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A coupling agent is an important treating agent whose field of application is growing steadily; it is mainly used as an additive for polymer composites. The most distinctive feature of the coupling agent molecule is that it contains two groups of different chemical nature: one is an inorganic-philic group that reacts readily with inorganic surfaces, and the other is an organic-philic group that can react chemically with synthetic resins or other polymers or form hydrogen bonds to dissolve into them. Coupling agents are therefore called “molecular bridges”: they improve the interfacial interaction between inorganic and organic materials, greatly enhancing the properties of composites — physical, electrical, thermal and optical properties among others. In the rubber industry, coupling agents increase the abrasion resistance and ageing resistance of products such as tyres, rubber sheets, hoses and footwear, and can reduce NR usage, thereby lowering cost. There are many types of coupling agents, mainly silane, titanate and aluminate coupling agents; silane and titanate coupling agents are currently the most widely used.

1. Silane Coupling Agents

Silane coupling agents were the first coupling agents to be researched and used. Thanks to their unique properties and the continuous introduction of new products, their application fields have gradually expanded and they have become an important branch of the organosilicon industry. They are a category of organosilicon products that has developed rapidly in recent years, with a great variety of types and novel structures — more than one hundred products with known structures alone. Around 1945, companies such as Union Carbide (UC) and Dow Corning in the United States developed and published a series of silane coupling agents with typical structures. In 1955, UC first proposed amino-containing silane coupling agents. From 1959 onwards, a series of modified amino silane coupling agents appeared one after another. Peroxy-containing silane coupling agents appeared in the early 1960s, and silane coupling agents with diazo and azide structures appeared at the end of the 1960s, greatly enriching the variety of silane coupling agents. In recent decades, the development of glass-fibre reinforced plastics has stimulated research into and development of various coupling agents. The synthesis and application of modified amino silane, peroxy silane and azide silane coupling agents were the main achievements of this period. China began developing silane coupling agents in the mid-1960s. The Institute of Chemistry of the Chinese Academy of Sciences began developing γ-functional silane coupling agents, and Nanjing University simultaneously began developing α-functional silane coupling agents.

1.1 Structure and Mechanism of Action

The general formula of silane coupling agents is RnSiX(4−n), where R is a non-hydrolysable organic functional group that can bond with the polymer. Depending on the nature of the polymer, R should have a strong affinity for or reactivity with the polymer molecule — for example, methyl, vinyl, amino, epoxy, mercapto and acryloxypropyl groups. X is a hydrolysable group: it decomposes upon contact with aqueous solutions, moisture in the air or water adsorbed on the surface of inorganic substances, and reacts readily with inorganic surfaces. Typical X groups include alkoxy, aryloxy, acyl and chloro groups; the most commonly used are methoxy and ethoxy, which generate methanol and ethanol respectively as by-products of the coupling reaction. Because chlorosilanes generate corrosive hydrogen chloride as a by-product of the coupling reaction, they should be used with discretion. In recent years, silane coupling agents of higher relative molecular mass and with special functional groups have developed rapidly — such as octenyl, dodecyl, and silanes containing peroxy, ureido, carbalkoxy and cationic alkyl groups. When silane coupling agents are used to treat carbon fibre surfaces, the silanol groups produced by hydrolysis of the methylsilyloxy end groups of the coupling agent bond with the hydroxyl functional groups on the carbon fibre surface; as a result, the tensile strength and modulus of the composite increase and the void content falls. In studies of the treatment of glass fibre surfaces with alkylchlorosilane coupling agents, it was found that using a silane group able to react with the resin to treat glass fibres in polyester glass-reinforced plastics can more than double the strength. Treating glass fibre surfaces with alkylchlorosilane hydrolysates enables chemical bonding with the resin. This was the first time the state of a surface treatment agent at the interface was explained from the molecular point of view. Because silane coupling agents carry these two types of chemical groups in the molecule, they can react with hydroxyl groups on inorganic materials while interacting with the long molecular chains of organic materials, achieving a coupling effect.

The mechanism of action can be roughly divided into the following three steps: (1) the X group hydrolyses to a hydroxyl group; (2) the hydroxyl group forms hydrogen bonds with hydroxyl groups present on the inorganic surface, or dehydrates to form ether bonds; (3) the R group combines with the organic material.

1.2 Applications

In using silane coupling agents, each particular application needs to be tested and pre-selected to obtain the best effect. The following summarises the silane coupling agents used for different materials, based on general rules and testing experience. Silane coupling agents are generally used as a very dilute aqueous or aqueous-ethanol solution; they may also be dissolved in water alone, but an aqueous acetic acid solution of mass fraction 0.001 should first be prepared to improve solubility and promote hydrolysis. They can also be used as non-aqueous solutions, such as solutions in methanol, ethanol, propanol or benzene; and they can be used directly. The dosage of the silane coupling agent is related to its type and the surface area of the filler, i.e. silane dosage (g) = [filler dosage (g) × filler surface area (m²·g⁻¹)] / minimum covering area of the silane (m²·g⁻¹). If the surface area of the filler is unknown, the amount of silane coupling agent can be set at about 1% of the filler quantity.