News

The functions and effects of silane coupling agents are now well understood and widely recognised. However, why such a tiny amount of coupling agent at the interface can have such a significant effect on the properties of a composite still has no complete coupling mechanism to explain it. Considerable research has been devoted to the mechanism of coupling agents at the interface between two materials of different natures, giving rise to explanations such as chemical bonding and physical adsorption. Among them, the chemical bonding theory is the oldest and, to date, the most successful theory.

1. Chemical Bonding Theory

This theory holds that the coupling agent contains a chemical functional group that can react with the silanol groups on the surface of glass fibres or with the surface molecules of other inorganic fillers to form covalent bonds. In addition, the coupling agent contains another, different functional group that bonds with the polymer molecule, achieving good interfacial bonding. The coupling agent thus acts as a bridge connecting the inorganic phase and the organic phase.

The chemical bonding theory is illustrated below using a silane coupling agent as an example — for instance, aminopropyltriethoxysilane. When it is first used to treat an inorganic filler (such as glass fibre), the silane first hydrolyses to a silanol; the silanol groups then undergo a dehydration reaction with the inorganic filler surface to form chemical bonds, as shown in the reaction scheme below:

Hydrolysis of the silane groups — reaction of the resulting hydroxyl groups with the inorganic filler — when the silane-treated inorganic filler is used to prepare the composite, the Y group in the coupling agent interacts with the organic polymer, finally building the bridge between the inorganic filler and the organic material.

There are many types of silane coupling agents, and different Y groups in the general formula suit different polymer types. This is because group Y reacts selectively with polymers. For example, silane coupling agents containing vinyl and methacryloxy groups are particularly effective with unsaturated polyester resins and acrylic resins — the unsaturated double bond of the coupling agent reacts chemically with the unsaturated double bond of the resin in the presence of initiators and promoters. However, coupling agents with these two groups show little effect with epoxy and phenolic resins, because their double bond does not take part in the curing reaction of epoxy or phenolic resins. Conversely, silane coupling agents bearing epoxy groups are particularly effective with epoxy resins; and because the epoxy group can react with the hydroxyl groups of unsaturated polyester, epoxy-containing silanes are also suitable for unsaturated polyesters. Amino-containing silane coupling agents are effective with epoxy, phenolic, melamine and polyurethane resins, while silanes containing –SH groups are the grades most widely used in the rubber industry.

Through the two reactions above, silane coupling agents use chemical bonding to improve the adhesion between the polymer and the inorganic filler in composites, greatly enhancing their properties. How effective is the coupling treatment? It can be characterised by calculating the theoretical bonding force. According to the adhesion theory of interface chemistry, the secondary-bond adhesion per unit area between the adhesive and the adhered material is mainly accounted for by dispersion forces.

2. Wetting Effect and Surface Energy Theory

In 1963, Zisman, reviewing what was then known about the surface chemistry and surface energy relevant to adhesion, concluded that good wetting of the adhered material by the liquid resin is of prime importance in the manufacture of composites. If complete wetting is achieved, the physical adsorption of the resin onto the high-energy surface will provide a bonding strength higher than the cohesive strength of the organic resin.

3. Deformable Layer Theory

To relieve the interfacial stresses that arise during cooling of the composite from the difference in thermal shrinkage between the resin and the filler, it is desirable that the resin interface adjacent to the treated inorganic material be a flexible, deformable phase, giving the composite maximum toughness. The surface of the filler treated with the coupling agent may preferentially absorb a particular compounding ingredient of the resin; the uneven curing across the interphase region may produce a flexible resin layer far thicker than the multimolecular layer of coupling agent between polymer and filler. This layer is called the deformable layer. It can relax interfacial stresses and prevent the propagation of interfacial cracks, thereby improving interfacial bonding strength and raising the mechanical properties of the composite.

4. Restrained Layer Theory

In contrast to the deformable layer theory, the restrained layer theory holds that the resin within the region of the inorganic filler should have a modulus somewhere between that of the inorganic filler and that of the matrix resin, and that the function of the coupling agent is to “tighten” the polymer structure within the interphase region. From the performance of the reinforced composite, achieving maximum adhesion and hydrolysis resistance requires a restrained layer at the interface.

As for titanate coupling agents, their bonding with the organic polymer in thermoplastic systems and in filled thermosetting composites is mainly based on the compatibility and intertwining of long-chain alkyl groups, together with the formation of covalent bonds with the inorganic filler. The hypotheses above reflect the coupling mechanism of coupling agents from different theoretical standpoints. In practice, several mechanisms usually act together.