Friday 17 June 2022

Flame Retardants - Why Do We Need Them and What Are The Major Systems?

Hello and welcome to a new blog post. Today with the topic of flame retardants, starting with an overview and then discussing as an example effective flame retardants for Polyamides. 

Why do we need flame retardants in plastics?

In general, adding flame retardants to your polymer compound formulation helps to prevent the immediate start of fire or slowing the growth of fire of your material. This in turn helps to fulfill a certain burning classification such as the UL V0 at a certain material thickness. The material requirement list of your application should consider flame rating needs since it will be easier later during polymer material selection to not miss such an important detail. 

Do all polymers need them?

Aliphatic polymers need them to achieve a desired level of UL V0. Semi-aromatic polymers such as PPS do not need them since the benzene rings enable an intrinsic flame retardancy. As a rule of thumb the higher the aromatic amount (benzene) the better the flame retardancy level of your polymer compound. 

Overview of the 3 major systems

There are three major systems used in the plastics industry: nitrogen-phosphorus systems, halogenated systems, and metal-hydroxide systems.

Nitrogen-phosphorus systems are halogen free and show a lower smoke emission compared to halogenated flame retardants. Furthermore they do not decrease the mechanical properties of your base polymer too much. Usually, adding the flame retardants results in a lowering of properties. Downside of this system is the narrow production window, water solubility and poss surface aesthetics. 

Halogenated systems are very good flame retardants and can be used at low concentration levels, together with a wide production window. Major disadvantage is the use of halogens (pay attention to local regulations) and during combustion it develops a lot of smoke emissions, together with the release of free radicals. Also, stabilization against weathering is not possible. 

Metal-hydroxide systems are halogen free and stabilization towards weathering is possible. During combustion, this system only releases water. Downside is the high concentration of flame retardant needed, and lower mechanical properties as a result. 

Figure 1 summarizes the advantages and disadvantages of the different flame retardant systems. 

Figure 1: Comparison of the advantages and disadvantages of the different flame retardant systems

Example: Use of flame retardants in Polyamide PA 6

Alumina Trihydrate (ATH) is a widely used flame retardant and can be a starting point for Polyamide 6. The decomposition temperature of ATH is around 180°C and it releases water.  However, the compounding and processing temperature of PA 6 is between 230°C and 290°C which leads to an activation of the decomposition of ATH. Therefore we need an alternative to safely bring PA 6 onto a certain flame rating. The solution is in Magnesium Hydroxide (MDH) which has a decomposition temperature of 330°C. Apart of MDH, boron zinc oxide and organophosphorus salt can be used for high performance Polyamides such as PPA and PARA. 

Thanks for reading and #findoutaboutplastics



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