Flame Retardancy By Endothermic Degradation
One type of flame retardant is a chemical that degrades endothermically. Examples of these types of chemicals are magnesium hydroxide, aluminum hydroxide and many different hydrates. An endothermic reaction is one where heat is absorbed. Systems in which an endothermic reaction is taking actually undergo a relative decrease in temperature. Hence, endothermic degradation is the breakdown of a chemical while absorbing the heat from the surrounding materials and environment. For example, while a magnesium hydroxide degradation reaction is absorbing heat, the surrounding material is cooling down. This inhibits the combustion process.
The hydroxides and hydrates used as flame-retardants only degrade once they reach their decomposition temperature. For magnesium hydroxide, the decomposition temperature ranges from 250 to 340 degrees Celsius. For aluminum hydroxide, the decomposition temperature is much lower at approximately 230 degrees Celsius. Hydrates usually have similar decomposition temperatures. These temperatures are relatively low when you think about the operating temperatures of plastic manufacturing lines. Molten plastic often reaches temperatures anywhere from 200 to 275 degrees Celsius in an extruder. Some of these chemicals would start to degrade in the extruder and that could be a big problem so you must be careful to consider your operating parameters when choosing a flame retardant.
[...] are poor heat conductors and often contain hydrates. In the previous post, I talked about how hydrates can be flame retardants in their own right due to their endothermic degradation. Once the hydrates have decomposed, the [...]
thanks very much for this interseting information .
Please this my comment , i observed when I add some metal hydroxide to PS, the glass transition (Tg) of PS material is decreased from 103 C* to 97 C* , this may decrease the thermal resistance of the polymer
as i expect the Tg must increase due to some force acting by the additives on the polymer chain to reduce the movement of the polymer chain .
Please i like to know your comment in this point.
Best Regards.
Sincerely
Hayder Alrashid
email ; h.rashid@aplaco.com.sa
Mr. Alrashid,
Thank you for your comment and question. A quick literature scan suggests that the Tg of polystyrene is 100ºC ± 2K. It is possible that you’ve been observing some natural variation in PS’s Tg.
However, let’s assume that the addition of magnesium hydroxide or some other metal hydroxide does have an impact on Tg. I would guess that a chemically inert additive would lower the glass transition temperature.
From a thermodynamic perspective, the introduction of an inert additive would disrupt the crystalline nature of pure PS. The higher entropic state of the system and the increase in free volume allow the polymer strands to start sliding passed one another at lower temperatures.