1、 Overview of coupling agents
Coupling agent is a kind of organosilicon compound with special structure. In its molecules, it also has reactive groups which can be combined with inorganic materials (such as glass, cement, metal, etc.) and organic materials (such as synthetic resin, etc.). The commonly used theories are chemical bond theory, surface wetting theory, deformation layer theory, binding layer theory and so on. When coupling agent is used as surface modifier to fill plastics with inorganic fillers, its dispersion and adhesion can be improved.
2、 Type of coupling agent
Coupling agents mainly include organic chromium coupling agent, organic silicon coupling agent and titanate coupling agent. Organosilicon coupling agent is often used in adhesives. Its general formula is rsix3, in which R is an organic group, such as - C6H5, - CH = CH2, etc., which can combine with resin; X is a hydrolyzable group, such as - OCH3, - OC2H5, - Cl, etc.
3、 Coupling agent action process
According to the coupling process of coupling agent, B? Arkles proposed a four-step reaction model, namely:
① The six group connected with silicon atom is hydrolyzed to produce SiOH;
② Dehydration and condensation of Si Oh resulted in the formation of siloxane containing Si oh;
③ SiOH in oligosiloxane forms hydrogen bond with Oh on the surface of substrate;
④ In the process of heating and curing, covalent bond is formed with the substrate along with dehydration reaction.
It is generally believed that only one of the three silicon hydroxyl groups generated by hydrolysis of silane coupling agent on the interface is bonded to the surface of the substrate; the remaining two Si OH groups are either condensed or free with the Si OH groups in other silanes. Therefore, through the silane coupling agent, the interface of the 2 materials with different properties can be coupled, so as to improve the performance of the composite and increase the bonding strength, and obtain a new composite with excellent and reliable performance. Silane coupling agent is widely used in rubber, plastics, adhesives, sealants, coatings, glass, ceramics, metal anticorrosion and other fields. At present, silane coupling agent has become one of the indispensable auxiliaries in the material industry.
The function and effect of silane coupling agent have been recognized and affirmed by people, but why a very small amount of coupling agent on the interface has such a significant impact on the performance of the composite, there is no complete set of coupling mechanism to explain. The mechanism of coupling agent on the interface between two kinds of materials with different properties has been studied, and the explanations of chemical bonding and physical absorption have been put forward. Among them, the chemical bonding theory is the oldest but also considered to be a relatively successful theory so far.
4、 Theory of coupling agent action
1. Theory of chemical combination
According to the theory, the coupling agent contains a kind of chemical functional group, which can form covalent bond with the silanol group on the surface of glass fiber or the molecule on the surface of other inorganic fillers; in addition, the coupling agent also contains a kind of different functional group which can bond with the polymer molecule to obtain a good interface bond, and the coupling agent acts as a bridge between the inorganic phase and the organic phase 。
The following is an example of silane coupling agent to illustrate the chemical bond theory. For example, aminopropyl triethoxysilane, when it is used to treat inorganic fillers (such as glass fiber, etc.) first, the silane is hydrolyzed to silanol, and then the silanol group reacts with the surface of inorganic fillers for dehydration and chemical bond connection. The specific process is as follows:
The R group in the coupling agent will interact with the organic polymer, and finally build a bridge between the inorganic filler and the organic material.
There are many kinds of silane coupling agents. Different R groups in the general formula make different kinds of polymers suitable for the coupling agents. This is because Group R has selectivity for the reaction of polymers. For example, silane coupling agents containing vinyl and methacryloxy groups are particularly effective for unsaturated polyester resin and acrylic resin. The reason is that the unsaturated double bond in the coupling agent and the unsaturated double bond in the resin have chemical reaction under the action of initiator and accelerant. However, when the coupling agent containing these two groups is used in epoxy resin and phenolic resin, the effect is not obvious, because the double bond in the coupling agent does not participate in the curing reaction of epoxy resin and phenolic resin. However, the silane coupling agent with epoxy group is particularly effective for epoxy resin. Because the epoxy group can react with the hydroxyl group in the unsaturated polyester, the silane with epoxy group is also suitable for the unsaturated polyester, while the silane coupling agent with amine group is effective for the epoxy, phenolic, melamine, polyurethane and other resins. Silane coupling agents containing - SH are widely used in rubber industry.
2. Wetting effect and surface energy theory
In 1963, when Zsman reviewed the known aspects of surface chemistry and surface energy related to adhesion, he concluded that in the manufacturing of composite materials, the good infiltration of liquid resin into the adherends is of primary importance. If the complete infiltration can be obtained, the physical adsorption of resin on the high energy surface will provide the bonding strength higher than the cohesive strength of organic resin.
3. Theory of deformable layer
In order to alleviate the interfacial stress caused by the different thermal shrinkage between the resin and the filler when the composite is cooled, it is hoped that the resin interface adjacent to the treated inorganic material is a flexible deformable phase, so the toughness of the composite is the largest. The surface of the inorganic material treated by coupling agent may choose a certain mixture in the absorption resin, and the uneven solidification of the inter phase area may lead to a flexible resin layer which is much thicker than the multi molecular layer between the polymer and the filler of coupling agent. This layer is called deformable layer, which can relax the interface stress and prevent the expansion of the interface crack, thus improving the interface bonding strength and improving the mechanical properties of the composite.
4. Theory of constraint layer
In contrast to the theory of deformable layer, the theory of constrained layer holds that the resin in the region of inorganic filler should have a certain modulus between inorganic filler and matrix resin, and the function of coupling agent is to "tighten" the polymer structure in the region of interphase. From the point of view of the properties of the reinforced composite, in order to obtain the maximum adhesion and hydrolysis resistance, it is necessary to have a constraint layer at the interface.
As for titanate coupling agent, its combination with organic polymer in thermoplastic system and thermosetting composite containing fillers is mainly based on the solubility and intertwining of long-chain alkyl, and forms covalent bond with inorganic fillers. All the above hypotheses reflect the coupling mechanism of coupling agents from different theoretical aspects. In the actual process, it is often the result of several mechanisms working together.