Particle-stabilized water-in-water emulsions were prepared by mixing dextran and poly(ethylene oxide) (PEO) in water and adding cellulose nanocrystals (CNC). The CNC formed a layer at the surface of the dispersed droplets formed by the PEO-rich phase. Excess CNC partitioned to the continuous dextran phase. Aggregation of CNC at different rates was induced by adding NaCl between 10 and 100 mM. In the presence of more than 2 g/L CNC, fast aggregation led to the formation of an emulsion gel showing no signs of creaming. Confocal laser scanning microscopy showed that the emulgels were formed by a continuous network of CNC in which the randomly distributed droplets were embedded. The gel stiffness was measured with oscillatory shear rheology and found to increase strongly with increasing CNC concentration ( C). The dispersed droplets were elastically active and increased the gel stiffness at low C. However, up to C = 10 g/L, the yield stress was too small to inhibit the flow when the gels were tilted. At C < 2 g/L, creaming was observed until the network of connected droplets became sufficiently dense to be strong enough to resist buoyancy.
Emulsion polymerization provides a sustainable way to produce latex polymers for coatings and adhesives thanks to the use of water as a dispersion medium. This synthesis approach can be even more attractive if synthetic surfactant can be replaced by biobased solid particles as a stabilizer, through what is known as a "Pickering effect". Herein, latex dispersions with solid content up to 35 wt% were successfully produced by emulsion polymerization using starch nanocrystals (SNCs) as a sole stabilizer and H2O2/citric acid as a redox-initiator. The effect of the SNC modification with vinyltriethoxysilane (VTES) on the colloidal properties of the polymer dispersion and performance of the resulting nanocomposite film were investigated.As an application of this approach, pressure-sensitive adhesive (PSA) dispersions have been prepared via Pickering emulsion polymerization in the presence of 8 wt% SNCs. The use of VTES-SNCs has a beneficial impact on the performance of PSAs with improved peel strength and wettability. The possibility to use SNCs as a stabilizer to replace synthetic surfactants in emulsion polymerization opens new avenues for the application of SNCs as biobased Pickering stabilizers to produce latex for coatings, adhesives, inks, and textiles.
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