Application area

Fillers, Pigments & Fibers

The application of coupling agent used in fillers, pigments and fibers

The most important function of the coupling agent is to make the surface of the inorganic material have organic reactivity through the action of "molecular bridge", so that the surface of the inorganic material is organically functionalized, thereby coupling and connecting the polymer (resin, rubber, plastic) and inorganic material (filler), and then improve the mechanical and electrical properties of the resin-based composite material or improve the adhesion of the resin coating to the inorganic substrate and the polar organic substrate.

As customers' requirements for product performance become higher and higher, surface modification of inorganic mineral powders, pigments and fibers has become more and more common, and product processing has become more refined and differentiated. There are also more and more inorganic mineral powders, inorganic mineral fibers and pigments which are surface-modified with silane coupling agents and Titanium (aluminum) coupling agent. Various inorganic mineral powders, pigments, fibers and abrasives such as Glass fiber, basalt fiber, glass microbead, glass phosphorous film, glass powder, silicon dioxide, silicon micropowder, wollastonite, quartz, mica, diatomaceous earth, kaolin, talc, aluminum hydroxide, magnesium hydroxide, oxidation Aluminum, asbestos, aluminum silicate, aluminum silver powder, calcium carbonate, barium sulfate, various metal powders, various metal oxides, ultramarine blue, titanium dioxide, iron oxide, zinc antimony white, zinc oxide, carbon black, lead chrome yellow, cadmium yellow, chromium oxide green, lead chrome green, iron blue, cobalt blue, diamond, corundum, silicon carbide, etc. are all suitable for surface modification with a coupling agent.

Suitability of coupling agent for different fillers and pigments

The silane coupling agent mainly functions by chemical bonding and physical adsorption of the silicon hydroxy group formed by hydrolysis with a reactive group (mainly a hydroxyl group) on the surface of the material, so the organofunctional silane has a good bonding force to the material has silicon hydroxy group or a high surface energy. And there is substantially no modification effect on a material having no hydroxyl group, non-polarity and low surface energy on the surface. Low surface energy materials can be surface treated by corona, flame, sand blasting, Acid and alkali washing, primer coating, etc. to improve surface energy.

Matching of silane coupling agent with inorganic pigment filler or substrate


The titanium (aluminum) coupling agent reacts with the surface of the filler or substrate by a so-called "protonation reaction", which does not require a hydroxyl group on the surface of the filler (the silane coupling agent has this limitation) and does not require Pre-hydrolysis or filler surface requires residual water (this is required for silane coupling agents). Titanium (aluminum) coupling agent is basically suitable for all types of fillers and pigments, regardless of whether or not the surface contains hydroxyl groups. It can also have good effect on the fillers which cannot be modified by silane coupling agents such as carbon black, graphite, calcium carbonate and barium sulfate.

Method for modifying inorganic powder and fiber by coupling agent

1、Dry modification

The dry modification is a surface coating treatment of the powder by using a high-speed stirring device with an appropriate amount of a silane coupling agent or a titanium (aluminate) coupling agent. The coupling agent may be directly added, or may be added after being diluted with a solvent (such as an alcohol), and the method of adding is preferably sprayed. Stir for 3 to 15 minutes, and adjust according to equipment conditions and stirring speed. In order to complete the reaction, it is recommended to dry at 104 to 121 °C.

2、wet modification

For some fillers produced by precipitation methods, such as precipitated silica, aluminum hydroxide, various whiskers. The silane or water-soluble titanate coupling agent can be formulated into an aqueous solution, added to the slurry of the powder, and the silane is adsorbed on the surface of the powder, and the chemical bonding is usually completed in the subsequent dehydration and drying process. For glass fibers, basalt fibers, the silane coupling agent is usually added to the sizing agent, and the fibers are dried after the sizing agent is wetted.

The silane addition

In theory, the formula for the amount of silane is calculated as follows :


The approximate specific surface area of some common filler is shown in the following table. The specific surface area of certain filler can be inquired for the product specification of the filler.:

Filler or fiber

Specific surface area(m2/g)

Electronic grade fiberglass


Quartz sand








Calcium silicate


Minimum wetting area of the commonly used silane:

Silane model

Minimum wetting area (m2/g)





























It should be noted that the calculation result of the theoretical dosage of the above silane is often different from the actual use amount, this is only for a reference value. Users need to optimize the design according to equipment conditions, cost constraints and performance requirements, and determine the modification conditions, process parameters and silane additions that best meet the requirements and the best cost performance through multiple tests. In general, 0.5% to 1.5% of the amount of powder is a relatively common range of addition, and 1% of the addition is suitable for most applications.

For powder materials without internal porosity and without strong adsorption, the empirical data in below Table 10 can be used:

Powder particle size

Silane addition

≤ 1 Micron


1-10 Micron


10-20 Micron


Particle size similar or thicker to sea sand

≤ 0.1%

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