Coupling Agent Selection of Different Fillers and How to ...

1. Select the appropriate coupling agent according to different fillers

According to recent findings, the choice of coupling agents is crucial for optimizing the properties of composite materials. Different fillers, such as silica or glass fibers, require specific coupling agents to achieve the best results in adhesion and performance.

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Coupling agents can be categorized into silane coupling agents, titanate coupling agents, and other types based on their chemical structures.

Generally, the functional groups at both ends of the coupling agent react with the dispersed phase of the filler and the matrix polymer, leading to varying coupling effects depending on the type of filler used. For instance, silane coupling agents demonstrate a significant impact on materials like silica, aluminum oxide, glass fiber, clay, silicate, and silicon carbide. In contrast, they show less effectiveness on asbestos, titanium dioxide, and carbon black among others.

For fillers with thiol groups on the surface, silane coupling agents tend to provide substantial coupling effects, while for calcium, magnesium, and barium compounds such as carbonates and sulfates, the effect is minimal. Employing a double coating method, where filler particles are treated with a hydrosol of silica before applying a silane coupling agent, results in improved coupling effectiveness.

Both titanate and aluminate coupling agents can modify the filler surface, but the results can vary widely.

For instance, titanate coupling agents work well with fillers like calcium carbonate, barium sulfate, and aluminum hydroxide, while their efficacy decreases with mica, silica, and talcum powder. Conversely, there is negligible effect on graphite and certain pigments.

2. How to choose the correct coupling agent?

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It is important to assess the moisture content of the filler before selecting a coupling agent. The ideal agent should correspond to the moisture level observed. For dry fillers, a monoalkoxy type coupling agent is preferable, whereas a chelation type or monoalkoxy pyrophosphate type is suited for wet fillers.

Factors such as the melting point, molecular weight, and chemical makeup of the polymer must also be taken into account. In the case of thermosetting polymers, curing temperature and mechanism are critical. Typically, finer particle fillers exhibit better coupling effects than coarser ones, with ultrafine fillers presenting unique advantages.

The dosage of coupling agents generally ranges from 0.5% to 3% of the treated material's weight, with an optimal range of 1% to 1.5%. An excess can diminish performance, while insufficient amounts lead to incomplete encapsulation. It’s essential to conduct dosage tests to ensure both effectiveness and cost efficiency.

Due to its small required quantity, titanate coupling agents must be evenly distributed within the filler to achieve their intended effect.

A coupling agent is a chemical that enhances adhesion between two phases in composite materials, which comprise multiple distinct constituents not chemically bound together. Two relevant examples include:

1. 1.

   Resins reinforced with glass fibers;

2. 2.

   Thermosetting resins and thermoplastics with particulate fillers.

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