Experimental and modeling study of the strength properties of clay-starch composite filled papers

To increase the filler content in paper without sacrificing paper properties, clay-starch composites, made from a simple precipitation method, were used in papermaking. Compared to untreated clay, the clay-starch composite filled paper improved paper tensile strength up to 100-200%. Contrary to untreated clays that usually reduce the ZDT (z-directional tensile strength) of papers, the clay-starch composite actually increases the ZDT. The significant improvements in the paper strengths are attributed to the formation of a fiber-starch-filler sandwich structure, which avoids direct contact between filler and fiber. The specific shear bond strengths (expressed as specific bond strengths) between fillers and fibers were measured and used as the main variables for the modeling. It was also found that the fiber-fiber specific strength is weaker than the fiber-starch specific bond strength. The tensile strength (expressed as breaking length) was modeled by using a modified Page equation. The experimental results agreed very well with the modeling results.