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[塑料] 塑料阻燃深度总结(九)硼酸盐阻燃剂

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发表于 2013-10-20 09:19:01 | 显示全部楼层 |阅读模式
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1.3.2.5 硼酸盐阻燃剂
44、Water-soluble borates such as sodium borate (borax) and boric acid have long been used to flame-retard cellulosic materials (e.g., paper boards, wood, and some technical textiles). On the other hand, water-insoluble and more thermally stable zinc borates have found use in thermoplastics. The mechanism of fire retardant action of these two types of borates is quite different.

长期以来,水溶性硼酸盐(如硼酸钠(硼砂)、硼酸)常被用于阻燃纤维素材料(如纸张、木制品、工业纺织品);而水不溶性且热稳定性较好的硼酸锌常被用于热塑性塑料。这两类硼酸盐的阻燃机理是截然不同的。
45It is believed that soluble borates can esterify the OH groups of cellulose and
promote char formation similar to that of phosphorus. For example, a comparison of the performance of ammonium pentaborate, which decomposes and releases boric acid, and ammonium polyphosphate, which releases polyphosphoric acid, showed some similarity. Borates and boric acid also release some water, which provides a heat sink. Sodium borate and boric acid or anhydride or their mixtures are low-melting solids. Their viscous glassy melts can cause intumescence by evolved decomposition gases, mostly water, or they can just cover the surface of the pyrolyzing polymer or char, healing cracks and providing a barrier to heat and decomposition products.

类似于磷系阻燃剂的作用,水溶性硼酸盐可与纤维素的-OH发生酯化反应并促进成炭。例如,对比五硼酸铵(分解释放硼酸)和APP(释放聚磷酸)的作用机理发现,二者之间存在一定的相似之处。另外,硼酸盐和硼酸也可释放一定量的水,起到热阻隔的作用。硼酸钠与硼酸、硼酸酐或其混合物熔点较低;一定温度时,它们可形成粘性玻璃化熔体,分解释放出的气体(主要为水蒸气)可致其发生膨胀,使它们可覆盖于裂解聚合物或炭层的表面以“修补”炭层裂缝,不能并阻止热量与裂解可燃性产物的释放。
46Several grades of zinc borates are commercially available, which release different amounts of water. Although in formulas for borates, water is often shown as a water of hydration, in fact, borates are rather complex hydroxide salts. Upon heating and polymer combustion, zinc borates dehydrate endothermically, and vaporized water absorbs heat and dilutes   oxygen   and   gaseous  flammable  components.  For  example,  zinc  borate  
2ZnO·3B 2O3·3.5H2O, known as Fire-brake ZB (U.S. Borax), loses about 13.5 wt% water at 290 to 450?C and absorbs 503 J/g. Thermogravimetric curves of thermal decomposition of various borates are shown in Figure 1.4. Zinc borates are often used in halogen-containing systems and most often in PVC. In PVC, zinc borates significantly increase the amount of char formed during combustion. Zinc borates react with hydrogen chloride released from the thermal decomposition of PVC. Then zinc chloride catalyzes dehydrohalogenation and promotes cross-linking. This leads to an increase in char yield, and, even more important , a significant decrease in smoke formation. At sufficiently high temperatures, zinc borate can melt to produce a glassy layer, but this usually does not happen in s mall flames. Instead, zinc borate sinters and helps improve the insulating properties of the char and inhibits afterglow combustion.

许多种硼酸锌(脱水量不同)都是可以进行工业化应用的。尽管硼酸盐的化学式为水合物,但是硼酸盐是一种结构复杂的盐。加热时,硼酸锌发生脱水吸热反应,水蒸气可吸热并稀释氧气与可燃气体。例如,牌号为Firebrake ZB(美国硼砂)的硼酸锌化学式为2ZnO·3B2O3·3.5H2O,290℃升温至450℃的过程中可脱去13.5%的水,吸热量为503J/g。图1.4为三种工业硼酸锌热热失重曲线。硼酸锌通常用于卤系阻燃系统,最常用于PVC,因燃烧时硼酸锌可显著提高PVC的成炭量。硼酸锌可与PVC的热分解产物HCl反应生成ZnCl,后者可催化PVC脱除卤化氢并促进交联,从而提高PVC的成炭率,而更为重要的是有效降低了生烟量。相反,在足够高的温度下,硼酸锌可在聚合物表面形成熔体层,但这通常在小火中难以发生。相反,结块硼酸锌可提高炭层的隔热性并有效地抑制阴燃。

47、Zinc borate can also change the oxidative decomposition pathway of halogen-free polymers. It is not completely clear if this is happening because of an inhibition effect of boron oxides toward the oxidation of hydrocarbons or the oxidation of graphite structures in the char, or is due purely to the formation of a protective sintered layer. In combination with ATH, zinc borate creates a porous ceramiclike residue, which has much better insulative properties than those of pure anhydrous alumina. It was shown that zinc borate accelerates dehydration of magnesium hydroxide and creates a ceramiclike structure with dehydrated MgO.

研究表明,硼酸锌也可以改变无卤聚合物的热氧降解方式,但尚未确定是硼氧化物对烯烃聚合物或炭层中石墨化结构发生氧化的抑制作用所引起的,或单单是由于形成了具有热阻隔性的结块层。同ATH复配使用时,硼酸锌可形成多孔的类陶瓷残留物,这比单一的无水氧化铝隔热性更佳。研究发现,硼酸锌可加速MH的脱水,并与无水MgO形成类陶瓷结构。



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