The increase of filler content in paper has been driven by reducing papermaking cost, improvements in optical properties, printability, and the water removal efficiency during papermaking. However, as filler loading is increased, paper strengths, bulk, and stiffness are usually decreased. To increase the proportion of filler in papers without sacrificing paper properties, various methods such as preflocculation, lumen loading, chemically treated filler with polymer and filler-wood fine composites have been explored. Although many approaches have been studied for improving the paper strength at high filler contents, except for the preflocculation method, none of these technologies has delivered a practical solution to paper manufactures.

Traditionally, starch has been used in wet-end papermaking applications because of its relatively low price and its ability to improve paper strength. Although starch can improve the try strength of papers, there are some problems related to high starch addition in wet end papermaking process. For example, the retention of raw starch in a pulp furnish is very poor. In order to enhance the adsorption of starch to anionically charged wood fiber surface, the cationic groups are usually introduced to starch's backbones. However, such chemical modification of starch significantly increases the cost of papermaking and the amount of cationic starch on wood fibers is limited by its maximum adsorption amount. Furthermore, if the retention of starch cannot be well controlled, unretained starch will accumulate in the whitewater to create pitch, slime, and sticky problems.

Recently, we reported a method for modification of precipitated calcium carbonate (PCC) or clay with coated starch. The results indicated that the strength of PCC or clay filled sheets could be significantly improved if the filler surface was coated by starch. In this work, we suggest an easier and lower cost method for coating starch on filler for paper strength improvement.

Different aggregates of clay-starch composites were prepared by starch-fatty acid complexation. These composites were used as a papermaking filler to improve the physical properties of the paper, filler retention, and the paper sizing. The solubility of the starch-fatty acid complex in water at different temperatures was measured. The results indicated that the starch-fatty acid complexes have very low solubility in water below 70oC, which can be easily coated on clay surface to modify clay-fiber bonding ability. The clay-starch composite filled handsheets showed that paper strength could increase more than 100~200% compared to untreated clay. It was found that ZDT of the handsheet decreased as the clay content increased when unmodified clay was used, but it increased when the starch-fatty acid composite modified filler was used. It was also found that the presence of fatty acids in the complex increased the water-repellant property of the handsheet, which can be used to aid in sizing during papermaking. Filler distribution and bonding characteristics between the composite and fiber were investigated using Scanning Electron Microscopy (SEM).