INCREASING TENSILE-STRENGTH WITHOUT LOSING OPACITY

Papermakers improve the tensile strength of paper by beating pulp fibers in a refiner. The beating process fibrillates the fibers, increasing the number of potential bonding sites and thus enhancing the strength of the finished sheet. However, this occurs at the expense of sheet opacity-there is less fiber surface area to scatter the light reflecting off the sheet surface. One way to sidestep this problem is to increase bonding without fibrillating the fibers. This report shows that bonding can be increased by depositing positively charged latex particles on unbeaten fibers. When sheets made from these fibers are formed and dried, the latex particles coalesce to form a film that enhances the bonding between fibers without diminishing the sheet's opacity. The treatment also makes the fibers water repellent, so the latex performs as both a sizing agent as well as a bonding agent. Alince speculates that the latex film increases sheet strength without changing sheet opacity because it brings fiber surfaces into molecular contact without changing the area of optical contact.