Mineral fillers are used in paper making to provide structure, surface finish and a bond between plant fibers, making the paper stronger. A recent study has shown that Egyptian talc, modified with phthalic anhydride and urea, provides a better bond between the fibers and higher resistance to water (Maha M. Ibrahim et al, doi:10.1016/j.carbpol.2008.07.007).
Papers consist of a network of bonded cellulose plant fibers, forming an extremely complicated structure. Fillers of fine mineral particles, such as clay, calcium carbonate and titanium dioxide, as well as a number of other substances, are added to enhance the properties of papers to strengthen and stabilise the structure and provide a suitable surface for writing or printing. Different substances are used to achieve brightness or opacity as well as texture.
Talc possesses ideal qualities for paper making but, being chemically inert, produces a weak bond with the paper fibers. During the study, made by researchers at the National Research Center, Giza and Helwan University, Cairo, the mechanical properties of paper were improved by the modification of Egyptian talc to form amphiphilic filler that has two different end groups, hydrophilic and hydrophobic.
The modified talc was used as internal treatment which led to high water resistance, due to the orientation of the hydrophobic groups to face the water face, and better mechanical properties due to the formation of a better fiber-filler-fiber bond. The modification produces a phthalimide which acts as a amphiphilic copolymer, forming a bond between the NH group of the modified talc and the OH group of the cellulosic fiber. This results in a good fiber-to-filler-to-fiber bond, increasing paper strength and consistency. The formation of the plasticized mat over the cellulosic fiber reduces water penetration.
“It was clear that modification of the talc plays a role in improvement of the bond between fibers and filler, where the performance of the modified talc depends on the nature of the group present after modification,” concludes Dr Maha Ibrahim, leader of the Egyptian team. Commenting on follow-up work, he says, “Our next step will be to produce indicator paper for humidity by modifying the kaolin with transition metals. Another modification for the kaolin and talc will be carried out to change its nature for enhancing the mechanical properties of the produced paper.”