To decrease the reaction to fire of a highly flammable plastic, polypropylene (PP), the concept of intumescence was applied. Two intumescent systems were designed on the basis of different mechanisms: physical expansion with expandable graphite (EG) and chemical expansion with modified ammonium polyphosphate (AP). Fire behavior of PP containing EG, AP, or an AP/EG mixture with a total loading of 10 wt% was evaluated by cone calorimetry at 35 kW m−2. Thermocouples allowed measuring the temperature at the backside or inside samples over time and evaluating the thermal barrier of these intumescent materials. Two grades of AP (difference in composition) and several grades of EG (difference in expansion characteristics) were compared. Mixing AP and EG does not create a synergistic effect in studied conditions. Contrarily, the incorporation of small amount of EG in PP-AP modifies heat transfer in the coating, creating a strong anisotropy. Graphite worms are trapped vertically into the expanded AP, which increases the transverse heat conductivity (lower efficiency of the thermal barrier) and decreases the fire performance. This phenomenon disappears in thicker specimens. While a higher expansion volume of graphite worms improves fire performances of PP with only small amount of EG (1 wt%), this effect is not noticeable with AP/EG mixtures.

中文翻译：

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膨胀聚丙烯：物理和化学膨胀之间的相互作用

为了减少高度易燃塑料聚丙烯 (PP) 对火的反应，应用了膨胀的概念。根据不同的机理设计了两种膨胀系统：可膨胀石墨 (EG) 的物理膨胀和改性聚磷酸铵 (AP) 的化学膨胀。通过锥形量热法在 35 kW m-2 下评估含有 EG、AP 或总负载为 10 wt% 的 AP/EG 混合物的 PP 的燃烧行为。热电偶允许随时间测量样品背面或内部的温度，并评估这些膨胀材料的热障。比较了两个等级的AP（成分不同）和几个等级的EG（膨胀特性不同）。在研究条件下，混合 AP 和 EG 不会产生协同效应。相反，在 PP-AP 中加入少量 EG 会改变涂层中的传热，产生强烈的各向异性。石墨蠕虫被垂直困在膨胀的 AP 中，这增加了横向热导率（热障效率较低）并降低了防火性能。这种现象在较厚的试样中消失。虽然石墨蠕虫的更高膨胀体积仅使用少量 EG (1 wt%) 提高了 PP 的防火性能，但这种效果对于 AP/EG 混合物并不明显。