氟橡胶是一种具有优异性能的高分子材料,含有氟原子的C-F键性能很高,同时氟原子有极大的吸附效应。这种特殊的分子结构赋予了氟橡胶优异的耐热性、耐药品性、耐溶剂性、耐氟化性、耐真空性、耐油性、耐老化等多种性能。氟橡胶最早应用于航空领域,但应用最广泛的是在汽车领域,占应用总量的60%~70%。因此,确保选择合适的氟橡胶是十分重要的。
氟橡胶分为氟碳橡胶、氟硅橡胶、氟化磷腈橡胶等种类,每种氟橡胶根据化学组成的不同,性能也不尽相同。目前,常见的氟橡胶有氟橡胶23、氟橡胶26、氟橡胶246、氟橡胶TP、偏氟醚橡胶、全氟醚橡胶和氟硅橡胶等七类。
通过机理分析,我们得知氟元素是已知的化学元素中电负性最强的元素,C-F键能很高。氟原子还能形成C-C键的保护屏障,赋予了含氟高分子弹性体C-C键的化学惰性。氟橡胶的键能增加,耐热性和耐腐蚀性都得到提高。同时,由于含氟化合物化学键的自由旋转能大为增加,使氟碳弹性体分子的刚性增强,柔性和耐低温性能有所下降。
通过对比试验,我们发现氟橡胶具有低气体透过性、良好的力学性能、耐高温性能较好、在短期内能够耐受高温,但耐受低温性能一般。常见橡胶的气体透过系数可以在表4中查看。 氟橡胶在汽车领域中的应用占了很大的比例。
在满足所需交联度的条件下,氟橡胶生产中应尽量减少硫化剂的使用量,以避免对耐热性能的影响。虽然增加补强剂的用量可提高其机械强度和电性能,但用量过多也会影响其耐热性。因此,氟橡胶的生产工艺中,氟含量、分子量、分子量分布、硫化剂浓度等系数的差异也往往是造成氟橡胶间特性差异的主要原因。表5-9及图6可见这些系数与氟橡胶特性之间的关系。
表5展示了含氟量与各特性之间的关系,图6则显示了氟含量与耐甲醇、耐丙酮之间的关系。表6、表7和表9分别展示了硫化方法、分子量和硫化剂与硫化促进剂与特性间的关系。表8则展示了分子量分布的影响。
氟橡胶中常见的有四丙氟橡胶(FEPM)、全氟醚橡胶(FFKM)和偏氟乙烯系氟橡胶(FKM)。FEPM和FKM由于分子结构的不同,其对不同化学物质的耐久性也不同。对碱、胺具有优异的耐久性的FEPM对于其他FKM无法使用的用途中可发挥其优势,同时还具有耐热性和电气绝缘性。而FKM则对汽油性的耐久性和低温柔软性较好。根据性能要求,需分开使用。图7展示了FEPM与FKM的比较,表10则展示了Kalrez与FKM在不同介质中的体积增加率。FFKM由于其主链中四氟乙烯被氟化,具有抗高温及化学药品的稳定的结构,可抵抗1600多种化学物质的腐蚀,因此在工业上具有各种特殊应用。图8则展示了FFKM与FKM的比较,图9则展示了Kalrez常用牌号类型。
氟橡胶因其优异的性能已经得到了越来越广泛的应用,随着人们对其制造工艺的不断改进和应用的深入研究,未来,这一综合性能最佳的密封材料势必在更多的领域得到推广应用。
Summary keywords: 氟橡胶、硫化剂、分子量、分子量分布、FEPM、FKM、FFKM
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