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[塑料] 塑料阻燃性深度总结(二)聚合物的燃烧

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发表于 2013-10-20 10:24:22 | 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 />
1.2.2 聚合物的燃烧
6 Ignitability depends to a large extent on how quickly the surface can be raised to the ignition temperature. Special consideration has to be given to polymers that melt before thermal decomposition. Usually, at a low heat exposure, melting precedes ignition and the polymer can or  , removing heat from the surface.This phenomenon is beneficial for flame retardancy of uncharrable polymers. On the other hand, at a higher heat exposure, ignition may occur before the surface is heated to sufficient depth for the melted material to flow, and such polymers may ignite relatively easy.  
材料的易燃性在很大程度上取决于材料表面温度 上升到燃点的速度 人们异常关注熔融温度低于热降解温度的聚合物。通常暴露于较小热源时,如果材料在被点燃之前为熔融态,则可通过 流动   熔滴 来降低表面热量。这十分有益于非成炭类聚合物的阻燃。相反,暴露于较大热源时,材料在可为熔融流动态之前即被点燃,该类聚合物则相对易燃。
7The possibility of extinguishing a polymer flame depends on the mechanism of thermal decomposition of the polymer. Whereas   ignition   of a polymer correlates primarily with the initial temperature of decomposition, steady combustion is related to the tendency of the polymer to yield a char, which is produced at the expense of combustible volatile fragments.
聚合物的自熄性取决于其热降解机理,其易燃性则相关于其热降解的初始温度,稳定燃烧则与其成炭性相关,成炭性越好,可燃性挥发物则越少。
8Four general mechanisms are important for thermal decomposition of polymers: (1) random chain scission, in which the polymer backbone is randomly split into smaller fragments; (2) chain-end scission, in which the polymer depolymerizes from the chain ends; (3) elimination of pendant groups without breaking of the backbone; and (4) cross-linking. Only a few polymers decompose pre-dominantly through one mechanism; in many cases a combination of two ormore mechanisms is in effect. For example, polyethylene and polypropylene tend primarily to decompose via random chain scission, which in the case of polyethylene is also accompanied by some cross-linking. Poly(methyl methacrylate) and polystyrene tend to depolymerize, poly(vinyl chloride) primarily undergoes elimination of pendant groups (dehydrochlorination), and polyacrylonitrile crosslinks. In terms of flammability, random scission and depolymerizationpolymers are
usually more flammable than polymers that crosslink or remove pendant groups. Cross-linking leads to precursors of char and as a result, to lower flammability. Elimination of pendant groups results in double bonds, which can also give crosslinks or lead to aromatization.
对于聚合物而言,以下四个机理十分重要:①无规断链,聚合物主链随机断裂为链段碎片;②链端断裂,聚合物从链端开始裂解;③无主链破坏的端基消除;④交联。大多数情况下,聚合物的热降解遵循两种或两种以上上述机理,极少数只遵循其中单个机理。比如,PEPP主要通过无规断链的方式裂解,但PE的裂解还伴随一些交联反应的发生;PMMAPS受热时将发生解聚反应;PVC主要发生端基消除反应(脱HCl);聚丙烯腈(PAN)热分解时则发生交联。对可燃性而言,热分解时发生无规断链和解聚的聚合物通常比发生交联或端基消除的聚合物更加易燃。交联可促进成炭,从而降低可燃性。端基消除导致形成双键,可促进发生交联或芳构化。
9、In general, polymers with aromatic or heterocyclic groups in the main chain are less combustible than polymers with an aliphatic backbone. Polymers with short flexible linkages between aromatic rings tend to crosslink and char. These polymers are thermally stable and show relatively good flame retardancy. For example, bisphenol  A – based polycarbonate, phenol formaldehyde resins, and polyimides are self-extinguishing and show either a V-2 or V-1 rating in the UL-94 test. On the other hand, polymers with    relatively long flexible (aliphatic) linkages are still relatively combustible despite aromatics in the backbone. Examples of these polymers are poly(ethylene terephthalate), poly(butylene terephthalate), polyurethanes, and bisphenol A – based epoxy resin.

一般而言,主链上含芳环或杂环基团的聚合物比脂肪族聚合物更为难燃。芳环间较短的柔性键可发生交联并成炭,此类聚合物具有较好的热稳定性和阻燃性。例如在UL94测试中,双酚A型聚碳酸酯、苯酚甲醛树脂、聚酰亚胺都可以自熄并达到V-2或V-1级。但是,含有较长柔性键(脂肪链)的聚合物的聚合物,虽主链上有芳环却仍相对易燃;例如PET、PBT、PU、双酚A型环氧树脂。

10、Charring of polymers proceeds through various stages: (1) cross-linking, (2)aromatization, (3) fusion of  aromatics, and (4) graphitization. The ability of a polymer to perform in one or several of these stages leading to char formation depends primarily on the polymer structure. However, this performance can be improved significantly by the use of flame retardants. Although many polymers tend to cross-link at early stages of thermal decomposition, this does not necessarily result in char formation. Char is formed only if the cross-linked polymer contains aromatic fragments and/or conjugated double bonds and is prone to aromatization during thermal decomposition.

聚合物的成炭分为多个步骤,依次为:①交联;②芳构化;③芳族稠环化;④石墨化。聚合物的成炭性主要取决于其结构,但是同时也可以通过使用阻燃剂来获得提高。尽管多种聚合物在热分解初始阶段就趋于交联,但这并不足以导致成炭。炭层的形成条件是:热分解过程中,交联聚合物含有芳环碎片及/或共轭双键,并趋于芳构化。

11、Fused aromatic rings in the char tend to assemble into small stacks, which are precursors of graphite. These pregraphitic domains are embedded in the amorphous char. This type of char, called turbostratic char , is usually formed at 600 to 900?C, temperatures typically found on the surface of burning polymers. Char
that contains more pregraphitic domains is more stable to thermal oxidation and therefore less likely to burn away and expose the polymer surface to the heat of the flame. On the other hand, highly graphitized chars are rigid and may have cracks, which do not retard diffusion of combustible materials to the flame. The best-performing char would be amorphous uncracked char with a requisite pregraphitic domain content.

炭层中的芳香族稠环趋于形成小型堆叠结构,称为石墨化前体。这些前体镶嵌于无定形化炭层中形成“无规炭层”,该结构通常于聚合物表面温度为600-900℃间时形成。炭层中含有的石墨前体越多,其热稳定性就越好,因此似乎也较难燃尽,并降低聚合物表面暴露于火焰热流中的概率。但另一方面,高度石墨化的炭层十分坚硬且可能存在裂缝,导致无法有效阻止可燃材料暴露于火焰。因此,性能最为优异的炭层应是含有适量石墨化前体的无定形无裂缝炭层。


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