Self-extinguishing Mechanism of Flame-Retardant Polymers

Overview

• Post By : Kumar Jeetendra


• Source: Amipolymer


• Date: 04 Jun,2020

Silicone elastomer based products like tube, hose, profile gaskets, O-rings, & sheets are widely used specially in the area like medical, pharmaceuticals, biopharma, cell-culture treatment, food & beverages & engineering. For engineering applications somewhere, we required flame retardant product for such type of work areas like Electronics, Automotive, wire & Cables.

Flame Retardant Polymers are highly resistant to degradation at elevated temperatures. Retardant is defined as a material that has been chemically treated to self-extinguish

There are different kind of polymers such as Fluoropolymers- PTFE (Polytetrafluoro Ethylene) & FEP (Fluorinated Ethylene Propylene). These polymers are Plastics based Flame-retardants. Flame resistance of silicone rubber can be improved by admixing with flame retardant additives such as the well-known halogens and metal hydrates or the lesser-known platinum, rhodium, and iridium complexes. Platinum is particularly useful because it serves as both a flame retardant and a catalyst for addition cure of silicone rubber.

Unlike hydrocarbon-based polymers materials with hydrogen bonded to oxygen, Fluoro based materials are less likely to burn thanks to fluorine, which is difficult to associate with oxygen when it comes out.
Furthermore, in fluorine materials, the C-C bond formed by –CF 2- is stronger than the C-C bond by -CH 2- and thus can withstand attacks, trying to break the CC bond, making it difficult to burn.

The most important flame-retardants systems used act either in the gas phase where they remove the high-energy radicals H and OH from the flame or in the solid phase, where they shield the polymer by forming a charred layer and thus protect the polymer from being attacked by oxygen and heat.

• To understand how flame-retardants works, it is first necessary to see how materials burn.

The Fire Cycle- Flame retardants interact with the fire cycle in order to stop or delay it. They act at different stages depending on their chemical basis-

(1) Any energy source (heat or a small flame) can be the initial ignition source.
(2) Energy transmitted by the ignition source to the polymer creates a degradation when pyrolysis takes place.
(3) Flammable gases, which are emitted to the gas phase. In the condense phase, the result is an inert carbonised material, called char.
(4) Pyrolysis is a process that degrades the polymers long-chain molecules into smaller hydrocarbon molecules, known as flammable gases.
(5) In the gas phase, flammable gases are mixed with oxygen from the air. The proper mix of oxygen and fuel is reached in the combustion zone.

(6) A perfect combustion would theoretically produce H2O & CO2. In real life, incomplete combustion products are also emitted during a fire (OC, PAHs, HCN, etc).
(7) Energy Emitted during exothermic reactions is transmitted to the polymer & reinforces pyrolysis. This allow the reaction to sustain itself.

The polymers that are most efficient at resisting combustion are those that are synthesized as intrinsically fire-resistant. However, these types of polymers can be difficult as well as costly to synthesize. Modifying different properties of the polymers can increase their intrinsic fire-resistance; increasing rigidity or stiffness, the use of polar monomers, and/or hydrogen bonding between the polymer chains can all enhance fire-resistance. Polyimides, polybenzoxazoles (PBOs), polybenzimidazoles, and polybenzthiazoles (PBTs) are examples of polymers made with aromatic heterocycles. Polymers made with aromatic monomers have a tendency to condense into chars upon combustion, decreasing the amount of flammable gas that is released.

Apart from Linear & ladder polymer (Cyclic aromatic components the flame retardant depends on the incorporations of in the condensed phase such as metal hydroxides (aluminum trihydrate, or ATH, magnesium hydroxide, or MDH, and boehmite), metal oxides and salts (zinc borate and zinc oxide, zinc hydroxystannate), as well as expandable graphite and some nanocomposites such as Nano dispersed montmorillonite(MMT) clay in the polymer matrix. Later, organomodified clays, TiO2 nanoparticles, silica nanoparticles, layered double hydroxides, carbon nanotubes.

FKM, PTFE, FEP, PFA are several polymers which is having excellent flame resistance due to halogen structure in Polymer. Polymer industries have started formulating various fluoropolymers for innovative engineering, chemical, API and pharma applications.