Role of the aggregation behavior of hydrophobic particles in paper surface hydrophobation

Artikel i vetenskaplig tidskrift, 2015

Three types of hydrophobic particles with different charge, i.e. cationic (SP+), anionic (SP-) and amphoteric (SPA) have been synthesized by emulsion polymerizations and evaluated in paper surface hydrophobation performance (frequently referred to as surface sizing). The surface sizing evaluation was done according to a well-established process, i.e. mixing of the particles with negatively charged starch followed by application in a conventional puddle size press using a fine paper grade containing calcium carbonate filler as model system. Prior to the application, the particles were characterized by light scattering and the surface charge was determined by particle charge density titrations and ζ-potential measurements. The SP+ particles were determined to be 30. nm in diameter while SP- and SPA particles were around 65. nm. Their colloidal behavior in the presence of anionic starch differed. The SP+ particles formed aggregates via bridging flocculation up to a charge ratio of 1:1 of starch:particles. At higher starch content the aggregates were partly redispersed. The SP- and SPA particles showed no sign of aggregation in the presence of anionic starch. In addition, the sizing performance of the different particles was evaluated by assessing the decrease of water uptake in a surface treated paper as well as water contact angle measurements on the paper surface. All three types of particles decreased the water penetration. However, the SP+/starch mixtures showed superior performance, which was attributed to a stronger sensitivity to the high electrolyte concentration usually found in the vicinity of the paper surface when the semi-soluble minerals composing the filler are exposed to water.