Flame Retardant Principle of Flame Retardant Fabric of Cotton

Flame Retardant Principle of Flame Retardant Fabric of Cotton
1, the endothermic effect of any combustion in a short period of time the heat released is limited, if in a short period of time to absorb the heat emitted by a part of the heat, then the flame temperature will be reduced, the radiation to the burning surface and role in The heat of the combustible molecules has been reduced into free radicals will be reduced, the combustion reaction will be a certain degree of inhibition. In the high temperature conditions, the flame retardant has a strong endothermic reaction to absorb part of the heat released from combustion, reduce the temperature of the combustible surface, effectively inhibit the formation of combustible gases to prevent 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 absorbs more heat before reaching the thermal decomposition temperature, thus improving its flame retardancy. This type of flame retardant to give full play to its combination of water vapor when a large number of endothermic characteristics, improve its own flame retardant capacity.
2, covering the role
In the combustible material by adding flame retardant, the flame retardant at high temperatures can form a glass or stable foam cover, isolated oxygen, with insulation, oxygen, to prevent the escape of combustible gas out to achieve flame retardant purpose. Such as organic phosphorus flame retardant when heated to produce more stable structure of the cross-linked solid material or carbonized layer. The formation of the carbonized layer on the one hand can prevent further pyrolysis of the polymer, on the other hand to prevent the internal thermal decomposition of the product into the gas phase involved in the combustion process.
3, inhibition of chain reaction
According to the theory of combustion chain reaction, it is necessary to maintain the free radicals required for combustion. Flame retardants can act on the gas phase combustion zone to capture the free radicals in the combustion reaction, thereby preventing the flame from propagating, reducing the flame density of the combustion zone and eventually lowering the combustion reaction rate until it terminates. Such as halogen-containing flame retardants, its evaporation temperature and polymer decomposition temperature is the same or similar, when the polymer heat decomposition, the flame retardant also volatile. At this time, the halogen-containing flame retardant and the thermal decomposition product are simultaneously in the gas-phase combustion zone, and the halogen can capture the free radicals in the combustion reaction, thereby preventing the flame from propagating, reducing the flame density of the combustion zone and finally lowering the combustion reaction rate The
4, non-combustible gas asphyxiation effect
When the flame retardant is heated, it can decompose the noncombustible gas, and the concentration of the combustible gas decomposed by the combustible material is diluted below the lower combustion limit. But also on the combustion zone of oxygen concentration has a dilution effect, to prevent the combustion continue to achieve the role of flame retardant.
5, the mechanism of combustion and flame retardant
In order to understand how existing textile flame retardants work and more importantly - how to develop future flame retardants, the key is to explore more deeply the mechanism of fiber-forming polymer combustion.
5.1 Flame retardant strategy
In the combustion, fuel (from thermal degradation or pyrolysis of fibers), heat (from ignition and combustion) and oxygen (from air) play a major role as a component. In order to interrupt this mechanism, people put forward five ways (a) ~ (e). The flame retardant may function in one or more of these modes. The following lists the various stages and associated flame retardant effects:
A) heat removal;
B) increase the decomposition temperature;
C) reduce the formation of combustible volatiles, increase the amount of carbon;
D) reduce contact with oxygen or dilute the flame;
E) interfere with the flame chemical reaction and / or increase the fuel ignition temperature (Tc);
Melting and / or degreasing and / or dewatering requires the addition of large amounts of heat (e.g., in the backcoat containing inorganic and organophosphate formulations, aluminum hydroxide or hydrated alumina).
Are generally not used as flame retardants; and are more common in inherent refractory and heat resistant fibers such as aramid fibers. Cellulose and wool in the majority of phosphorus, nitrogen-containing flame retardants; in the wool of heavy metal complexes. Hydrated and some charcoal flame retardants can release water; halogen-containing flame retardants can release hydrogen halide. Halogen-containing flame retardants, often combined with antimony oxide. As can be seen from the above, some class of flame retardants can play a role in a variety of ways, most of the effective examples are so. In addition, certain flame retardant formulations can produce liquid intermediate which can wet the surface of the fiber and thus become a barrier to heat and oxygen barrier - widely accepted borate-borate mixture can be in this way Play a role. In addition, it can promote charcoal. In order to simplify the classification of different ways of chemical flame retardancy, the terms 'condensed' phase and 'gas or vapor' phase activities can be used to distinguish them. Both are composite items, the former includes the above (a ~ c) way, which includes (d) and (e) way. The physical mechanism usually works at the same time by including the formation of a coating to remove oxygen and / or heat (mode d), increase the heat capacity (mode a), and dilute or cover the flame with a non-flammable gas (mode d).
5.2 Thermoplastic
Whether the fiber can soften and / or melt (as defined by the physical conversion temperature in Table 3) determines whether it is thermoplastic. Thermoplastic due to its associated physical changes can seriously affect the behavior of flame retardants. Conventional thermoplastic fibers (e.g., polyamides, polyesters, and polypropylenes) shrink away from igniting the flames to avoid being ignited: this causes them to exhibit flame retardancy on the 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, i.e. molten polymers are melted onto non-thermoplastic cotton and ignited. Similar effects can also be seen on composite textiles consisting of thermoplastic and non-thermoplastic components.
With this effect, there is a problem with droplets (usually flame droplets) that can remove the heat of the flame front and cause the flame to extinguish (and thus' pass the 'vertical flame test') So that the surface below it (such as carpet or skin) burning or secondary ignition.
Most flame retardants that are applied to conventional synthetic fibers during mass production or as finishing agents are usually function in both ways by enhancing melt dripping and / or promoting flame drop. So far, any means can not reduce the thermoplasticity and greatly promote the formation of charcoal, the flame retardant treatment of cellulose (including viscose fiber) is the case.
5.3 Flame retardant mechanism and charcoal
Flame retardants acting in the gas phase in the form of (d) and / or (e) have the advantage that they reduce the ignition tendency and contribute to the flame extinguishment of the textile fiber-forming polymer. This is because once the thermal product or the fuel produced by thermal degradation reacts with oxygen in the flame, its chemical properties become very similar. Therefore, such as disrupting oxygen ((e) way) or generate interference free radicals ((f) way) both ways can undoubtedly ensure the effectiveness of flame retardants.
According to cost and benefit, antimony-halogen flame retardants are the most successful flame retardants in the field of bulk polymers and backcoat textiles. Unlike fiber-reactive and reactive fiber-based flame retardants for cellulosic fibers, they are usually used only as a backcoat agent by means of a resin binder. In the case of textiles, most antimony-halogen systems consist of antimony trioxide and bromine-containing organic molecules (such as tetrabromobiphenyl oxide (DBDPO) or hexabromocyclotridecane (HBCD)). Once heated, these substances will release HBr and Br. base. The two will interfere with the chemical reaction of the flame.
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