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[塑料] 塑料阻燃深度总结(六)磷系阻燃剂

发表于 2013-10-20 09:27:05 | 显示全部楼层 |阅读模式


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本帖最后由 846082533 于 2015-12-31 14:59 编辑 <br /><br />磷系阻燃剂
28Phosphorus-based flame retardants are the second most widely used class of flame retardants. Recent efforts in the development of new flame retardants have shifted strongly toward phosphorus and other halogen-free systems. Among phosphorus-based flame retardants, one should distinguish (1) elemental red phosphorus, (2) inorganic phosphates, (3) numerous organic phosphorus-based products, and (4) chlororganophosphates. Although many phosphorus flame retardants exhibit general modes of action, there are specifics for each class mentioned above.
29、It is generally accepted that phosphorus flame retardants are significantly more
effective in oxygen- or nitrogen-containing polymers, which could be either heterochain polymers or polymers with these elements in pendant groups. Effective phosphorus flame retardants are more specific than halogen-based products to certain polymers. This relates to the condensed-phase mechanism of action, where the phosphorus flame retardant reacts with the polymer and is involved in its charring.
30The flame retardancy of cellulose has bee n studied in great detail, which gave
good insight for understanding the inter action of phosphorus flame retardants with polymers containing hydroxyl groups. Phosphorus flame retardants, in the form of either acids derived from decomposition of ammonium phosphate salts or of phosphate esters, react (esterify or transesterify) with the hydroxyl groups of the cellulose. Upon further heating, phosphorylated cellulose under-goes thermal decomposition and a significant amount of char is formed at the expense of combustible volatile products that would be produced by virgin cel-lulose. Some nitrogen-containing com pounds, such as urea, dicyandiamide, and melamine, will accelerate phosphorylation of cellulose through formation of a phosphorus – nitrogen intermediate, and thus synergize the flame retardant action of phosphorus. Phosphorus – nitrogen synergism is not a general phenomenon but depends on the structure of the phosphorus and nitrogen flame retardants as well as the polymer structure.
31Similar to cellulose, phosphate esters can transesterify other polymers. For example, polycarbonates can undergo rearrangement during thermal decomposition, where phenolic OH groups are formed which then become the target for attack by aromatic phosphate esters Thus, phosphorus is grafted on the polymer chain. Char will be formed upon thermal decomposition of this grafted polymer. Similar phosphorylation chemistry was found for polyphenylene ether (PPE; a component of a PPE/HIPS blend), which also tends to rearrange upon heating and form phenolic OH groups.


32If the polymer cannot be involved in the charring because of the absence of reactive groups, a highly charring coadditive is used in combination with the phosphorus flame retardant. The coadditive is usually a polyol, which can undergo phosphorylation similar to that of cellulose ; pentaerythritol is a typical example of such a polyol. Melamine can be used in conjunction with this system as well. These combinations of flame retardants are called intumescent systems because they form a viscous swollen char on the surface of the burning polymer. The char impedes the heat flux to the polymer surface and retards diffusion of volatile pyrolysis products to the flame. This mechanism of action is mostly physical because the polymer itself is not necessarily involved in the charring process,
but its volatilization is retarded significantly.


  33Phosphorus flame retardants can remain in the solid phase and promote charring or volatilize into the gas phase, where they act as potent scavengers of H·or OH? radicals. Volatile phosphorus compounds are among the most effective inhibitors of combustion. A recent study showed that phosphorus at the same molar concentration is, on average, five times more effective than bromine and 10 times more effective than chlorine.  磷系阻燃剂可以通过两种方式发挥阻燃作用:第一种是在凝聚相中促进成炭,第二种是在气相中作为H·或 OH?自由基的强力捕获剂。气相作用磷系阻燃剂是一种十分高效的阻燃剂。近来有研究表明,在相同添加量时,磷系阻燃剂的阻燃效率平均是溴系阻燃剂的5倍以及氮系阻燃剂的10倍。
34If conditions are right, phosphorus-based molecules can volatilize and are oxidized, producing active radicals in the flame. On the other hand, phosphorus flame retardants tend to react with the polymer or to oxidize to phosphoric acid in the condensed phase. This favors mostly condensed-phase mechanisms. It is challenging to design a phosphorus-based flame retardant that will volatilize into the flame at relatively low temperatures but will not be lost during polymer processing. 特定条件下,在气相中磷系阻燃剂可挥发或被氧化生成活性自由基,或者可在凝聚相中与聚合物发生反应或被氧化生成磷酸。后者即为磷系阻燃剂主要的凝聚相作用机理。现在,研发在较低温度下即可挥发生成自由基捕捉剂,但同时在聚合物加工过程中不发生降解的磷系阻燃剂仍是一个难题。

35Red phosphorus is the most concentrated source of phosphorus for flame retardancy. In fact, it is very effective in some polymers, such as thermoplastic polyesters or polyamides, where self-extinguishing UL-94 V-0 performance can be achieved at loadings of less than 10 wt%. Despite the apparent chemical simplicity of this additive, its mechanism of action is not completely understood. Most researchers agree that in oxygen- or nitrogen-containing polymers, red phosphorus reacts with the polymer and induces char formation. Although there is a belief that red phosphorus is oxidized and hydrolyzed by water before it Formation of the residue is considered to be a condensed-phase contribution of melamine, whereas ammonia evolution dilutes the flame with noncombustible
红磷是一种极为重要的磷系阻燃剂(上海北京顺德注:翻译错误,the most concentrated是浓度最大的意思)。实际上,在一些聚合物(如热塑性聚酯或尼龙)中它是十分高效的,添加量少于10%即可使聚合物达到UL94V-0级。尽管红磷化学成分单一,但人们仍然尚未完全了解其阻燃机理。多数学者较为赞同的观点是,在含氧或者含氮聚合物中,红磷可与聚合物发生反应促进成炭。尽管有学者认为,红磷在与聚合物反应前会发生氧化或水解,但同时也有强有力的证据表明红磷可在干燥的惰性气氛中直接与聚酯或PA发生反应。也有一些研究表明,PA6中的红磷为自由基作用机理的阻燃。红磷在烯烃类聚合物(如聚烯烃、PS)中的阻燃效果不佳。学者们认为,这是因为红磷在烯烃类聚合物中将降解为白磷(P4),后者可挥发而作用于气相。

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