Dynamic mechanical properties of paper containing a polyacrylamide dry-strength resin additive and its distribution within a fiber wall: Effect of the application method

Abstract

Laboratory handsheets made from lightly beaten hardwood kraft pulp containing various amounts of cationic-type polyacrylamide (PAM) dry-strength resin were prepared by both internal and external application methods. We performed the internal application by adding a dilute aqueous PAM solution to a pulp fiber suspension, whereas we performed the external application by dipping a dry paper (handsheet) into an aqueous PAM solution and further squeezing it out with or without the standard wet press. Attenuated total reflection/Fourier transform infrared analysis combined with a gradual etching method was used to clarify the difference in the depth profiles of PAM within a fiber wall between these application methods. For the internal application method, PAM existed, on the whole, mainly close to the fiber surface and slightly distributed from the fiber surface toward the center of the fiber wall. On the other hand, for the external application method, PAM existed mainly on the fiber surface and around the fiber-to-fiber bonds. Dynamic mechanical properties of the papers containing PAM were measured at temperatures ranging from 100 to 300°C and at the various frequencies. In case of addition by the internal method, no characteristic viscoelasticity of PAM appeared. On the other hand, external method addition at the nearly same retention level of PAM gave the characteristic viscoelasticity. These findings suggest following things: when cationic-type PAM dry-strength resin was applied by the internal method, the PAM distributes within the fiber wall in a molecularly dispersed state. The induced state of PAM within the fiber wall, with no existence of a phase of PAM, led to the disappearance of the viscoelasticity of the PAM. On the other hand, the external application method brought about the PAM distribution around the fiber-to-fiber bonds and over the fiber surface. The induced state of PAM, the existence of a kind of PAM phase, led to the appearance of the viscoelasticity of the PAM itself. Dynamic mechanical analysis may be a good method for examining whether PAM is molecularly distributed or making aggregates in paper and other polymer composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008