There are many types of coupling agents, which can be classified into four categories based on their chemical structure and composition: organic chromium complexes, silanes, titanates, and aluminum compounds. Today, the editor will introduce the mechanism of action of one of the silane coupling agents as follows.
The application and effectiveness of silane coupling agents have been recognized and affirmed by people, but there is currently no complete coupling mechanism to explain why the extremely small amount of coupling agents at the interface has such a significant impact on the properties of composite materials. The mechanism of action of coupling agents at the interface between two materials with different properties has been extensively studied, and explanations such as chemical bonding and physical adsorption have been proposed. The chemical bonding theory is the oldest but still considered one of the most successful theories to date.
1. Chemical bonding theory
This theory suggests that coupling agents contain a chemical functional group that can interact with silanol groups on the surface of glass fibers or molecules on the surface of other inorganic fillers to form covalent bonds; In addition, coupling agents also contain a different functional group that bonds with polymer molecules to achieve good interfacial bonding. Coupling agents act as bridges between inorganic and organic phases.
The following uses silane coupling agents as an example to illustrate the theory of chemical bonds. For example, when using aminopropyltriethoxysilane to first treat inorganic fillers (such as glass fibers), the silane first hydrolyzes into silanol, and then the silanol group undergoes dehydration reaction with the surface of the inorganic filler, forming a chemical bond connection. The reaction equation is as follows:
Hydrolysis of functional groups in silane—— Reaction between hydroxyl groups and inorganic fillers after hydrolysis—— When inorganic materials treated with coupling agents are filled to prepare composite materials, the Y group in the coupling agent will interact with the organic polymer, ultimately building a bridge between the inorganic filler and the organic matter.
There are many types of silane coupling agents, and the types of polymers suitable for coupling agents vary depending on the Y group in the formula. This is because the Y group has selectivity in the reaction of polymers. For example, silane coupling agents containing ethylene and methacryloyloxy groups are particularly effective for unsaturated polyester resins and acrylic resins. The reason is that the unsaturated double bonds in the coupling agent and the resin undergo chemical reactions under the action of initiators and promoters. However, coupling agents containing these two functional groups have limited effectiveness when used for epoxy resins and phenolic resins, as the double bonds in the coupling agent do not participate in the curing reaction of epoxy resins and phenolic resins. However, silane coupling agents with epoxy groups are particularly effective for epoxy resins, and because epoxy groups can react with hydroxyl groups in unsaturated polyesters, silane containing epoxy groups is also suitable for unsaturated polyesters; And silane coupling agents containing amino groups are effective for resins such as epoxy, phenolic, melamine, polyurethane, etc. Silane coupling agents containing - SH are widely used in the rubber industry.
Through the above two reactions, silane coupling agents improve the adhesion between high polymers and inorganic fillers in composite materials through chemical bonding, greatly improving their performance. So, what is the treatment effect of coupling agents? It can be characterized by the calculation of theoretical bonding force.
According to the bonding theory of interface chemistry, the adhesion force of the secondary bond between the adhesive and the adherend per unit area mainly considers the dispersion force.
2. Infiltration effect and surface energy theory
In 1963, ZISMAN, while reviewing the known aspects of surface chemistry and surface energy related to bonding, concluded that in the manufacturing of composite materials, good wetting of the adhered material by liquid resin is of paramount importance. If complete wetting can be achieved, the physical adsorption of high-energy surfaces by the resin will provide bonding strength higher than the cohesive strength of organic resin.
3. Deformable layer theory
In order to alleviate the interface stress caused by the different thermal shrinkage rates between the resin and filler during the cooling of composite materials, it is desirable for the resin interface adjacent to the treated inorganic material to be a flexible and deformable phase, so as to maximize the toughness of the composite material. The surface of inorganic materials treated with coupling agents may preferentially absorb a certain compounding agent in the resin, and the uneven curing of the interphase region may result in a much thicker flexible resin layer than the multi molecular layer between the polymer and filler of the coupling agent. This layer is called the deformable layer, which can relax the interface stress, prevent the expansion of interface cracks, thus improving the bonding strength of the interface and enhancing the mechanical properties of the composite material.
4. Constraint layer theory
Compared to the deformable layer theory, the constrained layer theory suggests that the resin in the inorganic filler region should have a modulus between the inorganic filler and the matrix resin, and the function of the coupling agent is to "tightly bind" the polymer structure in the interphase region. From the perspective of the performance of the enhanced composite material, in order to achieve maximum adhesion and hydrolysis resistance, a constraint layer is required at the interface.
As for titanium ester coupling agents, their binding with organic polymers in thermoplastic systems and thermosetting composites containing fillers is mainly based on the solubility and entanglement of long-chain alkyl groups, and they form covalent bonds with inorganic fillers. The above assumptions reflect the coupling mechanism of coupling agents from different theoretical perspectives. In practical processes, it is often the result of several mechanisms working together. For more knowledge on coupling agents, please feel free to consult
