To understand how different FR technologies work, you first need to know how combustion occurs. Combustion is the scientific word for burning. In a combustion reaction a substance reacts with oxygen from the air and transfers energy to the surroundings as light and heat. The products of a combustion reaction are called oxides. The combustion cycle starts when fabric is exposed to a high temperature energy source like a fire or arc flash. The thermal energy quickly starts to decompose the fabric which causes the material to start burning and releases both flammable and non flammable gases when the flammable gas is mixed with oxygen. It ignites and you have combustion! Combustion generates more heat energy and the cycle continues.
So, how do the various fr fabric technologies work to break this combustion cycle? Aramid base fabrics require such a high temperature to decompose, that the typical cycle never gets started. You can see the dyes flash off but the fabric never starts the actual combustion cycle. Fabrics like what we use for Knox FR clothing that uses flame retardants containing phosphorus and nitrogen, have an increased production of char and non flammable byproducts which break the cycle at an early stage. Aramid also create char in this early stage.
Another way to break the cycle is with technologies that reduce access to oxygen through displacement or dilution. In the case of halogen containing flame retardants which are often used in combination with materials such as antimony oxides, the combustion cycle is broken through free radical scavenging of the oxygen. This is typical of fabrics with modacrylic fibers. All of these combustion stopping technologies are successfully used in today's flame resistant clothing but that doesn't mean all flame resistant clothing is created equal. Be sure to check out more information on how to select the right FR clothing for the specific hazards in your work.