke when incompletely burned (crude MDI > MDI > TDI). From the above, it can be concluded that when one formula decreases the amount of TDI and increases the amount of MDI, the thermal stability of the foam will be enhanced. The thermal weight loss index is likely to pass the British standard test, but the smoke density, which is not easy to pass, will increase. At this point, it is advisable to appropriately increase the amount of melamine cyanurate to reduce smoke density.

The higher the molecular weight of the polyether, the worse the thermal stability, but the better the fire resistance. In the production of high-rebound flame-retardant foam, the amount of flame retardant added is only two-thirds that of regular-density flame-retardant foam, yet the flame retardancy remains very good and does not ignite. However, high-rebound flame-retardant foam is more difficult to pass the British standard test than regular foam (thermal weight loss is difficult to pass).

Flame retardants are not very stable when heated. Since the British standard test emphasizes thermal weight loss, the amount of flame retardant in the formula is the minimum required to pass the flame retardancy test.

When both TDI and water content in the formula decrease while methane content increases, the foam is less likely to ignite. The decrease in intrinsic properties due to the reduction of hard segments results in decreased thermal stability, thus reducing the ability to pass the thermal weight loss index.

When the foam density decreases, the TDI content increases, and both smoke density and thermal stability increase.

Inorganic materials like calcium carbonate and barium sulfate do not decompose when heated during British standard tests, but their addition does not improve the foam's properties, so they are not used in the British standard formula.

Besides selecting raw materials, achieving a balance is also crucial when meeting British standards. For example, both TDI and flame retardants, if given too much or too little, make it difficult to pass the test. Foaming is a balanced science, adjusting the formula is about seeking balance, and selecting raw materials is also about seeking balance.