**3.2 Coupling agent**

Coupling agents are amphoteric structural compounds, which can be divided into silane coupling agents, titanate coupling agents, aluminate coupling agents, etc. One end group of the coupling agent can react with the hydroxyl group on the surface of TiO2 particles to form a strong chemical bond. The other can react with the polymeric matrix. Consequently, two kinds of materials of different polarity, TiO2, and a polymer, are closely combined to give the composite material excellent comprehensive performance.

Silane coupling agents were first developed and used widely to modify TiO2. In the reaction, organic silicon is adsorbed on TiO2, and the molecule part reacts with the hydroxyl group on the surface of TiO2 to prevent the aggregation of particles. Wang et al. [35] reported the modification of TiO2 by three kinds of silane coupling agents (KH550, KH570, and HDTMS). The chemical structure and the reaction mechanism with TiO2 are shown in **Figure 5**(**a**). The results showed that TiO2 modified by silane coupling agent had small particle size, improved hydrophobicity, and low surface energy (**Figure 5**(**b**, **c**)). Furthermore, compared with raw TiO2 and KH550 coated TiO2, HDTMS-coated TiO2 and KH 570 coated TiO2 had excellent dispersion stability as white pigments in blue light curing inks (**Figure 5**(**d**)).

Sabzi et al. [36] used aminopropyl trimethoxyl silane (APS) as a coupling agent to modify TiO2. The results showed that silane coupling agents could significantly improve the dispersion of TiO2 in polyurethane composite and the mechanical properties of composite. Xuan et al. [37] reported the modification of TiO2 by vinyltrimethoxyl silane (A171) and the reinforcement of modified TiO2 on wheat straw fiber/polypropylene composite. The modified TiO2 could effectively improve the tensile, flexural, and impact resistance as well as the UV light stability of the composite. However, the thermal stability of the coupling agents is poor. This leads to the degradation of the organic layer on the surface of TiO2. Finally, the color and whiteness of TiO2 are changed.

**Figure 5.**

*(a) The scheme of reaction between silane coupling agents with different chemical structures and TiO2 particle, (b) particle size distribution of raw and modified TiO2, (c) contact angle of raw and modified TiO2-water interface, (d) dispersion of raw and modified TiO2 [35].*

### **3.3 Polymer**

The above two organic treatment methods depend on the reaction of small molecular modifiers with the surface groups of TiO2. In contrast, a modification with a macromolecule uses the polymer to coat the TiO2 particles directly or the reactive monomer to polymerize on the surface of TiO2 particles. In the coating with polymers, there is no interaction between the polymeric groups and TiO2, but the polymer induces a steric hindrance [38]. As a result, the dispersion of TiO2 in the subsequent polymer matrix is improved. TiO2 shows good pigmentary properties. The reaction mechanism and classification of polymer coating modification are collected in **Table 4**.

Man et al. [40] used the microcapsule method to modify TiO2. The *in situ* polymerization of acrylic monomer on the surface of TiO2 particles obtained the core-shell structure of modified TiO2. This core-shell structure TiO2 showed improved dispersion in organic media and excellent UV shielding ability. Olad et al. [41] used polyaniline (PANI) to modify TiO2 through *in situ* polymerization. The results showed that PANI was successfully implanted on the surface of TiO2, effectively inhibiting the aggregation of TiO2 nanoparticles.

In the "Anchor positioning" coating method, the polymers used are named hyperdispersant, which Schofield first proposed in the 1980s. Compared with the structure of traditional dispersants, such as surfactant SDS, with the hydrophilic and lipophile groups, hyper-dispersants have two completely different groups: anchoring group and


### **Table 4.**

*Modification by polymer coating [39].*

solvent group. The anchoring groups are anchored on the particles'surface by singlepoint or multipoint anchoring or co-anchoring with a surface synergist (**Figure 6**). At the same time, the solvent chain is extended in a nonaqueous system to provide steric stability. Therefore, the particles are stably dispersed (**Figure 7**). So, the hyperdispersants have a unique dispersion effect on the nonaqueous system.

There are a lot of different anchoring groups, and solvent chains can be designed to synthesize hyper-dispersant (**Tables 5** and **6**). Thus, hyper-dispersants with different effects can be designed and synthesized by selecting different anchoring groups and solvent chains.

Schaller et al. [43] modified TiO2 with poly(acrylic acid)-polystyrene block copolymer hyper-dispersants. It is proved that the end group of polymers will form some bond interactions with TiO2 particles, which improves the stability between polymer and TiO2 particles and then improves the dispersion of TiO2 in water.

### **Figure 6.**

*The anchorage form of hyper-dispersant on particle surface: (a) single-point, (b) multipoint, and (c) co-anchoring with a surface synergist.*

**Figure 7.** *The scheme of particle dispersion by using hyper-dispersant.*


**Table 5.**

*Electronegativity and section width of the anchoring group [42].*

Zhang et al. [44] synthesized three hyper-dispersants: nonterminated, carboxylterminated, and polyethylene imine-grafted poly(hydroxyl carboxylic acid) ester. It is found that polyethylene imine-grafted hyper-dispersant has the best dispersion performance in nano-TiO2/resin solution dispersion systems.

A novel acrylic polyester hyper-dispersant containing methacrylic acid (MAA), butyl acrylate (BA), and 3-pentadecylphenyl acrylate (PDPA) was polymerized by Liu et al. [45]. This hyper-dispersant was used to disperse TiO2 in a nonpolar solvent system. The results showed that the viscosity and particle size of suspensions were affected by monomer ratio and molecular weight. The optimum monomer ratio and molecular weight were MAA: BA: PDPA = 1:10:1.2 (wt%) and 6000, respectively. Liu et al. [46] further reported the effects of acrylic polyester hyper-dispersant on the


**Table 6.** *Solubility parameters of some polymer structural units [42].*

dispersion of TiO2 in different organic solvents. The results showed that acrylic polyester hyper-dispersant adsorption onto TiO2 is spontaneous and physical.
