Nano-TiO₂ Particles Stabilized Pickering Emulsions Of Reactive Sizing Agents

The primary particle size of nano-TiO₂ is about 25 nm and the mean size of its aggregate is approximately 260 nm when nano-TiO₂ particles are dispersed in water. The isoelectic point of nano-TiO₂ particles in aqueous dispersion is pH 6.4. The pH of particle dispersion is less than 6.4, the particle is thus positively charged. In the present paper the emulsification behaviors of reactive sizing agents AKD and ASA have been investigated using nano-TiO₂ as solid particle emulsifier. The prepared AKD and industry-specific ASA emulsions stabilized by nano-TiO₂ are applied in internal sizing and their sizing performance is exploited. The results show that the AKD emulsion can hardly stabilized by nano-TiO₂ particle alone or by chitosan with a lower dosage. However, the mixture of chitosan and nano-TiO₂ can well stabilize the O/W AKD emulsion when the oil/water ratio is 1:5 and the concentration of chitosan is 2.0 %. Meanwhile when the concentration of nano-TiO₂ is lower than 2.2 wt%, the mean size of droplets decrease with the increase of the concentration of nano-TiO₂; while the mean size of emulsion droplet keep constant when the particle concentration is higher than 2.2 wt%,. The AKD emulsion stabilized by nano-TiO₂ and chitosan has high sizing efficiency. When the particle concentration is lower than 2.2 wt%, the sizing degree of the paper increases with the increase of nano-TiO₂ concentration.Nano-TiO₂ particle can stable industry-specific ASA alone. When the content of ASA is lower than 50%, the prepared ASA emulsion is very stable. The mean diameter of ASA emulsion decreases with the increase of nano-TiO₂ concentration and reaches the minmum at 2.0 wt% of nano-TiO₂ concentration. The emulsion can impart a high sizing degree to handsheet and the sizing degree rises abruptly when the charge level of ASA increases from 0.4% to 0.5%. The emulsions with smaller droplet size perform better in sizing development. Thus the appropriate increase of nano-TiO₂ concentration can ameliorate the sizing efficiency of ASA emulsion. However the stability of the prepared emulsion to hydrolysis is very poor and within 50 min the emulsion loses nearly half of its sizing performance.Nano-TiO₂ particle can effectively stabilize reagent grade ASA. When the prepared emulsions are centrfuged at 3000 rpm for 15 min, the emulsions are stable to deemulsificaiton, creaming and sedimentation. The W/O emulsion can invert to O/W emulsion with increasing the mass fraction of water phase. The particle initial location, homogenization speed and particle concentration show remarkable effects on the required mass fraction of water phase for phase inversion. When the nano-TiO₂ particles are initially located in ASA, the mass fraction of water phase for phase inversion is lower than that when nano-TiO₂ particle initially dispersed in water. The results of diffusion experiment of nano-TiO₂ particle through ASA/water interface and infrared spectral analysis indicate that nano-TiO₂ particle has been modified in situ at ASA/water interface by alkenyl succinic acid which is the hydrolysate of ASA. As a consequence the particles are more"hydrophobical"when they are initially dispersed in water. Higher homogenization speed provides more energy to invert to the emulsion from W/O to O/W at a relative low mass fraction of water phase. While at higher particle concentration, the invertion from W/O to O/W ocurrs at higher mass fraction of water phase due to the relative high viscosity of the system.