Characterization and Application of TiO2/ZnO Nanoparticles as Pigments in Matt-Type Water-Based Paint

     This study investigates the characteristics of TiO₂/ZnO nanoparticles and the application of TiO₂/ZnO in matt-type water-based paint as a pigment. This study aims to determine the quality of TiO₂-ZnO pigments in water-based paints in terms of whiteness, hiding power, dispersibility, and gloss. Matt-type water-based paint has been made by mixing water and additives substances in a high-speed mixer to result in a mill base paste, which is mixed with the pigment of TiO₂/ZnO (25:75), CaCO₃ filler, water, and additives, then filtered and mixed again with the C-817 binder. The characterization results showed that TiO₂/ZnO nanoparticles contained TiO₂ of 18.62% and ZnO of 77.49%, with an average particle diameter of 151.54 nm. TiO₂/ZnO has a crystal size of 28.4 nm and a crystallinity degree of 72.3%. The application of TiO₂/ZnO nanoparticles as a pigment in matt-type water-based paint resulted in good dispersion and hiding power and achieved whiteness of 82.2 and gloss at 60° of 2.00 better than TiO₂ of 81.8 and gloss at 60° of 1.77.

Matt-type water-based paint; Pigment; TiO2; TiO2/ZnO; ZnO

Paint is a complex mixture consisting of resins, pigments, solvents, fillers, and other additives used to coat the surface of a material to beautify, strengthen, or protect the material. There are two types of paint, namely water-based paint and solvent-based paint. Water-based paint commonly used is the matt type which gives an even and non-reflective finish. This matt water-based paint is the most popular choice because it is easy to apply on smooth interior surfaces. The matt-type application is generally used on ceilings and walls with an even finish and a sheen level of less than 10%, giving it a non-reflective appearance (Chen et al., 2022; Peruchi et al., 2021).

In the paint industry, the main white pigment popularly used is titanium dioxide (TiO₂) because of its whiteness, high coverage, refractive index, covering power, achromatic force, good dispersion, and resistance to ultraviolet (UV) rays (Islam et al., 2020; Karakas and Çelik, 2018). These advantages have caused TiO_2­ to be widely developed and researched and become the main choice in its application as a pigment compared to zinc oxide (ZnO), lithopone, and others (Gao et al., 2022; Costa et al., 2017). 

The TiO₂ cannot be obtained naturally but is extracted from ilmenite ore. TiO₂ can also be synthesized using the sol-gel method (Solanki et al., 2021; Yuwono et al., 2014), sonochemical (Rosales et al., 2021),  hydrothermal, solvothermal (Mamaghani, Haghighat, Lee, 2019; Sofyan et al., 2018, 2019), microwave (Li et al., 2021), co-precipitation (Bhogaita and Devaprakasam, 2021), and direct oxidation (Daraee et al., 2018). However, the synthesis of TiO₂ is limited by the environmental pollution generated by industrial processes, the shortage of titanium resources, and the high selling price. Therefore, it is necessary to develop TiO₂ by mixing it with other white pigments with better or almost similar quality to reduce the high demand for TiO₂ nanoparticles (Razali et al., 2022;  Isfa et al., 2022; Dell’Edera et al., 2021; George et al., 2021).

Zinc oxide (ZnO) is the main white pigment that has long been used in the organic paint industry because of its environmentally friendly nature. ZnO is naturally found in the form of the mineral zincite. The advantages of ZnO as a pigment are that it is stable against UV rays (does not change color), long lasts on both water-based and oil-based paints, and increases color retention. Compared with the other white pigments, ZnO has the least tendency to turn yellow (Papp et al., 2022; Ma et al., 2019; Adiwibowo, Ibadurrohman, Slamet, 2018). Based on the advantages of ZnO as a pigment, it is necessary to develop research related to ZnO combined with TiO₂ to result in TiO₂/ZnO nanoparticles and implement the use of matt-type water-based paints. The novelty in this work is related to the application of TiO₂/ZnO mixtures as a pigment in matt-type water-based paints.

A few previous studies related to the development of TiO₂/ZnO nanoparticles have been reported, including Miklecic et al. (2015), which studied the effect of TiO₂/ZnO nanoparticles on the properties of waterborne polyacrylate coatings in outdoor conditions, where the existence of ZnO nanoparticles reduced coating flow time, increased pH, and decreased elongation. In contrast, the TiO₂ increased the glass transition so that the combination of TiO₂/ZnO increased color stability. El-Kader et al. (2021) observed morphological, structural, and antibacterial on TiO₂/ZnO (50:50) nanocomposites from Hibiscus rosa-senensis extract, which produced an increase in the crystal size growth of ZnO with uneven distribution and irregular particle shape. It is different with the crystal size growth of TiO₂ was inhibited the distribution particles and shape were evenly and uniform. Baudys et al. (2015) conducted a weathering test and photocatalytic activity on self-cleaning acrylic paint based on ZnO and TiO₂, where the results showed that the paint sample TiO₂-based pigment increased photoactivity during exposition on the QUV panel, while the paint sample ZnO-based ones showed a high initial photocatalytic activity but decreased during exposition in the QUV panel. Lv et al. (2019) studied radiation stability in solar reflective coatings using TiO₂ and ZnO pigments, where the particle size affects the stability of the paint coating from radiation. Using TiO₂/ZnO mixture as pigments in acrylic paints has achieved better photocatalytic activity than pure TiO2 and ZnO (Song et al., 2021; Jašková, Hochmannová, Vytrasova, 2013). The objective of this study is to characterize TiO₂/ZnO nanoparticles and observe the quality improvement of TiO₂/ZnO nanoparticles applied as pigments in matt-type water-based paints. 

2.1. Materials

    Titanium dioxide rutile (SR-2377) was obtained from Shandong Dongjia Group Co., Ltd. Shandong, China. Zinc oxide (ZnO) was supplied by Evergreen Chemical Factory Co., Ltd. Shaanxi, China. Calcium carbonate (Omyacarb-6 GD) as a filler was obtained from PT. Camco Omya Indonesia, Jakarta, Indonesia.

   The styrene-acrylic binder (C-817) and super-plasticizer (SPC) were received from PT. Inawan Chemtex Sukses Abadi, Jakarta, Indonesia. The optical brightener agent (OBA) was supplied by PT. Graha Jaya Chemical, Jakarta, Indonesia. Dispersing agents, anti-foaming agents, wetting agents, coalescent agents, antimicrobial agents (biocide), thickening agents, and ethylene glycol are used for commercial water-based paint recipes.

2.2. Nanoparticles Characterization

    The chemical composition of the pigments (TiO₂, ZnO, and TiO₂-ZnO) was analyzed by X-Ray Fluorescence (XRF) - PANalytical Epsilon 3. The pigments' crystal structure, size, and crystallinity degree were determined by X-Ray Diffraction (XRD) - PANalitycal X'Pert PRO MPD. Morphological of the pigments were characterized by Field Emission - Scanning Electron Microscope (FE-SEM) - FEI Inspect F50. The TiO₂-ZnO (25:75) nanoparticle pigments characterized were made from a mixing process using a planetary mixer (KNS-60 LB).

2.3. Manufacturing Process of Matt-Type Water-Based Paint

    Water, thickening agent, wetting agent, dispersing agent, antimicrobial agent, anti-foaming agent, and ethylene glycol were added to a high-speed mixer according to Table 1, stirred using a zirconium ball mill at 2000 rpm for 1 hour, filtered with a sieve (mesh 200) and allowed to cool at room temperature. The results obtained are in the form of mill-base paste. The mill base paste, pigment (TiO₂, ZnO, or TiO₂-ZnO), optical brightener agent, super-plasticizer, CaCO₃ filler, water, and coalescing agent were added into a high-speed mixer according to Table 1, stirred at 800 rpm for 30 min, filtered with a sieve (mesh 200), and dispersibility of the pigment was observed. Next, the C-817 binder was added and stirred at 100 rpm for 15 min. The results obtained are matt-type water-based paint with various pigments, referred to as a P sample (Karakas and Celik, 2018; Somtürk et al., 2016).

Figure 1 Preparation and application of TiO₂/ZnO in matt-type water-based paint

2.4.  Application of Nanoparticles in Matt-Type Water-Based Paint

       Matt-type water-based paint is applied to the surface of the panel paper using a rod coating (100), allowed for 24 h, and the obtained result was visually observed the hiding power in panel paper. Next, whiteness and gloss were measured using a whiteness meter (BGD 585) and a gloss meter (BGD 516).

3.1. XRF Analysis of Pigments

     The results of XRF analysis on TiO₂, ZnO, and TiO₂/ZnO nanoparticles (Table 2) displayed that rutile had a TiO₂ concentration of 94.57%. Zinc oxide nanoparticles had a ZnO concentration of 96.11%. In the TiO₂-ZnO mixture, the concentration of TiO₂ is 18.62%, and ZnO is 77.49%. These results indicate that the mixing of TiO₂/ZnO with a composition of 25:75 showed no significant difference with the concentration of TiO₂/ZnO in the mixture, which means that there is no chemical interaction. The concentration of TiO₂ rutile was not significantly different from previous research (Sisca et al., 2021; Miklecic et al., 2015).

Table 2 Chemical Composition of TiO₂, TiO₂/ZnO, and ZnO

The crystallinity degree, size, and shape crystal of TiO₂, TiO₂/ZnO, and ZnO were analyzed by XRD. The XRD pattern is displayed in Figure 2. The characteristic peaks of TiO₂ at 2 are 27.43°, 36.08°, 39.21°, 41.26°, 44.09°, 54.36°, 56.67°, 62.81°, 64.11°, 69.06°, 69.86°, 82.40°, and 89.63°. The peaks are suitable to the characteristic peak of rutile (01-087-0290), which has a crystallinity degree of 24.0 % and a crystal size of 29.8 nm with a tetragonal shape. This result also corresponds to previous reports (Razali et al., 2022; Daniyal, Akhtar, Azam, 2019).

The characteristic peaks of TiO₂/ZnO at 2? are 27.41°, 31.74°, 34.40°, 36.23°, 44.00°, 47.54°, 54.33°, 56.61°, 62.87°, 66.40°, 67.99°, 69.10°, 72.47°, 76.99°, 89.62°, 92.71°, 95.34°, and 98.57°. The peaks are suitable to the characteristic peak of TiO₂ rutile (01-072-4814) and zincite (03-065-3411), which has a crystallinity degree of 72.3 %, a crystal size of 28.4 nm with tetragonal and hexagonal shapes. This result corresponds to crystals of TiO₂/ZnO (El-Kader et al., 2021; Mazabuel-Collazos, Gómez, Rodríguez-Páez, 2019).

The characteristic peaks of ZnO at 2are 31.70°, 34.36°, 36.19°, 47.49°, 56.56°, 62.83°, 66.37°, 67.92°, 69.05°, 72.58°, 76.90°, 81.43°, 89.61°, 92.65°, 95.24°, and 98.55°. The peaks are suitable to the characteristic peak of zincite (01-075-7917), which has a crystallinity degree of 64.1 % and a crystal size of 25.4 nm with a hexagonal shape. This result corresponds to previous reports (El-Kader et al., 2021).

In TiO₂/ZnO pigment, there was a decrease in crystal size, which is not significantly different from TiO₂ rutile, so TiO₂/ZnO can achieve a good whiteness and is not significantly different when applied to water-based paint. The high crystallinity degree of TiO₂/ZnO can increase the gloss of water-based paints compared to TiO₂ pigments.

Figure 2 XRD of (a) TiO₂; (b) TiO₂/ZnO; and (c) ZnO

3.3.   SEM Analysis of Pigments

The morphological analysis results of nanoparticles using FE-SEM with a magnification of 10,000x (10 µm) and 100,000x (1 µm) (Figure 3) has displayed that all pigments have very small sizes (<200 nm), smooth, and uniform shapes (Mazabuel-Collazos, Gómez, Rodríguez-Páez, 2019; Karakas and Çelik, 2018; Mikle?i? et al., 2015).

The nanoparticle diameter distribution was measured using the ImageJ and originLab application based on the SEM image result (Figure 4). The nanoparticles diameter of TiO₂ is 80.98 – 469.04 nm with an average of 182.07 nm, TiO₂/ZnO is 67.61 – 446.20 nm with an average of 151.54 nm, and ZnO is 57.93 – 300.62 nm with an average of 127.96 nm. These results indicate that the diameter of the three pigments is smaller than that of a sieve (mesh 200) and suitable for use in the preparation of matt-type water-based paints because the pigments can be ensured that evenly distributed, which is indicated by the absence of pigments left on the sieve (El-Kader et al., 2021; Miklecic et al., 2015).

Figure 4 Diameter distribution of (a) TiO₂; (b) TiO₂/ZnO, and (c) ZnO

3.4.   Application of Nanoparticles as Pigments in Matt-Type Water-Based Paint Against Dispersibility, Hiding Power, Whiteness, and Gloss

Note:

*    Evaluation of hiding power results is carried out based on visual observations of the paint coating on panel paper, which refers to the standard color of white and black.

**   Evaluation of pigment dispersibility was carried out based on the number of nanoparticles left on the sieve (mesh 200), which were observed visually.

Figure 5 A Hiding Power of (a) P1 (TiO₂) & P2 (TiO₂/ZnO); (b) P1 (TiO₂) & P3 (TiO₂/ZnO/OBA/SPC); and (c) P1 (TiO₂) & P4 (ZnO/OBA)

Figure 6 (a) Whiteness and (b) Gloss of Water-Based Paint with Various Pigments

Reflection angles of 20° and 60° in sample P2 have resulted in better gloss values ??than other samples (Figure 6b), which is influenced by the high crystallinity degree of TiO₂/ZnO nanoparticles. Meanwhile, at a reflection angle of 85°, sample P3 has resulted in a significant increase in the gloss value of 6.57, which was influenced by the presence of OBA and SPC in sample P3.

In terms of dispersibility, hiding power, whiteness, and gloss, it is known that sample P1 using TiO₂/ZnO nanoparticles as a pigment has better quality than sample P1 when applied to matt-type water-based paint. The amount of pigment in water-based paint formulas commonly for the matt-type is 7-10%, semi-gloss is 20-25%, and gloss is above 35%. With a gloss value below 10%, the water-based paint was categorized as a matt-type.

The TiO₂/ZnO nanoparticles have been successfully combined with the composition (25:75), which resulted in a crystallinity degree of 72.3%, a crystal size of 28.4 nm with tetragonal and hexagonal shapes. These nanoparticles contain 18.62% TiO₂ and 77.49% ZnO compounds, with an average particle diameter of 151.54 nm. TiO₂/ZnO nanoparticles also have been successfully applied as pigment to a matt-type water-based paint which achieved good dispersibility and hiding power and resulted in a whiteness of 0.48% and a gloss at 60° of 12.99% were better than TiO₂. In the future, a TiO₂/ZnO mixture will be made using a chemical method applied to a matt-type water-based paint with a composition according to the current works.

    The author would like to thank PT. Inawan Chemtex Sukses Abadi for the opportunity to conduct research and use the facilities in the Technical Department Laboratory.

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