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In the construction industry, titanates are mainly used in building coatings, insulation materials and glass. As a transparent material, titanates can serve as a transparent insulation layer during building construction, protecting the interior environment of the building while improving its energy efficiency.

Titanate coupling agents are a class of multifunctional additives developed by Kenrich Petrochemicals, USA, in the 1970s. They combine the functions of dispersants, wetting agents, adhesion promoters, curing catalysts, crosslinkers, rust inhibitors and flame retardants. Compared with the corresponding alternatives, titanate coupling agents offer an excellent cost-performance ratio, and are of great significance for upgrading coating quality and promoting the development of the coatings industry. Titanate coupling agents differ according to the hydrophilic groups and lipophilic groups attached to the central titanium atom. In terms of chemical structure they can be divided into three types: monoalkoxy, chelate and coordinate. Among the three types, monoalkoxy titanate coupling agents have the largest number of grades, contain various functional groups and characteristics, suit the widest range of applications, and are low in price and high in output — giving them a broad market space in coatings applications.

1. Types and Coupling Mechanism

Types

Among monoalkoxy titanate coupling agent products, most grades (for example those containing isopropoxy groups) have poor water resistance, with the exception of the ethanolamino and pyrophosphate types, and are suitable for the surface treatment of dry pigments and fillers for use in water-free solvent-borne coatings. The representative products now being vigorously developed and used in solvent-borne coatings are mainly of three types: monoalkoxy fatty-acid ester type, monoalkoxy phosphate type and monoalkoxy pyrophosphate type.

1.2 Coupling Mechanism

The main coupling theories for titanate coupling agents are the chemical bonding theory, the coordination theory and the deformable layer theory. The effect of a titanate coupling agent should be attributed to its influence on the interface — that is, it can form chemical bridge bonds between the inorganic filler and the organic polymer. It is generally held that only one isopropoxy group is the hydrolysable group that couples with the inorganic material, so a monomolecular layer can form on the surface of the inorganic pigment or filler. Furthermore, because of its own chemical structural features, the titanate coupling agent has a surface-modifying effect.

2. Method of Use and Dosage

2.1 Method of Use

Monoalkoxy titanate coupling agents react chemically on the surface of pigments and fillers to form a monomolecular layer, allowing every molecule to act as a bridge — hence a small dosage achieves a large effect. To distribute the small amount of coupling agent uniformly over the pigment and filler surface, an effective method of use must be adopted to give full play to its function.

2.1.1 Pretreatment Method

Pretreating the pigment and filler surface with the titanate coupling agent beforehand is somewhat troublesome but gives a better effect. The procedure is as follows:

(1) dilute the coupling agent with a small amount of an aromatic solvent such as toluene or xylene; (2) mix the coupling agent solution with the pigment/filler in a high-speed mixer, raise the temperature to about 90°C under high-speed stirring (about 1000 r/min) and stir for about 0.5 h (or 15 min) so that every pigment and filler particle is coated with a layer of coupling agent; discharge and store for later use; (3) use the modified pigment/filler to formulate the coating by the normal paint-making process.

2.1.2 Direct Addition Method

The direct addition method adds the coupling agent directly during paint manufacture. First dissolve the weighed coupling agent in a solvent such as mineral spirits or xylene, disperse it into the organic binder, add the pigment/filler and disperse at high speed, then formulate the coating in the usual way. It should be noted that titanate coupling agents are generally added directly before the grinding step; because titanate coupling agents react readily with additives and resins bearing hydroxyl or carboxyl groups, substances containing such reactive groups must be added after grinding, otherwise the effect of the titanate coupling agent will be impaired.

2.2 Dosage

The dosage of titanate coupling agent is still to a large extent based on experience or semi-empirical rules. Most manufacturers consider that the dosage generally lies in the range of 0.25%–2%.

In theory, the dosage of titanate coupling agent should make all of its hydrophilic reactive groups react with the hydroxyl groups or protons supplied by the pigment/filler; there is no need to use an excess. In practice, however, the optimum dosage must be decided according to the particle size of the filler and the condition of its surface functional groups. A viscosity method can be used to determine the relationship between viscosity and coupling agent dosage; the point of maximum viscosity drop corresponds to the optimum titanate dosage.

3. Possible Hydrolysis–Condensation Reactions

The alkoxy groups of titanates are reactive groups that readily undergo hydrolysis, alcoholysis, acidolysis and condensation reactions. In the monoalkoxy titanate coupling agent i-C₃H₇OTiOX₃, X is a long-carbon-chain fatty acid ester, phosphate ester or pyrophosphate ester group. Because the titanium atom carries three long-carbon-chain X groups that do not hydrolyse readily, they partially shield the isopropoxy group sterically, reducing its reactivity and making hydrolysis difficult. Even so, the reactivity of the isopropoxy group still exists; for example, KR-TTS-type coupling agents may undergo hydrolysis–condensation reactions as follows:

Hydrolysis–Condensation Reaction

If no isopropoxy reactive group remains in the hydrolysis–condensation products, they can no longer undergo coupling reactions with inorganic pigments and fillers. Monoalkoxy titanate coupling agents must therefore be kept sealed and dry during storage and use to prevent hydrolysis and loss of effectiveness.

4. Examples of Product Applications

Since the early 1970s, when Kenrich Petrochemicals of the USA developed monofunctional titanate coupling agents, countries around the world have competed to invest in the research, development and production of titanates — including DuPont (USA), Tioxide (UK), Dynamit Nobel (Germany) and Ajinomoto (Japan). China began developing titanate coupling agents in the 1980s and, after more than 20 years of effort, has gradually approached the advanced international level in terms of product variety and performance, while holding a certain price advantage in many products. Taking three products manufactured by Nanjing Herun Coupling Agent Co., Ltd. as examples, the following shows the application possibilities and broad prospects of monoalkoxy titanate coupling agents in coatings.

4.1 Monoalkoxy Fatty-Acid Type GR-100

GR-100 is a monoalkoxy fatty-acid titanate coupling agent; the corresponding overseas grade is KR-TTS (Kenrich Petrochemicals, USA). As a solid titanate coupling agent, when used to treat fillers it offers high reactivity, good dispersion and good heat stability, and solves the stickiness and yellowing that occur when aluminates are used to treat fillers — making it an excellent surface treatment agent for inorganic materials. In coatings practice, adding GR-100 to treat fillers such as calcium carbonate, talc, titanium dioxide, aluminium hydroxide, diatomite, clay, carbon black and ferrite improves the mechanical properties of the product, reduces mechanical wear, raises the heat stability, surface gloss and dimensional stability of the material, allows higher filling levels, lowers viscosity, reduces resin consumption and cost, and at the same time imparts better corrosion resistance to the coating, lowers the baking temperature and shortens the baking time.

4.2 Monoalkoxy Phosphate Type GR-102

GR-102 is a monoalkoxy phosphate type titanate coupling agent; the corresponding overseas grade is KR-12 (Kenrich Petrochemicals, USA). GR-102 is suitable as a PVC flexibiliser with good heat stability and flame retardancy, and readily undergoes transesterification. In alkyd coatings it lowers the baking temperature and shortens the baking time; it disperses pigments well and provides good anti-settling, and its effect in preventing the settling and caking of acrylic baking enamels is particularly marked. Used to treat inorganic fillers such as calcium carbonate, barium sulphate and talc, it improves the dispersion and adhesion of the filler in the coating, improves processing flow and raises the mechanical strength of the coating.

4.3 Monoalkoxy Pyrophosphate Type GR-201

GR-201 is a pyrophosphate type titanate coupling agent; the corresponding overseas grade is KR-38S (Kenrich Petrochemicals, USA). It is compatible with semi-polar and polar materials and has flame-retardant properties. When used to treat inorganic fillers such as calcium carbonate and talc in filled resins, it improves processing rheology, permits high filler loading and raises mechanical performance. In coating production, this coupling agent improves the dispersibility of pigments, enhances the hiding power and tinting strength of pigments, and improves the corrosion resistance, water resistance and fire resistance of the coating. In addition, it can be used in coating formulations as a coupling agent between inorganic fillers and resins, strengthening the bonding between filler and resin, improving the uniformity, stability and anti-settling of fillers in the coating, and greatly improving the film’s resistance to whitening and its surface finish.

5. Conclusion

Monoalkoxy titanate coupling agents are a class of multifunctional additives with the highest output and enormous development potential among titanate coupling agents. With their diverse functional groups, numerous product grades, broad applicability and low price, they hold broad prospects in the research, development and application of coatings.