Encapsulation of submicron-sized TiO2 via in-situ emulsion polymerization to enhance the dispersion stability and reflectance

Titanium dioxide (TiO₂) is widely used as a white pigment in water-based paint and printing ink formulations because of its high brightness, whiteness, and refractive index (2.7 at 599 nm) [1]. Additionally, the exceptional physicochemical properties of titanium dioxide have made it a highly sought-after material in various industries. Its versatile nature has led to its use in the production of paper, plastic, synthetic fibers, pharmaceuticals, and cosmetics [2,3].

     Due to its exceptional light scattering, hiding power, and reflectivity, titanium dioxide is massively used in the paint industry as a white pigment to formulate cool coatings [4].  Coatings with high solar reflectivity are commonly referred to as cool coatings. The use of white coatings on the exterior surfaces of buildings assists in reflecting a significant portion of the incoming sunlight during the day. This reflection helps to minimize heat accumulation on the surface, consequently, reduces heat transfer into the building. As a result, mitigate the urban heat island effect, and enhance the overall thermal comfort of urban living [5]. This white coating also indirectly contributes towards reducing CO₂ emissions and mitigates global warming [6].

     The prime limitation to the efficient use of TiO₂ is its higher density (4.3 g/cm³) which causes it to settle 50 to 80 times faster than organic pigments in dispersion media, and aggregates during the drying process. The agglomeration dramatically reduces the light scattering efficiency of TiO₂, besides adversely affects on mechanical, electrical, and other properties of the coating [4]. Several techniques can be employed to enhance TiO₂ dispersion and minimize aggregation, including the use of steric extenders or spacers, or various types of dispersants. However, those strategies have many limitations, necessitating extensive reformulation. Encapsulation of TiO₂ with a polymeric shell has emerged as a promising solution to prevent agglomeration of the particles during the drying process. The polymeric shell acts as a barrier and hinders the formation of agglomerates [7]. Besides, the encapsulation with selective (co-)polymers is expected to result in superior ultimate dispersion of TiO₂ pigment both in water and organic media due to the more compatible surfaces that make more preferable to formulate coatings. However, a great challenge remains in the encapsulation of pigment particles in the water-based system.

     Numerous approaches have been investigated to encapsulate TiO₂ particles in a water-based system, but they represent several drawbacks and are quite challenging. For example: requires low concentration in the layer-by-layer (L-b-L) deposition method; mini-emulsion polymerization is challenging to encapsulate submicron-sized TiO₂;   RAFT agents cause coloration of TiO₂ in RAFT polymerization; and in conventional emulsion polymerization, chemical surface treatment of TiO₂ is required before encapsulation [8,9]. So, one promising solution is the direct encapsulation method using emulsion polymerization, which is a straightforward, cost-effective, convenient, and environmentally friendly approach.

Therefore, in this study, we will encapsulate TiO₂ (250-350 nm) by in-situ emulsion polymerization using different types of monomers. After that, the dispersion stability in both aqueous and organic media, and the reflectance of the encapsulated particles will be studied.

Research Gap:

Generally, the encapsulation of TiO₂ is conducted by emulsion polymerization after chemical surface treatment of TiO₂ in an organic media, which is time-consuming, costly, and harmful to the environment due to the use of organic solvent [9,10]. Besides, most of the time, the dispersion stability of synthesized hybrid particles is not clearly described. Therefore, this research project will minimize these stated research lackings.

• Research Project