Whether a fiber can soften and / or melt (defined by the physical conversion temperature in Table 3) determines whether it is thermoplastic. Thermoplastics can severely affect the behavior of flame retardants due to their associated physical changes. Traditional thermoplastic fibers (for example, polyamides, polyesters, and polypropylene) leave the ignition flame as soon as they shrink, preventing them from being ignited: this makes them flame-retardant on the surface. In fact, if shrinkage is blocked, they will burn violently. This so-called stent effect can be seen on polyester-cotton and similar blended fabrics, where molten polymer melts onto non-thermoplastic cotton and is ignited. Similar effects can also be seen on composite textiles consisting of thermoplastic and non-thermoplastic ingredients. With the above effects comes the problem of molten droplets (usually flame droplets). Although this dripping can remove the heat of the flame front and cause the flame to extinguish (thus it can 'pass' the vertical flame test), it can Burn or re-ignite the surface underneath (such as carpet or skin). Most of the environmentally friendly flame retardants applied to traditional synthetic fibers during mass production or as finishing agents usually work by enhancing melt dripping and / or promoting flame extinguishing. So far, no method has been able to reduce thermoplasticity and promote carbon formation to a large extent, as is the case with flame-retardant cellulose (including viscose fibers).