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胶粘剂/胶黏剂为什么能粘接的胶接理论之配位键理论

胶接理论之配位键理论

配位键理论认为,强的黏附作用来源于胶黏剂分子与被粘物在界面上生成的配位键(氢键就是一种特殊的配位键)粘接时,胶涂覆在被粘物表面后,受被粘物表面的吸引,黏剂开始润湿被粘物材料表面,同时胶分子向被粘物材料移动。在移动过程中,胶分子中带电荷部分(通常是带未共享电子或x电子的基团)逐渐向被粘物材料带相反电荷部分靠近,当这两部分距离小于0.35nm时,就结合形成配价键。配价键的形式依据胶与被粘物材料的不同而不同,常见的有含有未共享电子对的胶与金属形成的配价键和胶与被粘物材料之间含有未共享电子对,或给电子部分与缺电子部分或氢离子形成电荷转移配价键。

黏附性来源于界面上的配位键,生成配位键既需要提供未共享电子对的一方,又需要接受电子对的一方。就是说比较理想的粘接应当是,当被粘物材料是电子供给体则应采用电子接受体材料的胶黏剂进行粘接;当被粘物材料是电子接受体,则应采用电子供给体的胶黏剂进行粘接。如果在粘接中,胶黏剂与被粘物均能提供电子对或均为接受电子对的一方,则粘接就很难成功。聚四氟乙烯材料之所以难以粘接就是典型的例子,因为聚四氟乙烯可以提供电子对,而一般的胶黏剂大多可提供电子对,这样两者都能提供电子对,胶接时不能产生黏附的配位键,这就是聚四氟乙烯难粘的原因之一。

另外,金属之所以容易粘接是因为其中的金属原子都有空的价轨道,能够接受某些未共享电子对或π键,生成配位键,胶黏剂分子能有效地提供未共享电子对或π键,跨过界面生成配位键。再例如,环氧树脂之所以具有很好的粘接性,从最简单的分子结构看,它有4个氧原子2个苯环,从配位键机理分析,它应对金属有很好的黏附性,因为4个氧原子有8个配位能力很强的未共享电子对,还有2个具有共轭π键体系的苯环。另外分子中没有大的烷基链构成位阻,这就使它成为粘接金属材料的佼佼者。

配价键理论似乎比其他胶接理论能够更好地、更本质地解释各种粘接现象,在道理上说得通,实践中能得到验证,但绝不能否定其他理论,而是现有理论的深入探讨和补充。

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Coordination bond theory of bonding theory

According to the coordination bond theory, the strong adhesion comes from the coordination bond (hydrogen bond is a kind of special coordination bond) formed on the interface between the adhesive molecule and the adherent. When the adhesive is coated on the surface of the adherent, it is attracted by the surface of the adherent, and the adhesive begins to wet the surface of the adherent, and the adhesive molecule moves to the adherent. In the process of migration, the charged part (usually the group with unshared electrons or x-electrons) in the glue molecule gradually approaches to the opposite charged part of the adherent material. When the distance between the two parts is less than 0.35nm, they combine to form a valence bond. The form of valence bond is different according to the difference between the glue and the bonded material. The common ones are the valence bond formed between the glue and the metal with unshared electron pair, the valence bond formed between the glue and the bonded material with unshared electron pair, or the charge transfer valence bond formed between the electron donor part and the electron deficient part or hydrogen ion.

The adhesiveness comes from the coordination bond on the interface, and the formation of the coordination bond requires both the party who does not share the electron pair and the party who accepts the electron pair. That is to say, the ideal bonding method is to use the adhesive of the electron acceptor material when the adherent material is the electron donor; When the adherent material is the electron acceptor, the adhesive of the electron donor should be used for bonding. If both the adhesive and the adherent can provide or receive electron pairs in the bonding process, the bonding is difficult to succeed. Polytetrafluoroethylene is a typical example of why it is difficult to bond, because polytetrafluoroethylene can provide electron pairs, while most adhesives can provide electron pairs, so both can provide electron pairs, and there is no adhesive coordination bond when bonding, which is one of the reasons why polytetrafluoroethylene is difficult to bond.

In addition, the reason why metals are easy to bond is that the metal atoms have empty valence orbitals, which can accept some unshared electron pairs or electrons π The adhesive molecules can effectively provide unshared electron pairs or bonds π The coordination bond is generated across the interface. For another example, epoxy resin has good adhesion. From the simplest molecular structure, it has four oxygen atoms and two benzene rings. From the analysis of coordination bond mechanism, it has good adhesion to metals, because four oxygen atoms have eight unshared electron pairs with strong coordination ability, and two have conjugation π The benzene ring of the bond system. In addition, there is no large alkyl chain in the molecule to form steric hindrance, which makes it a leader in bonding metal materials.

It seems that the valence bond theory can better and more essentially explain all kinds of bonding phenomena than other bonding theories. It is reasonable and can be verified in practice, but it can not be determined by other theories, but is the in-depth discussion and supplement of existing theories.

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