1. Endothermic effect

The heat released by any combustion in a relatively short period of time is limited. If part of the heat released by the fire source can be absorbed in a relatively short period of time, the flame temperature will be lowered, radiating to the combustion surface and acting on the vaporized The heat of pyrolysis of combustible molecules into free radicals will decrease, and the combustion reaction will be suppressed to a certain extent. Under high temperature conditions, the flame retardant undergoes a strong endothermic reaction, absorbing part of the heat released by combustion, reducing the surface temperature of combustibles, effectively inhibiting the generation of combustible gases and preventing the spread of combustion. The flame retardant mechanism of Al(OH)3 flame retardant is to increase the heat capacity of the polymer so that it can absorb more heat before reaching the thermal decomposition temperature, thereby improving its flame retardant performance. This kind of flame retardant gives full play to its large heat absorption characteristics when combined with water vapor, and improves its own flame retardant ability.

After adding the flame retardant to the combustible material, the flame retardant can form a glassy or stable foam covering layer at high temperature, insulate oxygen, have the functions of heat insulation, oxygen insulation, and prevent combustible gas from escaping, so as to achieve flame retardancy purpose. For example, organic phosphorus-inhibiting flame retardants can produce cross-linked solid substances or carbonized layers with a more stable structure when heated. The formation of the carbonized layer can prevent the polymer from further pyrolysis, and on the other hand, it can prevent the thermal decomposition products inside it from entering the gas phase to participate in the combustion process.

3. Inhibit chain reaction

According to the chain reaction theory of combustion, free radicals are needed to maintain combustion. Flame retardants can act on the gas phase combustion zone to capture free radicals in the combustion reaction, thereby preventing the spread of flames, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction speed until it stops. For example, halogen-containing flame retardants have the same or similar evaporation temperature as the decomposition temperature of the polymer. When the polymer is decomposed by heat, the flame retardant will also volatilize at the same time. At this time, the halogen-containing flame retardant and the thermal decomposition products are in the gas phase combustion zone at the same time, and the halogen can capture the free radicals in the combustion reaction, thereby preventing the spread of the flame, reducing the flame density in the combustion zone, and finally reducing the combustion reaction speed until it stops. .

4. Non-combustible gas asphyxiating effect

Flame retardants decompose incombustible gas when heated, and dilute the concentration of combustible gas from combustibles to below the lower combustion limit. At the same time, it also dilutes the oxygen concentration in the combustion zone, prevents the combustion from continuing, and achieves the effect of flame retardancy.

5 Mechanism of combustion and flame retardancy

The mechanism of combustion and flame retardancy

In Section 3, we discussed the basic thermal parameters that determine the inherent combustion behavior of textile fibers. In order to understand how existing textile flame retardants work and, more importantly, how to develop future flame retardants, the key is to explore the combustion mechanism of fiber-forming polymers more deeply.

5.1 Flame retardant strategy

In the process of textile combustion mechanism (as a feedback mechanism), fuel (from thermal degradation or pyrolysis of fiber), heat (from ignition and combustion) and oxygen (from air) all play a role as the main components. In order to interrupt this mechanism, five ways (a) ~ (e) have been proposed. Flame retardants can work in one or more of these ways. The following are the various stages and related flame retardant effects:

"Melting and/or degradation and/or dehydration need to absorb a large amount of heat (for example, inorganic and organic phosphorus preparations, aluminum hydroxide or hydrated aluminum oxide in the back coating). Usually not used by flame retardants; it is more common in inherently fire-resistant and heat-resistant fibers (such as aramid fibers). Most phosphorus and nitrogen-containing flame retardants in cellulose and wool; heavy metal complexes in wool. Hydrated and certain carbon-promoting flame retardants can release water; halogen-containing flame retardants can release hydrogen halide. Halogen-containing flame retardants, often combined with antimony oxide. It can be seen from the above that certain types of flame retardants can function in a variety of ways, and most effective examples are like this. In addition, certain flame retardant formulations can produce a liquid phase intermediate that can wet the surface of the fiber, thereby becoming a barrier to heat and oxygen-the widely accepted borate-boric acid mixture can be used in this way Play a role. In addition, it can also promote char formation. In order to simplify the classification of different ways of chemical flame retardant behavior, the terms'condensed' phase and'gas or vapor' phase activity can be used to distinguish them. Both are compound terms. The former includes the above-mentioned (a~c) methods, and the latter includes (d) and (e) methods. Physical mechanisms usually work at the same time. These mechanisms include removing oxygen and/or heat by forming a coating (mode d), increasing heat capacity (mode a), and diluting or covering the flame with non-flammable gas (mode d).

5.2 Thermoplastic

Whether the fiber can soften and/or melt (defined by the physical transition temperature in Table 3) determines whether it has thermoplasticity. Thermoplasticity can seriously affect the behavior of flame retardants due to its related physical changes. Traditional thermoplastic fibers (for example, polyamide, polyester, and polypropylene) can leave the ignition flame as soon as they shrink, thereby avoiding being ignited: this makes them appear flame retardant on their surface. In fact, if the contraction 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 in composite textiles composed of thermoplastic and non-thermoplastic components.

With the above effect comes the problem of droplets (usually flame droplets). Although this droplet can remove the heat of the flame front and prompt the flame to extinguish (thus it can'pass' the vertical flame test), it can Burn or reignite the underlying surface (such as carpet or skin).

Most flame retardants applied to traditional synthetic fibers during mass production or as finishing agents usually work in two ways: enhancing melt dripping and/or facilitating flaming melt droplets to extinguish. So far, no means can reduce the thermoplasticity and promote the formation of charcoal in a large amount. This is the case with cellulose (including viscose fiber) treated with flame retardant.

5.3 Flame retardant mechanism and char formation

”Flame retardants that act in the gas phase in the manner (d) and/or (e) all have the advantage that they reduce the tendency to ignite and help the flame extinguishing of textile fiber-forming polymers. This is because once the volatile products or fuel produced by thermal degradation react with oxygen in the flame, their chemical properties will become very similar. Therefore, two methods such as cutting off oxygen ((e) method) or generating interfering free radicals ((f) method) can undoubtedly ensure the effect of flame retardants.

In terms of cost and benefit, antimony-halogen flame retardants are the most successful flame retardants in the field of bulk polymers and back-coated textiles. Unlike phosphorus and nitrogen-containing fiber-reactive durable flame retardants used for cellulose fibers (see below), they can generally only be used as back coating agents with the aid of resin binders. As far as textiles are concerned, most antimony-halogen systems are composed of antimony trioxide and bromine-containing organic molecules such as decabromodiphenyl oxide (DBDPO) or hexabromocyclotridecane (HBCD). Upon heating, these substances release HBr groups and Br. base. These two will interfere with the chemical reaction of the flame according to the diagram below. In the schematic diagram: R, CH2, H and OH groups are part of the flame oxidation chain reaction, which consumes fuel (RCH3) and oxygen.