Developing cellulose-based opacifiers by chemical methods

Abstract

Opacifiers are added to different materials, such as coatings, paints, cosmetics and plastics to make them opaque. Currently the most common and effective opacifier is titanium dioxide, due to its high refractive index. According to previous research, causing porosity on low refractive index polymers, such as cellulose, will increase the light scattering properties of the polymer.

This thesis studied the development cellulose-based opacifiers that could possibly substitute the harmful TiO2 in the future. Microcrystalline cellulose (MCC) was treated with sodium hydroxide (NaOH) and some samples were additionally treated with probe ultrasonication. The aim was to introduce porosity roughly in the size range of half of the wavelength of visible light (200–350 nm) and with the porosity to increase the light scattering and whiteness of MCC. Parameters such as concentration, time and temperature of the NaOH treatment and ultrasonication time were changed to create 8 different final samples. These final samples were then studied along with untreated MCC and TiO2 as reference with different analysis methods, to figure out the porosity, pore size distribution and optical properties, and how the different treatment conditions had affected them.

NaOH treatment alone and combined with ultrasonication were both able to create porosity in the range of 0–1000 nm, also in the optimal range of 200–350 nm. Most treated cellulose samples had substantially better optical properties compared to untreated MCC. Better results were achieved using a 14 % NaOH solution compared to a 20% solution, and when the NaOH treatment was performed at room temper-ature rather than in an ice bath. In addition, a correlation between increasing po-rosity and increasing opacity was found. However, TiO2 was still superior compared to cellulosic opacifiers, but with further optimization of treatment conditions, the performance of cellulose-based opacifiers could still be increased.

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Materials

3.1 Materials

MCC PURA Series™: 0–100 µm, 100–200 µm and 200–1000 µm particle sizes (Nordic Bioproducts Group Oy) Sodium hydroxide ≥98%, pellets, GPR RECTAPUR® (VWR Chemicals) Etax A, min 94 w-% (Anora Group Oyj) Kiva 70 furniture lacquer (Tikkurila Oyj) TiO2 slurry, 69% solids, density 2000 kg/m3, 380 mPas BR100, pH 8.18 (Kronos)

Eveliina Palo, Master’s Programme in Chemical, Biochemical and Materials Engineering, Developing cellulose-based opacifiers by chemical methods, Copyright ©2025 Eveliina Palo, 2025, https://aaltodoc.aalto.fi/items/eef4330d-55bb-4b55-8e78-e4e79b5f4887