Comprehensive Study on Mechanism of Paper Strength Development by the Addition of a Cationic Polyacrylamide Dry Strength Resin

In order to understand in detail mechanism of strength development by addition of a cationic polyacrylamide-based dry strength resin (PAM) at the internal application method, effects of PAM content and beating degree of fibers on the mechanical properties of papers made from bleached hardwood kraft pulp and further depth profile of PAM within a fiber wall were comprehensively investigated. The PAM retention in this study was quite high at lower addition levels under ca. 1% irrespective of beating degree, but decreased with decrease of beating degree at higher addition level. Measurements of an attenuated total reflection⁄Fourier transform infrared (ATR⁄FT-IR) combined with sputter etching and an electron spectroscopy for chemical analysis (ESCA) indicated that PAM tended to be distributed more inner side of fiber wall with increase in degree of beating. The retained PAM increased mechanically bonded area between fibers and further in some extent increased force holding capacity per bonded area for the papers from unbeaten and lightly beaten pulps. On the other hand for the papers from heavily beaten pulps, PAM addition caused a further increase of optical contact but no further increase of the mechanically bonded area. The increase in bonding strength per unit mechanically bonded area was a major cause of the strength development by PAM addition for the papers from lightly beaten pulps. The marginal effect of PAM addition on development of tensile strength for the papers from heavily beaten pulps should be related to the result that PAM content on and⁄or nearby the surface of fibers decreased with increase in beating degree. The increase in tearing strength by PAM addition for the papers from unbeaten and lightly beaten pulps would be derived from mainly an increase in bonding strength per unit mechanically bonded area rather than the increase of the bonded area. On the other hand, the retained PAM for the well-bonded papers from moderately and heavily beaten pulps gave constant tearing strength or a rather decrease in the strength, irrespective of the increase in bonded area probably due to stress concentration.