Effect of ink types and printing processes on flotation deinking

Examination of ink types reveals that newsprint oil-based offset-cold and offset-heat inks contain substantial oil (45 to 60%) and resin (5 to 35%), water-based ink contains water (40%) and resin (polystyrene, 30%). These inks are liquids with densities around 1 g/cm3, having various viscosities which range from 10 to 35 Pa s at 25 °C. On the other hand, photocopy and laser-print toners are solid powders with particle diameters of about 20 u.m, densities of 1 to 1.5 g/cm3, and a composition which consists primarily of styrene-acrylate copolymers (60 to 90%). Except water-based ink, which is water soluble, all inks in their initial state tend to be hydrophobic. As such it should be possible to separate them from hydrophilic cellulose fibers by flotation. During the printing process, however, changes in ink properties occur. Newsprint oil-based offset-cold ink is simply absorbed by pores in the paper; water-based ink and oil-based offset-heat ink are physically dried by evaporation at a temperature less than 60 °C. For these inks there is little change in the surface chemical properties and so their hydrophobic character is retained. On the other hand, photocopy and laser-print toner ink particles are thermally/photothermally fused and bonded to the cellulose fiber during the printing process. Due to exposure to heat, light, and oxygen (air), the toner particles undergo polymerization (fusion) and oxidation with the subsequent formation of peroxide bonds. The fusion due to polymerization causes bonding to the paper fibers and the creation of larger toner particle sizes. The oxidation creates a greater polarity at the toner particle surface. These effects account for the poor efficiency in the flotation deinking of office waste. It is evident that ink types and printing methods can have a significant effect on the efficiency of flotation deinking operations.