Papermaking potential of novel structured PCC fillers with enhanced refractive index

While much effort has been focused on optical properties imparted by particle size distribution, shape, and packing of inorganic particles, the refractive index contrasts within the paper matrix have received less attention. PCC fillers can be used as tools for increasing the refractive index contrast, which further improves paper optical properties. We studied the papermaking potential of novel PCC fillers with an enhanced refractive index. PCC samples were fabricated by conventional carbonation method in the presence of refractive-index enhancing additives. We had previously found that introduction of ZnO as fine spots, including nano-sized structures, on host PCC surfaces improved the effective refractive index and optical performance of PCC. Now, we have studied the papermaking potential of this approach in experiments with handsheets using moderate Zn modification. In addition to Zn, we studied the effect of Sr as an additive and found that Sr inclusion alters the host particle morphology and crystal structure. We report effects of such modifications on light scattering coefficient, brightness, opacity, tensile strength, bulk, and porosity of paper, both before and after calendering. Our results suggest beneficial optical properties, as well as combination of light scattering with strength properties, especially in the case of Sr modified filler. Zn modified fillers opened the sheet structure, while Sr structuring closed the sheets. Calendering resulted in reduced bulk with all fillers, while optical properties were maintained with the Sr modified PCC, but reduced benefit was observed in the combination of light scattering and strength. Calendering conditions should therefore be optimized for these fillers. A higher modification degree of the fillers should also be considered to achieve a more significant refractive index contrast.